Electrical transport in disordered, crystalline phase-change materials

• Elektrischer Transport in ungeordneten, kristallinen Phasenwechselmaterialien

Reindl, Johannes; Wuttig, Matthias (Thesis advisor); Mazzarello, Riccardo (Thesis advisor)

Aachen (2020, 2021)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2020

Abstract

In the coming years global energy consumption will increase, caused also by the increased demand of electricity to power more and more computational devices. In order to conserve limited natural resources, computing needs to become more energy efficient. To reach this goal, it is needed to break new grounds conceptualizing novel computing devices more similar to the blue prints from nature like neurons in the brain. Phase-change materials (PCMs) are already an important class of materials in optical data storage and as storage class memory in computers and offer a high contrast in optical reflectivity and electrical resistance between a stable amorphous and crystalline state, which can be rapidly and reversibly switched between. In this dissertation the focus will be on the investigation of the electrical transport in disordered, crystalline PCMs. Along the pseudo-binary line of PbTe and Sb$_{2}$Te$_{3}$ new alloys are found in a rock-salt like meta-stable crystalline phase, which are solid solutions of their constituents. A structural transition upon thermal annealing to a proposed thermodynamically stable hexagonal phase, similar to PCMs as GeSbTe-alloys is not found to occur. Instead, a segregation into the parent compounds at very high annealing temperatures takes place. A change in the majority carrier type from n- to p-type conduction is found upon thermal annealing the crystalline thin films in the meta-stable rock-salt like structure. A toy model is developed to describe the experimental results of measurements of the resistance, the carrier concentration, the Seebeck coefficient and density of states with tunneling spectroscopy, which describes the carrier type transition. An excess of Pb and Sb situated on intrinsic vacancy sites is hypothesized to be responsible for the initial n-type conduction, which is also based on the defect formation energies derived in density-functional theory calculations. The metal-insulator transition in SnSb$_{2}$Te$_{4}$ is investigated with electrical resistance measurements at cryogenic temperatures. Different models are used to describe the hopping transport in the insulating phase, with either Mott variable range hopping or Efros-Shklovskii hopping and their evolution close to the quantum critical point is investigated. The correlation length on the insulating side is utilized to characterize the transition point, with respect to different ordering parameters such as the room temperature conductivity, a Zero-Kelvin extrapolation of the conductivity and the Ioffe-Regel parameter. It is found that the results of a scaling analysis of the localization length with respect to the Ioffe-Regel parameter deviates from the literature of doped semiconductors. Furthermore, the magnetoresistance sign at low temperatures is found to change from negative in the insulating regime to positive close to the transition. The metallic side of the transport regime is investigated with measurements on very thin films. A resistance minimum at elevated temperatures not expected from classical transport theory is discovered, which scales with the annealing temperature and the initial resistance of the device. The inelastic scattering of electrons is investigated with measurements of the magnetoconductance and analyzed with the established theory of weak-antilocaliztion. The inelastic scattering rate is evaluated with respect to a constant, an electron-electron and an electron-phonon contribution. It is found that the relevance of electron-electron scattering at lowest temperatures persists even in regions where the diffusive picture of electron transport loses its validity. The change of the magnetoconductance sign is investigated in very thin films of SnSb$_{2}$Te$_{4}$. Measurements of the dependency on the angle between electrical current and magnetic field reveal a transition from an anisotropic magnetoconductance in the diffusive regime to an isotropic magnetoconductance in the insulating regime. The region of the sign change is close to a resistance of $R=(2\pi^2 \hbar)/\mathrm{e}^2$, where a change of the magnetoconductance sign occurs for parallel fields at higher temperatures than for perpendicular field alignment. The isotropic positive magnetocondcutance (negative magnetoresistance) in the insulating regime of SnSb$_{2}$Te$_{4}$ is attributed to a novel spin memory effect, where the suppression of spin-correlations between localized sites with a magnetic field increases the conductance. The effect is found to increase in strength with increasing disorder, in contrast to weak-antilocalization which decreases with increasing disorder.