Synthesis and Characterization of Manganese Based Thin Films Using Pulsed Laser Deposition Technique

Abstract

Thin films of un-doped manganese oxide and cobalt doped manganese oxide targets have been deposited on silicon surface under different conditions of deposition temperature and oxygen pressure using Pulsed Laser Deposition (PLD) technique. Films were characterized using XRD and FTIR for structural parameters, FESEM, AFM and optical microscope for surface roughness and morphology studies, Stylus Profiler for thickness measurement, EDX and XPS for elemental composition and oxidation state determinations. It has been found that at low oxygen pressure, a single crystalline phase of manganese oxide (Mn3O4) has been obtained, found to be stable at higher deposition temperatures but new planes appeared with variation in temperatures. These variations in temperature also affect thickness of the films directly. The bandgap was the largest (2.51 eV) and thickness was least approximately 323 nm obtained at 600 nm. When oxygen pressure increased, films became polycrystalline and each film has multiple manganese oxide phases. At lower deposition temperature, oxygen rich phases (MnO2 and Mn2O3) were dominant and at higher deposition temperature Mn3O4 was dominant but MnO2 was also found at 800 nm unexpectedly. Films were found to be smooth and thin with increase in deposition temperature. The largest bandgap (3.03 eV) and least thickness of the films (nearly 100 nm) was seen at 800 nm. On further increase in oxygen pressure, films were found to be polycrystalline at lower deposition temperatures having Mn2O3 and Mn3O4 phases which transform into single phase (Mn3O4) at 800 nm. Crystallite size and texture of the planes including other structural parameters also vary due to variation in deposition conditions but no new phases were found at these conditions. Elemental ratio also varies under different conditions. The largest band gap in this case was found to be 2.28 eV at 800 nm. Thickness of the films shows dependence on oxygen pressure and at low oxygen pressure it was higher than the films at higher oxygen pressure. Bandgap was also affected by oxygen pressure. Higher deposition temperature and oxygen pressure is suitable to obtained higher bandgap. Results from cobalt doped manganese oxide thin films showed that no phase of cobalt oxide was found (using XRD technique) but the crystallinity of films was poor as compared to undoped manganese oxide thin films, with multiple phases of manganese oxide having increased FHWM values. Surface morphology of these thin films was also different from undoped manganese oxide thin films deposited under the same conditions of substrate temperature and oxygen pressure. Thickness of the films deposited at room temperature was higher than the films deposited at higher temperatures. But at higher deposition temperature variation in film thickness was quite random. EDX analysis confirmed the presence of cobalt oxide in thin films but XPS analysis only showed presence of cobalt oxide fractionally only in few samples. This shows that doping of cobalt oxide in manganese oxide can affect structure, surface and optical properties but cannot be used to grow crystals and was neither uniformly distributed.