Study On Adsorption And Diffusion On AlN Surface With Incorporation Of Carbon Impurity

AlN single crystal thin film is an important semiconductor material with wide band gap,high breakdown voltage,high temperature resistance and corrosion resistance.It is a vital material for manufacturing blue and violet light emitting diodes and high power electronic devices.MOCVD is the main method to manufacture AlN crystal film.The surface morphology,impurity composition and defects of AlN films are determined by surface reaction.The incorporation of C impurity in the film will form a deep energy level between the conduction and the valence band of AlN crystal,causing recombination effect on carriers thus affect the photoelectric properties of the film.If the C impurities are doped with cation site(Al atom in AlN crystal),it will cause a large degree of dislocation in the crystal and affect the growth quality of the film.Therefore,it is very important to study the reaction mechanism between adsorbed particles and surface in MOCVD.Based on the density functional theory of quantum chemistry,this thesis studies the adsorption and diffusion of the MMAl on the surface,especially when it forms C_ad-Al_suron the surface after adsorption,and C impurities are incorporated into the surface.The main research methods are as follows:firstly,the AlN(2?2)periodic plane is established,and the adsorption energy and diffusion barrier of the adsorbed particles on two kinds of AlN polar surfaces(0001 and 000-1)are calculated.On the AlN(0001)polar surface,in order to explore the effect of the initial trace introduction of C impurity on the subsequent adsorption and diffusion of MMAl,the adsorption energy,diffusion barrier,bonding and charge transfer to the nearest adsorption site were calculated when the surface C hybrid ratio was 0.01%and0.02%,respectively.The bond population and differential charge density are calculated and analyzed;On the AlN(000-1)polar surface,the adsorption energy and diffusion barrier of MMAl were calculated under ground state and actual growth conditions.The main conclusions are as follows:(1)Through the observation and analysis of the surface structure after adsorption,it is found that in addition to the four traditional adsorption sites(Top site,Br site,H3 site,T4site),there is a new adsorption site-horizontal adsorption site.When the adsorption configuration is(Al_ad-t4,C_ad-Al_sur),the adsorption energy is-3.24e V which is larger than these of the vertical adsorption configurations.It shows that the horizontal adsorption is more likely to occur than the vertical adsorption when MMAl reaches the surface.After horizontal adsorption,the bond length of Al_ad-C_adbecomes longer,the bond energy decreases,and there is a tendency to fracture.The strong covalent bond can be formed between C_adand Al_suron the surface,which also indicates that C impurity is easy to be incorporated into the film during horizontal adsorption,resulting in the pollution of the film and the decrease of flim quality.(2)By adjusting the atomic composition ratio of Al atom at the adsorption site where C impurity is easy to incorporated into,the horizontal adsorption energy is obtained under the following nine conditions:5%,1%,0.5%,0.1%,0.05%,0.04%,0.03%,0.02%and 0.01%,respectively.The results show that the adsorption energy increases rapidly and then decreases when the mixed proportion of C impurity is 0.01%,which indicates that the initial introduction of trace C impurity will promote the subsequent introduction of C impurity at first,and then hinder it;In order to further clarify the effect of C incorporation into the lattice on the adsorption of AlN surface,C doping was used which is to replace the alsur on the surface of AlN with C atoms.The effect of C incorporation into the lattice on the adsorption behavior of MMAl was investigated.The results show that the surface has a large repulsion energy for horizontally placed MMAl,which is-1.14e V.This shows that with the increase of C hybrid ratio,the effect of horizontal placement of MMAl goes through a process of promotion,hindrance and repulsion.(3)The adsorption and diffusion of MMAl on the ideal surface of AlN(000-1)polar surface are also calculated.The results show that MMAl is more easily adsorbed on H3 and T4 sites,and the adsorption energies of the two adsorption sites are similar,which are higher than those on AlN(0001)polar surface,and the diffusion barrier is also higher than that on AlN(0001)polar surface;On the stable reconstructed surface covered by H,MMAl is also easily adsorbed on T4 site,and the ground state stable reconstructed surface is more easily adsorbed on T4 site far away from the covered h due to the repulsion of H;However,the horizontal adsorption of MMAl does not occur on the reconstructed surface under the actual growth conditions,which indicates that H₂can not only transport the reaction source gas(also called MO source),but also clean the surface and avoid the introduction of C impurity to a certain extent.In addition,the diffusion barrier of MMAl on the reconstructed surface is lower,which indicates that the adsorbed molecules are easier to diffuse on the reconstructed surface.