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.     

用Ξ形四能级模型研究非对称耦合量子阱非定域激子复合发光

基于Ξ形四能级模型运用密度矩阵方程研究了非对称量子阱中非定域激子复合发光特性.理论结果表明：非定域激子复合发光具有双峰特征，两峰相对于中心跃迁频率的红移和蓝移量与电子和空穴的振荡频率密切相关.与单量子阱相比，这种频率移动对外加电场相当敏感，即当外加反向电场作小的变化时，两峰有较大的移动，表现强量子限域斯塔克效应.这意味着利用非对称量子阱在新一代高速调制器和光开关中具有潜在的应用价值. 关键词： 非对称耦合量子阱 共振隧穿 非定域激子 量子限域斯塔克效应     

用深能级瞬态谱方法研究赝晶Si/Ge0.25Si0.75/Si单量子阱的价带偏移

应变的Ge_xSi_1-x层和未应变的硅层间的能带偏移主要是价带偏移。量子阱中载流子的热发射能与界面的能带偏移有着密切的关系。本文用深能级瞬态谱(DLTS)研究分子束外延生长的p型Si/Ge_0.25Si_0.75/Si单量子阱的价带偏移,阱宽为15nm,考虑到电场的影响和量子阱中第一子能级的位置,对从DLTS得到的热发射能进行适当的修正,可以计算出Si/Ge_0.25Si_0.75/S 关键词：     

Theoretical modeling for quantum-confined Stark effect due to internal piezoelectric fields in GaInN strained quantum wells

Recent experimental investigations revealed that the biaxial stress in thin InGaN layers grown on thick GaN layer induces a large piezoelectric field along [0001] orientation that causes red-shift in optical transitions and reduction in oscillator strengths because of spatial separation of the electron and hole wave functions. In this Letter based on theoretical modeling we determined the well width z-dependent effect on red-shifted quantum-confined Stark effect (QCSE) in GaN/In_xGa_1 − xN (x=0.13) strained quantum well structures. Analyses are based on the solution of Schrödinger equation in a finite well including the internal piezoelectric electric field (F) due to the strained polarization as the perturbation potential. Our theoretical results show: (1) the red-shift in optical transition has a quadratic well-width form as it is for infinite wells (Davies, 1998) [1], (2) assuming the model based on a carrier effective mass dependence on the width of quantum wells, m^∗(z), fits the experimental data (Takeuchi et al., 1997) [2] much more accurate compare to the model with constant effective mass, m^∗.     

Modified excitonic resonances in GaAs/AlAs multiple quantum well heterostructures with ultrathin barriers

Excitonic resonance structures in GaAs/AlAs multiple quantum well heterostructures with varying barrier-layer thicknessesL _B down to 1.3 nm are investigated for two sets of samples with the nominal well widths ofL _Z =9.2 and 6.4 nm, by 2K photoluminescence excitation spectroscopy. The observed resonance energies of then=1 heavyhole (1 hh) and light-hole (1 lh) free excitons imply that quantum confinement effects persist at least down to the decreased barrier-layer thickness ofL _B =1.3 nm. This result is inconsistent with the red shifts expected from the simple well-coupling theory within the one-band Kronig-Penney model at the point. Instead, blue shifts of 6–8 meV (8–17 meV) are observed for the 1 hh (1 lh) excitonic resonance peaks whenL _B is decreased from 10 to 2 nm. A relative decrease of the oscillator strength of the 1 lh transition compared to the 1 hh transition is also observed asL _B is decreased. These results manifest important effects of the indirect-gap barrier material for the actual wavefunction matching across the interface and the breakdown of the envelope function approach to GaAs/AlAs quantum well heterostructures with ultrathin barriers.     

应变Si1－xGex/(111)Si空穴有效质量模型

利用应变Si_1-xGe_x/(111)Si材料价带E(k)-k关系,研究获得了沿不同晶向的空穴有效质量,并在此基础上,建立了空穴各向同性有效质量模型.结果表明,与弛豫材料相比,应变Si_1-xGe_x/(111)Si材料价带带边空穴有效质量各向异性更加显著,带边空穴各向同性有效质量随Ge组分明显减小.该研究成果可为Si基应变PM 关键词： 1-xGe_x')" href="#">应变Si_1-xGe_x 空穴有效质量 价带     

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.     

Dependence of the activation energies on the well width in CdTe/ZnTe strained single quantum wells

Photoluminescence (PL) measurements on the CdTe/ZnTe strained single quantum wells grown by using the molecular beam epitaxy technique showed that the sharp excitonic peaks corresponding to the transition from the first electronic subband to the first heavy-hole (E₁–HH₁) were shifted to lower energy with increasing well width. The (E₁–HH₁) interband transitions were calculated by using an envelope function approximation taking into account the strain effects, and the values were in reasonable agreement with those obtained from the (E₁–HH₁) excitonic transitions of the PL spectra. The activation energies of the confined electrons in the CdTe quantum well were obtained from the temperature-dependent PL spectra, and their values increased with increasing CdTe well width due to the quantum confinement effect. The present results can help to improve the understanding of the activation energies dependent on the CdTe well width in CdTe/ZnTe single quantum wells.     

Physics of fluctuation processes in downscaled silicon MOSFETs

Physical mechanics of fluctuation processes in advanced submicron and decananometer MOSFETs (metal-oxide-semiconductor field-effect transistors) including the ultra-thin film SOI (siliconon-insulator) devices using strained silicon films are reviewed. The review is substantially based on the results obtained by the authors. It is shown that the following drastic changes occur in the nature and parameters of noise in such devices as a result of their downscaling when the gate oxide thickness and the channel length and width are decreased, the SOI substrates are used, the silicon film thickness is reduced, the film doping level is varied, the strained silicon films are employed, etc. Firstly, the Lorentzian components can appear in the current noise spectra. Those components are due to (i) electron tunneling from the valence band through the gate oxide in the SOI MOSFETs of a sufficiently thin gate oxide (LKE-Lorentzians); (ii) Nyquist fluctuations generated in the source and drain regions near the back Si/SiO₂ interface in the SOI MOSFETs (BGI Lorentzians); (iii) electron exchange between the channel and some single trap in the gate oxide of the transistors with sufficiently small length and width of the channel (RTS Lorentzians). Secondly, the 1/f-noise level can increase due to (i) the appearance of recombination processes near the Si/SiO₂ interface activated by the currents of electron tunneling from the valence band; (ii) an increase in the trap density in the gate oxide of the devices fabricated on the biaxially tensile-strained silicon films; (iii) the contribution of the 1/f fluctuations of the current flowing through the gate oxide as a result of electron tunneling from the conduction band. At the same time, the 1/f-noise level may decrease due to a decrease in the trap density in the gate oxide of the transistors fabricated on the uniaxially tensile-strained silicon films. Moreover, a 1/f ^1.7 component may appear in the noise spectra for the transistors of a sufficiently thin gate oxide, whose component is due to charge fluctuations on the defects located near the interface between the gate polysilicon and the gate oxide.     

Tensile-strained 1.3 μm InGaAs/InGaAlAs quantum well structure of high temperature characteristics

A new tensile strained InGaAs/InGaAlAs quantum well structure in the 1.3 μm wavelength region is proposed for high temperature characteristics via quantum well band structure and optical gain calculations. To obtain such features, a tensile-strained InGaAs/InGaAlAs quantum well structure, which emits light dominated by TM polarization, is considered. This proposed structure has very high temperature characteristics (T ₀ > 130 K) due to its high density of state at the first transition edge. This results clearly show the potential of tensile strained quantum well structure usage for the high temperature operation of quantum well semiconductor lasers.     

Screening influence on the Stark effect of impurity states in strained wurtzite GaN/AlxGa1-xN heterojunctions under pressure

The screening effect of the random-phase-approximation on the states of shallow donor impurities in free strained wurtzite GaN/Al x Ga 1 x N heterojunctions under hydrostatic pressure and an external electric field is investigated by using a variational method and a simplified coherent potential approximation.The variations of Stark energy shift with electric field,impurity position,Al component and areal electron density are discussed.Our results show that the screening dramatically reduces both the blue and red shifts as well as the binding energies of impurity states.For a given impurity position,the change in binding energy is more sensitive to the increase in hydrostatic pressure in the presence of the screening effect than that in the absence of the screening effect.The weakening of the blue and red shifts,induced by the screening effect,strengthens gradually with the increase of electric field.Furthermore,the screening effect weakens the mixture crystal effect,thereby influencing the Stark effect.The screening effect strengthens the influence of energy band bending on binding energy due to the areal electron density.     

Wannier–Stark effect in strongly coupled three quantum well structures and superlattices

The Wannier–Stark effect has been investigated in strongly coupled three quantum well structures and 23.5 period superlattices consisting of 25Å wide GaAs wells separated by Al_0.25Ga_0.75As barriers of the same width. A blue shift associated with the Wannier–Stark localization of heavy holes states is observed for the first time, followed by a second blue shift at higher electric fields due to the electron localization. This is contrary to the widely reported single blue shift of the order of (e+hh)/2, where e and hh are the electron and heavy hole miniband widths respectively, produced by Wannier–Stark localization in superlattices.     

InxGa1-xAs-GaAs应变层量子阱结构的流体静压光荧光研究

本文报道了对分子束外延(MBE)生长的In_0.25Ga_0.75As-GaAs应变层量子阱结构在77K下的压力光荧光(PL)研究的结果。流体静压力从0到50kbar.,给出了In_0.25Ga_0.75As-GaAs应变层量子阱的Γ谷压力系数,实验观察到了量子阱中能级与势垒GaAs中X谷的能级交叉。通过对其压力行为的分析,给出了In_0.25Ga_0.75As-GaAs异质结的导带与价带跃变比:Q_c=△E_c:△E_v=0.68:0.32。对(InGa)As-GaAs应变量子阱常压下的理论分析与实验符合很好。本文也对Al_0.3Ga_0.70As-GaAs量子阱进行了讨论。 关键词：     

Optically induced level anticrossing in undoped GaAs/AlGaAs coupled double quantum wells

We report a photoluminescence detected anticrossing of the energy levels in an undoped asymmetric coupled-double-quantum-well buried in a p-i-n structure. Due to the built-in electric field, the quantum wells are tilted in such a way that the symmetric energy level is higher than that of the antisymmetric one in the conduction band. Keeping the laser excitation energy below the barrier, with increasing laser power, the level anticrossing and the quantum confined Stark effect were observed due to decreasing built-in electric field by the photogenerated electron and hole pairs.     

Blue and red Stark shifts in single Si/SiO2quantum wells

The quantum-confined Stark shift (QCSS) of the ground and the first excited electron energy states in single Si/SiO₂quantum wells is calculated. The QCSS for the ground state is always negative (blue), while the QCSS for the first excited state is positive, in general, but it can turn out to be negative when the electric field intensity becomes high enough. A remarkable reduction (∼50%) of the Stark shifts is obtained when the existence of nonabrupt interfaces as thin as 10 Å is taken into account. The nonabrupt interfaces can also change the sign of the Stark shift in the case of the excited states.     

应变Si/(001)Si1-xGex空穴有效质量各向异性

基于应变Si/（001）Si_1-xGe_x材料价带E（k）-k关系模型,研究获得了其沿不同晶向的空穴有效质量.结果表明,与弛豫材料相比,应变Si/（001）Si_1-xGe_x材料价带带边（重空穴带）、亚带边（轻空穴带）空穴有效质量在某些k矢方向变化显著,各向异性更加明显.价带空穴有效质量与迁移率密切相关,该研究成 关键词： 应变Si 价带 空穴有效质量     

MBE GaAs1-xSbx/GaAs应变层量子阱的光调制反射光谱

本文采用光调制反射光谱技术研究了MBE GaAs_1-xSb_x/GaAs应变层量子阱。通过实验分析和理论上对受应力作用后能带结构的估算,确认在这一系统中流体静压力作用引起的能带结构变化主要出现在导带上,同时也证实了GaAs_1-xSb_x/GaAs应变层量子阱属于第Ⅱ型量子阱结构。实验结果与理论估算符合很好。 关键词：     

正弦平方势与带电粒子沟道效应的能带结构

在量子力学的框架内描述了带电粒子与晶体相互作用，利用正弦平方势把沟道粒子的Schr?dinger方程化为Mathieu方程，根据Bloch定理讨论了系统能量分布，并用摄动法求解了方程的低阶不稳定区及其禁带宽度，系统自动呈现出了能带结构，再现了粒子束同晶体相互作用的周期性特征.而这一点正是其他相互作用势不曾有的. 关键词： 沟道效应 能带结构 正弦平方势 摄动法     

极化诱导的内建电场对Mn δ掺杂的GaN/AlGaN量子阱居里温度的调制

采用传输矩阵方法分析极化诱导的内建电场对Mn δ掺杂的GaN/Al_xGa_1-xN量子阱居里温度(T_C)的调制作用.通过解薛定谔方程计算出在不同的内建电场条件下半导体量子阱局域态内的基态空穴能级和波函数分布情况，并在此基础上确定量子阱内Mn δ掺杂情况下T_C随内建电场的变化趋势，分析了不同量子阱结构引起的内建电场分布变化及其对T_C的影响.在耦合双量子阱中通过调节左右阱的不对称性可以得到T_C近3倍的增长. 关键词： GaN 量子阱 内建电场 居里温度     

InGaN/GaN多量子阱热退火的拉曼光谱和荧光光谱

通过拉曼光谱及荧光光谱测量研究了采用低压金属有机化学沉积(MOCVD)方法生长的InGaN／GaN多量子阱高温快速热退火处理对量子阱光学性质的影响。观测到退火后InGaN／GaN量子阱的拉曼光谱E2,A1(LO)模式的峰位置出现了红移,而且该振动峰的半高宽也有微小变化。温度升高退火效果更明显。退火使量子阱内应力部分消除,同时In,Ga原子扩散出现相分离使拉曼谱表现出变化。在常温和低温下的光荧光谱表明,退火处理的量子阱发光主峰都出现了红移;而且低温退火出现红移,退火温度升高相对低温退火出现蓝移;同时在低温荧光光谱里看到经过退火处理后原发光峰中主峰旁边弱的峰消失了。讨论了退火对多量子阱光学性质的影响。