Slide #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)

Slide #Goutam Koley

Surface potential patterning using mask

UV light

20 m circle

quartz mask

HFET Sample

(35% Al in barrier,

44 nm AlGaN layer)

Slide #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

3.5 m

G. Koley et al. JAP (2004)

Slide #Goutam Koley

Measurements in GaN based transistorsAFM scanning probe

Biasing Probes

Slide #Goutam Koley

Surface morphology and potential profiles in dc biased

transistorsGate SourceDrain

Morphology Surface Potential

Vd = 2V,

Vg = -1.5 V

Slide #Goutam Koley

Measurement of transients

+ve dc bias

Probe tip

-ve dc bias

or square pulse

Measurement setup schematic

A A

20

resistor

AFM scanning probe

Biasing Probes

Source

Gate Drain Gate Drain Source

Goutam KoleyG. Koley et al. IEEE Trans. Electron Dev. 50, 886 (2003)

Slide #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)

Slide #Goutam Koley

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.

Slide #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

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

Slide #Goutam Koley

Application of capacitance microscopy

Cross-sectional measurement in a MOSFET under actual

operation

Slide #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