microcantilevers iii cantilever based sensors: 1 the cantilever based sensors can be classified into...
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Microcantilevers IIICantilever based sensors:
1
The cantilever based sensors can be classified into three groups
(i)General detection of any short range forces: Scanning probe microscopy
(ii)Detection of mass attachment: based on changes in frequency response
(iii)Detection of molecular adsorption: based on changes in surface stress of a functionalized cantilever. This can also be detected based on resonance amplitude change.
(iv)Detection of radiation (IR or nuclear): based on deflection of a bimaterial cantilever, or from deflection changes
(v)Detection of charges or electric or magnetic fields: Based on resonance frequency or amplitude changes
Note that although cantilevers are highly sensitive to stress changes, but resonance frequency based changes are the most sensitive due to quality factor enhancements.
Goutam Koley
Kelvin probe techniqueKelvin probe technique measures surface charge, surface potential, and surface work function. The advantages are quantitative nature, and ease of operation.
Si
Eg,Sip+ Silicon
Ec
Ev,EF,Si
semi
EF,semi
qs
qVcon = qVdc = Si + Eg,Si - (semi + qs)
Probe Tip Semiconductor Sample
Evac
EC
qVcon
G. Koley and M. G. Spencer, J. Appl. Phys. 90, 337 (2001)
Goutam Koley
Mathematical Model for SKPM
tVVVd
CF accondc sin)(
2
0
2
4sin
2
1
d
QQVtVV
d
CF tips
conacdctotal
2FFFF dctotal
G. Koley and M. G. Spencer, J. Appl. Phys. 90, 337 (2001)
Goutam Koley
SKPM Measurement System
LOCK-INCIRCUITRY
CONTROLLER
SURFACE POTENTIAL IMAGE
MORPHOLOGY IMAGE
CONTROLLER
AMPLITUDE DETECTOR
PO
SIT
ION
DE
TE
CT
OR
LASER
SC
AN
NE
R
OSCILLATOR
Vacsint
Vdc
Vdc
SAMPLE
Z
G. Koley and M. G. Spencer J. Appl. Phys. 90, 337 (2001)
Goutam Koley
Electronic characterization of dislocations
Morphology Potential
0.1 V /Div
10 nm /Div
Sur
f. P
oten
tial
G. Koley and M. G. Spencer, Appl. Phys. Lett. 78, 2873 (2001)
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Surface potential patterning using mask
UV light
20 m circlequartz mask
HFET Sample(35% Al in barrier,44 nm AlGaN layer)
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Spatial resolution of charge storage
• UV exposure through a mask of 1, 2, 5, 10 and 20 m squares
• Spatial resolution on the order of ~1-2 m
3.5 m
G. Koley et al. JAP (2004)
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Measurements in GaN based transistorsAFM scanning probe
Biasing Probes
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Surface morphology and potential profiles in dc biased
transistorsGate SourceDrain
Morphology Surface Potential
Vd = 2V, Vg = -1.5 V
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The Cantilever effect
RRVV
KVV
surfsurf
dcmeas
1
zCzCR tipcanti
Smaller R results in larger measurement accuracy
Goutam KoleyG. Koley et al. Appl. Phys. Lett. 79, 545 (2001)
Goutam Koley
Measurement of transients
+ve dc bias
Probe tip
-ve dc biasor square pulse
Measurement setup schematic
A A20 resistor
AFM scanning probe
Biasing Probes
Source
Gate Drain
Gate Drain
Source
Goutam KoleyG. Koley et al. IEEE Trans. Electron Dev. 50, 886 (2003)
Goutam Koley
Drain current variations due to stress
Type II: continuous stress at Vd = 20V, Vg = -12 V for 2 mins
100 m HFET100 m HFET
Vd = 1 V, Vg = 0 V during measurement.
Type I: pulsed stress at Vd = 5V, Vg = pulsed from 0 to -10 V
Goutam KoleyG. Koley et al. IEEE Trans. Electron Dev. 50, 886 (2003)
Goutam Koley
Correlation between drain current and surface potential
Device stressed at Vd = 20 V, Vg = -12 V for 2 mins
Surface potentialDrain current
100 m HFET 100 m HFET
Vd = 1 V, Vg = 0 V during measurement.
Goutam KoleyG. Koley et al. IEEE Trans. Electron Dev. 50, 886 (2003)
Goutam Koley
• Stressed at Vg = -12V, Vd = 20 V for 2 mins
• Maximum variation observed ~0.3 m from the gate edge
• Charges take a long time to reach equilibrium value
Potential variation with distance and time
150 m HFET
Goutam KoleyG. Koley et al. IEEE Trans. Electron Dev. 50, 886 (2003)
Goutam Koley
Surface conductivity measurements
(a) Morphology, (b) conductivity, and (c) overlap of the surface morphology and conductivity images
Goutam Koley
Scanned gate microscopyScanned gate microscopy is useful to determine the variation of conductivity along a thin channel, and where direct measurement of conductivity is difficult
(a) Experimental set up for SGM, (c) the SGM image of a single-walled CNT bundle for Vtip = 1 V; Black corresponds to very high resistance
Refer to handouts for scanning thermal microscopy
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Scanning capacitance microscopyScanning capacitance technique actually measures the dC/dV signal which is
inversely proportional to doping. The advantages of this technique include a large measurement range (1015 – 1020 cm-3), and resolution of <10 nm
For capacitance measurement a low frequency ac voltage is applied to the sample. The ac voltage periodically changes the tip-sample capacitance. The sensor produces a high frequency signal to measure very small capacitance changes.
dC
dV
q
CN
AlGaNVC 0
3
Cz AlGaN
VC
0
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Application of capacitance microscopy
Cross-sectional measurement in a MOSFET under actual operation
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Applications to GaN samples
• The dC/dV decreases around the dislocations indicating the reduction in the background carrier concentration
Morphology image Capacitance image
C-V curve