Growth, Optical Absorption, and Photoresponse of Copper Oxide Thin Films and Nanocavities
Parry, James P.
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Copper oxide, Cu 2 O, is one of the most studied semiconductors having been used in devices dating back to the 1920’s. The material received additional study recently as an absorbing material in solar cells and hydrogen evolution reactions. The thickness of Cu 2 O in those devices is often hundreds of nanometers to over one micron thick. This work studies the use of thin film interference to enhance the optical absorption and photoresponse in very thin Cu 2 O films. The first section focuses on the growth of single phase Cu 2 O by reactive sputtering. The impact of synthesis parameters including sputtering rate, substrate temperature, oxygen flow, and post-growth annealing on deposited copper oxide films were studied. Other copper oxide phases, Cu 4 O 3 and CuO were evident when oxygen was added to the post-deposition annealing chamber atmosphere. Very thin Cu 2 O films were deposited on sputtered Al films by reactively sputtering Cu with DC power at 50W, Ar/O 2 flow 30/16 sccm, substrate deposition temperature at 150°C, followed by vacuum annealing at 350°C. Thin film interference in Cu 2 O grown on Al films was observed to highly enhance the absorption of films below 100nm thickness. 70nm of Cu 2 O on Al absorbed 96% of incident light at 548nm. The absorption resonance wavelength increased with increasing copper oxide thickness, demonstrating the tunability of the resonance maximum. Thin layers of Al 2 O 3 , 15nm or less, between the Al and Cu 2 O films modified the total absorption but not in a coherent manner. The optical absorption of nanocavities consisting of Al/Al 2 O 3 50nm/Cu 2 O were synthesized, optical absorption and photoresponse measured. The photoresponse of the synthesized nanocavities to light from a solar simulator was enhanced for short and long time scales, 1–3 minutes and several hours respectively. The photocurrent of 60nm Cu 2 O nanocavities doubled during a 2.5hr light soak, which was not enough to saturate the photoconductivity. Persistent photoconductivity recorded post-illumination in the nanocavities showed dependence on the light soak duration. The longer persistent photoconductivity time decay constant for a 3min light soak was τ 2 =117s, while for a 141min light soak the longer time decay constant was τ 2 =1904s.