Polarization Offsets Research Articles

Simulated Structural and Electronic Properties of Cation-Disordered ZnGeN 2 and its Interface with GaN

Cation disorder is increasingly being studied as a means of tuning properties in electronic materials. Through Monte Carlo simulations and first-principles calculations, we determine the ranges of elastic moduli and polarization constants in $\mathrm{Zn}\mathrm{Ge}\mathrm{N}$${}_{2}$ as a function of ordering. We use heterostructure calculations to demonstrate a disorder-dependent type-I or type-II band offset between relaxed $\mathrm{Ga}\mathrm{N}$ and strained $\mathrm{Zn}\mathrm{Ge}\mathrm{N}$${}_{2}$ in the polar [001] direction. Modeling polarization and band offsets, we then use these calculations to comment on which values are desirable for device design, which parameters are obtainable through targeted growth techniques, and the challenges that remain in realizing a device and understanding cation disorder in heterostructures. Although the elastic and polarization properties change little with the order parameter, the band characteristics undergo dramatic shifts, creating a type-I band offset in partially disordered material.

Band parameters of group III–V semiconductors in wurtzite structure

The properties of most III–V semiconductor materials in the wurtzite structure are not known because of their metastable character. However, recent advances in the growth of III–V wurtzite nanorods open new perspectives for applications. In this work, we present a systematic computational study of bulk wurtzite III–V semiconductors, using predictive ab initio methods, to provide a necessary base knowledge for studying the nanostructures. The most important physical properties of bulk systems, i.e., lattice constants, elasticity, spontaneous polarization, piezoelectricity, band structures, deformation potentials, and band offsets, have been studied. Comparison with the available experimental and theoretical data shows the high credibility of our results. Moreover, we provide a complete set of parameters for a six-band k⋅p model, which is widely used for simulating devices based on semiconductor heterostructures.

Tuning band alignment at a semiconductor-crystalline oxide heterojunction via electrostatic modulation of the interfacial dipole

We demonstrate that the interfacial dipole associated with bonding across the SrTiO3/Si heterojunction can be tuned through space charge, thereby enabling the band alignment to be altered via doping. Oxygen impurities in Si act as donors that create space charge by transferring electrons across the interface into SrTiO3. The space charge induces an electric field that modifies the interfacial dipole, thereby tuning the band alignment from type-II to type-III. The transferred charge, resulting in built-in electric fields, and change in band alignment are manifested in electrical transport and hard x-ray photoelectron spectroscopy measurements. Ab initio models reveal the interplay between polarization and band offsets. We find that band offsets can be tuned by modulating the density of space charge across the interface. Functionalizing the interface dipole to enable electrostatic altering of band alignment opens new pathways to realize novel behavior in semiconducting heterojunctions.

Nonuniform Absorption and Scattered Light in Direct-Drive Implosions Driven by Polarization Smoothing.

Laser-direct-drive symmetric implosions on OMEGA illuminate a target with 60 laser beams and are designed to produce spherical implosions. Each beam is smoothed using orthogonal polarizations obtained by passing the laser beams through distributed polarization rotators (DPRs). Observations of light scattered from OMEGA implosions do not show the expected symmetry and have much larger variation than standard predictions. For the first time, we have quantified the scattered-light nonuniformity from individual beams and identified the DPRs as the source of the enhanced nonuniformity. An instrument was invented that isolated and measured the variation in the intensity and polarization of the light scattered from each OMEGA beam. The asymmetric intensity and polarization measurements are explained when the on-target offsets between the two orthogonal polarizations produced by the DPRs are modeled using a 3D cross-beam energy transfer (CBET) code that tracks the polarizations of each beam. The time-integrated nonuniformity in laser absorption and scattered light due to CBET and the DPR polarization offsets during high-performance OMEGA implosions is predicted to be significant and dominated by low spherical harmonic mode numbers. The nonuniformity is predicted to be greatly reduced by replacing the DPRs with new optics that create smaller offsets.

Photonic-based dual-chirp microwave waveforms generation with multi-carrier frequency and large time-bandwidth product

Photonic-based dual-chirp microwave waveform generation with multi-carrier frequencies and large time-bandwidth product (TBWP) is proposed. In the approach, a polarization modulator is employed to introduce parabolic phase difference into the injected optical signals. After heterodyne detection in balanced photodetector, dual-chirp microwave waveforms with multi-carrier frequencies are generated. To remedy the TBWP limitation, approach based on splitting parabolic waveform is employed to increase the bandwidth of the generated waveforms while controlling the temporal duration unchanged. The generator features multi-band generation ability and large TBWP performance. Simulations show that bandwidth of the generated signals are increased by N/2 times after splitting the parabolic waveform into N pieces. Four-band dual-chirp signals with bandwidth of 7.32 GHz and TWBP of 6000 are generated. Auto-correlations of the generated waveforms exhibit good pulse compression performance. Ambiguity functions of the generated signals are analyzed, polarization offsets and approach limitations are also investigated.

Enhanced injection efficiency and light output in bottom tunnel-junction light-emitting diodes.

Recently, the use of bottom-TJ geometry in LEDs, which achieves N-polar-like alignment of polarization fields in conventional metal-polar orientations, has enabled enhancements in LED performance due to improved injection efficiency. Here, we elucidate the root causes behind the enhanced injection efficiency by employing mature laser diode structures with optimized heterojunction GaN/In0.17Ga0.83N/GaN TJs and UID GaN spacers to separate the optical mode from the heavily doped absorbing p-cladding regions. In such laser structures, polarization offsets at the electron blocking layer, spacer, and quantum barrier interfaces play discernable roles in carrier transport. By comparing a top-TJ structure to a bottom-TJ structure, and correlating features in the electroluminescence, capacitance-voltage, and current-voltage characteristics to unique signatures of the N- and Ga-polar polarization heterointerfaces in energy band diagram simulations, we identify that improved hole injection at low currents, and improved electron blocking at high currents, leads to higher injection efficiency and higher output power for the bottom-TJ device throughout 5 orders of current density (0.015-1000 A/cm2). Moreover, even with the addition of a UID GaN spacer, differential resistances are state-of-the-art, below 7 × 10-4 Ωcm2. These results highlight the virtues of the bottom-TJ geometry for use in high-efficiency laser diodes.

Analysis of polarization offsets observed for temperature-graded ferroelectric materials

A transverse Ising model in the framework of the mean field approximation is developed to analyze the polarization offsets phenomena in temperature-graded ferroelectric materials. A function of two-spin exchange interaction strength has been introduced to describe the ferroelectric distortion due to the distribution of temperature gradients in materials. Comparisons of the computational results with the experimental data reveal some fundamental factors in the formation of polarization offsets. It is shown that ferroelectric distortion has influenced much on polarization offsets in temperature-graded ferroelectric materials. When quantum fluctuation effect as well as ferroelectric distortion is considered, we have successfully reproduced the experimental observations qualitatively, especially for the indistinguishable polarization offsets from the background at small temperature gradients, which were not successfully reproduced in prior theoretical studies.

Nonuniform cosmological birefringence and active galactic nuclei

Cosmological birefringence, a rotation by an angle $\alpha$ of the polarization of photons as they propagate over cosmological distances, is constrained by the cosmic microwave background (CMB) to be $|\alpha|\lesssim1^\circ$ ($1\sigma$) out to redshifts $z\simeq1100$ for a rotation that is uniform across the sky. However, the rotation angle $\alpha(\theta,\phi)$ may vary as a function of position $(\theta,\phi)$ on the sky. Here I discuss how a position-dependent rotation can be sought in current and future AGN data. An upper limit $\VEV{\alpha^2}^{1/2} \lesssim 3.7^\circ$ to the scatter in the position-angle--polarization offsets in a sample of only N=9 AGN already constrains the rotation spherical-harmonic coefficients to $(4\pi)^{-1/2} \alpha_{lm}\lesssim 3.7^\circ$ and constrains the power spectrum for $\alpha$ in models where it is a stochastic field. Future constraints can be improved with more sources and by analyzing well-mapped sources with a tensor-harmonic decomposition of the polarization analogous to that used in CMB polarization and weak gravitational lensing.

Photovoltaic effects in BiFeO3

We report a photovoltaic effect in ferroelectric BiFeO3 thin films. The all-oxide heterostructures with SrRuO3 bottom and tin doped indium oxide top electrodes are characterized by open-circuit voltages ∼0.8–0.9 V and external quantum efficiencies up to ∼10% when illuminated with the appropriate light. Efficiencies are at least an order of magnitude larger than the maximum efficiency under sunlight (AM 1.5) thus far reported for ferroelectric-based devices. The dependence of the measured open-circuit voltage on film thickness suggests contributions to the large open-circuit voltage from both the ferroelectric polarization and band offsets at the BiFeO3/tin doped indium oxide interface.

Polarization offset of homogeneous Bi3.15Nd0.85Ti3O12 ferroelectric thin films

In this report, the polarization offset usually observed in compositionally graded ferroelectric thin films was observed in homogeneous Bi3.15Nd0.85Ti3O12 thin films sandwiched in Pt electrodes. The observed polarization offset was studied as functions of temperature and of the driving voltage amplitude using a modified Sawyer–Tower circuit. The polarization offset is obvious only at temperatures above 150 °C and increases with increasing testing temperature and amplitude of driving voltage. The reported phenomena demonstrate that the composition gradient is not a necessity for such a polarization offset. The strong temperature and voltage dependence indicates that the polarization offsets may be related to thermionic charge injection, which is asymmetric to top and bottom electrodes.

Polarization offsets of compositionally graded Nd-substituted Bi4Ti3O12 ferroelectric thin films

The vertical offsets of hysteresis loops of compositionally graded Nd-substituted Bi4Ti3O12 (BNdT) ferroelectric films were studied as functions of the Nd gradient, the temperature, and the amplitude of driving voltage. We show that graded BNdT films may exhibit no offsets at room temperature and both graded and nongraded BNdT films exhibit pronounced offsets at elevated temperatures. The offset does not have a direct relation with the composition gradient but increases as the maximum voltage is increased. It is verified that the offset increases with the increase in asymmetry of leakage currents at positive and negative biases.

Polarization Offsets of Non Graded and Graded Ferroelectric Films

ABSTRACT Shifts of hysteresis loops along polarization axis are observed on non graded Pb(Zrx,Ti1-x)O3 ferroelectric films using a Sawyer-Tower circuit. The offsets become significant providing that the sense capacitor is large enough and the input resistance of the measurement set up is artificially increased. The offsets, consisting in DC voltages rather than DC polarizations, are thus not the exclusive feature of graded ferroelectric devices (GFD's). Our observations are consistent with the charging up of the sense capacitor by asymmetrical leakage currents in the film and strongly suggest that rectifying effects are probably involved in most of the vertical offsets observed on ferroelectric thin film capacitors. Assuming blocking contacts at the electrode—ferroelectric interfaces, we propose that the composition gradient in GFD's produces a variation of the band gap energy leading to potential barrier with different heights at the two interfaces. Such asymmetry of the energy band diagram is consistent with our hypothesis of asymmetrical leakage currents.

Rectifying semiconductor-ferroelectric polarization loops and offsets in Pt–BaTiO 3–ZnO–Pt thin film capacitor structures

Electrical and polarization hysteresis measurements on Pt–BaTiO 3–ZnO–Pt heterostructures, grown by pulsed laser deposition on (001)Si, are reported. The layers were deposited without breaking the vacuum using a switchable target holder. Offsets of hysteresis loops along the polarization axis with increasing sweeping voltage, time-dependent charging, and rectifying behavior are observed. A simple electrical circuitry can be used to model the observed hysteresis behavior, where the interface between the wurtzite-structure ZnO and the pervoskite-structure BaTiO 3 is seen as the origin of a space charge accumulation region. Coupling between spontaneous wurtzite and switchable ferroelectric polarization is discussed.

Explicit expressions for the dynamic polarization behavior in ferroelectrics with symmetric/asymmetric electrical conductivity

We model the dynamic polarization behavior of ferroelectric films by an analytical approach. Using a parallelogramlike P-E hysteresis model, explicit expressions were obtained for describing the D-E loops of ferroelectric films as would be measured from a Sawyer-Tower circuit. By introducing the consideration of electrical conduction, resistive losses inflate the ferroelectric loop to some extent. In addition, an asymmetric conduction will result in polarization offsets with the magnitude and direction depending on the asymmetric conductivities of the materials. The inflated loop, as well as the offset phenomena, which have been observed experimentally [e.g., L. Zheng, C. Lin, W.-P. Xu, and M. Okuyama, J. Appl. Phys. 79, 8634 (1996)], will be discussed by means of our modeled results. Some simple exact formulas have been derived and the effects of electric, dielectric, and ferroelectric parameters, as well as the applied field on the “apparent” polarization were also examined.

Polarization imprint in ordered arrays of epitaxial ferroelectric nanostructures

Sub 50-nm-high BaTiO3 epitaxial nanostructures were fabricated into well-ordered arrays on SrTiO3:Nb (001) single-crystal substrates using pulsed laser deposition through monolayer masks of monodisperse latex spheres (1 or 0.5 μm) and postdeposition annealing. Imprint was investigated in nanostructures of various dimensions using piezoresponse force microscopy. Piezoresponse domain imaging revealed a preferential downward prepolarization state for the as-prepared nanostructures, and consistently piezoresponse hysteresis loops exhibited negative polarization offsets. The offsets are suggested to be due to locked nonswitchable domains in an interface near region with a thickness of 7–14 nm.

Time-dependent space-charge-limited conduction as a possible origin of the polarization offsets observed in compositionally graded ferroelectric films

We investigated the effects of free space charges on hysteresis-loop measurement of compositionally graded ferroelectrics and found that they are quite likely to be responsible for the “polarization offsets” observed in experiments. Taking into account conduction by those free space charges, or time-dependent space-charge-limited conduction, our computer simulation of compositionally graded lead zirconate titanate, which is placed in the Sawyer–Tower circuit and driven by an alternating applied voltage, produced shifting of measured hysteresis loops where the shift magnitudes are comparable with published experimental data. It also produced the key features as observed in experiments: The “offsets:” (a) have a monotonous increase with electric-field amplitude, (b) change in direction when the composition gradient is inverted, and (c) develop like the typical charging-up of a capacitor. All these results suggest that time-dependent space-charge-limited conduction is a possible origin of the polarization offsets observed in compositionally graded ferroelectrics.

Asymmetrical leakage currents as a possible origin of the polarization offsets observed in compositionally graded ferroelectric films

Offsets of hysteresis loops along the polarization axis have been observed on a step graded Pb(Zr,Ti)O3 (PZT) thin film using a Sawyer–Tower (ST) circuit. However, the same effect may be artificially reproduced by adding adequate resistors and diodes in parallel with a nongraded PZT thin film. The hypothesis that the offsets were mainly due to the asymmetrical charging up of the standard capacitor used in the ST circuit, allows us to establish that the graded ferroelectric sample behaves as a kind of rectifying device. It is concluded that the presence of asymmetrical leakage currents in compositionally graded devices may allow the elucidation of the origin of the offsets often observed in these structures. Correlatively, it is demonstrated that such offsets do not represent an abnormal static polarization but a dc voltage. The Em4 power law dependence of the offsets (where Em is the amplitude of the electric field) was found to be attributable to the nonlinear increase of the net leakage current.

Abnormal ferroelectric properties of compositionally graded Pb(Zr,Ti)O3 thin films with LaNiO3 bottom electrodes

We report abnormal ferroelectric properties of compositionally graded Pb(Zr,Ti)O3 thin films on LaNiO3-coated SiO2/Si substrates, where Pb(Zr,Ti)O3 and LaNiO3 films were prepared by a metalorganic decomposition technique and a sol–gel technique, respectively. It was found that the hysteresis loops of the Pb(Zr,Ti)O3 graded films measured by the conventional Sawyer–Tower method shifted along the polarization axis, i.e., they showed polarization offsets when applied by an alternating electric field. The polarization offsets were 82.5 μC/cm2 at 270 kV/cm and 62.5 μC/cm2 at 185 kV/cm for the up-graded film and the down-graded film, respectively. The absolute magnitude of the polarization offsets was closely related to the magnitude of the driving electric field, and the direction of the polarization offsets depended on the direction of the composition gradient with respect to the substrate. Analysis indicated that the polarization offsets did not originate from asymmetric contact effects, oxygen vacancies, alignment of defect dipoles, and/or electron injection. These results showed that polarization offsets in hysteresis loops were the intrinsic characteristic of the compositionally graded Pb(Zr,Ti)O3 thin films.

Composition gradient optimization and electrical characterization of (Pb, Ca)TiO3 thin films

Compositionally graded (Pb, Ca)TiO3 thin films were prepared by a monoethanolamine-modified sol–gel technique on platinum-coated silicon substrates at the annealing temperature of 600 °C. The composition gradient of the films was greatly improved by a modified annealing method. The dielectric constants, for up-graded and down-graded films annealed at 600 °C for 60 min, were found to be 469 and 355, respectively. Both were larger than those reported for conventional (Pb, Ca)TiO3 thin films. The compositionally graded films had large polarization offsets in hysteresis loops when excited by an alternating electric field. The more smooth the composition gradient of the graded film, the larger the polarization offset. This was consistent with a theoretical model reported previously by Mantese and coworkers [Appl. Phys. Lett. 71, 2047 (1997)]. The magnitude of polarization offset displayed a power-law dependence on the electric field, and the direction of the offset depended on the direction of the composition gradient with respect to the substrate. Both up-graded and down-graded films had good leakage current characteristics.

Dielectric and ferroelectric properties of compositionally graded (Pb,La)TiO3 thin films on Pt/Ti/SiO2/Si substrates

Compositionally graded (Pb,La)TiO3 thin films were prepared on platinum-coated silicon substrates by a sol–gel technique. The crystalline orientation and surface morphology of the graded films were closely related to the deposition sequence of the film layer. The dielectric constants, for up-graded and down-graded films annealed at 600 °C for 60 min, were found to be 765 and 374, respectively. The compositionally graded films had large polarization offsets in hysteresis loops when driven by an alternating electric field. The magnitude of polarization offsets displayed a power-law dependence on the electric field, and the direction of the offsets depended on the direction of the composition gradient with respect to the substrate. The offset, 250 μC/cm2 at the driving electric field of 250 kV/cm, was obtained. These results showed that the sol–gel technique was a very promising route for the realization of compositionally graded ferroelectric thin films and the compositionally-graded (Pb,La)TiO3 thin films had excellent dielectric properties and abnormal ferroelectric properties which can be used in various microelectronic devices.