Crystallinity Evolution of MOCVD-Grown $β$-Ga$_2$O$_3$ Films Probed by In Situ HT-XRD under Different Reactor Heights

The crystallinity of $β$-Ga$_2$O$_3$ thin films grown by metal-organic chemical vapor deposition (MOCVD) is strongly influenced by reactor design and the resulting growth environment. In this work, we investigate the role of reactor height on the crystallinity evolution of MOCVD-grown $β$-Ga$_2$O$_3$ films by directly comparing long- and short-chamber showerhead configurations. Structural evolution was probed by in situ high-temperature X-ray diffraction (HT-XRD) as the MOCVD-grown films were heated from 25~$^\circ$C to 1100~$^\circ$C. Temperature-dependent XRD reveals a consistent redshift of the $β$-Ga$_2$O$_3$($-201$) reflection after HT-XRD heating and subsequent cooling to room temperature for both reactor geometries, indicating a similar thermally driven strain response. Quantitative rocking-curve analysis shows a non-monotonic temperature dependence of the ($-201$) full width at half maximum (FWHM), with minimum values of approximately 2.03$^\circ$ and 2.72$^\circ$ for the short- and long-chamber films, respectively, reflecting differences in mosaic alignment established during growth. Atomic force microscopy further shows that short-chamber-grown films exhibit smoother surfaces, with root-mean-square roughness values of approximately 7.7nm before and 7.3nm after HT-XRD heating, compared to 19.3nm and 12.3~nm, respectively, for long-chamber-grown films. Overall, these results indicate that reactor height influences the initial crystalline and morphological templates of $β$-Ga$_2$O$_3$ films and modulates their elevated-temperature structural response, providing practical insights for optimizing MOCVD reactor design for high-quality $β$-Ga$_2$O$_3$ growth.

💡 Research Summary

This paper investigates how the vertical spacing between the showerhead and substrate—referred to as the reactor ceiling height—in a metal‑organic chemical vapor deposition (MOCVD) system influences the crystalline quality and surface morphology of β‑Ga₂O₃ thin films. Two showerhead‑type reactors were fabricated with markedly different ceiling heights: a short‑chamber configuration (≈136.5 mm) and a long‑chamber configuration (≈195.3 mm). All other growth parameters were held constant: α‑Al₂O₃ (0001) substrates, tri‑ethyl‑gallium (TEGa) and O₂ precursors, N₂ carrier gas, growth temperature 850 °C, chamber pressure 60 Torr, susceptor rotation 150 rpm, and a 40‑minute deposition time.

The authors first derive the relationship between ceiling height and key transport quantities—gas residence time (τ_res) and diffusion length (L_D)—using established mass‑transport models. A larger ceiling height increases the effective reactor volume, thereby lengthening τ_res and L_D, which promotes gas‑phase reactions and precursor depletion before reaching the substrate. Conversely, a reduced height shortens τ_res, suppresses parasitic gas‑phase chemistry, and enhances the flux of active species to the growth surface, albeit at the cost of reduced homogenization across the wafer.

To probe the structural response of the films, in‑situ high‑temperature X‑ray diffraction (HT‑XRD) was performed while heating the samples from 25 °C to 1100 °C in air, followed by cooling to room temperature. Both reactor geometries displayed a consistent red‑shift of the β‑Ga₂O₃ (‑201) reflection after the thermal cycle, indicating an increase in the out‑of‑plane lattice spacing that the authors attribute to partial relaxation of growth‑induced strain. No new phases appeared, and peak intensities remained essentially unchanged, confirming phase stability throughout the heating sequence.

Rocking‑curve (ω‑scan) analysis revealed a non‑monotonic temperature dependence of the full width at half maximum (FWHM) of the (‑201) peak. The short‑chamber film achieved a minimum FWHM of ~2.03°, whereas the long‑chamber film’s minimum was ~2.72°. The lower FWHM for the short‑chamber sample suggests that the initial mosaic spread was smaller, likely because the shorter residence time limited precursor depletion and allowed more uniform nucleation. As temperature increased, the FWHM first decreased (reflecting strain relaxation and defect re‑configuration) and then rose again at higher temperatures, possibly due to thermally activated defect generation or lattice distortion.

Surface morphology was examined by atomic force microscopy (AFM) over 5 × 5 µm² areas. Prior to HT‑XRD, the short‑chamber film exhibited a root‑mean‑square (RMS) roughness of ~7.7 nm, while the long‑chamber film was considerably rougher at ~19.3 nm. After the high‑temperature cycle, the short‑chamber film’s RMS changed only marginally to ~7.3 nm, indicating that the smoother surface was robust against thermal exposure. The long‑chamber film’s RMS decreased to ~12.3 nm, showing some surface relaxation but still remaining significantly rougher than its short‑chamber counterpart. These observations imply that the shorter ceiling height promotes smoother films, likely through enhanced surface diffusion and more uniform nucleation during growth.

Φ‑scan measurements of the (113) reflection after HT‑XRD showed sixfold periodic peaks for both samples, confirming the presence of multiple in‑plane rotational domains—a known consequence of the monoclinic β‑Ga₂O₃ lattice grown on the hexagonal α‑Al₂O₃ substrate. The high‑temperature treatment did not alter this domain structure, indicating that domain formation is set during the initial epitaxy rather than during post‑growth annealing.

By benchmarking the FWHM and RMS values against literature data for β‑Ga₂O₃ grown on various substrates, the authors demonstrate that the short‑chamber configuration yields films whose crystalline quality and surface smoothness approach those of the best homoepitaxial layers reported, despite being heteroepitaxial on sapphire.

In summary, the study provides compelling evidence that reactor ceiling height is a critical, previously under‑appreciated design parameter in MOCVD of β‑Ga₂O₃. A reduced height shortens gas residence time, limits unwanted gas‑phase reactions, and yields a more favorable initial mosaic template and smoother surface. These advantages persist during subsequent high‑temperature exposure, leading to lower FWHM and minimal surface roughening. The findings offer practical guidance for optimizing MOCVD reactor geometry alongside traditional growth‑parameter tuning, thereby advancing the fabrication of high‑quality ultra‑wide‑bandgap β‑Ga₂O₃ layers for power electronics, UV photodetectors, and radiation‑hard devices.