Abstract
The group IVB TM elemental oxides, TiO2, ZrO2 and HfO2, have emerged as candidate materials for advanced gate dielectrics for scaled Si microelectronics. Additionally, complex oxides, comprised of TM oxides and ordinary oxides, or TM and rare earth (RE) atom oxides are also being considered by the microelectronics community in the context of combining microprocessor and memory Si chip functions with additional types of functionality derived from complex oxides. This functionality includes ferroelectric and/or ferromagnetic storage or switching, which are generally enabled by Jahn-Teller (J-T) effects. The properties and reliabilities of both elemental and complex TM oxides are closely correlated with intrinsic TM-atom bonding defects, where J-T local bonding distortions are expected to be important. Defect centers can also be associated with impurity atoms, e.g., TM atoms that are not a constituent of the host TM oxide. J-T distortions in defect centers can manifest themselves in two ways: (1) adversely, as traps and/or charged defects that reduce carrier transport, or (2) positively, as centers which provide a pathway to control of nano-grain symmetry and thin film morphology, and promote changes in long range order as required for ferroelectric or ferromagnetic behavior.
This chapter will address two issues relevant to J-T structural distortions in elemental oxides: (1) cooperative J-T distortions in group IVB TM elemental oxides; and (2) localized J-T distortions in defect states in deposited thin film nanocrystalline TM elemental oxides. Each of these issues is addressed at two levels: (1) experimental determination of electronic structure, including valence band and final states, and band edge defects based on synchrotron O K edge X-ray absorption and soft X-ray photoelectron spectroscopies, and spectroscopic ellipsometry in the visible and vacuum UV; and (2) energy level diagrams based either on ab-initio calculations, or symmetry adapted linear combinations (SALC’s) of TM and oxygen atomic states, including “text book” models that include the SALC’s as a basis set.
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Acknowledgements
The author acknowledges the contributions of his most recent Ph.D. students, Sanghyun Lee, Hyuntak Seo, Joseph P Long, and Jinwoo Kim, and his most recent post doctoral fellows, Les Fleming, Kwun-Bum Chung and Relja Vasic in contributing to the film deposition and spectroscopy studies. He also acknowledges the important discussions relative to theory with Professors Jerry Whitten and Mike Whangbo of the Department of Chemistry at NCSU. He also acknowledges collaborations relative to (1) vis-VUV SE with Professor Dave Aspnes at NCSU, and (2) depth resolved cathodolumiscence with Professor Len Brillson of The Ohio State University. He also acknowledges an ongoing collaboration with Professor Darrell Scholm of Pennsylvania State University, soon to be moving to Cornell University in preparing complex oxides. On going collaboration in XAS and SXPS studies with Marc Ulrich of ARO, and an adjunct faculty at NC State is also gratefully acknowledged. Finally, it is a pleasure to acknowledge collaborations with Jan Luning and Dennis Nordlund of the Stanford Synchrotron Research Laboratory (SSRL) for their assistance in mentoring me, my graduate students, and post doctoral fellows in the acquisition of XAS data, and the mounting of samples as well at Beam-Line 10–1 of SSRL. The research results presented in this review have been supported in part by the Semiconductor Research Corporation, the Air Force Office of Scientific Research sponsorship of a MURI program that is Administered and Lead by Vanderbilt University, the National Science Foundation, and the Defense Threat Reduction Agency. Finally, the author appreciates a collaboration with Professors Theodore H. Geballe and Robert M. White that dates back to the mid-1979’s when the author was a Senior Research Fellow and Laboratory Manager at the Xerox Palo Alto Research Center (Xerox PARC), and Professor White was a Principal Scientist and Area Manager at Xerox PARC as well. This collaboration addressed transition metal dichalcogenides in groups VB, and VIB as well as group IVB and introduced me to the first, second and third row transition metal dichalcogenides, and the differences that a few additional electrons could make in the infra-red effective charges.
The features above the top of the valence band in the SXPS spectrum are associated with localized defects in the annealed film, and band-tail defects in the as-deposited film. There are inherent differences in the matrix elements for transitions for band to band, and impurity to impurity absorptions that result in differences in the band edge and defect absorption constants, a, and e2, as deduced from SE measurements. Therefore the relative strength in spectra must be adjusted according in the context of the N-sum rule [13]. The direct spectral estimates are factors of ∼ 20 –50-fold higher than the actual defect densities, bringing the spectroscopic results into good agreement with electrical I–V and C–V results. Typical defect densities in the high-temperature annealed films are in the regime of 1012 cm-2 or equivalently 1018 cm-3.
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Lucovsky, G. (2009). Long Range Cooperative and Local Jahn-Teller Effects in Nanocrystalline Transition Metal Thin Films. In: Köppel, H., Yarkony, D., Barentzen, H. (eds) The Jahn-Teller Effect. Springer Series in Chemical Physics, vol 97. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-03432-9_24
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DOI: https://doi.org/10.1007/978-3-642-03432-9_24
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