AlN nucleation and buffer layers have been grown with different Al∕N flux ratio by molecular beam epitaxy on sapphire. Thick AlN on top of a nucleation layer grown with Al∕N flux ratio=1.0 exhibited deep hexagonal holes (diameter 100–200 nm, density ∼109cm−2). Investigation of the nucleation layer surface revealed that the holes were formed already during the nucleation. The formation of holes in AlN buffer layers could be avoided by using a N-rich (Al∕N=0.5) nucleation layer. It is demonstrated that the holes in AlN buffer layers can be effectively avoided choosing a N-rich nucleation layer growth and then switch to Al∕N flux ratio =1.0 for buffer layer growth. The two-step AlN growth gave high quality AlN, with excellent crystalline quality and smooth surface.
The sapphire surface, used for growth of GaN by molecular beam epitaxy, was studied by reflection high-energy electron diffraction during the in situ pretreatment and initial growth. We investigated the effect of Ga cleaning, nitridation, and growth of the AlN nucleation layer on the lattice constant. One Ga-cleaning cycle restored the surface with a higher diffraction density. Nitridation changed the lattice constant instantaneously. The lattice constant was restored if the nitridation was switched off. For the AlN nucleation layer, the first monolayer was strained, followed by first a rapid and then a slow relaxation.
We have studied the influence of Al content, AlGaN layer thickness, and unintentional background doping by oxygen on the two-dimensional electron gas (2DEG) density in AlGaN/GaN heterostructures. Hall measurements were made on samples grown with molecular beam epitaxy. The 2DEG densities in the range 2-3 ϫ 10 13 cm Ϫ2 were measured. A one-dimensional Schrödinger-Poisson model was used to describe the heterostructure. The calculations gave twodimensional electron densities in accordance with measured values. The electron density is very sensitive to the Al concentration in the AlGaN layer, whereas the sensitivity to layer thickness is small. Our simulations also showed that the two-dimensional concentration increased 50% when the free-carrier concentration changed from 10 15 cm Ϫ3 to 10 18 cm Ϫ3 . The relation between donor concentration and free-carrier concentration was found to agree when using oxygen ionization energy as a parameter.
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