Showing papers by "Youdou Zheng published in 2022"

1.95-kV Beveled-Mesa NiO/β-Ga 2 O 3 Heterojunction Diode With 98.5% Conversion Efficiency and Over Million-Times Overvoltage Ruggedness

Abstract: The technical progress of Ga2O3 power diodes is now stuck at a critical point where a lack of performance evaluation and reliability validation at the system-level applications seriously limits their further development and even future commercialization. In this letter, by implementing beveled-mesa NiO/Ga2O3 p–n heterojunction diodes (HJDs) into a 500-W power factor correction (PFC) system circuit, high conversion efficiency of 98.5% with 100-min stable operating capability has been demonstrated. In particular, rugged reliability is validated after over 1 million times dynamic breakdown with a 1.2-kV peak overvoltage. Meanwhile, superior device performance is achieved, including a static breakdown voltage (BV) of 1.95 kV, a dynamic BV of 2.23 kV, a forward current of 20 A (2 kA/cm2 current density), and a differential specific on -resistance of 1.9 mΩ·cm2. These results indicate that Ga2O3 power HJDs are developing rapidly with their own advantages, presenting the enormous potential in high-efficiency, high-power, and high-reliability applications.

Light-activated ultrasensitive NO2 gas sensor based on heterojunctions of CuO nanospheres/MoS2 nanosheets at room temperature

Abstract: MoS 2 -based gas sensor has become an effective platform to detect NO 2 gas. However, room-temperature NO 2 gas sensor based on MoS 2 are facing low sensitivity and poor response/recovery behaviors because of low-reacting activity between gas-sensing materials and NO 2 gas at room temperature. Purposefully, a room-temperature NO 2 gas sensor based on CuO/MoS 2 heterojunctions with excellent sensitivity and fast response/recovery speeds is successfully realized. More attractively, the CuO/MoS 2 -3 sensor exhibits remarkable response (8.98) and short response/recovery time of 18.5/53.5 s to 10 ppm NO 2 at room temperature under red light illumination, which is obviously better than that under dark. The improved sensitivity and fast response/recovery behaviors are attributed not only to the charge separation efficiency derived from the effect of heterojunctions, but also to the charge flow on the sensing layer in the context of NO 2 /MoS 2 /CuO interaction under red light illumination. In addition, the fabricated sensor has low detection limit (50 ppb), excellent reversibility and high selectivity to NO 2 against other gases. After 40 days, the CuO/MoS 2 -3 sensor still maintains a relatively stable response with the fast response time (<18.5 s) under red light illumination, ensuring a long-term and fast detection to NO 2 in practical applications. • A room-temperature NO 2 gas sensor based on CuO/MoS 2 heterojunction is demonstrated for the first time. • CuO/MoS 2 -3 sensor possesses an ultra-fast response/recovery speed under red light illumination. • After 40 days, CuO/MoS 2 -3 sensor under red light illumination still maintains stable performance and fast response time. • The sensing mechanism of CuO/MoS 2 -3 sensor under red light illumination to NO 2 gas is deduced in detail.

Majority and Minority Carrier Traps in NiO/β-Ga2O3 p+-n Heterojunction Diode

Abstract: Identifying defects/traps is of vital importance for the implementation of high-performance Ga₂O₃ power devices. In this work, majority and minority carrier traps in beta-gallium oxide ( $\beta $ -Ga₂O₃) have been investigated and identified by means of deep level transient spectroscopy (DLTS) in Ni/ $\beta $ -Ga₂O₃ Schottky barrier diode (SBD) and NiO/ $\beta $ -Ga₂O₃ p⁺-n heterojunction diode (HJD). For both diodes, a dominant energy level of majority carrier (electron) trap states is determined to be ${E}_{C}-$ (0.75–0.79) eV with a concentration of (2.4–4.1) $\times 10^{{13}}$ cm $^{-{3}}$ . Meanwhile, an additional trapping level at ${E}_{V} +0.14$ eV with a concentration of 1.2 $\times 10^{{14}}$ cm $^{-{3}}$ yield is present in NiO/ $\beta $ -Ga₂O₃ bipolar HJD but absent in the Ni/ $\beta $ -Ga₂O₃ SBD unipolar counterpart. The detection of such minority carrier traps originates from the hole injection through trap-assisted tunneling (TAT) from $\text{p}^{+}$ -NiO to $\beta $ -Ga₂O₃. The bias- and frequency-dependent DLTS characteristics identify that such shallow-level minority carrier traps are located in the $\beta $ -Ga₂O₃ bulk region rather not interfacial states at the NiO/ $\beta $ -Ga₂O₃ heterointerface. The identification of both majority and minority carrier traps in this work may shed light on the in-depth understanding of carrier transport mechanisms in Ga₂O₃-based unipolar and bipolar power devices.

Highly responsive and selective ppb-level NO2 gas sensor based on mesoporous Pd-functionalized CuO/rGO at room temperature

Abstract: For the first time, Pd nanoparticle-functionalized, rGO nanosheet-modified 3D rod-like CuO (Pd-CuO/rGO) with nanowire hierarchical structure was successfully synthesized by a facile hydrothermal strategy. Interestingly, the developed Pd-CuO/rGO sensor exhibited...

1.2 kV/25 A Normally off P-N Junction/AlGaN/GaN HEMTs With Nanosecond Switching Characteristics and Robust Overvoltage Capability

Abstract: With a reverse p-n junction in the gate stack design, this work demonstrates a 1.2 kV/25 A normally off p-n junction/AlGaN/GaN high-electron mobility transistor (PNJ-HEMT). Benefiting from the robust gate terminal, the PNJ-HEMT exhibits a large gate breakdown voltage of 18.2 V and a positive threshold voltage of 1.7 V, enabling a wide gate-bias window. Thereafter, with an applicable V GS of 10 V, the transient switching characteristics in nanosecond timescale (11.7-ns rise time and 10.5-ns fall time) and notable immunity to dynamic R on degradation, as well as record-high dynamic breakdown voltage (1.62 kV) under transient overvoltage have been demonstrated. In particular, rugged reliability is validated after over 1-million times dynamic breakdown with a 1.5-kV peak overvoltage. To the best of our knowledge, this is the first demonstration of high- V GS (10 V) GaN HEMT's circuit-level operating capability with considerable reliability, and has well exceeded the V GS limit of 5–7 V in conventional p -GaN gate-terminal devices, thus possessing great potentials in high-power, high-frequency, and high-reliability applications.

Responsibility optimization of a high-speed InP/InGaAs photodetector with a back reflector structure.

Abstract: Top-illuminated PIN photodetectors (PDs) are widely utilized in telecommunication systems, and more efforts have been focused on optimizing the optical responsibility and bandwidth for high-speed and capacity applications. In this work, we develop an integrated top-illuminated InP/InGaAs PIN PD with a back reflector by using a microtransfer printing (µ-TP) process. An improved µ-TP process, where the tether of silicon nitride instead of photoresist, is selected to support an underetched III-V device on an InP substrate before transfer. According to theoretical simulations and experimental measurements, the seamless integration of the PD with a back reflector through µ-TP process makes full use of the 2nd or even multiple reflecting light in the absorption layer to optimize the maximum responsibility. The integrated device with a 5 µm square p-mesa possesses a high optical responsibility of 0.78 A/W and 3 dB bandwidth of 54 GHz using a 500 nm i-InGaAs absorption layer. The present approach for top-illuminated PIN PDs demonstrates an advanced route in which a thin intrinsic layer is available for application in high-performance systems.

M-Plane α-Ga₂O₃ Solar-Blind Detector With Record-High Responsivity-Bandwidth Product and High-Temperature Operation Capability

Abstract: We report on high-performance solar-blind photodetectors fabricated on the m-plane $\alpha $ -Ga₂O₃ epilayer grown by laser molecular beam epitaxy using the segment target approach. Benefited from the improved epitaxial quality with an $\alpha $ -(Al_0.24Ga_0.76)₂O₃ intermediate layer, the $\alpha $ -Ga₂O₃ detector shows a low dark current of 0.23 pA, a UV/visible rejection ratio of over $10^{{5}}$ , a photo-to-dark-current ratio of ${4.4}\times {10}^{{7}}$ , a peak responsivity of 132.6 A/W, a short response time of 97 $\mu \text{s}$ , and a specific detectivity of ${1.23}\times {10}^{{15}}$ Jones at the bias of 10 V. High-temperature operation robustness is also demonstrated up to 423 K, exhibiting the reduction of dark current, a maintained high responsivity (33.9 A/W) and a speedy response (90 $\mu \text{s}$ ), which is on the frontier among the state-of-the-art Ga₂O₃ solar-blind detectors with a record-high responsivity-bandwidth product. The temperature-dependent transient photocurrent feature implies that high gains result from the enhanced carrier lifetime due to the surface bending. It suggests that the m-plane $\alpha $ -Ga₂O₃ detectors are promising for real-time monitoring to high-speed objectives with weak radiation flux in harsh environments.

Identification and Suppression of Majority Surface States in the Dry-Etched β-Ga2O3.

Abstract: Surface treatment after dry etching is vital to enhance the surface quality of the material and thus improve device performance. In this Letter, we identified the majority surface states induced by the dry etching of β-Ga2O3 and optimized surface treatments to suppress these electrically active defects with the improved performance of Schottky barrier diodes. Transient spectroscopies suggested that the majority traps (EC-0.75 eV) related to divacancies (VGa-VO) were enhanced in the concentration of 3.37 × 1014 cm-3 by dry etching and reduced to 0.90 × 1014 cm-3 by the combined means of oxygen annealing and piranha solution treatment. The trap evolution is supported by the suppressed donor-acceptor pair radiative recombination related to oxygen vacancies, the improved carrier transport (negligible hysteresis current-voltage and unity ideality factor), and the reduced surface band bending. These findings provide a straightforward strategy to improve surface quality for the further performance improvement of Ga2O3 power diodes.

Over 1200 V Normally-OFF p-NiO Gated AlGaN/GaN HEMTs on Si With a Small Threshold Voltage Shift

Abstract: In this letter, we demonstrated a normally-off AlGaN/GaN HEMT using p-NiO as a gate stack combined with a recess structure. The fabricated HEMT exhibits a positive threshold voltage of 1.73 V, a saturation output current of 524 mA/mm, a small subthreshold swing of 79.7 mV/dec and a maximum transconductance as high as 143 mS/mm. This is the first time to demonstrate the breakdown characteristics of a p-NiO gate HEMT with a high breakdown voltage of 1205 V and a low specific ON-resistance of 2.22 $\text{m}\Omega \cdot $ cm², yielding a competitive Baliga’s figure-of-merit of 0.65 GW/cm². The instability evaluation of ${V}_{\text {TH}}$ by step stress and pulse transfer curves shows that the p-NiO gate HEMT has a negligible ${V}_{\text {TH}}$ shift in the entire measured gate bias range, which can be attributed to the counteraction between the electron trapping-induced positive $\text{V}_{\text {TH}}$ shift and hole accumulation induced-negative ${V}_{\text {TH}}$ shift. It is well understood in terms of the carrier transport model based on the large band discontinuity at the interface of the p-NiO/AlGaN type-II heterojunction, which is further verified by transient gate current spectra.

Self-Powered Solar-Blind Photodetectors Based on Vertically Aligned GaN@Ga2O3 Core–Shell Nanowire Arrays

Abstract: The self-powered photodetector has recently received wide attention as a fundamental component of energy-saving optoelectronic systems. In this study, the GaN@Ga2O3 core–shell nanowire arrays (NAs) were used as the photoanode for the self-powered solar-blind photoelectrochemical-type photodetectors (PEC-PDs). The vertically aligned GaN@Ga2O3 core–shell NAs on the GaN template was fabricated by the inductively coupled plasma etching combined with the thermal oxidation process. Under 255 nm illumination without an external power supply, the device exhibits a maximum responsivity of 93.48 mA/W, yielding a high external quantum efficiency of 45.54%, which shows one of the best values among the reported solar-blind Ga2O3-based PEC-PDs. Furthermore, the device shows a fast response speed (τr = 25 ms, τd = 5 ms) and good stability. Such excellent performance under zero bias may benefit from the superior light-absorbing capability of the vertical NAs, the good solid/liquid contact realized by the nanostructures, and the efficient photogenerated carrier separation driven by the double built-in electric field. This work provides a simple and feasible route to construct high-performance solar-blind Ga2O3-based PEC-PDs.

Gate-Controlled NiO/Graphene/4H-SiC Double Schottky Barrier Heterojunction Based on a Metal-Oxide-Semiconductor Structure for Dual-Mode and Wide Range Ultraviolet Detection

Abstract: Based on a metal-oxide-semiconductor (MOS) structure, a double Schottky barrier junction (SBJ) made of NiO/graphene/4H-SiC is built and employed in ultraviolet (UV) detection. The hole concentration of NiO can be modulated as depleted or accumulated states with gate voltages, which allows the device to work in dual-mode when used as a photodetector. In this work, a negative gate bias causes the device to operate as a photoconductive detector with gain due to the negligible Schottky barrier, whereas a zero or positive gate bias makes it work as a Schottky photodiode. The device has a high responsivity of 103.3 A/W and a gain of 490.8 despite the low light intensity (261 nm laser @ 30.19 μW/cm2) at VDS = 5 V and VGS = −3 V. The NiO layer and SiC substrate both serve as UV absorption materials and produce photogenerated carriers, and the device has a wide UV response range from 240 to 400 nm with a gain of 80.34 when VDS = −3 V and VGS = 0 V at 240 nm. The above findings suggest that this MOS-based NiO/graphene/4H-SiC double SBJ has a great prospect in practical UV detection.

Evaluation on Temperature-Dependent Transient VT Instability in p-GaN Gate HEMTs under Negative Gate Stress by Fast Sweeping Characterization

Abstract: In this work, temperature-dependent transient threshold voltage (VT) instability behaviors in p-GaN/AlGaN/GaN HEMTs, with both Schottky gate (SG) and Ohmic gate (OG), were investigated systematically, under negative gate bias stress, by a fast voltage sweeping method. For SG devices, a concave-shaped VT evolution gradually occurs with the increase in temperature, and the concave peak appears faster with increasing reverse bias stress, followed by a corresponding convex-shaped VT recovery process. In contrast, the concave-shaped VT evolution for OG devices that occurred at room temperature gradually disappears in the opposite shifting direction with the increasing temperature, but the corresponding convex-shaped VT recovery process is not observed, substituted, instead, with a quick and monotonic recovery process to the initial state. To explain these interesting and different phenomena, we proposed physical mechanisms of time and temperature-dependent hole trapping, releasing, and transport, in terms of the discrepancies in barrier height and space charge region, at the metal/p-GaN junction between SG and OG HEMTs.

RF performance enhancement in sub-μm scaled β-Ga2O3 tri-gate FinFETs

Abstract: In this Letter, we report on the enhanced radio frequency (RF) performance in sub-micrometer scaled β-Ga2O3 tri-gate FinFETs. With a 200-nm-thick β-Ga2O3 bulk channel and a 0.35 μm gate length, the FinFETs exhibit an improved current-gain cutoff frequency of 5.4 GHz and a maximum oscillation frequency of 11.4 GHz, which are 20% and 58% improved with respect to the planar counterpart, respectively. The improved RF performance results from the enhanced gate control capability and the suppressed short-channel effects, as evidenced by the improved pinch-off characteristics, the improved transconductance, and the suppressed output conductance. It suggests that the tri-gate multi-fin architecture is a promising strategy to break the scaling limitation of the gate-channel aspect ratio toward high-performance β-Ga2O3 RF MOSFETs.

Trap-mediated bipolar charge transport in NiO/Ga2O3 p+-n heterojunction power diodes

Abstract: The construction of p-NiO/n-Ga2O3 heterojunction becomes a popular alternative to overcome the technological bottleneck of p-type Ga2O3 for developing bipolar power devices for practical applications, whereas the identification of performance-limiting traps and the bipolar transport dynamics are still not exploited yet. To this end, the fundamental correlation of carrier transport, trapping and recombination kinetics in NiO/β-Ga2O3 p+-n heterojunction power diodes has been investigated. The quantitative modeling of the temperature-dependent current-voltage characteristics indicates that the modified Shockley-Read-Hall recombination mediated by majority carrier trap states with an activation energy of 0.64 eV dominates the trap-assisted tunneling process in the forward subthreshold conduction regime, while the minority carrier diffusion with near-unity ideality factors is overwhelming at the bias over the turn-on voltage. The leakage mechanism at high reverse biases is governed by the Poole-Frenkel emissions through the β-Ga2O3 bulk traps with a barrier height of 0.75 eV, which is supported by the identification of majority bulk traps with the energy level of EC − 0.75 eV through the isothermal capacitance transient spectroscopic analysis. These findings bridge the knowledge gap between bipolar charge transport and deep-level trap behaviors in Ga2O3, which is crucial to understand the reliability of Ga2O3 bipolar power rectifiers.

Enhancement of the light output efficiency and thermal stability of AlGaN-based deep-ultraviolet light-emitting diodes with Ag-nanodot-based p-contacts and an 8-nm p-GaN cap layer.

Abstract: The efficiency of AlGaN-based deep-ultraviolet light-emitting diodes (DUV LEDs) is limited by the high absorption issue of the p-GaN contact layer or poor contact properties of the transparent p-AlGaN contact layer. Enhancement of the light output efficiency and thermal stability of DUV LEDs with an emission wavelength of 272 nm was investigated in this work. Ag nanodots on an 8-nm p-GaN cap layer were used to form ohmic contact, and Al and Mg reflective mirrors were employed to enhance the light output power (LOP) of DUV LEDs. However, serious deterioration of LOP occurred after the high-temperature process for the LEDs with Al and Mg reflective mirrors, which can be attributed to the damage to the ohmic contact properties. A Ti barrier layer was inserted between the Ag/p-GaN and Al layers to prevent the degeneration of ohmic contact. The wall-plug efficiency (WPE) of DUV LEDs fabricated by the Ag-nanodot/Ti/Al electrode is 1.38 times that of LEDs fabricated by adopting a thick Ag layer/Ti/Al at 10 mA after a high-temperature process. The Ag-nanodot/Ti/Al electrode on thin p-GaN is a reliable technology to improve the WPE of DUV LEDs. The experimental and simulated results show that the ohmic contact is important for the hole-injection efficiency of the DUV LEDs when p-GaN is thin, and a slight increase in the contact barrier height will decrease the WPE drastically. The results highlighted the importance of thermally stable ohmic contacts to achieve high-efficiency DUV LEDs and demonstrated a feasible route for improving the LOP of DUV LEDs with a thin p-GaN cap layer and stable reflective electrodes.

Step-flow growth of Al droplet free AlN epilayers grown by plasma assisted molecular beam epitaxy

Abstract: We have investigated an Al modulation epitaxy (AME) method to obtain step-flow growth of Al droplet free AlN layers by plasma assisted molecular beam epitaxy (MBE). At the usual growth temperature of (Al)GaN/AlN heterostructures, Al atom migration and desorption rate are very low and consequently it is very difficult to avoid the formation of Al droplets on AlN growth front by conventional MBE growth method. By adopting the AME growth method, such a difficulty has been effectively overcome and step flow growth mode of AlN has been clearly observed. By optimizing the AME growth time sequence, namely, AlN growth time and N radical beam treatment time, Al droplet free AlN layers with step flow growth characteristics have been obtained, with atomic flat surfaces and an average atomic step width of ∼118 nm at 970 °C–1000 °C, which is still suitable to grow (Al)GaN/AlN heterostructures by MBE.

A Monolithic Optoelectronic Integrated Circuit of InP p-i-n/Double HBTs Through Transfer-Printing Fabrication

Abstract: In this letter, we present a photoreceiver front-end circuit that consists of an InP p-i-n photodetector (PIN PD) on an InP transimpedance amplifier (TIA) with double HBTs (DHBTs), in which the key integrated fabrication route of a transfer-printing (TP) process is introduced. Prior to integration, PDs and TIAs are individually fabricated on their respective wafers. Then, a discrete PD with only active layers is transferred onto the destination on a TIA wafer, thereafter, an on-chip coplanar waveguide connection is integrated. The PIN PD and DHBT TIA exhibit excellent performance, characterized by a responsivity of 0.51 A/W at $1.55~\mu \text{m}$ with a bandwidth of 20 GHz and a transimpedance gain of 36 dB $\Omega $ with a bandwidth of 27 GHz, respectively. Finally, the monolithically integrated circuit demonstrates a better frequency response by maximally reducing the parasitic effect, with an open clear eye diagram under a 20 Gb/s nonreturn-to-zero pattern, which evidently manifests its ability for ultrahigh-rate data operation. The present work, by employing a TP process, overcomes the issues in integrating discrete optoelectronic devices and proposes a unique approach for monolithic optoelectronic integrated circuit fabrication.

4H-SiC δn-i-p Extreme Ultraviolet Detector With Gradient Doping-Induced Surface Junction

Abstract: Extreme ultraviolet (EUV) detectors are key components required in many critical applications. In this work, a high-performance 4H-SiC $\delta \text{n}$ -i-p EUV detector with gradient doping-induced surface junction is designed and fabricated. The detector demonstrates ultra-low leakage current and excellent photo-response uniformity across the ~6 mm² photo-sensitive area. Under photovoltaic operation mode, the detector exhibits high responsivity of 0.104 A/W and corresponding quantum efficiency of 960%@13.5 nm, which are close to the theoretical limit. Meanwhile, the equivalent noise power of the device under 0 V bias is determined to be $2.35\times {10}^{\text {-12}}$ W by 1/f noise test, proving that the device has a very low noise level and corresponding high detectivity. In addition, the high-temperature working capability and radiation-resistant performance of the EUV detector are preliminarily verified.

Al0.1Ga0.9N p-i-n Ultraviolet Avalanche Photodiodes With Avalanche Gain Over 106

Abstract: We report Al_0.1Ga_0.9N p-i-n ultraviolet (UV) avalanche photodiodes (APDs) based on sapphire substrates with a record-high gain over $2 \times 10^{6}$ . The devices fabricated with various mesa diameters present consistent avalanche behaviors and identical dark current distributions over multiple ${I}-{V}$ scans, which are comparable to the GaN APDs grown on free-standing GaN substrates. The quadratic fitting of the dark currents versus the mesa sizes reveals that the leakage current at the breakdown voltage is a mixture of surface leakage and bulk leakage, and the surface component is comparable to that of the bulk. Additionally, KOH surface treatment and SiO₂ passivation are proven to be very effective in suppressing the leakage current and achieving robust avalanche performance.

InGaN/GaN Nanorod Arrays for a Hybrid Nanolaser

Abstract: Lasers with small size have demonstrated great potential in numerous applications including communication, optical computing, detection, displays, and optical logic circuits. In this study, hybrid plasmonic nanolasers with metal pad structures based on the InGaN/GaN nanorod are designed and fabricated to investigate the lasing modes and polarization modulation. Dominant coupling of the surface plasmon mode has been achieved by optimizing the hybrid nanolaser structures, which significantly enhances the electric field concentration, leading to an ultralow threshold (∼1.19 W cm–2) plasmonic multimode lasing. Based on the theoretical and experimental results, it is proposed that the suitable plasmonic structural parameters could provide wave-vector matching and phase compensation to form a strong plasmon resonator, yielding a low radiative loss and high gain for the laser. These InGaN/GaN nanorod arrays for the hybrid nanolaser not only provide a solution to the ultralow-threshold nanorod-based plasmonic lasers but also advocate the prospect of the greater potential of nanoscale arrays for luminescence and displays. These findings and understandings provide vital insights into the developments of electrically driven plasmonic nanolasers and may contribute to the realization of nanolaser-based display arrays and optical on-chip integration for the next generation of logic circuits.

Design of a quantum-spin sensor with sub-micron resolution and enhanced optical read-out ability by the nitrogen-vacancy centers in diamond

Abstract: To solve the inefficiency of optical out-coupling of the nitrogen-vacancy (NV) luminescence in bulk diamond, we propose a quantum-spin sensor based on the NV centers in diamond. The sensor is able to map out the microwave magnetic field or temperature and other distributions on a sub-micron scale. We demonstrate the implementation of a nanowire (NW) array fabricated from a regular diamond sensing layer and nano-antennas as photonic waveguides fabricated on the backside of a diamond substrate. By tuning the parameters, we used numerical modeling to find an optimal range of the NW’s parameters that allow for a large collection efficiency of the emitted photons. By systematic optimization, we have observed an optical read-out efficiency strengthened by a factor of ~ 30. Since the optimized parameters for the sensing device are in the order of hundreds of nanometers, the sensor can achieve an enhanced sensitivity with sub-micron resolution. Graphical abstract

Current transport dynamics and stability characteristics of the NiO x based gate structure for normally-off GaN HEMTs

Abstract: The application of p-type oxide typified with NiO x as cap layer in AlGaN/GaN high electron mobility transistor for normally-off operation has specific benefits, including etching free fabrication process, high hole density and elimination of Mg dopants diffusion effects. This work presents a device configuration exploration combining the p-NiO x gate cap layer and a thin AlGaN barrier layer. Calculation method for the threshold voltage has been discussed, which obtains good consistence with the experimental measurements. In addition, a nitrogen based post-annealing process was developed to improve the film stoichiometry for elevated gate controllability, realizing normally-off operation with enhanced channel conduction capability. The current transport dynamics in the gate stack as coupled with the NiO x /AlGaN interface states have also been studied, where a deep level trap was recognized in dominating the gate current characteristics and the gate stability performance under different forward gate bias conditions.

Low-temperature characteristics and gate leakage mechanisms of LPCVD-SiNx/AlGaN/GaN MIS-HEMTs

Abstract: In this paper, we systematically investigated the static properties and gate current mechanism of low-pressure chemical vapor deposition-SiNx/AlGaN/GaN metal–insulator–semiconductor-high-electronmobility-transistor (MIS-HEMTs) at cryogenic temperature range from 10 K to 300 K. It is found that the threshold voltage of the device shows a positive shift due to the decreased carrier concentration at low temperature, and both the maximum transconductance and ON-resistance are improved at the low temperatures because of the enhanced electron mobility. Under very low electric field, the gate leakage exhibits ohmic conduction. With increasing forward gate bias, the dominant gate leakage mechanism at temperature below150 K gradually transits into trap-assisted tunneling, participating with a deep trap energy level of 0.73 eV in the SiNx dielectric, to Fowler–Nordheim (FN) tunneling. In contrast, the dominant gate leakage mechanism at temperature above 150 K transits from Poole–Frenkel emission, showing a low trap barrier height of 56 meV in the SiNx dielectric, to Fowler–FN tunneling with increasing forward gate bias. Under high reverse gate bias, carrier-limited gate current becomes the dominated gate leakage mechanism.

Preparation of β-Ga2O3 films on off-angled sapphire substrates and solar-blind ultraviolet photodetectors

Abstract: β-Ga2O3 films have been prepared on (0001) sapphire substrates with different off-axis angles toward $?> <112ˉ0> by sol–gel method and the solar-blind ultraviolet photodetectors (PDs) with metal-semiconductor-metal structure have been fabricated on these films. The effects of off-angled sapphire substrates on the surface morphology and crystal quality of β-Ga2O3 films have been systematically investigated. All the results indicate that to some extent the larger off-angled sapphire substrate could lead to the smoother surface morphology and higher crystal quality of β-Ga2O3 films. Reduction in the numbers of in-plane rotational domains for β-Ga2O3 films on off-angled substrates may result in the above-mentioned improvement, which have been confirmed by x-ray diffraction ϕ-scan. β-Ga2O3 film grown on ∼7° off-angled sapphire substrate has ( 400 ) preferred orientation with the single-domain growth mode. Besides, the performance of β-Ga2O3 PDs fabricated on different off-angled substrates show the same trend with changing the off-axis angles. The β-Ga2O3 PD fabricated on ∼7° off-angled sapphire substrate has excellent optical characteristics with large photocurrent to dark current ratio of 430.53 and high UV–visible rejection ratio of 1.42 × 103. These results would be helpful for improving the crystal-quality of hetero-epitaxy β-Ga2O3 films by using off-angled sapphire substrates and fabricating the cost-effective mass-producible solar-blind ultraviolet PDs by sol–gel method.

Flip-Chip AlGaN/GaN Schottky Barrier Diode Using Buried-Ohmic Anode Structure With Robust Surge Current Ruggedness and Transient Energy Sustaining Capability

Abstract: Robust reliability is essential for electronic devices against inductive transient shocks in power switching applications. In this work, rugged surge current tolerance and transient energy sustaining capability are simultaneously demonstrated in AlGaN/GaN Schottky barrier diodes (SBDs) for the first time, which greatly fills the research gap toward the device reliability requirements for inductive switching applications. Such robustness is attributed to the combined advantages of the uniquely designed buried-ohmic anode structure and efficient thermal management enabled by substrate thinning and flip-chip packaging techniques. The resultant device exhibits a high surge current density of 3.4 kA/cm2 (42 A) and a critical transient dissipating energy density of 1.5 J/cm2 (18.6 mJ). All these values are the highest reported in AlGaN/GaN SBDs. In particular, the superior switching performance with nanosecond reverse recovery time is achieved under a 400-V operating condition with a fast di/dt of 200 A/ $\mu \text{s}$ , implying the desired functionality of the proposed device architecture. This work, thus, makes a significant step in reaching the promise of AlGaN/GaN SBDs for high-reliability and high-power applications.