α-Ga2O3 Thin Films Research Articles

Synthesis of nitrogen-doped crystalline Ga2O3 thin films via trimethylgallium doped NH3/H2/N2/O2 premixed stagnation flames

This study presents a novel and cost-effective technique for fabricating nitrogen-doped crystalline α-Ga2O3 and β-Ga2O3 thin films under atmospheric pressure conditions through flame synthesis for the first time. We employ metal–organic ammonia (NH3)-assisted flame synthesis (MO-NAFS) in a quasi-one-dimensional trimethylgallium (TMG) doped NH3/H2/N2/O2 flat flame to achieve a one-step deposition of Ga2O3 onto c-plane α-Al2O3 substrates. We found that hydrogen (H2) addition directly impacts the crystallinity of the Ga2O3 thin film. When only NH3 is employed as the fuel, a thin-film single-crystal nitrogen-doped β-Ga2O3 was observed. Blending the NH3 with 5 % of H2 yields a single-crystal nitrogen-doped α-Ga2O3, while a further increase of H2 fraction up to 10 % results in a mixture of both α-Ga2O3 and β-Ga2O3 crystals. Remarkably, the nitrogen doping through MO-NAFS was found to persistently result in an N/O atomic ratio as high as 1.12, as determined by the XPS analysis. This approach paves the way to a simple and scalable means for producing crystalline nitrogen-doped Ga2O3 thin films with potential control of crystal phase composition through parameter adjustments.

High-performance solar-blind photodetector based on Si-doped α-Ga2O3 thin films grown by mist chemical vapor deposition

α-Ga2O3 has gained increasing attention in the field of photodetectors because of its wide bandgap and exceptional chemical and physical properties. Si-doped α-Ga2O3 single-crystal epitaxial films were successfully deposited on sapphire substrates using an affordable and non-vacuum-based mist chemical vapor deposition (Mist-CVD) technique. An interdigital metal-semiconductor-metal (MSM) photodetector with excellent performance was prepared by using the thin film. At 254 nm illumination and a bias voltage of 20 V, the photodetector has a high light-to-dark current ratio of 3.24×106, a responsivity of 3.23×102 A/W, a detectivity of 8.80×1015 Jones, and an external quantum efficiency of 1.58×105%. The results show that Si-doped α-Ga2O3 thin films have great potential for application in high-performance photodetectors.

Structural characterization of threading dislocation in α-Ga2O3 thin films on c- and m-plane sapphire substrates

We discuss the structure of threading dislocations in α-Ga2O3 thin films grown on c- and m-plane sapphire substrates. The thickness-dependent threading dislocation density in both films directly affects the electrical properties of the films including carrier concentration and mobility. Two distinct types of threading dislocations are identified for each of the c- and m-plane α-Ga2O3 thin films. The c-plane α-Ga2O3 thin film shows Burgers vectors of 1/3[11¯01] and 1/3[112¯0], while the m-plane α-Ga2O3 thin film displays Burgers vectors of 1/3[21¯1¯0] and 1/3[11¯01]. This paper presents a detailed structure of the threading dislocations in α-Ga2O3, which has been little disclosed thus far mainly due to the difficulty in synthesizing the metastable α-Ga2O3.

Ge doping of α-Ga2O3 thin films via mist chemical vapor deposition and their application in Schottky barrier diodes

We performed Ge doping of α-Ga2O3 thin films grown on m-plane sapphire substrates using mist chemical vapor deposition. Although the typical growth rate was high at 4 μm/h, the resultant α-Ga2O3 thin films exhibited high crystallinity. We controlled the carrier density in the range of 8.2 × 1016–1.6 × 1019 cm−3 using bis[2-carboxyethylgermanium(IV)]sesquioxide as the Ge source. The highest mobility achieved was 66 cm2 V−1 s−1 at a carrier concentration of 6.3 × 1017 cm−3. Through secondary ion mass spectrometry analysis, a linear relationship between the Ge concentration in the α-Ga2O3 thin films and the molar ratio of Ge to Ga in the source solution was established. The quasi-vertical Schottky barrier diode fabricated using the Ge-doped α-Ga2O3 thin films exhibited an on-resistance of 7.6 mΩ cm2 and a rectification ratio of 1010. These results highlight the good performance of the fabricated device and the significant potential of Ge-doped α-Ga2O3 for power-device applications.

Demonstration of vertical schottky barrier diodes based on α-Ga2O3 thin films enabled by corundum structured rh-ITO bottom electrodes

Demonstration of vertical schottky barrier diodes based on α-Ga2O3 thin films enabled by corundum structured rh-ITO bottom electrodes

Revealing the influence of aerosol particle state on the high-quality α-Ga2O3 single crystal thin film growth by Mist CVD epitaxy

There is little research on α-Ga2O3 single crystal film because it is a metastable phase of Ga2O3 crystal and difficult for controllable fabrication. In this paper, highly crystallized Ga2O3 thin films of poly-crystalline, α-Ga2O3 and β-Ga2O3 were prepared by modulating the growth temperature from 400 °C to 700 °C by mist-assisted CVD (Mist CVD) method. The results show that films grown at 525 °C owes the best crystalline quality of α-Ga2O3 film. To further improve the crystallinity of α-Ga2O3 thin film, a systematic simulation revealing the distribution changes of aerosol particle size, flow field, and temperature gradient in the reaction chamber were explored. The optimal growth region C with uniform particle size, well-distribute temperature gradient and stable flow field is determined. Meanwhile, the experimental results of α-Ga2O3 thin films grown at different positions in the reaction chamber was proved to consistent with the simulation results. Furthermore, the optimal single crystallized α-Ga2O3 thin films with smooth surface morphology and optical property were obtained. The PL spectrum operated at room temperature show the α-Ga2O3 film with low defect density and high crystalline quality. It shows that the single crystallized α-Ga2O3 is orientated grown on sapphire substrate with epitaxial relationship of α-Ga2O3 (0006)//c-Al2O3 (0006). Although, slight lattice mismatch is observed on the selected area electron diffraction pattern of the interface between sapphire and α-Ga2O3 film, it is ideal for further application in devices. This work reveals the key point for α-Ga2O3 single crystal film growth by mist CVD is to construct a steady and orderly environment through controlling aerosol particle state.

Heteroepitaxial growth of a-, m-, and r-plane α-Ga2O3 thin films on rh-ITO electrodes for vertical device applications

α-Ga2O3 has garnered significant attention as a promising next-generation material for applications in power-switching and deep ultraviolet optoelectronics owing to its ultrawide bandgap. However, fabricating α-Ga2O3-based devices with a vertical structure is challenging because α-Ga2O3 requires heteroepitaxial growth on a heterogeneous substrate, typically insulating sapphire. In this study, we investigated the heteroepitaxial growth of a-, m-, and r-plane α-Ga2O3 on rhombohedral indium tin oxide (rh-ITO) electrodes via mist chemical vapor deposition. X-ray diffraction (XRD) 2θ-ω and φ scan analyses revealed that a-, m-, and r-plane α-Ga2O3 epitaxial thin films were successfully grown on rh-ITO bottom electrodes when α-Fe2O3 buffer layers were inserted. No phase separation or in-plane rotation was observed in the XRD results of the α-Ga2O3 thin films. The α-Fe2O3 buffer layers facilitated the preferential growth of α-Ga2O3 on various planes of rh-ITO. The surface morphology of the a-, m-, and r-plane α-Ga2O3, examined via field emission scanning electron microscopy, differed depending on the crystal plane. The results obtained in this study have potential applications in the development of vertical-structured devices with α-Ga2O3 thin films.

Controlled Crystallinity of a Sn-Doped α-Ga2O3 Epilayer Using Rapidly Annealed Double Buffer Layers.

Double buffer layers composed of (AlxGa1-x)2O3/Ga2O3 structures were employed to grow a Sn-doped α-Ga2O3 epitaxial thin film on a sapphire substrate using mist chemical vapor deposition. The insertion of double buffer layers improved the crystal quality of the upper-grown Sn-doped α-Ga2O3 thin films by blocking dislocation generated by the substrates. Rapid thermal annealing was conducted for the double buffer layers at phase transition temperatures of 700-800 °C. The slight mixing of κ and β phases further improved the crystallinity of the grown Sn-Ga2O3 thin film through local lateral overgrowth. The electron mobility of the Sn-Ga2O3 thin films was also significantly improved due to the smoothened interface and the diffusion of Al. Therefore, rapid thermal annealing with the double buffer layer proved advantageous in achieving strong electrical properties for Ga2O3 semiconductor devices within a shorter processing time.

Exploring heteroepitaxial growth and electrical properties of α-Ga2O3 films on differently oriented sapphire substrates

This study explores the epitaxial relationship and electrical properties of α-Ga2O3 thin films deposited on a-plane, m-plane, and r-plane sapphire substrates. We characterize the thin films by X-ray diffraction and Raman spectroscopy, and elucidate thin film epitaxial relationships with the underlying sapphire substrates. The oxygen vacancy concentration of α-Ga2O3 thin films on m-plane and r-plane sapphire substrates are higher than α-Ga2O3 thin film on a-plane sapphire substrates. All three thin films have a high transmission of over 80% in the visible and near-ultraviolet regions, and their optical bandgaps stay around 5.02–5.16 eV. Hall measurements show that the α-Ga2O3 thin film grown on r-plane sapphire has the highest conductivity of 2.71 S/cm, which is at least 90 times higher than the film on a-plane sapphire. A similar orientation-dependence is seen in their activation energy as revealed by temperature-dependent conductivity measurements, with 0.266, 0.079, and 0.075 eV for the film on a-, m-, r-plane, respectively. The origin of the distinct transport behavior of films on differently oriented substrates is suggested to relate with the distinct evolution of oxygen vacancies at differently oriented substrates. This study provides insights for the substrate selection when growing α-Ga2O3 films with tunable transport properties.

PLD of α-Ga2O3 on m-plane Al2O3: Growth regime, growth process, and structural properties

Phase-pure α-Ga2O3 thin films with high surface quality and crystallinity have been grown on m-plane sapphire using pulsed laser deposition (PLD). Therefore, the influence of growth temperature, oxygen background pressure, and film thickness on the structural properties is investigated to determine the growth window for phase-pure corundum structured α-Ga2O3. Samples were analyzed using x-ray diffraction (XRD), atomic force microscopy (AFM), and spectroscopic ellipsometry measurements. A distinct growth window in favor of phase-pure (10.0)-oriented α-Ga2O3 for growth temperatures above 480 °C and low oxygen partial pressures p(O2) of 3 × 10−4 mbar is identified. The growth rate increases significantly with increasing oxygen pressure. Furthermore, it shows an Arrhenius-like decrease for lower temperatures, caused by the increasing desorption of volatile Ga2O suboxides. It was found that for thicker layers, the growth of monoclinic β-Ga2O3 is promoted likely facilitated by the c-facets of the α-Ga2O3 grains. This leads to a (010)-oriented island growth corroborated by stripe-like features in AFM scans and a corresponding in-plane orientation confirmed by XRD ϕ-scans. For oxygen partial pressures above 3 × 10−4 mbar and Tg < 540 °C, the formation of mixed (10.0) α-Ga2O3 and spinel-defective (110)-γ-Ga2O3 manifesting as defective inclusions was observed independent of the layer thickness d. A corresponding p(O2) − d − T phase diagram for the growth of Ga2O3 on m-plane sapphire by PLD is provided.

Vertical self-powered ultraviolet photodetector using α-Ga2O3 thin films on corundum structured rh-ITO electrodes

Corundum-structured gallium oxide (α-Ga2O3), which exhibits an ultra-wide bandgap of 5.3 eV, has been exploited in optoelectronics devices. We demonstrate a vertical-type photodetector using α-Ga2O3. Corundum-structured rhombohedral indium tin oxide (rh-ITO), which has high conductivity, was used for a bottom electrode to realize a vertical structure. An α-Ga2O3 epitaxial film was successfully grown on rh-ITO thin films with an α-Fe2O3 buffer layer via mist chemical vapor deposition. The vertical α-Ga2O3 photodetector is sensitive to the UV-C region, and its photoresponsivity is 9.7 × 10-4 A/W at 220 nm without any bias voltage. This study contributes to vertical-structure device applications using corundum-structured oxides.

Corundum-structured α-Fe2O3 substrates for α-Ga2O3 epitaxial growth

The present study demonstrated the growth of α-Ga2O3 epitaxial thin films on α-Fe2O3 substrates via mist chemical vapor deposition. XRD analysis revealed that c-plane α-Ga2O3 thin films were epitaxially grown on a c-plane α-Fe2O3 substrate. The full width at half maximum for asymmetric diffraction (10–14) of α-Ga2O3 using α-Fe2O3 was lower than that obtained using α-Al2O3. Moreover, the XRD pole figure results revealed that α-Ga2O3 on α-Fe2O3 exhibited no in-plane rotations. We believe that α-Fe2O3 wafers with small lattice mismatches are promising substrates for α-Ga2O3 without rotational domains.

Strain relaxation and dislocation annihilation in compositionally graded α-(AlxGa1-x)2O3 layer for high voltage α-Ga2O3 power devices

α-Ga2O3 of the corundum structure and the large bandgap of 5.3 eV has attracted great interest because it can be grown on a sapphire (α-Al2O3) substrate with the same crystal structure. However, the lattice mismatch (4.3%) and the different thermal expansion coefficients between α-Ga2O3 and the sapphire substrate induce crystalline defects and thermal strain, leading to a high density of threading dislocations and degraded electrical properties of the α-Ga2O3 films grown directly on the substrate. Herein, to circumvents these issues, compositionally graded α-(AlxGa1-x)2O3 layers are adopted to reduce threading dislocations for a high quality of epitaxial α-Ga2O3 films. The evolution of strain relaxation and the inclination of threading dislocations in graded α-(AlxGa1-x)2O3 layers are confirmed by reciprocal space mapping (RSM) and transmission electron microscopy (TEM). Through RSM and TEM studies, we confirmed that compressive strain enhances the inclination of dislocations, and therefore, the dislocations merge and annihilate in the graded α-(AlxGa1-x)2O3 layers. Moreover, owing to dislocations annihilation in the graded α-(AlxGa1-x)2O3 layers, the calculated density of threading dislocations in α-Ga2O3 films with a graded α-(AlxGa1-x)2O3 layer is reduced by 64.9% compared with that of α-Ga2O3 films deposited directly grown on a bare sapphire substrate. Furthermore, a fabricated lateral-structure Schottky diodes reveals enhanced breakdown voltages and forward current density due to the improved crystalline quality using the graded α-(AlxGa1-x)2O3 layer. This study provides an attractive approach for obtaining high-quality epitaxial α-Ga2O3 thin films for high voltage power devices.

Growth of Metastable α-Ga<sub>2</sub>O<sub>3</sub> Epitaxial Thin Film on Flexible Synthetic Mica by Insertion α-Fe<sub>2</sub>O<sub>3</sub> Buffer Layer

Metastable α-Ga2O3 thin films with a corundum structure were grown epitaxially on flexible synthetic mica. The κ-Ga2O3 thin films were grown in a temperature range of 450‒600 °C, without a buffer layer. In contrast, the α-Ga2O3 thin films were grown in a wider temperature range of 350‒600 °C, by inserting corundum-structured α-Fe2O3 buffer layers. X-ray diffraction (XRD) rocking curve measurements revealed that the α-Ga2O3 thin film grown at 425 °C had the highest degree of crystallinity. Cross-sectional transmission electron microscopy (TEM) observations and an XRD φ scan revealed that the epitaxial relationship between the thin film and the substrate via the buffer layer was (0001) α-Ga2O3 [11‒20] || (001) synthetic mica [100]. Furthermore, when TEM observation was performed close to the surface of the α-Ga2O3 thin film at a high magnification, a lattice image derived from the out-of-plane (0001)-plane orientation was observed. However, it was also revealed that the α-Ga2O3 thin films did not have a single crystal structure but rather an in-plane domain structure. By conducting selected area electron diffraction (SAED) at the interface area, it was determined that the α-Fe2O3 buffer layer was polycrystalline. This implies that α-Ga2O3 thin films were epitaxially grown while forming the in-plane domains on the polycrystalline buffer layers. The results of this study indicate the flexible applications of α-Ga2O3 thin films, which have significant potential for use as power sources and deep-ultraviolet devices.

Self-powered solar-blind α-Ga2O3 thin-film UV-C photodiode grown by halide vapor-phase epitaxy

A compact self-powered solar-blind UV-C photodiode was demonstrated using an ultra-wide bandgap (UWBG) α-Ga2O3 thin film as a wavelength-selective absorber layer. The UWBG-based Schottky junction architecture renders the use of low-performance and bulky solar-blind UV bandpass filters unnecessary. High-quality α-Ga2O3 thin films with a thickness of 1.25 µm were grown on a (0001) sapphire substrate via the halide vapor-phase epitaxy technique. The self-powered solar-blind UV-C photodetector based on the Ni/α-Ga2O3 Schottky junction exhibited excellent responsivity (1.17 × 10−4 A/W), photo-to-dark current ratio (1.12 × 105), and reproducibility, as well as fast rise/decay characteristics without persistent photoconductivity upon exposure to UV-C radiation (254 nm wavelength). The relationship between light intensity (I) and photocurrent (P) was modeled by I ∼ P0.69, indicating the high-quality of the halide vapor-phase epitaxy-grown α-Ga2O3 thin film. Upon exposure to natural sunlight, the fabricated solar-blind photodetector showed excellent solar blindness with sensitivity to UV-C radiation and did not require an external power source. Therefore, this UWBG α-Ga2O3 thin-film Schottky barrier photodiode is expected to facilitate the development of a compact and energy-independent next-generation UV-C photodetector with solar blindness.

Metalorganic chemical vapor deposition of α-Ga2O3 and α-(AlxGa1−x)2O3 thin films on m-plane sapphire substrates

Single α-phase (AlxGa1−x)2O3 thin films are grown on m-plane sapphire (α-Al2O3) substrates via metalorganic chemical vapor deposition. By systematically tuning the growth parameters including the precursor molar flow rates, chamber pressure, and growth temperature, the epitaxial growth of high-quality phase pure α-(AlxGa1−x)2O3 films (0 ≤ x ≤ 1) is demonstrated with smooth surface morphologies and alloy homogeneities by comprehensive material characterization. The asymmetrical reciprocal space mapping reveals fully relaxed films for α-(AlxGa1−x)2O3 films with x ≤ 0.5. The coherent growth of α-(AlxGa1−x)2O3/α-Al2O3 superlattice structures is demonstrated with abrupt interfaces and uniform Al distribution for higher Al compositions at x = 0.78 in the α-(AlxGa1−x)2O3 layer. The influence of growth parameters, such as growth temperature and chamber pressure, on the phase stabilization and Al incorporation in the α-(AlxGa1−x)2O3 films is investigated. While lower growth temperatures facilitate the phase stabilization of α-Ga2O3 thin films, lower chamber pressure leads to higher Al incorporation in α-(AlxGa1−x)2O3 films. High resolution x-ray photoelectron spectroscopy was utilized for determining the Al compositions and bandgaps of α-(AlxGa1−x)2O3. Furthermore, the evolution of the valance and conduction band offsets at α-Al2O3/α-(AlxGa1−x)2O3 heterojunctions is evaluated with the variation of Al compositions, which reveals the formation of type-I (straddling) band alignment between α-Al2O3 and α-(AlxGa1−x)2O3.

Sub- μ m features patterned with laser interference lithography for the epitaxial lateral overgrowth of α-Ga2O3 via mist chemical vapor deposition

The low growth rate of mist chemical vapor deposition normally requires a long growth time to achieve coalescence in the epitaxial lateral overgrowth of α-Ga2O3 thin films on sapphire substrates. To address this issue, sub-μm features were patterned using laser interference lithography. Periodical stripes with a ∼590-nm pitch allowed the overgrowth of crack-free, void-free, and continuous thin films, while typical growth conditions using a low carrier gas flow rate and a low Ga precursor concentration were maintained. Coalescence was achieved even with a short growth time of <30 min and a low film thickness of <500 nm. Transmittance and x-ray diffraction spectra show that the film was predominantly in α-phase. Transmission electron microscopy (TEM) images reveal cup-top-like α-Ga2O3 regions of low dislocation density on the SiOx mask. Selected area electron diffraction and high-resolution TEM analyses confirm that an α-Ga2O3 layer was formed even on the top of the SiOx mask. Interestingly, the dislocations formed on the window areas did not bend toward the center of the masks; rather, a dislocation bending outward from the center was observed. This suggests the occurrence of early coalescence and/or atomic rearrangement.

The effect of HCl on the α-Ga2O3 thin films fabricated by third generation mist chemical vapor deposition

α-Ga2O3 thin films were grown on a c-plane sapphire substrate by a third generation mist chemical vapor deposition system. The surface roughness was measured by atomic force microscopy, and the composition of Ga2O3 was measured by x-ray photoelectron spectroscopy. It was found that HCl affects the growth rate, purity, and surface roughness of α-Ga2O3 films. The growth rate increased with the HCl supply. The thickness, surface roughness, and chemical state analyses indicate that three growth modes occurred depending on the Ga supply rate and HCl/Ga supply ratio and the purity was improved by optimizing the HCl/Ga supply ratio and Ga supply rate.

Leidenforst Motion: Leidenfrost Motion of Water Microdroplets on Surface Substrate: Epitaxy of Gallium Oxide via Mist Chemical Vapor Deposition (Adv. Mater. Interfaces 6/2021)

The Leidenforst motion of diluted water micro-droplets on hot flat substrates enables the growth of α-Ga2O3 thin films via mist chemical vapor deposition. The incident probability, incident angle, lifetime, and gliding distance of the microdroplets at surface vicinity determine the growth behaviors of the epilayer. More details can be found in article number 2001895 by Minh-Tan Ha, Si-Young Bae and co-authors.

Strain Relaxation and Dislocation Annihilation in Compositionally Graded α-(Al xGa 1-x) 2O 3 Layer for High Voltage α-Ga 2O 3 Power Devices

α-Ga2O3 of the corundum structure and the large bandgap of 5.3 eV has attracted great interest because it can be grown on a sapphire (α-Al2O3) substrate with the same crystal structure. However, the lattice mismatch (4.3%) between α-Ga2O3 and the sapphire substrate induces a high density of threading dislocations, which act as breakdown leakage paths and lead to deterioration of the crystallinity and electrical properties of the α-Ga2O3 films grown directly on the substrate. Herein, to circumvent this issue compositionally graded α-(AlxGa1-x)2O3 layers are adopted to reduce threading dislocations for a high quality of epitaxial α-Ga2O3 films. The evolution of strain relaxation and the inclination of threading dislocations in graded α-(AlxGa1-x)2O3 layers are confirmed by reciprocal space mapping (RSM) and transmission electron microscopy (TEM). Through RSM and TEM studies, we confirmed that compressive strain enhances the inclination of dislocations and therefore, the dislocations merge and annihilate in the graded α-(AlxGa1-x)2O3 layers. The calculated density of threading dislocations in an α-Ga2O33 films with a graded α-(AlxGa1-x)2O3 buffer layer is reduced by 64.9% compared with that of an α-Ga2O3 films deposited directly grown on a bare sapphire substrate. Furthermore, a fabricated lateral-structure Schottky diode reveals enhanced breakdown voltages and forward current density due to the improved crystalline quality by using the graded α-(AlxGa1-x)2O3 buffer layer. This study provides an attractive approach for obtaining high-quality epitaxial α-Ga2O3 thin films for high voltage power devices.