Debarati Bhattacharya

A study of the CuO phase formation during thin film deposition by molecular beam epitaxy

Abstract The kinetics of CuO growth under molecular beam epitaxial (MBE) conditions has been investigated. The evaporation of Cu and its deposition onto Si(111) substrate maintained at 823 K was carried out using an electron beam heated source. For the oxidation of Cu, sources of both molecular and atomic oxygen species were employed. The films were characterized by electron spectroscopy for chemical analysis (ESCA), X-ray diffraction (XRD), infrared (IR) transmission and scanning electron microscopy (SEM). The application of a fairly high flux of molecular oxygen (3.4×1020 molecules/(m2 s)) and O2 to Cu flux ratio of ∼250 during the deposition was found to be insufficient to convert a detectable amount of Cu into Cu+/Cu2+ state. On the other hand, Cu2O films could be grown with relative ease by maintaining atomic oxygen flux of 1.6 times the stoichiometric value. In contrast, the kinetics of CuO formation has been found to be quite slow. For atomic oxygen to copper flux ratio of ∼80, only ∼95% of the copper was found to be in fully oxidized state.

Nanostructured PdO Thin Film from Langmuir–Blodgett Precursor for Room-Temperature H2 Gas Sensing

Nanoparticulate thin films of PdO were prepared using the Langmuir-Blodgett (LB) technique by thermal decomposition of a multilayer film of octadecylamine (ODA)-chloropalladate complex. The stable complex formation of ODA with chloropalladate ions (present in subphase) at the air-water interface was confirmed by the surface pressure-area isotherm and Brewster angle microscopy. The formation of nanocrystalline PdO thin film after thermal decomposition of as-deposited LB film was confirmed by X-ray diffraction and Raman spectroscopy. Nanocrystalline PdO thin films were further characterized by using UV-vis and X-ray photoelectron spectroscopic (XPS) measurements. The XPS study revealed the presence of prominent Pd(2+) with a small quantity (18%) of reduced PdO (Pd(0)) in nanocrystalline PdO thin film. From the absorption spectroscopic measurement, the band gap energy of PdO was estimated to be 2 eV, which was very close to that obtained from specular reflectance measurements. Surface morphology studies of these films using atomic force microscopy and field-emission scanning electron microscopy indicated formation of nanoparticles of size 20-30 nm. These PdO film when employed as a chemiresistive sensor showed H2 sensitivity in the range of 30-4000 ppm at room temperature. In addition, PdO films showed photosensitivity with increase in current upon shining of visible light.

Design and development of an in-line sputtering system and process development of thin film multilayer neutron supermirrors

Neutron supermirrors and supermirror polarizers are thin film multilayer based devices which are used for reflecting and polarizing neutrons in various neutron based experiments. In the present communication, the in-house development of a 9 m long in-line dc sputtering system has been described which is suitable for deposition of neutron supermirrors on large size (1500 mm × 150 mm) substrates and in large numbers. The optimisation process of deposition of Co and Ti thin film, Co/Ti periodic multilayers, and a-periodic supermirrors have also been described. The system has been used to deposit thin film multilayer supermirror polarizers which show high reflectivity up to a reasonably large critical wavevector transfer of ∼0.06 Å(-1) (corresponding to m = 2.5, i.e., 2.5 times critical wavevector transfer of natural Ni). The computer code for designing these supermirrors has also been developed in-house.

Optimal electron irradiation as a tool for functionalization of MoS2: Theoretical and experimental investigation

We demonstrate the utility of electron irradiation as a tool to enhance device functionality of graphene-analogous MoS2. With the help of first-principles based calculations, vacancy-induced changes of various electronic properties are shown to be a combined result of crystal-field modification and spin-orbital coupling. A comparative theoretical study of various possible vacancy configurations both in bulk and monolayer MoS2 and related changes in their respective band-structures help us to explain plausible irradiation induced effects. Experimentally, various structural forms of MoS2 in bulk, few layered flakes, and nanocrystals are observed to exhibit important modification of their magnetic, transport, and vibrational properties, following low doses of electron irradiation. While irradiated single crystals and nanocrystals show an enhanced magnetization, transport properties of few-layered devices show a significant increase in their conductivity, which can be very useful for fabrication of electronic...

Probing molecular packing at engineered interfaces in organic field effect transistor and its correlation with charge carrier mobility.

Surface engineering of SiO2 dielectric using different self-assembled monolayer (SAM) has been carried out, and its effect on the molecular packing and growth behavior of copper phthalocyanine (CuPc) has been studied. A correlation between the growth behavior and performance of organic field effect transistors is examined. Depth profiling using positron annihilation and X-ray reflectivity techniques has been employed to characterize the interface between CuPc and the modified and/or unmodified dielectric. We observe the presence of structural defects or disorder due to disorientation of CuPc molecules on the unmodified dielectric and ordered arrangement on the modified dielectrics, consistent with the high charge carrier mobility in organic field effect transistors in the latter. The study also highlights the sensitivity of these techniques to the packing of CuPc molecules on SiO2 modified using different SAMs. Our study also signifies the sensitivity and utility of these two techniques in the characterization of buried interfaces in organic devices.

Ion beam studies in strained layer superlattices

Abstract The potential device application of semiconductor heterostructures and strained layer superlattices has been highlighted. Metal organic chemical vapour deposition grown In 0.53 Ga 0.47 As/InP lattice-matched structure has been irradiated by 130 MeV Ag 13+ and studied by RBS/Channelling using 3.5 MeV He 2+ ions. Ion irradiation seems to have induced a finite tensile strain in the InGaAs layer, indicating thereby that ion beam mixing occurs at this energy. Other complementary techniques like high resolution XRD and STM are needed to conclude the structural modifications in the sample.

Depth distribution of Mn in Mn doped GaAs using secondary ion mass spectrometry

Secondary ion mass spectrometry (SIMS) was used for depth distribution analysis of Mn in GaAs formed by ion implantation of Mn in GaAs single crystal wafers at an implant energy of 300 keV, followed by laser pulse heating to anneal out the radiation damage. Mn doped GaAs, due to its ferromagnetic behavior, has potential applications in spintronic devices. Knowledge of the quantitative depth distribution, diffusion and segregation of these dopant atoms in the host material are necessary for fabrication of these devices. The depth distribution of Mn was carried out using oxygen and cesium primary ion beams at various impact energies. The obtained depth profiles of Mn demonstrated that Mn was uniformly distributed in as-implanted and single shot laser annealing samples, whereas in case of double shot annealing, segregation effect of Mn atoms toward the decaying trail of the distribution curve was observed, thanks to extremely high detection sensitivity of SIMS. Relative sensitivity factors of Mn in GaAs were...

Identification of a kinetic length scale which dictates alloy phase composition in Ni-Al interfaces on annealing at low temperatures

Ni-aluminides are an important class of intermetallics from technological point of view. Ni-Al phase diagram has been studied in detail experimentally as well as theoretically. It is known that if annealed at low temperature, the first alloy phase is usually NiAl3 according to Benes rule. It is also understood that heat of formation may get modified by local densities of the constituents forming the alloy. In this regard, it is important to identify a kinetic length scale for defining “local density” in a system. We have deposited ultrathin multilayers of Ni and Al of layer thickness in tens of nanometres with Ni:Al stoichiometric ratio as 3:1 and 1:3, respectively. Considering these stoichiometry, Ni3Al and NiAl3 are the thermodynamically favoured alloy phases in these samples. We used x-ray reflectivity, polarized neutron reflectivity, x-ray diffraction, and secondary ion mass spectroscopy to follow the alloy formation after annealing and identified the alloy phases at interfaces with nanometre resolut...

Characterization of Ni/Zr system by x-ray reflectivity measurements

A study of the Ni/Zr system has been undertaken through x-ray reflectivity measurements of Ni-Zr alloy layers and a trilayer sample Ni/Zr/Ni, deposited by a home-built D.C. magnetron sputtering unit. Ni-Zr alloys are known to have high glass-forming ability in a wide range of concentrations and play a significant role in the study of bulk amorphous metallic materials. It was observed that the Ni-Zr alloy formed by co-sputtering was amorphous, and that no interfacial alloys were obtained in the analysis of the trilayer sample.

Micro-structural characterization of low resistive metallic Ni germanide growth on annealing of Ni-Ge multilayer

Nickel-Germanides are an important class of metal semiconductor alloys because of their suitability in microelectronics applications. Here we report successful formation and detailed characterization of NiGe metallic alloy phase at the interfaces of a Ni-Ge multilayer on controlled annealing at relatively low temperature ∼ 250 °C. Using x-ray and polarized neutron reflectometry, we could estimate the width of the interfacial alloys formed with nanometer resolution and found the alloy stoichiometry to be equiatomic NiGe, a desirable low-resistance interconnect. We found significant drop in resistance (∼ 50%) on annealing the Ni-Ge multilayer suggesting metallic nature of alloy phase at the interfaces. Further we estimated the resistivity of the alloy phase to be ∼ 59μΩ cm.