Piezo-Phototronic Effect Controlled Dual-Channel Visible light Communication (PVLC) Using InGaN/GaN Multiquantum Well Nanopillars

Enhanced Photoluminescence of Flexible InGaN/GaN Multiple Quantum Wells on Fabric by Piezo-Phototronic Effect

Fabric-based wearable electronics are showing advantages in emerging applications in wearable devices, Internet of everything, and artificial intelligence. Compared to the one with organic materials, devices based on inorganic semiconductors (e.g., GaN) commonly show advantages of superior characteristics and high stability. Upon the transfer of GaN-based heterogeneous films from their rigid substrates onto flexible/fabric substrates, changes in strain would influence the device performance. Here, we demonstrate the transfer of InGaN/GaN multiple quantum well (MQW) films onto flexible/fabric substrates with an effective lift-off technique. The physical properties of the InGaN/GaN MQWs film are characterized by atomic force microscopy and high-resolution X-ray diffraction, indicating that the transferred film does not suffer from huge damage. Excellent flexible properties are observed in the film transferred on fabric, and the photoluminescence (PL) intensity is enhanced by the piezo-phototronic effect, which shows an increase of about 10% by applying an external strain with increasing the film curvature to 6.25 mm^-1. Moreover, energy band diagrams of the GaN/InGaN/GaN heterojunction at different strains are illustrated to clarify the internal modulation mechanism by the piezo-phototronic effect. This work would facilitate the guidance of constructing high-performance devices on fabrics and also push forward the rapid development of flexible and wearable electronics.

Piezo-phototronic effect on photocatalysis, solar cells, photodetectors and light-emitting diodes

The piezo-phototronic effect (a coupling effect of piezoelectric, photoexcitation and semiconducting properties, coined in 2010) has been demonstrated to be an ingenious and robust strategy to manipulate optoelectronic processes by tuning the energy band structure and photoinduced carrier behavior. The piezo-phototronic effect exhibits great potential in improving the quantum yield efficiencies of optoelectronic materials and devices and thus could help increase the energy conversion efficiency, thus alleviating the energy shortage crisis. In this review, the fundamental principles and challenges of representative optoelectronic materials and devices are presented, including photocatalysts (converting solar energy into chemical energy), solar cells (generating electricity directly under light illumination), photodetectors (converting light into electrical signals) and light-emitting diodes (LEDs, converting electric current into emitted light signals). Importantly, the mechanisms of how the piezo-phototronic effect controls the optoelectronic processes and the recent progress and applications in the above-mentioned materials and devices are highlighted and summarized. Only photocatalysts, solar cells, photodetectors, and LEDs that display piezo-phototronic behavior are reviewed. Material and structural design, property characterization, theoretical simulation calculations, and mechanism analysis are then examined as strategies to further enhance the quantum yield efficiency of optoelectronic devices via the piezo-phototronic effect. This comprehensive overview will guide future fundamental and applied studies that capitalize on the piezo-phototronic effect for energy conversion and storage.

Dual-wavelength visible photodetector based on vertical (In,Ga)N nanowires grown by molecular beam epitaxy

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.

Seeing pressure in color based on integration of highly sensitive pressure sensor and emission tunable light emitting diode

We demonstrate a highly sensitive, low-cost, environmental-friendly pressure sensor derived from a wool-based pressure sensor with wide pressure sensing range using wool bricks embedded with a Ag nano-wires. The easy fabrication and light weight allow portable and wearable device applications. Wth the integration of a light-emitting diode possessing multi-wavelength emission, we illustrate a hybrid multi-functional LED-integrated pressure sensor that is able to convert different applied pressures to light emission with different wavelengths. Due to the high sensitivity of the pressure sensor, the demonstration of acoustic signal detection has also been presented using sound of a metronome and a speaker playing a song. This multi-functional pressure sensor can be implemented to technologies such as smart lighting, health care, visible light communication (VLC), and other internet of things (IoT) applications.

High-performance visible light photodetectors based on inorganic CZT and InCZT single crystals

Herein, the optoelectrical investigation of cadmium zinc telluride (CZT) and indium (In) doped CZT (InCZT) single crystals-based photodetectors have been demonstrated. The grown crystals were configured into photodetector devices and recorded the current-voltage (I-V) and current-time (I-t) characteristics under different illumination intensities. It has been observed that the photocurrent generation mechanism in both photodetector devices is dominantly driven by a photogating effect. The CZT photodetector exhibits stable and reversible device performances to 632 nm light, including a promotable responsivity of 0.38 AW-1, a high photoswitch ratio of 152, specific detectivity of 6.30 × 1011 Jones, and fast switching time (rise time of 210 ms and decay time of 150 ms). When doped with In, the responsivity of device increases to 0.50 AW-1, photoswitch ratio decrease to 10, specific detectivity decrease to 1.80 × 1011 Jones, rise time decrease to 140 ms and decay time increase to 200 ms. Moreover, these devices show a very high external quantum efficiency of 200% for CZT and 250% for InCZT. These results demonstrate that the CZT based crystals have great potential for visible light photodetector applications.

Piezotronics and Piezo-phototronics of Third Generation Semiconductor Nanowires

With the fast development of nanoscience and nanotechnology in the last 30 years, semiconductor nanowires have been widely investigated in the areas of both electronics and optoelectronics. Among them, representatives of third generation semiconductors, such as ZnO and GaN, have relatively large spontaneous polarization along their longitudinal direction of the nanowires due to the asymmetric structure in their c-axis direction. Two-way or multiway couplings of piezoelectric, photoexcitation, and semiconductor properties have generated new research areas, such as piezotronics and piezo-phototronics. In this review, an in-depth discussion of the mechanisms and applications of nanowire-based piezotronics and piezo-phototronics is presented. Research on piezotronics and piezo-phototronics has drawn much attention since the effective manipulation of carrier transport, photoelectric properties, etc. through the application of simple mechanical stimuli and, conversely, since the design of new strain sensors based on the strain-induced change in semiconductor properties.

Enhanced bandwidth of white light communication using nanomaterial phosphors

The bandwidth of white light emitting diodes (WLEDs) is an important factor that affects most of the system performances in visible light communication (VLC). It is mainly limited by the down-conversion phosphors. We propose in this paper to employ nanomaterial phosphors with short fluorescence lifetime and high quantum yield in VLC. The white-emitting device of bandwidth-based lifetime was fabricated by using several kinds of nanophosphors with different fluorescence lifetimes. Moreover, we proposed two theoretical models to analyze the factors that affect bandwidth. Compared with the commercial YAG-based WLEDs, the bandwidth of nanophosphor-based WLEDs can be improved over three times and close to the blue excitation sources. Our study indicates that nanophosphors can become promising fluorescent materials in VLC, and provides a new direction for developing wide-bandwidth VLC systems.

Anomalous electroluminescent blue-shift behavior induced by well widths variance and localization effect in InGaN/GaN multi-quantum wells

Twelve InGaN MQW LED samples with varying well thickness grown via metal-organic chemical vaper deposition (MOCVD) are investigated. It is observed from electroluminescence (EL) measurement that at low current densities, the peak energy shifts to blue with increasing current, and when the current change by fixed increment, the peak energy shifts to blue end to different extent among samples. This blue shift was expected to be stronger when the well thickness increases, however, for well widths above 5 nm we observe a decrease in emission energy. Since no relaxation was detected from reciprocal space mapping (RSM), the deteriorated homogeneity is found to be responsible for this phenomenon. Temperature dependent photoluminescence (TDPL) results analyzed by band-tail model fitting show that the localization effect gets more prominent with increasing well thickness. It is found that elevating the growth temperature of active region from 710°C to 750°C significantly improves the homogeneity of InGaN layer.

Electric-field driven photoluminescence probe of photoelectric conversion in InGaN-based photovoltaics

The spatial distribution of electric field in photovoltaic multiple quantum wells (MQWs) is extremely important to dictate the mutual competition of photoelectric conversion and optical transition. Here, electric-field-driven photoluminescence (PL) in both steady-state and transient-state has been utilized to directly investigate the internal photoelectric conversion processes in InGaN-based MQW photovoltaic cell. As applying the reversed external electric field, the compensation of the quantum confined stark effect (QCSE) in InGaN QW is beneficial to help the photoabsorbed minor carriers drift out from the localized states, whereas extremely weakening the PL radiative recombination. A directly driven force by the reversed external electric field decreases the transit time of photocarriers drifting in InGaN QW. And hence, the overall dynamic PL decay including both the slow and fast processes gradually speeds up from 19.2 ns at the open-circuit condition to 3.9 ns at a negative bias of -3 V. In particular, the slow PL decay lifetime declines more quickly than that of the fast one. It is the delocalization of photocarriers by electric-field drift that helps to further enhance the high-efficiency photoelectric conversion except for the tunneling transport in InGaN-based MQW photovoltaics. Therefore, it can be concluded that the electric-field PL probe may provide a direct method for evaluating the photoelectric conversion in multilayer quantum structures and related multijunction photovoltaic cells.

Piezo-Phototronic Matrix via a Nanowire Array

Piezoelectric semiconductors, such as ZnO and GaN, demonstrate multiproperty coupling effects toward various aspects of mechanical, electrical, and optical excitation. In particular, the three-way coupling among semiconducting, photoexcitation, and piezoelectric characteristics in wurtzite-structured semiconductors is established as a new field, which was first coined as piezo-phototronics by Wang in 2010. The piezo-phototronic effect can controllably modulate the charge-carrier generation, separation, transport, and/or recombination in optical-electronic processes by modifying the band structure at the metal-semiconductor or semiconductor-semiconductor heterojunction/interface. Here, the progress made in using the piezo-phototronic effect for enhancing photodetectors, pressure sensors, light-emitting diodes, and solar cells is reviewed. In comparison with previous works on a single piezoelectric semiconducting nanowire, piezo-phototronic nanodevices built using nanowire arrays provide a promising platform for fabricating integrated optoelectronics with the realization of high-spatial-resolution imaging and fast responsivity.

Low-Voltage Photodetectors with High Responsivity Based on Solution-Processed Micrometer-Scale All-Inorganic Perovskite Nanoplatelets

All-inorganic photodetectors based on scattered CsPbBr₃ nanoplatelets with lateral dimension as large as 10 µm are fabricated, and the CsPbBr₃ nanoplatelets are solution processed governed by a newly developed ion-exchange soldering mechanism. Under illumination of a 442 nm laser, the photoresponsivity of photodetectors based on these scattered CsPbBr₃ nanoplatelets is as high as 34 A W^-1 , which is the largest value reported from all-inorganic perovskite photodetectors with an external driven voltage as small as 1.5 V. Moreover, the rise and fall times are 0.6 and 0.9 ms, respectively, which are comparable to most of the state-of-the-art all-inorganic perovskite-based photodetectors. All the material synthesis and device characterization are conducted at room temperature in ambient air. This work demonstrates that the solution-processed large CsPbBr₃ nanoplatelets are attractive candidates to be applied in low-voltage, low-cost, ultra highly integrated optoelectronic devices.

Novel Green Luminescent and Phosphorescent Material: Semiconductive Nanoporous ZnMnO with Photon Confinement

A novel green luminescent and phosphorescent material of semiconductive nanoporous ZnMnO was synthesized by the thermal nucleation of nanopores in the 20-period Zn_0.93Mn_0.07O/Zn_0.65Mn_0.35O multilayer structure. Nanoporous ZnMnO showed an n-type semiconducting property and exhibited an extremely strong green light emission in its luminescence and phosphorescence characteristics. This arises from the formation of the localized energy level (i.e., green emission band) within the energy band gap and the confinement of photons. The results suggest nanoporous ZnMnO to have a great potential for the new type of semiconducting green phosphors and semiconductor light-emitting diodes with lower thresholds, producing an efficient light emission. In-depth analyses on the structural, electrical, and optical properties are thoroughly examined, and the formation mechanism of nanoporous ZnMnO and the origin of the strong green light emission are discussed.

Enhanced Luminescence Performance of Quantum Wells by Coupling Piezo-Phototronic with Plasmonic Effects

With a promising prospect of light-emitting diodes as an attractive alternative to conventional light sources, remaining challenges still cannot be addressed owing to their limited efficiency. Among the continued scientific efforts, significant improvement on the emission efficiency has been achieved via either piezo-phototronic effect-based strain modulation or resonant excitation of plasmons in metallic nanostructures. Here, we present the investigation on the coupling process between piezo-phototronic effect and localized surface plasmonic resonance for enhancing the photoluminescence of InGaN/GaN quantum wells coated with Ag nanoparticles. The underlying physical mechanism of experimental results originates from tuning plasmonic resonance controlled by the shift of emission wavelength via piezo-phototronic effect, and it is further confirmed with the support of theoretical calculations. As a result, our research provides an approach to the integration of plasmonics with piezo-phototronic effect and brings widespread applications to high-efficiency artificial lighting, on-chip integrated plasmonic circuits, subwavelength optical communication, and micro-optoelectronic mechanical systems.

Piezo-phototronic effect enhanced UV photodetector based on CuI/ZnO double-shell grown on flexible copper microwire

In this work, we present a facile, low-cost, and effective approach to fabricate the UV photodetector with a CuI/ZnO double-shell nanostructure which was grown on common copper microwire. The enhanced performances of Cu/CuI/ZnO core/double-shell microwire photodetector resulted from the formation of heterojunction. Benefiting from the piezo-phototronic effect, the presentation of piezocharges can lower the barrier height and facilitate the charge transport across heterojunction. The photosensing abilities of the Cu/CuI/ZnO core/double-shell microwire detector are investigated under different UV light densities and strain conditions. We demonstrate the I-V characteristic of the as-prepared core/double-shell device; it is quite sensitive to applied strain, which indicates that the piezo-phototronic effect plays an essential role in facilitating charge carrier transport across the CuI/ZnO heterojunction, then the performance of the device is further boosted under external strain.

Tuning carrier lifetime in InGaN/GaN LEDs via strain compensation for high-speed visible light communication

In recent years, visible light communication (VLC) technology has attracted intensive attention due to its huge potential in superior processing ability and fast data transmission. The transmission rate relies on the modulation bandwidth, which is predominantly determined by the minority-carrier lifetime in III-group nitride semiconductors. In this paper, the carrier dynamic process under a stress field was studied for the first time, and the carrier recombination lifetime was calculated within the framework of quantum perturbation theory. Owing to the intrinsic strain due to the lattice mismatch between InGaN and GaN, the wave functions for the holes and electrons are misaligned in an InGaN/GaN device. By applying an external strain that "cancels" the internal strain, the overlap between the wave functions can be maximized so that the lifetime of the carrier is greatly reduced. As a result, the maximum speed of a single chip was increased from 54 MHz up to 117 MHz in a blue LED chip under 0.14% compressive strain. Finally, a bandwidth contour plot depending on the stress and operating wavelength was calculated to guide VLC chip design and stress optimization.

Piezo-Phototronic Effect in a Quantum Well Structure

With enhancements in the performance of optoelectronic devices, the field of piezo-phototronics has attracted much attention, and several theoretical works have been reported based on semiclassical models. At present, the feature size of optoelectronic devices are rapidly shrinking toward several tens of nanometers, which results in the quantum confinement effect. Starting from the basic piezoelectricity equation, Schrödinger equation, Poisson equation, and Fermi's golden rule, a self-consistent theoretical model is proposed to study the piezo-phototronic effect in the framework of perturbation theory in quantum mechanics. The validity and universality of this model are well-proven with photoluminescence measurements in a single GaN/InGaN quantum well and multiple GaN/InGaN quantum wells. This study provides important insight into the working principle of nanoscale piezo-phototronic devices as well as guidance for the future device design.