Electronic transport in degenerate (100) scandium nitride thin films on magnesium oxide substrates

Abstract

Scandium nitride (ScN) is a degenerate n-type semiconductor with very high carrier concentrations, low resistivity, and carrier mobilities comparable to those of transparent conducting oxides such as zinc oxide. Because of its small lattice mismatch to gallium nitride (GaN), <1%, ScN is considered a very promising material for future GaN based electronics. Impurities are the source of the degeneracy. Yet, which specific impurities are the cause has remained in contention. ScN thin films of various thicknesses were grown on magnesium oxide substrates in a (001) orientation using reactive magnetron sputtering across a range of deposition conditions. X-ray diffraction was used to verify crystal orientation. Film thicknesses ranging from 39 to 85 nm were measured using scanning electron microscopy. The electronic transport properties of the films were characterized using Hall-effect measurements at temperatures ranging from 10 to 320 K. At 10 K, the electron concentration varies from 4.4 × 1020 to 1.5 × 1021 cm−3, resistivity from 2.1 × 10−4 to 5.0 × 10−5 Ω·cm, and Hall mobility from 66 to 97 cm2/V·s. Secondary ion mass spectroscopy (SIMS) was used to determine film compositions. Finally, density functional theory (DFT) was used to compute the activation energies for various point defects including nitrogen and scandium vacancies and oxygen and fluorine substituting for nitrogen. For both oxygen and fluorine substitution, the energies were negative, indicating spontaneous formation. Nevertheless, the combined results of the Hall, SIMS, and DFT strongly suggest that oxygen substitution is the primary mechanism behind the high carrier concentration in these samples.

High mobility and high thermoelectric power factor in epitaxial ScN thin films deposited with plasma-assisted molecular beam epitaxy

“…At room temperature, the ScN film having a thickness of 450 nm exhibited the highest mobility of 127 cm 2 /V s, with an associated n-type (electron) concentration of 8.6 Â 10 19 cm À3 , which resulted in a room-temperature electrical conductivity of 1759 Scm À1 . Such a high mobility obtained in PAMBE-deposited ScN is consistent with the previous reports 4,40,[48][49][50][51][52] of ScN growth by radical source MBE and is higher than the mobility obtained with previous sputterdeposited ScN (see Table I in supplementary material). It is important to note here that the measured mobility is high despite the presence of extended defects such as grain boundaries and dislocation networks that are known to scatter electrons strongly.…”

Semiconducting character of LaN: Magnitude of the bandgap and origin of the electrical conductivity

“…In comparison, rocksalt ScN and Sc-containing nitride alloys, compounds with similar chemistry as LaN, have been found to have negative formation energy for ON in defect calculations, 33 and O gets unintentionally incorporated during growth 38,39 and results in degenerate n-type doping. 40 Thus, an oxygen-free growth environment is necessary to prevent the undesired degenerate doping by substitutional O for LaN.…”

Molecular beam epitaxial growth of scandium nitride on hexagonal SiC, GaN, and AlN

“…A high electron carrier concentration has previously been reported in nominally undoped ScN, with possible causes being linked to nitrogen vacancies, Sc-N antisite defects, and atomic level concentrations of oxygen and fluorine originating from source and crucible material. Density functional theory (DFT) calculations have shown oxygen substitutional defects are lower in formation energy than the other defect mechanisms mentioned above [36] . Support for oxygen incorporating in the film and donating electrons comes from Scandium metal's high affinity for oxygen, as evidenced in its large negative enthalpy of formation for Sc2O3 from Ellingham diagrams [37] .…”