Amorphous Nitride Alloys as Hosts for Rare-Earth Luminescent Ions.
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Amorphous Nitride Alloys as Hosts for Rare-Earth Luminescent Ions. M. L. Caldwell, M. E. Little, C. M. Spalding, M. E. Kordesch1 and H. H. Richardson2 Condensed Matter and Surface Science Program Department of Physics and Astronomy1 Department of Chemistry and Biochemistry2 Ohio University, Athens, OH 45701, U.S.A. 1 [email protected] 2 [email protected] ABSTRACT Amorphous alloys of aluminum-gallium nitride doped with erbium (Er) were deposited at 300 K. The compositions ranged from 19% Al to 86% Al with optical band gaps varying linearly with composition from 3.4 eV (GaN) to 6.2 eV (AlN). The films were deposited on p-doped silicon (111) by a dc/rf dual gun system in a nitrogen/argon atmosphere at a pressure of 4.8 milli-Torr. After growth the films were thermally “activated” at 1070 K for 10 minutes in a nitrogen atmosphere. The cathodoluminescence emission intensities decreased linearly with Ga composition. This dependence suggests that the higher energy transitions in the Er ion are quenched by transitions to the conduction band of the alloys. INTRODUCTION Thin films (~200 nm) of amorphous AlN doped with Er, Tb, Cu, Mn and Cr have been grown by RF magnetron reactive sputtering at 77 and 300 K 1-9. The doped amorphous AlN films were “activated” at temperatures up to 1250 K in a nitrogen atmosphere and showed luminescence in the visible spectrum when excited with 2.85 kV electrons. Colors ranging from blue to red (420 nm – 690 nm) have been observed from the luminescence of Er, Tb, Cu, Mn and Cr 1-9. Electroluminescent devices have been fabricated using a glass substrate with an indium tin oxide transparent bottom electrode and an aluminum top electrode10. In these devices the phosphor showed luminescent efficiencies on the order of 0.03 lm/W. Amorphous AlxGa1-xN doped with rare-earth metals are promising phosphor materials because the bandgap of the host material can be tuned. The band-gap values range from 3.4 eV for GaN to 6.2 eV for AlN. Tuning the bandgap may alter the luminescent efficiency of the phosphor and could lead to more effective devices. Because amorphous films can be deposited on a variety of substrates, the luminescence in these materials may be technologically important. Infrared microscopy, time-resolved photoluminescence spectroscopy and x-ray diffraction have been used to characterize the phosphor activation process. Luminescent properties of amorphous nitride alloy films doped with rare-earth metal Er will be discussed.
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EXPERIMENTAL The films were grown by a dual-source rf/dc sputtering system at 300 K on substrates of p-doped silicon. Prior to the growth, the silicon substrates were cleaned by ultrasonic agitation in isopropyl alcohol, rinsed with distilled water, a 5-minute dip in 25% HF solution and final rinse in de-ionized water to remove all native oxides. After placing the samples in the chamber, the dc sputtering was performed with an Al target of 99.999% purity with a slug (3/16 in diameter) of Er inserted in the target. The rf sputtering was perfor
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