Nonequilibrium Carrier Dynamics Studied in Er,O-Codoped GaAs by Pump-Probe Reflection Technique
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Nonequilibrium Carrier Dynamics Studied in Er,O-Codoped GaAs by Pump-Probe Reflection Technique Yasufumi Fujiwara1, Kazuhiko Nakamura1, Shoichi Takemoto1, Yoshikazu Terai1, Masato Suzuki2, Atsushi Koizumi3, Yoshikazu Takeda3 and Masayoshi Tonouchi2 1
Department of Materials Science and Engineering, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan 2 Institute of Laser Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan 3 Department of Materials Science and Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan ABSTRACT Carrier dynamics in Er,O-codoped GaAs (GaAs:Er,O) have been systematically investigated by means of a pump and probe reflection technique with a mode-locked Ti:sapphire laser. In GaAs:Er,O, it has been found that the codoping produces a single atom configuration (Er-2O configuration) as an Er atom located at the Ga sublattice with two adjacent O atoms together with two As atoms, resulting in extremely strong Er luminescence. Time-resolved reflectivity of GaAs:Er,O exhibited an abrupt increase in amplitude, followed by a steep decrease to negative in less than 1 ps and then a gradual increase in approximately 100 ps. The steep decrease is due to bandgap renormalization. The gradual increase in reflectivity depended strongly on Er concentration, indicating that a trap induced by Er and O codoping plays an important role in dynamics of nonequilibrium carriers in GaAs:Er,O. INTRODUCTION Rare-earth (RE) doped semiconductors have gained significant attention as a promising new class of materials that emit light from the RE 4f shell by means of electrical injection, in which the energy of electron-hole pairs is transferred to the RE shell. The intra-4f shell transitions of RE ions give rise to sharp emission lines whose wavelengths are largely independent of both the host materials and temperature. This stability occurs because the filled outer 5s and 5p electron shells screen transitions within the inner 4f electron shell from the interaction with the host. The intra-4f shell transitions from the first excited state (4I13/2) to the ground state (4I15/2) of Er3+ ions at around 1.5 µm is of special interest because the wavelength matches the minimum loss region of silica fibers used in optical communications. We have intensively investigated organometallic vapor phase epitaxy (OMVPE) and luminescence properties of Er-doped III-V semiconductors [1]. In Er,O-codoped GaAs (GaAs:Er,O), the Er-related photoluminescence (PL) spectrum was dominated by seven emission lines under host-excited conditions at a low temperature [2]. The Er center has been identified as an Er atom located at the Ga sublattice with two adjacent O atoms (hereafter referred as Er-2O) together with two As atoms. Recently we have fabricated GaAs:Er,O homostructure light emitting diodes (LEDs) by OMVPE and successfully observed 1.5 µm electroluminescence (EL) due to an Er-2O center under forward bias at room temperatu
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