Distribution of Ge Self-Assembled Quantum Dots on Six Ge1 - x Buffer Layers

The effect of buried misfit dislocation on the distribution of Ge self-assembled quantum dots (SAQDs) grown on a relaxed SiGe buffer layer was investigated. The strain field of arrays of buried dislocations in a relaxed SiGe buffer layer provided preferential nucleation sites for quantum dots. Burgers vector analysis using plan-view transmission electron microscopy (TEM) verified that the preferential nucleation sites of Ge SAQDs depended on the Burgers vector direction of corresponding dislocations. The measurement of the lateral distance between SAQDs and dislocations together with crosssection TEM observation clarified that the location of SAQDs was at the intersection of the dislocation slip plane and the top surface. The misfit strain should be an additional factor governing the uniformity in size, shape and distribution of Ge SAQDs.

Local Structure of Ge/Si(100) Self-Assembled Quantum Dots

Local structure of uncapped and Si capped Ge quantum dots grown on Si(100) has been probed by X ray absorption fine structure spectroscopy. It is found that the uncapped Ge dots are partially oxidized and partially alloyed with Si. The amount of Ge present in the Ge phase is found to be about 20 30%. In the Si capped sample, Ge is found to be dissolved in silicon, the fraction of Ge atoms existing as pure Ge phase being not more than 10%.

Electric properties of Ge quantum dot embedded in Si matrix

The electric characteristics of Ge quantum dot grown by molecular beam epitaxy in Si matrix were investigated by admittance spectroscopy and deep level transient spectroscopy. The admittance spectroscopy measurements show that the activation energy of 0.341eV can be considered as the emitting energy of hole from the ground state of the quantum dot. And the capacitance variation with temperature of the sample shows a platform at various frequencies with reverse bias (0.5 V,) which indicates that the boundary of space charge region is located at the quantum dot layer where the large confined hole concentration blocks the further extension of space charge region. When the temperature increases from 120K to 200K, the holes in the dot emit out completely. The position of the platform shifting with the increase of the applied frequency shows the frequency effects of the charges in the quantum dot. The deep level transient spectroscopy results show that the charge concentration in the Ge quantum dot is a function of the pulse duration and the reverse bias voltage, the activation energy and capture cross-section of hole decrease with the increase of pulse duration due to the Coulomb charging effect. The valence-band offsets of hole in Ge dot obtained by admittance spectroscopy and deep level transient spectroscopy are 0.341 and 0.338eV, respectively.

Evolution of Self-Organized Ge Quantum Dots During Ultra High Vacuum Annealing

The evolution of self organized Ge quantum dots structure is investigated by scanning tunneling microscopy and atomic force microscopy during annealing treatment up to 700℃ in an ultra high vacuum(UHV) system.When the sample temperature rises to 630℃,a great amount of new dots emerge on the wetting layer,which are believed to be incoherent islands compared with the dislocation free coherent islands formed during molecular beam epitaxy growth.

Smaller Ge Quantum Dots Obtained by ArF Excimer Laser Annealing

Ge self-assembled quantum dots （SAQDs） are grown with a self-assembled UHV/CVD epitaxy system. Then, the as-grown Ge quantum dots are annealed by ArF excimer laser. In the ultra-shot laser pulse duration, -20ns, bulk diffusion is forbidden,and only surface diffusion occurs, resulting in a laser induced quantum dot （LIQD）. The diameter of the LIQD is 20-25nm which is much smaller than the as-grown dot and the LIQD has a higher density of about 6 ×10^10cm^-2. The surface morphology evolution is investigated by AFM.

Activation of silicon quantum dots and coupling between the active centre and the defect state of the photonic crystal in a nanolaser

A new nanolaser concept using silicon quantum dots （QDs） is proposed. The conduction band opened by the quantum confinement effect gives the pumping levels. Localized states in the gap due to some surface bonds on Si QDs can be formed for the activation of emission. An inversion of population can be generated between the localized states and the valence band in a QD fabricated by using a nanosecond pulse laser. Coupling between the active centres formed by localized states and the defect states of the two-dimensional （2D） photonic crystal can be used to select the model in the nanolaser.

Defect reduction in GaAs/Si film with InAs quantum-dot dislocation filter grown by metalorganic chemical vapor deposition

The growth of GaAs epilayers on silicon substrates with multiple layers of InAs quantum dots（QDs） as dislocation filters by metalorganic chemical vapor deposition（MOCVD） is investigated in detail.The growth conditions of single and multiple layers of QDs used as dislocation filters in GaAs/Si epilayers are optimized.It is found that the insertion of a five-layer InAs QDs into the GaAs buffer layer effectively reduces the dislocation density of GaAs/Si film.Compared with the dislocation density of 5×10^7 cm^-2 in the GaAs/Si sample without QDs,a density of 2×10^6 cm^-2 is achieved in the sample with QD dislocation filters.

Electronic states and shapes of silicon quantum dots

A curviform surface breaks the symmetrical shape of silicon quantum dots on which some bonds can produce localized electronic states in the bandgap. The calculation results show that the bonding energy and electronic states of silicon quantum dots are different on various curved surfaces, for example, a Si-O-Si bridge bond on curved surface provides localized levels in bandgap and its bonding energy is shallower than that on the facet. The red-shifting ofthe photoluminescence spectrum on smaller silicon quantum dots can be explained by the curved surface effect. Experiments demonstrate that silicon quantum dots are activated for emission due to the localized levels provided by the curved surface effect.

Temperature-Dependent Photoluminescence Characteristics of InAs/GaAs Quantum Dots Directly Grown on Si Substrates

The first operation of an electrically pumped 1.3μm InAs/GaAs quantum-dot laser was previously reported epitaxially grown on Si （100） substrate. Here the direct epitaxial growth condition of 1.3μm InAs/OaAs quantum on a Si substrate is further investigated using atomic force microscopy, etch pit density and temperature-dependent photoluminescence （PL） measurements. The PL for Si-based InAs/GaAs quantum dots appears to be very sensitive to the initial OaAs nucleation temperature and thickness with strongest room-temperature emission at 40000 （17Onto nucleation layer thickness）, due to the lower density of defects generated under this growth condition, and stronger carrier confinement within the quantum dots.

Investigation into the InAs/GaAs quantum dot material epitaxially grown on silicon for O band lasers

InAs/GaAs quantum dot(QD)lasers were grown on silicon substrates using a thin Ge buffer and three-step growth method in the molecular beam epitaxy(MBE)system.In addition,strained superlattices were used to prevent threading disloca-tions from propagating to the active region of the laser.The as-grown material quality was characterized by the transmission electron microscope,scanning electron microscope,X-ray diffraction,atomic force microscope,and photoluminescence spectro-scopy.The results show that a high-quality GaAs buffer with few dislocations was obtained by the growth scheme we de-veloped.A broad-area edge-emitting laser was also fabricated.The O-band laser exhibited a threshold current density of 540 A/cm^(2) at room temperature under continuous wave conditions.This work demonstrates the potential of large-scale and low-cost manufacturing of the O-band InAs/GaAs quantum dot lasers on silicon substrates.

Anti-Reflection Characteristics of Si Nanowires for Enhanced Photoluminescence from CdTe/CdS Quantum Dots

CdTe/CdS quantum dots（QDs） are fabricated on Si nanowires（NWs） substrates with and without Au nanoparticles（NPs）. The formation of Au NPs on Si NWs can be certified as shown in scanning electron microscopy images. The optical properties of samples are also investigated. It is interesting to find that the photoluminescence（PL） intensity of Cd Te/Cd S QD films on Si nanowire substrates with Au NPs is significantly increased,which can reach 8-fold higher than that of samples on planar Si without Au NPs. The results of finite-difference time-domain simulation indicate that Au NPs induce stronger localization of electric field and then boost the PL intensity of QDs nearby. Furthermore, the time-resolved luminescence decay curve shows the PL lifetime, which is about 5.5 ns at the emission peaks of QD films on planar, increasing from 1.8 ns of QD films on Si NWs to4.7 ns after introducing Au NPs into Si NWs.

Nonlinear Doping, Chemical Passivation and Photoluminescence Mechanism in Water-Soluble Silicon Quantum Dots by Mechanochemical Synthesis

A series of boron- and phosphorus-doped silicon wafers are used to prepare a series of doped silicon nanocrystals （nc-Si） by high-energy ball milling with carboxylic acid-terminated surface. The sizes of the nc-Si samples are demonstrated to be 〈 S nm. The doping levels of the nc-Si are found to be nonlinearly dependent on the original doping level of the wafers by x-ray photoelectron spectroscopy measurement. It is found that the nonlinear doping process will lead to the nonlinear chemical passivation and photoluminescence （I3L） intensity evolution. The doping, chemical passivation and PL mechanisms of the doped nc-Si samples prepared by mechanochemical synthesis are analyzed in detail.

PL Emission and Shape of Silicon Quantum Dots

The calculation results show that the bonding energy and electronic states of silicon quantum dots are different on various curved surfaces, for example, a Si-O-Si bridge bond on curved surface provides the localized levels in band gap and its bonding energy is shallower than that on facet. The red-shifting of PL spectra on smaller silicon quantum dots can be explained by curved surface effect. Experiments demonstrate that silicon quantum dots are activated for emission due to the localized levels provided in curved surface effect.

Si基Ge异质结构发光器件的研究进展

近年来,与Si的CMOS工艺相兼容的Ge/Si异质结构发光器件取得很多重要的进展。本文概述了Si基Ge异质结构发光器件的最新成果,如Ge/Si量子点发光二极管、Si衬底上的Ge发光二极管及激光器和Ge/SiGe多量子阱发光二极管,分别描述了这些器件的特点和增强其发光特性的途径。最后展望了Si基Ge异质结构发光器件的发展趋势,指出尽管Si基Ge异质结构发光器件获得了很大的发展,但是器件的发光效率仍然很低,离实用还有一定距离,还需要在材料和器件的结构方面有更多的创新。

SiO_2/Si(111)表面Ge量子点的生长研究

Si衬底用化学方法清洗后,表面大约残余1.0 nm厚SiO2薄膜。利用原子力显微镜(AFM)和反射高能电子衍射(RHEED)来研究温度和Ge蒸发厚度对在SiO2薄膜表面生长的Ge量子点的影响。实验结果表明,当衬底温度超过500℃时,SiO2开始与Ge原子发生化学反应,并形成与Si(111)表面直接外延的Ge量子点。在650℃时,只有Ge的厚度达到0.5nm时,Ge量子点才开始形成。

硼原子对Si(100)衬底上Ge量子点生长的影响

研究了硼原子对 Si( 1 0 0 )衬底上 Ge量子点自组织生长的影响 .硼原子的数量由 0单原子层变到 0 .3单原子层 .原子力显微镜的观察表明 ,硼原子不仅对量子点的大小 ,而且对其尺寸均匀性及密度都有很大影响 .当硼原子的数量为 0 .2单原子层时 ,获得了底部直径为 60± 5nm,面密度为 6× 1 0 9cm- 2 ,且均匀性很好的 Ge量子点 .另外 ,还简单讨论了硼原子对 Ge量子点自组织生长影响的机制 .

束流密度对Ge/Si量子点溅射生长的影响

采用离子束溅射技术在Si基底上自组织生长了一系列Ge量子点样品,研究了束流密度对Ge/Si量子点的尺寸分布和形貌演变的影响。原子力显微镜测试结果表明,随着束流密度的增加,量子点的面密度持续增大,其尺寸不断减小,量子点的形貌由圆顶形转变为过渡圆顶形。计算直径标准偏差的结果表明,当束流密度为0.86mA/cm2时,量子点的尺寸均匀性最佳。束流密度与沉积速率成正比,影响着表面吸附原子与其它原子相遇而形成晶核的能力。

光子晶体对nc-Ge/Si岛发光增强的模拟

在Si基集成光电子学的发展中，高效的Si基光源是人们不懈追求的目标。但是Si材料的间接带隙特性导致其发光效率低，更谈不上受激发射。于是人们探索了多种Si基材料体系来提高Si材料的发光效率，并在不同程度上取得了重要的进展。在众多的Si基发光材料体系中，Ge／Si量子点材料，不仅生长工艺与标准的CMOS工艺有很好的兼容性，而且发光波长能够覆盖重要的光通信波段即1．3—1．55μm，因此成为实现Si基发光器件的重要途径之一。但是目前这种材料的发光效率仍很低，所以提高其发光效率自然成为人们关注的焦点。如果将光子晶体引入到nc—Ge／Si材料中，它不仅可以改变材料本身的自发发射特性，而且可以改变发射的光子的提取效率，从而使材料的发光效率得到增强。提出了在Ge／Si量子点材料中引入光子晶体结构来提高其发光效率，包括光子晶体点缺陷腔结构和带边模式工作的完整光子晶体结构，并从理论上分析了发光效率提高的原理。针对发光波长在1．5μm附近的材料结构，模拟出了相应的光子晶体的结构参数。从模拟结果可以看出，对于缺陷腔的光子晶体结构，采用单点缺陷微腔很好地实现了单模运作，但是微腔内有源材料的体积很小，因此得到的发光效率很低。而采用耦合缺陷腔的结构和H2腔都增加了腔内有源区的体积。但是耦合腔与H2腔相比，谐振腔模减少，主谐振模式的峰值强度增加，更容易实现单模发光。因而更适用于提高nc—Ge/Si的发光效率。而带边模式工作的光子晶体结构，尺寸较大，不需引入缺陷，工艺上更容易实现。

Si(001)斜切衬底上Ge量子点的固相外延生长

研究了 Si(0 0 1)面偏 [110 ]方向 6°斜切衬底上 Ge量子点的固相外延生长 .实验结果表明 ,在 Si(0 0 1) 6°斜切衬底上固相外延生长 Ge量子点的最佳退火温度为 6 4 0℃ ,在斜切衬底上成岛生长的临界厚度低于在 Si(0 0 1)衬底成岛生长的临界厚度 ,6°斜切衬底上淀积 0 .7nm Ge即可成岛 ,少于 Si(0 0 1)衬底片上 Ge成岛所需的淀积量 .从 Ge量子点的密度随固相外延温度的变化曲线 ,得到 Ge量子点的激活能为 1.9e V,远高于 Si(111)面上固相外延 Ge量子点的激活能 0 .3e V.实验亦发现 ,在 Si(0 0 1)斜切衬底上固相外延生长的 Ge量子点较 Si(0 0 1)

Ge／Si(100)量子点生长与形态分布的研究

利用超高真空化学气相淀积系统，通过控制淀积量、温度、流量等生长参数，在n型Si（100）衬底上自组装生长了一系列Ge量子点样品，用原子力显微镜进行了表征与分析。系统地研究了生长参数对Ge岛形态分布的影响并分析了有序、高密度Ge岛的生长机理。结果表明，从二维向三维岛跃迁后，最初形成的高宽比（高度与底宽的比值）在0．04～0.06之间的小岛是一种在低温下可以与圆顶岛共存的稳定岛，两种岛的分布随淀积参数的变化而变化。在高温下小岛几乎消失，流量的变化对小岛的密度影响较小。实验中获得小岛的密度最高为2．6×10^10cm^-2，圆顶岛的密度最高为4．2×10^9cm^-2。