Nature and Excitation Mechanism of the Emission-dominating Minority Eu-center in GaN Grown by Organometallic Vapor-phase
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Nature and Excitation Mechanism of the Emission-dominating Minority Eu-center in GaN Grown by Organometallic Vapor-phase Epitaxy Jonathan Poplawsky1, Nathaniel Woodward1, Atsushi Nishikawa2, Yasufumi Fujiwara2, and Volkmar Dierolf1 1
Physics, Lehigh University, Bethlehem, Pennsylvania, U.S.A.
2
Division of Materials and Manufacturing Science, Osaka University, Osaka, Japan.
ABSTRACT In-situ doped Eu ions in GaN grown by Organometallic Vapor-phase Epitaxy (OMVPE) at different pressures were investigated under different excitation methods and through the use of the following experimental techniques: (1) resonant site-selective laser irradiation (2) electron beam excitation, and (3) a dual excitation using a combination of electron beam and laser irradiation. With these means, we have examined the difference in the excitation pathways that result from resonant laser and electron hole (e-h) pair excitation of Eu ions for two different distinct incorporation sites, which are responsible for most of the luminescence. We have obtained clear evidence that e-h pairs do not have the ability to excite all of the ions and that there is excitation trapping by defects involved in the Eu excitation. INTRODUCTION The RE doped GaN materials system has shown to be a promising candidate for LED’s, lasers, and displays [1-7]. RE’s have the ability to emit sharp spectral lines regardless of the host due to their shielded partially filled 4fn shell, while the wide band gap of GaN decreases the thermal quenching of the ions that correlate to a decreased emission at room temperature [8]. In particular, europium doped GaN is a candidate for the active layer used in a white LED structure to produce the red component of the white spectrum in order to have a pure semiconductor LED instead of one that uses phosphors to down convert. A problem that has been identified is that the emission intensity from excitation by e-h pair recombination is not very efficient and does not scale with the number of Eu centers incorporated into the sample [9]. This suggests that not all of the Eu ions are effectively excited by e-h pair recombination and hence do not contribute to the emission. In order to further improve performance, the excitation pathways and the nature of the most efficient Eu center type have to be better understood. To this end, we use simultaneous excitation of the ions by a visible laser and by an electron beam to study the relationship between resonant direct excitation of the ion itself and excitation of the ion after the creation of electronhole pairs. EXPERIMENT The two samples used in this experiment were grown by OMVPE at 1050o C under two different pressures (10 kPa and 100 kPa) and have a Eu concentration of 7x1019 cm-3 and 3x1019
21
cm-3 respectively. For more information on the growth of these samples refer to [3,7]. It has been reported that active layers grown under high-pressure (HP) exhibit the best performance in electroluminescence (EL) devices despite the lower concentration and emission spectra that exhibit more d
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