Radiation Damage in Sentaurus TCAD

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Radiation Damage in Sentaurus TCAD
David Pennicard University of Glasgow

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Overview

• Introduction to trap models
• Radiation damage effects and defects
• P-type damage model
• Some example simulations
• Sentaurus Device command file

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Radiation damage introduction

• High-energy particle displaces silicon atom from
  a lattice site
• Results in a vacancy and an interstitial
• Atom can have enough energy to displace more
  atoms
• After damage is caused, most vacancy-interstitial
  pairs recombine
• Left with more stable defect clusters, e.g.
  divacancy (V2)
• Defect clusters affected by annealing conditions
  impurities in the silicon
• Defect clusters give extra energy states (traps)
  in bandgap
• Increased leakage current
• Increased charge in depletion region (increase in
  effective p-type doping)
• Trapping of free carriers
• Can simulate this in Sentaurus Device by
  modelling behaviour of trap levels directly
• NB when dealing with different types and
  energies of particle irradiation, scale fluence
  (particles / cm2) by non-ionizing energy loss.
  Standard is 1MeV neutrons.

See M. Moll thesis, Hamburg 1999
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Traps in Sentaurus Device

• A statement added to the Physics section can
  describe the traps
• Parameters
• Acceptor trap has ve charge when occupied by
  electron, 0 charge when occupied by hole. (Donor
  has ve charge when occupied by hole)
• Level specifies how we describe energy level.
  Here, we give the energy below the conduction
  band. EnergyMid gives the energy difference
• Concentration given in cm-3
• Electron cross-section proportional to
  probability of electron moving between trap and
  conduction band - se
• Hole cross-section likewise, proportional to
  chance of carrier moving between valence band and
  trap level - sp

Physics (material"Silicon") Traps (
(Acceptor Level fromCondBand Conc1.613e15
EnergyMid0.42 eXsection9.5E-15
hXsection9.5E-14) )
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Traps in Sentaurus Device

• For each trap level, Sentaurus simulates
• Proportion of trap states occupied by electrons
  and holes
• NB not filled by electronoccupied by hole
• This affects charge distribution, and so has to
  be included in Poisson equations
• Rate of trapping / emission between conduction
  band and trap, and between valence band and trap
• These then have to be included in the carrier
  continuity equations

Poisson
Electron continuity
Hole continuity
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Increase in reverse leakage current
Leakage current increases with fluence,
independent of substrate type
Leakage current reduced by annealing
a3.9910-17A/cm3 after 80 mins anneal at 60C
(M. Moll thesis)
Also, temperature dependence. a normally given
for 20C
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Increase in leakage current

• 2 transitions involved
• Electron from valence band moves to empty trap,
  leaving a hole
• Electron in trap moves to conduction band, giving
  conduction electron
• Then, electron and hole are swept out of
  depletion region by field, avoiding recombination
• Rate of production limited by less frequent step
  (larger energy difference)
• Trap above midgap limited by rate of valence
  band-gttrap
• Traps below midgap likewise limited by
  trap-gtconduction band
• Rate drops rapidly with distance of trap from
  midgap
• Deep level traps dominate

Ec
Free electron produced
Trap
Emid
Hole produced
Ev
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Change in effective doping concentration
Effective p-type doping increases (giving type
inversion in n-type silicon) Dependent on
material, particularly oxygen content and
radiation type for p-type (n-type also has
donor removal effect) My models match
p-type Float Zone irradiated with protons
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Change in effective doping concentration
Additionally, have both beneficial annealing) in
short term, and reverse annealing in long
term Typically, test detectors after beneficial
annealing, to try to find stable damage level All
this implies very complicated defect behaviour!
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Change in effective doping concentration

• Charge state of defect depends on whether it
  contains electron or hole
• Acceptor -ve when occupied by electron
• Donor ve when occupied by hole
• Source of ve charge that gives effective p-type
  appears to be acceptors above midgap
• A small proportion of these traps are occupied by
  electrons
• Number of traps occupied once again is highly
  dependent on distance from bandgap
• Donors below bandgap can give ve charge, but
  relatively minor effect

Ec
Acceptor Trap
- -
Emid
Hole produced
Ev
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Charge trapping
Number of free carriers in device decays
exponentially over time Described by effective
lifetime Experimentally, effective lifetime
varies inversely with fluence (this has been
tested up to 1015neq/cm2)
G. Kramberger, Trapping in silicon detectors,
Aug. 23-24, 2006, Hamburg, Germany
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Charge trapping

• In equilibrium, traps above Emid are mostly
  unoccupied
• Free electrons in conduction band can fall into
  unoccupied trap states
• Likewise, traps below midgap contain electrons
  can trap holes in valence band
• Effect is less energy-dependent
• Similar equations for holes

Ec
Trap
Emid
Trap
Ev

• Afterwards, carrier can be released from trap
• If trap levels are reasonably close to midgap,
  detrapping is slow
• So, less effect on fast detectors for LHC

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University of Perugia trap models
IEEE Trans. Nucl. Sci., vol. 53, pp. 29712976,
2006 Numerical Simulation of Radiation Damage
Effects in p-Type and n-Type FZ Silicon
Detectors, M. Petasecca, F. Moscatelli, D.
Passeri, and G. U. Pignatel
Ec
Perugia P-type model (FZ)
-
- -
0
Ev

• 2 Acceptor levels Close to midgap
• Leakage current, negative charge (Neff), trapping
  of free electrons
• Donor level Further from midgap
• Trapping of free holes

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University of Perugia trap models

• Aspects of model
• Leakage current reasonably close to
  a4.010-17A/cm
• Depletion voltage matched to experimental
  results with proton irradiation with Float Zone
  silicon (M. Lozano et al., IEEE Trans. Nucl.
  Sci., vol. 52, pp. 14681473, 2005)
• Carrier trapping
• Model reproduces CCE tests of 300?m pad detectors
• But trapping times dont match experimental
  results

• Experimental trapping times for p-type silicon
  (V. Cindro et al., IEEE NSS, Nov 2006) up to
  1015neq/cm2
• ße 4.010-7cm2s-1 ßh 4.410-7cm2s-1
• Calculated values from p-type trap model
• ße 1.610-7cm2s-1 ßh 3.510-8cm2s-1

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Altering the trap models

• Priorities Trapping time and depletion behaviour
• Leakage current should just be sensible a
  2-10 10-17A/cm
• Chose to alter cross-sections, while keeping
  sh/se constant

Carrier trapping
Space charge
Modified P-type model
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Comparison with experiment

• Compared with experimental results with proton
  irradiation
• Depletion voltage matches experiment
• Leakage current is 30 higher than experiment,
  but not excessive

Comparison of Radiation Hardness of P-in-N,
N-in-N, and N-in-P Silicon Pad Detectors, M.
Lozano et al., IEEE Trans. Nucl. Sci., vol. 52,
pp. 14681473, 2005
a5.1310-17A/cm
a3.7510-17A/cm
Experimentally, a3.9910-17A/cm3 after 80 mins
anneal at 60C (M. Moll thesis)
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N on p strip detector CCE

• At high fluence, simulated CCE is lower than
  experimental value
• Looked at trapping rates using 1D sim as
  expected
• Trapping rates were extrapolated from
  measurements below 1015neq/cm2
• In reality, trapping rate at high fluence
  probably lower than predicted

PP Allport et al., IEEE Trans. Nucl. Sci., vol
52, Oct 2005
900V bias, 280?m thick
From ß values used, expect 25µm drift distance,
2ke- signal
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Example - Double-sided 3D detector

• Electrode columns etched from opposite sides of
  silicon substrate
• Short distance between electrodes
• Expect reduced depletion voltage and faster
  collection (less trapping)

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Example - Double-sided 3D at 1016 neq/cm2

• Plotted electric field in cross-section at 100V
  bias
• Where the columns overlap, (from 50?m to 250?m
  depth) the field matches that in the full-3D
  detector
• At front and back surfaces, fields are lower as
  shown below
• Region at back is difficult to deplete at high
  fluence

A.
1016neq/cm2, front surface
1016neq/cm2, back surface
A.
B.
100V
100V
Undepleted
B.
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Example - Collection with double-sided 3D

• Slightly higher collection at low damage
• But at high fluence, results match standard 3D
  due to poorer collection from front and back
  surfaces.

20 greater substrate thickness
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Sentaurus Device command file

• See Sentaurus/Seminar/RadDamage
• StripDetectorRadDamage_des.cmd
• StripDetectorRadDamage_Param_des.cmd
• Traps added to silicon
• Insert appropriate concentrations, or use a
  Fluence variable in Workbench

Physics (material"Silicon") Putting traps
in silicon region only Traps (
(Acceptor Level fromCondBand Conc_at_ltFluence1.613
gt_at_ EnergyMid0.42 eXsection9.5E-15
hXsection9.5E-14) (Acceptor Level
fromCondBand Conc_at_ltFluence0.9gt_at_
EnergyMid0.46 eXsection5E-15 hXsection5E-14
) (Donor Level fromValBand
Conc_at_ltFluence0.9gt_at_ EnergyMid0.36
eXsection3.23E-13 hXsection3.23E-14 ) )

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Sentaurus Device command file

• Extra variables can be added to Plot
• Warning trap models are sensitive to changes in
  the bandgap and temperature
• Dont change the effective intrinsic density
  model alters bandgap
• Likewise, keep using default 300K temp. (Strictly
  speaking this is slightly wrong, since the
  standard test temp should be 20C.)

Plot eTrappedCharge hTrappedCharge eGapSta
tesRecombination hGapStatesRecombination
Physics Standard physics models - no
radiation damage or avalanche etc. Temperature30
0 Mobility( DopingDep HighFieldSaturation
Enormal ) Recombination(SRH(DopingDep)) Effectiv
eIntrinsicDensity(Slotboom)
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Sentaurus Device command file

• Oxide charge increases after irradiation
• Electron-hole pairs produced in oxide holes
  become trapped in defects in oxide, giving
  positive charge
• Saturates fairly rapidly 1012cm-2 is a normal
  value after irradiation, though some papers claim
  up to 31012cm-2
• X-ray irradiation causes oxide charging, but
  little bulk damage
• Other points
• More complicated physics tends to give slower
  solving, and poorer convergence may need to
  alter solve conditions (smaller steps etc)
• For charge collection simulations, need to
  correct the integrated current to remove the
  leakage current
• CV simulations give strange results!

Physics(MaterialInterface"Oxide/Silicon")
Charge(Conc1e12)
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Example files

• See Sentaurus/Seminar/RadDamage
• StripDetectorRadDamage_des.cmd
• Basic MIP simulation at 1015neq/cm2
• This has already been run
• You can look at the output files in the same
  folder
• .dat files taken during IV ramp
• .dat files taken during the MIP transient
• .plt files
• StripDetectorRadDamage_Param_des.cmd
• _des.cmd file for a Workbench project
• Use parameter Fluence to control the radiation
  damage
• Uses if statements to omit Traps statement and
  use lower oxide charge if Fluence is zero
• Works with simple StripDetector.bnd/cmd files in
  Workbench folder
• Email d.pennicard_at_physics.gla.ac.uk

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