In situ transmission electron microscopy study of the crystallization of fast-growth doped Sb x Te alloy films
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Andrew Pauza Plasmon Data Systems Ltd., Hertsfordshire, SG8 6EN, United Kingdom (Received 11 November 2004; accepted 15 March 2005)
Crystallization of amorphous thin films composed of doped SbxTe with x ⳱ 3.0, 3.6, and 4.2 and constant dopant level was studied by in situ heating in a transmission electron microscopy. Magnetron sputtering was used to deposit 20-nm-thick films sandwiched between two types of 3-nm-thick dielectric layers on 25-nm-thick silicon-nitride membranes. One type of dielectric layer consists of ZnS–SiO2 (ZSO), the other of GeCrN (GCN). Crystallization was studied for temperatures in-between 150 and 190 °C. The type of dielectric layer turned out to strongly influence the crystallization process. Not only did the nucleation rate appear to depend sensitively on the dielectric layer type, but also the growth rate. The velocity of the crystalline/amorphous interface is about 5 times higher for the x ⳱ 4.2 film than for the x ⳱ 3.0 film if ZSO is used. In case of GCN, the interface velocity is about 2 times higher for the x ⳱ 4.2 film than for the x ⳱ 3.0 film. The activation energy for crystal growth is not significantly dependent on the Sb/Te ratio but is clearly different for ZSO and GCN—2.9 eV and 2.0 eV, respectively. The incubation time for the crystal nuclei formation is longer for ZSO than for GCN. Although the effects of the Sb/Te ratio and the dielectric layer type on the growth rates are strong, their effects on the nucleation rate are even more pronounced. A higher Sb/Te ratio results in a lower nucleation rate and the use of GCN instead of ZSO leads to higher nucleation rates.
I. INTRODUCTION
The crystallization rate in phase change optical recording, known from the rewritable CD and DVD formats, is becoming increasingly important because of the increasing demands on data-transfer rates. Crystallization is the rate-limiting process because amorphization is inherently a much faster process that in principle can be performed within femto-seconds.1 Ge2Sb2Te5 shows nucleationdominated crystallization; i.e., it nucleates easily and fast, but it shows only limited growth, with final sizes of the crystallites within a disk of 10–30 nm.2,3 With the ongoing decrease of the amorphous-mark sizes due to a decrease in laser wavelength and an increase in numerical aperture of the focusing lens, phase-change materials showing very fast growth (i.e., growth-dominated crystallization) tend to become more preferable than
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0228 J. Mater. Res., Vol. 20, No. 7, Jul 2005
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Ge2Sb2Te5, at least with respect to attainable datatransfer rates.3,4 If the bit size decreases, the distance a fast growing crystal has to proceed from the edge of the amorphous mark to its center decreases, and consequently the rewriting speed increases. For decreasing mark sizes, nucleation becomes less an issue since “nuclei” are always available at the edge of the mark. Therefore, phas
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