Growth and characterization of phase change materials grown by laser ablation : the case of germanium telluride.

Abstract

Phase change materials (PCM) represent a unique type of materials that can rapidly and reversibly switch between their amorphous and crystalline states. These two phases are characterized by significantly different physico-chemical properties such as electrical conductivity, optical reflectivity, mass density, and thermal conductivity. These differences as well as the repeatability and speed of the switching process make these materials very attractive for data storage and memory devices. Germanium telluride (GeTe) is a chalcogenide PCM which is particularly interesting for device applications because of its large resistivity window (4–5 orders of magnitude between the amorphous and the crystalline state), its the high crystallization speed (~20 ns) and its high crystallization temperature (~180 ºC). Typically, thin film deposition techniques such as magnetron sputtering and evaporation are used to growth of GeTe films, but alternatively, Pulsed Laser Deposition can prove to be a method of choice to grow such materials considering its simplicity in depositing high quality materials in a controllable and scalable manner. This thesis deals with PLD of GeTe thin films, and the study of their chemical and physical properties. All deposition runs were performed at room temperature and the effect of experimental growth conditions such as target to substrate distance, background pressure, laser energy, and deposition time on the properties of the films was investigated. The micro-structure of the films were characterized by X-Ray Diffraction (XRD), Rutherford Back-Scattering (RBS) and Scanning Electron Microscopy (SEM) whereas UV-VIS-NIR spectroscopy and temperature dependent resistivity measurements were used to determine their optical and electrical properties. Correlations between deposition conditions and film properties were elucidated and it was found that deposition at an argon background pressure of 1 × 10⁻⁴ mbar, a laser energy of 200 mJ and a target to substrate of 6 cm from the