Phase Change Materials: Chalcogenide (Optical Storage) Luca Lutterotti Functional and Smart Materials

Transcription

1 Phase Change Materials: Chalcogenide (Optical Storage) Luca Lutterotti Functional and Smart Materials

2 Introduction: history of optical storage Philips ( ) introduce the videodisc Philips and Sony made an agreement in1979 to develop the compact disc (CD) that will revolutionize the music market. In 1983 the CD (followed by the CD-R, CD-I and video-cd for multimedia application and video) is introduced on the market followed by the CD-ROM for computers in In 1996 appears the Digital Versatile Disc (DVD) followed by the DVD-RAM, DVD-R/RW, DVD+R/RW, double side or double layer. In 2002 the Blu-ray Disc, BD (Sony, Philips, LG, Samsung etc.) comes out and the HD-DVD (Panasonic, Microsoft, etc.) followed.

3 History of Optical Storage Media Year Product Capacity Trans. speed 1983 CD 74 min. music CD-R 74 min. music CD-ROM 650 MB 4.3 Mbps (1x) 1995 CD-RW MB DVD, DVD-RAM, DVD-R/RW, DVD+R/RW 2002 BD, BD-RE (23.3 GB), 2004 HD-DVD 4.7 GB (1 side) 11 Mbps (1x) 25 GB (1 layer) 50 GB (2 sides) 15 GB (1 layer) 30 GB (2 sides) 36 Mbps - 201x higher aperture lens GB (1 layer) -? DNA based? -

4 Evolution (roadmap)

5 Optical Drive

6 Optical Drive λ α Optical Data Storage Figure 1. Schematic drawings of an optical storage system. The upper image r

7 More important parameters Because they determine the capacity of the system: Wavelength of the laser (λ) Aperture number of the lens (NA=sinα)

8 Basic principles The data pattern in the trace corresponds to the bitstream of the channel and are converted by a decoder (reading) or encoder (writing) in/from user bitstream

9 Basic principles (contd.) The spots are zone of the material with different optical properties respect to the matrix. For example the detector can read a different beam intensity from the laser depending if it passes through a material with higher or lower reflectivity. Different layers can be written/read using a different laser focalization. What is really recorded is the modulation of the intensity caused by the interference of the spots sequence. The modulation is then analyzed/transformed into the real spot sequence.

10 1. Introduction 7 Basic principles: modulation

11 Laser focalization

12 Tracking (following the trace) 1. Introduction 13

13 Tracking (following the trace)

14 Tracking (following the trace)

15 Characteristics Parameter CD DVD BD Wavelength, nm Numerical aperture Track pitch, m Channel bit length, nm Shortest effect length, nm Modulation code Physical bit density, Gbit/inch 2 Reference disc velocity 1x CLV, m/s Substrate/cover thickness, mm Spot size, m Capacity per data layer, GB Transfer rate at 1x speed, Mbit/s / EFM / EFM PP

16 Materials for optical storage 1 layer composition

17 2 layers Materials for optical storage

18 Reflectance

19 How the spot are created Laser pulse controlled in time and power

20 Materials requirements storage requirement Material requirement material property Writability glass former melting point/layer design, appropriate optical absorption archival storage stable amorphous phase high activation energy, high crystallization temperature Readability large signal to noise ratio high optical contrast Erasability fast re-crystallization simple crystalline phase, low viscosity Cyclability Stable layer stack low stresses, low melting temperature

21 Chalcogenides for optical storage (or phase change materials) Year Composition 1971 Te-Ge-Sb-S 1974 Te-Ge-As 1983 Te-Ge-Sn-O 1985 Tn-Sn-Se, Ge-Se-Ga 1986 Te-Ge-Sn-Au, Sb 2 Se, In-Se, GeTe, Bi-Se-Sb, Pd-Te-Ge-Sn 1987 GeTe-Sb 2 Te 3,(Ge 2 Sb 2 Te 5, GeSb 2 Te 4 ), In-Se-Tl-Co 1988 In-Sb-Te, In 3 SbTe GeTe-Sb 2 Te 3 -Sb, Ge-Sb-Te-Pd, Ge-Sb-Te-Co, Sb 2 Te 3 -Bi 2 Se Ag-In-Sb-Te 2001 Ge-In-Sb-Te 2004 Ge-In-Sn-Sb

22 Glass transition V Liquid Undercooled liquid Glass Crystal T g T m T

23 Nucleation and growth Nucleation and growth energies: Growth rate (u) Nucleation rate (I) Crystallization rate (k) where

24 Nucleation and growth nucleation > growth growth > nucleation

25 Nucleation and growth

26 Phase change materials, type I Crystallization by nucleation and growth High activation energy

27 Phase change materials, type II Growth controlled crystallization

28 Materials simulation by DFT (Density Functional Theory) To investigate/screen wider ranges of compositions

29 DFT (AuSbTe2) Electron density The simulation considers the free electrons and in the band gap The system energy is calculated and minimized

30 Laser writing

31 Cooling effects Temperature measurement for a sequence of 6 writing pulses with slow and fast (BD stack)

32 Comparison DVD-BD

33 Bibliography E. R. Meinders, A. V. Mijiritsii, L. van Pieterson & M. Wuttig. Optical Data Storage. Phase-Change Media and Recording. Springer, Dordrecht, 2006, ISBN