Potential fluctuations, diffusion length and lateral photovoltage in hydrogenated amorphous silicon and silicon–germanium thin films

Abstract

Abstract The lateral photovoltage (LPV) has been measured between coplanar electrodes by illuminating hydrogenated amorphous silicon samples at various positions with a red laser spot. We find that the LPV decreases at higher temperatures and increases upon light soaking. Similar results are obtained for hydrogenated amorphous silicon–germanium alloys. The diffusion length of carriers in our samples is measured by the steady-state photocarrier grating technique and the LPV is calculated. The calculated LPV is much smaller than that experimentally measured. We propose that the potential fluctuations present in the samples might be responsible for the large LPV. We have measured the ambipolar diffusion lengths in a-Si : H and a-SiGe : H samples by the SSPG technique. The ambipolar diffusion length decreases with increasing germanium incorporation in the films while keeping the other deposition parameters the same. This decrease in the ambipolar diffusion length with increasing germanium incorporation is attributed to the increase in the DOS as evident from our CPM measurements. Moreover, for all the samples studied, we found that the diffusion length decreases with LS and this is also explained on the basis of the rise in the DOS. These findings are as expected and are in agreement with the published results (Ritter et al. 1987, Weiser and Ritter 1989, Sakata et al. 1997). A LPV is observed in all the samples studied. The LPV decreases upon increasing the temperature of measurement but increases upon LS. In all cases the magnitude of the LPV is found to be much larger than expected from the measured L. The observations are explained on the basis of the potential fluctuations present in the material. These arise from the heterogeneities, that is non-uniform distribution of hydrogen and variation in silicon and germanium concentrations from point to point in the a-SiGe: H film. In our model, the effect of these potential fluctuations is twofold. The electrons and holes become separated spatially in the presence of the potential fluctuations. This tends to reduce their recombination probability and might increase L. Secondly, there is an accumulation of localized charges in the states that coexist above the percolation edge with the extended states. Although these charges do not participate in conduction, they will give a contribution to the LPV. Other factors, for example band bending at the surface might also contribute by separating the carriers. Hence larger potential fluctuations are likely to give a large LPV, since both N and L in equation (14) are expected to be larger. Since the presence of a bias light is expected to reduce the potential fluctuations, the LPV should also be reduced. This explanation of the LPV in terms of potential fluctuations agrees with the observation that the value of L measured by SSPG decreases as the light intensity increases (Weiser and Ritter 1989). Further, we note that SSPG is always carried out in the presence of light and, therefore, is expected to give a smaller L than its value in the dark. In order to test this, we measured the LPV in the presence of bias light and found that the LPV decreases when the bias light is on (see figure 12). The increase in LPV after LS in all the samples can also be explained by this model. Since LS increases the potential fluctuations (Hauschildt et al. 1982, Agarwal 1996, Agarwal et al. 1996), the increase in the LPV is understandable. Thus we can explain all our data by the potential fluctuation model. A more quantitative analysis is not possible at this stage. It is, however, clear that a more heterogeneous sample should show a larger LPV since the potential fluctuations are more. This is an interesting observation which could be useful in comparing the quality of samples prepared under different conditions. This requires more experimentation.