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X-ray diffraction pattern of an undoped crystal. The inset is an azimuthal scan, exhibiting the expected threefold symmetry of the (015) diffraction peak.
FTIR transmittance spectra for both doped and undoped samples. Two Ca-doped samples have the smallest interband transition energy, . Samples with higher (-type) and lower (-type) Ca doping have a larger . This indicates that Ca is close to the compensate point. The inset is a picture of a cleaved Ca-doped crystal.
Interband transition energy, vs 3D carrier concentration, . Negative values in represent -type while positive values represent -type. The best fit of to the data results in and the reduced effective mass, . The insets show the sketches of interband transitions for - and -type samples. We use the same fitting parameters for both sides.
Temperature-dependent resistivity curves for both doped and undoped samples. One -type sample shows the insulating behavior. Its resistivity is two orders of magnitude greater than that of the metallic samples. The inset is the normalized resistivity vs .
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