Estimation of the dispersion and optical parameters of sprayed CdS thin fi lms chemical spray...

FONDAZIONE GIORGIO RONCHIhttp://ronchi.isti.cnr.it

Estratto da: Atti della Fondazione Giorgio RonchiAnno LXVIII, n. 1 - Gennaio-Febbraio 2013

Tip. L’Arcobaleno s.n.c. - Via Bolognese, 54 - Firenze2013

SAMI SALMAN CHIAD, NADIR FADHIL HABUBI

Estimation of the dispersion and optical parametersof sprayed CdS thin fi lms chemical spray pyrolysis

at different substrate temperatures

ANNO LXVIII GENNAIO-FEBBRAIO 2013 N. 1

A T T I DELLA «FONDAZIONE GIORGIO RONCHI»

EDITORIAL BOARD

Pubblicazione bimestrale - Prof. LAURA RONCHI ABBOZZO Direttore ResponsabileLa responsabilità per il contenuto degli articoli è unicamente degli Autori

Iscriz. nel Reg. stampa del Trib. di Firenze N. 681 - Decreto del Giudice Delegato in data 2-1-1953

Tip. L’Arcobaleno - Via Bolognese, 54 - Firenze - Febbraio 2013

Prof. Roberto BuonannoOsservatorio Astronomico di RomaMonteporzio Catorne, Roma, Italy

Prof. Ercole M. GloriaVia Giunta Pisano 2, Pisa, Italy

Prof. Franco GoriDip. di Fisica, Università Roma IIIRoma, Italy

Prof. Vishal GoyalDepartment of Computer SciencePunjabi University, Patiala, Punjab, India

Prof. Enrique Hita VillaverdeDepartamento de OpticaUniversidad de Granada, Spain

Prof. Irving KaufmanDepartment of Electrical EngineeringArizona State University, TucsonArizona, U.S.A.

Prof. Franco LottiI.F.A.C. del CNR, Via Panciatichi 64Firenze, Italy

Prof. Tommaso MaccacaroDirettore Osservatorio Astronomico di Brera,Via Brera 28, Milano

Prof. Manuel MelgosaDepartamento de OpticaUniversidad de Granada, Spain

Prof. Alberto MeschiariScuola Normale Superiore, Pisa, Italy

Prof. Riccardo PratesiDipartimento di FisicaUniversità di Firenze, Sesto Fiorentino, Italy

Prof. Adolfo PazzagliClinical PsychologyProf. Emerito Università di Firenze

Prof. Edoardo ProverbioIstituto di Astronomia e Fisica SuperioreCagliari, Italy

Prof. Andrea RomoliGalileo Avionica, Campi BisenzioFirenze, Italy

Prof. Ovidio SalvettiI.ST.I. del CNRArea della Ricerca CNR di Pisa, Pisa, Italy.

Prof. Mahipal SinghDeputy Director, CFSL, Sector 36 AChandigarh, India

Prof. Marija StrojnikCentro de Investigaciones en OpticaLeon, Gto Mexico

Prof. Jean-Luc TissotULIS, Veurey Voroize, France

Prof. Paolo VanniProfessore Emerito di Chimica Medicadell’Università di Firenze

Prof. Sergio VillaniLatvia State University, Riga, Lettonia

Estimation of the dispersion and optical parametersof sprayed CdS thin fi lms chemical spray pyrolysis

at different substrate temperatures

SAMI SALMAN CHIAD (*), NADIR FADHIL HABUBI (*)

SUMMARY. – CdS thin fi lms were prepared at different substrate temperatures using spray pyroly-sis technique. The absorbance, real and imaginary parts of dielectric constant and optical conductiv-ity has been measured for different substrate temperatures. The dispersion parameters proposed by Wemple and Di Domenico were also calculated as a function of substrate temperature which shows that the optical energy gap decreases from 2.42 eV to 2.35 eV as the substrate temperature increases. The Eo and Ed show the same trend of the optical energy gap. The value of Urbach tail increases with increasing substrate temperature, which proved that the crystal disorder increases by increasing substrate temperature.

Key words: Thin fi lms, Optical properties, Optical dispersion relation.

1. Introduction Extensive research has been done in the last two decades concerning CdS

thin fi lms, mainly due to wide application in large area of electronic devices such as in solar cells ,due to its wide and direct band gap transition, high index of re-fraction and n-type conductivity (1), photoelectrochemical application due to its proper band-edge positions for reduction/oxidation of water, high optical absorp-tion and high electron affi nity (2), cadmium sulfi de is photosensitive throughout the entire spectrum from infra-red down to ultraviolet, which makes it a potential and attractive semiconductor in the fi eld of optoelectronic like photo conductors, light emitting diodes, thin fi lms transistors (3), besides CdS gained particular at-tention as a window layer in various solar cell, which requires loss reduction for optical absorption of photons with the energies hν < 2.5 eV (4).

(*) Al_ Mustansiriyah University, College of Education, Physics Department, Baghdad, Iraq; e-mails. [email protected], [email protected]

ATTI DELLA “FONDAZIONE GIORGIO RONCHI” ANNO LXVIII, 2013 - N. 1

S.S. Chiad - N.F. Habubi132

The deposition of CdS fi lms has been explored by different techniques name-ly thermal evaporation, chemical bath deposition, chemical surface deposition, pulsed laser deposition, ion implantation, gradient re-crystallization and growth, radio-frequency magnetron sputtering and chemical spray pyrolysis (5-12).

The aim of this work is to produce CdS thin fi lms by means of spray pyrolysis at different substrate temperatures and to investigate their dispersion and some optical parameters dependent on substrate temperature.

2. Experimental details

The precursor solutions were prepared using cadmium chloride CdCl2 and Thiourea, in 1:1 volumetric proportion. The resulting solution (100 cc) was sprayed on a preheated substrate maintained at a different substrate temperature of 300, 350 and 400 °C. Chromel–Alumel thermocouple was used to record the substrate temperature, compressed air was used as a carrier gas. The cleaning procedures of substrates consist of washing in an ultrasonic bath with ethanol for 15 minutes, then two consecutive rinsing processes with deionized water were carried out. Finally, the substrate was dried in oven kept at 60 °C for 5 hours.

The spray rate was 10 ml/min. through the nozzle which ensures a uniform thickness, the average fi lm thickness was measured by weighting method and found to be about 0.2 + 0.05µm.

Transmittance and absorption spectra of these fi lms were recorded using a double beam Shimadzu spectrophotometer in the wavelength range 300-1100 nm.

3. Results and discussion

Figure 1 depicts the absorbance of the fi lm deposition at three substrate temperatures. It was noticed that the absorption edge of the fi lms is shifted to-wards high wavelength (red shifted) as the substrate temperature increases .This is mainly due to the formation of band tails, which causes a strong modifi cation of the joint density of states and consequently of the absorption spectrum (13).

The dielectric constant can be defi ned as:

[1]

The real ε1 and imaginary parts ε2 of dielectric constant are related to n and K0 values. Their values can be calculated using the following formulae (14):

[2]

[3]

ε = ε1 + iε2

ε1 = n2 − K02

ε2 = 2nK0

Estimation of the dispersion and optical parameters … 133

FIG. 3

Imaginary parts of dielectric constant for CdS thin fi lms vs wavelength.

Figures 2 and 3 present the dependence of the dielectric constant of the CdS fi lms on the wavelength. The real and imaginary parts follow the same pattern and the values of real part are higher than those of imaginary part.

FIG. 1

Absorption for CdS thin fi lms versus wavelength.

FIG. 2

Real parts of dielectric constant for CdS thin fi lms vs wavelength.


FIG. 4

Optical conductivity for CdS thin fi lms versus wavelength.

It clearly appears that the real and imaginary parts decrease with increasing substrate temperature. Optical conductivity σop is calculated from the relation (15):

[4]

where α is the absorption coeffi cient and c denotes the velocity of light.Figure 4 shows the variation of the optical conductivity with wavelength for

CdS thin fi lm deposited on different substrate temperatures. The increased of op-tical conductivity at lower wavelength is due to the high absorbance of CdS thin fi lms in that region. The optical conductivity decreases with increasing substrate temperature.

σop =

αnc4π

The dispersion in refractive index has been analyzed using the single oscilla-tor model expressed in the form (16)

[5]

where E0 is the single oscillator energy, which simulates all the electronic excita-tions involved and Ed is the dispersion energy.

In practice, the dispersion parameters E0 and Ed can be obtained by a plot of (n2–1)–1 vs (hν)2 as shown in Fig. 5. It is observed that the plots are approximately linear.

The values of E0 and Ed for different substrate temperature are listed in Table 1. Author Both E0 and Ed are decreasing if the temperature increases. The reason for this phenomenon is probably due to the decrease in refractive index with the decreasing of the degree of crystallinty of the fi lm. The oscillator energy E0 in a close approximation, it scales with the optical band in which E0≈ 2Eg, as suggested by WD model, the refractive index no at zero photon energy, which is

1

n2 −1=

E0

Ed

−1

E0Ed

(hν)2


defi ned by the infi nite wavelength dielectric constant ε∞ ≈ n02, can be deduced

from the dispersion relationship, i.e n2 = 1+E0/E0 (17). These values are listed in table (1). It can be seen that the optical band gap decreases from 2.42 eV to 2.35 eV as the substrate temperature increases from 300°C to 400°C. These results are in good agreement with the values reported previously for CdS thin fi lms (18-20).

FIG. 5

(n2–1)–1 plotted vsas a function of (hν)2.

The dispersion data of refractive index can also be fi tted by the following relation (16):

[6]

where λ is the wavelength of the incident photon, S0 is the average oscillator strength and λ0 values are obtained from a slope S0–1 and the intercept (S0λ0

–1) of the curves plotted in Fig. 6. Their values are listed in Table 1.

A measure of inter-band transition strength can be provided from M–1 and M-3 moments of the optical spectrum. The M–1

and M–3 moments are expressed as (21):

[7]

[8]

Table 1T °C E0 eV Ed eV Eopt eV n ε∞ S0×1013 m–2 λ0 nm M-1 ?? M–3 eV-2 ln α meV

300 4.83 13.80 2.42 1.96 3.85 3.56 264 2. 85 0.122 546

350 4.77 13.60 2.39 1.87 3.49 2.04 404 2.49 0.109 617

400 4.69 10.49 2.35 1.62 2.62 1.10 426 1.62 0.073 709

n2 −1 =S0λ

2

1−λ0

λ

⎛

⎝ ⎜

⎞

⎠ ⎟

2

E0

2 =M−1

M−3

E0

2 =M−1

M−3


These values too for different substrate temperatures are listed in Table 1, since the M–1 and M–3 moments are involved in the computation of E0 and Ed. The calculated values of M–1 is dimensionless whereas the dimensions of M–3 are (eV)–2.

It could be seen that the values of M–1 and M–3 decrease as the substrate tem-

perature increases. The width of the localized states available in the optical band gap of CdS fi lms affects the optical band gap structure and the optical transitions: this is called Urbach tail, and is related directly to a similar exponential tail for the density of state of either one of the two band edge (22). The Urbach tail of the as deposited fi lm can be obtained using the relation (23):

[9]

where E is the photon energy, α0 is constant and Eu is the Urbach energy which refers to the width of the exponential absorption edge

Figure 7 shows the variation of ln(α) versus photon energy , for CdS fi lms at different substrate temperatures.

FIG. 6

(n2–1)–1 as a function of 1/λ2.

FIG. 7

lnα as a fubction of hv.

α = α0 exp

EEu


This behavior corresponds to the optical transitions between occupied states in the valence band tail to unoccupied states at the conduction band edge. The Eu value was calcuted from the inverse slopes of the straight lines of Fig. 7. The obtained Eu values are given in Table 1. The Eu values change inversely to optical band gaps of the fi lms. The decrease in Eg is attributed to the increase of crystal disorder which leads to a redistribution of states from band to tail transitions (24).

4. Conclusions

The increasing of substrate temperature affects crystal disorder leading to a decrease in the optical energy gap and to an increase in the width of the localized energy states.

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ATTI DELLA “FONDAZIONE GIORGIO RONCHI” ANNO LXVIII, 2013 - N. 1

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