Jerzy M. Langer

CdF2:In: A novel material for optically written storage of information

We demonstrate that semiconducting CdF2 crystals doped with indium is an efficient medium for optical storage of information in static and dynamic regimes. A metastable phototransformation of 1018 cm−3 In centers from a localized deep state to a hydrogenlike shallow state leads to a change of the refractive index Δn of about 10−4 for the probe beam at the wavelength of 500 nm. The diffraction efficiency is temperature dependent due to spontaneous decay of the grating caused by thermal recovery of the In impurity from the metastable hydrogenic state to the localized ground state.

DEEP-SHALLOW TRANSFORMATION OF BISTABLE CENTERS IN SEMICONDUCTING CDF2 CRYSTALS

Abstract To study the nature of deep/shallow states of bistable In- and Ga-centers in semiconducting CdF 2 crystals, the quantum yield of the photochemical reaction of deep-shallow transformation and the kinetics of shallow-deep thermotransformation are studied. Both experiments prove the two-electron (negative- U ) nature of the deep state.

Energy structure and recombination for ZnS bulk crystals doped with Tb, Er and Eu

ZnS bulk crystals grown by the chemical transport method and doped with rare earth impurities have been studied. As will be shown, the most critical point of the whole sample preparation is the starting powder firing in a CS2 stream. The latter influences not only the RE3+ doping level in ZnS but also the incorporation of the RE3+ impurities on an active luminescence site. The special role of europium due to its 2+ charge state has been interpreted on the basis of photosensitive ESR measurements.

Temperature dependence of the Schottky barrier in Al/AlGaAs metal‐semiconductor junctions

The dependence on temperature and alloy composition of the Schottky barrier height of Al on AlxGa1−xAs metal‐semiconductor junctions for n‐ and p‐type substrates and 0<x<1 is reported. All the structures were grown by molecular beam epitaxy. The compositional dependence of the barrier heights is the same as that of the band offsets in GaAs/GaAlAs heterojunctions. The barrier height for the p‐type substrates is practically independent of temperature over the whole composition range, while for the n‐type substrates the temperature change of the Schottky barrier follows that of the energy gap. This observation questions validity of the class of models of the Schottky barrier formation based on the concept of a neutrality level. Such behavior can, however, be reconciled if localized defects, whose ground‐state wave function is of a bonding type are the source of the Fermi‐level pinning at the interface.

Direct spectral probing of energy storage in Si:Er by a free-electron laser

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.

Donor gettering in GaAs by rare‐earth elements

Optical and electrical measurements of GaAs layers grown by liquid phase epitaxy with different amounts of Yb metal (0–1000 ppm) added to the Ga solution are reported. The presence of Yb during growth causes strong suppression of all donor‐related optical transitions due to the effective removal of donors, as judged from Hall effect data. We have not found any appreciable increase of the background acceptor concentration during conductivity conversion from n‐type to p‐type, and thus conclude that dominant donor gettering occurs already in the Ga solution. No Yb3+ (4f 13)‐related emission was detected in the as‐grown GaAs layers.

Lasing in Rare-Earth-Doped Semiconductors: Hopes and Facts

Semiconductors doped with rare-earth (RE) elements have attracted a lot of attention as alternative materials for producing electrically pumpe d semiconductor lasers whose emission wavelength is very weakly dependent on temperature. This prospect is especially attractive in the case of indirect-gap Silicon, whose photonic applications as the material for light emitters still remain more of a hope than a reality. In view of a desirable emission wavelength at 1.5 μm, a lot of research has concentrated on Si:Er (see Coffa et al. for a recent review). It is generally recognized that doping with Er ions presents one of the most promising approaches to Silicon photonics. However, despiteintensive investigations, stimulated emission has not been conclusively demonstrated for Si.Er or for any other RE-doped semiconductor. This is in striking contrast to optical amplifiers and lasers based on various erbium-doped glasses. In this article, which builds on recent articles in MRS Bulletin on Silicon photonics, we will address the issues relevant to efficient light generation by semiconductors doped with RE elements in general, and specifically by Si:Er-based structures. The intraimpurity electronic structure of RE ions is dominate d by electron-electron and spin-orbit interactions within the 4 f shell. In the case of Er 3+ , they produce separated J-multiplets with 4 I 15/2 and 4 I 13/2 as the ground and the lowest-lying excited states, respectively. Due to the effective Screening of 4 f electrons by the outer electron Shells, the host has a very limited influence and changes only slightly the relative positions of the levels. Depending on a particular site symmetry, the even terms of the crystal field split the free-ion J-multiplets into the Stark components typically by several meV for the ground State. The energy-level diagram of an Er 3+ ion in a cubic crystal field is shown in Figure 1, where the energy transfer paths relevant for Si:Er are also schematically indicated. The odd terms of the crystal field potential admix the states of opposite parity to the 4 f 11 configuration of the Er 3+ ion, thereby introducing a certain degree of electric-dipole strength into the otherwise forbidden intra-4 f -shell transitions. This effect enhance s slightly the magnetic-dipole strength of the 4 I 15/2 ↔ 4 I 13/2 transition and is host- and site-dependent. There-fore, Er-related center s of different microstructure can be fairly easily identified.

On the determination of thermal ionization energy of deep centers from ESR data

It is shown that the skin effect due to thermal generation of free carriers may affect the ESR signal more than deep center depopulation. Experimental results for 0.33 eV deep Eu2+ donor in CdF2 crystals are presented, to show the way in which the thermal energy of deep centers is deduced from the ESR data.

Lattice relaxation, radiative and non-radiative deexcitation at localized defects

Abstract The influence of defect-lattice coupling on energy structure and recombination processes at localized defects is discussed. The emphasis is put on those processes in which the charge state of the defect changes. Among the topics discussed are: extrinsic self-trapping, negative U defects, metastable defect states, multiphonon recombination and recombination induced defect reactions.

Direct observation of localized impurity excited states degenerate with conduction band (CdF2: Eu)

Abstract On the basis of the coupled absorption, conductivity, thermoluminescence and ESR photoquenching experiments it is shown that all the excited 4f 6 5d states of Eu 2+ are degenerate with the conduction band of CdF 2 . This is a unique property of CdF 2 among other flourites.