CHARGE-TRANSPORT IN AMORPHOUS AND TETRAGONAL SEMICONDUCTING YBACUO FILMS

We have explored the charge transport mechanisms in six different YBaC uO semiconducting thin films in the temperature range of 70 K to room temperature. Two of the samples were deposited on LaAlO3 substrate and were tetragonal with the composition of YBa2Cu3O6.5 and YBa2Cu3O6.3. The other four were amorphous as-deposited on Si substrate with and wi thout a MgO buffer layer, and on an oxidized Si substrate with and wit hout a MgO buffer layer. Ail tested films exhibited semiconductor-type resistance vs. temperature characteristics with increasing resistance as the temperature was decreased. Around room temperature all six sam ples had thermally activated transport characteristics that was interp reted as activation of hole-like carriers from localized states around the Fermi level to extended states. As the temperature was decreased, two tetragonal samples went through a transition to a variable range hopping-like conduction. The amorphous ones remained within the therma lly-activated transport regime in the temperature range of 253 K to 31 8 K, with E-A approximate to 0.2 eV. (C) 1997 Elsevier Science Ltd.