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Photoelectrochemical (PEC) water splitting is considered as an ideal technology to produce hydrogen. Photogenerated carrier migration is one of the most important roles in the whole process of PEC water splitting. It includes bulk transfer inside of the photoelectrode and the exchange at the solid–liquid interface. The energy barriers during the migration process lead to the dramatic recombination of photogenerated hot carrier and the reducing of their redox capacity. Thus, an applied bias voltage should be provided to overcome these energy barriers, which brings the additional loss of energy. Plentiful researches indicate that some methods for the regulation of photogenerated hot carrier, such as p-n junction, unique transfer nanochannel, tandem nanostructure and Z-Scheme transfer structure et al., show great potential to achieve high-efficient PEC water overall splitting without any applied bias voltage. Up to now, many reviews have summarized and analyzed the methods to enhance the PEC or photocatalysis water splitting from the perspectives of materials, nanostructures and surface modification etc. However, few of them focus on the topic of photogenerated carrier transfer regulation, which is an important and urgent developing technique. For this reason, this review focuses on the regulation of photogenerated carriers generated by the photoelectrodes and summarizes different advanced methods for photogenerated carrier regulation developed in recent years. Some comments and outlooks are also provided at the end of this review.

High-performance AlGaN/GaN lateral Schottky barrier diodes (SBDs) with recess structure and dual metal nitride anodes were demonstrated. With high work-function and nonrecess structure, a NiN anode enhances the breakdown voltage (BV), while a TiN anode reduces the turn-on voltage ( ${V}_{ \mathrm{\scriptscriptstyle ON}}$ ) due to its low work-function and contact to the two-dimensional electron gas (2DEG) layer directly on a recess structure. As the length of the NiN anode ( ${L}_{r}$ ) on the nonrecess region decreases from 75 to 3 $\mu \text{m}$ , ${V}_{ \mathrm{\scriptscriptstyle ON}}$ is reduced from 0.56 to 0.30 V, while the reverse leakage current slightly increases from ${3} \times {10}^{-{4}}$ to ${2} \times {10}^{-{3}}$ A/cm2 at the bias of −10 V. The lateral AlGaN/GaN SBD with a ${L}_{r}$ of 3 $\mu \text{m}$ at a distance of cathode–anode ( ${L}_{\text {AC}}$ ) of 20 $\mu \text{m}$ achieves a high BV of 1.62 kV, an ultralow ${V}_{ \mathrm{\scriptscriptstyle ON}}$ of 0.30 V and a small capacitance of 6.0 pF at zero bias with little degradation on ON-resistance, indicating superior potential application in high-frequency and high-power devices.

AlGaN/gallium nitride (GaN) ion-sensitive field-effect transistors (ISFETs) were fabricated as pH sensors. The sensitivity of the AlGaN/GaN ISFETs was evolved with gate recess process and ammonium hydroxide (NH4OH) treatment. By performing the gate recess process, the threshold voltage ( ${V}_{\text {T}}$ ) of the ISFET increased from −3.33 to −0.31 V and the maximum conductance ( ${G}_{\text {M}}$ ) of the ISFET increased from 0.8 to 2 mS, with the current sensitivity of the pH sensor improving from 52.25 to $78.86~\mu \text{A}$ /pH. Further, after performing the ammonium hydroxide treatment, the ${V}_{\text {T}}$ of the ISFET increased from −0.33 to −0.14 V, with the current sensitivity of the pH sensor improving from 78.86 to $84.39~\mu \text{A}$ /pH. To characterize the surface conditions the X-ray photoelectron spectroscopy (XPS) was deployed. The results indicated that many nitrogen vacancies ( ${V}_{\text {N}}$ ) were introduced during the recess process, leading to a negative ${V}_{\text {T}}$ shift and a smaller potential sensitivity ( ${S}_{V}$ ), which can be improved by ammonium hydroxide treatment.

Vertical GaN Schottky barrier diode was fabricated as temperature sensor by using a thermally stable TiN anode. The current-voltage characteristics of the diode measured at various temperature present a zero-temperature coefficient (ZTC) bias point of approximately 0.6 V. At the voltage below the ZTC bias point (sub-threshold region), the forward voltage at a fixed current decreases linearly with the increasing temperature, resulting in a sensitivity of approximately 1.3 mV/K. In the reversely biased region, the leakage current also presents temperature-dependent behavior with a sensitivity of approximately 19.7 mA/K regardless of the bias. Those results can be interpreted by the thermionic emission model.

This article presents a terahertz integrated cavity bandpass filter based on cyclic etched silicon carbon (SiC) via-holes. Solutions are discussed in detail, including design, fabrication, and optimization. The fabrication procedures of the integrated filters are specially optimized, which are fully compatible with that of the common terahertz monolithic integrated circuit. The cyclic via-hole etching technique based on inductively coupled plasma (ICP) is proposed, which achieves via-holes with a controllable inclination on SiC substrate. Measurements show that the insertion loss of the cavity integrated filter is only 1.55 dB at 185 GHz, with a relative bandwidth of 9.7%. Two transmission zeros are introduced by the over-moded technique, achieving the out-of-band suppression of the filter higher than 40 dB. Compared with the traditional terahertz cavity filters, the integrated cavity on SiC substrate provides similar electric performances and better integration but with the one-thousandth of the volume.

This article presents a finger-type gallium nitride (GaN) Schottky barrier diode (SBD)-based microwave rectifier with medium-power capacity and wide power bandwidth. A complete solution including SBD design and fabrication, model extraction, circuit optimization, and demonstration is proposed. The finger-type anode and critical thickness epitaxial layer techniques are adopted to reduce the GaN SBD resistance to 1.9 $\Omega $ (0.011 $\text{m}\sf \Omega \cdot \text {cm}^{{2}}$ ) and achieve nearly constant junction capacitance over a wide range of voltages (17–25 V). Revised equivalent models including the effects of large pads, critical thickness epitaxial layer, and finger-type layout are proposed to describe the new features of the SBD. A 2.45-GHz microwave rectifier based on the GaN SBD is designed and measured, having a maximum power and an efficiency of 1.91 W and 78.5%, respectively. The high-efficiency power range (≥70% and ≥75%) is significantly extended to 14 and 10.8 dB. Finally, a wireless clinical examination prototype constructed by a rectifier, antennas, biomedical sensors, and a signal processer is proposed for demonstration. The resulting wireless electrocardiogram shown in computer is clear and stable.

In this work, we fabricate a semitransparent BiVO4 photoanode via the synergistic effect of indium (In) doping and anoxic annealing. An optimized photocurrent density of 3.6 mA cm–2 at 2 V versus r...

Temperature sensor using series GaN Schottky barrier diode (SBD) with TiN anode was fabricated and evaluated extensively. The diode presents good characteristics in a wide temperature range from 25 °C to 200 °C. The temperature dependent forward voltage of the conventional 8-finger SBD at a fixed current shows good linearity, resulting in a sensitivity of approximately 1.14 mV/K. On the other hand, the series SBD with two 8-finger diodes enhances the sensitivity by nearly two times. The enhancement in temperature sensitivity is interpreted using a series model, with the obtained parameters are comparable with the 8-finger SBD.

TiN/GaN Schottky barrier diodes with different anode diameters are fabricated to investigate the temperature sensing mechanism. All the circular diodes present good stability over a temperature range of 25 °C–200 °C. In the fully turn-on region, the sensitivity increases with the increasing diameter. Furthermore, the highest sensitivity of 1.22 mV/K is obtained for a 300- $\mu \text{m}$ -diameter device at current of 20 mA, taking into account the series resistance. In the subthreshold region, the forward current ( ${I}_{D}$ ) density determines the sensor sensitivity, in which a larger current density corresponds to a lower sensitivity. In addition, the strong dependence of the leakage current on the temperature indicates that the linearity of ln ( ${I}_{r}$ ) versus temperature can be also used for sensor applications.

In this paper, a terahertz integrated cavity band-pass filter (ICF) based on silicon carbide (SiC) has been proposed. The cascade triplet (CT) topology and over-moded cavity has been utilized to implement out-of-band suppression. Measurement showed that at the center frequency of 185GHz, insertion loss of the ICF is only 1.55dB, with a relative bandwidth of 9.7%. Two transmission zeros (TZs) has been produced, achieving the suppression of the upper side-band of higher than 40dB. Compared with a traditional cavity filter, the ICF is only one-thousandth of the volume with providing similar electric performance and better integration.

In this work, we investigated the oxide reliability of 4H-SiC (0001) MOS capacitors, the oxide was fabricated about 60 nm by thermal oxidation temperature at 1350°C, the oxides than annealed at different temperatures and times in diluted NO (10% in N2). The 4H-SiC MOS structure was analyzed by C-V and I-V measurement. Compared the J-E curves and Weibull distribution curves of charge-to-breakdown for fives samples under different annealing temperature and time, it shows that the high annealing temperature improves the electrical properties as the lifetime enhanced. The mode value of field-to-breakdown (EBD) for thermal oxides by post-oxide-annealing in NO for 30 min at 1350°C was 10.09 MV/cm, the charge-to-breakdown (QBD) of this sample was the highest in all samples, and the QBD value at 63.2% cumulative failure rate was 0.15 C/cm2. The QBD of the sample annealing at 1200°C for 120 min was 0.06 C/ cm2. The effects of NO annealing in high temperature enhance the lifetime of electrical properties and field-to-breakdown obviously. It can be demonstrated that the annealing temperature as high as 1300°C for 30 min can be used to accelerate TDDB of SiC MOS gate oxide.

A comparison study on electrical performance of Ge pMOSFETs with a GeO x passivation layer formed by ozone post oxidation (OPO) and plasma post oxidation (PPO) is performed. PPO and OPO were carried out on an Al2O3/n-Ge (001) substrate followed by a 5-nm HfO2 gate dielectric in situ deposited in an ALD chamber. The quality of the dielectric/Ge interface layer was characterized by X-ray photoelectron spectroscopy and transmission electron microscopy. The PPO treatment leads to a positive threshold voltage (V TH) shift and a lower I ON/I OFF ratio, implying a poor interface quality. Ge pMOSFETs with OPO exhibit a higher I ON/I OFF ratio (up to four orders of magnitude), improved subthreshold swing, and enhanced carrier mobility characteristics as compared with PPO devices. A thicker Al2O3 block layer in the OPO process leads to a higher mobility in Ge transistors. By comparing two different oxidation methods, the results show that the OPO is an effective way to increase the interface layer quality which is contributing to the improved effective mobility of Ge pMOSFETs.

We fabricate an AlGaN/GaN ion-sensitive field-effect transistor through the photoelectrochemical method for pH sensor application. The photoelectrochemical reaction transforms the AlGaN barrier into oxide, which can deplete the two demensional electron gas to achieve the normally-off operation. Besides, the oxide on the surface enhances the sensitivity to approximately 56.3 mV/pH, a value close to the room temperature Nernstian limit. Based on surface characterization results, the needle-like native oxide (a mixture of Al2O3 and Ga2O3) is transformed into a smooth Al2O3-dominated film. The enhanced surface status and transconductance of the normally-off device is regarded as the possible reason to effective improve the pH sensitivity.

The characteristics of GaN Schottky barrier diodes fabricated with TiN and Ni anodes were evaluated in the temperature range from 25 to 175 °C. The Schottky barrier height (ideality factor) increases (decreases) with increasing temperature for both kinds of diodes owing to the barrier height being nonhomogeneous. The GaN diode with TiN anode presents better interface quality and thermal stability is adopted for temperature sensing application. It demonstrated that the sensitivity of the TiN diode is approximately 1 mV/K and varies only slightly for all current levels.

In present study, copper oxide films were prepared at different sputtering powers (10–100 W) using magnetron reactive sputtering. The crystalline structure, surface morphologies, composition, and optical band gap of the as-grown films are dependent on sputtering power. As the sputtering power decreasing from 100 to 10 W, the composition of films changed from CuO to quasi Cu 2 O domination. Moreover, when the sputtering power is 10 W, a relative high hole carrier density and high-surface-quality quasi Cu 2 O thin film can be achieved. AlGaN/GaN HFETs were fabricated with the optimized p-type quasi Cu 2 O film as gate electrode, the threshold voltage of the device shows a 0.55 V positive shift, meanwhile, a lower gate leakage current, a higher ON/OFF drain current ratio of ∼10 8 , a higher electron mobility (1465 cm 2 /Vs), and a lower subthreshold slope of 74 mV/dec are also achieved, compared with the typical Ni/Au-gated HFETs. Therefore, Cu 2 O have a great potential to develop high performance p-type gate AlGaN/GaN HFETs.

An integrated ion beam sputter deposition module utilizing an anode layer ion source has been designed and fabricated. Silver oxide is deposited utilizing this ion beam sputter module. Experimental results show that Ag is fully oxidized only with oxygen partial flow rate equal to and higher than 0.8. With an anode voltage of 700 V, silver oxide with (1 1 1) preferred orientation was obtained. Increasing anode voltage to 1500 V results in (2 0 0) orientated silver oxide thin films. Two band gaps can be identified that increase from 1.0/2.6 eV to 2.0/2.8 eV as anode voltage increases from 700 to 1500 V, respectively. As anode voltage increases, XPS analysis shows that Ag 3d5/2 core level peak position shifts from 367.3 to 367.7 eV. This confirms that increased kinetic energy of sputtered atoms results in partial reduction of AgO to Ag2O. Fully oxidized silver oxide samples are likely to be p-type while the resistivity is in the range of 0.7–2.0 × 104 Ω·cm.

In this study, graphene quantum dot (GQD) modified g-C3N4 (GQDs/CN) composite photocatalysts were prepared. The photocatalytic ammonia degradation properties of the GQDs/CN composites were much higher than that of pure g-C3N4. When the amount of GQDs added reached 0.5 wt% the GQDs/CN composite showed the best performance for photocatalytic total ammonia nitrogen (TAN) removing, and a 90% TAN removing rate was achieved in 7 hours under visible light illumination (200 mW cm−2), which is approximately 3 times higher than that of pure g-C3N4. The increased photocatalytic property was contributed by the photon adsorption ability and electron transfer capacity, which were improved after GQD modification. The main photocatalytic end-product of TAN was NO3− which is a type of environmentally green ion. Further results indicated that the oxygen concentration and pH value of the reaction solution were very important for the photocatalytic ammonia degradation process. A better performance could be achieved under a higher oxygen concentration and pH value.

In this article, AlGaN/GaN heterostructure ion-sensitive field-effect transistors (ISFETs) were prepared and evaluated by thermal oxidation treatment on the AlGaN surface. The ISFETs were fabricated on the AlGaN/GaN heterostructure and then thermally oxidized with dry oxygen in 600, 700, and 800 °C, respectively. It indicates that the performance of the AlGaN/GaN heterostructure ISFETs, such as noise and sensitivity, has been improved owing to the thermal oxidation treatment process at different temperatures. The X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) results indicate that after thermal oxidation treatment at different temperatures, hydroxide who possesses high surface state density will transfer to oxide owing to the higher chemical stability of the latter. Moreover, a crystalline α-Al2O3 phase generated at 700 °C can not only provide a relatively smooth surface, but also improve the sensitivity to 57.7 mV/pH for the AlGaN/GaN heterostructure ISFETs, which is very close to the Nernstian limit.

Wet chemical deposition method by using copper salt doping approach is used to synthesize mixed phase (rutile and anatase) titanium dioxide (TiO 2 ) nanostructures. Pristine and Cu-doped TiO 2 photoanodes are analyzed for their physical properties. The results showed a significant influence of Cu-salt concentration in tuning the TiO 2 properties in terms of structural, morphological, porosity, optical and electrochemical properties. The Cu-free TiO 2 photoanode is rutile in structure whereas when Cu-doping level is increased, beyond 0.003 M, is transformed to mixed rutile-anatase structure. Presence of copper (Cu) in different weight percentages was confirmed from the surface energy dispersive X-ray analysis. Effect of Cu-doping on the power conversion efficiency of TiO 2 -based dye-sensitized solar cells is investigated. At 0.005 M Cu-doping level, both power conversion and incident-photon-to-current-conversion efficiencies of TiO 2 electrode are optimal i.e. 3.32% and 46% respectively; consistent to electron life time measurement where with increase in Cu-doping level from 0.001 to 0.005 M an electron life time of TiO 2 photoanode is increased from 0.1893 ms to 0.5967 ms.

The energy band alignment at the multilayer-MoS2/ZrO2 interface and the effects of CHF3 plasma treatment on the band offset were explored using x-ray photoelectron spectroscopy. The valence band offset (VBO) and conduction band offset (CBO) for the MoS2 /ZrO2 sample is about 1.87 eV and 2.49 eV, respectively. While the VBO was enlarged by about 0.75 eV for the sample with CHF3 plasma treatment, which is attributed to the up-shift of Zr 3d core level. The calculation results demonstrated that F atoms have strong interactions with Zr atoms, and the valence band energy shift for the d-orbital of Zr atoms is about 0.76 eV, in consistent with the experimental result. This interesting finding encourages the application of ZrO2 as gate materials in MoS2-based electronic devices and provides a promising way to adjust the band alignment.

In this paper, we prepare porous g-C3N4 by hydrothermal process, and then Rh atoms are anchored on the carbon nitride framework. After further phosphating, a novel single-sites-like Rh-phosphide (Rh-P) modified carbon nitride photocatalyst is fabricated. The photocatalytic hydrogen evolution rate of Rh-P modified carbon nitride is 33 times higher than that of pure C3N4. Structure analysis results show that Rh-P dots are dispersed with single-sites state on the surface of carbon nitride framework. Further theoretical calculation and the photoelectrochemical results reveal that Rh-P single-sites on carbon nitride provide photogenerated convergence centers to decrease the hydrogen evolution potential and enhance the free charge transfer capability. Thus, it improves the photocatalysis hydrogen evolution performance of the carbon nitride.

In this work, we fabricate a novel bismuth vanadate/two dimensional-carbon nitride/deoxyribonucleic acid (BiVO4/2D-C3N4/DNA) aptamer photoelectrochemical (PEC) sensor, and this sensor provides a record detection sensitivity area (5 × 10−7 μg/L - 10 μg/L) for Microcystin-LR (MC-LR). Meanwhile, except for MC-LR detection, this sensor presents highly sensitivity for tumor marker, heavy metal ion, antibiotic also by changing the DNA aptamer. Photo charge dynamic and theory calculation results reveal that 2D-C3N4 is a key material for multifunctional interface reconciliation of this PEC aptamer sensor. Firstly, it can serve as photogenerated hole oriented-transfer medium from the BiVO4 photoanode to the detective target; In addition, 2D-C3N4 with large area of π electron cloud can fix the DNA aptamer parallelly by π-π bonding with the nucleic acid in the DNA aptamer to shorten the hole transfer distance from the semiconductor to target. So that, a record MC-LR detection sensitivity has been achieved by the 2D-C3N4 modified BiVO4/DNA aptamer sensor.

In this paper, a novel gallium nitride (GaN)-based heterostructure Gunn diode is proposed for the first time to enhance the output characteristics of Gunn oscillation waveforms. A well-designed grooved anode contact is adopted to separate the long-channel diode into two short-channel diodes in parallel. If the grooved anode contact is positioned in the middle of the device, the output power nearly doubles in the grooved-anode diode compared with the single-channel ones, as does the output frequency. Based on the numerical results, the best output characteristics are obtained at the 2.0-&micro, m symmetrical grooved-anode diode, which produces nearly 5.48 mW of power at the fundamental frequency of 172.81 GHz, with 3.13% efficiency of power conversion. If the grooved anode contact is not positioned in the middle of the diode, the harmonic frequency would be enhanced. The GaN heterostructure grooved-anode Gunn diode has been demonstrated to be an excellent solid-state source of terahertz oscillator.

The paper confirmed the SiC/SiO2 interface state density obtained from the ultrahigh-temperature dry oxidation process on 4H-SiC Si-face (0 0 0 1) at up to 1550 °C without any other passivating techniques. Our results were consistent with those of previous reports. Furthermore, we also considered the reliability of SiO2, which is important for its practical application, by TDDB measurements. The optimal interface state density was obtained for the gate oxide formed at 1450 °C at EC − E = 0.2–0.6 eV, whereas the gate oxide was relatively the most reliable for the oxidation at 1250 °C. It suggests that the effects of oxidation temperature of 4H-SiC (0 0 0 1) had a trade-off between gate oxide reliability and SiC/SiO2 interface properties.

In this presentation, GaN Schottky barrier diodes (SBDs) with low access sheet resistance of 25 Ω/□ is fabricated for microwave power transmission. Turn-on voltage of about 0.43 V and ON resistance of 3.15 are achieved by introducing TiN Schottky electrode and low sheet resistance on the access layer. Breakdown voltage of 30 V and off capacitance of 7 pF are also obtained, indicating that the diode is suitable for the application in the microwave power transmission system.

GaN metal-oxide-semiconductor field-effect transistors (MOSFETs) on AlGaN/GaN heterostructure with a recess gate were fabricated and characterized. The device showed good pinch-off characteristics and a maximum field-effect mobility of 145.2 cm2·V−1·s−1. The effects of etching gas of Cl2 and SiCl4 were investigated in the gate recess process. SiCl4-etched devices showed higher channel mobility and lower threshold voltage. Atomic force microscope measurement was done to investigate the etching profile with different etching protection mask. Compared with photoresist, SiO2-masked sample showed lower surface roughness and better profile with stepper sidewall and weaker trenching effect resulting in higher channel mobility in the MOSFET.

GaN Schottky barrier diodes (SBDs) with low turn-on voltage are developed for microwave rectification. The diodes with reactively-sputtered TiN electrodes have a lower turn-on voltage compared with the diodes with Ni electrode, while the on-resistance, the reverse leakage current, and the reverse breakdown characteristics are comparable to each other. Theoretically, the SBDs with TiN electrodes can enhance the efficiency of a rectenna circuit at 2.45 GHz from 84% to 89% when the turn-on voltage decreases from 1.0 to 0.5 V.

GaN metal–oxide–semiconductor field-effect transistors (MOSFETs) with recessed gate on AlGaN/GaN heterostructure are reported in which the drain and source ohmic contacts were fabricated on the AlGaN/GaN heterostructure and the electron channel was formed on the GaN buffer layer by removing the AlGaN barrier layer. Negative threshold voltages were commonly observed in all devices. To investigate the reasons of the negative threshold voltages, different oxide thickness, etching gas and bias power of inductively-coupled plasma (ICP) system were utilized in the fabrication process of the GaN MOSFETs. It is found that positive charges of around 1 × 10 12 q/cm 2 exist near the interface at the just threshold condition in both silane- and tetraethylorthosilicate (TEOS)-based devices. It is also found that the threshold voltages do not obviously change with the different etching gas (SiCl 4 , BCl 3 and two-step etching of SiCl 4 /Cl 2 ) at the same ICP bias power level (20–25 W) and will become deeper when higher bias power is used in the dry recess process which may be related to the much serious ion bombardment damage. Furthermore, X-ray photoelectron spectroscopy (XPS) experiments were done to investigate the surface conditions. It is found that N 1s peaks become lower with higher bias power of the dry etching process. Also, silicon contamination was found and could be removed by HNO 3 /HF solution. It indicates that the nitrogen vacancies are mainly responsible for the negative threshold voltages rather than the silicon contamination. It demonstrates that optimization of the ICP recess conditions and improvement of the surface condition are still necessary to realize enhancement-mode GaN MOSFETs on AlGaN/GaN heterostructure.

To achieve thermally-stable Schottky electrode on gallium nitride (GaN) electron devices for gate-first process and high-temperature devices, Titanium nitride (TiN) film was synthesised by magnetron reactive sputtering as the Schottky contact on n-GaN and AlGaN/GaN heterostructure field-effect transistors (HFETs). To reduce the sheet resistance, a cap layer of W/Au was deposited on the films. The ideality factor and Schottky barrier height (SBH) of the circular Schottky contact on n-GaN were 1.07 and 0.59 eV, respectively. GaN Schottky diodes and AlGaN/GaN HFETs were fabricated by annealing the Ohmic contact (Ti/Al/Ti/Au) and Schottky contact simultaneously. No obvious change was observed in the forward characteristics of the GaN Schottky diode, when the sample was annealed at 850 � C for 1 min. For the AlGaN/GaN HFETs which were annealed at 850 � C, the HFETs operated very well when they were annealed at 850 � C for 1 or 3 mins. The TiN/W/Au Schottky

Gate oxide films on silicon carbide (SiC) markedly affect the performance of SiC metal-oxide-semiconductor field effect transistors (MOSFETs). We used current-voltage (I–V) measurements to investigate the influence of curvature induced stress/strain on the breakdown electric field and the charge-to-breakdown (QBD) at SiO2/SiC, defined here as time-zero dielectric breakdown and time-dependent dielectric breakdown, respectively. The curvature of the epitaxy wafers was characterized by a thin film stress measurement system. The compression/tensile curvature decreased EBD and QBD during the dry thermal oxidation process. Furthermore, first-principle calculations suggested that the energy levels of the samples were related to the lattice constants of the SiC crystal, indicating that stress mainly affected the SiO2/SiC interface. We suggest that a “stress free” oxide film might be the best choice for SiC-MOSFET applications.

The effect of temperature on the characteristics of gallium nitride (GaN) Schottky barrier diodes (SBDs) with TiN and Ni anodes is evaluated. With increasing the temperature from 25 to 175°C, reduction of the turn-on voltage and increase of the leakage current are observed for both GaN SBDs with TiN and Ni anodes. The performance after thermal treatment shows much better stability for SBDs with TiN anode, while those with Ni anode change due to more interface states. It is found that the leakage currents of the GaN SBDs with TiN anode are in accord with the thermionic emission model whereas those of the GaN SBDs with Ni anode are much higher than the model. The Silvaco TCAD simulation results show that phonon-assisted tunneling caused by interface states may lead to the instability of electrical properties after thermal treatment, which dominates the leakage currents for GaN SBDs with Ni anode. Compared with GaN SBDs with Ni anode, GaN SBDs with TiN anode are beneficial to the application in microwave power rectification fields due to lower turn-on voltage and better thermal stability.

Evaluation of electrical performance of AlGaN/GaN heterostructure field-effect transistors (HFETs) with titanium nitride (TiN) gate and its thermal stability was presented. The TiN film was formed by reactive sputtering in Ar and N 2 ambient by DC magnetron sputtering. No obvious degradation was found in the Schottky contact when the device was thermally treated at 600 °C for 1 h or 850 °C for several minutes. The sheet resistance of the TiN films was increased by thermal treatment. A possible reason is considered to be the oxidation during the annealing process. It shows that the electrical performance of the TiN-gate AlGaN/GaN HFET is thermally stable and can be a possible candidate for a gate material that endures a high-temperature process.

Technologies for microwave power transmission to small mobile equipment at 5.8 GHz are presented, including GaN Schottky barrier diode (SBD) for efficient and high power RF/DC conversion, rectenna circuits with voltage doubler pair to reduce signal reflection, and frequency modulation to control efficient charging condition in a microwave cavity box.

Schottky contacts of refractory metal nitrides formed by reactive sputtering on n-type gallium nitride (GaN) were electrically evaluated, including film resistivity, Schottky characteristics and thermal stability. For the metal nitrides of TiN x , MoN x and ZrN x , resistivities of 108.3, 159.0 and 270.0 μΩcm were obtained, respectively. Current–voltage ( I – V ) characteristics showed that the ideality factor varied from 1.03 to 1.16, while the Schottky barrier height (SBH) varied from 0.66 to 0.79 eV for the three kinds of Schottky contacts. Especially for the ZrN x contact, the ideality factor and SBH were improved after annealing at 800 °C for 30 s. Schottky contact utilizing a refractory metal nitride on GaN shows its potential to develop thermally stable GaN devices.

The reduction of gate leakage current in AlGaN/GaN metal–insulator–semiconductor (MIS) heterostructure field effect transistors (HFETs) is influenced by the resistance of the insulating layers and even more so by the increase in AlGaN layer resistance. Electrical performance of AlGaN/GaN HFETs with and without electron beam evaporated SiO X gate insulator layers indicates that the AlGaN layer resistance is increased seven orders of magnitude by oxide evaporation at negative gate bias. Also, the transconductance frequency dispersion of the MIS-HFET can be explained with a model in which the insulator is more conductive than the AlGaN layer, though they are both highly resistive. These results indicate that the deposition of the oxide reduces electron leakage through the AlGaN layer in the direction from the AlGaN surface to the GaN layer.

Structural and optical properties of AlGaN-based ultraviolet (UV) light-emitting diodes (LEDs) with a thin undoped GaN layer and thick n-type AlGaN/GaN superlattices have been investigated. Some V-defects were observed in the n-type superlattice layers. The V-defect density decreases enormously with increasing the undoped GaN thickness. Simultaneously, the output power of the UV-LEDs increases remarkably as the undoped GaN thickness increases. Since the threading dislocation density is the same for all the LEDs, the improved LED performance could be attributed to the largely reduced V-defect density.

By introducing Al 0.49 Ga 0.51 N/Al 0.16 Ga 0.84 N distributed Bragg reflectors (DBRs), the performance of an ultraviolet (UV) light–emitting diode (LED) with an emission wavelength of about 350 nm on a sapphire substrate was dramatically improved. The output power was enhanced by a factor of 2.3, compared to the conventional UV-LED with a similar emission wavelength. This enhancement due to the DBRs was larger than that occurring in InGaN-based blue LEDs also containing DBRs, in which the increase of optical power with a factor of 1.5 was reported compared to the InGaN-based blue LEDs without DBRs. This can be attributed to the introduction of DBRs into the UV-LED which can strongly suppress the so-called internal absorption issue, one of the key factors impeding the performance of UV-LEDs grown on sapphire substrates with emission wavelengths below 362 nm, the GaN bandgap.

The thermal stability of copper (Cu)-gate AlGaN/GaN high electron mobility transistors (HEMTs) was investigated by annealing the devices at 300 °C and 500 °C for 1 h, and at 700 °C for 30 min, respectively. The current–voltage performance of the devices such as drain current, transconductance, threshold voltage and gate leakage current has no obvious degradation up to an annealing temperature of 500 °C and a time of 1 h. Up to 500 °C and 1 h annealing, no Cu diffusion was found at the Cu and AlGaN interface by secondary ion mass spectrometry determination. At an annealing temperature of 700 °C and a time of 30 min, the devices failed due to Cu diffusion into the 2DEG channel. These results indicate that the Schottky contact and the device performance of a Cu-gate AlGaN/GaN HEMT are stable up to an annealing temperature of at least 500 °C and a time of 1 h. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Design and fabrication of monolithic blue LED series arrays that can be operated under high ac voltage are described. Several LEDs, such as 3, 7, and 20, are connected in series and in parallel to meet ac operation. The chip size of a single device is 150 μm x 120 μm and the total size is 1.1 mm x 1 mm for a 40 (20 + 20) LED array. Deep dry etching was performed as device isolation. Two-layer interconnection and air bridge are utilized to connect the devices in an array. The monolithic series array exhibit the expected operation function under dc and ac bias. The output power and forward voltage are almost proportional to LED numbers connected in series. On-wafer measurement shows that the output power is 40 mW for 40 (20 + 20) LED array under ac 72 V.

A high-performance AlGaN/GaN-based ultraviolet (UV) light-emitting diode (LED) is successfully fabricated on sapphire substrate by metal-organic-chemical-vapor-deposition technique. Generally, a p–n junction is grown on a thick GaN layer on sapphire substrate, which results in a strong internal-absorption effect. Simultaneously, a thick AlGaN cladding layer on the GaN layer also easily produces crack. In order to avoid the internal absorption, a thick AlGaN layer is immediately introduced on a thin low-temperature GaN buffer (LT GaN buffer) instead of a thick GaN layer, which successfully avoids crack formation. However, an enhanced lattice-mismatch of AlGaN/LT GaN buffer/sapphire compared with that of GaN/sapphire might result in an enhanced dislocation density, which leads to the degraded performance of UV-LED. An AlGaN/GaN supperlattice that is applied in UV-LED instead of the thick AlGaN layer strongly decreases the dislocation density, confirmed by transmission electron microscope. Furthermore, this AlGaN/GaN supperlattice successfully avoids crack formation. Consequently, the optical power of UV-LED is greatly increased. Based on the above results, we successfully fabricate a crack-free UV-LED with a high performance.

Oxygen vacancy on semiconductor has been usually considered as donor contributor which can improve the charge transfer capacity of the photoanode. However, oxygen vacancy has also been found to perform as recombination center for the photogenerated charges. Herein, electrochemical reduction method is employed to treat the surface of Mo-doped BiVO4 (BiMoVO) photoanode. Experimental data indicate that when the reduction potential is located at −0.8 V (vs Ag/AgCl), quasioxygen vacancy is formed on the (020) facet (only BiO bonds crack), the electron mobility and photo-electrochemical (PEC) current density of the BiMoVO photoanode are increased dramatically. However, with the reduction potential increasing to −1.2 V, oxygen vacancy is formed on the surface of (020) facet (both BiO and VO bonds crack simultaneously), the PEC current density is decreased obviously. Further density functional theory calculation data point out that a moderate level of reduction is a key factor for the adjustment of photoanode performance. Thus, these results demonstrate first that oxygen vacancy actually is not the positive factor to improve the PEC performance of a BiVO4 photoelectrode, but the quasioxygen vacancy forming on the surface of the active facet is.

In this study, titanium nitride (TiN) is synthesized using reactive sputtering for a self-aligned gate process. The Schottky barrier height of the TiN on n-GaN is around 0.5 to 0.6 eV and remains virtually constant with varying nitrogen ratios. As compared with the conventional Ni electrode, the TiN electrode presents a lower turn-on voltage, while its reverse leakage current is comparable with that of Ni. The results of annealing evaluation at different temperatures and duration times show that the TiN/W/Au gate stack can withstand the ohmic annealing process at 800°C for 1 or 3 min. Finally, the self-aligned TiN-gated AlGaN/GaN heterostructure field-effect transistors are obtained with good pinch-off characteristics.

Ohmic property for AlGaN/GaN high electron mobility transistor was investigated by insetting a highly Al-doped ZnO between the metal and AlGaN/GaN structure. The Al-doped ZnO was deposited by dc magnetron sputtering method and Ti/Al/Ni/Au was deposited on the ZnO by evaporation. Prior to the ZnO deposition, the surface of the samples was treated by O2 plasma, HCl and NH4OH, respectively. Good ohmic performance was obtained with contact resistance of 2.7 Ω mm even without annealing. The lowest contact resistance was 2.0 Ω mm after being annealed at 300 °C for the sample with HCl treatment before ZnO deposition.

Three dimensional diamond ultraviolet (UV) photodetector have been fabricated on diamond epitaxial layer through down-top approach, where diamond epitaxial layer was grown between metal electrodes. A thin diamond epitaxial layer was first grown on high-pressure high-temperature single crystal diamond substrate. Then, the diamond epitaxial layer was covered by interdigitated tungsten electrodes. Furthermore, another diamond epitaxial layer was grown on uncovered area. At last, UV-Ozone treatment was used to oxidize the surface. The optoelectronic performance of the photodetector was characterized, exhibiting a large responsivity and a repeatable transient response behavior. Moreover, down-top process is beneficial for the electrode conductivity stability. Also, an ohmic contact could be formed between tungsten and diamond during growth. The results indicate that down-top process is an efficient way for fabrication of three dimensional diamond photodetectors.