Reversal of Band-Ordering Leads to High Hole Mobility in Strained p-type Scandium Nitride

Low hole mobility of nitride semiconductors is a significant impediment in realizing their high-efficiency device applications. Scandium nitride (ScN), an emerging rocksalt indirect bandgap semiconductor, suffers from low hole mobility. Utilizing the ab initio Boltzmann transport formalism including spin-orbit coupling, here we show the dominating role of ionized impurity scattering in reducing the hole mobility in ScN thin films. We suggest a route to increasing the hole mobility by reversing the band ordering through strain engineering. Our calculation shows that the bi-axial tensile strain in ScN lifts the split-off hole band above the heavy hole and light hole bands, leading to a lower hole-effective mass and increasing mobility. Along with the impurity scattering, Fröhlich interaction also plays a vital role in the carrier scattering mechanism due to the polar nature of ScN. Increased hole mobility in ScN will lead to higher efficiencies in thermoelectric, plasmonics, and neuromorphic computing devices

Scandium Nitride as a Gateway III‐Nitride Semiconductor for both Excitatory and Inhibitory Optoelectronic Artificial Synaptic Devices

Abstract Traditional computation based on von Neumann architecture is limited by time and energy consumption due to data transfer between the storage and the processing units. The von Neumann architecture is also inefficient in solving unstructured, probabilistic, and real‐time problems. To address these challenges, a new brain‐inspired neuromorphic computational architecture is required. Due to the absence of resistance–capacitance delay, high bandwidth, and low power consumption, optoelectronic artificial synaptic devices are highly attractive. Yet, stable, scalable, and complementary metal–oxide–semiconductor (CMOS)‐compatible materials exhibiting both inhibitory and excitatory optoelectronic synaptic functionalities have not been demonstrated. Here, epitaxial CMOS‐compatible scandium nitride (ScN) optoelectronic artificial synaptic devices that emulate both inhibitory and excitatory biological synaptic activities are presented. The negative and positive persistent photoconductivity of undoped and magnesium‐doped ScN is equated to the inhibitory and excitatory synaptic plasticity, respectively, which leads to functionalities like learning–forgetting, frequency‐selective optical filtering, frequency‐dependent potentiation and depression, Hebbian learning, and logic‐gate operations. Temperature‐dependent photoresponse and photo‐Hall measurements reveal that scattering of photogenerated carriers from charged defect centers results in negative photoconductivity in undoped degenerate ScN. This work opens up the possibility of utilizing a group‐III epitaxial semiconducting nitride material with inhibitory and excitatory optoelectronic synaptic functionalities for practical neuromorphic applications

Detailed study of reactively sputtered ScN thin films at room temperature

To contemplate an alternative approach for the minimization of diffusion at high temperature, present findings impart viability of room-temperature deposited reactively sputtered ScN thin films. The adopted route endows precise control over the flow for a methodical structural phase evolution from Sc --> ScN and probe the correlated physical aspects of the highly textured ScN samples. In the nitrided regime i.e. at $R_{N_2}$= 2.5–100% flow, incorporation of unintentional oxygen defects was evidenced from surface sensitive soft x-ray absorption spectroscopy study, though less compared to their metal ( $R_{N_2}$ = 0%) and interstitial ($R_{N_2}$ = 1.6%) counterparts, due to higher Gibbs free energy for Sc-O-N formation with no trace of ligand field splitting around the O K-edge spectra. To eradicate the skepticism of appearance of N K-edge (401.6 eV) and Sc L-edge (402.2 eV) absorption spectra adjacent to each other, the first-ever Sc K-edge study has been adopted to validate complementary insight on the metrical parameters of the Sc-N system. Optical bandgaps of the polycrystalline ScN thin film samples were found to vary between 2.25 and 2.62 eV as obtained from the UV–vis spectroscopy, whereas, the nano-indentation hardness and modulus of the as-deposited samples lie between 15–34 GPa and 152–476 GPa, respectively following a linearly increasing trend of resistance to plastic deformations with an exception at 34 GPa in case of $R_{N_2}$ = 5% sample. Besides, contrary to other early 3d transition metal nitrides (TiN, VN, CrN), a comprehensive comparison of noticeably large homogeneity range in Sc-N has been outlined to apprehend the minuscule lattice expansion over the large $R_{N_2}$ realm

MBE deposited scandium nitride (ScN) for thermoelectric applications

Resumen del trabajo presentado en el Virtual Conference on Thermoelectrics (VCT), celebrado online del 21 al 23 de julio de 2020Scandium nitride (ScN) is an emerging rocksalt III-nitride semiconductor and has attracted significant interests in recent years for its potential thermoelectric applications, as a substrate for high-quality epitaxial GaN growth and as a semiconducting component for epitaxial singlecrystalline metal/semiconductor superlattices for thermionic energy conversion. In this poster, we show high mobility and high thermoelectric power factor in epitaxial ScN thin films deposited on MgO substrates with plasma-assisted molecular beam epitaxy (PAMBE). Structural and microscopic characterization showed epitaxial 002 oriented ScN film growth on MgO (001) substrates and the presence of extended defects including dislocations and grain boundaries. Electrical measurement exhibited high room-temperature mobility of 127 cm2 /Vs and temperaturedependent mobility from 50-400 K temperature range that is dominated by dislocation and grain boundary scattering. Due to the high mobility and a moderately large carrier concentration (8.6×1019 cm-3 ), ScN films exhibit high Seebeck coefficients (180 µV/K at 950 K) and a large thermoelectric power factor (2.3 × 10-3 W/m K2 at 500K). The thermal conductivity measured with a time-domain thermoreflectance (TDTR) measurement system exhibit a value of 7.33 W/m K at 500 K that results in a maximum ZT of 0.2. Further research on alloy formation as well as nanostructuring is currently underway to reduce the thermal conductivity and improve the ZT.Peer reviewe

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

Scandium nitride (ScN) is an emerging rock salt III-nitride semiconductor and has attracted significant interest in recent years for its potential thermoelectric applications as a substrate for high-quality epitaxial GaN growth and as a semiconducting component for epitaxial single-crystalline metal/semiconductor superlattices for thermionic energy conversion. Solid-solution alloys of ScN with traditional III-nitrides such as AlScN have demonstrated piezoelectric and ferroelectric properties and are actively researched for device applications. While most of these exciting developments in ScN research have employed films deposited using low-vacuum methods such as magnetron sputtering and physical and chemical vapor depositions for thermoelectric applications and Schottky barrier-based thermionic energy conversion, it is necessary and important to avoid impurities, tune the carrier concentrations, and achieve high-mobility in epitaxial films. Here, we report the high-mobility and high-thermoelectric power factor in epitaxial ScN thin films deposited on MgO substrates by plasma-assisted molecular beam epitaxy. Microstructural characterization shows epitaxial 002 oriented ScN film growth on MgO (001) substrates. Electrical measurements demonstrated a high room-temperature mobility of 127 cm/V s and temperature-dependent mobility in the temperature range of 50-400 K that is dominated by dislocation and grain boundary scattering. High mobility in ScN films leads to large Seebeck coefficients (-175 μV/K at 950 K) and, along with a moderately high electrical conductivity, a large thermoelectric power factor (2.3 × 10 W/m-K at 500 K) was achieved, which makes ScN a promising candidate for thermoelectric applications. The thermal conductivity of the films, however, was found to be a bit large, which resulted in a maximum figure-of-merit of 0.17 at 500 K.D.R., B.B., and B.S. acknowledge the International Center for Materials Science (ICMS) and Sheik Saqr Laboratory (SSL) in JNCASR for support. B.S. acknowledges the Science and Engineering Research Board (SERB) of the Government of India, Start-Up Research Grant No. SRG/2019/000613 for financial support. M.S.M.G. wants to acknowledge financial support from No. MAT2017-86450- C4-3-R and intramural CSIC Project No. 2D-MESES. M.G., V.B., and A.I.K.P acknowledge the facilities of Sydney Microscopy and Microanalysis at the University of Sydney. Y.R.K. and P.E.H. appreciate support from a MURI program through the Office of Naval Research, Grant No. N00014-18-1-2429