Hidden defects in silicon nanowires

Abstract

Recent publications have reported the presence of hexagonal phases in Si nanowires. Most of these reports were based on 'odd' diffraction patterns and HRTEM images , -'odd' means that these images and diffraction patterns could not be obtained on perfect silicon crystals in the classical diamond cubic structure. We analyze the origin of these 'odd' patterns and images by studying the case of various Si nanowires grown using either Ni or Au as catalyst in combination with P or Al doping. Two models could explain the experimental results : (i) the presence of an hexagonal phase or (ii) the presence of defects that we call 'hidden' defects because they cannot be directly observed on most images. We show that in many cases one direction of observation is not sufficient to distinguish between the two models. Several directions of observations have to be used. Secondly, conventional TEM images i.e. bright field two beam and dark field images, are of great value in the identification of 'hidden' defects. In addition, slices of nanowires perperpendicular to the growth axis can be very useful. In the studied nanowires no hexagonal phase with long range order is found and the 'odd' images and diffraction patterns are mostly due to planar defects causing superposition of different crystal grains. Finally, we show that in Raman experiments the defect rich NWs can give rise to a Raman peak shifted to 504-511 cm −1 with respect to the Si bulk peak at 520 cm −1 , indicating that Raman cannot be used to identify an hexagonal phase.

Visualising discrete structural transformations in germanium nanowires during ion beam irradiation and subsequent annealing

“…At the regions where these defects overlap a complex pattern is formed (marked with dashed boxes), that can be understood as high resolution Moiré patterns as suggested previously for Si nanowires. 35 Interestingly, the majority of the newly formed (111) stacking faults formed an ordered arrangement, similar to stacking faults identified in grown nanowires previously. 26,36 The crystallisation nucleated from separate crystal islands.…”

Visible and Infra-red Light Emission in Boron-Doped Wurtzite Silicon Nanowires

“…We are aware that this approach could result in overestimation of the doping concentration. In addition, the attribution of the optical phonons peak wavenumber is not straightforward because the wavenumber positions of the optical phonons peaks of wurtzite silicon are still an open issue1326272829.…”

Nanoindentation of Silicon and Germanium

“…Indeed, Pirouz et al (1990) also reported the formation of hd-Si during microindentation in the temperature range of 450-650°C and suggested that it resulted from the twinning of dc-Si. However, the presence of hd-Si in the twin bands is still under debate (Cayron et al, 2009;Hertog et al, 2012). There is further discrepancy in the early literature with Suzuki and Ohmura (1996) reporting that phase transformation persisted up to 500°C (with Vickers indentation), but beyond that temperature plastic deformation via dislocation activity (dislocation glide) occurred.…”