Optimization of Copper Schottky Contacts on Nanocrystalline ZnO thin films by Atomic Layer Deposition
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Optimization of Copper Schottky Contacts on Nanocrystalline ZnO thin films by Atomic Layer Deposition Mei Shen1, Triratna P. Muneshwar2, Ken Cadien2, Ying Y. Tsui1, Doug Barlage1 1 Deparment of Electrical and Computer Engineering, University of Alberta, Edmonton, T6G 1H9, Canada 2 Department of Chemical and Material Engineering, University of Alberta, Edmonton, T6G 1H9, Canada ABSTRACT Contact metallization is an essential obstacle for utilizing low temperature achievable polycrystalline ZnO in any discrete devices and integrated circuits. To develop ZnO based semiconductor devices with advanced feature of flexibility, transparency and compatibility with low temperature processing, rectifying junctions must be fully developed. In this work, nanoscale polycrystalline ZnO thin films are fabricated with via low temperature ( 3kT/q. The saturation current , determined by the extracted intercept of ln(I) vs V plot, is theoretically related to the physical properties of the device by I0
§ ) B · AA*T 2 exp ¨ ¸ © kT ¹
(2)
where כܣis the effective Richardson constant (theoretically = כܣ32 A cm-2 K-2 for ZnO assuming ݉ = כ0.27݉ )[5], ܣis the area and ߔ is effective Schottky barrier height. All devices show similar barrier heights in the range of 0.5-0.6 eV, ideality factor of more than 2, indicating that the current in these contacts are not simply thermionic emission.
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Table. 1 Electrical parameters for Cu contact metallized ZnO thin film Schottky diode. Deposition Deposition Remote Annealing Schottky ɸB-IV ݊ T (K) 373 K 403 K 403 K 403 K 403 K 473 K 473 K 473 K 473 K
Method PEALD PEALD PEALD PEALD PEALD PEALD PEALD PEALD PEALD
plasma (s) 5 s O2 5 s O2 5 s O2 5 s O2 5 s O2 5 s O2 15 s O2 30 s O2 30 s Ar
T (K) / / 373 K 473 K 573 K / / / /
Metal Cu Cu Cu Cu Cu Cu Cu Cu Cu
(eV) 0.55 0.56 0.56 0.57 0.60 0.54 0.58 0.56 0.59
2.3 2.6 2.6 2.6 2.4 2.7 2.7 2.6 2.7
I-V curves for devices with different ZnO deposition temperatures from 100 °C to 200 °C are depicted and compared in Fig. 1. These devices all have identical cleaning condition of 5s O2 plasma. All devices have a similar Schottky barrier (ɸB) around 0.55 eV, and a decreasing ݊ and increasing on/off ratio as deposition temperature decreased. The on/off ratio of the sample with 100 °C deposition temperature is increased by one order of magnitude compared to that of 200 °C deposition temperature. The set of 200 °C PEALD-ZnO devices pretreated with diverse plasma conditions present distinct I-V behaviors as show in Fig. 2, indicating the current transport is obviously affected by the interfacial layer formed that is altered by the duration (5s30s) of the plasma cleaning process. The most likely cause of this variation is the underlying oxidation of Cu surface. Copper oxide is a well-known p
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