Photoluminescence (PL) spectra of excitons bound to the isoelectronic defects ${\mathit{B}}_{71}^1$ (1.137 68 eV principal no-phonon line) created in phosphorus-doped silicon grown in a hydrogen atmosphere as a result of irradiation by thermal neutrons were investigated in magnetic fields up to 12 T and under uniaxial stress. The ${\mathit{C}}_{3\mathit{V}}$ symmetry of these defects was determined unambiguously from the dependences of the Zeeman splitting and the intensities of spectral components on magnetic-field orientation. The ground state of the bound exciton is split into a doublet with approximately 30 μeV energy separation. This splitting, which is not evident in the zero-field spectra because of the selection rules, results in the appearance of an additional spectral component in a magnetic field. Using group theoretical methods we constructed a Hamiltonian for excitons bound to the ${\mathit{B}}_{71}^1$ isoelectronic center, which takes into account electron-hole coupling and interaction with external perturbations. The phenomenological parameters of this Hamiltonian were determined from the optimal fit between theoretical and experimental dependences of the PL peak positions and their amplitudes on magnetic field and uniaxial stress. The proposed model of these bound excitons explains all of our experimental observations.

The cumulant-expansion and the thermodynamic-integration techniques, along with others, were considered for electrically neutral point-defect free energies of formation and migration. The thermodynamic integration method was found to be best. Free energies, enthalpies, and entropies of both formation and migration were evaluated as a function of temperature using the Tersoff3 potential-energy function for a variety of types of point defects. Equilibrium point-defect populations and diffusion coefficients have been evaluated as a function of temperature. Near the silicon melting temperature, Frenkel-defect formation was simulated to show that no activation barrier exists for point-defect recombination.

A small-angle x-ray-scattering (SAXS) and infrared-absorption study of microvoid dynamics and hydrogen evolution in a-Si:H is described. In films deposited on Al foil by rf sputtering, annealing at 350 °C and above sharply increased the integrated SAXS intensity Q, although the overall film density remained nearly constant. Incipient formation of microcrystalline Si domains by Si diffusing into the Al substrate is insufficient to account for the increased SAXS intensity. Rather, the close correlation between Q and the Si-bonded H content ${\mathit{C}}_{\mathrm{H}}$ suggests that it more likely results from migration of Si-bonded H to deep traps at the microvoid surfaces and its accumulation as ${\mathrm{H}}_2$ in the isolated voids. The density of the Si network then increases as its H content decreases, and the void sizes increase as hydrogen accumulates in them. These processes are discussed in relation to well-known characteristics of hydrogen diffusion in a-Si:H.

In contrast to the noncolumnar samples, the values of Q and ${\mathit{C}}_{\mathrm{H}}$ of the columnarlike film did not change during prolonged annealing (beyond 6 h) at 430 °C. This behavior is believed to result from migration of most of the Si-bonded H to the internal surfaces of that film, recombination to molecular ${\mathrm{H}}_2$, and its escape through the largely interconnected voids. Most of the remaining hydrogen was then probably bonded to the void surfaces in isolated sites which are not adjacent to a neighboring H atom. Annealing for 6 h at 430 °C also resulted in some crystallization sufficient to be detected by x-ray diffraction (XRD) and Raman scattering. Annealing for 18 h at that temperature sharply reduced the SAXS tilt-angle dependence of the columnarlike film. Finally, following 36 h at that temperature, the Raman and XRD spectra indicated that ∼50 vol. % of the film was still amorphous. Auger profiles yielded an Al concentration which increased from ∼2.2 at. % near the surface to ∼3.1 at. % near the substrate of that film. However, the sharp reduction of the tilt-angle dependence is probably not due to filling of the voids by the diffusing Al but rather to the relaxation and reduction of the columnarlike morphology.

We present experimental results obtained on a S-Cu-related metastable complex defect in Si, by optical detection of magnetic resonance (ODMR) at the X band and the K band. Two photoluminescence emissions arising from the bound-exciton (BE) recombination at the defect in two different configurations were monitored in the ODMR experiments. The spin-triplet nature of the lowest BE state for both BE’s was confirmed. The symmetry of each configuration has been determined to be monoclinic-I and triclinic, respectively. The unusually broad ODMR linewidth is argued to originate from unresolved hyperfine interaction with a copper atom involved in the defect, at which the primary bound particle (i.e., the hole) of the BE is highly localized. The configurational metastability has been demonstrated in the ODMR experiments.

X-ray-absorption near-edge-structure (XANES) spectra of the diluted-magnetic-semiconductor system ${\mathrm{Zn}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$S have been measured at the Zn ${\mathit{L}}_{\mathrm{I}\mathrm{I}\mathrm{I},\mathrm{I}\mathrm{I}}$ edge and of ${\mathrm{Zn}}_{1\mathrm{-}\mathit{x}}$${\mathit{M}}_{\mathit{x}}$S (M=Mn,Fe,Co) at the S K edge using the total-electron-yield mode. The Zn ${\mathit{L}}_{\mathrm{III}}$ edge results for ${\mathrm{Zn}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$S show a dramatic reduction in intensity of the first prominent feature, just above the edge, in the XANES spectra. Analysis of the S K-edge XANES spectra for ${\mathrm{Zn}}_{1\mathrm{-}\mathit{x}}$${\mathit{M}}_{\mathit{x}}$S revealed the appearance of a pre-edge peak, having linear x dependence in intensity and assigned to S 1s photoelectron excitations to an empty narrow band of S 3p–M 3d(${\mathit{t}}_2$) hybridized antibonding states. The rate of peak intensity with M concentration is associated with differences in the degree of p-d hybridization of states between the transition metal and sulfur. Features above the edge are generally found to decrease in intensity with an increase in transition-metal content in the alloys. The results are consistent with more S 3p–transition metal (M) 3d(${\mathit{t}}_2$) hybridized states and less S 3p–Zn 4(s,p) and S 3p–Zn 3d hybridized states becoming available as the transition-metal content increases in the alloys.

The influence of illumination on the thermal equilibrium temperature of the defect structure in undoped a-Si:H has been studied by measurements of the defect density versus temperature up to 290 °C. It was found that the light-induced excess carrier concentration leads to a decrease of the thermal equilibrium temperature depending on the carrier generation rate. This behavior is consistent qualitatively as well as quantitatively with a phenomenological model recently developed over a wide range of carrier generation rates.

Angle-resolved constant-initial-state spectroscopy from the valence-band maximum (VBM) as the initial state was used to determine the conduction-band energies at the Γ point up to 30 eV above the VBM for InSb. Structure in the spectra up to 20 eV could be assigned to particular interband transitions by comparison with empirical pseudopotential calculations. Autoionizing resonances due to surface core excitons have been observed. From their energies a surface-core-exciton binding energy of 0.5 eV for InSb has been determined. Both results are discussed in light of our previous work on the conduction-band states and surface core exditons of several III-V semiconductors.

Recent experiment and theoretical studies appear to show a great deal of complexity in the high-pressure behavior of tetrahedral semiconductors. We show that the structural stability of silicon and the order of its phase transitions is identical to that of a tetrahedrally coordinated network of rigid rods with spring-loaded joints. We further show how this parameter-free model resolves the recent controversy regarding the first high-pressure phase in silicon.

We propose a structural model of hydrogenated amorphous silicon carbide (a-SiC:H) at stoichiometric composition and low H content, based on the results of ab initio molecular-dynamics simulations. As found for a-SiC, the system has a complex structure which cannot be described as a distorted crystalline phase. Nevertheless, the degree of chemical order is larger in the hydrogenated network than in the pure network. H binds to both C and Si. H bound to C gives rise to ${\mathit{sp}}^3$ bonded monohydrated sites, thus favoring the formation of a tetrahedral network. H bound to Si forms diverse bonding configurations where weak and strong bonds can be identified. These results are in agreement with recent experiments. Furthermore, our findings for the evolution of Si-H and C-H bonding as a function of temperature are consistent with recent measurements of H desorption from Si-C alloys. Finally, we suggest an interpretation of x-ray and neutron-diffraction data based on analyses of several total distribution functions.

Generation of carriers in semiconductors by impact ionization is studied under the influence of a constant, arbibrarily high electric field. Using the density-matrix approach a system of equations for the coherent dynamics of electrons and holes in the presence of impact ionization and Auger recombination is derived, which extends the semiconductor Bloch equations by the inclusion of impact-ionization density-correlation functions as additional dynamic variables. From these equations we recover the pure (Zener) and the photon-induced (Franz-Keldysh) carrier tunneling rate and derive an expression for the field-assisted impact-ionization scattering rate. Different levels of approximation of the kinetic equations are discussed. It is shown that in contrast to the semiclassical treatment in the presence of an electric field, a fixed impact-ionization threshold does no longer exist, and the impact-ionization scattering rate is drastically enhanced around the semiclassical threshold by the intracollisional field effect. The close connection of field-assisted impact ionization to the Franz-Keldysh effect is emphasized.

The efficiency of two-phonon resonant Raman scattering in high magnetic fields is calculated for the intraband Fröhlich intraction of uncorrelated electrons and holes at the direct gap of a III-V or II-VI semiconductor with longitudinal optical (LO) phonons. We consider the case of different electron and hole effective masses, and take hole-mass anisotropy into account. Oscillations of the scattering efficiency with magnetic field and laser energy can be grouped into three types of resonances: incoming, outgoing, and intermediate ones, according to the respective real intermediate states at the bottom of the Landau bands. Calculated magneto-Raman intensity profiles and resonance fan plots are compared with experimental data for GaAs. Most of the observed resonances are classified.

Steady-state optical modulation spectroscopy (OMS) was carried out to obtain information on the gap states of p-type hydrogenated amorphous silicon (a-Si:H). Doping was achieved by adding diborane (${\mathrm{B}}_2$${\mathrm{H}}_6$) to the silane (${\mathrm{SiH}}_4$) gas in a conventional rf glow-discharge deposition. Measurements were performed at room temperature and at T≃20 K. The OMS spectra show clear differences with the spectra of undoped a-Si:H. These differences can be associated with the dopant states, lying in the valence-band tail, created by the boron doping. The numerical calculations, based on a model for the density-of-states distribution, take into account several possible optical transitions. The energy positions of the dopant states, ${\mathit{B}}_4^{\mathrm{-}}$ and ${\mathit{B}}_4^0$, and the doping-induced dangling-bond defects, ${\mathit{D}}_{\mathit{B}}^{+}$ and ${\mathit{D}}_{\mathit{B}}^0$, are determined. The dangling-bond states tend to shift further away from the valence-band edge with increasing doping concentration while the dopant states stay at the same energy position.

We have studied carrier-carrier scattering in photoexcited electron-hole plasmas in GaAs at plasma densities from ${10}^{15}$ to ${10}^{17}$ ${\mathrm{cm}}^{\mathrm{-}3}$, using numerical solution of the dynamically screened Boltzmann equation and classical molecular dynamics. The solution of the dynamically screened Boltzmann equation indicates that for excited electrons, scattering among the injected carriers is as important a scattering process as LO-phonon emission at densities greater than about 8×${10}^{15}$ ${\mathrm{cm}}^{\mathrm{-}3}$, and at ${10}^{17}$ ${\mathrm{cm}}^{\mathrm{-}3}$ the photoexcited electrons are nearly thermalized in 150 fsec. As a result of weaker screening, the interaction between carriers has a stronger effect in this case than when a low density of energetic electrons is immersed in a cool background plasma, where previous work has shown that carrier-carrier scattering becomes as significant as LO-phonon emission at a density of about 8×${10}^{16}$ ${\mathrm{cm}}^{\mathrm{-}3}$. We also find that classical molecular-dynamics calculations are dominated by nonphysical effects at short times, arising from the pointlike nature of the simulated carriers.

We have performed ab initio total-energy calculations of ground-state properties and migration paths of interstitial carbon in silicon. The ground state involves threefold-coordinated carbon and silicon atoms and its geometry suggests primarily p and sp bonding for carbon, rather than ${\mathit{sp}}^2$ one would naively expect. Examination of possible migration paths reveals that only three correspond to small ‘‘jumps’’ involving a single ‘‘bond breaking.’’ Of these, we predict that only one has a barrier of considerably lower energy (∼0.5 eV) and involves an intermediate ‘‘saddle-point’’ configuration of ${\mathit{C}}_2$ symmetry.

We study the effects of disorder on energy transport by confined optical-phonon modes in semiconductor superlattices. We investigate, in particular, whether it is possible to obtain Anderson localization of confined phonon modes by interface and barrier alloy disorder in superlattices like the GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As system. We develop a formalism for describing the transport of kinetic energy through the system and apply a diagrammatic perturbation theory that allows inclusion of disorder scattering. A simple model of the confined phonons shows that the strength of the disorder scattering is far too weak for localization to occur in realistic samples. We find, however, that a crossover from ballistic transport of energy by phonons to diffusive transport is expected as a function of increasing well width and alloy concentration. Possible implications for recent experiments are discussed.

The scanning tunneling microscope is used to study the boron-doped Si(111) surface as a function of annealing times and temperatures. The surface structure is found to be determined by the concentration of B. When the substitutional B concentration is less than 1% of the top 1×1 bilayer atoms, the surface is largely 7×7 but surrounded by adatom-covered 1×1 regions (which have higher B concentration). When the B concentration is more than 3%, the whole surface will be adatom-covered 1×1 regions including (√3 × √3 )R30° structures. The (√3 × √3 )R30° domains will increase with the B concentration. Because 7×7 can only exist in the region with low B concentration, the growth of 7×7 is slowed down. Further annealing at 560 °C can convert 2×2, c(4×2) into 7×7 and 9×9. Sides of the 7×7 domain preferentially grow along the three equivalent [112¯] directions. The adatom-covered 1×1 regions are bounded by faulted halves of the 7×7 domains. The dark sites of 7×7 are observed and counted. They are further interpreted in terms of a B substitution model. The pattern of bright and dark atoms in (√3 × √3 )R30° domains is analyzed and a criterion for a B stabilized Si-(√3 × √3 )R30° structure is obtained.

Radiative recombination from the ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As-GaAs heterostructure interface was investigated using photoluminescence and photoluminescence-excitation spectroscopy in modulation-doped and undoped samples. This emission is identified as H-band A, resulting from an indirect excitonlike transition in real space. The exciton is made up of a two-dimensional electron in the interface notch and a valence-band hole in the neutral region, having a binding energy of 1.8 meV. The H-band A exciton is directly excited by a free exciton making a vertical transition in real space. H-band A may be a distortion of the vertical direct free exciton in real space to an indirect free exciton in real space, in which case only one exciton is involved; or it could result from the direct excitation of the indirect exciton in real space by the vertical exciton through wave-function overlap, with energy and momentum being conserved. In the latter case two excitons are involved. While the exact coupling mechanism between the free exciton and the H-band A exciton is not well understood, two possibilities are suggested.

We report highly resolved photoluminescence excitation spectra of excitons in intrinsic pseudomorphic ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As/GaAs quantum wells (QW’s). The QW parameters are chosen such that only the ${\mathit{n}}_{\mathit{z}}$=1 electron and hole subbands are confined and the light-hole–heavy-hole exciton splitting exceeds the exciton binding energy. By using circularly polarized light the fine structure of excitonic transitions is well resolved in the spectra of quantum wells at magnetic fields H≤8 T. We observe a strong mixing of light- and heavy-hole excitons which causes optical transitions into high-angular-momentum exciton states and results in strong anticrossing effects. Two excited states of the exciton located at ≊1 and 2.5 meV above the 1s light-hole exciton state have been observed and are related to exciton states with the carrier-wave-function delocalized into the GaAs barriers.

In this work, optical intersubband transitions in conduction-band quantum wells have been reexamined in the multiband scheme. A generalized theory, with emphasis on the selection rules for the in-plane polarization, is developed in Kane’s k⋅P formalism. By taking into account the interband couplings, it is shown that the optical transition between any pair of electron subbands can occur with either the normal-to-plane polarized light or the in-plane polarized light. The characteristics of intersubband transitions depend upon whether the subband index differences Δn are odd integers (Δn=1,3,. . .) or even integers (Δn=2,4,. . .). In the case of Δn being even integers, intersubband transitions of both polarizations are allowed for electrons with a finite in-plane wave vector. In fact, the transition rates are proportional to the value of the in-plane wave vector, and the in-plane polarized transition is dominant. In the case of Δn being an odd integer, optical intersubband transitions of both polarizations can occur with a zero in-plane wave vector, but the normal-to-plane polarized transition is dominant. Consequences for device implementation such as a normal-incidence infrared photodetector that makes use of the allowed in-plane polarized optical transitions are discussed.

Using the x-ray standing-wave method, we have measured directly the thermal vibration amplitudes 〈${\mathit{u}}^2$〉 of symmetric As dimers on a Si(001)-(2×1) surface. For sample temperatures in the range 300 K ≤T≤650 K, the results are Debye-like. Above 650 K 〈${\mathit{u}}^2$〉 varies more rapidly, indicating the onset of defect-mediated processes. We also found that at room temperature the bond length of the As dimers is 2.58±0.04 Å and that, independent of temperature, they sit 1.40±0.01 Å above the top bulk-extrapolated silicon (004) plane. These results provide a critical test for theoretical structure calculations.