High Endurance Phase-Change Devices for Electrically Reconfigurable Optical Systems

University Of Pittsburgh, Pittsburgh PA

Investigators

Abstract

Award Title: High-endurance phase-change devices for electrically reconfigurable optical systems Non-Technical Abstract: Materials whose optical properties can be tuned electrically are essential to operations of many modern technologies that we rely on daily, for example, smartphone displays and fiber internet. For emerging applications in tunable optical components, high-speed computing, and advanced optical storage, a group of materials that can be reconfigured at the atomic level, known as “phase-change materials,” is particularly promising. When their atoms are arranged in either an ordered or disordered state, these materials exhibit a dramatic, stable, and reversible change in their optical properties, which could enable devices with very compact form-factor, low energy consumption and insensitivity to vibration. While many proof-of-concept optical devices have been demonstrated using phase-change materials, few can be controlled using electrical signals—a prerequisite for real world implementation. Additionally, these electrically controlled phase-change devices have shown poor endurance and switching cyclability, the cause of which is not well understood. The team proposes to address this challenge by first investigating the role of heat and mechanical expansion in the various layers comprising these devices using time-dependent optical and electrical measurements. Secondly, the team will use high resolution imaging techniques to study the role that migration of various types of atoms has on the reversibility of the phase-change material. Finally, the team will use these results to construct phase-change devices with improved reliability and explore the possibility of scaling them up to sizes needed for applications requiring larger tunable optical components. The team seeks to educate middle-/high-school students on topics related to novel materials in daily life from school districts with historically serving under-represented minorities, using a combination of interactive workshops and hands-on demos. This project also provides training for two graduate students in advanced device fabrication and characterization techniques, and hosts undergraduates from underrepresented groups during the summer months to broaden participation in STEM-related fields. Technical Abstract: Phase-change materials, such as Ge2Sb2Te5 and GeTe, are particularly promising for reconfigurable optical devices owing to their fast, dramatic, non-volatile, and reversible change in refractive index. Experimental demonstrations of reconfigurable smart windows, reflective displays, metasurfaces, and photonic devices for memory and computing have re-ignited interests in these materials. For phase-change devices with dimensions greater than the optical wavelength, an electro-thermal approach to switching is most promising, but limited prior work showed poor endurance and cyclability (1000 cycles or less) compared to the high endurances (greater than 10 million cycles) demonstrated for phase-change data storage. The team proposes that the endurance is limited by poorly matched thermal properties of materials within these devices, while the degrading optical contrast often observed is due to phase segregation and void formation in the phase-change layer. To validate this hypothesis, the project has three aims: (1) improve the lifetime of electro-thermal phase-change devices by properly matching the thermal expansion coefficients of the materials within the device layers; (2) reduce the cycling-induced degradation of optical contrast by reducing thermal gradients within the device and improving deposition conditions; and (3) identify the effects and limitations of scaling on phase-change optical devices. The proposed approach will overcome the limited cyclability of these electro-thermally switched phase-change devices by studying the thermal response of the device layers through complementary thermal-mechanical modelling, dynamic optoelectronic measurements, and advanced nano-characterization techniques. The insights gained by understanding and addressing the current limitations of electro-thermally controlled optical phase-change films are expected to be broadly applicable to such fields as tunable optical coatings, non-von Neumann computing, electrical-optical conversion, and reconfigurable photonic and RF systems. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.