Doping Effects on CdO Thin Films
- PDF / 50,239 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 79 Downloads / 229 Views
Doping Effects on CdO Thin Films X. Li, T. Barnes, C. DeHart, D. King, S. Asher, M. Young, T.A. Gessert, and T.J. Coutts National Renewable Energy Laboratory 1617 Cole Boulevard, Golden, CO 80401
ABSTRACT
This paper reports the properties of undoped CdO films and CdO films doped with the group VII element F and the group IV element Sn. The CdO films are made by low-pressure chemical-vapor deposition. We observe that undoped CdO films can achieve a carrier concentration of 1021 cm-3, apparently by controlling the intrinsic defect. However, the electron mobility of these films is only around 2 cm2 V-1 s-1. With fluorine doping, an electron mobility of ~260 cm2 V-1 s-1 has been achieved. However, low carrier concentration results because of the low solubility of F in CdO film. CdO films doped with both Sn and F demonstrate carrier concentrations of 1021 cm-3 and reasonable electron mobilities of around 20 cm2 V-1 s-1. Due to the small effective electron mass of CdO, a large Burstein-Moss shift is observed for films with high carrier concentration. The shift enables the fundamental absorption edges of undoped CdO films to reach 3.0 eV and 3.3 eV for films doped with both Sn and F.
INTRODUCTION
Cadmium oxide (CdO) is an n-type material with a bandgap of 2.2 eV [1]. This relatively narrow bandgap makes CdO a less popular transparent conducting oxide (TCO) than In2O3:SnO2 and SnO2. However, CdO has a relatively small effective electron mass (mc) and is thought to be degenerate even at comparatively low carrier concentration [2]. For degenerate materials, a small mc will produce a large Burstein-Moss (BM) shift of the fundamental optical absorption edge with increasing carrier concentration [3]. Thus, it is possible to enlarge the CdO optical bandgap by increasing the carrier concentration. The small mc should also enable a high electron mobility [4]. For typical TCO applications, large electron mobility is desirable because it allows the electronic and optical properties of the film to be improved [5]. Therefore, despite the relatively narrow bandgap, the superior electronic properties and potential variable bandgap may allow CdO to become a useful TCO material in certain applications. In this study, we investigate the effect of intrinsic defector and F and Sn extrinsic dopants on the electrical properties and bandgap widening of CdO. Our goal is to improve the optical properties of CdO by doping while maintaining its electronic properties. The Hall mobility and optical absorption spectrum are the major parameters used to assess the electrical and optical properties of the film.
F3.18.1
EXPERIMENTAL
The CdO films prepared for this study were made by low-pressure chemical-vapor deposition (CVD). We studied CdO films that were undoped, doped with the group VII element F, and doped with the group IV element Sn. Experimental detail are provided elsewhere [6]. The electrical properties were assessed using Hall measurements (Van der Pauw technique, BioRad Model HL5500). The optical properties (total transmittance [T] and
Data Loading...
