二維材料異質結構元件光電特性探討

Investigation of Photoelectric Characteristics of Two-Dimensional Material Heterostructure Devices

呂育瑄(Yu-Hsuan Lu)

指導教授 ： 吳肇欣

國立臺灣大學/電機資訊學院/光電工程學研究所/碩士(2021年)

https://doi.org/10.6342/NTU202103418

摘要

本實驗主要分成兩部分，其中一部分探討二硒化錫(SnSe2)、二硒化鎢(WSe2)及六方氮化硼(h-BN)異質結構之光電特性與空乏區電場分布情況。藉由機械式剝離及乾式轉印技術，將30 nm h-BN、5 nm WSe2及10 nm SnSe2依序堆疊在300 nm厚的二氧化矽基板上，其厚度分別為30 nm、5 nm、10 nm，並使用標準電子束微影技術製作出鈦金電極(10 nm/90 nm)。二硒化錫大小約為25 μm × 17 μm，二硒化鎢大小約為30 μm × 20 μm，兩材料重疊區域大約為10 μm × 20 μm。
元件在VDS = -1.6 V 時，其開關比可達106，次臨界擺幅(Subthreshold Swing)約0.54 V/dec。藉由物鏡將波長633 nm、強度5 nW雷射聚光在樣品表面，光點直徑約為1.5 μm，再搭配低雜訊電流放大器及鎖相放大器所組成之掃描光電流量測系統對元件做光電特性及空乏區電場分布情況研究。將雷射光點覆蓋整個樣品，量測元件短路電流(ISC)和開路電壓(VOC)隨不同VGS值的變化，在VGS = 0 V和VDS = -0.26 V時，可獲得最大光偵測率2.97×1012 Jones，在VGS = 40 V和VDS = 2 V時，最大光響應度為100 A / W。最後使用分光儀量測光電流頻譜。綜述上論，我們發現由二硒化鎢(WSe2)和二硒化錫(SnSe2)組成的異質結構具有出色光響應，可以用作光電感測器或光伏(photovoltaic)元件。
另一部分探討，不同絕緣層對於單層二硫化鉬電晶體的影響，首先先製作使用二氧化矽為絕緣層的背閘極單層二硫化鉬，加上不同的passivation比較電性變化，接著將背閘極絕緣層改為h-BN，再使用不同的passivation比較電性變化，最後製作上下閘極絕緣層均為h-BN，使用石墨烯做為電極的單層二硫化鉬元件，最大電流為150 uA，SS約為100 mV/dec，其開關比可達108，最後在利用C-V量測對絕緣層電容值進行修正，得到載子遷移率高達200 cm2/V·s。

關鍵字

二硒化鎢 ； 二硒化錫 ； 次臨界擺幅 ； 石墨烯 ； 二硫化鉬 ； 異質結構

並列摘要

This experiment is mainly divided into two parts. In the first part, we discusses the photoelectric characteristics and the electric field distribution of the depletion region of tin diselenide (SnSe2), tungsten diselenide (WSe2) and hexagonal boron nitride (h-BN) heterostructures. By mechanical exfoliation and dry transfer technique, 30 nm h-BN, 5 nm WSe2, and 10 nm SnSe2 are sequentially stacked on a Si substrate with a 300 nm thick silicon dioxide film on the top. We use the standard electron beam lithography and the thermal evaporation to form Ti/Au electrodes (10 nm/90 nm). The size of SnSe2 is about 25 μm × 17 μm and the size of WSe2 is about 30 μm × 20 μm. The overlapping area of SnSe2/ WSe2 is about 10 μm × 20 μm.
When the component is VDS = -1.6 V, its switching ratio can reach 106, and the subthreshold swing (SS) is about 0.54 V/dec. A laser with a wavelength of 633 nm and an intensity of 5 nW is focused on the surface of the sample by an objective lens, with a spot size of 1.5 μm. The scanning photocurrent measurement system is composed of a low-noise current amplifier and a lock-in amplifier for investigating the photoelectric characteristics and electric field distributions in the depletion region. Applying the larger laser spot which can be cover the entire sample, we measure the short-circuit current (ISC) and open-circuit voltage (VOC) of the devices with different VGS values. When VGS = 0 V and VDS = -0.26 V, the maximum detectivity is 2.97 × 1012 Jones. At VGS = 40 V and VDS = 2 V, the maximum photoresponsivity is 100 A/W. Moreover, we also check the photocurrent spectrum. In summary, we found that heterostructures composed of tungsten diselenide (WSe2) and tin diselenide (SnSe2) have excellent photoresponse and can be the poterntial canidate as a photodetecting or photovoltaic device.
The second part is focus on the influence of different insulating layers on single-layer molybdenum disulfide transistors. At first, we make a back gate single-layer molybdenum disulfide transistor with SiO2 as the insulator and cover by different passivations to compare the electrical changes of different devices. And then we change the back gate insulating layer to be h- BN and also apply different passivation to investigate the electrical changes. At last, we make a double-gate single-layer molybdenum disulfide device with both the upper and lower gate insulating h- BN layers and the graphene electrodes. The maximum current is 150 uA, the SS is about 100 mV. /dec, and the on-off ratio can be up to 108. The capacitance of the insulating layer is estimated by C-V measurement, and the carrier mobility is as high as 200 cm2/V·s.

並列關鍵字

Tungsten diselenide ； tin diselenide ； subthreshold swing ； graphene ； molybdenum disulfide ； heterostructure

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