鍺錫/氧化層介面之實驗特性與第一原理計算分析

Abstract

鍺(錫)因為其高載子遷移率的特性而具有成為金氧半場效電晶體(metal-oxide-semiconductor field-effect transistors, MOSFETs)通道材料的潛力，由於位在鍺錫表面的氧化層較不穩定，故鍺(錫)/氧化層介面品質有待改善。 在本論文中鍺錫/氧化層介面品質以氧化層電容、電容等效厚度、介面缺陷密度、頻率色散和遲滯判斷。首先使用快速熱氧化步驟來改善氧化層介面，假如在沉積高介電值氧化層前進行快速熱氧化步驟，GeSnOx中間層較厚且介面缺陷密度、頻率色散和遲滯效應大幅改善。使用二氧化鉿可以降低電容等效厚度，由於鍺錫/二氧化鉿介面品質比不上鍺錫/三氧化二鋁介面，中間層使用三氧化二鋁為較佳的選擇。利用氧氣電漿轟擊鍺錫/氧化層介面，相較於使用快速熱氧化其介面品質更加大幅地改善且中間層厚度也較薄，但是在氧氣電漿轟擊的過程中離子會殘留在氧化層內，故使用氧氣電漿會增強遲滯效應。 為了更全面地闡述鍺錫/氧化層介面特性，使用第一原理計算來模擬鍺(錫)/二氧化鍺(錫)介面其能態密度。首先，為了驗證模擬結果，本論文先模擬鍺錫能帶結構，錫比例的提高使得鍺錫能帶結構的直接能隙和間接能隙下降。在鍺/二氧化鍺介面附近原子間的鍵結情形影響著鍺/二氧化鍺介面模型之能態密度，假如在鍺/二氧化鍺介面附近有任何懸鍵，則會在能隙內形成缺陷能態。此外，鍺錫或二氧化鍺錫內錫原子的位置也影響鍺錫/二氧化鍺錫介面之能態密度，若錫原子靠近鍺錫/二氧化鍺錫介面，則會在能隙內產生缺陷能態。

Ge(Sn) is a promising channel material for metal-oxide-semiconductor field-effect transistors (MOSFETs) due to the high carrier mobility. However, the quality of Ge(Sn)/oxide interfaces is poor because the oxide on GeSn interfaces is less stable. In this thesis, the quality of GeSn/oxide interfaces is evaluated by oxide capacitance (Cox), capacitance equivalent thickness (CET), interface trap density (Dit), frequency dispersion, and hysteresis. First, a step of rapid thermal oxidation (RTO) is performed to improve oxide interfaces. If the RTO step is performed before the deposition of high-k dielectrics, the GeSnOx interfacial layer is thicker while the interface trap density, frequency dispersion, and hysteresis are greatly improved. Using HfO2 reduces CET while an interfacial layer of Al2O3 is suggested because of better GeSn/Al2O3 interfaces quality than GeSn/HfO2 interfaces. By oxygen plasma to bombard GeSn/oxide interfaces, interfaces quality is much improved compared to that by RTO with a thinner interfacial layer. However, using oxygen plasma enhances hysteresis because ions are incorporated into the oxide during the oxygen plasma bombardment. To fully characterize GeSn/oxide interfaces, a first-principles calculation on the density-of-states (DOS) of Ge(Sn)/Ge(Sn)O2 interfaces is performed. First, the GeSn band structures are simulated to justify the simulation results. As the Sn fraction increases, both direct and indirect bandgaps in GeSn band structure decrease. The bonding condition between atoms at Ge/GeO2 interfaces affects DOS of the Ge/GeO2 interfaces model. If there is any dangling bonds near Ge/GeO2 interfaces, defect states are induced within the bandgap. Furthermore, the position of the Sn atoms in GeSn or GeSnO2 also influences DOS of GeSn/GeSnO2 interfaces. If the Sn atoms locate near GeSn/GeSnO2 interfaces, it also creates defect states in the bandgap.