Combined Excitation Emission Spectroscopy of Europium ions in GaN and AlGaN films
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Combined Excitation Emission Spectroscopy of Europium ions in GaN and AlGaN films V.Dierolf1, Z. Fleischman1, and C, Sandmann1. A. Wakahara2, T. Fujiwara2, C. Munasinghe3. A. Steckl3, 1
Department of Physics and Center for Optical Technologies, Lehigh University, Bethlehem, PA 18015, USA 2 Toyohashi University of Technology 3 Nanoelectronics Laboratory, University of Cincinnati, Cincinnati, OH 45221-0030 ABSTRACT: Site-selective combined excitation emission spectroscopy studies have been performed on Eu-doped GaN and numerous sites have been identified. Relative numbers and broadening of these peaks has been investigated for different growth conditions and for increasing AL content of the AlxGa1-xN alloy. INTRODUCTION In a wide range of host materials, trivalent rare-earth RE3+ ions exhibit inner-shell(intra4f) transitions resulting in distinct optical absorption and emission with wavelengths ranging from the ultraviolet to the near infrared. Due the shielded nature of 4f electronic states, the transition energies are virtually independent of temperature and vary only slightly under changes of the crystal environment. This makes these ions very attractive as active ions in emitter and laser applications Optically pumped RE-based emitters have found many applications as phosphors for displays, solid-state lasers, and fiber-optic amplifiers [1]. However, to achieve electrically pumped RE emitters requires a semiconductor host. One of the most promising semiconductor hosts for RE emitters and lasing is the wide band-gap semiconductor GaN, which has been shown to produce outstanding “intrinsic” light emitting diodes and lasers. The wide band gap and the ability to tailor the band-gap through III-N alloying enable matching this materials system to many RE3+ ions. Electroluminescence from GaN doped with Er, Eu, Pr, Tb, Tm, and other REs has been reported [2]. Moreover, laser action in Eu-doped GaN has been reported [3] under optical excitation, rekindling the hope that an electrically-pumped rare doped semiconductor laser can be achieved. In this pursuit, it becomes important that all rare earth ions that are within the laser cavity are exited effectively. To this end, the presence of different incorporation sites complicates this goal and requires the detailed characterization of these sites in terms of their energy states and their capability to be excited electrically. In this contribution we address the first task by applying the site-selective technique of combined excitation emission spectroscopy to the Eu:GaN material system. EXPERIMENTAL TECHNIQUES In combined excitation-emission spectroscopy, the Eu-transitions are excited resonantly and the emission spectra are measured for a dense sequence of excitation wavelengths. The use of computer control and CCD-array detection for collection of the emission
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spectra, allows measurements of a large number of spectra within a short time. In this way a 2D-data set of emission intensities as function of excitation and emission wavelength is obtained. Depict
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