We report the study of infrared spectroscopy of intraband transitions in Ge/Si quantum dot superlattices. The superlattices, which were grown on (001) oriented Si substrates by a solid source molecular beam epitaxy system, are composed mainly of 20 or 30 periods of Ge dot layers and Si spacer films. The structural properties of them and of the uncapped Ge dots grown on the surfaces of some of them were tested by cross-sectional transmission electron and atomic force microscopes, respectively. It is found that the Ge quantum dots have flat lens-like shapes. Infrared absorption signals peaking in the mid-infrared range were observed using Fourier transform infrared and Raman scattering spectroscopy techniques. Experimental and theoretical analysis suggests that the mid-infrared response be attributed to intraband transitions within the valence band of the Ge quantum dots in the superlattices. The fact that the intraband absorption is strongly polarized along the growth axis of the superlattices signifies that the Ge quantum dots with flat lens-like shapes perform as Ge/Si-based quantum wells. This study demonstrates the application potential of these kinds of Ge/Si quantum dot superlattices for developing mid-infrared photodetectors.
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