Fabrication and evaluation of mis junctions on Zinc Oxide
Chen, Yen Jen
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Zinc oxide (ZnO) is a promising material for optoelectronic devices, with a wide bandgap and large exciton binding energy. Its high transparency in the visible wavelength region makes it an attractive material for visible-blind photodetectors. As a transparent high conducting oxide (TCO), ZnO can be doped with Al to tune its conductivity. Moreover, Al-doped ZnO (AZO) is relatively cheaper and more available compared to other TCO. AZO can form an n-n isotype heterojunction with silicon, which can be used for photodiodes and solar cells. Schottky contacts on ZnO are useful in many applications including photodetectors and metal semiconductor field-effect transistors. Combining a Schottky junction with a heterojunction of AZO/n-Si, photodiodes fabricated give high UV to dark contrast ratios. The study of metal-insulator-semiconductor (MIS) and Schottky ZnO photodiode and double junction photodiode was performed. ZnO films were deposited by RF sputtering, while AZO films were deposited by dual beam sputtering. MIS Schottky contacts were studied with different metals and different contacts. Au and Pd were chosen for their high work function, and Ag diodes gave even better results due to oxidation. Metal contacts were deposited using thermal evaporation and insulating layers Al 2 O 3 and HfO 2 deposition were achieved by atomic layer deposition. The oxide thicknesses were varied from 0 to 4 nm. I-V performance of dark, photo and UV illumination and characteristics of MIS Schottky photodiode and double junction were analyzed. For metal contact, Ag diodes gave the best performance, with a barrier height as high as 0.798 eV and a low dark saturation current of 1.20x10 -7 A/cm 2 with hafnium oxide as the insulating layer with a thickness of 2 nm. For oxide, HfO 2 with a thickness of 2 nm gave the most optimum performance, with low leakage current and higher turn on voltage, and a good UV response. Ag HfO 2 double junction diodes gave an ideality factor of as low as 1.74, J 0 of 2.79x10 -7 A/cm 2 , and a high barrier height 0.776 eV. For solar cell results, double junction solar cell gave the performance of V OC of 0.369 V and J SC of 16.2 x 10 -3 A/cm 2 .