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 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 mu 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.

Two-color optical spectroscopy is used to directly manipulate shallow levels available in Si:Er samples of different parameters. It is shown that Er photoluminescence can be quenched with a free-electron laser pulse applied shortly after the band-to-band excitation. For longer delay times between the visible and the infrared pulses an enhancenment of Er photoluminescence is observed. The effect is explained by the energy storage due to trapping of the photo generated carriers at shallow centers. These are subsequently ionized by the infrared beam and their recombination energy is transferred to the 4f-electron core of the Er3+ ion. In that way Er-related luminescence at 1.5μm can be generated by an infrared pulse applied within several milliseconds after the band-to-band excitation. By scanning the wavelength of the free-electron laser ionization spectra of the shallow centers participating in the energy storage are obtained providing their fingerprint. Presented results convincingly demonstrate power of the free-electron laser as a novel tool for spectroscopic applications in materials science of silicon.

Photoluminescence measurements have been made on float-zone and Czochralski-grown silicon samples which were doped with erbium by ion implantation. The characteristic luminescence spectra in the wavelength range between 1.5 and 1.6 μm were observed. Differences in the multiple line structure of the spectra indicated that the active luminescent centers have different symmetries and atomic structure. The dependence of the photoluminescence intensity on the laser excitation power and on the temperature was measured. Results are discussed on the basis of a physical model which includes the formation of free excitons, the binding of excitons to erbium ions, the excitation of 4f inner-shell electrons of the erbium ions, and their subsequent decay by light emission. To obtain a quantitative agreement between model analysis and experimental data the consideration of Auger processes by which erbium-bound excitons and erbium ions in excited state can decay by dissipating energy to conduction band electrons appears to be required. From the temperature dependence two activation energies are derived which are associated with the exciton binding energies and with an energy transfer process from excited erbium ions back to erbium-bound excitons, respectively. A good quantitative agreement can be obtained for suitable values of the model parameters. The luminescent properties of the samples of the different types of crystalline silicon are remarkably similar.

The photoluminescence intensity of erbium in silicon was measured as a function of laser excitation power and temperature. Results of these measurements are described on the basis of a physical model which includes the formation of free excitons, the binding of excitons to erbium ions, excitation of 4f-shell electrons of erbium ions and decay of excited erbium ions by light emission. An Auger energy transfer to free carriers by both erbium-bound excitons and excited erbium ions must be included in the model in order to obtain a quantitative agreement with experiment. From the temperature dependence two activation energies are derived, which are associated with the binding of excitons to erbium centers and with an energy transfer process from excited erbium ions back to erbium-bound excitons, respectively. The luminescence properties of the different types of Er-doped crystalline silicon are remarkably similar.

The intensity of the photoluminescence of erbium in silicon is analyzed by a model which takes into account the formation of free excitons, the binding of excitons to erbium ions, the excitation of inner-shell 4f electrons of erbium ions and their subsequent decay by light emission. Predictions of this model for the dependence of luminescence intensity on laser excitation power are compared with experimental observations. The results for float-zone and Czochralski-grown silicon, in which erbium is introduced by implantation with or without oxygen co-implantation, are remarkably similar. To obtain agreement between model analysis and experimental data it is necessary to include in the model terms describing energy dissipation by an Auger process of both the erbium-bound excitons and the erbium ions in excited state with free electrons in the conduction band. A good quantitative agreement is achieved.

The temperature dependence of the photoluminescence intensity of erbium-doped silicon was measured experimentally and modeled theoretically. Measurements were made on neat-zoned and Czochralski-grown silicon samples which were doped with erbium by ion implantation and on MBE-grown material. Results are discussed on the basis of a physical model in which following the formation of free electrons and holes by the exciting laser beam, the energy is transferred via the formation of free excitons and the binding of the excitons to erbium ions to the 4f electrons of the erbium ions, with their subsequent decay by light emission. In the analysis of the temperature dependence two activation energies emerge which are associated with the binding of excitons to erbium centers and with a transfer process from excited erbium ions back to erbium-bound excitons, respectively. The model provides good quantitative agreement with observations over the experimentally covered temperature range from 4 to nearly 200 K.

Results of a two-color spectroscopy in the visible and the mid-infrared on erbium-doped silicon (Si:Er) are presented. In the experiments, pulsed beam provided by a free-electron laser is directed on a sample under primary above-band-gap excitation with another laser. It is shown that the powerful infrared beam can be ionize carriers localized at shallow traps. Liberation of these carriers makes them available for excitation of erbium and thereby enhances the luminescence intensity. Identification of shallow levels responsible for the effect is discussed.

An enhancement of 1.5μm Si:Er photoluminescence by a mid-infrared pulse from a free-electron laser is investigated in detail. It is concluded that the effect is a consequence of defect-related energy storage in Si:Er samples. Carriers generated by a band-to-band excitation are participating not only in the excitation of Er luminescence via the excitonic mechanism, but are also trapped at various defect states. The infrared pulse photoionizes them, thus promoting extra carriers into the excitation channel of the Er3+ ion and leading to an additional luminescence. By scanning the wavelength of the free-electron laser ionization spectra of shallow centers participating in the energy transfer are obtained. The results also elucidate a special role of oxygen in Si:Er luminescence.

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 investigate the influence of infrared illumination using a free-electron laser on the photoluminescence of erbium-implanted silicon material. In addition to the earlier reported quenching of the Er-related photoluminescence due to dissociation of the intermediate excitation stage, two more features of the energy transfer mechanism are revealed. In the wavelength dependence of the quenching effect a local extreme is detected for a beam energy of approximately 100 meV. A possible origin of this effect is discussed. Further, the current experiment revealed the presence of non-radiative recombination centers which could transfer their energy to Er ions under the influence of the infrared beam. The centers were found to be characterized by extremely slow generation and decay kinetics. (C) 1999 Elsevier Science B.V. All rights reserved.