Ge/Si量子点生长的研究进展

回顾了近年来在Ge/Si量子点生长方面的研究进展.主要讨论了为了提高量子点空间分布有序性、增大量子点的密度、减小量子点的尺寸及改善其分布均匀性而采取的各种方法,如图形衬底辅助生长、表面原子掺杂及利用超薄SiO2层辅助生长等,以及Ge量子点的演变及组分变化.     

超高真空化学气相淀积自组织生长锗量子点

采用超高真空化学气相淀积系统制备了小尺寸、高密度、纵向自对准的Ge量子点．通过TEM和AFM对埋层和上层量子点的形貌和尺寸分布进行了研究，对生长的温度和时间进行了优化．采用硼预淀积的方法得到了尺寸分布小于3％的均匀的圆顶形Ge量子点．采用低温光荧光测量了多层量子点的光学特性．在10K的PL谱可以观察到明显的蓝移现象，表明量子点中较强的量子限制效应．量子点非声子峰的半高宽约为46meV，表明采用UHV／CVD工艺生长的多层量子点具有较窄的尺寸分布．     

斜切基片上溅射生长高密度小尺寸Ge纳米点的研究

采用离子束溅射技术,在斜切的单晶Si基底上生长了高密度的小尺寸Ge纳米点。系统研究了斜切基底上的表面台阶对Ge纳米点生长初期表面原子吸附行为的影响,以及斜切基片上Ge纳米点随原子沉积量的演变规律。实验结果表明,在斜切基片上原子级的表面台阶能有效地抑制吸附原子的表面扩散。因此,有利于Ge纳米点的形核,并抑制纳米点的过度长大,从而获得高密度、小尺寸的Ge纳米点。     

SiO2/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.     

SiO2/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.     

通过晶格失配调节有盖层张应变Ge量子点的光电特性

利用Ge与不同衬底形成的不同晶格失配度来调节有盖层的张应变Ge量子点的光电特性。通过有限元方法模拟并获得张应变Ge量子点内的应变分布,而后通过形变势理论和有效质量近似计算得到量子点的电子结构。与无盖层张应变Ge量子点相比,有盖层Ge量子点能保持更大的应变量。另外,随着量子点尺寸和晶格失配度的增大,导带Γ谷与导带L谷的能量差缩减,最终使Ge转变为直接带隙材料。直接带隙能量随着量子点尺寸的增大而减小。该研究结果表明张应变Ge量子点是制备包含激光器在内的Si基光源的理想材料,在未来光电子应用中有巨大潜力。     

温度对离子束溅射生长Ge/Si量子点的形貌影响

采用离子束溅射技术,在不同温度的Si(100)衬底上生长了一系列Ge量子点样品,利用AFM和Raman光谱对样品表面形貌和结构进行表征,结果表明,随着温度升高,量子点密度增大(750℃生长的量子点密度达到1.85×1010cm-2),均匀性变好及结晶性增强;但随生长温度升高,Si-Ge互混程度也同时加剧。     

一种新型Si电子束蒸发器的研制及其应用研究

我们成功地设计出一种新型的Si电子束蒸发器,并将它应用于Ge/Si(111)量子点的生长.由于采用悬臂式设计,它完全克服了高压短路的问题.电子束蒸发器的性能试验表明,稳定输出功率可以控制输出稳定的Si束流.应用这种电子束蒸发器可以在700 ℃,成功沉积出平整的单晶Si薄膜.进一步的试验表明,在这种缓冲层表面可以自组装生长出Ge量子点.     

埋层应变对溅射生长Ge量子点的影响

利用离子束溅射制备双层Ge/Si量子点,通过变化Si隔离层厚度和Ge沉积量研究了埋层应变场对量子点生长的影响。实验中观察到隔离层厚度较薄时,双层结构中第2层量子点形成的临界厚度减小,生长过程提前;此外,随着Ge沉积量的增加,第2层量子点密度维持在一定范围,分布被调制的同时岛均匀长大呈现单模分布。增大隔离层厚度,埋层岛的生长模式在第2层岛生长时得到复制。通过隔离层传递的不均匀应变场解释了量子点生长模式的变化。     

快速热退火对多层Ge量子点晶体质量的影响

使用超高真空化学气相淀积(UHV/CVD)设备在Si衬底上生长多层Ge量子点,用双晶X射线衍射(DCXRD)、拉曼光谱(Raman)等手段表征在不同条件下快速热退火的Ge量子点材料的组分、应力等特性,研究了快速热退火对多层Ge量子点晶体质量的影响.结果表明:随着退火温度的升高,量子点中Ge的组分下降,量子点应变的弛豫程...     

Influence of Si interdiffusion on carbon-induced growth of Ge quantum dots: a strategy for tuning island density

We have studied the epitaxial growth of self-assembled Ge quantum dots when a submonolayer of carbon is deposited on a Ge wetting layer (WL) prior to the growth of the dots. Using atomic-force microscopy combined with optical techniques like Raman and ellipsometry, we performed a systematic study of the role played by thermally activated Si interdiffusion on dot density, composition and morphology, by changing only the growth temperature T(WL) of the WL. Strikingly, we observe that higher dot densities and a narrower size distribution are achieved by increasing the deposition temperature T(WL), i.e.?by enhancing Si interdiffusion from the substrate. We suggest a two-stage growth procedure for fine tuning of dot topography (density, shape and size) useful for possible optoelectronic applications.     

Cooperative arrangement of self-assembled Ge dots on pre-grown Si mesas

In this work, we report on the study of one-dimensional (1-D) and two-dimensional (2-D) cooperative arrangements of self-assembled Ge dots grown on patterned Si (001) substrates. Selective epitaxial growth (SEG) of Si mesas was first performed to form Si mesas before Ge deposition. Self-assembled growths of Ge dots with variable Ge amount have been carried out. It is found that the cooperative arrangements of Ge dots are dependent on Ge amount deposited. The cooperative arrangements is attributed to the self-regulation of the dot size and position, which is driven by the minimization of the total free energy including strain energy, surface energy and deformation repulsive energy. This cooperative arrangement promoted by the energetically preferential nucleation on Si mesas allows us to control the placement of the self-assembled dots at the specific positions.     

生长温度对Si基Ge量子点VLP-CVD自组织生长的影响

对利用超低压化学气相淀积技术在Ｓｉ上自组织生长Ｇｅ量子点的特征进行了研究,发现生长温度对Ｇｅ量子点尺寸分布和密度的影响不同于分子束外延的结果,这种现象与ＶＬＰ－ＣＶＤ表面控制反应模式有关,实验表明,选择适当的生长温度可以在Ｓｉ上自组织生长具有窄尺寸分布和高密度和Ｇｅ量子点。     

Sb mediated formation of Ge/Si quantum dots: Growth and properties

The phenomenon of surfactant (Sb) mediated formation of Ge/Si(100) islands (quantum dots) by means of molecular beam epitaxy is discussed. The limited diffusivity of Si and Ge adatoms caused by the Sb layer leads to a reduction of the size of Ge islands, the increase in the island density, and the sharpening of the interfaces of Ge islands. Thereby, a thin Sb layer is considered to be a powerful tool that provides more freedom in designing Ge quantum dot features. Ge quantum dots, grown via a thin Sb layer and embedded coherently in a Si p-n junction, are revealed to be the origin of the intense photo- and electroluminescence in the spectral range of about 1.5 μm at room temperature.     

Ni induced lateral crystallization of high density Ge-dots/Si heterostructures

In this work, we propose a novel method for obtaining high-density Ge-dots/Si multilayered heterostructures. The high-density self-assembled Ge dots are firstly grown on a-Si layer using low-pressure chemical vapor deposition (LPCVD), and then low-temperature recrystallized by Ni based metal induced lateral crystallization (MILC). According to optical micrograph, microprobe Raman spectroscopy and transmission electron microscopy (TEM) observations, it has been found that the Ni induced lateral crystallized Si film has large leaf-like grains elongated along the MILC direction with (110) preference. The strain shift of Ge dots reveals the formation of high quality interface between the crystallized Si and Ge dot.     

A Ge/Si heterostructure nanowire-based double quantum dot with integrated charge sensor

One proposal for a solid-state-based quantum bit (qubit) is to control coupled electron spins on adjacent semiconductor quantum dots. Most experiments have focused on quantum dots made from III-V semiconductors; however, the coherence of electron spins in these materials is limited by hyperfine interactions with nuclear spins. Ge/Si core/shell nanowires seem ideally suited to overcome this limitation, because the most abundant nuclei in Ge and Si have spin zero and the nanowires can be chemically synthesized defect-free with tunable properties. Here, we present a double quantum dot based on Ge/Si nanowires in which we can completely control the coupling between the dots and to the leads. We also demonstrate that charge on the double dot can be detected by coupling it capacitively to an adjacent nanowire quantum dot. The double quantum dot and integrated charge sensor serve as an essential building block to form a solid-state qubit free of nuclear spin.     

Molecular ground hole state of vertically coupled GeSi/Si self-assembled quantum dots

We study the ground state of a hole confined in two vertically coupled GeSi/Si quantum dots as a function of the interdot distance and dot composition within the sp(3) tight-binding approach. Both quantum-mechanical tunneling and inhomogeneous strain distribution are included. For pure Ge dots, the strain is found to have two effects on the hole binding energy: (i)?reduction of the binding energy below the value of the single dot with increasing dot separation and (ii)?molecular bond breaking for intermediate interdot distances and posterior bond restoration at larger distance. Both effects are smeared upon Ge-Si intermixing.     

Influencing factors on the size uniformity of self-assembled SiGe quantum rings grown by molecular beam epitaxy

The size uniformity of self-assembled SiGe quantum rings, which are formed by capping SiGe quantum dots with a thin Si layer, is found to be greatly influenced by the growth temperature and the areal density of SiGe quantum dots. Higher growth temperature benefits the size uniformity of quantum dots, but results in low Ge concentration as well as asymmetric Ge distribution in the dots, which induces the subsequently formed quantum rings to be asymmetric in shape or even broken somewhere in the ridge of rings. Low growth temperature degrades the size uniformity of quantum dots, and thus that of quantum rings. A high areal density results in the expansion and coalescence of neighboring quantum dots to form a chain, rather than quantum rings. Uniform quantum rings with a size dispersion of 4.6% and an areal density of 7.8×10(8) cm(-2) are obtained at the optimized growth temperature of 640°C.     

Enhanced 400-600 nm photoresponsivity of metal-oxide-semiconductor diodes with multi-stack germanium quantum dots

Metal-oxide-semiconductor (MOS) diodes with zero-, one-?or three-layer Ge quantum dots (QDs) embedded in the gate oxide are fabricated for visible to near-ultraviolet photodetection. Ge dots are formed by thermally oxidizing one or three stacks of amorphous Si (a-Si)/polycrystalline-Si(0.87)Ge(0.13)/a-Si multi-layers that are sandwiched by SiO(2) barriers. The current-voltage characteristics of Ge QD MOS diodes exhibit strong rectification in darkness and feature significant current enhancement in the inversion mode when illuminated. Increasing the number of Ge QD layers from zero through one to three in the gate oxide improves the responsivity from 4.64 through 482 to 812?mA?W(-1) and enhances the corresponding quantum efficiency from 1.42 through 148 to 245%, respectively. The spectral response reveals a considerable blueshift in peak energies as the Ge dot size decreases from 9.1 to 5.1?nm, suggesting that the light absorption originates from the quantum confinement effect of Ge QDs. The temperature and bias dependences of the dark current indicate that the carrier transport mechanism involves percolation hopping.