利用有機金屬化學氣相沉積成長氮化鋁/氮化鎵高電子遷移率電晶體

Abstract

本研究利用有機金屬化學氣相沉積成長氮化鋁/氮化鎵高電子遷移綠結構。本研究分為三部分以達到具有平坦表面形貌的氮化鋁屏障層，第一部分為利用傳統氮化鋁鎵/氮化鋁/氮化鎵結構漸變成長為氮化鋁/氮化鎵，藉由改變氮化鋁鎵屏障層以及氮化鋁緩衝層的厚度以達到氮化鋁/氮化鎵結構，此部分可發現氮化鋁成長條件為最主要成長結構之因素，縱使在氮化鋁在傳統結構中對於電性有所提升，但因為氮化鋁層的粗糙度太大以及無法形成連續的薄膜；第二部分為利用預通三甲基鋁以增加氮化鋁中鋁原子的遷移長度，在此可觀察到在預通氣體時氮化鎵需暴露於高溫且高流氫氣之中，使氮化鎵表面造成嚴重腐蝕，並未對於氮化鋁/氮化鎵成長有顯著的感善。 第三部分藉由調變氮化鋁成長五三比例以達到平坦的氮化鋁/氮化鎵結構。本實驗中改變五三比例從7547至157，並觀察其表面形貌改變。在高五三比成長時，氮化鋁並未聚集成平面，另外在低五三比成長時出現較大的突起晶粒產生。最佳化的五三比例為314，具有平坦且連續的表面，由原子力顯微鏡觀察表面粗糙度為0.389 奈米；由穿透是顯微鏡觀察屏障層厚度為8 奈米。在電性表現部分，元件採用閘極長度 0.4 微米，源極致汲極為5 微米，可測量到最大汲極電流為 570 mA/mm，轉移電導為 240 mS/mm，截止崩潰電壓可超過 50V。高頻特性部分電流截止頻率為 20.2 GHz最大共振頻率 33.2GHz。這樣的結果說明在高頻應用上，目前研發的氮化鋁/氮化鎵高電子遷移率電晶體充滿潛力。

In this study, AlN/GaN structure were grown for high electron mobility transistor (HEMT) application by metal organic chemical vapor deposition (MOCVD) system. Three different experiment approaches were investigated to achieve an AlN barrier with smooth surface morphology. In the first approach, the AlN/GaN was modified from a typical HEMT structure of AlGaN/AlN/GaN. The thicknesses of the AlGaN barrier and AlN spacer layers were gradually decreased and increased, respectively, to achieve the AlN/GaN structure. The results suggested that the growth condition of AlN layer needed to be improved for a better surface morphology. Although the thin AlN spacer layer could improve the electrical properties of the AlGaN/AlN/GaN structure, but the AlN surface became too rough and discontinuous when its thickness became larger. In the second growth approach, TMAl pre-flow was used prior to the deposition of AlN barrier layer on GaN in order to enhance the migration distance of Al atoms for improving the AlN surface morphology. However, it was found that the H2 etching of GaN surface occurred during TMAl pre-flow when the NH3 source was not supplied into the growth chamber. As a result, the GaN surface was roughened with the formation of large etching pits. In the third growth approach, the V/III ratio of the AlN was varied from 7547 to 157 to investigate its effect on the film surface. At high V/III ratio, the AlN film was seemed un-coalesced. On the other hand, large protruded grains was observed on the AlN surface when very low V/III ratio (<314) was applied. The optimized V/III ratio occurred at approximately 314 with a continuous and smooth AlN film being achieved. The AlN barrier grown on the GaN layer exhibit a root-mean-square roughness of 0.389 nm for a thickness of about 8 nm. Finally, AlN/GaN HEMT devices were fabricated with gate length of 0.4 μm and source-drain spacing of 5 μm. From DC measurement, a maximum drain current density of 620 mA/mm, a peak transconductance of 258 mS/mm and an off-state breakdown voltage larger than 50 V were achieved. Moreover, a current-gain cutoff frequency of 20.6 GHz and a maximum oscillation frequency of 33.2 GHz had been achieved from the scattering parameters measurement. The results suggested that the AlN/GaN HEMT structure developed in the current study has the potential for high frequency applications.