Growth Of β-Ga2O3 Thin Films Research Articles

Orientation-dependent atomic-scale mechanism and defect evolution in β-Ga2O3 thin film epitaxial growth

β-Ga2O3 has been subjected to intense research interest as an ultrawide bandgap semiconductor. The epitaxial growth technique of β-Ga2O3 thin films plays a fundamental and vital role in Ga2O3-based device fabrication. In this work, the epitaxial growth mechanisms of β-Ga2O3 on four low-Miller-index facets, (100), (010), (001), and (2¯01), are systematically explored using large-scale machine-learning molecular dynamics simulations at the atomic scale. The simulations reveal that the migration of the face-centered cubic stacking O sublattice plays a dominant role in the different growth mechanisms between the (100)/(010)/(001) and (2¯01) orientations. The resultant complex combinations of the stacking faults and twin boundaries are carefully identified and show good agreement with experimental observations and ab initio calculations. Our results provide useful insights into the gas-phase epitaxial growth of β-Ga2O3 thin films and suggest possible ways to tailor its properties for specific applications.

Low-temperature homoepitaxial growth of β-Ga2O3 thin films by atmospheric pressure plasma-enhanced chemical vapor deposition technique

Low-temperature homoepitaxial growth of β-Ga2O3(-201) has been successfully demonstrated by the atmospheric pressure plasma-enhanced chemical vapor deposition technique. To search for low-temperature growth, temperature-dependent studies were carried out between 350 and 600 °C. A high N2 gas flow rate, low gallium source concentration, and high oxygen flow rate ratio played key roles in growing independent and homogeneous multiple nuclei of Ga2O3, leading to three-dimensional grain growth mode, single crystallinity, and the highest growth rate of ⁓0.17 µm/h at 350 °C. The highly reactive atmospheric pressure oxygen plasma actively led to epitaxial growth. The low thermal budget homoepitaxial growth is a record reduction reported thus far.

Heteroepitaxial growth of β-Ga2O3 thin films on single crystalline diamond (111) substrates by radio frequency magnetron sputtering

In this work, we demonstrate the first achievement in heteroepitaxial growth of β-Ga2O3 thin films on single crystalline diamond (111) wafers using RF magnetron sputtering. A single monoclinic (β-phase) structure with a monofamily {01} plane was obtained. XRD pole figure shows (02) and (002) textures of the (01) β-Ga2O3 plane parallel to (111) diamond with six distinct rotational domains, confirming successful epitaxial growth. Collectively, this research provides valuable insights into the epitaxial growth of β-Ga2O3 on diamond via sputtering, paving the way for scalable β-Ga2O3/diamond heterostructures for future electronic and optoelectronic applications with not only high performance but also effective self-thermal management.

Tutorial: Metalorganic chemical vapor deposition of β-Ga2O3 thin films, alloys, and heterostructures

β-phase gallium oxide (Ga2O3) is an emerging ultrawide bandgap (UWBG) semiconductor with a bandgap energy of ∼ 4.8 eV and a predicted high critical electric field strength of ∼8 MV/cm, enabling promising applications in next generation high power electronics and deep ultraviolet optoelectronics. The advantages of Ga2O3 also stem from its availability of single crystal bulk native substrates synthesized from melt, and its well-controllable n-type doping from both bulk growth and thin film epitaxy. Among several thin film growth methods, metalorganic chemical vapor deposition (MOCVD) has been demonstrated as an enabling technology for developing high-quality epitaxy of Ga2O3 thin films, (AlxGa1−x)2O3 alloys, and heterostructures along various crystal orientations and with different phases. This tutorial summarizes the recent progresses in the epitaxial growth of β-Ga2O3 thin films via different growth methods, with a focus on the growth of Ga2O3 and its compositional alloys by MOCVD. The challenges for the epitaxial development of β-Ga2O3 are discussed, along with the opportunities of future works to enhance the state-of-the-art device performance based on this emerging UWBG semiconductor material system.

Epitaxial growth of β-Ga2O3 thin films on SrTiO3 (1 1 1) and (1 0 0) substrates by chemical vapor deposition

Due to good lattice matching and bipolar doping behavior, wide bandgap SrTiO3 (STO) is a promising substrate for the heteroepitaxial growth of β-Ga2O3 to construct heterojunctions for (opto-)electronic device applications. However, the investigation of the growth of β-Ga2O3 film on the STO (111) substrate is still lacking. In this work, for the first time, the β-Ga2O3 (2¯ 01) thin films were epitaxially grown on the supercell matched STO (111) substrates by low pressure chemical vapor deposition. The effects of the source and growth temperatures on the surface roughness and crystalline quality of the β-Ga2O3 films were systemically investigated. By controlling the supply of Ga source, the heteroepitaxial β-Ga2O3 film grown under the optimized conditions exhibits a narrow X-ray diffraction rocking curve of 0.75° and a low root-mean-square roughness of 1.10 nm. Furthermore, the epitaxial growth of β-Ga2O3 films on the STO (100) substrate was also investigated. The heteroepitaxy of β-Ga2O3 films on STO lays the foundation for future device applications of β-Ga2O3-based heterojunctions.

Effect of off-axis substrate angles on β-Ga2O3 thin films and solar-blind ultraviolet photodetectors grown on sapphire by MOCVD

β-Ga2O3 thin films were heteroepitaxially grown on c-plane sapphire substrates with different off-axis angles toward <11‾00> direction by metalorganic chemical vapor deposition (MOCVD), where the crystal quality of the films and the deep-ultraviolet photodetectors (PDs) fabricated on these films were characterized. The effect of sapphire substrates with different off-axis angles on the β-Ga2O3 thin films and the optical performance of β-Ga2O3 PDs are investigated for the first time. (2‾01)-oriented β-Ga2O3 thin films were grown on sapphire substrates with off-axis angles of 0°, 1.5°, 4°, and 6°. The full width at half maxima (FWHM) of rocking curves decreased significantly with the increase of off-axis angles. The β-Ga2O3 thin films grown on the 6° off-axis angle substrate showed a FWHM of 0.55°, indicating excellent crystal quality. Substrate off-axis angles play a critical role in the nucleation process in the growth of β-Ga2O3 thin films. The step edge on the off-axis substrates serves as the preferential incorporation sites for Ga adatoms. The step terrace width is controlled by the off-axis angle and influences the growth modes depending on the relative magnitude of the step terrace width compared with the Ga adatom diffusion length. In addition, β-Ga2O3 photodetectors with a metal-semiconductor-metal (MSM) structure have been fabricated using these β-Ga2O3 thin films. The β-Ga2O3 PDs fabricated on 6° off-axis sapphire substrate showed excellent optical performance with a large photocurrent of 61 μA, an outstanding photo/dark current ratio of 2.3 × 103, and peak responsivity of 74.6 mA/W. These results can provide important guidance on heterogeneous epitaxy of β-Ga2O3 thin films and photodetectors on cost-effective mass-producible sapphire substrates by MOCVD.

Rapid homoepitaxial growth of (010) β-Ga2O3 thin films via mist chemical vapor deposition

We achieved rapid homoepitaxial growth of β-Ga2O3 thin films with smooth surfaces using a mist chemical vapor deposition (CVD) process. A smooth film was grown at a rate of 3.2 μm/h using a concentrated aqueous solution of the GaCl3 precursor. Homoepitaxial growth of a smooth (010) β-Ga2O3 thin film was achieved via mist CVD at 750 °C, and the growth of β-Ga2O3 in different orientations was not observed. The GaCl3 precursor was highly soluble in water and enabled the rapid growth of homoepitaxial β-Ga2O3 thin films with smooth surfaces.

MOCVD growth of high purity Ga2O3 epitaxial films using trimethylgallium precursor

We report on the growth of β-Ga2O3 thin films using trimethylgallium (TMGa) as a source for gallium and pure O2 for oxidation. The growth rate of the films was found to linearly increase with the increase in the molar flow rate of TMGa and reach as high as ∼6 μm/h at a flow rate of 580 μmol/min. High purity, lightly Si-doped homoepitaxial β-Ga2O3 films with a good surface morphology, a record low temperature electron mobility exceeding 23 000 cm2/V s at 32 K, and an acceptor concentration of 2 × 1013 cm−3 were realized, showing an excellent purity film. Films with room temperature (RT) electron mobilities ranging from 71 cm2/V s to 138 cm2/V s with the corresponding free carrier densities between ∼1.1 × 1019 cm−3 and ∼1.5 × 1016 were demonstrated. For layers with the doping concentration in the range of high-1017 and low-1018 cm−3, the RT electron mobility values were consistently more than 100 cm2/V s, suggesting that TMGa is suitable to grow channel layers for lateral devices, such as field effect transistors. The results demonstrate excellent purity of the films produced and confirm the suitability of the TMGa precursor for the growth of device quality β-Ga2O3 films at a fast growth rate, meeting the demands for commercializing Ga2O3-based high voltage power devices by metalorganic chemical vapor deposition.

Patterned growth of β-Ga2O3 thin films for solar-blind deep-ultraviolet photodetectors array and optical imaging application

Solar-blind deep-ultraviolet (DUV) photodetectors based on Ga2O3 have attracted great attention due to their potential applications for many military and civil purposes. However, the development of device integration for optoelectronic system applications remains a huge challenge. Herein, we report a facile method for patterned-growth of high-quality β-Ga2O3 thin films, which are assembled into a photodetectors array comprising 8 × 8 device units. A representative detector exhibits outstanding photoresponse performance, in terms of an ultra-low dark current of ∼0.62 pA, a large Ilight/Idark ratio exceeding 104, a high responsivity of ∼0.72 A W−1 and a decent specific detectivity of ∼4.18 × 1011 Jones, upon 265 nm DUV illumination. What is more, the DUV/visible (250/400 nm) rejection ratio is as high as 103 with a sharp response cut-off wavelength at ∼280 nm. Further optoelectronic analysis reveals that the photodetectors array has good uniformity and repeatability, endowing it the capability to serve as a reliable DUV light image sensor with a decent spatial resolution. These results suggest that the proposed technique offers an effective avenue for patterned growth of β-Ga2O3 thin films for multifunctional DUV optoelectronic applications.

Growth and characterization of β-Ga2O3 thin films by sol-gel method for fast-response solar-blind ultraviolet photodetectors

Monoclinic Ga2O3 (β-Ga2O3) has been considered as a promising candidate for solar-blind photodetector owing to its extremely wide bandgap. However, preparing homogeneous large-area thin films of β-Ga2O3 in a practical way still remains as an upfront challenge. Here we report the growth of β-Ga2O3 thin films on the Sapphire substrate via a low-cost sol-gel spin-coating method followed by a post-annealing process. The structures, surface morphologies and optical properties of the films were characterized using various techniques, including X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM) and ultraviolet–visible (UV–Vis) spectrophotometry. Our results showed that phase-pure β-Ga2O3 polycrystalline films could be formed by sol-gel technique when post-annealed at temperatures 700 °C and higher. All the films showed high transmittance of 90% in UV–Vis light with wavelength above 300 nm and the optical bandgap energy increased monotonously with the annealing temperature. The current-voltage (I–V) tests and time-resolved photoresponse measurements showed that the film annealed at 700 °C has the excellent photoresponse characteristic with a high Iphoto/Idark ratio (18.34) along with the shortest response time (0.10 s) and decay time (0.10 s), indicating that the sol-gel synthesized β-Ga2O3 films can have great potential for fabricating fast-response solar-blind ultraviolet photodetectors.

Homoepitaxial growth of β-Ga2O3 thin films by low pressure chemical vapor deposition

This paper presents the homoepitaxial growth of phase pure (010) β-Ga2O3 thin films on (010) β-Ga2O3 substrate by low pressure chemical vapor deposition. The effects of growth temperature on the surface morphology and crystal quality of the thin films were systematically investigated. The thin films were synthesized using high purity metallic gallium (Ga) and oxygen (O2) as precursors for gallium and oxygen, respectively. The surface morphology and structural properties of the thin films were characterized by atomic force microscopy, X-ray diffraction, and high resolution transmission electron microscopy. Material characterization indicates the growth temperature played an important role in controlling both surface morphology and crystal quality of the β-Ga2O3 thin films. The smallest root-mean-square surface roughness of ∼7 nm was for thin films grown at a temperature of 950 °C, whereas the highest growth rate (∼1.3 μm/h) with a fixed oxygen flow rate was obtained for the epitaxial layers grown at 850 °C.

Rf-plasma-assisted molecular-beam epitaxy of β-Ga2O3

Epitaxial growth of β-Ga2O3 thin films by the rf-plasma-assisted molecular-beam epitaxy technique is demonstrated. Growth on (1 0 0) β-Ga2O3 substrates leads to very smooth epilayers, while (2 0 1¯) and (1 0 0) oriented β-Ga2O3 films are obtained on (0 0 1) sapphire and (1 0 0) MgO substrates, respectively. Internal transmittance, refractive index and direct bandgaps are determined.