Photo Conductivity Measurement

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H. ARUL11PHD0036

Photoconductivity

Measurement1


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Introduction

Photoconductivity is an optical and electricalphenomenon in which a material becomes moreelectrically conductive due to the absorption of

electromagnetic radiation such as visible light,ultraviolet light, infrared light, or gamma radiation

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Photoconductivity AFM

Photoconductive atomic force microscopy is apowerful characterization tool to better understandthe complex optoelectronic and morphological

phenomenon at the Nano scale.

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Need for Photoconductive AFM:

1) To Minimize Mechanical noise and other interferences onthe cantilever.

2) To conduct studies on electron injection and chargetrapping effects.

3) To avoid discrepancies in the film morphology on the Nano-

scale level includesa)Low open circuit voltages

b)Heterogeneous interfacesc)Grain boundariesd)Phase-separated domains

4) The fundamental modification of AFM to pc-AFM is theaddition of an illumination source and an invertedmicroscope that focuses the laser to a nano meter-scalepoint directly underneath the conductive AFM tip

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Experimental Set up5


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Construction:

The light is focused on the device through the ITO(indium tin oxide) using an inverted opticalmicroscope and a sample is loaded in a closed air-tight cell flowed with dry nitrogen

The AFM probe can either sit on a specific point on asample surface to record the current as a function ofan applied bias or the probe can be scanned with afixed applied bias to provide a current map

Metal-coated silicon probes with varying workfunctions can be employed as the top nano electrodefor either hole or electron collection

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Continues.

The piezo-tube scanner is responsible for thedirection of tip displacement during a sampleanalysis, and is dependent on the mode of analysis.

The cantilever behaves as a spring and oscillates atits resonance frequency

The non-contact feedback loop is used to control thatchanges in the oscillations of the cantilever

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Procedure:8


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Procedure:

A) The tip is not in contact with anysubstrate so no deflection

B) The probe meets the substrateadvanced further onto thesample, cantilever bearing theprobe is deflected

C) The piezo drivers begins to

withdraw the probeD) The probe and substrate are

physically connected even theyare separated

E) The probe loses its contact and

jump back to its original position

Working mechanism: Approach andretraction

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Tripod - Piezo tube

The Tripod x, y and zcomponents are arrangedorthogonally to oneanother with their apex

attached to a movablepivot point

The Tripod designs the

voltage applied to thepiezo corresponding to theappropriate direction oftip displacement

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Continues

The sample and substrate are mounted on top ofthe z-piezo component. When the x and y piezocomponents are in use, the orthogonal deign

causes them to push against the base of the z-piezo, causing the z-piezo to rotate about a fixedpoint.

Applying voltage to the z-piezo causes the tube tomove up and down on its pivot point.

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Points to be taken care

1) The material that comprises the conductive tip andcantilever can be customized for a particularapplication

2) When modifying traditional AFM for pcapplication, all components must be combinedsuch that they do not interfere with one anotherand so that various sources of noise and

mechanical interference do not disrupt the opticalcomponents.

3) Acoustic vibrations should be minimized bykeeping vibration isolated table

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Advantages:

To study Nano scale photo physics using lightintensity dependence measurements

To visualize the phase separation of two

components To study the topological and photocurrent

properties of devices at nano scale

Manipulate Surface with Molecular Precision

Real Time Direct Structure-Function Studies

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Disadvantages:

Image distortions due to the induction of thermaldrift

Slow rate of scanning due to the Hysteresis of

Piezo-electric tube Resonant frequency affects the motion of cantilever

due to non-contact feed back setup

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References:

Balasubramanian .K.et al. Appl. Phys.Lett.2004,84, 24002402.

Sakaguchi, H. et al. J. Appl. Phys.2006,38: 3908

3911 D. C. Coffey.et al, O. G. Reid and D. S. Ginger.Nano

Lett. 7, 738 (2007).

Asylum research working manual for

Photoconductivity AFM

Wikipedia.org

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Photo Conductivity Measurement