Effect of Electron Irradiation and Excess Cd on Ion-Assisted Doping of p-CdTe Thin Films
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EFFECT OF ELECTRON IRRADIATION AND EXCESS Cd ON ION-ASSISTED DOPING OF p-CdTe THIN FILMS D. Kim, A.L. Fahrenbruch, A. Lopez-Otero, and R.H. Bube Department of Materials Science and Engineering Stanford University Stanford, California 94305-2205
ABSTRACT Ion-assisted doping of homoepitaxial p-CdTe films with low energy P ions (20 eV) deposited by vacuum evaporation has been investigated. In order to control the properties of the films, we applied low energy electron irradiation and Cd overpressure during the growth. We report the results of measurements of hole density, spectral response of Cr/p-CdTe Schottky barriers (to estimate the minority carrier diffusion length Ld). From L, we have found that the quality of the films is dependent on both the ion dose and the ion energy. By reducing the ion energy to 20 eV and applying electron irradiation and Cd overpressure, p-CdTe films with p = 1 x 1017 cm- 3 and Ld = 0.35 ptm were obtained. A p-CdTe film with p = 1016 cm- 3 was obtained with a low ion energy of 10 eV. INTRODUCTION CdTe is a promising material for thin film solar cells because of its optimum bandgap (1.5 eV) for absorbing the solar spectrum and its high absorption coefficient (>104 cm-1) which means that only a 2-3 g.m thickness is enough to absorb most of the incident light. The best efficiencies of todays' polycrystalline thin film CdS/CdTe solar cells range from 13 to above 14% [1]. However, high series resistance (R,) is still one of the major problems in improving the conversion efficiency of CdS/CdTe cells. The sources of R, are (1) the contact resistance of the metal to the p-CdTe layers, and (2) the bulk resistance of the CdTe layer outside the depletion region. To solve this problem, we have investigated high p-type doping in the CdTe layer obtained by various doping techniques: photon-assisted doping with various Cd/Te ratios, co-evaporation of compound dopant sources such as Cd 3As 2 [2], ion-assisted doping (lAD) with electron irradiation, and IAD with different Cd/Te ratios. Good control of the doping process may also lead to an increase of the open-circuit voltage (V0o) of the CdS/CdTe cells [3]. Ion-assisted doping (LAD) and ion-assisted growth (TAG) are now used to improve the kinetics of film growth by, for example, enhancing the sticking coefficient of the involved species to the substrate or enhancing the incorporation of the dopants. A brief overview of this subject is given elsewhere [4]. In TAD, a dopant such as P is co-evaporated with CdTe in a vacuum, becomes ionized by electron bombardment as the dopant vapor goes through an ionizer, and is accelerated by a grid towards the substrate, with an ion energy of 10-60 eV. Using lAD, Sharps et al. achieved p-type doping up to 2 x 1017 cm-3 using 60 eV P ions [5]. However, they found that the CdS/CdTe junctions based on the TAD CdTe films generally show poor photoelectronic properties, apparently due to the ion damage. It is the purpose of our investigation to characterize the quality of lAD films and to improve their photoelectronic prop
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