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