Time-resolved photoluminescence of delta-doped AlGaAs/GaAs heterostructures

The photoluminescence (PL) at low temperature of three delta-doped AlGaAs/GaAs heterostructures is investigated under continuous and pulsed excitations. The PL under continuous excitations allows the identification of the trapping centres and probes the carrier's transfer to the GaAs channel. The time-resolved photoluminescence (TRPL) inquires about carrier dynamics and gives radiative lifetimes of different levels. The DX level shows two time constants of the intensity decay relating the splitting of the valence band under impurity strains which reduce the crystal symmetry. The two time constants evolve with temperature and exhibit an increase near T=50 K.     

Two-carrier transition in radiative recombination of two-dimensional electrons in GaAs/AlGaAs

We observed photoluminescence (PL) and photoluminescence excitation (PLE) spectra due to shake-up processes of recombination of two-dimensional electrons and free excitons in a modulation-doped GaAs quantum well at He temperatures. One of the processes is that when an electron recombines with a hole, another electron is excited from the conduction band in GaAs to that in AlGaAs. The other process is that a hole is excited from an acceptor level or the valence band in GaAs to the valence band in AlGaAs during recombination. The electron process is observed in both PL and PLE spectra while the hole process only in the PL spectra. The excitation-intensity dependence of the peak intensity of hole-excited PL is almost quadratic, indicating three-carrier process in the shake-up process. The band offsets of the conduction and valence bands are estimated to be 220 and 146 meV, respectively.     

Carrier dynamics in type-II GaAsSb/GaAs quantum wells

Time-resolved photoluminescence (PL) characteristics of type-II GaAsSb/GaAs quantum wells are presented. The PL kinetics are determined by the dynamic band bending effect and the distribution of localized centers below the quantum well band gap. The dynamic band bending results from the spatially separated electron and hole distribution functions evolving in time. It strongly depends on the optical pump power density and causes temporal renormalization of the quantum well ground-state energy occurring a few nanoseconds after the optical pulse excitation. Moreover, it alters the optical transition oscillator strength. The measured PL lifetime is 4.5 ns. We point out the critical role of the charge transfer processes between the quantum well and localized centers, which accelerate the quantum well photoluminescence decay at low temperature. However, at elevated temperatures the thermally activated back transfer process slows down the quantum well photoluminescence kinetics. A three-level rate equation model is proposed to explain these observations.     

GaAs/AlGaAs异质结的界面束缚二维激子效应

本文讨论了GaAs/AlGaAs异质结界面的H线发光及其性能。用双晶X射线衍射及皮秒光致发光证明了H线与界面质量密切相关,并且有相似于激子跃迁的寿命行为。用限制于异质结界面势阱的二维电子(或空穴)与分布于GaAs边的三维空穴(或电子)组成的二维激子效应,解释了H线的实验结果。并讨论了不同外延生长的异质结与界面有关的发光行为。     

Temperature and density dependence of exciton lifetimes in GaAs/AlGaAs multiple quantum wells

The photoluminescence linewidths and excition lifetimes of free excitons in GaAs/AlGaAs multiple quantum wells were systematically investigated as a function of temperature, quantum well width, and carrier density. The experimental results showed that the exciton decay processes were strongly related to the linewidth of the exciton and the exciton binding energy.     

Photoluminescence in GaAs/AlGaAs quantum wells associated with excited confinement subbands

We report on photoluminescence emission features which are observed from GaAs/AlGaAs multiple quantum wells only at elevated temperatures (T>10K), using weak cw laser excitation. These features have energies higher than those of the heavy and light hole excitons and are associated with interband transitions between excited confinement conduction and valence subbands. Their energies are compared with values calculated from measured well dimensions and accepted band parameters for a series of samples with well widths between 80 and 375Å.     

Magneto-photoluminescence studies of manganese acceptors in GaAs/AlGaAs multiple quantum wells

We have investigated the photoluminescence associated with residual manganese acceptors in n-type, modulation doped, GaAs/AlGaAs multiple quantum wells. In a magnetic field the luminescence breaks into discrete lines attributed to transitions between conduction band Landau levels and manganese acceptor states. The polarization of the luminescence was studied as function of magnetic field. A simple model based on the spin exchange interaction between the holes and the manganese ions successfully describes the polarization data.     

AlGaAs capping effect on InAs quantum dots self-assembled on GaAs

The effects of AlGaAs capping on InAs quantum dots self-assembled on GaAs are investigated. It is observed that, the photoluminescence intensity becomes stronger up to twice when Al is incorporated into the cap layer. In the mean time, the full width at half maximum of the photoluminescence spectrum becomes narrower, the peak splitting between the ground and first excited exciton levels becomes wider, and the photoluminescence peak wavelength becomes longer. With considerations of the increased barrier height and the changed microstructures of the quantum dots induced by AlGaAs capping, the mechanisms of the observed improvements are discussed.     

Optically detected electron spin polarization and Hanle effect in AlGaAs/GaAs heterostructures

The spin polarization of optically created conduction electrons in p-type AlGaAs/GaAs heterostructures was observed via the degree of circular polarization of the photoluminescence. Application of a magnetic field perpendicular to the propagation of the light allows one to determine the spin relaxation time T₁ and the electron lifetime τ in the conduction band. By tilting the magnetic field with respect to an estimate of the effective nuclear field acting on the electrons can be obtained.     

Photoluminescence studies of confined states in AlGaAs/GaAs asymmetric quantum well

In the recombination spectra of AlGaAs/GaAs heterostructures, a peculiar and asymmetric photoluminescence (PL) band F has previously been reported [Aloulou et al., Mater. Sci. Eng. B 96 (2002) 14] to be due to recombinations of confined electrons from the two-dimensional electron gas (2DEG) formed at AlGaAs/GaAs interface in asymmetric quantum well (AQW). Detailed experiments are reported here on GaAs/Al_0.31Ga_0.69As/GaAs:δSi/Al_0.31Ga_0.69As/GaAs samples with different spacer layer thicknesses. We show that the band F is the superposition of two PL bands F′ and F″ associated, respectively, to AQW and a symmetric quantum well (SQW). In the low excitation regime, the F′ band present a blue shift (4.4 meV) followed by important red shift (16.5 meV) when increasing optical excitation intensity. The blue shift in energy is interpreted in terms of optical control of the 2DEG density in the AQW while the red shift is due to the narrowing of the band gaps caused by the local heating of the sample and band bending modification for relatively high-optical excitation intensity. Calculation performed using self-consistent resolution of the coupled Schrödinger–Poisson equations are included to support the interpretation of the experimental data.     

Temperature dependent time-resolved exciton luminescence in GaAs/AlGaAs quantum wires and dots

Transient photoluminescence of GaAs/AlGaAs quantum wires and quantum dots formed by strain confinement is studied as a function of temperature. At low temperature, luminescent decay times of the wires and dots correspond to the radiative decay times of localized excitons. The radiative decay time can be either longer or shorter than that of the host quantum well, depending on the size of the wires and dots. For small wires and dots (∼ 100 nm stressor), the exciton radiative recombination rate increases due to lateral confinement. Exciton localization due to the fluctuation of quantum well thickness plays an important role in the temperature dependence of luminescent decay time and exciton transfer in quantum wire and dot structures up to at least ∼ 80 K. Lateral exciton transfer in quantum wire and dot structures formed by laterally patterning quantum wells strongly affects the dynamics of wire and dot luminescence. The relaxation time of hot excitons increases with the depth of strain confinement, but we find no convincing evidence that it is significantly slower in quasi 1-D or 0-D systems than in quantum wells.     

Reduction in relaxation time due to ionized impurities in GaAs/AlGaAs quantum well structures

Time-resolved photoluminescence measurements in δ -doped GaAs/AlGaAs on the quantum well structures are performed to study effects of ionized impurities relaxation process of photoexcited carriers. It is theoretically shown that a thin quantum well with a δ -doping layer inserted in the barrier layer of double quantum wells enhances the impurity scattering rate significantly. Photoluminescence decay time in the δ -doped samples is found to decrease compared with the undoped samples.     

GaAs/AlGaAs量子点阵的制备及其荧光特性

采用电子束曝光和反应离子刻蚀的工艺,将GaAs/AlGaAs量子阱外延材料制成量子点阵,其光荧光谱显示出蓝移,并且蓝移量随着量子点直径尺寸的减少而增大。     

Photoluminescence investigation of Be-doped Npn AlGaAs/GaAs heterojunction bipolar transistor structures

Highly complex Npn AlGaAs/GaAs single heterojunction bipolar transistor (HBT) layers with Be-doped base were investigated by photoluminescence (PL) spectroscopy. Room temperature PL shows only a broad peak of GaAs due to thermalization; 15 K PL shows five peaks. The peak at 1.481 eV is from a p-type GaAs base, that at 1.517 eV is from a low-doped GaAs layer and that at 1.55 eV is from a high-doped GaAs collector. The that at 1.849 eV is due to bound exciton recombination in an AlGaAs emitter, and that at 1.828 eV is due to the acceptor-related transition from the AlGaAs layer. The integrated intensity ratio of these two peaks can be used to investigate the Be outdiffusion behavior, thus optimizing the growth conditions of base. The DC current gain of the HBT structure with different growth conditions was found to be in good agreement with the PL results.     

Kinetics of an exciton-trion system in shallow GaAs/AlGaAs quantum wells

The formation of three-particle charged exciton complexes (trions) in shallow GaAs/AlGaAs quantum wells in the temperature range 1.7–15 K has been investigated by luminescence spectroscopy and resonant light scattering. The effect of the photon energy and the intensity of additional above-barrier illumination on the trion formation kinetics has been analyzed. It is established that, upon intrawell excitation, illumination leads to the formation of trions when the light photon energy corresponds to the regions of effective formation of trions in the photoluminescence excitation spectra. It is shown that, with an increase in the illumination level, the trion concentration first increases and then reaches a plateau since the quantum well acquires an electric charge whose field equalizes the electron and hole capture rates.     

V形GaAs/AlGaAs量子线结构的微区光致发光谱研究

在室温下用显微光致发光的方法对单根V形GaAs/AlGaAs量子线进行了沿垂直于量子线方向的 空间分辨扫描测试,观察到各种量子结构的光致发光谱随空间位置的变化.在量子线区域附 近观察到来自量子线(QWR)、颈部量子阱(NQWL)和垂直量子阱(VQWL)等各种结构的发光,而 在距离量子线约1μm以远的发光光谱表现出侧面量子阱(SQWL)的发光.对全部发光光谱用高 斯线形进行了拟合,发现QWR和SQWL的发光包含了两个荧光峰,将它们分别归诸为电子到轻 、重空穴的跃迁.拟合后发光强度的空间变化直接确定了与量子线 关键词： V形GaAs/AlGaAs量子线 显微光致发光 空间分辨扫描     

非晶氧化硅薄膜带尾发光特性

采用等离子体增强化学气相沉积技术,通过改变CO2流量制备了不同氧含量的非晶氧化硅薄膜。利用紫外可见吸收谱、傅里叶红外吸收谱和稳态/瞬态光致发光谱等技术研究了薄膜的微观结构和光学特性。实验结果表明,随着氧含量的增加,薄膜的带隙增大,光致发光强度增加、峰值朝高能方向移动、光谱半峰全宽展宽。时间分辨光谱显示薄膜发光峰值处的衰减时间随氧含量的增加从6.2ns单调增加到21ns,而同一样品的发光寿命随发射波长能量增加而减小。综合分析光学吸收、发射及发光衰减特性表明,薄膜的发光机制主要归结为非晶材料带尾态之间的辐射复合。     

Auger decay of X-point excitons in a type II GaAs/AlGaAs superlattice

The origin and characteristics of the cross-interface photoluminescence observed in hydrostatic-pressure-induced, type II GaAs/AlGaAs superlattices is investigated. A low quantum efficiency luminescence feature with an Auger-limited lifetime, as evidenced by the observation of the “hot” Auger electrons recombining across the GaAs direct-gap, was observed. This feature has been found in a set of MOCVD-prepared superlattices, identical, except for the Al-mole-fraction of the barriers. The feature exhibiting Auger recombination is attributed to the recombination of a cross-interface exciton bound to a neutral donor in the AlGaAs. An impurity-bound-exciton seems necessary to explain the over 3-decade purely exponential decays observed in these experiments.     

Quantum efficiency of GaAs/AlGaAs thin films with two-dimensional photonic crystals

The optical properties of GaAs/AlGaAs thin films with photonic crystals were investigated by measuring their photoluminescence spectra. The spectral intensities, lifetimes, and quantum efficiencies decreased greatly compared with those in blank material without photonic crystals. The quantum efficiencies in the material were also calculated from spectral intensities and lifetimes and the quantum efficiencies calculated from those two methods agreed with each other to some extent.     

Time-resolved photoluminescence spectra of high-density InGaAs/AlGaAs quantum wire structures

We investigated the temperature dependence of the time-resolved photoluminescence (PL) spectra of high-density InGaAs/AlGaAs quantum wire (QWR) distributed-feedback laser structure on a submicron grating. A red-shift of peak in the time-resolved PL after photo-excitation was observed due to the relaxation of the photo-generated excitons from the entire QWR to localized centers at 10 K. On the other hand, at 60 K, no red-shift in the time-resolved PL spectra was observed since the localization centers are thermally activated and the excitons are delocalized.