β-Ga2O3 Schottky Barrier Diodes Research Articles

Electroconductive and photoelectric properties of Pt/(100) β-Ga2O3 Schottky barrier diode based on Czochralski grown crystal

Pt/(100) β-Ga2O3 Schottky barrier diodes were fabricated using a plate cleaved from the crystal grown by Czochralski method. Their electroconductive and photoelectric characteristics were studied. The following values were obtained: the Schottky barrier height (1.69/1.62/1.74 eV), ideality coefficient (1.09/1.14), saturation current density (9.91 × 10–15 A/cm2), diode series resistance (7.98 kΩ), and net donor concentration [(1.8–2.4) × 1018 cm–3]. The diodes demonstrate a high rectification ratio of 1010 at an applied voltage of ± 1 V and a relatively low experimental value of the leakage current density ∼10–11 A/cm2. These structures are solar-blind and also capable of operating in self-powered mode. The diodes are highly sensitive to short-wave ultraviolet radiation with a wavelength λ ≤ 265 nm. The maximum values of responsivity (20.4 A/W), external quantum efficiency (1.2 × 104%), and detectivity (9.6 × 1015 Hz0.5 × cm × W–1) of diodes were registered under exposure to irradiation at λ = 210 nm and at applied voltage of −1 V. The responsivity and external quantum efficiency values in the self-powered operation mode were 12.3 A/W and 7.2 × 103%, respectively. The diodes showed low rise and decay times in self-powered operation mode for photodiode based on Ga2O3: 14 and 30 ms, respectively.

Three-step field-plated β-Ga2O3 Schottky barrier diodes and heterojunction diodes with sub-1 V turn-on and kilovolt-class breakdown

A unique field termination structure combining a three-step field plate with nitrogen ion implantation to enhance the reverse breakdown performance of Pt/β-Ga2O3 Schottky barrier diodes (SBDs) and NiO/β-Ga2O3 heterojunction diodes (HJDs) is reported. The fabricated devices showed a low R on,sp of 6.2 mΩ cm2 for SBDs and 6.8 mΩ cm2 for HJDs. HJDs showed a 0.8 V turn-on voltage along with an ideality factor of 1.1 leading to a low effective on-resistance of 18 mΩ cm2. The devices also showed low reverse leakage current (<1 mA cm−2) and a breakdown voltage of ∼1.4 kV. These results offer an alternative, simpler route for fabricating high-performance kilovolt-class β-Ga2O3 diodes.

Current transport mechanism of lateral Schottky barrier diodes on β-Ga2O3/SiC structure with atomic level interface

Heterogeneous integration of β-Ga2O3 on highly thermal conductive SiC substrate by the ion-cutting technique is an effective solution to break the heat-dissipation bottleneck of β-Ga2O3 power electronics. In order to acquire high-quality β-Ga2O3 materials on SiC substrates, it is essential to understand the influence of the ion-cutting process on the current transport in β-Ga2O3 devices and to further optimize the electrical characteristics of the exfoliated β-Ga2O3 materials. In this work, the high quality of β-Ga2O3/SiC structure was constructed by the ion-cutting process, in which an amorphous layer of only 1.2 nm was formed between β-Ga2O3 and SiC. The current transport characteristics of Au/Pt/Ni/β-Ga2O3 Schottky barrier diodes (SBDs) on SiC were systematically investigated. β-Ga2O3 SBDs with a high rectification ratio of 108 were realized on a heterogeneous β-Ga2O3 on-SiC (GaOSiC) substrate. The net carrier concentration of the β-Ga2O3 thin film for GaOSiC substrate was down to about 8% leading to a significantly higher resistivity, compared to the β-Ga2O3 donor wafer, which is attributed to the increase in acceptor-type implantation defects during the ion-cutting process. Furthermore, temperature-dependent current–voltage characteristics suggested that the reverse leakage current was limited by the thermionic emission at a low electric field, while at a high electric field, it was dominated by the Poole–Frenkel emission from E3 deep donors caused by the implantation-induced GaO antisite defects. These results would advance the development of β-Ga2O3 power devices on high thermal conductivity substrate fabricated by ion-cutting technique.

8.7 A/700 V β-Ga2O3 Schottky barrier diode demonstrated by oxygen annealing combined with self-aligned mesa termination

β-Ga2O3 Schottky barrier diodes (SBDs) with low-defect epitaxial surface and effective termination are essential for realizing excellent blocking characteristics. This work systematically studied oxygen annealing at various temperatures, optimizing the epitaxial surface by reducing the surface roughness and dislocation density. Combined with mesa termination, the results showed that the breakdown voltage (V br) significantly increased from 845 V to 1532 V. The device with a 3 × 3 mm2 anode size was fabricated simultaneously, and the high forward currents of 8.7 A@2 V and V br > 700 V were achieved. This work shows a possible solution for the commercialization of β-Ga2O3 SBDs.

β-Ga2O3 Schottky barrier height improvement using Ar/O2 plasma and HF surface treatments

In this report, we show that Ar/O2 plasma exposure followed by HF treatment improves the Schottky barrier height (SBH) in β-Ga2O3 Schottky barrier diodes (SBDs) by nearly 0.3 eV, resulting in a breakdown voltage (VBR) gain of over 100 V on 2 × 1016 cm−3 doped substrates, without compromising the specific on-resistance. The SBH and VBR enhancement is observed on (2¯01) as well as (001) surfaces. Through extensive surface characterization, the Ar/O2 plasma exposure is shown to amorphize and increase surface oxygen vacancy concentration. HF treatment cleans the surface damage and passivates the surface through fluorine adsorption, leading to Fermi-level de-pinning and SBH improvement. Remarkably, however, the Ar/O2 plasma exposure enhances fluorine adsorption when compared to fluorine treatment alone, resulting in a more substantial improvement in SBH and VBR. Surface clean/treatment plays a critical and fundamental role in determining the quality of the metal/β-Ga2O3 interface. The improved surface treatment process demonstrated in this work can be easily integrated with various field termination methods that can help further improve the β-Ga2O3 SBD performance.

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.

Oxygen vacancies and local amorphization introduced by high fluence neutron irradiation in β -Ga2O3 power diodes

Beta phase gallium oxide (β-Ga2O3) is emerging as a promising material for space applications due to its unique properties and potential high performance in extreme environments. In this work, we systematically study the impact of β-Ga2O3 Schottky barrier diodes (SBDs) under a high fluence neutron irradiation to explore the degradation mechanism of the devices. After irradiated by neutrons with an average energy of 1–2 MeV and a dose rate of 1.3 × 1012 cm−2 s−1, SBDs with a homoepitaxial layer suffered serious performance degradation. The main manifestation of this degradation was a substantial increase in on-resistance, which rose from 3.9 to 3.5 × 108 mΩ·cm2 under the aforementioned irradiation conditions. The appearance of amorphous/polycrystalline striped lattice damage in the epitaxial layer as well as the presence of deep-level defects caused by oxygen vacancies are factors related to this phenomenon. The simulation revealed that the capture reaction of neutrons and Ga elements is the primary cause of neutron irradiation. This reaction generates high-energy beta- particles (β-particles) resulting in the formation of defects. This paper reveals the degradation mechanism of β-Ga2O3 SBDs under neutron irradiation and provides a possible design roadmap for radiation-resistant β-Ga2O3 power devices. Moreover, a high-temperature oxygen annealing process was implemented, which proved to be in restoring the device performance.

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.

Research on the β-Ga2O3 Schottky barrier diodes with oxygen-containing plasma treatment

This Letter reports two kinds of oxygen-containing plasma treated β-Ga2O3 Schottky barrier diodes (SBDs), including N2O plasma treatment and O2 plasma treatment, and the SBD without plasma is prepared for comparison. I–V characteristics, breakdown characteristics, and trap state characteristics of three devices have been studied. It is found that the turn-on voltage of SBDs with N2O plasma can reduce to 0.6 V, and the better current density of 750 A/cm2 and an on-resistance of 3.5 mΩ cm2 are obtained after the N2O plasma treatment. Moreover, the breakdown voltage of SBDs with N2O plasma is 50.2% higher than the conventional one, whose value reaches 323 V. In addition, the trap states' characteristics of the devices are studied, which show that the oxygen-containing plasma can reduce the deep level trap states density partly in the anode region, which can improve the surface quality effectively.

Large-area vertical-geometry Pt/(010) β-Ga2O3 Schottky barrier diodes and their temperature-dependent electrical properties

Herein, vertical-geometry Pt/(010) β-Ga2O3 Schottky barrier diodes (SBDs) with various large areas (2.25 × 10−2, 4 × 10−2 to 6.25 × 10−2 cm2) were fabricated on (010) orientation β-Ga2O3 substrates. The electrical properties of different area diodes and the uniformity of diodes with the same area were analyzed. The rectifying ratio of all diodes was determined to be larger than 108 at the bias of ±2 V. The current-voltage (I–V) characteristics of SBD with an area of 2.25 × 10−2 cm2 were investigated in detail from 300 to 450 K. The parameters of the diodes show a strong dependence on temperature, including ideality factor (n), on-resistance (Ron), turn-on voltage (Von), and the Schottky barrier (Φb). The results indicate that a dominant inhomogeneous Schottky barrier exists at the Pt/β-Ga2O3 interface. Such large-area devices exhibit that the value of n is close to 1 and the value of Φb was 1.09 eV at zero bias. In addition, the Von remains low and the stability remains good even after the tests at 450 K. These results indicate that we have successfully fabricated large-area SBDs with excellent performances. This work may contribute to further understanding of the performance of β-Ga2O3 diodes in large areas and harsh environments.

Homoepitaxial Si-doped Gallium Oxide films by MOCVD with tunable electron concentrations and electrical properties

Doping technology with stable carrier concentration regulation is a guarantee for optimizing β-Ga2O3-based high-power electronic devices and optoelectronic devices. In this study, β-Ga2O3 homoepitaxial epitaxial films with stable and tunable electron concentrations were grown on Fe-doped (010)-oriented β-Ga2O3 substrates by using metal-organic chemical vapor deposition (MOCVD). XRD shows that the basic bulk properties of β-Ga2O3 film, i. e. the structure and crystal orientation, are stable with low Si-doping, however, its electrical properties vary remarkably depending on concentrations of Si ions incorporated by Si source flow rate during the film growth process. Hall measurement is utilized to confirm high-quality β-Ga2O3 films with tunable electron concentration of 1017–1019 cm−3. As a consequence, the electron concentration-dependent behavior of β-Ga2O3 Schottky barrier diodes (SBDs) was studied. The β-Ga2O3 SBDs exhibited better forward current output electrical performance with reduction in built-in potential difference (Vbi) from 2.4 V to 0.3 V and on-resistance (Ron) from 257 Ω cm to 26 Ω cm. Our results suggest that stable and tunable electron concentration provides high-quality material assurance for constructing power electronic devices of Ga2O3.

X-ray topographic observation of dislocations in β-Ga2O3 Schottky barrier diodes and their glide and multiplication under reverse bias

We observed dislocations in β-Ga2O3 Schottky barrier diodes (SBD) using synchrotron X-ray topography (XRT) and studied their behaviors under forward or reverse bias. Several representative dislocation types were identified by observing the dislocation lines and their Burgers vectors. After comparing the XRT images taken before and after the bias was applied, we found that the dislocations were not static but could glide and multiply under a reverse bias of −140 V for 5 min. This was frequently observed near the dislocation bundles emanating from the end of a screw-type b-axis dislocation that had a Burgers vector of b∥[010]. The glide belonged to the 〈010〉{001} slip system. The glide and multiplication occurred even when the dislocations were not directly located in the SBD area being electrically stressed. It is considered that the b-axis screw dislocations acted as pathways of leakage current that was the driven force for dislocation glide and multiplication.

Interface engineering of p-type quaternary metal oxide semiconductor interlayer-embedded β-Ga2O3 Schottky barrier diode

Here, we propose a p-type copper aluminum oxide (p-CuAlO2) interlayer for the high breakdown and low leakage current of β-Ga2O3 Schottky barrier diodes (SBDs). The XPS and TEM analysis was employed to confirm the surface elemental compositions, chemical states, and microstructural properties of RF sputtered p-CuAlO2 thin films on β-Ga2O3. The electrical and carrier transport properties of Pt/β-Ga2O3 SBD and Pt/p-CuAlO2/β-Ga2O3 heterojunction (HJ) were studied by using current-voltage (I–V) and capacitance-voltage (C–V) measurements. In comparison to the SBD, the HJ displayed superior rectifying behavior, lower reverse leakage current, lower on-resistance, and a larger turn-on voltage. Higher barrier height (BH) was attained for the HJ compared to the SBD, which enabled the p-CuAlO2 interlayer to alter the barrier height. The reverse breakdown voltage of HJ increased by nearly 300 V compared to SBD and reached a maximum of 922 V. The BH values were extracted from the I–V, Hernandez, Cheung's, Mikhelashvili, Norde's, Chot, and surface potential methods found the values are comparable with one another, which indicates their steadiness and validity. The energy distribution profile of the interface state density (NSS) of the HJ decreases compared to the SBD, which indicates that the p-CuAlO2 interlayer facilitated the reduction of NSS of the Pt/β-Ga2O3 SBD interface. The forward log(I)-log(V) plot of the SBD and HJ reveals the ohmic nature at low voltage regions and space-charge limited conduction at higher voltage regions. Results reveal that the reverse leakage current conduction mechanism of the SBD and HJ showed that the Poole-Frenkel conduction mechanism is found to be governed in the lower voltage region, whereas the Schottky conduction mechanism is found to be directed in the higher voltage region, respectively. These outcomes point out that the p-CuAlO2 is a high potential candidate material for forming a heterojunction with β-Ga2O3.

β -Ga2O3 Schottky barrier diodes with 4.1 MV/cm field strength by deep plasma etching field-termination

In this work, we demonstrate a deep mesa etch design for efficient edge field termination in β-Ga2O3 Schottky barrier diodes (SBDs). The proposed design enabled parallel plate fields higher than 4.1 MV/cm with negligible change to the device ON characteristics. The effect of BCl3/Cl2-based dry etch on (100) and (010) etched vertical sidewalls is also analyzed. A remarkable anisotropy in depletion was observed for etch along (100) and (010) sidewalls. This work provides insight into the impact of etching on n-type Ga2O3 and shows a promising method to realize efficient field termination for high breakdown field strength SBDs.

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.

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

The elimination of killer defects, which are responsible for the reverse leakage current and breakdown at low voltage in β-gallium oxide (β-Ga2O3) Schottky barrier diodes (SBDs), is crucial for the commercialization of the power devices. We found probe-induced surface defects, which act as a reverse leakage current path in β-Ga2O3 SBDs. Each defect corresponds to a reverse leakage current of −0.725 μA at a reverse bias of −140 V. These defects are wrinkle shaped, which consists of a pair of the convex and concave structures, as observed by atomic force microscopy. The residual strain around the defects was observed as bright contrasts in the x-ray topography image. The surface defect comprised an 83 nm high convex and a 26 nm deep concave structure. A probe attachment at the pressure of 0.206 GPa induced the surface defect along with a reverse leakage current of −3.75 nA at a reverse voltage of −140 V.

Impact of thermal annealing on deep levels in nitrogen-implanted β-Ga2O3 Schottky barrier diodes

Understanding the properties of N-implanted β-Ga2O3 is fundamental for the optimization of doping and isolation structures based on gallium oxide. This paper reports an extensive analysis of the impact of thermal annealing on the concentration and properties of deep levels in N-implanted β-Ga2O3 Schottky barrier diodes by means of capacitance isothermal transient spectroscopy. Samples with annealing temperatures from 800 to 1200 °C were considered. The original results presented in this paper demonstrate the following: (a) The instability of current–voltage characteristics detected for all the samples under test can be attributed to the presence of three electron traps with activation energies of 0.6, 0.7, and 1 eV, consistent with previous reports in β-Ga2O3. (b) The detected traps are not the nitrogen level but intrinsic defects whose concentration is increased by the implantation process. (c) The concentration of deep levels decreases as the annealing temperature increases, demonstrating that the annealing process can effectively restore the quality of the material while keeping the conductivity decrease related to the presence of the nitrogen. Finally, (d) we demonstrate that the residual leakage and the turn-on voltage shift are correlated with the Arrhenius signature of the detected deep levels. An interpretation is proposed to explain the measurement results.

(Invited) Recent Progress on Anisotropic Electrical and Optical Studies in Ultrawide Bandgap β-Ga2O3 Electronics and Photonics

Ultrawide bandgap semiconductor beta-phase gallium oxide (β-Ga2O3) has recently garnered significant research interests in various electronic and photonic devices such as power devices, RF electronics, photodetectors, self-powered sensors, and high temperature and radiation-hardened devices. This is mainly driven by two factors. The first factor is β-Ga2O3’s advantageous material properties, including large bandgap (4.8 eV), high critical electric field (8 MV/cm), high device figure of merits, and excellent material stability in harsh environments. The second factor is that cost-effective bulk β-Ga2O3 substrates are commercially available, and high-quality β-Ga2O3 epilayers can be grown using various methods (e.g., MOCVD, MBE, and HVPE) for electronic and photonic device fabrication. One of the most important aspects to consider for β-Ga2O3 device design and fabrication is crystal orientation. Since β-Ga2O3 crystalizes in a highly asymmetric monoclinic structure, there are many selections available in terms of crystal orientations such as (010), (-201), (001), and (100). These different crystal orientations may lead to anisotropic electrical and optical properties in β-Ga2O3 electronics and photonics, which warrants a systematic study. This talk will discuss our recent studies of electrical and nonlinear optical properties in β-Ga2O3 with different crystal orientations and their effects on β-Ga2O3 devices. In the first part of the talk, a comparative investigation will be presented for vertical (010) and (-201) β-Ga2O3 Schottky barrier diodes, where their temperature-dependent forward and reverse I-V and C-V characteristics are compared. At forward bias, they showed different on-resistances, turn-on voltages, electron mobilities, and Schottky barrier heights. At reverse bias, different leakage currents, breakdown voltages, and leakage mechanisms were observed in the two devices. These discrepancies are ascribed to different atomic configurations and dangling bonds, and distinct surface band bending revealed by x-ray photoelectron spectroscopy (XPS) on the two surfaces of different crystal orientations. In addition, we also examined the deep-level defects in β-Ga2O3 with different crystal orientations using deep level transient spectroscopy (DLTS). The second part of the talk will focus on the nonlinear optical properties of (010) and (-201) β-Ga2O3, including two-photon absorption coefficient, Kerr nonlinear refractive index, and their polarization dependence. (-201) β-Ga2O3 has a smaller two-photon absorption coefficient, a larger Kerr refractive index, and strong in-plane nonlinear optical anisotropy. Interestingly, the two-photon absorption coefficient of β-Ga2O3 is 20x smaller than wide bandgap GaN, indicating its potential for low-loss waveguides, stable resonators, and integrated photonics. We recently demonstrated β-Ga2O3 optical waveguides with a low loss of 3.7 dB/cm in the ultraviolet to near infrared spectral region. More investigations are expected in this area as β-Ga2O3 electronics and photonics research progresses. And the choice of crystal orientations for the mass production of β-Ga2O3 devices rests on applications, device designs, and costs of substrate of a certain crystal orientation.

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.

A unified hybrid compact model of β-Ga2O3 Schottky barrier diodes for mixer and rectifier applications

A unified hybrid compact model of β-Ga2O3 Schottky barrier diodes for mixer and rectifier applications