Determination of the trap state density differences in hydrogenated amorphous silicon-germanium alloys
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. Braunsteina) and B. Alavi Department of Physics and Astronomy, University of California at Los Angeles, Los Angeles, California 90095
B. Nelson National Renewable Energy Laboratory, Golden, Colorado 80401 (Received 9 January 2004; accepted 2 August 2004)
Time-resolved photo- and thermoelectric effects (TTE) were used to determine simultaneously trap levels and trap state density differences in amorphous (a-SiGe:H) samples. In particular, the trap state density differences are obtained from the decay of the ambipolar charge distribution (i.e., stage II of the TTE transients). This type of spectroscopy has been applied for the first time to a-SiGe:H samples, and indeed trap states that seem to relate to concentration fluctuations, that is, Si(Ge) and Ge(Si) clusters, are observed.
I. INTRODUCTION
The time-resolved thermoelectric power (transient thermoelectric effects, TTE) method uses voltage transients for determining carrier relaxation times, drift mobilities, and thermal diffusion coefficients simultaneously and as a function of the temperature for a given semiconductor specimen.1–3 In this contribution, we have applied this method to the determination of carrier relaxation times in amorphous silicon-germanium (a-SiGe:H) samples. The possibility to determine and to correlate various parameters simultaneously for a given sample is especially valuable where solar materials like a-SiGe:H samples have to be optimized. Using the TTE method, the transport parameters are measured in the dark, because of the spatial spread connected with the carrier and heat diffusion, that is, the determination of these properties takes place outside of the illuminated region and light induced defects cannot obscure the measurements. On the other hand, using a separate steady state illumination of the dark part, the effect of light induced defects on the transport can be investigated. Furthermore, its been proposed that the amplitude of the stage II transient, which represents the decay of a thermally generated ambipolar charge distribution, is a measure of the trap state density differences.4 If this is correct, this particular quantity should be a very sensitive
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0014 48
http://journals.cambridge.org
J. Mater. Res., Vol. 20, No. 1, Jan 2005 Downloaded: 13 Mar 2015
indicator of any additional light-induced trap level distributions as well as a measure of the degree of structural or compositional disorder in a given semiconductor sample. II. THE TTE METHOD AS APPLIED TO SEMICONDUCTORS
Various time-resolved thermoelectric transients can be observed across the electrical contacts of a semiconductor sample when a light pulse is targeted on it asymmetrically, that is, near the one end, but avoiding the contact itself. With intrinsic semiconductors one then finds three transients: stage 1, under the light pulse, ambipolar excess carriers Dn, Dp are excited in the illuminated region and then diffuse into the dark sample part. Becau
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