Nature of gallium deep centres in lead telluride based semiconductors

Doped with Ga lead telluride was taken as a model object to explain the nature of group-III deep levels in IV-VI semiconductors and to elucidate the vapour phase doping mechanism. For this goal, interaction of various gallium-containing molecules with defect-free crystal as well as with native defects in PbTe was considered. Formation energies for different point defects created in PbTe as a result of interaction the Ga₂Te molecules, Ga₂ dimers and single Ga atoms with a host crystal were calculated using density functional theory. Particularly Ga_Pb and Ga_i together with formation of accompanied self interstitials Pb_i in various charge states were examined. In addition we propose the new type of defects - the impurity complex (2Ga)_Pb which looks like <111>-oriented gallium dumbbell. Calculations suggest the double donor behaviour and DX-like properties of this defect together with extremely low formation energy values. Namely, (2Ga)_Pb centres are preferably formed under Ga₂Te doping while (Ga₂)_Pb+Pb_i ones are formed under Ga₂ or Ga doping. In all cases, formation energies are negative and resulting defect concentration is determined by reaction kinetics only. Mechanisms of the lead vacancy compensation with the vapour phase doping are considered as well.     

Mn/PbTe(111)界面行为的光电子能谱研究

利用X光电子能谱(XPS)对Mn在PbTe(111)表面上沉积生长的界面性质进行了研究.研究表明Mn的沉积使衬底发生了原子尺度上的突变及金属/半导体界面的形成.从X光电子能谱的芯态能级峰来看,随着Mn膜的沉积Pb 4f峰的低结合能端出现了金属Pb的特征新峰,而Te 3d峰的高结合能端却出现了MnTe特征新峰.且随着Mn膜厚度的增加这些新峰变得越来越明显,当Mn膜厚度超过7 ML(monolayer)(即超过Pb,Te的探测深度)时,衬底信号峰完全消失,只剩下金属Pb和MnTe的芯态能级峰.Mn膜厚度继续增 关键词： PbTe半导体 界面形成 光电子能谱 偏析     

Optical and electrical properties of PbTe films grown by laser induced evaporation of pressed PbTe pellets

A simple and cheap method has been developed for the deposition of lead telluride thin films on glass substrates by pulsed Nd:YAG laser evaporation of lead telluride pellets made of high purity Pb and Te powders.Preliminary characterization of the crystallographic and optical properties of the films has been performed as a function of the substrate temperature.The influence of deposition conditions on the sheet resistance of these thin films has been studied. Both deposition temperature, nitrogen pressure during deposition, and addition of Ga and As impurities in the source pellets have been considered.     

125Te NMR chemical-shift trends in PbTe–GeTe and PbTe–SnTe alloys

Complex tellurides, such as doped PbTe, GeTe, and their alloys, are among the best thermoelectric materials. Knowledge of the change in ¹²⁵Te NMR chemical shift due to bonding to dopant or “solute” atoms is useful for determination of phase composition, peak assignment, and analysis of local bonding. We have measured the ¹²⁵Te NMR chemical shifts in PbTe-based alloys, Pb_1−xGe_xTe and Pb_1−xSn_xTe, which have a rocksalt-like structure, and analyzed their trends. For low x, several peaks are resolved in the 22-kHz MAS ¹²⁵Te NMR spectra. A simple linear trend in chemical shifts with the number of Pb neighbors is observed. No evidence of a proposed ferroelectric displacement of Ge atoms in a cubic PbTe matrix is detected at low Ge concentrations. The observed chemical shift trends are compared with the results of DFT calculations, which confirm the linear dependence on the composition of the first-neighbor shell. The data enable determination of the composition of various phases in multiphase telluride materials. They also provide estimates of the ¹²⁵Te chemical shifts of GeTe and SnTe (+970 and +400±150 ppm, respectively, from PbTe), which are otherwise difficult to access due to Knight shifts of many hundreds of ppm in neat GeTe and SnTe.     

Raman scattering for lead telluride-based thin film structures

For single-crystal epitaxial lead telluride films, Raman spectra obtained under conditions in which the intensity of incident radiation is minimized in order to suppress photo-initiated oxidative process are presented. The spectra were measured with an InVia Renishaw spectrometer at an exciting radiation wavelength of 514.5 nm and in-line focusing of a 20-μW beam. These measuring conditions allowed us for the first time to experimentally observe a large set of peaks for lead telluride, the positions of which were in line with the theoretical values of harmonics and combinatorial PbTe phonon modes. In order to demonstrate the possibilities of the methodology used, the picture of phonon modes for single-crystal and polycrystalline films of the Pb_{1 ? x} Eu _x Te (0.05 ? x ? 0.10) solution was additionally considered.     

Non-stoichiometry and magneto-optic properties of magnetic bubble thin films of TmIG substituted with bismuth and gallium

Thulium iron garnet thin films substituted with bismuth and gallium were prepared by LPE from a PbO.B₂O₃ flux. Radioactive isotope tracers were added to the melt as the chemical analysis technique. The deviation from the ratio (Pb+Bi+Tm)/(Fe+Ga)=0.6 of the layers was large and a proportion of the thulium ions possibly lie on the octahedral crystallographic sites. The concentration of Bi, Pb, and Ga as a function of the growth temperature was studied. The magneto-optic properties show that the layers are well suited for use in a magneto-optic device. A figure of merit of 2.5 degree per decibel at λ=560 nm was obtained and some magnetic bubble parameters are also presented.     

Structure,composition and optical properties of ZnO:Ga films electrodeposited on flexible substrates

Thin films of ZnO:Ga are electrodeposited on stainless steel from zinc chloride solutions containing gallium with a Ga/Zn molar ratio in the range 1–10%. The surface morphology, structure, composition, and the optical properties of these films are studied using SEM, XRD, EDX, XPS, photoluminescence and transmittance spectroscopy. Results reveal that films with embedded platelets can be grown with a Ga/Zn molar ratio up to 60%. In as-grown films, Ga is present in the form of oxide, hydroxide or hydroxychloride compounds which decompose by annealing. Films annealed at 400°C are composed only from ZnO and Ga₂O₃ phases. The incorporation of Ga in ZnO films does not make a considerable change in the optical transmittance and the photoluminescence properties of the films.     

Cyclic voltammetry study of electrodeposition of CuGaSe2 thin films on ITO-glass substrates

The electrodeposition mechanism of CuGaSe₂ (CGS) thin films on ITO substrates has been investigated using cyclic voltammetry technique. The cyclic voltammetric study was performed in unitary Cu, Ga and Se systems, binary Cu–Se, Ga–Se systems and ternary Cu–Ga–Se system. The electrodeposition metallic Ga from Ga unitary electrolytes is impossible due to its low reduction potential. No reduction peak was found for the reduction of Ga³⁺ to Ga in the cyclic voltammogram of unitary system. However, in the cyclic voltammogram of ternary Cu–Ga–Se system, reduction peak at −0.6 V was observed with addition of GaCl₃. Also, current density of the peak was increased with increasing concentration of GaCl₃. It is corresponded to the formation of gallium selenides and/or copper–gallium–selenium compounds. The contents of Ga in the films were significantly changed from −0.4 V to −0.6 V. SEM and XRD analysis also showed that surface morphology and crystalline phase of films were significantly changed with increasing Ga content.     

First-principles theory of competing order types, phase separation, and phonon spectra in thermoelectric AgPbmSbTe(m+2) alloys

Using a first-principles cluster expansion, we shed light on the solid-state phase diagram and structure of the recently discovered high-performance Pb-Ag-Sb-Te thermoelectrics. The calculated bulk thermodynamics favors the formation of coherent precipitates of ordered Ag(m)Sb(n)Te(m+n) phases immiscible with rocksalt PbTe, such as AgSbTe2. The solubility is high for Pb in AgSbTe2 and low for (Ag,Sb) in PbTe (8% vs 0.6% at 850 K). The differences in the phonon spectra of PbTe and AgSbTe2 suggest that these precipitates enhance the thermoelectric performance by lowering thermal conductivity.     

Far-infrared study of impurity local modes in gallium-doped PbTe

We present far-infrared reflection spectra and results of galvanomagnetic measurements of PbTe single crystals doped with gallium between 10 and 300 K. The analysis of the far-infrared reflection spectra was made by a fitting procedure based on the model of coupled oscillators. Together with the strong plasmon–phonon coupling we obtain three local modes of gallium at about 122, 166 and 192 cm⁻¹. The position of these modes depends of impurity center charge, and their intensity depends of temperature and of gallium concentration. Persistent photoconductivity effect was registered in the sample with 0.4 at.% Ga by galvanomagnetic and far-infrared measurements.     

On the type of superlattices in leadtintelluride system

It is predicted in the PbTePb_1?xSn_xTe superlattice (SL) that the bottom of the quantum well is situated at the PbTe layer in n-type specimens and the top at the Pb_1?xSn_xTe layer in p-type ones for x<0.23, and for x>0.23, SLs are of type II. These predictions are based upon an experimental tin concentration dependence of the band edge energies relative to the Fermi level pinned by amphoteric indium impurities in bulk Pb_1?xSn_xTe. In n-type PbTePb_0.8Sn_0.2Te SL, cyclotron resonances together with Hall coefficient and Shubnikov de Haas (SdH) oscillation experiments support the existence of the PbTe wells. The results of submillimeter cyclotron resonances for n-type PbSe_0.08Te_0.92Pb_0.8Sn_0.2Te and PbSePb_0.88Sn_0.12Se SLs are also presented.     

Characterization of ZnO:Ga and ZnO:N films prepared by PLD

The thin films of zinc oxide have been produced by the pulse laser deposition method at various levels of gallium and nitrogen doping. To obtain the n-type films we used gallium doping with concentration of gallium from zero up to 5 at %. The dependence of photoluminescence of the epitaxial ZnO:Ga films on the concentration of gallium doping has been studied. An optimum range of the n-type ZnO films doping with gallium has been determined to obtain highly effective films from the viewpoint of realizing p-n transitions. This range, on the one hand, defines the maximal PL amplitude and, on the other hand, specifies the minimal specific resistance that corresponds to an interval of 0.125–1.000 at % Ga. To produce the p-type ZnO:(Ga, N) films, the ZnO targets with the content of GaN from zero up to 2 at % were used. N₂O was used as a buffer gas. A difference is observed in the positions of the peaks of the emission lines of the photoluminescence spectra for the ZnO films, doped with gallium (Ga) and co-doped with gallium and nitrogen (N).     

Chemical characterization by XPS of Cu/Ge ohmic contacts to n-GaAs

Chemical composition of Cu/Ge layers deposited on a 1 μm thick n-type GaAs epitaxial layer (doped with Te to a concentration of 5 × 10¹⁸ cm⁻³) and its interface were examined ex situ by XPS combined with Ar⁺ sputtering. These measurements indicate a diffusion of Cu and Ge from the Cu/Ge layer towards GaAs and, also, an out-diffusion of Ga and As from the GaAs layer to the metallic films. The Auger parameter corrected Auger spectra and XPS spectra show only Cu and Ge metals in the in the Cu/Ge layer and in the interface.     

Nonequilibrium paramagnetic susceptibility of gallium impurity centers in lead telluride

The temperature dependences of the resistance and magnetic susceptibility are studied in gallium-doped lead telluride, which is characterized by a delayed photoconductivity effect, under various illumination conditions. After a sample is illuminated at low temperatures, the magnetic susceptibility is diamagnetic in the region of metallic delayed conductivity (for T<=0 K). In the region of thermodynamic equilibrium (T<70 K), where conductivity is activational, the magnetic susceptibility is likewise diamagnetic and essentially equals the low-temperature value. A paramagnetic susceptibility peak is observed in the transitional region (T∼50–70 K), where the conductivity is of a nonequilibrium character but the carriers are still nondegenerate. This peak increases in magnitude with the rate of measurements in the indicated temperature range. In addition, a paramagnetic variation of the susceptibility following the Curie law is observed with uncontrollable (weak) illumination from the cryostat cap at low temperatures (T<25 K). The interpretation of the observed dependences is based on notions of variable valence of gallium in lead telluride, while the appearance of a paramagnetic susceptibility peak is attributed to the presence of shallow localized levels of gallium in a trivalent state. Zh. éksp. Teor. Fiz. 114, 1859–1867 (November 1998)     

Ballistic transport in PbTe-based nanostructures

We describe our study of ballistic transport in nanostructures of lead telluride, PbTe. Submicron devices have been fabricated by electron beam lithography and chemical etching of 50 nm wide PbTe single quantum wells embedded between Pb_0.92Eu_0.08Te barriers grown by MBE on BaF₂. The electron concentration in the devices was tuned by the gate voltage applied across an interfacial p–n junction. The most important observation was zero-magnetic field conductance quantization (in multiplies of 2e²/h) in narrow constrictions of dimensions comparable to electron mean free path calculated from transport mobility. This indicates considerable relaxation of requirements for quantum ballistic transport in comparison with other materials. We argue that the huge static dielectric constant of PbTe (₀=1350 at 4.2 K) leads to suppression of the long-range Coulomb potentials of charged impurities and, thus, provides favorable conditions for the conductance quantization.     

Formation of Gallium-induced nanostructures on single crystal HOPG surface

The room temperature growth of gallium atoms on the highly oriented pyrolytic graphite (HOPG) surface has been performed. The gallium atoms were deposited by thermal evaporation method in an ultra high vacuum system at a base pressure 5 × 10⁻¹⁰ torr. The X-ray photo electron spectroscopy (XPS) studies had been performed to confirm the presence of gallium atoms on HOPG surface. Scanning tunneling spectroscopy (STM) technique was employed to study the surface morphology of the clean HOPG surface and gallium covered HOPG surfaces which recognize the formation of gallium induced nanostructures. The deconvoluted XPS core level spectra of C (1s) and Ga (3d) demonstrate the possible interaction between substrate and the adsorbate atoms. The STM analysis revealed that the gallium deposition on HOPG led to significant change in the surface morphology. It was observed that the Ga atoms adsorbed as layer structure on HOPG surface for low coverage while quasi one-dimensional chain like nanostructure (1 ± 0.2 nm) has been formed for higher Ga coverage. The nanostructured surfaces induced by Ga deposition are found to be stable and could be used as a template for the growth of metallic nanostructures.     

Recombination at mixed-valence impurity centers in PbTe(Ga) epitaxial layers

The photoconductivity kinetics in PbTe(Ga) epitaxial films prepared by the hot-wall method is studied. The recombination of nonequilibrium photoexcited electrons at low temperatures was found to proceed in two stages, with a period of relatively fast relaxation followed by delayed photoconductivity. The temperature at which delayed photoconductivity appears increases with decreasing film thickness. The relaxation rate over the period of fast relaxation depends on film thickness and is the lowest in the thinnest layers. In semi-insulating films, photoconductivity is always positive, whereas in samples with lower electrical resistivity positive and negative photoconductivities are observed to coexist. The data obtained are discussed in terms of a model in which the impurity gallium atom can be in more than one charged state.     

Ag2+δTe薄膜的低温纵向磁电阻效应

采用高真空电子束蒸发镀膜方法制备了不同结构的Ag-Te多层膜,通过热处理得到了不同自掺杂浓度δ的非磁性Ag_2+δTe薄膜.Ag_2.1Te薄膜在低温下有很大的负纵向磁电阻,并且出现电阻率和磁电阻的多重峰现象.这可能与Ag的掺杂度δ和测量温度有很大关系,用掺杂的能带理论解释了有关的多重峰现象. 关键词：     

X-ray characterization of precipitates in cerium-doped PbTe and SnTe crystals

The homogeneity, solid solubility, and chemical bonds in the new materials PbTe, SnTe doped with Ce were investigated. Scanning electron microscope observation and electron probe microanalysis carried out on PbTe crystals doped with Ce, revealed three types of Ce-rich precipitates with following compositions: CeTe₂, Ce₃Te₇, Ce₂Te₅ and small admixture of PbTe in precipitates. The solubility of Ce in PbTe matrix was estimated as 0.5±0.1 at. %. The solubility of PbTe in CeTe₂ and Ce₃Te₇ was found to be 3±0.5 at. %, but 7±0.5 at. % in the case of Ce₂Te₅. In SnTe crystal doped with Ce only one kind of precipitate with composition Ce₂SnTe₅ was found. Cerium solubility in SnTe matrix was estimated to be 1±0.25 at. %. According to our knowledge this is the first report of the identification of Ce₂SnTe₅ compound. The similar compounds Ce₂SnS₅ and Ce₂SnSe₅ are known. Received: 9 December 1998 / Accepted: 9 February 1999 / Published online: 28 April 1999     

Evidence for insulator–metal transition in amorphous chalcogenide Se–Ge–Te films

The temperature dependence of the dc conductivity and thermoelectric power was determined for five different amorphous chalcogenide Se–Ge–Te films, with Ge?=?3.0–22?at.%, Se?=?0–97?at.% and Te?=?0–97?at.%. The films were prepared by thermal evaporation of GeSe₄, GeTe₄ and GeSe₂Te₂ quenched bulk materials. Values of the activation energy calculated from the temperature dependence of both electrical conductivity and thermoelectric power showed a decrease with increasing Ge content in the Se–Ge films as well as with replacement of Te for Se in the Se–Ge–Te films. The results showed an Anderson transition, with the conductivity showing insulating behaviour on the Ge–Se side to metallic behaviour at the binary composition Ge–Te. The radius of localization was obtained for the different compositions investigated. The wave function associated with the charge carriers at the composition Ge_3.3Te_96.7 is non-localized. A minimum metallic conductivity of 237?±?5?(Ω?cm)^?1 was found.