Absorption edge and optical constants of Tl2Ga2S3Se crystals from reflection and transmission,and ellipsometric measurements

The optical properties of Tl₂Ga₂S₃Se layered crystalline semiconductors were investigated from transmission, reflection and ellipsometric measurements. The experimental results of the room temperature transmission and reflection measurements performed in the wavelength range of 400–1100 nm showed the presence of both indirect and direct transitions in the band structure of the crystals with 2.38 and 2.62 eV band gap energies.     

Intraband transitions in magnetoexcitons in coupled double quantum wells

A theory of far-infrared (FIR) magneto-optical intraband s→p ^± transitions of direct and indirect excitons in semiconductor coupled double quantum wells has been developed. The case of symmetric strained In_xGa_1−x As/GaAs quantum wells with nondegenerate valence band in the regime of both narrow and wide barriers has been analyzed. The energies and dipole matrix elements of transitions between the ground s and excited p ^± states in a quantizing magnetic field B>2 T and electric field ℰ perpendicular to the quantum well plane have been studied. The regimes of direct (in a weak electric field) and indirect (in a strong electric field) transitions, and the transition between the direct and indirect regimes, have been investigated. Zh. éksp. Teor. Fiz. 113, 1446–1459 (April 1998)     

Fundamental band edge optical absorption in Bi<Subscript>0.5</Subscript>Sb<Subscript>1.5</Subscript>Te<Subscript>3</Subscript>

Spectra of optical absorption in Bi_0.5Sb_1.5Te₃ films grown on mica and KBr substrates have been investigated for T = 145 and 300 K. The data obtained have been analyzed together with the data of investigations on the fundamental absorption edge for Bi₂Te₃ available in the scientific literature. It has been revealed that the interband absorption spectra for both Bi_0.5Sb_1.5Te₃ and Bi₂Te₃ represent a superposition of two components corresponding to direct and indirect allowed optical transitions. In this case, the least energy gap separating the valence band and the conduction band is direct for Bi_2−xSb_xTe₃ (x ≤ 1.5, T = 300 K). For Bi_0.5Sb_1.5Te₃ the temperature variation rates have been estimated for the thresholds of direct and indirect interband transitions. It has been shown that this solid solution is direct gap solution at T ≥ 145 K. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 50–52, July, 2008.     

Effect of Ga addition on optical properties of crystalline Ga20Se80 system

Ga_xSe_100−x (20 ≤ x ≤ 50) in polycrystalline form was prepared by direct fusion of stoichiometric proportions of pure elements. The spectral behavior of transmittance (T) and the reflectance (R) in the wavelength range 400–2500 nm for all films of different thicknesses were measured to obtain different optical parameters (refractive index, n, and absorption index, k). The study of inter-band transitions indicates that the existence of direct forbidden transitions and indirect forbidden transitions with energy gaps decrease with increasing Ga percentage.     

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.     

Self-absorption quantification in the case of SF6–N2 thermal plasma mixture

A simple and accurate method is presented to estimate quantitatively the self-absorption effect for the emission radiations from nitrogen in the case of SF₆–N₂ plasma mixture. The self-absorption phenomena of optically thick lines are treated parametrically on the basis of escape factor. The resonance escape factors for bound–bound transitions for the lines emitted by atomic nitrogen at 113.5, 119.96, 124.32, 149.26 and 174.53 nm are calculated assuming Voigt line shape. The escape factors for free-bound (continuum) radiation which have been the object of fewer investigations are also considered. The escape factors are calculated as a result of the solutions of the equation of radiative transfer which is independent from any special geometry. Assuming a constant plasma size the dependence of escape factors on the temperature, pressure and the nitrogen proportion in SF₆ plasma are also taken into account. These calculations may be useful in plasma modeling and diagnostics as well as the application of laser-induced plasma spectroscopy (LIPS) used in the quantitative analysis of elemental compositions.     

Optical and photoelectric properties of TlInS2 layered single crystals

Single crystals of the layered compound TlInS₂ were grown by direct synthesis of their constituents. The spectral and optical parameters have been determined using spectrophotometric measurements of transmittance and reflectance in the wavelength range 200–2500 nm. Absorption spectra of thin layers of TlInS₂ crystals are used to study the energy gap and the interband transitions of the compound in the energy region 2–2.4 eV. The dispersion curve of the refractive index shows an anomalous dispersion in the absorption region and a normal one in the transmitted region. The direct and indirect band gaps were determined to be 2.34 and 2.258 eV, respectively. Photoconductivity measurements at room temperature resolve the structure that can be identified with the optical transition.     

Fine structure in the absorption edge spectrum of NbS2Y2 (Y = Cl,Br, I)

High resolution absorption spectra of single crystals of NbS₂Y₂ (Y = Cl, Br, I), obtained at temperatures between 4.2 and 300 K revealed extensive fine structure in the absorption edge. This structure has been analysed and interpreted in terms of allowed indirect interband transitions, involving phonons corresponding to SS and NbS vibrations, followed by forbidden and by allowed direct transitions. From the shape of the absorption curve associated with the phonon branches of the indirect transitions a binding energy of 23 cm^? for indirect excitons is obtained. A binding energy of 28 cm^? for direct excitons is deduced from the exciton lines observed at the long-wavelength side of the direct transitions. A detailed interpretation of the optical transitions is given in terms of a molecular orbital diagram for the Nb₂S₄ clusters, present in these crystals.     

Photoconductivity of GaAsAlAs multiple quantum wells: Temperature dependent optical transitions

Employing temperature dependent photoconductivity, photoluminescence and photoreflectivity measurements, we have analyzed a GaAsAlAs multiple quantum well. The above optical techniques clearly resolve the fundamental inter-subband transitions, including heavy hole-light hole splittings. At T < 60K an anomalously high photoconductivity was discovered below the direct inter-subband transitions and is attributed to the presence of interface states. For T ≥ 100K the fundamental indirect Λ - J transition was discovered and associated with L0 (L) - phonon absorption. The energetic positions of the direct and the indirect gaps are in agreement with previous band structure calculations for GaAs/AlAs superlattices.     

Band structure enhancement of indirect transitions

Indirect and direct transitions in Ga_xIn_1?xP alloys has been studied in modulation spectroscopy, between 10 and 300 K. Deviation from simple indirect absorption has been studied and shown to be consistant with a model which takes into account band structure enhancement of the absorption coefficient at alloy composition values where direct and indirect edges occur, at nearly the same energy. For the first time we observe structures of Δε₁ corresponding to band structure enhanced indirect transitions.     

The Urbach tails and optical absorption in layered semiconductor TlGaSe<Subscript>2</Subscript> and TlGaS<Subscript>2</Subscript> single crystals

TlGaSe₂ and TlGaS₂ single crystals were grown by the modified Bridgman-Stockbarger method. We report the result of an experimental study of the optical absorption of TlGaSe₂ and TlGaS₂ crystals. The absorption measurements were performed in steps of 10 K. The direct and indirect band gaps for TlGaSe₂ and TlGaS₂ samples were calculated as a function of temperature. The phonon energies in TlGaSe₂ and TlGaS₂ crystals were calculated as (39±4) and (9±4) meV at 240 K, respectively. At 10 K, direct and indirect band gaps were found as 2.294 and 2.148 eV for TlGaSe₂, 2.547 and 2.521 eV for TlGaS₂ crystals, respectively. The abrupt changes were observed in the direct and indirect band gaps in the some temperature ranges. These changes were interpreted as phase transformation temperatures. The steepness parameters and Urbach energy for TlGaSe₂ and TlGaS₂ samples increased with increasing sample temperature in the range (10–320) K.     

Absorption Spectra of Single Crystals of EuGa2S4 and EuGa2S4:Co

The optical properties of EuGa₂S₄ and EuGa₂S₄:Co single crystals in a range of temperatures from 77 to 300 K are investigated. The single crystals are obtained by the Bridgman method and are characterized by tetragonal syngony. The behavior of the optical transitions in the photon energy range 1.70–2.45 eV and the temperature range 77–300 K is determined. It is established that in the energy range 1.77–1.90 eV absorption is associated with transitions of the Co²⁺ ion, while in the range 2.20–2.40 eV, with indirect allowed optical transitions.     

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.     

Die Temperaturabhängigkeit des Bandabstandes von Bleiselenid,gemessen an photoleitenden Schichten

The temperature dependence of spectral distribution of photoconductivity was measured on evaporated polycrystalline layers of lead-selenide in the range from 80 to 300 °K. The method ofBardeen, Blatt andHall was used, to calculate the band gap for direct and indirect transitions. A linear positive temperature coefficient was obtained for both transitions. The values areβ _dir=+(4.5±0.2) · 10^?4 eV/°K andβ _ind=+(3.0±0.2)· 10^?4eV/°K.     

Room temperature minority carrier lifetime and efficiency of p-type GaAs1−xPx

Using sine wave modulated laser excitation, the optical phase shift of the luminescence of vapor grown Zn doped GaAs_1?xP_x layers was measured. From such measurements the minority carrier lifetime can be derived. This lifetime was measured as a function of doping level for crystals with a composition sufficiently remote from the cross-over composition (0.30 < x < 0.35 to ascertain that only a small fraction of the minority carriers is in the indirect minima. In addition, luminescence efficiency measurements were made using cathode ray excitation. From the lifetime and efficiency results it is concluded that for p < 2 × 10¹⁹cm^-3 the lifetime, being between 1 and 3 ns, is dominated by nonradiative transitions. For the radiative transitions a rate constant of 4.5 × 10^-11cm³s^-1 was determined. This value compares reasonably well with values known for GaAs, about 7 × 10^-11cm³s^-1. The efficiency of a few strongly doped samples (p > 10¹⁹cm^-3) may be depressed by self-absorption of the luminescence. In addition, a few samples with a higher phosphorus content, up to x = 0.45, were measured. Their lifetimes being mainly determined by indirect transitions are also in the low nanosecond range. This stands in contrast with the longer lifetimes (up to 150 ns at p < 10¹⁷ cm^-3) of indirect bandgap material (x ≈ 0.8).     

Photocurrent spectroscopy of indirect transitions in Ge/Si multilayer quantum dots at room temperature

Lateral photoconductivity spectra of multilayer Ge/Si heterostructures with Ge quantum dots were studied in the work proposed at room temperature. The photocurrent with minimal energy 0.48-0.56 eV that is smaller than Ge band gap was observed from such structures at the geometry of waveguide excitation. Generation of the photocurrent with the limit energy 0.48-0.56 eV was explained by spatially indirect electron transitions from heavy hole states of SiGe valence band into Δ₂-valley of the conduction band of Si surrounding. It was found out that the limit energy of such transitions decreased, as the number of SiGe quantum dot layers increased.     

Electroreflectance of strontium titanate

The room temperature electroreflectance (ER) measurements for cubic SiTiO₃ are reported with particular emphasis on experimental conditions important for consistent reproducible lineshapes. This study includes for the first time a determination of the position of zero band perturbation (flatband) by making use of the even field nature of the ER lineshapes. The ER spectra are dependent upon dc bias, indicating the importance of a knowledge of the flatband position. A Kramers-Kronig analysis is performed and the lineshapes are reported in terms of ΔR/R, Δε₁ and Δε₂. The results in terms of Δε₂ show that the strongest direct transition occurs near 3.8 eV. The weak structure in the 3.4 eV region is due to either indirect transitions or weak direct transitions. Our unpolarized flatband ER spectra give rise to structures which for (110) samples are resolved into contributions from [001] and [12&#x0304;10] polarized spectra. In addition to the large oscillations observed above the band gap, weaker oscillatory structure is observed in the range 1.5 to 2.8 eV in the polarized spectra of (110) faced samples. The oscillations for [11&#x0304;0] polarization are 180° out of phase with those for [001] polarization.     

The low temperature spectral weight transfer problem in Kondo insulators

In this paper we address the problem of the spectral weight transfer in Kondo insulators (KI). We employ the X-boson approach for the periodic Anderson model, in the U →∞ limit. We calculate the two energy gaps of the system analytically: the indirect gap, Δ_ind = E_g ≃ E_mir, present in the density of states, and the direct one Δ_dir, associated with the minimum energy necessary to produce inter-band transitions. We find that the optical behavior of the system is governed by two energy scales: one of low frequency, characterized by E_g ≃ E_mir, in the mid-infrared region (MIR), which is a reminiscent of the heavy fermion E_mir peak, that appears in Kondo insulators as a broad maximum in the MIR region and that controls the low temperature transport properties, the gap opening in optical conductivity and the formation of the Drude peak, at ω = 0, in the intermediate temperature range. The other energy scale appears at high frequencies, and is characterized by the direct gap Δ_dir. According to our results, this peak controls the anomalous redistribution of spectral weight in the optical conductivity. We apply the theory in order to study the Kondo insulator FeSi, and we calculate the optical conductivity of the system and the spectral weight transfer in the optical conductivity.     

Interband transitions of semiconducting oxides determined from photoelectrolysis spectra

Measurements of the quantum efficiency of the photoelectrolysis of water, using semiconducting oxide anodes of high conductivity (≈ 1 Ωcm), are used to determine the energies of the interband transitions of the oxides. The previously determined values for the band gaps of TiO₂ and SrTiO₃ are confirmed, and new values are reported for Sr_0.5Ba_0.5Nb₂O₆ (3.28 eV indirect, 3.92 and 4.19 eV direct), WO₃ (2.62 eV indirect, 3.52 and 3.74 eV direct) and for Fe₂O₃ (1.88 eV indirect, 2.75 eV direct).     

Modulation of direct and indirect gaps in piezotransmission experiments

Piezomodulation spectroscopy of direct and indirect transitions in GaAs_1-xP_x is investigated. It is shown that, due to the different values of the deformation potential of the Γ_1c and X_1c minima, the strain induced modulation of these transitions are out of phase.