V. Yam

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.

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.

Growth kinetics of Ge crystals on silicon oxide by nanoscale silicon seed induced lateral epitaxy

In this paper, we present our studies on the growth kinetics of Ge crystals on silicon oxide by nanoscale seed induced lateral epitaxy. We propose a simple and reliable method based on standard local oxidation of silicon technique for creating nanoscale silicon seeds at the edge of thermally grown silicon oxide stripes of desired thickness. The growth of Ge from germane is initiated in the two silicon seed lines and evolves toward a complete wetting of the SiO2 stripe after coalescence. The wetting mechanism of SiO2 by Ge is strongly dependent on the seed orientation and closely related to the development of {111} facets. The coalescence of adjacent Ge crystals results in an improvement in the organization of the initial material. As a result, no defect is visible in the inner part of the structure. The observed defects are arrays of misfit dislocations standing along the seed lines, while only few dislocations are visible through the Ge crystal. Geometric phase analysis of high resolution transmission el...

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.

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.

Electrical properties of W/Si interfaces with embedded Ge/Si islands

In this work, we investigated electrical and morphological properties of W/p-type Si Schottky diodes with intentional inhomogeneities introduced by macroscopic Ge-islands embedded beneath the interface. The Si-cap layer thickness (or the island-distance to the interface) was progressively reduced by successive chemical etching cycles. Electrical characterizations were achieved through reverse current-voltage (I-V) at room temperature and forward I-V measurements as a function of the temperature. In parallel, Rutherford backscattering spectroscopy analyses were performed to follow the Si-cap/Ge islands chemical thinning down with increasing the number of etching cycles. In addition, the comparison of topographical and electrical properties of the etched silicon-cap layer was carried out by conductive atomic force microscopy analyses with a nanometer-scale resolution. Our results indicate that the areas on the top of islands exhibit lower resistance than those which covered the wetting layer. This lateral variation of resistance at the surface of the semiconductor may correspond to Schottky barrier height inhomogeneities observed on broad area I-V characteristics of Schottky contacts.

Mechanism of vertical correlation in Ge/Si(001) islands multilayer structures by chemical vapor deposition

The aim of this paper is to study the mechanism of vertical correlation of Ge/Si(001) islands in multilayered structures grown by ultrahigh vacuum chemical vapor deposition. We used in situ reflexion high energy electron diffraction, ex situ atomic force microscopy, photoluminescence spectroscopy, and high resolution and analytical field emission transmission electron microscopy in order to determine the morphological properties of the layered structures at each step of their formation. For Si spacer thickness below 90 nm, a roughness is observed above the top of each Ge island. The local composition of the multilayers measured by x-ray nanoanalysis shows an inhomogeneous Ge distribution in the wetting layers and in the islands. The roughness can then be explained by the existence of a difference of Si growth rate over Ge islands and Ge wetting layers. This roughness on the Si cap surface layer leads, on the one hand, to a preferential nucleation of Ge above each buried island, and consequently to the ver...

Polarized Raman Spectroscopy of Single Layer and Multilayer Ge/Si(001) Quantum Dot Heterostructures

Polarized Raman spectroscopy in backscattering geometry has been applied here for investigation of Ge/Si(001) quantum dot multilayer structures (ranging from 1 to 20 layers) grown by the Stranski-Krastanov technique. The characteristic Raman spectra of Ge dots have been obtained by taking the difference between the Raman spectra of the dots sample and the reference Si substrate, registered with the same polarization in the scattering channel. We found that Raman spectra of Ge dots obtained in such a manner are strongly polarized, in particular for Si-Ge (at ∼413 cm−1) and Ge-Ge (at ∼295 cm−1) vibrational modes. The dependence of peak intensity and peak position of Si-Ge and Ge-Ge modes versus the number of Ge dot layers, and versus the growth temperature for single layers, have been studied. The intermixing effect and stress have been obtained using the ratio of the integrated intensities and the peak positions of the aforementioned bands.