Study On Surface Reactions Of AlN And GaN In MOVPE Growth

In recent years,group III nitride semiconductor materials based on AlN and GaN have attracted great interest and gained extensive attention due to their potential applications in high temperature and high frequency,and have been widely used in the preparation of microelectronics and optoelectronic devices.Metal-organic chemical vapor epitaxy?MOVPE?is the main technique for growing AlN thin films and gan thin films^[1-4],including transport process,gas phase reaction and surface reaction.The first two determine the rate and concentration distribution of gaseous precursors?such as MMAl,NH₃,DMAlNH₂,MMGa,etc.?reaching the top of the substrate,while the latter determines the surface morphology,impurity composition and defects of thin film growth.However,it is still difficult to obtain high quality AlN and GaN thin films by growth.Despite a large number of experiments and theoretical studies,the growth mechanism of AlN and GaN is still controversial due to the complex surface reactions during the growth process.Density functional theory?DFT?of quantum chemistry in this paper is used to study is used to consider the possible decomposition and adsorption of NH₃ and H₂ on the surface of AlN or GaN to NH₂ and H.Adsorption and diffusion of the main adsorbed species MMGa on the AlN?0001?-Al surface?referred to as AlN surface?and GaN?0001?-Ga surface?referred to as GaN surface?mixed with NH₂ and H,and observed the mixed coverage of NH₂ and H on the surface The change of ratio affects the surface adsorption of three species.By comparing the surface adsorption energy,partial wave state density,charge change and surface diffusion energy barrier of adsorbed species under different coverages,the surface reaction mechanism in the growth of AlN and GaN was discussed.The main conclusions are as follows:?1?In the case of atomic layer epitaxy?ALD?,the adsorption and diffusion of MMAl under NH₂ and H mixed covering surfaces are studied.By comparing the surface adsorption energy,charge change and surface diffusion energy barrier of adsorption species under different covering degrees,it is found that the AlN?0001?-Al surface covered by MMAl mixed with NH₂and H,with the change of NH₂ and H covering degrees,MMAl is stably adsorbed at T4 and H3positions with similar adsorption probability.With the increase of NH₂ ratio and the decrease of H ratio,MMAl transfers charge to AlN surface after adsorption.At the same time,adsorption becomes relatively easy and diffusion becomes gradually difficult.Compared with before adsorption,the Al-C bond length in MMAl after adsorption is shortened and the bond energy is enhanced,which is not conducive to the separation of CH₃,resulting in an increased probability of introducing C impurity,indicating that MMAl may be one of the main reaction substances in growth and one of the main sources of introducing C impurity.If there is cover H on AlN surface,adsorbed MMAl will cause H atoms covered on the surface to tend to separate from AlN surface,which is favorable for subsequent growth.?2?The adsorption of AlN growth precursor DMAl on AlN?0001?-Al surface mixed with NH₂ and H under MOVPE was studied.It was found that with the increase of NH₂ ratio and the decrease of H ratio on AlN?0001?-Al surface,the electric charge was transferred to AlN surface from species after DMAlNH₂ adsorption.Compared with before adsorption,after DMAlNH₂adsorption,one of the Al_ads-C_adsds bonds grows in length,the bond energy becomes weak,and tends to be detached.When the coverage of surface H and NH₂ is 1:1,one CH₃ may be detached during adsorption and adsorbed to the surface.After DMAlNH₂ is adsorbed on the surface,new Al-N bonds will be generated with the surface,and the original Al-N bond energy on the surface will be weakened and the bond length will increase.Moreover,the intensity of Al_ads-N_surur generated with the surface after DMAlNH₂ adsorption will gradually decrease with the increase of the amount of Al_ads-N_surur generated with the surface.?3?The adsorption of GaN growth precursor MMGa on the GaN?0001?-Ga surface mixed with NH₂ and H under metal organic chemical vapor epitaxy?MOVPE?has been studied.The results show that when NH₂ and H are mixed to cover the GaN?0001?-Ga surface,with the change of NH₂ and H coverage,MMGa is stably adsorbed at positions T4 and H3 with similar adsorption probability.When the surface is covered with 75%NH₂ and 25%H at the same time,the adsorption energy of MMGa is the largest.Combining the density of states diagram with the differential charge density diagram,it is shown that Ga atoms in MMGa interact with N atoms in NH₂ on the surface,and charge transfers from MMGa to the surface.The 3s and 3p orbitals of Ga in MMGa hybridize with 2s and 2p orbitals of N in NH₂,forming Ga-N bonds during adsorption.Compared with that before adsorption,the Ga-C bond length in MMGa after adsorption is shortened and the bond energy is enhanced,which is not conducive to the separation of CH₃,resulting in an increased probability of introducing C impurity,indicating that MMGa may be one of the main reaction substances in growth and one of the main sources of introducing C impurity.