Dr. Scott R. Messenger

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Displacement Damage Dose Approach For Determining
Solar Cell Degradation In Space With Spenvis
Implementation
Dr. Scott R. Messenger SFA, Inc. (messenger_at_nrl.n
avy.mil)
SPENVIS GEANT4 workshop Faculty Club Leuven,
Belgium 3 - 7 October 2005
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Outline

• Introduction
• Space Solar Cell Degradation Calculations
• NASA JPL Equivalent Fluence Method
• NRL Displacement Damage Dose (Dd) Method
• Nonionizing Energy Loss (NIEL)
• Comparisons
• SPENVIS Implementation
• MULASSIS is the key
• Notes
• Future Work

S. Messenger, SPENVIS Workshop 2005
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The Problem
electrons
protons

• Omnidirectional, isotropic, energy spectrum in
  space
• Unidirectional, normally incident, monoenergetic
  irradiation of bare solar cells on the ground

Planar, slab geometry
S. Messenger, SPENVIS Workshop 2005
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Pmax Degradation Curves for GaAs/Ge Solar Cells
(JPL, 1991)
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The Solution

• Equivalent Fluence Method created by NASA Jet
  Propulsion Laboratory (JPL)
• Can be implemented through available FORTAN
  programs
• Is included in the SPENVIS web-suite (and others)
• Has widespread application and over 30 years of
  heritage
• Displacement Damage Dose Method (Dd) created by
  the US Naval Research Laboratory (NRL)
• Does not have widespread application due to lack
  of distributed computational tool
• Solar Array Verification and Analysis Tool
  (SAVANT) is available but only in beta-version
  (unfunded at present)
• This paper shows how the SPENVIS web-suite can be
  used to implement the Dd method

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JPL and NRL Methods

• NASA Jet Propulsion Laboratory (Pasadena, CA)
• Reduces mission space radiation effects to an
  equivalent 1 MeV electron fluence
• Read EOL power from measured 1 MeV electron curve
• US Naval Research Laboratory (Washington, DC)
• Calculate displacement damage dose, Dd, for
  mission
• Read EOL power from measured characteristic curve

S. Messenger, SPENVIS Workshop 2005
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JPL Method (Equivalent Fluence
Method)

• Summarized in two publications (developed in
  1980s)
• Solar Cell Radiation Handbook, JPL Publication
  82-69 (1982)
• GaAs Solar Cell Radiation Handbook, JPL
  Publication 96-9 (1996)
• Utilizes the concept of relative damage
  coefficients (RDCs)
• Reduces all damage to a 1 MeV electron equivalent
  fluence and uses 1 MeV electron data to get the
  EOL result
• Several computer programs (FORTRAN) are
  available
• EQFLUX (Si), EQGAFLUX (GaAs), and multijunction
  (MJ) cell
• Other programs (e.g. SPENVIS and Space Radiation)
  implement JPL method

S. Messenger, SPENVIS Workshop 2005
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JPL Equivalent Fluence Method
Measure PV Degradation Curves (4 electron and 8
proton energies)
Determine Incident Particle Spectrum (e.g. AP8)
Calculate Damage Coefficients for Isotropic
Particles w/ Coverglasses of Varied Thickness
Determine Damage Coefficients for Uncovered Cells
Calculate Equivalent 1 MeV Electron Fluence for
Orbit (EQGAFLUX)
1 MeV Electron Degradation Curve
Read Off EOL Values
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Electron Damage Coefficients
JPL Equivalent Fluence Method
Electron and Proton
Fluence Data (GaAs/Ge, 1991)
Proton Damage Coefficients
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Equivalent 1 MeV Electron Fluence
where the RDCs for a coverglass thickness t is
(for electrons)
where the energy loss is determined from
R(E) is the range
for protons, another term is included to account
for end-of-track effects
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JPL Equivalent Fluence Method
Initial Omnidirectional Spectrum
Proton Damage Coefficients
Equivalent 1 MeV Electron Fluence
1 MeV Electron Pmax Degradation
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JPL Model Pros/Cons

• Pros
• Heritage (developed in the 1980s)
• Widely available and already incorporated into
  many space radiation suites (SPENVIS, Space
  RadiationTM, etc.)
• Cons
• Much ground test data needed ()
• Requires 1 MeV electron AND 10 MeV proton data
• Currently available for Si (1982), GaAs/Ge
  (1996), MJ (1999)
• Program not particularly user friendly (FORTRAN)
• Several flags need to be set
• Entire calculation is technology specific (every
  design change needs requalification, )

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NRL Method (Displacement Damage Dose,
Dd)

• Summarized in
• Progress in PV Research and Applications 9,
  103-121 (2001)
• Appl. Phys. Lett. 71, 832 (1997)
• IEEE Trans. Nucl. Sci. 44, 2169 (1997)
• RDCs calculated from the nonionizing energy loss
  (NIEL)
• Determines degradation curve as a function of Dd
  and uses this curve to get the EOL result
• Particle transport through the coverglass
  calculated independently from RDC calculation
• Computer program (SAVANT) developed by NRL, NASA
  GRC, and OAI (unfunded at present) SPENVIS?

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NRL Displacement Damage Dose Method
Determine Incident Particle Spectrum (e.g. AP8,
AE8)
Choose Nonionizing Energy Loss (NIEL) Data
(Energy Dependence of Damage
Coefficients)
Calculate Slowed-Down Spectrum (SDS) (Shielding)
Measure Characteristic Degradation Curve vs. Dd
(DdNIELxFluence) (2 e- and 1 p
energy)
Calculate Dd for Mission (Integrate SDS with NIEL)
Read Off EOL Value
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NonIonizing Energy Loss
NIEL Rate at which energy is lost to nonionizing
events (UNITSMeV/cm or MeVcm2/g)
Lindhard partition factor
Differential scattering cross section for
displacements
Recoil energy
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NonIonizing Energy Loss

• Several calculations exist, all yielding similar
  results
• Notable NIEL calculations (p, e-, a, no, ions)
• NRL group (NSREC, 1986-2003)
• Van Ginneken, 1989
• NASA/JPL group (2000-2005, WINNIEL)
• CERN group (Huhtinen et al., 2000-2005)
• Akkerman and Barak, 2001
• Inguimbert Gigante (NEMO, 2005)
• Fischer and Thiel, U. Koln
• Especially good agreement over practical proton
  energies for solar cells in space (0.1-10 MeV)

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NIEL for Si (w/Neutron)
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NRL Displacement Damage Dose Method
Determine Incident Particle Spectrum (e.g. AP8,
AE8)
Choose Nonionizing Energy Loss (NIEL) Data
(Energy Dependence of Damage
Coefficients)
Calculate Slowed-Down Spectrum (SDS) (Shielding)
Measure Characteristic Degradation Curve vs. Dd
(Dd NIEL x Fluence) (1 p and 2
e- energies)
Calculate Dd for Mission (Integrate SDS with NIEL)
Read Off EOL Value
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Displacement Damage Dose (Dd)
Unit is MeV/g is analogous to ionizing dose
Rad(Si)
Protons n1 Electrons 1ltnlt2
Or, for a spectrum of particles, as that found in
space,
Slowed-down differential spectra
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NRL Displacement Damage Dose Method
Measured Data
Characteristic Curve
With NIEL

• Characteristic curve is independent of particle
• Calculated NIEL gives energy dependence of damage
  coefficients
• 4 empirically determined parameters (C,Dx,Rep,n)

S. Messenger, SPENVIS Workshop 2005
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NRL Displacement Damage Dose Method
Determine Incident Particle Spectrum (e.g. AP8,
AE8)
Choose Nonionizing Energy Loss (NIEL) Data
(Energy Dependence of Damage
Coefficients)
Calculate Slowed-Down Spectrum (SDS) (Shielding)
Measure Characteristic Degradation Curve vs. Dd
(DdNIELxFluence) (2 e- and 1 p
energy)
Calculate Dd for Mission (Integrate SDS with NIEL)
Read Off EOL Value
S. Messenger, SPENVIS Workshop 2005
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An Analytical Calculation Implementing the Dd
Approach

• Based on the Continuous Slowing Down
  Approximation (CSDA)
• The rate of energy loss equals that due to the
  total stopping power (i.e. no energy loss
  fluctuations, straggling)
• Particle transport governed by range data
• CSDA not expected to hold for electrons of low
  energy

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Analytical Proton Transport Model
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NRL Displacement Damage Dose Method
Determine Incident Particle Spectrum (e.g. AP8,
AE8)
Choose Nonionizing Energy Loss (NIEL) Data
(Energy Dependence of Damage
Coefficients)
Calculate Slowed-Down Spectrum (SDS) (Shielding)
Measure Characteristic Degradation Curve vs. Dd
(DdNIELxFluence) (2 e- and 1 p
energy)
Calculate Dd for Mission (Integrate SDS with NIEL)
Read Off EOL Value
S. Messenger, SPENVIS Workshop 2005
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NRL Displacement Damage Dose Method
Incident and SDS (Isotropic)
NonIonizing Energy Loss
Total Mission Dose
Pmax Degradation
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Cumulative Fraction of Dd
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SAVANT Dd Analysis Code
SAVANT Solar Array Verification and Analysis
Tool (NASA, NRL, OAI)
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Comparison of Results
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NRL Dd Model Pros/Cons

• Pros
• Few ground test measurements needed (3)
• Ground test particle energies can be conveniently
  chosen
• Uniform damage deposition required over active
  region
• Shielding algorithm is independent
• Allows for rapid analysis of emerging cell
  technologies
• Allows for easy trade studies
• Can combine data from different experiments
• Allows for alternate radiation particles
  (neutrons, alphas, etc.)
• Cons
• Lack of heritage (developed in the mid-1990s)
• More suited for sufficiently thin devices (few
  mm)
• Program currently not available to general public

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Why does the Dd Method work so well?
The energy dependence of the NIEL closely follows
the RDCs over practical energies considered for
space applications
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Proton NIEL Comparison vs. RDCs
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Electron NIEL Comparison vs. RDCs
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Effect of Low Energy Protons on Multijunction
(MJ) Solar Cells
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Monoenergetic, Unidirectional Irradiations
3J InGaP2/GaAs/Ge
T. Sumita, M. Imaizumi, S. Matsuda, T. Ohshima,
A. Ohi, and T. Kamiya, Proc. 19th EPVSEC, Paris,
2004.
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Proton-Induced QE Degradation in MJ Cells
100 keV protons
50 keV protons
400 keV protons
1 MeV protons
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Monoenergetic, Unidirectional Irradiations
Top cell degradation
Middle cell degradation
Results from SRIM 2003 v.26 (www.srim.org)
T. Sumita, M. Imaizumi, S. Matsuda, T. Ohshima,
A. Ohi, and T. Kamiya, Proc. 19th EPVSEC, Paris,
2004.

• Typical ground test conditions (not space
  conditions)
• Nonuniform vacancy distribution Bragg Peak at
  end of track
• Different energies can preferentially degrade one
  sub-junction
• This effect is not seen in 1 MeV Electron
  irradiation

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Spectrum, Omnidirectional Irradiation
Results from SRIM 2003 v.26 using special input
file (TRIM.DAT) which specifies random incident
angle and energy to simulate L2 spectrum (3 mil
SiO2)

• Representative of exposure in the space radiation
  environment
• The vacancy distribution profile is nearly
  uniform over active region

No special effects due to low energy protons
apparent!
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MJ Radiation Response Analysis Methodology

• Space radiation environment produces virtually
  uniform vacancy distribution throughout cell
• To reproduce this with a monoenergetic,
  unidirectionally incident particle, we need a
  fully penetrating proton (gt1 MeV)
• NO LOW ENERGY PROTON IRRADIATION NECESSARY
• Total damage induced in cell (i.e. total number
  of vacancies) in space can be quantified in terms
  of Displacement Damage Dose (Dd)
• Value of Dd is calculated by integrating the
  product of the slowed-down spectrum and the NIEL
  over energy
• Validation exists for several MJ technologies
• Enables quick and inexpensive qualification of
  new technologies
• SPENVIS Implementation Soon!!!

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SPENVIS Implementation
There are four basic components involved in this
calculation

• Incident differential radiation spectra (SPENVIS)
• Calculation of the slowed-down spectra after
  having passed through shielding (analytical,
  MULASSIS)
• Calculation of the total Dd for the mission
  (MULASSIS)
• Determination of the expected cell degradation
  (to be added, need characteristic curve info,
  i.e. C, Dx, n, Rep)

MULASSIS is the enabling tool!
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Walk Through SPENVIS Orbit Generation
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Walk Through SPENVIS Incident Particle Spectra
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Walk Through SPENVIS Shielding (Slowed Down
Spectra) and Equiv. Dd
x
x
x

• Fluence gives slowed down spectra
• NIEL option performs integration with NIEL to
  give mission Dd (not fully operational)

Run
x
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Calculations Made External to SPENVIS
Equivalent Value of Dd

• Slowed-down spectra exported as TXT file from
  MULASSIS
• Read into MS Excel and integrated with NIEL to
  give Dd
• Also calculated by in-house NRL program for
  comparison

electrons
protons
Proton Dd (MeV/g) Electron Dd (MeV/g)
MULASSIS 3.8E10 5.4E08
In-House Calc 3.3E10 6.0E08
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Thick Shielding Example
5093 km, circular, 57 degree, 1 year, 1000 mils
Al/Si
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Calculations Made External to SPENVIS Solar
Cell End-of-Life Power Output
Independent Variables
(c, Dx, n, Rep)
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Notes

• Mulassis agrees very well with the analytical
  slab geometry model for protons
• Mulassis allows for multiple interfaces and
  layers
• Effect of electrons usually minimal (However,
  MULASSIS is probably better since analytical
  model assumes CSDA)
• Could be extended for use with heavy ions and
  neutrons (NIEL is available for most cases)
• Could be used for other devices where
  displacement damage is an important damage
  mechanism (e.g. LED light output, CCD
  degradation, transistor gain, etc.)

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Future Work

• Continue to work with ESTEC, BIRA, and QINETIQ to
  further implement the method and perform
  benchmark tests
• Develop characteristic radiation degradation
  curves for current state-of-the-art solar cell
  technologies
• Develop capabilities for other devices and
  irradiation particles

S. Messenger, SPENVIS Workshop 2005