Engineered optical and electronic properties in semiconductor nanowire networks

Semiconductor nanowires are promising building blocks for next-generation optoelectronic devices, but understanding and controlling their optical and electronic properties at the nanoscale remains a major challenge. β-Ga₂O₃ and SnO₂ are wide-bandgap oxide semiconductors with well-established applications in UV photodetectors and power electronics. Integrating them into crossed nanowire architectures opens new possibilities for device miniaturisation and multifunctionality, while raising critical questions about how dopants and interfaces influence light emission and charge transport at the nanometre scales.

Although β-Ga₂O₃ and SnO₂ have been extensively studied as individual materials, the combined effects of simultaneous doping and nanojunction formation within their networks remain largely unexplored. How are dopants spatially distributed? How do they locally affect the optical response or electronic structure? Traditional characterisation techniques typically average over large sample volumes and lack the resolution to resolve these nanoscale effects. The ID16B beamline at the ESRF offers a unique opportunity to address these questions through correlated chemical and optical mapping with nanometric resolution.

This study investigated self-organised multiwire networks composed of a central Sn-doped β-Ga₂O₃ nanowire intersected by Ga-doped SnO₂ wires. A correlative synchrotron-based multimodal approach was implemented, combining X-ray fluorescence (XRF), X-ray excited optical luminescence (XEOL), and X-ray absorption near-edge spectroscopy (XANES), to enable spatially resolved chemical and spectroscopic mapping of individual nanowires and their junctions.

The results revealed several key insights. XRF confirmed that the central wire is Sn-doped β-Ga₂O₃, while the intersecting wires are primarily Ga-doped SnO₂, with no evidence of secondary phases or precipitates (Figure 1).

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Fig. 1: (Left) XRF maps of Ga (K-lines, shown in red) and Sn (L-lines, shown in green), revealing that the central wire is Sn-doped β-Ga₂O₃, while the intersecting wires are primarily Ga-doped SnO₂. (Right) Average XEOL spectra recorded over the Sn-doped β-Ga₂O₃ central wire (top) and Ga-doped SnO₂ crossed wires (bottom). (DAP: donor − acceptor pair recombination, V_O: oxygen vacancies, V_Ga: gallium vacancies, STHs: self-trapped holes).

XEOL hyperspectral mapping showed that the junctions exhibit a sharp enhancement of UV luminescence, suggesting a spatially confined modulation of emission intensity (Figure 2).

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Fig. 2: XEOL intensity maps integrated over selected energy windows at 1.9, 2.5, 3.0, and 3.5 eV (±0.15 eV). The orange and green emissions (1.9−2.5 eV) predominantly originate from the crossed SnO₂ wires, while the blue and UV signals (3.0−3.5 eV) are mainly concentrated in the central Ga₂O₃ wire. A pronounced enhancement in UV luminescence is observed at the wire junctions (see arrows), indicating localised optical modulation.

Additionally, Sn doping in β-Ga₂O₃ leads to the appearance of a broad orange band and a notable increase in the blue emission, both uncommon in undoped material. XANES measurements, supported by first-principles calculations, revealed that Sn substitutes Ga in β-Ga₂O₃, introducing donor levels near the conduction band, while Ga replaces Sn in SnO₂, generating acceptor states (Figure 3). The copresence of donor and acceptor defects is responsible for the spectral modifications observed, particularly the enhancement of the blue emission and the formation of the orange band in the Sn-doped β-Ga₂O₃ wire.

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Fig. 3: (Left) Ga K-edge XANES spectrum acquired from Ga-doped SnO₂ wires in the multiwire architecture (red line), compared with simulated XANES spectra calculated using the FDMNES code (black lines), modelling Ga as either an interstitial (Ga_i) or as a substitutional dopant replacing Sn (Ga_Sn) in the lattice, as shown in the unit cell model below. (Right) Sn K-edge XANES spectrum from Sn-doped β-Ga₂O₃ central wire (green line), shown alongside FDMNES simulations where Sn replaces Ga in either octahedral or tetrahedral coordination sites (black lines) within β-Ga₂O₃, as shown in the unit cell model below. The comparison reveals that Sn predominantly substitutes for Ga in octahedral sites within β-Ga₂O₃, while Ga replaces Sn in octahedral sites in SnO₂, confirming site-specific dopant incorporation in both oxide hosts. Ga, Sn, and O atoms are represented as red, green, and blue circles, respectively.

Together, these results provide direct experimental evidence of how site-specific doping and nanoscale junctions influence light-matter interactions in oxide nanowire systems. They highlight how controlled doping and nanoscale architecture can be used to tailor the optoelectronic response of oxide nanostructures. This approach provides a pathway to engineer spatially selective emission in nanophotonic devices such as deep-UV LEDs, nanoscale sensors, and photodetectors. It also establishes a platform to correlate nanoscale structural and spectroscopic features, offering predictive control of optical properties via local chemistry and crystallography.

More broadly, the results demonstrate the value of synchrotron nanoprobe techniques in uncovering structure–property relationships in complex, low-dimensional materials where dopant and/or defect interactions are critical.
 

Principal publication
Engineered Optical and Electronic Properties in β‑Ga₂O₃/SnO₂ Nanowire Networks, J. Dolado et al., Nano Lett. 25(29), 11299–11307 (2025); https://doi.org/10.1021/acs.nanolett.5c02409