A. Dhar

The effect of substrate temperature on the properties of ITO thin films for OLED applications

Indium tin oxide (ITO) thin films have been deposited by rf magnetron sputtering on glass substrates at different substrate temperatures. The structural, electrical and optical properties of these films have been investigated to obtain optimum values for resistivity, optical transmittance and surface smoothness. The film deposited at a substrate temperature of 300 °C shows good conductivity, optical transmittance, crystallinity and surface smoothness. These ITO films were used to fabricate organic light emitting diodes (OLEDs). The dc current–voltage (I–V) studies on ITO/PEDOT:PSS/MEH-PPV/Al test structures show better rectifying behaviour on a smoother ITO substrate.

Charge storage and photoluminescence characteristics of silicon oxide embedded Ge nanocrystal trilayer structures

Metal-oxide-semiconductor capacitors with a trilayer structure consisting of the cap gate oxide, sputtered SiGe layers and thermally grown tunnel oxide were fabricated on p-Si substrates. The trilayer structures were rapid thermal annealed at 1000u200a°C in nitrogen atmosphere for different durations. Cross-sectional transmission electron micrographs revealed the complete isolation of Ge nanocrystals in the sandwiched structure annealed for a longer duration. The optical and charge storage characteristics of trilayer structures were studied through photoluminescence spectroscopy and capacitance-voltage measurements, respectively. Under optimized annealing conditions, an enhancement of the charge storage capability of nanocrystals was observed in agreement with the optical emission characteristics.

Multilevel conductance switching in organic memory devices based on AlQ3 and Al/Al2O3 core-shell nanoparticles

Multilevel conductance switching in organic memory devices based on AlQ3 and Al/Al2O3 core-shell nanoparticles is demonstrated. The effect of middle Al layer thickness and the size of the nanoparticles on device performance are investigated. The high-resolution transmission electron micrographs revealed the formation of core-shell nanoparticles. The device has shown a series of conductance states. These states are nonvolatile in nature and can be accessed by applying proper programming voltage above a threshold voltage. Field-induced transfer of charge carriers between AlQ3 and aluminum core is proposed to be responsible for conductance switching.

Characteristics of CdS nanowires grown in a porous alumina template using a two-cell method

CdS nanowires have been grown by a two-cell chemical method using a porous alumina template. Field-emission scanning and transmission electron microscopy show the growth of nanowires of diameters ranging from 100 to 110 nm and 50 to 60 nm for two different templates. The surface topography of the template filled with CdS has been studied using atomic force microscopy. The microstructural properties of the nanowires have been studied by high resolution x-ray diffraction. UV–visible spectroscopy of nanowires exhibits a clear blue shift due to quantum confinement. The photoluminescence spectra show green emission due to the defect states of the nanowires and the resonance Raman spectra revealed the stoichiometric phase of CdS with good crystallinity.

Improved charge injection characteristics of Ge nanocrystals embedded in hafnium oxide for floating gate devices

Metal-oxide-semiconductor capacitors with a trilayer structure consisting of Ge+HfO2 layers sandwiched between HfO2 tunnel and cap oxides were fabricated on p-Si substrates. Ge nanocrystals embedded in SiO2 were also studied for comparison. Cross-sectional transmission electron micrographs revealed the formation of spherical shaped Ge nanocrystals. The optical and charge storage characteristics of trilayer structures were studied through photoluminescence spectroscopy and capacitance-voltage measurements, respectively. An enhancement of the charge injection capability into nanocrystals was observed for the device with HfO2 as tunnel and cap oxide. The optical emission characteristics support the carrier confinement in Ge nanocrystals embedded in oxide matrices.

Electrical and interfacial characteristics of ultrathin ZrO2 gate dielectrics on strain compensated SiGeC/Si heterostructure

Ultrathin ZrO2 gate dielectrics have been deposited on strain-compensated Si0.69Ge0.3C0.01 layers by rf magnetron sputtering. High-resolution transmission electron microscopy along with energy-dispersive x-ray spectroscopy and x-ray photoelectron spectroscopy show the formation of a polycrystalline ZrO2 and an amorphous Zr–germano–silicate interfacial layer between the deposited oxide and SiGeC films. A dielectric constant of 17.5 for ZrO2 and 7.0 for an interfacial-silicate layer have been calculated from the high-frequency capacitance–voltage measurements. These dielectrics show an equivalent oxide thickness as low as 1.9 nm for ZrO2 and 2.0 nm for the interfacial silicate layer. An extremely low leakage current density of ∼9×10−8u2009A/cm2 at a gate voltage of −1.0 V, breakdown field of 7 MV/cm and moderate interface state density of 6×1011u2009cm−2u200aeV−1 have been obtained for the fabricated capacitors.

Nonvolatile Memristive Switching Characteristics of TiO

Nickel nanocrystal (Ni-NC)-embedded titanium dioxide films have been deposited for nonvolatile resistive switching memory devices. The polycrystalline behavior of the films has been observed from the X-ray diffraction spectra. Tiny isolated Ni-NCs with an average size of 4 nm are observed for the 1000 °C, 5-min annealed samples. Stable, bipolar, nonvolatile, and bistable resistive switching states are observed for the optimized annealed Ni-NC-embedded devices with a low SET and RESET voltage of 0.8 and -0.2 V, respectively. A high resistance ratio (>;10), good stability, and retention properties are observed for the nanocrystal sample. The role of thin Ni-NC layer on memory switching stability is discussed.

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Pentacene based junctions with Al on both p-Si(1 0 0) and indium tin oxide (ITO) substrates have been analysed using impedance spectroscopy (100 Hz–20 MHz) and dc-current versus voltage measurements. Conduction mechanisms match well with the space charge limited current model with exponential trap charge distributions. Impedance studies reveal the Schottky behaviour for both the devices. The differences between p-Si/pentacene/Al and ITO/pentacene/Al devices have been explained by modelling equivalent circuits, with one and two RC elements, respectively. The switch-over field from ohmic conduction to space charge limited conduction, charge concentration as a function of voltage and bias dependent series resistance have been estimated for both the cases.

Films Embedded With Nickel Nanocrystals

We report on the observation of intraband near infrared (∼3.1u2002μm) and mid infrared (∼6.2u2002μm) photocurrent response at room temperature using Ge/Si self-assembled quantum dots grown by molecular beam epitaxy. Due to the bimodal size distribution and SiGe intermixing, distinguishable photoluminescence transitions are observed at 10 K, below and above the optical band gap of bulk Ge. The observed redshift in photocurrent with increasing temperature has been explained by the excitonic electric field originated due to infrared excitation at low temperatures. A good correlation between the spectral photocurrent response and photoluminescence of the quantum dots has been established.

Studies on conduction mechanisms of pentacene based diodes using impedance spectroscopy

Ferroelectric SrBi2Ta2O9 (SBT) thin films have been deposited by the radio-frequency magnetron sputtering technique on bare p-Si as well as on HfO2 insulating buffer p-Si. XRD patterns revealed the formation of a well-crystallized SBT perovskite thin film on the HfO2 buffer layer. The electrical properties of the metal–ferroelectric–insulator–semiconductor (MFIS) structure were characterized by varying thicknesses of the HfO2 layer. The MFIS structure exhibits a maximum clockwise C–V memory window of 1.60 V when the thickness of the HfO2 layer was 12 nm with a lower leakage current density of 6.20 × 10−7 A cm−2 at a positive applied voltage of 7 V. However, the memory window reaches a maximum value of 0.7 V at a bias voltage of ±5 and then decreases due to charge injection in the case of the insulating buffer layer thickness of 3 nm. The density of oxide trapped charges at/near the buffer layer–ferroelectric interface is studied by the voltage stress method. Capacitance–voltage (C–V) and leakage current density (J–V) characteristics of the Al/SBT/HfO2/Si(1 0 0) capacitor indicate that the introduction of the HfO2 buffer layer prevents interfacial diffusion between the SBT thin film and the Si substrate effectively and improves the interface quality. Furthermore, the Al/SBT/HfO2/Si structures exhibit excellent retention characteristics, the high and low capacitance values clearly distinguishable for over 1 h and 30 min. This shows that the proposed Al/SrBi2Ta2O9/HfO2/Si structure is ideally suitable for high performance ferroelectric memories.