Epitaxial growth and characterizations of cubic ZnxMg1-xO and ZnO on MgO substrates

• Main Authors: Cheng-Ying James Lu, 呂政穎
• Other Authors: Liu-wen Chang
• Format: Others
• Language: en_US
• Published: 2015
• Online Access: http://ndltd.ncl.edu.tw/handle/80833023517032385730

博士 === 國立中山大學 === 材料與光電科學學系研究所 === 103 === This thesis provides an overall experiments and discussions of depositing rock-salt (RS) type ZnO/ZnxMg1-xO epilayers and heterostructures in a plasma assisted molecular beam epitaxy system. The experiments support regardless of processing and characterizing the RS type ZnO/ZnxMg1-xO. A discussion of the processing was given, the growth of RS type ZnO/ZnxMg1-xO films is limited to low beam equivalent pressure ratios of Zn and growth rates. The ZnO/ZnxMg1-xO films are characterized to have specific lattice parameters, band gap energies, and the film quality with different Zn contents. In addition, several models are proposed to discuss the RS type ZnO/ZnxMg1-xO, such as critical thicknesses, strain relaxation, limitation of the Zn content. The total phenomena are then summarized to give a simple growth model. This thesis therefore reports three major results. In the first part, the growth were focused on the ZnO on (100)MgO substrate at 600 oC by plasma-assisted molecular beam epitaxy. Nonpolar (10-10)-oriented ZnO was grown epitaxially which was composed of four variant domains having an orientation relationship with the substrate as: (10-10)ZnO//(100)MgO and &;lt;1-213&;gt;ZnO~//&;lt;011&;gt;MgO with a +/-1.5o deviation. By introducing a RS type Zn0.6Mg0.4O buffer layer, the lattice mismatch was eliminated almost completely based on the extended coincidence lattice model. The crystal quality is therefore improved and the epilayer reveal better photoluminescence characteristics. Following, the ZnxMg1-xO epitaxial layers were prepared with different beam equivalent pressure ratios by plasma-assisted molecular beam epitaxy on MgO (100) substrate. X-ray diffraction characterization revealed that both samples retain the RS structure and the lattice parameters are 0.4256 and 0.4278 nm for x = 0.52 and 0.83, respectively. In addition, the epilayers are with low FWHM values (0.30o ~ 0.47o) of the (200) rocking curves. The epilayer surfaces are flat having a root mean square roughness of 1.0 nm. According to reciprocal space map and transmission electron microscopy analyses, the epitaxial strains have been partly relaxed at film thicknesses of 110 ~ 130 nm. In fact, a further relaxation of the strain when preparing the TEM specimen from the Zn0.8Mg0.2O epitaxial layer triggers back transformation of the RS to the wurtzite structure. The band gap energy of the Zn0.8Mg0.2O epitaxial layer was found to be as low as 4.73 eV. In the last part, RS type ZnO/ZnxMg1-xO/MgO heterostructures are deposited on lattice-matched MgO substrates. The results revealed six heterostructure samples (excluding sample H-ZMO-5) are facing phase transformation from rock-salt to wurtzite phase during the heterostructure or capping layer growth. In addition, the energy gaps that measured are similar to the values from the simulations. It is worth noting that RS type ZnO is obtained by embedded MgO layers to form a periodic heterojunctions in very thin layers. Besides, the wurtzite ZnO/MgO alternative layers are observed in the heterostructures. The wurtzite MgO sandwiched between ZnO layers was to stabilize as very thin layers. The dual phase of ZnO/MgO epilayers occurred in different areas within a sample. The interface of these areas provide a good opportunity to solve the orientation relationship between wurtzite structure and rock-salt structure, which has the same results as that derived in the (10-10)-oriented ZnO/MgO epilayer as: (10-10)wurtzite//(100)rock-salt and &;lt;1-213&;gt;wurtzite ~//&;lt;011&;gt;rock-salt. In addition, the band gap engineering by the present process has narrowed down the energy to 4.5 eV, i.e. extending toward lower UV region.