利用有機金屬化學氣相沉積法成長氧化鎵鋅磊晶膜之金氧半場效電晶體製作及其特性研究

Study on MOSFETs Fabrication and Related Properties of ZnGa2O4 Epilayers Grown by MOCVD

鄭力中(Li-Chung Cheng)

指導教授 ： 洪瑞華

國立交通大學/電機學院/電子工程系所/碩士(2018年)

摘要

本研究利用有機金屬化學氣相沉積(MOCVD)系統在藍寶石基板上成長氧化鎵鋅(ZnGa2O4)薄膜，並將此薄膜應用於金氧半場效電晶體(MOSFET)上，先是利用長晶時間的不同以調變氧化鎵鋅薄膜厚度，對於不同厚度的樣品進行電性及物性量測及分析探討，繼而討論缺陷形成與薄膜厚度間的關係，發現缺陷雖為此氧化物薄膜導電原因卻也造成電性上遲滯現象(Hysteresis)變為嚴重。最後，再取出具轉導值最大者之樣品，進行高溫退火改變元件特性，目的為減少介電層與氧化鎵鋅薄膜間的缺陷，並發現此舉能使Vth接近0 V，並尋找出特定退火溫度下元件特性最佳者參數，進行製程上的改善及著手可靠度量測分析。
此研究第一部分為薄膜厚度調控，實驗中嘗試沉積四種不同薄膜厚度，分別為70、50、40、25 nm製作成場效電晶體，發現當薄膜厚度小於40 nm時元件將從空乏型(Depletion-mode, D-mode)轉為增強型(Enhancement-mode, E-mode)場效電晶體，並於實驗結果中提出模型進行解釋，認為通道內的空乏區形成不僅是由於金屬閘極與氧化鎵鋅通道間功函數差異導致，更將因缺陷及通道內載子濃度不同所致，此現象將導致電晶體工作模式(Operational mode)改變。然而，針對薄膜元件電性量測分析搭配物性分析，探討不同薄膜厚度的結晶特性、表面形貌及原子鍵結對於元件之影響進行說明及解釋，發現不同薄膜厚度下的元件其崩潰電壓於全關狀態(off-state)時差異甚大，70 nm元件其崩潰電壓為223 V，但於25 nm元件中崩潰電壓可達400 V。認為此差異與元件中的缺陷有莫大關係，因此於下列部分中針對缺陷及製程方法進行分析改善。
第二部分為研究缺陷與薄膜厚度間的關係，先是找尋出結晶性和氧空缺與薄膜厚度間的關係，研究中顯示薄膜厚度越厚，結晶性越好，導電度越佳；然而此氧化物薄膜導電機制是由於氧空缺數量多寡所決定，氧空缺同時也是一種缺陷，氧空缺數量越多將導致元件Vth飄移越嚴重，可靠度不佳。然而，晶格不匹配(Dislocation)亦是一種缺陷，當薄膜厚度越薄，所受到晶格不匹配影響就越大，薄膜厚度需要大過一定程度，氧化鎵鋅薄膜才會處於晶格完整型態。
第三部分為高溫退火對於元件的影響，本實驗取出轉導值最大的70 nm元件，為了消除缺陷造成的電性影響，嘗試四種退火溫度變化，分別為500、600、700、800oC，退火時間皆持續30秒，實驗結果中發現500oC退火溫度下得到的特性最佳；接著進行製程改善，試著改變先於退火溫度500oC下持續30秒再鍍上金屬閘極，稱為閘極後鍍(Gate-last)，再觀察與閘極先鍍(Gate-first)元件特性的變化進行比較。研究結果先是發現隨著退火溫度的增加，其氧空缺於薄膜中的比例將減少，影響到元件飽和電流與臨界電壓；接著發現製程步驟的更換會對於可靠度產生影響，由於金屬閘極鎳會擴散進氧化層中，導致元件可靠性下降，因此閘極後鍍能有較好的可靠度特性。本研究顯示元件於低熱退火溫度及閘極後鍍製程下能有較好的可靠度，並於此部分中利用變溫量測及偏壓測試，提出利用此薄膜材料製程的功率元件所涉及的優缺點。

關鍵字

有機金屬化學氣相沉積 ； 氧化鎵 ； 氧化鎵鋅 ； 金氧半場效電晶體 ； 工作模式 ； 元件缺陷 ； 遲滯現象 ； 元件可靠度分析

並列摘要

In this study, the properties of ZnGa2O4 thin-film grown on the sapphire substrate by MOCVD had been investigated and applied to devices applications. At first, the different thin-film thickness devices utilized various growth time to control. Next, the collocation with electrical and physical properties had been proposed. The relationship between the defects and thin-film thickness which would generate hysteresis phenomenon to be severe had also been discussed in this article. At last, in order to eliminate the defects in the interface between dielectric layer and semiconductor, rapid thermal annealing process had been utilized. The measurement and model explanation would be discussed in this study.
Part I includes the device mechanism from depletion to enhancement mode for ZnGa2O4 metal-oxide semiconductor field-effect transistors (MOSFETs) grown on the sapphire substrate by metal-organic chemical-vapor deposition was studied. It was found that the thickness of the ZnGa2O4 thin-film would affect the operational mode of the MOSFETs. Under the low-voltage operation (VDS = 0.5 V), the transistors exhibited a high on/off ratio from 107 to 104, low subthreshold swing from 150 to 330 mV/dec, high field-effect mobility from 4.2 to 0.054 cm2/V-s and threshold voltages from -17.8 to 4.1 V (using constant current = 1 nA). These electrical properties all depend on the thickness of the ZnGa2O4 thin-film transistors. The crystallinity of different ZnGa2O4 thin-film thicknesses investigated by x-ray diffraction (XRD) data indicates that the crystallinity also plays a crucial role to influence the devices properties. Finally, the E-mode ZnGa2O4 thin-film transistor with off-state breakdown voltage over 400 V is fabricated.
Part II contains the content of how the electrons transport in ZnGa2O4 thin-film and figure out the defects analysis of ZnGa2O4 material. The study shows that the electrical properties are strongly related to the amount of oxygen vacancies which can also serve as the defect center. However, it is a trade-off between the conductivity and reliability issue. Through this study, the kinds of defects in oxide-based material ZnGa2O4 had been investigated. The crystallinity, oxygen vacancies, and the dislocation between ZnGa2O4 and the sapphire substrate would impact on the degradation of ZnGa2O4 devices.
Part III shows how to improve the reliability of ZnGa2O4 devices. Through rapid thermal annealing (RTA), which could eliminate the interface traps and bulk traps in ZnGa2O4 devices. Nevertheless, there exists a critical problem in device’ properties when executing the RTA process, it is nickel gate diffusion. When nickel gate diffuses into oxide layer, it would affect the devices’ properties. Hence, the gate-last (G-L) devices which annealing before metal gate deposition had been proposed. In order to investigate the reliability issue of ZnGa2O4 devices, the stress and temperature issue had also been discussed in this part. It shows that the devices under low annealing temperature and gate-last process is much more reliable. Also, the advantages and disadvantages of ZnGa2O4 devices are proposed in this study.

並列關鍵字

MOCVD ； Ga2O3 ； ZnGa2O4 ； MOSFET ； Operational Mode ； Trapped charge ； Hysteresis phenomenon ； Reliability

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