Abstract
Phase change materials (PCMs) store information by exploiting substantial property contrast between amorphous and crystalline states. PCMprogrammed memory exhibits non-volatility, fast switching speed, and high scalability, thus being most developed for applications in data storage and programmable photonics. However, the main drawback with PCM-based data storage is the high RESET power , which limits the integration of PCMs in electronic and photonic devices. Therefore, the aim of this thesis is to design and develop PCMs that can be efficiently switched with low energy. To overcome this drawback and improve the energy efficiency in switching, the central hypothesis is that the low-energy switching can be accessed by crystal structural optimization and thermal engineering. Switching performance depends on the crystal quality. Thus, I optimized the Sb2Te3 crystal quality using pulsed laser deposition (PLD), and compared the crystal quality of sputtered and pulsed laser deposited Sb2Te3 films using a combination of statistical design of experiments and density functional theory. The seed layer was the most significant factor for PLD grown films, while the crystal quality was sensitive to the deposition temperature in sputtering. Nonadiabatic quantum molecular dynamics simulations showed that this difference originates from the density of excited atoms in the plasma, leading to different adatom diffusivity. These results explain the wide range of Sb2Te3 and superlattice crystal qualities observed in the literature and indicate that plasma-based deposition methods are suitable for growing high quality chalcogenide PCM crystals. Strained interfacial phase change material (iPCM) superlattices can switch substantially more efficiently than their alloy counterparts. Herein, I designed a novel structure to further improve the switching efficiency by lowering the thermal conductivity through Ti doping. The structure retained (0 0 l) crystallographic orientation and showed a substantial decrease in the thermal conductivity when a specific Ti content was added into Sb2Te3 (i.e., the scaffold in the iPCM). I demonstrated that the consumed energy decreased in both optical and electrical switching. This energy efficiency improvement is due to thermal energy being better confined in the PCM during interfacial structural transitions. Moreover, the DF/MD simulation showed the GeTe layers start premeltdisordering at a lower temperature than the Ti Sb2Te3 scaffold, indicating interfacial phase transitions in the doped structure. This thermally engineered superlattice structure is applicable in a new generation of energy-efficient phase change memories. Given that highly oriented iPCM superlattices need a careful preparation, we demonstrated a facile approach to improve the energy efficiency by inserting a 2D layer between the PCM and the substrate. I found that the 2D layer radically reduced the laser power during the amorphization process. 2D layers efficiently confine energy within the PCM by utilizing the high thermal boundary resistance induced by the van der Waals (vdW) bonded layers. Therefore, I proposed that 2D materials with stable vdW interfaces can be used to improve the thermal efficiency of PCM-tuned reprogrammable photonics. Importantly for Si photonics devices, the waveguide simulations confirmed that the 2D layer does not affect the propagating mode. This simple additional 2D layer produced a substantial energy efficiency improvement without degrading the optical performance of PCMs. In addition, I optimized our PLD system to grow high quality chalcogenide PCMs. I optimized the beam profile, spot size, and uniformity. After this basic optimization, I obtained high quality layered Sb2Te3 crystals. The PLD system is now stable and capable of producing layered Sb2Te3 crystals. Based on the quantified crystal quality dataset, a statistical principal component analysis revealed the correlation between crystal quality and growth parameters, guiding the further optimization. This thesis addresses the issue of how to improve the switching energy efficiency of PCMs. This work has showed that in order to create efficient memory, one of the best ways is to use a thermally optimized superlattice, which is modified to have a low thermal conductivity. This can viably achieved by doping Sb2Te3 with Ti. Therefore, this work paves the way for energy efficient data storage and programmable photonic devices.