Strong Piezoelectric Field Research Articles

Enhancing the piezocatalytic performance for H2 evolution by constructing BiOCl/UiO-66 heterostructure

Developing highly efficient piezocatalytic system for hydrogen production is crucial in addressing the current energy crisis. Herein, we synthesized BiOCl/UiO-66 heterostructure piezocatalyst using facile one-step solvothermal method and evaluated its performance for piezocatalytic H2 evolution under ultrasonic vibration. Our findings reveal that BiOCl/UiO-66 heterostructure demonstrates excellent catalytic activity, yielding high piezocatalytic H2 evolution rate of 650.9 μmol g−1 h−1, significantly surpassing that of pure BiOCl (176.5 μmol g−1 h−1) and UiO-66 (352.6 μmol g−1 h−1). Furthermore, the BiOCl/UiO-66 presents good stability throughout cyclic experiments. The improved catalytic performance of BiOCl/UiO-66 is mainly attributed to the formation of heterostructure, which can induce more active sites and the strong piezoelectric field that aids in charge carriers separation. A possible mechanism of BiOCl/UiO-66 heterostructure is proposed. This work offers practical approach to boost the hydrogen production efficiency of Bi-based piezocatalyst.

Strain modulation effect of superlattice interlayer on InGaN/GaN multiple quantum well

The strong piezoelectric field in InGaN/GaN heterostructure quantum wells severely reduces the light emission efficiency of multiple quantum well (MQW) structures. To address this issue, a strain modulation interlayer is commonly used to mitigate the piezoelectric polarization field and improve the luminescence performance of the devices. To investigate the influence and mechanism of strain modulation in the InGaN/GaN superlattice (SL), epitaxial wafers with an n-type InGaN/GaN SL interlayer sample, and their corresponding control samples are prepared. The measured temperature-dependent photoluminescence (PL) spectra of the epitaxial wafers, show that the introduction of an SL interlayer leads to a shorter-wavelength emission and enhancement of internal quantum efficiency. As the temperature increases, a blue shift of the PL peak is observed. However, for the sample with an SL interlayer, the blue shift of the PL peak with temperature increasing is relatively small. Electroluminescence (EL) experiments indicate that the introduction of an SL interlayer significantly increases the integrated intensity of the EL peak and reduces its full width at half maximum. These phenomena collectively indicate that the incorporation of a superlattice interlayer can partly suppress the quantum-confined Stark effect (QCSE) that affects the light emission efficiency. Theoretical calculations show that the introduction of a superlattice strain layer before growing an active multiple quantum well can weaken the polarization-induced built-in electric field in the active quantum well, reduce the tilt of the energy band in the multiple quantum well active region, increase the overlap of electron and hole wave functions, enhance the emission probability, shorten the radiative recombination lifetime, and promote competition between radiative recombination and non-radiative recombination, thereby achieving higher recombination efficiency and improving light emission intensity. This study provides experimental and theoretical evidence that the strain modulation SL interlayer can effectively improve the device performance and offer guidance for optimizing the structural design of devices.

Bright Emission at Reverse Bias After Trailing Edge of Driving Pulse in Wide InGaN Quantum Wells

In group‐III‐nitride quantum wells (QWs), a strong piezoelectric field is formed. A built‐in potential from the p–n junction is working in the opposite direction depending on an externally applied voltage. Furthermore, the electric field in the QW can be partially screened by a high charge carrier density. Herein, the influence of these effects on recombination and spectrum for over 10 nm wide InGaN QWs is investigated. The very low overlap of electrons and holes would suggest inefficient devices. However, it is shown that thick QWs can be more effective and reach high optical gain. This can be explained by the screening of the electric field, resulting in a high overlap of excited electron and hole states that enable lasing. Herein, a pulsed electrical excitation scheme is used, where carrier injection at forward bias and predominant recombination at zero or reverse bias are separated in time. The interplay between the piezoelectric field and the built‐in potential on carrier recombination in dependence on an external bias voltage is observed. In particular, a strong increase of the radiative recombination rate after the trailing edge of the driving pulse is observed.

Factors Affecting Terahertz Emission from InGaN Quantum Wells under Ultrafast Excitation

InGaN quantum wells (QWs) grown on c-plane sapphire substrate experience strain due to the lattice mismatch. The strain generates a strong piezoelectric field in QWs that contributes to THz emission under ultrafast excitation. Physical parameters such as QW width, period number, and Indium concentration can affect the strength of the piezoelectric field and result in THz emission. Experimental parameters such as pump fluence, laser energy, excitation power, pump polarization angle, and incident angle can be tuned to further optimize the THz emission. This review summarizes the effects of physical and experimental parameters of THz emission on InGaN QWs. Comparison and relationship between photoluminescence properties and THz emission in QWs are given, which further explains the origin of THz emission in InGaN QWs.

Ball milling induced strong polarization electric fields in Cu3B2O6 crystals for high efficiency piezocatalysis

Harvesting mechanical energy by piezocatalyst from ultrasonication offers an unprecedented opportunity for environment remediation. However, the piezocatalytic efficiency is seriously hindered by current catalysts and the harvest of energy. Herein, for the first time, by agitation of a kind of borate like Cu3B2O6 (CBO) material through ball milling (BM) could decompose pollutants efficiently. The efficiency of CBO for Rhodamine B under BM is 6.4 times that of benchmark BaTiO3, and 10.4 times that of CBO under ultrasonication. Moreover, CBO also exhibits an excellent performance for the inactivation of Escherichia coil K-12 and a good catalytic cycle stability. The high catalytic efficiency can be attributed to the formed strong piezoelectric field in CBO and abundant reactive species generated under BM. The results give an insight into the development of high efficient piezocatalytic reactions based on the piezocatalysts and BM combined strategy.

Atomistic analysis of piezoelectric potential fluctuations in zinc-blende InGaN/GaN quantum wells: A Stillinger-Weber potential based analysis

In this paper we investigate strain and local polarization field effects in zinc-blende indium gallium nitride (InGaN) alloys and quantum wells. To do so we parametrize and establish a Stillinger-Weber potential with parameters fitted to hybrid functional density functional theory data. The developed model gives very good agreement with quantities to which it has not been fitted, such as Kleinman parameters of cubic III-N materials or the composition dependence of the lattice constant in InGaN alloys. Equipped with this model, we extract the composition dependence of elastic constants ${C}_{11}$ and ${C}_{12}$ in InGaN alloys, including bowing parameters for these quantities, which may form input for continuum-based calculations. Furthermore, applying this model to InGaN alloys and wells reveals that random alloy fluctuations can lead to strong local strain field fluctuations. Building on this information, we present a model that allows for the calculation of connected local built-in field fluctuations at the microscopic level, accounting for first- and second-order piezoelectric effects. The approach is general and can be applied to any zinc-blende III--V alloy or heterostructure investigated in the frame of semiempirical models (e.g., valence force field models) targeting strain fields on an atomistic level. Here, building on our Stillinger-Weber potential we show that local strain fluctuations in zinc-blende InGaN quantum wells can lead to strong piezoelectric built-in field fluctuations. This contribution has been widely overlooked in previous theoretical studies of these systems. Finally, we briefly discuss the impact of these polarization field fluctuations on carrier localization effects in such quantum well systems.

Disorder effects in nitride semiconductors: impact on fundamental and device properties

AbstractSemiconductor structures used for fundamental or device applications most often incorporate alloy materials. In “usual” or “common” III–V alloys, based on the InGaAsP or InGaAlAs material systems, the effects of compositional disorder on the electronic properties can be treated in a perturbative approach. This is not the case in the more recent nitride-based GaInAlN alloys, where the potential changes associated with the various atoms induce strong localization effects, which cannot be described perturbatively. Since the early studies of these materials and devices, disorder effects have indeed been identified to play a major role in their properties. Although many studies have been performed on the structural characterization of materials, on intrinsic electronic localization properties, and on the impact of disorder on device operation, there are still many open questions on all these topics. Taking disorder into account also leads to unmanageable problems in simulations. As a prerequisite to address material and device simulations, a critical examination of experiments must be considered to ensure that one measures intrinsic parameters as these materials are difficult to grow with low defect densities. A specific property of nitride semiconductors that can obscure intrinsic properties is the strong spontaneous and piezoelectric fields. We outline in this review the remaining challenges faced when attempting to fully describe nitride-based material systems, taking the examples of LEDs. The objectives of a better understanding of disorder phenomena are to explain the hidden phenomena often forcing one to use ad hoc parameters, or additional poorly defined concepts, to make simulations agree with experiments. Finally, we describe a novel simulation tool based on a mathematical breakthrough to solve the Schrödinger equation in disordered potentials that facilitates 3D simulations that include alloy disorder.

Atomic ordering and bond relaxation in optical spectra of self-organized InP/GaInP2 Wigner molecule structures

We used transmission electron microscopy, Raman, and photoluminescence spectroscopy to identify the effect of CuPt-type GaP-InP atomic ordering (AO) on the structural and emission properties of self-organized (SO) InP/GaInP2 Wigner molecule (WM) quantum dot (QD) structures. We found that the correlation of AO and SO growth results in the formation of InP/GaInP2 QD/AO-domain (QD/AOD) core-shell composites. This observation shows that intrinsic WMs in this system emerge due to a strong piezoelectric field generated by AODs, which induces QD doping and a built-in magnetic field. We found that the bond relaxation of AODs leads to a decrease in the emission energy of WMs of 80 meV. The photoluminescence spectra of single WMs having an emission energy ∼1.53 eV are presented here, the lowest one reported for this system.

The electron–hole overlap in the parabolic quantum well light emitting diode is much superior to the rectangular: even to that of a staggered quantum well

The strong piezoelectric field in the InxGa1−xN/GaN quantum well (QW) LEDs, separates the electrons and holes spatially, which decreases the luminescence. Various shapes and compositions of such QWs are studied to improve the performance. We have studied the transition energy (TE), overlap of electron and hole wave functions, band structures and field distributions of the parabolic QWs (PQW) through the self consistent solutions of Schrodinger and Poisson equations. The shape of the PQW is varied along with the compositions and dopings. The square of the overlap of electron and hole wave functions i.e. the transition probability (TP) is strikingly increased, compared to the rectangular QW and it is even higher than the symmetrically staggered QW. At a particular current density, for the same TE, the TP of the PQW increases more than two times that of the rectangular QWs. An important feature, desirable for the QW LEDs emerge. The change of the TE with increase in the current density is minimized. A brief theory, computational procedures and the results will be presented in details with suitable discussions.

High hole concentration in Mg-doped AlN/AlGaN superlattices with high Al content

We studied hole generation in Mg-doped AlN/Al0.75Ga0.25N superlattices (SLs) with an average Al content of 0.8. High hole concentrations on the order of 1018 cm−3 were obtained in the SLs. The temperature dependence of the hole concentration indicated an effective acceptor ionization energy of 40–67 meV, which is much lower than that of Mg-doped Al0.8Ga0.2N alloy (>400 meV). The hole concentration increased with increasing SL period thickness and became almost constant at about 30 nm. These results indicate that the band bending caused by strong spontaneous and piezoelectric polarization fields enhances the ionization of the Mg acceptors.

Screening effects of photogenerated carriers on terahertz radiation from coherent GaAs-like longitudinal optical phonons in (11n)-oriented GaAs/In0.1Al0.9As strained multiple quantum wells

We investigated terahertz radiation from coherent GaAs-like longitudinal optical (LO) phonons in (11n)-oriented GaAs/In0.1Al0.9As strained multiple quantum wells for clarifying the screening effects of photogenerated carriers. We observed the intense quasi-monochromatic terahertz wave from the coherent GaAs-like LO phonon, which originates from the initial polarization enhanced by the strong piezoelectric field. The intensity of the coherent GaAs-like LO-phonon band exhibited a saturation behavior as the pump power was increased. We evaluated the saturation behavior in terms of excitation efficiency of the terahertz wave from the coherent GaAs-like LO phonon using the parameter of unit-power intensity. From the pump-power dependence of the unit-power intensity, we conclude that the screening effect of high density photogenerated carriers on the piezoelectric field causes saturation of the terahertz-wave intensity from the coherent GaAs-like LO phonon.

Combined electrical and resonant optical excitation characterization of multi-quantum well InGaN-based light-emitting diodes

We present a comprehensive study of the emission spectra and electrical characteristics of InGaN/GaN multi-quantum well light-emitting diode (LED) structures under resonant optical pumping and varying electrical bias. A 5 quantum well LED with a thin well (1.5 nm) and a relatively thick barrier (6.6 nm) shows strong bias-dependent properties in the emission spectra, poor photovoltaic carrier escape under forward bias and an increase in effective resistance when compared with a 10 quantum well LED with a thin (4 nm) barrier. These properties are due to a strong piezoelectric field in the well and associated reduced field in the thicker barrier. We compare the voltage ideality factors for the LEDs under electrical injection, light emission with current, photovoltaic mode (PV) and photoluminescence (PL) emission. The PV and PL methods provide similar values for the ideality which are lower than for the resistance-limited electrical method. Under optical pumping the presence of an n-type InGaN underlayer in a commercial LED sample is shown to act as a second photovoltaic source reducing the photovoltage and the extracted ideality factor to less than 1. The use of photovoltaic measurements together with bias-dependent spectrally resolved luminescence is a powerful method to provide valuable insights into the dynamics of GaN LEDs.

Piezoelectric polarization and quantum size effects on the vertical transport in AlGaN/GaN resonant tunneling diodes**Project supported by the Deanship of Scientific Research of University of Dammam (Grant No. 2014137).

In this work, the electronic properties of resonant tunneling diodes (RTDs) based on GaN-AlxGa(1−x)N double barriers are investigated by using the non-equilibrium Green functions formalism (NEG). These materials each present a wide conduction band discontinuity and a strong internal piezoelectric field, which greatly affect the electronic transport properties. The electronic density, the transmission coefficient, and the current–voltage characteristics are computed with considering the spontaneous and piezoelectric polarizations. The influence of the quantum size on the transmission coefficient is analyzed by varying GaN quantum well thickness, AlxGa(1−x)N width, and the aluminum concentration xAl. The results show that the transmission coefficient more strongly depends on the thickness of the quantum well than the barrier; it exhibits a series of resonant peaks and valleys as the quantum well width increases. In addition, it is found that the negative differential resistance (NDR) in the current–voltage (I–V) characteristic strongly depends on aluminum concentration xAl. It is shown that the peak-to-valley ratio (PVR) increases with xAl value decreasing. These findings open the door for developing vertical transport nitrides-based ISB devices such as THz lasers and detectors.

Polarization‐field control in nitride light emitters

Polarization‐field control in nitride light emitters

Fabrication and characterisation of non-polar M-plane AlN crystals and LEDs

Aluminium nitride crystals and devices grown along the polar c-axis direction are affected by the strong spontaneous and piezoelectric polarisation fields. It is necessary to grow non-polar plane aluminium nitride crystals in order to solve the problem. The M-plane surface-oriented aluminium nitride single crystals were grown by physical vapour transport on a resistance heating furnace, which was designed by ourselves and included two heaters on sublimation region and crystalline region, respectively. The M-plane aluminium nitride light-emitting diode was further fabricated and characterised. The current–voltage characteristic showed a Schottky diode behaviour. A white light emission from the biased diode can be clearly seen by naked eyes in a dark place at room temperature. The ultra-wide electroluminescent spectrum covered from deep UV (200 nm) to infrared (1100 nm) and centred at 496 nm (2·5 eV). At last, the emission mechanism was discussed.

Influence of piezoelectric fields on InGaN based intermediate band solar cells

As it is practically infeasible to fabricate multiple-junction InGaN based tandem solar cells due to an intrinsic limit, intermediate-band solar cells (IBSCs) provide an alternative option for the fabrication of single-junction solar cells with their performance potentially equivalent to that of multiple-junction solar cells. InGaN quantum dots (QD) could be used for designing an IBSC structure. More importantly, it is well-known that there exist very strong piezoelectric fields in an InGaN/GaN system with a high indium composition, which becomes more pronounced for InGaN based QDs. The built-in piezoelectric fields can lead to a significant increase in the open circuit voltage and thus improved performance of solar cells, which has not yet been considered in designing III-nitride based solar cells so far. An optimized InGaN based QD-IBSC structure has been designed, combining the major advantages from the IBSC structure and the benefits due to the strong piezoelectric fields. A conversion efficiency, open-circuit voltage and short-circuit current have been calculated, and a highest conversion efficiency of 55.4% is obtained. The combination of the single-junction IBSC structure and the piezoelectric fields paves the way for the fabrication of InGaN based single-junction solar cells with ultra-high energy efficiency.

Investigation on Polarization Induced Electro-Optical Property of GaN LED Using TEM-EBIC Combined with Cathodoluminescenc

Strained InGaN layers grown on a c-plane sapphire is known to have strong spontaneous and piezoelectric polarization field, where the discontinuity of polarization at heterojunction is to induce bounded surface charges [1]. These polarization-related phenomenon can affect the internal electric field of active region and induce the quantum-confined stark effect (QCSE) at quantum well. QCSE results in the spatial separation of electrons and holes in the quantum well, blue shift, and increase of carrier density [2]. To reduce the polarization induced QCSE, efforts were made by growing GaN-InGaN on semi-polar and non-polar substrates [3]. In this study, we report the polarization induced electro-optical property of polar, semi-polar, and non-polar GaN-InGaN LED in nano-scale.Electron-Beam Induced Current imaging (EBIC) in Transmission Electron Microscopy (TEM-EBIC) enables us to map the electrical response from region to region in nano-scale. When the EBIC is to combine with cathodoluminescence (CL) and microstructural analysis in transmission electron microscopy(TEM), emission characteristics can be directly correlated with the electrical properties and microstructure, possibly linked with the piezoelectric polarization and electrical junction property.Electrical responses with electron beam were mapped with microstructure and the point-to-point mapping of local emission characteristics using home-built TEM-EBIC/CL holder (Fig. 1). The holder was equipped with liquid nitrogen cooling finger to minimize the carrier diffusion. Spectrum collection and the manipulation of the spectrum was carried out using home-built software from the panchromatic and monochromatic spectrum. Ion-milling system, instead of using focused ion beam, was used to avoid the artifacts of unintentional noise coming from the surface recombination current. First, we measured I-V curve of 3 TEM samples of the polar, semi-, and non-polar specimen to acquire the built-in voltage, breakdown voltage, and leakage current. And then, we acquired STEM-EDS on each specimen to identify microstructure and thickness of active region. Second, EBIC signal was obtained from each points of target analysis area under reverse bias condition, which enables us to estimate the depletion region width and the polarization field across on the p-n junction. We also observed that the polarization filed was decreased from the sample grown on the c-plane to the one on the m-plane as expected. The minority carrier diffusion length showed dependency with polarization, which could arise from the space charge induced by polarization. CL spectra showed a tendency of blue-shift of emission in c-plane specimen as injection beam current increased, which might be originated from the bandgap fluctuation by the quantum-confined stark effect (QCSE) with strong polarization. CL spectra were obtained to confirm the polarization direction with the increase of reverse bias voltage. Peak shifts in CL might come from the cancellation of internal polarization with the external bias. Built-in voltage was also estimated with the polarization.[1] Fabio Bernardini et al., Phys. Rev. B 56, R10024 (1997)[2] Shigefusa F. Chichibu et al., Nature materials 5, 810 (2006)[3] Pan et al., APEX 5, 062103 (2012) Number 42.

Strain- and surface-induced modification of photoluminescence from self-assembled GaN/Al0.5Ga0.5N quantum dots: strong effect of capping layer and atmospheric condition

We report on the influence of a capping layer on the photoluminescence properties of self-assembled GaN quantum dots grown on an Al0.5Ga0.5N template. Self-assembled GaN quantum dots show a large quantum confined Stark shift and long carrier recombination time due to strong built-in spontaneous and piezoelectric polarization fields. Nevertheless, owing to strong carrier localization and suppressed nonradiative processes, these quantum dots have a high-quantum efficiency even at room temperature. Here, we show that the capping thickness has an important role on the optical properties of the GaN quantum dots. The radiative and nonradiative recombination processes of quantum dots are strongly affected by adjusting the capping thickness, and the GaN quantum dots with 12 monolayers-thick Al0.5Ga0.5N capping layer show a remarkably high internal quantum efficiency of more than 80% at room temperature. We also studied photoluminescence quenching and enhancement for surface (uncapped) quantum dots caused by photoadsorption and photodesorption of oxygen.

Investigation of properties of InGaN-based vertical-type solar cells with emission wavelengths in ultraviolet-blue-green regions

We investigate the properties of InGaN-based vertical-type solar cells having wavelengths ranging from the ultraviolet to green regions. It is well known that InGaN-based solar cells require a high indium composition to obtain high conversion efficiency. However, although InGaN-based solar cells with a high indium composition have been fabricated, their conversion efficiency has not sufficiently increased. Therefore, to further understand carrier transport, we measured the bias-dependent external quantum efficiency. For vertical-type green solar cells with a high indium composition, we confirmed that they have a higher short circuit current than other samples tested due to their broader overlapping region with the solar spectrum, though their fill factor remained low due to their high barrier height and strong piezoelectric field, which caused a reduction in the carrier tunneling rate.

Displacement Damage Effects in AlGaN/GaN High Electron Mobility Transistors

We present the results of a radiation damage experiment on AlxGa1-xN/GaN high electron mobility transistors. The basic mechanism underlying the observed high radiation tolerance appears to be a strong internal piezoelectric field near the two-dimensional electron gas that causes scattered carriers to be reinjected.