Phase Change Materials from the Thermal Point of View

Abstract

Materials with a structural phase transition are interesting as emerging devices for neuromorphic and in-memory computing. Most prominent are non-volatile phase-change memory devices (PCMs) based on chalcogenides that utilize a transition between amorphous and crystalline states. Persistent states with different resistance are set by controlling the relation between heat generation and dissipation. In contrast, materials with an insulator-to-metal transition (IMT) exhibit a structural phase change upon heating but return to the original state when cooling back to room temperature. This behaviour can be exploited for oscillators rather than memories. As an example, $ VO_{2} $ transitions from a monoclinic to a rutile crystalline structure accompanied by a resistance change of up to 5 orders of magnitude. Processes in these materials are inherently tied to temperature changes and thermal transport. In PCMs, the (re)crystallization is driven by Joule heating to a certain temperature and subsequent (slow) cooling, leading to a stark change in resistance. In $ VO_{2} $, the IMT can be induced by temperature rise due to self-heating in the highly resistive state, which is highly non-uniform due to inhomogeneities of the material. The complex interplay between resistive heating, heat spreading, and the thermal phase-transition correlated with a strong change of resistance makes investigation challenging. Current filaments that form are similar to resistive oxide memories. Thermal imaging can elucidate the thermal processes in resistance switching devices. A valuable characterization technique for this task is Scanning Thermal Microscopy (SThM) due to its high spatial and temperature resolution. Moreover, it can image original devices in-operando and give quantitative values in a non-destructive, non-interfering manner. The SThM method determines the local temperature by measuring the heat flow between the device and an AFM-like cantilever with an integrated heating and sensing element [1]. We have developed and advanced the SThM method for devices with non-linear and volatile characteristics and applied it to various applications. For example, we were able to quantify local temperature distributions caused by heat-dissipating current filaments at sub-10 nm resolution in $ VO_{2} $ and resistive RAM devices in different voltage-dependent states. It was found that the form and position of the current filament in a lateral, polycrystalline $ VO_{2} $ device varies when the voltage is changed to switch between insulating and metallic state. This explains the undesirable varying oscillator characteristics during operation and suggests the development of new device designs. The temperature of vertical resistive RAM elements was imaged through the top electrode, revealing the position of a current filament which stays stable during cycling and does not change when switching between high and low resistive state. The dimension of the filament and the heat spreading radius can be estimated from the thermal image, which can be utilized to determine the minimum device size and the maximum packing density of those memories. In conclusion, the thermal imaging method developed here can be used to map the temperature distribution in a variety of devices during their operation, including those showing volatile and non-volatile switching. The interpretation of the resulting temperature maps can reveal thermal processes and guide design decisions and fabrication processes. REFERENCES 1. F. Menges et al., Rev. Sci. Instrum. 87, 074902 (2016)