利用有機金屬化學氣相沉積成長高功率氮化鋁鎵/氮化鎵高電子遷移率電晶體結構於藍寶石基板

Abstract

氮化鎵的高電子遷移率電晶體因為其能產生大電流以及承受高崩潰電壓的能力，在高功率的應用上有極大的潛力。然而，眾所周知，在藍寶石基板上用兩階段成長法(Two-step growth)的氮化鎵薄膜(使用低溫氮化鎵緩衝層)於接近氮化鎵與藍寶石基板介面處。存在一導電層，使之成為低電阻率的氮化鎵薄膜。一種較佳的解決方法是利用高溫氮化鋁緩衝層而非低溫氮化鎵緩衝層。 在本研究中，利用有機金屬化學氣相沉積成長氮化鋁鎵/氮化鎵高電子遷移率電晶體於藍寶石基板上．藉由不同的緩衝層結構：傳統低溫氮化鎵緩衝層，高溫氮化鋁緩衝層以及高溫氮化鋁緩衝層和低溫氮化鋁成核層。針對這些不同方式成長的氮化鎵薄膜試片，進行材料與電性的分析。經由霍爾量測，發現使用高溫氮化鋁緩衝層以及低溫氮化鎵緩衝的氮化鎵試片的片電阻分別為1202 Ω/sq 以及 464 Ω/sq。此外，經由高溫緩衝層與低溫氮化鋁成核層成長的氮化鎵試片，其片電阻超過霍爾量測儀器的極限：〖"10" 〗^"6" "Ω/sq" 。使用低溫氮化鎵緩衝層成長的氮化鎵的低阻值特性可以由在接近氮化鎵與藍寶石接面的高導電層來解釋，此導電層主要是由藍寶石基板中的氧經擴散的方式而來。另一方面，氮化鎵試片成在高溫氮化鋁緩衝層和低溫氮化鋁成核層上與氮化鎵試片成在高溫氮化鋁緩衝層上，經由氫氧化鉀水溶液的蝕刻，由表面形貌發現到此兩試片在氮化鎵極性上的不同，前者的氮化鎵是鎵極性而後者是混和極性(鎵極性與氮極性)，經由二次離子質譜儀分析發現到混和極性的氮化鎵在成長過程中有高度吸附氧的趨勢。另外，為了改善氮化鎵晶體品質與表面形貌，提出使用低溫氮化鎵作為介面層在氮化鎵與氮化鋁緩衝層間，實驗結果發現，使用低溫氮化鎵作為介面層於氮化鎵與鋁極性氮化鋁緩衝層間，能有效的改善氮化鎵的晶體品質與表面形貌。而使用低溫氮化鎵作為介面層於氮化鎵與氮極性氮化鋁緩衝層間的氮化鎵試片，其低溫氮化鎵的厚度對改善表面形貌並與促進氮極性氮化鎵的成長發揮了重大的作用。 探討氮化鋁緩衝層厚度對於氮化鋁鎵/氮化鎵高電子遷移率電晶體的崩潰電壓的影響。經由兩端點測試法(Two-terminal method)測試發現到崩潰電壓隨著氮化鋁厚度增加而增加。對於150nm厚的氮化鋁緩衝層的氮化鎵試片，其緩衝層崩潰電壓在兩電極間距為5μm與20μm下分別為460V與2590V，最後，將此氮化鋁鎵/氮化鎵試片做成高電子遷移率電晶體元件，其閘極長度為1μm，閘極寬度為50μm，源極到汲極間距為7μm。經由直流電性量測，得到飽和電流為536mA/mm，轉導為138 mS/mm與靜態崩潰電壓(off-state breakdown voltage)為250V的結果。

GaN-based high electron mobility transistors (HEMTs) have great potential for high power applications due to its capability to handle large device current and large breakdown voltage. However, it is also well known that GaN film grown on sapphire using conventional two-step growth (with low-temperature GaN (LT-GaN) buffer) has low resistivity because of the presence of a highly conductive layer at the GaN/sapphire interface. One potential method to improve the GaN film resistivity is to replace the LT-GaN buffer with a high-temperature AlN (HT-AlN) buffer. In this study, AlGaN/GaN HEMT structures were grown on sapphire substrates by using metal organic chemical vapor deposition (MOCVD) method. Different buffer structures, i.e the conventional low-temperature GaN (LT-GaN) buffer, high temperature AlN (HT-AlN) buffer and HT-AlN buffer with LT-AlN nucleation layers were adopted to investigate the structural and electrical properties of the GaN films. Hall effect measurement results show that low resistance GaN films were achieved when either LT-GaN or HT-AlN buffers were used. The sheet resistances of these samples were 1202 Ω/sq and 464 Ω/sq, respectively. On the other hand, the sheet resistance of GaN film grown on HT-AlN buffer with the LT-AlN nucleation layer was over the Hall system measurement limit (>〖10〗^6Ω/sq). The low resistivity of GaN film grown on LT-GaN buffer can be explained by the presence of a highly conductive layer at the GaN/sapphire interface due to out-diffusion of O atoms from the substrate. On the other hand, the morphology difference upon KOH etching revealed that GaN film grown on HT-AlN with and without a LT-AlN nucleation have different polarities. While the former sample had Ga-face polarity, the latter had a mixture of polarities (Ga- and N-faces). The SIMS results reveal that the low resistivity of the mixed face GaN layer was due to the high tendency of this material to absorb O impurity during the MOCVD growth. Furthermore, in order to improve the crystal quality and morphology of GaN, the LT-GaN was proposed as the interlayer between GaN and AlN buffer layer. The results proved that the LT-GaN interlayer could effectively improve both the crystal quality and morphology for the GaN grown on the Al-face AlN buffer layer. And for the GaN grown on the N-face AlN buffer layer, the LT-GaN thickness plays an important role in improving the surface morphology and also promotes the growth of Ga-face GaN material. The effects of AlN buffer thickness on the breakdown voltage of AlGaN/GaN HEMT were also investigated. The two-terminal method showed that the buffer breakdown voltage increased with the AlN buffer thickness. For a 150-nm thick AlN buffer layer, the buffer layer breakdown voltages were 460V and 2590V for spacing of 5μm and 20 μm, respectively. Finally, HEMT device with gate length of 1μm, gate width of 50μm and source-drain spacing of 7μm was fabricated on this AlGaN/GaN structure. The DC measurement showed a saturated drain current of 536mA/mm, a transconductance of 138 mS/mm and an off-state breakdown voltage of 250V were obtained.