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Crystallization Kinetics in Antimony and Tellurium Alloys Used for Phase Change Recording

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Crystallization Kinetics in Antimony and Tellurium Alloys Used for Phase Change Recording Crystallization kinetics in antimony and tellurium alloys used for phase change recording Von der Fakult¨at fu¨r Mathematik, Informatik und Naturwissenschaften der Rheinisch-Westf¨alischen Technischen Hochschule Aachen zur Erlangung des akademischen Grades eines Doktors der Naturwissenschaften genehmigte Dissertation vorgelegt von Diplom-Physiker Johannes Andreas Kalb aus Neuss Berichter: Universit¨atsprofessor Dr. Matthias Wuttig Universit¨atsprofessor Dr. Frans Spaepen Tag der mundlichen¨ Prufung:¨ 10. Februar 2006 Diese Dissertation ist auf den Internetseiten der Hochschulbibliothek online verfugbar.¨ ii Abstract Modern computers usually employ several types of data storage devices. Most fre- quently, magnetic and optical storage media are used. The latter have become of great importance throughout the last decade: nowadays a significant amount of data is stored on compact discs (CDs) and digital versatile discs (DVDs). A few years ago, rewritable CDs and DVDs have become commercially available and are widely used these days. In these storage media, a thin film of an antimony (Sb) or tellurium (Te) alloy is locally and reversibly switched by laser heating between the amorphous and the crystalline state. These states can be distinguished optically by their difference in reflectivity. Due to the reversibility of the phase transformation, rewritable CDs and DVDs are also called phase change media. The corresponding Sb and Te alloys are frequently termed phase change materials. Recently, phase change materials have also shown high potential for the development of non-volatile electronic phase change random access memories. In this application, a current pulse provides the heat that is necessary to induce the phase transformation between the amorphous and the crystalline state, which can be distinguished by their difference in electrical conductivity. First prototypes of this memory type are currently developed by the industry and demonstrate fast non-volatile data storage. There are good prospects that these memories finally replace current data storage devices in modern computers. In order to accomplish this, however, it is highly necessary to understand the phase transformation between the amorphous and the crystalline phase for Sb and Te alloys. This thesis makes a contribution to a fundamental understanding of the crystallization kinetics of amorphous and liquid phase change materials. The results should help to optimize both optical and electronic phase change media in terms of data transfer rates and scalability. iii iv ABSTRACT In one project of this study, ex situ atomic force microscopy (AFM) in combi- nation with a high-precision furnace was identified as a powerful and accurate tool to determine isothermal crystallization parameters in thin films as a function of time and temperature. This method was employed for a systematic study of crystalliza- tion kinetics in sputtered amorphous Ag0.055In0.065Sb0.59Te0.29 (hereafter: AgIn-Sb2Te), Ge4Sb1Te5, Ge2Sb2Te5, and Ge1Sb2Te4 thin films used for phase change recording. The temperature dependence of the crystal nucleation rate and the crystal growth velocity, which are the two fundamental quantities involved in crystallization, were de- termined between around 90 and 190 by direct observation of crystals. Using these quantities, the critical work for crystalline cluster formation could be calculated. The time dependence of the nucleation rate was also investigated. The knowledge of these crystallization parameters provides the basis to model crystallization and therefore to optimize data transfer rates. Ex situ transmission electron microscopy (TEM) was used to study the crystal morphology in these alloys. Tilting of plan view samples revealed that each crystallized growth formation is a bent single crystal. Cross-sectional TEM showed that only heterogeneous (not homogeneous) crystal nucleation occurs. These findings help to interpret the nucleation parameters obtained from the experiment mentioned above. In general, all alloys exhibited similar crystal growth characteristics, but the crystal nucleation behavior of AgIn-Sb2Te differed remarkably from that of the GeSbTe alloys. These observations provide an explanation for the different re-crystallization mecha- nisms observed for these materials under operating conditions. They also demonstrate that in particular the crystal nucleation rate is of great importance to control crystal- lization kinetics and therefore data transfer rates in phase change media. In a second project, sputtered amorphous films of the compositions mentioned above were studied by differential scanning calorimetry (DSC). Upon continuous heating, a heat release due to structural relaxation of the amorphous phase between 0.5 and 1.0 kJ/mol was observed. This value depends on the thermal history of the sample. Pre-annealing of the amorphous phase revealed the glass transition temperature within 10 K of the crystallization temperature upon continuous heating at 40 K/min. The knowledge of the glass transition temperature is of fundamental importance to under- stand and interpret crystal nucleation rates and crystal growth velocities. v In a third project, droplets of molten alloys of composition Ge12Sb88, AgIn-Sb2Te, Ge4Sb1Te5 and Ge2Sb2Te5, surrounded by a molten dehydrated B2O3 flux, were under- cooled to 40–80 K below their liquidus temperature in a differential thermal analyzer (DTA). The crystal-melt interfacial energy, which is the most important parameter for the calculation of the crystal nucleation rate, was determined from the nucleation tem- perature using the classical nucleation theory. This gave values of around 0.20 times the heat of fusion per atom in the interface for all alloys. This value should be a lower limit since it was not established that nucleation was homogeneous in the experiments. The steady-state nucleation rate was calculated between the liquidus and glass transi- tion temperature and was higher for the GeSbTe alloys than for the Sb-rich alloys. This explains the different crystallization behavior of these materials under operating condi- tions. Nevertheless, the nucleation rates appear too high to allow amorphization under operating conditions for the highest achievable cooling rates. Therefore, in conclusion, it is the presence of an incubation time for nucleation that makes amorphization and therefore phase change recording possible in both optical and electronic phase change media. vi ABSTRACT Kurzfassung Ubersetzung¨ des englischen Originaltitels: Kristallisationskinetik in Antimon- und Tellur-Verbindungen, die zur Datenspeicherung in Phasenwechselmedien eingesetzt werden. In modernen Computern werden ublicherweise¨ verschiedene Arten von Datenspei- chern eingesetzt. Sehr h¨aufig werden magnetische und optische Speichermedien be- nutzt. Letztere haben innerhalb des vergangenen Jahrzehnts an großer Bedeutung gewonnen: Heute wird ein Großteil der anfallenden Datenmenge auf CDs (englisch: compact disks) und DVDs (englisch: digital versatile disks) gespeichert. Seit eini- gen Jahren sind auch wiederbeschreibbare CDs und DVDs kommerziell erh¨altlich und sind heutzutage weit verbreitet. In diesen Datenspeichern wird ein dunner¨ Film einer Antimon- oder Tellur-Verbindung (Abk.: Sb- oder Te-Verbindung) durch Laser-Heizen lokal und reversibel zwischen dem amorphen und kristallinen Zustand hin- und zuruck-¨ geschaltet. Diese Zust¨ande k¨onnen aufgrund ihrer verschiedenen Reflektivit¨atskoeffi- zienten optisch unterschieden werden. Wegen der Umkehrbarkeit des Phasenubergangs¨ werden wiederbeschreibbare CDs und DVDs auch Phasenwechselmedien genannt. Die entsprechenden Sb- und Te-Verbindungen werden h¨aufig als Phasenwechselmaterialien bezeichnet. Kurzlich¨ haben Phasenwechselmaterialien auch hohes Potenzial zur Entwicklung nicht fluchtiger¨ elektronischer Datenspeicher mit wahlfreiem Zugriff (englisch: non- volatile electronic phase change random access memories) gezeigt. In dieser Anwen- dung wird die Joulsche W¨arme, die zur Phasentransformation zwischen dem amorphen und kristallinen Zustand ben¨otigt wird, durch einen Strompuls lokal in das Material eingekoppelt. Die beiden Zust¨ande unterscheiden sich stark durch ihre elektrische Leit- f¨ahigkeit und k¨onnen daher auf diese Art ausgelesen werden. Erste Prototypen dieses vii viii KURZFASSUNG Speichertyps werden derzeit industriell entwickelt und demonstrieren schnelle nicht fluchtige¨ Datenspeicherung. Die Aussichten, dass dieser Speichertyp in der Zukunft g¨angige Datenspeicher in modernen Computern ersetzt, sind sehr gut. Um diesen Schritt jedoch zu erreichen, ist es zwingend notwendig, die Phasentransformation zwi- schen amorpher und kristalliner Phase in Sb- und Te-Verbindungen besser zu verste- hen. Diese Doktorarbeit tr¨agt zu einem grundlegenden Verst¨andnis der Kristallisa- tionskinetik amorpher und flussiger¨ Phasenwechselmaterialien bei. Die Ergebnisse sind eine wichtige Hilfe fur¨ die Optimierung von Datenspeicherraten und der Skalierbarkeit sowohl in optischen als auch in elektronischen Phasenwechselmedien. In einem Projekt dieser Arbeit wurde Ex situ“–Rasterkraftmikroskopie in Kombi- ” nation mit einem Hochpr¨azisionsofen als eine leistungsf¨ahige und genaue Methode iden- tifiziert, um isotherme Kristallisationsparameter in dunnen¨ Filmen als Funktion von Zeit und Temperatur zu bestimmen. Diese Methode wurde benutzt, um dunne¨ gesput- terte amorphe Filme der Zusammensetzungen Ag0.055In0.065Sb0.59Te0.29 (im Folgenden AgIn-Sb2Te genannt), Ge4Sb1Te5, Ge2Sb2Te5 und Ge1Sb2Te4, die in Phasenwechsel-
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