Ultrafast Dynamics of Phase Change Materials

Phase-change memory technology relies on the electrical and optical properties of certain materials, such as multi-component chalcogenides, changing substantially when the atomic structure of the material is altered by laser or electrical heating. For example, laser switching Ge2Sb2Te5 (GST) alloy from its amorphous phase with localized covalent bonding to its delocalized, resonantly bonded meta- stable cubic crystalline phase decreases the resistivity by three orders of magnitude and significantly increases the reflectivity across the visible spectrum. Switching can occur on very short time scales of < 1 ps. Re-amorphisation occurs if the material is heated above its melting temperature and rapidly quenched. Despite widespread use (e.g. in rewritable optical discs), a lot of the underlying physics is controversial in both the ‘simple’ alloy and in interfacial PCMs (IPCMs), such as Sb2Te3/GeTe superlattices which have been designed to lower the threshold switching energy [1]. IPCMs also have high and low resistance phases but these are both crystalline and the structural transformation is associated with a change in the coordination number of Ge atoms. In this project you will use intense, single cycle pulses of ~ 100 fs duration THz radiation to drive the switching [2] and study the switching dynamics in strained superlattice IPCMs using near infrared and THz pump-probe spectroscopy. Of particular interest are the nature of the THz driven transition (is it intrinsically thermal or non-thermal?) and the dynamical differences (fundamental switching speed and energy) under different degrees of built-in biaxial strain (controlled during material growth by choice of Sb2Te3 layer thickness). The thin film materials for this work will be provided in a collaboration with Rob Simpson’s group in the Singapore University of Technology and Design which is pioneering work on strained IPCMs [3]. These fascinating materials display a wide range of behaviors which offer ample opportunities for broadening the project. For example, Sb2Te3 is a topological insulator and the superlattices show room temperature phenomena such as giant magnetoresistance [4] and anomalous magneto-optical Kerr-rotation [5] associated with strong spin-orbit coupling and topological features of the interfaces. Because the interface bandstructure possesses Dirac cones, iPCMs can also interact with long-wavelength light so that they are also of interest for THz devices such as detectors, modulators and tunable metamaterials for chemical sensing.

You will have access to extensive laser spectroscopy facilities together with the opportunity to benefit from in-house expertise in the underlying theory and the wider interactions and training opportunities offered by the group’s affiliations with the Bath-Bristol Centre for Doctoral Training in Condensed Matter Physics.