Computational Study Of AlN MOVCD Reaction,Surface Doping And Vacancy Based On DFT

Aluminum nitride?AlN?is a promising semiconductor material.It is often used as photoelectric materials,buffer layers,etc.It is especially suitable for GaN growth on Si substrate.AlN is usually prepared by metal-organic-chemical-vapor deposition?MOCVD?.The growth process is essentially a chemical reaction process,including gas phase reaction and surface reaction.Surface reaction is the centre of the whole process of vapor deposition.This dissertation was devoted to the study of microscopic reaction and surface condition of AlN.The reaction of AlN MOCVD was studied based on the density functional theory of quantum chemistry.The gas phase reaction,surface adsorption,and effects of doping and vacancy on the surface were calculated and analyzed by using CASTEP module in Materials Studio.The main researches are as follows:?1?Starting from the gas-phase reaction,the stable configuration of trimethylaluminium?TMA?was demonstrated firstly.Then the gas-phase reaction of the most stable dimer TMA(C_2v)and NH₃ was emphatically studied.The complex Al?CH₃?₃:NH₃?simplified as TMA:NH₃?and monomer m-TMA were produced,with the major changes in reaction process provided.The electronic structures and optical properties of the product TMA:NH₃,such as density of states,Mulliken charge and bond population,band gap,absorption coefficient and reflectivity,were further investigated.?2?The adsorption growth of AlN on Si substrate was investigated.Considering the different adsorption cases,the AlN molecule was placed horizontally or vertically above the Si?111?surface,respectively.According to the lowest adsorption energy,the preferential adsorption was found at the N_top-Al_bridgeridge site in the horizontal configuration,with an adsorption energy of-6.0515 eV.The hollow site was preferred when N atom was down in vertical configuration and had an adsorption energy of-4.6458 eV.Meanwhile,the bridge site was favorable when Al atom was down,with an adsorption energy of-2.9270 eV.The parameters of electronic structure at these preferential adsorption sites,such as partial density of states,electron density difference,Mulliken population were further studied.It was demonstrated by data in various aspects that,electron transfer and bonding occurred between AlN and topmost Si,forming molecular chemisorption.?3?The models of AlN bulk,?0001?-Al terminated surface and?000????-N terminated surface were constructed.The energy and geometric parameters,electronic structure,magnetism and some optical properties of the three models were compared and analyzed.It was found that the energy of bulk was lower than those of the surface systems,and the energy of?000????surface was lower than that of?0001?.The topmost Al atoms moved outward in the?0001?surface,while the topmost N atoms moved inward in the?000????surface.From bulk to?0001?and?000????surface,the band gap decreased in turn,?0001?-Al surface approached zero,?000????-N plane was gapless.AlN bulk was nonmagnetic.The?0001?and?000????systems were both ferrimagnetic.And the magnetic moment was 2.34?_B and 2.23?_B,respectively.For absorption coefficient and reflectivity,those of AlN bulk were significantly greater than those of surface systems.The absorption coefficient was up to 248.11×10³ cm^-11 and the reflectivity was up to0.921 in the AlN bulk.The absorption coefficient and reflectivity of?0001?surface were slightly higher than those of?000????surface.?4?The effects of different C-doping concentration in the top layer of AlN?000????-N terminated surface were studied,with emphasis on the electronic structure,magnetism and some optical properties.It was found that with the increase of C-doping concentration,the energy increased gradually and the system tended to be unstable.The C-Al bond length was longer than that of N-Al,which increased the surface activity.The C-doped systems were all ferrimagnetic.When C-doping concentration was 100%,the top layer was nonmagnetic,and the rest of the top layer were magnetic.The C-doping concentration had little effect on the optical properties of absorption and reflection.The maximum absorption peak(92.49×10³ cm^-1)and reflectivity?0.151?both appeared for 75%C-doped surface.?5?The effects of the vacancy in the top layer of AlN?0001?and?000????surfaces were studied.The electronic structure,magnetism and some optical properties of the vacant system were focused on.It was found that the energy of?0001?vacant system was lower than that of?000????,and thus the vacant?0001?one was more stable.Compared with saturated atoms,the density of states and bond population were increased,bond length was shortened,the electrons gained or lost were slightly less,and the covalence was enhanced in the atoms with broken bonds of the second layer.With vacancy,the band gap were both zero,showing metallic.The?0001?and?000????surfaces and systems were all nonmagnetic,while the corresponding intact systems were ferrimagnetic.Vacancy had little effect on the absorption and reflection of AlN systems.Compared with the vacant?0001?system,the vacant?000????one had higher maximum absorption coefficient and maximum reflectivity.In this dissertation,the microscopic characteristics of AlN MOCVD reaction and defected surface were obtained.The reaction process at the initial stage of AlN growth and the properties of the AlN defected surface were revealed.The results obtained will help to understand the MOCVD reaction mechanism and the causes&effects of AlN defected surface,enrich and improve the scientific content of the theory of micro-growth of high-quality AlN films,and provide reference and theoretical basis for production.