Epitaxial Growth of AlGaN/GaN Heterostructure by Plasma-Assisted Molecular Beam Epitaxy for High Electron Mobility Transistor Applications
Chang, Edward Yi
|關鍵字:||氮化鋁鎵/氮化鎵;高電子遷移率電晶體;電漿輔助式分子束磊晶;AlGaN/GaN;High Electron Mobility Transistor;Plasma-Assisted Molecular Beam Epitaxy|
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.
|Appears in Collections:||Thesis|
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