Electrical transport and switching in phase change materials

Phase change materials, typically composed out of Ge, Sb and Te alloys, are materials that can switch very fast between two stable states, the amorphous and the crystalline phase. The optical contrast of the two stable phases led to the application in optical data storage such as compact disks (CD), digital versatile disks (DVD) and Bluray disk. Due to the pronounced contrast in resistivity between the amorphous and the crystalline phase, phase change materials can also be used in electrical data storage devices. Since the two phases are stable at moderate temperatures, phase change memory is non-volatile, i.e. it does not need a power supply to sustain the information. Additionally, phase change memory is faster than the well-established Flash memory and exhibits a better cyclability and scalability. Therefore, phase change memory has become one of the most promising candidates for future non-volatile memory applications and also a candidate to bridge the gap between fast dynamic random access memory (DRAM) and slow non-volatile memory types. A precondition for fast switching from the low conductive amorphous phase into the high conductive crystalline phase is a phenomenon called threshold switching. At a characteristic threshold field the amorphous phase becomes highly conductive entering the so called amorphous on-state. Once the phase change memory is in the high conductive amorphous on-state, a high current can flow which enables Joule heating and therefore the phase transition. In fast memory applications like DRAM, threshold switching and the transient dynamics of the amorphous on-state are very important but poorly understood at the present time. Therefore, in this work the switching behavior of phase change materials and the dynamics of the amorphous phase have been studied. Phase change bridge devices of various width and length have been produced using several phase change materials as the active component. A very fast switching material, Ge15Sb85, has been found to switch within 10 ns. The pronounced differences (e.g. in crystallization temperature and crystallization speed) between the amorphous-as-deposited and amorphous-as-melt-quenched phases have been studied and explained with a difference in geometry of the environment of the active area. No evidence for an intrinsic difference has been found leading to the conclusion that it is adequate to study intrinsic properties of the amorphous-as-deposited phase which is much easier to obtain especially in thin film experiments. Furthermore, threshold switching in various phase change materials has been studied experimentally and linked to a generation-recombination statistics model in a multiple band transport picture. The simulations could reproduce all features observed in the experiment, and it was found that the threshold switching field varies strongly between different phase change materials which was linked to the difference in bandgap and defect states. Another requirement for a memory technology to succeed in the market is the potential to store multiple bits in a single memory cell (multi level storage). Thus, different states of resistance need to be stored in a phase change memory cell for multi level storage, which is achieved by producing amorphous marks of different size. Phase change materials are facing a challenge in this approach because of the resistance drift phenomenon. The resistivity at room temperature in the amorphous phase is increasing with time which would cause a memory cell drifting from one state into another. Therefore, it is important to achieve a better understanding of the amorphous phase to prevent resistance drift or at least describe it so that it can be handled in memory cells. For this reason, in this work the temperature dependence of several transport properties such as conductivity, field effect mobility and Seebeck coefficient has been studied. Based on optical absorption experiments a model of the density of states for the amorphous phase was developed. Considering hopping in localized states and transport in extended states, the experimentally observed transport properties were modeled. It is shown that band transport is predominant and that the resistance drift phenomenon can be explained by an increase of the bandgap. This result was confirmed by the absorption measurements.

Grantor dc:publisher

Publikationsserver der RWTH Aachen University

Year dc:date

2010

Author dc:creator

• Krebs, Daniel

Contributors dc:contributor

• Wuttig, Matthias

• info:eu-repo/classification/ddc/530
• Halbleiter
• Halbleiterspeicher
• Halbleitertechnologie
• Nichtflüchtiger Speicher
• Speicherzelle
• RAM
• IBM
• Speicher <Informatik>
• Elektronischer Transport
• Schaltvorgang
• Phase-Change-Technologie
• Schwellenspannung
• Rekombination
• Physik
• Widerstand
• Widerstandsdrift
• Amorph
• Threshold Feld
• IBM Almaden Research Center
• amorphous
• Generation-Recombination-Model
• Threshold Switching
• resistance drift
• on-state
• bridge cells

Statement dc:rights

• info:eu-repo/semantics/openAccess

Language dc:language

eng

Identifier

https://publications.rwth-aachen.de/search?p=id:%22RWTH-CONV-113872%22

OAI record oai:identifier

https://publications.rwth-aachen.de/oai2d?verb=GetRecord&metadataPrefix=oai_dc&identifier=oai%3Apublications.rwth-aachen.de%3A51594