Volume 92, Issue 10, 15 November 2002
Index of content:
92(2002); http://dx.doi.org/10.1063/1.1515376View Description Hide Description
We investigated near-infrared-to-blue upconversion from thuliumdoped in tellurite glasses upon continuous wave excitation near 800 nm. We observed an enhancement of over two orders of magnitude of the upconverted emission at when neodymium ions were codoped with ions. For comparison, using a codoped fluorozirconate glass as a reference material we observed a 40-fold enhancement of the blue emission. Analysis of the blue emission for samples with different doping levels of ions showed that energy transfer between and is the mechanism responsible for the enhancement in upconversion.
92(2002); http://dx.doi.org/10.1063/1.1513884View Description Hide Description
Mathematical analysis for diffusion-limited growth of a spherical crystal in an infinite matrix under linear heating conditions has been performed, and the exact and approximated solutions of the problem have been obtained. The approximated expression derived for evolution of the growing crystal radius at constant heating rate q formally coincides with the well-known parabolic law derived for isothermal conditions if the real time is replaced by with Q being the activation energy of diffusion. Despite a relatively large difference between the exact and approximated values of the crystal size, the accuracy of the volume fraction crystallized is within 1%. A good agreement of the calculated results with the relevant experimental data for the Fe–B metallic glasses implies the validity of the simple analytical relations derived for description of the diffusion-limited crystal growth in metallic glasses at constant rate heating.
Correlation between the type of threading dislocations and photoluminescence characteristics at different doping concentrations of Si in GaN films92(2002); http://dx.doi.org/10.1063/1.1516261View Description Hide Description
At various doping concentrations of Si in GaN films, the correlation between the type of dislocations and photoluminescence(PL)characteristics was investigated. A different broadening behavior of symmetric and asymmetric Bragg peaks as a function of carrier concentration provided qualitative evidence that the type of threading dislocations generated in GaN layers was strongly dependent on the doping levels of Si. PLspectra in conjunction with x-ray rocking curve measurements suggested that the yellow luminescence associated with deep levels was more strongly related with edge dislocations than screw or mixed ones.
92(2002); http://dx.doi.org/10.1063/1.1515372View Description Hide Description
We report on the growth of epitaxialthin films on -cut single crystals. The use of double buffer layers allows a single in-plane orientation of and results in superior superconducting properties. In particular, surface resistance values of 1.4 mΩ have been measured at 8 GHz and 65 K. The attainment of such low values of constitutes a key step toward the incorporation of high materials as electrodes in photonic and acoustic devices.
Comment on “Ion-assisted pulsed laser deposition of aluminum nitride thin films” [J. Appl. Phys. 87, 1540 (2000)]92(2002); http://dx.doi.org/10.1063/1.1515948View Description Hide Description
In a recent article [J. Appl. Phys. 87, 1540 (2000)] Lu et al. have reported on the achievement of good quality crystalline aluminum nitride(AlN)films deposited on Si〈100〉 substrates. The films were prepared in the 200–800 °C temperature range by the nitrogen-ion-assisted laser ablation of one AlN target. According to their experimental results, the infrared absorption bands due to Al–N bonds display a frequency shift of approx. as a consequence of the residual stress present in the AlN films. Interestingly, and despite such a high stress, the phonon vibration modes do not exhibit any appreciable shift. Actually, most of the phonon frequencies reported by Lu et al. perfectly match those observed in crystalline silicon. The analyses of Lu et al. probably contain inaccuracies regarding the true crystalline quality of their AlN films and is the subject of this comment.