Thermal Characterization of GeTe PCM based Reconfigurable Devices

Nanophotonic devices with adjustable optical response can be achieved through phase change materials (PCMs) such as Germanium Tellurium alloy (GeTe). GeTe can be rapidly melt-quenched or annealed between amorphous and crystalline states with large contrast of electrical and optical properties, making it a good candidate in reconfigurable electromagnetic devices, optical devices, and memory devices. However, challenges exist on the thermal aspect of GeTe PCM, where the cooling rate high as 1 K/ns from the melting point of 1000 K is required for quenching process. The stringent thermal requirements possess high demands on both heating power and heat dissipation ability. Currently several different heating approaches are adopted to achieve phase change in GeTe based nanophotonic devices, including hot plate, pulsed laser, and integrated electrical heater. However, reversible switching of GeTe covering a large area remains difficult, and the device-level understanding of the thermal and electrical properties is still elusive for the GeTe reconfigurable devices.<br/><br/>In this work, device level thermal modelling is conducted for a representative GeTe switching heater device, which includes a Si substrate, an AlN insulation layer, an electrical heater made from tungsten, an Al2O3 separation layer, and the GeTe phase change material on the top. Frequency domain thermoreflectance (FDTR) method is adopted for thermal characterization, where thermal properties of a multi-layered structure can be determined through pump-probe laser heating and curve fitting to an analytical heat conduction model. The most thermally sensitive material parameters, including thermal conductivity and heat capacity of Si, AlN and GeTe layers, as well as the thermal boundary resistance between AlN and tungsten heater, are characterized through FDTR with delicately designed multi-layered samples.<br/><br/>Based on the thermal measurement results, an actual GeTe switching heater device is designed with 8-unit heater layout covering the phase change region of 25×12.5 μm². A finite element model is established for the heater device to predict the steady state and transient thermal responses. To validate the thermal measurement results and the finite element model, the heater device is also fabricated and experimentally tested. The steady state thermal response is measured through a thermal mapping system; the transient response is reflected through a phase change test with pulse voltage input. Both the steady state temperature profile and the amorphized GeTe region under pulse input show high agreement between simulation and experiment, proving the accuracy of the thermal measurement and the finite element model. This work extends the thermal understanding of the GeTe based reconfigurable devices, and provides a general workflow on comprehensive thermal modelling of nanofabricated heater structures.