Tuning Photoresponse of Ultrathin ZnO Films Using Few Angstrom Al2O3 Overlayers
To understand the phenomena further, we report here temperature-based (80K-320K) conductance and photoresponse (Ion/Ioff) properties of 6.1 nm ZnO. Further, we track these parameters as a function of ALD Al2O3 ‘overlayers’ deposited on the same ZnO film. The temperature based Ion/Ioff is shown in Figure 1, left and the maximum Ion/Ioff is shown as a function of ALD Al2O3 overlayers in Figure 1, right. Based on these data, the Ion/Ioff ratio initially increases with the number of ALD Al2O3 layers and maximizes at 7 cycles of Al2O3 overlayer. It then decreases for additional ALD Al2O3 cycles. A maximum Ion/Ioff of 11.4x is obtained with 7 cycles of ALD Al2O3.
Activation energy (Ea) for conduction will be estimated using conductance vs. temperature curves.[iv] Accompanying data using XPS and photoluminescence will structurally probe the surface modification of the ZnO due to addition of the ALD Al2O3monolayers. These results lead to insights into the passivation mechanisms of ultrathin ZnO films with applications in a wide variety of electronic and optoelectronic devices.
Figure 1. Left - The Ion/Ioff vs. temperature (left) and, Right - The maximum Ion/Ioff plotted with number of ALD Al2O3 cycles. (*Ion/Ioffis defined by the light conductance over the dark conductance.)
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[iv] Wu, F., Tian, L., Kanjolia, R., Singamaneni, S., Banerjee, P. ACS Appl. Mater. Interfaces, 2013, 5, 7693-7697.