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Electronic characterization of La_[0:33]Sr_[0:67]FeO₃ films grown by molecular beam epitaxy
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Electronic characterization of La_[0:33]Sr_[0:67]FeO₃ films grown by molecular beam epitaxy

Robert C. Devlin
Master of Science (M.S.), Drexel University
18 Jun 2013
DOI:
https://doi.org/10.17918/etd-4235
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Abstract

Electronic Structure Materials Science Thin Films
Increasing computational speed and power has been contingent upon the scaling of silicon transistors; however, these components are reaching fundamental physical limits in terms of size. Transition metal oxides have emerged as candidate materials for next generation, multifunctional electronic devices due to useful phenomena such as metal-to-insulator transitions. Crucial to the realization of these materials in devices, however, is the elucidation of fundamental electronic properties, specifically in thin films, and the mechanisms driving observed electronic phase transitions. This thesis examines the electronic properties of La_[1/3]Sr_[2/3]FeO₃ (LSFO) thin films which undergo an abrupt phase transition from a highly conductive phase to an insulating phase. This phase transition is accompanied by an order of magnitude jump in resistivity over a 10K temperature range and an ordering of charge along the [111] direction. Strain effects on the electronic phase transition in LSFO films were explored through electronic transport and synchrotron x-ray diffraction of films grown by molecular beam epitaxy on substrates which imparted increasing tensile strain. It was found that the different substrates drastically alter the electronic transport of thin films, with increasing strain diminishing the abruptness of the transition. Synchrotron measurements confirmed that, in spite of the changes in transport properties, there is still an ordering of charge below the electronic phase transition and that the direction of this ordering is still along the [111] independent of strain state. Carrier behavior in LSFO films was also explored by fitting transport data to different conduction models and measuring carrier concentrations and mobilities. The conduction mechanism fits show that transport in LSFO films is dominated by weakly insulating behavior due to small polaron conduction at high temperatures and a change in conduction mechanism below the transition temperature to variable range hopping. The Hall effect measurements revealed near-constant carrier concentrations in the high temperature phase followed by a nearly two orders of magnitude decrease in free carriers in the low temperature phase. This decrease in carrier concentration was accompanied by an increase in carrier mobility by a factor of 10.

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