Articles written in Bulletin of Materials Science

    • Influence of AlN buffer layer on molecular beam epitaxy growth of wurtzite Al$_{1-x}$Sc$_x$N thin films


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      Wurtzite–Al$_{1-x}$Sc$_x$N thin films deposited by solid-state alloying of AlN with ScN exhibit high piezoelectric coefficient and large band gap that makes it a promising material for a variety of applications in piezo-electronics, electronic, acoustoelectric devices, etc. Research on epitaxial Al$_{1-x}$Sc$_x$N growth in wurtzite crystal structure is still at an early stage and achieving high scandium (Sc) concentrations in epitaxial films without any phase separation or secondaryphase formation is still a critical challenge. Moreover, as most of the reports of wurtzite–Al$_{1-x}$Sc$_x$N growth thus far relies on low-vacuum growth techniques, such as magnetron sputtering that are prone to large impurities and contaminants detrimental for device applications, high-vacuum deposition techniques, such as molecular beam epitaxy method needs to be developed. In this paper, we report the epitaxial growth of wurtzite–Al$_{1-x}$Sc$_x$N on sapphire (Al$_2$O$_3$) substrates under different Sc fluxes using ultra-high vacuum plasma-assisted molecular beam epitaxy. To prevent ScN phase separation, a 30 nm AlN buffer layer is deposited in situ on GaN epilayers as well as Al$_2$O$_3$ substrates that result in phase-pure wurtzite–Al$_{1-x}$Sc$_x$N thin films without any phase separation or secondary phase formation. The structural and compositional analyses performed with high-resolution X-ray diffraction (HRXRD) and secondary ion mass spectroscopy(SIMS), reveal epitaxial wurtzite–Al$_{1-x}$Sc$_x$N growth with 0001 orientations on (0001) Al$_2$O$_3$ substrates and the presence of cubic ScN. Demonstration of phase-pure Al$_{1-x}$Sc$_x$N on AlN buffer layers will enable the development of devices with improved efficiencies.

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