Disorder and electrical transport in phase change materials
Aachen / Publikationsserver der RWTH Aachen University (2013) [Dissertation / PhD Thesis]
Page(s): IX, 147, XLV S. : Ill., graph. Darst.
Phase-change materials (PCMs) are characterized by a high optical and electrical contrast between an amorphous and a crystalline phase. Both phases are stable at room temperature on a timescale of years. Hence, these materials can be employed in optical and electrical data storage technologies, in which the ability of fast and reversible switching between the two phases is exploited. While the application in rewritable optical data storage is well established, PCM-based electrical memories are treated as a potential successor of Flash and DRAM, combining non-volatility with high writing speed, good scalability and energy efficiency. The charge-transport mechanism in the amorphous phase of phase-change materials, especially the origin of the phenomena "threshold switching" and "resistance drift" is presently subject of controversy. In one part of this thesis, the charge transport in amorphous phase-change materials is analyzed by means of Hall-effect measurements. A measurement setup designated for this purpose is described. The setup makes use of a periodically modulated magnetic field. This way, the Hall mobility of amorphous PCMs can be measured with unprecedented precision. At room temperature, Hall mobilities between -0.03 cm^2/Vs and -0.12 cm^2/Vs are obtained for five different PCMs. The negative signs are in contrast to positive signs seen in thermopower measurements. In addition, a pronounced temperature dependence of the Hall mobility is found. The relevant activation energies are determined quantitatively for three of the five compounds and compared to the theoretical predictions of the small-polaron model. The observation of a sign anomaly and the existence of an activation energy of the Hall mobility are in accordance with the small-polaron model. The quantitative values however are, when combined with thermopower data of a different thesis, in contrast to the predictions of that model. The data are discussed in the context of the Standard Transport Model as well. Furthermore, this work demonstrates that disorder is highly relevant not only in the amorphous phase, but also in the crystalline phase. While the PCM GeTe is a p-type degenerate semiconductor with a metallic resistance-vs-temperature curve, some other phase-change materials such as Ge1Sb2Te4 display such an behavior only after annealing at temperatures high above the crystallization temperature. Annealing at lower temperatures leads to a relatively high resistivity with a negative temperature coefficient. This is attributed to a high degree of disorder, especially due to the random distribution of intrinsic vacancies. Moreover, insulating behavior is observed in cases of extremely high disorder. Hence, a metal-insulator transition takes place. For insulating samples, variable-range hopping is observed at low temperatures, with a crossover between Mott's law and the Efros-Shklovskii law. Temperature-dependent Hall-effect measurements show that the observations can neither be explained by non-degeneracy nor by ionized-impurity scattering. Another observation is the change in sign and shape of the magnetoresistance curves, which apparently coincides with the metal-insulator transition. This is taken as evidence for a change in the transport mechanism. All observations can be reproduced for similar materials. Noticeable, the sign change of the temperature coefficient of resistivity can be reliably described by the Ioffe-Regel rule, i.e. it takes place when the product of Fermi wave vector and mean free path equals one. This transition is found to be independent of the crystallographic phase. Even in the most metallic samples, a resistance minimum is observed at about 15 K. This regime is studied in the last chapter of this thesis by means of ultra-thin Hall-bar samples. A suitable sample preparation process is described. The dependences of resistance on temperature and magnetic field can be consistently explained by considering the mechanisms of weak antilocalization and disorder-enhanced electron-electron interaction. Elastic and inelastic scattering mechanisms are quantified.
- URN: urn:nbn:de:hbz:82-opus-47761
- REPORT NUMBER: RWTH-CONV-144194