Elsevier

Materials & Design

Volume 101, 5 July 2016, Pages 152-159
Materials & Design

Morphology dependent PL quenching of multi-zone nanoporous silicon: Size variant silicon nanocrystallites on a single chip

https://doi.org/10.1016/j.matdes.2016.03.161Get rights and content

Highlights

  • Fabrication of multi-zone structured nano-porous silicon on a single chip is presented.

  • Combinatorial effect of laser beam and spiral platinum electrode is observed.

  • Each zone in spiral structure has a unique and distinct morphology.

  • Sensing study enables preparation of optimized nano-porous silicon for desirable applications.

Abstract

Fabrication and optical sensing studies of single plane multi-zone nano-porous silicon (n-PS) prepared using laser-induced electrochemical etching technique is presented. Single plane multi-zone pertains to lateral formation of zones in silicon bearing step pore- and nano-crystallites size on the same plane. Different morphologies assembled in the form of a spiral shaped pattern on n-PS. Such novel pattern was a consequence of coupling between laser illumination and spiral shaped counter electrode used during silicon etching. The morphology analysis was done by recording profiles of characteristic LO phonon mode of silicon Raman spectra. The asymmetry and Raman peak shift helped in discerning the multi-zone morphology and it was complimented by Scanning Electron Microscopy (SEM) and photoluminescence (PL) studies. A wide range alcohol concentration (10–200 ppm) was tested using PL quenching technique. A direct correlation between morphology of n-PS and ethanol sensitivity was ascertained. Maximum sensing response of 40% was achieved at the centre and it decreased towards the periphery of the spiral pattern. The present simple and low cost fabrication technique allows fabrication of multiple morphologies on a single chip in one step and enables preparation of optimized n-PS for desirable applications.

Introduction

The unique features of nano-porous silicon (n-PS) are known through its encompassing assortment of applications in field of sensors, optical devices, waveguides, solid state electronics, bio-applications etc., where pore morphology plays an important role in deciding its specificity [1], [2]. Since the discovery of luminescence in n-PS by L.T. Canham [3] a large number of publications, investigating the formation mechanism of pores ranging from nano- to macro scale have appeared in the literature. In case of wet etching, n-PS is electrochemically etched from bulk crystalline silicon in a mixture of HF and ethanol while the control of physical parameters of the films, in particular porosity and thickness, is achieved by adjustment and optimization of the electrochemical parameters (HF concentration, current density, and time) [2], [4], [5], [6], [7]. The electrochemical process for formation of n-PS is reproducible, fast, and inexpensive.

The illumination of the samples during or after the etching process is an optional etching parameter and can be utilized for tuning the size of pores and silicon needles in the nano-porous structure [6]. In order to obtain quantum sized structures even on doped silicon substrates, an additional pore enlargement step is generally performed [8]. Unfortunately, for increasing the pore size/density by common methods such as chemical dissolution, n-PS suffers a serious setback, i.e. skeleton impairment and the PL disappearance [1]. Authors have made an attempt to utilize illumination parameter in designing the morphology of n-PS, without causing any damage to the n-PS surface. The technological applications of n-PS demand a serious thought on engineering the pore morphology, in particular, creating zones of different pore sizes on a single chip in a single attempt to commensurate with the requirements of specific applications such as sensors, display panels or integrated circuits with optoelectronic devices on board. No matter what the procedure is, the methodology for obtaining n-PS surface bearing a single pore size on an entire chip is fully established [9], but no report has been found so far that describes creation of step variant pore- and nano-crystallite sizes on a single chip.

n-PS has been notably used as chemical and biosensor [7], [10]. Sensing activities have been linked to its morphology [2], chemical structure [6] and functionalization [10], [11]. PL quenching is one of the most sensitive techniques for detection of chemical species, and the sensing mechanism is based on generation of non-radiative centres when chemical analytes interact with n-PS surface [6], [12], [13], [14], [15].

In this study, authors report the formation of single-plane multi-zone n-PS structure and its application as chemical sensor. Step pore- and silicon nano-crystallite sizes are observed on a single chip due to the combinatorial effect of laser beam and the electric displacement field produced by spiral shaped platinum electrode. This sample was exposed to ethanol in the range of 10–200 ppm. High sensitivity was observed at the centre of spiral pattern formed on the n-PS surface and least near to its periphery. The sensing response was linked to morphological distribution on n-PS surface. The detailed analysis of the surface morphology was done by Raman, PL (Photoluminescence) and SEM (Scanning Electron Microscopy) studies. For the past decades, SEM has been used as one of the useful tools to study the n-PS morphology. However, formation of step nano-crystallite size on the same n-PS sample, its study by Raman and PL spectroscopy and its sensing analysis has not been done so far. A special emphasis on Raman analysis by using confocal micro-Raman spectrometer fitted with mapping system and Phonon Confinement Model is being presented.

The proposed method demands attention of researchers for viewing the formation of variable morphology of n-PS using a novel method. From the point of having light source of multiple wavelengths or sensing different target species on a single chip this technique seems to be new, low cost, simple and flexible. Such morphologies pave the way for e-Nose applications also. Besides, there is enough scope to optimize the layer thickness, pore size and porosity by tuning its formation parameters and these are in pipeline of the authors' future work.

Section snippets

Material and methods

During electrochemical etching, four holes are required to dissolve one silicon atom in acidic solution for making nano-porous silicon (n-PS). In this technique, generation of holes is done by external DC current supply whereas in photo-induced electrochemical etching, an additional light illumination enhances e-h pair generation rates. Since p-type silicon has excess of holes, thus light illumination is not a necessity; however, in case of n-type silicon it is indispensable to supplement the

Morphology studies

Etching principle is based on oxidation and dissolution of target species and it can be done by various techniques [5], [16], [17], [18], [19], [20], [21], [22]. In our laser-induced electrochemical etching setup, in addition to both front- and back illumination employed a convex lens placed after front illumination laser which led to beam divergence so as to make it Gaussian. This Gaussian shape of the laser beam profile created a variable photon flux/photon density on the silicon surface.

Conclusion

Nano-porous silicon samples with a step variation in pore- and nano-crystallite size on a single plane of silicon wafer were fabricated using photo-electrochemical anodization technique where a specific design of the counter electrode is the key concern. The combined effect of field emanating from the laser beam and the design of counter electrode on silicon wafer was very pronounced and it resulted in formation of impression of counter (platinum) electrode along with

Acknowledgement

Authora,b gratefully acknowledges the financial support provided by the Department of Science & Technology, Govt. of India, through its INSPIRE Grant No. IFA-12 ENG-13.

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