Physical Behavior Of Impurity-induced In GaN Surface Areas:First-principles Study

GaN material has a broad prospect because of its unique and excellent photo-electromagnetic-heat comprehensive performance in the field of research andapplication of semiconductor. Surface characteristics have important influence on itsperformance, and the study of GaN surface could improve the quality of crystalgrowth technology, so as to realize the performance of the process of crystal growthand more effective controlling of crystal.In this paper, first-principles calculations are performed to study the energetic,atomic structures and magnetic properties of impurity (two types elements wereconsidered here: Al or Co) doped in the GaN (0001) surface. Our research resultscould improve the technology of crystal growth and spin injection, which couldprovide the correct theory basis and guidance. Main research results are as follows:(1) First-principles calculations are performed to study the energetics and atomicstructures of aluminum adsorption and incorporation at clean and Ga-bilayerGaN(0001) surfaces. We find that favorable adsorption site changes from T4to T1asAl coverage increased to1monolayer on the clean GaN(0001) surface, and atwo-dimensional hexagonal structure of Al overlayer appears. It is interesting that Alatoms both prefer to concentrate in one deeper Ga layer of clean and Ga-bilayerGaN(0001) surface respectively, while different structures could be achieved in abovesurfaces. For the case of clean GaN(0001) surface, corresponding to N-rich andmoderately Ga-rich conditions, a highly regular superlattice structure composed ofwurtzite GaN and AlN becomes favorable. For the case of Ga-bilayer GaN(0001)surface, corresponding to extremely Ga-rich conditions, the Ga bilayer is found to besustained stable in Al incorporating process, leading to an incommensurate structuredirectly. Furthermore, our calculations provide an explanation for the spontaneousformation of ordered structure and incommensurate structure observed in growingAlGaN films. The calculated results are attractive for further development of growthtechniques and excellent AlGaN/GaN heterostructure electronic devices. (2) Energetic and magnetic properties of Co incorporation in clean GaN(0001)and Ga-bilayer GaN(0001) surfaces are systematically studied from first-principlescalculations. It is found that Co atoms show strong surface site preference in bothsurfaces. Moreover, for clean GaN(0001) surface(corresponding to N-rich conditions),CoN/GaN interface could be formed, while CoGax/GaN interface would be possiblein Ga-bilayer GaN(0001) surface(corresponding to extremely Ga-rich conditions).More interestingly, we find that the magnetic coupling between Co atoms in cleanGaN(0001) surfaces presents stable antiferromagnetic behavior. While, in Ga-bilayerGaN(0001) surface, the antiferromagnetism and ferromagnetism transition would beexpected to occur as Co concentration increased. We suggested that ferromagneticstate is due to the formation of tightly-coupled CoGa bilayer with hybridization ofCo-3d and Ga-4p, and the magnetic properties are related to the growth condition andsurface topography. Such a tunable magnetism might allow Co:GaN system to havepotential applications in spin injection devices.