Selective Growth of Rectifying Contacts on ZnO Thin Films and Their Applications Toward Photovoltaics

Résumé du livre

Renewable energy sources draw unanimous attention in 21st century as the available natural sources are gradually being depleted. This alarming situation has prompted scientific communities to emerge with different forms of renewable energy sources; solar energy being the most popular so far among all these. The basis of photovoltaic devices is the formation of a junction among two different types of material. In the present study, properties of rectifying junctions on oxide based semiconductor zinc oxide(ZnO) had been thoroughly investigated. ZnO is probably the most explored oxide based semiconductor because of its noble properties such as a wide bandgap of 3.34 eV and exciton binding energy of 60 meV at room temperature. However, inherent high carrier concentration is a hurdle to produce a rectifying contact for ZnO. Schottky barriers are formed only on moderately doped semiconductors. Hence, ZnO films grown using pulsed laser deposition were subjected to adequate annealing in ambient oxygen pressure to diminish oxygen vacancies. In return, this suitably controlled the carrier concentration. Structural characterization was carried using X-ray diffraction (XRD) and optical properties were further studied by UV-VIS spectroscopy. The XRD study confirmed a preferred orientation along c-axis for these films. Carrier concentrations were calculated by Hall measurement, using the van-der Pauw configuration. Annealing in oxygen scaled down the carrier concentration from 1020 cm-3 to 1015 cm-3. The band gap could be modulated from 3.60 eV to 3.24 eV with presence of oxygen. Finally the device performance had been concluded using I-V characterization. With extensive annealing, i.e. decrease in carrier concentration the junction's behavior switched from ohmic to a rectifying one. This rectifying contact forms the basis of future photovoltaic devices.