Thin films of Mn-Doped zinc oxide diluted magnetic semiconductor materials for spintronics

Abstract

This dissertation reports the synthesis, characterization and processing of Mn-doped ZnO thin films for their potential use in the next generation spintronics devices. The idea of spintronics prompted the evolution of a new class of materials known as diluted magnetic semiconductors (DMS), where a fraction of atoms in the host semiconductor is substituted by ferromagnetic atoms of rare earth or transition metals to provide magnetic functionality. The II-VI based (ZnO) systems doped with Mn are the most well studied systems in this area. ZnO is considered to have higher solubility of Mn with the possibility of exhibiting room temperature ferromagnetism and therefore it is considered as an ideal candidate for spintronics device applications. However, one of the main obstacles in creating high quality ZnO-based spintronics devices is the presence of native defects which alter the exchange interactions of Mn in ZnO and yield inconsistent results related to room temperature ferromagnetism. Therefore, it is of prime scientific interest to reduce the concentration and contribution of structural defects towards ferromagnetic response. Un-doped and Mn-doped ZnO, prepared through wet chemical method, are studied in this thesis by a variety of synthesis and growth parameters like deposition techniques, doping concentration, ambient gas pressures, and in-situ and post-deposition annealing temperature for their possible effects in tailoring the materials composition and concentration of structural defects for good quality ferromagnetic thin films. <br><br>The nanocrystalline ZnO powders with 2 and 5 at.% Mn doping concentration were ferromagnetic at room temperature while the 10 at.% Mn-doped sample exhibited paramagnetic behavior. The absence of deep level emissions from structural defects for powder with 5 at.% Mn content indicated that ferromagnetism for this sample is not due to structural defects (oxygen vacancies and zinc interstitials) rather induced intrinsically by the incorporation of Mn²⁺ ions. The temperature-dependent tailoring of structural defects at different post-deposition annealing temperatures (500-800 °C) was carried out in un-doped and Mn-doped ZnO thin films. A consistent decrease in Auger Zn L₃M_4,5M_4,5 peak intensity (zinc interstitials) along with the temperature-dependent increase in the concentration of oxygen interstitials in turn favors the p-type conductivity in thin films of un-doped ZnO. Reduced deep level emission spectra in doped thin films with declined photoluminescence (PL) ratio validated the quenching of the intrinsic structural defects thereby improving the optical transparency in doped samples. Thin films deposited using spin coating technique revealed the absence of any impurity phases or nanoclustering of oxides of Mn in any of the samples. The presence of oxygen interstitials in deep level emission PL spectra revealed the origin of ferromagnetic ordering from Mn-defect pair exchange coupling.<br><br>Finally, the temperature dependent variation in structural defects in ZnO:Mn thin films was investigated under different Ar:O2 admixture gas pressures and in-situ annealing temperatures with the primary focus on understanding the relationship between the defect concentration, material composition and ferromagnetic properties. Structural analyses convincingly excluded the existence of any impurity phase and revealed that there was an optimum substrate temperature of 450 °C under controlled oxygen processing conditions at which the thin film showed stronger texture, better optical quality and improved stoichiometry for better ferromagnetic response, a pre-requisite for spintronics applications. <br><br>To conclude, this thesis presents a systematic study on the synthesis and processing of doped and un-doped ZnO thin films to tailor the materials composition and concentration of structural defects to obtain high quality ferromagnetic thin films.