n-型鍺錫電晶體製作與分析

Abstract

鍺(錫)材料具有高遷移率的特性，非常有潛力成為下一代電晶體(MOSFETs)的通道材料。鍺錫的電洞遷移率能透過施加壓縮應力提升;另一方面，當錫比例高達8-11%或施加拉伸應歷時，鍺錫材料則會轉變為直接能隙，這種能隙轉變能增加聚集在Γ能帶的電子數量，減少等效電子質量，進而提高電子遷移率。雖然p-型鍺錫電晶體已經有相當成熟的研究，n-型鍺錫電晶體的研究相對較少。 本論文探討了鍺錫材料的電晶體製作及特性。鍺錫材料的能隙較小，導致使用該材料製作電晶體時容易產生漏電流，而採用檯面結構來控制源/汲極與基板的PN接面，與傳統平面電晶體相比，能有效地減少漏電流。接著製作無接面鍺錫多閘極電晶體，減少摻雜擴散或退火等熱製程步驟，可提高鍺錫的熱穩定性。最後製作拉伸應變鍺錫檯面結構電晶體，並與鬆弛和壓縮應變的元件進行比較，觀察不同應變對於材料中電子遷移率的影響。

GeSn is a promising channel material for next-generation transistors due to its high carrier mobility. Compressive stresses can increase the hole mobility of GeSn, while the electron mobility can be boosted by transforming GeSn into a direct-bandgap material. As the Sn composition is as high as 8 - 11% or subjected to tensile stresses, GeSn becomes a direct bandgap material, which increases the number of electrons accumulated in the Γ-valley. Thus, the electron effective mass is reduced, leading to a higher electron mobility. Although great progress has been made for p-type GeSn transistors, research on n-type GeSn transistors is relatively scarce. This thesis investigates the fabrication and characteristics of n-type GeSn transistors. The small bandgap of GeSn leads to large leakage current in transistors. Using a mesa structure to control the PN junctions between the source/drain and the substrate, the leakage current is effectively reduced compared to conventional planar transistors. Then junctionless GeSn transistors are demonstrated to improve the thermal budget of GeSn by reducing the steps of thermal processes such as dopant diffusion or annealing. Tensile strained GeSn mesa transistors are characterized and compared to relaxed and compressive-strained devices.