Spontaneous Polarization Effects Research Articles

Non-Polar Gallium Nitride for Photodetection Applications: A Systematic Review

Ultraviolet photodetectors have been widely utilized in several applications, such as advanced communication, ozone sensing, air purification, flame detection, etc. Gallium nitride and its compound semiconductors have been promising candidates in photodetection applications. Unlike polar gallium nitride-based optoelectronics, non-polar gallium nitride-based optoelectronics have gained huge attention due to the piezoelectric and spontaneous polarization effect–induced quantum confined-stark effect being eliminated. In turn, non-polar gallium nitride-based photodetectors portray higher efficiency and faster response compared to the polar growth direction. To date, however, a systematic literature review of non-polar gallium nitride-based photodetectors has yet to be demonstrated. Hence, the objective of this systematic literature review is to critically analyze the data related to non-polar gallium nitride-based photodetectors. Based on the pool of literature, three categories are introduced, namely, growth and fabrication, electrical properties, and structural, morphological, and optical properties. In addition, bibliometric analysis, a precise open-source tool, was used to conduct a comprehensive science mapping analysis of non-polar gallium nitride-based photodetectors. Finally, challenges, motivations, and future opportunities of non-polar gallium nitride-based photodetectors are presented. The future opportunities of non-polar GaN-based photodetectors in terms of growth conditions, fabrication, and characterization are also presented. This systematic literature review can provide initial reading material for researchers and industries working on non-polar gallium nitride-based photodetectors.

Enabling Distributed Intelligence with Ferroelectric Multifunctionalities.

Distributed intelligence involving a large number of smart sensors and edge computing are highly demanded under the backdrop of increasing cyber‐physical interactive applications including internet of things. Here, the progresses on ferroelectric materials and their enabled devices promising energy autonomous sensors and smart systems are reviewed, starting with an analysis on the basic characteristics of ferroelectrics, including high dielectric permittivity, switchable spontaneous polarization, piezoelectric, pyroelectric, and bulk photovoltaic effects. As sensors, ferroelectrics can directly convert the stimuli to signals without requiring external power supply in principle. As energy transducers, ferroelectrics can harvest multiple forms of energy with high reliability and durability. As capacitors, ferroelectrics can directly store electrical charges with high power and ability of pulse‐mode signal generation. Nonvolatile memories derived from ferroelectrics are able to realize digital processors and systems with ultralow power consumption, sustainable operation with intermittent power supply, and neuromorphic computing. An emphasis is made on the utilization of the multiple extraordinary functionalities of ferroelectrics to enable material‐critical device innovations. The ferroelectric characteristics and synergistic functionality combinations are invaluable for realizing distributed sensors and smart systems with energy autonomy.

Effective Piezo-Phototronic Enhancement of Flexible Photodetectors Based on 2D Hybrid Perovskite Ferroelectric Single-Crystalline Thin-Films.

2D hybrid perovskites are very attractive for optoelectronic applications because of their numerous exceptional properties. The emerging 2D perovskite ferroelectrics, in which are the coupling of spontaneous polarization and piezoelectric effects, as well as photoexcitation and semiconductor behaviors, have great appeal in the field of piezo-phototronics that enable to effectively improve the performance of optoelectronic devices via modulating the electro-optical processes. However, current studies on 2D perovskite ferroelectrics focus on bulk ceramics that cannot endure irregular mechanical deformation and limit their application in flexible optoelectronics and piezo-phototronics. Herein, we synthesize ferroelectric EA4 Pb3 Br10 single-crystalline thin-films (SCFs) for integration into flexible photodetectors. The in-plane multiaxial ferroelectricity is evident within the EA4 Pb3 Br10 SCFs through systematic characterizations. Flexible photodetectors based on EA4 Pb3 Br10 SCFs are achieved with an impressive photodetection performance. More importantly, optoelectronic EA4 Pb3 Br10 SCFs incorporated with in-plane ferroelectric polarization and effective piezoelectric coefficient show great promise for the observation of piezo-phototronic effect, which is capable of greatly enhancing the photodetector performance. Under external strains, the responsivity of the flexible photodetectors can be modulated by piezo-phototronic effect with a remarkable enhancement up to 284%. Our findings shed light on the piezo-phototronic devices and offer a promising avenue to broaden functionalities of hybrid perovskite ferroelectrics.

Finite-size correction for slab supercell calculations of materials with spontaneous polarization

The repeated slab approach has become a de facto standard to accurately describe surface properties of materials by density functional theory calculations with periodic boundary conditions. For materials exhibiting spontaneous polarization, we show that the conventional scheme of passivation with pseudo hydrogen is unable to realize a charge-neutral surface. The presence of a net surface charge induces via Gauss’s law a macroscopic electric field through the slab and results in poor size convergence with respect to the thickness of the slab. We propose a modified passivation method that accounts for the effect of spontaneous polarization, describes the correct bulk limits and boosts convergence with respect to slab thickness. The robustness, reliability, and superior convergence of energetics and electronic structure achieved by the proposed method are demonstrated using the example of polar ZnO surfaces.

AlGaN/GaN asymmetric graded-index separate confinement heterostructures designed for electron-beam pumped UV lasers.

We present a study of undoped AlGaN/GaN separate confinement heterostructures designed to operate as electron beam pumped ultraviolet lasers. We discuss the effect of spontaneous and piezoelectric polarization on carrier diffusion, comparing the results of cathodoluminescence with electronic simulations of the band structure and Monte Carlo calculations of the electron trajectories. Carrier collection is significantly improved using an asymmetric graded-index separate confinement heterostructure (GRINSCH). The graded layers avoid potential barriers induced by polarization differences in the heterostructure and serve as strain transition buffers which reduce the mosaicity of the active region and the linewidth of spontaneous emission.

Flat bands, strains, and charge distribution in twisted bilayer h−BN

We study the effect of twisting on bilayer $h\text{\ensuremath{-}}\mathrm{BN}$. The effect of lattice relaxation is included; we look at the electronic structure, piezoelectric charges, and spontaneous polarization. We show that the electronic structure without lattice relaxation shows a set of extremely flat in-gap states similar to Landau levels, where the spacing scales with twist angle. With lattice relaxation we still have flat bands, but now the spectrum becomes independent of twist angle for sufficiently small angles. We describe in detail the nature of the bands and we study appropriate continuum models, at the same time explaining the structure of the in-gap states. We find that even though the spectra for both parallel and antiparallel alignment are very similar, the spontaneous polarization effects only occur for parallel alignment. We argue that this suggests a large interlayer hopping between boron and nitrogen.

Ultrathin HfAlO ferroelectrics enhancing electron transport and perovskite solar cell performance

Ferroelectric materials are promising for solar energy conversion due to the unique spontaneous polarization effect, leading to effective control of electron-hole recombination and potentially high power conversion efficiency of perovskite solar cells. Herein, the ultrathin ferroelectric hafnia alumina (HfAlO) films were obtained by atomic layer deposition and applied to modify the electron transport layer of PSCs. The XPS and AFM characterizations indicate that the Hf0.39Al0.10O films feature smooth, dense, and compact surface morphology. For the devices incorporating the ultrathin HfAlO films, the oriented ferroelectric effect-induced spontaneous polarization and internal electric field are crucial factors to enhance the charge separation during charge transfer. Accordingly, the short circuit current, open-circuit voltage, and power conversion efficiency (PCE) are increased by 11.8%, 9.0%, and 29.8%, respectively. Moreover, the HfAlO incorporating devices retained ~ 80% of the original PCE after 500 h ageing, much better than that of the pristine devices showing the retention of only ~ 50% of the original PCE.

Spin transport in polarization induced two-dimensional electron gas channel in c-GaN nano-wedges

A two-dimensional electron gas (2DEG), which has recently been shown to develop in the central vertical plane of a wedge-shaped c-oriented GaN nanowall due to spontaneous polarization effect, offers a unique scenario, where the symmetry between the conduction and valence band is preserved over the entire confining potential. This results in the suppression of Rashba coupling even when the shape of the wedge is not symmetric. Here, for such a 2DEG channel, relaxation time for different spin projections is calculated as a function of donor concentration and gate bias. Our study reveals a strong dependence of the relaxation rate on the spin-orientation and density of carriers in the channel. Most interestingly, relaxation of spin oriented along the direction of confinement has been found to be completely switched off. Upon applying a suitable bias at the gate, the process can be switched on again. Exploiting this fascinating effect, an electrically driven spin-transistor has been proposed.

Electron–optical-phonon scattering rates in cubic group III-nitride crystals: path-integral corrections to Fermi golden rule matrix elements

III-nitride semiconductors with a cubic crystal structure have shown promise in enhancing efficiency in photonic and optoelectronic applications. The recent interest in cubic III-nitrides has arisen due to the inability to realize enhanced efficiency in optoelectronic applications of the wurtzite phase due to spontaneous polarization effects, crystal defects due to growth on lattice mismatched substrates, and also due to the requirement to fabricate normally-off transistors for high-mobility transistors. Cubic III-nitride materials are characterized by the strong coupling of carriers to optical phonons in which the standard perturbative approach—based on first order perturbation theory—breaks down. In this paper we determine the necessary corrections to the Fermi golden rule electron–optical-phonon matrix elements for selected cubic III-nitrides via the nonperturbative Thornber–Feynman path-integral techniques. Specifically, we report electron transport parameters such as the threshold electric field, threshold velocity, mobility and runaway length for BN, AlN, GaN and InN. 72.10.Di, 72.15.Lh, 72.80.Ey.

Hybrid functional investigation of band offsets for non-polar, Ga-polar and Al-polar interfaces in GaN/AlN heterojunction

We investigated band alignment of non-polar and polar GaN/AlN heterojunction by using density functional theory with the Heyd–Scuseria–Ernzerhof hybrid functional. In the heterojunction model, AlN was considered as a substrate while strained GaN was grown on top. The deformation potential of GaN, were included to calculate the unstrained band offsets. For polar heterojunction, the artificial effects from an undesired interface was removed by using vacuum insertion and dipole correction. We found that the unstrained valence band offsets (VBOs) are 0.92 eV, 1.23 eV and 1.09 eV for non-polar, Ga-polar and Al-polar interfaces respectively. The variation of VBOs can be explained from the difference in both deformation and dipole potential at the interfaces. Moreover, the additional energy shift of polar VBO from non-polar VBO were extracted as 0.33 eV from considering the effect of spontaneous polarization.

Enhanced Photovoltaic Performances of La-Doped Bismuth Ferrite/Zinc Oxide Heterojunction by Coupling Piezo-Phototronic Effect and Ferroelectricity.

Ferroelectric materials have drawn widespread attention due to their switchable spontaneous polarization and anomalous photovoltaic effect. The coupling between ferroelectricity and the piezo-phototronic effect may lead to the design of distinctive photoelectric devices with multifunctional features. Here, we report an enhancement of the photovoltaic performances in the ferroelectric p-type La-doped bismuth ferrite film (BLFO)/n-type zinc oxide (ZnO) nanowire array heterojunction by rationally coupling the strain-induced piezoelectricity in ZnO nanowires and the ferroelectricity in BLFO. Under a compressive strain of -2.3% and a 10 V upward poling of the BLFO, the open-circuit voltage (VOC) and short-circuit current density (JSC) of the device increase by 8.4% and 54.7%, respectively. Meanwhile, the rise (/decay) time is modulated from 153.7 (/108.8) to 61.28 (/74.86) ms. Systematical band diagram analysis reveals that the promotion of photogenerated carriers and boost of the photovoltaic performances of the device can be attributed to the modulated carrier transport behaviors at the BLFO/ZnO interface and the superposed driving forces arising from the adding up of the piezoelectric potential and ferroelectric polarization. In addition, COMSOL simulation results of piezopotential distribution in ZnO nanowire arrays and the energy band structure change of the heterojunction further confirm the mechanisms. This work not only presents an approach to design high-performance ferroelectric photovoltaic devices but also further broadens the research scope of piezo-phototronics.

Non-Polar Wurtzite (1120) GaN/AlN Quantum Dots for Highly Efficient Opto-Electronic Devices

In III-nitride quantum dots (QDs), optical transition rate is very low because of the large built-in electrostatic field caused by the spontaneous polarization (SP) and piezoelectric (PZ) effects. In this work, we study the screening potential which is a solution of the self-consistent Hartree equation taking into account the built-in electrostatic field and its effect on light emission characteristics of non-polar wurtzite (WZ) (112¯0) GaN/AlN QD. It is found that the light emission intensity of the non-polar (112¯0) GaN/AlN QD structure is expected to be about four times larger than that of the c-plane (0001) GaN/AlN QD structure because the y-polarized matrix elements in the non-polar QD are larger than that in the c-plane QD. These predictions indicate that non-polar GaN/AlN QD structure have strong potential for highly efficient opto-electronic devices.

Highly tunable, polarization-engineered two-dimensional electron gas in ε-AlGaO3/ε-Ga2O3 heterostructures

We report on the modeling of polarization-induced two-dimensional electron gas (2DEG) formation at ε-AlGaO3/ε-Ga2O3 heterointerface and the effect of spontaneous polarization (Psp) reversal on 2DEG density in ε-Ga2O3/ε-AlGaO3/ε-Ga2O3 double heterostructures. Density-functional theory (DFT) is utilized to calculate the material properties of ε-Ga2O3 and ε-AlGaO3 alloys. Using Schrödinger–Poisson solver along with DFT calculated parameters, the 2DEG density is calculated as a function of barrier type and thickness. By optimizing the layer thicknesses of ε-Ga2O3/ε-AlGaO3/ε-Ga2O3 heterostructures, charge contrast ratios exceeding 1600 are obtained. This computational study indicates the high potential for ε-Ga2O3-based heterostructure devices for non-volatile memories and neuromorphic applications.

About the Extreme Problem of Identifying the Properties of Ferroelectrics

Ferroelectricity is a phenomenon exhibited by crystals with a spontaneous polarization and hysteresis effects associated with dielectric changes when an electric field is given. Ferroelectricity is a characteristic of certain materials that have a spontaneous electric polarization, which can be reversed by the application of an external electric field. The ferroelectric sensor is a simple device, which can measure and produce some form of the output of mechanical, electrical and optical product. Today, developmental sensors use computing, communications and connectivity to the web, mobile smart devices and integration clouds added to the sensor capabilities. Dependence of the switching current of the polarization of the ferroelectric on time and the electric field, the law of conservation of the full dipole moment in the ferroelectric crystal make up a system of equations that define the dependence of the switching current on the time and the electric field. The one-step implicit iterative Rosenbrock method for the solution of the Cauchy problem for stiff systems of ordinary differential equations is applied. It is based on the generalized Runge–Kutta schemes with up to four stages. Coordinate descent is based on the idea that the minimization of a multivariable function can be achieved by minimizing it along one direction at a time, i.e., solving univariate optimization problems in a loop. Golden-section search is used for the function of a single variable.

Spontaneous Polarization Effect and Photocatalytic Activity of Layered Compound of BiOIO3.

Internal polarized electric field is found to be an effective and available strategy to separate photogenerated electron-hole pairs. By this method, the efficiency of photocatalytic reactions can be obviously enhanced. Here, the layered compound of BiOIO3 with spontaneous polarization was synthesized by a simple hydrothermal method. Taking another bismuth compound BiOI as a counterpart, which has a similar layered structure, the spontaneous polarization effects of BiOIO3 were analyzed and confirmed. The photocatalytic activity of BiOIO3 and BiOI were evaluated by the degradation of methyl orange. Methyl orange was almost completely photocatalytically decomposed by BiOIO3 and BiOI in 40 and 90 min, respectively. The separation and transfer behaviors of photogenerated electron-hole pairs were investigated by a series of photoelectrochemical characterizations. It is further proved the separation and transmission efficiency of BiOIO3 are higher than those of BiOI. According to the results of density of theory calculations, the internal polarized electric field in BiOIO3 is ascribed to the spatial asymmetry of the IO3 group, which is estimated to ∼1.5 × 1010 V/m. Under the action of this internal polarized electric field, the photogenerated electrons and holes would transfer along opposite directions, i.e., photogenerated electrons and holes respectively gather at the Bi/I side and O side. Additionally, superoxide radicals (•O2-) and holes (h+) are produced during the degradation process, which are responsible for the high visible-light photocatalytic activity. Finally, the cyclic degradation test proves that its photocatalytic performance has long-term stability. Therefore, BiOIO3 polar material can be used as one of the alternative materials for efficient photocatalytic reaction.

A Rigorous Investigation of Electrostatic and Transport Phenomena of GaN Double-Channel HEMT

This paper presents a comprehensive investigation of electrostatics and transport characterization of GaN double-channel (DC) MOS-HEMT. Upon derivation of a polynomial analytical expression establishing a relationship between the Fermi level and the 2-D electron gas density (2DEG), a relationship between the sheet carrier density and applied gate voltage has been obtained. To confirm the validity of the model in both subthreshold and strong inversion regions, the charge density profile and capacitance–voltage profile have been attained from self-consistent simulation incorporating the quantum mechanical effect. The impact of GaN channel thickness on the conduction band profile as well as charge density profile has also been investigated. A 2-D analytical model for current–voltage characteristics of GaN-DC-MOS-HEMT has been developed for the first time. Assuming velocity saturation of electrons in 2DEG, the effects of spontaneous and piezoelectric polarization at the heterointerface, field dependent mobility, and the parasitic source and drain resistance have been taken into account in evolution of this model. The ON-resistance extracted from the analytical model was found to be 7.1 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Omega \cdot $ </tex-math></inline-formula> mm, which is in close proximity with experimental results. A steep subthreshold swing of 79 mV/dec was determined from the current–voltage characteristics with an ON– OFF drain current ratio of the order of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">9</sup> , which holds the promising application for enhancement mode operation with minimal leakage current. For corroboration, the derived results were compared with experimental data acquired from the literature, thereby enhancing the reliability of this model.

Effect of degree of strain relaxation on polarization charges of GaN/InGaN/GaN hexagonal and triangular nanowire solar cells

The innovative contribution of this paper is to derive polarization charges in hexagonal and triangular GaN/InGaN/GaN core/shell/shell nanowire solar cells considering effect of degree of strain relaxation (R) and appropriate stiffness coefficients. The crystal orientation angle, φ and non-linear effect of spontaneous polarization are tailored with strain calculations for a better precision of polarization charges in nanowire type devices. The article also formulated the effect of polarization charges while InGaN layers are grown above or below GaN layer in both types of nanowires depending on the lattice expansion or compression. The model accounts an innovative concept of polarization charges distribution with respect to growth of crystal orientation and strain relaxation. It is observed that nanowire solar cell with triangular geometry could be good enough for efficient generation of power as compared to hexagonal nanowire solar cell. This concept of one triangular nanowire solar cell exhibits an efficiency of 3.18% with 90.34% fill factor under 1 Sun AM1.5 illumination with 20% ‘In’ composition.

Temperature Dependence of InGaN / GaN Multiple Quantum Well Solar Cells

Elements based on nitride family and more specifically, those of the GaN/InGaN family, present a great interest for renewable energy conversion systems as they offer huge potentialities in photovoltaic applications, as example for the realization of concentration solar cells. This interest is linked to the possibility to enlarge the part of the solar energetic spectrum that a solar cell is able to capture thanks to the possibilities of growing multilayers cells with various band-gap and thus, with various absorption bands. In this paper, the performance of InGaN/GaN multiple quantum well MQW solar cells with respect to the N-polar orientation. is investigated. In order to obtain the exact conversion efficiency, the temperature effect as well as the spontaneous and piezoelectric polarization effect are taking into consideration in model. The results reveal that the increase of temperature decreases significantly the MQW solar cell efficiency.Our results and discussion would be helpful in designing and fabricating high efficiency InGaN/GaN solar cell in experiment.

Synthesis of BaTiO3/α-S8 composite for enhanced photocatalytic activity under UV/Solar light in comparison with α-S8 and BaTiO3 photocatalysts: Effect of spontaneous polarization

The BaTiO3/α-S8 composite is fabricated via grinding method combined with melt-diffusion strategy. Non-centrosymmetric crystal structure of BaTiO3 induces a permanent spontaneous polarization which drives the electrons and holes to the opposite surfaces. This resultant surface polarization causes band bending based on C+/C− domain. Under the UV light, the C+ domain of BaTiO3 influences the energy bands to bend downwards and drives the CB electrons towards the CB of α-S8 and the VB holes of BaTiO3 react with the surface adsorbed hydroxyl anions. Under the illumination of solar light, the charge carriers are generated only in the α-S8. The photogenerated electrons from the CB of α-S8 are trapped by the surface oxygen molecules and the holes move upwards to the VB of BaTiO3, as this energy level is located at 0.08 V higher than the VB of α-S8. The composite exhibited enhanced photocatalytic activity for the degradation of fast red dye under UV/solar light due to the efficient vectorial charge carrier separation. The band bending at the junction of two semiconductors results in a spatially distinct redox chemistry. This favors the enhancement in the photocatalytic activity of the BaTiO3/α-S8 composite.

Tunable Fermi level of graphene modified by azobenzene molecules

Carbon-based nanomaterials, especially graphene, are considered to be the most hopeful alternatives to silicon in the near future of pushing to its limits. However, the zero band gap of graphene suppresses its application, it needs to develop a convenient method to modulate its Fermi level and band gap. Herein, we report a general approach to regulate Fermi level of graphene precisely by designed azobenzene molecules with different dipole moment and dipole orientation. The Raman spectra results demonstrate the π-π interaction between the azobenzene molecules and graphene, which result in the modulation of the Fermi level of graphene. Besides, based on the field effect transistor characteristic measurement, we also observe the Fermi level adjustment for the hole/electron doping to graphene from the spontaneous polarization effect of the azobenzene molecules, further speculating the regulation of electronic structure and providing a new route for changing the electrical properties of graphene.