Effect of Surface Treatments in Nanocrystalline Silicon
- PDF / 215,680 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 58 Downloads / 209 Views
F3.51.1
Effect of Surface Treatments in Nanocrystalline Silicon
N. P. Mandal, S. Dey and S. C. Agarwal Department of Physics, Indian Institute of Technology Kanpur, 208016, India
ABSTRACT Exposure to ammonia (NH3) increases the dark current (DC) in porous silicon (PS), but evaporated selenium (Se) deposited on PS decreases DC. Photoluminescence (PL) measurement shows that there are two types of centers. PL in one region of PS (Peak ~ 800nm) initially increases with the NH3 exposure and then decreases. But the PL from another region of PS has a peak at ~ 780nm and it decreases continuously with the NH3 exposure. Dipping PS in water and drying in air shifts the PL peak at 800 nm to 744 nm. Atomic Force Microscopy (AFM) shows that the as prepared sample has wires of diameters 2.4nm, 3.4nm, 4.6nm and bigger. However, in the AFM images of the water treated sample the wires of diameters 3.4nm and 4.6nm are absent. The PL results are explained using the AFM data and the John-Singh model of quantum confinement.
INTRODUCTION Porous Silicon (PS) is a form of nanocrystalline silicon, and shows photoluminescence (PL) in visible at room temperature [1]. This is caused by the quantum confinement of the charge carriers [2] in the nanometer size Si crystallites. Koch et al [3] emphasized the existence of electronic surface states; and suggested that the dominant PL emission involves these surface states [3]. The large surface to volume ratio of PS plays an important role, as the surface provides the competing defect centers. This opens the possibility of application of the PS as gas sensors [4-8]. Fukuda et al [9] reported that PL was quenched after dipping PS in NH3. Hou et al. [10] found a large blue shift of the PL after boiling water treatment. Dittrich et al. [11] reported that the PL was enhanced and blue shifted after water treatment. These indicate that surface plays an important role in PS. There are several studies on changes on dark current (DC) upon exposure to various gases, e.g., NO2, water vapor, methanol vapor etc. [4,12,13]. For example, Boarino et al. [12] find an increase in DC after exposure to NO2, which they attributed to an activation of PS dopants. We report here the effect of NH3 on free standing PS. Exposure to NH3 increases DC, whereas a thin film of selenium deposited on PS decreases it. Photoluminescence has one peak whose position depends on the area of PS being probed. We identified two peaks from two different regions of the sample; these behave differently upon exposure to ammonia. PL at ~800 nm initially increases upon NH3 exposure, but then decreases. In contrast, the PL at 780 nm continuously decreases with the NH3 exposure. All changes in DC and PL after NH3 exposure can be erased by annealing at 1500 C (1hr). A blue shift of the PL peak is observed after rinsing with water. AFM studies show a structural change upon rinsing with water. We find that using John-Singh model [14] of quantum confinement; AFM data can explain the PL results before and after water treatment.
F3.51.2
EXPERIMENTAL
Data Loading...
