ZnO and GaN have been investigated for optoelectronic devices including UV-LEDs and TFTs. However, they are, in general, grown on c-sapphire or YSZ single crystal substrates but these have large lattice mismatchings with ZnO and GaN (Da/a > 10 %), which leads to a three-dimensional spiral grain growth and high-density defects. Therefore a lattice matched substrate or buffer layer is important to reduce the defects and to improve carrier transport properties [1]. Homologous series compounds such as ScAlMgO₄ (SCAM) with the chemical formula of RMO₃(AO)₃ have good lattice matching with ZnO and GaN (Da/a ~ 0.1 %).

Here we report the fabrication of single-crystalline thin films of SCAM on (111)-oriented YSZ single crystals by PLD and their application to buffer layers for ZnO and GaN. The SCAM films were fabricated by reactive-solid phase epitaxy (R-SPE) using bilayer composed of amorphous-SCAM / thin epitaxial ZnO template (~10 nm) [2].

In preliminarily experiments, it was found that 20mol% excess addition of Sc₂O₃ was necessary to obtain pure SCAM films. To obtain atomically-flat SCAM films, 40% Zn was added with respect to Mg in the PLD target. Finally, step-and-terraced epitaxial films were obtained with the relationships of [000l]_SCAM // [111]_YSZ in out-of-plane and [11-20]_SCAM // [1-10]_YSZ in in-plane. The SCAM layers were applied to buffer layers for growing ZnO and GaN films. The SCAM buffer layer enhanced the lateral growth of the ZnO layer to > 1 micrometer and resulted in step and flow growth at 800 ^oC. For GaN films grown by N₂ source RF-MBE on the SCAM, residual carrier concentration was reduced to ~ 4.0×10¹⁷ cm^-3 and Hall mobility was increased to ~140 cm²/Vs compared to ~2.0×10¹⁸ cm^-3 and ~120 cm²/Vs for GaN on Al₂O₃ (0001) a low temperature GaN buffer layer.

[1] A. Ohtomo et al., Appl. Phys. Lett. 75, 17 (1999).  [2] H. Ohta et al., Adv. Funct. Mater.13, 2 (2003).

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