Peculiarity of Porous Silicon Formed in the Transition Regime
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ABSTRACT Electrochemical anodization in the transition regime, between porous silicon formation region and electropolishing region, of monocrystalline silicon was investigated. Using this process bright and stable photoluminescence could be obtained on a very large range of substrate resistivities: p=12 -0.005 f2cm for p-type silicon and p = 20-0.001 f2cm for n-type substrates. Photoluminescence spectra, Fourier Transform IR (FTIR) absorbance and X-Ray Diffraction (XRD) measurements are reported. Investigations showed that anodic silicon suboxide was formed on the surface. The porous structure obtained in the transition regime is suggested to consist of silicon crystallites built inside an anodic oxide. INTRODUCTION Porous silicon (PS) is actually investigated for its exhibited strong visible photoluminescence (PL) [1] and electroluminescence [2,3,4], offering the possibility to develop all silicon based optoelectronic components. The PL phenomenon, observed under UV lamp or laser exposure of electrochemically etched silicon, was at first explained in terms of quantum effects, occurring in the nano-sized structures of PS formed during the anodization process [1]. The principal evidence supporting quantum model is the observed blue shift of PL spectra as the porosity increases, attributed to the size reduction. It is a fact that reaching the quantum confinement size in porous features leads to PL[1], but the increase in PL quantum efficiency can take its origin from a reduction of nonradiative processes by a PS surface passivation [5]. Surface effects involving different atomic structures, like hydride complexes [6], or Si:O:H compounds derived by siloxene [7] were reported. Other authors [8] suggested that oxidized silicon play an important role in the light emission phenomenon and in its stability. The first PL structures were fabricated on the base of p- type silicon. This because for PS formed on p- doped wafers, the sizes of pores and porous walls are in the range of 10-60 A for a porosity of 50% [1]. For p', n- and n' silicon the pore diameter and the interpore spacing are usually larger [9..11]. For this reason, to obtain luminescent PS on these substrates, an additional pore enlargement process is sometimes needed. Increasing pore dimensions by chemical etching [12] damages the needles structure, destroying it when porosity exceeds 80%, while the use of thermal [13] or anodic [5] oxidation allows a safer method to reduce silicon volumes. Zhang et al. [14] showed that silicon can be anodized in HF aqueous solutions in three different regimes: (i) porous silicon formation, (ii) electropolishing (no pores are formed on the silicon surface, which is covered by an homogeneous silicon oxide), and (iii) a transition regime from one to another. In the transition regime both pores and silicon oxide are formed
[14,15].
In this paper porous silicon obtained by electrochemical anodization in transition regime on a large range of substrate resistivities (even on degenerate silicon), showing bright and stable 357
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