ISSN 1063-7826, Semiconductors, 2007, Vol. 41, No. 8, pp. 953–957. © Pleiades Publishing, Ltd., 2007. Original Russian Text © A.S. Vorontsov, L.A. Osminkina, A.E. Tkachenko, E.A. Konstantinova, V.G. Elenskiœ, V.Yu. Timoshenko, P.K. Kashkarov, 2007, published in Fizika i Tekh- nika Poluprovodnikov, 2007, Vol. 41, No. 8, pp. 972–976. 953 1. INTRODUCTION Porous silicon (PS) produced by electrochemical etching of silicon single-crystal (c-Si) wafers in hydrof- luoric-acid-based solutions under certain conditions of formation presents ensembles of interconnected silicon nanocrystals [1]. Depending on the pore size, PS can be subdivided into three groups: microporous (≤2 nm), fine-porous (2–50 nm), and macroporous (≥50 nm) sil- icon [2]. The mesoporous silicon layers, as a rule, are characterized by sizes of the constituent silicon nanoc- rystals (nc-Si) on the order of 10–50 nm and by the developed specific surface that can be as large as 200 m 2 cm –3 [3]. In nc-Si in mesoporous silicon layers, mobile (free or quasi-free) equilibrium charge carriers can exist, with the concentration commonly lower than that in the wafer from which the layer was produced [4]. It has been established that adsorption of chemi- cally active molecules (NO 2 , C 5 H 5 N, NH 3 , etc.) induces a substantial change in the concentration of charge car- riers [5–8] and in the electrical conductivity [9, 10] of mesoporous silicon layers. A similar effect can be expected also upon the adsorption of iodine molecules (I 2 ) that exhibit the properties of acceptors. This offers a simple way of controlling the concentration of free charge carriers in the nc-Si ensembles. It is known that the interaction of I 2 molecules with the silicon surface gives rise to anion radicals [11, 12]. This process is I 2 – not accompanied by the formation of defects of silicon dangling bonds [13] that are the trapping centers for free charge carriers and, in turn, can limit the concen- tration of charge carriers [14]. In this context, the pur- pose of this study is to examine the effect of adsorption of I 2 molecules on the concentration of free charge car- riers and defects in mesoporous silicon by infrared (IR) and electron spin resonance (ESR) spectroscopy. 2. EXPERIMENTAL In this study, we examined the PS samples formed on the p-type c-Si:B (100) wafers with the concentra- tion of dopants 2 × 10 19 cm –3 (samples 1, mesoporous silicon) and 1 × 10 16 cm –3 (samples 2, microporous sil- icon). In addition, we studied the PS samples formed on the n-type c-Si:As (100) wafers with the concentration of dopants 2 × 10 19 cm –3 (samples 3, mesoporous sili- con). To etch the samples, we used the solution of hydrofluoric acid and ethanol, HF (48%) : C 2 H 5 OH, in the 1 : 1 ratio. The current density and the duration of etching were 50 mA cm –2 and 20 min, respectively. The PS films were exfoliated from the wafer by increasing the current up to 500 mA cm –2 for a short time. The thickness of the samples was measured with the use of an optical microscope. The thicknesses of samples 1, 2, and 3 were 50, 56, and 42 μm, respectively. The poros- ity of the samples was determined by the gravimetric technique to be at the levels of about 60% for samples 1 Modification of the Properties of Porous Silicon on Adsorption of Iodine Molecules A. S. Vorontsov^, L. A. Osminkina, A. E. Tkachenko, E. A. Konstantinova, V. G. Elenskiœ, V. Yu. Timoshenko, and P. K. Kashkarov Moscow State University (Physics Department), Moscow, 119992 Russia ^e-mail: vorontsov@vega.phys.msu.ru Submitted October 23, 2006; accepted for publication November 3, 2006 Abstract—Infrared spectroscopy and electron spin resonance measurements are used to study the properties of porous silicon layers on adsorption of the I 2 iodine molecules. The layers are formed on the p- an n-Si single- crystal wafers. It is established that, in the atmosphere of I 2 molecules, the charge-carrier concentration in the layers produced on the p-type wafers can be noticeably increased: the concentration of holes can attain values on the order of ~10 18 –10 19 cm –3 . In porous silicon layers formed on the n-type wafers, the adsorption-induced inversion of the type of charge carriers and the partial substitution of silicon–hydrogen bonds by silicon–iodine bonds are observed. A decrease in the concentration of surface paramagnetic defects, P b centers, is observed in the samples with adsorbed iodine. The experimental data are interpreted in the context of the model in which it is assumed that both deep and shallow acceptor states are formed at the surface of silicon nanocrystals upon the adsorption of I 2 molecules. PACS: 68.43.-h, 78.30.-j, 78.55.Mb DOI: 10.1134/S1063782607080167 AMORPHOUS, VITREOUS, POROUS, ORGANIC, AND MICROCRYSTALLINE SEMICONDUCTORS; SEMICONDUCTOR COMPOSITES