Publikationsserver der RWTH Aachen University
Electronic switching in phase-change materials
Abstract
dc:descriptionPhase-change materials (PCM) possess a unique property contrast between their crystalline and amorphous phases. Differences in resistivity of some orders of magnitude can be observed between both phases. Also, the reflectivity shows a remarkable contrast, and it allows the application of thin phase-change layers in optical media, such as CD, DVD and Bluray disk, to enable rewritable storage of information. Although both phases are stable for decades at room temperature, it is possible to switch between the phases in nanoseconds at elevated temperatures. This striking combination of stability and rapid transition, together with the pronounced resistivity contrast, make PCMs one of the most promising candidates for future, non-volatile, electronic memory. In this work, three physical aspects concerning such a memory have been investigated using custom made setups to cope with the challenges of sub-nanosecond timescales and resistances of hundreds of Gigaohms. Speed limitations of write and rewrite operations are a crucial topic, if PCMs shall be able to compete with the established concepts in electronic devices. Two classes of storage devices are used in modern computer systems, so far. On the one hand, there is the fast, but volatile memory close to the processor unit, like the dynamic (DRAM) and the static (SRAM) random access memory. On the other hand, there are slower, but non-volatile storages, like hard disk drive, flash, and optical media. In terms of speed, there is a gap of several orders of magnitude between memory and storage concepts. Phase-change memory could close this gap and establish a new storage class memory, and maybe, even allow to build a non-volatile memory device, which could replace the volatile DRAM. Investigations regarding the memory switching speed in phase-change memory will be presented in this work, and reveal that the phase transitions can be accomplished within a few nanoseconds. This demonstrates the potential of PCMs to compete with DRAM in terms of speed. The second topic in this work are transient phenomena, like threshold switching, which occur when PCMs are treated with electrical pulses. Threshold switching describes a sudden decrease of the material’s resistivity. This effect can be observed in amorphous PCMs, if the applied electrical field exceeds a threshold value. In this work, results will be presented which describe both the resistance drop during the threshold switching and the life time of this high conductive state. While present publications use a characteristic field strength for each PCM to describe the occurrence of the threshold switch, the results of this work suggest the definition of a field dependent delay time, which predicts the sudden change of conductivity. Besides the extraordinary behavior of disordered semiconductors at high electric fields, there is a further effect at low fields, which has a tremendous influence on phase-change memory applications: the resistance drift. This effect describes the time dependent increase of the resistivity of amorphous PCMs. In this work, experimental data will be presented which demonstrate the similarity of this effect in both unstructured films and memory devices. These data have been used to modify an existing model which explains the drift’s origin. The reported dependency of the drift behavior on the activation energy for conduction can be confirmed and a new aspect of this dependency will be presented.
Degree
thesis:*- Grantor dc:publisher
- Publikationsserver der RWTH Aachen University
- Year dc:date
- 2012
Author and committee
dc:creator, dc:contributor.*- Author dc:creator
-
- Bruns, Gunnar
- Contributors dc:contributor
-
- Wuttig, Matthias
Subjects
dc:subject × 11Rights
dc:rights- Statement dc:rights
-
- info:eu-repo/semantics/openAccess
- Language dc:language
- eng
Identifiers
dc:identifier.*- OAI identifier oai:identifier
- oai:publications.rwth-aachen.de:61329