Crystallization Kinetics, Structural and Optical Properties of In-Se-Bi Bulk and Thin Films for Reversible Phase Change Memory Applications
Wanjiku, Muchira Irene
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A detailed study of crystallization kinetics and structural properties of In-Se-Bi alloys has been carried out in this research. Kissinger’s analysis method was employed to investigate crystallization kinetics of the material sample. Electrical, morphological and optical characteristics have also been studied. Bulk samples of In2Se3 doped with Bi were synthesized using melt quenching technique. The elemental composition of the bulk samples was realized using Field Emissions Scanning Electron Microscope (FE-SEM) attached with Energy Dispersive Spectroscopy (EDS). To investigate thermal properties of the samples, Differential Scanning Calorimetry (DSC) technique was employed. DSC runs were done at 5, 10, 15 and 20K/min heating rates in dry nitrogen ambient at 200 ml per minute flow rate in isothermal conditions. X-ray diffractometer (XRD) technique was used to determine the structural properties of the samples. Copper target was used as the X-ray source of wavelength 1.54 (CuKα1). The 2θ spectrum scan ranged from 5 to 90o at a speed of 2oper minute. The XRD results showed the as-deposited samples were amorphous which upon annealing changed to polycrystalline. To deposit In-Se-Bi thin films, Pulsed laser deposition (PLD) technique was used. Electrical characteristics tests were carried by four point probe using Keithly 237 source meter interfaced with a computer. Electrical resistivity was found to decrease from 85.19 MΩcm to 22.96 MΩcm with increase in percentage bismuth. Elemental mapping carried out on the thin film samples using FE-SEM equipped with EDS revealed irregular distribution of spherical particles. Bulk samples showed agglomeration of particles without a definite shape. To determine optical band-gap, Kubelka-Munk function was applied on Ultraviolet Visible Spectroscopy (UV-VIS) reflectance spectrum. The optical band gap values were higher for the doped samples as compared with the un-doped. The values ranged from 1.36 eV to 1.30 eV for as-synthesized powder samples. There was decrease in optical band gap with increase in bismuth concentration. Similar findings were made for annealed powder samples where band gap values decreased from 1.85 eV to 1.79 eV as percentage bismuth increased. Photoluminescence (PL) properties were studied by use of He-Cd laser of wavelength 325 nm. Emission increased with increase in bismuth concentration for both as prepared and annealed powder samples. X-ray photoelectron spectroscopy (XPS) output showed that bismuth was present in all doped samples. Activation energy of the samples was determined using Kissinger equation. It was found to decrease from 0.231 eV to 0.172 eV with increase in percentage bismuth for values with 0% Bi to 6% Bi concentration. However, activation energy increased with bismuth concentration for higher bismuth concentration of 8% Bi and 10% Bi. XPS studies confirmed elemental composition of the samples where for indium 3d5/2 peak was at 445.0 eV and 3d3/2 peak was at 452.5 eV binding energies. The two peaks are due to spin-orbit splitting of the d orbitals. XPS spectrum for selenium showed a peak for 3d orbitals at 54.3 eV binding energy. Bismuth exhibited two asymmetrical peaks at 4f5/2 and 4f 7/2 respectively. In conclusion, addition of Bismuth to the binary alloy of indium and selenium enhanced its qualities for use in phase change memory applications in terms of low power consumption and less switching time due to decrease in resistivity and activation energy.