Sarath Witanachchi

Evidence that blue luminescence of oxidized porous silicon originates from SiO2

We have analyzed red and blue luminescence from porous silicon as a function of oxidation parameters and feature dimension determined with an atomic force microscope. We have found correlation between blue luminescence intensity and the increase in feature size caused by oxidation. We have further shown that blue luminescence, is identical, with respect to spectrum and fast decay, to that of high microelectronic quality SiO2 grown on crystalline silicon using dry oxygen plus an organic chlorine compound. Thus, we conclude that blue luminescence originates from SiO2 film rather than from the silicon nanocrystals in the porous material. Intensity enhancement, as compared to SiO2 on crystalline wafers, comes from the gigantic surface area of porous silicon.

Role of the oxygen atomic beam in low-temperature growth of superconducting films by laser deposition

An oxygen jet placed near the target during plasma‐assisted laser deposition produces a strong atomic oxygen beam with kinetic energies of 5.6 eV, simultaneous with the laser‐induced atomic beams of Ba, Cu, and Y from the target. All atomic beams can be well characterized by a supersonic expansion mechanism. The behavior of the velocity distributions was studied as a function of the distance from the target and laser energy fluence. A target‐substrate separation of 7 cm was found to be optimum in terms of producing the best as‐deposited films. At that distance, the velocity distributions of all atomic beams become nearly the same.

Laser evaporation deposition of superconducting and dielectric thin films

We describe here the deposition of superconducting and dielectric thin films by the laser evaporation technique. The characterization of this process, and possible optimization with regards to wavelength and pulse duration of the laser will be discussed.

Growth of highly oriented CdS thin films by laser-evaporation deposition

CdS films have been grown by laser‐evaporation deposition in a clean vacuum environment. The films are highly oriented with a c axis perpendicular to the surface, and are optically smooth and homogeneous. These high quality films should be useful in nonlinear integrated optics applications.

Dual‐laser ablation for particulate‐free film growth

A novel dual laser ablation process that leads to particulate‐free film growth is presented. A pulsed CO2 laser and an excimer (KrF) laser have been spatially overlapped on a Y2O3 target with a temporal delay between the pulses. The particulate density of the films grown by this method are at least three orders of magnitude smaller than the particulate density of a single excimer laser ablated film of similar thickness. In addition, a time‐of‐flight ion probe study indicates a sixfold enhancement of the plume species kinetic energies under dual‐laser ablation. The degree of the plume excitation is observed to depend strongly on the delay between the laser pulses.

Magnetic anisotropy and field switching in cobalt ferrite thin films deposited by pulsed laser ablation

We report the observation of contrasting magnetic behavior in cobalt ferrite (CFO) thin films deposited on single crystalline magnesium oxide (MgO) and strontium titanate (STO). Epitaxial films on MgO (100) with a lattice mismatch of 0.35% showed out-of-plane anisotropy whereas the films on STO (100) with a lattice mismatch of 7.4% displayed in-plane anisotropy. Stress anisotropy calculated from angle-dependent x-ray diffraction analysis confirmed that the change in anisotropy originates from the lattice mismatch. An additional low-field switching characteristic is observed in the M-H loops of the CFO films, which became prominent with lowering temperature as also evidenced from the rf transverse susceptibility measurements. The obtained results revealed that the low field switching is associated with the film-substrate interface.

Growth of CdSxSe1-x thin films by laser evaporation deposition

Optical quality thin films of CdSxSe1−x were deposited on quartz for various values of x. It was found that these films were polycrystalline in structure, with a high degree of orientation of the c axis. An empirical relationship between the band gap and the lattice constant for the binary system CdS‐CdSe was also obtained.

Effect of initial plasma geometry and temperature on dynamic plume expansion in dual-laser ablation

Recent experiments have revealed the capability of large-area, uniform film growth using dual-laser ablation. The mechanism of this dynamic plume expansion is investigated in this letter. We report the critical role played by the initial geometry and temperature of the plasma in the subsequent expansion under dual-laser ablation. Initial plasma dimensions in the dual-laser ablation of ZnO are quantified by gated intensified charge-coupled detector-array imaging and combined with a hydrodynamic theoretical expansion model to yield radial thickness profiles for the deposited films. Comparisons with ellipsometric film thickness profiles indicate that the primary factors responsible for increased expansion of the dual-laser ablated plume are an extension of the initial plasma dimension in the axial direction as well as enhanced plasma temperature.

Nanocolumnar interfaces and enhanced magnetic coercivity in preferentially oriented cobalt ferrite thin films grown using oblique-angle pulsed laser deposition.

Highly textured cobalt ferrite (CFO) thin films were grown on Si (100) substrates using oblique-angle pulsed laser deposition (α-PLD). X-ray diffraction and in-depth strain analysis showed that the obliquely deposited CFO films had both enhanced orientation in the (111) crystal direction as well as tunable compressive strains as a function of the film thicknesses, in contrast to the almost strain-free polycrystalline CFO films grown using normal-incidence PLD under the same conditions. Using in situ optical plume diagnostics the growth parameters in the α-PLD process were optimized to achieve smoother film surfaces with roughness values as low as 1-2 nm as compared to the typical values of 10-12 nm in the normal-incidence PLD grown films. Cross-sectional high resolution transmission electron microscope images revealed nanocolumnar growth of single-crystals of CFO along the (111) crystallographic plane at the film-substrate interface. Magnetic measurements showed larger coercive fields (∼10 times) with similar saturation magnetization in the α-PLD-grown CFO thin films as compared to those deposited using normal-incidence PLD. Such significantly enhanced magnetic coercivity observed in CFO thin films make them ideally suited for magnetic data storage applications. A growth mechanism based on the atomic shadowing effect and strain compression-relaxation mechanism was proposed for the obliquely grown CFO thin films.

Role of epitaxy in controlling the magnetic and magnetostrictive properties of cobalt ferrite-PZT bilayers

Epitaxial thin films of cobalt ferrite (CFO) single layer and CFO–lead zirconium titanate (PZT) bilayers were deposited on single crystal MgO (1 0 0) and SrTiO3 (STO) (1 0 0) substrates by pulsed laser deposition. The structural properties were characterized using x-ray diffraction and atomic force microscopy. The magnetic properties of the as-grown thin films were measured at 10 and 300 K in both parallel and perpendicular magnetic fields. The CFO–PZT bilayer films showed enhanced or reduced values of magnetization as compared with those of the CFO single layer films depending on the substrate of deposition. A strain compression–relaxation mechanism was proposed in order to explain the structure–property relationships in the CFO–PZT bilayer thin films.