02-09-22 G.Dubey, at Watlabs 1

Materials Chemistry and Physics of Ultra Thin Films

for Nanotechnolgy

G.Dubey

02-09-22 G.Dubey, at Watlabs 2

IntroductionThe Purpose of this discussion details recent work done on Silicon (100) wafers and Organic 0.05 M Polypyrrole, in conjuction with deposited copper, Cu(s), and to shed some light on both the quantitative and qualitative results we have so far encountered. This discussion might also help us brainstorm about what future and continuing work should be focussed on in this field.

Just to formally introduce myself, hi I’m Girjesh Dubey, an undergraduate assistant here at Watlabs for the Summer Term

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Topics of DiscussionMorphology of Polypyrrole and Copper Nano Structure Formation by ElectrodepostionResistivity/Conductivity of Ultra Thin FilmsFuture Suggestions for Research

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Morphology Of Thin Film and Nanoparticle Copper

Polypyrrole ( a semiconducting polymer made from light sensitivemonomer called Pyrrole) can be polymerized electrochemically or by photon induction. Induced light polymerization has not been wellestablished, and for our purposes we are using electrochemical techniques to synthesize the polymer. PPY however, must be deposited on an electrode, as a thin film, in the Three Electrode System, and so cannot be isolated from some chosen substrate. Our chosen substrate has been a p - doped 1 side polished Silicon (100) wafer, which produces and interesting combination (PPY+Si) of two semiconducting materials. The choice of electrode might be supposedly “arbitrary”, meaning that a Gold, Platinum, Nickel, Germanium electrode, etc. etc., should not affect the outcome of the deposition. However, both the theory and results have yet to be quantified.

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Diamond Structure of Silicon and Germanium

Polished SideUnpolished Side

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Chemical Structure of PPY film to be Deposited on Silicon

Alternating Double bonds in the aromatic structure give rise to a conjugated polymer, permitting electron transfer and thus film conductivity

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Electrochemical Experimental Setup For PPY and Cu(s) Depostion

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Si(100) Etching Procedures…..????

Polypyrrole does not absorb onto bare, un-etched silicon because the surface is too flat.

We thus etch our substrate for three main reasons:

Using Dilute HF solutions, roughen the surface, for better depostion results

Etching with HF should in theory terminate the silicon surface with hydrogen, giving the desired native crystal surface structure

Etching also provides a means of (hopefully) removing insulating SiO2 , which should affect both deposition and resistivity measurements

02-09-22 G.Dubey, at Watlabs 9

Film Thickness Calculation

When we apply a potential in the electrochemical cell, we measure the final charge Q= it, after polymerization.

Thus mass of PPY = (it/nF)xM, where M is the molar mass

So Volume of PPY = (it/nFρ)xM, where ρ is the density

And since V=Axh, the film thickness, h=itM /nFρA

So for a desired thickness, we measure the Area of the wafer, and control the amount of charge

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Morphology Of Polypyrrole/Copper On Silicon Substrates

Electrochemically deposited Ppy has exhibited some predictable surface morphology including Cauliflower, Branching, and Small Beading forms as shown:

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In thinner Films (less than 100nm), we notice distributions of small bubbles scattered along the

surface

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In Thicker Films, Horn and Tube Like Structures Found

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Copper Nano Crystal Morphology

Copper Nanocrystals Deposited Electrochemically In a solution of Copper(II) SulphateSeveral different shapes, sizes, and number densities

seen when deposited on the Polypyrrole, and bare SiliconWe have seen, perfect cubic, spherical, triangular,

irregular, hexagonal, and fractal forms of copper varying from micro to nano scale range.

By controlling the applied potential in depostion, we want to know if the shape and postion of copper crystals depends on the surface morphology of PPY and possibly even the Silicon Substrate surface

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Resitivity of Thin Films (PPY)We are interested in measuring various dielectric parameters of PPY, specifically the active resistivity, ρ, when we apply an external potential to the combined PPY/Si systemWe hope one day to produce a temperature dependent resistivity function for polypyrrole film itself, deposited on silicon, a combined semiconductor modelFurthermore, we hope one day to measure the effect of electrochemically deposited copper nano crystals on the dielectric function for these materials.

By definition, resistance through materials vary with length andarea, but the resistivity is a unique parameter per material. The resistance through a cylindrical conductor of length l and cross-section area A, has resistance R=ρl/A, where the potential is applied across the cross section.

We applied the same principle to bare 1x1cm2 Silicon itself ( known ρ=8x10-5 Ωm), using a two point probe and discovered a 374% difference from theoretical value!

Both the contact resistance and pressure inconsistence caused this dramatic result, so we turned to a four-point probe setup.

Four Point Four Point Resistivity Resistivity Methods:Methods:In this setup, we apply an external potential with known amperage, across two contact points in the sample, then measure the potential across the other two contact points

There are many advantages over a conventional two point method: Firstly, results will be more accurate, as the method eliminatescontact resistance as a source of error. Next, many different geometries can be invoked, with different probe spacing, depending on the sample geometry. Furthermore, it is the most recent and widely used method for resistance measurements of thin films andsemiconductors.

There are general formulas that yield the resistivity, but they depend on the distance between contacts and their arrangement on the surface of the sample.

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The Vander Pauw Method

The Vander Pauw Method is a special case of the 4-point probe resistivity method, in which all 4 contact points are arbitrarily placed at the periphery of the sample of known thickness, and the resistivity will be independent of the contact spacing and geometry.

A B

IBCVAD

CD

Two successive measurements on alternating probes are to be carried out, and then the resistivity, ρ can be extracted numericallyas follows:

e-πhR1/ ρ + e-πhR2/ ρ =1Note for the special case where R1=R2, ρ is immediately ρ =πhR / ln2

Two Proposed Mechanisms For Electron Flow:

1. Pure Parallel Circuit:

RSiPPYRPPY

Sie- flow

Film

Sie- flow

Film

RsiRppy

Rsi +RppyRT =

Rsi +Rppy

RsiRppyRT = nRi +

Uh oh I think Serge Is Ready To ask me something…