标题: | 以电浆辅助式分子束磊晶成长应用于高电子迁移率电晶体之氮化铝镓/氮化镓异质结构 Epitaxial Growth of AlGaN/GaN Heterostructure by Plasma-Assisted Molecular Beam Epitaxy for High Electron Mobility Transistor Applications |
作者: | 黄延仪 Wong, Yuen-Yee 张翼 Chang, Edward Yi 材料科学与工程学系 |
关键字: | 氮化铝镓/氮化镓;高电子迁移率电晶体;电浆辅助式分子束磊晶;AlGaN/GaN;High Electron Mobility Transistor;Plasma-Assisted Molecular Beam Epitaxy |
公开日期: | 2010 |
摘要: | 本论文利用电浆辅助式分子束磊晶在蓝宝石基板上成长应用于高电子迁移率电晶体元件制作之氮化铝镓/氮化镓异质结构。我们首先探讨氮化铝缓冲层成长条件对氮化镓薄膜缺陷结构的影响。在较低温度成长的氮化铝缓冲层上,由于表面比较粗糙,氮化镓薄膜□的刃线差排密度会降低,但螺旋线差排密度会增加。这是因为粗糙的氮化铝表面有助于弯曲刃线差排的成长方向,促进刃差排的交互作并减低其密度。另一方面,粗糙的氮化铝表面因为有许多让螺旋差排形成的成核点,导致螺旋差排的密度增加。进一步的试验也发现刃线差排与氮化铝缓冲层的厚度息息相关。大或小于最佳的厚度(15奈米),都会造成氮化镓薄膜的应力上升并增加刃线差排的密度。 为了有效的降低差排密度,我们利用了在镓贫乏成长条件下成长的氮化镓缓冲层。镓贫乏的氮化镓缓冲层表面有许多小平台及沟槽,沟槽□的斜壁提供了很好的方法以弯曲刃差排的成长方向并促进差排得交互作用。通过提升差排的互相结合及消灭效应,镓贫乏缓冲层能更有效的降低刃差排的密度。另,镓贫乏缓冲层也能有效的抑制螺旋差排的产生。因此把镓贫乏的氮化镓缓冲层成长在平滑表面的氮化铝缓冲层(可利用高温成长)上,就能有效的减少氮化镓薄膜□的所有差排密度。为了修复粗糙的氮化镓缓冲层表面,我们也开发了氮化镓的迁移促进磊晶技术。这方法是由交替沉积镓和氮原子于试片表面上来完成。结合镓贫乏的氮化镓缓冲层及迁移促进磊晶技术,我们利用分子束磊晶成长出低差排密度(~2x108 cm-2)以及拥有平滑表面的氮化镓薄膜。 最后,我们也研究了不同差排缺陷对氮化铝镓/氮化镓异质结构的电性特性影响。从霍尔量测中发现,其异质界面通道□的二维电子迁移率主要受限于刃差排的密度。这是因为刃差排缺陷趋向于捕捉电子,形成库伦散射中心并减缓通道□的电子迁移率和增加通道的阻值,所以刃差排将降低电子元件的电流密度和操作频率。另一方面,从萧基二极体的量测可得知,螺旋差排像有如让纵向电流流通的路径,非常不利于闸极的逆向偏压漏电流,导致元件的崩溃电压变差。因此,要制作高品质的高电子迁移率电晶体元件,氮化铝镓/氮化镓材料□的各种差排密度必须降低。 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. |
URI: | http://140.113.39.130/cdrfb3/record/nctu/#GT079418853 http://hdl.handle.net/11536/40806 |
显示于类别: | Thesis |
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