在矽基板上使用石墨烯對光響應度的增益

Amplification of Photoresponse Si using Graphene

黃少翾(Shao-Xuan Huang)

指導教授 ： 林浩雄

共同指導教授 ： 鄭鴻祥(Hung-Hsiang Cheng)

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/NTU202100105

摘要

由於矽的成本低，被廣泛使用在矽基光電系統，因此矽的製程技術已經很成熟，在這些已確定的基礎上，大家想要去實現超出目前矽的光電偵測器的能力範圍。因此其他四族材料，例如鍺(Germanium)和鍺錫(GeSn)合金，也已應用於四族化合物。這些材料通常生長在矽基板上，但是由於晶格失配，這些異質結構光偵測器的光響應度通常比單晶光偵測器的光響應度弱。為了實現這些異質結構光偵測器的應用，必須增強薄膜的光響應度，以滿足光電檢測的要求。通過在晶圓頂部摻入二維材料石墨烯，可以增強晶圓的光響應度。將該機制描述為電荷轉移過程，其中半導體中的光激發載子被轉移到石墨烯層，並且由於石墨烯的載子遷移率比半導體大，因此產生的光電流比在半導體大。
　　在本論文中，首先將p-type Si和n-type Si熱氧化生長極薄的二氧化矽 (silicon dioxide, SiO2)介電層 (dielectric layer)，其介電層厚度約2nm，微影製程後蒸鍍電極，沒有石墨烯 (graphene)的形成光導體，有石墨烯的類似MOS電容，比較二氧化矽/矽基板 (SiO2/Si)和石墨烯/二氧化矽/矽基板 (Graphene/SiO2/Si)的光電流。為了得到最好的特性，我們盡量避免接觸電阻影響光電流，我們找出如何製作p-type Si和n-type Si歐姆接觸 (ohmic contact)的方法，p-type Si電子束蒸鍍鋁 (aluminum, Al)退火後得到歐姆接觸的特性，接觸電阻約為11.066Ω；n-type Si作磷離子佈植後電子束蒸鍍金 (gold, aurum, Au) /鈦 (titanium, Ti)後直接得到歐姆接觸的特性，接觸電阻約為8.295Ω。接著我們架設光電量測系統量測精準的光電流 (photocurrent)，量測到Graphene/oxide/p-Si的光電流會大於oxide/p-Si，在電壓0.8V光強200μW時大約放大2~6倍，而Graphene/oxide/n-Si的光電流會小於oxide/n-Si，原因是矽與石墨烯接觸後能帶彎曲形成類似MOS的元件。當我們選擇短波長光源時，因為SiO2很薄和石墨烯轉移的方式會經過水，所以石墨烯為p型，電子穿隧過去會抵銷石墨烯端的電洞濃度，造成放大倍率變小。最後我們改變光源位置量測石墨烯光電流對光源位置的靈敏度。建立了定性定量分析方法，數值結果與功率和波長相關數據吻合。

關鍵字

歐姆接觸 ； 光導體 ； MOS電容器 ； 石墨烯 ； 電流漂移密度 ； 光電流 ； 穿隧 ； 過量載子生命期

並列摘要

Because of the low cost of silicon, it is widely used in Si-based optoelectronic systems. Therefore, silicon process technology is fully developed. Based on these established foundations, people want to achieve beyond the current silicon photodetector capabilities. Therefore, other Group IV materials, such as germanium (Ge) and germanium tin (GeSn) alloys, have also been applied to Group IV compounds. These materials are usually grown on silicon substrates, but due to lattice mismatch, the photoresponsivity of these heterostructure photodetectors is usually weaker than that of single crystal photodetectors. In order to realize the application of these heterostructure photodetectors, the responsivity of the film must be enhanced to meet the requirements of photoelectric detection. By doping the two-dimensional material graphene on the top of the wafer, the responsivity of the wafer can be enhanced. This mechanism is described as a charge transfer process in which photoexcited carriers in the semiconductor are transferred to the graphene layer, and since the carrier mobility of graphene is greater than that of semiconductors, the resulting photocurrent is greater than in semiconductors.
In this thesis, firstly, p-type Si and n-type Si are thermally oxidized to grow a very thin silicon dioxide (SiO2) dielectric layer with a thickness of about 2nm. After the lithography process and evaporation of the electrode, there is no graphene to form a photoconductor, but there is graphene is similar to MOS capacitors. Compare silicon dioxide/silicon (SiO2/Si) and graphene/silicon dioxide/silicon (Graphene/SiO2/Si) photocurrent.
In order to get the best characteristics, we try to avoid the contact resistance affecting the photocurrent. We find out how to make p-type Si and n-type Si ohmic contacts. The p-type Si electron beam vapor deposition aluminum (Al) annealed to obtain the characteristics of ohmic contact, the contact resistance is about 11.066Ω. The n-type silicon is implanted with phosphorus ions, and the electron beam evaporates the gold/titanium (Au/Ti) to directly obtain the characteristics of ohmic contact, and the contact resistance is about 8.295Ω. After that, we set up a photoelectric measurement system to measure the accurate photocurrent through a lock-in amplifier remove noise. The photocurrent of Graphene/oxide/p-Si will be greater than oxide/p-Si, which is approximately 2~6 times when the voltage is 0.8V and the light intensity is 200μW. While the photocurrent of Graphene/oxide/n-Si will be less than oxide/n-Si. Because the energy band of silicon and graphene bends to form a MOS-like device. When we choose a short-wavelength light source, because the SiO2 is very thin and the way of graphene transfers through water, the graphene is p-type. Therefore, electrons tunneling will recombine the hole concentration at the graphene, resulting in a smaller amplification. Finally, we change the position of the light source to measure the sensitivity of the graphene photocurrent to the position of the light source. A qualitative and quantitative analysis method was established, and the numerical results were consistent with the power and wavelength related data.

並列關鍵字

Ohmic contact ； Photoconductor ； MOS capacitor ； Graphene ； Drift current density ； Photocurrent ； Tunneling ； Excess carrier lifetime

參考文獻

[1] J. Michel, J. Liu, and L. C. Kimerling, "High-performance Ge-on-Si photodetectors," Nat. Photonics, vol. 4, pp. 527, Jul. 2010.

[2] D. Zhang, C. Xue, B. Cheng, S. Su, Z. Liu, X. Zhang, G. Zhang, C. Li, and Q. Wang, "High-responsivity GeSn short-wave infrared p-i-n photodetectors," Appl. Phys. Lett., vol. 102, pp. 141111, Apr. 2013.

[3] B. R. Conley, J. Margetis, W. Du, H. Tran, A. Mosleh, S. A. Ghetmiri, J. Tolle, G. Sun, R. Soref, B. Li, H. A. Naseem, and S. Q. Yu, "Si based GeSn photoconductors with a 1.63 A/W peak responsivity and a 2.4 μm long-wavelength cutoff," Appl. Phys. Lett., vol. 105, pp. 221117, Dec. 2014.

[4] R. Soref, "Enabling 2 μm communications," Nat. Photonics, vol. 9, pp. 358, May 2015.

[5] T. Pham, W. Du, H. Tran, J. Margetis, J. Tolle, G. Sun, R. A. Soref, H. A. Naseem, B. Li, and S. Q. Yu, "Systematic study of Si-based GeSn photodiodes with 2.6 µm detector cutoff for short-wave infrared detection," Opt. Express, vol. 24, pp. 4519, Feb. 2016.