Odd electron diffraction patterns in silicon nanowires and silicon thin films explained by microtwins and nanotwins

Cyril Cayron,Christopher Secouard,Martien Den Hertog,Laurence Latu-Romain,Jean-Luc Rouviere,Emmanuelle Rouviere,Céline Mouchet,Jean-Pierre Simonato

doi:10.1107/s0021889808042131

Abstract

Odd electron diffraction patterns (EDPs) have been obtained by transmission electron microscopy (TEM) on silicon nanowires grown via the vapour-liquid-solid method and on silicon thin films deposited by electron beam evaporation. Many explanations have been given in the past, without consensus among the scientific community: size artifacts, twinning artifacts or, more widely accepted, the existence of new hexagonal Si phases. In order to resolve this issue, the microstructures of Si nanowires and Si thin films have been characterized by TEM, high-resolution transmission electron microscopy (HRTEM) and high-resolution scanning transmission electron microscopy. Despite the differences in the geometries and elaboration processes, the EDPs of the materials show great similarities. The different hypotheses reported in the literature have been investigated. It was found that the positions of the diffraction spots in the EDPs could be reproduced by simulating a hexagonal structure with c/a = 12(2/3)(1/2), but the intensities in many EDPs remained unexplained. Finally, it was established that all the experimental data, i.e. EDPs and HRTEM images, agree with a classical cubic silicon structure containing two microstructural defects: (i) overlapping Σ3 microtwins which induce extra spots by double diffraction, and (ii) nanotwins which induce extra spots as a result of streaking effects. It is concluded that there is no hexagonal phase in the Si nanowires and the Si thin films presented in this work.

Highlights

Si nanowires (NWs) are low-dimensional objects with promising potential in the new, emerging semiconductor industries
The microstructural observations were performed by conventional TEM with a JEOL 2000FX (200 kV), by high-resolution transmission electron microscopy (HRTEM) with a JEOL 4000EX (400 kV) and by high-resolution scanning transmission electron microscopy (HRSTEM) with an FEI Titan (300 kV) equipped with a condenser Cs corrector and a high-angle annular dark-field (HAADF) detector
From the electron diffraction patterns (EDPs) acquired on untilted nanowires and indexed by the diamond structure, we could establish that the growth direction is generally h111i, quite often h112i and occasionally h110i

Summary

Introduction

Si nanowires (NWs) are low-dimensional objects with promising potential in the new, emerging semiconductor industries. Si NWs could constitute the interconnects and functional components of the future microelectronic industry (Cui & Lieber, 2001) They can be integrated into photovoltaic solar cells (Tian et al, 2007) or thermoelectric devices (Hochbaum et al, 2008; Boukai et al, 2008) to enhance power conversion efficiencies and have a role to play in the new energy industries. Polycrystalline Si thin films have been developed for more than 15 years for logic circuit and videographic applications (Brotherton et al, 1991), and for first- and second-generation photovoltaic solar cells (Aberle, 2006) Both Si NWs and Si thin films are developed at CEA– Grenoble for new energy applications.

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