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.     

The Peierls system in a light field

This paper studies the behavior of the low-temperature phase of the Peierls system in a quasimonochromatic time-independent random light field whose frequency is much lower than the frequency of the lower edge of band-to-band transitions. The density matrix method in the dipole approximation is used to derive equations for the band gap. A dependence between the band gap and the light-field intensity is established in an approximation in which the concentration of the nonequilibrium electrons in the conduction band is low. The possibility of, and the conditions for, the existence of a light-induced semiconductor-semiconductor phase transition and cavityless optical bistability with increasing absorption are established. Zh. éksp. Teor. Fiz. 111, 1398–1409 (April 1997)     

Nuclear and ion spins in semiconductor nanostructures

Spin interactions are studied between conduction band electrons in GaAs heterostructures and local moments, specifically the spins of constituent lattice nuclei and of partially filled electronic shells of impurity atoms. Nuclear spin polarizations are addressed through the contact hyperfine interaction resulting in the development of a method for high-field optically detected nuclear magnetic resonance sensitive to 10⁸ nuclei. This interaction is then used to generate nuclear spin polarization profiles within a single parabolic quantum well; the position of these nanometer-scale sheets of polarized nuclei can be shifted along the growth direction using an externally applied electric field. In order to directly investigate ion spin dynamics, doped GaMnAs quantum wells are fabricated in the regime of very low Mn concentrations. Measurements of coherent electron spin dynamics show an antiferromagnetic exchange between s-like conduction band electrons and electrons localized in the d-shell of the Mn impurities, which varies as a function of well width.     

氧化锌量子点浅施主能级增强绿色光致发光光谱的实验证明

利用溶胶-凝胶法制备了粒径在5 nm左右的氧化锌量子点,通过测量氧化锌量子点光致发光光谱、吸收光谱以及荧光寿命得到氧化锌量子点绿光光谱可分为两种且对应不同发光机理。较高能量的绿光光谱是电子由导带底跃迁至氧空穴辐射产生的,而较低能量的则归因于电子由浅施主能级跃迁到氧空位,这种浅施主能级能够增强绿光发射且激发光能量稍小于带隙时绿光光谱相对强度达到最大。其次,实验得到氧化锌量子点的蓝光光谱是由于激发电子从锌间隙能级跃迁至价带产生。该研究提出并分析了氧化锌量子点绿光光谱的两种发光机制,可为其在光学方面的应用提供参考。     

First-principles study on the structural and electronic properties of ultrathin ZnO nanofilms

Using first-principles density-functional calculations, we have studied the structural and electronic properties of ultrathin ZnO {0001} nanofilms. The structural parameters, the charge densities, band structures and density of states have been investigated. The results show that there are remarkable charge transfers from Zn to O atoms in the ZnO nanofilms. All the ZnO nanofilms exhibit direct wide band gaps compared with bulk counterpart, and the gap decreases with increased thickness of the nanofilms. The decreased band gap is associated with the weaker ionic bonding within layers and the less localization of electrons in thicker films. A staircase-like density of states occurs at the bottom of conduction band, indicating the two-dimensional quantum effects in ZnO nanofilms.     

公度双壁碳纳米管层间耦合对其场发射特性影响的研究

采用紧束缚能带理论，利用所提出的考虑卷曲效应的紧束缚能量哈密顿量，建立了公度双壁碳纳米管(DWNT)的能带结构模型；基于碳纳米管(CNT)发射电流与其能带结构的相关性，定量分析了公度DWNT的层间耦合作用对其场发射电流的影响.结果表明：在层间耦合作用下，DWNT的带结构中部分简并能级发生劈裂，同时使禁带宽度发生改变.前一个因素增加了电子发射的通道，后一个因素改变价带中参与发射的电子数量，导致在一定外电场下，DWNT与其外层的SWNT相比，场发射电流有一定程度的增加，且半导体性管发射电流增幅比金属性管大，在 关键词： 公度双壁碳纳米管 能带结构 层间耦合作用 卷曲效应     

Energy distributions of field-emitted electrons from silicon: Evidence for surface states

Energy distributions of field-emitted electrons were obtained from field evaporated and annealed silicon tips. Electrons are emitted from levels below the Fermi level but above the valence band. The shape of the distributions depends on the crystallographic orientation of the emitting surface. The results are intepreted in terms of emission from a band of surface states which overlaps with the band gap.     

Cyclotron resonance study of conduction electrons in n-type indium antimonide under a strong magnetic field—I: Thermal equilibrium case

The far IR cyclotron resonance of conduction electrons is investigated in n-type indium antimonide in the quantum regimes, ?ω_ck_BT and ?ω_c?k_BT. The resonance peak position, half width, and the degree asymmetry in the line shape are studied as a function of temperature. In analyzing the experimental data, the three band model has been employed together with modern theoretical results of electron scattering by ionized impurities in the presence of a strong magnetic field. It is found that, for example for an experiment at 84 μm, the Une width depends very little on temperature between 4.2 and 45 K where the ionized impurity scattering is dominant, and increases rapidly with temperature above 45 K where the onset of phonon scattering is expected. Further details of the ionized impurity scattering were investigated by using three different laser wavelengths 84, 119 and 172μm. The line width at the phonon-limited temperature region depends very little on magnetic field and sample. The temperature dependence of the band gap was also determined by analysis of the resonance peak shift.     

缺陷对单层MoS2电子结构的影响

为了研究缺陷对单层MoS2的电子结构, 本文基于密度泛函理论框架下的第一性原理, 采用数值基组的方法计算了MoS2的Mo位缺陷、S位缺陷的能带结构和态密度.结果发现：Mo位缺陷、S位缺陷的MoS2的能带结构中的价带顶与导带底都在Q点, 为直接带隙材料; 其中Mo位缺陷体的禁带区域都出现5条新能级, S位缺陷体的禁带区域出现了3条新能级; 缺陷体能带结构的能量下降与体系中未成键的电子有关.对于态密度而言, Mo位缺陷体的费米能级处出现了峰值, 表明Mo位缺陷会对其光电性质带来影响.同时分析电荷分布发现, Mo缺陷周围存在着负电荷聚集的现象, S缺陷周围存在正电荷聚集的现象.     

缺陷对单层MoS_2电子结构的影响

为了研究缺陷对单层MoS_2的电子结构,本文基于密度泛函理论框架下的第一性原理,采用数值基组的方法计算了MoS_2的Mo位缺陷、S位缺陷的能带结构和态密度.结果发现:Mo位缺陷、S位缺陷的MoS_2的能带结构中的价带顶与导带底都在Q点,为直接带隙材料;其中Mo位缺陷体的禁带区域都出现5条新能级,S位缺陷体的禁带区域出现了3条新能级;缺陷体能带结构的能量下降与体系中未成键的电子有关.对于态密度而言,Mo位缺陷体的费米能级处出现了峰值,表明Mo位缺陷会对其光电性质带来影响.同时分析电荷分布发现,Mo缺陷周围存在着负电荷聚集的现象,S缺陷周围存在正电荷聚集的现象.     

Auger relaxation processes in semiconductor nanocrystals and quantum wells

Auger recombination rates in mesoscopic semiconductor structures have been studied as a function of energy band parameters and heterostructure size. It is shown that nonthreshold Auger processes stimulated by the presence of heteroboundaries become the dominant nonradiative recombination channel in nanometer size semiconductor structures. The size dependence of luminescence quantum yields in nanostructures and microcrystals are discussed. Auger-like collisions of electrons and heavy holes are shown to serve as “accelerators” of thermalization processes in semiconductor quantum dots.     

Modulation of band gap by normal strain and an applied electric field in SiC-based heterostructures

The structure and electronic properties of the WS₂/SiC van der Waals (vdW) heterostructures under the influence of normal strain and an external electric field have been investigated by the ab initio method. Our results reveal that the compressive strain has much influence on the band gap of the vdW heterostructures and the band gap monotonically increases from 1.330 to 1.629 eV. The results also imply that electrons are likely to transfer from WS₂ to SiC monolayer due to the deeper potential of SiC monolayer. Interestingly, by applying a vertical external electric field, the results present a parabola-like relationship between the band gap and the strength. As the E-field changes from to ?0.50 +0.20 V/Å, the band gap first increases from zero to a maximum of about 1.90 eV and then decreases to zero. The significant variations of band gap are owing to different states of W, S, Si, and C atoms in conduction band and valence band. The predicted electric field tunable band gap of the WS₂/SiC vdW heterostructures is very promising for its potential use in nanodevices.     

限域量子态调控研究

电子、激子和声子等量子态在固体中的行为早已被人们所熟知. 然而,当体系的尺寸只有纳米量级的时候,已有的固体理论常常不能适用,需要新的低维物理理论的建立. 我们系统研究了低维体系限域量子态（包括电子、激子和声子）的行为对环境、应力、压力及光的响应和性质的调控. 较早认识到低维体系之显著的表面-体积比对量子态性质调控之有效性,系统地揭示了低维体系的一系列由表面和应力决定的新颖性质,证明了低维体系的表面和应力效应同量子限域效应同等重要. 本文概况了如下五个方面的结果：（1）一种使用应力效应调控电子能带结构的方法和（2）一种使用表面效应调控电子能带结构的方法（这两个方法都可将低维体系能带从间接能隙调控至直接能隙能带结构）;（3）一种低维体系表面掺杂方法,该方法将在低维体系掺杂中取代传统方法;（4）量子点表面诱导的光致异构现象;（5）基于表面自催化半导体低维结构的形成机理. 希望我们的研究工作有助于促进低维体系在光电子、纳电子、环境、能源、生物和医学等领域的应用.     

Inelastic scattering of hot electrons in n-GaAs/AlAs types I and II multiple quantum wells doped with silicon

We have studied the dependence of hot electron scattering rate on temperature in n-GaAs/AlAs types I and II multiple quantum wells. For a sample with well width 37 Å, which is on the borderline between types I and II band alignment, the increase of the temperature in the range 6–80 K leads to the strong decrease of the hot electron scattering rate. We explain this result by ionization of donors and transfer of cold electrons from the Γ-valley of GaAs to the X-valley of AlAs.     

Resonant Raman scattering of discrete hole states in self-assembled Si/Ge quantum dots

We report the first resonant electronic Raman spectroscopy study of discrete electronic transitions within small p-doped self-assembled Si/Ge quantum dots (QDs). A heavy hole (hh) to light hole (lh) Raman transition with a dispersionless energy of 105 meV and a resonance energy of the hh states to virtually localised electrons at the direct band gap of 2.5 eV are observed. The hh–lh transition energy shifts to lower values with increasing annealing temperature due to significant intermixing of Si and Ge in the QDs. Structural parameters of the small Si/Ge dots have been determined and introduced into 6-band k·p valence band structure calculations. Both the value of the electronic Raman transition of localised holes as well as the resonance energy at the E₀ gap are in excellent agreement with the calculations.     

Temperature and magnetic field dependence of superconductivity in nanoscopic metallic grains

We study pairing correlations in ultrasmall superconductor in the nanoscopic limit by means of a toy model where electrons are confined in a single, multiply degenerate energy level. We solve the model exactly to investigate the temperature and magnetic field dependence of number parity effect (dependence of ground state energy on evenness or oddness of the number of electrons). We find a different parity effect parameter to critical temperature ratio (4 rather than 3.5) which turns out to be consistent with exact solution of the BCS gap equation for our model. This suggests the equivalence between the parity effect parameter and the superconducting gap. We also find that magnetic field is suppressed as temperature increases.     

The quantum acoustomagnetoelectric field in a quantum well with a parabolic potential

The acoustomagnetoelectric (AME) field in a quantum well with a parabolic potential (QWPP) has been studied in the presence of an external magnetic field. The analytic expression for the AME field in the QWPP is obtained by using the quantum kinetic equation for the distribution function of electrons interacting with external phonons. The dependence of the AME field on the temperature T of the system, the wavenumber q of the acoustic wave and external magnetic field B for the specific AlAs/GaAs/AlAs is achieved by using a numerical method. The problem is considered for both cases: The weak magnetic field region and the quantized magnetic field region. The results are compared with those for normal bulk semiconductor and superlattices to show the differences, and we use the quantum theory to calculate the AME field in the QWPP.     

Optically biased photoreflectance spectroscopy of a GaAs epitaxial layer and an AlGaAs/GaAs quantum well

We propose optically biased photoreflectance (OBPR) spectroscopy, which is performed by continuous illumination of a secondary monochromatic light on a sample with conventional photoreflectance (PR), as a useful tool to investigate the internal electric fields dependence of the PR signals associated with band to band and quantum level transitions. Line shape of the PR signal has a strong dependence on the internal electric field. In OBPR, if a secondary incident light is absorbed, the internal electric field is suppressed by the photo-generated electron–hole pairs. On the other hand, if the secondary light is not absorbed, the internal electric field is not affected. Through the OBPR investigation of a GaAs epitaxial layer and an AlGaAs/GaAs quantum well, we are able to obtain an absorption like spectrum by performing a wavelength scan of a secondary monochromatic light. The results of OBPR measurements at each PR peak position show the contribution of the electric fields modification by the photo-generated carriers in each layers to the PR signals that are related to band gap and quantum level transitions.     

Electro-optical properties of zigzag and armchair boron nitride nanotubes under a transverse electric field: Tight binding calculations

The electro-optical properties of zigzag and armchair BNNTs in a uniform transverse electric field are investigated within tight binding approximation. It is found that the electric field modifies the band structure and splits band degeneracy where these effects reflect in the DOS and JDOS spectra. A decrease in the band gap, as a function of the electric field, is observed. This gap reduction increases with the diameter and it is independent of chirality. An analytic function to estimate the electric field needed for band gap closing is proposed which is in good agreement with DFT results. In additional, we show that the larger diameter tubes are more sensitive than small ones. Number and position of peaks in DOS and JDOS spectra for armchair and zigzag tubes with similar radius are dependent on electric field strength.     

Simulation of IPV effect in In-doped c-Si with optimized indium concentration and layer thickness

Impurity photovoltaic effect(IPV) is one of the attempts to improve efficiency of solar cells and is the idea of exploiting three step generation via impurity states within the band gap to utilize sub-band gap photons. The three transitions are of electrons from valence band (VB) to conduction band (CB), valence band to impurity level and impurity level to conduction band. In the present simulation, we have used the p⁺nn⁺ structure in order to achieve higher photogenerated current and efficiency without loosing the open circuit voltage. Compared to other group-III elements in silicon solar cell, Indium is the most suitable material to achieve higher benefit in IPV. In this simulation, the model of IPV is considered to achieve the maximum benefit from the impurity state in a solar cell. To simulate we have used the one dimensional simulation program, SCAPS-1D. Again light trapping is an important part of IPV solar cell that has been considered in this simulation. Using IPV we have numerically demonstrated, an increase in efficiency, by 2.79% over that without-IPV effect and a 3.23% increase over the efficiency, 30.9% as reported by Schmeits and Mani [1].