Boron Doping Effects on the Electro-optical Properties of Zinc Oxide Thin Films Deposited by Low-Pressure Chemical Vapor
- PDF / 612,038 Bytes
- 7 Pages / 612 x 792 pts (letter) Page_size
- 37 Downloads / 261 Views
0928-GG12-05
Boron Doping Effects on the Electro-optical Properties of Zinc Oxide Thin Films Deposited by Low-Pressure Chemical Vapor Deposition Process Jerome Steinhauser, Sylvie Faÿ, Romain Schlüchter, Seung Yeop Myong, Evelyne Vallat, and Christophe Ballif Institute of microtechnology, University of Neuchâtel, Breguet 2, Neuchatel, 2000, Switzerland
ABSTRACT Boron-doped zinc oxide (ZnO) films deposited by low-pressure chemical vapor deposition (LPCVD) technique are used as transparent conductive oxide (TCO) to contact thinfilm silicon solar cells. In this paper, the effect of boron introduced as a dopant during the ZnO film growth. These films are highly transparent in the visible range, whereas in the near infrared region their transmittance decreases with the increase of boron content due to free carrier absorption (FCA). A shift of the fondamental band gap is also observed, explain by the combination of both Burnstein-Moss and band gap narrowing effects. The resistivity decreases by about one order of magnitude with the increase of the doping ratio ([B2H6]/[DEZ]) from 0 to 2. This resistivity drop is mainly due to an increase of the free carrier concentration. In low doped samples, the Hall mobility increases with grain size, whereas it shows no grain size dependence in highly doped layers. This suggest that the scattering by grain-boundary is the main limiting factor for electronic transport in low doped ZnO samples, whereas in highly doped ZnO films, the electronic transport is controlled by ionized impurity scattering within the grains. INTRODUCTION Transparent Conductive Oxides (TCO) are an essential part of thin-film silicon solar cells. In order to contact the solar cell and let the light enter into the absorbing part of the cell, they have to exhibit good electrical (high conductivity) and optical (high transmittance) properties. In addition to these characteristics, they also have to scatter the light at the TCO-cell interface in order to increase the effective absorption of light within the active layer of the cell. This aspect is especially important in the case of microcrystalline silicon (µc-Si:H) thin-film solar cells, because of their relatively low optical absorption coefficient in the red and near-infrared (NIR) spectral range.[1,2] Boron-doped zinc oxide (ZnO:B) layers deposited by Low Pressure Chemical Vapour Deposition (LP-CVD) technique have been developed at the Institute of Microtechnology (IMT) of the University of Neuchâtel [3]. This material is a promising candidate for thin-film solar cell technology, especially because of its low cost, and because of the wide availability of its constituent raw materials. Furthermore, the LP-CVD technology is well suited for large-scale device fabrication [4,5]. Zinc oxide is a direct band gap, usually n-type, conductive oxide with a hexagonal wurtzite structure. The LP-CVD polycrystalline ZnO layers are highly transparent: 2µm thick films with a sheet resistance of 8Ω sq have a total transmittance around 80% averaged in the spectral range of 400-9
Data Loading...
