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

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

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

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

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

””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

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

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

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:

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).

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)

Additional Additional light light excitationexcitation: FB state: FB state

FBVV

Gas sensor layer

CPD

Vibrating capacitor (Kelvin) and SPV (surface photovoltage) methodVibrating capacitor (Kelvin) and SPV (surface photovoltage) method

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

Semiconductors in gas sensitive Semiconductors in gas sensitive structuresstructures

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

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)

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 ,,

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

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)

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.

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

Selective chemical sensing with potential mapping

360K

460K

Material gradient

Tem

per

atu

re g

rad

ien

tPd Ag Au Pt V Pt SnO2

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

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

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)

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

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

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

AFM and Kelvin Force MicroscopyAFM and Kelvin Force Microscopy

Morphology

potential distribution

Semiconductor (WO3) gas sensor nanograins

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