A.A. Melo

Current measurement on MeV energy ion beams

Abstract It is a crucial problem to obtain the number of incident particles in MeV energy ion beam techniques. To solve this problem a simple but powerful method, the transmission Faraday cup is suggested. A rotating beam chopper periodically detects the beam current. To eliminate the disturbing current of the secondary electrons it is surrounded on both sides by a Faraday cup. Additional mechanical shields as well as electrostatic and magnetic fields keep away the charged particles different from the ions of the incident beam. The (p, γ) reaction and He RBS test measurements show that the particle number reproducibility is better than 1% and 2%, respectively. The reproducibility in the charge measurement was found to be ~ 0.6%.

Incorporation and stability of erbium in sapphire by ion implantation

Abstract Precise results on the lattice site location and stability of Er implanted sapphire using the RBS/channeling technique are reported. The Er ions were implanted into 〈0001〉 and 〈01 1 0〉 cut samples of α-Al2O3 single crystals, at room and liquid nitrogen temperatures, with 200 keV energy at fluences between 8 × 1013 and 4 × 1015 Er+/cm2. The implantation of 8 × 1013 Er+/cm2 (0.01 at%) at room temperature leads to the incorporation of 70% of the Er ions near the free octahedral site (0.8 A displaced) along the c-axis. From the remaining fraction of Er ions, at least 20% can be in a tetrahedral site. At liquid nitrogen temperature the fluence of 6 × 1014 Er+/cm2 amorphizes sapphire, while at room temperature a fluence an order of magnitude higher produces only a damaged buried layer. The annealing at temperatures higher than 1200°C leads to the recrystallization of the amorphous layer, but the Er ions segregate to the surface or precipitate. For the samples implanted at room temperature, the annealing leads to a higher incorporation of Er in the sapphire lattice and only a small fraction segregates to the surface at 1500°C.

LATTICE SITE AND PHOTOLUMINESCENCE OF ERBIUM IMPLANTED IN ALPHA -AL2O3

Single-crystal sapphire (α–Al 2 O 3 ) was implanted at room temperature with 200 keV erbium ions to a fluence of 8 × 10 13 cm –2 . Ion channeling using 1.6 MeV He + shows that the crystal suffers little damage for this low dose implant. Angular scans through axial and planar directions in the crystal indicate that 70% of the Er atoms reside on displaced octahedral sites in the α–Al 2 O 3 lattice. As pure Al 2 O 3 has a high density of free octahedral sites, this explains why high concentrations of Er can be dissolved in this material. Smaller fractions of Er are found on tetrahedral (20%) and random (10%) sites. The samples exhibit strongly peaked photoluminescence spectra around 1.5 μm, due to intra-4 f transitions in Er 3+ , indicating the existence of well-defined sites for the luminescing Er 3+ ions. It is concluded that the octahedral site is the dominating optically active site in the lattice.

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.

A four-detector spectrometer for e−−γ PAC on-line with the ISOLDE-CERN isotope separator

Abstract A four-detector e−−γ spectrometer has been installed on-line with the ISOLDE isotope separator. The spectrometer consists of two magnetic lenses for detection of conversion electrons, and two BaF2 scintillators for γ-ray detection. The spectrometer has been equipped with a 20 kV pre-acceleration system which enables detection of conversion electrons down to 2 keV. Implantation and measurement can be performed simultaneously on a large temperature range by heating or cooling the sample holder. The advantages of using the e−−γ PAC technique on-line at ISOLDE are discussed.

Lattice location of Er in GaAs and Al0.5Ga0.5As layers grown by MBE on (100) GaAs substrates

Abstract The lattice site location of Er in semiconducting materials received considerable interest because its 4 I 13 2 −4 I 15 2 intra-4f shell luminescence, occuring at a wavelength of 1.54 μm, falls in the minimum absorption window for silica-based fibers. In the present work, results of a study of the lattice location of Er in GaAs:Er and Al 0.5 Ga 0.5 As:Er using the Rutherford backscattering/channeling method are presented. The GaAs:Er and Al 0.5 Ga 0.5 As:Er layers were approximately 1.6 μm thick and were grown by conventional elemental source molecular beam epitaxy on (100) GaAs substrates. These samples, with Er concentrations of the order of 10 19 to 10 20 cm −3 , presented intense photoluminescence. The RBS/channeling results show that Er atoms are located in the tetrahedral interstitial site in GaAs:Er, and about 70% of the Er atoms are slightly displaced from that site in Al 0.5 Ga 0.5 As:Er, the rest being in substitutional sites. For comparison an epitaxial Al 0.5 Ga 0.5 As:Er layer with Er concentration of the order of 10 18 cm −3 which does not show photoluminescence has been analysed. In this case there is clear evidence that Er is substitutionally located.

Interface and structure characterization of co/re multilayers using rbs channeling and hyperfine interactions

Abstract Cobalt/rhenium multilayers were prepared by rf and dc sputtering and studied using RBS and hyperfine interaction techniques. The layer thickness is determined with a precision of 3A, and the interface intermixing is less than 5Aup to 773 K. Solid state reactions occur for higher temperatures. The hyperfine field value confirms the hcp structure of the Co. A small shift of the hyperfine field measured as function of the cobalt layer thickness is explained by the effect of lattice stresses.

Diffusion and corrosion behaviour of tungsten-implanted Aluminium and the Al12W phase

Abstract Depth profiles of tungsten implanted into aluminium were studied as a function of the annealing temperatures using the Rutherford backscattering technique. The formation of the Al12W metastable alloy starts at a temperature of 450° C. Electrochemical corrosion studies show a shift in the corrosion potential of + 600 mV for the as-implanted samples relative to the nonimplanted ones. This shift is 50 mV lower for samples where the Al12W metastable alloy has been formed. However, the implanted samples show higher corrosion rates than the nonimplanted ones.

Oxygen gettering by hafnium implanted in beryllium: A 〈0001〉 Hf‐O dumbbell?

The interaction of hafnium implanted into beryllium single crystals with diffusing oxygen was studied using hyperfine interaction and Rutherford backscattering channeling techniques. It was observed that oxygen is trapped at hafnium in a well defined lattice position. The formation of a 〈0001〉 Hf‐O mixed dumbbell in the tetrahedral interstitial cage of the beryllium lattice is suggested to explain the experimental results.

Hyperfine fields of mercury in single‐crystalline cobalt

The combined hyperfine interaction of mercury in a hexagonal close‐packed cobalt single crystal was measured using the e−−γ perturbed angular correlation technique with the 197Hg and 199Hg probes, without applied magnetic field. The magnetic and quadrupole coupling constants of the 5/2− levels in both 197Hg and 199Hg were measured with extremely high precision, ωL( 197Hg)=951(9) Mrad/s, ωL (199Hg)=1039(10) Mrad/s, vQ (197Hg)=3.7(5) MHz and vQ (199Hg)=32(2) MHz. Using the well‐known g factors and quadrupole moments of the respective levels, the magnetic hyperfine field, ‖Hhf(HgCo)‖=581(12) kG, and the electric field gradient, ‖Vzz(HgCo)‖ =2.0(3)×1017 V/cm2, were derived. The importance of using these hyperfine probes for studying the microscopic structure of cobalt‐based multilayers is introduced and discussed.