High thermoelectric power factor in ambient-stable semiconducting rare-earth ErN thin films

Abstract

Erbium nitride (ErN) is an emerging semiconducting rare-earth pnictide with unique electronic and magnetic properties. ErN has attracted significant interest for spin superlattices and spintronic devices and as a second-stage regenerator for Gifford-McMahon cryo-coolers. Solid-solution alloys of ErN with III-nitride semiconductors such as GaN have been studied extensively for use in solid-state lasers, amplifiers, and light-emitting devices operating in the retina-safe and fiber-optic communication wavelength window of 1.54 μm. However, due to the high affinity of Er toward oxygen, ErN is prone to oxidation in ambient conditions. To date, no reports on the deposition of the high-quality ErN thin film and its thermoelectric properties have been published. In this Letter, semiconducting ErN thin films are deposited inside an ultrahigh-vacuum chamber and capped with thin (3 nm) AlN layers to stabilize it in ambient conditions. Structural, optical, and electronic characterization reveals that ErN thin films (a) grow with (111) and (002) orientations on (0001) Al2O3 and (001) MgO substrates with sharp and abrupt ErN-substrate interfaces, (b) demonstrate a direct bandgap of 1.9 eV, and (c) exhibit a high carrier concentration in the range of 4.3 × 1020 to 1.4 × 1021 cm-3. Thermoelectric measurements show a moderately high Seebeck coefficient of -72.6 μV/K at 640 K and a maximum power factor of 0.44 × 10-3 W/m K2 at 486 K. Demonstration of an ambient-stable semiconducting ErN thin film and its high thermoelectric power factor marks significant progress in rare-earth pnictide research and will help develop ErN-based spintronic and thermoelectric devices.