探索二維氧化鋅奈米結構傳輸機制與其狀態密度

Abstract

本實驗中，我們對二維結構的氧化鋅奈米片(ZnO nanosheets)做電性傳輸與外觀特性的討論，利用電子束微影技術與熱蒸鍍技術，製成兩種不同結構的氧化鋅奈米片元件，各別用於揭露氧化鋅奈米片本質電性傳輸行為與探索氧化鋅奈米片之能態密度。實驗中，我們使用兩點量測的電極結構探索氧化鋅奈米片的變溫電性，利用送電流訊號量測電壓訊號的方式估計其電阻值的大小，而未退火前的氧化鋅奈米片室溫電阻值過高，造成量測氧化鋅奈米片本質電性的困難，為了獲得較低的電阻率，因此進一步地採用高真空熱退火550 ℃持續24小時，使氧化鋅奈米片的載子濃度提高，其室溫電阻率相較於未退火處理前的樣品下降二至三個數量級。觀察熱退火後的氧化鋅奈米片樣品電阻率隨溫度的變化關係中，我們發現隨著溫度的降低，氧化鋅奈米片電性傳輸行為在300至30 K的廣溫度範圍內，嚴格的遵守二維Mott變程式跳躍傳輸機制，透過實驗我們可以得知退火處理後的氧化鋅奈米片具有相當高的平均能態密度約 eV-1 cm-2。更進一步地，藉由變程跳躍傳輸擬和，其室溫下的跳躍能量與跳躍長度分別被揭露為100 meV與40 nm。另一方面，透過氧化鋅奈米片穿隧結元件微分電導的觀察，我們發現該二維氧化鋅奈米片有別於一般三維半導體氧化鋅材料，並沒有觀察到能隙結構(energy gap)的存在；更有趣的是，我們發現穿隧電流對電壓的微分電導值呈現階梯式的變化趨勢，推測其成因來自於電子在氧化鋅奈米片厚度的維度中傳導受限所致，在有限小的厚度中，電子的波動效應增強，只有某些特定的特徵能量可以存在於氧化鋅奈米片之中；而階梯狀微分電導的電壓值對應了特徵能量的數值，代表電子只可存在於某些特定的特徵能量中，若氧化鋅奈米片的厚度有些微的變化，會使臺階產生相當明顯的變化。

In this study, electron transport properties and density of states of ZnO nanosheet have been investigated by two-probe and tunneling junction measurements, respectively. ZnO nanosheets with an average thickness 17 nm were taken as a two-dimentional material from the viewpoint of electron microscopes and their Fermi wavelength. Prior to the device fabrication, ZnO nanosheets were subjected to thermal annealing at 550 ℃ for 24 hours in a high vacuum to raise their carrier concentrations so as to reduce their resistivities by two orders of magnitude. Then, by using electron-beam lithography and thermal evaporation techniques, we made two types of nanodevices. Devices having Ti/Au electrodes making a direct Ohmic contact on ZnO nanosheets were categorized as Type I. Type I devices reveal a variation in room-temperature (RT) resistivity from 106 to 108 Ω and expose intrinsic electron transport in ZnO nanosheets. For Type II devices, we deposit Al2O3 film between ZnO nanosheet and one Ti/Au electrode to make a tunneling junction structure for probing density of states in ZnO nanosheets. Data of Type I devices can be well-described by two-dimensional Mott’s variable range hopping at temperatures ranging from 300 K to 30 K. These devices have hopping energy and hopping distance of 100 meV and 40 nm at RT, and have a high average density of states eV-1 cm-2. On the other hand, Type II devices show data of dI/dV spectrum to examine the density of states. Surprisingly, the semiconductor bandgap structure of the ZnO nanosheets was not obtained, whereas a metallic feature of stair-like density of states was observed. The stair-like density of state could come from electron quantization in the vertical direction of ZnO nanosheets.