Doping and Sn-alloying are very important in Ge research, since they can be used to drive this material into direct gap conditions. Both perturbations have similar effects on the optical dielectric function: they redshift and broaden the critical point structures, and they also reduce the strength of the optical transitions. On the other hand, there are some fundamental differences between them, since doping leads to Pauli blocking of transitions, which does not occur when Ge is substituted by an iso-electronic impurity such as Sn.
Quite recently we documented the existence of special phase-filling singularities in the dielectric function of n-type Ge associated with optical transitions to states at the Fermi level. We also developed a theoretical formalism that allows us to compute this contribution to the dielectric function, and clearly distinguish between the phase-filling and alloying aspects of the dopant contribution to the dielectric function. Here we use these advanced tools to carry out a systematic comparison of the dielectric function of doped Ge with that of Sn-alloyed Ge.
Jose Menendez, Chi Xu, and John Kouvetakis, "Comparative study of the effects of doping and Sn-alloying on the band structure of Ge (Conference Presentation)," Proc. SPIE 10537, Silicon Photonics XIII, 105370P (Presented at SPIE OPTO: January 31, 2018; Published: 14 March 2018); https://doi.org/10.1117/12.2289424.5751504668001.
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