Influence of O2 Flow Rate on the Properties of Ga2O3 Growth by RF Magnetron Sputtering
About
The influence of the O2 flow rate on the properties of gallium oxide (Ga2O3) by RF magnetron sputtering was studied. X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), transmittance spectra, and photoluminescence (PL) spectra have been employed to study the Ga2O3 thin films. With the increase in oxygen flow rate, both the crystal quality and luminescence intensity of the Ga2O3 samples first decrease and then enhance. All these observations suggested that the reduction in the oxygen defect density is responsible for the improvement in the crystal quality and emission intensity of the material. Our results demonstrated that high-quality Ga2O3 materials could be obtained by adjusting the oxygen flow rate.
Introduction
Recently, gallium oxide (Ga2O3) and related compounds (AlxGa2-XO3, (InxGa1-x)2O3) have brought about the widespread attention, attributed to their outstanding electrical and photoelectric properties, such as wide fundamental bandgap (4.5–5 eV), a high off-state breakdown voltage of 755 V, high dielectric constant values from 10.2 to 14.2, and high mobility of 2790 cm2 V−1 s−1. At present, Ga2O3 has been widely used in solar blind ultraviolet detection, high power switching, metal oxide semiconductor field effect transistors (MOSFET), high-temperature gas sensors, and many other fields.
Ga2O3 usually exists in six different polymorphic structures (α, β, γ, δ, ε, and k). Among these, β-Ga2O3 is considered as the most stable phase and can be converted from other phases at high temperatures. So far, many growth modes were used to develop β-Ga2O3, including RF Sputter, MBE, MOCVD, chemical vapor deposition, etc. RF magnetron sputtering is a comparatively economical deposition technique that has adequate control over stoichiometry and uniformity of the film compared to the above techniques. Until now, the effects of growth parameters, for instance, substrate temperature, oxygen/argon partial pressures, and sputtering power, on the properties of Ga2O3 have been studied the most. However, hardly any reports about the effect of the O2 flow rate at fixed Ar flow rate on the structure and optical properties of β-Ga2O3 thin films deposited by the reactive RF magnetron sputter. Since oxygen deficiency in the growth process can induce oxygen vacancies in Oxide semiconductor materials, oxygen vacancies affect the optical and electrical properties of oxide semiconductor films. Therefore, it is of great significance to study the effect of the O2 flow rate on the characteristics of Ga2O3 thin films deposited by RF magnetron sputtering.
In this work, β-Ga2O3 has been grown by RF magnetron sputtering, and the effect of O2 flow rate on the structure and optical characteristic of β-Ga2O3 have been studied in detail. The improvement of UV emission properties was observed in the β-Ga2O3 samples with an increased O2 flow rate. The mechanism of enhanced luminescence in the β-Ga2O3 film was an in-depth study by careful inspection of the PL spectrum combined with XRD results. It is anticipated that this work will provide a meaningful step toward the fabrication of high-quality β-Ga2O3 thin films.
Materials and Methods
Ga2O3 samples were grown on single-polished c-plane (0006) sapphire substrates using an RF magnetron sputtering system. A sintered ceramic Ga2O3 target of 99.99% purity was employed as the target. Before growth, the sapphire substrates were first cleaned with ultrasonic vibration in ethanol and then in high purity water. The argon gas flow rate was set at 30 sccm and pressure at 0.8 Pa. Sputtering power was adjusted to 80 w. The distance between the sample and target was 760 mm. The base pressure of vacuum chamber reached 5.6 × 10−4 Pa. The Ar flow rate was kept constant at 30 sccm. In the experiments, the O2 flow rate was set as 0 sccm, 1 sccm, 2 sccm, 4 sccm, respectively. The influence of other parameters was minimized.
The structure of Ga2O3 was characterized by an XRD technique (XRD, X’ Pert, Philips, Eindhoven, The Netherlands). The morphologies of samples were conducted on a field-emission scanning electron microscopy (FE-SEM, ZEISS Merlin Compact, Oberkochen, Germany). Photoluminescence (PL) spectrums were investigated by Zolix responsivity measurement system (λ = 266 nm) as the excitation source (DSR600, Zolix, Beijing, China).
Results & Conclusions
Shows the result of the X-ray diffraction of the Ga2O3 films growth with various O2 flow rates. The diffraction peaks located at 29.7°, 37.6°, and 58.4° originate from the 400, 402, and 603 of the β-Ga2O3, respectively. For the sample without the O2 flow rate, 400, 402, and 603 of the β-Ga2O3 diffraction peak coexisted; this suggests that the sample was polycrystalline. With the O2 flow rate increased from 0 to 4 sccm, the diffraction peak intensity of the 400 β-Ga2O3 decreased, while the intensity of both the 402 and 603 of β-Ga2O3 diffraction peak increased. Both of these two diffractions belong to the 201 plane family of the monoclinic Ga2O3. The above result illustrates that highly 201-textured β-Ga2O3 samples have been prepared and the orientation of crystal is gradually enhanced when oxygen flow increased. Furthermore, the full width at half maximum (FWHM) values of the 402 β-Ga2O3 peaks are 1.00°, 1.10°, 1.06°, and 0.96° for samples with the O2 flow rate increased from 0 to 4 sccm, respectively. The FWHM value is dependent on the O2 flow rate, and the results suggest a higher O2 flow rate results in improved crystal quality. The minimal FWHM is obtained at 4 sccm of the O2 flow rate, which means the grain size is the largest. The combined results of the XRD peak intensity and the FWHM value of the samples show that higher O2 flow rates lead to better quality.
In summary, in terms of the effect of oxygen flow on the structure, optical l properties of the Ga2O3 films have been investigated by XRD, EDX, AFM, transmission spectra, and PL spectra. With the increase in the oxygen flow rate, both the crystal quality and luminescence intensity of the sample first decreased and then enhanced. All these observations suggested that the reduction in the oxygen defect density is responsible for the improvement in the crystal quality and emission intensity of the material, however, there have been no reports about O2 flow rate on the properties of the Ga2O3 growth by RF magnetron sputtering. Our results were similar to those obtained by other techniques and the specific control of various experimental operating parameters. Vu found that the performance of β-Ga2O3-based photodetectors with a higher oxygen partial are better than those prepared at lower oxygen pressures. Wang et al. studied the influence of oxygen flow ratio on the performance of Sn-doped Ga2O3 films by RF magnetron sputtering; they found the sample with higher oxygen flow ratio displays an enhanced performance. Shen’s study revealed oxygen annealing will enhance the performance of β-Ga2O3 solar-blind photodetectors grown by ion-cutting process. Our results demonstrated that high-quality gallium oxide materials can be obtained by adjusting the oxygen flow rate.