Mechanisms of Initialization of Doped Sb-Te Phase-Change Media
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Mechanisms of Initialization of Doped Sb-Te Phase-Change Media Samantha J. Towlson1, Clifford A. Elwell1*, Clare E. Davies2 & A. Lindsay Greer1 1 University of Cambridge, Dept. of Materials Science and Metallurgy, Cambridge, UNITED KINGDOM 2 Plasmon Data Systems Ltd, Melbourn, UNITED KINGDOM *corresponding author. ABSTRACT Laser initialization of the chalcogenide optical-recording medium Ag-In-Sb-Te is investigated using transmission electron microscopy of the resulting microstructure. Initialization beam power and velocity are varied. The average inhomogeneous strain of the chalcogenide is estimated from X-ray peak broadening. At high power and low velocity a clearly defined columnar grain structure with low strain is produced, typical of directional solidification. At low power and high velocity the initialized structure has a high density of defects and high strain; this is attributed to crystallization from the amorphous rather than the liquid state. The beam power and linear velocity of laser initialization may therefore be used to control the microstructure. INTRODUCTION Chalcogenide-based phase-change materials are of interest as the active layer in re-writable CDs and DVDs. As-deposited films of the low-reflectivity amorphous phase are initialized to form a crystalline layer in which data marks can be written by local melting and amorphization. The materials can be divided into two categories based on the mechanism of mark erasure [1]. With nucleation-dominated systems such as Ge-Sb-Te, erasure occurs by crystal nucleation and subsequent growth throughout the amorphous mark [1]. In contrast, erasure in growthdominated systems such as Ag-In-Sb-Te occurs without nucleation, by growth from the crystalline film surrounding the mark [1]. The microstructures in the latter type of medium have not been widely studied and are the subject of the present work. Reading the data on a disc often shows increased error rates for the first few overwrite cycles. After this jitter bump [2], error rates are low until the cyclability limit is reached. Reducing the jitter bump and extending cyclability are key aims which may be achieved by optimizing the initialization. The crystallization of Ag-In-Sb-Te materials has previously been studied mainly by differential scanning calorimetry or by conventional thermal annealing [3-6]. Laser initialization of active layers, however, involves much higher lateral thermal gradients (~109 K m–1), possibly producing grain structures and phases quite different from those in annealed bulk samples [3]. Rapid solidification is known to lead to unusually high defect densities in crystalline materials (vacancies, dislocations, stacking faults [7]). EXPERIMENTAL DETAILS Re-writable CD-RW thin-film stacks were produced by sputter deposition on to polycarbonate discs (patterned into the usual lands and grooves [8]). The stack sequence is
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substrate/ZnS-SiO2 (91 nm)/ Ag-In-Sb-Te (27 nm)/ ZnS-SiO2 (170 nm)/ Al-alloy (36 nm); followed by a protective lacquer. The as-deposited Ag-In-Sb-
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