External rf substrate biasing during a-Si:H film growth using the expanding thermal plasma technique
- PDF / 142,311 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 92 Downloads / 174 Views
A9.21.1
External rf substrate biasing during a-Si:H film growth using the expanding thermal plasma technique A.H.M. Smets, W.M.M. Kessels and M.C.M. van de Sanden Department of Applied Physics, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands
ABSTRACT The interchangeability of ion bombardment and deposition temperature during a-Si:H film growth at high deposition rates (10-42 Å/s) by means of the expanding thermal plasma has been studied. The ion bombardment is generated by applying an external rf bias voltage on the substrate. It is shown that the opto-electronic performance of the a-Si:H films improves considerably when a moderate rf substrate bias voltage (~20-60 V) is applied, i.e. the photo response increases two orders of magnitude up to 106. Furthermore, it is also revealed that the additional energy supplied to the growth surface by the ion bombardment, makes a reduction of the deposition temperature by ~100 oC possible, while preserving good material properties. On the basis of the results obtained, three effects caused by the rf substrate bias can be distinguished: creation of an additional growth flux, a reduction of the void incorporation, and an increase in the vacancy density. INTRODUCTION In the recent years it has been demonstrated that the remote expanding thermal plasma (ETP) is a deposition method, which has easily access to ultrahigh deposition rates (up to 100 Å/s) of aSi:H. However, to obtain good a-Si:H film properties at these high deposition rates, relatively high substrate temperatures Tdep are required (typically Tdep=400 oC for 70 Å/s) [1,2]. In previous work, we have speculated that the need for higher substrate temperatures originates from the competition between the deposition rate and the surface diffusion processes during growth [2,3]. At present the maximum processing temperature of a pin-device is limited by the stability of the a-Si:C:H boron doped p-layer (Tdep~180-200 oC). This means that at present, a reasonable pin performance can be obtained up to an intrinsic a-Si:H deposition rate of 10 Å/s using the ETP. Possible solutions for applying higher deposition rates in solar cells are the development of a temperature stable p-layer [4], implementation of a high rate intrinsic film in a nip-device (as demonstrated by Mahan et al. using the hot wire technique up to growth rates of 130 Å/s [5]), or the reduction of the substrate temperature by the supply of additional energy to the surface during growth by ion bombardment. In this contribution we will explore to which extent the substrate temperature can be interchanged with additional energy supplied to the surface by ion bombardment. Several authors have shown that ion bombardment during the growth of Si-based films results in densification, film stress, and improved optical properties [6-8]. Molecular dynamics (MD) simulations of ion assisted thin film growth have revealed that these ion-induced modifications of the microstructure are a result of locally enhanced surface diffusion and removal
Data Loading...
