High-Quality AlGaN/GaN Grown on Sapphire by Gas-Source Molecular Beam Epitaxy using a Thin Low-Temperature AlN Layer
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heterostructures exhibiting mobilities of 2000 cm2/Vs, 9000 cm2/Vs, and 11000 cm2/Vs with sheet densities of 1 x1013 cm-2, 1 x1013 cm-2, 7 x1012 cm-2 at 300 K, 77 K, and 4.2 K, respectively, have been grown on conducting 6H-SiC substrates by metalorganic chemical vapor deposition (MOCVD) [6]. The highest attained mobility for similar structures grown on sapphire by MOCVD is 10300 cm2/Vs with an electron sheet density of 6.2 x 10 12 cm-2 at 1.5 K in spite of the large lattice mismatch [7]. Recently, latticematched homoepitaxy has been demonstrated by the use of a GaN template grown on sapphire by MOCVD for a plasma-assisted molecular beam epitaxy (PA-MBE) growth process, resulting in mobilities as high as 1150 cm2/Vs, 24000 cm2/Vs, and 51700 cm2/Vs with sheet electron densities of 1.4 x 1013 cm -2, 2.5 x 1012 cm -2, 2.2 x 1012 cm -2, at 300 K, 77 K, and 13 K, respectively, although the template layer was shown to exhibit a high background doping [8, 9]. In contrast, AlGaN/GaN heterostructures grown directly on highly mismatched (0001)-oriented sapphire substrates have also been achieved by molecular beam epitaxy (MBE) [10, 11], resulting in much lower mobilities: 1211 cm2/Vs, and 5660 cm2/Vs with electron sheet densities of 4.9 x 1012 cm-2 and 5 x 1012 cm-2 at 300 K and 77 K, respectively. However, for organometallic vapor phase epitaxial (OMVPE) growth of GaN on sapphire, the insertion of one or more low-temperature-grown (LT) -AlN or LTGaN interlayers within the high-temperature-grown GaN has recently been shown to result in a reduction in threading dislocation density for subsequently grown epilayers [12]. More specifically, a defect filtering process was observed as a large portion of the stress-induced threading dislocations originating from the GaN/sapphire interface were terminated at the LT-AlN and LT-GaN interlayers, resulting in improved quality for the epilayers grown on the buffer structure. Although experimental data indicates that higher quality growth can be achieved on 6H-SiC, it remains highly desirable to obtain highquality growth of GaN-based heterostructures on sapphire due to substrate availability, low cost, and high resistivity. In this work, we report on the incorporation of a single LT-AlN interlayer within the GaN buffer of a AlGaN/GaN HEMT structure grown directly on sapphire (0001) by ammonia gas source MBE (GS-MBE), resulting in a significant enhancement in the structural and electrical characteristics of the subsequently grown heterostructure. AlGaN/GaN structures exhibiting reduced background doping and Hall mobilities of 2100, 10310 and 12200 cm2/Vs with carrier sheet densities of 6.1 x 1012, 6.0 x 1012, and 5.8 x 1012 cm-2 at 300 K, 77 K, and 0.3 K, respectively, confirm the effectiveness of the buffer layer structure. The existence of a high-density two-dimensional electron gas is verified by magetotransport measurements performed at 0.3 K exhibiting Shubnikov-de Haas oscillations for fields as low as 3 T and a negatively-sloped magnetoresistance, indicating a low background dop
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