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Optical properties of CuAlX2 (X=Se, Te) thin films obtained by annealing of copper, aluminum and chalcogen layers sequentially deposited
Thin Solid Films 371 Ž2000. 195᎐200
Optical properties of CuAlX 2 Ž Xs Se, Te. thin films obtained by annealing of copper, aluminum and chalcogen layers sequentially deposited U
C.O. El Moctara , K. Kambasb, S. Marsillaca, , A. Anagnostopoulosb, J.C. Bernede ` a, K. Benchouckc a
Equipes couches minces et materiaux nou¨ eaux EPSE-FSTN, 2 Rue de la Houssiniere, BP 92208, 44322, Nantes, cedex 3, France b Physics Department, Aristotle Uni¨ ersity of Thessaloniki, Greece c LPMCE, Uni¨ ersite´ d’Oran Es-Senia, Oran, Algeria Received 29 July 1999; received in revised form 11 April 2000; accepted 11 April 2000
Abstract Thin layers of copper, aluminum and chalcogen sequentially deposited by evaporation are annealed to synthesize CuAlX2 ŽXs Se, Te. films. The films crystallized in the chalcopyrite structure. For CuAlSe2 , three characteristic energy gaps of 2.66Ž6., 2.78Ž2., and 2.91Ž5. eV were obtained from an analysis of the optical transmission spectra in the wavelength range 350᎐800 nm. The energies of the spin-orbit and the crystal field were found to be, respectively, 162 meV and y143 meV. Reflectivity measurements in the far infrared yield four mode frequencies for CuAlSe2 and CuAlTe2. 䊚 2000 Elsevier Science S.A. All rights reserved. Keywords: Chalcogens; Optical properties; Reflection spectroscopy
1. Introduction In the last years, one of the two trends in the research activities which try to develop high-efficiency thin film solar cells wbased on CuInSe2 ŽCIS. thin film absorbers with the use of CdS-like buffer layers deposited by CBD methodx is to realize more environmental-friendly device of Cd-free buffer layers w1,2x. I᎐III᎐VI2 compounds are ternary isoelectronic analogs of the II᎐VI binary compounds. They crystalU
Corresponding author. Tel.: q33-2-5112-5530; fax: q33-2-51125528. E-mail address: [email protected] ᎐ nantes.fr ŽS. Marsillac..
lize in the chalcopyrite structure, which is closely related to that of zinc blend. All these materials have direct band gaps w3x. The wide gap ternary I᎐III᎐VI2 and their polycrystalline thin films alloys, because of their optical and structural properties, could be expected as new alternative Cd-free buffer layers. Furthermore, these compounds could be obtained by a similar physical preparation process as the absorbers’ one. CuAlSe2 and CuAlTe2 semiconductors are two of the wide-gap, 2.67 eV w4x and 2.06 eV w5x, respectively, belonging to the I᎐III᎐VI2 features. It is well known and proven to be very difficult w6᎐12x to grow highquality CuAlVI2 compounds due to the existence of chemically active Al in the matrix. We have succeeded
0040-6090r00r$ - see front matter 䊚 2000 Elsevier Science S.A. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 0 . 0 0 9 8 5 - 8
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in growing good quality CuAlTe2 w11x and CuAlSe2 w12x polycrystalline thin films by using a simple low-cost technique. The purpose of this paper is to present some optical characteristics, in the visible and near UV range, of CuAlSe2 films. Moreover, some far infrared measurements will also be presented for both CuAlSe2 and CuAlTe2 . It has been mentioned that the band gap energy of I᎐III᎐VI2 compounds shows slight shifts depending on crystal growth conditions w13,14x and crystal anneal conditions w15x. Some of them have been explained in terms of deviation from ideal stoichiometry and of formation of band tail state and traps. The optical absorption spectra of polycrystalline chalcopyrite ternary compounds can be so considerably influenced by grain boundary effects w16,17x, which prevents an exact determination of the optical transition energies characteristic of these compounds. The band gap energy could then be used as a measure of the crystal quality. 2. Experimental details The process used to obtain thin CuAlX2 ŽXs Se, Te. films has been described earlier w11,12x and will be recalled shortly here. The substrates were soda lime glass chemically cleaned and rinsed in de-ionized water. The depositions were done in a vacuum of 10y4 Pa. The substrate temperature during deposition was 300 K. Copper, Al and X of 99.999% purity were deposited in the sequence, earlier discussed w11,12 x, CurAlrXr AlrCur . . . rX. The evaporation rate and the thickness were measured in situ by an HF quartz monitor. A rotating substrate holder allows to put alternatively the samples above the different crucibles. The layer thicknesses Ž0.6᎐1.2 m. were calculated to achieve a near stoichiometric composition. As discussed in the earlier paper w12x, to prevent aluminum oxidation, the deposition conditions have been optimized and the oxygen contamination has been reduced below 5 at.%. The CuAlX2 films were then synthesized by annealing under an argon flow. The best results have been obtained after 30 min at 858 K for CuAlSe2 w12x and at 673 K for CuAlTe2 w11x. At the end of process the films were chemically etched with a KCN solution Ž0.1 M. to eliminate the binary phases Cu2y ␦ Se. The thickness of the thin film was measured at the end of the process using SEM cross-section and the SEM Afore software developed by JEOL. 3. Experimental results and discussion All the films discussed in the present work have been characterized by X-ray diffraction and microprobe analysis before any optical investigation. They are crys-
Fig. 1. X-Ray diffraction spectra of Ža. a CuAlSe2 film; and Žb. a CuAlTe2 film.
tallized in the expected chalcopyrite structure. The films are textured with a preferential orientation along the Ž112. direction ŽFig. 1.. It should be noted that for CuAlSe2 films the full width at half maximum ŽFWHM. of the peaks is of the same order as that of the reference powder, which corroborates the good crystalline quality of the films. 3.1. Transmission measurements The transmission of the CuAlSe2 film was measured Žby steps of 1 nm. at room temperature using a carry 2300 UV-visible᎐near IR spectrophotometer. Fig. 2 shows the transmission, T, in the range 350᎐800 nm for two different film thicknesses. The absorption coefficient ␣ can be calculated from the transmission value T using the following expression w18x: Ts
Ž 1 y R . 2 exp Ž y␣ t . I s I0 1 y R2 exp Ž y2␣ t .
Ž1.
C.O. El Moctar et al. r Thin Solid Films 371 (2000) 195᎐200
Fig. 2. Transmission, T, as a function of the wavelength for two CuAlSe2 samples of different thicknesses.
where I and I0 are the transmitted and incident light intensity, respectively; R is the film reflectivity; and t the thickness. For high values of ␣ t the second term in the denominator of Eq. Ž1. can be neglected Že.g. if ␣ t s 1 and R s 0.3 the error is less than 2%.. Thus Eq. Ž1. is simplified to: T s Ž 1 y R . 2 exp Ž y␣ t .
Ž2.
The error made by taking Eq. Ž2. instead of Eq. Ž1. is much less than the errors in thickness measurements which dominate the experimental accuracy. Since the measurements were made on two samples of different thicknesses t1 and t2 , the absorption coefficient is simply given by: ␣s
T 1 ln 1 t2 y t1 T2
ž /
Ž3.
CuAlSe2 being a direct band gap semiconductor, the plot of Ž ␣ h .2 vs. the photon energy h is reported ŽFig. 3., because, for allowed direct band gap transition, the absorption coefficient can be related to the photon energy by:
197
supposed that the absorption spectrum is due to several interband transitions with different characteristic energies. The three threshold energies values of these interband transitions can be deduced from the intercept of the straight line issued from each absorption domain and the photon energy axis by extrapolation. The values deduced from Fig. 3 are 2.66Ž6. eV, 2.78Ž2. eV and 2.91Ž5. eV. The optical transition with the lowest energy must obviously be identified with the fundamental absorption edge of CuAlSe2 . This transition energy is in good agreement with the literature value of 2.67 eV at room temperature w4x. The structure of the fundamental absorption edge of the chalcopyrite compounds is known to be distinguished by the occurrence of three interband transitions. These are usually assigned as A, B, and C according to the increasing energy from the threefold degeneracy of p-like ⌫15 of the valence band. This band is completely lifted due to simultaneous perturbations of the tetragonal configuration of the crystal field and of the spin-orbit interaction w3x ŽFig. 3, inset.. The transitions energies values found in the present study are quasi-similar with those found for the A, B, and C transitions of epitaxial growth CuAlSe2 films measured by photoreflectance w19,20x. On another hand, the values of A, B and C transitions for the bulk crystal CuAlSe2 measured by reflection experiments were reported to be A Ž2.72 eV., B Ž2.86 eV. and C Ž3.01 eV. at 78 K w4x and A Ž2.65 eV., B Ž2.88 eV. and C Ž3.02 eV. at 110 K w21x. Therefore, even if measurements have been done with unpolarized light, the A, B and C transitions can be put in evidence from the absorption spectrum. It has been shown w21x that the polarization selection rule is greatly relaxed in CuAlSe2 . The energies of the crystal field splitting ⌬CF and the spin orbit splitting ⌬SO were calculated from the application of the quasicubic model w3x. The obtained values are ⌬SO s 162 meV and ⌬CF s y143 meV.
Ž ␣ h . 2 s AŽ h y Eg . where A is a constant and Eg is the optical energy gap. It can be seen from Fig. 2 that interference effects can be neglected and that three pronounced structures appear in the spectrum. These structures are also present in the other spectra of the absorption coefficient and of the Ž ␣ h .2 plot ŽFig. 3.. It can be observed that as well as the steep increase in ␣ at photon energies h in the range 2.6᎐2.7 eV there is a distinct change in the slope of the absorption coefficient curve near 2.75 eV and a well-pronounced nearly step-like additional increase in the absorption coefficient at photon energies of approximately 2.9 eV. Consequently it must be
Fig. 3. Plot of Ž ␣ h .2 vs. h for CuAlSe2 Žinset: band diagram..
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C.O. El Moctar et al. r Thin Solid Films 371 (2000) 195᎐200
Fig. 4. Reflectivity curve of CuAlSe2 .
The residual absorption below the threshold energy value could be related to some morphological inhomogeneity of the films Žgrain boundaries, etc.. andror to a band tail effect due to some levels introduced in the band gap by oxygen impurities or deviation from stoichiometry. In the case of CuAlTe2 , no structure is visible probably due to the higher contamination of the film Žapprox. 10% of oxygen.. The band gap has been extrapolated from the curve Ž ␣ h .2 vs. h and a value of approximately 2.3 eV has been found. This high value, compared to the reference, is probably due to the effect of oxygen. 3.2. Far infrared reflecti¨ ity measurements
cmy1 . The reflectivity is related to the Fresnel equation by: 1 y ' Ž . Rs 1 q ' Ž .
ž
2
/
where Ž . s 1Ž . q i2 Ž . is the complex dielectric constant. The real part, 1Ž ., and imaginary part, 2 Ž ., are related to the optical constants by: 1Ž . s n2 Ž . y k2 Ž . 2 Ž . s 2 nŽ . k Ž .
Fig. 4 shows the reflectivity spectrum of CuAlSe2 thin films in the frequency region 100᎐600 cmy1 . The spectra were measured by a Bruker IFS-113u, FTIR extended spectrometer using an unpolarized light, at room temperature. The ternary compound CuAlSe2 , as well as CuAlTe2 , are semiconductors with chalcopyrite . structure Žspace group I42 d ᎐D12 2 d and the body centered tetragonal lattice contains two formula units with 21 optical and three acoustical vibrational modes characterized by the following representation w22x:
where nŽ . is the refraction index and k Ž . the extinction coefficient. The synthesized reflectivity is given by:
⌫ s A1 q 2 A2 q 3 B1 q 3 B2 q 6 E
⌰s
Group theory analysis predicts nine infrared active modes, three of B2 symmetry and six of E Ždoubly degenerated.. In Fig. 4, we observe two relatively strong high frequency bands at 367 cmy1 and 345 cmy1 and two weak low frequency bands at 183 cmy1 and 161
From these data, the real and imaginary parts of the dielectric function could be determined, as well as the optical constants n and k. The results are shown in Figs. 5 and 6. We observed similar results for CuAlTe2 films which have been prepared by the same method as
Rs
Ž n y 1 . 2 y k2 Ž n y 1 . 2 q k2
As the measurements give the reflection amplitude RŽ . at an angular frequency , the phase angle ⌰Ž . can be obtained by the Kramers᎐Kronig equation: 2
⬁
H0
ln'R Ž ⬘ . d⬘ ⬘2 y 2
C.O. El Moctar et al. r Thin Solid Films 371 (2000) 195᎐200
199
Fig. 5. Plots of the real and imaginary parts of the dielectric function of CuAlSe2 .
the CuAlSe2 films. A striking similarity exists between the infrared spectra. We measured again four bands at frequencies 320, 298, 161 and 142 cmy1 . The analogy of these spectra means that both films have almost the same structure and Te atoms simply replace the Se atoms during crystallization. We observe also that all the frequencies of CuAlTe2 films are shifted compared to those of CuAlSe2 films toward the lower-wavelength side. Tellurium atoms are heavier than Se atoms and consequently the parameters of Al᎐Se fragments have changed with respect to Al᎐Te. In this case, the frequencies of all vibrational modes decrease when Se atoms are substituted by Te.
Another feature we can observe in these spectra is their simplicity, despite the very large number of atoms that the unit cell contains. The spectra should contain many vibrational modes with many degeneracies and overlaps. So we must assume that not only the basic structure of both films are fairly similar but also that, because of the bands, amorphous matter is present. Trying to explain the simplicity of the spectra, we can assume that because of the peculiarities of the structure, mentioned above, the unit cell can be reduced. The normal modes at such a reduced cell will span through the whole Brillouin zone and so the complete cell considered as a superstructure of the smallest cell
Fig. 6. Plots of the real and imaginary parts of the refraction index of CuAlSe2 .
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C.O. El Moctar et al. r Thin Solid Films 371 (2000) 195᎐200
would imply a folding of the Brillouin zone leading the large number of normal modes at k s 0 in a narrow frequency range. So, due to many overlappings, the bands decrease in number and increase in broadness. 4. Conclusion After X-ray diffraction and microprobe analysis, the films crystallized in the chalcopyrite structure were characterized by optical measurements. The splitting into three separate bands A, B and C of the valence band, due to the simultaneous interactions of the noncubic crystal field and the spin-orbit, can be directly put in evidence when careful transmission measurements are used. This result is in good agreement with the fact that the polarization selection rule is greatly relaxed in CuAlSe2 w21x. It is shown by FIR-reflectivity measurements that similar spectra are obtained in the case of CuAlSe2 and CuAlTe2 . The simplicity of their spectra can be explained by many overlappings of vibrating bands, which explains their broadness. Acknowledgements This work was supported by CMEP 95 MDU 337 ŽFrance᎐Algeria. which is gratefully acknowledged. References w1x D. Hariskos, R. Herbeholz, M. Ruckh, U. Ruhle, R. Schaffler, H.W. Schock, in: W. Freielsleben, W. Palz, H.A. Ossenbrink, P. Helm ŽEds.., Proceedings of the 13th European Photovoltaic Solar Energy Conference and Exhibition Acropolis, Convention Center, Nice, France, October 23᎐27, 1995, p. 1995.
w2x K. Kushiya, S. Kuriyagawa, T. Nii, I. Sugiyama, T. Kase, M. Sato, H. Takeshita, in: W. Freielsleben, W. Palz, H.A. Ossenbrink, P. Helm ŽEds.., Proceedings of the 13th European Photovoltaic Solar Energy Conference and Exhibition Acropolis, Convention Center, Nice, France, October 23᎐27, 1995, p. 2016. w3x J.L. Shay, J.H. Wernick, Ternary Chalcopyrite Semiconductors: Growth, Electronic Properties, and Applications, Pergamon, Oxford, 1975. w4x M. Bettini, Solid State Commun. 13 Ž1973. 599. w5x W.N. Honeyman, K.H. Wilkinson, J. Phys. D, Appl. Phys. 4 Ž1971. 1182. w6x K. Hara, T. Shinozawa, J. Yoshino, H. Kukimoto, J. Cryst. Growth 93 Ž1988. 771. w7x K. Sugiyama, K. Mori, H. Miyake, J. Cryst. Growth 113 Ž1991. 390. w8x Y. Morita, T. Naruzawa, Jpn. J. Appl. Phys. 30 Ž1991. L1238. w9x S. Chichibu, A. Iwai, H. Higuchi, J. Cryst. Growth 126 Ž1993. 635. w10x H. Miyake, M. Yamada, K. Sugiyama, J. Cryst. Growth 153 Ž1995. 180. w11x K. Benchouck, C. El Moctar, S. Marsillac, J.C. Bernede, J. ` Pouzet, N. Barreau, M. Emziane, J. Mater. Sci. 34 Ž1999. 1847. w12x C. El Moctar, S. Marsillac, J.C. Bernede, ` A. Conan, K. Benchouck, H. Khellil, Phys. Stat. Sol. Ža. 179 Ž1999. 213. w13x S. Shirakata, S. Isomura, J. Appl. Phys. 70 Ž1991. 7051. w14x S. Shirakata, K. Murakami, S. Isomura, Jpn. J. Appl. Phys. 28 Ž1989. 1728. w15x T.M. Hsu, J.S. Lee, H.L. Hwang, J. Appl. Phys. 68 Ž1990. 283. w16x D. Sridevi, K.V. Reddy, Thin Solid Films 141 Ž1986. 157. w17x W. Horig, H. Neumann, V. Savelen, J. Lagzedonis, B. Schumann, G. Kuhn, Cryst. Res. Technol. 24 Ž1989. 823. w18x R. Bischel, F. Levy, Thin Solid Films 124 Ž1985. 75. w19x S. Shirakata, S. Chichibu, S. Matsumoto, S. Isomura, Jpn. J. Appl. Phys. 32 Ž1993. L167. w20x S. Shirakata, S. Chichibu, J. Appl. Phys. 79 Ž1996. 2043. w21x N. Yamamato, Jpn. J. Appl. Phys. 15 Ž1976. 1909. w22x I.P. Kaminow, E. Buehler, J.H. Wernick, Phys. Rev. B 2 Ž1970. 960.
Optical properties of CuAlX 2 Ž Xs Se, Te. thin films obtained by annealing of copper, aluminum and chalcogen layers sequentially deposited U
C.O. El Moctara , K. Kambasb, S. Marsillaca, , A. Anagnostopoulosb, J.C. Bernede ` a, K. Benchouckc a
Equipes couches minces et materiaux nou¨ eaux EPSE-FSTN, 2 Rue de la Houssiniere, BP 92208, 44322, Nantes, cedex 3, France b Physics Department, Aristotle Uni¨ ersity of Thessaloniki, Greece c LPMCE, Uni¨ ersite´ d’Oran Es-Senia, Oran, Algeria Received 29 July 1999; received in revised form 11 April 2000; accepted 11 April 2000
Abstract Thin layers of copper, aluminum and chalcogen sequentially deposited by evaporation are annealed to synthesize CuAlX2 ŽXs Se, Te. films. The films crystallized in the chalcopyrite structure. For CuAlSe2 , three characteristic energy gaps of 2.66Ž6., 2.78Ž2., and 2.91Ž5. eV were obtained from an analysis of the optical transmission spectra in the wavelength range 350᎐800 nm. The energies of the spin-orbit and the crystal field were found to be, respectively, 162 meV and y143 meV. Reflectivity measurements in the far infrared yield four mode frequencies for CuAlSe2 and CuAlTe2. 䊚 2000 Elsevier Science S.A. All rights reserved. Keywords: Chalcogens; Optical properties; Reflection spectroscopy
1. Introduction In the last years, one of the two trends in the research activities which try to develop high-efficiency thin film solar cells wbased on CuInSe2 ŽCIS. thin film absorbers with the use of CdS-like buffer layers deposited by CBD methodx is to realize more environmental-friendly device of Cd-free buffer layers w1,2x. I᎐III᎐VI2 compounds are ternary isoelectronic analogs of the II᎐VI binary compounds. They crystalU
Corresponding author. Tel.: q33-2-5112-5530; fax: q33-2-51125528. E-mail address: [email protected] ᎐ nantes.fr ŽS. Marsillac..
lize in the chalcopyrite structure, which is closely related to that of zinc blend. All these materials have direct band gaps w3x. The wide gap ternary I᎐III᎐VI2 and their polycrystalline thin films alloys, because of their optical and structural properties, could be expected as new alternative Cd-free buffer layers. Furthermore, these compounds could be obtained by a similar physical preparation process as the absorbers’ one. CuAlSe2 and CuAlTe2 semiconductors are two of the wide-gap, 2.67 eV w4x and 2.06 eV w5x, respectively, belonging to the I᎐III᎐VI2 features. It is well known and proven to be very difficult w6᎐12x to grow highquality CuAlVI2 compounds due to the existence of chemically active Al in the matrix. We have succeeded
0040-6090r00r$ - see front matter 䊚 2000 Elsevier Science S.A. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 0 . 0 0 9 8 5 - 8
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C.O. El Moctar et al. r Thin Solid Films 371 (2000) 195᎐200
in growing good quality CuAlTe2 w11x and CuAlSe2 w12x polycrystalline thin films by using a simple low-cost technique. The purpose of this paper is to present some optical characteristics, in the visible and near UV range, of CuAlSe2 films. Moreover, some far infrared measurements will also be presented for both CuAlSe2 and CuAlTe2 . It has been mentioned that the band gap energy of I᎐III᎐VI2 compounds shows slight shifts depending on crystal growth conditions w13,14x and crystal anneal conditions w15x. Some of them have been explained in terms of deviation from ideal stoichiometry and of formation of band tail state and traps. The optical absorption spectra of polycrystalline chalcopyrite ternary compounds can be so considerably influenced by grain boundary effects w16,17x, which prevents an exact determination of the optical transition energies characteristic of these compounds. The band gap energy could then be used as a measure of the crystal quality. 2. Experimental details The process used to obtain thin CuAlX2 ŽXs Se, Te. films has been described earlier w11,12x and will be recalled shortly here. The substrates were soda lime glass chemically cleaned and rinsed in de-ionized water. The depositions were done in a vacuum of 10y4 Pa. The substrate temperature during deposition was 300 K. Copper, Al and X of 99.999% purity were deposited in the sequence, earlier discussed w11,12 x, CurAlrXr AlrCur . . . rX. The evaporation rate and the thickness were measured in situ by an HF quartz monitor. A rotating substrate holder allows to put alternatively the samples above the different crucibles. The layer thicknesses Ž0.6᎐1.2 m. were calculated to achieve a near stoichiometric composition. As discussed in the earlier paper w12x, to prevent aluminum oxidation, the deposition conditions have been optimized and the oxygen contamination has been reduced below 5 at.%. The CuAlX2 films were then synthesized by annealing under an argon flow. The best results have been obtained after 30 min at 858 K for CuAlSe2 w12x and at 673 K for CuAlTe2 w11x. At the end of process the films were chemically etched with a KCN solution Ž0.1 M. to eliminate the binary phases Cu2y ␦ Se. The thickness of the thin film was measured at the end of the process using SEM cross-section and the SEM Afore software developed by JEOL. 3. Experimental results and discussion All the films discussed in the present work have been characterized by X-ray diffraction and microprobe analysis before any optical investigation. They are crys-
Fig. 1. X-Ray diffraction spectra of Ža. a CuAlSe2 film; and Žb. a CuAlTe2 film.
tallized in the expected chalcopyrite structure. The films are textured with a preferential orientation along the Ž112. direction ŽFig. 1.. It should be noted that for CuAlSe2 films the full width at half maximum ŽFWHM. of the peaks is of the same order as that of the reference powder, which corroborates the good crystalline quality of the films. 3.1. Transmission measurements The transmission of the CuAlSe2 film was measured Žby steps of 1 nm. at room temperature using a carry 2300 UV-visible᎐near IR spectrophotometer. Fig. 2 shows the transmission, T, in the range 350᎐800 nm for two different film thicknesses. The absorption coefficient ␣ can be calculated from the transmission value T using the following expression w18x: Ts
Ž 1 y R . 2 exp Ž y␣ t . I s I0 1 y R2 exp Ž y2␣ t .
Ž1.
C.O. El Moctar et al. r Thin Solid Films 371 (2000) 195᎐200
Fig. 2. Transmission, T, as a function of the wavelength for two CuAlSe2 samples of different thicknesses.
where I and I0 are the transmitted and incident light intensity, respectively; R is the film reflectivity; and t the thickness. For high values of ␣ t the second term in the denominator of Eq. Ž1. can be neglected Že.g. if ␣ t s 1 and R s 0.3 the error is less than 2%.. Thus Eq. Ž1. is simplified to: T s Ž 1 y R . 2 exp Ž y␣ t .
Ž2.
The error made by taking Eq. Ž2. instead of Eq. Ž1. is much less than the errors in thickness measurements which dominate the experimental accuracy. Since the measurements were made on two samples of different thicknesses t1 and t2 , the absorption coefficient is simply given by: ␣s
T 1 ln 1 t2 y t1 T2
ž /
Ž3.
CuAlSe2 being a direct band gap semiconductor, the plot of Ž ␣ h .2 vs. the photon energy h is reported ŽFig. 3., because, for allowed direct band gap transition, the absorption coefficient can be related to the photon energy by:
197
supposed that the absorption spectrum is due to several interband transitions with different characteristic energies. The three threshold energies values of these interband transitions can be deduced from the intercept of the straight line issued from each absorption domain and the photon energy axis by extrapolation. The values deduced from Fig. 3 are 2.66Ž6. eV, 2.78Ž2. eV and 2.91Ž5. eV. The optical transition with the lowest energy must obviously be identified with the fundamental absorption edge of CuAlSe2 . This transition energy is in good agreement with the literature value of 2.67 eV at room temperature w4x. The structure of the fundamental absorption edge of the chalcopyrite compounds is known to be distinguished by the occurrence of three interband transitions. These are usually assigned as A, B, and C according to the increasing energy from the threefold degeneracy of p-like ⌫15 of the valence band. This band is completely lifted due to simultaneous perturbations of the tetragonal configuration of the crystal field and of the spin-orbit interaction w3x ŽFig. 3, inset.. The transitions energies values found in the present study are quasi-similar with those found for the A, B, and C transitions of epitaxial growth CuAlSe2 films measured by photoreflectance w19,20x. On another hand, the values of A, B and C transitions for the bulk crystal CuAlSe2 measured by reflection experiments were reported to be A Ž2.72 eV., B Ž2.86 eV. and C Ž3.01 eV. at 78 K w4x and A Ž2.65 eV., B Ž2.88 eV. and C Ž3.02 eV. at 110 K w21x. Therefore, even if measurements have been done with unpolarized light, the A, B and C transitions can be put in evidence from the absorption spectrum. It has been shown w21x that the polarization selection rule is greatly relaxed in CuAlSe2 . The energies of the crystal field splitting ⌬CF and the spin orbit splitting ⌬SO were calculated from the application of the quasicubic model w3x. The obtained values are ⌬SO s 162 meV and ⌬CF s y143 meV.
Ž ␣ h . 2 s AŽ h y Eg . where A is a constant and Eg is the optical energy gap. It can be seen from Fig. 2 that interference effects can be neglected and that three pronounced structures appear in the spectrum. These structures are also present in the other spectra of the absorption coefficient and of the Ž ␣ h .2 plot ŽFig. 3.. It can be observed that as well as the steep increase in ␣ at photon energies h in the range 2.6᎐2.7 eV there is a distinct change in the slope of the absorption coefficient curve near 2.75 eV and a well-pronounced nearly step-like additional increase in the absorption coefficient at photon energies of approximately 2.9 eV. Consequently it must be
Fig. 3. Plot of Ž ␣ h .2 vs. h for CuAlSe2 Žinset: band diagram..
198
C.O. El Moctar et al. r Thin Solid Films 371 (2000) 195᎐200
Fig. 4. Reflectivity curve of CuAlSe2 .
The residual absorption below the threshold energy value could be related to some morphological inhomogeneity of the films Žgrain boundaries, etc.. andror to a band tail effect due to some levels introduced in the band gap by oxygen impurities or deviation from stoichiometry. In the case of CuAlTe2 , no structure is visible probably due to the higher contamination of the film Žapprox. 10% of oxygen.. The band gap has been extrapolated from the curve Ž ␣ h .2 vs. h and a value of approximately 2.3 eV has been found. This high value, compared to the reference, is probably due to the effect of oxygen. 3.2. Far infrared reflecti¨ ity measurements
cmy1 . The reflectivity is related to the Fresnel equation by: 1 y ' Ž . Rs 1 q ' Ž .
ž
2
/
where Ž . s 1Ž . q i2 Ž . is the complex dielectric constant. The real part, 1Ž ., and imaginary part, 2 Ž ., are related to the optical constants by: 1Ž . s n2 Ž . y k2 Ž . 2 Ž . s 2 nŽ . k Ž .
Fig. 4 shows the reflectivity spectrum of CuAlSe2 thin films in the frequency region 100᎐600 cmy1 . The spectra were measured by a Bruker IFS-113u, FTIR extended spectrometer using an unpolarized light, at room temperature. The ternary compound CuAlSe2 , as well as CuAlTe2 , are semiconductors with chalcopyrite . structure Žspace group I42 d ᎐D12 2 d and the body centered tetragonal lattice contains two formula units with 21 optical and three acoustical vibrational modes characterized by the following representation w22x:
where nŽ . is the refraction index and k Ž . the extinction coefficient. The synthesized reflectivity is given by:
⌫ s A1 q 2 A2 q 3 B1 q 3 B2 q 6 E
⌰s
Group theory analysis predicts nine infrared active modes, three of B2 symmetry and six of E Ždoubly degenerated.. In Fig. 4, we observe two relatively strong high frequency bands at 367 cmy1 and 345 cmy1 and two weak low frequency bands at 183 cmy1 and 161
From these data, the real and imaginary parts of the dielectric function could be determined, as well as the optical constants n and k. The results are shown in Figs. 5 and 6. We observed similar results for CuAlTe2 films which have been prepared by the same method as
Rs
Ž n y 1 . 2 y k2 Ž n y 1 . 2 q k2
As the measurements give the reflection amplitude RŽ . at an angular frequency , the phase angle ⌰Ž . can be obtained by the Kramers᎐Kronig equation: 2
⬁
H0
ln'R Ž ⬘ . d⬘ ⬘2 y 2
C.O. El Moctar et al. r Thin Solid Films 371 (2000) 195᎐200
199
Fig. 5. Plots of the real and imaginary parts of the dielectric function of CuAlSe2 .
the CuAlSe2 films. A striking similarity exists between the infrared spectra. We measured again four bands at frequencies 320, 298, 161 and 142 cmy1 . The analogy of these spectra means that both films have almost the same structure and Te atoms simply replace the Se atoms during crystallization. We observe also that all the frequencies of CuAlTe2 films are shifted compared to those of CuAlSe2 films toward the lower-wavelength side. Tellurium atoms are heavier than Se atoms and consequently the parameters of Al᎐Se fragments have changed with respect to Al᎐Te. In this case, the frequencies of all vibrational modes decrease when Se atoms are substituted by Te.
Another feature we can observe in these spectra is their simplicity, despite the very large number of atoms that the unit cell contains. The spectra should contain many vibrational modes with many degeneracies and overlaps. So we must assume that not only the basic structure of both films are fairly similar but also that, because of the bands, amorphous matter is present. Trying to explain the simplicity of the spectra, we can assume that because of the peculiarities of the structure, mentioned above, the unit cell can be reduced. The normal modes at such a reduced cell will span through the whole Brillouin zone and so the complete cell considered as a superstructure of the smallest cell
Fig. 6. Plots of the real and imaginary parts of the refraction index of CuAlSe2 .
200
C.O. El Moctar et al. r Thin Solid Films 371 (2000) 195᎐200
would imply a folding of the Brillouin zone leading the large number of normal modes at k s 0 in a narrow frequency range. So, due to many overlappings, the bands decrease in number and increase in broadness. 4. Conclusion After X-ray diffraction and microprobe analysis, the films crystallized in the chalcopyrite structure were characterized by optical measurements. The splitting into three separate bands A, B and C of the valence band, due to the simultaneous interactions of the noncubic crystal field and the spin-orbit, can be directly put in evidence when careful transmission measurements are used. This result is in good agreement with the fact that the polarization selection rule is greatly relaxed in CuAlSe2 w21x. It is shown by FIR-reflectivity measurements that similar spectra are obtained in the case of CuAlSe2 and CuAlTe2 . The simplicity of their spectra can be explained by many overlappings of vibrating bands, which explains their broadness. Acknowledgements This work was supported by CMEP 95 MDU 337 ŽFrance᎐Algeria. which is gratefully acknowledged. References w1x D. Hariskos, R. Herbeholz, M. Ruckh, U. Ruhle, R. Schaffler, H.W. Schock, in: W. Freielsleben, W. Palz, H.A. Ossenbrink, P. Helm ŽEds.., Proceedings of the 13th European Photovoltaic Solar Energy Conference and Exhibition Acropolis, Convention Center, Nice, France, October 23᎐27, 1995, p. 1995.
w2x K. Kushiya, S. Kuriyagawa, T. Nii, I. Sugiyama, T. Kase, M. Sato, H. Takeshita, in: W. Freielsleben, W. Palz, H.A. Ossenbrink, P. Helm ŽEds.., Proceedings of the 13th European Photovoltaic Solar Energy Conference and Exhibition Acropolis, Convention Center, Nice, France, October 23᎐27, 1995, p. 2016. w3x J.L. Shay, J.H. Wernick, Ternary Chalcopyrite Semiconductors: Growth, Electronic Properties, and Applications, Pergamon, Oxford, 1975. w4x M. Bettini, Solid State Commun. 13 Ž1973. 599. w5x W.N. Honeyman, K.H. Wilkinson, J. Phys. D, Appl. Phys. 4 Ž1971. 1182. w6x K. Hara, T. Shinozawa, J. Yoshino, H. Kukimoto, J. Cryst. Growth 93 Ž1988. 771. w7x K. Sugiyama, K. Mori, H. Miyake, J. Cryst. Growth 113 Ž1991. 390. w8x Y. Morita, T. Naruzawa, Jpn. J. Appl. Phys. 30 Ž1991. L1238. w9x S. Chichibu, A. Iwai, H. Higuchi, J. Cryst. Growth 126 Ž1993. 635. w10x H. Miyake, M. Yamada, K. Sugiyama, J. Cryst. Growth 153 Ž1995. 180. w11x K. Benchouck, C. El Moctar, S. Marsillac, J.C. Bernede, J. ` Pouzet, N. Barreau, M. Emziane, J. Mater. Sci. 34 Ž1999. 1847. w12x C. El Moctar, S. Marsillac, J.C. Bernede, ` A. Conan, K. Benchouck, H. Khellil, Phys. Stat. Sol. Ža. 179 Ž1999. 213. w13x S. Shirakata, S. Isomura, J. Appl. Phys. 70 Ž1991. 7051. w14x S. Shirakata, K. Murakami, S. Isomura, Jpn. J. Appl. Phys. 28 Ž1989. 1728. w15x T.M. Hsu, J.S. Lee, H.L. Hwang, J. Appl. Phys. 68 Ž1990. 283. w16x D. Sridevi, K.V. Reddy, Thin Solid Films 141 Ž1986. 157. w17x W. Horig, H. Neumann, V. Savelen, J. Lagzedonis, B. Schumann, G. Kuhn, Cryst. Res. Technol. 24 Ž1989. 823. w18x R. Bischel, F. Levy, Thin Solid Films 124 Ž1985. 75. w19x S. Shirakata, S. Chichibu, S. Matsumoto, S. Isomura, Jpn. J. Appl. Phys. 32 Ž1993. L167. w20x S. Shirakata, S. Chichibu, J. Appl. Phys. 79 Ž1996. 2043. w21x N. Yamamato, Jpn. J. Appl. Phys. 15 Ž1976. 1909. w22x I.P. Kaminow, E. Buehler, J.H. Wernick, Phys. Rev. B 2 Ž1970. 960.