Raman study of laser-induced heating effects in free-standing silicon nanocrystals.

Lihao Han,Miro Zeman,Arno H M Smets

doi:10.1039/c5nr00468c

Abstract

This paper demonstrates that free-standing silicon nanocrystals (Si NCs) have significantly different thermal conductivity properties compared to Si NCs embedded in a host matrix. The temperatures of Si NCs under laser illumination have been determined by measuring the ratio of the Anti-Stokes to Stokes intensities of the first order Si-Si transverse optical (TO) phonon mode. It is found that large free-standing Si NCs are easily heated up to ∼953 K by the laser light. The laser heating effects are reversible to a large extent, however the nature of the free-standing Si NCs is slightly modified after intensive illumination. The free-standing Si NCs can even be easily melted when exposed to a well-focused laser beam. Under these conditions, the blackbody radiation of the heated Si NCs starts to compete with the detected Raman signals. A simplified model of the heating effects is proposed to study the size dependence of the heated free-standing Si NCs with increasing laser power. It is concluded that the huge red-shift of the Si-Si TO mode observed under intensive laser illumination originates from laser-induced heating effects. In contrast, under similar illumination conditions Si NCs embedded in matrixes are hardly heated due to better thermal conductivity.

Highlights

Nanocrystals (NCs), known as quantum dots (QDs), exhibit unique physical, mechanical and electrical properties since their excitons are confined in all three spatial dimensions.[1,2] NCs made of a variety of direct and indirect semiconductor materials have promising applications in the novel design of light emitting diodes (LEDs),[3] batteries,[4] solar cells[5,6] and water splitting devices.[7]
Three types of samples of silicon nanocrystals (Si NCs) embedded in a host material have been studied as well: Si NCs in a hydrogenated amorphous silicon carbide (a-SiC:H) matrix; Si NCs in a hydrogenated amorphous silicon (a-Si:H) matrix, generally referred to as hydrogenated nanocrystalline silicon; and Si NCs in a hydrogenated silicon oxide (a-SiOx:H), often referred to as nanocrystalline silicon oxide
Under mild laser illumination conditions, the shift of the firstorder Si–Si mode peak of the small Si NCs (2 nm < d < 10 nm) in the Raman spectrum is lower by a few wavenumbers than the shift observed for the bulk c-Si ω0 = 520.5 cm−1, as Nanoscale observed by many others.[5,23,37,38,39]

Summary

Introduction

Nanocrystals (NCs), known as quantum dots (QDs), exhibit unique physical, mechanical and electrical properties since their excitons are confined in all three spatial dimensions.[1,2] NCs made of a variety of direct and indirect semiconductor materials have promising applications in the novel design of light emitting diodes (LEDs),[3] batteries,[4] solar cells[5,6] and water splitting devices.[7] For example, NCs might open routes to new photovoltaic (PV) concepts conquering the Shockley–Queisser limit[8] of single-junction solar cell devices, using mechanisms like multiple exciton generation (MEG)[9] and down conversion by space-separated quantum cutting (SSQC).[10] In this contribution we focus on NCs based on silicon (Si), which is the most dominant material in the semiconductor industry due to its abundance, relatively low-cost processing and resistance against water.[5,11,12,13,14,15] These Si NCs can be either free-standing or embedded in a host matrix, such as amorphous silicon carbide (a-SiC:H), amorphous silicon (a-Si:H) and amorphous silicon oxide (a-SiOx:H).

Methods

Results

Conclusion