Above and below Bandgap Excitation of Er-defect Complexes and Isolated Er in Er-implanted GaN
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Above and below Bandgap Excitation of Er-defect Complexes and Isolated Er in Erimplanted GaN A.Braud1, M.Abouzaid1, M.Wojdak1, J.L. Doualan1, R. Moncorge1, B. Pipeleers2, A.Vantomme2 1
CIRIL – ISMRA, 6 Boulevard maréchal Juin, 14050 Caen cedex, France Instituut voor Kern-en Stralingsfysica, Departement Natuurkunde, Celestijnenlaan 200 D, 3001 LEUVEN, Belgium 2
ABSTRACT Photoluminescence (PL) spectra, luminescence dynamics and luminescence saturation of 3+ 4 the Er I13/2 → 4I15/2 transition in Er-implanted GaN samples at 7K are investigated. Under below-gap excitation, different Er centers are identified . Several Er centers are clearly excited via local defects or impurities. Er-defect complexes excited by above or below bandgap light are compared. Luminescence dynamics study shows that the 4I13/2 manifold has a shorter lifetime when Er ions are part of Er-defect complexes than when Er ions are isolated from any defect. The saturation of Er luminescence is investigated for the different types of Er center corresponding to specific excitation wavelengths. INTRODUCTION Rare-earth doped GaN is a very promising candidate for the development of new electroluminescent devices because of the unique optical properties of rare-earth doped materials combined with the wide bandgap of GaN [1]. This wide-bandgap leads to a reduced rare-earth emission quenching compared to other semiconductors such as InP, GaAs or Si [2]. The exact mechanisms leading to the excitation of rare-earth ions in rare-earth doped semiconductors are still not clearly understood. The most commonly supposed process involves first the creation of free electrons and free holes by above bandgap radiation [3]. A free electron or hole is then captured by a carrier trap and a hole or electron is attracted to the trap by Coulomb interaction, resulting in the formation of an electron-hole pair. The energy of the electron-hole pair recombination is subsequently transferred to the rare-earth ion. The work presented in this paper shows that the carrier trap involved in the formation of electron-hole pairs after above bandgap excitation exhibits spectroscopic features similar to the ones of local defects which can be excited below bandgap.
EXPERIMENTAL DETAILS The GaN wafer prepared by metalorganic chemical vapor deposition (MOCVD) was mounted on a two-axis goniometer and subsequently implanted with 80 keV Er ions at room temperature with a dose of either 2.5×1014 or 5×1014 Er/cm2. During implantation, the GaN axis was either aligned with the ion beam ('channeled' implantation) or tilted by 10° ('random' implantation). Subsequently, the samples were annealed in a tube furnace at 950 °C for 30 minutes under flowing nitrogen using unimplanted GaN as a proximity cap.
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The Er-implanted samples were mounted in a APD liquid helium cryostat and cooled down to 7 K. PL and PLE studies were performed by exciting the GaN:Er samples with a CW tunable Ti:Sapphire laser , Ar laser or He-Cd laser . For luminescence decay measurements the laser source was modulate
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