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Effect of substrate temperature on the structure and optical properties of CdTe thin film
Solar Energy Materials & Solar Cells 76 (2003) 339–346
Effect of substrate temperature on the structure and optical properties of CdTe thin film R. Sathyamoorthya,*, Sa.K. Narayandassb, D. Mangalarajb a
Department of Physics, Kongunadu Arts and Science College, Coimbatore-641 029, India b Department of Physics, Bharathiar University, Coimbatore-641 046, India
Abstract The present work deals with the preparation, structure and optical characterization of cadmium telluride (CdTe) thin films. These films are formed by vacuum evaporation on the well-cleaned glass substrates. The compositional analyses are made by energy dispersive analysis by X-ray. The thicknesses of the samples are measured by multiple beam interferometry. The samples are prepared at different substrate temperatures. The X-ray diffraction has been employed to study the structure of the film. The structures of the samples are found to be crystalline and the crystallite size increases with the increase of substrate temperature. The d-spacing and lattice parameters of the samples are calculated and the results are also discussed. Optical characteristics of the CdTe samples have been analyzed using spectrophotometer in the wavelength range of 400–800 nm. The transmittance is found to decrease with the increase of film thickness. The transmittance falls steeply with decreasing wavelength. It reveals that CdTe films are having considerable absorption throughout the wavelength region (400–800 nm). The optical band gap energy has been evaluated from the plot of a2 vs. hn: Two possible direct transitions have been observed for all the CdTe films in visible region. The observed allowed transition may be attributed due to the spin orbit splitting of the valence band. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Thin film; CdTe; Substrate temperature; Optical properties
1. Introduction Cadmium telluride (CdTe) thin films have drawn considerable interest in recent years due to their wide use in the fabrication of photoconductors, space charge *Corresponding author. E-mail address: [email protected] (R. Sathyamoorthy). 0927-0248/03/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 0 2 4 8 ( 0 2 ) 0 0 2 8 6 - 6
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limited diodes, photovoltaic devices, transistors and infrared (IR) detectors [1]. The optical constants of thin films of semi-conducting materials depend on the condition of preparation such as rate of evaporation and the substrate temperature during the deposition process. These parameters have to be controlled right from the initial stages of deposition. This paper reports the dependence of structure and optical properties of CdTe film on the film thickness and substrate temperature.
2. Experimental details Rectangular films of CdTe (37 mm 25 mm) were grown from Blazers evaporation grade CdTe (99.999%) using a molybdenum boat, under a vacuum of 2 105 Torr on glass substrates. Thicknesses of the films were measured by making use of Tolansky’s technique (Fizeau fringes). Structure of the films was studied using an X-ray diffractometer (Phillips/pw1010, pw1051). Copper Ka/Cobalt Ka radiation was employed in these experiments and a 2y spectrum from 01 to 801 was recorded for all the films. The composition of the film was studied using energy dispersive analysis by X-ray (EDAX). Optical transmission studies were made on the film grown on glass substrates using spectrophotometer (Pye Unicom UV–VIS 8800).
3. Results and discussion Films grown on glass substrates at different substrate temperature were used for structural and optical studies. 3.1. Structural properties X-ray diffraction (XRD) studies were made on CdTe thin films deposited on glass substrates to determine their structure and to identify the components and phases. Diffraction spectra for various films and for bulk powder are shown in Fig. 1. The spectrum of CdTe powder exhibits sharp peaks at 2y equal to 23.71, 39.31 and 46.61 which corresponds to diffraction from (1 1 1), (2 2 0) and (3 1 1) planes of the cubic phase, respectively. Both the peak height and peak position are in good agreement with ASTM data (16–770) for cubic CdTe, whereas the as-grown films (Ts ¼ 373 K) showed preferential growth of film crystallites corresponding to textured (1 1 1) and (2 2 0) growth. The (1 1 1) direction is the close packing direction of the zinc blende structure and this type of ordering is often observed in polycrystalline films [2,3] grown on heated amorphous substrates. A weak peak at 27.81 along with pronounced peaks have been observed for the film coated at low substrate temperature (Ts ¼ 373 K). This suggests the excess of metallic Te [4] in the as-grown CdTe films. On the other hand the presence of peaks only at 23.81 39.31, 46.61 have been observed with higher substrate temperature (Ts ¼ 573 K, Fig. 2).
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Fig. 1. XRD patterns of (a) CdTe bulk sample, and (b) CdTe film deposited at Ts ¼ 373 K.
Fig. 2. XRD patterns of the CdTe film coated at Ts ¼ 573 K.
The absence of a peak at 27.81 for films deposited at Ts ¼ 573 K is due to the reevaporation of metallic Te leading to the formation of stoichiometric CdTe films. A similar observation has been made by Makoto Takahashi et al. [4] for CdTe films.
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Table 1 Effect of substrate temperature on structural parameters Sample
2y
( Calculated (d) A
Probable hkl
( a¼b¼cA
CdTe bulk powder (Cu Ka)
23.700 39.100 46.600 56.800
3.749 2.309 1.949 1.629
111 220 311 400
6.494 6.531 6.464 6.484
CdTe film coated at 373 K (Cu Ka)
23.700 39.100
3.749 2.309
111 220
6.494 6.531
CdTe film coated at 573 K (Co Ka)
27.523 45.925 51.517 76.628
3.762 2.294 2.060 1.444
111 220 311 331
6.516 6.488 6.832 6.294
Table 1 Effect of substrate temperature on structural parameters
Crystallite sizes were determined from the XRD data using the Scherer formula [5] l ¼ lDcosy; where l is the grain size, l is the wave length of the X-rays and D is the width of the peak at full-width half-maximum. The value of l for the as-grown film was found to ( The calculated d spacing and lattice parameter along with the probable hkl be 450 A. values were presented in Table 1. ( is in good agreement with the ASTM data for The calculated value of a0 (6.48 A) cubic CdTe. 3.2. Composition analysis The EDAX studies were made on the CdTe films to determine their composition. An energy dispersive system was employed in the present studies in order to determine the various elements present in the films and their concentration and only Cd and Te were found to be present. It is seen that films coated at room temperature contains (Cd: 46.6%, Te: 54.4%) excess Te whereas films coated at lower substrate temperature (Ts ¼ 373 K) contain (Cd: 48%, Te: 52%) a small excess of Te which decreases with the increase of substrate temperature ðTs ¼ 573 KÞ (Cd: 50%, Te: 50%). 3.3. Optical properties Transmission characteristics of CdTe films have been given in Fig 3. The figure clearly indicates that the transmission is found to decrease with increasing film thickness. The transmittance falls steeply with decreasing wavelength. This reveals that the CdTe films are having considerable absorption throughout the wavelength region (400–800 nm) studied. This is in good agreement with the earlier investigation [6].
R. Sathyamoorthy et al. / Solar Energy Materials & Solar Cells 76 (2003) 339–346
343
Fig. 3. Transmittance curves of CdTe films.
The extinction coefficient ðkf Þ and refractive index ðnf Þ were evaluated in the wavelength range 400–800 nm using the relation kf ¼ 2:303llogð1WT0 Þ=4pt; where l is the wavelength, T0 is I=I0 ; and I is intensity of light. Fig. 4 shows the variation of kf and nf with wavelength for CdTe films. An inconsistency has been observed in the dependence of extinction coefficient on wavelength and thickness. The refractive indices were calculated using Manifacier’s formula [7] by the iterative method: T0 ¼
16na ng n2 expðatÞ ; R21 þ R22 expð2atÞ þ 2R1 R2 expðatÞcosð4pnt=lÞ
where R1 =ðn þ na Þðng þ 1Þ; R2 =ðn na Þðng 1Þ are the absorption coefficients, t is the thickness of the film, n; na ; ng are the refractive indices of the film, air and substrate, respectively. The d refractive index increases with increase of incident photon energy and thickness for CdTe film. This prevailing trend is in good agreement with the earlier investigations for CdTe films [8]. The optical band gap energy for CdTe film has been evaluated from the a2 vs. hn plot (Fig. 5). Two possible direct transitions have been observed for all the CdTe films in the visible region. The result is that there were two direct optical band gap energies for a single film. The optical band gap energy results observed for CdTe films (Ts ¼ 573 K) of various thicknesses are given in Table 2.
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Fig. 4. Dependence of nf and kf on wavelength for the CdTe film.
( Fig. 5. Variation of a2 with hg for CdTe film coated at 473 K (t ¼ 2460 A).
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Table 2 The optical band gap energies for CdTe films deposited at Ts ¼ 573 K ( Film thickness (A)
1090 2460 4270
Band gap energy (eV) Eg1
Eg2
1.75 1.65 1.45
2.18 2.03 1.78
Table 2 The optical band gap energies for CdTe films deposited at Ts ¼ 573 K Table 3 The optical band gap energies for CdTe films deposited at Ts ¼ 473 K ( Film thickness (A)
770 1060 1610 2190
Band gap energy(eV) Eg1
Eg2
1.5 1.55 1.54 1.58
1.87 2.3 2.22 2.3
Table 3 The optical band gap energies for CdTe films deposited at Ts ¼ 473 K Table 4 The optical band gap energies for CdTe films deposited at Ts ¼ 373 K ( Film thickness (A)
1450 2460 3690
Band gap energy (eV) Eg1
Eg2
1.58 1.56 1.50
2.13 1.95 2.07
Table 4 The optical band gap energies for CdTe films deposited at Ts ¼ 373 K
The observed (two direct) allowed transitions may be attributed to the spin orbit splitting of the valance band. Similar observations have been made by Thutupalli et al. [9] for CdTe films. The observed two optical band gap energies (Eg1 and Eg2 ) decrease with increase in film thickness. The decrease in optical band gap energy with increase in film thickness is due to the increased grain size of the higher thickness films. The ad atom mobility also increases as the substrate temperature increases, which also results in the crystalline size and crystallinity of the films. This is in good agreement with the observations made by several other workers. In order to make a detailed study on the effect of substrate temperature on the optical band gap energy for CdTe, films of varying thicknesses have been prepared at different substrate temperatures (373, 473 and 573 K). It is observed that CdTe films deposited at 573 K only possess the right stoichiometry (Cd=50% and Te=50%) and the optical band gap energy decreases with increase in film thickness. The film deposited at other
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substrate temperatures contain varying composition and the optical band gap energy which: (i) increases with increase in film thickness in the case of films deposited at 473 K (Table 3) and (ii) varies erratically with film thickness in case of films deposited at 373 K (Table 4).
4. Conclusion As-grown films of CdTe were polycrystalline with the small grain size. Films deposited at higher substrate temperature improved the grain size as well as mobility of the carriers. The optical transition in CdTe films was found to be allowed and direct. The effect of the substrate temperature and thickness on the optical band gap energy has also been reported.
References [1] [2] [3] [4] [5] [6] [7] [8] [9]
G. Entine, M.R. Sqcillante, H.B. Serreze, E. Clarke, IEEE Trans. Nucl. Sci. NS-28 (1981) 558. S. Saha, U. Pal, B.K. Samantaray, A.K. Chaudhuri, Thin Solid Films 164 (1988) 85. R.D. Gould, C.J. Bowler, Thin Solid Films 164 (1988) 281. Makato Takahashi, Kohei Vasaki, Hideaki Kila, Yoshi Kazu Suzuki, J. Appl. Phys. 58 (1985) 4292. L.I. Maissel, R. Glang, Hand Book of Thin Film Technology, McGraw-Hill, New York, 1980. J. Aranda, J.L. Morenza, J. Estere, J.M. Codia, Thin Solid Films 120 (1984) 23. J.C. Manifacier, J. Gasiot, J.P. Fillard, J. Phys. E 9 (1976) 1002. A.A. El-Shazly, H.T. El-Shavi, Thin Solid Films 78 (1981) 287. G.K.M. Thutupalli, S.G. Tomlin, J. Appl. Phys. D 9 (1976) 128.
Effect of substrate temperature on the structure and optical properties of CdTe thin film R. Sathyamoorthya,*, Sa.K. Narayandassb, D. Mangalarajb a
Department of Physics, Kongunadu Arts and Science College, Coimbatore-641 029, India b Department of Physics, Bharathiar University, Coimbatore-641 046, India
Abstract The present work deals with the preparation, structure and optical characterization of cadmium telluride (CdTe) thin films. These films are formed by vacuum evaporation on the well-cleaned glass substrates. The compositional analyses are made by energy dispersive analysis by X-ray. The thicknesses of the samples are measured by multiple beam interferometry. The samples are prepared at different substrate temperatures. The X-ray diffraction has been employed to study the structure of the film. The structures of the samples are found to be crystalline and the crystallite size increases with the increase of substrate temperature. The d-spacing and lattice parameters of the samples are calculated and the results are also discussed. Optical characteristics of the CdTe samples have been analyzed using spectrophotometer in the wavelength range of 400–800 nm. The transmittance is found to decrease with the increase of film thickness. The transmittance falls steeply with decreasing wavelength. It reveals that CdTe films are having considerable absorption throughout the wavelength region (400–800 nm). The optical band gap energy has been evaluated from the plot of a2 vs. hn: Two possible direct transitions have been observed for all the CdTe films in visible region. The observed allowed transition may be attributed due to the spin orbit splitting of the valence band. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Thin film; CdTe; Substrate temperature; Optical properties
1. Introduction Cadmium telluride (CdTe) thin films have drawn considerable interest in recent years due to their wide use in the fabrication of photoconductors, space charge *Corresponding author. E-mail address: [email protected] (R. Sathyamoorthy). 0927-0248/03/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 0 2 4 8 ( 0 2 ) 0 0 2 8 6 - 6
340
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limited diodes, photovoltaic devices, transistors and infrared (IR) detectors [1]. The optical constants of thin films of semi-conducting materials depend on the condition of preparation such as rate of evaporation and the substrate temperature during the deposition process. These parameters have to be controlled right from the initial stages of deposition. This paper reports the dependence of structure and optical properties of CdTe film on the film thickness and substrate temperature.
2. Experimental details Rectangular films of CdTe (37 mm 25 mm) were grown from Blazers evaporation grade CdTe (99.999%) using a molybdenum boat, under a vacuum of 2 105 Torr on glass substrates. Thicknesses of the films were measured by making use of Tolansky’s technique (Fizeau fringes). Structure of the films was studied using an X-ray diffractometer (Phillips/pw1010, pw1051). Copper Ka/Cobalt Ka radiation was employed in these experiments and a 2y spectrum from 01 to 801 was recorded for all the films. The composition of the film was studied using energy dispersive analysis by X-ray (EDAX). Optical transmission studies were made on the film grown on glass substrates using spectrophotometer (Pye Unicom UV–VIS 8800).
3. Results and discussion Films grown on glass substrates at different substrate temperature were used for structural and optical studies. 3.1. Structural properties X-ray diffraction (XRD) studies were made on CdTe thin films deposited on glass substrates to determine their structure and to identify the components and phases. Diffraction spectra for various films and for bulk powder are shown in Fig. 1. The spectrum of CdTe powder exhibits sharp peaks at 2y equal to 23.71, 39.31 and 46.61 which corresponds to diffraction from (1 1 1), (2 2 0) and (3 1 1) planes of the cubic phase, respectively. Both the peak height and peak position are in good agreement with ASTM data (16–770) for cubic CdTe, whereas the as-grown films (Ts ¼ 373 K) showed preferential growth of film crystallites corresponding to textured (1 1 1) and (2 2 0) growth. The (1 1 1) direction is the close packing direction of the zinc blende structure and this type of ordering is often observed in polycrystalline films [2,3] grown on heated amorphous substrates. A weak peak at 27.81 along with pronounced peaks have been observed for the film coated at low substrate temperature (Ts ¼ 373 K). This suggests the excess of metallic Te [4] in the as-grown CdTe films. On the other hand the presence of peaks only at 23.81 39.31, 46.61 have been observed with higher substrate temperature (Ts ¼ 573 K, Fig. 2).
R. Sathyamoorthy et al. / Solar Energy Materials & Solar Cells 76 (2003) 339–346
341
Fig. 1. XRD patterns of (a) CdTe bulk sample, and (b) CdTe film deposited at Ts ¼ 373 K.
Fig. 2. XRD patterns of the CdTe film coated at Ts ¼ 573 K.
The absence of a peak at 27.81 for films deposited at Ts ¼ 573 K is due to the reevaporation of metallic Te leading to the formation of stoichiometric CdTe films. A similar observation has been made by Makoto Takahashi et al. [4] for CdTe films.
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342
Table 1 Effect of substrate temperature on structural parameters Sample
2y
( Calculated (d) A
Probable hkl
( a¼b¼cA
CdTe bulk powder (Cu Ka)
23.700 39.100 46.600 56.800
3.749 2.309 1.949 1.629
111 220 311 400
6.494 6.531 6.464 6.484
CdTe film coated at 373 K (Cu Ka)
23.700 39.100
3.749 2.309
111 220
6.494 6.531
CdTe film coated at 573 K (Co Ka)
27.523 45.925 51.517 76.628
3.762 2.294 2.060 1.444
111 220 311 331
6.516 6.488 6.832 6.294
Table 1 Effect of substrate temperature on structural parameters
Crystallite sizes were determined from the XRD data using the Scherer formula [5] l ¼ lDcosy; where l is the grain size, l is the wave length of the X-rays and D is the width of the peak at full-width half-maximum. The value of l for the as-grown film was found to ( The calculated d spacing and lattice parameter along with the probable hkl be 450 A. values were presented in Table 1. ( is in good agreement with the ASTM data for The calculated value of a0 (6.48 A) cubic CdTe. 3.2. Composition analysis The EDAX studies were made on the CdTe films to determine their composition. An energy dispersive system was employed in the present studies in order to determine the various elements present in the films and their concentration and only Cd and Te were found to be present. It is seen that films coated at room temperature contains (Cd: 46.6%, Te: 54.4%) excess Te whereas films coated at lower substrate temperature (Ts ¼ 373 K) contain (Cd: 48%, Te: 52%) a small excess of Te which decreases with the increase of substrate temperature ðTs ¼ 573 KÞ (Cd: 50%, Te: 50%). 3.3. Optical properties Transmission characteristics of CdTe films have been given in Fig 3. The figure clearly indicates that the transmission is found to decrease with increasing film thickness. The transmittance falls steeply with decreasing wavelength. This reveals that the CdTe films are having considerable absorption throughout the wavelength region (400–800 nm) studied. This is in good agreement with the earlier investigation [6].
R. Sathyamoorthy et al. / Solar Energy Materials & Solar Cells 76 (2003) 339–346
343
Fig. 3. Transmittance curves of CdTe films.
The extinction coefficient ðkf Þ and refractive index ðnf Þ were evaluated in the wavelength range 400–800 nm using the relation kf ¼ 2:303llogð1WT0 Þ=4pt; where l is the wavelength, T0 is I=I0 ; and I is intensity of light. Fig. 4 shows the variation of kf and nf with wavelength for CdTe films. An inconsistency has been observed in the dependence of extinction coefficient on wavelength and thickness. The refractive indices were calculated using Manifacier’s formula [7] by the iterative method: T0 ¼
16na ng n2 expðatÞ ; R21 þ R22 expð2atÞ þ 2R1 R2 expðatÞcosð4pnt=lÞ
where R1 =ðn þ na Þðng þ 1Þ; R2 =ðn na Þðng 1Þ are the absorption coefficients, t is the thickness of the film, n; na ; ng are the refractive indices of the film, air and substrate, respectively. The d refractive index increases with increase of incident photon energy and thickness for CdTe film. This prevailing trend is in good agreement with the earlier investigations for CdTe films [8]. The optical band gap energy for CdTe film has been evaluated from the a2 vs. hn plot (Fig. 5). Two possible direct transitions have been observed for all the CdTe films in the visible region. The result is that there were two direct optical band gap energies for a single film. The optical band gap energy results observed for CdTe films (Ts ¼ 573 K) of various thicknesses are given in Table 2.
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Fig. 4. Dependence of nf and kf on wavelength for the CdTe film.
( Fig. 5. Variation of a2 with hg for CdTe film coated at 473 K (t ¼ 2460 A).
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345
Table 2 The optical band gap energies for CdTe films deposited at Ts ¼ 573 K ( Film thickness (A)
1090 2460 4270
Band gap energy (eV) Eg1
Eg2
1.75 1.65 1.45
2.18 2.03 1.78
Table 2 The optical band gap energies for CdTe films deposited at Ts ¼ 573 K Table 3 The optical band gap energies for CdTe films deposited at Ts ¼ 473 K ( Film thickness (A)
770 1060 1610 2190
Band gap energy(eV) Eg1
Eg2
1.5 1.55 1.54 1.58
1.87 2.3 2.22 2.3
Table 3 The optical band gap energies for CdTe films deposited at Ts ¼ 473 K Table 4 The optical band gap energies for CdTe films deposited at Ts ¼ 373 K ( Film thickness (A)
1450 2460 3690
Band gap energy (eV) Eg1
Eg2
1.58 1.56 1.50
2.13 1.95 2.07
Table 4 The optical band gap energies for CdTe films deposited at Ts ¼ 373 K
The observed (two direct) allowed transitions may be attributed to the spin orbit splitting of the valance band. Similar observations have been made by Thutupalli et al. [9] for CdTe films. The observed two optical band gap energies (Eg1 and Eg2 ) decrease with increase in film thickness. The decrease in optical band gap energy with increase in film thickness is due to the increased grain size of the higher thickness films. The ad atom mobility also increases as the substrate temperature increases, which also results in the crystalline size and crystallinity of the films. This is in good agreement with the observations made by several other workers. In order to make a detailed study on the effect of substrate temperature on the optical band gap energy for CdTe, films of varying thicknesses have been prepared at different substrate temperatures (373, 473 and 573 K). It is observed that CdTe films deposited at 573 K only possess the right stoichiometry (Cd=50% and Te=50%) and the optical band gap energy decreases with increase in film thickness. The film deposited at other
346
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substrate temperatures contain varying composition and the optical band gap energy which: (i) increases with increase in film thickness in the case of films deposited at 473 K (Table 3) and (ii) varies erratically with film thickness in case of films deposited at 373 K (Table 4).
4. Conclusion As-grown films of CdTe were polycrystalline with the small grain size. Films deposited at higher substrate temperature improved the grain size as well as mobility of the carriers. The optical transition in CdTe films was found to be allowed and direct. The effect of the substrate temperature and thickness on the optical band gap energy has also been reported.
References [1] [2] [3] [4] [5] [6] [7] [8] [9]
G. Entine, M.R. Sqcillante, H.B. Serreze, E. Clarke, IEEE Trans. Nucl. Sci. NS-28 (1981) 558. S. Saha, U. Pal, B.K. Samantaray, A.K. Chaudhuri, Thin Solid Films 164 (1988) 85. R.D. Gould, C.J. Bowler, Thin Solid Films 164 (1988) 281. Makato Takahashi, Kohei Vasaki, Hideaki Kila, Yoshi Kazu Suzuki, J. Appl. Phys. 58 (1985) 4292. L.I. Maissel, R. Glang, Hand Book of Thin Film Technology, McGraw-Hill, New York, 1980. J. Aranda, J.L. Morenza, J. Estere, J.M. Codia, Thin Solid Films 120 (1984) 23. J.C. Manifacier, J. Gasiot, J.P. Fillard, J. Phys. E 9 (1976) 1002. A.A. El-Shazly, H.T. El-Shavi, Thin Solid Films 78 (1981) 287. G.K.M. Thutupalli, S.G. Tomlin, J. Appl. Phys. D 9 (1976) 128.