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epartment of Physics, University of California,MaterialsSciences Division, Lawrence Berkeley Laboratory, Berkeley, CA 94720-7300, USA * Weizmann Institute of Science, Department of ChemicalPhysics, Rehovot, Israel 0 Technische UniversitdtMfinchen, Physik-DepartmentE16, D-85 747 Garching, Germany

ABSTRACT Fourier transform infrared spectroscopy is used to determine the time evolution of oxygen incorporation onto the surface of silicon nanocrystals. Oxygen concentrations up to one monolayer are investigated. The temporal progress of surface oxidation of Si nanocrystals in porous silicon shows a linear dependence on the square root of the oxidation time. This is similar to the oxidation of bulk Si and mesoporous silicon.

INTRODUCTION Porous silicon (PSi) is produced from bulk Si by electrochemical etching in a HF solution. The resulting material is an interconnected network of Si nanocrystals. The remaining silicon skeleton has a large 2 3 surface to volume ratio (from 300 to 1000 m /cm ). For samples with porosities of - 70 % a significant fraction of the atoms in the Si network are surface atoms. Immediately after the etching, the surface is terminated by Si-Hx bonds (x = 1,2,3). During prolonged storage under ambient conditions, oxygen is gradually incorporated into the samples. This slow oxidation process, known as ageing [1], has a pronounced effect on the luminescence properties [1]. Therefore, in order to obtain samples with well defined optical properties, it is desirable to control the level of oxidation. Rapid thermal oxidation has been previously applied to PSi samples, with good results regarding the luminescence properties. However, the fast oxidation at high temperatures results in damage to the brittle Si skeleton. Heating of PSi at moderate temperatures (below -230'C) accelerates the oxidation process, stabilizes the Si structure, and allows better control of the amount of incorporated oxygen. In this paper we use Fourier transform infrared spectroscopy (FTIR), which has previously been applied to investigate the chemical composition of Psi samples [2,3], as a tool to study the oxidation process of PSi layers at moderate temperatures.

SAMPLES AND ANALYSIS TECHNIQUE The samples used in this work are prepared from (100) p-type, B-doped, 1 O2cm Si wafers. Before anodization an ohmic back contact is provided by B implantation. Anodization is carried out in a 1:1 vol. 2 mixture of ethanol:HF (49%wt. in water). The etching current density is 30 mA/cm . Typical sample from the cell, samples are removed anodization the of 2 ptm. Following thicknesses are in the range washed with propanol and left to dry under ambient conditions. The freshly prepared samples show very weak luminescence. Thereafter the specimens are heated at 200 OC in ambient conditions for various time intervals. The IR spectra are taken in the range of 400 cm" to 4000 cml using a Bomem FTIR spectrometer. The IR light illuminates the sample in normal incidence. The spectra are normalized to the transmission of an unetched part of the Si wafer