- 學位論文
氮化鎵上氮化鋁薄膜式表面聲波元件特性研究
Investigation of thin film surface acoustic wave devices of AlN/GaN/Sapphire
摘要
表面聲波元件近年來成為相當熱門的研究領域，因為元件能夠應用在通訊產業，另外也因其高靈敏度適用在各類的感測器上，由於傳統的表面聲波元件材料無法與半導體製程整合，於是利用壓電薄膜製做層狀結構表面聲波元件變成相當受人矚目。 本文利用迴旋濺鍍系統在氮化鎵（GaN）基板上沉積氮化鋁(AlN)薄膜以製作表面聲波元件。氮化鎵為具有壓電性之半導體材料,但其相對較大之導電率,及不可避免之殘留摻雜使得其表面聲波元件一直難以實用. 氮化鋁薄膜具有高表面聲波波速、良好的壓電特性及高機電偶合係數等特性，相當適合用來作為表面聲波元件。由於氮化鋁薄膜與氮化鎵間晶格不匹配(lattice mismatch)較小(~2.4%)，因此能在氮化鎵基板上沉積結晶性較佳的氮化鋁薄膜，經過X光繞射儀分析證實氮化鋁薄沉積在氮化鎵基板上為磊晶薄膜。在氮化鎵基板上沉積氮化鋁薄膜能改善氮化鎵表面聲波元件之特性並與半導體、光電產業做結合，此為研究氮化鋁薄膜相當重要的原因之ㄧ。本篇論文主要是探討沉積不同厚度之氮化鋁薄膜對元件插入損耗(insertion loss)、旁帶抑制(sidelobe)、聲波波速(Vp)及機電偶合係數(K2)的影響，另外將分別針對氮化鎵基板的頻散效應做探討。最後則是對氮化鎵基板與氮化鋁薄膜做溫度實驗，藉由量測其中心頻率的變化，計算出基板與薄膜個別的頻率溫度係數。 由實驗結果可知，我們能藉著增加氮化鋁薄膜的厚度來改善元件插入損耗的問題，且提高元件的旁帶抑制以獲得更佳的濾波特性。另外分別量測到氮化鋁薄膜與氮化鎵基板的頻率溫度係數，氮化鎵基板的頻率溫度係數為-63.8 ppm/℃，氮化鋁薄膜的頻率溫度係數為-54.8 ppm/℃。我們發現沉積氮化鋁薄膜於氮化鎵基板上具有溫度補償之功用，藉由增加氮化鋁薄膜之厚度比較可得知氮化鋁薄膜之頻率溫度係數為正值，此結果將對於氮化鋁於氮化鎵層狀結構表面聲波元件之應用有很大貢獻。
並列摘要
In recent years, surface acoustic wave devices have become an attractive research field because the devices can be applied in communication field, and it suits to be sensors for it’s high sensitivity. Using piezoelectric thin films for layered structure SAW devices is popular. AlN thin films have been deposited on GaN/Sapphire for SAW devices by using Helicon sputtering system. GaN is a semiconductor with piezoelectric properties. However, its relatively large electrical conductivity and unavoidable residual dopants during the thin film growth have been the obstacles for the application of SAW devices made on GaN. AlN is the promising material for SAW devices due to its excellent piezoelectricity, high surface acoustic wave velocity, substantial electromechanical coupling coefficient, good thermal stability. Due to the small lattice mismatch between AlN and GaN, epitaxial AlN films can be deposited on GaN. Superior SAW characteristics have been obtained by deposition AlN on GaN. The thin film layered structure for SAW devices, with the combination of AlN and GaN may, in future, bring about the development of high frequency components which integrate and utilize their semiconducting, optoelectronic properties. The effects of thickness of AlN thin films have been discussed in terms of the insertion loss、sidelobe rejection、phase velocity and electromechanical coupling coefficient. The TCF of GaN/sapphire and AlN/GaN/Sapphire have been measured as well. The results showed that the loss and sidelobe rejection of the SAW devices can be improved by increasing the thickness of AlN films. The TCF of AlN on GaN was found to be positive, and by increasing the thickness of AlN, the TCF of the combination of AlN and GaN/Sapphire be compensated, which may contribute to the applications of this layered structure SAW devices.