Quantum well shape modification using vacancy generation and rapid thermal annealing

The effect of rapid thermal annealing (RTA) on the shapes of GaAs/AlGaAs quantum wells (QWs) has been investigated by monitoring exciton energies using low temperature photoluminescence and photoluminescence excitation spectroscopies. After RTA, large changes in exciton energies were observed only in regions of the samples in which excess surface vacancies were generated, either by capping with a thin layer of SiO₂ or by low-energy ion implantation. These changes were interpreted as resulting from modifications of the shapes of the as-grown QWs from abrupt or square to gradual (rounded) due to enhanced interdiffusion of well/barrier atoms. For single QWs there was an increase in exciton energy whose magnitude depended on the width of the well, its distance from the surface of the wafer, the annealing temperature and the total number of surface vacancies available. From studies of coupled QWs, there was clear evidence of asymmetry in the heterostructure after RTA. Although both techniques of vacancy generation yield substantial QW shape modifications, the ion implantation technique has the advantages of being highly reproducible and of being compatible with any material system.     

MOCVD的InGaAs/GaAs应变层量子阱的低温光致发光研究

本文中利用MOCVD方法得到了高质量的InGaAs/GaAs应变层量子阱材料,4.3nm量子阱10KPLFWHM仅为3.49meV.通过对样品荧光谱在变激发强度,变温时的峰位、峰形研究,我们发现,合金组分起伏散射是样品低温荧光谱展宽的主要原因,因而是MOCVD生长中应该首先解决的问题,实验结果还表明,在讨论低温PL谱形时,必需考虑光生载流子由随机起伏势中高能位置向低能位置的迁移过程。     

Si离子注入对分子束外延Si1-xGex/Si量子阱发光特性的影响

对分子束外延生长带边激子发光的Si_1-xGe_x/Si量子阱结构，通过Si离子自注入和不同温度退火，观测到深能级发光带和带边激子发光的转变.Si离子注入量子阱中并在600℃的低温退火，形成链状或小板式的团簇缺陷，它导致深能级发光带的形成，在850℃的高温退火后重新观测到带边激子发光.这种团簇缺陷的热离化能约为0.1eV，比Si中空穴或填隙原子缺陷的热激活能(约0.05eV)高.这表明早期文献中报道的深能级发光带是由类似的团簇缺陷产生的. 关键词：     

Partial intermixing of strained InGaAs/GaAs quantum wells

Shallow ion implantation and rapid thermal annealing (RTA) was used to modify the optical properties of strained InGaAs/GaAs quantum wells (QWs). After RTA, QW exciton energies, determined from peak positions of the photoluminescence spectra, shifted significantly to higher energies in the implanted areas, whereas they remained basically unaffected in the unimplanted regions. The magnitudes of the energy shifts depend on the well width, RTA temperature and ion implantation fluence. The shifts were interpreted as arising from modification of the shapes of the as-grown QWs due to diffusion of In out of the well material. This process is enhanced by diffusion of vacancies generated near the sample surface by ion implantation. QWs with compositions near the critical thickness exhibit different behaviour from that of fully pseudomorphic layers, due to the presence of dislocations in these layers.     

非对称双量子阱中载流子耦合的温度依赖性

非对称双量子阱中载流子动力学过程的温度依赖性研究,对于实现室温下高效的量子阱光电器件有着非常重要的意义。随着温度的升高,量子阱中的载流子被热激发到势垒层中后,部分载流子会被量子阱再俘获,即在不同的量子阱之间发生了载流子的耦合。利用耦合多量子阱载流子动力学模型,模拟了非对称双量子阱中载流子耦合的温度依赖性。研究表明,在不同温度下,各量子阱的光致荧光强度比强烈地依赖于量子阱的激活能差。光致荧光强度比的最大值与激活能差成指数关系,而其所对应的温度与激活能差成线性关系。     

Kinetics of indirect photoluminescence in GaAs/AlxGa1−x As double quantum wells in a random potential with a large amplitude

The kinetics of indirect photoluminescence of GaAs/Al_xGa_1−x As double quantum wells, characterized by a random potential with a large amplitude (the linewidth of the indirect photoluminescence is comparable to the binding energy of an indirect exciton) in magnetic fields B≤12 T at low temperatures T≥1.3 K is investigated. It is found that the indirect-recombination time increases with the magnetic field and decreases with increasing temperature. It is shown that the kinetics of indirect photoluminescence corresponds to single-exciton recombination in the presence of a random potential in the plane of the double quantum wells. The variation of the nonradiative recombination time is discussed in terms of the variation of the transport of indirect excitons to nonradiative recombination centers, and the variation of the radiative recombination time is discussed in terms of the variation of the population of optically active excitonic states and the localization radius of indirect excitons. The photoluminescence kinetics of indirect excitons, which is observed in the studied GaAs/Al_xGa_1−x As double quantum wells for which the random potential has a large amplitude, is qualitatively different from the photoluminescence kinetics of indirect excitons in AlAs/GaAs wells and GaAs/Al_xGa_1−x As double quantum wells with a random potential having a small amplitude. The temporal evolution of the photoluminescence spectra in the direct and indirect regimes is studied. It is shown that the evolution of the photoluminescence spectra corresponds to excitonic recombination in a random potential. Zh. éksp. Teor. Fiz. 115, 1890–1905 (May 1999)     

组合注入质子导致不对称耦合双量子阱截面混合效应研究

关键词：     

Enhancement of Photoluminescence Intensity of GaInNAs/GaAs Quantum Wells by Two-Step Rapid Thermal Annealing

We investigate the effect of rapid thermal annealing on InGaNAs/GaAs quantum wells. At optimized annealing temperatures and times, the greatest enhancement of the photoluminescence intensity is obtained by a special two-step annealing process. To identify the mechanism affecting the material quality during the rapid thermal annealing, differential temperature analysis is applied, and temperature- and power-dependent photoluminescence is carried out on the samples annealed under different conditions. Our experiment reveals that some composition redistribution or other related ordering processes may occur in the quantum-well layer during annealing. Annealing at a lower temperature for a long time primarily can remove defects and dislocations while annealing at a higher temperature for a short time primarily homogenizes the composition in the quantum wells.     

Optical and electronic properties of single modulation doped GaAs/AlGaAs V-grooved quantum wire modified by ion implantation

1 Introduction Recently, there is considerable interest in the fabrication and study of quasi-one di-mensional quantum wires (QWRs) due to their potential application for novel optoelec-tronic devices such as QWR laser array [1,2] etc. Among the various techniques devel-oped for producing quasi-one dimensional (quasi-1D) QWRs, the self-organized growth on patterned substrates has been proven to be one of the most promising methods, due to the simplicity of fabrication[3―5]. The QWR is fa…     

Theory of indirect exciton photoluminescence in elevated quantum trap

Inspired by an experiment of indirect excitons photoluminescence (PL) in elevated quantum trap (High et al., 2009), we theoretically investigate the energy relaxation and nonlinear interactions of indirect excitons in coupled quantum wells. It is shown that, when increasing the laser power, the intensity reversion of two PL peaks is due to the phonon necklace effect. In addition, we use a nonlinear Schrödinger equation including attractive two-body, repulsive three-body interactions and the excitation power dependence of energy distribution to understand the exciton states. This model gives a natural account for the PL blue shift with the increase of the excitation power. This study thus provides an alternative way to understand the underlying physics of the exciton dynamics in coupled potential wells.     

In-situ monitoring of optical near-field material processing by electron microscopes

A GaN-to-InGaN interface modification by predeposition of an ultrathin In-rich InGaN incomplete layer followed by a thin triangular InGaN well layer was employed to overcome the negative effects of polarization field on light emission efficiency of InGaN/GaN quantum wells as well as to improve the crystalline quality by avoidance of a significant strain generation and enhanced surfactant effect. Further, the interface modification induced energy band structure engineering reduces the spatial separation of electrons and holes, and thus increases the carrier recombination rate. The improvement in crystalline quality, localized potential fluctuation, and energy band engineering contribute to the significant increase of green emission of the InGaN/GaN quantum wells.     

离子注入诱导的具有两个不同发射波长的混合双量子阱结构

叙述了磷离子注入方法诱导具有不同发射波长的InGaAsP双量子阱结构的混合,并通过光致发光谱和断面透射电子显微术对量子阱混合的程度进行了研究。在特定条件下快速热退火处理后,光致发光谱显示,在离子注入剂量低于7×10¹¹/cm²情况下,两个阱的谱峰能保持较好的分离,注入剂量从10¹¹ /cm²增大到10¹²/cm²的过程中,两个阱的带隙蓝移值都似乎存在一个极大值,并且在同样的条件下,上阱(发射波长为1.52 μm)的带隙蓝移值较下阱(发射波长为1.59 μm)大些。当离子注入剂量达到10¹²/cm²时,上阱的谱峰近乎消失,双阱光致发光谱出现了一个谱峰。用断面透射电子显微术对原生长样品与带隙蓝移具有极大值的退火样品进行微结构比较,结果显示,对比原生长样品,退火样品的晶格原子基本得到修复,但阱与垒间的界面显得模糊,这说明离子注入导致两阱完全混合。     

反应磁控溅射ZnO/MgO多量子阱的光致荧光光谱分析

采用反应射频磁控溅射方法，在Si(001)基片上制备了具有高c轴择优取向的ZnO/MgO多量子阱.利用X射线反射、X射线衍射、电子探针，光致荧光光谱等表征技术，研究了ZnO/MgO多量子阱的结构、成份和光致荧光特性.研究结果表明，多量子阱的调制周期在1.85—22.3 nm之间，所制备的多量子阱具有量子限域效应，导致了室温光致荧光峰的蓝移，并观测到了量子隧穿效应引起的荧光效率下降.建立了基于多声子辅助激子复合跃迁理论的室温光致荧光光谱优化拟合方法，通过室温光致荧光光谱拟合发现，ZnO/MgO比ZnO/ZnMgO多量子阱具有更大的峰位蓝移，探讨了导致光致荧光光谱展宽的可能因素. 关键词： ZnO/MgO 多量子阱 磁控溅射 光致荧光 量子限域效应     

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.     

Inter-island energy transfer and in-plane exciton migration in AlGaAs/GaAs quantum wells detected by exciton dynamics

We present the results of optical, steady-state and time-resolved studies of photoluminescence and photoluminescence excitation in high-quality Al_0.3Ga_0.7As/GaAs quantum wells in which the presence of large (larger than the exciton radius) atomically flat islands can be inferred, identical to the case of interrupted MBE growth. Migration of excitons towards lower-lying energy states induced by local potential fluctuations and/or progressive localisation has been revealed and the transition rate between quantum well regions 24 to 25 monolayers thick has been derived to be 290 ps⁻¹.     

Time-resolved spectroscopy of excitonic transitions in ZnO/(Zn, Mg)O quantum wells

We report on the optical spectroscopy of a series of ZnO/(Zn, Mg)O quantum wells of different widths, using time-resolved photoluminescence. The samples were grown by molecular beam epitaxy on ZnO templates, themselves deposited on sapphire substrates. The barriers consist of Zn_0.78Mg_0.22O layers. The presence of large internal electric fields in these quantum wells results in a competition between quantum confinement and the quantum confined Stark effect as the quantum well width is varied. A transition energy lying 0.5 eV below the ZnO excitonic gap was observed for the widest of our wells. The PL spectra of the wide quantum wells were obtained using time-integrated photoluminescence, taking a great care with screening effects induced by their very slow dynamics. The effect of the built-in electric field on the excitonic properties was investigated. The excitonic fine structure is shown to depend strongly on the enhancement or suppression of the exchange interaction as a function of the quantum well width.     

GaAs/GaAlAs多量子阱的光致荧光诊断

利用光致荧光技术对GaAs/GaAlAs多量子阱质量进行了诊断。讨论了量子阱厚度涨落,铝成份涨落,各种缺陷和非故意掺杂等对量子阱光致荧光谱的影响,并反过来,又由光致荧光谱来推断引起量子阱质量退化的原因。在一定程度上为分子束外延工艺的改进提供了依据。 关键词：     

Analysis of localization effect in blue-violet light emitting InGaN/GaN multiple quantum wells with different well widths

Four blue-violet light emitting InGaN/GaN multiple quantum well(MQW) structures with different well widths are grown by metal–organic chemical vapor deposition. The carrier localization effect in these samples is investigated mainly by temperature-dependent photoluminescence measurements. It is found that the localization effect is enhanced as the well width increases from 1.8 nm to 3.6 nm in our experiments. The temperature induced PL peak blueshift and linewidth variation increase with increasing well width, implying that a greater amplitude of potential fluctuation as well as more localization states exist in wider wells. In addition, it is noted that the broadening of the PL spectra always occurs mainly on the low-energy side of the PL spectra due to the temperature-induced band-gap shrinkage, while in the case of the widest well, a large extension of the spectral curve also occurs in the high energy sides due to the existence of more shallow localized centers.     

Intrawell and interwell transfer of excitons in growth-interrupted quantum wells

Intrawell and interwell transfers of excitons are observed by a temperature-dependent continuous-wave photoluminescence study of growth-interrupted single quantum wells. The intrawell transfer among the interface localization areas suggests a thermodynamic equilibrium between energy relaxation via LO-phonon emission and thermal population via phonon absorption. Thermal population is dominant in wider wells while relaxation is clearly observable in a four-monolayer narrow well at low temperatures. Interwell transfer of excitons also occurs between two narrow wells.     

Near-field mechanism of photoluminescence excitation in quantum well heterostructures

We report a study into the process of energy transfer between quantum wells divided by 30-nm-thick opaque barriers. It was experimentally observed that the intensity of a photoluminescence signal from a quantum well increased by 15% under resonant excitation of exciton transition in the adjacent quantum well. The quantum wells were 30 nm apart. A radiative mechanism of energy transfer in the near-field region of emitting exciton is proposed. Within this theoretical model, the efficiency of the energy transfer decreases by a power law with greater distance between the quantum wells. The theory is found to be in qualitative agreement with the experimental results.