Polarization-modulated magnetoreflection in magnetic semiconductors: CdCr2S4 and EuSe

The polarization-modulated (PM) magnetoreflection technique has been used to study magnetic ordering effects in CdCr₂S₄ and EuSe. The energy gap for direct band-band transitions in CdCr₂S₄ has been measured to be E_G = 2.3 eV, and the exchange splitting of the valence band to be about 0.03 eV. Good agreement with thin film absorption measurements is obtained in the temperature dependence of spectral structure, observed at energies less than E_G, associated with crystal field and charge transfer transitions. No strongly blue-shifting peak is observed with magnetic order. PM magnetoreflection spectra of the E₁ peak of EuSe show a direct manifestation of the spin alignment from the ferrimagnetic to ferromagnetic state of this crystal at 2 K in external fields up to 16 kOe. An attempt is made to explain resonant Raman scattering in EuSe (observed by other workers) in terms of the field-induced shift of one of the polarized E₁ reflectivity components into coincidence with the 5145 Å argon-ion laser line.     

Infrared modulated reflectance spectra of n and p type gallium antimonide and n type indium antimonide

We have observed the modulated reflectance spectra of n and p type GaSb at 300, 80, and 5 K from 0.56 to 2 eV. The modulated reflectance of intrinsic n type InSb was measured at 80 K from 0.2 to 2 eV. The “dry sandwich” vapor deposition technique was used to make the electroreflectance (ER) samples. The low-temperature spectrum of the undoped p type GaSb sample shows three peaks at the band edge that could be associated with transitions from the top of the valence band, the light (0.903 eV) and heavy (1.014eV) hole state Fermi levels to the conduction band. The energies of the observed peaks are in agreement with the Fermi level determination from Hall effect and Faraday rotation measurements. This modulation mechanism is based on band population effects. The ER signal of InSb under flatband condition at 80 K has five half oscillations at the direct band gap. The contribution of piezoelectric strain to ER is present since the dc bias required to achieve flatband condition is different at the band gap than at E₁. The ER signal corresponding to the direct gap energy E₀ and to the spin-orbit energy E₀ + Δ₀ was determined in the n and p type samples of GaSb at different temperatures. We have measured the intrinsic energy gap in GaSb at room temperature. E_g = 0.74 eV. The corresponding spin-orbit splitting was found to be Δ₀ = 0.733 ± 0.002 eV.     

Theoretical studies on the structural, electronic, and optical properties of Ag2HgSnSe4

We investigate the electronic structure of Ag₂HgSnSe₄ in a wurtzite-stannite structure with the first principles method. This crystal is a direct band-gap compound. In addition the dielectric function, absorption coefficient, reflectivity, and energy-loss function are studied using the density functional theory in the generalized gradient approximation. We discuss the optical transitions between the valence bands and the conduction bands in the spectrum of the imaginary part of the dielectric function at length. We also find a very high absorption coefficient and a wide absorption band for this material. The prominent structures in the spectra of reflectivity and the energy-loss function are discussed in detail.     

Isotope effects on the electronic critical points of germanium: Ellipsometric investigation of the and transitions

Within the past years the optical excitations of electrons have been measured for semiconductor samples of different isotope compositions. The isotope shift observed have been compared with calculations of the effects of electron-phonon interaction on the electronic band structure. While qualitative agreement has been obtained, some discrepancies remain especially concerning the E₁ and transitions. We have remeasured the effect of isotope mass on the E₁ and transitions of germanium with several isotopic compositions. The results, obtained by means of spectroscopic ellipsometry, confirm that the real part of the gap self-energies induced by electron-phonon interaction is larger than found from band structure calculations, while the imaginary part agrees with those calculations, which are based on a pseudopotential band structure and a bond charge model for the lattice dynamics. Our results agree with predictions based on the measured temperature dependence of the gaps. We compare our data for E₁ and with results for the lowest direct (E₀) and indirect (E_g) gaps. The measured values of and increase noticeably with increasing isotope mass. Similar effects have been observed in the temperature dependence of in and . A microscopic explanation for this effect is not available. Received: 6 March 1998 / Revised: 27 April 1998 / Accepted: 15 May 1998     

Intraband and interband optical transitions in Zn3AS2

Absorption measurements were made on single crystals and thin films of Zn₃As₂; within the photon energy range of 0.12–1.16 eV at temperatures of 300, 80 and 5 K and reflectivity was measured in the range of 1.0–5.5 eV at 300 K. Absorption below the fundamental edge has been interpreted as a process involving three mechanisms: (i) free-carrier absorption, (ii) intraband transitions between levels in the valence band, and (iii) direct transitions from valence levels to the acceptor level/band. The fundamental absorption edge has been ascribed to direct interband transitions from three valence levels to one conduction level. An isotropic three-level Kane band model has been used to interpret the experimental data, modified by introducing the light-hole level split from the heavy-hole level due to the tetragonal crystal field. A reasonable fit of the model to the experimental results has been obtained in the region of both intraband and interband absorption for the following set of parameters: E_g = 0.985 eV, Δ_SO = 0.30 eV, Δ_CF = 0.05 eV, m^*_hh = 0.36 m₀ and P = 4 × 10^?10eVm (at 300 K). A proposed Zn₃As₂ energy-band model near the Γ point is described to interpret the observed absorption.     

Optical spectra of CsI,CsBr, ZuGeP2, and SbSI

We report here the study of the normal reflectivity spectra R and the corresponding derivative spectra (1/R) dR/dE of SbSI between 2 and 6 eV. The spectra show very strong anisotropy. We have also tried to use the change of the wavelength modulation spectrum to monitor the change in the band structure of SbSI due to the phase transition at 292 °C (D_2h¹⁶→C_2v⁹). Careful examination of the spectra at various temperatures around T_c shows no observable change. We report also the wavelength-modulated reflectivity spectra of the chalcopyrite crystal ZnGeP₂ at 5 K between 2 and 6 eV, for E∥ C and E⊥ C. The experimental results are compared with the theoretical band structure calculation by de Alvarez and Cohen. We also present measurements on CsBr, CsI in the vacuum uv (between 5 and 8 eV) at 1.8K, showing noticeable difference with previous reflectivity measurements at 77 K.     

Directional correlations in the decay of 1200y 166Ho

γγ-directional correlations have been measured between the ground state band transitions 6→4, 4→2 and 2→0 and the γ-rays depopulating the levels of the gamma vibrational band and the negative parity states of¹⁶⁶Er. Thereby angular momenta could be assigned to the latter states andM2/E1 multipole mixtures determined for the depopulating γ-transitions. TheE2/M1 mixing ratios were obtained for a series ofI _γ→I _g transitions up toI _γ=8⁺. These are used to investigate the angular momentum dependence of theE2/M1 admixtures.     

Optical characterization of gallium antimonide highly doped with manganese

Gallium antimonide crystals highly doped with Mn were prepared by a liquid-phase-electroepitaxy growth method. The crystals exhibited high hole concentrations up to 6×10¹⁸ cm⁻³. Photoluminescence (PL) and transmission techniques were used for their investigation. Spectral line-shapes typical for highly doped semiconductors were observed. The lines revealed the features corresponding to band gap narrowing and valence-band filling phenomena. Values of the band-gap narrowing ΔE_g and the degree of the valence-band filling ΔE_F were estimated from the PL spectra. The ionization energy of the Mn acceptor E_i was estimated to be approximately 15.1-15.6 meV. At low temperatures, the PL maxima shifted relatively strongly towards higher energy with temperature. The shifts most probably resulted from a dramatic change in the electron density of states near the bottom of the conduction band. The extent of low-energy tails of the PL bands correlates with the doping levels. The transmission spectra exhibited an absorption band centred at around 774-780 meV. The band most probably originated in electron transitions from the level of spin-orbit splitting to the top of the valence band.     

Physical properties of NxTiO2 prepared by sol–gel route

The compounds N_xTiO₂(x=0, 0.05, 0.1, 0.2) with the anatase structure have been synthesized by Sol–Gel method using Tri-ethyl Amine as nitrogen source and their optical, electrical and electrochemical properties are investigated. The electrical conductivity and thermoelectric power are measured in the temperature rang 300–600 K. The samples exhibit p-type behavior in contrast to TiO₂. The doped-samples exhibit two optical transitions (2.35≤E_h−Vis(eV)≤2.55; 1.97≤E_l−Vis (eV)≤2.06) directly allowed in the visible region, while only one transition is observed in UV region (E_UV∼3.00 eV). Pure TiO₂ shows direct band gap transition of 3.17 eV. The results confirm experimentally the calculations of Di. Valentin et al. [42]. The transitions E_h−Vis and E_l−Vis are attributed respectively to the promotion of electrons from the localized N 2p and π^? N–O bond to the conduction band. In all cases, E_UV is associated to the forbidden band energy. Though that the conductivity is generally improved by doping process, only N_0.05TiO₂ and N_0.1TiO₂ shows an enhanced mobility. The mechanism of conduction takes place by small polaron hopping. The band edge positions of N_xTiO₂ (x=0, 0.05, 0.1, 0.2) at room temperature is predicted from the obtained physical properties. This study proves experimentally the principal role of nitrogen in doping process and permits the electronic states localization associated with N-impurities in TiO₂ anatase.     

Refractive index, oscillator parameters and temperature-tuned energy band gap of Tl4In3GaS8-layered single crystals

Transmission and reflection measurements in the wavelength region 450-1100 nm were carried out on Tl₄In₃GaS₈-layered single crystals. The analysis of the room temperature absorption data revealed the presence of both optical indirect and direct transitions with band gap energies of 2.32 and 2.52 eV, respectively. The rate of change of the indirect band gap with temperature dE_gi/dT=-6.0×10⁻⁴ eV/K was determined from transmission measurements in the temperature range of 10-300 K. The absolute zero value of the band gap energy was obtained as E_gi(0)=2.44 eV. The dispersion of the refractive index is discussed in terms of the Wemple-DiDomenico single-effective-oscillator model. The refractive index dispersion parameters: oscillator energy, dispersion energy, oscillator strength and zero-frequency refractive index were found to be 4.87 eV, 26.77 eV, 8.48×10¹³ m⁻² and 2.55, respectively.     

Electronic excitations and luminescence of SrMgF4 single crystals

The electronic and crystal structures of SrMgF₄ single crystals grown by the Bridgman method have been investigated. The undoped SrMgF₄ single crystals have been studied using low-temperature (T = 10 K) time-resolved fluorescence optical and vacuum ultraviolet spectroscopy under selective excitation by synchrotron radiation (3.7–36.0 eV). Based on the measured reflectivity spectra and calculated spectra of the optical constants, the following parameters of the electronic structure have been determined for the first time: the minimum energy of interband transitions E _g = 12.55 eV, the position of the first exciton peak E _n = 1 = 11.37 eV, the position of the maximum of the “exciton” luminescence excitation band at 10.7 eV, and the position of the fundamental absorption edge at 10.3 eV. It has been found that photoluminescence excitation occurs predominantly in the region of the low-energy fundamental absorption edge of the crystal and that, at energies above E _g , the energy transfer from the matrix to luminescence centers is inefficient. The exciton migration is the main excitation channel of photoluminescence bands at 2.6–3.3 and 3.3–4.2 eV. The direct photoexcitation is characteristic of photoluminescence from defects at 1.8–2.6 and 4.2–5.5 eV.     

Optical constants of Cu(In0.7Ga0.3)5Se8 and Cu(In0.4Ga0.6)5Se8 crystals

Variable angle spectroscopic ellipsometry has been applied to characterize the optical constants of bulk Cu(In_0.7Ga_0.3)₅Se₈ and Cu(In_0.4Ga_0.6)₅Se₈ crystals grown by the Bridgman method. The spectra were measured at room temperature over the energy range 0.8-4.4 eV. Adachi’s model was used to calculate the dielectric functions as well as the spectral dependence of complex refractive index, absorption coefficient, and normal-incidence reflectivity. The calculated data are in good agreement with the experimental ones over the entire range of photon energies. The parameters such as strength, threshold energy, and broadening, corresponding to the E₀, E_1A, and E_1B interband transitions, have been determined using the simulated annealing algorithm.     

Optical properties of amorphous CuInSe2

Optical absorption spectra of polycrystalline and amorphous CuInSe₂ thin films were measured at room temperature in the photon energy range from 0.8 to 2.1 eV. In amorphous CuInSe₂ the absorption coefficient follows the relation α(hv) = A(hv?E₀)/hv characteristic of optical transitions between extended states in both the valence and conduction band. The optical gap of E₀ = 1.38 ± 0.01 eV is larger than the fundamental gap energy of E_g = 1.01 ± 0.01 eV in crystalline CuInSe₂. A comparison of the results for CuInSe₂ with those for ZnSe is given.     

Optical properties of the alkaline-earth fluorohalides matlockite-type structure SrFX (X=Cl, Br, I) compounds

The optical properties of the SrFX (X=Cl, Br, I) compound have been reported using the full potential linearized augmented plane wave (FP-LAPW) method as implemented in the WIEN2K code. We employed the generalized gradient approximation (GGA), which is based on exchange-correlation energy optimization to calculate the total energy. Also we have used the Engel-Vosko GGA formalism, which optimizes the corresponding potential for band structure calculations. Our calculations show that the valence band maximum (VBM) and conduction band minimum (CBM) are located at Γ resulting in a direct energy gap. We present calculations of the frequency-dependent complex dielectric function ε(ω) and its zero-frequency limit ε₁(0). We find that the value of ε₁(0) increases on decreasing the energy gap. The reflectivity spectra and absorption coefficient have been calculated and compared with the available experimental data.     

稀掺杂GaNxAs1－x(x≤0.03)薄膜的调制光谱研究

利用室温下压电调制反射光(PzR)谱技术系统测量了N掺杂浓度为0.0%—3%的分子束外延生长GaN_xAs_1-x薄膜，并对图谱中所观察的光学跃迁进行了指认.在GaN_0.005As_0.995和GaN_0.01As_0.99薄膜的PzR谱中观察到此前只在椭圆偏振谱中才看到的N掺杂相关能态E₁+Δ₁+Δ_N.当N掺杂浓度达到 关键词： 压电调制反射光谱(PzR) xAs_1-x薄膜')" href="#">GaN_xAs_1-x薄膜 分子束外延(MBE)     

Investigation of the optoelectronic properties of columbite ZnNb2O6 crystal

A high-quality ZnNb₂O₆ single-crystal grown by optical floating zone method has been used as a research prototype to analyze the optoelectronic parameters by measuring the absorption coefficient and transmittance spectra along the b-axis from 200 nm to 1000 nm at room temperature. The optical interband transitions of ZnNb₂O₆ have been determined as a direct transition with a band gap of 3.84 eV. The refractive index, extinction coefficient, and real and imaginary parts of the complex dielectric constants as functions of the wavelength for ZnNb₂O₆ crystal are obtained from the measured absorption coefficients and transmittance spectra. In the Urbach tail of 3.16–3.60 eV, the validity of the Cauchy–Sellmeier equation has also been evaluated. Using the single effective oscillator model, the oscillator energy E_o is found to be 4.77 eV. The dispersion energy E_d is 26.88 eV and ZnNb₂O₆ crystal takes an ionic value.     

Rotational analysis of the 7390- and 7937-Å bands of NO2 by means of Fourier transform spectroscopy

We have extended to higher N and to K_a = 3 and 4 the rotational analysis of the 7390-Å band of NO₂ performed by K. E. Hallin and A. J. Merer (Canad. J. Phys.55, 2101–2112 (1977)). The lines belong to a perturbed parallel band for which Hallin and others have proposed the vibrational assignment (2 13 1)-(0 0 0) within the electronic ground state. These authors presumed that this band borrows its intensity through a vibronic coupling (spin-orbit and/or Coriolis coupling) from the stronger (0 2 0)-(0 0 0) band of the $\text{A}\text{?}\text{-}\text{X}\text{?}$ electronic system at 7460 Å. We have observed about 900 transitions belonging to the K_a = 0, 1, 2, 3, 4 subbands of the (2 13 1)-(0 0 0) band for N values going up to about 23, and 300 lines of the “hot” band (2 13 1)-(0 1 0). We have also looked for spin-orbit-induced transitions and we have detected about 400 transitions with ΔN ≠ ΔJ. Among them ΔN = ±2 transitions with ΔK_a = 0 or ± 2 have been observed, indicating that N and K_a are no longer good quantum numbers, and demonstrating clearly the existence of rovibronic interactions perturbing the upper levels of the transitions.     

Optical coefficients of Hg1 − x − y Cd x Eu y Se crystals

Optical properties of Hg_{1 ? x ? y} Cd _x Eu _y Se crystals grown by the Bridgman method have been investigated based on the independent reflectance and transmittance measurements, which were performed on a Nicolet 6700 spectrometer at T = 300 K in the wavelength range 0.9 ≤ λ ≤ 26.6 μm. The values of refractive index n, absorption index k, and absorption coefficient α have been determined for the crystals studied. Based on the dependences α = f(hν), the presence of direct allowed interband optical transitions in the crystals is established and the band-gap values are determined. The influence of temperature on the transmittance and band gap are investigated in the range T = 114–300 K.     

The band structure and optical properties of the CdxHg1-xTe mixed crystals

The band structure of the Cd_xHg_1?xTe mixed crystals have been computed on the basis of tight-binding interpolation scheme and virtual crystal approximation. The bowing parameters of main energy gaps have been calculated as well as composition dependence of position of E₁ and E₁ + Δ₁ maxima in reflectivity spectra. Good agreement with previously reported experimental data has been found.     

Piezobirefringence in GaP and InP

We report measurements of piezobirefringence in GaP and InP in the region below the direct gap. For GaP this includes the region above the indirect gap where the sample is opaque. The experimental data are compared with theoretical calculations of the contributions to the piezobirefringence arising from the E₀, E₁ and E₂ transitions.