Electron states in inversion layers on the small-gap semiconductor Hg1−x Cd x Te

Summary The subband structure of the narrow-gap semiconductor Hg_1−x Cd _x Te is investigated theoretically at finite temperature by a modified sixbandk·p model based on the effective-mass approximation. A study of the complete surface energy spectrum is reported. The mobile, bound and resonant states are treated by considering energy states in a large quantum box. The theory is applied to cases with strong interband interaction when the resonant character of the subband states is prominent. The authors of this paper have agreed to not receive the proofs for correction.     

Resonant tunneling GaAs/AlGaAs quantum well structures for p-i-n photovoltaic cells

This study is devoted to the development of resonant-tunneling structures of quantum wells implementing resonant matching of lower subbands of size quantization in an electric field of the p-i-n junction of photovoltaic elements. The method for controlling the lower subband position in quantum wells by introducing a series of the tunnel-transparent barriers into a quantum well is proposed. The possibility of varying the level position in deep quantum wells in a wide range up to the continuous spectrum is demonstrated on a grown model structure; in this case, agreement between calculated and experimental subband positions is achieved.     

Influence of N cluster states on band dispersion in GaInNAs quantum wells

We use a modified band-anticrossing (BAC) model to investigate the band dispersion in a GaN_xAs_1-x/AlGaAs quantum well (QW) as a function of hydrostatic pressure. The band edge mass increases considerably more quickly with pressure than in the case of a GaAs/AlGaAs QW, and the subband separation also decreases significantly. We predict that the strong anticrossing interaction between the GaAs host conduction band and isolated N levels will inhibit tunnelling through the QW for a range of energy above the isolated N levels. The energy of N resonant states depends strongly on details of the local environment, giving a broader calculated distribution of N states in GaInNAs compared to GaNAs.     

The role of orbital ordering in the formation of electron structure in undoped LaMnO3 manganites in the regime of strong electron correlations

The electron structure of undoped LaMnO₃ and slightly doped La_1?x Sr_xMnO₃ manganites has been calculated within the framework of a generalized tight binding method with explicit allowance for strong intraatomic electron correlations. According to the results of these calculations, the ground state in orbitally disordered undoped LaMnO₃ ferromagnets would be metallic despite the Mott-Hubbard correlation gap in the spectrum of quasiparticles. Owing to the orbital ordering, the insulating state is stabilized in both antiferromagnetic and paramagnetic phases. In-gap states of a polaron nature with a spectral weight proportional to the dopant concentration have been found near the top of the valence band in La_1?x Sr_xMnO₃. As the doping level increases, a metal state appears in the ferromagnetic phase, which has a metallic character for one spin subband and an insulating character for the other subband (representing the so-called half-metallic state).     

AlxGa1-x N/GaN调制掺杂异质结构的子带性质研究

通过低温和强磁场下的磁输运测量研究了Al_0.22Ga_0.78N/GaN调制掺杂异质结构中2DEG的子带占据性质和子带输运性质.在该异质结构的磁阻振荡中观察到了双子带占据现象，并发现2DEG的总浓度随第二子带浓度的变化呈线性关系.得到了该异质结构中第二子带被2DEG占据的阈值电子浓度为7.3×10¹²cm^-2.采用迁移率谱技术得到了不同样品的分别对应于第一和第二子带的输运迁移率.发现当样品产生应变弛豫时第一子带的电子迁移 关键词： AlGaN/GaN异质结 二维电子气 子带占据 输运迁移率     

Observation of localized modes in TbOx and TbAl2 by resonance absorption of γ-quanta

In order to investigate excitations in TbO_x and TbAl₂, the nuclear resonant absorption of the 58 keV Mössbauer line supplied by¹⁵⁹Dy sources was studied.Localized states observed in both compounds are partly attributed to excitations of the terbium ion within theJ=6 state multiplet. In the case of TbO_x, this is confirmed by an investigation of the Schottky anomalies in the specific heat. In TbAl₂, the observed excitation states agree with values calculated by using parameters from magnetization data. Other localized states appearing in TbO_x are assigned to vibrations of the terbium ion. The cut off frequencies of the terbium oscillations are determined to be 18 meV (TbO_x) and 15 meV (TbAl₂) in agreement with conclusions from Debye Waller factors also investigated in the present experiments.     

STRUCTURE STUDY OF MODULATION-DOPED Cd1-xMnxTe: In/CdTe STRAINED LAYER MULTIPLE QUANTUM WELLS

The photomodulated reflectivity (PR) spectroscopy of modulation-doped diluted mag-netic semiconductor Cd_1-xMn_xTe: In/CdTe multiple quantum wells has been measured at 20-300K. Several spectral features associated with intersubband transitions have been found. The band structure of Cd_1-xMn_xTe: In/CdTe has been calculated by the Hartree self-consistent method. The results show that the theory is in agreement with experiments. In addition, an abnormal transition intensity ratio of 22H (the second heavy hole subband to the second electroa subband) to 11H (the first heavy hole subband to the first electron subband) caused by electron filled effect has been reported. At low temperature, a feature associated with Fermi level is observed, which has not been reported before.     

Resonant Raman scattering in ultrafine CdSxSe1−x colloidal particles

Vibrational spectra of ultrafine (～1.8 nm) CdS _x Se_1?x colloidal nanoparticles (NPs) are studied by resonant Raman scattering (RRS). The detected difference of the shape and spectral position of the longitudinal optical vibration band in comparison with the spectrum of slightly larger NPs (≥ 2 nm) is explained by the dominance of surface atoms as their fraction increases above 50%. The correlation of experimental results with ab initio calculated vibrational spectra and with vibration anharmonicity enhancement as the NP size decreases is discussed.     

p—HgCdTe反型层中子能带电子的基态能量

用变分自洽方法求解了ｐ－ＨｇＣｄＴｅ金属－绝缘体－半导体（ＭＩＳ）结构Ｎ型反型层子能带的基态能量Ｅ０及其与表面电子浓度的关系。计算中考虑了窄禁带半导体Ｈｇ１－ｘＣｄｘＴｅ带间相互作用所引起的非抛物带结构、波函数平均效应、共振缺陷态、Ｚｅｎｅｒ隧穿、以及电场在屏蔽长度内的衰减等因素，导出了子能带基态能量Ｅ０的计算公式并获得了与实验符合较好的结果。     

p-HgCdTe反型层中子能带电子的基态能量

用变分自洽方法求解了ｐ－ＨｇＣｄＴｅ金属－绝缘体－半导体（ＭＩＳ）结构Ｎ型反型层子能带的基态能量Ｅ₀及其与表面电子浓度的关系，计算中考虑了窄禁带半导体Hg_1-xCd_xTe带间相互作用所引起的非抛物带结构、波函数平均效应、共振缺陷态、Ｚｅｎｅｒ隧穿、以及电场在屏蔽长度内的衰减等因素，导出了子能带基态能量Ｅ₀的计算公式并获得了与实验符合较好的结果。     

The effect of hydrostatic pressure on subband structure and optical transitions in modulation-doped quantum well

Within the framework of effective-mass approximation, we have calculated theoretically the effects of hydrostatic pressure and doping concentration on subband structure and optical transitions in modulation-doped GaAs/Al_xGa_1−xAs quantum well for different well widths. The electronic structure of modulation-doped quantum well under the hydrostatic pressure is determined by solving the Schrödinger and Poisson equations self-consistently. The results obtained show that intersubband transitions and the subband energy levels in the modulation-doped quantum well can be significantly modified and controlled by the well width, donor concentration and hydrostatic pressure.     

Theoretical investigation of intersubband transition in AlxGa1−xN/GaN/AlyGa1−yN step quantum well

A modified self-consistent method is introduced for the design of Al_xGa_1−xN/GaN step quantum well (SQW) with the position and energy-dependent effective mass. The effects of nonparabolicity are included. It is shown that the nonparabolicity effect is minute for the lowest subband energy level and grows in size for the higher subband states. The effects of nonparabolicicty have significant influence on the transition energies and the oscillator strengths and should be taken into account in the investigation of the optical transitions. The strong asymmetric property introduced by the step quantum well magnifies the weak intersubband transition from the ground state to the third state (1→3). It is shown that in an appropriate scope, the intersubband transition (1→3) has the comparable oscillator strength with transition from the ground state to the second one (1→2), which suggests the possible application of the two-color photodetectors. The results of this work should provide useful guidance for the design of optically pumped asymmetric quantum well lasers and quantum well infrared photodetectors (QWIPs).     

Bound to continuum intersubband transition optical properties in the strain reducing layer-assisted InAs quantum dot structure

In this paper, the impact of wetting layer, strain reducing layer and dot height on the electronic, linear and nonlinear optical properties of bound to continuum states transitions are investigated in a system of InAs truncated conical shaped quantum dot covered with the In_xGa_1−x As strain reducing layer. The electronic structure, containing two main states of S and wetting layer states (WL), was calculated by solving one electronic band Hamiltonian with effective-mass approximation. The results reveal that the presence of the strain reducing layer in the structure extends the quantum dot emission to longer wavelength which is reported as a red-shift of the photoluminescence (PL) peak in the experimental measurement. This study also highlights the possibility of improving the intersubband optical properties based on the significant size-dependence of the three layer dot matrix by employing the strain reducing and wetting layers. According to this simulation, relatively tall dots on the thick wetting layer introduce the optimized structure size for practical applications to meet the SRL assisted enhanced dot structure.     

Resonant photoemission spectroscopy of Cu(InGa)Se2 materials for solar cells

The electron structure of CuIn_{1 ? x} Ga _x Se₂ single crystals is determined via resonant photoemis-sion and the main regularities of its transformation upon varying concentration x from 0 to 1 are established. The dependence of the shape of valence band spectra on the photon energy is studied. Integral photoemission intensities are shown to be determined by atomic photoionization cross sections. Processes of the direct and two-step creation of photoelectrons accompanying photoemission and the participation of internal states in the spectra of electrons from valence bands are studied. Two-hole final states in photoemission are obtained upon threshold excitation of the Cu 2p level. The strong interaction of holes leads to the multiplet splitting of these states. Partial densities of the components’ states are determined using the energy dependence of atomic photoionization cross sections.     

Tunneling time correction to the intersubband optical absorption in a THz laser-dressed GaAs/Al<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Ga<Subscript>1-</Subscript><Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>As quantum well

Tunneling effect on the intersubband optical absorption in a GaAs/Al _x Ga_{1- x} As quantum well under simultaneous presence of intense non-resonant laser and static electric fields is theoretically investigated. Based on the shooting method the quasi-stationary energy levels and their corresponding linewidths are obtained. By considering the joint action of the two external fields the linear absorption coefficient is calculated by means of Fermi’s golden rule and taking into account the intersubband relaxation. We found that: (i) the linewidth broadening due to the electron tunneling has an appreciable effect on the absorption spectrum; (ii) a constant relaxation time adopted in the previous studies could not be justified even for moderate electric fields, especially in the laser dressed wells. Our model predicts that the number of absorption peaks can be controlled by the external applied fields. While in the high-electric fields the excited states become unbounded due to a significant tunneling of the electrons, for high laser intensities and low/moderate electric fields the absorption spectrum has a richer structure due to the laser-generated resonant states. The possibility of tuning the resonant absorption energies by using the combined effects of the static electric field and the THz coherent radiation field can be useful in designing new optoelectronic devices.     

Effect of interface roughness on the density of states of finite barrier height quantum wells

We calculate the density of states of a 2D electron gas in finite barrier height quantum wells with the explicit inclusion of the interface roughness effect. By using Feynman path-integral method, the analytic expression is derived. The results show that the 2D density of states is dependent on the RMS of the fluctuation potential. The interface roughness causes localized states below the subband edge. We also apply the theory to model the finite barrier height quantum wells in Al_xGa_1?xAs/GaAs.     

The atomic and electron structure of ZrO2

The atomic structure of amorphous and crystalline zirconium dioxide (ZrO₂) films is studied using X-ray diffraction and extended X-ray absorption fine structure techniques. The electron structure of ZrO₂ is experimentally determined using X-ray and UV photoelectron spectroscopy, and the electron energy band structure is theoretically calculated using electron density functional method. According to these data, the valence band of ZrO₂ consists of three subbands separated by an ionic gap. The upper subband is formed by the O2p states and Zr4d states; the medium subband is formed by the O2s states; and the narrow lower subband is formed predominantly by the Zr4p states. The bandgap width in amorphous ZrO₂, as determined using the electron energy loss spectroscopy data, amounts to 4.7 eV. The electron band structure calculations performed for a cubic ZrO₂ phase point to the existence of both light (0.3m ₀) and heavy (3.5m ₀) holes, where m ₀ is the free electron mass. The effective masses of band electrons in ZrO₂ fall within (0.6–2.0)m ₀.     

Resonant acceptor states and terahertz stimulated emission of uniaxially strained germanium

The stimulated emission spectrum of uniaxially strained p-Ge is presented. The energy spectrum of the states of a shallow acceptor in Ge under uniaxial compression is calculated. The threshold pressure at which the acceptor state split off from the ground state becomes resonant is found. The pressure dependence of the width of this resonant level is calculated. The stimulated emission lines are identified. In particular, it is shown that the principal emission peak corresponds to the transition of holes from the resonant 1s (1s _r) state to the local p _±1 state. The probabilities of optical transitions are calculated. A mechanism of population inversion due to the intense resonant scattering of hot holes with an energy corresponding to the position of the 1s _r level is proposed. Zh. éksp. Teor. Fiz. 115, 89–100 (January 1999)     

Influence of a magnetic field on the Jahn-Teller band effect in a conducting ferromagnet

A simplified model of the Jahn-Teller band effect in a conducting ferromagnet with two degenerate subbands with the peak density of states of itinerant electrons is considered. It is found that the martensite transition temperature in a narrow-band conductor as a function of the position of the Fermi level near the peak of the energy density of states varies nonmonotonically in the narrow spin electron subband. The magnetic field dependence of the martensite-austenite structural phase transition temperature in the ferromagnet is analyzed. The developed theory and calculated data for the electron density of states in Ni₂MnGa are used as the basis for estimating the variation of the martensite transition temperature with the magnetic field (?T _m /?H), which demonstrates a satisfactory agreement with experimental data for the Heusler alloy Ni_2+x Mn_1?x Fe_yGa_1?y .     

Magnetotransport, optical, and electronic subband properties in AlxGa1−xN/AlN/GaN heterostructures

The Shubnikov-de Haas (S-dH) results at 1.5 K for Al_xGa_1−xN/AlN/GaN heterostructures and the fast Fourier transformation data for the S-dH data indicated the occupation by a two-dimensional electron gas (2DEG) of one subband in the GaN active layer. Photoluminescence (PL) spectra showed a broad PL emission about 30 meV below the GaN exciton emission peak at 3.474 eV that could be attributed to recombination between the 2DEG occupying in the AlN/GaN heterointerface and photoexcited holes. A possible subband structure was calculated by a self-consistent method taking into account the spontaneous and piezoelectric polarizations, and one subband was occupied by 2DEG below the Fermi level, which was in reasonable agreement with the S-dH results. These results can help improve understanding of magnetotransport, optical, and electronic subband properties in Al_xGa_1−xAs/AlN/GaN heterostructures.