| 摘要: | 本論文的內容主要為利用分子束磊晶成長氧化鋅系列(Zinc-oxide based)材料，為了使氧化鋅材料更廣泛被應用同時開發新穎的氧化物材料，本研究分別就n型高導電率氧化鋅薄膜製備、未摻雜氧化鋅薄膜缺陷分析以及氧硒化鋅三元材料成長進行系統化的探討。
N型高導電率氧化鋅薄膜相當有潛力可應用於元件之透明導電層(Transparent conductive layer)，其主要施體雜質為三族元素如鋁、鎵和銦元素，而以銦摻雜氧化鋅(In-doped ZnO)的研究相當少，且特性也不佳，因此本研究以分子束磊晶成長銦摻雜氧化鋅，分析其磊晶品質、電氣與光學特性並評估其應用在透明導電層的潛力。結果顯示銦摻雜氧化鋅相當輕易可藉由摻雜將載子濃度提高至1020 cm-3，但由於銦的原子尺寸較大，使得高摻雜下，氧化鋅磊晶品質與光譜上有明顯的衰化，而濃度雖然有達到減併態(Degenerate)的程度，但弱侷限態導致變溫量測時僅呈現半導體載子傳導型態，因此電阻率僅止於3.4×10-3 Ω?cm，輔以熱退火來提升材料品質，可觀察到銦摻雜氧化鋅由半導體傳導轉變成金屬傳導型態，而導電率可降至1×10-3 Ω?cm，惟相較於其他摻雜的氧化鋅特性上並非最佳。
本研究亦探討了鎵摻雜氧化鋅(Ga-doped ZnO)高導電率薄膜之特性，提出氧化鋅成長後結合鋅覆蓋和退火技術，有效抑制了文獻中提到熱退火後因鋅原子熱揮發(Thermal desorption)導致類受體(Acceptor-like)缺陷的形成。實驗結果顯示鎵摻雜氧化鋅電阻率可降至9×10-5 Ω?cm。相較於他人文獻中的記載，以此退火方式成長的氧化鋅不論在任何濃度下都能有較低的補償比(Compensation ratio)，且可維持較低的材料阻值，因此對於鎵摻雜氧化鋅應用於透明導電層上是相當重要技術。
另一方面，由於具高能隙的氧化鋅相當適合應用在透明電晶體(Transparent thin film transistor, TTFT)，但氧化鋅的本質缺陷(Intrinsic defect)對電氣特性的影響會造成元件特性不穩定，因此本論文中亦包括針對未摻雜氧化鋅與鎵摻雜氧化鋅系統性研究的結果。在其本質缺陷對電氣特性的影響研究觀察到，未摻雜與鎵摻雜氧化鋅在氮氣與氧氣環境中進行熱退火後產生相異的電氣特性，未摻雜氧化鋅在氧氣環境中退火後顯現出較佳的電性，而鎵摻雜氧化鋅在氮氣環境中退火則表現出穩定的特性。由光激光譜(Photoluminescence)中的缺陷訊號消長推測，氧化鋅電氣特性的變化可歸因於本質缺陷在不同退火環境中的產生與消弭，退火後的未摻雜氧化鋅主要來受到氧空缺(Vacancies)、反位(Anti-sites)和氧間隙(Interstitials)原子所影響，而鎵摻雜氧化鋅在退火過程中則是受到氧間隙原子所主導；針對不同的應用，未摻雜與鎵摻雜氧化鋅則將採用不同的退火環境。
在氧化鋅新穎材料開發方面，本研究探討了氧硒化鋅三元材料之磊晶成長與特性分析。以控制氧流量的方式，於砷化鎵基板上成長出具不同氧含量的氧硒化鋅薄膜，同時首先成長出氧含量達10 %的氧硒化鋅，其光學能隙能由硒化鋅的2.8 eV縮小至2.2 eV，其磊晶品質、光譜特性、載子動態行為等都一起被納入討論，其中重要突破是開發出以氯摻雜成長的n型氧硒化鋅，使該材料能夠被應用於在許多光電元件中。
本研究顯示，氧化鋅材料在應用於透明導電層、電子元件和太陽能電池上，相當具有潛力。
In order to manifest ZnO material into application and develop novel oxide-based materials, I have systematically investigated on three research areas over the past few years, including the development of n-type ZnO-based conductive layers, the influences of point defects on electrical properties in ZnO, and demonstration of ZnSeO alloys by plasma-assisted molecular beam epitaxy (MBE).
Highly conductive n-type ZnO has potential to employ into transparent conductive layers (TCL) and Al, Ga, and In are major dopant sources for achieving n-type characteristics. Al and Ga doped ZnO films have been extensively studied, while few efforts on the growth and characterization of In-doped ZnO films have been reported. Therefore, a serious of In-doped ZnO films was grown by MBE and the characteristics in respect of crystal quality, electrical, and optical properties were investigated and estimated for TCL applications. For electrical characteristics, In-doped ZnO films can easily increase their electron concentration over 1020 cm-3. However, the large size mismatch between Zn and In leads to a degradation on crystal quality and photoluminescence. Even though a degenerated concentration is achieved, no metallic transport behavior was seen and attributed a weak-localization effect caused by atomic disorder arrangement. Thus, when annealing process is introduced to improve crystal quality, a transition from semiconductor to metallic behavior can be observed. The resistivity can be reduced to 1×10-3 Ω?cm, and it is still not good enough compared to Al and Ga doped ZnO films.
The characteristics of Ga-doped ZnO films were also investigated, following an introduction of in-situ post thermal annealing under Zn flux. It’s found that this method effectively suppress the thermal desorption of Zn and the formation of acceptor-like defects. The resistivity of Ga-doped ZnO can reach as low as 9×10-5 Ω?cm in this study. Compared to reported findings, Ga-doped ZnO shows lower compensation ratio (NA/ND) at any electron concentration. Ga-doped ZnO with low compensation ratio and low resistivity are achieved simultaneously. In-situ annealing in Zn flux might become an important technology to manifest Ga-doped ZnO into transparent electrodes.
On the other hand, ZnO with large band gap is also suitable for transparent thin film transistor (TTFT). However, the influences of intrinsic defects on electrical properties might lead to unstable output characteristics in devices. Therefore, undoped and Ga-doped ZnO films grown by MBE were prepared for systematic investigations on the influences of native defects on their electrical properties. Distinct electrical properties of the undoped and Ga-doped ZnO films are observed after thermal treatments in nitrogen and oxygen ambient. It is found that the undoped ZnO films show improved characteristics when annealed in oxygen ambient, and Ga-doped ZnO films exhibit stable characteristics when annealed in nitrogen ambient. As revealed by defect emission in photoluminescence, the variation of electrical properties could be attributed to the generation and annihilation of native defects, depending on the ambient of treatments. It is concluded that the annealed undoped ZnO films are affected mainly by oxygen vacancies, antisite oxygen and oxygen interstitials, while the annealed Ga-doped ZnO films are dominated by oxygen interstitials. For specific applications, undoped and Ga-doped ZnO are annealed in different environments.
For development of novel oxide materials, characteristics of ZnSeO alloy were investigation in this thesis. A method for growing ZnSeO alloys on GaAs substrates by carefully controlling the oxygen flow rates was exploited. ZnSeO with varying oxygen content and even near 10 % oxygen was first demonstrated. The optical band gap can be reduced from 2.8 eV (ZnSe) to 2.2 eV. The characteristics including structural properties, optical properties, carrier dynamic and etc. are discussed in this chapter. For the first time, the n-type ZnSeO doped by Cl is obtained. Combined with band gap tuning, it can cover a wide range of wavelength region used in optical devices.
The dissertation shows that the ZnO-based materials with high Ga doping exhibit low resistivity, high transparency, and high stability. Meanwhile, the clarification of ZnO native defects shows very useful information in usage of electronic devices. A development of ZnSeO alloys makes a new opportunity for optoelectronic applications. According to the characteristics above-mentioned, ZnO-based materials show great potential to realize in transparent electrode, electronic devices, and PVs. |
