Liquid‐Metal Fabrication of Ultrathin Gallium Oxynitride Layers with Tunable Stoichiometry

Abstract

The synthesis of nanometer‐thick (≈3 nm) gallium oxynitride (GaO_xN_y) layers with a variable stoichiometry is reported. The approach primarily exploits the liquid metal chemistry (LMC) technique and promises easier integration of 2D materials onto photonic devices compared to traditional top‐down and bottom‐up methods. The fabrication follows a two‐step process, involving first liquid metal‐based printing of a nanometer‐thick layer of gallium oxide (Ga₂O₃), followed a plasma‐enhanced nitridation reaction. Control over nitridation parameters (plasma power, exposure time) allows adjustment of the GaO_xN_y layer's composition, granting access to compounds with distinct optical properties (e.g., a 20% index variation), as demonstrated by ellipsometry and density functional theory (DFT) simulations. DFT provides a microscopic understanding of the effect of the bond polarization and crystallinity on the optical properties of GaO_xN_y compounds. These findings expand the knowledge of ultrathin GaO_xN_y alloys, which are poorly studied with respect to their gallium nitride (GaN) and Ga₂O₃ counterparts. They also represent an essential step toward integrating such 2D materials into photonic chips and offer new opportunities to improve the performance of hybrid optoelectronic devices.

Introduction

Since the isolation of graphene in 2004, several other atomically thin materials, referred to as 2D materials^[] have emerged, offering new optoelectronic properties,^[] which can complement silicon photonics.^[] Incorporating 2D materials into silicon-based devices has several potential advantages, for instance, the enhancement of light absorption^[] or the improvement of light emission efficiency.^[] Although the design of the underlying dielectric photonic devices can be optimized to enhance the low-light–matter interaction with these ultra-thin materials, their integration into host devices remains a technological challenge that depends on both the material and the host device. Developing new fabrication techniques for 2D materials, wherein integration and synthesis could be seamlessly achieved and independently of the host substrate, would greatly facilitate the development of hybrid optoelectronic devices. Among 2D materials, Ga₂O₃ and GaN stand out due to their interesting optoelectronic properties.

Wide-bandgap Ga₂O₃ films have found applications in electronics,^[] photonics,^[] sensing,^[] resistive switching,^[] and bioinspired technologies.^[] Moreover, liquid gallium serves as a solvent with promising potential for growing metallic crystals at the nanoscale.^[] Additionally, a study by Mayyas. M, et al. demonstrated the direct expulsion of metals from Ga-based alloys (GaSn, GaIn, and GaZn) by inducing interfacial perturbations through electrochemistry.^[] This concept can be applied to process liquid alloys, offering a method for producing metallic nanostructures with broad applications. Recently, research on 2D surface oxide films of liquid gallium alloys has been set up to realize economically affordable techniques for synthesizing and extracting high-quality 2D semiconductors.^[] The choice of the synthesis technique profoundly impacts the morphology, the phase, and, therefore, the Ga₂O₃ electronic structure.^[] Many liquid-phase methods based on hydrothermal, microwave, and other solvothermal approaches have been proposed for synthesizing Ga₂O₃.^[] However, such techniques generally require extended processing time of several days for acceptable synthesis yields.

The other material of interest for this work, that is, GaN, has become one of the main materials for the fabrication of optoelectronic devices, such as high-efficiency light-emitting diodes,^[] lasers,^[] photodetectors,^[] and solar cells,^[] due to its bulk wide bandgap (E_g = 3.4 eV).^[] This property is very profitable for optoelectronic applications.^[] For example, it has been theoretically predicted that 2D GaN monolayer can emit light in the deep ultraviolet range, indicating a potential application in sterilization and water purification.^[] However, traditional fabrication techniques, such as mechanical exfoliation, cannot be applied to produce 2D GaN due to the wurtzite structure of the GaN bulk, which holds covalent bonds in all three directions^[] (i.e., it is not a layered van dew Waals crystal as most group IV or transition metal dichalcogenides). Among all the fabrication techniques, chemical vapor deposition (CVD) and elemental epitaxial methods are the most common methods for the deposition of GaN thin films.^[] The epitaxial method is yet inherently limited by high cost, it does allow control of the formation of thin films, but only on carefully selected substrates. On the other hand, more affordable CVD methods cannot produce thin films with only a few unit-cell thicknesses, due to inherent nucleation limitations. Therefore, developing a substrate-independent, scalable process for synthesizing highly crystalline 2D GaN nanosheets across large areas would grant access to 2D GaN at a low cost.

Here, we propose a fast and low-cost synthesis approach for the growth of ultrathin Ga₂O_3, GaN, and gallium oxynitride (GaO_xN_y) layers, involving relatively low temperatures (<320 °C), which is fully compliant with the integration of 2D materials in photonic devices. This technique relies on a two-step process: the first step uses the ‘liquid metal chemistry’ (LMC) method, which has been recently developed^[] and proven effective for synthesizing large-scale ultrathin oxide and metallic layers with various compositions depending on the liquid metal precursor.^[] The second step involves a microwave plasma-enhanced nitridation reaction. The efficiency of LMC technique has been demonstrated for metals and metal alloys present in liquid form below ≈350 °C.^[] Using the LMC technique, we synthesize here ultrathin Ga₂O₃ nanosheets from the self-limiting oxide layer that forms on the surface of liquid gallium near room temperature in a few seconds. Next, our ultrathin Ga₂O₃ nanosheets undergo a microwave plasma-assisted nitridation reaction, eventually resulting in ultrathin GaN layers. We demonstrate, through X-ray photoelectron spectroscopy (XPS) measurements, that this two-step process grants us access to intermediate GaO_xN_y compounds, with tunable optical properties, ranging between those of Ga₂O₃ and GaN. This study also expands our knowledge on GaO_xN_y compounds, which have been scantily explored compared to their GaN and Ga₂O₃ counterparts.^[] Considering that Ga₂O₃ and GaN are widely used in luminescent and power devices, the characterization of their alloys fosters the development of materials with tailored properties and provides new opportunities to improve the device performance. We anticipate that this technique allows for the synthesis of ultrathin GaO_xN_y nanosheets that can be deposited directly onto flat substrates and silicon photonic devices with embedded optical waveguides. This technology thus paves the way toward the realization of hybrid photonic circuits that locally exploit 2D materials to adjust or enhance the optical properties of mature silicon photonic chips.

Fabrication of the Ultra-Thin Films and Nitridation Process

Our LMC-based synthesis relies on a two-step process, exploiting 1) the squeeze printing of liquid gallium, which results in ultrathin Ga₂O₃, and 2) its conversion into GaO_xN_y or GaN via a plasma-assisted nitridation treatment.

The growth process started by placing a droplet (≈1 mm in diameter) of liquid gallium (melting point. 29.8 °C) on an atomically flat centimeter-scale SiO₂/Si substrate. Following the Cabrera–Mott oxidation model depicted in Figure , an ultrathin and self-limiting oxide layer formed on the liquid metal surface once it was exposed to the ambient atmosphere. Cabrera–Mott oxidation occured at the surface of liquid metals in the presence of oxygen at low/moderate temperatures. Electrons from the Ga metal tunneled through the growing oxide shell, resulting in a self-generated electric field called the Mott field.^[] This field promoted the diffusion of metal and oxygen ions into the oxide shell, leading to oxide growth. A glass slide was firmly pressed onto the droplet, effectively squeezing the droplet into the shape of a thin metal film. Upon removal of the top glass substrate, a large and continuous ultrathin gallium oxide film, which reached lateral dimensions exceeding several centimeters, remained on the SiO₂/Si sample. In order to remove the remaining liquid metal microdroplets, we used a solvent-assisted mechanical cleaning protocol based on ethanol heated to 75 °C. The SiO₂/Si sample with the deposited ultrathin Ga₂O₃ sheet was submerged in hot ethanol. Any metal inclusions can be entirely removed by wiping the SiO₂/Si wafer with a polyurethane foam swab while submerged, whereas the Ga₂O₃ sheet remained anchored on the substrate.^[]

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Subsequently, our gallium oxide sheets were converted into gallium oxynitride or gallium nitride using a microwave-activated plasma reaction (Figure ). By controlling the plasma parameters (see methods), in particular the time and power, we can either fully transform the oxide film into nitride or reach stabilized intermediate GaO_xN_y phases between Ga₂O₃ and GaN.

Results

X-ray Photoelectron Spectroscopy (XPS) of Ga₂O₃ and GaN Layers

To ascertain the quantitative conversion of Ga₂O₃ to GaN film, we analyzed the samples using XPS and we compared the relevant spectra before and after the nitridation process, using the plasma parameters indicated in the Experimental Section. Figure shows the XPS spectra of a doublet in the Ga 2p region centered at ≈1118.19 and ≈1144.8 eV corresponding to 2p_3/2 and 2p_1/2, respectively, in good agreement with the spectral values for bulk and ultrathin Ga₂O₃ and GaN.^[] As anticipated, these signatures do not exhibit significant changes between Ga₂O₃ and GaN, with the two peaks observed at the same positions for both samples. Indeed, the electronic configuration of gallium remains similar in both compounds, with only the anion (oxygen or nitrogen) being replaced during the transformation process. As a result, the Ga 2p doublet peaks serve as a reliable marker for the presence of gallium in both Ga₂O₃ and GaN films. To distinguish between the two materials and investigate the extent of the conversion, we next compared the O 1s peak for Ga₂O₃ and GaN (i.e., before and after the nitridation process). In Figure (Ga₂O₃), we can see a double peak for O 1s: the peak at ≈530.9 eV can be assigned to the O—Ga—O bonds in gallium oxide, and the O 1s peak at binding energy ≈532.2 eV can be ascribed to the Si—O bonding from the SiO₂ substrate layer underneath.^[] In contrast, after the nitridation process, only one single O 1s peak is detected on Figure , centered at ≈532.2 eV. The disappearance of the shorter energy peak thus confirms the effectiveness of the nitridation process for fully converting Ga₂O₃ into GaN. (See Figure S1, Supporting Information).

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Tailoring the GaO_xN_y Composition via Plasma-Assisted Nitridation

The presence of residual oxygen can significantly affect the structural, electrical, and optical properties of the material. In this context, during the nitridation process, a careful control of the plasma conditions, such as power, pressure, and gas flow, enables us to tailor the amount of the oxygen substitution. In order to explore how the oxygen substitution can be controlled, the nitridation process was conducted under various powers and time duration. First, the plasma was generated for 10 min at different powers of 100, 150, and 200 W. As displayed in Figure , XPS results show that by increasing the power, the atomic percentage of oxygen in the sample decreases, while the percentage of nitrogen increases, demonstrating a more effective replacement of oxygen with nitrogen bonds to gallium. By increasing the power to 200 W, the complete removal of oxygen was achieved. Alternatively, Figure presents the XPS results of five samples with different nitridation times varying from 0 to 10 min. During this experiment, power was kept constant at 150 W. Over time, as plasma exposure continues, the Ga₂O₃ film is gradually converted to GaN. This evidence is proved by the atomic ratios measured through XPS, showing a decrease in the oxygen percentage to gallium bond from 57.2 to 2.5 at% and an increase in the rate of nitrogen bonds to gallium from 0 to 46.5 at%. (See ).This confirms that the control of either the power and/or time duration of the plasma-assisted nitridation process allows us to reliably obtain intermediate compositions of GaO_xN_y films.

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AFM and TEM Structural Characterization of the GaN and Ga₂O₃ Layers

The uniformity of the synthesized ultrathin GaN sheets was checked through atomic force microscopy (AFM) on the deposited layer. Figure shows the results from the measurement. After the full nitridation process, GaN formed a continuous and atomically flat nanosheet, ≈3 ± 0.2 nm in thickness. This thickness was found to be similar to that of the initial Ga₂O₃, showing that the nitridation does not affect the thickness of the film. In general, the ultrathin sheets feature minimal cracks and holes, and a relatively homogeneous thickness across large distances, which is compatible with the typical size of integrated optical devices as sought for optoelectronic application prospects. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) were utilized to further characterize the morphology and assess the crystallographic properties of the gallium sheets before and after nitridation. Gallium oxide was directly deposited onto a Si₃N₄ TEM grid (Figure ). Figure shows that no crystalline structure is visible in the TEM maps of the ultrathin Ga₂O₃, confirming that the latter has an amorphous structure. The combination of low temperature and rapid growth kinetics during the squeeze printing method leads to the formation of amorphous Ga₂O₃ structures within seconds. Annealing of the obtained Ga₂O₃ at 370 °C is shown to have negligible effects on its crystalline properties in which Ga₂O₃ remains amorphous when exposed to these temperatures.^[] However, in this work, the Ga₂O₃ deposited on Si₃N₄ TEM grid was converted to GaN using the same plasma treatment (utilizing a mixed gas plasma of H₂ and N₂, we operate at 320 °C). It is worth noting that plasma-assisted synthesis is known to yield crystalline GaN at approximately this temperature.^[] Ultrathin nanosheets can now be found during TEM imaging, with the sheets being highly translucent, indicating the thin nature of the final GaN sample (Figure ). HRTEM confirmed the crystalline nature of the GaN samples with a lattice spacing of ≈0.189 nm, corresponding to the (102) planes of wurtzite phases.

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Ellipsometry Characterization of the Ultrathin GaN, Ga₂O₃, and GaO_xN_y Layers

The knowledge of the Ga₂O₃ and GaN refractive index is essential to properly design gallium-based optoelectronic devices. Although the optical properties of Ga₂O₃ thin films (down to sev. 10's of nm) have been investigated,^[] there remains a lack of experimental measurements of the properties of ultrathin Ga₂O₃ sheets with a few nanometers thickness. Similarly, for GaN, the available data focus on either bulk or thin layers (mentioned in Table ) and are not directly applicable to ultrathin (≈3 nm) GaN sheets.^[] Therefore, the properties of both materials in their 2D form remain elusive. Spectroscopic ellipsometry (SE) has been used to study bulk optical properties of Ga₂O₃^[] and Ga^[] In recent years, SE has emerged as a valuable tool also for investigating the optical properties of 2D materials.^[] Here, we characterized the Ga₂O₃ and GaN ultrathin films as well as GaO_xN_y intermediate compounds with SE. The substrate used for all the ultrathin films was SiO₂/Si with a 3 nm-thick silica layer. The optical indices of Ga₂O₃ and GaN were determined by fitting the I_s and I_c parameters of the SE data measured before or after the full nitridation process. For both cases, the dispersion of the 3 nm-thick layer was accounted for by a Tauc–Lorentz formula with adjusted parameters. The model also included the 3 nm-thick SiO₂ native oxide of the SiO₂/ Si host substrate.

Table 1 Comparison of refractive index measurement for Ga₂O₃ and GaN at different thicknesses

Material	Thickness	n	λ [nm]	References
Ga2O3	Bulk	1.84–1.88	980	[]
Ga2O3	67.9 nm	1.84	632	[]
Ga2O3	31.9–2468 nm	1.89	295–826	[]
Ga2O3	30 nm	1.87	632	[]
Ga2O3	89 nm	1.85	632	[]
Ga2O3	3 nm	1.89	632	This work
GaN	Bulk	2.34	632	[]
GaN	2.3 μm	2.42	632	[]
GaN	1.5 μm	2.33	632	[]
GaN	1.3 μm	2.37	632	[]
GaN	1.06 μm	2.34	632	[]
GaN	532 nm	2.1	1200	[]
GaN	3 nm	2.31	632	This work

Figure shows the refractive index (n) and the extinction coefficient (k) of the Ga₂O₃ layer. The resulting refractive index is found to be 1.89 at 632 nm and 1.865 at 980 nm wavelength. Despite the reduced thickness of our films (≈3 nm), these values are very close to those measured for Ga₂O₃ 68 nm-thick films (n = 1.84 at 632 nm),^[] or Ga₂O₃ 800 nm-thick films (n = 1.82 at 600 nm).^[]

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Figure displays the n and k spectra of GaN measured after the full nitridation process. The extracted refractive index was changed to 2.318 at 632 nm and 2.1 at 1200 nm, showing that the nitridation process was effective in turning the oxide films into GaN with distinct optical properties. Again, this refractive index value is only slightly lower than the values found in the literature for bulk GaN crystals (n = 2.34 at 632 nm), metal–organic chemical vapor-deposited GaN layers (n = 2.33 at 632 nm) on sapphire from another study,^[] and GaN epilayers (n ≈ 2.42 at 632 nm) from another study^[] Moreover, the refractive index at 1200 nm is consistent with another study^[] for GaN films with 532 nm thickness. A summary of the comparison of our work and the literature on Ga₂O₃ and GaN measured refractive index (n) at different thicknesses and wavelengths (λ) is reported in

Table Considering that our measured value for 3 nm-thick films is in the same range as for bulk and thick films, we conclude that the refractive index of these materials remains roughly constant regardless of changes in film thickness. In addition, by comparing the refractive index of Ga₂O₃ and GaN at the same wavelength, we note that the relatively high-index difference (≈0.42), featuring a significant ≈20% variation could be relevant for adjusting the optical properties of silicon photonic devices on top of which this ultrathin material is deposited.

In order to measure the effective index of the intermediate GaO_xN_y layers, the related ellipsometry spectra were fit using an effective medium model (Bruggeman). This model considers that the layer on top of the SiO₂/Si substrate consists of a composite (mixed) GaN/Ga₂O₃ layer, with each material following the Tauc–Lorentz dispersion formula extracted from the previous measurements (i.e., for the GaN and Ga₂O₃ layer). For these intermediate GaO_xN_y compounds, only the percentage ratio of GaN to Ga₂O₃ in the mixed layer is used to fit the data, while the thickness (3 nm) and the corresponding dispersion relation for either the GaN to Ga₂O₃ layer were kept fixed, in accordance with the results of

Figure This model allows us to extract the effective refractive index, extinction coefficient of the GaO_xN_y layer, as well as the “ratio” of GaN in the modeled composite layer. Figure shows the measurement of the effective index of the intermediate GaO_xN_y compounds. Panel a (panel b) presents the evolution of n and k as the plasma power (or time duration) is increased. The evident variations observed on these curves strongly depend on the degree of nitridation of the oxide film. These results indicate that the optical properties of the ultrathin GaO_xN_y film can be reliably adjusted by the plasma parameters in a controlled manner, as would be useful for photonic device applications. Figure shows the increase of refractive index at 1550 nm wavelength as a function of the plasma power and time adopted during the plasma nitridation treatment, respectively. These data show an increase of the fitted percentage of GaN in the modeled layer as a function of the plasma parameters, which is consistent with the XPS measurements in Figure . This re-emphasizes the effectiveness of the plasma treatment to turn the oxide layer into GaN in a gradual and controlled manner.

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DFT Calculation

First-principles simulations were carried out to corroborate the optical properties of GaN and Ga₂O₃ and to unravel the effect of crystallinity and atomic disorder on the optical response of those materials.The theoretical calculations were performed within the framework of density functional theory (DFT) as implemented in the Quantum Espresso (QE) package (see ).^[] In order to get rid the statistical complexity of dealing with amorphous systems, in this work, we considered the electronic and optical properties only of Ga₂O₃ crystal in the monoclinic phase. Even though this may appear as a too crude approximation, this choice is justified by the well-known observation that the polar nature of the bonds in metal oxides (with respect, e.g., to the covalent character of Si or Ge) makes the system almost insensitive to structural distortions.^[] As a consequence, the optical properties of most amorphous oxides are very similar to their crystalline counterpart.

Figure shows the bulk unit cells of Ga₂O₃ (panel a) and GaN (panel b) that were considered in the calculations. The optimized lattice parameters for monoclinic β-Ga₂O₃ crystal are a = 12.42 Å, b = 3.08 Å and c = 5.87 Å, and b = 103.74°, while those for the wurtzite GaN are a = b = 3.22 Å and c = 5.24 Å, in good agreement with previous DFT calculations.^[]

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The density of states (DOS) and band structure of β-Ga₂O₃ and GaN are shown in Figure , respectively. The well-known underestimation of the bandgap (E_g) due to the standard DFT functionals has been corrected by including a Hubbard-like potential on each chemical species, within the DFT + U frameworks.^[] Both materials have a direct bandgap, namely, E_g = 2.93 for GaN and E_g = 4.89 eV for β-Ga₂O₃. The calculated values are very close to those calculated at Heyd-Scuseria-Ernzerhof (HSE) level, and in very good agreement with experimental data, confirming the numerical accuracy of the methodology used in our calculations.^[] The projected DOS plots indicate that in both cases the top of the valence band has mainly a nonmetal, that is, O, N, character, with a small contribution from Ga sp-orbitals. The bottom of the conduction band has an almost equal contribution of O (N) and Ga orbitals. The different polarity of the Ga—O and Ga—N bonds is mainly responsible for the different bandgaps and thus of the different optical properties. Further localized states close to the mobility gap might appear in the amorphous structure of Ga₂O₃.^[] This does not change the overall scenario described below about the optical response of the metal-oxide system.

Figure shows the refractive indices and extinction coefficients spectra of Ga₂O₃ (panel a) and GaN (panel b) nanolayers obtained from the SE experiments compared with the DFT results of the corresponding bulk crystals. The theoretical absorption edge is at λ = 253 nm (λ = 423 nm) for Ga₂O₃ (GaN), respectively, which corresponds to the valence-to-conduction bandgap. Despite the numerical differences (mostly probably related to the effect of the residual presence of defects and impurities), the agreement between the experimental and DFT optical indices brings two important conclusions: 1) the experimental data for GaN well fit the results for crystalline (i.e., ordered) phase. This corroborates the analysis related to the structural quality of the ultrathin layer and the effectiveness of the nitridation process. The good agreement observed also in the Ga₂O₃ case confirms a posteriori our initial choice of considering the monoclinic crystal structure, instead of the experimental amorphous one. 2) The experimental data for nanosheets well fit the results for extended bulk (i.e., extended) materials. This confirms the observation that ultrathin films fast recover the bulk/thick film properties, getting rid of strong localization and or major surface effects.

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Conclusion

We reported the synthesis of ultrathin (≈3 nm thick) GaO_xN_y films, with an adjusted composition between that of Ga₂O₃ and GaN, using an easy and low-cost two-step process, relying on the liquid metal-based van der Waals squeeze printing technique followed by a plasma-assisted nitridation process. XPS analysis indicates that the use of a reactive N₂/H₂ plasma in the latter step is key to obtain an ultrathin GaN layer from replacing oxygen atoms with nitrogen ones in the squeeze-printed Ga₂O₃ film. Furthermore, we showed that, through adjusting plasma power and exposure time of the nitridation process, we could achieve tailored compositions of the GaO_xN_y ultrathin layers, with intermediate optical properties between those of Ga₂O₃ and GaN. We characterized the composition and optical properties of the resulting nanometer-thick Ga₂O₃, GaN, and GaO_xN_y ultrathin films. DFT calculations of Ga₂O₃ and GaN were conducted to compare the optical indices obtained by ellipsometry and give further evidence of the nature of the resulting thin films. The comparison between the optical index measurement conducted on these ultrathin films and those extracted from the literature shows that the refractive index of these ultrathin Ga₂O₃ and GaN materials does not depend much on the film thickness, at least down to 3 nm, as was investigated here.

The proposed synthesis method is scalable and fast, as well as compatible with low-temperature (<320 °C) processes, granting the access to large-area ulrathin Ga₂O₃ and GaN nanosheets. This offers new opportunities for the direct deposition of these ultrathin films onto chip-based photonic devices. The measurement of the effective index of the GaO_xN_y ultrathin film across the visible and near-IR range shows that a significant and controlled 20% variation of the ultrathin film refractive index can be achieved by tuning the composition. This can be exploited for adjusting the properties of optical devices based on these materials. In the longer term, this work provides a pathway toward the integration of 2D materials to create efficient hybrid photonic chips and locally adjust or functionalize an otherwise passive optical circuit.

Experimental Section

Transformation of 2D Ga₂O₃ to 2D GaN Sheets via Microwave Plasma-Assisted Nitridation Process

This transformation was afforded by a commercially available Plasma-Therm Vision 310 plasma-enhanced chemical vapor deposition (PECVD) equipment. The synthesis was carried out in a microwave plasma-enhanced CVD reactor at a substrate temperature of ≈320 °C using a plasma consisting of N₂ (99.5%) and H₂ (0.5%). The pressure in the plasma chamber was 1 Torr. The plasma was generated with radio frequency power of 150 W, while the concentration of the H₂/N₂ gas mixture was controlled by setting flow rate of the gases to 200 sccm/1 sccm, respectively. A small amount of H₂ plasma was used to improve the nitridation process by reducing the oxide of the nanosheets. During the transformation to GaN, the samples were exposed to the plasma for 10 min. While this specific plasma treatment recipe led to a full conversion of the oxide film into nitride, a slight change in the plasma power and time duration enabled us to reach intermediate GaO_xN_y phases between Ga₂O₃ and GaN.

Material Characterization

XPS analysis was conducted on materials grown on SiO₂/Si substrates using a Thermo Scientific K-alpha XPS spectrometer equipped with a monochromatic Al Kα source (hv = ≈1486.7 eV). The analyzer was operated with a pass energy of 50 eV to record the core-level spectra from a 300 μm spot size and 50 number of scans for each sample. A low-energy electron flood gun was utilized to remove the surface charging effect of the synthesized material without an ion gun. All the elements were adjusted with the C—C bond of carbon at 284.68 eV.All peaks were fit using Thermo Scientific Advantage Software V5.9. The scanned areas were automatically calculated using the Advantage software V5.9 to verify the atomic ratios and compositions. AFM image was obtained using a Park NX10 AFM. Gwyddion 2.56 software was utilized for AFM image processing and analysis. The high-resolution TEM measurements were performed using a JEOL JEM-F200 TEM with acceleration voltages of 200 kV. The Gatan micrograph 3.4. Software Package was used for TEM/HRTEM analysis. Thermally and mechanically robust Si3N4 TEM (Ted Pella, 21 587-10) membranes were used to develop the TEM samples that were prepared by directly printing the gallium oxide sheet onto TEM membrane and subsequent PECVD. The Si3N4 TEM grid allows experiments requiring temperatures up to 1000 °C, which suits the synthesis process presented in this work. The Si3N4 TEM grids also featured pores which were completely empty and allowed analysis of the suspended flakes through the pore without contribution of the TEM grid. The ellipsometric characterization was performed using a Jobin Yvon-UVISEL spectroscopic, covering a wavelength range from 260 to 2100 nm at 70° of incidence angle.

DFT Calculation

The theoretical calculations were performed within the framework of DFT as implemented in QE package. The exchange-correlation term was described by generalized gradient approximation –Perdew–Burke–Ernzerhof functional.^[] The electron−ion interactions were treated within the optimized norm-conserving Vanderbilt pseudopotentials and the energy cutoff was fixed at 95 Ry for expanding the planewaves.^[] A Γ-point-centered Monkhorst–Pack mesh of 13 × 11 × 1 grid was used to perform the integration over Brillouin zone with a Marzari–Vanderbilt smearing of 0.15 eV. All atomic positions were allowed to relax until the forces acting on each atom were less than 0.02 eV Å⁻¹ and the convergence threshold of the energy change was set at 10⁻⁵ eV.

Undesired DFT bandgap underestimation was corrected by including system-tailored Hubbard-like correction to each atomic species. The adopted values resulted from the self-consistent pseudo-hybrid Hubbard density functional approach implemented in the ACBN0 approach.^[] In the case of wurtzite GaN, the optimized values were U_3d(Ga)^GaN = 19.18 eV and U_2p(N) = 3.92 eV. The unitary cell of monocliclic Ga₂O₃ included two set of inequivalent Ga and O atoms. The corresponding U values were U_3d(Ga₁)^Ga2O3 = 17.85 eV, U_3d(Ga₂)^Ga2O3 = 18.08 eV, U_2p(O₁) = 7.89 eV, and U_2p(O₂) = 8.10 eV. Further accuracy tests on the effects of the inclusion of Hubbard-like correction on the bandgap as well as the dielectric and vibrational properties of semiconductors can be found in other studies.^[]

The optical properties were evaluated from first principles, using the ${\epsilon }_x$ code, contained in the QE package. The code implements a band-to-band single-particle approach based on a generalized Drude−Lorentz formulation of the macroscopic dielectric function $\widehat{\epsilon }={\epsilon }_1+i{\epsilon }_2$.^[] The refractive index (n) and extinction coefficient (k) were straightforwardly obtained from the algebraic transformation of the real (ε₁) and the imaginary (ε₂) part of the dielectric function: $n={\{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.[{({\epsilon }_1^2+{\epsilon }_2^2)}^{1/2}+{\epsilon }_1]\}}^{1/2}$, $k={\{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.[{({\epsilon }_1^2+{\epsilon }_2^2)}^{1/2}-{\epsilon }_1]\}}^{1/2}$.

Acknowledgements

The authors thank the facilities and technical assistance of the Institute Nanotechnology De Lyon (INL) and RMIT Micro Nano Research Facility (MNRF) and the assistance of RMIT Microscopy and Microanalysis Facility (RMMF). The authors also thank The I3E ECLAUSion project that supported this project under funding from the European Union's Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement no 801512 and ALphFA. The authors also acknowledge TD and KCN funding received from the Australian research council via the DECRA program (DE190100100).

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.

References

N. Mounet, M. Gibertini, P. Schwaller, D. Campi, A. Merkys, A. Marrazzo, T. Sohier, I. E. Castelli, A. Cepellotti, G. Pizzi, N. Marzari, Nat. Nanotechnol. 2018, 13, 246.

J. Cheng, C. Wang, X. Zou, L. Liao, Adv. Opt. Mater. 2019, 7, 1800441.

K. S. Novoselov, Science 2004, 306, 341.

P. Miró, M. Audiffred, T. Heine, Chem. Soc. Rev. 2014, 43, 6537.

D. Akinwande, C. J. Brennan, J. S. Bunch, P. Egberts, J. R. Felts, H. Gao, R. Huang, J. S. Kim, T. Li, Y. Li, K. M. Liechti, N. Lu, H. S. Park, E. J. Reed, P. Wang, B. I. Yakobson, T. Zhang, Y. W. Zhang, Y. Zhou, Y. Zhu, Extreme Mech. Lett. 2017, 13, 42.

S. Z. Butler, S. M. Hollen, L. Cao, Y. Cui, J. A. Gupta, H. R. Gutiérrez, T. F. Heinz, S. S. Hong, J. Huang, A. F. Ismach, E. Johnston-Halperin, M. Kuno, V. V. Plashnitsa, R. D. Robinson, R. S. Ruoff, S. Salahuddin, J. Shan, L. Shi, M. G. Spencer, M. Terrones, W. Windl, J. E. Goldberger, ACS Nano 2013, 7, 2898.

P. Sun, K. Wang, H. Zhu, Adv. Mater. 2016, 28, 2287.

T. J. Kim, J. S. Byun, Y. D. Kim, Y. C. Chang, H. J. Kim, J. Korean Phys. Soc. 2008, 53, 1575.

Y. Wei, R. Yang, Natl. Sci. Rev. 2019, 6, 324.

A. Splendiani, L. Sun, Y. Zhang, T. Li, J. Kim, C. Y. Chim, G. Galli, F. Wang, Nano Lett. 2010, 10, 1271.

K. S. Novoselov, A. Mishchenko, A. Carvalho, A. H. Castro Neto, Science 2016, 353, aac9439.

D. Deng, K. S. Novoselov, Q. Fu, N. Zheng, Z. Tian, X. Bao, Nat. Nanotechnol. 2016, 11, 218.

F. Xia, H. Wang, D. Xiao, M. Dubey, A. Ramasubramaniam, Nat. Photonics 2014, 8, 899.

C. H. Liu, Y. C. Chang, T. B. Norris, Z. Zhong, Nat. Nanotechnol. 2014, 9, 273.

J. Guo, J. Li, C. Liu, Y. Yin, W. Wang, Z. Ni, Z. Fu, H. Yu, Y. Xu, Y. Shi, Y. Ma, S. Gao, L. Tong, D. Dai, Light: Sci. Appl. 2020, 9, 29.

D. K. Sarfo, R. R. Taylor, A. P. O. Mullane, ACS Appl. Electron. Mater. 2020, 2, 2921.

B. R. Tak, S. Kumar, A. K. Kapoor, D. Wang, X. Li, J. Phys. D: Appl. Phys. 2021, 54, 453002.

M. Kim, S. Seo, Mol. Cryst. Liq. Cryst. 2019, 685, 40.

M. Mas-torrent, ACS Appl. Electron. Mater. 2020, 2, 3093.

Y.G. Park, Y. G. Lee, J. Jang, S. M. Yun, E. Kim, J. U. Park, Adv. Healthc. Mater. 2021, 10, 2002280.

M. B. Ghasemian, F. Allioux, S. Cai, P. Koshy, N. Gaston, K. Kalantar-zadeh, Science 2022, 1124, 1118.

M. Mayyas, M. Mousavi, M. B. Ghasemian, R. Abbasi, H. Li, M. J. Christoe, J. Han, Y. Wang, C. Zhang, A. Rahim, J. Tang, J. Yang, D. Esra, R. Jalili, F. Allioux, A. P. O. Mullane, K. Kalantar-zadeh, ACS Nano 2020, 14, 14070.

M. Karbalaei, F. Verpoort, S. Zhuiykov, Appl. Mater. Today 2022, 27, 101461.

R. Binions, C. J. Carmalt, I. P. Parkin, K. F. E. Pratt, G. A. Shaw, Chem. Mater. 2004, 16, 2489.

G. Park, W. Choi, J. Kim, Y. C. Choi, Y. H. Lee, C. Lim, Cryst. Growth Des. 2000, 220, 494.

J. Q. Hu, Q. Li, X. M. Meng, C. S. Lee, S. T. Lee, J. Phys. Chem. B 2002, 106, 9536.

G. Sinha, K. Adhikary, S. Chaudhuri, Opt. Mater. 2007, 29, 718.

S. Music, S. Popovic, M. Ristic, Mater. Lett. 2005, 59, 1227.

B. Geng, L. Zhang, G. Meng, T. Xie, X. Peng, Y. Lin, Cryst. Growth Des. 2003, 259, 291.

K. Girija, Semicond. Sci. Technol. 2013, 28, 035015.

H. S. Qian, P. Gunawan, Y. X. Zhang, G. F. Lin, J. W. Zheng, R. Xu, Cryst. Growth Des. 2008, 8, 1282.

D. Li, J. Mater. Chem. A 2013, 1, 12417.

Y. Hou, J. Zhang, Z. Ding, L. Wu, Powder Technol. 2010, 203, 440.

J. Zhang, Z. Liu, C. Lin, J. Lin, Cryst. Growth Des. 2005, 280, 99.

Y. Quan, D. Fang, X. Zhang, S. Liu, K. Huang, Mater. Chem. Phys. 2010, 121, 142.

C. R. Patra, Y. Mastai, A. Gedanken, J. Nanopart. Res. 2004, 6, 509.

X. Liu, G. Qiu, Y. Zhao, N. Zhang, R. Yi, J. Alloys Compd. 2007, 439, 275.

T. Katona, Nat. Photonics 2008, 2, 77.

H. Matsubara, Science 2008, 319, 445.

X. H. Wang, L. W. Guo, H. Q. Jia, Z. G. Xing, Y. Wang, X. J. Pei, J. M. Zhou, H. Chen, Appl. Phys. Lett. 2009, 94, 2007.

S. Strite, J. Vac. Sci. Technol., B: Microelectron. Nanometer Struct. –Process., Meas., Phenom. 1992, 10, 1237.

A. Avramescu, T. Lermer, J. Müller, S. Tautz, D. Queren, S. Lutgen, U. Strau, Appl. Phys. Lett. 2009, 95, 071103.

M. Asif Khan, Appl. Phys. Lett. 1998, 2917, 3.

C. J. Neufeld, N. G. Toledo, S. C. Cruz, M. Iza, S. P. DenBaars, U. K. Mishra, Appl. Phys. Lett. 2008, 93, 91.

F. A. Ponce, D. P. Bour, Nature 1997, 386, 351.

Z. Y. Al Balushi, K. Wang, R. K. Ghosh, R. A. Vilá, S. M. Eichfeld, J. D. Caldwell, X. Qin, Y. C. Lin, P. A. Desario, G. Stone, S. Subramanian, D. F. Paul, R. M. Wallace, S. Datta, J. M. Redwing, J. A. Robinson, Nat. Mater. 2016, 15, 1166.

N. Sanders, D. Bayerl, G. Shi, K. A. Mengle, E. Kioupakis, Nano Lett. 2017, 17, 7345.

M. A. Moram, M. E. Vickers, Rep. Prog. Phys. 2009, 72, 036502.

T. Kuykendall, P. Pauzauskie, S. Lee, Y. Zhang, J. Goldberger, P. Yang, Nano Lett. 2003, 3, 1063.

X. Chen, Adv. Mater. 2003, 15, 419.

J. Goldberger, R. He, Y. Zhang, S. Lee, H. Yan, H. J. Choi, P. Yang, Nature 2003, 422, 599.

A. Usui, Jpn. J. Appl. Phys., Part 2 1997, 36, 899.

T. Sasaki, T. Matsuoka, J. Appl. Phys. 1988, 64, 4531.

A. Zavabeti, J. Z. Ou, B. J. Carey, N. Syed, R. Orrell-Trigg, E. L. H. Mayes, C. Xu, O. Kavehei, A. P. O’Mullane, R. B. Kaner, K. Kalantar-Zadeh, T. Daeneke, Science 2017, 358, 332.

T. Daeneke, K. Khoshmanesh, N. Mahmood, I. A. De Castro, D. Esrafilzadeh, S. J. Barrow, M. D. Dickey, K. Kalantar-Zadeh, Chem. Soc. Rev. 2018, 47, 4073.

P. Aukarasereenont, A. Goff, C. Kim Nguyen, C. F. McConville, A. Elbourne, A. Zavabeti, T. Daeneke, Chem. Soc. Rev. 2022, 51, 1253.

M. Martin, R. Dronskowski, J. Janek, K. D. Becker, D. Roehrens, J. Brendt, M. W. Lumey, L. Nagarajan, I. Valov, A. Börger, Prog. Solid State Chem. 2009, 37, 132.

A. Goff, P. Aukarasereenont, C. K. Nguyen, R. Grant, N. Syed, A. Zavabeti, A. Elbourne, T. Daeneke, Dalton Trans. 2021, 50, 7513.

K. A. Messalea, N. Syed, A. Zavabeti, A. Jannat, P. Aukarasereenont, C. K. Nguyen, M. X. Low, S. Walia, B. Haas, C. T. Koch, N. Mahmood, K. Khoshmanesh, K. Kalantar-zadeh, T. Daeneke, ACS Nano 2021, 15, 16067.

A. Zavabeti, P. Aukarasereenont, H. Tuohey, N. Syed, A. Jannat, A. Elbourne, K. A. Messalea, B. Y. Zhang, B. J. Murdoch, J. G. Partridge, M. Wurdack, D. L. Creedon, J. van Embden, K. Kalantar-Zadeh, S. P. Russo, C. F. McConville, T. Daeneke, Nat. Electron. 2021, 4, 277.

N. Syed, A. Zavabeti, K. A. Messalea, E. Della Gaspera, A. Elbourne, A. Jannat, M. Mohiuddin, B. Y. Zhang, G. Zheng, L. Wang, S. P. Russo, D. Esrafilzadeh, C. F. McConville, K. Kalantar-Zadeh, T. Daeneke, J. Am. Chem. Soc. 2019, 141, 104.

N. Syed, A. Stacey, A. Zavabeti, C. K. Nguyen, B. Haas, C. T. Koch, D. L. Creedon, E. Della Gaspera, P. Reineck, A. Jannat, M. Wurdack, S. E. Bamford, P. J. Pigram, S. A. Tawfik, S. P. Russo, B. J. Murdoch, K. Kalantar-Zadeh, C. F. McConville, T. Daeneke, ACS Nano 2022, 16, 5476.

R. S. Datta, N. Syed, A. Zavabeti, A. Jannat, M. Mohiuddin, M. Rokunuzzaman, B. Yue Zhang, M. A. Rahman, P. Atkin, K. A. Messalea, M. B. Ghasemian, E. Della Gaspera, S. Bhattacharyya, M. S. Fuhrer, S. P. Russo, C. F. McConville, D. Esrafilzadeh, K. Kalantar-Zadeh, T. Daeneke, Nat. Electron. 2020, 3, 51.

K. A. Messalea, B. J. Carey, A. Jannat, N. Syed, M. Mohiuddin, B. Y. Zhang, A. Zavabeti, T. Ahmed, N. Mahmood, E. Della Gaspera, K. Khoshmanesh, K. Kalantar-Zadeh, T. Daeneke, Nanoscale 2018, 10, 15615.

N. Syed, A. Zavabeti, J. Z. Ou, M. Mohiuddin, N. Pillai, B. J. Carey, B. Y. Zhang, R. S. Datta, A. Jannat, F. Haque, K. A. Messalea, C. Xu, S. P. Russo, C. F. McConville, T. Daeneke, K. Kalantar-Zadeh, Nat. Commun. 2018, 9, 3618.

G. Schön, J. Electron. Spectros. Relat. Phenom. 1973, 2, 75.

N. Elkashef, R. S. Srinivasa, S. Major, S. C. Sabharwal, K. P. Muthe, Thin Solid Films 1998, 333, 9.

M. Wurdack, T. Yun, E. Estrecho, N. Syed, S. Bhattacharyya, M. Pieczarka, A. Zavabeti, S. Y. Chen, B. Haas, J. Müller, M. N. Lockrey, Q. Bao, C. Schneider, Y. Lu, M. S. Fuhrer, A. G. Truscott, T. Daeneke, E. A. Ostrovskaya, Adv. Mater. 2021, 33, 2005732.

M. Wurdack, T. Yun, E. Estrecho, N. Syed, S. Bhattacharyya, M. Pieczarka, A. Zavabeti, S. Y. Chen, B. Haas, J. Müller, M. N. Lockrey, Q. Bao, C. Schneider, Y. Lu, M. S. Fuhrer, A. G. Truscott, T. Daeneke, E. A. Ostrovskaya, Adv. Mater. 2021, 33, 2005732.

P. Motamedi, N. Dalili, K. Cadien, J. Mater. Chem. C 2015, 3, 7428.

Z. Liu, T. Yamazaki, Y. Shen, T. Kikuta, N. Nakatani, Y. Li, Sens. Actuators, B 2008, 129, 666.

J. T. Yan, C. T. Lee, Sens. Actuators, B 2009, 143, 192.

M. F. Al-Kuhaili, S. M. A. Durrani, E. E. Khawaja, Appl. Phys. Lett. 2003, 83, 4533.

P. Marie, X. Portier, J. Cardin, Phys. Status Solidi A 2008, 205, 1943.

M. E. Lin, B. N. Sverdlov, S. Strite, H. Morkoc, A. E. Drakin, Electron. Lett. 1993, 29, 1759.

H. Amano, N. Watanabe, N. Koide, I. Akasaki, Jpn. J. Appl. Phys. 1993, 32, L1000.

G. Yu, H. Ishikawa, T. Egawa, T. Soga, J. Watanabe, T. Jimbo, M. Umeno, Jpn. J. Appl. Phys., Part 2 1997, 36, 2202.

T. Kawashima, H. Yoshikawa, S. Adachi, S. Fuke, K. Ohtsuka, J. Appl. Phys. 1997, 82, 3528.

G. Yu, H. Ishikawa, M. Umeno, T. Egawa, J. Watanabe, T. Jimbo, T. Soga, Appl. Phys. Lett. 1998, 72, 2202.

T. Onuma, Jpn. J. Appl. Phys. 2016, 55, 1202B2.

M. Rebien, Appl. Phys. Lett. 2016, 250, 10.

A. Segura, L. Artús, R. Cuscó, R. Goldhahn, M. Feneberg, Phys. Rev. Mater. 2017, 1, 024604.

A. You, M. A. Y. Be, I. In, J. Appl. Phys. 2020, 686, 77.

J. W. Roberts, P. R. Chalker, B. Ding, R. A. Oliver, J. T. Gibbon, L. A. H. Jones, V. R. Dhanak, L. J. Phillips, J. D. Major, F. C. Massabuau, J. Cryst. Growth 2019, 528, 125254.

A. R. A. Zauner, Mater. Res. 1999, 2, 419.

E. S. Hellman, Mater. Res. 1999, 2, 1.

P. Boher, in Proc. 203rd Electrochemical Society Meeting, Paris, 27 April–2 May 2003, Symposium J2, p. 258.

T. Yang, S. Goto, M. Kawata, K. Uchida, A. Niwa, J. Gotoh, Jpn. J. Appl. Phys., Part 2 1998, 37, L1105.

A. Biswas, D. Bhattacharyya, N. K. Sahoo, B. S. Yadav, S. S. Major, J. Alloys Compd. 2008, 103, 083541.

S. J. Yoo, Q. H. Park, Nanophotonics 2022, 11, 2811.

S. Funke, U. Wurstbauer, B. Miller, A. Matković, A. Green, A. Diebold, C. Röling, P. H. Thiesen, Appl. Surf. Sci. 2017, 421, 435.

F. K. Shan, G. X. Liu, W. J. Lee, G. H. Lee, I. S. Kim, B. C. Shin, J. Appl. Phys. 2005, 98, 023504.

O. M. Bordun, I. Y. Kukharskyy, B. O. Bordun, V. B. Lushchanets, J. Appl. Spectrosc. 2014, 81, 771.

S. Scandolo, P. Giannozzi, C. Cavazzoni, S. de Gironcoli, A. Pasquarello, S. Baroni, Z. Kristallogr. – Cryst. Mater. 2005, 220, 574.

B. S. Zou, V. V. Volkov, Z. L. Wang, Chem. Mater. 1999, 11, 3037.

J. Robertson, Phys. Status Solidi B 2008, 245, 1026.

S. Poncé, Phys. Rev. Res. 2020, 2, 033102.

H. Qin, Materials 2017, 10, 1419.

L. A. Agapito, S. Curtarolo, M. B. Nardelli, Phys. Rev. X 2015, 5, 011006.

Q. Yan, P. Rinke, A. Janotti, M. Scheffler, C. G. Van De Walle, M. Foundry, L. Berkeley, Am. Phys. Soc. 2014, 1, 125118.

D. Mora-Fonz, Adv. Electron. Mater. 2020, 6, 1900760.

J. P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 1996, 77, 3865.

M. Schlipf, F. Gygi, Comput. Phys. Commun. 2015, 196, 36.

A. Calzolari, M. B. Nardelli, Sci. Rep. 2013, 3, 2999.

P. Gopal, M. Fornari, S. Curtarolo, L. A. Agapito, L. S. I. Liyanage, M. B. Nardelli, Phys. Rev. B: Condens. Matter Mater. Phys. 2015, 91, 245202.

A. Calzolari, A. Ruini, A. Catellani, ACS Photonics 2014, 1, 703.

M. Passlack, E. F. Schubert, W. S. Hobson, M. Hong, N. Moriya, S. N. G. Chu, K. Konstadinidis, J. P. Mannaerts, M. L. Schnoes, G. J. Zydzik, J. Appl. Phys. 1995, 77, 686.

M. Rebien, W. Henrion, M. Hong, J. P. Mannaerts, M. Fleischer, Appl. Phys. Lett. 2002, 81, 250.

A. Ortiz, J. C. Alonso, E. Andrade, C. Urbiola, J. Electrochem. Soc. 2001, 148, F26.

A. R. A. Zauner, M. A. C. Devillers, P. R. Hageman, P. K. Larsen, S. Porowski, Mater. Res. 1998, 3, e17.