The Influence of Deposition Conditions on the Electronic Properties of a-Si:H Prepared in Expanding Thermal Plasma
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0910-A07-04
The Influence of Deposition Conditions on the Electronic Properties of a-Si:H Prepared in Expanding Thermal Plasma Monica Brinza, and Guy J. Adriaenssens Halfgeleiderfysica, University of Leuven, Celestijnenlaan 200D, Leuven, 3001, Belgium
ABSTRACT A number of a-Si:H samples prepared in an expanding thermal plasma under varying conditions were examined by means of time-of-flight transient photocurrent measurements. A high deposition temperature allows high deposition rates while achieving high hole mobility and mobility-lifetime product. Lower hole mobility but higher µτ product result from slower deposition at lower temperature. Electron µτ products are uncharacteristically low for all samples due to pronounced deep trapping. RF biasing of the substrate does not improve the results.
INTRODUCTION A high deposition rate of a-Si:H is desirable since it ultimately leads to a reduction in the price of mass-produced solar cell modules. The expanding thermal plasma (ETP) method, developed at Eindhoven University of Technology, allows for such high deposition rates. Previous time-of-flight (TOF) studies of ETP a-Si:H [1] showed that ETP samples deposited at 6-7 nm/s have high hole mobilities, provided that the deposition temperature is around 400-450 °C. A field-independent hole drift mobility measured in ETP samples, which is characteristic to equilibrium transport but in this case was obtained on the basis of dispersive current transients, was explained by a valence band tail dominated by a Gaussian component that is superposed to the traditionally exponential tail [2]. The present study looks at samples deposited over a wider range of conditions as regards both the deposition rate and an additional ion bombardment of the growing surface. Earlier studies [3] indicated that the ion bombardment, which is realized through an external rf biasing of the substrate, increases the material photo-response. However, this increase is mainly due to a reduction in the dark current, the photocurrent being only slightly affected.
EXPERIMENTAL DETAILS TOF measurements were performed in a standard experimental set-up with 540 nm excitation as described in previous papers [1]. The samples are deposited on Corning 7059 glass and sandwiched between Cr contacts, the top one being semi-transparent. Table I lists some of the characteristic parameters.
Table I. Characteristic parameters of the samples, where L is the sample thickness, TDep is the deposition temperature, RDep is the deposition rate and E-ref the Eindhoven reference code. Sample
L (µm)
1 2 3 4
3.7 2.8 2.9 3.4
TDep (°C) 400 250 250 250
µτh (cm2/V) 6 ×10-9 9 ×10-9 4 ×10-9 < 2 ×10-9
Rf bias
RDep (nm/s) 9.2 0.77 3.2 3.8
No No No Yes
µτe (cm2/V) 3 ×10-9 2 ×10-9 4 ×10-9 3 ×10-9
E-ref DT764 DT765 DT771 DT879
Due to the presence of almost non-dispersive transients in certain samples, the transit time is defined as the moment when the photocurrent dropped to 50% of its extrapolated pre-transit value. In comparison with the other most used definition, where the
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