Transcript Literature review of Conduction in Polymer - Indico

Last Measurement on GEM and
Literature review of Conduction in
Polymers
Gabriele Croci (CERN)
GDD Meeting
February, the 21st 2008
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Leakage Current in 10x10 GEM
ZOOM
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Leakage Current in Cu covered Kapton
Foil (GEM 10X10 without holes)
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Comparison Holes/No Holes
The major effect seems to come from surface conduction
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REFERENCES
[1] P. Keith Watson, “The transport and Trapping of Electrons in Polymers”, IEEE Transaction on
Dielectrics and Electrical Insulation, Vol. 2 No 5. October 1995
[2] John J. Simmons, “Poole-Frankel Effect and Schottky Effect in Metal-Insulator-Metal Systems”,
Physical Review, Volume 155,3, 15 March 1967
[3] E. Motyl, “Electrode Effects and Electrical Conduction in Polyimide Kapton HN Films”, IEEE
Internation Conference on Conduction and Breakdown in Solid Dielectrics, June 22-25 1998
[4] J-P Salveat et al “Onset and growth of conduction in polyimide Kapton induced by swift heavyion irradiation”, Physical Review B, Volume 55, Number 10, 1 March 1997-II
[5] A. Rose, “Space-Charge Limited Currents in Solids”, Physical Review Volume 97, Number 6,
March 15, 1955
[6] Edward J. Yadlowsky, Robert C. Hazelton, “Radiation Induced conduction in Kapton H Film”,
IEEE Transactions on Nuclear Science, Volume 35, No 4, August 1988
[7] R.G. Filho et al, “Induced conductivity Of Mylar and Kapton Irradiated by X-Rays”, IEEE
Transactions on Electrical Insulation Volume EI-21 No. 3, June 1986
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General Polymer Description
• A polymer is a substance composed of
molecules with large molecular mass
composed of repeating structural units, or
monomers, connected by covalent chemical
bonds
• Poliymide (Kapton, dielectric used in GEM)
belongs to the polymer family
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General statements about Conduction
in Polymers
• Polymers conductivity can be due to the small
number of low-mobility charge carriers and to
the high trap density [1]
• The traps can play a very important role in the
carrier recombination process; they can trap
carriers and release them in a successive time [1]
• Mobility changes of several order of magnitude
with respect the free (without traps) mobility
• Dependence on temperature, applied electric
field and particle (e-, p+, X-rays, Ions..) irradiation
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Energy Band Diagram in a Polymer [1]
• Slight difference from organized structure like
metals or semiconductors
• The conduction band edge is substituted by the
mobility edge and we can keep the concept of
valence band
• The trap levels are usually between this two
states
• Tentative to discover the energy distribution of
these trapping states injecting electrons inside
the polymer
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Some possible origins of trapping
centers
•
•
•
•
•
•
Impurities in the material [2]
Presence of Radicals in the polymer
Chemical structure of polymer chain
Open covalent (0,C) bounds
Regions of free volumes
….
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Charge Trapping and Decay (1)[1]
• The model described in [1] does not take into
account retrapping after a charge is released by a
trap: this holds for thin polymers
• The current flowing in the polymer is a function
of the energy density of the traps
• Definition of a trapping parameter α=1/μτE
(μ:mobility, τ:characteristic time, E: electric field
in the polymer)
• Electron is shallow states are rapidly detrapped
and are driven more deeply in the material by the
field
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Charge Trapping and Decay (2)
• The charge detrapped
can contribute to the
conduction and can
accumulate on the
surface of the polymer
• Measurement of
Surface Potentials (Vs)
with time
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Other Possible Conduction
Mechanisms [2],[3],[4]
•
•
•
•
•
Poole-Frenkel effect
Schottky Effect
Hopping
Tunneling
Space charge limited currents
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Poole-Freknel Effect
• P-F: field-assisted thermal
ionization; lowering of a
Coulombic potential
barrier with an electric
field; it is associated with
the lowering of a trap
barrier in the bulk
• Change of work function:
W W- eβPFE½
• Change of conductivity:
σ=σ0exp(βPFE½/kT)
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Schottky Effect
• Very similar to Poole-Frenkel Effect. It is the
attenuation of a metal-insulator barrier arising
from electrode image force interaction. It is a
surface effect
• Change of conductivity similar to P-F
σ=σ0exp(βsE½/kT)
• βPF=2βs
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Hopping Models[4]
• Presence of π-conjugated bonds; phonon assisted tunneling
between localized states
• Two basic processes: local jumping between adjacent sites
and “percolation”
• A hop between two localized electronic states occurs when
the atomic vibratory motion changes the relaive energy of
the localized states
• Two kinds of hops
– Adiabatic: large electron energy transfer between states; jump
rate not limited by electron energy transfer or distance between
sites
– Non Adiabatic: low electron energy transfer; jump rate limited
by transfer energy and distances
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Tunneling and Space Charge Limited
Current
• Tunneling is the quantum effect of passing
through a barrier also if the energy is not
enough to overcame the barrier itself
• Space charge limited current[5]: maximum
current that can flow in a built-up capacitor
charged with static charges. The current can
be enhanced by PF effect. Current density has
a voltage square dependence
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Radiation Induced Conduction in
Kapton H Film [6]
• 8 μm thick kapton irradiated by 45 KeV
penetrating electrons
• I-V characteristic depends on the voltage
applied to the irradiated sample:
– Low Voltage (<50V): Ohmic regime, Linear I-V
Characteristic
– Intermediate Voltage (50V<V<700V): Space Charge Limited
Current (SCL) regime, I proportional to V2
– High Voltage (>700V): Trap Filled limit regime (TFL), I
exponentially proportional to V
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Conduction Model [6]
• This is the math form of previous statements
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V
V
V 2 hV kT
J  Aohmic  BSLC 3  CTFL e
L
L
L
It is possible to see three different regimes at different voltage
values for current density
The constant A,B,C take into account all the parameters of the
material and of the irradiation; h take into account also the
energy gap over which the traps are distributed
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Induced Conductivity of Mylar and
Kapton Irradiated by X-Rays [7]
• Kapton Samples of 80 mm diameter with
thickness varying from 6 to 75 μm were
irradiated with W X-Rays for several hours
• Electric field (of different intensity) were
applied to the samples
• They saw a variation of the Kapton
conductivity
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Induced Conductivity of Mylar and
Kapton Irradiated by X-Rays [7] (2)
From my calculation and considering the rate we are using in our lab to test GEM, we
are very close to the black curve
Next week I will perform this kind of measurement irradiating a 10x10 GEM for
several hours to see if there is a variation of the conductivity
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