Udo Scherz

Density-functional theory of sp-bonded defects in III/V semiconductors

Publisher Summary This chapter discusses the density-functional theory of sp-bonded defects in III/V semiconductors. The requirements of semiconductor technology for accurate control of defect concentrations, concentration profiles, defect electronic structure, diffusion properties, formation and reaction energies, and atomic structures at interfaces have stimulated extensive experimental and theoretical research. Native or intrinsic defects are imperfections of the perfect crystal that do not involve impurity atoms and that are in fact unavoidable in principle. However, their concentration can be controlled by the temperature, pressure, and environment (partial pressure) of the crystal. The growth of crystals under the conditions of controlled non-stoichiometry leads to different native defect concentrations. The chapter describes the electronic properties and formation energies of vacancies, antisites, and self-interstitials. Native defects are related to a deviation from stoichiometry, and it is important to understand which of them dominates the given chemical potentials of the cations and anions outside the crystal.

JAHN-TELLER EFFECT OF CU2+ IN II-VI COMPOUNDS

— 2T2) zero-phonon lines which have been investigated as a function of temperature and in magnetic fields up to 15 T. As a result, an unique termscheme for Cu2+ in the sulfide host compounds is presented. We interpret the observed Cu2+ transitions in ZnS and CdS crystals by a moderate Jahn-Teller coupling of the electronic states to a local vibrational mode of E symmetry and a Huang-Rhys factor of S = 0.8 for ZnS and S = 1.1 for CdS. We report on parameter-free calculations of the magnetic-field splitting of Cu2+ centers in II—VI compounds, which show a general agreement with the observed spectra. For ZnS : Cu2+ the agreement between the calculated and observed g values is excellent, whereas some differences for the 2T2 ground-state g factors occur. This is due to the neglect of the T, mode coupling in the Jahn-Teller calculation. The local vibrational modes of copper in ZnS and CdS are investigated and fitted in a cluster calculation using the valence-force model of Kane including Coulomb forces. An E mode at 32.4 meV is found to be the dominant coupling mode in ZnS whereas a T, mode at 21.3 meV is obtained for CdS.

Ab InitioCalculation of Vibrational Modes in Semiconductors

We calculate vibrational modes in semiconductors using density-functional theory in the local density approximation and norm-conserving Troullier-Martins pseudopotentials. We obtain the vibration frequencies from a direct calculation of the dynamical matrix by using the supercell method with plane wave expansion. In order to show the feasibility of the method we here present the results for the phonon-dispersion curves of GaAs, ZnSe and cubic GaN crystals, which are in very good agreement with neutron-scattering experiments and second-order Raman spectroscopy.