Synthesis and characterization of silver doped zinc oxide thin films for optoelectronic devices

The synthesis and properties of Ag-doped ZnO thin films were examined. Epitaxial films of 0.6 at.% Ag doped ZnO grown at moderately low temperatures (300°C to 500°C) by pulsed laser deposition yielded p-type material as determined by room temperature Hall measurements. Carrier (hole) concentrations ranging on the order mid-1015 cm-3 to mid-1019 cm-3 were realized. Growth at higher temperatures yielded n-type material, suggesting that the Ag substitution yielding an acceptor state is metastable. Photoluminescence measurements showed strong near-band edge emission with little to no mid-gap emission. The stability of the Ag-doped films was examined as well. Persistent photoconductivity was observed. ZnO buffer layers drastically improved the surface morphology of films thicker than 1.0 microm.;Photoluminescence studies showed that Ag inclusion resulted in smaller non-radiative relaxation rates over surface states, which lead to UV emission enhancement. Room temperature PL measurements also showed a suppression of ZnO visible luminescence suggesting that Ag does not occupy interstitial sites or an antisite. Low temperature and temperature dependent PL spectroscopy revealed strong and dominant emissions originating from free electron recombination to Ag-related acceptor states around 3.31eV. The A°X emission at 3.352 eV was also observed at low temperatures. Enhancement of the PL intensity with increasing grain size was observed. The nature of the acceptor related emissions was confirmed. The acceptor energy was estimated to be 124 meV. Weak deep level emission at low temperatures indicated that in the p-type ZnO:Ag native donor and acceptor defects are suppressed suggesting the observed acceptor related PL emissions and hole concentration are from the Ag in ZnO instead of native defects. High temperature ZnO buffers and lattice matched MgCaO buffers helped improve the UV emission of the Ag doped films. The room temperature PL spectrum of Ag-doped ZnO was compared to that of undoped, P-doped, Ga-doped, and Ag-Ga- codoped ZnO. The Ag-doped ZnO films showed superior optical properties.;Finally, the fabrication and properties of rectifying Ag-doped ZnO/Ga-doped ZnO thin film junctions were reported. A rectifying behavior was observed in the I-V characteristic, consistent with Ag-doped ZnO being p-type and forming a p-n junction. The turn on voltage of the device was 3.0 V under forward bias. The reverse bias breakdown voltage was approximately 5.5 V. The highest light emission output power measured was 5.2x10-8 mW. At excitation currents of 10 mA, the applied voltage was approximately 2.0 V. After each measurement the light intensity decreased and the junction became Ohmic. The instability appears to be related to surface conduction and perhaps hydrogen incorporation. Finally, deposition of layers in reversed order (Ag-doped ZnO on bottom, Ga-doped ZnO on top) did not result in rectifying I-V characteristics. The reason for this is unclear but may relate to the differing growth temperatures used for the two layers.