Optical parameters and photoluminescence of cadmium-doped epitaxial Pb1−xSnxTe films

Barium fluoride (111) cleaved surfaces have been used with the hot-wall method to grow epitaxial Pb_1−xSn_xTe films from source materials either undoped or doped with 0.1, 0.5, or 1 at. % cadmium and having x=0.2. The absorption in the wavelength range 2–13 μm has been measured together with the spectral dependence of the photoluminescence. The cadmium affects the optical width of the forbidden band and the high-frequency dielectric constant, as well as the photoluminescence intensity. The optical absorption spectra of the doped epitaxial films contain additional absorption bands due to the impurity level in the forbidden band. The position of the impurity level has been determined and the shift in it relative to the bottom of the conduction band as the temperature is reduced. Materials Science Institute, National Academy of Sciences of Ukraine. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 8–11, May, 1996.     

Control of acceptor doping in MOCVD HgCdTe epilayers

The acceptor doping of mercury cadmium telluride (HgCdTe) layers grown by MOCVD are investigated. (111)HgCdTe layers were grown on (100)GaAs substrates at 350°C using horizontal reactor and interdiffused multilayer process (IMP). TDMAAs and AsH3 were alternatively used as effective p-type doping precursors. Incorporation and activation rates of arsenic have been studied. Over a wide range of Hg_1−xCd_xTe compositions (0.17 < x < 0.4), arsenic doping concentration in the range from 5×10¹⁵ cm⁻³ to 5×10¹⁷ cm⁻³ was obtained without postgrowth annealing. The electrical and chemical properties of epitaxial layers are specified by measurements of SIMS profiles, Hall effect and minority carrier lifetimes. It is confirmed that the Auger-7 mechanism has decisive influence on carrier lifetime in p-type HgCdTe epilayers.     

Effect of compressive and tensile stresses in GaMnAs layers on their magnetic properties

The effect of elastic stresses (compressive, tensile) on the magnetic properties of epitaxial GaMnAs layers prepared by laser deposition of solid-state targets in a gas atmosphere on different buffer sublayers (In _x Ga_{1 − x} As and In _x Ga_{1 − x} P) and substrates (GaAs, InP) has been investigated. It has been established from the investigations of magnetic-field dependences of the Hall resistance that all layers exhibit ferromagnetic properties with the Curie temperature ∼50 K. It has been shown that, in the case of tensile stresses in GaMnAs layers (In _x Ga_{1 − x} As and In _x Ga_{1 − x} P buffers and InP substrate), the anomalous Hall effect shape demonstrates a predominant orientation of the easy-magnetization axis in the growth direction, unlike the GaMnAs layers prepared on a GaAs substrate (with compressive stresses), which demonstrate the predominance of the component of the magnetization vector in the layer plane.     

Temperature features of electrical resistivity of two-dimensional electrons in scattering by a correlated distribution of impurity ions in doped thin layers

Spatial correlations of impurity ions in doped thin layers at finite temperatures have been considered in the model of hard spheres on a plane. It has been shown that, in systems with separate doping, the correlations in the arrangement of impurity ions are weakened by thermal motion of electrons at sufficiently low temperatures (below the liquid-helium temperature). The temperature dependences of the electrical resistivity of degenerate two-dimensional electrons in heterostructures with separate doping have been investigated using the Al _x Ga_{1 − x} As/GaAs heterostructure as an example.     

Hall effect and pinning regimes in thin films doped with nanoparticles

Hall effect and flux pinning in YBa₂Cu₃O_6+x (YBCO) thin films doped with BaZrO₃ (BZO) nanoparticles is investigated. The results show that sign reversal of the Hall coefficient from positive hole-like to negative electron-like occurs in vortex-liquid regime of undoped and BZO-doped YBCO films. With increasing BZO concentration the amplitude of the negative Hall effect is suppressed while the temperature position of the anomalous Hall effect does not depend significantly on doping level. In addition, it is shown that Hall conductivity increases non-monotonically with increasing BZO doping. These results support a model where BZO at low doping concentration induces point pinning centres turning to strong columnar pinning defects in films doped with 4% BZO.     

Dynamical transport behavior in electron-doped manganites La0.4Ca0.6(Mn1−x Ru x )O3

Measurements of the dynamical electrical transport behavior are performed on electron-doped manganites La_0.4Ca_0.6(Mn_1−x Ru _x )O₃ (x=0 and 0.02). An undoped sample possesses a robust charge-ordered antiferromagnetic ground state, and only a positive resistivity relaxation can be observed. However, a low-temperature negative relaxation behavior arises after inducing a few ferromagnetic orders to the charge-ordered matrix by tiny Ru doping. We assigned this difference to the dynamical competition between ferromagnetic metallic and charge-ordered insulating phases. Consistently, for a doped sample, the crossover from positive to negative resistivity relaxation behavior ensues around T=115 K, which is just below the ferromagnetic Curie temperature.     

Transmission spectra of epitaxial layers of Pb1 ? xEuxTe (0 ≤ x ≤ 0.37) solid solutions in the frequency range 7–4000 cm?1

The spectra of epitaxial Pb_{1 − x} Eu _x Te (0 ≤ x ≤ 0.37) solid solution layers grown on BaF₂ and Si substrates have been investigated over a wide frequency range 7–4000 cm⁻¹ at temperatures of 5–300 K. Apart from the phonon and impurity absorption lines, the absorption in a local mode in PbEuTe layers of substrates and buffer layers has been observed in the frequency range 110–114 cm⁻¹. As the temperature decreases from 300 to 5 K, the transverse phonon mode softens from 33.0 to 19.5 cm⁻¹.     

Magnetic and band parameters of (3HgS)1–x(Al2S3)x (x = 0.5) crystals doped with manganese

Investigations of magnetic susceptibility (χ) of (3HgS)_1–x (Al₂S₃) _x (x = 0.5) crystals doped with manganese investigated by the Faraday method in the range of Т = 77–300 K and Н = 0.25–4 kOe demonstrate that peculiar magnetic properties can be due to the presence of clusters of the Mn–S–Mn–S type in these crystals. Based on the investigation of kinetic coefficients of the crystals performed for Т = 77–300 K and Н = 0.5–5 kOe, it is established that the crystals demonstrate conductivity of the n-type, and their electric conduction only very slightly depends on the temperature and shows a maximum, the Hall coefficient is independent of the temperature, while the temperature dependences of mobility exhibit a maximum. Using the concentration dependence of the effective electron mass at the Fermi level, the width of the forbidden band, the matrix element of interatomic interaction, and the effective mass on the conduction-band bottom are obtained. Based on investigations by optical methods, the presence of direct optical interband transitions in the crystals is established and the value of the optical forbidden band (at Т = 300 K) is determined.     

Mobile line in the acceptor photoluminescence spectrum of “pure” GaAs

A new line is observed in the photoluminescence spectra of epitaxial layers of undoped GaAs. The line is recorded in the region of the band-acceptor transitions with a delay relative to the excitation pulse, and with time after the excitation pulse it shifts substantially (by up to 15–18 meV) in the long-wavelength direction. The characteristics of the line attest to the possibility that small, highly doped, local regions with extended density-of-states tails can form in undoped GaAs. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 1, 81–85 (10 January 1997)     

Deposition and properties of epitaxial ZnxCd1−xS ON GaAs

We describe a method for growing epitaxial layers of zinc sulfide and cadmium sulfide solid solutions on the A-side of GaAs (111) by means of separated sulfide sources in an open hydrogen iodide system. The effects of substrate temperature, amount of doping from substrate elements, and source temperatures on photoluminescence from epitaxial films of Zn_xCd_1–xS on GaAs are determined. By varying the growth conditions it is possible to primarily obtain a single band in the emission spectrum. The presence of a larger number of photoluminescence bands indicates that the solid solution is nonhomogeneous. The composition of the solid-solution films was determined through empirical relationships based on spectra from standard solutions. X-ray analysis was used to confirm the compositions.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 10–13, February, 1982.     

Investigation of low-temperature electrical conduction mechanisms in highly resistive GaN bulk layers extracted with Simple Parallel Conduction Extraction Method

The electrical conduction mechanisms in various highly resistive GaN layers of Al _x Ga_1−x N/AlN/GaN/AlN heterostructures are investigated in a temperature range between T=40 K and 185 K. Temperature-dependent conductivities of the bulk GaN layers are extracted from Hall measurements with implementing simple parallel conduction extraction method (SPCEM). It is observed that the resistivity (ρ) increases with decreasing carrier density in the insulating side of the metal–insulator transition for highly resistive GaN layers. Then the conduction mechanism of highly resistive GaN layers changes from an activated conduction to variable range hopping conduction (VRH). In the studied temperature range, ln (ρ) is proportional to T ^−1/4 for the insulating sample and proportional to T ^−1/2 for the more highly insulating sample, indicating that the transport mechanism is due to VRH.     

Study of structural and electronic transport properties of Ce-doped LaMnO3

The structural and electronic transport properties of La_1−x Ce _x MnO₃ (x=0.0–1.0) have been studied. All the samples exhibit orthorhombic crystal symmetry and the unit cell volume decreases with Ce doping. They also make a metal-insulator transition (MIT) and transition temperature increases with increase in Ce concentration up to 50% doping. The system La_0.5Ce_0.5MnO₃ also exhibits MIT instead of charge-ordered state as observed in the hole doped systems of the same composition.     

Effect of Mg doping on the electrical properties of SnO2 nanoparticles

Sol-gel derived Mg doped tin oxide (Sn_1−xMg_xO₂) nanocrystals were synthesized with x ranging between 0.5 and 7 at. %. Characteristic single phase tetragonal structure of pure and doped samples was obtained and doping saturation was inferred by X-ray diffraction analysis. Structural, morphological and phase informations were obtained by high resolution transmission electron microscope, field emission scanning electron microscope and X-ray photoelectron spectroscopy respectively whereas bonding information was obtained from Fourier transformed infrared spectroscopy. Measurement of different electrical parameters with frequency (200 Hz-10⁵ Hz) has been carried out at room temperature. Ultrahigh dielectric constant and metallic AC conductivity were observed for undoped tin oxide and the profiles reflected highly sensitive changes in the atomic and interfacial polarizability generated by doping concentrations. Relaxation spectra of tangent loss of any sample did not show any loss peak within the frequency range. Both the grain and grain boundary contributions are observed to increase as the doping concentration increased. Results of first principle calculation based on density functional theory indicated effective Fermi level (E_F) suppression due to Mg doping which is responsible for the experimentally observed conductivity variation. AC conductivity was found to depend strongly on the doping concentration and the defect chemistry of the compound. Mg doped SnO₂ may find applications as a low loss dielectric and high density energy storage material.     

Mixed conductivity of zircon-type Ce1−xAxVO4±δ (A=Ca, Sr)

Incorporation of alkaline-earth cations into the zircon-type lattice of Ce_1−xA_xVO_4+δ (A=Ca, Sr; x=0−0.2) was found to significantly increase the p-type electronic conductivity and to decrease the Seebeck coefficient, which becomes negative at x≥0.1. The oxygen ionic conductivity is essentially unaffected by doping. The ion transference numbers of Ce_1−xA_xVO_4+δ in air, determined by the faradaic efficiency measurements, are in the range from 2×10⁻⁴ to 6×10⁻³ at 973–1223 K, increasing when temperature increases or alkaline-earth cation content decreases. The results on the partial conductivities and Seebeck coefficient suggest the presence of hyperstoichiometric oxygen, responsible for ionic transport, in the lattice of doped cerium vanadates. The activation energies for the electron-hole and ionic conduction both decrease on doping and vary in the ranges 39–45 kJ/mol and 87–112 kJ/mol, respectively. Paper presented at the 9th EuroConference on Ionics, Ixia, Rhodes, Greece, Sept. 15–21, 2002.     

The dynamics of accumulation of electrically active radiation defects on implantation of graded-band-gap HgCdTe films grown by MBE

The dynamics of accumulation of electrically active radiation defects under ion doping of epitaxial Cd _x Hg _1−x Te films is studied for various distributions of film composition in the implantation region. The epitaxial films were irradiated by boron ions at room temperature in the continuous regime, with the dose ranging within 10¹¹−3·10¹⁵ cm⁻², energy — 20–150 keV, and ion current density — j = 0.001–0.2 μA·cm⁻². It is found that the natural logarithm of the introduction rate of electrically active radiation defects linearly depends on the epitaxial-film composition in the range of mean projected path of implanted ions. An analysis of the experimental data shows that the dynamics of accumulation of electrically active radiation defects is determined by the epitaxial-film composition in the implantation region. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 25–28, September, 2006.     

Suppression of electron correlations in the superconducting alloys of Rh17−xIrxS15

We have studied the effect of Iridium doping (Rh_17?xIr_xS₁₅) in the rhodium sites of the strongly correlated superconductor Rh₁₇S₁₅. Even at low levels of doping (x = 1 and 2) we see a drastic change in the superconducting properties as compared to those of the undoped system. We deduce that there is a reduction in the density of states at the Fermi level from reduced Pauli susceptibility and Sommerfeld coefficient in the doped samples. Moreover, the second magnetization peak in the isothermal magnetization scan (‘fishtail’) which was very prominent in the magnetization data of the undoped crystal is suppressed in the doped samples. The temperature dependence of resistivity of the doped crystals show a remarkably different behavior from that of the undoped crystal with the appearance of a minima at lower temperatures, the position of which is fairly constant at different fields. Our data supports the notion that Iridium, which is a bigger atom than rhodium expands the lattice thereby, reduces the electron correlations that existed due to the interaction between closer lying rhodium atoms in the undoped system.     

Imperfection of the Sm sublattice and valence instability in compounds based on SmB6

The magnetic susceptibility, electrical resistance, specific heat, and thermal expansion coefficient of SmB₆, Sm_0.8B₆, and Sm_1–x La_xB₆ (x=0.1 and 0.2) are measured in the temperature range T=4–300 K. The dispersion curves of the acoustic phonon branches in lanthanum-doped samples are studied. A combined analysis of the results confirms the existence of an activation gap in the electron density of states in both nonstoichiometric and lanthanum-doped compounds. The anomalies in the electronic component of the thermal expansion coefficient are associated to a considerable degree with the temperature variation of the valence and, like the magnetic susceptibility, reflect features of the f-electron excitation spectrum. It is found that lanthanum doping does not lead to significant changes in the anomalies in the phonon spectrum of SmB₆. It is established that the homogeneous intermediate-valent state of the samarium ion is fairly stable and is maintained when the perfection of the Sm sublattice is violated. Zh. éksp. Teor. Fiz. 115, 1024–1038 (March 1999)     

Formation of multilayer strained-layer heterostructures by liquid epitaxy. II. Simulation of the fabrication of heterostructures based on indium-arsenic-antimony-bismuth solid solutions

The formation of InAs_1−x−y Sb_xBi_y/InSb and InAs_1−x−y Sb_xBi_y/InSb_yBi_y strained-layer heterostructures by “capillary” LPE is simulated. The laws governing the dependence of the gap width E _g and the thickness d of the epilayers on the conditions of the process are revealed. It is shown that because of the sharp increase in the rate of epitaxial deposition as the LPE temperature is raised, the successful growth of epilayers of subcritical thickness is possible only up to T<550 K. The influence of the rate of laminar flow of the liquid in the growth channel in a relaxation regime and in a continuous pumping regime on the uniformity of the distribution of E _g and d in the epitaxial heterostructures is analyzed. Effective combinations of parameters for carrying out the process, which ensure the achievement of E _g ≈0.1 eV (77 K) in the active layers with variable-band-gap layers of minimal thickness, are established. Zh. Tekh. Fiz. 67, 50–56 (July 1997)     

Studies on transport properties of copper doped tungsten diselenide single crystals

During recent years, transition metal dichalcogenides of groups IVB, VB and VIB have received considerable attention because of the great diversity in their transport properties. 2H-WSe₂ (Tungsten diselenide) is an interesting member of the transition metal dichalcogenide (TMDC's) family and known to be a semiconductor useful for photovoltaic and optoelectronic applications. The anisotropy usually observed in this diamagnetic semiconductor material is a result of the sandwich structure of Se-W-Se layers interacting with each other, loosely bonded by the weak Van der Waals forces. Recent efforts in studying the influence of the anisotropic electrical and optical properties of this layered-type transition metal dichalcogenides have been implemented by doping the samples with different alkali group elements. Unfortunately, little work is reported on doping of metals in WSe₂. Therefore, it is proposed in this work to carry out a systematic growth of single crystals of WSe₂ by doping it with copper in different proportions i.e. Cu_xWSe₂ (x=0, 0.5, 1.0) by direct vapour transport technique. Transport properties like low and high temperature resistivity measurements, high pressure resistivity, Seebeck coefficient measurements at low temperature and Hall Effect at room temperature were studied in detail on all these samples. These measurements show that tungsten diselenide single crystals are p-type whereas doped with copper makes it n-type in nature. The results obtained and their implications are discussed in this paper.     

Fabrication and characterisation of microscale air bridges in?conductive gallium nitride

Fabrication and electrical characterisation of microscale air bridges consisting of GaN heavily doped with silicon is described. These were made from GaN–AlInN–GaN epitaxial trilayers on sapphire substrates, in which the AlInN was close to the composition lattice matched to GaN at ∼17% InN fraction. The start of the fabrication sequence used inductively coupled plasma etching with chlorine chemistry to define mesas. In situ monitoring by laser reflectometry indicated an AlInN vertical etch rate of 400 nm/minute, ∼70% of the etch rate of GaN. Processing was completed by lateral wet etching of the AlInN in hot nitric acid to leave GaN microbridges supported between anchor posts at both ends. Deposition of Ti–Au contact pads onto the anchor posts allowed study of the electrical characteristics. At low applied voltages, vertical conduction through the undoped AlInN layers was minimal in comparison with the current path through the Si:GaN bridges. Typical structures showed highly linear current-voltage characteristics at low applied voltages, and had resistances of 1050 Ω. The observed resistance values are compared with the predicted value based on materials parameters and an idealised geometry. The microbridges showed damage from Joule heating only at current densities above 2×10⁵ A cm⁻².