Abstract
Phase change materials (PCMs) are a special class of predominantly chalco-genide alloys that exhibit fast and reversible structural switching between amor-phous and crystalline states. This structural change results in dramatic changes to their optical and electrical properties. PCMs are, therefore, important for op-tical and electrical data storage technologies. Phase change electrical memory is non-volatile, has a fast switching speed, and is highly scalable, thus it is a potential successor to the current FLASH memory technology. The main tech-nological challenge is reducing the high energy consumption during the RESET (amorphisation) process. It is hypothesised that the switching energy can be reduced by engineering and optimising the crystal structure. Hence, the aim of this thesis is to design energy ef?cient phase change switching alloys and super-lattices. To do this an optical pump-probe laser system was ?rst developed to characterise the phase transition energy and switching rate. The laser system is capable of automated operation, and can sequentially characterise phase change alloys and superlattices by performing both transient and post-pulse measure-ments. The phase change material’s crystal structure and texture were optimised using fractional factorial design and response surface analysis, which are statis-tical design of experiments techniques more commonly used in product design. The in?uence of the growth parameters on the crystal quality was investigated, and their signi?cance was analysed. This approach provided highly textured superlattices that when incorporated into prototype memory cells, resulted a reduced programming voltage of just 0.6 V, a substantial improvement over the 3.6 V required by the Ge2Sb2Te5 alloy. It was also hypothesised that Bi2Se3 and Bi2Te3 could reduce the switching energy of phase change superlattices via straining the GeTe layers. Hence, laser switching GeTe–Bi2Se3 and GeTe–Bi2Te3 superlattices were also investigated and compared with GeTe–Bi2Se3 composi-tion spread alloys. In the composition spread alloy experiments, the pure Bi2Se3 alloy showed photoexcitation without a structural phase transition, while GeTe only showed a phase transition without photoexcitation. However, incorporat-ing GeTe into the alloy allowed both a structural phase transition and photoex-citation. This implies that the GeTe–Bi2Se3 alloys phase separated into GeTe and Bi2Se3. The associated superlattices did not show any optical change, which is due to the negligible optical contrast at the probe laser wavelength. In addition to these material engineering and design studies, a volatile switching effect was discovered in the crystalline Ge2Sb2Te5 phase change alloy. The electrical resis-tance of the Ge2Sb2Te5 memory cell was found to increase by a factor of three for less than 5 ns during a voltage pulse. An analytical model was developed to explain this transient behaviour as a balance between carrier scattering by phonons and carrier excitation. The effect of external electric ?elds on Ge2Sb2Te5 thin ?lms was also investigated by terahertz time-domain spectroscopy. Increas-ing the Ge2Sb2Te5 crystallinity increased the number of charge carriers and con-sequently increased photoabsorption and photoconductivity. This thesis shows that the switching performance of phase change materials can be substantially improved by controlling the texture, superlattice structure, and strain of the phase change material.