Investigation of localized levels in Na-intercalated GaSe single crystal

Abstract

To identify sodium related and intrinsic structural defect levels in Na-intercalated GaSe single crystal; photoconductivity (PC), current–voltage characteristics (CVC) and electrical conductivity (EC) were investigated. Spectral distribution of PC reveals two bands; the main peak of the first band which lies at 2.052 eV is ascribed to indirect excitonic transitions whereas the weak peak of the second band which lies at 1.90 eV is attributed to optical transitions from the acceptor level A 1 of sodium related defects with ionization energy 0.152±0.002 eV to the indirect conduction band. Temperature dependence of dark current in space-charge limited current (SCLC) and Ohmic regions indicates the presence of a single discrete trapping level A 1 located at energy E t=0.153±0.002 eV above the valence band, this value is in good agreement with that obtained from the temperature dependence of the trapping factor θ; with E t=0.154 eV and N t=3×10 10 cm −3. This trapping level which acts as hole traps originates from sodium related defect acceptor level A 1. Furthermore, electrical conductivity measurement reveals two additional acceptor levels A 2 and A 3 with activation energies 0.322 and 0.560 eV, indicating that conduction in GaSe is extrinsic. In the light of the experimental results an energy diagram of the forbidden gap of Na-intercalculated GaSe has been proposed.