A review on photoelectrochemical cathodic protection semiconductor thin films for metals

Photoelectrochemical(PEC) cathodic protection is considered as an environment friendly method for metals anticorrosion. In this technology, a n-type semiconductor photoanode provides the photogenerated electrons for metal to achieve cathodic protection. Comparing with traditional PEC photoanode for water splitting, it requires the photoanode providing a suitable cathodic potential for the metal, instead of pursuit ultimate photon to electric conversion efficiency, thus it is a more possible PEC technology for engineering application. To date, great efforts have been devoted to developing novel n-type semiconductors and advanced modification method to improve the performance on PEC cathodic protection metals. Herein, recent progresses in this field are summarized. We highlight the fabrication process of PEC cathodic protection thin film, various nanostructure controlling, doping, compositing methods and their operation mechanism. Finally, the current challenges and future potential works on improving the PEC cathodic protection performance are discussed.

Temperature-Dependent Characteristics of GaN Schottky Barrier Diodes with TiN and Ni Anodes

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℃, 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 Ti N 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.

Band alignment between NiO_(x) and nonpolar/semipolar GaN planes for selective-area-doped termination structure

Band alignment between NiO_(x) and nonpolar GaN plane and between NiO_(x) and semipolar GaN plane are measured by x-ray photoelectron spectroscopy. They demonstrate that the maximum value of the valence band in the unintentional-doped a-plane, m-plane, and r-plane GaN are comparable to each other, which means that all the substrates are of n-type with similar background carrier concentrations. However, the band offset at the NiO_(x)/GaN interface presents obvious crystalline plane dependency although they are coated with the same NiO_(x) films. By fitting the Ga 3 d spectrum obtained from the NiO_(x)/GaN interface, we find that relatively high Ga–O content at the interface corresponds to a small band offset. On the one hand, the high Ga–O content on the GaN surface will change the growth mode of NiO_(x). On the other hand, the affinity difference between Ga and O forms a dipole which will introduce an extra energy band bending.

Regulation of the photogenerated carrier transfer process during photoelectrochemical water splitting:A review

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.

Normally-off AlGaN/GaN heterojunction field-effect transistors with in-situ AlN gate insulator

AlGaN/GaN heterojunction field-effect transistors(HFETs)with p-GaN cap layer are developed for normally-off operation,in which an in-situ grown AlN layer is utilized as the gate insulator.Compared with the SiNxgate insulator,the AlN/p-GaN interface presents a more obvious energy band bending and a wider depletion region,which helps to positively shift the threshold voltage.In addition,the relatively large conduction band offset of AlN/p-GaN is beneficial to suppress the gate leakage current and enhance the gate breakdown voltage.Owing to the introduction of AlN layer,normally-off p-GaN capped AlGaN/GaN HFET with a threshold voltage of 4 V and a gate swing of 13 V is realized.Furthermore,the field-effect mobility is approximately 1500 cm^(2)·V^(-1)·s^(-1)in the 2DEG channel,implying a good device performance.

Effect of anode area on the sensing mechanism of vertical GaN Schottky barrier diode temperature sensor

Effect of anode area on temperature sensing ability is investigated for a vertical GaN Schottky-barrier-diode sensor.The current-voltage-temperature characteristics are comparable to each other for Schottky barrier diodes with different anode areas,excepting the series resistance.In the sub-threshold region,the contribution of series resistance on the sensitivity can be ignored due to the relatively small current.The sensitivity is dominated by the current density.A large anode area is helpful for enhancing the sensitivity at the same current level.In the fully turn-on region,the contribution of series resistance dominates the sensitivity.Unfortunately,a large series resistance degrades the temperature error and linearity,implying that a larger anode area will help to decrease the series resistance and to improve the sensing ability.

Design of vertical diamond Schottky barrier diode with junction terminal extension structure by using the n-Ga_(2)O_(3)/p-diamond heterojunction

A novel junction terminal extension structure is proposed for vertical diamond Schottky barrier diodes(SBDs) by using an n-Ga_(2)O_(3)/p-diamond heterojunction. The depletion region of the heterojunction suppresses part of the forward current conduction path, which slightly increases the on-resistance. On the other hand, the reverse breakdown voltage is enhanced obviously because of attenuated electric field crowding. By optimizing the doping concentration, length, and depth of n-Ga_(2)O_(3), the trade-off between on-resistance and breakdown voltage with a high Baliga figure of merit(FOM)value is realized through Silvaco technology computer-aided design simulation. In addition, the effect of the work functions of the Schottky electrodes is evaluated. The results are beneficial to realizing a high-performance vertical diamond SBD.

Fabrication of CuO_x thin-film photocathodes by magnetron reactive sputtering for photoelectrochemical water reduction

The CuO_x thin film photocathodes were deposited on F-doped Sn O_2 (FTO)transparent conducting glasses by alternating current(AC)magnetron reactive sputtering under different Ar:O_2 ratios.The advantage of this deposited method is that it can deposit a CuO_x thin film uniformly and rapidly with large scale.From the photoelectrochemical(PEC)properties of these CuO_x photocathodes,it can be found that the CuO_x photocathode with Ar/O_2 30:7 provide a photocurrent density ofà3.2 m A cm^(à2)under a bias potentialà0.5 V(vs.Ag/Ag Cl),which was found to be twice higher than that of Ar/O_2 with 30:5.A detailed characterization on the structure,morphology and electrochemical properties of these CuO_x thin film photocathodes was carried out,and it is found that the improved PEC performance of CuO_x semiconductor photocathode with Ar/O_230:7 attributed to the less defects in it,indicating that this Ar/O_230:7 is an optimized condition for excellent CuO_x semiconductor photocathode fabrication.

GaN metal-oxide-semiconductor field-effect transistors on AIGaN/GaN heterostructure with recessed gate

GaN metal-oxide-semiconductor field-effect transistors （MOSFETs） on AIGaN/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 CI2 and SiCI4 were investigated in the gate recess process. SiCI4-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.