CN1945862A - Semiconductor LED structure with high extracting efficiency and its preparing method - Google Patents

Abstract

A semiconductor light-emitting diode structure with high extraction efficiency and a preparation method thereof belong to the technical field of optoelectronic device manufacturing, and are suitable for semiconductor LEDs of various wavelengths. Although the existing LED has high internal quantum efficiency, its external quantum efficiency is very low, and only a small part of photons can be emitted from the LED. The present invention proposes to grow a compound anti-reflection film structure composed of ITO transparent conductive film and _Six N _y dielectric film on the LED epitaxial wafer. The optical thickness of the ITO transparent conductive film is an integral multiple of half of the LED emission wavelength, and _Six N _y The optical thickness of the dielectric film is an odd multiple of a quarter of the LED emission wavelength, and the refractive index of the Six _{N y} dielectric film is the root of the refractive index of the uppermost semiconductor material of the LED epitaxial wafer. This design can achieve good current expansion while minimizing the interface reflectivity to achieve the best anti-reflection effect, greatly improving the external quantum efficiency, and increasing the light intensity by more than 130%.

Description

High extraction efficiency semiconductor light-emitting diode structure and preparation method thereof

1. Technical field

The invention belongs to the technical field of optoelectronic device manufacturing, and relates to a structure and a preparation method for improving the extraction efficiency of semiconductor light-emitting diodes (LEDs), which are suitable for semiconductor LEDs with various wavelengths (red light, blue light, green light, etc.).

2. Background technology

Semiconductor light-emitting diodes are energy-saving, environmentally friendly and long-life light-emitting devices. LED energy consumption is 10% of incandescent lamps and 50% of fluorescent lamps. LED adopts solid packaging, firm structure, life span of 100,000 hours, 10 times that of fluorescent lamps and 100 times that of incandescent lamps. In terms of environmental protection, replacing incandescent or fluorescent lamps with LEDs does not require the use of glass vacuum packaging, and there is no pollution of toxic gas and mercury. And it has a wide range of uses, and can be applied to various articles in daily life, such as indicator lights or light sources of various home appliances. In recent years, due to the development trend of multi-color and high brightness, it has been applied in urban landscape lighting, road and traffic instructions, indoor and outdoor decorative lighting, full-color large-screen display, car lights and other fields.

In order to obtain high-brightness light-emitting diodes, the key is to improve the external quantum efficiency of the device. However, the n-type electrode of the chip is a surface electrode, which is much larger than the upper electrode, so when it is forward biased, the injected carriers cannot spread as quickly and uniformly as in the metal, but flow into the semiconductor material. Radial expansion, if the current expansion is not good, the injected current is mainly concentrated under the electrode, so that a large number of photons emitted from the active area under the electrode are blocked by the electrode and cannot be emitted and lost inside the LED. In addition, the difference in refractive index between the semiconductor material used to prepare the light-emitting diode and air is large, resulting in a small light emission angle and high interface reflectivity. When light is incident on the interface between two substances with refractive indices n ₁ and n ₂ , the incident angle θ ₁ and refraction angle θ ₂ obey Snell's law, that is, n ₁ sinθ ₁ = n ₂ sinθ ₂ . The semiconductor material of light-emitting diodes has a high refractive index, for example, n ₁ = 3.6, n ₂ is air, the critical angle at the junction of this crystal and air is θ ₁ = sin ^-1 (1/n ₁ ) ≈ 16.2°, the incident Total reflection is formed when the angle is larger than the critical angle. As far as composite luminescence is emitted uniformly within the 4π solid angle, the light within the critical angle only accounts for $\frac{1}{2}(1-\cos {\mathrm{\θ}}_1)\≈0.02.$ Part of the light within the critical angle will be reflected back to the interior by the surface. For example, when the light is vertically incident, the reflectance is (n ₁ -1) ² /(n ₁ +1) ² ≈0.32. The light that is reflected back will be absorbed inside the LED if it cannot exit other surfaces. A large amount of light is lost inside the LED, and only a small amount of light can be emitted to the outside, resulting in low external quantum efficiency of the LED. Low external quantum efficiency means low extraction efficiency and low light intensity of the LED.

3. Contents of the invention

The purpose of the present invention is to propose a composite anti-reflection film structure composed of ITO transparent conductive film and _{Six Ny} dielectric film grown on the LED epitaxial wafer, while achieving good current expansion, the interface reflectivity can be reduced to the lowest to achieve the best Excellent anti-reflection effect, thereby greatly improving the external quantum efficiency.

Although a single layer of indium tin oxide (ITO) transparent conductive film can achieve the current spreading effect, it is difficult to achieve the best anti-reflection effect. The condition of an ideal single-layer anti-reflection coating is that the optical thickness of the coating layer is an odd multiple of a quarter wavelength, and its refractive index is the square root of the product of the refractive index of the incident medium and the substrate. The refractive index of the ITO transparent conductive film is about 1.7. If the incident medium is air, the refractive index of the air is 1. It can be seen that only when the base material is 2.89 and the optical thickness of the ITO transparent conductive film is an odd multiple of a quarter wavelength , in order to achieve the best anti-reflection effect. However, the refractive index of the semiconductor material on the surface of the red light-emitting diode is about 3.5. Therefore, in order to achieve the best anti-reflection effect while achieving a good current spreading effect, we designed a thin GaP layer prepared by ITO transparent The composite anti-reflection film formed by the conductive film and the _{Six Ny} dielectric film can improve the extraction efficiency of the light emitting diode. The optical thickness of the ITO transparent conductive film is designed to be an integer multiple of half the LED emission wavelength, the optical thickness of the Six _{N y} dielectric film is an odd multiple of a quarter of the LED emission wavelength, and the refractive index of the _Six N _y dielectric film is It is the root of the refractive index of the uppermost semiconductor material of the LED epitaxial wafer. When the optical thickness of a film layer is an integer multiple of one-half wavelength, this film layer will become a dummy layer, that is, it has no effect on the reflectivity of the central wavelength, and the ITO film cannot be used as the best In the case of the anti-reflection coating, we design its optical thickness to be an integer multiple of half the wavelength, so that it has no effect on the reflectivity of the emission wavelength of the light-emitting diode on which the ITO film is grown, and then use the plasma Enhanced chemical vapor deposition (PECVD) equipment re-grows a layer of Si _x N _y dielectric film (amorphous film) on ITO, so that its refractive index is the square root of the refractive index of the semiconductor material under the ITO transparent conductive film, and the optical thickness is quarter an odd multiple of one of the wavelengths. The required refractive index of the Six _{N y} dielectric film is obtained by adjusting growth parameters using PECVD equipment. When the process parameters are determined, the growth rate remains basically unchanged, and the physical thickness of the Six _{N y} dielectric film can be controlled by controlling the growth time. This method has a simple process and is easy to implement.

The basic structure of the epitaxial wafer of the LED is to grow N-type semiconductor 7 , multi-quantum well active region 6 , P-type semiconductor 5 , and GaP layer 4 on the substrate 8 in sequence. The composite anti-reflection film composed of ITO transparent conductive film and Six _{N y} dielectric film designed by us is grown on the uppermost semiconductor surface of the epitaxial wafer. The optical thickness of the ITO transparent conductive film 3 is designed to be an integer multiple of 1/2 of the LED emission wavelength, the optical thickness of the Six _{N y} dielectric film 2 is an odd multiple of a quarter of the LED emission wavelength, and the _Six N _y dielectric film 2 The refractive index is the root of the refractive index of the uppermost semiconductor material of the LED epitaxial wafer.

The invention provides a semiconductor light-emitting diode structure with high extraction efficiency, including an LED epitaxial wafer, which is characterized in that an ITO transparent conductive film with an optical thickness of an integral multiple of 1/2 wavelength is sequentially grown on the surface of the uppermost semiconductor material of the LED epitaxial wafer 3 and the _Six N _y dielectric film 2 whose optical thickness is an odd multiple of 1/4 wavelength, and the refractive index of the Six _{N y} dielectric film 2 is the root of the refractive index of the uppermost semiconductor material of the LED epitaxial wafer, and the P electrode 1 The bottom is in direct contact with the ITO transparent conductive film 3 , and the sidewall of the P electrode 1 is in direct contact with the _{Six Ny} dielectric film 2 .

The present invention is characterized in that it adopts following technological process:

1) Prepare LED epitaxial wafers;

2) On the surface of the uppermost semiconductor material of the LED epitaxial wafer, grow an ITO transparent conductive film 3 whose optical thickness is an integral multiple of 1/2 the LED emission wavelength, which can well realize the current spreading effect;

3) On the ITO transparent conductive film 3, the Six _{N y} dielectric film 2 was prepared by plasma enhanced vapor phase chemical deposition (PECVD), and the Six _{N y} dielectric film 2 was prepared by using silane and nitrogen, and its optical thickness was 1/4 An odd multiple of the LED emission wavelength, the refractive index is the root of the refractive index of the uppermost semiconductor material of the LED epitaxial wafer, and the _{Six Ny} dielectric film 2 and the ITO transparent conductive film 3 work together to form a composite anti-reflection film, which can achieve the best Anti-reflection effect;

4) Preparation of the P electrode 1: Spray glue on the surface of the Six _{N y} dielectric film 2, photolithography, and develop, etch the _Six N _y dielectric film 2 at the electrode position, then evaporate the metal electrode on it, and ultrasonically peel it off, leaving only The metal at the lower electrode position forms the P electrode 1, so that the bottom of the P electrode 1 is in direct contact with the ITO transparent conductive film 3, and the sidewall of the P electrode 1 is in direct contact with the _Six N _y dielectric film 2;

5) Then the substrate 8 of the LED epitaxial wafer is thinned, and metal is evaporated on the substrate 8 to form an N electrode 9;

6) Cleavage.

The composite anti-reflection film structure composed of ITO transparent conductive film 3 and _{Six Ny} dielectric film 2 is not only suitable for red LEDs, but also for blue and green GaN-based LEDs. It plays an important role in GaN-based LEDs. Good current expansion effect can achieve the best anti-reflection effect at the same time. For GaN-based LEDs, because it is very difficult to prepare GaN substrates, sapphire is usually used as the substrate, and then GaN buffer layers, N-type GaN materials, multi-quantum active regions, and P-type GaN materials are grown sequentially on the sapphire substrates. . Due to the poor conductivity and thermal conductivity of the sapphire substrate, both the N-type electrode and the P-type electrode are prepared on the upper surface. When the ITO transparent conductive film is used for the current spreading layer of GaN-based LEDs, the optical thickness is also an integer multiple of half of the LED emission wavelength. Then grow a _{Six Ny} dielectric film 2 on the ITO, _the optical thickness is an odd multiple of a quarter of the LED emission wavelength, and the refractive index of the Six _Ny dielectric film 2 is the refractive index of the P-type semiconductor material of the GaN-based LED square root of . GaN-based blue and green LEDs are compared with red LEDs. Although the preparation process of LED epitaxial wafer materials and devices is different, the ITO transparent conductive film 3 and Six _{N y} dielectric film 2 are grown on the LED epitaxial wafer. The design principles of the composite AR coating structure are the same. The optical thickness of the ITO transparent conductive film 3 is designed to be an integer multiple of 1/2 of the LED emission wavelength, the optical thickness of the Six _{N y} dielectric film 2 is an odd multiple of a quarter of the LED emission wavelength, and the _Six N _y dielectric film 2 The refractive index is the root of the refractive index of the uppermost semiconductor material of the LED epitaxial wafer. GaN-based LED devices can be prepared by the following process: GaN-based LED epitaxial wafers are first evaporated on the uppermost semiconductor material of the LED epitaxial wafers with an ITO evaporation system, and an ITO transparent conductive film whose optical thickness is an integer multiple of 1/2 the LED emission wavelength , and then usually use photoresist or SiO ₂ as a mask, and use ICP etching to etch the P-type GaN semiconductor material in sequence, the multi-quantum active region, until the N-type GaN material, and then respectively in the N-type GaN material and P The N electrode and the P electrode are respectively prepared on the GaN-type _GaN material _{, and the Six N y dielectric film} 2 is grown again. The root of the refractive index of the uppermost semiconductor material of the base LED epitaxial wafer. Then photolithography is used to expose the positions of the N electrode and the P electrode. The bottom of the P electrode is in direct contact with the ITO transparent conductive film, and the sidewall of the P electrode is in direct contact with the _{Six Ny} dielectric film 2 . The N electrode is not in contact with the side wall of the LED, and the bottom of the N electrode is in direct contact with the N-type GaN semiconductor material.

Both the ITO transparent conductive film 3 and the _{Six Ny} dielectric film 2 have high transmittance in the visible and near-infrared ranges, and the difference in the refractive index between the semiconductor material of the optoelectronic light-emitting device and air is the root cause of the interface reflectance , so in order to reduce the interface reflectivity and achieve the best antireflection effect, this composite antireflection coating is also suitable for other optoelectronic light-emitting devices to reduce surface reflectivity and improve light output. The design principle of this composite anti-reflection film is also the same. The optical thickness of the ITO transparent conductive film 3 is an integer multiple of 1/2 of the LED emission wavelength, and the optical thickness of the Six _{N y} dielectric film 2 is 1/4 of the LED emission wavelength. An odd multiple of the wavelength, the refractive index of the _{Six Ny} dielectric film 2 is the square root of the refractive index of the uppermost semiconductor material of the epitaxial wafer of the optoelectronic light-emitting device.

The structural design and process method of the above invention have the following advantages:

1) The use of ITO transparent conductive film 3 whose optical thickness is an integral multiple of 1/2 of the LED emission wavelength can well realize the current expansion effect;

2) The refractive index of the Six _{Ny dielectric} film 2 used is the root of the refractive index of the uppermost semiconductor material of the LED epitaxial wafer, and the optical thickness is an odd multiple of a quarter of the LED emission wavelength, which can be combined with the ITO transparent conductive film 3 Form a composite anti-reflection film to achieve the best anti-reflection effect;

3) Using PECVD to prepare the Six _{N y} dielectric film 2, the refractive index of the Six _{N y} dielectric film 2 can be flexibly adjusted to the required value by adjusting the growth process parameters, which is easy to implement;

4) The composite film formed by ITO transparent conductive film 3 and Six _{N y} dielectric 2 film can well realize current expansion and achieve the best anti-reflection effect. In theory, the surface reflectance can be reduced to 0, and the LED can be improved. The light extraction efficiency is high. After growing the composite anti-reflection film formed by the ITO transparent conductive film 3 and the _{Six Ny} dielectric film 2 on the device, the light intensity can be increased by more than 130%.

4. Description of drawings

Figure 1 is a cross-sectional view of an LED with a composite anti-reflection film composed of an ITO transparent conductive film and a Six _{N y} dielectric film. 1-P electrode, 2- _Six N _y dielectric film, 3-ITO transparent conductive film, 4- GaP layer, 5-P-type semiconductor, 6-multiple quantum well active region, 7-N-type semiconductor, 8-substrate, 9-N electrode;

5. Specific implementation

Comparative example:

1) Prepare a red LED epitaxial wafer, that is, grow an N-type semiconductor 7, a multi-quantum well active region 6, a P-type semiconductor 5, and a GaP layer 4 sequentially on a substrate 8;

2) Preparation of P electrode 1: Clean the sample with 50nm GaP layer 4 with acetone, absolute ethanol and deionized water, put it into the metal electrode sputtering system chamber, sputter AuZnAu, and then protect it with photoresist , using a metal corrosion solution to corrode the part other than the P electrode 1, leaving only the metal at the electrode position to form the P electrode 1;

3) Then substrate 8 is thinned, and metal AuGeNi is evaporated on substrate 8 to form N electrode 9;

4) Cleavage.

Only 8 tube cores of 50nmGaP are tested using light intensity testing equipment. Under the constant current of 20mA, the average voltage is 2.22V, and the average light intensity is 49mcd.

Example:

1) Preparing a red LED epitaxial wafer, that is, growing an N-type semiconductor 7, a multi-quantum well active region 6, a P-type semiconductor 5, and a GaP layer 4 sequentially on a substrate 8, and the uppermost semiconductor material of the LED epitaxial wafer is GaP layer 4;

2) Put the sample cleaned with acetone, absolute ethanol and deionized water into the ITO electron beam evaporation station, grow an ITO transparent conductive film 3 with an optical thickness of 1/2 wavelength on the GaP layer 4, and the growth temperature is 190 ℃, the oxygen flow rate is 3sccm, and the evaporation rate is 0.2nm/s; (In the actual preparation process, according to the conditions of your own equipment, you can flexibly adjust the growth process parameters such as growth temperature and oxygen flow rate to prepare the required ITO transparent conductive film3);

3) Using PECVD equipment to grow _{Six Ny} dielectric film 2 on the ITO transparent conductive film 3: the deposition temperature is 300°C, the power of the 13.56MHz high-frequency excitation source is 20W, and silane and high-purity nitrogen gas with a purity of 5% are introduced, Their flows are 30sccm and 900sccm respectively, and the optical thickness of the grown Six _{Ny dielectric} film 2 is a quarter of the LED emission wavelength. _Six N _y dielectric film 2; (In the process of preparing dielectric film by PECVD, the growth process parameters such as temperature, gas flow rate, and high-frequency excitation source power can be flexibly adjusted to prepare the required _Six N _y dielectric film 2) ;

4) Preparation of P electrode 1: Spin glue, photolithography, and develop on the Six _{N y} dielectric film 2, etch the Six _{N y} dielectric film 2 at the electrode position, then evaporate the metal electrode Cr/Au on it, and ultrasonically peel it off , leaving only the metal at the electrode position to form the P electrode 1, so that the bottom of the P electrode 1 is in direct contact with the ITO transparent conductive film 3, and the sidewall of the P electrode 1 is in direct contact with the _Six N _y dielectric film 2;

5) Then the substrate 8 is thinned, and the metal AuGeNi is steamed on the substrate 8 to form an N electrode 9;

6) Cleavage.

Using the light intensity test equipment, 8 tube cores grown on the 50nm GaP layer 4 with an ITO transparent conductive film with an optical thickness of 1/2 wavelength and a Six _{N y} dielectric film with an optical thickness of 1/4 the LED emission wavelength were tested. Under 20mA constant current, the average voltage is 1.94V, and the light intensity is 118.6mcd. Compared with the LED with only 50nmGaP, the light intensity is increased by 142%, and the voltage is reduced by 12.6%. This shows that the design and process improve the light output of the LED without deteriorating the electrical characteristics of the device.

Those skilled in the art know that the materials used to prepare blue and green LED epitaxial wafers are different from those of red LED epitaxial wafers. The design principles of the ITO transparent conductive film 3 and the composite anti-reflection film formed by the Six _{N y} dielectric film 2 on the blue and green LED devices are consistent, and the ITO transparent conductive film is grown on the uppermost semiconductor material of the structured LED epitaxial wafer. It is the same as the _Six N _y dielectric film 2, and the process of preparing the ITO transparent conductive film and the Six _{N y} dielectric film 2 on the blue and green LEDs is the same.

Claims (2)

1, a kind of semiconductor LED structure of high extracting efficiency, comprise the LED epitaxial wafer, it is characterized in that on the superiors' semiconductor material surface of LED epitaxial wafer growing optics thickness successively is that ITO nesa coating (3) and the optical thickness of 1/2 wavelength integral multiple is the Si of 1/4 wavelength odd-multiple_xN _yDeielectric-coating (2), and Si_xN _yThe refractive index of deielectric-coating (2) is the evolution of the refractive index of LED epitaxial wafer the superiors semi-conducting material, and the bottom of P electrode (1) directly contacts with ITO nesa coating (3), sidewall and the Si of P electrode (1)_xN _yDeielectric-coating (2) directly contacts.

2, the preparation method of the semiconductor LED structure of high extracting efficiency according to claim 1 is characterized in that, may further comprise the steps:

1) preparation LED epitaxial wafer;

2) growing optics thickness is the thick ITO transparency conducting layer (3) of 1/2 wavelength on the superiors' semiconductor material surface of LED epitaxial wafer;

3) on ITO transparency conducting layer (3), use the standby Si of plasma enhanced chemical vapor deposition legal system_xN _yDeielectric-coating (2) uses silane and nitrogen to prepare Si_xN _yDeielectric-coating (2), its optical thickness are 1/4th LED emission wavelengths, and refractive index is the evolution of refractive index on the superiors' semiconductor material surface of LED epitaxial wafer;

4) preparation of P electrode (1): at Si_xN _yThe surperficial whirl coating of deielectric-coating (2), photoetching, development erode the Si of electrode position_xN _yDeielectric-coating (2), then evaporated metal electrode in the above, ultrasonic peeling off, the metal that only stays electrode position forms P electrode (1), so that the bottom of P electrode (1) directly contacts sidewall and the Si of P electrode (1) with ITO nesa coating (3)_xN _yDeielectric-coating (2) directly contacts;

5) then with substrate (8) attenuate of LED epitaxial wafer, steam metal at substrate (8) and form N electrode (9);

6) cleavage.