CA2008899C - Electrically pumped vertical cavity laser - Google Patents
Electrically pumped vertical cavity laserInfo
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- CA2008899C CA2008899C CA002008899A CA2008899A CA2008899C CA 2008899 C CA2008899 C CA 2008899C CA 002008899 A CA002008899 A CA 002008899A CA 2008899 A CA2008899 A CA 2008899A CA 2008899 C CA2008899 C CA 2008899C
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S5/00—Semiconductor lasers
- H01S5/10—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region
- H01S5/18—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities
- H01S5/183—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities having only vertical cavities, e.g. vertical cavity surface-emitting lasers [VCSEL]
- H01S5/18361—Structure of the reflectors, e.g. hybrid mirrors
- H01S5/18375—Structure of the reflectors, e.g. hybrid mirrors based on metal reflectors
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/09—Processes or apparatus for excitation, e.g. pumping
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L2224/00—Indexing scheme for arrangements for connecting or disconnecting semiconductor or solid-state bodies and methods related thereto as covered by H01L24/00
- H01L2224/01—Means for bonding being attached to, or being formed on, the surface to be connected, e.g. chip-to-package, die-attach, "first-level" interconnects; Manufacturing methods related thereto
- H01L2224/42—Wire connectors; Manufacturing methods related thereto
- H01L2224/47—Structure, shape, material or disposition of the wire connectors after the connecting process
- H01L2224/48—Structure, shape, material or disposition of the wire connectors after the connecting process of an individual wire connector
- H01L2224/4805—Shape
- H01L2224/4809—Loop shape
- H01L2224/48091—Arched
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S5/00—Semiconductor lasers
- H01S5/02—Structural details or components not essential to laser action
- H01S5/0206—Substrates, e.g. growth, shape, material, removal or bonding
- H01S5/0207—Substrates having a special shape
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S5/00—Semiconductor lasers
- H01S5/02—Structural details or components not essential to laser action
- H01S5/022—Mountings; Housings
- H01S5/0225—Out-coupling of light
- H01S5/02251—Out-coupling of light using optical fibres
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S5/00—Semiconductor lasers
- H01S5/10—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region
- H01S5/18—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities
- H01S5/183—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities having only vertical cavities, e.g. vertical cavity surface-emitting lasers [VCSEL]
- H01S5/18305—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities having only vertical cavities, e.g. vertical cavity surface-emitting lasers [VCSEL] with emission through the substrate, i.e. bottom emission
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S5/00—Semiconductor lasers
- H01S5/10—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region
- H01S5/18—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities
- H01S5/183—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities having only vertical cavities, e.g. vertical cavity surface-emitting lasers [VCSEL]
- H01S5/18308—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities having only vertical cavities, e.g. vertical cavity surface-emitting lasers [VCSEL] having a special structure for lateral current or light confinement
- H01S5/18322—Position of the structure
- H01S5/18327—Structure being part of a DBR
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S5/00—Semiconductor lasers
- H01S5/10—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region
- H01S5/18—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities
- H01S5/183—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities having only vertical cavities, e.g. vertical cavity surface-emitting lasers [VCSEL]
- H01S5/18361—Structure of the reflectors, e.g. hybrid mirrors
- H01S5/18377—Structure of the reflectors, e.g. hybrid mirrors comprising layers of different kind of materials, e.g. combinations of semiconducting with dielectric or metallic layers
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S5/00—Semiconductor lasers
- H01S5/40—Arrangement of two or more semiconductor lasers, not provided for in groups H01S5/02 - H01S5/30
- H01S5/42—Arrays of surface emitting lasers
- H01S5/423—Arrays of surface emitting lasers having a vertical cavity
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- Physics & Mathematics (AREA)
- Electromagnetism (AREA)
- Optics & Photonics (AREA)
- Condensed Matter Physics & Semiconductors (AREA)
- General Physics & Mathematics (AREA)
- Engineering & Computer Science (AREA)
- Plasma & Fusion (AREA)
- Semiconductor Lasers (AREA)
- Lasers (AREA)
- Optical Elements Other Than Lenses (AREA)
Abstract
Abstract of the Disclosure An electrically pumped vertical cavity laser depends upon reflection as between an unaided DBR reflector on one side of the cavity and a metal-supplemented DBR
reflector on the other. Placement of the shorter supplemented DBR on the p-conductivity type side of the cavity reduces the resistance of the electrical series pump path. Permitted use of an active region of a thickness of 1µm or less in the lasing direction results in low lasing threshold. The structural approach is of significance for laser integration in integrated circuits, whether electro-optic or all-optic.
reflector on the other. Placement of the shorter supplemented DBR on the p-conductivity type side of the cavity reduces the resistance of the electrical series pump path. Permitted use of an active region of a thickness of 1µm or less in the lasing direction results in low lasing threshold. The structural approach is of significance for laser integration in integrated circuits, whether electro-optic or all-optic.
Description
ELECTRICALLY PUMPED VERTICAL CAVITY LASER
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. Back~round of the Invention `:
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1. Field of the Invention ' The invention is concerned with electrically pumped vertical cavity laser 5 structures designed with a view to low lasing threshold current. While discrete devices are contemplated, an important aspect is in terms of integratable structures now made practical because of low threshold (and consequently, lowered heating). 2. Description of the Prior Art A number of recent developments have directed attention to the need for - 10 inexpensive reliable lasers - both discrete and integrate~ As important as the laser is in so many specialties - optical communications, medical diagnosis, weaponry, basic scientific inquiry, to name a few - development has lagged in very meaningful terms.
The void resulting from unavailability of high density, small dimension, integrated devices is particularly noteworthy.
Advantages that would flow from introduction of a truly integratable ; laser e.g. permitting element-to-element communicadon on a chip, chip-t~chip ,, communicadon, and performance of switching, amplification, and other functions : now dependent on semiconductor elements are important and varied. It is estimated that computation speed could be doubled by use of all-optic circuitry. Projection 20 displays and laser printers utilizing high density laser arrays would profoundly affect the industry.
~; Optical interconnect is generally contemplated in terms of inclusion in integrated circuits constructed of material suitable for performing electronic as well as optical functions. Use of e.g. m-vs and other compound semiconductors of 25 appropriate bandgap values will likely serve in Opto Electronic Integrated Circuits.
Optical functions may play an important role in conjunction with silicon technology as well. Silicon circuitry likely to condnue its electronic dominance for some time may be provided with optical capability by superimposition of chips of other material; for example, to provide for chip-to-chip communication.
Rapid introduction of fiber optics has already revolutionized long distance communications. Availability of inexpensive lasers will extend fiber optics both to the loop plant and to terminal connection (both in industry and the home).
Other implications include steerable beams for pointing, and high power arrays.
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; 2008899 Such potendal uses have not been overlooked. Intense worldwide effort has resulted in very significant advances. Recent work has produced devices of small size benefidng from sophisticated fabricadon techniques such as _olecular B
eam Epitaxy and Metal Qrganic Chemical Vapor Deposidon. MBE, in particular, 5 must be credited for emergence of layered structures requiring dimensional control, , dimensiona1 homogenity, freedom from defects and surface smoothness, all in terms of a fraedon of a wavelength (of contemplated radiadon). Surface emitting vertical lasers may udlize such layered struetures in the acdve region (e. g. in the form of quantum wdls), as well as for defining the eavity (in the form of Distr~buted _ragg 10 Refleetors.
A major obstaele to more general implementadon of the laser is s temperature inerease due to heat generadon attending lasing. This is pardeularly true for integradon in whieh the problem is only aggravated for the higher paeking densides whieh is a major thrust for integradon. For any given level of efficiency of -; 15 operadon, need for heat dissipadon deereases as lasing threshold decreases. Lasing "., threshold is in turn dependent - generally linearly dependent - on the volume of aedve material. Major effort has been in the direedon of deereasing this volume.The eavity strueture is an important determinant for the thiekness of aetive material between the mirrors. The uldmate requirement for any lasing 20 strueture is for a refleedvity/loss eharaeterisde which accommodates per-pass gain x~ for the stimulated radiadon. Recendy announced work appears to have carried this :' to the uldmate limit in permitdng lasing with but a single quantum well in a surface - emitdng strueture. ("Applied Physies Letters", vol. 55 (24), pp. 2473-2475 Deeember 11, 1989). The work depends upon use of DBR mirrors of near-perfect 25 refleedvity (reflecdvity of 99.9% for a laser wavelength of 980 nm resulted from use of DBR mirrors eaeh of approximately 20 periods). While the first experiments were based on opdeally pumping, the authors immediately realized implications in terrns of eleetrically pumping.
- Ebetrieal pumping, the form most eommereially signifieant devices will 30 take, intmduees probloms. The DBR, ebarly dhe best approach so far, does not intrinsieally have the desirable eleetrieal eharaeterisdcs for serving in the series . eleetrieal pump padh. Opdmized Bragg struetures of appropriate transparency, refraedve index (n) and layer-t~layer index difference (~n) introduces a level of I2R
Ioss and headng to result in need for inereased volume of aedve material (and still 35 further headng). Beyond some level, headng is too great to be compensated by ;, further volume inerease. Modificadon of the DBRs to create the needed pn junction requires doping whieh implieidy results in seattering centers, and, therefore, in ::, . ., ....
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2~8899 further loss (requiring further increase in volume of acdve material). The critical consideration in many design terms is the thickness dimension in the emission direcdon since, aside from diffraction effects, both loss and gain are generallylinearly dependent on lateral dimensions - usually on lateral area.
, S One line of endeavor represented, for exarnple, by an article in "Electronics Letters," vol. 24, no. 15, pp. 928, 929 (July 21, 1988) seeks to avoid the problem by providing distinct electrical and opdcal paths. In the cited article, a S
urface Emitting _aser having an acdve region of 3 ~m thickness and 30 ~m diameter was operated room temperature, pulsed at a threshold current, Id~ = 200 mA, 10 equivalent to a current density of 30kA/cm2. The cavity in this instance used but a ; single DBR and depended on a convendonal single-surface reflector on the other side. The authors' determinadon to put thc DBR on the n-type side was no doubt , dictated by the greater electrical resistance introduced by p-type doping. Other approaches also dependent upon use of distinct current and opdcal paths have used lS various means for injecdng pumping currents laterally, e.g. at acdve material -u mi~or interfaces. "Applied Physics Letters", vol. Sl, no. 21, pp. 1655-57 (Nov. 23, 1987).
; It may be concluded that absence of truly low threshold current devices, for example, as required for ICs, is due, in one instance, to implicitly high resistance ^. 20 (for devices using coincident current and optical paths) and in the other, to fabricadon difficulq as well as device funcdon problems (for devices using disdnct current and opdcal paths).
Summary of the Invention All aspects of thc invention depend upon a pardcular laser structure. The structure is 25 generally classified as a verdcal caviq laser and is somedmes referred to as "surface emitdng" to differentiate it from the edge emitdng qpe of device. Lasers of the invendon are capable of low lasing threshold which derives from use of relatively thin acdvc regions (regions of a thickness of 1 ~m or less), in turn permitted by excellent reflectivity-loss characteristics dependent upon use of DBR mirrors on both 30 sides of the laser cavity. Electrical pumping in accordance with the inventive teaching depends upon current passage through both DBR mirrors in the same . general verdcal direcdon as that of the sdmulated laser beam (in the terrninology S used cu~ent passage is referred to as "verdcal" in the same sense used in defining `r'' lasing direcdon in the "vertical" caviq laser.
.'~J' 35 While all laser structures of the invention udlize two DBRs, one of the two has a lesser number of periods (a lesser number of paired layers) than the other so that this DBR is, in and of itself, inadequate to attain the desired cavity ~i, ^. - 3 -...,;, .~, ::.
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reflectivity. This "deficient" DBR is supplemented by a conventional reflector -generally a conventional metallic reflector - to result in desired reflectivity greater than about 98% (in most instances in a reflecdvity of greater than 99%). The resulting "hybrid" mirror is on the p-conducdvity type side since such placement is 5 most effecdve in reducing the series resistance of the pump path.
A primary aspect of the invendve teaching generally takes the form of what would ordinarily be regarded as a distordon in the DBR portion of the hybrid ;~. mirror. In general the DBR layer closest to the convendonal reflector is of a thic~ness either less than or greater than the usual quarter wavelength (or integral :.: 10 number) considered required in a functioning DBR. This layer, thinner than the other Bragg layers if of the index of refracdon of the lesser index layer, thicker if of the greater, serves to maximize reflectivity. As discussed more fully in the Detailed ' Descripdon deviation from ~4 layer thickness derives from the fact that metallic reflectors are not "ideal" but Mther evidence a characteristic penetration depth which 15 results in some destructive interference as between reflected radiadon from the front and "back" of the final Bragg layer (the layer closest to the conventional mirror ;, which in effect is altered in thickness due to this characterisdc of the metallic " reflector. If there is need to interpose a "contact" layer to assure ohmic contact between the convendonal and DBR milTors, the thickness of this "phase matching"
. 20 layer is further adjusted accordingly.
cr aspects of the laser structure of the invention are known. Such consideradons apply to the materials of the DBRs both from the standpoint of optical reRecdvity and electrical conducdvity. As with other such structures free carriers introduced into the actdve region are desirably confined by "confinement" layers of 25 sufSciendy increased bandgap to prevent escape. Substrate material, like other materials of the structure, substandally defect-free, are as nearly transparent to emitted laser energy as consistent with other requirements.
Structures of the invention may be discrete or may be included in ICs -dther OEICs or aU-opdc. Whib both electrical and optical paths are through the 30 DBRs - generally through all layers of both DBRs, there are versions of the ,.; invendon in which paths are not invariably identical. In one version the entirety of : the Snal DBR of the non-hybrid reflector does not contact substrate, but a portion is ?~ bared, possibly by etching, e.g. to facilitate coupling to an optical fiber. Another version makes use of a hole in the metallic mirror - for example, for monitoring laser ?.~ 35 operadon.
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Z(~08899 Brief Description of the Drawin~
FIG. 1 is a schematic elevational view depicting a laser structure typical of those i included in every apparatus of the invention. FIG. 2, on coordinates of intensity on the ordinate and wavelength on the abscissa shows intensity dependence on emission , 5 wavelength for four different pump levels, one below and three above threshold, for ,~
, an exemplary device of the invention. FIG. 3 is a simplified elevational view of a ;~ structure such as that depicted in FIG. 1 but providing for possible variations including direct fiber coupling with the emitting surface of the active region of the laser cavity. Discussion of this figure entails use of the fiber as a permanent part of `'Jr 10 the apparatus as well as its use in testing. FIG. 4 is a perspective view of a portion of an integrated circuit including an array of lasers in accordance with the invention.
FIG. 5, also in perspective, depicts an integrated circuit containing a laser structure such as included in FIG. 4 together with electronic drive circuitry.
Detailed D~scription . 15 Drawin~
The device of FIG. 1 includes an active layer 1 embraced by conSnement layers 2 and 3. Lower reflector 4 is a distributed Bragg reflector made - up of alternating layers 5 and 6 of high and low refracdve index respectively. Since - this lower DBR serves as the n-type side of the pn juncdon layers are doped with n-20 typc signiScant impurity. In comrnon with usual DBR structures layers 5 and 6 are each of a thickness dimension equal to an integral number of quarter wavelengths -generally one quarter wavelength (~J4) - for the wavelength of stimulated emission (as measured in the material of which the DBR is constructed). As discussed in detail further on, the number of 5-6 pairs (the number of periods) is generally 25 sufficient to provide reflecdvity of at least 98% and usually greater than 99% - a condidon which in at least one example herein is accomplished by use of 23 periods.
Substrate 7, is of necessary crystalline perfecdon and of composidon and thickness otherwise suitable to fabricadon (e.g. epitaxial growth) and to operation.
Upper DBR 8 made of alternating, generally ~14 thick layers 9 and 10 of 30 p-type conductivity rnaterial of high and low refracdve index, n, respectively has a -~ Iesser number of periods than that of DBR 4. Uppermost Bragg layer 11 is the -~ "phase-matching" layer designed to maximize construcdve interference. For the structure shown the material of convendonal mirror 12 does not provide dependable ohmic contact to layer 11. This deficiency is cured by provision of contact layer 13.
.~ 3S Layer 11 is of a thickness smaller than or larger than ~J4 depending upon whether it ~ is constructed of the lesser or greater of the material of layers 9 and 10 respecdvely.
-:'. Actual thickness of layer 11 is cridcally dependent on the phase delay introduced by . . .
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penetration of laserradiation into mirror layer 12 as fur~her varied by layer 13 if present.
"`\ Certain structural aspects are not discussed in detail. For example, the particular device shown in FIG. 1 has a hybrid mirror portion which is reduced in 5 area relative to the remainder of the device. It has been found convenient to fabricate alrays by deposition of continuous layers over the endre area to be ~,occupicd by lasers. Separation of individual lasers is accomplished by etching to the depth of the upper confinement layer 2 while defining mesa structures of dimension 14. Of course, alternative approaches are known and may be preferred .10 depcnding on a number of factors: composition of material to be etched, density of -~Iascr structures intendcd, etc. For example, a variant on etch-scparation makes use of -ion bombardment to rcndcr scparadng material less conducting. Circuit design may dictate separation of thc lower Bragg mirrors as well. Of course, device fabricadon ~ ~need not rely on etching but may be thc consequence of as-grown structures e.g. by ,~15 use of selective deposition (perhaps by masking). It is not the purposc to make this part of the dcscription exhaustive. Fabrication approaches suitable for the many~aspccts of thc invendon afe wcll known and othcrs are cmerging.
:Invendvc devices cxcmplified by FIG. 1 owe significance to decreased .need for heat dissipadon. Actual operadon at room tcmperature is obtainable for~i20 duty cycles up to 100% (for CW operadon). Nevcrthclcss, there are circumstances where it may be desirable to introduce cooling. Such circumstances, for example,include deliberately pumping above thrcshold to realize higher power output as well as design criteria which may call for e. g. dcliberately decrcased mirror reflectivity or deliberatcly inc eascd acdve layer thickness. Such structurcs may profit by use of 25 hcat sinks perhaps as aidcd by thcrmoelectric cooling mcans. Such variations are considercd well within the skill of thc ardsan and are not discusscd in dctail.
Similarly, electrical circuitry is rcprcsented in FIG. 1 only schematically by means of clectfodc 15 (attached to mctallic mirror layer 12 and electrode 16 which talccs thc form of a deposited metallic layer on thc underside of substrate 7) in 30 an actual cxample use was made of a l,SOOA thick gold-germanium alloy laycr.
In FIG. 2 Curves 20, 21, 22 and 23 represent cmission spectra for pump currents of 20mA, 25mA, 30mA and 40mA as applied to the device of Example 1.
This devicc uscs a l5~1m diameter, Q5 nm thick activc laycr of GaAs lasing at 0.862311m. Curvc 20, below threshold is incrcased in amplitude tenfold to better35 show observcd dctails of the spontaneous emission. Curves 21, 22 and 23 all above thrcshold manifest stimulated emission as spikes 24, 25 and 26. To better show enhancement in intensity - to prevent overlap of the spikes which are all at the same "~,............... .
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2~)08899 wavelength - the figure is presented as a three-dimensional plot.
FIG. 3 depicts a variant of the device of FIG. 1. Taking on a ` contemplated discrete form, the structure shown represents features which may be incorporated separately or in combination in the variety of devices contemplated -S whether integrated or discrete. The first feature provides for insertion of optical fiber 17 through substrate hole 18 to bring it into engagement with the lower DBR
(to serve a device function or for test purposes - e.g. during fabrication). While as depicted, direct contact is indicated, other considerations, largely in terms of material and structural properties, may dictate an interposed transparent 1ayer not shown. The 10 device is generally similar to that of FIG. 1 consisting of active region 31 confinement layers 32 and 33 lower Bragg reflector 34, made up of alternadng high ' and low index layers 35 and 36, on substrate 37. The upper rnirror is a hybrid structure consisting of Bragg portion 38 composed of high and low index layers 39, -' 40 and 41. Phase matching is accomplished by appropriate dimensioning of 15 layer 41. Electrical contact as between phase-matching layer 41 and mirror 42 is assured by means of contact 1ayer 43. Wire electrode 45 and ground electrode 46 complete the structure electrica11y.
Active layer 31 is made up of laser quantum wells 31(a) separated by spacers 31(b) and 31(c). As in known Muld Quantum _ell structures, spacers 31(b)20 are designed to place the QWs at high energy positions within the stimulated laser standing wave. Thinner spacers 31(c) are needed to define the individual QW
structutes themselves within the trios depicted. Always useful in terms of increased efficiency reladve to bulk materia1, MQW structures as depicted, represent a pardcularly useful feature in the terms of this invendon since headng due to (useless) 25 spontaneous radiadon, in turn, due to camer tecombinadon in low or zero energy positdons is avoide~ The overall effect is sdll further lowered lasing thresholdcurrent (still further lowered need for heat dissipadon). A similar structure may `, provide for convendonal bullc wells, each of thickness equal to a trio of QWs posidoned by spacers 31(b).
i ~ 30 The schemadc representadon includes yet another feature, a hole 47 baring a small central region of upper Bragg mirror 38. By maintaining hole 47 small - of the order of one or two wavelengths (of sdmulated emission) - the cavity is perturbed very little. The small amount of radiadon emitted through hole 47 is coherent but is subject to significant edge diffracdon. One purpose that may be 35 served is monitoring simply to assure that the system is operadve (that laser-~ radiadon is being produced at the substrate side of the cavity).
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' ~ ~:00~3899 FIG. 4 depicts an array of lasers 51 which may be of the detailed design ` ~ shown in any of FIGS. 1 and 3. As in those figures, lasers 51 are supported on a substrate, in this instance 52, upon which, and in at least some instances, they were grown, for example, by Molecular Beam Epitaxy. Lasers depicted in this figure asS well as in FIG. S are exaggerated in dimension in the lasing direcdon. In general, they are only a few microns in hdght.
; \ FIG. S shows a pordon of an OEIC. The portion shown consists simply,. of laser 53 together with drive electronics 54 both on common substrate 55. The arrangement is schematically shown as connected via lead 56 with the electrical 10 circuit being completed by a common substrate lead.
Desi~n Consideradons , 1. General The basic thrust of the invendon is that of an expediently fabricated, low threshold ` (and, therefore, integratable) laser structure, and discussion is largely in such terms.
` 15 The gencral teaching does have addidonal uses c.g. in terms of higher power devices.
The advantage in this generic sense concerns the use of the æymmetric Bragg cavity . in which a phase-matched convendonal metallic mir or supplements the shorter DBR. Design of high power devices may retain the 99~% reflecdvity for both cavity , ends, as desired for low threshold devices, or design consideradons might dictate a 20 lesser reflecdvity, likely primarily on the n- conducdvity side of the cavity. Also, the acdve layer may be increased in thickness beyond the minimum permitted by the cavity characterisdcs.
Lateral dimensions of the laser structure depend upon funcdon. Low current threshold values are meaningfully considered in terms of current per unit 25 area so that the same heat dissipadon advantage is in fact realizcd independent of individual devicc area assuming reasonably close packing. For IC use, design criteria generally suggest small spot size, in turn, corresponding with an emitting surface of lateral dimensions 10 or l5~m or smaller.
Following descripdon in this secdon is, in most respects, in terms of 30 likely greatest interest - in terms of low threshold.
2. Acdve Re~ion While the absolute minimum thickncss in final detail dcpends upon thc precise nature of thc materials used (and in turn upon operational charactcrisdcs such as : lascr wavcbngth) studies to date suggest use of acdvc regions of a thickncss no 35 greater than about l~lm. Structure opdmizadon for material systcms most intcnsely r~", studied suggest a prcferred maximum active region thickness of 0.5~m. "Thickness"
rcfers to the endre dimcnsion betwcen confinemcnt layers (or between DBRs if ,~, ,,, .:,, .~,. .
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- . , -~ . . ~ , , ,. Z00~899 conSnement layers are not used, whether of uniform bulk material or of spaced wells - either QWs or conventional. For the material system described in Example 1, high duty cycle, up to CW, is permitted for active layer thickness of 0.25~m. In all systems studied, round-trip gains are easily sufficient to permit lasing with the 5 described Bragg-hybrid Bragg cavides. For opdmally designed MQWs and also for spaced convendonal wells, such gains are attainable for acdve layer thicknesses about the same as for bulk layers, but since a significant part of the active region is made up of spacers, there is less heat generated for given output intensity. A
.', representative MQW structure is describcd in conjunction with FIG. 3. That 10 structure depends on three trios of quantum wells so posidoned by means of spacers as to place them at energy peaks in the standing wave. With spacers, those separadng thc trios, those at the two mirror boundaries, and those between individual wells of each trio, the total thickness is about 0.311m. Examples 2, 3, 5 and 7 depend upon MQW active regions.
From the compositional standpoint, emitdng material in the acdve layer (bullc or well material) is constructed of semiconductor material having a direct bandgap accommodadng the desired emission wavelength. While far more general use is expected, near term use is likely in terms of wavelengths of communicadons interest. Such wavelengths, 0.8611m, 1.311m and 1.55~1m are obtainable by use of20 well-understood compound semiconductors: GaAs at 0.8611m and InGaAsP at the ; other wavelengths. Strained InGaAs may be used at l.OIlm. The invendve devices - are, of course, usable over a much larger wavelength range as permitted by a variety i of semiconductor materials.
It is useful to describe aspccts of the structure in specific exemplary 25 terms. In terms of the structure of Example 1, as indicated, the acdve region is gallium arsenide, GaAs. Composidons noted in following subsecdons are in the same terms - in fact, as used in Example 1.
Another matter, not appropriately discussed in detail, may be mentioned ~ briefly. The effecdve cavity dimension, determinative of the size of the standing ;` 30 wave and, therefore, of emission wavelength, while based primarily on the thickness of the acdve region as discussed, is in fact somewhat thicker due to radiation penetradon, likely through the endrety of confinement layers if present, and to some ,penetradon depth within the DBRs as well. At this dme, determination of precise.~,''?dimensions corresponding with specified wavelength, is accomplished empirically '~35 (by building a first structure, measuring emission ~, adjusdng some dimension -perhaps that of the confinement layer - again measuring, etc).
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200~3899 3. Confinement La~lers Nature and function of confinement layers is discussed elsewhere. See H. C. Casey, Jr. and M. B. Panish, "Heterostructure Lasers: Part A Fundamental Principles", (Academic, N.Y. 1978) Sec. 4.6 (Carrier Confinement) pp. 245-253). Its 5 primary purpose is to confine free carriers to the acdve region. Composition is chosen to yield an energy gap Eg larger than that of the active region (generally a gap differendal of 25 % is adequate. See H. C. Casey, Jr. and M. B. Panish, "Heterostructure Lasers: Part A Fundamental Principles", (Academic, N.Y. 1978) ~~ Sec. 4.6 (Carrier Confinement) pp. 245-253. Thickness of the confinement layer is - 10 sufficient to prevent appreciable tunneling (tunneling is of little consequence for thicknesses greater than about lOOA). Thickness of the confinement layer is not critical - layer thickness of about lOOOA was used in Example 1. The confinementlayer may serve an additional and impo tant funcdon for the heterojunctions generally contemplated - that of minimizing back injection of minority carriers.It is generally preferred that the laser-functioning pn junction be on the end type side of the cavity. Under these circurnstances, the active region is of p-conducdvity type. In structures tested, the confinement layer is doped to the same conducdvity tyype as that of the adjacent DBR - in this instance, is of n-type conducdvity. Such placernent of the functioning junction takes advantage of the fact s~ 20 that electron mobility is inherently greater than that of hole mobility - a design ~; preference of increasing significance for increasing cavity length..~ The desire for maximum ~n from the opdcal standpoint dictates use of ; .j the higher index Bragg material at an air interface and choice of the material yielding :; the greatest ~n at other interfaces (e. g. at the Bragg-acdve region interface).
;.~ 25 However, electrical consideradons - minirnizadon of electrical resistance -somedmes dictates less ~n values.
~'. The nature and funcdon of the confinement layers has been set forth.
While separate confinement layers are usefully employed for the particular material systems of the examples, this is not invariant. Confinement, as well as back 30 injecdon, are both dependent on bandgap of such layers relative to the adjacent :.~ acdve region. Needed increased gap reladve to the active region, as well as appropriate displacement of the relevant band edge values, may be inherent in .~ materials other vise suitable in construcdon of the DBR - on either or both sides of c~ the caviq. Elimination of the need for one or both confinement layers may be of .~ 35 considerable benefit in enhancing efficiency.
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1. Field of the Invention ' The invention is concerned with electrically pumped vertical cavity laser 5 structures designed with a view to low lasing threshold current. While discrete devices are contemplated, an important aspect is in terms of integratable structures now made practical because of low threshold (and consequently, lowered heating). 2. Description of the Prior Art A number of recent developments have directed attention to the need for - 10 inexpensive reliable lasers - both discrete and integrate~ As important as the laser is in so many specialties - optical communications, medical diagnosis, weaponry, basic scientific inquiry, to name a few - development has lagged in very meaningful terms.
The void resulting from unavailability of high density, small dimension, integrated devices is particularly noteworthy.
Advantages that would flow from introduction of a truly integratable ; laser e.g. permitting element-to-element communicadon on a chip, chip-t~chip ,, communicadon, and performance of switching, amplification, and other functions : now dependent on semiconductor elements are important and varied. It is estimated that computation speed could be doubled by use of all-optic circuitry. Projection 20 displays and laser printers utilizing high density laser arrays would profoundly affect the industry.
~; Optical interconnect is generally contemplated in terms of inclusion in integrated circuits constructed of material suitable for performing electronic as well as optical functions. Use of e.g. m-vs and other compound semiconductors of 25 appropriate bandgap values will likely serve in Opto Electronic Integrated Circuits.
Optical functions may play an important role in conjunction with silicon technology as well. Silicon circuitry likely to condnue its electronic dominance for some time may be provided with optical capability by superimposition of chips of other material; for example, to provide for chip-to-chip communication.
Rapid introduction of fiber optics has already revolutionized long distance communications. Availability of inexpensive lasers will extend fiber optics both to the loop plant and to terminal connection (both in industry and the home).
Other implications include steerable beams for pointing, and high power arrays.
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; 2008899 Such potendal uses have not been overlooked. Intense worldwide effort has resulted in very significant advances. Recent work has produced devices of small size benefidng from sophisticated fabricadon techniques such as _olecular B
eam Epitaxy and Metal Qrganic Chemical Vapor Deposidon. MBE, in particular, 5 must be credited for emergence of layered structures requiring dimensional control, , dimensiona1 homogenity, freedom from defects and surface smoothness, all in terms of a fraedon of a wavelength (of contemplated radiadon). Surface emitting vertical lasers may udlize such layered struetures in the acdve region (e. g. in the form of quantum wdls), as well as for defining the eavity (in the form of Distr~buted _ragg 10 Refleetors.
A major obstaele to more general implementadon of the laser is s temperature inerease due to heat generadon attending lasing. This is pardeularly true for integradon in whieh the problem is only aggravated for the higher paeking densides whieh is a major thrust for integradon. For any given level of efficiency of -; 15 operadon, need for heat dissipadon deereases as lasing threshold decreases. Lasing "., threshold is in turn dependent - generally linearly dependent - on the volume of aedve material. Major effort has been in the direedon of deereasing this volume.The eavity strueture is an important determinant for the thiekness of aetive material between the mirrors. The uldmate requirement for any lasing 20 strueture is for a refleedvity/loss eharaeterisde which accommodates per-pass gain x~ for the stimulated radiadon. Recendy announced work appears to have carried this :' to the uldmate limit in permitdng lasing with but a single quantum well in a surface - emitdng strueture. ("Applied Physies Letters", vol. 55 (24), pp. 2473-2475 Deeember 11, 1989). The work depends upon use of DBR mirrors of near-perfect 25 refleedvity (reflecdvity of 99.9% for a laser wavelength of 980 nm resulted from use of DBR mirrors eaeh of approximately 20 periods). While the first experiments were based on opdeally pumping, the authors immediately realized implications in terrns of eleetrically pumping.
- Ebetrieal pumping, the form most eommereially signifieant devices will 30 take, intmduees probloms. The DBR, ebarly dhe best approach so far, does not intrinsieally have the desirable eleetrieal eharaeterisdcs for serving in the series . eleetrieal pump padh. Opdmized Bragg struetures of appropriate transparency, refraedve index (n) and layer-t~layer index difference (~n) introduces a level of I2R
Ioss and headng to result in need for inereased volume of aedve material (and still 35 further headng). Beyond some level, headng is too great to be compensated by ;, further volume inerease. Modificadon of the DBRs to create the needed pn junction requires doping whieh implieidy results in seattering centers, and, therefore, in ::, . ., ....
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2~8899 further loss (requiring further increase in volume of acdve material). The critical consideration in many design terms is the thickness dimension in the emission direcdon since, aside from diffraction effects, both loss and gain are generallylinearly dependent on lateral dimensions - usually on lateral area.
, S One line of endeavor represented, for exarnple, by an article in "Electronics Letters," vol. 24, no. 15, pp. 928, 929 (July 21, 1988) seeks to avoid the problem by providing distinct electrical and opdcal paths. In the cited article, a S
urface Emitting _aser having an acdve region of 3 ~m thickness and 30 ~m diameter was operated room temperature, pulsed at a threshold current, Id~ = 200 mA, 10 equivalent to a current density of 30kA/cm2. The cavity in this instance used but a ; single DBR and depended on a convendonal single-surface reflector on the other side. The authors' determinadon to put thc DBR on the n-type side was no doubt , dictated by the greater electrical resistance introduced by p-type doping. Other approaches also dependent upon use of distinct current and opdcal paths have used lS various means for injecdng pumping currents laterally, e.g. at acdve material -u mi~or interfaces. "Applied Physics Letters", vol. Sl, no. 21, pp. 1655-57 (Nov. 23, 1987).
; It may be concluded that absence of truly low threshold current devices, for example, as required for ICs, is due, in one instance, to implicitly high resistance ^. 20 (for devices using coincident current and optical paths) and in the other, to fabricadon difficulq as well as device funcdon problems (for devices using disdnct current and opdcal paths).
Summary of the Invention All aspects of thc invention depend upon a pardcular laser structure. The structure is 25 generally classified as a verdcal caviq laser and is somedmes referred to as "surface emitdng" to differentiate it from the edge emitdng qpe of device. Lasers of the invendon are capable of low lasing threshold which derives from use of relatively thin acdvc regions (regions of a thickness of 1 ~m or less), in turn permitted by excellent reflectivity-loss characteristics dependent upon use of DBR mirrors on both 30 sides of the laser cavity. Electrical pumping in accordance with the inventive teaching depends upon current passage through both DBR mirrors in the same . general verdcal direcdon as that of the sdmulated laser beam (in the terrninology S used cu~ent passage is referred to as "verdcal" in the same sense used in defining `r'' lasing direcdon in the "vertical" caviq laser.
.'~J' 35 While all laser structures of the invention udlize two DBRs, one of the two has a lesser number of periods (a lesser number of paired layers) than the other so that this DBR is, in and of itself, inadequate to attain the desired cavity ~i, ^. - 3 -...,;, .~, ::.
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reflectivity. This "deficient" DBR is supplemented by a conventional reflector -generally a conventional metallic reflector - to result in desired reflectivity greater than about 98% (in most instances in a reflecdvity of greater than 99%). The resulting "hybrid" mirror is on the p-conducdvity type side since such placement is 5 most effecdve in reducing the series resistance of the pump path.
A primary aspect of the invendve teaching generally takes the form of what would ordinarily be regarded as a distordon in the DBR portion of the hybrid ;~. mirror. In general the DBR layer closest to the convendonal reflector is of a thic~ness either less than or greater than the usual quarter wavelength (or integral :.: 10 number) considered required in a functioning DBR. This layer, thinner than the other Bragg layers if of the index of refracdon of the lesser index layer, thicker if of the greater, serves to maximize reflectivity. As discussed more fully in the Detailed ' Descripdon deviation from ~4 layer thickness derives from the fact that metallic reflectors are not "ideal" but Mther evidence a characteristic penetration depth which 15 results in some destructive interference as between reflected radiadon from the front and "back" of the final Bragg layer (the layer closest to the conventional mirror ;, which in effect is altered in thickness due to this characterisdc of the metallic " reflector. If there is need to interpose a "contact" layer to assure ohmic contact between the convendonal and DBR milTors, the thickness of this "phase matching"
. 20 layer is further adjusted accordingly.
cr aspects of the laser structure of the invention are known. Such consideradons apply to the materials of the DBRs both from the standpoint of optical reRecdvity and electrical conducdvity. As with other such structures free carriers introduced into the actdve region are desirably confined by "confinement" layers of 25 sufSciendy increased bandgap to prevent escape. Substrate material, like other materials of the structure, substandally defect-free, are as nearly transparent to emitted laser energy as consistent with other requirements.
Structures of the invention may be discrete or may be included in ICs -dther OEICs or aU-opdc. Whib both electrical and optical paths are through the 30 DBRs - generally through all layers of both DBRs, there are versions of the ,.; invendon in which paths are not invariably identical. In one version the entirety of : the Snal DBR of the non-hybrid reflector does not contact substrate, but a portion is ?~ bared, possibly by etching, e.g. to facilitate coupling to an optical fiber. Another version makes use of a hole in the metallic mirror - for example, for monitoring laser ?.~ 35 operadon.
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Z(~08899 Brief Description of the Drawin~
FIG. 1 is a schematic elevational view depicting a laser structure typical of those i included in every apparatus of the invention. FIG. 2, on coordinates of intensity on the ordinate and wavelength on the abscissa shows intensity dependence on emission , 5 wavelength for four different pump levels, one below and three above threshold, for ,~
, an exemplary device of the invention. FIG. 3 is a simplified elevational view of a ;~ structure such as that depicted in FIG. 1 but providing for possible variations including direct fiber coupling with the emitting surface of the active region of the laser cavity. Discussion of this figure entails use of the fiber as a permanent part of `'Jr 10 the apparatus as well as its use in testing. FIG. 4 is a perspective view of a portion of an integrated circuit including an array of lasers in accordance with the invention.
FIG. 5, also in perspective, depicts an integrated circuit containing a laser structure such as included in FIG. 4 together with electronic drive circuitry.
Detailed D~scription . 15 Drawin~
The device of FIG. 1 includes an active layer 1 embraced by conSnement layers 2 and 3. Lower reflector 4 is a distributed Bragg reflector made - up of alternating layers 5 and 6 of high and low refracdve index respectively. Since - this lower DBR serves as the n-type side of the pn juncdon layers are doped with n-20 typc signiScant impurity. In comrnon with usual DBR structures layers 5 and 6 are each of a thickness dimension equal to an integral number of quarter wavelengths -generally one quarter wavelength (~J4) - for the wavelength of stimulated emission (as measured in the material of which the DBR is constructed). As discussed in detail further on, the number of 5-6 pairs (the number of periods) is generally 25 sufficient to provide reflecdvity of at least 98% and usually greater than 99% - a condidon which in at least one example herein is accomplished by use of 23 periods.
Substrate 7, is of necessary crystalline perfecdon and of composidon and thickness otherwise suitable to fabricadon (e.g. epitaxial growth) and to operation.
Upper DBR 8 made of alternating, generally ~14 thick layers 9 and 10 of 30 p-type conductivity rnaterial of high and low refracdve index, n, respectively has a -~ Iesser number of periods than that of DBR 4. Uppermost Bragg layer 11 is the -~ "phase-matching" layer designed to maximize construcdve interference. For the structure shown the material of convendonal mirror 12 does not provide dependable ohmic contact to layer 11. This deficiency is cured by provision of contact layer 13.
.~ 3S Layer 11 is of a thickness smaller than or larger than ~J4 depending upon whether it ~ is constructed of the lesser or greater of the material of layers 9 and 10 respecdvely.
-:'. Actual thickness of layer 11 is cridcally dependent on the phase delay introduced by . . .
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penetration of laserradiation into mirror layer 12 as fur~her varied by layer 13 if present.
"`\ Certain structural aspects are not discussed in detail. For example, the particular device shown in FIG. 1 has a hybrid mirror portion which is reduced in 5 area relative to the remainder of the device. It has been found convenient to fabricate alrays by deposition of continuous layers over the endre area to be ~,occupicd by lasers. Separation of individual lasers is accomplished by etching to the depth of the upper confinement layer 2 while defining mesa structures of dimension 14. Of course, alternative approaches are known and may be preferred .10 depcnding on a number of factors: composition of material to be etched, density of -~Iascr structures intendcd, etc. For example, a variant on etch-scparation makes use of -ion bombardment to rcndcr scparadng material less conducting. Circuit design may dictate separation of thc lower Bragg mirrors as well. Of course, device fabricadon ~ ~need not rely on etching but may be thc consequence of as-grown structures e.g. by ,~15 use of selective deposition (perhaps by masking). It is not the purposc to make this part of the dcscription exhaustive. Fabrication approaches suitable for the many~aspccts of thc invendon afe wcll known and othcrs are cmerging.
:Invendvc devices cxcmplified by FIG. 1 owe significance to decreased .need for heat dissipadon. Actual operadon at room tcmperature is obtainable for~i20 duty cycles up to 100% (for CW operadon). Nevcrthclcss, there are circumstances where it may be desirable to introduce cooling. Such circumstances, for example,include deliberately pumping above thrcshold to realize higher power output as well as design criteria which may call for e. g. dcliberately decrcased mirror reflectivity or deliberatcly inc eascd acdve layer thickness. Such structurcs may profit by use of 25 hcat sinks perhaps as aidcd by thcrmoelectric cooling mcans. Such variations are considercd well within the skill of thc ardsan and are not discusscd in dctail.
Similarly, electrical circuitry is rcprcsented in FIG. 1 only schematically by means of clectfodc 15 (attached to mctallic mirror layer 12 and electrode 16 which talccs thc form of a deposited metallic layer on thc underside of substrate 7) in 30 an actual cxample use was made of a l,SOOA thick gold-germanium alloy laycr.
In FIG. 2 Curves 20, 21, 22 and 23 represent cmission spectra for pump currents of 20mA, 25mA, 30mA and 40mA as applied to the device of Example 1.
This devicc uscs a l5~1m diameter, Q5 nm thick activc laycr of GaAs lasing at 0.862311m. Curvc 20, below threshold is incrcased in amplitude tenfold to better35 show observcd dctails of the spontaneous emission. Curves 21, 22 and 23 all above thrcshold manifest stimulated emission as spikes 24, 25 and 26. To better show enhancement in intensity - to prevent overlap of the spikes which are all at the same "~,............... .
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2~)08899 wavelength - the figure is presented as a three-dimensional plot.
FIG. 3 depicts a variant of the device of FIG. 1. Taking on a ` contemplated discrete form, the structure shown represents features which may be incorporated separately or in combination in the variety of devices contemplated -S whether integrated or discrete. The first feature provides for insertion of optical fiber 17 through substrate hole 18 to bring it into engagement with the lower DBR
(to serve a device function or for test purposes - e.g. during fabrication). While as depicted, direct contact is indicated, other considerations, largely in terms of material and structural properties, may dictate an interposed transparent 1ayer not shown. The 10 device is generally similar to that of FIG. 1 consisting of active region 31 confinement layers 32 and 33 lower Bragg reflector 34, made up of alternadng high ' and low index layers 35 and 36, on substrate 37. The upper rnirror is a hybrid structure consisting of Bragg portion 38 composed of high and low index layers 39, -' 40 and 41. Phase matching is accomplished by appropriate dimensioning of 15 layer 41. Electrical contact as between phase-matching layer 41 and mirror 42 is assured by means of contact 1ayer 43. Wire electrode 45 and ground electrode 46 complete the structure electrica11y.
Active layer 31 is made up of laser quantum wells 31(a) separated by spacers 31(b) and 31(c). As in known Muld Quantum _ell structures, spacers 31(b)20 are designed to place the QWs at high energy positions within the stimulated laser standing wave. Thinner spacers 31(c) are needed to define the individual QW
structutes themselves within the trios depicted. Always useful in terms of increased efficiency reladve to bulk materia1, MQW structures as depicted, represent a pardcularly useful feature in the terms of this invendon since headng due to (useless) 25 spontaneous radiadon, in turn, due to camer tecombinadon in low or zero energy positdons is avoide~ The overall effect is sdll further lowered lasing thresholdcurrent (still further lowered need for heat dissipadon). A similar structure may `, provide for convendonal bullc wells, each of thickness equal to a trio of QWs posidoned by spacers 31(b).
i ~ 30 The schemadc representadon includes yet another feature, a hole 47 baring a small central region of upper Bragg mirror 38. By maintaining hole 47 small - of the order of one or two wavelengths (of sdmulated emission) - the cavity is perturbed very little. The small amount of radiadon emitted through hole 47 is coherent but is subject to significant edge diffracdon. One purpose that may be 35 served is monitoring simply to assure that the system is operadve (that laser-~ radiadon is being produced at the substrate side of the cavity).
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' ~ ~:00~3899 FIG. 4 depicts an array of lasers 51 which may be of the detailed design ` ~ shown in any of FIGS. 1 and 3. As in those figures, lasers 51 are supported on a substrate, in this instance 52, upon which, and in at least some instances, they were grown, for example, by Molecular Beam Epitaxy. Lasers depicted in this figure asS well as in FIG. S are exaggerated in dimension in the lasing direcdon. In general, they are only a few microns in hdght.
; \ FIG. S shows a pordon of an OEIC. The portion shown consists simply,. of laser 53 together with drive electronics 54 both on common substrate 55. The arrangement is schematically shown as connected via lead 56 with the electrical 10 circuit being completed by a common substrate lead.
Desi~n Consideradons , 1. General The basic thrust of the invendon is that of an expediently fabricated, low threshold ` (and, therefore, integratable) laser structure, and discussion is largely in such terms.
` 15 The gencral teaching does have addidonal uses c.g. in terms of higher power devices.
The advantage in this generic sense concerns the use of the æymmetric Bragg cavity . in which a phase-matched convendonal metallic mir or supplements the shorter DBR. Design of high power devices may retain the 99~% reflecdvity for both cavity , ends, as desired for low threshold devices, or design consideradons might dictate a 20 lesser reflecdvity, likely primarily on the n- conducdvity side of the cavity. Also, the acdve layer may be increased in thickness beyond the minimum permitted by the cavity characterisdcs.
Lateral dimensions of the laser structure depend upon funcdon. Low current threshold values are meaningfully considered in terms of current per unit 25 area so that the same heat dissipadon advantage is in fact realizcd independent of individual devicc area assuming reasonably close packing. For IC use, design criteria generally suggest small spot size, in turn, corresponding with an emitting surface of lateral dimensions 10 or l5~m or smaller.
Following descripdon in this secdon is, in most respects, in terms of 30 likely greatest interest - in terms of low threshold.
2. Acdve Re~ion While the absolute minimum thickncss in final detail dcpends upon thc precise nature of thc materials used (and in turn upon operational charactcrisdcs such as : lascr wavcbngth) studies to date suggest use of acdvc regions of a thickncss no 35 greater than about l~lm. Structure opdmizadon for material systcms most intcnsely r~", studied suggest a prcferred maximum active region thickness of 0.5~m. "Thickness"
rcfers to the endre dimcnsion betwcen confinemcnt layers (or between DBRs if ,~, ,,, .:,, .~,. .
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- . , -~ . . ~ , , ,. Z00~899 conSnement layers are not used, whether of uniform bulk material or of spaced wells - either QWs or conventional. For the material system described in Example 1, high duty cycle, up to CW, is permitted for active layer thickness of 0.25~m. In all systems studied, round-trip gains are easily sufficient to permit lasing with the 5 described Bragg-hybrid Bragg cavides. For opdmally designed MQWs and also for spaced convendonal wells, such gains are attainable for acdve layer thicknesses about the same as for bulk layers, but since a significant part of the active region is made up of spacers, there is less heat generated for given output intensity. A
.', representative MQW structure is describcd in conjunction with FIG. 3. That 10 structure depends on three trios of quantum wells so posidoned by means of spacers as to place them at energy peaks in the standing wave. With spacers, those separadng thc trios, those at the two mirror boundaries, and those between individual wells of each trio, the total thickness is about 0.311m. Examples 2, 3, 5 and 7 depend upon MQW active regions.
From the compositional standpoint, emitdng material in the acdve layer (bullc or well material) is constructed of semiconductor material having a direct bandgap accommodadng the desired emission wavelength. While far more general use is expected, near term use is likely in terms of wavelengths of communicadons interest. Such wavelengths, 0.8611m, 1.311m and 1.55~1m are obtainable by use of20 well-understood compound semiconductors: GaAs at 0.8611m and InGaAsP at the ; other wavelengths. Strained InGaAs may be used at l.OIlm. The invendve devices - are, of course, usable over a much larger wavelength range as permitted by a variety i of semiconductor materials.
It is useful to describe aspccts of the structure in specific exemplary 25 terms. In terms of the structure of Example 1, as indicated, the acdve region is gallium arsenide, GaAs. Composidons noted in following subsecdons are in the same terms - in fact, as used in Example 1.
Another matter, not appropriately discussed in detail, may be mentioned ~ briefly. The effecdve cavity dimension, determinative of the size of the standing ;` 30 wave and, therefore, of emission wavelength, while based primarily on the thickness of the acdve region as discussed, is in fact somewhat thicker due to radiation penetradon, likely through the endrety of confinement layers if present, and to some ,penetradon depth within the DBRs as well. At this dme, determination of precise.~,''?dimensions corresponding with specified wavelength, is accomplished empirically '~35 (by building a first structure, measuring emission ~, adjusdng some dimension -perhaps that of the confinement layer - again measuring, etc).
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200~3899 3. Confinement La~lers Nature and function of confinement layers is discussed elsewhere. See H. C. Casey, Jr. and M. B. Panish, "Heterostructure Lasers: Part A Fundamental Principles", (Academic, N.Y. 1978) Sec. 4.6 (Carrier Confinement) pp. 245-253). Its 5 primary purpose is to confine free carriers to the acdve region. Composition is chosen to yield an energy gap Eg larger than that of the active region (generally a gap differendal of 25 % is adequate. See H. C. Casey, Jr. and M. B. Panish, "Heterostructure Lasers: Part A Fundamental Principles", (Academic, N.Y. 1978) ~~ Sec. 4.6 (Carrier Confinement) pp. 245-253. Thickness of the confinement layer is - 10 sufficient to prevent appreciable tunneling (tunneling is of little consequence for thicknesses greater than about lOOA). Thickness of the confinement layer is not critical - layer thickness of about lOOOA was used in Example 1. The confinementlayer may serve an additional and impo tant funcdon for the heterojunctions generally contemplated - that of minimizing back injection of minority carriers.It is generally preferred that the laser-functioning pn junction be on the end type side of the cavity. Under these circurnstances, the active region is of p-conducdvity type. In structures tested, the confinement layer is doped to the same conducdvity tyype as that of the adjacent DBR - in this instance, is of n-type conducdvity. Such placernent of the functioning junction takes advantage of the fact s~ 20 that electron mobility is inherently greater than that of hole mobility - a design ~; preference of increasing significance for increasing cavity length..~ The desire for maximum ~n from the opdcal standpoint dictates use of ; .j the higher index Bragg material at an air interface and choice of the material yielding :; the greatest ~n at other interfaces (e. g. at the Bragg-acdve region interface).
;.~ 25 However, electrical consideradons - minirnizadon of electrical resistance -somedmes dictates less ~n values.
~'. The nature and funcdon of the confinement layers has been set forth.
While separate confinement layers are usefully employed for the particular material systems of the examples, this is not invariant. Confinement, as well as back 30 injecdon, are both dependent on bandgap of such layers relative to the adjacent :.~ acdve region. Needed increased gap reladve to the active region, as well as appropriate displacement of the relevant band edge values, may be inherent in .~ materials other vise suitable in construcdon of the DBR - on either or both sides of c~ the caviq. Elimination of the need for one or both confinement layers may be of .~ 35 considerable benefit in enhancing efficiency.
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4. Distributed Bra~ Reflectors Design criteria of DBRs are well known. See M. Born and E. Wolf, "Principles of Opdcs", (Pergammon, N.Y. 1964) p. 51 (Principles of Multilaya - Reflectors). See J. P. Van der Ziel and M. Ilegems, "Applied Optics", vol. 14, no.
S 11, Nov. 1975 (MBE Growth). They consist of alternating layers of material of '~, diffaing refracdve index - are generally discussed in terms of "periods" each consisdng of a pair of high and low index material. For the constructive reinforcement of radiadon reflected from successive juncdons, layer thicknesses are an integral number of quarter wavelengths (for the desired coherent radiation as10 meæured in the laya material). To minimize DBR thickness, layers are of ~14 thickness. Minimizadon is useful electrically to minimiæ series resistance, and from the opdcal standpoint, it results in minimiæd scattering (scattering centers correspond with positions of n or p-type dopant atoms3 as well as in minimized insertion loss from stadsdca11y distributed defect sites, etc. Minimum DBR
15 thickness, still corresponding with ~J4 thick layers also minimizes diffracdon losses (greata path lengdl aggravates any edge diffracdon effects otherwise present).
Index values are chosen to yield the maximum attainable value of ~n commensurate with other device criteria. Such criteria include the electrical property, series resistance, and the various opdcal properties. Optimal device design 20 may be quite sophisdcated. As an exampk, one tested device made advantageous ` ;; use of deliberately reduced ~n in the p-type DBR thereby requiring an increase in the number of periods to achieve the dcsired 99+% reflectivity. The primary objecdve., was to reduce barrier hdght at the Bragg juncdons to lessen headng. In this instance, the overall I2R headng was reduced even though the path length was 25 increased Design of the hybrid mirror is complex. Here, permitted loss is most ;;:7~ importantly in oerms of significant p-type resistance (resistance of p-type material is ~,i two to three orders of magnitude greaoer than that of n-type material in the DBR
structure). Generally, the objecdve is a composite reflecdvity sufficient for lasing . 30 under the desired condidons. In terms of the usual inventive thrust, that is, in terrns of low lasing threshold, reflectivity again of ~ 99~o or greater is desired. Structures as used in Examples 1 through 7 depend upon hybrid mirrors of ~period DBRs as ~,,. supplemented by the metallic mir or for yielding this reflectivity.
Reflecdvity of the hybrid mirror is cridcally dependent upon the `~ 35 thickness of the DBR layer adjacent the metallic reflector. It has been indicated that ~ use of unadjusted layer thickness results in destrucdve interference to the extent that :~ reflecdvity is significantly reduced. Under the usua11y preferred design approach in .. .. .
~ , .. . ..
.
:, . ~ . . . ; .
;~0~8899 ..
which the affected DBR layer is of the lesser index valuee, phase matching requires thinning, generally of a value of approximately 25% relative to that of the next DBR
Iaya of lesser index. For structures in which the DBR layer to be adjusted is of the greater DBR index, phase matching requires that it be thickened, generally to an5 extent of approximately 50% relative to the nearest DBR layer of greater index. The explanation is in the difference in phase shift as between the back side reflected wave and the front side reflected wave as dependent upon whether the juncdon is with a preceding DBR layer of high or low index.
There is yet another advantage due to the hybrid mirror structure. The 10 effect of the metallic reflector is to flatten spectral response relative to that of the DBR portion. Where cavity optimization or other consideradon leads to use of different composidons in the two DBRs, spectral flattening increases spectral overlap and alleviates one more design complicadon.
Selecdon of appropriate metallic material for construction of the hybrid 15 mirror is based on a number of factors which need not be discussed in detail.Basically, spectral properties of metals sufficient for selection for use at different emission wavelengths are well known. Other properdes of consequence have to do c with stability e.g. against migradon, e.g. of alloying ekments as caused during heat-, processing (which while useful electrically, may impair reflectivity due to . 20 accompanying interfacial roughening), effectiveness of electrical contact (in the :; instance of FIG. 1 giving rise to use of a contact layer intermediate the metal and the DBR), etc. Both unalloyed silver and gold were used sadsfactorily in structures described in the examples.
~:~ Again, the n-type DBR is generally designed to yield close to perfect 25 reflecdvity for low threshold devices. For the material system of Example 1, twenty pairs were used yielding the desired 99+%.
DBR design has been in terms of constancy of both high and low index layer composidons. A variety of circumstances, intendonal or accidental, may ~, dictate otherwise.
` 30 5. TheSubstrate ~; In general, the substrate does not serve a nccessary device function.
This is especially true in terms of opdcal funcdoning. Reason for being, in most, instances, is to serve as an appropriate body upon which fabrication of the functional device depends - generally in terrns of epitaxy with all of the characteristics and 35 requirements that this imposes. In otha terms, the substrate, since likely present, may serve as a support and may also serve in some secondary electrical sense - e.g.
. for biasing via an electrode on its underside. In funcdoning terms, therefore, the '`' :"
., .: . . . .
... . . .
:` ' " ' :. .
2~0~3899 substrate is merely a part of the structure to be tolerated. From the optical standpoint, use of appropriately broad bandgap material assures transparency for' waveleng;hs corresponding with included energy levels. From the electrical standpoint, the desire is for the best possible conductivity for the usual instance in S which biasing of the supported structure includes the substrate as part of the series path.
By far the most significant characteristic of the substrate, for fabrication approaches usually contemplated, is in terms of crystal parameters and perfection required to assure sufficient absence of defects in epitaxial grown layers.
Choice of substrate is on bases generally understood to those knowledgeable in crystal growth. Substrate materials used in the exarnples in the following section are reasonably illustrative.
6. The Examples Introduction Examples set forth in tabular form are generally representative of - operation at wavelengths of communicadons interest. The purpose is to teach composition and design criteria for fabrication. Specific criteria, both structural and operational, depend upon specific objectives. For example, devices may be operated, in pulse mode by use of pulse generators, using well known designs. Duty cycle . 20 may be varied. Duty cycle is de&ed as the fraction of time that pulse biasing is non-zero (consistent with usual convention, pulses for this purpose are treated as , though square). Devices may be operated at very small duty cycles up to and including 100% (or condnuous _ave"). As stated above, an advantage of the "; invendon in terms of near-term use contemplates thc feasibiliq of high duq cycles 25 due to feasibiliq of operadon at low threshold current. In these terms, a preferred aspect of the invendon is for devices permissive of duty cycles of at least about 50%
(easily attainabb for reported structures having threshold current densities, Jd"oflOkA/cm2). In this respect, too, tabular examples are only representative.
That is, many of the observadons reported in the text are based on room temperature f,` 30 operadon. While avoidance of cooling is certainly economically desirable, there are many instances where e.g. heat sinking in combinadon with thermoelectric coolingmay permit operadon at powa levels, duty cycles, or even wavelength ranges not immediately attainable for specific material systems/device designs without cooling.
..:.
c Concentradon on wavelengths within the general range of 35 0.84~1m-l.55llm is, from the standpoint of present development, most easily accomplished in either of the III-V material systems GaAs or InP. The first, usable over the shorter end of the wavelength range is easily altered in accordance with the ~' :.` ' ~ . . .
~ ' ~
~ , .
Z~)08899 - present state of the art to rçsult in electrical doping and composidonal modification to produce high and/or low refracdve indices for useful Bragg operation. Examples 1 and 2 depend upon pardal subsdtudon, specifica11y 10 At % of Al for Ga for thehigher index of the pair and upon complete subsdtudon (of Al for Ga for the lower 5 index). Confinement layers used in the relevant examples are based on 30 At %
~- subsdtudon of Al for Ga to increase bandgap. Specific composidons used are based on extensive widely acknowledged experience. For example, pardal subsdtution by aluminum in the high index Bragg layer in examples 1 and 2 is designed to lessenabsorption relative to unmodified GaAs for emission of the wavelength range 10 indicated.
~- The examples are to a certain extent based on practical consideradons.
For example, as noted above, unmodified GaAs is significantly absorbing for laser emission within the wavelength range 0.84~0.8711m, and accordingly, an i absorpdon problem is introduced by use of this material in the substrate (for usually 15 contemplated operadon in which the beam is emitted through the substrate). It is , clear that a substrate of A1O.lGaO9As would alleviate this problem. As a pracdcal !;'' matter, wafers of this rnaterial of suitable crystalline perfecdon are not readily ; available. Etching to pernut direct access to emission is the alternadve suggested.
Dimensions and certain structural details set forth in the table are 20 exemplary only. In general, these considerations have been discussed. For example, for consistency, acdve layers are generally set forth either as bulk material layers of thickness 0.2511m or as MQWs consisdng of 9 wells and appropriate spacers as discussed in conjuncdon with FIG. 3. MQW structures of examples 2, 5 and 7 s: depend upon direct energy gaps characterisdc of the reported materials in their 25 equilibrium state. Inclusion of example 3 is to illustrate attainment of needed gaps by delibcrate introducdon of crystallographic strain. In this instance, strain due to crystalline mismatch is introduced into the InO.15GaO.8sAs which, based on experimental observadon, is reduced to a thickness of 80A (reladve to the usual lOOA thickness both of the GaAs wells in this example and of unstrained wells of the 30 other examples). The purpose of thinning, while mildly disadvantageous from a:~ funcdoning standpoint, is to minimize crystalline damagc of the growing layer due ~ to mismatch. Generally, specific MQW structures described are only representative -;. device funcdoning objecdves may dictate greater or lesser numbers of wells. As discussed, in conjuncdon with FIG. 3, useful well structures, sdll retaining some 35 advantage over uniform bullc layers, may make use of conventional wells of bulk material (of wells appreciably larger than ~ lOOA required for true quantum welloperadon).
!
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~ Z0~8899 For the purposes of this description, it is considered adequate to discuss QWs as having a thickness of ~ 100 A. In fact, this thickness is quite appropriate for semiconductor materials generally contemplated. It is beyond the necessary scope of this description to discuss this critical dimension in terms of electron wave 5 funcdons (in terms of deBroglie wavelengths).
Actual device design entails a number of addidonal considerations.
Study has indicated that a metal mirror alone may under best condidons yield a reflecdvity of about 95%. Using gold, as an example, proper phase matching may result in total reflecdvides for the hybrid mirror of 98.2%, 99.1% and 99.5%, 10 respecdvely for three pair, five pair and seven pair hybrid pordons. By comparison, a pure DBR in the same general system requires twenty pairs for 99.5% reflecdvity at an air interface.
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S 11, Nov. 1975 (MBE Growth). They consist of alternating layers of material of '~, diffaing refracdve index - are generally discussed in terms of "periods" each consisdng of a pair of high and low index material. For the constructive reinforcement of radiadon reflected from successive juncdons, layer thicknesses are an integral number of quarter wavelengths (for the desired coherent radiation as10 meæured in the laya material). To minimize DBR thickness, layers are of ~14 thickness. Minimizadon is useful electrically to minimiæ series resistance, and from the opdcal standpoint, it results in minimiæd scattering (scattering centers correspond with positions of n or p-type dopant atoms3 as well as in minimized insertion loss from stadsdca11y distributed defect sites, etc. Minimum DBR
15 thickness, still corresponding with ~J4 thick layers also minimizes diffracdon losses (greata path lengdl aggravates any edge diffracdon effects otherwise present).
Index values are chosen to yield the maximum attainable value of ~n commensurate with other device criteria. Such criteria include the electrical property, series resistance, and the various opdcal properties. Optimal device design 20 may be quite sophisdcated. As an exampk, one tested device made advantageous ` ;; use of deliberately reduced ~n in the p-type DBR thereby requiring an increase in the number of periods to achieve the dcsired 99+% reflectivity. The primary objecdve., was to reduce barrier hdght at the Bragg juncdons to lessen headng. In this instance, the overall I2R headng was reduced even though the path length was 25 increased Design of the hybrid mirror is complex. Here, permitted loss is most ;;:7~ importantly in oerms of significant p-type resistance (resistance of p-type material is ~,i two to three orders of magnitude greaoer than that of n-type material in the DBR
structure). Generally, the objecdve is a composite reflecdvity sufficient for lasing . 30 under the desired condidons. In terms of the usual inventive thrust, that is, in terrns of low lasing threshold, reflectivity again of ~ 99~o or greater is desired. Structures as used in Examples 1 through 7 depend upon hybrid mirrors of ~period DBRs as ~,,. supplemented by the metallic mir or for yielding this reflectivity.
Reflecdvity of the hybrid mirror is cridcally dependent upon the `~ 35 thickness of the DBR layer adjacent the metallic reflector. It has been indicated that ~ use of unadjusted layer thickness results in destrucdve interference to the extent that :~ reflecdvity is significantly reduced. Under the usua11y preferred design approach in .. .. .
~ , .. . ..
.
:, . ~ . . . ; .
;~0~8899 ..
which the affected DBR layer is of the lesser index valuee, phase matching requires thinning, generally of a value of approximately 25% relative to that of the next DBR
Iaya of lesser index. For structures in which the DBR layer to be adjusted is of the greater DBR index, phase matching requires that it be thickened, generally to an5 extent of approximately 50% relative to the nearest DBR layer of greater index. The explanation is in the difference in phase shift as between the back side reflected wave and the front side reflected wave as dependent upon whether the juncdon is with a preceding DBR layer of high or low index.
There is yet another advantage due to the hybrid mirror structure. The 10 effect of the metallic reflector is to flatten spectral response relative to that of the DBR portion. Where cavity optimization or other consideradon leads to use of different composidons in the two DBRs, spectral flattening increases spectral overlap and alleviates one more design complicadon.
Selecdon of appropriate metallic material for construction of the hybrid 15 mirror is based on a number of factors which need not be discussed in detail.Basically, spectral properties of metals sufficient for selection for use at different emission wavelengths are well known. Other properdes of consequence have to do c with stability e.g. against migradon, e.g. of alloying ekments as caused during heat-, processing (which while useful electrically, may impair reflectivity due to . 20 accompanying interfacial roughening), effectiveness of electrical contact (in the :; instance of FIG. 1 giving rise to use of a contact layer intermediate the metal and the DBR), etc. Both unalloyed silver and gold were used sadsfactorily in structures described in the examples.
~:~ Again, the n-type DBR is generally designed to yield close to perfect 25 reflecdvity for low threshold devices. For the material system of Example 1, twenty pairs were used yielding the desired 99+%.
DBR design has been in terms of constancy of both high and low index layer composidons. A variety of circumstances, intendonal or accidental, may ~, dictate otherwise.
` 30 5. TheSubstrate ~; In general, the substrate does not serve a nccessary device function.
This is especially true in terms of opdcal funcdoning. Reason for being, in most, instances, is to serve as an appropriate body upon which fabrication of the functional device depends - generally in terrns of epitaxy with all of the characteristics and 35 requirements that this imposes. In otha terms, the substrate, since likely present, may serve as a support and may also serve in some secondary electrical sense - e.g.
. for biasing via an electrode on its underside. In funcdoning terms, therefore, the '`' :"
., .: . . . .
... . . .
:` ' " ' :. .
2~0~3899 substrate is merely a part of the structure to be tolerated. From the optical standpoint, use of appropriately broad bandgap material assures transparency for' waveleng;hs corresponding with included energy levels. From the electrical standpoint, the desire is for the best possible conductivity for the usual instance in S which biasing of the supported structure includes the substrate as part of the series path.
By far the most significant characteristic of the substrate, for fabrication approaches usually contemplated, is in terms of crystal parameters and perfection required to assure sufficient absence of defects in epitaxial grown layers.
Choice of substrate is on bases generally understood to those knowledgeable in crystal growth. Substrate materials used in the exarnples in the following section are reasonably illustrative.
6. The Examples Introduction Examples set forth in tabular form are generally representative of - operation at wavelengths of communicadons interest. The purpose is to teach composition and design criteria for fabrication. Specific criteria, both structural and operational, depend upon specific objectives. For example, devices may be operated, in pulse mode by use of pulse generators, using well known designs. Duty cycle . 20 may be varied. Duty cycle is de&ed as the fraction of time that pulse biasing is non-zero (consistent with usual convention, pulses for this purpose are treated as , though square). Devices may be operated at very small duty cycles up to and including 100% (or condnuous _ave"). As stated above, an advantage of the "; invendon in terms of near-term use contemplates thc feasibiliq of high duq cycles 25 due to feasibiliq of operadon at low threshold current. In these terms, a preferred aspect of the invendon is for devices permissive of duty cycles of at least about 50%
(easily attainabb for reported structures having threshold current densities, Jd"oflOkA/cm2). In this respect, too, tabular examples are only representative.
That is, many of the observadons reported in the text are based on room temperature f,` 30 operadon. While avoidance of cooling is certainly economically desirable, there are many instances where e.g. heat sinking in combinadon with thermoelectric coolingmay permit operadon at powa levels, duty cycles, or even wavelength ranges not immediately attainable for specific material systems/device designs without cooling.
..:.
c Concentradon on wavelengths within the general range of 35 0.84~1m-l.55llm is, from the standpoint of present development, most easily accomplished in either of the III-V material systems GaAs or InP. The first, usable over the shorter end of the wavelength range is easily altered in accordance with the ~' :.` ' ~ . . .
~ ' ~
~ , .
Z~)08899 - present state of the art to rçsult in electrical doping and composidonal modification to produce high and/or low refracdve indices for useful Bragg operation. Examples 1 and 2 depend upon pardal subsdtudon, specifica11y 10 At % of Al for Ga for thehigher index of the pair and upon complete subsdtudon (of Al for Ga for the lower 5 index). Confinement layers used in the relevant examples are based on 30 At %
~- subsdtudon of Al for Ga to increase bandgap. Specific composidons used are based on extensive widely acknowledged experience. For example, pardal subsdtution by aluminum in the high index Bragg layer in examples 1 and 2 is designed to lessenabsorption relative to unmodified GaAs for emission of the wavelength range 10 indicated.
~- The examples are to a certain extent based on practical consideradons.
For example, as noted above, unmodified GaAs is significantly absorbing for laser emission within the wavelength range 0.84~0.8711m, and accordingly, an i absorpdon problem is introduced by use of this material in the substrate (for usually 15 contemplated operadon in which the beam is emitted through the substrate). It is , clear that a substrate of A1O.lGaO9As would alleviate this problem. As a pracdcal !;'' matter, wafers of this rnaterial of suitable crystalline perfecdon are not readily ; available. Etching to pernut direct access to emission is the alternadve suggested.
Dimensions and certain structural details set forth in the table are 20 exemplary only. In general, these considerations have been discussed. For example, for consistency, acdve layers are generally set forth either as bulk material layers of thickness 0.2511m or as MQWs consisdng of 9 wells and appropriate spacers as discussed in conjuncdon with FIG. 3. MQW structures of examples 2, 5 and 7 s: depend upon direct energy gaps characterisdc of the reported materials in their 25 equilibrium state. Inclusion of example 3 is to illustrate attainment of needed gaps by delibcrate introducdon of crystallographic strain. In this instance, strain due to crystalline mismatch is introduced into the InO.15GaO.8sAs which, based on experimental observadon, is reduced to a thickness of 80A (reladve to the usual lOOA thickness both of the GaAs wells in this example and of unstrained wells of the 30 other examples). The purpose of thinning, while mildly disadvantageous from a:~ funcdoning standpoint, is to minimize crystalline damagc of the growing layer due ~ to mismatch. Generally, specific MQW structures described are only representative -;. device funcdoning objecdves may dictate greater or lesser numbers of wells. As discussed, in conjuncdon with FIG. 3, useful well structures, sdll retaining some 35 advantage over uniform bullc layers, may make use of conventional wells of bulk material (of wells appreciably larger than ~ lOOA required for true quantum welloperadon).
!
. . .
"' ` .
. , `' "; ' . ` ~ ' ' ':, . ' ' ~'.: '~ ~ ' ' r,, : ~ , .. ~ . , ,, .
~ Z0~8899 For the purposes of this description, it is considered adequate to discuss QWs as having a thickness of ~ 100 A. In fact, this thickness is quite appropriate for semiconductor materials generally contemplated. It is beyond the necessary scope of this description to discuss this critical dimension in terms of electron wave 5 funcdons (in terms of deBroglie wavelengths).
Actual device design entails a number of addidonal considerations.
Study has indicated that a metal mirror alone may under best condidons yield a reflecdvity of about 95%. Using gold, as an example, proper phase matching may result in total reflecdvides for the hybrid mirror of 98.2%, 99.1% and 99.5%, 10 respecdvely for three pair, five pair and seven pair hybrid pordons. By comparison, a pure DBR in the same general system requires twenty pairs for 99.5% reflecdvity at an air interface.
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Claims (28)
1. Apparatus comprising at least one electrically pumped vertical cavity laser consisting essentially of an active region within a cavity dependent for cavitation upon two Distributed Bragg Reflectors, each including successive pairs of high and low refractive index material, one of such DBRs being of n-type conductivity and the other being of p-type conductivity, together with means forelectrically pumping, said means including passage of electrical current through at least a substantial number of such pairs in both DBRs characterized in that the p-type DBR has a lesser number of such pairs but is supplemented by a metallic reflector and in that the DBR layer closest to the metallic reflector, referred to as the phase matching DBR layer, is adjusted in thickness relative to that of the next DBR layer of corresponding refractive index to accommodate effective thickness variation due to emission penetration into adjoining material, adjustment increasing reflectivity by lessening destructive interference, to result in a "hybrid" reflector, in which laser cavity reflectivity is due to reflectivity of at least 98% at each cavity end.
2. Apparatus of claim 1 in which the thickness of the active region is a maximum of 1µm in the lasing direction.
3. Apparatus of claim 2 in which reflectivity is at least 98% at each cavity end.
4. Apparatus of claims 1, 2 or 3 in which the active region consists essentially of bulk material.
5. Apparatus of claims 1, 2 or 3 in which the active region consists essentially of at least one well of active material positioned by means of spacer material so as to substantially coincide in position with an energy peak of a cavitated standing wave during operation of said laser.
6. Apparatus of claim 5 in which the said well is a quantum well.
7. Apparatus of claim 5 in which the said well is of sufficient thickness in the lasing direction that it evidences essentially bulk characteristics.
8. Apparatus of claim 1 in which the said active region includes at least two wells and at least three spacers to position wells at at least two energy peaks of a cavitated standing wave during operation of the said laser.
9. Apparatus of claim 1 in which the said laser includes a confinement layer adjacent at least one surface of the said active region for the purpose ofincreasing confinement of free electrical carriers within the said active region, the said confinement layer having an energy bandgap which is greater than that of the DBR layer adjacent the confinement layer.
10. Apparatus of claim 9 in which the said confinement layer is the same conductivity type as that of the adjacent DBR and in which the confinement layerpresents a substantial energy barrier to lessen back injection of minority electrical carriers.
11. Apparatus of claim 10 in which the said confinement layer is on the n-type side of the cavity and in which the energy barrier is the consequence of a confinement layer band edge which is substantially different from that of the adjacent portion of the active region.
12. Apparatus of claim 1 including a substrate supporting the said cavity.
13. Apparatus of claim 12 in which at least the initial portion of the supported structure is epitaxially grown on the said substrate.
14. Apparatus of claim 13 in which epitaxial growth is by MBE.
15. Apparatus of claim 14 in which the said substrate evidences substantial absorption for laser emission and in which a hole is provided in thesubstrate for accessing laser emission.
16. Apparatus of claim 15 including an optical fiber passing through such hole.
17. Apparatus of claim 12 in which the DBR closest to the substrate is of n-type conductivity.
18. Apparatus of claim 17 in which the electrical pump path includes an electrode on the underside of the substrate.
19. Apparatus of claim 18 in which the said electrical pump path includes the metallic reflector.
20. Apparatus of claim 19 in which there is a contact layer intermediate the said metallic reflector and the underlying DBR, the said contact layer assuring ohmic contact.
21. Apparatus of claim 12 comprising an integrated circuit which includes a multiplicity of said electrically pumped lasers.
22. Apparatus of claim 21 in which the said integrated circuit additionally includes electronic elements.
23. Apparatus of claim 22 in which electrical pumping involves functioning of an included electronic element.
24. Apparatus of claim 1 in which the phase matching DBR layer is of said low refractive index and in which its thickness in the emission direction is less relative to that of the next low index DBR layer.
25. Apparatus of claim 24 in which the thickness of the phase matching DBR layer is about 25% less than that of the next low index DBR layer.
26. Apparatus of claim 1 in which the phase matching DBR layer is of high refractive index and in which its thickness in the emission direction is increased relative to that of the next high index DBR layer.
27. Apparatus of claim 26 in which the thickness of the phase matching DBR layer is about 50% larger than that of the next high index DBR layer.
28. Apparatus of claim 1 in which the said DBRs are of different chemical composition resulting in differing spectral reflectivity and in which spectral reflectivity is flattened by inclusion of the said metallic reflector, thereby increasing cavity efficiency relative to that of a structure excluding the said metallic reflector.
Applications Claiming Priority (3)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US07/343,760 US4991179A (en) | 1989-04-26 | 1989-04-26 | Electrically pumped vertical cavity laser |
| US343,760 | 1989-04-26 | ||
| SG44894A SG44894G (en) | 1989-04-26 | 1994-03-26 | Electrically pumped vertical cavity laser |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| CA2008899A1 CA2008899A1 (en) | 1990-10-26 |
| CA2008899C true CA2008899C (en) | 1994-02-01 |
Family
ID=26663995
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002008899A Expired - Lifetime CA2008899C (en) | 1989-04-26 | 1990-01-30 | Electrically pumped vertical cavity laser |
Country Status (8)
| Country | Link |
|---|---|
| US (1) | US4991179A (en) |
| EP (1) | EP0395315B1 (en) |
| JP (1) | JPH07105572B2 (en) |
| KR (1) | KR940001793B1 (en) |
| CA (1) | CA2008899C (en) |
| DE (1) | DE69006087T2 (en) |
| HK (1) | HK108194A (en) |
| SG (1) | SG44894G (en) |
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| DE69132764T2 (en) * | 1990-11-02 | 2002-07-11 | Norikatsu Yamauchi | Semiconductor device with reflective layer |
| US5313324A (en) * | 1991-03-27 | 1994-05-17 | Massachusetts Institute Of Technology | Solid state optical converter |
| JPH04333290A (en) * | 1991-05-08 | 1992-11-20 | Nec Corp | Vertical resonator type area emission laser and area input-output photoelectric fusion element |
| US5625636A (en) * | 1991-10-11 | 1997-04-29 | Bryan; Robert P. | Integration of photoactive and electroactive components with vertical cavity surface emitting lasers |
| US5206872A (en) * | 1991-11-01 | 1993-04-27 | At&T Bell Laboratories | Surface emitting laser |
| US5258990A (en) * | 1991-11-07 | 1993-11-02 | The United States Of America As Represented By The Secretary Of The United States Department Of Energy | Visible light surface emitting semiconductor laser |
| US5404373A (en) * | 1991-11-08 | 1995-04-04 | University Of New Mexico | Electro-optical device |
| FR2685098B1 (en) * | 1991-12-12 | 1995-02-10 | Andre Schiltz | METHOD OF MOUNTING AND OPTICAL COUPLING ON A SUBSTRATE AND SUBSTRATE PROVIDED WITH OPTICAL FIBER. |
| US5408105A (en) * | 1992-02-19 | 1995-04-18 | Matsushita Electric Industrial Co., Ltd. | Optoelectronic semiconductor device with mesa |
| US5325386A (en) * | 1992-04-21 | 1994-06-28 | Bandgap Technology Corporation | Vertical-cavity surface emitting laser assay display system |
| US5245622A (en) * | 1992-05-07 | 1993-09-14 | Bandgap Technology Corporation | Vertical-cavity surface-emitting lasers with intra-cavity structures |
| US5244749A (en) * | 1992-08-03 | 1993-09-14 | At&T Bell Laboratories | Article comprising an epitaxial multilayer mirror |
| DE4240706A1 (en) * | 1992-12-03 | 1994-06-09 | Siemens Ag | Surface emitting laser diode |
| JPH06347734A (en) * | 1993-06-11 | 1994-12-22 | Nec Corp | Surface type optical switch |
| US5475701A (en) * | 1993-12-29 | 1995-12-12 | Honeywell Inc. | Integrated laser power monitor |
| US5432809A (en) * | 1994-06-15 | 1995-07-11 | Motorola, Inc. | VCSEL with Al-free cavity region |
| FR2724056B1 (en) * | 1994-08-23 | 1996-11-15 | France Telecom | OPTICAL, OPTOELECTRONIC OR PHOTONIC COMPONENT CONTAINING AT LEAST ONE LATERAL CONFINED OPTICAL CAVITY AND PROCESS FOR ITS REALIZATION |
| US6243407B1 (en) | 1997-03-21 | 2001-06-05 | Novalux, Inc. | High power laser devices |
| US5960024A (en) | 1998-03-30 | 1999-09-28 | Bandwidth Unlimited, Inc. | Vertical optical cavities produced with selective area epitaxy |
| US6493372B1 (en) | 1998-04-14 | 2002-12-10 | Bandwidth 9, Inc. | Vertical cavity apparatus with tunnel junction |
| US6760357B1 (en) | 1998-04-14 | 2004-07-06 | Bandwidth9 | Vertical cavity apparatus with tunnel junction |
| US6535541B1 (en) | 1998-04-14 | 2003-03-18 | Bandwidth 9, Inc | Vertical cavity apparatus with tunnel junction |
| US6493373B1 (en) | 1998-04-14 | 2002-12-10 | Bandwidth 9, Inc. | Vertical cavity apparatus with tunnel junction |
| US6487231B1 (en) | 1998-04-14 | 2002-11-26 | Bandwidth 9, Inc. | Vertical cavity apparatus with tunnel junction |
| US5991326A (en) | 1998-04-14 | 1999-11-23 | Bandwidth9, Inc. | Lattice-relaxed verticle optical cavities |
| US6487230B1 (en) | 1998-04-14 | 2002-11-26 | Bandwidth 9, Inc | Vertical cavity apparatus with tunnel junction |
| US6493371B1 (en) | 1998-04-14 | 2002-12-10 | Bandwidth9, Inc. | Vertical cavity apparatus with tunnel junction |
| US6226425B1 (en) | 1999-02-24 | 2001-05-01 | Bandwidth9 | Flexible optical multiplexer |
| US6233263B1 (en) | 1999-06-04 | 2001-05-15 | Bandwidth9 | Monitoring and control assembly for wavelength stabilized optical system |
| US6275513B1 (en) | 1999-06-04 | 2001-08-14 | Bandwidth 9 | Hermetically sealed semiconductor laser device |
| US6577658B1 (en) * | 1999-09-20 | 2003-06-10 | E20 Corporation, Inc. | Method and apparatus for planar index guided vertical cavity surface emitting lasers |
| NL1015714C2 (en) * | 2000-07-14 | 2002-01-15 | Dsm Nv | Process for crystallizing enantiomerically enriched 2-acetylthio-3-phenylpropanoic acid. |
| US6696308B1 (en) * | 2000-10-27 | 2004-02-24 | Chan-Long Shieh | Electrically pumped long-wavelength VCSEL with air gap DBR and methods of fabrication |
| US6671304B2 (en) | 2001-08-28 | 2003-12-30 | The United States Of America As Represented By The Secretary Of The Navy | Amplitude-modulated laser for high-bandwidth communications systems |
| DE102004057802B4 (en) * | 2004-11-30 | 2011-03-24 | Osram Opto Semiconductors Gmbh | Radiation-emitting semiconductor component with intermediate layer |
| DE102006004591A1 (en) * | 2005-09-29 | 2007-04-05 | Osram Opto Semiconductors Gmbh | Radiation-emitting semiconductor chip |
| EP1974423A4 (en) * | 2006-01-20 | 2010-06-09 | Massachusetts Inst Technology | Surface-emitting fiber laser |
| EP1995794A4 (en) * | 2006-03-10 | 2011-08-31 | Panasonic Elec Works Co Ltd | Light-emitting device |
| US7772615B2 (en) * | 2007-08-10 | 2010-08-10 | Connector Optics | Anti stark electrooptic medium and electrooptically modulated optoelectronic device based thereupon |
| JP6450743B2 (en) | 2013-03-15 | 2019-01-09 | プレビウム リサーチ インコーポレイテッド | Variable laser array system |
| EP2967468A4 (en) * | 2013-03-15 | 2017-03-08 | Praevium Research, Inc. | Widely tunable swept source |
| JP2017216348A (en) * | 2016-05-31 | 2017-12-07 | 三菱電機株式会社 | End face emission type semiconductor laser |
| CN113675726A (en) * | 2021-10-21 | 2021-11-19 | 福建慧芯激光科技有限公司 | Epitaxial structure of high-speed vertical cavity surface emitting laser |
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| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4163953A (en) * | 1977-07-07 | 1979-08-07 | Northern Telecom Limited | Double heterostructure laser for direct coupling to an optical fiber |
| US4309670A (en) * | 1979-09-13 | 1982-01-05 | Xerox Corporation | Transverse light emitting electroluminescent devices |
| JPS58180080A (en) * | 1982-04-15 | 1983-10-21 | Mitsubishi Electric Corp | Semiconductor laser device |
| NL8602653A (en) * | 1986-10-23 | 1988-05-16 | Philips Nv | SEMICONDUCTOR LASER AND METHOD OF MANUFACTURE THEREOF. |
| US4873696A (en) * | 1988-10-31 | 1989-10-10 | The Regents Of The University Of California | Surface-emitting lasers with periodic gain and a parallel driven nipi structure |
| US4999842A (en) * | 1989-03-01 | 1991-03-12 | At&T Bell Laboratories | Quantum well vertical cavity laser |
-
1989
- 1989-04-26 US US07/343,760 patent/US4991179A/en not_active Expired - Lifetime
-
1990
- 1990-01-30 CA CA002008899A patent/CA2008899C/en not_active Expired - Lifetime
- 1990-04-20 DE DE69006087T patent/DE69006087T2/en not_active Expired - Lifetime
- 1990-04-20 EP EP90304256A patent/EP0395315B1/en not_active Expired - Lifetime
- 1990-04-25 KR KR1019900005802A patent/KR940001793B1/en not_active IP Right Cessation
- 1990-04-25 JP JP2107735A patent/JPH07105572B2/en not_active Expired - Lifetime
-
1994
- 1994-03-26 SG SG44894A patent/SG44894G/en unknown
- 1994-10-06 HK HK108194A patent/HK108194A/en not_active IP Right Cessation
Also Published As
| Publication number | Publication date |
|---|---|
| JPH02302085A (en) | 1990-12-14 |
| EP0395315B1 (en) | 1994-01-19 |
| JPH07105572B2 (en) | 1995-11-13 |
| SG44894G (en) | 1995-03-17 |
| CA2008899A1 (en) | 1990-10-26 |
| KR900017241A (en) | 1990-11-15 |
| US4991179A (en) | 1991-02-05 |
| HK108194A (en) | 1994-10-14 |
| EP0395315A2 (en) | 1990-10-31 |
| EP0395315A3 (en) | 1991-09-11 |
| DE69006087D1 (en) | 1994-03-03 |
| DE69006087T2 (en) | 1994-05-26 |
| KR940001793B1 (en) | 1994-03-05 |
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