Amit Kumar Das

Effect of disorder on carrier transport in ZnO thin films grown by atomic layer deposition at different temperatures

We have grown ∼200 nm thick ZnO films on (0001) sapphire substrates using atomic layer deposition at different substrate temperatures ranging from ∼150 to 350 °C. X-ray diffraction and photoluminescence spectra of these films showed that crystalline and compositional native defects were strongly dependent on the substrate temperature. Room temperature Hall measurement showed that all the films were degenerate with carrier concentration exceeding the Motts critical density nc required for metallic conduction. The lowest value of room temperature resistivity ∼3.6 × 10−3 Ω cm was achieved for the film deposited at ∼200 °C, which had an estimated carrier concentration ∼5.7 × 1019 cm−3 and mobility ∼30 cm2/V s. The films deposited both below and above ∼200 °C showed increased resistivity and decreased mobility presumably due to the intensified defects and deteriorated crystalline quality of these films. To investigate the effect of disorder on the underlying charge transport mechanisms in these films, the ele...

Studies on temperature dependent semiconductor to metal transitions in ZnO thin films sparsely doped with Al

For a detailed study on the semiconductor to metal transition (SMT) in ZnO thin films doped with Al in the concentration range from 0.02% to 2%, we grew these films on (0001) sapphire substrates using sequential pulsed laser deposition. It was found that the Al concentration in the films increased monotonically with the ratio of ablation durations of the alumina and ZnO targets used during the deposition. Using x-ray photo electron spectroscopy, it was found that while most of the Al atoms occupy the Zn sites in the ZnO lattice, a small fraction of the Al also gets into the grain boundaries present in the films. The observed SMT temperature decreased from ∼270 to ∼50 K with increase in the Al concentration from 0.02% to 0.25%. In the Al concentration range of ∼0.5% to 2%, these doped ZnO films showed metallic behavior at all the temperatures without undergoing any SMT. A theoretical model based on thermal activation of electrons and electron scatterings due to the grain boundaries, ionic impurities, and p...

Thickness dependent metal-insulator transition and dimensional crossover for weak localization in Si0.02Zn0.98O thin films grown by pulsed laser deposition

Metal to insulator transition was observed in Si0.02Zn0.98O (SZO) films, grown by pulsed laser deposition on sapphire substrates, as the thicknesses of the films were reduced from ∼40 to 15 nm. The SZO film with thickness of ∼40 nm showed typical metallic behavior in temperature dependent resistivity measurements. On the contrary, the SZO film with thickness of ∼15 nm was found to exhibit strong localization where the transport at low temperature was dominated by variable range hopping conduction. In the intermediate thickness regime, quantum corrections were important and a dimensional crossover from 3D to 2D weak localization occurred in the SZO film with thickness of 20 nm.

Effect of Mg diffusion on photoluminescence spectra of MgZnO/ZnO bi-layers annealed at different temperatures

Mg0.29Zn0.71O/ZnO (MZO/ZnO) bilayers were grown on sapphire substrates by pulsed laser deposition at ~ 400 C and were subsequently annealed at different temperatures ranging from ~ 500 to 900 C to allow diffusion of Mg across the interface. The diffusion of Mg was confirmed from secondary ion mass spectroscopy (SIMS) and Photo-absorption measurements on the samples. As a result of Mg diffusion the photoluminescence (PL) spectra of the bilayers got significantly modified. Whereas the as grown structure had two distinct near band edge UV luminescence peaks corresponding to the ZnO and MZO layer, the one annealed at ~ 900 C had a single near band edge PL peak corresponding to a complete intermixed layer. At the intermediate annealing temperature of ~800 C the PL peak position corresponding to the MZO layer red-shifted while that corresponding to the ZnO layer blue-shifted, implying a diffused interface and partial intermixing. A theoretical model has been developed and numerically simulated to correlate the PL spectra of the bilayers with the Mg diffusion profile across the interface, as measured by SIMS.

Surface functionalized carbon nanotube with polyvinylidene fluoride: Preparation, characterization, current-voltage and ferroelectric hysteresis behaviour of polymer nanocomposite films

A comparative study of current-voltage characteristics and ferroelectric properties of polyvinylidene fluoride (PVDF) based nanocomposite films with pure multiwall carbon nanotubes (MWCNTs) and surface functionalized MWCNTs by different type of functionalize groups (-COOH, -OH, -NH2) as filler, was reported. The flexible nanocomposite films based on PVDF and surface modified MWCNTs were fabricated via solution casting method. The current voltage characteristics suggests that –COOH and –OH functionalize MWCNT loading PVDF films are semiconducting in nature whereas raw MWCNT and –NH2 functionalize MWNCT loading PVDF films are comparatively conducting in nature. Experimental data of current density-electric field were fitted with different existing theoretical models. It is observed that just by changing the functionalization group the electrical conductivity of the composite films significantly changes which is discernible from the current-voltage characteristic. From ferroelectric study the energy density ...

AC electrical transport properties and current-voltage hysteresis behavior of PVA-CNT nanocomposite film

Polyvinyl alcohol (PVA) – Carbon nanotube (CNT) composite has been prepared and its electric modulus, ac conductivity, impedance spectroscopy and current-voltage characteristics have been studied, at and above room temperature, to understand the prevailing charge transport mechanism. Non-Debye type relaxation behavior was observed with activation energy of 1.27 eV whereas correlated barrier hopping was found to be the dominant charge transport mechanism with maximum barrier height of 48.7 meV above room temperature. The sample, under ±80 V applied voltage, exhibits hysteresis behavior in its current - voltage characteristics.

Influence of functional group on the electrical transport properties of polyvinyl alcohol grafted multiwall carbon nanotube composite thick film

We present a novel technique to obtain a higher or lower value of dielectric constant due to the variation of a functional group on the surface of multiwall carbon nanotube (MWCNTs) for a polyvinyl alcohol (PVA) grafted MWCNT system. We have prepared PVA grafted pristine and different types of functionalized (-COOH, -OH, and -NH2) MWCNT nanocomposite films. The strong interfacial interaction between the host PVA matrix and nanofiller is characterized by different experimental techniques. The frequency variation of the electrical transport properties of the composite films is investigated in a wide temperature range (303 ≤ T ≤ 413 K) and frequency range (20 Hz ≤ f ≤ 1 MHz). The dielectric constant of the amine (-NH2) functionalized MWCNT incorporated PVA film is about 2 times higher than that of the pristine MWCNT embedded PVA film. The temperature variation of the dielectric constant shows an anomalous behaviour. The modified Cole-Cole equation simulated the experimentally observed dielectric spectroscopy at high temperature. The ac conductivity of the composite films obeys the correlated barrier hopping model. The imaginary part of the electric modulus study shows the ideal Debye-type behaviour at low frequency and deviation of that at high frequency. To illustrate the impedance spectroscopy of the nanocomposite films, we have proposed an impedance based battery equivalent circuit model. The current-voltage characteristic shows hysteresis behaviour of the nanocomposite films. The trap state height for all composite films is evaluated by simulating the current density-electric field data with the Poole-Frenkel emission model. This investigation opens a new avenue for designing electronic devices with a suitable combination of cost effective soft materials.

Electrical conductivity and current-voltage hysteresis behavior of BiFeO3 doped PVA nanocomposite film

Multiferroic Bismuth Ferrite (BFO) nanoparticles of average crystallite size 52 nm and BFO doped PVA nanocomposite film have been synthesized by sol gel method. Variation of DC conductivity follows Arrhenius formalism with activation energies 0.22 eV and 0.14 eV whereas Mott’s VRH model provides the good fit with average hopping energy 3.56 eV at 340 K. Correlated barrier hopping was found to be the dominant charge transport mechanism with maximum barrier height 55 meV above room temperature. Current-voltage study of this sample exhibits hysteresis loop under ±50 V applied voltage.