中紅外光光源與偵測器在熱影像儀器、溫室氣體檢測上有重要的應用。然而目前市面上較成熟、以砷化鎵材料製作的中紅外波段半導體雷射皆相當昂貴而限制其應用。因此，近年來以鍺錫等材料發展相對經濟性的中紅外光光電元件之研究即相當受到重視，譬如在鍺緩衝矽基板上鍍鍺錫薄膜即已成為一個熱門的研究領域。在一個高錫含量的鍺錫薄膜中，其能隙可隨著其錫含量的增加而減少，並在錫含量超過大約9%時，會從間接能隙變成直接能隙。然而因為錫在鍺內的熱力學平衡溶解率僅不到1%，因此產生一個高錫含量的鍺錫合金鍍膜有其相對的技術挑戰。此外，在鍺緩衝基板上鍍鍺錫薄膜時，也會面臨到產生負面應變效應(adverse strain effect)與穿孔錯位缺陷(threading dislocation)的問題。
由於鍺和錫本身的熔點小於1000度，若單純使用脈衝雷射沉積法(pulsed-laser deposition)製作1微米以上的鍺錫薄膜將導致薄膜表面出現過多的鍺與錫液滴(particulate)，進而使製作出的半導體元件有過高的漏電流與能階改變等造成元件效能降低，甚至失效的問題。因此，本研究目標為發展一套以化學氣相沉積法為主、使用四氯化鍺(GeCl4)和四氯化錫(SnCl4)為前驅物(precursor)和氫氣為反應物(reactant)在矽基板上製作鍺薄膜以及鍺錫薄膜的製程。實驗結果顯示，相對於以脈衝雷射沉積法製作之1微米鍺薄膜，其鍺液滴密度達10^6~10^7 cm^(-2)，使用化學氣相沉積法為主所製作的1微米鍺薄膜，其液滴密度即可低於10^4 cm^(-2)。化學氣相沉積法使用GeCl4當作前驅物會使的薄膜成長方式都是島狀成長模式，薄膜都是由晶柱排列所構成，在化學氣相沉積法的過程中加入掃描式連續光雷射處理也無法改變薄膜的成長方式，使薄膜有好的表面平整度。
本研究將進一步發展電漿誘發化學氣相沉積法，藉由氫電漿與前驅物GeCl4和SnCl4反應，並且在基板沉積鍺或鍺錫薄膜，以此來避免產生島狀生長模式，本研究成功在RF功率200 W，腔體總壓0.2 torr的條件下沉積出非晶柱組成的鍺薄膜，並且鍍率高達28 nm/min。另外在RF功率100 W，腔體總壓1 torr的條件下成功沉積表面粗糙度跟鍺薄膜相同的鍺錫薄膜，而鍺錫薄膜的錫含量有達到10 %，鍍率則是14 nm/min。

The use of mid-infrared (MIR) light sources and detectors are significant in thermal imaging instruments and greenhouse gas detection. To date, most of the matured mid-infrared semiconductor lasers/detectors are made by gallium arsenide materials with high costs that can substantially hinder the widespread use of them. In recent years, there has been a considerably push to achieve economical MIR optoelectronic components with germanium-tin (GeSn) alloy, such as depositing GeSn films on germanium (Ge) buffered silicon substrates to fabricate optoelectronic devices with small band gaps. In this way, the value of band gap decreases at a higher Sn content in a GeSn film. More importantly, the GeSn alloy can be modified from an indirect band gap into a direct band gap when Sn content exceeds 9%. However, depositing such films is typically challenging since the equilibrium solubility of Sn in Ge is less than 1%. The adverse strain effect and threading dislocations can also violate the growth of GeSn film on Ge-buffered substrate.
When using pulsed-laser deposition (PLD) to produce GeSn films, the low melting points < 1000 degrees for Ge and Sn can inevitably result in the formation of Ge and Sn particulates on surfaces; consequently, device performances degrade with high leakage currents and distortions of energy bands. Therefore, the goal of this research is to develop chemical vapor deposition (CVD) methods that use germanium tetrachloride and tin tetrachloride as the precursor and hydrogen as the reactant for fabricating Ge and GeSn films on the silicon substrates. With CVD, results show that a particulate density smaller than the observation limit of 10^4 cm^(-2) can be achieved for 1-µm-thick Ge films. In contrast, the particulate density is about 10^6~10^7 cm^(-2) when producing 1-µm-thick Ge films by PLD. However, due to the Volmer-Weber growth mode (VW mode) occurred in CVD with GeCl4 precursors, these Ge films are produced with prominent crystal columns that cannot even be inhibited when scanning continuous wave laser annealing is applied in conjunction with the CVD process.
To enable the growth of Ge/GeSn films without VW mode, the setup is transformed into plasma-induced chemical vapor deposition (PICVD); whereby the hydrogen plasma interacts with the GeCl4 and/or SnCl4 precursors then activates the formation of Ge/GeSn films. While setting 200-W RF power and 0.2-torr backing pressure, Ge film can grow at the rate of 28 nm/min without prominent crystal columns inside. Using 100-W RF power and 1-torr backing pressure, GeSn film with a Sn content at 10% and the growth rate of 14 nm/min can also be produced with similar surface roughness as the Ge film accordingly.

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