States of antimony and tin atoms in lead chalcogenides

It is shown by Mössbauer spectroscopy of the ¹¹⁹Sb(^119m Sn) isotope that impurity antimony atoms in PbS, PbSe, and PbTe lattices are distributed between cation and anion sublattices. In n-type samples, the greatest part of antimony is located in the anion sublattice; in hole ones, in the cation sublattice. The tin atoms formed as a result of radioactive decay of ¹¹⁹Sb (antisite state) are electrically inactive in the anion sub-lattice of PbS and PbSe, while, in the cation sublattice, they form donor U ^? centers. Electron exchange between the neutral and doubly ionized tin U ^? centers via the allowed band states is observed. The tin atoms formed after radioactive decay of ¹¹⁹Sb are electrically inactive in the anion and cation sublattices of PbTe.     

Antistructural defects in PbTe-type semiconductors

Mössbauer emission spectroscopy on the isotope ^119m Te(^119m Sn) is used to identify tin impurity centers in PbTe and PbS lattices. Using the emission variant of Mössbauer spectroscopy makes it possible to stabilize the tin impurity atoms in the anion sublattice, i.e., to obtain antistructural defects. The charge state of the tin impurity atoms displaced from the anion sublattice is found to depend on the position of the Fermi level.     

Local symmetry and electronic structure of tin atoms in (Pb<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Sn<Subscript>x</Subscript>)<Subscript>1−<Emphasis Type="Italic">z</Emphasis></Subscript>In<Subscript>z</Subscript>Te lattices

Mössbauer spectroscopy of the ¹¹⁹Sn isotope has shown that the superconducting transition in (Pb_1?xSn_x)_1?zIn_zTe solid solutions is not accompanied by a change in the local symmetry of tin atoms or in their electronic structure.     

Determining the position of antimony impurity atoms in PbS by 119Sb(119m Sn) emission Mössbauer spectroscopy

Emission Mössbauer spectroscopy based on the isotope ¹¹⁹Sb(^119m Sn) is used to show that the location of antimony impurity atoms in the PbS lattice depends on the conductivity type of the material: in n-type samples the antimony is localized primarily on the anion sublattice, while in p-type material it is primarily on the cation sublattice. It is noteworthy that when the ^119m Sn center appears in the anion PbS sublattice (i.e., as an antisite defect) after radioactive conversion of ¹¹⁹Sb, its charge state does not depend on the position of the Fermi level. When the ¹¹⁹Sb-center is in the cation sublattice of PbS, it acts like an electrically active substitutional impurity: in n-type samples the spectrum corresponds to the neutral state of a donor center (^119m Sn²⁺), while in p-type material it corresponds to the doubly ionized state of this center (^119m Sn⁴⁺).     

Two-electron germanium centers with a negative correlation energy in lead chalcogenides

It is shown that the charge state of the ⁷³Ge antisite defect that arises in anionic sublattices of PbS, PbSe, and PbTe after radioactive transformation of ⁷³As does not depend on the position of the Fermi level, whereas the ⁷³Ge center in cationic sublattices of PbS and PbSe represents a two-electron donor with the negative correlation energy: the Mössbauer spectrum for the n-type samples corresponds to the neutral state of the donor center (Ge²⁺), while this spectrum corresponds to the doubly ionized state (Ge⁴⁺) of the center in the p-type samples. In partially compensated PbSe samples, a fast electron exchange between the neutral and ionized donor centers is realized. It is shown by the method of Mössbauer spectroscopy for the ¹¹⁹Sn isotope that the germanium-related energy levels are located higher than the levels formed in the band gap of these semiconductors by the impurity tin atoms.     

Statistics of electrons in PbS with <Emphasis Type="Italic">U</Emphasis> centers

The dependence of the concentration of two-electron tin centers in the intermediate charge state Sn³⁺ in PbS on the correlation energy is obtained using the Gibbs distribution. It is shown that this state cannot be observed by Mössbauer spectroscopy on an ¹¹⁹Sn isotope (due to insufficient sensitivity), but it can manifest itself in the temperature dependence of the hole density in Pb_1?x?ySn_xNa_yS solid solutions.     

On the density-of-states effective mass and charge-carrier mobility in heteroepitaxial films of bismuth telluride and Bi<Subscript>0.5</Subscript>Sb<Subscript>1.5</Subscript>Te<Subscript>3</Subscript> solid solution

It is shown that the Seebeck coefficient α, the power factor α²σ, and the density-of-states effective mass m/m ₀ in heteroepitaxial films of Bi0.5Sb1.5Te3 solid solution are higher than the corresponding characteristics of bulk thermoelectric materials. The elevated values and weak temperature dependences of these parameters lead to a rise in the parameter proportional to the effective mass, the charge-carrier mobility, and the figure of merit. The character of change in α, α²σ, and m/m ₀ is determined by the peculiarities of the mechanism of charge-carrier scattering, the anisotropy of the constant-energy surface, and the possible influence of topological surface states of Dirac fermions in the films.     

Alternating current conductivity and electron spin resonance of the Cd<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Fe<Subscript>x</Subscript>Te alloys

The results of measurements of frequency dependences of conductivity and electron spin resonance (ESR) of the Cd_{1 ? x} Fe_xTe alloys (0.01 ≤ x ≤ 0.05) at room temperature are presented. It is found that, in the composition range 0.003 ≤ x ≤ 0.05, a new asymmetric ESR absorption line with g ≈ 3.9 emerges, and its hysteresis manifests itself while the samples are magnetized. The line with g ≈ 3.9 is attributed to the charge state 3⁺ of the Fe atoms, which occurs in tetrahedra with three Fe atoms.     

Two-electron tin centers arising in glassy chalcogenides of arsenic due to nuclear reactions

Impurity ^119m Sn atoms arising as a result of radioactive decay of parent ^119mm Sn atoms in the structure of the glasses As₂S₃, As₂Se₃, and As₂Te₃ are part of the glass composition in the form of structural units corresponding to tetravalent tin. The impurity ^119m Sn atoms formed as a result of radioactive decay of ¹¹⁹Sb atoms in the structure of the As₂S₃ and As₂Se₃ glasses are localized at the arsenic sites and play the role of two-electron centers with a negative correlation energy. For the As₂Te₃ glass, similarly formed ^119m Sn atoms are electrically inactive. The greatest part of the daughter ^119m Sn atoms arising after radioactive decay of parent ^119m Te atoms are located at the chalcogen sites and are electrically inactive in the As₂S₃, As₂Se₃, and As₂Te₃ glasses. A significant recoil energy of daughter atoms in the case of the ^119m Te radioactive decay brings about the appearance of the ^119m Sn displaced atoms.     

Mechanism of the Generation of Donor–Acceptor Pairs in Heavily Doped <Emphasis Type="Italic">n</Emphasis>-ZrNiSn with the Ga Acceptor Impurity

The nature of the mechanism of the simultaneous generation of donor–acceptor pairs under heavy doping of n-ZrNiSn intermetallic semiconductor with the Ga acceptor impurity is established. Such spatial arrangement in the crystal lattice of ZrNiSn_1–xGa _x is found when the rate of movement of the Fermi level εF found from calculations of the density distribution of electron states coincides with that experimentally established from dependences lnρ(1/T). It is shown that when the Ga impurity atom (4s²4p¹) occupies the 4b sites of Sn atoms (5s²5p²), structural defects of both acceptor nature and donor nature in the form of vacancies in the 4b site are simultaneously generated. The results are discussed in the scope of the Shklovskii–Efros model of a heavily doped and compensated semiconductor.     

Charge transport in silver chalcogenides in the region of phase transition

Data on the σ(T), R(T), and U(T) dependences in Ag₂Te, Ag₂Se, and Ag₂S in the region of the phase transition are analyzed. It is found that the phase transition in Ag₂Te is accompanied by a decrease in the electron concentration and this transition in Ag₂Se is accompanied by an increase in this concentration. The concentration of intrinsic charge carriers in Ag₂Te decreases by a factor of 4 as a result of the phase transition and increases by a factor of 2 in Ag₂Se. The effect of variation in the energy-band parameters in the region of phase transition on the electron mobility is considered. It is established that, in Ag₂Te and Ag₂S, electrons are scattered by optical phonons in the region of the phase transition, while electrons are scattered by acoustic phonons in the α and β phases. It is assumed that the anomalously large increase in σ and U in Ag₂S as a result of the phase transition is caused by an increase in the concentration n and a simultaneous decrease in σ _g and m _n ^* by a factor of about 2.     

Energy spectrum of charge carriers in Ag<Subscript>2</Subscript>Te

On the basis of investigations of the temperature and concentration dependences of kinetic coefficients (the Hall coefficientR, the electrical conductivity σ, and thermopower α₀) in n-type Ag₂Te, it is established that Ag atoms in Ag₂Te create the shallow donor levels located at a distance of (0.002?7 × 10^?5 T) eV from the bottom of the conduction band. It is shown that silver telluride has n-type conductivity starting with the deficiency of Ag ≥ 0.01 at % in the stoichiometric composition, and it is practically impossible to achieve the stoichiometric composition in Ag₂Te.     

Electron exchange between neutral and ionized germanium centers in PbSe

It is shown that the charge state of the antisite ⁷³Ge defect formed in the anion sublattice of PbSe as a result of radioactive transformation of ⁷³As does not depend on the Fermi level position. In contrast, the ⁷³Ge center in the cation sublattice of PbSe is an electrically active substitutional impurity: the electron energy spectrum corresponds to the neutral state of the donor center (Ge²⁺) in the n-type samples and to the doubly ionized states of this center (Ge⁴⁺) in the p-type samples. For partially compensated samples, fast electron exchange between the neutral and ionized donor centers is observed.     

Determining and Controlling the Magnesium Composition in CdTe/CdMgTe Heterostructures

The relationships between Mg composition, band gap, and lattice characteristics are investigated for Cd_1?x Mg _x Te barrier layers using a combination of cathodoluminescence, energy dispersive x-ray spectroscopy, variable angle spectral ellipsometry, and atom probe tomography. The use of a simplified, yet accurate, variable angle spectral ellipsometry analysis is shown to be appropriate for fast determination of composition in thin Cd_1?x Mg _x Te layers. The validity of using high-resolution x-ray diffraction for CdTe/Cd_1?x Mg _x Te double heterostructures is discussed. The stability of CdTe/Cd_1?x Mg _x Te heterostructures are investigated with respect to thermal processing.     

Formation and annealing of radiation defects in tin-doped <Emphasis Type="Italic">p</Emphasis>-type germanium crystals

The effect of tin on the formation and annealing of radiation defects in p-type germanium crystals irradiated with 6-MeV electrons at a temperature of 80 K is studied. It is shown that acceptor complexes SnV with a hole ionization enthalpy of 0.16 eV are dominant in irradiated Ge:(Sn, Ga) crystals after their heating to a temperature of 300 K. These complexes disappeared as a result of the annealing of irradiated crystals in the temperature range 30–75°C. Annealing of irradiated crystals at temperatures in the range 110–150°C brings about the formation of deep-level centers with a donor level at E _v + 0.29 eV; this center is presumably related to a complex consisting of a tin atom and an interstitial gallium atom.     

Radiation-induced bistable centers with deep levels in silicon <Emphasis Type="Italic">n</Emphasis> <Superscript>+</Superscript>–<Emphasis Type="Italic">p</Emphasis> structures

The method of deep level transient spectroscopy is used to study electrically active defects in p-type silicon crystals irradiated with MeV electrons and α particles. A new radiation-induced defect with the properties of bistable centers is determined and studied. After keeping the irradiated samples at room temperature for a long time or after their short-time annealing at T ～ 370 K, this defect does not display any electrical activity in p-type silicon. However, as a result of the subsequent injection of minority charge carriers, this center transforms into the metastable configuration with deep levels located at E_V + 0.45 and E_V + 0.54 eV. The reverse transition to the main configuration occurs in the temperature range of 50–100°C and is characterized by the activation energy ～1.25 eV and a frequency factor of ～5 × 10¹⁵ s^–1. The determined defect is thermally stable at temperatures as high as T ～ 450 K. It is assumed that this defect can either be a complex of an intrinsic interstitial silicon atom with an interstitial carbon atom or a complex consisting of an intrinsic interstitial silicon atom with an interstitial boron atom.     

Features of the Current Flow in Injection Structures Based on PbSnTe:In Films

This paper reports a study of the current–voltage (I–V) features of the p-i-p structures based on Pb_1–xSn_xTe:In films with the tin content х ≈ 0.31 in which the metal–insulator transition occurs. It is shown that the photocurrent is the hole current under light interband excitation, and electron trapping is dominant. The experimental and theoretical data are compared.     

Features of conductivity mechanisms in heavily doped compensated V<Subscript>1–<Emphasis Type="Italic">x</Emphasis> </Subscript>Ti<Subscript> <Emphasis Type="Italic">x</Emphasis> </Subscript>FeSb Semiconductor

The crystal and electronic structure and also the energy and kinetic properties of n-VFeSb semiconductor heavily doped with the Ti acceptor impurity are investigated in the temperature and Ti concentration ranges of T = 4.2–400 K and N_A^Ti ≈ 9.5 × 10¹⁹–3.6 × 10²¹ cm^–3 (x = 0.005–0.20), respectively. The complex mechanism of the generation of acceptor and donor structural defects is established. It is demonstrated that the presence of vacancies at Sb atomic sites in n-VFeSb gives rise to donor structural defects (“a priori doping”). Substitution of the Ti dopant for V in VFeSb leads simultaneously to the generation of acceptortype structural defects, a decrease in the number of donor defects, and their removal in the concentration range of 0 ≤ x ≤ 0.03 via the occupation of vacancies by Sb atoms, and the generation of donor defects due to the occurrence of vacancies and an increase in their number. The result obtained underlies the technique for fabricating new n-VFeSb-based thermoelectric materials. The results are discussed in the context of the Shklovsky–Efros model for a heavily doped compensated semiconductor.     

Characterization of HgCdTe Films Grown on Large-Area CdZnTe Substrates by Molecular Beam Epitaxy

Recent advances in growth of Hg_1?x Cd _x Te films on large-area (7 cm × 7.5 cm) CdZnTe (CZT) substrates is presented. Growth of Hg_1?x Cd _x Te with good uniformity on large-area wafers is achieved using a Riber 412 molecular beam epitaxy (MBE) tool designed for growth of Hg_1?x Cd _x Te compounds. The reactor is equipped with conventional CdTe, Te, and Hg sources for achieving uniform exposure of the wafer during growth. The composition of the Hg_1?x Cd _x Te compound is controlled in situ by employing a closed-loop spectral ellipsometry technique to achieve a cutoff wavelength (λ _co) of 14 μm at 78 K. We present data on the thickness and composition uniformity of films grown for large-format focal-plane array applications. The composition and thickness nonuniformity are determined to be <1% over the area of a 7 cm × 7.5 cm wafer. The films are further characterized by Fourier-transform infrared spectroscopy, optical microscopy, and Hall measurements. Additionally, defect maps show the spatial distribution of defects generated during the epitaxial growth of the Hg_1?x Cd _x Te films. Microdefect densities are in the low 10³ cm^?2 range, and void defects are below 500 cm^?2. Dislocation densities less than 5 × 10⁵ cm^?2 are routinely achieved for Hg_1?x Cd _x Te films grown on CZT substrates. HgCdTe 4k × 4k focal-plane arrays with 15 μm pitch for astronomical wide-area infrared imagers have been produced using the recently developed MBE growth process at Teledyne Imaging Sensors.     

Mobility of minority charge carriers in <Emphasis Type="Italic">p</Emphasis>-HgCdTe films

Temperature dependences of electron mobility in p-Hg_1?xCd_xTe films (x=0.210–0.223) grown by molecular beam epitaxy are investigated. In the temperature range 125–300 K, mobility was found by the mobility-spectrum method, and for the range 77–125 K, it was found using a magnetophotoconductivity method suggested in this study. The method is based on the measurement of the magnetic-field dependence of photoconductivity. The magnetic field is parallel to the radiation and normal to the sample surface. The electron mobility is determined using the simple expression μ _n [m²/(V s)]=1/B _H [T]. Here, B _H is the induction of the magnetic field corresponding to a half-amplitude of the photoconductivity signal under zero magnetic field. In the temperature range 100–125 K, the results obtained by the magnetophotoconductivity and mobility-spectrum methods coincide. For the samples investigated, the electron mobility at 77 K is in the range 5–8 m²/(V s).