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)

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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

Goutam Koley

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

Goutam Koley

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