Graduate Thesis Or Dissertation
 

Electronic behavior of purified Selenium-rich alloys

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  • A great concern about the effect of oxygen in Se samples has been solved by a purification method called the distillation-with-Al method. The electrical conductivity of liquid Se prepared by this purification method shows Arrehnius behavior with a single activation energy and positive thermopower within the experimental temperature range. This indicates that Se samples are oxygen-free. Therefore, we have the opportunity to study the electronic behavior for Se-rich Se[subscript 100-x]Te[subscript x], Se[subscript 100-x],T1[subscript x], and Se[subscript 100-x]As[subscript x], alloys. Measurements on σ and S of Se[subscript 100-x]Te[subscript x] have been made from pure Se to pure Te. In the Se-rich side, a large drop in S at small x and low temperatures, and a fast increase in lna at large x and high temperatures are observed. In this composition range, the electronic behavior is analyzed in three regimes. The primary transport is the Maxwell-Boltzmann transport at the mobility edge of the valence band. For x≤20 and at low temperatures, the behavior is described by a two-band transport mechanism with contributions from the mobility edge of the valence band and a donor band. For x≥40 and at high temperatures, the electronic transport is in the Fermi-Dirac range, reflecting movement of the Fermi energy into the valence band. In the Te-rich side, the behavior of these alloys changes from semiconducting to metallic. A transition to metallic behavior is shown by the studies of the electrical conductivity, thermopower and magnetic susceptibility at high temperatures for x=50 and 60. In the metallic range (x≥70), we infere the shape of the density of states at the Fermi energy from the dependence of S on σ. We also analyze the joint behavior of S, σ, and x[subscript p] to deduce information about the behavior of the Fermi energy E[subscript f] and related quantities as T and x are changed, and to evaluate the parameters describing the electronic structure. Reproducible behavior of σ and S is observed from 0<x≤4 for Se[subscript 100-x]T1[subscript x] as the result of the absence of oxides. Plots of σ[superscript 1/2] vs x serve as indirect quantitative evidence for nearly complete removal of oxygen from Se samples. Measurements of σ and S in Se[subscript 100-x]As[subscript x] alloys were made at 0<x≤25. The behavior of σ and S in Se[subscript 100-x]As[subscript x] alloys is similar to the behavior of σ and S in Se[subscript 100-x]Te[subscript x] alloys.
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