CN103094427A - Method for improving AlGaN-based-ultraviolet (UV)-light-emitting diode (LED) luminous efficiency by utilizing of double-faced patterned substrate - Google Patents

Abstract

The invention discloses a method for improving the luminous efficiency of AlGaN-based UV-LEDs by using a double-sided patterned substrate. This method can effectively reduce the stress and dislocation density in AlGaN-based LEDs, and obtain high-quality, atomically flat surfaces. AlN template, and then get a high-quality AlGaN epitaxial layer, thereby improving the internal quantum efficiency of the LED. In addition, since the reverse side of the substrate is etched with a sub-wavelength grating with a symmetrical square pattern, the transmission of light can be increased and non-polarized light can be obtained. The patterned sapphire substrate technology also has the advantages of simple process, low cost, enhanced heat dissipation and avoiding crystal damage.

Description

A method for improving the luminous efficiency of AlGaN-based UV-LEDs by using double-sided patterned substrates

technical field

The invention belongs to the technical field of semiconductors, in particular to a method for improving the luminous efficiency of an AlGaN-based UV-LED by using a double-sided patterned substrate. This method can effectively combine the patterning of the front side of the substrate and the two-dimensional subwavelength grating on the back side to greatly improve the internal quantum efficiency and light emission rate of AlGaN-based UV-LEDs, thereby greatly improving the luminous efficiency of UV-LEDs. .

Background technique

Ultraviolet light-emitting diode (UV-LED) is a semiconductor device that emits ultraviolet bands under current drive. The research and development of such devices has become a focus in the field of wide-bandgap semiconductor optoelectronic devices, and it is also a research hotspot in the use of wide-bandgap semiconductor materials to realize ultraviolet light sources. At present, commonly used ultraviolet light sources include mercury lamps, xenon lamps, and fluorescent lamps. However, these lamps are bulky, have high operating voltage and are not very environmentally friendly, so they are not very convenient to use. In contrast, aluminum-gallium-nitride (AlGaN)-based UV-LED is a semiconductor solid light source with small size, light weight, long life, high efficiency and low operating voltage. Therefore, it has broad application prospects in the fields of national defense technology, computer data storage, biomedicine, anti-counterfeiting identification, environmental monitoring and public health. AlGaN material with high aluminum (Al) composition is an important material for preparing light-emitting devices in the deep ultraviolet band, and plays an important role in the military and civilian markets. In terms of civil use, UV-LED light sources have important applications in lighting, antivirus, medical treatment, printing, and high-density information storage.

In the past ten years, reports on UV-LEDs have emerged, especially since M. Khan et al. of South Carolina State University in the United States adopted pulsed atomic layer metal-organic chemical vapor deposition (MOCVD) technology and superlattice in 2005. The emission wavelength of UV-LED is constantly moving towards the short wave direction. In 2006, Yoshitaka Taniyasu et al. of NTT Corporation of Japan reported an AlN-based UV-LED with a light emission wavelength of 210 nm. Since then, people have obtained UV-LEDs with emission wavelengths in the range of 230nm-280nm by adjusting the Al composition. Although people can realize UV-LEDs in the 210-400nm band by adjusting the Al composition, but with the increase of the Al composition, the difficulty from material growth to device preparation also increases accordingly. Under the drive of 40mA direct current, the 210nm UV-LED has a luminous power of only 0.02mW, and its external quantum efficiency is lower than 10 ^-5 %. Therefore, improving luminous efficiency and power has become a development goal of UV-LED.

AlGaN-based UV-LED is a wide bandgap electro-optic conversion device. The conversion process includes three steps, firstly, electrons and holes are injected into the active area, secondly, electrons and holes radiate and recombine in the active area to emit light, and finally light is emitted from the surface of the device. To obtain high luminous efficiency, there must be enough electrons and holes drifting to the active region for radiative recombination under the action of the electric field, and the radiative recombination light should be emitted to the device surface as much as possible. However, the dislocations introduced by the lattice mismatch between sapphire and the epitaxial layer become electron or hole trap centers or non-radiation centers, resulting in a decrease in internal quantum efficiency. Some teams adopt the method of epitaxial UV-LED on patterned substrate or patterned AlN, in order to obtain good crystal quality by controlling the growth of buffer layer (Buffer). In 2008, Amano et al. reported that the strip pattern was etched on the aluminum nitride (AlN) Buffer, and then the method of lateral epitaxy increased the luminous efficiency of UV-LED by 27 times (the same as that of the non-patterned phase Compare). However, this kind of UV-LED generally adopts a front-emitting packaging method, so it is difficult to avoid the problems caused by the low thermal conductivity of sapphire. Moreover, the p-type gallium nitride (GaN) on the front side has a strong absorption effect on ultraviolet light, which makes the efficiency of emitting light from the front side low. For this reason, many experts use UV-LEDs with a flip-chip (filp-chip) structure. However, the refractive index (~1.8) of the sapphire substrate is larger than that in the air, and the light is easy to form total reflection at this interface and is not easy to exit, resulting in a decrease in the extraction rate of light and low luminous power. In order to improve the light extraction efficiency of AlGaN-based UV-LEDs, various researches have been carried out, such as surface roughening, surface coating with Bragg gratings, and use of photonic crystals. However, the surface roughening treatment only improves the light extraction efficiency by increasing the scattering of light at the interface, and its effect is not very obvious, and the outgoing light is relatively divergent. Although the Bragg grating coating on the surface can improve the output of light, the design of this film system is limited by the refractive index of the material, and the coated film is easy to wear and fall off, and it is not good for heat dissipation. Photonic crystals guide light based on the photonic band gap, which can improve the light extraction rate, but the preparation cost is relatively high, most of the outgoing light is polarization-dependent, and there are also disadvantages such as wear and shedding. Therefore, it is imperative to develop a method that can effectively improve the crystal quality of AlGaN materials and improve the light extraction efficiency in the process of UV-LED research and development.

Contents of the invention

The purpose of the present invention is to solve the problems of poor crystal quality and low light extraction rate in the above-mentioned UV-LED preparation process, and propose a method for improving the luminous efficiency of AlGaN-based UV-LEDs by using double-sided patterned sapphire substrates.

The technical solution of the present invention is: a method for improving the luminous efficiency of AlGaN-based UV-LEDs by using a double-sided patterned substrate, the steps of which are: step 1: depositing a layer of silicon dioxide film on a sapphire substrate; step 2: using light The photoresist pattern array is prepared by engraving technology, and its pattern unit is rectangular;

Step 3: Using a photoresist pattern array as a mask, use a mixture of hydrofluoric acid, ammonium fluoride and water to etch a silicon dioxide film with a patterned structure; Step 4: use a patterned silicon dioxide film As a mask, use a mixture of sulfuric acid and phosphoric acid to wet etch the sapphire substrate to etch the pattern onto the sapphire substrate; step 5: use hydrofluoric acid solution to remove the remaining silicon dioxide film, and use deionized water to Clean the sapphire substrate; step 6: grow a low-temperature nucleation layer on the patterned sapphire substrate by metal-organic chemical vapor deposition, and then increase the temperature and change the III/V ratio to obtain a high-temperature AlN buffer layer; step 7: Using the method of pulsed atomic layer epitaxy, grow another layer of high-temperature AlN layer;

Step 8: growing n-type doped AlGaN on the high-temperature AlN layer; Step 9: epitaxially growing the required multi-quantum well layer and p-type AlGaN electron blocking layer and p-type AlGaN and p-type GaN layers on the n-type AlGaN; Step 10: Etching a symmetrical rectangular pattern on the back of the sapphire substrate by standard nanoimprinting process.

The silicon dioxide film described herein has a thickness of 50 nanometers to 2.5 microns.

Wherein the photoresist pattern array is rectangular, and the size and pitch of pattern units are 0.2 micron-1 micron.

The mixed volume ratio of sulfuric acid and phosphoric acid used is 1:3.

The etching temperature described therein is 350°C-450°C, and the etching time is 30 seconds-20 minutes. When growing the low-temperature AlN layer, the gas pressure is 40torr, the growth temperature is 570°C to 720°C, and the thickness is 50nm.

The thickness of the low-temperature AlN layer is 50nm, the thickness of the high-temperature layer is 1.5μm, and the thickness of the pulsed epitaxy is 100nm.

The multi-quantum well layer described therein is _{AlxGa1 -xN} / _{AlyGa1 -yN} , and the emission wavelength is 280nm.

The depth of the symmetrical two-dimensional rectangular pattern on the back of the sapphire is 430nm-630nm, the side length of the square is 80nm-160nm, and the distance of a period is 200nm.

The invention provides a method for improving the luminous efficiency of AlGaN-based UV-LEDs by using a double-sided patterned substrate. This method can effectively reduce the stress and dislocation density in AlGaN-based UV-LEDs, and obtain high-quality, surface atomic-level A flat AlN template can be used to obtain a high-quality AlGaN epitaxial layer, thereby improving the internal quantum efficiency of UV-LEDs. In addition, since the reverse side of the substrate is etched with a sub-wavelength grating with a symmetrical square pattern, the transmission of light can be increased and non-polarized light can be obtained. The patterned sapphire substrate technology also has the advantages of simple process, low cost, enhanced heat dissipation and avoiding crystal damage.

The advantages of the present invention are: (1) There are graphics in a specific direction on the front side, which can control the growth of AlN Buffer, thereby reducing the stress and defects caused by lattice mismatch in the Buffer layer, and obtaining high-quality, atomically flat surface. AlN layer; (2) The pattern on the front of the substrate is in the shape of a Mitsubishi cone, so that the refractive index of the epitaxial material changes continuously between the refractive index of AlN and the refractive index of sapphire, so that the reflection of light at the interface between AlN and sapphire (3) Subwavelength gratings are etched on the reverse side of the substrate. The period of the grating is smaller than the wavelength of the emitted ultraviolet light. By rationally designing the etching thickness and duty cycle, the enhancement of the 0th-order diffraction peak can be achieved, and no other-order diffracted light appears, thereby realizing the anti-reflection of light. Through theoretical calculation, the transmittance of this kind of grating can reach more than 99%; (4) The subwavelength grating is a symmetrical rectangular pattern grating, which can avoid the birefringence effect caused by the general asymmetrical subwavelength grating. It is still unpolarized light; (5) The reverse side of the substrate is an etched microstructure, which is good for heat dissipation. the

Description of drawings

Figure 1 is a schematic cross-sectional view of the sapphire substrate after photolithography;

FIG. 2 is a schematic cross-sectional view of a silicon dioxide film 2 etched by a mixture of hydrofluoric acid, ammonium fluoride and water using the photoresist array 3 as a mask;

Figure 3 uses the silicon dioxide film after etching as a mask to etch the sapphire substrate along the (10-10) direction (a crystal plane direction) with a mixture of sulfuric acid and phosphoric acid (volume ratio of 3:1) 1. Schematic diagram of the cross-section after the graphics are transferred to the sapphire substrate;

Figure 4 is a schematic cross-sectional view of dilute hydrofluoric acid removing the residual silicon dioxide film 1 and cleaning the sapphire;

Fig. 5 continues to etch the sapphire substrate 1 with the mixed solution of sulfuric acid and phosphoric acid, forming a triangular structure in section;

Figure 6 is a flat surface formed after the epitaxial low-temperature AlN nucleation layer and high-temperature AlN on the sapphire substrate;

Figure 7 shows n-type AlGaN, AlGaN/AlGaN multiple quantum wells, p-type barrier layer and p-type GaN that continue to be epitaxial on a flat AlN Buffer layer;

Figure 8 is the interface after etching the two-dimensional square subwavelength grating on the reverse side of sapphire;

Fig. 9 is a bottom view of the sapphire after etching a two-dimensional square subwavelength grating;

In the figure: 1 is sapphire substrate, 2 is silicon dioxide film, 3 is photoresist pattern, 4 is AlN Buffer layer, 5 is n-type AlGaN, 6 is multiple quantum well, 7 is electron blocking layer, 8 is p type AlGaN, 9 is a p-type GaN layer, and 10 is an air gap.

Detailed ways

Please refer to Fig. 1-shown in Fig. 9, the present invention comprises the following steps:

Step 1: Deposit a layer of silicon dioxide film 2 on the sapphire substrate 1 by chemical vapor deposition (CVD) (as shown in Figure 1), the thickness of the silicon dioxide is 50 nanometers to 1.5 microns;

Step 2: Prepare a photoresist pattern array 3 by using photolithography technology, and its pattern unit is rectangular; the pattern unit spacing is 0.5 micron-1 micron;

Step 3: Using the photoresist pattern array 3 as a mask, use a mixture of hydrofluoric acid, ammonium fluoride and water to etch a silicon dioxide film with a pattern structure (as shown in Figure 2);

Step 4: Using the patterned silicon dioxide film 2 as a mask, use a mixture of sulfuric acid and phosphoric acid to wet-etch the sapphire substrate 1, and etch the pattern onto the sapphire substrate (as shown in Figure 3); the The mixing volume ratio of sulfuric acid and phosphoric acid is 1:3, the etching temperature is between 350-450°C, and the etching time is 30 seconds to 20 minutes;

Step 5: Use hydrofluoric acid solution to remove the residual silicon dioxide film 2, and clean the sapphire substrate with deionized water (as shown in Figure 4);

Step 6: Further etch to make the cross-section of the figure triangular, then clean the substrate and dry it (as shown in Figure 5), the etching time is 1min-5min;

Step 7: Using the metal-organic chemical vapor deposition method, first grow a low-temperature AlN nucleation layer on the patterned sapphire substrate 1 under the conditions of 40torr and 570°C-720°C, and then raise the temperature to 1100°C , first grow 900nm AlN when the molar flow ratio (Ⅴ/III) of Group III source and ammonia (NH ₃ ) is 120000; then reduce the Ⅴ/III ratio to 30000 and grow 700nm AlN;

Step 8: Using the method of pulsed atomic layer epitaxy, grow another layer of high-temperature AlN layer 4; the growth temperature is 1050°C, the pulse period is 0.3min, the flow rate of NH ₃ is 1500sccm, and the growth thickness is 100nm (as shown in Figure 6 Show);

Step 9: grow n-type doped AlGaN on the high-temperature AlN layer, and epitaxially grow the required multi-quantum well layer, p-type electron blocking layer and p-type GaN layer on the n-type AlGaN (as shown in Figure 7);

Step 10: Etch a symmetrical rectangular pattern on the back of the sapphire substrate (as shown in Figure 8 and Figure 9) using a standard inductive ion beam etching (ICP) process; the symmetrical two-dimensional square pattern on the back of the sapphire The depth is 430nm-630nm, the side length of the square is 80nm-160nm, and the distance of a cycle is 200nm.

The technical characteristics of the present invention will be further elaborated below through specific examples.

This embodiment is a method for improving the luminous efficiency of an AlGaN-based UV-LED by using a double-sided patterned substrate.

First, a 200nm silicon dioxide film 2 is deposited on a 2-inch c -plane sapphire substrate 1 using chemical vapor deposition (CVD) technology, and then a periodic photoresist pattern display 3 is made using conventional photolithography technology, and the pattern unit is rectangular , the spacing is 500nm, and the cross-sectional view after photolithography is shown in Figure 1;

Then use the photoresist array as a mask, and use a mixture of hydrofluoric acid, ammonium fluoride and water to etch a silicon dioxide film with a pattern structure (as shown in Figure 2);

Then, using the silicon dioxide film 2 with a pattern structure as a mask plate, the c -plane sapphire substrate 1 is etched at 450°C for 10 minutes with a mixture of sulfuric acid and phosphoric acid (volume ratio: 1:3). The figure is shown in Figure 3;

Use dilute hydrofluoric acid to wet-etch away the remaining silicon dioxide film 2, and the cross section is shown in Figure 4;

Continue to etch the patterned sapphire substrate 1 with a mixture of sulfuric acid and phosphoric acid at 450°C to form a triangular prism on the surface, as shown in Figure 5 in cross section;

Put the patterned sapphire substrate that has been cleaned and dried into the MOCVD equipment, grow the AlN nucleation layer at 620°C for 4.4min, and then grow the high-temperature AlN layer at 1100°C for 30min. Finally, at 1050°C, a 100nm high-temperature AlN layer was regrown by pulsed atomic layer epitaxy. The section is shown in Figure 6;

On high-temperature AlN, grow n-type AlGaN, 10 cycles of AlGaN/AlGaN multiple quantum wells, p-type AlGaN electron blocking layer, p-type AlGaN, and p-type GaN. Its section is shown in Figure 7;

Finally, a symmetrical two-dimensional square sub-wavelength grating was prepared on the reverse side of the sapphire by a standard nanoimprint process. The depth of the pattern is 430nm-630nm, the side length of the square is 80nm-160nm, and the distance of a period is 200nm. The cross-section is shown in Figure 8, and the bottom view is shown in Figure 9.

The above examples describe the method of improving the luminous efficiency of AlGaN-based UV-LEDs by using double-sided patterned sapphire substrates. Since the front pattern is etched by wet method, the surface is relatively smooth, which is beneficial to the subsequent crystal growth. Moreover, pulsed atomic layer epitaxy is also used on the high-temperature AlN layer to obtain a good AlN surface morphology. In addition, the nanoimprinting process is used on the back side, which can precisely control the etching depth and width, so the deviation of the parameters of the prepared subwavelength grating from the design value is small, so that the anti-reflection can be well achieved, and the AlGaN substrate can be further improved. Luminous efficiency of UV-LED.

Claims (8)

1. one kind is utilized two-sided patterned substrate to improve AlGaN base UV-LED luminous efficiency method, its step:

Step 1: deposition layer of silicon dioxide film on Sapphire Substrate;

Step 2: utilize photoetching technique to prepare the photoetching offset plate figure array, its graphic element is rectangle;

Step 3: make mask with the photoetching offset plate figure array, utilize the mixed liquor of hydrofluoric acid, ammonium fluoride and water, etch the silica membrane with graphic structure;

Step 4: as mask plate, utilize the mixed liquor wet etching Sapphire Substrate of sulfuric acid and phosphoric acid with silica membrane with figure, with pattern etching on Sapphire Substrate;

Step 5: utilize hydrofluoric acid solution to remove remaining silicon dioxide film, and with deionized water, Sapphire Substrate is cleaned up;

Step 6: utilize the metal-organic chemical vapor deposition equipment method, at patterned Grown on Sapphire Substrates low temperature nucleating layer, then the method for raise temperature and conversion III/ V ratio obtains high temperature AlN resilient coating;

Step 7: utilize the method for pulsed atomic layer epitaxy, regrowth one deck high temperature AlN layer;

Step 8: the AlGaN of growing n-type doping on high temperature AlN layer;

Step 9: extension goes out required multiple quantum well layer and p-type AlGaN electronic barrier layer and p-type AlGaN and p-type GaN layer on N-shaped AlGaN;

Step 10: utilize the nano-imprint process of standard, etch again symmetrical rectangular patterns at the Sapphire Substrate back side.

2. a kind ofly according to claim 1 utilize two-sided patterned substrate to improve AlGaN base UV-LED luminous efficiency method, it is characterized in that: the thickness of described silicon dioxide film is 50 nanometers-2.5 micron.

3. a kind ofly according to claim 1 utilize two-sided patterned substrate to improve AlGaN base UV-LED luminous efficiency method, it is characterized in that: described photoetching offset plate figure array is rectangle, and the size of graphic element and spacing are 0.2 micron-1 micron.

4. a kind ofly according to claim 1 utilize two-sided patterned substrate to improve AlGaN base UV-LED luminous efficiency method, it is characterized in that: sulfuric acid used and phosphoric acid mixed volume are than being 1:3.

5. a kind ofly according to claim 1 utilize two-sided patterned substrate to improve AlGaN base UV-LED luminous efficiency method, it is characterized in that: described etching temperature is at 350 ℃-450 ℃, etch period is 30 seconds-20 minutes, during growing low temperature AlN layer, air pressure is 40torr, growth temperature is 570 ℃ to 720 ℃, and thickness is 50nm.

6. a kind ofly according to claim 1 utilize two-sided patterned substrate to improve AlGaN base UV-LED luminous efficiency method, it is characterized in that: the thickness of described low temperature AI N layer is 50nm, and the thickness of heat zone is 1.5 μ m, and the thickness of pulsed extension is 100nm.

7. a kind ofly according to claim 1 utilize two-sided patterned substrate to improve AlGaN base UV-LED luminous efficiency method, it is characterized in that: described multiple quantum well layer is Al _xGa _1-xN/Al _yGa _1-yN, emission wavelength are 280nm.

8. a kind ofly according to claim 1 utilize two-sided patterned substrate to improve AlGaN base UV-LED luminous efficiency method, it is characterized in that: the symmetrical expression two-dimensional rectangle figure degree of depth at the described sapphire back side is 430nm-630nm, the foursquare length of side is 80nm-160nm, and the one-period distance is 200nm.

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