Growth and characteristics of HgGaInS4 single crystals

Single crystals of the HgGaInS₄ layered compound were grown by the iodine transport technique. Results of their optical, photoelectric, and radiative properties' study are presented. The band gap and the binding energy of holes on the sensitizing centres were determined to be E_g = 2.41 eV and E_a = 0.2 eV, respectively. A presence of quasi-continuously distributed states was stated which are responsible for the exponential segment of the absorption edge and which take part in the radiative recombination.     

Optical absorption and photoelectric properties of Ga2In4S9 single crystals

Presented in this paper are the results obtained from the investigation of absorption edge and photoconductivity of layered Ga₂In₄S₉ single crystals as well as from the investigation of the photovoltaic effect in Pt—Ga₂In₄S₉ surface-barrier structures. We determined the magnitude of the direct band gap to be E_g = 2.72 eV and the position of the sensitizing centres to be ≈︁0.15 eV above the valence band top. In the regions of the photon energies smaller than E_g the absorption band edge is described by an exponential dependence with a temperature independent slope.     

Effect of Neutron Irradiation and Annealing on the Intensity of the Copper Related 1.01 eV Emission Band in n-type GaAs

Effect of neutron irradiation (E = 2 MeV, ϕ ≤ 10¹⁵ n/cm²) and subsequent annealing (T ≤ 700 °C, t = 30 min) on the intensity of the copper-related peaked at hvm =1.01 eV emission band in n-type GaAs (n₀ = 2 × 10¹⁸ cm⁻³) is studied. A strong irradiation-induced increase of the above emission intensity was observed testifying about the irradiation-stimulated growth in the concentration of copper-related 1.01 eV radiative centres (Cu_GaV_As pairs). A model is presented to explain this effect.     

Energetic Band Location of Rhenium-doped MoSe2 Single Crystals to Assess their Usefulness in PEC Solar Cell Fabrication

Single crystals of MoSe₂ and rhenium-doped MoSe₂ have been grown by direct vapour transport technique and characterized by optical and electrochemical studies. The transmission spectra show the crystals of MoSe₂ and rhenium-doped MoSe₂ to be direct band gap semiconductors with band gap 1.6 eV and 1.2 eV, respectively. Mott-Schottky plots have been drawn to determine the type of semiconductor and its parameters, N_d, E_C and E_V.     

Refractive index dispersion of spinel Zn2TiO4 single crystal

The transmittance and absorption spectra of a high‐quality Zn₂TiO₄ single crystal have been measured along the a‐axes at room‐temperature in the range of 200‐1000 nm. The wavelength dependent refractive index, extinction coefficient, real and imaginary parts of the complex dielectric constant of the Zn₂TiO₄ crystal have been derived from the measured T and α spectra. By fitting the refractive index spectrum, the Sellmeier dispersion equation of the Zn₂TiO₄ crystal has been obtained. The validity of Cauchy‐Sellmeier equation has been evaluated in the energy range of 2.90–3.20 eV representing the Urbach tail. Applying the single‐effective oscillator model, the dispersion energy E_d and the oscillator energy E₀ have been determined as 18.76 and 5.05 eV, respectively. The obtained dispersion energy E_d takes on ionic crystal value. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth and Characterization of CdAl2S4 and CdAl2Se4 Single Crystals

Single crystals of CdAl₂S₄ and CdAl₂Se₄ showing high transparency were grown by the chemical vapour transport method. Their composition was proven by microprobe analysis. Structural investigations were done by Rietveld refinements and are in good agreement with known structure data. From transmittance and reflectance measurements the energy of the band gap WBS estimated. Assuming a direct nature of the corresponding optical transition the following values were obtained: E_g = 3.82 eV (RT), E_g = 3.94 eV (85 K) for CdAl₂S₄ and E_g = 2.95 eV (RT), E_g = 3.07 eV (85 K) for CdAl₂Se₄ respectively.     

Growth mechanism in atomic layer epitaxy (III) reevaporation of Cd and Te from CdTe (111) surfaces and thick elemental deposits monitored by quadrupole-mass spectrometry

Quadrupole-mass spectroscopical (QMS) studies on isothermal reevaporation of Cd and Te species from the CdTe (111) surface have been performed in two extreme cases. The first concerns the reevaporation of thick, bulk-like non-crystalline Cd and Te films deposited in high vacuum at room temperature on the (111) surface, whereas in the second case evaporation of the constituent species from the bare single crystalline (111) surface has been investigated. The fluxes of the species desorbing in high vacuum (10⁻⁶ Pa) have been monitored with QMS and the desorption temperatures have been measured with a thermocouple mounted as near as possible to the sample surface. The following values E_a(Cd)_bulk = 1.13 ± 0.12 eV, E_a(Te)_bulk = 1.64 ± 0.18 eV and E_a(Cd)₍₁₁₁₎ = 1.13 ± 0.06 eV, E_a(Te)₍₁₁₁₎ = 1.92 ± 0.13 eV of the activation energies for these two cases have been determined from the slopes of the Arrhenius plots. Using these experimental values, the numbers of atomic bonds N_Cd and N_Te occurring in the atomic aggregates of quasi-gas molecules forming the near surface quasi-gas transition layers have been estimated. For Cd quasi-gas molecules 2 ≤ N_Cd ≤ 5, whereas for Te molecules 3 ≤ N_Te ≤ 10. However, no prediction concerning the number of atoms creating the quasi-gas molecules could be made on the basis of the QMS investigations. It has also been shown that Cd atoms evaporate from the bare single crystalline CdTe (111) surface with an activation energy that is equal (in the limits of the experimental error) to the activation energy for sublimation of Cd atoms from pure, non-crystalline, bulk Cd pieces. The analogous activation energy measured for Te atoms is about 20% larger than that of the relevant sublimation process. This result confirms the fact that Te atoms are bound much stronger in the CdTe crystal lattice than Cd atoms.     

Charge transport in chlorine doped amorphous Se:Te xerographic photoreceptor films

Time-of-flight drift mobility experiments were carried out on a-Se_1?xTe_x (x = 0-0.1) with chlorine as an additive up to 70 atomic parts per million to investigate the charge transport mechanism in these xerographically important photoreceptor films. Hole drift mobility-temperature data indicate that hole transport in a-Se:Te alloys is controlled by a relatively discrete set of Te-introduced shallow traps (probably Te₁^? centres) at ～ 0.43 eV above E_v whose concentration increases nearly linearly with Te addition. Chlorine doping generates an additional set of traps (probably Cl₀^? centres) around the same energy as Te-induced traps in a similar fashion to the effect of Cl addition to a-Se. Electron drift mobility-temperature data for a-Se:Te alloys containing no Cl can also be interpreted by assuming Te-introduced electron traps at ～ 0.49 below E_c. There was no electron transport observable in Cl-doped a-Se:Te alloys.     

MAEAM investigation of the structural stability and theoretical strength of Fe crystals under uniaxial loading

The structural stability and theoretical strength of BCC crystal Fe under uniaxial loading have been investigated with the modified analytic embedded‐atom method (MAEAM). Even if an orthorhombic path is applied, the deformation is spontaneous along the tetragonal path till Milstein modified Born criterion B₂₂‐B₂₃>0 is violated at λ₁=0.9064 in the compressive region. The branched orthogonal path with lower compressive stress σ₁ and energy E is preferred over the conventional tetragonal Bain path. A stress‐free FCC phase with the local maximum energy of ‐4.2186eV appearing either in compressive region (orthorhombic path) at λ₁=0.8923 or in tensile region (tetragonal path) at λ₁=1.2619 is unstable and would slip spontaneously into its near neighbor stress‐free mBCT phase with the local minimum energy of ‐4.2270eV. The initial BCC phase with the lowest energy of ‐4.280eV is the most stable in correspondence with the actual behavior of Fe. Furthermore, the stable region ranges from ‐79.7eV/nm³ to 30.6eV/nm³ in the theoretical strength or from 0.9064 to 1.1788 in the stretch λ₁ correspondingly. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

A density functional theory study on multiple exciton generation in lead chalcogenides

Abstract

Quantum confined structure-based solar cell is promising two folds increment of the maximum theoretical photovoltaic conversion efficiency i.e., > 60% in comparison with that of the bulk analogs e.g., silicon-based and dye sensitized solar cell (ca. 32% of maximum theoretical efficiency). The key to the significant increment is the ability of the fluorophore to exhibit multiple exciton generation upon absorption photon with sufficient energy. Small size of lead chalcogenides (PbS, PbSe, PbTe) crystals have been reported and proven experimentally could exhibit this unique property. We have investigated few clusters of narrow bandgap lead chalcogenides nanocrystals i.e., (PbS)n, (PbSe)n and (PbTe)n; which n?=?4 - 80. The cluster models were optimized using quantum chemical calculations to the lowest energy geometry at B3LYP/lanl2dz level of theory. The predicted realistic (PbS)₈₀, (PbSe)₅₀, and (PbTe)₇₄ clusters with the size, and bandgap of 4.58?nm (2.00?eV), 4.03?nm (1.51?eV), and 4.84?nm (1.55?eV) are smaller than that of their exciton Bohr radius i.e., 5.01, 13.1, and 24.8?nm respectively. Therefore, the occurrence of multi exciton generation in the clusters is hypothesized upon absorption of photon with E_photon = 2E_g.     

Investigations on the growth kinetics of GP zones and a′R-precipitates in Al-Zn alloys

X-ray small-angle scattering (XSAS) and resistivity (R) measurements were particularly done with an Al-Zn (15 at·,%) alloy in rather wide ranges of both the ageing time t_a and ageing temperature T_a, in order to obtain information on the dependence of the growth exponent m of the l = β₀t^m growth law and the activation energy E_act on t_a and T_a. The XSAS-measurements yielded that within the range of the GUINIER radius r_G between 1 nm and 2 nm the growth is essentially retarded (m < 0.1) and for r_G > 2 nm m depends on T_a ranging from 0.15 to 0.23 with a maximum at 175°C. Reasons for these effects are discussed. The differences between the m-values obtained by means of XSAS-and TEM-measurements are explained by the distinctions of the two methods applied. The E_act taken from XSAS- and R-measurements show a remarkable increase with t_a. At the beginning of the decomposition E_act = (0.49 ± 0.05) eV holds well explainable by the migration of quenched-in VZn pairs, but at the end E_act = (1.05 ± 0.07) eV was found. This value was also obtained from TEM-investigations (growth of the length). It fits well the E_act of ZnV pairs in thermal equilibrium at T_a.     

Space-Charge-Limited Current Effects in p-Type CuIn0.8Ga0.2Se2/In Schottky Diodes

Schottky diodes, fabricated by local in diffusion of indium onto p-type single crystals of CuIn_0.8Ga_0.2Se₂ have been shown to exhibit space-charge-limited current effects under forward bias. Two regions are clearly observed in the current-voltage characteristics of the device; the first one associated with the saturation-velocity regime and the second region that corresponds to the ballistic regime with a V^3/2 dependence (Child-Langmuir law). The observed characteristic was used to calculate the semiconductor potential (E_i ≈︂ 0.15 eV). By means of photovoltaic effects the barrier height (φ_B ≈︂ 0.74 eV) and the ideality factor (n = 1.7) of the diode were also obtained.     

Determination of optical parameters of Ga0.75In0.25Se layered crystals

The optical properties of the Ga_0.75In_0.25Se crystals have been investigated by means of transmission and reflection measurements in the wavelength range of 380–1100 nm. The analysis of the results performed at room temperature revealed the presence of optical indirect transtions with band gap energy of 1.89 eV. The variation of the band gap energy as a function of temperature was also studied in the temperature range of 10–300 K. The rate of change of band gap energy (γ = –6.2 × 10^–4 eV/K) and absolute zero value of the band gap (E_gi(0) = 2.01 eV) were reported. The wavelength dependence of the refractive index was analyzed using Wemple and DiDomenico, Sellmeier and Cauchy models to find the oscillator energy, dispersion energy, oscillator strength and zero‐frequency refractive index values. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of Fast Electron Irradiation and Annealing on the Luminescence in GaAs(Zn) crystals. Irradiation-induced 1.26 and 1.39 eV Emission Bands

The effect of 2.2 MeV electron irradiation and subsequent annealings on the photoluminescence in zinc-doped p-type GaAs crystals is studied and analyzed. Rather strong emission bands peaked at hv_m (77 K) near 1.26 eV (induced by electron irradiation) and 1.39 eV (induced by annealing of irradiated crystals) are observed. Evidence is presented that the 1.26 and 1.39 eV emission bands occur due to radiative electronic transitions in As_iZn_Ga and V_AsZn_Ga pairs induced by irradiation and annealing of irradiated crystals, accordingly. The observed variations in the intensities of the 1.26 and 1.39 eV emission bands upon irradiation and subsequent annealings of GaAs(Zn) crystals are explained in terms of irradiation and annealing-induced variations in the amount of 1.26 and 1.39 eV radiative centres resulting from: a) the effective interaction of mobile radiation-induced defects in the arsenic sublattice with zinc atoms leading to the formation of As_iZn_Ga and V_AsZn_Ga pairs; b) the thermal dissociation of As_iZn_Ga and V_AsZn_Ga pairs on individual components.     

Investigation of the effect of temperature on the positron annihilation in solid and liquid tin

The temperature dependence of the counting rate, F(T), has been measured in tin. A sudden increase in the counting rate, connected with the creation of vast defects acting as positron traps, has been observed on passing through the melting point. Both the F(T) and angular correlation curves indicate that the defected areas decay with increasing temperature. The kinetics of this effect has been discussed and the vacancy formation energy, being equal to E_ν = 0.50 ± 0.02 eV, has been determined.     

The amorphous Sb1−xTex system

Amorphous films of Sb_1?xTe_x, where 0.11 ? x ? 0.86, have been prepared by coevaporation. Crystallization temperatures occur near room temperature for all compositions and appear to depend on film thickness. The optical and transport properties have been investigated as a function of temperature. Optical band gaps at room temperature, E_g, ranged between 0.3 and 0.7 eV and decreased non-linearly with temperature. For the composition Sb₂Te₃, E_g ≈ 0.7 eV, in contrast to the semimetallic character of the crystalline form. All compositions were p-type with the Fermi level close to the middle of the gap. The results have been interpreted in terms of the chemical bonding.     

Irradiation plus annealing-induced 1.20 eV emission band in p-type GaAs

The effect of 2.2 MeV electron irradiation and subsequent annealings on the luminescence of tellurium-compensated p-type GaAs crystals is studied and analyzed. A comparatively strong emission band peaked at hv_m near 1.20 eV (induced by radiative electron recombination in Te_AsV_Ga pairs) appears in irradiated annealed (at T ≧ 250 °C) compensated p-type GaAs. The data obtained testify about the following: a) electron irradiation of GaAs creates stable (at T ≦ 200 °C) defects not only in the arsenic sublattice of GaAs, but in its gallium sublattice too; b) radiation plus annealing (r.p.a.)-induced Te_AsV_Ga pairs are characterized by a rather high (compared to that for grown-in Te_As V_Ga pairs) probability of electron radiative transitions in them. The electrical characterization of the r.p. a.-induced 1.20 eV radiative centres (Te_AsV_Ga pairs) is given.     

Electrical Characterization of Irradiation plus Annealing – induced VAsZnGa Pairs in p-type GaAs (Zn) Crystals

Effect of fast electron irradiation (E =2.2 Mev, ϕ_c = 1 × 10¹⁶ el/cm²) and subsequent annealings (T = 150 to 350 °C, t = 10 to 600 min) of zinc-doped p-type GaAs crystals on the formation and dissociation of V_AsZn_Ga, pairs is studied. An analysis of the formation and dissociation kinetics of V_AsZn_Ga pairs permitted to find the diffusion coefficient of radiation-induced arsenic vacancies D(D = 1.5 × 10⁻¹⁸, 1 × 10⁻¹⁷ and 5 × 10⁻¹⁷ cm²/s at 150, 175 and 200 °C accordingly), their migration energy ϵ_m(ϵ_m = 1.1 eV), the binding energy of V_AsZn_Ga, pairs ϵ_b(ϵ_b = 0.5 eV), and also their dissociation energy ϵ_d(ϵ_d = 1.6 eV).     

New experimental results on nucleation of Au on NaCl: Dependence on deposition rate and time

Systematic experiments have shown that saturation density, nucleation rate, and condensation coefficient are considerably smaller than previously reported. The low condensation coefficient and the linear deposition rate dependence of the nucleation rate show that the model usually applied for the theoretical explanation of the nucleation process needs some revision. From measured condensation coefficients we have estimated the diffusion length of adatoms, x_s = (14.4 ± 2.4) nm, and the difference between desorption and diffusion energy, E_a – E_sd = (0.31 ± 0.01) eV. Possible reasons for the deviation of the new results from the earlier ones are discussed.     

Wide band-gap organic molecules containing benzodithiophene and difluoroquinoxaline derivatives for solar cell applications

Abstract

Two new semiconducting organic small molecules, namely BDTQ-BDT(EH) and BDTQ-BDT(OC), were prepared by attaching electron accepting 2,3-didodecyl-6,7-difluoro-5,8-di(thiophen-2-yl)quinoxaline (DTQ) unit on 2,6-position of electron donating 4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b']dithiophene (BDT(EH)) and 4,8-bis(octyloxy)benzo[1,2-b:4,5-b']dithiophene (BDT(OC)) units. Molecule BDTQ-BDT(EH) showed higher thermal stability (5% weight loss temperature, T_d “349 ^оC), slightly lower band-gap (E_g “2.10?eV) and deeper highest occupied molecular orbital energy level (HOMO “–5.36?eV) level compared to those (T_d “336 ^оC, E_g “2.11?eV, and HOMO “–5.30?eV, respectively.) of the molecule BDTQ-BDT(OC). The organic solar cells (OSCs) made with the synthesized molecules as an electron donor and [6,6]-phenyl C₇₁ butyric acid methyl ester (PC₇₀BM) as an electron acceptor gave a maximum power conversion efficiency (PCE) of 1.20% and 0.83%, respectively, for BDTQ-BDT(EH) and BDTQ-BDT(OC). This study confirmed that the substituents attached on the 4,8-position of BDT unit greatly alter the properties of the resulting molecules.