在矽基板使用石墨烯之光導體探討

Investigation of graphene on silicon hybrid photoconductor

黃晨凱(Chen-Kai Huang)

指導教授 ： 鄭鴻祥

國立臺灣大學/電機資訊學院/電子工程學研究所/碩士(2021年)

https://doi.org/10.6342/NTU202102925

摘要

近年來二維材料已被廣泛研究與各種半導體結構做結合，其中最被廣泛研究的莫過於石墨烯，單層石墨烯的高電子遷移率是我們想要使用來突破現有光電領域中光偵測器的光電流及光響應度的材料之一，而石墨烯藉由濕式轉移(wet transfer)至晶圓頂部時，石墨烯表面吸附水氣及氧原子變成原生p型參雜，使石墨烯的費米能階(fermi level)從狄拉克點(dirac point)掉到價帶，載子濃度(carrier concentration)也發生改變。
為了計算載子濃度及費米能階確切的變化量，本論文在p-type Si和n-type Si上熱氧化成長一層非常薄的二氧化矽(SiO2)，厚度為2.7 nm，經過微影製程與蒸鍍金屬電極後，確認熱氧化層良好絕緣效果避免大量漏電流影響之後的閘極電壓調變效果，蓋上石墨烯後元件結構由上而下分別為(graphene/SiO2/Si)類似MOS電容並加上背電極(back gate)進行調變，閘極電壓改變時石墨烯費米能階移動，狀態密度(density of state)變少導電性變差，電阻變大，當p-type Si閘極電壓為0.9 V電阻最大，此時費米能階位於狄拉克點狀態密度最低，經由定量分析計算費米能階在狄拉克點下方0.313 eV，載子濃度為7.191x1012cm-2，電壓繼續增加電阻變大代表費米能階已經提升到導帶，n-type Si電阻最大值則發生在1 V費米能階在狄拉克點下方0.3297eV，載子濃度為7.98x1012 cm-2，證明石墨烯為p型，照光後由於能帶彎曲p-Si聚集光激發電子使石墨烯費米能階往下移動、n-Si聚集光激發電洞使石墨烯費米能階往上移動，最後改變波長與光強度量測光電流，光強度越強光電流會越大但會隨光強度上升趨近飽和，而光電流大小與波長關係則跟矽的光響應度有關在800-900 nm時光響應度最高光電流最大，1180 nm左右光電流變得非常小，因為此時光子能量已經小於矽的能隙無法繼續激發產生電子電洞對。
關鍵字：石墨烯、光電流、費米能階、狄拉克點、載子濃度、光響應度

關鍵字

石墨烯 ； 光電流 ； 費米能階 ； 狄拉克點 ； 載子濃度 ； 光響應度

並列摘要

In recent years, two-dimensional materials have been extensively studied and combined with various semiconductor structures. Graphene is the most widely studied material. The high electron mobility of single-layer graphene is one of the materials we want to use to break through the photocurrent and photoresponsivity of photodetectors in the existing optoelectronic field. When the graphene is transferred to the top of the wafer by wet transfer, the surface of the graphene adsorbs hydroxide ions in the water to become a native p-type dopant so that the fermi level in the graphene moves from the Dirac point falls into the valence band, and the carrier concentration also changes.
In order to calculate the exact change of the carrier concentration and the Fermi level, this paper thermally oxidized to grow a very thin silicon dioxide (SiO2) on p-type Si and n-type Si with a thickness of 2.7 nm. After the lithography process and the vapor deposition of the metal electrode, it is confirmed that the thermal oxide layer has a good insulation effect to avoid the influence of a large amount of leakage current and the gate voltage adjustment effect. After the graphene is covered, the component structure is separated from top to bottom (graphene/SiO2/Si) is similar to MOS capacitor and added with back gate for modulation. When the gate voltage changes, the graphene Fermi level moves, the density of state decreases, the conductivity decreases, and the resistance increases. Resistance is the largest when the p-type Si gate voltage is 0.9 V. Fermi level is located at the Dirac point andwith the lowest density of state. Calculated by quantitative analysis, the Fermi level is 0.313 eV below the Dirac point, and the carrier concentration is 7.191x1012 cm-2. The voltage continues to increase and the resistance becomes larger, which means that the Fermi level has increased to the conduction band. The maximum value of n-type Si resistance occurs when the 1V Fermi level is 0.3297 eV below the Dirac point, and the carrier concentration is 7.98x1012 cm-2, which proves that graphene is p-type donpant. Under the lihgt illumination, due to the band bending, p-Si gathers light-excited electrons to move the graphene Fermi level downwards, and n-Si gathers light-excited holes to move the graphene Fermi level upwards. Finally, change the wavelength and light intensity to measure the photocurrent. The stronger the light intensity, the higher the photocurrent will be, but it will approach saturation as the light intensity rises. The relationship between photocurrent and wavelength is related to the photoresponsivity of silicon. The highest photocurrent is at wavelenght 800-900 nm, and the photocurrent becomes very small around 1180 nm, because the photon energy is smaller than the energy gap of silicon and cannot continue to excite electron-hole pairs.

並列關鍵字

graphene ； photocurrent ； Fermi level ； Dirac point ； carrier concentration ； light responsivity

參考文獻

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