Features of the fine structure of the fundamental absorption edge in low-doped semiconductor quantum wells with allowance for the Coulomb gap

Influence of the Coulomb gap on the fine structure of the fundamental absorption edge in quantum wells is studied. Using the interpolation formula, which adequately describes the effect of the Coulomb gap in quantum wells under conditions of low doping regardless of the compensation degree, an explicit expression is obtained for the absorption coefficient related to transitions from the valence level of size quantization to the impurity band. The dependence of the absorption coefficient of a quantum well on the levels of doping and compensation is also studied.     

Charge depletion in delta wells

Delta wells can become charge-depleted through background doping, or as part of asymmetric n-i-p-i structures. For moderate depletions, the electronic properties of the residual 2DEGs in these cases are similar. For moderate depletion by background doping, the charge removed from the well varies inversely with the square-root of the background intensity, independent of the strength of the delta well. Depletion of delta wells is found to procede in a ‘top-down’ fashion in a 2-dimensional analogue to atomic ionization.     

势垒硅掺杂对GaN基LED极化电场及其光电性能的影响

势垒硅掺杂对InGaN量子阱中的电场及LED器件的光电性能有着重要的影响。采用6×6 K·P方法计算了不同势垒硅掺杂浓度对量子阱中电场的变化,研究表明当势垒硅掺杂浓度>1e¹⁸ cm^-3时,阱垒界面处的电场强度会变大,这主要是由于硅掺杂浓度过高导致量子阱中界面电荷的聚集。进一步发现随着势垒掺杂浓度的升高,总非辐射复合随之增加,其中俄歇复合增加,而肖克莱-霍尔-里德复合随之减少,这是由于点陷阱的增大形成了缺陷能级。电流电压曲线表明势垒掺杂可有效改善GaN基LED的工作电压,这归于掺杂浓度的提高改善了载流子的传输特性。当掺杂浓度为1e¹⁸ cm^-3时,获得了较高的内量子效率,这主要是由于适当的势垒掺杂降低了量子阱中界面电荷的损耗。     

Influence of delta doping on intersubband transition and absorption in AlGaN/GaN step quantum wells for terahertz applications

Effects of delta doping location and density on intersubband transitions in AlGaN/GaN step quantum wells for terahertz (THz) applications have been investigated by solving Schrödinger and Poisson equations self-consistently. It shows that delta doping near the GaN well/AlGaN step well interface causes a blue-shift, while delta doping in the barrier or near barrier/GaN well and barrier/step well interfaces cause a red-shift first and then a blue-shift with increasing doping density. The shifts are attributed to the combination of many body effect and internal field modulation effect, and can be more than 200% or 70% of the e₁–e₂ transition energy, as for blue-shift or red-shift, respectively. In addition, the influences of delta-doping location and density on the absorption coefficient are also investigated in detail. Delta doping at the middle of a layer is found much more desirable over uniform-doping in order to improve the absorption coefficient, especially in the step well.     

Mobility of carrier in the single-side and double-side doped square quantum wells

We present a theoretical study of the effect from doping of quantum wells (QWs) on enhancement of the mobility in one-side (1S) and two-side (2S) doped square infinite quantum well. Within the variational approach, we introduce the enhancement factor defined by the ratio of the overall mobility in the 2S doped square quantum wells to that in the 1S doped counterpart with the same sheet carrier density and interface profiles. The enhancement is fixed by the sample parameters such as well width and sheet carrier density. We propose two-side doping as an efficient way to upgrade the quality of QWs. Our theory is able to well reproduce the recent experimental data about low-temperature transport of electrons and holes in one-side and two-side doped square QWs.     

The variation of electronic properties with the doping concentration of modulation-doped AlxGa1−xAs–GaAs double quantum wells

In this paper we have calculated the sub-band structure and the confinement potential of modulation-doped Ga_1−xAl_xAs–GaAs double quantum wells as a function of the doping concentration. The electronic properties of this structure were determined by self-consistent solutions of the Schrödinger and Poisson equations. To understand the effects of doping concentration on band bending, sub-band energies, and sub-band populations, the doping concentration on one right side of the structure is decreased while holding it constant on the left side. We found that at low doping concentrations on the right side, the effects of the doping concentration are more pronounced on band bending and sub-band populations.     

Hydrostatic pressure dependence of negative-donor-ion singlet and singlet-like bound magnetoplasmon transitions in doped GaAs/AlGaAs quantum wells

Applied hydrostatic pressure modifies the Coulomb bound states of a quasi-two-dimensional electron gas in quantum wells by increasing the effective mass and by tuning the free electron density. Here, we explore these mechanisms by measuring the effects of pressure on the cyclotron resonance, the D⁰ 1s → 2p⁺ transition, and the D⁻-singlet and singlet-like transitions in low-and high-density, modulation-doped GaAs quantum wells. For low doping density, detailed calculations employing a pressure-dependent electron mass agree well with the observed magnetic field and pressure dependencies. For high doping density and low fields, the blue-shift of the D⁻-singlet-like transition at fields below 8 T decreases with applied pressure as anticipated, due to loss of free electrons via the Γ–X crossover. However, near 7.5 T, this singlet-like transition exhibits an anomalous branching for pressures above 4 kbar, which indicates the presence of a resonant level and obscures the blue-shift at high fields.     

弱耦合GaAs/AlGaAs/InGaAs双势阱隧穿结构的磁隧穿特性研究

研究了低温(15 K)条件下弱耦合GaAs/AlGaAs/InGaAs双势阱结构的纵向磁隧穿特性. 研究表明,器件在零偏压下处于共振状态. 通过分析不同偏压下的磁电导振荡曲线,可以得到双量子阱中的基态束缚能级随偏压的变化规律,从而可以确定隧穿电流峰对应的隧穿机制. 所得结果可为弱耦合双量子点器件的制备提供基础. 关键词： 双量子阱 隧穿结构 磁电导振荡     

双三角量子阱中不对称性及掺杂浓度对电子拉曼散射的影响(英文)

在有效质量近似下,从理论上研究了非对称双三角量子阱的拉曼散射。推导了导带子带间电子跃迁的微分散射截面表达式,以GaAs/AlxGa1-xAs材料为例进行了数值计算。结果表明,散射光谱不仅与掺杂浓度有关,而且与双量子阱的不对称性有关,随着量子阱不对称性的增加或掺杂浓度的减少,散射峰发生了红移。本工作对设计新型微电子和光电子器件有一定的指导意义。     

量子阱中光生载流子的瞬态衰减过程与发光效率

提出了量子阱系统中包括非辐射复合效应和载流子屏蔽效应在内的光生载流子瞬态复合过程的唯象模型．结果表明，荧光衰退时间与样品质量、掺杂浓度以及激发光强度有着密切的联系 关键词：     

Dynamical control of self-trapping of two weakly coupled Bose-Einstein condensates with a time-modulated nonlinearity

We investigate the effect of a periodic nonlinearity on the self-trapping transitions of two weakly coupled Bose-Einstein condensates in a double well potential. By using an averaging method, the equations of motion of the slow dynamics are derived to analyze the self-trapping behavior. A new type of the tunneling dynamics, characterized by alternate appearance of self-tapping in the two wells, is observed when certain conditions are satisfied.     

On the polarization self-screening effect in multiple quantum wells for nitride-based near ultraviolet light-emitting diodes

The tilted energy band in the multiple quantum wells(MQWs) arising from the polarization effect causes the quantum confined Stark effect(QCSE) for [0001] oriented III-nitride-based near ultraviolet light-emitting diodes(NUV LEDs). Here, we prove that the polarization effect in the MQWs for NUV LEDs can be self-screened once the polarization-induced bulk charges are employed by using the alloy-gradient In_xGa_(1-x)N quantum barriers. The numerical calculations demonstrate that the electric field in the quantum wells becomes weak and thereby flattens the energy band in the quantum wells, which accordingly increases the spatial overlap for the electron-hole wave functions. The polarization self-screening effect is further proven by observing the blueshift for the peak emission wavelength in the calculated and the measured emission spectra. Our results also indicate that for NUV LEDs with a small conduction band offset between the quantum well and the quantum barrier,the electron injection efficiency for the proposed structure becomes low. Therefore, we suggest doping the proposed quantum barrier structures with Mg dopants.     

Thermal interdiffusion in InGaAs/GaAs and GaAsSb/GaAs strained quantum wells as a function of doping density

We studied the thermal stability and interdiffusion of InGaAs/GaAs and GaAsSb/GaAs single quantum wells as a function of temperature for both Be and Si doping at various doping concentrations. The interdiffusion was monitored using the photoluminescence from the ground states of the valence- and conduction-band quantum wells. Using a Green's function method to solve the diffusion equation, assuming Fick's law behaviour, the evolution of the well shape during annealing was determined, and Schrödinger's equation was solved for this well shape to provide the ground-state energy levels of the system using the diffusion constant as the only fitting parameter. The validity of this model as applied to both systems is discussed.Strained Layer Structures Research Group.     

On intersubband absorption of radiation in delta-doped QWs

The results of calculation of intersubband absorption coefficients for either center-, or edge-delta-doped with Phosphorus 10 nm and 20 nm-wide Si_0.8Ge_0.2/Si/Si_0.8Ge_0.2 quantum wells are presented. It is shown, that the absorption for delta-doped structures differs substantially from that of a pure rectangular or uniformly doped ones. There are two main features for delta-doped quantum wells. The first one is the blue-shift for optical transitions between first and others (more pronounced), and second and others (less pronounced) space quantized energy levels. The second one is that edge doping changes the symmetry of the quantum well and forbidden optical transitions for the rectangular structure become now allowed. The influences of temperature, quantum well width, and impurity concentration on the optical absorption are studied. It is shown that the most dramatic changes in comparison with rectangular quantum wells are for wider investigated edge-doped structures with bigger number of ionized impurities.     

Magnetic-field-dependent excitation transfer in quantum wells of diluted magnetic semiconductor

We studied the excitation transfer in double quantum wells of a diluted magnetic semiconductor using a scanning near-field optical microscope at 7 K in external magnetic fields up to 9 T. In each quantum well, local energy minima are generated by local fluctuation of layer thickness and doping concentration of magnetic components. Excitons relax into the local energy minima and transfer between the minima via near-field optical interactions even across quantum wells toward stable sites at which to localize. We measured the intensity maps of near-field photoluminescence with spatial resolution estimated to be 30 nm under varying external magnetic fields. The measurement position reproducibility was confirmed by scanning tunneling microscope images. Analysis of the maps derived the magnetic-field dependence of the typical size of exciton-localization sites for each quantum well. Based on these results, we investigated the excitation transfer between the two quantum wells lying in different layers of the double quantum well system, and showed that the exciton transfer takes place at the two specific applied magnetic-field intensities that result in the crossing of Zeeman-split energy levels of the two different wells. We concluded that both the localization and the inter-quantum-well transfer of excitons are able to be controlled by an external magnetic field. This provides the basis for functional devices operating without any wiring.     

n doping effect modeling in 1.3 μm GaN0.58yAs1−1.58yBiy/GaAs quantum wells

The effect of n doping on the band structure of lattice-matched GaNAsBi/GaAs quantum wells was investigated using a self-consistent calculation combined with the 16-band anti-crossing model. Bi/N incorporation and doping effects can offer a huge potential to engineer the electronic band structure of such materials suitable for the design of photodetectors and emitters operating at 1.3 µm. The increase of the doping density induces a blue-shift of the fundamental transition energy in the doping range between 6×10¹⁷ and 5×10¹⁸ cm⁻³. The absorption spectra dependence on the well width are discussed. To maintain the fundamental transition fixed at the wavelength 1.3 µm, we have adjusted the Bi composition for the well width range between 4.5 and 10 nm with respect of the confinement conditions.     

Electroluminescence from Si/SiGe quantum cascade emitters

Intersubband electroluminescence results are presented from Si/SiGe quantum cascade emitters at 3.2 THz and at temperatures up to 150 K. The effect of adding doping into the active quantum wells was studied in addition to reduced barrier widths from previous measurements. While the current through the sample is increased by the addition of doping, the emitted power is reduced through additional free carrier absorption and Coulombic scattering. Free electron laser measurements confirm the intersubband transitions in the quantum wells of the cascade devices and produce non-radiative lifetimes of 20 ps between 4 and 150 K.     

1．55μmInGaAsP及其相应量子阱结构的LP-MOCVD生长

本文研究了用低压金属有机化合物汽相外延(LP-MOCVD)技术在(100)InP衬底上生长InGaAsp体材料及InGaAP/InP量子阱结构材料的生长条件。三甲基镓(TM63)、三甲基铟(TMh)和纯的砷烷(A8H3)、磷烷(PH3)分别用作Ⅲ族和Ⅴ族源,在非故意掺杂情况下,InGaAsP材料的载流子浓度为3.6×10¹⁵cm^-3;在液氦温度和室温下,与InP晶格匹配的InGaAsP光致发光半峰宽分别为19.2meV和63meV;对外延层的组分及厚度均匀性分别进行了转靶X光衍射仪,低温光致发光和扫描电子显微镜分析,对不同阱宽的量子阱结构材料测出了由于量子尺寸效应导致光致发光波长随阱宽增加而红移现象。     

Binding energies of donor impurities in modulation-doped double quantum wells under an electric field

In this study, we have investigated theoretically the binding energies of shallow donor impurities in modulation-doped GaAs/Al_0.33Ga_0.67As double quantum wells (DQWs) under an electric field which is applied along the growth direction for different doping concentrations as a function of the impurity position. The electronic structure of modulation-doped DQWs under an electric field has been investigated by using a self-consistent calculation in the effective-mass approximation. The results obtained show that the carrier density and the depth of the quantum wells in semiconductors may be tuned by changing the doping concentration, the electric field and the structure parameters such as the well and barrier widths. This tunability gives a possibility of use in many electronic and optical devices.