Formation of relaxed SiGe on the buffer consists of modified SiGe stacked layers by Si pre-intermixing

High-quality relaxed SiGe films on Si (0 0 1) have been demonstrated with a buffer layer containing modified SiGe (m-SiGe) islands in ultra-high vacuum chemical vapor deposition (UHV/CVD) system. The m-SiGe islands are smoothened by capping an appropriate amount of Si and the subsequent annealing for 10 min. This process leads to the formation of a smooth buffer layer with non-uniform Ge content. With the m-SiGe-dot multilayer as a buffer layer, the 500-nm-thick uniform Si_0.8Ge_0.2 layers were then grown. These m-SiGe islands can serve as effective nucleation centers for misfit dislocations to relax the SiGe overlayer. Surface roughness, strain relaxation, and crystalline quality of the relaxed SiGe overlayer were found to be a function of period's number of the m-SiGe-dot multilayer. By optimizing period number in the buffer, the relaxed Si_0.8Ge_0.2 film on the 10-period m-SiGe-dot multilayer was demonstrated to have a threading dislocation density of 2.0 × 10⁵ cm⁻² and a strain relaxation of 89%.     

Role of Ge interlayer in the growth of high-quality strain relaxed SiGe layer with low dislocation density

High-quality strain relaxed SiGe layer has been fabricated on Si using a thin Ge interlayer grown at 330 °C. The properties of SiGe layers with and without the low-temperature Ge interlayer are compared. The results indicate that the Ge interlayer plays an important role in the preparation of SiGe layer. The strain relaxed low-temperature Ge interlayer with coalesced island surface, acting as a stable and compliant template, could remove the cross-hatch misfit dislocation lines on surface and promote the strain relaxation in the SiGe layer homogeneously.     

Evolution of stress and strain relaxation of Ge and SiGe alloy films on Si(0 0 1)

The growth of Ge and SiGe alloy films on Si substrates has attracted considerable interest in the last years because of their importance for optoelectronic devices as well as Si-based high speed transistors. Here we give a short overview on our recent real time stress measurements of Ge and SiGe alloy films on Si(0 0 1) performed with a sensitive cantilever beam technique and accompanied by structural investigations with atomic force microscopy. Characteristic features in the stress curves provide detailed insight into the development and relief of the misfit strain. For the Stranski–Krastanow system Ge/Si(0 0 1) as well as for SiGe films with Si contents below 20%, the strain relaxation proceeds mainly into two steps: (i) by the formation of 3D islands on top of the Ge wetting layer; (ii) via misfit dislocations in larger 3D islands and upon their percolation.     

Magnetism of Fe clusters and islands on Pt surfaces

Clusters and islands of Fe atoms have been prepared by noble gas buffer layer assisted growth as well as by standard molecular beam epitaxy on Pt substrates. Xe buffer layers have been utilized to promote the formation of compact, relaxed Fe clusters with narrow size distribution. Without the Xe buffer, strained Fe islands with a characteristic misfit dislocation network are formed. Magnetization loops obtained by magneto-optical Kerr effect measurements reveal that in-plane easy magnetization axis is only found for the relaxed clusters, pointing out the important role of epitaxial lattice deformations for the magnetic anisotropy. PACS 61.46.+w; 68.37.Ef; 36.40.Cg; 75.75.+a     

Thermal annealing effects on a compositionally graded SiGe layer fabricated by oxidizing a strained SiGe layer

Thermal annealing effects on a thin compositionally graded SiGe buffer layer on silicon substrate fabricated by oxidizing a strained SiGe layer are investigated with X-ray diffraction, ultraviolet Raman spectra and atomic force microscopy. Interestingly, we found that the surface roughness and the threading dislocation densities are kept low during the whole annealing processes, while the Ge concentration at the oxidizing interface decreases exponentially with annealing time and the strain in the layer is only relaxed about 66% even at 1000 °C for 180 min. We realized that the strain relaxation of such a compositionally graded SiGe buffer layer is dominated by Si-Ge intermixing, rather than generation and propagation of misfit dislocations or surface undulation.     

Surface evolution and three-dimensional shape changes of SiGe/Si(0 0 1) islands during capping at various temperatures

The authors use a combination of atomic force microscopy and selective wet chemical etching of the Si capping layer to investigate both the surface and the three-dimensional SiGe/Si(0 0 1) island shape changes during capping at various temperatures. Different evolution paths are identified depending on the capping temperature. During the early stages of Si overgrowth at 450 °C, a moderate SiGe alloying occurs near the island apex. In the later stages, island burying begins through lateral growth of pyramid-like structures, which consist of pure Si. A comparison with previous overgrowth studies allows us to clarify the role of the initial island size in determining the surface evolution above buried islands. Island dissolution with material transfer to the wetting layer dominates upon capping at 580 °C. Finally, when the temperature during growth and capping is identical, the islands become flatter and wider indicating that the system starts to evolve towards an energetically preferred SiGe quantum well.     

Morphology of alkali halide thin films studied by AFM

Thin layers of alkali halides were investigated by atomic force microscope (AFM). The studied systems were: LiBr/KBr(0 0 1) with −16.7% misfit, LiF/Si(0 0 1) with +4.4% misfit, LiBr/LiF(0 0 1) with +36.8% misfit and NaCl/Si(0 0 1) with +46.5% misfit. The results show that the surface morphology strongly depends on the temperature of layer formation. The alkali halides deposited on the foreign substrate at elevated temperatures or at room temperature and subsequently annealed form preferentially 3D islands leaving uncovered substrate areas between them. It is suggested that Ostwald ripening takes place at elevated temperatures.     

Ordering of Ge nanocrystals using FIB nanolithography

Formation and ordering of Ge nanocrystals (NC) are studied on Si(0 0 1) and SiO₂/Si(0 0 1) substrates patterned by focused ion beam (FIB). In both cases we use a three step process consisting of FIB milling of hole patterns with various periodicities, ex-situ substrate cleaning to remove Ga contamination and Ge NC growth by molecular beam epitaxy (MBE). We show that Ge NC can be ordered between or inside the holes on patterned Si(0 0 1) substrates and inside the holes on patterned SiO₂/Si(0 0 1) substrates.     

High-density-plasma (HDP)-CVD oxide to thermal oxide wafer bonding for strained silicon layer transfer applications

Direct wafer bonding between high-density-plasma chemical vapour deposited (HDP-CVD) oxide and thermal oxide (TO) has been investigated. HDP-CVD oxides, about 230 nm in thickness, were deposited on Si(0 0 1) control wafers and the wafers of interest that contain a thin strained silicon (sSi) layer on a so-called virtual substrate that is composed of relaxed SiGe (∼4 μm thick) on Si(0 0 1) wafers. The surfaces of the as-deposited HDP-CVD oxides on the Si control wafers were smooth with a root-mean-square (RMS) roughness of <1 nm, which is sufficiently smooth for direct wafer bonding. The surfaces of the sSi/SiGe/Si(0 0 1) substrates show an RMS roughness of >2 nm. After HDP-CVD oxide deposition on the sSi/SiGe/Si substrates, the RMS roughness of the oxide surfaces was also found to be the same, i.e., >2 nm. To use these wafers for direct bonding the RMS roughness had to be reduced below 1 nm, which was carried out using a chemo-mechanical polishing (CMP) step. After bonding the HDP-CVD oxides to thermally oxidized handle wafers, the bonded interfaces were mostly bubble- and void-free for the silicon control and the sSi/SiGe/Si(0 0 1) wafers. The bonded wafer pairs were then annealed at higher temperatures up to 800 °C and the bonded interfaces were still found to be almost bubble- and void-free. Thus, HDP-CVD oxide is quite suitable for direct wafer bonding and layer transfer of ultrathin sSi layers on oxidized Si wafers for the fabrication of novel sSOI substrates.     

Thermal stability of SiGe films on an ultra thin Ge buffer layer on Si grown at low temperature

The thermal stability of SiGe films on an ultra thin Ge buffer layer on Si fabricated at low temperature has been studied. The microstructure and morphology of the samples were investigated by high-resolution X-ray diffraction, Raman spectra and atomic force microscopy, and using a diluted Secco etchant to reveal dislocation content. After thermal annealing processing, it is observed that undulated surface, threading dislocations (TDs) and stacking faults (SFs) appeared at the strained SiGe layer, which developed from the propagation of a misfit dislocation (MD) during thermal annealing, and no SFs but only TDs formed in strain-relaxed sample. And it is found that the SiGe films on the Ge layer grown at 300 °C has crosshatch-free surface and is more stable than others, with a root mean square surface roughness of less than 2 nm and the threading dislocation densities as low as ∼10⁵ cm⁻². The results show that the thermal stability of the SiGe films is associated with the Ge buffer layer, the relaxation extent and morphology of the SiGe layer.     

X-ray diffraction study of the composition and strain fields in buried SiGe islands

We report on studies of strain and composition of two-dimensionally ordered SiGe islands grown by molecular beam epitaxy using high resolution x-ray diffraction. To ensure a small size distribution of the islands, pit-patterned (001) Si wafers were used as substrates. The Si wafers were patterned by optical lithography and reactive ion etching. The pits for island growth are ordered in regular 2D arrays with periods ranging from 500 to 1000 nm along two orthogonal 〈110〉 directions. After the growth of a Si buffer layer, 5 to 9 monolayers of Ge are deposited, leading to the formation of islands with either dome- or barn shape, depending on the number of monolayers deposited. The Si capping of the islands is performed at low temperatures (300^○C) to avoid intermixing and thus strain relaxation. Information on the surface morphology obtained by atomic force microscopy (AFM) was used to set up models for three-dimensional Finite Element Method (FEM) simulations of the islands including the patterned Si substrate. In the model, special attention was given to the non uniform distribution of the Ge content within the islands. The FEM results served as an input for calculating the diffracted x-ray intensities using kinematical scattering theory. Reciprocal space maps around the vicinity of symmetric (004) and asymmetric (113) and (224) Bragg peaks were recorded in coplanar geometry. Simulating different germanium gradients leads to altered scattered intensity distribution and consequently information on this quantity is obtained for both dome- and barn-shaped islands as well as on the strain fields.     

Tuning strain relaxation by surface morphology: Surfactant-mediated epitaxy of germanium on silicon

In this paper, we summarize the results on the surfactant-mediated epitaxy (SME) of germanium on (0 0 1) and (1 1 1) silicon substrates. Then, we discuss, how the surfactant-controlled development of micro-facets determines the strain relaxation process. We place particular emphasis on the different types of strain-compensating dislocation networks that form at the Ge/Si(0 0 1) interface in epitaxy with and without Sb as a surfactant. At elevated temperatures, high Sb-coverage promotes the generation of a regular array of edge type misfit dislocations, which allows for abrupt instead of gradual strain relaxation in the initial stage of growth.     

Impact of misfit dislocations on wavefront distortion in Si/SiGe/Si optical waveguides

Silicon-rich SiGe alloys represent a promising platform for the development of large-area single-mode optical waveguides to be integrated in silicon-based optical circuits. We find that SiGe layers epitaxially grown on Si successfully guide radiation with a 1.55 μm wavelength, but, beyond a critical core thickness, their optical properties are strongly affected by the clustering of misfit dislocations at the interface between Si and SiGe, leading to a significant perturbation of the local refractive index. Transmission electron microscopy and micro-Raman spectroscopy, together with finite-element simulations, provide a complete analysis of the impact of dislocations on optical propagation.     

Fabrication of well-defined individual dislocations in SiGe as a novel one-dimensional system

We show a new way to fabricate well-defined individual dislocations in SiGe. We started with a fully pseudomorphic but metastable SiGe layer grown on Si(0 0 1) by molecular beam epitaxy. Next, elongated (1 mm) mesa stripes with various widths (0.5–3 μm) were fabricated by a combination of isotropic and anisotropic etching. For smaller stripes, elastic relaxation of the strained SiGe layer can occur, transforming the originally biaxial strained layer into uniaxial strained subsystems. Subsequent strain relaxation caused by high temperature treatments leads to the formation of individual dislocation along the mesa stripes. The number of parallel dislocation can be adjusted by the original strain (Si:Ge ratio and layer thickness) and the mesa widths. We were able to fabricate structures with exactly one dislocation. Finally, contact pads were added to the stripes enabling the electrical characterization of individual dislocation.     

Effect of strain relaxation of oxidation-treated SiGe epitaxial thin films and its nanomechanical characteristics

In this study, we examined the effect of high-temperature oxidation treatment on the SiGe epitaxial thin films deposited on Si substrates. The X-ray diffraction (XRD), atomic force microscopy (AFM), and nanoindentation techniques were employed to investigate the crystallographic structure, surface roughness, and hardness (H) of the SiGe thin films, respectively. The high-temperature oxidation treatment led to Ge pileup at the surface of the SiGe thin films. In addition, strain relaxation occurred through the propagation of misfit dislocations and could be observed through the cross-hatch pattern (800-900 °C) and SiGe islands (1000 °C) at the surface of the SiGe thin films. Subsequent hardness (H) measurement on the SiGe thin films by continuous penetration depth method indicated that the phenomenon of Ge pileup caused a slightly reduced H (below 50 nm penetration depth), while relaxation-induced defects caused an enhanced H (above 50 nm penetration depth). This reveals the influence of composition and defects on the structure strength of high-temperature oxidation-treated SiGe thin films.     

Photoluminescence of Ge nanostructures grown by gas source molecular beam epitaxy on silicon (1 1 8) surface

In this paper, we present a photoluminescence (PL) study of Si/Ge/SiGe/Si structures grown by gas source molecular beam epitaxy on an (1 1 8) undulated surface with various Ge coverage. Nucleation and growth of Ge films is obtained by the Stranski–Krastanov mechanism. The influence of the substrate orientation on the changeover 2D–3D growth mode is investigated. Furthermore, we show the use of growing an SiGe wetting layer to control the uniformity of the Ge island size. The PL signal related to the Ge islands is found to be highly dependent of the power excitation and is observed up to room temperature.     

Growth and morphology of Cobalt thin films on Pd(1 1 1)

We report on the growth of thin Co films on Pd(1 1 1) at three different temperatures 180 K, 300 K, and 550 K. The structure and morphology was determined by scanning tunneling microscopy and low energy electron diffraction. The growth mode was found to vary with temperature. For 180 K and 300 K, we observed a tendency to double layer growth for the initial layers while at elevated temperatures, the initial film grows in single layer. For most conditions, non-ideal three-dimensional growth was observed. Two-dimensional growth was only found for growth temperature of 550 K and coverages above 5 ML. Depending on temperature, the Co islands at low coverages exhibit three principally different shapes: dendritic at 180 K, hexagonal at 300 K and triangular at 550 K. For growth at 550 K and coverages above 5 ML, the islands changed to an irregular shape. This transition is most likely responsible for the transition to 2D growth. Further, the large strain is relaxed by the creation of a dislocation network with mixed fcc and hcp stacking. Depending on the temperature and coverage, a hexagonal or a triangular network was observed. Finally, we have investigated the effect of annealing Co films prepared at 180 K and 300 K. Heating to 490 K leads to coarsening and intermixing.     

Ion-beam-assisted MBE growth of semi-spherical SiGe/Si nano-structures

Semi-spherical SiGe/Si nano-structures of a new type are presented. Epitaxial islands of 30–40 nm in base diameter and 11 nm in height and having a density of about 6×10¹⁰ cm^-2 were produced on (001) Si by molecular beam epitaxial growth of Si/Si_0.5Ge_0.5 layers with in situ implantation of 1-keV As⁺ ions. It was found by cross-section transmission electron microscopy that the islands have a complicated inner structure and consist of a micro-twin nucleus and semi-spherical nano-layers of various SiGe compositions. The nature of the surface patterning is interpreted by stress relaxation through implantation-induced defects. Received: 12 July 2001 / Accepted: 4 September 2001 / Published online: 2 October 2001     

In-plane and out-of-plane shape transitions of heteroepitaxially self-assembled nanostructures

In this work, shapes and shape transitions of several types of self-assembled heteroepitaxial nanostructures, as observed in in situ scanning tunneling microscopy experiments during growth, are examined in the framework of several equilibrium and kinetic models. In particular, heteroepitaxial TiSi₂ and CoSi₂ islands on Si(1 1 1) are shown to behave in accordance with generalized Wulff-Kaishew theorem of equilibrium strained and supported crystal shapes. More specifically, these silicide nanocrystals exhibit out-of-plane thickening shape transition by increasing their vertical aspect ratio with growth, as long as they are strained, and inverse (flattening) transition upon relaxation by misfit dislocations. On the other hand, heteroepitaxial Ge and CoSi₂ islands on Si(0 0 1) are well-known for their in-plane anisotropic elongation. Plausible energetic and kinetic reasons for such elongation, based on the unique nucleation features of Ge-hut/Si(0 0 1) and non-planar CoSi₂-hut/Si(0 0 1) interface, are discussed.     

Si epitaxial growth on LaAlO3(0 0 1)

We report on the growth of Si on c(2 × 2) reconstructed LaAlO₃(0 0 1) surfaces at high substrate temperature (700 °C) by molecular beam epitaxy. An initial Volmer-Weber mode is evidenced using reflection high energy electron diffraction (RHEED), X-ray photoelectron diffraction (XPD) and atomic force microscopy. After the deposition of a few monolayers, the islands coalesce. Using X-ray photoelectron spectroscopy, we demonstrate that Si islands exhibit an abrupt interface with the LaAlO₃ substrate without formation of silicate or silica. Finally, combined RHEED and XPD measurements show the epitaxial growth of Si with a unique Si(0 0 1)//LaAlO₃(0 0 1) and Si<1 0 0>//LAO<1 1 0> relationship.