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Ultrathin-barrier AlN/GaN heterostructures grown by rf plasma-assisted molecular beam epitaxy on freestanding GaN substrates

We report the structural and electrical properties of ultrathin-barrier AlN/GaN heterostructures grown on freestanding GaN substrates by rf plasma-assisted molecular beam epitaxy. Structures with barrier thicknesses between 1.5 nm and 7.5 nm were grown and characterized. We observe that AlN/GaN structures with barriers of 3.0 nm exhibit the highest Hall mobility, approximately 1700 cm2/Vs. Furthermore, the Hall mobility is much diminished in heterostructures with AlN barriers thicker than 4.5 nm, coincident with the onset of strain relaxation.
Highlights
• Rf-MBE growth of AlN/GaN high electron mobility transistors.
• AlN/GaN HEMTs on free standing GaN substrates.
• Effect of barrier thickness on structural and electronic properties.
 
Fig. 1. Atomic force micrographs showing the surfaces of five ultrathin-barrier AlN/GaN HEMT structures with AlN barrier thicknesses indicated. Isolated cracks appearing as thin white lines are visible in the micrographs of the 4.5 nm and 6.0 nm barrier surfaces, while extensive cracking is observed in the 7.5 nm barrier surface. Z-ranges are 3–5 nm, exclusive of visible debris, and rms roughnesses are 0.4–0.7 nm for all micrographs shown except the 3.0 nm barrier sample surface, which exhibited a z-range of 10 nm and an rms roughness of 1.4 nm.
 
Source: Journal of Crystal Growth
 
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