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Electronic characterization of dislocations
10 nm
/Div
0.1 V
/Div
Morphology
G. Koley and M. G. Spencer, Appl. Phys. Lett. 78, 2873 (2001)
Potential
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Goutam Koley
Surface potential patterning using mask
UV light
20 m circle
quartz mask
HFET Sample
(35% Al in barrier,
44 nm AlGaN layer)
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Goutam Koley
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
Relative inverse surface barrier (eV)
0.38
0.37
0.36
0.35
0.34
3.5 m
0.33
0.32
G. Koley et al. JAP (2004)
0.31
0.3
60
62
64
66
68
Distance(m)
70
72
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Goutam Koley
Measurements in GaN based transistors
AFM scanning probe
Biasing Probes
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Goutam Koley
Surface morphology and potential profiles in
dc biased transistors
Drain
Gate
Morphology
Source
Vd = 2V,
Vg = -1.5 V
Surface Potential
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Goutam Koley
Measurement of transients
AFM scanning probe
Measurement setup schematic
Probe tip
Gate
Drain
Source
Biasing Probes
G. Koley et al. IEEE Trans. Electron Dev. 50, 886 (2003)
Source
Gate
Drain
A
A
20 
resistor
-ve dc bias
or square pulse
+ve dc bias
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Goutam
Goutam
Koley Koley
Potential variation with distance and time
• 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
Surface Potential (V)
-0.5
150 m HFET
-1
0 min
0.5 min
1 min
1.5 min
2 min
3 min
4 min
5 min
8 min
11 min
18 min
28 min
before stress
-1.5
-2
-2.5
-3
-3.5
0
0.2
0.4
0.6
0.8
Distance from gate edge (m)
G. Koley et al. IEEE Trans. Electron Dev. 50, 886 (2003)
Slide #
Goutam
Goutam
Koley Koley
1
Surface conductivity measurements
(a) Morphology, (b) conductivity,
and (c) overlap of the surface
morphology and conductivity
images
Slide #
Goutam Koley
Scanned gate microscopy
Scanned 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.
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Goutam Koley
Scanning capacitance microscopy
Scanning 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
N C V 
C3
q 0 AlGaN
dV
dC
zC V 
 0 AlGaN
C
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.
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Goutam Koley
#
Application of capacitance microscopy
Cross-sectional measurement in a MOSFET under actual
operation
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Goutam Koley
Applications to GaN samples
Morphology image
Capacitance image
C-V curve
• The dC/dV decreases around the dislocations indicating the reduction in
the background carrier concentration
Slide #
Goutam Koley