Overcoming metal-rich surface chemistry limitations of ScAlN for high electrical performance heterostructures.

While metal-rich ScAlN epitaxy has traditionally led to mixed phase films by controlling the surface chemistry with transient metal doses utilizing a pulsed method of molecular beam epitaxy, phase-pure, metal-rich epitaxy of ScAlN was demonstrated, showing improved structural and electrical characteristics. The effects of substrate temperature and III/V ratio were studied, and an x-ray diffraction figure of merit and surface roughness as low as 225 arcsec and 0.68 nm, respectively, were demonstrated. A significant catalytic effect is observed with the use of Sc in metal-rich conditions, resulting in varied growth rates with substrate temperature and Sc surface coverage. This catalytic effect results in complications when selecting synthesis conditions and for in situ monitoring and can be accounted for improved phase purity. The variation of growth rates with Sc surface coverage introduces non-linearities to the transient initiation stage of growth but also introduces a feedback stabilization of the surface chemistry. Accounting for these complexities, a Sc_0.2Al_0.8N high electron mobility transistor (HEMT) heterostructure is demonstrated with a sheet resistance of 152 Ω/□, a mobility of 700 cm²/Vs, and a sheet charge of 5.9 × 10¹³ cm⁻². [ABSTRACT FROM AUTHOR]