High conductivity of n-type β-Ga₂O₃(010) thin films achieved through Si doping by mist chemical vapor deposition

Recently, wide-bandgap semiconductors (WBGSs) have garnered significant attention for power switching applications due to their reduced energy consumption. Among WBGSs, SiC (3.3 eV) and GaN (3.4 eV) have emerged as prominent candidates, and significant research efforts are directed toward transitioning them to the commercialization phase. In this context, Ga₂O₃, a semiconductor characterized by a much larger bandgap of ∼4.8 eV and a high breakdown voltage of ∼8 MV/cm,¹ has garnered attention as a promising candidate.

The existence of Ga₂O₃ polymorphs was discovered by Roy et al. in 1952,² and the authors revealed five crystal polymorphs of Ga₂O₃: α, β, γ, δ, and κ(ε). Among the five polymorphs, monoclinic β-Ga₂O₃, which is the most thermodynamically stable phase of five, has garnered the most attention because large-diameter bulk crystals of β-Ga₂O₃ can be obtained more easily than the other wide bandgap materials. Bulk crystals of β-Ga₂O₃ can be grown via melting methods, such as edge-defined film-fed growth,³ floating zone,⁴ Czochralski,⁵ and vertical Bridgman techniques.⁶ In addition, the bandgap of β-Ga₂O₃ can be tuned by incorporating In₂O₃ and Al₂O₃ to obtain β-(In_xGa_1−x)₂O₃ and β-(Al_xGa_1−x)₂O₃, respectively.^7,8 Using these bandgap-tuning techniques, various heterostructures based on β-Ga₂O₃, such as quantum wells⁹ and superlattices,¹⁰ have been developed. Furthermore, due to the promising characteristics of β-Ga₂O₃, various devices based on β-Ga₂O₃, such as the Schottky barrier diode,¹¹ metal–oxide–semiconductor field-effect transistor (FET),¹² and modulation-doped FET,¹³ have been developed.

For developing various devices based on β-Ga₂O₃, it is critical to control its carrier concentration. Accordingly, several epitaxial growth methods, such as molecular beam epitaxy (MBE),¹⁴ pulsed laser deposition (PLD),¹⁵ metal–organic chemical vapor deposition (MOCVD),¹⁶ low-pressure chemical vapor deposition (LPCVD),¹⁷ halide vapor phase epitaxy (HVPE),¹⁸ and mist chemical vapor deposition (CVD)¹⁹ are utilized for fabricating high-quality β-Ga₂O₃ thin films with controllable carrier concentration in the range of 10¹⁵–10²⁰ cm⁻³. Among these epitaxial growth methods, the mist CVD method for growing films of oxide semiconductors, such as Ga₂O₃,¹⁹ In₂O₃,²⁰ ZnO,²¹ and VO₂,²² was developed by our group. The mist CVD utilizes a solution of the film precursor for growing thin films under atmosphere pressure. Thus, the mist CVD technique can be performed under environmental conditions and is economical. Moreover, mist CVD enables the doping and alloying of materials through the simultaneous incorporation of the dopant and alloy precursors in the solution. Using this approach, we have previously demonstrated the growth of In₂O₃:Sn,²³ α-Ga₂O₃:F,²⁴ β-Ga₂O₃:Ge,²⁵ (Al_xGa_1−x)₂O₃,²⁶ (In_xGa_1−x)₂O₃,²⁷ and ZnMgO.²⁸ 

Group 14 elements are recognized as n-type dopants that occupy the Ga sites in the β-Ga₂O₃ lattice.²⁹ Studies have indicated that Sn prefers the octahedral Ga(II) site, whereas Ge and Si favor the tetrahedral Ga(I) site. Moreover, the incorporation of Sn and Ge into Ga(II) sites results in defect states close to the conduction band minimum to maintain the substitutional configuration. Conversely, the Si dopant does not form any defect states.³⁰ In addition, among the group 14 dopants, Si exhibits the lowest activation energy (36 meV), facilitating complete activation at room temperature. Furthermore, Zhang et al. reported that Si-doped β-Ga₂O₃ thin films have a notably high electron mobility, particularly at a Si doping concentration of 0.5%.^29,31 The authors claimed that the large energy gap between the orbitals of Si 3s and Ga 4s prevents their mixing at the conduction band minimum although Si 3s has the same symmetry as Ga 4s. This phenomenon subsequently contributes to elevated electron mobility. Consequently, Si emerges as the most favorable dopant, exhibiting a shallow donor level in β-Ga₂O₃. Although Si-doped α-Ga₂O₃ heteroepitaxial thin films have been previously fabricated through mist CVD,³² the growth of Si-doped β-Ga₂O₃ homoepitaxial thin films using mist CVD remains unexplored. Therefore, in this study, we extend this methodology to produce Si-doped β-Ga₂O₃ homoepitaxial thin films using the same precursor solution employed in the mist CVD of Si-doped α-Ga₂O₃. Typically, Si-doped α-Ga₂O₃ films fabricated using mist CVD contain numerous dislocations due to heteroepitaxial growth with a large lattice mismatch, preventing the realization of high mobility at a high carrier concentration. Therefore, this study focused on the homoepitaxial growth of Si-doped β-Ga₂O₃ thin films with fewer dislocations on a β-Ga₂O₃ substrate to investigate the possibility of enhancing the carrier mobility.

Si-doped β-Ga₂O₃ thin films were grown on Fe-doped semi-insulating (010) β-Ga₂O₃ substrates (Novel Crystal Technology) via mist CVD. First, we prepared a 0.5 mol/l GaCl₃ solution in deionized water and a 0.5 mol/l C₆H₁₂ClNSi solution in aqueous HCl for use as Ga and Si precursor solutions, respectively. To control the Si concentration in β-Ga₂O₃ thin films, the amount of C₆H₁₂ClNSi in the mixed precursor solution was varied from 0.001% to 0.7% with respect to GaCl₃. The mixed precursor solution was atomized to form mist using ultrasonic transducers (2.4 MHz), and the mist was transported using N₂ as the carrier gas at 10 l/min rate to a heated quartz tube with the substrate. The substrate was then heated up to 750 °C in a furnace and the growth time was set as 20 min based on the time used for growing high-quality Ge-doped β-Ga₂O₃ thin films via mist CVD.²⁵ The thicknesses of all films grown with different Si doping contents were ∼200 nm, as measured using a step profiler (Bruker, Dektak XT-3). Thus, the growth rate was ∼10 nm/min.

To investigate the effect of Si doping on the crystal structure of the homoepitaxial β-Ga₂O₃ thin films grown by mist CVD, their crystal structures were analyzed via x-ray diffraction (XRD, Bruker, D8 Discover). In specific, 2θ-ω and rocking curves were recorded using Cu Kα radiation (λ = 1.5405 Å). The primary x rays were generated by a sealed x-ray tube with 40 kV-40 mA loading power, then paralleled and monochromated by the Göbel multilayer x-ray mirror and two-bounce Ge (004) channel-cut monochromator. The diffracted x rays were analyzed by the one-bounce Ge (220) analyzer and collected by the scintillation counter. As illustrated in Fig. 1(a), all the Si-doped β-Ga₂O₃ samples provided a single peak at ∼60.9°, indicating that the thin films were homoepitaxially grown along (010) plane, and no other peaks of other planes or polymorphs of Ga₂O₃ were observed in the XRD 2θ-ω scanning. Figure 1(b) shows the XRD rocking curves of the different samples, along with their full-width at half-maximum (FWHM) values. The slight right shoulder of the peaks in Fig. 1(b) was observed. We believe that the lattice constant of β-Ga₂O₃ decreased with heavy Si doping, as Si (IV) ions, smaller than Ga (III) ions,³³ replace them. As a result, the diffraction peaks shifted to the higher angle side. Furthermore, due to the close match between the lattice constants of β-Ga₂O₃ substrates and heavily Si-doped β-Ga₂O₃ thin films, there was a small tensile strain in-plane,³⁴ resulting in the occurrence of compressive strain along the [010] direction. It is considered that these factors collectively contributed to the shift of the diffraction peaks to the higher angle side. Figure 1(b) also reveals that the FWHM increased with an increasing Si content in the precursor solution and, thus, the film, which is consistent with the results of Si-doped β-Ga₂O₃ thin films grown via PLD.³⁴ This phenomenon can also be attributed to the different atomic radii of Si and Ga and the incorporation of Si at Ga(II) sites. Although the FWHM of the (020) rocking curve of the doped film changed with the Si concentration in the precursor solution, the FWHM value of 0.1% Si-doped β-Ga₂O₃ thin film obtained in this study (∼40 arc sec) is equivalent to that of Si-doped β-Ga₂O₃ thin films grown via PLD.³⁴ Thus, we conclude that Si-doped β-Ga₂O₃ thin films with high crystallinity were grown via mist CVD.

Next, to investigate the effect of Si doping on the surface roughness of the Si-doped β-Ga₂O₃ thin films, we investigated their surface morphologies via atomic force microscopy (AFM, SII, NanoNavi) in the dynamic force mode. We obtained AFM images of 2 × 2 µm² areas of the films and calculated the root mean square (rms) roughness from these images. Figure 2 displays the AFM images of the Si-doped β-Ga₂O₃ thin films with different Si contents. All surfaces exhibited the striped pattern often observed for homoepitaxially grown (010) β-Ga₂O₃ thin films. Furthermore, the rms roughness values of all Si-doped β-Ga₂O₃ thin films were less than 0.5 nm, indicating the minimal effect of Si doping on the surface roughness for the dopant concentration range used in this study. A small roughness is essential for producing heterostructured devices, such as high electron mobility transistors. Our promising findings are expected to pave the way for β-Ga₂O₃ based hetero-structured applications via cost-effective mist CVD.

To investigate the electrical properties of the mist CVD Si-doped β-Ga₂O₃ thin films, we performed Hall effect measurements using the Van der Pauw method. To obtain ohmic contacts with the thin films, some Ti/Au (40/40 nm) electrodes with a diameter of 600 µm were heating-deposited on the thin films without preprocessing, and then welding In on the Ti/Au electrodes. Figure 3 illustrates the carrier concentration of the film against the Si concentration in the precursor solution used for mist CVD. The data reveal that the carrier concentration of the Si-doped β-Ga₂O₃ thin film can be controlled in the range of 3.85 × 10¹⁸ to 2.58 × 10²⁰ cm⁻³ by varying the Si content in the precursor solution from 0.001% to 0.7% with respect to Ga. The highest carrier concentration achieved in this study is equivalent to that of Ge-doped β-Ga₂O₃ thin films grown by mist CVD. Remarkably, the minimum controllable carrier concentration obtained in this study is considerably higher than that of the β-Ga₂O₃ thin films grown by other growth techniques, such as MOCVD,³⁵ LPCVD,¹⁷ and HVPE.¹⁸ A possible reason for the outstanding carrier concentrations of our films is that Si from the quartz tube used for mist CVD was incorporated into the β-Ga₂O₃ thin films, as described in our previous report.²⁴ In practice, further investigation is needed to clarify the mechanism of the low carrier concentration region because the effect of Cl, which can act as a dopant should be considered. Figure 4 shows the relationship between the Hall mobility and carrier concentration of the Si-doped β-Ga₂O₃ thin films grown by mist CVD. For comparison, the previously published results of Ge- and Sn-doped β-Ga₂O₃ thin films produced using mist CVD are also shown.^25,36 Compared with the mobilities of the Sn-doped and Ge-doped β-Ga₂O₃ thin films, the homoepitaxial Si-doped β-Ga₂O₃ thin films grown in this study exhibit higher mobilities across the entire range of carrier concentration. This is because Si, which has the lowest ionization energy among the group 14 elements in β-Ga₂O₃,³⁷ exhibits a high activation efficiency in β-Ga₂O₃ thin films. Furthermore, both Sn and Ge incorporated at Ga(II) sites form localized defect states close to the conduction band minimum to maintain substitutional configuration, whereas Si does not form any defect states.³⁰ Considering these characteristics of Si, we infer that Si is the most favorable n-type dopant for β-Ga₂O₃ thin films produced using mist CVD for achieving high mobility. The variation in the mobility with carrier concentration of Ge- and Sn-doped β-Ga₂O₃ exhibits a typical single-crystal behavior, with the mobility decreasing consistently with increasing carrier concentration owing to impurity scattering. Conversely, the doping of Si into β-Ga₂O₃ resulted in increased mobility at a high carrier concentration. This result is consistent with the report by Zhang et al., who claimed that Si-doped β-Ga₂O₃ thin films exhibit high electron mobility at ∼0.5% Si doping.^29,31 With regard to films with higher carrier concentrations, the mobilities of the Si-doped β-Ga₂O₃ thin films with carrier concentrations exceeding 1.0 × 10²⁰ cm⁻³ are considerably higher than those of the films with Sn and Ge dopants. In addition, our sample achieved the highest conductivity of 2368 S/cm (57.2 cm²/V s of the Hall mobility at 2.58 × 10²⁰ cm⁻³) because the mobility reached ∼60 cm²/V s of the Hall mobility at a carrier concentration of ∼2 × 10²⁰ cm⁻³. In the case of Ga₂O₃, due to the limitation in activation efficiency, there exists an upper limit of the carrier concentration. In contrast, the mobility of the β-Ga₂O₃ thin films produced via mist CVD did not show the trend of the intrinsic electron mobility limit of β-Ga₂O₃ in the range of the smaller carrier concentration.³⁸ Therefore, Si-doped β-Ga₂O₃ with high mobility at a higher carrier concentration holds great promise, such as in reducing the contact resistance in device applications. We speculate that Fe diffused into the thin films from the Fe-doped β-Ga₂O₃ substrate affected the electron mobility via carrier compensation because the thicknesses of the samples were ∼200 nm. Certain studies have investigated Fe diffusion from Fe-doped β-Ga₂O₃ substrates into thin films grown on them via secondary ion mass spectrometry (SIMS), revealing that Fe from the substrate diffused into thin films to a depth of more than 200 nm.^39,40 Meng et al. reported that high-mobility β-Ga₂O₃ thin films with greater than 100 cm²/V s mobility at smaller carrier concentrations of less than 10¹⁸ cm⁻³ were obtained with a fast growth rate of MOCVD to achieve >1 µm of the film thickness to reduce the effect of Fe diffusion.⁴¹ In addition, by inserting a 300 nm buffer layer, a high hall mobility was achieved in Si-doped β-Ga₂O₃ thin films grown via metal-organic vapor phase epitaxy.⁴² To comprehensively investigate the negative impact of Fe diffusion on the hall mobility of Si-doped β-Ga₂O₃ thin films grown by mist CVD, we plan to perform SIMS measurements in the future. To date, MOCVD,⁴³ MBE,⁴⁴ and PLD³⁴ have been the predominant techniques for achieving high conductivity characterized by high carrier concentration and mobility through Si doping in β-Ga₂O₃ thin films. However, this study demonstrated that cost-effective mist CVD can yield comparable high conductivity equivalent that obtained through MOCVD and PLD. Furthermore, mist CVD has the advantage of being able to easily perform alloying and additional co-doping, making it easy to pursue further possibilities in the high carrier concentration region for device applications.

In conclusion, we demonstrated the homoepitaxial growth of Si-doped β-Ga₂O₃(010) films via mist CVD for developing β-Ga₂O₃-based power devices and rapid switching devices. The surfaces of the Si-doped β-Ga₂O₃ films were very smooth, with less than 0.5 nm of rms roughness, and the surface flatness was not affected by the Si content in the film. Moreover, we could control the carrier concentration of the sample in the range of 3.85 × 10¹⁸ to 2.58 × 10²⁰ cm⁻³ by varying the Si content in the precursor solution. Overall, the Hall mobilities of our films are higher than those of the other n-type doped β-Ga₂O₃ thin films grown via mist CVD. The sample exhibits the highest conductivity of 2368 S/cm (Hall mobility = 57.2 cm²/V s at 2.58 × 10²⁰ cm⁻³ of carrier concentration). These results are expected to promote the application of β-Ga₂O₃ films grown via mist CVD in practical devices.

This work was supported by the JST FOREST Program (Grant No. JPMJFR222M, Japan) and by JSPS KAKENHI (Grant No. JP22H01527).

We would like to thank Editage (www.editage.jp) for English language editing.