Influence of Thermally-Induced Defects on the Transient Photoconductivity in a-Si:H
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INFLUENCE OF THERMALLY-INDUCED DEFECTS ON THE TRANSIENT PHOTOCONDUCTIVITY INa-Si:H
A.WERNER* AND M. KUNST
Hahn-Meitner-institut, Bereich Strahlenchemie, D-1000 Berlin 39, Federal Republic of Germany ABSTRACT Contactless transient photoconductivity measurements in intrinsic and n-type a-Si:H were performed at 220 oc and at room temperature where also rapidly quenched films were investigated. In undoped samples the measurements suggest thermally induced structure changes which lead to an increase of the effective electron lifetime. In n-type samples the effective electron lifetime decreases with temperature, what may be explained by an increase of the hole emission rate with temperature. INTRODUCTKIN In the last years evidence has been accumulated that an increase of the density of free charge carriers in hydrogenated amorphous silicon (a-Si:H) leads to an increase of structural disorder. More specifically this phenomenon has been formulated as "the intrinsic tendency of a-Si:H to reduce the concentration of free carriers by forming dangling bond-defects" 1 . The dependence of a number physical properties of a-Si:H on the doping level was the first observation that was explained by structural changes of the material upon a change of the free carrier density. Later on, the influence of electrically and optically induced charge carriers was interpreted in the same way 2 . Recently also changes of the a-Si:H lattice with temperature were reported 3 . In this case a connection between these changes and the diffusion of hydrogen in a-Si:H has been proposed 4 . Because in all these cases the density of free charge carriers is increased, a correlation between this increase and changes of a-Si:H structure was proposedI.
In general, the influence of the temperature on physical properties of a-Si:H is investigated by heating of the film (between 120 oC and 250 oc), whereafter the film is quenched to room temperature. Then the physical properties of the quenched film are compared to those of the same or an identical film after slow cooling. It seems interesting to measure also directly the properties of a-Si:H at higher temperature, and compare to those of the quenched films. This enables the separation between structural changes of a-Si:H at higher temperature with relatively long equilibrium times and changes of rate parameters with temperature. In this work the influence of the temperature on the transient photoconductivity is investigated with a contactless microwave conductivity technique. EXPERIMENTAL The a-Si:H films (about 1 pIm thick) have been prepared in a conventional glow-discharge system by decomposition of silane, where in some cases phosphine has been added to achieve n-type doping. The quenching experiments have been done by dipping the samples heated up to 220 oc in liquid nitrogen and then heating up to room temperature. The experimental set-up for the microwave measurements was identical to that described elsewhereb. The transient photoconductivity in intrinsic and n-type a-Si:H is due to mobile electrons
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