1. University of Idaho ECE Research Colloquium March 8, 2007 Multi-Layer Phase-Change Electronic Memory Devices Kris Campbell Associate Professor Dept. of Electrical and Computer Engineering & Dept. of Materials Science and Engineering Boise State University

2. Introduction Chalcogenide-based memories – why do we need a new memory technology? Types of chalcogenide resistive memories – ion conducting and phase-change Chalcogenide memory stack structures Tuning the phase-change memory operating parameters With materials Electrically Summary

3. What is a Chalcogenide Material? A Chalcogenide material contains one of the Group VI elements S, Se, or Te (O is usually omitted). Some examples of chalcogenides: GeS – germanium sulfide SnSe – tin selenide ZnTe – zinc telluride

4. Energy generation (solar cells) Photodetectors Environmental pollutant detection Energy storage (batteries) Memory (CD’s, electronic) Chalcogenide materials are key to many new technology developments Uses of Chalcogenide Materials

5. Why Are New Memory Technologies Under Development? Could replace both DRAM and Flash memory types DRAM has reached a size scaling limitation and is volatile Flash is prone to radiation damage, is high power, and has a short cycling lifetime Radiation resistant Scalable Low power operation Reconfigurable electronics applications Potential for multiple resistance states (means multiple data states in a single bit)

6. How Does a Chalcogenide Material Act as a Memory? Chalcogenide materials can be used as resistance variable memory cells: Logic ‘0’ state: R cell > 200 kΩ Logic ‘1’ state: R cell = 200 Ω to 100 kΩ The resistance ranges vary quite a bit depending upon the material used. ‘0’‘0’ ‘1’‘1’ 1 MΩ Write, V w V 10 kΩ V Erase, V e OFFON

7. ON and OFF State Distributions Resistance values in the ON and OFF states have a distribution of values; Threshold voltages or programming currents for ON and OFF states also have a distribution of possible values.

8. Single Bit Test Structure Top down view Device is here

9. Types of Chalcogenide Resistive Memory Ion-Conducting Ions (e.g. Ag + and Cu + ) are added to a chalcogenide glass Application of electric field causes formation of a conductive channel through glass (Kozicki, M.N. et al., Microelectronic Engineering 63, 485 (2002)) Thermally Induced Phase Change Crystalline to amorphous phase change; low R to high R shift High current heats material to cause phase change (S.R. Ovshinsky, Phys. Rev. Lett. 21, 1450 (1968))

10. Ion-Conducting Memories Resistance variable memory based on Ag + mobility in a chalcogenide glass; Ag is photodoped into a Ge x Se 100-x based chalcogenide glass (x<33). Ag Ge 30 Se 70 Visible light (Ge 40 Se 60 ) 33 (Ag 2 Se) 67 Developed by Axon Technologies (http://www.axontc.com)http://www.axontc.com

11. Ion-Conducting Memories - Operation A positive potential applied to the Ag electrode writes the bit to a low resistance state; A negative potential applied to the Ag-containing electrode erases the bit to a high resistance state. - +

12. Ion-Conducting Chalcogenide-Based Memories Example material: Ge 30 Se 70 photodoped with Ag Ag (Ge 30 Se 70 ) 67 Ag 33 W V From Kozicki, et al. NVMTS, Nov. 2004.

13. Why is Glass Stoichiometry Important For Photodoping? Glasses in region I phase separate and form Ag 2 Se. Glasses in region II will not phase separate Ag 2 Se but will put Ag on the glass backbone. Photodoped Ge 30 Se 70 will form 32% Ge 40 Se 60 and 68% Ag 2 Se. Mitkova, M.; et al., Phys. Rev. Lett. 83 (1999) 3848-3851.

14. Traditional Ion-Conducting Structure vs Stack Structure Ag Ge 30 Se 70 Bottom electrode Ag 2+x Se Top electrode Ge 40 Se 60 Bottom electrode Traditional Ion-Conducting Memory Structure Stacked Layer Ion-Conducting Memory Structure

15. Ag 2 Se-Based Ion-Conducting Memory (Instead of Photodoping with Ag) ‘1’ Low R ‘0’ High R VwVw VeVe

16. Ion-Conducting Memory Improvement Ag 2 Se can be replaced with other metal- chalcogenides. Examples: SnSe, PbSe, SnTe, Sb 2 Se 3 The Ge-chalcogenide must contain Ge-Ge bonds. GeSe-based materials are more stable than S or Te containing materials.

17. Ion-Conducting Memory Improvement Eliminate Ag photodoping Use a metal-chalcogenide layer above a Ge x Se 100-x glass with carefully selected stoichiometry ‘1’ Low R ‘0’ High R VwVw VeVe Metal Chalcogenide

18. Ion-Conducting Memory Research Projects Investigate operational mechanism: Influence of metal in the Metal-Se layer. Role of redox potential Glass – rigid or floppy Type of mobile ion (e.g. Ag or Cu) Effects of these on memory properties: switching speed power data retention resistance distribution thermal tolerance

19. What Are Phase-Change Materials? Materials that change their electrical resistance when they are switched between crystalline and glassy (disordered) structures. A well-studied example is Ge 2 Sb 2 Te 5 (referred to as GST). Figure modified from Zallen, R. “The Physics of Amorphous Solids” John- Wiley and Sons, New York, (1983) 12. Low Resistance High Resistance

20. Thermally Induced Phase Change Creates Low R State Creates High R State

21. Phase Change Memory IV Curve One programming voltage polarity. Current requirement can be high. Voltage application must go beyond V T before switching will occur. Polycrystalline

22. Traditional Phase Change Structure Compared to a Stack Structure Bottom electrode Top electrode Ge 2 Sb 2 Te 5 Top electrode SnTe Bottom electrode GeTe Traditional Phase Change Memory Structure Stacked Phase Change Memory Structure

23. Phase-Change Memory Multi-Layer Stack Structures Tested Devices consist of a core Ge-chalcogenide (Ge-Ch) layer and a metal chalcogenide layer (M- Ch). Properties wanted: Flexible operational properties; tunable via materials selection or operating method Multiple resistance states Low power Large cycling lifetime Device Dimensions: 0.25 um via

24. Initial Devices Tested Initial devices tested consisted of the stacks: (1) GeTe/SnTe (2) Ge 2 Se 3 /SnTe (3) Ge 2 Se 3 /SnSe It was found that the material layers used had a significant effect on device operation.* * Campbell, K.A.; Anderson, C.M. Microelectronics Journal, 38 (2007) 52-59.

25. GeTe/SnTe TEM Image W SnTe Si 3 N 4 GeTe W

26. Electrical Characterization Methodology Perform a current sweep with the top electrode potential either at a +V or a - V. Perform limited cycling endurance measurements on single bit structures.

27. Initial Electrical Characterization GeTe/SnTe Structure, +V +V is on the electrode nearest the SnTe Layer (top electrode)

28. Initial Electrical Characterization GeTe/SnTe Structure, - V -V is on the electrode nearest the SnTe layer (top electrode) Snap back at a higher V and higher I than the +V case.

29. Initial Electrical Characterization Ge 2 Se 3 /SnTe Structure

30. Initial Electrical Characterization Ge 2 Se 3 /SnSe Structure No switching!

31. Initial Electrical Characterization Ge 2 Se 3 /SnSe Structure A 30nA pre-condition (+V), Followed by -V Switching!

32. Movement of Sn Ions into Ge 2 Se 3 Activates Operation +V drives Sn 2+ or Sn 4+ ions into the lower glass layer, thus allowing it to phase change. -V will not produce phase change since Sn ions do not move into lower glass. An activation (pre-conditioning) step of +V at very low current (nA) will alter the Ge 2 Se 3 material, thus allowing phase change operation to occur with –V.

33. Initial Results Summary GeTe/SnTe – phase change switching, +/-V Ge 2 Se 3 /SnTe – phase change switching, +/-V Ge 2 Se 3 /SnSe – phase change switching, +V; -V switching only possible after +V, low current conditioning. Sn ions were moved into the Ge-Ch layer during +V operation. Te ions were moved into Ge-Ch layer during -V operation.

34. Tuning the Switching Properties By selection of stack structure, we can create a device with selective operation (on only when activated). Operational mode depends on the voltage polarity used with the device. Can we tune the switching properties by altering the metal used in the metal chalcogenide layer or the electrode materials?

35. Tuning Operating Parameters with Materials Ge-Ch stoichiometry: Ge-Ge bonds provide a thermodynamically favorable pathway for ion incorporation. Metal-Ch: The redox potential, ionic radii, oxidation state, and coordination environment properties of the metal will impact the ability of the metal ion to migrate into and incorporate into the Ge-Ch material. Addition of other metal ions: What happens upon the addition of small amounts of Cu or Ag?

36. Testing the Lower Glass and Metal Ion Influence We have subsequently tested the following stacks: (1) GeTe/ZnTe – metal ion influence (2) GeTe/SnSe – lower glass influence (3) Ge 2 Se 3 /SnSe/Ag – metal ion (4) GeTe/SnSe/Ag – metal ion and lower glass (5) Ge 2 Sb 2 Te 5 (GST)/SnTe – lower glass Resistance switching is observed in all stacks – but switching properties are different.

37. Current-Voltage Curves of Stack Structures +V applied

38. Effects of M-Ch Layer on Switching +V applied

39. How are the Electrical Properties Altered by Addition of Ag? Devices were tested with: Ge 2 Se 3 /SnSe/Ag GeTe/SnSe/Ag

40. Ge 2 Se 3 /SnSe/Ag Device – Multistate Resistance Behavior

41. GeTe/SnSe/Ag Device – Some Multistate Behavior

42. Metal Ion Effects Summary The metal ion influences the possible multiple resistance states. Metal ion allows phase change switching in cases where the Ge-Ch normally does not switch. We can use the metal ion to alter the voltage needed to initiate ‘snap back’ for phase change operation or alter the switching currents. Under investigation: Switching speed and cycle lifetime Temperature dependence Resistance state retention Resistance stability of multistate behavior.

43. Electrical Characterization – Lifetime Cycling Single bit testing is not ideal, however it does provide insight into how the material stack might perform over many cycles. Agilent 33250A Arbitrary Waveform Generator Agilent Oscilloscope Micromanipulator R load PCRAM Device R load is typically 10 kΩ to 1 kΩ depending on the material under study.

44. Electrical Characterization – Lifetime Cycling – GeTe/SnTe GeTe/SnTe – initial tests show bits cycle > 2 million times. Input (red) and V across load resistor (black)

45. Electrical Characterization – Lifetime Cycling – Ge 2 Se 3 /SnTe Ge 2 Se 3 /SnTe – initial tests show more consistent cycling than GeTe/SnTe structures. Input (red) and V across load resistor (black) Current through device (calculated by V load /R load )

46. Electrical Characterization – Lifetime Cycling –Ge 2 Se 3 /SnSe > 1e6 cycles Operation up to 135 °C.

47. Ge 2 Se 3 /SnSe/Ag Device Cycling T = 135°C; R load = 1kΩ

48. GeTe/SnSe/Ag Device Cycling T = 30°C; R load = 1.5kΩ

49. Materials Questions We Need To Ask How are switching parameters altered by the materials and stack structure? Influence of Ge-Ch structure on switching? Properties of the M-Ch work function? Metal ion properties? How well does it ‘fit’ into the glass structure? How mobile is the ion and what energy is required to cause it to move? Adhesion to electrodes? Knowing these answers will allow optimization for device electrical property tuning.

50. Tuning Operating Parameters Electrically Can we find electrical probing techniques that will: Enable well separated resistance states? Improve data retention and temperature dependence? Create a wide dynamic range of allowed resistance values in a programmed state? What are the operating limitations in order to avoid losing the resistance state while in use in a circuit?

51. Multiple Resistance States – Challenges Resistance range can vary as a function of: Programming current Temperature Programming pulse parameters Retention time of the resistance value can also vary as a function of these parameters. How well does the resistance state get retained during operation as a ‘resistor’ in a circuit? Quite often, due to the nature of the amorphous materials, the resistance values have a large spread. This overlap prevents reliable use of multistate programming with these materials. Can we use electrical techniques to help?

52. Example of Poor Programming Resistance Distributions: GeTe/SnSe

53. Electrical Control: Reverse Potential Programming Provides Multiple Resistance States +V -V

54. Electrical Control Summary Multistate resistance programming possible by programming with negative and positive potentials in the Ge-Ch/M-Ch stack structure. Electrically controlled activation of stack structure allows a device to be ‘turned on’ when it is needed.

55. Summary Using Stacked Layers, we have more device operational flexibility… We can control and tune operational parameters: Threshold voltage, programming current, speed, retention, endurance Value of resistance states Number of possible resistance states We can electrically control device function Electrically activated devices Larger dynamic range between resistance states

56. Acknowledgements Collaborators: Prof. Jeff Peloquin, Boise State University – synthesis of materials. Mike Violette, Micron Technology – equipment loan and use of analytical facilities for thin film characterization (SEM, ICP, TEM). Prof. Santosh Kurinec, Rochester Institute of Technology – characterization of thin film stacks using XRD, RBS, Raman; development of CMOS-based test array for materials stacks. Students: Morgan Davis, Becky Munoz, Chris Anderson, Daren Wolverton. Funding: This research was partially supported by a NASA Idaho EPSCoR grant, NASA grant NCC5-577.

58. Phase-Change Memory Radiation Resistance OFF state : Complete crystallization is not induced by SEE or TID. Localized crystallization can occur.* ON state : Even if some regions in the crystalline material are disturbed by SEE or TID, the crystallinity in the rest of the cell will keep R low. Phase-Change Memory * El-Sayed, S.M. Nuclear Instruments and Methods in Physics Research B 225 (2004) 535-543.

59. Ion-Conducting Memory Radiation Resistance OFF State : Material is disordered, SEE or TID will not affect it. ON State : Ag filling the conductive channel would have to be completely displaced from contact with either electrode. Ion-Conducting Memory