Ordering of Ge islands on Si(001) substrates patterned by nanoindentation

Spatial organization of Ge islands, grown by physical vapor deposition, on prepatterned Si(001) substrates has been investigated. The substrates were patterned prior to Ge deposition by nanoindentation. Characterization of Ge dots is performed by atomic force microscopy and scanning electron microscopy. The nanoindents act as trapping sites, allowing ripening of Ge islands at those locations during subsequent deposition and diffusion of Ge on the surface. The results show that island ordering is intrinsically linked to the nucleation and growth at indented sites and it strongly depends on pattern parameters.     

Ge-rich islands grown on patterned Si substrates by low-energy plasma-enhanced chemical vapour deposition

Si(1-x)Ge(x) islands grown on Si patterned substrates have received considerable attention during the last decade for potential applications in microelectronics and optoelectronics. In this work we propose a new methodology to grow Ge-rich islands using a chemical vapour deposition technique. Electron-beam lithography is used to pre-pattern Si substrates, creating material traps. Epitaxial deposition of thin Ge films by low-energy plasma-enhanced chemical vapour deposition then leads to the formation of Ge-rich Si(1-x)Ge(x) islands (x > 0.8) with a homogeneous size distribution, precisely positioned with respect to the substrate pattern. The island morphology was characterized by atomic force microscopy, and the Ge content and strain in the islands was studied by μRaman spectroscopy. This characterization indicates a uniform distribution of islands with high Ge content and low strain: this suggests that the relatively high growth rate (0.1 nm s(-1)) and low temperature (650?°C) used is able to limit Si intermixing, while maintaining a long enough adatom diffusion length to prevent nucleation of islands outside pits. This offers the novel possibility of using these Ge-rich islands to induce strain in a Si cap.     

Kinetic vs. strain-induced growth instabilities on vicinal Si(001) substrates

The vicinal Si(001) surfaces are known to exhibit both kinetically and thermodynamically driven instabilities during overgrowth with Si or SiGe. Here, we present a comparative study that allows the discrimination of kinetic effects and strain-induced equilibrium effects. Under kinetic growth conditions, layers with low Ge content become smoother than their homoepitaxial Si counterparts. In contrast, for layers with high Ge content, hut cluster formation is identified as the dominant mechanism of the epilayer to relax strain. We find no evidence for one-dimensional strain-induced step bunching on small angle miscut substrates. Our results strongly suggest that kinetic or kinetically limited processes rather than thermodynamic effects determine the morphology of Si/SiGe interfaces at 550°C.     

Stranski-Krastanow growth of germanium on silicon nanowires

There have been extensive studies of germanium (Ge) grown on planar silicon (Si) substrates by the Stranski-Krastanow (S-K) mechanism. In this study, we present S-K growth of Ge on Si nanowires. The Si nanowires were grown at 500 degrees C by a vapor-liquid-solid (VLS) method, using silane (SiH4) as the gaseous precursor. By switching the gas source from SiH4 to germane (GeH4) during the growth and maintaining the growth conditions, epitaxial Ge islands deposited on the outer surface of the initially formed Si nanowires. Transmission electron microscopy (TEM), scanning TEM, and energy-dispersive X-ray spectroscopy techniques were utilized to identify the thin wetting layer and the three-dimensional Ge islands formed around the Si core nanowires. Cross-sectional TEM verified the surface faceting of the Si core nanowires as well as the Ge islands.     

Metal-induced assembly of a semiconductor island lattice: Ge truncated pyramids on Au-patterned Si

We report the two-dimensional alignment of semiconductor islands using rudimentary metal patterning to control nucleation and growth. In the Ge on Si system, a square array of submicron Au dots on the Si (001) surface induces the assembly of deposited Ge adatoms into an extensive island lattice. Remarkably, these highly ordered Ge islands form between the patterned Au dots and are characterized by a unique truncated pyramidal shape. A model based on patterned diffusion barriers explains the observed ordering and establishes general criteria for the broader applicability of such a directed assembly process to quantum dot ordering.     

Ge island assembly on metal-patterned Si: truncated pyramids, nanorods, and beyond

The organization of semiconductor nanostructures into functional macroassemblies remains a fundamental challenge in nanoscience and nanotechnology. In the context of semiconductor epitaxial growth, efforts have focused on the application of advanced substrate patterning strategies for the directed assembly quantum-dot islands. We present a comprehensive investigation on the use of simple metal patterns to control the nucleation and growth of heteroepitaxial islands. In the Ge on Si model system, a square array of metal dots induces the assembly of Ge islands into an extensive two-dimensional lattice. The islands grow at sites between the metal dots and are characterized by unique shapes including truncated pyramids and nanorods, which are programmed prior to growth by the choices of metal species and substrate orientation. Our results indicate that ordering arises from the metal-induced oxidation of the Si surface; the oxide around each metal dot forms an array of periodic diffusion barriers that induce island ordering. The metals decorate the island surfaces and enhanced the growth of particular facets that are able to grow as a result of significant intermixing between deposited Ge and Si substrate atoms.     

Chemical bond manipulation for nanostructure integration on wafer scale

In this paper, we have briefly summarized our activity in the area of chemical bond manipulation for the integration of nanostructures on a full wafer scale. Chemical bond manipulation involves a judicious combination of surface phenomena: reactions or diffusion, and growth process such as molecular beam epitaxy (MBE). Here, we present our results on oxidation, metallization and nitridation and their role in the formation of nanostructures. We find that oxygen changes the bonding partner from Ge to Si and this phenomenon can be controlled by controlling the annealing temperature. We have employed this phenomenon for the fabrication of novel, multiperiod Si/SiO₂/Ge layered structure which exhibits interesting light emitting properties. Further, by making use of selective diffusion of cobalt atoms through Ge layers it is possible to incorporate metallic features into Ge quantum dots. Moreover, it is possible to fabricate Si nanopillars through high temperature reaction of nitric oxide. NO molecules dissociate on the surface and nitrogen atoms thus produced form nitride islands. These islands act as protective masks for the etching of Si by the oxygen atoms, through the desorption of SiO species. Occurrence of these two simultaneous processes result in the formation of nanometre-sized Si pillars capped by silicon nitride. All these results emphasize the fact that we can extend information obtained through traditional surface science experiments for the fabrication of novel structures on a full wafer scale.     

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.     

Ge–Si intermixing in Ge quantum dots on Si

We have provided direct evidence for the presence of considerable Si–Ge intermixing in strained and unstrained Ge quantum dots deposited on Si(001) and Si(111). The local structure around Ge was probed by using Ge K-edge X-ray absorption spectroscopy; complementary evidence for intermixing was provided by AFM and STM studies. These results implied that the strain energy in the dots was reduced by Si atoms diffusing into the dots, resulting in a modified form of Stranski–Krastanov growth.     

Gold-catalyzed oxide nanopatterns for the directed assembly of Ge island arrays on Si

The heteroepitaxial growth of Ge on Au-patterned Si(001) is investigated using in situ spectromicroscopy. Patterning of a hydrogen-terminated Si surface with a square array of Au dots followed by brief exposure to air leads to the spontaneous, local oxidation of Si. The resulting oxide nanopattern limits the surface migration of Au during annealing up to 600 degrees C, resulting in complete preservation of the Au pattern. Subsequent deposition of Ge induces a redistribution of Au across the surface even as the oxide nanopattern persists. As a result, the oxide pattern drives the growth of Ge islands into an ordered assembly, while Au decorates the surfaces of the Ge islands and modifies their shape.     

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。     

Removal of stacking faults in Ge grown on Si through nanoscale openings in chemical SiO2

Nucleation and eventual coalescence of Ge islands, grown out of 5 to 7 nm diameter openings in chemical SiO₂ template and epitaxially registered to the underlying Si substrate, have been shown to generate a low density of threading dislocations (?10⁶ cm^− 2). This result compares favorably to a threading dislocation density exceeding 10⁸ cm^− 2 in Ge films grown directly on Si. However, the coalesced Ge film contains a relatively high density of stacking faults (5 × 10⁷ cm^− 2), and subsequent growth of GaAs leads to an adverse root-mean-square roughness of 36 nm and a reduced photoluminescence intensity at 20% compared to GaAs grown on Ge or GaAs substrates. Herein, we find that annealing the Ge islands at 1073 K for 30 min before their coalescence into a contiguous film completely removes the stacking faults. However, the anneal step undesirably desorbs any SiO₂ not covered by existing Ge islands. Further Ge growth results in a threading dislocation density of 5 × 10⁷ cm^− 2, but without any stacking faults. Threading dislocations are believed to result from the later Ge growth on the newly exposed Si where the SiO₂ has desorbed from areas uncovered by Ge islands. The morphology and photoluminescence intensity of GaAs grown on the annealed Ge is comparable to films grown on GaAs or Ge substrates. Despite this improvement, the GaAs films grown on the annealed Ge/Si exhibit a threading dislocation density of 2 × 10⁷ cm^− 2 and a minority carrier lifetime of 67 ps compared to 4 to 5 ns for GaAs on Ge or GaAs substrates. A second oxidation step after the high temperature anneal of the Ge islands is proposed to reconstitute the SiO₂ template and subsequently improve the quality of Ge film.     

Growth of Ge islands on Si substrates

We present a study of Ge islands formation on Si(100) substrates using grazing-incidence small-angle X-ray scattering (GISAXS) and atomic force microscopy (AFM). Samples were prepared by magnetron sputtering of a 5 nm thick Ge layer in a very high vacuum on Si(100) substrate held at different temperatures. The vertical cut (perpendicular to the surface) of the experimental 2D GISAXS pattern has been fitted using a Guinier approximation. The optimum temperature for the islands formation was 650 °C. At this temperature, islands grow in conical shape with very similar dimensions; however, inter-island distances varied significantly.     

Si_3N_4/Si表面Ge生长过程的STM研究

利用STM和LEED分析了Ge在Si3 N4 /Si(111)和Si3 N4 /Si(10 0 )表面生长过程的结构演变。在生长早期 ,Ge在两种衬底表面上都形成高密度的三维纳米团簇 ,这些团簇的大小均在几个纳米范围内 ,并在高温退火时体积增大。当生长继续时 ,Ge的晶体小面开始显现。在晶态的Si3 N4 (0 0 0 1) /Si(111)表面 ,Ge的 (111)晶向的小面生长比其他方向优先。最后在大范围内形成以 (111)方向为主的晶面。相反 ,在非晶的Si3 N4 表面 ,即Si3 N4 /Si(10 0 ) ,Ge晶体的高指数侧面生长较顶面快 ,最终形成金字塔形的岛结构。对这样的表面生长过程进行了探讨并给出了合理的物理解释     

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.     

硅、锗材料在Si(100)表面的生长研究

利用扫描隧道显微镜和超高真空实验装置系统进行了Si(10 0 )表面生长Si,Ge的实验研究。分析了所生成表面的形貌、结构等物理性质。研究表明 :Si在Si(10 0 )表面的同质生长可以形成纳米结构薄膜。Ge在Si(10 0 )表面生长形成规则的三维小岛。而在Si/Ge/Si(10 0 )多层膜上生长则形成大小二种三维岛。研究表明大岛具有Ge/Si/Ge的壳层结构     

Microphotoluminescence and perfect ordering of SiGe islands on pit-patterned Si(001) substrates

We show that both the morphology and the optoelectronic properties of SiGe islands growing in the pits of periodically pre-patterned Si(001) substrates are determined by the amount of Ge deposited per unit cell of the pattern. Pit-periods (p) ranging from 300 to 900 nm were investigated, and Ge growth was performed by molecular beam epitaxy (MBE) at temperatures of 690 and 760?°C. The ordered SiGe islands show photoluminescence (PL) emission, which becomes almost completely quenched, once a critical island volume is exceeded. By atomic force and transmission electron microscope images we identify the transition from pyramid-shaped to dome-shaped islands with increasing p. Eventually, the nucleation of dislocations in the islands leads to PL quenching. Below a critical Ge coverage a narrowing and a blue shift of the PL emission is observed, as compared to islands grown on a planar reference area of the same sample.     

Raman study of the strain and H<Subscript>2</Subscript> preconditioning effect on self-assembled Ge island on Si (001)

An investigation of the microscopic mechanisms of Ge self-assembling island growth is of great importance for future optoelectronic applications of quantum dot nanostructures. In this study, two sets of self-assembled germanium islands on Si (001) substrate, with and without preconditioning using a high-temperature hydrogenation step on their nucleation and subsequent temporal evolution, were grown by low-pressure chemical vapor deposition (LPCVD). The average germanium concentration, mean diameter of Ge crystalline regions and the strain inside the germanium quantum dots are characterized with high resolution micro-Raman spectroscopy (μRS). Both the intensity and peak position of the Si–Si vibration mode at about 520.07 cm⁻¹ in the Raman spectra have been used as a reference to separate the germanium Raman signal from the overlapping localized Si–Si optical phonon at ∼300 cm⁻¹.In the absence of preconditioning, both the island size and germanium composition increase steadily as a function of deposition time. However, on the H₂ preconditioned surface, the nucleation and growth rates are greatly increased during the first stages and slow down significantly after deposition for 10 s.Our results indicate that the compressive strain inside the islands acts as a barrier for Ge adatoms to diffuse from the wetting layer into the islands. For the growth times used in this study, for both sets of samples with and without H₂ preconditioning, the normalized rate of increase of the Ge concentration (%Δ [Ge]/Δ t) decreases by ∼0.13/s for a 1% compressive strain increase. The H₂ preconditioning can initially increase the density of island nucleation sites, but cannot accelerate the Ge island growth. It tends to lower %Δ [Ge]/Δ t by 0.015/s instead. The decreased strain due to surface roughing is the principal reason why the Ge islands grow so rapidly at the beginning on the H₂ preconditioned samples.     

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.     

Thermally induced morphology evolution of pit-patterned Si substrate and its effect on nucleation properties of Ge dots

We demonstrate the effect of the pre-growth heat treatment process on the nucleation properties of Ge dots grown on pit-patterned Si(001) substrates. The prefabricated 200 nm diameter pits inherently evolve into truncated inverted pyramids (TIPs) with (110) base edges and a 7°-9° sidewall slope during heat treatment; this morphology transformation is robust against variations in shape and orientation of the pit patterns. Uniform Ge dots with an areal density of 4 × 10(9) cm(-2) were obtained on the Si substrates having TIPs. Each TIP contains four aligned Ge dots locating symmetrically with respect to (110). These dots exhibit an elliptical dome shape with major axis oriented along (100). The nucleation position, shape and spatial orientation of these Ge dots coincide with the calculated surface chemical potential distribution of the TIP.