# Exploring Structural Changes in Ge-Te Amorphous Films Through Small-Angle Neutron Scattering
## 1 Introduction  
GeTe-based materials, known for their remarkable phase-change behaviour [1] and thermoelectric [2, 3] performance, are widely explored for advanced applications in non-volatile memory devices and energy-harvesting systems. Phase-Change Materials (PCMs) were first conceptualised in the late 1960s, with [PERSON]'s pioneering work [4] highlighting their potential for data storage applications. These materials exhibit unique properties, such as the ability to switch between crystalline and amorphous states, each with distinct optical and electrical characteristics. The crystalline phase typically shows high electrical conductivity and high optical reflectivity, whereas the amorphous phase has high resistivity and low reflectivity. This transformation, driven by external energy inputs like laser pulses or electrical currents, occurs on the nanosecond scale and is fully reversible [5]. Prominent PCM compositions include telluride-based materials, such as Ge\({}_{2}\)Sb\({}_{2}\)Te\({}_{5}\), Ge\({}_{5}\)Sb\({}_{2}\)Te\({}_{4}\), and GeTe [6, 7, 8], which have been widely utilised in optical storage devices like CD-RW, DVD-RW, and Blu-ray Discs due to their fast switching capabilities, durability, and scalability [9]. Current research focuses on advancing PCMs for electrical memory technologies, such as Phase-Change Random-Access Memory (PC-RAM), which offers advantages such as faster speeds, greater energy efficiency, and enhanced durability compared to conventional flash memory. One of the defining characteristics of PCMs is their ability to crystallise rapidly from the amorphous phase, generating a marked contrast in electrical properties between the two states. However, the interaction between the amorphous and crystalline structures remains a critical area of research. These structures must be sufficiently distinct toensure the stability of the amorphous phase and reliable data retention at operating temperatures yet topologically similar enough to enable rapid crystallisation within nanoseconds. Storage density is another crucial aspect of PC-RAM technology. One approach to enhancing storage density is the development of multi-level cells (MLCs) [10], where the resistance of a cell can be precisely adjusted by modulating the volume of its amorphous phase. However, this requires a stable amorphous state with time-independent resistance. A significant challenge in this area is resistance drift [11; 12; 13], a phenomenon in which the resistivity of amorphous telluride materials, such as GeTe and Ge\({}_{0.15}\)Te\({}_{0.85}\), increases over time, potentially leading to data degradation in MLCs. During the last decades, different techniques have been used to study the atomic and local structures of amorphous Ge-Te, including the extended X-ray absorption fine structure (EXAFS) at the Ge and Te K-edges [14; 15; 16], anomalous X-ray scattering [17], time-of-flight neutron diffraction [18], X-ray absorption fine structure spectroscopy [16], and numerical simulations [19; 20; 21; 22]. Previous studies on amorphous Ge\({}_{x}\)Te\({}_{1-x}\) (\(0.10<x\leq 0.50\)) films, produced via co-sputtering, have identified three distinct regimes of resistance drift, with notable transitions occurring at approximately 25 at.% and 35 at.% Ge. Ge-rich glasses (\(x>0.25\)) exhibit high drift coefficients, with values ranging from 0.13 for Ge\({}_{0.50}\)Te\({}_{0.50}\) to \(\approx\)0.29 for compositions near the stiffness threshold. In contrast, the drift coefficient decreases with lower Ge content in Ge-poor compositions (\(x\leq 0.25\)) [23]. These anomalies may result from microstructural or topological changes, although the possibility of phase separation near the critical composition thresholds cannot be ruled out. Recent structural analyses of co-evaporated or sputtered Ge\({}_{x}\)Te\({}_{1-x}\) (\(0.10<x\leq 0.50\)), utilising techniques such as X-ray diffraction and EXAFS combined with reverse Monte Carlo (RMC) modelling, have provided valuable insights into the atomic-scale structure [24; 25; 26].