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

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.