Study on β-Ga 2 O 3 Films Grown with Various VI / III Ratios by MOCVD

β-Ga2O3 films were grown on sapphire (0001) substrates with various O/Ga (VI/III) ratios by metal organic chemical vapor deposition. The effects of VI/III ratio on growth rate, structural, morphological, and Raman properties of the films were systematically studied. By varying the VI/III ratio, the crystalline quality obviously changed. By decreasing the VI/III ratio from 66.9 × 103 to 11.2 × 103, the crystalline quality improved gradually, which was attributed to low nuclei density in the initial stage. However, crystalline quality degraded with further decrease of the VI/III ratio, which was attributed to excessive nucleation rate.


Introduction
β-Ga 2 O 3 , the most stable phase of Ga 2 O 3 , shows great potential because of its excellent material properties.It is a wide bandgap (WBG) semiconductor with band gap of ~4.9 eV, breakdown field of 8 MV cm -1 and Baliga's figure of merit of 3444 at room temperature, which offers more advantages in high-efficiency power device application than SiC and GaN [1].Moreover, its high transparency in UV wavelength range, and excellent thermal and chemical stability also have great application potential in flat panel displays, UV detectors, and high-temperature gas sensors [2][3][4][5][6].There are several ways to produce a β-Ga 2 O 3 film, which include molecule beam epilayer (MBE) [7], metal organic chemical vapor deposition (MOCVD) [8], halide vapor phase epitaxy (HVPE) [4], chemical vapor deposition (CVD) [9], magnetron sputtering [10], and thermal oxidation [11].Conventional CVD methods [12][13][14], especially MOCVD have several advantages, including excellent reproducibility and capability for scale-up to high-volume production [15].Impressive studies on the growth of β-Ga 2 O 3 by MOCVD have been recently reported.Lv et al. investigated the epitaxial relationship between β-Ga 2 O 3 and sapphire substrates [16].Zhuo et al. studied the control of the crystal phase composition of the Ga 2 O 3 thin film [17].Sbrockeyet al. demonstrated the large-area growth of β-Ga 2 O 3 films using rotating disc MOCVD reactor technology [15].Alema et al. studied the growth rates of β-Ga 2 O 3 epitaxial films by close coupled showerhead MOCVD [18].Takiguchi et al. studied β-Ga 2 O 3 epitaxial films obtained by low temperature MOCVD [19].Chen et al. investigated the effect of growth pressure on the characteristics of β-Ga 2 O 3 films grown on GaAs (100) substrates [20].However, the crystalline quality of heteroepitaxial β-Ga 2 O 3 films has not been able to meet the requirements of device fabrication so far.
In this paper, β-Ga 2 O 3 films were grown by MOCVD on sapphire (0001) substrates with various VI/III ratios.In addition, the effects of VI/III ratio on growth rate, structural, morphological, and Raman properties were systematically studied.By varying the ratio, the crystalline quality of the films was effectively improved.

Materials
High purity O 2 (purity, 5 N) and trimethylgallium (TMGa, 6 N in purity, Nata Opto-electronic Material Co., Nanjing, China) and were used as oxidant and organometallic source, respectively.High purity Ar (purity, 6 N) worked as a carrier gas.

Preparation
The β-Ga 2 O 3 films were grown on sapphire (0001) substrates by MOCVD.The equipment was modified from an Emcore D180 MOCVD (Emcore, Alhambra, CA, USA).The close coupled showerhead method is used; the highest growth temperature of the MOCVD was 1150 • C. Before the growth process, the substrates were cleaned sequentially by acetone, ethanol, deionized water in an ultrasonic bath, and then dried with N 2 .The growth pressure and substrate temperature were kept at 20 mbar and 750 • C during the whole growth process, respectively.High purity O 2 was injected into the reaction chamber with a fixed flow rate of 1200 sccm.TMGa was stored in a stainless steel bubbler, maintained at 1 • C. The pressure inside the bubbler were kept at 900 Torr.Ar carrier gas passed through the TMGa bubbler and delivered the TMGa vapor to the reactor.To obtain β-Ga 2 O 3 films grown with various VI/III ratios, the flow rates of Ar carrier gas were varied from 5 sccm to 60 sccm (5 sccm, 15 sccm, 30 sccm, 45 sccm, 60 sccm).The growth time was 30 min.

Characterization
The structural properties of β-Ga 2 O 3 films were investigated by X-ray diffractometer (XRD, Rigaku, Ultima IV, Tokyo, Japan, λ = 0.15406 nm, graphite filter).The morphological properties of the β-Ga 2 O 3 films were studied by field emission scanning electron microscopy (FESEM, JSM-7610F, JEOL, Tokyo, Japan) and atomic force microscopy (AFM, Veeco, Plainview, NY, USA).Raman properties of the films was analyzed by a Raman spectrometer (HORIBA, LABRAM HR EVO, Kyoto, Japan) using a wavelength of λ = 633 nm laser.The thicknesses of the films were measured by a thin film analyzer (F40, Filmetrics, San Diego, CA, USA).

Results and Discussion
The molar flow rates in the experiments can be calculated by Equations ( 1)-(3) [21,22]: where P MO is the vapor pressure of TMGa, a = 8.07, b = 1703, T is the thermodynamic temperature of TMGa, where n MO is the molar flow rate of TMGa, F is the flow rate of carrier gas, V m = 22414 cm 3 /mol, P bub is the pressure inside the bubbler, where n O is the molar flow rate of O 2 , F O is the flow rate of O 2 .The VI/III ratios in the experiments are shown in Table 1.

Growth Rate Analysis
To investigate the growth rates, the thicknesses of the samples were measured by a thin film analyzer.The sample obtained with VI/III ratio of 5.6 × 10 3 is unsuitable for such analysis due to its excessively rough surface [18].The growth rate showed a strong dependence on the VI/III ratio (Figure 1).Because the flow rate of oxygen was a constant, the growth rate was mainly limited by the flow rate of organometallic source.By increasing the flow rates of Ar carrier gas from 5 sccm to 45 sccm, the VI/III ratio decreased from 66.9 × 10 3 to 7.4 × 10 3 , and the growth rate improved from 0.26 to 1.98 µm/h.

Growth Rate Analysis
To investigate the growth rates, the thicknesses of the samples were measured by a thin film analyzer.The sample obtained with VI/III ratio of 5.6 × 10 3 is unsuitable for such analysis due to its excessively rough surface [18].The growth rate showed a strong dependence on the VI/III ratio (Figure 1).Because the flow rate of oxygen was a constant, the growth rate was mainly limited by the flow rate of organometallic source.By increasing the flow rates of Ar carrier gas from 5 sccm to 45 sccm, the VI/III ratio decreased from 66.9 × 10 3 to 7.4 × 10 3 , and the growth rate improved from 0.26 to 1.98 μm/h.

XRD Analysis
Figure 2 shows the XRD θ-2θ scan patterns of β-Ga2O3 films grown with various VI/III ratios.For the film grown with VI/III ratio of 66.9 × 10 3 , except the diffraction peaks of Al2O3 substrate, only three peaks located at 18.76°, 38.10° and 58.84° could be observed, which related to β-Ga2O3 (-201), (-402), and (-603).It indicated that the thin film consisted of pure β-Ga2O3.By decreasing the VI/III ratio from 66.9 × 10 3 to 11.2 × 10 3 , the three peaks of β-Ga2O3 were strengthened and sharpened.The crystallite sizes along the direction vertical to (-201) plane of the samples obtained with the VI/III ratios of 66.9 × 10 3 , 22.3 × 10 3 , and 11.2 × 10 3 were calculated to be 11.2, 12.2, and 17.5 nm, respectively (by Scherrer equation).Larger crystallite sizes indicated lower defect density and an improvement of crystalline quality.Lower VI/III ratio was helpful to reduce the nuclei density in the initial stage of deposition process and enlarge the size of islands in the subsequent stage, which indicated that less defects occurred in island coalescence [23,24].However, further decreasing the VI/III ratio caused crystalline quality degradation.For the film grown with VI/III ratio of 7.4 × 10 3 , the intensities of the three β-Ga2O3 peaks declined, and peaks related to β-Ga2O3 (401), (-601), (601), and (-801) were observed, indicating the polycrystalline structure of the film.The change in crystalline structure is caused by excessive nucleation rate with this VI/III ratio.At this nucleation rate, the deposited particles were unable to migrate to the appropriate lattice positions, and the films grew and oriented in unsuitable directions, which caused random growth.As for the sample obtained with VI/III ratio of 5.6 × 10 3 , the change in crystalline structure was obvious-15 peaks of β-Ga2O3 showed up.The  The crystallite sizes along the direction vertical to (-201) plane of the samples obtained with the VI/III ratios of 66.9 × 10 3 , 22.3 × 10 3 , and 11.2 × 10 3 were calculated to be 11.2, 12.2, and 17.5 nm, respectively (by Scherrer equation).Larger crystallite sizes indicated lower defect density and an improvement of crystalline quality.Lower VI/III ratio was helpful to reduce the nuclei density in the initial stage of deposition process and enlarge the size of islands in the subsequent stage, which indicated that less defects occurred in island coalescence [23,24].However, further decreasing the VI/III ratio caused crystalline quality degradation.For the film grown with VI/III ratio of 7.4 × 10 3 , the intensities of the three β-Ga 2 O 3 peaks declined, and peaks related to β-Ga 2 O 3 (401), (-601), (601), and (-801) were observed, indicating the polycrystalline structure of the film.The change in crystalline structure is caused by excessive nucleation rate with this VI/III ratio.At this nucleation rate, the deposited particles were unable to migrate to the appropriate lattice positions, and the films grew and oriented in unsuitable directions, which caused random growth.As for the sample obtained with VI/III ratio of 5.6 × 10 3 , the change in crystalline structure was obvious-15 peaks of β-Ga 2 O 3 showed up.The crystallite sizes of the films grown with VI/III ratio of 7.4 × 10 3 and 5.6 × 10 3 were calculated to be 14.

AFM Analysis
To investigate the effects of VI/III ratios on the surface morphology of β-Ga2O3 films, AFM was carried out; the images are shown in Figure 3.The surface roughness of the films depended highly on the VI/III ratios.For the film grown with the VI/III ratios from 66.9 × 10 3 to 11.2 × 10 3 , root-meansquare (RMS) surface roughness increased from 3.71 to 7.83 nm.The hillocks on the surfaces enlarged and decreased in density, in good agreement with the XRD analysis.By decreasing the VI/III ratio to 7.4 × 10 3 , the surface roughness had little change, while the morphology changed greatly.Many wheat-like structures were observed, which means that excessive nucleation rate hindered particle migration and caused random growth.For the film grown with VI/III ratio of 5.6 × 10 3 , the roughness increased greatly, even reaching 56.3 nm (seven times higher than that of any other film), in accordance with its XRD pattern (Figure 2).

AFM Analysis
To investigate the effects of VI/III ratios on the surface morphology of β-Ga 2 O 3 films, AFM was carried out; the images are shown in Figure 3.The surface roughness of the films depended highly on the VI/III ratios.For the film grown with the VI/III ratios from 66.9 × 10 3 to 11.2 × 10 3 , root-mean-square (RMS) surface roughness increased from 3.71 to 7.83 nm.The hillocks on the surfaces enlarged and decreased in density, in good agreement with the XRD analysis.By decreasing the VI/III ratio to 7.4 × 10 3 , the surface roughness had little change, while the morphology changed greatly.Many wheat-like structures were observed, which means that excessive nucleation rate hindered particle migration and caused random growth.For the film grown with VI/III ratio of 5.6 × 10 3 , the roughness increased greatly, even reaching 56.3 nm (seven times higher than that of any other film), in accordance with its XRD pattern (Figure 2).

FESEM Analysis
According to XRD analysis of all the films, the sample obtained with VI/III ratio of 11.2 × 10 3 was measured by FESEM. Figure 4 shows the top and cross-sectional views of FESEM images of the sample.The surface with minor defects is in accordance with the AFM image in Figure 3.The relatively smooth cross-sectional image indicates high film quality.In addition, the thickness measured by the cross-sectional view images is about 0.68 μm.

FESEM Analysis
According to XRD analysis of all the films, the sample obtained with VI/III ratio of 11.2 × 10 3 was measured by FESEM. Figure 4 shows the top and cross-sectional views of FESEM images of the sample.The surface with minor defects is in accordance with the AFM image in Figure 3.The relatively smooth cross-sectional image indicates high film quality.In addition, the thickness measured by the cross-sectional view images is about 0.68 µm.

FESEM Analysis
According to XRD analysis of all the films, the sample obtained with VI/III ratio of 11.2 × 10 3 was measured by FESEM. Figure 4 shows the top and cross-sectional views of FESEM images of the sample.The surface with minor defects is in accordance with the AFM image in Figure 3.The relatively smooth cross-sectional image indicates high film quality.In addition, the thickness measured by the cross-sectional view images is about 0.68 μm.

Raman Analysis
Figure 5 presents the Raman spectra of β-Ga 2 O 3 films grown with various VI/III ratios.For comparison, the Raman spectra of the sapphire substrates is also shown in this figure.Except for the peaks related to the substrates, only one Raman peak related to β-Ga 2 O 3 was observed.For the film grown with VI/III ratio of 66.9 × 10 3 , due to poor crystalline quality and a smooth surface, only one peak related to β-Ga 2 O 3 was clearly observed.By decreasing the VI/III ratio, due to the change in crystalline quality and roughness, more peaks related to β-Ga 2 O 3 showed up, which were gradually enhanced.However, when the VI/III ratio was decreased to 5.6 × 10 3 , owing to the excessively rough surface of the obtained sample, its surface area increased and its Raman spectrum changed greatly.Ten peaks related to β-Ga 2 O 3 showed up and the peaks were enhanced greatly.The 10 peaks were divided into three categories [25][26][27]-the peaks located at 115, 147, 171, and 201 cm −1 were attributed to libration and translation of tetrahedral-octahedra chains; the peaks located at 322, 349, and 476 cm −1 were attributed to deformation of GaO 6 octahedra, and the peaks located at 631, 655, and 768 cm −1 were attributed to stretching and bending of GaO 4 tetrahedra.The Raman results confirmed that all the obtained films consisted of pure β-Ga 2 O 3 .
Coatings 2019, 9, x FOR PEER REVIEW 6 of 8 peaks related to the substrates, only one Raman peak related to β-Ga2O3 was observed.For the film grown with VI/III ratio of 66.9 × 10 3 , due to poor crystalline quality and a smooth surface, only one peak related to β-Ga2O3 was clearly observed.By decreasing the VI/III ratio, due to the change in crystalline quality and roughness, more peaks related to β-Ga2O3 showed up, which were gradually enhanced.However, when the VI/III ratio was decreased to 5.6 × 10 3 , owing to the excessively rough surface of the obtained sample, its surface area increased and its Raman spectrum changed greatly.
Ten peaks related to β-Ga2O3 showed up and the peaks were enhanced greatly.The 10 peaks were divided into three categories [25][26][27]-the peaks located at 115, 147, 171, and 201 cm −1 were attributed to libration and translation of tetrahedral-octahedra chains; the peaks located at 322, 349, and 476 cm −1 were attributed to deformation of GaO6 octahedra, and the peaks located at 631, 655, and 768 cm −1 were attributed to stretching and bending of GaO4 tetrahedra.The Raman results confirmed that all the obtained films consisted of pure β-Ga2O3.

Conclusions
In summary, β-Ga2O3 films were grown on sapphire (0001) substrates with various VI/III ratios by MOCVD.By varying the VI/III ratio, the crystalline quality obviously changed.For the film grown with VI/III ratios from 66.9 × 10 3 to 11.2 × 10 3 , the crystalline quality improved gradually, attributed to low nuclei density in the initial stage.However, further decreasing the VI/III ratio caused degradation of crystalline quality, and the morphological and Raman properties changed greatly, which was attributed to excessive nucleation rate.This work offers a feasible way to improve the crystalline quality of heteroepitaxial β-Ga2O3 films and is beneficial for device fabrication.

Figure 1 .
Figure 1.Growth rates of the samples obtained with various VI/III ratios.

Figure 1 .
Figure 1.Growth rates of the samples obtained with various VI/III ratios.

Figure 2
Figure2shows the XRD θ-2θ scan patterns of β-Ga 2 O 3 films grown with various VI/III ratios.For the film grown with VI/III ratio of 66.9 × 10 3 , except the diffraction peaks of Al 2 O 3 substrate, only three peaks located at 18.76 • , 38.10 • and 58.84 • could be observed, which related to β-Ga 2 O 3 (-201), (-402), and (-603).It indicated that the thin film consisted of pure β-Ga 2 O 3 .By decreasing the VI/III ratio from 66.9 × 10 3 to 11.2 × 10 3 , the three peaks of β-Ga 2 O 3 were strengthened and sharpened.The crystallite sizes along the direction vertical to (-201) plane of the samples obtained with the VI/III ratios of 66.9 × 10 3 , 22.3 × 10 3 , and 11.2 × 10 3 were calculated to be 11.2, 12.2, and 17.5 nm, respectively (by Scherrer equation).Larger crystallite sizes indicated lower defect density and an improvement of crystalline quality.Lower VI/III ratio was helpful to reduce the nuclei density in the initial stage of deposition process and enlarge the size of islands in the subsequent stage, which indicated that less defects occurred in island coalescence[23,24].However, further decreasing the VI/III ratio caused crystalline quality degradation.For the film grown with VI/III ratio of 7.4 × 10 3 , the intensities of the three β-Ga 2 O 3 peaks declined, and peaks related to β-Ga 2 O 3 (401), (-601), (601), and (-801) were observed, indicating the polycrystalline structure of the film.The change in crystalline structure 2 and 21.3 nm, respectively.Coatings 2019, 9, x FOR PEER REVIEW 4 of 8 crystallite sizes of the films grown with VI/III ratio of 7.4 × 10 3 and 5.6 × 10 3 were calculated to be 14.2 and 21.3 nm, respectively.

Figure 5
Figure 5 presents the Raman spectra of β-Ga2O3 films grown with various VI/III ratios.For comparison, the Raman spectra of the sapphire substrates is also shown in this figure.Except for the

Figure 5
Figure 5 presents the Raman spectra of β-Ga2O3 films grown with various VI/III ratios.For comparison, the Raman spectra of the sapphire substrates is also shown in this figure.Except for the

Table 1 .
The VI/III ratios at various flow rates of Ar carrier gas.