Quantum Chemical Calculation Study On Reaction Mechanism In AlGaN MOCVD Growth

The group?nitride semiconductor material AlGaN is the basis for the preparation of UV and deep-UV microelectronics and optoelectronic devices,which has been widely concerned.Metal organic chemical vapor deposition?MOCVD?is an important method for the growth of AlGaN films,which includes three stages:the gas transport,gas-phase reactions and surface reactions.However,it is still difficult to obtain high quality AlGaN films efficiently.Gas-phase and surface reactions are the key steps for film growth,which are hindered by the existing problems undoubtedly.Therefore,comprehensive understanding of the reaction mechanism and the construction of a complete reaction path can provide theoretical guidance for the optimization of growth,which has important scientific significance and practical value.In this dissertation,the research status of AlGaN grown by MOCVD at home and abroad in recent 30 years is reviewed in detail.It is found that there are still some problems such as low growth rate,low Al composition in the film,difficulty in p-type doping of high Al composition AlGaN and poor surface morphology of the film.Then,the reaction mechanism of the key factors influencing the film growth is studied,specifically quantum chemical theory calculations,and combining chemical kinetics and thermodynamics theory were used to analyze the gas phase and surface reaction mechanism.The main work and achievements of this paper are summarized as follows:?1?The gas-phase oligomerization reaction path during the AlGaN MOCVD growth was calculated by quantum chemistry,the final stable products of oligomers was determined theoretically.The former viewpoint is modified,which is considered that pure gas-phase AlN/GaN molecules will not be formed theoretically by the polymerization reaction path.In addition,it was found that DMAlNH₂ has a stronger oligomerization ability than DMGaNH₂,meanwhile the presence of DMAlNH₂ can promote DMGaNH₂ to form heterogeneous oligomers,leading to the consumption of Ga source precursors.?2?The nucleation path of gas-phase nanoparticles during the AlGaN MOCVD growth was calculated by quantum chemistry,the formation mechanism of the nanoparticles was revealed.It is found that the trimers are the main source of nanoparticle nucleus,which is embodied in the easier self-polymerization of the decomposition products of[DMXNH₂]₃ through coordination bond or polymerization with amides to form nano-sized molecules.These molecules are likely to develop into nanoparticles with X-N bond as the backbone.In addition,the"Gas-phase Scavenging Effect"proposed by the predecessors has been successfully explained by the following mechanism:Al-containing substances have a stronger polymerization capacity than Ga-containing substances,which are easy to polymerize Ga-containing substances to form heteropolymers with the effect of scavenging on Ga source precursors.?3?The reaction path of p-dopant Cp₂Mg during the AlGaN MOCVD growth was calculated by quantum chemistry.The complete gas-phase reaction mechanism of Cp₂Mg was proposed for the first time,which has been verified by previous experimental phenomena and results.The formation process of Mg-N-H complex is revealed theoretically,and the"Memory Effect"of p-type doping is explained.There are two types of competing addition path and decomposition path.For adduct reaction path in the low temperature range,Cp₂Mg might be combined with NH₃ to form complex Cp₂Mg:NH₃ or Cp₂Mg:?NH₃?₂.The decomposition paths mainly includes the self-decomposition path,hydrogenolysis path and ammonolysis path.The reaction energy barrier and decomposition temperature are high although the formation of favorable doping product Mg atoms by the self-decomposition pathway,which is theoretically difficult to proceed.Due to the participation of H or NH₂ radicals in the hydrogenolysis and ammonolysis paths,the reaction energy barrier and decomposition temperature are relatively low,but the favorable product is the complex which is not conducive to doping.?4?The intrinsic and doped structure of AlGaN with different Al compositions were investigated by the plane wave pseudo-potential method based on density functional theory.It was found that the main factor to influence the stability is the Al composition instead of the position of atoms.The better stability of the crystal structure with high Al composition can be explained by the stronger Al-N bond energy,meanwhile its increase has a"Squeezing Effect"on the AlGaN crystal,which results in the enhanced bond energy of Ga-N,Mg-N and Si-N bonds.In addition,the surface structures of AlGaN?0001?and?0001^<sub>?surfaces with different Al compositions under different covering species were studied.It was revealed that the coordination number of surface atoms,the uniformity of the surface electron distribution and the surface bond strength are crucial influencing factors in the surface stability.For the AlGaN?0001?surfaces with the same Al composition,NH₂ and NH₃covering exhibits better surface stability than that of N and NH covering,and the stability becomes better with the surface Al composition.However,the stability of H-covered AlGaN?000^<sub>1?surfaces deteriorates as the surface Al composition increasing.?5?Based on the first principle pseudo-potential plane wave method of density functional theory,the influence of Al composition on surface reactions of group?and doped precursors on NH₂ covered AlGaN?0001?surfaces was studied,meanwhile the adsorption structures,the bonding mechanism and adsorption energies of the precursors are obtained.It was found that chemical adsorption occurs only at two adsorption sites of T4 and H3;the bonding mechanism of the X-N bonds formed on surfaces is X?sp?hybrid orbital and N?p?orbital to provide bonding electrons;The surface Al composition and the adsorbed positions have no effect on the bonding mechanism,but influence the surface X-N bond energy;the Ga rich?0001?surface and the H3 adsorption site are more beneficial for adsorption growth;The atomic precursors,Al-containing precursors and n-type doped precursors are easier to grow by adsorption.