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Applied Physics Letters

Direct evidence of trap-mediated excitation in GaN:Er3+ with a two-color experiment

Loïc Bodiou, Alain Braud

Published Oct 13, 2008• 11 citations
Publisher

Abstract

We report a direct confirmation of the generally proposed rare-earth excitation model in GaN. The intermediate step involving the formation of excitons bound to rare-earth ions prior to the rare-earth excitation is demonstrated. Using a two-color experiment, we show that the erbium photoluminescence decay following a pulsed laser excitation is quenched when an additional continuous-wave (cw) laser is applied. As the two beams excite the sample, the cw beam perturbs the intermediate step of the Er excitation mechanism either by photoionization of Er-related trapped carriers or by inducing an Auger effect between the trapped carriers and free carriers.

DOI
10.1063/1.2999585

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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•Jun 30, 2003•0 citations
We report a direct confirmation of the generally proposed rare-earth excitation model in GaN. The intermediate step involving the formation of excitons bound to rare-earth ions prior to the rare-earth excitation is demonstrated. Using a two-color experiment, we show that the erbium photoluminescence decay following a pulsed laser excitation is quenched when an additional continuous-wave (cw) laser is applied. As the two beams excite the sample, the cw beam perturbs the intermediate step of the Er excitation mechanism either by photoionization of Er-related trapped carriers or by inducing an Auger effect between the trapped carriers and free carriers.
We report on verification of the proposed energy transfer mechanism responsible for photoluminescence of rare earth (RE) ions in semiconductors. Using two-color spectroscopy in the visible and the midinfrared regions (with a free-electron laser) we demonstrate reversal of the most important step in the excitation process. In that way, formation of the intermediate state bridging atomic states of the RE ion core and extended orbitals of a semiconducting host is explicitly confirmed and its characteristic energy spectroscopically determined. The study is performed for InP:Yb. It is argued, however, that the conclusions are valid for all semiconductor:RE systems, including the notorious Si:Er.
Two-color optical spectroscopy is used to directly manipulate shallow levels available in Si:Er samples of different parameters. It is observed that the excitation energy provided by a laser operating in the visible can be temporarily stored by trapping photogenerated carriers at shallow centers available in the material. Subsequently, this energy can be transferred to the 4f4f4f-electron core of the Er3+{\mathrm{Er}}^{3+}Er3+ ion in a trap ionization process induced by a mid-infrared pulse from a free-electron laser. In that way, Er-related luminescence at $1.5 \ensuremath{\mu}\mathrm{m}$ can be generated by an infrared pulse applied within several milliseconds after the band-to-band excitation pulse. By scanning the wavelength of the free-electron laser, ionization spectra of the shallow centers participating in the energy transfer are obtained, allowing for their identification. On that basis, the involvement of thermal donors is suggested. The results demonstrate that excitation of Er ions in Si is a multichannel energy transfer process where shallow centers play an important role.
Photoluminescence experiments have been carried out on erbium doped Ga0.55Al0.45As under continuous wave and pulsed laser excitation. For the first time the emissions arising from the two first Er3+ excited states 4I11/2, 4I13/2 and their temperature dependence have been systematically studied. From these results the nature and the excitation-deexcitation processes of these photoluminescences are discussed. With the help of these spectroscopic data we give an estimate of the stimulated emission cross section and the laser applications of such rare-earth doped III-V materials are examined.
We report on experimental verification of the proposed energy transfer mechanism responsible for photoluminescence of rare earth ions in semiconductors. Using two-color optical spectroscopy in the visible and the mid-infrared region, we demonstrate reversal of the most important step in the excitation process. This “back-transfer ” effect was supported experimentally only in an indirect manner by measurements on thermal quenching of PL intensity and lifetime. In the current study on InP:Yb, formation of the intermediate state bridging atomic states of the RE ion and extended orbitals of a semiconducting host is explicitly confirmed and its characteristic energy spectroscopically determined. This finding will allow optimization of the efficiency of semiconductor:RE systems for applications.
In this work we have developed a kinetics model of energy transfer from the host lattice to the localized core excited states of rare-earth isoelectronic structured traps (REI traps). The presence of low-lying empty core orbitals in rare-earth impurities introduces new excitation and recombination phenomena. To adequately describe the energy transfer to a REI trap, the buildup and decay kinetics of rare-earth luminescence, we consider six separate states of the REI impurity (unoccupied, electron occupied, electron occupied excited, exciton occupied, excited electron occupied, and excited exciton occupied). The energy-transfer processes occur through an Auger mechanism where the recombination energy of the bound electron with a free hole is transferred nonradiatively to the core states, or energy can be transferred from the bound exciton on a REI trap to the core states. If the initial and final states are not resonant (in both mechanisms), the energy mismatch must be accommodated by emission or absorption of phonons. Furthermore we discuss details of several quenching processes, which are incorporated into the kinetics equations. We derive two sets of differential equations for semi-insulating and n-type semiconductors governing the kinetics of rare-earth luminescence. Equations have been solved by a numerical method to derive the time dependence of the rise and decay kinetics as a function of excitation intensity. The numerically simulated luminescence rise and decay times show a good overall quantitative agreement with experimental data obtained for InP:Yb, over a wide range of generation rates.
In spite of a small and indirect bandgap, silicon features properties that are potentially very attractive for optical applications. The unsurpassed level of impurity control leads to suppression of nonradiative recombination paths and provides for a very long minority carrier lifetime. Up to the present, observation of an optical memory effect has not been reported for crystalline silicon (c-Si). In contrast, it has been observed in GaAs and GaN. In this study, we have used c-Si doped with erbium (Er) ions, since the trivalent Er3+ ion emits at a wavelength of 1.54 μm that is suitable for telecommunication applications. In a two-color experiment, utilizing a Nd:YAG pulse excitation as a writing beam and a free-electron laser (FEL) as a reading beam, we revealed that Si exhibits optical memory and afterglow effects at low temperatures T<50 K. Both effects are, in fact, manifestations of the properties of Si itself, conveniently revealed by the optical dopant. Detailed investigations of the FEL-induced emission at 1.54 μm show that this excitation mechanism is governed by the release of a single type of charge carrier. This implies that hole and electron capture events necessary for the Er excitation could, in principle, be separated in time. Therefore it appears possible to combine optical and electrical write/read procedures for the reported memory effect.