R.B. Tokas

Achieving superior band gap, refractive index and morphology in composite oxide thin film systems violating the Moss rule

The interrelation between energy gap and high frequency refractive index in semiconductors and dielectrics is manifested by an inverse law which is popularly known as the Moss rule. This semi-empirical relationship is based on the fundamental principle that in a dielectric medium all energy levels are scaled down by a factor of the square of the dielectric constant. Such a rule is obeyed by most pure semiconductors and dielectrics with a few rare violations in composite materials which display several interesting parametric and microstructural evolutions. The present results are based on some specific oxide composite thin films involving Gd2O3/SiO2 and ZrO2/SiO2 codeposited systems that have displayed a superior refractive index and energy gaps violating the semi-empirical Moss rule. Also, morphological supremacy is also distinctly noticed in these composites. The novel microstructural and polarizability properties of such composite systems were probed through multi-mode atomic force microscopy and phase modulated spectroscopic ellipsometry using refractive index modelling, autocorrelation and height–height correlation functional analyses. These binary composite thin films have shown their potential as well as the possibility of meeting expectations in satisfying the challenging optical coating requirements of the deep ultraviolet spectral region.

Surface roughness and interface width scaling of magnetron sputter deposited Ni/Ti multilayers

Using an indigenously built r.f. magnetron sputtering system, several single layer Ti and Ni films have been deposited at varying deposition conditions. All the samples have been characterized by Grazing Incidence X-ray Reflectivity (GIXR) and Atomic Force Microscopy to estimate their thickness, density, and roughness and a power law dependence of the surface roughness on the film thickness has been established. Subsequently, at optimized deposition condition of Ti and Ni, four Ni/Ti multilayers of 11-layer, 21-layer, 31-layer, and 51-layer having different bilayer thickness have been deposited. The multilayer samples have been characterized by GIXR and neutron reflectivity measurements and the experimental data have been fitted assuming an appropriate sample structure. A power law correlation between the interface width and bilayer thickness has been observed for the multilayer samples, which was explained in the light of alternate roughening/smoothening of multilayers and assuming that at the interface ...

Deposition and characterization of titania-silica optical multilayers by asymmetric bipolar pulsed dc sputtering of oxide targets

Titania?silica (TiO2/SiO2) optical multilayer structures have been conventionally deposited by reactive sputtering of metallic targets. In order to overcome the problems of arcing, target poisoning and low deposition rates encountered there, the application of oxide targets was investigated in this work with asymmetric bipolar pulsed dc magnetron sputtering. In order to evaluate the usefulness of this deposition methodology, an electric field optimized Fabry Perot mirror for He?Cd laser (? = 441.6?nm) spectroscopy was deposited and characterized. For comparison, this mirror was also deposited by the reactive electron beam (EB) evaporation technique. The mirrors developed by the two complementary techniques were investigated for their microstructural and optical reflection properties invoking atomic force microscopy, ellipsometry, grazing incidence reflectometry and spectrophotometry. From these measurements the layer geometry, optical constants, mass density, topography, surface and interface roughness and disorder parameters were evaluated. The microstructural properties and spectral functional characteristics of the pulsed dc sputtered multilayer mirror were found to be distinctively superior to the EB deposited mirror. The knowledge gathered during this study has been utilized to develop a 21-layer high-pass edge filter for radio photoluminescence dosimetry.

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.

Oblique angle deposition of HfO2 thin films: quantitative assessment of indentation modulus and micro structural properties

Oblique angle deposition of oxides is used routinely for fabricating inhomogeneous thin films with proven correlation between refractive index and the angle of deposition. Inhomogeneous layers play a key role in the development of rugate interference devices for photo-physical applications. Such obliquely deposited thin films show high porosity which is a critical issue related to their density, mechanical and environmental stability. Hence, it is important to investigate elastic properties of such film in addition to optical properties. Using atomic force acoustic microscopy, we report indentation modulus of HfO2 thin films deposited at angles 80, 68, 57, 40 and 0 degree with normal to substrate plane on Si (100) substrate. Such films were measured to have indentation modulus of 42 GPa for extreme obliquely deposited film and indentation modulus increases with decrease in angle to become highest with a value of 221 GPa for normally deposited films. We also report microstructural properties and density of films measured by FESEM and grazing angle x-ray reflectometer, respectively. Both indentation modulus and density depict a nonlinearly decreasing behavior with angle of deposition. Variation of density is again confirmed from FESEM cross-sectional morphology.

Optical properties of electron beam evaporated ZrO2:10 %SiO2 thin films: dependence on structure

ZrO2:10%SiO2 thinfilms have been deposited on fused silica substrate by reactive electron beam co-evaporation technique at different oxygen partial pressure. The structural analysis shows tetragonal phase with residual tensile stress in the films. The intensity of the tetragonal t(110) phase are found increasing with increasing oxygen pressure. The optical band gap is found increasing from 5.06 eV to 5.28 eV because of increasing crystalinity of monoclinic phase, while the film grain size remains almost constant with increase of oxygen pressure, concludes that the crystallite or grain size has no effect on the optical properties of the films. The dispersion of the refractive index is discussed in terms of single oscillator Wimple-DiDomenico model. The dispersion energy parameter better known as structural order parameter are found increasing with the intensity of t(110) phase. It is observed that films having higher value of order parameter show lower surface roughness which concludes that the local microstructure ordering can predominantly influence the grain morphology which in turn can lead to better surface for higher value of order parameter.

Investigations on Substrate Temperature-Induced Growth Modes of Organic Semiconductors at Dielectric/semiconductor Interface and Their Correlation with Threshold Voltage Stability in Organic Field-Effect Transistors

Influence of substrate temperature on growth modes of copper phthalocyanine (CuPc) thin films at the dielectric/semiconductor interface in organic field effect transistors (OFETs) is investigated. Atomic force microscopy (AFM) imaging at the interface reveals a change from layer+island to island growth mode with increasing substrate temperatures, further confirmed by probing the buried interfaces using X-ray reflectivity (XRR) and positron annihilation spectroscopic (PAS) techniques. PAS depth profiling provides insight into the details of molecular ordering while positron lifetime measurements reveal the difference in packing modes of CuPc molecules at the interface. XRR measurements show systematic increase in interface width and electron density correlating well with the change from layer + island to coalesced huge 3D islands at higher substrate temperatures. Study demonstrates the usefulness of XRR and PAS techniques to study growth modes at buried interfaces and reveals the influence of growth modes of semiconductor at the interface on hole and electron trap concentrations individually, thereby affecting hysteresis and threshold voltage stability. Minimum hole trapping is correlated to near layer by layer formation close to the interface at 100 °C and maximum to the island formation with large voids between the grains at 225 °C.

Achieving omnidirectional photonic band gap in sputter deposited TiO2/SiO2 one dimensional photonic crystal

The multilayer structure of TiO2/SiO2 (11 layers) as one dimensional photonic crystal (1D PC) has been designed and then fabricated by using asymmetric bipolar pulse DC magnetron sputtering technique for omnidirectional photonic band gap. The experimentally measured photonic band gap (PBG) in the visible region is well matched with the theoretically calculated band structure (ω vs. k) diagram. The experimentally measured omnidirectional reflection band of 44u2005nm over the incident angle range of 0°-70° is found almost matching within the theoretically calculated band.

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.

Spectroscopic ellipsometry investigations of optical anisotropy in obliquely deposited hafnia thin films

In present work, HfO2 thin films have been deposited at various oblique incidences on Si substrates by electron beam evaporation. These refractory oxide films exhibited anisotropy in refractive index predictably due to special columnar microstructure. Spectroscopic ellipsometry being a powerful tool for optical characterization has been employed to investigate optical anisotropy. It was observed that the film deposited at glancing angle (80°) exhibits the highest optical anisotropy. Further, anisotropy was noticed to decrease with lower values of deposition angles while effective refractive index depicts opposite trend. Variation in refractive index and anisotropy has been explained in light of atomic shadowing during growth of thin films at oblique angles.