Systematic study of the influence of contacts on CPM results

Abstract

The constant photocurrent method (CPM) 1 is now widely used for the basic characterization of amorphous hydrogenated silicon (a-Si:H). For detailed deconvolution procedures 2 reliable data are of great importance. We therefore studied the influence of different contacts on the measured spectra of identical samples. Undoped a-Si:H samples were covered with coplanar contacts made from NiCr, Ag, or Ti. Different contact spacings were used to investigate the influence of scattering in the samples 3 . An additional n + layer between a-Si:H and metal was used on some samples to ensure good ohmic contacts as a reference. The influence of SF 6 plasma etching (necessary for removal of the n + layer) was also tested. We found that for all the above arrangements, the subbandgap shoulder increases with increasing distance of coplanar contacts 3 . However, significant differences were found in the magnitude and slope of the shoulder for different contact metals. These differences are not related to contact ohmicity (addition of n + layer has negligible influence). The preliminary results indicate that this effect can be at least partially explained by the reflection of light at the contact edges, whose height and shape can be critical in the subbandgap region.