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Study of CdTe and ZnTe single-crystal growth from vapour phase and investigation of the grown crystals
Journal of Crystal Growth 242 (2002) 41–44
Study of CdTe and ZnTe single-crystal growth from vapour phase and investigation of the grown crystals G. Ilchuka,*, V. Ukrainetza, A. Danylova, V. Maslukb, J. Parlagb, O. Yaskovc b
a National University Lvivska Polytechnika, Lviv, Ukraine Institute of Electronic Physics of Ukrainian National Academy of Science, Uzhorod, Ukraine c Kyivstar GSM, JSV, Lviv, Ukraine
Received 9 March 2002; accepted 21 March 2002 Communicated by T. Hibiya
Abstract Results of a preliminary investigation of cadmium telluride (CdTe) single-crystal gamma-spectra for crystals grown by vapour-phase deposition method of chemical transport reaction with NH4X (X=Cl, Br, I) as transferors by means of g- and neutron-activation analysis methods are presented. Conditions and techniques of activation experiments as well as criteria of analytical g-line choice for identification of chemical elements in the compounds are discussed in detail. Preliminary data from g-activation analysis (Eg ¼ 12 MeV) of CdTe single crystal stoichiometry are obtained. r 2002 Elsevier Science B.V. All rights reserved. PACS: 82.80.E; 81.10.A Keywords: A1. Nucleation; A2. Growth from vapor; B1. Cadmium compounds; B1. Zinc compounds; B2. Semiconducting cadmium compounds; B2. Semiconducting II–VI materials
1. Introduction Cadmium telluride (CdTe) and zinc telluride (ZnTe) were proved to be perspective materials for optoelectronics applications. These materials are widely used for electron-optical modulation of laser radiation, in technology of fabrication of broad spectrum range photoreceivers based on CdxHg1xTe, ZnxHg1xTe solid solutions, for direct conversion of solar radiation energy into electrical energy, for registration of X-ray and gamma-radiations, etc. To grow a compound with *Corresponding author. E-mail address: [email protected] (G. Ilchuk).
controlled properties is a challenging problem of modern semiconductor material research and an integral part of system investigations with the aim of novel semiconductor-device development. For a long time, we have been developing the method of chemical transport reaction (CTR) with NH4Cl, NH4Br and NH4I substances as transferors to grow single crystals of binary compounds of A2B6 group and their solid solutions as well. The method is based on the mathematical simulations of gas phase content and mass transfer [1]. Semiinsulating single crystals with dislocation density 102 cm2 were grown [2]. Optoelectronic studies show that the growth process is accompanied by Cl, Br and I halogen inclusions into a
0022-0248/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 2 ) 0 1 3 4 6 - 5
42
G. Ilchuk et al. / Journal of Crystal Growth 242 (2002) 41–44
CdTe single crystal [3,4]. The problem of control of deviation level from stoichiometry for a grown single crystal as well as inclusion of halogens into the crystals during the growth process becomes urgent. To solve the problem we used g- and neutron-activation techniques for the analysis of the crystals.
2. Experimental procedures Activation techniques were realized on the electron accelerator-microtron M30 in IEF of National Academy of Sciences of Ukraine. The accelerator allows to vary the energy of accelerated electrons in the range 1–25 MeV when current is in the order of 2–5 mkA. That is, at the pointed conditions, the flow of photoneutrons can achieve a level of 109–1010 n/s. To obtain more reliable results of neutron- and g-activation analysis of Cd–Zn–Te /Br, I, ClS, activation of Cd, Zn, Te, B and I samplesmonitors was carried out in identical conditions. Irradiation time (activation time) of the samples (Tir ) was arranged according to the half-life period (T1=2 ) of samples-monitors to satisfy the condition Tir > 5T1=2 : It allowed to obtain saturation of the resulting isotope. Measuring time (Tm ) was chosen so that the statistical error of analytical peak measurement did not exceed 5%. During the process of activation, samples were positioned in the irradiation unit, that contains braking (1.5–3 mm in size) Ta-target or a photoneutron Be–Pb converter, placed at accelerated electron beam axis (Ee ¼ 12 MeV; Ie ¼ 527 mA) in the 4p-polyethylene moderator. There were the standards for comparison also. The monitor of secondary emission was used to control the electron dose. To control g2n dose the activation monitor positioned in the irradiation unit was used.
3. Results and discussion Identification of chemical elements was carried out on gamma-spectrometry complex with ‘‘SBS40’’ spectrometer and coaxial semiconducting
Ge(Li)-detector of high definition with volume 100 cm3. Library of the spectrometer had more than 390 radioisotopes. Detector was placed into a combine protection house with the layers of copper (8 mm), aluminum (3 mm), cadmium (1 mm), and lead (95 mm), and was cooled by liquid nitrogen. Resolution of gamma-spectrometer was 2.01 keV for the line 122.06 keV of Co-57 and 3.63 for the line 1332.50 keV of Co-60. The efficiency of measurements was controlled due to the special standard volume source Eu-152. The main nuclear-physical constants, namely, analytical lines of original and (n,g) reaction product nuclides necessary for their identification in ZnxCd1xTe /Br, I, ClS are shown in Table 1. The presented data allow to conclude the following: to identify radionuclides for semiconductor compounds of ZnxCd1xTe /Br, I, ClS type direct and indirect techniques can be used. So for Te, considering the reaction Te-131 ) I-131 as possible, one can carry out the identification by line 364.5 keV. There are different approaches to stoichiometry control of compounds such as CdTe, based on the presence of isotopes with short and long half-life periods: (a) analysis of intensive lines of nuclides with short half-life period, such as Cd-111m, Te131, when relaxation time of their activities is taken into account; and (b) analysis of long halflife period nuclides. In the latter case, when time of measurement is less than min{T1=2 (Cd-114)}, there is no sense to consider the nucleon decay characteristics, and we can perform an analysis of CdTe stoichiometry content by the ratio between intensities of analytical g-lines 336.3 (Cd-115) and 364.5 keV (I-131) that correspond to long half-life period radionuclides. For g-activation technique, when nuclear reactions (g; n) take place, it is necessary to carry out the time analysis of activities and g-spectral dependences for the investigated substance and corresponding monitors as well. One should assume that selected samples-monitors have natural mixture of radionuclides for the selected chemical elements. As for the previous case, there are different strategies of analysis by long and short half-life period isotopes of Cd and Te.
G. Ilchuk et al. / Journal of Crystal Growth 242 (2002) 41–44
43
Table 1 Main nuclear-physical constants of nuclides of chemical elements necessary for their identification in ZnxCd1xTe /Br, I, ClS samples Elements
Abundance
Product
Half-life period, T1=2
Analytical line, Eg (keV) (quantum yield, %)
Cd-114
28.93
Cd-115
53.46 h
336.24 (45.9) 527.90 (27.4)
Cd-110 Zn-68 Br-81
12.32 18.80 49.31
Cd-111m Zn-69m Br-82
48.70 min 13.76 h 35.30 h
245.4 (94.0) 438.63 (94.8) 554.35 (70.8) 619.11 (43.4) 776.52 (83.5)
I-127 Cl-37
100 24.23
I-128 Cl-38
24.99 min 37.24 min
442.94 (16.9) 1642.7 (31.9) 2167.41 (42.4)
Te-130
33.87
Te-131 Te-131 ) I-131
25.0 min
149.72 (68.9) 452.32 (12.2) 364.50 (78.0)
CdTe-1andTe-2,Cd-3 monitors 10000
Count
To determine the stoichiometry control possibility in CdTe single crystals, preliminary studies were carried out by means of braking (bremsstrahlung) beams of g-radiation with a maximum energy of 12 MeV as well as beams of turmoil neutrons as activators. Comparison of g-spectrum of the activated CdTe single crystal, grown by CTR method, and a spectrum of samples-monitors, measured 4 h after activation by braking radiation with maximum energy Eg ¼ 12 MeV is shown in Fig. 1. Measuring time Tm was equal to 10 min. It is obvious that some lines of CdTe g-spectrum contain contributions from relevant regions of gspectra for Cd and Te and, therefore, are not suitable for identification. Authors assume that other impurities can be included into the CdTe matrix and affect its resulting g-spectrum. The above-mentioned factors, as well as a varying sensitivity of Ge(Li)-detector for different regions of g-spectrum cause its characteristic fine structure, expressed in Fig. 1. As the analytical lines of CdTe g-spectrum for Cd identification, the Cd-115 isotope line with energy 492 keV (1192 channel) as well as In-115m isotope line with energy 336 keV (780 channel) which is the product of b-decay reaction for Cd-115 can be chosen. The ratio of the
8.07 days
1000
-500 418 keV, Te-127
100
1.
459,6 keV, Te-129 487 keV, Te-129 -50
492 keV Cd-115
2.
336 keV, In-115m
10
3.
1 500
1000 1500 Channels
2000
2500
Fig. 1. Gamma-spectra of activated CdTe single crystals grown by CTR method and samples-monitors of Cd and Te (purity level 99.9999%) measured 4 h after activation by braking irradiation (Eg ¼ 12 MeV; Tm ¼ 10 min).
line intensities allows to determine the stoichiometry level of the grown CdTe single crystals. Let us note that the analysis corresponds to the strategy of chemical content analysis owing to long-living radioisotopes with half-life period T1=2 from 10 h to 2 days. We determined that for a number of CdTe samples, synthesized with NH4Cl and NH4I as transferors, the ratio of intensities of
44
G. Ilchuk et al. / Journal of Crystal Growth 242 (2002) 41–44
the lines varies within the limit of 10% for a measurement series. It shows the satisfactory stoichiometry of samples and confirms the perspectives of nuclear-physical methods for stoichiometry control.
4. Conclusions The preliminary investigations of gamma-spectra of activated CdTe single crystals grown by CTR method as well as Cd and Te monitors show good perspectives of nuclear-physical method for identification of Cd and Te chemical elements in CdTe crystals and for control of stoichiometry. Analytical lines for identification of the elements were determined.
References [1] G.A. Ilchuk, Zinc telluride transport in the ZnTe–NH4I system, Inorg. Mater. 36 (2000) 668. [2] V.O. Ukrainets, G.A. Ilchuk, N.A. Ukrainets, Yu.V. Rud’, V.I. Ivanov-Omskii, Electrical properties of Schottky diodes using high-resistivity CdTe crystals, Tech. Phys. Lett. 25 (1999) 642. [3] G.A. Ilchuk, N.A. Ukrainets, V.I. Ivanov-Omskii, Yu.V. Rud’, V.Yu. Rud’, Optoelectronic effects in semi-insulating CdTe single crystals and structures based on them, Semiconductors 33 (1999) 518. [4] G.A. Ilchuk, N.A. Ukrainets, V.O. Ukrainets, B.I. Datsko, R.O. Zabrodsky, Yu.V. Rud’, Alternative method of doping of cadmium telluride by halogens Cl, Br, I, International Workshop on Advances in Growth and Characterization of II–VI Semiconductors, Wurzburg, . Germany, 1999, p. 38.
Study of CdTe and ZnTe single-crystal growth from vapour phase and investigation of the grown crystals G. Ilchuka,*, V. Ukrainetza, A. Danylova, V. Maslukb, J. Parlagb, O. Yaskovc b
a National University Lvivska Polytechnika, Lviv, Ukraine Institute of Electronic Physics of Ukrainian National Academy of Science, Uzhorod, Ukraine c Kyivstar GSM, JSV, Lviv, Ukraine
Received 9 March 2002; accepted 21 March 2002 Communicated by T. Hibiya
Abstract Results of a preliminary investigation of cadmium telluride (CdTe) single-crystal gamma-spectra for crystals grown by vapour-phase deposition method of chemical transport reaction with NH4X (X=Cl, Br, I) as transferors by means of g- and neutron-activation analysis methods are presented. Conditions and techniques of activation experiments as well as criteria of analytical g-line choice for identification of chemical elements in the compounds are discussed in detail. Preliminary data from g-activation analysis (Eg ¼ 12 MeV) of CdTe single crystal stoichiometry are obtained. r 2002 Elsevier Science B.V. All rights reserved. PACS: 82.80.E; 81.10.A Keywords: A1. Nucleation; A2. Growth from vapor; B1. Cadmium compounds; B1. Zinc compounds; B2. Semiconducting cadmium compounds; B2. Semiconducting II–VI materials
1. Introduction Cadmium telluride (CdTe) and zinc telluride (ZnTe) were proved to be perspective materials for optoelectronics applications. These materials are widely used for electron-optical modulation of laser radiation, in technology of fabrication of broad spectrum range photoreceivers based on CdxHg1xTe, ZnxHg1xTe solid solutions, for direct conversion of solar radiation energy into electrical energy, for registration of X-ray and gamma-radiations, etc. To grow a compound with *Corresponding author. E-mail address: [email protected] (G. Ilchuk).
controlled properties is a challenging problem of modern semiconductor material research and an integral part of system investigations with the aim of novel semiconductor-device development. For a long time, we have been developing the method of chemical transport reaction (CTR) with NH4Cl, NH4Br and NH4I substances as transferors to grow single crystals of binary compounds of A2B6 group and their solid solutions as well. The method is based on the mathematical simulations of gas phase content and mass transfer [1]. Semiinsulating single crystals with dislocation density 102 cm2 were grown [2]. Optoelectronic studies show that the growth process is accompanied by Cl, Br and I halogen inclusions into a
0022-0248/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 2 ) 0 1 3 4 6 - 5
42
G. Ilchuk et al. / Journal of Crystal Growth 242 (2002) 41–44
CdTe single crystal [3,4]. The problem of control of deviation level from stoichiometry for a grown single crystal as well as inclusion of halogens into the crystals during the growth process becomes urgent. To solve the problem we used g- and neutron-activation techniques for the analysis of the crystals.
2. Experimental procedures Activation techniques were realized on the electron accelerator-microtron M30 in IEF of National Academy of Sciences of Ukraine. The accelerator allows to vary the energy of accelerated electrons in the range 1–25 MeV when current is in the order of 2–5 mkA. That is, at the pointed conditions, the flow of photoneutrons can achieve a level of 109–1010 n/s. To obtain more reliable results of neutron- and g-activation analysis of Cd–Zn–Te /Br, I, ClS, activation of Cd, Zn, Te, B and I samplesmonitors was carried out in identical conditions. Irradiation time (activation time) of the samples (Tir ) was arranged according to the half-life period (T1=2 ) of samples-monitors to satisfy the condition Tir > 5T1=2 : It allowed to obtain saturation of the resulting isotope. Measuring time (Tm ) was chosen so that the statistical error of analytical peak measurement did not exceed 5%. During the process of activation, samples were positioned in the irradiation unit, that contains braking (1.5–3 mm in size) Ta-target or a photoneutron Be–Pb converter, placed at accelerated electron beam axis (Ee ¼ 12 MeV; Ie ¼ 527 mA) in the 4p-polyethylene moderator. There were the standards for comparison also. The monitor of secondary emission was used to control the electron dose. To control g2n dose the activation monitor positioned in the irradiation unit was used.
3. Results and discussion Identification of chemical elements was carried out on gamma-spectrometry complex with ‘‘SBS40’’ spectrometer and coaxial semiconducting
Ge(Li)-detector of high definition with volume 100 cm3. Library of the spectrometer had more than 390 radioisotopes. Detector was placed into a combine protection house with the layers of copper (8 mm), aluminum (3 mm), cadmium (1 mm), and lead (95 mm), and was cooled by liquid nitrogen. Resolution of gamma-spectrometer was 2.01 keV for the line 122.06 keV of Co-57 and 3.63 for the line 1332.50 keV of Co-60. The efficiency of measurements was controlled due to the special standard volume source Eu-152. The main nuclear-physical constants, namely, analytical lines of original and (n,g) reaction product nuclides necessary for their identification in ZnxCd1xTe /Br, I, ClS are shown in Table 1. The presented data allow to conclude the following: to identify radionuclides for semiconductor compounds of ZnxCd1xTe /Br, I, ClS type direct and indirect techniques can be used. So for Te, considering the reaction Te-131 ) I-131 as possible, one can carry out the identification by line 364.5 keV. There are different approaches to stoichiometry control of compounds such as CdTe, based on the presence of isotopes with short and long half-life periods: (a) analysis of intensive lines of nuclides with short half-life period, such as Cd-111m, Te131, when relaxation time of their activities is taken into account; and (b) analysis of long halflife period nuclides. In the latter case, when time of measurement is less than min{T1=2 (Cd-114)}, there is no sense to consider the nucleon decay characteristics, and we can perform an analysis of CdTe stoichiometry content by the ratio between intensities of analytical g-lines 336.3 (Cd-115) and 364.5 keV (I-131) that correspond to long half-life period radionuclides. For g-activation technique, when nuclear reactions (g; n) take place, it is necessary to carry out the time analysis of activities and g-spectral dependences for the investigated substance and corresponding monitors as well. One should assume that selected samples-monitors have natural mixture of radionuclides for the selected chemical elements. As for the previous case, there are different strategies of analysis by long and short half-life period isotopes of Cd and Te.
G. Ilchuk et al. / Journal of Crystal Growth 242 (2002) 41–44
43
Table 1 Main nuclear-physical constants of nuclides of chemical elements necessary for their identification in ZnxCd1xTe /Br, I, ClS samples Elements
Abundance
Product
Half-life period, T1=2
Analytical line, Eg (keV) (quantum yield, %)
Cd-114
28.93
Cd-115
53.46 h
336.24 (45.9) 527.90 (27.4)
Cd-110 Zn-68 Br-81
12.32 18.80 49.31
Cd-111m Zn-69m Br-82
48.70 min 13.76 h 35.30 h
245.4 (94.0) 438.63 (94.8) 554.35 (70.8) 619.11 (43.4) 776.52 (83.5)
I-127 Cl-37
100 24.23
I-128 Cl-38
24.99 min 37.24 min
442.94 (16.9) 1642.7 (31.9) 2167.41 (42.4)
Te-130
33.87
Te-131 Te-131 ) I-131
25.0 min
149.72 (68.9) 452.32 (12.2) 364.50 (78.0)
CdTe-1andTe-2,Cd-3 monitors 10000
Count
To determine the stoichiometry control possibility in CdTe single crystals, preliminary studies were carried out by means of braking (bremsstrahlung) beams of g-radiation with a maximum energy of 12 MeV as well as beams of turmoil neutrons as activators. Comparison of g-spectrum of the activated CdTe single crystal, grown by CTR method, and a spectrum of samples-monitors, measured 4 h after activation by braking radiation with maximum energy Eg ¼ 12 MeV is shown in Fig. 1. Measuring time Tm was equal to 10 min. It is obvious that some lines of CdTe g-spectrum contain contributions from relevant regions of gspectra for Cd and Te and, therefore, are not suitable for identification. Authors assume that other impurities can be included into the CdTe matrix and affect its resulting g-spectrum. The above-mentioned factors, as well as a varying sensitivity of Ge(Li)-detector for different regions of g-spectrum cause its characteristic fine structure, expressed in Fig. 1. As the analytical lines of CdTe g-spectrum for Cd identification, the Cd-115 isotope line with energy 492 keV (1192 channel) as well as In-115m isotope line with energy 336 keV (780 channel) which is the product of b-decay reaction for Cd-115 can be chosen. The ratio of the
8.07 days
1000
-500 418 keV, Te-127
100
1.
459,6 keV, Te-129 487 keV, Te-129 -50
492 keV Cd-115
2.
336 keV, In-115m
10
3.
1 500
1000 1500 Channels
2000
2500
Fig. 1. Gamma-spectra of activated CdTe single crystals grown by CTR method and samples-monitors of Cd and Te (purity level 99.9999%) measured 4 h after activation by braking irradiation (Eg ¼ 12 MeV; Tm ¼ 10 min).
line intensities allows to determine the stoichiometry level of the grown CdTe single crystals. Let us note that the analysis corresponds to the strategy of chemical content analysis owing to long-living radioisotopes with half-life period T1=2 from 10 h to 2 days. We determined that for a number of CdTe samples, synthesized with NH4Cl and NH4I as transferors, the ratio of intensities of
44
G. Ilchuk et al. / Journal of Crystal Growth 242 (2002) 41–44
the lines varies within the limit of 10% for a measurement series. It shows the satisfactory stoichiometry of samples and confirms the perspectives of nuclear-physical methods for stoichiometry control.
4. Conclusions The preliminary investigations of gamma-spectra of activated CdTe single crystals grown by CTR method as well as Cd and Te monitors show good perspectives of nuclear-physical method for identification of Cd and Te chemical elements in CdTe crystals and for control of stoichiometry. Analytical lines for identification of the elements were determined.
References [1] G.A. Ilchuk, Zinc telluride transport in the ZnTe–NH4I system, Inorg. Mater. 36 (2000) 668. [2] V.O. Ukrainets, G.A. Ilchuk, N.A. Ukrainets, Yu.V. Rud’, V.I. Ivanov-Omskii, Electrical properties of Schottky diodes using high-resistivity CdTe crystals, Tech. Phys. Lett. 25 (1999) 642. [3] G.A. Ilchuk, N.A. Ukrainets, V.I. Ivanov-Omskii, Yu.V. Rud’, V.Yu. Rud’, Optoelectronic effects in semi-insulating CdTe single crystals and structures based on them, Semiconductors 33 (1999) 518. [4] G.A. Ilchuk, N.A. Ukrainets, V.O. Ukrainets, B.I. Datsko, R.O. Zabrodsky, Yu.V. Rud’, Alternative method of doping of cadmium telluride by halogens Cl, Br, I, International Workshop on Advances in Growth and Characterization of II–VI Semiconductors, Wurzburg, . Germany, 1999, p. 38.