High Temperature Phase Change Materials

Chalcogenide thin films that exhibit a reversible transition between crystalline and amorphous phases has been the important material platform for data storage and emerging neuromorphic computing applications. For the past decades, a wide variety of phase change alloys, including GST (Ge₂Sb₂Te₅) – the most commonly used for phase change memory, have been studied in a general effort of increasing the speed performance or energy efficiency of modern electronics and computing hardware. However, the (semiconductor) chip shortage is still one of the biggest challenges in the nation, and in particular, a high temperature application domain has strong demands for novel scientific and engineering approaches to develop materials of the highest reliability and stability. For example, the use of high temperature electronics (operating at above 125 °C) continues to grow in an automotive industry, and thus, heat-resistive metals, plastics, and semiconductors need to be further researched and developed. Phase change materials are identified as an essential semiconducting component of high temperature electronics for data storage, sensing, and energy harvesting and conversion.<br/><br/>Here in this work, we performed a systematic study on phase change materials for far-reaching applications in the high temperature domain. It has been understood that GeSb possesses the high potential to become the next-generation platform for high temperature electronics due to its relatively high crystallization temperature (T_C). We synthesized 50 nm-thick Ge₁₄Sb₈₆ thin films by using the RF sputtering technique, and carefully measured the resistivity vs. temperature characteristics. It is found that T_C of our Ge₁₄Sb₈₆ thin film reaches the value as high as 230 °C, which is in good agreement with the literature. This is best attributed to the growth-driven crystallization kinetics of GeSb (i.e., crystallization tends to start occurring at the crystalline-amorphous interfaces). However, it still remains unanswered if GeSb can satisfy the multiple requirements of contemporary high temperature electronics because a holistic study that further investigates GeSb beyond its high T_Cis still missing. For example, GeSb is known for relatively slow (SET) switching speed, and the topic of enhancing the switching speed of GeSb while still affording the high T_Cremains unexplored. Therefore, our work is expected to greatly advance the field by exploring the performance of GeSb in multiple aspects, including the crystallization temperature, crystallization (SET switching) speed, data retention, and reliability.<br/><br/>It is highlighted that our experimental approach is based around the idea that doping is key to best engineer the phase change materials and find the optimal balance among properties desired for high temperature applications. In this work, we fabricated a wide variety of phase change materials such as (undoped) GeSb, Ti-doped GeSb, and Al-doped GeSb, and found that Al-doped GeSb achieves the T_Cvalue of greater than 230 °C, excellent data retention (10-year data retention @ around 140 °C), and switching speed of a few tens of nanoseconds at a moderate amount of current applied. We’re currently investigating the effect of other dopants as well, such as Hf and Ta, while performing comparative studies and analyses with reference samples that consist of undoped or doped conventional GST thin films. Therefore, our study features a holistic approach to investigate both phase change materials of GST and GeSb that are either undoped or doped by various transition metal elements in terms of high temperature application-specific performance criteria.