Structural properties of Eu doped GaN and its relation with luminescence properties
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Structural properties of Eu doped GaN and its relation with luminescence properties Hyungjin Bang1, Takahiro Maruyama1, 2, Shigeya Naritsuka1, 2, and Katsuhiro Akimoto3 21st Century COE program “Nano Factory”, Meijo University, 1-501 Shiogamaguchi, Tempaku-ku, Nagoya 468-8502, Japan 2. Department of materials science and engineering, Meijo University, 1-501 Shiogamaguchi, Tempaku-ku, Nagoya 468-8502, Japan 3. Institute of applied physics, University of Tsukuba, Tennodai 1-1-1, Tsukuba, Ibaraki, 3058573, Japan 1.
ABSTRACT The Structural properties of Europium (Eu) doped GaN and its relation with optical properties were studied. Concentration quenching of the intensity of the Eu related luminescence observed when Eu concentration exceeds 3 at.%. In situ reflection high-energy electron diffraction (RHEED), transmission electron microscopy (TEM), and X-ray diffraction (XRD) were carried out to study this luminescence quenching and it was discovered that there is close relationship between the luminescence intensity at 622 nm and structural properties. The cause of the concentration quenching is likely related to the polycrystalline growth as well as to the EuN formation. INTRODUCTION InGaN led to commercialization of blue, green light emitting diodes (LEDs) and laser diodes (LDs), and still plays pivotal roles in those optical applications. Red emission based on GaN, however, has not been realized commercially, due to the difficulty of increasing In content of InGaN. Recently Eu doped GaN attracts considerable interests since it yields red luminescence (622nm) which is not available in InGaN based structures so far. We have previously reported single crystalline growth of Eu-doped GaN and nearly-temperature-independent red luminescence at 622nm originates from the intra 4f-4f transition of the Eu3+ ion [1]. And the red luminescence was analyzed and determined to be generated through a trap-level-mediated energy transfer from the host GaN, and the external emission efficiency was estimated to be about 0.18 at room temperature when the Eu concentration was about 2 at.% [2, 3]. These results revealed that Eu-doped GaN could be a potential material for an active layer in LED or LD. However, the luminescence intensity abruptly decreased when the Eu concentration exceeded 2 at.%. The cause of this luminescence quenching is not well understood. In this study, structural properties of Eu-doped GaN were examined by reflection highenergy electron diffraction (RHEED), X-ray diffraction (XRD) measurements, and the transmission electron microscope (TEM) observation in order to study the cause of the concentration quenching of Eu-related luminescence in Eu-doped GaN. EXPERIMENTAL The Eu-doped GaN thin films were grown on sapphire (0001) substrates by gas-source molecular beam epitaxy (GSMBE) using uncracked NH3 gas with 6N purity as the nitrogen
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source. Metallic Ga with 6N purity and Eu with 3N purity were evaporated from conventional Knudsen effusion cells. Details of the growth sequences are given elsewhere [4-
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