Refereed Articles
High Mobility AlGaN/GaN Heterostructures Grown by Plasma-Assisted Molecular Beam Epitaxy on Semi-Insulating GaN Templates Prepared by Hydride Vapor Phase Epitaxy
  We report on an extensive study of the growth and transport properties of the two-dimensional electron gas (2DEG) confined at the interface of AlGaN/GaN heterostructures grown by molecular beam epitaxy (MBE) on thick, semi-insulating GaN templates prepared by hydride vapor phase epitaxy (HVPE). Thick (~20 µm) GaN templates are characterized by low threading dislocation densities (~5×108 cm–2) and by room temperature resistivities of ~108 Omega cm. We describe sources of parasitic conduction in our structures and how they have been minimized. The growth of low Al containing (x<=0.05) AlxGa1-xN/GaN heterostructures is investigated. The use of low Al containing heterostructures facilitates the study of the 2DEG transport properties in the previously unexplored regime of carrier density ns<=2×1012 cm–2. We detail the impact of MBE growth conditions on low temperature mobility. Using an undoped HVPE template that was residually n type at room temperature and characterized an unusually low dislocation density of ~2×108 cm–2, we have grown an Al0.05Ga0.95N/GaN heterostructure with a record mobility of 75 000 cm2/V s at sheet density of 1.5×1012 cm–2 and T = 4.2 K. The same heterostructure design grown on a semi-insulating HVPE template yielded a peak mobility of 62 000 cm2/V s at a density of ns = 1.7×1012 cm–2 and T = 4.2 K. The observation of the fractional quantum Hall effect at filling factor nu= 5/3 in the AlGaN/GaN system is reported. It is also demonstrated that thick semi-insulating GaN templates grown by HVPE are a viable substrate for the growth of high electron mobility transistors. Typical Al0.25Ga0.75N/GaN heterostructures exhibit room temperature density of 1.0×1013 cm–3 and mobility of ~1500 cm2/V s. The dc and rf characteristics of transistors grown by MBE on a HVPE template are presented.  
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    High Mobility AlGaN/GaN Heterostructures Grown by Plasma-Assisted Molecular Beam Epitaxy on Semi-Insulating GaN Templates Prepared by Hydride Vapor Phase Epitaxy.  M. Manfra, N. Weimann, J. Hsu, L. Pfeiffer, K. West, S. Syed, H. Stormer, W. Pan, D. Lang, S. Chu, G. Kowach, A. Sergent, J. Caissie, K. Molvar, L. Mahoney, R. Molnar, J. Appl. Phys. 2002, 92, 338.