Direct Electronic Transitions Research Articles

Optical properties of Tm2S3 thin films

The reflection and absorption spectra of Tm2S3 single-phase, crystalline films prepared by vacuum-thermal evaporation from two independent sources were studied in the range 1.2–5.2 eV. From the frequency dependence of the absorption coefficient, it was found that indirect and direct interband electron transitions with the respective energies 1.2eV and 2.5 eV contribute to the formation of the absorption edge.

Elasto-optical study of stabilized cubic zirconia.

The piezo-optical coefficients ${\mathrm{\ensuremath{\pi}}}_{11\mathrm{\ensuremath{-}}}$${\mathrm{\ensuremath{\pi}}}_{12}$ and ${\mathrm{\ensuremath{\pi}}}_{44}$ of cubic yttria-stabilized zirconia are measured as a function of wavelength, temperature, and yttria concentration. The results show that the piezo-optical coefficients increase nonlinearly with temperature. The dispersion of the elasto-optical constants ${\mathit{n}}^{4}$${\mathit{p}}_{\mathit{i}\mathit{j}}$ for 20-wt % yttria composition is determined and attributed to a direct energy-gap electronic transition.

Electron escape and photoemission in the threshold region.

The formula giving the probability of electron escape is generalized to the case of electron excitation involving two conduction bands in the vicinity of the Brillouin-zone boundary when the gap between the bands is of critical importance. The escape probability in this case can be significantly larger than that found for excitation from the occupied states lying well within the first Brillouin zone. It is also shown that the Fowler law (commonly used to describe photoemission in the threshold region) is not restricted to the usually accepted assumption of indirect, momentum nonconserving, electron transitions but is a consequence of the linear dependence of the escape function on the electron energy near threshold and is valid for both direct and indirect electron transitions.

Evidence for Fano interference in spin polarization of the 6-eV satellite in Ni valence-band photoemission.

Spin-polarized photoemission spectra have been measured on the ferromagnetic Ni(110) single crystal. Evidence for the Fano interference effect in the spin polarization of the 6-eV satellite of the valence band has been obtained. The results indicate that the interference effect between the direct 3d electron transition and the 3p core electron excitation followed by the MVV super-Coster-Kronig process to form the two-d-hole bound state is important to explain the behavior of the resonant 6-eV satellite. It is also found that the spin polarization is still large under an off-resonant condition, and this indicates that the direct 3d transition from the majority-spin state is effective to form the two-hole singlet bound state.

Preparation and properties of Ge microcrystals in MgF 2 layers

Ge particles were successfully incorporated into MgF 2 films by the multi source PVD technique. The dependence of the optical band gap energy E g on Ge concentration in the MgF 2 matrix was investigated before and after annealing. In as-deposited films a strong increase of E g at low Ge concentrations points to the existence of a quantum-size effect. Annealing at 500°C yields enlarged Ge crystallites on a 10 nm scale connected with a decrease of E g and a split of the energy dependence of the absorption constant indicating the coexistence of non-direct and direct electronic transitions.

Thermal Stability of the Structural and Electronic Properties of Strain Symmetrized Sim Gen Superlattices and Ungraded, Nearly Relaxed Si1−x Gex

ABSTRACTIn this study, photoreflectance (PR) spectroscopy and x-ray rocking curves measurements were used to study the variation in strain configuration, defect propagation, structural properties and direct electronic transition energies in Sim Gen superlattices (SL) and nearly relaxed Si1−x Gex buffer layers grown on < 100 > Si as a function of annealing temperature. The in-plane (a│) and perpendicular (a┴) lattice constants of the alloy buffer layers are found to vary only slightly with anneal temperature, TA, up to a temperature To. For TA To, the in-plane strain changed from roughly zero a│ ≈ a┴ (relaxed) or a┴ > a│ (compressive) to a┴ > a│ (tensile). This change in strain configuration is believed to be caused by the difference in thermal expansion coefficients between the epilayer and the Si substrate. The anneal temperature T0 is also correlated with the disappearance of higher order x-ray harmonics from the SL. This point was also correlated with a large energy shift and broadening of the PR spectra from the SL. The shift in energy of the PR spectra is explained in terms of the interdiffusion of Si and Ge at SL heterointerfaces, and to a lesser degree, the strain induced by the above mentioned difference in thermal expansion coefficients. The PR spectra of the alloy E0 transitions are also observed to shift to higher energy with increasing TA.

Near-band gap transitions in the surface photovoltage spectra for GaAs, GaP and Si surfaces

The spectral surface photovoltage dependences of the clean GaAs(100) surface as well as the real GaP(100) and Si(111) surfaces have been investigated in the vicinity of the energy band-gaps. The spectral regions have been determined, where optical band-to-band direct (GaAs, GaP) and indirect (GaP, Si) electron transitions are dominant. From the graphical analysis the photothresholds of these processes have been estimated, consistent with the literature data.

Near-band gap transitions in the surface photovoltage spectra for GaAs, GaP and Si surfaces

Abstract The spectral surface photovoltage dependences of the clean GaAs(100) surface as well as the real GaP(100) and Si(111) surfaces have been investigated in the vicinity of the energy band-gaps. The spectral regions have been determined, where optical band-to-band direct (GaAs, GaP) and indirect (GaP, Si) electron transitions are dominant. From the graphical analysis the photothresholds of these processes have been estimated, consistent with the literature data.

Wavelength optimization and power dependence of optically controlled modulation in InGaAsP/InP

We investigate wavelength and power dependence of optically controlled transmission modulation based on dynamic band filling in InGaAsP (lambda(g) = 1.3 microm). A maximum modulation depth of 42% is observed at a 1280-nm wavelength for 5-mW control power. The modulation depth slightly decreases with increasing test power. A theoretical analysis based on carrier rate equations and direct electronic transitions between parabolic bands provides a good description of the experiments.

All-optical modulation at 1.3 ?m wavelength in the inGaAsP/InP system

All-optical, normal-to-surface modulation in InGaAsP epitaxial layers, lattice matched to InP, is investigated. Close to the gap wavelength of 1.3 μm a transmission increase under optical excitation is observed. A modulation depth of 34% is achieved for 0.8 mW pump power at 790 nm wavelength. The modulation frequency is limited by excess carrier lifetime. A 3 dB frequency of 80 MHz is achieved with a 10 dB decrease at 400 MHz. A lateral electric field enlarges the bandwidth but decreases the modulation depth. For weak excitation the experiments are well described in terms of direct electronic transitions between parabolic bands or in terms of simple band filling. The devices are well suited for parallel optical data processing.

Optically controlled transmission of InGaAsP epilayers

We investigate the transmission of InGaAsP epilayers lattice matched to InP substrates under optical excitation at normal incidence to the surface. Near the gap wavelength (lambda = 1.3 microm) we observe a transmission change of 20% for a 0.5-mW pump beam at a wavelength lambda(2) = 790 nm. Switching dynamics are shown to be limited by carrier lifetimes of tau approximately 2 nsec. A simple analysis based on parabolic bands and direct electronic transitions provides a good description of the experiments.

A fractional integral representation for interband dielectric function of critical point electrons in various dimensional crystals

The dielectric function for direct interband transitions of critical point electrons in an n-dimensional crystal has been derived as the n/2-th iterated integrals of the dielectric function of a harmonic oscillator. A generalized expression for the derivatives of interband dielectric function has also been presented by fractional integration. The results present an explicit physical picture and provide a new approach to the investigations of optical properties of low dimensional and anisotropic crystals.

Optical absorption and dangling bonds in damaged silicon

We have examined the optical absorption and spin-resonance signal characteristics of thin layers of silicon damaged by an abrasive process at room temperature. We find an Urbach-like dependence of the subgap absorption and a g=2.0055 isotropic spin-resonance signal characteristic of silicon dangling bonds. Changes in the optical and spin-resonance data caused by thermal anneals in air, or in atomic hydrogen, indicate that direct electronic transitions of the dangling bond defect are not responsible for the observed optical-absorption spectra. The doping level dependence of these spectra is consistent with the hypothesis that electronic transitions arising from disorder-induced band-tail states cause the subgap absorption.

ELECTROREFLECTANCE OF THIN ZINC SULFIDE FILMS GROWN BY ATOMIC LAYER EPITAXY AND ELECTRON BEAM EVAPORATION TECHNIQUES

We have measured electroreflectance spectra of polycrystalline ZnS:Mn thin films grown using atomic layer epitaxy and electron beam evaporation techniques for electroluminescent devices. From the Eo direct electron transition observed at 3.74 eV and 3.705 eV it is concluded that the ZnS thin films grown by atomic layer epitaxy are predominantly hexagonal whereas the crystal structure of the films grown by electron beam evaporation is close to polytype 4H or 6H.

Theory of the photon drag effect in tellurium

A detailed microscopic theory of the photon drag effect due to CO2 laser induced direct electronic intervalence band transitions in tellurium is presented based on the solution of Boltzmann’s transport and the resulting current equations. The numerical evaluation of the theoretically derived expression is shown to agree favorably with the reported experimental results.

Photon emission from rare gas ion bombardment of metal surfaces

Metal surfaces (Mg, Cu, Zr, Mo) are bombarded with He +, Ne + and Ar + in the energy range of 400 eV to 8 keV. Radiation from scattered projectiles and sputtered target particles is observed between 200 and 700 nm. It is shown that most of the radiating particles originate from surface collisions. Auger neutralization, resonance tunneling and direct electron transitions are the important electronic processes involved.

Numerical behaviour of transmission and derivative traces due to direct electronic transitions near van hove singularities

AbstractAnalytical forms of the complex dielectric function ε(ω) near each of the singular (Van Hove) critical points are given for the allowed and forbidden transitions at arbitrary light polarizations. By numerically simulating ε(ω) the characteristic traces of the transmission T(ω) and its derivatives dT/dω, d2T/dω2 have been obtained over a range of refractive index n, k values. From these it has been found possible to tabulate the possible Van Hove singularities corresponding to a given characteristic trace.

Population of the higher-lying vibrational levels of the nitrogen B3π state in a glow discharge

The absolute populations of the higher-lying vibrational levels of the B3π state of the nitrogen molecule in a glow discharge have been determined for pressures from 0.1 to 1.0 torr and current densities from 80 to 200 mA/cm2. The absolute concentrations of molecules at levels v′ = 5, 6, ..., 12 under these conditions are of the order of 109–1011 cm−3. The most probable processes leading to a population of the B3π state are thought to be direct electronic impact, cascade transitions, and collisions of normal molecules with molecules in the singlet state.

Intrinsic Optical Absorption of Gallium Phosphide between 2.33 and 3.12 eV

Single ribbons of gallium phosphide have been synthesized which are very thin (down to ∼0.35 μ) and yet possess relatively large cross-sectional areas with uniform thickness so that high-resolution optical-absorption measurements can be extended well above the lowest energy direct interband electronic transition (Γ15→Γ1). The Γ15→Γ1 exciton energy gap is 2.873 eV below 25°K, and the internal binding energy of the direct exciton is about 5 meV. Direct transitions involving hole states in the split-off valence band can be clearly seen at low temperatures, and the spin-orbit splitting is 82±1 meV. Weak structure corresponding to indirect transitions at the Γ15→X3 energy gap is also observed at low temperatures, and the separation of the X3 and X1 conduction-band minima is 0.29±0.01 eV, in excellent agreement with the value obtained from interconduction-band absorption spectra. The temperature coefficient of the Γ15→Γ1 direct energy gap is − (5.2±0.1)×10−4 eV(°K)−1 near 300°K, more than twice the value for the Γ15→X1 indirect gap.

Impurity-Sensitive Infrared Absorption in n-Type α-SiC

Optical measurements were made on several crystals of n-type α-SiC in the ranges 1 to 5µ (Range I)and 15 to 30µ (Range II) at 300°K and at 500°K. The absorption coefficient α is nearly proportional to the square of wavelength, increases with increasing donor concentration N D and decreases with the rise in temperature in Range I. These results can be explained by the direct transition of electrons from donor levels to conduction band rather than the intraband transitions of the free carriers. The relative magnitudes of α in Ranges I and II support the above conclusion. In Range II, the value of α increases with the increase in N D and depends weakly upon λ; the electrical susceptibility has a positive sign. The direct transition still seems to contribute appreciably in Range II.