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.     

Study of Semiconductor Super Thin Heterostructures with Synchrotron Radiation X-Ray Standing Wave Technique

The X ray standing wave experiment method is established with the double-crystal monochromator and precision 2-circle goniometer at Beijing Synchrotron Radiation Facility. It is used combined with the X-ray diffraction, to investigate the heterostructure of super thin Ge atomic layer within Si crystals. The results show that the Ge _x Si_1－x alloy layer with average x=0.13 was formed in the Si crystal sample due to the segregation of Ge atoms during the preparation. Due to the diffusion of Ge atoms to the crystal surface, the Ge_x Si_1－x alloy layer was disappeared and nearly pure Ge layer was formed on the Si crystal surface after annealing at 650℃.     

On the use of the thermal lens effect as a thermo-optical spectroscopy of solids

The annealing behaviour of C-, N-, Mg-, Al- and P-implanted Si and Ge has been investigated by the (p,γ) resonance broadening method. In the cases of C- and Mg-implanted Ge, and N- and Al-implanted Si, fast out-diffusion and blocking of the implanted atoms at the sample surface was observed. Partial out-diffusion and blocking of nitrogen implanted in Ge took place. In the cases of Mg-implanted silicon and P-implanted germanium no blocking occurred and a decrease in the number of implanted atoms took place. In C-implanted Si retarded surface enrichment was observed, whereas in P-implanted Si clear diffusion into the bulk took place and no out-diffusion was observed.     

Surface mobility difference between Si and Ge and its effect on growth of SiGe alloy films and islands

Based on first-principles calculations of surface diffusion barriers, we show that on a compressive Ge(001) surface the diffusivity of Ge is 10(2)-10(3) times higher than that of Si in the temperature range of 300 to 900 K, while on a tensile surface, the two diffusivities are comparable. Consequently, the growth of a compressive SiGe film is rather different from that of a tensile film. The diffusion disparity between Si and Ge is also greatly enhanced on the strained Ge islands compared to that on the Ge wetting layer on Si(001), explaining the experimental observation of Si enrichment in the wetting layer relative to that in the islands.     

硅(001)图形衬底上锗硅纳米线的定位生长

纳米线的定位生长是实现纳米线量子器件寻址和集成的前提.结合自上而下的纳米加工和自下而上的自组装技术,通过分子束外延生长方法,在具有周期性凹槽结构的硅(001)图形衬底上首先低温生长硅锗薄膜然后升温退火,实现了有序锗硅纳米线在凹槽中的定位生长,锗硅纳米线的表面晶面为(105)晶面.详细研究了退火温度、硅锗的比例及图形周期对纳米线形成与否,以及纳米线尺寸的影响.     

Ge quantum dots growth on nanopatterned Si(0 0 1) surface: Morphology and stress relaxation study

The self-organized growth of germanium quantum dots on square nanopatterned Si(0 0 1) substrates is investigated by scanning tunnelling microscopy (STM) and grazing incidence X-ray diffraction (GIXRD) techniques. A regular surface patterning in the 10-100 nm period range is obtained by etching an interface dislocation network obtained by the controlled molecular bonding of Si substrates. The depth of the silicon surface profile is increased by a double etching process. Growth experiments are performed by solid source molecular beam epitaxy (MBE), and for deep trenches, germanium growth conditions are optimized to obtain one Ge dot per Si mesa. It is shown that the trench depth and the mesa profile strongly affect the dot size and its coincidence with the initial regular surface network. Anomalous GIXRD measurements are performed to highlight the Ge elastic relaxation and intermixing during heteroepitaxial growth. We report a significant modification in the stress state of Ge dots as a function of thermal annealing after growth.     

Room temperature oxidation of Cu(3)Ge films grown on Si and Si(1-x)Ge(x) substrates

Room temperature oxidation of Cu3Ge films grown on Si, Si(0.85)Ge(0.15) and Si(0.52)Ge(0.48) substrates, respectively, at a temperature of 200-300 degrees C was studied using transmission electron microscopy (TEM) in conjunction with energy dispersive spectrometry (EDS) and scanning electron microscopy (SEM). For Cu(3)Ge films grown at 200 degrees C and subsequently exposed in air for 1 week oxide protrusions and oxide networks appeared in the film surface and grain boundaries of Cu(3)Ge, respectively. At room temperature O from air and Si from the substrate, diffused along the grain boundaries of Cu(3)Ge to react with Cu(3)Ge grains, initiating the Cu(3)Si-catalyzed oxidation. Cu(3)Ge films are superior to Cu(3)(Si(1-x)Gex) films in retarding Cu(3)Si-catalyzed oxidation. Annealing at 300 degrees C allowed Si diffusion from the substrate into the Cu(3)Ge overlayer to form Cu(3)(Si(1-x)Gex), enhancing the Cu(3)Si-catalyzed oxidation rate. In the present study, Cu(3)Ge films grown on Si(0.52)Ge(0.48) at 200 degrees C show the best resistance to room temperature oxidation because higher Ge concentration in the substrate and lower temperature annealing can more effectively retard Si diffusion from the substrate into the Cu(3)Ge overlayer, and hence reduce the Cu(3)Si-catalyzed oxidation rate.     

Ge/Pb/Si(111)生长中Ge原子沿团簇边缘扩散的三维Monte-Carlo模拟

用动态Monte-Carlo方法对Ge在单层表面活性剂Pb覆盖的Si(111)表面上沿团簇边缘扩散进行三维模拟.重点讨论Ge原子是否沿团簇边缘扩散,沿边缘扩散时的最大扩散步数及最近邻原子数对三维生长的影响,并计算薄膜表面粗糙度研究三维生长模式.模拟表明Ge沿团簇边缘扩散的行为对薄膜生长模式的影响很大,同时讨论了ES势对三维生长模式的影响.     

Growth dynamics studied by RHEED during Si/Ge epitaxy from gaseous hydrides

Si and Ge epitaxial growth from disilane and germane in a gas-source molecular beam epitaxy (GSMBE) system is followed in situ by reflection high-energy electron diffraction (RHEED) intensity oscillations. During Ge homoepitaxy, the growth rate on a Ge(100) substrate is found to be limited by surface hydrogen desorption below 350°C and by hydride adsorption above this temperature. Ge heteroepitaxy on Si results in incomplete layer growth leaving exposed Si at the surface during the initial stages of growth. Therefore, a gradual change in the observed Si surface concentration is seen as growth proceeds. Si heteroepitaxy on Ge follows the Volmer-Weber growth mode and proceeds via island formation. This, combined with Ge surface segregation, results in a slow decrease of the Ge surface population at the growth front. During heteroepitaxial growth, hydride reaction rates differ on Si and Ge surfaces, and therefore a changing concentration of the surface species is manifest as a gradual change in the observed oscillation frequency. This effect, observed during the early stages of growth, shows strong temperature dependence, consistent with previous observations on SiGe alloys. Following several layers of growth, however, the surfaces become rough. The influence of this roughness on the oscillation frequency is also discussed.     

Features of atomic processes at the formation of a wetting layer and nucleation of three-dimensional Ge islands on Si(111) and Si(100) surfaces

The intermediate stages of the formation of a Ge wetting layer on Si(111) and Si(100) surfaces under quasiequilibrium grow conditions have been studied by means of scanning tunneling microscopy. The redistribution of Ge atoms and relaxation of mismatch stresses through the formation of surface structures of decreased density and faces different from the substrate orientation have been revealed. The sites of the nucleation of new three-dimensional Ge islands after the formation of the wetting layer have been analyzed. Both fundamental differences and common tendencies of atomic processes at the formation of wetting layers on Si(111) and Si(100) surfaces have been demonstrated. The density of three-dimensional nuclei on the Si(111) surface is determined by changed conditions for the surface diffusion of Ge adatoms after change in the surface structure. Transition to three-dimensional growth on the Si(100) surface is determined by the nucleation of single {105} faces on the rough Ge(100) surface.     

Growth mechanism and morphology of Ge on Pb covered Si(111) surfaces

We study the mechanism and surface morphology in epitaxial growth of Ge on Pb covered Si(111) using a scanning tunneling microscope (STM). We find that Ge adatoms have a very large diffusion length at room temperature. The growth is close to perfect layer-by-layer for the first two bilayers. Surface roughness increases gradually with the film thickness, but no 3D islands are found at room temperature. For growth at 200°C, 3D Ge islands appear after completion of the second bilayer. At room temperature, we believe, the Pb layer enhances surface diffusion and the descending-step motion of Ge adatoms, but the ascending-step motion is hindered and thus 3D island growth is suppressed.     

The microstructure of vertically coupled quantum dots ensembles by EXAFS spectroscopy

The microstructure of samples containing Ge/Si, GaN/AlN, and InAs/AlAs (Ge, GaN, and InAs quantum dots (QDs) in the Si, AlN, and AlAs matrices, respectively) multilayer heterostructures has been investigated by EXAFS spectroscopy. The effect of the effective thicknesses of Si (or AlN) barrier layers, number of Ge (or GaN) layers in a sandwich, and annealing temperature on the size and shape of Ge (or GaN) clusters, interfacial diffusion in these systems, and formation of three-dimensional ordered ensembles of QDs has been established.     

Efficient Visible Photoluminescence from Self-Assembled Ge QDs Embedded in Silica Matrix

Measuring the growth parameters of Ge quantum dots(QDs) embedded in SiO_2/Si hetero-structure is prerequisite for developing the optoelectronic devices such as photovoltaics and sensors. Their optical properties can be tuned by tailoring the growth morphology and structures, where the growth parameters' optimizations still need to be explored. We determine the effect of annealing temperature on surface morphology, structures and optical properties of Ge/SiO_2/Si hetero-structure. Samples are grown via rf magnetron sputtering and subsequent characterizations are made using imaging and spectroscopic techniques.     

Identification of Ge/Si intermixing processes at the Bi/Ge/Si(111) surface

The chemical contrast between Si and Ge obtained by scanning tunneling microscopy on Bi-covered Si(111) surfaces is used as a tool to identify two vertical Ge/Si intermixing processes. During annealing of an initially pure Ge monolayer on Si, the intermixing is confined to the first two layers approaching a 50% Ge concentration in each layer. During epitaxial growth, a growth front induced intermixing process acting at step edges is observed. Because of the open atomic structure at the step edges, relative to the terraces, a lower activation barrier for intermixing at the step edge, compared to the terrace, is observed.     

Surfactants: Perfect heteroepitaxy of Ge on Si(111)

The hetero growth of Ge on Si results in formation of 3D clusters with an uncontrolled defect structure. Introduction of a monolayer of a surfactant completely changes the growth mode to a 2D-layer growth (Frankvan der Merwe) with a continuous and smooth Ge film on Si(111). The surfactant is not incorporated but segregates and floates on the growing Ge film. The saturation of the dangling bonds of the semiconductor reduces the surface free energy and drives the strong segregation. The effect on the growth process is the selective change of activation energies which are important for diffusion and the mobility of the Ge. Up to a thickness of 8 MLs (MonoLayers) the misfit-related strain of the pseudomorphic Ge film is relaxed by formation of a micro rough surface. This allows a partial relaxation of the Ge towards its bulk lattice constant which would not be possible for a flat and continuous film. For thicker Ge films the misfit of 4.2% is relieved by a periodic dislocation network, which is confined to the Si-Ge interface. Ge-films thicker than 20 MLs are free of defects and completely relaxed to the Ge bulk lattice constant: a model system for perfect heteroepitaxial growth.     

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.     

Kinetic step bunching instability during surface growth

We study the step bunching kinetic instability in a growing crystal surface characterized by anisotropic diffusion. The instability is due to the interplay between the elastic interactions and the alternation of step parameters. This instability is predicted to occur on a vicinal semiconductor surface Si(001) or Ge(001) during epitaxial growth. The maximal growth rate of the step bunching increases like F4, where F is the deposition flux. Our results are complemented with numerical simulations which reveal a coarsening behavior in the long time evolution for the nonlinear step dynamics.     

Surface structure of GaAs(211)

The atomic structures of the two inequivalent (211) surfaces of GaAs have been investigated by LEED. Both surfaces, prepared by etching and heat-cleaning or ion-sputtering and annealing, are unstable and develop large (110) facets which exhibit the atomic geometry of the (110) GaAs surface. These facets entirely cover the surface. Three sets of facets, making 30°, 30° and 54° angles with the (211) plane, are detected on one surface. Only two sets, making 30° angles with the (211) plane, are detected on the other. The reasons for this difference are not understood at this time. The LEED study of Si(211) and Ge(211) shows that the Si surface is flat whereas the Ge surface exhibits reconstructed (311) facets. The structural difference between the (211) surfaces of GaAs and Ge and the facetting of the compound are invoked to explain the problems encountered in the MBE growth of GaAs on Ge(211).     

Effect of adsorbed Sn on Ge diffusivity on Si(111) surface

The effect of adsorbed Sn as a surfactant on Ge diffusion on a Si(111) surface has been studied by Low Energy Electron Diffraction and Auger Electron Spectroscopy. The experimental dependence of Ge diffusion coefficients on the Si(111) surface versus temperature in the presence of adsorbed Sn atoms has been measured in the range from 300 to 650°C. It has been shown that at a Sn coverage of about 1 monolayer the mobility of Ge atoms increases by several orders of magnitude.      

Germanium growth on Br-terminated Si(1 0 0)

The consequences of Ge deposition on Br-terminated Si(1 0 0) were studied with scanning tunneling microscopy at ambient temperature after annealing at 650 K. One monolayer of Br was sufficient to prevent the formation of Ge huts beyond the critical thickness of 3 ML. This is possible because Br acts as a surfactant whose presence lowered the diffusivity of Ge adatoms. Hindered mobility was manifest at low coverage through the formation of short Ge chains. Further deposition resulted in the extension and connection of the Ge chains and gave rise to the buildup of incomplete layers. The deposition of 7 ML of Ge resulted in a rough surface characterized by irregularly shaped clusters. A short 800 K anneal desorbed the Br and allowed Ge atoms to reorganize into the more energetically favorable “hut” structures produced by conventional Ge overlayer growth on Si(1 0 0).