KNbO3 Films Research Articles

Enhanced ultraviolet photo-detecting by constructing TiO2/KNbO3 heterojunction

The demands of the ultraviolet (UV) radiation monitoring is rising in fields of human health and survival surroundings. Among many types of UV detectors, the photoelectrochemical (PEC) type UV detectors based on solar sensitized cells have gained a lot attention because of their fast response, low cost, simple manufacturing process, self-powered and so on. The heterojunction can accelerate the separation and migration of photogenerated carriers and then improve the photoresponse of UV detectors. In this work, the nanostructure KNbO3 (KNO) films were modified by chemical bath deposition to form a TiO2/KNO heterojunction. Compared with the UV detectors based on pure KNO and TiO2, the responsivity of the developed PEC UV photodetectors based on TiO2/KNO reached 11.99 mA W−1 under UV illumination of 35 mW cm−2 @ 365 nm, which is higher than that of the pure KNO UV detector (1.26 mA W−1) and the TiO2 UV detector (3.96 mA W−1). Besides, the lifetime of photogenerated carrier, response time and recovery time of the detector are all promoted. The results can be attributed to the formation of a TiO2/KNO heterojunction, in which the generated built-in electric field can promote the separation of photogenerated carriers and hinder the recombination of photogenerated electron–hole pairs, thus improving the detection performance of the UV detectors.

Polarization-gradient KNbO3 film with a large photovoltaic current

Potassium niobate (KNbO3, KNO) has been intensively investigated for photovoltaic performance because of its non-toxicity and excellent nonlinear optical properties. The corresponding photovoltaic current density, however, remains very low due to a wide bandgap. Chemical doping and strain engineering strategies have been employed to tailor the band structure to enhance photovoltaic current density. Nevertheless, the original current density is still at a level of several tens of nA/cm2, significantly limiting device applications. In this work, we report a lattice-gradient KNO film on (100) single-crystal 0.7 wt. % Nb doped SrTiO3 (NSTO) substrate processed by annealing, generating a polarization-gradient that allows us to generate a large current density via a built-in field. The film exhibits a remarkable short-circuit current density (Jsc) of 58.63 µA/cm2 under the 375 nm irradiation of 500 mW/cm2 light intensity, where the corresponding responsivity (117.26 µA/W) is ∼3.82 times higher than those of reported KNO-based materials. It was revealed that the annealing process driven interfacial structure evolution from disorder to atomic-scale smoothness, accompanied by the transformation of the polarization shielding mechanism. After this process, an intriguing lattice-gradient throughout the film was established to have a uniform polarization direction, possibly accounting for the improved photovoltaic current density of KNO film. These findings may trigger interest in developing KNO as a potential key material for lead-free optoelectronic or photodetector devices.

Solution epitaxy of single-crystal and single-domain KNbO3 film with a great photovoltaic current

As a classic ferroelectric material, orthorhombic perovskite KNbO3 demonstrates a large electro-optical coefficient, high refractive index, and excellent nonlinear optical property. These make single-crystal KNbO3 films being long-pursued for lead-free sensor or optoelectronic device applications. However, it remains a challenge to obtain high-quality single-crystal KNbO3 films because of the volatility of potassium at a high growth temperature. In this work, single-crystal KNbO3 films with a tunable thickness have been successfully fabricated by a low-temperature (∼220 °C) hydrothermal epitaxy on cubic single-crystal Nb:SrTiO3 substrates. Interestingly, the KNbO3 film was investigated to adopt a single-domain structure with a polarization direction pointing to the substrate. In-depth characterization reveals an intriguing disordered region of approximately 0.6–1.2 nm because of partial Nb substitution of Ti near the interface, giving rise to the coexistence of Ti3+ and Ti4+ ions. Such region is proposed to release a large interface strain energy from a large lattice misfit (∼2.3%) between {011}KNbO3 and {100}Nb:SrTiO3 and reduce interfacial electrostatic energy via screening of the positive polar surface of KNbO3. This allows to drive a hydrothermal epitaxy of single-crystal and single-domain KNbO3 without forming a typical dislocation. The films demonstrate a great photovoltaic short-circuit current of 15.65 μA/cm2 under the 375 nm laser irradiation of 500 mW/cm2 light intensity, and the corresponding photoresponsivity is the highest among the emerged KNbO3-based materials. Such improvement in the photoresponsivity has been attributed to a successful low-temperature solution epitaxy of single-crystal and single-domain KNbO3 films, where K deficiency can be significantly suppressed. These findings suggest that a solution epitaxy strategy is effective in producing high-quality ferroelectric films for lead-free optoelectronic or self-powered photodetector device applications.

Electrocaloric Effect of (110) Oriented KNbO₃ Film.

The room temperature electrocaloric effect is researched for (110) oriented KNbO₃ film based on Landau-Devonshire theory. The phase map with different ferroelectric states is built at room temperature with the considerations of thermodynamic equilibrium conditions and minimum of thermodynamic potential. Five ferroelectric structural phases are obtained theoretically. The negative in-plane misfit strains are conducive to form the tetragonal c phase and the positive strains are in favor of the stability of tetragonal a1 and a₂ phases. The electrocaloric effect relies on both misfit strain and electric field. Moreover, large electrocaloric effect is achieved in the orthorhombic phases.

Study of piezoelectric behaviour of sputtered KNbO3 nanocoatings for flexible energy harvesting

Novel potassium niobate (KNbO3) nanocoatings were sputtered on polyethylene naphthalate (PEN) substrates at different sputtering voltages and concentrations of sputtering gas. The relations growth time - thickness and sputtering voltage-deposition speed were established and the vacuum deposition modes were optimized. This was realized, in order to correlate the microstructure, surface morphology and the growing conditions for the nanosized KNbO3 films. Uniform coatings with variety of thicknesses were obtained without overheating degradation of the substrates at process parameters Us and PAr varying from 0.6 kV to 0.9 kV and from 1 × 10−2 Torr to 9 × 10−3 Torr, respectively. New Al/KNbO3/Al energy harvesting element was fabricated and tested on tension-compression cycles. For the samples, showing the lowest surface roughness of the KNbO3 film (3.6%), maximum piezoelectric voltage of 431 mV was measured at loading up to 3 kg. Basic electric parameters, such as capacitance, contact and bulk resistance of the piezoelectric element, involving KNbO3 film, produced at the optimum sputtering conditions, were measured as a function of the applied mass load. Output electrical power of 2.324 μW was obtained at maximum mechanical loading, which is comparable with the results for lead-containing thin film harvesters. Theoretical analysis of the novel elements was made based on their dielectric behaviour in terms of polarization type. They were connected to passive power management system for operation as an independent power supply for low-power electronics.

Orientation control of KNbO3 film grown on glass substrates by Ca2Nb3O10− nanosheets seed layer

ABSTRACT KNbO3 films have been deposited by pulsed laser deposition on glass substrates covered by Ca2Nb3O10− nanosheets used as seed layer, for the purpose of promoting (001) preferential orientation. Nanosheets have been prepared by the exfoliation process of HCa2Nb3O10 phase that is obtained by cation exchange of the Dion-Jacobson KCa2Nb3O10 phase in an acidic solution. Electron diffraction in transmission electron microscopy performed on KCa2Nb3O10 and HCa2Nb3O10 powders, and on Ca2Nb3O10− nanosheets revealed formation of local superstructures that are not detected by X-ray diffraction due to the weakness of superstructure reflection intensities. Nanosheets were deposited on substrates using the Langmuir-Blodgett method. A smooth covering of the surface was shown by atomic force microscopy. 200 nm thick KNbO3 films have been grown by pulsed laser deposition on Ca2Nb3O10−-NS/glass and on (001)SrTiO3 substrates for comparison. Substrates covered with nanosheets induced highly textured (001)-oriented thin film, which possesses a similar microstructure as epitaxial film on single crystalline strontium titanate substrate.

Piezoelectric and pyroelectric properties of (001) oriented KNbO3 film

A phenomenological Landau-Devonshire theory is used to investigate the “misfit strain-temperature” phase diagram and physical properties of (001) oriented KNbO3 film. Three ferroelectric phases, such as c phase, aa phase, and γ phase, occur in the phase diagram. In-plane compressive misfit strain is beneficial to form c phase and tensile strain makes aa phase stable. Physical properties of KNbO3 film largely depend on misfit strain and electric field. At room temperature, γ phase of KNbO3 film with the tensile misfit strain has larger piezoelectric and pyroelectric coefficients than that of c phase with the compressive strain. Large piezoelectric and pyroelectric coefficients are 180 pm/V and 8.5 × 10−4 C/m2K respectively at the electric field of 100 KV/cm and misfit strain of 6.7 × 10−3, which are comparable with the experimental results of Pb(ZrxTi1−x)O3 film. Therefore, choosing the appropriate substrates can be used to improve the physical properties of KNbO3 film. In addition, the applied electric field decreases the dielectric, piezoelectric and pyroelectric properties of (001) oriented KNbO3 film.

Large modulation of interface magnetization and interface magnetoelectric effect in SrRuO3|KNbO3 oxide heterostructures: Prediction from first-principles study

The interface magnetization and its modulation is explored in oxide heterostructures within the framework of density functional theory. In particular, modulation in interface magnetization is investigated in prototype heterostructure SrRuO3|KNbO3(0 0 1) consisting ferromagnetic SrRuO3 and nonmagnetic KNbO3 as the thickness of KNbO3 is varied. The computed interfacial magnetization is found to alternate between a higher and a lower value with increasing KNbO3 thickness in the heterostructure. The average change in interface magnetization is computed to be ∼−60% as KNbO3 film changes from non-stoichiometric to stoichiometric and ∼+60% as it changes from stoichiometric to non-stoichiometric due to variation in thickness. The magnetic moment per interface cell changes from ∼2.27 μB for non-stoichiometric KNbO3 to ∼1.31 μB for stoichiometric KNbO3 in the heterostructure. The modulation in interface magnetization arises due to spin-dependent screening of effective charge of KNbO3 unit cells by accumulated free carriers at the interface. Large modulation in interface magnetization is also obtained for other heterostructures consisting magnetic substructure and ionized planes at the interface. In addition, the alteration in interface magnetization due to ferroelectric polarization reversal in ferromagnetic|ferroelectric SrRuO3|KNbO3 (0 0 1) superlattice is explored. The resulting interface magnetoelectric effect as well as control of interface magnetization using ionized planes may have interesting implications for technological applications.

Synaptic Plasticity and Metaplasticity of Biological Synapse Realized in a KNbO3 Memristor for Application to Artificial Synapse.

Amorphous KNbO3 (KN) films were grown on a TiN/SiO2/Si substrate to synthesize a KN memristor as a potential artificial synapse. The Pt/KN/TiN memristor exhibited typical and reliable bipolar switching behavior with multiple resistance levels. It also showed the transmission properties of a biological synapse, with a good conductance modulation linearity. Moreover, the KN memristor can emulate various biological synaptic plasticity characteristics including short-term plasticity, long-term plasticity, spike-rate dependent plasticity, paired-pulse facilitation, and post-tetanic potentiation by controlling the number and rate of the potentiation spike. Spike-timing-dependent plasticity (STDP), which is an essential property of biological synapses, is also realized in the KN memristor. The synaptic plasticity of the KN memristor can be explained by oxygen vacancy movement and oxygen vacancy filaments. The metaplasticity of biological synapses was also implemented in the KN memristor, including the metaplasticity of long-term potentiation and depression, and of STDP. Therefore, the KN memristor could be used as an artificial synapse in neuromorphic computing systems.

Phase diagram and physical properties of (110) oriented KNbO3 film

Based on the phenomenological Landau-Devonshire theory, the phase diagram and physical properties of (110) oriented KNbO3 film are investigated. The “misfit strain-misfit strain” phase diagram at room temperature is constructed in the frame of eighth-order thermodynamic potential. Three tetragonal phases and two orthorhombic phases are stable in phase diagram due to the strain anisotropy. The large in-plane compressive or tensile misfit strains favor the tetragonal phases. Moreover, both the tensile misfit strain u1 (or u2) and compressive strain u2 (or u1) induce the orthorhombic phases. Large piezoelectric coefficient is obtained in orthorhombic phases which have the small in-plane tensile misfit strains. It makes (110) oriented KNbO3 film suitable for applications in electromechanical devices.

Low-Temperature-Grown KNbO3 Thin Films and Their Application to Piezoelectric Nanogenerators and Self-Powered ReRAM Device.

Amorphous KNbO3 (KN) film containing KN nanocrystals was grown on TiN/SiO2/Si substrate at 350 °C. This KN film showed a dielectric constant (εr) and a piezoelectric strain constant (d33) of 43 and 80 pm/V at 10 V, respectively, owing to the existence of KN nanocrystals. Piezoelectric nanogenerators (PNGs) were fabricated using KN films grown on the TiN/polyimide/poly(ethylene terephthalate) substrates. The PNG fabricated with the KN film grown at 350 °C showed an open-circuit output voltage of 2.5 V and a short-circuit current of 70 nA. The KN film grown at 350 °C exhibited a bipolar resistive switching behavior with good reliability characteristics that can be explained by the formation and rupture of the oxygen vacancy filaments. The KN resistive random access memory device powered by the KN PNG also showed promising resistive switching behavior. Moreover, the KN film shows good biocompatibility. Therefore, the KN film can be used for self-powered biomedical devices.

Structural and electrical properties of KNbO3 thin film grown on a Pt/Ti/SiO2/Si substrate using the RF magnetron sputtering method

A homogeneous KNbO3 (KN) thin film was grown on a Pt/Ti/SiO2/Si substrate using the RF sputtering method. A K2.88Nb5O15 secondary phase was formed in the KN film when the deposition and the annealing temperatures were greater than room temperature (RT) and 800 °C, respectively, owing to the evaporation of K2O. On the other hand, KNb3O8 and K3Nb5.45O15 secondary phases were formed in the KN films when the annealing temperature was less than 800 °C or the annealing time at 800 °C was shorter than 90 min. A homogeneous KN thin film was formed when it was deposited at RT and subsequently annealed at 800 °C for 90 min under the K2O atmosphere. This KN film exhibits a relative permittivity of 884 with a dissipation factor of 6.71% at 100 kHz. The leakage current density of 1.06 × 10−6 A/cm2 at 0.1 MV/cm and a breakdown field of 1.5 MV/cm were observed from this film. This film showed a saturation polarization of 21.9 μC/cm2, with a remnant polarization of 8.3 μC/cm2, and a piezoelectric strain constant of 125 pm/V.

Effect of epitaxy on interband transitions in ferroelectric KNbO3

Very large lattice strain and strain-induced polarization are achieved in KNbO3 using epitaxial growth of a thin KNbO3 film onto a (001)-oriented SrTiO3 single-crystal substrate. We demonstrate experimentally that epitaxy produces dramatic changes of interband transitions in the film compared to those of a reference KNbO3 crystal: the energies of transitions change, some transitions are substantially suppressed and new ones appear in the film. A comparison of the experimental observations with theoretical calculations points to yet unexplored phenomena. Our results indicate that optical refraction and electro-optical coefficients of ferroelectric films can be controlled by epitaxial growth, which is of importance for emerging photonic and optoelectronic applications.

Two-dimensional electron gas in the KNbO3:Y ultrathin film

First-principles calculations are used to study KNbO3:Y ultrathin film. The conducting two-dimensional electron gas (2DEG) is found in KNbO3:Y film, whereas SrTiO3:Y superlattice is insulating [Science 331, 886 (2011)]. The carrier density of 2DEG of KNbO3:Y film is twice that of LaTiO3/SrTiO3 superlattice, which is in agreement with the theoretical results of KNbO3:La superlattice. It is found that the charge densities of different monolayers are modulated by the polarization effect of KNbO3. We predict that 2DEG may be in other rare-earth atoms-doped KNbO3 film.

Ferroelectric and piezoelectric properties of KNbO3 films deposited on flexible organic substrate by hydrothermal method

KNbO3 films were synthesized at 120–240 °C by a hydrothermal method. The deposition amount of KNbO3 films decreased with decreasing deposition temperature because the deposition rate decreased together with the increase in the starting time of precipitation. The KNbO3 phase was ascertained at temperature as low as 120 °C in films deposited on (100)cSrRuO3//(100)SrTiO3 substrates. On the other hand, powders prepared at 150 °C included the Nb2O5 phase as well as the KNbO3 phase. In addition, powders prepared at 120 °C consist of the K3Nb7O19 phase instead of the KNbO3 phase. KNbO3 films were deposited at 150 °C on LaNiO3/Pt/Ti/polysulfone substrates. Films consisting of the polycrystalline KNbO3 phase were obtained, and their ferroelectricity and piezoelectricity were observed. The remanent polarization and piezoelectric coefficient were 4.2 µC/cm2 and 20–32 pm/V, respectively.

Lead- and alkali-metal-free BaTiO3–Bi(Mg0.5Ti0.5)O3–BiFeO3 solid-solution thin films with high dielectric constant prepared on Si substrates by solution-based method

Lead- and alkali-metal-free BaTiO3–Bi(Mg0.5Ti0.5)O3–BiFeO3 solid-solution thin films were prepared on (111)cSrRuO3/(111)Pt/TiO2/SiO2/(100)Si substrates by chemical solution deposition (CSD) and their crystal structure and dielectric properties were investigated. The lattice spacing as a function of z/(x + z) in xBaTiO3–0.1Bi(Mg0.5Ti0.5)O3–zBiFeO3 indicated the existence of phase boundaries (pseudocubic/rhombohedral) in the range of z/(x + z) = 0.33–0.56, where the relatively high relative dielectric constant, εr, of above 800 was obtained. On the other hand, dielectric loss, tan δ, of below 0.2 was confirmed in the range z/(x + z) = 0–0.87, which rapidly increased toward z/(x + z) = 1.0. The relatively high εr values of these films deposited on Si substrates by a solution-based process suggest that they can be used as alternative to Pb(Zr,Ti)O3, KNbO3, and (Bi1/2Na1/2)TiO3-based films.

Low Temperature Preparation of KNbO3Films by Hydrothermal Method and Their Characterization

ABSTRACTKNbO3films were prepared at 100 - 240°C on (100)cSrRuO3//(100)SrTiO3substrates by hydrothermal method using KOH and Nb2O5as source materials. The incubation time before starting deposition and the deposition rate after starting deposition increased and decreased with decreasing deposition temperature, respectively. Epitaxial {100}c-oriented KNbO3films with 300 nm thick were successfully obtained at 100°C on (100)cSrRuO3//(100)SrTiO3substrates for 144 h. We observed the typical butterfly-shape strain curves originated from the piezoelectricity for the first time for KNbO3films deposited down to 120°C.

Lead-Free Oxide Thin Films for Gas Detection

In view to develop gas sensors, a first generation of lead-free thin films was deposited by different techniques on commercial Si and Al2O3 substrates. During our research project, the correlation between the micro structure of films, the structure of the embedded sensors and the applied temperature range, is being studied. In the first year, doped and undoped BaTiO3, KNbO3 and ZnO thin films have been deposited by sol-gel and PLD techniques. BT and BST films have shown a polycrystalline structure with very fine and regular grains, while disoriented grains with an average size ranging from 50 to 200 nm were observed on the KNbO3 film surface, and ZnO films exhibited a columnar growth. All films were characterized and finally embedded to make semiconductor gas sensors which have been tested under different gases. In this first generation of sensors, ZnO sensors have shown encouraging preliminary results under CO and H2S gases. Keywords : thin films, lead-free oxide, sensor, gas.

Lead-Free Oxide Thin Films for Gas Detection

In view to develop gas sensors, a first generation of lead-free thin films was deposited by different techniques on commercial Si and Al2O3 substrates. During our research project, the correlation between the micro structure of films, the structure of the embedded sensors and the applied temperature range, is being studied. In the first year, doped and undoped BaTiO3, KNbO3 and ZnO thin films have been deposited by sol-gel and PLD techniques. BT and BST films have shown a polycrystalline structure with very fine and regular grains, while disoriented grains with an average size ranging from 50 to 200 nm were observed on the KNbO3 film surface, and ZnO films exhibited a columnar growth. All films were characterized and finally embedded to make semiconductor gas sensors which have been tested under different gases. In this first generation of sensors, ZnO sensors have shown encouraging preliminary results under CO and H2S gases. Keywords : thin films, lead-free oxide, sensor, gas.

Effect of substrate on phase formation and surface morphology of sol-gel lead-free KNbO3, NaNbO3, and K0.5Na0.5NbO3 thin films

AbstractEnvironmentally acceptable lead-free ferroelectric KNbO3 (KN) or NaNbO3 (NN) and K0.5Na0.5NbO3 (KNN) thin films were prepared using a modified sol-gel method by mixing potassium acetate or sodium acetate or both with the Nb-tartrate complex, deposited on the Pt/Al2O3 and Pt/SiO2/Si substrates by a spin-coating method and sintered at 650°C. X-ray diffraction (XRD) analysis indicated that the NN and KNN films on the Pt/SiO2/Si substrate possessed a single perovskite phase, while NN and KNN films on the Pt/Al2O3 substrate contained a small amount of secondary pyrochlore phase, as did KN films on both substrates. Scanning electron microscopic (SEM) and atomic force microscopic (AFM) analyses confirmed that roughness R q of the thin KNN/Pt/SiO2/Si film (≈ 7.4 nm) was significantly lower than that of the KNN/Pt/Al2O3 film (≈ 15 nm). The heterogeneous microstructure composed of small spherical and larger needle-like or cuboidal particles were observed in the KN and NN films on both substrates. The homogeneous microstructure of the KNN thin film on the Pt/SiO2/Si substrate was smoother and contained finer spherical particles (≈ 50 nm) than on Pt/Al2O3 substrates (≈ 100 nm). The effect of different substrates on the surface morphology of thin films was confirmed.