應變矽/鍺反轉層電子遷移率計算與非晶矽基薄膜太陽能電池

Abstract

在本論文中，分為兩大主題，分別探討與金氧半電晶體及太陽能發電工業相關技術。其一為應變矽及應變鍺反轉層之電子遷移率計算；另一為非晶矽基太陽能電池的高效率開發與相關材料或元件特性分析。 第一部分：長久以來，互補式金氧半工業藉由提昇電晶體性能達到更大操作電流與更快操作速度來使元件面積不斷微縮。然而在元件不斷微縮到已接近物理極限的情況下，所需之製程複雜度及成本也隨之大幅增加。已知提昇閘極電容或載子遷移率皆可提高操作電流；其中，對通道施加應力(藉由不同晶格大小基板提供應力、製程中產生應力或於製作完成後外加機械應力等)來提昇載子遷移率已廣泛應用於先進奈米製程中。本論文中針對n通道矽金氧半電晶體在不同基板位向、不同通道方向施予各種不同應力造成之反轉層電子遷移率變化做深入探討，已瞭解各種情況下之差異，並找出做佳的基板、通道、應力條件，使達電子遷移率最大化。其次，由於鍺先天具有較矽高之遷移率，使用鍺或矽鍺通道亦為提昇遷移率的方法之一，本論文中亦針對n通道鍺金氧半電晶體在不同基板位向、不同通道方向施予各種不同應力造成之反轉層電子遷移率變化做深入探討最佳化條件。 第二部分：近年來由於空氣污染及地球暖化等問題，使再生能源產業蓬勃發展，盼能在未來取代以化石燃料發電，以使地球永續發展。在各種不同的再生能源中，太陽能電池以其發電時無污染性及幾乎全球皆可利用，而成為極具發展潛力的選項。傳統上佔主流之結晶矽太陽能電池雖較薄膜型有較高之效率，然而其提鍊多晶矽之高溫製程亦需使用大量能源，延長了其成品之能源回收時間；而需要以矽為基板使大面積模組製程較為不易。非晶矽基太陽能電池之吸收層仍以矽為原料，同樣不具毒性，且不需矽基板，又能以較低溫製程，故為薄膜型太陽能電池中最具商業發展價值之選項。然而其穩定效率仍和結晶矽型有一段差距，且在製作完成後照光數百小時內會有本質上的劣化造成效率損失。故在本論文中，乃針對非晶矽型太陽能電池之高效率開發(以非晶/微晶/微晶三層結構)及劣化率的下降做探討，以期提昇非晶矽基太陽能電池之競爭力，俾對地球永續發展有所助益。

In this dissertation, two important topics are investigated and discussed for MOSFET (metal-oxide-semiconductor field-effect transistor) and photovoltaic technologies, respectively. One is the electron mobility calculation in the strained silicon or strained germanium inversion layers. The other is the performance improvement together with material and device characterization of amorphous silicon based solar cells. Part I: It is the goal for the CMOS (complementary metal-oxide-semiconductor) industry enabling large-scale decrease in chip area and improving transistor performance by scaling down the devices to give higher drive current and higher circuit speed. However, the device scaling down requires a complicated process improvement and high cost, especially when the approaching of the physical limits. Both the higher gate capacitance Cg and the higher carrier mobility μ can improve the drive current Id since Id ~ Cg∙μ. Thus, mobility enhancement offers an alternative way to further improve the drive current. Several various techniques such as substrate strain, process strain, and mechanical/package strain have been proposed to give strain into the silicon channel. In order to find out the optimal strain condition which gives highest electron mobility for silicon channels, the electron mobility in the silicon inversion layer is comprehensively studied for various substrate orientations, various channel directions, and various stress conditions. Furthermore, it was well known that bulk germanium (Ge) substrate offer 2x higher mobility for electrons and 4x higher mobility for holes as compared to Si. Thus, the strain induced electron mobility change in Ge channel inversion layer is also comprehensively studied. Part II: In recent years, the air pollution and global warming issues resulting from the mass consumption of fossil fuels have attracted more and more attention due to the aim for sustainability of the ecosystem on earth. The development of clean energy resources is thus an important challenge in these years. Among the wide variety of renewable energy, solar cells (also called photovoltaics, PV), which is pollution free in use and almost available everywhere in the world, is the most promising candidate. Although conventional crystalline silicon solar cells have higher efficiency, higher energy consumption during process and smaller substrate area give bottlenecks for this technique in the future from the ecological and economical point of view. Amorphous silicon based thin film solar cells, which can give large-scale applications and shorter energy pay-back time (EPBT), are the most commercially available thin film PV technology with abundant and non-toxic absorber material. However, the still limited stable efficiency has to be improved to compete with other PV technologies. In this dissertation, some material property and device structure are investigated to improve the initial and stable efficiency of a-Si based thin film solar cells.