Growth of Highly Conductive n-Type Al_0.7Ga_0.3N Film by Using AlN Buffer with Periodical Variation of V / III Ratio

High quality and highly conductive n-type Al_0.7Ga_0.3N films are obtained by using AlN multi-step layers (MSL) with periodical variation of V/III ratios by low-pressure metalorganic chemical vapour deposition (LP-MOCVD). The full-width at half-maximum (FWHM) of (0002) and (10^-15) rocking curves of the Si-doped Al_0.7Ga_0.3N layer are 519 and 625 arcsec, respectively. Room temperature (RT) Hall measurement shows a free electron concentration of 2.9×10¹⁹cm^-3, and mobility of 17.8cm²V^-1s^-1, corresponding to a resistivity of 0.0121Ωcm. High conductivity of the Si-doped AlGaN film with such high Al mole fraction is mainly contributed by a remarkable reduction of threading dislocations (TDs) in AlGaN layer. The TD reducing mechanism in AlN MSL growth with periodical variation of V / III ratio is discussed in detail.