1. Introduction
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 crystal β-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 growing β-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, the β-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]. Ma et 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 epitaxial β-Ga2O3 films can be improved by controlling the crystal domains. Oshima et 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 of β-Ga2O3 films from six-fold in-plane rotational domain growth to single quadrilateral-domains growth[22].
In this paper, we report β-Ga2O3 epitaxial layers on Al2O3 substrates with different off-axis angles. The structural properties of β-Ga2O3 were studied, and it was found that the surface topography of β-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.
2. Experimental
2.1 Film growth
Crystal β-Ga2O3 thin films were grown on c-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 growing β-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.
2.2 Characterization method
The as-grown film thickness was approximately 300 nm with the growth rate about 0.7 μm/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.
3. Results and discussion
3.1 Effects of off-axis angles of Al2O3 substrates on crystal quality
The crystal quality of the gallium oxide thin films grown on sapphire substrates was characterized by HR-XRD in the ω–2θ configuration, as shown in Fig. 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. Zhuo et 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 of β-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 (
To explain the role of buffer layer and mis-cut substrate angle, HR-TEM experiments were carried out. As shown in Fig. 3(a), the step is clearly visible along the <

3.2 Effects of off-axis angles of Al2O3 substrates on β-Ga2O3 film surface morphology
Fig. 4 shows the AFM image of β-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 of β-Ga2O3 films was also characterized by SEM, as demonstrated in Fig. 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].
3.3 PL properties of β-Ga2O3 films on off-angled Al2O3 substrates
PL spectrum is an effective method to investigate the defects of as-grown β-Ga2O3 thin films. Fig. 6 shows the room temperature (297 K) PL spectra of β-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 in Figs. 6(b)–6(d). Varley et al. reported that self-trapped holes (STHs) were widespread in β-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 VO donor and VGa or VGa–VO complex acceptors[27, 28]. The radiative recombination luminescence from the conduction band and valence band of β-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 in Fig. 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.
4. Conclusion
In summary, β-Ga2O3 thin films were successfully grown on c-plane mis-cut Al2O3 substrates. Our results showed that the quality of the β-Ga2O3 thin films was improved by step-flow growth, and the FWHM of the (
Acknowledgements
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).