General aspects of the vapor growth of semiconductor crystals - a study based on DFT simulations of the NH3/NH2 covered GaN(0001) surface in hydrogen ambient

摘要

Vapor growth of semiconductors is analyzed using recently obtained dependenceof the adsorption energy on the electron charge transfer between the surfaceadsorbed species and the bulk [Krukowski et al. J. Appl. Phys. 114 (2013)063507, Kempisty et al. ArXiv 1307.5778 (2013)]. Ab initio calculations wereperformed to study the physical properties of GaN(0001) surface in ammonia-richconditions, i.e. covered by mixture of NH3 molecules and NH2 radicals. TheFermi level is pinned at valence band maximum (VBM) and conduction band minimum(CBM) for full coverage by NH3 molecules and NH2 radicals, respectively. Forthe crossover content of ammonia of about 25% monolayer (ML), the Fermi levelis unpinned. It was shown that hydrogen adsorption energy depends on the dopingin the bulk for the unpinned Fermi level, i.e. for this coverage. Surfacestructure thermodynamic and mechanical stability criteria are defined andcompared. Mechanical stability of the coverage of such surfaces was checked bydetermination of the desorption energy of hydrogen molecules. Thermodynamicstability analysis indicates that initally equilibrium hydrogen vapor partialpressure steeply increases with NH3 content to attain the crossover NH3/NH2coverage, i.e. the unpinned Fermi level condition. For such condition theentire range of experimentally accessible pressures belongs showing that vaporgrowth of semiconductor crystals occurs predominantly for unpinned Fermi levelat the surface, i.e. for flat bands. Accordingly, adsorption energy of mostspecies depends on the doping in the bulk that is basis of the possiblemolecular scenario explaining dependence of the growth and the doping ofsemiconductor crystals on the doping in the bulk