Physics
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Item In Situ Growth and Doping Studies of Topological Insulator Bismuth Selenide(2015) Hellerstedt, John Thery; Fuhrer, Michael; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The past decade has borne witness to the rapid development of a new field of theoretical and experimental condensed matter physics commonly referred to as "topological insulators". In a (experimentalist's) single sentence a topological insulator can be thought of as material that behaves like an empty metal box: the i-dimensional material (i = 2,3) is insulating, but conducting states exist at the (i-1)-dimensional boundaries. These edge or surface states possess non-trivial properties that have generated interest and excitement for their potential utility in solving practical problems in spin electronics as well as the creation of condensed matter systems for realizing and testing new physics. The most extensively studied materials systems with these properties suffer a major drawback in that the interior of the metal box is not "empty" (insulating) but instead filled with metal. The goal of this work has been to understand why the box is full instead of empty, and explore a particular pathway to making it empty. To address these outstanding questions in the literature pertaining to the persistent n-doping of topological insulator Bi2Se3, a custom apparatus was developed for combined epitaxial thin film growth with simultaneous, in situ measurement of transport characteristics (resistivity, Hall carrier density and mobility). Bi2Se3 films are found to be n-doped before exposure to ambient conditions and this doping appears to be interfacial in origin. This work and methodology was extended to study the efficacy of an amorphous MoO3 capping layer. MoO3 acts as an electron acceptor, p-doping the Bi2Se3 up to a |∆n| = 1x10^13 cm−2 change in carrier density. Complimentary X-ray photoemission spectroscopy (XPS) on bulk crystals showed that this was enough to put the Fermi level into the topological regime. Thin films were too highly n-doped initially to reach the topological regime, but the same magnitude change in doping was observed via the Hall effect. Finally, a Bi2Se3 film with a 100 nm capping layer of MoO3 was vented to ambient, and found to retain a stable doping for days of ambient exposure, demonstrating the effectiveness of MoO3 for passivation.Item Broadband In-plane Relative Permittivity Characterization of Ruddlesden-Popper Sr(n+1)Ti(n)O(3n+1) Thin Films(2010) Orloff, Nathan Daniel; Takeuchi, Ichiro; Booth, James C.; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)We present a broadband on-wafer measurement technique for the characterization of the in-plane complex relative permittivity of a thin-film test wafer and a companion substrate test wafer from 100 Hz to 40 GHz, and potentially to 110 GHz. From 100 Hz to 300 MHz, the approach uses an ensemble of interdigitated capacitors with different interdigitated active lengths l = (0.100 mm, 0.325 mm, 0.875 mm, 1.835 mm, 2.9 mm) fabricated on both test wafers. Within this regime, from 100 Hz to 1 MHz, the measurements were performed with an inductance-capacitance-resistance meter. From 1 MHz to 300 MHz, the scattering parameters of the set of interdigitated capacitors were measured with a radio frequency vector network analyzer. In the high frequency regime, 300 MHz to 40 GHz, we measure scattering parameters of a set of coplanar waveguides of active lengths l = (0.420 mm, 1.270 mm, 2.155 mm, 3.22 mm, 3.993 mm, 5.933 mm) fabricated on the test wafers. We extracted the capacitance and conductance of the interdigitated capacitors and coplanar waveguides on the test wafers for the appropriate frequency regimes. We then obtained a mapping function from 2D finite element simulations that relates the change in capacitance of the thin-film test wafer relative to the companion substrate test wafer to the real part of the in-plane relative permittivity. The imaginary part of the in-plane relative permittivity was obtained from the real part of the in-plane relative permittivity and the in-plane loss tangent. We applied this broadband dielectric spectroscopy technique to explore the frequency-dependent relative permittivity of unstrained Ruddlesden-Popper series Srn+1TinO3n+1(n=1, 2, 3) thin films as a function of temperature and dc electric field. At room temperature, the in-plane relative permittivities (K11) obtained for Srn+1TinO3n+1(n=1, 2, 3) were 42 plus/minus 3, 54 plus/minus3, and 77 plus/minus2, respectively, and were independent of frequency. At low temperatures, K11 increased with a behavior consistent with an incipient ferroelectric, and paraelectric behavior developed in Sr4Ti3O10(n=3). In 2004, J. H. Haeni, et al. showed that SrTiO3 (n = infinity) on DyScO3 (110) undergoes a ferroelectric to paraelectric phase transition around room temperature. As a means to understand the origins of the loss and tunability in strained SrTiO3 (n = infinity), we performed our broadband dielectric spectroscopy technique on epitaxial thin-films of Ruddlesden-Popper series Srn+1TinO3n+1(n=2, 3, 4, 5, 6) on the rare-earth scandate substrates, DyScO3 (110) and GdScO3 (110). For these thin films, DyScO3 (110) and GdScO3 (110) corresponded to biaxial tensile strain of approximately 1% and 1.7%, respectively. The thin films were 50 nm thick on DyScO3 (110) and 25 nm thick on GdScO3 (110), which ensured uniform strain throughout the film. We report the dependence of the critical temperature, tunability, and loss tangent on series number and strain at 1 MHz. We also examined the broadband frequency dependent dielectric properties of these thin films as a function of temperature, electric field, series number and strain.