Isotopic constitution of semiconductors: manifestations in their electronic and vibrational spectra

Abstract

Semiconductors are prime examples of crystals which can be grown with unprecedented purity, imperfections being introduced deliberately in a controlled manner. However, even the chemically purest semiconductor is isotopically disordered as dictated by the natural abundance of elements. Diamonds have 98.9% ¹²C and 1.1% ¹³C; silicon consists of 92.23% ²⁸Si, 4.67% ²⁹Si and 3.1% ³⁰Si whereas germanium exhibits a large isotopic disorder originating in 21.23% ⁷⁰Ge, 27.66%⁷²Ge, 7.73% ⁷³Ge, 35.94% ⁷⁴Ge and 7.44% ⁷⁶Ge! The isotopic composition of single crystal diamonds, controlled by the starting material used for growth by chemical vapor deposition followed by high temperature growth, provides a splendid opportunity to address the subtle manifestations of zero-point motion in their Brillouin and Raman spectra. Isotopically pure Ge shows mass dependent small shifts in its electronic band structure which can however be detected in the signatures observed in its modulated reflection and transmission spectra. The local modes of oxygen in isotopically pure Ge are observed by infrared spectroscopy as exceptionally sharp lines with striking fine structure. Substitutional incorporation of the lighter elements (e.g., Mg, Mn, Ca, ...) in the II-VI semiconductors leads to sharp local modes (including isotopic shifts) in their infrared spectra.