QUASI-PARTICLE BAND-STRUCTURE CALCULATIONS FOR C, SI, GE, GAAS, AND SIC USING GAUSSIAN-ORBITAL BASIS-SETS

We report state-of-the-art first-principles calculations of the quasip article energies of prototype homo-polar and heteropolar covalent semi conductors described in terms of the electron self-energy operator. Th e wave functions are calculated within density-functional theory using the local-density approximation and employing nonlocal, norm-conservi ng pseudopotentials. The self-energy operator is evaluated in the GW a pproximation. Employing the plasmon-pole approximation for the frequen cy dependence of the dielectric matrix epsilon(G),(G)'(q omega), its s tatic part is fully calculated within the random-phase approximation ( RPA) as well as by using a number of different models. All calculation s are carried out employing localized Gaussian orbital basis sets. Thi s will turn out to be very useful for detailed studies of the quasipar ticle properties of more complex systems such as bulk defects includin g lattice relaxation and reconstructed surfaces with large unit cells or interfaces, which are otherwise computationally too demanding. Usin g an s,p,d,s double dagger basis set of 40 Gaussian orbitals for Si, f or example, yields already convergent results in excellent agreement w ith tl;e results of a 350-plane-wave calculation in the corresponding plane-wave representation. Most of our results for Si, diamond, Ge, an d GaAs are in very good agreement with experimental data and with avai lable plane-wave GW calculations. To our knowledge, our results for Si C are the first quasiparticle energies reported so far for this import ant material of high current technological interest. Also in this case we find very good agreement with the available experimental data exce pt for Epsilon(L(lc)). We believe that this deviation may be attribute d to experimental uncertainties. In particular, we discuss and scrutin ize the applicability of six different models for the static dielectri c matrix epsilon(G),(G)'(q,O) in the GW approximation ranging from the simple Hartree-Fock expression over diagonal models to nondiagonal mo dels that take the local fields within the inhomogeneous electronic ch arge density into account. Some of the nondiagonal models are shown to yield results in very good agreement with the full RPA results.