“In vivo” STM studies of the growth of Germanium and Silicon on Silicon

A scanning tunneling microscope (STM) capable of imaging during crystal growth from the vapour is described. This method (MBSTM) opens the possibility to follow the growth process of semiconductor molecular beam epitaxy (MBE) in vivo. The ability of the microscope to access the evolution of specific features during growth is demonstrated by images of the Si homoepitaxy. The transition from initial multilayer to layer-by-layer growth was imaged in Si(1 1 1) homoepitaxy. In Si/Si(1 0 0) homoepitaxy the fractional coverage of non-equivalent terraces was studied as function of coverage and a theoretically predicted transient growth mode was observed. In Ge on Si(1 1 1) heteroepitaxy the nucleation of 3D Ge islands was observed. When 3D islands occurred on the surface an etching of the 2D Stranski-Krastanov layer was observed.     

Alignment of Ge nanoislands on Si(111) by Ga-induced substrate self-patterning

A novel mechanism is described which enables the selective formation of three-dimensional Ge islands. Submonolayer adsorption of Ga on Si(111) at high temperature leads to a self-organized two-dimensional pattern formation by separation of the 7 x 7 substrate and Ga/Si(111)-(square root[3] x square root[3])-R30 degrees domains. The latter evolve at step edges and domain boundaries of the initial substrate reconstruction. Subsequent Ge deposition results in the growth of 3D islands which are aligned at the boundaries between bare and Ga-covered domains. This result is explained in terms of preferential nucleation conditions due to a modulation of the surface chemical potential.     

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.     

Initial surface roughening in Ge/Si(001) heteroepitaxy driven by step-vacancy line interaction

The initial surface roughening during Ge epitaxy on Si(001) is shown to arise from an effective repulsion between S(A) surface steps and dimer vacancy lines (VLs). This step-VL interaction gradually inactivates a substantial fraction of adatom attachment sites at the growth front, causing a rapid increase in the rate of two-dimensional island nucleation. The mutual repulsion hinders the crossing of S(A) surface steps over VLs in the second layer, thus organizing the developing surface roughness into a periodic array of anisotropic 2D terraces. Isolated (105) facets forming at specific sites on this ordered template mediate the assembly of first 3D Ge islands.     

Evolution of self-assembled Ge/Si island grown by ion beam sputtering deposition

The effect of temperature and Ge coverage on the evolution of self-assembled Ge/Si islands grown by ion beam sputtering deposition is studied. Atomic force microscopy and Raman spectroscopy are used to analyze the island morphology and the intermixing between Si and Ge. The experiments are presented in two aspects. First, when the temperature is increased, intermixing is promoted, resulting in the reappearance of low aspect ratio islands. Second, a different evolution pathway is observed, in which short islands initially don’t grow along the constant ratio of 11:1 (diameter:height) and the islands always grow faster in vertical direction. In summary, the interdiffusion, surface diffusion, and amount of Ge determines the evolution of Ge/Si islands.     

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.     

Ultrasmall Ge islands with low diameter-to-height aspect ratio on Si(1 0 0)-(2 × 1) surfaces

Scanning tunneling microscopy (STM) and high resolution cross-sectional transmission electron microscopy (XTEM) studies have been used to investigate the formation of Ge nanocrystals grown on Si(1 0 0)-(2 × 1) surfaces by molecular beam epitaxy (MBE). We observe relatively high density of Ge islands where small ‘pyramids’, small ‘domes’ and facetted ‘domes’ of various sizes co-exist in the film. As revealed from XTEM images, a large fraction of islands, especially dome-shaped Ge islands have been found to have an aspect ratio of ∼1 (diameter):1 (height). Observation of truncated-sphere-shaped Ge islands with a narrow neck contact with the wetting layer is reported.     

Minute SiGe quantum dots on Si(001) by a kinetic 3D island mode

We investigated the initial growth stages of Si(x)Ge(1-x)/Si(001) by real time stress measurements and in situ scanning tunneling microscopy at deposition temperatures, where intermixing effects are still minute (< or =900 K). Whereas Ge/Si(001) is a well known Stranski-Krastanow system, the growth of SiGe alloy films switches to a 3D island mode at Si content above 20%. The obtained islands are small (a few nanometers), are uniform in shape, and exhibit a narrow size distribution, making them promising candidates for future quantum dot devices.     

Real time observation of GeSi/Si(001) island shrinkage due to surface alloying during Si capping

The Si capping of Ge/Si(001) islands was observed by in situ time-resolved transmission electron microscopy. During the initial stages of the Si deposition, islands were observed not only to flatten but also to shrink in volume. This unexpected shrinkage is explained by taking into account the intermixing of the deposited Si with the wetting layer and a consequently induced diffusion of Ge from the islands into the wetting layer. A model of the capping process which takes into account Ge diffusion is presented which is in good agreement with the experimental data.     

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.     

Initial stages of Ge epitaxy on Si(111) under quasi-equilibrium growth conditions

The results of the structural and morphological studies of Ge growth on a Si(111) surface at the initial stages of epitaxy by means of scanning tunneling microscopy and high-resolution transmission electron microscopy are presented. Epitaxy of Ge has been performed in the temperature range of 300 to 550°C under the quasi-equilibrium growth conditions and low deposition rates of 0.001–0.01 bilayers per minute. The stages of the formation and decay of the nanoclusters as a result of the redistribution of the Ge atoms into two-dimensional pseudomorphic Ge islands before the formation of the continuous wetting layer have been experimentally detected. The positions of the preferable nucleation of three-dimensional Ge islands on the wetting layer formed after the coalescence of the two-dimensional islands have been analyzed. The c2 × 8 → 7 × 7 → c2 × 8 phase transitions due to the lateral growth of the islands and the plastic relaxation of the misfit strains occur on the surface of the three-dimensional Ge islands when their strain state changes. The misfit dislocations gather at the interface and two types of steps lower than one bilayer are formed on the surface of the three-dimensional islands during the relaxation process.     

Strain-driven morphology of Si1-xGex islands grown on Si(100)

A study has been carried out on the morphology and structure of three-dimensional (3D) SiGe islands grown by molecular beam epitaxy (MBE) on Si(100) substrates. Samples of Si1-xGex alloys have been prepared to investigate the effects either of the alloy composition or of the growth temperature. Atomic force microscopy (AFM) evidenced the growth of 3D islands and transmission electron microscopy (TEM) demonstrated wetting layer growth on Si(100), independently on the deposition conditions. Energy dispersive spectroscopy (EDS) micro-analyses carried out on cross-sections of large Si1-xGex islands with defects allowed a measurement of the Ge distribution in the islands. To the best of our knowledge, these have been the first experimental evidences of a composition change inside SiGe islands. The interpretation of the experimental results has been done in terms of strain-enhanced diffusion mechanisms both of the growing species (Si and Ge) and of small islands.     

Photoemission studies of the surfactant-aided growth of Ge on Te-terminated Si(100)

The interactions of Ge adatoms with a Si(100) surface terminated by an ordered layer of Te have been studied in detail using XPS, SXPS, STM and LEED. It has been demonstrated that the Te layer has a surfactant action on the growth mode of the Ge in that the two dimensional growth regime is extended to at least 200 Å and the Te is seen to segregate to the growing Ge surface. The surface reconstruction of the Ge layer changes from (1 × 1) in the initial stages to (2 × 2) as growth proceeds and the surface population of Te is reduced. SXPS line shape analysis has indicated that the initial stages of Ge incorporation are characterised by the formation of small islands above those surface Si sites not fully coordinated with Te. Continued growth of such islands is, however, restricted due to their high surface free energy with respect to the surrounding Te-terminated areas. Ge atoms therefore site-exchange with Te atoms in bridge sites, thus becoming incorporated onto the Si lattice and displacing the Te to bridge sites on the growing surface. In this manner islanding is prevented and two-dimensional growth continues beyond the critical thickness. No evidence is seen for any significant incorporation of the Te within the growing Ge layer.     

Temperature dependent low energy electron microscopy study of Ge growth on Si(1 1 3)

We investigated the initial Ge nucleation and Ge island growth on a Si(1 1 3) surface using low energy electron microscopy and low energy electron diffraction. The sample temperature was varied systematically between 380 °C and 590 °C. In this range, a strong temperature dependence of the island shape is observed. With increasing temperature the Ge islands are elongated in the direction. Simultaneously, the average island size increases while their density decreases. From the Arrhenius-like behaviour of the island density, a Ge adatom diffusion barrier height of about 0.53 eV is deduced.     

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.     

Solid phase epitaxy of Ge films on CaF2/Si(1 1 1)

At room temperature deposited Ge films (thickness < 3 nm) homogeneously wet CaF₂/Si(1 1 1). The films are crystalline but exhibit granular structure. The grain size decreases with increasing film thickness. The quality of the homogeneous films is improved by annealing up to 200 °C. Ge films break up into islands if higher annealing temperatures are used as demonstrated combining spot profile analysis low energy electron diffraction (SPA-LEED) with auger electron spectroscopy (AES). Annealing up to 600 °C reduces the lateral size of the Ge islands while the surface fraction covered by Ge islands is constant. The CaF₂ film is decomposed if higher annealing temperatures are used. This effect is probably due to the formation of GeF_x complexes which desorb at these temperatures.     

STM study of successive Ge growth on “V”-stripe patterned Si (0 0 1) surfaces at different growth temperatures

The self-assembly process of Ge islands on patterned Si (0 0 1) substrates is investigated using scanning tunneling microscopy. The substrate patterns consist of one-dimensional stripes with “V”-shaped geometry and sidewalls inclined by an angle of 9° to the (0 0 1) surface. Onto these stripes, Ge is deposited in a step-wise manner at different temperatures from 520 °C to 650 °C. At low temperature, the Ge first grows nearly conformally over the patterned surface but at about 3 monolayers a strong surface roughening due to reconstruction of the surface ridges as well as side wall ripple formation occurs. At 600 °C, a similar roughening takes place, but Ge accumulates within the grooves such that at a critical thickness of 4.5 monolayers, 3D islands are formed at the bottom of the grooves. This accumulation process is enhanced at 650 °C growth, so that the island formation starts about 1 monolayers earlier. At 600 and 650 °C, all islands are all aligned at the bottom of the stripes, whereas at 550 °C Ge island form preferentially on top of ridges. The experimental observations are explained by the strong temperature dependence of Ge diffusion over the patterned surface.     

C-induced Ge dots: enhanced light-output from Si-based nanostructures

Pre-deposition of a fraction of a monolayer of C on an Si (0 0 1) substrate causes the formation of extremely small islands after the growth of only 2 monolayers (ML) of Ge. We demonstrate that these CGe dots exhibit particularly intense photoluminescence (PL) compared to a variety of different but comparable structures. Although grown at low temperatures (460°C), the CGe islands show a ten times more intense PL signal than conventionally grown self-assembled Ge islands, grown at 700°C. We show that the initial stage of CGe dot formation is likely to be governed by strain compensation effects. In a series of samples, where we have kept the total C and Ge amounts constant but varied the deposition sequence, we show that the specific CGe dot growth order of pre-grown low surface mobility C, followed by high surface mobility Ge leads to a distinct nanostructure within the SiGeC material system, exhibiting typical ‘dot-like' and intense PL. An almost strain compensated 50 layer stack of CGe dots is shown to emit intense PL at 0.99 eV.     

INCREASING THE PHOTOLUMINESCENCE INTENSITY OF Ge ISLANDS BY CHEMICAL ETCHING

Self-assembled Ge islands were grown on Si(100) substrate by Si₂H₆-Ge molecular beam epitaxy. After being subjected to chemical etching, it is found that the photoluminescence from the etched Ge islands became more intense and shifted to the higher-energy side compared to that of the as-deposited Ge islands. This behaviour was explained by the effect of chemical etching on the morphology of the Ge islands. Our results demonstrate that chemical etching can be a way to change the luminescence property of the as-deposited islands.     

Electronic properties and interfacial coupling in Pb islands on single-crystalline graphene

Introducing metal thin films on two-dimensional (2D) material may present a system to possess exotic properties due to reduced dimensionality and interfacial effects. We deposit Pb islands on single-crystalline graphene on a Ge(110) substrate and studied the nano- and atomic-scale structures and low-energy electronic excitations with scanning tunneling microscopy/spectroscopy (STM/STS). Robust quantum well states (QWSs) are observed in Pb(111) islands and their oscillation with film thickness reveals the isolation of free electrons in Pb from the graphene substrate. The spectroscopic characteristics of QWSs are consistent with the band structure of a free-standing Pb(111) film. The weak interface coupling is further evidenced by the absence of superconductivity in graphene in close proximity to the superconducting Pb islands. Accordingly, the Pb(111) islands on graphene/Ge(110) are free-standing in nature, showing very weak electronic coupling to the substrate.