Tunable Device Applications Research Articles

High tunability in (1 1 0)-oriented BaZr0.2Ti0.8O3 (BTZ) lead-free thin films fabricated by pulsed laser deposition

Using a pulsed laser deposition method the BaZr0.2Ti0.8 O3 (BTZ) lead–free thin films with a thickness of ~250nm were grown on FTO, ITO and Pt–Si substrates, respectively. The analysis results of microstructural, dielectric properties and leakage current reveal that the thin films deposited on Pt–Si substrates are oriented growth along the (1 1 0) direction and exhibit the optimal performance characteristics. Calculations of figure of merit (FoM) and dielectric tunability display a maximum value of ~42.8 and ~68.5% at E = 400kV/cm at room temperature, respectively. The excellent tunable properties, high dielectric constant (~635@ 100kHz) and low leakage current density of (9.3 × 10–8A/cm2 at 400kV/cm) make the (1 1 0)–oriented BaZr0.2Ti0.8 O3 thin film to be an attractive material for applications of tunable devices.

Electric field involved transport at elevated temperature in nanocrystalline silicon carbide nitride (nc-SiCN) thin films for harsh environment applications

The present study represents a systematic temperature dependent charge transport and dielectric properties of nanocrystalline silicon carbide nitride (nc-SiCN) thin films grown on Pt/Ti/SiO2/Si substrate. A large negative temperature coefficient of resistance (TCR) ranging from 6200 to 2300 ppmK-1 in the temperature range 300–773 K, suggests that the nc-SiCN thin films could be useful for futuristic thermal-based sensors. The current density vs. electric field (J-E) characteristics was measured at different temperatures (300–673 K). Detailed J-E analysis revealed an ohmic conduction at the low applied electric field (<65 kV/cm) within the entire temperature range. However, at high electric field (>65 kV/cm), space charge limited conduction (SCLC) mechanism was found to be dominating in low measurement temperature (300–473K), whereas, a transition from SCLC mechanism to Poole-Frenkel mechanism was observed with further increment in the temperature beyond 473 K. The temperature invariant dielectric tunability (nr ∼10%) and low zero electric field leakage current density (J ∼10−7A/cm2) at 673 K temperature, demonstrates the feasibility of nc-SiCN thin films for tunable device applications in the high-temperature and harsh environment.

Design of negative refractive index metamaterial with water droplets using 3D-printing

We numerically and experimentally demonstrate a negative refractive index (NRI) behavior in combined water droplets and photosensitive resin materials operating in the microwave regime. The NRI is achieved over a very wide frequency range in 10.27–15 GHz with bandwidth of 4.63 GHz. The simulated results approximately agree with the experimental results. The negative index band can be controlled by water droplet radius. The proposed metamaterial production process is simple and may have potential applications in broadband tunable devices.

Metasurfaces Based on Phase-Change Material as a Reconfigurable Platform for Multifunctional Devices.

Integration of phase-change materials (PCMs) into electrical/optical circuits has initiated extensive innovation for applications of metamaterials (MMs) including rewritable optical data storage, metasurfaces, and optoelectronic devices. PCMs have been studied deeply due to their reversible phase transition, high endurance, switching speed, and data retention. Germanium-antimony-tellurium (GST) is a PCM that has amorphous and crystalline phases with distinct properties, is bistable and nonvolatile, and undergoes a reliable and reproducible phase transition in response to an optical or electrical stimulus; GST may therefore have applications in tunable photonic devices and optoelectronic circuits. In this progress article, we outline recent studies of GST and discuss its advantages and possible applications in reconfigurable metadevices. We also discuss outlooks for integration of GST in active nanophotonic metadevices.

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

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

Tunable performance of BaZr0.2Ti0.8O3 thin films prepared by pulsed laser deposition

Abstract BaZr 0.2 Ti 0.8 O 3 (BZT) thin films were deposited at various oxygen pressures (0–60 Pa) on Pt/TiO x /SiO 2 /Si substrates by using pulsed laser deposition. The crystallinity of the thin films initially improves but subsequently deteriorates with increasing oxygen pressure. The tunable performance of BZT thin films is associated with crystal structure and oxygen vacancies. The theoretical mechanisms for the relative permittivity and dielectric loss as a function of electric field are proposed. The frequency response of dielectric properties is also investigated and the variation mechanisms are supposed. Eventually, BZT thin films fabricated under 15 Pa have the highest tunability (70.3% at 400 kV/cm) with the largest relative permittivity of 486, the lowest loss tangent of 0.021 and the maximum commutation quality factor of 7744.8. The results imply that BZT thin films are potential candidates for tunable device applications.

Geometrical influence of conducting fillers on the dielectric tunable properties of antiferroelectric ceramic/conducting filler/polystyrene composites under low electric field

(Pb0.87Ba0.1La0.02)(Zr0.7Sn0.24Ti0.06)O3 (PBLZST) antiferroelectric ceramic/conducting filler (carbon nanopowder, graphene nanosheets or carbon nanotubes)/polystyrene (PS) composites were prepared by solution-mixing method and molded by hot-pressing process. All composites exhibited high room-temperature tunability under low electric field (200 V/mm) due to the introduction of conducting filler, which increased the effective electric field applied on the ceramics particles. For different carbon conducting fillers, their shapes are different and thus they have different specific surface areas, resulting in different mass ratios for obtaining highest dielectric tunability. The maximum room-temperature dielectric tunability of three-phase composites with different conducting fillers were respectively 17.5% (1.0 wt% carbon nanopowder), 17% (0.4 wt% carbon nanotubes) and 20% (0.6 wt% graphene nanosheets) at 200 V/mm, which were two or three times of the tunability of PBLZST/PS composites without any conducting fillers. Besides, all composites showed low dielectric loss and permittivity that met the practical application requirements. All these make three-phase composites obtained in this work promising candidates for tunable devices application.

Structures and dielectric performances of Mn/Y alternately doped BST films prepared by a novel preheating process

Mn (2%, 5%, 8%)/Y (1%) alternately doped BST multilayer thin films have been prepared by a modified sol-gel method on Pt/Ti/SiO2/Si substrates. The effects of Mn doping contents on the structures and dielectric properties of the films are studied by XRD, XPS, FESEM, EDS, and C–V. Meanwhile, a graded preheating (GP) process is designed to modify the conventional preheating (CP) process. It is found that for the two processes, the doped Mn2+ ions first enter into Ti sites and then into Sr sites with an increase of doping content, but the GP process mainly promotes Mn2+ ions to enter into Ti sites independence of the doping content. Therefore, the GP process leads to a more obvious enhancement on nonlinear dielectric properties, in particular a more marked increase in tunability and a more evident decrease in dielectric loss of the BST films. It is implied that the GP process is an effective route to improve the dielectric properties of BST films for the applications of tunable microwave devices. The effects of the CP process with different temperatures on the BST films and the modified mechanisms are also discussed.

Dielectric properties of compositionally graded Ba1−xLaxTi1−x/4O3 thick films

Compositionally graded thick films based on lanthanum-substituted barium titanate solid solutions Ba1−xLaxTi1−x/4O3 (BLT, x=0.02, 0.04, 0.06) are prepared by tape casting and their phase structures, surface morphologies, dielectric and tunability properties have been examined. Compared with single-layered BLT film, compositionally graded BLT02/04/06 film with three-layered sandwich-structure has better temperature stability in dielectric and tunability properties. Curie peak is broadened, dielectric loss is reduced, and more importantly, it has moderate permittivity and high tunability (>35%) in a temperature range of 25–75°C, which is beneficial for practical applications of tunable devices.

BMN-based transparent capacitors with high dielectric tunability

This paper covers the fabrication of a novel transparent capacitor with high dielectric tunability using Sb–doped SnO2/Ag/Bi1.5MgNb1.5O7/W–doped SnO2 structure and its observed dielectric tunable characteristics. This kind of Bi1.5MgNb1.5O7-based transparent capacitor shows an average optical transmittance of ∼80% (including the substrate) in the visible light range (380–780 nm), and a dielectric tunability of ∼18% (0.83 MV/cm) at 100 kHz, while the dielectric constant and loss tangent are about 87 and 0.0032 at zero bias field, respectively. The transparent capacitor exhibits a leakage current density of approximately 5.8 × 10−7 A/cm2 at 0.4 MV/cm. We believe that Bi1.5MgNb1.5O7-based capacitors are a promising candidate for transparent electric-field tunable device applications.

Low loss and highly tunable (Ba,Sr)(Ti,Mn)O3/(Ba,Sr)TiO3 bilayered films for electrically tunable microwave device applications

Ba0.05Sr0.95TiO3 (BST), 5 mol% Mn-doped BST (Mn-BST) and bilayered Mn-BST/BST films with a thickness of about 100 nm were fabricated by using pulsed laser deposition technique on MgO single crystal substrates, and their microwave dielectric properties were comparatively investigated. It was confirmed that all the BST-based films were crystallized in pure single-oriented perovskite phase. There appeared nanosized pores in the BST film; after addition of Mn, however, the film surface morphology became denser while exhibiting an increased roughness, and ultimately, a smooth and dense morphology was achieved by the design of bilayered Mn-BST/BST film. Coherently, the microwave dielectric properties of BST films were enhanced by Mn doping and more significantly enhanced in the bilayered Mn-BST/BST film. As a result, for the bilayered film, a very low loss tangent of 0.017–0.007 and a high dielectric tunability of 70.9% under E = 36 V/μm were simultaneously obtained at 79 K and L-band frequency (1–2 GHz), thereby resulting in an extremely high commutation quality factor of 14492. The excellent microwave dielectric properties obtained in the Mn-BST/BST bilayered film originate not only from the Mn-doping effect, but are also ascribed to the pre-deposition of BST layer, which serves as a good template for the growth of Mn-BST film.

Fabrication, characterisation and dielectric properties of KH550 modified BST/PVDF nanocomposites with high dielectric strength

Ba0.6Sr0.4TiO3 (BST)/poly(vinylidene fluoride) (PVDF) 0–3 nanocomposites with BST nanoparticle contents of 10–40 vol% were prepared by tape casting method. The BST nanoparticles with ∼60 nm crystal size and accurate atomic ratio were synthesised by sol–gel method. The effect of the BST nanoparticle contents on the microstructure and dielectric properties of BST/PVDF nanocomposites were investigated. The BST/PVDF nanocomposites system declares that feasibility of applying low contents nanoparticles to form a homogeneous distribution in polymer matrix, higher dense interface connectivity, thereby providing a significant improvement to the dielectric strength and energy storage density; meanwhile, the nanocomposites possess high dielectric tunability. In this study, the maximum dielectric strength ∼287.12 kV/mm and energy storage density ∼5.15 J/cm3 were obtained in 10 vol% BST/PVDF nanocomposites, respectively. Moreover, the nanocomposite possesses high dielectric tunability ∼62.6% at 58 kV/mm, which proposed the nanocomposites suitable for tunable microwave devices, supercapacitors and energy storage device applications at high-voltage environment.

Tunable plasmon-induced transparency in a grating-coupled double-layer graphene hybrid system at far-infrared frequencies.

A grating-coupled double-layer graphene hybrid system is proposed to investigate the plasmon-induced transparency effect at far-infrared frequencies. Based on the guided mode resonance principle, a diffractive grating is used to couple the normally incident waves and excite the plasmonic resonances on two graphene films separated by a spacer, thereby avoiding the need for patterning graphene. It is found that the origin of the observed transparency window transforms from Autler-Townes splitting to electromagnetically induced transparency with the increase of the separation distance between the two graphene films. The tunability of this hybrid system is also investigated via varying the Fermi energy in graphene. The proposed hybrid system has potential applications in tunable switches, sensors, and slow light devices and may open up new avenues for constructing easy-to-fabricate graphene-based plasmonic devices.

Electrically Tunable Scattering from Devitrite–Liquid Crystal Hybrid Devices

Devitrite is normally an unwanted crystalline impurity in the soda‐lime‐silica glass making process. Thin needles formed by heterogeneous nucleation of devitrite on the glass surface provide unique birefringence properties for potential applications in tunable optical devices. Here, devitrite and a liquid crystal are combined to create an electrically variable optical diffuser. The magnitude and scattering angle of the transmitted light propagating through the diffuser are tuned by varying the voltage between the graphene and indium tin oxide electrodes on either side of the liquid crystal. The threshold voltage to switch the transmitted light from a predominantly horizontal diffusion to a random order is 3.5 V. Angle‐resolved measurements show broad diffusion angles of transmitted light with a maximum deflection of ±60°. The dynamically tunable devitrite‐liquid crystal hybrid devices may advance the development of currently less viable technologies including beam shaping and automatic light transmission control.

Photoresponse of composites of zinc oxide and poly(3-hexythiophene) under selective UV and white-light illumination

We investigate charge transport in UV sensing devices based on organic-inorganic semiconductor composites with the metal-semiconductor-metal (MSM) structure. Composite materials of zinc oxide (ZnO) nanoparticles and poly(3-hexylthiophene) (P3HT) were prepared by drop-casting their colloidal mixture in chloroform onto low-cost interdigitated copper electrodes. The current-voltage characteristics of the devices were investigated under both dark and illuminated conditions in the UV–visible range. The highest photoresponse was observed for an optimal P3HT:ZnO ratio of 1:8 w/w in the wavelength range between 310 and 380 nm. The dynamic response was investigated by pulsing a 365 nm UV light with a long period to reveal the response time of 4 s and the recovery time of less than 1 s. The photoresponse of the materials was also investigated for a shorter period of UV pulsing, using a rotating chopper. The response time and recovery time for the short UV pulse were found to be approximately 20 m and 25 m, respectively. The dual response times should stem from the presence of two types of semiconductor materials, namely ZnO with a high electron mobility and P3HT with a moderate hole mobility. To probe the charge generation and transport mechanisms, we further investigate the photoresponse using UV pulsing under background white light of different intensities, and vice versa. The background white light was found to deteriorate the UV photoresponse of the materials. On the other hand, the background UV illumination produced an anomalous photoresponse pattern with the white light pulsing. Understanding the charge transport mechanisms for composite materials is highly important for future applications in low-cost UV sensors and tunable optoelectronic devices.

Dielectric relaxation and pinning phenomenon of (Sr,Pb)TiO3 ceramics for dielectric tunable device application.

The behavior of ferroelectric domain under applied electric field is very sensitive to point defects, which can lead to high temperature dielectric relaxation behaviors. In this work, the phases, dielectric properties and ferroelectric switching behavior of strontium lead titanate ceramics were investigated. The structural characterization is confirmed by X-ray diffraction. The high dielectric tunability and high figure of merit of ceramics, especially Sr0.7Pb0.3TiO3 (SPT), imply that SPT ceramics are promising materials for tunable capacitor applications. Oxygen vacancies induced dielectric relaxation phenomenon is observed. Pinched shape hysteresis loops appeared in low temperature, low electric field or high frequency, whereas these pinched hysteresis loops also can become normal by rising temperature, enhancing electric field or lowering frequency. The pinning and depinning effect can be ascribed to the interaction between oxygen vacancies and domain switching. A qualitative model and a quantitative model are used to explain this phenomenon. Besides, polarization and oxygen treated experiment can exert an enormous influence on pinning effect and the machanisms are also discussed in this work.

Unique dielectric tunability of Pb0.99[(Zr0.6Sn0.4)0.94Ti0.06]0.98Nb0.02O3 antiferroelectric ceramics

The tunable dielectric properties of Pb0.99[(Zr0.6Sn0.4)0.94Ti0.06]0.98Nb0.02O3 antiferroelectric ceramics were investigated, and high relative tunability of 49% was obtained at 25 °C under a low bias electric field of 50 kV/cm. Abrupt changes and a significant hysteresis in dielectric constant and dielectric loss against bias electric field were observed, which are very different from the previously reported antiferroelectric materials. The unique dielectric tunability is attributed to the square-shaped double hysteresis loop and indicates the possible applications in some special tunable devices, such as an electrically-controlled switch. Pb0.99[(Zr0.6Sn0.4)0.94Ti0.06]0.98Nb0.02O3 ceramics also exhibit unique dielectric tunability at −5 °C. Abrupt changes in dielectric constant and dielectric loss were observed when the bias electric field increased to 31 kV/cm for the fresh sample, which is similar to the antiferroelectric-like dielectric tunability at 25 °C. However, the dielectric tunability was ferroelectric-like in the following measurement. This response is consistent with the hysteresis loop and can be explained by the electric field-assisted irreversible antiferroelectric-ferroelectric phase transition.

Active tuning of epsilon-near-zero point of hyperbolic metamaterial at visible and near-infrared regimes

An active method of continuously tuning the effective permittivity of a hyperbolic metamaterial at visible and near-infrared spectra is introduced in this letter. A transparent conducting oxide, whose optical properties can be altered accurately by applying an external voltage, is inserted into a traditional metal–dielectric multilayer structure to increase the degree of freedom for tuning the epsilon-near-zero point by shifting the topological transition point. The tuning effect is explained in detail by combining the relationship between permittivity and voltage with wavevector analysis. This may promote other active applications of hyperbolic metamaterials and ultrafast tunable optoelectronic devices.

Effect of Rb-doping on the dielectric and tunable properties of Ba0.6Sr0.4TiO3 thin films prepared by sol-gel

There is growing interest in BaxSr1−xTiO3 (BST) thin films due to their dielectric tunability and low dielectric loss. Doping small concentrations of elements is an effective way to increase the dielectric tunability and decrease the dielectric loss of BaxSr1−xTiO3 thin films. Many doping elements have been studied to date, but this does not include the use of rubidium (Rb). In this study, we reported here the effect of Rb-doping on the surface morphology, dielectric and tunable properties of Ba0.6Sr0.4TiO3 thin films prepared by the sol-gel method. It was observed that the tunability of Ba0.6Sr0.4TiO3 thin films was increased greatly from 46.53% to 63.37% at 377kV/cm when Rb was doped at a concentration of 4mol%. The observed improvements originate from increased grain size. Doping effects included occupation of the A site of the composites by Rb+ ions, and also enhanced grain growth during the annealing process, due to the relatively low melting point of rubidium oxide. (Ba0.6Sr0.4)96%Rb4%TiO3 thin films exhibited the largest grain size and a surface root-mean-square roughness (RMS) of 2.409nm, the smallest among the various doping concentrations of rubidium used. They also exhibited the best dielectric characteristics (figure of merit value 31.53 at 2MHz). Thus, Rb doping can be considered a promising way to optimize the dielectric properties of BST thin films for their potential application in microwave tunable devices.

Polarization-insensitive tunable multiple electromagnetically induced transparencies analogue in terahertz graphene metamaterial

A graphene-based metamaterial structure composed of multilayer graphene/dielectric stacking configuration is proposed, which achieves multiple analogue of electromagnetically induced transparencies (EIT) effect at terahertz frequencies. Using the phase-coupling scheme, a theoretical model is established to study the EIT-like effect of the proposed structure, and the theoretical calculations coincide well with the numerical simulated results. By varying the Fermi energy level of the graphene, the EIT-like windows can be dynamically tuned in a wide range of terahertz spectra. Particularly, since the symmetry of the structure, the EIT-like effect is polarization-insensitive and can be performed very well in a large incident angles, nearly 80° for both transverse electric and transverse magnetic waves. The proposed structure has potential applications in tunable terahertz chip-integrated optical devices, especially for dynamic multi-band filters, sensors, modulators and nonlinear devices.