Optical spectroscopy studies of ion bombarded gallium arsenide and vanadium carbide thin films

Abstract This thesis gives a report on two projects involving the use of the optical techniques of surface Brillouin scattering and Raman spectroscopy to study thin layered materials. In the first case, the thin layered material is formed from ion bombardment of a semiconductor which results in an amorphous layer on a single crystal substrate. The second case involved the study of the elastic properties of vanadium carbide deposited by RF magnetron sputtering on silicon carbide and silicon substrates. There have been a number of reports on the recrystallization of ion implanted GaAs in which it has been noted that it is not a single step process as observed in Ge and Si. These studies have used ion channelling and Raman spectroscopy techniques to establish that the nature of the damaged gallium arsenide layer depends on the implantation conditions such as substrate temperature, implantation ion energy, and fluence. In the present work, the investigation of the recrystallization of ion implanted GaAs using the two optical spectroscopic techniques of surface Brillouin scattering (SBS) and Raman spectroscopy has been carried out for the first time. Single crystals of (100) gallium arsenide were implanted with 100 keV argon ions at liquid nitrogen temperature using ion fluences of 2 × 1014 ions/cm2 and 1 × 1015 ions/cm2 on samples denoted A and B respectively. Additionally, sample C was also implanted with 100 keV argon ions but at an elevated temperature of 50 oC. Raman spectroscopy and SBS measurements are in good agreement for samples A and B showing recrystallization below 300 oC in both cases. Raman measurements on sample A, implanted at the lowest fluence, showed that before annealing was carried out a completely amorphous layer was not achieved. In the early stages of isochronal annealing on both samples A and B, the transverse optical peak is observed to have higher intensity compared to the longitudinal optical peak. At higher annealing temperatures, the longitudinal peak attains higher intensity. Changes in the Rayleigh surface acoustic wave frequency from SBS measurements on Sample C during isochronal annealing are observed from 100 oC and saturation is obtained at about 450 oC. The Raman results show that recrystallization commences above 450 oC and saturates at about 600 oC implying complete recrystallization of the overlayer. Vanadium carbide is a refractory material known for its high hardness, corrosion resistance, high melting point, and high electrical conductivity. There have been a number of studies on the properties of vanadium carbides of different compositions and structures but to the best of the author’s knowledge, there has never been a study on the elastic properties of the thin deposited films. Thin films of vanadium carbide have been grown using RF magnetron sputtering with varying thicknesses on SiC and Si substrates. Different deposition times were used in order to have films with varying thicknesses. X-ray reflectometry, X-ray diffraction, Rutherford backscattering, and atomic force microscopy, have been used to determine the film characteristics such as thickness, roughness, density, and stoichiometry. Surface Brillouin scattering was then carried out. The SBS results were used together with a least squares fitting approach to determine the elastic constants of the VC films computationally. A Green’s function approach was used to verify the results of the fitting procedure.EM201

Spatial dispersion in phonon focusing

The ballistic phonon flux emanating from a point–like heat source in a crystal shows strong directional dependence. This effect is called phonon focusing and is measured using a technique called phonon imaging. In situations where long wavelength phonons are involved, the observations can be explained on the basis of classical continuum elasticity theory. Dispersion i.e. the variation of velocity with wavelength, sets in when the phonon wavelengths become comparable to the natural scale of length of the material, the lattice constant. This has a significant effect in the phonon focusing pattern and causes shorter wavelength phonons to lag behind longer wavelength ones and the dispersion relation i.e. the relation between angular frequency ω and wave number k becomes non-linear. A number of studies have used lattice dynamics models to explain the observed dispersive phonon images. Measured phonon images are not entirely satisfactorily reproduced by any of these lattice dynamics models and the different models tend to predict somewhat different focusing patterns. In this thesis, we set out to explain the observed dispersive phonon focusing patterns of cubic crystals by using a modification of continuum elasticity theory. This is done by including third and fourth order spatial derivatives of the displacement field in the wave equation. The coefficients of these higher order terms are the dispersive elastic constants. They are determined through optimized fitting to frequency versus wave vector data extracted from neutron scattering experiments for the acoustic modes in symmetry directions of a number of cubic crystals. Our approach is limited to the first onset of spatial dispersion and does not apply to near Brillouin zone boundary phonons. It is also applicable to crystals of any symmetry but in this thesis we focus on crystals of cubic symmetry. We report results on two crystals with a centre of inversion, Ge and Si, and two crystals without a centre of inversion, InSb and GaAs

Osteopathology and insect traces in the Australopithecus africanus skeleton StW 431

We present the first application of high-resolution micro computed tomography in an analysis of both the internal and external morphology of the lumbar region of StW 431 – a hominin skeleton recovered from Member 4 infill of the Sterkfontein Caves (South Africa) in 1987. The lumbar vertebrae of the individual present a number of proliferative and erosive bony processes, which were investigated in this study. Investigations suggest a complex history of taphonomic alteration to pre-existing spinal degenerative joint disease (SDJD) as well as post-mortem modification by an unknown insect. This study is in agreement with previous pathological diagnoses of SDJD which affected StW 431 and is the first time insect traces on this hominin are described. The results of this analysis attest to the complex series of post-mortem processes affecting the Sterkfontein site and its fossil assemblages

The inner craniodental anatomy of the Papio specimen U.W. 88-886 from the Early Pleistocene site of Malapa, Gauteng, South Africa

Cercopithecoids represent an essential component of the Plio-Pleistocene faunal assemblage. However, despite the abundance of the cercopithecoid fossil remains in African Plio-Pleistocene deposits, the chronological and geographic contexts from which the modern baboons (i.e. Papio hamadryas ssp.) emerged are still debated. The recently discovered Papio (hamadryas) angusticeps specimen (U.W. 88-886) from the Australopithecus sediba-bearing site of Malapa, Gauteng, South Africa, may represent the first modern baboon occurrence in the fossil record. Given the implication of U.W. 88-886 for the understanding of the papionin evolutionary history and the potential of internal craniodental structures for exploring evolutionary trends in fossil monkey taxa, we use X-ray microtomography to investigate the inner craniodental anatomy of this critical specimen. Our goal is to provide additional evidence to examine the origins of modern baboons. In particular, we explore (i) the tissue proportions and the dentine topographic distribution in dental roots and (ii) the endocranial organization. Consistent with the previous description and metrical analyses of its external cranial morphology, U.W. 88-886 shares internal craniodental anatomy similarities with Plio-Pleistocene and modern Papio, supporting its attribution to Papio (hamadryas) angusticeps. Interestingly, average dentine thickness and distribution in U.W. 88-886 fit more closely to the extinct Papio condition, while the sulcal pattern and relative dentine thickness are more like the extant Papio states. Besides providing additional evidence for characterizing South African fossil papionins, our study sheds new light on the polarity of inner craniodental features in the papionin lineage.https://www.wits.ac.za/esi/palaeontologia-africanapm2020Anatom

The inner craniodental anatomy of the Papio specimen U.W. 88-886 from the Early Pleistocene site of Malapa, Gauteng, South Africa

Cercopithecoids represent an essential component of the Plio-Pleistocene faunal assemblage. However, despite the abundance of the cercopithecoid fossil remains in African Plio-Pleistocene deposits, the chronological and geographic contexts from which the modern baboons (i.e. Papio hamadryas ssp.) emerged are still debated. The recently discovered Papio (hamadryas) angusticeps specimen (U.W. 88-886) from the Australopithecus sediba-bearing site of Malapa, Gauteng, South Africa, may represent the first modern baboon occurrence in the fossil record. Given the implication of U.W. 88-886 for the understanding of the papionin evolutionary history and the potential of internal craniodental structures for exploring evolutionary trends in fossil monkey taxa, we use X-ray microtomography to investigate the inner craniodental anatomy of this critical specimen. Our goal is to provide additional evidence to examine the origins of modern baboons. In particular, we explore (i) the tissue proportions and the dentine topographic distribution in dental roots and (ii) the endocranial organization. Consistent with the previous description and metrical analyses of its external cranial morphology, U.W. 88-886 shares internal craniodental anatomy similarities with Plio-Pleistocene and modern Papio, supporting its attribution to Papio (hamadryas) angusticeps. Interestingly, average dentine thickness and distribution in U.W. 88-886 fit more closely to the extinct Papio condition, while the sulcal pattern and relative dentine thickness are more like the extant Papio states. Besides providing additional evidence for characterizing South African fossil papionins, our study sheds new light on the polarity of inner craniodental features in the papionin lineage.https://www.wits.ac.za/esi/palaeontologia-africanapm2020Anatom

The inner craniodental anatomy of the Papio specimen U.W. 88-886 from the Early Pleistocene site of Malapa, Gauteng, South Africa

Cercopithecoids represent an essential component of the Plio-Pleistocene faunal assemblage. However, despite the abundance of the cercopithecoid fossil remains in African Plio-Pleistocene deposits, the chronological and geographic contexts from which the modern baboons (i.e. Papio hamadryas ssp.) emerged are still debated. The recently discovered Papio (hamadryas) angusticeps specimen (U.W. 88-886) from the Australopithecus sediba-bearing site of Malapa, Gauteng, South Africa, may represent the first modern baboon occurrence in the fossil record. Given the implication of U.W. 88-886 for the understanding of the papionin evolutionary history and the potential of internal craniodental structures for exploring evolutionary trends in fossil monkey taxa, we use X-ray microtomography to investigate the inner craniodental anatomy of this critical specimen. Our goal is to provide additional evidence to examine the origins of modern baboons. In particular, we explore (i) the tissue proportions and the dentine topographic distribution in dental roots and (ii) the endocranial organization. Consistent with the previous description and metrical analyses of its external cranial morphology, U.W. 88-886 shares internal craniodental anatomy similarities with Plio-Pleistocene and modern Papio, supporting its attribution to Papio (hamadryas) angusticeps. Interestingly, average dentine thickness and distribution in U.W. 88-886 fit more closely to the extinct Papio condition, while the sulcal pattern and relative dentine thickness are more like the extant Papio states. Besides providing additional evidence for characterizing South African fossil papionins, our study sheds new light on the polarity of inner craniodental features in the papionin lineage.https://www.wits.ac.za/esi/palaeontologia-africanapm2020Anatom

The atlas of StW 573 and the late emergence of human-like head mobility and brain metabolism

Functional morphology of the atlas reflects multiple aspects of an organism’s biology. More specifically, its shape indicates patterns of head mobility, while the size of its vascular foramina reflects blood flow to the brain. Anatomy and function of the early hominin atlas, and thus, its evolutionary history, are poorly documented because of a paucity of fossilized material. Meticulous excavation, cleaning and high-resolution micro-CT scanning of the StW 573 (‘Little Foot’) skull has revealed the most complete early hominin atlas yet found, having been cemented by breccia in its displaced and flipped over position on the cranial base anterolateral to the foramen magnum. Description and landmark-free morphometric analyses of the StW 573 atlas, along with other less complete hominin atlases from Sterkfontein (StW 679) and Hadar (AL 333-83), confirm the presence of an arboreal component in the positional repertoire of Australopithecus. Finally, assessment of the cross-sectional areas of the transverse foramina of the atlas and the left carotid canal in StW 573 further suggests there may have been lower metabolic costs for cerebral tissues in this hominin than have been attributed to extant humans and may support the idea that blood perfusion of these tissues increased over the course of hominin evolution.The DST-NRF for sponsoring the Micro-XCT facility at Necsa, and the DST-NRF and Wits University for funding the microfocus X-ray CT facility in the ESI. The Ghent University Special Research Fund (BOF-UGent) for the financial support of the Centre of Expertise UGCT (BOF.EXP.2017.0007), the Sterkfontein excavations and MicroCT scanning work have been provided by National Research Foundation and by PAST.http://www.nature.com/srepam2021Anatom

What is South African early Homo? New insights from the molar endostructural signature

International audienc

The atlas of StW 573 and the late emergence of human-like head mobility and brain metabolism

International audienceFunctional morphology of the atlas reflects multiple aspects of an organism’s biology. More specifically, its shape indicates patterns of head mobility, while the size of its vascular foramina reflects blood flow to the brain. Anatomy and function of the early hominin atlas, and thus, its evolutionary history, are poorly documented because of a paucity of fossilized material. Meticulous excavation, cleaning and high-resolution micro-CT scanning of the StW 573 (‘Little Foot’) skull has revealed the most complete early hominin atlas yet found, having been cemented by breccia in its displaced and flipped over position on the cranial base anterolateral to the foramen magnum. Description and landmark-free morphometric analyses of the StW 573 atlas, along with other less complete hominin atlases from Sterkfontein (StW 679) and Hadar (AL 333-83), confirm the presence of an arboreal component in the positional repertoire of Australopithecus. Finally, assessment of the cross-sectional areas of the transverse foramina of the atlas and the left carotid canal in StW 573 further suggests there may have been lower metabolic costs for cerebral tissues in this hominin than have been attributed to extant humans and may support the idea that blood perfusion of these tissues increased over the course of hominin evolution