CN104659174A - Method for improving light emitting property of LED by using laser radiated gallium nitride epitaxial wafer as substrate of LED - Google Patents

Abstract

The invention relates to a method for improving the luminous performance of LEDs based on gallium nitride epitaxial wafers by irradiating them with laser light, which belongs to the field of material preparation. In the present invention, the GaN epitaxial wafer (the surface is p-type) is pretreated to remove the contamination and organic residues on the surface, and the surface lattice defects are improved. A 248nm excimer laser is used to irradiate the GaN epitaxial wafer, and the single pulse energy The density is 0.15J/cm ² -0.6J/cm ² . After depositing metal electrodes on the surface of the sample by magnetron sputtering, conduct electrical tests such as carrier concentration and surface resistivity of some metal-semiconductor ohmic contacts. After testing, the electrical properties of the sample changed significantly after irradiation, indicating that the ohmic contact was greatly improved after irradiation. The GaN epitaxial wafers before and after irradiation were manufactured by a process-based LED process, and then the LED characteristic parameters such as forward voltage, reverse leakage current, and light output power were tested, and all of them were improved to varying degrees.

Description

A method for improving the light-emitting performance of LEDs based on gallium nitride epitaxial wafers by laser irradiation

technical field

The invention belongs to the field of material preparation.

Background technique

Wide bandgap semiconductors represented by materials such as gallium nitride and silicon carbide are emerging materials with rich physical properties, and many of their electrical and optical properties far exceed those of the previous generation of semiconductor materials. Wide bandgap semiconductors are playing an increasingly important role in current production and life, especially gallium nitride materials play a pivotal role in the human lighting industry, and are widely used in the new generation of lighting LED devices.

The bandgap of GaN material is direct bandgap type, and its bandgap width is 3.39eV at room temperature, which belongs to the category of wide bandgap semiconductors, and generally has a high quantum efficiency during carrier recombination. The GaN material has a direct band gap, which means that it has a wide band gap, and is very suitable for making blue, green and ultraviolet light-emitting devices (such as LEDs) and ultraviolet light detection devices. Its electron saturation rate, breakdown electric field, thermal conductivity, and band gap are higher than those of Si and GaAs, so it can be used in charge-coupled devices, high-speed memories, high-power devices, and photodetectors that require relatively low dark current. Of course, GaN and other direct bandgap materials can also produce short-wavelength high-power lasers. Secondly, this kind of material has high dielectric strength, which is suitable for the development of corresponding high-power amplifiers, switching devices and diodes. In addition, its relative permittivity is relatively low, and it can be used to make millimeter-wave amplifier series capacitors. These excellent properties make it have great application potential in high temperature, high frequency, optoelectronics, high power and radiation resistance.

GaN materials have great room for performance improvement. First of all, from the perspective of the substrate for epitaxial growth of GaN materials, the most ideal substrate for GaN growth is GaN single crystal material. If it is used as the substrate material for GaN growth, the epitaxial film can be greatly improved. Improve the crystal quality, reduce the dislocation density, increase the working life of the device, improve the luminous efficiency, and thus increase the working current density of the device. However, it is very difficult to prepare GaN bulk single crystal, and there is no effective method so far. The most common substrate for GaN growth today is sapphire, Al ₂ O ₃ . Its advantages are good chemical stability, no absorption of visible light, moderate price, and relatively mature manufacturing technology. However, the lattice mismatch between the sapphire substrate and the GaN material is large, resulting in a large dislocation density in the epitaxially grown GaN material, which affects the crystal quality and many electrical and optical properties of the material. Because of this, the epitaxial growth of high-quality GaN materials is very difficult, and the development of its growth technology is also very slow. Until the 1990s, with the continuous improvement of material growth and device process level, as well as the adoption of buffer layer technology and the breakthrough of p-type doping technology, the research on GaN-based materials and their epitaxial growth became unprecedentedly active. It has become a hotspot of research and investment in the global semiconductor field.

Secondly, even GaN crystals grown by various methods inevitably contain a certain amount of nitrogen vacancy defects (V _N ), which will lead to the fact that non-doped GaN materials are all n-type semiconductors, and the background electron concentration is between 10 ¹⁴ -4x10 ¹⁶ cm ^-3 or so, showing high conductivity; while p-type GaN is generally a compensation material, and generally uses Mg element as the acceptor doping impurity. At present, the controllable doping concentration is only between 10 ¹¹ -10 ²⁰ cm ^-3 . This means that the electrical properties of epitaxially grown p-type GaN materials have a large room for improvement, and the unsatisfactory electrical properties directly lead to the unsatisfactory luminous performance of LEDs based on GaN materials. The output power and the electro-optical conversion efficiency can only be limited to a relatively low level, and the electro-optical conversion efficiency can only reach about 15%-20% in industrial production. .

LED is a professional term abbreviated from the prefixes of English light emitting diode, namely "light emitting diode". It is an energy conversion device that converts electrical energy into light energy. It is a semiconductor p-n junction under an appropriate forward bias voltage. A device that emits light by spontaneous emission. At present, how to improve the luminous performance of GaN-based blue LEDs is one of the global academic hotspots. The reason is that the research on the luminescent materials of blue LEDs started late, and its material properties have not reached the expected level.

Since the advent of laser technology, it has been widely used in research fields such as the preparation of new materials and the modification of irradiated materials due to its advantages of high energy, easy operation, good controllability and difficulty in introducing pollution. Especially with the application of short-wavelength high-peak power lasers such as excimer lasers, the idea of material cold processing is becoming more and more important. It mainly studies the changes of electrical, optical, and magnetic properties of materials under laser irradiation, involving semiconductor materials, dielectric Electrical materials, high temperature superconducting materials and magnetic materials. The advantages of using the method of excimer laser irradiation for surface modification are mainly that the laser has a high energy density light output; the output energy density, pulse frequency and pulse number and other parameters are precisely controllable; Selectivity and repeatability. The present invention proposes to irradiate the GaN epitaxial wafer structure by using laser irradiation method under different atmospheres and light pulse parameters. Taking advantage of the variable and precisely controllable irradiation atmosphere and parameters, different conditions and parameters are tried to determine the The electrical properties of GaN epitaxial wafers and the most obvious parameter settings for improving the luminous performance of LEDs based on GaN epitaxial wafers.

Contents of the invention

The purpose of the present invention is to provide a method for improving the electrical properties of GaN epitaxial wafers and various luminous performances of LEDs based on GaN under different irradiation atmospheres and irradiation parameters.

The purpose of the present invention is achieved through the following technical solutions:

1) After immersing the GaN epitaxial wafer in an organic solvent for ultrasonic cleaning for 5 minutes, rinse; after immersing in an alcohol solvent for ultrasonic cleaning for 5 minutes, rinse; after immersing in deionized water for ultrasonic cleaning for 6-10 minutes, rinse, and dry;

2) Place the GaN epitaxial wafer sample processed in step 1) on the target stage, adjust the optical path, and the wavelength of the laser is 248nm;

3) The sample is irradiated with a pulsed laser with a single pulse energy density of 0.15J/cm2-0.6J/cm2, a frequency of 3Hz-10Hz, and a pulse number of 60-120. [0011] Further, it is characterized in that the gallium nitride epitaxial wafer sample is a multilayer epitaxial structure sample whose surface layer is a p-type GaN thin film.

Further, it is characterized in that said laser is a KrF excimer laser.

In the above step 3), the laser irradiation of the GaN epitaxial wafer is carried out under standard atmosphere, nitrogen and oxygen atmosphere respectively. And by changing the laser irradiation parameters (pulse energy density, number of irradiation pulses, and pulse frequency) in different atmospheres, the electrical and optical properties of GaN and the luminous performance of the encapsulated LED can be improved to varying degrees.

For the samples obtained according to the above process method, after depositing Ni (30nm)/Au (100nm) metal electrodes on the surface of the sample by magnetron sputtering, the samples were tested for carrier concentration, surface resistivity and other metal-semiconductor ohm measurements. Electrical testing of contacts. The results show that under nitrogen atmosphere, laser irradiation with appropriate irradiation parameters can increase the carrier concentration of the sample by up to two orders of magnitude, and the surface resistivity can also be reduced by one order of magnitude at the same time.

The irradiated gallium nitride epitaxial wafer sample obtained according to the above process method, after cleaning, evaporating ITO, step (mesa) photolithography, ICP etching, deglue, ITO alloy, HF acid cleaning, patterned pad ) Low-power LED process flow such as lithography sputtering Ti/Au, stripping Ti/Au, alloy, probe station test, etc. In this step, the LED luminous performance is tested, mainly including reverse leakage current, light output power, light LED characteristic parameters such as output efficiency.

The invention is based on the photochemical and photothermal reactions between the excimer laser and the GaN semiconductor material, which causes changes in the physical properties of the material surface when irradiated. The method of laser irradiation modification has great flexibility. It can make the irradiated material have the best electrical and optical properties by changing the irradiation parameters and atmosphere, and has a certain effect on the luminous performance of the packaged LED. improvement.

Compared with the existing semiconductor modification technology, mainly rapid thermal annealing, the present invention has obvious innovations:

1. The method of laser irradiating GaN epitaxial wafers has simple process operation, short time period, precise and controllable laser irradiation parameters, and can process specific local surfaces. The electro-optic properties of the sample and the luminous performance of the packaged LED can be improved to the greatest extent by adjusting the laser irradiation parameters and changing the irradiation atmosphere.

2. After modification by laser irradiation, the electro-optic properties of the sample are significantly improved. And it is combined with the LED luminescence test, and more combined with the actual problems of production and life to improve the luminous performance of blue LEDs represented by gallium nitride LEDs.

3. Using laser as a high-energy radiation source, the laser power and pulse number are small, the process is simple, and the energy consumption is low; the irradiation can be completed within 1 minute, and the modification speed is fast.

4. Using laser irradiation as a material modification method does not affect the material properties of other structures under the surface of the gallium nitride epitaxial wafer, and does not cause environmental pollution.

Description of drawings

Figure 1 is a physical picture of the LED lamp bead after the gallium nitride epitaxial wafer sample has undergone the LED packaging process, wherein Figure 1(a) is a top view of the LED lamp bead, and Figure 1(b) is a side view of the LED lamp bead.

Figure 2(a) and Figure 2(b) respectively show the reverse leakage current of the packaged LED device after the epitaxial wafer samples are irradiated by lasers with different irradiation power densities and different pulse numbers. It can be seen that the reverse leakage current of LEDs made of irradiated epitaxial wafers has been reduced to varying degrees compared with those before irradiation, indicating that the electrical properties of LEDs after packaging are improved compared with those before irradiation. The degree of improvement of the irradiation parameters is also different, which also shows that the LED device can obtain the best electrical properties by changing the irradiation parameters.

Fig. 3(a) and Fig. 3(b) respectively show the change of the luminous performance of the packaged LED device after the epitaxial wafer samples are irradiated by laser with different irradiation power densities and different pulse numbers. It can be seen that the output power of LED light after irradiation has been significantly improved compared with that before irradiation, indicating that the luminous performance of LED after laser irradiation treatment has been improved to varying degrees compared with that before irradiation.

Detailed ways

The specific implementation manners of the present invention will be further described in detail below in conjunction with the accompanying drawings.

Example 1:

Cut the gallium nitride epitaxial wafer into square samples with a size of about 1cm×1cm each with a glass knife; immerse in acetone solvent for ultrasonic cleaning for 5 minutes, then rinse; immerse in alcohol solvent for ultrasonic cleaning for 5 minutes, then rinse; immerse in deionized water After ultrasonic cleaning for 10 min, rinse and dry. Put the pretreated gallium nitride epitaxial wafer on the target stage, and irradiate it with an excimer laser with a wavelength of 248nm in air, oxygen, and nitrogen environments (the air pressure is a standard atmospheric pressure); the pulse laser frequency 3Hz, a single pulse energy density of 0.5J/cm ² , and a pulse number of 90. The out-of-focus amount is 4cm, and the compound eye structure is used to shape the outgoing laser light. The spot area acting on the sample is 1cm ² , that is, a square spot with a size of 1cm×1cm. The samples obtained before and after irradiation were all deposited Ni (30nm)/Au (100nm) metal electrodes on the surface of the samples by magnetron sputtering.

Carrier concentration, surface resistivity and carrier mobility were tested for the samples obtained through the above steps. The results show that the ohmic contact characteristics of the sample surface after irradiation will change compared with those before irradiation. In these sets of data, it can be seen that under different irradiation atmospheres, the electrical properties of the surface of the above three materials are different. Changes before irradiation, the electrical properties of samples irradiated in a nitrogen atmosphere improved the most, which can be attributed to the fact that when the single pulse energy or the number of irradiation pulses is large, the excessive oxide formation on the surface is not beneficial to The electrical properties of its surface are improved.

Example 2:

Cut the gallium nitride epitaxial wafer into square samples with a size of about 1cm×1cm each with a glass knife; immerse in acetone solvent for ultrasonic cleaning for 5 minutes, then rinse; immerse in alcohol solvent for ultrasonic cleaning for 5 minutes, then rinse; immerse in deionized water After ultrasonic cleaning for 10 min, rinse and dry. Place the pretreated gallium nitride epitaxial wafer on the target stage, and irradiate it with an excimer laser with a wavelength of 248nm in an atmospheric environment; the pulse laser frequency is 3Hz, and the single pulse energy density is 0.2-0.6J/ cm ² , the number of pulses is 60, 90, 120. The out-of-focus amount is 4cm, and the compound eye structure is used to shape the outgoing laser light. The spot area acting on the sample is 1cm ² , that is, a square spot with a size of 1cm×1cm. After cleaning, evaporating ITO, step (mesa) lithography, ICP etching, glue removal, ITO alloy, HF acid cleaning, patterned pad lithography sputtering Ti/Au, stripping Ti/Au, alloy, probe Needle table test and other low-power LED process flow In this step, the LED luminous performance is tested, mainly including LED characteristic parameters such as reverse leakage current, light output power, and luminous efficiency.

Table 1 shows the changes of carrier concentration, surface resistivity, and carrier mobility before and after irradiation under different irradiation atmospheres. It can be seen that under different irradiation atmospheres, the electrical properties of the surfaces of the above three materials have changed from those before irradiation, and the electrical properties have improved the most under nitrogen. The improvement of electrical properties after irradiation will change the luminous properties of the packaged LED.

Claims (5)

1. adopt laser irradiation gallium nitride epitaxial slice to improve a method for the LED luminescent properties being substrate with it, it is characterized in that, comprise the following steps:

1) gallium nitride epitaxial slice is immersed acetone solvent ultrasonic cleaning after 5 minutes, rinse; Immerse spirit solvent ultrasonic cleaning after 5 minutes, rinse; After immersion deionized water for ultrasonic cleans 10 minutes, rinse, dry;

2) by through step 1) the GaN epitaxy sheet sample that processed is placed on target platform, adjustment light path, and laser wavelength is 248nm;

3) pulse laser single pulse energy metric density 0.15J/cm is adopted ²-0.6J/cm ², frequency 3Hz-10Hz, umber of pulse is 60-120 irradiation sample.

2. method according to claim 1, is characterized in that gallium nitride epitaxial slice sample to be top layer is the multilayer epitaxial structure sample of p-type GaN film.

3. method according to claim 1, is characterized in that said laser is KrF excimer laser.

4. method according to claim 1, it is characterized in that by changing laser pulse energy metric density, irradiance pulse number or pulse frequency, or change irradiation atmosphere change gallium nitride electrical properties, to improve with gallium nitride epitaxial slice the luminescent properties of the LED being substrate further.

5. method according to claim 1, is characterized in that said irradiation carries out respectively under standard atmosphere, nitrogen or oxygen atmosphere.