Pulsed Laser Deposition and Characterization of AlN, TiO2, and Zn1-xMgxO (0≤x≤0.7) Semiconductor Thin Films
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The focus of work presented in this thesis was to understand and optimize the pulsed laser deposition (PLD) process to fabricate thin films of three important wide band gap semiconductors, AlN, TiO2 and Zn1-xMgxO (0DxD0.7). These three materials were chosen due to their potential for opto-electronic applications in the UV region. AlN thin films were deposited onto Si (100) at room temperature in N2 ambient using PLD technique. The use of pure Al metal as target in the present work, instead of AlN ceramic pellets has made the PLD system simplified. Highly (002) oriented wurtizite AlN thin films were grown in low N2 pressure regime (0.05 mbar to 0.10 mbar) while (101) oriented wurtzite AlN thin films were obtained in high pressure regime (10 mbar to 100 mbar). The emission spectrum of laser ablated Al plasma in N2 ambient was studied to understand the deposition process. TiO2 thin films were deposited using PLD in Ar and O2 ambient. As-deposited thin films of TiO2 were having anatase phase which were converted to rutile phase with annealing. TiO2 films deposited in O2 ambient converted into rutile nanorods upon annealing. TiO2 films deposited in Ar ambient preserved the anatase phase upon annealing and showed nanocone structures. The Ti:O ratio was found to be more stochiometric for films deposited in O2 ambient as compared to Ar ambient. ZnO thin films were deposited using PLD technique on to Si (100) substrates in O2 ambient. From the spectroscopic measurements of laser induced ZnO plasma, O2 pressure and target substrate distance was optimized for the deposition of c-axis oriented stochiometric ZnO thin films.Bandgap of ZnO was tuned by alloying with MgO. Zn1-xMgxO composite thin films were deposited via PLD onto (0001) sapphire substrates in O2 ambient. Wurtzite structure was obtained for Zn1-xMgxO alloy films up to 34 % Mg concentration and cubic phase for 60 % and above of Mg concentration. Excitonic emission was observed from the c-axis oriented Zn1-xMgxO (0.0DxD0.34) thin films. Effect of Mg concentration and substrate temperature on the exciton band gap, exciton binding energy, and broadening parameter was studied from the absorption spectra. The annealing of the sapphire substrate prior to the deposition enhanced columnar growth and crystallinity of the Zn1-xMgxO films. The second harmonic generation, MPA induced UV emission and Non-linear absorption co-efficient, nonlinear refractive index and third order nonlinear susceptibility in Zn1-xMgxO thin films were studied. The NLO properties of these films were found to be decreasing with increase in Mg content. Vertically oriented ZnO nanorods were hydrothermally grown onto pulsed laser deposited ZnO seed layer. The effect of pre-heating time, seed layer and growth time on the structural and optical properties of these nanorods was discussed. Nonlinear absorption coefficient and nonlinear refractive index were estimated using z-scan technique and found to be enhanced significantly compared to that of pulsed laser deposited ZnO seed layer. Lasing action and harmonic generation in ZnO NRs having high aspect ratio was realized via multiphoton absorption process.
Supervisor: Alika Khare