ScxAl1-xN thin films were grown on freestanding GaN and SiC substrates using an Omicron PRO-75 RF-plasma MBE system, equipped with an Al effusion cell and an e-beam evaporator to supply Sc. Several series of samples were grown to investigate the impact of growth temperature (360–890 °C) and III-V ratio (0.6–1.1) on ScxAl1-xN crystal quality and composition. The measured ScN fraction was constant between 360–810 °C and increased at 890 °C due to Al re-evaporation from the growth surface. X-ray diffraction (XRD) rocking curve full width at half maximum values were below 300 arcsec for 80-nm-thick Sc0.18Al0.82N thin films for growth temperatures between 520–730 °C, indicating a wide growth window and high crystal quality. Surface rms roughness measured by atomic force microscopy was generally under 1 nm and as low as 0.7 nm at a growth temperature of 730 °C. XRD measurements indicated single crystalline phase epitaxial ScxAl1-xN for N-rich samples (III/V ratio < 1) and the emergence of additional phases for samples grown metal-rich.
In addition to having high spontaneous and piezoelectric polarization, ScxAl1-xN also has a relatively low etch rate in Cl2-based dry etching commonly used for GaN and AlN. The etch selectivity is as high as 11.2 relative to AlN and 18.6 relative to GaN. The etched surface remains smooth with no increase in the rms roughness or evidence of pitting or micromasking. There are several etch methodologies that allow selective etching of Al-containing layers relative to GaN, but this is the first demonstration of a conventional dry etch chemistry with a working ESL relative to AlN, leading to a variety of applications in AlN-based electronic devices and deep-UV optoelectronics.
Using a 25-nm-thick Sc0.14Al0.86N barrier layer in a GaN-based HEMT structure, we demonstrate the first ScAlN-barrier HEMTs with ns as high as 3.4 × 13 cm-2 and mobility of 910 cm2\V∙s, resulting in a sheet resistance of 213 Ω/□. Reducing the ScAlN barrier thickness to only 3 nm results in an ns of 2.0 × 1013 cm‑2 with a mobility of 1060 cm2/V∙s. Both devices included both AlN and GaN interlayers to improve the mobility. These results demonstrate the potential for ScAlN as a barrier material in a highly-scaled, high charge density HEMT for high power millimeter-wave amplifiers.
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 M. A. Caro, S. Zhang, T. Riekkinen, M. Ylilammi, M. A. Moram, O. Lopez-Acevedo, et al., "Piezoelectric coefficients and spontaneous polarization of ScAlN," J. Phys.: Condens. Matter, vol. 27, p. 245901, 2015.