# p-Type $\beta$-Ga2O3 Homoepitaxial Films with Superior Electrical Transport Properties

Zeyu Chi, Corinne Sartel, Vincent Sallet, Bruno Berini, Yves Dumont, Yunlin

Zheng, Leonid Chernyak, Miquel Vellvehí, Amador Pérez-tomás, Ekaterine

Chikoidze

► To cite this version:

Zeyu Chi, Corinne Sartel, Vincent Sallet, Bruno Berini, Yves Dumont, et al. p-Type $\beta$-Ga2O3 Homoepitaxial Films with Superior Electrical Transport Properties. Advanced Electronic Materials, 2025, 10.1002/aelm.202500190. hal-05230264

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# *p*-Type *β*-Ga₂O₃ Homoepitaxial Films with Superior Electrical Transport Properties

Zeyu Chi, Corinne Sartel, Vincent Sallet, Bruno Berini, Yves Dumont, Yunlin Zheng, Leonid Chernyak, Miquel Vellvehí, Amador Pérez-Tomás, and Ekaterine Chikoidze*

This work reports high structural quality and exceptional electrical transport properties of homoepitaxial *β*-Ga₂O₃ thin films grown by Metal–Organic Chemical Vapor Deposition (MOCVD) on (010)- and (-201)-oriented substrates. (010) *β*-Ga₂O₃ samples exhibit mobility of up to 69.4 cm² (V·s)⁻¹ and stable hole concentrations ≈2.4 × 10¹⁷ cm⁻³ from 370 to 700 K. Structural and morphological studies, including XRD, AFM, and STEM, confirm high epitaxial quality, absence of extended defects and minimal strain. (-201) *β*-Ga₂O₃ layer, which is simultaneously grown, exhibits typical *p*-Ga₂O₃ behavior with observed deep level defects. The hole mobility ranging from 26 to 36 cm² V⁻¹·s⁻¹ is measured between 420 and 700 K. Comparison of (010) and (-201) orientations reveals distinct anisotropic electrical properties. The findings emphasize the free motion of holes in *β*-Ga₂O₃ and the critical role of crystallographic orientation.

## 1. Introduction

Over the past decade, Gallium Oxide (Ga₂O₃) has attracted significant interest for the next-generation of power electronics

and optoelectronics. Ga₂O₃ exhibits a collection of outstanding electronic properties such as ultra-wide bandgap (UWBG) of 4.6–4.9 eV, high breakdown electrical field larger than 8 MV cm⁻¹,[1,2] availability of 6-inch wafers, excellent control of *n*-type doping with concentrations ranging from 10¹⁶ to 10²⁰ cm⁻³,[3–7] and room-temperature electron mobility exceeding 200 cm² (V·s)⁻¹.[8] Additionally, electronic devices such as field effect transistors,[9,10] Schottky barrier diodes,[11] *p*-NiO/*n*-Ga₂O₃ heterojunction diodes,[12,13] and 2D transistors[14] showed the great potential of this material for advancements in power electronics. Besides, *β*-Ga₂O₃ has shown significant promise in solar-blind ultraviolet sensors, especially in the UVC region, due to both its UWBG and superior radiation hardness compared to GaN.[15–19] Solar-blind photodetectors are insensitive to infrared, visible, and near-UV light, but respond to ultraviolet radiation with wavelengths below 300 nm, which is completely absorbed by the atmospheric ozone layer, enabling selective detection of UVC sources, leading to a low false-alarm rate.[17,20–22]

To realize the full functionality of any emerging electronic technology based on UWBG semiconductors, both *n*- and *p*-type conductivity (i.e., bipolarity) should be attained. Till recently, a drawback with bipolar Ga₂O₃ technology was related to the absence of robust homoepitaxial *p*-type layers possessing high conductivity at room temperature. This obstacle has led to heterostructures mainly using polycrystalline *p*-NiO as *p*-type oxides.[12,23–25] Several publications have demonstrated the native defect-related hole conductivity in *β*-Ga₂O₃ films grown on sapphire. The origin of the free holes in undoped *p*-Ga₂O₃ was attributed to the Ga vacancies with deep ionization energy *E*ᵢ = 1.12 eV[26,27] and levels associated with native defects.[28–30] Efficient *p*-type doping of *β*-Ga₂O₃ layers with Zn,[1,28,31] N,[32] and P (by implantation) was later reported.[33,34] Very recently, *β*-Ga₂O₃-based *p*-*n* homojunctions grown on *c*-sapphire[35–38] and Si/SiO₂[39] substrates have been demonstrated.

It is worth emphasizing that significant advancements have been made in the growth of high quality *n*-type homoepitaxial *β*-Ga₂O₃ thin-films on substrates with (-201), (100), (010), and (001) crystallographic orientations using MOCVD,[5,40–42] MOVPE,[43–45] LPCVD,[46] HVPE,[47,48] MBE.[49] However, demonstrations of Ga₂O₃ homo *p*-*n* junctions are still handful. To efficiently address the urgent demand for *p*-*n*

Z. Chi, C. Sartel, V. Sallet, B. Berini, Y. Dumont, E. Chikoidze
Groupe d'Etude de la Matière Condensée (GEMA C)

CNRS
Université Paris-Saclay
Université de Versailles St-Quentin en Yvelines
45 Av. des Etats-Unis, Versailles 78035 Cedex, France
E-mail: ekaterine.chikoidze@uvsq.fr

Y. Zheng
Institut des Nano Sciences de Paris (INSP)
CNRS UMR7588
Sorbonne Université
4 place Jussieu, Paris 75252, France

L. Chernyak
Department of Physics
University of Central Florida
Orlando, FL 32816, USA

M. Vellvehí, A. Pérez-Tomás
Institute of Microelectronics of Barcelona (IMB-CNM-CSIC)
C/ dels Til.lers s/n, Campus UAB, Bellaterra, Barcelona 08193, Spain

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/aelm.202500190

© 2025 The Author(s). Advanced Electronic Materials published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

DOI: 10.1002/aelm.202500190
<figure><img src="image_2.png" /><figcaption>**Figure 2.** Electrical transport properties of homoepitaxially-grown β-Ga₂O₃ (010) sample: a) Temperature-dependent resistivity b) Hall voltage as a function of applied magnetic field at 500 K. c) Hall hole concentration and d) Hall hole mobilities versus temperature.</figcaption></figure>

remains nearly invariant at ≈2.4 × 10¹⁷ cm⁻³ across the measured temperature range (Figure 2c), suggesting that the density of thermally activated carriers does not significantly change above room temperature. As shown in Figure 2d, the Hall mobility shows a decrease from 69.4 to 39.8 cm² (V·s)⁻¹ with increasing temperature from 370 to 700 K. In β-Ga₂O₃, optical phonon (OP), acoustic deformation potential (ADP), ionized impurity (II), and neutral impurity (NI) scattering mechanisms are generally considered when analyzing carrier mobility, each with different temperature-dependent behaviors.[5,50–52] As given in reference,[52] OP scattering has a major impact at room and elevated temperatures and it follows μₚ₀ ≈ 1/T dependence. At lower temperatures, the carrier mobility is limited by impurities, following μᵢ₁ ≈ T³/². At the same time, NI and ADP scatterings have minor influence on mobility. Our experimental mobility dependence on T qualitatively agrees with the previously reported optical phonon scattering in β-Ga₂O₃.[5,52] Moreover, Ma et. al.[51] demonstrated a significant increase in ADP-related mobility after N doping. The electrical conductivity shows temporal (we have performed measurements within 5 and 14 months intervals) and temperature (cycling up to 750 K) stability. While samples with (–201) crystallographic orientations show usual behavior of semiconducting material, the extraordinary properties observed for (010) Ga₂O₃ merit detailed structural studies, discussed in the paragraph below.

### 3. Structural Properties

To understand the origin of superior electrical properties for (010) β-Ga₂O₃ thin films, detailed structural, chemical, and morphological

studies were carried out. The techniques such as X-ray diffraction (XRD), atomic force microscopy (AFM), and scanning transmission electron microscopy (STEM) were employed.

X-ray reflectivity (XRR) indicates a surface roughness (σ) of 2.0 nm (Figure 3a), versus 0.3 nm for the as-received substrate evaluated prior to growth. As was expected, the θ/2θ scan displays only the 020 reflection with no additional reflections, indicating complete homoepitaxy and high crystalline quality of the film (Figure 3b). The rocking curve of the reflection shows no notable difference from the substrate, with a full width at half maximum (FWHM) of 0.013° for the (010) lattice planes, which is very close to the value of 0.010° for the substrate (see inset 3 (b)). The small FWHM value of several tens of arcsec indicates the high crystallinity of thin films deposited on β-Ga₂O₃ substrates.

Surface atomic force microscopy (AFM) images taken for three different scales ranging from 200 nm to 1 µm, along with surface root mean square (RMS) roughness, of homoepitaxial β-Ga₂O₃ (010) film are shown in Figure 3c–e. The grooves, elongated along [001] direction (cf. Figure 11c–e), were also observed in β-Ga₂O₃ films grown by grown by MBE,[49,53] MOCVD,[5] LPCVD.[46] The RMS of the surface depends on the growth rate[54] and temperature,[42,53] and can also be reduced by such pretreatment as Ar annealing.[41] In reference,[54] the surface roughness decreased from 5.87 to 0.62 nm for β-Ga₂O₃ films grown by MOCVD on (010) Ga₂O₃ substrates at 800–1000 °C, while the growth rate was reduced from 1.6 to 0.9 µm h⁻¹.

Figure 4 shows the Raman spectra recorded in 130–800 cm⁻¹ range using 514.5 nm excitation for (010) β-Ga₂O₃ single crystal and homoepitaxially grown layers. All these samples exhibit
<figure><img src="image_3.png" /><figcaption>**Figure 3.** X-ray a) reflectivity and b) diffraction of homoepitaxial β-Ga₂O₃ (010) film (in red) compared with the scans of the substrates alone (in blue). σ in (a): surface roughness; inset in (b): rocking curves of 020 reflection where Δω is the deviation from the angle of incidence of θ/2θ. c–e): Surface AFM images taken for three different scales, with an overall RMS of 2.3 nm.</figcaption></figure>

the well-known active Raman phonon modes for (010) β-Ga₂O₃ at 170, 201, 321, 347, 417, 476, 631, 652, and 767 cm⁻¹.[55,56] The presence of strain in materials changes the phonon frequencies by compressing or stretching the lattice, leading to blue or redshifts.[57] In this work, for epilayers under test, there is almost no deviation in Raman shift relative to the substrate, indicating that the as-grown layers have strain and extended defect levels comparable with the single crystal substrates.[58] Different peak intensities in different orientations for individual phonon modes are attributed to the low symmetry of β-Ga₂O₃ and the effects of birefringence.[59]

High-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) images are shown in **Figure 5** and reveal that homoepitaxial growth occurred without the gen-

eration of extended defects or dislocations. In particular, no V-shape “sympetalous” defects, like those observed by,[44] have been found within the lamella. Such defects nucleate from nanotubes or dislocations in the substrate and further extend like inverted polycrystalline pyramids as the growth proceeds. The absence of extended defects in our sample suggests a low etch pit density in the supplied β-Ga₂O₃ (010) substrate and/or growth process that prevents propagation of surface defects through the epilayer. The interface with the substrate is hardly seen in Figure 5b, where only a very weak contrast can be distinguished at low magnification, indicating the successful homoepitaxy. Energy-dispersive X-ray spectroscopy (EDS) mappings carried out on the interface area did not reveal any contamination by extrinsic elements. Figure 5c with zoomed detail in the inset shows a HAADF image with high resolution taken in the epilayer. Note-worthy, a very similar image was taken in the substrate area (not shown here). The lattice of Ga atomic columns is clearly observed, and it fits perfectly with the Carine software simulation of the β-Ga₂O₃ crystal along the [001] zone axis, as illustrated in Figure 5d.

Although no line or planar defects were found within the field of view in the lamella, point defects such as vacancies, intersterials, or complexes involving several of them, are expected in β-Ga₂O₃. Interestingly, from the HAADF-STEM intensities, Johnson et al. identified point defect complexes consisting of one Ga interstitial paired with two Ga vacancies, and directly correlated them with DFT calculations.[60] Shortly after, the same group investigated the structural properties of (Al₀.₄Ga₀.₆)₂O₃ layers grown on β-Ga₂O₃ by atomic resolution STEM from [001] zone axis, and found out many cases where an additional Ga atomic

<figure><img src="image_4.png" /><figcaption>**Figure 4.** Raman spectra of homoepitaxial β-Ga₂O₃ (010) film (in blue) and substrate (in black).</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>**Figure 5.** STEM analysis of a Ga₂O₃ epilayer grown on (010) Ga₂O₃ substrate, with [001] zone axis: a) low magnification HAADF image of the lamella covering layer and substrate regions, b) HAADF images at the interface, c) HAADF image with atomic scale resolution taken in the layer (in insert, detail showing additional bright contrasts indicated by arrows), and d) Carine simulation of the Ga (green atoms) and O (red atoms) lattices observed in β-Ga₂O₃ along [001] zone axis.</figcaption></figure>

column (bright spot) is located in between two Ga1 positions.[61] These extra atomic columns, appearing as bright spots, were interpreted as interstitial Ga atoms, based on the reduced intensity of the adjacent Ga1 columns, which indicated associated divacancy-interstitial complexes.

In this work, STEM is also performed across [001] zone axis and we note the same occurrence of additional contrasts between two Ga1 atomic columns, as indicated by the blue arrow in the insert of Figure 5c, so that they are attributed to columns of interstitial Ga. This results in a significant density of short segments of three bright spots aligned along the [100] crystallographic direction in both epilayer and substrate areas. In addition, it appears that other additional spots with weaker brightness can be found at other positions where oxygen atomic columns are expected, as indicated by the orange arrows (Figure 5c). Their low intensity is explained by the HAADF-STEM conditions which are sensitive to the atomic number Z, therefore enhancing the gallium lattice at the expense of the oxygen lattice. Such observation was also reported by the reference.[62]

In conclusion, the (010) β-Ga₂O₃ sample exhibits a well-defined monophasic structure without extended defects. The interface is structurally stable and free of impurities.

## 4. Discussion

Homoepitaxial (–201) and (010) β-Ga₂O₃ thin films, which are free of extrinsic impurities and extended defects were successfully grown by MOCVD and demonstrated significantly different electrical transport properties. While p-type (–201) β-Ga₂O₃ layer showed a typical semi-insulating behavior, p-type (010) β-Ga₂O₃ thin film demonstrates hole concentration ≈2.4 × 10¹⁷ cm⁻³, which is almost invariant above room-temperature. This highlights the strong anisotropy in electronic properties in β-Ga₂O₃, which arises from different crystallographic orientations with distinct surface terminations, influencing, in turn, the thermodynamics of native defect formation. Indeed, (010) plane contains gallium atoms in tetrahedral (Ga1) and octahedral (Ga2) sites and oxygen atoms with a ratio of Ga:O = 2:3; the (–201) plane is exclusively composed of either Ga1, Ga2 or oxygen atoms, leading to different dangling bond density and surface energy.[30,63]

High hole concentration for (010) β-Ga₂O₃ thin film corroborate the recent key observation of this exact sample by electron beam-induced current (EBIC) technique.[64] Mainly, large minority carrier diffusion lengths ($L_e$) was measured to be ≈1 µm at
<table><caption>**Table 1.** Brief summary of parameters for several *p*-type oxide layers reported in the literature.</caption><thead><tr><th>Materials</th><th>Structure</th><th>E<sub>g</sub> [eV]</th><th>Substrate</th><th>Growth technique</th><th>p [cm<sup>-3</sup>]</th><th>μ [cm<sup>2</sup> V<sup>-1</sup>·s<sup>-1</sup>]</th><th>Refs.</th></tr></thead><tbody><tr><td>SnO</td><td>Tetragonal</td><td>—</td><td>Si/SiO<sub>2</sub></td><td>Sputtering</td><td>5.4 × 10<sup>18</sup></td><td>8.8</td><td>[74]</td></tr><tr><td>SnO</td><td>Tetragonal</td><td>≈2.7</td><td>Soda-lime glass</td><td>Sputtering</td><td>2.2 × 10<sup>17</sup></td><td>18.7</td><td>[75]</td></tr><tr><td>CuAlO<sub>2</sub></td><td>Delafossite</td><td>3.5</td><td>c-Al<sub>2</sub>O<sub>3</sub></td><td>Solid-state reaction</td><td>1.3 × 10<sup>17</sup></td><td>10.4</td><td>[76]</td></tr><tr><td>LiGa<sub>5</sub>O<sub>8</sub></td><td>Spinel cubic</td><td>5.36</td><td>c-Al<sub>2</sub>O<sub>3</sub></td><td>Mist-CVD</td><td>1.9 × 10<sup>15</sup></td><td>20.1</td><td>[77]</td></tr><tr><td>Ba<sub>2</sub>BiTaO<sub>6</sub></td><td>Perovskite</td><td>>4.5</td><td>MgO</td><td>PLD</td><td>10<sup>14</sup></td><td>30</td><td>[78]</td></tr><tr><td>Cu<sub>2</sub>O</td><td>Cubic</td><td>2.17</td><td>MgO</td><td>PLD</td><td>1.7 × 10<sup>14</sup></td><td>90</td><td>[79]</td></tr><tr><td>Cu<sub>2</sub>O</td><td>Cubic</td><td>—</td><td>Glass</td><td>Sputtering</td><td>1.0 × 10<sup>14</sup></td><td>256</td><td>[80]</td></tr></tbody></table>

room temperature, which is much longer than that for holes found in *n*-type β-Ga₂O₃ (*L*ᵢ≈100–300 nm).[65–68] The activation energy of acceptor traps was estimated at ≈100 meV, and concentration *N*<sub>A</sub> ≈1.8 × 10¹⁸ cm⁻³. Moreover, the structural analysis through XRD confirmed exceptional crystal quality, with a FWHM of 46.8 arcsec for the (020) lattice plane, comparable to the substrate. Raman spectroscopy demonstrated minimal shifts in Raman active phonon modes compared to the substrate, indicating low strain and extended defect levels. EDS showed that the interface does not contain extrinsic impurities. STEM imaging showed no extended defects or dislocations, affirming the high-quality epitaxy. Based on existing knowledge of the native acceptor centers,[28,29,69] collectively, we think that the origin of hole conductivity might be related to V<sub>Ga</sub>-V<sub>O</sub> complex-like defects, strongly enhanced during the growth on (010) substrate.

The important finding in this work is related to the hole mobility, reflecting the nature of free holes. According to the theoretical calculations, self-trapping of holes is expected in many oxides whose valence bands are dominated by O2p orbitals, leading to flat valence band. It further leads to a large effective mass which, combined with strong electron-phonon coupling, favors lattice distortion and thus the hole localization, resulting in low hole mobility. At the same time, it should be recalled that a family of *p*-type oxide semiconductors with non-polaronic conduction, i.e., with free holes (μ > 1 cm² s⁻¹) are known.[70] **Table 1** summarizes *p*-type oxide semiconductors with free hole mobilities of ≈10 cm² V⁻¹·s⁻¹ and beyond, including their experimentally reported preparation methods and other electrical transport properties. Indeed, discrepancies between experiments and theoretical predictions for semiconductor oxides, as far as electronic properties are concerned, are not unique in β-Ga₂O₃.[71,72] For example, in the reference,[73] authors pointed out that early interpretations of small polaron hopping in *p*-type spinel Rh₂ZnO₄, were based on experimental misinterpretations and limitations of theoretical calculations. Their experiments showed inconsistency with small polaron hopping which were further explained by corrected computational methods, revealing that the self-trapping energy (*E*<sub>ST</sub>) is positive, and suggesting the band conduction with heavy, but free, holes in a narrow Rh·t<sub>2g</sub> valence band.

For β-Ga₂O₃, theoretical predictions suggested that hole conductivity may have a polaronic character, as holes are expected to be self-trapped.[81–83] However, the EBIC measurements of the diffusion length of minority holes in *n*-type β-Ga₂O₃,[65–68] along with several other experimental works on Ga₂O₃//Al₂O₃ thin films[31,32,34] combined with the present observation of high

mobility values, do not match with the small polaronic character of hole conductivity.[84] These experimental findings suggest that holes in β-Ga₂O₃ may move freely and are not localized. This discrepancy between experimental observations and theoretical predictions, related to the small polaron conduction in β-Ga₂O₃, highlights the need for further theoretical refinement.

## 5. Conclusion

In this work, we demonstrated that β-Ga₂O₃ (010) and (–201) layers simultaneously grown by MOCVD exhibit *p*-type conductivity with pronounced anisotropic electrical transport properties, strongly dependent on the substrate’s crystallographic orientation. Our measurements revealed that the (–201) layer has typical *p*-Ga₂O₃ behavior with deep level defects. At the same time, the hole mobility for this layer ranges from 26 to 36 cm² V⁻¹·s⁻¹ for the temperatures between 420 and 700 K. The (010) layer, on the other hand, exhibited high free hole concentrations (2.4 × 10¹⁷ cm⁻³) at 370 K with high mobility of 69.4 cm² V⁻¹·s⁻¹. High crystalline quality and the absence of extrinsic impurities and extended defects for the layers, grown on the substrates of both orientations, were confirmed by structural and compositional studies.

## 6. Experimental Section

**Sample Preparation:** The β-Ga₂O₃ epilayers were grown simultaneously on β-Ga₂O₃ (010) and (–201) oriented substrates purchased from Novel Crystal Technology, by horizontal Metal–Organic Chemical Vapor Deposition (MOCVD) set-up. Trimethylgallium (TMGa) and high-purity oxygen were used as the precursors for gallium and oxygen, with 5.5 µmol min⁻¹ and 600 sccm flow rates, respectively. Purified argon was used as carrier gas. The pressure and temperature in the growth chamber were maintained at 37 Torr and 775 °C. The thicknesses of (010) and (–201) oriented samples were determined to be ≈1150 and ≈1000 nm, respectively.

**X-Ray Diffraction:** X-ray diffraction (XRD) data were collected on a 5-circle diffractometer (Rigaku SmartLab) with Cu K<sub>α</sub> radiation from a rotating anode. The incident beam was parallelized by a parabolic mirror, reducing the angular divergence to ≈0.01°. Further reduction of the beam divergence was achieved by a channel-cut 2-reflection Ge(220) monochromator, which selected also Cu K<sub>α1</sub> radiation. The reflected beam was counted by a NaI scintillation detector. The film roughness was measured by X-ray reflectivity (XRR) and the out-of-plane (OP) orientation and strain by θ/2θ scans and rocking curves (ω-scans).

**Raman:** Raman spectra were recorded using a Renishaw Invia Reflex micro-Raman spectrometer at room temperature. The samples were excited using a cw Modu-Laser Stellar-REN laser emitting at 514.5 nm with a power of 2–4 mW. The reflecting microscope objective was 50× with
a NA 0.75; the excitation spot diameter was 1 µm. The back-scattered light was dispersed by a mono chromator with a spectral resolution of 1.4 cm⁻¹. The light was detected by a charge-coupled device. The typical accumulation time was 30 s. Raman shifts were calibrated using an optical phonon frequency (520.5 cm⁻¹) of a silicon monocrystal. Several reference Raman spectra for β-Ga₂O₃ single crystal substrates and sapphire c-cut substrates have been also measured.

**AFM:** The films surface morphology was investigated using a Bruker Dimension ICON AFM operating in Peak-Force tapping mode. A ScanAsyst Air model tip was used with a 0.4N m⁻¹ spring constant and a nominal tip radius of 2 nm. The image resolution was 512 pixels × 512 pixels.

**TEM:** The crystalline structure of the Ga₂O₃ epilayer was characterized with atomic scale resolution by using high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM, ThermoFisher Spectra microscope operating at 300 kV). The system includes energy dispersive X-ray spectroscopy (EDS) enabling chemical analysis and mapping.

**Electrical Measurements:** Hall effect measurements were performed in a van der Pauw configuration by custom designed high impedance measurement set-up with magnetic fields of -1.6-1.6 T perpendicular to the film plane. A current source supplies a DC current (2 nA-100 µA) and the exact supplied value is measured with a pico-ammeter. For each switching configuration, monitored with a Keithley 7152 switching system, the adequate statistics is adjusted (typically from 3 to 15) during the measurements. The voltages are recorded using two high impedance Keithley 6517 electrometers and then are subtracted via a Keithley 2700 multimeter. The Hall difference of voltage dynamics is a few tens of microvolts to few tens of volts.

## Acknowledgements

The present work is a part of "GALLIA" International Research Project, CNRS, France. GEMaC colleagues acknowledge financial support of French National Agency of Research (ANR), project "GOPOWER", CE-50 N0015-01. The research at UCF was supported in part by NSF (ECCS2310285; ECCS2341747; ECCS 2427262), US-Israel BSF (award # 2022056) and NATO (awards # G6072; G6194). The authors would like to acknowledge Dr. Bernat Mundet for the TEM inspection at ALBA Synchrotron, Dr. José Santiso for the Carine simulations, Dr. Belén Ballesteros, and Dr. Francisco Belarre for the TEM sample preparation at Catalan Institute of Nanoscience and Nanotechnology (ICN2).

## Conflict of Interest

The authors declare no conflict of interest.

## Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

## Keywords

electrical transport properties, free hole conduction, gallium oxide, p-type, ultra-wide bandgap

Received: March 21, 2025

Revised: August 5, 2025

Published online:

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