In a recent article published in the journal ACS Applied Energetic Materialsthe researchers discussed the usefulness of irradiation-induced multifunctional defects to improve the thermoelectric characteristics of scandium nitride thin films.
Study: Multifunctional defects induced by irradiation to improve the thermoelectric properties of scandium nitride thin films. Image credit: Lionel Alvergnas/Shutterstock.com
For more than two decades, traditional Group III nitride semiconductors and their solid-solution alloys have transformed modern optoelectronic and power electronic device technologies. Wurtzite nitride semiconductors are intriguing for application in several sectors due to their wide tunable band gap, high quantum efficiency for light emission and high breakdown voltage.
However, Group III(A) nitride semiconductors are not well suited for some upcoming applications. Scandium Nitride (ScN), a Group III(B) transition metal nitride (TMN), has many advantages over Group III(A) nitride semiconductors.
For ScN thermoelectric applications, techniques that minimize thermal conductivity without changing the power factor will be beneficial. According to recent theoretical calculations, the insertion of native defects such as Sc and N vacancies in ScN should result in asymmetric peaks in the electron densities of near-Fermi level (DOS) states. However, integrating such native defects into ScN is difficult due to their thermodynamic instability and high formation energy. Radiation-induced damage, on the other hand, can introduce point defects in thin films, as reported recently for a Bi2You3 film with He ion irradiation.
HRXRD peak convolution for irradiated ScN. The main ScN peak appears at 2θ = 40.12° and a shoulder peak occurs due to lattice expansion caused by defects created by irradiation. Image Credit: Rao, D et al., ACS Applied Energy Materials
About the study
In this study, the authors used lithium-ion irradiation to induce native defects in ScN thin films produced by molecular beam epitaxy (MBE) and investigated the impact of these native defects on the thermoelectric properties of ScN. . The versatility of irradiation-induced defects in adapting the thermoelectric characteristics of transition metal nitride semiconductors has been demonstrated.
The team investigated the thermoelectric characteristics of nominally single-crystal ScN thin films deposited by MBE. Plasma-assisted molecular beam epitaxy has been used to deposit ScN thin films on MgO(001) substrates. Penetration depth of Li atoms and irradiation-induced damage in ScN films were simulated using the ion-stop-and-scope tool in matter. High-resolution X-ray diffraction (HRXRD) has been used to study structural changes caused by irradiation.
Researchers measured thermoelectric properties such as electrical resistivity, Seebeck coefficient and power factor as a function of temperature in the range of 400 to 1000 K using the Linseis LSR-3. The transmission and reflection spectra were used to study variations in the optical band gap. Finally, a time-domain thermo-reflectance approach was used to determine the thermal conductivity as a function of temperature.
The thermoelectric figure of merit (zT) of irradiated ScN is slightly higher than pristine ScN at high temperature, but the change is within error bars. Image Credit: Rao, D et al., ACS Applied Energy Materials
The Seebeck coefficient of irradiated ScN thin films increased significantly, as predicted by theoretical models. At normal temperature, phonon scattering by irradiation-induced defects reduced the thermal conductivity by more than half to 7 ± 1 W/(mK). Despite lower electrical conductivity due to defect diffusion, the irradiated ScN thin films had a high power factor of approximately (1−2) × 10−3 W/(m K2) in the 300-950 K range and a slight increase in total zJ.
The results revealed that irradiation-induced defects in ScN increased the Seebeck coefficient while significantly reducing the thermal conductivity. However, an increase in the Seebeck coefficient was followed by a decrease in electrical conductivity. In the 300-950 K range, the thermoelectric power factor of the irradiated films remained strong. Additionally, annealing the irradiation films restored structural and optical characteristics and simultaneously maintained thermoelectric properties.
The heat conduction of ScN was also reduced by 50% due to phonon scattering by the irradiation-induced defects. The irradiated films had a high power factor in the temperature range of 300–700 K, despite their lower electrical conductivity. Additionally, when the irradiation films were annealed, the optical characteristics were restored while the electrical conductivity and Seebeck coefficient were maintained.
The contribution of the temperature-dependent grating to the thermal conductivity (K grating) of pristine and irradiated ScN films with two assays is presented. Thermal conductivity data are fitted with the Umklapp diffusion model. Image Credit: Rao, D et al., ACS Applied Energy Materials
In conclusion, this study discussed the usefulness of irradiation-induced defects in ScN thin films to improve their Seebeck coefficient and decrease thermal conductivity. HRXRD was used to examine the point defects created by irradiating 35 keV Li ions on the ScN thin films deposited by MBE. The ScN defects caused the grating to expand, resulting in a broadening of the shoulder peak near the 002 diffraction peak.
ScN retained its epitaxy and tight crystal structure despite an irradiation dose of 1 x 1015 ions/cm2, according to scanning transmission electron microscopy (STEM) imaging. At all temperatures, the projected formation of unequal densities of states near the Fermi energy increased the Seebeck coefficient by more than 12%.
The results demonstrated the versatility of irradiation-induced defects in maintaining and enhancing the thermoelectric properties of ScN, which can be applied to other material systems.
Rao, D., Chowdhury, O., Pillai, AIK, et al. Multifunctional defects induced by irradiation to improve the thermoelectric properties of scandium nitride thin films. ACS Applied Energetic Materials (2022). https://pubs.acs.org/doi/10.1021/acsaem.2c00485