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.     

Enhanced Performance of a Self‐Powered Organic/Inorganic Photodetector by Pyro‐Phototronic and Piezo‐Phototronic Effects

Self‐powered photodetectors (PDs) have long been realized by utilizing photovoltaic effect and their performances can be effectively enhanced by introducing the piezo‐phototronic effect. Recently, a novel pyro‐phototronic effect is invented as an alternative approach for performance enhancement of self‐powered PDs. Here, a self‐powered organic/inorganic PD is demonstrated and the influences of externally applied strain on the pyro‐phototronic and the photovoltaic effects are thoroughly investigated. Under 325 nm 2.30 mW cm^‐2 UV illumination and at a ‐0.45% compressive strain, the PD's photocurrent is dramatically enhanced from ≈14.5 to ≈103 nA by combining the pyro‐phototronic and piezo‐phototronic effects together, showing a significant improvement of over 600%. Theoretical simulations have been carried out via the finite element method to propose the underlying working mechanism. Moreover, the pyro‐phototronic effect can be introduced by applying a ‐0.45% compressive strain to greatly enhance the PD's response to 442 nm illumination, including photocurrent, rise time, and fall time. This work provides in‐depth understandings about the pyro‐phototronic and the piezo‐phototronic effects on the performances of self‐powered PD to light sources with different wavelengths and indicates huge potential of these two effects in optoelectronic devices.     

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

Visible light communication (VLC) simultaneously provides illumination and communication via light emitting diodes (LEDs). Keeping a low bit error rate is essential to communication quality, and holding a stable brightness level is pivotal for illumination function. For the first time, a piezo‐phototronic effect controlled visible light communication (PVLC) system based on InGaN/GaN multiquantum wells nanopillars is demonstrated, in which the information is coded by mechanical straining. This approach of force coding is also instrumental to avoid LED blinks, which has less impact on illumination and is much safer to eyes than electrical on/off VLC. The two‐channel transmission mode of the system here shows great superiority in error self‐validation and error self‐elimination in comparison to VLC. This two‐channel PVLC system provides a suitable way to carry out noncontact, reliable communication under complex circumstances.     

Recent Progress on Piezotronic and Piezo‐Phototronic Effects in III‐Group Nitride Devices and Applications

Wurtzite‐structured III‐group nitrides, like GaN, InN, AlN, and their alloys, present both piezoelectric and semiconducting properties under straining owing to the polarization of ions in a crystal with non‐central symmetry. The piezoelectric polarization charges are created at the interface when a strain is applied. As a result, a piezoelectric potential (piezopotential) is produced, which is used as a “gate” to tune/control the charge transport behavior across a metal/semiconductor interface or a p‐n junction. This is called as piezotronic effect. A series of piezotronic devices and applications have been developed, such as piezotronic nanogenerators (NGs), piezotronic transistors, piezotronic logic devices, piezotronic electromechanical memories, piezotronic enhanced biochemical, and gas sensors and so on. With the flourished development of piezotronic effect, the piezo‐phototronic effect, as the three‐way coupling of piezoelectric polarization, semiconductor properties, and optical excitation, utilizes the piezopotential to modulate the energy band profile and control the carrier generation, transportation, separation, and/or recombination for improving performances of optoelectronic devices. This paper intends to provide an overview of the rapid progress in the emerging fields of piezotronics and piezo‐phototronics, covering from the fundamental principles to devices and applications. This study will provide important insight into the potential applications of GaN based electronic/optoelectronic devices in sensing, active flexible/stretchable electronics/optoelectronics, energy harvesting, human‐machine interfacing, biomedical diagnosis/therapy, and prosthetics.

     

Progress in Piezo‐Phototronic‐Effect‐Enhanced Light‐Emitting Diodes and Pressure Imaging

Wurtzite materials exhibit both semiconductor and piezoelectric properties under strains due to the non‐central symmetric crystal structures. The three‐way coupling of semiconductor properties, piezoelectric polarization and optical excitation in ZnO, GaN, CdS and other piezoelectric semiconductors leads to the emerging field of piezo‐phototronics. This effect can efficiently manipulate the emission intensity of light‐emitting diodes (LEDs) by utilizing the piezo‐polarization charges created at the junction upon straining to modulate the energy band diagrams and the optoelectronic processes, such as generation, separation, recombination and/or transport of charge carriers. Starting from fundamental physics principles, recent progress in piezo‐phototronic‐effect‐enhanced LEDs is reviewed; following their development from single‐nanowire pressure‐sensitive devices to high‐resolution array matrices for pressure‐distribution mapping applications. The piezo‐phototronic effect provides a promising method to enhance the light emission of LEDs based on piezoelectric semiconductors through applying static strains, and may find perspective applications in various optoelectronic devices and integrated systems.     

Site‐Specific Self‐Assembled Liquid‐Gated ZnO Nanowire Transistors for Sensing Applications

A scalable bottom‐up solution‐based approach for the site‐specific realization of ZnO nanowire (ZnO‐NW)‐based field‐effect transistors for sensing applications in liquids is reported. The nanowires are grown across predefined electrodes patterned by photolithography. Site specificity is attained by the use of nanoparticles acting as seeds. Using integrated on‐chip microchannels and microfabricated gate electrodes, electrochemically gated ZnO‐NW network transistors functioning in liquids are demonstrated. The optimized devices are rendered sensitive to pH through chemical functionalization. The unique combination of the sensitivity, site specificity, scalability, and cost effectiveness of the technique opens up avenues for the routine realization of one‐dimensional nanostructure‐based chemical and biosensors for analytical and diagnostic applications.     

Piezo‐Phototronic Effect on Selective Electron or Hole Transport through Depletion Region of Vis–NIR Broadband Photodiode

Silicon underpins nearly all microelectronics today and will continue to do so for some decades to come. However, for silicon photonics, the indirect band gap of silicon and lack of adjustability severely limit its use in applications such as broadband photodiodes. Here, a high‐performance p‐Si/n‐ZnO broadband photodiode working in a wide wavelength range from visible to near‐infrared light with high sensitivity, fast response, and good stability is reported. The absorption of near‐infrared wavelength light is significantly enhanced due to the nanostructured/textured top surface. The general performance of the broadband photodiodes can be further improved by the piezo‐phototronic effect. The enhancement of responsivity can reach a maximum of 78% to 442 nm illumination, the linearity and saturation limit to 1060 nm light are also significantly increased by applying external strains. The photodiode is illuminated with different wavelength lights to selectively choose the photogenerated charge carriers (either electrons or holes) passing through the depletion region, to investigate the piezo‐phototronic effect on electron or hole transport separately for the first time. This is essential for studying the basic principles in order to develop a full understanding about piezotronics and it also enables the development of the better performance of optoelectronics.