用電子傳輸與場效應探索氧化鋅錫奈米線 電子系統的相變化

Abstract

大約在1998年開始有人研究氧化鋅錫的薄膜，2004年開始有人研究氧化鋅錫的奈米線。氧化鋅錫由氧化鋅和氧化錫合成，相對於氧化鋅，氧化鋅錫對可見光的敏感性較低。實驗使用的氧化鋅錫(ZTO)奈米線，線徑約20-120奈米，線長約數個微米，利用電子束微影、熱蒸鍍系統及真空中熱退火系統製程場效電晶體。 我們藉由兩點量測法研究其電性傳輸性質，量測在300K-80K不同溫度下的電阻率，發現氧化鋅錫為半導體性質(電阻率隨溫度下降提高)。但我們發現電阻率較大的樣品有相變化，其電阻率在約240K-200K並不隨溫度下降而上升。我們利用三維變程跳躍模型解釋ZTO的電性傳輸，發現有相變化的樣品是在相變化前後侷域長度或狀態密度改變了。 我們利用背向閘極偏壓量測其場效應，得出不同溫度下的遷移率。發現兩種情形，一種是遷移率隨溫度下降亦下降，另一種是遷移率隨溫度下降先下降之後會上升，並在約240K-200K上升到極大值，之後再下降。我們利用電阻率和遷移率求得載子濃度，發現不管有沒有相變化，載子濃度對溫度的關係圖趨勢相同，因此我們認為相變化主要來自於遷移率，而非載子濃度。由於跳躍傳輸的假設以及遷移率與侷域長度有關，我們推測相變化為侷域長度的改變。我們利用電阻率和遷移率求得載子濃度隨溫度的變化，以此估算侷域長度的變化率。

ZTO (Zn2SnO4) is a composite made of SnO2 and ZnO. ZTO has been reported to have high electron mobility, high electrical conductivity and low visible absorption. ZTO materials used in this study are nanowires with a diameter in the range between 20 and 120 nm and a length of several micrometers. Standard electron-beam lithography and thermal coater were used to fabricate ZTO-nanowire field-effect transistor (FET) devices. The as-made devices were post-annealed to improve their performances. We studied electron transport and field-dependent properties of ZTO nanowires in the temperature range from 300K to 80K. Electron transport measurements reveal the semiconducting behavior of all the ZTO nanowires. The model of three-dimension variable-range hopping can suitably describe the transport data. In addition, the high-resistivity nanowires unveil a phase transition in electron transport in the temperature range of 200 - 240 K. The mobility measurements of ZTO nanowires in the temperature range of 80 - 300 K were also carried out using FET devices. It is confirmed that the high-resistivity nanowires have a maximum mobility in the same transitional temperature range (200 - 240 K). We argued that the phase transition comes from the change of electron’s scattering mechanism as well as the localization length according to our measurements of electron transport and field-effect transistors of ZTO nanowires.