Application of p electron theory to predict new materials for rewritable optical recording

Phase change recording is based upon the reversible phase transformation of a chalcogenide Te alloy between the crystalline and amorphous states using a focused laser. The formation of the micron-sized bit of information is distinguished from the difference in the optical properties of the two states. Until now most alloys utilised in optical data storage are based on ternary GeSbTe and the quartenary AgIn-Sb2Te alloys. To ensure that optical data storage remains competitive, new alloys with superior performance should be designed. However the design of new phase change alloys is hampered by the lack of a detailed theory for material selection. In the past the search for new alloys for optical data storage applications has been based on trial and error strategies. These approaches were expensive and time consuming especially when the exploration of the vast composition phase space is to be done. Therefore this work proposes some guidelines based on a newly developed theory to select and design new phase change materials with superior performance. These guidelines are based on the critical material requirements such as fast erasure, sufficient optical contrast and single isotropic structure specified for any suitable phase change media. To this end Au19In26Te55, Au18Sn23Te59, Au18Sb23Te59, In52Sb19Te29, and Ag18Sn26Te56 alloys have been prepared by combinatorial material synthesis approach to establish guidelines for selecting new phase change alloys. In optical data storage, recrystallization stands out as the technologically important process since it is the time limiting step. Suitable phase change media therefore require short recrystallization times in the order of nanoseconds. Therefore recrystallization has been performed on these alloys to determine their suitability as phase change media. Investigations on Au18Sb23Te59, and In52Sb19Te29 have shown recrystallization times of 110 and 76 ns, respectively thus making these alloys suitable for optical recording. While on one hand, long crystallization times in the order of 1 microsecond have been observed for the Au18Sn23Te59 no recrystallization was evident for the Au19In26Te55 alloy. This as will be mentioned later is attributed to the low optical contrast of this alloy. Even though the recrystallization time is significant for any suitable phase change media, not much information about the data transfer rate as well as the storage density can obtained from this parameter alone. Such information is obtained from the mechanisms of recrystallization since these mechanisms are crucial to improve the data transfer rates and storage densities. The classification of phase change alloys on the basis of nucleation and growth dominated alloys is technologically essential especially for alloys whose recrystallization times are dependent on the size of the bit. However the classification is hampered by the lack of a theory that separates these mechanisms for various alloy compositions. As a step towards establishing a criterion to distinguish these mechanisms, far field setup experiments on Au18Sb23Te59 and In52Sb19Te29 alloys have been performed to establish their mechanisms of recrystallization. The static tester results have shown that these alloys are growth-dominated. Using arguments based on previous experiments which have demonstrated that alloys with Tg/Tm < 0.5 re-crystallize through nucleation. The recrystallization measurements can be correlated with the Tg/Tm values for these alloys which have been determined to 0.52 and 0.59, respectively. From this results, it is seen that the Tg/Tm ratio presents a simple approach to distinguish recrystallization mechanisms for phase change alloys. In addition to fast recrystallization, sufficient optical contrast is a basic requirement for suitable phase change media. To this end, the optical properties of the above alloys have been determined in the as deposited and crystalline states. In addition the density change of these alloys have been determined upon crystallization. A high optical contrast is reported for the Au18Sn23Te59, Au18Sb23Te59 and In52Sb19Te29 alloys. Such high optical contrasts are coupled to a considerable density increase of at least 4% upon crystallization which allows phase change recording for the Au18Sn23Te59, Au18Sb23Te59 and In52Sb19Te29 alloys. However a low optical contrast of less than 1 % was established for the Au19In26Te55 alloy. This is related to a lower density contrast of 2% which leads to little or no change in the optical properties of this alloy. The low optical contrast could be the reason of not observing recrystallization in the Au19In26Te55 alloy. Suitable phase change materials have been previously established to crystallize frequently to a cubic structure which allows optical recording. To correlate the fast recrystallization and sufficient optical contrast, structure investigations were performed by X-ray diffraction. Our results have shown that the Au18Sn23Te59, Au18Sb23Te59, In52Sb19Te29 alloys which could be recrystallized, are cubic in structure. However, a chalcopyrite structure is reported for the Au19In26Te55 alloy. These results therefore indicate that the cubic structure is the preferred structure for most suitable phase change alloys. To understand the differences in these structures, the average number of the s and p valence electrons was analysed for each alloy. Hence it is seen that alloys with chalcopyrite structure have Nsp = 4 whereas cubic structures are characterised with Nsp > 4. It therefore follows that the average electron number provides an easy and fast criterion for predicting suitable phase change alloys. The speed with which new alloys can be designed is determined to a great extent by the successful predictions by theoretical calculations of their crystal structures. Towards this end DFT calculations have been applied to determine the ground state structure of these alloys. The results have correctly predicted the thermodynamic structures of these alloys with the exception of Ag18Sn26Te56 alloy. Now that the area to look for suitable materials is identified as that of cubic structures, it is predicted that the scope of the search can be reduced further by DFT calculations to predict the most stable of the six possible candidates identified as constituents of the cubic structure.

Grantor dc:publisher

Publikationsserver der RWTH Aachen University

Year dc:date

2004

Author dc:creator

• Wamwangi, Daniel Muturi

Contributors dc:contributor

• Wuttig, Matthias

• info:eu-repo/classification/ddc/530
• Optischer Speicher
• Telluride
• Strukturelle Phasenumwandlung
• Dichtefunktionalformalismus
• Elektronentheorie
• Physik
• Phase Change
• Structure
• combinatorial material synthesis

Statement dc:rights

• info:eu-repo/semantics/openAccess

Language dc:language

eng

Identifier

https://publications.rwth-aachen.de/search?p=id:%22RWTH-CONV-123610%22

OAI identifier oai:identifier

oai:publications.rwth-aachen.de:62011