characterization technique

7/30/2019 characterization technique

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Objective:

The aim of this experiment is to investigate a dummy cell using the

FFT impedence spectroscopy and then on the basis of results evaluate the

electrical components used in the dummy cell and arrangement ofcomponents in dummy cell. Also, use the impedence spectroscopy at

dynamic electrochemical system (oxidation of p-Si) for measuring the

dielectric properties of the system and then observing the dependence of

these properties on the frequency of the alternating current.

Theory:

Impedence (Z)is the resistance to the flow of alternating current. It

is the ratio of voltage Uand current I. Here Uand Iare the functions of

time and frequency.

Here Um is the amplitude of voltage, Im is amplitude of current, is

frequency and is phase shift between voltage and current. Z contain real

and imaginary part. Impedence is independent of frequency only for pure

ohmic resistors. But capacitor and inductors impedence depends upon

frequency. [1]

Cis capacitance of capacitor and L is inductance of inductor. Phase

shift in case of resistor is 0 but for capacitor phase shift is -90 and for

inductor phase shift is 90. A nyquist plot will obtain if we draw

imaginary part of impedence against real part of impedence. A Bode plot

can also be drawn for any quantity against frequency.


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Fig 1: Nyquist plot for Capacitor[1]

Fig 2: Bode Plot for Capacitor[1]

Similarly Nyquist and Bode plot for resistor, Inductor and for any

combination of these components can also be drawn. [1]

Faraday Law of Electrolysis can be used to measure the valance

number (z) of the material.

z =

z is valance number, I current, t is time, M is molar mass, F is

Faraday constant and m is mass.[2]

Apparatus and Material:

1. Dummy cell

2. Electrolyte(water + acetic acid with 1:1)

3. p-Silicon

4. FFT Impedence Spectrometer Elypor-02, ET & Te


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Procedure:

Firstly, FFT impedence spectroscopy was used to determine the

components and arrangement of components of Dummy cell. Dummy cell

was composed of differnet electrical components like resistor, capacitorand inductor etc. Hence a static voltage of 1V was applied to dummy cell

for 5 minutes and then impedence of dummy cell was calculated.

Secondly, FFT impedence spectroscopy was used to characterize

the dynamic electochemical system. The electrochemical system consist of

p-Si sample in contact with electrolyte. This leads to the oxidation reaction

which consiquently form SiO2. Electrolyte was given negative potential as

compared to silicon substrate, while the reference electrode was also

imersed in the electrlyte. For this reaction a constant current of 1mA was

applied.

Finally, Negative potential of -1.4 was applied at the oxidized

sample of p-Si to check the impedence of elecctrochemical system in

reversed biased direction.

Results and Discussion:

Dummy Cell

In graph 1, Nyquist plot is shown for the dummy cell. Graph 2 is

between Zreal and Frequency. With increase in frequency, real part of

impedence is decreasing. Where as in graph 3, which is between

imaginary part of impedence and frequency, with increase in frequency,

Zim is first decreasing and then increasing.


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Graph 1: Nyquist Plot of Dummy Cell

Graph 2: Bode plot of Dummy cell

Graph 3: Bode Plot of Dummy Cell

0

100

200

300

400

500

600

0 200 400 600 800 1000 1200

Zreal [Ohm]

-Zim

[Ohm]

0

200

400

600

800

1000

1200

0 1 2 3 4 5

log(f) [Hz]

Z real

-600

-500

-400

-300

-200

-100

0

0 1 2 3 4 5

log(f) [Hz]

Z im

[Ohm]


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From these 3 graphs, one can estimate that the dummy cell is

composed of a capacitor and a resistor in parallel manner this is then

connected with an other resistance and inductor in series.

Figure: Components of Dummy Cell

With increase in frequency, impendence of capacitor will decrease

and then more current will pass through capacitor which will

consequently short circuit the parallel resistance. That is the reason that

we are getting a decreasing curve in graph. From graph 2, one can

estimate that the series resistance is of 100 Ohm and parallel resistance is

of 1000 Ohm, Because

At very high frequency

Zreal = Rser = 100 Ohm

And at very low frequency, when capacitor will act as insulator then

Zreal = Rser + Rpara = 1100 Ohm Rpara = 1000 Ohm

Inductor connected in series will not influence because real part of

inductors impedence is zero. Graph 3 is showing relation between Zim of

dummy cell and frequency. With increase in frequncy, first impendence

decreases but after getting a minimum point it starts increasing with more

increase in frequency.

Oxidation

Graph 4 is showing the increase in series resistance with time. This

series resistance is the resistance of electrolyte. With passing time, there is


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not much change in the resistance of the electrolyte. Hence this small

change of resistance in system can be ignored.

Graph 4: Series Resistance change with time

Graph 5 is showing increase in parallel resistance with time. Here

parallel resistance is actually the resistance of electrolyte which is inside

the pores of oxide layers and reaching to the Si substrate for doing

oxidation. With increase in thickness of layer, pores are getting narrower

and longer so resistance is increasing. 2 peaks in graph can be seen, which

might be due to any noise in the system so we can ignore these.

Graph 5: Parallel resistance change with Time

660

680

700

720

740

760

780

800

0 10 20 30 40

time [min]

Rs[Ohm]

0

500

1000

1500

2000

2500

3000

3500

4000

4500

0 10 20 30 40

Rp[Ohm]

time [min]


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Graph 6 is showing decrease in capacitance with time. As time is

passing, oxide layer thickness is increasing which in turn decreases the

capacitance.

Cis the capacitance, A is the area, r is the relative staticpermittivity, 0 is the electric constant (0 8.8541012 F m1) and d is thethickness of oxide.[3]

Graph 6: Change in capacitance with time

As we have pores in our oxide so actuall area is 3 times the area of thesubstrate getting oxidized. Hence

Aeffective = 3*A = 3* (1cm2) = 3cm2, r = 3.9. Now we can calculate d (oxide

layer thickness)

Graph 7: Thickness of Oxide layer Vs time

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 10 20 30 40

time [min]

C[F]

0

50

100

150

200

0 10 20 30 40

Thickness

[nm]

time [min]
http://en.wikipedia.org/wiki/Relative_static_permittivityhttp://en.wikipedia.org/wiki/Relative_static_permittivityhttp://en.wikipedia.org/wiki/Vacuum_permittivityhttp://en.wikipedia.org/wiki/Vacuum_permittivityhttp://en.wikipedia.org/wiki/Relative_static_permittivityhttp://en.wikipedia.org/wiki/Relative_static_permittivity


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z =

z is valance number, Icurrent, t is time,Mis molar mass of SiO2, F

is Faraday constant and m is mass of SiO2.

z = 5

Here is etch rate which is zero in our case, is growth rate, Jox iscurrent density and ds/dt is change in oxide thickness with time. From

start value of is 0.423 (nm/s)(cm2/mA) and from end it is 0.207

(nm/s)(cm2/mA)

Negative potential:

After oxidation, negative potential of 1.4V was applied to the

substrate of p-Si which has developed oxide layer. Graph 8 is showing

Nyquist plot in which we have a semi circle in start. This semi circle is due

to the parallel impedence of oxide layer and electrolyte in pores of oxide

layer. Semi circle starts from some value of Zreal showing the impedence

value of electrolyte. After Semi circle there is a sudden increase in thegraph which is dur to the formation of space charge region or the

deplition zone at the bottom of p-Si substrate because electron from

negative potential will combine with holes of p-Si and form deplition zone

which will act as capacitor.

Figure 3: Negative potential cause formation of Space charge Region

Oxide Layer


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Graph 8: Nyquist plot at negative potential

Graph 9 is showing the decrease in Zreal with increase in frequency,

because at at high frequency impedence of capacitor gets lower andcurrent face less resistance.

Graph 9: Bode plot of Impedence real part

Graph 10 is showing the decrease in imaginary part of impedence

with increase in frequency this first decrease is due to decrease in the

capacitance of the depletion region and then the second semi circle is

obtained due to the parallel impedence of oxide layer and electrolyte in

series with impedence of electrolyte.

0

1000

2000

3000

4000

5000

6000

0 2000 4000 6000 8000

-Zim

[Ohm]

Zreal [Ohm]

0

1000

2000

3000

4000

5000

6000

7000

0 1 2 3 4 5

log(f) [Hz]

Zreal

[Ohm]


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Graph 10: Bode Plot of Impedence imaginary part

Conclusion:

FFT Impedence Spectroscopy is the reliable technique to analyze

the static as well as dynamic electrochemical system. It can give precise

thickness of oxide layer with time. Hence one can easily study the

oxidation phenomenon and apply it in multi fields.

References:

1. Lab manual M208- FFT Impdence Spectroscopy

2. "

Faraday's Electrochemical Laws and the Determination ofEquivalent Weights".Journal of Chemical Education 31 (May): 226

232

3. College physics by Raymond, vol 10, page # 362

Questions and Answers:

Questtion # 1:

What is the role of reference electrode?

Answer:

Reference elecctrode was used to reduce the ohmic losses in

dynamic electrochemical system.

0

1000

2000

3000

4000

5000

6000

0 1 2 3 4 5

log(f) [Hz]

- Zim

[Ohm]


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Question # 2:

What Nyquist plot would you expect from a circuit containing an

inductor and resistor in parallel configuration?

Answer:

If we connect a inductor in parallel with a resistor then Impedence

will be

Hence, when freqency will be zero then impedence of indutor will

be zero and it will short circuit the resistor, making total Z=0 Ohm. But

when frequency will be infinity then inductor will act as insulator and

Z=Rpara. So Bode plot will be like as given below

Figure 4: Bode plot For Parallel connected Inductor and Resistor

Question # 3:

In which system can the following Nyquist plot occur? Which is the

equivalent circuit.


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Answer:

For the above given graph, equivalent circuit will be like given

below

Fig 5: Equivalent Circuit

Here this should be kept in mind that C1 and C2 should not be

equal to each other. Because If it will happen then both semi circle will

overlap and we will not be able to distinguish them.

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characterization technique