矽/鍺金屬-絕緣層-半導體光偵測器

Abstract

本論文中，我們利用金屬-絕緣層-半導體穿隧二極體來製作光偵測器。此金屬-絕緣層-半導體結構可降低暗電流。針對中遠紅外光，我們建構出矽鍺/矽量子點紅外光偵測器。針對近紅外光，我們利用晶圓黏合與聰明切技術得到單晶薄膜鍺光偵測器，並利用模擬軟體設計最佳化之單晶薄膜太陽電池。 首先，我們在量子點紅外光偵測器及量子井紅外光偵測器中加入極薄摻雜。比起未經摻雜的量子點紅外光偵測器，極薄摻雜之量子點紅外光偵測器可在3.5-5 μm 得到新的吸收區域。至於經極薄摻雜的量子井紅外光偵測器，因為其侷限能量較極薄摻雜之量子點紅外光偵測器小，截止波長延伸到7 μm，且有較大的響應度。 若在矽鍺/矽量子點紅外光偵測器的矽間隔層中引入極薄摻雜，我們則可得到一個寬帶頻譜。我們發現極薄摻雜在矽的價帶形成很淺的量子井，此量子井可運用在長波長紅外光的偵測。頻譜幾乎涵蓋大氣層可穿透的紅外光波段，所以利用此元件來達成寬帶偵測是可行的。利用計算，與其他元件的比較，以及光激發光的頻譜，我們可以分別指出量子點與極薄摻雜量子井中的躍遷與物理機制。 另一方面，利用晶圓黏合與聰明切技術可製作出絕緣層上鍺的金屬-絕緣層-半導體光偵測器，此晶圓黏合方法是個將光學與電子元件整合在同一個基座上的可行技術。因為鍺具有比矽小的能帶間隙，所以可偵測850 nm，1.3 μm 及1.55 μm 的紅外光。使用1.3 μm 厚的鍺層，可成功在1.3 μm 的紅外光波段達到0.23 A/W 的響應度。絕緣層上鍺元件利用高功函數的金屬(鉑)作為閘極金屬，成功的降低了暗電流，並藉由外加機械應力來提升光電流。值得注意的是暗電流幾乎不會隨著應力而變大。 最後，我們也成功的將單晶薄膜鍺轉移到玻璃基座上。雖然在聰明切的過程中，鍺會因為佈植過程而產生缺陷，但我們可藉由化學蝕刻將缺陷區去除，且將表面粗糙度降成4 nm。經過蝕刻的元件在可見光的光電流可提升成1.85 倍，暗電流還能降低至三十分之ㄧ。同樣的玻璃基板上鍺元件也被測試是否可運用在太陽電池的應用上。我們探討了此元件低效率的原因，並進一步利用模擬軟體設計出最佳化的結構，利用四層3 nm 厚的鍺之矽/鍺/矽薄膜結構可達到15.7%的效率。在未來可利用模擬結果及已有的製程技術來製作高效率單晶薄膜太陽電池。

In this dissertation, the Si/Ge metal-insulator-semiconductor (MIS) tunneling diodes are utilized as photodetectors, and it is proven that the MIS structure can reduce the dark current. We have demonstrated mid- and long- wavelength infrared detection by MIS SiGe/Si quantum dot infrared photodetectors (QDIPs). On the other hand, single crystalline thin-film structures obtained by wafer bonding and smart-cut can be applied to MIS near-infrared detectors and solar cells. First, δ doping is introduced in the QDIPs and quantum well infrared photodetectors (QWIPs). The δ doping in QDIPs provides QDs with a sufficient hole concentration for infrared excitation. Compared to the un-doped QDIP, a new absorption region at 3.5-5 μm is observed. Due to the smaller confinement energy of the δ-doped SiGe QWIP as compared with the δ-doped SiGe QDIP, the cut-off wavelength extends to 7 μm and a larger responsivity is achieved. The broadband absorption of MIS SiGe/Si QDIPs is demonstrated using the boron δ doping in Si spacers. Shallow QWs can be formed in the valence band due to the boron δ doping in Si spacers and contribute to the long-wavelength infrared detection. The broadband spectrum covers most of the atmospheric transmission windows for infrared, so the broadband detection is feasible using this device. Calculations, comparison with other δ-doped QDIPs/QWIPs, and PL spectrum are studied to identify the transitions in QDs and δ-doping wells. On the other hand, Ge-on-insulator MIS detectors are fabricated by wafer bonding and smart-cut. Wafer bonding is an enabling technology to integrate both optical devices and electronic devices on the same substrate. Due to the small bandgap of Ge, the 850 nm, 1.3 μm, and 1.55 μm infrared can be detected. The responsivity of 0.23 A/W at the wavelength of 1.3 μm has been achieved using n-type Ge with the thickness of 1.3 μm. The large work function metal (Pt) is used for the gate electrode to reduce the dark current. External mechanical strain can further enhance the photocurrent with only slight degradation on the dark current. Finally, the single crystalline thin-film Ge on glass is also demonstrated. The implantation damage of transferred Ge on glass is removed by chemical etching, and the surface roughness is reduced to 4 nm. The defect removal reduces the dark currentby a factor of 30, and increases the visible-light photocurrent by a factor of 1.85. The GOG MIS structure is also tested for solar cell applications. The reason for low efficiency is discussed, and then the optimized structures are designed by simulation. An outstanding enhancement on efficiency can be achieved with the Si/Ge/Si structure. With four-layer 3-nm-thick Ge in the Si/Ge/Si solar cell, the efficiency will be as high as 15.7 %. Based on the simulation and technology, high efficiency thin film solar cells can be demonstrated in the future.