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Optical manipulation of excitation paths in RE-doped semiconductors

M.A.J. Klik, T. Gregorkiewicz

Published Jan 1, 2002• 0 citations
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Abstract

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.

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Kinetics and quenching mechanisms of photoluminescence in Yb-doped InP

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H. J. Łożykowski

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Deactivation of the photoionization excitation mechanism of rare-earth intraion emission in ZnS

Krzysztof Swiatek, M. Godlewski

A new, very efficient excitation mechanism of the rare-earth (RE) intraion emission was observed recently. This is a process in which the RE ion undergoes ionization and the RE intraion emission is induced when the ionized carrier is recaptured by the RE. The capture process proceeds via an intermediate state due to RE bound exciton (BE), from which the energy is transferred to the RE excited state. In this letter we discuss mechanisms which limit the efficiency of this excitation channel. The first is the thermal dissociation of the RE BE, which reduces the efficiency of the energy transfer thus quenches the RE intraion emission. It is also indicated that an efficient energy transfer from RE Be to iron Fe2+ centers competes with the transfer of the BE energy to RE core states. Our results mean that the practical utilization of the above excitation mechanism of emission in the RE-activated ZnS devices may be hampered by the common contamination of this material with iron.

Applied Physics Letters•May 28, 1990•14 citations
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.
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.
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.
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.
The photoluminescence spectra of Yb-implanted InP samples were studied under pulsed and cw excitations using an Ar+-ion laser (above band-gap excitation) at different temperatures and excitation intensities. The photoluminescence spectrum of the 4f transitions 2F5/2–2F7/2 consists of a sharp peak at 1001 nm and broader peaks in the spectral range between 1002.5 and 1010 nm. Time-resolved spectra were recorded at different temperatures. Using a tunable Ti:sapphire laser (above and below band-gap excitation) the photoluminescence excitation spectra were investigated for different emission lines. Study of the rise and decay time under intrinsic excitation (band to band) show that the rise time is a function of excitation intensity, which reflects an indirect excitation process for Yb3+. The experimental data are explained using a kinetics model of energy transfer from the host lattice to the localized core excited states of rare-earth isoelectronic structured traps. The numerically simulated luminescence rise and decay times show a good general quantitative agreement with experimental data, over a wide range of generation rates. The photoluminescence spectra and decay time were also studied as a function of temperature. A quenching mechanism of ytterbium luminescence involving Yb and Fe ions is proposed. Finally, the electric-field quenching of InP:Yb photoluminescence is investigated.
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.
A new, very efficient excitation mechanism of the rare-earth (RE) intraion emission was observed recently. This is a process in which the RE ion undergoes ionization and the RE intraion emission is induced when the ionized carrier is recaptured by the RE. The capture process proceeds via an intermediate state due to RE bound exciton (BE), from which the energy is transferred to the RE excited state. In this letter we discuss mechanisms which limit the efficiency of this excitation channel. The first is the thermal dissociation of the RE BE, which reduces the efficiency of the energy transfer thus quenches the RE intraion emission. It is also indicated that an efficient energy transfer from RE Be to iron Fe2+ centers competes with the transfer of the BE energy to RE core states. Our results mean that the practical utilization of the above excitation mechanism of emission in the RE-activated ZnS devices may be hampered by the common contamination of this material with iron.