Fabrication of MgIn 2 O 4 Thin Films With Low Resistivity on MgO (100) Surface by PLD Method
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R. NOSHIRO*, K. UEDA*, H. HOSONO*, and H. KAWAZOE**
*Materials & Structures Laboratory, Tokyo Institute of Technology, Nagatsuta, Midori-ku, Yokohama 226-8503 **R&D Center, HOYA Corp., Musashino, Akishima 196-8510, Japan e-mail: [email protected] ABSTRACT Thin films of MgIn2O 4 spinel, which is a recently discovered TCO material, were deposited on MgO (100) surface by PLD. The thin films were prepared under low oxygen partial pressure to enhance formation of oxygen vacancies, from which carrier electrons were generated. X-ray analyses and AFM observations suggest epitaxial growth of the grains with diameter of 100-200nm. The grains showed strong orientations both along the normal of the thin film and in plane. Epitaxial growth of the spinel was also confirmed by high-resolution transmission electron microscopic observations. The lattice image of the interface region suggests formation of structural imperfections such as dislocations, grain boundaries and amorphous phase in significant fraction. Strong optical absorption due to electron carriers was detected in near infrared region. Very large Burnstein-Moss shift was observed in ultraviolet region, and the optical band gap was estimated to be 4.3eV. DC conductivity observed was 4.5xl0 3Scm-1', which is the highest value reported for the material so far. Concentration and Hall mobility of carrier electrons were found to be 2.1x10 21cm- 3 and 14 cm 2 V-ls', respectively. INTRODUCTION In the flat panel industries developing a transparent electrode thin film with higher conductivity is a current issue. A promising approach would be refinement of the fabrication process conditions of ITO thin films. Actually very high conductivity of 1.4x 104 Scm-1 was reported in the laboratory made ITO thin films [1-4]. We have proposed an alternative approach to the problem, finding new and highly conductive materials having a potential of replacing ITO. Spinel lattice was selected, because it contains a linear chain of edge sharing MO 6 octahedra running along [110]. The linear chain is expected to play a role of highway for carrier electrons. MgIn2O4 15-8], ZnGa2O4 [9] and CdGa 20 4 [10] spinel oxides were found to be a new n-type TCO material. Conductivity of Mgln2O4 polycrystalline thin films deposited by RF sputtering on glass substrates [7,8] was changed drastically in 10 orders of magnitude from 10-tScm- 1 to 102 ScM-1 upon H+- or Lit-implantation at room temperature without post annealing. However, Hall mobility of the thin films remained around 2-3 cm 2V- ts-1, which was almost one order of magnitude smaller than that of the doped ITO thin films. The most probable cause of the low mobility was supposed to be the scattering at grain boundaries. The purpose, therefore, of the present study is to examine whether higher conductivity of the spinel oxide is observable or not by fabricating the thin film with higher crystallinity. Mgln2O4 thin films are grown epitaxially on (100) surface of a single crystalline MgO substrate [11]. Electron carriers were generated from oxygen
