Formation of Porous Silicon

In the exponential region where PS forms, the slope of the logarithmic current density versus potential plot, i.e., the Tafel plot, is typically about 60mV/decade for p type and heavily doped n types of silicon samples as shown in Table 5.3. 

The typical characteristics of the i-V curves of p-Si of different doping levels and orientations are essentially identical except for a slightly higher current density of the 

FIGURE 8.1. Successive i-V curves made on the same p-Si sample in 1% HF with a potential sweep rate of 2 mV/s. After Zhang et al (Reproduced by permission of The Electrochemical Society, Inc.) 

The i-V curve of n-Si under a high illumination intensity, when the reaction is no longer limited by the availability of photogenerated carriers, is identical to that for p-Si except for a shift along the potential axis. As for p-Si, formation of PS on n-Si occurs only below the critical current, Ji. The /-Vrelationship at a current density much lower than the saturation photocurrent density is similar to that observed onp-Si. When the saturation photocurrent density is lower than the critical current density, the current is limited by the saturation photocurrent. 

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A detailed and comprehensive review on all aspects of the fundamental and applied electrochemistry of silicon/electrolyte interface was provided in a recently published book.1 The objective of this paper is to provide a conceptual analysis of the mechanisms for the morphology and formation of porous silicon using the large body of the information assembled in the book and to provide an integrated view of the formation mechanisms that can be coherent with the various morphological features on the... 

Formation of porous silicon is an anodic dissolution process, which consists of carrier transport in the semiconductor, electrochemical reactions at the interface, and mass transport of the reactants and reaction products in the electrolyte. There are a... 

The electrochemistry of silicon is highly important as a tool for surface treatment and the formation of porous silicon. Under the reverse bias (anodic for n-type, cathodic for p-type) of silicon immersed in an electrolyte, a space charge layer is formed near the electrode surface, in which the concentration of the charge carriers differs from that of the bulk material. The width of this space charge layer depends on the type and density of dopants in the material and the potential bias. 

Silicon exhibits a diverse range of electrochemical phenomena, such as current oscillation, anisotropic etching, formation of porous silicon, etc. Each of these phenomena has extremely rich details that are governed by complex relationships between structures and properties of silicon electrodes on the one hand and between properties and experimental conditions on the other. The silicon/electrolyte interface is a complex system in which a great many variables are interacting with each other in a great many ways." ... 

The complexity of the system implies that many phenomena are not directly explainable by the basic theories of semiconductor electrochemistry. The basic theories are developed for idealized situations, but the electrode behavior of a specific system is almost always deviated from the idealized situations in many different ways. Also, the complex details of each phenomenon are associated with all the processes at the silicon/electrolyte interface from a macro scale to the atomic scale such that the rich details are lost when simplifications are made in developing theories. Additionally, most theories are developed based on the data that are from a limited domain in the multidimensional space of numerous variables. As a result, in general such theories are valid only within this domain of the variable space but are inconsistent with the data outside this domain. In fact, the specific theories developed by different research groups on the various phenomena of silicon electrodes are often inconsistent with each other. In this respect, this book had the opportunity to have the space and scope to assemble the data and to review the discrete theories in a global perspective. In a number of cases, this exercise resulted in more complete physical schemes for the mechanisms of the electrode phenomena, such as current oscillation, growth of anodic oxide, anisotropic etching, and formation of porous silicon. 

Another problem in application of the basic theories is associated with surface geometry. Most theories are developed to describe the relationships among the area-averaged quantities such as charge density, current density, and potentials assuming a uniform electrode surface. In fact, the silicon surface may not be uniform at the micrometer, nanometer, or atomic scales. There can be great variations in the distribution of reactions from extremely uniform, for example, in electropolishing, to extremely nonuniform, for example, in the formation of porous silicon. 

R. C. Frye, The formation of porous silicon and its applications to dielectric isolation, Mater. Res. Soc. Symp. Proc. 33, 53, 1984. 

A. M. Thonisson, M. G. Berger, R. Arens-Fisher, O. Gliick, M. Kruger, and H. Liith, Illumination-assisted formation of porous silicon. Thin Solid Films 276, 21, 19%. 

Fig. 4 [7, 8], At anodic potentials near J the electrode behavior is characterized by an exponential dependence of current on potential and by the uneven dissolution of silicon surface leading to the formation of porous silicon (PS) [9]. The values of the characteristic currents J to J4 are a function of electrolyte composition but are largely independent of doping. At potentials more positive than the second plateau current J4, current oscillation may occur [8]. The surface resulting from dissolution at potentials higher than the second peak is brightly smooth, while that produced between the first and second peak is relatively less smooth [10].

FTIR has been used to study the sihcon/electrolyte interface. The formation of porous silicon on n-Si during photoetching in fluoride media can readily be followed since the hydrogen-terminated surface is identified by the Si-H stretch bands centred around 2100 cm (Peter et ah, 1989 Peter et ah, 1990a). Similarly, the transition from a hydrogen-terminated to an oxide-covered surface during electropolishing has been followed by in-situ infrared spectroscopy (da Fonseca et al, 1996 and 1997). 

Punzon-Quijoma E, Torres-Costa V, Manso-Silvan M, Martin-Palma RJ, Climent-Font A (2012) MeV Si ion beam implantation as an effective patterning tool for the localized formation of porous silicon. Nucl Instrum Methods Phys Res 282 25-28 Ruano GD, Ferron J, Koropecki RR (2009a) Reversible ion induced modification of consequent secondary electron emission in porous silicon. Open Surf Sci J 1 46 Ruano GD, Ferron J, Koropecki RR (2009b) Secondary electron emission in nanostmctured porous silicon. J Phys Conf Ser 167 012006... 

The formation of porous silicon by metal-assisted chemical etch (MACE) was discovered in 1997 when patterned aluminum on silicon rapidly induced selective formation of porous silicon layers in stain-etch conditions (Dimova-Malinovska et al. 1997). 

Although silicon nanocrystals are now more commonly prepared by a variety of means which are easier to scale up, e.g., pyrolysis of silanes (Xuegeng et al. 2004), thermal treatment of silsesquioxanes (Hessel et al. 2006, 2010), and from reactions of molecular silicon compounds (Wilcoxon et al. 1999 Bley and Kauzlarich 1996), this review will concentrate on routes which proceed via the formation of porous silicon. More general reviews of silicon nanocrystals Irom physics and chemistry perspectives are available (Shirahata 2011 Kang et al. 2011 Heitmann et al. 2005). Derivatization of porous silicon and SiNCs usually relies on the chemistry of the hydrogen-terminated silicon surface, which shares some of the organic reactivity of hydrosilanes (Buriak 2002). Reaction with alcohols results in Si-O-C bonded monolayers (Sweryda-Krawiec et al. 1999), but these are suseeptible to hydrolysis under ambient conditions. Alternately, addition of surface Si-H aeross a C = C double bond produces Si-C bonded monolayers, which are very stable. 

Ogata YH, Koyama A, Harraz FA, Salem MS, Sakka T (2007) Electrochemical formation of porous silicon with medium sized-pores. Electrochemistry 75 270-272 Raman NK, Anderson MT, Brinker CJ (1996) Template-based approaches to the preparation of amorphous, nanoporous silicas. Chem Mater 8(8) 1682-1701 Rumpf K, Granitzer P, Poelt P, Allbu M (2011) Double-sided porous silicon template for metal deposition. Phys Stat Sol (c) 8(6) 1808-1811... 

There are two types of technology of formation of porous silicon layer on silicon solar cells (1) the thin porous silicon is formed in the final step on the surface of ready Si solar cell with metal contacts and (2) the relatively thick porous silicon layer is fomied prior to emitter diffusion and metal contact deposition. In the first case, which is more applied, the thickness of porous layer (70-150 nm) must be less than the depth of n -p (or p -n)-junction (300-800 nm), and the duration of electrochemical etching is short (about 5-15 s). 

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