Search

Charge doping in transition metal dichalcogenide is currently a subject of high importance for future electronic and optoelectronic applications. Here, we demonstrate chemical doping in the CVD-grown monolayer (1L) of WS2 by a few commonly used laboratory solvents by investigating the room temperature photoluminescence (PL). The appearance of distinct trionic emission in the PL spectra and quenched PL intensities suggests n-type doping in WS2. The temperature-dependent PL spectra of the doped 1L-WS2 reveal a significant enhancement of trions emission intensity over the excitonic emission at low temperature, indicating the stability of trion at low temperature. The temperature-dependent exciton-trion population dynamic has been modeled using the law of mass action of trion formation. These results shed light on the solution-based chemical doping in 1L-WS2 and its profound effect on the photoluminescence which is essential for the control of optical and electrical properties for optoelectronic applications. [ABSTRACT FROM AUTHOR]

Highly oriented 0.90[PbZr0.53Ti0.47]0.10[La0.80Sc0.20]O3-δ (PLZTS) thin films deposited on La0.67Sr0.33MnO3 (LSMO) coated MgO (100) substrates were grown by pulsed laser deposition technique. Temperature dependent dielectric measurements on metal-ferroelectric-metal Pt/PLZTS/LSMO thin film capacitors were carried out at several frequencies which exhibit high dielectric constants (450–580) with the diffuse peak around 400 K, and the diffusivity parameter γ was obtained as 1.96 for 100 kHz data. The slim polarization-electric field hysteresis loop was observed with less remanent polarization (∼7–10 μC/cm2) indicating its relaxor behavior. Temperature dependent Raman spectra measured between 80 and 550 K show softening of the symmetric E(LO2) band that disappeared at 300 K, corroborating the tetragonal-cubic phase transition. From the analysis of the measured hysteresis loops, the recovered energy density Ure ∼ 19 J/cm3 with the efficiency η ∼ 66% was estimated, suggesting its possible application in energy density capacitors. [ABSTRACT FROM AUTHOR]

We report structural, optical, temperature and frequency dependent dielectric, and energy storage properties of pulsed laser deposited (100) highly textured BaZrxTi1-xO3 (x=0.3, 0.4, and 0.5) relaxor ferroelectric thin films on La0.7Sr0.3MnO3/MgO substrates which make them potential lead-free capacitive energy storage materials for scalable electronic devices. A high dielectric constant of ~1400–3500 and a low dielectric loss of<0.025 were achieved at 10 kHz for all three compositions at ambient conditions. Ultrahigh stored and recoverable electrostatic energy densities as high as 214±1 and 156±1 J/cm3, respectively, were demonstrated at a sustained high electric field of ~3MV/cm with an efficiency of 72.8±0.6% in an optimum 30% Zr substituted BaTiO3 composition. [ABSTRACT FROM AUTHOR]

Metal-insulator-metal (MIM) capacitors comprised of amorphous Si:SrTiO3-Al2O3-Si:SrTiO3 multi-dielectric architecture have been fabricated employing a combination of pulsed laser and atomic layer deposition techniques. The voltage linearity, temperature coefficients of capacitance, dielectric and electrical properties upon thickness were studied under a wide range of temperature (200-400 K) and electric field stress (±1.5 MV/cm). A high capacitance density of 31 fF/µm², a low voltage coefficient of capacitance of 363 ppm/V², a low temperature coefficient of capacitance of <644 ppm/K, and an effective dielectric constant of ~133 are demonstrated in a MIM capacitor with ~1.4 nm capacitance equivalent thickness in a ~40 nm thick ultra high-k multi-dielectric stack. All of these properties make this dielectric architecture of interest for next generation, highly scaled MIM capacitor applications. [ABSTRACT FROM AUTHOR]

Nanostructured materials, such as carbon nanotubes, are excellent cold cathode emitters. Here, we report comparative field emission (FE) studies on topographically tailored few layer MoS2 films consisting of h0001i plane perpendicular (⊥) to c-axis (i.e., edge terminated vertically aligned) along with planar few layer and monolayer (1L) MoS2 films. FE measurements exhibited lower turn-on field Eto (defined as required applied electric field to emit current density of 10 µA/cm²) ~4.5 V/lm and higher current density ~1mA/cm², for edge terminated vertically aligned (ETVA) MoS2 films. However, Eto magnitude for planar few layer and 1L MoS2 films increased further to 5.7 and 11 V/µm, respectively, with one order decrease in emission current density. The observed differences in emission behavior, particularly for ETVA MoS2 is attributed to the high value of geometrical field enhancement factor (β), found to be ~1064, resulting from the large confinement of localized electric field at edge exposed nanograins. Emission behavior of planar few layers and 1L MoS2 films are explained under a two step emission mechanism. Our studies suggest that with further tailoring the microstructure of ultra thin ETVA MoS2 films would result in elegant FE properties. [ABSTRACT FROM AUTHOR]

We studied switchable photovoltaic and photo-diode characteristics of Pt/(Bi0.9Sm0.1)(Fe0.97Hf0.03)O3/ LaNiO3 (Pt/BSFHO/LNO) heterostructures integrated on Si (100). The directions of photocurrent (JSC) and rectification are found to be reversibly switchable after applying external poling voltages. In pristine state, metal-ferroelectric-metal capacitor Pt/BSFHO/LNO shows JSC ~32 µA/cm² and VOC ~ 0.04 V, which increase to maximum value of JSC ~ 303 (-206) µA/cm² and VOC ~-0.32 (0.26) V after upward (downward) poling at ±8 V. We believe that Schottky barrier modulation by polarization flipping at Pt/BSFHO interface could be a main driving force behind switchable photovoltaic and rectifying diode characteristics of Pt/BSFHO/LNO heterostructures. [ABSTRACT FROM AUTHOR]

In this research, BiFeO3 (BFO) films are integrated into BFO/SrTiO3 (STO)/BFO nanolaminates (BSB-NLs) featuring nanometer-scale thickness of BFO and STO layers. By introducing the STO layer in between two BFO layers, the leakage current density is reduced by two orders of magnitude with respect to relatively high leakage currents of current single BFO layers, i.e., from 10-5 A/cm² to 10-7 A/cm². The BSB-NL also shows very high piezoelectric response, which is ~5 times higher than that of the pure BFO with the same thickness. The highly strained state of the BFO layers concurrently with the chemical/crystallographic state of the interfaces between the BFO and STO layers contribute to the very high values of piezoresponse and very low leakage current observed in the BSB-NLs. [ABSTRACT FROM AUTHOR]

We report the photovoltaic properties of doped ferroelectric [Bi0.9La0.1][Fe0.97Ti0.02Zr0.01]O3 (BLFTZO) thin films. Polycrystalline BLFTZO films were fabricated on Pt/TiO2/SiO2/Si substrates by pulsed laser deposition technique. Al-doped ZnO transparent top electrodes complete the ZnO:Al/BLFTZO/Pt metal-ferroelectric-metal capacitor structures. BLFTZO showed switchable photoresponse in both polarities. The open circuit voltage (VOC) and short circuit current (JSC) were found to be ~0.022 V and ~650 μA/cm², respectively after positive poling, whereas significant difference in VOC ~ 0.018V and JSC ~ 700 μA/cm² was observed after negative poling. The observed switchable photocurrent and photovoltage responses are explained on the basis of polarization flipping in BLFTZO due to the applied poling field. [ABSTRACT FROM AUTHOR]

We report the switchable photovoltaic effects in graphene/BiFeO3/Pt heterostructures. Pure phase polycrystalline BiFeO3 films were deposited on Pt/TiO2/SiO2/Si substrates by pulse laser deposition. A bilayer graphene was transferred onto the BiFeO3 film which serves as transparent conducting electrodes. The heterostructures showed switchable photovoltaic effect depending on ferroelectric polarization directions indicating depolarization field induced separation of photogenerated carriers. The open circuit voltage (VOC) and short circuit current density (JSC) were measured to be ~ 110 mV, ~92 μA/cm² in positive polarity and similar values were obtained when the polarity was reversed. The JSC and VOC also showed rapid response (< 100 ms) as a function of light exposure time. [ABSTRACT FROM AUTHOR]

We report a remarkable photovoltaic effect in pulsed laser deposited multiferroic aurivillius phase Bi5FeTi3O15 (BFTO) thin films sandwiched between ZnO:Al transparent conductive oxide top electrode and SrRuO3 bottom electrode fabricated on amorphous fused silica substrates. The structural and micro structural properties of these films were analysed by X-ray diffraction and atomic force microscopy techniques. The films were showing a photo sensitive ferroelectric behaviour with a notable apparent polarization in the range of 10-15 μC/cm². These films also exhibited a switchable photoresponse and this parameter was observed to be sensitive to polarisation field and the polarization direction. The device shows a large ON/OFF photo current ratio with an open circuit voltage of 0.14 V. The photo response at zero bias of this BFTO based heterostructures showed rapid increase to a saturation value of 6 μA at zero bias. [ABSTRACT FROM AUTHOR]

Multilevel resistive switching was observed in random access memory device using amorphous SmGdO3 (SGO) ternary oxide thin films. Non-volatile and stable 4-level resistance states with sufficient margin of resistance ratios were observed by varying compliance current which was attributed to compliance current dependent variation in size of conducting filaments. As fabricated Pt/SGO/Pt devices exhibited excellent switching parameters such as stable resistance ratios of reset (ON) to set (OFF) states, non-overlapping switching voltages, excellent data retention, and endurance. Temperature dependent variation of resistances of ON and OFF states of the device was studied to elucidate current conduction and resistive switching mechanisms. [ABSTRACT FROM AUTHOR]

Systematic studies are presented on the effects of cantilever buckling in vector piezoresponse force microscopy (V-PFM) imaging of polarization domains in thin-film based (001)-oriented BiFeO3 nanostructures, as observed through the coupling of out-of-plane and in-plane PFM images. This effect is a strong function of the laser spot position on the cantilever, being strongest at the free end, and insignificant at 60% of the cantilever length from the pivot point. This finding provides a unique approach to V-PFM imaging of ferroelectric polarization domains, yielding three dimensional PFM images without sample rotation in the plane. [ABSTRACT FROM AUTHOR]

Use of multilayer dielectric thin films provides opportunities to optimize properties for different applications. In this work, the changes in the leakage current in Ba0.8Sr0.2TiO3 thin films upon introduction of ZrO2 layers of different thicknesses are studied. Unusual changes in the leakage current and the transition field with variation in the number and thickness of the ZrO2 layers are observed. Profile of the oxygen concentration across the sample, as determined by x-ray photoelectron spectroscopy, shows that the oxygen diffusion length controls the modulation in the concentration of the charged oxygen vacancies, and hence the depletion layer thicknesses, at the interfaces. A qualitative model for the observed behavior is provided. [ABSTRACT FROM AUTHOR]

SrBi[sub 2]Ta[sub 2]O[sub 9] (SBT) thin films with different concentrations of Ca at the Sr-site were grown on platinized silicon substrates. A systematic shift of diffraction lines towards higher diffraction angle confirmed the decrease in lattice parameter and attributed to the smaller ionic radii of Ca than for Sr. The grain size of the films was found to be increased upon Ca incorporation in SBT. The dielectric constant of the SBT films was systematically decreased with increasing Ca contents and it was attributed to lower dielectric permittivity of CaBi[sub 2]Ta[sub 2]O[sub 9] system. The ferroelectric properties were gradually increased up to 25% Ca at the Sr site, above which the ferroelectric properties started degrading. The maximum value of remanent polarization (∼23.8 μC/cm[sup 2]) of the films was obtained at 20% Ca substituted SBT. The systematic increase in coercive field was attributed to the higher electronegativity of Ca. The films showed minimal fatigue degradation (<23%) after 10[sup 10] switching cycles. Substitution of Ca (up to 25%) in SBT thin films did not show any pronounced influence on leakage current characteristics. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Non-c-axis-oriented SrBi[sub 2]Nb[sub 2]O[sub 9] (SBN) thin films were grown on Pt/TiO[sub 2]/SiO[sub 2]/Si substrates using the pulsed-laser-deposition technique. X-ray diffraction results confirmed the textured growth of SBN thin films along (115) and (200) orientations. The increase in the value of the dielectric permittivity and decrease in the tangential loss of the films with an increase in annealing temperature were attributed to grain size dependence. SBN thin films annealed at 750 °C exhibited a maximum value of the dielectric constant of ∼346 with a dissipation factor of 0.02. Thin films with certain deposition parameters exhibited the highest remanent polarization (P[sub r]) and coercive field, 25.7 µC/cm² and 198 kV/cm, respectively. There was minimal (<20%) degradation in the switchable polarization (P[sup *] - P[sup ˆ]) after 10[sup 10] switching cycles. At lower field, the leakage current follows ohmic behavior, and at higher field, up to 100 kV/cm, the leakage current density was about 5 × 10[sup -7] A/cm². [ABSTRACT FROM AUTHOR]

We have studied the leakage current conduction mechanism of pulsed-laser-deposited SrBi[sub 2]Nb[sub 2]O[sub 9] (SBN) thin films on platinized silicon substrates. The time-dependent dc leakage current densities of SBN thin films do not follow Curie-von Schweidler power law. Instead the contribution of conduction current is predominantly electronic. At lower fields, the leakage current follows the ohmic behavior, and it increases exponentially for higher fields. The leakage current density of the SBN thin films was studied at elevated temperatures, and the data were fitted with the Schottky emission model. The effective Richardson's constant was calculated to be about 8.7 × 10[sup -6] A/cm² K². The Schottky barrier height of the SBN thin films was estimated to be 1.37 eV. [ABSTRACT FROM AUTHOR]

Bismuth-layered ferroelectric thin films of Sr[sub 1-x]A[sub x]Bi[sub 2]Ta[sub 2]O[sub 9], with composition x=0 and 0.2, were fabricated by using the pulsed-laser deposition technique. Structural characterization of the films by x-ray diffraction and atomic force microscopy, revealed that the films are polycrystalline in nature with average grain size of 180 nm. The films displayed spherical grains with a surface roughness of 12 nm. The ferroelectric measurements of Sr[sub 0.8]Ba[sub 0.2]Bi[sub 2]Ta[sub 2]O[sub 9], SrBi[sub 2]Ta[sub 2]O[sub 9], and Sr[sub 0.8]Ca[sub 0.2]Bi[sub 2]Ta[sub 2]O[sub 9] showed saturated hysteretic behavior with remanent polarization (2P[sub r]) of 23.5, 17.9, 14 μC/cm[sup 2] and coercive field (E[sub c]) of 31.06, 74.2, 86.3 kV/cm for a maximum applied electric field of 360 kV/cm. Films exhibited minimal (<=17%) degradation of polarization for up to 10[sup 10] switching cycles. It was observed that the coercive field decreased with increase in the ionic size of partially substituted cations. The leakage current density of films were found to be of the order of ∼10[sup -8] A/cm[sup 2] for up to a breakdown field of about 75 kV/cm. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Thin films of CaBi[sub 2]Ta[sub 2]O[sub 9] (CBT) were deposited on Pt/TiO[sub 2]/SiO[sub 2]/Si substrates using the pulsed laser deposition technique at temperatures ranging from 500 to 700 °C. The presence of (115) and (00[UNDRLINE]10[/UNDRLINE]) orientations confirm the phase formation at the lower temperature (500 °C). Microstructure evolution of CBT films with oxygen pressure of 100-200 mTorr at a substrate temperature of 650 °C shows that the films deposited at lower pressure have a relatively smaller grain size and less surface roughness. The films grown at 650 °C exhibited a maximum polarization of (2P[sub m]) 17 μC/cm[sup 2], remanent polarization of (2P[sub r]) 8 μC/cm[sup 2] and coercive field of (E[sub c]) 128 kV/cm, with fatigue endurance up to 10[sup 10] switching cycles. The higher dielectric constant (∼115 at 100 kHz) with a relatively lower dissipation factor (0.02) at higher growth temperature (700 °C) was explained by the increased grain size. The higher leakage current density (∼10[sup -7] A/cm[sup 2]) at higher deposition temperature is attributed to the interfacial diffusion of the film and the substrate. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]