Polarization-induced Internal Electric Fields Research Articles

Solution-Grown MAPbBr3 Single Crystals for Self-Powered Detection of X-rays with High Energies above One Megaelectron Volt.

Perovskite single crystals are actively studied as X-ray detection materials with enhanced sensitivity. Moreover, the feasibility of using perovskites for self-powered devices such as photodetectors, UV detectors, and X-ray detectors can significantly expand their application range. In this work, the charge carrier transport and photocurrent properties of MAPbBr3 single crystals (MSCs) are improved by the mechanochemical surface treatment using glycerin combined with an additional electrode design that forms an ohmic contact. The sensitivity of MSC-based detectors and pulse shape generated by X-rays are enhanced at various bias voltages. The synthesized MSC detectors generate direction-dependent photocurrents, which indicate the presence of a polarization-induced internal electric field. In addition, photocurrent signals are produced by X-rays with energies greater than 1 MeV under a zero-bias voltage. This work demonstrates a high application potential of perovskites as self-powered detectors for X-rays with energies exceeding 1 MeV.

Self-powered dual-wavelength polarization-sensitive photodetectors based on ZnO/BiFeO3 heterojunction

The ferroelectric-photovoltaic (FE-PV) devices have received growing attention for the FE polarization-induced internal electric field to facilitate the separation and transport of the photogenerated electron-hole (e-h) pairs in the FE semiconductors. However, the low photocurrent output of the FE-PV devices is still a key challenge for the application and development of these devices. Here, ZnO/BiFeO3 (BFO) heterojunction photodetectors (PDs) are designed to utilize the built-in electric field at the heterojunction interface of ZnO nanoparticles (NPs)/BFO and the spontaneous FE polarization field of BFO to promote the separation and transport of the photogenerated carriers. ZnO nanorod arrays (NRs) grown on ZnO NPs provide a directional transport channel for the photogenerated electrons. ZnO/BFO heterojunction devices demonstrate a self-powered photoresponse in the ultraviolet (UV) and visible (vis) dual-wavelength. The devices show a responsivity of 1.32 mA/W, response time (τrise/τdecay) of 12.6/44.9 ms at 420 nm light illumination. In addition, the polarization imaging function of ZnO/BFO PDs is explored by extending the polarization photocurrent mapping values of a single PD unit to a complex array of optical information image processing utilizing the polarization-sensitive properties of BFO without external bias.

A Systematic Exploration of InGaN/GaN Quantum Well-Based Light Emitting Diodes on Semipolar Orientations -=SUP=-*-=/SUP=-

Light-emitting diodes (LEDs) based on group III-nitride semiconductors (GaN, AlN, and InN) are crucial elements for solid-state lighting and visible light communication applications. The most widely used growth plane for group III-nitride LEDs is the polar plane (c-plane), which is characterized by the presence of a polarization-induced internal electric field in heterostructures. It is possible to address long-standing problems in group III-nitride LEDs, by using semipolar and nonpolar orientations of GaN. In addition to the reduction in the polarization-induced internal electric field, semipolar orientations potentially offer the possibility of higher indium incorporation, which is necessary for the emission of light in the visible range. This is the preferred growth orientation for green/yellow LEDs and lasers. The important properties such as high output power, narrow emission linewidth, robust temperature dependence, large optical polarization ratio, and low-efficiency droop are demonstrated with semipolar LEDs. To harness the advantages of semipolar orientations, comprehensive studies are required. This review presents the recent progress on the development of semipolar InGaN/GaN quantum well LEDs. Semipolar InGaN LED structures on bulk GaN substrates, sapphire substrates, free-standing GaN templates, and on Silicon substrates are discussed including the bright prospects of group III-nitrides. Keywords: Group III-nitride semiconductor, semipolar, light-emitting diodes, InGaN/GaN quantum well.

Piezoelectric polarization-induced internal electric field manipulation of the photoelectrochemical performance in Nd, Co codoped BiFeO3

Investigation of the synergistic mechanism of element doping and piezoelectric polarization to improve the catalytic activity of BiFeO3.

Polarization-induced internal electric field to manipulate piezo-photocatalytic and ferro-photoelectrochemical performance in bismuth ferrite nanofibers

Developing lead-free ferroelectrics BiFeO3 with polarized electric field for tuning charge-transport properties in piezo-photocatalytic and ferro-photoelectrochemical (PEC) is highly desired but also challenging, especially defects such as impurity phases and oxygen vacancies lead to the weak polarization and large leakage current of BiFeO3. Here, we used a facile electrospinning strategy to modify BiFeO3 nanofibers by A-site Pr ion and B-site Mn ion co-doping. In this way, the concentrations of oxygen vacancies and valence of Fe3+ to Fe2+ were significantly inhibited, and the morphotropic phase boundary (MPB) of the rhombohedral (R) to tetragonal (T) phase was obtained, resulting in better ferroelectric performances and lower leakage current. Thus, BiPrFeMnO3 nanofibers was able to generate a large piezoelectric potential through magnetic stirring (piezoelectric effect) and light irradiation (photocatalytic effect), resulting in superior piezo-photocatalytic performance with a degradation rate of 0.1352 min−1 for rhodamine B, which was 8.29, 4.3 and 4.2 times higher than that of BiFeO3, BiPrFeO3 and BiFeMnO3, respectively. In addition, optimized PEC performance by controlling the polarization state was observed in BiPrFeMnO3. The photocurrent could be effectively tuned by more than 16 times (8.2 −131.2 μA·cm−2 at 0 V vs Ag/AgCl) under irradiation of simulated sunlight by tuning the poling voltage between + 4 and − 4 V. Meanwhile, the onset potential switched from − 0.16 to − 0.18 V, which was favorable for the PEC reactions. Our present work gives a clear understanding of the role of ferroelectric polarization and solar energy conversion and provides a way to develop highly efficient piezo-/ferroelectric nanomaterials.

Recent Developments in Semipolar InGaN Laser Diodes

Group III-nitride semiconductors (GaN, AlN, and InN) are attractive materials for a wide range of electronic and photonic applications. The most widely employed growth plane for III-nitrides is the polar plane, characterized by the presence of a polarization-induced internal electric field in heterostructures. To eliminate the deleterious effects of polarization, III-nitride devices grown on nonpolar and semipolar orientations have become a major area of research. In addition to the reduction in the polarization-induced internal electric field, semipolar orientations potentially offer the possibility of higher indium incorporation, which is necessary for the emission of light in the visible range and is the preferred growth orientation for green/yellow light-emitting diodes and lasers. This review presents the recent progress on the development of semipolar InGaN quantum well laser diodes. The developments of laser diodes in three different semipolar planes such as (11–22), (20–21), and (20–2–1) planes are discussed including the bright prospects of group III-nitrides.

Polarization-Induced Internal Electric Field to Manipulate Piezo-Photocatalytic and Ferro-Photoelectrochemical Performance in Bismuth Ferrite Nanofibers

Polarization-Induced Internal Electric Field to Manipulate Piezo-Photocatalytic and Ferro-Photoelectrochemical Performance in Bismuth Ferrite Nanofibers

Recent Advances of Ferro-, Piezo-, and Pyroelectric Nanomaterials for Catalytic Applications

Ferro-, piezo-, and pyroelectric materials are emerging as potential candidates for converting various forms of primary energy from the ambient environment (e.g., sunlight, mechanical, and thermal energy) into secondary energy (e.g., chemical energy). Despite the relatively short investigation time, much progress has been made related to this field. This review covers the fundamental principles of coupling ferro-, piezo-, and pyroelectric effects with different catalytic reactions; the crucial role of a polarization-induced internal electric field in charge separation and transport in these materials is discussed. We particularly focus on recent notable examples of using these three types of nanostructured materials for a variety of catalytic applications in the fields of renewable energy production (e.g., water splitting and CO2 reduction), environmental remediation (e.g., organic pollutant decontamination), and materials synthesis (e.g., selective growth/deposition and organic synthesis). Finally, we conclude this review by proposing critical challenges and future perspectives for developing ferro-, piezo-, and pyroelectric nanomaterial-based catalysts for efficient energy harvesting.

Internal-Field-Enhanced Charge Separation in a Single-Domain Ferroelectric PbTiO3 Photocatalyst.

Ferroelectric materials with spontaneous polarization-induced internal electric fields have drawn increasing attention in solar fuel production due to the intrinsic polarized structure. However, the origination of charge separation in these materials at the nano/microlevel is ambiguous owing to the complexity of the multielectric fields. Besides, the observed charge separation ability is far from theoretical expectation. Herein, by spatially resolved surface photovoltage spectroscopy, it is clearly demonstrated that the depolarization field in single-domain ferroelectric PbTiO3 (PTO) nanoplates is the main driving force for charge separation and it can effectively drive photogenerated electrons and holes to the positive and negative polarization facets, respectively. Moreover, the charge separation ability of PTO nanoplates increases with increasing particle size along the polarization direction, due to the increasing potential difference between the opposite polarization facets. Furthermore, this driving force for charge separation directly contributes to the enhancement of the photocatalytic hydrogen evolution reaction activity in ferroelectrics. Finally, it is proved that the screening field compensates part of the depolarization field and can be diminished by adding a dielectric layer on the ferroelectric surface. These findings demonstrate the importance of increasing the depolarization field and decreasing the screening field for efficient charge separation in ferroelectric semiconductor photocatalysts.

Electrical Polarization in AlN/GaN Nanodisks Measured by Momentum-Resolved 4D Scanning Transmission Electron Microscopy.

We report the mapping of polarization-induced internal electric fields in AlN/GaN nanowire heterostructures at unit cell resolution as a key for the correlation of optical and structural phenomena in semiconductor optoelectronics. Momentum-resolved aberration-corrected scanning transmission electron microscopy is employed as a new imaging mode that simultaneously provides four-dimensional data in real and reciprocal space. We demonstrate how internal mesoscale and atomic electric fields can be separated in an experiment, which is verified by comprehensive dynamical simulations of multiple electron scattering. A mean difference of 5.3±1.5 MV/cm is found for the polarization-induced electric fields in AlN and GaN, being in accordance with dedicated simulations and photoluminescence measurements in previous publications.

Bias-Controlled Optical Transitions in GaN/AlN Nanowire Heterostructures.

We report on the control and modification of optical transitions in 40× GaN/AlN heterostructure superlattices embedded in GaN nanowires by an externally applied bias. The complex band profile of these multi-nanodisc heterostructures gives rise to a manifold of optical transitions, whose emission characteristic is strongly influenced by polarization-induced internal electric fields. We demonstrate that the superposition of an external axial electric field along a single contacted nanowire leads to specific modifications of each photoluminescence emission, which allows to investigate and identify their origin and to control their characteristic properties in terms of transition energy, intensity and decay time. Using this approach, direct transitions within one nanodisc, indirect transitions between adjacent nanodiscs, transitions at the top/bottom edge of the heterostructure, and the GaN near-band-edge emission can be distinguished. While the transition energy of the direct transition can be shifted by external bias over a range of 450 meV and changed in intensity by a factor of 15, the indirect transition exhibits an inverse bias dependence and is only observable and spectrally separated when external bias is applied. In addition, by tuning the band profile close to flat band conditions, the direction and magnitude of the internal electric field can be estimated, which is of high interest for the polar group III-nitrides. The direct control of emission properties over a wide range bears possible application in tunable optoelectronic devices. For more fundamental studies, single-nanowire heterostructures provide a well-defined and isolated system to investigate and control interaction processes in coupled quantum structures.

A comparative study of efficiency droop and internal electric field for InGaN blue lighting-emitting diodes on silicon and sapphire substrates

We investigated the efficiency droop and polarization-induced internal electric field of InGaN blue light-emitting diodes (LEDs) grown on silicon(111) and c-plane sapphire substrates. The efficiency droop of the LED sample grown on silicon substrates was considerably lower than that of the identically fabricated LED sample grown on sapphire substrates. Consequently, the LED on silicon showed higher efficiency at a sufficiently high injection current despite the lower peak efficiency caused by the poorer crystal quality. The reduced efficiency droop for the LED on silicon was attributed to its lower internal electric field, which was confirmed by reverse-bias electro-reflectance measurements and numerical simulations. The internal electric field of the multiple quantum wells (MQWs) on silicon was found to be reduced by more than 40% compared to that of the MQWs on sapphire, which resulted in a more homogenous carrier distribution in InGaN MQWs, lower Auger recombination rates, and consequently reduced efficiency droop for the LEDs grown on the silicon substrates. Owing to its greatly reduced efficiency droop, the InGaN blue LED on silicon substrates is expected to be a good cost effective solution for future lighting technology.

Terahertz intersubband photodetectors based on semi-polar GaN/AlGaN heterostructures

Terahertz intersubband photodetectors are developed based on GaN/AlGaN quantum wells grown on a free-standing semi-polar (202¯1¯) GaN substrate. These quantum wells are nearly free of the polarization-induced internal electric fields that severely complicate the design of nitride intersubband devices on traditional c-plane substrates. As a result, a promising bound-to-quasi-bound THz photodetector design can be implemented. Pronounced photocurrent peaks at the design frequency near 10 THz are measured, covering frequencies that are fundamentally inaccessible to existing arsenide intersubband devices due to reststrahlen absorption. This materials system provides a favorable platform to utilize the intrinsic advantages of nitride semiconductors for THz optoelectronics.

Mid-infrared Photoconductive Response in AlGaN/GaN Step Quantum Wells.

AlGaN/GaN quantum structure is an excellent candidate for high speed infrared detectors based on intersubband transitions. However, fabrication of AlGaN/GaN quantum well infrared detectors suffers from polarization-induced internal electric field, which greatly limits the carrier vertical transport. In this article, a step quantum well is proposed to attempt solving this problem, in which a novel spacer barrier layer is used to balance the internal electric field. As a result, a nearly flat band potential profile is obtained in the step barrier layers of the AlGaN/GaN step quantum wells and a bound-to-quasi-continuum (B-to-QC) type intersubband prototype device with detectable photocurrent at atmosphere window (3–5 μm) is achieved in such nitride semiconductors.

Pseudo-square AlGaN/GaN quantum wells for terahertz absorption

THz intersubband transitions are reported down to 160 μm within AlGaN/GaN heterostructures following a 4-layer quantum well design. In such a geometry, the compensation of the polarization-induced internal electric field is obtained through creating a gradual increase in polarization field throughout the quantum “trough” generated by three low-Al-content layers. The intersubband transitions show tunable absorption with respect to doping level as well as geometrical variations which can be regulated from 53 to 160 μm. They also exhibit tunnel-friendly designs which can be easily integrated into existing intersubband device architectures.

Probing the internal electric field in GaN/AlGaN nanowire heterostructures.

We demonstrate the direct analysis of polarization-induced internal electric fields in single GaN/Al0.3Ga0.7N nanodiscs embedded in GaN/AlN nanowire heterostructures. Superposition of an external electric field with different polarity results in compensation or enhancement of the quantum-confined Stark effect in the nanodiscs. By field-dependent analysis of the low temperature photoluminescence energy and intensity, we prove the [0001̅]-polarity of the nanowires and determine the internal electric field strength to 1.5 MV/cm.

Screening of the quantum-confined Stark effect in AlN/GaN nanowire superlattices by germanium doping

We report on electrostatic screening of polarization-induced internal electric fields in AlN/GaN nanowire heterostructures with germanium-doped GaN nanodiscs embedded between AlN barriers. The incorporation of germanium at concentrations above 1020 cm–3 shifts the photoluminescence emission energy of GaN nanodiscs to higher energies accompanied by a decrease of the photoluminescence decay time. At the same time, the thickness-dependent shift in emission energy is significantly reduced. In spite of the high donor concentration, a degradation of the photoluminescence properties is not observed.

Intraband Absorption in Self-Assembled Ge-Doped GaN/AlN Nanowire Heterostructures

We report the observation of transverse-magnetic-polarized infrared absorption assigned to the s-p(z) intraband transition in Ge-doped GaN/AlN nanodisks (NDs) in self-assembled GaN nanowires (NWs). The s-p(z) absorption line experiences a blue shift with increasing ND Ge concentration and a red shift with increasing ND thickness. The experimental results in terms of interband and intraband spectroscopy are compared to theoretical calculations of the band diagram and electronic structure of GaN/AlN heterostructured NWs, accounting for their three-dimensional strain distribution and the presence of surface states. From the theoretical analysis, we conclude that the formation of an AlN shell during the heterostructure growth applies a uniaxial compressive strain which blue shifts the interband optical transitions but has little influence on the intraband transitions. The presence of surface states with density levels expected for m-GaN plane charge-deplete the base of the NWs but is insufficient to screen the polarization-induced internal electric field in the heterostructures. Simulations show that the free-carrier screening of the polarization-induced internal electric field in the NDs is critical to predicting the photoluminescence behavior. The intraband transitions, on the other hand, are blue-shifted due to many-body effects, namely, the exchange interaction and depolarization shift, which exceed the red shift induced by carrier screening.

Interaction between Si doping and the polarization-induced internal electric field in the AlGaN/GaN superlattice

AlGaN/GaN superlattices (SLs) with and without Si doping exhibit very different properties. Because of the difference between the dielectric constants of AlGaN and GaN, the wells of the SL are depleted in the undoped structure. With increased Si doping in the GaN wells, the depletion effect will vanish, and the accumulation of electrons will compensate for the polarization-induced internal electric field (PIIEF) in the AlGaN barrier, which is followed by disturbance of the PIIEF in the GaN wells due to electron overflow from the ground state E0 to the first excited state E1. This leads to a decrease in E1-E0.

Midinfrared intersubband absorption in GaN/AlGaN superlattices on Si(111) templates

We report on the observation of midinfrared intersubband absorption in Si-doped GaN/AlGaN superlattices grown by plasma-assisted molecular-beam epitaxy on semi-insulating GaN-on-Si(111) templates. TM-polarized absorption attributed to transition between the first two electronic levels in the quantum wells peaked in the range from 2 to 9 μm. The relative spectral width remains around 20% in the whole midinfrared spectral range. Doping is predicted to have a large influence on the intersubband absorption energy due to screening of polarization-induced internal electric field.