Trapping levels and anisotropy of electrical conductivity of Na-intercalated GaS single crystals

To investigate hole trapping levels in Na-intercalated GaS single crystals, temperature dependence of electrical conductivity (EC) and current-voltage characteristics (CVC) were studied. Analysis of the experimental results i ndicates that temperature dependence of EC reveals two hole trapping levels located at energies (0.68 +/- 0.02) and (0.53 +/- 0.02) eV above the valen ce band. It is suggested that, the first level originates from sodium relat ed defects and the second level is ascribed to the presence of Al impuritie s and/or structural native defects. Anisotropy of EC was discussed in detai l. Furthermore, the concentration of hole traps was calculated from the tem perature dependence of the trapping factor (theta), N-t = 2.5 x 10(16) cm(- 3) along the c-axis and 9.2 x 10(18) cm-3 for E perpendicular to c. The cal culated value of E-t lies between (0.69 +/- 0.02) and (0.66 +/- 0.02) eV an d is consistent with that obtained from the temperature dependence of stead y state dark current, free holes concentration and EC measurements. Debye l ength and the height of the potential barrier at Ag : GaS contact were dete rmined at different temperatures. From the above mentioned analysis it is s uggested that the conduction in Na-intercalated GaS:Na is an extrinsic and thermally activated process with activation energy increasing with the appl ied voltage. It is also concluded that hole traps are exponentially distrib uted in the band gap of these crystals. (C) 1999 Elsevier Science B.V. All rights reserved.