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Blue and violet light-emitting devices based on III-nitrides caused an ongoing revolution in general lighting. One of the highly deliberated discussions in this field is devoted to the problem call...

Semiconductor photocatalysis as a desirable technology shows great potential in environmental remediation and renewable energy generation, but its efficiency is severely restricted by the rapid recombination of charge carriers in the bulk phase and on the surface of photocatalysts. Polarization has emerged as one of the most effective strategies for addressing the above-mentioned issues, thus effectively promoting photocatalysis. This review summarizes the recent advances on improvements of photocatalytic activity by polarization-promoted bulk and surface charge separation. Highlighted is the recent progress in charge separation advanced by different types of polarization, such as macroscopic polarization, piezoelectric polarization, ferroelectric polarization, and surface polarization, and the related mechanisms. Finally, the strategies and challenges for polarization enhancement to further enhance charge separation and photocatalysis are discussed.

Realizing single-mode-lasing output while being able to dynamically select and regulate a specified resonant mode could bring revolutionary impact for laser technology, on-chip data communication, and optical sensing/switches. Here, we demonstrate a single-mode lasing achieved by the piezoresistive and piezoelectric polarization synergistic effect on an epoxy-encapsulated ZnO microresonator. Based on relative shifts of gain spectrum and the resonant wavelength, the lasing mode in a hexagonal ZnO rod can be selected and regulated dynamically within a certain range. The relationship between the corresponding applied strain and the tunable refractive index is analyzed in depth and discussed systematically. Our studies open up exciting avenues for constructing optical mode-phase modulator, high-sensitive optical switches and color-perceived optical sensing.

Recently, there has been an increasing research interest in the emerging fields of piezophotonics, which is the great interesting physics responsible for numbers of important technologies such as light source, smart sensor and mechanoelectronics. Piezophotonic effect is the coupling between the piezoelectric polarization and the photonic excitation in crystal that has a non-central symmetry. The strain-induced piezopotential can stimuli the photon emission without additional energy excitation such as light and electricity, which also offers great new opportunities of manipulating and fabrication of flexible optoelectronic devices. In this review, we will give a detailed description of the piezophotonic effect including its theoretical fundamental and practical applications. The piezophotonic light emission in doped ZnS CaZnOS, SrAl2O4 and LiNbO3 attract great interesting during the past years, and researchers have executed many scientific inquiries into flexible/stretchable optoelectronic devices. Until now, significant breakthroughs have been achieved on piezophotonic e-signature system, visible wearable electronic devices and multi-physical coupling devices. Certainly, rapid innovations in this field will be quite significant to the future of human life.

International audience; We report piezo-phototronic effects on the electrical properties of a Schottky contact metal/ZnO(n) solar cell. The piezo-phototronic effect is a three way coupling effect of piezoelectric, semiconductor and photonic properties occurring in non-central symmetric semiconductor materials when an applied strain to a semiconductor generates a piezoelectric potential controlling the carrier generation, transport, separation and/or recombination at the junction. The purpose of the present study is to determinate the influence of piezoelectric polarization on the electrical characteristics of this solar cell. Results show that the stress causes an increase of the electric field in the region where the piezoelectric fields are located. The sign of the stress proves that if it is positive (extensive strain), the effective barrier height increases which reduces the flow of electrons and thereafter is considered as a limiting factor for photovoltaic conversion, However, if the stress is negative (compression strain), a significant improvement of photovoltaic efficiency is obtained. Using the piezoelectric effects created by an external stress, our study provides not only a theoretical understanding about the piezo-phototronic effects on the characteristics of a solar cell but also assists for the possible design improvement allowing to increase.

The quasi-normally-off operation of AlGaN/GaN HEMTs was realized using strain engineering. The dual-layer silicon nitride (SiNx) stressors applied compression to the gate region in devices, and the introduced compression translated to piezoelectric polarization charges and hence, increased the threshold voltage of devices. The devices with dual-layer stressors showed quasi-normally-off characteristics and the threshold voltage of 0.17 V. Besides, the comb-gate structure was introduced to suppress the short channel effects. The on-state performance of the comb-gate devices, including on-resistance $(\pmb{R}_{\pmb{on}})$ and saturation current $(\pmb{I}_{\pmb{d},\pmb{sat}})$ , was benchmarked against that of p-GaN gate HEMTs from a commercial foundry. The dual-layer stressor and comb-gate structure further improved the feasibility of strain engineering as a practical approach in the pursuit of the normally-off operation of AIGaN/GaN HEMTs in power electronics applications.

Two-dimensional (2D) Janus semiconductors with mirror asymmetry can introduce novel properties, such as large spin-orbit coupling (SOC) and normal piezoelectric polarization, which have attracted a great interest for their potential applications. Inspired by the recently fabricated 2D ferromagnetic (FM) semiconductor CrI3, a stable 2D (in x-y plane) antiferromagnetic (AFM) Janus semiconductor Fe2Cl3I3 with normal sublattice magnetization (m//z) is obtained by density functional theory calculations. By applying a tensile strain, the four magnetic states sequentially occur: AFM with m//z of sublattice, AFM with m//xy of sublattice, FM with m//xy, and FM with m//z. Such novel magnetic phase diagram driven by strain can be well understood by the spin-spin interactions including the third nearest-neighbor hoppings with the single-ion anisotropy, in which the SOC of I atoms is found to play an essential role. In addition, the electric polarization of Fe2Cl3I3 preserves with strain due to the broken inversion symmetry. Our results predict the rare Janus material Fe2Cl3I3 as an example of 2D semiconductors with both spin and charge polarizations, and reveal the highly sensitive strain-controlled magnetic states and magnetization direction, which highlights the 2D magnetic Janus semiconductor as a new platform to design spintronic materials., Comment: 8 figures

Third-generation semiconductors, such as ZnO and GaN, exhibit strong piezoelectric polarization due to the lack of inversion symmetry. The piezotronic effect observed in these semiconductors was proposed for tuning carrier transport in electronic devices by utilizing the induced piezoelectric potential as a virtual gate. This novel concept allows effective interactions between micro-/nanoelectronic devices and external mechanical stimuli. Piezotronics provide a promising approach for designing future electronic devices beyond Moore’s Law with potential for developing smart sensors, environment monitoring systems, human–machine interaction elements, and other transducers. In this article, we review recent progress in piezotronics using one-dimensional materials, heterojunctions, and large-scale arrays. We provide guidance for future piezotronic devices based on these materials.

By using three-point bending tests and numerical simulation, influence of polarization on the electromechanical properties of GaN piezoelectric semiconductive ceramics (PSCs) were investigated in this paper. The results show that the piezoelectricity of GaN PSCs can be attained through a special polarization treatment. For polarized samples under loading, because piezoelectric polarization charges and the electric field are concentrated at high-strain positions, their bending strength increases by 7%. Polarization results in a nearly 55% improvement of the electrical current transport capacity. Due to piezoelectricity, the electric displacement of polarized samples is also largely changed. It is shown that there is a strong correlation between polarization and electromechanical properties of PSCs. These findings highlight the influence of polarization on the electromechanical performance of PSCs, and also imply some potentials for their applications.

Series of GaN-based blue laser diodes (LDs) with different InxGa1-xN quantum barrier (QB) and lower waveguide (LWG) layers are investigated by using the two-dimension simulator LASTIP. It is found that a decrease slope efficiency is resulted when the indium content of InxGa1-xN quantum barrier (QB) layers is higher than about 1%, which is caused by the significantly increase of electron leakage current due to the piezoelectric polarization effect in high indium content InxGa1-xN last QB (LQB) layer. Therefore, an asymmetric MQW with a thin thickness of LQB is designed to lower the piezoelectric polarization effect and to reduce the electron leakage current. Meanwhile, a new LD structure with high InxGa1-xN LWG is also proposed to further reduce the optical loss. The two ways are useful to improve the slope efficiency.

Transition-metal dichalcogenides (TMDCs), such as MoS2, lack their inversion center in monolayers, exhibiting in-plane piezoelectricity at a nanoscale thickness. In conventional piezoceramics devices, the operating mechanism has been well established that piezocharges appear at crystal edges and how these charges act in capacitor structures. Although TMDC monolayers are expected to possess a unique system due to their semiconductor nature, strong interaction with contact metals alters physical properties predominantly. In this study, we identify the position of piezocharges in MoS2 generators based on the displacement current under dynamic strain. The present results provide new insights for future device engineering.

An inserted novel polarization-graded AlGaN back barrier structure is designed to enhance performances of In0.17Al0.83N/GaN high electron mobility transistor (HEMT), which is investigated by the two-dimensional drift-diffusion simulations. The results indicate that carrier confinement of the graded AlGaN back-barrier HEMT is significantly improved due to the conduction band discontinuity of about 0.46 eV at interface of GaN/AlGaN heterojunction. Meanwhile, the two-dimensional electron gas (2DEG) concentration of parasitic electron channel can be reduced by a gradient Al composition that leads to the complete lattice relaxation without piezoelectric polarization, which is compared with the conventional Al0.1Ga0.9N back-barrier HEMT. Furthermore, compared to the conventional back-barrier HEMT with a fixed Al-content, a higher transconductance, a higher current and a better radio-frequency performance can be created by a graded AlGaN back barrier.

Strain-gated flexible optoelectronics are reported based on monolayer MoS2 . Utilizing the piezoelectric polarization created at the metal-MoS2 interface to modulate the separation/transport of photogenerated carriers, the piezophototronic effect is applied to implement atomic-layer-thick phototransistor. Coupling between piezoelectricity and photogenerated carriers may enable the development of novel optoelectronics.

AlGaN/GaN heterostructure was observed using scanning nonlinear dielectric microscopy, which can measure both carrier and polarization profile in AlGaN/GaN heterostructure. As a result, GaN spontaneous polarization and AlGaN polarization which was sum of spontaneous polarization and piezoelectric polarization were clearly distinguished. Moreover, two dimensional electron gas was observed at the AlGaN/GaN interface. These results show that scanning nonlinear dielectric microscopy is useful method for evaluation of two dimensional electron gas profile and polarization profile in AlGaN/GaN heterostructure.

In this work, piezoelectric ZnO microwire was utilized to form heterojunctions with perovskite as flexible photovoltaic devices, and has a power conversion efficiency of 0.0216%. By employing the piezo-phototronic effect, the strain-induced piezoelectric polarization charges at the vicinity of ZnO/perovskite interface can modulate the transport and separation processes of photo-generated carriers within the photovoltaic device and enhance the performances of ZnO/perovskite solar cells. The corresponding open-circuit voltage (Voc), short-circuit current (Isc) and efficiency of the solar cells were improved by 25.42%, 629.47% and 1280% (from 0.0216% to 0.298%) through piezo-phototronic effect, respectively. Physical working mechanism behind the observed results was carefully investigated by using energy band diagram. This study provides a promising approach to effectively enhance the overall performances of flexible solar cells for various applications.

We propose and demonstrate a novel scheme of semi-open-loop polarization control (SOL-PC), which controls the state of polarization (SOP) with high accuracy and uniform high speed. For any desired SOP, we first adjust the initial SOP using open-loop control (OLC) based on the matrix model of a three-unit piezoelectric polarization controller, and quickly move it close to the objective one. Then closed-loop control (CLC) is performed to reduce the error and reach precisely the desired SOP. The response time is three orders faster than that of the present closed-loop polarization control, while the average deviation is on par with it. Finally, the SOL-PC system is successfully applied to realize the suppression of the polarization mode dispersion (PMD) effect and reduce the first-order PMD to near zero. Due to its perfect performance, the SOL-PC energizes the present polarization control to pursue an ideal product that can meet the future requirements in ultrafast optical transmission and quantum communication.

The asymmetrical structures were created by inserting a low-indium-content layer between the QW and barrier to form a step-like quantum barrier (QB) at one side of QW. The optical effects of the inserting layer on QW emission were investigated with low-temperature photoluminescence (PL) and time-resolved PL (TRPL). The inserted layer partially relaxed the strain within QW layer and induced about 25 nm red-shift in the PL emission compared with conventional QW, while the presence of localization centers around QW affected the emission mechanism and increased the radiative decay time. Furthermore, the position of the inserted layer played different roles in the changed structures, and whilst the n-side step-barrier exhibited strong localization in the energy levels of the inserted layer, the p-side step-barrier showed stronger localization center for the QW levels.

We have demonstrated the first carrier density model for AlGaN channel with AlN buffer using spontaneous and piezoelectric polarization comparison with experimental and theoretical results. From the results we proved that the formation of 2DEG in undoped structure relied both on spontaneous and piezoelectric polarization. The electron distribution of Al concentration (0 < x < 0.5) was measured for both AlGaN channel and barrier. Barrier thickness assumed between 20 and 25 nm for validating the experimental results. The carrier concentration was observed at the specific interface of the N- and Ga-face by assuming x1, x2 = 0. The model results are verified with previously reported experimental data.

In this paper, the influence of strain distribution, Indium concentration and growth orientation on spontaneous and piezoelectric polarization charges of GaN/InGaN/GaN core/shell/shell nanowire solar cell are investigated. The model represents a nonlinear dependence of polarization effect on crystal orientation of triangular nanowire solar cell. It is observed that the effect of polarization charges are suppressed along ±62°, and ±118° (semi-polar plane) orientation of nanowire due to cancellation of spontaneous and piezoelectric charges. Moreover, the theoretical calculation provides a unique set of crystal planes for triangular nanowire solar cell, which is highly suitable for solar photovoltaic applications.

High sensitivity of pressure/strain sensors is the key to accurately evaluating external mechanical stimuli and could become more important in future generations of human–machine interfaces and artificial skin. Here we report the study of a two-terminal piezotronic transistor based on ZnO twin nanoplatelets (TNPT). Owing to the mirror symmetrical structure of ZnO twin nanplatelet, compressive pressure-induced positive piezoelectric polarization charges created at both metal–semiconductor interfaces can simultaneously lower both Schottky barrier heights and thus significantly modulate the carrier transport. Our device exhibits the highest pressure sensitivity of 1448.08–1677.53 meV/MPa, which is more than ∼20 times larger than the highest value reported previously, and a fast response time of

The piezoelectric polarization effect is dominant at the every GaN/InGaN interface in the MQW structure. In this paper we show that, how this piezoelectric polarization effect is useful in improving the open circuit voltage of GaN/InGaN MQW solar cell. For this purpose we have design a mathematical model of GaN/InGaN MQW solar cell and calculate the values of open circuit voltage with and without piezoelectric polarization effect. For simplicity of the model we are neglecting the current clouding effect, composition fluctuation and electro migration effect present in the device. Here we also assuming that, all the layers of GaN and InGaN material are terminated by Ga-face. Results indicate that the open circuit voltage of MQW solar cell improves by ~6.28% (at indium fraction in quantum well is 0.2 and quantum well thickness is 1 nm) and by ~16.77% (at indium fraction in quantum well is 0.2 and quantum well thickness is 3 nm) by using piezoelectric polarization effect.

The piezo-phototronic effect is about the enhanced separation, transport, and recombination of the photogenerated carriers using the piezoelectric polarization charges present in piezoelectric-semiconductor materials. Here, it is presented that the piezo-phototronic effect can be effectively applied to improve the relative conversion efficiency of a flexible solar cell based on n-ZnO/p-SnS core-shell nanowire array for 37.3% under a moderate vertical pressure. The performance of the solar cell can be effectively enhanced by a gentle bending of the device, showing its potential for application in curly geometries. This study not only adds further understanding about the concept of increasing solar energy conversion efficiency via piezo-phototronic effect, but also demonstrates the great potential of piezo-phototronic effect in the application of large-scale, flexible, and lightweight nanowire array solar cells.

Low-dimensional piezoelectric semiconductor nanomaterials, such as ZnO and GaN, have superior mechanical properties and can be integrated into flexible devices that can be subjected to large strain. More importantly, the coupling between piezoelectric polarization and semiconductor properties (for example, electronic transport and photoexcitation) in these materials gives rise to unprecedented device characteristics. This has increased research interest in the emerging fields of piezotronics and piezo-phototronics, which offer new means of manipulating charge-carrier transport, generation, recombination or separation in the controlled operation of flexible devices through the application of external mechanical stimuli. We review the recent progress in advancing our fundamental understanding and in realizing practical applications of piezotronics and piezo-phototronics, and provide an in-depth discussion of future research directions. Piezotronics and piezo-phototronics offer new means of implementing adaptive electronics and optoelectronics, taking advantage of the coupling between piezoelectric polarization and semiconductor properties in piezoelectric semiconductor nanomaterials. This Review discusses the recent progress in piezotronics and piezo-phototronics, as well as future research directions.

High power InGaN-based blue LEDs have been commercialized despite the presence of a high dislocation density in the heteroepitaxial structures. Traditional trial-and-error techniques are very inefficient for improving the process of MOVCD. We analyze the effects of spontaneous and piezoelectric polarization on spontaneous emission rate of blue LEDs on SiC substrate by adjusting screening values and barrier doped concentration. The simulation results indicate to improve characteristics of blue LEDs on SiC substrate, interfacial charges for piezoelectric and polarization should get equilibrium with crystal defects and barrier doped concentration. There is an optimization point.

GaN-based LEDs is applied to illumination and visible-light communication. In this study, we research the impact of electron blocking layer (EBL) with different aluminum (Al) contents of 20% and 15% on the modulation bandwidth of high-power LEDs. LEDs with 15% Al in EBL shows higher radiative recombination rate and hole injection efficiency. Moreover, LED with 15% Al in EBL exhibits the modulation bandwidth of 25.5 MHz versus 23.5 MHz for LED with 20% Al at 300 mA. This finding demonstrates the influence of piezoelectric polarization field and low hole injection efficiency caused by EBL on modulation bandwidth of high-power LEDs.

We investigate the effects of internal polarization-related drift and diffusion on the emitted THz radiation from m -plane (100), a -plane (110), and c -plane (0001) nitride semiconductors. Enhanced THz radiation is observed from c -plane InN/InGaN multiple quantum wells as compared to c -plane bulk InN at 800 nm excitation wavelength. THz generation in the quantum well structure is due to surface normal transport in electric fields due to the termination of spontaneous and piezoelectric polarization at the well/barrier interfaces. From high stacking fault density nonpolar GaN, we observe further enhanced THz emission as compared to stacking fault free m -plane GaN. THz generation from the high stacking fault density m -plane GaN is attributed to in-plane transport in built-in fields due to stacking fault-terminated internal polarization. A similar effect is observed in m -plane as well as a -plane InN. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The optical properties of blue-violet InGaN light-emitting diodes under normal and reversed polarizations are numerically studied. The best light-emitting performance under normal and reversed polarization is obtained in a single quantum-well structure and double quantum-well structure, respectively. The key factors responsible for these phenomena are presumably the carrier concentration distribution and the amount of carriers in quantum wells. The turn-on voltage of light-emitting diodes under reversed polarization is lower than that of light-emitting diodes under normal polarization, due mainly to lower potential heights for electrons and holes in the active region.

Polarization is a crucial issue for electrical and optical characteristics of the UV AlGaInN light-emitting diodes. The effect of spontaneous and piezoelectric polarization on optical characteristics of the UV AlGaInN light-emitting diodes is investigated numerically. The simulation results indicate that the polarization-related effect for the UV AlGaInN light-emitting diodes is dominated by the piezoelectric polarization. When the UV AlGaInN light-emitting diodes are without piezoelectric polarization, the optical performance is enhanced effectively due to improved overlap of electron and hole wavefunctions, reduced electron leakage current, and increased density of holes in the active region.

Optical properties of ZnO-based single nanorods are probed by cathodoluminescence (CL) measurements at T = 5 K. We observe a variation of the ZnO near band edge CL by three orders of magnitude along the nanorod axis, accompanied by a spectral blueshift of 10-30 meV. This indicates a rather poor structural quality of the nanorod bottom part, close to the substrate. ZnO/ZnMgO quantum wells grown on top of ZnO nanorods are found to exhibit much stronger confinement effects as compared to their two-dimensional counterparts, suggesting a reduced spontaneous and piezoelectric polarization effects.

Flexible self-powered sensing is urgently needed for wearable, portable, sustainable, maintenance-free and long-term applications. Here, we developed a flexible and self-powered GaN membrane-based ultraviolet (UV) photoswitch with high on/off ratio and excellent sensitivity. Even without any power supply, the driving force of UV photogenerated carriers can be well boosted by the combination of both built-in electric field and piezoelectric polarization field. The asymmetric metal-semiconductor-metal structure has been elaborately utilized to enhance the carrier separation and transport for highly sensitive UV photoresponse. Its UV on/off ratio and detection sensitivity reach to 4.67 × 10(5) and 1.78 × 10(12) cm·Hz(0.5) W(1-), respectively. Due to its excellent mechanical flexibility, the piezoelectric polarization field in GaN membrane can be easily tuned/controlled based on piezo-phototronic effect. Under 1% strain, a stronger and broader depletion region can be obtained to further enhance UV on/off ratio up to 154%. As a result, our research can not only provide a deep understanding of local electric field effects on self-powered optoelectronic detection, but also promote the development of self-powered flexible optoelectronic devices and integrated systems.

We describe the optical properties of semi/non-polar InGaN and AlGaN quantum wells. In semipolar (11¯22) InGaN QWs, spatially uniform but spectrally broad emissions are observed. This finding is interpreted with consideration of the exciton migration length shortened by the fast radiative recombination lifetime due to the reduced electric field. Non/semipolar AlGaN QWs are also fabricated. In the semipolar (1¯102) AlGaN QWs, the radiative recombination lifetimes faster than that in the (0001) QW are confirmed experimentally. As a consequence, much stronger emission is achieved from the semipolar AlGaN QWs at room temperature

We review the recent progress of zinc oxide (ZnO) nanowire sensors with ohmic-contacted and Schottky-contacted configurations and the enhancement of the performances of Schottky-contacted ZnO NW sensors (SCZNSs) by the piezotronic effect. Comparing with the traditional ohmic-contacted ZnO NW sensors (OCZNSs), the SCZNSs have higher sensitivities and faster responses controlled by the barrier height at the metal-semiconductor (M-S) interface. The piezotronic effect was applied to tune the Schottky barrier height (SBH) with the strain-induced piezoelectric polarization charges at the interface of the M-S contact. The piezotronic effect can thus improve the detection limitation, sensitivity, and response time of the SCZNSs in different applications, such as UV detection, gas and bio/chemical sensing. These piezotronic-enhanced SCZNSs may find potential applications in human-machine interfacing and flexible electronics skin technologies.

The effect of piezoelectric polarization on GaN-based light emitting diodes (LEDs) with different kinds of prestrain layers between the multiple quantum wells (MQWs) and n-GaN layer is studied and demonstrated. Compared with the conventional LED, more than 10% enhancement in the output power of the LED with prestrain layer can be attributed to the reduction of polarization field within MQWs region. In this study, we reported a simple method to provide useful comparison of polarization fields within active region in GaN-based LEDs by using temperature-dependent electroluminescence (EL) measurement. The results pointed out that the polarization field of conventional LED was stronger than that of the others due to larger variation of the wavelength transition position (i.e., blue-shift change to red-shift) from 300 to 350 K, and thus the larger polarization field must be effectively screened by injecting more carriers into the MQWs region.

We report on recombination kinetics, by means of time resolved photoluminescence technique, on GaN quantum discs embedded in strain free AlGaN nanocolumn. The emission of the 100 A QD shows a very long time decay (about 20 ns), denoting a small radiative rate; in contrast, fast decay time are observed in the case of 20 A disc (∼450 ps). At the same time a large blue shift of the PL band is found in the 100 A QD when increasing the power excitation while the PL of the 20 A QD does not show any energy shift. The whole phenomenology can be interpreted in terms of piezoelectric polarization fields inside the strained GaN QDs embedded in relaxed AlGaN nanocolumn (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The electronic properties of inversion domains in a GaN-based lateral polarity heterostructure were investigated using micro-Raman spectroscopy. The piezoelectric polarization of each domain was calculated from strain determined via Raman scattering. The free carrier concentration and electron mobility were deduced from the longitudinal optical phonon-plasmon coupled mode. The electron concentration in the N-face domain was slightly higher than that in the Ga-face domain. It appears that during growth, a larger number of donor impurities may have been incorporated into the N-face domain than into the Ga-face domain. Author has checked copyright AD 14/01/2014

The strain-induced piezoelectric polarization and the spontaneous polarization can be reduced effectively using the applied electric field in the CdZnO/ZnMgO quantum well (QW) structure with high Cd composition. That is, optical properties as a function of internal and external fields in the CdZnO/ZnMgO QW with various applied electric field result in the increased optical gain due to the fact that the QW potential profile is flattened as a result of the compensation of the internal field by the reverse field as confirmed. These results demonstrate that a high-performance optical device operation can be realized in CdZnO/MgZnO QW structures by reducing the droop phenomenon.

We performed detailed studies of the effect of polarization on III-nitride solar cells. Spontaneous and piezoelectric polarizations were assessed to determine their impacts upon the open circuit voltages (VOC) in p–i(InGaN)–n and multi-quantum well (MQW) solar cells. We found that the spontaneous polarization in Ga-polar p–i–n solar cells strongly modifies energy band structures and corresponding electric fields in a way that degrades VOC compared to non-polar p–i–n structures. In contrast, we found that piezoelectric polarization in Ga-polar MQW structures does not have a large influence on VOC compared to non-polar MQW structures. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Exploiting piezoelectric effect to engineer material interface has been confirmed as a promising way to optimize the performance of optoelectronic devices. Here, by using this effect, we have greatly improved the photoresponse of the fabricated ZnO/Au Schottky junction based self-powered UV detector. A 440% augment of photocurrent, together with 5× increased sensitivity, was obtained when the device was subjected to a 0.580% tensile strain. The enhancement can be attributed to the facility separation and extraction of photoexcites due to the formation of the stronger and expanding built-in field, which is a result of charge redistribution induced by piezoelectric polarization at the ZnO/Au interface. This study not only can strengthen the understanding of piezoelectric effects on energy devices but also can be extended to boost performances of optoelectronic devices made of piezoelectric semiconductor materials.

The CdZnO/ZnMgO QW structure with high Cd composition is found to have smaller optical gain because the strain-induced piezoelectric polarization and the spontaneous polarization in the well increase with the inclusion of Cd. The internal field is reduced due to the additional polarization of opposite direction by Li in the CdZnO/ZnMgO QW structure. These results show that Li:CdZnO-based QW lasers are promising candidates for optoelectric applications in visible and UV regions.

A key property of the nitrides is the fact that they possess large spontaneous and piezoelectric polarization fields that allow a significant tailoring of the carrier dynamics and optical properties of nitride devices. In this paper, based on first-principles calculations of structural and electronic properties of bulk nitrides and their heterostructure, we investigate the potential of this novel material class for modern device applications by performing self-consistent Monte Carlo simulations. We demonstrate that the internal electric fields have a significant and favorable influence on the transistor characteristics.

A comprehensive investigation was carried out on n-type ZnO nanowires for studying the temperature dependence of the piezotronic effect from 77 to 300 K. In general, lowering the temperature results in a largely enhanced piezotronic effect. The experimental results show that the behaviors can be divided into three groups depending on the carrier doping level or conductivity of the ZnO nanowires. For nanowires with a low carrier density (10(17)/cm(3) at 77 K), the pieozotronic effect is dominant at low temperature for dictating the transport properties of the nanowires; an opposite change of Schottky barrier heights at the two contacts as a function of temperature at a fixed strain was observed for the first time. At a moderate doping (between 10(17)/cm(3) and 10(18)/cm(3) at 77 K), the piezotronic effect is only dominant at one contact, because the screening effect of the carriers to the positive piezoelectric polarization charges at the other end (for n-type semiconductors). For nanowires with a high density of carriers (10(18)/cm(3) at 77 K), the piezotronic effect almost vanishes. This study not only proves the proposed fundamental mechanism of piezotronic effect, but also provides guidance for fabricating piezotronic devices.

Piezoelectricity is closely related with the performance and application of piezoelectric devices. It is a crucial issue to understand its detailed fundamental for designing functional devices with more peculiar performances. Basing on the first principles simulations, the ZnO piezoelectric tunnel junction is taken as an example to systematically investigate its piezoelectricity (including the piezopotential energy, piezoelectric field, piezoelectric polarization and piezocharge) and explore their correlation. The comprehensive picture of the piezoelectricity in the ZnO tunnel junction is revealed at atomic scale and it is verified to be the intrinsic characteristic of ZnO barrier, independent of its terminated surface but dependent on its c axis orientation and the applied strain. In the case of the ZnO c axis pointing from right to left, an in-plane compressive strain will induce piezocharges (and a piezopotential energy drop) with positive and negative signs (negative and positive signs) emerging respectively at the left and right terminated surfaces of the ZnO barrier. Meanwhile a piezoelectric polarization (and a piezoelectric field) pointing from right to left (from left to right) are also induced throughout the ZnO barrier. All these piezoelectric physical quantities would reverse when the applied strain switches from compressive to tensile. This study provides an atomic level insight into the fundamental behavior of the piezoelectricity of the piezoelectric tunnel junction and should have very useful information for future designs of piezoelectric devices.

The methods used to make the Film Bulk Acoustic Resonators (FBAR) tunable is reviewed. Intrinsically tunable FBARs make use ferroelectrics in ferroelectric and paraelectric (non-polar) phases have tunabilities up to 5% and more. Extrinsically tunable resonators are based on traditional piezoelectric FBARs and make use of external varactors and inductors. The low tunability (

The performance of 2DEG in AlGaN/GaN HEMTs was investigated using the self-consistent solution of Poisson and Schrodinger equations, which in the simulation, the effects of the spontaneous and piezoelectric polarization were included. An optimized structure with cap layer yielded an increase in effective barrier height, with little penalty in sheet carrier concentration. The structure due to AlN interlayer was investigated by using the energy band calculation.

GaN material has a very good potential in today's semiconductor world because of its highlighted characters like wide band gap and high saturation velocity. The combined effect of piezoelectric polarization and spontaneous polarization provides a high electron concentration called 2DEG in Al 0.3 Ga 0.7 N/GaN interface which allows higher mobility of carriers in interface than in bulk GaN material. 2-D simulation of power Al 0.3 Ga 0.7 N/GaNHFET device is carried out here and the effect of different parameters are analysed and studied using Sentaurus TCAD software. Prameters like I d V g , I d V d , g m etc are obtained. Several analysis are done with source to gate length, passivation layer etc.

The aim of this work is to study the potential offered by microwave power in the device AlGaN/GaN HEMT by studying the thermal effect and self heating on the output conductance taking into account the effects of spontaneous and piezoelectric polarization.

Strained II–VI and III–V semiconductor heterostructures in general are expected to have a piezoelectric polarization with a non-zero component in the plane of the interfaces which generates lateral piezoelectric fields. We use InAs structures synthesized on vicinal (110)-GaAs surfaces as a model system to study such fields for the first time. These lateral fields manifest themselves in strong blueshifts of the photoluminescence peaks and an energy dependence of the radiative lifetime. Our experiments unambiguously prove the existence of these electric fields and furthermore demonstrate their tremendous impact on the electronic and optical properties of our InAs/GaAs-samples.