WO2011046244A1

WO2011046244A1 - Iii-nitride surface grating reflector

Info

Publication number: WO2011046244A1
Application number: PCT/KR2009/005996
Authority: WO
Inventors: 전헌수; 이준희; 안성모; 장호준
Original assignee: 서울대학교산학협력단
Priority date: 2009-10-16
Filing date: 2009-10-16
Publication date: 2011-04-21
Also published as: KR20120079113A; KR101377397B1

Abstract

Disclosed is a III-nitride surface grating reflector. As one embodiment, the III-nitride surface grating reflector comprises: a substrate; and a III-nitride layer which is disposed on the one side of the substrate, and is formed with a structure of a one-dimensional diffraction grating pattern on the surface, wherein the structure of the grating pattern has an uneven cross section on which crests and troughs are arranged periodically, and among the lights incident from the inside of the III-nitride layer, a first light that passes through the crests and the second light that passes through the troughs mutually cause destructive interference whereby the incident lights are reflected on the surface of the structure of the grating pattern.

Description

Group III nitride surface grating reflector

FIELD The present disclosure generally relates to surface grating reflectors, and more particularly to group III nitride surface grating reflectors.

In many optoelectronic devices, particularly surface emitting semiconductor lasers (VCSELs) and resonant cavity light emitting diodes (RCLEDs), reflectors are an important part of the material, typically using metals such as Ag, Al, or high To obtain the reflectance, a distributed Bragg reflector (hereinafter referred to as 'DBR') composed of periodic layers of two materials with different refractive indices is used. However, in the short wavelength region, the metal has a higher absorption, so the mirror using metal cannot provide high reflectance. In the case of DBR, the group III nitride material system, which is representative of short-wavelength light emitting materials, has a large difference in the lattice constant between group III nitride and AlN, but a small difference in refractive index, which requires a very special technique to produce a high reflectance DBR. There is a disadvantage that the thickness becomes thick. It is also conceivable to use a dielectric DBR instead of a semiconductor DBR, but in that case, it is difficult to inject electricity or dissipate heat.

One aspect of the disclosed technology includes a substrate and a group III nitride layer disposed on one surface of the substrate and having a one-dimensional diffraction grating pattern structure formed on a surface thereof, wherein the grating pattern structure has a concave-convex shape in which floors and valleys are periodically disposed. The incident light enters the surface of the lattice pattern structure by having a cross-section of the first light transmitted through the floor portion and the second light passing through the valley portion among the incident light incident from the inside of the III-nitride. It provides a group III nitride surface grating reflector that is reflected at.

Another aspect of the disclosed technology provides a light emitting device comprising the group III nitride surface lattice reflector.

Another aspect of the disclosed technology includes a substrate and a III-nitride layer disposed on one surface of the substrate and having a lattice pattern structure formed on the surface, wherein the lattice pattern structure is two-dimensionally periodically arranged with a parquet portion and a valley portion. The grating has a polygonal shape, and among the incident light incident from the inside of the group III nitride, the first light passing through the floor portion and the second light passing through the valley portion cancel each other, thereby causing the incident light to form the grating pattern structure. It provides a group III nitride surface grating reflector that is reflected at the surface of.

Another aspect of the disclosed technology provides a light emitting device comprising the group III nitride surface grating reflector.

According to an embodiment of the group III nitride surface grating reflector of the present disclosure, the grating pattern having a concave-convex shape in cross section includes a structure in which the grating pattern is periodically repeated. It has a high reflectance for incident light in a particular wavelength band, and particularly exhibits excellent reflectance for TE polarized light. In addition, the wavelength band of incident light showing high reflectivity of about 0.8 or more is wide, about 90 nm.

Further, according to another embodiment of the group III nitride surface grating reflector of the present disclosure, it includes a two-dimensional lattice pattern structure in which the ridge portion and the valley portion are periodically arranged two-dimensionally. In this case, the incident light may have a high reflectance regardless of whether it is TE polarized light or TM polarized light.

1A is a perspective view illustrating a group III nitride surface grating reflector of the present disclosure.

1B is a diagram showing a cross section of the GaN surface lattice reflector of the present disclosure.

2 is a flowchart illustrating a method of manufacturing a GaN surface grating reflector according to an embodiment of the present disclosure.

FIG. 3 is a schematic diagram illustrating a holographic lithography system for fabricating a GaN surface grating reflector of the present disclosure.

4 is a scanning electron micrograph of a group III nitride surface grating reflector patterned using holographic lithography.

FIG. 5 is a diagram showing an electric field distribution when a transverse electric field (TE) polarized incident light having a wavelength of 450 nm is incident on a GaN surface grating reflector.

FIG. 6 is a graph showing reflection spectra according to the polarization direction calculated by the RCWA method with respect to the preferable Group III nitride grid pattern structure. FIG.

7 is a diagram illustrating a change in the zeroth order reflectance of incident light according to the period of the grating or the height of the grating calculated by the RCWA method.

8 is a view showing a change in the zero-order reflectance of the incident light according to the fill factor of the grating or the tilt angle of the grating calculated by the RCWA method.

FIG. 9 is a reflectance measurement system for incident light traveling toward air or vacuum inside a Group III nitride layer of a GaN surface grating reflector.

10 is a graph showing the reflectance of a GaN surface grating reflector measured using a reflectance measuring system.

Hereinafter, exemplary embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. However, the technology disclosed herein is not limited to the embodiments described herein and may be embodied in other forms. It is merely to be understood that the embodiments introduced herein are provided to enable those skilled in the art to fully convey the spirit and spirit of the present disclosure to those skilled in the art. In the drawings, the width, thickness, or shape of structures are enlarged in order to clearly express various layers (or layers), regions, and shapes. The drawings have been described at the point of view of the observer, and if portions of layers, films, regions, etc. are expressed as being “above” or “above” other portions, as well as “immediately above” or “immediately above”, This includes the case where there is another part in the middle. Like reference numerals designate like elements throughout the specification.

1A is a perspective view illustrating a group III nitride surface grating reflector of the present disclosure. Referring to FIG. 1A, in the group III nitride surface grating reflector 100, a group III nitride layer 104 is disposed on one surface of the substrate 102. Group III nitrides may be, for example, GaN, AlGaN or InGaN. For simplicity, hereinafter, the group III nitride surface lattice reflector 100 will be described for GaN in the group III nitride. Here, a lattice pattern structure P is formed on the surface of the GaN layer 104. The grid pattern structure P has a concave-convex cross section in which the floor portion 110 and the valley 120 portion are periodically arranged. That is, the lattice pattern structure P has a form such that a plurality of bars are arranged in parallel on at least a part of the surface of the GaN layer 104. In the grid pattern structure (P), period (Λ) is the sum of the width (w ₂₎ of the width (w ₁₎ with the bone parts 120 in the top portions 110.

The surface of the grid pattern structure P may be in contact with air or placed in a vacuum.

The substrate 102 may be, for example, a substrate made of sapphire, silicon, gallium nitride, gallium arsenide, silicon carbide, zinc oxide, or glass.

When light having a specific wavelength is incident from inside the GaN, the GaN surface grating reflector 100 may have a high reflectance at the surface of the grating pattern structure P. Although not bound by a specific theory, the reason why the GaN surface grating reflector of the present disclosure reflects incident light can be explained as follows.

1B is a diagram showing a cross section of the GaN surface lattice reflector of the present disclosure. Referring to FIG. 1B, when the incident light 130 passing through the GaN layer 104 passes through the grating pattern structure P, the wavelength of the incident light 130 and the period Λ of the grating pattern structure P are shown. As a result, light may be diffracted at various angles. Properly designed grating patterns can suppress high order diffraction in which incident light transmits or reflects at an angle different from the incident angle, and ensures that only zero-order diffraction exists where the incident light transmits or reflects at the same angle as the incident angle. The incident light can have a wavelength of about 300 nm to about 700 nm.

At this time, when the phase difference between the first light 130-1 and the second light 130-2 passing through the floor portion 110 and the valley portion 120 becomes π, the light passing through the grating cancels interference in the air. May cause The phase difference may be determined according to structural parameters that determine the geometry of the lattice pattern structure P. FIG. Thus, by appropriately adjusting structural parameters such as floor height (h) or filling factor (F), it is possible to fully reflect incident waves of a particular wavelength passing through the GaN layer 104. The fill factor is the ratio of the width of the floor (w ₁ ) to the period of the lattice (Λ).

According to the exemplary embodiment of the present disclosure, the GaN lattice pattern of the GaN surface lattice reflector may be implemented as a two-dimensional lattice pattern structure without being limited to a structure in which the valleys 120 extend in one direction. That is, the two-dimensional lattice pattern structure may be implemented in a form in which the floor portion and the valley portion are periodically arranged in two dimensions. As an example the grid may be triangular or square in shape.

Hereinafter, a method of manufacturing a GaN surface grating reflector according to an embodiment will be described. 2 is a flowchart illustrating a method of manufacturing a GaN surface grating reflector according to an embodiment of the present disclosure. Referring to FIG. 2, a GaN layer is formed on a substrate in S200. The substrate may be, for example, a sapphire substrate. The GaN layer may be grown to a thickness of several microns using, for example, metal-organic chemical-vapor-deposition (hereinafter referred to as MOCVD).

In S210, a SiO ₂ layer is formed on the GaN layer. The SiO ₂ layer may be grown by, for example, plasma-enhanced chemical vapor deposition. The SiO ₂ layer is used as a mask for forming a lattice pattern of the GaN layer.

In S220, a Cr layer is formed on the SiO ₂ layer. The Cr layer is used as a mask for etching SiO ₂ layer. The Cr layer may be grown by e-gun evaporation, for example.

In S230, the Cr layer is made of a line pattern using holographic lithography and etched to form a lattice pattern.

3 is a schematic representation of a holographic lithography system for fabricating a GaN surface grating reflector of the present disclosure. Holographic lithography makes it easier to fabricate one- and two-dimensional periodic IC nanostructures in large areas at a fraction of the cost and time required compared to e-beam lithography. Referring to FIG. 3, a holographic lithography system includes a helium-cadmium laser generator 350,

mirrors

330a, 330b, and 330c, a shutter 340, an optical expander 320, a lens 310, and a sample stage 360. Include. The laser 305 emitted from the 325 nm helium-cadmium (He-Cd) laser generator 350 is reflected by the mirror 330b and passed through the shutter. For example, the shutter 340 may adjust an exposure time of a laser irradiated to the sample 300 as an electronic shutter. The laser passing through the shutter 340 is reflected by the mirror 330b and spreads through the optical expander 320. The laser spreads through the optical expander 320 is made into parallel light with the collimator 310 and then incident to the sample stage 360. The laser directly irradiated onto the sample 300 and the laser reflected from the mirror 330c installed perpendicular to the sample 300 enter the sample 300 to interfere with the surface of the sample 300 to form a one-dimensional grating pattern. By rotating the sample stage 360 to adjust the incident angles of the two lasers, the period of the grating pattern can be easily adjusted. The grating can be patterned over a large area of the sample 300 using holographic lithography using the interference of two lasers.

Referring back to FIG. 2, the SiO ₂ layer is referred to as reactive ion etching (RIE) in S240. A GaN surface lattice reflector can be fabricated by forming a lattice pattern in the GaN layer by inductively-coupled plasma RIE (hereinafter referred to as " ICPRIE ") using a lattice SiO ₂ layer in S250. Can be.

4 shows a scanning electron micrograph of a GaN surface grating reflector patterned using holographic lithography as described above. 4A is a photograph observed from the side of the GaN surface grating reflector, and FIG. 4B is a photograph observed from the top of the GaN surface grating reflector. From Figs. 4A and 4B, the lattice structure having a periodic pattern can be observed.

The reflectance measurement for the GaN surface grating reflector made by the above-described method can be performed in the following manner.

FIG. 5 is a diagram showing an electric field distribution when a transverse electric field (TE) polarized incident light having a wavelength of 450 nm is incident on a GaN surface grating reflector. TE polarized light means light in which the electric field of light and the valley direction ('l' in Fig. 1 (a)) of the lattice pattern structure are parallel. The distribution of the electric field may be analyzed using a finite-difference time-domain (hereinafter referred to as 'FDTD') method. Referring to (a) of FIG. 5, each number is a value representing a relative intensity of an electric field of light. Coordinates with the same intensity of the electric field are shown by connecting lines. The electric field is strongly concentrated locally on the surface of the GaN grid pattern. Areas 'A' and 'B' are light passing through the floor and valley of the grid respectively. Referring to FIG. 5B, each light has the same phase before passing through the grid pattern. However, the phase difference occurs by π between the light passing through the grid floor and the light passing through the valley of the grid while passing through the grid pattern. As a result, the sum of the electric fields of light passing through the 'A' and 'B' regions respectively becomes zero. Therefore, it can be seen that the above description is consistent with the above description that a high reflection can be obtained by the destructive interference phenomenon of light passing through the floor and valley portions of the lattice pattern structure. As the phase difference approaches π, the reflectance increases, and the phase difference may be determined by structural parameters of the grating pattern structure. Examples of the structural parameters may include the period (Λ), the filling rate (F), the height (h), and the inclination angle (θ) of the lattice described above with reference to FIG. 1. Here, the angle of inclination (θ) of the lattice refers to the angle formed between the normal line across the valley portion of the lattice and the lattice side. For example, when the angle of inclination θ of the lattice is 0 degrees, the cross section of the lattice pattern structure becomes a rectangular shape, and when it is 0 degrees or more, it becomes a trapezoidal shape. The structural parameters can be variously modified according to the required use.

The period Λ should be shorter than the wavelength λ of the incident light. Preferably λ / 2 <Λ <λ. For example, when the wavelength of the incident light is about 450 nm, the preferred period Λ may be about 400 nm to about 450 nm.

The height h of the floor portion should be around λ / 2 (n _GaN −1) when the fill factor is about 0.5 (λ: wavelength of incident light, n _GaN : refractive index of GaN). Preferably, λ / 4 (n _GaN −1) <h <λ / (n _GaN −1). For example, when the wavelength of the incident light is about 450 nm, the height h may be about 80 to about 150 nm, preferably about 95 nm to about 125 nm.

The filling rate F may be about 0.1 to about 0.7, preferably about 0.2 to about 0.45. The inclination angle θ of the grating may be about 0 to about 40 degrees, preferably about 0 Degrees to about 30 degrees.

When satisfying each range, the reflectance may be about 90% or more.

As a result of careful calculation by rigorous coupled-wave analysis (RCWA) for various structural parameters, an example of a preferred GaN lattice pattern structure, for example, wavelength λ of about 450 nm The reflectance can be close to 1 for period (Λ) = about 419 nm, fill factor (F) = about 0.4, height (h) = about 114 nm, and tilt angle (θ) = about 0 degrees have.

FIG. 6 is a graph showing reflection spectra according to the polarization direction calculated by the RCWA method with respect to the preferable Group III nitride grid pattern structure. FIG. In this RCWA calculation, it is assumed that the dispersion of reflection index according to the wavelength of group III nitride follows the Selmeier equation. '0th R', '1st R', and '2nd R' refer to the zeroth order diffraction reflectivity, the first order diffraction reflectance, and the second order diffraction reflectance of the incident light, respectively. The zeroth-order diffraction reflectance means the reflectance of incident light when the incident angle and the reflection angle of the incident light are the same. The diffraction reflectivity of the first order or more means the reflectance of incident light when the incident angle and the reflection angle of the incident light are not the same. "0th T" and "1st T" mean 0th diffraction transmittance and 1st diffraction transmittance of incident light, respectively. The zero-order diffraction transmittance means the transmittance of incident light when the incident direction of the incident light and the transmission direction are parallel. The first or more diffraction transmittance means the transmittance of incident light when the incident direction of the incident light and the transmission direction are not parallel. Referring to FIG. 6 (a), the diffraction reflectance of the 0th order is close to 1 near the wavelength of about 450 nm with TE polarized light. It can also be seen that the diffraction reflectivity of the first order or higher of the TE polarization is effectively suppressed. With TE polarized light, a reflector having a fairly wide band can be obtained, with a spectral band having a zeroth order diffraction reflectance of about 80% or more and a spectral band of about 90% having a zeroth order diffraction reflectance of about 60%. This is wider than the typical gain of InGaN quantum wells, which are commonly used as short wavelength semiconductor materials. Referring to FIG. 6B, the reflectivity of the 0th order is close to 0 in the vicinity of the wavelength of 450 nm with TM polarized light. TM polarized light refers to light in which the magnetic field of light and the valley direction of the lattice pattern structure ('l' in FIG. 1A) are parallel. It can also be seen that the first order diffraction reflectivity of the TM polarization is not effectively suppressed. From the above results, it can be seen that the group III nitride surface lattice reflector of the present disclosure can act as a good reflector selectively for TE polarization.

FIG. 7 is a diagram illustrating a change in the zeroth order diffraction reflectance of incident light according to the period of the grating or the height of the grating calculated by the RCWA method. Coordinates with the same zero-order diffraction reflectance are connected to each other to form a contour line. Referring to FIG. 7A, a change in reflectance according to the period of the grating is illustrated. The period of the grid is the width of one floor plus the width of the valleys. It can be seen that as the period of the grating increases, the region having the highest reflectance-the region having the reflectance of about 0.9 or more-moves toward the long wavelength and the reflectance of the incident light decreases. It can be seen that the change in reflectance according to the change in the period of the grating is not large compared to other parameters.

Referring to FIG. 7B, the change in reflectance according to the height h of the grating is shown. It can be seen that the region with a high reflectivity-the region with a reflectance of about 0.9 or more-appears around the 114 nm of the grating height.

8 is a view showing a change in the zero-order reflectance of the incident light according to the fill factor of the grating or the tilt angle of the grating calculated by the RCWA method. Coordinates with the same zero-order diffraction reflectance are connected to each other to form a contour line. Referring to FIG. 8A, the change in reflectance according to the filling factor F of the grating is shown. In the case of the filling ratio of the lattice, the reflectance is high at the optimum value of about 0.4 or less, but the reflectance rapidly decreases from about 0.45. In addition, areas with high reflectance are also relatively narrow compared to other parameters.

Referring to FIG. 8B, a change in reflectance according to the inclination angle θ of the grating is shown. The angle of inclination of the grid refers to the angle formed between the normals of the valleys of the grid and the sides of the grid. It can be seen that the change in reflectance of the grating angle from 0 ° to about 30 ° is not relatively large compared to other parameters. Inferred from the results shown in FIGS. 7 and 8, the fill factor of the grating is a relatively important variable in determining the reflectance.

9 is a reflectance measurement system for incident light traveling toward air or vacuum inside a GaN layer of a GaN surface grating reflector. 9, the reflectance measuring system 900 includes a xenon lamp 910, a lens 920, a pinhole 930, a polarizer 940, a beam splitter 960, and a spectrometer 970. Light emitted from the xenon lamp 910 and passed through the lens 920 is incident to the substrate of the sample 950 using the beam separator 960. Using the polarizer 940 and the pinhole 930, only light in a specific polarization direction may be incident perpendicularly to the sample 950. The light reflected from the sample 950 may be incident on the spectrometer 970 to measure reflectance according to the wavelength. The measured results can be normalized using an aluminum mirror.

10 is a graph showing the reflectance of a GaN surface grating reflector measured using a reflectance measuring system. 'TE-exp' is the result of measuring reflectance using a reflectance measuring system for TE polarized light. 'TE-RCWA' is the result of calculating the reflectance using the RCWA method for TE polarized light. 'TM-exp' is the result of measuring reflectance using a reflectivity measuring system for TM polarized light. 'TM-RCWA' is the result of calculating the reflectance using the RCWA method for TM polarized light. Referring to FIG. 10, it can be seen that the reflectance of TE polarized light having a wavelength of about 450 nm is close to about 1, which is very high. When the wavelength of the TE polarized incident light is about 0 nm or more and about 510 nm or less, the reflectance may be about 80% or more. On the other hand, the reflectance of TM polarized light is almost 0.4 or less. Thus, it can be seen that the GaN surface grating reflector is an excellent reflector for TE polarization. Meanwhile, in the case of the group III nitride surface grating reflector including the two-dimensional lattice structure in which the ridge portion and the valley portion are periodically two-dimensionally arranged, the incident light may have a high reflectance regardless of whether TE or TM polarization is incident.

The GaN surface grating reflector of the present disclosure may be mounted in a GaN based optical device that requires high reflectance. For example, the GaN surface grating reflector may be mounted and applied to a resonant-cavity light-emitting diode or a vertical-cavity light-emitting diode.

As described above, according to one embodiment of the group III nitride surface grating reflector of the present disclosure, a cross-sectional grating pattern having a concave-convex shape includes a structure in which a periodic pattern is repeated. It has a high reflectance for incident light in a particular wavelength band, and particularly exhibits excellent reflectance for TE polarized light. In addition, the wavelength band of incident light showing high reflectivity of about 0.8 or more is wide, about 90 nm.

As described above, the disclosed technology has been described in detail with reference to various embodiments, but a person having ordinary skill in the art to which the disclosed technology belongs, the spirit and scope of the disclosed technology defined in the appended claims Various modifications may be made to the disclosed technology without departing from it. Therefore, changes in the future embodiments of the disclosed technology will not be able to escape the technology of the disclosed technology.

Claims

A group III nitride layer disposed on one surface of the substrate and having a structure of a one-dimensional diffraction grating pattern formed on a surface thereof,

The grid pattern structure has a concave-convex cross section in which the floor portion and the valley portion are periodically arranged,

Group III in which the incident light is reflected on the surface of the lattice pattern structure by canceling out interference between the first light penetrating the ridge portion and the second light penetrating the valley portion among the incident light incident from the inside of the Group III nitride Nitride surface grating reflector.
The method of claim 1,

The group III nitride surface group grating reflector is one of GaN, AlGaN or InGaN.
The method of claim 1,

The incident light has a wavelength of 300 nm to 700 nm group III nitride surface grating reflector.
The method of claim 1,

The period Λ of the grating pattern structure is a group III nitride surface grating reflector wherein λ / 2 <Λ <λ (λ is the wavelength of incident light).
The method of claim 1,

The height h of the ridge portion of the lattice pattern structure is λ / 4 (n group III nitride -1) <h <λ / (n group III nitride -1). Wavelength, n group III nitride : refractive index of group III nitride).
The method of claim 1,

A group III nitride surface grating reflector having a fill factor (F) of the grating pattern structure of 0.2 to 0.7.
The method of claim 1,

A group III nitride surface grating reflector having an inclination angle [theta] of the grating of the grating pattern structure being 0 to 40 degrees.
A light emitting element comprising the group III nitride surface lattice reflector of any one of claims 1 to 7.
A group III nitride layer disposed on one surface of the substrate and having a lattice pattern structure formed thereon;

The lattice pattern structure is the floor portion and the valley portion is periodically arranged in two dimensions and the lattice is a polygonal form,

Group III in which the incident light is reflected on the surface of the lattice pattern structure by canceling out interference between the first light penetrating the ridge portion and the second light penetrating the valley portion among the incident light incident from the inside of the Group III nitride Nitride surface grating reflector.
The method of claim 9,

The group III nitride surface group grating reflector is one of GaN, AlGaN or InGaN.
The method of claim 9,

And said incident light is a transverse electric field (TE) polarization in which the induced electric field direction of said incident light is parallel to the longitudinal direction of said valley portion.
The method of claim 9,

And said incident light is a transverse electric field (TM) polarized light in which the magnetic field direction of said incident light is parallel to the longitudinal direction of said valley portion.
13. A light emitting device comprising the group III nitride surface grating reflector of any one of claims 9-12.