GaNAs/GaAs量子阱的激子局域化以及退局域化

我们利用光荧光(PL)以及时间分辨光谱(TRPL)研究了用MBE生长在GaAs衬底上的GaNAs/GaAs量子阱的激子局域化以及退局域化.研究发现,在低温下用连续光(Cw)激发,由于GaNAs中势振荡所产生的局域激子发光是所测量到光谱的主要发光来源.然而在脉冲激发下,情况完全不同.在高载流子密度激发或者高温下GaNAs/GaAs量子阱中例外,一个高能端的PL峰成为了主要的发光来源.通过研究,我们将这个新的发光峰指认为量子阱中非局域激子复合的PL峰.这个发光峰在温度和激发强度的变化过程中与局域激子相互竞争.我们相信这一过程也是许多文献所报道的在InGaN和AlGaN等氮化物中经常观测到的发光峰位随温度"S"形变化的主要根源.     

GaNAs/GaAs量子阱的激子局域化以及退局域化

我们利用光荧光(PL)以及时间分辨光谱(TRPL)研究了用MBE生长在GaAs衬底上的GaNAs/GaAs量子阱的激子局域化以及退局域化。研究发现,在低温下用连续光(CW)激发,由于GaNAs中势振荡所产生的局域激子发光是所测量到光谱的主要发光来源。然而在脉冲激发下,情况完全不同。在高载流子密度激发或者高温下GaNAs/GaAs量子阱中例外,一个高能端的PL峰成为了主要的发光来源。通过研究,我们将这个新的发光峰指认为量子阱中非局域激子复合的PL峰。这个发光峰在温度和激发强度的变化过程中与局域激子相互竞争。我们相信这一过程也是许多文献所报道的在InGaN和AlGaN等氮化物中经常观测到的发光峰位随温度“S”形变化的主要根源。     

不同In含量InGaN/GaN量子阱材料的变温PL谱

通过对不同In含量的InGaN/GaN量子阱材料的变温光致发光(PL)谱进行实验分析,得出样品激活能和PL谱峰值能量随温度变化的S形曲线中拐点温度与In含量的关系。说明对于我们的样品,这种S形曲线并不是来源于量子限制Stark效应(QCSE),而是与量子阱中In团簇有关。对比结果表明,含In量越多的材料其局域的能量越大,由热扰动脱离局域所需要的温度越高。     

关于InGaN/GaN多量子阱结构内量子效率的研究

利用金属有机物化学气相沉积技术在蓝宝石衬底(0001)面生长了InGaN/GaN多量子阱结构,并测量了其荧光(PL)光谱的峰位能量和发光效率对温度和注入载流子密度的依赖性.结果显示,该样品PL的峰位能量对温度的依赖性是"S形"的(降低-增加-降低),并且最大发光效率出现在50 K左右.前者反映了InGaN阱层中势能的非均一性和载流子复合的局域特征,后者则表明了将极低温度下的内量子效率设定为100%的传统界定方法应当被修正.进一步的研究结果显示,发光效率不仅是温度的函数,同时也是注入载流子密度的函数.为此我们对传统的基于PL光谱测量来确定某结构(或器件)内量子效率的方法进行了修正:在不同温度下测量发光效率对注入载流子密度的依赖性,并将发光效率的最大值设为内量子效率是100%,这样,其他温度点和注入载流子密度点所对应的内量子效率也就随之确定.     

InGaN/GaN多量子阱LED载流子泄漏与温度关系研究

通过测量光电流,直接观察了InGaN/GaN量子阱中载流子的泄漏程度随温度升高的变化关系。当LED温度从300K升高到360K时,在相同的光照强度下,LED的光电流增大,说明在温度上升之后,载流子从量子阱中逃逸的数目更多,即载流子泄漏比例增大。同时,光电流的增大在激发密度较低的时候更为明显,而且光电流随温度的增加幅度与激发光子的能量有关。用量子阱-量子点复合模型能很好地解释所观察到的实验现象。实验结果直接证明,随着温度的升高,InGaN/GaN量子阱中的载流子泄漏将显著增加,而且在低激发密度下这一效应更为明显。温度升高导致的载流子泄漏增多是InGaN多量子阱LED发光效率随温度升高而降低的重要原因。     

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。     

InGaN/GaN多量子阱蓝光LED外延片的变温光致发光谱

利用MOCVD在Al_2O_3(0001)衬底上制备InGaN/GaN MQW结构蓝光LED外延片。以400 mW中心波长405 nm半导体激光器作为激发光源,采用自主搭建的100～330 K低温PL谱测量装置,以及350～610 K高温PL测量装置,测量不同温度下PL谱。通过Gaussian分峰拟合研究了InGaN/GaN MQW主发光峰、声子伴线峰、n-GaN黄带峰峰值能量、相对强度、FWHM在100～610 K范围的温度依赖性。研究结果表明:在100～330 K温度范围内,外延片主发光峰及其声子伴线峰值能量与FWHM温度依赖性,分别呈现S与W形变化;载流子的完全热化分布温度约为150 K,局域载流子从非热化到热化分布的转变温度为170～190 K;350～610 K高温范围内,InGaN/GaN MQW主发光峰峰值能量随温度变化满足Varshni经验公式,可在MOCVD外延生长掺In过程中,通过特意降温在线测PL谱,实时推算掺In量,在线监测外延片生长。以上结果可为外延片的PL发光机理研究、高温在线PL谱测量设备开发、掺In量的实时监测等提供参考。     

GaAs间隔层厚度对InAs/GaAs量子点分子光学性质的影响

采用光致荧光发射谱(PL)和时间分辨荧光发射谱(TRPL)研究了GaAs间隔层厚度对自组装生长的双层InAs/GaAs量子点分子光学性质的影响.首先,测量低温下改变激发强度的PL谱,底层量子点和顶层量子点的PL强度比值随激发强度发生变化,表明两层量子点之间的耦合作用和层间载流子的转移随着间隔层厚度变大而变弱.接着测量改变温度的PL谱,量子点荧光光谱峰值位置(Emax)、半峰全宽及积分强度随温度发生变化,表明GaAs间隔层厚度直接影响到量子点内载流子的动力学过程和量子点发光的热淬灭过程.最后,TRPL测量发现60mL比40mL间隔层厚度样品的载流子隧穿时间有明显延长.     

InGaAs/GaAs应变量子阱的光谱研究

分别用光致发光谱（PL），光伏谱（PV）及时间分辨谱（TRPL）的方法，测量了应变InGaAs/GaAs单量子阱和多量子阱在不同温度下的光谱，发现单量子阱与多量子阱有不同的光学4性质。多量子阱PL谱发光峰和PV谱激子峰的强度与半高宽都比单量子阱的大，但单量子阱的半高宽随着温度的升高增大很快，这是由激子－声子耦合引起的，通过时间分辨谱研究发现了量子阱子能级之间的跃迁，多量子阱的发光寿命明显比单量子阱的长，我们利用形变势模型对量子阱的能带进行了计算，很好地解释了实验结果。     

In_(0.2)Ga_(0.8)As/GaAs单量子阱PL谱温度特性及其机制

测量了不同阱宽In0.2Ga0.8As/GaAs单量子阱的PL谱的峰值波长和荧光谱线半峰全宽随温度的变化。利用Varshni公式对实验峰值波长进行拟合,得到了新的参数。结果表明,无位错应变量子阱带隙仍具有其体材料的特性:荧光谱线半峰全宽随温度升高迅速展宽,这主要归因于声子关联作用增强和激子热离化为自由载流子所致;阱宽越窄荧光峰值能量越高,将其与量子尺寸效应的理论计算结果进行了比较。文中还考察了谱线半峰全宽和阱宽的关系,利用合金无序对这一现象进行了解释。     

Influence of the quantum-confined Stark effect on the temperature-induced photoluminescence blueshift of In GaN/GaN quantum wells in laser diode structures

Measurements of the excitation power-dependence and temperature-dependence photoluminescence(PL) are performed to investigate the emission mechanisms of In Ga N/Ga N quantum wells(QWs) in laser diode structures. The PL spectral peak is blueshifted with increasing temperature over a certain temperature range. It is found that the blueshift range was larger when the PL excitation power is smaller. This particular behavior indicates that carriers are thermally activated from localized states and partially screen the piezoelectric field present in the QWs. The small blueshift range corresponds to a weak quantum-confined Stark effect(QCSE) and a relatively high internal quantum efficiency(IQE) of the QWs.     

Reversible Carriers Tunnelling in Asymmetric Coupled InGaN/GaN Quantum Wells

Temperature-dependent photoluminescence （PL） and time resolved photoluminescence （TRPL） are performed to study the PL characteristics and carrier transfer mechanism in asymmetric coupled InGaN/GaN multiple quantum wells （AS-QWs）. Our results reveal that abnormal carrier tunnelling from the wide quantum well （WQW） to the narrow quantum well （NQW） is observed at temperature higher than about lOOK, while a normal carrier tunnelling from the NQW to the WQW is observed at temperature lower than 100 K. The reversible carrier tunnelling between the two Q Ws makes it possible to explore new types of temperature sensitive emission devices. It is shown that PL internal quantum efficiency （IQE） of the NQW is enhanced to about 46% due to the assistant of the abnormal carrier tunnelling.     

Emission and strain in InGaAs/GaAs quantum wells with InAs quantum dots obtained at different temperatures

The photoluminescence (PL), its temperature dependence and X ray diffraction (XRD) have been studied in the symmetric In_0.15Ga_0.85As/GaAs quantum wells (QWs) with embedded InAs quantum dots (QDs), obtained with the variation of QD growth temperatures (470–535 °C). The increase of QD growth temperatures is accompanied by the enlargement of QD lateral sizes (from 12 up to 28 nm) and by the shift non monotonously of PL peak positions. The fitting procedure has been applied for the analysis of the temperature dependence of PL peaks. The obtained fitting parameters testify that in studied QD structures the process of In/Ga interdiffusion between QDs and capping/buffer layers takes place partially. However this process cannot explain the difference in PL peak positions.     

Exciton diffusion dynamics in quantum nanostructures on V-groove patterned substrates

Time-resolved photoluminescence (PL) measurements were performed for two different quantum nanostructures on V-groove patterned substrates; SiGe/Si quantum wells (QWs) on V-grooved Si substrates and AlGaAs spontaneous vertical quantum wells on V-grooved GaAs substrates. Anomalous behaviours of the PL, such as the decrease of the decay time of the SiGe (111) QWs and the (111)A AlGaAs layer, were fully explained by taking the exciton diffusion towards the bottom of the V-groove into account, showing that the exciton diffusion driven by the spatial nonuniformity of the alloy compositions and/or geometry of the substrates is a key to controlling the PL properties of the nanostructures.     

InxGa1-xAs/GaAs量子阱应变量对变温光致发光谱的影响

利用变温光致发光(PL)研究了In0.182Ga0.818As/GaAs应变及应变补偿量子阱在77～300 K温度范围内的发光特性.随着温度T的升高,PL峰位向低能方向移动.在应力作用下In0.182Ga0.818As/GaAs量子阱的价带顶轻空穴带和重空穴带发生了劈裂.通过理论计算推导应变随温度变化对InxGa1-x...     

Reasons of emission inhomogeneity in InAs quantum dot structures

The photoluminescence (PL) inhomogeneity has been studied in InAs quantum dots (QDs) embedded in the symmetric In_0.15Ga_0.85As/GaAs quantum wells (QWs) with QDs grown at different temperatures. It was shown that three reasons are responsible for the emission inhomogeneity in studied QD structures: (i) the high concentration of nonradiative recombination centers in the capping In_0.15Ga_0.85As layer at low QD growth temperatures, (ii) the QD density and size distributions for the structure with QD grown at 510 ^°C, (iii) the high concentration of nonradiative recombination centers in the GaAs barrier at higher QD growth temperatures.     

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.     

Theoretical study of quantum-confined band-edge shifts and radiative lifetimes in oxidized Si(001) quantum films: comparison with experiment for Si/SiO2 quantum wells

The electronic structure and radiative lifetimes of Si(001) quantum films terminated by SiO₄ tetrahedra, which simulate Si/SiO₂ quantum wells (QWs), are calculated by the extended Hückel-type non-orthogonal tight-binding method. It is found that calculated band-gap widenings and radiative lifetimes account for band-edge shifts and photoluminescence (PL) peak shifts and lifetimes measured in amorphous-Si/SiO₂ QWs, suggesting that quantum confinement effects on the extended band-edge states in the amorphous-Si layer are responsible for the observed results. However, it is shown that band-edge shifts and PL energies and lifetimes observed in crystalline-Si/SiO₂ QWs cannot be reproduced properly by the interface model proposed in this study, implying that further studies are needed on the atomic structure of the crystalline-Si/SiO₂ interface.