KR101061803B1 - Group III nitride thin film lattice reflector - Google Patents

Abstract

A group III nitride thin film lattice reflector is disclosed. In an embodiment, the group III nitride thin film lattice reflector includes a substrate and a buffer layer disposed on the substrate, and a first layer of group III nitride including an air gap disposed on the buffer layer, and on the first layer. First light passing through the grating of incident light perpendicularly to the grating structure from the outside, including a second layer of group III nitride comprising a parallel grate-like thin film grating structure in which the grating and air holes are periodically arranged And the second light passing through the air hole cancel each other and cause the incident light to be reflected on the surface of the grating structure.

Description

III-nitride membrane grating reflector

The present disclosure relates generally to thin film lattice reflectors, and more particularly to group III nitride lattice reflectors.

Recently, many studies have been conducted to make reflectors using group III nitrides. In reflectors using a group III nitride semiconductor series, there have been many studies on distributed Bragg reflectors (DBRs). Among them, many studies on AlN / GaN reflectors have been conducted, but the mismatch between the lattice is large, and a special growth method is needed to overcome this. However, since the difference between the refractive indices of AlN and GaN (Δn = 0.3) is not large, the reflection area is small and more than 20 pairs of thick structures must be made. To overcome the problems of AlN / GaN DBR, dielectric DBR was also used. Alternatively, DBR using air and GaN has been studied. In this case, since the difference in refractive index is about 1.5, the cutoff band is wide and the reflectance is high. However, these structures are thick and hard to make good crystals.

In conclusion, reflectors using group III nitrides have had many problems. To solve these problems, a reflector having a new structure is required.

According to an aspect of the present disclosure, a substrate and a buffer layer are disposed on the substrate, and a first layer of group III nitride including an air gap is disposed on the buffer layer, and a lattice and an air hole are periodically disposed on the first layer. Passing through the air hole and the first light passing through the grating of the incident light incident perpendicularly to the grating structure from the outside, including a second layer of III-nitride comprising a parallel grate-type thin film grating structure arranged in The second light may cause mutual interference to provide a group III nitride thin film lattice reflector in which the incident light is reflected on the surface of the lattice structure.

Another aspect of the present disclosure provides a light emitting device including the group III nitride thin film lattice reflector.

The foregoing provides only optional concepts in a simplified form for the details that follow. This disclosure is not intended to limit the main or essential features of the claims or to limit the scope of the claims.

1 is a diagram briefly explaining the principle of a thin film lattice reflector.
2 is a diagram schematically illustrating a group III nitride thin film lattice reflector as an embodiment of the present disclosure.
3 is a view illustrating a method of manufacturing a group III nitride thin film lattice reflector as an embodiment of the present disclosure.
4 is a scanning electron microscope (SEM) photograph of a group III nitride thin film lattice reflector of the present disclosure.
FIG. 5 is a micro-reflectance measurement setup for measuring reflectance characteristics of a group III nitride thin film lattice reflector of the present disclosure.
FIG. 6 shows a TE wave (transverse electric field: polarized light in parallel with the longitudinal direction of the grating) and a TM magnetic wave (transverse magnetic field) entering the group III nitride thin film lattice reflector of the present disclosure using microwave reflecting equipment. Reflectance is measured for each polarization direction and electric field perpendicular to each other).

Hereinafter, exemplary embodiments disclosed herein will be described in detail with reference to the accompanying drawings. Unless otherwise indicated in the text, like reference numerals in the drawings indicate like elements. The illustrative embodiments described above in the detailed description, drawings, and claims are not meant to be limiting, other embodiments may be utilized, and other changes may be made without departing from the spirit or scope of the technology disclosed herein. Those skilled in the art can arrange, configure, combine, and designate the components of the present disclosure, that is, the components generally described herein and described in the figures, in a variety of different configurations, all of which are expressly devised and It will be readily understood that they form part of. In order to clearly express various layers (or layers), regions, and shapes in the drawings, the width, length, thickness, or shape of the components may be exaggerated.

When one component is referred to as "connecting" to another component, it may include the case in which the one component is directly connected to the other component, as well as an additional component interposed therebetween. have.

When one component is referred to as "positioning to" another component, it may include a case in which one component is directly disposed on the other component, as well as a case where additional components are interposed therebetween.

1 is a diagram briefly explaining the principle of a thin film lattice reflector. The physical principle of the thin film lattice reflector is briefly explained. When the incident light 110 passes through the grating 100 having a period smaller than the wavelength, the first-order diffraction transmission disappears and only the zero-order diffraction transmission remains. Zero-order diffraction transmission means transmission of incident light when the incident direction of the incident light and the transmission direction are parallel. The first-order diffraction transmission means transmission of incident light when the incident direction of the incident light and the transmission direction are not parallel. At this time, the phase of incident light 120 traveling to a portion having a relatively low refractive index (for example, air or vacuum) and the incident light 110 traveling to a portion 100 having a relatively high refractive index (eg, an AlGaN thin film lattice) The thin film lattice structure can be made so that the phases of the? Are different by π. At this time, the incident light is physically zero-order diffraction transmission is 0 in some areas. In this state, since the transmission energy of incident light becomes zero, only zero-order reflection of incident light remains. The zeroth order reflection refers to the reflection of incident light when the incident angle and the reflected angle of the incident light are the same. After all, theoretically designing the grating structure appropriately for the wavelength of the incident light can reflect all the energy of the incident light.

2 is a diagram schematically showing a group III nitride thin film lattice reflector as an example of the present disclosure. Referring to FIG. 2A, the III-nitride lattice reflector includes a parallel grate-type lattice structure in which the air holes 244 and the thin film lattice 242 are periodically arranged, and an air gap below the thin film lattice structure. (235). A diagram showing a cross section taken of the group III nitride thin film lattice reflector in the direction A-A 'is shown in Fig. 2B. Referring to FIG. 2B, the group III nitride thin film lattice reflector 200 of the present disclosure includes a buffer layer 220, a first layer 230, and a second layer 240. The buffer layer 220 may be disposed on the substrate. The substrate may be, for example, a sapphire substrate. The buffer layer 220 may be, for example, an n-type GaN layer. The buffer layer 220 may be formed to be thick with a thickness of about 4 μm in order to reduce the effect of mismatch due to the difference between the substrate and the lattice constant. The first layer 230 may be formed on the buffer layer. The first layer 230 includes an air gap 235. For example, the first layer 230 may be a sacrificial layer 230 having a superlattice structure having at least one pair of an In _x Ga _1-x N layer and an In _y Ga _1-y N layer (0 < x <1, 0 <y <1, x ≠ y). The second layer 240 is disposed on the first layer 230. The second layer 240 may have a parallel grate structure in which the grating 242 and the air holes 244 are periodically arranged. The second layer 240 may be, for example, an Al _z Ga _1-z N layer (0 <z <1). Proper design of the thickness t of the second layer 240, i.e. the thickness t of the grating, the filling factor of the grating structure, and the grating period p, can lead to the lattice structure outside the group III nitride thin film grating reflector. The phases of the first light A passing through the grate-type grating 242 and the second light B passing through the air hole 244 of the grating structure are equal to π, so that the mutually incident incident light cancels each other. Cause Therefore, the incident light may be 100% reflected at the grating structure surface. The fill factor of the lattice structure is the ratio (w / p) of the width w of the lattice to the lattice period p. The conditions under which total reflection of incident light occurs are as follows.

(n ₀ : refractive index of air, n ₁ : refractive index of grating, λ: wavelength of incident light, k: wave vector (= 2π / λ))

Rigorous Coupled-Wave Analysis (hereinafter referred to as "RCWA") can be used to determine the optimal conditions for the thickness t of the second layer, the lattice period, and the fill factor of the lattice structure. For example, if the wavelength of the incident light is about 480 nm, the optimized conditions obtained by the RCWA method are 430 nm in the lattice period and 55% in the lattice structure. For example, when the wavelength of the incident light is about 300 nm to about 600 nm, the thickness of the second layer may be about 100 nm or more and about 300 nm or less, and the lattice period may be about 200 nm or more and about 600 nm or less. In addition, the filling rate of the lattice structure may be about 30% or more and about 70% or less. Hereinafter, as a description of FIG. 3, a process of manufacturing a group III nitride reflector is described as an embodiment of the present disclosure.

3 is a diagram illustrating a method of manufacturing a group III nitride lattice reflector as an embodiment of the present disclosure. Referring to FIG. 3A, an n-type GaN layer 220, which is a complete layer, is formed on a substrate 210, and an In _x Ga _1-x N layer and an In _y Ga _1-y N layer are formed thereon. A sacrificial layer 230 of at least one superlattice structure may be formed (0 <x <1, 0 <y <1, x ≠ y). As an example, seven pairs of In _0.04 Ga _0.96 N layers and In _0.08 Ga _0.92 N layers may be formed, each having a thickness of about 14 nm. The n-type GaN layer 210 and the sacrificial layer 230 may be formed using, for example, a MOCVD method. Based on the RCWA calculation result, the Al _z Ga _1-z N layer 240 may be formed with a thickness of about 180 nm (0 <z <1). For example, the Al _z Ga _1-z N layer may be an Al _{0.15 5} Ga _0.85 N layer. The SiO ₂ layer 250 and the Cr layer 260 are grown on the Al _z Ga _1-z N layer 240 thus grown. In (b) of FIG. 3, a parallel grate pattern having a period of 430 nm and a filling rate of 55% of a lattice structure was formed on the Cr layer 260 using holography lithography. First, a grate pattern parallel to the Cr layer 260 is formed by using reactive ion etching (hereinafter, referred to as 'RIE'). In (c) of FIG. 3, a mesa having a width of 15 μm and a length of 50 μm is made to a SiO ₂ layer by using a general optical lithography method and RIE, and Inductively coupled plasma etching (hereinafter referred to as “ICP”) is performed. A mesa is formed to the sacrificial layer 230 using the n-type GaN layer 220. In (d) of FIG. 3, photolithography and etching are performed so that the lattice structure of the SiO ₂ layer 250 is positioned at a width of 10 μm × 10 μm in the center of the mesa. In FIG. 3E, the sacrificial layer 230 is exposed by forming a lattice structure on the Al _z Ga _1-z N layer 240 using RIE and ICP. Thereafter, the Ti / Au electrode 225 was formed on the mesa peripheral n-type GaN layer 220 by a general lift-off method. In FIG. 3F, band gap selective photoelectrochemical etching is performed, in which a 1000 W xen lamp filtered with gallium nitride (GaN) is exposed to generate a carrier in the sacrificial layer 225. And react with 0.008M HCl etching solution. At this time, the excessively generated electrons are ejected to the Ti / Au electrode 225. In this way, the air gap 235 is completed in the sacrificial layer 230.

4 is a scanning electron microscope (SEM) photograph of a group III nitride thin film lattice reflector of the present disclosure. Referring to FIG. 4A, a grate-type thin film grating is formed periodically, and the length l of the grating is about 10 μm. An air gap is formed below the grating and the thickness of the air gap is about 200 nm. FIG. 4B is a scanning microscope photograph of the plane of the parallel grating thin film lattice structure reflector. It can be seen that the lattice structure is clearly formed.

FIG. 5 is a micro-reflectance measurement setup for measuring reflectance characteristics of a group III nitride lattice reflector of the present disclosure. Referring to FIG. 5A, the micro reflector 500 may include a light source 510, a visible light transmitting filter 520, an aperture 530, a polarizer 540, a beam separator 550, and a microscope objective lens 560. ), A slit 580. The light source 510 used a 600W broadband light emitting xenon (Xe) lamp, and the light emitted from the xenon lamp passed through the visible light (wavelength of about 390nm to about 750nm) transmission filter 520 and about 1mm diameter aperture 530. The polarization state of the light is adjusted using the polarizer 540 mounted to the rotating stage. Light directed to the grating reflector 570 through the beam splitter 550 enters the grating reflector 570 through the microscope objective lens 560. Light reflected from the grating reflector 570 is directed back to the spectroscope 580 through the microscope objective lens 560. Light directed to the spectrometer 580 passes through the aperture 530 and the slit 580. Next, a spectroscope 580 having a 0.27m focal length and an electron coupled device (hereinafter, referred to as 'CCD') cooled with liquid nitrogen are reflected from the lattice reflector 570 and incident on the spectroscope 580. The characteristics of the light were measured. FIG. 5B is a diagram illustrating a measurement position of the grating 570 of the grating reflector selected by using the aperture 530 and the slit 580 in front of the spectroscope. The incident angle of light incident to the spectrometer 580 through the diaphragm 530 is about 0 ° <θ <about 15 ° in FIG. 5A.

FIG. 6 shows a TE wave (transverse electric field: polarization parallel to the longitudinal direction of the grating and an electric field parallel to the group III nitride thin film lattice reflector using microwave reflecting equipment) and a TM wave (transverse magnetic field); Reflectance is measured for each polarization direction and electric field perpendicular to each other). Referring to FIG. 6, each figure is shown as an RCWA result. The conditions of RCWA assume that the thickness of the GaN layer of the lattice reflector is 4 [mu] m and all components have no absorption.

A microscope objective lens with a numerical aperture of 0.85 receives light having a large range of incidence angles and reflection angles (about 0 ° <θ <about 58 °). On the other hand, the direction component parallel to the longitudinal direction of the grating of the light traveling vector components can be ignored, the interval between the slits in Figure 5 (b) is about 100 μm, the incident angle of the incident light that can be incident on the spectrometer is about 0 ° in This is because it is about 1 °. On the other hand, the range of the incident angle θ of the incident light that is perpendicular to the longitudinal direction of the grating is determined by the aperture hole in front of the spectrometer. In this case, the incident angle θ of incident light that can be incident on the spectrometer is about 0 ° or more and about 15 ° or less. The reflectance spectrum R (λ) can be obtained by integrating the reflectance spectrum R (λ, θ) with respect to the incident angle θ.

Where θ ₀ is the maximum angle of incidence angle θ, about 15 °. λ is the wavelength of incident light. The reflectance spectra R (λ, θ) were obtained by the RCWA method, where θ = 0 °, 1 °, 2 °,... , 15 ° and the integration was performed by discrete methods. RCWA results are shown in Figs. 6 (a) and 6 (b). In order to minimize the effect of the angle formed by the direction perpendicular to the longitudinal direction of the grating, an experiment was conducted in which the laser was incident perpendicularly to the longitudinal direction of the grating using several lasers having different wavelengths. Although the experimental apparatus is the same as that of FIG. 5 (a), a light source is used as a laser, the numerical aperture of the microscope objective lens is 0.13, and the incident angle θ is almost vertical at about 0 ° <θ <about 7.5 °. As a measuring instrument, a Si-photodetector was used instead of a spectrometer. The measurement results are shown in (c) and (d) of FIG. 6 and drawn together for comparison with the RCWA results. Referring to (c) and (d) of FIG. 6, it can be seen that the RCWA result and the measurement result are almost identical. The wavelength band of incident light having a reflectance of 0.8 or more is about 100 nm or more, and it can be seen that the group III nitride lattice reflector is an excellent reflector for TE waves in a wide wavelength range (about 400 nm to about 550 nm).

The group III nitride thin film lattice reflector of the present disclosure can be mounted in a light emitting device that requires high reflectance. For example, when a group III nitride thin film lattice reflector is applied to an LED, a polarization state can be controlled and an LED can be manufactured.

As described above, according to one embodiment of the group III nitride thin film lattice reflector of the present disclosure, a grating structure is arranged in parallel and periodically. 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 a high reflectance of 0.8 or more is about 100 nm. In addition, when a light emitting device using the same is manufactured, a light source having good luminous efficiency and high color purity can be obtained.

From the above, various embodiments of the present disclosure have been described for purposes of illustration, and it will be understood that various modifications are possible without departing from the scope and spirit of the present disclosure. And the various embodiments disclosed are not intended to limit the present disclosure, the true spirit and scope will be presented from the following claims.

Claims (10)

A parallel grate having a substrate and a buffer layer disposed on the substrate, a first layer of group III nitride comprising an air gap on the buffer layer, and a lattice and air holes periodically arranged on the first layer The first light passing through the grating and the second light passing through the air hole among the incident light incident from the outside perpendicular to the grating structure, including a second layer made of a group III nitride including the type thin film grating structure, are mutually different. A group III nitride thin film lattice reflector wherein the incident light is reflected off the lattice structure surface by causing destructive interference. The method of claim 1,
The buffer layer is a group III nitride thin film lattice reflector is an n-type GaN layer. The method of claim 1,
The first layer is a group III nitride thin film lattice reflector having a superlattice structure of at least one pair of In _x Ga _1-x N layers and In _y Ga _1-y N layers (0 <x <1, 0 < y <1, x ≠ y). The method of claim 1,
The second layer is a group III nitride thin film lattice reflector (0 <z <1), which is an Al _z Ga _1-z N layer. The method of claim 1,
The incident light has a wavelength of 300 nm to 600 nm group III nitride thin film lattice reflector. The method of claim 1,
The thickness t of the second layer is λ / {2 (n ₁ -n ₀ )} group III nitride thin film lattice reflector (λ: wavelength of incident light in vacuum, n ₀ : refractive index of air, n ₁ : group III Refractive index of nitride). The method of claim 1,
The group III nitride thin film lattice reflector having a thickness of the second layer of 100 nm or more and 300 nm or less. The method of claim 1,
The group III nitride thin film lattice reflector having a period of the lattice structure is 200 nm or more and 600 nm or less. The method of claim 1,
The fill factor of the lattice structure is a group III nitride thin film lattice reflector of 30% or more and 70% or less. A light emitting device comprising the group III nitride thin film lattice reflector of claim 1.

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