The third-generation semiconductor material has material performance advantages that cannot be compared with silicon materials. Judging from the characteristics of bandwidth, thermal conductivity, breakdown electric field and other characteristics that determine the performance of the device, the third-generation semiconductor is better than that of silicon materials. Therefore, the introduction of the third-generation semiconductor can well solve the shortcomings of silicon materials today and improve the device. Heat dissipation, conduction loss, high temperature, high frequency and other characteristics are known as a new engine in optoelectronics and microelectronics industries.
Among them, GaN has wide application and is considered to be one of the most important semiconductor materials after silicon. Compared with the silicon-based power devices widely used at present, GaN power devices have higher critical electric field strength, lower open-state resistance, and faster switching frequency, which can achieve higher system efficiency and work at high temperatures.
Difficulties of homogeneous epitaxy
The links of the GaN semiconductor industry chain are: substrate → GaN material extension → device design → device manufacturing. Among them, the substrate is the foundation of the entire industrial chain.
As a substrate, GaN is naturally the most suitable substrate material for growing as a GaN epitaxial film. Homogeneous epitaxial growth can fundamentally solve the problem of lattice mismatch and thermal mismatch encountered by the use of heterogeneous substrate materials, minimize the stress caused by differences in properties between materials during the growth process, and can grow a high-quality GaN epitaxial layer that cannot be compared with the heterogeneous substrate. For example, high-quality gallium nitride epitaxial sheets can be grown with gallium nitride as a substrate. The internal defect density can be reduced to one-thousandth of the epitaxial sheet with sapphire substrate, which can effectively reduce the junction temperature of LEDs and increase the brightness per unit area by more than 10 times.
However, at present, the substrate material commonly used in GaN devices is not a single crystal of GaN. The main reason is that it is a word: Difficult! Compared with conventional semiconductor materials, the growth of GaN monocrystals is slow, and the crystal is difficult to grow and costly.
GaN was first synthesized in 1932, when gallium nitride was synthesized from NH3 and pure metal Ga. Since then, although there have been many positive studies on gallium nitride monocrystalline materials, because GaN cannot be melted at atmospheric pressure, it is decomposed into Ga and N2 at high temperature, and the decomposition pressure at its melting point (2300°C) is as high as 6GPa. It is difficult for the current growth equipment to withstand such high pressure at the GaN melting point. Therefore, the traditional melt method cannot be used for the growth of GaN monocrystals, so heterogeneous epitaxy can only be selected on other substrates. At present, GaN-based devices are mainly based on heterogeneous substrates (silicon, silicon carbide, sapphire, etc.), making the development of GaN single crystal substrates and homogeneous epitaxial devices lag behind the application of heterogeneous epitaxial devices.
Several substrate materials
Sapphire
Sapphire (α-Al2O3), also known as corundum, is the most commercially used LED substrate material, occupying a large share of the LED substrate market. In early use, the sapphire substrate reflects its unique advantages. The GaN film grown is comparable to the dislocation density of the film grown on the SiC substrate, and the sapphire is grown by melt technology. The process is more mature. It can obtain a lower cost, larger size and high-quality single crystal, which is suitable for industrial development. Therefore, It is the earliest and most widely used substrate material in the LED industry.
Silicon carbide
Silicon carbide is a group IV-IV semiconductor material, which is currently a second only sapphire LED substrate material in market share. SiC has a variety of crystal types, which can be divided into three categories: cubic (such as 3C-SiC), hexagonal (such as 4H-SiC) and diamond (such as 15R-SiC). Most crystals are 3C, 4H and 6H, of which 4H and 6H-SiC are mainly used as GaN substrates.
Silicon carbide is very suitable for being an LED substrate. However, due to the high-quality growth, large-size SiC single crystal is difficult, and SiC is a layered structure, which is easy to cleate, and the machining performance is poor. It is easy to introduce step defects on the substrate surface, which affects the quality of the epitaxial layer. The price of SiC substrate of the same size is dozens of times that of sapphire substrate, and the high price limits its large-scale application.
Monocrystalline silicon
Silicon material is the most widely used and mature semiconductor material at present. Due to the high maturity of monocrystalline silicon material growth technology, it is easy to obtain low-cost, large size (6-12 inches) and high-quality substrate, which can greatly reduce the cost of LEDs. Moreover, because silicon monocrystalline has been widely used in the field of microelectronics, the direct integration of LED chips and integrated circuits can be realized by using monocrystalline silicon substrate, which is conducive to the miniaturization of LED devices. In addition, compared with the most widely used LED substrate, Sapphire, monocrystalline silicon has some advantages in performance: high thermal conductivity, good electrical conductivity, vertical structures can be prepared, and is more suitable for high-power LED preparation.
Summary
In recent years, the market has put forward increasing requirements for the performance of GaN devices, especially for high-current density devices (such as lasers) and high-power and high-voltage-voltage-resistant electronic devices. For example, the dislocation density of long-life high-power lasers cannot exceed the 105cm-2 order. Due to well-known shortcomings of heterogeneous epitaxy, such as lattice mismatch, high dislocation density caused by thermal expansion coefficient mismatch, mosaic crystal structure, biaxial stress and wafer warping, the performance of the device is significantly limited by the quality of the substrate structure. Obviously, the ideal solution to this problem is still a breakthrough in the preparation technology of gallium nitride monocrystalline.
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