Abstract
Binary and ternary chalcogenides have recently attracted much attention due to their wide range of applications including phase-change memory materials, topological insulators, photonic switches, and thermoelectrics. These applications require a precise control of the number and mobility of charge carriers. Here, an unexpected charge-carrier transition in ternary compounds from the PbTe-Sb2 Te3 pseudo-binary line is reported. Upon thermal annealing, sputtered thin films of PbSb2 Te4 and Pb2 Sb2 Te5 undergo a transition in the temperature coefficient of resistance and in the type of the majority charge carriers from n-type to p-type. These transitions are observed upon increasing structural order within one crystallographic phase. To account for this striking observation, it is proposed that the Fermi energy shifts from the tail of the conduction band to the valence band because different levels of overall structural disorder lead to different predominant types of native point defects. This view is confirmed by an extensive computational study, revealing a transition from excess cations and SbPb for high levels of disorder to PbSb prevailing for low disorder. The findings will help fine-tune transport properties in certain chalcogenides via proper thermal treatment, with potential benefits for memories, thermoelectric material optimization, and neuromorphic devices.
Highlights
We assume considerably more Pb and Sb atoms than expected from the nominal composition. These excess atoms can occupy vacant lattice sites, leading to a decreased number of intrinsic, cationic vacancies. This assumption is supported by measurements of the film density with X-ray reflectometry (XRR) as shown in Figure 2c: The average density of films deposited with different working pressures exceeds the nominal density, which is derived from the X-ray diffraction (XRD) lattice constant assuming an ideal, rock-salt-like lattice containing 25% and 20% vacancies on the cation sites in PbSb2Te4 and Pb2Sb2Te5, respectively
Grazing incidence XRD measurements on thin films generally do not allow a quantitative evaluation of the peak intensities, the more pronounced intensity of the peaks associated with PbTe in Pb2Sb2Te5 compared to PbSb2Te4 strengthens the hypothesis of phase separation into the constituents, since a higher PbTe content in Pb2Sb2Te5 is expected
Regarding p-type defects, the additional Te atom occupying an empty site (TeV) and the Pb atom replacing an Sb atom (PbSb) are the least expensive in the disordered phase, but have formation energies that are positive for all growth conditions and are much less favorable than the three n-type defects discussed above: Even when considering the chemical potentials at point B, which most strongly penalize the addition of Sb while favoring the addition of Te, the energy difference between TeV and SbV is still greater than 0.3 eV for PbSb2Te4 and greater than 0.4 eV for Pb2Sb2Te5
Summary
Inspired by the stoichiometric formulas of the well-studied phase-change materials GeSb2Te4 and Ge2Sb2Te5, thin films of PbSb2Te4 and Pb2Sb2Te5 are deposited from stoichiometric targets. These excess atoms can occupy vacant lattice sites, leading to a decreased number of intrinsic, cationic vacancies This assumption is supported by measurements of the film density with X-ray reflectometry (XRR) as shown in Figure 2c: The average density of films deposited with different working pressures exceeds the nominal density, which is derived from the XRD lattice constant assuming an ideal, rock-salt-like lattice containing 25% and 20% vacancies on the cation sites in PbSb2Te4 and Pb2Sb2Te5, respectively. Further annealing experiments of newly generated samples on SiO2 capped with (ZnS)80:(SiO2), conducted in an Ar protective gas atmosphere, exhibit additional XRD peaks for higher annealing temperatures (250–300 °C); see Figure S2, Supporting Information These peaks are not associated with the cubic phase of the alloys but stem from a separation into hexagonal Sb2Te3 and rock-salt-like PbTe. No transition to hexagonal PST is observed. Grazing incidence XRD measurements on thin films generally do not allow a quantitative evaluation of the peak intensities, the more pronounced intensity of the peaks associated with PbTe in Pb2Sb2Te5 compared to PbSb2Te4 strengthens the hypothesis of phase separation into the constituents, since a higher PbTe content in Pb2Sb2Te5 is expected
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