Signature of the Weak Bond-Dangling Bond Conversion Process in a-Si:H As Seen by Total Photoelectron Yield Spectroscopy
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SIGNATURE OF THE WEAK BOND-DANGLING BOND CONVERSION PROCESS IN a-Si:H AS SEEN BY TOTAL PHOTOELECTRON YIELD SPECTROSCOPY
W. GRAF, K. LEIHKAMM, J.RISTEIN, L. LEY Universitct Erlangen-Narnberg, Institut ftr Technische Physik, Erwin-Rommel-Str. 1, D-W-8520 Erlangen, Germany
ABSTRACT Photoelectron Yield Spectroscopy is one of the most direct methods to study the density of gap states of of gap amorphous semiconductors. We have made use of it to investigate the transient changes of the density 2 states of a-Si:H upon illumination with above-band-gap light. An excitation density of 50 mW/cm of 532 rum light from a frequency doubled Nd-YAG laser was modulated with frequencies between 0.1 Hz and 10 KHz and the transient response of the photoelectron yield signal was monitored by gated electron counting or a lock-in amplifier. From this we deduce a reversible increase of the occupied density of defect states under illumination 1 3 with a maximum Ag - 1017 cm- eVWat about 0.3 eV below the Fermi energy and a decrease below EF- 0.75 eV, i. e. in the region of deep tail states. The effect exhibits a wide spectrum of time constants involved. Based on additional transient surface photo voltage measurements as well as numerical simulations we argue that the observed redistribution of states is not due to a light induced change in surface band bending or due to a mere excitation of electrons from occupied to empty states. Instead, we interpret the observed effect as a conversion of weak to dangling bonds, i.e. a precursor of the Staebler-Wronski Effect, lacking however the final step of hydrogen stabilisation necessary for the creation of metastable defects. INTRODUCTION Total Photoelectron Yield Spectroscopy (Yield) is now a well established method to determine near surface electronic properties of amorphous semiconductors. The Yield spectrum Y(ho)) is defined as the number of electrons emitted from a solid into the vacuum per incident photon of energy ho). The large dynamic range (107: 1) and the relatively high energy resolution (AE- 0 the ODOS would decrease in the energy range below EF where we observe an increase. However, as was pointed out by Cohen et al. [41 a band of near surface defect states with negative correlation energy can in principle give the observed signal below EF when electrons are removed from these states. The situation is depicted in fig. 6 where we have indicated the negative U defect band such that the resulting difference spectrum most nearly corresponds to the observed signal. Note, that this explanation requires the negative U defect band to be situated further below EF than IUI. There are, however, several problems with this explanation. Firstly, in order to describe the measured ODOS additional positive U defect states near EF are required. Their contribution to the light induced signal would reduce if not overcompensate the positive signal due to the depopulation of the negative U defects (curve a in figure 6b). Secondly, in either case the excited electrons are expected to occupy previously empty
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