Spontaneous Polarization Field Research Articles

Impact of defect distribution on IrOx/ZnO interface doping and Schottky barriers

We used depth-resolved cathodoluminescence spectroscopy (DRCLS) to measure the nature and spatial distribution of native point defects at Zn- and O-polar ZnO interfaces with iridium oxide (IrOx) and their impact on Schottky barrier formation. IrOx and other metal oxides exhibit higher Schottky barriers than their pure metal counterparts, consistent with wider depletion regions and potentially useful for ohmic contacts to p-type semiconductors. DRCLS with I-V and 1/C2-V barrier height and carrier profile measurements showed high zinc vacancy VZn and CuZn defect densities that compensate free carrier densities, increase depletion widths, and form higher effective barriers than Ir/ZnO contacts. Zn-polar versus O-polar ZnO interfaces with IrOx exhibit 40% higher VZn + CuZn interface segregation and lower carrier densities within a wider depletion region, accounting for the significantly higher (0.89 vs. 0.67 eV) barrier heights. Both the depth of VZn density segregation and the Zn-deficient layer thickness measured microscopically match the depletion width and applied electric fields comparable to spontaneous polarization fields across similar layers displaying analogous defect segregation. These results account for the difference in polarity-dependent segregation due to the electric field-driven diffusion of native defects near ZnO interfaces.

Polarization Enhanced Charge Transfer: Dual-Band GaN-Based Plasmonic Photodetector

Here, we report a dual-band plasmonic photodetector based on Ga-polar gallium nitride (GaN) for highly sensitive detection of UV and green light. We discover that decoration of Au nanoparticles (NPs) drastically increases the photoelectric responsivities by more than 50 times in comparition to the blank GaN photodetector. The observed behaviors are attributed to polarization enhanced charge transfer of optically excited hot electrons from Au NPs to GaN driven by the strong spontaneous polarization field of Ga-polar GaN. Moreover, defect ionization promoted by localized surface plasmon resonances (LSPRs) is also discussed. This novel type of photodetector may shed light on the design and fabrication of photoelectric devices based on polar semiconductors and microstructural defects.

Simulation and Experimental Data of P-E Hysteresis Loop in BNT and BKT

Hysteresis loop measurements performed both experimentally for Bismuth Sodium Titanate (BNT) and Bismuth Potassium Titanate (BKT) samples using modified Sawyer-Tower Circuit. The experiment showed that the P-E hysteresis had main parameters for BNT, remnant polarization (Pr = 27 μC/cm2), spontaneous polarization (Ps = 35 μC/cm2) and coercive electric field (Ec = 60 kV/cm) and for BKT the remnant polarization was (Pr = 5.2 μC/cm2), spontaneous polarization (Ps = 30 μC/cm2) and coercive electric field (Ec = 4.72 kV/cm). These three parameters (remnant polarization, spontaneous polarization, coercive electric field) were used in a simulated software depending on the mathematical model for the polarization in ferroelectric materials. The simulation software predicted the value of applied electric field required to perform the P-E hysteresis experiment varying with Pr, Ps, Ec. The results of the simulation exhibited agreement with the experimental data. The last prediction could help the researchers in studying the ferroelectric hysteresis loop, especially for those studying a fatigue behaviour or studying the effect of electric field cycle on the hysteresis loop.

Ab initio and experimental studies of polarization and polarization related fields in nitrides and nitride structures

Spontaneous and piezoelectric polarization in the nitrides is analyzed. The slab model was designed and proved to be appropriate to obtain the spontaneous polarization in AlN, GaN and InN. The spontaneous polarization and polarization related electric fields in AlN, GaN and InN were determined using DFT slab calculations. The procedure generates single value of spontaneous polarization in the nitrides. It was shown that Berry phase polarization may be applied to determination of spontaneous polarization by appropriate addition of polarization induced electric fields. The electric fields obtained from slab model are consistent with the Berry phase results of Bernardini et al. The obtained spontaneous polarization values are: 8.69*10-3 C/m2, 1.88*10-3 C/m2, and 1.96*10-3 C/m2 for AlN, GaN and InN respectively. The related Berry phase polarization values are 8.69*10-2 C/m2, 1.92*10-2 C/m2, and 2.86*10-2 C/m2, for these three compounds, respectively. The GaN/AlN multiquantum wells (MQWs) were simulated using ab intio calculations. The obtained electric fields are in good agreement with those derived from bulk polarization values. GaN/AlN MQWs structures, obtained by MBE growth were characterized by TEM and X-ray measurements. Time dependent photoluminescence measurements were used to determine optical transition energies in these structures. The PL obtained energies are in good agreement with ab initio data confirming overall agreement between theoretical and experimental data.

Magneto-electric features of BaFe9.5Al1.5CrO19-CaCu3Ti4O12 nanocomposites

BaFe9.5Al1.5CrO19-xCaCu3Ti4O12 nanocomposites were prepared by a solid-state technique. The X-ray diffraction and field emission scanning electron microscopy were employed for evaluation of structural and morphological features. The electrical properties of samples were evaluated by ferroelectric tester. The vibrating sample magnetometer was employed for the analysis of magnetic properties of specimens. The microwave characteristics of BaFe9.5Al1.5CrO19-30CaCu3Ti4O12 by controlling matching thickness were also characterized by a vector network analyzer. The CaCu3Ti4O12 and nanocomposites exhibited ferroelectric behavior. The obtained results confirmed that the dielectric constant of CaCu3Ti4O12 did not undergo an abrupt variation with an increase in temperature and frequency. The spontaneous polarization and coercive field declined by addition of CaCu3Ti4O12. The coercive field was increased due to the surface anisotropy in BaFe9.5Al1.5CrO19-CaCu3Ti4O12 interface.

Surface and finite size effects impact on the phase diagrams, polar, and dielectric properties of (Sr,Bi)Ta2O9 ferroelectric nanoparticles

In the framework of the thermodynamic approach Landau-Ginzburg-Devonshire (LGD) combined with the equations of electrostatics, we investigated the effect of polarization surface screening on finite size effects of the phase diagrams, polar, and dielectric properties of ferroelectric nanoparticles of different shapes. We obtained and analyzed the analytical results for the dependences of the ferroelectric phase transition temperature, critical size, spontaneous polarization, and thermodynamic coercive field on the shape and size of the nanoparticles. The pronounced size effect of these characteristics on the scaling parameter, the ratio of the particle characteristic size to the length of the surface screening, was revealed. Also our modeling predicts a significant impact of the flexo-chemical effect (that is a joint action of flexoelectric effect and chemical pressure) on the temperature of phase transition, polar, and dielectric properties of nanoparticles when their chemical composition deviates from the stoichiometric one. We showed on the example of the stoichiometric nanosized SrBi2Ta2O9 particles that except the vicinity of the critical size, where the system splitting into domains has an important role, results of analytical calculation of the spontaneous polarization have a little difference from the numerical ones. We revealed a strong impact of the flexo-chemical effect on the phase transition temperature, polar, and dielectric properties of SryBi2+xTa2O9 nanoparticles when the ratio Sr/Bi deviates from the stoichiometric value of 0.5 within the range from 0.35 to 0.65. From the analysis of experimental data, we derived the parameters of the theory, namely, the coefficients of expansion of the LGD functional, the contribution of flexo-chemical effect, and the length of the surface screening.

Spontaneous polarization induced electric field in zinc oxide nanowires and nanostars

We report on the detection mechanism of spontaneous polarization using electrostatic force microscopy in zinc oxide nanowires and nanostars grown by vapor-liquid-solid technique. Optical and structural properties are investigated in detail to understand the complex ZnO nanostructures comprehensively. Calculations are carried out to estimate the electric field from the change in interleave amplitude induced by the electrostatic force due to the spontaneous polarization effects. Attraction of the probe between the tip and the sample varies for different structures with a stronger attraction for nanostars as compared to nanowires. Strength of electric field is dependent on the orientation of nanowires and nanostars c-axis with measured magnitude of electric field to be ∼107 V/m and 108 V/m respectively. This technique presents a unique detection mechanism of built-in spontaneous polarization and electric field from polar ZnO nanowires with applications in voltage gated ion channels, nano-bio interfaces, optoelectronic and photonic devices.

Effect of effective mass and spontaneous polarization on photocatalytic activity of wurtzite and zinc-blende ZnS

Semiconductor zinc sulphide (ZnS) has two common phases: hexagonal wurtzite and cubic zinc-blende structures. The crystal structures, energy band structures, density of states (DOS), bond populations, and optical properties of wurtzite and zinc-blende ZnS were investigated by the density functional theory of first-principles. The similar band gaps and DOS of wurtzite and zinc-blende ZnS were found and implied the similarities in crystal structures. However, the distortion of ZnS4 tetrahedron in wurtzite ZnS resulted in the production of spontaneous polarization and internal electric field, which was beneficial for the transfer and separation of photogenerated electrons and holes.

Investigation of the GaN-based light-emitting diodes with engineered energy band in InGaN multiple quantum wells

There is intense spontaneous polarization field in wurtzite nitride materials due to the lack of inversion symmetry. In addition, when the materials are strained, piezoelectric polarization filed will be produced. Hence, large piezoelectric and spontaneous polarizations is appeared in strained InGaN/GaN multiple quantum wells (MQWs), which is necessary materials for GaN-based blue light-emitting diodes (LEDs). The large polarization field will cause band bending in InGaN/GaN multiple quantum wells (MQWs), which caused by polarization effect results in electron-hole separation and reduces the efficiency of GaN-based light-emitting diodes (LEDs). In this paper, according to the polarization-doped theory, we engineer the energy band of InGaN MQWs by grading In composition to weaken band bending. Meanwhile, according to the polarization-doped theory,the polarization field in the graded InGaN also can induce high density hole in the quantum well. The electrical and optical performances of the conventional LED and the LED with engineered energy band are compared experimentally. Compared with the conventional LED, the optical output power and the EQE of the LED with engineered energy band are enhanced. These are ascribed to less energy band bending by engineering the energy band of InGaN MQWs, which is proved by less EL peak position energy blue shift with injection current increasing and less PL peak position energy blueshift with reverse voltage increasing for the LED with engineered energy band. The calculated results indicate that, the band bending of the LED with engineered energy band is weakened, and the hole concentration in the graded InGaN quantum wells is enhanced. Then the overlap of the electron-hole wave function is enhanced, which results in improved radiative recombination rate.

Finite size effects in ferroelectric-semiconductor thin films under open-circuit electric boundary conditions

General features of finite size effects in the ferroelectric-semiconductor film under open-circuit electric boundary conditions are analyzed using Landau-Ginzburg-Devonshire theory and continuum media electrostatics. The temperature dependence of the film critical thickness, spontaneous polarization, and depolarization field profiles of the open-circuited films are found to be significantly different from the characteristics of short-circuited ones. In particular, we predict the re-entrant type transition boundary between the mono-domain and poly-domain ferroelectric states due to reduced internal screening efficiency and analyzed possible experimental scenarios created by this mechanism. Performed analysis is relevant for the quantitative description of free-standing ferroelectric films phase diagrams and polar properties. Also our results can be useful for the explanation of the scanning-probe microscopy experiments on free ferroelectric surfaces.

Second-order nonlinear optical properties in a strained InGaN/AlGaN quantum well under the intense laser field

In this work, the optical rectification and the second harmonic generation coefficients in a strained InGaN/AlGaN quantum well are studied. Impacts of the spontaneous and piezoelectric polarization fields on the potential profile are taken into account. The energy levels and wave functions are calculated using the fourth-order Runge–Kutta method and optical properties are obtained using the compact density matrix approach. Effects of intense laser field, In composition, Al composition, the well width and barrier width on the second-order nonlinear optical properties are investigated. Results reveal that the confinement potential is considerably affected by the laser field and internal electric field. Results also indicate that the resonant peaks experience a red-shift with increasing the laser field strength and barrier width. Moreover, the resonant peaks suffer a blue-shift with the increase in In and Al compositions.

Graphene-GaN Schottky diodes

The electrical characteristics of graphene Schottky contacts formed on undoped GaN semiconductors were investigated. Excellent rectifying behavior with a rectification ratio of ∼107 at ±2 V and a low reverse leakage current of 1.0 × 10−8 A/cm2 at −5 V were observed. The Schottky barrier heights, as determined by the thermionic emission model, Richardson plots, and barrier inhomogeneity model, were 0.90, 0.72, and 1.24 ± 0.13 eV, respectively. Despite the predicted low barrier height of ∼0.4 eV at the graphene-GaN interface, the formation of excellent rectifying characteristics with much larger barrier heights is attributed to the presence of a large number of surface states (1.2 × 1013 states/cm2/eV) and the internal spontaneous polarization field of GaN, resulted in a significant upward surface band bending or a bare surface barrier height as high as of 2.9 eV. Using the S parameter of 0.48 (measured from the work function dependence of Schottky barrier height) and the mean barrier height of 1.24 eV, the work function of graphene in the Au/graphene/GaN stack could be approximately estimated to be as low as 3.5 eV. The obtained results indicate that graphene is a promising candidate for use as a Schottky rectifier in GaN semiconductors with n-type conductivity.

Preparation, Characterization, Dielectric, Piezoelectric and Ferroelectric Properties of Yttrium and Praseodymium Doped Lead Titanate Ceramics

Rare earth double doped lead titanate with general formula Pb1-x Prx Ti1-y YyO3 where x (praseodymium) = 0.02, 0.04, 0.06, 0.08 and y (yttrium) = 0.02 was synthesized by solid state reaction technique. X-ray diffraction analysis confirms single phase formation with perovskite structure having tetragonal phase. Micro-structural analysis shows formation of dense micro-structure with grain size calculated using average line intercepted method. Dielectric behavior as a function of temperature was found to decrease with Pr substitution. The diffusivity in the dielectric behavior was observed in all the compositions. Piezoelectric coefficient d33 and g33 was determined with increase in compositions of Pr substituent. The ferroelectric property was observed at room temperature and the value of remanent polarization (Pr), spontaneous polarization (Ps) and (EC) coercive field was calculated.

Intense laser field effects on the electron Raman scattering in a strained InGaN/GaN quantum well

The differential cross-section for an intersubband electron Raman scattering process in a strained InGaN/GaN quantum well in the presence of an intense laser field is studied. In the effective-mass approximation, the electronic structure is calculated by taking into account the effects of spontaneous and piezoelectric polarization fields on the confinement potential. Effects of laser field strength, indium composition and the well width on the differential cross-section of the strained quantum well are investigated. Results show that the position and the magnitude of the peaks of emission spectra considerably depend on the laser field strength as well as structural parameters.

Loparite-(Ce), a natural ferroelectric with the perovskite-type structure

We report on the results of studies of dielectric properties of loparite-(Ce), a naturally occurring member of the perovskite-type family of materials, in the temperature range of 80–400 K. A strong frequency dispersion attributable to disorder and a dielectric anomaly have been observed in the vicinity of T = 157 K. The behavior of the inverse dielectric permittivity 1/ε′(T) has been found to be well described by the Curie–Weiss law with parameters C = 2.2 × 105 K−1 and Tm = 157 K. Studies of a dielectric hysteresis demonstrated the presence of ferroelectric loops below Tm ≈ 157 K, which points to the existence of a ferroelectric phase transition at Tm. The behaviors of spontaneous polarization, remanent polarization, and coercive field indicate that the ferroelectric phase transition is of the first-order type.

Molecular Dynamics Simulations of Piezoelectric Materials for Energy Harvesting Applications

The previously proposed polarizable charge equilibrium (PQEq) force field model is parameterized for studying lead titanate (PT), lead zirconate (PZ), and their alloys: lead zirconate titanate (PZT). Several molecular dynamics (MD) simulations are performed to assess the degree of accuracy of the model. The phase transition temperatures, which are generally inaccurate in MD, are shown to be similar to experimental measurements. Also, the calculation of the ferroelectric hysteretic behavior, including the spontaneous polarization, saturated polarization and coercive fields, with extended MD is shown to give a qualitatively correct comparison between PT and PZT. The accuracy of the electronic properties in PQEq leads to direct application to a range of interesting problems such as enhanced properties of piezo- and ferro-electric nanostructures and the kinetics of domain walls in these materials.

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.

Intense laser field effects on the linear and nonlinear intersubband optical properties in a strained [formula omitted] quantum well

In this work, the linear, the third-order nonlinear and the total optical absorption coefficients and refractive index changes of a strained InGaN/GaN quantum well are investigated numerically. In the effective-mass approximation, the electronic energy levels and the corresponding wave functions are calculated by taking into account the effects of spontaneous and piezoelectric polarization fields on the conduction band edge. Effects of intense laser field, In composition and the well width on the optical properties of the strained quantum well are studied. Results indicate that the laser field as well as the strain induced piezoelectric field considerably affects the confining potential of the quantum well. Results also show that the resonant peaks experience a red-shift (blue-shift) with the increase in the laser field intensity and well width (In composition).

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.

Effects of spontaneous and piezoelectric polarization fields on the electronic and optical properties in GaN/AlN quantum dots: multimillion-atomsp3d5s*tight-binding simulations

GaN quantum dots QDs grown on AlN substrates are strong candidates for UV and near-infrared applications. The wurtzite crystal symmetry in these dots induces internal fields arising from the following: i crystal atomicity;ii strained active region; iii piezoelectricity; and iv spontaneous polarization pyroelectricity. Accurate modeling of electronic and optical properties of these QDs must capture the interplay of these atomistic and long-range fields and the size quantization on an equal footing. In this work, single-particle electronic structure and interband optical transition rates of a GaN/AlN QD grown along the c-axis are studied using a coupled molecular mechanics-atomistic 20-band sp3d5s* tight-binding VFF-TB framework. To calculate piezoelectricity, a recently reported model that takes into account both the linear and the nonlinear dependence of polarization on the strain tensors has been employed. The simulated GaN/AlN dot is realistically sized containing ~3 million atoms and of hexagonal disk shape having height and base length of 4.1 and 17nm, respectively. It is found that, in contrast to the well-studied InN/GaN systems, the pyroelectric potential in GaN/AlN dot is larger than the piezoelectric counterpart, and the effects of piezoelectric and pyroelectric fields add up. The internal fields result in a large redshift in the electronic states near the Brillouin zone center known as quantum confined Stark effect, pronounced non-degeneracy in the excited states, strongly suppressed optical transition, and anisotropic emission spectra. Copyright © 2014 John Wiley & Sons, Ltd.