Johannes Pollmann

The ideal (111), (110) and (100) surfaces of Si, Ge and GaAs; A comparison of their electronic structure

Abstract In this paper we report all surface band structures and some layer densities of states for the ideal (111), (110) and (100) surfaces of Si, Ge and GaAs in comparison. The bulk materials are described by the best available empirical tight-binding Hamiltonians. The surface problem for semi-infinite solids is solved exactly using the Koster-Slater scattering-theoretic technique. The results for the different surfaces and the different materials are compared and characteristic properties stemming from particular surface geometries or varying ionicity are identified unambiguously. The calculations were carried out using first-nearest-neighbour as well as first- and second-nearest-neighbour bulk Hamiltonians. The sensitivity of the surface band structures with respect to the used bulk Hamiltonians is discussed, For some of the eleven different surfaces these are the first surface band structure calculations on the basis of a realistic tight binding bulk description.

Atomic, electronic, and vibronic structure of semiconductor surfaces

A brief review on recent progress in the theory of electronic, structural, and vibronic properties of semiconductor surfaces is presented with particular emphasis on the empirical and selfconsistent scattering theoretical method for semiinfinite systems. The current knowledge of the Si(001) (2×1) surface is discussed in detail. The Ge(001) (2×1) surface, as well as, the clean and the Ge-covered GaAs(110) surfaces are addressed, in addition. In the discussion of the results it is shown, that the scattering theoretical method is an extremely versatile tool for calculating electronic surface properties unambiguously with high spectral resolution concerning energy, wavevector, layer-index and orbital type. Currently used approaches for calculating the total energy, Hellmann-Feynman forces and optimal structure models are summarized. Using the total energy as a starting point, the calculation of atomic force constants and surface phonon spectra is exemplified.

Upper bounds for the ground-state energy of the exciton-phonon system

Abstract Upper bounds for the ground-state energy of the exciton-phonon system are calculated variationally for all ratios of the polaron radii to the exciton radius, whereby the constituents of the exciton are no longer treated as two independent polarons. The numerical results compare satisfactory with experimental data even for the thallous halides.

Elastic displacement field of point defects in anisotropic cubic crystals

The elastic displacement field of point defects in cubic crystals is calculated for weak anisotropy by second order perturbation theory and by a variational procedure. The results are compared with numerical calculations for Cu. Further analytical approximations are given for the volume change in an infinite crystal and for the interaction energy of two point defects.

Angle-resolved photoemission from Si(100): Identification of bulk band transitions

Abstract We have measured normal-emission photoelectron spectra from Si(100)−(2 × 1) using excitation with NeI, HeI, and HeII radiation. The angular dependence of the NeI data was investigated along the ΓXWK bulk mirror plane. We demonstrate that bulk-band transitions can be clearly distinguished from surface-state emission. A new surface band could be identified around the J′ point of the surface Brillouin zone. All results, including the bulk features, may be consistently interpreted on the basis of calculated bulk and surface states.

Angle-resolved photoemission from Si(100): direct versus indirect transitions

We have measured angle-dependent photoelectron spectra from Si(100)-(2 × 1) along the ΓXWK bulk mirror plane after excitation with HeI radiation. Emission features originating from bulk bands can be clearly distinguished from surface-band emission. However, besides direct (primary cone) bulk band transitions also one-dimensional (k‖ resolved) density-of-states (DOS) emission contributes appreciably to the spectra. Both contributions are still interpretable in terms of the bulk three-dimensional valence band structure, and critical point energies X4V = −3.1 ± 0.3 eV, W2V = −4.1 ± 0.2 eV, K2V = −3.1 ± 0.3 eV and K1V = −4.8 ± 0.3 eV could be deduced. Hydrogenation of the surface neither quenches the direct transitions nor the one-dimensional DOS emission. This rules out reconstruction-induced effects as a primary reason for the strong appearance of the latter one. Instead we make responsible an appreciable surface disorder, which manifests itself in defect levels observed just at the Fermi edge.

Ab initio calculations of structural and electronic properties of prototype surfaces of group IV, III–V and II–VI semiconductors

Abstract In this brief review, structural and electronic properties of technologically important semiconductor surfaces are presented and discussed with particular emphasis on most recent ab initio results for semi-infinite and supercell geometries. Most of the results are based on the local density approximation of density functional theory but GW quasiparticle band structures and results of calculations incorporating self-interaction-corrections are included as well. A general picture of the surface reconstruction or relaxation behaviour is developed and the resulting electronic properties of prototype surfaces of diamond-, zincblende- and wurtzite-structure crystals are discussed. The systems addressed comprise reconstructed (001) surfaces of diamond, Si, Ge and SiC, the relaxed (110) surface of SiC and GaAs and nonpolar (10 1 0) surfaces of wurtzite-structure SiC, ZnO and CdS. A comparing discussion of the relaxed surfaces of SiC, GaAs and of II–VI compound semiconductors allows to address the physical origins of the relaxation behaviour of these compounds and to identify characteristic differences and similarities in their relaxation behaviour related to the specific heteropolarity or ionicity of these systems. Our results show excellent agreement with a whole body of experimental data.

Theory of adsorption: Ordered monolayers from Na to Cl on Si(001) and Ge(001)

First principles calculations of clean and adsorbate-covered surfaces of Si(001) and Ge(001) are reported. Chemical trends in the adsorption of ordered Na, K, Ge, As, Sb, S, Se and Cl overlayers are discussed. The calculations are based on the local-density approximation and employ non-local, norm-conserving pseudopotentials together with Gaussian orbital basis sets. The semi-infinite geometry of the substrate is properly taken into account by employing our scattering theoretical method. From total-energy minimization calculations we obtain optimal surface reconstructions which show asymmetric dimers for Si(001), Ge(001) and Ge:Si(001). For As:Si(001), Sb:Si(001) and Sb:Ge(001), we find symmetric adatom dimers in the equilibrium geometries. S or Se adlayers are found to be adsorbed in bridge positions forming a (1×1) unit cell with a geometry very close to the configuration of a terminated bulk lattice. Cl atoms adsorb on top of the dangling bonds of symmetric Si dimers residing in the first substrate-surface layer. Our calculations for Na:Si(001) and K:Si(001) confirm valley-bridge site adsorption for half monolayer coverage. For full monolayer alkali-metal coverage, adsorption in pedestal and valley-bridge positions is found to be energetically most favourable. The calculated optimal adsorption configurations are in excellent agreement with a whole body of recent experimental data on surface-structure determination. For these structural models, we obtain electronic surface band structures which agree very good with a wealth of data from angle-resolved photoemission spectroscopy investigations.

Microscopic approach to the quantum size effect in superlattices

Abstract A new method for the microscopic calculation of electronic properties of superlattices is introduced and used to study the effects of carrier confinement in GaAs-Ga1−xAlxAs heterostructures. Our approach allows to evaluate the superlattice band structure and layer densities of states for arbitrary thickness of the GaAs and of the Ga1−xAlxAs slabs. We discuss the quantum size effect, the subband dispersion, the effective electron masses and the layer density of states near the conduction band edge for varying periodicity length and varying barrier thickness. Our results confirm simple potential well calculations and agree very good with experimental data.

Theory of semiconductor heterojunctions

Abstract A short review of characteristic electronic properties of heterojunction interfaces is given. Band edge discontinuities and interface band structures for lattice-matched junctions are discussed in detail. The examples presented include non-polar and polar junctions as well as overlayer systems. The results of involved calculations are interpreted in terms of simple physically appealing pictures by directly relating the changes in bonds across an interface to the resulting bands in the interface band structure. The meaning of the results for the transport properties of semiconductor heterojunctions is briefly assessed.