The Influence of Annealing Temperature on the Interfacial Heat Transfer in Pulsed Laser Deposition-Grown Ga₂O₃ on Diamond Composite Substrates

Round 1

Reviewer 1 Report

The manuscript entitled “The Influence of Annealing Temperature on the Interfacial Heat Transfer in PLD-grown Ga₂O₃ on Diamond Composite Substrates” (corresponding authors Hong-Ping Ma and Qing-Chun Zhang) submitted to the Journal of Carbon Research proposes preparation and characterization of samples of gallium oxide semiconductor films with improved thermal properties.

Ga₂O₃ films are grown by pulsed laser desorption on AlN films deposited by magnetron sputtering on diamond substrates and subsequently annealed. High-resolution X-ray diffraction, atomic force microscopy, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, and are used for characterization of the samples to derive structural data, surface morphology, and sectional information. The effect of annealing temperature on the microstructure of the Ga₂O₃ films is discussed. In addition, time-domain thermal reflection technique is used to characterize thermal conductivity of the samples, which is related to crystallinity and defect concentration. A parameter is introduced to score the thermal performance of the semiconductor films. The samples characterized in this study have the best value of this parameter among other realizations in the literature.

The manuscript reports progress with production and characterization of semiconductor films. I suggest publication of the manuscript in the Journal of Carbon Research.

I indicate below some corrections and comments that I suggest to take into account in the final version of the manuscript.

1. Lines 166-169 : describe the pioneer-one apparatus setup and verify the operating wavelength of the Ti-Sapphire femtosecond laser

2. Figure 1 : the positions of the (-201) peaks are the same at different annealing temperatures but the positions of the (-603) peaks are not: please explain why.

3. Figure 3 (f) : on the same planar interface measurement are quoted together indications for two different crystallographic planes - (111) and (400),- with different spacings. Explain which is the crystallographic plane of the surface and correlate with interplanar spacings. 

4. Section 3.3 : describe the thermal model that is used in the fitting by introducing all parameters.

No additional comments.

Author Response

Reviewer #1:

The manuscript entitled “The Influence of Annealing Temperature on the Interfacial Heat Transfer in PLD-grown Ga₂O₃ on Diamond Composite Substrates” (corresponding authors Hong-Ping Ma and Qing-Chun Zhang) submitted to the Journal of Carbon Research proposes preparation and characterization of samples of gallium oxide semiconductor films with improved thermal properties.

Ga₂O₃ films are grown by pulsed laser desorption on AlN films deposited by magnetron sputtering on diamond substrates and subsequently annealed. High-resolution X-ray diffraction, atomic force microscopy, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, and are used for characterization of the samples to derive structural data, surface morphology, and sectional information. The effect of annealing temperature on the microstructure of the Ga₂O₃ films is discussed. In addition, time-domain thermal reflection technique is used to characterize thermal conductivity of the samples, which is related to crystallinity and defect concentration. A parameter is introduced to score the thermal performance of the semiconductor films. The samples characterized in this study have the best value of this parameter among other realizations in the literature.

The manuscript reports progress with production and characterization of semiconductor films. I suggest publication of the manuscript in the Journal of Carbon Research.

I indicate below some corrections and comments that I suggest to take into account in the final version of the manuscript.

Comment 1: Lines 166-169: describe the pioneer-one apparatus setup and verify the operating wavelength of the Ti-Sapphire femtosecond laser.

Reply: We are grateful for your professional questioning. Accordingly, we have incorporated the TDTR configuration into the manuscript as follows:

TDTR is a high-precision and high temporal resolution measurement technique, widely used to study the thermal properties of various materials, including the thermal conductivity (κ) and heat capacity of single-layer and multi-layer films, and liquid materials, as well as the thermal boundary conductance (TBC) at solid-solid, solid-liquid, and microstructural interfaces [1]. In this study, the AUTINST Pioneer-01 Time-Domain Thermal Reflectance (TDTR) system was employed to quantify the κ of Ga₂O₃, as well as the TBC of Ga₂O₃/A lN and AlN/diamond interfaces. A schematic representation of a typical TDTR setup is depicted in Figure 1. Specifically, the sample is rapidly heated by irradiating its surface with a stronger laser pulse (pump pulse). Subsequently, weaker pulses (probe pulses) are applied to the same location at varying time delays, and the intensity of the reflected probe pulses is measured. By analyzing both the amplitude and phase shift of these reflected pulses, the temperature decay is tracked, thus determining the material’s reflectivity changes. Through further analysis of the reflectance variation and fitting the data to a thermal model, key parameters such as thermal conductivity, specific heat capacity, and TBC are extracted. Additionally, by minimizing the discrepancy between the TDTR signal and a heat transfer model, the required thermal performance metrics can be accurately estimated [2, 3].

 

Figure 1. Schematic of the typical TDTR setup. The acronyms EOM and PBS stand for electro-optic modulator and polarizing beam splitter, respectively.

In addition, the wavelength of the femtosecond laser used in the TDTR system is determined to be 1064 nm.

 

Comment 2: Figure 1: the positions of the (-201) peaks are the same at different annealing temperatures but the positions of the (-603) peaks are not: please explain why.

Reply: I am grateful for your professional inquiry. In the course of XRD measurements, it has been observed on occasion that certain peaks belonging to the same crystal face group in the sample are shifted. The related forming mechanism is, however, complex. A number of factors may be responsible for this phenomenon, including alterations to the crystal structure and the impact of stress states during the preparation of the sample. Specifically, the following factors should be considered:

2.1 Crystal structure changes:

The occurrence of the phenomenon in question can be attributed to the presence of doped atoms. The introduction of impurity atoms larger than the dimensions of the host atoms may result in alterations to the lattice parameters, leading to an expansion or contraction of the interplanar spacing. In such a scenario, if the doped atoms occupy the same position as the original atoms in the material, it may result in an increase in the lattice constant, which in turn could lead to a minor angle shift of the diffraction peak. Figure 2 illustrates that as the concentration of the Sn dopant increases, the D-spacing strain also increases, resulting in a slight shift of the diffraction peaks.

 

Figure 2. (a) XRD patterns and (b) D-spacing strain of β-Ga₂O₃ nanostructures with different Sn concentrations. [4]

The other one is attributed to lattice distortion. In addition to doping atoms, other factors such as dislocations, slip, climb, and so forth may also cause lattice distortion, which in turn affects the interplanar spacing and the position of diffraction peaks. These microscopic defects can cause changes in lattice parameters, leading to shifts in XRD peaks.

2.2 The influence of stress state

One is attributed to the presence of residual stress within the material. The residual stress present within the material has the potential to induce a shift in diffraction peaks. The origin of this stress may be attributed to thermal stress, mechanical stress, or other factors. A change in stress state can affect the lattice constant, leading to a shift in XRD peaks. The application of an appropriate annealing temperature can enhance the crystallinity of Ga₂O₃ while reducing partial residual stress. Nevertheless, excessive annealing temperatures can induce additional stress, which may result in a deterioration of Ga₂O₃ crystallinity, as illustrated in Figure 3. The illustration inserted in Figure 3(b) demonstrates that Ga₂O₃ is approaching a state of stress-free behaviour at 700 ℃, with the peak shift reflecting the variations in stress.

 

Figure 3. (a) XRD ω−2θ scan and (b) FWHM of rocking curve for β-Ga₂O_­3 films grown at different TB. Inset shows the XRD ω scan for films. [5]

The other one is due to macroscopic stress. Macroscopic stress, such as tensile or compressive stress, may also result in lattice distortion, which in turn affects the position of XRD peaks. Under conditions of stress, crystals may undergo contraction or expansion, which in turn gives rise to alterations in the parameters of the lattice. To illustrate, the considerable lattice and thermal mismatch between Ga₂O₃ and Si gives rise to an apparent internal stress, which consequently results in the inferior growth quality observed in Ga₂O₃ films grown on a Si substrate. Buffer layers have been incorporated into the structure to mitigate the effects of the lattice mismatch. Figure 4 illustrates that the incorporation of buffer layers mitigates the stress associated with the mismatch, resulting in a minor shift of the diffraction peak of Ga₂O₃.

Figure 4. XRD peak positions for the (401) crystal plane of the annealed Ga₂O₃ films directly on (100), (110) and (111) oriented Si substrates and those on Al₂O₃ or HfO₂ buffer layer on Si substrates, respectively. [6]

The above section considers the potential causes of XRD peak shift. In our study, a slight shift was observed, particularly at 700°C, where the maximum shift occurred. As illustrated in Figure 5, the peak shift was approximately -0.3° for (-603). The negative sign indicates a shift towards a smaller diffraction angle. The presence of a considerable amount of noise has obscured the information pertaining to the peak shift, rendering it challenging to ascertain the precise value of the (-201) peak shift. While thermal treatment releases internal stress, a few vacancies are also filled with O and N impurities. Consequently, the XRD peak position undergoes a complex variation, as illustrated in Figure 5.

 

Figure 5. XRD patterns of Ga₂O₃ on diamond composite substrates at different annealing temperatures.

Comment 3: Figure 3 (f): on the same planar interface measurement are quoted together indications for two different crystallographic planes – (111) and (400), - with different spacings. Explain which is the crystallographic plane of the surface and correlate with interplanar spacings. 

Reply: I am most grateful to you for your meticulous review. As evidenced by the XRD results presented in the manuscript, Ga₂O₃ displays a preferred orientation of (-201). Furthermore, the Ga₂O₃ sample prepared exhibits polycrystalline characteristics, as evidenced by the TEM images of additional regions presented in Figure 6. The two aforementioned crystal planes exhibit weak peaks that are challenging to detect by XRD, thereby hindering the ability to ascertain which crystal plane is present on the surface.

   

Figure 6. TEM images of Ga₂O₃ on diamond composite substrates.

Comment 4: Section 3.3: describe the thermal model that is used in the fitting by introducing all parameters.

Reply: We are grateful to the reviewer for their professional commentary. The thermal model employed in the fitting is founded upon a one-dimensional heat transport model. A one-dimensional heat transport model is a theoretical construct that is employed to examine the transfer of heat in a single direction. The model is founded upon the fundamental principle of heat conduction, which describes the transfer of heat from regions of high temperature to those of low temperature. In circumstances where a primary direction of heat transfer is present, it is appropriate to formulate a one-dimensional form of the heat equation. Moreover, when the time scale for alterations in boundary conditions and sources is considerable in comparison to the time scale over which the thermal system attains equilibrium, the analysis can be regarded as being in a steady state. 

In the one-dimensional conduction model, an elemental volume, ΔV, situated between spatial locations x and x + Δx is employed for a heat transfer process that is in a steady state. The first law of thermodynamics is applied to the elemental volume, with consideration given to a scenario in which there is no internal generation (Figure 7).

 

Figure 7. Schematic of differential volume in which steady one-dimensional heat equation is developed [7].

References:

[1] David G. Cahill, Paul V. Braun, Gang Chen, et al. Nanoscale thermal transport. II. 2003–2012, APPLIED PHYSICS REVIEWS, 1,011305 (2014).

[2] David G. Cahill. Analysis of heat flow in layered structures for time-domain thermoreflectance, REVIEW OF SCIENTIFIC INSTRUMENTS, Vol. 75, No. 12, 5119-5122 (2004).

[3] Aaron J. Schmidt, Xiaoyuan Chen, and Gang Chen. Pulse accumulation, radial heat conduction, and anisotropic thermal conductivity in pump-probe transient thermoreflectance, REVIEW OF SCIENTIFIC INSTRUMENTS 79, 114902 (2008).

[4] H. Ryou, T. H. Yoo, et al. Hydrothermal Synthesis and Photocatalytic Property of Sn-doped β-Ga₂O₃ Nanostructure. ECS Journal of Solid State Science and Technology, 2020, 9, 045009.

[5] P. Ma, J. Zheng, et al. Two-step growth of β-Ga₂O₃ on c-plane sapphire using MOCVD for solar-blind photodetector. Journal of Semiconductors, 2024, 45, 022502.

[6] X. Liu, J. Wang, et al. Improved Ga₂O₃ films on variously oriented Si substrates with Al₂O₃ or HfO₂ buffer layer via atomic layer deposition. Micro and Nanostructures, 2024, 193, 207925.

[7] Carslaw, H.S., and Jaeger, J.C., “Conduction of heat in solids,” Oxford: Clarendon Press, 1959, 2nd Ed., Vol. 1,1959

 

Author Response File: Author Response.docx

Reviewer 2 Report

The authors present an experimental study of the heat transfer in PLD-grown Ga2O3. The work is interesting and has a potential for practical application. The methods applied are appropriate; therefore, the conclusions made are expected to be reliable. Thus, I can recommend the manuscript for publication after a minor revision. 

My specific comments are the following:

1. Maybe I’ve missed it in the text, but why the temperature 700 ℃ leads to the maximum thermal conductivity of the Ga2O3/diamond interface? What are the mechanisms behind this?

2. What irregular mixtures are formed at temperatures above 800 ℃ and how do they affect phonon scattering?

3. A short comment about the influence of oxygen vacancies on the thermal properties of Ga2O3 is suggested.

 

Author Response

Reviewer #2:

The authors present an experimental study of the heat transfer in PLD-grown Ga2O3. The work is interesting and has a potential for practical application. The methods applied are appropriate; therefore, the conclusions made are expected to be reliable. Thus, I can recommend the manuscript for publication after a minor revision.

My specific comments are the following:

Comment 1: Maybe I’ve missed it in the text, but why the temperature 700 ℃ leads to the maximum thermal conductivity of the Ga₂O₃/diamond interface? What are the mechanisms behind this?

Reply: We would like to express our gratitude to the reviewer for their meticulous consideration. Firstly, the atomic arrangement of the interface, the roughness of the interface, and other factors are significant influencing factors on the transmission of phonons at the interface [1,2]. The post-treatment process, such as annealing, has been demonstrated to effectively improve the quality of the interface, thereby enhancing the thermal boundary conductance. As can be observed in the TEM images presented in Figure 6 of the manuscript, the disordered layers at the interface exhibit a gradual tendency to become ordered as the annealing temperature increases. However, at temperatures exceeding the optimal range, the formation of low thermal conductivity amorphous layers near the interface and a mixture that suppresses interface phonon transmission becomes increasingly prevalent.

Comment 2: What irregular mixtures are formed at temperatures above 800 ℃ and how do they affect phonon scattering?

Reply: We greatly appreciate your question about the rigor of our work. As mentioned in Comment 1, it has been reported that thermal boundary conductance (TBC) is influenced by interface roughness, atomic arrangement, and the complexity of element composition. As shown in Figure 1, the phonon scattering probability increases with the increase of roughness and becomes stable gradually, thus promoting an increase in thermal boundary conductance.

 

Figure 1. (A-C) TDTR amplitude signals and best fits as a function of delay time. (D) TDTR values of TBC under different area ratios (positive proportion with roughness). The mechanism of the influence of interface morphology on phonon scattering is shown in the dashed box [2].

 

Figure 2. Theoretical analysis of the influence of interface roughness on boundary phonon scattering. [3,4]

However, excessive roughness can result in severe boundary phonon scattering, which in turn suppresses the transmission of interface phonons through the interface. It has been demonstrated that even between lattice-matched crystalline materials, there exist non-uniform transition layers that behave as an effective atomic-scale interface roughness with a root-mean-square (rms) height Δ, as illustrated in Figure 2. This effective interface roughness results in the randomisation of phonon momentum and the emergence of interface resistance in cross-plane transport.

Our findings indicate that AlN exhibits a notable tendency to diffuse towards Ga₂O₃ at elevated temperatures, resulting in the formation of a Ga₂O₃-AlN mixture. Concurrently, the interface roughness rises, intensifying boundary phonon scattering and consequently reducing the thermal boundary conductance.

Comment 3: A short comment about the influence of oxygen vacancies on the thermal properties of Ga₂O₃ is suggested.

Reply: Thanks for your professional suggestions. We have added it in the manuscript, as follows:

On the one hand, the introduction of vacancies results in a mass disorder relative to β-Ga₂O₃ with the perfect unit cell, due to the missing O atom at the vacancy site. This contributes to a change in the effective reduced mass. while atomic level defects result in the scattering of phonons due to differences in mass or the generation of strain fields. The scattering cross-sections follow Rayleigh scattering in different phonon modes. Furthermore, the existence of vacancy sites has been observed to increase phonon scattering, suppress the intensity of vibrational modes, and reduce the phonon mean free paths, which ultimately leads to a reduction in thermal conductivity.

On the other hand, vacancy defects can intensify the scattering effect between phonons and defects, expand the phonon dispersion curve of the material (Figure 3), abbreviate the phonon relaxation time, while impede the participation rate of phonon vibration modes, thereby reducing the thermal conductivity.

 

Figure 3. Thermal conductivity as a function of oxygen vacancy as predicted from MD simulations [5].

References:

[1] Prediction of metal/semiconductor interface thermal conductance using a mixed mismatch model. Acta Physica Sinica, 2023.

[2] Regulated Thermal Boundary Conductance between Copper and Diamond through Nanoscale Interfacial Rough Structures. ACS Appl. Mater. Interfaces, 2023.

[3] S. Mei and I. Knezevic. Thermal conductivity of III-V semiconductor superlattices. Journal of applied physics, 2015, 118 (17), 175101.

[4] Jesús Carrete et al. almaBTE: a solver of the space-time dependent Boltzmann transport equation for phonons in structured materials, Computer physics communications, 2017-11, Vol.220, p.351-362.

[5] J. Munshia, A. Roy, et al. Effect of vacancy defects on the thermal transport of β-Ga₂O₃. MOLECULAR SIMULATION, 2021, 47, 12, 1017–1021.

Author Response File: Author Response.docx

Reviewer 3 Report

1. In the manuscript, the authors concluded that “After annealing in N2 and Ar

atmospheres, the detected diffraction peaks of Ga₂O₃ are enhanced. Additionally, the

peak enhancement is more pronounced and the FWHM is narrower in N2, indicating

that high-temperature annealing in N2 is more effective at improving the crystallinity

of Ga₂O₃.” The authors should provide a clear explanation why the crystallinity

of Ga2O3 is improved more significantly in an N2 atmosphere compared to an Ar

atmosphere during annealing.

2. The authors conducted XPS analysis to verify the chemical composition and

elemental state of the Ga₂O₃ thin films. In Figure 5(a), the survey spectrum of Ga₂O₃

shows some additional peaks. The authors should clearly identify and label all the

peaks present in the spectrum.

3. While explaining the reduction in oxygen-vacancy defects, the authors mention

partial Ga-N bond formation. However, there is no evidence of Ga-N bond formation

in the XPS data. Author should justify it.

4. From the XPS analysis, the authors concluded that increasing the annealing

temperature in Ga₂O₃ reduces oxygen-vacancy defects from 20.6% to 9.9%. The

authors attributed this reduction primarily to the formation of Ga-N bonds. However,

this explanation is questionable, as molecular nitrogen is relatively inert and does not

easily react or incorporate into solid materials like Ga₂O₃ under typical annealing

conditions. Nitrogen doping in Ga₂O₃ typically requires more reactive forms of

nitrogen or specialized techniques. Therefore, the authors should carefully redo the

material synthesis experiments and re-evaluate the properties of the synthesized

materials.

1. In the manuscript, the authors concluded that “After annealing in N2 and Ar

atmospheres, the detected diffraction peaks of Ga₂O₃ are enhanced. Additionally, the

peak enhancement is more pronounced and the FWHM is narrower in N2, indicating

that high-temperature annealing in N2 is more effective at improving the crystallinity

of Ga₂O₃.” The authors should provide a clear explanation why the crystallinity

of Ga2O3 is improved more significantly in an N2 atmosphere compared to an Ar

atmosphere during annealing.

2. The authors conducted XPS analysis to verify the chemical composition and

elemental state of the Ga₂O₃ thin films. In Figure 5(a), the survey spectrum of Ga₂O₃

shows some additional peaks. The authors should clearly identify and label all the

peaks present in the spectrum.

3. While explaining the reduction in oxygen-vacancy defects, the authors mention

partial Ga-N bond formation. However, there is no evidence of Ga-N bond formation

in the XPS data. Author should justify it.

4. From the XPS analysis, the authors concluded that increasing the annealing

temperature in Ga₂O₃ reduces oxygen-vacancy defects from 20.6% to 9.9%. The

authors attributed this reduction primarily to the formation of Ga-N bonds. However,

this explanation is questionable, as molecular nitrogen is relatively inert and does not

easily react or incorporate into solid materials like Ga₂O₃ under typical annealing

conditions. Nitrogen doping in Ga₂O₃ typically requires more reactive forms of

nitrogen or specialized techniques. Therefore, the authors should carefully redo the

material synthesis experiments and re-evaluate the properties of the synthesized

materials.

Author Response

Reviewer #3:

Comment 1: In the manuscript, the authors concluded that “After annealing in N2 and Ar atmospheres, the detected diffraction peaks of Ga₂O₃ are enhanced. Additionally, the peak enhancement is more pronounced and the FWHM is narrower in N2, indicating that high-temperature annealing in N2 is more effective at improving the crystallinity of Ga₂O₃.” The authors should provide a clear explanation why the crystallinity of Ga2O3 is improved more significantly in an N2 atmosphere compared to an Ar atmosphere during annealing.

Reply: We are grateful for your inquiry regarding the rigor of our work. The alteration in the intensity ratio of the two sub-peaks associated with Ga-O bonds suggests a reduction in oxygen vacancy concentration following the annealing process. It is reported [1] that the degree of oxygen vacancy reduction is influenced by the prevailing atmospheric conditions. In an Ar atmosphere, as illustrated in Figure 1, the inert gas maintains relative stability, resulting in a very similar annealing effect on the thin film as that observed in untreated thin films. During the N₂ atmospheric annealing process, a proportion of the N atoms in the film occupy vacant oxygen sites. Furthermore, the N atoms within the lattice capture available oxygen atoms to some extent, thereby promoting atomic rearrangement and crystallization.

 

Figure 1. XPS analysis of Ga2O3 samples annealed under different atmospheric conditions. [1]

Comment 2: The authors conducted XPS analysis to verify the chemical composition and elemental state of the Ga₂O₃ thin films. In Figure 5(a), the survey spectrum of Ga₂O₃ shows some additional peaks. The authors should clearly identify and label all the peaks present in the spectrum.

Reply: I am grateful for your professional questioning regarding the XPS analysis. The Figure 5(a) in the manuscript has been revised and the additional Ga₂O₃ peaks have been labelled as shown in Figure 2.

Figure 2. Revised Figure 5 in the manuscript.

Comment 3: While explaining the reduction in oxygen-vacancy defects, the authors mention partial Ga-N bond formation. However, there is no evidence of Ga-N bond formation in the XPS data. Author should justify it.

Reply: We greatly appreciate your question about the rigor of our work. Due to the proportion of N impurities occupying the O site is very small, there is no GaN diffraction peak on XRD measurements or Ga-N or O-N sub peaks on XPS analysis. In the manuscript, as the temperature increases, O vacancies decrease, and Ga-N bonding analysis is just a partial reason. The basis of this analysis can refer to the conclusions of previous reports [2,3]. As shown in Figure 3, after high-temperature annealing, the peak position of Ga3d gradually shifts towards lower binding energies, indirectly reflecting the possible formation of Ga-N bonds [4,5]. Meanwhile, as mentioned in the manuscript, there may be emptied air in the annealing furnace tube, and a higher oxygen content in the air can significantly reduce O vacancies. At the same time, annealing in N2 environment can also suppress the O vacancy defects, and the effect can be likely even stronger than that in oxygen rich environment [6]. As shown in the Figure 4, it is reported [4] that annealing under O₂ and N₂ atmospheres produces lower oxygen vacancy concentrations under nitrogen atmospheres. Herein, O vacancy concentration is 37.5% for as-grown samples, 20.3% and 13.6% for O₂ and N₂ annealed samples, respectively. In the manuscript, we used annealing furnace tubes without a vacuum pump equipped. Before annealing, N2 will be continuously introduced to exhaust as much air as possible from the furnace tube. However, compared to the method of first applying negative pressure vacuum and then introducing N2 for annealing, the annealing atmosphere in our work is considered to be a mixed gas. These findings are also where future work needs to constantly explore and optimize.

 

Figure 3. (A) Ga 3d peak shift as annealing temperature increases in N2. [2] (B) Ga 3d peak shift as N2 flow increase when preparing Ga₂O₃ using RF sputtering. [3] (C) XPS analysis results in our work.

Figure 4. O 1s XPS spectra of Ga₂O₃ films annealed under different atmospheres. [4]

Comment 4: From the XPS analysis, the authors concluded that increasing the annealing temperature in Ga₂O₃ reduces oxygen-vacancy defects from 20.6% to 9.9%. The authors attributed this reduction primarily to the formation of Ga-N bonds. However, this explanation is questionable, as molecular nitrogen is relatively inert and does not easily react or incorporate into solid materials like Ga₂O₃ under typical annealing conditions. Nitrogen doping in Ga₂O₃ typically requires more reactive forms of nitrogen or specialized techniques. Therefore, the authors should carefully redo the material synthesis experiments and re-evaluate the properties of the synthesized materials.

Reply: We are grateful for your inquiry regarding the rigor of our work. The reviewer's suggestion is indeed accurate. As an inert gas, N₂ is typically resistant to decomposition under typical conditions. However, research has demonstrated that at elevated temperatures, some N₂ will undergo decomposition and form bonds with Ga atoms. As illustrated in the accompanying figure, the concentration of oxygen vacancy defects was found to be significantly lower in the presence of nitrogen at 800°C compared to that observed in oxygen environments. Similarly, in 2021, Xiaohu Hou et al. [2] obtained comparable results, indicating that as the annealing temperature rises, the concentration of oxygen vacancies declines in an N₂ environment, as illustrated in Figure 12. At 900 ℃, as the proximity increases and oxygen vacancies gradually decrease, the Ga 3d peak shifts gradually towards lower binding energy. This suggests the formation of Ga-N bonds.

Furthermore, there are mature applications for high-temperature N₂ decomposition in N-doping in semiconductors, such as SiC homoepitaxy [5,6], which can achieve highly uniform and concentrated doping by introducing pure N2 at 1500-1650 ℃. Given the strong correlation between doping concentration and temperature, it can be postulated that the formation of Ga-N bonds through N doping is a contributing factor.

 

Figure 5. Influence of defect and doping engineering on V_O concentration and bond states in the GaO_X films. a) O1s and b) Ga2p3/2/Ga2p1/2/Ga3d core-level spectra of the pristine a-GaOX, N-500, N-700, and N-900 GaO_X films. Both V_O concentration and Ga peak position change with the annealing temperature. c) Summary of the subtle changes, including recrystallization, nanopore formation, and introduction of N doping in the GaO_X films at different annealing conditions. [2]

References:

[1] Chen W J, et al. Influence of annealing pretreatment in different atmospheres on crystallization quality and UV photosensitivity of gallium oxide films, RSC advances, 2024, 14(7): 4543-4555.

[2] Xiaohu Hou, et al. High-Performance Harsh-Environment-Resistant GaO_X Solar-Blind Photodetectors via Defect and Doping Engineering, Advanced Materials, 2021, 2106923.

[3] Yanfang Zhang et al. Anion Engineering Enhanced Response Speed and Tunable Spectral Responsivity in Gallium-Oxynitrides-Based Ultraviolet Photodetectors. ACS Appl. Electron. Mater. 2020, 2, 808−816.

[4] Young-Jae Lee, et al. Influence of Annealing Atmosphere on the Characteristics of Ga2O3/4H-SiC n-n Heterojunction Diodes, materials, 2019, 13, 434.

[5] Zhuorui Tang et al. Influence of Temperature and Flow Ratio on the Morphology and Uniformity of 4H-SiC Epitaxial Layers Growth on 150 mm 4 Off-Axis Substrates, Crystals, 2023, 13, 62.

[6] Zhuorui Tang et al. Study on the Surface Structure of N-Doped 4H-SiC Homoepitaxial Layer Dependence on the Growth Temperature and C/Si Ratio Deposited by CVD, Crystals, 2023, 13, 193.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

The authors took into account all reviewer’s comments and improved the manuscript. I would like to thank the authors for their professionalism.

I recommend again the publication of this work.

No comments.

Reviewer 2 Report

The authors performed an appropriate revision on the previous version of the manuscript and, I think, it gained more rigor and clarity. 

Thus, I can recommend the manuscript for publication in its present form.

Reviewer 3 Report

Authors have addressed all the queries raised by reviewer. Manuscript may be accepted in current version.

Authors have addressed all the queries raised by reviewer. Manuscript may be accepted in current version.