Synthesis and characterization of wide bandgap cubic ZnO

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Synthesis and characterization of wide bandgap cubic ZnO


Author: Lau, Chun-hei
Title: Synthesis and characterization of wide bandgap cubic ZnO
Degree: M.Phil.
Year: 2008
Subject: Hong Kong Polytechnic University -- Dissertations.
Thin films.
Zinc oxide thin films.
Pulsed laser deposition.
Department: Dept. of Applied Physics
Pages: xii, 141 leaves : ill. ; 30 cm.
Language: English
InnoPac Record:
Abstract: Crystalline magnesium zinc oxide (MZO) thin films of about 300 nm to 500 nm thick were fabricated on MgO (0 0 1) and LAO (0 0 1) substrates by pulsed laser deposition method. The MZO ablation target has a nominal composition of Mg0.5Zn0.5O. It has been revealed that the Zn content in the deposited films is substantially less than that of the target and it varies according to the growth conditions. X-ray diffraction and transmission electron microscopic studies show that a single cubic phase MZO (c-MZO) is obtained in films grown on these two types of substrates at deposition temperatures between 300oC and 600oC. The x-ray 360o phi-scan results suggest excellent epitaxial relation between the films and the substrates. The phenomenon of 45o in-plane twisted growth of c-MZO film on LAO is observed. This is attributed to the result of maximized lattice matching. Films grown on MgO substrates, however, show the typical cube-on-cube epitaxy. Indium doped c-MZO films exhibit n-type semiconduction. The electrical conduction in these films is attributed to the free electrons generation via the indium (In) dopant, and therefore is highly sensitive to the indium content. Epitaxial c-MZO films with resistivity of 5.5x10-2 cm were obtained when doped with more than 20 at. % of indium. All indium doped c-MZO (c-MZO :In) films have an optical transmittance of over 80 % for the whole visible spectrum. It has been revealed that the c-MZO band gap energy is roughly equal to 4eV and can be tuned by simply changing the indium concentration. Therefore these epitaxial films can be used as variable UV filters and transparent conducting oxide (TCO) for various optoelectronics applications. In the present study we have also fabricated all-oxides p-n junctions. They are formed by depositing n-type indium doped c-MZO and p-type LiNiO (LNO) films in succession on the lattice matched substrates. Both the c-MZO:In and LNO have similar cubic structures and can be grown epitaxially on MgO or LAO substrates. I-V rectifying characteristics at room temperature were evaluated. Excellent rectifying properties have been demonstrated. Temperature dependence studies suggested non-ideal p-n junction characteristics. Both the c-MZO and LNO are optically transparent, and we have therefore produced a transparent all-oxide cubic p-n junction, which is believed to be very useful for future oxide electronics and spintronics. For device applications, the potential of integrating these p-n junctions onto Si substrates are of prime importance. In the present work we have grown indium doped c-MZO on silicon with TiN buffer layer. The TiN layer, apart from assisting the epitaxial growth of the oxide heterostructures, also serves as a high conduction electrical lead for the p-n junction. Excellent current rectifying characteristics have been demonstrated. Our results thus provide strong experimental support for developing a workable all-oxide integrated p-n junction.

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