Effects of disorder in chalcogenides: interesting concepts and applications

Transcription

1 Effects of disorder in chalcogenides: interesting concepts and applications from Optical Data Storage to Novel Electronic Memories Matthias Wuttig RWTH Aachen University, Germany ICANS 26, Aachen,

2 Questions: 1) What are Phase Change Materials? 2) Why are they used in Optical Data Storage and Novel Electronic Memories? 3) What is a universal memory? 4) What is resonance bonding? 5) Why is it important for phase change materials? 6) How does the treasure map for Phase Change Materials look like? 7) What have Profs. Mott and Anderson been arguing about? 8) Lessons to learn from Rene Descartes? 9) and from Watson? 2 2

3 Question 1) What are Phase Change Materials? 3

4 4 The principle of phase change recording Amorphous phase Short high laser or current pulse (reset pulse) Low reflectivity High resistance Liquid phase Laser power or electric current Laser power or electric current Time Long low laser or current pulse (set pulse) Time 1 0 Crystalline phase High reflectivity Low resistance Free Enthalpy energy ( H Free or (Atomic) volume) Volume Under cooled Amorphous Crystalline Tg Crystallization Tm Melting Super heated Temperature (K) M. Wuttig and N. Yamada, Nature Materials 6, 824 (2007) Concept first suggested by S. Ovshinksy in (1968)

5 5 Answer 1) Phase Change Materials (for data storage): are materials that can be rapidly switched between two metastable states (usually an amorphous and a crystalline state), which significantly differ in their optical and electrical properties

6 6 Question 2) Why are they used in Optical Data Storage and Novel Electronic Memories? see answer 1) Phase Change Materials (for data storage): are materials that can be rapidly switched between two metastable states (usually an amorphous and a crystalline state), which significantly differ in their optical and electrical properties, and hence can be used to store information

7 Application potential: Date Storage (optical and electronic) Typical structure of rewriteable DVD (BD-ray disc) Courtesy of N. Yamada (Panasonic) Switching between amorphous and nanocrystalline state 7

8 Unique property correlation in phase change materials (Scientific challenge) Pronounced structural difference between amorphous and crystalline state nevertheless: fast crystallization (time scale ns) Structure Phase Change Materials Kinetics Properties Threshold switching unique electronic transport Pronounced optical contrast They enable new storage solutions, including NVMs! 8

9 Question 3) What is a universal memory? 9

10 Development of Electronic Memory Technologies Market volume: 50 Billion $ Goal: Universal Memory M.Wuttig, Nature Materials 4, 265 (2005) 10

11 11 Answer 3) A universal memory combines the speed of DRAM with the non-volatility of FLASH Latest announcement: Intel and Micron introduce Xpoint memory (July 2015)

12 12 Phase Change Memories Recent announcements: Samsung and Micron ship PC-RAMs to OEMs (Mobile Phones). Simple principle! Speed? Scalability? Power consumption? all will depend on PC material Goal: Design PC materials Understand which materials to choose from

13 Question 4a) Where is the property contrast coming from? 13

14 Goal: Understanding properties: Structure Property Relationship? 14 Loss of long range order sufficient to explain property contrast?

15 Property contrast between amorphous and crystalline state SiO 2 Pronounced No difference difference in optical in properties! XRD spectrum! 15

16 Structural difference between amorphous and crystalline state for Ge 2 Sb 2 Te 5 (long range order) (a) (b) 200nm I. Friedrich et al., Thin Solid Films 38, 239 (2001) Absence of long range order in amorphous state 16

17 Structural difference between amorphous and crystalline state (short range order) GaAs Ga edge EXAFS Ge 2 Sb 2 Te 5 As edge A. Kolobov et al., Nature Materials 3, 703 (2004) Pronounced change of local structure for PC media 17

18 18 Phase Change Materials: Surprising crystal structures - High vacancy concentrations PC alloys: pseudobinary materials Ge 1 Sb 2 Te 4 High vacancy concentration

19 Change in energy upon atom removal M. Wuttig et al., Nature Materials 6, 122 (2007) Atom removal (vacancy formation) lowers the energy! 19

20 Analysis of occupied orbitals Calculation by M. Gillesen, R. Dronskowski (AC/ RWTH Aachen) M. Wuttig et al., Nature Materials 6, 122 (2007) Solid has too many electrons removal of atoms reduces energy 20

21 Origin of strong change in absorption Absorption given by Fermi's golden rule: No significant changes in jdos Matrix elements define optical contrast! This explains how DVD works slide 16/18 W. Welnic et al., Physical Review Letters 98, (2007) 21

22 22 How about simplicity, speed and insight? Investigate phase change alloys for photons with energies below Eg

23 Typical case: covalent semiconductors 23

24 The surprise: Phase Change Materials (Ge 1 Sb 2 Te 4 ) 24

25 Data analysis ε E g amorphous crystal % increase amorphous crystal % decrease Ge 1 Sb 2 Te Ge 1 Sb 1 Te GeTe Ge 2 Sb 2 Te Ge 15 Sb Ge 2 Sb 1 Te Ge 3 Sb 4 Te AIST Te Se Amorphous phase predictable (if density and stoichiometry known) Crystalline phase not predictable from those numbers The crystalline state provides the challenge! 25

26 The true electronic structure: Two limiting cases Cross section through (100) plane 26

27 The true electronic structure: Resonance Bonding Cross section through (100) plane 27

28 Fingerprints of Resonance Bonding High optical dielectric constant ɛ Small band gap High Born effective charge Anharmonic potential Soft TO modes large static dielectric constant ɛ st Resonance bonding explains optical contrast Resonance bonding only for a subset of chalcogenides (treasure map) 28

29 Difference amorphous crystalline state in PCMs: Crystalline state: Resonance bonding, leading to high electronic polarizability, small band gap, lattice distortions, high effective charges, soft modes, etc. Amorphous state: B. Huang, J. Robertson, PRB 81, 1(R), (2010) Resonance bonding impossible due to lack of medium range order, only ordinary covalent bonding, lower electronic polarizabilty, larger band gap, etc. Resonance bonding is only possible under unique conditions 29

30 30 Answer 4a) The optical property contrast stems from resonance bonding (which exists in the crystal but not the amorphous state)

31 31 Question 5) Why is resonance bonding important for Phase Change Materials? Answer 5) It helps to identify phase change materials

32 Question 6) How does the treasure map for Phase Change Materials looks like? 32

33 Introduce Coordinates for a Map for Phase-Change Materials Littlewood two coordinates measures of ionicity and covalency Generalization for non-binaries based on atomic radii derived from pseudopotential calculations ionicity ~ size difference covalency ~ s-p-splitting treat materials as effective binaries average cations = A average anion= B St. John and Bloch, PRL Vol. 33 Nr. 18 (1974) Littlewood, J. Phys. C.: Solid St. Phys. 13 (1980) Phillips, Solid. State. Communications Vol. 22 (1977) Chelikowsky and Phillips, Phys. Rev. B Vol. 17 Nr. 6 (1978) D. Lencer, M. Salinga, B. Grabowski, T. Hickel, J. Neugebauer, M. W., Nature Materials 7, 972 (2008) 33

34 Treasure Map D. Lencer, M. Salinga, B. Grabowski, T. Hickel, J. Neugebauer, M. Wuttig Nature Materials 7, 972 (2008) 34

35 D. Lencer et al., Nature Materials 7, 972 (2008) 35 Treasure Map