Optical propagation loss reduction in ZnO thin films under CO 2 laser treatment
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A. Ferrari Dipartimento di Elettronica, Universita Roma I, Italy, and GNEQP, GNSM, CNEQP L. S. Qian Changchun Institute of Optics and Fine Mechanics, Academia Sinica, People's Republic of China F. Quaranta and A. Valentini Dipartimento di Fisica, Universita Bari, Via Amendola 163, Bari, Italy
(Received 16 February 1990; accepted 21 May 1990) The reduction of propagation losses in ZnO thin film waveguides for integrated optics after CO2 laser treatment is studied. RHEED patterns show that the measured decrease of optical losses is accompanied by an increase of ordering after the laser treatment. The reordering depends on the initial film state and the laser irradiation conditions.
I. INTRODUCTION A few years ago it was shown that annealing with a CO2 laser resulted in a large decrease of propagation losses in a number of thin film optical waveguides deposited on different substrates.12 In particular, in the case of ZnO waveguides, Bertolotti et a/.3 confirmed these results showing that for both ZnO films deposited on Corning glass or silicon oxide substrates a large decrease in propagation loss was obtained which was larger for lower order modes. No clear explanation of the operating mechanism was given, although Dutta2 ascribed it to a reduction of interface scattering. To look closer at the explanation of the observed effect, the experiment was repeated using optical waveguides made of ZnO films deposited on Corning glass or sapphire substrates. Laser treatment was performed with a cw scanning CO2 laser, and the influence of scanning velocity was also examined. Reflection high energy electron diffraction (RHEED) of the treated surface was also performed. The results show that an optimum scanning velocity exists for loss reduction, and that in this case a marked reordering of the material occurs.
substrate distance about 60 mm; deposition rate about 0.5 A/s. The surface morphology and RHEED patterns were observed with an AEI EMG6 electron microscope at 60 kV. The analysis yields information about the structure of the samples within a depth of a few tens of nanometers. The ZnO was deposited as a thin film in general with a polycrystalline columnar structure with a preferred orientation of the crystallites, such that the caxis is perpendicular to the plane of the substrate.4 The waveguiding properties were studied with a one-prism coupling technique, using a rutile prism. The analysis was performed by observing the m-lines distribution, and the waveguide attenuation was obtained by the in-plane light scattering. The setup for this measurement is shown in Fig. 1, in which a He-Ne polarized laser beam (A = 6328 A), after passing through a
II. MATERIAL AND EXPERIMENTAL SETUP
ZnO films were prepared using a rf diode sputtering system using Corning glass 7059 or c-axis oriented sapphire substrates. A commercially available sintered ZnO target (diameter 100 mm) of 5 N purity was used. The deposition conditions were substrate temperature: 300 °C; sputtering gas pressure: 2.5 x 10"3 mbar; gas mixture composition: 28% O-
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