E Rita
University of Lisbon
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Featured researches published by E Rita.
Journal of Applied Physics | 2005
A.J. Neves; M. C. Carmo; M.J. Soares; Marco Peres; J Wang; E. Alves; E Rita; Ulrich Wahl
We report the observation of slow emission bands overlapped with the near-band-edge steady-state luminescence of nominally undoped ZnO crystals. At low temperatures the time-resolved spectra are dominated by the emission of several high-energy bound exciton lines and the two-electron satellite spectral region. Furthermore, two donor-acceptor pair transitions at 3.22 and 3.238eV are clearly identified in temperature-dependent time-resolved spectroscopy. These donor-acceptor pairs involve a common shallow donor at 67meV and deep acceptor levels at 250 and 232meV.
Journal of Applied Physics | 2003
C. Boemare; M.J. Soares; E Rita; E. Alves
We report Fe3+-related emission in ion-implanted ZnO single crystals. Iron ions were implanted at room temperature with 100 keV and a fluence of 1×1016 Fe+/cm2, and were submitted to annealing treatments in vacuum and in air. After implantation, the damage raises the minimum yield (χmin) from 2% to 50%. Annealing in an oxidizing atmosphere leads to a reduction of the implantation damage, which is fully recovered after annealing at 1050 °C with a χmin∼3% in the implanted region. With extrinsic excitation, red Fe-related emission is observed at low temperatures. The intensity is dependent on the annealing conditions. For samples annealed in air, the luminescence can be detected up to 120 K. When a comparison is made between unimplanted and post-implanted annealed samples, noticeable changes on near-band-edge and deep-level photoluminescence spectra are observed. A thermally populated structured green emission could be observed in the sample annealed in air, as shown by the temperature-dependent photolumines...
Applied Physics Letters | 2004
B. L. de Vries; V. Matias; André Vantomme; Ulrich Wahl; E Rita; E. Alves; A. M. L. Lopes; J. G. Correia
The lattice location of low-dose implanted Er in GaN, GaN:O, and GaN:C was investigated using the emission channeling technique. The conversion electrons emitted by the probe isotope 167mEr give direct evidence that the majority (≈90%) of Er atoms are located on substitutional Ga sites for all samples. Annealing up to 900 °C does not change these fractions, although it reduces the Er root-mean-square (rms) displacements. The only visible effect of oxygen or carbon doping is a small increase in the rms displacements with respect to the undoped sample.
Applied Physics Letters | 2007
Ulrich Wahl; J. G. Correia; J. P. Araújo; E Rita; J.C. Soares
The authors have determined the lattice location of implanted arsenic in GaN by means of conversion electron emission channeling from radioactive As73. They give direct evidence that As is an amphoteric impurity, thus settling the long-standing question as to whether it prefers cation or anion sites in GaN. The amphoteric character of As and the fact that AsGa “antisites” are not minority defects provide additional aspects to be taken into account for an explanantion of the so-called miscibility gap in ternary GaAs1−xNx compounds, which cannot be grown with a single phase for values of x in the range of 0.1
MRS Proceedings | 2007
Ulrich Wahl; J. G. Correia; E Rita; A. Marques; E. Alves; J.C. Soares
We have determined the lattice location of ion implanted As in ZnO and GaN by means of conversion electron emission channeling from radioactive 73 As. In contrast to what one might expect from its nature as a group V element, we find that As does not occupy substitutional O sites in ZnO but in its large majority substitutional Zn sites. Arsenic in ZnO is thus an interesting example for an impurity in a semiconductor where the major impurity lattice site is determined by atomic size and electronegativity rather than its position in the periodic system. In contrast, in GaN the preference of As for substitutional cation sites is less pronounced and about half of the implanted As atoms occupy Ga and the other half N sites. Apparently, the smaller size-mismatch between As and N and the chemical similarity of both elements make it feasible that As partly substitutes for N atoms.
MRS Proceedings | 2002
E Rita; Ulrich Wahl; A. M. L. Lopes; J. P. Araújo; J. G. Correia; E. Alves; J.C. Soares
In this work we report on the lattice site location of rare earths in single-crystalline ZnO by means of the emission channeling (EC) technique. Following low dose (3×10 13 at/cm 2 ) 60 keV ion implantation of the precursor isotope 169 Yb, a position-sensitive electron detector was used to monitor the angular distribution of the conversion electrons emitted from 169 Tm * as a function of the annealing temperature up to 600oC in vacuum. An additional annealing at 800oC in flowing O2 was performed. The EC measurements revealed that around 95-100% of the rare earth atoms occupy substitutional Zn sites up to an annealing temperature of 600oC/vacuum. After the 800oC/O2 annealing, the emission channeling effects decreased considerably.
Nuclear Instruments & Methods in Physics Research Section B-beam Interactions With Materials and Atoms | 2003
E. Alves; E Rita; Ulrich Wahl; J. G. Correia; J. C. Soares; C. Boemare
Superlattices and Microstructures | 2006
A.J. Neves; M. C. Carmo; M.J. Soares; Marco Peres; E. Alves; E Rita; Ulrich Wahl
Nuclear Instruments & Methods in Physics Research Section B-beam Interactions With Materials and Atoms | 2006
Ulrich Wahl; J. G. Correia; E Rita; J. P. Araújo; J.C. Soares
Nuclear Instruments & Methods in Physics Research Section B-beam Interactions With Materials and Atoms | 2006
E Rita; E. Alves; Ulrich Wahl; J. G. Correia; M.J. Soares; A.J. Neves; Marco Peres