高效率二維材料異質接面光電晶體

Abstract

在此論文中，我們成功將傳統的矽基材料結合石墨烯與二硫化鉬這兩種二維材料結合並應用於光偵測器上。這些元件展現出低漏電流(nA)、高光電轉換效率、直接轉換成光電壓，不像一般圖像轉換器複雜，且在以石墨烯作為基極的異質接面電晶體中，此元件達到11583超高電流增益效果。 在以石墨烯與矽所做成的光電二極體中，因為數層的石墨烯能具有90%的高穿透度再加上高的載子傳輸速度，因此石墨烯同時能做為導電電極又能與n型矽基板形成蕭特基接面作為光偵測的區域。在整個元件中，不論在電性或光學上的表現，數層石墨烯都扮演了重要的角色。在此研究中，石墨烯與n型矽基板元件中，能在不加偏壓下達到95 mA/W光電流響應度；在石墨烯與n型矽基板元件中間成長14 nm的二氧化矽元件，在189.5 Lux微弱的照射下，能輕易直接轉換成90 mV的光電壓。 在傳統的雙極性電晶體(BJT)元件中，越短的基極寬度，載子擴散到集極的距離就越短，就能減少載子復合的機率，並獲得越高的電流增益效果，但如此一來就很難避免元件被擊穿的問題。在此研究中，我們提出以二維電子材料形成的異質接面光電晶體能在低電壓操作並且達到11583倍的超高電流增益效果，而在傳統元件中，操作電壓往往需要1 V以上而且電流增益效果也才大約200左右。而在此元件中，藉由重參雜n 型矽 (射極)/石墨烯 (基極)/n 型矽 (射極)的能帶圖安排，即使我們在重參雜n型矽基板與一般n型矽基板中夾了一層4 Å的石墨烯也不會有被擊穿的問題。從能帶上的設計中我們能發現，空乏區將會落在矽基板的區域也就是石墨烯會與射極與集極的矽基板形成蕭特基接面。此外，藉由此設計能使元件達到超高電流增益效果同時又不會被電流擊穿，而此概念也從我們的電性量測中獲得驗證。 在二硫化鉬的異質接面光電晶體中，其結構為金 (射極)/二硫化鉬 (基極)/n型矽基板 (集極)，並在金與二硫化鉬接面形成蕭特基接面、二硫化鉬與n型矽基板形成pn接面。根據能帶圖的分析，由於金與二硫化鉬形成較高能障而抑制暗電流的增加，此外，我們在二硫化鉬上鍍上夠薄的20 nm金膜使得光能穿透金到達下面的兩個界面，分別是金與二硫化鉬和二硫化鉬與n型矽基板的界面，而達到更高的光電流輸出。因此，在金與二硫化鉬界面產生光激發電子能到達二硫化鉬，使之電子能態提高，並降低其界面的能障。在加上二硫化鉬只有4 nm，因此電子能從射極發射並快速通過二硫化鉬到達n型矽基板，使電流增大。如此一來，此元件能在59 Lux極低的光照射強度下，就能增加到相較於暗電流來說，5個級數的電流，同時達到30倍的光增益效果。 在這研究中，我們提出了三種類型的光偵測器，分別是石墨烯與矽所做成的光電二極體、石墨烯的異質接面光電晶體 (電晶體)、二硫化鉬的異質接面光電晶體 (電晶體)，而這些元件能應用於現今所需的電子與光電元件應用之中。

In this thesis, combining two dimensional material-based (Graphene or MoS2) with conventional Si-based photo sensors, photodiode or phototransistor, are demonstrated and well examined successfully. We provide low leakage (nA), high conversion capability of optical-to-electrical characteristic, directly photo voltage output in graphene/n-Si photodiode without peripheral complex circuit like CMOS image sensor, and ultrahigh current gain of 11,583 of graphene-based heterojunction transistor. In the graphene-based photodiode, few-layer graphene with optical transmittance of 90% and high mobility benefits to be the conducting electrode, except forming the Schottky junction (optical detection region) between few-layer graphene and n-type Si. Therefore, the few-layer graphene plays an important role for multi-functions no matter what in electrical or optical performance. We demonstrate the high photoresponsivity of 95 mA/W under zero bias and directly conversion capability of current-to-voltage output by inserting 14 nm SiO2 between few-layer graphene and n-type Si simply to provide photo voltage of 90 mV under intensity of 189.5 Lux in this work. In conventional bipolar junction transistor (BJT), the narrower base width is, the higher current gain becomes because of lower recombination possibility during carrier diffusion in neutral base region, in addition to device punch through issue. In this study, we provide the first two dimensional material (graphene and MoS2) heterojunction phototransistor to achieve ultrahigh current gain of 11,583 under low bias (current gain of ~200 in conventional BJT under 1 V). Critically, device punch through due to narrow base (~4 Å) can be depressed by arranging the energy band diagram among n++-Si (emitter)/graphene (base)/n-Si (collector) properly. According to the designed band diagram, the distribution of depletion region will locate at n++-Si (emitter) and n-Si (collector) because the interface between graphene and n++-Si and that between graphene and n-Si are Schottky junctions. Therefore, the device design concept benefits ultrahigh current gain without punching through. Moreover, the electrical properties of current gain and I-V current get agreement with measurement results. In MoS2-based heterojunction transistor (phototransistor) case, the structure Au (emitter)/MoS2 (base)/n-Si (collector) indicate a Schottky junction and a p-n junction among Au-MoS2 and MoS2-n-type Si, respectively. According to the analysis of band diagram, the higher barrier height between Au and MoS2 profit to suppress device dark current. Additionally, we deposit gold film as thin as 20 nm which light can penetrate into both detection junctions, the interface between gold and MoS2 and that between MoS2 and n-Si, respectively, to achieve higher photo current output. Besides, photo generation electrons from interface between gold and MoS2 can move to MoS2 and raise the fermi level of MoS2 leading to lower barrier height. Owing to 4 nm of MoS2, electrons can quickly pass through MoS2 to n-Si resulting in current amplification. In this way, it can reach 5 orders ratio between dark current and photo current under illumination as low as 59 Lux without bias. In this study, we demonstrate three kinds of photodetectors which are graphene-based photodiode, graphene-based heterojunction phototransistor (transistor), and MoS2-based heterojunction phototransistor (transistor), respectively. These devices can be provided to the demanded applications in electronics or optoelectronics.