Direct measurement of Urbach tail and gap state absorption in CuGaSe2 thin films by photothermal deflection spectroscopy and the constant photocurrent method

The applicability of photothermal deflection spectroscopy PDS and the constant photocurrent method CPM to chemical vapor deposited CVD and physical vapor deposited PVD CuGaSe2 CGSe thin films is investigated. Process dependent variations in direct band gap energies, Urbach tail widths, bulk and surface defect densities of the films are given as a function of composition. Differences in the PDS and CPM absorption spectra of more than one order of magnitude are found. Using the standard a Si theory of PDS and CPM analysis, a two layer system consisting of a defect rich near surface layer and a less defective bulk layer is found in device quality films. Optical improvements due to the Ga Se annealing step as a second stage of the growth process result in a reduced bulk defect density while the near surface defect density remains unchange