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Self-powered ultraviolet (UV) photodetectors (PDs) are promising and essential for the applications of next-generation optoelectronic devices. This work proposes and demonstrates a self-powered pho...

Owing to its broadband absorption, ultrafast recovery time, and excellent saturable absorption feature, graphene has been recognized as one of the best candidates as a high-performance saturable absorber (SA). However, the low absorption efficiency and reduced modulation depth severely limit the application of graphene-based SA in ultrafast fiber lasers. In this paper, a single-layer graphene saturable absorber mirror (SG-SAM) was coated by a quarter-wave SiO

Gallium nitride-based (GaN-based) nanowire films are promising for various devices requiring high flexibility and/or high transparency, such as detectors and wearable intelligent electronics. In this work, a cost-effective and novel electrochemical procedure has been designed to detach films with (Al,Ga)N nanowires (NWs). Besides the ability of selectively etching, the nitric acid (HNO3) is found to protect the (Al,Ga)N NWs without too much etching, which is better than the potassium hydroxide (KOH). Therefore, a film comprising a dense ensemble of (Al,Ga)N NWs with both good flexibility and high transparency can be achieved successfully by this lift-off technology. In addition, it is demonstrated that the HNO3 etching can improve the photoelectric response of ultraviolet (UV) photodetectors significantly. Compared to that of UV photodetectors without HNO3 etching, the peak responsivity of UV photodetectors with HNO3 etching is enhanced by about 166% and the corresponding decay time becomes much faster. The removal of the bottom aluminium nitride (AlN) layer and Al-rich (Al,Ga)N shell, as well as the decrease of the surface states and defects can contribute to the improvements.

Due to the wide applications of blue and red photodetectors, dual-wavelength (blue/red) photodetectors are promising for future markets. In this work, a dual-wavelength photodetector based on vertical (In,Ga)N nanowires and graphene has been fabricated successfully. By using the transparent graphene, both blue and red responses can be clearly detected. The rise time of response can reach 3.5 ms. Furthermore, the underlying mechanism of double responses has also been analyzed. The main reason contributing to the dual-wavelength response could be the different diameters of nanowires, leading to different In components within (In,Ga)N sections.

Beryllium (Be) may be a substitute for GaN p-type dopant magnesium due to its smaller atomic size and higher electronegativity. However, Be has been shown to be a deep acceptor in recent experiments. By using the hybrid functional based on density functional theory (DFT), we predict two acceptors with symmetric-broken configurations (SB1 and SB2) in neutral charge state of Be substitutional doping, and analyze their optical and electronic properties. The ionization energy and emission peak of the symmetric-broken configuration SB2 are almost the same as the ground-state (neutral charge state) configuration SB1. Our work provides a comprehensive understanding of the deep acceptor behavior of Be in wurtzite GaN.

We propose and demonstrate a cost-effective and facile electrochemical procedure to detach films with (In,Ga)N nanowires from the original Si substrate used for growth. Without ultraviolet illumina...

The charge carrier concentration profile is a critical factor that determines semiconducting material properties and device performance. Dielectric force microscopy (DFM) has been previously developed to map charge carrier concentrations with nanometer-scale spatial resolution. However, it is challenging to quantitatively obtain the charge carrier concentration, since the dielectric force is also affected by the mobility. Here, we quantitative measured the charge carrier concentration at the saturation mobility regime via the rectification effect-dependent gating ratio of DFM. By measuring a series of n-type GaAs and GaN thin films with mobility in the saturation regime, we confirmed the decreased DFM-measured gating ratio with increasing electron concentration. Combined with numerical simulation to calibrate the tip–sample geometry-induced systematic error, the quantitative correlation between the DFM-measured gating ratio and the electron concentration has been established, where the extracted electron concentration presents high accuracy in the range of 4 × 1016 – 1 × 1018 cm−3. We expect the quantitative DFM to find broad applications in characterizing the charge carrier transport properties of various semiconducting materials and devices.

Metal modulation epitaxy (MME) is a technique in which metal beams (Al, Ga, In, and Mg) are switched on and off in short periods in an RF MBE system while a continuous nitrogen plasma beam is kept on. We systematically studied the effect of periodic duty cycling on the morphology, crystalline quality, Mg doping concentration, and electrical properties of GaN:Mg films grown by MME. When the metal shutter duty cycling is 20 s open/10 s close, the sample has smooth surface with clear steps even with Mg doping concentration higher than 1 × 1020 cm−3. The RMS roughness is about 0.5 nm. The FWHM of (002) XRD rocking curve is 230 arcsec and the FWHM of (102) XRD rocking curve is 260 arcsec. As result, a hole concentration of 5 × 1018 cm−3 and a resistivity of 1.5 Ω·cm have been obtained. The hole concentration increases due to the incorporation of surface accumulated Mg dopants into suitable Ga substitutional sites with minimal formation of compensatory defects.

Due to the requirements of oceanography exploration and detection, self-powered photodetectors (PDs) with low-power consumption are essential for the next-generation optoelectronic applications. In this work, we successfully demonstrate a self-powered photoelectrochemical (PEC) PD in seawater based on the (In,Ga)N/GaN core-shell heterojunction nanowires. Compared to those of the PD in pure water, it is found that the upward and downward overshooting features of current can be the key reason contributing to the much faster response speed of the PD in seawater. Thanks to the enhanced response speed, the rise time of PD can be reduced more than 80%, and the fall time remains only 30% by applying in seawater instead of pure water. The key factors of generating these overshooting features should be the instantaneous temperature gradient, carrier accumulation and elimination on the semiconductor/electrolyte interfaces at the moments of light on and off. By the analysis of experimental results, the Na+ and Cl- ions are proposed to be the main factors affecting the PD behavior in seawater, which can enhance the conductivity and accelerate the oxidation-reduction reaction significantly. This work paves an effective way to develop the new self-powered PDs for the wide applications in under-seawater detection and communication.

The atomic structure and surface chemistry of GaP/Si(100) heterostructure with different pre-layers grown by molecular beam epitaxy are studied. It is found that GaP epilayer with Ga-riched pre-layers on Si(100) substrate has regular surface morphology and stoichiometric abrupt heterointerfaces from atomic force microscopes (AFMs) and spherical aberration-corrected transmission electron microscopes (ACTEMs). The interfacial dynamics of GaP/Si(100) heterostructure is investigated by X-ray photoelectron spectroscopy (XPS) equipped with an Ar gas cluster ion beam, indicating that Ga pre-layers can lower the interface formation energy and the bond that is formed is more stable. These results suggest that Ga-riched pre-layers are more conducive to the GaP nucleation as well as the epitaxial growth of GaP material on Si(100) substrate.

Underwater wireless optical communication (UWOC) is a wireless communication technology using visible light to transmit data in an underwater environment, which has wide applications. Based on lift-off (In,Ga)N nanowires, this work has proposed and successfully demonstrated a self-powered photoelectrochemical (PEC) photodetector (PD) with excellent transmissivity. The transparent functionality of the PD is critical for 360° omnidirectional underwater detection, which was realized by detaching the (In,Ga)N nanowires from the opaque epitaxial substrates to the indium tin oxide (ITO)/glass. It was also found that the insulating SiO2 layer can enhance the photocurrent by about 12 times. The core–shell structure of the nanowires is beneficial for generating carriers and contributing to the photocurrent. Furthermore, a communication system with ASCII code is set to demonstrate the PD detection in underwater communication. This work paves an effective way to develop 360° omnidirectional PDs for the wide applications in UWOC system and underwater photodetection.

We report on the room-temperature wafer-bonded InGaP/GaAs//InGaAsP/InGaAs four-junction solar cell grown by all solid-source molecular beam epitaxy (MBE). The material growth and structure optimization of multi-junction solar cell were investigated. The effect of growth temperature on phosphide-related material of the solar cell was discussed. The lattice-matched InGaP/GaAs tandem cell on GaAs substrate and InGaAsP- or InGaAsP/InGaAs-tandem cell on InP substrate were grown separately by MBE. Room-temperature wafer bonding technique was used to integrate the subcells into multi-junction solar cells. The current limiting of multi-junction cells was analysed and the device structure of four-junction solar cell was optimized. The best wafer-bonded four-junction solar cell reached a conversion efficiency of 42% under concentration.

We investigated the growth and characterization of InGaN films with high In content up to 51.9% obtained by MBE MME-growth. Comparing the structural and optical properties of InGaN films grown using common growth method and MME technique, we could acquire the good quality of InGaN sample by MME-growth method with lower dislocation density, better composition uniformity and smoother surface. These experimental results suggest that MME has an enormous potential to promote the quality of high In-content InGaN materials and the development of InGaN-based devices. Besides, further study shows the process of MME growth, the parameters of material growth have opposite effects on the background carrier concentration and carrier mobility.

Highly efficient, lightweight, and flexible III-V compound multi-junction solar cells play an important role in the wearable electronic field. GaInP/GaAs/InGaAs inverted metamorphic (IMM) solar cells directly grown on an N-doped GaAs substrate by a GaInP etch stop layer were fabricated. The AlGaInAs graded buffer layer is grown to relax the strain of InGaAs with GaAs. Most misfit dislocations are localized at the internal interfaces of the graded buffer and do not appear to propagate. Then, the solar cells are fabricated by the copper-plated process, and Cu thin films serve as not only the flexible support but also the P-type electrode. The device had a total thickness of less than 35 μm, a mass density of less than 0.03 g/cm2, and can exhibit excellent bendability. Under the AM1.5G solar illumination, the thin film solar cell has the best photovoltaic conversion efficiency of 33.13% with the Voc of 2.86 V, a Jsc value of 13.55 mA/cm2, and a FF of 85.49%. This method can be used to prepare flexible thin film solar cells simply and efficiently.

The mechanical properties of Al1−xGaxN epitaxial layers grown on GaN and AlN substrates were studied by using density functional theory and anharmonic elastic calculation method. Accurate theoretical calculation of nonlinear elastic responses is a basic and important scientific question, which is the key to understand and further explore the potential synthesis and design of new solids. In this paper, detailed numerical calculations were carried out for the nonlinear mechanical properties of wurtzite GaN and AlN compounds. The variation of strain energy and stress of disordered Al1−xGaxN epitaxial layer with gallium composition was analyzed. We found that in the epitaxial structure of AlN substrate, with the increase of gallium content, the influence of nonlinear effect on internal stress reached 29.4%, which proved the great influence of nonlinear mechanical response on semiconductor alloy material and heterostructure.

In this paper, the selective-area growth (SAG) of GaN-based nanocolumns (NCs) on silicon (Si) has been investigated experimentally. A low-cost and facile method has been proposed and implemented to fabricate the SAG NCs, which contains the processes of nanosphere lithography (NSL) and plasma-assisted molecular beam epitaxy (MBE). SAG NCs grown by this method indicates a higher ability to modulate the NC distributions than those grown by the conventional patterned method. According to photoluminescence results, InGaN/GaN NCs grown on Si nanopillars produced by NSL and etching show the more uniform indium (In) distribution than those grown on the openings of patterned Si substrate. Furthermore, according to the kinetic processes during MBE growth, it is proposed that the SAG of NCs results from the combination of the shadow effect of Si nano-pillars and the growth suppression of titanium (Ti) film.

Deep ultraviolet (UV) emission at 260 nm has been obtained from high Al content AlGaN/AlN multi-quantum well (MQW) structures by plasma-assisted molecular beam epitaxy (PA-MBE) on AlN template on sapphire substrates. The effect of III/V ratio and growth temperature on structural and optical properties of AlGaN was studied. It was observed that Al content in AlGaN material increased with increasing Al flux ratio or growth temperature. By altering growth conditions, AlGaN with high Al content of 47–80% was obtained. Further studies indicate that AlGaN layers have excellent surface morphology and optical properties grown in slightly metal-rich condition. As for AlGaN/AlN MQW, the emission wavelength was tuned by controlling the thickness of AlGaN quantum well, and deep ultraviolet emission below up to 260 nm was obtained from AlGaN (2.4 nm)/AlN(6.9 nm) MQW at room temperature.

Photocatalytic conversion of CO

In this paper, a solar-blind ultraviolet photodetector (PD) based on the graphene/vertical Ga2O3 nanowire array heterojunction was proposed and demonstrated. To the best of our knowledge, it is the first time that vertical Ga2O3 nanowire arrays have been realized. Ga2O3 nanowires were obtained by thermally oxidizing GaN nanowires grown by molecular beam epitaxy on n-doped Si substrate. Then, a monolayer graphene film was transferred to Ga2O3 nanowires to form the graphene/vertical Ga2O3 nanowire array heterojunction and transparent electrodes. The fabricated device exhibited a responsivity (R) of 0.185 A/W and rejection ratio (R258 nm/R365 nm) of 3×104 at the bias of −5 V. Moreover, the fast response times of this PD were 9 and 8 ms for the rise and decay times under 254 nm illumination, respectively, which are attributed to the unique properties of nanowire arrays and the graphene/vertical Ga2O3 nanowire array heterojunction structure.

We present a study of recombination dynamics and lifetimes of minority carriers in a GaInP/GaAs heterostructure single-junction solar cell by using a variable-temperature time-resolved photoluminescence (TRPL) technique. It was found that the minority carriers, i.e., electrons, in the p-type GaInP base layer have super long lifetimes of ∼220 μs under the excitation of nanosecond laser pulses. On the basis of a newly developed model for time-resolved luminescence of a localized-state ensemble [e.g., Z. C. Su and S. J. Xu, Sci. Rep. 7, 13 (2017)], it is revealed that as much as 38% of photons emitted by localized carriers may be involved in the photon recycling in the GaInP base layer. The effective photon recycling is believed to be an important factor that causes the super long lifetime of the minority carriers, and can help reduce the radiative recombination loss of photogenerated carriers in heterostructure based solar cells.

High Al-content AlGaN epilayers were grown on AlN template by using indium (In) surfactant with plasma-assisted molecular beam epitaxy (PA-MBE), and deep ultraviolet emission at 235 nm was obtained at room temperature. The effects and mechanisms of In-surfactant on the crystalline quality and optical properties of AlGaN were investigated. It was found that In-surfactant could facilitate two-dimensional AlGaN growth by reducing activation barrier for Al/Ga atoms to cross steps and effectively increasing the migration rate on the growth surface, and thus improve surface morphology and decrease defect density. The photoluminescence measurements revealed that the optical properties were remarkably improved by adopting In as surfactant, and phase separation was also effectively eliminated. Furthermore, the concentration of impurities including oxygen and silicon was decreased, which is attributed to higher defects formation energy for these impurities with In-surfactant assisted epitaxy growth.

AlN/GaN resonant tunneling diodes (RTDs) were grown separately on freestanding GaN (FS-GaN) substrates and sapphire substrates by plasma-assisted molecular-beam epitaxy (PA-MBE). Room temperature negative differential resistance (NDR) was obtained under forward bias for the RTDs grown on FS-GaN substrates, with the peak current densities (J p) of 175–700 kA/cm2 and peak-to-valley current ratios (PVCRs) of 1.01–1.21. Two resonant peaks were also observed for some RTDs at room temperature. The effects of two types of substrates on epitaxy quality and device performance of GaN-based RTDs were firstly investigated systematically, showing that lower dislocation densities, flatter surface morphology, and steeper heterogeneous interfaces were the key factors to achieving NDR for RTDs.