Excitation-Wavelength Dependent and Time-Resolved Photoluminescence Studies of Europium Doped GaN Grown by Interrupted G
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Excitation-Wavelength Dependent and Time-Resolved Photoluminescence Studies of Europium Doped GaN Grown by Interrupted Growth Epitaxy (IGE) Ei Ei Nyein1, Uwe Hömmerich1,*, Chanaka Munasinghe2, Andrew J. Steckl2, and John M. Zavada3 1 Department of Physics, Hampton University, Hampton, VA 23668 2 Department of Electrical and Computer Engineering, University of Cincinnati, OH 45221 3 U.S. Army Research Office, Research Triangle Park, NC 27709 *E-mail: [email protected] ABSTRACT The emission properties of Eu doped GaN thin films prepared by interrupted growth epitaxy (IGE) were investigated through excitation-wavelength dependent and time-resolved photoluminescence (PL) studies. Under above-gap excitation (333-363 nm) large differences were observed in the Eu3+ PL intensity and spectral features as a function of Ga shutter cycling time. The overall strongest red Eu3+ PL intensity was obtained from a sample grown with a Gashutter cycling time of 20 minutes. The main Eu3+ emission line originating from 5D0 ! 7F2 transition was composed of two peaks located at 620 nm and 622 nm, which varied in relative intensity depending on the growth conditions. The room-temperature emission lifetimes of the samples were non-exponential and varied from ~50 µs to ~200 µs (1/e lifetimes). Under resonant excitation at 471 nm (7F0->5D2) all samples exhibited nearly identical PL spectra independent of Ga shutter cycling time. Moreover, the Eu3+ PL intensities and lifetimes varied significantly less compared to above-gap excitation. The excitation wavelengths dependent PL results indicate the existence of different Eu3+ centers in GaN: Eu, which can be controlled by the Ga shutter cycling time. INTRODUCTION Rare-earth (RE) doped III-V compound semiconductors have received significant attention due to their applications in light emitting devices and for their unique optical and electrical characteristics [1,2]. In recent years, RE ions have been introduced into GaN for optoelectronic devices operating in the ultraviolet (UV), visible, and infrared (IR) spectral region [1,2]. Eu3+ doping of GaN has been of particular interest because of its efficient red emission at ~ 622 nm used in electroluminescence (EL) devices [2]. In addition, Eu3+ exhibits a relatively simple energy diagram, which makes Eu3+ an excellent spectroscopic probe [3-5]. The ground state level 7F0 and the lowest emitting level 5D0 of Eu3+ are non-degenerate. Therefore, the optical transition 5D0 ! 7F0 can provide important information on different Eu3+ centers in the GaN lattice. The main aim of this research is to develop compact, efficient, and bright EL devices based on RE doped GaN thin films. To achieve this goal, the materials need to be optimized for several growth parameters: e.g. RE concentration, growth temperature, and III/V ratio. In this work, a set of GaN:Eu samples was prepared by a new growth technique called “Interrupted Growth Epitaxy” (IGE) [6]. During IGE the group III (Ga) shutter is closed for a certain time interval, which allows the GaN
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