Conduction Band Transitions Research Articles

Excited-state absorption spectra and gain measurements of CaF_2:Sm^2+

We have measured the pump-probe spectrum for CaF2:Sm2+ over a wide (360–1020 nm) spectral range. We observed gain throughout most of the 730-nm emission band, although there is some evidence of excited-state absorption (ESA) in this region. A much stronger ESA was observed near 500 nm. The temperature dependence of the ESA signal clearly indicates that the absorption is associated with divalent samarium. Because of the width and strength of this transition, we assign the ESA to a 4f55d → conduction band transition. We calculated the oscillator strength for this ESA band to be 0.10 and rationalized its magnitude based on an intensity-borrowing mechanism that involves the transition from the valence band to the conduction band.

Resonant Raman effect and Fano distortion in the stage-2 graphite donor intercalation compound C/Rb.

Raman scattering of the C-C vibration with ${\mathit{E}}_{2\mathit{g}2}$ symmetry in stage-2 C/Rb has been studied with different incident photon energies. Resonant enhancement occurs for laser energies in the blue region, close to the threshold for the valence- to conduction-band electronic transition ${\mathrm{\ensuremath{\Omega}}}_{\mathit{T}}$. The results were fitted with a two-band resonance enhancement model and the fitting procedure yields ${\mathrm{\ensuremath{\Omega}}}_{\mathit{T}}$=2.6\ifmmode\pm\else\textpm\fi{}0.15 eV, with a broadening of 0.13 eV. In the red spectral region, the Raman line exhibits a Fano-type distortion, indicating an interaction between the discrete one-phonon excited state, and a continuum which is attributed to electronic excitations.

Novel use of resonant tunneling structures for optical and IR modulators

The basic concepts and some preliminary calculations are presented showing the feasibility of high speed optical and infrared modulators based on resonant tunneling structures. A modulator based on a conventional resonant tunneling structure can be operated in the infrared region by using the intersubband transitions and in the optical region by using the valence band to conduction band transitions in the quantum well of the double barrier structure. A novel resonant tunneling structure is proposed as a modulator in which the quantum well conduction band at zero bias is below the collector layer conduction band. This deep well resonant tunneling structure can allow intersubband transitions with higher energy than its conventional counterpart and with a careful design may even result in population inversion and gain. The band-to-band transitions are also possible in the deep well resonant tunneling modulator. Due to the inherent negative resistance of the device which persists to ultrahigh frequencies, it is possible to operate the device as a self-oscillating modulator.

Bistability of magnetic polarons bound to acceptors in a wurtzite semimagnetic semiconductor

The bistable properties of magnetic polarons bound to acceptor impurities (${A}^{0}$-BMP's) in ${\mathrm{Cd}}_{0.95}$${\mathrm{Mn}}_{0.05}$Se have been studied by time-resolved spectroscopy of D-A pair luminescence under two different conditions: (1) action of a magnetic field, in the Faraday configuration and above-band-gap optical excitation; (2) optical orientation of the magnetic moment of ${A}^{0}$-BMP's through a site-selection mechanism, by absorption of circularly polarized light tuned to the tail-of-acceptor--conduction-band transitions. The first experiments give clear indication of two different regimes: (a) At high temperature (T>5 K), the polarization rate ${P}_{c}$ of D-A luminescence increases with time delay and the evolution of ${P}_{c}$ is faster when T increases. This reflects the thermally activated orientation of ${A}^{0}$-BMP's by jumping over a potential barrier. (b) At low T (T<4 K) ${P}_{c}$ decreases with time delay, which reflects the recombination of holes with spin-polarized electrons on donors. In the second kind of experiment, the time evolution of ${P}_{c}$ is not influenced by spin-dependent recombination and is related only to the thermal relaxation of ${A}^{0}$-BMP orientation. At 1.7 K, ${P}_{c}$ is constant for at least 5 \ensuremath{\mu}s, which shows that ${A}^{0}$-BMP's are frozen. As T increases, the time-integrated value of ${P}_{c}$ decreases because of a progressive unfreezing of the polarons. Both experiments are consistent with a wide distribution of activation energies, presumably related to alloying effects. The distribution is centered around 10 meV, as predicted by a model of anisotropic ${A}^{0}$-BMP's proposed by Bhattacharjee [Phys. Rev. B 35, 9108 (1987)].

Light hole interband transitions in HgTe-HgCdTe superlattices

The light hole to conduction band optical transition has been identified in the room-temperature absorption spectra of several high quality HgTe-HgCdTe superlattices, in addition to the familiar heavy hole to conduction band transitions. The observation of the light hole transition, coupled with a more accurate determination of the superlattice layer thicknesses, allows the superlattice band gap and the HgTe-HgCdTe valence band offset to be determined more precisely than previously possible. A two-band tight-binding model was used to calculate the transition energies to compare with the optical data. The valence band offset for the HgTe-Hg0.15 Cd0.85 Te interface was determined to be 300±25 meV.

Growth and characterization of CuInSe 2 films by close-space evaporation

CuInSe 2 thin films were prepared on mica substrates using an evaporation technique which involved varying the distance between the source and the substrate. Films evaporated on substrates at distances less than 1 cm from the source exhibited preferential orientation in the (112) direction. The grain size increased from 0.5 to 1.0 μ m as the source substrate distance decreased from 3 to 1 cm. The conductivity increased with a decrease in the distance between the source and substrate. Electron probe microanalysis of these films showed that films coated on substrates farther away from the source had a slight selenium deficiency and an excess of indium. Three characteristic energy gaps of 1.01, 1.25 and 2.4 eV were obtained from an analysis of the optical absorption spectrum. The optical transition probability for valence band to conduction band transitions was estimated to be 10.91 eV which gives an admixture of copper d states to the valence band of 32%.

Femtosecond hot carrier energy redistribution in GaAs and AlGaAs

Excited carrier dynamics in GaAs and Al .2Ga .8As are investigated using femtosecond pump and continuum probe techniques. Absorption saturation measurements provide evidence for transient spectral hole burning due to split-off as well as heavy and light hole valence to conduction band transitions. The initial nonthermal carrier distribution thermalizes and assumes a broad energy distribution on a femtosecond time scale.

Resonant Raman scattering in stage-1 graphite acceptor intercalated compounds: C-AsF5.

A Raman scattering cross section for stage-1 graphite acceptor intercalated compounds is calculated within a two-dimensional tight-binding electronic band model. It is shown that the two-band resonant Raman scattering process is much more efficient than the possible three-band process. The experimental results on stage-1 C-${\mathrm{AsF}}_{5}$ are fitted to the two-band model and the fitting procedure yields the threshold for the valence- to conduction-band transition ${\ensuremath{\omega}}_{T}$=2.45 eV, and the broadening constant \ensuremath{\Gamma}=0.02 eV.

Variational calculation of polarization of quantum-well photoluminescence.

We calculate for the first time the circular polarization of GaAs/${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Al}}_{\mathrm{x}}$As quantum-well photoluminescence taking into account the true nonparabolic valence-band structure. Versus the excitation energy, we find high positive polarization for the transition n=1 heavy-hole--to--conduction-band transition and negative polarization at the onset of n=1 light-hole--to--conduction-band transition in qualitative agreement with experiments. We also study the ${\mathrm{Ga}}_{\mathrm{y}}$${\mathrm{In}}_{1\mathrm{\ensuremath{-}}\mathrm{y}}$As/InP system and predict no negative polarization in this case.

Optical absorption due to a soliton lattice in trans-polyacetylene

The optical absorption due to a charged soliton lattice in trans -(CH) x is calculated using the exact electronic wavefunctions and band structure. It is shown that the matrix element for transitions from the valence band to the conduction band vanishes; this is due to the order parameter changing sign after half the soliton-lattice period. The optical conductivity is evaluated numerically for a soliton lattice constant of 30 trans -(CH) x lattice units, corresponding to the first ordered phase for the case of Na doping. The resulting absorption consists of two sharp peaks, a singular one due to transitions between the edges of the soliton and conduction bands and a second one due to transitions from the center of the soliton band to an almost parallel region in wave-vector space of the conduction band. We show that the sum rule is satisfied despite the suppression of valence-band to conduction-band transitions. The results account for the observation that the valence-band to conduction-band transition does not go to higher energy with doping. Although we have not done detailed calculations taking into acount the many effects that remove the singularities and smear out the peaks in the actual sample, our theory can account for the very broad midgap absorption observed even at the lowest doping concentrations.

Optical absorption of a soliton lattice and applications to trans-polyacetylene.

X-ray measurements and electrochemical data have established that doping of trans-polyacetylene with more than \ensuremath{\sim}0.5 at. % of an alkali metal results in ordered phases. On the evidence of very low spin susceptibility up to \ensuremath{\sim}6 at. %-Na doping, the ordered phase is a regular array of solitons, i.e., a soliton lattice. We have calculated the optical conductivity of such a soliton lattice employing the correct electronic band structure and wave functions. In contrast to earlier calculations we find that the matrix element vanishes for transitions to the conduction band from the valence band and the lower half of the soliton band; the vanishing is due to the order parameter changing sign after half a soliton-lattice period. The integrals for the optical absorption due to transitions from the upper half of the soliton band to the conduction band were evaluated numerically for the impurity concentration, corresponding to the ordered phase for Na doping, 6.67 at. %, or a soliton-lattice period of 15 trans-polyacetylene lattice constants. The resulting absorption consists of two sharp peaks, a singular one due to transitions between the edges of the soliton and conduction bands and a second due to transitions from the center of the soliton band to an almost parallel region in wave-vector space of the conduction band. We show that the sum rule is satisfied despite the suppression of the valence-band to conduction-band transitions. To compare with experimental data it is necessary to add the contributions of the highly doped ordered portion of the sample to the essentially undoped portion. The results account for the observation that the valence-band to conduction-band transition does not go to higher energy with doping, as predicted by earlier theories. Although we have not done detailed calculations taking into account the many effects that remove the singularities and smear out the absorption for an actual sample, it is clear that our theory can account for the very broad midgap absorption observed at even the lowest doping concentrations.

Valence band structure of CuGa(S1−zSez)2 alloys

Measurements of wavelength-modulated reflectance have been made in the temperature range 20–300 K on single-crystal samples of the alloy system CuGa(S1−zSez)2 and values of the valence to conduction band transition energies EA, EB and EC determined as a function of temperature. It has been shown that if the quasi-cubic model is used in the analysis of these data to determine values of spin-orbit and crystal-field splittings and their variation with temperature, the results are inconsistent with those from X-ray diffraction measurements of lattice parameters and tetragonal distortion. This is because the quasi-cubic model does not properly allow for the fractional d-character of the valence bands of these materials. The results have therefore been analyzed in terms of a recently developed theoretical model which takes into account the effects of p–d hybridization on both spin-orbit and crystal-field splittings. The analysis gives values of a dimensionless parameter M/E which is a measure of the interaction between the p and d states forming the valence bands and which determines the fractional d-like character [Formula: see text] of those bands. The variations of M/E and [Formula: see text] with both temperature and composition are considered in some detail.

Valence band structure of some chalcopyrite compounds

It is shown that if the quasi-cubic model is used in the analysis of the variation of the valence to conduction band transition energies as a function of temperature, the values of spin-orbit and crystal-field splittings and their temperature variations so determined are inconsistent with those from X-ray diffraction measurements of lattice parameters and tetragonal distortion. Therefore the previously published results for five compounds have now been analyzed in terms of a recently developed theoretical model which takes into account the effects of p-d hybridization on both spin-orbit and crystal-field splittings. The analysis gives values of a dimensionless parameter M/E which is a measure of the interaction between the p and d states forming the valence bands and which determines the fractional d-like character (1 − α ) of those bands. The variations of M/E and (1 − α ) with temperature have been determined for each compound considered.

Acceptor spectra of AlxGa1-xAs-GaAs quantum wells in external fields: Electric, magnetic, and uniaxial stress.

We have used the variational method to calculate the acceptor binding energies in GaAs-${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As quantum wells with and without the application of electric, magnetic, and uniaxial stress fields. The calculation includes the coupling of the top four valence bands of both materials in the multiband effective-mass approximation. To ensure the convergence of the calculation, a large number of basis functions which are made up of the s-like, p-like, or d-like spatial states multiplied by j=(3/2) spinors are used for the expansion of the acceptor wave function. Because the quantum well and external field potentials reduce the point-group symmetry from ${T}_{d}$ to ${D}_{2d}$, the bulk ${\ensuremath{\Gamma}}_{8}$ acceptor ground state splits into ${\ensuremath{\Gamma}}_{6}$ and ${\ensuremath{\Gamma}}_{7}$ states. The ${\ensuremath{\Gamma}}_{6}$ state is predominantly derived from the heavy-hole subband and the ${\ensuremath{\Gamma}}_{7}$ state is predominantly derived from the light-hole subband. The magnetic field further splits the ${\ensuremath{\Gamma}}_{6}$ and ${\ensuremath{\Gamma}}_{7}$ states. We have calculated the energies of the ${\ensuremath{\Gamma}}_{6}$ state and the ${\ensuremath{\Gamma}}_{7}$ state for both center doped and edge doped quantum wells as well as the ${\ensuremath{\Gamma}}_{6}$ and ${\ensuremath{\Gamma}}_{7}$ valence-subband edges for various barrier heights as functions of well width. In recent studies the photoluminescence resulting from the acceptor levels to conduction-band transition in molecular-beam-epitaxy-grown GaAs-${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As superlattices have been measured. Our theoretical results are in excellent agreement with these experimental data.

Spin glass phase transition in Hg 1− kMn kTe semimagnetic semiconductors

Low field interband Faraday rotation and a.c. magnetic susceptibility are investigated in Hg 1− k Mn k Te and Hg 1− k− x Mn k Te semimagnetic semiconductors ( k⩽0.5). The spin glass phase transition is experimentally manifested by characteristic cusps in χ( T) and kinks in θ( T). The transition temperatures obtained from both types of experiments are well correlated which enable us to determine the phase diagram for Hg 1− k Mn k Te system. The spin glass phase transition occurs at low temperature in quaternary alloys than in Hg 1− k Mn k Te alloys with identical Mn composition. This implies that the antiferromagnetic interaction between Mn 2+ ions due to the virtual valence to conduction band transitions plays a significant role in the mechanism responsible for the spin glass formation.

Optical Properties of CuInSe2 Thin Films Prepared by R.F. Sputtering

AbstractThe optical absorption of r.f. sputtered CuInSe2 thin films was studied in the photon energy range from 1 to 3 eV. The gap energy and the spin‐orbit splitting are found to be (1.01 ± 0.01) eV and (0.24 ± 0.02) eV, respectively. From the photon energy dependence of the absorption coefficient it is concluded that the heavy and light hole bands are parabolic whilst the split‐off band contains terms linear in the wavevector. The optical transition probability for valence band‐ to‐ conduction band transitions is estimated to be (10.8 ± 1.0) eV which yields an admixture of copper d states to the valence band of (30 ± 8) %.

Non‐Γ Deep Levels and the Conduction Band Structure of Ga1−xAlxAs Alloys

AbstractThe electrical properties of the Ga1−xAlxAs alloys are strongly controlled by the emergence of deep levels in crystals which have x values near to the direct—indirect conduction band transition composition. The activation energy of the deep levels and the energy of the L conduction band minima with respect to the lowest energy Γ minimum (0.19 ≦ x ≦ 0.43) and × minima (0.43 < 〈x ≦ 0.61), are determined from the temperature dependence (77 K ⪅ T ⪅ 750 K) of the Hall electron concentration on high‐purity crystals.It is shown that the lowest energy indirect minima is L in Ga1−xAlxAs for alloy compositions in the range 0.19 ≦ x ≦ 0.37. The variation in the energy separation ΔEΓL at 300 K between the Γ and L minima is best described by the equation ΔEΓL = (0.285 — 0.595x) eV for 0.19 ≦ x ≦ 0.47. The Γ—L cross‐over composition is, thus, determined to be x = 0.47. For x ⪆ 0.25, a deep level dominates the electrical characteristics of the crystals. The activation energy of the deep level with respect to the Γ minimum increases with x, being (0.170 ± 0.005) eV at x = 0.44, but with respect to the L minima remains almost constant with a value of ≤0.200 ± 0.005≤ eV for 0.19 ⪅ x = 0.44. For x > 0.44, the energy of this level below the × minima decreases monotonically reaching a value of (0.106 ± 0.005) eV at x = 0.78. Heavily compensated shallow donor levels are also present in the alloys. Similar measurements on samples with compositions in the range 0.23 ≦ x ≦ 0.32 and under hydrostatic pressure have shown that the deep level is mainly associated with the indirect minima L and × although the Γ minimum is the lowest in energy.

Time-resolved pulsed DC electroluminescence studies in ZnS:Mn, Cu-powder Phosphors

Time-resolved studies of pulsed dc electroluminescence from ZnS:Mn, Cu-powder phosphors are reported. Emission bands from the orange Mn <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> transition and a very broad band extending from 1.5 to 3.6 eV are detected. The broad band, attributed to higher conduction band transitions of hot electrons, is used as a probe of avalanching in the devices. Study of the onset of the Mn <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> luminescence provides evidence for direct impact excitation of Mn <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> ions. Current densities much higher than expected for uniform current flow are deduced from the rate of increase of luminescence rise transients.

Theory of Core Excitons in Semiconductors

AbstractCore exciton binding energies are calculated for the Ga 3d‐X6 conduction band transition inGa‐V compounds and the Si 2p‐Δ6 transition in Si. Various corrections to the Wannier‐Mott theory are considered. Realistic core hole charge distributions, q‐ and ω‐dependent dielectric screening and an‐isotropic electron effective masses are taken into account. In the case of Ga‐V compounds, binding energies from the calculation are considerably larger than the Wannier‐Mott values, and close to experimental data. Smaller changes result for Si. The large discrepancies between theory and experiment cannot be resolved in this case.

Preparation and Optical Properties of ZnIn2Se4 Crystals

Absorption edge, photoresponse, and reflectance measurements have been carried out on ZnIn2Se4 crystals grown by means of chemical transport reactions. Indirect and direct energy gaps were determined as Egi=1.74 eV and Egd=1.92 eV at LNT. Reflectance measurements have been done in the 1.5–25 eV energy range. The Kramers-Kronig analysis has been performed and optical constants of studied semiconductor have been claculated. Reflectance data (R) and calculated imaginary part (ε2) of dielectric function allowed to make some conclusions about interband transitions. Singularities in R, ε2 and valence electron concentration per atom (neff) were basis for conclusion that d band–conduction band transitions at energy about 12.5 eV have been observed.