Interface engineering by piezoelectric potential in ZnO-based photoelectrochemical anode

Through a process of photoelectrochemical (PEC) water splitting, we demonstrated an effective strategy for engineering the barrier height of a heterogeneous semiconductor interface by piezoelectric polarization, known as the piezotronic effect. A consistent enhancement or reduction of photocurrent was observed when tensile or compressive strains were applied to the ZnO anode, respectively. The photocurrent variation is attributed to a changed barrier height at the ZnO/ITO interface, which is a result of the remnant piezoelectric potential across the interface due to a nonideal free charge distribution in the ITO electrode. In our system, ～1.5 mV barrier height change per 0.1% applied strain was identified, and 0.21% tensile strain yielded a ～10% improvement of the maximum PEC efficiency. The remnant piezopotential is dictated by the screening length of the materials in contact with piezoelectric component. The difference between this time-independent remnant piezopotential effect and time-dependent piezoelectric effect is also studied in details.     

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.     

Piezotronic Tunneling Junction Gated by Mechanical Stimuli

Tunneling junction is used in many devices such as high‐frequency oscillators, nonvolatile memories, and magnetic field sensors. In these devices, modulation on the barrier width and/or height is usually realized by electric field or magnetic field. Here, a new piezotronic tunneling junction (PTJ) principle, in which the quantum tunneling is controlled/tuned by externally applied mechanical stimuli, is proposed. In these metal/insulator/piezoelectric semiconductor PTJs, such as Pt/Al₂O₃/p‐GaN, the height and the width of the tunneling barriers can be mechanically modulated via the piezotronic effect. The tunneling current characteristics of PTJs exhibit critical behavior as a function of external mechanical stimuli, which results in high sensitivity (≈5.59 mV MPa^?1), giant switching (>10⁵), and fast response (≈4.38 ms). Moreover, the mechanical controlling of tunneling transport in PTJs with various thickness of Al₂O₃ is systematically investigated. The high performance observed with these metal/insulator/piezoelectric semiconductor PTJs suggest their great potential in electromechanical technology. This study not only demonstrates dynamic mechanical controlling of quantum tunneling, but also paves a way for adaptive interaction between quantum tunneling and mechanical stimuli, with potential applications in the field of ultrasensitive press sensor, human–machine interface, and artificial intelligence.     

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.

     

Regulation of Charge Carrier Dynamics in ZnO Microarchitecture‐Based UV/Visible Photodetector via Photonic‐Strain Induced Effects

A feasible, morphological influence on photoresponse behavior of ZnO microarchitectures such as microwire (MW), coral‐like microstrip (CMS), fibril‐like clustered microwire (F‐MW) grown by one‐step carrier gas/metal catalyst “free” vapor transport technique is reported. Among them, ZnO F‐MW exhibits higher photocurrent (I_Ph) response, i.e., I_{Ph/ZnO F‐MW} > I_{Ph/ZnO CMS} > I_{Ph/ZnO MW}. The unique structural alignment of ZnO F‐MW has enhanced the I_Ph from 14.2 to 186, 221, 290 µA upon various light intensities such as 0 to 6, 11, 17 mW cm^?2 at λ_{405 nm}. Herein, the nature of the as‐fabricated ZnO photodetector (PD) is also demonstrated modulated by tuning the inner crystals piezoelectric potential through the piezo‐phototronic effect. The I_Ph response of PD decreases monotonically by introducing compressive strain along the length of the device, which is due to the synergistic effect between the induced piezoelectric polarization and photogenerated charge carriers across the metal–semiconductor interface. The current behavior observed at the two interfaces acting as the source (S) and drain (D) is carefully investigated by analyzing the Schottky barrier heights (Φ_SB). This work can pave the way for the development of geometrically modified strain induced performances of PD to promote next generation self‐powered optoelectronic integrated devices and switches.     

Fundamental theory of piezotronics

Due to polarization of ions in crystals with noncentral symmetry, such as ZnO, GaN, and InN, a piezoelectric potential (piezopotential) is created in the crystal when stress is applied. Electronics fabricated using the inner-crystal piezopotential as a gate voltage to tune or control the charge transport behavior across a metal/semiconductor interface or a p-n junction are called piezotronics. This is different from the basic design of complimentary metal oxide semiconductor (CMOS) field-effect transistors and has applications in force and pressure triggered or controlled electronic devices, sensors, microelectromechanical systems (MEMS), human-computer interfacing, nanorobotics, and touch-pad technologies. Here, the theory of charge transport in piezotronic devices is investigated. In addition to presenting the formal theoretical frame work, analytical solutions are presented for cases including metal-semiconductor contact and p-n junctions under simplified conditions. Numerical calculations are given for predicting the current-voltage characteristics of a general piezotronic transistor: metal-ZnO nanowire-metal device. This study provides important insight into the working principles and characteristics of piezotronic devices, as well as providing guidance for device design.     

Gate‐Tunable Graphene–WSe2 Heterojunctions at the Schottky–Mott Limit

Metal–semiconductor interfaces, known as Schottky junctions, have long been hindered by defects and impurities. Such imperfections dominate the electrical characteristics of the junction by pinning the metal Fermi energy. Here, a graphene–WSe₂ p‐type Schottky junction, which exhibits a lack of Fermi level pinning, is studied. The Schottky junction displays near‐ideal diode characteristics with large gate tunability and small leakage currents. Using a gate electrostatically coupled to the WSe₂ channel to tune the Schottky barrier height, the Schottky–Mott limit is probed in a single device. As a special manifestation of the tunable Schottky barrier, a diode with a dynamically controlled ideality factor is demonstrated.     

Diode-Like Selective Enhancement of Carrier Transport through Metal–Semiconductor Interface Decorated by Monolayer Boron Nitride

2D semiconductors such as monolayer molybdenum disulfide (MoS₂) are promising material candidates for next-generation nanoelectronics. However, there are fundamental challenges related to their metal–semiconductor (MS) contacts, which limit the performance potential for practical device applications. In this work, 2D monolayer hexagonal boron nitride (h-BN) is exploited as an ultrathin decorating layer to form a metal–insulator–semiconductor (MIS) contact, and an innovative device architecture is designed as a platform to reveal a novel diode-like selective enhancement of the carrier transport through the MIS contact. The contact resistance is significantly reduced when the electrons are transported from the semiconductor to the metal, but is barely affected when the electrons are transported oppositely. A concept of carrier collection barrier is proposed to interpret this intriguing phenomenon as well as a negative Schottky barrier height obtained from temperature-dependent measurements, and the critical role of the collection barrier at the drain end is shown for the overall transistor performance.     

Anisotropic Thermal Boundary Resistance across 2D Black Phosphorus: Experiment and Atomistic Modeling of Interfacial Energy Transport

Interfacial thermal boundary resistance (TBR) plays a critical role in near‐junction thermal management of modern electronics. In particular, TBR can dominate heat dissipation and has become increasingly important due to the continuous emergence of novel nanomaterials with promising electronic and thermal applications. A highly anisotropic TBR across a prototype 2D material, i.e., black phosphorus, is reported through a crystal‐orientation‐dependent interfacial transport study. The measurements show that the metal–semiconductor TBR of the cross‐plane interfaces is 241% and 327% as high as that of the armchair and zigzag direction‐oriented interfaces, respectively. Atomistic ab initio calculations are conducted to analyze the anisotropic and temperature‐dependent TBR using density functional theory (DFT)‐derived full phonon dispersion relation and molecular dynamics simulation. The measurement and modeling work reveals that such a highly anisotropic TBR can be attributed to the intrinsic band structure and phonon spectral transmission. Furthermore, it is shown that phonon hopping between different branches is important to modulate the interfacial transport process but with directional preferences. A critical fundamental understanding of interfacial thermal transport and TBR–structure relationships is provided, which may open up new opportunities in developing advanced thermal management technology through the rational control over nanostructures and interfaces.     

Correlative High‐Resolution Mapping of Strain and Charge Density in a Strained Piezoelectric Multilayer

A key to strain engineering of piezoelectric semiconductor devices is the quantitative assessment of the strain‐charge relationship. This is particularly demanding in current InGaN/GaN‐based light‐emitting diode (LED) designs as piezoelectric effects are known to degrade the device performance. Using the state‐of‐the‐art inline electron holography, we have obtained fully quantitative maps of the two‐dimensional strain tensor and total charge density in conventional blue LEDs and correlated these with sub‐nanometer spatial resolution. We show that the In_0.15Ga_0.85N quantum wells are compressively strained and elongated along the polar growth direction, exerting compressive stress/strain on the GaN quantum barriers. Interface sheet charges arising from a polarization gradient are obtained directly from the strain data and compared with the total charge density map, quantitatively verifying only 60% of the polarization charges are screened by electrons, leaving a substantial piezoelectric field in each In_0.15Ga_0.85N quantum well. The demonstrated capability of inline electron holography provides a technical breakthrough for future strain engineering of piezoelectric optoelectronic devices.     

Transport at the Epitaxial Interface between Germanium and Functional Oxides

Combining functional oxides with conventional semiconductors provides the potential for transformational advancement of microelectronic devices. Harnessing the full spectrum of oxide functionalities requires current transport between the oxide and the semiconductor. This aspect is addressed by controlling the electronic barrier at an interface between a ferroelectric oxide, BaTiO₃, and germanium. For the aligned conduction bands of germanium and BaTiO₃, as measured by spectroscopy, current transport is controlled by the barrier between the top metal electrode and the bands of the BaTiO₃. Capacitance–voltage analysis of metal–oxide–semiconductor devices further shows that the semiconductor's Fermi level can be moved by a field effect. These results demonstrate a viable approach for electronically bridging the functionalities of oxides directly with a common semiconductor.     

A Triode Device with a Gate Controllable Schottky Barrier: Germanium Nanowire Transistors and Their Applications

Electrical contacts often dominate charge transport properties at the nanoscale because of considerable differences in nanoelectronic device interfaces arising from unique geometric and electrostatic features. Transistors with a tunable Schottky barrier between the metal and semiconductor interface might simplify circuit design. Here, germanium nanowire (Ge NW) transistors with Cu₃Ge as source/drain contacts formed by both buffered oxide etching treatments and rapid thermal annealing are reported. The transistors based on this Cu₃Ge/Ge/Cu₃Ge heterostructure show ambipolar transistor behavior with a large on/off current ratio of more than 10⁵ and 10³ for the hole and electron regimes at room temperature, respectively. Investigations of temperature‐dependent transport properties and low‐frequency current fluctuations reveal that the tunable effective Schottky barriers of the Ge NW transistors accounted for the ambipolar behaviors. It is further shown that this ambipolarity can be used to realize binary‐signal and data‐storage functions, which greatly simplify circuit design compared with conventional technologies.     

Non-stoichiometry and electronic properties of interfaces

The paper gives an overview on the influence of point defects on electronic properties of interfaces including band alignment (barrier heights) and transport properties. As examples interfaces between metals and the II–VI semiconductors CdTe and ZnTe are presented. In addition untypical phenomena at semiconductor heterocontact formation at In₂S₃/ZnO and CuInSe₂/CdS interfaces is described. It is suggested that the barrier heights as well as the transport properties at both interfaces are strongly affected by defects, which are either present because of non-stoichiometry of the materials or introduced by contact formation due to chemical interactions.     

Band Structure Engineering at Heterojunction Interfaces via the Piezotronic Effect

Engineering the electronic band structure using the piezopotential is an important aspect of piezotronics, which describes the coupling between the piezoelectric property and semiconducting behavior and functionalities. The time‐independent band structure change under short‐circuit condition is believed to be due to the remnant piezopotential present at the interface, a result of the finite charge‐screening depth at the interface. A series of materials, including metals, semiconductors and electrolytes, are selected to investigate the interfacial band structure engineered by remnant piezopotential when they are in contact with a strained piezoelectric semiconductor. The remnant piezopotential at the interface can switch the junction between Ohmic and Schottky characters, enhance charge combination/separation, regulate barrier height, and modulate reaction kinetics. The difference between the regular time‐dependent, pulse‐type piezopotential and constant remnant piezopotential is also discussed in detail using a ZnO‐based photoelectrochemical anode as an example. The piezotronic effect offers a new pathway for engineering the interface band structure without altering the interface structure or chemical composition, which is promising for improving the performance of many electronics, optoelectronics, and photovoltaic devices.     

Optimal design of laminated piezocomposite energy harvesting devices considering stress constraints

Energy harvesting devices are smart structures capable of converting the mechanical energy (generally, in the form of vibrations) that would be wasted otherwise in the environment into usable electrical energy. Laminated piezoelectric plate and shell structures have been largely used in the design of these devices because of their large generation areas. The design of energy harvesting devices is complex, and they can be efficiently designed by using topology optimization methods (TOM). In this work, the design of laminated piezocomposite energy harvesting devices has been studied using TOM. The energy harvesting performance is improved by maximizing the effective electric power generated by the piezoelectric material, measured at a coupled electric resistor, when subjected to a harmonic excitation. However, harmonic vibrations generate mechanical stress distribution that, depending on the frequency and the amplitude of vibration, may lead to piezoceramic failure. This study advocates using a global stress constraint, which accounts for different failure criteria for different types of materials (isotropic, piezoelectric, and orthotropic). Thus, the electric power is maximized by optimally distributing piezoelectric material, by choosing its polarization sign, and by properly choosing the fiber angles of composite materials to satisfy the global stress constraint. In the TOM formulation, the Piezoelectric Material with Penalization and Polarization material model is applied to distribute piezoelectric material and to choose its polarization sign, and the Discrete Material Optimization method is applied to optimize the composite fiber orientation. The finite element method is adopted to model the structure with a piezoelectric multilayered shell element. Numerical examples are presented to illustrate the proposed methodology. Copyright © 2015 John Wiley & Sons, Ltd.     

Cross‐Plane Carrier Transport in Van der Waals Layered Materials

The mechanisms of carrier transport in the cross‐plane crystal orientation of transition metal dichalcogenides are examined. The study of in‐plane electronic properties of these van der Waals compounds has been the main research focus in recent years. However, the distinctive physical anisotropies, short‐channel physics, and tunability of cross layer interactions can make the study of their electronic properties along the out‐of‐plane crystal orientation valuable. Here, the out‐of‐plane carrier transport mechanisms in niobium diselenide and hafnium disulfide are explored as two broadly different representative materials. Temperature‐dependent current–voltage measurements are preformed to examine the mechanisms involved. First principles simulations and a tunneling model are used to understand these results and quantify the barrier height and hopping distance properties. Using Raman spectroscopy, the thermal response of the chemical bonds is directly explored and the insight into the van der Waals gap properties is acquired. These results indicate that the distinct cross‐plane carrier transport characteristics of the two materials are a result of material thermal properties and thermally mediated transport of carriers through the van der Waals gaps. Exploring the cross‐plane electron transport, the exciting physics involved is unraveled and potential new avenues for the electronic applications of van der Waals layers are inspired.     

Ultrahigh Piezoelectric Properties in Textured (K,Na)NbO3‐Based Lead‐Free Ceramics

High‐performance lead‐free piezoelectric materials are in great demand for next‐generation electronic devices to meet the requirement of environmentally sustainable society. Here, ultrahigh piezoelectric properties with piezoelectric coefficients (d₃₃ ≈700 pC N^?1, d₃₃* ≈980 pm V^?1) and planar electromechanical coupling factor (k_p ≈76%) are achieved in highly textured (K,Na)NbO₃ (KNN)‐based ceramics. The excellent piezoelectric properties can be explained by the strong anisotropic feature, optimized engineered domain configuration in the textured ceramics, and facilitated polarization rotation induced by the intermediate phase. In addition, the nanodomain structures with decreased domain wall energy and increased domain wall mobility also contribute to the ultrahigh piezoelectric properties. This work not only demonstrates the tremendous potential of KNN‐based ceramics to replace lead‐based piezoelectrics but also provides a good strategy to design high‐performance piezoelectrics by controlling appropriate phase and crystallographic orientation.     

Mapping Energy Levels for Organic Heterojunctions

An organic semiconductor thin film is a solid‐state matter comprising one or more molecules. For applications in electronics and photonics, several distinct functional organic thin films are stacked together to create a variety of devices such as organic light‐emitting diodes and organic solar cells. The energy levels at these thin‐film junctions dictate various electronic processes such as the charge transport across these junctions, the exciton dissociation rates at donor–acceptor molecular interfaces, and the charge trapping during exciton formation in a host–dopant system. These electronic processes are vital to a device's performance and functionality. To uncover a general scientific principle in governing the interface energy levels, highest occupied molecular orbitals, and vacuum level dipoles, herein a comprehensive experimental research is conducted on several dozens of organic–organic heterojunctions representative of various device applications. It is found that the experimental data map on interface energy levels, after correcting variables such as molecular orientation‐dependent ionization energies, consists of three distinct regions depending on interface fundamental physical parameters such as Fermi energy, work function, highest occupied molecular orbitals, and lowest unoccupied molecular orbitals. This general energy map provides a master guide in selection of new materials for fabricating future generations of organic semiconductor devices.