Glass Relaxation Effects on the Performance of Phase-Change Photonic Devices and the Control Strategies (Invited)

Objective Optical information storage has been realized using non-volatile photonic memories by integrating chalcogenide phase-change materials (PCMs) on the silicon photonic platform, which offers the advantages of low power consumption, large bandwidth, and high parallelism for in-memory photonic computing, thereby meeting the demands for high computational power and energy efficiency. However, the spontaneous structural relaxation of amorphous PCMs leads to changes in their optical properties, thereby causing programming variations in the devices. In this work, we systematically investigate the temperature- and time-dependent structural relaxation effects on the optical properties of amorphous germanium telluride (GeTe) thin films and the performance of GeTe-based waveguide devices. It was found that prolonging the holding time or increasing the heating temperature facilitates amorphous relaxation, leading to a gradual increase in the transmittance of the amorphous PCM device. We propose strategies to reduce the impact of structural relaxation, which may enhance the reproducibility of phase-change photonic devices. Methods GeTe and ITO thin films were deposited via magnetron sputtering using a JGP-560 SKY system. The composition and thickness of the films were characterized by energy-dispersive X-ray spectroscopy (EDS, X Flash 7, Bruker). An electrical measurement system based on a source meter (2636B, Keithley) and a hot stage (mK200, Instec) was employed to investigate the electrical properties of the films. X-ray diffraction (XRD) analysis was performed using a D8 ADVANCE diffractometer (Bruker) to examine the crystal structure of the films. The refractive index and extinction coefficient were measured with a UVISEL PLUS spectroscopic ellipsometer (Horiba), and the experimental data were fitted using the CODE software (https://wtheiss.com). Silicon photonic waveguides with a thickness of 220nm were fabricated through a multi-project wafer (MPW) service. A post-fabrication process was developed to etch away part of the silica upper cladding so as to facilitate the integration of PCM thin films on the silicon waveguides. A 20-nm-thick GeTe and a 20-nm-thick ITO thin film were deposited by magnetron sputtering (JGP-560 SKY), and the PCM devices were fabricated via a lift-off process. Optical measurements on transmission spectra and all-optical switching were conducted using a fiber-to-chip coupling setup. Pump and probe lights were coupled into two grating couplers connected to a waveguide, enabling counter-propagation of the pump and probe lights within the waveguide. The probe light transmitted through a PCM device was extracted using an optical fiber circulator. The probe light was further separated from the pump light by a band-pass filter. The optical power of the probe light was monitored using a photodetector. Results and Discussions Stoichiometric GeTe thin films were prepared via magnetron sputtering. Both amorphous and crystalline GeTe films were characterized by EDS, XRD, resistance-temperature measurements, and refractive index measurements. Measurements of refractive indices were performed on GeTe films subjected to structural relaxation treatments, including storage at room temperature for various durations (up to 4 weeks) and heating at various temperatures (up to 130°C) for 1 hour. We found that both an increase in storage time and a rise in heating temperature enhance the structural relaxation of amorphous GeTe. The structural relaxation effect reduces the extinction coefficient of amorphous GeTe in the optical telecommunication band. Heterogeneous integration of phase-change materials onto silicon photonic waveguides fabricated through multi-project wafer (MPW) runs has been achieved via our developed back-end-of-line (BEOL) fabrication process. Subsequently, experimental transmission measurements, binary switching, and multi-level switching were carried out for the GeTe-based photonic devices without structural relaxation treatments, demonstrating the function of photonic memory. Furthermore, transmission measurements were performed on the amorphous GeTe waveguide devices before and after structural relaxation treatments for different durations at room temperature and heating at various temperatures. Structural relaxation led to enhanced optical transmission and deviations in device logic values. As the time duration extended to 4 weeks, the transmittance of the amorphous GeTe device at 1540nm increased from 76.7 to 89.1. As the heating temperature rose to 130°C, the transmittance of the amorphous GeTe device at 1540nm increased from 76.7 to 87.2. Under identical pumping conditions, the reduced optical absorption of the amorphous GeTe resulted in a weakened photothermal effect, and thus a reduced switching contrast in all-optical pump-probe testing. Based on density functional theory calculations, the effects of Peierls distortion on the electronic structures of crystalline and amorphous GeTe were studied. Finally, we proposed several strategies to regulate structural relaxation, which could help improve the programming consistency of phase-change photonic devices. We applied stronger laser pulses or annealing treatment to the aged devices and obtained the fully crystalline state again. Subsequent laser programming using the initial laser parameters resulted in a consistent programming window. Therefore, these approaches can indeed minimize the impact of structural relaxation effects. Conclusions This work focused on the impact of structural relaxation on the optical properties of GeTe and the performance of waveguide devices. During the structural relaxation process, the bandgap was widened, leading to a reduction in the extinction coefficient. Our experimental results demonstrated that both an increase in time duration and annealing temperature enhance the structural relaxation of amorphous GeTe. The time-dependent and temperature-dependent structural relaxation resulted in increased optical transmission of the GeTe-based waveguide devices. With a weakened photothermal effect, the switching contrasts were reduced when using the same pump pulse settings. We proposed several suggestions to mitigate the structural relaxation effects. These regulation strategies can help improve the programming consistency and robustness for scalable and large-scale photonic computing.