Electrical Transport in Mesoporous Silicon Layers
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575 Mat. Res. Soc. Symp. Proc. Vol. 358 01995 Materials Research Society
MEASUREMENTS The electrical characterization techniques we used include temperature-dependent dark and photo-conductivity, thermopower, time-resolved photoconductivity and the Steady-State Photocarrier Grating technique (SSPG). The dark conductivity was measured using both the two- and four-probe technique in order to study the influence of the contacts. The behavior of majority carriers was investigated by photoconductivity measurements. For this case the samples were excited by HeNe-laser irradiation with intensities up to 10 mW/cm 2 . For the time-resolved measurements the laser beam was modulated using a Pockels cell, with time resolution of 200 ns. The sign of the majority carriers was determined by thermopower measurements. The thermopower was induced by a small temperature difference of about 5PC in order to avoid degradation. It was measured in a DC mode using either an electrometer or null current detection. The behavior of the minority carriers was deduced from SSPG measurements. This method was first applied by Ritter et al. to measure the ambipolar diffusion length in amorphous Si (a-Si:H) [6]. We discussed the application of this method to PS and its interpretation in a previous paper [5]. RESULTS AND DISCUSSION
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1000/T (K-1) Fig. 1: The temperature dependence of the dark conductivity of a meso-PS sample in vacuum and in methanol vapor. The activation energy is lowered in the presence of the methanol vapor, while the conductivity prefactor a0 stays the same. Fig.1 depicts the temperature dependence of the dark conductivity of one of the meso-PS samples. For samples kept in vacuum, the conductivity shows Arrhenius behavior with an
576
activation energy of 0.49 eV and a conductivity prefactor of - 102 (12 cm)-1 . When methanol is introduced into the chamber the activation energy decreases, but the prefactor is not affected. Fig.2 shows the temperature dependence of the mobility-lifetime products (gr) for both the majority and minority carriers (while the sample is in vacuum), estimated from photo-conductivity and SSPG measurements, respectively. Note that (Wri)maj and (.tC)min do not differ very much, in agreement with the intrinsic-like nature of the meso-PS. Both have only a weak temperature dependence, especially in comparison to that of the dark conductivity. Since the free carrier lifetime does not change very much in this temperature regime, we have to conclude that the free carrier mobilities of both carrier types do not change strongly with temperature. The activated behavior of the dark conductivity is therefore a result of a thermal activation of carriers, rather than a strong change in their diffusivity.
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