Al 0.26 Ga 0.74 N ∕ Al N ∕ Ga N heterostructures with 1-nm-thick AlN interfacial layers were grown on 100-mm-diam epitaxial AlN/sapphire templates and sapphire substrates by metalorganic vapor phase epitaxy. It was found that AlN/sapphire templates significantly enhanced the electron mobility of the two-dimensional electron gas (2DEG) confined at the GaN channel. This can be explained by the high-crystal-quality GaN channel realized by the use of epitaxial AlN/sapphire templates as substrates. The very high Hall mobilities of approximately 2100cm2∕Vs at room temperature and approximately 17000cm2∕Vs at 77K with a 2DEG density of approximately 1×1013∕cm2 were uniformly obtained for AlGaN∕AlN∕GaN heterostructures on 100-mm-diam epitaxial AlN/sapphire templates. The Hall mobility of AlGaN∕AlN∕GaN heterostructures on epitaxial AlN/sapphire templates reached a very high value of 25500cm2∕Vs at 15K.

High-electron-mobility transistors (HEMTs) have been demonstrated on both AlN/sapphire templates and sapphire substrates, and the dc characteristics of the fabricated devices were examined at room temperature. Better dc characteristics with high extrinsic transconductances and drain current densities were observed in the HEMTs grown on AlN/sapphire templates when compared with the HEMTs on sapphire substrates. Extrinsic transconductances of 214 and 137 mS/mm for Wg/Lg=15/2 μm HEMTs on AlN/sapphire templates and HEMTs on sapphire substrates were achieved, respectively. The enhancement of dc characteristics with small variations in threshold voltage (⩽130 mV) is due to the reduction of dislocation density (1.5×108 cm−2). The decrease of dark spot density has been confirmed in the GaN grown on AlN/sapphire templates using cathodoluminescence measurements. The advantage of using AlN/sapphire templates is that low dislocation density GaN layers at a high temperature can be grown without using low-temperature-grown GaN buffer layers.

The strain dependence of the free-exciton resonance energies in AlN epilayers is presented and the values are analyzed using an appropriate Hamiltonian assuming equibiaxial stress for the wurtzite crystal structure in order to obtain valence band parameters. Based on the results, we study the strain dependence of the valence band ordering, optical transition probability, and free-exciton binding energy. As a result of these calculations, the following strain-free values are obtained for the energy gap, averaged dielectric constants, and ordinary and extraordinary dielectric constants: Eg=6.095 eV at T=11 K, ϵ=7.87, ϵ⊥=7.33, and ϵ∥=8.45, respectively. A brief discussion of the valence band ordering in bulk AlxGa1−xN is also presented.

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