US20030012240A1 - Semiconductor laser and method of manufacturing the same - Google Patents
Semiconductor laser and method of manufacturing the same Download PDFInfo
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- US20030012240A1 US20030012240A1 US09/453,546 US45354699A US2003012240A1 US 20030012240 A1 US20030012240 A1 US 20030012240A1 US 45354699 A US45354699 A US 45354699A US 2003012240 A1 US2003012240 A1 US 2003012240A1
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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
- H01S5/20—Structure or shape of the semiconductor body to guide the optical wave ; Confining structures perpendicular to the optical axis, e.g. index or gain guiding, stripe geometry, broad area lasers, gain tailoring, transverse or lateral reflectors, special cladding structures, MQW barrier reflection layers
- H01S5/22—Structure or shape of the semiconductor body to guide the optical wave ; Confining structures perpendicular to the optical axis, e.g. index or gain guiding, stripe geometry, broad area lasers, gain tailoring, transverse or lateral reflectors, special cladding structures, MQW barrier reflection layers having a ridge or stripe structure
- H01S5/227—Buried mesa structure ; Striped active layer
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B82—NANOTECHNOLOGY
- B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
- B82Y20/00—Nanooptics, e.g. quantum optics or photonic crystals
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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
- H01S2304/00—Special growth methods for semiconductor lasers
- H01S2304/04—MOCVD or MOVPE
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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
- H01S5/20—Structure or shape of the semiconductor body to guide the optical wave ; Confining structures perpendicular to the optical axis, e.g. index or gain guiding, stripe geometry, broad area lasers, gain tailoring, transverse or lateral reflectors, special cladding structures, MQW barrier reflection layers
- H01S5/22—Structure or shape of the semiconductor body to guide the optical wave ; Confining structures perpendicular to the optical axis, e.g. index or gain guiding, stripe geometry, broad area lasers, gain tailoring, transverse or lateral reflectors, special cladding structures, MQW barrier reflection layers having a ridge or stripe structure
- H01S5/2205—Structure or shape of the semiconductor body to guide the optical wave ; Confining structures perpendicular to the optical axis, e.g. index or gain guiding, stripe geometry, broad area lasers, gain tailoring, transverse or lateral reflectors, special cladding structures, MQW barrier reflection layers having a ridge or stripe structure comprising special burying or current confinement 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
- H01S5/20—Structure or shape of the semiconductor body to guide the optical wave ; Confining structures perpendicular to the optical axis, e.g. index or gain guiding, stripe geometry, broad area lasers, gain tailoring, transverse or lateral reflectors, special cladding structures, MQW barrier reflection layers
- H01S5/22—Structure or shape of the semiconductor body to guide the optical wave ; Confining structures perpendicular to the optical axis, e.g. index or gain guiding, stripe geometry, broad area lasers, gain tailoring, transverse or lateral reflectors, special cladding structures, MQW barrier reflection layers having a ridge or stripe structure
- H01S5/227—Buried mesa structure ; Striped active layer
- H01S5/2275—Buried mesa structure ; Striped active layer mesa created by etching
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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
- H01S5/30—Structure or shape of the active region; Materials used for the active region
- H01S5/34—Structure or shape of the active region; Materials used for the active region comprising quantum well or superlattice structures, e.g. single quantum well [SQW] lasers, multiple quantum well [MQW] lasers or graded index separate confinement heterostructure [GRINSCH] lasers
- H01S5/343—Structure or shape of the active region; Materials used for the active region comprising quantum well or superlattice structures, e.g. single quantum well [SQW] lasers, multiple quantum well [MQW] lasers or graded index separate confinement heterostructure [GRINSCH] lasers in AIIIBV compounds, e.g. AlGaAs-laser, InP-based laser
- H01S5/34306—Structure or shape of the active region; Materials used for the active region comprising quantum well or superlattice structures, e.g. single quantum well [SQW] lasers, multiple quantum well [MQW] lasers or graded index separate confinement heterostructure [GRINSCH] lasers in AIIIBV compounds, e.g. AlGaAs-laser, InP-based laser emitting light at a wavelength longer than 1000nm, e.g. InP based 1300 and 1500nm lasers
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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
- H01S5/30—Structure or shape of the active region; Materials used for the active region
- H01S5/34—Structure or shape of the active region; Materials used for the active region comprising quantum well or superlattice structures, e.g. single quantum well [SQW] lasers, multiple quantum well [MQW] lasers or graded index separate confinement heterostructure [GRINSCH] lasers
- H01S5/343—Structure or shape of the active region; Materials used for the active region comprising quantum well or superlattice structures, e.g. single quantum well [SQW] lasers, multiple quantum well [MQW] lasers or graded index separate confinement heterostructure [GRINSCH] lasers in AIIIBV compounds, e.g. AlGaAs-laser, InP-based laser
- H01S5/3434—Structure or shape of the active region; Materials used for the active region comprising quantum well or superlattice structures, e.g. single quantum well [SQW] lasers, multiple quantum well [MQW] lasers or graded index separate confinement heterostructure [GRINSCH] lasers in AIIIBV compounds, e.g. AlGaAs-laser, InP-based laser with a well layer comprising at least both As and P as V-compounds
Definitions
- the present invention relates to a semiconductor laser and a method of manufacturing the same, more particularly, a semiconductor laser employed in optical fiber communication and having a buried heterostructure and a method of manufacturing the same.
- a buried heterostructure is employed in the semiconductor laser used in the optical fiber communication.
- Such buried heterostructure is employed to inject a current to the active layer efficiently, and there are a buried heterostructure using a pn junction and a buried heterostructure using a semi-insulating layer.
- the buried heterostructure using the pn junction is suitable for the high temperature operation.
- the semiconductor laser having the pn-junction buried heterostructure has a structure shown in FIG. 1, for example.
- an active layer 2 of InGaAsP and a first p-type cladding layer 3 of p-InP are formed on an n-type InP substrate 1 .
- Layers from the first p-type cladding layer 3 to an upper area of the n-InP substrate 1 are formed like a mesa shape to form a mesa portion.
- the active layer 2 in the mesa portion is formed as a stripe shape having a width of about 1 to 1.5 ⁇ m.
- the buried heterostructures are provided on both sides of the mesa portion.
- a p-type buried layer 4 of p-InP and an n-type current blocking layer 5 of n-InP are formed in the buried regions. Then, a second p-type cladding layer 6 formed of p-InP and a p-type contact layer 7 formed of p-InGaAs are formed in sequence on the n-type current blocking layer 5 and the first p-type cladding layer 3 .
- a p-side electrode 8 is formed on the p-type contact layer 7 and an n-side electrode 9 is formed under the InP substrate 1 .
- the manufacturing method of the semiconductor laser having such buried heterostructure comprises the steps of forming the buried heterostructures by growing the active layer 2 and the first p-type cladding layer 3 on the n-InP substrate 1 , and forming a substntial stripe shape layers by etching from the first p-type cladding layer 3 to the InP substrate 1 by using a mask, and then forming the p-type buried layer 4 and the n-type current blocking layer 5 on both sides of the substantial stripe shape layers.
- a quantum well structure or a strained-layer quantum well structure is employed as the active layer in many cases.
- the active layer shown hereinafter means not only the quantum well structure consisting of a well layer and a barrier layer but also a structure including the quantum well structure and upper and lower light guiding layers provided to put the quantum well structure between them.
- both sides of the active layer 2 are buried by the p-type buried layers 4 , and such layers are connected to the p-type cladding layers 3 , 6 formed directly on the active layer 2 .
- the leakage current which flows from the p-type cladding layers 3 , 6 to the n-type InP substrate 1 via the p-type buried layers 4 , via routes indicated by arrows in FIG. 1, is generated in the high temperature operation. Since the leakage current depends on an interval between the active layer 2 and the n-type current blocking layer 5 , a distance between the active layer 2 and the n-type current blocking layer 5 must be narrowed into about 0.2 ⁇ m, for example, in order to reduce the leakage current. In addition, such distance must be fabricated with good controllability in order to achieve the uniform laser characteristic.
- an innermost point of the n-type current blocking layer 5 is set on an edge of a top surface of the mesa portion, but an angle ⁇ of the bottom surface of the n-type current blocking layer 5 spreads in the neighborhood of the active layer 2 at a gentle angle of about 30 degrees relative to the horizontal direction. Therefore, the distance between the active layer 2 and the n-type current blocking layer 5 is abruptly increased downward, so that a width of the area through which the leakage current flows is excessively widely increased.
- the angle ⁇ of the bottom face of the n-type current blocking layer 5 depends on an angle of an upper surface of the p-type buried layer 4 formed under the n-type current blocking layer 5 .
- the (111) facet which has a slow growth rate appears at the beginning of growth in the crystal growth of the p-type buried layers 4 , and then such bottom face having a gentle angle of about 30 degrees appears to start its growth on the (111) facet because of the dependence of the growth rate on facet orientation.
- a position and an angle of such bottom face are very sensitive to a height of the mesa portion, a lower shape of the mesa portion, change in the growth rates in respective face orientations due to the change in the growth conditions, etc.
- an angle of the side surfaces of the active layer which is formed on the mesa-type first cladding layer is set in the range of 70 to 90 degrees relative to the upper surface of the first cladding layer, then one end of the current blocking layer is brought into contact to an upward extension from the side surface fo the active layer, and then an angle of the facet of the current blocking layer which extends downward from the one end below the active layer is substantially inclined by 55 degrees.
- the passage area for the leakage current which flows from the second cladding layer located over the active layer to the buried layer is made small to thus reduce the leakage current.
- the current-optical output power characteristic can be made uniform at the time of high temperature and high output power.
- Such semiconductor laser manufacturing method can be attained by forming the active layer and the lower layer portion of the second cladding layer in sequence on the first cladding layer, then forming the mesa portion by patterning the layers from the lower layer portion of the second cladding layer to the upper layer portion of the first cladding layer by using the dry etching, and then forming the current blocking layer on the buried layer by controlling the growth of the buried layer such that the (111) facet exists from the side area of the active layer to the lower side thereof.
- the (111) facet of the buried layer has an inclination of about 55 degrees relative to the substrate surface.
- formation of another facet on the (111) facet can be prevented previously by leaving the facet, which is formed in parallel with the side surface of the mesa portion, under the (111) facet of the buried layer.
- the angle of the side surfaces of the active layer which is formed on the mesa-type first cladding layer is set in the range of 70 to 90 degrees relative to the upper surface of the first cladding layer, then one end of the current blocking layer is brought into contact to an upward extension from the side surface, and then the angle of the facet of the current blocking layer which extends downward from the one end is substantially inclined by 55 degrees, and then the angle of the other facet of current blocking layer which is formed to side of the active layer is set larger than an angle of the side surfaces of the active layer but smaller than 90 degrees on both sides of the active layer.
- Such semiconductor laser manufacturing method can be attained by forming the active layer and the lower layer portion of the second cladding layer in sequence on the first cladding layer, then forming the mesa portion by patterning the layers from the lower layer portion of the second cladding layer to the upper layer portion of the first cladding layer by using the dry etching, and then controlling the growth of the buried layer such that the (111) facet exists over the active layer and the facet which is substantially parallel with the active layer appears below the (111) facet.
- a film thickness of the buried layer between the first cladding layer and the current blocking layer must be sufficiently thick to prevent turn ON of a parasitic thyristor while reducing a film thickness of the buried layer which is to be grown on the side surface of the active layer.
- a desired film thickness is formed on the side surface of the active layer by using a chlorine containing gas in growth of the buried layer, and then an introduced amount of the chlorine containing gas is increased. Accordingly, the film thickness formed on the substrate surface can be increased locally.
- FIG. 1 is a sectional view showing a semiconductor laser in the prior art
- FIGS. 2A to 2 G are sectional views each showing structures of a buried layer employed in a semiconductor laser of an embodiment of the present invention according to respective growing processes;
- FIGS. 3A to 3 C are sectional views showing growing steps for layers of a first example of a semiconductor laser according to the embodiment of the present invention.
- FIG. 3D is a sectional view showing the first example of the semiconductor laser according to the embodiment of the present invention.
- FIG. 4 is a sectional view showing a second example of the semiconductor laser according to the embodiment of the present invention.
- FIGS. 5A to 5 C are sectional views showing manufacturing steps for a third example of a semiconductor laser according to the embodiment of the present invention.
- FIG. 5D is a sectional view showing the third example of the semiconductor laser according to the embodiment of the present invention.
- FIGS. 2A to 2 G show steps carried out until formation of a buried layer of a semiconductor laser according to an embodiment of the present invention is completed.
- an n-type buffer layer (n-type cladding layer) 22 formed of n-InP of 300 to 1000 nm film thickness, a MQW (multi quantum well) active layer 23 formed of undoped InGaAsP of 200 to 300 nm film thickness, and a first p-type cladding layer 24 formed of p-InP of 250 to 700 nm film thickness are formed on a (100) facet of an n-InP substrate 21 by the MOVPE method.
- the InP is grown by using trimethylindium (TMIn) and phosphine (PH 3 ) as a material gas.
- TMIn trimethylindium
- PH 3 phosphine
- the InGaAsP is grown by using TMIn, PH 3 , arsine (AsH 3 ), and triethylgallium (TEGa) as a material gas.
- DMZn dimethylzinc
- SiH 4 silane
- the MQW active layer 23 comprises, for example, five periodical well layers of InGaAsP having a thickness of 6 nm and 1% compressive strain, barrier layers each formed between the well layers and formed of InGaAsP whose lattice matches InP and which has a thickness of 10 nm, and light guiding layers each formed on and under the quantum well structure which consists of well and barrier layers and formed of InGaAsP of 100 nm thickness.
- Each of the barrier layers and the light guiding layers is formed of InGaAsP whose bandgap wavelength is 1.1 ⁇ m. Accordingly, a semiconductor laser whose wavelength band is 1.3 ⁇ m can be formed.
- a layer structure of the MQW active layer 23 is not limited to such structure.
- the active layer may be formed of an InGaAsP system which can provide a 1.55 ⁇ m band, a 1.48 ⁇ m band, and other wavelength bands.
- An impurity concentration of the n-InP buffer layer 22 is about 5 ⁇ 10 17 atoms/cm 3
- an impurity concentration of the p-InP cladding layer 24 is about 5 ⁇ 10 17 atoms/cm 3 .
- Both the n-InP buffer layer 22 formed of n-InP under the active layer 23 and the n-InP substrate 21 act as an n-type cladding layer.
- a dielectric film e.g., an SiO 2 film, having a thickness of about 0.3 ⁇ m is formed on the p-InP first p-type cladding layer 24 by the CVD method. Then, the dielectric film is patterned into a narrow stripe 25 , which has a width of about 1.5 ⁇ m and extends along the ⁇ 011> direction, by the photolithography method. The dielectric stripe 25 is used as a mask.
- the p-InP cladding layer 24 to the n-InP substrate 21 which are not covered with the mask 25 , are etched up to a depth of almost 2 to 3 ⁇ m to thus form a mesa portion 26 under the mask 25 .
- the mesa portion 26 is a projection which has a mesa-like sectional shape and a stripe-like planar shape.
- the etching is performed by the RIE (Reactive Ion Etching) method, and employs an ethane type gas, e.g., a mixed gas of C 2 H 6 , O 2 , and H 2 .
- this p-InP buried layer 27 will be grown according to following steps.
- the mask 25 is not extended from a top surface of the mesa portion 26 like a visor. Also, side surfaces of the mesa portion 26 are inclined sharply to have a 70 degrees or more relative to the horizontal direction (i.e., the substrate surface).
- the p-InP buried layer 27 is formed by the second MOVPE method.
- overgrowth of InP on the mask 25 can be prevented by introducing TMIn and PH 3 as a material gas into a reaction chamber of a MOVPE equipment and also introducing methyl chloride (CH 3 Cl) together with the material gas into the reaction chamber, so that (111) facets can be grown from both edges of the top surface of the mesa portion 26 .
- CH 3 Cl methyl chloride
- the (111) facets appear downward from the edges of the top surface of the mesa portion 26 , and two facets which parallel substantially the side surfaces of the mesa portion 26 appear on both sides of the mesa portion 26 , other two facets which have a gentle angle of about 30 degrees relative to the substrate surface appear in the neighborhood of the bottom portion of the mesa portion 26 , and (100) facets appear on the substrate surface on both sides of the mesa portion 26 .
- the (111) facets extend obliquely downward to spread much more, while the two facets which parallel the side surfaces of the mesa portion 26 are shortened along the side surfaces. Also, with the progress of the growth, the two facets, which substantially paralleled both sides of the mesa portion 26 at the beginning of the growth, become close gradually to the vertical direction relative to the substrate surface.
- the other two facets of the buried layer 27 each is grown from near the bottom of the mesa portion 26 to have the gentle angle of about 30 degrees, erode the (111) facets according to the furthermore progress of the growth of the p-InP buried layer 27 , therefore the (111) facets are shortened.
- These appearances of above shapes are due to difference in the growth rates in respective facet orientations.
- the semiconductor laser described in the following shows an example wherein the growth of the p-InP buried layer 27 is stopped at the point of time when the facets, which are formed in parallel with the side surfaces of the mesa portion 26 , of the p-InP buried layer 27 disappear, and then a current blocking layer is formed.
- the (111) facets, 30-degrees inclined faces, and the (100) facets of the p-InP buried layer 27 whose growth has been completed are exposed in sequence along the direction from the edges of the top surface of the mesa portion 26 to the bottom.
- a thickness of the active layer 23 is about 0.3 ⁇ m
- a thickness of the first p-type cladding layer 24 is 0.4 ⁇ m
- a height of the mesa portion 26 is about 2 ⁇ m
- an angle of the side surface of the active layer 23 of the mesa portion 26 is about 83 degrees.
- a film thickness of the flat portion of the p-InP buried layer 27 is 0.7 ⁇ m
- an angle of the (111) facet of the p-InP buried layer 27 is about 55 degrees relative to the horizontal line.
- the bottom surface of the n-InP current blocking layer 28 has the same shape as the upper surface of the p-InP buried layer 27 . Also, one end of the n-InP current blocking layer 28 is positioned over the active layer 23 by 0.4 ⁇ m along an extension of the side surface of the mesa portion 26 .
- a film thickness of the p-InP buried layer 27 is about 0.4 ⁇ m at the lower end of the side surface of the active layer 24 .
- a shortest distance from an upper end of the active layer 24 to the n-InP current blocking layer 28 is about 0.19 ⁇ m.
- the thickness of the first p-type cladding layer 24 and an inclination of the side surface of the mesa portion 26 may be adjusted such that this shortest distance is present in the range of 0.1 to 0.3 ⁇ m.
- a film thickness of the n-InP current blocking layer 28 is set to about 0.9 ⁇ m in the flat region, the (111) facet as the bottom surface of the n-InP current blocking layer 28 , which is a boundary between the p-InP buried layer 27 and the n-InP current blocking layer 28 , extend below the active layer 24 from its one end which is closest to the first p-type cladding layer 24 .
- an upward-inclined surface having (111) facet, a flat surface, and a downward-inclined surface appear in sequence from its one end which contacts to the side surface of the mesa portion 26 .
- a second p-type cladding layer 29 which is formed of p-InP to have a film thickness of about 1.5 ⁇ m, is formed on the n-InP current blocking layer 28 and the first p-type cladding layer 24 . Then, an intermediate layer 30 formed of p-InGaAsP having a 0.2 ⁇ m film thickness and a contact layer 31 formed of p + -type InGaAs having a 0.5 ⁇ m film thickness are formed on the second p-type cladding layer 29 .
- a p-side electrode 32 made of Ti/Pt/Au is formed on the contact layer 31 , and then an n-side electrode 33 made of AuGe/Au is formed on the lower surface of the n-InP substrate 21 .
- the n-InP current blocking layer 28 is positioned closely to the active layer at a distance of less than 0.2 ⁇ m.
- the region in which the first p-type cladding layer 24 , the second p-type cladding layer 29 , and the p-InP buried layer 27 are coupled can be narrowed rather than the prior art, and thus a leakage current which is passed through such region can be reduced.
- the growth condition of the buried layer 27 , etc. are set such that the (111) facet, which appears in the neighborhood of the active layer 23 , of the p-InP buried layer 27 can be extended below the active layer 23 , the (111) facet can be positioned on both sides of the active layer 23 even if a height of the mesa portion 26 is slightly changed due to the error in fabrication.
- a distance between the n-InP current blocking layer 28 and the active layer 23 is substantially decided by a distance from the active layer 23 to the top surface of the mesa portion 26 (i.e., the first p-type cladding layer 24 ), and the error in the distance between the n-InP current blocking layer 28 and the active layer 23 in fabrication only depends on the deviation of the angle of the side surface of the mesa portion 26 . Accordingly, a shortest width of the region through which the leakage current flows, i.e., a shortest distance of the clearance between the n-InP current blocking layer 28 and the active layer 23 does not depend on the growth conditions of the films and displacement of the height of the mesa portion 26 in fabrication. As a result, a magnitude of the leakage current can be reduced stably, the current-optical output power characteristic can be made uniform at the time of high temperature and high output power, and reproducibility can be improved.
- the upper (111) facet of the n-InP current blocking layer 28 is inclined by the angle of about 55 degrees and thus the uppermost portion of the (111) facet is positioned higher than the mesa portion 26 .
- the upper portions of the n-InP current blocking layer 28 which are located on both sides of the mesa portion 26 , are made narrower toward the active layer 23 like a taper-shape, the injection current can be collected effectively into the upper area of the active layer 23 because the film thickness of the n-InP current blocking layer 28 is increased suddenly large in the neighborhood of the active layer 23 .
- the side surfaces of the mesa portion 26 are formed extremely close to the vertical direction such as about 83 degrees, the distance between the n-InP layers (the n-InP buffer layer 22 and the n-InP substrate 21 ) under the active layer 23 and the n-InP current blocking layer 28 can be spread widely downward.
- a pnpn thyristor which consists of the p-type cladding layers 24 and 29 , the n-InP current blocking layer 28 , the p-InP buried layer 27 , and the n-InP layers (the n-InP buffer layer 22 and the n-InP substrate 21 ) formed as the lower portion of the mesa portion 26 is difficult to turn ON.
- a second example of the semiconductor laser is characterized by a structure in which, since the height of the mesa portion 26 is set higher like about 2.5 ⁇ m, the p-InP surfaces which are parallel with the side surfaces of the mesa portion can still remain on the p-InP buried layer 27 at the stage where the growth of the p-InP buried layer 27 is completed. In other words, in the second example, the growth of the p-InP buried layer is stopped at the stage shown in FIG. 2E.
- the n-InP current blocking layer 28 is formed on the p-InP buried layer 27 , then the mask 25 is removed, and then the second p-type cladding layer 29 , the p-InGaAsP intermediate layer 30 , and the contact layer 31 formed of p + -type InGaAs are formed on the n-InP current blocking layer 28 and the first p-type cladding layer 24 by the MOVPE method. Then, a semiconductor laser having a structure shown in FIG. 4 can be obtained by forming the p-side electrode 32 and the n-side electrode 33 .
- the p-InP cannot grow on the (111) facet of the p-InP buried layer 27 even when the growth rate of the p-InP buried layer 27 is slightly varied according to any change in the conditions.
- the thickness control of the p-InP film on the active layer 23 i.e., the control of the distance between the n-InP current blocking layer 28 and the active layer 23 can be made more easily without fail.
- the growth of the p-InP buried layer 27 is stopped in the situation between FIG. 2E and FIG. 2F, and then the n-InP current blocking layer 28 is formed.
- the growth of the p-InP buried layer 27 is stopped in the situation close to FIG. 2D, and then steps of growing the n-InP current blocking layer, etc. are employed.
- the (111) facet appears obliquely downward from the edges of the top surface of the mesa portion 26 , faces which are almost parallel with the side surfaces of the mesa portion 26 appear on both sides of the active layer 23 , and 30-degrees inclined faces and (100) facets appear under the active layer 23 .
- upper faces of the p-InP buried layer 27 which are substantially parallel with the active layer 23 are inclined larger than the side surfaces of the active layer 23 but smaller than a perpendicular angle to the substrate surface.
- a height of the mesa portion 26 is about 2 ⁇ m
- a thickness of the active layer 23 of the mesa portion 26 is about 0.3 ⁇ m
- a lower end of the active layer 23 is positioned over the bottom of the mesa portion 26 by about 1.3 ⁇ m.
- a film thickness of the flat portion of the p-InP buried layer 27 is 0.6 ⁇ m
- an angle of the (111) facet of the p-InP buried layer 27 is about 55 degrees relative to the substrate surface (horizontal surface).
- a thickness of the p-InP buried layer 27 is about 0.2 ⁇ m on the side surfaces of the active layer 23 of the mesa portion 26 . In case the p-InP buried layer 27 having such profile is grown, following conditions are needed.
- the mesa portion 26 should be formed by dry etching such that the side surfaces are formed almost vertically with respect to the substrate surface. This is because the film growth proceeds quickly on the face which is positioned close to a (211) facet and thus, if gently inclined surfaces shown in FIG. 1 appear on the side surfaces of the mesa portion when the mesa portion is formed by the wet etching, the faces of the p-InP buried layer 27 which are almost parallel with the side surfaces of the mesa portion 26 quickly disappear.
- the p-InP buried layer 27 is merely grown on the side surfaces of the active layer 23 to have a film thickness of about 0.2 ⁇ m, the p-InP buried layer 27 cannot be formed thick on the horizontal surface (the substrate surface) by the simple method using the material gas only.
- the p-InP buried layer 27 has a thickness of 0.2 ⁇ m on the active layer 23 and has a thickness of 0.6 ⁇ m on the horizontal surface of the n-InP substrate 21 .
- TMIn, PH 3 as well as CH 3 Cl are introduced by a very small amount (the partial pressure is almost 14 mTorr) into the reaction chamber as the material gas, then the p-InP layer of about 0.2 ⁇ m thickness is grown on the side surfaces of the mesa portion 26 and the horizontal surface, then an introduced amount of CH 3 Cl is increased up to the partial pressure of about 92 mTorr, and then the p-InP of about 0.4 ⁇ m thickness is grown.
- the p-InP buried layer 27 having a sectional shape shown in FIG. 5A can be formed, so that the pnpn thyristor structure in which only the p-InP layer on the side surfaces of the mesa portion 26 is formed thin can be provided.
- the lower end of the faces, which are almost parallel with the side surfaces of the mesa portion 26 , of the p-InP buried layer 27 is positioned substantially on an extension of the bottom surface of the active layer 23 .
- an n-InP current blocking layer 28 a is grown on the p-InP buried layer 27 on both sides of the mesa portion 26 .
- a shape of the bottom surface of the n-InP current blocking layer 28 a is similar to the upper surface of the p-InP buried layer.
- its one end of the n-InP current blocking layer 28 a contacts the edges of the top surface of the first p-type cladding layer 24 , and the (111) facets of the n-InP current blocking layer 28 a appear obliquely upward and downward from such one end respectively.
- the n-InP current blocking layer 28 a is almost parallel with the side surface of the active layer 23 and is separated from such side surface by 0.2 ⁇ m in the region where the n-InP current blocking layers 28 a are opposed to the side surfaces of the active layer 23 .
- the leakage current which flows from the n-InP current blocking layers 28 a into the n-InP layer formed under the active layer 23 through the narrow p-InP buried layer 27 can be reduced.
- a second p-InP cladding layer 29 a which has a film thickness of about 1.5 ⁇ m is formed on the n-InP current blocking layer 28 a and the first p-InP cladding layer 24 . Then, the intermediate layer 30 formed of p-InGaAsP having a 0.2 ⁇ m thickness and the contact layer 31 formed of p + -type InGaAs having a 0.5 ⁇ m thickness are formed on the second p-InP cladding layer 29 a.
- the p-side electrode 32 made of Ti/Pt/Au is formed on the contact layer 31 , and then the n-side electrode 33 made of AuGe/Au is formed on the lower surface of the n-InP substrate 21 .
- the semiconductor laser has such a structure that a part of the surface of the p-InP buried layer 27 being located on both sides of the active layer 23 is formed in almost parallel with the side surfaces of the mesa portion 26 (the active layer 23 ).
- the shortest portion of the distance between the active layer 23 and the n-InP current blocking layer 28 a is not limited to one point on the side surface of the active layer 23 but spreads onto overall area of the side surface of the active layer 23 , the advantage of reducing the leakage current can be increased rather than the semiconductor laser in the first example, and the current-optical output power characteristic can be made uniform at the time of high temperature and high output power.
- the distance between the active layer 23 and the n-InP current blocking layer 28 a cannot be automatically decided, and therefore such distance is controlled by the grown film thickness of the p-InP buried layer 27 .
- controllability of the growth rate on a particular facet which is formed prior to the formation of the p-InP buried layer 27 is superior to the position control of the face which newly appears during growth in the prior art.
- the side surfaces of the mesa portion 26 is decided prior to the formation of the p-InP buried layer 27 .
- control of the film growth on the side surfaces is inferior to the growth control of the film on the perfectly flat substrate surface, controllability such as almost 0.01 ⁇ m can be achieved.
- uniformity of the film thickness can be remarkably improved rather than the prior art.
- the mesa angle dependency of the growth rate of the p-InP buried layer 27 does not appear in the range of several angles.
- minute fluctuation of the mesa angle can be absorbed by the growth of the p-InP buried layer 27 .
- the present invention may be applied to an optical device having a similar buried heterostructure such as a DFB (distributed feedback) laser or a DBR (distributed Bragg reflector) laser having the diffraction grating, a narrow radiation angle laser in which a taper waveguide is integrated, a semiconductor optical amplifier, or the like.
- a DFB distributed feedback
- DBR distributed Bragg reflector
- an angle of the side surfaces of the active layer which is formed on the mesa-type first cladding layer is set in the range of 70 to 90 degrees relative to the upper surface of the first cladding layer, then one end of the current blocking layer is brought into contact to an upward extension of the side surface, and then an angle of the facet of the current blocking layer which extends downward from the one end below the active layer is substantially inclined by 55 degrees.
- the passage area for the leakage current which flows from the second cladding layer located over the active layer to the buried layer is made small to thus reduce the leakage current, and also the current-optical power characteristic can be made uniform at the time of high temperature and high output power.
- the angle of the side surfaces of the active layer which is formed on the mesa-type first cladding layer is set in the range of 70 to 90 degrees relative to the upper surface of the first cladding layer, then one end of the current blocking layer is brought into contact to an upward extension of the side surface, and then the angle of the facets of the current blocking layer which extends downward from the one end is substantially inclined by 55 degrees, and then the angle of other facet of the current blocking layer which is formed on side of the active layer is set larger than an angle of the side surfaces of the active layer but smaller than 90 degrees on both sides of the active layer. Therefore, the shortest distance between the current blocking layer and the active layer can be set along overall side surfaces of the active layer, and thus an area of the buried layer between them is narrowed. As a result, the leakage current which is passed through such area can be further reduced.
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Abstract
Description
- 1. Field of the Invention
- The present invention relates to a semiconductor laser and a method of manufacturing the same, more particularly, a semiconductor laser employed in optical fiber communication and having a buried heterostructure and a method of manufacturing the same.
- 2. Description of the Prior Art
- As the application area of the optical fiber communication is spread from the trunk line system of the communication network to the subscriber system, an operation of the semiconductor laser as a light source is required in the wide temperature range circumstances. In particular, good laser characteristics must be attained at the high temperature at which an operating current is increased. At the same time, a required amount of the semiconductor laser is now increased.
- Therefore, a structure for achieving the semiconductor laser which is operable up to the high temperature with good uniformity and a method of manufacturing the same are requested.
- Normally a buried heterostructure is employed in the semiconductor laser used in the optical fiber communication. Such buried heterostructure is employed to inject a current to the active layer efficiently, and there are a buried heterostructure using a pn junction and a buried heterostructure using a semi-insulating layer. The buried heterostructure using the pn junction is suitable for the high temperature operation.
- The semiconductor laser having the pn-junction buried heterostructure has a structure shown in FIG. 1, for example.
- In FIG. 1, an active layer2 of InGaAsP and a first p-type cladding layer 3 of p-InP are formed on an n-type InP substrate 1. Layers from the first p-type cladding layer 3 to an upper area of the n-InP substrate 1 are formed like a mesa shape to form a mesa portion. The active layer 2 in the mesa portion is formed as a stripe shape having a width of about 1 to 1.5 μm. The buried heterostructures are provided on both sides of the mesa portion.
- A p-type buried
layer 4 of p-InP and an n-typecurrent blocking layer 5 of n-InP are formed in the buried regions. Then, a second p-type cladding layer 6 formed of p-InP and a p-type contact layer 7 formed of p-InGaAs are formed in sequence on the n-typecurrent blocking layer 5 and the first p-type cladding layer 3. - In addition, a p-side electrode8 is formed on the p-
type contact layer 7 and an n-side electrode 9 is formed under the InP substrate 1. - The manufacturing method of the semiconductor laser having such buried heterostructure comprises the steps of forming the buried heterostructures by growing the active layer2 and the first p-type cladding layer 3 on the n-InP substrate 1, and forming a substntial stripe shape layers by etching from the first p-type cladding layer 3 to the InP substrate 1 by using a mask, and then forming the p-type buried
layer 4 and the n-typecurrent blocking layer 5 on both sides of the substantial stripe shape layers. - In the recent optical communication laser, a quantum well structure or a strained-layer quantum well structure is employed as the active layer in many cases. The active layer shown hereinafter means not only the quantum well structure consisting of a well layer and a barrier layer but also a structure including the quantum well structure and upper and lower light guiding layers provided to put the quantum well structure between them.
- As particular reports concerning the above-mentioned structure, there are Kondo et al., 1995 Autumn Meeting the Japan Society of Applied Physics 27p-ZA-5 and Chino et al., 1997 Spring Meeting the Japan Society of Applied Physics 30p-NG-11.
- However, in the buried heterostructure semiconductor laser, it is important that the leakage current which is not passed through the active layer must be reduced in order to achieve the good characteristics at the high temperature.
- In the laser having a pn buried heterostructure shown in FIG. 1, both sides of the active layer2 are buried by the p-type buried
layers 4, and such layers are connected to the p-type cladding layers 3, 6 formed directly on the active layer 2. - Therefore, the leakage current which flows from the p-
type cladding layers 3, 6 to the n-type InP substrate 1 via the p-type buriedlayers 4, via routes indicated by arrows in FIG. 1, is generated in the high temperature operation. Since the leakage current depends on an interval between the active layer 2 and the n-typecurrent blocking layer 5, a distance between the active layer 2 and the n-typecurrent blocking layer 5 must be narrowed into about 0.2 μm, for example, in order to reduce the leakage current. In addition, such distance must be fabricated with good controllability in order to achieve the uniform laser characteristic. - However, in the prior art structure, an innermost point of the n-type
current blocking layer 5 is set on an edge of a top surface of the mesa portion, but an angle θ of the bottom surface of the n-typecurrent blocking layer 5 spreads in the neighborhood of the active layer 2 at a gentle angle of about 30 degrees relative to the horizontal direction. Therefore, the distance between the active layer 2 and the n-typecurrent blocking layer 5 is abruptly increased downward, so that a width of the area through which the leakage current flows is excessively widely increased. - The angle θ of the bottom face of the n-type
current blocking layer 5 depends on an angle of an upper surface of the p-type buriedlayer 4 formed under the n-typecurrent blocking layer 5. In other words, the (111) facet which has a slow growth rate appears at the beginning of growth in the crystal growth of the p-type buriedlayers 4, and then such bottom face having a gentle angle of about 30 degrees appears to start its growth on the (111) facet because of the dependence of the growth rate on facet orientation. A position and an angle of such bottom face are very sensitive to a height of the mesa portion, a lower shape of the mesa portion, change in the growth rates in respective face orientations due to the change in the growth conditions, etc. - Therefore, even if the p-type buried
layer 4 is formed by the MOVPE (metal organic vapor-phase epitaxy) method which is said to have good controllability, it is difficult to fabricate uniformly the position of the n-typecurrent blocking layer 5 with respect to the active layer 2 with good reproducibility. - It is an object of the present invention to provide a semiconductor laser capable of implementing a narrow distance between an active layer and current blocking layers formed over a substrate with good controllability, and a method of manufacturing the same.
- According to the present invention, an angle of the side surfaces of the active layer which is formed on the mesa-type first cladding layer is set in the range of 70 to 90 degrees relative to the upper surface of the first cladding layer, then one end of the current blocking layer is brought into contact to an upward extension from the side surface fo the active layer, and then an angle of the facet of the current blocking layer which extends downward from the one end below the active layer is substantially inclined by 55 degrees.
- Therefore, since the buried layers existing on both sides of the active layer are narrowed, the passage area for the leakage current which flows from the second cladding layer located over the active layer to the buried layer is made small to thus reduce the leakage current. As a result, the current-optical output power characteristic can be made uniform at the time of high temperature and high output power.
- Such semiconductor laser manufacturing method can be attained by forming the active layer and the lower layer portion of the second cladding layer in sequence on the first cladding layer, then forming the mesa portion by patterning the layers from the lower layer portion of the second cladding layer to the upper layer portion of the first cladding layer by using the dry etching, and then forming the current blocking layer on the buried layer by controlling the growth of the buried layer such that the (111) facet exists from the side area of the active layer to the lower side thereof.
- In this case, the (111) facet of the buried layer has an inclination of about 55 degrees relative to the substrate surface. In addition, formation of another facet on the (111) facet can be prevented previously by leaving the facet, which is formed in parallel with the side surface of the mesa portion, under the (111) facet of the buried layer.
- Also, according to another present invention, the angle of the side surfaces of the active layer which is formed on the mesa-type first cladding layer is set in the range of 70 to 90 degrees relative to the upper surface of the first cladding layer, then one end of the current blocking layer is brought into contact to an upward extension from the side surface, and then the angle of the facet of the current blocking layer which extends downward from the one end is substantially inclined by 55 degrees, and then the angle of the other facet of current blocking layer which is formed to side of the active layer is set larger than an angle of the side surfaces of the active layer but smaller than 90 degrees on both sides of the active layer.
- Therefore, since the shortest distance between the current blocking layer and the active layer can be set along overall side surfaces of the active layer, an area of the buried layer between them is narrowed. As a result, the leakage current which is passed through such area can be further reduced.
- Such semiconductor laser manufacturing method can be attained by forming the active layer and the lower layer portion of the second cladding layer in sequence on the first cladding layer, then forming the mesa portion by patterning the layers from the lower layer portion of the second cladding layer to the upper layer portion of the first cladding layer by using the dry etching, and then controlling the growth of the buried layer such that the (111) facet exists over the active layer and the facet which is substantially parallel with the active layer appears below the (111) facet.
- In this case, a film thickness of the buried layer between the first cladding layer and the current blocking layer must be sufficiently thick to prevent turn ON of a parasitic thyristor while reducing a film thickness of the buried layer which is to be grown on the side surface of the active layer. For this reason, there may be provided such a method that a desired film thickness is formed on the side surface of the active layer by using a chlorine containing gas in growth of the buried layer, and then an introduced amount of the chlorine containing gas is increased. Accordingly, the film thickness formed on the substrate surface can be increased locally.
- By utilizing the present invention as above, a size of the buried layer in the region where the leakage current which has an influence upon the variation in the characteristics at the high temperature flows can be fabricated with good reproducibility. Therefore, the present invention can largely contribute to the highly-uniformed characteristic of the high-temperature operation optical communication semiconductor laser.
- FIG. 1 is a sectional view showing a semiconductor laser in the prior art;
- FIGS. 2A to2G are sectional views each showing structures of a buried layer employed in a semiconductor laser of an embodiment of the present invention according to respective growing processes;
- FIGS. 3A to3C are sectional views showing growing steps for layers of a first example of a semiconductor laser according to the embodiment of the present invention;
- FIG. 3D is a sectional view showing the first example of the semiconductor laser according to the embodiment of the present invention;
- FIG. 4 is a sectional view showing a second example of the semiconductor laser according to the embodiment of the present invention;
- FIGS. 5A to5C are sectional views showing manufacturing steps for a third example of a semiconductor laser according to the embodiment of the present invention; and
- FIG. 5D is a sectional view showing the third example of the semiconductor laser according to the embodiment of the present invention.
- Therefore, an embodiment of the present invention will be explained with reference to the accompanying drawings hereinafter.
- FIGS. 2A to2G show steps carried out until formation of a buried layer of a semiconductor laser according to an embodiment of the present invention is completed.
- First, as shown in FIG. 2A, an n-type buffer layer (n-type cladding layer)22 formed of n-InP of 300 to 1000 nm film thickness, a MQW (multi quantum well)
active layer 23 formed of undoped InGaAsP of 200 to 300 nm film thickness, and a first p-type cladding layer 24 formed of p-InP of 250 to 700 nm film thickness are formed on a (100) facet of an n-InP substrate 21 by the MOVPE method. - The InP is grown by using trimethylindium (TMIn) and phosphine (PH3) as a material gas. The InGaAsP is grown by using TMIn, PH3, arsine (AsH3), and triethylgallium (TEGa) as a material gas. Also, dimethylzinc (DMZn) is employed as a p-type dopant, and silane (SiH4) is employed as an n-type dopant.
- The MQW
active layer 23 comprises, for example, five periodical well layers of InGaAsP having a thickness of 6 nm and 1% compressive strain, barrier layers each formed between the well layers and formed of InGaAsP whose lattice matches InP and which has a thickness of 10 nm, and light guiding layers each formed on and under the quantum well structure which consists of well and barrier layers and formed of InGaAsP of 100 nm thickness. Each of the barrier layers and the light guiding layers is formed of InGaAsP whose bandgap wavelength is 1.1 μm. Accordingly, a semiconductor laser whose wavelength band is 1.3 μm can be formed. - In this case, a layer structure of the MQW
active layer 23 is not limited to such structure. Also, the active layer may be formed of an InGaAsP system which can provide a 1.55 μm band, a 1.48 μm band, and other wavelength bands. - An impurity concentration of the n-
InP buffer layer 22 is about 5×1017 atoms/cm3, and an impurity concentration of the p-InP cladding layer 24 is about 5×1017 atoms/cm3. - Both the n-
InP buffer layer 22 formed of n-InP under theactive layer 23 and the n-InP substrate 21 act as an n-type cladding layer. - After the first film growth described above has been finished, a dielectric film, e.g., an SiO2 film, having a thickness of about 0.3 μm is formed on the p-InP first p-
type cladding layer 24 by the CVD method. Then, the dielectric film is patterned into anarrow stripe 25, which has a width of about 1.5 μm and extends along the <011> direction, by the photolithography method. Thedielectric stripe 25 is used as a mask. - Then, as shown in FIG. 2B, the p-
InP cladding layer 24 to the n-InP substrate 21, which are not covered with themask 25, are etched up to a depth of almost 2 to 3 μm to thus form amesa portion 26 under themask 25. Themesa portion 26 is a projection which has a mesa-like sectional shape and a stripe-like planar shape. The etching is performed by the RIE (Reactive Ion Etching) method, and employs an ethane type gas, e.g., a mixed gas of C2H6, O2, and H2. - Then, surfaces of compound semiconductor layers, which are damaged by the dry etching, are removed by the acid treatment such as sulfuric acid, etc. Then, a p-InP buried
layer 27 is grown on depressed areas on both sides of themesa portion 26. - As shown in FIGS. 2C to2G, this p-InP buried
layer 27 will be grown according to following steps. - At first, the
mask 25 is not extended from a top surface of themesa portion 26 like a visor. Also, side surfaces of themesa portion 26 are inclined sharply to have a 70 degrees or more relative to the horizontal direction (i.e., the substrate surface). - Under such condition, the p-InP buried
layer 27 is formed by the second MOVPE method. In growing this p-InP buriedlayer 27, overgrowth of InP on themask 25 can be prevented by introducing TMIn and PH3 as a material gas into a reaction chamber of a MOVPE equipment and also introducing methyl chloride (CH3Cl) together with the material gas into the reaction chamber, so that (111) facets can be grown from both edges of the top surface of themesa portion 26. - More particularly, as shown in FIG. 2C, as for the p-InP buried
layer 27, the (111) facets appear downward from the edges of the top surface of themesa portion 26, and two facets which parallel substantially the side surfaces of themesa portion 26 appear on both sides of themesa portion 26, other two facets which have a gentle angle of about 30 degrees relative to the substrate surface appear in the neighborhood of the bottom portion of themesa portion 26, and (100) facets appear on the substrate surface on both sides of themesa portion 26. - As shown in FIG. 2D, by keeping the growth of the p-InP buried
layer 27, the (111) facets extend obliquely downward to spread much more, while the two facets which parallel the side surfaces of themesa portion 26 are shortened along the side surfaces. Also, with the progress of the growth, the two facets, which substantially paralleled both sides of themesa portion 26 at the beginning of the growth, become close gradually to the vertical direction relative to the substrate surface. - As shown in FIG. 2E, with the further progress of the growth of the p-InP buried
layer 27, the two faces, which substantially paralleled both sides of themesa portion 26, disappear. - As shown in FIGS. 2F and 2G, the other two facets of the buried
layer 27, each is grown from near the bottom of themesa portion 26 to have the gentle angle of about 30 degrees, erode the (111) facets according to the furthermore progress of the growth of the p-InP buriedlayer 27, therefore the (111) facets are shortened. These appearances of above shapes are due to difference in the growth rates in respective facet orientations. - In the course of the above growth of the p-InP buried
layer 27, not-mentioned faces and transition regions are appeared in the region where an angle of the face is changed (corner portions), but they are omitted from the figures. - According to above difference in shapes in the course of the growth of the p-InP buried
layer 27, it is possible to form a semiconductor laser having a structure described in the following. In this case, in three following examples of the semiconductor laser, explanation will be made by omitting processes needed until themesa portion 26 is formed. - The semiconductor laser described in the following shows an example wherein the growth of the p-InP buried
layer 27 is stopped at the point of time when the facets, which are formed in parallel with the side surfaces of themesa portion 26, of the p-InP buriedlayer 27 disappear, and then a current blocking layer is formed. - More particularly, as shown in FIG. 3A, the (111) facets, 30-degrees inclined faces, and the (100) facets of the p-InP buried
layer 27 whose growth has been completed are exposed in sequence along the direction from the edges of the top surface of themesa portion 26 to the bottom. - In this case, a thickness of the
active layer 23 is about 0.3 μm, a thickness of the first p-type cladding layer 24 is 0.4 μm, a height of themesa portion 26 is about 2 μm, and an angle of the side surface of theactive layer 23 of themesa portion 26 is about 83 degrees. In addition, a film thickness of the flat portion of the p-InP buriedlayer 27 is 0.7 μm, and an angle of the (111) facet of the p-InP buriedlayer 27 is about 55 degrees relative to the horizontal line. - Under such conditions, as shown in FIG. 3B, when an n-InP
current blocking layer 28 is formed on the p-InP buriedlayer 27 on both sides of themesa portion 26, the bottom surface of the n-InPcurrent blocking layer 28 has the same shape as the upper surface of the p-InP buriedlayer 27. Also, one end of the n-InPcurrent blocking layer 28 is positioned over theactive layer 23 by 0.4 μm along an extension of the side surface of themesa portion 26. - In this case, a film thickness of the p-InP buried
layer 27 is about 0.4 μm at the lower end of the side surface of theactive layer 24. A shortest distance from an upper end of theactive layer 24 to the n-InPcurrent blocking layer 28 is about 0.19 μm. The thickness of the first p-type cladding layer 24 and an inclination of the side surface of themesa portion 26 may be adjusted such that this shortest distance is present in the range of 0.1 to 0.3 μm. - If a film thickness of the n-InP
current blocking layer 28 is set to about 0.9 μm in the flat region, the (111) facet as the bottom surface of the n-InPcurrent blocking layer 28, which is a boundary between the p-InP buriedlayer 27 and the n-InPcurrent blocking layer 28, extend below theactive layer 24 from its one end which is closest to the first p-type cladding layer 24. As the upper surface of the n-InPcurrent blocking layer 28, an upward-inclined surface having (111) facet, a flat surface, and a downward-inclined surface appear in sequence from its one end which contacts to the side surface of themesa portion 26. - After the growth of such n-InP
current blocking layer 28 has been finished and then themask 25 has been removed, the process is shifted to the third crystal growth steps. - As shown in FIG. 3C, in the third MOVPE crystal growth, a second p-
type cladding layer 29, which is formed of p-InP to have a film thickness of about 1.5 μm, is formed on the n-InPcurrent blocking layer 28 and the first p-type cladding layer 24. Then, anintermediate layer 30 formed of p-InGaAsP having a 0.2 μm film thickness and acontact layer 31 formed of p+-type InGaAs having a 0.5 μm film thickness are formed on the second p-type cladding layer 29. - Next, as shown in FIG. 3D, a p-
side electrode 32 made of Ti/Pt/Au is formed on thecontact layer 31, and then an n-side electrode 33 made of AuGe/Au is formed on the lower surface of the n-InP substrate 21. - In this case, above-mentioned film thicknesses are values in the flat regions unless they are not particularly mentioned.
- With the above, a basic structure of the semiconductor laser has been completed.
- According to the above semiconductor laser, since the surface which is close to the
mesa portion 26 of the n-InPcurrent blocking layer 28 has an angle of 55 degrees relative to the horizontal surface, the n-InPcurrent blocking layer 28 is positioned closely to the active layer at a distance of less than 0.2 μm. - Accordingly, the region in which the first p-
type cladding layer 24, the second p-type cladding layer 29, and the p-InP buriedlayer 27 are coupled can be narrowed rather than the prior art, and thus a leakage current which is passed through such region can be reduced. - In addition, since the growth condition of the buried
layer 27, etc. are set such that the (111) facet, which appears in the neighborhood of theactive layer 23, of the p-InP buriedlayer 27 can be extended below theactive layer 23, the (111) facet can be positioned on both sides of theactive layer 23 even if a height of themesa portion 26 is slightly changed due to the error in fabrication. - Therefore, a distance between the n-InP
current blocking layer 28 and theactive layer 23 is substantially decided by a distance from theactive layer 23 to the top surface of the mesa portion 26 (i.e., the first p-type cladding layer 24), and the error in the distance between the n-InPcurrent blocking layer 28 and theactive layer 23 in fabrication only depends on the deviation of the angle of the side surface of themesa portion 26. Accordingly, a shortest width of the region through which the leakage current flows, i.e., a shortest distance of the clearance between the n-InPcurrent blocking layer 28 and theactive layer 23 does not depend on the growth conditions of the films and displacement of the height of themesa portion 26 in fabrication. As a result, a magnitude of the leakage current can be reduced stably, the current-optical output power characteristic can be made uniform at the time of high temperature and high output power, and reproducibility can be improved. - In addition, in the first example, the upper (111) facet of the n-InP
current blocking layer 28 is inclined by the angle of about 55 degrees and thus the uppermost portion of the (111) facet is positioned higher than themesa portion 26. In this fashion, if the upper portions of the n-InPcurrent blocking layer 28, which are located on both sides of themesa portion 26, are made narrower toward theactive layer 23 like a taper-shape, the injection current can be collected effectively into the upper area of theactive layer 23 because the film thickness of the n-InPcurrent blocking layer 28 is increased suddenly large in the neighborhood of theactive layer 23. - However, there is no need that the upper portions of the n-InP
current blocking layer 28 should be always risen on both sides of themesa portion 26. - Also, as described above, since the side surfaces of the
mesa portion 26 are formed extremely close to the vertical direction such as about 83 degrees, the distance between the n-InP layers (the n-InP buffer layer 22 and the n-InP substrate 21) under theactive layer 23 and the n-InPcurrent blocking layer 28 can be spread widely downward. Hence, a pnpn thyristor which consists of the p-type cladding layers 24 and 29, the n-InPcurrent blocking layer 28, the p-InP buriedlayer 27, and the n-InP layers (the n-InP buffer layer 22 and the n-InP substrate 21) formed as the lower portion of themesa portion 26 is difficult to turn ON. - As a result, if only the shortest distance between the n-InP
current blocking layer 28 and theactive layer 23 is considered, it is possible to control such shortest distance in a gentle spreading mesa shape in which respective layers which are lower than theactive layer 23 are formed by the wet etching. However, from the viewpoint of the current blocking characteristic of the thyristor, it is possible to say that themesa portion 26 having the side surfaces which are formed almost along the vertical direction, as formed by the dry etching, is preferable, like the first example. - A second example of the semiconductor laser is characterized by a structure in which, since the height of the
mesa portion 26 is set higher like about 2.5 μm, the p-InP surfaces which are parallel with the side surfaces of the mesa portion can still remain on the p-InP buriedlayer 27 at the stage where the growth of the p-InP buriedlayer 27 is completed. In other words, in the second example, the growth of the p-InP buried layer is stopped at the stage shown in FIG. 2E. - After such p-InP buried
layer 27 has been formed, like the first example, the n-InPcurrent blocking layer 28 is formed on the p-InP buriedlayer 27, then themask 25 is removed, and then the second p-type cladding layer 29, the p-InGaAsPintermediate layer 30, and thecontact layer 31 formed of p+-type InGaAs are formed on the n-InPcurrent blocking layer 28 and the first p-type cladding layer 24 by the MOVPE method. Then, a semiconductor laser having a structure shown in FIG. 4 can be obtained by forming the p-side electrode 32 and the n-side electrode 33. - As described above, if the p-InP faces which are parallel with the side surfaces of the
mesa portion 26 still remains on a part of the p-InP buriedlayer 27 before the p-InP buriedlayer 27 is grown, the p-InP cannot grow on the (111) facet of the p-InP buriedlayer 27 even when the growth rate of the p-InP buriedlayer 27 is slightly varied according to any change in the conditions. As a result, the thickness control of the p-InP film on theactive layer 23, i.e., the control of the distance between the n-InPcurrent blocking layer 28 and theactive layer 23 can be made more easily without fail. - In the foregoing first example, the growth of the p-InP buried
layer 27 is stopped in the situation between FIG. 2E and FIG. 2F, and then the n-InPcurrent blocking layer 28 is formed. - On the contrary, in a third example, the growth of the p-InP buried
layer 27 is stopped in the situation close to FIG. 2D, and then steps of growing the n-InP current blocking layer, etc. are employed. - More particularly, as shown in FIG. 5A, as for the surface shape of the buried
layer 27 after its growth has been finished, the (111) facet appears obliquely downward from the edges of the top surface of themesa portion 26, faces which are almost parallel with the side surfaces of themesa portion 26 appear on both sides of theactive layer 23, and 30-degrees inclined faces and (100) facets appear under theactive layer 23. In this case, upper faces of the p-InP buriedlayer 27, which are substantially parallel with theactive layer 23 are inclined larger than the side surfaces of theactive layer 23 but smaller than a perpendicular angle to the substrate surface. - In this case, a height of the
mesa portion 26 is about 2 μm, a thickness of theactive layer 23 of themesa portion 26 is about 0.3 μm, and a lower end of theactive layer 23 is positioned over the bottom of themesa portion 26 by about 1.3 μm. Also, a film thickness of the flat portion of the p-InP buriedlayer 27 is 0.6 μm, and an angle of the (111) facet of the p-InP buriedlayer 27 is about 55 degrees relative to the substrate surface (horizontal surface). In addition, a thickness of the p-InP buriedlayer 27 is about 0.2 μm on the side surfaces of theactive layer 23 of themesa portion 26. In case the p-InP buriedlayer 27 having such profile is grown, following conditions are needed. - First, it is preferable that the
mesa portion 26 should be formed by dry etching such that the side surfaces are formed almost vertically with respect to the substrate surface. This is because the film growth proceeds quickly on the face which is positioned close to a (211) facet and thus, if gently inclined surfaces shown in FIG. 1 appear on the side surfaces of the mesa portion when the mesa portion is formed by the wet etching, the faces of the p-InP buriedlayer 27 which are almost parallel with the side surfaces of themesa portion 26 quickly disappear. - Also, because the p-InP buried
layer 27 is merely grown on the side surfaces of theactive layer 23 to have a film thickness of about 0.2 μm, the p-InP buriedlayer 27 cannot be formed thick on the horizontal surface (the substrate surface) by the simple method using the material gas only. - Then, when the film thickness of the flat portion of the p-InP buried
layer 27 becomes thin on the n-InP substrate 21, the pnpn thyristor formed on both sides of the mesa portion is turned ON easily to thus increase the leakage current. Therefore, in the present structure, such a method is employed that the p-InP buriedlayer 27 has a thickness of 0.2 μm on theactive layer 23 and has a thickness of 0.6 μm on the horizontal surface of the n-InP substrate 21. - As the actual method, like the first example, TMIn, PH3 as well as CH3Cl are introduced by a very small amount (the partial pressure is almost 14 mTorr) into the reaction chamber as the material gas, then the p-InP layer of about 0.2 μm thickness is grown on the side surfaces of the
mesa portion 26 and the horizontal surface, then an introduced amount of CH3Cl is increased up to the partial pressure of about 92 mTorr, and then the p-InP of about 0.4 μm thickness is grown. - If a flow rate of CH3Cl is increased gradually at the time when the p-InP layer constituting the buried
layer 27 is grown, the growth rate is abruptly lowered on the side surfaces of themesa portion 26 in contrast to the growth rate on the substrate surface, and finally the p-InP layer is seldom grown on the side surfaces of themesa portion 26 but only the thickness of the p-InP layer is increased on the substrate surface. According to such method, the p-InP buriedlayer 27 having a sectional shape shown in FIG. 5A can be formed, so that the pnpn thyristor structure in which only the p-InP layer on the side surfaces of themesa portion 26 is formed thin can be provided. - In this case, the lower end of the faces, which are almost parallel with the side surfaces of the
mesa portion 26, of the p-InP buriedlayer 27 is positioned substantially on an extension of the bottom surface of theactive layer 23. - As shown in FIG. 5B, after the p-InP buried
layer 27 has been formed as above, an n-InPcurrent blocking layer 28 a is grown on the p-InP buriedlayer 27 on both sides of themesa portion 26. A shape of the bottom surface of the n-InPcurrent blocking layer 28 a is similar to the upper surface of the p-InP buried layer. In this case, like the first example, its one end of the n-InPcurrent blocking layer 28 a contacts the edges of the top surface of the first p-type cladding layer 24, and the (111) facets of the n-InPcurrent blocking layer 28 a appear obliquely upward and downward from such one end respectively. - Therefore, the n-InP
current blocking layer 28 a is almost parallel with the side surface of theactive layer 23 and is separated from such side surface by 0.2 μm in the region where the n-InP current blocking layers 28 a are opposed to the side surfaces of theactive layer 23. - According to such structure, the leakage current which flows from the n-InP current blocking layers28 a into the n-InP layer formed under the
active layer 23 through the narrow p-InP buriedlayer 27 can be reduced. - After the growth of such n-InP
current blocking layer 28 a has been finished and then themask 25 has been removed, the process is shifted to the third crystal growth steps. - As shown in FIG. 5C, in the third crystal growth, a second p-InP cladding layer29 a which has a film thickness of about 1.5 μm is formed on the n-InP
current blocking layer 28 a and the first p-InP cladding layer 24. Then, theintermediate layer 30 formed of p-InGaAsP having a 0.2 μm thickness and thecontact layer 31 formed of p+-type InGaAs having a 0.5 μm thickness are formed on the second p-InP cladding layer 29 a. - Next, as shown in FIG. 5D, the p-
side electrode 32 made of Ti/Pt/Au is formed on thecontact layer 31, and then the n-side electrode 33 made of AuGe/Au is formed on the lower surface of the n-InP substrate 21. - With the above, a basic structure of the semiconductor laser has been completed.
- According to the above, the semiconductor laser has such a structure that a part of the surface of the p-InP buried
layer 27 being located on both sides of theactive layer 23 is formed in almost parallel with the side surfaces of the mesa portion 26 (the active layer 23). - Accordingly, since the shortest portion of the distance between the
active layer 23 and the n-InPcurrent blocking layer 28 a is not limited to one point on the side surface of theactive layer 23 but spreads onto overall area of the side surface of theactive layer 23, the advantage of reducing the leakage current can be increased rather than the semiconductor laser in the first example, and the current-optical output power characteristic can be made uniform at the time of high temperature and high output power. Unlike the first example, the distance between theactive layer 23 and the n-InPcurrent blocking layer 28 a cannot be automatically decided, and therefore such distance is controlled by the grown film thickness of the p-InP buriedlayer 27. - However, controllability of the growth rate on a particular facet which is formed prior to the formation of the p-InP buried
layer 27 is superior to the position control of the face which newly appears during growth in the prior art. - In other words, the side surfaces of the
mesa portion 26 is decided prior to the formation of the p-InP buriedlayer 27. Although control of the film growth on the side surfaces is inferior to the growth control of the film on the perfectly flat substrate surface, controllability such as almost 0.01 μm can be achieved. As a result, uniformity of the film thickness can be remarkably improved rather than the prior art. - Also, in the third example, the mesa angle dependency of the growth rate of the p-InP buried
layer 27 does not appear in the range of several angles. In addition, minute fluctuation of the mesa angle can be absorbed by the growth of the p-InP buriedlayer 27. - Although the explanation has been made using the Fabry-Perot type semiconductor laser in above three examples, it is a matter of course that the present invention may be applied to an optical device having a similar buried heterostructure such as a DFB (distributed feedback) laser or a DBR (distributed Bragg reflector) laser having the diffraction grating, a narrow radiation angle laser in which a taper waveguide is integrated, a semiconductor optical amplifier, or the like.
- As described above, according to the present invention, an angle of the side surfaces of the active layer which is formed on the mesa-type first cladding layer is set in the range of 70 to 90 degrees relative to the upper surface of the first cladding layer, then one end of the current blocking layer is brought into contact to an upward extension of the side surface, and then an angle of the facet of the current blocking layer which extends downward from the one end below the active layer is substantially inclined by 55 degrees. Therefore, since the buried layers existing on both sides of the active layer are narrowed, the passage area for the leakage current which flows from the second cladding layer located over the active layer to the buried layer is made small to thus reduce the leakage current, and also the current-optical power characteristic can be made uniform at the time of high temperature and high output power.
- In addition, according to another present invention, the angle of the side surfaces of the active layer which is formed on the mesa-type first cladding layer is set in the range of 70 to 90 degrees relative to the upper surface of the first cladding layer, then one end of the current blocking layer is brought into contact to an upward extension of the side surface, and then the angle of the facets of the current blocking layer which extends downward from the one end is substantially inclined by 55 degrees, and then the angle of other facet of the current blocking layer which is formed on side of the active layer is set larger than an angle of the side surfaces of the active layer but smaller than 90 degrees on both sides of the active layer. Therefore, the shortest distance between the current blocking layer and the active layer can be set along overall side surfaces of the active layer, and thus an area of the buried layer between them is narrowed. As a result, the leakage current which is passed through such area can be further reduced.
Claims (18)
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JP34684298A JP3907854B2 (en) | 1998-12-07 | 1998-12-07 | Semiconductor laser and manufacturing method thereof |
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US (1) | US6512783B1 (en) |
JP (1) | JP3907854B2 (en) |
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US6697407B2 (en) * | 2001-02-28 | 2004-02-24 | Anritsu Corporation | Semiconductor laser attaining high efficiency and high power, and method of manufacturing the same |
US20050245089A1 (en) * | 2001-02-15 | 2005-11-03 | Fujitsu Quantum Devices Limited | Process of manufacturing a semiconductor device |
US20070091957A1 (en) * | 2005-08-11 | 2007-04-26 | Mitsubishi Denki Kabushiki Kaisha | Semiconductor laser device and method for manufacturing the same semiconductor laser device |
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US20120309121A1 (en) * | 2011-05-31 | 2012-12-06 | Sumitomo Electric Industries, Ltd. | Method of making semiconductor optical integrated device |
US8798110B2 (en) | 2010-04-27 | 2014-08-05 | Sumitomo Electric Device Innovations, Inc. | Optical semiconductor device and method of manufacturing optical semiconductor device |
US20170207604A1 (en) * | 2016-01-14 | 2017-07-20 | Sumitomo Electric Device Innovations, Inc. | Process of forming semiconductor optical device and semiconductor optical device |
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US4841536A (en) * | 1985-04-12 | 1989-06-20 | Hitachi, Ltd. | Semiconductor laser devices |
US5260230A (en) * | 1991-07-12 | 1993-11-09 | Nippon Telegraph And Telephone Corporation | Method of manufacturing buried heterostructure semiconductor laser |
JP3034688B2 (en) | 1992-04-27 | 2000-04-17 | 株式会社日立製作所 | Semiconductor device |
JP2823476B2 (en) * | 1992-05-14 | 1998-11-11 | 三菱電機株式会社 | Semiconductor laser and method of manufacturing the same |
US5441912A (en) * | 1993-07-28 | 1995-08-15 | The Furukawa Electric Co., Ltd. | Method of manufacturing a laser diode |
JP3729210B2 (en) | 1994-07-26 | 2005-12-21 | 富士通株式会社 | Manufacturing method of semiconductor device |
KR0146714B1 (en) * | 1994-08-08 | 1998-11-02 | 양승택 | Fabrication method of buried heterojunction laser diode |
JP3421140B2 (en) | 1994-08-23 | 2003-06-30 | 三菱電機株式会社 | Method of manufacturing semiconductor laser device and semiconductor laser device |
US5764842A (en) * | 1995-03-23 | 1998-06-09 | Hitachi, Ltd. | Semiconductor guided-wave optical device and method of fabricating thereof |
JPH09237940A (en) * | 1995-12-28 | 1997-09-09 | Mitsubishi Electric Corp | Semiconductor device and manufacture thereof |
-
1998
- 1998-12-07 JP JP34684298A patent/JP3907854B2/en not_active Expired - Fee Related
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1999
- 1999-12-03 US US09/453,546 patent/US6512783B1/en not_active Expired - Lifetime
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US6697407B2 (en) * | 2001-02-28 | 2004-02-24 | Anritsu Corporation | Semiconductor laser attaining high efficiency and high power, and method of manufacturing the same |
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US20070091957A1 (en) * | 2005-08-11 | 2007-04-26 | Mitsubishi Denki Kabushiki Kaisha | Semiconductor laser device and method for manufacturing the same semiconductor laser device |
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Also Published As
Publication number | Publication date |
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JP3907854B2 (en) | 2007-04-18 |
DE19958275B4 (en) | 2007-12-20 |
DE19958275A1 (en) | 2000-09-28 |
FR2786939B1 (en) | 2006-12-01 |
US6512783B1 (en) | 2003-01-28 |
FR2786939A1 (en) | 2000-06-09 |
JP2000174389A (en) | 2000-06-23 |
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