Effect of Light Soaking on Hot Wire Deposited a-Si:H Films
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DAEWON KWON, AND J. DAVID COHEN University of Oregon, Department of Physics, Eugene, OR 97403 BRENT P. NELSON, AND EUGENE IWANICZKO National Renewable Energy Laboratory, Golden, CO 80401
ABSTRACT We present the results of studies on the defect properties and the effect of light soaking for various hot wire deposited (HW) films. We employ junction capacitance measurements together with the transient photocapacitance spectroscopy to measure the deep defect densities in as-grown state (state A) and in light soaked state (state B). Good agreement is found between the defect densities measured from both measurements. The HW film with a hydrogen content of 10 - 12 at.% shows physical characteristics and defect densities similar to conventional PECVD films. The HW films with hydrogen content, CH, in the range 2 - 9 at.% show a smaller defect density in state B than the defect density of the film with higher CH. However, the film with a hydrogen level of less than 1 at.% exhibits markedly inferior physical properties.
INTRODUCTION It has been reported that device-quality a-Si:H films can be produced by the hot-wire technique [1 ,2 ,3 1 which have the same or the better performance than the films grown by plasma enhanced CVD (PECVD) technique. Hydrogen is required to passivate the excess dangling bonds in good quality films. In the conventional PECVD technique, the hydrogen concentration is almost always in the range of 10 - 15 at.% for the best quality material. It has been suggested that this level of hydrogen is necessary to relieve the film strain. The possibility of producing a-Si:H films with significantly lower hydrogen concentration and good performance is important for several reasons. First, it helps to identify the critical concentration of hydrogen needed for passivating dangling bonds as well as relieving the film strain. Second, it offers a means to reduce the optical gap thus improving solar cell efficiencies. Finally, reducing the hydrogen concentration may offer the possibility of suppressing the creation of metastable defects, since many authors believe that the hydrogen is closely related to this degradation process [4,5,6 1. In this paper, we measure the defect densities for various a-Si:H films grown by the HW technique in as-grown states as well as in state B to determine whether such low hydrogen concentration HW films really can exhibit the same or the better physical characteristics than the films grown by the conventional PECVD technique.
EXPERIMENTAL METHODS A set of four a-Si:H films were deposited on stainless steel substrates at various substrate temperatures (290 - 400 'C) by the HW technique. Deposition at different temperatures results in H concentrations varying over the range 0.1 - 12 at.%. After film growth, we evaporate semi301 Mat. Res. Soc. Symp. Proc. Vol. 377 ©1995 Materials Research Society
HW290 HW325 HW360 HW400
S
290 325 Tý(°C) 360 400
1.45 1.83 Thickness([.tm) 2.38 1.69
10- 12 7- 9 Cn(% 2- 3
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