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Physical review. B, Condensed matter

Energy transfer between shallow centers and rare-earth ion cores: Er3+ ion in silicon

T. Gregorkiewicz, Du Thi Xuan Thao, Jerzy M. Langer, Herman H. P. Th. Bekman, Mikhail S. Bresler, Jürgen Michel, Lionel C. Kimerling

Published Feb 15, 2000• 40 citations
Publisher

Abstract

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 $4f$-electron core of the ${\mathrm{Er}}^{3+}$ 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.

DOI
10.1103/physrevb.61.5369

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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•Jun 30, 2003•0 citations
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\ensuremathμm1.5 \ensuremath{\mu}\mathrm{m}1.5\ensuremathμm 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.
Participation of shallow states in the energy transfer processes between the rare-earth ions and a host semiconductor matrix is discussed. It is argued that shallow levels link the atomic-like states of the inner core of the rare-earth ion with the band structure of the host crystal. For the two most investigated systems InP:Yb and Si:Er the important role of these states at various stages of excitation and de-excitation mechanisms is shown. Models of the energy transfer process with a formation of a shallow intermediate state are discussed. It is shown that the formation of such a state is essential for the RE ion core activation. Experimental results supporting the involvement of shallow states are reviewed. Temperature dependencies of the photoluminescence activation and decay time constants are presented; these show an important role of shallow doping in the activation of Auger processes relevant to the mechanism of rare-earth ion photoluminescence. For the Si:Er system direct evidence for the two-stage excitation mechanism is discussed.
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.
Energy transfer in erbium doped optical waveguides based on silicon This thesis describes the energy transfer processes\noccurring in materials that can be used for the fabrication of silicon compatible optical integrated circuits, operating at 1.54 mm.The thesis consists of three parts: Part I discusses energy transfer in optical waveguide amplifiers on silicon. The waveguide amplifiers use an intra-4f transition of the rare earth ion erbium occurring at 1.54 mm. To achieve significant amplification, high\n Er concentrations (> 10 20 cm -3 ) are required. At these concentrations Er ions can interact, resulting in co-operative\n upconversion, in which one excited Er ion is excited into a higher energy state, while a neighboring Er ion returns to the ground\n state. In Part I this and other gain limiting effects are investigated. In particular, it is shown that the atomic-scale Er distribution\n is of crucial importance for the performance of Er doped waveguide amplifiers. Part II discusses the energy transfer processes\n occurring in Er doped silicon. It is shown that Er in silicon can be excited by electron-hole pair recombination in an\n impurity-Auger effect. This effect may be used to fabricate an electrically pumped Er doped waveguide amplifier. The reverse\n process can also occur: an excited Er ion can de-excite and generate an electron-hole pair in the silicon, due to a non-radiative\n backtransfer process. This process has an efficiency of 70% at room temperature. This observation enables the fabrication of an\n Er doped photo-detector in a Si waveguide operating at 1.54 mm. The performance of such a detector is discussed. Part III\n discusses the interaction between Er and silicon nanocrystals embedded in SiO2. Si nanocrystals efficiently absorb visible light,\n resulting in the generation of quantum confined electron-hole pairs. The electron-hole pairs efficiently transfer energy to Er.\n Due to the energy transfer, the effective Er absorption cross section is five orders of magnitude larger than for direct optical\n excitation. Silicon nanocrystals may thus be used to increase the pumping efficiency of an Er doped waveguide amplifier. It is\n shown that a single nanocrystal can excite ~1 Er ion at a time and that an optical gain of 0.6 dB/cm may be achieved in a typical\n nanocrystal sensitized waveguide amplifier. Due to the broad absorption band of Si nanocrystals, a broad band light source can\n be used as pump source in such a nanocrystal sensitized Er doped waveguide amplifier.
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 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 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.