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AlN epilayer properties (120 nm thick) grown by ammonia assisted molecular beam epitaxy on c-sapphire substrates with different low temperature AlN buffer layers (LT-BL) have been studied. The role of the LT-BL on the AlN structural and optical properties was investigated as a function of the LT-BL thickness and growth temperature. Optimum growth conditions were identified with LT-BL thickness of 3 nm and growth temperature between 480 °C and 520 °C. It was shown that by optimizing these conditions, a reduction of both mixed and edge threading dislocation densities up to 75% is achieved. The impact of the growth temperature of the AlN epilayer was also studied showing an additional improvement of the AlN crystal and morphological properties while growing at higher temperature. A correlation between the epilayer strain and the PL emission was also investigated. Finally, an Al 0.7 Ga 0.3 N:Si doped layer was grown on the top of the optimized AlN template showing a smooth surface with monoatomic steps and a roughness ∼0.2 nm, confirming the potential of such templates for the fabrication of AlGaN based heterostructures. [ABSTRACT FROM AUTHOR]

AlN thin films, grown on (0001) sapphire substrates by molecular beam epitaxy (MBE), were annealed at high temperature (up to 1650 °C) in flowing N 2 . X-ray diffraction (XRD) studies, combined with Williamson-Hall and Srikant plots, have shown that annealing leads to a strong reduction of both edge and mixed threading dislocation densities, as confirmed by transmission electron microscopy (TEM) images, up to 75%. Moreover, it is found that annealing at high temperatures allows the relaxation of the tensile strain in the AlN film due to the growth process. In addition, the morphological properties of the films were determined by atomic force microscopy (AFM) and show that the annealing conditions have a strong impact on the surface morphology and roughness. Finally, an annealing at 1550 °C for 20 min appears as an ideal tradeoff to enhance the structural properties while preserving the initial AlN surface morphology. [ABSTRACT FROM AUTHOR]

• Selective sublimation of p-GaN is developed to fabricate enhancement-mode HEMTs. • AlGaN regrowth drastically reduces access resistances in enhancement-mode HEMTs. • Sublimation and regrowth are combined to co-integrate E/D-mode GaN HEMTs. In the present study, the selective sublimation of the p-GaN cap layer of Al(Ga)N/GaN HEMTs is developed to replace the commonly used dry etching with no risk of damage in the barrier layer in order to fabricate enhanced mode transistors. Thanks to this approach, enhancement-mode transistors are fabricated with a threshold voltage between 0 V and +1.5 V depending on the barrier layer aluminum molar fraction and thickness. Furthermore, we show the benefit of the combination of selective sublimation with the regrowth of AlGaN to reduce access resistance in these transistors which can be co-integrated with depletion-mode devices fabricated in the same process in areas where p-GaN has been totally evaporated. [ABSTRACT FROM AUTHOR]