β-Ga2O3 Single Crystal Substrates Research Articles

Temperature-dependent capacitance-voltage characteristics of β-Ga2O3 Schottky barrier diodes with (001) epitaxial grown layer using MOCVD

We investigate the thin film quality of the epitaxial layer on Sn-doped (001)-oriented β-Ga2O3 single crystal substrates. The homoepitaxial layer is grown by metal organic chemical vapor deposition (MOCVD) at 700 °C, 800 °C, 900 °C, and 1000 °C. To evaluate the grown epitaxial layer at different temperatures, the depletion layer of the Schottky barrier diode (SBD) is studied using the impedance-phase angle (Z-theta) via capacitance-voltage measurements. In addition, the Z-theta value is correlated with X-ray diffraction and transmission electron microscopy. The results show that the epitaxial layer grown at 900 °C has the best epitaxial layer quality. It is presumed that Z-theta measurements are a feasible and reliable approach to evaluate the epitaxial layer in Ga2O3-related devices.

Large-Scale β-Ga2O3 Trench MOS-Type Schottky Barrier Diodes with 1.02 Ideality Factor and 0.72 V Turn-On Voltage

β-Ga2O3 Schottky barrier diodes (SBDs) suffer from the electric field crowding and barrier height lowering effect, resulting in a low breakdown voltage (BV) and high reverse leakage current. Here, we developed β-Ga2O3 trench MOS-type Schottky barrier diodes (TMSBDs) on β-Ga2O3 single-crystal substrates with halide vapor phase epitaxial layers based on ultraviolet lithography and dry etching. The 1/C2−V plots are deflected at 2.24 V, which is caused by the complete depletion in the mesa region of the TMSBDs. A close-to-unity ideality factor of 1.02 and a low turn-on voltage of 0.72 V are obtained. This is due to the low interface trap density in the metal/semiconductor interface of TMSBDs, as confirmed by the current–voltage (I–V) hysteresis measurements. The specific on-resistance calculated with the actual Schottky contact area increases as the area ratio (AR) increases because of the current spreading phenomenon. Furthermore, the reverse leakage current of the TMSBDs is smaller and the BV is increased by 120 V compared with the regular SBD. This work paves the way for further improving the overall performance of β-Ga2O3 TMSBDs.

Band alignment of h-BN/β-Ga2O3 heterostructure grown via ion beam sputtering deposition

Integrating two-dimensional ultra-wide band gap hexagonal boron nitride (h-BN) on β-Ga2O3 surface into van der Waals heterostructures is of great interest for developing novel high-power devices and optoelectronic devices because of their unique properties. The energy band alignment at the heterointerface is critical for device design, however, the band alignment of h-BN/β-Ga2O3 heterojunction has not been investigated to date. In this work, the h-BN/β-Ga2O3 heterostructure is constructed by directly growing h-BN few-layer on the β-Ga2O3 single crystal substrate using ion beam sputtering deposition method. The high-quality few-layer h-BN with the abrupt interface and smooth surface allow for the accurate determination of band alignment at the h-BN/β-Ga2O3 heterointerface by X-ray photoelectron spectroscopies. The valence and conduction band offsets are determined to be 0.47 and 1.42 eV for the h-BN/β-Ga2O3 heterostructure, respectively, with a type-Ⅱ staggered band alignment. Furthermore, the electronic structures of h-BN/β-Ga2O3 heterostructure are also investigated experimentally and theoretically. These results indicate that the h-BN/β-Ga2O3 heterostructure has great potential in (opto)electronic devices.

Lateral β-Ga2O3 Schottky barrier diode fabricated on (–201) single crystal substrate and its temperature-dependent current–voltage characteristics

Lateral β-Ga2O3 Schottky barrier diodes (SBDs) each are fabricated on an unintentionally doped (-201) n-type β-Ga2O3 single crystal substrate by designing L-shaped electrodes. By introducing sidewall electrodes on both sides of the conductive channel, the SBD demonstrates a high current density of 223 mA/mm and low specific on-resistance of 4.7 mΩ⋅cm2. Temperature-dependent performance is studied and the Schottky barrier height is extracted to be in a range between 1.3 eV and 1.35 eV at temperatures ranging from 20 °C to 150 °C. These results suggest that the lateral β-Ga2O3 SBD has a tremendous potential for future power electronic applications.

The Influence of the Preheating Temperature of the (−2 0 1) β-Ga2O3 Substrates on c-Plane GaN Epitaxial Growth

In this paper, we demonstrate the direct epitaxial growth of c-plane GaN on a preheated (−2 0 1) β-Ga2O3 single-crystal substrate with no interlayer or pre-patterning processes by using the atmospheric pressure metalorganic chemical vapor deposition method. The results show that high-temperature preheating (>500 °C) can modify the surface morphology of the substrate so that the crystalline quality of the grown GaN layer can be improved. With higher preheated temperatures, the grown GaN layer reveals smaller FWHM (full width at half-maximum) of the X-ray rocking curve. In addition, we find that the photoluminescence spectra of the GaN layers reveal their narrowest linewidth at a preheated temperature of 800 °C. These results support improvements of crystalline quality and provide optimization of a c-GaN grown epitaxially on the preheated (−2 0 1) β-Ga2O3 substrates for further device fabrication.

High temperature (500 °C) operating limits of oxidized platinum group metal (PtOx, IrOx, PdOx, RuOx) Schottky contacts on β -Ga2O3

Very high temperature operation β-Ga2O3 Schottky contacts were fabricated on moderately doped 2¯01 β-Ga2O3 single crystal substrates using four different types of intentionally oxidized platinum group metal (PGM) Schottky contacts (SCs), i.e., PtOx, IrOx, PdOx, and RuOx (x ∼ 2.0, 2.2, 1.1, and 2.4, respectively) formed by reactive rf sputtering of plain-metal targets in an oxidizing plasma. All four types of oxidized PGM SCs showed rectification ratios (at ± 3 V) of more than 10 orders of magnitude up to 300 °C, with almost no measurable increase in reverse leakage current density (Jrev) from that at room temperature. From 350 to 500 °C, a measurable increase in Jrev was observed, which was consistent with the thermionic emission of charge carriers over the respective image force (IF) lowered Schottky barriers. Despite this increase, PtOx(IrOx)[PdOx]{RuOx} SCs showed large rectification ratios (at ± 3 V) of 6 × 106(8 × 106)[5 × 105]{2 × 104} and IF-corrected barrier heights of 2.10(2.10)[1.90]{1.60} ± 0.05 eV, respectively, while operating at 500 °C. The significantly lower 500 °C barrier height of the RuOx SCs was due to the thermal reduction of RuOx to Ru that occurred above 400 °C. In contrast, the Schottky barriers of IrOx, PtOx, and PdOx SCs were thermally stable while operating at 500 °C, indicating significant potential for their use in very high temperature rectifying devices.

Observation of dislocations in β-Ga2O3 single-crystal substrates by synchrotron X-ray topography, chemical etching, and transmission electron microscopy

To reveal dislocations in β-Ga2O3 and categorize them by their Burgers vectors, we have carried out a comprehensive dislocation characterization by synchrotron X-ray topography (XRT), chemical etching, and transmission electron microscopy (TEM). An important type of dislocation confirmed by XRT is the screw-type dislocations extending in the 〈010〉 direction at a density of about 4.2 × 104 cm−2. These dislocations had length in millimeters and seldom bent to other directions. Chemical etching using molten KOH + NaOH at 200 °C formed hillocks with multiple crystallographic facets on the (010) surface. These hillocks showed various geometrical features that indicated the direction of the dislocation lines. TEM observation confirmed that the hillocks were related to dislocations. Finally, the feasibility of using TEM weak-beam dark-field images and large-angle convergent-beam electron diffraction to identify dislocation Burgers vectors was examined, which seemed to be problematic due to the low-symmetry of the monoclinic crystal structure of β-Ga2O3.

Forward bias degradation and thermal simulations of vertical geometry β-Ga2O3 Schottky rectifiers

Vertical geometry β-Ga2O3 Schottky rectifiers of various sizes were deliberately stressed at a high forward current density level until a sudden decrease of reverse bias breakdown voltage was observed. The diodes were fabricated on an Sn-doped (n = 3.6 × 1018 cm−3) (001) β-Ga2O3 single crystal substrate with a 10 μm epilayer grown by halide vapor phase epitaxy with a carrier concentration of 3.5 × 1016 cm−3. The forward bias stressing caused reverse breakdown degradation and thermally induced failure on both the Ni/Au Schottky contact and the epitaxial layer due to the low thermal conductivity of Ga2O3. The resulting temperature distributions at forward bias under different current conditions were simulated using 3D finite element analysis. The temperature profile at the surface during the rectifier turn-on period shows a strong dependence with crystalline orientation, evidenced by infrared camera measurements. The maximum junction temperature rise occurs at the center of the metal contact and is in the range of 270–350 °C.

Fabricating GaN-based LEDs on (−2 0 1) β-Ga2O3 substrate via non-continuous/continuous growth between low-temperature undoped-GaN and high-temperature undoped-GaN in atmospheric pressure metal-organic chemical vapor deposition

This study demonstrates two approaches to the growth of GaN-based LEDs on (−2 0 1)-oriented β-Ga2O3 single crystal substrates using metal-organic CVD under atmospheric pressure. One approach induces non-continuous growth between low-temperature undoped-GaN (u-GaN) and high-temperature u-GaN, whereas the other approach induces continuous growth. We observed the following reduction in the FWHM of X-ray diffraction rocking curves: GaN (0 0 2) on (−2 0 1) β-Ga2O3 substrate (from 464 to 342 arcsec) and GaN (1 0 2) (from 886 to 493 arcsec). An LED with six pairs of InGaN/GaN multiple quantum wells was successfully fabricated on the (−2 0 1) β-Ga2O3 single crystal substrate.

High-temperature (350 °C) oxidized iridium Schottky contacts on β -Ga2O3

Oxidized iridium (IrOx) Schottky contacts (SCs) with excellent high temperature stability were fabricated on 2¯01 β-Ga2O3 single crystal substrates. These IrOx:β-Ga2O3 SCs were operated at temperatures from 24 to 350 °C with only a very small increase in reverse leakage current, while maintaining extremely high rectification ratios (at ±3 V) of more than 10 orders of magnitude at all temperatures, including 350 °C. This remarkable high temperature performance was due to their very high and thermally stable rectifying barriers that, after an initial heat-related improvement, were characterized by zero-bias effective barrier heights of 2.05 ± 0.02 eV and ideality factors of 1.05–1.10, which were almost unchanged by further repeated operation at 350 °C. The reverse leakage current density at 350 °C was only ∼2.3 × 10−9 A cm−2 (∼3.0 pA) at −3 V and ∼7.5 × 10−8 A cm−2 (∼100 pA) at −100 V. These IrOx:β-Ga2O3 SCs represent a significant improvement in high-temperature β-Ga2O3 SC performance, with considerable potential for the fabrication of high temperature β-Ga2O3 rectifying diodes, deep UV photodetectors, and metal-semiconductor field effect transistors.

Direct comparison of plain and oxidized metal Schottky contacts on β-Ga2O3

High quality Ru, Ir, Pd, Pt, Ag, and Au Schottky contacts (SCs) were fabricated on 2¯01 β-Ga2O3 single crystal substrates via rf sputtering under inert and oxidizing plasma conditions. The oxidized SCs exhibited significantly higher rectifying barriers and, with the exception of gold oxide, significantly improved high temperature performance, with more than 12 orders of magnitude of stable rectification at 180 °C. With the exception of Ag, the image-force-corrected laterally homogeneous barrier heights of the plain metal SCs were pinned close to 1.3 eV, irrespective of the metal work function, with the Fermi level at the SC interface close to the predicted VO (2+/0) transition level of fourfold coordinated oxygen vacancies. The equivalent barrier heights of the oxidized SCs were consistently 0.5–0.8 eV higher than their plain metal counterparts, lying in the range of 1.8–2.5 eV, with the increase attributed to the passivation of interfacial oxygen vacancies and a significant increase in the work function of the oxidized metals. The highest Schottky barriers for both the plain and oxidized metal SCs involved Ag, which may be linked to the relative ease of its unintentional and intentional oxidation, respectively. The very high rectifying barriers and the thermal stability of oxidized Schottky contacts to β-Ga2O3 indicate their potential for high temperature device applications.

Schottky barrier diode based on β-Ga2O3 (100) single crystal substrate and its temperature-dependent electrical characteristics

The Pt/β-Ga2O3 Schottky barrier diode and its temperature-dependent current-voltage characteristics were investigated for power device application. The edge-defined film-fed growth (EFG) technique was utilized to grow the (100)-oriented β-Ga2O3 single crystal substrate that shows good crystal quality characterized by X-ray diffraction and high resolution transmission electron microscope. Ohmic and Schottky electrodes were fabricated by depositing Ti and Pt metals on the two surfaces, respectively. Through the current-voltage (I-V) measurement under different temperature and the thermionic emission modeling, the fabricated Pt/β-Ga2O3 Schottky diode was found to show good performances at room temperature, including rectification ratio of 1010, ideality factor (n) of 1.1, Schottky barrier height (ΦB) of 1.39 eV, threshold voltage (Vbi) of 1.07 V, ON-resistance (RON) of 12.5 mΩ·cm2, forward current density at 2 V (J@2V) of 56 A/cm2, and saturation current density (J0) of 2 × 10−16 A/cm2. The effective donor concentration Nd − Na was calculated to be about 2.3 × 1014 cm3. Good temperature dependent performance was also found in the device. The Schottky barrier height was estimated to be about 1.3 eV–1.39 eV at temperatures ranging from room temperature to 150 °C. With increasing temperature, parameters such as RON and J@2V become better, proving that the diode can work well at high temperature. The EFG grown β-Ga2O3 single crystal is a promising material to be used in the power devices.

Communication—Electrical Characterization of β-Ga2O3 Single Crystal Substrates

We have electrically investigated deep-level defects in unintentionally-doped β-Ga2O3 single crystal substrates fabricated by an edge-defined film-fed growth method, employing capacitance-voltage (C-V) and steady-state photo-capacitance spectroscopy (SSPC) techniques. From C-V measurements, n-type residual carriers with their concentration of ∼8.7 × 1016 cm−3 were almost uniformly distributed over the measured depth region of 188 – 336 nm from the β-Ga2O3 surface. SSPC measurements revealed four dominant deep-level defects located at ∼0.8, ∼2.04, ∼2.71, and ∼3.87 eV below the conduction band, in addition to near-band-edge emissions at ∼4.49 eV. These deep-level defects tended to be more densely present with shallowing the probing depth range.

The orientational relationship between monoclinic β-Ga2O3 and cubic NiO

The orientational relationship between β-Ga2O3 and NiO was studied by X-ray diffraction measurements and cross-sectional high resolution transmission electron microscopy. A β-Ga2O3 thin film was formed on a (100) NiO layer on a (100) MgO substrate by gallium evaporation in an oxygen plasma. It was found that the resulting β-Ga2O3 had a four-fold domain structure satisfying both (100) β-Ga2O3 ‖ (100) NiO and (010) β-Ga2O3 ‖ {011} NiO. A γ-Ga2O3 layer was observed at the interface between the β-Ga2O3 and the NiO. An NiO film was also formed on a (100) β-Ga2O3 single-crystal substrate by the sol–gel method. An epitaxial (100) NiO film was formed on a (100) β-Ga2O3 substrate, and satisfied (011) NiO ‖ (010) β-Ga2O3. The crystal orientations of β-Ga2O3 on (100) NiO and NiO on (100) β-Ga2O3 can be explained using atomic arrangement models of the (100) plane of NiO and the (100) plane of β-Ga2O3.

Determination of Vickers microhardness in β-Ga2O3 single crystals grown from their own melt

The results of microhardness measurements of β-Ga2O3 single crystals for (001) crystallographic face are reported. The crystals were grown by the free crystallization with the Garnet-2M equipment. Microhardness values were determined by the Vickers method at varying loads. A four-sided diamond pyramid was used as an indenter. The average value of gallium oxide microhardness was equal to 8.91 GPa. We have carried out comparison of the values obtained with the microhardness for the other wide bandgap semiconductors epitaxial GaN layers grown on 6H-SiC and GaP layers grown on GaP:S. The findings are usable for machining process development of β-Ga2O3 single crystal substrates. In particular, silicon carbide and electrocorundum may be recommended for β-Ga2O3 machine processing.

Solar-blind photodiodes composed of a Au Schottky contact and a β-Ga2O3 single crystal with a high resistivity cap layer

We fabricated Ga2O3 photodiodes composed of a Au Schottky contact and a β-Ga2O3 single-crystal substrate with a sol–gel prepared high resistivity cap layer. The photodiodes with the cap layer showed solar-blind photosensitivity under both forward and reverse biases in contrast to conventional Schottky photodiodes. Finally, we proposed energy band diagram of the i-n junction to determine the photodetection mechanism of our photodiodes. The photoconductive device model explained the high responsivity of over 1 A/W at forward bias. In this model, the cap layer behaves like a photoconductor, and the substrate behaves like an electrode that replenishes electrons.

Wet Etching of β-Ga2O3 Substrates

Wet etching of (100)-oriented β-Ga2O3 single crystal substrates was carried out using H3PO4 and H2SO4. The etching reactions followed the Arrhenius equation, but the etching in H2SO4 at a high temperature of 190 °C was disturbed by the formation of sulfates on the surface. Considering the higher etching rate over the temperature range of 100–194 °C, H3PO4 is more preferable as an etchant for β-Ga2O3. Although the isotropic etching led to side etching, a grid pattern in the order of µm was successfully fabricated. These results indicate that this simple and low-cost wet etching using H3PO4 is suitable for isolating devices or patterning structures on β-Ga2O3 substrates.

Demonstration of GaN/InGaN Light Emitting Diodes on (100) β-Ga2O3 Substrates by Metalorganic Chemical Vapour Deposition

The growth and fabrication of GaN/InGaN multiple quantum well (MQW) light emitting diodes (LEDs) on (100) β-Ga2O3 single crystal substrates by metal-organic chemical vapour deposition (MOCVD) technique are reported. x-ray diffraction (XRD) θ – 2θ scan spectroscopy is carried out on the GaN buffer layer grown on a (100) β-Ga2O3 substrate. The spectrum presents several sharp peaks corresponding to the (100) β-Ga2O3 and (004) GaN. High-quality (0002) GaN material is obtained. The emission characteristics of the GaN/InGaN MQW LED are measurement. The first green LED on β-Ga2O3 with vertical current injection is demonstrated.

Growth of c-GaN films on the nitridated β-Ga2O3 substrates using RF-MBE

Cubic GaN is successfully grown on β-Ga2O3 by molecular beam epitaxy for the first time. Prior nitridation of the (100) β-Ga2O3 single-crystal substrate by exposure to electron cyclotron resonance nitrogen plasma causes the formation of a surficial (001) c-GaN layer, upon which homo-epitaxial growth of c-GaN can be achieved by radio-frequency molecular beam epitaxy. The epitaxial relationship is confirmed by electron microscopy to be (001) c-GaN // (100) β-Ga2O3, [110] c-GaN // [010] β-Ga2O3, and [1-10] c-GaN // [001] β-Ga2O3.

透明導電性基板としての酸化ガリウム単結晶の育成と評価

Large size single crystals of β-Ga2O3 with 1 inch in diameter have been grown by the floating zone technique. Wafers have been cut and fine polished in the (100), (010) and (001) planes. These were highly transparent in the visible and near UV, as well as electrically conductive, indicating the potential use of β-Ga2O3 as a substrate for optoelectric devices operating in the visible/near UV and with vertical current flow. Epitaxial growth of nitride compounds by the metalorganic vapor phase epitaxy (MOVPE) technique is demonstrated on β-Ga2O3 single crystal substrates. High-quality (0001) GaN epi-layers with a narrow bandedge luminescence are obtained using a low temperature conductive buffer layer. InGaN multi-quantum well (MQW) structures were also successfully grown. The first blue light-emitting diode (LED) on β-Ga2O3 with vertical current injection is demonstrated.