The relative stabilities of hydrogen at (or near) the tetrahedral interstitial and bond-centered sites as well as that of hydrogen dimers in the molecular and bond-centered–antibonding configurations are calculated at the ab initio level in molecular clusters for c-C, Si, Ge, and α-Sn. The trends show that the lowest-energy configurations change as one goes down the Periodic Table. The relative stability of the possible equilibrium sites affects which charge states of H are likely to be realized in a given host. This in turn affects the diffusion properties of H and its interactions with dopants and other defect centers. The trends in equilibrium geometries and relative stabilities show that silicon is a particular case among group IV hosts in which both isolated interstitials and both dimer states are close to each other in energy. We also examine some properties of two charge states of molecular hydrogen in Si in order to determine the key features of their electron paramagnetic resonance spectra.

In ZnS ${\mathrm{the}}^3$${\mathit{T}}_1$(F)${\mathrm{-}}^3$${\mathit{T}}_1$(P) transition of isolated ${\mathrm{Ni}}^{2+}$ causes structured near infrared absorption and emission bands. Crystals doped in the sub-ppm region exhibit exceptionally sharp lines suited to study the dependence on external fields. Here the fine structure of the perfectly cubic and one axial ${\mathrm{Ni}}^{2+}$ center is studied. A secure assignment of the observed optical transitions is achieved on the basis of high-resolution absorption and emission investigations at various crystal temperatures and in magnetic fields up to 15 T. The experimental results give evidence for intermediate dynamical Jahn-Teller effects in both the ${}_{}{}^3{}_{}{}^{}$${\mathit{T}}_1$(F) ground and the ${}_{}{}^3{}_{}{}^{}$${\mathit{T}}_1$(P) excited state, which are attributed to the coupling to local ${\mathit{T}}_2$-type modes. A detailed comparison shows good agreement between the experimental data for ZnS:Ni and those of the well understood system CdS:Ni. A Fano-type structure in the ${}_{}{}^3{}_{}{}^{}$${\mathit{T}}_1$(F)${\mathrm{-}}^3$${\mathit{T}}_1$(P) absorption is explained by the coupling of a distinct local mode of the Ni center to a continuum of phonon states. Additionally, the interaction of low-frequency acoustical phonons with the Ni center is investigated. No thermal equilibrium is established in the excited ${}_{}{}^3{}_{}{}^{}$${\mathit{T}}_1$(P) state during its lifetime, which gives rise to hot emission lines.

A systematic study of ground-state properties of cubic and hexagonal silicon carbide polytypes (3C-, 6H-, 4H-, and 2H-SiC) is reported using well converged density-functional calculations within the local-density approximation and norm-conserving, fully separable, soft, ab initio pseudopotentials. Equilibrium results are obtained for the lattice parameters, atomic positions, bond lengths and angles, cohesive energies, and the bulk modulus. The internal degrees of freedom, i.e., atomic relaxations, are fully taken into account. The results are discussed in comparison with experimental data. We derive trends with the hexagonality for the molecule volume and the energetic ordering of the polytypes. Driving forces for the polytypism and the atomic relaxations are discussed.

We have performed simulations of liquid phases of phosphorus using the molecular-dynamics–density-functional method. Starting from a system of 26 ${\mathrm{P}}_4$ tetrahedra in a simple cubic unit cell of constant volume, we determine radial, angular, and defect distribution functions for the molecular liquid and for polymeric high temperature liquid red P. Comparison with available experimental data and earlier calculations shows that the structures of the amorphous and liquid polymeric phases are very similar. The polymerization of molecular phosphorus is a complex process that may be viewed as a defect-induced chain reaction, in which the elevated temperature and the environment of the tetramers play decisive roles.

The plane-wave pseudopotential approach to density-functional theory (DFT) in the local-density approximation has been applied to investigate a variety of ground-state properties of the 3C, 2H, and 4H polytypes of silicon carbide. The linear-response theory within DFT has been used to obtain lattice-dynamical properties of cubic SiC such as the phonon-dispersion curves, phonon eigenvectors, elastic and Grüneisen constants, as well as the thermal expansion coefficient and specific heat within the quasiharmonic approximation. Finally, we present some results for phonon-dispersion curves in the hexagonal 2H (wurtzite) and 4H structure. These results are analyzed and discussed in view of further applications to temperature-dependent properties.

We report systematic specific-heat measurements of crystalline Si doped with P in the concentration range enclosing the metal-insulator transition, in the temperature range from 40 mK to 3 K and in magnetic fields up to 7 T. The data can be interpreted in terms of a phonon contribution, a term linear in T due to itinerant electrons, which persists even on the insulating side, and an excess contribution ΔC varying sublinearly with temperature. The strong magnetic field dependence of ΔC suggests an interpretation in terms of local magnetic moments. We discuss a theoretical model for the metallic phase, which provides a quantitative description of the concentration of local moments. We show that the temperature power law in zero field can be explained by the Kondo effect.

The development of the Anderson impurity model for the case of the symmetry-inverted band-edge heterojunction that was considered in the framework of the effective Dirac model has been suggested. The Anderson impurity due to interaction with the heterojunction and the band states is shown to give the impurity level inside the band gap for the proper model parameters, its energy being changed with the impurity-atom location relative to the interface plane.

Raman scattering from amorphous silicon (a-Si) produced by self-ion-implantation reveals an exciting wavelength-dependent scattering-intensity ratio of the optical- and the acoustic-phonon-like peaks. The results show that this wavelength-dependent scattering originates from a depth-dependent disordering. The surface part (∼200 Å) of the samples demonstrates weaker long-range disordering, while the internal part demonstrates stronger long-range disordering. After annealing at 500 °C for an hour this inhomogeneity disappears. This is explained as the elimination of certain defects that strongly influence the longer-range order of the lattice. These defects relax spontaneously at the surface during the ion implantation.

Optical studies of the neutral electron-hole (eh) magnetoplasma in undoped $\frac{{\mathrm{In}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}}{\mathrm{GaAs}}$ quantum wells (QW's) have been performed in magnetic fields up to 12 T. Three experimental situations were investigated over a wide range of plasma densities with subband splittings that were smaller, slightly larger, and much larger than the cyclotron energy. For dense ($n_{\mathrm{eh}}\approx 5\times {10}^{12}$ ${\mathrm{cm}}^{-2\mathrm{}}$) and hot ($T\approx 200-300$ K) photoexcited plasmas, the zeroth Landau-level (LL) transition energies of both sub-bands are in agreement with results of random-phase-approximation calculations neglecting excitonic effects. The stronger renormalization of the first subband is mainly due to the difference in the screened exchange contribution to the renormalization of the band gap. Excitonic corrections become important with decreasing density. They appear first near the Fermi edge and influence the initial or the final state of the optical transitions, depending on the filling of the corresponding LL's. In narrow QW's they lead to a much stronger renormalization of the zeroth LL in the second subband. The excitonic effects decrease with increasing QW width and decreasing magnetic field. At 4 T and a QW width of 30 nm they are negligible already at $n_{\mathrm{eh}}\approx 5\times {10}^{11}$ ${\mathrm{cm}}^{-2\mathrm{}}$. In this case, the strong difference between the renormalization of the zeroth and the first LL observed at small plasma densities is connected with the much smaller binding energy of the $2s$ magnetoexcitons in the empty QW.

Electron-spin resonance and photoluminescence (PL) experiments have been performed on porous silicon. Results indicate the presence of oxygen shallow donors of binding energy in the 0.1-eV range, which show a distinct correlation with the intensity of the red PL observed in porous silicon. It is suggested that the shallow donors become paramagnetic by photoinduced capture of carriers created by light absorption in the silicon crystallites. A model is suggested that links the presence of the shallow donors to the presence of nonbridging oxygen hole centers, which are thought to be responsible for the intense red PL reported in porous silicon.

Interband optical transitions in a semiconductor superlattice induced by an intense optical wave in the presence of a uniform electric field are analyzed. Both the oscillating electric field of the optical wave and the uniform electric field are directed perpendicular to the heterolayers. The superlattice potential barriers are modeled by a periodical chain of δ functions. Quasienergetic time-dependent states are used. The explicit dependence of the coefficient of the multiphoton absorption on the frequency and magnitude of the oscillating electric field, the superlattice parameters, and on the magnitude of the uniform electric field is obtained. The importance of a sufficiently strong uniform electric field, which causes Wannier-Stark localization of the electrons and holes, is emphasized. It has been shown that this localization increases with the magnitude of both the uniform and oscillating electric fields. The main influence of the intense oscillating field is found to be in the narrowing of the energy minibands. Under localization conditions, the electroabsorption multiphoton spectrum consists of a sequence of intense steps such that the number of steps depends upon the number of photons and increases with this number. The effective red boundary of the spectrum shifts towards longer wavelengths as the magnitude of the uniform electric field increases. The form of the spectrum is shown to depend upon the parity of the number of photons involved. Estimates for the GaAs/${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Al}}_{\mathit{x}}$As superlattice are given.

The modulation-doped ordered-${\mathrm{GaInP}}_2$/disordered-${\mathrm{GaInP}}_2$ homojunction is presented. Capacitance-voltage (CV) profiling techniques, temperature-dependent Hall and resistivity measurements, cross-sectional transverse electron micrographs (TEM), and high-field magnetotransport have been used to characterize this structure grown by metal-organic vapor-phase epitaxy. The CV measurements showed a narrow profile at the homointerface with an order of magnitude reduction in carrier density within 3 nm. Typical two-dimensional behavior was observed from Hall data showing sheet carrier densities as high as 3.6×${10}^{13}$ ${\mathrm{cm}}^{\mathrm{-}2}$ without carrier freeze-out, and constant mobilities around 900 ${\mathrm{cm}}^2$ ${\mathrm{V}}^{\mathrm{-}1}$ ${\mathrm{s}}^{\mathrm{-}1}$ below T=100 K. The 300-K channel conductivity of this junction is 3.2×${10}^{\mathrm{-}3}$ ${\mathrm{\Omega }}^{\mathrm{-}1}$, which is higher than reported for other two-dimensional electron gases. By proper choice of the substrate orientation, domains of only the (111¯) ordering variant were present. TEM showed elongated shapes of average thickness 3.5–6 nm and length 75 nm in the (011) plane. By using Hall bars with different current directions, an asymmetry is observed for the contributions to the scattering mechanisms which determine the mobility: ‘‘mesoscopic’’ interface-roughness scattering for T<100 K and cluster scattering for 100<T<300 K. Polar optical scattering at T>300 K indicates strong electron-phonon coupling. This asymmetry shows that the domain length in the (011) plane is larger than that in the (011¯) plane. The magnetoresistance ${\mathrm{\rho }}_{\mathit{x}\mathit{x}}$ and the Hall resistance ${\mathrm{\rho }}_{\mathit{x}\mathit{y}}$ show oscillations in reciprocal magnetic field involving an excited subband i with ${\mathit{n}}_{2\mathrm{D}}^{\mathit{i}}$=7.6×${10}^{11}$ ${\mathrm{cm}}^{\mathrm{-}2}$, where 2D denotes two dimensional. The ${\mathrm{\rho }}_{\mathit{x}\mathit{y}}$ versus B curve shows features of a slight parallel conduction.

Lateral piezoelectric fields exist in almost all strained III-V semiconductor quantum wells. We first discuss the origin of the lateral fields and point out their most important features. Then the controlled creation of lateral piezofields is demonstrated using a series of (110) InAs/GaAs quantum-wire structures. After verification of the structural perfection by high-resolution x-ray diffraction and transmission electron microscopy an optical study unambiguously demonstrates the existence of lateral fields. By increasing the excitation density in photoluminescence experiments we observe pronounced blueshifts of the luminescence lines simultaneously with a clear reduction of the linewidths. Additional evidence comes from time-resolved measurements where a strong dependence of the radiative lifetime on energy is observed. These results are supported by a study of strained structures with other orientations. In (211), (311), and (210) samples we find evidence of lateral piezofields created by interface fluctuations. Our results show that the impact of lateral piezoelectric fields has to be taken into account in any fundamental analysis of strained III-V semiconductor heterostructures. Furthermore, lateral piezoelectric fields represent a different concept to achieve artificial materials with strong optical nonlinearities.

Surface states of doped polymers are investigated by means of the Green-function technique. We consider only the particular case in which the surface is created right next to the impurity. It is shown that the surface state that resulted from cutting the double bond is qualitatively different from that created by cutting the single bond. In the former case, there is a midgap state and two localized states outside the energy bands, while in the latter case there are only two localized states, which may occur outside the energy bands or in the energy band gap depending on the sign of the impurity potential.

A Monte Carlo computer-simulation study of the growth of submonolayer coverages of Sb atoms on the GaAs(110) surface is reported. The Monte Carlo model incorporates explicitly the substrate-overlayer tetrahedral lattice structure and considers atom-atom interactions out to second-nearest-neighbor distances. The surface configuration-dependent dissociative chemisorption of molecular Sb is also included in the model. Atom-atom interaction energies employed in the simulation of diffusion events are obtained from the application of an empirical tight-binding total-energy calculational procedure to selected atom configurations, providing an internally consistent set of energy parameters. Simulated growth as a function of temperature in the range 300–500 K is discussed in detail for simulation parameters corresponding to typical experimental growth conditions. Details of the simulated growth may be understood by a consideration of the diffusion activation barriers involved at different stages of the growth. The model predicts a predominance of odd- over even-numbered-atom Sb chains at submonolayer coverages. Changes in the interface resulting from subsequent annealing after deposition of submonolayer Sb coverages are also briefly discussed.

We have calculated the Ge 3d core-level shifts on the Ge/Si(100)-(2×1) surface using the final-state pseudopotential theory. We find that the core levels of the up and down atoms within the asymmetric Ge dimer are separated by 0.54 eV at 1-ML Ge coverage, 0.43 eV at 2-ML Ge coverage, and 0.40 eV at the clean Ge(100) surface. Such a large core-level shift represents a substantial charge asymmetry within the Ge dimer. The present results agree well with recent x-ray photoemission spectroscopy (XPS) data on the Ge(100) surface, but disagree with XPS data on the adsorbed Ge/Si(100) surface.

Room-temperature Raman spectroscopy was used to characterize n-type-doped ${\mathrm{In}}_{0.53}$${\mathrm{Ga}}_{0.47}$As and ${\mathrm{In}}_{0.52}$${\mathrm{Al}}_{0.48}$As layers by observing coupled longitudinal-optical (LO) -phonon-plasmon mode spectra as a function of carrier density. Carrier densities determined from the high-frequency ${\mathit{L}}_{+}$ coupled mode shift in the Raman spectra were compared to carrier densities determined from electrochemical capacitance-voltage profiling measurements. In ${\mathrm{In}}_{0.53}$${\mathrm{Ga}}_{0.47}$As with carrier densities n≤8×${10}^{17}$ ${\mathrm{cm}}^{\mathrm{-}3}$ accurate determination of carrier densities from the Raman spectra was hindered by Landau damping and small ${\mathit{L}}_{+}$ mode dispersion. Better agreement between the two techniques was observed in ${\mathrm{In}}_{0.53}$${\mathrm{Ga}}_{0.47}$As and ${\mathrm{In}}_{0.52}$${\mathrm{Al}}_{0.48}$As for carrier densities of about 8×${10}^{17}$ ${\mathrm{cm}}^{\mathrm{-}3}$ and higher, doping levels at which Landau damping is less pronounced. Surface band bending was evaluated from changes in Raman intensity as a function of carrier density of dipole-allowed LO-phonon modes originating in the surface space-charge region. Values for the surface built-in potential determined from the Raman spectra were found to be smaller than those obtained from electrical measurements. The discrepancy is attributed to screening of the surface dipole by photogenerated carriers.

The orbital magnetic susceptibility of ballistic microstructures is considered within the independent-electron model. Using semiclassical theory, specifically Gutzwiller’s trace formula, the finite-size corrections to the Landau susceptibility are expressed in terms of the classical periodic orbits. It is found that these finite-size corrections can be much larger than the bulk susceptibility in the quantum-coherent regime. It is demonstrated that the orbital susceptibility is a sensitive probe of quantum chaos, having a larger amplitude in integrable than in completely chaotic ballistic microstructures. The approach is applied to the square billiard. While the predictions for the amplitude and the magnetic-field dependence are consistent with recent experimental results by Lévy et al., the theory predicts a faster decrease with temperature than observed experimentally.

High-resolution x-ray-diffraction experiments show that the lattice parameter of porous-silicon layers expands when wetted by an alcohol or an alkane. This phenomenon is nearly reversible when the alkane is removed while there is a time-dependent drift during alcohol wetting. The experimental results obtained for several alkanes and for two types of samples (p and ${\mathit{p}}^{+}$ type) reveal that the magnitude of the lattice-parameter change is correlated with the size of the nanocrystallites rather than with the nature of the alkane. We propose that the lattice expansion is due to a change of the porous-silicon surface stress induced by wetting.

Auger photoelectron coincidence spectroscopy has been used to study the intrinsic and extrinsic satellite contributions to the Ga ${\mathit{L}}_{23}$${\mathit{M}}_{45}$${\mathit{M}}_{45}$ Auger line of GaAs. Data have been collected for the Ga ${\mathit{L}}_{23}$${\mathit{M}}_{45}$${\mathit{M}}_{45}$ Auger line in coincidence with the 2${\mathit{p}}_{3/2}$ and 2${\mathit{p}}_{1/2}$ photoelectron lines from GaAs. The Ga 2${\mathit{p}}_{3/2}$ photoelectron line has also been collected in coincidence with the satellite on the lower kinetic-energy side of the Ga ${\mathit{L}}_{23}$${\mathit{M}}_{45}$${\mathit{M}}_{45}$ Auger line. The relative contributions of the intrinsic loss processes is shown to be negligible, as expected, whereas the extrinsic loss function is shown to be a significant consideration when using these lines in quantitative analysis. No evidence is seen for a difference in the response of the Ga ${\mathit{L}}_{23}$${\mathit{M}}_{45}$${\mathit{M}}_{45}$ final-state terms to the presence of a 3d hole at the surface and at the bulk as has been reported in the literature for the As ${\mathit{M}}_{45}$VV line.