表面氧化參雜效應於高性能硒化銦光電晶體特性之研究

Surface Oxidation Doping Effect on High Performance Indium Selenide Phototransistor

張益仁(Yih-Ren Chang)

指導教授 ： 陳俊維

國立臺灣大學/工學院/材料科學與工程學研究所/碩士(2017年)

https://doi.org/10.6342/NTU201701987

摘要

硒化銦為III-VI族金屬硫化物的一員，同時其具有層狀堆疊的結構亦為二維半導體材料之一。硒化銦晶體的單層結構在水平方向與石墨烯相同為六邊型結構而垂直方向由兩個硒原子與兩個銦原子以硒-銦-銦-硒的順序堆疊而成。其層與層之間與其他二維材料相同，皆僅由微弱的凡得瓦作用力鍵結在一起，硒化銦的層間距為0.8奈米。在早期的硒化銦塊材研究中發現其在常溫下具有相對高的本質載子遷移率~1000 cm2V-1s-1以及較小的電子有效質量(~0.15m0)，加上硒化銦本身具有能隙使其具有作為電晶體元件應用的潛力。此外單層硒化銦能帶結構中所具有的非直接能隙特性會隨著材料厚度增加而轉換到直接能隙，且多層硒化銦的能隙大小坐落於約1.3電子伏特，能夠吸收紅外光以上波段的光，使多層硒化銦具有更多的光電元件應用空間。
然而硒化銦本身於大氣下並不夠穩定，暴露在充滿水氣及氧氣的大氣下，其表面十分容易自發性的生成粗糙的氧化層，並使其本質所具有的優秀光電性質衰退、造成無法控制的p型摻雜近而影響其後續相關研究及實際應用的發展。因此尋找硒化銦易於在大氣下氧化的解決方案成為了其未來光電元件應用發展的重要課題。
在本論文的第一部份(第四章)，我們首先比較水氣對於硒化銦表面氧化的影響，並發現到在無水純氧環境下生成的氧化層不但平坦且具有防止內層材料繼續氧化的效果。此外這層氧化層甚至具有可以改善硒化銦與電極接觸界面的功能。在硒化銦表面具有無水氧化層的情況之下，加上藉由選擇材料厚度的方式來優化電晶體元件，我們可以得到現有報導中表現最高的硒化銦電晶體元件，為未來奠下基石。
本論文的第二部份(第五章)中我們則利用在前一章中發現到的氧化層誘發p型摻雜概念，藉由電漿氧化的方式來調控硒化銦表面氧化程度來形成不同強度及深度的p型摻雜區域，進而在多層硒化銦中誘發垂直電場促使光激發電子電洞對被分離的趨勢增強。除了藉由調控表面氧化p型摻雜的程度我們可以優化元件並在背電極驅動的多層硒化銦光偵測器中得到極高的光響應外其亦有表面鈍化效果。

關鍵字

硒化銦 ； 二維半導體 ； 表面鈍化 ； 表面氧化p型摻雜 ； 薄膜電晶體 ； 光偵測器

並列摘要

Indium selenide (InSe) is one of the member of III-VI groups metal chalcogenide family, which also belongs to two-dimensional (2D) semiconductors. Similar to other 2D materials, InSe crystals are also a layered material with each layer composed of four covalently bonded atoms in form of Se–In–In–Se. Each InSe quadruple layer has a honeycomb shape lattice and bonds with each other by van der Waals interactions as the layer spacing d is about 0.8nm.
In previous researches on bulk form InSe, it has been reported that InSe exhibits a small electron effective mass (about 0.15m0) and its Hall mobility isnear 103 cm2V-1s-1 at room temperature. Couple with the existence of band gap, InSe has a great potential in transistor applications. Beside the impressing intrinsic electrical transport properties, some recent researches also report that InSe thin flakes possess unique nature of thickness-dependent bandgap. For example, the direct-to-indirect bandgap transition as the flake thickness decreasing in InSe and the bandgap variation with thickness. The multilayer InSe usually exhibit with a direct band gap less than 1.3eV which is appropriate to absorb light with wavelength shorter than IR region, making it a candidate for optoelectronic application.
However, InSe is sensitive to moisture and oxidation occurs spontaneously as exposed to the air. Such oxidation can go deep into the inner layers due to the loose structure of the resulting oxides and leads to an uncontrollable p-doping in transistor operations which would degrade the device performance. It is utmost important issue to solve the oxidation problem for future InSe device development.
In the first part of this research, we first compare the difference between wet oxide and dry oxide, then it was found that the self-limiting dry oxide can effectively retard further oxidation of the inner crystalline InSe layers to preserve InSe crystal from degradation. Furthermore, the dry oxide can also serve as depinning layer in their FET structure. Through controlling the surface oxidation of InSe with different thickness we can obtain an optimized record high transistor performance which pave the way toward the development of high-performance InSe FETs with vertical geometry shirking down to atomic scale.
In the second part of this research, with the oxidation layer induced p-type doping in mind, a novel method to create vertical band bending has been developed. By oxygen plasma treatment, a surface oxidation layer was formed to serve as acceptor which can provide hole doping to the top InSe layers and therefore, lead to a doping gradient in vertical direction. Within the built-in potential induced by doping gradient, we could obtain an ultrahigh photo responsivity in InSe phototransistors since the charge separation ability was largely enhanced. What’s more, the thin oxidation layer could retard the further oxidation on InSe as well, which is utmost important character for InSe devices future development.

並列關鍵字

Indium Selenide (InSe) ； 2D semiconductor ； Surface passivation ； Surface oxidation p-type doping ； Thin film transistor ； Photodetector

參考文獻

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