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Preparation and photoluminescence characterization of high-purity CdTe single crystals: purification effect of normal freezing on tellurium and cadmium telluride
Journal of Crystal Growth 236 (2002) 165–170
Preparation and photoluminescence characterization of high-purity CdTe single crystals: purification effect of normal freezing on tellurium and cadmium telluride S.H. Song, J. Wang*, M. Isshiki Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 1-1, Katahira 2-chome, Aobaku, Sendai 980-8577, Japan Received 14 November 2001; accepted 13 December 2001 Communicated by T. Nishinaga
Abstract Extremely high-purity CdTe single crystals have been obtained by the traditional vertical Bridgman technique, beginning with the refining of tellurium material by the normal freezing method. The purification effect of normal freezing on tellurium has been confirmed to be very effective. This effect was also used to prepare extremely high-purity CdTe single crystals. The crystals were characterized by low temperature high-resolution photoluminescence (PL) spectroscopy. Only a sharp peak at 1.5896 eV was detected in the PL spectrum. The full-width at half-maximum is o0.31 meV, which indicates that the crystal is of extremely high purity. To our knowledge, this is the purest CdTe single crystal prepared by the Bridgman method. r 2002 Elsevier Science B.V. All rights reserved. PACS: 68.55.Ln; 81.10.Bk Keywords: A1. Impurities; A2. Growth from melt; B1. Cadmium compounds; B2. Semiconducting II–VI materials
1. Introduction CdTe is a direct-band-gap II–VI compound semiconductor. Due to its distinctive physical properties, it is a promising material for g-ray and X-ray detectors and as substrate for Hg1xCdxTe, solar cell and other optical devices. For this reason, CdTe has received much attention in the past [1]. The majority of CdTe applications
*Corresponding author. Fax: +81-22-217-5139. E-mail address: [email protected] (J. Wang).
have several severe requirements for the crystal. In general, a high quality crystal with controllable electrical properties is needed. Particularly, for adjusting the electrical properties by doping, a high-purity crystal is required. However, control of the electrical properties of CdTe has not yet been fully achieved. This is because the CdTe crystals used suffer from native defects and residual impurities. Although there is a general agreement on the importance of residual impurities, native defects and their interaction [2–6], a systematic study on controlling them is still lacking. For this purpose, in the present paper
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 1 ) 0 2 3 9 5 - 8
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we report our results of preparing high-purity crystals for the first time. Since photoluminescence (PL) is very sensitive for obtaining information about impurities and defects in semiconductors, it is often used for characterizing high-purity material. In the past years, numerous investigations of PL properties have been performed on CdTe crystals [7,8]. These results have contributed much to identify impurities and native defects in this material, although there are some disagreements still unresolved. For example, the origin of the dominant emission line located around 1.5896 eV in the 4.2 K PL spectrum of high-purity as-grown p-type CdTe single crystals, which is generally recognized as a recombination emission of excitons bound to a neutral acceptor (A0 ; X ), has not been clarified. Some studies have shown that the line can be attributed to a substitutional Cu (CuCd) [9,10], while others have considered it to be due to Cdvacancy (VCd) [11,12], or due to a complex defect composed of a Cd-vacancy and a donor-impurity (VCd–D) [13]. There is also a report that it is associated with a complex defect involving two donor-type impurities (VCd–2D) [14]. The broadening of emission lines caused by residual impurities makes analysis difficult in crystal of usual purity. Therefore, high-purity crystals are more essential for clarifying these problems and others related to the residual impurities by PL measurement. In this paper, taking advantage of the suitable segregation coefficients of a majority of impurities in tellurium (Te) [1,15], the tellurium material used for CdTe crystal growth was firstly purified with the normal freezing method. After that, high-purity CdTe single crystals were obtained by the Bridgman method using the highly purified Te as one of the source materials, in addition to extremely high-purity Cd. In this step, the purification effect of normal freezing on CdTe was also clarified and attributed to be the main factor in obtaining extremely high-purity CdTe crystals. 4.2 K PL was used for clarifying the purification effect. High-resolution PL was used to characterize the highly purified CdTe single crystals.
2. Experiments 2.1. Refining of tellurium For purifying the tellurium material, a quartz tube with a diameter of 9 mm was used. After careful cleaning, the quartz ampoule was vacuumbaked at 1373 K for 20 h. About 50 g Te was charged into the ampoule, which was then pumped and sealed at 10–4 Pa. The starting material was originally 6 N Te. The apparatus used is similar to that for CdTe crystal growth with the vertical Bridgman method. The ampoule was directly raised to 800 K, and kept for more than 20 h, then descended with a velocity of 3.3 mm/h. The temperature gradient near the interface where Te started solidifying was 3 K/cm. The resulting Te ingot was divided into three approximately equal parts as shown in Fig. 1. The portions of Te1, Te2 and Te3 were used for CdTe crystal growth, respectively. 2.2. Crystal growth of CdTe by the vertical Bridgman method All CdTe single crystals were grown in quartz ampoules by the unseeded vertical Bridgman method. The ampoule treatment was the same as described in Section 2.1, except that the ampoules were carbon-coated and vacuum-baked once more using the same procedure. The amount of Te excess, o102 at% [16] in the starting material, was adopted to prevent the ampoule from break-
Fig. 1. Schematic diagram of purified Te ingot and corresponding CdTe crystal ingots.
S.H. Song et al. / Journal of Crystal Growth 236 (2002) 165–170
ing, because of the high pressure of Cd at the growth temperature. High-purity Cd, which is of extremely high purity with residual resistivity ratio of above 20,000 prepared by the overlap-zonemelting method [17], was used as another starting material for all crystal growth. All CdTe single crystals were grown with the same apparatus. The growth rate was 2 mm/h. The temperature gradient near the interface where CdTe started solidifying was 5 K/cm [18]. The CdTe ingots were cleaved into wafers for PL measurement. 2.3. PL measurement The PL measurement was performed at 4.2 K with the sample mounted on a holder in a strainfree manner. Excitation was accomplished with 18 mW/cm2 680 nm light from a semiconductor laser. The PL spectra were dispersed with a 1.0 m spectrometer and detected by a thermoelectrically cooled GaAs photo-multiplier tube. The whole system was controlled by a computer, which also served for data displaying, analysis and storage. All samples were measured on the cleaved surface (1 1 0) without any surface treatment.
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It is known that trace analysis of the impurities in Te is difficult; therefore, in this study we use an indirect method to examine the Te purity. For clarifying the purification effect of normal freezing on Te, three CdTe ingots were grown as shown in Fig. 1, using Te1, Te2 and Te3 as source material. Three specimens of CdTe were picked from each CdTe ingot at the closed g value (g ¼ 0:4870:03), and denoted as CdTe/Te1, CdTe/Te2 and CdTe/Te3. Here the g represents the solidification fraction of the CdTe ingot. PL spectra of these samples are shown in Fig. 2. For comparison, the PL intensity in each spectrum is normalized to the intensity of (A0 ; X ) peak at 1.5896 eV [20]. Fig. 2 shows the PL spectra of CdTe single crystal grown using different portions of the refined Te ingot. In the overall view, the three PL spectra are similar. There is a very strong and sharp emission line (A0 ; X ), originating from an exciton bound to a neutral acceptor, accompanied by its first- and second-order longitudinal optical (LO) phonon replicas in each of the PL spectra. The 1.45 eV donor–acceptor pair (DAP) emission
3. Results and discussion 3.1. Purification effect of normal freezing on tellurium (Te) In CdTe, the electrically active impurities are IA (Li, Na and K), IIIB (Al, Ga and In), VB (N, P, etc.), VIIB (Cl, Br and I) and IB (Cu, Ag and Au) elements in the periodic table. IIIB and VIIB elements serve as substitutional donors, which also through forming complexes with cadmium vacancies conversely act as acceptors. Others serve as substitutional acceptors. The IA and IB elements are more complicated since these elements have relatively smaller atomic or ionic radii, and faster diffusibility, which might go off the lattice position to serve as interstitial donors. Some of these elements (for example, Cu and Ag) form complex defects with native defects and cause the ageing effect on the CdTe crystal [19].
Fig. 2. PL spectra of CdTe single crystals grown with different portion of the refined Te ingot.
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bands with the LO phonon replicas are also detected. For CdTe, the LO energy is 21.3 meV [21]. The exciton emission spectrum is often used as an indicator of crystal quality and purity [22]. The present PL spectra show that all our CdTe samples are of relatively high quality and purity. In all the three PL spectra, the edge emission that is associated with impurities cannot be detected. This also suggests that the crystals are of high purity. However, for these CdTe crystals, there is still significant difference in purity as will be discussed in the following. By comparing the DAP emission bands it can be seen that the crystal (CdTe/Te3) gives the strongest intensity. The other two crystals give almost similar intensity of DAP emission. These results indicate that normal freezing method is a very effective for purifying Te. It is clarified that almost all the impurities associated with the DAP emission band have been swept over into the end of the Te ingot. On the other hand, the donor bound exciton emission (D0 ; X ) at 1.5931 eV is observed in the PL spectrum (CdTe/Te3). This result indicates that the normal freezing method is more effective for removing the donor-type impurities. Francou et al. [8] have given a detailed study of the donor-type impurities, which all have the ionization energy of about 14 meV and result in (D0 ; X ) emission at 1.593 eV in the PL spectrum. The emission line at 1.5815 eV denoted as Z with its LO phonon replicas can also be observed in the PL spectra of CdTe/Te3 and CdTe/Te2. The emission intensity shows a clear dependence on the portion of the Te used. It is reported [20] that the emission line Z can be associated with an Agrelated complex defect (X2 Ag). It could be considered, furthermore, that the hump Z 1 at the lower PL energy side of the Z emission might also be related to complex defects associating with impurities. As a matter of fact, in Ag-doped CdTe single crystals, a series of complex defects related to Ag have been observed in the PL energy range of 1.5809–1.5837 eV [23]. It has also been reported for a copper-doped CdTe crystal that Cu forms similar complex defects easily [10]. By comparing intensities of the Z emission band it can be concluded that the normal freezing method is
effective for removing acceptor-type impurities in tellurium, as well as the donor-type impurities. 3.2. Purification effect of normal freezing on CdTe The normal freezing method must lead to a purification effect on CdTe crystals, because most of the impurities also have suitable segregation coefficients in CdTe [1]. Fig. 3 shows the PL spectra for four pieces of CdTe samples. These samples are picked from different g values in the same CdTe ingot grown with the purest part of the Te ingot (Te1) as source material. The PL intensities of each spectrum are normalized to (A0 ; X ) at 1.5896 eV. It can be seen that, for all the samples the (D0 ; X ) and Z emission lines (see Fig. 2) are not detected. This again gives evidence of how effective is the purification effect on Te. However, the DAP emission bands show that there still are impurities remaining in these samples. The DAP emission intensities of the samples (g ¼ 0:46; 0.75 and 0.98) become stronger with increase of the solidification fraction (g). This
Fig. 3. PL spectra dependence on solidification fraction (g) of the CdTe single crystal grown with Te1 as source material.
S.H. Song et al. / Journal of Crystal Growth 236 (2002) 165–170
indicates that the residual impurities were gradually concentrated into the fraction of the CdTe ingot left behind. It is found that the purest CdTe can be obtained at a g value of around 0.5. The PL spectrum of the specimen with g ¼ 0:24 shows the highest DAP emission intensity. This sample is picked from the fraction close to the head part of the crystal ingot, where a greater number of grain boundaries and twins were generated due to spontaneous nucleation, superheating and supercooling during crystal growth [18]. These phenomena exist for almost all CdTe crystals growth by Bridgman method. It is well known that impurities are easily concentrated in the dislocation region: this might be the reason why the sample (g ¼ 0:24) shows the higher concentration of impurities. Although there is also report that aluminium (Al) has a segregation coefficient greater than unity in CdTe [1], it seems not the main reason for the result here.
3.3. High-resolution PL spectrum of highly purified CdTe single crystal High-resolution PL spectroscopy is often used to characterize semiconductor material. Especially, it is usually used to find emissions near the bandedge energy region since there are a great number of emission lines. These emission lines might get together to overlap and make only the dominating lines observable in a ‘normal’ PL spectrum. This situation holds more significantly for CdTe single crystals because of its relatively narrower band gap energy. Fig. 4 shows the typical high-resolution PL spectrum of a high-purity CdTe sample obtained in the present study. Only a sharp and strong emission line of (A0 ; X ) at 1.5896 eV is observed. No other emissions such as PTe, Li/NaCd, and (D0 ; X )/(Dþ ; X ) (related to substitutional donors) [24,25] can be detected. The full-width at halfmaximum of the (A0 ; X ) line is 0.31 meV. This is the narrowest value ever reported for CdTe single crystals grown by the Bridgman method. All of the above indicate that the sample is of high purity and quality.
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Fig. 4. High-resolution PL spectrum of a high-purity CdTe single crystal in the near band-edge region.
4. Conclusions The segregation effect of residual impurities in Te and CdTe was observed. It is confirmed that the normal freezing method is very effective for purifying tellurium and cadmium telluride. In this study, highly purified CdTe single crystals were obtained by unseeded vertical Bridgman method using highly purified elemental source material.
Acknowledgements This work was performed under the interuniversity cooperative research program of the Institute for Materials Research, Tohoku University.
References [1] K. Zanio, Semiconductors and semimetals 13 (1978) 164. . [2] F.A. Kroger, Rev. Phys. Appl. 12 (1977) 205. [3] P. Siffert, J. Berger, C. Scharager, A. Cornet, R. Stuck, R.O. Bell, H.B. Serreze, F.V. Wald, IEEE Trans. Nucl. Sci. 23 (1976) 159. [4] R.N. Bhargava, J. Crystal Growth 59 (1982) 15. [5] M. Samimi, B. Bigliari, M. Hage-Ali, J.M. Koebel, P. Siffert, Phys. Status Solidi A 100 (1987) 251. [6] R.E. Kremer, W.B. Leigh, J. Crystal Growth 86 (1988) 490. [7] E. Molva, J.L. Pautrat, K. Saminadayar, G. Milchberg, N. Magnea, Phys. Rev. B 30 (1984) 3344.
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[8] J.M. Francou, K. Saminadayar, J.L. Pautrat, Phys. Rev. B 41 (17) (1990) 12035. [9] J. Krustok, J. Madasson, K. Hjelet, H. Collan, J. Mater. Sci. Lett. 14 (1995) 1490. [10] J.P. Chamonal, E. Molva, J.L. Pautrat, Solid State Commun. 43 (1982) 801. [11] C.E. Barnes, C. Kikuchi, Radiat. Effects 26 (1975) 105. [12] F.J. Bryant, H.J. Totterdell, Radiat. Effects 9 (1971) 115. [13] S. Seto, A. Tanaka, Y. Masa, S. Dairaku, M. Kawashima, Appl. Phys. Lett. 53 (1988) 1532. [14] H.Y. Shin, C.Y. Sun, Mater. Sci. Eng. B 52 (1998) 78. [15] L. Kuchar, J. Drapala, J. Lunacek, J. Crystal Growth 161 (1996) 94. [16] U. Becker, P. Rudolph, R. Boyn, M. Wienecke, I. Utke, Phys. Status Solidi A 120 (1990) 653. [17] Y. Ishikawa, Y. Bailing, K. Mimura, T. Tomizono, M. Isshiki, J. Mining Mater. Process Inst. Japan 110 (1994) 1175.
[18] P. Rudolph, Prog. Cryst. Growth Charact. 29 (1994) 275. [19] B. Monemar, E. Molva, Le Si Dang, Phys. Rev. B 33 (2) 1986) 1134. [20] C.H. Wang, K.Y. Cheng, S.J. Yang, J. Appl. Phys. Lett. 46 (10) (1985) 962. [21] E. Molva, J.P. Chamonal, J.L. Pautrat, Phys. Status Solidi (b) 109 (1982) 635. [22] D.E. Cooper, J. Bajaj, P.R. Newman, J. Crystal Growth 86 (1988) 544. [23] H. Zimmermann, R. Boyn, P. Rudolph, J. Bollmann, A. Klimakow, R. Krause, Mater. Sci. Eng. B 16 (1993) 139. [24] Z.C. Feng, A. Mascarenhas, W.J. Choyke, J. Luminescence 35 (1986) 329. [25] J.L. Pautrat, J.M. Francou, N. Magnea, E. Molva, K. Saminadayar, J. Crystal Growth 72 (1985) 194.
Preparation and photoluminescence characterization of high-purity CdTe single crystals: purification effect of normal freezing on tellurium and cadmium telluride S.H. Song, J. Wang*, M. Isshiki Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 1-1, Katahira 2-chome, Aobaku, Sendai 980-8577, Japan Received 14 November 2001; accepted 13 December 2001 Communicated by T. Nishinaga
Abstract Extremely high-purity CdTe single crystals have been obtained by the traditional vertical Bridgman technique, beginning with the refining of tellurium material by the normal freezing method. The purification effect of normal freezing on tellurium has been confirmed to be very effective. This effect was also used to prepare extremely high-purity CdTe single crystals. The crystals were characterized by low temperature high-resolution photoluminescence (PL) spectroscopy. Only a sharp peak at 1.5896 eV was detected in the PL spectrum. The full-width at half-maximum is o0.31 meV, which indicates that the crystal is of extremely high purity. To our knowledge, this is the purest CdTe single crystal prepared by the Bridgman method. r 2002 Elsevier Science B.V. All rights reserved. PACS: 68.55.Ln; 81.10.Bk Keywords: A1. Impurities; A2. Growth from melt; B1. Cadmium compounds; B2. Semiconducting II–VI materials
1. Introduction CdTe is a direct-band-gap II–VI compound semiconductor. Due to its distinctive physical properties, it is a promising material for g-ray and X-ray detectors and as substrate for Hg1xCdxTe, solar cell and other optical devices. For this reason, CdTe has received much attention in the past [1]. The majority of CdTe applications
*Corresponding author. Fax: +81-22-217-5139. E-mail address: [email protected] (J. Wang).
have several severe requirements for the crystal. In general, a high quality crystal with controllable electrical properties is needed. Particularly, for adjusting the electrical properties by doping, a high-purity crystal is required. However, control of the electrical properties of CdTe has not yet been fully achieved. This is because the CdTe crystals used suffer from native defects and residual impurities. Although there is a general agreement on the importance of residual impurities, native defects and their interaction [2–6], a systematic study on controlling them is still lacking. For this purpose, in the present paper
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 1 ) 0 2 3 9 5 - 8
166
S.H. Song et al. / Journal of Crystal Growth 236 (2002) 165–170
we report our results of preparing high-purity crystals for the first time. Since photoluminescence (PL) is very sensitive for obtaining information about impurities and defects in semiconductors, it is often used for characterizing high-purity material. In the past years, numerous investigations of PL properties have been performed on CdTe crystals [7,8]. These results have contributed much to identify impurities and native defects in this material, although there are some disagreements still unresolved. For example, the origin of the dominant emission line located around 1.5896 eV in the 4.2 K PL spectrum of high-purity as-grown p-type CdTe single crystals, which is generally recognized as a recombination emission of excitons bound to a neutral acceptor (A0 ; X ), has not been clarified. Some studies have shown that the line can be attributed to a substitutional Cu (CuCd) [9,10], while others have considered it to be due to Cdvacancy (VCd) [11,12], or due to a complex defect composed of a Cd-vacancy and a donor-impurity (VCd–D) [13]. There is also a report that it is associated with a complex defect involving two donor-type impurities (VCd–2D) [14]. The broadening of emission lines caused by residual impurities makes analysis difficult in crystal of usual purity. Therefore, high-purity crystals are more essential for clarifying these problems and others related to the residual impurities by PL measurement. In this paper, taking advantage of the suitable segregation coefficients of a majority of impurities in tellurium (Te) [1,15], the tellurium material used for CdTe crystal growth was firstly purified with the normal freezing method. After that, high-purity CdTe single crystals were obtained by the Bridgman method using the highly purified Te as one of the source materials, in addition to extremely high-purity Cd. In this step, the purification effect of normal freezing on CdTe was also clarified and attributed to be the main factor in obtaining extremely high-purity CdTe crystals. 4.2 K PL was used for clarifying the purification effect. High-resolution PL was used to characterize the highly purified CdTe single crystals.
2. Experiments 2.1. Refining of tellurium For purifying the tellurium material, a quartz tube with a diameter of 9 mm was used. After careful cleaning, the quartz ampoule was vacuumbaked at 1373 K for 20 h. About 50 g Te was charged into the ampoule, which was then pumped and sealed at 10–4 Pa. The starting material was originally 6 N Te. The apparatus used is similar to that for CdTe crystal growth with the vertical Bridgman method. The ampoule was directly raised to 800 K, and kept for more than 20 h, then descended with a velocity of 3.3 mm/h. The temperature gradient near the interface where Te started solidifying was 3 K/cm. The resulting Te ingot was divided into three approximately equal parts as shown in Fig. 1. The portions of Te1, Te2 and Te3 were used for CdTe crystal growth, respectively. 2.2. Crystal growth of CdTe by the vertical Bridgman method All CdTe single crystals were grown in quartz ampoules by the unseeded vertical Bridgman method. The ampoule treatment was the same as described in Section 2.1, except that the ampoules were carbon-coated and vacuum-baked once more using the same procedure. The amount of Te excess, o102 at% [16] in the starting material, was adopted to prevent the ampoule from break-
Fig. 1. Schematic diagram of purified Te ingot and corresponding CdTe crystal ingots.
S.H. Song et al. / Journal of Crystal Growth 236 (2002) 165–170
ing, because of the high pressure of Cd at the growth temperature. High-purity Cd, which is of extremely high purity with residual resistivity ratio of above 20,000 prepared by the overlap-zonemelting method [17], was used as another starting material for all crystal growth. All CdTe single crystals were grown with the same apparatus. The growth rate was 2 mm/h. The temperature gradient near the interface where CdTe started solidifying was 5 K/cm [18]. The CdTe ingots were cleaved into wafers for PL measurement. 2.3. PL measurement The PL measurement was performed at 4.2 K with the sample mounted on a holder in a strainfree manner. Excitation was accomplished with 18 mW/cm2 680 nm light from a semiconductor laser. The PL spectra were dispersed with a 1.0 m spectrometer and detected by a thermoelectrically cooled GaAs photo-multiplier tube. The whole system was controlled by a computer, which also served for data displaying, analysis and storage. All samples were measured on the cleaved surface (1 1 0) without any surface treatment.
167
It is known that trace analysis of the impurities in Te is difficult; therefore, in this study we use an indirect method to examine the Te purity. For clarifying the purification effect of normal freezing on Te, three CdTe ingots were grown as shown in Fig. 1, using Te1, Te2 and Te3 as source material. Three specimens of CdTe were picked from each CdTe ingot at the closed g value (g ¼ 0:4870:03), and denoted as CdTe/Te1, CdTe/Te2 and CdTe/Te3. Here the g represents the solidification fraction of the CdTe ingot. PL spectra of these samples are shown in Fig. 2. For comparison, the PL intensity in each spectrum is normalized to the intensity of (A0 ; X ) peak at 1.5896 eV [20]. Fig. 2 shows the PL spectra of CdTe single crystal grown using different portions of the refined Te ingot. In the overall view, the three PL spectra are similar. There is a very strong and sharp emission line (A0 ; X ), originating from an exciton bound to a neutral acceptor, accompanied by its first- and second-order longitudinal optical (LO) phonon replicas in each of the PL spectra. The 1.45 eV donor–acceptor pair (DAP) emission
3. Results and discussion 3.1. Purification effect of normal freezing on tellurium (Te) In CdTe, the electrically active impurities are IA (Li, Na and K), IIIB (Al, Ga and In), VB (N, P, etc.), VIIB (Cl, Br and I) and IB (Cu, Ag and Au) elements in the periodic table. IIIB and VIIB elements serve as substitutional donors, which also through forming complexes with cadmium vacancies conversely act as acceptors. Others serve as substitutional acceptors. The IA and IB elements are more complicated since these elements have relatively smaller atomic or ionic radii, and faster diffusibility, which might go off the lattice position to serve as interstitial donors. Some of these elements (for example, Cu and Ag) form complex defects with native defects and cause the ageing effect on the CdTe crystal [19].
Fig. 2. PL spectra of CdTe single crystals grown with different portion of the refined Te ingot.
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bands with the LO phonon replicas are also detected. For CdTe, the LO energy is 21.3 meV [21]. The exciton emission spectrum is often used as an indicator of crystal quality and purity [22]. The present PL spectra show that all our CdTe samples are of relatively high quality and purity. In all the three PL spectra, the edge emission that is associated with impurities cannot be detected. This also suggests that the crystals are of high purity. However, for these CdTe crystals, there is still significant difference in purity as will be discussed in the following. By comparing the DAP emission bands it can be seen that the crystal (CdTe/Te3) gives the strongest intensity. The other two crystals give almost similar intensity of DAP emission. These results indicate that normal freezing method is a very effective for purifying Te. It is clarified that almost all the impurities associated with the DAP emission band have been swept over into the end of the Te ingot. On the other hand, the donor bound exciton emission (D0 ; X ) at 1.5931 eV is observed in the PL spectrum (CdTe/Te3). This result indicates that the normal freezing method is more effective for removing the donor-type impurities. Francou et al. [8] have given a detailed study of the donor-type impurities, which all have the ionization energy of about 14 meV and result in (D0 ; X ) emission at 1.593 eV in the PL spectrum. The emission line at 1.5815 eV denoted as Z with its LO phonon replicas can also be observed in the PL spectra of CdTe/Te3 and CdTe/Te2. The emission intensity shows a clear dependence on the portion of the Te used. It is reported [20] that the emission line Z can be associated with an Agrelated complex defect (X2 Ag). It could be considered, furthermore, that the hump Z 1 at the lower PL energy side of the Z emission might also be related to complex defects associating with impurities. As a matter of fact, in Ag-doped CdTe single crystals, a series of complex defects related to Ag have been observed in the PL energy range of 1.5809–1.5837 eV [23]. It has also been reported for a copper-doped CdTe crystal that Cu forms similar complex defects easily [10]. By comparing intensities of the Z emission band it can be concluded that the normal freezing method is
effective for removing acceptor-type impurities in tellurium, as well as the donor-type impurities. 3.2. Purification effect of normal freezing on CdTe The normal freezing method must lead to a purification effect on CdTe crystals, because most of the impurities also have suitable segregation coefficients in CdTe [1]. Fig. 3 shows the PL spectra for four pieces of CdTe samples. These samples are picked from different g values in the same CdTe ingot grown with the purest part of the Te ingot (Te1) as source material. The PL intensities of each spectrum are normalized to (A0 ; X ) at 1.5896 eV. It can be seen that, for all the samples the (D0 ; X ) and Z emission lines (see Fig. 2) are not detected. This again gives evidence of how effective is the purification effect on Te. However, the DAP emission bands show that there still are impurities remaining in these samples. The DAP emission intensities of the samples (g ¼ 0:46; 0.75 and 0.98) become stronger with increase of the solidification fraction (g). This
Fig. 3. PL spectra dependence on solidification fraction (g) of the CdTe single crystal grown with Te1 as source material.
S.H. Song et al. / Journal of Crystal Growth 236 (2002) 165–170
indicates that the residual impurities were gradually concentrated into the fraction of the CdTe ingot left behind. It is found that the purest CdTe can be obtained at a g value of around 0.5. The PL spectrum of the specimen with g ¼ 0:24 shows the highest DAP emission intensity. This sample is picked from the fraction close to the head part of the crystal ingot, where a greater number of grain boundaries and twins were generated due to spontaneous nucleation, superheating and supercooling during crystal growth [18]. These phenomena exist for almost all CdTe crystals growth by Bridgman method. It is well known that impurities are easily concentrated in the dislocation region: this might be the reason why the sample (g ¼ 0:24) shows the higher concentration of impurities. Although there is also report that aluminium (Al) has a segregation coefficient greater than unity in CdTe [1], it seems not the main reason for the result here.
3.3. High-resolution PL spectrum of highly purified CdTe single crystal High-resolution PL spectroscopy is often used to characterize semiconductor material. Especially, it is usually used to find emissions near the bandedge energy region since there are a great number of emission lines. These emission lines might get together to overlap and make only the dominating lines observable in a ‘normal’ PL spectrum. This situation holds more significantly for CdTe single crystals because of its relatively narrower band gap energy. Fig. 4 shows the typical high-resolution PL spectrum of a high-purity CdTe sample obtained in the present study. Only a sharp and strong emission line of (A0 ; X ) at 1.5896 eV is observed. No other emissions such as PTe, Li/NaCd, and (D0 ; X )/(Dþ ; X ) (related to substitutional donors) [24,25] can be detected. The full-width at halfmaximum of the (A0 ; X ) line is 0.31 meV. This is the narrowest value ever reported for CdTe single crystals grown by the Bridgman method. All of the above indicate that the sample is of high purity and quality.
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Fig. 4. High-resolution PL spectrum of a high-purity CdTe single crystal in the near band-edge region.
4. Conclusions The segregation effect of residual impurities in Te and CdTe was observed. It is confirmed that the normal freezing method is very effective for purifying tellurium and cadmium telluride. In this study, highly purified CdTe single crystals were obtained by unseeded vertical Bridgman method using highly purified elemental source material.
Acknowledgements This work was performed under the interuniversity cooperative research program of the Institute for Materials Research, Tohoku University.
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