Tuning Photoresponse of Ultrathin ZnO Films Using Few Angstrom Al2O3 Overlayers

Ultra-thin ZnO films can be deposited with precise thickness control, good conformality and uniformity by atomic layer deposition (ALD). This strategy has been widely used in optical devices and solar cells.[i] However, ZnO with a thickness less than 10 nm – where surface effects dominate electronic transport properties, has been found to possess a high degree of electronic defects.[ii] These defects originate on the surface as unterminated, dangling bonds and degrade electronic and optoelectronic transport properties.[iii]

To understand the phenomena further, we report here temperature-based (80K-320K) conductance and photoresponse (I_on/I_off) 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 I_on/I_off is shown in Figure 1, left and the maximum I_on/I_off is shown as a function of ALD Al₂O₃ overlayers in Figure 1, right. Based on these data, the I_on/I_off 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 I_on/I_off of 11.4x is obtained with 7 cycles of ALD Al₂O₃.

Activation energy (E_a) 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 I_on/I_off vs. temperature (left) and, Right - The maximum I_on/I_off plotted with number of ALD Al₂O₃ cycles. (*I_on/I_offis defined by the light conductance over the dark conductance.)

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[i] a) Lee, D.-J., Kim, H.-M., Kwon, J.-Y., Choi, H., Kim, S.-H., Kim, K.-B. Adv. Funct. Mater. 2011, 21, 448-455. b) Wang, J.-C., Weng, W.-T., Tsai, M.-Y., Lee, M.-K., Lee, M.-k., Horng, S.-F. Perng, T.-P., Kei, C.-C., Yu, C.-C. Meng, H.-F. J. Mater. Chem. 2010, 20, 862-866.

[ii] a) Cheun, H., Fuentes-Hernandez, C., Zhou, Y., Potscavage, W. J., Kim, S.-J., Shim, J., Dindar, A., Kippelen, B. J. Phys. Chem. C 2010, 114, 20713-20718. b) Na, J.-S., Scarel, G., Parsons, G. N. J. Phys. Chem. C 2010, 114, 383-388.

[iii]  Cheun, H., Fuentes-Hernandez, C., Zhou, Y., Potscavage, W. J., Kim, S.-J., Shim, J., Dindar, A., Kippelen, B. J. Phys. Chem. C 2010, 114, 20713-20718.

[iv] Wu, F., Tian, L., Kanjolia, R., Singamaneni, S., Banerjee, P. ACS Appl. Mater. Interfaces, 2013, 5, 7693-7697.

See more of: Energy Applications I
See more of: P1: Atomic Layer Deposition Applications 10
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