不同淀积厚度InAs量子点的喇曼散射

利用喇曼散射方法在77K温度下对不同淀积厚度的InAs/GaAs量子点材料进行了研究.在高于InAs体材料LO模的频率范围内观察到了量子点的喇曼特征峰,分析表明应变效应是影响QD声子频率的主要因素.实验显示,随着量子点层淀积厚度L的增加,InAs量子点的声子频率由于应变释放发生红移.在加入InAlAs应变缓冲层的样品中,类AlAs声子峰随L增大发生了蓝移,从侧面证实了InAs量子点层的应变释放过程.     

Ge/Si超晶格喇曼谱研究

我们用喇曼光散射的方法,观察了应变层超晶格的折迭声学声子,从而验证了材料的超晶格多层结构.将实验中观察到的频率和理论计算值进行比较表明,所研究样品的标称值和喇曼实验是基本相符合的.根据光学限制模的频率移动,估算了超晶格样品中Ge层和Si层的应变分布.     

多量子阱的声子喇曼散射理论

<正> 近年来多量子阱的声子喇曼散射,在实验方面已有许多富有成果的工作。为了提供系统的理论基础,推动实验的进一步深入,我们系统地研究了多量子阱喇曼散射的微观理论。本文将介绍理论的部分内容,侧重说明理论基础和讨论区别于体材料最具特色的一些结果。 在微观理论中,最便于表征喇曼散射的是喇曼张量(以下具体讨论Stokes散射):     

CdTe/ZnTe超晶格的多声子共振喇曼谱研究

用共振喇曼散射研究了CdTe/ZnTe应变层超晶格的多声子谱.实验结果表明,我们首次观察到了多达10级的ZnTeLO的多声子喇曼散射,和反映超晶格结构的子带跃迁介入多声子共振喇曼散射过程的实验现象.     

多量子阱中激子作为中间态的喇曼散射微观理论

<正> 鉴于准二维激子在半导体多量子阱的光学过程中所起的决定性的作用,可以预期,在共振喇曼散射中,激子应是主要的中间态。但由于多量子阱电子结构、激子、声子的研究只是在近几年才有比较深入的理解,迄今尚无系统的喇曼散射的微观理论。本文将给出二维各向同性近似和电偶极近似下的多量子阱喇曼散射微观理论。     

应变对PbSe材料晶格振动的影响

在晶格失配的BaF2衬底上用分子束外延技术生长了不同厚度的PbSe单晶薄膜,PbSe外延薄膜的喇曼光谱测量到位于136～143cm-1之间的布里渊区中心纵光学声子(LO)振动,位于83～88cm-1之间的纵光学声子与横光学声子的耦合模(LO-TO)振动,以及位于268～280cm-1之间的2LO声子振动.而且PbSe薄膜的LO声子频率随薄膜厚度的不同明显移动,随着薄膜的厚度减小声子频率线性增大,这是由外延膜与衬底之间的失配应力不同引起的.为了理解PbSe声子振动模喇曼活性的物理原因,还比较分析了PbSe体单晶的喇曼光谱,同样,PbSe体单晶样品也呈现出喇曼活性的散射峰.     

应变对PbSe材料晶格振动的影响

在晶格失配的BaF2衬底上用分子束外延技术生长了不同厚度的PbSe单晶薄膜,PbSe外延薄膜的喇曼光谱测量到:位于136～143cm-1之间的布里渊区中心纵光学声子(LO)振动,位于83～88cm-1之间的纵光学声子与横光学声子的耦合模(LO-TO)振动,以及位于268～280cm-1之间的2LO声子振动.而且PbSe薄膜的LO声子频率随薄膜厚度的不同明显移动,随着薄膜的厚度减小声子频率线性增大,这是由外延膜与衬底之间的失配应力不同引起的.为了理解PbSe声子振动模喇曼活性的物理原因,还比较分析了PbSe体单晶的喇曼光谱,同样,PbSe体单晶样品也呈现出喇曼活性的散射峰.     

半导体超晶格中光学声子的喇曼散射研究

报道了在室温和非共振条件下,(GaAs)_n(AlAs)_n超晶格结构的喇曼散射测量结果。在适当的散射配置下,观察到分别限制在GaAs和AlAs层中的LO和TO声子模。根据线性链模型,把喇曼散射测量到的光学声子限制模的频率按q=2πm/[α_0(n+1)]展开所得到的光学声子色散曲线,与体材料GaAs和AlAs的光学声子色散曲线符合良好。     

GaP_(1-x)N_x混晶的喇曼散射谱

在室温下测试了Ga P1 - x Nx(x=0 .0 5 %～3.1% )混晶的喇曼散射谱.在一级喇曼散射谱中观测到了Ga P的L O(Γ)模和强度较弱的禁戒TO(Γ)模以及N的局域模(495 cm- 3) .在N组分较高的一组样品(x=1.3%～3.1% )中,还观察到了位于Ga P的L O(Γ)模和TO(Γ)模之间的由N导致的L O(N)模的喇曼频移(387cm- 1 ) ,其强度随着N浓度的增加而增强.在二级喇曼散射谱中,除了观测到布里渊区中心的声子散射峰2 L O(Γ)外,还观测到了布里渊区边界的声子散射峰2 L O(L )、2 TO(X)以及L O(L ) +TO(X) .且边界散射峰的强度比中心散射峰更强.另外在组分x=0 .6 %和x=0 .81%的样品     

IN1—x—yGaxAlyAs四元混晶的喇曼散射

报道了ＭＢＥ生长的Ｉｎ１－ｘ－ｙＧａｘＡｌｙＡｓ四元混晶中光学声子的喇曼散射实验结果．光学声子模的频率与强度的组分关系表明Ｉｎ１－ｘ－ｙＧａｘＡｌｙＡｓ四元混晶中有３种光学声子模，即类ＩｎＡｓ、类ＧａＡｓ和类ＡｌＡｓ模．喇曼光谱的偏振分析表明３种光学声子在退偏振条件下是喇曼活性的，而在偏振条件下是喇曼非活性的．由于混晶中的无序效应，可观察到泄漏的ＴＯ模叠加在ＬＯ的低能侧使喇曼峰显现出非对称形状．     

Transition Mechanism of InAs Quantum Dot to Quantum Ring Revealed by Photoluminescence Spectra

The transition mechanism of InAs quantum dot (QD) to quantum ring (QR) was investigated. After the growth of InAs QDs, a thin layer of GaAs was overgrown on the InAs QD and the sample was annealed at the same temperature for a period of time. It was found that the central part of the InAs islands started to out diffuse and formed ring shape only after a deposition of a critical thickness (1 ~ 2 nm) of GaAs capped layer depending on the size of InAs QDs. This phenomenon was revealed by photoluminescence measurement and atomic force microscopy image. It is suggested that the strain energy provided by the GaAs overgrown layer is responsible for the InAs to diffuse out of the island to form QR.     

InAs/GaAs系列量子点研究

为了获得波长长、均匀性好和发光效率高的量子点,采用分子束外延(MBE)技术和S-K应变自组装模式,在GaAs(100)衬底上研究生长了三种InAs量子点。采用MBE配备的RHEED确定了工艺参数:As压维持在1.33×10-5Pa;InAs量子点和In0.2Ga0.8As的生长温度为500℃;565℃生长50nmGaAs覆盖层。生长了垂直耦合量子点(InAs1.8ML/GaAs5nm/InAs1.8ML)、阱内量子点(In0.2Ga0.8As5nm/InAs2.4ML/In0.2Ga0.8As5nm)和柱状岛量子点(InAs分别生长1.9、1.7、1.5ML,停顿20s后,生长间隔层GaAs2nm)。测得对应的室温光致发光(PL)谱峰值波长分别为1.038、1.201、1.087μm,半峰宽为119.6、128.0、72.2nm、相对发光强度为0.034、0.153、0.29。根据PL谱的峰位、半峰宽和相对发光强与量子点波长、均匀性和发光效率的对应关系,可知量子点波长有不同程度的增加、均匀性越来越好、发光效率显著增强。     

Persistent IR photoconductivity in InAs/GaAs structures with QD layers

Persistent IR photoconductivity in InAs/GaAs structures with layers of QDs with a p-and n-type conductivity was studied. At the initial stage, after the illumination is switched off, the relaxation of photoconductivity follows a logarithmic law. The relaxation time depends on temperature; it decreases as temperature increases. A simple model of photoconductivity relaxation, based on thermal activation of carriers from the QD layer, is proposed. The model is consistent with the experimental data.     

Room-temperature optical absorption in the InAs/GaAs quantum-dot superlattice under an electric field

Electroluminescence and absorption spectra of a ten-layer InAs/GaAs quantum dot (QD) superlattice built in a two-section laser with sections of equal length is experimentally studied at room temperature. The thickness of the GaAs spacer layer between InAs QD layers, determined by transmission electron microscopy, is ∼6 nm. In contrast to tunnel-coupled QDs, QD superlattices amplify the optical polarization intensity and waveguide absorption of the TM mode in comparison with the TE mode. It is found that variations in the multimodal periodic spectrum of differential absorption of the QD superlattice structure are strongly linearly dependent on the applied electric field. Differential absorption spectra exhibit the Wannier-Stark effect in the InAs/GaAs QD superlattice, in which, in the presence of an external electric field, coupling of wave functions of miniband electron states is suppressed and a series of discrete levels called the Wannier-Stark ladder states are formed.     

Functionalized Self‐Assembled InAs/GaAs Quantum‐Dot Structures Hybridized with Organic Molecules

Low‐dimensional III–V semiconductors have many advantages over other semiconductors; however, they are not particularly stable under physiological conditions. Hybridizing biocompatible organic molecules with advanced optical and electronic semiconductor devices based on quantum dots (QDs) and quantum wires could provide an efficient solution to realize stress‐free and nontoxic interfaces to attach larger functional biomolecules. Monitoring the modifications of the optical properties of the hybrid molecule–QD systems by grafting various types of air‐stable diazonium salts onto the QD structures surfaces provides a direct approach to prove the above concepts. The InAs/GaAs QD structures used in this work consist of a layer of surface InAs QDs and a layer of buried InAs QDs embedded in a wider‐bandgap GaAs matrix. An enhancement in photoluminescence intensity by a factor of 3.3 from the buried QDs is achieved owing to the efficient elimination of the dangling bonds on the surface of the structures and to the decrease in non‐radiative recombination caused by their surface states. Furthermore, a narrow photoluminescence band peaking at 1620 nm with a linewidth of 49 meV corresponding to the eigenstates interband transition of the surface InAs QDs is for the first time clearly observed at room temperature, which is something that has rarely been achieved without the use of such engineered surfaces. The experimental results demonstrate that the hybrid molecule–QD systems possess a high stability, and both the surface and buried QDs are very sensitive to changes in their surficial conditions, indicating that they are excellent candidates as basic sensing elements for novel biosensor applications.     

Influence of bismuth doping of InAs quantum-dot layer on the morphology and photoelectronic properties of Gas/InAs heterostructures grown by metal-organic chemical vapor deposition

GaAs/InAs quantum dot (QD) heterostructures prepared by metalloorganic chemical vapor deposition (MOCVD) are investigated. It is established that the introduction of isovalent bismuth doping during the growth of InAs QD layer results in the suppression of the nanocluster coalescence and favors the formation of more uniform QDs. Bismuth itself is virtually not incorporated into the dots, its role being mainly in limiting the migration mobility of atoms at the surface of the growing layer. A method for investigating the morphology of buried layers of InAs QDs in GaAs matrix by atomic-force microscopy is developed; it relies on the removal of the cap layer by selective chemical etching. The photoluminescence (PL) and photoelectric sensitivity spectra of the fabricated heterostructures and their relation to the morphology of the QD layer are studied. In doped structures, PL and selective photosensitivity owing to the QDs are observed at a wavelength of 1.41 µm with the linewidth of 43 meV at room temperature. Some of the morphological features and photoelectronic properties of the MOCVD-grown heterostructures are related to the formation of a transitional layer at the GaAs/InAs QD interface due to the diffusion-induced mixing of the components.     

晶格畸变分析及其在半导体量子点结构中的应用

在适当的成像条件下可以直接从高分辨透射电子像（HRTEM）以接近原子级的分辨率进行晶格畸变分析。本综述介绍晶格畸变分析（LADIA）程序包及其在半导体自组装量子点（QD）系统中的应用。对多层InP小QD系统中畸变分布的分析表明：光致发光（PL）能量峰位的红移和QD中的应变弛豫直接相关。在慢速生长的InAs大QD系统中应变引起的元素互溶是PL峰位蓝移的主要因素。多层系统中QD的垂直叠合可解释为间隔层厚度低于临界值时生长前沿的横向张应变的作用。研究了生长以后不同条件快速退火对QD稳定性的影响,观测到垂直叠合的InPQD中出现各向异性的退火不稳定性。     

InGaAs/GaAs quantum dot heterostructures for 3–5 μm IR detectors

Specific features in the formation of InAs quantum dots (QD) by MOCVD were studied in relation to the growing time or equivalent thickness of the InAs layer. TEM and photoluminescence studies have shown that, as the growing time of QDs in a GaAs matrix becomes longer, both the size and shape of the QDs are modified; namely, the aspect ratio increases. Selectively doped multilayer InGaAs/GaAs QD structures were fabricated, and photoconductivity in the IR range was studied for lateral and vertical electron transport. Under a normal incidence of light, intraband photoconductivity in the mid-IR range, 2.5–5 μm, was observed at temperatures of up to 110 K.     

Theoretical and experimental study of the effect of InAs growth rate on the properties of QD arrays in InAs/GaAs system

The dependence of properties of quantum dot (QD) arrays in an InAs/GaAs system on the InAs growth rate has been investigated theoretically and experimentally. The derived kinetic model of the formation of coherent nanoislands allows the calculation of the average size, surface density of islands, and wetting layer thickness as functions of the growth time and conditions. Optical properties of InAs/GaAs QDs have been studied for the case of two monolayers (ML) of the material deposited at different growth rates. Predictions of the theoretical model are compared with the experimental data. It is shown that with two ML of the deposited material the characteristic lateral size of QDs decreases and the thickness of the residual wetting layer increases with rising growth rate.     

Annealing behavior of InAs/GaAs quantum dot structures

We investigate the annealing behavior of InAs layers with different thicknesses in a GaAs matrix. The diffusion enhancement by strain, which is well established in strained quantum wells, occurs in InAs/GaAs quantum dots (QDs). A shift of the QD luminescence peak toward higher energies results from this enhanced diffusion. In the case of structures where a significant portion of the strain is relaxed by dislocations, the interdiffusion becomes negligible, and there is a propensity to generate additional dislocations. This results in a decrease of the QD luminescence intensity, and the QD peak energy is weakly affected.