Nanosecond Probing of Phase Transition Properties in Chalcogenides Using Embedded Heater-Thermometer

Phase-change materials (PCM) are technologically attractive materials for non-volatile memory, neuromorphic computing, optoelectronics, and recently, radiofrequency (RF) applications. Chalcogenide PCMs present a dissimilar resistance between the crystalline (low-resistivity) and amorphous phase (high-resistivity). The phase transition is thermally activated, and its kinetics can span orders of magnitude in time, down to sub-nanosecond timescale. Hence, proper probing of the thermal actuation is required to understand the fundamental material properties of PCMs, particularly the physics of melt-quench processes in nanoscale devices, interface vs. bulk effects, drift, and threshold voltage phenomena. Research efforts have been made to characterize PCM using a microthermal stage (MTS) [1]. The MTS structure used either a top Pt heater for the thermal actuation of a lateral Ge2Sb2Te5 (GST) device or a Pt heater surrounding the GST. Thus, the MTS allowed for probing with thermal time scales on the order of microseconds or longer.
Four-terminal, in-line, indirectly heated phase-change switches (IPCS) have been proposed for high-performance RF applications, thanks to their state-of-the-art figure-of-merit (FOM)=1/(2πRONCOFF) [2, 3]. A typical IPCS consists of two RF ports in-line with the PCM (i.e., the RF path), separated by a small gap, and two terminals for thermal actuation using an embedded heater. The heater runs transversely to the direction of the RF path, under the PCM. The heater and the RF path are separated by a thin electrically isolating film (e.g., Si3N4). GeTe has been so far the material of choice for RF applications due to its low resistivity in the crystalline phase and relatively large resistivity contrast with the amorphous state, but recently an Sb7Te3 IPCS was demonstrated with similar FOM and improved energy efficiency [3].
In this study, we report on the use of IPCS device structure as a platform for electro-thermal characterization of phase transition properties of PCM in nanoscale films at nanosecond temporal resolution. The isolation between the heater and the PCM layer electrically decouples the heating pulse and PCM probing. Thus, the phase transition, as well as temperature-dependent properties up to temperatures of ~1100 K, can be measured during heating pulse application with ns resolution. Furthermore, since the heater is buried underneath the PCM, our setup allows for 1) steady-state PCM temperature validation using scanning thermal microscopy (SThM) from the top surface, 2) separation between contribution of contact (interface) and bulk effects by varying overlap and underlap between the heater and the PCM contacts, and 3) test different materials of interest without significant change in the fabrication process. We use GeTe as a prototype material with known melting temperature, crystallization kinetics, and surface treatment for low ohmic contacts [4]. Measurements are carried out using RF probes for the heater and the PCM to reduce the overshoot and ringing of short-pulses and produce clean waveforms for nanosecond probing. The experimental results are compared to a detailed finite element electro-thermal model to extract key material properties. Overall, our platform can be used to uncover the intriguing kinetics of crystallization and amorphization in chalcogenides and other amorphous semiconductors.