Ferromagnetism in diluted Pb1 ? x ? y Ge x Cr y Te magnetic semiconductors

Magnetic properties of Pb_{1 − x − y} Ge _x Cr _y Te alloys (x = 0.02–0.20, y = 0.01–0.08) were studied. It was detected that the magnetic susceptibility of alloys consists of two contributions: the paramagnetic Curie-Weiss part (the temperature region is T < 50 K) caused, probably, by the Cr³⁺ ion paramagnetism, and the high-temperature ferromagnetic part (T < 300 K). The dependences of the concentration of magnetic centers on the composition of the matrix were obtained from the paramagnetic and ferromagnetic contributions. It was shown that a decrease in the concentration of paramagnetic centers can be, at least qualitatively, explained by the transformation of the electronic structure as the germanium concentration increases. A phenomenological model was suggested, which explains the behavior of magnetic properties as the chromium content increases, and possible mechanisms of ferromagnetic ordering in studied alloys are discussed. Original Russian Text ? E.P. Skipetrov, M.G. Mikheev, F.A. Pakpour, L.A. Skipetrova, N.A. Pichugin, E.I. Slyn’ko, V.E. Slyn’ko, 2009, published in Fizika i Tekhnika Poluprovodnikov, 2009, Vol. 43, No. 3, pp. 316–323.     

Properties and structure of (As2Se3)1 ? z (SnSe2) z ? x (Tl2Se) x and (As2Se3)1 ? z (SnSe) z ? x (Tl2Se) x glasses

Tin is stabilized in the bivalent and tetravalent states in the structure of (As₂Se₃)_{1 − z} (SnSe₂) _{z − x} (Tl₂Se) _x and (As₂Se₃)_{1 − z} (SnSe) _{z − x} (Tl₂Se) _x glasses. The presence of bivalent tin in the structural network of a glass does not give rise to extrinsic conductivity. Dependences of density, microhardness, and the glass-transition temperature on the composition of the glasses are interpreted using a model according to which the structure of the glasses is composed of structural units that correspond to As₂Se₃, AsSe, TlAsSe₂, Tl₂Se, SnSe, and SnSe₂ compounds. Original Russian Text ? G.A. Bordovsky, A.V. Marchenko, E I. Terukov, P.P. Seregin, T.V. Likhodeeva, 2008, published in Fizika i Tekhnika Poluprovodnikov, 2008, Vol. 42, No. 11, pp. 1353–1356.     

Effect of Atmosphere on n -Type Hg1– x Cd x Te Surface after Different Wet Etching Treatments: An Electrical and Structural Study

The impact on surface recombination velocity (SRV) and minority carrier lifetime of three wet etchants are examined here. The etchants are 60DI +  10HBr + 1H₂O₂ (by volume), 1% bromine–methanol, and 2% bromine–lactic acid. After initial etching, the HgCdTe surfaces were exposed in specific time steps to the atmosphere. At each step in the process, the effect of each etchant and of exposure to atmosphere was determined by photoconductive decay (PCD) measurements as well as by x-ray photoelectron spectroscopy (XPS). The PCD results showed that the 60DI + 10HBr + 1H₂O₂ etchant, after exposure to atmosphere, led to a relatively small change in SRV compared to the changes that occurred for the two bromine-based etchants. In addition, XPS measurements showed that there were no neutral tellurium inclusions in the case of 60DI + 10HBr + 1H₂O₂ and that TeO₂ formed strictly out of tellurium bonded to HgCdTe. Moreover, TeO₂ formed at a much slower rate on surfaces etched with HBr compared with those etched with the bromine-based etchants. A model is proposed in terms of band alignment at HgCdTe-Te⁰ and HgCdTe-TeO₂ interfaces to explain the different surface recombination mechanisms that are due to different etchants and to atmospheric exposure.     

Composition and parameters of domains resulting from spinodal decomposition of quaternary alloys in epitaxial GaInP/Ga x In1 ? x As y P1 ? y /GaInP/GaAs(001) heterostructures

Structural and optical properties of two- and three-layer epitaxial heterostructures containing GaInP/Ga _x In_{1 − x} As _y P_{1 − y} quaternary alloy layers were studied. Domain formation due to spinodal decomposition of the quaternary alloy was detected in three-layer heterostructures. As a result, an additional long-wavelength band appears in the photoluminescence spectra, and an additional doublet of the line appears in X-ray diffraction patterns of the (006) line. The domain composition was determined on the basis of Vegard’s law and the Kouphal equation. Original Russian Text ? E.P. Domashevskaya, N.N. Gordienko, N.A. Rumyantseva, B.L. Agapov, P.V. Seredin, L.A. Bityutskaya, I.N. Arsent’ev, L.S. Vavilova, I.S. Tarasov, 2008, published in Fizika i Tekhnika Poluprovodnikov, 2008, Vol. 42, No. 9, pp. 1086–1093.     

Single-Crystal Investigations on Quaternary Clathrates Ba8Cu5SixGe41?x (x?=?6, 18, 41)

Type I clathrates have been considered as promising thermoelectric materials due to their special structural characteristics: the “rattling” guest atoms in the larger of the two cages of the clathrate I structure are frequently held responsible for the low lattice thermal conductivity. By single-crystal x-ray diffraction, we investigated the quaternary clathrates Ba₈Cu₅Si _x Ge_41−x (x = 6, 18, 41). Rietveld refinements confirmed that the clathrates in this system crystallize with cubic primitive symmetry, in the type I clathrate structure, and that no phase transitions occur in the temperature range investigated (100 K to 300 K). We derive the concentration dependencies of the Debye temperature, the Einstein temperatures, the static disorder parameters, and the size of the two cages and argue that these dependencies underpin the previously assumed different bonding character of the Ba guest atoms in the larger and smaller cages. The linear thermal expansion coefficients for the samples are derived.     

Flexibility of p–n Junction Formation from SWIR to LWIR Using MBE-Grown Hg(1–x)CdxTe on Si Substrates

In this paper, we show the versatility of using molecular-beam epitaxy (MBE) for the growth of the mercury cadmium telluride (HgCdTe) system. Abrupt composition profiles, changes in doping levels or switching doping types are easily performed. It is shown that high-quality material is achieved with Hg_(1–x)Cd _x Te grown by MBE from a cadmium mole fraction of x = 0.15 to x = 0.72. Doping elements incorporation as low as 10¹⁵ cm⁻³ for both n-type and p-type material as well as high incorporation levels >10¹⁸ cm⁻³ for both carrier types were achieved. X-ray curves, secondary-ion mass spectrometry (SIMS) data, Hall data, the influence of doping incorporation with cadmium content and growth rate, etch pit density (EPD), composition uniformity determined from Fourier-transform infrared (FTIR) transmission spectro- scopy, and surface defect maps from low to high x values are presented to illustrate the versatility and quality of HgCdTe material grown by MBE. All data presented in this work are from layers grown on silicon (112) substrate.     

Synthesis,Structure and Electrical Properties of(SnO2)x(In2O3)1?x(x=0.5–1) Nanocomposites

The experimental results of synthesizing thin films (<1 μm thick) of (SnO₂) _x (In₂O₃)_{1 − x} (x = 0.5–1 wt) nanocomposites fabricated by high-frequency magnetron sputtering of metal-oxide targets in a controlled Ar + O₂ atmosphere are presented. The films, deposited on hot substrates (400°C), are studied by the X-ray diffraction analysis, atomic-force microscopy, and optical and electrical methods. The effect of the synthesis conditions and film composition on the size of crystalline grains, band gap, and the concentration and mobility of free charge carriers was determined. It is shown that films of the composition (SnO₂) _x (In₂O₃)_{1 − x} with x = 0.9 are the most promising for applications in gas sensorics.     

Dependence of photoluminescence spectra of epitaxial Pb1 ? xEuxTe (0 ≤ x ≤ 0.1) alloy layers on conditions of growth

The photoluminescence spectra of the Pb_{1 − x} Eu _x Te (0 ≤ x ≤ 0.1) alloy are studied at low temperatures. Epitaxial layers were grown by molecular-beam epitaxy at different temperatures. Along with the basic line corresponding to interband radiative recombination, additional emission lines are observed in the low-energy region of the spectra. Some nonuniformities are observed at the sample surface (within an area smaller than 1% of the total surface area). The concentration and size (1–10 μm) of the nonuniformities decrease with increasing temperature of growth. The additional emission lines are attributed to local nonuniformities in the layer. The dependence of the band gap of the Pb_1−x Eu _x Te (0 ≤ x ≤ 0.1) alloy on the composition parameter x is determined at 77.4 K. The dependence is nonlinear and adequately described by the relation E _g [eV] = 0.213 + 4.8x − 18.4x ².     

Structure and High-Temperature Thermoelectric Properties of the n-Type Layered Oxide Ca2?xBix?δMnO4?γ

We have investigated the effects of Bi doping on the crystal structure and high-temperature thermoelectric properties of the n-type layered oxide Ca₂MnO_4−γ . The electrical conductivity σ and the absolute value of the Seebeck coefficient S were, respectively, found to increase and decrease with Bi doping. The thermal conductivity κ of doped Ca₂MnO_4−γ is relatively low, 0.5 W/m K to 1.8 W/m K (27°C to 827°C). Consequently, the ZT value, ZT = σS ² T/κ, increases with Bi doping. The maximum ZT is 0.023 for Ca_1.6Bi_0.18MnO_4−γ at 877°C, which is ten times higher than that of the end member, Ca₂MnO_4−γ . The increase of ZT mainly results from the considerable increase of σ, which can be explained in terms of structural change. The␣Mn-O(1) and the Mn-O(2) distances in the c-direction and ab-plane, respectively, increase with increasing Bi concentration, indicating that the valence state of Mn ions decreases with the increase of electron carriers in the CaMnO₃ layers. In addition, the Mn-O(2)-Mn bond angle increases linearly with Bi doping, leading to an improvement of the electron carrier mobility.     

Dispersion of the refractive index of epitaxial Pb1 ? xEuxTe (0 ≤ x ≤ 1) alloy layers below the absorption edge

The transmittance spectra of epitaxial Pb_{1 − x} Eu _x Te (0 ≤x ≤ 0.1) alloy layers are exploited to study the dispersion of their refractive index in the spectral range from 650 to 8000 cm⁻¹ (below the absorption edge). The refractive index and the position of the absorption edge as functions of the composition parameter of the alloys are determined at two temperatures, 80 and 295 K. The refractive index is calculated in the context of the classic wave concepts of propagation of electromagnetic radiation. The experimentally determined dispersion dependences are described by the empiric Sellmeier expression of the second order. From analysis of the transmittance of the layers, it follows that the band gap of the epitaxial Pb_{1 − x} Eu _x Te alloys increases with increasing temperature at x < 0.5 and decreases at x > 0.5.     

基于正交实验法的混合相VOx薄膜最佳制备参数研究

为研究磁控溅射的衬底温度、氧氩比、沉积时间和工作气体流量对混合相VOx薄膜电学性能的影响,采用正交实验法,设计了4因素4水平16组实验,实施实验并记录样品电阻温度系数(TCR)和方阻值,分析薄膜电学性能随不同因素不同水平的变化趋势;然后,结合均值和方差分析以及XPS分析,得到不同因素影响混合相VOx薄膜电学性能程度的大小依次为:工作气体流量,衬底温度,氧氩比,沉积时间。最后,得出制备混合相VOx薄膜的优选参数:溅射电流0.3A,衬底温度270℃,氧氩比2.8%,沉积时间20min,工作气体流量120cm³/min,测试结果显示,其TCR为-2.65%/K,方阻为1102.1kΩ/□。     

Optical properties of quaternary GaNxAsyP1 ? x ? y semiconductor alloys

The optical properties of quaternary GaN _x As _y P_{1 − x − y} semiconductor alloys grown on a GaP (100) substrate surface are studied by photoluminescence spectroscopy in the temperature range 20–300 K and by photoluminescence excitation spectroscopy at liquid-nitrogen temperature. The measurements are carried out for the GaN _x As _y P_{1 − x − y} alloys, for which the nitrogen and arsenic molar fractions x and y range from 0.006 to 0.012 and from 0.00 to 0.18, respectively. A comparative analysis of the data is conducted, and the dependences of the energy position of the photoluminescence peak on the composition of the quaternary alloy are established. From the studies of photoluminescence in the range 20–300 K, it is found that the temperature dependence of the position of the photoluminescence peak substantially differs from the behavior described by Varshni’s expression.     

Simulation of relaxation times and energy spectra of the CdTe/Hg1 ? xCdxTe/CdTe quantum well for variable valence band offset, well width, and composition x

The dependences of relaxation times and energy spectrum of the CdTe/Hg_{1 − x} Cd _x Te/CdTe quantum well (QW) on its parameters were simulated in the cadmium molar fraction range 0 < x < 0.16. It was found that the x increase from 0 to 0.16 changes electron wave function localization in the QW. A criterion for determining the number of interface levels of localized electrons depending on QW parameters was obtained. The effect of a sharp (by two orders of magnitude) increase in the relaxation time of localized electrons was detected at small QW widths and x close to 0.16.     

Drift velocity of electrons in quantum wells of selectively doped In0.5Ga0.5As/AlxIn1 ? xAs and In0.2Ga0.8As/AlxGa1 ? xAs heterostructures in high electric fields

The field dependence of drift velocity of electrons in quantum wells of selectively doped In_0.5Ga_0.5As/Al _x In_{1 − x} As and In_0.2Ga_0.8As/Al _x Ga_{1 − x} As heterostructures is calculated by the Monte Carlo method. The influence of varying the molar fraction of Al in the composition of the Al _x Ga_{1 − x} As and Al _x In_{1 − x} As barriers of the quantum well on the mobility and drift velocity of electrons in high electric fields is studied. It is shown that the electron mobility rises as the fraction x of Al in the barrier composition is decreased. The maximum mobility in the In_0.5Ga_0.5As/In_0.8Al_0.2As quantum wells exceeds the mobility in a bulk material by a factor of 3. An increase in fraction x of Al in the barrier leads to an increase in the threshold field E _th of intervalley transfer (the Gunn effect). The threshold field is E _th = 16 kV/cm in the In_0.5Ga_0.5As/Al_0.5In_0.5As heterostructures and E _th = 10 kV/cm in the In_0.2Ga_0.8As/Al_0.3Ga_0.7As heterostructures. In the heterostructures with the lowest electron mobility, E _th = 2–3 kV/cm, which is lower than E _th = 4 kV/cm in bulk InGaAs.     

Optical properties of quantum-confined heterostructures based on GaPxNyAs1 ? x ? y alloys

The results of calculations of the band gap in GaP _x N _y As_{1 − x − y} alloys and the estimated parameter of hybridization of the conduction band in GaP and the localized level of nitrogen are reported. The optical properties of quantum-confined heterostructures based on GaP _x N _y As_{1 − x − y} alloys synthesized on the GaP (100) substrate surface are studied by photoluminescence measurements in the temperature range of 15–300 K. The heterostructures consist of GaP_0.814N_0.006As_0.18 quantum wells separated by GaP barrier layers. The well width and the barrier thickness are 5 nm. Heterostructures with different numbers of periods are considered. On optical excitation of the structures, an intense photoluminescence line is observed in the spectral range 620–650 nm. The photoluminescence spectra of the GaP_0.814N_0.006As_0.18/GaP quantum wells are profoundly broadened because of the inhomogeneity of the quaternary alloy in composition. It is established that the increase in the number of quantum well layers from 10 to 25 does not results in degradation of the photoluminescence properties of the heterostructures. The results of the study support the view that it is possible to produce efficient optoelectronic devices on the basis of GaP _x N _y As_{1 − x − y} alloys.     

Thermoelectric Properties of Ag-Doped n-Type (Bi2?xAgxTe3)0.96?(Bi2Se3)0.04 Pseudobinary Alloys

n-Type thermoelectric powders of (Bi_2−x Ag _x Te₃)_0.96−(Bi₂Se₃)_0.04 (0 ≤ x ≤ 0.05) have been synthesized by mechanical alloying and then consolidated by spark plasma sintering. The analysis results show that the grain size of pure Bi, Te, Ag, and Se powders is decreased to about 1 μm to 0.5 μm after they are mechanically alloyed for 2 h. The power factor of bulk material increases with increasing Ag-doping content, while the trend for the lattice thermal conductivity is the opposite. Bulk (Bi_0.99Ag_0.04)₂(Te_0.96Se_0.04)₃ after milling for 12 h exhibits a higher power factor, lower thermal conductivity, and thus a higher ZT of 0.74 at 373 K.     

Quaternary (FeIn2S4)x(MnIn2S4)1 ? x alloys and photosensitive structures on their basis

Using directional crystallization of the melt of the (FeIn₂S₄) _x (MnIn₂S₄)_{1 − x} alloy, homogeneous crystals of a similar atomic composition are grown over the entire range of compositions 1 ≥ x ≥ 0. It is established that the crystals of the continuous series of quaternary alloys in the range x = 0–1 crystallize in the spinel structure and lattice parameter a linearly depends on x. It is established that it is possible to obtain In(Al)/(FeIn₂S₄) _x (MnIn₂S₄)_1–x photosensitive structures. Room-temperature spectra of relative quantum efficiency of photoconversion of the In(Al)/(FeIn₂S₄) _x (MnIn₂S₄)_{1 − x} structures fabricated for the first time are obtained. From the analysis of these spectra, activation energies of direct and indirect band-to-band transitions for the crystals of the (FeIn₂S₄) _x (MnIn₂S₄)_1–x alloys are determined and the dependence of these parameters on the composition of the position-disordered phases of mentioned alloys is discussed. It is concluded that the crystals of the (FeIn₂S₄) _x (MnIn₂S₄)_{1 − x} alloys can be used in broadband photoconverters of optical radiation.     

Electrical and Thermal Properties of the Ca3?xGdxCo4O9+δ System

The effects of Gd substitution on the thermoelectric (TE) properties of Ca₃Co₄O_9+δ have been systematically investigated from 25 K to 335 K. Partial substitution of Gd in Ca₃Co₄O_9+δ results in an increase of thermopower and resistivity, and a decrease of thermal conductivity. A maximum dimensionless figure of merit (ZT) of 0.028 was achieved at 335 K for Ca_2.4Gd_0.6Co₄O_9+δ , which is about one order of magnitude larger than that for Ca₃Co₄O_9+δ . The investigation demonstrates that the TE performance of the Ca₃Co₄O_9+δ system can be improved through Gd doping.     

Electronic and Optical Properties of MgxZn1?xO and BexZn1?xO Quantum Wells

As a preparatory step toward establishing reliable numerical design tools for ZnO-based optoelectronic devices, we have reassessed the available information on material parameters relevant for the simulation of light-emitting diodes (LEDs) with active regions including ZnO, MgZnO, and BeZnO layers. The impact of different approximations for the electronic structure and the interface polarization charge on the optical properties of bulk ZnO and ZnO/MgZnO quantum wells has been evaluated, and a consistent set of parameters has been used not only for systematic comparison of ZnO/MgZnO and ZnO/BeZnO single quantum well structures but also for the first simulation of a realistic ZnO/BeZnO multiple quantum well LED.     

Influence of Cadmium Composition on CH4–H2-Based Inductively Coupled Plasma Etching of Hg1?xCdxTe

In this paper, inductively coupled plasma etching of Hg_1−x Cd _x Te in CH₄–H₂-based chemistry is studied. This work is focused on the effects of substrate temperature, ion energy, and alloy composition on etch rate and surface composition. A strong influence of substrate temperature is shown. The etch rate is multiplied by more than a factor of 3 when the temperature is increased from 5°C to 35°C. A purely physical Cd removal mechanism is ruled out using x-ray photoelectron spectroscopy data from samples etched at different temperatures. Under the conditions of very low ion energy, an etching mechanism limited by the supply of active species from the plasma predicts an Hg_1−x Cd _x Te etch rate evolution that fits very well with our data.