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Crystal growth of Cd1−xZnxTe by the traveling heater method in microgravity on board of Foton-M4 spacecraft
Journal of Crystal Growth ∎ (∎∎∎∎) ∎∎∎–∎∎∎
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Crystal growth of Cd1 xZnxTe by the traveling heater method in microgravity on board of Foton-M4 spacecraft E.B. Borisenko a,n, N.N. Kolesnikov a, A.S. Senchenkov b, M. Fiederle c a
Institute of Solid State Physics, Russian Academy of Sciences, Chernogolovka, Russia Research and Development Institute for Launch Complexes, Moscow, Russia c Materials Science Center, University of Freiburg, Freiburg, Germany b
art ic l e i nf o
a b s t r a c t
Communicated by Dr Francois Dupret
Cadmium zinc telluride crystals were grown using the traveling heater method (THM) under microgravity conditions on board of Foton-M4 spacecraft, and a reference crystal was grown on Earth under gravity conditions. Structure, chemical and phase compositions of these crystals, their optical characteristics and microhardness were compared. It can be concluded that the THM growth in microgravity has a positive effect on CZT crystals, since they have more homogeneous composition and their structural perfection is improved as compared with the crystals grown under terrestrial conditions, which results in improvement of electric and optical characteristics. & 2016 Elsevier B.V. All rights reserved.
Keywords: A2. Microgravity conditions A2. Single crystal growth A2. Traveling solvent zone growth B2. Semiconducting cadmium compounds B2. Semiconducting II-VI materials
1. Introduction
2. Experimental
Crystal growth of CdTe and Cd1 xZnxTe (CZT) bulk specimens is developed for needs in high-resolution ionizing-radiation detectors. These crystals, especially ternary solutions doped with 6– 10 at% Zn, are very efficient for detector performance, which can be executed even at room temperature, without cooling. These materials are attractive due to their high atomic number, high specific resistivity associated with low leakage current, wide band gap, which provide high resolution and long-term stability of X-ray and gamma-ray detectors. They are also conventionally used in IR laser optics. The purpose of this work is to grow perfect bulk crystals, with low density of structural defects and homogeneous stoichiometric content. Why we consider crystal growth in microgravity advantageous as compared to the terrestrial THM growth? Structural defects, structural and chemical heterogeneities, pores, grain boundaries appear due to gravitational convection. In space, in microgravity, diffusion and convection are approached to stationary processes far closer than under terrestrial conditions. The THM grown crystals from Foton-M4 spacecraft and terrestrial ones are considered in this study. The parameters of the crystal growth derived from the diffusion and the convection mixing models conform to chosen experimental parameters under microgravity conditions [1].
Using of the traveling heater method (THM) [1] provides the solution-in-melt growth, which results in purification from foreign impurities and more homogeneous composition than in the case of the traditional Bridgman (HPVB) or High Pressure Vertical Zone Melting (HPVZM) methods [2]. But more than this, since a crystal is grown from solution in melt via Te liquid zone, the growing temperature was lowered considerably as compared to the melt growth, which allowed the experiment on board of the Foton-M4 spacecraft launched for two-months trip in space. A CZT feed and a single crystalline CdTe seed were loaded in a quartz ampoule, the traveling Te-rich liquid zone was installed. The initial feed had Cd0.9Zn0.1Te composition with 4 at% Te excess over the expected final crystal content. The CdTe (Cd 5N, Te 5N) seed contained about 2 at% Se. The solvent zone width was 26 mm, the estimated temperature on the interface was E 780 °C. The pulling rate was 0.3 mm/h, by 10 times lower than in the case of HPVB or HPVZM methods. The cooling rate was 50 °C/h.
n
Corresponding author. E-mail addresses: [email protected] (E.B. Borisenko), [email protected] (A.S. Senchenkov), michael.fi[email protected] (M. Fiederle).
3. Results and discussion The grown crystals were 32 mm in diameter and E42–43 mm in length. Crystals with high resistivity were obtained upon growth from the Te-rich solution. Specific resistivity of the samples was measured using probe technique. The values were measured in four points in each sample and averaged. The 3-mm thick CZT plates were cut and polished in 14% bromine-methanol, and 4-mm Ag contacts were made for the
http://dx.doi.org/10.1016/j.jcrysgro.2016.08.063 0022-0248/& 2016 Elsevier B.V. All rights reserved.
Please cite this article as: E.B. Borisenko, et al., (2016), http://dx.doi.org/10.1016/j.jcrysgro.2016.08.063i
2
E.B. Borisenko et al. / Journal of Crystal Growth ∎ (∎∎∎∎) ∎∎∎–∎∎∎
Fig. 1. XRD pattern of the crystal grown on board of Foton-M4, lines of the calculated Cd0.94Zn0.06Te data are superimposed.
Fig. 3. Rocking curves from two spots of Cd0.94Zn0.06Te crystal grown on board of Foton-M4. A combination-crystal detector scheme, a silicon monochromator in (111) reflection, CuKα1 radiation was used. The slit for the incident X-ray beam was 100 1000 mm.
measurements. The resistivity was measured near zero point at applied voltage from 20 to þ 20 V. In the samples grown under normal gravitation conditions average specific resistivity was 6 108 Ω cm. In the samples grown under microgravity conditions the specific resistivity was 109 Ω cm. This is increase in resistivity by about 1.7 due to microgravity conditions. It should be noted that a spread of data along a sample axis in the terrestrial samples was up to an order of magnitude, while in the orbital samples there was no more than two times difference in resistivity along a sample. The resistivity value obtained in CZT grown on Foton-M4 is comparable with average resistivity in commercial CZT [3], and the advantage of the growth in microgravity is in obtained uniformity of resistivity reached in the sample. According to X-ray diffraction (XRD) data obtained on a Siemens D500 X-ray diffractometer, using CuKα1 radiation, the crystal grown in microgravity has a cubic phase with sphalerite lattice, a ¼6.458 Å, which coincides precisely with the unit cell parameter calculated for this composition using the Vegard rule for solid solutions (Fig. 1). It is clearly seen from the Laue patterns, obtained using CuKα1 radiation (Fig. 2) that the crystal grown in microgravity is a single crystal, this is also confirmed by a rocking curve from a sample in (220) reflection (Fig. 3). Very narrow deviation from Bragg
Fig. 4. Image of Cd0.94Zn0.06Te crystals grown by THM method: (a) in Space; (b) on Earth.
Fig. 2. Laue pattern of (100) plane of the Cd0.94Zn0.06Te crystal grown on board of Foton-M4.
reflection δθ1 ¼39.5 arcsec, δθ2 ¼53 arcsec for two different tests, and very high reflection coefficient point to high crystal perfection of the sample grown on board of Foton-M4 (Fig. 4a). Oppositely to this, the sample grown under terrestrial conditions was a polycrystal (Fig. 4b), though with the same unit cell
Please cite this article as: E.B. Borisenko, et al., (2016), http://dx.doi.org/10.1016/j.jcrysgro.2016.08.063i
E.B. Borisenko et al. / Journal of Crystal Growth ∎ (∎∎∎∎) ∎∎∎–∎∎∎
3
the crystals grown on board of Foton-M4 provides decrease in particle size to 50 mm from 200-μm Te precipitates typical of the crystals grown in gravity. Density of the particles measured metallographically was 1.7 103 and 2 103, respectively. As compared to Redlen THM grown crystals or commercial Bridgman CZT crystals [4], Te inclusions in our crystals were considerably bigger, but their concentration was by 103 lower than in the referred papers. This is explained by differences in growth and cooling conditions in both cases. The IR transmission spectra of 3-mm thick Cd0.94Zn0.06Te plates measured on Specord-M80 spectrophotometer show that transmission reaches 70% in the crystals grown in microgravity and 60% in the crystals grown in gravity. IR transmission in the range from 200 to 4000 cm 1 wavenumbers is by 10% higher in the crystals grown on Foton-M4 board than in the crystals grown in normal conditions. This result shows that Te inclusions at the measured low density do not impede IR light transmission, which has an increment due to higher uniformity of CZT composition in the orbit-grown crystals. The measured Vickers microhardness HV under load 50g at room temperature is 640 MPa for the terrestrial crystal and 540 MPa for the orbital crystal. Spread of the data is about 6% for the former crystal, and less than 2% for the latter crystal. We suppose that microhardness of the terrestrial crystal is higher due to its more stressed structure containing differently oriented grains.
4. Conclusions
Fig. 5. Dependencies of elemental composition distribution in Cd0.94Zn0.06Te crystals along the growth axis: (a) grown in Space; (b) grown under terrestrial conditions.
parameter as the crystal grown on board of Foton-M4. The surface of the terrestrial sample in Fig. 4b is grained; the grain boundaries are visible. Electron probe microanalysis (EPMA) data confirm that both in the terrestrial sample and in the sample grown in microgravity, the composition tends asymptotically to the constant value of Cd0.94Zn0.06Te as the crystals grow, however, axial distribution of the components (Fig. 5a) in the space-grown crystal reaches higher homogeneity at shorter distance from the seed, which points to more stable growth than in the terrestrial crystal (Fig. 5b). As regards excess selenium, it crystallizes first in the initial part of the ingot due to the highest melting point of CdSe compound in the system containing Cd, Zn, Se, Te. There is always a drawback in the melt-grown CZT, which cannot be avoided even if slower and more stable solution-in-melt method is applied. In particular, it means tellurium inclusions, which precipitate in as-grown crystals. However, we have found that microgravity has a positive effect on size and concentration of Te particles. Excess Te is crystallized predominantly in the primary part of the solidified ingot grown from the solution in melt, its concentration decreases as the traveling heater moves, and the crystal composition tends asymptotically to Cd0.94Zn0.06Te. The effect of microgravity on maximum size of Te inclusions in
We can conclude that the THM growth of CZT in microgravity results in production of the crystals with more homogeneous composition and structure as compared to the crystals grown under terrestrial conditions, which provides two-time increase and higher stability of resistivity all over the sample contrary to the THM grown crystals in normal conditions. The reached value 109 Ω cm is a competitive magnitude for CZT used for ionizing radiation detectors. Growth in microgravity also has a positive effect on Te particles, which show four-fold reduction of their size and slight decrease in density with respect to the ground-grown crystals. This brings to 10% increase in IR transmission, which reaches 70% in the space-grown CZT.
Acknowledgements The authors are grateful to I. A. Smirnova for the rocking curves presented in this work. The work was partially financially supported by Russian Federal Space Agency (Project no. 962-12).
References [1] A.S. Senchenkov, M. Fiederle, N.N. Kolesnikov, CZT crystal growth by THM in microgravity – preparation of experiments for FOTON-M4 mission, in: Proceedings of the 65th International Astronautical Congress, Toronto, IAC-14A2.4.7, 29.09–3.10.2014. [2] N.N. Kolesnikov, V.V. Kveder, R.B. James, D.N. Borisenko, M.P. Kulakov, HPVB and HPVZM shaped growth of CdZnTe, CdSe, and ZnSe crystals, in: R.B. James, L.A. Franks, A. Burger, E.W. Westbrook, R.D. Durst (Eds.), Proc. of SPIE X-Ray and Gamma-Ray Detectors and Applications, SPIE 2002, Vol. 4784, 2002, pp. 93–104. [3] U.N. Roy, S. Weiller, S. Stein, A. Gueorgiev, Unseeded growth of CdZnTe:In by THM technique, in: R.B. James, L.A. Franks A. Burger (Eds.), Proc. Of SPIE Hard X-Ray, Gamma-Ray, and Neutron Detector Physics XI, SPIE 2009, Vol. 7449, 2009, pp. 74490U-1. [4] H. Chen, S.A. Awadalla, J. Mackenzie, R. Redden, G. Bindley, A.E. Bolotnikov, G. S. Camarda, G. Carini, R.B. James, Characterization of Traveling Heater Method (THM) grown Cd0.9Zn0.1Te Crystals, IEEE Trans. Nucl. Sci. 54 (4) (2007) 811–816.
Please cite this article as: E.B. Borisenko, et al., (2016), http://dx.doi.org/10.1016/j.jcrysgro.2016.08.063i
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Crystal growth of Cd1 xZnxTe by the traveling heater method in microgravity on board of Foton-M4 spacecraft E.B. Borisenko a,n, N.N. Kolesnikov a, A.S. Senchenkov b, M. Fiederle c a
Institute of Solid State Physics, Russian Academy of Sciences, Chernogolovka, Russia Research and Development Institute for Launch Complexes, Moscow, Russia c Materials Science Center, University of Freiburg, Freiburg, Germany b
art ic l e i nf o
a b s t r a c t
Communicated by Dr Francois Dupret
Cadmium zinc telluride crystals were grown using the traveling heater method (THM) under microgravity conditions on board of Foton-M4 spacecraft, and a reference crystal was grown on Earth under gravity conditions. Structure, chemical and phase compositions of these crystals, their optical characteristics and microhardness were compared. It can be concluded that the THM growth in microgravity has a positive effect on CZT crystals, since they have more homogeneous composition and their structural perfection is improved as compared with the crystals grown under terrestrial conditions, which results in improvement of electric and optical characteristics. & 2016 Elsevier B.V. All rights reserved.
Keywords: A2. Microgravity conditions A2. Single crystal growth A2. Traveling solvent zone growth B2. Semiconducting cadmium compounds B2. Semiconducting II-VI materials
1. Introduction
2. Experimental
Crystal growth of CdTe and Cd1 xZnxTe (CZT) bulk specimens is developed for needs in high-resolution ionizing-radiation detectors. These crystals, especially ternary solutions doped with 6– 10 at% Zn, are very efficient for detector performance, which can be executed even at room temperature, without cooling. These materials are attractive due to their high atomic number, high specific resistivity associated with low leakage current, wide band gap, which provide high resolution and long-term stability of X-ray and gamma-ray detectors. They are also conventionally used in IR laser optics. The purpose of this work is to grow perfect bulk crystals, with low density of structural defects and homogeneous stoichiometric content. Why we consider crystal growth in microgravity advantageous as compared to the terrestrial THM growth? Structural defects, structural and chemical heterogeneities, pores, grain boundaries appear due to gravitational convection. In space, in microgravity, diffusion and convection are approached to stationary processes far closer than under terrestrial conditions. The THM grown crystals from Foton-M4 spacecraft and terrestrial ones are considered in this study. The parameters of the crystal growth derived from the diffusion and the convection mixing models conform to chosen experimental parameters under microgravity conditions [1].
Using of the traveling heater method (THM) [1] provides the solution-in-melt growth, which results in purification from foreign impurities and more homogeneous composition than in the case of the traditional Bridgman (HPVB) or High Pressure Vertical Zone Melting (HPVZM) methods [2]. But more than this, since a crystal is grown from solution in melt via Te liquid zone, the growing temperature was lowered considerably as compared to the melt growth, which allowed the experiment on board of the Foton-M4 spacecraft launched for two-months trip in space. A CZT feed and a single crystalline CdTe seed were loaded in a quartz ampoule, the traveling Te-rich liquid zone was installed. The initial feed had Cd0.9Zn0.1Te composition with 4 at% Te excess over the expected final crystal content. The CdTe (Cd 5N, Te 5N) seed contained about 2 at% Se. The solvent zone width was 26 mm, the estimated temperature on the interface was E 780 °C. The pulling rate was 0.3 mm/h, by 10 times lower than in the case of HPVB or HPVZM methods. The cooling rate was 50 °C/h.
n
Corresponding author. E-mail addresses: [email protected] (E.B. Borisenko), [email protected] (A.S. Senchenkov), michael.fi[email protected] (M. Fiederle).
3. Results and discussion The grown crystals were 32 mm in diameter and E42–43 mm in length. Crystals with high resistivity were obtained upon growth from the Te-rich solution. Specific resistivity of the samples was measured using probe technique. The values were measured in four points in each sample and averaged. The 3-mm thick CZT plates were cut and polished in 14% bromine-methanol, and 4-mm Ag contacts were made for the
http://dx.doi.org/10.1016/j.jcrysgro.2016.08.063 0022-0248/& 2016 Elsevier B.V. All rights reserved.
Please cite this article as: E.B. Borisenko, et al., (2016), http://dx.doi.org/10.1016/j.jcrysgro.2016.08.063i
2
E.B. Borisenko et al. / Journal of Crystal Growth ∎ (∎∎∎∎) ∎∎∎–∎∎∎
Fig. 1. XRD pattern of the crystal grown on board of Foton-M4, lines of the calculated Cd0.94Zn0.06Te data are superimposed.
Fig. 3. Rocking curves from two spots of Cd0.94Zn0.06Te crystal grown on board of Foton-M4. A combination-crystal detector scheme, a silicon monochromator in (111) reflection, CuKα1 radiation was used. The slit for the incident X-ray beam was 100 1000 mm.
measurements. The resistivity was measured near zero point at applied voltage from 20 to þ 20 V. In the samples grown under normal gravitation conditions average specific resistivity was 6 108 Ω cm. In the samples grown under microgravity conditions the specific resistivity was 109 Ω cm. This is increase in resistivity by about 1.7 due to microgravity conditions. It should be noted that a spread of data along a sample axis in the terrestrial samples was up to an order of magnitude, while in the orbital samples there was no more than two times difference in resistivity along a sample. The resistivity value obtained in CZT grown on Foton-M4 is comparable with average resistivity in commercial CZT [3], and the advantage of the growth in microgravity is in obtained uniformity of resistivity reached in the sample. According to X-ray diffraction (XRD) data obtained on a Siemens D500 X-ray diffractometer, using CuKα1 radiation, the crystal grown in microgravity has a cubic phase with sphalerite lattice, a ¼6.458 Å, which coincides precisely with the unit cell parameter calculated for this composition using the Vegard rule for solid solutions (Fig. 1). It is clearly seen from the Laue patterns, obtained using CuKα1 radiation (Fig. 2) that the crystal grown in microgravity is a single crystal, this is also confirmed by a rocking curve from a sample in (220) reflection (Fig. 3). Very narrow deviation from Bragg
Fig. 4. Image of Cd0.94Zn0.06Te crystals grown by THM method: (a) in Space; (b) on Earth.
Fig. 2. Laue pattern of (100) plane of the Cd0.94Zn0.06Te crystal grown on board of Foton-M4.
reflection δθ1 ¼39.5 arcsec, δθ2 ¼53 arcsec for two different tests, and very high reflection coefficient point to high crystal perfection of the sample grown on board of Foton-M4 (Fig. 4a). Oppositely to this, the sample grown under terrestrial conditions was a polycrystal (Fig. 4b), though with the same unit cell
Please cite this article as: E.B. Borisenko, et al., (2016), http://dx.doi.org/10.1016/j.jcrysgro.2016.08.063i
E.B. Borisenko et al. / Journal of Crystal Growth ∎ (∎∎∎∎) ∎∎∎–∎∎∎
3
the crystals grown on board of Foton-M4 provides decrease in particle size to 50 mm from 200-μm Te precipitates typical of the crystals grown in gravity. Density of the particles measured metallographically was 1.7 103 and 2 103, respectively. As compared to Redlen THM grown crystals or commercial Bridgman CZT crystals [4], Te inclusions in our crystals were considerably bigger, but their concentration was by 103 lower than in the referred papers. This is explained by differences in growth and cooling conditions in both cases. The IR transmission spectra of 3-mm thick Cd0.94Zn0.06Te plates measured on Specord-M80 spectrophotometer show that transmission reaches 70% in the crystals grown in microgravity and 60% in the crystals grown in gravity. IR transmission in the range from 200 to 4000 cm 1 wavenumbers is by 10% higher in the crystals grown on Foton-M4 board than in the crystals grown in normal conditions. This result shows that Te inclusions at the measured low density do not impede IR light transmission, which has an increment due to higher uniformity of CZT composition in the orbit-grown crystals. The measured Vickers microhardness HV under load 50g at room temperature is 640 MPa for the terrestrial crystal and 540 MPa for the orbital crystal. Spread of the data is about 6% for the former crystal, and less than 2% for the latter crystal. We suppose that microhardness of the terrestrial crystal is higher due to its more stressed structure containing differently oriented grains.
4. Conclusions
Fig. 5. Dependencies of elemental composition distribution in Cd0.94Zn0.06Te crystals along the growth axis: (a) grown in Space; (b) grown under terrestrial conditions.
parameter as the crystal grown on board of Foton-M4. The surface of the terrestrial sample in Fig. 4b is grained; the grain boundaries are visible. Electron probe microanalysis (EPMA) data confirm that both in the terrestrial sample and in the sample grown in microgravity, the composition tends asymptotically to the constant value of Cd0.94Zn0.06Te as the crystals grow, however, axial distribution of the components (Fig. 5a) in the space-grown crystal reaches higher homogeneity at shorter distance from the seed, which points to more stable growth than in the terrestrial crystal (Fig. 5b). As regards excess selenium, it crystallizes first in the initial part of the ingot due to the highest melting point of CdSe compound in the system containing Cd, Zn, Se, Te. There is always a drawback in the melt-grown CZT, which cannot be avoided even if slower and more stable solution-in-melt method is applied. In particular, it means tellurium inclusions, which precipitate in as-grown crystals. However, we have found that microgravity has a positive effect on size and concentration of Te particles. Excess Te is crystallized predominantly in the primary part of the solidified ingot grown from the solution in melt, its concentration decreases as the traveling heater moves, and the crystal composition tends asymptotically to Cd0.94Zn0.06Te. The effect of microgravity on maximum size of Te inclusions in
We can conclude that the THM growth of CZT in microgravity results in production of the crystals with more homogeneous composition and structure as compared to the crystals grown under terrestrial conditions, which provides two-time increase and higher stability of resistivity all over the sample contrary to the THM grown crystals in normal conditions. The reached value 109 Ω cm is a competitive magnitude for CZT used for ionizing radiation detectors. Growth in microgravity also has a positive effect on Te particles, which show four-fold reduction of their size and slight decrease in density with respect to the ground-grown crystals. This brings to 10% increase in IR transmission, which reaches 70% in the space-grown CZT.
Acknowledgements The authors are grateful to I. A. Smirnova for the rocking curves presented in this work. The work was partially financially supported by Russian Federal Space Agency (Project no. 962-12).
References [1] A.S. Senchenkov, M. Fiederle, N.N. Kolesnikov, CZT crystal growth by THM in microgravity – preparation of experiments for FOTON-M4 mission, in: Proceedings of the 65th International Astronautical Congress, Toronto, IAC-14A2.4.7, 29.09–3.10.2014. [2] N.N. Kolesnikov, V.V. Kveder, R.B. James, D.N. Borisenko, M.P. Kulakov, HPVB and HPVZM shaped growth of CdZnTe, CdSe, and ZnSe crystals, in: R.B. James, L.A. Franks, A. Burger, E.W. Westbrook, R.D. Durst (Eds.), Proc. of SPIE X-Ray and Gamma-Ray Detectors and Applications, SPIE 2002, Vol. 4784, 2002, pp. 93–104. [3] U.N. Roy, S. Weiller, S. Stein, A. Gueorgiev, Unseeded growth of CdZnTe:In by THM technique, in: R.B. James, L.A. Franks A. Burger (Eds.), Proc. Of SPIE Hard X-Ray, Gamma-Ray, and Neutron Detector Physics XI, SPIE 2009, Vol. 7449, 2009, pp. 74490U-1. [4] H. Chen, S.A. Awadalla, J. Mackenzie, R. Redden, G. Bindley, A.E. Bolotnikov, G. S. Camarda, G. Carini, R.B. James, Characterization of Traveling Heater Method (THM) grown Cd0.9Zn0.1Te Crystals, IEEE Trans. Nucl. Sci. 54 (4) (2007) 811–816.
Please cite this article as: E.B. Borisenko, et al., (2016), http://dx.doi.org/10.1016/j.jcrysgro.2016.08.063i