Abstract
1. Introduction
β-Ga2O3, a semiconductor with a bandgap energy of 4.6–4.9 eV at 300 K[
Many techniques have been employed to prepare Ga2O3 thin films, including sol-gel methods[
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 (
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 films. 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 [
Figure 1.(Color online) (a) RHEED patterns before and after the deposition of
Meanwhile, the RHEED patterns were observed periodically every 60° rotation starting at the [
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)[
FWHM of the XRD rocking curve obtained from the diffraction peak of (
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[
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[
Figure 2.(Color online) (a) XRD 2
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 (
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 (
Figure 3.(Color online) A growth diagram for the Ga2O3 MBE growth. The Ga flux dependent growth rate of Ga2O3 grown at different temperatures.
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 (
Figure 4.(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
Figure 5.(Color online) AFM surface morphology of
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 (VO) and acceptors might be created by gallium vacancies (VGa) or gallium–oxygen vacancies pair (VGa–VO)[
Figure 6.(Color online) RT-CL spectra of (a) 400-nm-thick
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](
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).
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