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Applied Physics Letters

Three-center Auger excitation mechanism of ytterbium intrashell emission in ZnS

Krzysztof Swiatek, A. Suchocki, M. Godlewski

Published Jul 2, 1990• 10 citations
Publisher

Abstract

A new, very efficient excitation mechanism of Yb intrashell 4f-4f emission in ZnS is reported. We show that the energy transfer from donor-acceptor pairs to Yb sites leads to Yb ionization (Yb3+⇒Yb2+), followed by capture of ionized carriers via the Yb3+ external excited state (bound exciton state), an energy transfer to the Yb3+ 4f shell, and Yb3+ 4f-4f radiative recombination. Since three centers (donor, acceptor, and Yb) participate in the energy transfer process in which Yb is first ionized, the above excitation mechanism is of three-center Auger transition nature. A similar excitation mechanism is expected for Eu, Sm, and Tm in ZnS, i.e., for these rare earth (RE) ions which can change their charge state from 3+ to 2+ in this lattice. For wider band-gap sulfides, such as CaS or SrS, this mechanism can be of importance also for other REs, such as, for example, Pr, Dy, and Er.

DOI
10.1063/1.103572

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Journal of Applied Physics•Oct 15, 1994•20 citations
A new, very efficient excitation mechanism of Yb intrashell 4f-4f emission in ZnS is reported. We show that the energy transfer from donor-acceptor pairs to Yb sites leads to Yb ionization (Yb3+⇒Yb2+), followed by capture of ionized carriers via the Yb3+ external excited state (bound exciton state), an energy transfer to the Yb3+ 4f shell, and Yb3+ 4f-4f radiative recombination. Since three centers (donor, acceptor, and Yb) participate in the energy transfer process in which Yb is first ionized, the above excitation mechanism is of three-center Auger transition nature. A similar excitation mechanism is expected for Eu, Sm, and Tm in ZnS, i.e., for these rare earth (RE) ions which can change their charge state from 3+ to 2+ in this lattice. For wider band-gap sulfides, such as CaS or SrS, this mechanism can be of importance also for other REs, such as, for example, Pr, Dy, and Er.
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.
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.
The characteristics of impact excitation of the 1.54-μm emission due to intrashell transitions of 4f electrons of erbium atoms (4I13/2 →4 I15/2) doped into InP were investigated in comparison with the photoluminescence (PL) emission of the same peak from the same sample. The thermal quenching of this Er-related electroluminescence (EL) emission was found to be very small in contrast to the large thermal quenching of the PL emission. The EL emission intensity at room temperature was half that at 77 K, and the temperature dependence of the emission energy and the spectral width was very small. Due to almost temperature-independent nature of the impact excitation process, the quenching of the EL emission intensity reflected directly that of the fluorescence efficiency of this intrashell transitions of Er 4f electrons. Using this result, the temperature dependence of the PL energy transfer efficiency was also obtained. It was also found that the fine structure of the 1.54-μm emission was different between EL and PL. It was speculated that Er atoms on different lattice sites, which were in different crystal fields, were excited depending on excitation processes.
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.