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In this work, we demonstrate, for the first time, Al0.35GaN/GaN/Al0.25GaN double heterostructure field effect transistors on 200 mm Si(111) substrates. Thick crack-free Al0.25GaN buffer layers are achieved by optimizing Al0.75GaN/Al0.5GaN intermediate layers and AlN nucleation layers. The highest buffer breakdown voltage reaches 1380 V on a sample with a total buffer thickness of 4.6 µm. According to Van der Pauw Hall measurements, the electron mobility is 1766 cm2 V-1 s-1 and the electron density is 1.16×1013 cm-2, which results in a very low sheet resistance of 306±8 Ω/square.

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High-temperature (HT) annealing effects on the evolution of strain in AlN films grown on sapphire have been investigated. It is found that there is a significant transition behavior from tensile to compressive strain in AlN before and after HT annealing at an optimal temperature of 1700 °C. Based on a microstructural analysis, it is clarified that the HT annealing will result in the (1) disappearance of grains that account for the tensile stress before HT annealing, (2) generation of a new interface that has little influence on the lattice constant upper/below this interface, and (3) regular 8/9 arrangement of misfit dislocation at the AlN/sapphire interface that relieves almost all stress associated with lattice mismatch. It is thus deduced that the remnant compressive strain in AlN after HT annealing mainly comes from the cooling down process due to thermal mismatch between sapphire and AlN. This understanding of the annealing effect is certainly of great significance in AlN materials science and technology.

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