Amorphization kinetics of Ge2Sb2Te5 thin film induced by ion implantation R' De Bastiani tutor: Prof

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Amorphization kinetics of Ge2Sb2Te5 thin film
induced by ion implantationR. De
Bastianitutor Prof.ssa M.G. Grimaldi
National council of research-Institute of
microelectronics and microsystems (CNR-IMM)
INFM
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Outline

• Introduction
• GST characterizations RBS, PIXE, XRD, SEM,
  optical measurements
• Study of amorphization kinetics in GST film
  induced by ion irradiation
• Conclusions

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Chalcogenide-based memories why do we need a
new memory technology?
Size scaling limitation
2007 65 nm
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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
• GST Chalcogenide Phase Change Memory Technology
• Used in CD-RW and DVD-RW applications
• Could replace both DRAM and Flash memory types
• Scalable
• Low power operation
• Excellent retention (10 years 120C)
• Fast read, fast write (10-40 ns)
• Large cycling lifetime 1012 write/erase cycles
• Rapid, reversible transition between two
  metastable states

Amorphous State Low reflectance High resistance
Polycrystalline State High reflectance Low
resistance
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Thermally Induced Phase Change

• Chalcogenide materials can be used as resistance
  variable memory cells
• Three modes of operation
• Reset (logic 0 state) Applying current ?
  Heating above Tm (632 ?C) ? Melt ? rapid cool ?
  amorphous
• Set (logic 1 state) Applying current ? Heating
  above Tx (142 ?C) ? slow cool ? crystalline
• Read Low voltage is applied, current determined
  by resistance of the phase
• -Amorphous state high resistance (R gt 100 MO)
  low current
• -Crystalline state low resistance (R 100 kO)
  high current

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Experimental samples characterization
P100 W PAr5x10-3 mbar
Ge2Sb2Te5
Ge2Sb2Te5 film deposited using rf sputtering from
a single target, on a 100 nm thick SiO2 film,
deposited by CVD on a Si substrate
Stoichiometry Thickness Structural
characterization Optical constants
RBS measurements SEM analysis XRD
analysis Reflectivity measurements
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RBS measurements

• Composition of as-deposited film was determined
  by means of Rutherford backscattering analyses
  using a 2.7 MV 4He beam (normal incidence,
  scattering angle of 165) to separate the
  overlapping Sb and Te signals.

• The sample was simulated using the stoichiometry
  
• 2.4x1017at.cm-2 Ge2Sb2Te5 / 106 nm SiO2 / Si
  substrate

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SEM Analysis
Cross section
Plan view
SEM 7.2x10-6 cm
RBS 2.4x1017at./cm2
RBS measurements in conjunction with SEM
measurements were used to determine the thickness
and density of GST layer
GST density (fcc) 3.33 x1022 at./cm3
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XRD measurements
X-ray diffraction was employed to characterize
the film structure before and after amorphization
by means of a diffractometer operating with Cu Ka
(l0.15406 nm). Geometric configurations Grazy
angle X-ray diffraction (GID) Diffraction
condition 2dsin? n? (d is the
interplanar distance)

• Grazy angle X-ray diffraction
• Incidence angle 0.5 deg
• It is able to probe the material up to a
  specific depth
• Diffraction from planes perpendicular to the
  surface
• Low Si substrate contribution

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Metastable Crystal structures of Ge2Sb2Te5
Anion site is wholly occupied by only Te, and the
cation site is randomly occupied by Ge, Sb, and
20 vacancies
fcc structure Fm3m lattice parameter 6.01 Å, 20
vacancies in Ge Sb sublattice
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Optical measurements
He-Ne laser ?633 nm
GST(crystal) n 3.2 k 3.75
GST(amorphous) n 4 k 2.1
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Time resolved reflectivity
Ion beam
He-Ne Laser
Window
Detector
Mirror
Control box
Sample

• Samples were bombarded with Ar (90 250 keV) or
  Sb (50 120 keV) ions at room
• (R.T.) and liquid nitrogen temperature (LN2)
• The phase transition was monitored with a He-Ne
  Laser

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Amorphization induced by ion bombardment
Ai is the area of the amorphous generated by a
single ion track
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Phase change induced by ion bombardment
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Amorphization induced by ion bombardment
70 nm 170 nm
70 nm 170 nm
GST SiO2 Si substrate
GST SiO2 Si substrate

• Sb ion 50-120 KeV projected range 20-45 nm
• Large energy density released in dense collision
  cascades with respect to Ar ion.
• The collision cascade is assumed as an ellipsoid
  centered at the projected range

• Ar ion energy range 90-250 KeV projected range
  80-200 nm
• Dilute collision cascades in the GST film
• Uniform damage
• The ion energy is released along the track in a
  cylindrical volume

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Srim2006 simulations
Elastic collision produce amorphization
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Threshold fluence Vs Energy density
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Conclusions

• The fluence for amorphization and the elastic
  displacement threshold during ion irradiation has
  been determined in a large range of ion energy
  and mass at R.T. and LN2 temperature
• Amorphization is ruled by the elastic interaction
  
• The threshold fluence for the amorphization
  decreases with increasing the energy density
  released in the collision cascade
• The amorphization threshold depends on the
  collision cascade type (dilute or dense)

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Pubblicazioni 1) L. Romano, A.M. Piro, R. De
Bastiani, M.G. Grimaldi, E. Rimini, Group III
impurities Si interstitials interaction caused
by ion irradiation, Nuclear Instruments and
Methods B 242 (2006) 646. 2) Lucia Romano,
Riccardo De Bastiani, Cristina Miccoli, Gabriele
Bisognin, Enrico Napolitani, Davide De Salvador,
Maria Grazia Grimaldi, Carrier mobility and
strain effect in heavily doped p-type Si
Materials Science and Engineering B N. 135, pag
220223 (2006) 3) R. De Bastiani, A.M. Piro, M.G.
Grimaldi, E. Rimini, Amorphization kinetics of
Ge2Sb2Te5 thin film induced by ion implantation
Nuclear Instruments and Methods in Physics
Research B N. 257, pag. 572576 (2007) 4) F.
Ruffino, R. De Bastiani, M. G. Grimaldi, C.
Bongiorno, F. Giannazzo, F. Roccaforte, C.
Spinella, V. Raineri, Self-organization of Au
nanoclusters on the SiO2 surface induced by
200keV-Ar irradiation, Nuclear Instruments and
Methods in Physics Research B N. 257, pag.
810-814 (2007). 5) Liliana Caristia, Giuseppe
Nicotra, Corrado Bongiorno, Nicola Costa,
Sebastiano Ravesi, Salvo Coffa, Riccardo De
Bastiani, Maria Grazia Grimaldi, Corrado
Spinella, The influence of hydrogen and nitrogen
on the formation of Si nanoclusters embedded in
sub-stoichiometric silicon oxide layers,
Microelectronics Reliability N. 47, pag 777780
(2007) 6) Riccardo De Bastiani, Alberto Maria
Piro, Salvatore Lombardo, Maria Grazia Grimaldi,
Emanuele Rimini, Effects of mass, energy and
temperature on amorphization in ion implanted
Ge2Sb2Te5 thin films, Materials Today Magazine,
submitted.
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Sb energy loss
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Phase change induced by ion bombardment
70 nm 170 nm
GST SiO2 Si substrate
Sb ion 50 KeV The projected range is into the
film thickness Very High energy release