The influences of thickness of spacing layer and the elastic anisotropy on the strain fields and band edges of InAs/GaAs conical shaped quantum dots

Based on the continuum elastic theory, this paper presents a finite element analysis to investigate the influences of elastic anisotropy and thickness of spacing layer on the strain field distribution and band edges (both conduction band and valence band) of the InAs/GaAs conical shaped quantum dots. To illustrate these effects, we give detailed comparisons with the circumstances of isolated and stacking quantum dot for both anisotropic and isotropic elastic characteristics. The results show that, in realistic materials design and theoretical predication performances of the optoelectronic devices, both the elastic anisotropy and thickness of the spacing layer of stacked quantum dot should be taken into consideration.     

The electronic structure of InAs/GaSb(001) superlattices-two dimensional effects

Detailed calculations of the two dimensional effects in the electronic structure of InAs/GaSb(001)superlattices are presented for the first time. Comparison of the calculated thickness dependence of the superlattice band gap with optical absorption measurements shows that, at the Γ-point, the conduction band edge of InAs lies about 60 meV below the valence band edge of GaSb. Eigenfunctions of the highest light and heavy hole bands, and the lowest two conduction bands exhibit spatially confined nature in the GaSb and InAs regions respectively, thus establishing the two-dimensional nature of these bands. The calculated conduction band effective mass in the plane of the superlattice near the Γ-point is found to be enhanced by a factor of 2.5 over the bulk InAs value and compares very well with the appropriate mass extracted from recent magnetoresistance measurements.     

A minimal basis semi-ab initio approach to the band structures of semiconductors

The band structures of 32 of the most important semiconductor crystals are calculated using an efficient, minimal basis, orthogonalized LCAO method. These include the diamond structure of C, Si, Ge, α-Sn; the zinc blende structure of β-SiC, BN, BP, AlP, AlAs, AlSb, GaP, GaAs, GaSb, InP, InAs, InSb, β-ZnS, ZnSe, ZnTe, CdS, CdTe; the wurtzite structure of AlN, GaN, ZnO, α-ZnS, CdS, CdSe; the sodium chloride structure of CdO, GeTe, SnTe and trigonal Se and Te. The calculations, which involve diagonalizations of small size matrix equations yield results having the following characteristics: (1) satisfactory valence bands and lower conduction bands and bulk densities of states; (2) the gap sizes and the locations of valence band maximum and conduction band minimum in agreement with experiment; (3) reasonable values of fractional ionicity and electron and hole effective masses. These are achieved by fine-tuning the exchange parameters in the construction of the potentials. Application of this approach to the study of the electronic structures of disordered and other complex semiconductor systems is also discussed.     

Quantum dots formed in InSb/AlAs and AlSb/AlAs heterostructures

The crystal structure of new self-assembled InSb/AlAs and AlSb/AlAs quantum dots grown by molecularbeam epitaxy has been investigated by transmission electron microscopy. The theoretical calculations of the energy spectrum of the quantum dots have been supplemented by the experimental data on the steady-state and time-resolved photoluminescence spectroscopy. Deposition of 1.5 ML of InSb or AlSb on the AlAs surface carried out in the regime of atomic-layer epitaxy leads to the formation of pseudomorphically strained quantum dots composed of InAlSbAs and AlSbAs alloys, respectively. The quantum dots can have the type-I and type-II energy spectra depending on the composition of the alloy. The ground hole state in the quantum dot belongs to the heavy-hole band and the localization energy of holes is much higher than that of electrons. The ground electron state in the type-I quantum dots belongs to the indirect X_XY valley of the conduction band of the alloy. The ground electron state in the type-II quantum dots belongs to the indirect X valley of the conduction band of the AlAs matrix.     

Selective optical doping to predict the performance and reveal the origin of photocurrent peaks in quantum dots-in-a-well infrared photodetectors

Resonant optical pumping across the band gap was used as artificial doping in InAs/In_0.15Ga_0.85As/GaAs quantum dots-in-a-well infrared photodetectors. Through efficient filling of the quantum dot energy levels by simultaneous optical pumping into the ground states and the excited states of the quantum dots, the response was increased by a factor of 10. Low temperature photocurrent peaks observed at 120 and 148 meV were identified as intersubband transitions emanating from the quantum dot ground state and the quantum dot excited state, respectively by a selective increase of the electron population in the different quantum dot energy levels.     

Electronic structure of the Sr2MgSi2O7:Eu persistent luminescence material

The electronic structures of the distrontium magnesium disilicate (Sr₂MgSi₂O₇(:Eu²⁺)) materials were studied by a combined experimental and theoretical approach. The UV-VUV synchrotron radiation was applied in the experimental study while the electronic structures were investigated theoretically by using the density functional theory. The structure of the valence and conduction bands and the band gap energy of the material as well as the position of the Eu²⁺ 4f ground state were calculated. The calculated band gap energy (6.7 eV) agrees well with the experimental value of 7.1 eV. The valence band consists mainly of the oxygen states and the bottom of the conduction band of the Sr states. The calculated occupied 4f ground state of Eu²⁺ lies in the energy gap of the host though the position depends strongly on the Coulomb repulsion strength. The position of the 4f ground state with respect to the valence and conduction bands is discussed using the theoretical and experimental evidence available.     

窄禁带半导体材料中共振缺陷态测量

运用量子电容谱测量技术，在窄禁带半导体材料InSb和HgCdTe价带和导带中分别发现了两个共振缺陷态．根据建立的实验模型研究了这些共振缺陷态的特性 关键词：     

电子相互作用对聚乙炔中极化子电子态的影响

本文根据SSH和Hubbard模型研究了电子相互作用对聚乙炔中极化子的电子态的影响。结果发现:1.电子相互作用使能隙中出现了一个新的电子束缚态,它是靠近价带的浅能级;2.电子相互作用使得价带下面的一个电子束缚态消失,同时在导带上面出现了两个电子束缚态。 关键词：     

Local electronic structure near Mn acceptors in InAs: surface-induced symmetry breaking and coupling to host states

We present low-temperature scanning tunneling spectroscopy measurements on Mn acceptors in InAs in comparison with tight-binding calculations. We find a strong (001)-mirror asymmetry of the bound hole wave function close to the (110) surface. In addition, multiple acceptor-related peaks are observed and are attributed to a spin-orbit splitting of the acceptor level. Because of the p-d exchange interaction the local density of states near the acceptors is enhanced in the valence band and suppressed in the conduction band. We also observe signs of anisotropic scattering of the conduction band states by neutral acceptors.     

Subband states inn-inversion layers on small-gap semiconductors

Subband states inn-inversion layers on small-gap semiconductors are subject to the coupling between valence and conduction band (nonparabolicity effects). In order to account for this coupling we study different models: two of them are based on Kane's 6×6 and 8×8 bulk-Hamiltonians, the third one takes into account higher order terms of the electron momentum in a 2×2 conduction band Hamiltonian. We perform selfconsistent calculations for these models with parameters characteristic for InSb and HgCdTe and electron concentrationsN _S , for which up to two subbands are occupied. The calculated subband separations and Fermi energies are independent of the models only if the same energy band dispersion is used and depend strongly on the applied boundary conditions.Work supported in part by the Deutsche Forschungsgemeinschaft     

Band mixing in narrow gap semiconductors

The interaction of conduction and valence bands in narrow gap semiconductors such as InSb and HgCdTe influences the position and width of subband energy levels in space-charge layers. While a nonzero width can only occur if electrons from the conduction band can tunnel into approximately degenerate states of the valence band the level shifts due to band mixing are always present. We present a Green's function treatment which allows in a simple way to discuss the dependence of band mixing effects on the parameters of thek·p-Hamiltonian in particular the band gap. The essential qualitative feature of the level shifts is adecrease of subband energy separation withdecreasing effective mass. This agrees with recent experimental results for Hg_1-x Cd _x Te.     

Ⅰ类和Ⅱ类准周期半导体超晶格的电子子带和波函数

本文将包迹函数近似推广用于计算有理数近似下,垂直于超晶格轴的波矢K_⊥不等于零时,准周期半导体超晶格(QSS)的电子子带和波函数,对K_⊥=0的情形,分别计算了Ⅰ类的GaAs/Al_xGa_1-xAs和Ⅱ类的InAs/GaSb QSS的电子子带和波函数,直至代序数m=9和6。对于价带对导带影响强的InAs/GaSb QSS,分别计算了m=5和6时电子子带随K_⊥的变化关系。并提出了利用本文结果计算Ⅰ类的GaAs/Al_xGa_1-xAs QSS带间集体激发的具体方法。     

Strain effects on optical properties of pyramidal InAs/GaAs quantum dots

Strain distribution and optical properties in a self-assembled pyramidal InAs/GaAs quantum dot grown by epitaxy are investigated. A model, based on the theory of linear elasticity, is developed to analyze three-dimensional induced strain field. In the model, the capping material in the heterostructure is omitted during the strain analysis to take into account the sequence of the fabrication process. The mismatch of lattice constants is the driving source of the induced strain and is treated as initial strain in the analysis. Once the strain analysis is completed, the capping material is added back to the heterostructure for electronic band calculation. The strain-induced potential is incorporated into the three-dimensional steady-state Schrödinger equation with the aid of Pikus–Bir Hamiltonian with modified Luttinger–Kohn formalism for the electronic band structure calculation. The strain field, the energy levels and wave functions are found numerically by using of a finite element package FEMLAB. The energy levels as well as the wave functions of both conduction and valence bands of quantum dot are calculated. Finally, the transition energy of ground state is also computed. Numerical results reveal that not only the strain field but also all other optical properties from current model show significant difference from the counterparts of the conventional model.     

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.     

Experimental determination of the constants of absolute volume deformation potentials at band edges in semiconductors

A method is proposed for determining the constants of absolute volume deformation potentials at edges of the conduction and valence bands in semiconductors. This method is based on (i) the volume-concentration effect, (ii) the concept that the energy of deep-lying strongly localized impurity centers does not depend on the hydrostatic pressure, and (iii) the use of experimental data on the electrical resistivity and Hall coefficient. For Ge, GaAs, InAs, and InSb semiconductors, the constants of absolute volume deformation potentials at edges of the conduction and valence bands are determined from our results and data available in the literature.     

Cracking self-assembled InAs quantum dots

We present a cross-sectional scanning tunneling microscopy (X-STM) investigation of InAs quantum dots in a GaAs matrix. The structures were grown by molecular beam epitaxy (MBE) at a low growth rate of 0.01 ML/s and consist of five layers of uncoupled quantum dot structures. Detailed STM images with atomic resolution show that the dots consist of an InGaAs alloy and that the indium content in the dot increases towards the top. The analysis of the height versus base-length relation obtained from cross-sectional images of the dots shows that the shape of the dots resembles that of a truncated pyramid and that the square base is oriented along the [010] and [100] directions. Using scanning tunneling spectroscopy (STS) we determined the onset for electron tunneling into the conduction and out of the valence band, both in the quantum dots and in the surrounding GaAs matrix. We found equal voltages for tunneling out of the valence band in GaAs or InGaAs whereas tunneling into GaAs occurred at higher voltages than in InGaAs.     

第一原理研究界面弛豫对InAs/GaSb超晶格界面结构、能带结构和光学性质的影响

通过广义梯度近似的第一原理全电子相对论计算, 研究了不同界面类型InAs/GaSb超晶格的界面结构、电子和光吸收特性. 由于四原子界面的复杂性和低对称性, 通过对InAs/GaSb超晶格进行电子总能量和应力最小化来确定弛豫界面的结构参数. 计算了InSb, GaAs型界面和非特殊界面(二者交替)超晶格的能带结构和光吸收谱, 考察了超晶格界面层原子发生弛豫的影响.为了证实能带结构的计算结果, 用局域密度近似和Hartree-Fock泛函的平面波方法进行了计算. 对不同界面类型InAs/GaSb超晶格的能带结构计算结果进行了比较, 发现界面Sb原子的化学键和离子性对InAs/GaSb超晶格的界面结构、 能带结构和光学特性起着至关重要的作用.     

Coupled ultrathin InAs layers in GaAs as a tool for the determination of band offsets

We have determined the band offsets at the highly strained InAs/GaAs heterointerface by photoluminescence excitation (PLE) measurements of the symmetric and antisymmetric states in two coupled ultrathin InAs layers embedded in a GaAs matrix. The conduction band offset ΔE_ccould be separated from the valence band offsets, since in a 32 monolayer (ML) barrier sample, the splitting between the heavy-hole exciton transitions is solely determined by ΔE_c. Knowing ΔE_c, the heavy-hole (hh) and light-hole (lh) band offsets ΔE_hhand ΔE_lhcould subsequently be determined from the coupling-induced shift and splitting in samples with a 16, 8 and 4 ML barrier. We find a conduction band offset of 535 meV, a conduction band offset ratio ofQ_c= 0.58 and a strain induced splitting between the hh and lh subbands of 160 meV.     

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.     

Mapping of the hole wave functions of self-assembled InAs-quantum dots by magneto-capacitance–voltage spectroscopy

We have performed magneto-capacitance–voltage spectroscopy for the valence band states of InAs quantum dots embedded in a p-type Schottky diode. By choosing the right measurement frequency and applying an in-plane magnetic field, we were able to map the k-space wave functions corresponding to the individual charging peaks. The wave functions belonging to the first two charging peaks show no nodes as expected for an s-like ground state. In contrast, nodes are observed for the next four charging peaks supporting the identification as excited states with finite orbital angular momentum. Peaks 3 and 4 show different wave functions compared to peaks 5 and 6, which points to different angular momenta for this two pairs of charging peaks.