| Summary: | 碩士 === 國立虎尾科技大學 === 光電與材料科技研究所 === 96 === To accomplish high-performance ZnO-based optoelectronic devices, the formation of high quality metal electrode contacts is essential. A superior rectifying junction with metals and low-resistance ohmic contacts onto the ZnO surface was the best mechanism that promoted their use in diode, UV detector, gas sensor, piezoelectric transducer, and optical applications . There were many reports addressing the mechanisms for the difficulties in the formation of ZnO-based Schottky diodes, including surface morphology, environment activity and subsurface defects . A variety of ZnO surface treatment methods, such as chemical preparation with acid, plasma or irradiation treatments, and surface passivation via a chemical solution were demonstrated to removal the interfacial states of the metal Schottky contact to ZnO. To date, magnetron sputtering is a commonly used system for deposition crystalline ZnO films in application on the optoelectronic devices. However, there were very limited reports on Schottky contacts of ZnO, especially for that of sputtered-ZnO thin films.
In this study, the 2 μm-thick undoped-ZnO film was deposited onto silicon substrate using rf magnetron sputtering system and then annealed at 700oC for 30 min under oxygen ambient to achieve a superior c-axis orientation with oxygen-terminated crystalline structure. The films were undoped but show n-type conduction (~ 3.83 ? 1011 cm-3). Ni/Au and Al were respectively, employed to form Schottky and ohmic contact on the ZnO-based structures. These contact metals patterned directly by lift-off of evaporated films onto the ZnO film was denoted as the conventional Schottky diode (sample A), whereas that of the Schottky contact surface processed with an additive oxygen plasma treatment at 270 W for 10 min prior to metal deposition was classified as sample B.
In addition to the conventional Schottky diode structure, another set of multilayer Schottky diode structure (sample C) with a homogeneity ITO-ZnO cosputtered layer (~ 250 nm) deposited onto the undoped-ZnO film also prepared to improve the ohmic contact performance. The ITO-ZnO cosputtered film at an atomic ratio of 90% [Zn / (Zn + In) at.%] was annealed at 300oC for 30 min under oxygen ambient and possessed an electron carrier concentration of 7.01 ? 1018 cm-3 . Detail structures of the conventional and multilayer Schottky diodes structures are illustrated in Fig. Carrier concentration and hall mobility of the deposited films were measured by the van der Pauw method. The crystalline structures and surface morphologies were examined by XRD and AFM measurements. Current-voltage properties of these Schottky diode structures were characterized using semiconductor parameter analyzer (HP4156C).
A comparison for the I-V characteristic of Ni/Au Schottky contacts to the undoped-ZnO surface with and without oxygen plasma process is shown. Both the reverse and forward currents of the conventional Schottky diode were markedly reduced after processing with an additive oxygen plasma treatment. In addition, the ratios of the forward to leakage current measured at -2 and 2 V were also increased from 4.78 (sample A) to 14.25 (sample B), indicating a better rectifying behavior. The convention Schottky diode had a high ideality factor (n) of 2.47, meaning that the existence of multiple current pathways other than thermionic emission. In contrast, the ideality factor and barrier height (ΦB) were evaluated to be 1.92 and 0.82 eV, respectively. The reduction in the ideality factor as well as the increase in the barrier height performances was consisted with the report that addressed to be the donor-like defect passivation of the oxygen radical in the plasma diffused into the host lattice in the ZnO films surface . Although the rectifying behavior had been significant improved through the oxygen plasma treatment on the Schottky contact surface, the forward current was too small due to the poor ohmic contact behavior. The specific contact resistance was greatly decreased to 1.44 ? 10-3 ? cm2 with a homogeneity ITO-ZnO cosputtered film deposited onto the undoped-ZnO film.
Elimination of surface carbon- and hydrogen-related contaminations as well as the compensation of the oxygen-related vacancies reduced not only the defect-assisted tunneling of electron but also the net carrier concentration at the Ni/ZnO surface leading to the better Schottky diodes performance.
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