Investigations for Sn diffusion in Pb1−xSnxTe and PbTe

By annealing Pb_1−xSn_xTe and PbTe isothermally in a quartz ampoule Sn diffused from Pb_1−xSn_xTe into PbTe. The profiles obtained have been investigated by means of an electron beam microanalyser, and the coefficients of diffusion have been determined at various temperatures. The diffusion of Sn can be explained by the expressions: D_PbSnTe = 1.5 · 10⁻¹ exp (−1.8 eV/kT) cm² s⁻¹ (0,14 < x < 0,18) D_PbTe = 5,5 · 10⁻⁴ exp (−1.5 eV/kT) cm² s⁻¹. N-type layers are observed at the surface of Pb_1−xSn_xTe specimens.     

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.     

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.     

Zum Wachstum von AIIIBV-Halbleitern aus nichtstöchiometrischen Schmelzen (II) Dotierung von GaAs und Ga1−xAlxAs mit Zink

Doping of GaAs and Ga_1−xAl_xAs with Zn in the LPE process was studied by a radioanalytical method. It is found that Zn diffuses from the Ga-(Al)-As-Zn-solution into the n-GaAs substrate before epitaxial growth starts. This “pre-diffusion” and the following diffusion of Zn out of the epitaxial layer into the substrate results in three regions with distinct Zn-graduation in the vicinity of the pn-junction. There are no striking differences for GaAs/GaAs and Ga_1−xAl_xAs/GaAs structures. The Zn concentration in GaAs epitaxial layers decreases exponentially from the substrate up to the layer surface. From this profile the temperature dependence of the Zn segregation coefficient is calculated. At 900°C a value k_eff = 5,2 · 10⁻² is found. The doping profile in Ga_1−xAl/As layers is more complex. It is influenced by the changings of temperature during the growth of the layer and by the nonuniform Aldistribution over the layer thickness.     

Absorption and electrical conductivity of CaMoO4 crystals

The electrical conductivity σ of CaMoO₄ crystals was investigated between room temperature and 850°C. The mobility v_v and the diffusion coefficient D_v of the O⁻ ion vacancies have been derived from σ: v_vT = 3300 exp (−1.52 eV/kT) cm² K/Vs, D_v = (0.1…1) × exp (−1.52 eV/kT) cm²/s. An absorption occuring in crystals which are reduced or X-irradiated at low temperatures is dichroitic and caused by Mo⁵⁺ ions. For measurement in c direction the oscillator strength of the 680 nm absorption band is found to be about 0.1.     

Crystallizaton Characteristics of the Alternating Tetrafluoroethylene-Ethylene Copolymer

The alternating tetrafluoroethylene-ethylene (TFE-E) copolymer has been studied with respect to its crystallization by using differential scanning calorimetry and optical microscopy. The value of the specific surface energy σ at the melt/crystal interface is calculated to be about 2 · 10⁻³ J/m². The nucleating activity Φ of TiO₂ particles introduced into TFE-E copolymer is estimated to be approximately 0.9. The temperature dependence of the nucleation rate and of the linear growth velocity are constructed in the whole region from the melting temperature T_m to the vitrification temperature T_g. The minimum cooling rate for the formation of a crystal-free TFE-E copolymer glass is calculated and its value amounts to 10⁴ K · min⁻¹.     

Localized valence states in the forbidden gap of non-crystalline tetracene

Employing the thermally stimulated current technique the existence of three distinct gaussian trap distributions in vapor-deposited tetracene layers has been established. In layers formed at T_f ? 130 K the valence band is split into a distribution of localized states, the width being σ ? 0.06 eV, and the density N_t equal to the molecular density. Upon increasing T_f from 130 to 180 K, both N_t and σ decrease, indicating a decrease of structural disorder. For $\text{T}{}_{\mathrm{<mtext>f</mtext>}}\text{? 180}\text{K}$ the center of the trap distribution is at E₀ = 0.07 eV above the valence band. At T_f = 180 K an optimum quasi amorphous structure is formed which still lacks long-range order, but in which short-range order is present to a degree that in local regions a narrow valence band can be established. Trap distributions centered near 0.4 and 0.7 eV with a width of 0.07 and 0.1 eV, respectively, and containing about 10¹⁵ states/cm^?3, are almost independent of film formation condition, and are probably also of structural origin.     

Molecular beam epitaxy of Ca1−xRxF2+x (R=Nd, Er) layers: study of RHEED pattern and lattice mismatch

Ca_1−xNd_xF_2+x and Ca_1−xEr_xF_2+x layers were grown on CaF₂(1 1 1) substrates at 600 and 550°C, respectively, by molecular beam epitaxy. Reflection high-energy electron diffraction (RHEED) investigation revealed that Ca_1−xNd_xF_2+x layers have two types of surface structure, namely (1×1) and ( )R30⁰ with hexagonal symmetry, depending on Nd mole fraction, while Ca_1−xEr_xF_2+x layers have three types of surface structure, namely (1×1) and (2×2) with hexagonal symmetry, and a triple rotated domain structure based on a rectangular cell depending on Er mole fraction. The lattice mismatch of the epilayers and substrate, which is important for applications involving buffer layers, was measured by X-ray rocking curve (XRC) analysis.     

The dependence of surface morphology of GaSb- and AlxGa1–xSb epitaxial layers on the conditions of LPE growth

The present work shows the dependence of surface morphology of the GaSb and Al_xGa_1–xSb epitaxial layers on the conditions of LPE growth. It is found that for LPE growth at 500 °C a supersaturation of 5—10 °C and a cooling rate of 0.24—0.4°C/min for GaSb epitaxial layers and 0.8—1.2 °C/min for Al_xGa_1–xSb epitaxial layers is necessary to obtain a flat and smooth surface.     

Temperature‐tuned band gap energy and oscillator parameters of Tl2InGaSe4 semiconducting layered single crystals

The optical properties of Tl₂InGaSe₄ layered single crystals have been studied through the transmission and reflection measurements in the wavelength range of 500‐1100 nm. The analysis of room temperature absorption data revealed the presence of both optical indirect and direct transitions with band gap energies of 1.86 and 2.05 eV, respectively. Transmission measurements carried out in the temperature range of 10‐300 K revealed that the rate of change of the indirect band gap with temperature is γ = – 4.4 × 10^‐4 eV/K. The absolute zero value of the band gap energy was obtained as E_gi(0) = 1.95 eV. The dispersion of the refractive index is discussed in terms of the single oscillator model. The refractive index dispersion parameters: oscillator wavelength and strength were found to be 2.53 × 10^–7 m and 9.64 × 10¹³ m^–2, respectively. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The Process of Formation of Epitaxial Cadmium and Zinc Oxide Films by Interaction of Single-Crystal AIIBVI Layers with Oxygen

The results of electron microscopic and electron diffraction investigation of the surface during annealing of single-crystal A^IIB^VI (CdS, CdSe, ZnSe, ZnS) films in the presence of oxygen are reported. It is shown that the behaviour of islands of cadmium oxide formed by chemical interaction between O₂ and the CdS, CdSe substrate in the process of growth resembles that of the nuclei in film condensation. CdO films become “continuous” more quickly, if the size of blocks of substrate-epitaxial A^IIB^VI film is larger. Epitaxial ZnO films formed on the surface of epitaxial ZnS and ZnSe layers are of a wurtzite structure or a cubic structure with the parameter a = 4.5 Å.     

Growth kinetics of RF-sputtered nickel films in the beginning stages

By the analysis of X-ray reflectivity angle dependence the existence of a characteristic nucleus height h in the island stage of nickel growth have been found. As the condensation time τ rises, the value of h increases according to a linear law, and the portion of substrate surface occupied by nuclei is described by a curve with saturation. The formation latent period of the fraction with preferential size h is extremely small, that, in principle, allows to produce smooth surfaces, even if the effective thickness is only about several atomic layers. The kinetic characteristics are identical both at growing on monocrystalline silicon and amorphous SiO_x substrates.     

Deep levels and growth conditions of LPE GaAs crystals

Deep levels in LPE GaAs were studied in relation to growth conditions. The residual deep levels in LPE layers are hole traps 0.50 and 0.65 eV above the top of the valence band. Their concentrations are of the order of 10¹³ cm^-3 for growth temperatures between 650 and 840°C, and increase with increasing growth temperature. The activation energy of incorporation is about 0.7 eV, which is very close to that of an arsenic vacancy. It is also noted that their concentrations near the grown surface are independent of the growth temperature. This indicates that the hole traps undergo an annealing effect after the growth period. From the experimental results for Fe and oxygen doping, each impurity acts as a deep level only when it makes a complex defect with a gallium vacancy, otherwise they are shallow levels, The distribution coefficients at 750°C are 1 × 10^-7 and 1.1 × 10^-5 for the deep and Fe and the shallow acceptor, respectively.     

Optical Properties of Pyrochlore Lead Scandium Tantalate Crystals

Optically homogeneous Pb₂Sc_0.5Ta_1.5O_6.5 crystals with pyrochlore structure are prepared by the method of high-temperature solution growth (HTSG). The refractive index in the visible region of the spectrum is measured. From the transmission spectra in the visible and infrared region the optical transmission window is estimated. From the fundamental absorption edge in the visible region of the spectrum the optical bandgap is determined corresponding to the intrinsic absorption. From the behaviour of the absorption coefficient α(hν) and α^1/2(hν) and the Raman spectrum permit to suggest that the absorption edge below 2.75 eV is due to indirect transitions.     

Temperature dependence of electrical conductivity and hall effect of Ga2Se3 single crystal

Electrical conductivity (σ) and Hall coefficient (R_H) of single crystal grown from the melt have been investigated over the temperature range from 398 K to 673 K. Our investigation showed that our samples are p-type conducting. The dependence of Hall mobility an charge carrier concentration on temperature were presented graphically. The forbidden energy gap was calculated and found to be 1.79 eV. The ionization energy of impurity level equals 0.32 eV approximately. At 398 K the mobility equals to 8670 cm² V⁻¹ s⁻¹ and could described by the law μ = aTⁿ (n = 1.6) in the low temperature range. In the high temperature range, adopting the law μ = bT^–m (as m = 1.67), the mobility decreases. This result indicates that in the low temperature range the dominant effect is scattering by ionized impurity atoms, whereas in the high temperature range the major role is played by electron scattering on lattice vibrations (phonons). At 398 K the concentration of free carriers showed a value of about 1.98 × 10⁷ cm⁻³.     

Preference for fcc atom stacking observed during growth of defect‐free LJ3281 clusters

Growth simulations of 3‐dimensional Lennard–Jones (LJ) clusters/nuclei from the vapor phase were carried out in order to estimate the role of kinetic and energetic effects in determining the internal structure of the clusters. The simulated growth was realized from N = 3281 up to ca. 10000 atoms at two reduced growth temperatures T^* = 0.3 and 0.5 and the vapor atom concentration n_v = 0.002 (in reduced units of LJ system). During growth at the low temperature, clusters evolve from octahedral to an irregular shape, while at the higher one they become spherical. Statistics of newly created structural units shows a strong preference for the formation of fcc units with their number approximately 3‐4 times larger than those of the hcp units. The possible explanation of this preference is by: (1) a larger surface diffusion leading to near spherical shape of clusters, (2) an energetic effect caused by different interaction energy of two fcc or hcp layers located on the neighboring {111} planes, and (3) an entropic effect in the form of disappearance of hcp island on the close‐packed fcc layers. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Hydrothermal Liquid Phase Epitaxy of Gallium Orthophosphate on Quartz Crystal Substrates

Gallium orthophosphate (GaPO₄) layers for surface acoustic wave (SAW) and sensor applications have been grown on quartz crystal substrates with sizes of about 30 x 30 mm² by hydrothermal liquid phase epitaxy (HLPE). The growth of epitaxial GaPO₄ layers is difficult because of a strong tendency for twinning. Besides, a retrograde solubility and an intense chemical aggressiveness of the solution has to be considered. Nevertheless, we found an effective crystal growth technique to deal with these problems using large and qualitatively good substrate crystals of quartz. The most important step of the epitaxy is the formation of an interlayer between the quartz substrate and the GaPO₄ deposit. Epitaxial layers with thickness up to 500 μm were obtained and characterised by means of X‐ray techniques.     

Calculation of Diffusion-limited Growth of Hg1−x Cdx Te Epilayers on CdTe from Te-rich Melts by Step-Cooling LPE

Calculations of layer thicknesses and composition profiles in Hg_1−xCd_xTe layers on CdTe substrates for the growth from Te-rich melts have been carried out for liquidus temperatures of 460 °C, 480 °C, and 500 °C. This has been made on the basis of the multicomponent diffusion model of SMALL and GHEZ and the solid-liquid phase relation of BRICE . It could be shown that growth velocity increases only slightly with rising liquidus temperature. On the other hand, the interdiffusion velocity of Hg and Cd in the solid increases remarkably at a higher temperature. Therefore, to get layers with a constant x-value a higher supersaturation of the melt is necessary. The x-value decreases with rising supercooling by about 0.003 K⁻¹. To demonstrate the thermodynamically and kinetically advantageous properties of CdTe as substrate material, comparative calculations for a “hypothetical” HgTe substrate have been involved.     

Effect of temperature and isomorphic atom substitution on optical absorption edge of TlInS2xSe2(1‐x) mixed crystals (0.25 ≤ x ≤ 1)

The optical properties of the TlInS_2xSe_2(1‐x)mixed crystals (0.25 ≤ x ≤ 1) have been investigated through the transmission and reflection measurements in the wavelength range of 400–1100 nm. The optical indirect band gap energies were determined by means of the analysis of the absorption data. It was found that the energy band gaps decrease with the increase of selenium atoms content in the TlInS_2xSe_2(1‐x)mixed crystals. The transmission measurements carried out in the temperature range of 10–300 K revealed that the rates of change of the indirect band gaps with temperature are γ = –9.2×10^–4 eV/K, –6.1×10^–4 eV/K, –4.7×10^–4 eV/K and –5.6×10^–4 eV/K for TlInS₂, TlInS_1.5Se_0.5, TlInSSe and TlInS_0.5Se_1.5 crystals, respectively. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

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)