1.
Introduction

β-Ga₂O₃, a semiconductor with a bandgap energy of 4.6–4.9 eV at 300 K^{[1, 2]}, exhibits a great breakdown electric field of 6–8 MV/cm^{[3, 4]} and high transparency in the deep ultraviolet (UV) and visible wavelength region. Therefore, β-Ga₂O₃ is attracting interest for solar-blind UV photodetectors^{[1, 5]}, gas sensors^[6], transparent conducting films for electrodes on a variety of Schottky barrier diode (SBD)^{[4, 7, 8]}, metal–semiconductor field effect transistors (MESFETs)^[9], metal oxide semiconductor field-effect transistors (MOSFETs)^{[4, 9]}, high dielectric oxide or active material for FET device and so on^[10–12].?Another important feature of Ga₂O₃ is that its substrate can be fabricated at low cost from bulk single crystal using the same methods such as floating zone (FZ)^[13] and the edge-defined film-fed growth (EFG)^{[2, 14, 15]} employed for manufacturing sapphire substrate, which provides a potential in mass production at low cost.



Many techniques have been employed to prepare Ga₂O₃ thin films, including sol-gel methods^{[16, 17]}, magnetron sputtering^{[18, 19]}, metal-organic chemical vapor deposition (MOCVD)^[20–22], pulsed laser deposition (PLD)^[23–25] and molecular beam epitaxy^[26–28]. Among them, MBE is an ideal technology for the growth of high quality and high purity Ga₂O₃ thin films due to ultrahigh vacuum growth environment and precise controllability on the growth parameters, though the growth rate is lower than some other growth methods such as MOCVD^[22]. However, it is still a challenge to grow high quality β-Ga₂O₃ thin film on sapphire substrate, due to the large lattice mismatch between the β-Ga₂O₃ and sapphire.



In this paper, we report the systematical study on growth of β-Ga₂O₃ thin film on (0001) Al₂O₃ substrate by PA-MBE. It was found that crystal quality and surface flatness were improved with increasing growth temperature up to 730 °C, with a best full width at half maximum (FWHM) of XRD ω-rocking curve of ($ bar{2}01$) plane and root mean square (RMS) roughness of 0.68° and 2.04 nm, respectively. Room temperature cathodoluminescence measurement shows an emission at ~417 nm, which is most likely originated from the recombination of acceptor or donor–acceptor pair (DAP).

2.
Experimental methods

Ga₂O₃ thin films are grown on (0001) plane sapphire substrate by PREVAC PA-MBE. The Ga beam is supplied by a conventional Knudsen-cell and the flux is modified by the temperature of Ga-cell. O atoms are supplied by a radio frequency plasma cell for oxygen gas, with a constant oxygen flux of 2 sccm and a forward plasma power of 280 W. The typical growth time is 200 min for all ?lms. Reflection high-energy electron diffraction (RHEED) was used to monitor the growth process. The crystal structure and surface morphology were analyzed by X-ray diffraction (XRD), scanning electron microcopy (SEM) and atomic force microscopy (AFM). Cathodoluminescence (CL) measurement was performed at room temperature to characterize emission of the Ga₂O₃ thin films.

3.
Results and discussion

3.1
Structural characterization

The whole growth procedure was in-situ monitored by RHEED. Fig. 1(a) shows the evolution of RHEED patterns before and after the deposition of Ga₂O₃ and displays that the films are single crystal. The typical RHEED patterns of sapphire (0001) plane along [$1bar{1}00 $] and [$11bar{2}0 $] azimuths were clearly observed before the growth of Ga₂O₃. As soon as the deposition of Ga₂O₃ starts, the bright fringes of sapphire were gradually transformed into faint streaky patterns. Fig. 1(a) (iii) and (iv) exhibit the RHEED patterns of Ga₂O₃ recorded at the same electron injection directions as those in Fig. 1(a) (i) and (ii), respectively. Compared with the patterns from sapphire substrate, not only the pattern intensity dimmed, but also the fringe spacing changed, indicating that new crystal structure is appeared. Taking into account the rectangle arranged in-plane atoms, one can easily predict that the RHEED patterns shown in Fig. 1(a) (iii) and (iv) were collected along the [010] and [102] azimuth ofβ-Ga₂O₃, respectively. And the epitaxial relationship between sapphire and Ga₂O₃ should be [010]($bar{2}01$)β-Ga₂O₃||[$1bar{1}00 $](0001)Al₂O₃ and [102]($ bar{2}01$)β-Ga₂O₃||[$11bar{2}0 $](0001)Al₂O₃ with the lattice mismatch value of 4.2% and 10.7%, respectively^{[10, 22]}.






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Figure1.
(Color online) (a) RHEED patterns before and after the deposition of β-Ga₂O₃ films. (b) XRD in-plane ? scan for the β-Ga₂O₃ film grown at substrate temperature of 630 °C.




Meanwhile, the RHEED patterns were observed periodically every 60° rotation starting at the [$1bar{1}00 $] azimuth of Al₂O₃, which agrees well with the in-plane XRD scan result for the film grown at 630 °C shown in Fig. 1(b). The six peaks that appear every 60° indicate 6-fold in-plane rotational symmetry. Considering the fact that monoclinic β-Ga₂O₃ {$bar{2}01 $} planes originally have 2-fold in-plane rotational symmetry, it can be concluded that the grown film contains in-plane rotational domains. The appearance of rotational domains is due to the 3-fold rotational symmetry of the c-sapphire surface; that is, the originally 2-fold β-Ga₂O₃ epitaxially grew in the three different directions at same rates, resulting in the 6-fold rotational symmetry^{[26, 29]}.



Fig. 2(a) presents the XRD 2θ–ω scan of the Ga₂O₃/Al₂O₃ sample grown at 630 °C. Comparing with the standard PDF card (No.: 43-1012)^[29], the three diffraction peaks located at 18.98°, 38.48° and 59.28° can be ascribed to ($bar{2}01$), ($bar{4}02$), and ($bar{6}03$) planes of β-Ga₂O₃, respectively^[30–32]. The result indicates that the film deposited on (0001) Al₂O₃ substrate is pure β-Ga₂O₃ with single orientation along the [$bar{2}01$] direction and further confirms the predicted epitaxy relationship from the RHEED patterns.



FWHM of the XRD rocking curve obtained from the diffraction peak of ($bar{2}01$) plane was used to represent the crystal quality. The FWHM of ($bar{2}01$) of this sample is about 1.45°. According to the 3 × 3 μm² AFM image shown in Fig. 2(b), the corresponding RMS roughness is about 4.43 nm, indicating that the crystalline quality is not satisfied. In order to improve the crystal quality and surface flatness of Ga₂O₃ epilayer, the growth conditions need to be optimized.

3.2
Growth conditions optimization

To optimize the efficient Ga to O atoms ratio, the growth temperature was maintained at 630 °C where the desorption of Ga adatoms nearly negligible, while the oxygen flux was kept constant at 2 sccm with forward plasma power of 280 W. The thickness of the β-Ga₂O₃ film was obtained by fitting the X-ray reflectivity (XRR) curve and further verified by scanning electron microscope (SEM) measurement. Even though the growth rate of β-Ga₂O₃ is low, it is clearly observed from the growth diagram shown in Fig. 3 that the growth rate increases almost linearly with increasing Ga flux up to 1.0 × 10^?6 mbar at a fixed growth temperature of 630 °C and then the growth rate tends to decrease in the higher Ga flux. During the growth of Ga₂O₃, there is a competition between Ga₂O₃ (epilayer) and Ga₂O (gas). The reactions for the layer growth and suboxide formations are^[33]:

$ 2{ m{Ga}}left( { m{g}} ight) + 3{ m{O}}left( { m{g}} ight) to { m{G}}{{ m{a}}_2}{{ m{O}}_3}left( { m{s}} ight){ m{ }}, $

(1)

$ 2{ m{Ga}}left( { m{g}} ight) + 1{ m{O}}left( { m{g}} ight) to { m{G}}{{ m{a}}_2}{{ m{O}}_1}left( { m{g}} ight){ m{ }}. $

(2)

The labels g and s denote the gas and solid phase, respectively. The redundant Ga and O adatoms will form Ga₂O, then desorb from the surface and does not contribute to the epitaxy of Ga₂O₃. This tendency has also been observed in other groups^{[26, 33]}, indicating that the growth rate is limited by the minority atom species on the growing surface and therefore the behavior in Fig. 3 can be classified into two regions of O-rich and Ga-rich. Since in Ref. [34], the growth around the stoichiometric region has resulted in the best quality, the optimum Ga flux was taken to be 1.0 × 10^?6 mbar.






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Figure3.
(Color online) A growth diagram for the Ga₂O₃ MBE growth. The Ga flux dependent growth rate of Ga₂O₃ grown at different temperatures.




To explore the influence of growth temperature, several samples were grown at different temperatures ranging from 630 to 830 °C and the efficient Ga/O flux ratio was set at stoichiometric region. The XRD 2θ–ω scan spectra are presented in Fig. 4, Three diffraction peaks are observed in addition to the substrate’s peaks at 630 °C, 680 °C and 730 °C, which can be ascribed to the diffraction peaks from ($bar{2}01 $), ($bar{4}02 $) and ($bar{6}03 $) planes of β-Ga₂O₃ with single orientation along the [$bar{2}01 $] direction. Surprisingly, both of the ($bar{2}01 $) and ($bar{6}03 $) diffraction peaks of β-Ga₂O₃ disappear when the growth temperature is above 730 °C, and only the ($bar{4}02 $) peak survives but tends to be annihilated. The absence of ($bar{2}01 $) and ($bar{6}03 $) peaks and the faint ($bar{4}02 $) peak indicate the degradation of crystal structure and the reduction of growth rate due to the relatively large decomposition rate at higher growth temperature region. At higher temperature (780 °C and 830 °C), the stress caused by different coefficients of thermal expansion between film and substrate destroys the epitaxial growth, and the structure of the film changes to somehow polycrystalline^[22]. The results imply that the film grown at 730 °C has optimized crystallization with an out-plane relationship of β-Ga₂O₃($ bar{2}01$)||Al₂O₃(0001).



Luckily, all of the diffraction peaks from β-Ga₂O₃ completely existed at lower growth temperature region (630–730 °C). Besides, the XRD peak intensity gradually increase and the line width gradually decrease with increasing growth temperature, indicating the improvement of crystal quality.



Fig. 4(b) shows FWHM of XRD ω-rocking curve for ($bar{2}01 $) plane of β-Ga₂O₃ layers grown at different temperatures. Apparently, the FWHM value monotonically decreases with increasing growth temperature, and the values are 1.45°, 0.82°, and 0.68° for 630, 680, and 730 °C, respectively. The improvement of crystal quality with increasing growth temperature is a common phenomenon for materials epitaxy, but limited up to 730 °C for β-Ga₂O₃ in our MBE system.






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Figure4.
(Color online) (a) XRD patterns of Ga₂O₃ films deposited on (0001) sapphire substrates with different substrate temperatures. (b) The growth temperature dependent FWHM of XRD ω-scan for ($bar{2}01$) plane of Ga₂O₃ films.




Fig. 5 shows surface morphology of Ga₂O₃ grown at different temperatures. Clearly, with increasing growth temperature, the RMS roughness in a scanned area of 3 × 3 μm² gradually decreases from 4.43 nm (630 °C) to 1.32 nm (780 °C). Therefore, higher growth temperature is beneficial to improve surface flatness. Taking into account the AFM image shown in Fig. 2(b) (grown at 630 °C), all of the Ga₂O₃ surfaces were covered with crystal grains rather than steps. The large lattice mismatch (4.2% and 10.7%) between Ga₂O₃ and sapphire should be responsible for this phenomenon. Statistical analysis shows that the grain size significantly decreases with increasing growth temperature, while the grain density changes to the opposite direction. The evolution of grain size and density mainly result from the enhancement of adatoms diffusion length and the reduction of critical nucleation size with increasing growth temperature. Considering the growth rate, crystal quality, and surface flatness, the optimized growth temperature in our MBE system is 730 °C, and the corresponding Ga₂O₃ epilayer exhibits a FWHM of ($bar{2}01 $) about 0.68° with a RMS value ~2.04 nm.






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Figure2.
(Color online) (a) XRD 2θ–ω scan of Ga₂O₃/ Al₂O₃ grown at 630 °C. (b) Surface morphology investigated by AFM in a scanned area of 3 × 3 μm².






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Figure5.
(Color online) AFM surface morphology of β-Ga₂O₃ deposited at different substrate temperatures. (a) 630 °C. (b) 680 °C. (c) 730 °C. (d) 780 °C.

3.3
Optical characterization

Room temperature cathodeluminescence (RT-CL) have been performed on the surface and cross section of Ga₂O₃/Al₂O₃ and Al₂O₃ substrate. The spectra in Fig. 6 shows a strong broad UV-blue and a weak red emission band centered at 325, 417 and 650 nm. Unfortunately, we did not find any band edge emission (240–270 nm) from these samples. Comparing with the emission behavior of sapphire substrate shown in Fig. 6(c), the emission peak in Fig. 6(b) around 325 and 650 nm mainly results from the defects in sapphire while the emission with peak at 417 nm comes from β-Ga₂O₃ film. This blue emission is due to recombination of electron on donor and hole at acceptor or donor–acceptor pair (DAP). These donors might be dominated by oxygen vacancies (V_O) and acceptors might be created by gallium vacancies (V_Ga) or gallium–oxygen vacancies pair (V_Ga–V_O)^[35–37]. It is clear that the quality of β-Ga₂O₃ film is not good enough and further study should be done to improve it.






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Figure6.
(Color online) RT-CL spectra of (a) 400-nm-thick β-Ga₂O₃ film, (b) cross sectional β-Ga₂O₃ thin film, (c) Al₂O₃ substrate.

4.
Conclusions

In conclusion, β-Ga₂O₃ thin film has been grown on (0001) sapphire substrate by PA-MBE, and the epitaxial relationship is confirmed as [010]($bar{2}01$)β-Ga₂O₃||[$01bar{1}0 $](0001)Al₂O₃. Crystalline quality and surface flatness have been improved with increasing growth temperature with the best FWHM of XRD ω-scans of ($bar{2}01 $) plane and RMS roughness of 0.68° and 2.04 nm, respectively. RT-CL spectra suggests that the β-Ga₂O₃ thin film exhibits a strong defects-related emission at around 417 nm and further improvement of crystalline quality is needed.

?

Acknowledgements



This work was supported by the National Key R&D Program of China (No. 2018YFB0406502) and the National Natural Science Foundation of China (Nos. 61734001, 61521004).