射頻磁控濺鍍p型氧化亞錫薄膜及場效電晶體特性之研究

Abstract

本論文討論在不同生長條件下，氧化亞錫薄膜其晶相與結晶度、透光度、表 面粗糙度與導電性的分析。並且以氧化亞錫薄膜作為主動層製作p 型透明薄膜電 晶體。 氧化亞錫的薄膜薄膜濺鍍製程主要以錫為靶材，並調控氧分壓進行氧化膜中 氧量的控制。在薄膜晶相與結晶度分析中，發現當隨濺鍍時氧通量比例提升，或 濺鍍壓力增加時，薄膜中的氧原子比例亦會提升，薄膜中的錫金屬相隨之減少， 但過多的氧量會使薄膜中的氧化亞錫開始改變化學態形成氧化錫，使薄膜不易結 晶。進行氮氣退火時，薄膜在200℃退火具有較大的結晶晶粒，隨溫度提升晶粒 逐漸縮小。300℃以上退火溫度因高於氧化亞錫化學穩定態，結晶度明顯下降。在 光穿透率部分，氧通量增加與濺鍍壓力提升皆會使薄膜中的金屬錫含量減少而使 透光率提升，薄膜光學能隙亦從2.7 eV 開始持續隨著薄膜氧比例提升而增加，最 高可至3.45 eV，表現出錫氧化物在氧量調變下從氧化亞錫(2.7 eV)至氧化錫(3.9 eV)的轉換過程。表面型態可看出薄膜表面有明顯錫顆粒析出，並有些微結晶，隨 氧量增加錫顆粒析出減少，同時薄膜趨向平整，進入非晶相狀態。250℃以上退火 溫度因高於金屬錫熔點，薄膜表面錫顆粒有明顯增加。氧化亞錫薄膜氧比例增加 使薄膜缺乏錫更容易產生錫空缺，此外，氧化亞錫晶粒粒徑較大，使得薄膜載子 濃度與載子遷移率都逐漸提升，但過多氧量會使薄膜接近非晶相而讓導電度將急 遽降低，低溫200℃退火因氧化亞錫結晶度較高，有助於載子遷移率的提升，但 缺陷過多使載子濃度普遍偏高，300℃載子遷移率略低，但應用於電晶體主動層時 載子濃度可有效控制。 本實驗使用氧化亞錫薄膜作為主動層，成功製備出下閘極結構 p 型場效薄膜 電晶體。其電流開關比最佳可達到3.15× 103，飽和區場效遷移率在 0.153 cm2/Vs 左右。

We investigated the crystallinity, optical transmittance, surface morphology and electrical properties of tin monoxide (SnO) thin film under various deposition conditions. The SnO films were applied to the active layers of p-type thin film transistors. SnO films were sputter-deposited under various O2/Ar flow ratios and sputtering pressure without intentional heating. Higher oxygen content reduced the content of β-Sn phase, leading to higher purity of tin monoxide. As the O2/Ar flow ratios reaches 4.38%, excessive oxygen would partially transform Sn(II)O state into tin(IV) dioxide(SnO2). Under this circumstance, SnO phase was eliminated and the film is turned into amorphous state. The transmittance of SnO thin film increases with sputtering atmosphere pressure and O2/Ar ratio in sputtering atmosphere, owing to the reduction of light-scattering caused by tin particles in SnO films. The optical band gap (Eg) increases from 2.73 eV to 3.45 eV as the sputtering pressure increases from 1 mtorr to 4 mtorr, indicating the transformation of SnO (Eg = 2.7 eV) to SnO2 (Eg = 3.9 eV). Nano-scale grains and scattered tin particles were observed on the surface of SnO thin films. For higher oxygen ratio during sputtering, film surface became smoother and less tin particles were observed on the film surface, indicating the transformation of SnO and SnO2 into amorphous Sn-O states. In addition, more tin particles were detected on the film surface when the post-annealing temperature is raised beyond 250 ℃. This is because the metallic tin could melt and gather as nanoparticles at the temperature over 230℃. SnO thin films were also evaluated by Hall measurement. Higher mobility and greater carrier concentration could be achieved by raising the oxygen content of SnO thin films. By raising the oxygen content, the grain size usually becomes larger; more tin vacancies and oxygen interstitials were formed. But too much oxygen content could cause the detrimental of p-type property. 200℃ post-annealed SnO reveals better Hall mobility of 5.33 because of larger grain sizes. But the carrier concentration was also raised such that the current could not be modulated when the film is used as the TFT’s active layer. The carrier concentration of SnO was reduced by a 300℃ post-annealing process. The TFT with 300℃ post-annealed SnO active layer shows better modulation characteristics at a cost of lower hall mobility of 3 cm2/Vs. With a deposition conditions of 3 mtorr sputtering pressure O2/Ar ratio of 3.75% and post-annealing temperature of 300℃, we have demonstrated a bottom gate SnO thin-film transistor with on/off ratio of 3.15× 103 and saturated field-effect mobility of 0.153 cm2/Vs.