Showing papers on "Gallium nitride published in 2018"

The 2018 GaN power electronics roadmap

Abstract: Gallium nitride (GaN) is a compound semiconductor that has tremendous potential to facilitate economic growth in a semiconductor industry that is silicon-based and currently faced with diminishing returns of performance versus cost of investment. At a material level, its high electric field strength and electron mobility have already shown tremendous potential for high frequency communications and photonic applications. Advances in growth on commercially viable large area substrates are now at the point where power conversion applications of GaN are at the cusp of commercialisation. The future for building on the work described here in ways driven by specific challenges emerging from entirely new markets and applications is very exciting. This collection of GaN technology developments is therefore not itself a road map but a valuable collection of global state-of-the-art GaN research that will inform the next phase of the technology as market driven requirements evolve. First generation production devices are igniting large new markets and applications that can only be achieved using the advantages of higher speed, low specific resistivity and low saturation switching transistors. Major investments are being made by industrial companies in a wide variety of markets exploring the use of the technology in new circuit topologies, packaging solutions and system architectures that are required to achieve and optimise the system advantages offered by GaN transistors. It is this momentum that will drive priorities for the next stages of device research gathered here.

Growth of 2D GaN Single Crystals on Liquid Metals

Abstract: Two-dimensional (2D) gallium nitride (GaN) has been highly anticipated because its quantum confinement effect enables desirable deep-ultraviolet emission, excitonic effect and electronic transport properties. However, the currently obtained 2D GaN can only exist as intercalated layers of atomically thin quantum wells or nanometer-scale islands, limiting further exploration of its intrinsic characteristics. Here, we report, for the first time, the growth of micrometer-sized 2D GaN single crystals on liquid metals via a surface-confined nitridation reaction and demonstrate that the 2D GaN shows uniformly incremental lattice, unique phonon modes, blue-shifted photoluminescence emission and improved internal quantum efficiency, providing direct evidence to the previous theoretical predictions. The as-grown 2D GaN exhibits an electronic mobility of 160 cm2·V–1·s–1. These findings pave the way to potential optoelectronic applications of 2D GaN single crystals.

Demonstration of Constant 8 W/mm Power Density at 10, 30, and 94 GHz in State-of-the-Art Millimeter-Wave N-Polar GaN MISHEMTs

Abstract: This paper reports on state-of-the-art millimeter-wave power performance of N-polar GaN-based metal–insulator–semiconductor high-electron-mobility transistors at 30 and 94 GHz. The performance is enabled by our N-polar deep recess structure, whereby a GaN cap layer is added in the access regions of the transistor to simultaneously enhance the access region conductivity while mitigating dc-to-RF dispersion. The impact of lateral scaling of the drain access region length is examined using the tradeoff between breakdown voltage and small-signal gain. Load-pull measurements are presented at 94 GHz, corresponding to the target device operating frequency in W-band, where the device demonstrated a peak power-added efficiency (PAE) of 28.8% at 16 V and record-high maximum output power density of 8 W/mm at 20 V. Additional load-pull measurements at 30 and 10 GHz demonstrate the viability of this device across a wide frequency range where the peak power remained constant at 8 W/mm and with peak PAEs of 56% and 58%, respectively.

GaN/NbN epitaxial semiconductor/superconductor heterostructures.

Abstract: Epitaxy is a process by which a thin layer of one crystal is deposited in an ordered fashion onto a substrate crystal. The direct epitaxial growth of semiconductor heterostructures on top of crystalline superconductors has proved challenging. Here, however, we report the successful use of molecular beam epitaxy to grow and integrate niobium nitride (NbN)-based superconductors with the wide-bandgap family of semiconductors-silicon carbide, gallium nitride (GaN) and aluminium gallium nitride (AlGaN). We apply molecular beam epitaxy to grow an AlGaN/GaN quantum-well heterostructure directly on top of an ultrathin crystalline NbN superconductor. The resulting high-mobility, two-dimensional electron gas in the semiconductor exhibits quantum oscillations, and thus enables a semiconductor transistor-an electronic gain element-to be grown and fabricated directly on a crystalline superconductor. Using the epitaxial superconductor as the source load of the transistor, we observe in the transistor output characteristics a negative differential resistance-a feature often used in amplifiers and oscillators. Our demonstration of the direct epitaxial growth of high-quality semiconductor heterostructures and devices on crystalline nitride superconductors opens up the possibility of combining the macroscopic quantum effects of superconductors with the electronic, photonic and piezoelectric properties of the group III/nitride semiconductor family.

Room-temperature continuous-wave electrically pumped InGaN/GaN quantum well blue laser diode directly grown on Si

Abstract: Current laser-based display and lighting applications are invariably using blue laser diodes (LDs) grown on free-standing GaN substrates, which are costly and smaller in size compared with other substrate materials.1–3 Utilizing less expensive and large-diameter Si substrates for hetero-epitaxial growth of indium gallium nitride/gallium nitride (InGaN/GaN) multiple quantum well (MQW) structure can substantially reduce the cost of blue LDs and boost their applications. To obtain a high crystalline quality crack-free GaN thin film on Si for the subsequent growth of a blue laser structure, a hand-shaking structure was formed by inserting Al-composition step down-graded AlN/AlxGa1−xN buffer layers between GaN and Si substrate. Thermal degradation in InGaN/GaN blue MQWs was successfully suppressed with indium-rich clusters eliminated by introducing hydrogen during the growth of GaN quantum barriers (QBs) and lowering the growth temperature for the p-type AlGaN/GaN superlattice optical cladding layer. A continuous-wave (CW) electrically pumped InGaN/GaN quantum well (QW) blue (450 nm) LD grown on Si was successfully demonstrated at room temperature (RT) with a threshold current density of 7.8 kA/cm2.

Ultrathin-Barrier AlGaN/GaN Heterostructure: A Recess-Free Technology for Manufacturing High-Performance GaN-on-Si Power Devices

Abstract: (Al)GaN recess-free normally OFF technology is developed for fabrication of high-yield lateral GaN-based power devices. The recess-free process is achieved by an ultrathin-barrier (UTB) AlGaN/GaN heterostructure that features a natural pinched-off 2-D electron gas channel. The top–down manufacturing technique overcomes the challenges in etching of AlGaN barrier with well-controlled depth and uniformity, which is especially attractive for fabrication of normally OFF GaN-based high-electron-mobility transistors (HEMTs) and metal–insulator–semiconductor HEMTs (MIS-HEMTs) on large-size Si substrate. With SiNx passivation grown by low-pressure chemical-vapor deposition, on-resistance of the UTB-AlGaN/GaN-based power devices can be significantly reduced. High-uniformity low-hysteresis normally OFF HEMTs and Al2O3/AlGaN/GaN MIS-HEMTs are successfully demonstrated on the UTB AlGaN/GaN-on-Si platform. It is also a compelling technology platform for manufacturing high-performance GaN-based lateral power diodes, and normally OFF p-(Al)GaN heterojunction field-effect transistors.

A polarization-induced 2D hole gas in undoped gallium nitride quantum wells

Abstract: The long-missing polarization-induced two-dimensional hole gas is finally observed in undoped gallium nitride quantum wells. Experimental results provide unambiguous proof that a 2D hole gas in GaN grown on AlN does not need acceptor doping, and can be formed entirely by the difference in the internal polarization fields across the semiconductor heterojunction. The measured 2D hole gas densities, about $4 \times 10^{13}$ cm$^{-2}$, are among the highest among all known semiconductors and remain unchanged down to cryogenic temperatures. Some of the lowest sheet resistances of all wide-bandgap semiconductors are seen. The observed results provide a new probe for studying the valence band structure and transport properties of wide-bandgap nitride interfaces, and simultaneously enable the missing component for gallium nitride-based p-channel transistors for energy-efficient electronics.

High Efficiency Si Photocathode Protected by Multifunctional GaN Nanostructures

Abstract: Photoelectrochemical water splitting is a clean and environmentally friendly method for solar hydrogen generation Its practical application, however, has been limited by the poor stability of semiconductor photoelectrodes In this work, we demonstrate the use of GaN nanostructures as a multifunctional protection layer for an otherwise unstable, low-performance photocathode The direct integration of GaN nanostructures on n+-p Si wafer not only protects Si surface from corrosion but also significantly reduces the charge carrier transfer resistance at the semiconductor/liquid junction, leading to long-term stability (>100 h) at a large current density (>35 mA/cm2) under 1 sun illumination The measured applied bias photon-to-current efficiency of 105% is among the highest values ever reported for a Si photocathode Given that both Si and GaN are already widely produced in industry, our studies offer a viable path for achieving high-efficiency and highly stable semiconductor photoelectrodes for solar water splitting with proven manufacturability and scalability

Understanding of MoS 2 /GaN Heterojunction Diode and its Photodetection Properties

Abstract: Fabrication of heterojunction between 2D molybdenum disulfide (MoS2) and gallium nitride (GaN) and its photodetection properties have been reported in the present work. Surface potential mapping at the MoS2/GaN heterojunction is done using Kelvin Probe Force Microscopy to measure the conduction band offset. Current-voltage measurements show a diode like behavior of the heterojunction. The origin of diode like behavior is attributed to unique type II band alignment of the heterojunction. The photocurrent, photoresponsivity and detectivity of the heterojunction are found to be dependent on power density of the light. Photoresponse investigations reveal that the heterojunction is highly sensitive to 405 nm laser with very high responsivity up to 105 A/W. The heterojunction also shows very high detectivity of the order of 1014 Jones. Moreover, the device shows photoresponse in UV region also. These observations suggest that MoS2/GaN heterojunction can have great potential for photodetection applications.

Gallium nitride nanowire as a linker of molybdenum sulfides and silicon for photoelectrocatalytic water splitting.

Abstract: The combination of earth-abundant catalysts and semiconductors, for example, molybdenum sulfides and planar silicon, presents a promising avenue for the large-scale conversion of solar energy to hydrogen. The inferior interface between molybdenum sulfides and planar silicon, however, severely suppresses charge carrier extraction, thus limiting the performance. Here, we demonstrate that defect-free gallium nitride nanowire is ideally used as a linker of planar silicon and molybdenum sulfides to produce a high-quality shell-core heterostructure. Theoretical calculations revealed that the unique electronic interaction and the excellent geometric-matching structure between gallium nitride and molybdenum sulfides enabled an ideal electron-migration channel for high charge carrier extraction efficiency, leading to outstanding performance. A benchmarking current density of 40 ± 1 mA cm−2 at 0 V vs. reversible hydrogen electrode, the highest value ever reported for a planar silicon electrode without noble metals, and a large onset potential of +0.4 V were achieved under standard one-sun illumination.

An industry-ready 200 mm p-GaN E-mode GaN-on-Si power technology

Abstract: Enhancement mode 650V rated p-GaN gate HEMTs are fabricated on 200 mm p+ Si substrates by using an industrial, Au-free process. The devices show true e-mode performance, with a high V t of 2.8 V, low off-state leakage current and are dynamic R DS-ON free over the complete V DS and temperature range. High temperature reverse bias (HTRB) testing is done on-wafer and after packaging. For the first time, 650 V e-mode power HEMTs realized on 200 mm Si substrates, show industry ready device performance and pass 1008 hour reliability testing, at V GS =0 V, V DS =650 V.

Alkali-metal-adsorbed g-GaN monolayer: ultralow work functions and optical properties.

Abstract: The electronic and optical properties of alkali-metal-adsorbed graphene-like gallium nitride (g-GaN) have been investigated using density functional theory. The results denote that alkali-metal-adsorbed g-GaN systems are stable compounds, with the most stable adsorption site being the center of the hexagonal ring. In addition, because of charge transfer from the alkali-metal atom to the host, the g-GaN layer shows clear n-type doping behavior. The adsorption of alkali metal atoms on g-GaN occurs via chemisorption. More importantly, the work function of g-GaN is substantially reduced following the adsorption of alkali-metal atoms. Specifically, the Cs-adsorbed g-GaN system shows an ultralow work function of 0.84 eV, which has great potential application in field-emission devices. In addition, the alkali-metal adsorption can lead to an increase in the static dielectric constant and extend the absorption spectrum of g-GaN.

Parasitic capacitance Eqoss loss mechanism, calculation, and measurement in hard-switching for GaN HEMTs

Abstract: Gallium Nitride enhancement-mode high electron mobility transistors (GaN E-HEMTs) can achieve relatively high-efficiency and high-frequency in hard-switching mode. One particular reason is that GaN E-HEMTs obtain zero reverse-recovery loss and also a zero reverse-recovery period. For silicon (Si) MOSFETs, it has been a well-known issue that their Q rr is too big to switch the transistor in hard-switching mode. Researchers have made extensive efforts to calculate the reverse-recovery loss. However, few of them pay attention to the Q oss , as the Q rr dominates in the turn-on switching loss for Si MOSFETs. For GaN HEMTs, the absence of the Q rr makes the Q oss noticeable, although the value of the Q oss for GaN HEMTs is still the smallest among both Si and Silicon Carbide (SiC) MOSFETs. This paper focus on the Eqoss loss in GaN HEMTs. The Eqoss loss mechanism, detailed calculation and detailed measurement method for GaN HEMTs are provided. In addition, the theoretical results are verified by the double-pulse test at different junction temperatures and gate resistances.

720-V/0.35-m $\Omega \cdot$ cm 2 Fully Vertical GaN-on-Si Power Diodes by Selective Removal of Si Substrates and Buffer Layers

Abstract: This letter demonstrates a novel technology to fabricate fully vertical GaN-on-Si power diodes with state-of-the-art performance. Si substrate and buffer layers were selectively removed and the bottom cathode was formed in the backside trenches extending to an n+-GaN layer. A specific differential ON-resistance of 0.35–0.4 $\text{m}\Omega \cdot $ cm2 (normalized to the total device area) and a breakdown voltage of 720 V were demonstrated in this novel fully vertical GaN-on-Si p-n diode, rendering a Baliga’s figure of merit over 1.5 GW/cm2. These results set a new record performance in all vertical GaN power diodes on foreign substrates, and demonstrate the feasibility of making fully vertical GaN-on-Si power diodes and transistors by selective removal of Si substrates and buffer layers.

Gate Conduction Mechanisms and Lifetime Modeling of p-Gate AlGaN/GaN High-Electron-Mobility Transistors

Abstract: The gate conduction mechanisms in p-gallium nitride (GaN)/AlGaN/GaN enhancement mode transistors are investigated using temperature-dependent dc gate current measurements. In each of the different gate voltage regions, a physical model is proposed and compared to experiment. At negative gate bias, Poole–Frenkel emission (PFE) occurs within the passivation dielectric from gate to source. At positive gate bias, the p-GaN/AlGaN/GaN “p-i-n” diode is in forward operation mode, and the gate current is limited by hole supply at the Schottky contact. At low gate voltages, the current is governed by thermionic emission with Schottky barrier lowering in dislocation lines. Increasing the gate voltage and temperature results in thermally assisted tunneling (TAT) across the same barrier. An improved gate process reduces the gate current in the positive gate bias region and eliminates the onset of TAT. However, at high positive gate bias, a sharp increase in current is observed originating from PFE at the metal/ p-GaN interface. Using the extracted conduction mechanisms for both devices, accurate lifetime models are constructed. The device fabricated with the novel gate process exhibits a maximum gate voltage of 7.2 V at ${t}_{\textsf {1}\%}=\textsf {10}$ years.

Characterisation and Modeling of Gallium Nitride Power Semiconductor Devices Dynamic On-State Resistance

Abstract: Gallium nitride high-electron-mobility transistors (GaN-HEMTs) suffer from trapping effects that increases device on-state resistance ( $R_\mathrm{DS(on)}$ ) above its theoretical value. This increase is a function of the applied dc bias when the device is in its off state, and the time which the device is biased for. Thus, dynamic $R_\mathrm{DS(on)}$ of different commercial GaN-HEMTs are characterised at different bias voltages in the paper by a proposed new measurement circuit. The time-constants associated with trapping and detrapping effects in the device are extracted using the proposed circuit and it is shown that variations in $R_\mathrm{DS(on)}$ can be predicted using a series of RC circuit networks. A new methodology for integrating these $R_\mathrm{DS(on)}$ predictions into existing gallium nitride-high-electron-mobility transistors models in standard SPICE simulators to improve model accuracy is then presented. Finally, device dynamic $R_\mathrm{DS(on)}$ values of the model is compared and validated with the measurement when it switches in a power converter with different duty cycles and switching voltages.

A High-Bandwidth Integrated Current Measurement for Detecting Switching Current of Fast GaN Devices

Abstract: Gallium nitride (GaN) devices are suitable for high-frequency power converters due to their excellent switching performance. To maximize the performance of GaN devices, it is necessary to study the switching characteristics, which requires measuring the switching current. However, GaN devices have a fast switching speed and are sensitive to parasitic parameters, so the current measurement should have a high bandwidth and should not introduce excessive parasitic inductance into the power converters. Traditional current measurements are difficult to meet these requirements, especially for fast GaN devices. This paper presents a high-bandwidth integrated current measurement for detecting the switching current of fast GaN devices. By effectively utilizing the parasitic inductance in the circuit, a single-turn coil is embedded in the printed circuit board. This coil could pick up a sufficiently strong voltage signal, which is then processed to reconstruct the switching current. Moreover, corrections are carried out to further improve the accuracy. The current measurement has a small insertion impedance and a high bandwidth with a small influence on the parasitic inductance of the converter. The accuracy of the current measurement is experimentally verified by a 40 V GaN-based double pulse test circuit with a load current up to 25 A.

Design of Low-Inductance Switching Power Cell for GaN HEMT Based Inverter

Abstract: In this paper, an ultralow inductance power cell is designed for a three-level active neutral point clamped (3L-ANPC) based on 650-V gallium nitride (GaN) high electron mobility transistor (HEMT) devices. The 3L-ANPC topology with GaN HEMT devices and the selected modulation scheme suitable for wide-bandgap devices are presented. The commutation loops, which mainly contribute to voltage overshoots and increase of switching losses, are discussed. The ultralow inductance power cell design based on a four-layer printed circuit board with the aim to maximize the switching performance of GaN HEMTs is explained. The design of gate drivers for the GaN HEMT devices is presented. Parasitic inductance and resistance of the proposed design are extracted with finite-element analysis and is discussed. Common-mode behaviors based on the simulation program with integrated circuit emphasis (SPICE) model of the converter are analyzed. Experimental results on the designed 3L-ANPC with output power of up to 1 kW are presented, which verifies the performance of the proposed design in terms of ultralow inductance.

Electronic structure and optical properties of novel monolayer gallium nitride and boron phosphide heterobilayers.

Abstract: Motivated by the increasing number of studies on optoelectronic applications of van der Waals (vdW) heterostructures, we have investigated the electronic and optical properties of monolayer gallium nitride (MGaN) and boron phosphide (MBP) heterobilayers by using first-principle calculations based on density functional theory. We have ensured the dynamical stability of the structures by considering their binding energies and phonon spectra. We show that the magnitude and status (direct or indirect) of the band gap are strongly dependent on the stacking pattern of the heterobilayers. Furthermore, we have investigated the band splittings in the presence of an external electric field which show the effect of the field on the band alignment of the structure. We have also shown the band gap, charge redistributions and work function of the structures are highly dependent on the magnitude and direction of the electric field such that its magnitude yields indirect to direct band gap transitions around |E⊥| ∼ 0.7 V A−1 at the Γ point and the order of the band gap is varied according to the direction of the electric field. Moreover, we examine optical properties of MGaN/MBP heterobilayers as part of DFT calculations. The band gap and work function being tunable with changes in the external field together with the prominent absorption over the UV range make the MGaN/MBP heterobilayer a feasible candidate for optoelectronic applications.

Vertical GaN-on-Si MOSFETs With Monolithically Integrated Freewheeling Schottky Barrier Diodes

Abstract: We demonstrate for the first time the monolithic integration of vertical GaN MOSFETs with freewheeling Schottky barrier diodes (SBD), based on a 6.7- $\mu \text{m}$ -thick n-p-n heterostructure grown on 6-inch silicon substrates by metal organic chemical vapor deposition. The anode of the SBD is integrated in the source pad of the MOSFET and the cathode is directly connected to the MOSFET drain through the bottom n+-GaN layer, eliminating the need of any metal wire interconnection. This monolithic integration scheme offers reduced footprint, minimized parasitic components, and simplified packaging. The integrated MOSFET-SBD showed enhancement-mode operation with a threshold voltage of 3.9 V, an ON/ OFF ratio of over 108 and a dramatic improvement in reverse conduction, without degradation in on-state performance from the integration of the SBD. The integrated GaN-on-Si vertical SBD exhibited excellent performance, with a specific on-resistance of 1.6 $\text{m}\Omega \cdot $ cm2, a turn-on voltage of 0.76 V, an ideality factor of 1.5, along with a breakdown voltage of 254 V.

Wide Bandgap Devices in Electric Vehicle Converters: A Performance Survey

Abstract: This paper introduces a unique quantified study about using low-losses fast-switching wide bandgap (WBG) devices, i.e., gallium nitride (GaN) and silicon carbide (SiC), over traditional Silicon (Si) devices in the switching of dc/dc converters, focusing on electric vehicles’ (EVs) machine drive and battery charger. A detailed model of the power train of a Nissan Leaf was developed in PSIM software, with WBG semiconductors’ capability. The model was simulated one time using GaN semiconductors and another time using SiC devices. Simulation results are quantified and a comparison between different semiconductors in terms of total losses and efficiency is presented. The developed PSIM model can also be extended to other EVs like Chevy Volt. A proof of concept prototype for a Nissan Leaf dc/dc converter was built in the laboratory and results were collected. Componentwise experimental results are presented and their correlation with simulation findings is demonstrated. In addition, experimental results of the overall power train test bench are found to be matched with the simulation results on a system level as well.

GaN Integration Technology, an Ideal Candidate for High-Temperature Applications: A Review

Abstract: In many leading industrial applications such as aerospace, military, automotive, and deep-well drilling, extreme temperature environment is the fundamental hindrance to the use of microelectronic devices. Developing an advanced technology with robust electrical and material properties dedicated for high-temperature environments represents a significant progress allowing to control and monitor the harsh environment regions. It may avoid using cooling structures while improving the reliability of the whole electronic systems. As a wide bandgap semiconductor, gallium nitride is considered as an ideal candidate for such environments, as well as in high-power and high-frequency applications. We review in this paper the main reasons that offer superiority to GaN devices over better-known technologies such as silicon (Si), silicon-on-insulator, gallium arsenide (GaAs), silicon germanium (SiGe), and silicon carbide (SiC). The theory of operation and main challenges at high temperature are discussed, notably those related to materials and contacts. In addition, the main limitations of GaN, including the technological (thermal and chemical) and intrinsic (current collapse and device self-heating) features are provided. In addition, the GaN devices recently developed for high-temperature applications are examined.

Self-Assembled UV Photodetector Made by Direct Epitaxial GaN Growth on Graphene.

Abstract: Hybrid systems based on the combination of crystalline bulk semiconductors with 2D crystals are identified as promising heterogeneous structures for new optoelectronic applications. The direct integration of III–V semiconductors on 2D materials is very attractive to make practical devices but the preservation of the intrinsic properties of the underlying 2D materials remains a challenge. In this work, we study the direct epitaxy of self-organized GaN crystals on graphene. We demonstrate that severe metal–organic chemical vapor deposition growth conditions of GaN (chemically aggressive precursors and high temperatures) are not detrimental to the structural quality and the charge carrier mobility of the graphene base plane. Graphene can therefore be used both as an efficient sensitive material and as a substrate for GaN epitaxy to make a self-assembled UV photodetector. A responsivity as high as 2 A W–1 is measured in the UV-A range without any further postprocessing compared to simple deposition of contact...

Electron Trapping in Extended Defects in Microwave AlGaN/GaN HEMTs With Carbon-Doped Buffers

Abstract: This paper investigates AlGaN/GaN high-electron mobility transistors (HEMTs) fabricated on epistructures with carbon (C)-doped buffers. Metalorganic chemical vapor deposition is used to grow two C-doped structures with different doping profiles, using growth parameters to change the C incorporation. The C concentration is low enough to result in n-type GaN. Reference devices are also fabricated on a structure using iron (Fe) as dopant, to exclude any process related variations and provide a relevant benchmark. All devices exhibit similar dc performance. However, pulsed ${I}$ – ${V}$ measurements show extensive dispersion in the C-doped devices, with values of dynamic ${R}_{ \mathrm{\scriptscriptstyle ON}}~3$ –4 times larger than in the dc case. Due to the extensive trapping, the devices with C-doped buffers can only supply about half the output power of the Fe-doped sample, 2.5 W/mm compared to 4.8 W/mm at 10 GHz. In drain current transient measurements, the trap filling time is varied, finding large prevalence of trapping at dislocations for the C-doped samples. Clusters of C around the dislocations are suggested to be the main cause for the increased dispersion.

Anchoring MWCNTs to 3D honeycomb ZnO/GaN heterostructures to enhancing photoelectrochemical water oxidation

Abstract: Gallium nitride (GaN) is one of the ubiquitously known photoanode for photoelectrochemical water splitting (PEC-WS) due to its tunable band gap and favorable band edge positions. However, the unavoidable surface defects in GaN induces surface Fermi level pinning and surface band bending which severely reduces its PEC conversion efficiency. Constructing heterostructure is the key to approaching better charge separation efficiency and light harvesting ability for PEC-WS. Considering this, we have fabricated ternary heterostructure of GaN/ZnO/MWCNTs photoanode by combining metal organic chemical vapour deposition (MOCVD), hydrothermal and ‘dip and dry’ methods. FE-SEM results showed the formation of 3D hierarchical honeycomb structure of ZnO on GaN thin film surface when MWCNTs are added into hydrothermal reaction. We investigate the advantage of ZnO honeycomb structure in enhancing the solar PEC-WS performance of GaN photoanode. The synergy of incorporating MWCNTs has resulted into improvement in surface morphology, electron transportation and light harvesting capability. The as obtained ternary heterostructure exhibits photocurrent (Jph) of 3.02 mA/cm2 at 0 V versus Pt electrode under 1-sun light illumination which is about 2.58 times higher than that of pristine GaN photoanodes (Jph = 1.14 mA/cm2).

Stable and Reversible Lithium Storage with High Pseudocapacitance in GaN Nanowires.

Abstract: In this work, gallium nitride (GaN) nanowires (NWs) were synthesized by chemical vapor deposition (CVD) process. The hybrid electrode showed the capacity up to 486 mAh g–1 after 400 cycles at 0.1 A g–1. Even at 10 A g–1, the reversible capacity can stabilize at 75 mAh g–1 (after 1000 cycles). Pseudocapacitive capacity was defined by kinetics analysis. The dynamics analysis and electrochemical reaction mechanism of GaN with Li+ was also analyzed by ex situ XRD, HRTEM, and XPS results. These results not only cast new light on pseudocapacitance enhanced high-rate energy storage devices by self-assembled nanoengineering but also extend the application range of traditional binary III/V semiconductors.