Gallium Oxide as a Host for Rare Earth Elements
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Gallium Oxide as a Host for Rare Earth Elements J. F. Muth1, P. Gollakota1, A. Dhawan1, H. L. Porter1, Y. N. Saripalli2, L.M. Lunardi1 Department of Electrical and Computer Engineering 2 Department of Materials Science and Engineering North Carolina State University, Raleigh, NC 27695 1
ABSTRACT Pulsed Electron Beam Deposition (PEBD) and Pulsed Laser Deposition (PLD) were used to grow Gallium Oxide (Ga2O3) thin films on double sided polished sapphire substrates. At 850oC substrate temperature, smooth single crystal β-Ga2O3 films were obtained, which were confirmed with measurements by AFM of RMS surface roughness of about 1 nm. When characterized under electron beam excitation, the films exhibited different responses. For example: Europium doped films emitted intense red emission from 5D0 to 7Fj transitions while exhibiting weak broad emission from 300 to 500 nm. In contrast, Erbium doped films emitted strong emission from 300 to 500 nm peaked at 360 nm that was attributed to defects in the host matrix. Green emission from the Erbium transitions was observed at 528 and 550 nm. Films with different rare earth compositions varying from 0.1 % to 0.4 % were also prepared. High quality natural waveguides were formed with the deposited Ga2O3 films on the lower refractive index substrate sapphire. This was confirmed by measuring the refractive index by prism coupling and sharp coupling spectra. INTRODUCTION The optical properties of rare earth elements have long attracted interest due to their narrow emission lines in the visible, resulting in their use as phosphors in display applications and the infrared for optical fiber amplifiers in optical communications. Previously, GaN has been used as a host for rare earth elements with large bandgap (3.4 eV) and sufficient carrier mean free path.1 However, in this study we investigate the possibility of using a material with even wider band gap. β-gallium oxide, is a transparent conducting oxide (TCO), naturally n-type, and has a band gap of ~4.7 eV, among the largest values of any of the known TCO’s to date. The crystal structure is monoclinic with a = 12.23 ± 0.02 Å, b = 3.04 ± 0.01 Å, c = 5.80 ± 0.01 Å, β = 103.7 ± 0.3o, and the space group is C2/m (C2h3).2 Gallium Oxide has been grown by a variety of methods including the floating zone method,3,4 and through the Verneuil method.5 Thin films of β-gallium oxide have been grown through pulsed laser deposition (PLD),6 and electron-beam evaporation7 and spray pyrosisis8. While rare earth doped Gadolinium Oxide9,10 and Gd2EuTe2O4 monoclinic crystals11 have been produced , analyzed and serve as a reference for comparing the emission of rare earth ions in monoclinic crystals, there has been little work done on doping Gallium Oxide thin films with rare earths.8 EXPERIMENTAL DETAILS The targets were prepared by mixing the rare earth element oxides with Ga2O3, both in powder forms, pelletizing into 1-inch diameter disks at 5000 PSI, and sintering in air at 1400o C for 5 h. After sintering, the final target gets rather densified
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