13 Revista Latinoamericana de Metalurgia y Materiales, Vo/. 10 Nos. 1 & 2 (1990)

Optical properties of cadmium telluride thin film in the infrared region

A. H. Eid, S. Mahmoud III1dM. M. SayedPhysics Department, National Research Centre, Cairo , A. R. E .The values ofrefractive index, extinction coefficient, dielectric constant, optical conductivity and sorne physical dataof cadmiurn telluridefilms were eslÍmated inthe infraredregion, using transmission technique. The transmission is close to 70%. Theindexof refraction wasdetermined to be - 2.65 and [he extinctioncoefficientis very small. The valueofO.2fortherefl~tivity coefficient "R" was obtained by plotting transrnission againstthickness for various wavelengths and extrapolatingto zero thickness.It is seen thar our values are in a satisfactory agreernent with those given in the literature . The results shows, presence of maximum in E" E" el

and 0, al about 0.06, 0.08 and 0.197 eVoThese peaks may be attributed to transitions from the valence band to an acceptorlevel.

1. INTRODUCTIONStudying theoptical properties ot a semiconductíng materialis one of the most familiar method to gain an adequateknowledgeaboutits band structure. Absorption by boundandby free electrons are both significant in semiconductors. Thefonner gives rise lo the intense absorption to the shortwavelength side of the main absorption edge, while at longwavelengths, free carrier absorption becomes significant. Inan ideal semiconductor at zero temperature, the valence bandwould be completely full, so that an electron could not beexcited to a higher energy state within the bando The onlypossible absorption is that of quanta sufficiently energetic forthe electrons to be excited acrossthe forbidden zero unit lo theempty conduction bando In practice the resulting absorptíonspectrum is a continuum of intense absorption at shortwavelengths, bounded by amoreor less steep ahsorption edgebeyond which the rnaterials are relatively transparent, Theintensity of the absorption due to direct transitions in thesimple case, is detennined primarily by the numbers ofoccupied states in the valence band and unoccupied stares inthe conduction which are energetically within .. hy" of eachother, and on the transition probability [1].The optical properties of Cd'Te in the near and far infraredhave investigated by many workers [2]. Mitsuishi [3] meas-ured the reflectance as a function of wavelength from 400mmto200mmat90K.Hegotapeak:at71.4~.FisherandF[m[4]reponed a reflectance peak: at 68~, and two absorptionbands at 35~ and 4O¡.unfor measurements rnade al roomtemperature.

They suggested the tormer (35¡.un) as the second hannonic ofthe fundamental optical absorption frequency. Some authors[2] determined the dielectric constant of CdTe in the farinfrared from 1000 um to 132 um by means of interferencefringes. The reported value of Ea is 9.7 ± 0.3. The highfrequency dielectric constant of CdTe was determined byMarple [5] as Eoo=7.21. The.index of refraction ínthe ínfraredwas determined to be 2.61[6].Near-IR transmission spectra 01'vacuum evaporated CdTe thinfilms grown at different temperatures and rates were studied,as well as the effect of thennal annealing on the spectra Thecondition for the preparation of films with a high transmissionand a well-defined absorption edge were determined, Theabsorption coefficient and optical band gap of the filrns wereobtained from the transmission spectra, The optical band gapis associated with direct interband transitions [7].

2. EXPERIMENTAL TECHNIQUEThe investigated cadmium telluride layers were obtained byvacuum sublimation onto KBr discs of highly controlledweight and thickness for IR regíon, The KBr discs wereprepared by pressing about 200 mg. KBr in a special dísc undervacuum at about ISOkp/cm pressure by means of a hydraulicpress. Theproduceddíscwas 1.2 cm in diameter andaboutü.l Smm thickness . The film thickness varied Irorn 300 to 900 nmand the sublirnation rate was fixed at 7 nm / s, in vacuum of1Q-4Pa.The film thickness was measured by Tolansky interfer-ence method [8]. The transmissíon spectra were obtained with

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-,.- .' ••••••••• '- .. - •. - (bl/ .. »: .' './' -'-'-- . .- /". • .•.•••••. ( e I

11"'"-' --'--'. / ./ '-" •••.••••.••./ .-

_._. »:" r··-·.,~-"..,"'"<. i ./

"-"-"~'r'''/' ...••..... - •.

...... ',_:

.......

~n~ __ ~ ~ __ ~~I ~ __ ~~I~~ __ ~~ __ ~~~~ __ ~~~~ __ ~'~~--JO 10 20 30 ~o), I r»

Fig.I- Percent transmission VS. wavelength for Cdl'e films of different.thickness (a) d= 460.7 nm (b)d=663.4l1m (C)d=819I1m

LatinsmericanIoumal ofMetallurgy and Materials. Vol. lONos. 1 & 2 (1990)

the help of a4220 Beckmann double beam spectrophoiometerwith an accuracy of 1% used in the infrared region. Theoptical transmission measurements were made over thespectral range from 2.5 to 40 um for infrared region.

3. RESUL TS AND DISCUSSIONThe variation in the transmittance "T' of CdTe films ofdifferent thicknesses onto KBr discs with the wavelength inmicron is shown in figure (1). The transmittance shows apronounced dip at about 6.3, 15 and 22.2 um, moreover, thetransmittance decreases and increases gradually as the filmthickness and wavelength íncreases respectívely. The trans-minance is close 10=70%. Lorimor et al. [9] observed that thetransmittance from 1 to 22.2 urn is essentially constant at =65 % for three different thicknesses of CdTe films. But below25 um only two prevíously reponed transmíssíon mínimawere observed.In calculating the absorption coefficients, corrections weremade for reflections .The retlectivity coefficient "R", used inthe calculations of "K" and "a", was obtained by plottingtransmission against thickness for various wavelengths (2. 6,5, 10, 141lffi) and extrapolating to zero thickness accordingto the formula [10]:

T = (l-R) . e.acl (1)The dat.a is shown in figure (2). The value of 0.2 for "R",calculated from the present work (2.5 -40 um) agrees wellwith the value of 0.2 obtained by Davis et al. [6] and Lorimoretal. [9].A~well,itisconf1l1l1edbythevalueof O.21obtainedby Marple et al [l1J using the reflection technique and thevalue of O.207 published by Daniclewicz et al [2].

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In figure (3) the values of n, k, a for various thicknesses areplotted against photon energy, It ~sclear that the extinction"K" and absorption "a" coefficient decreases gradually withincreases 01' photon energy and thickness, but the refractiveindex "n" increases with the increase of photon energy . Theabsorption peak at about 0.197 eV may be attributed totransitions from the valence band to an acceptor level . Theoptical absorption of p-type CdTe samples at photon ener-gíes less than Eg has been measured by Vul et al. [13] andby Capek et al. [14]. The author's obtained the absorptionpeak at about 0.2 eV, which is not strongly temperaturedependent. 1 t is attributed to transitions from the valenceband to an acceptor level, while the temperature dependentabsorption at lower energies is attributed to interband free-carrier transitions from the light-hole valence to the heavy-hole valence bando Also i t is found that the values of theextinction coefficient "K" are small and ranges between 0.07to 0.9 and the value of the refractive index is about 2.65. 1t is seen that our values are in a satísfactory agreement with(hose given in the literature [9, 12 ,15]. The index ofrefraction in the infrared was determined inter ferometrícallyto be 2.61[6].De Nobel and Hofman [16] repon a low- frequency dielectricconstant of 11.0 ± 0.3 on CdTe which corresponds to arefractive index of about 3.32. A comparison of this value withthe optical refractive index (2.65) determined in the presentwork suggests that bonding in CdTe is somewhat ionic. Thisconclusion, as well as the value of the optical refractive index,is at variance with the observations of Garlick et al. [17] but issupported by earlier estimates of de Nobel [18] and by recent

102-

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O 200 1000 600thlc kne s s Inm

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Fig.2- Percent transmission VS. thickness.

-r-íe x 1<1u., 0.5Io..,tj

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15

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Revista Latinoamericana de Metalurgia y Materiales, Vol. 10 Nos. 1 & 2 (l990)

1.1

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Fig. 3- Dependence of the extinction coefficient (K), refractive index (11) and the absorption coefficient (a) on the photon energy(a)d=460.1Il1n (b)d= 663.4/111I (c)d= 8191l11l (dt d» 460.71l11l

~ ~ __~ ~~~ ~~ ~~~--'-~~~o0.0 0.1 0.2 O.l 0.4 O.S~....I '!V

Fig.4- Dependence of the dieleetric constant. .•and optical conductivity 011 photon-energy d = 460.7 nm.

Latin/vnerican Iovmal of Metallurgy and Materials, Vol. 10 Nos. 1 & 2 (1990)

detailed measurements ofFisher and Fan [4].In ionie crystals there is a strong absorption band at longwavelengths which arises from the oscillation of the atoms orions, Although the inter atomíc torees in ionie crystals arecomparable with eleetronic forces in atoms, the faet that theionie masses are some 10" times greater than electronicmasses means that the absorption bands lie at very longwavelengths .

The detaíled variation of real, El and imaginary "E2" parts ofthe dielectrie and optical conductivity "0'1" with photonenergy, for the 460.7 nm sample are shown in Figure (4) .The

value of "E2" decrease gradually as the photon energy in-

creases moreover, the values of "El" and "al" rise graduallyas the photon energy increases .Figure( 4) shows the presenceofmaximum in~\, E

2and o.arabout O.Oó,0.08and 0.197 eY.

The impurity states in semiconductor always exist and thereis no way to avoid their role on the optical and eleetriealpropertíes of any semiconductor. The transítíon of an electron .either trom tne matrix to the impurity acceptor or from theimpurity "donor" to the matrix is directly related to theionization of the irnpurity atom. The hydrogen type modelyield an expression of the form [19]:

E =.l.l..52... Z . (m*/m) eV (2)t

where 13.52 eV is the energy of ionization 01' a free hydrogenatom. To differentiate between impurities, their ionizationenergies have to replaee that of the hydrogen free atom. It isclear from equation (2), that the ionization energy of theempded impurity atom is 1/etimes less than the value of thefree atom.Since CdTe is partially ionie (ionicity of 0.717) [20], thepolarization of the ions under the applied electric fieldeomponentoftheelectro-magnetic wave will reduce the valueof the static dielectric constant, Accordingly, an effective

value " Ee" should replace the static "E" and the optical one"Eo" ano is given by:

..L= ..L + ~. (l/Eo - l/E) (3)

ea e 16The ionization energy of the empeded impurity atom ís thengiven by:

E = E (impurity free atom ), Z . (m*/m) eV (4)te

introducing theknown values ofE (irnpuríty), ee, m*/m anothe value of Z, E may turn out to be sorne hundredths orthousandths fractions of an electron volt. These fractions stillhave to be reduced less due to the reduction of the impurityionization energy by increasing the concentration of thealready ionized impurities.Accordingly the impurities which form theír own shallowlevels either just-below the bottom of the conduction band orjust above the top of the valance band, with a slow rise internperature will be the main source of charge free carríers,This imply that the optical and electrical properties of theCd'Ie filrns is controlled mainly by the concentration of the

16

already exísted.ímpuritíes during thermal sublimation of thefilm under vacuum.

ACKNOWLEDGEMENTThe authors like to acknowledge Prof. Dr. M. Z. Zidan for biskind coopemtion and interest.

REFERENCES1.T. S. Moss, "Optical properties of serniconductors" London Butterworths,(1961). .2. O. M. Stafsudd, F. A. Haak, and K. Radisavlzevic, J.Opt. Soc, Amer.,57(1967) 1475. .3, A. Mitsuishi, Phys. Soco .Tapan, 16(1%1) 533;4. P. Fisher and H. y .Fan, Bull. Am.Phys, Soco 4(1959) 409.5. D. T. F. Marple, 1.AppL Phys. 35 (1964) 539.6. P. W. Davis and T. S. Shilliday, Phys. Rev. 118 (1960) 1020.7. J. Aranda, J. L.Morenza, J. EsteveandJ.M. Codina, 1llÍnSolid Film s, 120(1984) 23.8. S "Tolansky, "Introduction to Interferometry", Longmans Green and Co.London 2nd Edition (1973) 157.9. O. G. Lorimor and W.G. Spitzer, J.AppL Phys. 36(1965) 1841.10. N. V. Agrinskaya, E .N .Arkad'eva and O .A. Matveev, SOY. Phys.Semiconductors (USA), 4 (1970) 370.11. D. T. F. Marple and B. Segall, Bull, Amer. Phys, Soco 9 (1964) 223.12. E. J. Danielwicz and P. D. Colernan, AppL Opt., 13 (1974) 1164.13. B. M. Vul, V. M. Salman and V.A. Chapnin, SOY.Phys. Semi conductor, 4(1970) 52.14. V. Capek, K. Zimmerman, C. Konak., M. Popova and P. Polivka, Phys.State. Sol. (b) 56 (1973) 729,15.1. E.Kiefer, T. A.Nussmeier and F. Goodwin, IEEE, 1.Quantum Electronique8 (1972) 173.16. D. de MobeJ and D. Hofrnan, Physica, 22 (1956)252.17.G.F. J. Garlick,.T.M. HoughandR.A.Fatchally, Proc.Phys, Soc., 72 (1958)92.'\.18. D. de MobeJ, Philips Research Reps. 14 (1959) 320.19. P. S. Kireev, Semiconductor physics 1975.20. A, J. Strauss, Revue De physique Appliquee, 12 (1977) 167.