分子束外延生长InGaN/AlN量子点的组分研究

报道了分子束外延生长的绿光波段InGaN/AlN量子点材料,并综合考虑InGaN量子点的应变弛豫,以及应力和量子限制斯塔克效应对量子点发光波长的影响,提出了一种结合反射式高能电子衍射原位测量与光致荧光测量确定InGaN量子点组分的方法.     

InGaN量子点的诱导生长和发光特性研究

降低InGaN的维数是提高GaN基发光器件发光效率的一种非常有效的方法,本文的工作主要集中在高密度InGaN量子点的生长和分析上。在MOCVD设备上,经过钝化和低温两个特殊工艺条件,在高温CaN表面生长了一层低温岛状GaN．形成表面形貌的起伏,进而导致表面应力的不均匀分布。在这一层低温岛状GaN的诱导性作用下生长并形成InGaN量子点。通过原子力显微镜、透射电子显微镜和光致发光谱对其徽观形貌和光学性质进行了观察和研究。从原子力显微镜以及透射电子显微镜观察得到的结果表明：InGaN量子点为平均直径约30nm、高度约25nm、分布较均匀的圆锥,其密度约10^11cm^-2。室温下,InGaN量子点材料的PL谱强度大大超出相同条件生长的InGaN薄膜材料。这些现象表明,用InGaN量子点代替普通InGaN薄膜．有望获得发光效率更高的GaN基发光器件。     

钝化低温法生长多层InGaN量子点的结构和光学特性

采用一种新方法生长多层InGaN/GaN量子点,研究所生长样品的结构和光学特性。该方法采用了低温生长和钝化工艺,所以称之为钝化低温法。第一层InGaN量子点的尺寸平均宽度40nm,高度15nm,量子点密度为6.3×10¹⁰/cm²。随着层数的增加,量子点的尺寸也逐渐增大。在样品的PL谱测试中,观察到在In(Ga)As材料系中普遍观察到的量子点发光的温度特性---超长红移现象。它们的光学特性表明:采用钝化低温法生长的纳米结构中存在零维量子限制效应。     

SrTiO_3(001)衬底上多层FeSe薄膜的分子束外延生长

近几年,由于用分子束外延法在SrTiO_3衬底表面制备的单层FeSe具有很高的超导转变温度而引发了极大的研究热潮,随之而来的是对多层FeSe薄膜日益增长的研究兴趣.但目前还没有对成功生长高质量多层FeSe薄膜的详细报道.本文利用高能电子衍射仪(RHEED)实时监控在不同生长条件下制备的多层FeSe薄膜,发现在FeSe薄膜的生长初期,RHEED图像的强度演化基本符合台阶密度模型的描述特征,即台阶密度ρ正相关于衍射条纹强度.FeSe(02)衍射条纹的强度在第一生长周期内呈现稳定而明显的峰型振荡,而且不受高能电子掠射角的影响,最适合用来标定FeSe薄膜的厚度.结合扫描隧道显微镜对FeSe薄膜质量的原位观察,确定了制备多层FeSe薄膜的最佳生长条件,为FeSe薄膜的物性研究提供了重要的材料基础.     

反射式高能电子衍射实时监控的分子束外延生长GaAs晶体衬底温度校准及表面相变的研究

以反射式高能电子衍射(RHEED)作为实时监测工具,根据GaAs(100)表面重构相与衬底温度、As₄等效束流压强之间的关系,对分子束外延(MBE)系统中衬底测温系统进行了校准,这种方法也适用于其他的MBE系统.为生长高质量的外延薄膜材料、研究InGaAs表面粗糙化及相变等过程提供了实验依据. 关键词： 分子束外延 反射式高能电子衍射 表面重构 温度校准     

分子束外延生长ZnSe自组织量子点光、电行为研究

分别应用光致发光、电容电压和深能级瞬态傅里叶谱技术详细研究ZnSe自组织量子点样品的光学和电学行为.光致发光温度关系表明ZnSe量子点的光致发光热猝火过程机理.两步猝火过程的理论较好模拟和解释了相关的实验数据.电容电压测量表明样品表观载流子积累峰出现的深度(样品表面下约100nm处)大约是ZnSe量子点层的位置.深能级瞬态傅里叶谱获得的ZnSe量子点电子基态能级位置为ZnSe导带下的011eV,这与ZnSe量子点光致发光热猝火模型得到的结果一致.     

分子束外延生长ZnSe自组织量子点光、电行为研究

分别应用光致发光、电容-电压和深能级瞬态傅里叶谱技术详细研究ZnSe自组织量子点样品的光学和电学行为.光致发光温度关系表明ZnSe量子点的光致发光热猝火过程机理.两步猝火过程的理论较好模拟和解释了相关的实验数据.电容-电压测量表明样品表观载流子积累峰出现的深度(样品表面下约100nm处)大约是ZnSe量子点层的位置.深能级瞬态傅里叶谱获得的ZnSe量子点电子基态能级位置为ZnSe导带下的0.11eV,这与ZnSe量子点光致发光热猝火模型得到的结果一致.     

InGaN量子阱的微观特性

采用VASP程序包模拟计算InGaN量子阱的能带,精细展示了量子阱实空间能带结构。计算结果表明,In原子所在区域出现局域束缚态,导带底与价带顶的简并能级发生分裂,同时量子阱沿垂直结面方向存在分立的能级。此外,针对影响能带的In组分波动、能带弯曲等问题进行探讨,以准确描述其电子行为,从而深入系统地了解InGaN/GaN量子阱的电学光学等特性。     

高能电子衍射研究H钝化偏角Si衬底上Si,GexSi1-x的分子束外延生长模式

用高能电子衍射(RHEED)研究H钝化偏角Si衬底上Si,Ge_xSi_1-x材料的分子束外延(MBE)生长模式,发现经低温处理的H钝化Si衬底上要经过10nm左右的Si生长才能获得比较平整的表面.Si,Ge_xSi_1-x外延时的稳定表面均以双原子台阶为主,双原子台阶与单原子台阶并存的结构.Si双原子台阶上的Si二聚体列(dimerrow)取向垂直于台阶边缘,而Ge_xSi_1-x双原 关键词：     

在Si-Ge晶体外延生长中的RHEED花样研究

在晶体衍射理论基础上,利用运动学理论解释了反射式高能电子衍射（reflection high energy electron diffraction, RHEED）在Si-Ge晶体外延生长过程中不同阶段出现的花样．尤其研究了晶体岛状生长之后出现的RHHED透射式衍射花样,并给出了相应的解释．解释了硅锗外延生长过程中的多晶环图案和孪晶衍射图案的含义,并给出各个生长阶段演化的工艺条件． 关键词： 硅锗外延生长 反射式高能电子衍射 表面重构 透射式衍射花样     

Dynamics of single InGaN quantum dots

Decay dynamics for single InGaN quantum dots are presented using time-resolved photoluminescence. The recombination is shown to be characterized by a single exponential decay, in contrast to the non-exponential recombination dynamics seen in the 2D wetting layer. The lifetimes of single dots in the temperature range 4–60 K decrease with increasing temperature. Different dots show similar lifetimes of 2 ns.     

Lateral electron transport through single InAs quantum dots grown by molecular beam epitaxy

The transport properties of single InAs quantum dots (QDs) grown by molecular beam epitaxy have been investigated by metallic leads with nanogaps. It was found that the uncapped InAs QDs grown on the GaAs surfaces show metallic conductivities, indicating that even the exposed QDs are not depleted. On the contrary, it was found that no current flows through the exposed wetting layers. For the case of the QDs covered with GaAs capping layers, clear Coulomb gaps and Coulomb staircases have been observed at 4.2 K.     

GaN quantum dots by molecular beam epitaxy

The conditions to grow GaN quantum dots (QDs) by plasma-assisted molecular beam epitaxy will be examined. It will be shown that, depending on the Ga/N ratio value, the growth mode of GaN deposited on AlN can be either of the Stranski–Krastanow (SK) or of the Frank–Van der Merwe type. Accordingly, quantum wells or QDs can be grown, depending on the desired application. In the particular case of modified SK growth mode, it will be shown that both plastic and elastic strain relaxation can coexist. Growth of GaN QDs with N-polarity will also be discussed and compared to their counterpart with Ga polarity.     

Local structure of uncapped and capped InGaN/GaN quantum dots

The local structure around the indium atoms in uncapped and capped In_xGa_1?xN quantum dots has been studied by In K‐edge extended X‐ray absorption fine structure (EXAFS) spectroscopy. The samples were grown by metal organic vapour phase epitaxy. The EXAFS was successfully applied to study the structural properties of buried quantum dots which are not optically active. The analysis revealed that capping the quantum dots with GaN does not affect the bond distances of the In—N and In—Ga, but makes the In—In distance shorter by 0.04 Å.     

Self-assembly of InAs quantum dots on GaAs(001)by molecular beam epitaxy

Currently, the nature of self-assembly of three-dimensional epitaxial islands or quantum dots (QDs) in a lattice-mismatched heteroepitaxial growth system, such as InAs/GaAs(001) and Ge/Si(001) as fabricated by molecular beam epitaxy (MBE), is still puzzling. The purpose of this article is to discuss how the self-assembly of InAs QDs in MBE InAs/GaAs(001) should be properly understood in atomic scale. First, the conventional kinetic theories that have traditionally been used to interpret QD self-assembly in heteroepitaxial growth with a significant lattice mismatch are reviewed briefly by examining the literature of the past two decades. Second, based on their own experimental data, the authors point out that InAs QD self-assembly can proceed in distinctly different kinetic ways depending on the growth conditions and so cannot be framed within a universal kinetic theory, and, furthermore, that the process may be transient, or the time required for a QD to grow to maturity may be significantly short, which is obviously inconsistent with conventional kinetic theories. Third, the authors point out that, in all of these conventional theories, two well-established experimental observations have been overlooked: i) A large number of “floating” indium atoms are present on the growing surface in MBE InAs/GaAs(001); ii) an elastically strained InAs film on the GaAs(001) substrate should be mechanically unstable. These two well-established experimental facts may be highly relevant and should be taken into account in interpreting InAs QD formation. Finally, the authors speculate that the formation of an InAs QD is more likely to be a collective event involving a large number of both indium and arsenic atoms simultaneously or, alternatively, a morphological/structural transformation in which a single atomic InAs sheet is transformed into a three-dimensional InAs island, accompanied by the rehybridization from the sp²-bonded to sp³- bonded atomic configuration of both indium and arsenic elements in the heteroepitaxial growth system.     

MBE生长PbSe/PbSrSe量子阱结构的光致中红外发光的研究

研究了分子束外延技术生长的PbSe/PbSrSe多量子阱结构的中红外光致荧光现象.高分辨率X射线衍射（HRXRD）谱观察到了多量子阱所特有的多级卫星峰,表明量子阱界面陡峭.变温光致荧光谱测量显示量子阱结构对电子空穴有强的限制效应,在相同温度下,量子阱样品的荧光峰峰位相对PbSe体材料有一定的蓝移.发现量子阱样品的荧光强度同温度有关,温度从150 K上升到230 K时,荧光强度逐渐增大,温度继续升高,荧光强度缓慢下降,但在高于室温时,仍能观察到较强的荧光发射,这说明该量子阱结构材料具有应用于室温工作的中红外 关键词： PbSe/PbSrSe多层量子阱（MQWs） 光致中红外荧光 高分辨X射线衍射（HRXRD）     

Self-assembled GaAs/AlGaAs quantum dots by molecular beam epitaxy and in situ AsBr3 etching

We report on a new method to produce self-assembled, unstrained, GaAs/AlGaAs quantum dots (QDs) with large confinement energy. First we create nanoholes on a GaAs surface by growing InAs islands on GaAs(0 0 1), overgrowing them with GaAs and etching the surface in situ with AsBr₃ gas. Then we transfer the holes to an AlGaAs surface, fill them with GaAs and overgrow them with AlGaAs. In this way we obtain GaAs inclusions in an AlGaAs matrix. We investigate the optical properties of such QDs by photoluminescence spectroscopy and their morphology by atomic force microscopy. We show that the QD size can be tuned and emission with inhomogeneous broadening down to 8.9 meV can be achieved.