Heteroepitaxial Growth of α‑Ga₂O₃ by MOCVD on a‑, m‑, r‑, and c‑Planes of Sapphire

Abstract

Ga₂O₃ thin films were deposited simultaneously on (112̅0) a-plane, (101̅0) m-plane, (0001) c-plane, and (011̅2) r-plane sapphire substrates using metal–organic chemical vapor deposition (MOCVD) and characterized by X-ray diffraction (XRD) and atomic force microscopy (AFM). The different surface energy and strain conditions imposed by each sapphire plane make the choice of substrate orientation critical to the stabilization of the α-phase. β-Ga₂O₃ nucleation was found to be preferential over α-Ga₂O₃ on sapphire orientations with <11̅00> α-Al₂O₃ present (c- and a-planes) when grown under the same conditions. In contrast, α-Ga₂O₃ is preferred during the initial stages of growth on the r- and m-plane, although suppression of island growth is required to prevent the formation of inclined facets on which β-Ga₂O₃ might nucleate. Transmission electron microscopy (TEM) provided a direct confirmation of this growth for r-plane substrates. Classical nucleation theory was applied to rationalize these observations and guide the search for the growth window of α-Ga₂O₃. As a result, decreasing the VI/III ratio and increasing the TEGa flow rate were found to be effective in realizing phase-pure α-Ga₂O₃ on a-plane sapphire by MOCVD with good structural quality (62 arcsec full width half-maxima of X-ray rocking curve), though the equivalent growth on c-plane substrates yielded mixed-phase β- and κ-Ga₂O₃another metastable phase of Ga₂O₃, instead. Growth on the m-plane resulted in the smoothest surface morphology and thickest phase-pure α-Ga₂O₃ film, indicating that it is the most promising substrate orientation for future device manufacturing.

Introduction

Corundum α-Ga₂O₃ (space group R3̅c) boasts the widest bandgap of all Ga₂O₃ polymorphs (5.1–5.3 eV) and is isostructural with α-Al₂O₃, commonly known as sapphire. This permits not only the growth of high-quality domain-matched α-Ga₂O₃ layers on economical substrates (sapphire being available at significantly lower cost than native Ga₂O₃ substrates used for high-quality β-Ga₂O₃ growth) but also extensive bandgap engineering by alloying with Al over the full composition range from x = 0 to 1 in α-(Al _x Ga_1–x )₂O₃, leading to a bandgap range of 5.1–8.8 eV. − Such compositional control is generally more difficult in the monoclinic phase, where β-(Al _x Ga_1–x )₂O₃ films typically suffer from local segregation of Al and Ga (e.g., on (001) and (2̅01) β-Ga₂O₃ substrates) , or phase segregation of β- and γ-Ga₂O₃ (e.g., on (010) β-Ga₂O₃ substrates) , toward high Al content. Up to 99% Al composition β-(AlGa)₂O₃ films have been achieved on (100) β-Ga₂O₃ substrates, but the film quality degrades with increasing Al content. In contrast, α-(AlGa)₂O₃ films do not degrade or even improve in crystallinity toward Al-rich regimes. , Finally, it is worth highlighting that, similar to β-Ga₂O₃, n-type conductivity of α-Ga₂O₃ films can be achieved by Si, − Sn, , or Ge doping, , with controllable carrier density over the range of 10¹⁷–10¹⁹ cm^–3 and electron mobility up to 98.7 cm² V^–1 s^–1 having been reported. These qualities make α-Ga₂O₃ an attractive material for the fabrication of high-breakdown power devices like Schottky diodes, ,, field-effect transistors, , and optoelectronic devices such as solar blind photodetectors.

Despite these advantages, the growth of α-Ga₂O₃ is challenging because of its metastability. Thin films of phase-pure α-Ga₂O₃ can transform into the most thermodynamically stable polymorph β-Ga₂O₃, typically at temperatures above 600–650 °C. − Up to now, mist chemical vapor deposition (mist-CVD) ,,, and halide vapor phase epitaxy (HVPE) − have proven to be the most effective methods capable of realizing thick layers of α-Ga₂O₃ on c-plane sapphire; this might be due to the presence of HCl that acts as a catalyst and aids the formation of α-Ga₂O₃. For epitaxial techniques without HCl such as pulsed laser deposition (PLD), molecular beam epitaxy (MBE), or standard metal organic chemical vapor deposition (MOCVD), Schewski et al. found that only a pseudomorphic interlayer of α-Ga₂O₃ could be stabilized on c-plane sapphire before transitioning to (2̅01) β-Ga₂O₃ as soon as the film relaxes. Although only a few monolayers of α-Ga₂O₃ were stabilized, this study demonstrated that substrate-imposed strain conditions are a key factor in the stabilization of α-Ga₂O₃. A later study by Schowalter et al. showed that the thin pseudomorphic interlayer is actually α-(AlGa)₂O₃ with ∼65% Al, rather than pure α-Ga₂O₃ as previously assumed, suggesting that Ga–Al interdiffusion between the film and substrate may occur during its formation. Nonetheless, the influence of epitaxial stress and strain on the Ga₂O₃ phase selection remains significant and cannot be overlooked. Hence, a-, m-, and r-plane-oriented sapphire were subsequently investigated by other authors as a growth substrate for α-Ga₂O₃. − Nevertheless, due to different growth kinetics across the range of epitaxial techniques reported in the literature, it is not fully clear how sapphire orientation alone affects the assumed phase of Ga₂O₃. For instance, in MBE growth on m-plane sapphire, it was found that β-Ga₂O₃ started to grow on a-plane facets of α-(InGa)₂O₃ film as the thickness exceeded 50 nm. In contrast, a recent MOCVD study reported that growth on m-plane yields mixed-phase α and β at 100 nm thickness, attributed to β-Ga₂O₃ growing on a-plane facets on the α-Ga₂O₃ layer. In PLD growth, using the same substrate orientation, phase-pure α-Ga₂O₃ films are only possible up to 220 nm before β-Ga₂O₃ started to nucleate on c-plane facets instead.

In this work, to unambiguously determine the influence of sapphire substrate orientation on Ga₂O₃ epitaxy by MOCVD, which is the industry-preferred growth technique, growth must be performed simultaneously on all orientations in the same reactor. The following four sapphire orientations were studied: (112̅0) a-plane, (101̅0) m-plane, (0001) c-plane, and (011̅2) r-plane. Systematic investigation of Ga₂O₃ crystallographic structure, material quality, and growth mode for each sapphire substrate orientation provides crucial insight toward achieving high-quality α-Ga₂O₃ growth by MOCVD.

Experimental Details

Thin films of Ga₂O₃ were simultaneously grown on (11̅20) a-plane, (101̅0) m-plane, (0001) c-plane, and (011̅2) r-plane sapphire substrates using an Agnitron Agilis 100 MOCVD reactor. All sapphire substrates were diced into 10 × 7.5 mm² rectangular samples and cleaned using solvents, buffered oxide etch, and a 30 min DI water rinse with ultrasonification before being loaded into the reactor chamber. Triethylgallium (TEGa) and high-purity O₂ gas (99.9999%) were used as Ga and O precursors with argon (Ar) carrier gas. TEGa flow rate, VI/III ratio, reactor pressure, and growth temperature were held constant at 19.92 μmol/min, 450, 20 Torr, and 500 °C, unless stated otherwise. The surface morphology of the resulting Ga₂O₃ layers was characterized using tapping mode atomic force microscopy (AFM), and crystallographic characteristics (crystal structure, quality, and orientation) were studied by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Layer thicknesses were measured by X-ray refractory (XRR) or optical reflectometry. Reflectometry data were fitted using α- and β-Ga₂O₃ refractive indices reported by Johnston et al. All XRD and XRR measurements were performed using a Philips X’pert diffractometer with a Cu Kα source.

Results and Discussion

Growth time was varied between 2 and 14 min to produce sets of Ga₂O₃ films of various thicknesses on the a-, m-, c-, and r-plane. Figure shows XRD 2θ-ω scans of all 16 samples grown. In each growth, the thicknesses of the layers were measured to be approximately 13, 20, 50, and 100 nm for all four substrates, i.e., the average growth rate (∼430 nm/h) was found to be independent of crystal orientation. We denote the 16 films as X₁–X₄, where X indicates the substrate orientation (A: a-plane, M: m-plane, C: c-plane, and R: r-plane), and the subscript corresponds to the film thickness (1:13 nm, 2:20 nm, 3:50 nm, and 4:100 nm). For example, A₃ refers to the film grown on a-plane sapphire with a 50 nm thickness.

1.

XRD scans of 13–100 nm thick Ga₂O₃ films deposited on (a) c-plane sapphire, (b) a-plane sapphire, (c) r-plane sapphire (skew-symmetric geometry with azimuth aligned to [211̅0], tilt ∼25°), and (d) m-plane sapphire substrates, under the same growth conditions of T = 500 °C, TEGa ∼20 μmol/min, VI/III = 450, and p = 20 Torr. The inset of (c) shows XRD scans of the same r-plane samples in symmetric geometry. Note that a few spectra show aluminum peaks at 2θ = 38.2°, 44.5°, and 82.3° arising from the diffractometer stage on which samples were directly placed during the scan.

For growth on the c-plane (0001) sapphire (Figure a), Ga₂O₃ nucleated as the β-phase and continued to grow into pure β-Ga₂O₃ films with increasing growth time: only {2̅01} β-Ga₂O₃ peaks (2θ = 18.9°, 38.3°, and 59.1°) were observed. For growth on a-plane sapphire (11̅20), shown in Figure b, a mixture of α- and β-Ga₂O₃ was detected, as indicated by the presence of both (112̅0) α-Ga₂O₃ (2θ = 36.0°) and (2̅01) β-Ga₂O₃ peaks in Figure b. As the film grows, the intensity of the (2̅01) β-Ga₂O₃ peak increases at a faster rate than the (112̅0) α-Ga₂O₃ peakthe latter remaining roughly constant as the film thickness exceeded 50 nm, indicating that growth is predominantly β-Ga₂O₃ for thicker films.

For layers grown on r-plane (011̅2) sapphire, the (022̅4) α-Ga₂O₃ peak at 2θ = 50.2° in the symmetric scan (zero tilt) was always present (Figure c inset), but from 50 nm thickness onward, the (2̅01) β-Ga₂O₃ peak begins to appear in the skew-symmetric scan (25° tilt) aligned to a {112̅3} plane of the sapphire (Figure c). Note that the (112̅3) α-Ga₂O₃ reflection has a lower relative intensity than the (022̅4) α-Ga₂O₃ reflection due to the former’s weaker diffracting power. Likewise, (4̅02) β-Ga₂O₃ is not visible in Figure c because of a weaker diffracting power than that of (2̅01) β-Ga₂O₃, which already has a low intensity. These XRD observations indicate that the film on the r-plane begins as predominantly α-Ga₂O₃ and becomes a mixed phase at some point during epitaxial growth, likely due to β-Ga₂O₃ growing on exposed facets of α-Ga₂O₃ growth islands. This can occur via (2̅01) β-Ga₂O₃ nucleating on {112̅3} (n-plane) facets of α-Ga₂O₃, but not exclusively, as later transmission electron microscopy (TEM) analysis revealed the presence of additional orientations of the β-Ga₂O₃ crystallite. So far, we have assumed that no bulk α → β transition took place within the initial few monolayers during growth, which is reasonable considering that the growth temperature was fixed at 500 °Cwell within the stability limit of α-Ga₂O₃. Such a transformation would also heavily suppress the (011̅2) α-Ga₂O₃ peaks, which was not observed. In fact, the (022̅4) α-Ga₂O₃ peak increases in intensity as the film grows thicker. Note that since (2̅01) β-Ga₂O₃ growing on n-plane facets does not result in a detectable peak in symmetric geometry, caution must be exercised when determining the phase composition of Ga₂O₃ grown on the r-plane. In addition, as previously discussed, a similar growth mode was also observed in MBE growth where the (2̅01) β-Ga₂O₃ reflections were found on c-plane facets instead.

Finally, only growth on the m-plane (101̅0) sapphire resulted in phase-pure α-Ga₂O₃ films. The peak attributed to (303̅0) α-Ga₂O₃ at 2θ = 64.8° can be seen in Figure d. In terms of α-Ga₂O₃ crystal quality, the full width at half-maximum (fwhm) of the ω-scan rocking curve was measured to be 0.42° for 50 nm thick layers on the a-plane (at (112̅0)), 0.73° for the m-plane (at (303̅0)), and 1.18° for the r-plane (at (022̅4)).

Investigation of surface morphology (imaged by AFM, Figure ) of Ga₂O₃ grown on a-, m-, c-, and r-plane sapphire substrates reveals that from 13 nm layer thickness onward, growth proceeds through 3D islands for c-, a-, and r-plane substrates. On average, the growth island size on c- and a-planes (24–26 nm wide at 13 nm thickness) is larger than the island size on m- and r-planes (15–16 nm wide at 13 nm thickness). The increasing size and height of the growth islands manifest as a monotonic increase in surface roughness with film thickness (Figure ). Notably, the morphological evolution of the film on c- and a-planes is quite similar, with almost identical surface roughening rates as a function of deposited thickness and island sizes. This correlates with the dominance of β-Ga₂O₃ growth on these substrate orientations, especially after 50 nm for the a-plane as shown in the XRD results in Figure . In contrast, for the r-plane, although the growth started with the nucleation of α-Ga₂O₃ nanoislands, as the film exceeds 50 nm, large islands began to emerge on the surface of the samples grown on r-plane sapphire (Figure k); these subsequently grow into elongated islands running along one direction (Figure l), resulting in a distinct increase in surface roughness. It is speculated that these elongated islands are a result of β-Ga₂O₃ nucleating on inclined facets of α-Ga₂O₃ islands as previously discussed, since the emergence of these features coincides with the onset of (2̅01) β-Ga₂O₃ peaks in the skew-symmetric XRD scan of the r-plane, seen in Figure c. Interestingly, at higher growth temperatures only (650 °C), a similar growth mode can be observed on the m-plane, which will be discussed below.

2.

AFM scans (2 × 2 μm² area) of Ga₂O₃ films grown on (a–d) a-plane (films A₁–A₄), (e–h) c-plane (films C₁–C₄), (i–l) r-plane (films R₁–R₄), and (m–p) m-plane (films M₁–M₄) sapphire substrates, with thicknesses between 13 and 100 nm (T _gr = 500 °C). Note that film names have the format X _i , where X indicates the substrate orientation (A: a-plane, M: m-plane, C: c-plane, and R: r-plane), and the subscript i corresponds to the film thickness (1:13 nm, 2:20 nm, 3:50 nm, and 4:100 nm). The surface roughness of all 16 samples is plotted in Figure .

3.

Root-mean-square (RMS) surface roughness of 13–100 nm thick Ga₂O₃ films grown on a-, m-, c-, and r-planes (T _gr = 500 °C).

For growth on m-plane substrates (Figure m–p) at 500 °C, before reaching 20 nm film thickness, it appears that the surface fully coalesces with subnanometer surface roughness. When the epilayer reaches 50 nm thickness, however, new growth hillocks (40–80 nm wide) emerge and are sparsely spaced-out on a very smooth coalesced surface of RMS roughness ∼0.6 nm (Figure o), with RMS roughness comparable to the film at a thickness of 20 nm (Figure n). These islands subsequently expand to 70–130 nm width (Figure p). Hence, it is most likely that the growth mode of α-Ga₂O₃ on the m-plane is layer + island (Stranski-Krastanov), consistent with observations reported by Li et al. and Jinno and Okumura. Compared to the Ga₂O₃ films grown at 500 °C discussed so far, Figure shows the XRD scan of m-plane Ga₂O₃ films grown at a higher growth temperature of 650 °C and their respective AFM scans. As the film grows past 60 nm, XRD 2θ peaks associated with β-Ga₂O₃ (2θ = 60.9°) begin to appear near the K_β sapphire peak (Figure a), coinciding with the emergence of elongated islands running along [0001] in AFM scans (Figure c,d). Since similar surface features and XRD peaks have been observed in growth by PLD, they have been attributed to (2̅01) β-Ga₂O₃ nucleating onto c-plane facets of α-Ga₂O₃ growth islands, which act as nucleation sites for β-Ga₂O₃ at higher growth temperatures only.

4.

(a) XRD scans of 50–100 nm thick Ga₂O₃ films grown on m-plane sapphire at 650 °C. AFM scan over a 5 × 5 μm² area of the (b) 50 nm thick, (c) 60 nm thick, and (d) 100 nm thick film.

The in-plane crystallographic relationships among α-Ga₂O₃, β-Ga₂O₃, and the sapphire were confirmed using XRD skew-symmetric ϕ-scans. In Figure a, every α-Ga₂O₃ peak on the a-, m-, and r-plane is matched to its sapphire equivalent, signifying one single rotational domain. On the other hand, the six peaks over 360° of β-Ga₂O₃ represent six rotational domains. For growth on c-plane sapphire, this is the well-known epitaxial relationship <010>(2̅01) β-Ga₂O₃ || <101̅0>(0001) α-Al₂O₃ as depicted by the model in Figure b, which shows similar hexagonal oxygen arrangement for both (0001) α-Al₂O₃ and (2̅01) β-Ga₂O₃. However, unlike the c-plane, atoms on the a-plane of sapphire do not have 3-fold rotation symmetry. Thus, the epitaxial relationship must be slightly different despite the same number of β-Ga₂O₃ rotational domains. Noting that β-Ga₂O₃ domains are only approximately 60° apart, and that [11̅01] and [11̅01̅] form a 57.6° angle with [11̅00] on either side such that [11̅00] is the bisecting line, the epitaxial relationship is <010>(2̅01) β-Ga₂O₃ || [11̅00]/<11̅01>(112̅0) α-Al₂O₃ (depicted in Figure c). Based on the spacing between neighboring oxygen atoms, the lattice mismatch between β-Ga₂O₃ and a-plane sapphire is approximately 6.5% for <010> β-Ga₂O₃ || [11̅00] α-Al₂O₃ and 12.0% for <010> β-Ga₂O₃ || <11̅01> α-Al₂O₃.

5.

(a) Skew-symmetric ϕ-scans of Ga₂O₃ films on a-, m-, c-, and r-plane substrates. Visual model of the oxygen atom arrangement of (2̅01) β-Ga₂O₃ overlaid on that of (b) c-plane sapphire and (c) a-plane sapphire. Red spheres represent oxygen atoms on sapphire while blue ones represent oxygen atoms on β-Ga₂O₃ (generated with Visualization for Electronic and Structural Analysis (VESTA) software).

Our observations so far suggest that there are two general cases of Ga₂O₃ thin film growth on sapphire substrates (Figure ). In case (a), mixed α- and β- or pure β-grains directly nucleate on the substrate on which there are atoms aligned along <11̅00> α-Al₂O₃ (e.g., c- and a-planes). Eventually, β-grains of Ga₂O₃ grow over α-grains, and β-Ga₂O₃ becomes the only phase growing. The proposed growth mode of case (a) substrates is consistent with transmission electron microscopy (TEM) observations reported by various authors. , In contrast, in case (b), for sapphire planes which do not have atoms aligned along a <11̅00>, such as m-plane and r-plane substrates, a layer of phase-pure α-Ga₂O₃ is stabilized first. However, if growth proceeds via 3D islands, as the α-Ga₂O₃ film develops, the β-phase can start nucleating on inclined facets of α-Ga₂O₃ islands, resulting in significant surface roughening. This occurs after a certain thickness only, since larger grains provide more suitable facets for β-Ga₂O₃ nucleation. Hence, suppressing island growth and achieving a smooth surface morphology will be crucial to growing phase-pure α-Ga₂O₃ on case (b) substrate orientations by MOCVD. For m-plane substrates, such a growth mode has been consistently demonstrated across a range of epitaxy methods. − However, as previously noted, the specific facet that emerges, which is likely governed by growth kinetics, might vary between different methods.

6.

Schematic diagram of the structure evolution of the Ga₂O₃ epilayer in two cases: (a) In-plane <11̅00> α-Al₂O₃ is present. β-Ga₂O₃ can grow directly on the substrate. (b) In-plane <11̅00> α-Al₂O₃ is not present. α-Ga₂O₃ grows first, but β-Ga₂O₃ can nucleate through growth island facets.

To confirm that the structural evolution of Ga₂O₃ films grown on r-plane substrates is consistent with the proposed growth mode of case (b) substrates, transmission electron microscopy (TEM) images of sample R₃ (T _gr = 500 °C, t = 50 nm) are shown in Figure . A sharp film–substrate interface can be clearly observed in both low- and high-magnification images of Figure a,b. The high-resolution TEM (HR-TEM) image of the film in Figure b reveals two distinct regions: a lower α-Ga₂O₃ region with similar lattice spacing (d-spacing) to the substrate and an upper β-Ga₂O₃ region with larger d-spacing, consistent with the fact that the β-phase has lower density. While the precise α-/β-phase boundary is somewhat indistinct, possibly due to being slightly off edge-on orientation, the β-phase appears to nucleate on top of inclined facets of α-phase crystallites rather than directly on sapphire, appearing as (200) β-Ga₂O₃ lattice planes (with a d-spacing of approximately 0.6 nm) oriented ∼15° to the (011̅2) α-Ga₂O₃ planes in Figure b. Note that the dotted white line depicting the boundary in Figure b is an approximation since the phase boundary may not be sharply defined: phase overlap and deviation from the line are possible. Local fast Fourier transforms (FFTs) of the substrate and regions where α- and β-phases exist separately further confirm the identity and relative orientation of the crystals. Both α-Ga₂O₃ and sapphire show diffraction patterns (DPs) consistent with the [211̅0] zone axis of corundum crystals (Figure d,e). The FFT of the β-Ga₂O₃ region (Figure c) is consistent with the diffraction pattern (DP) of the [010] zone axis. Notably, the Bragg spots of (200) β-Ga₂O₃ are misaligned by ∼15° relative to the (011̅2) spots of the corundum crystals, which explains why the β-crystallite observed in Figure b is not visible in standard symmetric 2θ-ω scans. Therefore, TEM analysis of the film grown on the r-plane substrate shows that, in addition to (2̅01) β-Ga₂O₃ growing on (112̅3) α-Ga₂O₃ facets found in XRD, β-Ga₂O₃ can also grow as (200) β-Ga₂O₃ on inclined α-Ga₂O₃ facets. Since in the DPs in Figure c,d, {202̅} β-Ga₂O₃ and {01̅4} α-Ga₂O₃ Bragg spots are approximately aligned on the same axis, we speculate that the crystallite consisting of (200) β-Ga₂O₃ planes (in Figure b) arises from the similar d-spacings of {202̅} β-Ga₂O₃ planes (d-spacing ∼2.8 Å) and {01̅4} α-Ga₂O₃ planes (d-spacing ∼2.6 Å). Nonetheless, a more detailed TEM investigation will be needed to exhaustively categorize β-phase grains and fully explain their origins on r-plane sapphire. Current TEM and XRD analyses collectively support the proposed structural evolution, wherein β-Ga₂O₃ nucleates on inclined α-Ga₂O₃ facets during growth and not directly on the r-plane-oriented substrates (i.e., Figure case (b) substrates).

7.

(a) Cross-sectional bright-field (TEM) micrograph of sample R₃ (T _gr = 500 °C, t = 50 nm). (b) HR-TEM image of a magnified region of sample R₃. Dotted white lines indicate the approximate locations of α-Ga₂O₃/sapphire and α-/β-Ga₂O₃ interfaces, inferred by contrast and local diffraction pattern. Fast Fourier transforms (FFT) of selected regions in (b) are shown for (c) β-Ga₂O₃ region (dotted yellow square), corresponding to the [010] zone axis; (d) α-Ga₂O₃ region (dotted green square), corresponding to the [211̅0] zone axis; and (e) sapphire substrate (dotted red square), corresponding to the [211̅0] zone axis. Reference simulated DPs are provided in Supporting Information.

Overall, under the tested growth conditions thus far, we observed that corundum planes belonging in case (b) tend to favor the growth of α-Ga₂O₃, stabilizing at least an initial corundum-phase layer. In particular, the m-plane was found to be most conducive to achieving phase-pure α-Ga₂O₃ films. In contrast, planes belonging in case (a) tend to have undesirable direct nucleation of β-crystallites. Since growth kinetics were the same on each substrate, the variance in the phase composition evolution of the films must arise chiefly from different strain conditions imposed on the epilayer. Indeed, the growth of α-Ga₂O₃ on lattice-matched sapphire is an example of epitaxy-induced phase transformation, in which in-plain epitaxial strain experienced by very thin epilayers modifies the ground state energy such that typically metastable phases, such as α-Ga₂O₃, become the most stable. The key difference between the substrates in case (a) and case (b) appears to be the presence (or lack thereof) of atoms aligned along <11̅00> in the in-plane direction. To qualitatively understand why sapphire substrates with <11̅00> in-plane (i.e., case (a)) often favors the growth of β-Ga₂O₃ over that of α-Ga₂O₃, let us consider the heterogeneous nucleation of a 3D Ga₂O₃ island. Adopting a similar argument outlined by Jesser, the Gibbs free energy difference ΔG _β–α between a spherical β- and α-nucleates of the same radius R is

$$\mathrm{\Delta }{G}_{\beta -\alpha }=\mathrm{\Delta }{G}_{\beta }-\mathrm{\Delta }{G}_{\alpha }=\frac{2}{3}\pi R^3\mathrm{\Delta }{G}_{\mathrm{V}}+2\pi R^2{\gamma }_{\beta -\alpha }+\pi R^2{\sigma }_{\mathrm{m},\beta -\alpha }+E_{\mathrm{s},\beta -\alpha }$$ 1

The nucleation of α-Ga₂O₃ is favored if ΔG _β–α > 0. The first term of eq , which accounts for the difference in condensation energy (per unit volume) ΔG _V between β- and α-nucleates, is independent of substrate orientation and always negative since β is the most stable phase. The second term accounts for the difference in surface energies γ_β‑α. Assuming that the four corundum planes considered in this study compete with common monoclinic planes such as (2̅01) and (100), density functional theory calculations suggested that γ_β‑α is most likely negative for all four substrates. The third term accounts for the difference in misfit dislocation energy σ _m,β–α. During the very initial stages of growth, this term is negligible because islands can be fully strained. The last term, which accounts for the difference in strain energy E _s,β–α, is the only positive term since α-Ga₂O₃ is lattice-matched to sapphire. Hence, if the magnitude of E _s,β–α is greater than that of $\frac{2}{3}\pi R^3\mathrm{\Delta }{G}_{\mathrm{V}}+2\pi R^2{\gamma }_{\beta -\alpha }$ , α-Ga₂O₃ growth should be initially favored. Past studies have estimated that, if the nearest-neighbor mismatch between the stable phase (β-Ga₂O₃) and the substrate (α-Al₂O₃) is much larger (greater than 10–20%) compared to the mismatch between the metastable phase (α-Ga₂O₃) and the substrate (ideally as close to zero as possible), then growth islands forming on the substrate prefer to assume the metastable phase. , However, for sapphire orientations in which the in-plane epitaxial relationship <010> β-Ga₂O₃ || <101̅0> α-Al₂O₃ is possible, such as c-plane, a-plane, or n-plane, the mismatch between α-Ga₂O₃ and sapphire is comparable to the mismatch between β-Ga₂O₃ and sapphire. Based on the arrangement of oxygen atoms, the distance between nearest neighboring atoms along [11̅00] is 3.02 Å for α-Ga₂O₃ and 2.86 Å for α-Al₂O₃, resulting in a mismatch of 5.6%. In comparison, the mismatch of <010> β-Ga₂O₃ || < 11̅00> α-Al₂O₃ is 6.5%. This is far from the ideal case considered in theory. As such, to minimize free energy, growth islands prefer to assume the β-phase: the energy cost of inducing more in-plane strain in the system is lower than the cost of assuming the α-phase.

Nonetheless, according to eq , it is possible to enhance the nucleation of the metastable α-Ga₂O₃ over β for case (a) substrates. Solving for R by setting $\frac{\partial (\mathrm{\Delta }{G}_{\beta -\alpha })}{\partial R}=0$ gives a constant critical radius R* above which growth islands will exclusively prefer the stable phase over the metastable phase. This signifies that, to successfully grow metastable polymorphs, the island size must be kept below R*. In MOCVD growth, the island size can be reduced by increasing Ga supersaturation via a combination of higher TEGa flow rate and lower VI/III ratio. This theory was tested for a-plane and c-plane substrates using a TEGa flow rate of ∼23 μmol/min (15% increase from 20 μmol/min), a VI/III ratio of 200 (125% reduction from 450), and a total pressure of 10 Torr (halved from 20 Torr) while maintaining the growth temperature at 500 °C. Note that pressure was reduced here to minimize the probability of homogeneous β-Ga₂O₃ nucleation from the gas phase, which can occur under high TEGa flow, despite higher chamber pressure increasing supersaturation. The XRD 2θ-ω scan of the resulting 70 nm thick Ga₂O₃ film on a-plane sapphire (Figure a) shows peaks arising from α-Ga₂O₃ and the substrate only without any {2̅01} β-Ga₂O₃ peaks. In terms of crystallinity, ω-scan rocking curves measured at the (112̅0) α-Ga₂O₃ peak show excellent quality, achieving a fwhm of 62 arcsec, comparable to α-Ga₂O₃ grown on the c-plane by mist-CVD. , This marked improvement is owed to the film being phase-pure α-Ga₂O₃, unlike the mixed-phase film obtained using the initial growth conditions. However, phase-pure α-Ga₂O₃ growth on a c-plane substrate still proves challenging. Under the same conditions for which phase-pure α-Ga₂O₃ was grown on a-plane sapphire, the film grown on the c-plane is mixed-phase β and κ-Ga₂O₃ instead. In the XRD 2θ-ω scan in Figure b, a new set of peaks appear to the right of the {2̅01} β-Ga₂O₃ peaks, which can be attributed to κ-Ga₂O₃. It is difficult to distinguish the first set of {2̅01} β-Ga₂O₃ and {002}­κ-Ga₂O₃ peaks around 2θ = 19°, but the separation is more noticeable toward higher 2θ angles, for example, (6̅03) β-Ga₂O₃ at 2θ = 59.1° versus (006) κ-Ga₂O₃ at 2θ = 59.8°. Nonetheless, our results demonstrate that increasing TEGa flow rate and decreasing VI/III ratio (with preferably low chamber pressure) can favor the growth of metastable Ga₂O₃ (α- or κ-) on substrates with <11̅00> α-Al₂O₃ present (tested for c- and a-plane substrates), in agreement with nucleation theory of metastable polymorphs. In addition, our observations for growth on c-plane substrates are consistent with a previous growth MOCVD study, in which κ-Ga₂O₃ was stabilized on c-plane sapphire by lowering VI/III ratios, and similar diffraction patterns were observed. Interestingly, pushing growth conditions into metal-rich regimes (i.e., low VI/III ratio for MOCVD) seems to favor the growth metastable polymorphs of Ga₂O₃ (α- or κ-) on sapphire substrates with <11̅00> α-Al₂O₃ for MBE , and PLD as well.

8.

XRD scans of Ga₂O₃ films grown on (a) a-plane sapphire (with the inset showing the ω-scan rocking curve of the (112̅0) α-Ga₂O₃ peak) and (b) c-plane sapphire under the same growth conditions of T = 500 °C, TEGa∼ 23 μmol/min, VI/III = 200, and p = 10 Torr. A Ge(220) 4-bounce monochromator was used to obtain these scans.

Conclusions

By performing simultaneous growth of Ga₂O₃ using MOCVD, on a-, m-, c-, and r-plane sapphire substrates, and systematically characterizing the resulting Ga₂O₃ films by XRD and AFM, we have shown that the structural evolution of the Ga₂O₃ film depends on the presence of <11̅00> α-Al₂O₃ in the substrate. For substrate orientations with <11̅00> α-Al₂O₃, such as c- and a-planes, β-Ga₂O₃ tends to directly nucleate at the start of growth, making the film either phase-pure β-Ga₂O₃ or mixed-phase α- and β-Ga₂O₃. On the other hand, for substrate orientations without <11̅00> α-Al₂O₃, such as m- and r-planes, α-Ga₂O₃ tends to nucleate first without any sizable β-Ga₂O₃ grains present, but 3D island growth needs to be suppressed to prevent the emergence of facets that act as nucleation sites for β-crystallites. Such a growth mode was directly observed for growth on r-plane substrates via TEM. Despite differences in the growth mechanism, the growth rate of the Ga₂O₃ was found to be mostly independent of the substrate orientation. Finally, classical nucleation theory was adopted to gain insight on the phase selectivity and help identify favorable conditions for α-Ga₂O₃ epitaxy on sapphire substrates with <11̅00> α-Al₂O₃. As a result, we have demonstrated phase-pure α-Ga₂O₃ (with 62 arcsec rocking curve fwhm) can be grown on a-plane substrates by MOCVD under conditions with higher Ga supersaturation, but for c-plane substrates so far, only mixed-phase β- and κ-Ga₂O₃ films could be obtained. These results serve as a valuable foundation for optimizing scalable α-Ga₂O₃ growth toward future device fabrication.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.cgd.5c00183.

• Simulated DPs of the [010] zone of β-Ga₂O₃ and the [211̅0] zone of corundum crystals (PDF)

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

The authors declare no competing financial interest.