N/GaN multi-quantum well structures using high resolution scanning transmission electron microscopy and x-ray diffraction*

Wurtzite Sc_xAl_1-xN has recently emerged as an alternative to Al(Ga,In)N materials due to unique ferroelectric properties and the potential for lattice-matching to c-plane GaN. The exact Sc-composition for lattice-matching to GaN, however, is still a subject of debate as it has been reported in the literature for Sc compositions ranging from 9 to 18%. In this study, single layer ScAlN and multi-quantum well (MQW) Sc_xAl_1-xN/GaN (x ~ 0.13 – 0.18) structures grown by plasma-assisted molecular beam epitaxy are investigated with high-angle annular dark field scanning transmission electron microscopy, energy dispersive X-ray spectroscopy (EDX), and high-resolution x-ray diffraction (XRD). Sc-compositions determined by EDX are consistent with measurements by Rutherford back-scattering spectrometry.  XRD reciprocal space maps are used to determine Sc_xAl_1-xN in-plane lattice constant and to assess its strain state.  Coherently strained ScAlN films on GaN are observed for x = 13-18%. The results are used to develop a technique for quantifying Sc-composition from symmetric ω - 2θ XRD spectra, and to refine theoretical band-structure calculations. The Sc_xAl_1-xN/GaN MQWs exhibit strong and narrow near-infrared intersubband absorption that is promising for optoelectronic applications.