A New Era for Semiconductors: Real-Time Formation of Ultrathin Gallium Oxide Dielectrics

February 02, 2026 – Researchers from the University of Nottingham (UK), Ulm University (Germany), and the Universidad de Cádiz (Spain) have captured the real-time formation of crystalline gallium oxide (Ga₂O₃) at the atomic scale on conducting graphene. Using in operando near-ambient pressure X-ray photoelectron spectroscopy and advanced scanning transmission electron microscopy, the team revealed the growth mechanisms of these ultrathin dielectric layers. The resulting oxide exhibits a wide band gap of 4.5 eV, large band offsets (>1.5 eV) relative to graphene, and breakdown electric fields of 5–30 MV/cm—properties that make it a promising building block for scalable, miniaturized nanoelectronics.

The relentless drive to miniaturize electronic components continues to push modern technology toward greater functionality and energy efficiency.^[1] As device dimensions shrink ever further, the integration of two-dimensional semiconductors (2DSEMs) with compatible 2D dielectrics has become essential for achieving effective electrical isolation and precise gate control.^[2] While hexagonal boron nitride (h-BN) serves as a widely used dielectric, its relatively low static dielectric constant (κ < 5)^[3] limits its suitability for low-power applications.^[4],[5] This has created an urgent need for high-κ dielectrics (κ > 10) that can efficiently store charge and provide electrostatic shielding at reduced operating voltages.^{[4],[6]–[8]} Among the emerging candidates, gallium oxide (β-Ga₂O₃)—a van der Waals layered material—stands out as a thermodynamically stable option^[4],[7],[8] that can be produced through the direct oxidation of two-dimensional precursors.^[9]

To investigate this material in detail, the research team fabricated gallium selenide (GaSe) van der Waals layers—typically two to three layers thick—on graphene/SiC substrates within an ultra-high vacuum (UHV) cluster designed for epitaxy and in situ characterization (EPI2SEM). This integrated facility combines molecular beam epitaxy (MBE) with a suite of analytical tools, including low-energy electron diffraction (LEED), scanning probe microscopy (SPM), and X-ray photoelectron spectroscopy (ESCA).^[10]

After initial characterization, the scientists transferred the samples via a UHV suitcase to a near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) system (Fig. 1a). Standardized flag-style sample holders (~1 cm²) enabled seamless transfer between instruments without atmospheric exposure, preserving surface integrity. This UHV-linked workflow allowed the team to monitor in real time the chemical evolution of the surface under controlled oxygen exposure (up to 2 mbar) while gradually raising the sample temperature from 20 °C to 410 °C.

The NAP-XPS instrument featured an analysis chamber with an ultra-high vacuum base pressure of 10^–10 mbar, equipped with interchangeable near-ambient pressure cells that docked directly onto the analyzer entrance. During O₂ exposure, the researchers tracked the core-level spectra for O 1s, Se 3d, Ga 3d, and C 1s in situ. The evolution of these spectral signals as a function of binding energy and temperature is visualized in a color plot (Fig. 1b), with representative spectra shown in Fig. 1c.

The measurements revealed that above approximately 380 °C, a pronounced O 1s peak appeared at 531 eV, alongside a weak spin-orbit doublet at 538 eV arising from gas-phase O₂.^[11] At the same time, the Se 3d signal at 55 eV faded—evidence that selenium desorbed while oxygen chemisorbed onto the surface. As the Se 3d peak weakened and shifted to lower binding energies, the Ga 3d line moved toward higher binding energy, suggesting that GaSe passed through an intermediate phase—likely Ga₂Se₃^[12]—before reaching full oxidation. Importantly, the C 1s signal, which comprises contributions from the SiC substrate (283 eV), the C-buffer layer (286 eV), and the graphene (285 eV),^[13] remained essentially unchanged throughout the process, confirming that the underlying graphene/SiC template retained its structural integrity.

After oxidation, the team transferred the samples within the UHV cluster for angle-resolved XPS analysis at both normal emission (NE) and grazing emission (GE) geometries (Fig. 1d). By comparing the NE data—which probes deeper into the film—with the more surface-sensitive GE data, the researchers calculated Ga 3d to O 1s peak ratios. These ratios proved consistent across both measurement geometries, confirming a uniform distribution of gallium and oxygen throughout the oxide layer.

To promote crystallization, the scientists subjected the oxide films to a series of in situ annealing steps in UHV at temperatures up to 800 °C. XPS analysis showed that this thermal treatment had negligible effects on the overall film stoichiometry. Spectral fitting did, however, identify a minor sub-oxide component (Ga₂O) coexisting with the dominant Ga₂O₃ phase. Quantitative analysis indicated that the as-oxidized film consisted of approximately 92% Ga₂O₃, a value that rose to 95% after the 800 °C anneal—confirming the high phase purity and thermal stability of the synthesized dielectric.

The team then examined the structural properties of the ultrathin Ga₂O₃ and its interface with graphene using a combination of microscopy techniques. Top-down scanning electron microscopy (SEM) imaging (Fig. 2a) revealed extended islands covering approximately 80% of the surface, while tapping-mode atomic force microscopy (AFM) (Fig. 2b) indicated an average island height of 1.5 ± 0.7 nm.

Cross-sectional STEM analysis revealed a 1 nm thick oxide layer with regions of both partial and full crystallinity sitting atop the graphene and C-buffer layers (see Figs. 2c–d). High-angle annular dark-field (HAADF) STEM intensity line scans resolved the atomic stacking sequence, with distinct intensity modulations corresponding to the SiC substrate, the C-buffer layer, the graphene sheet, and the gallium atoms within the oxide.

The researchers found that the interfacial distance between graphene and the primary gallium layer measured approximately 3.5 Å, while three gallium layers within the oxide exhibited an interlayer spacing of about 2.6 Å. These experimental values agreed closely with theoretical predictions for 2D β-Ga₂O₃ along the [100] direction^[6] and with established data for various van der Waals oxides along the c-axis.^[4],[7]

The coexistence of oxide domains with different degrees of crystallinity caused a measurable loss of long-range order, as revealed by LEED (Fig. 2e). In the pristine GaSe/graphene/SiC heterostructures, the LEED pattern showed distinct diffraction spots from the hexagonal lattices of GaSe, graphene, and SiC. The GaSe-related spots formed a ring, indicating randomly oriented GaSe grains within the layer plane.

After oxidation, the LEED pattern was dominated by the sharp diffraction spots of the graphene and SiC substrate against a diffuse background. The researchers attributed this diffuse signal to inelastic electron scattering from the newly formed, partially disordered oxide layer. Energy-dispersive X-ray (EDX) spectroscopy (Fig. 2f) further confirmed the chemical composition of the oxide/graphene/SiC heterostructure.

To complement the in operando investigations, the team studied the oxidation of GaSe/graphene/SiC heterostructures of varying thickness in a tube furnace under a controlled O₂/Ar atmosphere. Specifically, they analyzed 30 nm-thick GaSe layers annealed for 30 minutes at 400 °C, 600 °C, and 800 °C, tracking the structural evolution by HAADF-STEM (Figs. 3a–d).

At 400 °C, unlike the thin two- to three-layer films, the 30 nm-thick GaSe underwent only partial oxidation, yielding a mixture of cubic Ga₂Se₃ and amorphous gallium oxide (a-Ga₂O₃) (Fig. 3bi). Integrated differential phase contrast (iDPC) imaging confirmed that the graphene and buffer layers remained intact at this stage (Figs. 3aii–bii). At 600 °C, the GaSe layer was fully oxidized, and crystalline domains appeared near the graphene interface (Fig. 3ci), though iDPC analysis revealed the onset of slight structural distortion in the graphene (Fig. 3cii). At 800 °C, the GaSe converted entirely into crystalline β-Ga₂O₃ (Fig. 3di), but the high-temperature processing induced significant interfacial roughness in the underlying graphene (Fig. 3dii).

The new findings demonstrated that oxidation conditions could be precisely tuned to engineer distinct structural phases, including crystalline oxide domains with atomically clean interfaces to graphene. The interface remained robust and well-preserved at oxidation temperatures up to approximately 600 °C. The oxidation process also induced crystallographic distortions and localized bond-length contractions within the lattice, promoting oxygen accumulation at the graphene surface—a phenomenon of critical importance for producing high-quality, functionalized graphene.^[14],[15] By bridging in operando NAP-XPS observations with ex situ tube furnace synthesis, the methodology established a versatile and scalable pathway for manufacturing high-performance 2D-semiconductor/oxide heterostructures.

The quality of the oxide layer and its atomically abrupt interface with graphene enabled the team to probe the electronic band structure using angle-resolved photoemission spectroscopy (ARPES). The scientists compared the heterostructure before and after oxidation of two to three layers of GaSe on graphene/SiC (see Fig. 4). In the as-grown samples, the ARPES spectra clearly resolved six Dirac cones characteristic of n-doped graphene on SiC, along with the distinctive inverted Mexican hat-shaped valence band (VB) of the 2L GaSe layer.^[10]

From the energy band dispersion along high-symmetry directions of the graphene Brillouin zone, the authors estimated that the Fermi level lay approximately 0.4 eV above the Dirac point, corresponding to an electron density of ~2 × 10¹³ cm^–2 (based on a Fermi velocity of 10⁶ m/s).^[16] This elevated carrier concentration was attributed to donor states originating from dangling bonds (Si and C) in the SiC buffer layer.^[17]

After oxidation in the NAP-XPS chamber (up to 400 °C) followed by UHV annealing at 800 °C, the Dirac cones and characteristic energy dispersion of the graphene remained well-preserved. The GaSe energy bands, however, were entirely replaced by a weakly dispersed valence band centered at 1.5 eV below the Fermi level (Fig. 4b)—clear evidence that the GaSe had been successfully converted into the intended oxide phase.

The scientists observed that the valence band structure of the synthesized ultrathin oxide closely matched electronic profiles reported for both bulk β-Ga₂O₃^[18],[19] and ε-Ga₂O₃,^[20] as well as theoretical predictions for monolayer β-Ga₂O₃^[6] and layered Ga_xO_y phases.^[7],[8] The investigators noted that for both 2D and 3D β-Ga₂O₃, the VB maximum is predicted to be slightly offset from the Brillouin zone center Γ, but the energy difference of approximately 40 meV^[6] fell within the resolution limits of the ARPES measurements.

The ARPES data revealed no detectable hybridization between the electronic states of the oxide and the underlying graphene. While charge redistribution at the oxide/graphene interface could in principle open an energy gap in the Dirac cone through hybridization, such effects are only expected to be significant at interlayer distances below 3 Å.^[21] In these heterostructures, the measured spacing of approximately 3.5 Å was large enough to preserve the integrity of the graphene Dirac cone.

Complementary ultraviolet photoemission spectroscopy (UPS) data (Fig. 4c) pointed to the formation of an interfacial charge dipole. The work function increased from the pristine graphene value of approximately 3.8 ± 0.1 eV^[10] to 4.0 ± 0.1 eV for the Ga₂O₃/graphene system and about 4.1 ± 0.1 eV for GaSe/graphene, corresponding to built-in potentials of roughly 0.3 eV and 0.2 eV at the graphene interface, respectively.

Because the conduction band (CB) states of the oxide are unoccupied, they could not be accessed by ARPES. To locate the CB edge and determine the transport band gap, the researchers conducted photoconductivity studies. For these measurements, GaSe layers of varying thickness were oxidized in a furnace at approximately 800 °C—a temperature at which the graphene contribution to the overall conductivity becomes negligible, enabling direct characterization of the dielectric. The photodetector devices consisted of interdigitated gold electrodes with a 50 μm finger spacing, defining a total active sensing area of 1 mm² (Fig. 5a).

The photoresponse measurements revealed that each sensor exhibited high electrical insulation in the dark, with photocurrent and responsivity increasing markedly with photon energy at constant incident power (see Figs. 5b,c). For the thicker oxide layers, a sharp rise in photocurrent appeared at approximately 4.5 eV. The thinnest layers, by contrast, were dominated by the SiC substrate signal at around 3.5 eV. To isolate the oxide’s intrinsic properties, the team tested an oxide on graphene/sapphire, which confirmed a consistent photocurrent onset at approximately 4.5 eV.

The near-independence of the band-edge absorption from layer thickness supported theoretical predictions that the CB states in Ga₂O₃ tend to be localized near the surface.^[6] The combined UPS, ARPES, and photoconductivity results yielded a comprehensive energy band diagram (see Fig. 5d) showing the Fermi level, the VB and CB edges, and the work functions for graphene, GaSe/graphene, and Ga₂O₃/graphene. The analysis revealed substantial band offsets relative to the graphene Dirac cone: Δ_VB > 2 eV and Δ_CB > 1.5 eV.

Such large band offsets are essential for effective electrical insulation and dielectric gating in graphene-based electronics, because they suppress leakage currents caused by carrier emission and tunneling into the dielectric.^[22] To assess the dielectric strength of the oxide, the team employed conductive atomic force microscopy (C-AFM), a technique that probes dielectric breakdown with high spatial resolution while simultaneously mapping the surface topography. In this configuration, a platinum-coated silicon AFM tip served as a local electrode, forming a capacitor with the graphene/SiC substrate (Fig. 6a).

In voltage-ramping experiments, the team observed that the leakage current remained in the nanoampere range until an abrupt jump of approximately 10 μA occurred at V ≈ –4 V, indicating local dielectric breakdown (see Fig. 6b). The current dropped back to zero once the bias was removed, confirming the localized nature of the event. AFM images taken before and after breakdown revealed localized damage in the oxide morphology (see Figs. 6c,d).

The team measured the breakdown electric field at multiple sites, obtaining values between 5 and 30 MV/cm for oxide thicknesses ranging from 1 to 5 nm. The data showed a clear thickness-dependent trend in which the breakdown field decreased with increasing thickness (see Figs. 6e,f)—behavior consistent with conventional amorphous (SiO₂) and layered (h-BN) dielectrics.^[23] The spread in values suggested the presence of structural imperfections such as local cracks. Because the oxide contained both crystalline and amorphous domains (as observed by STEM, see Fig. 2), the C-AFM measurements represented an integrated breakdown response, and a direct correlation between local crystal phase and breakdown field remains a topic for future study.

The breakdown field values obtained in the new study ranked among the highest reported to date for β-Ga₂O₃ films,^[24],[25] highlighting the exceptional quality of the oxide/graphene interface and its potential for highly scaled, miniaturized electronics.

Although the optical characterization of ultrathin oxides on graphene/SiC is complicated by the dominant optical response of the SiC substrate—which renders traditional techniques such as ellipsometry unreliable—the combined ARPES and photoconductivity data offered a robust alternative (see Figs. 4, 5). The experimental results aligned closely with theoretical band structure calculations^[6] and the high-energy absorption edges reported for bulk and thin-film β-Ga₂O₃.^[26]

The demonstrated high-energy sensitivity makes the large-area sensors developed in the new work particularly attractive for deep-UV photonics.^[27] Ultimately, the synthesis and integration strategy presented by the researchers provides a versatile platform for a wide range of future applications in both advanced nanoelectronics and high-performance optoelectronics.

Resource:
Rahman, K., Bradford, J., Alghamdi, S. A., Dewes, B. T., Alzeer, A. A. M., Cottam, N. D., Shiffa, M., Cheng, T. S., Novikov, S. V., Makarovsky, O., Mellor, C. J., Mundszinger, M., Biskupek, J., Kaiser, U., Gonzalez, D., Ben, T., O’Shea, J. N., Patanè, A. (2026).
In operando synthesis of an ultrathin dielectric based on crystalline gallium oxide.
Commun. Mater. 7, 78.
https://doi.org/10.1038/s43246-026-01086-0