鍺錫合金之電子傳輸特性與能帶結構模擬

Abstract

近年來，鍺錫材料越來越受到關注，這是由於鍺錫合金在錫濃度高於 6~11 %時有機會轉變為直接能隙材料，除了能應用於高效能的光電元件，鍺錫同時擁有高電子與電洞遷移率，且可與現今的積體電路技術相容，因此極具潛力成為下一代場效電晶體(MOSFET)的通道(channel)材料。為了有效利用直接能隙鍺錫的優點，已有許多文獻探討其光電特性，然而對於其電特性的相關研究較少，因此，本論文將利用變溫霍爾量測與數值贋勢法計算深入研究鍺錫的能隙與電特性。 我們以減壓化學氣相沉積法製備含有錫比例為8 %、12 %的壓縮應變、鬆弛、拉伸應變之內摻雜鍺錫磊晶，並將磊晶製作成霍爾棒元件進行變溫霍爾量測，探討錫比例與應變對於鍺錫的電子遷移率之影響。在8 %鍺錫中，以拉伸應變鍺錫的遷移率最高，壓縮應變鍺錫的遷移率最低；在鬆弛鍺錫中，以高錫比例的12 %鍺錫之遷移率較高，推測原因為施加拉伸應變或提高錫比例有助於讓鍺錫由間接能隙轉變為直接能隙，使具有較小等效質量的Γ電子的比例增加。 為了驗證實驗結果，在論文的第二部分使用Sentaurus TCAD套件以數值贋勢法模擬鍺錫能帶結構，並計算L谷與Γ谷的等效電子質量與電子於兩谷中的分布比例，以研究錫比例與應變對電子分布情形的影響。在錫比例0 %至20 %的無應變鍺錫中，L點的等效質量約為Γ點的15~40倍。不同應變條件中，拉伸應變鍺錫之Γ電子最多，而不同錫比例的鍺錫中，12 %鍺錫的Γ電子較多，因此預期其電子遷移率較高，此結果與實驗數據吻合。

Germanium-tin (GeSn) alloys have attracted much attention recently because of its direct-bandgap characteristics for high-performance optoelectronic devices. Furthermore, GeSn is very promising for the next-generation channel material MOSFET applications due to its high electron and hole mobilities and compatibility with Si VLSI technology. Despite the potential of direct-bandgap GeSn, there is few experimental results about electrical transport reported while much optical data is available. Therefore, the band structures and electrical properties of GeSn were investigated in this thesis. In this thesis, electron transport properties in epitaxial GeSn films under different strain conditions such as compressive-strain, strain-relaxation, and tensile-strain are investigated. The epitaxial films are epitaxially grown by reduced pressure chemical vapor deposition and the Sn fractions are 8 and 12 %. Hall bar devices are fabricated and characterized at 4 ~ 300 K. Among all strain conditions, the electron mobility of tensile-strained GeSn is the highest. For strain-relaxed GeSn, the electron mobility is higher as the Sn fraction increases. Both could be attributed to a higher electron population in the Γ valley, where electrons have a lower effective mass and high mobility. Applying tensile stress on GeSn films or increasing the Sn fraction reduces the energy difference between the Γ valley and the L valley. As a result, more electrons populate in the Γ valley, leading to a higher mobility due to the smaller effective mass. To characterize the electron transport properties in n-GeSn, we perform TCAD simulation of GeSn band structures. Effective masses and electron populations at Γ and L valleys are calculated. With Sn fractions of 0 ~ 20 %, the effective mass at the L valley is 15 ~ 40 times larger than that at the Γ valley. Among all strain conditions, tensile-strained GeSn shows the highest electron populations in the Γ valley. Moreover, for strain-relaxed GeSn, the electron population in the Γ valley increases as the Sn fraction increases. Both simulation results support the experimental data.