以金屬有機化學氣相沉積方式成長氮化鋁緩衝層應用於氮化鋁(銦)鎵/氮化鎵高電子遷率電晶體

Abstract

本論文利用有機化學氣相沉積方式於藍寶石基板上磊晶成長氮化鋁緩衝層應用於高電子遷移率電晶體之氮化鋁(銦)鎵/氮化鎵異質結構。首先，我們將氮化鎵材料磊晶成長在由低溫(520 °C)及高溫(1000 °C)結合而成之氮化鋁緩衝層上，藉由使用不同厚度的低溫氮化鋁材料來探討其對於氮化鎵薄膜材料特性的影嚮。在無任何低溫氮化鋁材料於緩衝層結構中時，氮化鎵薄膜擁有一低片電阻464 ohm/sq 並且在氮化鎵材料表面可觀察到許多微小孔洞。此外，當在氮化鋁緩衝層結構使用一層厚度為5 nm的低溫氮化鋁材料時，可使得後來成長的氮化鎵薄膜中擁有大於106 ohm/sq之片電阻值，實驗結果指出，這層低溫氮化鋁材料可以改善氮化鎵表面形貌並且有效的在氮化鎵材料中抑制反極性區域的產生且降低氧原子的摻入。然而，當低溫氮化鋁材料的厚度增加為10 nm時，氮化鎵材料的表面形貌及晶體品質都會因為低溫氮化鋁材料中的晶粒錯位而產生嚴重的劣化。 為了更進一步的最佳化氮化鋁緩衝層結構，我們也調整了在緩衝層結構中高溫氮化鋁材料的厚度。結果指出，氮化鋁緩衝層表面會隨著增加高溫氮化鋁材料厚度而變得粗糙，並且使得氮化鋁緩衝層表面晶粒尺寸變大。這樣子的表面使得氮化鎵材料在磊晶成長初期能夠以二維模式來進行，這樣的成長模式能有效地抑制氧原子的摻入，氮化鎵材料本身的阻值也因此獲得改善。此外，晶體品質也因為緩衝層表面擁有較低的晶粒密度而有所提升。因此，在此緩衝層上所製造出來的氮化銦鋁/氮化鎵高電子遷移率電晶體的元件特性如輸出電流、轉移電導及崩潰電壓都能隨著增加高溫氮化鋁材料的厚度而得到改善。 最後，我們也研究了在藍寶石基板上不進行氮化(nitridation)步驟以及不在氮化鋁緩衝層中使用低溫氮化鋁材料之下，探討在不同氮化鋁緩衝層沉積溫度下，對於氮化鎵材料特性的影嚮。由實驗結果得知，在使用較高溫度成長氮化鋁緩衝層時，其沉積在此氮化鋁緩衝層上的氮化鎵表面充滿了許多粗糙的島狀結構，同時呈現許多相反極性的區域。此外，在氮化鎵材料中的螺旋差排密度及漏電流也因為溫度的提升而有所增加。氮化鎵材料中，反極性區域的產生是來在自於成長氮化鋁緩衝層初期時，由於氨氣瞬間的注入，而在藍寶石基板上引起了氮化效果。藉由使用二階段溫度方式來成長氮化鋁緩衝層，藍寶石基板的氮化可以有效的被抑制。使得，氮化鎵材料成長於這樣的緩衝層上面時，能夠得平坦的表面，單一極性，高晶體品質及高阻值等等特性，並且能在此緩衝層上成功地製作出氮化鋁鎵/氮化鎵高電子遷移率電晶體。

The AlGa(In)N/GaN heterostructure for the high electron mobility transistor applications were grown on sapphire substrate with AlN buffer layer by metal-organic vapor deposition (MOCVD). The AlN buffer was consisted of low-temperature and high-temperature growth conditions. The effect of the low-temperature AlN layer thickness on the properties of GaN film were first investigated. When GaN was grown without the LT-AlN nucleation layer, the GaN layer had low sheet resistance of 464 ohm/sq and the surface was decorated with pitted region. On the other hand, when a LT-AlN layer with thickness of 5 nm was inserted, a GaN layer with sheet resistance higher than 106 ohm/sq was achieved. This thin nucleation layer also improved the GaN morphology, suppressed inversion domain formation, and reduced oxygen (O) impurity incorporation. However, the surface morphology and quality of the GaN crystal were degraded severely when the LT-AlN thickness was increased to 10 nm due to the formation of disorientated grains in the LT-AlN layer. For further optimizing the structure of AlN buffer layer, the investigation transferred to the thickness of HT-AlN layer. As revealed by atomic force microscope analysis, a rougher surface and larger grain size were observed with a thicker buffer layer. The larger grains promoted the two-dimensional (2D) growth mode of the GaN layer at the initial growth stage. This suppressed oxygen incorporation at the GaN/HT-AlN interface and thus improved the resistivity of the GaN layer. Moreover, the lower grain density also resulted in enhanced GaN crystal quality of the GaN layer. As a consequence, the electrical properties of the InAlN/GaN HEMT device, such as output current, transconductance and off-state breakdown voltage, were improved by increasing the HT-AlN buffer layer thickness. Finally, the sapphire nitridation and the LT-AlN layer were omitted during the growth. At relatively higher AlN buffer growth temperature, the surface morphology of subsequent grown GaN layer was decorated with island-like structure and revealed the mixed-polarity characteristics. In addition, the density of screw TD and leakage current in the GaN film was also increased. The occurrence of mixed-polarity GaN material result could be from unintentional nitridation of the sapphire substrate by ammonia (NH3) precursor at the beginning of the AlN buffer layer growth. By using two-step temperature growth process for the buffer layer, the unintentional nitridation could be effectively suppressed. The GaN film grown on this buffer layer exhibited a smooth surface, single polarity, high crystalline quality and high resistivity. AlGaN/GaN high electron-mobility transistor (HEMT) devices were also successfully fabricated by using the two-step AlN buffer layer.