Electron Paramagnetic Resonance of Material Properties and Processes
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ELECTRON PARAMAGNETIC RESONANCE OF MATERIAL PROPERTIES AND PROCESSES*
K. L. BROWER Sandia National Laboratoriest,
Albuquerque, New Mexico 87185,
USA
ABSTRACT The purpose of this paper is to demonstrate, primarily for the non-specialist and within the context of new and recent achievements, the diagnostic value of electron paramagnetic resonance (EPR) in the study of material properties and processes. I have selected three EPR studies which demonstrate the elegance and uniqueness of EPR in atomic defect studies and exemplify unusual achievements through the use of new techniques for material measurement and preparation. A brief introduction into the origin, interaction, and detection of unpaired electrons is included.
INTRODUCTION This series of papers focuses on EPR studies of materials such as semiconductors, glasses, catalysis, amorphous Si:H, ... which are of relevance in today's technology. Our purpose and approach are intended to demonstrate, primarily for the benefit of the non-specialist and within the context of new and recent achievements, the diagnostic value of EPR in the study of material properties and processes. Following a brief introduction into some of the basic aspects of EPR in defect studies, I will highlight the results of EPR studies on 1) nitrogen in silicon by Brower and Peercy, 2) the boron interstitial in silicon by Troxell and Watkins--here the ideas of Anderson's negative U energy and possibly the Bourgoin mechanism are exemplified, and 3) Si-Si0 2 interface states
by Poindexter,
Caplan,
Deal,
and Razouk.
EPR IN DEFECT STUDIES Origin of Unpaired Electrons It is convenient to characterize the electronic structure of a semiconductor or insulator in terms of a band structure in which, ideally, the valence band is filled with electrons and the conduction band, which is separated from the The chemical bonding of these - 1023 valence band by an energy gap, is empty. 3 valence electrons/cm is usually such that they are spin paired according to the Pauli principle. Under these conditions the solid is diamagnetic and yields a null EPR spectrum. Imperfections such as vacancies, interstitials, impurities, ... in solids often have localized states with one or more localized energy levels within the bandgap. Whether an unpaired electron is trapped in one of the localized states depends in the case of semiconductors on the position of the Fermi level and in the case of insulators on the availability of electrons or holes. The effects of ionizing irradiation, n- or p-type doping, light illumination, temperature, etc., will sometimes induce paramagnetism in existing defects. Also, defects *This article was sponsored by the U. S. Department of Energy, Basic Energy Sciences, under Contract DE-AC04-76-DP00789. tA U. S. Department of Energy facility.
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