Vinh Le Thanh

Low-temperature formation of Si(001) 2×1 surfaces from wet chemical cleaning in NH4F solution

In situ reflection high-energy electron diffraction and thermal desorption spectroscopy were used to investigate the surface structure and chemistry of Si(001) surfaces after wet chemical etching in dilute HF and ammonium fluoride (NH4F) solutions. The HF-etched surfaces were found to be rough and terminated by mono-, di-, and tri-hydride species, in good agreement with previous vibrational spectroscopy measurements. The surface roughness observed after HF treatment appears to arise from surface defects such as adstructures and kinks left on the surface after the removal of the oxide layers. The Si(001) etching processes in NH4F solution were found to be highly anisotropic, and the etching time in the solution was found to be the key parameter which determined the chemistry and the morphology of the surface. We show that for a limited etching time (∼1 min), a flat and bulk-like dihydride-terminated Si(001) surface could be formed and this surface was found to transform, upon annealing in ultrahigh vacuum,...

New insight into the kinetics of Stranski-Krastanow growth of Ge on Si(001)

Abstract In situ reflection high-energy electron diffraction along with atomic force microscopy and photoluminescence spectroscopy have been combined to investigate the kinetic pathway of the transition from two-dimensional (2D) to islanding growth in the Ge/Si(0xa00xa01) system. By performing experiments in the dynamic growth regime and under growth interruption, we can identify two main steps in the kinetics of Ge/Si(0xa00xa01) growth. First, the 2D wetting layers (WLs) are found to undergo a morphological instability well before reaching the critical thickness. This instability appears to be strain driven and is the driving force leading to the formation of intermediate clusters between 2D layers and 3D macroscopic islands. By controlling the degree of instability of the 2D layers, we show that islands with different sizes, shapes and optical properties can be formed. Second, when the Ge coverage reaches the critical thickness, 3D macroscopic islands are found to be spontaneously formed by consuming Ge from the 2D layers without needing to proceed via a precursor phase. Energetically, the spontaneous nucleation of 3D islands is a favorable process because it allows a reduction by a factor of almost two of the elastic energy of the remaining WL through the reduction of its thickness.

Strain-driven modification of the Ge/Si growth mode in stacked layers : a way to produce Ge islands having equal size in all layers

The Ge/Si growth process in standard stacked layers of self-assembled Ge/Si(001) islands was studied using in situ reflection high-energy electron diffraction, transmission electron microscopy and photoluminescence spectroscopy. It was found that the decrease of the Ge critical thickness in the upper layers of a stacked layer is the main parameter which leads to the increase of the upper island size and height. Such an evolution of the Ge critical thickness could be explained by an accumulation of elastic strain induced by the lower Ge islands and wetting layers in the Si spacer layers. This result opened the ways to the realization of stacked layers in which the islands have equal size in all layers even for stacked intervals being reduced down to about 1 nm.

Fabrication of SiGe quantum dots: a new approach based on selective growth on chemically prepared H-passivated Si(100) surfaces

Abstract We report, in this paper, on a new method to produce SiGe quantum dots on Si(100) surfaces. Starting from the fact that the adsorption of hydride molecules (SiH4, GeH4) requires free adsorption sites on the surface, the basic idea of our approach is to limit the number of sites for molecular adsorption. We show that etching of Si(100) surfaces in ammonium fluoride (NH4F) solution initially produces a flat and dihydride-terminated Si(100) surface and that longer etching leads to the formation of microscopic (111) facets which are regularly distributed along the surface. Hydrogen atoms are found to desorb completely from surface dihydrides at ∼400°C while those from monohydride-terminated (111) facets remain stable up to 650°C. Thus, for growths carried out in the temperature range of 400–650°C, the adsorption of hydride molecules occurs only on the sites that have been previously terminated by dihydride species, i.e. free of hydrogen. Compared with islands formed by the strain-induced growth mode transition, we demonstrate, by using this new approach, that SiGe islands with better uniformity and much smaller sizes (down to ∼200 A) can be achieved.

Nucleation and growth of self-assembled Ge/Si (001) quantum dots in single and stacked layers

In situ reflection high-energy electron diffraction, atomic force microscopy, transmission electron microscopy and photoluminescence spectroscopy have been combined to analyze the Ge/Si (001) growth process in single and stacked layers. In a single layer, the existence of intermediate clusters between two-dimensional layers and three-dimensional islands is established. These clusters are shown to be metastable both in view of shape and optical properties. In stacked layers, the decrease of the Ge critical thickness in the upper layers is found to be the main parameter, leading to the increase of the island size and height. Such an evolution of the Ge critical thickness could be explained by elastic strain fields induced by lower Ge layers in the Si spacer layers. These results open the ways to the realization of stacked layers in which the islands have equal size in all layers and offer a promising opportunity for studying a real effect of electronic coupling between islands.

Effect of the bimodal size distribution on the optical properties of self-assembled Ge/Si(001) quantum dots

Abstract In situ reflection high-energy electron diffraction, atomic force microscopy and photoluminescence spectroscopy have been combined to investigate the effects of a bimodal size distribution and of the pyramid/dome transition on the optical properties of related Ge/Si layers. It is shown that the wetting layers are inhomogeneous in thickness due to the lateral diffusion of Ge from 2D layers towards islands, while no change is observed in the island-related photoluminescence. These results obtained indicate that 3D islands are, in their early nucleation stages, formed by consuming Ge from 2D layers, and that island luminescence energy is not sensitive to the vertical confinement inside islands.

On the formation of self-assembled Ge/Si(0 0 1) quantum dots

Abstract In situ reflection high-energy electron diffraction (RHEED) along with atomic force microscopy (AFM) and photoluminescence spectroscopy (PL) have been used to investigate the Ge/Si(0xa00xa01) growth process in an ultrahigh-vacuum chemical-vapor deposition (UHV-CVD) system. The existence of an intermediate phase between entirely pseudomorphic 2D layers and 3D macroscopic islands is established. This phase is found to occur before the onset of the 2D to 3D growth mode transition determined from RHEED. It consists of square based pyramidal clusters, and is found to be metastable against the formation of 3D macroscopic islands. The formation of this phase can be explained by a local strain relaxation of the Ge wetting layer surface before reaching the equilibrium critical thickness.

Superlattices of self-assembled Ge/Si(0 0 1) quantum dots

The effect of vertical ordering in superlattices of self-assembled Ge/Si(0 0 1) quantum dots was investigated by a combination of structural and optical characterizations via in situ reflection high-energy electron diffraction (RHEED), transmission electron microscopy (TEM), atomic force microscopy (AFM) and photoluminescence (PL) spectroscopy. We show that the vertical ordering observed in quantum-dot superlattices is characterized not only by the alignment of islands along the growth direction but also by a reduction of the critical thickness. The better the vertical ordering is, the more pronounced the reduction of the critical thickness will be. Such an evolution of the critical thickness could be explained by elastic strain fields induced by buried islands and propagate through the spacer layers. An important result issued from this work is the realization of superlattices in which dots can have equal size in all layers. On the other hand, experiments performed on the transformation of the island shape versus the spacer layer thickness suggest that preferential nucleation induced by surface roughness may be the main mechanism responsible for the vertical ordering observed in quantum-dot superlattices.

A metastable (√3 × √3)R30° reconstruction of the Si(111) surface, induced by silicon adatoms

Abstract Reflection high-energy electron diffraction (RHEED) studies of Si(111) growth using silane reveal that the growing surface exhibits two equilibrium structures as a function of growth temperature: a 1 × 1:H structure for temperatures below 600°C, and (7 × 7) for temperatures higher than 600°C. Starting from the (1 × 1):H surface, two structural pathways are identified upon H desorption when silane flux is interrupted. In the growth temperature range 570–600°C, the (1 × 1):H surface transforms directly to the (7 × 7) surface. At lower growth temperatures, a metastable √3 × √3)R30° structure is observed as an intermediate step during the transformation from the (1 × 1) to the (7 × 7) surface. The formation of the (√3 × √3)R30° structure can be explained by the redistribution of excess Si adatoms present on the surface during growth.

Schottky-barrier height inhomogeneities controlled by buried Ge/Si quantum dots

In this work, we investigated W/p-type Si Schottky contacts with intentional inhomogeneities beneath the interface. These inhomogeneities are related to the presence of Ge-dots located just below the contact. The size and the density of the inhomogeneities can be controlled either through the deposition conditions (Ge-coverage, here) er the thickness of the Si-cap layer. Electrical characterizations of contacts were achieved through current-voltage measurements in a temperature range 100-300 K. These experimental results are compared to numerical simulations using the Atlas-Silvaco package. To describe the contact, we have chosen a cylindrical geometry. The Schottky current takes into account the contributions of small circular patches of lower Schottky barrier height (SBH) embedded in a large area of uniform higher SBH. Our results evidence a correlation between the parameters of the Ge-dots (size, density, distance to the interface) and those of the patches introduced in the model The well-known linear correlation between SBHs and ideality factors, ΦB(n), is observed for all the samples.