Tantalum nitride (Ta3N5) is a promising semiconductor for solar-driven photoelectrochemical (PEC) water splitting, but its performance is limited by intrinsic defects. Here, we investigate the effect of titanium (Ti) doping (0–10 at%) on the structural, compositional, and optoelectronic properties of Ta3N5 thin films. At low concentrations (<2 at%), Ti4+ preferentially substitutes Ta at four-coordinated sites, enhancing nitrogen incorporation and suppressing defect states associated with under-coordinated Ta. This leads to improved carrier dynamics and prolonged electron–hole lifetimes. Higher doping levels (≥3.5 at%) result in occupation of three-coordinated sites, inducing increase in the oxygen content, lattice distortion, and defect formation that deteriorate carrier lifetimes. PEC measurements reveal that optimized Ti doping significantly reduces charge transfer resistance and nearly seven-fold increase in the photocurrent. These findings underscore the importance of controlled Ti doping for defect engineering and band structure tuning to boost the PEC performance of Ta3N5 thin films.

The self-induced formation of core-shell InAlN nanorods (NRs) is addressed at the mesoscopic scale by density functional theory (DFT)-resulting parameters to develop phase field modeling (PFM). Accounting for the structural, bonding, and electronic features of immiscible semiconductor systems at the nanometer scale, we advance DFT-based procedures for computation of the parameters necessary for PFM simulation runs, namely, interfacial energies and diffusion coefficients. The developed DFT procedures conform to experimental self-induced InAlN NRs' concerning phase-separation, core/shell interface, morphology, and composition. Finally, we infer the prospects for the transferability of the coupled DFT-PFM simulation approach to a wider range of nanostructured semiconductor materials.

Currently, self-induced InAlN core-shell nanorods enjoy an advanced stage of accumulation of experimental data from their growth and characterization as well as a comprehensive understanding of their formation mechanism by the ab initio modeling based on Synthetic Growth Concept. However, their electronic and optical properties, on which most of their foreseen applications are expected to depend, have not been investigated comprehensively. GW and the Bethe-Salpeter equation (BSE) are regarded as the state-of-the-art ab initio methodologies to study these properties. However, one of the major drawbacks of these methods is the computational cost, much higher than density-functional theory (DFT). Therefore, in many applications, it is highly desirable to answer the question of how well approaches based on DFT, such as e.g. the local density approximation (LDA), LDA-1/2, the modified Becke-Johnson (mBJ) and the Heyd-Scuseria-Ernzerhof (HSE06) functionals, can be employed to calculate electronic and optical properties with reasonable accuracy. In the present paper, we address this question, investigating how effective the DFT-based methodologies LDA, LDA-1/2, mBJ and HSE06 can be used as approximate tools in studies of the electronic and optical properties of scaled down models of core-shell InAlN nanorods, thus, avoiding GW and BSE calculations.

Cu2O has emerged as a promising material for sustainable hydrogen production through photoelectrochemical (PEC) water splitting, while inefficient charge separation remains one of the main challenges hindering its development. In this work, a new architecture of InAlN/Cu2O heterojunction photocathode is demonstrated by combining n-type InAlN and p-type Cu2O to improve the charge separation efficiency, thus enhancing PEC water-splitting performance. The Pt/InAlN/Cu2O photoelectrode exhibits a photocurrent density of 2.54 mA cm(-2) at 0 V versus reversible hydrogen electrode (V-RHE), which is 3.21 times higher than that of Cu2O (0.79 mA cm(-2) at V-RHE). The enhanced PEC performance is explained by the larger built-in potential V-bi of 1.43 V formed at the InAlN/Cu2O p-n junction than that in the single Cu2O photocathode (V-bi &lt; 0.77 V), which improves the separation of the photogenerated carriers and thus relieves the bottlenecks of charge-transfer kinetics at the electrode bulk and electrode/electrolyte interface. In this work, an avenue is opened for designing III-nitrides/Cu2O heterojunction toward solar energy conversion.

Ga2O3 is a promising ultrawide-bandgap semiconductor for high-voltage and high-power applications, yet achieving reliable p-type electrical conductivity remains a significant challenge. We utilized halide vapor phase epitaxy growth to synthesize epitaxial layers of beta-phase Ga2O3 doped with Zn, which can serve as a suitable acceptor. Thin-film samples with Zn doping concentrations of 1.7 x 1019 and 2.5 x 1020 ions/cm3 were confirmed as single phases of monoclinic beta-Ga2O3 by X-ray diffraction. To determine the location of Zn ions within the beta-Ga2O3 lattice, we employed X-ray absorption near-edge structure (XANES) in conjunction with first-principles density functional theory calculations. Theoretical XANES spectra for Zn substitutions in the tetrahedral and octahedral Ga sites in beta-Ga2O3, as well as a precipitation of ZnGa2O4 spinel, were compared with the experimental data. The experimental XANES spectra of the Zn L 3 edge were reproduced well by theoretical spectra of Zn ions occupied at cationic positions at the tetrahedral coordinated site.

Zinc aluminogallate, Zn(AlxGa1−x)2O4 (ZAGO), a single-phase spinel structure, offers considerable potential for high-performance electronic devices due to its expansive compositional miscibility range between aluminum (Al) and gallium (Ga). Direct growth of high-quality ZAGO epilayers however remains problematic due to the high volatility of zinc (Zn). This work highlights a novel synthesis process for high-quality epitaxial quaternary ZAGO thin films on sapphire substrates, achieved through thermal annealing of a ZnGa2O4 (ZGO) epilayer on sapphire in an ambient air setting. In-situ annealing x-ray diffraction measurements show that the incorporation of Al in the ZGO epilayer commenced at 850 °C. The Al content (x) in ZAGO epilayer gradually increased up to around 0.45 as the annealing temperature was raised to 1100 °C, which was confirmed by transmission electron microscopy (TEM) and energy dispersive x-ray spectroscopy. X-ray rocking curve measurement revealed a small full width at half maximum value of 0.72 °, indicating the crystal quality preservation of the ZAGO epilayer with a high Al content. However, an epitaxial intermediate �–(AlxGa1−x)2O3 layer (� - AGO) was formed between the ZAGO and sapphire substrate. This is believed to be a consequence of the interdiffusion of Al and Ga between the ZGO thin film and sapphire substrate. Using density functional theory, the substitution cost of Ga in sapphire was determined to be about 0.5 eV lower than substitution cost of Al in ZGO. Motivated by this energetically favorable substitution, a formation mechanism of the ZAGO and AGO layers was proposed. Spectroscopic ellipsometry studies revealed an increase in total thickness of the film from 105.07 nm (ZGO) to 147.97 nm (ZAGO/AGO) after annealing to 1100 °C, which were corroborated using TEM. Furthermore, an observed increase in the direct (indirect) optical bandgap from 5.06 eV (4.7 eV) to 5.72 eV (5.45 eV) with an increasing Al content in the ZAGO layer further underpins the formation of a quaternary ZAGO alloy with a tunable composition.

Gallium nitride (GaN) epitaxial films on sapphire (Al2O3) substrates have been grown using reactive magnetron sputter epitaxy with a liquid Ga target. Threading dislocations density (TDD) of sputtered GaN films was reduced by using an inserted high-quality aluminum nitride (AlN) buffer layer grown by reactive high power impulse magnetron sputtering (R-HiPIMS) in a gas mixture of Ar and N2. After optimizing the Ar/N2 pressure ratio and deposition power, a high-quality AlN film exhibiting a narrow full-width at half-maximum (FWHM) value of the double-crystal x-ray rocking curve (DCXRC) of the AlN(0002) peak of 0.086° was obtained by R-HiPIMS. The mechanism giving rise the observed quality improvement is attributed to the enhancement of kinetic energy of the adatoms in the deposition process when operated in a transition mode. With the inserted HiPIMS-AlN as a buffer layer for direct current magnetron sputtering (DCMS) GaN growth, the FWHM values of GaN(0002) and (10 1‾ 1) XRC decrease from 0.321° to 0.087° and from 0.596° to 0.562°, compared to the direct growth of GaN on sapphire, respectively. An order of magnitude reduction from 2.7 × 109 cm−2 to 2.0 × 108 cm−2 of screw-type TDD calculated from the FWHM of the XRC data using the inserted HiPIMS-AlN buffer layer demonstrates the improvement of crystal quality of GaN. The result of TDD reduction using the HiPIMS-AlN buffer was also verified by weak beam dark-field (WBDF) cross-sectional transmission electron microscopy (TEM).

By addressing precursor prevalence and energetics using the DFT-based synthetic growth concept (SGC), the formation mechanism of self-induced InAlN core–shell nanorods (NRs) synthesized by reactive magnetron sputter epitaxy (MSE) is explored. The characteristics of In- and Al-containing precursor species are evaluated considering the thermal conditions at a typical NR growth temperature of around 700 °C. The cohesive and dissociation energies of In-containing precursors are consistently lower than those of their Al-containing counterparts, indicating that In-containing precursors are more weakly bonded and more prone to dissociation. Therefore, In-containing species are expected to exhibit lower abundance in the NR growth environment. At increased growth temperatures, the depletion of In-based precursors is even more pronounced. A distinctive imbalance in the incorporation of Al- and In-containing precursor species (namely, AlN/AlN+, AlN2/AlN2+, Al2N2/Al2N2+, and Al2/Al2+ vs InN/InN+, InN2/InN2+, In2N2/In2N2+, and In2/In2+) is found at the growing edge of the NR side surfaces, which correlates well with the experimentally obtained core–shell structure as well as with the distinctive In-rich core and vice versa for the Al-rich shell. The performed modeling indicates that the formation of the core–shell structure is substantially driven by the precursors’ abundance and their preferential bonding onto the growing edge of the nanoclusters/islands initiated by phase separation from the beginning of the NR growth. The cohesive energies and the band gaps of the NRs show decreasing trends with an increment in the In concentration of the NRs’ core and with an increment in the overall thickness (diameter) of the NRs. These results reveal the energy and electronic reasons behind the limited growth (up to ∼25% of In atoms of all metal atoms, i.e., InxAl1–xN, x ∼ 0.25) in the NR core and may be qualitatively perceived as a limiting factor for the thickness of the grown NRs (typically <50 nm).

Tritantalum pentanitride (Ta₃N₅) semiconductor is a promising material for photoelectrolysis of water with high efficiency. Ta₃N₅ is a metastable phase in the complex system of TaN binary compounds. Growing stabilized single-crystal Ta₃N₅ films is correspondingly challenging. Here, we demonstrate the growth of a nearly single-crystal Ta₃N₅ film with epitaxial domains on c-plane sapphire substrate, Al₂O₃(0001), by magnetron sputter epitaxy. Introduction of a small amount ~2% of O₂ into the reactive sputtering gas mixed with N₂ and Ar facilitates the formation of a Ta₃N₅ phase in the film dominated by metallic TaN. In addition, we indicate that a single-phase polycrystalline Ta₃N₅ film can be obtained with the assistance of a Ta₂O₅ seed layer. With controlling thickness of the seed layer smaller than 10 nm and annealing at 1000 °C, a crystalline β phase Ta₂O₅ was formed, which promotes the domain epitaxial growth of Ta₃N₅ films on Al₂O₃(0001). The mechanism behind the stabilization of the orthorhombic Ta₃N₅ structure resides in its stacking with the ultrathin seed layer of orthorhombic β-Ta₂O₅, which is energetically beneficial and reduces the lattice mismatch with the substrate.

Direct growth of orthorhombic Ta3N5-type Ta-O-N compound thin films, specifically Ta3-xN5-yOy, on Si and sapphire substrates with various atomic fractions is realized by unbalanced magnetron sputtering. Low-degree fiber-textural Ta3-xN5-yOy films were grown through reactive sputtering of Ta in a gas mixture of N-2, Ar, and O-2 with keeping a partial pressure ratio of 3:2:0.1 in a total working pressure range of 5-30 mTorr. With increasing total pressure from 5 to 30 mTorr, the atomic fraction of O in the as-grown Ta3-xN5-yOy films was found to increase from 0.02 to 0.15 while that of N and Ta decrease from 0.66 to 0.54 and 0.33 to 0.31, respectively, leading to a decrease in b lattice constant up to around 1.3%. Metallic TaNx phases were formed without oxygen. For a working pressure of 40 mTorr, an amorphous, O-rich Ta-N-O compound film with a high O fraction of similar to 0.48, was formed, mixed with non-stoichiometric TaON and Ta2O5. By analyzing the plasma discharge, the increasing O incorporation is associated with oxide formation on top of the Ta target due to a higher reactivity of Ta with O than with N. The increase of O incorporation in the films also leads to a optical bandgap widening from similar to 2.22 to similar to 2.96 eV, which is in agreement with the compositional and structural changes from a crystalline Ta3-xN5-yOy to an amorphous O-rich Ta-O-N compound.