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Graphene

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Aina Marquez

on 22 September 2015

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Transcript of Graphene

Graphene
Christian Koch, Aina Marquez
IFAE, July/September 2015

GOAL
Graphene Detectors in Particle Physics
Charged particle (ion) sensors
Photosensors
Graphene: flat monolayer of carbon atoms packed into a 2D layout
Interesting electrical properties
Semiconductor behaviour, with valence and conduction bands meeting at a single point, the Dirac Point
Dirac Point
(max R)
Resistance depends on cone cross-sectional area

Smaller area implies greater resistance

Electrical Noise
First step towards a graphene detector: characterization of electrical noise
Two main kinds:
White noise
Spectrum of a random signal with constant PSD
White Noise
Pink noise
Frequency spectrum for which the PSD is inversely proportional to the frequency of the signal
Measurements
Comparison of PSD for same R resistor and graphene layer (Hf substrate)
Each sweep measures f and V for the back gated Drain-Source signal.
Two setups; compare DC and Pulsed for final detector
Procedure
DC Voltage Sweeps
3 to 6 takes, 5min each
300kS at 1kHz
50mV to 1500mV
Pulsed Sweeps
3 takes, 5min each
300kS at 1kHz
50μs to 950μs
Long Sweeps
Single takes, 30-120min
Different voltages
Both setups
1800kS at 1kHz

Plot Results
Conclusions
Issues
Coming soon
DC Voltage Sweep 1V R=1kΩ
f (log scale)
PSD
Power Spectral Density is the frequency response of a random or periodic signal
Distributes the average power as a function of frequency
Average of the Fourier transform magnitude squared over a large time interval
Error Estimation
1/f observed in both graphene and the resistor
Resistor α≈1
Graphene α>1, usually α≈1.2
Not originated in power sources
No difference between direct and pulsed current
Hysteresis observed: Impurities? Quantum Dots? Water? ...
Systematic method: lowering the voltage to 0 before each measurement
Bibliography conclusions
Small collection on graphene noise
Even smaller on 1/f in graphene
No results on α (p2) value
Considerable amount of papers about influence of impurities in graphene resistance (especially water)
Humidity rises electrical resistance of graphene. Experiments should be carried in Nitrogen. Graphene resistance increased from 1kΩ to 6.2kΩ
Quantum dots affect conductivity
In the past, observed diode properties on Hf substrate
Observed hysteresis during DC Voltage Sweeps in Graphene
Short term
1. Pulsed sweeps with shaper readout
2. Nitrogen medium and temperature monitoring while measuring

Long term
3. New, purer graphene without Quantum Dots
4. SiO substrate
Why Graphene?
DC Voltage Sweep
Pulsed Sweep
DC Voltage Sweep 1V Graphene
f (log scale)
Pulse Width Sweep 500µs R=50Ω
f (log scale)
Pulse Width Sweep 500µs Graphene
f (log scale)
Tutors: Federico Sánchez, José Gabriel Macías, Thorsten Lux

When graphene is doped with Quantum Dots, trapped charged particles shift the Dirac Point because of electrical field alterations.
Error can be estimated with many measurements or dividing a single long one
Average and RMS of PSD
White Noise Sample
p0≈11μV
ADC≈9μV
PSD (log scale)
PSD (log scale)
PSD (log scale)
PSD (log scale)
DC Source PSD
Systematic measurement methodology developed
2
α
Full transcript