At present, Group III compound semiconductor materials, silicon carbide and oxide semiconductor materials are the mainly third-generation semiconductor materials. Among them, Group III compound semiconductor materials are commonly gallium nitride materials and aluminum nitride materials; oxide semiconductor materials mainly include zinc oxide, gallium oxide and perovskite etc.
Since its large band gap and the advantages of high breakdown electric field, high thermal conductivity, high electron saturation rate, and strong anti-radiation ability, the third-generation semiconductor is suitable for making high-voltage, high-frequency, high-current-resistant devices, and can reduce the power consumption of the device.
1. The Development History of Gallium Nitride
The development of GaN is relatively late. In 1969, the Japanese scientists like Maruska used hydride vapor deposition technology to deposit a large area of gallium nitride film on the surface of the sapphire substrate. However, because of the poor quality of the material and the difficulty of P-type doping, it was once considered to have no application prospects.
Gallium nitride is the material with the highest conversion efficiency of electro-optical and photoelectric theoretically so far, which has the features of large forbidden band width, high breakdown voltage, large thermal conductivity, high saturated electron drift speed and strong radiation resistance.
Metal organic chemical vapor deposition (MOCVD), hydride vapor phase epitaxy (HVPE), and molecular beam epitaxy (MBE) are the main methods for the epitaxial growth of gallium nitride. The fundamental chemical principle of MOCVD for the growth of gallium nitride is to pass Ga (CH) 3 in vapor state and gaseous NH3 into the reaction chamber, a series of reactions is occurred in a high temperature environment. Finally a GaN epitaxial layer is formed on the surface of the substrate.
2. Methods for the Epitaxial Growth of Gallium Nitride Materials
2.1 Preparation Process of the MOCVD for GaN Epitaxial Growth
MOCVD technology was first putted forward by Manasevit in 1968. With the improvement of raw material purity and process, this method has gradually become the main growth process for the second-generation semiconductor material (gallium arsenide and phosphorus indium), and the third-generation semiconductor (gallium nitride materials). In 1993, Nakamura and other scientists of Nichia Chemical used the MOCVD method to achieve the preparation of InGaN indium gallium nitride epitaxial layers with high-quality, which indicates the significance of MOCVD in the third generation of semiconductor materials.
The Advantages of MOCVD:
Firstly, the reactants enter the reaction chamber in gaseous form, and the thickness, composition and carrier density of the epitaxial material can be controlled by controlling the flow of various gases accurately;
Secondly, the gas flow in the reaction chamber is fast, and a steep heterojunction interface can be obtained by changing the gas;
Thirdly, high quality crystal is obtained with fewer impurities;
Lastly, the equipment is relatively simple, which is conducive to large-scale industrial production.
2.2 Hydride Gas Phase Epitaxy Process for GaN Epitaxial Wafer
Practically, the initial growth method of gallium nitride material was hydride vapor phase epitaxy HVPE, which was originally used by Maruska and other scientists to make gallium nitride epitaxial layers. The HVPE reaction is usually performed at atmospheric pressure in a hot quartz reactor. The chemical reaction is that gaseous hydrogen chloride reacts with metallic gallium in a low temperature environment to generate gaseous gallium chloride. Then Gallium chloride reacts with gaseous ammonia in a high temperature environment, forming a gallium nitride film. The by-products of this reaction, hydrogen chloride and hydrogen, can be recovered in gaseous form.
The preparation of gallium nitride by HVPE requires chemical reactions, a low-temperature reaction and a high-temperature reaction. Therefore, the HVPE reactor should divide the reaction chamber into a low-temperature zone and a high-temperature zone. Meanwhile, many parameters need to be adjusted in this process to achieve controllable and deposition of GaN thin films.
In the 1970s and 1980s, the HVPE method was extensively used for the growth of gallium nitride materials, but many defects were found in the application of HVPE. The prepared gallium nitride has a large number of crystal defects, the crystal quality is poor, and there are spatial parasitic reactions. Operating the HVPE method under normal pressure, a large number of gallium nitride particles caused by parasitic reaction will be deposited on the outlet of gallium chloride gas, the growth surface and the surface of the quartz glass tube wall in the reactor. Parasitic gallium nitride will not only consume gallium chloride, reducing the growth rate, resulting in damage to the gallium chloride pipeline, but also cause crystal defects.
Moreover, the doping cannot control well, and P-type doping is hard to achieve through this method. However, after the 1990s, because of its relatively simple equipment, HVPE was re-emphasized by the industry. The advances in technology have made a faster rate of gallium nitride growing by HVPE, and it is easy to produce large-area films with a better film uniformity.
2.3 The Method of MBE for Epitaxial Growth of GaN
Except for the MOCVD, MBE molecular beam epitaxy has become an important growth method for gallium nitride materials. The MBE is an epitaxial growth method for growing high-quality crystal films on the surface of a substrate, and this method should be carried out in a high vacuum or even an ultra-high vacuum environment.
Although the MBE growth rate is usually no more than 1 micron/hour, which means growing a single atomic layer per second or longer, it is easy to achieve precise control of film thickness, structure and composition, and it is easy to achieve the heterostructure and quantum structure of the steep interface;
The low epitaxial growth temperature reduces the lattice defects introduced by the different thermal expansion coefficients on the interface;
Compared to the chemical processes of HVPE and MOCVD, MBE is a physical deposition process, so it is no need to consider the impurity pollution caused by chemical reactions.