氧化鋅薄膜與金屬電極接觸特性之研究

Study of metal electrode contacts to Zinc Oxide thin films

徐政維(Cheng-Wei Sheu)

指導教授 ： 劉代山

國立虎尾科技大學/電機資訊學院/光電與材料科技研究所/碩士(2008年)

摘要

本研究利用射頻磁控濺鍍系統，分別沉積未摻雜氧化鋅薄膜及n型氧化鋅-氧化銦錫共濺鍍薄膜，並利用這些薄膜製作蕭特基二極體元件，研究初期，為使所製備之氧化鋅薄膜具有較佳的結晶特性並抑制薄膜內部之缺陷濃度，將所沉積之試片於氧氣環境中進行高溫熱處理製程。研究結果顯示，當熱處理溫度達700 ℃時，氧化鋅薄膜具有較佳的結晶特性，因此利用此薄膜結構條件來製作蕭特基二極體。針對蕭特基接觸研究結果顯示，以此結構所製作之蕭特基二極體元件，由於金屬與氧化鋅薄膜間有較多的表面態位密度，使得元件整流特性不佳，為了改善元件的整流特性，本研究利用適當的氧氣電漿表面鈍化處理技術，鈍化未摻雜氧化鋅薄膜的表面態位密度，結果顯示薄膜表面經氧氣電漿處理後，元件逆向飽和電流獲得明顯的改善，且蕭特基位障從0.75 eV上升至0.83 eV，而理想因子也從未經表面電漿處理時的1.87下降至1.34，顯示適當氧氣電漿處理確實可以有效改善表面態位密度所造成蕭特基元件特性劣化的影響。此外，為了進一步改善使用單層氧化鋅薄膜製作蕭特基元件時，由於歐姆接觸特性不佳而造成順偏導通電流較小的缺點，本研究在未摻雜氧化鋅薄膜表面再沉積一層具有較高濃度之氧化鋅-氧化銦錫薄膜並製作歐姆接觸電極，藉此改善元件的順向導通電流特性。研究結果顯示，雙層結構之蕭特基元件可以保有與單層蕭特基元件相同的蕭特基位障與理想因子，更可大幅提升順向導通電流與逆向飽和電流之比值，進一步優化蕭特基元件之整流特性。

關鍵字

氧化銦錫 ； 氧化鋅

並列摘要

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.

並列關鍵字

ITO ； ZnO

參考文獻

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