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Growth and characterization of Cd1−xZnxTe thin films prepared from elemental multilayer deposition
Applied Surface Science 256 (2010) 4879–4882
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Growth and characterization of Cd1−x Znx Te thin films prepared from elemental multilayer deposition Rajiv Ganguly a , Sumana Hajra a , Tamosha Mandal a , Pushan Banerjee b,∗ , Biswajit Ghosh b a b
Institute of Engg. & Management, Saltlake Electronic Complex, Kolkata 700091, India School of Energy Studies, Jadavpur University, Kolkata 700032, India
a r t i c l e
i n f o
Article history: Received 18 December 2009 Received in revised form 23 February 2010 Accepted 23 February 2010 Available online 3 March 2010 PACS: 78.67.Pt 81.15.Dj 78.70.Ck 78.55.−m
a b s t r a c t Cd1−x Znx Te is a key material for fabrication of high-energy radiation detectors and optical devices. Conventionally it is fabricated using single crystal growth techniques. The method adopted here is the deposition of elemental multilayer followed by thermal annealing in vacuum. The multilayer structure was annealed at different temperatures using one to five repetitions of Cd–Zn–Te sequence. X-ray diffraction pattern for the multilayer with five repetitions revealed that annealing at 475 ◦ C yielded single-phase material compared to other annealing conditions. EDX spectroscopy was carried out to study the corresponding compositions. Photoluminescence properties and change of resistance of the multilayer under illumination were also studied. The resistivity of the best sample was found to be a few hundreds of cm. © 2010 Elsevier B.V. All rights reserved.
Keywords: CZT Multilayer Vapor deposition X-ray diffraction Luminescence
1. Introduction Cd1−x Znx Te (CZT) is the basic element in the fabrication of solid state detectors for X-ray, gamma ray, cosmic ray and infrared electro-magnetic radiation detection observed in medicine, astronomy, and high-energy physics [1]. The physical properties of CZT such as constituent atoms with high atomic number, a sufficiently large tunable energy band gap (from the visible to the infrared portion of the electro-magnetic spectrum) to minimize leakage currents at room temperature, high detective quantum efficiency, good charge transport, high resistivity and high intrinsic mobilitylifetime () products for electrons and holes give it an attractive candidature as a good detector material [2]. However, despite the tremendous promise of this material, problems clearly exist. CZT crystals are difficult to grow in large sizes and with ultra high purity. There is a need to further lower the leakage currents in detector grade material and also to increase the efficiency of charge collection. Uniform CZT single crystals can be obtained by using conventional or modified Bridgman method [3,4], the trav-
∗ Corresponding author. Tel.: +91 33 24146823; fax: +91 33 24146853. E-mail address: b [email protected] (P. Banerjee). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.02.076
eling heater technique [5,6] and the temperature gradient solution growth [7–9]. Several methods have been used to prepare CdZnTe films, such that molecular beam epitaxy [10], liquid phase epitaxy [11], electro-deposition [12], close space vapor transport [13], laser ablation [14], thermal evaporation [15,16], sputtering [17], and metal-organic chemical vapor deposition (MOCVD) [18]. Multilayer method of deposition was also tried by diffusion of elemental Zn into CdTe [19]. Here we have tried to fabricate CZT thin film using all elemental precursors—Cd, Zn and Te. As an alternative to single crystals, this method of multilayer elemental deposition in vacuum can be utilized for the production of large-area polycrystalline CZT thin films. 2. Experimental methods The substrate used in the present study was chromium coated sodalime glass slides, because cadmium and zinc were found to stick poorly over glass. Chromium was found earlier to act as a good inter-adhesive layer between glass and thin films [20]. So, prior to deposition of Cd, Zn and Te, a thin (∼10 nm) layer of chromium was deposited over glass slides at a substrate temperature of 190 ◦ C, using thermal evaporation of chromium in high vacuum. Then pure Cd, Zn and Te raw materials were taken in separate quartz crucibles
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R. Ganguly et al. / Applied Surface Science 256 (2010) 4879–4882
Table 1 Calculated individual thickness required for deposition of elemental layers. No. of repetitions
1 2 3 4 5
Thickness (nm) of Cd
Zn
Te
185.5 92.8 61.8 46.4 37.1
132.4 66.2 44.1 33.1 26.5
582.1 291.0 194.0 145.5 116.3
(acting as effusion sources) and placed individually (for individual element depositions) in the physical vapor deposition (PVD) chamber. At a high vacuum (10−5 mbar), the crucible was heated to suitable temperature to evaporate the corresponding element placed inside. Stacked multilayers comprising of Cd, Zn and Te thin films – Cd at the bottom and Te at the top – were consecutively deposited onto chromium coated unheated sodalime glass substrates of size 7.4 cm × 2.4 cm. Tellurium was kept at the top so as to ease the future electrical contact fabrication to the structure. Cd was preferred as the layer just above chromium because it was found that keeping Zn after chromium had resulted in peeling off of the entire multilayer after annealing. The required thicknesses of the individual layers were calculated so as to keep an equal molar proportion of cadmium and zinc, and are given in Table 1. The thickness of all the layers was controlled within ±5 nm of the required values and measured during evaporation using a digital thickness monitor (DTM-101, Hindhivac, India). The process was carried out for one to five numbers of Cd–Zn–Te repetitions and the individual thickness was so adjusted as to keep the total thickness of the films around 900 nm. The evaporated multilayers were annealed in a vacuum of 10−5 mbar for 1 h, at temperatures of 425, 450 and 475 ◦ C (i.e. three sets), because for the formation of tellurides from elements, temperature greater than 400 ◦ C is required. The annealed structures were then tested structurally using Bruker D8 powder X-ray diffrac-
tometer, Hitachi S3200 scanning electron microscope (Department of Materials Science, University of Surrey, UK); and optically using PerkinElmer LS-55 luminescence spectrometer (UGC-DAE CSR, Kolkata) through an excitation at 490 nm at room temperature. The change of resistance of the multilayer with time under illumination (photoresponse) was also recorded using vacuum evaporated silver contacts placed 1 cm apart over the film surface. Electronic parameters were determined using Van der Pauw Hall measurement technique with silver dots evaporated at the corners of the square shaped film of side 1 cm. 3. Results and discussion 3.1. Structural analysis using XRD, EDX and SEM Fig. 1 shows the X-ray diffraction pattern for five of the multilayers, from which it becomes evident that the multilayer structure with five repetitions annealed at 475 ◦ C (will be referred as sample 5L hereafter) showed single-phase behaviour with no dominant phase other than CZT present. While on reducing the no. of repetitions or the annealing temperature, peaks for ZnTe and especially unreacted Te (JCPDS card no. 040555) appeared. For Cd1−x Znx Te, the d-value of XRD peak for a particular plane lies between that for pure CdTe and pure ZnTe. For this purpose JCPDS card no. 150770 was taken for CdTe with lattice parameter aCdTe = 6.4810 Å and card no. 150746 for ZnTe with lattice parameter aZnTe = 6.1026 Å [21]. The average value of “x” for sample 5L was determined from the observed d-values three XRD peaks using the following formula (Vegard’s law): d(Cd1−x Znx Te) = (1 − x) · d(CdTe) + x · d(ZnTe) Using the above formula, the value of “x” was found to be 0.40. As only this particular sample turned out to be single phase among all, analysis of the subsequent properties was carried out only for this sample.
Fig. 1. XRD pattern for multilayers.
R. Ganguly et al. / Applied Surface Science 256 (2010) 4879–4882
4881
Fig. 2. EDX spectrum for sample 5L.
Table 2 Composition of sample 5L from EDX. Element
Weight%
Atomic%
Zn K Cd L Te L
13 29 58
22 28 50
The composition of sample 5L, as determined from EDX (Fig. 2), is shown in Table 2. Using this table, the value of “x” was determined as 0.43, which is in close agreement with the value obtained from XRD. The SEM micrograph for sample 5L is shown in Fig. 3. It is evident that the particles are smoothly and uniformly distributed at the surface, the particle size being less than 100 nm. 3.2. Luminescent property The PL spectrum of sample 5L is shown in Fig. 4. In the photoluminescence spectrum two distinct peaks are visible—a lower intensity broader peak centered about 662 nm (1.87 eV) and another higher intensity sharper one at around 740 nm (1.67 eV). The origin of the peaks can be explained from the property of tellurium clusters as an iso-electronic exciton trap in II–VI compounds, as reported earlier by several authors. Roessler et al. [22] found in CdS:Te that for concentration of Te around 1018 cm−3 , the PL spectra (with excitation of 435 nm) showed a higher energy band around 600 nm. As the Te doping was raised gradually, lower energy band appeared and eventually dominated the spectrum for Te concentrations above 1020 cm−3 . The high-energy band was attributed to an exciton trapped at an isolated Te atom on a sulfur site, whereas the lower energy band resulted from trapping by two Te atoms on nearest neighbour sulfur sites. Cuthbert and Thomas
Fig. 3. SEM for sample 5L.
Fig. 4. PL spectra for sample 5L.
[23] also showed that in case of CdS1−x Tex the spectrum with low tellurium concentrations was due to radiative recombination of an exciton bound to a Te atom. Similarly, tellurium was found as isoelectronic exciton traps in several other II–VI compounds, such as CdSe [24], ZnSe1−x Tex [25] as well as in ZnSe–ZnTe superlattice [26] and CdTe/CdS combination [27]. Thus, tellurium related photoluminescence peaks were observed not only for systems with Te dopant but also where Te was one of the chief elements. However, no earlier report on room temperature PL was noticed by us with Cd–Zn–Te multilayer system adopted here, with excitation at 490 nm. So, we cannot get a direct evidence regarding the origin of the peaks at those particular positions reported by us. But in view of the discussions made by the above authors, it can be inferred here that the higher energy band around 662 nm possibly originated from a bound hole and electron recombining at a tellurium atom. The 740 nm band corresponds to the radiative decay of a hole and electron bound to two Te atoms at nearest neighbour sites. It is to be noted here that earlier workers [22,23] also reported the generation of two PL peaks due to Te traps—the position of the peaks being dependent on the excitation wavelength. 3.3. Change in resistance under illumination Sample 5L were kept under a tungsten halogen lamp with 100 mW/cm2 intensity and the changes in the resistance (measured across two silver contacts over them) were recorded as a function time of illumination up to 3 min. The dark resistances were taken as that at time “0”. The normalized values of the resistances were plotted by taking the dark resistance as unity and have been shown in Fig. 5 against the time of illumination. When the film was illuminated, the measured resistance fell with time and after some time the change in resistance was very small. This was due to the fact that the rate of photogeneration of carriers decreased with time. Also the process of recombination took place with respect to time, thereby decreasing the value of photocurrent. Ultimately a situation was obtained where the process of generation of charge carrier and recombination reached equilibrium under constant illumination. This resulted in a nearly flat profile with time at the end. The resistance at the end of recording was about 25% to that at dark (time = 0). However, the response under illumination is rather slow and this is because the charge carriers had to cross a large number of grain boundaries to flow from one contact to the other, undergoing frequent scattering and recombination.
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R. Ganguly et al. / Applied Surface Science 256 (2010) 4879–4882
Table 3 Parameters from Hall measurement. Sample 5L
Bulk resistivity ( cm)
Carrier density (cm−3 )
Hall mobility (cm2 V−1 S−1 )
Conductivity type
416
1.0 × 10
0.001
p
19
Acknowledgements One of the authors (Pushan Banerjee) gratefully acknowledges the support provided by Council of Scientific and Industrial Research, Govt. of India for carrying out this research. The help extended by Dr. Abhijit Saha of UGC-DAE CSR, Kolkata center, towards taking PL measurements is also thankfully acknowledged. References
Fig. 5. Photoresponse for sample 5L.
3.4. Electronic properties using Hall effect The result of the Van der Pauw Hall measurement (carried out at a magnetic field of 5 kG) of sample 5L is shown in Table 3. The resistivity was of the order of a few hundreds of cm with p-type conductivity, and the carrier density was in the range of 1019 cm−3 . 4. Conclusion Cd1−x Znx Te thin films have been prepared from multilayer structures comprising of Cd, Zn and Te. The structures were annealed at various temperatures with different number of Cd–Zn–Te repetitions. Results of X-ray diffraction proved that the multilayer annealed at 475 ◦ C with five repetitions of Cd–Zn–Te showed best behaviour in regard to single-phase nature. The values of “x” determined from XRD and EDX matched closely. SEM exhibited smooth and uniform particle distribution at the surface of the film. Further works in this area are on the way to improve the performance of the film and its application in large-area sensing devices.
[1] O. Zelaya-Angel, J.G. Mendoza-Alvarez, M. Becerril, N. Navaro-Contreras, N. Tirado-Nejia, J. Appl. Phys. 95 (2004) 6284. [2] G. Li, X. Zhang, W. Jie, Semicond. Sci. Technol. 20 (2005) 86. [3] C.M. Greaves, B.A. Brunett, J.M. Van Scyoc, T.E. Schlesinger, R.B. James, Nucl. Instrum. Methods Phys. Res. Sec. A 458 (2001) 96. [4] Z.F. Li, W. Lu, G.S. Huang, J.R. Yang, L. He, S.C. Shen, J. Appl. Phys. 90 (2001) 260. [5] R. Triboulet, T.N. Duy, A. Durand, J. Vac. Sci. Technol. A 3 (1985) 95. [6] K. Oettinger, D.M. Hoffmann, A.L. Efros, B.K. Meyer, M. Salk, K.W. Benz, J. Appl. Phys. 71 (1992) 4523. [7] H.Y. Shin, C.Y. Sun, Mater. Sci. Eng. B 41 (1996) 345. [8] W.C. Chou, F.R. Chen, T.Y. Chinagg, H.Y. Shin, C.Y. Sun, C.M. Lin, K. Chern-Yu, C.T. Tsai, D.S. Chuu, J. Cryst. Growth 169 (1996) 747. [9] P.Y. Tseng, C.B. Fu, M.C. Kuo, C.S. Yang, C.C. Huang, W.C. Chou, Y.T. Shih, H.Y. Hsin, S.M. Lan, W.H. Lan, Mater. Chem. Phys. 78 (2002) 529. [10] J.H. Dinan, S.B. Qadri, J. Vac. Sci. Technol. A 3 (1985) 851. [11] B. Pellicary, J.P. Chamonal, G.L. Destefanis, L. DiCiocio, Proc. SPIE 865 (1988) 22. [12] B.M. Basol, V.K. Kapur, M.L. Ferris, J. Appl. Phys. 66 (1989) 1816. [13] J. Gonzalez-Hernandez, O. Zelaya, J.G. Mendoza-Alvarez, E. Lopez-Cruz, D.A. Pawlick, D.D. Allred, J. Vac. Sci. Technol. A 9 (1991) 550. [14] A. Aydinli, A. Compaan, G. Contreras-Puente, A. Mason, Solid State Commun. 80 (1991) 465. [15] Y.J. Park, K.H. Kim, Y.H. Na, T.R. Jung, S.U. Kim, IEEE Nucl. Sci. Symp. Conf. Rec. 7 (2004) 4425. [16] R. Sharma, L. Dori, S. Chandra, K. Misra, Proc. SPIE 3975 (2000) 1278. [17] M. Becerril, H. Silva-Lopez, O. Zelaya-Angel, Revista Mexican de Fisica 50 (2004) 588. [18] T.L. Chu, S.S. Chu, C. Ferekides, J. Britt, J. Appl. Phys. 71 (1992) 5635. [19] G.G. Rusu, M. Rusu, M. Girtan, Vacuum 81 (2007) 1476. [20] B. Ghosh, D.P. Chakraborty, M.J. Carter, Semicond. Sci. Technol. 11 (1996) 1358. [21] S. Stolyarova, F. Edelman, A. Chack, A. Berner, P. Werner, N. Zakharov, M. Vytrykhivsky, R. Beserman, R. Weil, Y. Nemirovsky, J. Phys. D Appl. Phys. 41 (2008) 065402. [22] D.M. Roessler, J. Appl. Phys. 41 (1970) 4589. [23] J.D. Cuthbert, D.G. Thomas, J. Appl. Phys. 39 (1968) 1573. [24] O. Goede, W. Heimbrodt, Phys. Status Solidi B 110 (1982) 175. [25] K. Dhese, J. Goodwin, W.E. Hagston, J.E. Nicholls, J.J. Davier, B. Cockayne, P.J. Wright, Semicond. Sci. Technol. 7 (1992) 1210. [26] J.J. Davies, Semicond. Sci. Technol. 3 (1988) 219. [27] D.H. Levi, B.D. Fluegel, R.K. Ahrenkiel, A.D. Compaan, L.M. Woods, Proceedings of the 25th IEEE Photovoltaic Specialists Conference, Washington, DC, USA, 1996, p. 913.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Growth and characterization of Cd1−x Znx Te thin films prepared from elemental multilayer deposition Rajiv Ganguly a , Sumana Hajra a , Tamosha Mandal a , Pushan Banerjee b,∗ , Biswajit Ghosh b a b
Institute of Engg. & Management, Saltlake Electronic Complex, Kolkata 700091, India School of Energy Studies, Jadavpur University, Kolkata 700032, India
a r t i c l e
i n f o
Article history: Received 18 December 2009 Received in revised form 23 February 2010 Accepted 23 February 2010 Available online 3 March 2010 PACS: 78.67.Pt 81.15.Dj 78.70.Ck 78.55.−m
a b s t r a c t Cd1−x Znx Te is a key material for fabrication of high-energy radiation detectors and optical devices. Conventionally it is fabricated using single crystal growth techniques. The method adopted here is the deposition of elemental multilayer followed by thermal annealing in vacuum. The multilayer structure was annealed at different temperatures using one to five repetitions of Cd–Zn–Te sequence. X-ray diffraction pattern for the multilayer with five repetitions revealed that annealing at 475 ◦ C yielded single-phase material compared to other annealing conditions. EDX spectroscopy was carried out to study the corresponding compositions. Photoluminescence properties and change of resistance of the multilayer under illumination were also studied. The resistivity of the best sample was found to be a few hundreds of cm. © 2010 Elsevier B.V. All rights reserved.
Keywords: CZT Multilayer Vapor deposition X-ray diffraction Luminescence
1. Introduction Cd1−x Znx Te (CZT) is the basic element in the fabrication of solid state detectors for X-ray, gamma ray, cosmic ray and infrared electro-magnetic radiation detection observed in medicine, astronomy, and high-energy physics [1]. The physical properties of CZT such as constituent atoms with high atomic number, a sufficiently large tunable energy band gap (from the visible to the infrared portion of the electro-magnetic spectrum) to minimize leakage currents at room temperature, high detective quantum efficiency, good charge transport, high resistivity and high intrinsic mobilitylifetime () products for electrons and holes give it an attractive candidature as a good detector material [2]. However, despite the tremendous promise of this material, problems clearly exist. CZT crystals are difficult to grow in large sizes and with ultra high purity. There is a need to further lower the leakage currents in detector grade material and also to increase the efficiency of charge collection. Uniform CZT single crystals can be obtained by using conventional or modified Bridgman method [3,4], the trav-
∗ Corresponding author. Tel.: +91 33 24146823; fax: +91 33 24146853. E-mail address: b [email protected] (P. Banerjee). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.02.076
eling heater technique [5,6] and the temperature gradient solution growth [7–9]. Several methods have been used to prepare CdZnTe films, such that molecular beam epitaxy [10], liquid phase epitaxy [11], electro-deposition [12], close space vapor transport [13], laser ablation [14], thermal evaporation [15,16], sputtering [17], and metal-organic chemical vapor deposition (MOCVD) [18]. Multilayer method of deposition was also tried by diffusion of elemental Zn into CdTe [19]. Here we have tried to fabricate CZT thin film using all elemental precursors—Cd, Zn and Te. As an alternative to single crystals, this method of multilayer elemental deposition in vacuum can be utilized for the production of large-area polycrystalline CZT thin films. 2. Experimental methods The substrate used in the present study was chromium coated sodalime glass slides, because cadmium and zinc were found to stick poorly over glass. Chromium was found earlier to act as a good inter-adhesive layer between glass and thin films [20]. So, prior to deposition of Cd, Zn and Te, a thin (∼10 nm) layer of chromium was deposited over glass slides at a substrate temperature of 190 ◦ C, using thermal evaporation of chromium in high vacuum. Then pure Cd, Zn and Te raw materials were taken in separate quartz crucibles
4880
R. Ganguly et al. / Applied Surface Science 256 (2010) 4879–4882
Table 1 Calculated individual thickness required for deposition of elemental layers. No. of repetitions
1 2 3 4 5
Thickness (nm) of Cd
Zn
Te
185.5 92.8 61.8 46.4 37.1
132.4 66.2 44.1 33.1 26.5
582.1 291.0 194.0 145.5 116.3
(acting as effusion sources) and placed individually (for individual element depositions) in the physical vapor deposition (PVD) chamber. At a high vacuum (10−5 mbar), the crucible was heated to suitable temperature to evaporate the corresponding element placed inside. Stacked multilayers comprising of Cd, Zn and Te thin films – Cd at the bottom and Te at the top – were consecutively deposited onto chromium coated unheated sodalime glass substrates of size 7.4 cm × 2.4 cm. Tellurium was kept at the top so as to ease the future electrical contact fabrication to the structure. Cd was preferred as the layer just above chromium because it was found that keeping Zn after chromium had resulted in peeling off of the entire multilayer after annealing. The required thicknesses of the individual layers were calculated so as to keep an equal molar proportion of cadmium and zinc, and are given in Table 1. The thickness of all the layers was controlled within ±5 nm of the required values and measured during evaporation using a digital thickness monitor (DTM-101, Hindhivac, India). The process was carried out for one to five numbers of Cd–Zn–Te repetitions and the individual thickness was so adjusted as to keep the total thickness of the films around 900 nm. The evaporated multilayers were annealed in a vacuum of 10−5 mbar for 1 h, at temperatures of 425, 450 and 475 ◦ C (i.e. three sets), because for the formation of tellurides from elements, temperature greater than 400 ◦ C is required. The annealed structures were then tested structurally using Bruker D8 powder X-ray diffrac-
tometer, Hitachi S3200 scanning electron microscope (Department of Materials Science, University of Surrey, UK); and optically using PerkinElmer LS-55 luminescence spectrometer (UGC-DAE CSR, Kolkata) through an excitation at 490 nm at room temperature. The change of resistance of the multilayer with time under illumination (photoresponse) was also recorded using vacuum evaporated silver contacts placed 1 cm apart over the film surface. Electronic parameters were determined using Van der Pauw Hall measurement technique with silver dots evaporated at the corners of the square shaped film of side 1 cm. 3. Results and discussion 3.1. Structural analysis using XRD, EDX and SEM Fig. 1 shows the X-ray diffraction pattern for five of the multilayers, from which it becomes evident that the multilayer structure with five repetitions annealed at 475 ◦ C (will be referred as sample 5L hereafter) showed single-phase behaviour with no dominant phase other than CZT present. While on reducing the no. of repetitions or the annealing temperature, peaks for ZnTe and especially unreacted Te (JCPDS card no. 040555) appeared. For Cd1−x Znx Te, the d-value of XRD peak for a particular plane lies between that for pure CdTe and pure ZnTe. For this purpose JCPDS card no. 150770 was taken for CdTe with lattice parameter aCdTe = 6.4810 Å and card no. 150746 for ZnTe with lattice parameter aZnTe = 6.1026 Å [21]. The average value of “x” for sample 5L was determined from the observed d-values three XRD peaks using the following formula (Vegard’s law): d(Cd1−x Znx Te) = (1 − x) · d(CdTe) + x · d(ZnTe) Using the above formula, the value of “x” was found to be 0.40. As only this particular sample turned out to be single phase among all, analysis of the subsequent properties was carried out only for this sample.
Fig. 1. XRD pattern for multilayers.
R. Ganguly et al. / Applied Surface Science 256 (2010) 4879–4882
4881
Fig. 2. EDX spectrum for sample 5L.
Table 2 Composition of sample 5L from EDX. Element
Weight%
Atomic%
Zn K Cd L Te L
13 29 58
22 28 50
The composition of sample 5L, as determined from EDX (Fig. 2), is shown in Table 2. Using this table, the value of “x” was determined as 0.43, which is in close agreement with the value obtained from XRD. The SEM micrograph for sample 5L is shown in Fig. 3. It is evident that the particles are smoothly and uniformly distributed at the surface, the particle size being less than 100 nm. 3.2. Luminescent property The PL spectrum of sample 5L is shown in Fig. 4. In the photoluminescence spectrum two distinct peaks are visible—a lower intensity broader peak centered about 662 nm (1.87 eV) and another higher intensity sharper one at around 740 nm (1.67 eV). The origin of the peaks can be explained from the property of tellurium clusters as an iso-electronic exciton trap in II–VI compounds, as reported earlier by several authors. Roessler et al. [22] found in CdS:Te that for concentration of Te around 1018 cm−3 , the PL spectra (with excitation of 435 nm) showed a higher energy band around 600 nm. As the Te doping was raised gradually, lower energy band appeared and eventually dominated the spectrum for Te concentrations above 1020 cm−3 . The high-energy band was attributed to an exciton trapped at an isolated Te atom on a sulfur site, whereas the lower energy band resulted from trapping by two Te atoms on nearest neighbour sulfur sites. Cuthbert and Thomas
Fig. 3. SEM for sample 5L.
Fig. 4. PL spectra for sample 5L.
[23] also showed that in case of CdS1−x Tex the spectrum with low tellurium concentrations was due to radiative recombination of an exciton bound to a Te atom. Similarly, tellurium was found as isoelectronic exciton traps in several other II–VI compounds, such as CdSe [24], ZnSe1−x Tex [25] as well as in ZnSe–ZnTe superlattice [26] and CdTe/CdS combination [27]. Thus, tellurium related photoluminescence peaks were observed not only for systems with Te dopant but also where Te was one of the chief elements. However, no earlier report on room temperature PL was noticed by us with Cd–Zn–Te multilayer system adopted here, with excitation at 490 nm. So, we cannot get a direct evidence regarding the origin of the peaks at those particular positions reported by us. But in view of the discussions made by the above authors, it can be inferred here that the higher energy band around 662 nm possibly originated from a bound hole and electron recombining at a tellurium atom. The 740 nm band corresponds to the radiative decay of a hole and electron bound to two Te atoms at nearest neighbour sites. It is to be noted here that earlier workers [22,23] also reported the generation of two PL peaks due to Te traps—the position of the peaks being dependent on the excitation wavelength. 3.3. Change in resistance under illumination Sample 5L were kept under a tungsten halogen lamp with 100 mW/cm2 intensity and the changes in the resistance (measured across two silver contacts over them) were recorded as a function time of illumination up to 3 min. The dark resistances were taken as that at time “0”. The normalized values of the resistances were plotted by taking the dark resistance as unity and have been shown in Fig. 5 against the time of illumination. When the film was illuminated, the measured resistance fell with time and after some time the change in resistance was very small. This was due to the fact that the rate of photogeneration of carriers decreased with time. Also the process of recombination took place with respect to time, thereby decreasing the value of photocurrent. Ultimately a situation was obtained where the process of generation of charge carrier and recombination reached equilibrium under constant illumination. This resulted in a nearly flat profile with time at the end. The resistance at the end of recording was about 25% to that at dark (time = 0). However, the response under illumination is rather slow and this is because the charge carriers had to cross a large number of grain boundaries to flow from one contact to the other, undergoing frequent scattering and recombination.
4882
R. Ganguly et al. / Applied Surface Science 256 (2010) 4879–4882
Table 3 Parameters from Hall measurement. Sample 5L
Bulk resistivity ( cm)
Carrier density (cm−3 )
Hall mobility (cm2 V−1 S−1 )
Conductivity type
416
1.0 × 10
0.001
p
19
Acknowledgements One of the authors (Pushan Banerjee) gratefully acknowledges the support provided by Council of Scientific and Industrial Research, Govt. of India for carrying out this research. The help extended by Dr. Abhijit Saha of UGC-DAE CSR, Kolkata center, towards taking PL measurements is also thankfully acknowledged. References
Fig. 5. Photoresponse for sample 5L.
3.4. Electronic properties using Hall effect The result of the Van der Pauw Hall measurement (carried out at a magnetic field of 5 kG) of sample 5L is shown in Table 3. The resistivity was of the order of a few hundreds of cm with p-type conductivity, and the carrier density was in the range of 1019 cm−3 . 4. Conclusion Cd1−x Znx Te thin films have been prepared from multilayer structures comprising of Cd, Zn and Te. The structures were annealed at various temperatures with different number of Cd–Zn–Te repetitions. Results of X-ray diffraction proved that the multilayer annealed at 475 ◦ C with five repetitions of Cd–Zn–Te showed best behaviour in regard to single-phase nature. The values of “x” determined from XRD and EDX matched closely. SEM exhibited smooth and uniform particle distribution at the surface of the film. Further works in this area are on the way to improve the performance of the film and its application in large-area sensing devices.
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