The Nd-nanocluster Coupling Strength and its Effect in Excitation/de-excitation of Nd 3+ Luminescence in Nd-doped Silico
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The Nd-nanocluster coupling strength and its effect in excitation/de-excitation of Nd3+ luminescence in Nd-doped silicon-rich silicon oxide Se-Young Seo∗ and Jung H. Shin Department of Physics, Korea Advanced Institute of Science and Technology (KAIST), 3731~Kusung-dong, Yusung-gu, Taejon, Korea ABSTRACT The Nd-nanocluster Si (nc-Si) coupling strength and its effect in excitation/de-excitation of Nd luminescence in Nd-doped silicon-rich silicon oxide (SRSO) is investigated. Nd-doped SRSO thin films, which consist of nc-Si embedded inside a SiO2 matrix, were prepared by electron-cyclotron-resonance plasma enhanced chemical vapor deposition (ECR-PECVD) of 3+
SiH4 and O2 with co-sputtering of Nd and subsequent anneal at 950 °C. Efficient Nd3+ luminescence with moderate temperature quenching is observed. Based on the temperature dependence of Nd3+ luminescence lifetime, a coupling strength between nc-Si and Nd that is strong enough to result in efficient excitation of Nd3+ via quantum confined excitons while weak enough to result in a small back-transfer rate is identified as the key to Nd3+ luminescence. INTRODUCTION Rare earth (RE) doping of Si has gathered much attention as a possible way of realizing a Sibased photonic material that can enable Si-compatible microphotonics [1]. In particular, RE doping of SRSO that consists of nc-Si embedded inside an SiO2 matrix has proven successful in obtaining efficient RE luminescence [2-4]. While the exact details are not yet known, many Conduction Band
E0
Exciton formation
Excitation
Exciton dissociation
Backtransfer
4
4F3/2 I15/2 4 I13/2 4 I11/2
4
I9/2
Valence Band
Si nanoclusters
Exciton
4f shell of Nd
Figure 1. Exciton mediated excitation of rare-earth ions in SRSO. ∗
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results suggest that the exciton-mediated excitation mechanism originally proposed for RE ions in bulk semiconductors [5] applies to RE ions in SRSO as well. That is, carriers generated in ncSi form excitons, which then decay by Auger-exciting the RE ions. A schematic description of this exciton-mediated excitation mechanism is given in Fig. 1. Note, however, that such excited RE ions can, with help of phonons, decay non-radiatively by creating an exciton. This process, labeled "back-transfer" in Fig. 1, has been identified to be a major factor contributing to the thermal quenching of RE luminescence that has plagued many RE-doped semiconductors [6,7]. One proven method to suppress back-transfer is using a wide-bandgap host material [8], since a wide bandgap tends to increase E0, the energy mismatch between the exciton recombination energy and the RE intra-4f transition energy (See Fig. 1), thereby increasing the energy barrier against back-transfer. Or the bandgap of a narrow bandgap semiconductor suc
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