Epitaxial β-Ga2O3 Research Articles

Microstructure and optical properties of β-Ga2O3 thin films fabricated by pulsed laser deposition

β-Ga2O3 has attracted extensive attention in the fields of optoelectronics and high-power electric devices due to its excellent electrical properties and thermal stability. Growth of epitaxial β-Ga2O3 films with good crystallinity is challenging since it is difficult to effectively control the impurities in the films. Here, β-Ga2O3 thin films were epitaxially grown on MgO (100) substrates via pulsed laser deposition system. The microstructure and optical properties of the β-Ga2O3 thin films were systematically characterized by combining high-resolution X-ray diffraction, X-ray photoelectron spectroscopy, ultraviolet–visible spectrophotometer and advanced transmission electron microscopy. Effects of the growth temperature and O2 partial pressure on the crystallinity, valance states of Ga ions and band gap of the β-Ga2O3 thin films were investigated. The light absorption wavelength of the β-Ga2O3 thin films ranges from 220 nm to 260 nm, which is close to the theoretical value. The epitaxial orientation between β-Ga2O3 film and MgO substrate is β-Ga2O3 (100) [02¯1] // MgO (100) [010]. A thin layer of γ-Ga2O3 exists in the interfacial region.

Unraveling evolution of microstructural domains in the heteroepitaxy of β -Ga2O3 on sapphire

Addressing microstructural domain disorders within epitaxial β-Ga2O3 is critical for phase engineering and property improvement, whereas the associated evolution of β-Ga2O3 heteroepitaxial domains remains largely unexplored. In this Letter, we conducted a quantitative investigation of microstructural domains in (−201)-oriented epitaxial β-Ga2O3 films grown on (0001) sapphire using halide vapor-phase epitaxy technique with a β-(Al0.57Ga0.43)2O3 buffer layer. The distinct split of x-ray diffraction rocking curves for (−201) β-Ga2O3 grown below 950 °C was observed, indicative of domain tilt disorders. As quantitatively assessed by transmission electron microscopy, the domain tilt angle significantly decreases from 2.33° to 0.90° along the [132] zone axis and from 2.3° to 0.56° along the [010] zone axis, respectively, as the growth temperature is elevated from 850 to 1100 °C. The reduction in tilt disorders is accompanied by the decrease in in-plane domain twist. It indicates that the elimination of small-angle domain boundaries is energetically favorable at high growth temperature above 1000 °C. The quantitative investigation on the evolution of domain disorders in β-Ga2O3 shed light on the pathway to improve epitaxial quality for cutting-edge power electronic and optoelectronic device applications.

Killer defect responsible for reverse leakage current in halide vapor phase epitaxial (011) β -Ga2O3 Schottky barrier diodes investigated via ultrahigh sensitive emission microscopy and synchrotron x-ray topography

In this study, we identify the killer defect responsible for the reverse leakage in the halide vapor phase epitaxial (011) β-Ga2O3 Schottky barrier diode via ultrahigh sensitive emission microscopy, synchrotron x-ray topography, and scanning transmission electron microscopy. A polycrystalline defect was found to be causing a leakage current of −5.1 μA at a reverse bias of −50 V. They were distributed across the wafer with a density ranging from 10 to 103 cm−2. Cross-sectional scanning electron microscopy of the polycrystalline defect revealed domains with various crystal orientations accompanied by a (100)-oriented micro-crack and dislocations along the [010] direction.

Observation of comet-shaped defect as killer defect in halide vapor phase epitaxial (001) β-Ga2O3 and its impact on Schottky barrier diodes

We observed killer defects that served as reverse leakage current paths for halide vapor phase epitaxial (001) β-Ga2O3 Schottky barrier diodes, which resulted in a leakage current of −0.46 μA at reverse bias of −100 V. Synchrotron X-ray topography revealed comet-shaped contrasts extending along [010] which are induced from the strain field surrounding the defects. They consisted of perfect (100)-cracks and horizontal-oriented (001) cracks. The cross-sectional scanning transmission electron microscopy observation showed (100)-cracks on the surface and dislocations along [100] beneath the surface. This defect was speculated to be generated from damage during the chemical-mechanical polishing.

Uniformity improvement of thickness and net donor concentration in halide vapor phase epitaxial β-Ga2O3 wafers prepared on miscut angle substrates

High uniformity of the thickness and net donor concentrations (N d-N a) in 100 mm diameter β-Ga2O3 (001) epitaxial wafer prepared by halide vapor phase epitaxy was demonstrated. An epitaxial wafer grown on a substrate with a miscut angle near 0° generated a specific epitaxial region with a lower growth rate and a higher N d-N a than the rest of the region, which deteriorated the wafer uniformity. In contrast, epitaxial wafers prepared on substrates with a miscut angle of –0.1° suppressed the occurrence of the specific epitaxial region, and an average thickness after chemical mechanical polishing of 10.6 μm with 5% tolerance and an average N d-N a without intentionally doping of 1.1 × 1016 cm−3 within 7% tolerance were successfully obtained.

Characterization of dislocation of halide vapor phase epitaxial (001) β-Ga2O3 by ultrahigh sensitive emission microscopy and synchrotron X-ray topography and its influence on Schottky barrier diodes

This study elucidates the dislocation responsible for the leakage current in a halide vapor phase epitaxial (001) β-Ga2O3 Schottky barrier diode. A high reverse leakage current of −0.98 μA was observed at −100 V as an emission pattern via ultrahigh-sensitive emission microscopy. This was attributed to the presence of dislocation. Moreover, the Burgers vector of this dislocation was determined to be ⫽ 〈12〉 from the g·b invisibility criteria via synchrotron X-ray topography observation. This dislocation was speculated to be induced by the relaxation of the strain field surrounding the void and subsequently propagated to the surface. Cross-sectional scanning transmission electron microscopy revealed multiple threading dislocations along the (100) plane.

Solar-blind avalanche photodetector based on epitaxial Ga2O3/La0.8Ca0.2MnO3 pn heterojunction with ultrahigh gain

Ga2O3-based avalanche photodetectors (APDs) have gained increasing attention because of their excellent photoelectric conversion capability in the UV solar-blind region. Integrating high-quality epitaxial Ga2O3 with p-type semiconductor remains an open challenge associated with the integration difficulty on alleviating its defects and dislocations. Herein, we construct an APD consisting of epitaxial β-Ga2O3/La0.8Ca0.2MnO3 heterostructure. The pn junction APDs exhibit a high responsivity of 568 A/W as well as an enhanced avalanche gain of up to 3.0×105 at a reverse bias voltage of 37.9 V. The integration capability demonstrated in this work provides exciting opportunities for further development of high-performance Ga2O3-based electronics and optoelectronics.

Epitaxial Growth of β-Ga2O3 Thin Films on Si with YSZ Buffer Layer.

We report the epitaxial growth of (2̅01)-oriented β-Ga2O3 thin films on a (001) Si substrate using the pulsed laser deposition technique employing epitaxial yttria-stabilized zirconia (YSZ) buffer layers. Epitaxial β-Ga2O3 thin films possess a biaxial compressive strain on YSZ single-crystal substrates while they exhibit a biaxial tensile strain on YSZ-buffered Si substrates. Post-annealing improves the crystalline quality of β-Ga2O3 thin films. High-resolution X-ray diffraction analyses reveal that the epitaxial (2̅01) β-Ga2O3 thin films on Si have eight in-plane domain variants to accommodate the large difference in the crystal structure between monoclinic β-Ga2O3 and cubic YSZ. The results provide a pathway to integrate epitaxial β-Ga2O3 thin films on a Si gold standard substrate, which will expand the application scope beyond high-power electronics.

Critical review of Ohmic and Schottky contacts to β-Ga2O3

Over the last decade, beta-phase gallium oxide (β-Ga2O3) has developed an extensive interest for applications such as high-power electronics. Due to its ultrawide bandgap of ∼4.8 eV and predicted breakdown field of ∼8 MV/cm along with its ability to be grown from the melt, this material demonstrates immense promise for high-voltage switching. The pace of development for β-Ga2O3 over these past ten years has been rapid, and ample new information has been generated on metal/β-Ga2O3 interfaces. With the advent of high-quality melt-grown substrates and epitaxial layers, low ideality factors (<1.1) and high Schottky barrier heights (>2.2 eV) have been demonstrated for Schottky contacts. For Ohmic contacts, specific contact resistivities of the common Ti/Au metallization are routinely reported in the 10−5–10−6 Ω cm2 range. This critical and comprehensive review focuses on the fundamental physics of Ohmic and Schottky contacts to bulk and epitaxial β-Ga2O3 in the published literature. It centers on the influence of surface treatments and defects on electrical contacts, Ohmic contacts, and Schottky contacts to β-Ga2O3. Native upward band bending is observed on β-Ga2O3, and the influence of wet and dry etching on band bending along with Schottky barrier height and ideality factor is discussed. Work on Ohmic contacts concentrates on the conventional Ti/Au anneal but additional Ohmic metallizations such as conductive oxides and others are treated as well. Schottky contacts are examined with specific focus on Fermi level pinning, thermal stability of Schottky metallizations, and Schottky barrier inhomogeneity.

Activation of implanted Si, Ge, and Sn donors in high-resistivity halide vapor phase epitaxial β-Ga2O3:N with high mobility

Activation of implanted donors into a highly-resistive, nitrogen-doped homoepitaxial β-Ga2O3 has been investigated. Nitrogen acceptors with the concentration of ∼1017 cm−3 were incorporated during epitaxial growth yielding low-doped (net donor concentration <1014 cm−3) films subsequently implanted with Si, Ge, and Sn. Upon Ohmic contact formation to the implanted regions, sheet resistance values of 314, 926, and 1676 Ω/sq were measured at room temperature for the Si-, Ge-, and Sn-implanted samples, respectively. Room temperature Hall measurements resulted in sheet carrier concentrations and Hall mobilities of 2.13 × 1014 /93, 8.58 × 1013/78, and 5.87 × 1013/63 cm2/(V s), respectively, for these three donor species. Secondary ion mass spectroscopy showed a volumetric dopant concentration of approximately 2 × 1019 cm−3 for the three species, resulting in carrier activation efficiencies of 64.7%, 40.3%, and 28.2% for Si, Ge, and Sn, respectively. Temperature-dependent Hall effect measurements ranging from 15 to 300 K showed a nearly constant carrier concentration in the Si-implanted sample, suggesting the formation of an impurity band indicative of degenerate doping. With a bulk carrier concentration of 1.3 × 1019 cm−3 for the Si implanted sample, a room temperature mobility of 93 cm2/(V s) is among the highest reported in Ga2O3 with a similar carrier concentration. The unimplanted Ga2O3:N regions remained highly resistive after the surrounding areas received implant and activation anneal. These results open the pathway for fabricating Ga2O3 devices through the selective n-type doping in highly resistive epitaxial Ga2O3.

Identification of dislocation responsible for leakage current in halide vapor phase epitaxial (001) β-Ga2O3 Schottky barrier diodes investigated via ultrahigh-sensitive emission microscopy and synchrotron X-ray topography

Dislocation responsible for leakage current in a halide vapor phase epitaxial (HVPE) (001) β-Ga2O3 Schottky barrier diode (SBD) was explored via ultrahigh-sensitive emission microscopy, synchrotron X-ray topography, and scanning transmission electron microscopy (STEM). Light emission due to the reverse leakage current was observed at the dislocation position with a leakage current of −0.98 μA at −100 V. Moreover, the dislocation is visible for the g vectors of 605 and 225; however, it is invisible for and . The b was determined to be from the g·b invisible criteria. This dislocation was determined mixed-type.

Dynamic Switching of 1.9 A/1.76 kV Forward Current NiO/β-Ga2O3 Rectifiers

The switching performance of unpackaged vertical geometry NiO/β-Ga2O3 rectifiers with a reverse breakdown voltage of 1.76 kV (0.1 cm diameter, 7.85 × 10−3 cm2 area) and an absolute forward current of 1.9 A fabricated on 20 μm thick epitaxial β-Ga2O3 drift layers and a double layer of NiO to optimize breakdown and contact resistance was measured with an inductive load test circuit. The Baliga figure-of-merit of the devices was 261 MW.cm−2, with differential on-state resistance of 11.86 mΩ.cm2. The recovery characteristics for these rectifiers switching from forward current of 1 A to reverse off-state voltage of −550 V showed a measurement-parasitic-limited recovery time (trr) of 101 ns, with a peak current value of 1.4 A for switching from 640 V. The reverse recovery time was limited by extrinsic parasitic and thus does not represent the intrinsic device characteristics. There was no significant dependence of trr on switching voltage or forward current.

Highly conductive epitaxial β-Ga2O3 and β-(Al x Ga1−x )2O3 films by MOCVD

We report on the epitaxial growth of highly conductive degenerated Ge or Si doped β-Ga2O3 and β-(Al x Ga1−x )2O3 films by MOCVD. Highly conductive homoepitaxial β-Ga2O3 layers with record conductivities of 2523 and 1581 S cm−1 were realized using Si and Ge dopants. Highly conductive Si doped β-(Al x Ga1−x )2O3 films were also grown. The incorporation of Si decreased with the increase in Al content and layer thickness. Record high conductivity of 612 S cm−1 was attained for coherently strained β-(Al0.12Ga0.88)2O3.

Heteroepitaxial β-Ga2O3 thick films on sapphire substrate by carbothermal reduction rapid growth method

The heteroepitaxial β-Ga2O3 thick films were rapidly grown on various oriented sapphire substrates using carbothermal reduction method. The β-Ga2O3 films were prepared in our home-made vertical dual temperature zone furnace. The growth direction as well as surface morphology showed the strong dependence on the orientation of the sapphire substrate. The fastest growth rate was obtained reaching approximate 15 μm h−1 on c-plane sapphire substrate according to the average 30 μm thickness of β-Ga2O3 films grown for 2 h measured by cross-section scanning electron microscope. The Raman spectra indicated the pure-phase β-Ga2O3 films without obvious strain. The bandgap for grown films were in range of 4.6–4.7 eV confirmed by x-ray photoelectron spectra and Tauc plot from absorption spectra. Secondary ion mass spectrometry was used to check the impurities indicating a limited amount of residual carbon inside the films even though graphite as the reducing agent. The results in this work give promising alternative method of rapid epitaxial β-Ga2O3 thick films for the application on high-power electronic devices.

High Stability Flexible Deep-UV Detector Based on All-Oxide Heteroepitaxial Junction

Transparent flexible electronics constitute a significant research field. Flexible deep-ultraviolet (UV) detectors have received much attention due to their potential in the applications of healthcare, communications, astronomy, and environment monitoring. Recent studies have investigated a variety of flexible photodetectors but show that the transparent, flexible, chemical, and thermal stability performances of these detectors cannot meet the requirements for practical applications. In this study, we demonstrate transparent flexible deep-UV detectors based on the combination of high-quality epitaxial n-type β-Ga2O3 and p-type NiO films as a photodiode on a flexible muscovite substrate. The electrical current of this heterojunction is increased over a 1000 times for on/off ratio under 265 nm wavelength illumination with a reasonable response (<1 s). Moreover, these photodetectors also exhibit good thermal stability as well as excellent mechanical flexibility. Our results exhibit the superior performance of the oxide-based solar-blind deep-UV detectors for advanced flexible sensing and smart applications.

Epitaxial growth of β-Ga2O3 by hot-wall MOCVD

The hot-wall metalorganic chemical vapor deposition (MOCVD) concept, previously shown to enable superior material quality and high performance devices based on wide bandgap semiconductors, such as Ga(Al)N and SiC, has been applied to the epitaxial growth of β-Ga2O3. Epitaxial β-Ga2O3 layers at high growth rates (above 1 μm/h), at low reagent flows, and at reduced growth temperatures (740 °C) are demonstrated. A high crystalline quality epitaxial material on a c-plane sapphire substrate is attained as corroborated by a combination of x-ray diffraction, high-resolution scanning transmission electron microscopy, and spectroscopic ellipsometry measurements. The hot-wall MOCVD process is transferred to homoepitaxy, and single-crystalline homoepitaxial β-Ga2O3 layers are demonstrated with a 2̄01 rocking curve width of 118 arc sec, which is comparable to those of the edge-defined film-fed grown (2̄01) β-Ga2O3 substrates, indicative of similar dislocation densities for epilayers and substrates. Hence, hot-wall MOCVD is proposed as a prospective growth method to be further explored for the fabrication of β-Ga2O3.

Epitaxial growth of β-Ga2O3 on SrTiO3 (001) and SrTiO3-buffered Si (001) substrates by plasma-assisted molecular beam epitaxy

Thin Ga2O3 films were deposited by plasma-assisted molecular beam epitaxy on SrTiO3 (001) and SrTiO3-buffered Si (001) substrates. Examination using reflection-high-energy electron diffraction, x-ray diffraction, and transmission electron microscopy shows a consistent picture of (100)- and (1¯12)-oriented β-Ga2O3 grains. The structural alignments are β-Ga2O3[010] || STO ⟨110⟩ and β-Ga2O3 [021] || STO ⟨100⟩, respectively, each with four in-plane rotational domain variants. Successful integration of epitaxial β-Ga2O3 with Si could enable major opportunities for monolithically integrated Ga2O3 technology by serving as a high-quality seed layer for further epitaxial growth.

Line-shaped defects: Origin of leakage current in halide vapor-phase epitaxial (001) β -Ga2O3 Schottky barrier diodes

We observed line-shaped defects in halide vapor-phase epitaxial (001) β-Ga2O3 SBDs. Light emission patterns, representing the reverse leakage current, were observed at these line-shaped defects. In atomic force microscopy observations, the line-shaped defects appeared as grooves with a typical depth and width of 113 and 520 nm, respectively. Such defects corresponded to a leakage current of −0.23 μA at −200 V. Simulation results indicated that the electric field, with an intensity of 1.3 MV/cm, was highly concentrated at the bottom of the line defects. According to the cross-sectional scanning transmission electron microscopy analysis, the line-shaped defects were generated from a dislocation network beneath the crystal near the surface.

High-performance β-Ga2O3 solar-blind ultraviolet photodetectors epitaxially grown on (110) TiO2 substrates by metalorganic chemical vapor deposition

High-quality (100)-oriented β-Ga2O3 films were successfully grown on (110) TiO2 substrates by metalorganic chemical vapor deposition (MOCVD) for the first time. Crystal structure, chemical composition, and surface morphology of the epitaxial β-Ga2O3 films were systematically investigated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and atomic force microscope (AFM). The epitaxial relationship between the β-Ga2O3 film and the TiO2 substrate was identified as β-Ga2O3 (100)//TiO2 (110) with β-Ga2O3 [001]//TiO2 [11‾0] and β-Ga2O3 [010]//TiO2 [001]. These material characterizations confirmed that the β-Ga2O3 film grown at 950 °C exhibited the highest crystalline quality and the smoothest surface. Furthermore, these β-Ga2O3 films were then used to fabricate solar-blind photodetectors (PDs) based on a metal-semiconductor-metal (MSM) structure. Due to the β-Ga2O3 film's best material and surface morphology grown at 950 °C, the β-Ga2O3 PDs showed the best performance with a large photocurrent of 85.3 μA, a responsivity of 2.56 A W−1, and a detectivity of 5 × 1011 Jones. This work explores a new TiO2 substrate for the heterogeneous epitaxy of β-Ga2O3 films, and opens the door for the development of β-Ga2O3 photonics and electronics on cost-effective mass-producible substrates.

Solid-phase epitaxial crystallization of β-Ga2O3 thin film by KrF excimer laser irradiation from backside of NiO (111)-buffered α-Al2O3 (0001) substrate at room temperature

We have investigated the solid-phase epitaxial crystallization of β-Ga2O3 thin films on NiO (111) (3 nm thick)-buffered α-Al2O3 (0001) substrates prepared by pulsed KrF excimer laser irradiation onto amorphous Ga2O3 thin films from the backside of the substrate at room temperature. The results of x-ray diffraction, reflection high-energy electron diffraction, and transmission electron microscopy indicated that the solid-phase crystallization of the amorphous Ga2O3 thin film to epitaxial β-Ga2O3 film started from the film/substrate interface toward the film surface with an average growth rate of about 0.1 nm thickness per one pulse irradiation. The optical bandgap of the epitaxial β-Ga2O3 thin films (∼80 nm thick) was estimated to be 4.9 eV from the UV/Vis transmittance measurement.