N-polar ScAlN and HEMTs grown by molecular beam epitaxy

Molecular beam epitaxy of single-phase wurtzite N-polar Sc_xAl_1−xN (x ∼ 0.11–0.38) has been demonstrated on sapphire substrates by locking its lattice-polarity to the underlying N-polar GaN buffer. Coherent growth of lattice-matched Sc_0.18Al_0.82N on GaN has been confirmed by x-ray diffraction reciprocal space mapping of the asymmetric (105) plane, whereas lattice-mismatched, fully relaxed Sc_0.11Al_0.89N and Sc_0.30Al_0.70N epilayers exhibit a crack-free surface. The on-axis N-polar crystallographic orientation is unambiguously confirmed by wet chemical etching. High electron mobility transistors using N-polar Sc_0.18Al_0.82N as a barrier have been grown on sapphire substrates, which present the existence of well confined two-dimensional electron gas. A Hall mobility of ∼564 cm²/V s is measured for a 15-nm-thick Sc_0.18Al_0.82N barrier sample with a sheet electron concentration of 4.1 × 10¹³ cm⁻², and the corresponding sheet resistance is as low as 271 Ω/sq. The polarity-controlled epitaxy of Sc_xAl_1−xN provides promising opportunities for applications in high-frequency and high-power electronic and ferroelectric devices.