Study On Surface Adsorption And Diffusion In AlN MOVPE Growth

AlN is an important third generation semiconductor material with wide direct bandgap,radiation resistance,high temperature resistance and high breakdown field strength.It can be used to fabricate laser diode,high brightness LED,UV photovoltaic device and so on.MOVPE is the most widely technique for the growth of AlN thin films.During the growth of AlN by MOVPE,the surface reactions determine the defect distribution,impurity content and surface roughness of the films.Therefore,it is important to understand the surface reaction mechanism of AlN-MOVPE in order to prepare high-quality thin films to meet the requirements of devices.In this paper,based on density functional theory of quantum chemistry,the adsorption and diffusion of three main surface precursors,MMAl,DMAlNH₂ and Al?NH₂?₃,on ideal,H-covered and NH₂-covered AlN?0001?-Al surfaces?referred to as AlN surfaces for short?were calculated.Firstly,three periodic models of AlN surface are established by using Material Studio.The electronic exchange-correlation energy is calculated by PW91 method in GGA.Then,the adsorption energy,PDOS and Mulliken chemical bond population of MMAl,DMAlNH₂ and Al?NH₂?₃ at different adsorption sites on different surfaces were analyzed,and the stable adsorption sites and bonding principles of particles on the surface were revealed.Finally,according to the stable adsorption sites calculated above,the diffusion barriers of species on the surface were calculated.The main conclusions are as follows:?1?Comparing the adsorption energies of three species on different surfaces,it is found that the species containing Al-N bonds?DMAlNH₂ and Al?NH₂?₃?are easier to adsorb on the ideal AlN surface than those containing only Al-C bonds?MMAl?,while on the AlN surface covered by H and NH₂,it is the opposite.The results show that Al-rich surface is helpful for the adsorption of species containing Al-N bonds,while H-rich and NH₂-rich surfaces inhibit the adsorption of Al-N bonds.?2?The adsorption of MMAl on three surfaces is molecular adsorption,and all of them are adsorbed at H3 and T4 sites with higher surface symmetry?also at Br and Top sites on ideal surface?.The adsorption of DMAlNH₂ on the H-and NH₂-covered AlN surfaces is also molecular adsorption,whereas decomposition adsorption is easy to occur on ideal surfaces.The adsorption of Al?NH₂?₃ on three surfaces are molecular adsorption.?3?On the H-covered AlN surface,both DMAlNH₂ and Al?NH₂?₃ may attract a surface H to form-NH₃ group,and thus the surface H coverage is changed from 1ML to0.75ML,which indicates that the adsorption of DMAlNH₂ and Al?NH₂?₃ tend to form an H-covered surface of 0.75ML.?4?For the adsorption of the species containing C atoms on three surfaces,it was found that C atoms do not participate in surface bonding when MMAl is adsorbed alone.when DMAlNH₂ is adsorbed on H-and NH₂-covered surfaces,the C atoms do not participate in bonding as well.However,on ideal surfaces,one of the Al-C bonds in DMAlNH₂ may break,and forms CH₃,which is adsorbed onto an surface Al atom.?5?The diffusion barrier of MMAl on ideal AlN surface is obviously smaller than that of DMAlNH₂ and Al?NH₂?₃,which indicates that Al-rich surface is beneficial to the diffusion of MMAl.On the NH₂-covered AlN surface,the diffusion barrier of MMAl is obviously higher than that on the other two surfaces,indicating that the NH₂-rich surface inhibits the diffusion of particles containing only Al-C bonds.The diffusive barriers of DMAlNH₂ and Al?NH₂?₃ on three surfaces from large to small are as follows:ideal surface,NH₂-covered surface and H-covered surface,indicating that Al-rich surface is the most disadvantageous to the diffusion of species containing Al-N bond,followed by NH₂-rich surface,and H-rich surface is helpful to their diffusion.