Ga液滴沉积量对GaAs/GaAs(001)同心量子双环形貌的影响

通过液滴外延法制备了GaAs/GaAs(001)同心量子双环(Concentric Quantum Double Rings,CQDRs),研究了Ga液滴沉积量对CQDRs的影响.研究结果发现:随着Ga液滴沉积量的增加,CQDRs密度降低,内环高度增高,外环高度降低,中心孔洞深度增加.CQDRs内环拟合结果表明,Ga液滴沉积量少于0.92ML(Monolayer,ML)时无法成环;外环拟合结果显示,在本实验条件下,形成外环的最小Ga液滴沉积量为3.1ML.拟合结果与实验结果一致,相关研究结果对液滴外延法制备GaA s同心量子双环具有指导意义.     

Ga液滴沉积速率对GaAs/GaAs(001)量子双环形貌的影响

采用液滴外延法在GaAs (001)衬底上制备同心量子双环(concentric quantum double rings, CQDRs),利用原子力显微镜表征其表面形貌,并研究Ga液滴沉积速率对CQDRs的影响.研究结果发现,随着Ga液滴沉积速率的增加, CQDRs的密度增加,内外环半径均降低.根据成核理论中最大团簇密度和Ga液滴沉积速率之间的关系拟合出临界成核原子数目为5,表明在Ga液滴形成阶段时稳定的Ga原子晶核至少包含5个Ga原子;根据成核理论和拟合结果绘制成核过程状态转化图以深入理解Ga液滴形成过程.相关研究结果对液滴外延法制备密度可控的GaAs同心量子双环具有一定的指导意义.     

基于液滴外延技术变Al、Ga组分对GaAs表面液滴生长形貌的影响

液滴外延技术不仅适用于晶格失配,也适用于晶格匹配材料系统,且易于制备低维半导体结构,如低密度量子点、环等.本文研究了液滴外延法在GaAs表面进行不同Al、Ga组分的量子点生长.在实验中用反射式高能电子衍射仪(Reflection High Energy Electron Diffraction, RHEED)对样品进行原位监控.通过控制Al、Ga液滴的沉积速率来控制液滴同时沉积在衬底上形成的组分.研究发现,随着Al组分的增加,量子点逐渐变得密集,润湿角变低.在Al组分增高超过0.5之后,出现了大小不一的量子点,且量子点密度出现指数型增长.对此进行研究分析,给出了一个经验公式,并就现象进行了解释.     

In液滴低温时在不同沉积量下的形貌演变分析

本文研究了低温时不同沉积量的In液滴在GaAs(001)衬底上的形貌特征.在衬底温度为160℃时,对In液滴形貌进行观察和分析并根据经典的成核理论解释了不同沉积量下In液滴纳米结构的形成机制和In液滴形貌随沉积量的演变规律;通过对液滴数量,直径和高度以及液滴周围出现扩散圆盘的直径和高度进行统计,结合液滴形貌与沉积量的相关理论公式以及本文实验中所得数据,拟合计算出In液滴产生扩散圆盘的最小沉积量约为3.3 ML,表明In液滴的沉积量大于3.3 ML时才会形成圆盘.相关研究结果对液滴外延法制备InAs纳米结构具有指导意义.     

In液滴在GaAs(001)表面扩散行为的研究

量子点的物理与光电性质主要依赖于其尺寸及密度参数,而量子点的密度、高度等参数又控制着原子在衬底上的成核行为。本文采用液滴外延法在GaAs(001)表面生长金属In液滴,研究了In液滴的扩散运动与衬底温度和沉积速率之间的关系,研究发现,随着衬底温度的升高和沉积速率的降低,In液滴尺寸增大密度却降低。通过得到的实验数据,拟合关于In液滴密度与衬底温度和沉积速率的曲线,分析了量子环的生长机制,并根据原子的表面迁移行为,进一步分析其表面原子扩散机理。     

液滴外延下生长参数对InAs纳米结构形貌的影响

利用液滴外延法在GaAs(001)衬底表面制备InAs量子点,通过控制变量分别研究沉积速率、沉积量对In液滴在GaAs表面生长过程中的影响.使用原子力显微镜(Atomic Force Microscope, AFM)表征InAs纳米结构形貌,得出结论：(1)沉积速率主要通过影响In液滴成核率来控制液滴的密度,即随着沉积速率的增大,In原子在衬底表面的成核率增加,InAs量子点密度增加,实验符合生长动力学经典成核理论.(2)沉积量的改变主要影响液滴的熟化过程,即随着沉积量的增大,可参与生长的活跃的In原子增加,促进了液滴熟化,使得扩散坍塌的原子数量增加,导致在InAs纳米结构中出现多量子点现象.     

零砷压下Ga液滴在AlGaAs表面扩散行为的研究

本文研究了Ga液滴在Al_(0.4)Ga_(0.6)As表面的扩散行为.在衬底温度为380℃时,在Al_(0.4)Ga_(0.6)As薄膜上沉积3 MLGa液滴;在零砷压下,通过改变退火时间(0 s, 150 s, 300 s, 450 s, 600 s)观察液滴形貌变化,利用剖面线、坑洞和液滴的比例变化分析发现液滴高度随时间延长越来越低,直至形成坑洞,Ga液滴内部的原子在Al_(0.4)Ga_(0.6)As表面上首先向外扩散,而后与表面As原子结合成环,约在退火时间到500 s时扩散模式逐步变化为向下溶蚀.利用公式计算出最初Ga覆盖率约为2.6 ML,并且在380℃下Ga液滴在Al_(0.4)Ga_(0.6)As表面的液滴消耗速率为0.0065 ML/s.     

低温下InAs纳米结构在GaAs(001)表面形成机制的研究

改变生长工艺、控制并调整液滴中原子扩散机制是对复杂纳米结构制备的关键途径,并且对基于液滴外延方法研究半导体纳米结构十分重要.本文在不同衬底温度,不同As压下在GaAs(001)上沉积相同沉积量(5 monolayer)的In液滴并观察其表面形貌的变化.原子力显微镜图像显示,液滴晶化后所形成的扩散"盘"且呈现一定的对称性.随着衬底温度的增高,圆盘半径逐渐扩大,扩散圆盘中心出现了坑.而随着As压的增高,所形成的液滴密度增加,以液滴为中心所形成的扩散圆盘宽度逐渐减小.基于经典的成核扩散理论对实验数据拟合得到:GaAs(001))表面In原子在[110]和[110]晶向上的扩散激活能分别为(0.62 ± 0.01) eV和(1.37 ± 0.01) eV,且扩散系数D_0为1.2 × 10~(-2) cm~2/s.对比其他研究小组的结果证实了理论的正确性.实验中得到的In原子的扩散激活能以及In液滴在GaAs(001)上扩散机理,可以为InAs纳米结构特性的调制提供实验指导.     

GaSb/GaAs复合应力缓冲层上自组装InAs量子点的生长

研究了GaSb/GaAs复合应力缓冲层上自组装生长的InAs量子点.在2ML GaSb/1ML GaAs复合应力缓冲层上获得了高密度的、沿［100］方向择优分布量子点.随着复合应力缓冲层中GaAs层厚度的不同,量子点的密度可以在1.2×10¹⁰cm^-2和8×10¹⁰cm^-2进行调控.适当增加GaAs层的厚度至5ML,量子点的发光波长红移了约25nm,室温下PL光谱波长接近1300nm. 关键词： 自组装量子点 分子束外延 Ⅲ-Ⅴ族化合物半导体     

原子氢辅助分子束外延GaAs(331)A表面形貌演化

研究了GaAs高指数面(331)A在原子氢辅助下分子束外延形貌的演化.原子力显微镜测试表明：在常规分子束外延情况下,GaAs外延层台阶的厚度和台面的宽度随衬底温度的升高而增加,增加外延层厚度会导致台阶的密度和台面的宽度增加然后饱和.而在原子氢辅助分子束外延情况下,当GaAs淀积量相同时GaAs外延层台阶的密度增大宽度减小.认为这是由于原子氢的作用导致Ga原子迁移长度的减小.在GaAs(331)A台阶基底上生长出InAs自组织纳米线,用光荧光测试研究了其光学各项异性特征.     

Extremely Low Density InAs Quantum Dots with No Wetting Layer

Extremely low density InAs quantum dots （QDs） are grown by molecular beam droplet epitaxy. The gallium deposition amount is optimized to saturate exactly the excess arsenic atoms present on the GaAs substrate surface during growth, and low density InAs/GaAs QDs （4× 10^6 cm^-2） are formed by depositing 0.65 monolayers （MLs） of indium. This is much less than the critical deposition thickness （1.7 ML）, which is necessary to form InAs/GaAs QDs with the conventional Stranski-Krastanov growth mode. The narrow photoluminescence linewidth of about 24 meV is insensitive to cryostat temperatures from IO K to 250K. All measurements indicate that there is no wetting layer connecting the QDs.     

Electron and hole states in a quantum ring grown by droplet epitaxy: Influence of the layer inside the ring opening

The electronic structure of the conduction and valence bands of a quantum ring containing a layer inside the ring opening is modeled. This structure (nanocup) consists of a GaAs nanodisk (the cup’s bottom) and a GaAs nanoring (the cup’s rim) which encircles the disk. The whole system is embedded in an (Al,Ga)As matrix, and its shape resembles realistic ring structures grown by the droplet epitaxy technique. The conduction-band states in the structure are modeled by the single-band effective-mass theory, while the 4-band Luttinger–Kohn model is adopted to compute the valence-band states. We analyze how the electronic structure of the nanocup evolves from the one of a quantum ring when the size of either the nanodisk or the nanoring is changed. For that purpose, (1) the width of the ring, (2) the disk radius, and (3) the disk height are separately varied. For dimensions typical for experimentally realized structures, we find that the electron wavefunctions are mainly localized inside the ring, even when the thickness of the inner layer is 90% of the ring thickness. These calculations indicate that topological phenomena, like the excitonic Aharonov–Bohm effect, are negligibly affected by the presence of the layer inside the ring.     

溶液液滴蒸发变干的环状沉积

液体蒸发驱动的颗粒自组装现象在许多的工业技术中有重要应用. 本文利用显微镜观测含有颗粒物质的液滴变干后留在固体表面的颗粒形成的环状沉积图案. 采用微米粒径的SiO₂小球水溶液液滴蒸发变干模拟咖啡环的形成过程, 结果发现液滴蒸发过程中接触线的钉扎是环状沉积的必要条件. 在液滴蒸发过程中颗粒随着补偿流不断的向液滴边缘移动, 聚集在接触线处形成环. 液滴蒸发变干后残留在液滴内部的颗粒数随颗粒质量分数的增加而增加, 可以达到单层的颗粒排列. 而玻璃衬底上的颗粒环在颗粒质量分数很小时, 形成单层排列, 且一排一排地生长. 蒸发过程中颗粒环由于液滴边缘的尺寸限制向液滴中心缓慢移动. 这会导致液滴中不同大小颗粒的分离. 关键词： 液滴 接触线 蒸发 颗粒     

Influence of Ga coverage on the sizes of GaAs quantum dash pairs grown by high temperature droplet epitaxy

We investigate the formation of GaAs quantum dash pairs with different coverages by droplet epitaxy. The GaAs quantum dash pairs of various sizes are fabricated by high temperature droplet epitaxy. Dual‐sized quantum dash pairs are observed along $[01\bar 1]$ orientation. Depending on the Ga cov‐ erage, the width of the quantum dash pairs can be tuned from ～100 nm to ～300 nm while keeping the height in the range of 4 nm to 10 nm. The coverage dependence of quantum dash pairs is also confirmed with photoluminescence measurement. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Evolution of surface morphology and photoluminescence characteristics of 1.3-μm In0.5Gao.5As/GaAs quantum dots grown by molecular beam epitaxy

Evolution of surface morphology and optical characteristics of 1.3-μm In0.5Gao.5As/GaAs quantum dots (QDs) grown by molecular beam epitaxy (MBE) are investigated by atomic force microscopy (AFM) and photoluminescence (PL). After deposition of 16 monolayers (ML) of In0.5Ga0.5As, QDs are formed and elongated along the [110] direction when using sub-ML depositions, while large size InGaAs QDs with better uniformity are formed when using ML or super-ML depositions. It is also found that the larger size QDs show enhanced PL efficiency without optical nonlinearity, which is in contrast to the elongated QDs.     

Influence of Ga(Al)As substrates on surface morphology and critical thickness of InGaAs quantum dots

Influence of Ga(Al)As substrates on surface morphology of InGaAs quantum dots and critical thickness of In_0.5Ga_0.5As film grown by molecular beam epitaxy is investigated. The In_0.5Ga_0.5As quantum dots are grown on (001) surfaces of GaAs and Al_0.25Ga_0.75 A at 450 °C, scanning tunneling microscope images show that the size of quantum dots varied slightly for 10 ML of In_0.5Ga_0.5As grown on GaAs and Al_0.25Ga_0.75As surfaces. Reflection high energy electron diffraction (RHEED) is used to monitor the growth of 4 monolayers (ML) In_0.5Ga_0.5As on Al_0.25Ga_0.75As and GaAs surfaces during deposition. The critical thickness is theoretically calculated by adding energy caused by surface roughness and heat from substrate. The calculations show that the critical thickness of In_0.5Ga_0.5As grown on GaAs and Al_0.25Ga_0.75As are 3.2 ML and 3.8 ML, respectively. The theoretical calculation agrees with the experimental results.     

Magneto-photoluminescence study of InGaAs quantum dots fabricated by droplet epitaxy

We have successfully measured magneto-photoluminescence of InGaAs quantum dots (QDs) fabricated by droplet epitaxy with highly dense Ga droplets, termed separated-phase enhance epitaxy with droplets (SPEED). In the low magnetic field region, the PL peak energy shift increases linearly with square of the magnetic field. From the estimated Bohr radii for the QDs, it is found that the size of the QDs in the lateral direction is 2.7-times larger than that in the vertical direction. Moreover, using high magnetic region for estimating the detailed lateral size, the cyclotron radius around 25 T in the QDs becomes equal to the lateral size of the QDs. The cyclotron radius of 5.1 nm at 25 T suggests that the size of the InGaAs QDs in lateral direction might be as small as about 10 nm.