n型多孔硅刻蚀时间的效应

Porous silicon layers have been prepared from n-type silicon wafers of (100) orientation. SEM, FTIR and PL have been  used to characterize the morphological and optical properties of porous silicon. The inflfluence of varying etching time in  the anodizing solution, on structural and optical properties of porous silicon has been investigated. It is observed that  pore size increases with etching time and attain maximum for 20 minutes and then decreases. The PL spectrum peak shifts  towards the higher energy side, which supports the quantum confifinement effect in porous silicon. The FTIR shows that  the Si-Hn peaks are observed at the surface of the PS layer and these chemical species also give raise the PL in PS.

Silicon has historically been the dominant material  for electronics, while work in optoelectronics has  relied almost entirely on III – V compound materials  such as GaAs and InP. The primary reason for this  dichotomy of materials systems has been the tenet  that light emission from silicon is impractical  because of its indirect band gap structure [1].  However the observation of room temperature  visible photoluminescence1 (PL) in porous silicon  has demonstrated the potential for practical,  effificient silicon based emitters for optoelectronic  applications [2,3]. As a result the porous silicon  (PS) layers fabricated by electrochemical etching,  followed by pore widening treatment to decrease  the pore wall dimension, have become subjects for  intensive investigation [1,4]. Porous Silicon can  exhibit a large variety of morphologies and particle  size. It has been reported that the luminescence  of the PS is ascribed to the quantum confifinement  effect (QCE) as well as to the presence of Si-Hn bond near the surface of nano crystallites. However  the exact luminescence mechanisms and relevant  nanostructures still remain unclear [5]. So intensive  studies on PS continues.

By adjusting the formation parameters, porous  silicon with large range of porosities and  morphologies can be produced, which can be tuned  for several applications. In all PS applications,  information about the pore size and their distribution  and surface chemistry and their dependence on the  fabrication conditions plays a decisive role [6].  Correlation of observed physical properties with  the morphology of PS fifilms and the relationship  between PS morphology and preparation parameters  is necessary. Principal parameters controlling macro  pore formation depend on the properties of silicon  substrate (crystal orientation, doping), anodizing  solution and temperature [6]. For a given substrate  at a fifixed temperature, the pore formation will be  determined by the properties of anodizing solution  (HF concentration, current density, etching time and  bath illumination condition). In the present work, an attempt has been made to study the relationship  linking the morphology, optical properties and  fabrication conditions of porous silicon formed  using n-type material.

Porous Silicon samples were prepared from n-type  Silicon wafers (100) orientation with a resistivity of  50 Ω cm. The samples have been prepared by keeping  the HF: ethanol ratio and current density constant  at 1:2 and 30 mA/cm2 respectively and varying  the etching time and during anodization the wafer  was illuminated by a halogen lamp of 300 W. After  anodization the porous silicon samples were rinsed  in ethanol. The etching time used for the anodizing  process has a strong inflfluence on the porous silicon  properties [7]. So the etching time is to be varied  to study its inflfluence on the PS characteristics. The  experimental data on the relationship between the  etching time and the morphological and the optical  properties are discussed in the following sections.

Fig1

Thus the study shows that the pore size and hence  the porosity of PS depends on the etching time i.e.,  the etching time is a parameter controlling the pore size hence the porosity of the PS. It can be further  noted that even when the etching time is increased  by 10 minutes beyond the etching time of 20 minutes  which gives the maximum pore size, some of the  pore walls alone break, exposing a small portion of  the next layer; a further increase of 10 minutes in  etching time (totally 40 minutes) is needed to break  most of the thin pore walls and expose most part of  the next layer. This can be interpreted to mean that  the thin pore walls are strong. Such a conclusion is  in excellent agreement with the report of Lehmann  and Gosele [3] on the strength of the thin pore  walls.

The photoluminescence (PL) spectra of the n-type  porous silicon formed using different etching times  of 10, 20, 30 and 40 minutes are displayed in fifigure  2. It can be seen that room temperature PL in the  visible region has been obtained for all the etching  times used. The PL intensity maximum has been  found to occur at 624 nm and 619 nm for the etching  times of 10 and 20 minutes respectively, indicating  that there is a blue shift of the PL maximum with  etching time.