Dark conductivity, steady state photoconductivity and space charge limited conduction of Cu and Al doped amorphous As2Se3
Wafula, Brian A.
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Electronic transport has been investigated in amorphous As2Se3 doped with 1 at % Cu and 1 at. % Al through measurements of dark conductivity, photoconductivity and space charge limited (SCL) conduction. Dark conductivity and SCL conduction were studied in the 253 - 373K temperature range and photoconductivity in the 233K - 343K temperature range. Through the thermal EMF test, all the samples were found to be p-type. The majority charge carriers (holes) were the same both in the dark and under illumination. The electrical conductivity was found to be thermally activated and showed two types of conduction mechanisms, that of hopping conduction in the localized states at low temperature and conduction in the extended states at high temperature. The activation energy E6 and the pre-exponential coefficient 60 of the conductivity expression =°exp (E/KBT were determined. For the conduction in the extended states E=1.03±0.02eV and °=4.90x104-1cm-1 for pure As2Se3, 0.88±0.0eV and 1.34x103-1cm-1 for Cu doped As2Se3, and 0.65±-0.02eV and 7.35x103-1cm-1 for Al doped As2Se3. In the hopping conduction region, E=0.58±0.02eV and °=1.23x10-2-1cm-1 for pure As2Se3, 0.36±0.02eV and 1.371x10-6-1cm-1 for Cu doped As2Se3 and 0.62±0.03eV and 3.72x10-5-1cm-1 for Al doped As2Se3. Activation energies from photoconductivity measurements for = 6332A° and E+ = 0.32±0.01eV, 0.25±0.01eV and 0.22±0.01eV and for = 5292A° of visible spectrum were determined. In high temperature region above the maximum where the dark conductivity is more than the photoconductivity the activation energies for = 6332A° were E- = 0.34±0.01eV, 0.28±0.02eV and 0.20±0.01eV and for x - 529A°, E- = 0.47±0.01eV, 0.33±0.01eV and 0.20±0.01 for pure As2Se3, Cu doped and Al doped As2Se3 respectively. In the intermediate temperature region immediately below the maximum where photoconductivity is higher than the dark conductivity E+ = 0.32±0.01eV, 0.25±0.01eV and 0.22±0.01eV and for = 529A0, E+ = 0.50±0.02eV, 0.26±0.01eV and 0.25±0.01eV for pure As2Se3, Cu doped and Al doped As2Se3 respectively. The maximum photoconductivity for = 6332A° occurred at 313K for pure As2Se3 and at 303K for both Cu and Al doped As2Se3. SCL conduction measurements showed that the samples had both deep and shallow traps confined in single or multiple discrete energy levels in the energy gap. Trap concentration for pure sample, Nt, was 3.49 x 1019/cm3. The shallow traps lie 0.27±0.03eV deep in the energy gap. The results have been explained on the basis of CFO, dangling bonds and small polaron models.