Ulrich Wahl

High-temperature annealing and optical activation of Eu-implanted GaN

Europium was implanted into GaN through a 10nm thick epitaxially grown AlN layer that protects the GaN surface during the implantation and also serves as a capping layer during the subsequent furnace annealing. Employing this AlN layer prevents the formation of an amorphous surface layer during the implantation. Furthermore, no dissociation of the crystal was observed by Rutherford backscattering and channeling measurements for annealing temperatures up to 1300°C. Remarkably, the intensity of the Eu related luminescence, as measured by cathodoluminescence at room temperature, increases by one order of magnitude within the studied annealing range between 1100 and 1300°C.

Near-band-edge slow luminescence in nominally undoped bulk ZnO

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.

Emission channeling studies of Pr in GaN

We report on the lattice location of Pr in thin film, single-crystalline hexagonal GaN using the emission channeling technique. The angular distribution of β− particles emitted by the radioactive isotope 143Pr was monitored by a position-sensitive electron detector following 60 keV room temperature implantation of the precursor isotope 143Cs at a dose of 1×1013 cm−2 and annealing up to 900 °C. Our experiments provide direct evidence that Pr is thermally stable at substitutional Ga sites.

Lattice location and thermal stability of implanted Fe in ZnO

The emission channeling technique was applied to evaluate the lattice location of implanted Fe59 in single-crystalline ZnO. The angular distribution of β− particles emitted by Fe59 was monitored with a position-sensitive electron detector, following 60 keV low dose (2.0×1013cm−2) room-temperature implantation of the precursor isotope Mn59. The emission patterns around the [0001], [1102],[1101], and [2113] directions revealed that following annealing at 800 °C, 95(8)% of the Fe atoms occupy ideal substitutional Zn sites with rms displacements of 0.06-0.09 A.

Implantation site of rare earths in single-crystalline ZnO

The lattice location of rare-earth 167mEr in single-crystalline hexagonal ZnO was studied by means of the emission channeling technique. Following 60-keV, room-temperature implantation of the precursor isotope 167Tm at doses of 1.3–2.8×1013 cm−2 and annealing up to 900 °C, the angular distribution of conversion electrons emitted by the radioactive isotope 167mEr was measured by a position-sensitive electron detector. The conversion electron emission patterns from 167mEr around the [0001], [1_102], [1_101], and [2_113] directions give direct evidence that the large majority of Er atoms (75%–90%) occupies substitutional Zn sites.

Direct evidence for Sb as a Zn site impurity in ZnO

The lattice location of ion implanted antimony in zinc oxide has been determined by means of β− emission channeling from the radioactive S124b isotope. Following 30 keV implantation of S124b into a single-crystalline ZnO sample to a fluence of 1×1014 cm−2, the angular-dependent emission rate of β− particles around several crystallographic directions was measured with a position-sensitive Si detector. The majority of Sb was found to occupy Zn sites, with the possible fraction on O sites being at maximum 5%–6%.

Electron emission channeling with position-sensitive detectors

Abstract Electron emission channeling allows direct lattice location studies of low doses of radioactive atoms implanted in single crystals. For that purpose the anisotropic emission yield of conversion electrons from the crystal surface is measured, most conveniently by use of position-sensitive detectors. We discuss characteristic features of this method, including quantitative data analysis procedures, which are achieved by fitting simulated two-dimensional emission distributions for different lattice sites to the experimental patterns. The capabilities of this approach are illustrated by the case of rare earth atoms (Er, Tm, Yb) in Si, where we were able to do lattice location experiments down to implanted doses which are 150 times lower compared to previous RBS studies.

EMISSION CHANNELING STUDIES OF LI IN SEMICONDUCTORS

Abstract The emission channeling and blocking technique is a direct method for lattice location of radioactive atoms in single crystals. In the case of α-emitting isotopes, position-sensitive detection of emitted particles has provided an efficient means of carrying out a large number of experiments, and Monte Carlo simulations of the channeling effect allow for rather accurate identification and quantitative determination of occupied lattice sites. This work reviews recent results on the lattice sites of Li in elemental and III–V semiconductors, including Si, GaAs, GaP, InP and InSb, obtained by emission channeling and blocking following ion implantation of 8Li at temperatures between 30 and 700 K. Relevant properties of Li in these semiconductors are also briefly reviewed, and emission channeling and blocking is discussed in relation to other experimental methods which give direct information on lattice sites and atomic configurations of defects in semiconductors. The experimental methods described in this work are also suitable to study other probe-atom-host-crystal systems (possible examples are Li and Na in II–VI semiconductors or diamond, Er and other rare-earth elements in semiconductors or metals, etc.) and an outlook on ongoing and future experiments is given.

Lattice location study of implanted In in Ge

We report on emission channeling experiments to determine the lattice location and the thermal stability of implanted I111n atoms in Ge. The majority of the In atoms was found on the substitutional site, which is a thermally stable site at least up to 500 °C. We also found strong indication that directly after implantation, a fraction of the implanted I111n atoms occupies the bond-centered (BC) site. This fraction disappears after annealing at 300 °C. From comparison with ab initio calculations, electrical studies, and perturbed angular correlation experiments, the In atoms on the BC site can be related to In-vacancy and In-self-interstitial defect complexes. The activation energy for dissociation of this BC related defect was found to be below 1.6 eV.

Lattice sites of ion implanted Li in diamond

Radioactive Li ions were implanted into natural IIa diamonds at temperatures between 100 and 900 K. Emission channeling patterns of α‐particles emitted in the nuclear decay of 8Li(t1/2=838 ms) were measured and, from a comparison with calculated emission channeling and blocking effects from Monte Carlo simulations, the lattice sites taken up by the Li ions were quantitatively determined. A fraction of 40(5)% of the implanted Li ions were found to be located on tetrahedral interstitial lattice sites, and 17(5)% on substitutional sites. The fractions of implanted Li on the two lattice sites showed no change with temperature, indicating that Li diffusion does not take place within the time window of our measurements.