鈦/鋁金屬與氧化鋅-氧化銦錫共濺鍍薄膜歐姆接觸及其應用於蕭特基二極體之研究

Ti/Al ohmic contacts to ZnO-ITO cosputtered films and its applications to Schottky diodes

蔡富鈞(Fu-Chun Tsai)

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(2009年)

摘要

本研究於室溫下利用射頻磁控式共濺鍍系統，選擇氧化鋅與氧化銦錫為共濺鍍靶材，在藍寶石基板上沉積共濺鍍薄膜（Zn/(Zn+In) = 80 at.%），並利用熱處理製程製作具有電子濃度為1017 ~ 1018 cm−3之氧化鋅-氧化銦錫薄膜，在薄膜上分別沉積鋁以及鋁/鈦歐姆接觸電極，藉由半導體參數分析儀、X光繞射分析儀、歐傑電子能譜儀之縱深分佈以及X光光電子能譜儀量測熱退火前後歐姆接觸特性變化及機制。結果顯示由於鈦原子較鋁原子更容易與界面氧原子產生鍵結，使共濺鍍薄膜表面產生較多的氧空位缺陷，進而造成薄膜表面電子濃度提升，因此利用鈦/鋁金屬在共濺鍍薄膜上製作歐姆接觸，在熱退火前具有優異的接觸電阻率8.78 × 10−4 Ωcm2，在氮氣環境於不同溫度熱退火處理後，當退火溫度為200oC時具有最低接觸電阻率8.30 × 10−6 Ωcm2，這是由於更多氧化鋅-氧化銦錫薄膜表面之氧原子外擴散並與鈦原子產生鈦-氧（Ti-O）鍵結，而提高氧化鋅-氧化銦錫薄膜表面之電子濃度，進而大幅降低了鈦/鋁金屬與共濺鍍薄膜之歐姆接觸電阻率；而當熱退火溫度達到400oC時，由於鈦原子與鋁原子形成鈦鋁合金（Ti3Al），造成劣化了鈦/鋁金屬與共濺鍍薄膜之歐姆接觸電阻率特性。接著，再利用鈦/鋁金屬對氧化鋅-氧化銦錫共濺鍍薄膜作歐姆接觸以及鎳/金金屬對未摻雜氧化鋅薄膜對作蕭特基接觸，於藍寶石基板上製作氧化鋅蕭特基二極體，研究中可以發現，在氧化鋅薄膜表面未進行處理之蕭特基元件，其元件在外加2伏特之逆向偏壓下的漏電流為7.21 × 10−4 A、蕭特基位障為0.55 eV、理想因子為2.12；而對氧化鋅表面進行氧電漿以及雙氧水表面處理後，其蕭特基元件之漏電流分別降至3.26 × 10−8 A及1.42 × 10−10 A，且蕭特基位障提升至0.82 eV及0.89 eV，理想因子則分別為1.22及1.13；因此，藉由氧電漿以及雙氧水表面處理後，將可有效降低薄膜的表面態位密度，大幅提升蕭特基二極體的整流特性。

關鍵字

射頻磁控共濺鍍 ； 氧化鋅 ； 歐姆接觸 ； 蕭特基接觸

並列摘要

In this study, the ZnO-ITO films were grown by rf magnetron cosputtering deposition on sapphire substrate, using 2 in. diameter targets of ITO (purity 99.99% with In2O3 : SnO2 = 90 : 10 wt %) and ZnO (purity 99.99%). The cosputtered film at an atomic ratio of 80% [Zn / (Zn + In) at.%] was annealed under oxygen ambient and possessed an electron carrier concentration of 1017 ~ 1018 cm−3. Prior to lithography, the annealed ZnO-ITO films were ultrasonically degreased in acetone, methanol, and rinsed in deionized water for 5 min in each step. A standard photolithography technique was used to pattern transmission line method (TLM). Ti/Al and Al metal electrode were then deposited on the cosputtered films by electron beam evaporation. Using Current–voltage (I–V), glancing angle X-ray diffraction (GXRD), Auger electron spectroscopy (AES), and X-ray photoelectron spectroscopy (XPS) depth profiles analysis, a better understanding of the mechanism involved in a Ti/Al ohmic contact has been developed. It is shown that the as-deposited Ti/Al contacts produce a specific contact resistivity of 8.78×10−4 Ωcm2. The accumulation of oxygen vacancies (Vo) at the region near the surface of the ZnO-ITO films was found and the Ti-O bonds were formed more than the Al-O bonds. It is also noted that the carrier concentration of the Ti/Al metallization scheme is higher than the Al metallization scheme. However, annealed Ti/Al contacts exhibit linear current–voltage characteristics, indicating that high-quality ohmic contacts are formed. The Ti/Al scheme produces a lowest specific contact resistance of 8.30×10−6 Ωcm2 when annealed at 200oC for 60 sec under nitrogen ambient. The relative intensity of the Ti-O phases remained virtually unchanged with increasing temperature. For the 400oC sample, new phases, such as Ti3Al, were found to occur. The Ti3Al phases would lead to a high specific contact resistivity. Then, ohmic contacts to cosputtered films using Ti/Al metallization scheme, and its applications to Ni/Au Schottky diodes. It is shown that the Ni/Au contact to conventionally cleaned undoped ZnO surface showed 7.21 × 10−4 A leakage current to −2 V, a barrier height of 0.55 eV, and an ideality factor of 2.12. However, the contact produces a Schottky behavior, when the undoped ZnO surface is treated in oxygen plasma-treated. The Ni/Au contact to oxygen plasma-treated undoped ZnO surface showed 3.26 × 10−8 A leakage current to −2 V, a barrier height of 0.82 eV, and an ideality factor of 1.22. And the contact produces a Schottky behavior, when the undoped ZnO surface is treated in a boiling hydrogen peroxide solution at 100oC for 1 min. The Ni/Au contact to hydrogen peroxide-treated undoped ZnO surface showed 1.42 × 10−10 A leakage current to −2 V, a barrier height of 0.89 eV, and an ideality factor of 1.13. The reversed leakage current as well as the ideality factor of the conventional Schottky diodes were effectively reduced with oxygen plasma treatment and hydrogen peroxide treatment on the Schottky contact surface, respectively. Thus, elimination of the surface contaminations and the compensation of the oxygen-related vacancies were the mechanism to reduce the electron tunneling effect and led to the better Schottky diodes performance.

並列關鍵字

Rf magnetron cosputtering ； zinc oxide ； ohmic contact ； Schottky contact

參考文獻

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