Origin of Photodegradation in a-Si:H - Bond Breaking or Charge Trapplng?
- PDF / 334,004 Bytes
- 6 Pages / 414.72 x 648 pts Page_size
- 24 Downloads / 159 Views
ORIGIN OF PHOTODEGRADATION IN a-Si:H BOND BREAKING OR CHARGE TRAPPING?
TATSUO SHIMIZU, MINORU KUMEDA, QING ZHANG, JINYAN ZIIANG AND TOSHIHIRO OIITSUKA Faculty of Engineering, Kanazawa University, Kanazawa 920, Japan
ABSTRACT In order to clarify the microscopic origin of the photodegradation in a-Si:II, two kinds of approaches have been done. One approach is to carry out the light soaking both at room temperature (RT) and 77 K to cause the photodegradation and to investigate the recovery by thermal annealing. The other approach is to investigate the light-induced ESR in hydrogenated amorphous silicon-nitrogen alloys (a-Sii_-N,,:I1) and to see the xdependence down to x = 0. From the first approach, it is found that there is a very broad continuous distribution of annealing activation energies from a few eV through 1.4 eV. It appears less likely that such a wide distribution of annealing activation energy is related with the bond-breaking model. On the other hand, it is found friom the second approach that there exist two distinct origins for the light-induced spins, the fast process and the slow process, which are likely to be the carrier capture and the bond breaking, respectively. From the continuation of the observed behavior to a-Si:Il, that is, x = 0 in a-Sil N,:II, the RT photodegradation in a-Si:H appears to be the bond breaking.
INTRODUCTION Since the discovery of the photodegradation phenomena in a-Si:lt by Staebler and Wronski in 1977 [1], many efforts have been devoted to clarify the mechanism of the photodegradation. However, the problem is still not resolved. Here we make two kinds of approaches from different directions to clarify the microscopic origin of the photocreation of dangling bonds in a-Si:tI. One approach is to carry out the light soaking both at room temperature (RT) and 77 K to cause the photodegradation and investigate the recovery by thermal annealing. ESR is mainly used to study the photodegradation. The other approach is to investigate the light-induced ESR in hydrogenated amorphous silicon nitrogen alloys (a-Sil_,N,:It) and to see the x-dependence down to x = 0.
EXPERIMENTAL RESULTS AGAINST VARIOUS PROPOSED MODELS Two kinds of models have been proposed for the photocreation of neutral dangling bonds in a-Si:H. The first model is based on breaking of weak bonds followed by some rearrangement of the atoms for stabilizing created dangling bonds, which is considered to be accomplished by movement of H atoms [2]. However, the following experimental results appear to oppose this: (1) Light-induced defects are created at 4.2 K with the density comparable with one at RT [3]. (2) The separation between a dangling bond and a Hi atom should be larger than 0.4 nm judging from the results of pulsed ESR. [4]. (3) The separation between dangling bonds should be larger than 10 nm [4]. (4) The density of photocreated neutral dangling bonds is correlated well with the extent of flexibility of the amorphous network [5], suggesting that H atoms do not play a direct role in the photocreation of neutral
Data Loading...
