We report on the near-infrared intersubband (ISB) absorption properties of strain- free Sc0.14Al0.86N/GaN multiple quantum wells (MQWs) grown on c-plane GaN substrates by molecular beam epitaxy. These MQWs exhibit strong, sharp, and tunable absorption energies between 515 meV and 709 meV, for well widths ranging from 7 nm to 1.5 nm, respectively. Observation of ISB absorption in ultra-thin Sc0.14Al0.86N/GaN MQWs not only extends the near-infrared range accessible with Sc-containing nitrides but also highlights the challenges of growing nanometer-thick GaN quantum wells. We explore the effects of growth temperature on absorption characteristics and find that substrate temperatures above 600 degrees C significantly enhance ISB absorption intensity but also introduce an energy redshift for the narrowest wells. The redshift is attributed to increased interface roughness due to ScAlN surface morphology degradation at higher temperatures. Additionally, a comparison of experimental results with simulated band-structures indicates that the magnitude of net polarization rises faster with Sc-composition than previously suggested by theoretical calculations. This study advances the prospects of ScAlN/GaN heterostructures for novel photonic devices in the technologically important near-infrared range.(c) 2025 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement

Using comprehensive x-ray reciprocal space mapping, we establish the precise lattice-matching composition for wurtzite ScxAl1−xN layers on (0001) GaN to be x = 0.14 ± 0.01. 100 nm thick ScxAl1−xN films (x = 0.09–0.19) were grown in small composition increments on c-plane GaN templates by plasma-assisted molecular beam epitaxy. The alloy composition was estimated from the fit of the (0002) x-ray peak positions, assuming the c-lattice parameter of ScAlN films coherently strained on GaN increases linearly with Sc-content determined independently by Rutherford backscattering spectrometry [Dzuba et al., J. Appl. Phys. 132, 175701 (2022)]. Reciprocal space maps obtained from high-resolution x-ray diffraction measurements of the (101¯5) reflection reveal that ScxAl1−xN films with x = 0.14 ± 0.01 are coherently strained with the GaN substrate, while the other compositions show evidence of relaxation. The in-plane lattice-matching with GaN is further confirmed for a 300 nm thick Sc0.14Al0.86N layer. The full-width-at-half-maximum of the (0002) reflection rocking curve for this Sc0.14Al0.86N film is 106 arc sec and corresponds to the lowest value reported in the literature for wurtzite ScAlN films.

Wurtzite ScxAl1−xN/GaN (x = 0.13–0.18) multi-quantum wells grown by molecular beam epitaxy on c-plane GaN are found to exhibit remarkably strong and narrow near-infrared intersubband absorption in the technologically important 1.8–2.4 μm range. Band structure simulations reveal that, for GaN wells wider than 3 nm, the quantized energies are set by the steep triangular profile of the conduction band caused by intrinsic polarization fields. As a result, the intersubband transition energies provide unique and direct access to essential ScAlN polarization parameters. Measured infrared absorption indicates that the spontaneous polarization difference of the presumed lattice-matched Sc0.18Al0.82N/GaN heterostructure is smaller than the theoretically calculated value. The intersubband transition energies are relatively insensitive to the barrier alloy composition indicating negligible variation of the net polarization field in the probed 0.13–0.18 Sc composition range.

Tunable polymorphic structures to achieve novel properties are of great concern for energy-related applications. Herein, we demonstrate temperature-driven irreversible structural phase transition in LiDy(WO4)2 (b-LiDyW and a-LiDyW). A facile sol-gel method has been employed to achieve phase pure crystalline polymorphs at relatively lower temperatures and time. LiDy(WO4)2 crystallizes in a mono-clinic wolframite-type structure (space group, P21/n, No = 14) at ambient temperature and a Scheelite-like tetragonal structure (space group, I41/a, No = 88) upon heating at high temperature. Crystal structure analysis shows that Li+ and Dy3+ occupy the distinct site, and W forms a distorted WO6 oc-tahedron in the b-LiDyW phase. In contrast, Li+ and Dy3+ are statistically distributed on a dodecahedral S4 site sharing a similar crystallographic lattice site, and W forms a WO4 tetrahedron in the a-LiDyW phase. The Wolframite to Scheelite (b-> a) transformation is due to the increase in the crystal symmetry on the heating function where distorted WO6 transformed to free WO4 going from b-> a phase and coordination of W+6 is lowered. Density functional theory calculations at 0 K revealed that the b-LiDyW is energetically more favorable than a-LiDyW by 337.3 meV per formula unit. The co-substitution of the Yb3+-Er3+ pair in LiDy(WO4)2 lattice displays a concentration-dependent upconversion red emission when excited with 980 nm. The monoclinic phase of LiDy0.7Yb0.2Er0.1(WO4)2 shows an intense red emission at 656 nm due to inherent lower crystal symmetry. Distorted WO6 and chemically induced lattice distortions in b-LiDy0.7Yb0.2Er0.1(WO4)2 would have strongly influenced the coordination envi-ronment of the Er3+ to achieve red emission. This study presents the structural relationship among the tunable crystallographic phases of LiDy(WO4)2 with the observed red emission induced by Yb3+-Er3 upon 980 nm irradiation.(c) 2023 Elsevier Ltd. All rights reserved.

Recently, a new class of mixed polyanionic compounds with general formula Na3MPO4CO3 (M = Ni, Mn, Fe, Co, Cu), discovered through high-throughput computations have attracted much attention for its secondary battery applications and safety aspects. Here, we combine electrochemical measurements with inductively coupled plasma-optical emission spectrometer (ICP-OES), energy-dispersive X-ray (EDX), field emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), Raman spectroscopy, x-ray photoelectron spectroscopy (XPS), and an ab-initio DFT approach to unravel the dynamic self-limiting surface restructuring of Na3NiCO3PO4 in aqueous 1 M KOH/NaOH electrolyte. The etching of lattice CO32-, PO43- and Na serves as the key to trigger the surface reconstruction. This work establishes a fundamental understanding of the pseudocapacitance mechanism associated with surface self-reconstruction of mixed polyanionic compounds. The reconstruction-derived self-limiting dense Ni(OH)2 layers at the surface and its oxidation to NiOOH at anodic potentials can be attributed to the observed high-performance pseudocapacitive behavior. The surface reconstructed Na3NiCO3PO4 electrode exhibits high specific capacitance (2378.2 F g-1 at 1 A g-1). The assembled symmetric pseudocapacitor delivers a high energy density (57.2 Wh kg-1), power density (1500 W kg- 1) at 1 Ag-1 and long cycle life (13000 cycles) with 100% retention.

Erbium nitride (ErN) is an emerging rocksalt rare-earth semiconducting pnictide and has attracted significant interest in recent years for its potential applications in thermoelectric energy conversion, spintronic devices, and for the Gifford-McMahon cryocoolers. Due to the Er intra-4f electronic transition, Er-doped III-nitride semiconductors such as GaN, InGaN, etc. exhibit strong emission in the retina-safe and fiber optical communication wavelength window of 1.54 mu m that is researched extensively for developing solid-state lasers, amplifiers, and light-emitting devices. However, due to ErN's propensity for oxidation in ambient, high-quality ErN thin film growth has been challenging and an in-depth understanding of its electronic structure remains unanswered. In this work, the valence band electronic structure of ErN thin films is measured with normal as well as with resonant synchrotron-radiation photoemission spectroscopy. Photoemission measurements show a valence band maximum and Fermi energy difference of similar to 2.3 eV in ErN. First-principles density functional theory (DFT) calculations are performed not only to explain the valence band electronic structure but also to determine transport properties such as effective mass and deformation potentials. Strong localized Er-4f states are observed similar to 6-8 eV below the valence band maxima and the valence band edge is found to exhibit N-2p character. Resonant photoemission data corroborates the DFT calculations. To accurately capture the electronic structure in modeling, beyond generalized gradient approximation (GGA) methods such as (a) Heyd-Scuseria-Ernzerhof hybrid functional and (b) GGA-PU Hubbard correction schemes are utilized. Determination of the electronic structure of ErN marks significant progress in developing ErN-based electronic, optoelectronic, and thermoelectric devices.

Rare‐earth semiconducting mononitrides (RENs) are an emerging class of materials due to their unique electronic and magnetic properties originating from strongly localized 4f orbitals. Erbium nitride (ErN) is one of the most promising REN and attracts significant interest for spintronics, thermoelectric, and Gifford–McMahon cryo‐cooler applications. However, despite such progress, growth and characterization of the physical properties of ErN are rather challenging due to its propensity for oxidation, and no report on its thermal transport properties exists to date. Recently, high‐quality ErN thin films are deposited and are stabilized in ambient with thin capping layers. Herein this letter, first‐principles density functional perturbation theory to model the vibrational spectrum of ErN is utilized and the calculations with the phonon frequency measurements with inelastic Raman spectroscopy are verified. Consistent with its polar dielectric nature, ErN exhibits a longitudinal‐optical transverse‐optical phonon mode splitting at the Γ‐point with a separation of 333 cm−1. Time‐domain thermoreflectance is used to measure the low‐temperature (80 –300 K) thermal conductivity of ErN films. At room temperature, ErN films exhibit a low thermal conductivity of 1.16 ± 0.15 and 2 ± 0.2 W mK−1 on (001) MgO and (0001) Al2O3 substrates, respectively, making them attractive for thermoelectrics and thermal barrier coating applications.

Epitaxial metal/semiconductor superlattices with atomically sharp interfaces and tunable Schottky barrier heights have attracted significant interest in recent years for thermionic emission-based high-temperature thermoelectric devices, optical hyperbolic metamaterials, hot-electron photocatalysis, and optoelectronic heterostructures for visible-to-terahertz frequency range applications. ZrN/ScN is a demonstration of such epitaxial metal/semiconductor superlattices and exhibits atomically sharp lattice-matched interfaces, albeit with the presence of threading dislocations on MgO substrates. Along with its influence on structural integrity and atomic diffusion, the presence of such defects significantly impacts electron and phonon transport in these metamaterials with carrier trapping, scattering, shunt path, etc. Therefore, an in-depth analysis of the atomistic structure and the composition of such defects is extremely necessary to design devices with improved efficiencies. In this paper, high-resolution scanning transmission electron microscopy and atom-probe tomography are employed to determine the structure and three-dimensional (3D) spatial distribution of oxide defect clusters along the voids in ZrN/ScN superlattices. $\mathrm{Sc}{\mathrm{O}}^{+}$ and $\mathrm{Sc}{\mathrm{O}}^{++}$ ions are found to cluster predominantly along such 3D interface defects with zirconium and scandium atoms surrounding them. Defect regions are also found to be depleted of nitrogen atoms and rich with a high concentration of oxygen. The oxygen content was found to be higher inside the ScN layers compared to ZrN. First-principles modeling analysis verified the clustering of oxygen at high oxygen partial pressure and demonstrated a higher affinity of scandium towards oxygen than for zirconium towards oxygen. These results mark significant progress in understanding the atomic structure and composition of defects in nitride superlattices.

Point defects create exotic properties in materials such as defect-induced luminescence in wide-bandgap semiconductors, magnetism in nonmagnetic materials, single-photon emission from semiconductors, etc. In this article, oxygen defect formation in metallic TiN and semiconducting rock salt-(Al,Sc)N is investigated with a combination of first-principles density functional theory, synchrotron-based x-ray absorption spectroscopy (XAS) analysis, and scanning transmission electron microscopy–energy-dispersive x-ray spectroscopy mapping. Modeling results show that oxygen in TiN and rock salt-(Al,Sc)N prefers to be in the defect complex of substitutional and interstitial oxygen (nON + Oi) types. While in TiN, the preferential interstitial sites of oxygen in ON + Oi are at the tetrahedral site, in rock salt-(Al,Sc)N, a split interstitial site along the [111] direction was found to be energetically preferable. Simulations performed as a function of the oxygen partial pressure show that under experimental growth conditions, four oxygen atoms at the substitutional sites of nitrogen (4ON), along with four Ti atoms, decorate around an interstitial oxygen atom at the tetrahedral site (Oi) in the energetically favored configuration. However, in rock salt-(Al,Sc)N, n in nON + Oi was found to vary from two to four depending on the oxygen partial pressure. Theoretical predictions agree well with the experimentally obtained XAS results. These results are not only important for a fundamental understanding of oxygen impurity defect behavior in rock salt nitride materials but will also help in the development of epitaxial metal/semiconductor superlattices with efficient thermionic properties.

Undoped and Lithium (Li) doped Zinc Oxide (ZnO)films were prepared by Successive Ionic Layer Adsorption and Reaction (SILAR) technique using 0.1M Zinc Sulphate with Ammonia as the cationic solution. Doping is done at different atomic weight percentage (atm. wt%) 5, 10 and 15 by adding Lithium Chloride to the cationic solution. The samples were annealed at 500◦C for one hour after deposition. The UV-Visible Transmittance and Absorbance of the samples were recorded from 250nm to 900nm.The reflectance data of the samples were estimated and the plot of reflectance suggested that the quality of the films is good except for Li:ZnO film at 5atm.wt%.The Optical constants ’n’ and ’k’ were calculated for different incident wavelengths on the samples. The variation of optical constants with incident wavelengths suggested that the Li: ZnO sample at 5atm.wt% has relatively high values when compared to other samples. The Urbach energy (Ee) or width of band tail is calculated from the (lnα-hν) plot and the Optical bandgap energy of the samples were calculated fron the (αhν)2 − hν plot. The bandgap energy was observed to be high for the undoped samples and decreases to normal value for 5atm.wt% sample. The bandgap energy again increases with doping concentration. The Urbach energy is found to be increasing with increase in doping concentration. These variations could be explained using the amorphous-crystalline nature and defect related effects in the samples due to doping.

Aluminum scandium nitride (AlxSc1-xN) is an emerging III-nitride semiconductor that has attracted significant interest in recent years in surface and bulk acoustic resonators for its high piezoelectric coefficient and applications in high-power electronic devices. AlxSc1-xN stabilizes in the rock salt phase for x < 0.52 when deposited directly on (001) MgO substrates and has been utilized as a semiconductor in single-crystalline TiN/AlxSc1-xN metal/semiconductor superlattices for thermionic energy conversion, optical hyperbolic metamaterials, and the fundamental studies on heat and current transport in materials. However, due to the presence of oxygen impurities and native defects, such as nitrogen vacancies, sputter-deposited rock salt-AlxSc1-xN exhibits a high carrier concentration in the (2–4) × 1020 cm−3 range that leads to its Ohmic tunneling contact with metals and prevents observation of thermionic emission. In this Letter, we demonstrate that magnesium (Mg) acts as an efficient hole-dopant in r-AlxSc1-xN, increases its resistivity, and reduces its carrier concentration as a function of Mg concentration to as low as 1.4 × 1018 cm−3. A combination of spectroscopy, microscopy, and first-principles modeling demonstrate (a) epitaxial 001 oriented AlxSc1-xN:Mg growth for the first 35–75 nm and subsequent pyramidal growth with multiple in-plane orientations, (b) MgxNy to form a uniform and homogeneous solid solution with r-AlxSc1-xN without any precipitation, phase separation, or secondary phase formation, and (c) Mg-defect states are located deep inside the valence and conduction bands that leave behind a pristine r-AlxSc1-xN bandgap and band edges. The demonstration of Mg-hole doping in r-AlxSc1-xN marks significant progress in r-AlxSc1-xN thin film and superlattice-based devices.

We report here the influence of Mg and Si-doping during growth, on the morphology, structure, and optical properties of single-crystalline GaN nanorods (NRs) grown on Si substrates using plasma-assisted molecular beam epitaxy. Mg-doping is shown to enhance the lateral growth of the NRs, leading to a higher degree of coalescence. Si-doping during the nucleation stage of the growth enhances the mutual misorientation of the NRs. Strain profile measurements along the length of individual NR by transmission electron microscopy shows that the top regions are relaxed. Evaluation of carrier concentration by Raman spectroscopy reveals that Si-doping leads to an increase of carrier concentration from 1016 to 1017 cm-3, and the optimal Mg incorporation for the realisation of pdoping is confirmed by photoluminescence spectroscopy. These results will significantly help in understanding and tuning the structural and optical properties of GaN NRs through doping in the fabrication of NR based optoelectronic devices.

We study the native charge compensation effect in Mg doped GaN nanorods (NRs), grown by Plasma Assisted Molecular Beam Epitaxy (PAMBE), using Raman, photoluminescence (PL) and X-ray photoelectron spectroscopies (XPS). The XPS valence band analysis shows that upon Mg incorporation the E$_F$-E$_{VBM}$ reduces, suggesting the compensation of the native n-type character of GaN NRs. Raman spectroscopic studies on these samples reveal that the line shape of longitudinal phonon plasmon (LPP) coupled mode is sensitive to Mg concentration and hence to background n-type carrier density. We estimate a two order of native charge compensation in GaN NRs upon Mg-doping with a concentration of 10$^{19}$-10$^{20}$ atoms cm$^{-3}$. Room temperature (RT) PL measurements and our previous electronic structure calculations are used to identify the atomistic origin of this compensation effect.

We discuss the microstructural origin of enhanced radial growth in magnesium (Mg) doped single crystalline wurtzite gallium nitride (w-GaN) nanorods (NRs) grown by MBE, using electron microscopy and first-principles Density Functional Theory calculations. Experimentally, we observe that Mg incorporation increases the surface coverage of the grown samples as a consequence of an increase in the radial growth rate of the NRs. We also observe that the coalescence of NRs becomes prominent and the height at which coalescence between proximal rods occurs decreases with increase in Mg concentration. From first-principles calculations, we find that the surface free energy of the Mg doped surface reduces with increasing Mg concentration in the samples. The calculations further suggest a reduction in the adsorption energy and the diffusion barrier of Ga adatoms along [112¯0] on the side wall surface of the NRs as the underlying mechanism for the observed enhancement in the radial growth rate of GaN NRs. The physics and chemistry behind reduction of the adsorption energy of Ga ad-atoms on the doped surface are explained in the light of electronic structure of the relevant surfaces.

We address the mechanism of early stages of growth and shape transition of the unique nanowall network (NwN) of GaN by experimentally monitoring its morphological evolution and complementing it by first-principles calculations. Using atomic force and scanning electron microscopy, we observe the formation of oval shaped islands at very early stages of the growth which later transformed into tetrahedron shaped (3 faced pyramid) islands. These tetrahedron shaped islands further grow anisotropically along their edges of the (202¯1) facets to form the wall-like structure as the growth proceeds. The mechanism of this crystal growth is discussed in light of surface free energies of the different surfaces, adsorption energy, and diffusion barrier of Ga ad-atoms on the (202¯1) facets. By first-principles calculations, we find that the diffusion barrier of ad-atoms reduces with decreasing width of facets and is responsible for the anisotropic growth leading to the formation of NwN. This study suggests that formation of NwN is an archetype example of structure dependent attachment kinetic instability induced shape transition in thin film growth.

The resistance to flow of electrons in these amorphous semiconductors which have very high density of localized states may be controlled by hopping processes according to Lazarus. However, no theoretical calculations for the electrical resistance of these amorphous semiconductors have been carried out to the best of our knowledge. In the present work, we have tested an alternative approach which is based on the fundamental assumptions of Drickamer. It has been stated by Drickamer that the basic effects of pressure on a material are; (1) to decrease interatomic distance and (2) to increase overlap between adjacent electronic orbitals. Apart from these assumptions, Drickamer, however, has not put forward any formulation for the explanation of his measurements. A simple phenomenological formulation based on these fundamental assumptions has been tested successfully in case of some semiconductors as hydrogenated and non-hydrogenated amorphous silicon (a-Si: H and a-Si), germanium and selenium.

Platinum and Aluminium are widely used as a pressure calibration standard. The present proposal which intends to compare the efficiency of the four equations under close examination reports the V/VO versus P values derived from the new modified forms of Murnaghan equation NMME1, NMME2, Birch equation (BE) and Freund-Ingalls (FIE) obtained for the best agreement with the experimental data of Mc Queen et.al.