Improving Crystallization Temperature by Ge-Incorporation in Ge_xSb_yTe_z Phase Change Alloys and Heterostructures for Automotive Applications

The interest in Phase Change Memory (PCM) devices with improved crystallization temperature is increasing for automotive applications, where devices must be able to operate reliably at high temperatures (&gt; 160 °C) for at least ten years. Alloys like Ge₂Sb₂Te₅ (GST225) are already a standard to realize memory devices. However, one of the major disadvantages of GST225 single layers is the low crystallization temperature (T_c), which results in low thermal stability and limited data retention. An interesting option to increase T_c is to grow Ge-rich GST alloys. Here, several Ge_xSb₂Te₅ and Ge_xSb₂Te₃ layers were grown by RF-sputtering and their thermal stability was studied by X-ray diffraction (XRD) and Raman spectroscopy as a function of temperature. The compositional analysis was performed by X-ray fluorescence (XRF). The results showed that both T_cand Ge-segregation amount grow linearly with the Ge-content in the alloys. Among the different possibilities, our approach combines different phase change materials with opposite properties (Sb₂Te₃, Ge₂Sb₂Te₅ and Ge_xSb_yTe_z)<b>, </b>also introducing a confinement Ge layer to realize interesting novel test devices. Therefore Sb₂Te₃/Ge₂Sb₂Te₅/Ge heterostructures with a total thickness of 110 nm were deposited by RF sputtering and their thermal stability was analyzed by X-ray diffraction and Raman spectra, evidencing a slower crystallization dynamic (by 50°C) than in Ge₂Sb₂Te₅. After optimization, the heterostructures were deposited onto proper single-cell vehicles prepared on Si(001) substrates and metal contacts were nanolithographically (EBL) defined. The subsequent technical analysis (I-V, R-V, R-cycles graphs for the SET/RESET states) showed that the cell has a programming current of 1.2 mA and an endurance of 2x10⁵ cycles.<br/>This project has received funding from the European Union's Horizon 2020 research and innovation program under Grant Agreement No. 824957 (“BeforeHand:” Boosting Performance of Phase Change Devices by Hetero and Nanostructure Material Design).