M.R. da Silva

Micron-scale analysis of SiC/SiCf composites using the new Lisbon nuclear microprobe

Abstract A new nuclear microprobe has now been commissioned at ITN, Lisbon. This paper first describes the layout and modes of operation of the new microprobe, which is located on a 3.1 MV single-ended Van de Graaff accelerator. Within two days of first commissioning the microprobe, a spatial resolution of ∼1.5 μm was achieved for backscattering analysis, and a resolution of ∼1.0 μm for transmission work. The steps taken to produce this resolution, and the remaining factors, which further limit it are discussed. The first results from this microprobe for the spatially resolved analysis of SiC/SiCf ceramic composites are presented here. These materials have applications in fusion technology and structural changes were investigated after exposure to lithium orthosilicate and lithium titanate breeder materials in fusion relevant conditions. Ti and Cr rich precipitates could be found in the samples that were exposed to lithium titanate.

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.

Direct identification of interstitial Mn in heavily p-type doped GaAs and evidence of its high thermal stability

We report on the lattice location of Mn in heavily p-type doped GaAs by means of β− emission channeling from the decay of M56n. The majority of the Mn atoms substitute for Ga and up to 31% occupy the tetrahedral interstitial site with As nearest neighbors. Contrary to the general belief, we find that interstitial Mn is immobile up to 400 °C, with an activation energy for diffusion of 1.7–2.3 eV. Such high thermal stability of interstitial Mn has significant implications on the strategies and prospects for achieving room temperature ferromagnetism in Ga1−xMnxAs.

Diluted manganese on the bond-centered site in germanium

The functional properties of Mn-doped Ge depend to large extent on the lattice location of the Mn impurities. Here, we present a lattice location study of implanted diluted Mn by means of electron emission channeling. Surprisingly, in addition to the expected substitutional lattice position, a large fraction of the Mn impurities occupies the bond-centered site. Corroborated by ab initio calculations, the bond-centered Mn is related to Mn-vacancy complexes. These unexpected results call for a reassessment of the theoretical studies on the electrical and magnetic behavior of Mn-doped Ge, hereby including the possible role of Mn-vacancy complexes.

Lattice location and thermal stability of implanted nickel in silicon studied by on-line emission channeling

We have studied the lattice location of implanted nickel in silicon, for different doping types (n, n+, and p+). By means of on-line emission channeling, 65Ni was identified on three different sites of the diamond lattice: ideal substitutional sites, displaced bond-center towards substitutional sites (near-BC), and displaced tetrahedral interstitial towards anti-bonding sites (near-T). We suggest that the large majority of the observed lattice sites are not related to the isolated form of Ni but rather to its trapping into vacancy-related defects produced during the implantation. While near-BC sites are prominent after annealing up to 300–500 °C, near-T sites are preferred after 500–600 °C anneals. Long-range diffusion starts at 600–700 °C. We show evidence of Ni diffusion towards the surface and its further trapping on near-T sites at the Rp/2 region, providing a clear picture of the microscopic mechanism of Ni gettering by vacancy-type defects. The high thermal stability of near-BC sites in n+-type Si, ...

Mixed Zn and O substitution of Co and Mn in ZnO

The physical properties of an impurity atom in a semiconductor are primarily determined by the lattice site it occupies. In general, this occupancy can be correctly predicted based on chemical intuition, but not always. We report on one such exception in the dilute magnetic semiconductors Co- and Mn-doped ZnO, experimentally determining the lattice location of Co and Mn using {beta}{sup -}-emission channeling from the decay of radioactive {sup 61}Co and {sup 56}Mn implanted at the ISOLDE facility at CERN. Surprisingly, in addition to the majority substituting for Zn, we find up to 18% (27%) of the Co (Mn) atoms in O sites, which is virtually unaffected by thermal annealing up to 900 deg. C. We discuss how this anion site configuration, which had never been considered before for any transition metal in any metal oxide material, may in fact have a low formation energy. This suggests a change in paradigm regarding transition-metal incorporation in ZnO and possibly other oxides and wide-gap semiconductors.

Epitaxial regrowth of C- and N-implanted silicon and α-quartz

Abstract We have investigated the epitaxial regrowth and diffusion during thermal annealing of C- and N-implanted Si and α-quartz, utilising Rutherford Backscattering Spectroscopy in Channeling geometry (RBS-C) and nuclear reaction analysis (NRA). In both materials solid phase epitaxial regrowth was observed to occur at temperatures around 550°C (Si) and 1000°C (SiO 2 ). Epitaxial regrowth in Si was found to be strongly retarded compared to self-ion-implantation and the epitaxial recrystallisation front stopped at about the depth of maximum implant concentration even after annealing at 900°C. In fact, the implanted N-concentration profile in Si was found to be unchanged up to this temperature. This points at a chemical reaction between Si and the implanted C- and N-atoms, which obviously suppresses epitaxial recrystallisation. In SiO 2 almost no nitrogen could be detected in the N-implanted sample at T ≥900°C, i.e. no significant amount of impurity atoms was present during recrystallisation, which may hinder or suppress the regrowth process. A similar behavior is expected for the implanted C-atoms. In fact, epitaxial recrystallisation in the C-implanted α-quartz sample was found to be almost complete, except for a small amount of defects in the recrystallised matrix.

Lattice Location Studies on Hafnium, Thallium and Lead Implanted Magnesium Single Crystals

The lattice location of hafnium, thallium and lead implanted into magnesium crystals has been studied using the Rutherford backscattering channeling technique with the 1.2 MeV 4He+ beam of the Van de Graaff - accelerator of LNETI – Sacavem. The results showed complete substitutionality for the hafnium implantation.This position was not affected by annealing at 300°C. For lead and thallium it was found that only a fraction of the implanted ions occupy substitutional lattice sites. After a similar annealing treatment all the implanted ions are observed at random positions. It will be shown that these data are in good agreement with recent predictions of Sood (4) and Singh and Zunger (5). However, they cannot be explained with the extended Miedema scheme (1,2).

Study of a gold-implanted magnesium metastable system☆

Abstract Studies on the AuMg system formed by implanting a low fluence of gold ions in magnesium single crystals were carried out using the Rutherford backscattering-channelling technique. A distorted tetrahedral location was observed for gold. Furnace annealings in a helium atmosphere were carried out up to 525 K. The results show a fast segregation of gold to the MgMgO interface where it precipitates as metallic aggregates. The observed migration of gold to the surface with an activation energy of about 0.21 eV should indicate that the interstitial-vacancy mechanism is the dominant process of the impurity segregation.

Identification of the interstitial Mn site in ferromagnetic (Ga,Mn)As

We determined the lattice location of Mn in ferromagnetic (Ga,Mn)As using the electron emission channeling technique. We show that interstitial Mn occupies the tetrahedral site with As nearest neighbors (TAs) both before and after thermal annealing at 200 °C, whereas the occupancy of the tetrahedral site with Ga nearest neighbors (TGa) is negligible. TAs is therefore the energetically favorable site for interstitial Mn in isolated form as well as when forming complexes with substitutional Mn. These results shed new light on the long standing controversy regarding TAs versus TGa occupancy of interstitial Mn in (Ga,Mn)As.