Electron Transport and Conductton-Band-Tail States in a-Si:H Deposited with a Remote Hydrogen Plasma
- PDF / 338,665 Bytes
- 6 Pages / 414.72 x 648 pts Page_size
- 58 Downloads / 172 Views
ELECTRON TRANSPORT AND CONDUCTION-BAND-TAIL STATES IN a-Si:H DEPOSITED WITH A REMOTE HYDROGEN PLASMA
C. E. NEBEL%, R. A. STREET**, N. M. JOHNSON**, J. WALKER** *Institut ffir Physikalische Elektronik, Universit~t Stuttgart, Pfaffenwaldring 47, D-7000 Stuttgart 80, Germany **Xerox Palo Alto Research Center, 3333 Coyote Hill Rd., Palo Alto, California 94304, USA. ABSTRACT
Electron transport properties of a-Si:H prepared in a remote hydrogen plasma deposition reactor (RHPD) at TD = 400 °C were investigated in the temperature regime 110 K < T < 300 K by time-of-flight and post-transit spectroscopy experiments. Based on these data the conduction-band-tail state distribution was calculated. In the energy range 85 meV < EcE _ 350 meV) the tail smoothly passes over into the defect density which is approximately six orders of magnitude smaller than at the mobility edge. Comparisons with data deduced on conventionally prepared a-Si:H (RF-, DC-glow discharge) at TD = 230 'C show that electron transport and the conduction band tail of the RHPD material are comparable. INTRODUCTION
We have demonstrated recently, that the stability of a-Si:H thin films is improved by deposition at high temperatures (TD = 400 °C) in a remote hydrogen plasma deposition reactor (RHPD) [1,2]. It's principle features are the elimination of effects that arise from the interaction of the plasma and the growing film and isolation of a single primary gas-phase reaction to generate radicals for film deposition. The remote hydrogen deposition conditions were as follows: 200 sccm H2 , 10 sccm SiH 4 , microwave power of 400 W (2.45 GHz), and a chamber pressure of 0.5 Tort. Defect and dopant metastability investigations clearly identify an improved thermal stability. The initial defect density, determined by constant photocurrent spectroscopy (CPM) measurements is lowest for the 400 0C/RHPD film, the rate of light-induced defect generation is comparable for the RHPD and conventional GDfilms, and the 400 °C/RHPD film saturates at lower defect density than the simultaneously illuminated GD films. We have speculated that the hydrogen is more stably incorporated and/or that the density of weak Si-Si bonds is smaller in the high TD material. A decrease of the weak bond density should be manifest in the slope of the valence- (Urbach) and the conduction-band-tail. CPM experiments could not detect a significantly steeper Urbach tail. It is, however, important to note that the requisite decrease in slope is expected to be less than the accuracy of the CPM experiment. In this paper we present and discuss a detailed investigation of electron transport properties to deduce the conduction-band-tail state distribution of a-Si:H prepared by a RHPD at TD = 400 °C. Based on time-of-flight (TOF) and post-transit spectroscopy (PTS) experiments we explore the conduction-band-tail in the regime 85 meV < EC-E _ 500 meV, where E. is the mobility edge. After a brief introduction of experimental details we introduce data and calculate the tail state distribution. Finally a discuss
Data Loading...
