Silicon–Rich Silicon Oxide Thin Films Fabricated by Electro-Chemical Method

Abstract

Porous silicon (PS) has attracted increasing research interest in basic physics as well as applications since 1990 when Canham reported on the efficient visible photoluminescence (PL) of porous silicon (Canham, 1990). Structurally, PS consists of many pores and silicon residuals and usually can be described as a homogeneous mixture of silicon, air and, even silicon dioxide. Based on porosity, PS can be classified into three types: nano, mesoand macro-pores. In the case of PS nano-pores, the size of both the silicon residuals and the air voids (pores) can be in the range of few nanometers. The exciton Bohr radius in Si is around 4.3 nm, so that quantum confinement can occur and change the electronic structure of those silicon nanocrystals. On the other hand, because the value of porosity is directly linked to the effective index of refraction of the PS layer, this layer appears as an effective medium, where the refractive index has a tunable value between the index of refraction of bulk Si and that of the air (pores). Those changes in the electronic structure and refractive index of PS when compared with bulk Si make it fascinating as both a low-dimensional material and an optical one. The considerable and controllable changes in the electronic structure and refractive index of PS fabricated by electrochemical anodization make it a promising material for photonics in comparison with bulk silicon and/ or pure silica. Using the oxidation process in O2 environment at high temperature, the PS samples become siliconrich silicon oxides (SRSO), which has high chemical instability and avoids the aging of the PS that is important condition for optical devices such as planar optical waveguides, optical interference filters, micro-cavities, etc (Bettotti et al., 2002). During the last decade, Erbium (Er)-doped silicon-rich silicon oxide has attracted much interest due to its big potential application in Si-based optoelectronic devices for telecom and optical sensors. The Er-ions implanted in SRSO materials produce light emission at around wavelength range of 1540 nm, which corresponds to minimum light absorption in silica-based glass fibers. In this regard, a lot of studies have been carried out to improve the luminescence efficiency of this material. Such studies have revealed that co-implantation of Er and O2 induce a strong enhancement in the Er-ions related emission at range of 1540 nm. In first case, samples were prepared by co-implanting Si and Er into silica thin films or co-sputtering Si, Er2O3 and SiO2 on the silicon substrate (Shin et al., 1995). In second case, samples were prepared by implanting Er-ions into SiO2 films containing Si-nanocrystals (nc-Si) and/or by Er-ion electrochemical deposition on silicon-rich oxide (SRSO) layers. The room temperature luminescence emission at the range of 1540 nm from Er-electrochemically doped porous