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.
Erratum: "A high-speed variable-temperature ultrahigh vacuum scanning tunneling microscope" [Rev. Sci.In this article we introduce a novel scanning tunneling microscope ͑STM͒, which operates in a sample temperature range from 60 to at least 850 K. The most important new feature of this STM is that, while one selected part of the surface is kept within the microscope's field of view, the sample temperature can be varied over a wide range of several hundreds of degrees during actual imaging. The extremely low drift of the scanner and sample was achieved by the combination of a thermal-drift compensated piezoelectric scanner design with a newly developed sample stage. The design of the sample stage defines a fixed center from which thermal expansions, in all three directions, are forced outwards. The performance of the microscope is demonstrated for several surfaces including Au͑110͒, on which we follow one particular surface region over a temperature range of more than 270 K.
We conclusively establish a direct link between formation of an Er-related donor gap state and the 1.5 microm emission of Er in Si. The experiment is performed on Si/Si:Er nanolayers where a single type of Er optical center dominates. We show that the Er emission can be resonantly induced by direct pumping into the bound exciton state of the identified donor. Using two-color spectroscopy with a free-electron laser we determine the ionization energy of the donor-state-enabling Er excitation as E(D) approximately 218 meV. We demonstrate quenching of the Er-related emission upon ionization of the donor.
We use variable-temperature scanning tunneling microscopy (STM) combined with pattern recognition of STM images to demonstrate that the vicinal Ag(115) surface undergoes a true Kosterlitz-Thouless(KT)-type roughening transition. As predicted, the height correlation function remains finite below the roughening temperature and it evolves to a logarithmically diverging function at and above T R. From a quantitative analysis of the height fluctuations we derive a roughening temperature of T R 465 6 25 K. Recent diffraction experiments could not confirm a KT-type roughening transition on vicinal surfaces. We attribute this to small residual errors in local surface orientation, which overshadow the real thermally excited height fluctuations. [S0031-9007(99)08538-5]
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