R. Vianden

Photoluminescence and lattice location of Eu and Pr implanted GaN samples

Rare earth (RE) ions implanted GaN films were studied by optical spectroscopy and RBS techniques. Sharp emission lines due to intra-4f n shell transitions can be observed even at room temperature for the Eu 3+ and Pr 3+ .Th e photoluminescence spectra recorded by the above band gap excitation reveal dominant transitions due to the 5 D0- 7 F1,2,3 lines at 6004, 6211 and 6632 ( A for the Eu 3+ and 3 P0,1- 3 F2,3 at 6450 and 6518 ( A, respectively, for the Pr 3+ . We report on the temperature dependence of the intra-ionic emissions as well as on the lattice site location of the RE detailed angular scans through the / 00 0 1S and / 10 % S axial directions; which indicates that for Pr, complete substitutionality on the Ga sites was achieved while for Eu a Ga displaced site was found. r 2001 Elsevier Science B.V. All rights reserved.

SYSTEMATICS OF ELECTRIC FIELD GRADIENTS IN METALS

The intensive study of electric field gradients (EFG) at the site of atomic nuclei in noncubic metals has revealed several systematic trends, e.g. in the relation of the total EFG to the so called lattice EFG as well as in its variation with temperature. Numerous investigations have been carried out in order to test these systematics. The results will be reviewed and compared to the known trends. Progress in theoretical calculations of the EFG in pure and impurity — host systems will be discussed and compared to the latest available experimental data. Recent measurements of the EFG at metal surfaces and new calculations of the EFG at host sites in impure cubic metals may contribute to the understanding of the EFG in metals.

Optical doping of nitrides by ion implantation

A series of rare earth elements (RE) were implanted in GaN epilayers to study the lattice site location and optical activity. Rutherford backscattering spectrometry in the channeling mode (RBS/C) was used to follow the damage behavior in the Ga sublattice and the site location of the RE. For all the implanted elements (Ce, Pr, Dy, Er, and Lu) the results indicate the complete substitutionality on Ga sites after rapid thermal annealing at 1000°C for 2 min. The only exception occurs for Eu, which occupies a Ga displaced site. Annealing at 1200°C in nitrogen atmosphere at a pressure of 1GPa is necessary to achieve the complete recovery of the damage in the samples. After annealing the recombination processes of the implanted samples were studied by above and below band gap excitation. For Er implanted samples besides the 1.54 μm emission green and red emissions are also observed. Red emissions from 5D0 → 7F2 and 3P0 → 3F2 transitions were found in Eu and Pr implanted samples even at room temperature.

Lattice location of Tl and diffusion studies of Tl and Hf implanted in magnesium

Abstract The diffusion behavior of thallium and hafnium implanted in magnesium single crystals and the lattice location of thallium in magnesium were studied using the Rutherford backscattering/channeling technique. Annealing treatments in a helium atmosphere were carried out up to 875 K. The results obtained show that, in spite of both impurities being fully substitutional in the magnesium lattice, their diffusion coefficients are completely different, D Tl ⩾ (9 ± 7) × 10 −13 cm 2 s at 563 K, and D Hl = (12 ± 7) × 10 −15 cm 2 s at 875 K. A tentative explanation for these results is presented.

RBS/Channeling study of Er doped GaN films grown by MBE on Si(111) substrates

The influence of the Ga cell temperature on the quality of GaN films grown by MBE on p-Si(1 1 1) substrates was studied for cell temperatures (TGa) in the range from 865∞C to 922∞C using the RBS/Channeling technique. The films were in situ doped during growth with Er at a constant cell temperature. The films show a strong dependence of the crystalline quality on the Ga cell temperature with the best films grown at TGaa 915C. For temperatures TGa below 880∞C the films showed no channeling eAect. The thickness increases linearly with the temperature suggesting that changes in the Ga flux influence the growth process. The decrease of the Ga flux allows the incorporation of higher Er concentrations in the films. The data showed that a maximum value of about 0.35 at% was reached under the chosen growth conditions. The Er ions occupy mainly the Ga sublattice in the films with single crystalline quality. A comparison of the angular scans through the AE 0 0 01 ae and the AE 10 11 ae axes with Monte Carlo simulations leads to the conclusion that a majority (90%) of the Er ions occupies Ga sites. ” 2000 Elsevier Science B.V. All rights reserved.

Reversible changes in the lattice site structure for In implanted into GaN

The perturbed angular correlation method was employed to study the lattice environment of In implanted into GaN. It was found, after annealing the implantation induced damage, that 65% of the implanted atoms were situated in regular undisturbed Ga lattice sites. The remaining fraction showed an unusual behavior insofar as its lattice surroundings changed reversibly from undisturbed at temperatures above 600 K to strongly disturbed at low temperatures.

DEFECT-ACCEPTOR PAIRS IN GERMANIUM

The PAC probe111In was implanted into intrinsic, n-and p-doped germanium. After removing the radiation damage by thermal annealing the samples were irradiated with different lons to study defect-acceptor pairs by means of the PAC technique. In all samples a high electric field gradient could be observed, characterized by a quadrupole interaction frequency of υQ2=415MHz and an axial symmetry (η=0). Some samples show another frequency of υQ152.5 MHz and η=0. The results give evidence for an intrinsic defect trapped at the111In probe.

Vacancy trapping at111In in tungsten

TDPAC measurements were carried out in tungsten foils and single crystals using implanted111In as a probe nucleus. After damaging the bcc tungsten lattice by heavy ion implantation, electron- and proton-irradiation the trapping of three different defects in the temperature range 293–1,000 K was observed. They were characterized by quadrupole interaction frequencies of vQ1 = 142(2) MHz, vQ2 = 181(5) MHz and vQ3 = 263(5) MHz. The largest fraction of In nuclei experiencing electric field gradients corresponding to vQ1 and vQ2 was reached at app. 630 K whereas for vQ3 this maximum appeared at app. 850 K. The defect configurations producing the different electric field gradients could be identified as a monovacancy trapped in a nearest neighbour 〈111〉 position (vQ1), a double vacancy consisting of two adjacent vacancies in the nearest neighbour shell of the probe atom (vQ2) and possibly a larger cluster (vQ3).

The electric quadrupole interaction of the 134 keV state of197Hg in the metal hosts Cd, Zn, Sn and Ti studied by e−-γ TDPAC measurements

The electric quadrupole coupling constants of the lowest excited 5/2− state in197Hg have been measured by the e−-γ TDPAC method for the hosts Ti and Sn at room temperature and for Zn and Cd at various temperatures. The observed temperature dependences are approximately the same as those known for other impurities in the same host lattices. They are well described by a simpleT3/2 relation.

High temperature annealing of Er implanted GaN

Abstract Defect recovery, optical activation and diffusion of Er implanted GaN epilayers grown on sapphire were studied after annealing at 1000°C with proximity cap and 1200°C under nitrogen atmosphere at high pressure (1GPa). The erbium ions with 160 keV were implanted at room temperature to nominal fluences of 5×10 14 cm −2 and 5×10 15 cm −2 . Some samples were co-implanted with oxygen ions to study its influence on the Er behaviour. During implantation a large fraction of Er is incorporated in Ga sites of the GaN lattice for the samples implanted with lower dose. The implantation damage recovers almost completely after rapid thermal annealing (120 s) at 1000°C with proximity cap. The annealing has no influence on the Er profile. The increase of the annealing time leads to the degradation of the surface due to nitrogen loss. The samples implanted with higher fluence and exposed to the same annealing procedure display distinct behaviour depending on the presence of oxygen. In samples without oxygen, the recovery is faster and accompanied by the segregation of Er towards the surface. For samples containing oxygen the damage recovery proceeds slowly and the Er profile remains stable. Annealing at 1200°C in nitrogen atmosphere at a pressure of 1GPa promotes the complete recovery of the damage in the sample without oxygen. During this annealing, a fraction of Er diffuses into the bulk. After annealing the optical spectra reveal the presence of several sharp lines the intensity of which increases significantly with the annihilation of the implantation damage.