IWGO 2026 Session IWGO-MoM2: Epitaxial Growth and Doping Control I

The key to realizing the full potential of Ga₂O₃-based devices for medium voltage (>1kV) power switching applications lies in large part in the ability to grow thick (>20 microns) and simultaneously lightly doped (<1x10¹⁶ cm^-3) drift layers with high crystalline quality. To date, the only option for preparing such thick layers on diameter-scalable (001)-oriented freestanding Ga₂O₃ substrates is halide vapor phase deposition (HVPE). Our previous work focused on the development of thick homoepitaxial layers on 2” substrates by HVPE which could be controllably doped throughout the ranges of interest for power electronics device designers, i.e. from ~5x10¹⁵ cm^-3 to ~3x10¹⁶ cm^-3 [1]. In this work, we report on the growth of commercially relevant Ga₂O₃ epilayers grown on 100mm and 150mm substrates using HVPE. Figures below show a photograph of an as-grown ~20µm thick epilayer as well as a N_D-N_A free carrier concentration map showing good doping control in the range of ~1-2x10¹⁶ cm^-3 over a 100mm substrate. Initial results from similar epilayers grown on 150mm substrates will be presented as well as an outlook on the commercial landscape for thick epilayers of Ga₂O₃.

Acknowledgements
This technology was primarily supported by the Microelectronics Commons Program, a DoW initiative, under award number N00164-23-9-G059. The funders had no role in study design, data collection or analysis, or preparation of the manuscript. Distribution Statement A: Approved for public release. Distribution is unlimited.

[1] Leach et al. GOX 2025, Salt Lake City UT, USA, 5 August 2025.

β-Ga₂O₃ has emerged as a promising semiconductor for vertical power electronics. However, achieving high-quality thick β-Ga₂O₃ drift layers with smooth surface morphology and well-controlled low doping remains a long-standing challenge. In HVPE growth of thick (001) β-Ga₂O₃, post-growth chemical–mechanical polishing (CMP) is typically required to achieve adequate surface smoothness for device processing.

In this study, (011) β-Ga₂O₃ films up to 20 µm thick were grown on (011) β-Ga₂O₃ substrates at growth rates of 2.86–5.5 µm/h. Atomic force microscopy (AFM) measurements revealed smooth surfaces with RMS roughness of 0.48–1.05 nm (5x5 µm²), among the lowest reported for comparable thicknesses. X-ray diffraction (XRD) confirmed high crystalline quality, with rocking curve FWHM of 13.6 arcsec (on-axis) and 29.2 arcsec (off-axis), indicating low threading dislocation density. Dent-type defects increased with thickness but were mitigated by incorporating a buffer layer. Secondary ion mass spectrometry (SIMS) indicated low background levels of C, H, and Si, near the detection limit.

Substrate pre-treatment and growth conditions were optimized to suppress dent-type defects. The incorporation of acid treatment, in-situ annealing, buffer layers, and pulse-flow growth layer led to a reduction of defect density by over an order of magnitude. Schottky barrier diodes (SBDs) were fabricated on the MOCVD-grown (011) β-Ga₂O₃ drift layers, as shown in Fig. 1(a). The forward J–V characteristics exhibit near-ideal behavior with an ideality factor of η = 1.02, Schottky barrier height of ϕ_B = 1.47 eV, and turn-on voltage of V_on = 1.11 V (defined at 1 A/cm²). The forward current density exceeded 1250 A/cm² at 5 V. Reverse-bias measurements yielded a breakdown voltage of ~640–740 V, which is expected to be further improved through optimized field-management designs in future work.

Overall, these results demonstrate that high-quality thick (011) β-Ga₂O₃ drift layers can be achieved by MOCVD with effective defect control, enabling strong potential for high-performance vertical power devices.

One emerging ultrawide bandgap material that is closely related to Ga₂O₃ is ⍺-(Al_xGa_1−x)₂O₃, which has a tunable E_g ranging 5.4 – 8.6 eV. Despite theory predicting silicon to be a shallow n type donor over the range of 5.4 – 7.5 eV[1], achieving active donors has proven to be extremely difficult. This challenge has led to only molecular-beam epitaxy (MBE)[2], metal-organic chemical vapor deposition (MOCVD)[3], and mist chemical vapor deposition (CVD)[4] having produced any conductive films with x > 0.

We previously have demonstrated that an uncommon variant of MBE, suboxide MBE (S-MBE), can be utilized to grow ⍺-(Al_xGa_1−x)₂O₃ with excellent structural quality[5], as well as ⍺-Ga₂O₃ with record electronic properties[6]. In this work, we report a multistep S-MBE technique that reliably produces conductive Si-doped ⍺-(Al_xGa_1−x)₂O₃ thin films with S-MBE. The technique produces conductive ⍺-(Al_xGa_1−x)₂O₃ thin films with x ≤ 0.58 (E_g = 7.0 eV).

[1] D. Wickramaratne, J.B. Varley, & J.L. Lyons, Appl. Phys. Lett. 121, 042110 (2022).

[2] H. Okumura, and J.B. Varley, Jpn. J. Appl. Phys. 63(7),075502 (2024).

[3]H. Okumura et al., Jpn. J. Appl. Phys. 63(5), 055502 (2024).

[4] G.T. Dang, et al. 2020, AIP Adv. 10, 115019

[5] J. Steele, et al., APL Mater. 12(4), 041113 (2024).

[6] J. Steele, et al. APL Mater. 13, 101117 (2025).

The (100) surface of Ga₂O₃ is highly desirable from a device and epitaxy standpoint – bulk growth of (100) material is more scalable than (010), the surface is nearly lattice-matched to p-type partner NiO, and Al₂O₃ incorporates at higher concentrations without phase separation. More importantly, the impact ionization coefficients along the [100] direction are minimized while the dielectric constant is maximized, leading to the highest possible critical fields. This is advantageous compared to the current state of the art, (001), due to reduced surface defects and increased possible breakdown voltage. However, the epitaxial growth rate on (100) surfaces is less than 10% of other faces due to weak bonding and favorable desorption, and on-axis (100) growth easily forms twin domains. Recent demonstrations have shown growth rate improvements from 0.4 nm/min to 1.5 nm/min by growing on (100) wafers that are offcut 6° in the -c direction.¹ These films show step-flow growth from (20-1) step-edges and high electron mobility due to suppressed twins. Despite these exciting results, offcuts greater than 6° have not been explored due to the waste associated with grinding and polishing large offcuts.

In this talk we will discuss the molecular beam epitaxy (MBE) growth and properties of β-Ga₂O₃ grown on (100) substrates offcut in the -c direction up to 13.4°. These large offcuts are enabled by edge-fed film-defined growth (EFG) where the offcut is grown into the surface by pulling the crystal through the EFG die with the seed crystal rotated by the desired offcut angle. We will demonstrate that 13.4° offcut substrates still exhibit a terraced (100) surface, and that a >10x increase (>5 nm/min) in growth rate is achieved. Despite the large offcut angle we will demonstrate step-flow growth with RMS roughness values below 2 nm RMS, even after 600 nm of growth. As previously reported on lower offcuts, we observe reversal of substrate twin domains around the (001) direction at the substrate-epilayer interface. This increase in growth rate, along with careful control of impurity levels, leads to record-low (by MBE) unintentional doping densities of < 2E15 cm^-3 on 13.4° offcut wafers and < 5E15 cm^-3 on 11.1° offcut wafers. We will also demonstrate (Al_xGa_1-x)₂O₃ films grown on highly offcut substrates are monoclinic up to x = 0.33. This work establishes highly offcut (100) β-Ga₂O₃ as a viable and scalable alternative substrate orientation for power electronic devices.

The ultra-wide bandgap (∼ 4.8 eV) and high estimated breakdown field strength (8 MV/cm) of β-Ga₂O₃ make it a promising material for radio frequency applications [1]. One of the main material limitations that hinders device performance is a low maximum room temperature mobility (∼ 200 cm²/Vs) [2]. This is further exacerbated once dopants are introduced into the β-Ga₂O₃ lattice, as scattering from ionized and neutral impurities further reduces the electron mobility [2, 3]. One approach to increase the carrier mobility, while retaining high carrier densities, is to implement modulation doping in a heterostructure. The spatial separation of the impurity atoms from the charge carriers in a triangular quantum well results in the formation of a high mobility 2-dimensional electron gas (2DEG) channel [4, 5].

In this work, we utilize triethylaluminum (TEAl) and triethylgallium (TEGa) for the source of Al and Ga, respectively, to grow β-(Al_xGa_1−x)₂O₃/β-Ga₂O₃ heterostructures by MOCVD. TEAl and TEGa pyrolyze via β-hydrogen elimination [6], enabling minimal carbon incorporation at a relatively low substrate temperature of 650 °C, which is essential for mitigating the formation of compensating gallium vacancies while maintaining high crystalline quality and low surface roughness. As-grown films exhibit room-temperature mobilities as high as 165 cm²/Vs at a sheet density of ∼ 1.8 × 10¹² cm⁻². The films exhibits record low-temperature mobility for a β-(Al_xGa_1−x)₂O₃/β-Ga₂O₃ 2DEG as high ∼ 2914 cm²/Vs at 45 K.

Additionally, from Shubnikov–de Haas (SdH) oscillations below 5 K, we extract a 2DEG density of 1.7 × 10¹² cm⁻², in agreement with transport data. We extract a cyclotron effective mass, m^⋆= 0.308 ± 0.004 m_e, which agrees with the calculated conduction band non-parabolicity in β-(Al_xGa_1−x)₂O₃/β-Ga₂O₃ and the estimated position of E_F above the conduction band minimum [7]. This work affirms the viability of MOCVD using TEAl and TEGa for growing high-quality gallium oxide–based heterostructures.

References

[1] Higashiwaki et al., AAPPS Bull. 32(1), 2022.

[2] Ma et al., Appl. Phys. Lett. 109, 2016.

[3] Jena et al., Oxford Univ. Press, 2022.

[4] Shieh et al., J. Vac. Sci. Technol. B 12, 1994.

[5] Kudo et al., Jpn. J. Appl. Phys. 33, 1994.

[6] Smith et al., J. Inorg. Nucl. Chem. 29(3), 1967.

[7] Peelaers et al., Appl. Phys. Lett. 111, 2017.