电场调谐InAs量子点荷电激子光学跃迁

在低温5 K下, 采用光致发光光谱及外加偏压调谐量子点电荷组态研究了InAs单量子点的精细结构和对应发光光谱的偏振性、不同带电荷激子的圆偏振特性. 得出如下结果: 1) 指认InAs单量子点中不同荷电激子的发光光谱和对应的激子本征态的偏振特性; 2) 外加偏压可以调谐量子点的荷电激子的发光光谱; 3) 伴随着电子、空穴的能量弛豫, 电子的自旋弛豫时间远大于空穴的自旋弛豫时间. 关键词： InAs量子点 激子 荧光光谱 电场调谐     

MOCVD生长InxGa1-xN合金微观结构和光学特性

利用发光光谱、X射线衍射(XRD)、原子力显微(AFM)等实验方法对MOCVD生长的InxGa1-xN合金进行了研究。原子力显微图样表明样品表面出现纳米尺度为微岛状结构。样品PL和PLE谱表明,其主要吸收峰位于波长为365,474nm,发光峰的位置位于波长为545,493nm处,其中545nm发光峰半高宽较493nm发光峰宽,这两个峰分别起源于In(Ga)N浸润层和InGaN层发光,浸润层局域化激子和岛状微观结构弛豫特性是产生发光峰Stokes移动的重要原因。     

InxGa1-xN/InyGa1-yN多量子阱结构 发光特性的测量与分析

利用MOCVD方法在蓝宝石衬底上生长了In_xGa_1-xN/In_yGa_1-yN多量子阱结构外延层,并用变温光致发光(PL)光谱、选择激发光谱以及激发(PLE)光谱等手段研究了该结构的量子效率、多峰效应的起源以及峰位随温度变化等信息。变温PL光谱的结果表明:在温度从30 K变化到300 K时,其峰值强度只下降了1.36倍且发光波长发生了蓝移。通过选择激发光谱证明了其发光峰位的独立性。PLE结果表明了GaN和势垒层的Stokes位移很小,但是In_xGa_1-xN阱层的Stokes位移变化很大。同时,提出了一种可同时获得多个吸收边的数据处理方法。     

不同形态MEH-PPV的构象及其光学特性

通过对在固溶体、稀溶液、薄膜和纳米孔中MEH—PPV的PL和PLE谱的测量分析,研究不同形态下MEH-PPV分子链的构象及其对电子能带和光学性质的影响。在THF稀溶液中,MEH—PPV分子链基本上皆为分立态;在MEH—PPV薄膜中,分子链基本上皆为聚集态;在MEH-PPV／PS固溶体中,MEH—PPV分子链为聚集态和分立态两构象并存,聚集态的比例随MEH-PPV浓度的增加而升高;在多孔氧化铝模板纳米孔中,MEH—PPV分子链形成链束。分立态、聚集态和链束这三种不同构象的分子链具有不同的电子能带结构和光学性质。     

硅衬底阳极氧化铝膜的荧光发射研究

报道了用电子束蒸发技术在硅守底上沉积，并于１５ｗｔ％Ｈ２ＳＯ４，温度２５℃和４０Ｖ直流电压条件下阳极氧化铝薄的制备（膜厚约４００ｎｍ）。研究了该阳极氧化铝膜的红外吸收光谱（ＦＴＩＲ）、光致荧光光谱（ＰＬ）和荧光激发光谱（ＰＬＥ）。发现其荧光光谱在２８０￣５００ｎｍ范围内由三个主发射带组成，其峰值分别位于３１２ｎｍ，３６７ｎｍ和４４９ｎｍ。所有这三个ＰＬ带，经分析都与阳极氧化铝膜中的氧化铝膜中的氧空     

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.     

Thermal dissociation of excitons in a type-I GaAs/AlAs superlattice studied by time-resolved photoluminescence measurements

In order to investigate thermal properties of excitons, we have performed time-resolved photoluminescence (PL) measurements for a type-I (GaAs)₁₅/(AlAs)₁₅ superlattice. At low temperatures, PL signals show ordinal exponential time decay, whereas at high temperatures, the PL shows power decay. Such change of PL signals is understood by considering thermal dissociation of exciton into account. At low temperatures, recombination of bound excitons generates PL which shows exponential decay. At temperatures higher than the exciton binding energy, correlation between electrons and holes disappears. Recombination of free excitons causes PL which decays by a power function. By means of the least-squares fitting, we evaluate the portion of bound excitons, recombination time of bound and free excitons as functions of temperatures.     

Exciton luminescence and many-body effect of monolayer WS2 at room temperature

Monolayer transition metal dichalcogenides favor the formation of a variety of excitonic quasiparticles, and can serve as an ideal material for exploring room-temperature many-body effects in two-dimensional systems. Here, using mechanically exfoliated monolayer WS₂ and photoluminescence (PL) spectroscopy, exciton emission peaks are confirmed through temperature-dependent and electric-field-tuned PL spectroscopy. The dependence of exciton concentration on the excitation power density at room temperature is quantitatively analyzed. Exciton concentrations covering four orders of magnitude are divided into three stages. Within the low carrier concentration stage, the system is dominated by excitons, with a small fraction of trions and localized excitons. At the high carrier concentration stage, the localized exciton emission from defects coincides with the emission peak position of trions, resulting in broad spectral characteristics at room temperature.     

Resonant electronic energy transfer from excitons confined in silicon nanocrystals to oxygen molecules

We demonstrate efficient resonant energy transfer from excitons confined in silicon nanocrystals to molecular oxygen (MO). Quenching of photoluminescence (PL) of silicon nanocrystals by MO physisorbed on their surface is found to be most efficient when the energy of excitons coincides with triplet-singlet splitting energy of oxygen molecules. The dependence of PL quenching efficiency on nanocrystal surface termination is consistent with short-range resonant electron exchange mechanism of energy transfer. A highly developed surface of silicon nanocrystal assemblies and a long radiative lifetime of excitons are favorable for achieving a high efficiency of this process.     

调制掺杂ZnSe/BeTe Ⅱ型量子阱结构中的内秉电场和新型带电激子

报道了调制掺杂的ZnSe/BeTe/ZnSe Ⅱ型量子阱(type-Ⅱ QW)在低温(2—5 K)条件下的光致发光(PL),光致发光激发(PLE)和磁性光致发光(magneto-PL)光谱的实验结果.　观察到非掺杂样品的PL有两个很强的主发光峰而掺杂样品只有一个的奇异发光.　PL直线偏振度和PLE的测量结果都表明了这些空间间接型跃迁PL是来自两个异质结界面的贡献,非掺杂样品的两个主发光峰的分离则是起因于QW结构中的内秉电场(built-in electric field).在平行于QW生长方向的强磁场中, 关键词： 光致发光 二维电子气 带电激子 Ⅱ型量子阱     

Exciton localization behaviour in different well width undoped GaN/Al0.07Ga0.93N nanostructures

We report results from optical spectroscopy such as photoluminescence (PL) and time resolved photo-luminescence (TRPL) techniques from different well width MOCVD grown GaN/Al_0.07Ga_0.93N MQW samples. There is evidence of localization at low temperature in all samples. The decay time of all samples becomes non-exponential when the detection energy is increased with respect to the peak of the emission. Localization of carriers (excitons) is demonstrated by the “S-shape” dependences of the PL peak energies on the temperature. The time-resolved PL spectra of the 3-nm well multi quantum wells reveal that the spectral peak position shifts toward lower energies as the decay time increases and becomes red-shifted at longer decay times. There is a gradient in the PL decay time across the emission peak profile, so that the PL process at low temperatures is a free electron-localized hole transition.     

Magnetophotoluminescence study of GaAs/AlGaAs coupled double quantum wells with bimodal heterointerface roughness

This work reports on the results of magnetophotoluminescence (MPL) measurements carried out in a sample containing two Al_0.35Ga_0.65As/GaAs, coupled double quantum wells (CDQWs), with inter-well barriers of different thicknesses, which have the heterointerfaces characterized by a distribution of bimodal roughness. The MPL measurements were performed at 4 K, with magnetic fields applied parallel to the growth direction, and varying from 0 to 12 T. The diamagnetic shift of the photoluminescence (PL) peaks is more sensitive to changes in the confinement potential, due to monolayer variations in the mini-well thickness, rather than to the exciton localization at the local potential fluctuations. As the magnetic field increases, the relative intensities of the two peaks in each PL band inverts, what is attributed to the reduction in the radiative lifetime of the delocalized excitons, which results in the radiative recombination, before the excitonic migration between the higher and lower energy regions in each CDQW occurs. The dependence of the full width at half maximum (FWHM) on magnetic field shows different behaviors for each PL peak, which are attributed to the different levels and correlation lengths of the potential fluctuations present in the regions associated with each recombination channel.     

离子注入诱导量子阱界面混合效应的光致荧光谱研究

采用多元芯片方法获得了一系列不同离子注入剂量的GaAsAlGaAs非对称耦合量子阱单元,通过光致荧光谱测量,研究了单纯的离子注入导致的界面混合效应.荧光光谱行为与有效质量理论计算研究表明,Al原子在异质结界面的扩散在离子注入过程中已基本完成,而热退火作用主要是去除无辐射复合中心. 关键词： 量子阱 离子注入 光致荧光谱 界面混合     

Enhancing redshift phenomenon in time-resolved photoluminescence spectra of AlGaN epilayer

AlGaN epitaxial layer has been studied by means of temperature-dependent time-integrated photoluminescence(PL)and time-resolved photoluminescence(TRPL). An enhancing redshift phenomenon in TRPL spectra with increasing temperature was observed, and the localized excitons behaved like quasi two-dimensional excitons between 6 K and 90 K. We demonstrated that these behaviors are caused by a change in the carrier dynamics with increasing temperature due to the competition of carriers' localization and delocalization in the AlGaN alloy.     

用巯基乙酸作稳定剂制备CdSe纳米晶的光学性质

以巯基乙酸为稳定剂制备了CdSe纳米晶，通过尺寸选择沉淀得到2nm到3nm之间不同尺寸的纳米晶，利用室温光吸收，光致发光（PL）和光致发光激发（PLE）谱来研究了CdSe纳米团簇的光学性质。紫外－可见吸收谱给了具有清晰激光特征的尖锐吸收边，这表明样品的尺寸分布很窄。光致发光研究表明，样品有两个发射带，一个具有较高能量位于吸收边，来自电子－空穴对从最低激发态能级弛豫后的辐射复合，另一个低能发射带归属于基质与纳米晶界面存在的俘获中心。PLE谱中有2个吸收带，分别是S－S和P－P跃迁。最后还给出了不同激发能量下的发光特性。     

Exchange-induced splitting of radiative exciton levels in a single quantum wire

We report on polarization-resolved micro-photoluminescence experiments performed on a single GaAs/GaAlAs V-shaped quantum wire. At low temperature the micro-photoluminescence spectra are composed of sharp peaks corresponding to excitons localized in naturally formed quantum boxes. We observed splittings of the radiative doublet of these exciton levels into two linearly polarized states due to the exchange interaction. The exchange splittings are of the order of 100 μeV. A theoretical model of the exchange interaction on localized states in quantum wires is developed. It shows that the exchange splitting is characteristic of the uniaxial anisotropy of the localized states and thus related to their extension along the wire axis. The experimental results are discussed within the frame of this model.     

Magnetic-field-effects on photoluminescence polarization in type II GaAs/AlAs superlattices

Optical orientation and alignment in the presence of a magnetic field have been applied to study the fine structure of excitons in type II GaAs/AlAs superlattices. We have developed a theory of polarized photoluminescence taking into account the anisotropic exchange splitting of the radiative excitonic doublet. The observed effects of the longitudinal and transverse magnetic fields on the polarization of the exciton emission unambiguously confirm that the actual symmetry of the exciton is lower that D_2d and that there exist two classes of excitons with opposite signs of the anisotropic exchange splitting.     

液封坩埚下降法生长非掺杂InP单晶的特性研究

用液封坩埚下降(LE-VB)法沿〈100〉晶向成功地生长了非掺杂InP单晶。LE-VB晶体的4.2K光致发光谱包含束缚于中性浅受主上的激子发光、与Zn受主相关的施主-受主(DA)对发光及其声子伴线、以及与本征缺陷等有关的深能级发光三部分。通过与液封直拉(LEC)生长的籽晶的光致发光谱比较表明,在LE-VB晶体中,束缚于中性浅受主上的激子发光与籽晶中的相差不大;DA对发光的晶格弛豫比籽晶中的小;与本征缺陷等有关的深能级发光强度比籽晶中的弱。晶体的室温光致发光谱仅包含带-带发光,其发光强度形貌测试结果表明,LE-VB晶体的带-带发光强度比LEC籽晶的强。用Huber法对晶片腐蚀的结果表明,在LE-BV晶体中,位错密度仅为LEC籽晶中的三分之一。分析认为,在LE-VB晶体中,本征缺陷和位错等浓度较低,可能是其带-带发光强度比在LEC籽晶中强的物理起因。     

多孔硅发光峰温度行为的研究

实验研究了多孔硅（Ｐｏｒｏｕｓ Ｓｉｌｉｃｏｎ）光致发光峰随测量温度的变化,发现发光峰位随温度的移动与发光峰的能量有关。随温度升高,发光峰波长较长的样品它们的发光峰都移向高能,面发光峰能量较高的样品它们的发光峰都移向低能,发光峰波长位于７４０ｎｍ附近的样品,它们的发光峰与测量温度无关。对上术结果的起源作了讨论。     

Eu掺杂SiC_xO_y薄膜的Eu~(3+)发光机制

利用磁控溅射技术在低温250℃下制备Eu掺杂SiC_xO_y薄膜,研究薄膜的Eu~(3+) 发光激发机制。实验结果表明,薄膜的发光谱由来自基体材料的蓝光和来自Eu~(3+) 的红光组成;随着薄膜中Eu含量由0.19%增加到2.27%,其红光强度增加3倍左右,而蓝光逐渐减弱。Raman光谱及荧光瞬态谱分析表明,其蓝光由中立氧空位缺陷发光中心引起。结合薄膜的Eu~(3+) 激发光谱分析,SiC_xO_y∶Eu薄膜的红光增强源于薄膜中Eu~(3+) 离子浓度的增加和/或基体材料的中立氧空位缺陷发光中心与Eu~(3+) 离子的能量转移。