In recent years, Ga₂O₃ has gained considerable attention in transparent electrodes, solar-blind ultraviolet detectors, flame sensors and power devices^[1-4] thanks to its ultrawide bandgap (4.5–4.9 eV)^[5] and high critical breakdown field (8 MV/cm)^[6]. Monoclinic crystalβ-Ga₂O₃ has the highest thermal stability among the five phases of gallium oxide^[7], and its Baliga's figure of merit (BFOM) is 3.8 and 10.14 times that of the third-generation semiconductors GaN and 4H-SiC, respectively^[8]. Although homogeneous substrates are the ideal choice for growingβ-Ga₂O₃ films, the preparation process for large size gallium oxide wafer is immature and its cost is still expensive when compared with Al₂O₃ substrates. Moreover, the thermal conductivity of gallium oxide is relatively poor, with values of 15.4%, 4.7% and 1.2% for silicon, 4H-SiC and diamond, respectively. Hence, theβ-Ga₂O₃ obtained by heteroepitaxy is important for devices that generate amount of heat during operation.

There are several reports about the growth of high-quality Ga₂O₃ on foreign substrates^[9]. Maet al. tried to grow Ga₂O₃ on substrates such as SrTiO₃(100)^[10], epi-GaN/sapphire (0001)^[11], MgO(111)^[12] and MgAl₆O₁₀^[13] by using MOCVD. However, these thin films were obtained by heteroepitaxy and have multiple crystal domains, which will affect the transport properties of carriers^[14]. Among the different substrates, sapphire substrates are widely studied due to their low cost and smaller lattice mismatch with Ga₂O₃. Substantial effort has been devoted to grow high-quality Ga₂O₃ single crystals on sapphire substrates^[15-20]. However,β-Ga₂O₃ and sapphire belong to different crystal phases, and multiple crystal domains will appear because of random nucleation during growth. Using sapphire substrates with off-axis angle, it was shown that the crystallinity of epitaxialβ-Ga₂O₃ films can be improved by controlling the crystal domains. Oshimaet al. have grown Ga₂O₃ on off-axis substrates by HVPE and found that the (310) oriented domains disappeared when off-axis angles beyond 3°^[21]. It was also found that the off-angled sapphire substrates can change the growth mode ofβ-Ga₂O₃ films from six-fold in-plane rotational domain growth to single quadrilateral-domains growth^[22].

In this paper, we reportβ-Ga₂O₃ epitaxial layers on Al₂O₃ substrates with different off-axis angles. The structural properties ofβ-Ga₂O₃ were studied, and it was found that the surface topography ofβ-Ga₂O₃ films can be greatly improved by using a buffer layer and off-angled sapphire substrates. The ultraviolet to blue light emission related with oxygen and gallium vacancies was observed and the PL mechanism was investigated.

Crystalβ-Ga₂O₃ thin films were grown onc-plane Al₂O₃ substrates (Two-inch, single polished) with different off-angles (0°, 4°, 6° and 8°, corresponding to sample Nos. S0-S3) by using MOCVD (Aixtron Ltd) under the same conditions. For comparison, a Ga₂O₃ film (No. S0*) was also deposited without a buffer layer. Trimethylgallium (TMGa, 6N), which was stored in a stainless-steel bubbler maintained at 5 °C, and O₂ (purity, 5N) were used as precursors for gallium and oxygen, respectively. High-purity argon (Ar, 5N) was applied as the carrier gas to deliver TMGa vapor to the reactor chamber by passing through the TMGa bubbler. Prior to growingβ-Ga₂O₃ thin films at 960 °C with an O₂/TMGa source flow about 2000 sccm/30 sccm (standard cubic centimeter per minute), a 10 nm thick Ga₂O₃ buffer layer was deposited at 800 °C with an O₂/TMGa source flow about 1000 sccm/5 sccm. During the growth process, an excess oxygen source was used to avoid the formation of a large number of oxygen vacancies in the crystal. Finally, the metal organic source and oxygen cracking efficiency reached 9.5% and 0.7‰, respectively. The chamber pressure was maintained at 100 mbar during the entire growth process.

The as-grown film thickness was approximately 300 nm with the growth rate about 0.7μm/h. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) were used to characterize the surface morphology of the samples. HR-XRD and high-resolution transmission electron microscopy (HR-TEM) were carried out to characterize the crystal phase and quality. The room temperature photoluminescence (PL) was measured by using a 213 nm laser as excited source. The PL signal was guided into a monochromator and detected by photomultiplier.

The crystal quality of the gallium oxide thin films grown on sapphire substrates was characterized by HR-XRD in theω–2θ configuration, as shown inFig. 1. For the film grown on sapphire substrate (S0*) without a buffer layer, the XRD spectrum shows totally different pattern. By comparing to PDF #06-0529, the appearance of a diffraction peak centered at 46.43° may belong to the δ-Ga₂O₃ domains. Zhuoet al. reported that when Ga₂O₃ was first deposited on Al₂O₃ substrates, there were nuclei of various phases^[17]. This result also proves that the existence of the buffer layer promoted the growth ofβ-Ga₂O₃. However, for the films grown on Al₂O₃ substrates with a buffer layer, three sharp diffraction peaks located at 18.45°, 38.29° and 58.95° are observed, which belong to the ( 2¯01), ( 4¯02) and ( 6¯03) planes ofβ-Ga₂O₃ (compared to JCPDS No. 43-1012). These results show that the obtained gallium oxide films with a buffer layer are pureβ-Ga₂O₃ without any other gallium oxide phase. The preferential crystal growth direction is ( 2¯01) because the arrangement of oxygen atoms in theβ-Ga₂O₃ ( 2¯01) plane is equivalent to sapphire (0001)^[15].Fig. 2 shows the XRD rocking curves at the ( 2¯01) diffraction peak of the four samples. With the increasing substrates off-axis angles, the XRD peak intensity increases and the full width at half maximum (FWHM) of the ( 2¯01) diffraction peak is decreased, reaching 0.64° for the film deposited on an 8° off-angled substrate. To illustrate the effect of the buffer layer clearly,β-Ga₂O₃ film without a buffer layer was also grown on the Al₂O₃ substrate with 8° off-axis angles toward \lt 112¯0 \gt . Very weak X-ray diffraction peak belong to ( 2¯01), ( 4¯02) and ( 6¯03) was observed and the RMS value was 7.5 nm (the RMS value for the film with a buffer is 1.27 nm), from the AFM measurement. (Figures not shown here.) This indicates that the surface topography ofβ-Ga₂O₃ films grown on off-angled Al₂O₃ substrates can be improved by introducing a buffer layer prior to thick film growth.

To explain the role of buffer layer and mis-cut substrate angle, HR-TEM experiments were carried out. As shown inFig. 3(a), the step is clearly visible along the < 112¯0 \gt direction of the Al₂O₃ substrate. The ( 2¯01) plane distance of the epitaxial layer is calculated to be 0.466 nm by performing a Fourier transform on the region selected by the yellow dashed box inFig. 3(b). As the thickness of the epitaxial layer increases, the atoms are arranged more orderly and most dislocations are successfully constrained in the thin buffer layer.Figs. 3(c) and3(d) show the SAED patterns of buffer layer near and away the Ga₂O₃ films/Al₂O₃ interface, respectively. The SAED pattern consists of multiple sets of diffraction spots near the interface and became more regular in region A. This indicates that there were other phases of Ga₂O₃ in the initial growth period and the epitaxial layer gradually became pureβ-Ga₂O₃ as the film continued to grow. Thus, the great improvement in crystal quality is likely to happen because mis-cut sapphire can inhibit the appearance of the crystal domain by the strong in-plane orientation enhancement^[23] and the buffer layer can accommodate most defects.

Fig. 4 shows the AFM image ofβ-Ga₂O₃ films deposited on different off-axis Al₂O₃ substrates. The measured surface RMS roughness values of samples S0, S1, S2 and S3 are 6, 2.58, 3.55 and 1.27 nm, respectively. The film deposited on an 8° off-axis Al₂O₃ substrate has the lowest RMS roughness. The RMS value in this work is much lower than the reported values grown on off-angled Al₂O₃ substrates^[24]. There are many tiny domains on the surface of the as-grown sample, indicating that the growth changes from a two-dimensional plane growth to a three-dimensional island-like growth pattern. To further reveal the low RMS values of those grown on off-axis sapphire substrates, the surface morphology ofβ-Ga₂O₃ films was also characterized by SEM, as demonstrated inFig. 5. The surface domain of the films deposited on mis-cut sapphire substrates seems more closely due to step-flow growth^[25] and the film surface shows a corrugated shape, which is the phenomenon of step bunching^[26].

PL spectrum is an effective method to investigate the defects of as-grownβ-Ga₂O₃ thin films.Fig. 6 shows the room temperature (297 K) PL spectra ofβ-Ga₂O₃ thin films excited by a 213 nm laser. Compared with the samples grown on the off-axis substrates (S1, S2 and S3), the luminescence of the sample S0 grown on the normal Al₂O₃ substrate is significantly weaker, which is due to the large number of non-radiative recombination centers generated by the crystal domain interface. The PL spectra have a broad emission band from ultraviolet to blue. The broad emission can be divided into two emission peaks near 365 and 410 nm, as shown inFigs. 6(b)–6(d). Varleyet al. reported that self-trapped holes (STHs) were widespread inβ-Ga₂O₃ and localized mainly on a single O atom in the lattice with the shape characteristic of an O 2p orbital^[27]. The 365 nm peak is the process of radiative recombination between STHs and electrons to form STEs. This process has a strong electron-phonon coupling effect and local lattice distortion can cause the spectral broadening. The blue emission is overlapped at the UV band tail, which has been widely reported and is suggested to be related to donor-acceptor-pair recombination between V_O donor and V_Ga or V_Ga–V_O complex acceptors^[27,28]. The radiative recombination luminescence from the conduction band and valence band ofβ-Ga₂O₃ is not observed in all samples, which is likely to be due to the fast non-radiative transition to the self-trapped energy level. As shown inFig. 6(a), the PL intensity is the strongest for the S1 sample and decreases with increasing substrate off-axis angles. Thus, the decrease in PL intensity indicates that the density of O and Ga vacancies are reduced. This is in accordance with the improved crystal quality from the XRD measurement.

In summary,β-Ga₂O₃ thin films were successfully grown onc-plane mis-cut Al₂O₃ substrates. Our results showed that the quality of theβ-Ga₂O₃ thin films was improved by step-flow growth, and the FWHM of the ( 2¯01) plane of the film deposited on an 8° off-angled substrate was 0.64°. It was revealed that the dislocations were mainly blocked by the buffer as observed by TEM measurement. The surface RMS roughness can also be reduced for theβ-Ga₂O₃ films deposited on off-axis Al₂O₃ substrates, reaching about 1.27 nm. Theβ-Ga₂O₃ film had broad light emission from ultraviolet to blue, which was attributed to the oxygen vacancies and gallium vacancies in the films. This vacancy related PL intensity was decreased with increasing off-angle, showing the improved crystalline quality, which was consistent with the HR-XRD results. These findings are helpful for the fabrication of high performanceβ-Ga₂O₃ devices based on Al₂O₃ substrate in near future.

This work was supported in part by the National Key Research and Development Program of China (Grant No. 2018YFB2200500), the National Natural Science Foundation (Grant Nos. 62050073, 62090054, 61975196).