Epitaxial growth of Ge2Sb2Te5 thin films on BaF2 substrates at room temperature by pulsed laser deposition

Epitaxial Ge2Sb2Te5 (GST225) thin films, key materials for phase change memory, were successfully grown on the (111) surface of BaF2 single-crystal substrates at room temperature using pulsed laser deposition. Laser fluence and repetition rate played a crucial role in controlling the microstructure of the thin films. Epitaxial growth was achieved at fluences between 2 and 4 J cm−2 with a low repetition rate of 1 Hz. In contrast, a fluence of 1 J cm−2 combined with repetition rates of 1, 10, and 100 Hz, as well as fluences of 2–4 J cm−2 combined with repetition rates of 10 and 100 Hz, resulted in amorphous thin films. The crystalline GST225 layers exhibited a cubic structure with slightly expanded lattice parameters, suggesting a partial relaxation of lattice mismatch. X-ray pole figures and φ-scans revealed a well-aligned out-of-plane orientation, accompanied by rotational twin domains. The out-of-plane epitaxial relationship is determined to be GST225(111)||BaF2(111), whereas the in-plane relationship is characterized by two directions, i.e., GST225[110]||BaF2[110] and GST225[110]||BaF2[1–10]. Atomic-resolution scanning transmission electron microscopy confirmed the cubic phase and uncovered localized planar defects resembling van der Waals-like gaps. Overall, this study demonstrates the feasibility of growing high-quality, epitaxial phase change materials at room temperature, enabling prospects for low-power, high-speed memory devices, and next-generation reconfigurable photonic systems.