Pulsed Laser Deposition of ZnO Thin Films for Electronic and Optical Device Applications

  • Author / Creator
    Chowdhury, Fatema R
  • Zinc Oxide (ZnO) is a promising wide band gap semiconductor with exceptional electrical and optical properties. Thin film ZnO can be used for a wide variety of electronic and optoelectronic device applications. The first step on the way to achieving good quality devices is to grow optimum quality thin films on desired substrates. Pulsed laser deposition (PLD) is a popular technique to deposit complex structures and oxide semiconductors. The growth parameters of PLD have significant effects on thin film properties and qualities. The first part of this work presents the optimization of Zinc oxide (ZnO) film properties for thin film transistor (TFT) application. Thin films of ZnO were deposited by PLD under a variety of growth conditions. The oxygen pressure, laser fluence, substrate temperature, and annealing conditions were varied to optimize the growth conditions for a higher mobility and lower defect density thin films according to the device demand. Room temperature ZnO growths followed by air and oxygen annealing showed improvement in the (002) phase formation with a carrier concentration in the order of 1017-1018 cm-3 along with low mobility in the range of 0.01 - 0.1 cm2V-1s-1, while relatively low temperature growth (250 °C) of ZnO achieved a Hall mobility of 8 cm2V-1s-1 and a carrier concentration of 5 x 1014 cm-3. The low carrier concentration indicates that the number of defects have been reduced nearly by three orders of magnitude as compared to the room temperature annealed growths. Also, it was observed clearly that higher mobility had a strong correlation with the (002) crystal orientation. Higher mobility (18 cm2V-1s-1) was achieved with a 250 °C growth followed by in-situ oxygen annealing for an hour, however, the defect concentrations also went up to the order of 1019 cm-3. Higher temperature studies on different substrates like sapphire and SiO2/Si were also done with other optimized parameters. Temperatures higher than 250 °C increased other phase formations like (103). At 700 °C films were highly (103) oriented with hexagonal ZnO grain structures. The optical properties of ZnO have been studied for more than 60 years now but there are still unsolved puzzles about the origins and decay natures of different defect emissions which contribute to the wide visible band of the PL spectrum of ZnO. This visible emission and near band edge UV emission are also an indicator about the films radiative efficiency and crystal quality. In the second part of this project time integrated and time resolved photo luminescence measurements were done on a single crystal ZnO wafer and PLD thin film ZnO samples to understand the radiative nature of the thin films with respect to excitation parameters. The time resolved measurements were instrumentation limited for the thin film samples but for the single crystal the fast and slow components were 140±10 ps and 340±20 ps which is in agreement with literature. The fast decay time for thin film samples may be due to more non radiative traps present near the surface. The radiative efficiency in the visible range decreased with increased excitation density for continuous wave, ns and fs excitation. On the other hand, the UV emission efficiency increased with higher excitation density. For all the samples it was observed that excitation density close to their electrically measured defect density saturates the visible defect emission and enhances the near band edge UV emission. This may be due to the change in exciton formation rate and capture rate by the defect sites which are saturated with increased excitation density.

  • Subjects / Keywords
  • Graduation date
    Spring 2017
  • Type of Item
  • Degree
    Master of Science
  • DOI
  • License
    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.