微結晶矽材料於電子及光電元件之應用

Abstract

微結晶矽薄膜被廣泛應用在太陽能電池及薄膜電晶體之電子元件上。其主要歸咎於其材料相較於非晶矽材料具備高載子遷移率、高摻雜特性以及可吸收紅外光波段之特點。再者，微結晶矽材料其製作方式較易於與標準之工業製程相匹配。因此，於本研究中我們深入探討微結晶矽薄膜材料並將其應用於不同之電子元件。 於第一部分，我們利用了熱燈絲化學式氣相沉積系統成長一微結晶矽薄膜，並透過此系統開發一”漸變式p-型窗口層”以降低非晶相之孵化層。此方式，將可有效降低載子被復合的機率，提高其太陽能電池之轉換效率。透過特殊的漸變式不同濃度氫含量之p-型窗口層，由實驗證明其非晶相之孵化層可完全消除，並可同時改善本質層之結晶度增加其光子之吸收效能。另一方面，此結構因不同氫含量的差異，也可漸變式的達到抵抗光折射的特點，增加其入射光的吸收。因此，於太陽電池的電特性表現上達到短路電流19.7 mA/cm2 、開路電壓450 mV以及光電轉換效率4.5%。 第二部分，我們藉由感應耦合式電漿系統於低溫375度的條件下，開發一微結晶矽薄膜，並發展一微結晶矽薄膜電晶體。透過實驗的分析，我們發現微結晶矽薄膜電晶體的高載子遷移率，歸咎於通道層內較少的晶界缺陷，故有利於載子於通道中的傳輸。因此，經驗證此微結晶矽薄膜電晶體其電特性的表現，於臨界電壓為-0.64V，而其相對之載子遷移率則為370 cm2/V-s。同時，我們利用其元件所得到之高電子遷移率的特點，同時發展一具備雙金屬閘極結構之薄膜電晶體。此結構將可有效調控其臨界電壓，其調控之臨界電壓可由-1V調控至2.7V，將可有利於應用在IC電路上之電壓的調控。 最後，我們同時採用感應耦合式化學式氣相沉積系統開發微結晶鍺薄膜。透過最佳化的調變，如不同之氫稀釋比、rf電漿源以及氬氣流量等。我們發現所開之微結晶鍺薄膜其結晶度可達88%，且其相對尺寸透過XRD及SEM的分析約略微56 nm。另一方面，為了提高其結晶度及結晶尺寸，我們同時導入固態綠光脈衝雷射系統。透過其雷射結晶化的方式，將可有效增大其微結晶鍺的結晶尺寸並由微結晶鍺轉化為一多晶鍺材料。其結晶性可達92%，其中其結晶大小甚可達500奈米以上。因此透過本研究所開發之鍺材料，將可有利於發展高速之電子元件。

關鍵詞:微結晶矽、漸變式p型窗口層、熱燈絲化學氣相沉積系統、薄膜電晶體、雙金屬閘結構、感應耦合式化學氣相沉積系統。

Hydrogenated microcrystalline silicon (μc-Si:H) has attracted much interest in the application of electronic and photovoltaic devices, such as thin film transistors (TFTs) and solar cells. This is attributed to its superior electronic and photovoltaic properties. Compared with hydrogenated amorphous silicon (a-Si:H), μc-Si:H has a higher carrier mobility, better doping efficiency, higher absorption in the red and infrared wavelength ranges, and an enhanced carrier lifetime. Furthermore, the fabrication process of μc-Si:H is easy and compatible with the standard conventional process. In this thesis, we studied material properties of μc-Si:H thin films and the application for solar cells and TFT devices. First, we prepared thin film solar cells using μc-Si:H grown by hot-wire chemical vapor deposition (HWCVD). To decrease the a-Si:H phase incubation layer and enhance the cell efficiency, we develop a novel “graded p-type window layer (GPL)” structure to study the dependence of the cell performance on the hydrogen concentration in the multilayer p-type window layer. The a-Si:H incubation layer is absent in the HWCVD-prepared GPL structure, and the crystallinity of the intrinsic layer is greatly improved. Moreover, the GPL structure also leads to a nearly omni-directional antireflection. The μc-Si:H solar cell with the triple hydrogen dilution grades in the p-layer has a high photo-conversion efficiency of 4.5% as a result of the improvement in the electrical and the antireflection properties of the cell. The short circuit current (Jsc) and the open circuit voltage (Voc) of the solar cell are 19.7 mA/cm2 and 450 mV, respectively. Second, we fabricated μc-Si:H TFTs with a double-metal-gate structure, which exhibited a high electron-mobility (μFE) and an adjustable threshold voltages (Vth). The μc-Si:H channel and source/drain (S/D) regions of the multilayered TFT were deposited at 375oC by inductively coupled plasma chemical vapor deposition (ICP-CVD). The low grain-boundary defect density of the channel layer is responsible for the high μFE of 370 cm2/V-s, a steep subthreshold slope of 90 mV/decade, and a low Vth of -0.64 V. When biased with the double-gate driving mode, the device shows a tunable Vth value extending from -1 V up to 2.7 V. Finally, we fabricated hydrogenated microcrystalline germanium (μc-Ge:H) thin films by ICP-CVD by optimizing the deposition conditions, including the hydrogen dilution ratio, rf power and the Ar gas dilution rate. The μc-Ge:H thin film has a crystallinity about 90 %, and a grain size of 56 nm as determined by Raman scattering analysis and x-ray diffraction, respectively. In addition, a solid-state green light pulsed lasers was used to crystallize the µc-Ge:H thin films forming poly-Ge thin films with a larger grain size. The laser annealed poly-Ge thin films had a better crystallinity (92%) than the as-prepared µc-Ge:H thin film (90%). The poly-Ge thin films are suitable for the application of high speed electronic devices.

Keywords：Microcrystalline Si, Graded p-type window layer (GPL), Hot-wire chemical vapor deposition (HWCVD), Thin film transistors (TFT), Double-metal-gate, Inductively coupled plasma chemical vapor deposition (ICP-CVD).