Science on the past and present lives of GaN

At present, the third generation semiconductor materials mainly include triplex semiconductor materials, silicon carbide and oxide semiconductor materials, among which the common triplex semiconductor gallium nitride and aluminum nitride; The main oxide semiconductor materials are zinc oxide, gallium oxide and perovskite.

The third generation semiconductor material has large band gap, high breakdown electric field, high thermal conductivity, high electron saturation rate, strong radiation resistance and other advantages, so it is suitable for making high voltage, high frequency, high current resistance devices, but also can reduce the power consumption of devices.

Development of gallium nitride materials and preparation technology of MOCVD

Gallium nitride was developed late. In 1969, Japanese scientist Maruska et al. deposited a large area of gallium nitride film on the surface of sapphire substrate by hydride vapor deposition technology. However, due to the poor quality of the material and the difficulty of P-type doping, it was once considered that there was no application prospect.

Gallium nitride has the characteristics of large band-gap width, high breakdown voltage, high thermal conductivity, high saturated electron drift speed and strong anti-radiation ability. Gallium nitride is the material with the highest electro-optical and photoelectric conversion efficiency theoretically so far.

The epitaxy growth methods of gallium nitride mainly include metal-organic chemical vapor deposition MOCVD, hydride vapor phase epitaxy HVPE and molecular beam epitaxy MBE. The basic chemical principle of the growth of gallium nitride by MOCVD is that Ga (CH) 3 in the vapor state and NH3 in the gaseous state are passed into the reaction cavity, and a series of reactions occur under high temperature environment, and finally gallium nitride epitaxial layer is formed on the substrate surface:

MOCVD technology was first proposed by Manasevit in 1968. After that, with the improvement of raw material purity and process, the method gradually became the main growth process for the second generation of semiconductor materials represented by gallium arsenide and phosphorus indium and the three groups of semiconductor materials represented by gallium nitride. In 1993, Nakamura et al. from Nippon Chemical realized the preparation of InGaN indium gallium nitrogen epitaxial layer with high quality management by using MOCVD method, which shows the importance of MOCVD in the third generation semiconductor materials.

The advantage of MOCVD is that the reactants enter the reaction chamber in gaseous form, and the thickness, composition and carrier density of epitaxial materials can be controlled by precisely controlling various gas flows. Secondly, the gas flow in the reaction cavity is fast, and the steep heterojunction interface can be obtained by changing the gas. Third, the impurities obtained are less and the crystal quality is high. Fourth, the equipment is relatively simple, conducive to large-scale industrial production.

MOCVD plays an increasingly important role in the preparation of trigroup semiconductor materials. In terms of equipment supply, besides Aixtron of Germany and VECCO of the United States, Chinese and micro companies have also achieved a major breakthrough, and MOCVD has been replaced by domestic products.

Hydride gas phase epitaxy technology and its disadvantages

In fact, the original growth method of gallium nitride is hydride gas phase epitaxy HVPE, which was originally used by Maruska et al to make gallium nitride epitaxy layer. HVPE reaction is usually carried out in atmospheric pressure hot quartz reactor. The basic chemical reaction is gaseous hydrogen chloride and metal gallium under low temperature environment to produce gaseous gallium chloride, which then reacts with gaseous ammonia under high temperature environment to produce gallium nitride film. The by-products of the reaction, hydrogen chloride and hydrogen gas, can be recovered as gas.

The preparation of gallium nitride by HVPE requires two steps of chemical reaction: low temperature reaction and high temperature reaction. Therefore, the reaction cavity of HVPE reactor needs to be divided into low temperature zone and high temperature zone. At the same time, many parameters need to be adjusted during this process to achieve the control and deposition of gallium nitride film.

HVPE method was widely used in gallium nitride growth in 1970s and 1980s, but many defects of this method were found in application: The prepared gallium nitride has a large number of crystal defects and poor crystal quality, mainly due to the existence of spatial parasitic reaction. Because HVPE operates under atmospheric pressure, a large number of parasitic gallium nitride particles will be deposited on the outlet of gallium chloride gas in the reactor, the growth surface and the surface of quartz glass tube wall. The parasitic gallium nitride will not only consume gallium chloride, but also reduce the growth rate. Gallium chloride pipeline damage will also lead to crystal defects.

In addition, the method can not control doping well, and it is difficult to achieve P-type doping, so it was once abandoned. However, after the 1990s, HVPE was again paid attention to by the industry, because its equipment is relatively simple, in addition to the progress of technology, HVPE has a fast growth rate of gallium nitride, easy to produce a large area of film, film uniformity is also good.

At present, besides MOCVD, MBE molecular beam epitaxy has also become an important method for the growth of gallium nitride and other semiconductor materials. MBE is an epitaxial growth method for growing high-quality crystal films on the substrate surface, but it needs to be carried out in high vacuum or even ultra-high vacuum environment.

The advantages of MBE are: although the growth rate of MBE is usually not more than 1 micron/hour, equivalent to a second or longer time which only grows a single atomic layer, it is easy to achieve accurate control of film thickness, structure and composition, easy to achieve steep interface heterostructure and quantum structure, etc. Second, the low epitaxial growth temperature reduces the lattice defects introduced by different thermal expansion coefficients on the interface. Thirdly, compared with the chemical process of HVPE and MOCVD, MBE is a physical deposition process, so the impurity pollution caused by chemical reaction is not considered.