(Invited) Antireflective and Sub-Band Pumped Photoconductive Solution Processed ZnO and Al:ZnO Quasi-Superlattice Films

Abstract

Zinc oxide (ZnO) and Al-doped ZnO are important optoelectronic materials. ZnO in particular has a wide band gap (Eg ~ 3.3 eV at 300 K), large exciton binding energy (~66 meV) and especially for the variety of methods by which it can be processed. Moreover, ZnO is readily able to alloy with other metals in the oxide form and has a lattice that can facilitate interstitial doping using several metals, including Al and Sn. This gives ZnO a key role in optoelectronics, metal oxide thin films and thin film transistor (TFT) technologies. We demonstrate that crystalline, epitaxial-like and highly ordered ZnO and Al:ZnO (AZO) quasisuperlattice thin films can be achieved from a precursor liquid at relatively low temperature via spin-coating, which crystallize with near epitaxy with a pronounced c-plane texture. We also show the visible and near-infra red (VIS-NIR) light spectroscopy of ZnO and Al:ZnO multi-layered thin film structures grown on oxidized silicon substrates and also define the growth conditions and processing to provide tunable antireflection coatings of ZnO and AZO. Lastly, we show measurements demonstrating sub-band optical pumping of AZO QSLs that enable significant photoconductivity, whereas negligible photoconductance is observed in similarly grown ZnO QSLs. In AZO QSLs, conduction band electrons from valence band pumping at incident irradiation > Eg, are not required to initiate or enhance photoconductivity. The photoconductance scales with optical pump power and points to electron mobility possibly from transitions within the band gap of Al:ZnO or from localized photo-processes in Al-doped ZnO. References D. Buckley, D. McNulty, V. Z. Zubialevich, P. J. Parbrook and C. O'Dwyer, J. Vac. Sci. Technol. A, 35, 061517 (2017).D. Buckley, D. McNulty, V. Z. Zubialevich, P. J. Parbrook and C. O'Dwyer, ECS Trans., 77, 99 (2017).D. Buckley, R. McCormack and C. O'Dwyer, J. Phys. D: Appl. Phys., 50, 16TL01 (2017).D. Buckley, R. McCormack, D. McNulty, V. Z. Zubialevich, P. J. Parbrook and C. O'Dwyer, ECS Trans., 77, 75 (2017).