We have studied the properties of ZnO thin films grown by pulsed laser deposition (PLD) using a ZnO target under various background oxygen conditions. Substrate material included pre-nitridated (0001) sapphire (Al2O3) and quartz glass. The substrate temperature was varied between RT and 600 °C. The targets were ablated with either the IR line at 1064 nm or the 4th harmonic (266 nm) of a Nd:YAG laser. Various methods are employed to obtain the optical, structural and electrical film quality, like optical transmission spectroscopy, RBS, SIMS and XPS for composition, SEM and AFM for surface analysis, photoluminescence, UV-excited photoconductivity [1].
Depending on deposition temperature and background oxygen process gas pressure the sample resistivity varied over about 9 orders of magnitude. The dark conductivity temperature dependence indicated strong non-intentional doping. The RBS analysis of the samples was done at ITN with a 1.5 MeV He+ beam, at normal incidence with standard and annular detectors (scattering angle 140° and 180°, respectively). Both film thickness and film composition was determined. The RBS results indicate a slight predominance of oxygen in ZnO films. The composition of the films approaches stoichiometry with increasing film thickness. Photocurrent measurements at temperatures between 80 K and 300 K with excitation by the 325 nm line of a HeCd laser showed a strong persistent photoconductivity effect (PPC) indicating the presence of deep defects.
The results obtained so far show that the relatively simple PLD technique is capable of producing ZnO films with good structural and optical quality. The control of substrate temperature and background oxygen process gas pressure leads to a large variety of film properties. Thus PLD method proves to be a versatile technique for deposition of ZnO films prepared for specific applications like conducting transparent oxide layers, active layers for thin film transistor or sensor applications.
Acknowledgement:
The work at IST is supported by Fundaço para a Ciência e a Tecnologia (FCT).
[1] L. Bentes, R. Ayouchi, C. Santos, R. Schwarz, P. Sanguino, O. Conde, M. Peres, T. Monteiro, and O. Teodoro. ZnO Films Grown by Laser Ablation with and without Oxygen CVD. Submitted to E-MRS Spring Meeting 2006.
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