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The influence of the laser fluence on the transmission features of thin CdS films formed by laser ablation
Microelectronic Engineering 43–44 (1998) 695–700
The influence of the laser fluence on the transmission features of thin CdS films formed by laser ablation a, b e c d a B. Ullrich *, H. Sakai , N.M. Dushkina , H. Ezumi , S. Keitoku , T. Kobayashi a
Department of Physics, University of Tokyo, 7 -3 -1 Hongo, Bunkyo-ku, Tokyo 113, Japan Department of Electric Engineering, Hiroshima-Denki Institute of Technology, Hiroshima 739 -03, Japan c Department of Fundamental Education, Shimane College of Nursing, 151 Nishihayashigi-cho, Izumo, Shimane 693, Japan d Hiroshima Women’ s University, Hiroshima 734, Japan e Central Laboratory of Optical Storage and Processing of Information, Bulgarian Academy of Science, P.O. Box 95, Sofia 1113, Bulgaria b
Abstract Transmission threshold and steepness of the transmission edge of thin CdS films formed by laser ablation with different laser fluences (2, 4 and 5 J cm 22 ) were studied at 300 K. The threshold close to 514.5 nm is shifted to shorter wavelengths and the edge becomes steeper with increasing laser fluence. We show that the modification of the transmission features underlies the turn of the c-axis of the CdS films from a perpendicular to the surface oriented direction at 2 J cm 22 to a parallel orientation at 5 J cm 22 . 1998 Elsevier Science B.V. All rights reserved. Keywords: Laser ablation; Transmission of thin CdS films; Orientation of thin CdS films
1. Introduction Optically smooth thin CdS films are easily achieved by vacuum evaporation. This method, however, strongly influences the stoichiometry, producing donors, e.g., S vacancies, which lower the light sensitivity of the film by the increase of dark conductivity [1]. Beyond that, also the luminescence features of the CdS films suffer considerably under disturbed stoichiometry since the increase in the impurity concentration increases the nonradiative recombination rate reducing the radiation efficiency [2]. Without abandonment of optical smoothness, more intrinsic samples with improved optical features are expected by congruent evaporation using a UV laser beam for vaporizing the target material. Indeed, the use of pulsed lasers for vaporizing CdS has been proven to result in optically smooth thin films [3]. Furthermore, transmission and resistivity of the CdS films are tunable by pulse duration and energy density, respectively, of the laser beam [4]. Concerning *Corresponding author. Fax: 181-3-3814-9717; e-mail: [email protected] 0167-9317 / 98 / $19.00 Copyright 1998 Elsevier Science B.V. All rights reserved. PII: S0167-9317( 98 )00246-9
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B. Ullrich et al. / Microelectronic Engineering 43 – 44 (1998) 695 – 700
stoichiometry, it turned out that CdS films on substrates without lattice constant (glass) exhibit a balanced stoichiometry in contrast to films on substrates with the wrong lattice constant (quartz) [5]. In the current work, we demonstrate that transmission threshold and steepness of the transmission edge of the thin CdS films depend crucially on the energy density of the UV laser beam.
2. Experiment The samples were prepared by heating the target, which consisted of sintered pure (99.999%) CdS powder, by a Nd:YAG laser (355 nm) with a pulse width of 5 ns and a repetition rate of 10 Hz. Three samples were deposited on glass substrates using different laser fluences (2, 4 and 5 J cm 22 ). The distance between substrate and target was 3 cm. During the deposition processes the substrate temperature and the ambient pressure was kept at 2508C and 3 3 10 25 Torr, respectively. The optical experiments were carried out with the green line (514.5 nm, 10 mW) of an argon laser and a standard spectrophotometer. The orientation of the films was determined in the common manner by X-ray analysis. All experiments were carried out at 300 K.
3. Results and discussion Fig. 1 reveals the transmittance of the three investigated thin CdS films vs. wavelength. Since all spectra exhibit very well pronounced Fabry–Perot fringes, we conclude that the energy density of the laser does not influence the optical smoothness of the film surface. However, as shown in Figs. 2 and 3, both threshold of the effective transmittance near 514.5 nm and steepness of the transmission edge (dTr / dl) increase with the energy density of the excimer laser. On the other hand, the reflectance does not depend notably on the energy density. The above results imply either a drastic decrease of the donor concentration in the samples or an orientation change of the crystallographic plane. The former reason appears unlikely due to the former mentioned balanced stoichiometry [5]. Therefore, we have performed X-ray analysis of the samples. Fig. 4(a) shows the X-ray diffraction for the sample formed with 2 J cm 22 . The dominant 002 peak shows that the c-axis is perpendicular to the surface of the film. In Fig. 4(b,c), at 4 and 5 J cm 22 , respectively, the 002 peak almost vanishes and the (100), (101) and (110) peaks come up indicating the turn of the c-axis from perpendicular to parallel orientation. In order to interpret the increase of transmittance and dTr / dl, we bear in mind that CdS is a double refractive material. When a laser beam impinges CdS, the absorption depends on the orientation of the E vector of the laser beam with respect to the c-axis of the sample. At E parallel to c, the absorption coefficient becomes minimal and the transmittance increases. As a result, by turning the c-axis parallel to the surface, the amount of crystallites with E parallel to c increases and the whole thin CdS film appears more transparent and dTr / dl rises quickly below the gap. In the order to consider the different sample thicknesses and to prove the above argument quantitatively, we write down the basic equations for reflectance (Re) and transmittance (Tr) [2]: Re 5 Rh1 1 (1 2 R)2 exp(22as,p d)j
(1)
B. Ullrich et al. / Microelectronic Engineering 43 – 44 (1998) 695 – 700
697
Fig. 1. Transmittance vs. wavelength of the thin CdS films prepared with an energy density of: (a) 2 J cm 22 , (b) 4 J cm 22 and (c) 5 J cm 22 . From the distance of the fringes and the refractive index (52.2) of the film we found the thicknesses (a) 1.83 mm, (b) 1.07 mm and (c) 0.65 mm.
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B. Ullrich et al. / Microelectronic Engineering 43 – 44 (1998) 695 – 700
Fig. 2. Dependence of the transmittance and reflectance on the energy density at 514.5 nm. The figure shows the measured (Tr / exp and Re / exp) and calculated (Tr / theo and Re / theo) values. The lines are a guide for the eyes.
Tr 5 (1 2 R)2 exp(2as,p d)
(2)
where R (50.20) is the reflection coefficient, as,p is the absorption coefficient for the perpendicularly and parallel oriented material, respectively, and d is the thickness of the film. At 514.5 nm, ap 5 as / 800 [6]; by using this decrease in the absorption coefficient for the parallel oriented sample formed with 5 J cm 22 , and, straightforwardly, ap 5 as / 400 for the film formed with 4 J cm 22 , we found a reasonable agreement between Eqs. (1) and (2) and the measurements in Fig. 2. The reflectance is almost constant since at 514.5 nm the absorption of the thin film in both orientations is still so large that no considerable gain of the surface reflectance by interference with the substrate takes place. Sakai and coworkers [7] pointed out that in the ablated plume at energy densities $1 J cm 22 the
Fig. 3. First derivative of the transmittance with respect to the wavelength at 525 nm. The line is a guide for the eyes.
B. Ullrich et al. / Microelectronic Engineering 43 – 44 (1998) 695 – 700
699
Fig. 4. X-Ray patterns of the three thin CdS films formed with a fluence of: (a) 2 J cm 22 , (b) 4 J cm 22 and (c) 5 J cm 22 .
amount of fast particles (atoms and ions) decreases and that of slow particles (clusters and droplets) increases. Possibly, the clusters coming to the surface stick with the smoothest and largest area, i.e. the cleavage, on the substrate. Since the cleavage is parallel to the c-axis, the film then grows parallel
B. Ullrich et al. / Microelectronic Engineering 43 – 44 (1998) 695 – 700
700
to the surface. However, further experimental work is clearly needed to understand the mechanism, which turns the c-axis. 4. Conclusion For the first time, we have demonstrated that laser ablation enables the modification of the orientation of the c-axis of thin CdS films. As a consequence, the transmission properties are continuously tunable and the production of ‘‘custom-made’’ thin films by laser ablation is possible. Acknowledgements This work was performed under the management of Japan High Polymer Center. B.U. is an Industrial Technology Researcher employed by New Energy and Industrial Technology Development Organization. N.M.D. wants to thank the Ministry of Education, Science, Sports and Culture ¨ (Monbusho) for financial assistance. We wish to thank Dr. T. Loher for a number of helpful discussions. References [1] [2] [3] [4] [5] [6] [7]
C. Bouchenaki, B. Ullrich, J.P. Zielinger, et al., J. Opt. Soc. Am. B 8 (1991) 691. J. I. Pankove, Optical Processes in Semiconductors, Dover, New York, 1971. H.S. Kwok, J.P. Zheng, S. Witanachchi, et al., Appl. Phys. Lett. 52 (1988) 1095. H. Ezumi, S. Keitoku, Jpn. J. Appl. Phys. 32 (1993) L1783. B. Ullrich, H. Sakai, N.M. Dushkina, et al., Materials Science and Engineering B 47 (1997) 187. D. Dutton, Phys. Rev. 112 (1958) 785. H. Sakai, S. Keitoku, H. Ezumi, Jpn. J. Appl. Phys. 36 (1997) L409.
The influence of the laser fluence on the transmission features of thin CdS films formed by laser ablation a, b e c d a B. Ullrich *, H. Sakai , N.M. Dushkina , H. Ezumi , S. Keitoku , T. Kobayashi a
Department of Physics, University of Tokyo, 7 -3 -1 Hongo, Bunkyo-ku, Tokyo 113, Japan Department of Electric Engineering, Hiroshima-Denki Institute of Technology, Hiroshima 739 -03, Japan c Department of Fundamental Education, Shimane College of Nursing, 151 Nishihayashigi-cho, Izumo, Shimane 693, Japan d Hiroshima Women’ s University, Hiroshima 734, Japan e Central Laboratory of Optical Storage and Processing of Information, Bulgarian Academy of Science, P.O. Box 95, Sofia 1113, Bulgaria b
Abstract Transmission threshold and steepness of the transmission edge of thin CdS films formed by laser ablation with different laser fluences (2, 4 and 5 J cm 22 ) were studied at 300 K. The threshold close to 514.5 nm is shifted to shorter wavelengths and the edge becomes steeper with increasing laser fluence. We show that the modification of the transmission features underlies the turn of the c-axis of the CdS films from a perpendicular to the surface oriented direction at 2 J cm 22 to a parallel orientation at 5 J cm 22 . 1998 Elsevier Science B.V. All rights reserved. Keywords: Laser ablation; Transmission of thin CdS films; Orientation of thin CdS films
1. Introduction Optically smooth thin CdS films are easily achieved by vacuum evaporation. This method, however, strongly influences the stoichiometry, producing donors, e.g., S vacancies, which lower the light sensitivity of the film by the increase of dark conductivity [1]. Beyond that, also the luminescence features of the CdS films suffer considerably under disturbed stoichiometry since the increase in the impurity concentration increases the nonradiative recombination rate reducing the radiation efficiency [2]. Without abandonment of optical smoothness, more intrinsic samples with improved optical features are expected by congruent evaporation using a UV laser beam for vaporizing the target material. Indeed, the use of pulsed lasers for vaporizing CdS has been proven to result in optically smooth thin films [3]. Furthermore, transmission and resistivity of the CdS films are tunable by pulse duration and energy density, respectively, of the laser beam [4]. Concerning *Corresponding author. Fax: 181-3-3814-9717; e-mail: [email protected] 0167-9317 / 98 / $19.00 Copyright 1998 Elsevier Science B.V. All rights reserved. PII: S0167-9317( 98 )00246-9
696
B. Ullrich et al. / Microelectronic Engineering 43 – 44 (1998) 695 – 700
stoichiometry, it turned out that CdS films on substrates without lattice constant (glass) exhibit a balanced stoichiometry in contrast to films on substrates with the wrong lattice constant (quartz) [5]. In the current work, we demonstrate that transmission threshold and steepness of the transmission edge of the thin CdS films depend crucially on the energy density of the UV laser beam.
2. Experiment The samples were prepared by heating the target, which consisted of sintered pure (99.999%) CdS powder, by a Nd:YAG laser (355 nm) with a pulse width of 5 ns and a repetition rate of 10 Hz. Three samples were deposited on glass substrates using different laser fluences (2, 4 and 5 J cm 22 ). The distance between substrate and target was 3 cm. During the deposition processes the substrate temperature and the ambient pressure was kept at 2508C and 3 3 10 25 Torr, respectively. The optical experiments were carried out with the green line (514.5 nm, 10 mW) of an argon laser and a standard spectrophotometer. The orientation of the films was determined in the common manner by X-ray analysis. All experiments were carried out at 300 K.
3. Results and discussion Fig. 1 reveals the transmittance of the three investigated thin CdS films vs. wavelength. Since all spectra exhibit very well pronounced Fabry–Perot fringes, we conclude that the energy density of the laser does not influence the optical smoothness of the film surface. However, as shown in Figs. 2 and 3, both threshold of the effective transmittance near 514.5 nm and steepness of the transmission edge (dTr / dl) increase with the energy density of the excimer laser. On the other hand, the reflectance does not depend notably on the energy density. The above results imply either a drastic decrease of the donor concentration in the samples or an orientation change of the crystallographic plane. The former reason appears unlikely due to the former mentioned balanced stoichiometry [5]. Therefore, we have performed X-ray analysis of the samples. Fig. 4(a) shows the X-ray diffraction for the sample formed with 2 J cm 22 . The dominant 002 peak shows that the c-axis is perpendicular to the surface of the film. In Fig. 4(b,c), at 4 and 5 J cm 22 , respectively, the 002 peak almost vanishes and the (100), (101) and (110) peaks come up indicating the turn of the c-axis from perpendicular to parallel orientation. In order to interpret the increase of transmittance and dTr / dl, we bear in mind that CdS is a double refractive material. When a laser beam impinges CdS, the absorption depends on the orientation of the E vector of the laser beam with respect to the c-axis of the sample. At E parallel to c, the absorption coefficient becomes minimal and the transmittance increases. As a result, by turning the c-axis parallel to the surface, the amount of crystallites with E parallel to c increases and the whole thin CdS film appears more transparent and dTr / dl rises quickly below the gap. In the order to consider the different sample thicknesses and to prove the above argument quantitatively, we write down the basic equations for reflectance (Re) and transmittance (Tr) [2]: Re 5 Rh1 1 (1 2 R)2 exp(22as,p d)j
(1)
B. Ullrich et al. / Microelectronic Engineering 43 – 44 (1998) 695 – 700
697
Fig. 1. Transmittance vs. wavelength of the thin CdS films prepared with an energy density of: (a) 2 J cm 22 , (b) 4 J cm 22 and (c) 5 J cm 22 . From the distance of the fringes and the refractive index (52.2) of the film we found the thicknesses (a) 1.83 mm, (b) 1.07 mm and (c) 0.65 mm.
698
B. Ullrich et al. / Microelectronic Engineering 43 – 44 (1998) 695 – 700
Fig. 2. Dependence of the transmittance and reflectance on the energy density at 514.5 nm. The figure shows the measured (Tr / exp and Re / exp) and calculated (Tr / theo and Re / theo) values. The lines are a guide for the eyes.
Tr 5 (1 2 R)2 exp(2as,p d)
(2)
where R (50.20) is the reflection coefficient, as,p is the absorption coefficient for the perpendicularly and parallel oriented material, respectively, and d is the thickness of the film. At 514.5 nm, ap 5 as / 800 [6]; by using this decrease in the absorption coefficient for the parallel oriented sample formed with 5 J cm 22 , and, straightforwardly, ap 5 as / 400 for the film formed with 4 J cm 22 , we found a reasonable agreement between Eqs. (1) and (2) and the measurements in Fig. 2. The reflectance is almost constant since at 514.5 nm the absorption of the thin film in both orientations is still so large that no considerable gain of the surface reflectance by interference with the substrate takes place. Sakai and coworkers [7] pointed out that in the ablated plume at energy densities $1 J cm 22 the
Fig. 3. First derivative of the transmittance with respect to the wavelength at 525 nm. The line is a guide for the eyes.
B. Ullrich et al. / Microelectronic Engineering 43 – 44 (1998) 695 – 700
699
Fig. 4. X-Ray patterns of the three thin CdS films formed with a fluence of: (a) 2 J cm 22 , (b) 4 J cm 22 and (c) 5 J cm 22 .
amount of fast particles (atoms and ions) decreases and that of slow particles (clusters and droplets) increases. Possibly, the clusters coming to the surface stick with the smoothest and largest area, i.e. the cleavage, on the substrate. Since the cleavage is parallel to the c-axis, the film then grows parallel
B. Ullrich et al. / Microelectronic Engineering 43 – 44 (1998) 695 – 700
700
to the surface. However, further experimental work is clearly needed to understand the mechanism, which turns the c-axis. 4. Conclusion For the first time, we have demonstrated that laser ablation enables the modification of the orientation of the c-axis of thin CdS films. As a consequence, the transmission properties are continuously tunable and the production of ‘‘custom-made’’ thin films by laser ablation is possible. Acknowledgements This work was performed under the management of Japan High Polymer Center. B.U. is an Industrial Technology Researcher employed by New Energy and Industrial Technology Development Organization. N.M.D. wants to thank the Ministry of Education, Science, Sports and Culture ¨ (Monbusho) for financial assistance. We wish to thank Dr. T. Loher for a number of helpful discussions. References [1] [2] [3] [4] [5] [6] [7]
C. Bouchenaki, B. Ullrich, J.P. Zielinger, et al., J. Opt. Soc. Am. B 8 (1991) 691. J. I. Pankove, Optical Processes in Semiconductors, Dover, New York, 1971. H.S. Kwok, J.P. Zheng, S. Witanachchi, et al., Appl. Phys. Lett. 52 (1988) 1095. H. Ezumi, S. Keitoku, Jpn. J. Appl. Phys. 32 (1993) L1783. B. Ullrich, H. Sakai, N.M. Dushkina, et al., Materials Science and Engineering B 47 (1997) 187. D. Dutton, Phys. Rev. 112 (1958) 785. H. Sakai, S. Keitoku, H. Ezumi, Jpn. J. Appl. Phys. 36 (1997) L409.