(Invited) ScAlN: A Novel Barrier Material for High Power GaN-Based RF Transistors
Authors: Hardy Matthew T. , Downey Brian P. , Nepal Neeraj , Storm David F. , Katzer D. Scott , Meyer David J. 
Published: February 2020

GaN-based high-electron-mobility transistors (HEMTs) have established a ten-fold increase in power density over incumbent GaAs technology over a wide range of RF operating frequencies. The improvement in power density comes as a result of both increased breakdown voltage in the wide-bandgap nitrides and increased channel current available via polarization engineering. Next generation high-power RF HEMTs will require further increases in channel sheet charge density (n
s
) to improve output power while requiring thinner barrier layers for higher frequency operation in the millimeter-wave regime. Sc
x
Al1-x
N is a novel materials system consisting of an alloy between group-IIIa (transition metal) nitrides and group-IIIb nitrides, with a stable wurtzite crystal structure for x < 0.55 and a lattice-match to GaN with x ∼ 0.2. Thin films with x up to 0.43 have been reported having a factor of five enhancement of the piezoelectric response relative to AlN [1], and similar predicted enhancement in spontaneous polarization [2]. The calculated polarization discontinuity between a Sc0.2Al0.8N barrier layer and GaN channel gives a predicted n
s
as high as 5 × 1013 cm-2, a factor of five higher than a conventional Al0.25Ga0.75N channel device. In this talk we will show initial development of ScAlN for high-power HEMTs, focusing on molecular beam epitaxy (MBE) growth and materials characterization of ScAlN, plasma etching of ScAlN and its effectiveness as an etch stop layer (ESL), and dc characterization of ScAlN-barrier HEMTs.

Sc
x
Al1-x
N 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 Sc
x
Al1-x
N 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 Sc
x
Al1-x
N 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, Sc
x
Al1-x
N 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 n
s
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 n
s
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.

[1] M. Akiyama, K. Kano, and A. Teshigahara, "Influence of growth temperature and scandium concentration on piezoelectric response of scandium aluminum nitride alloy thin films," Appl. Phys. Lett., vol. 95, p. 162107, 2009.

[2] 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.