Defects in amorphous phase-change materials
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Daniel Krebs IBM Zürich Research Laboratory, 8803 Rüschlikon, Switzerland
Stephanie Grothe I. Physikalisches Institut, RWTH Aachen University, 52056 Aachen, Germany
Josef Klomfaß and Reinhard Carius IEF-5 Photovoltaik, Forschungszentrum Jülich, 52425 Jülich, Germany
Christophe Longeaud Laboratoire de Génie Electrique de Paris (CNRS UMR 8507), Supelec, Universités Paris VI et XI, Plateau de Moulon, 91190 Gif sur Yvette, France
Matthias Wuttiga) I. Physikalisches Institut, RWTH Aachen University, 52056 Aachen, Germany; and JARA Fundamentals of Future Information Technology, RWTH Aachen University, 52056 Aachen, Germany (Received 30 August 2012; accepted 12 March 2013)
Understanding the physical origin of threshold switching and resistance drift phenomena is necessary for making a breakthrough in the performance of low-cost nanoscale technologies related to nonvolatile phase-change memories. Even though both phenomena of threshold switching and resistance drift are often attributed to localized states in the band gap, the distribution of defect states in amorphous phase-change materials (PCMs) has not received so far, the level of attention that it merits. This work presents an experimental study of defects in amorphous PCMs using modulated photocurrent experiments and photothermal deflection spectroscopy. This study of electrically switching alloys involving germanium (Ge), antimony (Sb) and tellurium (Te) such as amorphous germanium telluride (a-GeTe), a-Ge15Te85 and a-Ge2Sb2Te5 demonstrates that those compositions showing a high electrical threshold field also show a high defect density. This result supports a mechanism of recombination and field-induced generation driving threshold switching in amorphous chalcogenides. Furthermore, this work provides strong experimental evidence for complex trap kinetics during resistance drift. This work reports annihilation of deep states and an increase in shallow defect density accompanied by band gap widening in aged a-GeTe thin films. I. INTRODUCTION
Phase-change materials (PCMs) exhibit a combination of extraordinary properties enabling their application in information storage.1 First, these materials show a drastic difference in resistivity or reflectivity between amorphous and crystalline phases.2,3 In addition, this class of materials exhibits very fast crystallization kinetics proceeding on a submicrosecond time scale. In some alloys such as germanium telluride (GeTe) the phase transitions can even be as fast as 1 ns.4 Hence, PCMs offer the possibility to create a universal memory combining nonvolatility with dynamic random access memory (DRAM)-like switching speeds.5,6 The successful application of PCM in electronic devices is
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Address all correspondence to this author. e-mail: [email protected] This paper has been selected as an Invited Feature Paper. DOI: 10.1557/jmr.2013.72 J. Mater. Res., Vol. 28, No. 9, May 14, 2013
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