Dangling-Bond Relaxation and Metastability in P-Type Hydrogenated Amorphous Silicon
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METASTABILITY
IN
P-TYPE
RICHARD S. CRANDALL, MARTIN W. CARLEN, KLAUS LIPS, AND YUEQIN XU National Renewable Energy Laboratory, Golden, Colorado 80401 ABSTRACT We discuss the subtle effects involved in observing slow dangling bond relaxation by studying capacitance transients in p-type hydrogenated amorphous silicon (a-Si:H). The data suggest that neutral dangling bonds are reversibly converted into metastable positive charged dangling bonds by hole trapping. These metastable positive dangling bonds reconvert to neutral dangling bonds upon annealing at elevated temperature. The annealing kinetics for this process are the same as those observed for annealing of quenched in conductivity changes in p-type a-Si:H. INTRODUCTION The discovery of the Staebler-Wronski effect' produced intense interest in metastable changes in hydrogenated amorphous silicon, a-Si:H. Light,' charge injection,' charge extraction,' or quenching from elevated temperature5 all produce these metastable changes. These metastable changes usually produce an increase in the density of neutral dangling bond defects, D, which is a threefold-coordinated Si atom. This deep D-defect in the mobility gap of a-Si:H controls most of its electronic properties, making it an important factor for thin-film electronic devices such as solar cells, transistors6 and light emitting diodes. Cohen et al. reported a slow relaxation process associated with electronic transitions from Ddefects in n-type a-Si:H and later Carlen et al.7 observed similar phenomena in p-type a-Si:H. By using junction capacitance transient measurements, Cohen et al. found that an electron's thermal release time, tre1,is proportional to its residence time in the defect. creis assumed equal to the filling pulse width, tp. The shape of the capacitance transients is close to a power law and extends to minutes at room temperature. In addition, the decays of the trapped charge are virtually independent of temperature. Similar results were found for holes in p-type a-Si:H by Carlen et al. However, considerable doubt was raised by Jackson and Johnson8 who attributed the unusual emission dynamics in n-type a-Si:H to a simple measurement artifact because they failed to observe the anomalous emission dynamics when probing an n-type Schottky junction with an ohmic back contact. Cohen et al .'S6 n-type sample was grown on a p-type crystalline substrate, Jackson and Johnson speculated that for short filling pulses this p-type back contact may not supply enough charge to fill all defect states..This might distort the transients and scale the transients vertically in signal height. However, Gardner and Cohen9 showed that all the defects could be filled in 100 lts which was the shortest u used. Considering the variety of contact materials and Fermi level positions used, it is hard to understand how the observation of slow relaxation effects in both n- and p-type material is traceable to exactly the same measurement artifact in both materials. Moreover, different experiments such as the influence of temperatur
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