We calculate the coherent electronic contributions to the third-order optical response ${\mathrm{\chi }}^{\mathrm{}\left(3\right)\mathrm{}}$(-ω;ω,Ω,-Ω) of bulk semiconductors in the independent-particle approximation using a simple two-band model. The formalism used to derive this response coefficient naturally accounts for all relevant contributions and, in contrast to existing results in the literature, leads to physically realistic, nondivergent expressions in the limits ω,Ω→0. Such well behaved infrared limits imply that the imaginary part of our ${\mathrm{\chi }}^{\mathrm{}\left(3\right)\mathrm{}}$ correctly describes the dispersion of nondegenerate absorption; indeed for Ω=0 our results are consistent with predictions from Franz-Keldysh theory. Complementing these results, we can now also unambiguously extract from the real part of ${\mathrm{\chi }}^{\mathrm{}\left(3\right)\mathrm{}}$ the below band gap, two-band model predictions for the nonlinear refractive index, the dc Kerr effect, and the virtual photoconductivity; all of these predict a finite, real ${\mathrm{\chi }}^{\mathrm{}\left(3\right)\mathrm{}}$(0;0,0,0) as physically expected for clean, cold semiconductors. Finally, our specific results help expose more general consequences of the gauge choice when employing common approximate band-structure models.

We study the bonding character, electronic structure, and defects of amorphous Si through first-principles molecular-dynamics simulations. We generate the disordered Si network from the quenching of incompletely melted samples and find more dangling bonds than floating bonds, while in previous liquid-quench simulations more floating-bond defects were generated. The structural parameters such as the pair-correlation and bond-angle distribution functions are in good agreement with experimental data. The electronic structures of the dangling and floating bonds in the amorphous network are examined by analyzing the angular momentum decomposition of the local density of states. The localized states in the gap are found to be mostly p-like at a threefold coordinated site, consistent with the picture that paramagnetic active defects in amorphous Si are due to dangling bonds.

We develop a tight-binding molecular-dynamics model for Si-H systems that incorporates relevant physics of charge-transfer and local-environment dependence of atomic interactions. Our model was fitted to silane, and yields electronic levels of disilane in good agreement with experiment and vibrational frequencies that are consistent with the experimental trend for ${\mathrm{SiH}}_{\mathit{n}}$ groups. The model describes well the formation energies and energy surfaces for the different charge states of H in c-Si, including the stability of the bond-centered site for positive and neutral charge states and the stability of the tetrahedral site for negative charge states. The model also describes well the structural and electronic properties of a-Si:H models. The present approach utilizes quantum-mechanical forces, incorporates important electronic effects, and is suitable for studying complex phenomena such as H diffusion and dynamics of c-Si or a-Si:H.

Band-edge luminescence in photoexcited CdSe is known to show several interesting features due to radiative decay of free and bound excitons and excitonic molecules (biexcitons). In this paper, we report our experiments on the study of the time evolution of some of these processes in CdSe, using up-conversion luminescence spectroscopy with a time resolution of 2.5 ps. We obtain time-dependent luminescence spectra at two e$hyh pair excitation densities (estimated to be ${\mathit{n}}_0$=8×${10}^{16}$ and 8×${10}^{17}$ ${\mathrm{cm}}^{\mathrm{-}3}$). The role of hot biexciton dynamics in determining the time evolution of luminescence is evident. At low density (${\mathit{n}}_0$=8×${10}^{16}$ ${\mathrm{cm}}^{\mathrm{-}3}$), the luminescence spectrum also reveals the time evolution of the bound exciton emission at long delays (>200 ps). In addition, we obtain time-resolved luminescence of the longitudinal-optical phonon-assisted Stokes sidebands of the free exciton. The exciton and biexciton lifetimes are deduced to be 600 and 10 ps, respectively, on the basis of a theoretical model describing their coupled dynamics.

Band-gap fluctuations due to precipitates in semiconducting solid solutions are examined theoretically. The precipitates are shown to produce long-range Coulomb-like changes in the solute concentration and, thus, modulate the band gap. The Coulombic nature of the effective field due to an individual precipitate makes the problem similar to the well known problem of band-gap fluctuations caused by charged impurities in a doped semiconductor. In particular, small statistical fluctuations in the precipitate’s concentration lead to large-scale band-gap fluctuations, which are shown to win over the standard Gaussian fluctuations even though the relative amount of matter in the precipitates is small. The implications of these findings for low-energy absorption tails in solid solutions are considered.

The electrical and optical properties of Si/${\mathrm{Si}}_{0.8}$${\mathrm{Ge}}_{0.2}$/Si p-type modulation-doped heterostructures grown on (001) Si using low-pressure chemical-vapor deposition are investigated by a variety of techniques. The thickness of the ${\mathrm{Si}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Ge}}_{\mathit{x}}$ quantum well was about 15 nm and the modulation-doping effect has been obtained by two remote boron-doped ∼10-nm-thick Si layers. We found a mobility enhancement at low temperatures in all modulation-doped heterostructures. Clear Shubnikov–de Haas oscillations and quantum Hall plateaus confirm the presence of a two-dimensional hole gas. From the Hall-effect measurements a hole mobility of 6870 ${\mathrm{cm}}^2$/V s at a sheet-hole concentration of 4.5×${10}^{11}$ ${\mathrm{cm}}^{\mathrm{-}2}$ at 50 mK was obtained, which is comparable with the best published values for x=0.2. The experimental results were compared with self-consistent calculations of the valence-band diagram and hole concentration. The values for the effective mass used for the calculations were obtained by solving the 6×6 Luttinger-Kohn Hamiltonian. The photoluminescence from the ${\mathrm{Si}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Ge}}_{\mathit{x}}$ quantum well shows excitonic behavior according to its variation with excitation power. A general trend of increasing photoluminescence intensity with the mobility of the two-dimensional hole gas was observed. The high hole mobility and strong photoluminescence reflect the good interfacial quality of the Si/${\mathrm{Si}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Ge}}_{\mathit{x}}$/Si heterointerfaces grown by low-pressure chemical-vapor deposition.

The dynamics of the infrared photoluminescence (PL) in silicon powder grown by plasma-enhanced chemical-vapor deposition (PECVD) of silane is reported. A nonlinear dependence of PL intensity on laser power of the form I∝${\mathit{P}}^{\mathit{n}}$ with n as high as 7 has been found, indicating a multistep-multiphoton excitation process. To confirm this hypothesis a very detailed theoretical and experimental analysis has been performed. As a result, the lifetimes of several levels in the excitation chain have been determined, as well as the optical cross section (σ). For the slowest level σ=3×${10}^{\mathrm{-}18}$ ${\mathrm{cm}}^2$ and the lifetime is as long as 400 ms. As the energy of the emitted photon is smaller than that of the excitation photon, a model involving a considerable nonradiative energy relaxation, together with a tunnel effect is proposed.

High-resolution electron-energy-loss-spectroscopy (HREELS) measurements have been performed on Si(111) surfaces heavily p-doped by the decomposition of adsorbed decaborane with subsequent diffusion more than ∼1000 Å below the surface. After thermal decomposition of the decaborane to produce B atoms on the surface the low-energy-electron-diffraction pattern shows a √3 × √3 periodicity due to 1/3 ML of boron in the second complete layer. The HREELS data have two strong features: (1) the B-Si dipole vibrational mode at 96 meV and a broad electronic surface-plasmon mode at ∼100 meV loss energy due to the free carriers in the region below the B-reconstructed surface layer. We have investigated the energy dependence of the plasmon mode in order to determine the possibility of using HREELS to determine the depth profile of the free carriers due to B diffusion into the region ∼50–500 Å below the surface. Unexpectedly, we find that kinematic factors play an important role in the energy range used, 1.5–28 eV, and thus limit the degree of quantitative information that can be obtained about the carrier depth profile from HREELS data in this low-energy range. An approximate depth profile is deduced from the well-established three-layer model (vacuum-surface-bulk layers) after correcting the plasmon peak position for the kinematic factors.

The resistivity ρ and thermoelectric power S of the doped Kondo semiconductor (${\mathrm{Ce}}_{1\mathrm{-}\mathit{x}}$${\mathrm{La}}_{\mathit{x}}$${)}_3$${\mathrm{Bi}}_4$${\mathrm{Pt}}_3$ are examined to determine the effects of doping on the narrow gap exhibited by this compound. The transport data provide evidence that the energy gap progressively disappears with increasing concentration of La. The T=0 transport-energy gap, as determined from either ρ or S, scales with the single-impurity Kondo energy scale ${\mathit{T}}_{\mathit{K}}$, as estimated from magnetic-susceptibility measurements, independently of x for x≤0.25. This result suggests that the gap arises from band hybridization that is driven by Kondo-like correlations rather than from single-electronic interactions.

The temperature dependence of the photoluminescence (PL) intensity of strained ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As/GaAs single-quantum-well (QW) structures grown by metalorganic vapor-phase epitaxy is investigated. We adopt the theoretical model recently proposed by Vening, Dunstan, and Homewood [Phys. Rev. B 48, 2412 (1993)] to describe the variation of the QW PL intensity with temperature. The Arrhenius behavior of the PL intensity at the highest temperatures investigated for each structure is shown to be due to the thermally activated escape of electron-hole pairs from the well. In each case, the deviation of the experimental data at intermediate temperatures is accounted for by assuming the dominant nonradiative recombination process to be the escape of the less-confined carrier species from the QW. The existing model is modified to include this process. The activation energies obtained are in reasonable agreement with the calculated heavy-hole confinement energies assuming a conduction-band offset ratio of ${\mathit{Q}}_{\mathit{c}}$≊0.83.

Using magneto-optical absorption in ZnSe/${\mathrm{Zn}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Se superlattices, we have observed a series of transitions involving above-barrier states of the conduction and valence bands, at the zone center and at the zone edge of the superlattice Brillouin zone. The Mn concentration in ${\mathrm{Zn}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Se layers (x∼0.04) has been specifically chosen such that the bowing of the band gap with x and the strain in the superlattice together give rise to conduction- and valence-band offsets, which are nearly zero in the absence of a magnetic field. Owing to the large Zeeman splittings of band edges that occur in the ${\mathrm{Zn}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Se layers, the band alignment in these superlattices can be externally tuned by an applied magnetic field, providing a unique laboratory for investigating above-barrier subbands in a superlattice defined by very small periodic-potential variations (intermediate-dimensionality regime). The magnetic-field tuning, together with calculations based on an eight-band k⋅p model, allow us to unambiguously identify the transitions, and to compare the observed transition energies with theoretical predictions.

The electronic and structural properties of the Si/GaP(110) interface with various thicknesses of Si overlayers on a GaP(110) semi-infinite substrate have been studied by using the self-consistent semi- empirical tight-binding method with the charge neutrality condition. In addition, we present an approach to calculating the variation of band offsets with the number of adlayers during the Si/GaP(110) interface formation. In the calculation, the influence of surface and interface dipoles are taken into account. The calculation shows that the band offset exists with the first Si adlayer on the GaP(110) substrate and increases continuously with the number of Si overlayers. When reaching five or six Si layers, the band offset is close to the final value, 0.43 eV, of the Si/GaP(110) interface of two semi-infinite crystals.

Valence-band spectra from the sputtered and annealed InSb(1¯ 1¯ 1¯)3×3 surface in normal emission and in the Γ¯ M¯, Γ¯ M¯’, and Γ¯ K¯ azimuths have been measured. Some possible surface bands have been identified, among them one that clearly reflects the 3×3 periodicity of the surface. Core-level spectra of Sb 4d and In 4d were deconvoluted revealing one surface-shifted component for Sb 4d and, surprisingly, no pronounced surface-shifted component for In 4d. Scanning tunneling microscopy images from this surface taken at different biases are presented. It is argued that the surface imaged at high bias is perturbed by the tip.

The linear optical properties of (Si${)}_{\mathit{n}}$/(Ge${)}_{\mathit{m}}$ strained-layer superlattices (SLS’s) grown on ${\mathrm{Si}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Ge}}_{\mathit{x}}$(001) substrates are studied within the framework of the tight-binding approximation. A systematic study of the influence of periodicity (n+m), composition (n/m), and strain of the superlattice on the ${\mathit{E}}_1$-like and ${\mathit{E}}_2$-like transitions of the dielectric function is presented. The near-gap absorption coefficient is calculated and the contribution of the pseudodirect transitions is investigated. The dielectric function ${\mathrm{\varepsilon }}_2$(ω) of Si/Ge SLS’s is compared with the average crystal dielectric function ${\mathrm{\varepsilon }}_2^{\mathit{a}}$(ω), as well as to the mean value of the dielectric function of the constituent materials, ${\mathrm{\varepsilon }}_2^{\mathit{m}}$(ω). For light polarization parallel to the growth plane, the dielectric functions of the SLS is better described by the ${\mathrm{\varepsilon }}_2^{\mathit{m}}$(ω) spectrum, even for thin SLS’s. For polarization along the growth axis it is closer to the ${\mathrm{\varepsilon }}_2^{\mathit{a}}$(ω) spectrum and only for quite thick SLS’s do we observe Ge-like and Si-like features. These results are also confirmed with the use of a decomposition technique of the SLS wave function.

We show that a thin ${\mathrm{CaF}}_2$ intralayer at the Au/Si(111) interface increases the Schottky barrier height, whereas all known growth properties of this intralayer would predict an interface dipole in the opposite direction. This unexpected result was obtained with internal photoemission and ultraviolet photoemission spectroscopy. Internal photoemission gave a Schottky barrier without intralayer of 0.81±0.02 eV, in full agreement with other authors’ results and confirmed by ultraviolet photoemission. After inserting the ${\mathrm{CaF}}_2$ intralayer, we found with ultraviolet photoemission a barrier of 1.0±0.1 eV, and a consistent internal photoemission-photocurrent threshold at 0.98±0.02 eV.

Studies on the surface evolution during vacuum annealing of GaAs(001) have been performed using scanning tunneling microscopy. The transition from an ordered (2×4)/c(2×8) reconstructed surface to an ordered mixed phase (2×6)/(3×6) reconstruction is observed as the annealing temperature is increased from 450 to 500 °C. The transition to the high-temperature phase is mediated by a transient disordered state. We discuss the origin and role of this state. At higher annealing temperatures, T∼520 °C, in the (2×6)/(3×6) phase, we find that terrace edges become unstable, and the surface evolves to a morphology similar to those observed in general pattern-forming systems. A mechanism based on diffusion-limited growth of surface vacancies is discussed.

A persistent spectral hole-burning phenomenon in nanometer-size semiconductor microcrystals (nanocrystals), namely, CdSe, ${\mathrm{CdS}}_{0.59}$${\mathrm{Se}}_{0.41}$, and CuCl, embedded in a crystal or glass was observed. When the spectrally narrow laser excites the inhomogeneously broadened absorption band of CdSe and CuCl nanocrystals, a narrow bleaching hole and induced absorption arise in the absorption spectra at 2 K and are present for more than a few hours after the laser irradiation. Long-lived bleaching was observed also in ${\mathrm{CdS}}_{0.59}$${\mathrm{Se}}_{0.41}$ nanocrystals together with photodarkening. A photophysical model explains the long-lived bleaching ascribed to the spectral antihole fairly well, but some part of the persistent spectral hole burning comes from the photochemical mechanism. Nanocrystals consisting of ${10}^3$–${10}^4$ atoms behave like molecules or ions in a matrix to give the persistent spectral hole-burning phenomenon. This means that the total system consisting of even one semiconductor nanocrystal and the matrix has more than one ground-state configuration. Not only the size distribution but also these ground-state configurations give inhomogeneous broadening to semiconductor nanocrystals.

Resonant Raman scattering is performed between 1.6 and 3.0 eV on a series of strain-symmetrized (Si${)}_{\mathit{n}}$(Ge${)}_{\mathit{n}}$ superlattices with n=4–12 and a (Si${)}_4$(Ge${)}_{12}$ superlattice pseudomorphic to the Ge substrate. The dependence of the resonance energies and the degree of localization of states on single-layer thickness and strain are investigated. The results are compared with band-structure calculations and electroreflectance results on similar samples. We observe strong transitions confined within the Ge layers below 2.5 eV and transitions that are attributed to the intermixed interface regions, since they appear independently of layer thickness for all phonon modes around 2.6 eV. The shift of the dominant Ge-like transitions with layer thickness is much weaker than calculated for the localized ${\mathit{E}}_0$ superlattice transitions. These resonance maxima cannot be explained by bulk Ge-like ${\mathit{E}}_1$ transitions, since their energy is below ${\mathit{E}}_1$ of strained bulk Ge. Folded acoustic modes show a pronounced upward shift of the energy of the resonance maximum with increasing mode index. Forbidden confined ${\mathrm{LO}}_{\mathit{m}}$ modes with even index m were observed under resonance conditions in nonpolar superlattices.

The contactless microwave technique was used to measure light-induced transients in the power absorption by thin films of polycrystalline CdSe. Because the rise time of the detector was 500 ns, the analysis was limited 1 μsec or longer. Measurement of these transients at a number of fixed frequencies across the ‘‘dark’’ resonance frequency made reconstruction of the difference signal possible. This signal, which represents the difference between the ‘‘dark’’ and ‘‘light’’ Lorentz resonance curves, was determined at various times during the decay. Analysis of these signals provided the changes in the real and imaginary parts of the dielectric constant as a function of time. The decays of these parameters were characterized by the sum of two exponential terms: ${\mathrm{\tau }}_1$=16.9 μsec and ${\mathrm{\tau }}_2$=261.5 μsec for the change in the imaginary part, and ${\mathrm{\tau }}_1$=18.5 μsec and ${\mathrm{\tau }}_2$=283 μsec for the change in the real part. The close agreement of these values indicates the simultaneous presence of both trapped and free electrons having identical decay times. A mechanism consistent with these results involves rapid equilibration between the free electrons and those in the two shallow traps. Decay from each trap is the rate limiting step, and the time to reach thermodynamic equilibrium must be less than 500 ns.

The initial electron densities in the two traps were 2.1×${10}^{18}$ and 1.0×${10}^{18}$ ${\mathrm{cm}}^{\mathrm{-}3}$, and the corresponding concentrations of the free electrons were 3.6×${10}^{17}$ and 1.4×${10}^{17}$ ${\mathrm{cm}}^{\mathrm{-}3}$. For the free electrons, the values for the drift mobility were 2.1 and 1.8 ${\mathrm{cm}}^2$/V s, respectively. Using the harmonic-oscillator model for trapped electrons, the trap energies were 0.096 eV for the fast decay and 0.152 eV for the slower decay. The initial changes in a real part of the dielectric constant were 44.9 and 11.9, respectively. Thus, the photoinduced concentrations of electrons in naturally existing shallow traps are sufficient to effect large changes in the dielectric constant on a short time scale.

The carrier-emission processes from quantum wells and from deep-level defects have been identified in deep-level transient spectroscopy (DLTS) measurements. The emissions from quantum wells contribute to a majority carrier peak, from which the valence-band offset Δ${\mathit{E}}_{\mathit{v}}$ at the heterointerface is derived. For ${\mathrm{Si}}_{0.67}$${\mathrm{Ge}}_{0.33}$/Si, our experimental result Δ${\mathit{E}}_{\mathit{v}}$=0.24 eV is comparable with the theoretical prediction and previous measurement. The emission of carriers from a high density of interfacial defects gives rise to a minority carrier signal in DLTS, which can be detected only by using an injection pulse with a relatively large pulse width. The partial relaxation of the misfit strain or the nucleation of dislocations may be responsible for the formation of interfacial defects.