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Effect of swift heavy ion irradiation on spray deposited CdX (X = S, Te) thin films
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 262 (2007) 209–214 www.elsevier.com/locate/nimb
Effect of swift heavy ion irradiation on spray deposited CdX (X = S,Te) thin films V.V. Ison a, A. Ranga Rao a, V. Dutta a
a,*
, D.K. Avasthi
b
Photovoltaic Laboratory, Centre for Energy Studies, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110 016, India b Inter University Accelerator Center, New Delhi 110 067, India Received 27 April 2007; received in revised form 4 June 2007 Available online 23 June 2007
Abstract Cadmium sulfide and cadmium telluride thin films are irradiated with high energy heavy ion beam to study the irradiation induced effects in these films. The polycrystalline thin film samples deposited by spray pyrolysis are irradiated with 60 MeV Oxygen ions using tandem Pelletron accelerator. The X-ray diffraction patterns exhibit a reduction in peak intensities in both CdS and CdTe films. The grain size decrease with fluence is observed for both CdS and CdTe films, with more decrease for CdTe films. The AFM results support this observation. The films show opposite trend in the variation of electrical resistivity with irradiation fluence. A decrease in resistivity is observed for CdS films due to an increase of carrier concentration arising by the creation of sulfur vacancies during the irradiation. The creation of sulfur vacancies is confirmed by XPS studies. The stoichiometric changes seen from XPS studies support this observation. An enhancement of grain boundary scattering due to the reduction of grain size leads to the increase of electrical resistivity for CdTe films. 2007 Elsevier B.V. All rights reserved. PACS: 61.10.Nz; 61.72.Lk; 61.72.Mm; 61.82.Fk Keywords: Spray pyrolysis; Polycrystalline thin films; Ion beam irradiation; CdS; CdTe
1. Introduction II–VI compound semiconductors are of extreme importance because of their applications in various optoelectronic and photovoltaic devices. Cadmium Telluride is used as an absorber layer in thin film solar cells and in X-ray and c-ray detectors [1]. CdS is found to be the best window layer for CdTe based solar cells. Structural, optical and electrical properties of these materials prepared using different techniques have been reported [2–15]. The high energy heavy ion irradiation is a useful tool in the controlled and localized modification of materials [16– 27]. The modifications in materials can be classified into three categories such as bulk effects, interface mixing and surface changes. The information regarding the basic pro*
Corresponding author. Tel.: +91 112659 1261; fax: +91 11686 2037. E-mail address: [email protected] (V. Dutta).
0168-583X/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.06.018
cesses involved in the modification of materials by swift heavy ion is contained in the resulting damage produced. When a heavy ion passes through a material it dissipates its energy in two different ways; nuclear stopping and electronic stopping. In nuclear stopping, the ions undergo elastic collisions with the atoms of the material and as a result of which atoms get displaced from their sites, creating vacancies. This process dominates at low energy of the ion beam (10–100 keV). In the higher energy region (10– 100 MeV), where the velocity of the heavy ion is comparable to the Bohr velocity of the valence electron, the incident ions make inelastic collisions with the atoms of the material and the atoms get excited or ionized. This process is called electronic stopping. This can create a cluster of point defects and can also lead to other phenomenon like crystallization, amorphization, phase transformation etc. In the energy range we have chosen for the heavy ion beam, the effects due to electronic stopping dominates over its nuclear counterpart.
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In this paper we report a comparative study on the effect of heavy ion irradiation on the structural, morphological and electrical properties of CdS and CdTe thin films deposited using spray pyrolysis. 2. Experimental Spray pyrolysis has been used for the thin film deposition, which involves atomization of the precursor solution and spray formation followed by the chemical reaction on a hot substrate. The precursor solution for CdS deposition is prepared by dissolving Cadmium Acetate (Cd(CH3COO)2) and Thiourea (NH2CSNH2) in double distilled water. A 0.06 M solution is used for spray deposition. For CdTe, the precursor solution is prepared by dissolving Cadmium Chloride (CdCl2,0.02 M) and Tellurium Dioxide (TeO2, 0.02 M) in a solution containing ammonia and double distilled water in the ratio 1:4. Hydrazine Hydrate (7.5 ml for 250 ml of the spray solution) is used as a reducing agent to the precursor solution to change the oxidation state of Tellurium ions from Te4+ to Te2 to favor the formation of CdTe. The pH of the solution is adjusted at 11.2 using HCl. Nitrogen is used as the carrier gas in both the depositions. The distance between the spray nozzle and the substrate is maintained at 17 cm. The substrate is kept at 350 C [3]. For obtaining a nearly oxide free film, CdTe deposition is done in a closed chamber. The deposition chamber is attached to a rotary vacuum pump. The chamber is evacuated partially before starting the deposition. After the evacuation, nitrogen gas is passed into the chamber for making the ambient nearly Oxygen free during deposition. The pumping followed by purging the chamber with nitrogen gas helps in getting Oxide free CdTe films. The samples (1 cm · 1 cm) are irradiated using 60 MeV O5+ ions with fluences 1 · 1012 and 1 · 1014 ions/cm2 using the 15 UD tandem Pelletron accelerator at Inter University Accelerator Centre, New Delhi. During irradiation the ion beam current is maintained constant around 2 particle nanoampere (pna). The ion beam is focused by an electro-magnetic scanner to a spot of about 1 mm diameter and then scanned over the whole area of the film. The beam energy is so chosen that no ions are implanted and the effects produced in the film are due to irradiation only. Both the unirradiated and irradiated films are characterized for studying the changes in structural, morphological, chemical and electrical properties. The structural characterization is done using a Philips XPERT-PRO (PW3040) glancing angle X-ray diffractometer with CuKa1 radiation ˚ as the X-ray source. Morphohaving wavelength 1.5406 A logical studies are done using atomic force microscopy (Multimode IIIa, Digital Instruments). The electrical measurements of the samples are carried out by measuring the resistance using a Keithley electrometer. X-ray photoelectron spectroscopy using MgKa (Model ESCA-750, Shimadzu) is used for chemical composition analysis. Talystep (Taylor–Hobson, UK) is used for thickness measurement.
3. Results and discussion 3.1. Structural studies The effect on the polycrystallinity of the unirradiated and irradiated films is analyzed using glancing angle X-ray diffraction. Fig. 1 shows the X-ray diffractogram of the unirradiated and irradiated CdS films. CdS is deposited in hexagonal phase preferably oriented along the (0 0 2) plane. It can be seen that the intensity of the peaks is decreasing with irradiation fluence. The decrease of peak intensity is due to the reduction in crystallinity of the material. We also observed a slight decrease of lattice constant ˚ to 4.138 A ˚ after irradiation at the highest flufrom 4.154 A 14 2 ˚ ence 1 · 10 ions/cm . The c value changes from 6.735 A ˚ to 6.769 A. X-ray diffractogram of the unirradiated and irradiated CdTe films is shown in Fig. 2. CdTe is deposited in cubic zincblende phase with preferred orientation along the (1 1 1) direction. Here also the peak intensities are decreas-
Fig. 1. X-ray diffractograms of unirradiated and irradiated CdS films.
V.V. Ison et al. / Nucl. Instr. and Meth. in Phys. Res. B 262 (2007) 209–214
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square fit of these points to h = p/2 [28]. From the figure it is clear that the lattice parameter for CdTe remains con˚ ) even after irradiation. The Dd/d versus Nelstant (6.478 A son–Reily function curves drawn indicate the variation of strain along different planes due to irradiation for CdS (Fig. 4) and CdTe films (Fig. 5) [29]. The d values are compared with the JCPDS data values [30]. Polycrystalline films usually show non-uniform stress in them. For the unirradiated and the sample irradiated at fluence 1 · 1012 ions/cm2 of CdS, the stress present is compressive. A decrease of stress is observed for the sample irradiated at highest fluence, which is the origin of the slight change of lattice parameters for this film. CdTe samples have very small stress and there is an insignificant change after irradiation. The average grain size of the unirradiated and irradiated CdS and CdTe samples are calculated using the
Fig. 2. X-ray diffractograms of unirradiated and irradiated CdTe films.
ing with irradiation fluence. The lattice parameter calculated from each diffraction peak for CdTe films is plotted against Nelson–Reily function (NRF) ð12 ½ðcos2 h= sin hÞþ ðcos2 h=hÞÞ as shown in Fig. 3. The exact lattice constant for the samples is calculated by extrapolating the least
Fig. 4. Dd/d versus NRF curves for the unirradiated and irradiated CdS films.
Fig. 3. Variation of lattice constant with fluence for CdTe films.
Fig. 5. Dd/d versus NRF curves for the unirradiated and irradiated CdTe films.
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Fig. 8. AFM images of CdTe films (a) unirradiated (b) irradiated with 1e14 fluence.
Fig. 6. Variation of the grain size of CdS and CdTe films with ion fluence.
Debye–Scherrer formula D = (0.9k)/(bcos h), where D is the diameter of the crystallites forming the films, k is the wavelength of the CuKa1 line, b is the full width at half maximum (FWHM) in radians and h is the Bragg angle. For CdS as well as CdTe, the grain size is decreasing with ion fluence (Fig. 6), with the reduction in the grain size more for higher ion fluence. The variation of grain size in CdTe films is larger compared to that in CdS. 3.2. Surface morphology The surface topographies of the unirradiated and irradiated CdS, CdTe films are analyzed by atomic force microscopy. The AFM images of the unirradiated and the irradiated CdS sample at fluence 1 · 1014 ions/cm2 are shown in Fig. 7. The reduction of particle size and the surface roughness are evident from the topography. An ˚ and 90 A ˚ , respectively, are average particle size of 100 A obtained for the unirradiated sample and for the sample irradiated with fluence 1 · 1014 ions/cm2. Fig. 8 shows the AFM images of the unirradiated sample and CdTe sample irradiated at fluence 1 · 1014 ions/cm2. These pictures also
Fig. 7. AFM images of CdS films (a) unirradiated (b) irradiated with 1e14 fluence.
show the reduction of particle size and surface roughness. The average particle size of the samples obtained from ˚ and 200 A ˚ , respectively. The the AFM pictures are 320 A morphological data supports the results of the X-ray diffraction analysis that show the decrease in the average grain size. 3.3. Electrical and XPS studies Fig. 9 gives the variation of electrical resistivity with fluence for CdS and CdTe films. The electrical resistivity of the CdS films decreases considerably with ion fluence. The grain size reduction observed in CdS films with fluence is small and therefore the decrease of resistivity would be mainly due to the increase in carrier concentration [26] which can occur by the way of the creation of Sulfur vacancies and due to the creation of defects that arises due to the electronic stopping. This increase of carrier concentration due to the creation of Sulfur vacancies is supported by the XPS studies. The XPS spectra of the unirradiated and the sample irradiated at fluence 1 · 1014 ions/cm2 of CdS is given in Fig. 10. The Cd to S ratio is increasing from
Fig. 9. Variation of the electrical resistivity of CdS and CdTe films with ion fluence.
V.V. Ison et al. / Nucl. Instr. and Meth. in Phys. Res. B 262 (2007) 209–214
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Fig. 10. XPS spectra of CdS films (a) unirradiated (b) irradiated with 1e14 fluence.
size with fluence from XRD study proves the reduction of crystallinity. A slight decrease of lattice parameter is observed in the case of CdS films with fluence, whereas it remains constant for CdTe films. An opposite variation of electrical resistivity is observed; with irradiation fluence, the electrical resistivity of CdS films is decreased and that of CdTe films is increased. Due to the creation of Sulfur vacancies and defects, there is an increase in carrier concentration in CdS films. The increase of grain boundary scattering leads to an increase of electrical resistivity for CdTe films. Acknowledgements
Fig. 11. Variation of composition ratio of CdS and CdTe films with ion fluence.
1.04 for unirradiated sample to 1.38 for sample irradiated at the highest fluence (Fig. 11). For CdTe films, the electrical resistivity is increasing with fluence. Since in these films there is a reduction of ˚ to 239 A ˚ , the increase of resistivity grain size from 299 A can be due to the enhanced grain boundary scattering [26]. The XPS analysis shows that the ratio Cd/Te remains nearly constant with ion fluence, which means that there is no significant change in the carrier concentration. 4. Conclusions CdS and CdTe polycrystalline thin films prepared by spray pyrolysis are irradiated with 60 MeV Oxygen ion beam at varying levels of fluence to study the irradiation induced changes. The decrease of peak intensities and grain
The help from Mr. D.C. Agarwal, R.B.S. College, Agra during beam-time is greatly appreciated. The authors thank Dr. D. Sakthi Kumar, Department of Applied Chemistry, Toyo University, Japan for his help in XPS studies. The authors wish to acknowledge Dr. Ambuj Tripathi and Ms. Indira Sulania for their support to carryout the AFM measurements. References [1] K.L. Chopra, P.D. Paulson, V. Dutta, Prog. Photovolt.: Res. Appl. 12 (2004) 69. [2] K. Vamsi Krishna, V. Dutta, Sol. Energy Mater. Sol. Cells 80 (2003) 247. [3] K. Vamsi Krishna, V. Dutta, P.D. Paulson, Thin solid films 444 (2003) 17. [4] S. Kumar, S.K. Sharma, T.P. Sharma, M. Husain, J. Phys. Chem. Solids 61 (2000) 1809. [5] K. Vamsi Krishna, V. Dutta, K.S.R. Koteswara Rao, Phys. Stat. Sol. (a) 198 (2003) 443. [6] E. Saucedo, V. Corregidor, L. Fornaro, A. Cuna, E. Dieguez, Thin Solid Films 471 (2005) 304. [7] J. Fritsche et al., Thin Solid Films 387 (2001) 161. [8] G. Khrypunov, A. Romeo, F. Kurdesau, D.L. Batzner, H. Zogg, A.N. Tiwari, Sol. Energy Mater. Sol. Cells 90 (2006) 664.
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[9] T.M. Razykov, S.Zh. Karazhanov, A.Yu. Leiderman, N.F. Khusainova, K. Kouchkarov, Sol. Energy Mater. Sol. Cells 90 (2006) 2255. [10] S. Lalitha, S.Zh. Karazhanov, P. Ravindran, S. Senthilarasu, R. Sathyamoorthy, J. Janabergenov, Physica B 387 (2007) 227. [11] D. Bhattacharyya, M.J. Carter, Thin Solid Films 288 (1996) 176. [12] B. Su, K.L. Choy, Thin Solid Films 359 (2000) 160. [13] M.E. Hernandez-Torres, R. Silva-Gonzalez, Sol. Energy Mater. Sol. Cells 90 (2006) 2241. [14] Jaehyeong Lee, Appl. Surf. Sci. 252 (2005) 1398. [15] Deokjoon Cha, Sunmi Kim, N.K. Huang, Mater. Sci. Eng. B 106 (2004) 63. [16] D.K. Avasthi, Nucl. Instr. and Meth. B 136–138 (1998) 729. [17] D.K. Avasthi, Vacuum 48 (1997) 1011. [18] J.P. Singh et al., Nucl. Instr. and Meth. B 156 (1999) 206. [19] G.K. Mehta, Nucl. Instr. and Meth. A 382 (1996) 335. [20] D. Kanjilal, Current Sci. 80 (12) (2001) 560. [21] O.P. Sinha, T. Shripathi, N.P. Lalla, P.C. Srivastava, Appl. Surf. Sci. 230 (2004) 222.
[22] W. Wesch, A. Kamarou, E. Wendler, Nucl. Instr. and Meth. B 225 (2004) 111. [23] A. Benyagoub, F. Couvreur, S. Bouffard, F. Levesque, C. Dufour, E. Paumier, Nucl. Instr. and Meth. B 175 (2001) 417. [24] M.S. Kamboj, R. Thangaraj, D.K. Avasthi, D. Kabiraj, Nucl. Instr. and Meth. B 211 (2003) 369. [25] M.S. Kamboj, G. Kaur, R. Thangaraj, D.K. Avasthi, J. Phys. D: Appl. Phys. 35 (2002) 477. [26] P.M. Ratheesh Kumar, C. Sudha Kartha, K.P. Vijayakumar, F. Singh, D.K. Avasthi, T. Abe, Y. Kashiwabe, G.S. Okram, M. Kumar, Sarvesh Kumar, J. Appl. Phys. 97 (2005) 013509. [27] P.C. Srivastava, S.P. Pandey, O.P. Sinha, D.K. Avasthi, K. Asokan, Nucl. Instr. and Meth. B 156 (1999) 105. [28] S. Lalitha, R. Sathyamoorthy, S. Senthilarasu, A. Subharayan, K. Natarajan, Sol. Energy Mater. Sol. Cells 82 (2004) 187. [29] P. Babu Dayal, B.R. Mehta, Y. Aparna, S.M. Sivaprasad, Appl. Phys. Lett. 81 (22) (2002) 4254. [30] JCPDS XRD data for cubic CdTe, file no. 15-770.
Nuclear Instruments and Methods in Physics Research B 262 (2007) 209–214 www.elsevier.com/locate/nimb
Effect of swift heavy ion irradiation on spray deposited CdX (X = S,Te) thin films V.V. Ison a, A. Ranga Rao a, V. Dutta a
a,*
, D.K. Avasthi
b
Photovoltaic Laboratory, Centre for Energy Studies, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110 016, India b Inter University Accelerator Center, New Delhi 110 067, India Received 27 April 2007; received in revised form 4 June 2007 Available online 23 June 2007
Abstract Cadmium sulfide and cadmium telluride thin films are irradiated with high energy heavy ion beam to study the irradiation induced effects in these films. The polycrystalline thin film samples deposited by spray pyrolysis are irradiated with 60 MeV Oxygen ions using tandem Pelletron accelerator. The X-ray diffraction patterns exhibit a reduction in peak intensities in both CdS and CdTe films. The grain size decrease with fluence is observed for both CdS and CdTe films, with more decrease for CdTe films. The AFM results support this observation. The films show opposite trend in the variation of electrical resistivity with irradiation fluence. A decrease in resistivity is observed for CdS films due to an increase of carrier concentration arising by the creation of sulfur vacancies during the irradiation. The creation of sulfur vacancies is confirmed by XPS studies. The stoichiometric changes seen from XPS studies support this observation. An enhancement of grain boundary scattering due to the reduction of grain size leads to the increase of electrical resistivity for CdTe films. 2007 Elsevier B.V. All rights reserved. PACS: 61.10.Nz; 61.72.Lk; 61.72.Mm; 61.82.Fk Keywords: Spray pyrolysis; Polycrystalline thin films; Ion beam irradiation; CdS; CdTe
1. Introduction II–VI compound semiconductors are of extreme importance because of their applications in various optoelectronic and photovoltaic devices. Cadmium Telluride is used as an absorber layer in thin film solar cells and in X-ray and c-ray detectors [1]. CdS is found to be the best window layer for CdTe based solar cells. Structural, optical and electrical properties of these materials prepared using different techniques have been reported [2–15]. The high energy heavy ion irradiation is a useful tool in the controlled and localized modification of materials [16– 27]. The modifications in materials can be classified into three categories such as bulk effects, interface mixing and surface changes. The information regarding the basic pro*
Corresponding author. Tel.: +91 112659 1261; fax: +91 11686 2037. E-mail address: [email protected] (V. Dutta).
0168-583X/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.06.018
cesses involved in the modification of materials by swift heavy ion is contained in the resulting damage produced. When a heavy ion passes through a material it dissipates its energy in two different ways; nuclear stopping and electronic stopping. In nuclear stopping, the ions undergo elastic collisions with the atoms of the material and as a result of which atoms get displaced from their sites, creating vacancies. This process dominates at low energy of the ion beam (10–100 keV). In the higher energy region (10– 100 MeV), where the velocity of the heavy ion is comparable to the Bohr velocity of the valence electron, the incident ions make inelastic collisions with the atoms of the material and the atoms get excited or ionized. This process is called electronic stopping. This can create a cluster of point defects and can also lead to other phenomenon like crystallization, amorphization, phase transformation etc. In the energy range we have chosen for the heavy ion beam, the effects due to electronic stopping dominates over its nuclear counterpart.
210
V.V. Ison et al. / Nucl. Instr. and Meth. in Phys. Res. B 262 (2007) 209–214
In this paper we report a comparative study on the effect of heavy ion irradiation on the structural, morphological and electrical properties of CdS and CdTe thin films deposited using spray pyrolysis. 2. Experimental Spray pyrolysis has been used for the thin film deposition, which involves atomization of the precursor solution and spray formation followed by the chemical reaction on a hot substrate. The precursor solution for CdS deposition is prepared by dissolving Cadmium Acetate (Cd(CH3COO)2) and Thiourea (NH2CSNH2) in double distilled water. A 0.06 M solution is used for spray deposition. For CdTe, the precursor solution is prepared by dissolving Cadmium Chloride (CdCl2,0.02 M) and Tellurium Dioxide (TeO2, 0.02 M) in a solution containing ammonia and double distilled water in the ratio 1:4. Hydrazine Hydrate (7.5 ml for 250 ml of the spray solution) is used as a reducing agent to the precursor solution to change the oxidation state of Tellurium ions from Te4+ to Te2 to favor the formation of CdTe. The pH of the solution is adjusted at 11.2 using HCl. Nitrogen is used as the carrier gas in both the depositions. The distance between the spray nozzle and the substrate is maintained at 17 cm. The substrate is kept at 350 C [3]. For obtaining a nearly oxide free film, CdTe deposition is done in a closed chamber. The deposition chamber is attached to a rotary vacuum pump. The chamber is evacuated partially before starting the deposition. After the evacuation, nitrogen gas is passed into the chamber for making the ambient nearly Oxygen free during deposition. The pumping followed by purging the chamber with nitrogen gas helps in getting Oxide free CdTe films. The samples (1 cm · 1 cm) are irradiated using 60 MeV O5+ ions with fluences 1 · 1012 and 1 · 1014 ions/cm2 using the 15 UD tandem Pelletron accelerator at Inter University Accelerator Centre, New Delhi. During irradiation the ion beam current is maintained constant around 2 particle nanoampere (pna). The ion beam is focused by an electro-magnetic scanner to a spot of about 1 mm diameter and then scanned over the whole area of the film. The beam energy is so chosen that no ions are implanted and the effects produced in the film are due to irradiation only. Both the unirradiated and irradiated films are characterized for studying the changes in structural, morphological, chemical and electrical properties. The structural characterization is done using a Philips XPERT-PRO (PW3040) glancing angle X-ray diffractometer with CuKa1 radiation ˚ as the X-ray source. Morphohaving wavelength 1.5406 A logical studies are done using atomic force microscopy (Multimode IIIa, Digital Instruments). The electrical measurements of the samples are carried out by measuring the resistance using a Keithley electrometer. X-ray photoelectron spectroscopy using MgKa (Model ESCA-750, Shimadzu) is used for chemical composition analysis. Talystep (Taylor–Hobson, UK) is used for thickness measurement.
3. Results and discussion 3.1. Structural studies The effect on the polycrystallinity of the unirradiated and irradiated films is analyzed using glancing angle X-ray diffraction. Fig. 1 shows the X-ray diffractogram of the unirradiated and irradiated CdS films. CdS is deposited in hexagonal phase preferably oriented along the (0 0 2) plane. It can be seen that the intensity of the peaks is decreasing with irradiation fluence. The decrease of peak intensity is due to the reduction in crystallinity of the material. We also observed a slight decrease of lattice constant ˚ to 4.138 A ˚ after irradiation at the highest flufrom 4.154 A 14 2 ˚ ence 1 · 10 ions/cm . The c value changes from 6.735 A ˚ to 6.769 A. X-ray diffractogram of the unirradiated and irradiated CdTe films is shown in Fig. 2. CdTe is deposited in cubic zincblende phase with preferred orientation along the (1 1 1) direction. Here also the peak intensities are decreas-
Fig. 1. X-ray diffractograms of unirradiated and irradiated CdS films.
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square fit of these points to h = p/2 [28]. From the figure it is clear that the lattice parameter for CdTe remains con˚ ) even after irradiation. The Dd/d versus Nelstant (6.478 A son–Reily function curves drawn indicate the variation of strain along different planes due to irradiation for CdS (Fig. 4) and CdTe films (Fig. 5) [29]. The d values are compared with the JCPDS data values [30]. Polycrystalline films usually show non-uniform stress in them. For the unirradiated and the sample irradiated at fluence 1 · 1012 ions/cm2 of CdS, the stress present is compressive. A decrease of stress is observed for the sample irradiated at highest fluence, which is the origin of the slight change of lattice parameters for this film. CdTe samples have very small stress and there is an insignificant change after irradiation. The average grain size of the unirradiated and irradiated CdS and CdTe samples are calculated using the
Fig. 2. X-ray diffractograms of unirradiated and irradiated CdTe films.
ing with irradiation fluence. The lattice parameter calculated from each diffraction peak for CdTe films is plotted against Nelson–Reily function (NRF) ð12 ½ðcos2 h= sin hÞþ ðcos2 h=hÞÞ as shown in Fig. 3. The exact lattice constant for the samples is calculated by extrapolating the least
Fig. 4. Dd/d versus NRF curves for the unirradiated and irradiated CdS films.
Fig. 3. Variation of lattice constant with fluence for CdTe films.
Fig. 5. Dd/d versus NRF curves for the unirradiated and irradiated CdTe films.
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Fig. 8. AFM images of CdTe films (a) unirradiated (b) irradiated with 1e14 fluence.
Fig. 6. Variation of the grain size of CdS and CdTe films with ion fluence.
Debye–Scherrer formula D = (0.9k)/(bcos h), where D is the diameter of the crystallites forming the films, k is the wavelength of the CuKa1 line, b is the full width at half maximum (FWHM) in radians and h is the Bragg angle. For CdS as well as CdTe, the grain size is decreasing with ion fluence (Fig. 6), with the reduction in the grain size more for higher ion fluence. The variation of grain size in CdTe films is larger compared to that in CdS. 3.2. Surface morphology The surface topographies of the unirradiated and irradiated CdS, CdTe films are analyzed by atomic force microscopy. The AFM images of the unirradiated and the irradiated CdS sample at fluence 1 · 1014 ions/cm2 are shown in Fig. 7. The reduction of particle size and the surface roughness are evident from the topography. An ˚ and 90 A ˚ , respectively, are average particle size of 100 A obtained for the unirradiated sample and for the sample irradiated with fluence 1 · 1014 ions/cm2. Fig. 8 shows the AFM images of the unirradiated sample and CdTe sample irradiated at fluence 1 · 1014 ions/cm2. These pictures also
Fig. 7. AFM images of CdS films (a) unirradiated (b) irradiated with 1e14 fluence.
show the reduction of particle size and surface roughness. The average particle size of the samples obtained from ˚ and 200 A ˚ , respectively. The the AFM pictures are 320 A morphological data supports the results of the X-ray diffraction analysis that show the decrease in the average grain size. 3.3. Electrical and XPS studies Fig. 9 gives the variation of electrical resistivity with fluence for CdS and CdTe films. The electrical resistivity of the CdS films decreases considerably with ion fluence. The grain size reduction observed in CdS films with fluence is small and therefore the decrease of resistivity would be mainly due to the increase in carrier concentration [26] which can occur by the way of the creation of Sulfur vacancies and due to the creation of defects that arises due to the electronic stopping. This increase of carrier concentration due to the creation of Sulfur vacancies is supported by the XPS studies. The XPS spectra of the unirradiated and the sample irradiated at fluence 1 · 1014 ions/cm2 of CdS is given in Fig. 10. The Cd to S ratio is increasing from
Fig. 9. Variation of the electrical resistivity of CdS and CdTe films with ion fluence.
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Fig. 10. XPS spectra of CdS films (a) unirradiated (b) irradiated with 1e14 fluence.
size with fluence from XRD study proves the reduction of crystallinity. A slight decrease of lattice parameter is observed in the case of CdS films with fluence, whereas it remains constant for CdTe films. An opposite variation of electrical resistivity is observed; with irradiation fluence, the electrical resistivity of CdS films is decreased and that of CdTe films is increased. Due to the creation of Sulfur vacancies and defects, there is an increase in carrier concentration in CdS films. The increase of grain boundary scattering leads to an increase of electrical resistivity for CdTe films. Acknowledgements
Fig. 11. Variation of composition ratio of CdS and CdTe films with ion fluence.
1.04 for unirradiated sample to 1.38 for sample irradiated at the highest fluence (Fig. 11). For CdTe films, the electrical resistivity is increasing with fluence. Since in these films there is a reduction of ˚ to 239 A ˚ , the increase of resistivity grain size from 299 A can be due to the enhanced grain boundary scattering [26]. The XPS analysis shows that the ratio Cd/Te remains nearly constant with ion fluence, which means that there is no significant change in the carrier concentration. 4. Conclusions CdS and CdTe polycrystalline thin films prepared by spray pyrolysis are irradiated with 60 MeV Oxygen ion beam at varying levels of fluence to study the irradiation induced changes. The decrease of peak intensities and grain
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