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Growth and characterization of mercury indium telluride single crystals
Materials Science and Engineering B 133 (2006) 129–131
Growth and characterization of mercury indium telluride single crystals Linghang Wang ∗ , Wanqi Jie, Gangqiang Zha, Gang Xu State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, People’s Republic of China Received 6 May 2006; received in revised form 6 June 2006; accepted 10 June 2006
Abstract A novel photoelectric single crystal, mercury indium telluride (MIT), with dimensions of 15 mm in diameter and 175 mm in length has been successfully grown by using the vertical Bridgman method (VB). The density of the MIT crystal was measured to be 6.248 g/cm−3 . The structural crystal quality was investigated by single-crystal X-ray diffraction (SXRD) experiments and shown to be a single phase and to have good quality. The optical properties were investigated by UV–NIR–IR analysis, which proved that the cut-off length is 1697 nm and the energy band-gap of MIT was authenticated to be 0.73 eV. The transmittance spectra from 2.5 to 25 m showed high middle and far-infrared transmittance, which was determined to be 50–55%. © 2006 Elsevier B.V. All rights reserved. Keywords: X-ray diffraction; Crystal growth; Mercury indium telluride; Semiconductor materials
1. Introduction The photoelectric single crystal of mercury indium telluride (MIT) is a novel material for near-infrared photovoltaic detectors, which is a equimolar solid solution of Hg3 Te3 and In2 Te3 and crystallizes in the defect zincblende structure [1–3]. HgTe itself is a semimetal in consequence of a small band overlap. The addition of In2 Te3 causes the energy overlap to decrease until it goes through zero, which results in the reduction of the carrier effective mass and the increase of the carrier mobility. Such narrow band gap compounds are useful in the fabrication of the infrared detectors. The MIT compound can be considered as being derived from HgTe by replacing half the mercury atoms by units of two In ions plus one vacancy, which leads to the high concentration of structural vacancies (about 1021 cm−3 ). The number of structural vacancies is proportional to the content of In2 Te3 and has large effects on the properties of the crystals. These defects reduce the electrical sensitivity to the introduced impurities, pin the Fermi level near the midgap due to the self-compensation effect and enhance the stability towards the ionzing radiation, which imply that the crystal is a promising material for the application of fast and efficient photodetectors.
∗
Corresponding author. Tel.: +86 29 884 86065; fax: +86 29 884 95414. E-mail address: [email protected] (L. Wang).
0921-5107/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2006.06.015
There are some previous reports concerning the reaction kinetics and the elastic behavior [4–7], with no details about the crystal growth and the optical properties. In our work, the crystals have been successfully grown by vertical Bridgman method (VB) and the optical properties of the MIT crystal were investigated. We are presenting the preliminary results of mercury indium telluride single crystal grown by vertical Bridgman method in this paper. 2. Experimental procedure The single crystals were grown by vertical Bridgman method with a quartz crucible (15 mm in diameter). The starting materials were high purity elements (of at least 7 N purity) of Te, In, Hg and were charged into the dried quartz crucible with a stoichiometric ratio corresponding to the compound to be grown, then evacuated to about 10−4 Torr and sealed. The crucible was designed to be sharp-pointed cone tip to form a seed crystal in the initial region and had been well treated with the aqua regia and acetone to acquire the clean inner surface and decrease the chance of side wall nucleation. The starting materials in the crucible were synthesized in a rocking furnace. The crystal growth was carried out in the same crucible in a vertical Bridgman furnace to avoid extraneous impurity pollution. Mixtures to be grown were fused in the VB furnace and annealed at 1040 K for 24 h to obtain homogeneous melts, then the crucible was gradually lowered from the high temperature zone to the cold zone in
130
L. Wang et al. / Materials Science and Engineering B 133 (2006) 129–131
Fig. 1. MIT single crystal grown by VB. Fig. 2. X-ray spectrum of an as-grown MIT wafer.
the temperature gradient of about 10 K/cm. The crucible translation speed was less than 1 mm/h. The as-grown MIT crystal of Φ15 mm × 175 mm with good quality and free crack is shown in Fig. 1. Single crystal X-ray diffraction data of MIT were recorded by using a D/max 2400 type X-ray diffractometer with Cu K␣ ˚ at room temperature in the 2θ range of radiation (λ = 1.54056 A) 30◦ –80◦ in steps of 0.02◦ . The accelerating voltage was 40 kV and the current was 30 mA. The as-grown MIT crystal was cut perpendicular to the Z direction into 1.0-mm thick wafers, then chemo-mechanically polished and etched in the Br–methanol solution for the study of the optical properties. The optical transmission spectra of the MIT crystal were recorded at room temperature by means of the SHIMADZU model UV-3150 UV–vis–NIR spectrophotometer with the wavelength from 0.3 to 3.0 m and the NICOLET NEXUS FT-IR spectrometer with the wavelength from 2.5 to 25 m.
3.2. Single crystal XRD analysis The structural crystal quality is usually characterized by the X-ray diffraction. Fig. 2 shows the single crystal X-ray diffraction spectrum of an as-grown MIT wafer. From Fig. 2, we can draw a conclusion that the wafer is a single crystal. The diffraction peak position is at 47.940◦ , which indicates that the cut plane is (3 1 1) facet. The FWHM is about 15.6 . The peak shape of diffraction curve is sharp and has good symmetry. The results show that the as-grown MIT crystal has good crystal quality. 3.3. Optical properties The UV–NIR–IR transmission spectrum of the mercury indium telluride single crystal is shown in Fig. 3. In the region of about 1.7 m, wavelength is found to be linearly correlated with the transmittance in the range of (10–60%) Tmax . The cut-off
3. Results and discussions 3.1. Density measurement The density of MIT single crystal has been found by experimental method. A block was cut from the as-grown crystal for the measurement. The density was measured to be 6.248 g/cm−3 by using the Archimedes’ method. The density of MIT single crystal was also determined theoretically from the crystallographic data using the following formula: ρ=
MZ N0 V
(1)
where M is the molecular weight, Z the number of molecules per unit cell, N0 the Avagadro number and V is the volume of the unit ˚ 3 , where the lattice constant of the cubic MIT cell, V = 249.21 A ˚ From Eq. (1), the density of MIT single crystal is a = 6.2930 A. crystal is calculated to be 6.268 g/cm−3 . It is found that the experimental density is in good agreement with the theoretical value.
Fig. 3. Transmission spectrum of MIT at room temperature.
L. Wang et al. / Materials Science and Engineering B 133 (2006) 129–131
wavelength can be approximately determined by the intersection of the tangent to this linear range and the wavelength axis, which is determined to be 1697 nm. From the formula Eg =
1.2396 λ∞
131
the range of 2.5–25 m. Therefore, the transmittance of the MIT crystal ascends with the increase of the wavelength. 4. Conclusions
(2)
where λ∞ is the cut-off wavelength. Eg was calculated to be 0.73 eV, which is in good agreement with that of the Refs. [8,9]. From Fig. 3, it can be seen that the transmittance of the MIT crystal is 50–55% in the region of 2.5–25 m, which indicates that the as-grown crystal has good quality and less deficiency. Because the MIT crystal is a direct energy band-gap semiconductor compound, the electrons in the valence band cannot intrinsically absorb the photons with wavelength in the range of middle and far infrared band. Therefore, the MIT crystal has high transmittance over a wide range of wavelength from 2.5 to 25 m. It can also be found that the transmittance of the MIT crystal increases with the increase of wavelength in this range, which is mostly due to the lattice absorption at low wavelength and free carriers absorption at high wavelength. Thanks to the vibration of the lattice, the translation of the anions and cations produces the electric dipole moment. The higher frequency aggravates the vibration of the lattice and leads to the increase of the electric dipole moment. The larger the electric dipole moment is, the stronger the absorption is. Therefore, the transmittance of the MIT crystal increases with the increase of the wavelength. Furthermore, the free carriers are scattered by the phonons in the field of the infrared radiation and transfer the energy to the lattice, which weaken the energy of the infrared radiation and reduce the intensity of the infrared light. With the increase of the wavelength, the transmittance of the MIT crystal decreases. However, the energy that the free carriers transfer to the lattice results in the stronger lattice absorption, which is dominant in
Using the vertical Bridgman method, we have successfully grown a new photoelectric single crystal, mercury indium telluride, with dimensions of 15 mm in diameter and 175 mm in length. The results of density measurement show that the density of the MIT crystal is 6.248 g/cm−3 . Single-crystal X-ray diffraction (SXRD) experiment confirms that the as-grown MIT crystal is a single phase and has good crystal quality. We studied the optical properties of the MIT crystal by UV–NIR–IR analysis. The results reveal that the cut-off length of the MIT crystal is 1697 nm and the energy band-gap of MIT was determined to be 0.73 eV. The transmittance spectra from 2.5 to 25 m show that the MIT crystal has high middle and far-infrared transmittance, which was determined to be 50–55%. Acknowledgements This work is supported by the National Natural Science Foundation of China (50336040) and graduate starting seed fund of Northwestern Polytechnical University (200612). References [1] [2] [3] [4] [5] [6] [7] [8] [9]
P.M. Spencer, B. Ray, Br. J. Appl. Phys. (J. Phys. D) 1 (1968) 299. D. Weitze, V. Leute, J. Alloys Compd. 236 (1996) 229. V. Leute, H.M. Schmidtke, J. Phys. Chem. Solids 4 (1988) 409. V. Leute, H.M. Schmidtke, J. Phys. Chem. Solids 11 (1988) 1317. G.A. Sauders, T. Seddon, J. Phys. Chem. Solids 31 (1970) 2495. T. Halling, G.A. Sauders, W.A. Lambson, Phys. Rev. B 26 (1982) 5786. G.A. Sauders, T. Seddon, J. Phys. Chem. Solids 37 (1976) 873. P.M. Spencer, Br. J. Appl. Phys. 15 (1964) 625. D.A. Wright, Br. J. Appl. Phys. 16 (1965) 939.
Growth and characterization of mercury indium telluride single crystals Linghang Wang ∗ , Wanqi Jie, Gangqiang Zha, Gang Xu State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, People’s Republic of China Received 6 May 2006; received in revised form 6 June 2006; accepted 10 June 2006
Abstract A novel photoelectric single crystal, mercury indium telluride (MIT), with dimensions of 15 mm in diameter and 175 mm in length has been successfully grown by using the vertical Bridgman method (VB). The density of the MIT crystal was measured to be 6.248 g/cm−3 . The structural crystal quality was investigated by single-crystal X-ray diffraction (SXRD) experiments and shown to be a single phase and to have good quality. The optical properties were investigated by UV–NIR–IR analysis, which proved that the cut-off length is 1697 nm and the energy band-gap of MIT was authenticated to be 0.73 eV. The transmittance spectra from 2.5 to 25 m showed high middle and far-infrared transmittance, which was determined to be 50–55%. © 2006 Elsevier B.V. All rights reserved. Keywords: X-ray diffraction; Crystal growth; Mercury indium telluride; Semiconductor materials
1. Introduction The photoelectric single crystal of mercury indium telluride (MIT) is a novel material for near-infrared photovoltaic detectors, which is a equimolar solid solution of Hg3 Te3 and In2 Te3 and crystallizes in the defect zincblende structure [1–3]. HgTe itself is a semimetal in consequence of a small band overlap. The addition of In2 Te3 causes the energy overlap to decrease until it goes through zero, which results in the reduction of the carrier effective mass and the increase of the carrier mobility. Such narrow band gap compounds are useful in the fabrication of the infrared detectors. The MIT compound can be considered as being derived from HgTe by replacing half the mercury atoms by units of two In ions plus one vacancy, which leads to the high concentration of structural vacancies (about 1021 cm−3 ). The number of structural vacancies is proportional to the content of In2 Te3 and has large effects on the properties of the crystals. These defects reduce the electrical sensitivity to the introduced impurities, pin the Fermi level near the midgap due to the self-compensation effect and enhance the stability towards the ionzing radiation, which imply that the crystal is a promising material for the application of fast and efficient photodetectors.
∗
Corresponding author. Tel.: +86 29 884 86065; fax: +86 29 884 95414. E-mail address: [email protected] (L. Wang).
0921-5107/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2006.06.015
There are some previous reports concerning the reaction kinetics and the elastic behavior [4–7], with no details about the crystal growth and the optical properties. In our work, the crystals have been successfully grown by vertical Bridgman method (VB) and the optical properties of the MIT crystal were investigated. We are presenting the preliminary results of mercury indium telluride single crystal grown by vertical Bridgman method in this paper. 2. Experimental procedure The single crystals were grown by vertical Bridgman method with a quartz crucible (15 mm in diameter). The starting materials were high purity elements (of at least 7 N purity) of Te, In, Hg and were charged into the dried quartz crucible with a stoichiometric ratio corresponding to the compound to be grown, then evacuated to about 10−4 Torr and sealed. The crucible was designed to be sharp-pointed cone tip to form a seed crystal in the initial region and had been well treated with the aqua regia and acetone to acquire the clean inner surface and decrease the chance of side wall nucleation. The starting materials in the crucible were synthesized in a rocking furnace. The crystal growth was carried out in the same crucible in a vertical Bridgman furnace to avoid extraneous impurity pollution. Mixtures to be grown were fused in the VB furnace and annealed at 1040 K for 24 h to obtain homogeneous melts, then the crucible was gradually lowered from the high temperature zone to the cold zone in
130
L. Wang et al. / Materials Science and Engineering B 133 (2006) 129–131
Fig. 1. MIT single crystal grown by VB. Fig. 2. X-ray spectrum of an as-grown MIT wafer.
the temperature gradient of about 10 K/cm. The crucible translation speed was less than 1 mm/h. The as-grown MIT crystal of Φ15 mm × 175 mm with good quality and free crack is shown in Fig. 1. Single crystal X-ray diffraction data of MIT were recorded by using a D/max 2400 type X-ray diffractometer with Cu K␣ ˚ at room temperature in the 2θ range of radiation (λ = 1.54056 A) 30◦ –80◦ in steps of 0.02◦ . The accelerating voltage was 40 kV and the current was 30 mA. The as-grown MIT crystal was cut perpendicular to the Z direction into 1.0-mm thick wafers, then chemo-mechanically polished and etched in the Br–methanol solution for the study of the optical properties. The optical transmission spectra of the MIT crystal were recorded at room temperature by means of the SHIMADZU model UV-3150 UV–vis–NIR spectrophotometer with the wavelength from 0.3 to 3.0 m and the NICOLET NEXUS FT-IR spectrometer with the wavelength from 2.5 to 25 m.
3.2. Single crystal XRD analysis The structural crystal quality is usually characterized by the X-ray diffraction. Fig. 2 shows the single crystal X-ray diffraction spectrum of an as-grown MIT wafer. From Fig. 2, we can draw a conclusion that the wafer is a single crystal. The diffraction peak position is at 47.940◦ , which indicates that the cut plane is (3 1 1) facet. The FWHM is about 15.6 . The peak shape of diffraction curve is sharp and has good symmetry. The results show that the as-grown MIT crystal has good crystal quality. 3.3. Optical properties The UV–NIR–IR transmission spectrum of the mercury indium telluride single crystal is shown in Fig. 3. In the region of about 1.7 m, wavelength is found to be linearly correlated with the transmittance in the range of (10–60%) Tmax . The cut-off
3. Results and discussions 3.1. Density measurement The density of MIT single crystal has been found by experimental method. A block was cut from the as-grown crystal for the measurement. The density was measured to be 6.248 g/cm−3 by using the Archimedes’ method. The density of MIT single crystal was also determined theoretically from the crystallographic data using the following formula: ρ=
MZ N0 V
(1)
where M is the molecular weight, Z the number of molecules per unit cell, N0 the Avagadro number and V is the volume of the unit ˚ 3 , where the lattice constant of the cubic MIT cell, V = 249.21 A ˚ From Eq. (1), the density of MIT single crystal is a = 6.2930 A. crystal is calculated to be 6.268 g/cm−3 . It is found that the experimental density is in good agreement with the theoretical value.
Fig. 3. Transmission spectrum of MIT at room temperature.
L. Wang et al. / Materials Science and Engineering B 133 (2006) 129–131
wavelength can be approximately determined by the intersection of the tangent to this linear range and the wavelength axis, which is determined to be 1697 nm. From the formula Eg =
1.2396 λ∞
131
the range of 2.5–25 m. Therefore, the transmittance of the MIT crystal ascends with the increase of the wavelength. 4. Conclusions
(2)
where λ∞ is the cut-off wavelength. Eg was calculated to be 0.73 eV, which is in good agreement with that of the Refs. [8,9]. From Fig. 3, it can be seen that the transmittance of the MIT crystal is 50–55% in the region of 2.5–25 m, which indicates that the as-grown crystal has good quality and less deficiency. Because the MIT crystal is a direct energy band-gap semiconductor compound, the electrons in the valence band cannot intrinsically absorb the photons with wavelength in the range of middle and far infrared band. Therefore, the MIT crystal has high transmittance over a wide range of wavelength from 2.5 to 25 m. It can also be found that the transmittance of the MIT crystal increases with the increase of wavelength in this range, which is mostly due to the lattice absorption at low wavelength and free carriers absorption at high wavelength. Thanks to the vibration of the lattice, the translation of the anions and cations produces the electric dipole moment. The higher frequency aggravates the vibration of the lattice and leads to the increase of the electric dipole moment. The larger the electric dipole moment is, the stronger the absorption is. Therefore, the transmittance of the MIT crystal increases with the increase of the wavelength. Furthermore, the free carriers are scattered by the phonons in the field of the infrared radiation and transfer the energy to the lattice, which weaken the energy of the infrared radiation and reduce the intensity of the infrared light. With the increase of the wavelength, the transmittance of the MIT crystal decreases. However, the energy that the free carriers transfer to the lattice results in the stronger lattice absorption, which is dominant in
Using the vertical Bridgman method, we have successfully grown a new photoelectric single crystal, mercury indium telluride, with dimensions of 15 mm in diameter and 175 mm in length. The results of density measurement show that the density of the MIT crystal is 6.248 g/cm−3 . Single-crystal X-ray diffraction (SXRD) experiment confirms that the as-grown MIT crystal is a single phase and has good crystal quality. We studied the optical properties of the MIT crystal by UV–NIR–IR analysis. The results reveal that the cut-off length of the MIT crystal is 1697 nm and the energy band-gap of MIT was determined to be 0.73 eV. The transmittance spectra from 2.5 to 25 m show that the MIT crystal has high middle and far-infrared transmittance, which was determined to be 50–55%. Acknowledgements This work is supported by the National Natural Science Foundation of China (50336040) and graduate starting seed fund of Northwestern Polytechnical University (200612). References [1] [2] [3] [4] [5] [6] [7] [8] [9]
P.M. Spencer, B. Ray, Br. J. Appl. Phys. (J. Phys. D) 1 (1968) 299. D. Weitze, V. Leute, J. Alloys Compd. 236 (1996) 229. V. Leute, H.M. Schmidtke, J. Phys. Chem. Solids 4 (1988) 409. V. Leute, H.M. Schmidtke, J. Phys. Chem. Solids 11 (1988) 1317. G.A. Sauders, T. Seddon, J. Phys. Chem. Solids 31 (1970) 2495. T. Halling, G.A. Sauders, W.A. Lambson, Phys. Rev. B 26 (1982) 5786. G.A. Sauders, T. Seddon, J. Phys. Chem. Solids 37 (1976) 873. P.M. Spencer, Br. J. Appl. Phys. 15 (1964) 625. D.A. Wright, Br. J. Appl. Phys. 16 (1965) 939.