The blue bronze K0.3MoO3: A new one-dimensional conductor

The optical reflectivity of the blue bronze K_0.3MoO₃ has been measured on single crystals for photon energies between 0.03 and 12 eV at temperatures from 10 to 300 K using polarized light. The data are interpreted that this compound is a one-dimensional conductor for temperatures above 180 K and that the metal-semiconductor transition at 180 K is due to a Peierls type transition, leading to a gap of 0.15 eV in the density of states.     

Excitonic absorption and Urbach's tail in bismuth sulfide single crystals

The absorption coefficient of bismuth sulfide single crystals has been measured through more than four orders of magnitude and in the range of energies from 1.25 to 1.70 eV. A detailed study as a function of temperature has been carried out from 29 to 300 K. An Urbach tail for low values of absorption has been found. This tail and its temperature evolution fit the expression for ionic materials. An excitonic region appears at low temperature and the shape of the exciton peak is Gaussian, which corresponds to a strong exciton-phonon coupling. The exciton binding energy is estimated (28±3 meV) and then the energy gap at 29 K is obtained (E _g =1.523±0.003 eV). The fundamental electronic transition has been found to be a strongly anisotropic allowed direct transition. From reflectivity measurements a localized level at 1.361 eV at 29 K has been found. The change of the gap with temperature is interpreted through an electron-phonon mechanism.     

Optical properties of the red bronze K0.33MoO3

The optical reflectivity of the red bronze K_0.33MoO₃ has been measured on single crystals in the spectral energy range between 0.03 and 12 eV at temperatures from 4 K to 300 K using polarized light. The optical constants have been determined by means of a Kramers-Kronig analysis; the data are interpreted that this compound is a 0.5 eV energy gap semiconductor with very strong anisotropy in the infrared and visible energy range.     

Tunneling study of the charge-density-wave state in 1T-TaS2 and 1T-TaSe2

The charge density wave induced energy gaps of 1T-TaS₂ and 1T-TaSe₂ have been investigated by tunneling measurements at temperatures between 4.2 and 320 K. For 1T-TaS₂, the energy gap 2Δ is about 0.4 eV in the nearly-commensurate phase, and the gap becomes as large as 1.0 eV in the commensurate phase. On the other hand, for 1T-TaSe₂, the energy gap is about 0.5 eV and almost unchanged in the whole temperature range studied. In addition, the two compounds show cusp-like zero bias anomaly at 4.2 K, which might be related to the Coulomb effects in a disordered system     

Optical band-gap of Zn3As2

Absorption measurements of single Zn₃As₂ crystals were made at temperatures 5, 80 and 300 K. Free-carrier absorption is interpreted in the simple classical model. Interband absorption shows contributions from Urbach-like excitations. The direct optical gap has been estimated as 0.99 eV at 300 K, 1.09 eV at 80 K and 1.11 eV at 5 K. The linear dependence of band-gap on temperature was found in the range 80–300 K with dE_g/dT = ? 4.55 × 10^?4eVK^?1.     

Temperature induced band gap shrinkage in Cu2GeSe3: Role of electron-phonon interaction

The ternary semiconducting compound Cu₂GeSe₃ has been investigated for optical properties with photoacoustic spectroscopy. Optical absorption spectra of Cu₂GeSe₃ is obtained in the range of 0.76-0.81 eV photon-energy at temperatures between 80 and 300 K. The thermal variation of band gap energy has been examined from the optical absorption spectra at different temperatures. The temperature induced band gap shrinkage has been explained on the basis of electron-phonon interaction. Varshni's empirical relation in conjunction with Vina and Passler model is taken into consideration for data fitting. The Debye temperature was calculated approximately as 240 K. The acoustic phonons with a characteristic temperature as 160 K corresponding to effective mean frequency have been attributed to the thermal variation of the energy gap.     

Luminescence and electronic excitations in KBe<Subscript>2</Subscript>BO<Subscript>3</Subscript>F<Subscript>2</Subscript> crystals

This paper reports on a study of the dynamics of electronic excitations in KBe₂BO₃F₂ (KBBF) crystals by low-temperature luminescent vacuum ultraviolet spectroscopy with nanosecond time resolution under photoexcitation by synchrotron radiation. The first data have been obtained on the kinetics of photoluminescence (PL) decay, time-resolved PL spectra, time-resolved PL excitation spectra, and reflection spectra at 7 K; the estimation has been performed for the band gap E _g = 10.6−11.0 eV; the predominantly excitonic mechanism for PL excitation at 3.88 eV has been identified; and defect luminescence bands at 3.03 and 4.30 eV have been revealed. The channels of generation and decay of electronic excitations in KBBF crystals have been discussed.     

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.     

Semiconducting properties of CuIn5S8 single crystals I. Electrical properties

The electrical resistivity and Hall coefficient of n-type CuIn₅S₈ single crystals were measured in the temperature range from 80 K–500 K. The energy gap at 0 K was determined to be 1.4 eV. The donor levels at 0.017 eV and 0.09 eV below the conduction band are identified. The mobility data are analysed assuming scatterings by acoustic and polar optical phonons and ionized impurities.     

Anomalous temperature dependence of the band gap in AgGaSe2 and AgInSe2

The lowest band gaps of AgGaSe₂ and AgInSe₂ single crystals in the temperature range from 90 to 300 K were determined from photoconductivity measurements. Below (above ≈ 120 K in AgInSe₂ and ≈ 125 K in AgGaSe₂ the temperature coefficient of the band gap is +5 × 10⁻⁴ eV/K (−1.5 × 10⁻⁴ eV/K) and +1.1 × 10⁻⁴ eV/K (−4.28 × 10⁻⁴ eV/K), respectively. The positive value is explained with the lattice dilation effect being the dominant mechanism for the band gap variation at the temperatures less than ≈ 120–125 K.     

Optical properties of Tl2In2Se3S layered single crystals

The optical properties of Tl₂In₂Se₃S layered single crystals have been analyzed using transmission and reflection measurements in the wavelength region between 500 and 1100 nm. The optical indirect transitions with a band gap energy of 1.96 eV and direct transitions with a band gap energy of 2.16 eV were determined from analysis of absorption data at room temperature. 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 4.67 eV, 45.35 eV, 1.38 × 10¹⁴ m ^{? 2} and 3.27, respectively. Transmission measurements were also performed in the temperature range 10–300 K. As a result of temperature-dependent transmission measurements, the rate of change in the indirect band gap with temperature, i.e. γ = ?5.6 × 10^?4 eV/K, and the absolute zero value of the band gap energy, E _gi(0) = 2.09 eV, were obtained.     

Characterization of (As.Te)1-xSex thin films

1-x Se_x thin films (where x=0.20, 0.23, 0.27, 0.32 and 0.44) have been studied. Increasing the Se content was found to increase the optical energy gap and the activation energy for conduction of the investigated films. The optical energy gap of the As_0.40Te_0.40Se_0.20 films was increased up to 1.21 eV by increasing the film thickness to ∼120 nm, while thermal annealing at 480 K reduced it down to 0.83 eV. The decrease of the optical gap is discussed on the basis of amorphous–crystalline transformations. Received: 07 July 1997/Accepted: 30 July 1997     

Resonance Raman scattering in InAs near the E1 gap

Measurements of resonance Raman scattering in InAs at 77°K near the E₁ gap have been extended to 2.73 eV. The peak in the resonance curve appears at about 2.66 eV, 70 meV above the optical gap, and gives a larger temperature shift of the resonance than previously reported. Resonance lineshapes are obtained for allowed TO and LO and for forbidden LO phonon scattering. The forbidden scattering intensities are consistent with selection rules predicted for linear q-dependent and/or surface electric field induced scattering mechanisms.     

Optical properties of epitaxial BiFeO3 thin films

Optical properties of epitaxial BiFeO₃ thin films grown via pulsed-laser deposition on (110) DyScO₃ substrates have been investigated. Their near-normal spectroscopic reflectivity was measured in the spectral range 2 to 14 eV at room temperature, while spectroscopic ellipsometry in the spectral range 1–6 eV was measured in the temperature range from 300 to 775 K. The optical response functions have been calculated and a direct optical gap was determined varying from 2.75 to 2.70 eV in this temperature range.     

Optical absorption in layered MnGaInS<Subscript>4</Subscript> single crystals

Optical absorption in MnGaInS₄ single crystals has been studied. Direct and indirect optical transitions are found to occur in the range of photon energies of 2.37–2.74 eV and in the temperature range of 83–270 K. The temperature dependence of the band gap has been determined; its temperature coefficients E _gd and E _gi are −5.06 × 10⁻⁴ and −5.35 × 10⁻⁴ eV/K, respectively. MnGaInS4 single crystals exhibit anisotropy in polarized light at the absorption edge; the nature of this anisotropy is explained.     

Surface photovoltage for real n- and p-type in the visible and near spectroscopy (110) CdTe surfaces infra-red spectral range

Surface photovoltage spectroscopy has been carried out on real n- and p-type (110) CdTe surfaces in the wavelength range 0.36-1 μm at room temperature (300 K), and at atmospheric pressure. The measurements show the existence of surface states at 1.3; 1.48, and 1.2; 1.46 eV within the energy gap of n- and p-type CdTe, respectively. Surface states greater than the energy gap at 2.24, 2.38, 2.68, and 3.1 eV have also been detected in n-type samples and at 1.66, 2.12, 2.69 eV in p-type samples.     

Electrical properties of microcrystalline Sc3N@C80 fullerene

The electrical properties of microcrystalline Sc₃N@C₈₀ fullerene with fcc structure are studied by measuring both d.c. conductivity temperature dependence and a.c. impedance. Below 450 K the Sc₃N@C₈₀ sample has an energy band gap of 1.71 eV, which does not depend on the strength of the applied electric field. But when the temperature is above 450 K, a phase transition which results in a small band gap of 1.22 eV occurs under electric field strengths larger than 1 kV/cm. We also found from Cole–Cole plots of a.c. impedance that the contact resistance at the Au/Sc₃N@C₈₀ interface is less than that at the Au/C₆₀ interface.     

Temperature dependence of electroluminescence from Si-based light emitting diodes with β-FeSi2 particles active region

Electroluminescence (EL) properties of Si-based light emitting diodes with β-FeSi₂ particles active region grown by reactive deposition epitaxy are investigated. EL intensity of β-FeSi₂ particles versus excitation current densities has different behaviors at 8, 77 K and room temperature, respectively. The EL peak energy shifted from 0.81 to 0.83 eV at 77 K with the increase of current density from 1 to 70 A/cm². Temperature dependence of the peak energy can be well fitted by semi-empirical Varshni's law with the parameters of α=4.34 e-4 eV/K and β=110 K. These results indicate that the EL emission originates from the band-to-band transition with the band gap energy of 0.824 eV at 0 K.     

Optical absorption in MnIn2S4 single crystals

Optical absorption in MnIn₂S₄ single crystals has been studied. Direct and indirect optical transitions are found to occur at photon energies of 1.90?C2.16 eV in the temperature range of 80?C342 K. The temperature dependence of the band gap is determined; its temperature coefficients E _gd and E _gi are found to be ?4.84 × 10^?4 and ?6.33 × 10^?4 eV/K, respectively. The electron-phonon interaction is the main mechanism of the temperature shift of the intrinsic-absorption edge. MnIn₂S₄ single crystals exhibit anisotropy in polarized light at the absorption edge in the temperature range of 90?C190 K; the nature of this anisotropy is explained.     

First observation of photoconductivity in the semiconducting phase of VO2

We report in this paper the first successful measurements of photoconductivity in the semiconducting phase of VO₂ single crystals. At low temperatures the frequency dependence of the photosensitivity (ps) exhibits an edge which gradually flattens at higher temperatures. The resulting mobility gap at 4.2 K amounts to 0.95 eV and thus differs from the optically determined band gap (0.65–0.75 eV).¹ These measurements, together with other known data suggest a “quasi-amorphous” electronic behavior of crystalline and semiconducting VO₂.