LED epitaxial sapphire substrate (substrate) details

The following figure is a cutaway view of sapphire; a top view of the crystal structure; a side view of the crystal structure; a structural diagram of Al2O3; a schematic view of the crystal surface of sapphire

Sapphire crystal surface diagram

The most commonly used GaN epitaxial is the non-polar surface of C-plane (0001), so the polarity of GaN will be determined by the process.

Sapphire (Al2O3) Characteristics Table

2 sapphire crystal growth method

There are two common methods for growing sapphire crystals:

1: Czochralski method (Czochralski method), referred to as CZ method. The raw material is heated to the melting point and then melted to form a molten soup, and then a single crystal seed crystal is used to contact the surface of the molten soup, and the solid solution of the seed crystal and the molten soup. The interface is too cold due to the temperature difference. The melt then begins to solidify on the surface of the seed crystal and grow and crystallize the single crystal of the same crystal structure. The seed crystal is pulled up at a very slow speed and rotates with a certain rotation speed. As the seed crystal rises upward, the melt gradually solidifies on the liquid-solid interface of the seed crystal, thereby forming an axisymmetric single. Crystal ingots.

2: Kyropoulos method (Kyropoulos method), referred to as KY method, the continent is called the bubble method. The principle is similar to Czochralskimethod. The raw material is heated to the melting point and then melted to form a melt. Single crystal seed crystals (SeedCrystal, also known as seed crystal rods) contact the surface of the molten soup, and begin to grow and crystallize the single crystal of the same crystal structure at the solid-liquid interface between the seed crystal and the molten soup. The seed crystal is extremely slow. The speed is pulled up, but the seed crystal is pulled up for a period of time to form a crystal neck. After the solidification rate of the melted soup and the seed crystal interface is stabilized, the seed crystal is no longer pulled up, and there is no rotation, only to control Cooling rate mode to make the single crystal gradually solidify from above, and finally solidified into a whole single crystal germanium.

The crystal growth diagrams of the two methods are as follows:

Schematic diagram of the Czochralski method

3 sapphire substrate processing

The raw material of the sapphire substrate is an ingot, and the ingot is processed from sapphire crystal. The related manufacturing process is as follows:

Sapphire crystal rod manufacturing process sapphire crystal rod processing

Growth: Crystal growth of large-size, high-quality single crystal sapphire crystals using a crystal growth furnace

Orientation: Ensure the correct position of the sapphire crystal on the crowbar table for easy pry bar processing

Crowbar: Sapphire crystal rods are extracted from sapphire crystals in a specific way

Barrel grinding: Cylindrical grinding of the ingot with a cylindrical grinder for accurate outer circle dimensional accuracy

Quality inspection: Ensure the quality of the ingot and the size and orientation of the ingot after the picking are in accordance with customer specifications.

Orientation: Accurately position the sapphire ingot on the microtome for precise slicing

Slicing: cutting a sapphire ingot into a thin wafer

Grinding: cutting the damaged layer caused by the wafer and improving the flatness of the wafer

Chamfer: Trim the edge of the wafer into an arc shape to improve the mechanical strength of the edge of the sheet and avoid defects caused by stress concentration

Polishing: Improve the roughness of the wafer to achieve the accuracy of the epitaxial wafer epitaxial level

Cleaning: Remove contaminants from the surface of the wafer (eg, dust particles, metals, organic contaminants, etc.)

Quality inspection: Inspection of wafer quality (flatness, surface dust particles, etc.) with high-precision testing instruments to meet customer requirements

4 sapphire substrate application types

The sapphire substrates used by the majority of epitaxial wafer manufacturers are divided into three types:

1:C-Plane sapphire substrate

This is the sapphire substrate surface commonly used by GaN for the growth of GaN. This is mainly because the sapphire crystal grows along the C-axis, the process is mature, the cost is relatively low, the physicochemical properties are stable, and the technology for epitaxial growth on the C-plane is mature and stable.

2: R-Plane or M-Plane sapphire substrate

It is mainly used to grow non-polar/semi-polar GaN epitaxial films to improve luminous efficiency. The GaN epitaxial film usually prepared on a sapphire substrate is grown along the c-axis, and the c-axis is the polar axis of GaN, resulting in GaN. A strong built-in electric field appears in the active layer quantum well of the base device, and the luminous efficiency is thus reduced. The non-polar surface GaN epitaxy is developed to overcome this physical phenomenon and improve the luminous efficiency.

3: Patterned Sapphire Substrate (PSS)

In the form of growth or etching, a nano-specific regular microstructure pattern is designed on the sapphire substrate to control the output light form of the LED, and at the same time reduce the difference between the GaN grown on the sapphire substrate. Discharge defects, improve epitaxial quality, and improve the internal quantum efficiency of LEDs and increase light extraction efficiency.

1:C-Plane sapphire substrate

The C-Plane sapphire substrate is a commonly used sapphire substrate. In 1993, Professor Akasaka Yoshi of Japan and Dr. Nakamura Shuji of Nichia Chemical at that time broke through the lattice mismatch (buffer layer) and p-type material of InGaN and sapphire substrate. After activation and other issues, Nichia was finally able to develop blue LEDs at the end of 1993. In the following years, Nichia Chemical used sapphire as a substrate, using InGaN materials, and continuously improved sapphire substrate and epitaxial technology through MOCVD technology. To improve the luminous efficiency of blue light, and at the same time, UV LEDs were developed in 1997. In 1999, blue-violet LED samples began to be shipped. In 2001, white LEDs were introduced. This laid the foundation for Nichia's leading position in the LED field.

Taiwan closely follows Japan's LED technology. Taiwan's LED development first purchased epitaxial wafer processing from Japan, and then bought MOCVD machines and sapphire substrates for epitaxy. Later, local Taiwanese manufacturers studied the growth and processing technology of sapphire crystals. Production, through independent research and development, to obtain LED patent authorization and other means to achieve sapphire crystal, substrate, epitaxial wafer production, epitaxial wafer processing and other independent production technology capabilities, step by step to establish Taiwan's important position in the LED upstream business.

At present, most of the blue/green/white LED products are products that are produced by MOCVD epitaxy using sapphire substrates represented by Japan and Taiwan. The sapphire substrate is very common, and the US CREE company uses SiC as the substrate. The representative LED products follow.

2: patterned sapphire substrate
(Pattern Sapphire Substrate referred to as PSS)

Designing micro- or nano-scale micro-structure-specific patterns on sapphire substrates by etching (in sapphire C-face dry etching/wet etching) to control the output light form of LEDs (on sapphire substrates) The concave-convex pattern produces a light scattering or refraction effect to increase the light extraction rate), and the GaN film grows on the patterned sapphire substrate to produce a lateral epitaxial effect, which reduces the difference in defects between GaN grown on the sapphire substrate, and improves The epitaxial quality improves the internal quantum efficiency of the LED and increases the light extraction efficiency. Brightness is increased by more than 70% compared to LEDs grown on general sapphire substrates. At present, Taiwan produces patterned sapphire with Sino-US twin, crystal, and mega crystal. 2/4 inch of the sapphire substrate is a mature product, the price is gradually stable, and the large size (such as 6/8 inch) ordinary sapphire substrate With the 2-inch patterned sapphire substrate in the growth period, the price is also high, and its manufacturer is also the main push large size and patterned sapphire substrate, while also actively increasing production capacity. At present, no manufacturer in the mainland can produce patterned sapphire substrates.

Figure 9: Nanopatterned sapphire substrate

3: R-Plane or M-Plane sapphire substrate

Generally, a GaN thin film grown on a C-plane sapphire substrate grows along its polar axis, ie, the c-axis direction, and the film has spontaneous polarization and piezoelectric polarization effects, resulting in a strong inside of the thin film (active layer quantum well). The built-in electric field (Quantum Confine Stark Effect, QCSE; history tank effect) greatly reduces the luminous efficiency of GaN films. GaN thin films grown on some non-C-plane sapphire substrates (such as R-plane or M-plane) and other special substrates (such as lithium aluminate; LiAlO2) are non-polar and semi-polar, as caused by the polarization field. The negative effects produced in the illuminating device will be partially or even completely improved. Traditional three-five nitride semiconductors are grown on c-plane sapphire substrates. If these compounds are grown on R-plane or M-Plane, the built-in electric field can be generated parallel to the epitaxial layer to increase the electron power. The probability of a hole in the compound. Therefore, the growth of the LED structure based on the nitride epitaxial film on the R-plane or M-Plane sapphire substrate can effectively solve the problem of low efficiency of the quantum efficiency of the LED compared to the conventional C-plane sapphire epitaxy. Increase the luminous intensity of the component. The latest news is said that non-polar LEDs can double the luminous efficiency of white light.

Since non-polar GaN has more potential than traditional c-axis GaN to produce high-efficiency components, many international manufacturers and research units have increased the research and production of such epitaxial technology. Therefore, the demand and requirements for R-plane or M-Plane sapphire substrates are correspondingly increased.

The non-polar surface refers to the surface in the normal direction of the polar surface, and the semi-polar surface is the surface between the polar surface and the non-polar surface.

5 main technical parameters of sapphire substrate

Due to different technologies and processes, epitaxial wafer manufacturers have different requirements for sapphire substrates, such as thickness and crystal orientation.

Below are some basic technical parameters for sapphire substrates produced by several manufacturers (using a mature C-plane 2 inch sapphire substrate as an example). More is that the epitaxial wafer manufacturers customize the sapphire substrate to meet the requirements of their own use according to their own technical characteristics and the quality requirements of the epitaxial wafers produced. That is, customer customization.

They are: A: Taiwan Taoyuan Zhaojing Technology Co., Ltd.

B: Taiwan Hsinchu Zhongmei Jingjing Products Co., Ltd.

C: American Crystal Systems

D: Russia Cradley Crystals

B: Technical parameters of C-side 2 inch sapphire substrate of Taiwan Zhongmei Jingjing Products Co., Ltd.

C: Technical parameters of C-side 2 inch sapphire substrate of Crystal Systems, USA

D: Russia Cradley Crystals C-side 2 inch sapphire substrate technical parameters

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