Infrared and Visible Dielectric Function of Electroplated Bi[sub 2±x]Te[sub 3±x] Films Determined by Spectroscopic Ellipsometry

Abstract

The pseudodielectric functions of electroplated Bi 2±x Te 3±x thin films with atomic bismuth content varying from 1.8 to 2.2 were determined using spectroscopic ellipsometry in the energy range of 0.03-3.10 eV. The experimental dielectric functions show differences with the literature bulk single-crystal data which may be explained by a ca. 50% porosity in the electroplated films. In the visible range, the optical constants did not seem to depend on the film composition while in the infrared range a rough tendency can be established: the real part of the dielectric function shifted to lower values with increasing bismuth content while the imaginary part shifted toward higher values. In the infrared range, Tauc-Lorentz combined with Drude dispersion relations were successfully used. The energy bandgap E g was found to be about 0.11 eV independent of the film composition. This result is related to the minor evolution of the film composition. Moreover, the fundamental absorption edge was described by an indirect optical band-to-band transition. The thickness of the thinner films were estimated, which are comprised between 1.77 and 1.99 m following the samples. Electrical resistivity and energy-independent scattering time were found to be related to the bismuth content.V-VI binary compounds such as Bi 2 Te 3 , Sb 2 Te 3 , and Bi 2 Se 3 are narrow bandgap semiconductors. They are becoming quite interesting as materials for thermoelectric devices. Due to their high figureof-merit at room temperature, bismuth telluride ͑Bi 2 Te 3 ͒ and its selenide and antimony-doped compounds are considered to be the best materials for thermoelectric applications. 1 Many deposition techniques, such as sputtering, 2-4 metallorganic chemical vapor deposition, 5,6 and molecular beam epitaxy, 7-9 have been used to obtain thin films of bismuth telluride compounds. These deposition techniques have as drawback a rather high production cost. Electrodeposition 10-12 may offer a low-cost film growth method with a high deposition rate. Several electrochemical processes have been developed, leading to different compositions of Bi 2 Te 3 binary compounds. 12,13 The stoichiometry of these thermoelectric films directly influences their electronic and transport properties.Due to both experimental interest and theoretical importance, the optical properties of Bi 2 Te 3 were studied using crystals or thin films. The first results were performed on ingots with transmittance and reflectance measurements methods. 14-18 More recently, thin films were characterized by classical optical methods. [19][20][21][22] Only one work 20 was reported on thin films obtained by metallorganic chemical vapor deposition using spectroscopic ellipsometry ͑SE͒ in the visible range. Ellipsometry is an optical method for surface analysis based on the measurement of the change of the polarization state of a light beam during reflection. 23 From ellipsometry, the dielectric function of bulk materials is obtained analytically, while for thin films appropriate fitting is needed. Th...

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Characterizations of bismuth telluride films from Mott‐Schottky plot and spectroscopic ellipsometry

“…For this case, an additional fit parameter was added: the thickness of the layer (d). For both cases, the DF were determined in the IR range and modeled by combining the Drude model with the Tauc-Lorentz (TL) model, providing excellent fitting to experimental data: [6] ε…”

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Characterization of Bi2Te3Thin Films Grown by Pulse Electroplating

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