Many ‘mathematical phenomena’ find application and sometimesspectacular physical illustration in the physics of light. Conceptssuch as fractals, catastrophe theory, knots, infinity, zero, and evenwhen 1+1 fails to equal 2, are needed to understand rainbows, twinklingstarlight, sparkling seas, and simple experiments on interference,polarization and focusing, and oriental magic mirrors. The lecturewill be based on pictures (slides and demonstrations), and is nontechnicalyet intellectual.
22/01/2009 at 3:00 pm
Dr. V. Venugopal, Department of Physics, University of Genova, Italy
Seminar of General Interest
Lecture Hall Block A, Institute of Physics
Document Date:
Defects and thermal stability of ultrathin Cu films on Ta characterized by helium ion implantation
Defects and thermal stability of ultrathin Cu films on Ta characterized by helium ion implantation
Abstract
The study of the growth, structure and thermal stability of an fcc film likeCu on a bcc substrate like Ta is important for understanding heteroepitaxyand for many technological applications. Such a system is vital to study theeffect of lattice and thermal mistmatch between the film and the substrateon defects and thermal stability of the films. Cu/Ta is one of the materialcombinations employed in the current IC-metallization, where Cu is theinterconnect material and Ta is used as diffusion barrier. Ultrathin Cufilms on bcc substrates have been studied as a model bimetallic catalyst.This talk explains the use of thermal helium desorption spectrometry (THDS)to characterize defects and thermal stability of ultrathin Cu films (5-200Å) deposited on Ta (110) and Ta(100) in ultrahigh vacuum (1×10-10 mbar) byelectron beam evaporation. Helium ion energy used is 1000 or 75 eV in orderstudy induced or native defects in the sample. THDS yields information aboutdefects in the subsurface region of a film, down to 10-100 Å below thesurface, depending on the incident helium energy. After collisional slowdownand thermalization, the implanted helium atoms in the sample diffuseinterstitially until they leave the sample or encounter a defect such asvacancies and vacancy clusters where they are trapped. On heating thesample, the helium is released from these defects at temperatures that arecharacteristic of the helium-defect dissociation energies.Some of the salient results that will be discussed are as follows. Heliumrelease from ultrathin Cu films will be compared to bulk Cu. Cu films onTa(110) and Ta(100) at room temperature are metastable and on heating, thefilms transform into islands. The temperature at which this takes place isstrongly dependent on the Cu film thickness and for a given thickness occursat a lower temperature on Ta(100) than on Ta(110). There is evidence for thestressed states of both the Cu films and the Ta substrates. Helium releasefrom monovacancies and vacancy clusters in Cu films on Ta(110) and Ta(100)was detected at ~750 K and ~800-1000 K respectively. The sublimation of theCu films from the Ta substrates could be observed by the release of retainedhelium at ~1300 K.
22/01/2009 at 4:00 am
Prof. Michael Berry, Department of Physics, University of Bristol Tyndall Avenue, Bristol BS8 1TL,
The quantum physics of identical particles is embodiedin the connection between spin and statistics (the Pauli principle).For many decades it has been thought that this can be understood onlywith ideas from relativity and quantum field theory. But in fact itcan be understood in terms of ordinary few-body nonrelativisticquantum mechanics, if this is reformulated to take proper accountof the geometry of the indistinguishability of particles. This canbe made plausible by conjuring tricks with a belt. The ‘Pauli sign’ (-1)^2Semerges as a geometric phase factor.
