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Prospects of doped Sb-Te phase-change materials for high-speed recording

L. van Pieterson, M. H. R. Lankhorst, M. van Schijndel, B. A. J. Jacobs and J. C . N. Rijpers Philips Research Laboratories, Prof: Holstluun 4, 5656 AA Eindhoven, The Netherlands,

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Introduction

One of the main aims in phase-change optical recording is to increase the recording speed of existing and future data-storage systems as CD-RW, DVD+RW, DVD-RAM and DVR (Blu- Ray). Essential for high-speed optical recording is to optimize the phase-change material. Due to their growth controlled crystallization mechanism, Sb-Te compositions doped with e.g. Ge, In and Ag have proven to be very suitable materials for obtaining high data-rates in high-density systems as DVD+RW and DVR [l]. These growth type materials combine excellent optical contrast with high crystallization rate and good amorphous stability, making recording up to linear velocities of about 20 m/s possible. This paper focuses on the aspects of doped Sb-Te materials for even higher recording speeds at which diminished amorphous mark stability, increased media noise and severe recrystallization during writing become important issues.

Recording strategy and recrystallization

Excellent recording performance can be achieved for doped Sb-Te materials. This is illustrated in Fig. 1, which shows the jitter and modulation for a DVD+RW.disc with In-doped Sb-Te material. Average multitrack jitter below 8% has been achieved in a broad speed range of lx- 4 . 8 ~ DVD+RW. Recording is possible with a conventional 1 T-strategy, but using relatively high powers (up to 25 mw) and short pulses to overcome recrystallization during recording of the amorphous marks. It is noted that the archival life of this disc is not sufficient yet.

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Fig. I: Average multitrack jilter and modulation for an In-doped Sb-Te phase-change disc for la-4.8~ DVD+RW conditions. A conventional IT stratea was used with short pulses and relatively high pon'ers (up to 25 mW).

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Crystallization rate

The complete erasure time (CET) is defined as the minimum duration of the erase pulse for complete crystallization of a recorded amorphous mark in a crystalline layer. In Fig. 2(a) the CET is plotted versus the mark radius for In and Ge doped Sb-Te phase-change materials. It is observed that the CET is dependent on the Sb/Te ratio, with the crystallization rate increasing with Sbme ratio. Also the dopant influences the CET; Sb-Te compositions doped with In are generally found to be faster than these materials doped with Ge. In-doped Sb-Te compositions with high Sb/Te ratio have been designed with 30 dB DC-erasibility of recorded marks at linear velocities well above 40 d s , as can be seen in Fig. 2(b). From these data we are confident that the criteria regarding the high crystallization rates can be met for the (near) future.

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Fig. 2: (a) Complete erasure time as a function ofmark size and (b) erasibility as a function of recording velocity for doped Sb-Te phase-change materials.

Archival life stability

Besides high crystallization rates at elevated temperatures, at room temperature the amorphous marks should be stable against spontaneous recrystallization for at least 100 years. This is called the archival life stability. One way to increase the archival life stability of Sb-Te phase-change materials is doping with elements like Ge or In. We estimated the archival life stability of written

Temperature PC)

Fig. 3: Archival life stability of wrinen amorphous marks for doped Sb-Te phase-change materials (markers represent the experimental data). The doned line shows the extrapolation to room temperature.

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amorphous marks from measurement of the crystallization temperature of the phase-change material. Fig. 3 shows the archival life stability for Sb-Te phase-change materials doped with In and Ge. It is clear that the dopant influences the stability against crystallization; addition of Ge leads to extremely good archival life stability, while doping with In gives insufficient amorphous stability. This might be caused by the relatively higher binding energy of Ge in the amorphous matrix that raises the crystallization temperature [2].

Media noise

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Up to now, noise of phase-change optical discs based on doped Sb-Te has not received much attention. Fig. 4(a) shows the noise spectra of DVD+RW discs with Ge-doped Sb-Te materials. It is observed that noise increases with Sb/Te ratio. Since crystallization speed also increases with Sb/Te ratio, noise induced jitter problems can be anticipated for high-speed recording. We related noise to jitter, phase-change material composition and phase-change stack structure. For high-speed Ge-doped Sb-Te compositions, the jitter was dominated by the media noise. We found that the media noise originates from reflection variations in the crystalline phase, possibly induced by grain boundaries or different crystal orientations. Fig. 4(b) shows the influence of doping Sb-Te with Ge, In or Ag. Whereas doping with Ge and In yields similar noise, media- noise seems to be lower when Ag is used as a dopant. However, as the crystallization speed of Ag-doped Sb-Te composition is lower than for the In-doped material, the best trade-off between speed and noise may still be found for the latter.

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Fig. 4: (a) Noise spectra for Ge-doped Sb-Te compositions measured on the DVD+RW recorder. The peak corresponds to the wobble. (b) Values for dRiR and CET for doped Sb-Te (SbiT~3.6) at a calculated amorphous mark radius of 125 nm.

Conclusions

Materials optimization is a very important aspect in high-speed optical recording. It is observed that for doped Sb-Te phase-change materials, a high crystallization rate at elevated temperatures is often accompanied by a low archival life stability and high media noise. Although crystallization rate can be increased easily by tuning the phase-change material composition, the involved low archival life stability and/or high media noise may be a drawback for the application of doped Sb-Te materials in (ultra) high-speed recording.

[I]: H. J. Borg et al. Jpn. J. Appl. Phys. 40 (2001) 1592 [2]: M. H. R. Lankhorst, J. Non-Cryst. Solids 297 (2002) 210

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