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					     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


                       University of Idaho
                     ECE Research Colloquium
                          March 8, 2007
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
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
Uses of Chalcogenide Materials
                        Memory
Energy generation       (CD‟s, electronic)
(solar cells)



                           Chalcogenide         Photodetectors
                         materials are key to
                        many new technology
                           developments
  Environmental
  pollutant detection
                                                Energy storage
                                                (batteries)
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)
 How Does a Chalcogenide Material
 Act as a Memory?
 Chalcogenide materials can be used as resistance variable
  memory cells:
      Logic „0‟ state: Rcell> 200 kΩ
      Logic „1‟ state: Rcell= 200 Ω to 100 kΩ
         The resistance ranges vary quite a bit depending upon the

          material used.
                 V                                    V

                                Write, Vw

         1 MΩ                                               10 kΩ

                                Erase, Ve


                 ‘0’                                  ‘1’
                OFF                                   ON
ON and OFF State Distributions
 Resistance values in the ON and OFF states have a
  distribution of values;
                      1.0



                      0.8
       Distribution




                               ON                      OFF
                      0.6



                      0.4



                      0.2
                            1k to 200k                1M to 1G


                                         Resistance

 Threshold voltages or programming currents for ON and
  OFF states also have a distribution of possible values.
Single Bit Test Structure
                                              Top electrode   Metal-
                                                              chalcogenide
                                Memory cell


                                                                       Insulator


                       Bottom electrode




      Device is here
                                                     Top down view
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))
      Ion-Conducting Memories

       Resistance variable memory based
                                                          Visible light
        on   Ag+
               mobility in a chalcogenide
        glass;
       Ag is photodoped into a GexSe100-x
                                                                 Ag

                                                              Ge30Se70
        based chalcogenide glass (x<33).



                                                         (Ge40Se60)33 (Ag2Se)67
Developed by Axon Technologies (http://www.axontc.com)
Ion-Conducting Memories - Operation
 A positive potential applied to the
  Ag electrode writes the bit to a low
  resistance state;
                                         +-

                                                 Ag electrode

                             V
                                              (Ge2Se3)33(Ag2Se)67




                                         -+
 A negative potential applied to the
  Ag-containing electrode erases the
  bit to a high resistance state.
Ion-Conducting Chalcogenide-Based
Memories
Example material: Ge30Se70 photodoped with Ag




                                                          Ag
                                                V
                                                    (Ge30Se70)67Ag33
                                                           W




      From Kozicki, et al. NVMTS, Nov. 2004.
    Why is Glass Stoichiometry Important
    For Photodoping?
                                                   Glasses in region I
                                                    phase separate and form
                                                    Ag2Se.
                                                   Glasses in region II will
                                                    not phase separate
                                                    Ag2Se but will put Ag
                                                    on the glass backbone.
                                                   Photodoped Ge30Se70
                                                    will form 32%
Mitkova, M.; et al., Phys. Rev. Lett. 83 (1999)
                                                    Ge40Se60 and 68%
3848-3851.                                          Ag2Se.
 Traditional Ion-Conducting Structure
 vs Stack Structure


        Ag                            Top electrode
                                       Ag2+xSe
      Ge30Se70
                                       Ge40Se60
  Bottom electrode                   Bottom electrode

Traditional Ion-Conducting   Stacked Layer Ion-Conducting
    Memory Structure               Memory Structure
                       Ag2Se-Based Ion-Conducting Memory
                                 (Instead of Photodoping with Ag)
                      20


                      15
                                             ‘1’                            +
                                           Low R                                W electrode
Current (microamps)




                      10
                                                                        V          Ag2+Se
                        5

                                                                                 Ge40Se60
                        0
                                                                                W electrode

                       -5
                                  ‘0’                                       -
                                High R                       Vw
                                                      Ve
                      -10
                         -0.4       -0.2      0.0          0.2    0.4
                                            Voltage
    Ion-Conducting Memory Improvement
   Ag2Se can be replaced
    with other metal-
    chalcogenides.                   +

        Examples: SnSe, PbSe,           W electrode
                                             Ag
         SnTe, Sb2Se3
                                 V          SnSe
   The Ge-chalcogenide
    must contain Ge-Ge                    Ge40Se60

    bonds.                               W electrode

   GeSe-based materials are         -
    more stable than S or Te
    containing materials.
    Ion-Conducting Memory Improvement
 Eliminate Ag photodoping

 Use a metal-chalcogenide layer above a GexSe100-x
     glass with carefully selected stoichiometry
                                                                20
            +
                                                                15
                                                                                       ‘1’
                           W electrode                                               Low R



                                          Current (microamps)
                               Ag                               10
V                             SnSe
                 Metal Chalcogenide                               5

                             Ge40Se60                             0

                                                                            ‘0’
                            W electrode
                                                                 -5
             -                                                            High R
                                                                                                Ve     Vw
                                                                -10
                                                                   -0.4       -0.2      0.0          0.2    0.4
                                                                                      Voltage
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
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 Ge2Sb2Te5 (referred to as GST).



                                                Figure modified from Zallen,
                                                     R. “The Physics of
                                                 Amorphous Solids” John-
                                                Wiley and Sons, New York,
                                                         (1983) 12.




             Low                High
             Resistance         Resistance
Thermally Induced Phase Change

               Creates High R State




                                      Creates Low R State
Phase Change Memory IV Curve

                          One
                           programming
                           voltage polarity.

                          Current
                           requirement can
                           be high.

                          Voltage
                           application must
                           go beyond VT
                           before switching
       Polycrystalline
                           will occur.
 Traditional Phase Change Structure
 Compared to a Stack Structure

      Top electrode            Top electrode
                                  SnTe
      Ge2Sb2Te5
                                  GeTe
     Bottom electrode         Bottom electrode



Traditional Phase Change   Stacked Phase Change
    Memory Structure         Memory Structure
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                       Device Dimensions:
                                       0.25 um via
      Large cycling lifetime
Initial Devices Tested
 Initial devices tested consisted of the stacks:
        (1) GeTe/SnTe
        (2) Ge2Se3/SnTe
        (3) Ge2Se3/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.
GeTe/SnTe TEM Image

                      GeTe
                       W
 W
SnTe

Si3N4
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.
               Initial Electrical Characterization
               GeTe/SnTe Structure, +V
              +V is on the electrode nearest the SnTe Layer (top electrode)
                   -4
              10

                   -5
              10

                   -6
              10
Current (A)




                   -7
              10

                   -8
              10

                   -9
              10

               -10
              10

               -11
              10
                        0.4   0.6   0.8     1.0   1.2   1.4
                                      Voltage
              Initial Electrical Characterization
               GeTe/SnTe Structure, -V
  -V is on the electrode nearest the SnTe layer (top electrode)
                   -4
              10

                   -5
              10

                   -6
              10
Current (A)




                   -7
              10

                   -8
              10

                   -9
              10

               -10
              10

               -11
              10
                        0.0   0.5   1.0         1.5   2.0   2.5
                                      Voltage (V)


Snap back at a higher V and higher I than the +V case.
Initial Electrical Characterization
Ge2Se3/SnTe Structure
                    -4
               10

                    -5
               10

                    -6
               10
 Current (A)




                    -7
               10

                    -8
               10
                                                           +V
                    -9                                     -V
               10

                -10
               10

                -11
               10
                         0.2   0.4   0.6       0.8   1.0   1.2   1.4
                                           Voltage
Initial Electrical Characterization
Ge2Se3/SnSe Structure
                   -4
              10

                   -5
              10

                   -6
              10
Current (A)




                   -7
              10

                   -8
              10
                                                  +V
                   -9                             -V
              10                                  No switching!
               -10
              10

               -11
              10
                        0   2   4             6        8          10
                                    Voltage
              Initial Electrical Characterization
              Ge2Se3/SnSe Structure
              A 30nA pre-condition (+V),                         Followed by -V
                   -7                                                -5
              10                                                10
Current (A)




                   -8




                                                  Current (A)
              10                                                     -7
                                                                10
                   -9
              10
                                                                     -9
                -10                                             10
              10                                                                  Switching!
                -11                                              -11
              10                                                10
                        0.0   1.0    2.0    3.0                           0.0   1.0    2.0     3.0
                              Voltage (V)                                       Voltage (V)

                                 (a)                                              (b)
Movement of Sn Ions into Ge2Se3
Activates Operation
 +V drives Sn2+ or Sn4+ 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 Ge2Se3
  material, thus allowing phase change operation to
  occur with –V.
Initial Results Summary

 GeTe/SnTe – phase change switching, +/-V
 Ge2Se3/SnTe – phase change switching, +/-V
 Ge2Se3/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.
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?
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?
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) Ge2Se3/SnSe/Ag – metal ion
      (4) GeTe/SnSe/Ag – metal ion and lower
  glass
      (5) Ge2Sb2Te5 (GST)/SnTe – lower glass
 Resistance switching is observed in all stacks –
  but switching properties are different.
Current-Voltage Curves of Stack
Structures
+V                10
                       -4


applied                -5
                  10

                       -6
                  10
    Current (A)




                       -7
                  10

                       -8
                  10                                       Ge2Se3/SnTe
                                                           Ge2Se3/SnSe
                       -9
                  10                                       GeTe/SnTe
                                                           GST/SnTe
                   -10                                     GST
                  10

                   -11
                  10
                            0.0   0.5   1.0   1.5    2.0   2.5   3.0     3.5
                                                Voltage
 Effects of M-Ch Layer on Switching
+V                       -4
                    10
applied
                         -5
                    10

                         -6
                    10
      Current (A)




                         -7
                    10

                         -8                               GeTe/ZnTe
                    10
                                                          GeTe/SnTe
                         -9
                    10

                     -10
                    10

                     -11
                    10
                              0.5   1.0   1.5    2.0      2.5   3.0   3.5
                                                Voltage
  How are the Electrical Properties
  Altered by Addition of Ag?
 Devices were tested with:
    Ge2Se3/SnSe/Ag
    GeTe/SnSe/Ag
                              W       +
                    Ag
                              Sn-ch
                              Ge-ch
                      Si3N4

                      W
                                      _
Ge2Se3/SnSe/Ag Device – Multistate
Resistance Behavior
               100


               80                        700
Current (A)




               60

                                        1K
               40


               20                       2K

                                        5K
                                        5K


                0
                 0.00   0.02   0.04   0.06 0.08 0.10   0.12   0.14
                                         Voltage (V)
GeTe/SnSe/Ag Device – Some Multistate
Behavior
                100


                80
 Current (A)




                60         1k


                40

                                        3k
                20


                 0
                  0.00   0.05    0.10    0.15   0.20    0.25   0.30   0.35
                                          Voltage (V)
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.
 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                                   Agilent Oscilloscope
       Generator            Micromanipulator

                                       PCRAM Device
                                            Micromanipulator
                                                                     Rload

              Rload is typically 10 kΩ to 1 kΩ
              depending on the material under study.
 Electrical Characterization – Lifetime
 Cycling – GeTe/SnTe
 GeTe/SnTe – initial tests show bits cycle > 2
  million times.




Input (red) and
V across load resistor (black)
 Electrical Characterization – Lifetime
 Cycling – Ge2Se3/SnTe
  Ge2Se3/SnTe – initial tests show more consistent
    cycling than GeTe/SnTe structures.




Input (red) and                  Current through device
V across load resistor (black)   (calculated
                                 by Vload/Rload)
 Electrical Characterization – Lifetime
 Cycling –Ge2Se3/SnSe
 > 1e6
                              8
  cycles
 Operation                           Erase
  up to 135                   6
                                                                   Vout
  °C.
              Amplitude (V)

                                                                   Vin

                              4
                                              Write


                              2

                                       Read           Read
                              0
                                  0             1           2      3
                                                       Time (ms)
         Ge2Se3/SnSe/Ag Device Cycling
         T = 135°C; Rload = 1kΩ
              1.5
                                                           Input
                                                      Response after given
                                                      number of cycles:
              1.0        Write                             10
                                                             1

                                                             2
                                                           10
                                                             3
                                                           10
                                                             4
                                                           10
              0.5                                          10
                                                             5
                           Read
Voltage (V)




                                                             6
                                                           10


              0.0
                                                          Read
              -0.5


              -1.0                      Erase

              -1.5
                     0    100        200        300              400
                                  Time (s)
GeTe/SnSe/Ag Device Cycling
T = 30°C; Rload = 1.5kΩ
               1.5                                           Input
                                                         Response after given
                                                         number of cycles:
                                                                1
                          Write                              10
               1.0                                           10
                                                                2

                                                                3
                                                             10
                                                                4
                                                             10
                                                                5
               0.5                Read                       10
 Voltage (V)




                                                                6
                                                             10


               0.0
                                                               Read
               -0.5

               -1.0
                                         Erase

               -1.5

                      0    100     200   300     400   500       600
                                         Time (s)
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.
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?
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?
Example of Poor Programming Resistance
Distributions: GeTe/SnSe
                                            Programming Current
                             + potential         100uA                                - potential
                                                 1mA
                                                                         5
                                                                        10
                         2                                                   8
                                                                             7
                     6                                                       6
                    10                                                       5




                                                    Resistance (Ohms)
Resistance (Ohms)




                         6                                                   4
                         4
                                                                             3
                         2
                                                                             2
                     5
                    10
                         6
                         4                                               4
                                                                        10
                                                                             8
                         2
                                                                             7
                                                                             6
                     4
                    10                                                       5

                             0    2   4   6  8                                   0    2   4   6  8
                                 Device Number                                       Device Number
Electrical Control: Reverse Potential
Programming Provides Multiple Resistance States
                       9
                      10

                       8
                      10
  Resistance (Ohms)




                                        100A max +V
                       7                Reverse potential 1mA max -V
                      10                OFF

                       6
                      10

                       5
                      10

                       4
                      10

                           0   2      4          6           8
                                   Device Number
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.
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
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.
    Phase-Change Memory Radiation
    Resistance
    Phase-Change Memory
                                                             ON state:
    OFF state:                                               Even if some regions in the crystalline
    Complete crystallization is not induced
                                                             material are disturbed by SEE or TID,
    by SEE or TID.
                                                             the crystallinity in the rest of the cell
    Localized crystallization can occur.*
                                                             will keep R low.
    Metal 2                                                    Metal 2



                    Rc1    Rc2                Chalcogenide                     Rc1    Rc2                 Chalcogenide

                                    Crystalline                                                 Crystalline

                    Ra1    Ra2                                                Ra1     Ra2
                                     Amorphous                                                  Amorphous


     Metal 1                                                    Metal 1




* El-Sayed, S.M. Nuclear Instruments and Methods
in Physics Research B 225 (2004) 535-543.
Ion-Conducting Memory Radiation
Resistance
    Ion-Conducting Memory
OFF State: Material is                             ON State: Ag filling the
disordered, SEE or TID will not                    conductive channel would have
affect it.                                         to be completely displaced from
                                                   contact with either electrode.

             +
                                                               +
                            Ag electrode
                                                                                 Ag electrode
V
                         (Ge2Se3)33(Ag2Se)67
                                               V
                                                                              (Ge2Se3)33(Ag2Se)67


              -
                                                                -

				
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