A-Si:H Device Characterization Using Transient Response Measurements

Abstract

a -Si:H device characterization using transient response measurementsRolf KonenkampHahn -Meitner -Institut Berlin, Bereich Strahlenchemie,Glienicker Str. 100, 1000 Berlin 39, Federal Republic of GermanyAbstractSeveral time -resolved experimental techniques have been applied to a -Si:H based thinfilms and diodes to determine carrier ranges, junction field profiles and charge storagetimes. The results on p -i -n type diodes are compared to a theoretical model of a -Si:H so-lar cells. Electrochemical surface treatment of Schottky- barrier type devices indicatesthat improved carrier transport is obtained for interfaces from which oxide related de-fects are removed. For the case of thin a -SiC:H alloy films it is shown that deep trappinggives rise to significant space charge build -up which can be used for charge storageapplications.IntroductionThere is now considerable interest in the use of amorphous silicon based diodes as ameans of producing inexpensive thin film devices. A detailed modelling of such devices hasrecently been achieved by Hack and Shur1,2, which lends itself to a consistent picture inthe understanding of the device physics, but a detailed comparison to experimentally de-termined parameters has so far not been carried out. In particular, there is a lack of ex-perimental data from thin device structures which can directly be interpreted in terms ofmicroscopic parameters such as carrier ranges, lifetimes and the density of localized sta-tes. We have applied a variety of transient and steady state techniques which are suitedto provide such information, and have investigated a range of thin sample structures withconsideration of both surface and bulk effects. Part of the experiments are carried outunder conditions where excess charge carriers of both types - electrons and holes - areinjected into the sample so that recombination processes determine the kinetic behavior.These experiments therefore bear close similarity to device operation conditions in solarcells. In the experiments performed to investigate surface and interface properties, onlyone carrier type is injected and used as a probe to determine carrier ranges or electricfield profile. In this paper we will show that transient techniques can be used for a de-tailed device characterization if the unique features due to trap controlled transport areaccounted for. Here we will present three techniques to discuss the influence of shallowstates on bulk and surface properties and we will point out a charge storage mechanism dueto the existence of deep states in