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ABSTRACT We have studied the degradation kinetics of undoped a-Si:H films which contain a significant fraction of silicon microcrystallites. The degradation rate is found to be exceptionally slow in the first stage of degradation, then the defect density follows the "normal" t1"3 rate and finally saturates. We present a model which relates this abnormal kinetics to the microcrystallites which are embedded in the amorphous matrix.

INTRODUCTION The Staebler-Wronski effect [1] is a major obstacle for the application of a-Si:H in solar cells. Considerable effort continues in order to develop a-Si:H materials with better stability. For example, it is known that a-Si:H solar cells grown under hydrogen dilution are more stable [2,3], and it has been argued that this is related to the onset of silicon microcrystallite formation within the amorphous material [4]. The more stable material can be thus viewed as an intermediate state between pure a-Si:H and microcrystlline silicon. However, up to now there has been no systematic study of the degradation kinetics throughout the transition region between these two limits. Recently we showed [5] that a-Si:H deposited by dc reactive magnetron sputtering in the presence of either hydrogen or deuterium includes a considerable density of microcrystallites embedded in the amorphous matrix. Evidence for the existence of these microcrystallites was found by Raman spectroscopy and by TEM micrographs. In addition, defect states associated with the microcrystallites were discovered using transient photocapacitance and transient photocurrent spectroscopies. This, and a previous report indicating a slow degradation in similar samples [6], suggest that such films are an appropriate system to study the effect of the presence of microcrystallites on the degradation kinetics of a-Si:H. In this work we examine the degradation kinetics of a-Si:H films grown by dc reactive magnetron sputtering. We show that the presence of the microcrystallites leads to a very unusual defect creation kinetics. We present a model which accounts for the experimental behavior and relates the degradation anomaly with the structure of the samples.

EXPERIMENTAL The a-Si:H samples used in this study were deposited by dc reactive magnetron sputtering of a 5"x12" planar Si target in an Ar + H2 plasma (with pressures of 1.5 and 0.6 mTorr, respectively). The p+ c-Si substrate was kept at a temperature of 230'C, and the growth rate was about 170,/min. CPM measurements indicate a band gap of 1.73 eV and an Urbach energy of 62 meV. More details about the growth procedure have been given elsewhere [7]. 729 Mat. Res. Soc. Symp. Proc. Vol. 507 © 1998 Materials Research Society

Semitransparent Pd dots were evaporated on top of the a-Si:H film, yielding contacts for the capacitance measurements. These measurements can thus be performed on either the back junction or the front Schottky barrier. Prior to the measurements the samples were annealed at 490K for 1 hour. Light soaking was performed using a tungsten-halogen lamp with a m