Shallow acceptors in strained Ge/Ge1−x Six heterostructures with quantum wells

The energies of localized acceptor states in quantum wells (strained Ge layers in Ge/Ge_1?x Si_x heterostructures) were analyzed theoretically in relation to the quantum well width and the impurity position in the well. The impurity absorption spectrum in the far IR range is calculated. Comparison of the results of the calculation with experimental photoconductivity spectra allows an estimation of the acceptor distribution in the quantum well to be made. In particular, it was concluded that acceptors may largely concentrate near the heterointerfaces. The absorption spectrum is calculated taking into account the resonance impurity states. This allows the features observed in the short-wavelength region of the spectrum to be interpreted as being due to transitions into the resonance energy levels “linked” to the upper size-quantization subbands.     

Specific features of self-compensation in Er x Sn1 − x Se solid solutions

The effect of doping and degree of compensation on the conductivity activation energy ΔE _i in Er _x Sn_{1 ? x} Se has been investigated. The carrier concentration decreases at a low doping level in the case of low and moderate compensations. It is found that, beginning with x ≥ 0.005 at % Er, carriers in the solid solutions under study change sign, depending on the amount of substituted erbium in the SnSe samples that is in equilibrium with the selenium phase. This phenomenon can be explained in terms of the concepts of the self-compensation of donors in the Sn sublattice.     

Optical transitions in Cd x Hg1 ? x Te-based quantum wells and their analysis with account for the actual band structure of the material

Quantum-confinement levels in a Cd _x Hg_{1 ? x} Te-based rectangular quantum well are calculated in the framework of the four-band Kane model taking into account mixing between the states of electrons and three types of holes (heavy, light, and spin-split holes). Comparison of the calculation results with experimental data on the photoluminescence of Cd _x Hg_{1 ? x} Te-based quantum wells suggests that optical transitions involving the conduction and light-hole bands are possibly observed in the spectra.     

Large-scale modelling of the phase transitions in KTa1−xNbxO3 perovskite solid solutions

We use quantum chemical method of the intermediate neglect of the differential overlap (INDO) based on the Hartree–Fock formalism for a large-scale modelling of the atomic and electronic structure of KNb_xTa_1−xO₃ (KTN) perovskite solid solutions. Results for periodic defect models (large unit cell) of 40 and 320 atoms are compared with 135-atom INDO cluster calculations. Periodic Nb impurities in KTaO₃ reveal clear off-center displacement beginning with the smallest calculated concentrations, so does an isolated Nb impurity in a cluster INDO calculation. The magnitude of Nb off-center displacement is close to the X-ray-absorption fine-structure observation (0.27 a.u.). In contrast, Ta impurities in KNbO₃ always remain on-center, due to higher ionicity of Ta as compared to Nb atom. Using the calculated energy gain due to the off-center displacements of Nb atoms for several concentrations of Nb in KTN, we construct the non-empirical Ginzburg–Landau-type functional for the excess energy. The coefficients of this functional are concentration dependent. This dependence allows to define the type of the concentration-induced phase transition in KTaO₃ alloyed by Nb.     

InGaAs/InP heterostructures with strained quantum wells and quantum dots (λ=1.5–1.9 µm)

Strongly strained In_xGa_1−x As/In_0.53Ga_0.47As/InP heterostructures with indium content x=0.69−1.0 in the active region were investigated experimentally and theoretically. Two types of structures were obtained by vapor-phase epitaxy from metalorganic compounds: 1) with isolated compression-strained quantum wells and 2) with self-organized nanosize InAs clusters (quantum dots). The temperature dependence of the quantum radiation efficiency of samples with quantum wells in the temperature range 77–265 K is characterized by T ₀=43 K. One reason for the low value of T ₀ is electron delocalization in the active region. The maximum radiation wavelength obtained in structures with quantum dots is 1.9 μm at 77 K. Fiz. Tekh. Poluprovodn. 33, 1105–1107 (September 1999)     

Lateral transport and far-infrared radiation of electrons in In x Ga1 − x As/GaAs heterostructures with the double tunnel-coupled quantum wells in a high electric field

It is shown that the far-infrared radiation of electrons in the selectively doped heterostructures with double tunnel-coupled quantum wells in high lateral electric fields strongly depends on the level of doping of the wells. At a high impurity concentration in a narrow well, higher than (1−2) × 10¹¹ cm⁻², the radiation is caused only by indirect intrasubband electron transitions. At a lower concentration, along with the indirect transitions, the direct intersubband transitions also contribute to the radiation. These transitions become possible in high electric fields due to the real-space electron transfer between the quantum wells.     

Low-temperature photoluminescence and X-ray diffractometry study of InxGa1−x As quantum wells

The structures grown by molecular-beam epitaxy with In_xGa_1?x As quantum wells (QWs) in GaAs were studied by X-ray diffractometry and low-temperature photoluminescence techniques. The inhomogeneity of the QW composition along the growth direction was established. Energy positions of the exciton recombination lines in the QWs with step-graded In distribution were calculated, and good agreement with the experimental data was obtained.     

Microdeformations of the crystal lattice of PbTe1 − x Br x solid solutions

Microdeformations of the crystal lattice of PbTe_{1 ? x} Br _x solid solutions depending on the bromine content are determined by analysis of the physical broadening of X-ray reflections. It is found that they increase with the introduction of bromine from 0.188% (for undoped PbTe) to 0.283% for PbTe_{1 ? x} Br _x at x = 0.02. The microstrains remain constant with a further increase in the bromine content.     

Superstructured ordering in Al x Ga1 − x As and Ga x In1 − x P alloys

Epitaxial heterostructures produced on the basis of Al _x Ga_{1 ? x} As and Ga _x In_{1 ? x} P ternary alloys by metal-organic chemical vapor deposition are studied. The composition parameter x of the alloys was ～0.50. By X-ray diffraction studies, scanning electron microscopy, atomic force microscopy, and photoluminescence spectroscopy, it is shown that superstructured ordered phases with the stoichiometry composition III_{1 ? η}III_{1 + η}V₂ can be formed. As a consequence of this effect, not only does the cubic crystal symmetry change to the tetragonal type in the new compound, but also the optical properties become different from those of disordered alloy with the same composition.     

Hopping transport in doped (Pb0.78Sn0.22)1−x InxTe solid solutions

Transport coefficients in (Pb_0.78Sn_0.22)_1?x In_xTe solid solutions with indium content x=0.03 and 0.05 additionally doped with acceptors (Tl) or donors (Cl) have been measured and analyzed. The Seebeck coefficient S is positive for x=0.05 in the low temperature range 77–200 K; its sign changes to negative when the Tl acceptor is added. This unusual behavior of the thermoelectric power can be attributed to hopping conduction at a nonmonotonic energy dependence of the density of localized states. The density-of-states function has been determined at x=0.03 and 0.05 from experimental data on the thermopower. Theoretical estimates of the Nernst-Ettingshausen coefficient are made for x=0.03 additionally doped with Cl. The estimates are based on taking into account, along with the hopping conduction, the contribution from electrons with energies above the mobility edge and on using the critical electrical conductivity exponent obtained in the percolation theory. The activation energies characterizing the temperature dependences of conductivity and Hall and Nernst-Ettingshausen coefficients are discussed and compared.     

Photoconductivity relaxation processes in Cu1−xZnxInS2 solid solutions

Photoconductivity and its relaxation in Cu_1−xZn_xIn₂S₄ alloys (x=0–16) single crystals were investigated in the temperature range 30–100 K. The long-lasting relaxation processes (τ~10³ s) and induced photoconductivity phenomena were identified. The main parameters characterizing the photoconductivity kinetic were determined. The observed phenomena were analyzed by taking into account effects of the trapping levels. It is shown that relaxation of the photoconductivity and induced photoconductivity are controlled by the multicenter recombination in which both 'fast' and 'slow' recombination centers take part.     

Industrial prospects of Pb1 − x Sn x Te:In with x > 0.3 solid solutions for photodetectors with extended sensitivity spectral range

Photoelectric properties of Pb_{1 ? x} Sn _x Te:In films with composition x > 0.3 in the temperature range from 4.2 to 80 K have been investigated. High sample sensitivity to black-body radiation has been discovered at the temperature of helium, and as the temperature of the radiation source decreases the sensitivity increases, which can be connected with the optical-frequency transition in the short-wavelength infrared and terahertz spectral range. The detectivity value D* = 8.2 × 10¹⁶ cm · Hz^1/2/W corresponding to the NEP = 3.1 × 10^?18 W/Hz^1/2, has been obtained at detector temperature 4.2 K and T _BBR = 15 K.     

Hubbard energy of two-electron tin centers in PbS1−zTez solid solutions

It is shown by Mössbauer spectroscopy of the isotope ¹¹⁹Sn that an isovalent tin impurity in PbS_1?z Te_z solid solutions is a two-electron donor with a negative correlation energy, where the energy levels associated with tin centers are situated against the background of the valence band continuum. The Hubbard energy U is estimated for impurity tin atoms in PbS_1?z Te_z (|U|>0.2 eV), which is found to be substantially higher than for analogous tin centers in PbS_1?z Se_z solid solutions (|U|=0.058 eV).     

Optical phonons in CdGa2S4x Se4(1 − x) alloys

The Raman spectra of CdGa₂S_4x Se_{4(1 ? x)} alloys (x = 0.1, 0.2, … 0.9) are studied. Both the singlemode and double-mode behavior of optical phonons are observed in CdGa₂S_4x Se_{4(1 ? x)} alloys. The observed optical mode at 138 cm^?1 is independent of the composition. It seems likely that this mode is the “breathing mode” and is caused by atomic motion in the anion sublattice relative to vacancies. It is shown that the high-frequency modes of symmetry B ₁(LO, TO) and B ₂(LO, TO) are caused by the in-phase motion of atoms in the anion sublattice along the tetragonal axis c relative to trivalent Ga atoms. The doubly degenerate symmetry modes E ₁(LO, TO) and E ₂(LO, TO) are caused by the in-phase motion of atoms in the anion sublattice relative to trivalent Ga atoms of the cation sublattice in the xy plane (dipole 2Ga–4C), where C is S, Se. The optical symmetry modes B ₃(LO, TO) and B ₄(LO, TO) are associated with the motion of atoms in the anion sublattice relative to Cd atoms along the tetragonal axis c. The doubly degenerate modes E ₃(LO, TO) and E ₄(LO, TO) are associated with the motion of atoms in the anion sublattice relative to Cd atoms (dipole Cd–4C). The low-frequency modes B ₅(LO, TO) and E ₅(LO, TO) are the analogs of acoustic phonons at the edge of the Brillouin zone of sphalerite.     

Photosensitivity of thin-film structures based on (CuInSe2)x(2ZnSe)1−x solid solutions

Polycrystalline (CuInSe₂)_x(2ZnSe)_1−x films (x=0.6–1.0) with p-type conductivity and a thickness of 0.5–0.9 μm were obtained by pulsed laser evaporation. It is shown that a chalcopyrite-sphalerite transition occurs in the above system for x=0.7. The obtained films were used to fabricate the photosensitive structure of the In/p-(CuInSe₂)_x(2ZnSe)_1−x and InSe(GaSe)/(CuInSe₂)_x(2ZnSe)_1−x types. Spectral dependences of photovoltaic-conversion quantum efficiency were studied, and the photosensitivity of the structures in relation to the type of energy barrier and the composition was analyzed. It is concluded that the structures under consideration can be used as broadband photovoltaic converters. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 34, No. 5, 2000, pp. 576–581. Original Russian Text Copyright ? 2000 by V. Rud’, Yu. Rud’, Bekimbetov, Gremenok, Bodnar’, Rusak.     

Magnetotransport in a semimetal channel in p-Ga1−x InxAsySb1−y /p-InAs heterostructures with various compositions of the solid solution

Magnetotransport properties of an electron channel at the heteroboundary in type II separated p-Ga_1?x In_xAs_ySb_1?y /p-InAs heterostructures grown by LPE (x=0.09–0.22) were studied in the temperature range of 77–300 K. It is shown that an electron channel, which is formed at the heteroboundary and has high mobility μ=(3–5)×10⁴ cm² V^?1 s^?1, exists throughout the whole composition range. The band diagram of the heterostructures under study is discussed, and some parameters of the electron channel are evaluated. It is found that the electron channel with high mobility persists up to room temperature. Type II GaInAsSb/p-InAs heterostructures can find application in new Hall sensor devices with an electron channel at the heteroboundary.     

Study of lifetimes and photoconductivity relaxation in heterostructures with Hg x Cd1 ? x Te/Cd y Hg1 ? y Te quantum wells

Carrier lifetimes in the continuum of the quantum well of a Hg _x Cd_{1 ? x} Te/Cd _y Hg_{1 ? y} Te hetero-structure were studied by terahertz pump-probe spectroscopy. It is found that the relaxation duration of the transmission signal is ??65 ps and is independent of the pump power. Such rapid relaxation in these structures is most likely determined by the interaction of holes with acoustic phonons due to a high density of states in the valence band and a larger effective mass compared with electrons. By the obtained data, the times of the interband nonradiative recombination of holes are determined. In this publication, we report the results of numerical calculation of the energy spectrum of the model structure, in which the possibility of obtaining population inversion at specified concentrations of nonequilibrium carriers is analyzed.     

Ionization of the sulfur-related DX Center in In 1 − x Ga x P in an electric field

Using deep-level transient spectroscopy and capacitance-voltage measurements, the effect of electric field on ionization of the sulfur-related DX center is investigated for n-In_{1 ? x} Ga _x P layers (x = 0.65) grown by vapor-phase epitaxy. It is shown that, upon increasing the electric field strength from 1.3 × 10⁴ to 1.9 × 10⁵ V/cm, the activation energy of the center decreases from ～0.38 to ～0.26 eV, and the ionization process is governed by multiphonon tunneling. By the estimated tunneling time of the defect, it is concluded that the DX center corresponds to a model of large lattice relaxation.     

Optical properties of InGaAs/InGaAlAs quantum wells for the 1520–1580 nm spectral range

The optical properties of elastically strained semiconductor heterostructures with InGaAs/InGaAlAs quantum wells (QWs), intended for use in the formation of the active region of lasers emitting in the spectral range 1520–1580 nm, are studied. Active regions with varied lattice mismatch between the InGaAs QWs and the InP substrate are fabricated by molecular beam epitaxy. The maximum lattice mismatch for the InGaAs QWs is +2%. The optical properties of the elastically strained InGaAlAs/InGaAs/InP heterostructures are studied by the photoluminescence method in the temperature range from 20 to 140°C at various power densities of the excitation laser. Investigation of the optical properties of InGaAlAs/InGaAs/InP experimental samples confirms the feasibility of using the developed elastically strained heterostructures for the fabrication of active regions for laser diodes with high temperature stability.     

Technology and electronic properties of PHEMT AlGaAs/In y(z)Ga1 − y(z)As/GaAs compositionally graded quantum wells

Graded In _y Ga_{1 ? y} As quantum well epitaxial technology is developed for engineering the band potential profile. The crystal structure of the samples is clarified by high-resolution X-ray diffraction. The influence of quantum-well bending on the crystal and electron transport properties is studied on one- and two-side δ-doped Al_0.23Ga_0.77As/In _y Ga_{1 ? y} As/Al_0.23Ga_0.77As PHEMT heterostructures. The highest InAs content gradient reached is 1.2%/nm for the mean InAs content y = 0. 2. Optimization of the InAs content grading leads to an increase in the electron mobility and concentration. This effect is related to the straightening and deepening of the quantum-well potential profile. In addition, the electron wavefunction shifts toward the quantum-well center, thus reducing electron scattering.