Translated Abstract
GaN-based semiconductors have been paid more and more attention, for their superior performance, along with the development of information technology and optoelectronic technology. High quality GaN single crystal would bring revolutionary development for the information storage technology and semiconductor lighting technology at present. Because of the low growth rate and polycrystallization of GaN crystals，it is difficult to obtain high quality and relatively cheap GaN single crystals.Therefore, how to grow high quality and relatively cheap gallium nitride single crystals has been one of hot research in the field of semiconductor.The main methods of GaN crystals growth include Hydride Vapor Phase Epitaxy (HVPE) method, High Pressure Solution (HPS) method, ammonothermal method, and Na flux method. We reach the conclusion that the Na flux method would be more probable to obtain high quality and relatively cheap GaN single crystals after a series of research and analysis.Centering round the growth condition of GaN crystals by Na flux method, I have completed the following work：1. A minitype unit of high pressure for GaN crystal growth has been designed and manufactured，after refer to the related research achievements at home and abroad. Some merits and demerits have been summarized after a series of tests. The volatility of Na and the oxidation of crucibles have been suppressed in this unit. However，the real conditions of GaN single crystal are difficult to be confirmed and there are disadvantageous for growth of large size GaN crystals. A solution for the unit improvement have been provided when considering the high pressure vessel’s high-temperature creep and high rate of heat dissipation resulting in the low temprature in the crucibles. The size of high pressure vessel is decreased and the hastelloy alloy is used as substitute for 304 stainless steel.2. The large scale high pressure unit for GaN crystals growth in our laboratory has been used. The problem that this unit could not work stably for a long time has been solved, when the nickel-chromium wire instead of Alchrome is used to manufacture the crystal growth furnance. A lot of GaN crystals were obtained in size about 1mm, when this high pressure unit can provide the conditon of GaN crystals growth3. Test experiments of temperature fields have been carried out in the large scale high pressure unit for GaN crystals growth，in order to explore the suitable conditions of GaN single crystal. Various furnace heating cores have been created on the basis of different power combinations. the temperature fields were tested using the furnance whose core consists of two parts of nickel-chromium wires, under the pressure of vacuum (0.01MPa),atmospheric pressure(0.1MPa), 2MPa, 3.5MPa, 5MPa, 6MPa and 7MPa and the temperature of 500℃，600℃，700℃，750℃，800℃，850℃ and 900℃。Moreover, the three pressure: acuum (0.01MPa), atmospheric pressure (0.1MPa) and 2MPa were focused on test. In addtion, curves of temperature fields have been made. I conclude that both temperature and pressure variation influence the temperature gradient of the crystal growth furnance. Under vacuum and atmospheric pressure, the variation of temperature fields of the crystal growth furnance is little with the temperature changes Meanwhile, under the high pressure (2MPa, 3.5MPa, 5MPa, 6MPa, 7MPa), the variation of temperature gradient of the crystal growth furnance varies widly. The variation of pressure has a huge influence on the temperature fields of the crystal growth furance, because of the changes of heat transfer mode. When the upper portion of the two parts of wires of the furnace core works only, the temperature gradient of the crystal growth furnance increases with temperature increasing but when the lower part works only, the temperature gradient decreases with the temperature increasing. When the upper part and the lower part of the two wires work together, if the controlled points of temperature at the upper part and the lower part of the crystal growth furnance remains constant , the temperature gradient of upper part of the crystal growth furnance decreases with the pressure increasing, while the temperature gradient of lower part increases to a limiting value with the pressure increasing. The available temperature gradient could be adjusted according to the results, for the future GaN crystals growth experiments.KEY WORDS: growth of GaN crystals Na flux method units tests of temperature fieldsTYPE OF THESIS: Applied Research
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