半导体自组织量子点量子发光机理与器件

介绍了自组织量子点单光子发光机理及器件研究进展.主要内容包括：半导体液滴自催化外延GaAs纳米线中InAs量子点和GaAs量子点的单光子发光效应、自组织InAs/GaAs量子点与分布布拉格平面微腔耦合结构的单光子发光效应和器件制备,单量子点发光的共振荧光测量方法、量子点单光子参量下转换实现的纠缠光子发射、单光子的量子存储效应以及量子点单光子发光的光纤耦合输出芯片制备等.     

双层堆垛长波长InAs/GaAs量子点发光性质研究

研究了双层堆垛InAs/GaAs/InAs自组织量子点的生长和光致发光(PL)的物理性质。通过优化InAs淀积量、中间GaAs层厚度以及InAs量子点生长温度等生长条件,获得了室温光致发光1391～1438nm的高质量InAs量子点。研究发现对量子点GaAs间隔层实施原位退火、采用Sb辅助生长InGaAs盖层等方法可以增强高密度(2×1010 cm-2)InAs量子点的发光强度,减小光谱线宽,改善均匀性和红移发光波长。     

自组织InAs/GaAs量子点垂直排列生长研究

利用自组织生长InAs/GaAs量子点的垂直相关排列机制，生长了上下两层用6.5nm GaAs间隔的InAs结构.下层InAs已经成岛，由于应力传递效应，上层InAs由二维生长向三维成岛生长的转变提前发生，临界厚度从1.7ML变成小于1.5ML.透射电子显微镜截面象显示形成上下两层高度差别很大的InAs量子点，但是由于两层量子点之间存在强烈的电子耦合，光致发光谱中只有与包含大量子点的InAs层相对应的一个发光峰.     

p型掺杂1.3μm InAs/GaAs量子点激光器的最大模式增益特性的研究

对p型掺杂13 μm InAs/GaAs量子点激光器的最大模式增益进行了实验和理论分析.实验上,测量了不同腔长激光器阈值电流密度与总损耗的对应关系,拟合出的最大模式增益为175 cm^-1,与相同结构非掺杂量子点激光器的最大模式增益一致.同时理论分析表明,p型掺杂对InAs/GaAs量子点激光器的最大模式增益并无影响,并且最大模式增益的计算结果与实验值相符.具有较小高度或高宽比的量子点能达到更高的最大模式增益,而较高的最大模式增益对p型掺杂13 μm InAs/GaAs自组织量子点激光器在光通信系统中的应用具有重要意义. 关键词： 最大模式增益 p型掺杂 InAs/GaAs量子点激光器     

不同组分InxGa1-xAs(0≤x≤0.3)覆盖层对自组织InAs量子点的影响

研究了不同In组分的In_xGa_1-xAs(0≤x≤0.3)覆盖层对自组织InAs量子点的结构及发光特性的影响.透射电子显微镜和原子力显微镜表明,InAs量子点在InGaAs做盖层时所受应力较GaAs盖层时有所减小,并且x＝0.3时,InGaAs在InAs量子点上继续成岛.随x值的增大,量子点的光荧光峰红移,但随温度的变化发光峰峰位变化不明显.理论分析表明InAs量子点所受应力及其均匀性的变化分别是导致上述现象的主要原因. 关键词： 量子点 盖层 应力 红移     

利用InAs/GaAs数字合金超晶格改进InAs量子点有源区的结构设计

利用分子束外延技术,通过InAs/GaAs数字合金超晶格代替传统的直接生长InGaAs层的方式,在GaAs(100)衬底上生长了InAs量子点结构并成功制备了1.3μm InAs量子点激光器.通过原子力显微镜和光致荧光谱测试手段,对传统生长模式和数字合金超晶格生长模式的两种样品进行了表征,研究发现采用32周期InAs/GaAs数字合金超晶格样品的量子点密度非常高,发光性能良好.通过与常规生长方式所制备激光器的性能对比,发现采用InAs/GaAs数字合金超晶格生长InAs量子点的有源区也可以得到高质量的激光器.利用该方式生长的InAs量子点激光器的阈值电流为24 mA,相应的阈值电流密度仅为75 A/cm2,最高工作温度达到120℃.InAs/GaAs数字合金超晶格既可以保证生长过程中源炉的温度保持不变,还可以对InGaAs层的组分实现灵活调控.不需要改变生长速度,通过改变InAs/GaAs数字合金超晶格的周期数以及InAs层和GaAs层的厚度,便可以获得任意组分的InGaAs,从而得到不同发光波长的激光器.这种生长方式对量子点有源区的结构设计和外延生长提供了新思路.     

金纳米颗粒光散射提高InAs单量子点荧光提取效率

采用光学方法确定InAs/GaAs单量子点在样品外延面上的位置坐标, 利用AlAs牺牲层把含有量子点的GaAs层剥离并放置在含有金纳米颗粒或平整金膜上, 研究量子点周围环境不同对量子点自发辐射寿命及发光提取效率的影响. 实验结果显示, 剥离前后量子点发光寿命的变化小于13%, 含有金纳米颗粒的量子点发光强度是剥离前的7倍, 含有金属薄膜的量子点发光强度是剥离前的2倍. 分析表明在金纳米颗粒膜上的量子点荧光强度的增加主要来自于金纳米颗粒对量子点荧光的散射效应, 从而提高量子点发光的提取效率.     

Mn离子注入InAs/GaAs量子点结构材料的光电性质研究

用高能离子注入(160keV)的方法对InAs/GaAs量子点结构进行掺杂，研究了不同退火工艺处理后量子点的光致发光和电学性能.相对于长时间退火，快速退火处理后的量子点发光通常较强.在相同的退火条件下，量子点发光峰位随着Mn注入剂量的增加，先是往高能量端快速移动，而后发光峰又往低能方向移动.后者可能是由于Mn原子进入InAs量子点，释放了InAs量子点中的应变所致.对于高注入剂量样品和长时间退火样品，变温电阻曲线在40 K附近会出现反常行为. 关键词： 离子注入 InAs/GaAs量子点 光致发光 团簇     

不同组分InxGa1-xAs(0≤x≤0.3)覆盖层对自组织InAs量子点的影响

研究了不同In组分的InxGa1-xAs(0≤x≤0.3)覆盖层对自组织InAs量子点的结构及发光特性的影响.透射电子显微镜和原子力显微镜表明,InAs量子点在InGaAs做盖层时所受应力较GaAs盖层时有所减小,并且x＝0.3时,InGaAs在InAs量子点上继续成岛.随x值的增大,量子点的光荧光峰红移,但随温度的变化发光峰峰位变化不明显.理论分析表明InAs量子点所受应力及其均匀性的变化分别是导致上述现象的主要原因.     

能级构型对InAs/GaAs量子点电磁感应透明介质中光孤子存储的影响

基于现有的实验,利用不同频率的光脉冲耦合到InAs/GaAs量子点的不同能级之间可形成梯形、Λ形和V形等3类量子点电磁诱导透明介质.继而研究这三类能级构型InAs/GaAs量子点电磁诱导透明介质中的光孤子形成和存储性质,结果表明,梯形和Λ形InAs/GaAs量子点体系不但可形成光孤子还可以实现光孤子的存储与读取,且其所存储光孤子的保真度比光存储的保真度高;但V形InAs/GaAs量子点体系却不能形成光孤子,这是由于体系的非线性效应非常弱.有趣的是在相同的实验参数下,Λ形InAs/GaAs量子点体系所存储的光孤子幅度比梯形所存储的光孤子幅度大.这为半导体量子点器件对所存储光孤子进行调幅操作提供了理论依据.     

Electric field effect in the spin dynamics of self-assembled InAs/GaAs quantum dots

We identify fundamental mechanisms of electron and hole dynamics in self-organized InAs/GaAs quantum dots (QDs) subject to vertical electric fields by photocurrent investigations. We propose a spin–flip mechanism involving a spin exchange between neighboring QDs. The spin–flip process is revealed in the photocurrent dynamics when the exciton population increases unexpectedly with reverse bias.     

Effects of p-type doping on the optical properties of InAs/GaAs quantum dots

The effects of p-type doping on the optical properties of self-organized InAs/GaAs quantum dots (QDs) were investigated by both micro-photoluminescence and degenerated pump–probe reflection measurements. As compared to undoped InAs/GaAs QDs, it was observed that the transitions between the ground and the first excited states of electrons and holes levels appeared at higher energies for p-doped InAs/GaAs QDs. In addition, the PL intensities for both undoped and p-doped QDs were found to decrease when the excitation power exceeded a critical value. The critical excitation power for p-doped QDs appeared to be much lower than that for undoped ones. In the pump–probe experiments, it was revealed that the value and sign of the differential reflectivity depends strongly on excitation wavelength. P-doped QDs exhibited a response behavior that is different from that of undoped ones. It is believed that the large build-in population of holes plays a crucial role in determining the transient reflection spectrum.     

Phonon-assisted polar exciton–transitions in self-organized InAs/GaAs quantum dots

Phonon-assisted exciton transitions are investigated for self-organized InAs/GaAs quantum dots (QDs) using selectively excited photoluminescence (PL) and PL excitation spectroscopy. The results unambiguously demonstrate intrinsic recombination in the coherent InAs/GaAs QDs and the absence of a Stokes shift between ground state absorption and emission. Phonon-sidebands corresponding to a phonon energy of 34 meV are resolved and Huang–Rhys parameters of 0.015 and 0.08 are found for phonon-assisted emission and absorption, respectively, which are about one order of magnitude larger than in bulk InAs. Calculations of the exciton–LO–phonon interaction based on an adiabatic approximation and realistic wave functions for ideal pyramidal InAs/GaAs QDs show this enhanced polar coupling to result from the particular confinement and the strain-induced piezoelectric potential in such strained low-symmetry QDs.     

Spin relaxation quenching in semiconductor quantum dots

We have studied the spin dynamics in self-organized InAs/GaAs quantum dots by time-resolved photoluminescence performed under strictly resonant excitation. At low temperature, we observe strictly no decay of both the linear and the circular luminescence polarization. This demonstrates that the carrier spins are totally frozen on the exciton lifetime scale.     

Highly reduced fine-structure splitting in InAs/InP quantum dots offering an efficient on-demand entangled 1.55-microm photon emitter

To generate entangled photon pairs via quantum dots (QDs), the exciton fine-structure splitting (FSS) must be comparable to the exciton homogeneous linewidth. Yet in the (In,Ga)As/GaAs QD, the intrinsic FSS is about a few tens microeV. To achieve photon entanglement, it is necessary to cherry-pick a sample with extremely small FSS from a large number of samples or to apply a strong in-plane magnetic field. Using theoretical modeling of the fundamental causes of FSS in QDs, we predict that the intrinsic FSS of InAs/InP QDs is an order of magnitude smaller than that of InAs/GaAs dots, and, better yet, their excitonic gap matches the 1.55 microm fiber optic wavelength and, therefore, offers efficient on-demand entangled photon emitters for long distance quantum communication.     

Radiative and non-radiative relaxation of excitons in strain-compensated quantum dots

We have investigated the population dynamics of excitons in strain-compensated InAs quantum dots (QDs) using a pump–probe technique under resonant excitation. Precise control of polarization directions of incident pulses enabled us to selectively estimate population lifetimes for two orthogonally polarized exciton ground states according to polarization selection rules. Measured decay times of the probe transmissions were highly dependent on the polarization directions of the exciton states. We found that the ratio of the decay times for the orthogonally polarized states is in quantitative agreement with the ratio of square of the transition dipole moments. This indicates that radiative recombination processes have a dominant effect on the population dynamics and that non-radiative and spin relaxations are negligible in our QDs. As a result, we can estimate the radiative lifetimes to be 1.0±0.1 and 1.7±0.2 ns for orthogonally polarized exciton ground states.     

InAs/GaAs量子点1.3 μm单光子发射特性

采用双层耦合量子点的分子束外延生长技术生长了InAs/GaAs量子点样品,把量子点的发光波长成功地拓展到1.3 μm.采用光刻的工艺制备了直径为3 μm的柱状微腔,提高了量子点荧光的提取效率.在低温5 K下,测量得到量子点激子的荧光寿命约为1 ns;单量子点荧光二阶关联函数为0.015,显示单量子点荧光具有非常好的单光子特性;利用迈克耳孙干涉装置测量得到单光子的相干时间为22 ps,对应的谱线半高全宽度为30 μeV,且荧光谱线的线型为非均匀展宽的高斯线型.     

Tuning vertically stacked InAs/GaAs quantum dot properties under spacer thickness effects for 1.3 μm emission

Coherent InAs islands separated by GaAs spacer (d) layers are shown to exhibit self-organized growth along the vertical direction. A vertically stacked layer structure is useful for controlling the size distribution of quantum dots. The thickness of the GaAs spacer has been varied to study its influence on the structural and optical properties. The structural and optical properties of multilayer InAs/GaAs quantum dots (QDs) have been investigated by atomic force microscopy (AFM), transmission electron microscopy (TEM), and photoluminescence (PL) measurements. The PL full width at half maximum (FWHM), reflecting the size distribution of the QDs, was found to reach a minimum for an inter-dots GaAs spacer layer thickness of 30 monolayers (ML). For the optimized structure, the TEM image shows that multilayer QDs align vertically in stacks with no observation of apparent structural defects. Furthermore, AFM images showed an improvement of the size uniformity of the QDs in the last layer of QDs with respect to the first one. The effect of growth interruption on the optical properties of the optimized sample (E30) was investigated by PL. The observed red shift is attributed to the evolution of the InAs islands during the growth interruption. We show the possibility of increasing the size of the QDs approaching the strategically important 1.3 m wavelength range (at room temperature) with growth interruption after InAs QD deposition.     

Hydrostatic pressure effects on exciton states in InAs/GaAs quantum dots

Based on the effective-mass approximation, the hydrostatic pressure effects on exciton states in InAs/GaAs self-assembled quantum dots (QDs) are studied by means of a variational method. Numerical results show that the exciton binding energy has a minimum with increasing dot height for any hydrostatic pressure. The interband emission energy increases when the hydrostatic pressure increases. In particular, we find that hydrostatic pressure has a remarkable effect on exciton states for small QD size. Our results are in agreement with experiment measurements.     

EFFECT OF DOPANT ON THE UNIFORMITY OF InAs SELF-ORGANIZED QUANTUM DOTS

Low-temperature photoluminescence studies have been performed on Si-doped and Bedoped self-organized InAs/GaAs quantum dot(QD) samples to investigate the effect of doping. When Si or Be is doped into the sample,a remarkable decrease in line-width is observed. We relate this phenomenon to a model that takes the Si or Be atoms as the nucleation centers for the formation of QDs. When Si or Be is doped, more smalll uniform quantum dots are formed. The result will be of significance for the application of self-organized InAs quantum dots in semiconductor devices.