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Title: Fabrication and characterization of insulating-film/phosphor-film/insulating-film heterostructures
Authors: Yeung, Kai-ming
Degree: M.Phil.
Issue Date: 2002
Abstract: Insulating-film/phosphor-film/insulating-film heterostructures are currently being investigated as potential structures for future ac thin film electroluminescent device (TFELD) applications. The insulating layer and the phosphor layer are of different kinds of materials possessing very different structural, electrical and optical properties. To optimize the performance of these heterostructures, a good understanding of the optical and electrical properties of each individual layer, namely, the insulating layer and the phosphor layer, is of paramount importance. The main objective of our study is to identify the important processing parameters in the fabrication of each individual layer used in these heterostructures. The effects of these processing parameters on the structural, electrical and optical properties of the individual layers were investigated. The correlation between the structural, optical and electrical properties was investigated. In this study, ZnS:Mn was used as the phosphor layer while for the insulating materials, two types of ceramic materials, BaTiO3 and PbZrxTi1-xO3 (PZT) were employed. Optimized processing parameters for growing these films by pulsed laser deposition (PLD) have been revealed. The crystallographic structures and impurities of the prepared films were studied by X-ray diffractometry (XRD) and Raman spectroscopy. The surface morphology, surface roughness, grain size and porosity were examined by scanning electron microscopy (SEM) and atomic force microscopy (AFM). From the growth of BaTiO3 films on LaAlO3 (LAO) substrates by PLD, structural characteristics were found to depend on film thickness. In general, (100)-oriented BaTiO3 films with different thickness were epitaxially grown on LAO substrates. All the films fabricated were smooth and crack free. The texture of the films as examined by XRD rocking curve varies with film thickness. The grain size and surface roughness were found to increase with thickness of the films. Raman scattering confirmed that these films possess tetragonal phase with good stoichiometry and free of impurities. For those BaTiO3 and PZT films grown on epitaxial LNSMO buffered LAO, they were epitaxially grown with (100) orientation. On the other hand, ZnS:Mn films grown on either Si or ITO coated glass substrate were polycrystalline in structure. These films were found to have different combinations of phase when fabricated by PLD under different deposition temperatures. Zinc blende structure dominated when fabricated under lower deposition temperatures of around 150C, and with higher deposition temperatures of around 450C a higher wurtzite phase ratio was obtained. For electrical properties, the dielectric constant and P-E loop characteristics of the BaTiO3 and PZT films, deposited on the LNSMO electrode buffered LAO, were measured. Only weak ferroelectric behaviour for BaTiO3 films as observed and the dielectric constant at 1kHz was around 1200. PZT films were confirmed to have ferroelectricity as shown in P-E measurements and the dielectric constant at 1 kHz was around 680. For optical properties, the optical constants of the ZnS:Mn layers were measured by spectroscopic ellipsometry (SE). Variations in the band gap energy of ZnS and the 4T1(G) - 6A1(S) transition energy of the doped Mn2+ ions were investigated by optical transmittance and photoluminescence (PL) measurements, respectively. The changes in the optical properties of the ZnS:Mn layer with different processing parameters were correlated to its structural characteristics. Lastly, heterostructures of phosphor layer and insulating layers were fabricated. Their performance in relation to different insulating materials was characterized by measuring their electroluminesent properties. However, this study indicated that the use of BaTiO3 and PZT insulating materials has no significant influence on the electroluminescence behaviour of the ZnS:Mn layer.
Subjects: Hong Kong Polytechnic University -- Dissertations
Thin layer chromatography
Thin film devices
Heterostructures -- Optical properties
Pages: v, 116 leaves : ill. ; 30 cm
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