Self-organization and ordering in nanocrystalline Si/SiO₂ superlattices

Abstract

The solid phase crystallization of nanometer-thick layers of disordered Si confined between layers of amorphous SiO₂ has been achieved using high temperature annealing. For ultrathin Si layers (∼1–$\text{3}\text{nm}$ thick) crystallization was not possible even after extensive annealing at temperatures up to 1100°C, because of the high strain fields introduced by the SiO₂ layers. However, for thicker layers (∼4–$\text{20}\text{nm}$ thick) a variety of Si nanocrystals ranging in shape from spheres to bricks could be spontaneously formed and, in suitable cases, oriented along the $\text{〈1}\text{1}\text{1〉}$ crystallographic direction. This formation of organized nanocrystals is an important step towards the construction of Si/SiO₂ quantum devices.

Introduction

The spontaneous formation of organized nanocrystals has been discovered in the heteroepitaxial growth and chemical synthesis of III–V and II–VI semiconductors [1], [2], [3], [4], [5]. A reproducible fabrication of size-controlled Si nanocrystals encapsulated by SiO₂ is complicated by the amorphous nature of SiO₂ and the mismatch in their lattice constants and thermal expansion coefficients. In previously reported Si nanocrystal fabrication [6], [7], [8], [9], [10], precise control over the nanocrystal shape and crystallographic orientation has never been attained. A preferred crystallographic orientation, which can be introduced by self-organization, is especially important for Si quantum dots because of the Si indirect-gap band structure and a strong anisotropy in the energy-momentum dispersion along different directions within the Brillouin zone [11]. An important step forward in the size control of Si nanocrystals was the introduction of precise growth techniques to produce ultrathin layers of disordered Si (d-Si) separated by amorphous SiO₂ (a-SiO₂) layers [12], [13]. Crystallization of these thin layers (∼1–$\text{3}\text{nm}$ thick) proved to be very difficult, and actually impossible for the thinnest layers, even after extended annealing at temperatures up to 1100°C [14]. This is because of the high strain fields introduced by the large lattice mismatch of Si and SiO₂ [14]. Only for larger layer thicknesses can crystallization be achieved [15], [16]. Here we report on self-organization and ordering in Si nanocrystals fabricated by the solid phase crystallization (SPC) of 4–$\text{20}\text{nm}$ thick Si layers confined between amorphous SiO₂ layers. Depending on the layer thickness and thermal processing, Si nanoparticles form in various shapes and in a preferred orientation.