P. Banerji

Embedded Ag-rich nanodots in PbTe: Enhancement of thermoelectric properties through energy filtering of the carriers

The concept of energy filtering of the carriers to control the thermoelectric properties of PbTe is experimentally applied in this present work. The energy barriers at the grain interfaces of the nanocomposites and the embedded Ag-rich nanodots within the grains are supposed to control the energy dependency of carrier scattering: that is what we mean by energy filtering of carriers. As a case study, vertical Bridgman grown bulk PbTe:undoped, PbTe:Ag crystals and nanocomposites of PbTe:Ag are used as samples. Thermoelectric properties of all the samples have been evaluated through temperature dependent electrical conductivity, Seebeck coefficient and room temperature Hall and thermal conductivity measurements. It is found that the PbTe:Ag nanocomposites has the highest power factor of 18.78×10−4 W m−1 K−2 with a room temperature thermal conductivity of 1.69 W m−1 K−1. The crystal structures of these samples show the effective potential barrier at the grain boundaries and embedded nanodots within the grains...

Temperature dependent electrical transport in p-ZnO/n-Si heterojunction formed by pulsed laser deposition

ZnO film, with urea as nitrogen source to dope its p-type, is deposited by pulsed laser on n-type (100) Si substrate to fabricate p-ZnO/n-Si heterojunctions. The current-voltage (I-V) characteristics of the heterojunction have been studied in the temperature range 140–300 K. The turn on voltage decreased with increasing temperature while the breakdown voltage is increased slightly. The forward current is greatly increased with increasing temperature, while the reverse current is increased nominally. Both the decrease in barrier height and the increase in ideality factor with decrease in temperature are indicative of deviation from the pure thermionic emission-diffusion mechanism. The ln(I0) versus 1/kT plot exhibits the linear portion corresponding to an activation energy of 0.07 eV. Temperature-dependent forward current measurements suggest that trap-assisted multistep tunneling is the dominant carrier transport mechanism in this heterojunction. C-V analysis indicates an abrupt interface and band bending...

Exploration of Zn resonance levels and thermoelectric properties in I-doped PbTe with ZnTe nanostructures.

Motivated by the theoretically predicted Zn resonant states in the conduction band of PbTe, in the present work, we investigated the effect of Zn substitution on the thermoelectric properties in I-doped n-type PbTe. The room temperature thermopower values show good agreement with the theoretical Pisarenko plot of PbTe up to a carrier concentration of 4.17 × 10(19) cm(-3); thus, the presence of Zn resonance levels is not observed. Because of the low solubility of Zn in PbTe, a second phase of coherent ZnTe nanostructures is observed within the PbTe host matrix, which is found to reduce the lattice thermal conductivity. The reduced lattice thermal conductivity in PbTe by ZnTe nanostructures leads to notable enhancement in the figure of merit with a maximum value of 1.35 at 650 K. In contrast to the recent literature, the carrier mobility is not found to be affected by the band offset between ZnTe nanostructures and PbTe. This is explained by the quantum tunneling of the charge carrier through the narrow offset barrier and depletion width and coherent nature of the interface boundary between the two phases, i.e., ZnTe and PbTe.

Dramatic enhancement of thermoelectric power factor in PbTe:Cr co-doped with iodine

We report thermoelectric properties of PbTe doubly doped with chromium (Cr) and iodine (I). Cr is found to create a local enhancement of density of states (DOS) in the conduction band of PbTe, and I is found to tune the position of the Fermi level. The coincidence of the Fermi level with the enhanced DOS is found to produce up to 135% enhancement in thermopower leading to a high power factor (PF) of 56.16 × 10−4 Wm−1K−2 at 500 K. Such high value of power factor can produce thermoelectric figure of merit (ZT) above 2.

Properties of indium doped nanocrystalline ZnO thin films and their enhanced gas sensing performance

Zinc oxide (ZnO) is one of the most promising semiconducting metal oxides, particularly for gas sensing applications. Impurity doped ZnO has offered much improved sensing performance, as compared to its undoped counterpart. In this work, undoped as well as indium doped nanocrystalline ZnO thin films are synthesized by a low cost chemical solution deposition route. X-ray diffraction patterns of the synthesized films reveal preferential orientation along the (002) plane. The surface and cross section morphology has clearly changed with the variation of indium content. Optical transmittance values increase with the increase in indium concentration and the band gap energy is found in the range 3.210 eV to 3.221 eV. PL spectra reveal three characteristic peaks, owing to band to band transition and defect level emissions. The effect of indium doping on the electrical parameters of ZnO is analyzed through Hall effect measurements at room temperature. The gas sensing characteristics of these sensors offer good reproducibility and stability towards various reducing gases, with an enhancement of response% and lower detection limit as compared to undoped ZnO. A doping level of 3 wt% of indium in ZnO is found to give optimum response and the lowest detection limit of hydrogen of 1 ppm or even lower. However, further increase in the doping concentration resulted in reduced sensing performance. This is attributed to the gas sensing mechanism related to the substitution of In3+ ions at Zn2+ ion sites enhancing the number of free charge carriers at the optimum level of indium. Through exploring this gas sensing mechanism, it is argued that the sensor performance can be dramatically improved by tailoring the indium concentration in ZnO for its practical application in various sectors as an effective gas sensor.

Studies on Al/ZrO2/GaAs metal-oxide-semiconductor capacitors and determination of its electrical parameters in the frequency range of 10 kHz–1 MHz

Aluminum (Al)/zirconium oxide (ZrO2)/GaAs metal-oxide-semiconductor (MOS) capacitors were fabricated on p-GaAs. The carrier concentration of n-GaAs was varied by metal organic chemical vapor deposition. The ZrO2 gate dielectrics were prepared by the sol-gel process and then spin-coated onto GaAs substrates. Three different thicknesses of the ZrO2 layer, viz., 25, 40 and 50 nm, were used to study the effect of oxide thickness on different MOS parameters. Sulfur (S) passivation of the GaAs surface was done to control the interface state densities before ZrO2 deposition. It was found that S passivation resulted in both low hysteresis and high accumulation capacitance of the device. Frequency dependent studies on the dielectric characterizations were made in the frequency range of 10 kHz–1 MHz. It was found that within this frequency range dispersion of the oxide capacitance was 2%/decade. It was observed that interface trap densities (Dit) increased with an increase in the carrier concentration of n-GaAs fro...

Aluminum/polyaniline/GaAs metal-insulator-semiconductor solar cell: Effect of tunneling on device performance

We report fabrication of GaAs based metal-insulator-semiconductor solar cell with polyaniline, a polymeric material, as an insulator. The photovoltaic response of Aluminum/Polyaniline/GaAs metal-insulator-semiconductor diode was measured in the air mass (AM) 1.0 and 1.5. The values of the open circuit voltage, short circuit current and the series resistance at AM 1.0 are measured to be 0.45 mA, 1.07 V and 26 Ω, respectively. It is found that the thickness of the polyaniline layer plays a crucial role in determining various properties of the device. The findings are explained on the basis of the tunneling probability.

Qualitative and quantitative differentiation of gases using ZnO thin film gas sensors and pattern recognition analysis.

In the present work we have grown highly textured, ultra-thin, nano-crystalline zinc oxide thin films using a metal organic chemical vapor deposition technique and addressed their selectivity towards hydrogen, carbon dioxide and methane gas sensing. Structural and microstructural characteristics of the synthesized films were investigated utilizing X-ray diffraction and electron microscopy techniques respectively. Using a dynamic flow gas sensing measurement set up, the sensing characteristics of these films were investigated as a function of gas concentration (10-1660 ppm) and operating temperature (250-380 °C). ZnO thin film sensing elements were found to be sensitive to all of these gases. Thus at a sensor operating temperature of ~300 °C, the response% of the ZnO thin films were ~68, 59, and 52% for hydrogen, carbon monoxide and methane gases respectively. The data matrices extracted from first Fourier transform analyses (FFT) of the conductance transients were used as input parameters in a linear unsupervised principal component analysis (PCA) pattern recognition technique. We have demonstrated that FFT combined with PCA is an excellent tool for the differentiation of these reducing gases.

Lead-free epitaxial ferroelectric material integration on semiconducting (100) Nb-doped SrTiO3 for low-power non-volatile memory and efficient ultraviolet ray detection

We report lead-free ferroelectric based resistive switching non-volatile memory (NVM) devices with epitaxial (1-x)BaTiO3-xBiFeO3 (x = 0.725) (BT-BFO) film integrated on semiconducting (100) Nb (0.7%) doped SrTiO3 (Nb:STO) substrates. The piezoelectric force microscopy (PFM) measurement at room temperature demonstrated ferroelectricity in the BT-BFO thin film. PFM results also reveal the repeatable polarization inversion by poling, manifesting its potential for read-write operation in NVM devices. The electroforming-free and ferroelectric polarization coupled electrical behaviour demonstrated excellent resistive switching with high retention time, cyclic endurance, and low set/reset voltages. X-ray photoelectron spectroscopy was utilized to determine the band alignment at the BT-BFO and Nb:STO heterojunction, and it exhibited staggered band alignment. This heterojunction is found to behave as an efficient ultraviolet photo-detector with low rise and fall time. The architecture also demonstrates half-wave rectification under low and high input signal frequencies, where the output distortion is minimal. The results provide avenue for an electrical switch that can regulate the pixels in low or high frequency images. Combined this work paves the pathway towards designing future generation low-power ferroelectric based microelectronic devices by merging both electrical and photovoltaic properties of BT-BFO materials.

The effect of chromium impurity on the thermoelectric properties of PbTe in the temperature range 100–600 K

In this work we have investigated the effect of the chromium (Cr) impurity on the thermoelectric properties of lead telluride (PbTe) bulk crystal with different Cr-content. The structural characterization of the crystals was done by x-ray diffraction and high-resolution transmission electron microscopic analyses. The incorporation of Cr in PbTe over the soluble limit has been found to replace Pb from the lattice site, forcing it to be precipitated into nanodots, with their dimensions in the range of 2–13 nm and their numbers increasing with the increase in Cr doping. The transport properties of the samples with different Cr-content have been evaluated through temperature dependent electrical resistivity, the Hall coefficient, and thermopower measurements in the temperature range of 100–600 K. The highest mobility is obtained as 1404 cm2/V s for a sample with a carrier concentration of 3.84 × 1018 cm−3 at 300 K. The high electron mobility and moderately higher value of the thermopower are expected to produ...