鍺錫磊晶薄膜之光學、穿隧與載子傳輸特性

Abstract

鍺錫(GeSn)材料因具有直接能隙和兼容於傳統矽製程的特性, 而在許多研究應用上受到廣大的矚目。由於在其直接能谷(Γ)的電子擁有小的等效質量，因此可利用直接能隙的鍺錫材料來製作高性能的光電子元件，而得到高電子遷移率和高穿隧機率等優點。本篇論文利用化學氣相沉積法磊晶高品質的鍺錫薄膜，並使用許多材料分析技術來觀測其材料性質，包含穿透式電子顯微鏡、原子力顯微術、二次離子質譜法、X光繞射與倒置空間圖譜技術。在鍺錫材料中，藉由增加其錫比例或施加拉伸應力來達到鍺錫直接能隙的條件。 直接能隙的鍺錫在許多材料特性上有顯著的進步與表現，例如量子穿隧、載子傳輸、與光學性質。本篇論文利用化學氣相沉積法磊晶高品質的鍺錫薄膜，並製作相關的鍺錫元件來探討上述的材料特性，並研究鍺錫中間接-直接能隙的轉換在材料性上的表現。為了增加鍺錫薄膜中的錫比例，我們探討磊晶成長過程中的各種條件，如氣體先驅物流量、磊晶溫度、和鬆弛緩衝層。在鍺錫完全應變於鍺虛擬基板的結構中，可磊晶錫比例高達18%的鍺錫薄膜，其薄膜受到相當大的壓縮應變達-2.4%。藉由成長與利用鍺錫鬆弛緩衝層，可使鍺錫薄膜的錫比例增加至最高24%。 接下來，利用光致發光頻譜量測鍺錫磊晶薄膜中直接能隙與相關的光學性質。根據室溫下光致發光頻譜的結果，鍺錫直接能隙的大小會隨著錫比例增加而持續減少。在錫比例24%的鍺錫薄膜中，其最窄的能隙達到只有0.295 eV，此對應的發光波長為4.2 微米。另外，量測低溫變溫(最低溫約15 K)光致發光頻譜的結果顯示，鍺錫間接-直接能隙轉換發生在錫比例8%附近。 為了研究鍺錫量子穿隧的特性，我們製作鍺錫穿隧二極體並具有良好的負微分電阻特性，藉由量測其峰值穿隧電流來探討鍺錫的能帶間穿隧效應特性。我們首次在室溫下在鍺錫穿隧二極體中展現負微分電阻特性。並且在應變鬆弛Ge0.925Sn0.075元件中，分別在室溫與4 K低溫下，得到極高的峰谷電流比例達到15倍與219倍。比較各種應變條件的Ge0.925Sn0.075的元件，藉由壓縮應變的鬆弛或更加施以拉伸應變，隨著直接能隙的穿隧效應逐漸的主導，其穿隧電流會隨之有顯著的增加。在拉伸應變鍺錫穿隧二極體中，分別在室溫與4 K低溫下達到325和545 kA/cm2的峰值穿隧電流密度，此數值為所有四族穿隧二極體中最高的紀錄。藉由分析峰值穿隧電流隨溫度的變化，以及聲子輔助穿隧頻譜的量測，分析得到隨著施加拉伸拉伸應力，愈來越多電子於處在直接能谷(Γ)中並擁有較小的等校質量，導致較高的能帶間的穿隧機率。 論文最後的部分，以霍爾量測來探討鍺錫薄中電子傳輸性質。其中在N型Ge0.82Sn0.18薄膜中，於75 K低溫下量到最高9,500 cm2/Vs的電子遷移率。藉由比較所有樣品在4K低溫下的電子遷移率，可以清楚地發現在錫比例8%以上的樣品中，電子遷移率會隨著錫比例增加而明顯的增高，並在錫比例18%的鍺錫薄膜中達到最高值。電子遷移率隨著錫比例的變化也可以利用鍺錫的間接-直接能隙轉換所解釋，因為在電子在直接能谷(Γ)比在間接能谷(L) 擁有較小的等效質量，所以隨著直接能谷的電子數量變多，鍺錫整體的電子遷移率也隨之增高。此外，此章節也包含完整的理論計算來模擬鍺錫電子遷移率隨許多因素的變化趨勢，並可以清楚地解釋電子遷移率因為間接-直接能隙轉換而增加的過程。

GeSn has drawn great attention due to its direct-bandgap characteristic and the compatibility with Si VLSI technology. While high-performance optoelectronic devices were fabricated using direct-bandgap GeSn, high carrier mobility and tunneling probability have been demonstrated owing to its small electron effective mass in the direct Γ valley. In this work, high-quality epitaxial GeSn films were grown by chemical vapor deposition (CVD), and characterized by various material metrology tools, such as transmission electron microscopy (TEM), atomic force microscopy (AFM), secondary ion mass spectrometry (SIMS), X-ray diffraction (XRD), and reciprocal space mapping (RSM). Increasing its Sn fraction or applying tensile stresses is used to achieve a direct bandgap in GeSn. In order to achieve a high Sn fraction in GeSn, the effects of the flow rates of precursors, growth temperature, and relaxed buffer are investigated. For compressive-strained GeSn films on a Ge virtual substrate (VS), the Sn fraction up to 18 % was demonstrated with a large compressive strain of -2.4 %. Using a GeSn relaxed buffer, the highest Sn fraction of ~ 24 % is achieved in compressive-strained GeSn films. Photoluminescence (PL) measurements were performed to characterize the bandgap energy in GeSn epitaxial films. At room temperature, the direct-bandgap energy of GeSn monotonically decreases with the Sn fraction. The smallest bandgap energy of 0.295 eV is demonstrated in the Ge0.76Sn0.24 film, corresponding to the wavelength of ~ 4.2 μm. The cryogenic (~ 15 K) PL spectra suggest that the indirect-to-direct bandgap transition in GeSn occurs at a Sn fraction around 8 %. Esaki tunnel diodes with negative differential resistance (NDR) were used to characterize the band-to-band tunneling (BTBT) properties in GeSn. The GeSn Esaki diodes with clear NDR at room temperature are presented for the first. In the strain-relaxed Ge0.925Sn0.075 Esaki diodes, extremely high peak-to-valley ratios (PVCRs) of 15 and 219 are achieved at RT and 4 K, respectively. The tunneling current is increased by strain relaxation and further enhanced as the GeSn epitaxial film is under tensile stresses since the direct BTBT dominates. High peak current densities of 325 and 545 kA/cm2 are demonstrated in the tensile-strained Ge0.925Sn0.075 diodes at RT and 4 K, respectively, which is the highest reported among group-IV Esaki diodes. Both temperature-dependence of the peak current density and phonon-assisted tunneling spectra suggest that by applying tensile stresses on GeSn, more electrons populate in the Γ valley, leading to a higher BTBT rate owing to its small electron effective mass. Last, electron transport in GeSn is investigated by Hall measurements. The highest Hall mobility is 9,500 cm2/Vs at 75 K in a n-type Ge0.82Sn0.18 film. At 4 K, the electron mobility increases with the Sn fraction as the Sn fraction is above 8 %. The mobility enhancement results from more electrons in the direct Γ valley, where electrons have a smaller effective mass than that in the L valley. The theoretical calculation on the GeSn electron mobility is also presented and suggests that the mobility enhancement is attributed to the indirect-to-direct bandgap transition.