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Changes in surface chemistry and wettability of femtosecond laser surface processed (FLSP) copper due to atmospheric exposure are quantified using X-ray photoelectron spectroscopy and contact angle measurements.

At high phonon temperature, defect-mediated electron-phonon collisions (supercollisions) in graphene allow for larger energy transfer and faster cooling of hot electrons than the normal, momentum-conserving electron-phonon collisions. Disorder also affects the heat flow between electrons and phonons at very low phonon temperature, where the phonon wavelength exceeds the mean free path. In both cases, the cooling rate is predicted to exhibit a characteristic cubic power law dependence on the electron temperature, markedly different from the T^4 dependence predicted for pristine graphene. The impact of defect-induced cooling on the performance of optoelectronic devices is still largely unexplored. Here we study the cooling mechanism of hot-electron bolometers based on epitaxial graphene quantum dots where the defect density can be controlled with the fabrication process. The devices with high defect density exhibit the cubic power law. Defect-induced cooling yields a slower increase of the thermal conductance with increasing temperature, thereby greatly enhancing the device responsivity compared to devices with lower defect density and operating with normal-collision cooling.

Point defects in SiC are an attractive platform for quantum information and sensing applications because they provide relatively long spin coherence times, optical spin initialization, and spin-dependent fluorescence readout in a fabrication-friendly semiconductor. The ability to precisely place these defects at the optimal location in a host material with nano-scale accuracy is desirable for integration of these quantum systems with traditional electronic and photonic structures. Here, we demonstrate the precise spatial patterning of arrays of silicon vacancy ($${V}_{Si}$$ V Si ) emitters in an epitaxial 4H-SiC (0001) layer through mask-less focused ion beam implantation of Li+. We characterize these arrays with high-resolution scanning confocal fluorescence microscopy on the Si-face, observing sharp emission lines primarily coming from the $${V1}^{{\prime}}$$ V 1 ′ zero-phonon line (ZPL). The implantation dose is varied over 3 orders of magnitude, leading to $${V}_{Si}$$ V Si densities from a few per implantation spot to thousands per spot, with a linear dependence between ZPL emission and implantation dose. Optically-detected magnetic resonance (ODMR) is also performed, confirming the presence of V2 $${V}_{Si}$$ V Si . Our investigation reveals scalable and reproducible defect generation.

Quantum technology is a field of significant interest that will benefit many applications including communications and sensing. SiC is a promising material for quantum applications such as quantum memories, due to point defects, specifically VSi, in the material, which result in long spin coherence times. We have found that no VSiare present in our epitaxially grown unintentionally and nitrogen-doped 4H-SiC with electron concentrations ranging from 1014to 1018cm-3. We create these vacancies using electron irradiation, in concentrations from single defects to ensembles. To utilize the defect luminescence for realistic applications, we have fabricated the SiC into photonic crystal arrays. We present the processing steps required to create photonic crystal cavities in SiC and subsequent challenges.

Emerging interdisciplinary fields of multiferroics and spin-optronics demand for new oxide materials that unfolds significant coupling between electric and magnetic order at room temperature, preferably with controlled optical properties. Rare-earth orthoferrites (RFeO3) have received much attention due to their coupled electric, magnetic and optic properties. Here we report structural, optical, temperature-dependent (83–800 K) Raman spectroscopy, electronic transition analysis and electric poling induced magnetic modulation in nanostructured SmFeO3 (SFO). Rietveld refinement of SFO confirmed the orthorhombic phase (space group D 2 h 16 , Pbnm) with expanded lattice parameters (ao = 5.3979 A

In recent years, ferroelectric random access memory has drawn considerable attention as promising replacement to both dynamic random access memory and flash memory. Specifically in metal–ferroelectric–insulator–semiconductor (MFIS)-based structures, bi-stable polarization of ferroelectric gate even in absence of power holds the resistance state of semiconductor-drain channel between two logic states and offers additional features of non-destructive readout and non-volatile storage capability. However, insulating layer in such structure leads to high depolarizing field across FE layer and in turn high-voltage operation. In the present work, comprehensive performance of low-voltage-driven MFIS device, i.e., Pt (40 nm)/BiFeO3 (265 nm)/DyScO3 (6 nm)/Si is evaluated for gate voltage stress (± 2 to ± 9 V) at different thermal agitation (200–400 K). Fat capacitance–voltage (C–V) hysteresis centered at zero bias with large memory window (ΔV FB) of 1.9 V at low operating voltage of ± 5 V, and stable data retention with distinguishable ON/OFF state values specifies strong charge storage potential of MFIS device in extreme conditions of ± 100 K. X-ray diffraction revealed polycrystalline and rhombohedral R3c phase of BiFeO3 film and out-of-plane piezoresponse force microscopy analysis showed the ultrafast domain switching with sharp contrast. Complete 180° phase reversal in hysteresis loop and bufferfly-shaped piezo-actuation amplitude loop further confirmed the enhanced ferroelectric properties of BiFeO3 thin films. Nonlinear J–V curves of MFIS structure were investigated to understand the device reliability and charge transport mechanism. These encouraging results are crucial for designing more reliable integrated MFIS-based non-destructive readout non-volatile memory devices.

The intercalation of epitaxial graphene on SiC(0001) with Ca has been studied extensively, yet precisely where the Ca resides remains elusive. Furthermore, the intercalation of Mg underneath epitaxial graphene on SiC(0001) has not been reported. Here, we use low energy electron diffraction, x-ray photoelectron spectroscopy, secondary electron cut-off photoemission and scanning tunneling microscopy to elucidate the physical and electronic structure of both Ca- and Mg-intercalated epitaxial graphene on 6H-SiC(0001). We find that Ca intercalates underneath the buffer layer and bonds to the Si-terminated SiC surface, breaking the C-Si bonds of the buffer layer i.e. 'freestanding' the buffer layer to form Ca-intercalated quasi-freestanding bilayer graphene (Ca-QFSBLG). The situation is similar for the Mg-intercalation of epitaxial graphene on SiC(0001), where an ordered Mg-terminated reconstruction at the SiC surface and Mg bonds to the Si-terminated SiC surface are formed, resulting in Mg-intercalated quasi-freestanding bilayer graphene (Mg-QFSBLG). Ca-intercalation underneath the buffer layer has not been considered in previous studies of Ca-intercalated epitaxial graphene. Furthermore, we find no evidence that either Ca or Mg intercalates between graphene layers. However, we do find that both Ca-QFSBLG and Mg-QFSBLG exhibit very low workfunctions of 3.68 and 3.78 eV, respectively, indicating high n-type doping. Upon exposure to ambient conditions, we find Ca-QFSBLG degrades rapidly, whereas Mg-QFSBLG remains remarkably stable., 58 pages, 10 figures, 4 tables. Revised text and figures

Metal films deposited on graphene are known to influence its electronic properties, but little is known about graphene’s interactions with very low work function rare earth metals. Here we report on the work functions of a wide range of metals deposited on n-type epitaxial graphene (EG) as measured by Kelvin Probe Force Microscopy (KPFM). We compare the behaviors of rare earth metals (Pr, Eu, Er, Yb, and Y) with commonly used noble metals (Cr, Cu, Rh, Ni, Au, and Pt). The rare earth films oxidize rapidly, and exhibit unique behaviors when on graphene. We find that the measured work function of the low work function group is consistently higher than predicted, unlike the noble metals, which is likely due to rapid oxidation during measurement. Some of the low work function metals interact with graphene; for example, Eu exhibits bonding anomalies along the metal-graphene perimeter. We observe no correlation between metal work function and photovoltage, implying the metal-graphene interface properties are a more determinant factor. Yb emerges as the best choice for future applications requiring a low-work function electrical contact on graphene. Yb films have the strongest photovoltage response and maintains a relatively low surface roughness, ~5 nm, despite sensitivity to oxidation.

A highly sensitive hydrogen gas sensor has been developed utilizing 3% copper-doped zinc oxide thin films. These films were deposited using pulsed laser deposition technique on quartz substrates with predeposited interdigitated platinum electrodes. The deposited films were characterized by X-ray diffraction and atomic force microscopy. The hydrogen sensing properties of the sensor were studied at three operating temperatures, viz., 50 °C, 100 °C, and 150 °C. We observed that the sensor shows good performance even at a low operating temperature of 50 °C for a very small concentration of hydrogen (10 ppm).

Temperature stability of voltage tunability and figure of merit (FOM) were investigated for the pulsed laser deposited Ba0.6Sr0.4TiO3 (BST0.6) thin films on platinum coated silicon [Pt(111)/TiOx/SiOx/Si(100)] substrates. The structural and micro-structural properties of these films were analyzed using x-ray diffraction (XRD) and atomic force microscopy (AFM), respectively. Metal-ferroelectric-metal (MFM) mono-dielectric layer stacks were fabricated using these films and their temperature and voltage dependent dielectric properties were quantified. The temperature dependence of dielectric properties were analysed using the modified Curie-Weiss law. The films showed a good dielectric tunability of about 40% at a bias field of 125 kV/cm. The determined figure of merits are found to be highly sensitive to the temperature changes. The observed ferroelectric-like behaviour in these films above the Curie point (Tc) has been attributed to the thermal expansion mismatch and the substrate induced misfit strain with...

Room temperature ferroelectricity in pulsed laser deposited rare-earth doped hafnium oxide (HfO2) thin films is discussed. Maximum values of remnant polarizations (Pr) ~13.5 and 12 μC/cm2 along with coercive fields (EC) ~334 and 384 kV/cm are observed in 6 mol. % of rare-earth (Sm or Gd) doped-HfO2 thin films (Sm:HfO2 and Gd:HfO2), respectively. Piezoresponse force microscopy measurements confirmed ferroelectric nature of films by showing phase hysteresis and butterfly amplitude loops. It is noticed that wake-up cycles improved the remnant polarization and found to be necessary for the forming of well saturated hysteresis loops. Our results showed potential toward realization of future highly scaled non-volatile ferroelectric memories.

We have investigated optical and band alignment properties of pulsed laser deposited amorphous thin films of bismuth based monoclinic pyrochlore Bi2Zn2/3−x/3Nb4/3−2x/3ZrxO7 (Zr–BZN) where x = 0.4 on quartz and silicon substrates, respectively. The optical parameters, such as complex refractive index (n − jk), energy bandgap (Eg), complex dielectric function ( e ′ - j e ″ ), and complex conductivity ( σ ′ - j σ ″ ) and associated dispersion parameters were estimated from the UV–Visible transmission spectra. The analysis of refractive index dispersion confirmed the Wemple–DiDomenico single-effective-oscillator model for the direct inter-band transition. The valence band of Zr–BZN is found to be ∼0.1 eV above that of silicon. The numerical values for conduction band offset ΔEC on silicon and optical bandgap Eg were estimated to be ∼2.46 eV and ∼3.46 eV, respectively for Zr–BZN samples. We determined a complete electron band offset dominated type II staggered band lineup of this high-k dielectric/semiconductor heterostructure, where a straight forward spatial confinement of electrons and holes is facilitated. These important results can play critical role and provide key insight for the practical applications of Zr–BZN material, especially in CMOS (complementary metal–oxide–semiconductor) logic and memory devices.

Resistive random access memories (ReRAMs) have emerged as a potential memory candidate that has the capability of fulfilling the future high density requirements due to their simple design, excellent scalability, high speed storage capacity, low power consumption, and CMOS process-flow compatibility. With this motivation pulsed laser-deposited amorphous GdGaO3 (GGO) thin films sandwiched between two Pt-electrodes were investigated to explore their resistive switching (RS) behaviour. Pt/GGO/Pt memory cells showed a stable resistance ratio of ~104 between low resistance state (LRS) and high resistance state (HRS) and non-overlapping switching voltages (set voltage VON ~1.2˗1.4 V and reset voltage VOFF ~0.5˗0.7 V) with a small variation of 5˗10%. To confirm the reliability of the Pt/GGO/Pt memory cells, cyclic endurance up to 60 set/reset cycles and data retention up to 103 seconds were measured. Only a unipolar RS mechanism in Pt/GGO/Pt memory cells was observed due to a conductive filamentary (thermo-chemical) mechanism in which, the change in resistance state (LRS↔HRS) of the cell was found to occur from rupture of conductive filaments consisting of metallic Gd atoms and oxygen vacancies. The charge transport mechanisms of the cell in LRS and HRS states were found to be dominated by Ohmic conduction and trap controlled space–charge limited current, respectively. This work reveals the potential of rare-earth gallates for non-volatile memory device applications.

Rare-earth oxides have attracted considerable research interest in resistive random access memories (ReRAMs) due to their compatibility with complementary metal-oxide semiconductor (CMOS) process. To this end we report unipolar resistive switching in a novel ternary rare-earth oxide LaHoO3 (LHO) to accelerate progress and to support advances in this emerging densely scalable research architecture. Amorphous thin films of LHO were fabricated on Pt/TiO2/SiO2/Si substrate by pulsed laser deposition, followed by sputter deposition of platinum top electrode through shadow mask in order to elucidate the resistive switching behavior of the resulting Pt/LHO/Pt metal-insulator-metal (MIM) device structure. Stable unipolar resistive switching characteristics with interesting switching parameters like, high resistance ratio of about 105 between high resistance state (HRS) and low resistance state (LRS), non-overlapping switching voltages with narrow dispersion, and excellent retention and endurance features were observed in Pt/LHO/Pt device structure. The observed resistive switching in LHO was explained by the formation/rupture of conductive filaments formed out of oxygen vacancies and metallic Ho atom. From the current-voltage characteristics of Pt/LHO/Pt structure, the conduction mechanism in LRS and HRS was found to be dominated by Ohm’s law and Poole-Frenkel emission, respectively.

The unipolar resistive switching characteristics of the amorphous LaLuO3 thin films deposited by pulsed laser deposition have been studied. Reliable and repeatable nonvolatile switching of the resistance of LaLuO3 films was obtained between two well defined states of low and high resistance with nearly constant resistance ratio ∼107 and non-overlapping switching voltages in the range of 0.66-0.83 V and 1.9-2.7 V respectively. The temperature dependent measurement revealed metallic and semiconducting behavior in low and high resistance states respectively. The switching between low and high resistance states was attributed to the change in the separation between oxygen vacancies in light of the correlated barrier hopping theory. The current conduction mechanism of the device in high-resistance state followed the Poole's law, whereas the conduction in low-resistance state was found to be dominated by percolation. The resistance of low and high resistance states of the film showed no obvious degradation for ...

A comprehensive study on the ternary dielectric, LaGdO3, synthesized and qualified in our laboratory as a novel high-k dielectric material for logic and memory device applications in terms of its excellent features that include a high linear dielectric constant (k) of ~22 and a large energy bandgap of ~5.6 eV, resulting in sufficient electron and hole band offsets of ~2.57 eV and ~1.91 eV, respectively, on silicon, good thermal stability with Si and lower gate leakage current densities within the International Technology Roadmap for Semiconductors (ITRS) specified limits at the sub-nanometer electrical functional thickness level, which are desirable for advanced complementary metal-oxide-semiconductor (CMOS), bipolar (Bi) and BiCMOS chips applications, is presented in this review article.

A comprehensive photogenerated carrier assisted etching investigation is carried out on the Si polar surface of high-quality p- and n-type (0001) 4H–SiC epilayers. The epilayers have intentional or unintentional doping densities in the 1014–1018 cm−3 range. Cyclic voltammetry and chronoamperometry studies under a non-focused above bandgap (280–400 nm) light illumination (≤0.66 Wcm−2) in a highly basic etching medium (1 wt% KOH solution, pH ∼12) complemented by Mott-Schottky characterization reveal a dependence of the etch voltage (within ±1 V) and etch rate (∼20–60 nm/min) on doping concentration and type. Our results demonstrate separate mutually exclusive etch voltage windows (∼± 0.3–0.7 V) in the diffusion-limited regime for a pair of doping concentrations with opposite conductivities to establish smooth preferential etching. These results provide insight in the design and fabrication of higher performance three dimensional SiC homo/hetero-junctions for various applications including photonic crystal cavities.

We report the observation of coherent phonon modes in zinc oxide nanostructures synthesized by arc exploding technique (AET). The band gap (Eg) is estimated to be 3.3 eV from UV-absorption spectra. The UV resonance Raman spectra (UVRRS) revealed the presence of multiple longitudinal optical (LO) phonon modes demonstrating the transformation of zinc to zinc oxide as a function of ultra-sonication time. The observed multiple A1(LO) Raman active phonons in ZnO nanostructures were attributed to atomic displacement parallel to wurtzite c-axis. Higher order harmonics and intensity variations in multiple LO phonon modes were interpreted using cascade model in resonant spectra. The appearance of E2 (low) at ~101 cm−1 was observed with higher processing time along with enhanced multiple LO phonon modes, indicating the oscillations of zinc sub-lattice. The Zn to ZnO evolution was also confirmed using Photoluminescence (PL) spectroscopy showing band-edge emission at 380 nm.

The room temperature crystal structure of potential high-k dielectric LaGdO3 has been analyzed by X-ray diffraction techniques and its structural evolution with temperature has been probed by Raman spectroscopy. Rietveld refinement of the ambient XRD data established the perfectly layered B-type monoclinic crystal structure with a=14.43 angstrom, b=3.68 angstrom, c=8.99 angstrom, and =100.57 degrees which is further validated by the 21 (14A(g) and 7B(g)) Raman-active vibrational modes with centro-symmetric space group C2h3 or C2/m. Atomic positions, coordination numbers, inter-ionic lengths, etc. were derived using Rietveld analysis. All prominent Bragg peaks were successfully indexed. Temperature dependent Raman spectra of LaGdO3 from 80 to 1400K were analyzed using the damped harmonic oscillator model. The softening and hardening of vibrational modes are reported. Above 900K, disappearance of all high frequency modes (>600cm(-1)) and merger of several mid frequency modes (between 200 and 500cm(-1)) into two distinct modes was observed, a direct evidence of a possible structural phase transition from monoclinic to tetragonal/pseudocubic.

Optical constants of pulsed laser ablated amorphous high-k LaGdO3 (LGO) thin films on quartz (0001) substrates were measured and analyzed. Refractive indices and extinction coefficients of the LGO layers were estimated to be in the range 2.05 - 2.29 and 0.004 - 0.017, respectively using envelope method. Optical constants were parameterized by analysis of the UV/Visible transmission spectra in the 300 - 750 nm spectral window employing Cauchy-Urbach dispersion model and confirmed the applicability of this model in this wavelength region.

Optical constants of pulsed laser ablated amorphous high-k LaGdO3 (a-LGO) thin films on quartz (0001) substrates were measured and analyzed. Refractive index (n ∼ 2.05‐2.29) and extinction coefficient (k ∼ 0.004‐0.017) were parameterized by analysis of the UV/Visible transmission spectra employing a Cauchy-Urbach dispersion model in the 300‐750 nm spectral window and confirmed the applicability of this model in this wavelength region. The composition and chemistry of these dielectric films were studied by utilizingATR-FTIRspectroscopy. Thededucedpinningfactor(S),0.5suggests thatLGO/Si is aninteracting interface. Theparameter S was applied in the Cowley-Sze relation to estimate the interface trap density (Dit )t o be 1.08× 10 13 states cm −2 eV −1 .A s grown LGO/p-Si heterojunction showed a downward Fermi-level (FL) pinning of 0.39 ± 0.02 eV. A complete Type I straddled band lineup of this gate dielectric/semiconductor heterostructure was determined by applying the charge neutrality level (CNL) model. The CNL of LGO was deduced to be 2.11 ± 0.05 eV above valence band maximum.

The electrical properties of LaGdO3 (LGO) high-k dielectric thin films prepared by pulsed laser deposition (PLD) on SiOx/p-Si substrate have been studied. The equivalent oxide thickness (EOT) and gate leakage current density (Jg) were determined on metal-oxide-semiconductor (MOS) structures. Capacitance-Voltage characteristics showed negligible hysteresis, ~8 mV. The extracted dielectric constant (k) of LGO layer from the EOT-physical thickness plot was ~21.6 ± 1.7. LGO layers with EOT of 3nm had a Jg ~ 3 x 10-8 A/cm2 at accumulation (Vg=VFB-1). The estimated interfacial trap density at the flatband voltage as determined by Lehovec method was in the range 1012 eV-1cm-2. N-channel LGO metal-oxide-semiconductor field effect transistors (MOSFET) have been fabricated and electrically characterized. The threshold voltage (Vt) was ~0.3V.

LaGdO3 (LGO) ceramics and thin films were prepared for high-k applications. Electrical properties of LGO ceramics were studied as a function of temperature and frequency using metal-insulator-metal (MIM) capacitor configuration. The dielectric constant and loss tangent at 100 kHz were 20 and 0.004 respectively at ambient conditions without any temperature and voltage dependence. Bulk properties are encouraging for high-k gate-oxide applications. Thin films of LGO were prepared on Si substrates by pulsed laser deposition. The electrical properties of thin films were investigated with metal–insulator–semiconductor (MIS) capacitors. The relative dielectric permittivity was 21.6 ± 1.7 and the equivalent oxide thickness was controlled by the SiOx interlayer. However, compared to the standard SiO2 capacitors of similar equivalent oxide thickness, less leakage currents were obtained. By controlling the interlayer growth between the high-k and silicon, EOT can be reduced further to meet the requirements of 22 nm ...

High-k DyScO 3 linear dielectric films were considered as a buffer layer for the metal-ferroelectric-insulator-semiconductor (MFIS) structures with Aurivillius Bi 3.25 Nd 0.75 Ti 3 O 12 ferroelectric films. The DyScO 3 films on Si substrates were amorphous and dense with a smooth surface morphology, showing negligible CV hysteresis and low leakage current. The remnant polarization of ∼20 μC/cm 2 , dielectric constant ∼400, and the loss tangent ∼0.04 were obtained for the ferroelectric Bi 3.25 Nd 0.75 Ti 3 O 12 film on Pt/TiO 2 /SiO 2 /Si substrates. The Pt/Bi 3.25 Nd 0.75 Ti 3 O 12 /DyScO 3 /Si MFIS capacitors showed a large memory window of 4.0 V and excellent retention up to 1000 s, encouraging results for practical applications in nonvolatile RAM.

Financial support from NSF Grant No. NSF-RII-0701525 was acknowledged. S.D. is thankful to DOD for doctoral fellowship under Grant No. W911NF-11-1-0204. S.P.P. is grateful to NSF for financial assistance under Grant No: NSF-EFRI RESTOR # 1038272. A systematic study on structural, microstructural, optical, dielectric, and electrical properties of phase-pure silicon-modified SrTiO3 polycrystalline electroceramics synthesized using high energy solid state reaction techniques is presented. The asymmetry and splitting in the x-ray diffractometer spectra and the observation of first order transverse optical TO2 and longitudinal optical LO4 modes in Raman spectra (nominally forbidden) revealed the distortion in the cubic lattice as a result of breaking of inversion symmetry due to doping. A bandgap Eg of 3.27 eV was determined for the sample by diffuse reflectance spectroscopy. A high dielectric constant of -400 and very low dielectric loss of -0.03 were obtained at 100 kHz near ambient conditions. The temperature dependence of the dielectric data displayed features of high temperature relaxor ferroelectric behavior as evidence of existence of polar nano-regions. The ac conductivity as a function of frequency showed features typical of universal dynamic response and obeyed a power law σac = σdc+Aωn . The temperature dependent dc conductivity followed an Arrhenius relation with activation energy of 123 meV in the 200–500 K temperature range. The linear dielectric response of Pt/SrSi0.03Ti0.97O3/Pt dielectric capacitors was well characterized. The measured leakage current was exceptionally low, 13 nA/cm2 at 8.7 kV/cm, revealing an interface blocked bulk conduction mechanism. Publisher PDF

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

A detailed study of the effect of off-center donor ion (Sc3+) substitution on structural, microstructural, optical, dielectric, electrical, and ferroelectric properties of morphotropic composition of lead zirconate titanate electroceramics with the stoichiometric formula Pb0.85Sc0.10Zr0.53Ti0.47O3 (PSZT) and synthesized using a high energy solid-state reaction technique was carried out. Powder x-ray diffractometry was used to identify the stabilized tetragonal phase (space group P4mm) with considerably reduced tetragonal strain, c/a= 1.005. An analysis of the thermal dependence of the Raman results indicated a smooth displacive (ferroelectric-paraelectric) phase transition as revealed by the observed disappearance of the soft modes A1 (1TO) and A1 (2TO) above 460 K. The dielectric response of Pt/PSZT/Pt metal-ferroelectric-metal capacitors was probed over a wide range of thermal excursions (85–600 K) and ac signal frequencies (102–106 Hz). Thermally activated dynamic and static conduction processes indica...

Pb(Zr 0.5 Ti 0.5 )O 3 and (Pb 0.9 Sr 0.1 )(Zr 0.5 Ti 0.5 )O 3 thin films were grown on Pt/TiO x /SiO 2 /Si substrates by chemical solution deposition. 10% Sr substituted PZT film showed high degree of (0 0 1) type preferential grain growth. Surface morphology revealed a clear correlation between preferred grain orientation and grain size. Room temperature dielectric constant was 1200 and 700 for the PZT and PSZT films, respectively. Dielectric loss reduced with Sr substitution. PZT film showed severe fatigue, and hence the polarization reduced to 20% of the initial value (24 μC/cm 2 ) after 10 8 cycles where as PSZT showed less fatigue, 75% of the initial polarization (12 μC/cm 2 ) was retained after 10 8 switching cycles.

Careful selection of pulsed laser deposition conditions was executed to achieve sub-nanometer EOT (equivalent oxide thickness) in amorphous LaGdO3 based high-k/metal gate stacks. The lowest EOTs attained were ∼5.4 A and 8.4 A with and without quantum mechanical correction, respectively. The electrical measurements yielded a high permittivity of 20.5 ± 2.4, a thin bottom interfacial layer of thickness 4.5 ± 1 A, and interface (cm−2 eV−1) and fixed (cm−2) charge densities of ∼1012. Analysis of temperature dependent leakage currents revealed that gate injection current was dominated by Schottky emission below 1.2 MV/cm and quantum mechanical tunneling above this field. The physical origin of substrate injection was found to be a combination of Schottky emission and trap assisted tunneling.

Nonvolatile unipolar resistive switching properties of the amorphous LaGdO3 thin films deposited by pulsed laser deposition have been studied. Reliable and repeatable switching of the resistance of LaGdO3 film was obtained between low and high resistance states with nearly constant resistance ratio ∼ 106 and non-overlapping switching voltages in the range of ∼0.6-0.75 V and 2.5-4 V respectively. The switching between low and high resistance states was attributed to the formation and rupture of conductive filaments using temperature dependent resistance measurements. The current conduction mechanisms of the LaGdO3 film in low and high resistance states were found to follow the Ohmic behavior and Poole-Frenkel emission respectively. The resistance of low and high resistance states of the film remained nearly constant for up to ∼ 104 seconds indicating good retention. The observed resistive switching characteristics of LaGdO3 thin films are promising for futuristic nonvolatile memories.

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. Postprint

Single phase delafossite p-type CuFeO2 (CFO) semiconductor was synthesized in bulk by modified solid state reaction technique. X-ray diffraction (XRD) and X-ray photo spectroscopy (XPS) studies suggest single phase CFO at room temperature. The energy dispersive X-ray spectroscopy (EDX) revealed that the atomic ratio of Cu and Fe is 1:1. The XPS spectra showed two intense Cu 2p3/2 and 2p1/2 peaks at 932.5 eV and 952 eV suggesting Cu is in +1 state. The temperature dependent Raman spectra of CFO displayed two intense modes at 349 cm-1 and 690 cm-1 at room temperature that matched with other delaffosite structures. The temperature dependent Raman spectra showed significant shift in both Raman active modes to lower frequency side. We observed the disappearance of pure CFO Raman active modes above 750 K and the appearance of new peaks related to CuO compounds, indicating disintegration of CFO starting above 750 K which almost completed above 1100 K. The temperature dependent thermo-gravimetric analysis indicates change in CFO mass above 750 K with wide range of differential thermo-gravimetric slope suggests disintegration started above 750 K and completed at 1100 K. Raman spectra, XPS, and XRD of disintegrated CFO matched well with the Raman spectra, XPS and XRD of CuO and CuFe2O4 confirmed its disintegration above 750 K in air.

Epitaxial thin films of multiferroics Bi1-xLaxO3 (x = 0.0, 0.05, 0.10, 0.15 and 0.20)and BiFe1-xCrxO3.....(x = 0.05, 0.10, 0.15 and 0.20) were grown on SrTiO3(100) and (110) substrates by pulsed laser deposition (PLD). X-ray diffraction (XRD)U was used to study the structure, orientation, and crystallinity of the film. The growth mechanism and microstructure of the films were studied using atomic force microscopy (AFM) and scanning electron microscopy (SEM). The magnetic properties (magnetization vs. temperature and M-H loops) of the films were carried out using a superconducting quantum interference device (SQUID)magnetometer and the electrcal properties (dielectric permittivity and leakage current) of the films were carried out using our in-house characterization facilities. The polarized Raman scattering was used to identify and understand the phonon vibrations and crystalstructure of the films. It is found that the films grown on STO(100) and (110) substrates are of monoclinic, contrary to the reported bulk-like rhombohedral and tetragonal structure. The temperature variation of Raman scattering was investigated to understand the dynamicc of the phase transitions (magnetic and ferroelectric). We observed phonon-magnon coupling near the magnetic transition (370 C) and a structural phase transition at 810C. The result will be discussed in details.

We studied the resistive memory switching in pulsed laser deposited amorphous LaHoO3 (LHO) thin films for non-volatile resistive random access memory (RRAM) applications. Nonpolar resistive switching (RS) was achieved in PtLHOPt memory cells with all four possible RS modes ( positive unipolar, positive bipolar, negative unipolar, and negative bipolar) having high RON and ROFF ratios (in the range of 104 to 105) and non-overlapping switching voltages (set voltage, VON 3.6 to 4.2 V and reset voltage, VOFF 1.3 to 1.6 V) with a small variation of about 5 to 8 percent. X ray photoelectron spectroscopic studies together with temperature dependent switching characteristics revealed the formation of metallic holmium (Ho) and oxygen vacancies (VO) constituted conductive nanofilaments (CNFs) in the low resistance state (LRS). Detailed analysis of current versus voltage characteristics further corroborated the formation of CNFs based on metal like (Ohmic) conduction in LRS. Simmons Schottky emission was found to be the dominant charge transport mechanism in the high resistance state.

We report photovoltaic response of highly transparent graphene/BiFe0.95Si0.05O3 (BFSiO)/ITO/glass derived from bottom-up spin coating technique. The device exhibits short-circuit-current (ISC 0.75 mA) with 1000 fold upsurge and open-circuit-voltage (VOC ∼ 0.45 V) under standard AM 1.5 illumination through graphene. In combination, ISC of 0.63 mA and VOC of 0.35 V for same illumination through ITO, reveals the prospects of harvesting indoor light. Also, crystallographic structure, red shift in band gap, leakage behavior, and ferroelectric characteristics of BFSiO thin films are reported. Reproducible transient response of ISC and VOC with quick switching (

The polarization bistability of the ferroelectric material offers the possibility to develop dense, energy efficient, fast and non volatile ferroelctric random access memory (FeRAM) with long retention and high endurance, as a promising replacement to both DRAM and Flash for the continued scaling. With this motivation, metal-ferroelectric-insulator-semiconductor (MFIS) structures were fabricated on p-Si semiconductor substrate, by first depositing a 6 nm thick amorphous DyScO3 high-k insulator layer using metal organic chemical vapor deposition (MOCVD), followed by a 265 nm thick polycrystalline BiFeO3 (BFO) ferroelectric layer employing radio frequency (RF) sputtering, and finally a 40 nm thick platinum metal layer by DC magnetron sputtering. Electrical properties of the developed MFIS (Pt/BiFeO3/DyScO3/SiOx/Si) stacks were characterized as a function of thermal agitation (200 K to 400 K) to evaluate their device performance. Fat hysteresis loop was observed in the frequency dependent (1 kHz-1 MHz) capacitance-voltage (C-V) measurement with a memory window (ΔVFB) of 0.8 V at an exceptionally very low sweep voltage of ± 2 V. The memory window increased to 2.2 V as the sweep voltage was raised to ± 4 V and beyond up to ± 8 V, indicating a highly stable device. Low leakage current (1.2 μA/cm2 at 4 V) and excellent data retention characteristics with distinguishable high and low capacitance values over the long time period of 106 sec, revealed strong charge storage potential of the fabricated device architecture. The non-linear hysteretic J-E curves of the MFIS structure were also investigated in detail as a function of temperature to understand the device reliability and the charge transport mechanisms. Piezoelectric force microscopy (PFM) and P-E hysteresis loop (domains reversal) studies were carried out to confirm the ferroelectric nature of the polycrystalline BFO thin films. The encouraging results achieved reveals that Pt/BiFeO3/DyScO3/SiOx/Si MFIS device architecture is a promising one for realization of FeRAM devices.

Enriched and versatile room temperature multiferroic, BiFeO3 has been widely appreciated for future generation optoelectronic devices including light detectors, terahertz emitters, tunable solar cells, micro-sensors, and thin-film transistors with additional degree of magnetic and electric control. Moreover, literature also highlights its adjustable electrical, optical, and ferroelectric properties by doping with selected elements. We hereby report substantial photovoltaic response of semitransparent graphene/semiconductor Si doped BiFeO3 hetero-structure fabricated on In2O3 doped SnO2 coated glass substrates using bottom up spin coating technique considering the commercial aspect. The short circuit current density Jsc and open circuit voltage VOC were measured as 75 mA/cm2 and 0.45 V, respectively, with approximately 3 orders of increase in current density under standard AM1.5 (100 mW/cm2) illumination at room temperature. For the first time crystal structure, optical band gap, leakage behavior, ferroelectric and magnetic characteristics of 5 mol% Si doped BiFeO3 polycrystalline thin films is investigated and reported. Quick and reproducible transient response of JSC and VOC with (ton,toff 0.95Si0.05O3 as a high performing multiferroic material with complementary graphene and ITO as electrodes for self-powered oxide electronics.

In resistive random access memory (ReRAM) device, an electroforming process is necessary to activate the resistive memory cell and requires an extra high-voltage source in the memory circuit which leads to high power consumption and complex circuit design. Therefore, electroforming free reproducibleresistive switching(RS) phenomenon in ReRAM is required to reduce the power consumption, to simplify the circuit design and to have better uniformity in the device performance. Herein, we present the non-volatile RS behaviour of pulsed laser deposited polycrystalline thin films of rare-earth oxides. The electroforming free stable unipolar RS behaviour was observed in Pt/RE2O3/Pt memory cells (RE = Sm and Gd).The Pt/RE2O3/Pt cells showed a high-resistance ratio in the range of ∼105-106, a good data retention as long as 104 s measured at 100 °C, a reliable endurance for up to 100 cycles, and non-overlapping switching voltages (SET voltage VON ~2.7˗3.2 V and reset voltage VOFF ~ 0.6˗0.9 V). RS mechanism in Pt/RE2O3/Pt memory cells can be explained on the basis of oxygen vacancies assisted conductive nano-filamentary (thermo-chemical) model. The dominant charge transport mechanism in the oxide films was ascribed to space-charge-limited-current (SCLC) conduction.

Recently, observation of switchable polarization-induced ferroelectric photovoltaic effect (FEPV) in BiFeO3 (BFO) has attracted great interest. In order to improve its FEPV properties, co-substituted [Bi0.9La0.1][Fe0.97Ta0.03]O3 (BLFTO) films were fabricated on Pt/TiO2/SiO2/Si substrates by pulsed laser deposition (PLD). The phase formation of the films was confirmed by X-ray diffraction, Raman spectroscopy, and XPS studies. The FEPV properties in La and Ta co-doped BiFeO3 (BFO) thin films were evaluated under illumination using sandwich configuration with a transparent conducting electrode (TCE) graphene as top electrode. The TCE electrode was transferred by chemical method on BLFTO thin films. The band gap of the films was determined to be 2.66 eV from spectrophotometric measurements, which is very close to that of pure BFO (2.7 eV). The ferroelectric nature of the films was characterized by P-E loop measurements, which indicated that leakage was still the prominent factor for not getting good polarization hysteresis. The piezo-force microscopy (PFM) studies however confirmed the ferroelectric nature of the films. The photovoltaic effect of the films was studied in both geometries, namely top-bottom and planar electrodes configurations. We will present aforementioned investigations and discuss possible photovoltaic applications of the above structure.

This work was supported by the DOE Grant DE-FG02-08ER46526 and NSF (Fellowship EPS 01002410). N. Ortega acknowledges financial support from the NSF-RII Grant R10701525 and Dr. Ashok Kumar was supported by DOD Grant W911NF-11-1-0204. The authors thank Josue Ortiz Morales (MCC) for his help in the XPS measurements. We describe systematic studies on Nd and Mn co-doped BiFeO3, i.e., (Bi0.95Nd0.05) (Fe0.97Mn0.03)O3 (BNFM) polycrystalline electroceramics. Raman spectra and X-ray diffraction patterns revealed the formation of rhombohedral crystal structure at room temperature, and ruled out structural changes in BiFeO3 (BFO) after low percentage chemical substitution. Strong dielectric dispersion and a sharp anomaly around 620K observed near the Neel temperature (TN similar to 643K of BFO) support strong magneto-dielectric coupling, verified by the exothermic peak in differential thermal data. Impedance spectroscopy disclosed the appearance of grain boundary contributions in the dielectric data in the region, and their disappearance just near the Neel temperature suggests magnetically active grain boundaries. The resistive grain boundary components of the BNFM are mainly responsible for magneto-dielectric coupling. Capacitive grain boundaries are not observed in the modulus spectra and the dielectric behavior deviates from the ideal Debye-type. The ac conduction studies illustrate short-range order with ionic translations assisted by both large and small polaron hopping. Magnetic studies indicate that the weak antiferromagnetic phase of BNFM ceramics is dominated by a strong paramagnetic response (unsaturated magnetization even at applied magnetic field of 7 T). The bulk BNFM sample shows a good in-plane magnetoelectric coupling (ME) coefficient. Publisher PDF

Financial support from DOE Grant No. DE-FG02-08ER46526 is acknowledged. S.P.P. is grateful to NSF for financial assistance under Grant No. NSF-EFRI RESTOR #1038272. Y.S. is thankful to IFN-NSF for doctoral fellowship under NSF-RII-0701525 grant. We report structural, optical, charge transport, and temperature properties as well as the frequency dependence of the dielectric constant of Ho2Hf2O7 (HHO) which make this material desirable as an alternative high-k dielectric for future silicon technology devices. A high dielectric constant of similar to 20 and very low dielectric loss of similar to 0.1% are temperature and voltage independent at 100 kHz near ambient conditions. The Pt/HHO/Pt capacitor exhibits exceptionally low Schottky emission-based leakage currents. In combination with the large observed bandgap Eg of 5.6 eV, determined by diffuse reflectance spectroscopy, our results reveal fundamental physics and materials science of the HHO metal oxide and its potential application as a high-k dielectric for the next generation of complementary metal-oxide-semiconductor devices. Publisher PDF

High-pressure synchrotron based angle dispersive x-ray diffraction (ADXRD) studies were carried out on SmGdO3 (SGO) up to 25.7 GPa at room temperature. ADXRD results indicated a reversible pressure-induced phase transition from ambient monoclinic to hexagonal phase at ∼8.9 GPa. The observed pressure-volume data were fitted with the third order Birch-Murnaghan equation of state yielding zero pressure bulk modulus B0 = 132(22) and 177(9) GPa for monoclinic (B-type) and hexagonal (A-type) phases, respectively. Pressure dependent micro-Raman spectroscopy further confirmed the monoclinic to hexagonal phase transition at about 5.24 GPa. The mode Gruneisen parameters and pressure coefficients for different Raman modes corresponding to each individual phases of SGO were calculated using pressure dependent Raman mode analysis.

This work was supported by the DOE-EPSCoR Grant No. DE-FG02-08ER46526. Acknowledgment is also due to NSF Grant No. #1002410 for providing fellowships to R.K.K., D.B., and J.S.Y. Doubly substituted [Bi0.9La0.1][Fe0.97Ta0.03]O3 (BLFTO) films were fabricated on Pt/TiO2/SiO2/Si substrates by pulsed laser deposition. The ferroelectric photovoltaic properties of ZnO:Al/BLFTO/Pt thin film capacitor structures were evaluated under white light illumination. The open circuit voltage and short circuit current density were observed to be ∼0.20 V and ∼1.35 mA/cm2, respectively. The band gap of the films was determined to be ∼2.66 eV, slightly less than that of pure BiFeO3 (2.67 eV). The PVproperties of BLFTO thin films were also studied for various pairs of planar electrodes in different directions in polycrystalline thin films. Publisher PDF