ELSEVIER Solar Energy Materials and Solar Cells 34 (1994) 533-539 sow rnwqwt~ma~ md ,~lat OWti Deep defect determination by the constant photocurrent method (CPM) in annealed or light soaked amorphous hydrogenated silicon (a-Si" H) Andreas Mettler, Nicolas Wyrsch, Michael Goetz, Arvind Shah Institute of Microtechnology IMT, University of Neuchatel, rue Breguet 2, CH-2000 Neuchatel, Switzerland Abstract We present systematic measurements of CPM on two independent series of slightly phosphorus and boron doped films. For "n-type" samples of both series, the CPM deep defect absorption is proportional to the square root of the gas dopant ratio. For these samples we discuss the influence of Fermi level on the CPM spectra. For slightly "p-type" samples, CPM deep defect absorption as evaluated by CPM becomes higher than the corresponding PDS-values. This fundamental problem can be traced back to the violation of two basic conditions necessary for a correct evaluation of the absorption from CPM measurements: (1) the power law exponent 3' (Rose factor) of the photoconductivity must be spectrally independent, and (2) the generation rate G, which corresponds to the CPM photocurrent, also has to be spectrally independent. Further, we compare the annealed and the "saturated" light soaked states of selected slightly doped samples and an undoped sample: the variations in the CPM deep defect absorption and in photoconductivity due to light-soaking are discussed. 1. Introduction The Constant Photocurrent Method (CPM) [1] measures the absorption spec- trum of amorphous, hydrogenated silicon (a-Si:H) and is well established as a tool to determine the deep defect density of undoped a-Si:H films in the annealed or light soaked states. Absorption at a well defined sub-bandgap energy (usually 1.2 eV) can be used to monitor the deep defect density [2]. In comparison with the Photothermal Deflection Spectroscopy (PDS) method, CPM is less sensitive to surface states. 0927-0248/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0927-0248(94)00074-3