cpd and other imaging technics for gas sensor mizsei, jános 18-28/05/2006 ustron budapest...
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Sample: 100 ft. 0.35 elektrodavalSize: ¤22.00mmPixel Size: 0.25mm
0.500 1.100
CPD and other imaging technics for gas sensor
MizseiMizsei,, János János
18-2818-28/0/055//20062006
UstronUstron
Budapest University of Technology and Economics,
Department of Electron Devices
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OutlineOutline
• Introduction: potentials in general
• Ideal (static) voltmeter. Do we really Do we really need contacts ?need contacts ?
• What is it for ? Applications...
• …extension of the application (x-y scanning, higher resolution, (Kelvin Force microscopy) etc...
• Summary
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IntroductionIntroduction: : thethe popotentialtential
• working ability of a point charge in r
• the electric field: force on the charge
• it is a general „boundary condition” in the electronics
• electrochemical potential:
• advantages: it can be easily measured in a broad range,
• excellent for characterisation of physical systems:
)(rE gradU
in
n
q
kTUU ln)()(' rr
q
kTD
)())('( nqDgradEnqgradUnqJ r
0UN
FUU
P
n p
in
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VOLTMETERVOLTMETERSS
unknownV
•”Handy” voltmeters: 20 M
•Electrometers: 1012-14 (?) (electron tubes, FET)
•Compensation: voltage measurements without current
•…without current. Do we really need
contacts ?
V
I0
I
if
VVunknown
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The ideal VOLTMETERThe ideal VOLTMETER: : RRinin==
Vi
How can we do that in practice?
Capacitive coupling + compensation:
„„Vibrating reed voltmeterVibrating reed voltmeter””
d
V
tE
tA
i
t
D00
Qt
CVtd
VA
ti
0
d
VE
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””Vibrating reed” voltmeterVibrating reed” voltmeter::• static capacitive coupling: it is not static capacitive coupling: it is not
applicable to transfer the information applicable to transfer the information about DC voltage (except: MOS FET)about DC voltage (except: MOS FET)
• solutionsolution:: non-static (vibrating) capacitor non-static (vibrating) capacitor
R=U
Phase sensitive
frquency selective current detector
tCUdt
dCU
dt
dQI
CUQ
cos
tCCC sin0 U
tC
I
cos U He
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cpdU
What elseWhat else? Potential directly from the surface, ? Potential directly from the surface, without contact.without contact.
U
Phase sensitive
frequency selective current detector
UU
I
tCUUdt
dCUU
dt
dQI
UUCQ
cpd
cpdcpd
cpd
0
cos)()(
)(
tCCC sin0
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TheThe CPD: CPD:
zero electric field between the plates !zero electric field between the plates !
CPD compensated:Lower work function (electron emission, positive surface charge)
Higher work function (negative charge on the surface 0ldE
A B
CPDUldEB
A
AB
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Current to Current to be detected:be detected:
6.0
3.0
0
0
0
0
d
d
d
d
d
d
-2.5
-2
-1.5
-1
-0.5
0
0.5
1
1.5
2
2.5
-10 -5 0 5 10
cos(x)/(((1+0.3*sin(x))*(1+0.3*sin(x))))
cos(x)/(((1+0.6*sin(x))*(1+0.6*sin(x))))
cos(x)
2
0
00
sin1
cos
)(
tdd
tdd
C
VVt
Qi cpd
td
dC
tdd
Ad
AC
sin1
1
sin
0
0
00
0
)( VVCQ cpd CapacitancCapacitance:e:
Charge:Charge:
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Up to date Up to date
equipment:equipment: •frequency selective amplifying, phase sensitive (multiply) demodulation
•feedback of the DC voltage (automatic compensation)
•optical excitation for surface photovoltage measurements
•digital realisation
•second harmonics detection and feedback for distance control
•surface mapping (x-y scan).
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The “ideal” energy diagram of a vibrating capacitor - The “ideal” energy diagram of a vibrating capacitor -
semiconductor systemsemiconductor system qq
WqCPDq b
gmsm )
2()(
Ideal: no surface traps around the Fermi-level, the surface index S=1
)2
()( bg
msm qW
CPDq
FBVV
lightdark CPDCPDCPD )()()(
dark (equilibrium)
light (non-equilibrium)
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Additional Additional light light excitationexcitation: FB state: FB state
FBVV
Gas sensor layer
CPD
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Vibrating capacitor (Kelvin) and SPV (surface photovoltage) methodVibrating capacitor (Kelvin) and SPV (surface photovoltage) method
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2)(2
1VV
d
CF cpd
tvtvVVvVVd
C
tvtvVVVVd
CtvVV
d
CF
cpdcpd
cpdcpdcpd
2cos2
1sin)(2
2
1)(
2
1
)sin(sin)(2)(2
1)sin(
2
1
222
222
VVcpd
)sin( tvV Vibration due to voltage on the tip: …stops when !!!Vibration due to voltage on the tip: …stops when !!!
22
22
1V
d
ACVW
2
22
2
12
CVd
Vd
A
d
WF
)sin( tv
)(V
)( VVcpd
Kelvin Force Microscopy: AFM + KelvinKelvin Force Microscopy: AFM + Kelvin
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Semiconductors in gas sensitive Semiconductors in gas sensitive structuresstructures
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Behaviour of the Behaviour of the semiconductor gas sensor materialssemiconductor gas sensor materials
Experimentally observed change in the Experimentally observed change in the work function and change in the work function and change in the resistance (logarithmic scale) as resistance (logarithmic scale) as function of partial pressure (root scale)function of partial pressure (root scale)
in
n
q
kTUU ln)()(' rr
nqR
1
n
n
q
kT
R
R
q
kT g
g
lnln
gR
R
q
kTU ln)(' r
nq
nq
R
R g
g
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Behaviour of the semiconductor gas sensor materialsBehaviour of the semiconductor gas sensor materials
Experimentally observed correlation between the work function Experimentally observed correlation between the work function VVKK and and change in the resistance (logarithmic scale, which shows change in the resistance (logarithmic scale, which shows VVRR linearly) linearly)
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Potential shift (change in the Potential shift (change in the CPD) due to chemical signal:CPD) due to chemical signal:
•dipole adsorption on the semiconductor surface
•dipole adsorption on the reference electrode
•charged particles (ions) on the semiconductor surface
•change in the bulk defect (donor, acceptor) concentration due to diffusion of the adsorbed atoms
•change in the composition (stoichiometry) of the semiconductor materials
adDii
TTTTDi
T
qNdepletionFLqn
Ush
UUch
Uch
L
UQ
,b
bbb
2);()sgn(2
2)sgn(
i
adTb n
NU ,ln
adadNq
ig nW ,,
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Semiconductor Semiconductor resistance/conductance response resistance/conductance response due to chemical signal: due to chemical signal:
•dipole adsorption on the semiconductor surface
•charged particles (ions) on the semiconductor surface
•change in the bulk defect (donor, acceptor) concentration due to diffusion of the adsorbed atoms
•change in the composition (stoichiometry) of the semiconductor materials
•dipole adsorption on the reference electrode
•NO
•YES, if the surface charge is balanced by the space charge layer in the semiconductor
•YES, usually at higher temperature
•YES, usually at higher temperature
•NO
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Non-ideal Non-ideal systemsystem
Large number of surface traps: Fermi-level pinning, the surface index S<1
Charged particles (ions) on the semiconductor surface:
counterpart of the charge is localized to the surface
charged particles form dipole layer:
CPD response: YES
potential barrier,
space charge, resistance response: NO
(usually at lower temperature)
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Activated semiconductor Activated semiconductor gas sensor surfacegas sensor surface
High number of Qss
Change of the charge in the surface/interface states (Qss) instead of the space charge layer in the semiconductor.
no conductivity response.
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ALE SnOALE SnO22 layers: CPD and resistance maps layers: CPD and resistance maps
Sample: alesno91Size: 50.00mm × 50.00mmPixel Size: 1.00mm
-0.100 0.600
33K 30K 46K 107 1011
34K 48K 133K 1012 1012
46K 71K 1012 1012 1012
97K 107 1011 1012 1011
182K 108 1012 1011 1011
Sample: alesno 72Size: 50.00mm × 50.00mmPixel Size: 1.00mm
-0.100 0.300
200 432 213
145 181 157
146 331 172
140 156
184 4M 182
Chemical pictures by vibrating capacitorChemical pictures by vibrating capacitor
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Selective chemical sensing with potential mapping
360K
460K
Material gradient
Tem
per
atu
re g
rad
ien
tPd Ag Au Pt V Pt SnO2
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Chemical pictures Chemical pictures (surface: Pd-Ag-Au-Pt-V-Pt-SnO(surface: Pd-Ag-Au-Pt-V-Pt-SnO22))
Pd Ag Au Pt V Pt SnO2
C
30mm
25m
m
1% H2 –in air NH4OH vapour (NH3) CHCl3 vapour C2H5OH vapour
460K
-0.2
0
0.2
0.4
0.6
0 20 40 60360K
-0.4
-0.2
0
0.2
0.4
0 20 40 60
VoltVolt
Pixel Pixel
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Porous silicon-pPorous silicon-p++Si as gas sensor Si as gas sensor materialmaterial
PSid
Extremely high amount of + charge in the porous Si
msPSiVV
msPSiVV FB
FBVV
Light excitation
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The charge balanceThe charge balance::
);()sgn(22)sgn( bbbb
0
FLqnU
shUU
chU
chL
Udx Dii
TTTTDi
T
d
PSi
PSi
FBVV
from vibrating capacitor (dark-light) or from the SPV (the saturated SPV signal is proportional with the potential barrier)
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KelviKelvinnmapsmaps
Sample: darkSize: 35.00mm × 57.00mmPixel Size: 1.00mm
0.000 0.600
Sample: l ightSize: 35.00mm × 57.00mmPixel Size: 1.00mm
-0.300 0.300
Sample: Size: 35.00mm × 57.00mmPixel Size: 1.00mm
0.000 0.600
Sample: PSi on p SiSize: 35.00mm × 57.00mmPixel Size: 0.50mm
-0.191 -0.012
- =
SPVSPVmapmap
10
20
50
100
200
500
Process time/s in 1.5/3.5 HF/C2H5OH mixture with 50 mA/cm2 current density (growth rate is ~0.07-0.1 micron/s)
P+ 0.015 ohmcm Si
inversion
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Surface conditions: thick and ultrathin oxide Surface conditions: thick and ultrathin oxide covered Si covered Si
Doping
Surface layer
n+ n p p+
Bare accumulation accumulation inversion (weak) depletion
Ultra thin oxide (weak) depletion,
near flat-band
depletion,
near intrinsic
inversion,
near intrinsic
(weak) depletion,
near flat-band
Thick oxide accumulation accumulation inversion (weak) depletion
V VQSS>0
Porous silicon on p+: (strong) depletion
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Atomic Force and Kelvin Force Microscopy: charged Atomic Force and Kelvin Force Microscopy: charged surfacesurface
700000V/m
700000V/m
Atomic Force: oxide step
Kelvin Force: surface potential
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AFM and Kelvin Force MicroscopyAFM and Kelvin Force Microscopy
Morphology
potential distribution
Semiconductor (WO3) gas sensor nanograins
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SummarySummary• Vibrating capacitor method included the high Vibrating capacitor method included the high
resolution version (Kelvin Force Microscopy)resolution version (Kelvin Force Microscopy)
• ExamplesExamples: analytical tool and sensor (chemical signal : analytical tool and sensor (chemical signal converter)converter)
• Conclusion: a lot of useful application possibilitiesConclusion: a lot of useful application possibilities