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Crystal defects and optical transitions in high purity, high resistivity CdTe for device applications
Materials Science and Engineering B91– 92 (2002) 353– 357 www.elsevier.com/locate/mseb
Crystal defects and optical transitions in high purity, high resistivity CdTe for device applications N. Armani *, C. Ferrari, G. Salviati, F. Bissoli, M. Zha, L. Zanotti CNR–MASPEC Institute, Parco Area delle Scienze, 37 /A 43010 Fontanini, Parma, Italy
Abstract The structural and optical properties of high purity undoped semi-insulating (SI) Cadmium Telluride (CdTe) crystals, grown by Physical Vapour Transport (PVT) and by modified Vertical Bridgman (BG) techniques have been studied. The samples were obtained from 7N source elements by direct synthesis followed by heat treatment to adjust the stoichiometry. The experimental measurements have been systematically performed using high resolution X-ray diffraction (HRXRD) and cathodoluminescence (CL) techniques, before and after the thermal annealing procedures. The CL emissions from low and high resistivity samples have been studied in order to correlate the CL bands to the presence and concentration of native defects in the crystals. Two broad luminescence bands have been found in addition to the expected emission related to the excitonic recombination. The nature of the defects involved in the transitions was studied by analysing the position of the energy levels and the relative intensity variation of the CL peaks. The effect of the thermal treatments, performed at different temperatures, on the optical and electrical properties of the specimens was also analysed. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Cadmium Telluride; Defect formation; Annealing; Cathodoluminescence; X-ray diffraction
1. Introduction Cadmium Telluride (CdTe) is a very attractive semiconducting material for a large number of applications in the field of X- and g-ray detectors and electro-optical devices. In order to obtain high resistivity material, required for electro-optic applications, stoichiometric and impurity-free crystals should be grown. Up to now, semi-insulating (SI) CdTe has been obtained by introducing Group III or VII elements (Cl, In, I, etc.) as dopants to create compensating defects in the crystal [1,2]. However, the intentional doping of the crystals may lead to lattice deformation and poor crystalline quality, due to segregation of elements in the grown crystal. The need for high quality and defect free CdTe, with high optical transmission, has motivated a systematic study on the formation of crystalline defects. The precipitation in the crystals is usually associated with the Te-rich condition of the melt-growth; in this case dislocations can also occur. However, precipitation can also * Corresponding author. E-mail address: [email protected] (N. Armani).
happen in PVT-grown crystals, despite the narrow range of deviation from stoichiometry and the vapour phase of elements do not allow the formation of inclusions. Thus, precipitates are unavoidable in CdTe and some attempts to remove them were made by annealing the specimens in a controlled pressure of Cadmium [3]. The SI nature of doped CdTe samples is due to a complex, created by the dopant atoms with a Cadmium vacancy (VCd), which compensates the excess of carriers. In the case of almost impurity free undoped crystals, like the present samples, the mechanism responsible for the compensation is different. During the growth it is impossible to avoid the formation of native defects, the concentration of which strictly depends on the deviation from the stoichiometry. Theoretical calculations supply data on the native defect concentration for different growth conditions [4,5]. Considering the Te-rich material and a growth temperature above 900 °C, the energetically most probable native defect is the VCd, even if the Tellurium vacancy (VTe) and the substitutional Tellurium on the Cadmium site (TeCd) can also be present. Theoretical calculations give the energy positions of the levels due to those defects with a large uncertainty; further, those data are not in good agreement with the experimental values [6].
0921-5107/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 0 1 ) 0 1 0 6 6 - 2
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N. Armani et al. / Materials Science and Engineering B91–92 (2002) 353–357
CdTe crystals were synthesised from 7N purity Cd and Te [7], then a thermal treatment to correctly adjust the Cd/Te stoichiometry ratio [8] was performed. The crystals obtained presented the required stoichiometry and purity useful for growth of SI CdTe from both the vapour phase and the melt. The effect of native defects and doping on the electrical and optical properties of CdTe crystals has been systematically studied in the past by means of the comparative techniques of cathodoluminescence (CL) and photo-induced current transient spectroscopy (PICTS) [9]. It has been possible to correlate a typical optical emission denoted as A-band [10] with three distinct electronic levels in the band gap. Further, luminescence-based analyses on doped crystals showed a correlation between the electrical effects due to the dopant and the optical emissions seen in CL and photoluminescence (PL) spectra [11,12]. The CL technique, by means of the possibility to acquire pan- and mono-chromatic images and by changing the energy of the impinging electron probe, can give information on the distribution of the luminescence in plane and in depth of the specimens and on the correlation between optical transitions and crystal defects [13,14].
2. Experimental The crystals studied were grown both by the Physical Vapour Transport (PVT) and the modified Bridgman (BG) techniques. A horizontal configuration was adopted to avoid the crystals from falling down during the growth because of poor sticking. Separate heating controllers (Eurothem 818) checked the three zones of the furnace and were able to keep the temperature fluctuation within 9 0.2 °C. Due to the poor heat
conductivity of CdTe, the applied temperature profile was intentionally designed to present a step temperature gradient at the growth front. A closed quartz ampoule was provided with a short capillary at the crystallisation end. The capillary was part of the core of a quartz bar (10 cm long and 1 cm in diameter), coaxial with the ampoule, which further favoured the heat release. The effect of the attached bar was evidenced by the convex shape of the growing interface, on which a number of facets appeared. The furnace was so designed to steadily move, with respect to the fixed growth ampoule, in a speed range from 1 to 10 mm per day. A 3 mm per day moving speed was applied in these growth experiments which yielded crystals up to 27 mm in diameter and several cm in length. The post-growth cooling rates were about 20–40 °C h − 1. The BG growth was carried out with the charge (70 g), encapsulated with B2O3, in an open quartz crucible inside a pressure chamber (5 atm Nitrogen). Crystal ingots made of large single grains were grown in about 36 h. After a first run of measurements, the CdTe crystals studied were annealed at 600 °C in a closed quartz ampoule, in an inert gas atmosphere, with a Cadmium counterpressure produced by Cd-rich CdTe powder. The resistivity of the crystals was analysed by Van der Pauw measurements at room temperature; the structural characterisation was performed by high resolution X-ray diffraction (HRXRD) and the optical studies by CL. A Philips diffractometer in the two crystal configuration with a (220) Ge monochromator was used to acquire the V–2q rocking curves. The optical analyses were performed with a Gatan MonoCL system mounted on a commercial SEM; the spectra as well as the panchromatic and monochromatic images were obtained at temperatures ranging from 77 to 300 K.
3. Results and discussion
Fig. 1. HRXRD rocking curves, using the CuKa1 (333) symmetrical reflection, of the high and low resistivity BG-grown samples (cz8, cz3), in comparison with a high resistivity PVT-grown specimen.
Different BG-grown crystals with resistivities (z) between 300 and 108 V cm, respectively, and PVT-grown samples with z 109 V cm have been investigated. The analysis on the crystalline quality of the samples has been carried out by acquiring the (333) symmetrical rocking curves with the CuKa1 wavelength. The diffraction profiles of high and low resistivity BG-grown and high resistivity PVT-grown samples are shown in Fig. 1. The BG SI specimen shows the most intense reflectivity and the lowest FWHM, indicating the better crystalline quality of the sample. A spatial mapping of the (333) reflection, built by acquiring the rocking curves in different positions of the specimen surface, shows a clear inhomogeneity of the crystal, with the angular
N. Armani et al. / Materials Science and Engineering B91–92 (2002) 353–357
Fig. 2. Low temperature (77 K) CL panchromatic image of the low resistivity BG-grown sample, taken at a primary electron beam energy of 25 keV.
deviation of the Bragg peak of about 300¦ and with the FWHM ranging from 70 to 130¦. In Fig. 2 a panchromatic image of the low resistivity sample is shown. A large density of dark spots of different dimensions, representing non-radiative recombination centres, can be observed. The dark spots could be related to crystal defects, like dislocations emerging to the (111) surface of the crystal, or precipitates.
355
In the low temperature (77 K) CL spectra, of Fig. 3, in addition to an intense near band edge (NBE) emission peaked at about 1.57 eV, a large emission band in the region of 0.9–1.2 eV and the so called A-band, centred at about 1.4 eV can be observed. In the low resistivity sample spectrum, the intensity of the A-band is strongly reduced, while the intensity of the 0.9–1.2 eV band is slightly increased. A deconvolution of the A-band reveals two distinct Gaussian peaks at 1.362 and 1.44 eV, respectively. The same fitting procedures have been applied to the CL emission band between 0.9 and 1.2 eV, revealing two Gaussian peaks at 1.02 and 1.12 eV, respectively. The nature of the levels responsible for the CL emissions was investigated by acquiring CL spectra at different injection powers and temperatures. By increasing the power density from 0.29 to 29 W cm − 2, the relative intensity of the 1.44 eV peak decreases with respect to the 1.362 eV peak, as shown in Fig. 4. Further, the position and the relative intensity of the 1.02 and the 1.12 eV peaks remains almost unchanged, while the intensity of the whole band increases with increasing the injection power. The transitions responsible for the large CL band in the region 0.9–1.2 eV, present in all the spectra, should be due to deep levels. As for the 1.02 eV emission, a donor level due to VTe or TeCd with an energy of 0.4 eV below the conduction band (CB) and an acceptor level due to VCd in the 0/ − 1 charge state, at 0.2 eV above
Fig. 3. CL spectra, at 77 K, of the same crystals studied with the HRXRD technique. In the inset of the graph is shown the magnification of the low energy spectral region.
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N. Armani et al. / Materials Science and Engineering B91–92 (2002) 353–357
Fig. 4. Comparison of the low temperature CL spectra, from a SI BG-grown sample, acquired at increasing injection power density from 0.29 to 29 W cm − 2.
shallow donors involved in the 1.44 eV transition have been ionised so promoting the carriers in the CB. In this case the carriers could recombine through the alternative path with a shallow acceptor, so increasing the 1.36 eV CL peak intensity. After the annealing procedures performed at temperatures of 300 and 600 °C a modification in the CL spectra was observed, as shown in Fig. 5. The deep level related CL band of the p-type specimen practically disappears, while the intensity of the socalled A-band increases. The precipitates present in the crystals were probably dissolved during the annealing, as demonstrated by analogous work [16] with the creation of the native defects responsible for the shallow levels involved in the 1.4 eV band. Transmission Electron Microscopy investigations are in progress in order to study the precipitates behaviour before and after thermal treatment.
4. Conclusions CdTe material prepared by 7N pure starting elements and with well-controlled stoichiometry has been studied by HRXRD and CL. The measurements on both low and high resistivity samples suggest that shallow levels and deep levels are related to intrinsic defects, so confirming their crucial role in the compensation mechanism providing the high resistivity in our CdTe crystals. Fig. 5. Effect of 600 °C thermal annealing on the low temperature CL spectra performed on a low resistivity p-type BG-grown sample.
References the valence band (VB) [4] can be taken into account. On the contrary, the 1.12 eV CL peak can be ascribed to the deep acceptor VCd in the −1/− 2 configuration, the energy of which has been measured with different experimental techniques in the range of 0.43 –0.47 eV above the VB [6,12]. In particular, the intensity of the 1.12 eV peak is higher in p-type crys− 1/ − 2 tals, suggesting that the VCd , most likely present in high concentration in this material, is involved in this transition. Concerning the 1.36 and 1.44 eV peaks, observed only in the SI crystals, they can be related to transitions involving a shallow donor and a shallow acceptor, ascribed to the presence of VCd and Cadmium interstitial (Cdi), the concentration of which depends on the position of the Fermi level in the gap. In the case of SI or n-type material [15], these two defects exist separately only if the EF is located far from the VB. Moreover, by increasing the power density, the behaviour of the CL peak intensities reveals that the
[1] D.V. Korbutyak, S.G. Krylyuk, P.M. Tkachuk, V.I. Tkachuk, N.D. Korbutyak, M.D. Raransky, J. Cryst. Growth 197 (1999) 659. [2] J. Lee, N.C. Giles, D. Rajavel, C.J. Summers, J. Appl. Phys. 78 (1995) 5669. [3] N.V. Sochinskii, E. Dieguez, U. Pal, J. Piqueras, P. Ferna´ ndez, F. Agullo´ -Rueda, Semicond. Sci. Technol. 10 (1995) 870 –875. [4] M.A. Berding, Phys. Rev. B 60 (1999) 8943. [5] D. Henning, M. Hanke, A. Kaschte, J. Cryst. Growth 101 (1991) 355. [6] P. Emanuelsson, P. Omling, B.K. Meyer, M. Wienecke, M. Schenk, Phys. Rev. B 47 (1993) 15578. [7] A. Zappettini, T. Go¨ ro¨ g, M. Zha, L. Zanotti, G. Zuccalli, C. Paorici, J. Cryst. Growth 214 – 215 (2000) 14. [8] M. Zha, F. Bisssoli, A. Zappettini, G. Zuccalli, L. Zanotti, C. Paorici, Phys. Stat. Sol. (b) 229 (2002) 15. [9] A. Castaldini, A. Cavallini, B. Fraboni, P. Ferna´ ndez, J. Piqueras, J. Appl. Phys. 83 (1998) 2121. [10] J.W. Allen, Semicond. Sci. Technol. 10 (1995) 1049. [11] D.M. Hoffmann, P. Omling, H.G. Grimmeiss, B.K. Meyer, K.W. Benz, D. Sinerius, Phys. Rev. B 45 (1992) 6247. [12] A. Castaldini, A. Cavallini, B. Fraboni, P. Fernandez, J. Piqueras, Phys. Rev. B 56 (1997) 14897. [13] C.C.R. Watson, K. Durose, J. Cryst. Growth 126 (1993) 325.
N. Armani et al. / Materials Science and Engineering B91–92 (2002) 353–357 [14] G. Salviati, P. Franzosi, M. Scaffardi, S. Bernardi, Appl. Phys. Lett. 65 (1994) 3257. [15] V. Babentsov, V. Corregidor, K. Benz, M. Fiederle, T. Feltgen,
357
E. Die´ guez, Nucl. Instr. Methods Phys. Res. A 458 (2001) 85. [16] N.V. Sochinskii, M.D. Serrano, E. Die´ guez, F. Aullo´ -Rueda, U. Pal, J. Piqueras, P. Ferna´ ndez, J. Appl. Phys. 77 (1995) 2806.
Crystal defects and optical transitions in high purity, high resistivity CdTe for device applications N. Armani *, C. Ferrari, G. Salviati, F. Bissoli, M. Zha, L. Zanotti CNR–MASPEC Institute, Parco Area delle Scienze, 37 /A 43010 Fontanini, Parma, Italy
Abstract The structural and optical properties of high purity undoped semi-insulating (SI) Cadmium Telluride (CdTe) crystals, grown by Physical Vapour Transport (PVT) and by modified Vertical Bridgman (BG) techniques have been studied. The samples were obtained from 7N source elements by direct synthesis followed by heat treatment to adjust the stoichiometry. The experimental measurements have been systematically performed using high resolution X-ray diffraction (HRXRD) and cathodoluminescence (CL) techniques, before and after the thermal annealing procedures. The CL emissions from low and high resistivity samples have been studied in order to correlate the CL bands to the presence and concentration of native defects in the crystals. Two broad luminescence bands have been found in addition to the expected emission related to the excitonic recombination. The nature of the defects involved in the transitions was studied by analysing the position of the energy levels and the relative intensity variation of the CL peaks. The effect of the thermal treatments, performed at different temperatures, on the optical and electrical properties of the specimens was also analysed. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Cadmium Telluride; Defect formation; Annealing; Cathodoluminescence; X-ray diffraction
1. Introduction Cadmium Telluride (CdTe) is a very attractive semiconducting material for a large number of applications in the field of X- and g-ray detectors and electro-optical devices. In order to obtain high resistivity material, required for electro-optic applications, stoichiometric and impurity-free crystals should be grown. Up to now, semi-insulating (SI) CdTe has been obtained by introducing Group III or VII elements (Cl, In, I, etc.) as dopants to create compensating defects in the crystal [1,2]. However, the intentional doping of the crystals may lead to lattice deformation and poor crystalline quality, due to segregation of elements in the grown crystal. The need for high quality and defect free CdTe, with high optical transmission, has motivated a systematic study on the formation of crystalline defects. The precipitation in the crystals is usually associated with the Te-rich condition of the melt-growth; in this case dislocations can also occur. However, precipitation can also * Corresponding author. E-mail address: [email protected] (N. Armani).
happen in PVT-grown crystals, despite the narrow range of deviation from stoichiometry and the vapour phase of elements do not allow the formation of inclusions. Thus, precipitates are unavoidable in CdTe and some attempts to remove them were made by annealing the specimens in a controlled pressure of Cadmium [3]. The SI nature of doped CdTe samples is due to a complex, created by the dopant atoms with a Cadmium vacancy (VCd), which compensates the excess of carriers. In the case of almost impurity free undoped crystals, like the present samples, the mechanism responsible for the compensation is different. During the growth it is impossible to avoid the formation of native defects, the concentration of which strictly depends on the deviation from the stoichiometry. Theoretical calculations supply data on the native defect concentration for different growth conditions [4,5]. Considering the Te-rich material and a growth temperature above 900 °C, the energetically most probable native defect is the VCd, even if the Tellurium vacancy (VTe) and the substitutional Tellurium on the Cadmium site (TeCd) can also be present. Theoretical calculations give the energy positions of the levels due to those defects with a large uncertainty; further, those data are not in good agreement with the experimental values [6].
0921-5107/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 0 1 ) 0 1 0 6 6 - 2
354
N. Armani et al. / Materials Science and Engineering B91–92 (2002) 353–357
CdTe crystals were synthesised from 7N purity Cd and Te [7], then a thermal treatment to correctly adjust the Cd/Te stoichiometry ratio [8] was performed. The crystals obtained presented the required stoichiometry and purity useful for growth of SI CdTe from both the vapour phase and the melt. The effect of native defects and doping on the electrical and optical properties of CdTe crystals has been systematically studied in the past by means of the comparative techniques of cathodoluminescence (CL) and photo-induced current transient spectroscopy (PICTS) [9]. It has been possible to correlate a typical optical emission denoted as A-band [10] with three distinct electronic levels in the band gap. Further, luminescence-based analyses on doped crystals showed a correlation between the electrical effects due to the dopant and the optical emissions seen in CL and photoluminescence (PL) spectra [11,12]. The CL technique, by means of the possibility to acquire pan- and mono-chromatic images and by changing the energy of the impinging electron probe, can give information on the distribution of the luminescence in plane and in depth of the specimens and on the correlation between optical transitions and crystal defects [13,14].
2. Experimental The crystals studied were grown both by the Physical Vapour Transport (PVT) and the modified Bridgman (BG) techniques. A horizontal configuration was adopted to avoid the crystals from falling down during the growth because of poor sticking. Separate heating controllers (Eurothem 818) checked the three zones of the furnace and were able to keep the temperature fluctuation within 9 0.2 °C. Due to the poor heat
conductivity of CdTe, the applied temperature profile was intentionally designed to present a step temperature gradient at the growth front. A closed quartz ampoule was provided with a short capillary at the crystallisation end. The capillary was part of the core of a quartz bar (10 cm long and 1 cm in diameter), coaxial with the ampoule, which further favoured the heat release. The effect of the attached bar was evidenced by the convex shape of the growing interface, on which a number of facets appeared. The furnace was so designed to steadily move, with respect to the fixed growth ampoule, in a speed range from 1 to 10 mm per day. A 3 mm per day moving speed was applied in these growth experiments which yielded crystals up to 27 mm in diameter and several cm in length. The post-growth cooling rates were about 20–40 °C h − 1. The BG growth was carried out with the charge (70 g), encapsulated with B2O3, in an open quartz crucible inside a pressure chamber (5 atm Nitrogen). Crystal ingots made of large single grains were grown in about 36 h. After a first run of measurements, the CdTe crystals studied were annealed at 600 °C in a closed quartz ampoule, in an inert gas atmosphere, with a Cadmium counterpressure produced by Cd-rich CdTe powder. The resistivity of the crystals was analysed by Van der Pauw measurements at room temperature; the structural characterisation was performed by high resolution X-ray diffraction (HRXRD) and the optical studies by CL. A Philips diffractometer in the two crystal configuration with a (220) Ge monochromator was used to acquire the V–2q rocking curves. The optical analyses were performed with a Gatan MonoCL system mounted on a commercial SEM; the spectra as well as the panchromatic and monochromatic images were obtained at temperatures ranging from 77 to 300 K.
3. Results and discussion
Fig. 1. HRXRD rocking curves, using the CuKa1 (333) symmetrical reflection, of the high and low resistivity BG-grown samples (cz8, cz3), in comparison with a high resistivity PVT-grown specimen.
Different BG-grown crystals with resistivities (z) between 300 and 108 V cm, respectively, and PVT-grown samples with z 109 V cm have been investigated. The analysis on the crystalline quality of the samples has been carried out by acquiring the (333) symmetrical rocking curves with the CuKa1 wavelength. The diffraction profiles of high and low resistivity BG-grown and high resistivity PVT-grown samples are shown in Fig. 1. The BG SI specimen shows the most intense reflectivity and the lowest FWHM, indicating the better crystalline quality of the sample. A spatial mapping of the (333) reflection, built by acquiring the rocking curves in different positions of the specimen surface, shows a clear inhomogeneity of the crystal, with the angular
N. Armani et al. / Materials Science and Engineering B91–92 (2002) 353–357
Fig. 2. Low temperature (77 K) CL panchromatic image of the low resistivity BG-grown sample, taken at a primary electron beam energy of 25 keV.
deviation of the Bragg peak of about 300¦ and with the FWHM ranging from 70 to 130¦. In Fig. 2 a panchromatic image of the low resistivity sample is shown. A large density of dark spots of different dimensions, representing non-radiative recombination centres, can be observed. The dark spots could be related to crystal defects, like dislocations emerging to the (111) surface of the crystal, or precipitates.
355
In the low temperature (77 K) CL spectra, of Fig. 3, in addition to an intense near band edge (NBE) emission peaked at about 1.57 eV, a large emission band in the region of 0.9–1.2 eV and the so called A-band, centred at about 1.4 eV can be observed. In the low resistivity sample spectrum, the intensity of the A-band is strongly reduced, while the intensity of the 0.9–1.2 eV band is slightly increased. A deconvolution of the A-band reveals two distinct Gaussian peaks at 1.362 and 1.44 eV, respectively. The same fitting procedures have been applied to the CL emission band between 0.9 and 1.2 eV, revealing two Gaussian peaks at 1.02 and 1.12 eV, respectively. The nature of the levels responsible for the CL emissions was investigated by acquiring CL spectra at different injection powers and temperatures. By increasing the power density from 0.29 to 29 W cm − 2, the relative intensity of the 1.44 eV peak decreases with respect to the 1.362 eV peak, as shown in Fig. 4. Further, the position and the relative intensity of the 1.02 and the 1.12 eV peaks remains almost unchanged, while the intensity of the whole band increases with increasing the injection power. The transitions responsible for the large CL band in the region 0.9–1.2 eV, present in all the spectra, should be due to deep levels. As for the 1.02 eV emission, a donor level due to VTe or TeCd with an energy of 0.4 eV below the conduction band (CB) and an acceptor level due to VCd in the 0/ − 1 charge state, at 0.2 eV above
Fig. 3. CL spectra, at 77 K, of the same crystals studied with the HRXRD technique. In the inset of the graph is shown the magnification of the low energy spectral region.
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N. Armani et al. / Materials Science and Engineering B91–92 (2002) 353–357
Fig. 4. Comparison of the low temperature CL spectra, from a SI BG-grown sample, acquired at increasing injection power density from 0.29 to 29 W cm − 2.
shallow donors involved in the 1.44 eV transition have been ionised so promoting the carriers in the CB. In this case the carriers could recombine through the alternative path with a shallow acceptor, so increasing the 1.36 eV CL peak intensity. After the annealing procedures performed at temperatures of 300 and 600 °C a modification in the CL spectra was observed, as shown in Fig. 5. The deep level related CL band of the p-type specimen practically disappears, while the intensity of the socalled A-band increases. The precipitates present in the crystals were probably dissolved during the annealing, as demonstrated by analogous work [16] with the creation of the native defects responsible for the shallow levels involved in the 1.4 eV band. Transmission Electron Microscopy investigations are in progress in order to study the precipitates behaviour before and after thermal treatment.
4. Conclusions CdTe material prepared by 7N pure starting elements and with well-controlled stoichiometry has been studied by HRXRD and CL. The measurements on both low and high resistivity samples suggest that shallow levels and deep levels are related to intrinsic defects, so confirming their crucial role in the compensation mechanism providing the high resistivity in our CdTe crystals. Fig. 5. Effect of 600 °C thermal annealing on the low temperature CL spectra performed on a low resistivity p-type BG-grown sample.
References the valence band (VB) [4] can be taken into account. On the contrary, the 1.12 eV CL peak can be ascribed to the deep acceptor VCd in the −1/− 2 configuration, the energy of which has been measured with different experimental techniques in the range of 0.43 –0.47 eV above the VB [6,12]. In particular, the intensity of the 1.12 eV peak is higher in p-type crys− 1/ − 2 tals, suggesting that the VCd , most likely present in high concentration in this material, is involved in this transition. Concerning the 1.36 and 1.44 eV peaks, observed only in the SI crystals, they can be related to transitions involving a shallow donor and a shallow acceptor, ascribed to the presence of VCd and Cadmium interstitial (Cdi), the concentration of which depends on the position of the Fermi level in the gap. In the case of SI or n-type material [15], these two defects exist separately only if the EF is located far from the VB. Moreover, by increasing the power density, the behaviour of the CL peak intensities reveals that the
[1] D.V. Korbutyak, S.G. Krylyuk, P.M. Tkachuk, V.I. Tkachuk, N.D. Korbutyak, M.D. Raransky, J. Cryst. Growth 197 (1999) 659. [2] J. Lee, N.C. Giles, D. Rajavel, C.J. Summers, J. Appl. Phys. 78 (1995) 5669. [3] N.V. Sochinskii, E. Dieguez, U. Pal, J. Piqueras, P. Ferna´ ndez, F. Agullo´ -Rueda, Semicond. Sci. Technol. 10 (1995) 870 –875. [4] M.A. Berding, Phys. Rev. B 60 (1999) 8943. [5] D. Henning, M. Hanke, A. Kaschte, J. Cryst. Growth 101 (1991) 355. [6] P. Emanuelsson, P. Omling, B.K. Meyer, M. Wienecke, M. Schenk, Phys. Rev. B 47 (1993) 15578. [7] A. Zappettini, T. Go¨ ro¨ g, M. Zha, L. Zanotti, G. Zuccalli, C. Paorici, J. Cryst. Growth 214 – 215 (2000) 14. [8] M. Zha, F. Bisssoli, A. Zappettini, G. Zuccalli, L. Zanotti, C. Paorici, Phys. Stat. Sol. (b) 229 (2002) 15. [9] A. Castaldini, A. Cavallini, B. Fraboni, P. Ferna´ ndez, J. Piqueras, J. Appl. Phys. 83 (1998) 2121. [10] J.W. Allen, Semicond. Sci. Technol. 10 (1995) 1049. [11] D.M. Hoffmann, P. Omling, H.G. Grimmeiss, B.K. Meyer, K.W. Benz, D. Sinerius, Phys. Rev. B 45 (1992) 6247. [12] A. Castaldini, A. Cavallini, B. Fraboni, P. Fernandez, J. Piqueras, Phys. Rev. B 56 (1997) 14897. [13] C.C.R. Watson, K. Durose, J. Cryst. Growth 126 (1993) 325.
N. Armani et al. / Materials Science and Engineering B91–92 (2002) 353–357 [14] G. Salviati, P. Franzosi, M. Scaffardi, S. Bernardi, Appl. Phys. Lett. 65 (1994) 3257. [15] V. Babentsov, V. Corregidor, K. Benz, M. Fiederle, T. Feltgen,
357
E. Die´ guez, Nucl. Instr. Methods Phys. Res. A 458 (2001) 85. [16] N.V. Sochinskii, M.D. Serrano, E. Die´ guez, F. Aullo´ -Rueda, U. Pal, J. Piqueras, P. Ferna´ ndez, J. Appl. Phys. 77 (1995) 2806.