Beer_Lambert Law

PROPERTJESOF PPTEOS

4.6 Ultraviolet-visible (Uv-vis) Optical Absorption Spectroscopic Analysis The optical energy gaps and /he aUowed direct transitions and allowed indirect

4.6.1 Beer_Lambert Law

For most spoctm Ihe solution obe}s Beer's Law. This states that the light absorbed is propmtional to Ihe number of absorbing molecules TIlls is only lrue for dilute solutions. A second law- Lambert's law tells us that the fraction of radiation absorbed is independent of the intensity of the radiation, Combining these two la",~

gives the Beer-Lambert law:

4.2 4.3

where IS 1, the intensity of the incident radiaIJon, I is the mtensity of the transmitted radiation, d is the path length of the absorbing spocies or thickness and a IS the absOJption co-efficient. The absorption spoctrum can be analyzed by Beer~Lamberl law which governs the absorption oflight by the molecules and states that, "When a beam of monochromatic radiation passes through a homogeneous absorbing medIUm the rate of decrease in inlensity of electromagnetic radiation in Uv-vis region with thickness of the absorbing medium IS proportional to the intensity of inCIdent radiation",

The intensity of transmittance is e"pressed as the inverse of intensity of absorbance.

The absorption 00 efficient n, can be calculated from the absorption data using the relation [4.3]

a

^2.30JA

d

4'

where A ⁼IOg,,(1; )is the Absorbance.

To estimate the form of absorption a random phase model is used where Ihe momentum selection rule is completely relaxed, 1be integraled density of statesN(E) has been used and defined by

••

N(E). J ^g(")dE

^4.5

Chapter4, Chemical,Thermaland Opticalpropertic:gof PPTEOS

The density of states per unit energy interval may be represented by

g(E)= ~L5(R~E.),

where

Vis

the volume,

E

is energy at which gM IS to be evaluated and EnlSthe energy of the nth state.

If g,.⁰⁰ F! and g,(E) ⁰⁰ (E - F;opt)",where energies are measured from the valance band mobility edge in the conduction band (mobility gap), and substituting these values mto an expression for the random phase approximation, the relationship obtamed v~ h (v) 00 (hv-Eof'l+l, where 12(v) is the imaginary part of the complex permittivity. If the density of states of both band edges is parabolic, then the photon energy dependence of the absorption becomes

aowv

²

I,(vy{hv-Eop,f.

So for higher photon energies the simplified general equation is

ahv =B(hv - E"",)" 4.6 where hv is the energy of absorbed light, n is the parameter connected WIth distribulion of the density of states and B is the propoTlJOnaiityfactor The index n equals Vi and 2 for allowed dIrect transition and indirect trlll1Silionenergy gaps respectively [18].

Thus, from the straight-line plots of (ahv)' versus hv and (<Ihv)U2versus hv the direct and indirect energy gaps respecting of insulators and lor dIelectrics can be determined.

4.6.2 Experimental procedure

UV-Vis spectrum of the TEOS monomer liquid and PPTEOS samples on glass shde (Marienfed, Germany) ofarea 18xl& mm2 were obtained ¹⁰absorption mode with a spectrophotometer Shimadzu UV-160A (Shimadzu Corporation, Tokyo, Japan) in the Wa\'e1ength range 200 to ROOnm at room temperature, The optical absorption was measured in the above wavelength region for the PPTEOS films on substrate agamst a blank co,'er glass slide as the reference,

4.6.3 Results llJ1d di£cussion

These Uv-vis spectra of PPTEOS for different thicknesses are presented in Fig. 4.9.

In these spectra, it is observed that the absorption increases with increasing lhlckness.

Chapter 4: Chemical. Thermal and Optical properties ofPPTEOS

700

800

500 600

400

•

16

)

1.2

w

•

06 ]

< OA

o I

200

300 400

500

d, (nm)

300

1.4

1.2

~ 1

+

m

'" •

^u

O'!

• •

~⁰

_•

06

f

~

od

<

f

02 +

J

200

Wavelength, 1..

(nm)

Fig. 4.9. Vanation of absorbance (ABS) with wavelength, 1c, for various PPTEOS thin films; 400 run, spectrum a; 350 nm, spectrum b; 300 run, spectrum c; 250 run, spectrum d at deposition power 40W and deposition time 90 minutes, (Inset: ABS vs d plot) at deposition po"er"'40W and deposition time 90 minutes.

FIg. 4.10 represents the absorption spectrum ofPPTEOS thin film of350 run thickness for funher analysis. It is seen that there is a sharp rise of the absorption in at around 315 Ill1l, above 480 nm absorption decreases slowly. It is seen that the absorption poW;:oflhe PPTEOS is not shifted to the higher wavelength compare to that ofTEOS.

Chaptcr 4: Chemical, ThennaI and Optical properties ofPPTEOS

0.8

"

• ,.,

0.6

• _< ^OA

w

,.,

m

'-

<

_{0 ~'-'-}

" _""

³⁰⁰

'"'

^>0,

c

OA

l!

₀ ^{•• (nm)}

•

0

<

0.2

o

'00 300

400

500 600 700 800

INavelength, '- (nm)

FIg. 410. Variation ofabsomance (ABS) WIth wavelength, A.,for PPTEOS thm film of thickness 350 nm at deposition power40W and deposition time 90 minutes (Inset' ABS vs wavelength of monomer) at deposlIion power '"

40W and deposlIion time 90 minutes,

the low wavelength side and !hen rapidly decreases up to about 480 run wi!h a peak The absorption co--d'ficient <1was calculated from the absorbance data of(FIg.4.1O.) using the eqn 4.4. The spectral dependence of <1as II function ofphotDn energy, hv, for the sample of thickness 350 run is shown in Fig.4,II. It is observed that in the low energy region the edges foHow linear fall for values of <1below about 15,000 cm''. This linear falling edge may either be due to lack oflong-range order or due to the presence of defects in the thin films [191

Opllcal band gap IS related to the electronic structure of II malenal and it is one of the

most sIgnificant parameter. The

a

\'S hv curve of Fig.4.11 can be titled to two

II

straight1ines of different slopes in the lower and higher photon energies. ThIs may

-(l

•

Chapter4, Chemical,ThermalandOpticalpropertiosofPPTEOS 68

indicate the presence of direct and lnl!lrect transitions PPTEOS films. The allowed direct transition energy gap E". can be evaluated from the plots of (0;IIv)2 as a function ofhv shown in Fig 4, 12. The E..Jwas determined from the intercept of the linear pari of the curve extrapolated to 7ero^(l in the photon energy a..us,

50 "',-", .. "-- .- .- .--- ---

o

"

²²Wavel&llllth,^2.2 l. (nm)^4,2

Fig 4.1 L Plot of absO'l'tion co-efficient,

a,

as a function of photon energy, hv, for PPTEOS thin film.

o ''''-';:

^{I '}

" "

Photon enelgY, hv (••V)

, ..

Fig. 4.12. (ahvi versus hv curve for PPTEOS dun film

\\ ..

Chapter 4: Chemical, Thormal and Optical properties ofPPTEOS 69

In Fig. 4.13 (tdw)'"

as

a ftmction ofhv was plotted to obtain the Eqi. The intercept of the e;l,uapolatoo curve to LefO(lin the photon energy axis

was

taken

as

the indirect

'"

,

{. , "

g

~>

<

" _"

o

f~-'.'~'~I

,

1

'.2 1.4 1.6 1.8 2 2,2 2.4

Photon energy, hv (eV)

Fig. 4.13. (ahv)1n versus hv c\ITI;efor PPTEOS lhin film.

1200

,

"

., 1000 o

-

• f

~

"

•

~

24 34 4.4

Photon energy, h••.(eV)

Fig, 4,14, Plot of extinction co-efficienl, K,

as

a function of hv for PPTEOS thin film.

Chapter4, Chemical,ThermalandOpticalpropertiesofPPTEOS

transition energy gap. The \'alue of allowed direct transition energy gap, Eq..,is 3.00 eY and that of allowed indirecttrarnition energy gap, E.,i,is 1.28 eY respeclJvely.

Physical processes that control the behavior of gap Slales in non-aystalline materials are Slructural disorder responsIble for the tail states and structural defects 1ll deep states [20]. The Tauc parameter, B is a measure of the steepness of band tail (Urbach region) denslly of states. Higher value ofB is due to less slructural disorder [21] The value ofB from FIg 4.13 for PPTEOS tltm films is found almost 442 cm"1'>(eVr11'>.

The extinction co-efficlent, K, was computed from

a

using tlte relation

a

⁼ 4~

'where" IS the wavelength. The variallon oftlte K for PPTEOS thin films 'with ltv is shown in Fig 4.14 II is seen from the plot that the increase ofK with the increase m photon energy indicating that the probability of electron transfers across the mobility gap rises with the photon energy,

Thus, it can be attributed from the EA and IR observations that the chemical structure of PPTEOS is little different than tltat of TEOS. It is seen from SEM that the surface of PPTEOS film is smooth, uniform and pinltole free. DTAfTGA imestigatioJlS show that the PPTEOS thm films are stable upto 4461<..From tN-Vis investiglltion Eq.. and Eq,are found to be 3,00 and 1,28 eY respectively, for PPTEOS, The calculated value ofTauc parameter, S, is 442 cm.tll (eVr'll. The dependence of K, on photon energy indiclltes that the probability of electron transfer across the mobility gap rises with the photon energy

References

1. N. Morosoff, "An (ntroduehon to Plasma Polymenzation", in "Plasma Deposition, Treatment, and Etching of Polymers", R. d' Agostino Ed, Academic Press, San DIego, CA, 1990.

2, N. Morosoff, "Surface Modificll1ion by Plasma Polymerization", m

"Innovations in materials processing", Plenum, New York, 1985.

•

Chapter4: Chemical,Thermaland OptiCalpropertiesofPPTEOS 71

3. J Taue., in "Optical Properlles of Solids", F, Abeles Ed., North"Holland, Amsterdam, 1972 chapS

4. H. Yasuda, "Glow Discharge Polymerization in Thin Film Processes", AcademiC,New York, 1978.

5, R. Liepins, K Sakaoku, J. AppL Polym Sci. 16(J972) 2633.

6, L. F, Thompson and G. Smohnsky, J. AppL Po!yrn, Sci. 16 (1972) 1179.

7 li Biederman, Vacuum31 (1981)^2H5,

8, H, Biederman, S. M. Ojhaand L. Holland, Thin Solid Films41(1975) 329.

9, M, R. Havens, K.G. Mayhan and W. J James, J. Appl. Polym Sci.22 (197~) 2799.

10. R- Liepins, M,Campbell, 1. S. Clements, J, Hammond and R. J. Fries, J. Vac SCI.Technol 18 (1981) 1218.

11. N. lnagaki, M, Itami IlIldK Katsuura, Int.J, Adhes. 2 (1982) 169.

12, R.M, Silverstein, G.C. Bassler, T C. Morrill, "Spectrometnc Identification of Orgamc Compounds", John Willey &Sons, New York, 1981.

13, K. Nakamura, M, Watanabe, M. Zhou, M, Fujishima, M Tsuchiya, T. Handa, S, Ishi" fl Noguchi, K. Kashiwagi and Y Yoshida, "Plasma polymerization of oobalt tetraphenylporphyrin and the functiona1ities of the thin films produced", Thin Solid Films 345 (1999) ')9-103.

14. R.T. Conley "Infrared Spectroscopy" Second Ed. AJI)TI and Bacon. Inc., London, 1975,

15. Premamoy Ghosh, "Polymer Science and Technology of Plastic and Rubbers", 4'" Ed., TATA McGrow Hill Pub. Co., New Delhi, 1996.

16. M.e Kim, S.H. Cho, J,G Han, B.Y Hong, Y,J. Kim, S.H. Yang and 1.H.

Boo,'Wgh-rate dep<lsitwn of plasma pol)Tller1zed Ihin films using PECVD method and characterizution of Iheir optical properties", Surf and Coat.

Techno!. 169-170 (2003) 595-599.

17. AM. Bhuiyan and S,V, Bhoraskar, '" Electrical, optical and ESR study of thin films of plasma-polymerized acrylonitrile", J. Mater. Sci, 24 (1989) 3091- 3094,

18. ARM. Shah Jalai, S. Ahmed, Ali Bhuiyan and M. Ibrahim, "UV-Vis absorption spe<:trosoopicstudies ofplasma-pol)merized m-xylene thin films", Thin Solid FlintS 288 (19%) 10M-III,

19. E A. Davis, N. F. Mot!, Philos; Mag, 22(1970)903.

Chapter 4: Chemical, Thermal and Optical properties QfPPTEOS 72

20. D, Dasgupta, F. Demichelis, C. F, J'irri, A. Tagliaferro, Phys, Rev. B 43(1991)2131

21. A Matsuda, T. Yarnoka, S. Wolff, M. Koyama, Y-Ymanishi, H. Kalooka, H, Matsuura, K. Tanaka,J.Appl. Phys 60(1986)4025.

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