Research On Transport Phenomena And Reactor Design In MOCVD System

Gallium nitride semiconductor material is one of the most importment wide-gap semiconductor materials.MOCVD(Metal Organic Chemical Vapor Deposition)equipment is the main method to grow GaN semiconductor materials.With the characters of high quality material,good stability and repeatability and large scale production etc,MOCVD equipment can not be replaced by other equipment.A typical MOCVD system is made up of many components and the chamber is the most importment core component.The simulation about the chamber structure and the growth kinetics in the process of material growth is an effective way in the design of chamber structure and in the research on how to produce high quality materials.The developing status,composition and structure,realization principles and main characters of domestic and international MOCVD system are presented at first in this dissertation.On the basis of that the development process of XIDIAN MOCVD-120 system is presented in detail.The key technologies about gas transport and control,chamber structure,growth temperature control method,the chamber pressure control method and equitment control system are particularly described.A chamber structure model and GaN chemical reaction model are built to simulate GaN growth process in the chamber.The chamber structure model is corresponding to the actual chamber structure and size.The chemical reaction model includes gas phase chemical reaction process and surface chemical reaction process.The gas phase chemical reaction process also includes adduct reaction and pyrolytic reaction.The surface reaction chemical reaction process includes the reactions that produce GaN and the etching reaction of hydrogen.On the basis of models established,simulation on the GaN growth process in XIDIAN MOCVD-120 system with FLUENT and CVDsim software is carried out.And the simulation results are analysised.The GaN growth process with different growth parameter is firstly simulated and the results are analysised.With the speed of gas entering into the chamber rising,the growth rate of GaN is increased first and then decreased,the thickness uniformity of GaN material become worse and chemical reactions in gas phase transport strengthen.With the chamber pressure increasing,the growth rate of GaN is increased first and then decreased.Adduct reaction in gas phase transport strengthen.If the temperature of gas entering into the chamber is too high or too low,eddy becomes obvious inside the chamber.With the growth temperature increasing,the growth rate is increased,the adduct reactions are decreased and pyrolytic reactions are increased.The pyrolytic reactions become obvious at temperature above 600?.The etching reaction of hydrogen is observed at temperature above 800?,which will decrease growth rate.With the ratio of V/III increasing,the growth rate of GaN is increased first and then decreased.The adduce reaction is increased and pyrolytic reactions is increased first and then decreased.The GaN growth process in chambers with defferent geometries is silumated after first part and the results are also analysised.In vertical chamber,the gas flow becomes stable when the spray header is elevated.At the same time,the adduct reaction in gas phase becomes obvious.The bigger size of gas inlet,the more stable gas flow obtained.But the usage of source material is decreased when the gas inlet is too large.With the diameter of chamber increasing,the adduct reaction strength and pyrolytic reaction is decreased.At the same time,the thickness uniformity is increased.Increasing the rotate speed of graphite within a limited range,the stability of gas flow and growth rate of GaN are all increased.The gas flow inside the chamber becomes stable when the temperature of chamber side wall increases.But the high temperature side wall can also keep the reaction materials focus around the side wall which will affect the GaN growth process.