Diffusion dynamics during the nucleation and growth of Ge/Si nanostructures on Si(111)

Monitoring the kinetic evolution of self-assembled SiGe islands grown by Ge surface thermal diffusion from a local source

In this paper we experimentally study the growth of self-assembled SiGe islands formed on Si(001) by exploiting the thermally activated surface diffusion of Ge atoms from a local Ge source stripe in the temperature range 600-700 °C. This new growth strategy allows us to vary continuously the Ge coverage from 8 to 0 monolayers as the distance from the source increases, and thus enables the investigation of the island growth over a wide range of dynamical regimes at the same time, providing a unique birds eye view of the factors governing the growth process and the dominant mechanism for the mass collection by a critical nucleus. Our results give experimental evidence that the nucleation process evolves within a diffusion limited regime. At a given annealing temperature, we find that the nucleation density depends only on the kinetics of the Ge surface diffusion resulting in a universal scaling distribution depending only on the Ge coverage. An analytical model is able to reproduce quantitatively the trend of the island density. Following the nucleation, the growth process appears to be driven mainly by short-range interactions between an island and the atoms diffusing within its vicinities. The islands volume distribution is, in fact, well described in the whole range of parameters by the Mulheran's capture zone model. The complex growth mechanism leads to a strong intermixing of Si and Ge within the island volume. Our growth strategy allows us to directly investigate the correlation between the Si incorporation and the Ge coverage in the same experimental conditions: higher intermixing is found for lower Ge coverage. This confirms that, besides the Ge gathering from the surface, also the Si incorporation from the substrate is driven by the diffusion kinetics, thus imposing a strict constraint on the initial Ge coverage, its diffusion properties and the final island volume.

Film-thickness-dependent conduction in ordered Si quantum dot arrays

In recent years, silicon nanostructures have been investigated extensively for their potential use in photonic and photovoltaic applications. So far, for silicon quantum dots embedded in SiO(2), control over inter-dot distance and size has only been observed in multiple bilayer stacks of silicon-rich oxides and silicon dioxide. In this work, for the first time the fabrication of spatially well-ordered Si quantum dots (QDs) in SiO(2) is demonstrated, without using the multilayer approach. This ordered formation, confirmed with TEM micrographs, depends on the thickness of the initially deposited sub-stoichiometric silicon oxide film. Grazing incidence x-ray diffraction confirms the crystallinity of the 5 nm QDs while photoluminescence shows augmented bandgap values. Low-temperature current-voltage measurements demonstrate film thickness and order-dependent conduction mechanisms, showing the transition from temperature-dependent conduction in randomly placed dots to temperature-independent tunnelling for geometrically ordered nanocrystals. Contrary to expectations from dielectric materials, significant conduction and photocarrier generation have been observed in our Si QDs embedded in SiO(2) demonstrating the possibility of forming initial film-thickness-controlled conductive films. This conduction via the silicon quantum dots in thick single layers is a promising result for integration into photovoltaic devices.

Improved infrared photoluminescence characteristics from circularly ordered self-assembled Ge islands

The formation of circularly ordered Ge-islands on Si(001) has been achieved because of nonuniform strain field around the periphery of the holes patterned by focused ion beam in combination with a self-assembled growth using molecular beam epitaxy. The photoluminescence (PL) spectra obtained from patterned areas (i.e., ordered islands) show a significant signal enhancement, which sustained till 200 K, without any vertical stacking of islands. The origin of two activation energies in temperature-dependent PL spectra of the ordered islands has been explained in detail.

Low temperature Ga surface diffusion from focused ion beam milled grooves

Ga diffusion from focused ion beam (FIB) milled grooves has been studied using x-ray photoemission electron microscopy (XPEEM) and mirror electron microscopy (MEM). We analyze the surface chemistry of the FIB structures measuring the Ga presence in the top layers of the milled grooves and morphological defects inside the grooves. The Ga is initially strictly confined to the grooves. However, annealing at temperatures as low as 150 degrees C leads to rapid and significant Ga surface diffusion from the FIB structures. The out-diffused Ga forms a thin layer extending up to several microns laterally in a non-regular pattern. The diffusion is significantly enhanced at small crystallites at the edges of the grooves. We explain the general behavior with an atomic scale model in which interstitial Ga in the milled areas diffuses out and substitutes silanol groups on the surface.

The influence of deposition temperature on the correlation of Ge quantum dot positions in amorphous silica matrix

We studied the structural properties of (Ge+SiO2)/SiO2 multilayer films, especially the influence of the deposition temperature and the parameters of subsequent annealing on the formation and spatial correlation of Ge quantum dots in an amorphous silica matrix. We showed that in-layer and inter-layer spatial correlations of the formed Ge quantum dots strongly depend on the deposition temperature. For suitable chosen deposition parameters, highly correlated dot positions in all three dimensions can be obtained. It is demonstrated that the degree of the spatial correlation of quantum dots influences the size distribution width, which further affects the macroscopic properties of the quantum dot arrays.

Global faceting behavior of strained Ge islands on Si

The evolution of crystallographic facets of strained heteroepitaxial Ge islands on Si is investigated. Islands growing on Si(001), (111), (110) and (113) are bound by an equilibrium set of facets that includes only shared stable surfaces between bulk Si and Ge--{105}, {113}, {15 3 23} and {111}. The formation of a stereographic map from these indices facilitates the prediction of Ge faceted-island shapes on any Si substrate at different stages of growth. The analysis presented here can be applied to other heteroepitaxial islanding systems where a finite set of shared equilibrium facets exists for the bulk starting materials.