Pulsed Laser Deposition of Stable Cubic ZnO/MgO Multilayers
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M5.10.1
Pulsed Laser Deposition of Stable Cubic ZnO/MgO Multilayers P. Bhattacharya, Rasmi R. Das, and Ram S. Katiyar Department of Physics, University of Puerto Rico, San Juan, PR 00931-3343
ABSTRACT Wide bandgap (~6 eV) ZnO/MgO multilayer thin films have been fabricated using pulsed laser deposition technique from their respective targets on c-plane Al 2O3 substrates. The thickness of single ZnO sublayer thickness was varied in the range of 0.75-2.5 nm with a fixed MgO sublayer thickness of 1 nm in order to achieve a total film thickness of ~300 nm. The structural transition from hexagonal to cubic phase was observed with the decrease in the thickness of single ZnO layer from 2.5 to 0.75 nm that resulted in the increase in bandgap from 3.5 to 6.2 eV. Mg contents of the films were increased from 40 % to 60 % for hexagonal and cubic phases, respectively. The surface roughness and grain structure were not influenced much with the increase of Mg concentration. Resistivity of the films was increased six orders of magnitude with the increase in Mg incorporation. The post annealing at 750° C did not show any significant change in the crystal structure and the optical properties. INTRODUCTION Artificial engineering of thin films by multilayer deposition has been attracted for the application of new functional devices [1]. Recently, ZnO and related materials are extensively investigated as a possible candidate for wide bandgap opto-electronic devices [2,3]. ZnO with a direct bandgap of 3.3 eV and excitonic energy of 60 meV, is a potential candidate for lasing action for the ultra violet region and photo-detection. The solid solution with MgO can produce wide bandgap MgxZ1-xO (MZO) thin films for application in quantum well related devices. Alloying ZnO with MgO has been made to fabricate ternary MgxZn1-xO, (MZO) which has been used as a barrier layer for quantum well devices based on ZnO and successfully demonstrated the quantum confinement effect [4,5]. Yang et al. [6] have reported ultraviolet photo-detector based on epitaxial MZO thin films on c-plane sapphire using metal-semiconductor-metal structure. One of the major advantages of ZnO –MgO system over group-III nitride is that one can tune the band gap as high as 8 eV, compared to 6.2 eV (AlN) for nitride alloys. One of the limiting factors for growth of MZO thin films is the solid solubility limit of MgO in ZnO, which is only 4%. However, Ohtomo et al. [7] had shown that epitaxial MZO thin films for up to 33% of Mg concentration could be grown by pulsed laser deposition (PLD) on c-plane Al 2O3 substrates. In most of the cases, the secondary phase is more pronounced at higher (>33%) Mg in MZO thin films. However, the binary phase diagram of ZnO and cubic MgO showed perfect solid solution for up to 40 at% of Mg [8]. Narayana et al.[9] have reported the epitaxial cubic MZO thin films with a composition of Mg0.8Zn0.2O on Si substrate without segregation of any secondary phase using TiN as a buffer layer. The fabrication of wide band gap (>5 eV) metastable MZO thin film
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