Study of deep hole trap levels associated with bias-induced metastabilities in Cu(In,Ga)Se2 thin films using isothermal capacitance transient spectroscopy

Abstract

The origins of bias-induced metastabilities in Cu(In,Ga)Se2 thin film samples were investigated using isothermal capacitance transient spectroscopy. We confirmed that these metastabilities are associated with deep hole trap levels, and examined these levels in detail. The trap has multiple charge states, tentatively assigned to a double acceptor, where the first (−1/0) acceptor level has an apparent activation energy of 0.72 eV and the second acceptor level (−2/−1) has an apparent activation energy of 0.51 eV, indicating negative-U-like properties. The metastable behavior becomes prominent on forward biasing of the junction, which was attributed to the slow hole-capture rate of the first (−1/0) acceptor level. This slow rate is probably because of a large lattice relaxation associated with the hole-capture process, as well as depletion of free holes because of the excess concentration of the deep level compared with the net concentration of shallow acceptors.