The role of AlN interlayer in Al_xGa_1-xN/GaN heterostructures with high x from 0.35 to 0.50 Grown on sapphire (0001)

We report the role of thin AlN interlayer in the fabrication of Al_xGa_1-xN/GaN heterostructures with high x from 0.35 to 0.50 on sapphire (0001) by MOVPE. After growing the AlN interlayer of 20 nm thick on GaN/sapphire (0001) epitaxy, the Al_xGa_1-xN epitaxial layers of 1.0 um thick were grown on it with increasing the flow rate of TMA. The measured Al mole fractions of Al_xGa_1-xN/GaN heteroepitaxy grown using the thin AlN interlayer from each TCD rocking curve are 0.35, 0.37, 0.45 and 0.50, respectively. As incorporation rate of Al in the Al_xGa_1-xN increases, the crystallinity is getting well and the RMS values scanned by AFM of their surfaces become lower. And also, the optoelectronic characteristics of those heteroepitaxy evaluated by cathodeluminescence(CL) are improved with the increase of x. To the contrary these trends, it is usually known that crystallinity, surface morphology and optical property of the Al_xGa_1-xN/GaN heteroepitaxy grown without AlN interlayer between them become worse with the increase of x above 0.2. Meanwhile, electrical resisitivities of Al_0.35Ga_0.65N, Al_0.37Ga_0.63N, Al_0.45Ga_0.55N and Al_0.50Ga_0.50N epitaxy, which were grown with the thin AlN interlayer, measured by four point probing method are 13.5, 18.1, 31.7 and 36.2 Mohm-cm, respectively. These resisitivities increase by degrees in spite of the advance of characteristic s with the raising of x.