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Carrier density control in Cu₂HgGeTe₄ and discovery of Hg₂GeTe₄via phase boundary mapping

Ortiz, Brenden R., Gordiz, Kiarash, Gomes, Lídia C., Braden, Tara, Adamczyk, Jesse M., Qu, Jiaxing, Ertekin, Elif, Toberer, Eric S.
Journal of materials chemistry A 2019 v.7 no.2 pp. 621-631
meta-analysis, prediction, semiconductors, solid solutions, surveys
The optimization and application of new functional materials depends critically on our ability to manipulate the charge carrier density. Despite predictions of good n-type thermoelectric performance in the quaternary telluride diamond-like semiconductors (e.g. Cu₂HgGeTe₄), our prior experimental survey indicates that the materials exhibit degenerate p-type carrier densities (>10²⁰ h⁺ cm⁻³) and resist extrinsic n-type doping. In this work, we apply the technique of phase boundary mapping to the Cu₂HgGeTe₄ system. We begin by creating the quaternary phase diagram through a mixture of literature meta-analysis and experimental synthesis, discovering a new material (Hg₂GeTe₄) in the process. We subsequently find that Hg₂GeTe₄ and Cu₂HgGeTe₄ share a full solid solution. An unusual affinity for CuHg and HgCᵤ formation within Cu₂HgGeTe₄ leads to a relatively complex phase diagram, rich with off-stoichiometry. Through subsequent probing of the fourteen pertinent composition-invariant points formed by the single-phase region, we achieve carrier density control ranging from degenerate (>10²¹ h⁺ cm⁻³) to non-degenerate (<10¹⁷ h⁺ cm⁻³) via manipulation of native defect formation. Furthermore, this work extends the concept of phase boundary mapping into the realm of solid solutions and clearly demonstrates the efficacy of the technique as a powerful experimental tool within complex systems.