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Effect of annealing on the optical properties of thermally evaporated ZnO films
Vacuum 61 (2001) 1}7
E!ect of annealing on the optical properties of thermally evaporated ZnO "lms S.A. Aly*, N.Z. El Sayed, M.A. Kaid Physics Department, Faculty of Science, Minia University, Minia, Egypt
Abstract Two zinc oxide samples with di!erent "lm thicknesses have been thermally evaporated on unheated glass substrate using ZnO high-purity powder. The "lms were subjected to post-deposition annealing in air at di!erent temperatures. X-ray di!raction con"rmed that the "lms in the as-deposited as well as after annealing are in the amorphous state. The optical properties were studied by transmittance and re#ectance measurements. The integrated visible (¹ ), and solar, ¹ , transmittance were calculated. The dependence of absorption coe$cient and the refractive index on wavelength before and after post-deposition heat treating was also reported. 2001 Elsevier Science Ltd. All rights reserved.
1. Introduction The best solar selective absorbers are possible with the combination of a metal re#ector and a semiconductor having bandgap in the range 0.5}1.2 eV [1]. Unfortunately, any semiconductor with a band gap in this range has a high refractive index which results in a high re#ectance of the incidence radiation and, thus, have poor solarabsorbing e$ciency [2]. The simplest approach to the reduction of re#ection loss is to deposit a thin antire#ection coating onto the selective surface [2]. One major need is to develop a coating of protective and antire#ective functions [3]. Re#ection losses are caused by optical interference from the boundary formed between media with di!erent optical properties. Antire#ection coating must has
* Corresponding author.
a refractive index approximately midway between that of air (n+1) and the solar selective absorber (n+2.3). A signi"cant increase of the solar absorptance was achieved when the solar absorber is coated with antire#ection coating [4]. The use of antire#ection coatings in case of heat mirrors is also very e!ective. Metal "lms with quite small thicknesses exhibit partial visible and solar transparency. Dielectric overlayers serve to protect and to partially antire#ect in the visible region, thereby, increasing transmission. Technologically, zinc oxide is a very important material. Its optical, electrical and acoustical properties have been exploited in a variety of applications. ZnO is a promising material for many di!erent applications such as solar cells, gas sensors, ultrasonic oscillation and transducers [5}10]. It is also used in optical waveguides and laser de#ectors [11], light modulators and optical sensors [12] and transparent conductive coatings
0042-207X/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 0 ) 0 0 4 1 5 - 2
2
S.A. Aly et al. / Vacuum 61 (2001) 1}7
[13]. It has a band gap of 3.3 eV which allows transmittance of almost all incident solar radiation [5]. The material can be made n-type conducting, hence, infrared re#ective, highly transparent in the visible region and also can be made into a material with low electrical resistivity by heavily doping with a trivalent impurity [14}16]. Undoped, nonstoichiometric and electrically conducting ZnO "lms have also been reported [17]. Thin "lms of ZnO were prepared using sputtering [16,18], vacuum evaporation [19], spray pyrolysis [5,20}22], sol}gel [23] and CVD [24,25] methods. This work aims to study the e!ect of postdeposition heat treatment on the optical properties of the prepared "lms and to correlate the dependence of the integrated visible and solar transmittance relating to the annealing temperatures (¹ ) to get the optimum properties of zinc oxide to be used as antire#ection coating.
2. Experimental work 2.1. Sample preparation ZnO samples were prepared on unheated substrates by thermal evaporation using a high-purity ZnO powder and a coating unit (type E306A Edwards Co.). The substrates were glass plates with rectangular shape, which were washed with alcohol, dried and then rubbed gently with cotton. The source to substrate distance was 16 cm apart. The heating "lament was a conventional molybdenum boat. The pressure was brought down until a vacuum of about 4;10\ Pa was achieved. The samples were subjected to a subsequent heat-treatment performed in air at di!erent annealing temperatures (¹ ), for a period of 1 h, then left to cool down to the room temperature before carrying out the required physical measurements. 2.2. Sample characterization The "lm thickness was optically measured by multiple beam Fizeau fringes method at re#ection using monochromatic light (Hg, "546 nm). Structure characteristics of the prepared "lms were examined by X-ray di!raction (XRD) using
JEOL-di!ractometr type PX160 with slow scanning speed (23/min.) and Ni-"ltered CuK radiation. ? X-ray di!ractograms show that the as-deposited as well as the annealed ZnO samples are in the amorphous state. 2.3. Optical measurements The transmittance and re#ectance measurements were carried out at normal incidence in the spectral range from 300 to 2400 nm using Shimadzu UV-3101PC:UV}VIS}NIR double-beam spectrophotometer, with a specular re#ection attachment of V}N type. The integrated visible, ¹ , and solar (¹ ), transmittance of the prepared samples are derived by integrating, in the corresponding wavelength range the measured transmittance (¹()), weighted by the solar spectrum G() at air mass2 (AM2) [26], thus, H ¹()G() d , (1) ¹ " H V H G() d H where x denotes visible or solar while and are the integration limits 380}780 and 300}2400 nm, respectively. The absorption coe$cient () was calculated from the following relation [27]:
1 (1!R() " ln , t ¹()
(2)
where R() and ¹() are, respectively, the spectral re#ectance and transmittance at wavelength and t is the "lm thickness.
3. Results and discussion 3.1. Transmittance and reyectance measurements The dependence of the measured transmittance and re#ectance curves on wavelength before (as-deposited state) and after annealing at di!erent temperatures (¹ ), for ZnO "lms is shown in Figs. 1 and 2 for thicknesses of 30 and 71 nm, respectively. A strong temperature-dependent transmittance can be easily observed over the used wavelength range. A poor transmittance in the
S.A. Aly et al. / Vacuum 61 (2001) 1}7
Fig. 1. The dependence of spectral transmittance and re#ectance for ZnO sample with a "lm thicknesses of 30 nm.
3
used to overcoat a metal, must exhibit high infrared transmittance in order to preserve the infrared re#ectance of the metal [28]. In contrast with transmittance, a small decrease in the re#ectance intensity from &30 to 10% with increasing ¹ for the sample of 71 nm, while the sample of 30 nm thickness possess a very low re#ectance values )10%. As stated above, XRD analysis showed no evidence of conversion from an amorphous to crystalline structure with annealing. So, one can attribute the increase of transmittance to the result of oxygen reaction with ZnO. The absorbed oxygen annihilates the oxygen vacancies, thus, reducing the density of these donor-like defects and carrier density [29]. Also, oxygen chemisorbs readily as an acceptor on ZnO surfaces [29}31] and pores in the "lms as O\ by accepting an electron from the occupied conduction band state [23]. Heating of the surface leads to the desorption of oxygen with the increase in annealing temperature [23]. 3.2. Integrated transmittance The annealing temperature dependence of ¹ and ¹ is illustrated in Fig. 3. The dependence indicated qualitatively the same behaviour for both thin (30 nm) and thick (71 nm) "lms but there are some di!erences in the measured values. A rapid
Fig. 2. The dependence of (a) spectral transmittance and (b) re#ectance for ZnO sample with a "lm thicknesses of 71 nm.
as-deposited state (before annealing) and an increase in value is achieved with increasing ¹ . It can be, also, noted that the transmittance in the NIR region, after annealing, is quite high. Films
Fig. 3. Integrated visible and solar transmittance of ZnO samples with "lm thicknesses of 30 and 71 nm as a function of the annealing temperature, ¹ .
4
S.A. Aly et al. / Vacuum 61 (2001) 1}7
increase to about 0.8 of ¹ and ¹ was observed up to 1503C for thinner "lm and in the temperature range of 200}3003C for thicker one, then no considerable change. However, in general, thinner "lms showed higher values. 3.3. Absorption coezcient and energy gap The absorption coe$cient () has been calculated using Eq. (2) and its dependence on wavelength at di!erent values of ¹ has been investigated for the two samples as shown in Fig. 4. An abrupt decrease up to 380 nm and then no appreciable change was observed as the wavelength increases. The dependence of the absorption coe$cient (at "500 nm) on ¹ for the sample of 71 nm thick ness is shown in Fig. 5. It is clear that possesses a high value in the as-deposited state, then decreases with increasing ¹ . The strong absorption and dispersion in the wavelength range indicates
Fig. 5. The dependence of absorption coe$cient () on the annealing temperature, ¹ , for ZnO sample with a "lm thick ness of 71 nm.
that the energy gap is located in that region (380 nm indicating that the energy gap is located in that region. The optical energy gap (E ) of the "lms was estimated from the optical measurements. The absorption coe$cient was found to follow the relation A(h!E ) , " h
(3)
where A is a constant. The plot of (h) versus the photon energy in the absorption region for the sample of 71 nm thickness shown in Fig. 6 indicates direct allowed transition before and after annealing at two di!erent temperatures. The energy gap can be determined from the extrapolation of the linear portion with the photon energy axis and its dependence on ¹ is illustrated in Fig. 7. It is noticed that E increases with increasing ¹ and its value at ¹ "4003C is around 3.63 eV in ac cordance with the E value of ZnO obtained by di!erent authors [5,23,32,33]. The variation in band gap energy may be related with the degree of non-stoichiometry in ZnO and the increase in its value could be attributed to the absorption of oxygen throughout the sample which produces a more nearly stoichiometric ZnO "lms. 3.4. Refractive index (n) Fig. 4. The dependence of absorption coe$cient () on wavelength for two ZnO samples with "lm thicknesses of: (a) 30 nm and (b) 71 nm.
The refractive index was determined using a technique developed by El-Kadry et al. [34]. Such
S.A. Aly et al. / Vacuum 61 (2001) 1}7
5
Fig. 6. (h) as a function of the photon energy (h) for ZnO sample of "lm thickness"71 nm in the as-deposited state and at di!erent ¹ .
Fig. 8. The dependence of refractive index (n) in the as-deposited state and at di!erent ¹ on wavelength for two ZnO samples with "lm thicknesses of: (a) 30 nm and (b) 71 nm.
Fig. 7. The dependence of E on the annealing temperature, ¹ , for ZnO sample with a "lm thickness of 71 nm.
technique involves bivarient search based on minimizing (R) and (¹) simultaneously, where R"R
!R , ¹"¹ !¹ .
(4)
The subscripts exp and calc refer to the experimental and calculated results, respectively, and both the transmittance T and re#ectance R are given by Murmansnn's exact equations [35,36]. The refractive indices spectrum of the two ZnO samples in the visible region is shown in Fig. 8. For the sample of 71 nm thickness, the refractive index, n, experienced a noticeable decrease up to +450 nm wavelength, followed by a slight decrease
Fig. 9. The dependence of refractive index (n) at wavelength of 500 nm on the annealing temperature, ¹ , for ZnO sample with a "lm thickness of 71 nm.
toward the end of the visible spectrum. The dependent of n (at "500 nm) on ¹ is depicted in Fig. 9. It is observed that the value of n decreases as ¹ increases up to 2503C, then remains almost constant. The values of n are 2.15, 1.91, 1.86 and
6
S.A. Aly et al. / Vacuum 61 (2001) 1}7
1.86 at ¹ "200, 250, 300 and 4503C, respective ly. However, in case of the sample of 30 nm thickness, no appreciable change of n with wavelength or annealing temperature is noticed. This behaviour can also be attributed to the stoichiometric stability at a certain ¹ which depends on the "lm thick ness. In general, one can observe that the average value of n after annealing is less than that of the bulk material (n"2).
4. Conclusion An increase in the spectral transmittance value of ZnO thin "lms (thickness"30}71 nm) is achieved by increasing the annealing temperature. The "lms showed no evidence of conversion from an amorphous to crystalline structure even at annealing temperatures up to 5003C. The visible as well as the solar transmittance were found to have reached a stable state at a certain annealing temperature. The optical energy gap was found to be a!ected by the annealing temperature and has been located around 3.65 eV at ¹ of 4003C. Films showed high values of the absorption coe$cient for (380 nm and abrupt decrease for wavelengths *380 nm with no appreciable changes as the wavelength increases. The absorption coe$cient possess a quite high value in the as-deposited state, then decreases with an increase in the annealing temperature for both the two ZnO samples of different "lm thickness. A rapid decrease of the refractive index n, was found with increasing ¹ up to a certain temper ature limit, which varies according to the "lm thickness, and then remains almost constant. Its average value at high ¹ was found to be smaller than that of the bulk material (n"2).
Acknowledgements The authors would like to thank and express their deepest appreciation to Prof. Dr. K. Abdel-Hady, Physics Department, Faculty of Science, Minia University and Prof. Dr. A. A. Ramadan, Physics Department, Faculty of Science, Helwan Univer-
sity, for their helpful discussions relevant to this work.
References [1] Reddy GB, Panadya DK, Chopra KL. Sol Energy Mater 1987;15:153. [2] Mattox DM, Kominiak GJ. J Vac Sci Technol 1975;12(1):182. [3] Lampert CM. Energy Res 1983;7:359. [4] Karl WB. Advances in solar energy, vol. 3. New York: Plenum Press, 1986. p. 9. [5] Krunks M, Mellikov E. Thin Solid Films 1995;270:33. [6] Belight K, Subhan MA, RuK lhe U, Duchemin S, Bougnot J. Proceedings of the 10th EC Photovoltaic Solar Energy Conference, Lisbon, Portugal, 1991. p. 613. [7] Pacs HR Jr., Pinho LMOC, Losch W. Proceedings of the 9th EC Photovoltaic Solar Energy Conference, Freiburg, Germany, 1991. p. 63. [8] Guillemoles JF, Lincot D, Cowache P, Vedel J. Proceedings of the 10th EC Photovoltaic Solar Energy Conference, Lisbon, Portugal, 1991. p. 609. [9] Olvera ML, Maldonado A, Asomoza R, Konagaiand M, Asomoza M. Thin Solid Films 1993;229:196. [10] Mazon C, Muci J, Sa-neto A, Ortiz-Conde A, Garcia FJ. Proceedings of the 22nd IEEE PV Spectroscopy Conference, 1991. p. 1156. [11] Faster NF, Pozgonyi GA. Appl Lett 1966;8:221. [12] Hickernell FS. IEEE Trans Sonics Ultrasonics SU 1985;32:621. [13] Ruth M, Tuttle J, Goral J, Nou" R. J Cryst Growth 1986;79:993. [14] Takai O, Futsuhara M, Shimizu G, Lungu CP, Nozue J. Thin Solid Films 1998;318:117. [15] Emilar K, Schieck R, Fiechter S. Appl Surf Sci 1993; 70:707. [16] Mahmood FS, Gould RD, Salih MH. Thin Solid Films 1995;270:376. [17] Nanto H, Minami T, Shooji S, Takata S. J Appl Phys 1986;55:1029. [18] Qiu CX, Shih I. Sol Energy Mater 1986;13:75. [19] Ma J, Ji F, Ma H, Li S. Thin Solid Films 1996;279:215. [20] Bahadur L, Hamadani M, Koenig JF, Chartier P. Sol Energy Mater 1986;14:107. [21] Aktaruzzaman AF, Sharma GL, Malhotra LK. Thin Solid Films 1991;198:67. [22] Lee C, Lim K, Song J. Sol Energy Mater Sol Cells 1996;43:37. [23] Tang W, Cameron DC. Thin Solid Films 1994;238:83. [24] Lau CK, Tiku SK, Lakin KM. J. Electrochem Soc 1980;127:1843. [25] Roth AP, Williams DF. J Appl Phys 1981;52:6685. [26] Ribbing CG. Introduction to solar selective coating. International School on Material Science and Solar Energy, Cairo and Alexandria, March 1983.
S.A. Aly et al. / Vacuum 61 (2001) 1}7 [27] Demichelis F, Kaniadakis G, Tagliferro A, Tresso E. J Appl Opt 1987;26:1737. [28] Lampert CM, Granqvist CG, (Eds.). Large area chromogenics: materials and devices for transmittance control. Bellingham: SPIE, 1990. [29] Zhang DH, Brodie DE. Thin Solid Films 1994;238:95. [30] Hirschwald WH. Acc Chem Res 1985;18:228. [31] Many A. CRC Critical Review of Solid State Science, May 1974, p. 515.
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[32] Ma J, Li S. Thin Solid Films 1994;237:16. [33] Bose S, Ray S, Barua AK. G Phys J Appl Phys 1996;29:1873. [34] Soliman SH, ElKadry N, Jamjoum O, ElNahass MM, Darwish HB. J Opt 1988;17:46. [35] Heavens OS. Optical properties of thin "lms, vol. 46. New York: Dover, 1965. [36] Heavens OS. Thin "lm physics. London: Methuen, 1970.
E!ect of annealing on the optical properties of thermally evaporated ZnO "lms S.A. Aly*, N.Z. El Sayed, M.A. Kaid Physics Department, Faculty of Science, Minia University, Minia, Egypt
Abstract Two zinc oxide samples with di!erent "lm thicknesses have been thermally evaporated on unheated glass substrate using ZnO high-purity powder. The "lms were subjected to post-deposition annealing in air at di!erent temperatures. X-ray di!raction con"rmed that the "lms in the as-deposited as well as after annealing are in the amorphous state. The optical properties were studied by transmittance and re#ectance measurements. The integrated visible (¹ ), and solar, ¹ , transmittance were calculated. The dependence of absorption coe$cient and the refractive index on wavelength before and after post-deposition heat treating was also reported. 2001 Elsevier Science Ltd. All rights reserved.
1. Introduction The best solar selective absorbers are possible with the combination of a metal re#ector and a semiconductor having bandgap in the range 0.5}1.2 eV [1]. Unfortunately, any semiconductor with a band gap in this range has a high refractive index which results in a high re#ectance of the incidence radiation and, thus, have poor solarabsorbing e$ciency [2]. The simplest approach to the reduction of re#ection loss is to deposit a thin antire#ection coating onto the selective surface [2]. One major need is to develop a coating of protective and antire#ective functions [3]. Re#ection losses are caused by optical interference from the boundary formed between media with di!erent optical properties. Antire#ection coating must has
* Corresponding author.
a refractive index approximately midway between that of air (n+1) and the solar selective absorber (n+2.3). A signi"cant increase of the solar absorptance was achieved when the solar absorber is coated with antire#ection coating [4]. The use of antire#ection coatings in case of heat mirrors is also very e!ective. Metal "lms with quite small thicknesses exhibit partial visible and solar transparency. Dielectric overlayers serve to protect and to partially antire#ect in the visible region, thereby, increasing transmission. Technologically, zinc oxide is a very important material. Its optical, electrical and acoustical properties have been exploited in a variety of applications. ZnO is a promising material for many di!erent applications such as solar cells, gas sensors, ultrasonic oscillation and transducers [5}10]. It is also used in optical waveguides and laser de#ectors [11], light modulators and optical sensors [12] and transparent conductive coatings
0042-207X/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 0 ) 0 0 4 1 5 - 2
2
S.A. Aly et al. / Vacuum 61 (2001) 1}7
[13]. It has a band gap of 3.3 eV which allows transmittance of almost all incident solar radiation [5]. The material can be made n-type conducting, hence, infrared re#ective, highly transparent in the visible region and also can be made into a material with low electrical resistivity by heavily doping with a trivalent impurity [14}16]. Undoped, nonstoichiometric and electrically conducting ZnO "lms have also been reported [17]. Thin "lms of ZnO were prepared using sputtering [16,18], vacuum evaporation [19], spray pyrolysis [5,20}22], sol}gel [23] and CVD [24,25] methods. This work aims to study the e!ect of postdeposition heat treatment on the optical properties of the prepared "lms and to correlate the dependence of the integrated visible and solar transmittance relating to the annealing temperatures (¹ ) to get the optimum properties of zinc oxide to be used as antire#ection coating.
2. Experimental work 2.1. Sample preparation ZnO samples were prepared on unheated substrates by thermal evaporation using a high-purity ZnO powder and a coating unit (type E306A Edwards Co.). The substrates were glass plates with rectangular shape, which were washed with alcohol, dried and then rubbed gently with cotton. The source to substrate distance was 16 cm apart. The heating "lament was a conventional molybdenum boat. The pressure was brought down until a vacuum of about 4;10\ Pa was achieved. The samples were subjected to a subsequent heat-treatment performed in air at di!erent annealing temperatures (¹ ), for a period of 1 h, then left to cool down to the room temperature before carrying out the required physical measurements. 2.2. Sample characterization The "lm thickness was optically measured by multiple beam Fizeau fringes method at re#ection using monochromatic light (Hg, "546 nm). Structure characteristics of the prepared "lms were examined by X-ray di!raction (XRD) using
JEOL-di!ractometr type PX160 with slow scanning speed (23/min.) and Ni-"ltered CuK radiation. ? X-ray di!ractograms show that the as-deposited as well as the annealed ZnO samples are in the amorphous state. 2.3. Optical measurements The transmittance and re#ectance measurements were carried out at normal incidence in the spectral range from 300 to 2400 nm using Shimadzu UV-3101PC:UV}VIS}NIR double-beam spectrophotometer, with a specular re#ection attachment of V}N type. The integrated visible, ¹ , and solar (¹ ), transmittance of the prepared samples are derived by integrating, in the corresponding wavelength range the measured transmittance (¹()), weighted by the solar spectrum G() at air mass2 (AM2) [26], thus, H ¹()G() d , (1) ¹ " H V H G() d H where x denotes visible or solar while and are the integration limits 380}780 and 300}2400 nm, respectively. The absorption coe$cient () was calculated from the following relation [27]:
1 (1!R() " ln , t ¹()
(2)
where R() and ¹() are, respectively, the spectral re#ectance and transmittance at wavelength and t is the "lm thickness.
3. Results and discussion 3.1. Transmittance and reyectance measurements The dependence of the measured transmittance and re#ectance curves on wavelength before (as-deposited state) and after annealing at di!erent temperatures (¹ ), for ZnO "lms is shown in Figs. 1 and 2 for thicknesses of 30 and 71 nm, respectively. A strong temperature-dependent transmittance can be easily observed over the used wavelength range. A poor transmittance in the
S.A. Aly et al. / Vacuum 61 (2001) 1}7
Fig. 1. The dependence of spectral transmittance and re#ectance for ZnO sample with a "lm thicknesses of 30 nm.
3
used to overcoat a metal, must exhibit high infrared transmittance in order to preserve the infrared re#ectance of the metal [28]. In contrast with transmittance, a small decrease in the re#ectance intensity from &30 to 10% with increasing ¹ for the sample of 71 nm, while the sample of 30 nm thickness possess a very low re#ectance values )10%. As stated above, XRD analysis showed no evidence of conversion from an amorphous to crystalline structure with annealing. So, one can attribute the increase of transmittance to the result of oxygen reaction with ZnO. The absorbed oxygen annihilates the oxygen vacancies, thus, reducing the density of these donor-like defects and carrier density [29]. Also, oxygen chemisorbs readily as an acceptor on ZnO surfaces [29}31] and pores in the "lms as O\ by accepting an electron from the occupied conduction band state [23]. Heating of the surface leads to the desorption of oxygen with the increase in annealing temperature [23]. 3.2. Integrated transmittance The annealing temperature dependence of ¹ and ¹ is illustrated in Fig. 3. The dependence indicated qualitatively the same behaviour for both thin (30 nm) and thick (71 nm) "lms but there are some di!erences in the measured values. A rapid
Fig. 2. The dependence of (a) spectral transmittance and (b) re#ectance for ZnO sample with a "lm thicknesses of 71 nm.
as-deposited state (before annealing) and an increase in value is achieved with increasing ¹ . It can be, also, noted that the transmittance in the NIR region, after annealing, is quite high. Films
Fig. 3. Integrated visible and solar transmittance of ZnO samples with "lm thicknesses of 30 and 71 nm as a function of the annealing temperature, ¹ .
4
S.A. Aly et al. / Vacuum 61 (2001) 1}7
increase to about 0.8 of ¹ and ¹ was observed up to 1503C for thinner "lm and in the temperature range of 200}3003C for thicker one, then no considerable change. However, in general, thinner "lms showed higher values. 3.3. Absorption coezcient and energy gap The absorption coe$cient () has been calculated using Eq. (2) and its dependence on wavelength at di!erent values of ¹ has been investigated for the two samples as shown in Fig. 4. An abrupt decrease up to 380 nm and then no appreciable change was observed as the wavelength increases. The dependence of the absorption coe$cient (at "500 nm) on ¹ for the sample of 71 nm thick ness is shown in Fig. 5. It is clear that possesses a high value in the as-deposited state, then decreases with increasing ¹ . The strong absorption and dispersion in the wavelength range indicates
Fig. 5. The dependence of absorption coe$cient () on the annealing temperature, ¹ , for ZnO sample with a "lm thick ness of 71 nm.
that the energy gap is located in that region (380 nm indicating that the energy gap is located in that region. The optical energy gap (E ) of the "lms was estimated from the optical measurements. The absorption coe$cient was found to follow the relation A(h!E ) , " h
(3)
where A is a constant. The plot of (h) versus the photon energy in the absorption region for the sample of 71 nm thickness shown in Fig. 6 indicates direct allowed transition before and after annealing at two di!erent temperatures. The energy gap can be determined from the extrapolation of the linear portion with the photon energy axis and its dependence on ¹ is illustrated in Fig. 7. It is noticed that E increases with increasing ¹ and its value at ¹ "4003C is around 3.63 eV in ac cordance with the E value of ZnO obtained by di!erent authors [5,23,32,33]. The variation in band gap energy may be related with the degree of non-stoichiometry in ZnO and the increase in its value could be attributed to the absorption of oxygen throughout the sample which produces a more nearly stoichiometric ZnO "lms. 3.4. Refractive index (n) Fig. 4. The dependence of absorption coe$cient () on wavelength for two ZnO samples with "lm thicknesses of: (a) 30 nm and (b) 71 nm.
The refractive index was determined using a technique developed by El-Kadry et al. [34]. Such
S.A. Aly et al. / Vacuum 61 (2001) 1}7
5
Fig. 6. (h) as a function of the photon energy (h) for ZnO sample of "lm thickness"71 nm in the as-deposited state and at di!erent ¹ .
Fig. 8. The dependence of refractive index (n) in the as-deposited state and at di!erent ¹ on wavelength for two ZnO samples with "lm thicknesses of: (a) 30 nm and (b) 71 nm.
Fig. 7. The dependence of E on the annealing temperature, ¹ , for ZnO sample with a "lm thickness of 71 nm.
technique involves bivarient search based on minimizing (R) and (¹) simultaneously, where R"R
!R , ¹"¹ !¹ .
(4)
The subscripts exp and calc refer to the experimental and calculated results, respectively, and both the transmittance T and re#ectance R are given by Murmansnn's exact equations [35,36]. The refractive indices spectrum of the two ZnO samples in the visible region is shown in Fig. 8. For the sample of 71 nm thickness, the refractive index, n, experienced a noticeable decrease up to +450 nm wavelength, followed by a slight decrease
Fig. 9. The dependence of refractive index (n) at wavelength of 500 nm on the annealing temperature, ¹ , for ZnO sample with a "lm thickness of 71 nm.
toward the end of the visible spectrum. The dependent of n (at "500 nm) on ¹ is depicted in Fig. 9. It is observed that the value of n decreases as ¹ increases up to 2503C, then remains almost constant. The values of n are 2.15, 1.91, 1.86 and
6
S.A. Aly et al. / Vacuum 61 (2001) 1}7
1.86 at ¹ "200, 250, 300 and 4503C, respective ly. However, in case of the sample of 30 nm thickness, no appreciable change of n with wavelength or annealing temperature is noticed. This behaviour can also be attributed to the stoichiometric stability at a certain ¹ which depends on the "lm thick ness. In general, one can observe that the average value of n after annealing is less than that of the bulk material (n"2).
4. Conclusion An increase in the spectral transmittance value of ZnO thin "lms (thickness"30}71 nm) is achieved by increasing the annealing temperature. The "lms showed no evidence of conversion from an amorphous to crystalline structure even at annealing temperatures up to 5003C. The visible as well as the solar transmittance were found to have reached a stable state at a certain annealing temperature. The optical energy gap was found to be a!ected by the annealing temperature and has been located around 3.65 eV at ¹ of 4003C. Films showed high values of the absorption coe$cient for (380 nm and abrupt decrease for wavelengths *380 nm with no appreciable changes as the wavelength increases. The absorption coe$cient possess a quite high value in the as-deposited state, then decreases with an increase in the annealing temperature for both the two ZnO samples of different "lm thickness. A rapid decrease of the refractive index n, was found with increasing ¹ up to a certain temper ature limit, which varies according to the "lm thickness, and then remains almost constant. Its average value at high ¹ was found to be smaller than that of the bulk material (n"2).
Acknowledgements The authors would like to thank and express their deepest appreciation to Prof. Dr. K. Abdel-Hady, Physics Department, Faculty of Science, Minia University and Prof. Dr. A. A. Ramadan, Physics Department, Faculty of Science, Helwan Univer-
sity, for their helpful discussions relevant to this work.
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