Disorder Control in Crystalline GeSb2Te4 and its Impact on Characteristic Length Scales

• Published in: Physica status solidi. PSS-RRL. Rapid research letters Vol. 13; no. 4
• Main Authors: Dück, Matthias M., Schäfer, Tobias, Jakobs, Stefan, Schön, Carl‐Friedrich, Niehaus, Hannah, Cojocaru‐Mirédin, Oana, Wuttig, Matthias
• Format: Journal Article
• Language: English
• Published: Berlin WILEY‐VCH Verlag Berlin GmbH 01-04-2019; Wiley Subscription Services, Inc
• Subjects: Crystal structure; Crystallinity; Current carriers; Deposition; disorder; electrical properties; Electrical resistivity; Grain boundaries; metal–insulator transition; Mica; Phase transitions; phase‐change materials; Scattering; structural properties; Substrates; Transport properties
• ISSN: 1862-6254; 1862-6270
• DOI: 10.1002/pssr.201800578

Crystalline GeSb2Te4 (GST) is a remarkable material, as it allows to continuously tune the electrical resistance by orders of magnitude without involving a structural phase transition or stoichiometric changes. While well‐ordered specimen are metallic, increasing amounts of disorder eventually lead to an insulating state with vanishing conductivity in the 0 K limit, but a similar number of charge carriers. Hence, GST provides ideal grounds to explore the impact of disorder on transport properties. Here, a sputter‐deposition process is employed that enables growing biaxially textured GST films with large grain sizes on mica substrates. The resulting films exhibit a systematic variation between metallic and truly insulating specimen upon varying deposition temperature. Transport measurements reveal that their electron mean free path can be altered by a factor of 20, while always remaining more than an order of magnitude smaller than the lateral grain size. This proves unequivocally that grain boundaries play a negligible role for electron scattering, while intra‐grain scattering, presumably by disordered vacancies, dominates. These findings underline that the insulating state and the system's evolution toward metallic conductivity are intrinsic properties of the material.
Precise disorder control in the phase‐change material GeSb2Te4 is achieved in samples deposited on mica substrates with near‐constant grain size and orientation independent of substrate temperature. By contrast, the electrical transport properties change drastically with deposition temperature and reveal a disorder‐dependent metal‐to‐insulator transition. The material system displays an exceptional interplay of length scales, with disorder being dominated by intra‐grain scattering.