鍺（錫）金氧半場效電晶體的低溫電子傳輸特性

Abstract

鍺錫 (GeSn) 合金因其具備直接能隙、高載子遷移率，以及與矽積體電路技術的相容性，在光電與電子應用領域中引起廣泛關注。儘管已有諸多研究展示出高性能的鍺錫元件，關於鍺錫由間接能隙轉變為直接能隙之機制，仍未獲得完整的理解。過往相關的實驗研究多半聚焦於光學特性，對於電性方面的探究則相對稀少。因此，本論文旨在探討鍺錫由間接能隙轉變為直接能隙對於電子傳輸特性造成的影響。

本研究使用化學氣相沉積 (CVD) 技術，成長高品質且應變鬆弛的鍺磊晶結構及錫原子比例分別為 4.5% 與 10.5% 的鍺錫磊晶結構。隨後製備鍺（錫） n 型金氧半場效電晶體 (n-MOSFETs)，並於 300 K 至 4 K 的溫度範圍內進行電性特性分析。在 300 K 時，隨著錫原子比例增加，元件的關閉電流上升而開態電流下降，推測原因分別為接面漏電流的增加與合金散射效應的加劇。在所有元件中，於積累區與空乏區皆觀察到異常高的閘極–通道電容，其成因為接面在交流訊號的驅動下提供了大量的多數載子（電洞）。本文提出一種方法，未來可用於校正關閉電流對閘極電容和等效遷移率的影響，適合應用於能隙小、遷移率高的電晶體，如鍺錫、砷化銦 (InAs) 及銻化銦 (InSb) n 型金氧半場效電晶體。當溫度自 300 K 降低至 4 K 時，隨著接面漏電流受到抑制，所有元件的關閉電流皆呈現單調遞減。然而，開態電流隨溫度的變化則呈現出較為複雜的行為。為便於分析，本研究採用分裂電容–電壓方法，以分離開態電流對電子濃度與遷移率的依賴性。在強反轉區中，閘極–通道電容對溫度的微幅變化，可歸因於反轉層電子在交流訊號驅動下的供應對溫度的變化。最後，本研究探討了電子的等效遷移率。隨著錫原子比例提升至 10.5%，遷移率隨溫度變化的趨勢由原先的單調遞減，轉變為在 100 K 至 20 K 之間出現異常上升的行為。此現象可歸因於直接能隙 Ge0.895Sn0.105 元件中的遷移率提升，此提升係由隨著溫度降低，Γ 谷中電子數量的增加所致。根據傳輸特性的結果可推論，鍺錫材料中由間接能隙轉變為直接能隙的轉換範圍，發生於錫原子比例介於 4.5% 與 10.5% 之間。

Germanium-tin (GeSn) alloys have attracted great attention for both optoelectronic and electronic applications due to their direct-bandgap characteristics, high carrier mobility, and compatibility with Si VLSI technology. While numerous studies have demonstrated high-performance GeSn devices, the understanding of indirect-to-direct bandgap transition in GeSn remains incomplete. Previous experimental studies have predominantly focused on optical aspects, with limited electrical data available. This thesis investigates the impact of the indirect-to-direct bandgap transition on electron transport properties in GeSn.

High-quality, strain-relaxed Ge, Ge0.955Sn0.045, and Ge0.895Sn0.105 epitaxial structures are grown using chemical vapor deposition. Ge(Sn) n-type metal-oxide-semiconductor field-effect transistors (n-MOSFETs) are fabricated and characterized at temperatures of 300 K to 4 K. At 300 K, the off-state current increases and the on-state current decreases with the Sn fraction due to enhanced junction leakage and alloy scattering, respectively. Abnormally high gate-to-channel capacitance is observed in the accumulation and depletion regimes across all devices due to a large supply of majority carriers (holes) through the junctions in response to the AC signals. A method is proposed for future calibration of the off-state leakage effect on CGC, and consequently, mobility extraction, which can be applied to small-bandgap, high-mobility transistors such as GeSn, InAs, and InSb n-MOSFETs. As the temperature decreases from 300 K to 4 K, the off-state current decreases monotonically across all devices due to the suppressed junction leakage. The on-state current, however, exhibits a complex trend with temperature. To facilitate analysis, the dependence of on-state current on both electron density and mobility is decoupled using a split C-V method. The slight temperature dependence of gate-to-channel capacitance in the strong inversion regime is a result of the temperature-depend-ent supply of inverted electrons in response to the AC signals. Lastly, the electron effec-tive mobility is investigated. As the Sn fraction increases up to 10.5 at%, the trend of mobility over temperature transitioned from a monotonic decrease to one exhibiting an anomalous upturn between 100 K and 20 K. This is attributed to the mobility enhance-ment in the direct-bandgap Ge0.895Sn0.105¬ device due to an increased electron population in the Γ-valley as the temperature decreases. Based on the transport results, the indirect-to-direct bandgap transition in GeSn is identified to occur between Sn fractions of 4.5 at% and 10.5 at%.