The effects of thermal annealing in self-assembled Ge/Si quantum dots

The effects of thermal annealing in Si base self-assembled Ge dots have been investigated by Raman spectra and PL spectra. An obvious Raman frequency shift under different annealing temperature can be observed. There are two main effects during the annealing procession: one is the inter-diffusion of the Si and Ge quantum dots; the other is the relaxation of the elastic strain. With the calculated results, PL blue shift can be related to strain relaxation effects, and/or a general decrease of Ge content due to the Ge-Si intermixing.     

Raman study of Si–Ge intermixing in Ge quantum rings and dots

The Ge/Si (1 0 0) nanostructures have been studied by atomic force microscopy (AFM) and Micro Raman optical spectroscopy. Two layers of Ge of total thickness 0.75 nm and Si cap with thickness 2.5 nm were deposited by the method of molecular beam epitaxy at the temperature range 640–700 °C. AFM shows both quantum dots and ring-shape Ge nanostructures. From the analysis of the intensity and energy shift of the Raman signal we have found that the average concentration of Ge decreases considerably from 44% to 27%, when the growth temperature increases, whereas the degree of strain relaxation remains roughly the same. This allows us to conclude that intermixing is a dominating mechanism for strain relaxation in processes of transformation of Ge quantum dots to quantum rings.     

Micro-Raman scattering by laser-modified structures with Ge/Si quantum dots

Structures with self-assembled Ge/Si quantum dots grown by molecular-beam epitaxy are exposed to pulsed radiation of a picosecond laser. Changes in the vibrational spectrum of nanostructures under an external action are studied by Raman spectroscopy. An analysis of the Raman spectra measured with a micron spatial resolution along the exposed region indicates a mixing of Ge and Si atoms and a change in the induced mechanical stresses in quantum dots.     

Phonons in Ge/Si superlattices with Ge quantum dots

Ge/Si superlattices containing Ge quantum dots were prepared by molecular beam epitaxy and studied by resonant Raman scattering. It is shown that these structures possess vibrational properties of both two-and zero-dimensional objects. The folded acoustic phonons observed in the low-frequency region of the spectrum (up to 15th order) are typical for planar superlattices. The acoustic phonon lines overlap with a broad emission continuum that is due to the violation of the wave-vector conservation law by the quantum dots. An analysis of the Ge and Ge-Si optical phonons indicates that the Ge quantum dots are pseudoamorphous and that mixing of the Ge and Si atoms is insignificant. The longitudinal optical phonons undergo a low-frequency shift upon increasing laser excitation energy (2.54–2.71 eV) because of the confinement effect in small-sized quantum dots, which dominate resonant Raman scattering.     

Optical phonons in Ge quantum dots obtained on Si(111)

The Raman light scattering from optical phonons of Ge quantum dots grown by molecular beam epitaxy on a Si(111) surface is studied. A series of Raman lines related to the quantization of phonon spectrum is observed. It is shown that phonon frequencies are adequately described in terms of the elastic properties and the dispersion of the optical phonons of bulk Ge. The strain experienced by the Ge quantum dots is estimated.     

Spectrum of electron-hole states of the Si/Ge structure with Ge quantum dots

The lateral photoconductivity spectra of Si/Ge multilayer structures with Ge quantum dots of various sizes are investigated. We observed optical transition lines between the hole levels of quantum dots and electronic states of Si. This enabled us to construct a detailed energy level diagram of the electron-hole spectrum of the Si/Ge structures. It is shown that the hole levels of Ge quantum dots are successfully described by the “quantum box” model using the actual sizes of Ge islands. It I found that the position of the longwavelength photosensitivity boundary of Si/Ge structures with Ge quantum dots can be controlled by changing the growth parameters.     

Vibrational density of states of hen egg white lysozyme

The resonant Raman scattering in GeSi/Si structures with GeSi quantum dots has been analyzed. These structures were formed at various temperatures in the process of molecular-beam epitaxy. It has been shown that Raman scattering spectra recorded near resonances with the E₀ and E₁ electronic transitions exhibit the lines of Ge optical phonons whose frequencies differ significantly from the corresponding values in bulk germanium. In the structures grown at low temperatures (300–400°C), the phonon frequency decreases with increasing excitation energy. This behavior is attributed to Raman scattering, which is sensitive to the size of quantum dots, and shows that quantum dots are inhomogeneous in size. In the structures grown at a higher temperature (500°C), the opposite dependence of the frequency of Ge phonons on excitation energy is observed. This behavior is attributed to the competitive effect of internal mechanical stresses in quantum dots, the localization of optical photons, and the mixing of Ge and Si atoms in structures with a bimodal size distribution of quantum dots.     

Infrared spectroscopy of intraband transitions in Ge/Si quantum dot superlattices

We report the study of infrared spectroscopy of intraband transitions in Ge/Si quantum dot superlattices. The superlattices, which were grown on (001) oriented Si substrates by a solid source molecular beam epitaxy system, are composed mainly of 20 or 30 periods of Ge dot layers and Si spacer films. The structural properties of them and of the uncapped Ge dots grown on the surfaces of some of them were tested by cross-sectional transmission electron and atomic force microscopes, respectively. It is found that the Ge quantum dots have flat lens-like shapes. Infrared absorption signals peaking in the mid-infrared range were observed using Fourier transform infrared and Raman scattering spectroscopy techniques. Experimental and theoretical analysis suggests that the mid-infrared response be attributed to intraband transitions within the valence band of the Ge quantum dots in the superlattices. The fact that the intraband absorption is strongly polarized along the growth axis of the superlattices signifies that the Ge quantum dots with flat lens-like shapes perform as Ge/Si-based quantum wells. This study demonstrates the application potential of these kinds of Ge/Si quantum dot superlattices for developing mid-infrared photodetectors.     

Resonant Raman scattering by strained and relaxed germanium quantum dots

This paper reports on the results of resonant Raman scattering investigations of the fundamental vibrations in Ge/Si structures with strained and relaxed germanium quantum dots. Self-assembled strained Ge/Si quantum dots are grown by molecular-beam epitaxy on Si(001) substrates. An ultrathin SiO₂ layer is grown prior to the deposition of a germanium layer with the aim of forming relaxed germanium quantum dots. The use of resonant Raman scattering (selective with respect to quantum dot size) made it possible to assign unambiguously the line observed in the vicinity of 300 cm^?1 to optical phonons confined in relaxed germanium quantum dots. The influence of confinement effects and mechanical stresses on the vibrational spectra of the structures with germanium quantum dots is analyzed.     

Energy Spectrum of Charge Carriers in Elastically Strained Assemblies of Ge/Si Quantum Dots

The results of studying the energy spectrum of electrons and holes localized in second-type Ge/Si heterostructures with Ge quantum dots are presented. In such structures, holes are localized at Ge quantum dots, and electrons, in three-dimensional quantum wells, which form in Si at the Ge—Si interface because of inhomogeneous deformations that appear as a result of the difference between the Ge and Si lattice constants. It is shown that changes in the deformations in the assembly of quantum dots as a result of a variation in their spatial arrangement significantly changes the binding energy of electrons, the position of their localization at quantum dots, the binding energy and wave-function symmetry of holes at double quantum dots (artificial molecules), and the exchange interaction of electrons and holes in the exciton composition. A practically important result of the presented data is the development of approaches to increase the luminescence quantum efficiency and the absorption coefficient in assemblies of quantum dots.     

Raman scattering of Ge dot superlattices

Self-organised Ge dot superlattices grown by molecular beam epitaxy of Ge and Si layers utilizing Stranski-Krastanov growth mode were investigated by Raman spectroscopy. An average size of Ge quantum dots was obtained from transmission electron microscopy measurements. The strain and interdiffusion of Ge and Si atoms in Ge quantum dots were estimated from the analysis of frequency positions of optical phonons observed in the Raman spectra. Raman scattering by folded longitudinal acoustic phonons in the Ge dot superlattices was observed and explained using of elastic continuum theory. Received 25 January 2000     

Resonant Raman scattering of discrete hole states in self-assembled Si/Ge quantum dots

We report the first resonant electronic Raman spectroscopy study of discrete electronic transitions within small p-doped self-assembled Si/Ge quantum dots (QDs). A heavy hole (hh) to light hole (lh) Raman transition with a dispersionless energy of 105 meV and a resonance energy of the hh states to virtually localised electrons at the direct band gap of 2.5 eV are observed. The hh–lh transition energy shifts to lower values with increasing annealing temperature due to significant intermixing of Si and Ge in the QDs. Structural parameters of the small Si/Ge dots have been determined and introduced into 6-band k·p valence band structure calculations. Both the value of the electronic Raman transition of localised holes as well as the resonance energy at the E₀ gap are in excellent agreement with the calculations.     

Hopping photoconduction and its long-time kinetics in a heterosystem with Ge quantum dots in Si

The effect of interband-transition-inducing illumination on the hole hopping conduction along a two-dimensional array of Ge quantum dots in Si was studied. It is found that the photoconductance has either positive or negative sign depending on the initial filling of quantum dots with holes. In the course of illumination and after switching off the light, long-time photoconduction kinetics was observed (10²?10⁴s at T=4.2 K). The results are discussed in terms of a model based on the spatial separation of nonequilibrium electrons and holes in a potential relief formed by positively charged dots. The effect of equalization of potential barrier heights as a result of photohole capture by the charged quantum dots during the process of illumination and relaxation is suggested as an additional factor for explaining the phenomenon of persistent conduction.     

离子束溅射自组装Ge/Si量子点生长的演变

采用离子束溅射技术,通过改变Ge的沉积量,在n型Si(100)衬底上自组装生长了一系列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,系统地研究了Ge量子点形貌、密度、尺寸大小以及Ge的结晶性和量子点中组分等随Ge沉积量的演变规律. 结果表明:Ge层从二维薄层向三维岛过渡过程中,没有观察到传统的由金字塔形向圆顶形量子点过渡,而是直接呈圆顶形生长;且随着Ge沉积量的增加,量子点密度先增大后减小,Ge的结晶性增强同时Ge/Si互混加剧,量子点中Si的组分增加. 关键词： 离子束溅射 量子点 表面形貌 Raman光谱     

Electron spins in an array of tunnel-coupled quantum dots

Mechanisms of electron spin relaxation in semiconductor arrays with tunnel-coupled quantum dots are reviewed. The contribution for anisotropic exchange interaction is shown for asymmetrical quantum dots having no inversion axis relative to their plane. The configuration of vertically coupled double Ge/Si quantum dots is found where anisotropic exchange coupling does not contribute to spin decoherence. It could be a basic configuration of spin-based quantum computation schemes.     

高速率沉积磁控溅射技术制备Ge点的退火生长研究

采用磁控溅射技术在Si衬底上以350?C沉积14 nm的非晶Ge薄膜,通过退火改变系统生长热能,实现了低维Ge/Si点的生长.利用原子力显微镜(AFM)和拉曼(Raman)光谱所获得的形貌和声子振动信息,对Ge点的形成机理和演变规律进行了研究.实验结果表明:在675?C退火30 min后,非晶Ge薄膜转变为密度高达8.5×109cm-2的Ge点.通过Ostwald熟化理论、表面扩散模型和对激活能的计算,很好地解释了退火过程中,Ge原子在Si表面迁移、最终形成纳米点的行为.研究结果表明用高速沉积磁控溅射配合热退火制备Ge/Si纳米点的方法,可为自组织量子点生长实验提供一定的理论支撑.     

小尺寸Si/Ge量子点内应变和组分的拉曼光谱表征

本文详细地研究了原始生长和退火处理后的Si/Ge量子点的拉曼光谱。我们观测到了Si/Ge量子点的一系列本征的拉曼振动模以及Ge-Ge模的LO和TO声子峰间4.2cm-1的频率劈裂。通过这些参数,我们自洽地确定了原始生长的平面直径为20nm和高为2nm的Si/Ge量子点内Ge的平均组分为80%,平均应变为-3.4%。分析清楚地表明了这种小尺寸的Si/Ge量子点内的应变仍遵从双轴应变,并且应变的释放主要由量子点和Si隔离层间Si-Ge原子互扩散决定。     

Ge dots on Si (1 1 1) and (1 0 0) surfaces with SiO2 coverage: Raman study

Germanium quantum dots formed on Si (1 1 1) and (1 0 0)-oriented surfaces coated with ultra-thin oxide layers are studied using Raman spectroscopy technique. Some structural properties (height, stoichiometry and mechanical stresses) of the dots were estimated from Raman data. For analysis of the experimental data, the Raman spectra of Ge nanoclusters containing some hundreds of Ge atoms were calculated numerically. The effects of the resonance enhancement of the intensity of Raman scattering in the Ge-nanoclusters–SiO₂–Si system were discussed. The influence of the lateral sizes of Ge nano-clusters on the frequencies of phonons localized in them was studied using numerical simulation. The influence of multi-layer growth on the structure of the Ge quantum dots was investigated.     

p型掺杂Si/Ge量子点的电子拉曼散射(英文)

在本文中我们首次报道了p型掺杂的自组织Si/Ge量子点中空穴能级子带间的电子拉曼散射,此电子跃迁的能量为105meV。Si/Ge量子点Ge Ge模的共振拉曼散射表明此空穴能级间的电子拉曼散射与Γ点附近的E0(≈2.52eV)发生了共振,而E1的能量小于2.3eV.变温实验和偏振实验进一步证实了我们的指认。所有观测的实验数据与6 bandk·p能带结构理论的计算结果吻合得很好。     

Excitons in Ge/Si double quantum dots

The spatial structure of excitons and the oscillator strength characterizing the intensity of interband optical transitions in vertically coupled Ge/Si quantum dots have been theoretically studied. It has been found that the probability of the exciton transition under certain conditions (the sizes of the quantum dots, the separation of the dots) can be much larger (up to a factor of 5) than the value for the case of single quantum dots. It is expected that the results will make it possible to approach the creation of efficient light-emitting and photoreceiving devices based on Si and Ge indirect-band semiconductors.