Showing papers by "Mukul Gupta published in 2013"

Influence of in-situ annealing ambient on p-type conduction in dual ion beam sputtered Sb-doped ZnO thin films

Abstract: Sb-doped ZnO (SZO) films were deposited on c-plane sapphire substrates by dual ion beam sputtering deposition system and subsequently annealed in-situ in vacuum and in various proportions of O2/(O2 + N2)% from 0% (N2) to 100% (O2). Hall measurements established all SZO films were p-type, as was also confirmed by typical diode-like rectifying current-voltage characteristics from p-ZnO/n-ZnO homojunction. SZO films annealed in O2 ambient exhibited higher hole concentration as compared with films annealed in vacuum or N2 ambient. X-ray photoelectron spectroscopic analysis confirmed that Sb5+ states were more preferable in comparison to Sb3+ states for acceptor-like SbZn-2VZn complex formation in SZO films.

Effect of growth temperature on structural, electrical and optical properties of dual ion beam sputtered ZnO thin films

Abstract: ZnO epitaxial thin films were grown on p-type Si(100) substrates by dual ion beam sputtering deposition system. The crystalline quality, surface morphology, optical and electrical properties of as-deposited ZnO thin films at different growth temperatures were studied. Substrate temperature was varied from 100 to 600 °C at constant oxygen percentage O2/(O2 + Ar) % of 66.67 % in a mixed gas of Ar and O2 with constant chamber pressure of 2.75 × 10−4 mBar. X-Ray diffraction analyses revealed that all the films had (002) preferred orientation. The minimum value of stress was reported to be −0.32 × 1010 dyne/cm2 from ZnO film grown at 200 °C. Photoluminescence measurements demonstrated sharp near-band-edge emission (NBE) was observed at ~375 nm along with deep level emission (DLE) in the visible spectral range at room temperature. The DLE Peak was found to have decrement as ZnO growth temperature was increased from 200 to 600 °C. The minimum FWHM of the NBE peak of 16.76 nm was achieved at 600 °C growth temperature. X-Ray photoelectron spectroscopy study revealed presence of oxygen interstitials and vacancies point defects in ZnO film grown at 400 °C. The ZnO thin film was found to be highly resistive when grown at 100 °C. The ZnO films were found to be n-type conducting with decreasing resistivity on increasing substrate temperature from 200 to 500 °C and again increased for film grown at 600 °C. Based on these studies a correlation between native point defects, optical and electrical properties has been established.

Effect of oxygen partial pressure on the behavior of dual ion beam sputtered ZnO thin films

Abstract: Undoped ZnO thin films were grown on p-type Si (1 0 0) substrates at different oxygen partial pressure by dual ion beam sputtering deposition system at a constant growth temperature of 400 °C. The crystallinity, surface morphology, optical, elemental and electrical properties of these ZnO thin films was studied. The minimum value of full-width at half-maximum of the θ-rocking curve obtained from x-ray diffraction of the ZnO (0 0 2) plane, was reported to be 0.1865° from ZnO film grown at 50% of (O2/(O2 + Ar))%. Crystalline property of ZnO films was observed to degrade with the increase in oxygen partial pressure. Photoluminescence measurements demonstrated sharp near-band-edge emission at ∼381 nm at room temperature. X-ray photoelectron spectroscopy study revealed presence of oxygen interstitials and vacancies as point defects in ZnO films. Electrical resistivity of ZnO was found to increase with the increase in oxygen partial pressure.

P-type conduction from Sb-doped ZnO thin films grown by dual ion beam sputtering in the absence of oxygen ambient

Abstract: Sb-doped ZnO (SZO) thin films were deposited on c-plane sapphire substrates by dual ion beam sputtering deposition system in the absence of oxygen ambient. The electrical, structural, morphological, and elemental properties of SZO thin films were studied for films grown at different substrate temperatures ranging from 200 °C to 600 °C and then annealed in situ at 800 °C under vacuum (pressure ∼5 × 10−8 mbar). Films grown for temperature range of 200–500 °C showed p-type conduction with hole concentration of 1.374 × 1016 to 5.538 × 1016 cm−3, resistivity of 66.733–12.758 Ω cm, and carrier mobility of 4.964–8.846 cm2 V−1 s−1 at room temperature. However, the film grown at 600 °C showed n-type behavior. Additionally, current-voltage (I–V) characteristic of p-ZnO/n-Si heterojunction showed a diode-like behavior, and that further confirmed the p-type conduction in ZnO by Sb doping. X-ray diffraction measurements showed that all SZO films had (002) preferred crystal orientation. X-ray photoelectron spectroscopy...

Evolution of structural and magnetic properties of amorphous CoFeB film with thermal annealing

Abstract: Evolution of structural and magnetic properties of amorphous Co68Fe14B18 thin film with thermal annealing has been studied. Initially, the film exhibits a structural relaxation as evidenced by annihilation of excess free volume and an increase in topological short range order. Annealing at 473 K results in precipitation of primary phase followed by formation of boride phase at a still higher temperature of 598 K. Iron preferentially precipitates out in the primary phase, resulting in the formation of bcc Co58Fe41. This suggests an affinity of Co towards B. Such affinity between Co and B is evidenced even in the as-deposited film, using hard x-ray photoelectron spectroscopy (HAXPES) measurements. As-deposited film exhibits an in-plane uniaxial magnetic anisotropy which disappears at a temperature well beyond crystallization temperature, suggesting that the origin of anisotropy is mainly a chemical short range order in the system. Variation in the coercivity with thermal annealing can be understood in terms of random anisotropy model. Precise measurement of Fe self-diffusion using neutron reflectivity shows that diffusion length associated with annihilation of excess free volume in the film is about 0.5 nm. This agrees with the length scale of structural fluctuations in amorphous alloys. Secondary ion mass spectrometry measurements show that thermal annealing results in depletion of B in the region of the interface with the substrate, with associated faster Fe diffusion in this region. This faster diffusion of Fe may be a possible cause of preferential crystallization of the film in the interfacial region as seen in some earlier studies.

Spin-phonon coupling in K_{0.8}Fe_{1.6}Se_{2} and KFe_{2}Se_{2}: Inelastic neutron scattering and ab initio phonon calculations

Abstract: We report measurements of the temperature dependence of phonon densities of states in K${}_{0.8}$Fe${}_{1.6}$Se${}_{2}$ using inelastic neutron scattering technique. While cooling down to 150 K, a phonon peak splitting \ensuremath{\sim}25 meV is observed and a new peak appears at 31 meV. The measurements support the recent Raman and infrared measurements, indicating a lowering of symmetry of K${}_{0.8}$Fe${}_{1.6}$Se${}_{2}$ upon cooling below 250 K. Ab initio phonon calculations have been carried out for K${}_{0.8}$Fe${}_{1.6}$Se${}_{2}$ and KFe${}_{2}$Se${}_{2}$. The comparison of the phonon spectra as obtained from the magnetic and nonmagnetic calculations show pronounced differences. We show that in the two calculations, the energy range of the vibrational contribution from both Fe and Se are quite different. We conclude that Fe magnetism is correlated to the phonon dynamics and plays an important role in stabilizing the structure of K${}_{0.8}$Fe${}_{1.6}$Se${}_{2}$, as well as that of KFe${}_{2}$Se${}_{2}$. The calculations highlight the presence of low energy optical vibrational modes in K${}_{0.8}$Fe${}_{1.6}$Se${}_{2}$ compared to KFe${}_{2}$Se${}_{2}$.

Investigation of dual ion beam sputtered transparent conductive Ga-doped ZnO films

Abstract: Ga-doped ZnO (GZO) transparent conducting films were deposited on sapphire (0001) substrates using dual ion beam sputtering deposition system. The impact of growth temperature on the structural, morphological, elemental, optical, and electrical properties was thoroughly investigated and reported. X-ray diffraction measurements explicitly confirmed that all GZO films had (002) preferred crystal orientation. The film deposited at 400 °C exhibited the narrowest full-width at half-maximum value of 0.24° for (002) crystalline plane and the lowest room temperature electrical resistivity of 4.11 × 10−3 Ω cm. The Raman spectra demonstrated the vibrational modes at 576 and 650–670 cm−1, associated with native oxygen vacancies and elemental Ga doping in ZnO lattice, respectively. All doped films showed an overall transmittance of above 95 % in the visible spectra. A correlation between structural, optical, elemental, and electrical properties with GZO growth temperature was established.

Dielectric properties of sol–gel synthesized SrTiO3/(Ba0.7Sr0.3)TiO3 and SrTiO3/Ba(Zr0.3Ti0.7)O3 thin film heterostructures

Abstract: The paper presents synthesis of Ba0.7Sr0.3TiO3 (BST), BaZr0.3Ti0.7O3 (BZT) and SrTiO3 (ST) thin films and their heterostructures using modified Pechini method. The La0.7Sr0.3MnO3 has been used as a conducting bottom layer to form metal ferroelectric metal capacitor. The thin films are spin coated on SiO2/n-Si(100) substrates. The thin films thus deposited are characterized for crystal structure, morphology, dielectric, complex impedance and admittance properties. Deposition of surface layer ST is observed to reduce loss tangent tan δ of BST and BZT thin films, still maintaining equivalent magnitude of figure of merit γ. The results on dielectric properties are analyzed in terms of the Maxwell–Wagner model and Koop’s phenomenological theory.

Selective Capacitive Sensor for Ammonia Hydroxide at Room Temperature

Abstract: Nanostructured yttria stabilized ZrO2 (YSZ) films are prepared by electron beam deposition and subsequently characterized by using X-ray diffraction, field emission scanning electron microscopy, and UV-Vis spectroscopy. A capacitive device with Al/YSZ/Al structure acts as a sensor for analytes like ammonia solution, acetone, and alcohols. Low powered read-out system based on an astable mode frequency generator is employed for monitoring the response continuously. The YSZ capacitive sensor operates at room temperature with a decrease in frequency on exposure to ammonia solution and the response varies linearly with concentrations (1%-30%). In contrast, highly volatile analytes like acetone, ethanol, and dichloromethane exhibit an increase in frequency after the exposures. Water exposure produces no significant change in the sensor response. The unique selectivity of YSZ towards ammonia solution is attributed to the modified dielectric constant of the YSZ films due to preferential binding of water molecules to the analyte.