Advanced Characterization of Emissive Materials: Photocathodes and MCPs
Determining Carrier Density through Measuring Resistivity Kathleen Broughton Ernesto Indacochea...
Transcript of Determining Carrier Density through Measuring Resistivity Kathleen Broughton Ernesto Indacochea...
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Determining Carrier Density through Measuring Resistivity
Kathleen BroughtonErnesto Indacochea
Klaus AttenhoferPhotocathodes Group
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Resistivity Measurement
Measurement of how strongly a material resists electrical flow High Resistivity (R ≥ 1 GΩ); Low Resistivity ( R < 1 GΩ)
Ρ = Ε / J = R l / A = 1/σ
Ρ = resistivityΕ = magnitude of electric fieldJ = magnitude of current densityR = electrical resistancel = length of materialA = cross-sectional area of material σ = conductivity
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Current – Voltage Curve
Standard I-V curve– Saturation current is temperature dependant
Perceived I-V curve– Create an internal electric field on material– Question as to whether or not the dopant are a surface barrier and if the electrons that pass though material are equivalent
Drude TheoryE = ρ * j ; E = electric field, ρ = resistivity, j = current densityj = σ * E σ = conductivity, σ = ne^2τ / m τ = relaxation time (avg. time since its last collision)
n = number of carriers, e = electrical charge, m = mass
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Standard I-V curve Perceived I-V curve of photocathode
j j
v
-e (J(h) + J(e))
j(V)=e(J(h)+J(e))(e^eV/kT -1)
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Sample Surface Measurements
Passage time through the bulk is much greater than just the surface Temperature Dependant Measurement can provide :
– Carrier density in bulk– Carrier density on surface– Activation energy (chemical potential) of defects and dopants – Work Function (comparison of dark and light measurement)
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Bulk
Surface
R (b)
R (s)V
I
Bulk
Surface
R(b) V
I
R (surface) << R (bulk)
Sample V/I = R(b) + R(s)
V/I = (R(b)*R(s)) / (R(b) + R(s))
R (s)
R (s)
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Low Resistivity Measurements (R <1 GΩ)
• 4 Wire Resistance Measurement• Test Current (I) is forced through the test resistance (R) • voltage (Vm) across DMM is measured through sense leads• Voltage drop across sense leads is negligible, so V(m) = V(r)
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High Resistivity Measurements (R ≥1 GΩ)
Guarding Approach– significantly reduces the leakage error – improves measurement accuracy
Voltage across R(L) is essentially zero Test current I(R) flows through R(S) Source resistance can accurately be determined
Source: Low Level Measurements Handbook. 6th Edition, Keithley.
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BNC and Triaxial Connectors
Triaxial Connector– Inner shield can be driven at guard
potential to reduce cable leakage and minimize circuit rise times
Source: Low Level Measurements Handbook. 6th Edition, Keithley.
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Chamber Set-up
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Triax
Floating BNC Connector
Chamber Wall Chamber Wall
Chamber
Signal
Zero Volt
Ground
Triax 1 Triax 2
SHV
SHVSample
Black-GroundYellow-SignalBlue-Reference PotentialRed-High Voltage
Triax / BNC Feedthrough DesignSwitchbox for Triax and SHV (safety feature) Sample holder (compatible with Igor’s)
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Conclusion
Literature Review– Basic understanding of conductivity (Drude Theory)– Theoretical understanding of conductivity measurements (Triax system)– Becoming familiar with literature search
Resistivity Measurement of Sample will provide– Carrier Density– Activation Energy of dopant and defects creating free carriers– Work Function with light
Chamber Design has started – Working on Triax / BNC Feedthrough Design– Conceptual work Sample Holder and Safety Features
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