Emission channeling studies of Indium Phosphide at low temperatures at CERN-ISOLDE

Tese de mestrado, Física (Física Nuclear e Partículas), Universidade de Lisboa, Faculdade de Ciências, 2009111In radioactive atoms were implanted into a single crystal of InP. After annealing for lattice recovery of implantation defects, the lattice site location of 111In/111Cd was studied with the emission channelling technique, from room temperature ( 300K) down to 50K at CERN-ISOLDE 2.This work aims to test a recently developed cooling station for emission channelling experiments. InP is a material with a relatively low Debye temperature, where significant changes of atomic vibrations are expected with temperature, thus providing an ideal test ground of the effects, which can be expected to influence the data, i.e., dechanneling from lattice vibration and changes of the root mean square displacement (r.m.s.) of the atomic position of the probe atom. In the future we intend to apply these studies to monitor individual impurities or lattice constituents, with temperature, upon phase transitions as well as studying lattice sites of dopants implanted at low temperature. 2"The On-Line Isotope Mass Separator ISOLDE is a facility dedicated to the production of a large variety of radioactive ion beams for a great number of different experiments" http://isolde.web.cern.ch/ISOLDE

Lattice site location of electrical dopant impurities in group-III nitrides

The group III nitrides (GaN, InN, AlN ) are semiconductors with a large band gap, which can be adjusted combining the different nitrides into alloys. Also these compounds have very interesting electrical and optical properties and these properties depend largely on the doping of the material. Due to these characteristics, and to add on their scientific interest, the nitride semiconductors have wide applications in light emitting diodes. Although in order to create a diode a semiconductor is doped to create a p-n junction and these semiconductors are naturally n-type, which means the p-type doping is quite challenging. The main p-type dopant used for these nitrides is magnesium (Mg). However the activation of Mg in the group III nitrides, and consequently its efficiency in the material, depends largely on the precise location it occupies in the compound crystal structure, i.e. its lattice site. The p-type doping occurs when Mg substitutes a Ga, In or Al atom in the crystal structure, if on the other hand it goes to an interstitial or a N substitutional site, the Mg can induce negatively harged defects acting as n-type dopant. The aim of this phd is to study the lattice site location of Mg in the group III nitrides. One of the most accurate techniques used to determine the lattice site location of an impurity in a crystal is emission channeling (EC). Radioactive isotopes of the element to study are used as probes. The isotopes are typically implanted in very low fluences (ppm) using the ISOLDE-CERN facility. Upon the radioactive decay, the emitted charged particles are channeled along the crystal high-symmetry directions, due to the periodically arranged positive nuclei centers. From the comparison of the experimental emitted particle anisotropy to the theoretically calculated patterns, along different orientations of the crystal, one can determine the projections of the probes precise location. This way we pin point the lattice site location of the impurity that was implanted, with a precision up to 0.02nm. In the framework of this phd, the short-lived (9.5min) isotope 27Mg will be implanted in single-crystal GaN, AlN and InN samples. Since the life time of the isotope is educed, a chamber will be used to implant and measure the EC patterns on-line with the ISOLDE beam line. There are many factors to study that can influence the location of Mg in this type of semiconductors, for example: the fluence of Mg implanted; the temperature of the sample during the implantation; the defects and presence of other impurities as oxygen and hydrogen; the proportion of Ga, In and Al in the nitride in the case of alloys . Also there are many factors that can influence the measurement of the lattice site location using EC, ergo this study will be complemented with characterization techniques, i. e. the crystalline structure, purity and mosaicity of the samples will be evaluated using RBS, HRXRD and other techniques available in Leuven. Moreover this study can be completed with a theoretical approach done using ab initio simulations to determine the Mg lattice site in crystal that would imply less energy. The EC simulations are calculated using approximations both in the potentials and in the phonon spectrum. This phd involves a large quantity of EC experiments and therefore to complement it an improvement on the simulations will be attempted. To remove the approximations of the simulations an ab initio approach will be used. This study will include some extra emission channeling experiments.status: publishe

Characterisation of microbial attack on archaeological bone

As part of an EU funded project to investigate the factors influencing bone preservation in the archaeological record, more than 250 bones from 41 archaeological sites in five countries spanning four climatic regions were studied for diagenetic alteration. Sites were selected to cover a range of environmental conditions and archaeological contexts. Microscopic and physical (mercury intrusion porosimetry) analyses of these bones revealed that the majority (68%) had suffered microbial attack. Furthermore, significant differences were found between animal and human bone in both the state of preservation and the type of microbial attack present. These differences in preservation might result from differences in early taphonomy of the bones. © 2003 Elsevier Science Ltd. All rights reserved