Theory of the electronic and optical properties of germanium-tin alloys

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O'Halloran, Edmond J.
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University College Cork
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The elemental group-IV materials silicon (Si) and germanium (Ge) see widespread use in conventional microelectronic and have been used to develop a wide range of passive photonic components. The presence of a fundamental indirect gap in both Si and Ge however, severely limits these materials application in the development of active photonic components, such as light emitting diodes or lasers. Due to the fact that the fundamental indirect band gap in Ge lies only 145 meV below it’s direct gap, there has recently been a surge of interest in engineering a direct band gap from Ge, with the approach of alloying Ge with fellow Group IV materials garnering particularly widespread attention. Theoretical and Experimental research suggest that with incorporation of just ∼ 3% Pb or ∼ 10% Sn in Ge a direct fundamental band gap can be achieved in materials that can be grown directly on Si substrates. Utilizing this approach of alloying Ge with a small amount of Sn, optical and electically pumped lasing has recently been demonstrated, A key requirement for the developement of more efficient active components utilizing these promising new alloys remains the need for a more complete understanding of the electronic band structure of these materials and the mechanism by which the band gap develops and transitions from indirect- to a fundamentally direct gap. Using first principles density functional theory (DFT) calculations we present a detailed comparison of the electronic structure evolution in Ge1−x(Sn,Pb)x alloys with increasing x content. Through our analysis of the alloy electronic structure we then identify and quantify the mechanisms responsible for the indirect- to direct-gap transition in Ge1−x(Sn,Pb)x alloys. Our results provide insight into the properties of these emerging group-IV alloys and quantify the impact that incorporation of small amounts of Sn or Pb has on optical transition rates and carrier mobilities.
Chemistry , Physics , Germanium , Group-iv , condensed matter
O'Halloran, E. J. 2020. Theory of the electronic and optical properties of germanium-tin alloys. PhD Thesis, University College Cork.