2O3 Schottky Research Articles

Rapid-thermal-annealing-induced microstructural evolution of Au/Ni/β-Ga2O3 Schottky diodes correlated with their electrical properties

This study investigated the rapid thermal annealing (RTA)-induced phase and microstructural evolution of Au/Ni/ β -Ga 2 O 3 Schottky diodes to demonstrate their effect on the diodes’ electrical properties in the temperature range of 400–700 °C. Annealing at 500 °C led to a reaction between the Au and Ni films, forming Au-rich Au 3 Ni and Ni-rich AuNi 3 phases along with a homogeneous AuNi alloy phase with a smooth surface and interface morphology. Only the Au 2 Ni phase formed after RTA at temperatures above 600 °C. Reactions did not occur between the Ni/Au metal contacts and the Ga 2 O 3 substrate, regardless of the RTA temperature. The Au/Ni/ β -Ga 2 O 3 Schottky diode annealed at 500 °C exhibited improved rectifying behavior with a large barrier height (1.22 eV) and large breakdown voltage ( V br ) of 460 V, which may be associated with the formation of Au-Ni alloy films with a smooth surface morphology and abrupt interface with the Ga 2 O 3 substrate. The Schottky diode behavior began to degrade upon RTA at 600 °C, which is attributed to the initiation of degradation of the surface and interface morphology of the Au 2 Ni alloy film. The deterioration of the Schottky behavior with the drastic increase of the reverse leakage current and sharp drop of V br upon RTA at 700 °C is attributed to the severe deterioration of the interface and surface morphologies, such as the roughened surfaces caused by agglomeration and void formation with the grain boundaries serving as high leakage current paths. • RTA-induced device performance of Au/Ni/ β -Ga 2 O 3 Schottky diode was investigated. • No reactions occurred between Ni/Au films and β -Ga 2 O 3 for all RTA temperatures. • RTA at 500 °C induced a significant reaction between the Au and Ni films. • High breakdown voltage of 460 V was obtained on annealing at 500 °C. • Above 600 °C, device performance deteriorated due to structural degradation.

Deep level defects in low-pressure chemical vapor deposition grown (010) β-Ga2O3

This study provides the full-bandgap evaluation of defect state distributions in beta phase gallium oxide (β-Ga2O3) grown by low-pressure chemical vapor deposition (LPCVD) on (010) β-Ga2O3 substrates at high growth of up to 20 µm/h. Deep-level optical spectroscopy and deep-level transient spectroscopy measurements applied to Ni/β-Ga2O3 Schottky diodes revealed the presence of a previously unreported defect state at EC-3.6 eV, which dominated the overall trap distribution in LPCVD grown material. However, states at EC-0.8 eV, EC-2.0 eV, and EC-4.4. eV were also detected, similar to prior studies on β-Ga2O3 grown by other methods, with similar or lower concentrations for the LPCVD samples. The EC-0.8 eV and EC-2.0 eV states were previously connected to residual Fe impurities and gallium vacancies, respectively. The total concentration of traps in the LPCVD material was on par with or lower than the state-of-the-art metal–organic chemical vapor deposition-grown materials despite the much higher growth rate, and the distribution of states showed negligible dependence on SiCl4 flow rate and doping concentration. These results demonstrate that the high growth rate of LPCVD-grown β-Ga2O3 is very promising for achieving thick, low defect density, and high-quality layers needed for multi-kV device applications.

Nanoscale Characterization of Chemical and Structural Properties of the Au/(100) β-Ga2O3 Interface

Au/β-Ga2O3 Schottky contacts deposited at room temperature via electron-beam evaporation are characterized electrically with current density–voltage (J–V) and capacitance–voltage (C–V) measurements. The ideality factor and Schottky barrier heights measured from J–V and C–V are determined to be 1.32, 1.37 eV, and 1.98 eV, respectively. Due to the peculiarities of the electrical properties of the Au/β-Ga2O3 Schottky contacts, scanning transmission electron microscopy (STEM) was performed. High-angle annular dark-field (HAADF)-STEM imaging reveals an inhomogeneous chemical reaction occurred at the metal–semiconductor interface. The reaction is signified by void formation 5–20 nm below the interface, Ga diffusion into Au at the interface, and Ga interstitial diffusion toward the interface. Energy-dispersive X-ray spectroscopy (EDS) mapping shows Ga diffusion into the Au contact region accompanied by a transitional region of 4–5 nm.

Elevated barrier height originated from electric dipole effect and improved breakdown characteristics in PtOx/β-Ga2O3 Schottky barrier diodes

The higher Schottky barrier height of PtOx/β-Ga2O3 Schottky barrier diode (SBD) was derived from the electric dipole effect of PtOx Schottky electrode. And the higher Schottky barrier height effectively improved the reverse breakdown characteristics of β-Ga2O3 SBD. In this work, PtOx/β-Ga2O3 and Pt/β-Ga2O3 SBDs were fabricated, and the Schottky barrier height of PtOx SBD increased with the increment of oxygen element component in PtOx electrode, which were all higher than the Schottky barrier height of Pt SBD. Kelvin probe force microscope measurement indicated that Fermi level pinning effect and the variation in work functions of Schottky electrodes were irrelevant to the higher barrier height. Moreover, with the increment of inserting PtOx layer thickness in Pt/PtOx/β-Ga2O3 SBDs, the Schottky barrier height increased from 1.32 eV to 1.82 eV. Bright-field scanning transmission electron microscopy image demonstrated that PtOx was mainly polycrystal with layer structure near the Schottky interface. The layer structure composed of Pt ions and O ions induced electric dipole effect, and the electric dipole effect led to the increase of Schottky barrier height for PtOx SBD. Lower leakage current density, higher breakdown voltages and more concentrated breakdown voltage distribution were obtained for PtOx SBDs. Furthermore, the barrier heights of PtOx SBDs gradually increased as the temperature raised, resulting in their reverse leakage current increased much more slowly with temperature than that of Pt SBD. The above results demonstrated that PtOx/β-Ga2O3 SBD had great potential in enhancing reverse blocking characteristics and high-temperature environment applications.

Thin-film temperature sensors based on LPD-fabricated β-Ga2O3 Schottky diodes

We report the synthesis of wide-bandgap β-Ga2O3 nanocrystalline thin films via the low-cost and non-vacuum-based liquid phase deposition (LPD) method. The morphological evolution of the nanocrystalline β-Ga2O3 grains was investigated as a function of the growth temperature, processing time, and pH value of the precursor solution. We successfully calcined gallium oxide hydroxide GaO(OH) through a 3-h annealing process at 800 °C to convert it into β-Ga2O3. We fabricated horizontal-structured Ni/β-Ga2O3 Schottky diodes and investigated the electrical characteristics pertinent to sensing temperature in the range of 100−800 K. The temperature sensitivity of the Ni/β-Ga2O3 Schottky-junction temperature sensors, defined as the temperature dependence of junction voltage at a fixed bias current of 10 µA, peaked at −2.924 mV/K in the range between 300 and 500 K. At room temperature, we measured a barrier height of 0.915 eV and a Richardson constant of 43.04 ± 0.01 A/cm2 K2 from the Ni/β-Ga2O3 Schottky junctions. These results indicate that the LPD-synthesized β-Ga2O3 material and devices hold promising potential for sensing applications especially at high temperatures.

Physical Operations of a Self-Powered IZTO/β-Ga2O3 Schottky Barrier Diode Photodetector.

In this work, a self-powered, solar-blind photodetector, based on InZnSnO (IZTO) as a Schottky contact, was deposited on the top of Si-doped β-Ga2O3 by the sputtering of two-faced targets with InSnO (ITO) as an ohmic contact. A detailed numerical simulation was performed by using the measured J–V characteristics of IZTO/β-Ga2O3 Schottky barrier diodes (SBDs) in the dark. Good agreement between the simulation and the measurement was achieved by studying the effect of the IZTO workfunction, β-Ga2O3 interfacial layer (IL) electron affinity, and the concentrations of interfacial traps. The IZTO/β-Ga2O3 (SBDs) was tested at a wavelength of 255 nm with the photo power density of 1 mW/cm2. A high photo-to-dark current ratio of and a photoresponsivity of 0.64 mA/W were obtained at 0 V as self-powered operation. Finally, with increasing power density the photocurrent increased, and a 17.80 mA/W responsivity under 10 mW/cm2 was obtained.

Betavoltaic cell based on Ni/β-Ga2O3 and 63Ni source

The parameters of betavoltaic cells based on a Ni/β-Ga2O3 Schottky barrier diode and beta-particle source containing 63Ni are evaluated. Monte Carlo simulation is used to calculate the depth-dependent generation rate of excess carriers produced by beta-radiation from 63Ni source. It is shown that the dependence can be described by the exponential function exp(−z/z0) with z0 equal to 1.3 μm. The short circuit current, open circuit voltage, and maximum power are calculated for the Schottky barrier with parameters obtained from the electron beam induced current studies of Ni Schottky barriers formed on n-type β-Ga2O3.

Uniformity characterization of SiC, GaN, and α-Ga2O3 Schottky contacts using scanning internal photoemission microscopy

Local variation within one dot electrode was characterized for Ni/SiC, Ni/GaN and Cu/Ti/α-Ga2O3 Schottky contacts by using scanning internal photoemission microscopy (SIPM). SIPM measurements were conducted for 200 μmϕ and 400 μmϕ dot electrodes with an energy resolution in determining Schottky barrier height (qϕ B) of less than 0.2 meV. All three kinds of Schottky contacts on the wide bandgap semiconductors exhibited good uniformity as small as less than 10 meV in a standard deviation of qϕ B. The Schottky barrier height and the standard deviation values are the basis for estimating device performances and designing large-area devices as a measurement-based physical value.

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.

Realization of highly rectifying Schottky barrier diodes and pn heterojunctions on κ-Ga2O3 by overcoming the conductivity anisotropy

Novel devices based on orthorhombic κ-Ga2O3 could enable solar blind infrared detection or high-electron mobility transistors with large two-dimensional electron gas densities. Here, we report on the current transport parallel to the growth direction of κ-Ga2O3 layers grown by pulsed-laser deposition on highly conductive Al-doped ZnO back contact layers. Besides ohmic Ti/Al/Au contact layer stacks, vertical Pt/PtOx/κ-Ga2O3 and Pd/PdOx/κ-Ga2O3 Schottky barrier diodes and NiO/κ-Ga2O3 and ZnCo2O4/κ-Ga2O3 pn-heterodiodes are investigated by current–voltage measurements. While a lateral current transport is severely suppressed to less than 10−9Acm−2 due to rotational domains, we record a significant current flow through the ohmic contacts in the vertical direction of >0.1Acm−2. The Schottky barrier diodes and the pn-heterojunctions exhibit rectification ratios of up to seven orders of magnitude. Room temperature current–voltage characteristics of diode ensembles as well as temperature-dependent measurements for selected Pt-based diodes reveal a mean barrier height of ϕBm≈2.1eV and ideality factors down to η≈1.3.

Highly sensitive solar-blind deep ultraviolet photodetector based on graphene/PtSe2/β-Ga2O3 2D/3D Schottky junction with ultrafast speed

There is an emerging need for high-sensitivity solar-blind deep ultraviolet (DUV) photodetectors with an ultra-fast response speed. Although nanoscale devices based on Ga2O3 nanostructures have been developed, their practical applications are greatly limited by their slow response speed as well as low specific detectivity. Here, the successful fabrication of two-/three-dimensional (2D/3D) graphene (Gr)/PtSe2/β-Ga2O3 Schottky junction devices for high-sensitivity solar-blind DUV photodetectors is demonstrated. Benefitting from the high-quality 2D/3D Schottky junction, the vertically stacked structure, and the superior-quality transparent graphene electrode for effective carrier collection, the photodetector is highly sensitive to DUV light illumination and achieves a high responsivity of 76.2 mA/W, a large on/off current ratio of ~ 105, along with an ultra-high ultraviolet (UV)/visible rejection ratio of 1.8 × 104. More importantly, it has an ultra-fast response time of 12 µs and a remarkable specific detectivity of ~ 1013 Jones. Finally, an excellent DUV imaging capability has been identified based on the Gr/PtSe2/β-Ga2O3 Schottky junction photodetector, demonstrating its great potential application in DUV imaging systems.

Modeling and analyzing temperature-dependent parameters of Ni/β-Ga2O3 Schottky barrier diode deposited by confined magnetic field-based sputtering

In this work, the temperature-dependent parameters of Ni/β-Ga2O3 Schottky barrier diode (SBD) were analyzed and modeled. The simulation is to elucidate the physical phenomenon behind this temperature dependence. At room temperature, the deviation of SBD parameters from the ideal case is due to the Schottky barrier height ( inhomogeneity. A model is developed for this inhomogeneity in which an interfacial defected layer (IDL) is formed. Defects (extrinsic states) are related to plasma and Ar atom bombardment used in the confined magnetic field-based sputtering to realize the Ni Schottky contact diffusion in β-Ga2O3. Ni diffuses, upon annealing, to compensate defects in this IDL. It was found that the Schottky barrier height () and threshold voltage decrease with increasing temperature. This decrease is related to intrinsic and extrinsic states (plasma and Ar bombardment). However, the ideality factor (η) increases which is related to the series resistance (R S) increase. The increase is related to the interfacial layer and nickel resistance increase with increasing temperature.

Modeling a Ni/β-Ga2O3 Schottky barrier diode deposited by confined magnetic-field-based sputtering

In this work, a detailed numerical simulation is carried out to model the current–voltage characteristics of a nickel/β-Ga2O3 Schottky barrier diode at different temperatures. These SBDs are produced using confined magnetic-field-based sputtering to deposit the nickel (Ni) Schottky contact of the diode. This method reduces the thickness of the defect area created by plasma and argon bombardment, and consequently, the electrical characteristics are less affected by temperature changes or annealing (i.e. the device is more stable). During annealing, Ni diffuses into β-Ga2O3. A model for this diffusion is proposed in this work, in which Ni diffusion reduces the defects produced by plasma and argon bombardment by filling the Ga vacancy. Furthermore, Ni diffusion produces a new interfacial compound, namely at the interface between the Ni and the β-Ga2O3. This new compound layer has different properties than those of β-Ga2O3, in particular, those of the bandgap and the affinity. Finally, the temperature-dependent current-density–voltage (J–V) characteristics are simulated, taking the proposed model into account. A good agreement with measured values is achieved, especially at low forward voltages, which demonstrates the soundness of the proposed model.

Defect-insensitive current–voltage characteristics of Schottky barrier diode formed on heteroepitaxial α-Ga2O3 grown by mist chemical vapor deposition

An n-type α-Ga2O3 layer was grown by mist chemical vapor deposition on a sapphire substrate, and a Ti/α-Ga2O3 Schottky barrier diode was fabricated. Although the α-Ga2O3 layer has a high threading dislocation density (larger than 109 cm−2), the ideality factor of 1.03 was obtained from the forward current–voltage characteristic in the range of 298 K–423 K, indicating the clear thermionic emission transport. The reverse current–voltage characteristic was also investigated, and the leakage current showed good agreement with the theoretical calculation based on the thermionic field emission model without any fitting parameter in the temperature range of 298 K–423 K.

Dynamic characteristics of PdCoO2/β-Ga2O3 Schottky junctions

A high-frequency diode is an essential component in electrical circuits providing the current rectification function for AC/DC converters, radio frequency detectors, and automotive inverters. Schottky barrier diodes based on wide-bandgap semiconductors are promising for the high-frequency applications owing to short reverse recovery time that minimizes the energy dissipation during the switching. In this study, we report dynamic characteristics of Schottky junctions composed of a layered oxide metal PdCoO2 and an n-type β-Ga2O3 substrate. Rectifying current–voltage characteristics with reasonably small hysteresis were demonstrated up to a high frequency of 3 MHz in the PdCoO2/β-Ga2O3 Schottky junctions. For the on-state to off-state switching with the current ramp rate of approximately −2 × 1010 A/scm2, the reverse recovery time was as short as 11 ns. The short reverse recovery time was constantly obtained in the operation temperature range of 25–350 °C, showing low-loss switching properties of the PdCoO2/β-Ga2O3 Schottky junctions. The Schottky barrier height of ∼1.78 eV and the ideality factor of ∼1.06 were maintained after the 108-times on–off switching cycles. The fast switching with less energy dissipation and high durability of the PdCoO2/β-Ga2O3 Schottky junctions would be suitable for application in high-frequency power devices operating at high temperature.

Temperature-dependent current-voltage characteristics of β -Ga2O3 trench Schottky barrier diodes

Temperature-dependent behavior of regular and trench Ni/β-Ga2O3 (001) Schottky barrier diodes (SBDs) was studied. Current–Voltage (I–V) characteristics, ideality factor, and barrier height of trench SBDs were compared with those of regular SBDs at temperatures ranging from 100 K to 650 K. The trench SBDs showed a superior performance to regular SBDs. At elevated temperatures (as high as 650 K), the trench SBDs maintained a high ON/OFF current ratio (105), which is four orders of magnitude higher than that in the regular diodes. The current–voltage characteristics of the trench SBDs were recovered when the sample was cooled to room temperature after high temperature measurements, whereas the I–V characteristics of the regular SBDs were degraded. The breakdown voltage (BV) was also measured on as-fabricated devices and after high temperature ramp up to 650 K. We observed a reduction in maximum achieved BV from 1084 V to 742 V on the trench SBDs and from 662 V to 488 V on regular SBDs, respectively, after temperature-dependent measurements.

Influence of Crystallographic Orientation on Schottky Barrier Formation in Gallium Oxide

Highly rectifying graphite/I²-Ga2O3 Schottky junctions have been prepared by a simple low-cost drop-casting process. The influence of two different crystal orientations on the current transport mechanism in the graphite-based Schottky junctions was investigated by direct-current (DC) and alternating-current (AC) electrical measurements. The nonideal behavior observed for both $$ \left\langle {\bar{2}01} \right\rangle $$ and $$ \left\langle {010} \right\rangle $$ crystallographic orientations can be explained by the lateral inhomogeneity of the junction related to the imperfection of the graphite/semiconductor interface. A lower density of interface states and their shorter time constants are reported for Schottky junctions formed on $$ \left\langle {\bar{2}01} \right\rangle $$ crystallographic plane, as reflected also by the higher effective barrier height and lower ideality factor.

Temperature-Dependent Electrical Characteristics of Ni/Au Vertical Schottky Barrier Diodes on β-Ga2O3 Epilayers

Temperature dependent current transport mechanism in Ni/β-Ga2O3 Schottky Barrier Diodes was studied using current-voltage (I-V) and capacitance-voltage (C-V) characterization techniques in the range of 78–350 K. Schottky barrier height ϕb0 and ideality factor ɳ from I-V characteristics were found to be 1.27 eV and 1.12, respectively, at room temperature. Plots of barrier height and ideality factor with inverse of temperature show strong temperature dependency and a deviation from barrier height obtained from C-V characteristics. The temperature dependence of barrier height and ideality factor assigned to barrier inhomogeneity at Ni/β-Ga2O3 interface, and modulated by the potential fluctuation model. Diode turn-on voltage and turn-on resistance at 300 K were found to be 1.08 eV and 7.80 mΩ-cm2, respectively. A large rectification ratio of the order of 1012 was obtained at room temperature and also the rectification ratio of the order of 109 was consistent over the whole temperature range (78–350 K).

High responsivity tin gallium oxide Schottky ultraviolet photodetectors

The authors report on high spectral responsivity (SnxGa1 − x)2O3 Schottky UV photodetectors grown by plasma-assisted molecular beam epitaxy on β-Ga2O3 substrates. Schottky devices exhibited peak responsivities ranging from 49 to 194 A/W, with peak responsivity and wavelength position increasing systematically for higher Sn concentration from x = 0.01 to 0.18. Dark currents for the devices ranged from <1 nA to 3 μA with rise and fall times in the 0.21–3 s time range, with slower response times likely due to photoconductive gain caused by trapped holes. Incorporation of up to 18% Sn into the tin gallium oxide (TGO) devices resulted in a redshift in the peak responsivity position, ranging from 5.19 to 4.86 eV, demonstrating tunability within the UV-C spectral region through Sn concentration adjustment. The authors believe this to be the highest reported responsivity for a planar Ga2O3-based Schottky photodetector to date, suggesting that TGO based UV-C Schottky detectors are an attractive approach toward deep-UV sensing applications.

Schottky Barrier Parameters and Low-Frequency Noise Characteristics of Au/Ni Contact to n-Type β-Ga2O3

An Au/Ni/β-Ga2O3 Schottky barrier diode was fabricated on an 8.6-μm-thick lightly doped drift region grown on heavily doped Ga2O3 substrate, and its electrical and low-frequency noise characteristics were investigated. The diode showed excellent rectifying behavior with the reverse current being saturated with the applied bias and a magnitude of ∼ 10−13 A. The diode exhibited a barrier height of 1.04 eV and a reasonably high reverse breakdown voltage of 540 V without employing any edge termination methods. The Schottky barrier parameters, such as barrier height, ideality factor, and series resistance, were obtained using a range of methods and found to be in close agreement with each other. The interface state density of the Au/Ni/β-Ga2O3 Schottky diode obtained from the capacitance–voltage (C–V) method was lower than that determined from the forward current–voltage (I–V) characteristics, attributed to the inhomogeneous distribution of interface states at the interface. An analysis of the forward log I–log V plot of the Au/Ni/β-Ga2O3 Schottky diode revealed ohmic-type conduction, and space-charge-limited current dominated the carrier transport mechanisms in lower and higher voltage ranges, respectively. For bias below 0.3 V, the low-frequency noise of the Au/Ni/β-Ga2O3 Schottky diode was a linear combination of two noise components associated with uniformly distributed bulk traps of β-Ga2O3 and the charge fluctuation in the generation-recombination center. On the other hand, for bias above 0.4 eV, the noise spectral density showed a 1/f2 dependence, indicating the domination of generation-recombination noise.