Raman spectromicroscopic study to understand microscopic level physics in low dimensional semiconductor

Abstract

Any technological advancement requires a sound knowledge of physical process and phenomena taking place in a material that is used in a particular technological evolution. Quantum size (or confinement) effect1 (QCE) is one of the main phenomena that enabled the scientific revolution to fabricate novel materials including Silicon nanostructures (SiNSs) which became the backbone of recent device development. Due to QCE, a material starts showing new properties which depend strongly on size in such a way that it becomes more prominent with decrease in size and are thus interesting to analyse. Various methods of fabricating SiNSs include electrodeposition, laser induced etching, chemical vapor deposition and many others, out of which metal assisted etching (MACE) is one of the most economical and simplest techniques. The SiNSs fabricated using MACE have interesting perturbations in their vibrionic, electronic and optical properties, due to the effect of QC. Along with, in heavily doped SiNSs, the matching of energies of electronic continuum with discrete phonon, gives rise to electron-phonon interaction, also called as Fano resonance. Raman spectroscopy, being a sensitive and non-destructive tool, can detect the subtle scale phenomena and is thus indulges in the analysis of such systems. The perturbations in SiNSs are manifested in terms of various Raman line-shapes, when analysed through theoretical Raman line-shape models, yields interesting interplays of two physical phenomena for example QCE & Fano resonance. This research work comprises of study to establish absorption spectroscopy as an easier and efficient technique for estimation of mean size and size distribution (SD) in SiNSs. This has been done by analysing a diffuse reflectance spectral line-shape by means of the proposed line-shape function developed by incorporating the size dependence of band gap. The model has been used for consolidate size analysis of two SiNWs samples prepared by metal induced etching technique. The estimated size and SD has been compared with the results obtained using Raman spectroscopy, a well-established technique for this purpose The research work reported here deals with the analysis of perturbed Raman line shape obtained from heavily doped p- & n type SiNSs. Cross sectional Raman mapping which has been done to understand the role of MACE in perturbing the size at microscopic level, reveals an interplay of size and two distinct physical phenomena namely, QCE & Fano resonance, with each other. The nature of complexities that arise in nano-crystalline via several ways like preparation methods can be easily assessed using Raman spectro-microscopy due to non-destructive assessment of sample along with requirement of only small amounts. To vividly understand the mechanism of etching that leads to formation of nano-crystalline Si, a cross-sectional Raman mapping has been done for the first time, that validates the time dependent etching mechanism in SiNSs. The interplay time dependent stay of different portions of wafer causes has been found to cause a inhomogeneity in SiNSs size leading to a presence of smaller sizes at the tip of the NSs as compared to the vicinity of the substrate. Presence of structural inhomogeneity and physical phenomena therein taking place at microscopic level is very difficult to identify simultaneously using a holistic technique. Raman microscopy has been developed here and established for this purpose and shown to successfully work on n- and p-type silicon nanowires, a well known system otherwise, prepared using chemical technique. A Raman microscopic image) not only shows the presence of inhomogeneity in the nanocrystallites’ size but also quantifies the size and its effect on microscopic quantum phenomena. Raman image has been shown to be a good blend of microscopic and spectroscopic technique. Another observation is the reduced doping in case of heavily doped p- and n-type nanostructures near the tip of the nanowire due to zonal migration of dopant atom (boron and phosphorus in p- & n- type respectively) and hence causing change in the Fermi level position. The size dependent electron-phonon interaction, which manifest itself in terms of asymmetrical Raman line shape has been analysed using cross-sectional Raman mapping. Perturbations in optoelectronic properties like band gap enhancement and PL from SiNSs have also been analysed by suitable theoretical modelling.