Strongly temperature-dependent free-energy barriers measured in a polycrystalline semiconductor

We measured temperature-dependent changes in free-energy barriers for emission of holes and electrons from metastable defects in polycrystalline CuIn1-xGaxSe2 using transient capacitance techniques. The free-energy barrier for holes increases by 17 meV, while the barrier for electrons decreases by 30 meV as the temperature increases from 380 to 400 K. The usual relation between thermal emission time and activation energy is reversed in this temperature range (we find a shorter emission time for the larger activation energy). Large entropy changes of +15 k(B) and -21 k(B) for the electron and hole emission, respectively, are calculated using transition-rate theory. These entropy terms in the free energy barrier account for the temperature dependences and the emission time inversion.