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Effect of Si ion irradiation on polycrystalline CdS thin film grown from novel photochemical deposition technique
ARTICLE IN PRESS
Physica B 355 (2005) 222–230 www.elsevier.com/locate/physb
Effect of Si ion irradiation on polycrystalline CdS thin film grown from novel photochemical deposition technique S. Soundeswarana, O. Senthil Kumara, P. Ramasamya, D. Kabi Rajb, D.K. Avasthib, R. Dhanasekarana, a
Crystal Growth Centre, Anna University, Sardar Patel Road, Chennai 600 025, India b Nuclear Science Centre, New Delhi 110067, India
Received 4 August 2004; received in revised form 14 October 2004; accepted 27 October 2004
Abstract CdS thin films have been deposited from aqueous solution by photochemical reactions. The solution contains Cd(CH3COO)2 and Na2S2O3, and pH is controlled in an acidic region by adding H2SO4. The solution is illuminated with light from a high-pressure mercury-arc lamp. CdS thin films are formed on a glass substrate by the heterogeneous nucleation and the deposited thin films have been subjected to high-energy Si ion irradiations. Si ion irradiation has been performed with an energy of 80 MeV at fluences of 1 1011, 1 1012, 1 1013 and 1 1014 ions/cm2 using tandem pelletron accelerator. The irradiation-induced changes in CdS thin films are studied using XRD, Raman spectroscopy and photoluminescence. Broadening of the PL emission peak were observed with increasing irradiation fluence, which could be attributed to the band tailing effect of the Si ion irradiation. The lattice disorder takes place at high Si ion fluences. r 2004 Elsevier B.V. All rights reserved. PACS: 82.50.Fv; 81.0.5Dz; 60.10.Nz; 78.30.Fs Keywords: A1. Raman spectra; A1. X-ray diffraction; A3. Photochemical deposition; B1. CdS
1. Introduction There is a recent interest to reduce the cost of solar modules for space applications. Radiation Corresponding author. Tel./fax: +91 44 2235 2774.
E-mail address: [email protected] (R. Dhanasekaran).
resistance is an important issue, since high-energy particles continuously damage the semiconductor lattice, degrading the solar cell performance and limiting the useful lifetime of the solar cell [1]. Investigation of defects produced by high-energy radiations in semiconductors has become an important area of research in view of optoelectronic devices. The major advantage of
0921-4526/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2004.10.095
ARTICLE IN PRESS S. Soundeswaran et al. / Physica B 355 (2005) 222–230
high-energy irradiation is precise control over the spatial distribution of defects. Experiments carried out using the high-energy heavy ion irradiation studies on GaAs and germanium resulted in autocrystallization by electronic energy loss and inelastic collisions [2]. Polycrystalline thin film solar cells of II–VI and I–III–VI2 compounds are potentially important because of their low cost, high efficiency and stable performance. CdTe/CdS, Cu(In,Ga)Se2 (called CIGS) solar cells are irradiated with proton and radiation stability of the solar cell is studied [3,4]. Only few reports are available on the influence of high-energy ion irradiation on II–VI compound semiconductors like CdS, CdSe, CdTe etc [5–7]. It would be interesting to carry out a detailed study on irradiation effects of MeV heavy ions on semiconductors thin films. The heterojunctions, based on CdS thin films, are very promising structures for solar cells because of suitable band gap, optical absorption and good stability. The compound semiconductor deposition from aqueous solutions becomes increasingly popular because it has the advantages of economy and the capability of largearea deposition. So far, chemical bath deposition technique (CBD) and electrochemical deposition technique (ECD) have been extensively studied [8,9]. Recently, a new technique for compound semiconductor deposition from aqueous solutions has been developed and called photochemical deposition technique (PCD) [10,11]. In the present study, the substrate is held in an aqueous solution containing thiosulfate ions and metal ions. The solution is illuminated with UV light (a highpressure mercury lamp). The sulfur atoms and solvated electrons are released from photo-excited thiosulfate ions, and a sulfide semiconductor film is deposited in the irradiated region of the substrate. We have used ultrasonic vibration instead of magnetic stirrer because ultrasonic waves can drive chemical reactions such as oxidation, reduction and decomposition [12–14]. The PCD process can be easily controlled by switching on/off the light. Thus, the PCD technique has better controllability than the electrodeposition and CBD techniques. The study of the structural and optical properties of CdS thin film with various irradiation concentrations is important to understand the basic material
223
properties for practical application. We are the first to report the high-energy irradiation studies on PCD-CdS thin films grown by photochemical deposition technique. 2. Experimental 2.1. Photochemical deposition of CdS thin film The CdS thin film deposition is carried out from an aqueous solution containing Cd(CH3COO)2 and Na2S2O3 made of de-ionized water. The various deposition parameters like concentrations of precursors, and pH of the growth solution have been varied and the optimum conditions for the good deposition of CdS have been investigated. CdS thin film was deposited from a bath containing 1–10 mM Cd(CH3COO)2 and 100 mM Na2S2O3. The pH of the solution is in the range of 3.0–4.5. Glass or indium-tin-oxide (ITO)-coated glass or a metal foil has been used as the substrate for deposition, which provides the nucleation centers needed for the film growth. Fig. 1 shows the schematic sketch of the photochemical deposition set-up used for the present investigation. It consists of a high-pressure mercury lamp with a provision for UV light illumination. The solution is taken in a glass container and the substrate used for deposition is kept immersed in the solution at a few millimeter depth from the surface of the solution. The deposition solution above the substrate is illuminated by UV illumination from the top as shown in the figure. The formation of the CdS is based on the excitation of S2O2 3 ions upon UV illumination, which release sulfur atoms and electrons. These sulfur atoms and electrons react with the cadmium metal ions present in the solution to form CdS compound. 2.2. PCD conditions and reactions The photochemical formation of CdS takes place through the following reactions: The S2O2 3 ions present in the solution absorb UV illumination and release sulfur atom according to the following equation: 2 S2 O2 3 þ hn ! S þ SO3 :
(1)
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UV l ight
Quartz lens
Substrate holder
CdS deposition Substrate Growth solution
Ultrasonic vibrations
Fig. 1. The schematic sketch of the photochemical deposition (PCD) set-up.
Also the S2O2 3 ions get excited by absorbing the UV illumination and release electrons according to the following equations: 2 2S2 O2 3 þ hn ! S4 O6 þ 2e ;
(2)
2 2 SO2 3 þ S2 O3 þ hn ! S3 O6 þ 2e ;
(3)
þ S2 O2 3 þ 2H ! S þ H2 SO3 :
(4)
The sulfur atoms and electrons combine with the Cd metal ions present in the solution and forms CdS according to the equation given below: Cd2þ þ S þ 2e ! CdS:
(5)
In a more acidic medium, the spontaneous release of S takes place according to Eq. (4). 2.3. Si ion irradiation on CdS thin film The PCD-CdS and CBD-CdS thin films are mounted on a vacuum shielded vertical sliding ladder having four rectangular faces. They are
irradiated under high vacuum (6 10–6 Torr) using the 80 MeV Si ion beam with beam current of 3 pnA (particle nanoampere) of fluences 1 1011, 1 1012, 1 1013 and 1 1014 ions/cm2 using the 15UD tandem pelletron accelerator at Nuclear Science Centre, New Delhi, India. The ion fluence is estimated by integrating the total charge accumulated on the sample which is kept in cylindrical electron suppresser geometry. Heating of the sample has been avoided during irradiation by giving low beam current (3 pnA). The samples were mounted on a thick copper block with proper thermal contact to it. In order to expose the entire target area, the beam is scanned by an electromagnetic scanner in an area of 1 cm2.
3. Characterization of photochemically deposited CdS thin film The structural and optical properties of the asdeposited and irradiated films were analyzed by
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different techniques. The structural analysis of the as-deposited and irradiated films is carried out by X-ray diffraction (XRD) analysis. Raman spectroscopic analysis is carried out in the frequency range of 100–500 cm1 using an Ar laser (488 nm) as the excitation source. Photoluminescence analysis is carried out using He–Cd laser. The optical absorption spectra of as-grown and Si-irradiated CdS samples were recorded using UV–VIS spectrometer (Shimadazu, Japan).
4. Results and discussion The XRD patterns of the as-deposited and Si ion irradiated film are shown in Fig. 2. The spectrum (2a) indicates that the as-deposited film is crystalline in nature. For the as-deposited film, there are three peaks which can be attributed to the CdS film. They are assigned to (1 1 1), (2 2 0), and (3 1 1) diffractions of cubic CdS. It is known that they are the strongest three diffraction peaks for cubic CdS. Therefore, the as-deposited film is
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cubic zincblende phase. XRD patterns of the asdeposited CdS thin film did not exhibit peaks corresponding to elemental Cd or S. The width of the XRD peak is slightly increased for fluence of 1 1014 Si ion irradiated CdS thin films. This might be attributed to the lattice damage caused during ion irradiation. For the lower fluences (1 1011 to 1 1013) there is no considerable change in XRD peaks. A new broad peak appears at 2y ¼ 38:41 in the diffraction pattern of the irradiated (1 1014 fluence) film, which can be attributed either to the (1 0 1) reflection of Cd or to the (2 0 0) reflection of CdO as the peak matches well for both the phases. Mady et al. [15] and Narayanan et al. [16] have investigated the effect of low-energy ion implantation in ZnS and CdS films. In XRD analysis they observed the formation of Zn and Cd clusters under nitrogen implantation. They reported that there is a possibility of loss of sulfur atoms, and the excess Cd or CdO could precipitate in the CdS matrix. They ruled out the formation of metal oxide because of the presence of vacuum (106 Torr).
Fig. 2. The X-ray diffraction patterns of the as-deposited and Si ion irradiated PCD-CdS thin films: (a) as-deposited, (b) 1 1011 ions/ cm2, (c) 1 1012 ions/cm2, (d) 1 1013 ions/cm2 and (e) 1 1014 ions/cm2.
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Avasthi et al. [17] reported that even at the high vacuum conditions (8 107 mbar), H 2O 14 (3 10 molecules) present in several mono-layers and heavy ion irradiation leads to breakage of water molecule due to electronic excitation. The dissociated oxygen molecule available at the surface of sample will be incorporated into the film. Fig. 3 shows the Raman spectra of the asdeposited and Si ion-irradiated PCD-CdS thin films. A Lorentzian line shape is fitted to the Raman spectra from which the peak position, FWHM of the Raman peak has been obtained. The peak observed at 300 cm1 is attributed to the (LO) phonon in CdS [18]. The FWHM of the peak is 20.2 cm1. But, the well-defined peaks observed from the XRD pattern indicate the crystalline nature of the films. The results of the Raman analysis show that the as-deposited film consists of CdS only. Hence the FWHM value in the present case is attributed to the polycrystalline nature of
Fig. 3. The Raman spectra of the as-deposited and Si ion irradiated PCD-CdS thin films: (a) as-deposited, (b) 1 1011 ions/cm2 & 1 1012 ions/cm2, (c) 1 1013 ions/cm2 and (d) 1 1014 ions/cm2.
as-deposited film. Low-energy ion irradiation (implantation) results in lattice damage, which would decrease the peak area, and peak height and increase the FWHM value [19]. In the present case, Raman peak position has been shifted minutely to higher wave numbers upon high-energy irradiation. This might be due to the replacement of sulpfur atoms by oxygen atoms in the CdS lattice during irradiation, which results in increased vibrational frequency of Cd–O bonds as compared to that of Cd–S bonds. Also the lattice damage is proportional to irradiation fluence. The FWHM does not increase considerably at a low fluence (1 1011 and 1 1012 ions/cm2) but increases continuously at higher fluences (1 1013 and 1 1014 ions/cm2). The decrease in peak height at higher fluences are likely to be due to the irradiation-induced defects. This increase in FWHM may more probably be due to irradiation-induced lattice disorder, which is supported by XRD analysis. The peak position is not changed for all the fluences. At higher fluences reduction of the area under the Raman peak is due to the formation of a new phase CdO. The chemical bath deposition conditions of CdS thin films have been discussed elsewhere [8]. These CBD-CdS thin films have been subjected to highenergy Si ion irradiation and it shows similar changes structural and optical properties like PCD-CdS thin films. We can conclude from the Raman spectra (Fig. 4) of the as-deposited and Si ion-irradiated CBD-CdS thin films that it follows similar changes like irradiated PCD-CdS thin films. For photoluminescence (PL) measurements, a He–Cd laser is employed as the excitation source, focused on the sample through a spherical lens. The photoluminescence spectra of as-deposited and irradiated films are shown in Fig. 5. The CdS thin films exhibit a luminescence peak near bandedge at 2.47 eV. The peak intensity decreases with increase of ion fluences. The peaks in 2.04–2.11 eV range have been associated with radiative transitions from donor levels, arising from Cd atoms located in interstitial sites, to the valence band [20]. The PL emission intensity gradually decreases with increase in irradiation fluence upto 1 1012 ions/ cm2 and, however, the band-edge PL emission
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Fig. 5. The Photoluminescence of the as-deposited and Si ion irradiated PCD-CdS thin films: (a) as-deposited, (b) 1 1011 ions/cm2, (c) 1 1012 ions/cm2, (d) 1 1013 ions/cm2 and (e) 1 1014 ions/cm2.
Fig. 4. The Raman spectra of the as-deposited and Si ion irradiated CBD-CdS thin films: (a) as-deposited, (b) 1 1011 ions/cm2 & 1 1012 ions/cm2, (c) 1 1013 ions/cm2 and (d) 1 1014 ions/cm2.
drastically decreases for 1 1013 and 1 1014 ions/ cm2. The high-energy radiation-induced defects are responsible for the observed change in PL spectra. Optical absorption spectra for as-deposited and Si ion irradiated CdS films have been recorded over the range 200–1000 nm and the absorption coefficient as a function of energy is shown in Fig. 6a. Band gap energy for the as-deposited and Si ion irradiated PCD-CdS thin film is obtained from the slope of ðahnÞ2 vs. band gap (eV) plot and is 2.47 eV at room temperature. The band gap value gradually decreases up to 2.0 eV for the fluence 1014 ions/cm2 upon Si irradiation. Fig. 6c shows as-deposited and Si ion irradiated CBD-CdS thin film is obtained from the slope of ðahnÞ2 vs. band gap (eV) plot. In both cases the high-energy irradiation-induced lattice damage creates defect energy levels below the conduction band and hence the band gap decreases. Also the lattice
damage is proportional to irradiation fluence and hence due to band tailing effect band gap decreases continuously with the increase of ion fluence. The high-energy irradiation induces band tailing effects which seriously affect the properties of most of the semiconductor optoelectronic devices [21,22]. Figs. 7a and b show the AES data of asdeposited and Si ion irradiated PCD-CdS thin films with fluence of 1 1014 ions/cm2. The AES analysis of the as-deposited PCD-CdS and CBDCdS thin film does not have any oxygen impurity. On the other hand, the CdS thin films, which have undergone Si ion irradiation with fluence of 1 1014 ions/cm2 show the presence of oxygen. This confirms the formation of CdO in CdS matrix after Si ion irradiation in higher fluence.
5. Conclusion Cadmium Sulfide thin films were grown by PCD technique. XRD and Raman analysis confirmed the crystalline formation of cubic CdS thin films. The as-deposited CdS thin films were irradiated with silicon ions at different fluences. The effect of Si ion irradiation on the structural and optical properties of PCD-CdS and CBD-CdS thin films were investigated. A new peak in XRD emerging at fluence of 1014 ions/cm2 is most likely due to
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(a)
(b)
a
1.0x104
b c d
(αhν))2 (eV/m)2
e
5.0x102
0.0 2. 0 (c)
2. 1
2. 2 2. 3 Energy (eV)
2. 4
2. 5
Fig. 6. (a) The ðahnÞ2 vs. band gap energy of as-deposited and Si ion irradiated PCD-CdS thin films. (b) Absorption coefficient vs. l plot of as-deposited and Si ion irradiated PCD-CdS thin film. (c) The ðahnÞ2 vs. band gap energy (eV) of as-deposited and Si ion irradiated CBD-CdS thin films.
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Intensity (arb. units)
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100
Intensity (arb. units)
0
200
300 400 Kinetic energy (eV)
500
600
Cd
S
O
0
100
200
300
400
500
600
Kinetic energy (eV) Fig. 7. The Auger electron spectrum of PCD-CdS thin films: (a) as-deposited and (b) Si ion irradiated 1 1014 ions/cm2).
CdO. The Raman peak position of the asdeposited CdS film appears at 300 cm1 due to scattering from CdS (LO) mode. The peak position remains constant at low fluences of Si ion irradiation. The FWHM of Raman peak increased at high fluences of Si ion. The decrease in the area under the Raman peak for the irradiated films was attributed to the irradiationinduced defects. CdS thin films are stable upto 1 1013 ions/cm2. The observed band gap variation due to Si ion irradiation might be profitably used in tailoring the properties of this semiconductor material.
Acknowledgements The authors are thankful to NSC for financial assistance and irradiation studies. One of the author O.S. thanks CSIR for the award of SRF.
References [1] M.E. Moussa, M.Y. Feteha, M.F.M. Hassan, Renewable Energy 23 (2001) 361. [2] V.N. Bhoraskar, Current Sci. 80 (2001) 12. [3] A. Romeo, D.L. Batzner, H. Zogg, A.N. Tiwari, Mat. Res. Soc. Symp. Proc. 668 (2001) H 3.3.1. [4] H.W. Schock, K. Bogus, Proceedings of the Second World PVSEC, 1998, p. 3586. [5] J.R. Tuttle, A. Szalaj, J. Ke, 28th IEEE Photovoltaics Specialists Conference, 2000, p. 15. [6] A. Jasenek, U. Rau, K. Weinert, I.M. Kotschau, G. Hanna, G. Voorwinden, M. Powalla, H.W. Schock, J.H. Werner, Thin Solid Films 387 (2001) 228. [7] S. Antohe, V. Ruxandra, H. Alexandru, J. Cryst. Growth 237–239 (2002) 1559. [8] S. Soundeswaran, O. Senthil Kumar, R. Dhanasekaran, Mater. Lett. 58 (2004) 2381. [9] S. Soundeswaran, O. Senthil Kumar, R. Dhanasekaran, P. Ramasamy, R. Kumaresen, M. Ichimura, Mater. Chem. Phys. 82 (2003) 253. [10] F. Goto, M. Ichimura, E. Arai, Jpn. J. Appl. Phys. 36 (1997) L1146.
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[11] M. Ichimura, F. Goto, Y. Ono, E. Arai, J. Cryst. Growth 198/199 (1999) 308. [12] M.M. Mdleleni, T. Hyeon, K.S. Suslick, J. Am. Chem. Soc. 120 (1998) 6189. [13] J.Y. Choi, K.-J. Kim, J.-B. Yoo, D. Kim, Solar Energy 64 (1998) 41. [14] G.Z. Wang, Y.W. Wang, W. Chen, C.H. Liang, G.H. Li, L.D. Zhang, Mater. Lett. 48 (2001) 269. [15] Kh.A. Mady, Z.S. Moustafa, M.S. Selim, A.A. Gabr, J. Mater. Sci. 23 (1988) 2403. [16] K.L. Narayanan, R. Rajaraman, M.C. Valsakumar, K.G.M. Nair, K.P. Vijayakumar, Mater. Res. Bull. 34 (1999) 1729. [17] D.K. Avasthi, W. Assmann, A. Tripathi, S.K. Srivastava, S. Ghosh, F. Gruner, M. Toulemonde, Phys. Rev. B 68 (2003) 153106.
[18] M. Ichimura, A. Usami, T. Wada, M. Funato, K. Ichino, Fujita Sz, Fujita Sg, Phys. Rev. B 46 (1992) 4273. [19] K.L. Narayanan, K.P. Vijayakumar, K.G.M. Nair, B. Sundarakkannan, R. Kesavamoorthy, Nucl. Instrum. Meth. B 160 (2000) 471. [20] J. Aguilar-Hernandez, G. Contreras-Puente, A. MoralesAcevedo, O. Vigil-Galan, F. Cruz-Gandarilla, J. VidalLarramendi, A. Escamilla-Esquivel, H. Hernandez Contreras, M. Hesiquio-Garduno, A. Arias-Carbajal, M. Chavarr’Wa-Castaneda, G. Arriaga-MejWal, Semicond. Sci. Technol. 18 (2003) 111. [21] M.S. Kamboj, G. Kaur, R. Thangaraj, D.K. Avasthi, J. Phys. D 35 (2002) 477. [22] K.L. Narayanan, K.P. Vijayakumar, K.G.M. Nair, N.S. Thampi, Physica B 240 (1997) 8.
Physica B 355 (2005) 222–230 www.elsevier.com/locate/physb
Effect of Si ion irradiation on polycrystalline CdS thin film grown from novel photochemical deposition technique S. Soundeswarana, O. Senthil Kumara, P. Ramasamya, D. Kabi Rajb, D.K. Avasthib, R. Dhanasekarana, a
Crystal Growth Centre, Anna University, Sardar Patel Road, Chennai 600 025, India b Nuclear Science Centre, New Delhi 110067, India
Received 4 August 2004; received in revised form 14 October 2004; accepted 27 October 2004
Abstract CdS thin films have been deposited from aqueous solution by photochemical reactions. The solution contains Cd(CH3COO)2 and Na2S2O3, and pH is controlled in an acidic region by adding H2SO4. The solution is illuminated with light from a high-pressure mercury-arc lamp. CdS thin films are formed on a glass substrate by the heterogeneous nucleation and the deposited thin films have been subjected to high-energy Si ion irradiations. Si ion irradiation has been performed with an energy of 80 MeV at fluences of 1 1011, 1 1012, 1 1013 and 1 1014 ions/cm2 using tandem pelletron accelerator. The irradiation-induced changes in CdS thin films are studied using XRD, Raman spectroscopy and photoluminescence. Broadening of the PL emission peak were observed with increasing irradiation fluence, which could be attributed to the band tailing effect of the Si ion irradiation. The lattice disorder takes place at high Si ion fluences. r 2004 Elsevier B.V. All rights reserved. PACS: 82.50.Fv; 81.0.5Dz; 60.10.Nz; 78.30.Fs Keywords: A1. Raman spectra; A1. X-ray diffraction; A3. Photochemical deposition; B1. CdS
1. Introduction There is a recent interest to reduce the cost of solar modules for space applications. Radiation Corresponding author. Tel./fax: +91 44 2235 2774.
E-mail address: [email protected] (R. Dhanasekaran).
resistance is an important issue, since high-energy particles continuously damage the semiconductor lattice, degrading the solar cell performance and limiting the useful lifetime of the solar cell [1]. Investigation of defects produced by high-energy radiations in semiconductors has become an important area of research in view of optoelectronic devices. The major advantage of
0921-4526/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2004.10.095
ARTICLE IN PRESS S. Soundeswaran et al. / Physica B 355 (2005) 222–230
high-energy irradiation is precise control over the spatial distribution of defects. Experiments carried out using the high-energy heavy ion irradiation studies on GaAs and germanium resulted in autocrystallization by electronic energy loss and inelastic collisions [2]. Polycrystalline thin film solar cells of II–VI and I–III–VI2 compounds are potentially important because of their low cost, high efficiency and stable performance. CdTe/CdS, Cu(In,Ga)Se2 (called CIGS) solar cells are irradiated with proton and radiation stability of the solar cell is studied [3,4]. Only few reports are available on the influence of high-energy ion irradiation on II–VI compound semiconductors like CdS, CdSe, CdTe etc [5–7]. It would be interesting to carry out a detailed study on irradiation effects of MeV heavy ions on semiconductors thin films. The heterojunctions, based on CdS thin films, are very promising structures for solar cells because of suitable band gap, optical absorption and good stability. The compound semiconductor deposition from aqueous solutions becomes increasingly popular because it has the advantages of economy and the capability of largearea deposition. So far, chemical bath deposition technique (CBD) and electrochemical deposition technique (ECD) have been extensively studied [8,9]. Recently, a new technique for compound semiconductor deposition from aqueous solutions has been developed and called photochemical deposition technique (PCD) [10,11]. In the present study, the substrate is held in an aqueous solution containing thiosulfate ions and metal ions. The solution is illuminated with UV light (a highpressure mercury lamp). The sulfur atoms and solvated electrons are released from photo-excited thiosulfate ions, and a sulfide semiconductor film is deposited in the irradiated region of the substrate. We have used ultrasonic vibration instead of magnetic stirrer because ultrasonic waves can drive chemical reactions such as oxidation, reduction and decomposition [12–14]. The PCD process can be easily controlled by switching on/off the light. Thus, the PCD technique has better controllability than the electrodeposition and CBD techniques. The study of the structural and optical properties of CdS thin film with various irradiation concentrations is important to understand the basic material
223
properties for practical application. We are the first to report the high-energy irradiation studies on PCD-CdS thin films grown by photochemical deposition technique. 2. Experimental 2.1. Photochemical deposition of CdS thin film The CdS thin film deposition is carried out from an aqueous solution containing Cd(CH3COO)2 and Na2S2O3 made of de-ionized water. The various deposition parameters like concentrations of precursors, and pH of the growth solution have been varied and the optimum conditions for the good deposition of CdS have been investigated. CdS thin film was deposited from a bath containing 1–10 mM Cd(CH3COO)2 and 100 mM Na2S2O3. The pH of the solution is in the range of 3.0–4.5. Glass or indium-tin-oxide (ITO)-coated glass or a metal foil has been used as the substrate for deposition, which provides the nucleation centers needed for the film growth. Fig. 1 shows the schematic sketch of the photochemical deposition set-up used for the present investigation. It consists of a high-pressure mercury lamp with a provision for UV light illumination. The solution is taken in a glass container and the substrate used for deposition is kept immersed in the solution at a few millimeter depth from the surface of the solution. The deposition solution above the substrate is illuminated by UV illumination from the top as shown in the figure. The formation of the CdS is based on the excitation of S2O2 3 ions upon UV illumination, which release sulfur atoms and electrons. These sulfur atoms and electrons react with the cadmium metal ions present in the solution to form CdS compound. 2.2. PCD conditions and reactions The photochemical formation of CdS takes place through the following reactions: The S2O2 3 ions present in the solution absorb UV illumination and release sulfur atom according to the following equation: 2 S2 O2 3 þ hn ! S þ SO3 :
(1)
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UV l ight
Quartz lens
Substrate holder
CdS deposition Substrate Growth solution
Ultrasonic vibrations
Fig. 1. The schematic sketch of the photochemical deposition (PCD) set-up.
Also the S2O2 3 ions get excited by absorbing the UV illumination and release electrons according to the following equations: 2 2S2 O2 3 þ hn ! S4 O6 þ 2e ;
(2)
2 2 SO2 3 þ S2 O3 þ hn ! S3 O6 þ 2e ;
(3)
þ S2 O2 3 þ 2H ! S þ H2 SO3 :
(4)
The sulfur atoms and electrons combine with the Cd metal ions present in the solution and forms CdS according to the equation given below: Cd2þ þ S þ 2e ! CdS:
(5)
In a more acidic medium, the spontaneous release of S takes place according to Eq. (4). 2.3. Si ion irradiation on CdS thin film The PCD-CdS and CBD-CdS thin films are mounted on a vacuum shielded vertical sliding ladder having four rectangular faces. They are
irradiated under high vacuum (6 10–6 Torr) using the 80 MeV Si ion beam with beam current of 3 pnA (particle nanoampere) of fluences 1 1011, 1 1012, 1 1013 and 1 1014 ions/cm2 using the 15UD tandem pelletron accelerator at Nuclear Science Centre, New Delhi, India. The ion fluence is estimated by integrating the total charge accumulated on the sample which is kept in cylindrical electron suppresser geometry. Heating of the sample has been avoided during irradiation by giving low beam current (3 pnA). The samples were mounted on a thick copper block with proper thermal contact to it. In order to expose the entire target area, the beam is scanned by an electromagnetic scanner in an area of 1 cm2.
3. Characterization of photochemically deposited CdS thin film The structural and optical properties of the asdeposited and irradiated films were analyzed by
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different techniques. The structural analysis of the as-deposited and irradiated films is carried out by X-ray diffraction (XRD) analysis. Raman spectroscopic analysis is carried out in the frequency range of 100–500 cm1 using an Ar laser (488 nm) as the excitation source. Photoluminescence analysis is carried out using He–Cd laser. The optical absorption spectra of as-grown and Si-irradiated CdS samples were recorded using UV–VIS spectrometer (Shimadazu, Japan).
4. Results and discussion The XRD patterns of the as-deposited and Si ion irradiated film are shown in Fig. 2. The spectrum (2a) indicates that the as-deposited film is crystalline in nature. For the as-deposited film, there are three peaks which can be attributed to the CdS film. They are assigned to (1 1 1), (2 2 0), and (3 1 1) diffractions of cubic CdS. It is known that they are the strongest three diffraction peaks for cubic CdS. Therefore, the as-deposited film is
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cubic zincblende phase. XRD patterns of the asdeposited CdS thin film did not exhibit peaks corresponding to elemental Cd or S. The width of the XRD peak is slightly increased for fluence of 1 1014 Si ion irradiated CdS thin films. This might be attributed to the lattice damage caused during ion irradiation. For the lower fluences (1 1011 to 1 1013) there is no considerable change in XRD peaks. A new broad peak appears at 2y ¼ 38:41 in the diffraction pattern of the irradiated (1 1014 fluence) film, which can be attributed either to the (1 0 1) reflection of Cd or to the (2 0 0) reflection of CdO as the peak matches well for both the phases. Mady et al. [15] and Narayanan et al. [16] have investigated the effect of low-energy ion implantation in ZnS and CdS films. In XRD analysis they observed the formation of Zn and Cd clusters under nitrogen implantation. They reported that there is a possibility of loss of sulfur atoms, and the excess Cd or CdO could precipitate in the CdS matrix. They ruled out the formation of metal oxide because of the presence of vacuum (106 Torr).
Fig. 2. The X-ray diffraction patterns of the as-deposited and Si ion irradiated PCD-CdS thin films: (a) as-deposited, (b) 1 1011 ions/ cm2, (c) 1 1012 ions/cm2, (d) 1 1013 ions/cm2 and (e) 1 1014 ions/cm2.
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Avasthi et al. [17] reported that even at the high vacuum conditions (8 107 mbar), H 2O 14 (3 10 molecules) present in several mono-layers and heavy ion irradiation leads to breakage of water molecule due to electronic excitation. The dissociated oxygen molecule available at the surface of sample will be incorporated into the film. Fig. 3 shows the Raman spectra of the asdeposited and Si ion-irradiated PCD-CdS thin films. A Lorentzian line shape is fitted to the Raman spectra from which the peak position, FWHM of the Raman peak has been obtained. The peak observed at 300 cm1 is attributed to the (LO) phonon in CdS [18]. The FWHM of the peak is 20.2 cm1. But, the well-defined peaks observed from the XRD pattern indicate the crystalline nature of the films. The results of the Raman analysis show that the as-deposited film consists of CdS only. Hence the FWHM value in the present case is attributed to the polycrystalline nature of
Fig. 3. The Raman spectra of the as-deposited and Si ion irradiated PCD-CdS thin films: (a) as-deposited, (b) 1 1011 ions/cm2 & 1 1012 ions/cm2, (c) 1 1013 ions/cm2 and (d) 1 1014 ions/cm2.
as-deposited film. Low-energy ion irradiation (implantation) results in lattice damage, which would decrease the peak area, and peak height and increase the FWHM value [19]. In the present case, Raman peak position has been shifted minutely to higher wave numbers upon high-energy irradiation. This might be due to the replacement of sulpfur atoms by oxygen atoms in the CdS lattice during irradiation, which results in increased vibrational frequency of Cd–O bonds as compared to that of Cd–S bonds. Also the lattice damage is proportional to irradiation fluence. The FWHM does not increase considerably at a low fluence (1 1011 and 1 1012 ions/cm2) but increases continuously at higher fluences (1 1013 and 1 1014 ions/cm2). The decrease in peak height at higher fluences are likely to be due to the irradiation-induced defects. This increase in FWHM may more probably be due to irradiation-induced lattice disorder, which is supported by XRD analysis. The peak position is not changed for all the fluences. At higher fluences reduction of the area under the Raman peak is due to the formation of a new phase CdO. The chemical bath deposition conditions of CdS thin films have been discussed elsewhere [8]. These CBD-CdS thin films have been subjected to highenergy Si ion irradiation and it shows similar changes structural and optical properties like PCD-CdS thin films. We can conclude from the Raman spectra (Fig. 4) of the as-deposited and Si ion-irradiated CBD-CdS thin films that it follows similar changes like irradiated PCD-CdS thin films. For photoluminescence (PL) measurements, a He–Cd laser is employed as the excitation source, focused on the sample through a spherical lens. The photoluminescence spectra of as-deposited and irradiated films are shown in Fig. 5. The CdS thin films exhibit a luminescence peak near bandedge at 2.47 eV. The peak intensity decreases with increase of ion fluences. The peaks in 2.04–2.11 eV range have been associated with radiative transitions from donor levels, arising from Cd atoms located in interstitial sites, to the valence band [20]. The PL emission intensity gradually decreases with increase in irradiation fluence upto 1 1012 ions/ cm2 and, however, the band-edge PL emission
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Fig. 5. The Photoluminescence of the as-deposited and Si ion irradiated PCD-CdS thin films: (a) as-deposited, (b) 1 1011 ions/cm2, (c) 1 1012 ions/cm2, (d) 1 1013 ions/cm2 and (e) 1 1014 ions/cm2.
Fig. 4. The Raman spectra of the as-deposited and Si ion irradiated CBD-CdS thin films: (a) as-deposited, (b) 1 1011 ions/cm2 & 1 1012 ions/cm2, (c) 1 1013 ions/cm2 and (d) 1 1014 ions/cm2.
drastically decreases for 1 1013 and 1 1014 ions/ cm2. The high-energy radiation-induced defects are responsible for the observed change in PL spectra. Optical absorption spectra for as-deposited and Si ion irradiated CdS films have been recorded over the range 200–1000 nm and the absorption coefficient as a function of energy is shown in Fig. 6a. Band gap energy for the as-deposited and Si ion irradiated PCD-CdS thin film is obtained from the slope of ðahnÞ2 vs. band gap (eV) plot and is 2.47 eV at room temperature. The band gap value gradually decreases up to 2.0 eV for the fluence 1014 ions/cm2 upon Si irradiation. Fig. 6c shows as-deposited and Si ion irradiated CBD-CdS thin film is obtained from the slope of ðahnÞ2 vs. band gap (eV) plot. In both cases the high-energy irradiation-induced lattice damage creates defect energy levels below the conduction band and hence the band gap decreases. Also the lattice
damage is proportional to irradiation fluence and hence due to band tailing effect band gap decreases continuously with the increase of ion fluence. The high-energy irradiation induces band tailing effects which seriously affect the properties of most of the semiconductor optoelectronic devices [21,22]. Figs. 7a and b show the AES data of asdeposited and Si ion irradiated PCD-CdS thin films with fluence of 1 1014 ions/cm2. The AES analysis of the as-deposited PCD-CdS and CBDCdS thin film does not have any oxygen impurity. On the other hand, the CdS thin films, which have undergone Si ion irradiation with fluence of 1 1014 ions/cm2 show the presence of oxygen. This confirms the formation of CdO in CdS matrix after Si ion irradiation in higher fluence.
5. Conclusion Cadmium Sulfide thin films were grown by PCD technique. XRD and Raman analysis confirmed the crystalline formation of cubic CdS thin films. The as-deposited CdS thin films were irradiated with silicon ions at different fluences. The effect of Si ion irradiation on the structural and optical properties of PCD-CdS and CBD-CdS thin films were investigated. A new peak in XRD emerging at fluence of 1014 ions/cm2 is most likely due to
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(a)
(b)
a
1.0x104
b c d
(αhν))2 (eV/m)2
e
5.0x102
0.0 2. 0 (c)
2. 1
2. 2 2. 3 Energy (eV)
2. 4
2. 5
Fig. 6. (a) The ðahnÞ2 vs. band gap energy of as-deposited and Si ion irradiated PCD-CdS thin films. (b) Absorption coefficient vs. l plot of as-deposited and Si ion irradiated PCD-CdS thin film. (c) The ðahnÞ2 vs. band gap energy (eV) of as-deposited and Si ion irradiated CBD-CdS thin films.
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Intensity (arb. units)
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100
Intensity (arb. units)
0
200
300 400 Kinetic energy (eV)
500
600
Cd
S
O
0
100
200
300
400
500
600
Kinetic energy (eV) Fig. 7. The Auger electron spectrum of PCD-CdS thin films: (a) as-deposited and (b) Si ion irradiated 1 1014 ions/cm2).
CdO. The Raman peak position of the asdeposited CdS film appears at 300 cm1 due to scattering from CdS (LO) mode. The peak position remains constant at low fluences of Si ion irradiation. The FWHM of Raman peak increased at high fluences of Si ion. The decrease in the area under the Raman peak for the irradiated films was attributed to the irradiationinduced defects. CdS thin films are stable upto 1 1013 ions/cm2. The observed band gap variation due to Si ion irradiation might be profitably used in tailoring the properties of this semiconductor material.
Acknowledgements The authors are thankful to NSC for financial assistance and irradiation studies. One of the author O.S. thanks CSIR for the award of SRF.
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