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Effect of superheating and fast cooling on Te inclusions of Cd0.9Zn0.1Te:In crystals grown by vertical gradient freezing
Journal of Crystal Growth 361 (2012) 5–10
Contents lists available at SciVerse ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Effect of superheating and fast cooling on Te inclusions of Cd0.9Zn0.1Te:In crystals grown by vertical gradient freezing H. Bensalah n, J. Crocco, V. Carcele´n, A. Black, Q. Zheng, J.L. Plaza, E. Die´guez Crystal Growth Laboratory, Universidad de Autonoma de Madrid, Material Physics Department of the Science Faculty, 28049 Madrid, Spain
a r t i c l e i n f o
abstract
Article history: Received 30 January 2012 Received in revised form 25 June 2012 Accepted 7 July 2012 Communicated by A. Burger Available online 24 July 2012
The goal of this paper is to study the effect of superheating and fast cooling conditions on the defects of CdZnTe crystals especially Te inclusions. The paper reports characterization of the crystal defects using IR microscopy and Fourier Transform Infrared spectroscopy. Three ingots of CdZnTe doped with Indium were grown by Vertical Gradient Freezing. Ingot A and B were grown with superheating temperatures of 26 1C and 56 1C respectively. In the last case the growth process was followed by a fast cooling. The results show that the size of Te inclusions was greatly reduced and the crystal quality was improved after higher superheating and fast cooling. & 2012 Elsevier B.V. All rights reserved.
Keywords: CZT VGF Superheating Fast cooling Te inclusions
1. Introduction The CdZnTe (CZT) is a key detector technology for X-ray and gamma-ray due to its high absorption efficiency, excellent spatial resolution, good energy resolution, and room temperature operation [1]. Despite these advantages, the use of CZT material is still limited by some macroscopic defects such as the presence of large Te inclusions, twins, grain boundaries, dislocations which are formed during crystallization of the liquid and cooling down of the crystallized bulk ingots. Each of these factors causes degradation of detector performance [2]. The formation of Te inclusions and precipitates was well explained by the retrograde solid solubility during cooling and can be strongly influenced by such factors as the melt stoichiometry and the instability of liquid–solid interface during the growth process [3,4]. Furthermore, because the overpressure of cadmium is much higher than that of tellurium under growth conditions, Te can condense and occupy Cd vacancies [5]. Several studies have shown that inclusions and precipitates may influence optical transmission, electrical recombination, and mobility-lifetime product [6,7]. Fiederle et al. found that the formation of Te-clusters is the essential reason for the polycrystalline structure [8].
n
Corresponding author. E-mail address: [email protected] (H. Bensalah).
0022-0248/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jcrysgro.2012.07.012
A large amount of work has been done in order to eliminate or avoid the formation of inclusions. Capper et al. demonstrated a reduction in the density of inclusions present in melt growth using the accelerated crucible rotation technique [9]. Post growth annealing under Cd vapor was found to be effective to reduce Te inclusions superior to 1 um but less effective to remove smaller ones ( o1 um) [10]. Szeles et al. reported that growth process under Cd excess corrects the stoichiometry which reduces the formation of Te inclusions [11]. The ionic character of the Cd–Te chemical bond makes CdTe melts highly associated close to the melting point, resulting in the presence of extremely organized particles affecting nucleation process and growth kinetics [12]. Therefore large superheating DT þ (DT þ ¼ T–Tm); where T is the maximum melt temperature before the crystal growth and (Tm is the melting point) is necessary to destroy the associated melt complexes [13]. The quality of the as grown CZT crystal is related to the temperature of the mother phase before the growth process starts [14]. In fact, Rudolph et al. demonstrated that slight superheating temperature affects the crystal growth by generating subgrains and twins, whereas after marked superheating temperature these defects disappear. Therefore, the crystal growth process could be improved by using a defined superheating and holding time before the crystallization. This paper is focused on the effect of superheating and fast cooling on Te inclusions.
6
H. Bensalah et al. / Journal of Crystal Growth 361 (2012) 5–10
2. Experimental details 2.1. Growth and samples preparation Three Cd1–xZnxTe ingots (x ¼10%) were grown by the Vertical Gradient Freeze (VGF) method using a 5-zone furnace [15]. The pBN crucible was filled with Te, Cd, Zn, 7N high purity as starting materials, in the order of their melting points. The concentration of Indium used for each experiment was 3 ppm. To measure thermal gradients applied throughout the growth process, type S thermocouples were placed below the nucleation tip of the ampoule and circumferentially along the axial direction of growth as shown in Fig. 1. A SiC pedestal was used in conjunction with the pBN crucible in order to improve the thermal dynamics at the start of crystal growth [16]. The ingots were obtained by different growth temperature processes. Presented in Fig. 2 are two different thermal ramps applied to Ingot A and Ingots B and C. In the first ramp, the temperature of the melt is increased to 26 1C over the melting point of CZT (Tm ¼1104 1C). In the second ramp, this temperature is increased to þ56 1C over the melting point of CZT to push the reaction further, and break apart secondary phases in the melt [17]. A picture of the as grown ingots is shown in Fig. 3. Ingot A was grown with a superheating temperature of 26 1C (superheating type 1). Ingot B was grown with a superheating temperature of 56 1C (superheating type 2), the temperature goes up until 1160 1C and then it attains 1130 1C. Ingot C was grown with the same conditions as those of ingot B but with a faster cooling rate applied after growth. The ingots were cooled to RT at a rate of 20 1C/h for ingots A and B and 110 1C/h for ingot C. To study the distribution of Te inclusions, the ingots were sliced into wafers perpendicular to the growth direction. The samples were first lapped and then polished using 3 mm Al2O3 powder [18].
Fig. 2. Temperature profile for superheating the melt by 26 1C (a) and 56 1C (b).
2.2. Characterization The distribution and size of second phase defects were examined using an IR transmission 90i ZEISS Nikon microscope and a DS-Qi1 cooled digital microscope camera. Fourier Transform Infrared (FTIR) spectroscopy is a technique which provides useful information about crystal quality. The FTIR spectrum (7000 cm to 500 cm 1) was obtained using a Bruker FTIR spectrometer IFS66 v with a resolution of 4 cm 1.
Fig. 3. As grown 25 mm diameter CZT ingots.
3. Results and discussion
Fig. 1. Thermocouples distribution and SiC pedestal used for the growth.
Fig. 3 shows the three CdZnTe:In ingots of 25 mm diameter. We can observe that the ingot A is more damaged than the other ingots and its top has a different shape. Moreover, the top of the ingot B did not exhibit the polycrystalline conical shaped region
H. Bensalah et al. / Journal of Crystal Growth 361 (2012) 5–10
7
Fig. 4. IR microscopy images and diameter distribution (mm) of the inclusions for samples from the first part till freezing of the ingots A, B and C successively.
which has high void content. This could be explained with increasing the temperature until 1160 1C and all the starting material melts. It seems that 1130 1C is not sufficient to complete the reaction between the Cd, Zn, and Te in the melt. Ingot C shows some bubbles along the surface which is due to the fast cooling, moreover this ingot did not show any cracks. IR microscopy is a convenient technique to visualize Te inclusions in CZT samples. As we know, inclusions are formed from pure Te which strongly absorb light and appear as dark spots in the IR images. Figs. 4 and 5 show the IR microscopy images and diameter distribution (mm) of Te inclusions, respectively. CZT samples from the ingot A contain larger Te inclusions than the samples from the ingot B which show a reduction in the size of Te inclusions. This behavior could be due to the superheating of 56 1C wherein the material remains at 1160 1C and Te inclusions were broken and then their size reduces. The ingot C presents the smallest Te inclusions due to the retrograde solubility of the phase diagram [19]. Presented in Fig. 6 is the distribution of Te inclusion diameters for the three ingots. One may observe that ingot A has very large inclusions or clusters in the first to freeze region, in excess of 160 mm. In the rest of Ingot A, the diameter drops to between 20 mm and 40 mm, however they are considered relatively large in comparison with those of ingots B and C. An example of these clusters is presented in Fig. 8. Ingot B, on the other hand, has a more or less constant inclusion size between 10 mm and 20 mm,
also the clusters are not present in the first to freeze region. Further improvements are made regarding ingot C grown with both superheating and fast cooling; we observe that the diameter distribution is more uniform throughout the crystal and does not exceed 4.6 mm [20]. Fig. 7 shows the Te density distribution along the three ingots. We observe that Ingot A has the least uniform distribution in inclusion density, with wide fluctuations in density between 4.5 105 cm3 and less than 1.0 105 cm3. Ingot B has a significantly lower density in the range 0.5–1.0 105 cm3, whereas Ingot C has elevated Te density in the range 3.5–4.5 105 cm3. Finally, to really demonstrate the difference between type 1 and type 2 superheating at macroscopic level, IR transmission mapping for two wafers is presented in Fig. 8a for Ingot A and Fig. 8b for ingot B. The figures show that the inclusions are much smaller in Ingot B, clearly demonstrating the problems with incomplete homogenization of type 1 superheating. The IR transmittance spectrum is a useful method to evaluate the quality of CZT crystals. The IR transmission samples harvested from each of the ingots is presented in Fig. 9. These images clearly show the effect of superheating on the transmission of the samples. The transmittance of intrinsic CZT crystals can be expressed as T¼
ð1RÞ2 ead ð1RÞ2 ead
8
H. Bensalah et al. / Journal of Crystal Growth 361 (2012) 5–10
Fig. 5. IR microscopy images and diameter distributions (mm) of the inclusions for samples from the last part till freezing of the ingots A, B and C successively.
Fig. 6. Diameter distribution of Te inclusions along the three ingots. Fig. 7. Density distribution of Te inclusions along the three ingots.
where R is the reflectivity, which has been shown to be 0.21, d is the sample thickness, and a is the absorption coefficient. Since R is fixed, the IR transmittance T is determined only by the absorption coefficient a for fixed d.
The samples harvested from the ingot A not only exhibit the lowest transmission, but this transmission decreases with increasing wavenumber because the large size of Te inclusions continues to scatter the incident IR light. The effect of superheating, which
H. Bensalah et al. / Journal of Crystal Growth 361 (2012) 5–10
9
Fig. 9. IR transmittance spectra for the CZT samples from the ingots A, B, and C.
Fig. 10. 5 cm diameter CdZnTe ingots grown using (a) Type 1 superheating (b) Type 2 superheating, 15 min and (c) Type 2 superheating, 5 h.
lattice parameter brought about by dislocations and Te precipitates/inclusions may help explain increased absorption. These second phase particles can change the lattice parameter and destroy the periodicity of the lattice, consequently enlarging the electric dipole moment [22]. Fig. 8. IR transmission microscopy images of 10 mm 10 mm CZT wafers. (a) superheating type 1 and (b) superheating type 2.
has been applied to ingot B, reduced the size of the inclusions and increases the transmission. However, these samples still contain relatively large inclusions, which helps explain the lower IR transmission i.e. less than 40%. The IR transmission was further improved with the implementation of both superheating and fast cooling processes. The IR transmission for these samples exceeds 60% for wavenumbers 1500 cm-1 to 7000 cm-1. Indeed, the samples harvested from ingot C exhibit transmission values near the theoretical limit [21]. This is a direct result of the smaller diameter inclusions within the sample, resulting in less scattering. Similar transmission data was also observed for other samples harvested from the same ingot. In addition to scattering, another possible reason for the decreased transmission is absorption. IR absorption of CZT may be attributed to lattice absorption and free-carrier absorption. The lattice absorption changes the electric dipole moment via the displacement of atoms. The larger the electric dipole moment is, the more the lattice absorption will be. The change in the local
4. Conclusion The effects of melt superheating and fast cooling on the ingot composition have been demonstrated. It was observed that superheating of the melt by 1104þ 26 1C was not sufficient to complete the reaction between Cd and Te leading to the formation of polycrystalline conical shaped regions which exhibit high void content. These differences between the superheating Types 1 and 2 were also observed in the 50 mm diameter ingots as shown in Fig. 10. The ingot grown using Type 1 superheating exhibited a similar conical polycrystalline region at the top of the ingot. To eliminate this feature, the second ingot was grown using Type 2 superheating protocol for 15 min. As can be seen, the volume of this conical region has been reduced. Therefore, the last ingot was grown using the same protocol, however the melt was held for 5 h this time. As can be seen, this polycrystalline conical region has been essentially eliminated. Increasing the superheating temperature of the melt to 1160 1C; however, substantially improved the material homogeneity. Infrared transmission measurements have been used to demonstrate how
10
H. Bensalah et al. / Journal of Crystal Growth 361 (2012) 5–10
this superheating has reduced the size of secondary phases within the CZT matrix. This study examined the effect of different growth temperature secondary phases and in turn CdZnTe properties. From the above results, one can observe that superheating temperature and fast cooling have a notable effect on Te inclusions. IR images showed that this ingot contains the smallest size of Te inclusions. The FTIR spectra are stable between 3500 cm 1 and 7000 cm 1 and linear at a high value of 62% which could be attributed to the smaller Te inclusions.
Acknowledgments This work was partially supported by the following projects: MAT 2009–08582, Spanish ‘‘Ministerio de Ciencia e Innovacion’’; FP7-SEC-2007-01, ‘‘Cooperation across Europe for Cd(Zn)Te based Security Instrument’’, COCAE References [1] C.M. Stahle, B.H. Parker, A.M. Parsons, L.M. Barbier, S.D. Barthelmy, N.A. Gehrels, D.M. Palmer, S.J. Snodgrass, J. Tueller, Nuclear Instruments and Methods in Physics Research A 436 (1999) 138–145. [2] T.E. Schlesinger, J.E. Toney, H. Yoon, E.Y. Lee, B.A. Brunett, L. Franks, R.B. James, Materials Science and Engineering 32 (2001) 103–189. [3] P. Rudolph, Crystal Research and Technology 38 (7–8) (2003) 542–554. [4] R.Triboulet P. Siffert, CdTe and Related Compounds; Physics, Defects, Heteroand Nano-structures, Crystal Growth, Surfaces and Applications, 2009.
[5] R Schwarz, K.W Benz, Journal of Crysral Growth 144 (1994) 150–156. [6] M.P. Bellmann, E.A Meese, L Arnberg, Journal of Crystal Growth 318 (2011) 239–243. [7] S. Sitharaman, R. Raman, L. Durai, Surendra Pal, Madhukar Gautam, Anjana Nagpal, Shiv Kumar, S.N. Chatterjee, S.C. Gupta, Journal of Crystal Growth 285 (2005) 318–326. [8] M. Fiederle, A. Fauler, V. Babentsov, J. Franc, J. Konrath, M. Webel, J. Ludwig, K.W. Benz, Nuclear Instruments and Methods in Physics Research A 509 (2003) 70–75. [9] J. Franc, R. Grill, P Hlıdek, E Belas, L. Turjanska, P. Hoschl, I. Turkevych, A.L. Toth, P. Moravec, H. Sitter, Semiconductor Science and Technology 16 (2001) 514–520. [10] A. Koyama, A. Hichiwa, R. Hirano, Journal of Electronic Material 28 (1999). [11] C. Szeles, S.E. Cameron, J.-O. Ndap, W.C. Chalmers, Advances in the crystal growth of semi-insulating CdZnTe for radiation detector applications, IEEE Transactions on Nuclear Science 49 (2002) 2535–2540. [12] Hans J. Scheel Tsuguo Fukuda, Crystal Growth and Technology, 2003. ¨ [13] M. Muhlberg, P. Rudolph, Journal of Crystal growth 128 (1993) 571–575. [14] P Rudolph, H.J Koh, N Schafer, T Fukuda, Journal of Crystal Growth 166 (1996) 578–582. [15] J. Crocco, H. Bensalah, Q. Zheng, F. Dierre, P. Hidalgo, J. Carrascal, O. Vela, J. Piqueras, E. Die´guez, IEEE Transactions on Nuclear Science 58 (2011). [16] J. Crocco, H. Bensalah, Q. Zheng, V. Carcelen, E. Dieguez, Journal of Crystal Growth, In press. [17] M Fiederle, A Fauler, A. Zwerger, IEEE Transactions on Nuclear Science 54 (2007). [18] H. Bensalah, J.L. Plaza, J. Crocco, Q. Zheng, V. Carcelen, A. Bensouici, E. Dieguez, Applied Surface Science 257 (2011) 4633–4636. [19] J.H. Greenberg, V.N. Guskov, Journal of Crystal Growth 289 (2006) 552–558. [20] The formation and elimination of inclusions in CdZnTe crystal, Proc. SPIE 6835, 68350I (2007); http://dx.doi.org/10.1117/12.757404. [21] Li. Guo-Qiang, Jie. Wan-Qi, Gu. Zhi, Hu A. Hui, Chinese Physics Letters 20 (2003). [22] Q. Zheng, F. Dierre, J. Crocco, V. Carcelen, H. Bensalah, J.L. Plaza, E. Dieguez, Applied Surface Science 257 (2011) 8742–8746.
Contents lists available at SciVerse ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Effect of superheating and fast cooling on Te inclusions of Cd0.9Zn0.1Te:In crystals grown by vertical gradient freezing H. Bensalah n, J. Crocco, V. Carcele´n, A. Black, Q. Zheng, J.L. Plaza, E. Die´guez Crystal Growth Laboratory, Universidad de Autonoma de Madrid, Material Physics Department of the Science Faculty, 28049 Madrid, Spain
a r t i c l e i n f o
abstract
Article history: Received 30 January 2012 Received in revised form 25 June 2012 Accepted 7 July 2012 Communicated by A. Burger Available online 24 July 2012
The goal of this paper is to study the effect of superheating and fast cooling conditions on the defects of CdZnTe crystals especially Te inclusions. The paper reports characterization of the crystal defects using IR microscopy and Fourier Transform Infrared spectroscopy. Three ingots of CdZnTe doped with Indium were grown by Vertical Gradient Freezing. Ingot A and B were grown with superheating temperatures of 26 1C and 56 1C respectively. In the last case the growth process was followed by a fast cooling. The results show that the size of Te inclusions was greatly reduced and the crystal quality was improved after higher superheating and fast cooling. & 2012 Elsevier B.V. All rights reserved.
Keywords: CZT VGF Superheating Fast cooling Te inclusions
1. Introduction The CdZnTe (CZT) is a key detector technology for X-ray and gamma-ray due to its high absorption efficiency, excellent spatial resolution, good energy resolution, and room temperature operation [1]. Despite these advantages, the use of CZT material is still limited by some macroscopic defects such as the presence of large Te inclusions, twins, grain boundaries, dislocations which are formed during crystallization of the liquid and cooling down of the crystallized bulk ingots. Each of these factors causes degradation of detector performance [2]. The formation of Te inclusions and precipitates was well explained by the retrograde solid solubility during cooling and can be strongly influenced by such factors as the melt stoichiometry and the instability of liquid–solid interface during the growth process [3,4]. Furthermore, because the overpressure of cadmium is much higher than that of tellurium under growth conditions, Te can condense and occupy Cd vacancies [5]. Several studies have shown that inclusions and precipitates may influence optical transmission, electrical recombination, and mobility-lifetime product [6,7]. Fiederle et al. found that the formation of Te-clusters is the essential reason for the polycrystalline structure [8].
n
Corresponding author. E-mail address: [email protected] (H. Bensalah).
0022-0248/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jcrysgro.2012.07.012
A large amount of work has been done in order to eliminate or avoid the formation of inclusions. Capper et al. demonstrated a reduction in the density of inclusions present in melt growth using the accelerated crucible rotation technique [9]. Post growth annealing under Cd vapor was found to be effective to reduce Te inclusions superior to 1 um but less effective to remove smaller ones ( o1 um) [10]. Szeles et al. reported that growth process under Cd excess corrects the stoichiometry which reduces the formation of Te inclusions [11]. The ionic character of the Cd–Te chemical bond makes CdTe melts highly associated close to the melting point, resulting in the presence of extremely organized particles affecting nucleation process and growth kinetics [12]. Therefore large superheating DT þ (DT þ ¼ T–Tm); where T is the maximum melt temperature before the crystal growth and (Tm is the melting point) is necessary to destroy the associated melt complexes [13]. The quality of the as grown CZT crystal is related to the temperature of the mother phase before the growth process starts [14]. In fact, Rudolph et al. demonstrated that slight superheating temperature affects the crystal growth by generating subgrains and twins, whereas after marked superheating temperature these defects disappear. Therefore, the crystal growth process could be improved by using a defined superheating and holding time before the crystallization. This paper is focused on the effect of superheating and fast cooling on Te inclusions.
6
H. Bensalah et al. / Journal of Crystal Growth 361 (2012) 5–10
2. Experimental details 2.1. Growth and samples preparation Three Cd1–xZnxTe ingots (x ¼10%) were grown by the Vertical Gradient Freeze (VGF) method using a 5-zone furnace [15]. The pBN crucible was filled with Te, Cd, Zn, 7N high purity as starting materials, in the order of their melting points. The concentration of Indium used for each experiment was 3 ppm. To measure thermal gradients applied throughout the growth process, type S thermocouples were placed below the nucleation tip of the ampoule and circumferentially along the axial direction of growth as shown in Fig. 1. A SiC pedestal was used in conjunction with the pBN crucible in order to improve the thermal dynamics at the start of crystal growth [16]. The ingots were obtained by different growth temperature processes. Presented in Fig. 2 are two different thermal ramps applied to Ingot A and Ingots B and C. In the first ramp, the temperature of the melt is increased to 26 1C over the melting point of CZT (Tm ¼1104 1C). In the second ramp, this temperature is increased to þ56 1C over the melting point of CZT to push the reaction further, and break apart secondary phases in the melt [17]. A picture of the as grown ingots is shown in Fig. 3. Ingot A was grown with a superheating temperature of 26 1C (superheating type 1). Ingot B was grown with a superheating temperature of 56 1C (superheating type 2), the temperature goes up until 1160 1C and then it attains 1130 1C. Ingot C was grown with the same conditions as those of ingot B but with a faster cooling rate applied after growth. The ingots were cooled to RT at a rate of 20 1C/h for ingots A and B and 110 1C/h for ingot C. To study the distribution of Te inclusions, the ingots were sliced into wafers perpendicular to the growth direction. The samples were first lapped and then polished using 3 mm Al2O3 powder [18].
Fig. 2. Temperature profile for superheating the melt by 26 1C (a) and 56 1C (b).
2.2. Characterization The distribution and size of second phase defects were examined using an IR transmission 90i ZEISS Nikon microscope and a DS-Qi1 cooled digital microscope camera. Fourier Transform Infrared (FTIR) spectroscopy is a technique which provides useful information about crystal quality. The FTIR spectrum (7000 cm to 500 cm 1) was obtained using a Bruker FTIR spectrometer IFS66 v with a resolution of 4 cm 1.
Fig. 3. As grown 25 mm diameter CZT ingots.
3. Results and discussion
Fig. 1. Thermocouples distribution and SiC pedestal used for the growth.
Fig. 3 shows the three CdZnTe:In ingots of 25 mm diameter. We can observe that the ingot A is more damaged than the other ingots and its top has a different shape. Moreover, the top of the ingot B did not exhibit the polycrystalline conical shaped region
H. Bensalah et al. / Journal of Crystal Growth 361 (2012) 5–10
7
Fig. 4. IR microscopy images and diameter distribution (mm) of the inclusions for samples from the first part till freezing of the ingots A, B and C successively.
which has high void content. This could be explained with increasing the temperature until 1160 1C and all the starting material melts. It seems that 1130 1C is not sufficient to complete the reaction between the Cd, Zn, and Te in the melt. Ingot C shows some bubbles along the surface which is due to the fast cooling, moreover this ingot did not show any cracks. IR microscopy is a convenient technique to visualize Te inclusions in CZT samples. As we know, inclusions are formed from pure Te which strongly absorb light and appear as dark spots in the IR images. Figs. 4 and 5 show the IR microscopy images and diameter distribution (mm) of Te inclusions, respectively. CZT samples from the ingot A contain larger Te inclusions than the samples from the ingot B which show a reduction in the size of Te inclusions. This behavior could be due to the superheating of 56 1C wherein the material remains at 1160 1C and Te inclusions were broken and then their size reduces. The ingot C presents the smallest Te inclusions due to the retrograde solubility of the phase diagram [19]. Presented in Fig. 6 is the distribution of Te inclusion diameters for the three ingots. One may observe that ingot A has very large inclusions or clusters in the first to freeze region, in excess of 160 mm. In the rest of Ingot A, the diameter drops to between 20 mm and 40 mm, however they are considered relatively large in comparison with those of ingots B and C. An example of these clusters is presented in Fig. 8. Ingot B, on the other hand, has a more or less constant inclusion size between 10 mm and 20 mm,
also the clusters are not present in the first to freeze region. Further improvements are made regarding ingot C grown with both superheating and fast cooling; we observe that the diameter distribution is more uniform throughout the crystal and does not exceed 4.6 mm [20]. Fig. 7 shows the Te density distribution along the three ingots. We observe that Ingot A has the least uniform distribution in inclusion density, with wide fluctuations in density between 4.5 105 cm3 and less than 1.0 105 cm3. Ingot B has a significantly lower density in the range 0.5–1.0 105 cm3, whereas Ingot C has elevated Te density in the range 3.5–4.5 105 cm3. Finally, to really demonstrate the difference between type 1 and type 2 superheating at macroscopic level, IR transmission mapping for two wafers is presented in Fig. 8a for Ingot A and Fig. 8b for ingot B. The figures show that the inclusions are much smaller in Ingot B, clearly demonstrating the problems with incomplete homogenization of type 1 superheating. The IR transmittance spectrum is a useful method to evaluate the quality of CZT crystals. The IR transmission samples harvested from each of the ingots is presented in Fig. 9. These images clearly show the effect of superheating on the transmission of the samples. The transmittance of intrinsic CZT crystals can be expressed as T¼
ð1RÞ2 ead ð1RÞ2 ead
8
H. Bensalah et al. / Journal of Crystal Growth 361 (2012) 5–10
Fig. 5. IR microscopy images and diameter distributions (mm) of the inclusions for samples from the last part till freezing of the ingots A, B and C successively.
Fig. 6. Diameter distribution of Te inclusions along the three ingots. Fig. 7. Density distribution of Te inclusions along the three ingots.
where R is the reflectivity, which has been shown to be 0.21, d is the sample thickness, and a is the absorption coefficient. Since R is fixed, the IR transmittance T is determined only by the absorption coefficient a for fixed d.
The samples harvested from the ingot A not only exhibit the lowest transmission, but this transmission decreases with increasing wavenumber because the large size of Te inclusions continues to scatter the incident IR light. The effect of superheating, which
H. Bensalah et al. / Journal of Crystal Growth 361 (2012) 5–10
9
Fig. 9. IR transmittance spectra for the CZT samples from the ingots A, B, and C.
Fig. 10. 5 cm diameter CdZnTe ingots grown using (a) Type 1 superheating (b) Type 2 superheating, 15 min and (c) Type 2 superheating, 5 h.
lattice parameter brought about by dislocations and Te precipitates/inclusions may help explain increased absorption. These second phase particles can change the lattice parameter and destroy the periodicity of the lattice, consequently enlarging the electric dipole moment [22]. Fig. 8. IR transmission microscopy images of 10 mm 10 mm CZT wafers. (a) superheating type 1 and (b) superheating type 2.
has been applied to ingot B, reduced the size of the inclusions and increases the transmission. However, these samples still contain relatively large inclusions, which helps explain the lower IR transmission i.e. less than 40%. The IR transmission was further improved with the implementation of both superheating and fast cooling processes. The IR transmission for these samples exceeds 60% for wavenumbers 1500 cm-1 to 7000 cm-1. Indeed, the samples harvested from ingot C exhibit transmission values near the theoretical limit [21]. This is a direct result of the smaller diameter inclusions within the sample, resulting in less scattering. Similar transmission data was also observed for other samples harvested from the same ingot. In addition to scattering, another possible reason for the decreased transmission is absorption. IR absorption of CZT may be attributed to lattice absorption and free-carrier absorption. The lattice absorption changes the electric dipole moment via the displacement of atoms. The larger the electric dipole moment is, the more the lattice absorption will be. The change in the local
4. Conclusion The effects of melt superheating and fast cooling on the ingot composition have been demonstrated. It was observed that superheating of the melt by 1104þ 26 1C was not sufficient to complete the reaction between Cd and Te leading to the formation of polycrystalline conical shaped regions which exhibit high void content. These differences between the superheating Types 1 and 2 were also observed in the 50 mm diameter ingots as shown in Fig. 10. The ingot grown using Type 1 superheating exhibited a similar conical polycrystalline region at the top of the ingot. To eliminate this feature, the second ingot was grown using Type 2 superheating protocol for 15 min. As can be seen, the volume of this conical region has been reduced. Therefore, the last ingot was grown using the same protocol, however the melt was held for 5 h this time. As can be seen, this polycrystalline conical region has been essentially eliminated. Increasing the superheating temperature of the melt to 1160 1C; however, substantially improved the material homogeneity. Infrared transmission measurements have been used to demonstrate how
10
H. Bensalah et al. / Journal of Crystal Growth 361 (2012) 5–10
this superheating has reduced the size of secondary phases within the CZT matrix. This study examined the effect of different growth temperature secondary phases and in turn CdZnTe properties. From the above results, one can observe that superheating temperature and fast cooling have a notable effect on Te inclusions. IR images showed that this ingot contains the smallest size of Te inclusions. The FTIR spectra are stable between 3500 cm 1 and 7000 cm 1 and linear at a high value of 62% which could be attributed to the smaller Te inclusions.
Acknowledgments This work was partially supported by the following projects: MAT 2009–08582, Spanish ‘‘Ministerio de Ciencia e Innovacion’’; FP7-SEC-2007-01, ‘‘Cooperation across Europe for Cd(Zn)Te based Security Instrument’’, COCAE References [1] C.M. Stahle, B.H. Parker, A.M. Parsons, L.M. Barbier, S.D. Barthelmy, N.A. Gehrels, D.M. Palmer, S.J. Snodgrass, J. Tueller, Nuclear Instruments and Methods in Physics Research A 436 (1999) 138–145. [2] T.E. Schlesinger, J.E. Toney, H. Yoon, E.Y. Lee, B.A. Brunett, L. Franks, R.B. James, Materials Science and Engineering 32 (2001) 103–189. [3] P. Rudolph, Crystal Research and Technology 38 (7–8) (2003) 542–554. [4] R.Triboulet P. Siffert, CdTe and Related Compounds; Physics, Defects, Heteroand Nano-structures, Crystal Growth, Surfaces and Applications, 2009.
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