Surface photovoltage spectroscopy of electronic surface states on cleaved germanium(111) surfaces

Surface photovoltage (SPV) measurements on UHV cleaved Ge(111) surfaces at 100 K are reported for photon energies 0.4 < ?ω < 1 eV. The SPV spectra are sensitive to surface treatment. Upon annealing to temperatures above 200°C, which is accompanied by a reconstruction change from the (2 × 1) to an (8) superstructure, the SPV spectrum shows 2 shoulders below band gap energy with threshold energies near 0.4 and 0.45 eV. These structures are interpreted in terms of electronic transitions from the valence band into empty surface state levels which are related to the (8) superstructure. Adsorbed oxygen and water vapor both cause new similar transitions from the valence band into empty surface states at 0.08 eV below the bottom of the conduction band.     

Optical and electrical properties of GaN: Si-based microstructures with a wide range of doping levels

The optical and electrical properties of silicon-doped epitaxial gallium nitride layers grown on sapphire have been studied. The studies have been performed over a wide range of silicon concentrations on each side of the Mott transition. The critical concentrations of Si atoms corresponding to the formation of an impurity band in gallium nitride (～2.5 × 10¹⁸ cm^?3) and to the overlap of the impurity band with the conduction band (～2 × 10¹⁹ cm^?3) have been refined. The maximum of the photoluminescence spectrum shifts nonmonotonically with increasing doping level. This shift is determined by two factors: (1) an increase in the exchange interaction leading to a decrease in the energy gap width and (2) a change in the radiation mechanism as the donor concentration increases. The temperature dependence of the exciton luminescence with participating optical phonons has been studied. The energies of phonon-plasmon modes in GaN: Si layers with different silicon concentrations have been measured using Raman spectroscopy.     

Indirect electronic transitions in alkali halate crystals-I (sodium and potassium chlorate crystals)

The long wavelength tail of the fundamental absorption in NaClO₃ and KClO₃ crystals has been analysed based on the theory of band to band transitions of Bardeen et al.[8] developed in the case of semi-conducting crystals. Evidence of phonon involvement in the transitions giving an indirect band gap is observed. The energies of the phonons involved in the process are the same for both the crystals, and agree well with combinations of prinicple frequencies of ClO₃^? ion, their overtones and also lattice phonons. The indirect band gap in these crystals varies with temperature more or less linearly and the rate of variation is ?3·8 × 10^?4 eV/K and ?5·0 × 10^?4 eV/K for sodium chlorate and potassium chlorate respectively.     

Analysis of the charge transfer mechanisms responsible for the current-voltage characteristics of Ca<Subscript>4</Subscript>Ga<Subscript>2</Subscript>S<Subscript>7</Subscript>: Eu<Superscript>3+</Superscript> single crystals

The current-voltage characteristics of Ca₄Ga₂S₇: Eu³⁺ single crystals are measured for the first time, and the processes affecting these characteristics are analyzed theoretically. It is demonstrated that Ca₄Ga₂S₇: Eu³⁺ single crystals are high-resistance semiconductors with a resistivity of ～10⁹ Ω cm and a relative permittivity of 10.55. The electrical properties of the studied materials are governed by traps with activation energies of 0.13 and 0.19 eV and a density ranging from 9.5×10¹⁴ to 2.7×10¹⁵ cm^?3. The one-carrier injection is observed in weak electric fields. In electric fields with a strength of more than 4×10³ V/cm, traps undergo thermal field ionization according to the Pool-Frenkel mechanism. At low temperatures and strong fields (160 K and 5×10⁴ V/cm), the electric current is most likely due to hopping conduction by charge carriers over local levels in the band gap in the vicinity of the Fermi level.     

Gap states in hydrogenated amorphous silicon: A comparison of photoemission and photoconductivity results

We report photoemission results from which we directly determined the density of states g(E) in the gap of a-Si:H between the top of the valence band E_v and the Fermi level. At 0.4 eV above E_v, g(E) was found to be ≈1×10²⁰ cm^-3 eV^-1 in the undoped film; P-doping increased g(E) in this region whereas annealing reduced it. The photoconductivity-derived optical absorption spectrum matched the shape of the photoemission spectrum, and thus supports the explanation that the photoconductivity shoulder at photon energies in the region of 1.3 eV is due to transitions from localized states above the valence band to the conduction band.     

Electrical conduction mechanism and carrier trapping in β-metal free phthalocyanine single crystals

The dark electrical conductivity of β-metal free phthalocyanine single crystals has been investigated over the temperature range 273–600°K, at a reduced pressure of 10^?7 torr. The results obtained are in accordance with the model proposed by Barbe and Westgate[5] for this material, in which the energy gap between the top of the valence band and the bottom of the conduction band is determined to be 2·00 eV. At temperatures below about 410°K, the conduction process is consistent with the presence of an electron trapping level located 0·32 eV below the conduction band edge, with a density of 7×10¹⁶ cm^?3, and a donor level of density 2×10⁷ cm^?3 at the same energy. Above about 410°K, there is evidence to suggest that the conduction process is intrinsic.     

InP掺杂超晶格的MOCVD生长与表征

用常压MOCVD技术生长了InP掺杂超晶格,所用源材料为TMIn和PH3.InP的载流子浓度低为8×1013cm-3,300K迁移率高达5744cm2/V·s.n型掺杂剂是DETe,p型为DMZn,两者在InP中的掺杂浓度均可达~1019cm-3.用不同激发强度下的低温PL谱,时间衰减,时间分辨PL谱和光反射(PR)谱技术表征了InP掺杂超晶格.文中给出了在不同激发强度下InP掺杂超晶格的4KPL谱,该样品由六层(n·p)组成,每层厚度200Å,掺杂浓度分别是1×1018cm-3和2×1018cm-3.发光峰值能量明显低于InP的禁带宽度,当激发强度增加时峰值能量移向高能边,这是真实空间中的间接跃迁特征.总PL信号衰减分成几步,每步都按指数规律,在4K情况下,衰减常数从6×10-8秒到7×10-4秒,具有明显间接带隙材料特征.InP掺杂超晶格中的量子尺寸效应用PR谱技术进行了研究.为描述InPnipi结构中的量子尺寸效应,引进了抛物型势阱模型,实验结果与根据引进模型所进行的计算相符.     

Photoluminescence observation of donor pairs in silicon

The asymmetric broadening with a tail on the lower energy side and the peak shift to lower energies are observed in the luminescence band due to bound excitons in silicon crystals which contain phosphorus impurities below the critical concentration for semiconductor-metal transition (8 × 10¹⁶ -3 × 10¹⁸ cm^-3). These behaviors are understood in terms of an exciton bound to a donor pair in silicon.     

Single-site approximation for the s–f model of antiferromagnetic semiconductors

For the s–f model of an antiferromagnetic semiconductor, the effect of the antiferromagnetic ordering of the localized spins on the conduction-electron state is investigated over a wide range of exchange strengths by combining the effective-medium approach with the Green's function in the 2×2 sublattice Bloch function representation. The band splitting due to the reduced magnetic Brillouin zone occurs below the Néel temperature. There is a marked effect of the thermal fluctuation of the antiferromagnetically ordered localized spins on the conduction electron at the energies near the top (bottom) of the lower- (higher-) energy subband.     

Self-consistent surface calculation of electron-hole drops in gallium phosphide

Self-consistent Kohn-Sham method is used to investigate the surface structure of electron-hole drops (EHD) in GaP. If the conduction bands are located on the X-point and have a degeneracy of 3, the surface tension is found to be 85 × 10^?4 erg cm^?2. In the presence of a “camel's-back” structure and a conduction band degeneracy of 6, the surface tension is calculated to be 130 × 10^?4 erg cm^?2. The surface charge on the EHD is found to be negative irrespective of whether the conduction band degeneracy is 3 or 6.     

Cr掺杂浓度对AlN半金属性影响的第一性原理研究

本文基于密度泛函理论的第一性原理方法, 在广义梯度近似(GGA)下计算了掺杂过渡金属Cr原子后AlN晶体自旋极化的能带结构、分态密度等性质. 结果表明, 半金属能隙随着掺杂浓度的增大而减小. 文中以掺杂浓度为12.5%的Cr-AlN(2×2×1)为例, 分析了其自旋极化的能带结构、分态密度和磁矩等性质, 发现Cr-3d电子对自旋向下子带导带底的能量位置起决定作用. 随着掺杂浓度的增大, Cr原子间相互作用增强, Cr-3d能带向两边展宽, 导致自旋向下子带导带底的能量位置下降, 从而半金属能隙变窄.     

Cr掺杂浓度对AlN半金属性影响的第一性原理研究

本文基于密度泛函理论的第一性原理方法, 在广义梯度近似(GGA)下计算了掺杂过渡金属Cr原子后AlN晶体自旋极化的能带结构、分态密度等性质. 结果表明, 半金属能隙随着掺杂浓度的增大而减小. 文中以掺杂浓度为12.5%的Cr-AlN(2×2×1)为例, 分析了其自旋极化的能带结构、分态密度和磁矩等性质, 发现Cr-3d电子对自旋向下子带导带底的能量位置起决定作用. 随着掺杂浓度的增大, Cr原子间相互作用增强, Cr-3d能带向两边展宽, 导致自旋向下子带导带底的能量位置下降, 从而半金属能隙变窄.     

锑化铟中载流子的复合过程

用定态光电导及光磁电的方法测量了p型n型InSb在85—290°K之间的电子及空穴的寿命。在室温附近,所有样品的截流子寿命都趋于同一值,在290°K时为3×10^-8秒。从寿命的绝对值及温度依赖关系,以及掺杂对寿命的影响,可以肯定在室温附近起主要作用的复合过程是带间碰撞复合过程。在200°K以下,p型样品中的电子寿命与空穴寿命有很大差别,表明有陷阱作用。用位于价带之上0.05eV的复合中心及位于导带之下0.11eV的电子陷阱能完满地解释200°K以下的寿命与温度的依赖关系。     

Electronic structure of QD arrays: application to intermediate-band solar cells

Intermediate band solar cells (IBSC) have been proposed as a potential design for the next generation of highly efficient photo-voltaic devices. Quantum nanostructures, such as quantum dots (QD), arranged in super-lattice (SL) arrays produce a mini-band (IB) that is separated by a region of zero density of states from other states in the conduction band. Additional absorption from the valence band to the IB and IB to the conduction band allows two photons with energies below the energy gap to be harvested in generating one electron-hole pair. We present a theoretical study of the electronic and optical properties of the IB formed by an InAs/GaAs QD array. The calculations are based on an 8-band k · p Hamiltonian, incorporating mixing between valence and conduction states, strain and piezoelectric field. Theoretical results of the the mini-band width variation with the period of the QD array in the z direction are presented. For one particular spacer distance, d _z = 4 nm, we report detailed variation of the optical dipole matrix elements through the mini-band and identify the character of the states involved. This approach captures the essential physics of the absorption processes in a realistic model of the IBSC structure and will be used to provide input parameters for predictive modelling of transport properties.     

An opportunity for directly estimating the characteristics of zero-point dynamics in polyethylene crystals

For large polyethylene crystallites (100 × 60 × 60 nm), the width of the Raman band at 1129 cm^?1 and the angular position of the x-ray equatorial 110 reflection were measured as a function of temperature over the range 5–300 K. It is found that the Raman bandwidth has an athermic (zero-point) component at low temperatures. This component is used to estimate the zero-point energies of torsional and bending vibrations of polyethylene molecules. These energies are close to those obtained from analyzing the x-ray diffraction data. It is concluded that the characteristics of zero-point dynamics can be determined directly from measuring the zero-point width of a Raman band.     

Shubnikov-de haas effect in n-CdSnAs2

Shubnikov-de Haas oscillations in the transverse magnetoresistance of single-crystalline n-type CdSnAs₂ have been recorded at temperatures between 2 and 25 K in magnetic fields up to 5T. The electron concentration of the samples ranged from 2 × 10¹⁷ to 2 × 10¹⁸ cm^?3. The angular dependences of the oscillation periods and cyclotron effective masses showed that the conduction band exhibits an energy dependent anisotropy, obeying the Kildal band structure model. For the low-temperature values of the band parameters we found: a band gap E_g = 0.30 eV, a spin-orbit splitting Δ = 0.50 eV, a crystal field splitting parameter δ = ?0.09 eV, and an interband matrix element P = 8.5 × 10^?8eV cm. This simple four-level model was found to be not adequate to describe quantitatively the observed electronic effective g-factor for a sample with low electron concentration.     

用MOS方法研究四元混晶In0.75Ga0.25As0.58P0.42中的深能级中心

采用MOS方法,在液相外延生长的n型In_0.75Ga_0.25As_0.58P_0.42混晶中,我们测出了两个位于导带下面0.20eV和0.48eV处的电子陷阱,对电子的俘获截面分别为1.4×10^-16cm²和3.8×10^-12cm²。关于采用MOS方式研究具有多层结构的化合物半导体材料的DLITS问题,本文也进行了讨论。 关键词：     

Electrical and optical properties of CuIn5S8 single crystals

Summary Several transport and optical properties have been studied onn-type CuIn₅S₈ single crystals. The energy gap at 0 K was determined from the electrical measurements to be 1.4 eV. An anisotropy of the magnetoresistance effect was found and it was suggested that the minima of the conduction band were located at points along the [100] directions ink-space. An optical-absorption band was found in an infrared region of (1÷1.6) μm and was attributed to the transitions from the lowest conduction band situated along the [100] directions to an upper conduction band. Paper presented at the ?V International Conference on Ternary and Multinary Compounds?, held in Cagliari, September 14–16, 1982.     

Excitonic instabilities,vacancies, and reconstruction of covalent surfaces

Two possibilities for explaining the superlattices of (111) surfaces of Si(7 × 7) and Ge(2 × 8) are considered. The equilibrium concentration of surface vacancies is related to the ratio of metallic and directional contributions to vacancy energies. The macroscopic thermo-dynamic explanation based on vacancy concentrations is preferred to an alternate microscopic one based on excitonic instabilities. A relation between anisotropy of surface energies and vacancy concentrations is proposed that predicts new superlattices for Si (100) 2 × 1, (110) 2 × 1, 5 × 1 and 4 × 5.     

Camel's back structure of the conduction band in GaP

A simple k · p theory based on Si and the ionic X gap is applied to the conduction band of GaP. It is found that recent evidence may indicate a location of the absolute minima away from the zone boundary. In the directions parallel to 〈100〉, the band structure is highly nonparabolic for carrier energies in the range 1–50 meV.