Luminescent Holmium Doped Amorphous AlN Thin Films for use as Waveguides and Laser Cavities.
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Luminescent Holmium Doped Amorphous AlN Thin Films for use as Waveguides and Laser Cavities. Muhammad Maqbool, H.H. Richardson, P.G. Van Patten and M. E. Kordesch Department of Physics & Astronomy and Condensed Matter and Surface Sciences Program, Ohio University, Athens, OH 45701, USA.
ABSTRACT Holmium doped AlN thin films have been deposited on flat silicon substrates and optical fibers of different diameters, in order to fabricate resonant cavities for lasers. In particular, the films on fiber substrates are expected to perform as cylindrical waveguides or "whispering gallery" mode cavities. The AlN films are deposited by reactive sputtering at liquid nitrogen temperature, using 100-200 Watts RF power, 5-8 mTorr nitrogen, using a metal target of Al and Ho. One micron thick films of AlN:Ho have been deposited on flat silicon substrates and 10 micron thick films on 80µm and smaller optical fibers. X-ray Diffraction and SEM studies show that films deposited on flat silicon are amorphous while those deposited on the fibers show columnar growth and some gain structure, most probably due to a temperature rise at the substrate during deposition. Cathodoluminescence (CL) emission is observed in thermally activated AlN:Ho in both crystalline and amorphous forms. An intense, narrow emission peak is observed at 549 nm. Less intense peaks are also observed at 362nm, 394 nm, 461 nm, and 659 nm. The most promising wavelength for our design is emission from the 5S2 → 5I8 transition at 549 nm.
INTRODUCTION Rare-earth (RE)-doped III-nitride semiconductor thin films are promising materials of interest due to their good electrical, optical, thermal and acoustical properties and their potentially significant applications in display technology and laser construction which are useful for medical and industrial purposes [1-3]. Amorphous nitride semiconductor films doped with rare-earth elements have also attracted interest because these semiconductors have many of the desirable qualities of the crystalline materials [4] and because of their visible luminescence [4-6]. These amorphous semiconductors may be more suitable for waveguides and cylindrical and spherical laser cavities because of the elimination of grain boundaries at low temperature growth. Emission from such amorphous semiconductors is observed in Er-doped amorphous AlN thin films [2,7,8]. The present work reports the green Cathodoluminescence (CL) emission from Holmium doped amorphous AlN. The films were grown using RF magnetron sputtering. It was also observed that amorphous AlN:Ho exhibits the same intensity of emission as does the crystalline AlN:Ho which makes amorphous AlN:Ho an equally important material. Future work is aimed at observation of the Ho transition at 2.1µm and lasing.
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EXPERIMENTAL WORK Thin films of AlN:Ho were prepared at room temperature and 100 K by RF magnetron sputtering of a holmium-aluminum bi-metal target in a pure nitrogen atmosphere in a cryopumped vacuum system. The RF power was varied between 100 and 200 watts. The aluminu
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