Dependence of carrier mobility on an electric field in gallium selenide crystals

The dependence of the mobility of charge carriers on voltage has been studied in undoped GaSe single crystals and crystals doped with gadolinium; the latter crystals have exhibited various values of dark resistivity (??_d.r ?? 10⁴?10⁸ ?? cm at 77 K) and of the doping level (N = 10^?5, 10^?4, 10^?3, 10^?2, and 10^?1 at %). It is established that the dependence of the charge-carrier mobility on the electric field applied to the sample E ?? 10² V/cm is observed in undoped high-resistivity GaSe crystals (??_d.r ?? 10⁴ ?? cm) and in lightly doped GaSe crystals (N ?? 10^?2 at %) in the region of T ?? 150 K. It is found that this dependence is not related to heating of the charge carriers by an electric field; rather, it is caused by elimination of drift barriers as a result of injection.     

A Novel Optical Thermometry Strategy Based on Diverse Thermal Response from Two Intervalence Charge Transfer States

In this work, a novel thermometry strategy based on the diversity in thermal quenching behavior of two intervalence charge transfer (IVCT) states in oxide crystals is proposed, which provides a promising route to design self‐referencing optical temperature sensing material with superior temperature sensitivity and signal discriminability. Following this strategy, uniform Tb³⁺/Pr³⁺:NaGd(MoO₄)₂ micro‐octahedrons are directionally synthesized. Originated from the diverse thermal responses between Tb³⁺‐Mo⁶⁺ and Pr³⁺‐Mo⁶⁺ IVCT states, fluorescence intensity ratio of Pr³⁺ to Tb³⁺ in this material displays excellent temperature sensing property in a temperature range from 303 to 483 K. The maximum absolute and relative sensitivity reaches as high as 0.097 K^?1 and 2.05% K^?1, respectively, being much higher than those of the previously reported optical thermometric materials. Excellent temperature sensing features are also demonstrated in the other Tb³⁺/Pr³⁺ codoped oxide crystals having d⁰ electron configured transition metal ions (Ti⁴⁺, V⁵⁺, Mo⁶⁺, or W⁶⁺), such as scheelite NaLu(MoO₄)₂ and NaLu(WO₄)₂, and monazite LaVO₄ and perovskite La₂Ti₃O₉, evidencing the universal validity of the proposed strategy. This work exploits an effective pathway for developing new optical temperature sensing materials with high performance.     

Effect of Initial Bulk Material Composition on Thermoelectric Properties of Bi2Te3 Thin Films

V₂VI₃ compounds and solid solutions based on them are known to be the best low-temperature thermoelectric (TE) materials. The predicted possibility of enhancement of the TE figure of merit in two-dimensional (2D) structures has stimulated studies of the properties of these materials in the thin-film state. The goal of the present work is to study the dependences of the Seebeck coefficient S, electrical conductivity σ, Hall coefficient R _H, charge carrier mobility μ _H, and TE power factor P = S ² σ of Bi₂Te₃ thin films on the composition of the initial bulk material used for preparing them. Thin films with thickness d = 200 nm to 250 nm were grown by thermal evaporation in vacuum of stoichiometric Bi₂Te₃ crystals (60.0 at.% Te) and of crystals with 62.8 at.% Te onto glass substrates at temperatures T _S of 320 K to 500 K. It was established that the conductivity type of the initial material is reproduced in films fairly well. For both materials, an increase in T _S leads to an increase in the thin-film structural perfection, better correspondence between the film composition and that of the initial material, and increase in S, R _H, μ _H, σ, and P. The room-temperature maximum values of P for the films grown from crystals with 60.0 at.% and 62.8 at.% Te are P = 7.5 × 10^?4 W/K² m and 35 × 10^?4 W/K² m, respectively. Thus, by using Bi₂Te₃ crystals with different stoichiometry as initial materials, one can control the conductivity type and TE parameters of the films, applying a simple and low-cost method of thermal evaporation from a single source.     

Atomic and molecular photostimulated desorption from complex ionic crystals

Calcium carbonate and sodium nitrate are isostructural crystals in which the covalently bonded polyatomic anions are also isoelectronic, however, the chemical properties of these crystals are distinctly different. In this study, we report results involving the photostimulated desorption (PSD) of neutral CO and O(³P) products from geologic calcite (CaCO₃) using low laser fluence at 193 nm. Product states are probed using (2+1) resonance enhanced multiphoton ionization for both neutrals. The CO products display a narrow angular distribution normal to the surface and a translational energy characterized, by a temperature (T=110K) significantly lower than that of the substrate (T=295K).¹ Atomic oxygen products display both broader angular and kinetic energy distributions than that of the CO fragments. Emission of O(³P) is seen at angles greater than 30° with respect to the surface normal. In contrast, recent PSD studies of product NO from single crystal sodium nitrate (NaNO₃) show product distributions with both thermal and hyper-thermal components.² Projected density of state plots computed using periodic Hartree-Fock theory indicate a strong overlap in the metal and carbonate bands of the low lying excited states in CaCO₃, while NaNO₃ was found to have a well-separated nitrate band in the excited state below the mixed metal/nitrate bands. The differences in the electronic structure of these materials may account for differences in the observed product distributions in the PSD of calcite and sodium nitrate.     

Growth of CdGeAs2 single crystals by the chemical vapor transport method

We have grown CdGeAs₂ single crystals by chemical vapor transport (CVT), a method not previously applied for this compound. The crystallographic data of this chalcopyrite (cell parametersa ₀ = 5.9456 ± 0.0001Å, c₀ = 11.2131 ± 0.0007Å) and its electrical transport properties are reported. Predominantly n-type crystals are obtained (at RTn = 1 · 10¹⁷cm?3, μⁿ = 2000 cm²(Vs)^?1). Vacuum heat treatment at 500° C yields a type conversion fromn- to p-type. In all p-type samples the minority carrier mobility is calculated to be larger than 10000 cm²(Vs)^?1.     

Interrelation of vibrational spectra of Ag3Ga5Se9, Ag3Ga5Te9 and Ag3In5Te9 single crystals: A consequence of trivalent cation and anion substitutions

Silver gallium (indium) ternary selenides (tellurides) single crystals were studied through infrared reflection in the frequency range of 80–500 cm⁻¹. These spectra presented four infrared-active modes for studied crystals. Spectral dependencies of optical constants were computed from reflectivity spectra. The frequencies of transverse and longitudinal optical modes, damping constants and oscillator strengths were also evaluated. By replacing light selenium anions by heavier tellurium ones in Ag₃Ga₅Se₉ crystal and by substitution of light gallium cations by heavier indium ones in Ag₃Ga₅Te₉ crystal all the bands shift towards low frequencies. The bands observed in IR spectra of studied crystals were assigned to various vibration types (valence and valence-deformation). Crystal structure and atomic composition ratio of the constituent elements in Ag₃Ga₅Se₉, Ag₃Ga₅Te₉ and Ag₃In₅Te₉ crystals were revealed by structural characterization techniques of X-ray diffraction and energy dispersive spectroscopy.     

On the Feasibility of Growing Pb[(Zn1/3Nb2/3)0.91Ti0.09]O3 Single Crystals

Pb[(Zn_1/3Nb_2/3)_0.91Ti_0.09]O₃ (PZNT91/9) single crystals were grown by two methods: from solution using PbO as a self‐fluxing agent (SC method) and directly from the melt without fluxing (MC method). In both growth methods, an allomeric Pb[(Mg_1/3‐Nb_2/3)_0.69Ti_0.31]O₃ (PMNT69/31) single crystal was used as a seed. X‐ray diffraction patterns of ground crystals showed that phase‐pure perovskite PZNT91/9 single crystals were successfully fabricated by the above two methods. The composition of the crystals obtained by both the SC and MC methods was analyzed using X‐ray fluorescence, which confirmed that the crystal composition is close to the nominal value, although volatilization of PbO and segregation during crystal growth are inevitable. The MC PZNT91/9 crystals exhibit excellent piezoelectric properties, with the piezoelectric constant, d₃₃, in the range of 1800–2200 pC N^–1. This value is comparable to that of the SC crystals. However, the MC crystals show an abnormal dielectric behavior. In contrast with the SC crystals, in the MC crystals a much broader dielectric peak appears in the dielectric response curves, accompanied by a much lower peak temperature of around 105 °C. Furthermore, frequency dispersion is apparent over a much wider temperature range (even more apparent than in pure relaxors), where a large, i.e., about 70 °C, full width at half maximum (FWHM) for the dielectric peaks is observed in the dielectric response. It is speculated that such an unusual phenomenon correlates with defects, microinhomogeneities, and polar regions in the as‐grown MC crystals. The origins of this abnormality have not been interpreted in detail until now. However, optical observation of the domain structure confirms that both the SC and MC crystals possess complex structural states.     

Electric and luminescence properties of GaAs-AIIBIVC 2 V single crystals

GaAs-A^IIB^IVC ₂ ^V single crystals are grown by crystallization from dilute gallium fluxed solutions. The electric and luminescence properties of the crystals obtained are investigated. It is shown that the technological process is accompanied by the standard doping of gallium arsenide and makes it possible to grow gallium arsenide single crystals whose optoelectronic properties are controlled by the A^IIB^IVAs₂ compound introduced into the fluxed solution. Fiz. Tekh. Poluprovodn. 33, 697–700 (June 1999)     

Epitaxial Growth of Large‐Scale Orthorhombic CsPbBr3 Perovskite Thin Films with Anisotropic Photoresponse Property

Inorganic cesium lead halide perovskite (CsPbX₃, X = Cl, Br, I) is a promising material for developing novel electronic and optoelectronic devices. Despite the substantial progress that has been made in the development of large perovskite single crystals, the fabrication of high‐quality 2D perovskite single‐crystal films, especially perovskite with a low symmetry, still remains a challenge. Herein, large‐scale orthorhombic CsPbBr₃ single‐crystal thin films on zinc‐blende ZnSe crystals are synthesized via vapor‐phase epitaxy. Structural characterizations reveal a “CsPbBr₃(110)//ZnSe(100), CsPbBr₃[?110]//ZnSe[001] and CsPbBr₃[001]//ZnSe[010]” heteroepitaxial relationship between the covering CsPbBr₃ layer and the ZnSe growth substrate. It is exciting that the epitaxial film presents an in‐plane anisotropic absorption property from 350 to 535 nm and polarization‐dependent photoluminescence. Photodetectors based on the epitaxial film exhibit a high photoresponsivity of 200 A W^?1, a large on/off current ratio exceeding 10⁴, a fast photoresponse time of about 20 ms, and good repeatability at room temperature. Importantly, a strong polarization‐dependent photoresponse is also found on the device fabricated using the epitaxial CsPbBr₃ film, making the orthorhombic perovskite promising building blocks for optoelectronic devices featured with anisotropy.     

Oxygen doped Ge crystals Czochralski-grown from the B2O3-fully-covered melt

Local vibrations of oxygen in Ge crystals grown from a melt fully covered by B₂O₃ were evaluated by Fourier-transform infrared spectroscopy. Ge single crystals containing oxygen were grown by the Czochralski method under various growth conditions. Oxygen concentrations in the crystals were determined to be in the range between 8.5 × 10¹⁵ and 5.5 × 10¹⁷ cm⁻³ from the infrared absorption at 855 cm⁻¹ originating in local vibration of Ge-O_i-Ge quasi-molecules. Absorption peaks relating to GeO_x, SiO_x and Si-O_i-Si were not detected in the as-grown crystals. The calibration coefficient for determining oxygen concentration in Ge crystals from the absorption peak intensity at 1264 cm⁻¹ was estimated to be 1.15 × 10¹⁹ cm⁻².     

Exact self-compensation of conduction in Cd<Subscript>0.95</Subscript>Zn<Subscript>0.05</Subscript>Te:Cl crystals in a wide range of Cd vapor pressures

The process of self-compensation in Cd_0.95Zn_0.05Te:Cl solid-solution crystals has been studied by annealing single crystals under a controlled Cd vapor pressure, with subsequent measurements of the Hall effect, photoluminescence, carrier lifetime and mobility, and photocurrent memory in the annealed crystals. By means of this annealing, conditions of thermal treatment that make it possible to fabricate low-conductivity samples with a low carrier density, 10⁷–10¹¹ cm^?3, are defined. In these samples, a p → n conduction inversion is observed at a higher free-carrier density (n, p ≈ 10⁹ cm^?3) and the dependence of the electron density on the Cd vapor pressure exhibits a more gentle slope than in the case of CdTe:Cl crystals. The obtained data are discussed in terms of a self-compensation model in which intrinsic point defects act as acceptors with deep levels. This level is attributed to a Zn vacancy, which remains active at high Cd pressure.     

White light emission from Dy3+-doped LiLuF4 single crystal grown by Bridgman method

Lithium lutetium fluoride （LiLuF4） single crystals doped with different Dy3＋ ion concentrations were grown by Bridgman method. The Judd-Ofelt （J-O） strength parameters （Ω2, Ω4, Ω6） of Dy3＋ in LiLuF4 crystal are calculated according to the measured absorption spectra and the J-O theory, by which the asymmetry of the Dy3＋：LiLuF4 single crystal and the possibility of attaining stimulated emission from 4F9/2 level are analyzed. The capability of the Dy3＋：LiLuF4 crystal in generating white light by simultaneous blue and yellow emissions under excitation with ultra- violet light is produced. The effects of excitation wavelength and doping concentration on chromaticity coordinates and photoluminescence intensity are also investigated. Favorable CIE coordinates, x=0.319 3 and y=0.349 3, can be obtained for Dy3＋ ion in 2.701% molar doping concentration under excitation of 350 nm.     

Liquid encapsulated czochralski pulling of InP crystals

The growth of bulk indium phosphide crystals via liquid encapsulated Czochralski pulling from both stoichiometric and nonstoichiometric melts is described. Nominally un-doped crystals with carrier concentration N_D-N_A = 6 × 10¹⁵ cm⁻³ and Hall mobilities of 4510 cm²/Vsec at room temperature were grown. Also, we prepared Zn-or Cd-doped p-type crystals in the range 10¹⁶ ≤ N_A-N_D ≤ 10¹⁸ cm⁻³ with Hall mobilities ≤ 130 cm²/Vsec and Sn-doped n-type crystals in the range 4 × 10¹⁷ ≤ N_A-N_D ≤ 10¹⁸ cm^-3 with Hall mobilities ≤ 2400 cm²/Vsec. The dislocation density of LEC pulled InP crystals is typically ~ 10⁴ cm⁻².     

Visible emission from ZnO doped with rare-earth ions

We report the results of a cathodoluminescence (CL) and photoluminescence (PL) study of ZnO-bulk single crystals and epilayer thin-film samples grown on a sapphire (0001) substrate and doped by implantation with rare-earth ions (RE³⁺): Pr³⁺, Dy³⁺, Ho³⁺, Er³⁺, Tm³⁺ (bulk crystals, co-doped with Li), Sm³⁺, Dy³⁺, and Er³⁺ (epilayers). The PL and PL excitation (PLE) spectra of polycrystalline ZnO doped with RE³⁺ ions (Nd³⁺, Dy³⁺, Er³⁺, and Tm³⁺) and codoped with Li⁺, Cl⁻, and N⁻ ions have also been studied.     

Solution Growth and Thermoelectric Properties of Single-Phase MnSi<Subscript>1.75−<Emphasis Type="Italic">x</Emphasis></Subscript>

We have investigated the crystal growth of single-phase MnSi_1.75−x by a temperature gradient solution growth (TGSG) method using Ga and Sn as solvents and MnSi_1.7 alloy as the solute, and measured the thermoelectric properties of the resulting crystals. Single-phase Mn₁₁Si₁₉ and Mn₄Si₇ crystals were grown successfully using Ga and Sn as solvents, respectively. The typical size of a grown ingot of Mn₁₁Si₁₉ was 2 mm to 4 mm in thickness and 12 mm in diameter, whereas Mn₄Si₇ had polyhedral shape with dimensions in the range of several millimeters. The single-phase Mn₁₁Si₁₉ has good electrical conduction (ρ = 0.89 × 10⁻³ Ω cm to 1.09 × 10⁻³ Ω cm) compared with melt-grown multiphase higher-manganese silicide (HMS) crystals. The Seebeck coefficient, power factor, and thermal conductivity were 77 μV K⁻¹ to 85 μV K⁻¹, 6.7 μW cm⁻¹ K⁻² to 7.2 μW cm⁻¹ K⁻², and 0.032 W cm⁻¹ K⁻¹, respectively, at 300 K.     

Component Overpressure Growth and Characterization of High-Resistivity CdTe Crystals for Radiation Detectors

Spectrometer-grade CdTe single crystals with resistivities higher than 10⁹ Ω cm have been grown by the modified Bridgman method using zone-refined precursor materials (Cd and Te) under a Cd overpressure. The grown CdTe crystals had good charge-transport properties (μτ _e = 2 × 10⁻³ cm² V⁻¹, μτ _h = 8 × 10⁻⁵ cm² V⁻¹) and significantly reduced Te precipitates compared with crystals grown without Cd overpressure. The crystal growth conditions for the Bridgman system were optimized by computer modeling and simulation, using modified MASTRAPP program, and applied to crystal diameters of 14 mm (0.55′′), 38 mm (1.5′′), and 76 mm (3′′). Details of the CdTe crystal growth operation, structural, electrical, and optical characterization measurements, detector fabrication, and testing using ²⁴¹Am (60 keV) and ¹³⁷Cs (662 keV) sources are presented.     

Effect of superstoichiometric components on the spectral and kinetic characteristics of the luminescence of ZnSe crystals with isovalent impurities

The spectral and kinetic parameters of the X-ray luminescence of ZnSe crystals doped with Zn, Se, and Te were investigated during the growth process at temperatures in the range 80–500 K, and also after annealing in Zn vapor. ZnSe crystals grown from a stoichiometric mixture, or mixture containing chalcogenide impurities, typically produce the minimum level of afterglow and a rapid rise of X-ray luminescence, as well as a shift of its peak from the infrared region toward shorter wavelengths after annealing in zinc. ZnSe crystals grown from material with excess of Zn have a relatively low X-ray luminescence yield and a substantial level of afterglow. It is assumed that the growth of Te-activated crystals is accompanied by the development of thermally stable complexes of the form V _ZnTe_Se that act as radiative recombination centers. The introduction of excess Zn into the initial mixture produces a reduction in the concentration of V _Zn and, hence, in the concentration of radiative recombination centers. It is shown that, for free-electron concentrations n<10¹⁸ cm⁻³, the afterglow time constant τ can be described as a function of n by a model of radiative recombination that involves a single impurity level, whereas for n>10¹⁸ cm⁻³, the time constant decreases with increasing n, which cannot be explained in terms of the simple model. It is suggested that radiative recombination centers of a new type are produced as a result of prolonged annealing in Zn vapor. Fiz. Tekh. Poluprovodn. 31, 1211–1215 (October 1997)     

Nature of the donor action of Gd impurity in crystals of lead and tin telluride

Electron spin resonance and the Hall effect are investigated in n-Pb_1−x Sn_xTe:Gd crystals grown from melt. It is found that there is no direct correlation between the free electron density and the density of the Gd³⁺ impurity in these crystals. The conclusion is drawn that the the electron conductivity of Pb_1−x Sn_xTe:Gd crystals is not caused by the Gd impurity but by intrinsic defects of the crystal lattice which have zero activation energy due to the Gd impurities. Fiz. Tekh. Poluprovodn. 32, 1331–1333 (November 1998)     

High-frequency method to determine SiC crystal conductivity

In this paper, we present a simple non-destructive method for testing SiC plate single crystals of any size and shape. The method is based on measuring the impedance changes of an inductive ferrite-cored coil due to placing the sample into the core gap. The method is valid for any SiC polytypes, though we used 6H one. Using this method we have obtained and discussed a conductivity as a function of doping level (N_d–N_a) for 6H–SiC Lely crystals. The conductivity measurements were carried out with alternating current of 747 kHz frequency. The sensitivity of the method is limited by minimal conductivity 1 Ω⁻¹ cm⁻¹ (that is corresponding to (N_d–N_a)∼2×10¹⁶ cm⁻³ for 6H–SiC : N Lely crystals).     

Energy Spectra of the Local States in the Forbidden Gap of Monoclinic TlInS 2x Se 2(1−x) Crystals

Near IR properties of the mixed TlInS_2xSe_2(1?x) have been studied previously by the present authors. In this work the temperature and frequency dependence's of the conductivity and the current-voltage characteristics (in relatively weak electric field), have been investigated for monoclinic TlInS_2xSe_2(1?x) crystals, which are perspective materials for IR applications. From the temperature dependence's of conductivity in the direction perpendicular to c- axis the band gap E^g = 2.22 eV was determined for β--TlInS₂ crystals. The impurity centres were determined located at 0.43, 0.73 eV and 0.35, 0.48, 1.12 eV for the direction of current i//c and i ⊥ c, respectively. The concentration of the centres located at 0.48 and 1.12 eV were calculated to be N_A ? N_D = 4.8 · 10⁹ cm^?3 and 1.9 · 10¹¹ cm^?3, respectively. It was found that in the solid solutions TlInS_2xSe_2(1?x) for 0.3 ≤ x ≤ 1, the conductivity follows the dependence σ (v) = σ₀·υ^s in the temperature range between 100 to 600 K. In the temperature range of 80-400 K charge bounce plays an important role in the conductivity mechanism. Occurrence of the deep and low-levels impurity centres and a “tail” of the density of energy states in TlInS_2xSe_2(1?x) crystals make them perspective for practical applications: switching and memory effects, N-type current-voltage characteristics, induced conductivity etc.