以電漿輔助式分子束磊晶成長應用於高電子遷移率電晶體之氮化鋁鎵/氮化鎵異質結構

Abstract

本論文利用電漿輔助式分子束磊晶在藍寶石基板上成長應用於高電子遷移率電晶體元件製作之氮化鋁鎵/氮化鎵異質結構。我們首先探討氮化鋁緩衝層成長條件對氮化鎵薄膜缺陷結構的影響。在較低溫度成長的氮化鋁緩衝層上，由於表面比較粗糙，氮化鎵薄膜□的刃線差排密度會降低，但螺旋線差排密度會增加。這是因爲粗糙的氮化鋁表面有助於彎曲刃線差排的成長方向，促進刃差排的交互作並減低其密度。另一方面，粗糙的氮化鋁表面因爲有許多讓螺旋差排形成的成核點，導致螺旋差排的密度增加。進一步的試驗也發現刃線差排與氮化鋁緩衝層的厚度息息相關。大或小於最佳的厚度（15奈米），都會造成氮化鎵薄膜的應力上升並增加刃線差排的密度。 爲了有效的降低差排密度，我們利用了在鎵貧乏成長條件下成長的氮化鎵緩衝層。鎵貧乏的氮化鎵緩衝層表面有許多小平臺及溝槽，溝槽□的斜壁提供了很好的方法以彎曲刃差排的成長方向並促進差排得交互作用。通過提升差排的互相結合及消滅效應，鎵貧乏緩衝層能更有效的降低刃差排的密度。另，鎵貧乏緩衝層也能有效的抑制螺旋差排的產生。因此把鎵貧乏的氮化鎵緩衝層成長在平滑表面的氮化鋁緩衝層（可利用高溫成長）上，就能有效的減少氮化鎵薄膜□的所有差排密度。爲了修復粗糙的氮化鎵緩衝層表面，我們也開發了氮化鎵的遷移促進磊晶技術。這方法是由交替沉積鎵和氮原子於試片表面上來完成。結合鎵貧乏的氮化鎵緩衝層及遷移促進磊晶技術，我們利用分子束磊晶成長出低差排密度（~2x108 cm-2）以及擁有平滑表面的氮化鎵薄膜。 最後，我們也研究了不同差排缺陷對氮化鋁鎵/氮化鎵異質結構的電性特性影響。從霍爾量測中發現，其異質界面通道□的二維電子遷移率主要受限於刃差排的密度。這是因爲刃差排缺陷趨向於捕捉電子，形成庫倫散射中心並減緩通道□的電子遷移率和增加通道的阻值，所以刃差排將降低電子元件的電流密度和操作頻率。另一方面，從蕭基二極體的量測可得知，螺旋差排像有如讓縱向電流流通的路徑，非常不利於閘極的逆向偏壓漏電流，導致元件的崩潰電壓變差。因此，要製作高品質的高電子遷移率電晶體元件,氮化鋁鎵/氮化鎵材料□的各種差排密度必須降低。

AlGaN/GaN heterostructure for the high electron mobility transistor applications were grown by plasma-assisted molecular beam epitaxy (PA-MBE) on the sapphire substrates. The effects of AlN buffer growth parameters on the defect structure on GaN film were first investigated. For GaN film grown on lower-temperature buffer, the density of screw threading dislocation (TD) was increased while the density of edge TD was decreased. The rough AlN surface helped to bend the growth direction of edge TDs and then reduced the dislocation density through recombination and annihilation processes. However, the screw TD was increased on the rough AlN buffer because this surface provided many nucleation centers for screw dislocation. Further examinations revealed that the edge TD was also closely related to the AlN buffer thickness which corresponding to the stress in GaN film. Total TD density could be minimized by optimizing the AlN buffer growth temperature and thickness. GaN buffer grown at Ga-lean condition was found useful to reduce the edge TD density in the GaN film significantly. The Ga-lean buffer, with inclined trench walls on its surface, provides an effectively way to bend the propagation direction and promotes the interaction of edge TDs in the GaN film. As a result, the edge TD density was reduced by approximately two orders of magnitude to 2x108 cm-2. The rough surface of Ga-lean buffer was recovered using migration enhanced epitaxy (MEE), a process of alternating deposition cycle of Ga atoms and N2 radicals, during the PA-MBE growth. By growing the Ga-lean GaN buffer on a smooth AlN buffer (achieved by high temperature), both the edge and screw TDs in the GaN film could be effectively reduced. Finally, the roles played by different types of TDs on the electrical properties of AlGaN/GaN heterostructure were studied. From the Hall measurement, the electron mobility in two-dimensional electron gas channel was mainly controlled by the edge TDs. The edge TD acted as Coulomb scattering centers inside the channel and reduced the carrier mobility and increased its resistance. On the other hand, from the Schottky barrier diode characterization, the screw TDs which acted at the current leakage path and was more deleterious to the gate reverse-bias leakage current of the AlGaN/GaN structure. As a result, the output current density and operating frequency of the HEMT devices were decreased by the edge TDs while the device breakdown voltage was degraded by the screw TDs. Therefore, for high performance HEMT device fabrication, both screw and edge TD densities in the AlGaN/GaN material have to be minimized.