Carrier-Induced Changes in The Gap States of Undoped a-Si:H Films
- PDF / 237,230 Bytes
- 4 Pages / 414.72 x 648 pts Page_size
- 80 Downloads / 135 Views
Y.E. CHEN, J.W. TSAI and H.C. CHENG Dept. of Electronics Engineering, National Chiao-Tung Univ.; Hsinchu, Taiwan, ROC
ABSTRACT Carrier-induced changes in the gap states of plasma-enhanced-chemical-vapordeposited (PECVD) undoped a-Si:H films are systematically studied using isothermal capacitance transient spectroscopy (ICTS) method via the novel structure we proposed previously. The density-of-state distribution g(E) and the energy dependence of electroncapture cross section oT,(E) of gap states in undoped a-Si:H films before and after injection of electrons are directly measured for the first time. Experimental results show that the density of g(E) increases but the (7,(E) has no obvious change after the electron injection. These indicate that the defects created during the electron injection are the same in type as those of the as-deposited films. In addition, it is found the shape of g(E) distorts after the injection, implying that the gap states with different energy levels are associated with distinct types of defects.
INTRODUCTION For the past twenty years much work has been focused on elucidating the origin of metastable changes in the properties of a-Si:H films which were discovered by Staebler and Wronski. 1-2 Besides being a most challenging materials science problem an understanding of this metastability has technological importance because it cause degradation of amorphous silicon thin film transistor. Metastable changes in the properties of a-Si:H films can be induced by a number of different factors, like light,1 2 and accumulation of carriers. 3-6 For all metastable changes, the creation of dangling bonds due to the breaking of weak Si-Si bonds is most widely accepted as the mechanism causing these degradation. However, many questions are still remained to be clarified, like if the metastable and the as-deposited dangling bond defects are the same or not. In earlier works, a novel Au/undoped a-Si:H/n-type low-resistivity c-Si structure which has the space charge region entirely covering the undoped a-Si:H film and penetrating into the c-Si substrate was proposed to replace the conventional Schottky configuration for determining the g(E) of gap states in undoped a-Si:H by isothermal capacitance transient spectroscopy (ICTS) method.7 In addition, by performing the pulse-filling measurements with the novel structure, the energy dependence of the electron-capture cross section •r(E) of gap states can be also obtained.8 In this paper, the changes of properties of gap states in undoped a-Si:H during electron injection is directly observed by measuring the g(E) and Gn(E) of gap states before and after forward bias stress, respectively, via the novel structure. It is found that the density of g(E) increases but the ay(E) has no, obvious change after the electron injection, indicating that the defects created during the electron injection and those of the as-deposited films before the injection are the same in type. 379 Mat. Res. Soc. Symp. Proc. Vol. 377 ©1995 Materials Research Society
SAMPLE PREPARATIO
Data Loading...
