Nanocrystalline porous silicon layers are prepared from p-doped Si(001) wafers by electrochemical etching in HF-ethanol solutions under constant current density, followed by in situ photochemical etching by using a Xe lamp. Thus, the samples are stabilized by either native or anodic oxidation. Transmission electron microscopy (TEM) revealed a double scale macroporous (in the range of 1.5-3 microns) and nanoporous structure, the latter consisting in networks of nanowires of about 5 nm diameter with several microns length. The nanowires are distributed orthogonal to the [001] axis, which is also the main symmetry axis of the macropores. Photoconduction (PC) measurements were interpreted in a quantum confinement model: electrons are trapped inside the nanowires, whereas the observed PC maxima are attributed to transitions between the resulting discrete energy levels [1]. However, for the accurate knowledge of electron confinement and phototransport parameters, the knowledge of the width of the native silica layer is crucial, for both precise determination of the true silicon nanowire thickness, where electrons are confined, with knowledge of the parameters of the confinement potential barrier, which are needed in a refinement of the model in study now. This is determined in the present work by X-ray photoelectron spectroscopy (XPS) measurements recorded after several steps of Ar+ sputtering of the samples, coupled with measurements recorded in the same conditions on flat native oxide formed on Si(001). The evolution of the Si 2p peaks corresponding to pure and oxidized silicon in the electron distribution curves (EDC) with the sputtering dose (Ar ions/cm2) are modelized by taking into account: (i) the average pore width; (ii) the average oxide thickness layer; (iii) the geometric parameters of the sputtering process. This allows the determination of the nanopore width, together with an estimate of the thickness of silica layers formed inside the nanopores. The value obtained for the nanopore width is in line with TEM observations and with the value derived from PC mesurements in the quantum confinement model, whereas the thickness of silica walls range from 1 to 1.5 nm.
[1] M.L. Ciurea, V. Iancu and I. Stavarache, J. Optoel. Adv. Mater. 8, 2156 (2006). |