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Optical properties of swift ion beam irradiated CdTe thin films
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Thin Solid Films 516 (2008) 5508 – 5512 www.elsevier.com/locate/tsf
Optical properties of swift ion beam irradiated CdTe thin films S. Chandramohan a , R. Sathyamoorthy a,⁎, P. Sudhagar a , D. Kanjilal b , D. Kabiraj b , K. Asokan b a
PG and Research Department of Physics, Kongunadu Arts & Science College, Coimbatore, Tamilnadu 641029, India b Inter-University Accelerator Centre, Aruna Asaf Ali Marg, New Delhi 110067, India Available online 13 July 2007
Abstract This paper reports the effect of swift (80 MeV) oxygen (O+6) ion irradiation on the optical properties of CdTe thin films grown by conventional thermal evaporation on glass substrates. The films are found to be slightly Te-rich in composition and irradiation results no change in the elemental composition. The optical constants such as refractive index (n), absorption coefficient (α) and the optical band gap energy show significant variation in their values with increase in ion fluence. Upon irradiation the band gap energy decreased from a value of 1.53 eV to 1.46 eV whereas the refractive index (n) increased from 2.38 to 3.12 at λ = 850 nm. The photoluminescence spectrum shows high density of native defects whose density strongly depends on the ion fluence. Both analyses indicate considerable defect production after swift ion beam irradiation. © 2007 Elsevier B.V. All rights reserved. Keywords: CdTe films; Ion irradiation; Optical constants; Band gap energy; Photoluminescence; Defect creation
1. Introduction High-energy electron, proton, neutron and ion irradiation of semiconductor diodes and solar cells has long been a topic of considerable interest in the field of semiconductor device fabrication. The inevitable damage production during the process of irradiation is used to study and engineer the defects in semiconductors. In a strong radiation environment in space, the electrical performance of solar cells is degraded due to direct exposure to energetically charged particles. A considerable amount of work has been carried out for the study of radiation damage in various solar cell materials and devices in the recent past [1–5]. In most cases high-energy heavy ions damage the material by producing a large amount of extended defects, but high-energy light ions are suitable for producing and modifying the intrinsic point defects [6]. The defects can play a variety of electronically active roles that affect the electrical and optical properties of a semiconductor. These defects give rise to additional discrete levels in the band gap and the Fermi level is pinned to one of these levels. ⁎ Corresponding author. Kongunadu Arts & Science College, G.N. Mills (post), Coimbatore 641029, Tamilnadu, India. Fax: +91 422 2644452. E-mail addresses: [email protected], [email protected] (R. Sathyamoorthy). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.07.057
CdTe has drawn considerable interest owing to its wide spread utility in a variety of electronic and optoelectronic devices such as solar cells, light emitting devices, infrared and gamma ray detectors, etc. Considerable amount of data are available on the irradiation effects of γ-rays, electrons and protons on CdTe films [7–9]. Batzner et al. [8] have studied the effect of low energy protons and high-energy electrons on CdTe/CdS solar cells and found that the devices were stable against irradiation. Electron irradiation is known to have less degradation effect than neutron irradiation of similar energy and dose, but have higher damage coefficient than for gamma rays. Manjunatha Pattabi et al. [9] have studied the effect of 8 MeV electron irradiation on the Au/CdTe Schottky diodes and found that at low doses the devices were stable against irradiation. Most of these studies were focused on the stability of the devices upon particle irradiation and no work has been reported on the modification of physical and electronic properties of CdTe films. Therefore, the present investigation is the first effort focused on the exploitation of ion irradiation as a tool to induce modifications in the optical properties of CdTe films. The results indicate significant modifications in the optical properties, principally a decrease in the optical band gap and the density of defective states, which are the critical parameters in determining the performance of the devices.
S. Chandramohan et al. / Thin Solid Films 516 (2008) 5508–5512
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2. Experimental details CdTe thin films of thickness 1 μm were deposited on chemically cleaned glass substrates using a conventional vacuum coating unit. The starting material (99.999% pure stoichiometric CdTe powder) was evaporated from a molybdenum boat at a pressure of 6 × 10− 6 mbar. The films were subjected to 80 MeV oxygen ions produced with 15 UD Pelletron tandem accelerator for different fluences in the range 1012–1014 ions/cm2 under a vacuum of 10− 6 mbar. The beam current was maintained b 3 pnA (particle nanoampere) to avoid heating effect during irradiation. The ion beam was focused to a spot of 10 mm diameter and then scanned over an area of 1 cm2 using magnetic scanner to achieve the dose uniformity across the sample area. The fluence values were measured by collecting the charge falling on the sample mounted on an electrically insulated sample ladder placed in secondary electron suppressed geometry. Ladder current was integrated with a digital current integrator and the charged pulses were counted using scalar counter. The projected range of 80 MeV ions in the films calculated using SRIM-2003 (Version 2003.26) software is about 50.56 μm, which is greater than the total thickness of the film. Thus, the bombarding ions pass through the entire film and deposit in the substrate. For this energy the electronic and nuclear energy loss values are 1.202 × 102 and 7.492 × 10− 2 eV/Å, respectively. The elemental analysis of the films was made by using Energy Dispersive X-ray Spectrometer (EDS), INCA Oxford, optionally attached with Scanning Electron Microscope (JEOL JSM 5600). The optical transmittance of the films before and after irradiation was measured using a doublebeam UV–Vis–NIR Spectrophotometer, model JASCO V-570 and the photoluminescence (PL) spectra were recorded at room temperature using He–Ne laser as an excitation source (λ = 632.8 nm). 3. Results and discussion The EDS analysis shows that both as grown and irradiated films are slightly Te-rich in composition and the change in Cd/Te ratio is almost independent of ion fluence (Table 1). The surface micrograph of the as grown film shown in Fig. 1(a) is an evidence for fine grains of nm size with almost smooth surface. The micrograph for a typical film irradiated at a fluence of 3 × 1013 ions/cm2 shows enhancement in the surface roughness.
Fig. 1. SEM micrographs of CdTe films (a) As grown; (b) irradiated at 3 × 1013 ions/cm2.
All the spectra reveal very pronounced interference effects for wavelength away from the fundamental absorption edge. Such behaviour of the transmission spectra is an evidence of the thickness uniformity of the films. The transmittance of the films get reduced to ∼50% after ion beam irradiation with a slight red shift in the fundamental absorption edge. The decrease in transmission could arise due to the increase in absorbance associated
3.1. Optical constants Fig. 2 shows the optical transmittance spectrum of as grown and irradiated CdTe films in the wavelength range 500–2500 nm. Table 1 Shows change in optical constants and PL band position with ion fluence Ion fluence
Cd/Te ratio
Refractive index
Band gap
PL band position
(Ions/cm2)
(±0.03)
(at λ = 850 nm)
(eV)
(eV)
As grown 3 × 1012 3 × 1013 1 × 1014
0.93 0.90 0.92 0.95
2.38 3.03 2.88 3.12
1.53 1.52 1.46 1.49
1.457 1.493 1.460 1.429
Fig. 2. Transmittance spectrum of as grown and irradiated CdTe films.
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with change in surface microstructure as will be discussed in the forthcoming section. The spectral refractive index (n) of the films was calculated from the transmittance spectra by using the simple and straightforward method developed by Manifacier's [10], which is applicable to any transmission spectrum showing appreciable interference fringes similar to the one observed in our present work. The refractive index of the films was calculated using the formulae n ¼ fN þ ðN 2 þ n2o n21 Þ1=2 g
1=2
ð1Þ
where no is the refractive index of air, n1 is the refractive index of the substrate and N ¼ ðn2o þ n21 Þ=2 þ 2no n1 ðTM Tm Þ=TM Tm
ð2Þ
where TM and Tm are the upper and lower extreme transmittance values for a given wavelength obtained from the envelope curves. The dispersion of n with wavelength for films before and after irradiation is shown in Fig. 3 from which many interesting remarks can be observed. The results show an abrupt increase of n with λ over the spectral range from 800 to 900 nm, and then become almost constant with increasing wavelength. This indicates that the refractive index of CdTe films either as deposited or after being irradiated exhibits anomalous dispersion in the spectral range 800–900 nm. The observed anomalous dispersion of n is more or less in agreement with the results observed by El-Kadry et al. [11]. As can be seen from Fig. 3 that the value of n is higher for films subjected to irradiation and shows consistent increase with increase in ion fluence. By taking in to account any two consecutive maxima or minima the thickness of the films can be determined using the n values calculated as discussed above. The results show that there is a decrease in the film thickness after irradiation. Therefore the observed increase of n for irradiated films could be due to the decrease in the film thickness because, a decrease in thickness of the film results in an increase in its density that, in turn, leads to a higher refractive index (Lorentz–Lorentz law) [12]. The estimated values of n at λ = 850 nm are given in the Table 1.
Fig. 3. Spectral dispersion of refractive index (n) with wavelength.
Fig. 4. Spectral and ion fluence dependence of absorption coefficient (α).
The observed values are in good agreement with the values reported for CdTe films [13–15]. The second optical constant of significant interest is the absorption coefficient (α), which is a measure of the ability of a semiconductor to absorb photons. In the region of high absorption, the value of α was determined using the simple relation [16] T ¼ AexpðadÞ
ð3Þ
where T is the transmittance, d is the film thickness and A is given by the expression A¼
16na ng ðn2 þ k 2 Þ ½ðna þ nÞ2 þ k 2 ½ðng þ nÞ2 þ k 2
where k is the extinction coefficient, na and ng are, respectively, the refractive indices of air and glass. A is found to be nearly unity at the absorption edge. Fig. 4 illustrates the variation of absorption coefficient as a function of wavelength for as grown and irradiated films. It is seen that for irradiated films, the spectra in the high absorption region (λ b 500 nm) shows abrupt increase of α whereas for as grown film the variation of α with λ seems to be constant in this region. The increase in the absorption after irradiation may be attributed either due to the creation of trap levels or to the increase in the scattering on the sample surface [17]. The increase in surface roughness after irradiation (Fig. 1) supports the later one. According to the theory of optical interband transitions (direct or indirect) in solids, near the absorption edge, the absorption coefficient varies with the photon energy hυ according to the expression [18] (αhυ) = A(hυ − Eg)n, where A is a constant and Eg is the optical band gap and n depends on the kind of optical transitions that prevail. Generally, for CdTe films, the above relationship is well obeyed, provided that n = 1/2. Fig. 5 shows a plot of (αhυ)2 vs. hυ from which the direct allowed transition energy was estimated by extrapolating the linear portion to intercept on the energy axis. The calculated values are given in the Table 1. As can be seen from the table that the band gap energy decreases gradually with increase in ion fluence. Similar decrease in the band gap energy during irradiation was reported
S. Chandramohan et al. / Thin Solid Films 516 (2008) 5508–5512
Fig. 5. Tauc plot showing the determination of optical band gap.
on other semiconductor thin films [19,20]. The observed decrease in the band gap might be due to the creation of shallow defect levels near the conduction band as a result of irradiation. It has also been observed from Fig. 4 that there is a step below the band gap energy value, which may be attributed to the spin orbit splitting of the valence band. 3.2. Photoluminescence properties The relatively open zincblende structure of CdTe should easily accommodate interstitial atoms in fact the atomic spacing in CdTe is larger than the spacing of most tetrahedral structures. Correspondingly, the cohesive strength of CdTe is smaller than that of most tetrahedral structures suggesting the energy of vacancy formation to be smaller and the concentration of vacancies to be relatively larger. Hence a considerable native defect density is expected in CdTe films irrespective of the growth process and post deposition treatment. Generally, PL studies have been directed towards the identification of such native defects and impurities, either through direct recombination associated with band to defect-type transitions or the recombination of excitons associated with defect centers. Fig. 6 shows the PL emission spectra of as grown and irradiated CdTe films. The spectrum of as grown film is dominated by a broad band with twin peaks at 1.436 and 1.457 eV. In general, the 1.457 eV band has been observed between 1.4 and 1.49 eV, usually ascribed to intrinsic defects/impurities and, consequently, is known as the defect band. This includes bandacceptor (e-A) transitions, donor–acceptor pair (DAP) recombination and internal transitions within highly localized defects. In the present work the 1.457 eV peak is attributed to the shallow DAP transitions, the recombination of free charge carrier from the conduction band with a carrier bound on an acceptor. The assignment of the peak to the DAP recombination is well supported by the earlier works on CdTe single crystals and films [21–23]. The peaks observed at 1.436, 1.415 and 1.391 eV are the longitudinal-optical (LO) phonon replicas of the 1.457 eV peak. The appearance of LO phonon replicas
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further confirms the assumption that the band observed at 1.457 eV is due to the shallow DAP emission because, in general, DAP transitions are accompanied by ‘n’ Stokes LO phonon replicas and the number of which depends on the electron-phonon coupling strength [24]. The excitonic emission is severely attenuated with the appearance of DAP emission, which is a signature of presence of high density of native defects in the as grown CdTe films. We have observed a weak band around 1.55 eV, which most probably related to cadmium vacancies (VCd) and could be associated with the transition from conduction band to VCd acceptor level [25,26]. This is in good agreement with the results obtained from EDS analysis. The major difference between the spectra of as grown and irradiated films is the shift in the position of the defect band and change in its emission intensity. For film exposed to 3 × 1012 ions/cm2 a broad feature centered at 1.492 eV is observed with a row of equidistant peaks at 1.471, 1.450 and 1.513 eV, whose energy separation is equal to the CdTe LO phonon energy of 21 meV [27,28]. The peak at 1.513 eV, the socalled ‘shoulder’ is identified as the zero-phonon line of the DAP recombination and the peaks at 1.471 and 1.450 eV are the first and second LO phonon replicas of the band at 1.492 eV. Another weak band centered at 1.553 eV can be related to cadmium vacancies. The position of the dominant broad band for films irradiated with 3 × 1013 and 1 × 1014 ions/cm2 was observed at 1.468 and 1.429 eV respectively, along with their phonon replicas. We observed a considerable reduction in the PL intensity after irradiation. Veeramani et al. [29] reported a similar reduction in PL intensity in CdTe single crystals subjected to 100 MeV Ag7+ ions and suggested that the SHI irradiation induced defects could act either as traps or as nonradiative recombination centres. The shift in the defect band position may be either due to the variation in the residual stress or in the band gap energy. The correlation between the shift in the PL peak position and the shift in the band gap energy is in good agreement with the work of Vamsi Krishna et al. [30], where the shift in the exciton peak position is reported due to the change in the band gap of CdTe.
Fig. 6. Photoluminescence spectrum of as grown and irradiated CdTe films.
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4. Conclusion The effect of 80 MeV oxygen ion irradiation on the optical constants and photoluminescence properties has been investigated. The composition of the film was unaffected whereas a strong suppression in the transmission was noticed after irradiation. The values of n and α are found to be fluence dependent showing considerable increase of absorption in the visible region due to irradiation induced defective states in the material. The decrease in direct optical band gap energy and appearance of second band gap after irradiation implies the creation of additional energy levels and spin orbit splitting of valence band, respectively. The PL spectra shows considerable amount of native defects in the as grown films and after irradiation the density of defects increased. From the overall observations it is to be noted that the irradiation of CdTe films led to the creation of defects due to which both the electronic structure and optical properties can be modified. Further investigations are required to find out the exact role of these defects in the process of photoconduction. Acknowledgements This work was supported by the Inter University Accelerator Centre (IUAC), New Delhi, India through the Project UFUP 34319. The authors cordially acknowledge the help extended by technical staff of Pelletron group during the irradiation experiment. The authors wish to acknowledge Dr. D.M. Phase, Scientist and Mr. Vinay Ahire, Junior engineer, UGC-DAE Consortium for Scientific Research, Indore Centre for EDA/SEM analysis. One of the authors (RS) gratefully acknowledges University Grants Commission (UGC), New Delhi for awarding UGC-Research Award [Project No. F-30-1/2004 (SA-II)]. References [1] P.C. Srivastava, S.P. Pandey, K. Asokan, Nucl. Instrum. Methods Phys. Res., B Beam Interact. Mater. Atoms 244 (2006) 166. [2] V.N. Bhoraskar, Curr. Sci. 80 (2001) 1567. [3] K. Weinert, M. Schwicken, U. Rau, 3rd World Conference on Photovoltaic Energy Conversion, Osaka, Japan, May 11–18, 2003, p. 697. [4] S.Zh. Karazhanov, Appl. Phys. Lett. 78 (2001) 24. [5] M. Imaizumi, M. Yamaguchi, S.J. Taylor, S. Matsuda, O. Kawasaki, T. Hisamatsu, Sol. Energy Mater. Sol. Cells 50 (1998) 237.
[6] D. Kabiraj, S. Ghosh, Nucl. Instrum. Methods Phys. Res., B Beam Interact. Mater. Atoms 212 (2003) 135. [7] A. Cavallini, B. Fraboni, W. Dusi, M. Zanarini, M. Hage-Ali, P. Siffert, J. Appl. Phys. 89 (2001) 4664. [8] D.L. Batzner, A. Romeo, M. Terheggen, M. Dobeli, H. Zogg, A.N. Tiwari, Thin Solid Films 451–452 (2004) 536. [9] Manjunatha Pattabi, Sheeja Krishnan, Ganesh, X. Mathew, Sol. Energy 81 (2007) 111. [10] J.C. Manificier, J. Gaslot, J.P. Fillard, J. Phys. E 9 (1976) 1002. [11] N. El-Kadry, A. Ashour, M.F. Ahmed, K. Abdel-Hady, Thin Solid Films 259 (1995) 194. [12] M.F. Al-Kuhaili, S.M.A. Durrani, E.E. Khawaja, J. Shirokoff, J. Phys. D: Appl. Phys. 35 (2002) 910. [13] Xavier Mathew, P.J. Sebastian, A. Sanchez, J. Campos, Sol. Energy Mater. Sol. Cells 59 (1999) 9. [14] T.H. Myers, S.W. Edwards, J.F. Schetzina, J. Appl. Phys. 52 (1981) 4231. [15] J. Aranda, J.L. Morenza, J. Esteve, J.M. Codina, Thin Solid Films 120 (1984) 23. [16] K. Prabakar, Sa.K. Narayandass, D. Mangalaraj, Physica, B 328 (2003) 355. [17] I. Muntele, P. Thevenard, C. Muntele, B. Chhay, R.L. Zimmerman, S. Sarkisov, D. Ila, Nucl. Instrum. Methods Phys. Res., B Beam Interact. Mater. Atoms 242 (2006) 512. [18] J.J. Pankove, Optical Process in Semiconductors, Dover publications, Inc., New York, 1971, p. 34. [19] Yatendra S. Chaudhary, Saif A. Khan, Rohit Shrivastav, Vibha R. Satsangi, Satya Prakash, D.K. Avasthi, Sahab Dass, Nucl. Instrum. Methods Phys. Res., B Beam Interact. Mater. Atoms 225 (2004) 291. [20] P.M. Ratheesh Kumar, Teny Theresa John, C. Sudha Kartha, K.P. Vijayakumar, Nucl. Instrum. Methods Phys. Res., B Beam Interact. Mater. Atoms 244 (2006) 171. [21] C.J. Bride, P. Dawson, P.D. Buckle, M.E. Ozsan, Semicond. Sci. Technol. 15 (2000) 975. [22] S. Seto, S. Yamada, K. Suzuki, Sol. Energy Mater. Sol. Cells 67 (2001) 167. [23] R. Ahmad-Bitar, H. Moutinho, Foad Abulfotuh, L. Kazmerski, Renew. Energy 6 (1995) 553. [24] M.O. Manasreh, in: Maria C. Tamargo (Ed.), II–VI Semiconductor Materials and Their Applications, vol. 12, Taylor & Francis, New York, 2002, p. 158, Chapter 4. [25] J. Aguilar-Hernandez, G. Contreras-Puente, J. Vidal-Larramendi, O. VigilGalan, Thin Solid Films 426 (2003) 132. [26] Xavier Mathew, J.R. Arizmendi, J. Campos, P.J. Sebastian, N.R. Mathews, Cristino R. Jimenez, Miguel G. Jimenez, R. Silva-Gonzales, M.E. HernandezTorres, Ramesh Dhere, Sol. Energy Mater. Sol. Cells 70 (2001) 379. [27] R. Ahmad-Bitar, D.E. Arafah, Sol. Energy Mater. Sol. Cells 51 (1998) 83. [28] B. Bhattacharya, M.J. Carter, Thin Solid Films 288 (1996) 176. [29] P. Veeramani, M. Haris, D. Kanjilal, K. Asokan, S. Moorthy Babu, J. Phys. D: Appl. Phys. 39 (2006) 2707. [30] K. Vamsi Krishna, V. Dutta, J. Appl. Phys. 96 (2004) 3962.
Thin Solid Films 516 (2008) 5508 – 5512 www.elsevier.com/locate/tsf
Optical properties of swift ion beam irradiated CdTe thin films S. Chandramohan a , R. Sathyamoorthy a,⁎, P. Sudhagar a , D. Kanjilal b , D. Kabiraj b , K. Asokan b a
PG and Research Department of Physics, Kongunadu Arts & Science College, Coimbatore, Tamilnadu 641029, India b Inter-University Accelerator Centre, Aruna Asaf Ali Marg, New Delhi 110067, India Available online 13 July 2007
Abstract This paper reports the effect of swift (80 MeV) oxygen (O+6) ion irradiation on the optical properties of CdTe thin films grown by conventional thermal evaporation on glass substrates. The films are found to be slightly Te-rich in composition and irradiation results no change in the elemental composition. The optical constants such as refractive index (n), absorption coefficient (α) and the optical band gap energy show significant variation in their values with increase in ion fluence. Upon irradiation the band gap energy decreased from a value of 1.53 eV to 1.46 eV whereas the refractive index (n) increased from 2.38 to 3.12 at λ = 850 nm. The photoluminescence spectrum shows high density of native defects whose density strongly depends on the ion fluence. Both analyses indicate considerable defect production after swift ion beam irradiation. © 2007 Elsevier B.V. All rights reserved. Keywords: CdTe films; Ion irradiation; Optical constants; Band gap energy; Photoluminescence; Defect creation
1. Introduction High-energy electron, proton, neutron and ion irradiation of semiconductor diodes and solar cells has long been a topic of considerable interest in the field of semiconductor device fabrication. The inevitable damage production during the process of irradiation is used to study and engineer the defects in semiconductors. In a strong radiation environment in space, the electrical performance of solar cells is degraded due to direct exposure to energetically charged particles. A considerable amount of work has been carried out for the study of radiation damage in various solar cell materials and devices in the recent past [1–5]. In most cases high-energy heavy ions damage the material by producing a large amount of extended defects, but high-energy light ions are suitable for producing and modifying the intrinsic point defects [6]. The defects can play a variety of electronically active roles that affect the electrical and optical properties of a semiconductor. These defects give rise to additional discrete levels in the band gap and the Fermi level is pinned to one of these levels. ⁎ Corresponding author. Kongunadu Arts & Science College, G.N. Mills (post), Coimbatore 641029, Tamilnadu, India. Fax: +91 422 2644452. E-mail addresses: [email protected], [email protected] (R. Sathyamoorthy). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.07.057
CdTe has drawn considerable interest owing to its wide spread utility in a variety of electronic and optoelectronic devices such as solar cells, light emitting devices, infrared and gamma ray detectors, etc. Considerable amount of data are available on the irradiation effects of γ-rays, electrons and protons on CdTe films [7–9]. Batzner et al. [8] have studied the effect of low energy protons and high-energy electrons on CdTe/CdS solar cells and found that the devices were stable against irradiation. Electron irradiation is known to have less degradation effect than neutron irradiation of similar energy and dose, but have higher damage coefficient than for gamma rays. Manjunatha Pattabi et al. [9] have studied the effect of 8 MeV electron irradiation on the Au/CdTe Schottky diodes and found that at low doses the devices were stable against irradiation. Most of these studies were focused on the stability of the devices upon particle irradiation and no work has been reported on the modification of physical and electronic properties of CdTe films. Therefore, the present investigation is the first effort focused on the exploitation of ion irradiation as a tool to induce modifications in the optical properties of CdTe films. The results indicate significant modifications in the optical properties, principally a decrease in the optical band gap and the density of defective states, which are the critical parameters in determining the performance of the devices.
S. Chandramohan et al. / Thin Solid Films 516 (2008) 5508–5512
5509
2. Experimental details CdTe thin films of thickness 1 μm were deposited on chemically cleaned glass substrates using a conventional vacuum coating unit. The starting material (99.999% pure stoichiometric CdTe powder) was evaporated from a molybdenum boat at a pressure of 6 × 10− 6 mbar. The films were subjected to 80 MeV oxygen ions produced with 15 UD Pelletron tandem accelerator for different fluences in the range 1012–1014 ions/cm2 under a vacuum of 10− 6 mbar. The beam current was maintained b 3 pnA (particle nanoampere) to avoid heating effect during irradiation. The ion beam was focused to a spot of 10 mm diameter and then scanned over an area of 1 cm2 using magnetic scanner to achieve the dose uniformity across the sample area. The fluence values were measured by collecting the charge falling on the sample mounted on an electrically insulated sample ladder placed in secondary electron suppressed geometry. Ladder current was integrated with a digital current integrator and the charged pulses were counted using scalar counter. The projected range of 80 MeV ions in the films calculated using SRIM-2003 (Version 2003.26) software is about 50.56 μm, which is greater than the total thickness of the film. Thus, the bombarding ions pass through the entire film and deposit in the substrate. For this energy the electronic and nuclear energy loss values are 1.202 × 102 and 7.492 × 10− 2 eV/Å, respectively. The elemental analysis of the films was made by using Energy Dispersive X-ray Spectrometer (EDS), INCA Oxford, optionally attached with Scanning Electron Microscope (JEOL JSM 5600). The optical transmittance of the films before and after irradiation was measured using a doublebeam UV–Vis–NIR Spectrophotometer, model JASCO V-570 and the photoluminescence (PL) spectra were recorded at room temperature using He–Ne laser as an excitation source (λ = 632.8 nm). 3. Results and discussion The EDS analysis shows that both as grown and irradiated films are slightly Te-rich in composition and the change in Cd/Te ratio is almost independent of ion fluence (Table 1). The surface micrograph of the as grown film shown in Fig. 1(a) is an evidence for fine grains of nm size with almost smooth surface. The micrograph for a typical film irradiated at a fluence of 3 × 1013 ions/cm2 shows enhancement in the surface roughness.
Fig. 1. SEM micrographs of CdTe films (a) As grown; (b) irradiated at 3 × 1013 ions/cm2.
All the spectra reveal very pronounced interference effects for wavelength away from the fundamental absorption edge. Such behaviour of the transmission spectra is an evidence of the thickness uniformity of the films. The transmittance of the films get reduced to ∼50% after ion beam irradiation with a slight red shift in the fundamental absorption edge. The decrease in transmission could arise due to the increase in absorbance associated
3.1. Optical constants Fig. 2 shows the optical transmittance spectrum of as grown and irradiated CdTe films in the wavelength range 500–2500 nm. Table 1 Shows change in optical constants and PL band position with ion fluence Ion fluence
Cd/Te ratio
Refractive index
Band gap
PL band position
(Ions/cm2)
(±0.03)
(at λ = 850 nm)
(eV)
(eV)
As grown 3 × 1012 3 × 1013 1 × 1014
0.93 0.90 0.92 0.95
2.38 3.03 2.88 3.12
1.53 1.52 1.46 1.49
1.457 1.493 1.460 1.429
Fig. 2. Transmittance spectrum of as grown and irradiated CdTe films.
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with change in surface microstructure as will be discussed in the forthcoming section. The spectral refractive index (n) of the films was calculated from the transmittance spectra by using the simple and straightforward method developed by Manifacier's [10], which is applicable to any transmission spectrum showing appreciable interference fringes similar to the one observed in our present work. The refractive index of the films was calculated using the formulae n ¼ fN þ ðN 2 þ n2o n21 Þ1=2 g
1=2
ð1Þ
where no is the refractive index of air, n1 is the refractive index of the substrate and N ¼ ðn2o þ n21 Þ=2 þ 2no n1 ðTM Tm Þ=TM Tm
ð2Þ
where TM and Tm are the upper and lower extreme transmittance values for a given wavelength obtained from the envelope curves. The dispersion of n with wavelength for films before and after irradiation is shown in Fig. 3 from which many interesting remarks can be observed. The results show an abrupt increase of n with λ over the spectral range from 800 to 900 nm, and then become almost constant with increasing wavelength. This indicates that the refractive index of CdTe films either as deposited or after being irradiated exhibits anomalous dispersion in the spectral range 800–900 nm. The observed anomalous dispersion of n is more or less in agreement with the results observed by El-Kadry et al. [11]. As can be seen from Fig. 3 that the value of n is higher for films subjected to irradiation and shows consistent increase with increase in ion fluence. By taking in to account any two consecutive maxima or minima the thickness of the films can be determined using the n values calculated as discussed above. The results show that there is a decrease in the film thickness after irradiation. Therefore the observed increase of n for irradiated films could be due to the decrease in the film thickness because, a decrease in thickness of the film results in an increase in its density that, in turn, leads to a higher refractive index (Lorentz–Lorentz law) [12]. The estimated values of n at λ = 850 nm are given in the Table 1.
Fig. 3. Spectral dispersion of refractive index (n) with wavelength.
Fig. 4. Spectral and ion fluence dependence of absorption coefficient (α).
The observed values are in good agreement with the values reported for CdTe films [13–15]. The second optical constant of significant interest is the absorption coefficient (α), which is a measure of the ability of a semiconductor to absorb photons. In the region of high absorption, the value of α was determined using the simple relation [16] T ¼ AexpðadÞ
ð3Þ
where T is the transmittance, d is the film thickness and A is given by the expression A¼
16na ng ðn2 þ k 2 Þ ½ðna þ nÞ2 þ k 2 ½ðng þ nÞ2 þ k 2
where k is the extinction coefficient, na and ng are, respectively, the refractive indices of air and glass. A is found to be nearly unity at the absorption edge. Fig. 4 illustrates the variation of absorption coefficient as a function of wavelength for as grown and irradiated films. It is seen that for irradiated films, the spectra in the high absorption region (λ b 500 nm) shows abrupt increase of α whereas for as grown film the variation of α with λ seems to be constant in this region. The increase in the absorption after irradiation may be attributed either due to the creation of trap levels or to the increase in the scattering on the sample surface [17]. The increase in surface roughness after irradiation (Fig. 1) supports the later one. According to the theory of optical interband transitions (direct or indirect) in solids, near the absorption edge, the absorption coefficient varies with the photon energy hυ according to the expression [18] (αhυ) = A(hυ − Eg)n, where A is a constant and Eg is the optical band gap and n depends on the kind of optical transitions that prevail. Generally, for CdTe films, the above relationship is well obeyed, provided that n = 1/2. Fig. 5 shows a plot of (αhυ)2 vs. hυ from which the direct allowed transition energy was estimated by extrapolating the linear portion to intercept on the energy axis. The calculated values are given in the Table 1. As can be seen from the table that the band gap energy decreases gradually with increase in ion fluence. Similar decrease in the band gap energy during irradiation was reported
S. Chandramohan et al. / Thin Solid Films 516 (2008) 5508–5512
Fig. 5. Tauc plot showing the determination of optical band gap.
on other semiconductor thin films [19,20]. The observed decrease in the band gap might be due to the creation of shallow defect levels near the conduction band as a result of irradiation. It has also been observed from Fig. 4 that there is a step below the band gap energy value, which may be attributed to the spin orbit splitting of the valence band. 3.2. Photoluminescence properties The relatively open zincblende structure of CdTe should easily accommodate interstitial atoms in fact the atomic spacing in CdTe is larger than the spacing of most tetrahedral structures. Correspondingly, the cohesive strength of CdTe is smaller than that of most tetrahedral structures suggesting the energy of vacancy formation to be smaller and the concentration of vacancies to be relatively larger. Hence a considerable native defect density is expected in CdTe films irrespective of the growth process and post deposition treatment. Generally, PL studies have been directed towards the identification of such native defects and impurities, either through direct recombination associated with band to defect-type transitions or the recombination of excitons associated with defect centers. Fig. 6 shows the PL emission spectra of as grown and irradiated CdTe films. The spectrum of as grown film is dominated by a broad band with twin peaks at 1.436 and 1.457 eV. In general, the 1.457 eV band has been observed between 1.4 and 1.49 eV, usually ascribed to intrinsic defects/impurities and, consequently, is known as the defect band. This includes bandacceptor (e-A) transitions, donor–acceptor pair (DAP) recombination and internal transitions within highly localized defects. In the present work the 1.457 eV peak is attributed to the shallow DAP transitions, the recombination of free charge carrier from the conduction band with a carrier bound on an acceptor. The assignment of the peak to the DAP recombination is well supported by the earlier works on CdTe single crystals and films [21–23]. The peaks observed at 1.436, 1.415 and 1.391 eV are the longitudinal-optical (LO) phonon replicas of the 1.457 eV peak. The appearance of LO phonon replicas
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further confirms the assumption that the band observed at 1.457 eV is due to the shallow DAP emission because, in general, DAP transitions are accompanied by ‘n’ Stokes LO phonon replicas and the number of which depends on the electron-phonon coupling strength [24]. The excitonic emission is severely attenuated with the appearance of DAP emission, which is a signature of presence of high density of native defects in the as grown CdTe films. We have observed a weak band around 1.55 eV, which most probably related to cadmium vacancies (VCd) and could be associated with the transition from conduction band to VCd acceptor level [25,26]. This is in good agreement with the results obtained from EDS analysis. The major difference between the spectra of as grown and irradiated films is the shift in the position of the defect band and change in its emission intensity. For film exposed to 3 × 1012 ions/cm2 a broad feature centered at 1.492 eV is observed with a row of equidistant peaks at 1.471, 1.450 and 1.513 eV, whose energy separation is equal to the CdTe LO phonon energy of 21 meV [27,28]. The peak at 1.513 eV, the socalled ‘shoulder’ is identified as the zero-phonon line of the DAP recombination and the peaks at 1.471 and 1.450 eV are the first and second LO phonon replicas of the band at 1.492 eV. Another weak band centered at 1.553 eV can be related to cadmium vacancies. The position of the dominant broad band for films irradiated with 3 × 1013 and 1 × 1014 ions/cm2 was observed at 1.468 and 1.429 eV respectively, along with their phonon replicas. We observed a considerable reduction in the PL intensity after irradiation. Veeramani et al. [29] reported a similar reduction in PL intensity in CdTe single crystals subjected to 100 MeV Ag7+ ions and suggested that the SHI irradiation induced defects could act either as traps or as nonradiative recombination centres. The shift in the defect band position may be either due to the variation in the residual stress or in the band gap energy. The correlation between the shift in the PL peak position and the shift in the band gap energy is in good agreement with the work of Vamsi Krishna et al. [30], where the shift in the exciton peak position is reported due to the change in the band gap of CdTe.
Fig. 6. Photoluminescence spectrum of as grown and irradiated CdTe films.
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4. Conclusion The effect of 80 MeV oxygen ion irradiation on the optical constants and photoluminescence properties has been investigated. The composition of the film was unaffected whereas a strong suppression in the transmission was noticed after irradiation. The values of n and α are found to be fluence dependent showing considerable increase of absorption in the visible region due to irradiation induced defective states in the material. The decrease in direct optical band gap energy and appearance of second band gap after irradiation implies the creation of additional energy levels and spin orbit splitting of valence band, respectively. The PL spectra shows considerable amount of native defects in the as grown films and after irradiation the density of defects increased. From the overall observations it is to be noted that the irradiation of CdTe films led to the creation of defects due to which both the electronic structure and optical properties can be modified. Further investigations are required to find out the exact role of these defects in the process of photoconduction. Acknowledgements This work was supported by the Inter University Accelerator Centre (IUAC), New Delhi, India through the Project UFUP 34319. The authors cordially acknowledge the help extended by technical staff of Pelletron group during the irradiation experiment. The authors wish to acknowledge Dr. D.M. Phase, Scientist and Mr. Vinay Ahire, Junior engineer, UGC-DAE Consortium for Scientific Research, Indore Centre for EDA/SEM analysis. One of the authors (RS) gratefully acknowledges University Grants Commission (UGC), New Delhi for awarding UGC-Research Award [Project No. F-30-1/2004 (SA-II)]. References [1] P.C. Srivastava, S.P. Pandey, K. Asokan, Nucl. Instrum. Methods Phys. Res., B Beam Interact. Mater. Atoms 244 (2006) 166. [2] V.N. Bhoraskar, Curr. Sci. 80 (2001) 1567. [3] K. Weinert, M. Schwicken, U. Rau, 3rd World Conference on Photovoltaic Energy Conversion, Osaka, Japan, May 11–18, 2003, p. 697. [4] S.Zh. Karazhanov, Appl. Phys. Lett. 78 (2001) 24. [5] M. Imaizumi, M. Yamaguchi, S.J. Taylor, S. Matsuda, O. Kawasaki, T. Hisamatsu, Sol. Energy Mater. Sol. Cells 50 (1998) 237.
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