High-performance transparent conductor materials are crucial for applications like light-emitting diodes (LEDs), photovoltaic cells, and optical detectors, requiring a balance of electrical conductivity and optical transparency. While research has focused on visible spectrum transparency, many applications demand shorter wavelengths, including solar-blind detection (240–280 nm), UV curing (260–320 nm), biomolecule sensing (250–400 nm), UV germicidal irradiation (260–280 nm), UV lithography (248 nm), UV phototherapy (UVB, 280–315 nm), photochemotherapy (UVA, 315–400 nm), and plant growth stimulation (UVB). The need for energy-efficient and long-lasting UV LEDs, as alternatives to mercury lamps, further drives this research. Current UV LEDs have significantly lower external quantum efficiencies (EQEs) (around 1%) compared to visible and UVA LEDs (45–96%), due to poor hole injection and high defect densities. The lack of high-performance UV transparent conductors, with comparable performance to ITO in the visible, is a major obstacle. ITO's absorption edge near 360 nm limits its UV applications. Other proposed materials, like oxide-metal-oxide heterostructures, TCOs, and ultrathin metallic films, also lack high deep-UV transmittance. While ultrawide bandgap semiconductors such as β-Ga2O3 and ZnGa2O4 show high UV transmittance, their conductivities are much lower than ITO. This necessitates costly flip-chip designs to improve EQEs. This paper explores SrNbO3 as a potential solution, leveraging its unique electronic properties to achieve both high conductivity and UV transparency.

Existing transparent conductors face limitations in the UV range. Indium tin oxide (ITO), the most common transparent conductor, exhibits strong absorption below 360 nm, hindering its use in UV applications. Other materials, such as oxide-metal-oxide heterostructures (e.g., ITO/Cu/ITO, IZO/Ag/IZO), transparent conducting oxides (TCOs) like Zn-In-Sn-O and Al/Ga-doped ZnO, and ultrathin metallic films (Ag, Ni, Cr), also show limited transmittance in the deep-UV region. Ultra-wide bandgap semiconductors like β-Ga2O3 and ZnGa2O4, while possessing high UV transmittance due to their large band gaps, suffer from orders of magnitude lower electrical conductivity compared to ITO. These limitations have driven the development of costly flip-chip designs for UV LEDs to improve light extraction, resulting in EQE improvements from 3-10% and up to 20% at 275nm. The research clearly highlights the need for novel materials with a superior combination of conductivity and UV transparency.

This study combined first-principles calculations (density functional theory, DFT, and dynamical mean-field theory, DMFT) with experimental investigations. DFT and DMFT calculations were performed to quantify SrNbO3's potential as a transparent conductor in the visible and UV regimes. A series of SrNbO3 films with varying thicknesses were grown on KTaO3 substrates using pulsed laser deposition. Electrical properties were characterized using Hall effect and conductivity measurements, determining sheet resistance (R□), resistivity, carrier concentration, and mobility. The dielectric function of SrNbO3 was measured using spectroscopic ellipsometry, enabling the extraction of the electron correlation strength using the extended Drude model. The figure of merit (ΦTC) for transparent conductors was calculated for both the visible and UV (260–320 nm) spectral ranges, considering the effects of film thickness and surface scattering on both electrical and optical properties. The detailed computational methods involved the Wien2K code for DFT calculations, and the EDMFT package for DFT+DMFT calculations, using specific parameters for on-site Coulomb interaction (U) and Hund's coupling (J). Experimental characterization involved X-ray diffraction (XRD) and X-ray reflectivity to determine film structure and thickness, and spectroscopic ellipsometry using multiple instruments to cover a wide spectral range (IR to UV). Electrical transport measurements were conducted using van der Pauw geometry, with Hall measurements to determine carrier properties. The Sommerfeld model was used to estimate the electron mean-free path.

DFT and DMFT calculations showed SrNbO3's band structure is similar to SrVO3, with three bands from the t2g manifold of the Nb 4d orbital intersecting the Fermi level, leading to metallic conduction. However, a larger energy gap (2.3 eV) between valence and conduction bands was observed in SrNbO3 compared to SrVO3 (1 eV), resulting in a blue-shifted interband absorption edge. The calculations also revealed a reduced correlation strength in SrNbO3 compared to SrVO3, indicated by a larger renormalization constant (Z ≈ 0.72 for SrNbO3 vs. Z ≈ 0.55 for SrVO3), leading to a smaller effective mass. Experimentally, SrNbO3 films (10–60 nm) showed sheet resistances between 67.5 Ω/sq and 7.3 Ω/sq, and resistivities between 6.9 × 10−5 Ω cm and 3.8 × 10−5 Ω cm. Carrier mobility was around 8 cm²/Vs, and carrier concentration was approximately 1 × 10²² cm⁻³, significantly higher than in conventional TCOs and UV transparent conductors. Spectroscopic ellipsometry measurements confirmed the blue-shifted absorption edge near 4.8 eV (≈260 nm), agreeing with DFT predictions. The figure of merit (ΦTC) for SrNbO3 in the visible (400–700 nm) range reached 5 × 10⁻³ Ω⁻¹ at 10 nm, exceeding SrVO3 and ITO at similar thicknesses. In the UV range (260–320 nm), ΦTC for 10 nm SrNbO3 was above 10⁻³ Ω⁻¹, comparable to ITO in the visible and significantly higher than other UV transparent conductors. The experimental renormalization constant (Z ≈ 0.89) was slightly higher than the theoretical value, suggesting a slight overestimation of the calculated correlation strength. The results demonstrate SrNbO3's superior performance as a UV transparent conductor.

The findings demonstrate that SrNbO3 is a highly promising material for UV transparent electrodes. The significantly improved performance in the UV spectral range compared to existing materials is directly linked to the energetic isolation of the conduction band and the resulting blue-shift of the absorption edge. The combination of high transparency and conductivity addresses a critical limitation in current UV LED technology. The relatively low electron mean-free path in SrNbO3 allows for aggressive thickness scaling, further enhancing its suitability for device fabrication. While the experimental renormalization constant deviates slightly from theoretical predictions, the overall agreement between theory and experiment validates the fundamental understanding of SrNbO3's electronic structure and optical properties. This material's superior performance is not simply a result of a single property improvement, but rather a synergistic effect between improved optical transparency and high carrier concentration that surpasses the performance of existing materials by orders of magnitude in the relevant UV spectral range.

This study successfully demonstrated SrNbO3 thin films as high-performance UV transparent conductors, particularly in the 260–320 nm range. DFT and DMFT calculations, along with experimental data, confirmed the material's superior properties, attributed to its energetically isolated conduction band and blue-shifted absorption edge. SrNbO3's exceptional combination of high transmittance and conductivity holds great promise for enhancing the efficiency and performance of UV LEDs. Future research could focus on optimizing film growth techniques to further reduce defect densities and improve material quality, potentially leading to even better performance and enabling advancements in various UV applications, including sanitation, biosensing, and phototherapy.

The study acknowledges that the relatively low residual resistivity ratios (RRR) of the SrNbO3 films indicate a significant contribution from temperature-independent scattering due to defects. This defect concentration likely affects both electrical and optical properties. Further optimization of the film growth process is needed to reduce defect densities and enhance the RRR values. The study primarily focused on the optical and electrical properties, and a comprehensive assessment of the material’s long-term stability and reliability in device applications would be beneficial. Finally, the theoretical calculations employed approximations, such as the use of specific parameters in the DMFT calculation, which might slightly affect the quantitative accuracy of predictions. Further refinements in theoretical modeling could be pursued.