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Cd-vacancy-related excitonic emission in CdTe
ARTICLE IN PRESS
Journal of Crystal Growth 257 (2003) 231–236
Cd-vacancy-related excitonic emission in CdTe S.H. Song*, J.F. Wang, M. Isshiki Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 1-1, Katahira 2-chome, Aobaku, Sendai 980-8577, Japan Received 1 May 2003; accepted 5 June 2003 Communicated by K. Nakajima
Abstract The dominant luminescence emission of excitons bound to a neutral acceptor defect (A0, X) in as-grown high purity CdTe single crystals has been studied by Cd-dip treatment, doping with Cu and Ag, and irradiation with g-rays from 60 Co. It has been demonstrated that, by Cd-dip treatment, the dominant (A0, X) emission can be annihilated to be a double-structure emission with their energy at 1.5896 and 1.5885 eV, respectively. Our doping experiments have unambiguously shown that (A0, X) at 1.5896 eV is associated with residual impurities of Cu and (A0, X) at 1.5885 eV with Ag. Further experiment with g-ray irradiation has revealed a new (A0, X) emission at 1.5892 eV in the double structure. We suggest that this emission is associated with Cd-vacancies introduced by g-rays. To our knowledge, this is the first report on the isolated Cd-vacancies showing (A0, X) emission with the energy position different from 1.5896 eV. r 2003 Elsevier B.V. All rights reserved. PACS: 78.55. m; 78.55.Et; 81.10.Fq; 85.40.Ry Keywords: A1. Defects; A2. Single crystal growth; B1. Cadmium compounds
1. Introduction Discussion about exciton luminescence emission with cadmium vacancies is closely interrelated with a dominant photoluminescence emission in high purity p-type CdTe crystals. The origin of the dominant luminescence emission near 1.5896 eV from as-grown CdTe single crystal continues to be a topic of great interest [1–11]. This is because the related defects are supposed to be mainly responsible for the p-type conductivity. Much of this activity has coincided with the emergence of CdTe as a very promising material for fabricating X-, *Corresponding author. Fax: +81-22-217-5139. E-mail address: [email protected] (S.H. Song).
g-ray detector and used as substrate for HgCdTe epitaxial films. Past investigations showed that the emission could be associated with the defects of Cdvacancies (VCd ) [1,2], substitutional copper (CuCd ) [3–7] or complexes consisting of VCd and donor-type impurities [8]. Based on the Cdannealing results of high purity CdTe sample prepared by physical vapor transport, Yang et al. [12] found that the emission line was formed by two origins related to VCd and CuCd : However, they failed to separate the two sub-lines because their energy difference is too small. As a matter of fact, the emission near 1.5896 eV is a superposition of several acceptor-bound exciton recombination lines. The energies of these
0022-0248/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0022-0248(03)01457-X
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acceptor-bound excitons for a variety of acceptortype impurities have been identified by Molva et al. [5]. Nevertheless, as well known that the cadmium vacancies do exist in as-grown CdTe and play an important role in self-compensation effect that strongly influences the transport properties of this material, so it is of significance to find the (A0 ,X) emission related to isolated VCd and determine its energy position. Our previous study [13] has showed that (A0,X) emissions related to complex defects involving Cdvacancy and one or two donor-type impurities exhibit different energy positions from 1.5896 eV. In the present investigation, with our Cd-dipped CdTe specimen firstly we will unambiguously show that the (A0, X) recombination line at 1.5896 eV is associated with copper, then by introducing Cdvacancies into the sample (A0, X) emission related to VCd can be found and the energy position will be determined by reference to the line at 1.5896 eV. Incidentally, since the Ag-related (A0,X) line [5,14] at 1.5885 eV has been observed in our Cd-dipped CdTe specimen with comparable intensity of that related to copper, the confirmation result about it by doping experiment will be also presented in this paper.
Room temperature irradiation of g-rays from Co was used to introduce cadmium vacancies (VCd ) into the CdTe specimen. The dose of g-rays was 1.7 107 Gy. Measurement of the photoluminescence was performed at 4.2 K with the sample immersed in the liquid helium. Light with the wavelength of 680 nm from a semiconductor laser was used to stimulate the sample. For wavelength-energy con( version, the relation E(eV) l(A)=12395.13 [4] was used. 60
3. Results and discussion Fig. 1 shows the typical PL spectra for an identical CdTe single-crystal specimen before (asgrown) and after Cd-dip treatment. The sharp strong peak at 1.5896 eV denoted as (A0, X)1.5896 from as-grown crystal is the often discussed PL emission. As mentioned in the introduction,
2. Experimental procedures As-grown and Cd-dip treated CdTe single crystals, with extremely high purity, were used for this investigation. Preparation details for these crystals can be found in other papers [15,16]. Background doping of Cu and Ag were performed at room temperature by dipping the CdTe specimens into a 1% aqueous solution of Cu(NO3)2 or AgNO3, respectively, and subsequent rinsing with deionized (DI) water. The dipping and rinsing time were changed to control the concentration of Cu or Ag diffusing into the specimens. The Cu(NO3)2 and AgNO3 are of 5N and 6N purity. After drying, the samples were kept in a vacuum desiccator. In order to impel much Ag to occupy the Cd lattice position, Agdoped CdTe sample was additionally treated by isothermal annealing in Te atmosphere at 973 K for 24 h.
Fig. 1. PL spectra of as-grown and Cd-dip annealing treated CdTe single crystals.
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for as-grown CdTe single crystal the peak in the energy range from 1.588 to 1.591 eV is a superposition of a variety of acceptor-bound exciton (A0, X) recombination lines associated with different defects, so in order to clarify the origin of (A0, X)1.5896 emission single crystals of very high purity and quality are needed. In addition, for identifying other luminescence emissions covered by the peak, intensity of (A0, X)1.5896 emission should be suppressed. For this purpose, as-grown specimen with extremely high purity was put into Cd-dip annealing treatment. As shown in the upper PL spectrum of Fig. 1, Cd-dipped CdTe single crystal only gives two peaks in the (A0, X) emission range between the energy of 1.588 and 1.591 eV. The two peaks are at 1.5896 and 1.5885 eV, respectively. A great number of research works have shown that the (A0, X) at 1.5896 eV is related to substitutional defects of CuCd [3–6] and (A0, X) at 1.5885 eV to AgCd [5,14]. Details of discussion about other peaks can be referred to our previous paper [16]. In the following focus will be given on the (A0, X) emission range. In the literature much identification work about (A0, X) peak at 1.5896 eV was performed on the as-grown CdTe single crystals by background doping of copper followed by complicated thermal treatment including of Te-atmosphere annealing [3,4]. Although these results showed that intensity of the (A0, X)1.5896 peak was strongly enhanced after Cu-doping, the PL spectra had to be recorded in the same PL measurement condition since there was no other peak for reference. Moreover, Teatmosphere annealing treatment was used in those experiments in order to create high concentration of Cd vacancies and consequently suppose that more Cu atom could enter the crystals and occupy Cd lattice position. However, because there is no reliable evidence supporting that Cd-vacanciesrelated (A0 ,X) emission exhibits different energy from 1.5896 eV, assignment that (A0, X)1.5896 peak originates from Cu has been accepted tentatively. For solving the problem mentioned above, our assignment about (A0, X)1.5896 peak was performed on the Cd-dip treated CdTe single crystal. Apparently, this crystal includes very low concentration of Cd vacancies. Cu-doping was achieved
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by dipping the crystal into the 1% aqueous solution of Cu(NO3)2 for 1-min, then rinsed with DI water. After this treatment we observed that obvious Cu-color was put on the sample surface. For contrast we also did the same experiment using our as-grown CdTe single crystal, but resulting sample did not show the Cu-color. On the other hand, by comparing PL intensity measured under same condition it was noticed that this sample indeed exhibited much stronger luminescence of (A0, X)1.5896 than no-doping asgrown sample. These results indicate that it is easier for Cu atom to enter the as-grown crystal than Cd-dip treated one, since the former crystal includes much higher concentration of Cd vacancies than the latter one. Fig. 2 gives the experimental results performed on our Cd-dip treated CdTe single crystal. The PL spectra were measured on the same position of an identical sample. As shown in the figure, PL spectra of Cu-doped sample were recorded after treated by Cu-doping for 1 day and 35 days, respectively. PL spectrum for the sample before Cu-doping also was shown for reference. The intensity of each PL spectrum was normalized with
Fig. 2. PL spectra for Cd-dip treated CdTe samples doped with Cu or not.
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the intensity of their free exciton (FE) peaks. It can be seen, by Cu-doping treatment, only the intensity of (A0, X) peak at 1.5896 eV was enhanced. Obvious enhancement of (A0, X)1.5896 peak can be noticed for the Cu-doped sample aged for 35 days at room temperature. This is because that the Cu atom absorbed on the sample surface could steadily incorporate into the sample by selfdiffusion, even at room temperature. Above experiment has unambiguously shown that the (A0, X)1.5896 peak is associated with Cu. However, with the experiment of thermal neutron irradiating on their CdTe crystals, Barnes et al. [2] found that the dominant bound exciton emission at 1.589 eV in their crystal could be also enhanced by thermal neutron irradiation and strongly suggested that the recombination emission was associated with exciton trapping at the VCd acceptor center. So (A0, X) emission related to VCd needs to be found. Our experiment was performed on the Cd-dip treated CdTe single crystal including very low concentration of Cd-vacancies. So in order to identify VCd -related (A0, X) emission, concentration of Cd vacancies should be increased. It has been reported that 60Co rays of 1.7 107 Gy can produce a great number of Frenkel defects since Compton scattering is a dominant process in that energy region and the energy transferred from g-rays to the crystal is sufficient to displace host atoms in CdTe crystal [17]. Based on this statement our irradiation experiment was undergone. Fig. 3 shows the PL spectrum for the sample irradiated by g-rays. Compared to PL spectrum of the sample before irradiation given in Fig. 1, a new emission peak has been superimposed near (A0, X)1.5896 peak. Meanwhile, there is no other special change can be found in the spectrum. This result is quite consistent with that demonstrated by Barnes et al. [2] and clearly shows that VCd acceptor center can give exciton recombination emission at 1.5892 eV. Note that doping of Li or Na might also induce the enhancement of (A0, X) at 1.5892 eV [5], but our experiment should be different from that case because special attention has been paid to prevent from contamination. There has ever been report [18,19] that CdTe:I film
Fig. 3. PL spectrum showing for the Cd-vacancies-related (A0, X) emission.
prepared by MBE exhibits strong (A0, X) emission at 1.5892 eV, but it was simply ascribed to generating from Li or Na impurities contaminated by diffusing out from the CdTe substrate. Up to now, one might wonder why the Cdvacancy-related (A0, X) emission at 1.5892 eV can not be observed in our as-grown CdTe since there should be a high concentration of Cd-vacancies in the crystal. It is true there are a great number of Cd-vacancies in our as-grown CdTe, however, they cannot all exist in isolated state because in CdTe these Cd-vacancies are easy to form complex defects with residual impurities during crystal growth. As shown in Fig. 1, (D0, X) emissions related to residual donor-type impurities were not observed in our as-grown CdTe, but they were well observed after Cd-dip treatment. Barnes et al. [20] introduced two complex defects consisting of Cdvacancy and one or two donor-type residual impurities, (VCd D) and (VCd 2D), and based on this consideration high resistivity CdTe was prepared. Fig. 1 indicates that in as-grown CdTe almost all of the donor-type residual impurities were formed to be complex defects with Cdvacancies and these complex defects could be disassociated by Cd-dip treatment. In our previous study with Al-doped CdTe [13], the hump peak at the energy higher than (A0, X)1.5896 has been found to be associated with complex defect of (VCd 2AlCd ). Besides, as well known, II–VI
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compound semiconductor including CdTe is heavily suffered from Cu contamination. Especially for as-grown CdTe with high concentration of Cdvacancies the crystal is much easier to be contaminated by Cu. So with a high quantum efficiency of Cu, PL spectrum of high purity p-type CdTe generally exhibits a dominant Cu-related (A0, X) emission at 1.5896 eV, whereas the emission of (A0, X) related to Cd-vacancies at 1.5892 eV is covered. On the other hand, in the case of Cd-dip treatment Cd-solvent gives two effects on the CdTe crystal: filling the Cdvacancies and extracting the impurities with high diffusion coefficient such as Cu and Ag [16]. So CdTe sample treated by Cd-dip annealing exhibits very weak (A0, X) emissions. It was on the basis mentioned above that (A0, X) emission related to Cd-vacancies could be determined by increasing the concentration of Cd-vacancies through g-ray irradiation. Now drawing attention to the (A0, X) emission peak at 1.5885 eV. As shown in Fig. 1, after Cd-dip treatment our crystal exhibits clear emission peak of (A0, X) at 1.5885 eV. Although attempt to do the assignment was performed by use of the Cd-dip treated sample, similar results to the Cu-doping experiments has not been achieved. So Ag-doping experiment was performed on our as-grown CdTe single crystal. In order to make much more Ag atom incorporate into the crystal, the sample was dipped in the 1% aqueous solution of AgNO3 for 24 h. After rinsed by DI water the sample was additionally put into Te atmosphere annealing for 24 h. Fig. 4 gives PL spectra of the sample, before and after Ag-doping. It can be seen that the (A0, X) emission peak at 1.5885 eV become dominant over the PL spectrum of Agdoped sample. The another characteristic evidence is the observation of DAP band with its zerophonon peak at 1.4980 eV. These results are in well agreement with former reports [3,5,14,21,22]. More clear observation of (A0, X) emission peak at 1.5885 eV can be seen in Fig. 5. Comparing to the PL spectrum of no-doping as-grown CdTe shown in Fig. 1, the peak becomes much weaker and broader. It is easy to imagine that the (A0, X) emission peak should be covered up, due to high concentration of Ag impurities in the sample.
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Fig. 4. PL spectrum from an as-grown CdTe sample doped with Ag.
Fig. 5. Near band-edge PL spectrum of as-grown CdTe sample doped with Ag.
4. Summary Our results unambiguously showed that the (A0, X)1.5896 peak is associated with Cu impurities.
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For the first time, we have observed the isolated Cd-vacancy-related (A0, X) emission that is meanwhile accompanied by the (A0, X)1.5896 peak. The energy position of (A0, X) associated with Cd-vacancies was determined to be 1.5892 eV. This study has made the origin of (A0, X)1.5896 clear. It was also demonstrated that (A0, X)1.5885 is associated with Ag impurities.
References [1] F.J. Bryant, H.J. Totterdell, Radiat. Effects 9 (1971) 115. [2] C.E. Barnes, C. Kikuchi, Radiat. Effects 26 (1975) 105. [3] J.P. Chamonal, E. Molva, J.L. Pautrat, Solid State Commun. 43 (1982) 801. [4] E. Molva, J.P. Chamonal, J.L. Pautrat, Phys. Stat. Sol. B 109 (1982) 635. [5] E. Molva, J.L. Pautrat, K. Saminadayar, G. Milchberg, N. Magnea, Phys. Rev. B 30 (1984) 3344. [6] E. Molva, L.S. Dang, Phys. Rev. B 27 (1983) 6222. [7] J. Krustok, J. Madasson, K. Hjelet, H. Collan, J. Mater. Sci. Lett. 14 (1995) 1490. [8] S. Seto, A. Tanaka, Y. Masa, S. Dairaku, M. Kawashima, Appl. Phys. Lett. 53 (1988) 1532.
[9] J. Krustok, J. Madasson, K. Hjelet, H. Collan, J. Mater. Sci. Lett. 14 (1995) 1490. [10] H.Y. Shin, C.Y. Sun, Mater. Sci. Eng. B 52 (1998) 78. [11] H.Y. Shin, C.Y. Sun, J. Crystal Growth 186 (1998) 354. [12] B. Yang, Y. Ishikawa, T. Miki, Y. Doumae, T. Tomizono, M. Isshiki, J. Crystal Growth 159 (1996) 171. [13] S.H. Song, J. Wang, Y. Ishikawa, S. Seto, M. Isshiki, J. Crystal Growth 1726 (2002) 237. [14] J. Hamann, A. Burchard, M. Deicher, T. Filz, S. Lany, V. Ostheimer, F. trasser, H. Wolf, ISOLDE Collaboration,, Th. Wichert, J. Crystal Growth 214/215 (2000) 207. [15] S.H. Song, J. Wang, M. Isshiki, J. Crystal Growth 236 (2002) 165. [16] S.H. Song, J.F. Wang, G.M. Lelav, L. He, M. Isshiki, J. Crystal Growth 252 (2003) 102. [17] S. Seto, A. Tanaka, F. Takeda, K. Matsuura, J. Crystal Growth 138 (1994) 346. [18] N.C. Giles, J. Lee, D. Rajavel, C.J. Summers, J. Appl. Phys. 73 (1993) 4541. [19] J. Lee, N.C. Giles, D. Rajavel, C.J. Summers, Phys. Rev. B 49 (1994) 1668. [20] C.E. Barnes, K. Zanio, J. Appl. Phys. 46 (9) (1975) 3959. [21] B. Monemar, E. Molva, L.S. Dang, Phys. Rev. B 33 (1986) 1134. [22] J.P. Chamonal, E. Molva, J.L. Pautrat, L. Revoil, J. Crystal Growth 59 (1982) 297.
Journal of Crystal Growth 257 (2003) 231–236
Cd-vacancy-related excitonic emission in CdTe S.H. Song*, J.F. Wang, M. Isshiki Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 1-1, Katahira 2-chome, Aobaku, Sendai 980-8577, Japan Received 1 May 2003; accepted 5 June 2003 Communicated by K. Nakajima
Abstract The dominant luminescence emission of excitons bound to a neutral acceptor defect (A0, X) in as-grown high purity CdTe single crystals has been studied by Cd-dip treatment, doping with Cu and Ag, and irradiation with g-rays from 60 Co. It has been demonstrated that, by Cd-dip treatment, the dominant (A0, X) emission can be annihilated to be a double-structure emission with their energy at 1.5896 and 1.5885 eV, respectively. Our doping experiments have unambiguously shown that (A0, X) at 1.5896 eV is associated with residual impurities of Cu and (A0, X) at 1.5885 eV with Ag. Further experiment with g-ray irradiation has revealed a new (A0, X) emission at 1.5892 eV in the double structure. We suggest that this emission is associated with Cd-vacancies introduced by g-rays. To our knowledge, this is the first report on the isolated Cd-vacancies showing (A0, X) emission with the energy position different from 1.5896 eV. r 2003 Elsevier B.V. All rights reserved. PACS: 78.55. m; 78.55.Et; 81.10.Fq; 85.40.Ry Keywords: A1. Defects; A2. Single crystal growth; B1. Cadmium compounds
1. Introduction Discussion about exciton luminescence emission with cadmium vacancies is closely interrelated with a dominant photoluminescence emission in high purity p-type CdTe crystals. The origin of the dominant luminescence emission near 1.5896 eV from as-grown CdTe single crystal continues to be a topic of great interest [1–11]. This is because the related defects are supposed to be mainly responsible for the p-type conductivity. Much of this activity has coincided with the emergence of CdTe as a very promising material for fabricating X-, *Corresponding author. Fax: +81-22-217-5139. E-mail address: [email protected] (S.H. Song).
g-ray detector and used as substrate for HgCdTe epitaxial films. Past investigations showed that the emission could be associated with the defects of Cdvacancies (VCd ) [1,2], substitutional copper (CuCd ) [3–7] or complexes consisting of VCd and donor-type impurities [8]. Based on the Cdannealing results of high purity CdTe sample prepared by physical vapor transport, Yang et al. [12] found that the emission line was formed by two origins related to VCd and CuCd : However, they failed to separate the two sub-lines because their energy difference is too small. As a matter of fact, the emission near 1.5896 eV is a superposition of several acceptor-bound exciton recombination lines. The energies of these
0022-0248/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0022-0248(03)01457-X
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acceptor-bound excitons for a variety of acceptortype impurities have been identified by Molva et al. [5]. Nevertheless, as well known that the cadmium vacancies do exist in as-grown CdTe and play an important role in self-compensation effect that strongly influences the transport properties of this material, so it is of significance to find the (A0 ,X) emission related to isolated VCd and determine its energy position. Our previous study [13] has showed that (A0,X) emissions related to complex defects involving Cdvacancy and one or two donor-type impurities exhibit different energy positions from 1.5896 eV. In the present investigation, with our Cd-dipped CdTe specimen firstly we will unambiguously show that the (A0, X) recombination line at 1.5896 eV is associated with copper, then by introducing Cdvacancies into the sample (A0, X) emission related to VCd can be found and the energy position will be determined by reference to the line at 1.5896 eV. Incidentally, since the Ag-related (A0,X) line [5,14] at 1.5885 eV has been observed in our Cd-dipped CdTe specimen with comparable intensity of that related to copper, the confirmation result about it by doping experiment will be also presented in this paper.
Room temperature irradiation of g-rays from Co was used to introduce cadmium vacancies (VCd ) into the CdTe specimen. The dose of g-rays was 1.7 107 Gy. Measurement of the photoluminescence was performed at 4.2 K with the sample immersed in the liquid helium. Light with the wavelength of 680 nm from a semiconductor laser was used to stimulate the sample. For wavelength-energy con( version, the relation E(eV) l(A)=12395.13 [4] was used. 60
3. Results and discussion Fig. 1 shows the typical PL spectra for an identical CdTe single-crystal specimen before (asgrown) and after Cd-dip treatment. The sharp strong peak at 1.5896 eV denoted as (A0, X)1.5896 from as-grown crystal is the often discussed PL emission. As mentioned in the introduction,
2. Experimental procedures As-grown and Cd-dip treated CdTe single crystals, with extremely high purity, were used for this investigation. Preparation details for these crystals can be found in other papers [15,16]. Background doping of Cu and Ag were performed at room temperature by dipping the CdTe specimens into a 1% aqueous solution of Cu(NO3)2 or AgNO3, respectively, and subsequent rinsing with deionized (DI) water. The dipping and rinsing time were changed to control the concentration of Cu or Ag diffusing into the specimens. The Cu(NO3)2 and AgNO3 are of 5N and 6N purity. After drying, the samples were kept in a vacuum desiccator. In order to impel much Ag to occupy the Cd lattice position, Agdoped CdTe sample was additionally treated by isothermal annealing in Te atmosphere at 973 K for 24 h.
Fig. 1. PL spectra of as-grown and Cd-dip annealing treated CdTe single crystals.
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for as-grown CdTe single crystal the peak in the energy range from 1.588 to 1.591 eV is a superposition of a variety of acceptor-bound exciton (A0, X) recombination lines associated with different defects, so in order to clarify the origin of (A0, X)1.5896 emission single crystals of very high purity and quality are needed. In addition, for identifying other luminescence emissions covered by the peak, intensity of (A0, X)1.5896 emission should be suppressed. For this purpose, as-grown specimen with extremely high purity was put into Cd-dip annealing treatment. As shown in the upper PL spectrum of Fig. 1, Cd-dipped CdTe single crystal only gives two peaks in the (A0, X) emission range between the energy of 1.588 and 1.591 eV. The two peaks are at 1.5896 and 1.5885 eV, respectively. A great number of research works have shown that the (A0, X) at 1.5896 eV is related to substitutional defects of CuCd [3–6] and (A0, X) at 1.5885 eV to AgCd [5,14]. Details of discussion about other peaks can be referred to our previous paper [16]. In the following focus will be given on the (A0, X) emission range. In the literature much identification work about (A0, X) peak at 1.5896 eV was performed on the as-grown CdTe single crystals by background doping of copper followed by complicated thermal treatment including of Te-atmosphere annealing [3,4]. Although these results showed that intensity of the (A0, X)1.5896 peak was strongly enhanced after Cu-doping, the PL spectra had to be recorded in the same PL measurement condition since there was no other peak for reference. Moreover, Teatmosphere annealing treatment was used in those experiments in order to create high concentration of Cd vacancies and consequently suppose that more Cu atom could enter the crystals and occupy Cd lattice position. However, because there is no reliable evidence supporting that Cd-vacanciesrelated (A0 ,X) emission exhibits different energy from 1.5896 eV, assignment that (A0, X)1.5896 peak originates from Cu has been accepted tentatively. For solving the problem mentioned above, our assignment about (A0, X)1.5896 peak was performed on the Cd-dip treated CdTe single crystal. Apparently, this crystal includes very low concentration of Cd vacancies. Cu-doping was achieved
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by dipping the crystal into the 1% aqueous solution of Cu(NO3)2 for 1-min, then rinsed with DI water. After this treatment we observed that obvious Cu-color was put on the sample surface. For contrast we also did the same experiment using our as-grown CdTe single crystal, but resulting sample did not show the Cu-color. On the other hand, by comparing PL intensity measured under same condition it was noticed that this sample indeed exhibited much stronger luminescence of (A0, X)1.5896 than no-doping asgrown sample. These results indicate that it is easier for Cu atom to enter the as-grown crystal than Cd-dip treated one, since the former crystal includes much higher concentration of Cd vacancies than the latter one. Fig. 2 gives the experimental results performed on our Cd-dip treated CdTe single crystal. The PL spectra were measured on the same position of an identical sample. As shown in the figure, PL spectra of Cu-doped sample were recorded after treated by Cu-doping for 1 day and 35 days, respectively. PL spectrum for the sample before Cu-doping also was shown for reference. The intensity of each PL spectrum was normalized with
Fig. 2. PL spectra for Cd-dip treated CdTe samples doped with Cu or not.
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the intensity of their free exciton (FE) peaks. It can be seen, by Cu-doping treatment, only the intensity of (A0, X) peak at 1.5896 eV was enhanced. Obvious enhancement of (A0, X)1.5896 peak can be noticed for the Cu-doped sample aged for 35 days at room temperature. This is because that the Cu atom absorbed on the sample surface could steadily incorporate into the sample by selfdiffusion, even at room temperature. Above experiment has unambiguously shown that the (A0, X)1.5896 peak is associated with Cu. However, with the experiment of thermal neutron irradiating on their CdTe crystals, Barnes et al. [2] found that the dominant bound exciton emission at 1.589 eV in their crystal could be also enhanced by thermal neutron irradiation and strongly suggested that the recombination emission was associated with exciton trapping at the VCd acceptor center. So (A0, X) emission related to VCd needs to be found. Our experiment was performed on the Cd-dip treated CdTe single crystal including very low concentration of Cd-vacancies. So in order to identify VCd -related (A0, X) emission, concentration of Cd vacancies should be increased. It has been reported that 60Co rays of 1.7 107 Gy can produce a great number of Frenkel defects since Compton scattering is a dominant process in that energy region and the energy transferred from g-rays to the crystal is sufficient to displace host atoms in CdTe crystal [17]. Based on this statement our irradiation experiment was undergone. Fig. 3 shows the PL spectrum for the sample irradiated by g-rays. Compared to PL spectrum of the sample before irradiation given in Fig. 1, a new emission peak has been superimposed near (A0, X)1.5896 peak. Meanwhile, there is no other special change can be found in the spectrum. This result is quite consistent with that demonstrated by Barnes et al. [2] and clearly shows that VCd acceptor center can give exciton recombination emission at 1.5892 eV. Note that doping of Li or Na might also induce the enhancement of (A0, X) at 1.5892 eV [5], but our experiment should be different from that case because special attention has been paid to prevent from contamination. There has ever been report [18,19] that CdTe:I film
Fig. 3. PL spectrum showing for the Cd-vacancies-related (A0, X) emission.
prepared by MBE exhibits strong (A0, X) emission at 1.5892 eV, but it was simply ascribed to generating from Li or Na impurities contaminated by diffusing out from the CdTe substrate. Up to now, one might wonder why the Cdvacancy-related (A0, X) emission at 1.5892 eV can not be observed in our as-grown CdTe since there should be a high concentration of Cd-vacancies in the crystal. It is true there are a great number of Cd-vacancies in our as-grown CdTe, however, they cannot all exist in isolated state because in CdTe these Cd-vacancies are easy to form complex defects with residual impurities during crystal growth. As shown in Fig. 1, (D0, X) emissions related to residual donor-type impurities were not observed in our as-grown CdTe, but they were well observed after Cd-dip treatment. Barnes et al. [20] introduced two complex defects consisting of Cdvacancy and one or two donor-type residual impurities, (VCd D) and (VCd 2D), and based on this consideration high resistivity CdTe was prepared. Fig. 1 indicates that in as-grown CdTe almost all of the donor-type residual impurities were formed to be complex defects with Cdvacancies and these complex defects could be disassociated by Cd-dip treatment. In our previous study with Al-doped CdTe [13], the hump peak at the energy higher than (A0, X)1.5896 has been found to be associated with complex defect of (VCd 2AlCd ). Besides, as well known, II–VI
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compound semiconductor including CdTe is heavily suffered from Cu contamination. Especially for as-grown CdTe with high concentration of Cdvacancies the crystal is much easier to be contaminated by Cu. So with a high quantum efficiency of Cu, PL spectrum of high purity p-type CdTe generally exhibits a dominant Cu-related (A0, X) emission at 1.5896 eV, whereas the emission of (A0, X) related to Cd-vacancies at 1.5892 eV is covered. On the other hand, in the case of Cd-dip treatment Cd-solvent gives two effects on the CdTe crystal: filling the Cdvacancies and extracting the impurities with high diffusion coefficient such as Cu and Ag [16]. So CdTe sample treated by Cd-dip annealing exhibits very weak (A0, X) emissions. It was on the basis mentioned above that (A0, X) emission related to Cd-vacancies could be determined by increasing the concentration of Cd-vacancies through g-ray irradiation. Now drawing attention to the (A0, X) emission peak at 1.5885 eV. As shown in Fig. 1, after Cd-dip treatment our crystal exhibits clear emission peak of (A0, X) at 1.5885 eV. Although attempt to do the assignment was performed by use of the Cd-dip treated sample, similar results to the Cu-doping experiments has not been achieved. So Ag-doping experiment was performed on our as-grown CdTe single crystal. In order to make much more Ag atom incorporate into the crystal, the sample was dipped in the 1% aqueous solution of AgNO3 for 24 h. After rinsed by DI water the sample was additionally put into Te atmosphere annealing for 24 h. Fig. 4 gives PL spectra of the sample, before and after Ag-doping. It can be seen that the (A0, X) emission peak at 1.5885 eV become dominant over the PL spectrum of Agdoped sample. The another characteristic evidence is the observation of DAP band with its zerophonon peak at 1.4980 eV. These results are in well agreement with former reports [3,5,14,21,22]. More clear observation of (A0, X) emission peak at 1.5885 eV can be seen in Fig. 5. Comparing to the PL spectrum of no-doping as-grown CdTe shown in Fig. 1, the peak becomes much weaker and broader. It is easy to imagine that the (A0, X) emission peak should be covered up, due to high concentration of Ag impurities in the sample.
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Fig. 4. PL spectrum from an as-grown CdTe sample doped with Ag.
Fig. 5. Near band-edge PL spectrum of as-grown CdTe sample doped with Ag.
4. Summary Our results unambiguously showed that the (A0, X)1.5896 peak is associated with Cu impurities.
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For the first time, we have observed the isolated Cd-vacancy-related (A0, X) emission that is meanwhile accompanied by the (A0, X)1.5896 peak. The energy position of (A0, X) associated with Cd-vacancies was determined to be 1.5892 eV. This study has made the origin of (A0, X)1.5896 clear. It was also demonstrated that (A0, X)1.5885 is associated with Ag impurities.
References [1] F.J. Bryant, H.J. Totterdell, Radiat. Effects 9 (1971) 115. [2] C.E. Barnes, C. Kikuchi, Radiat. Effects 26 (1975) 105. [3] J.P. Chamonal, E. Molva, J.L. Pautrat, Solid State Commun. 43 (1982) 801. [4] E. Molva, J.P. Chamonal, J.L. Pautrat, Phys. Stat. Sol. B 109 (1982) 635. [5] E. Molva, J.L. Pautrat, K. Saminadayar, G. Milchberg, N. Magnea, Phys. Rev. B 30 (1984) 3344. [6] E. Molva, L.S. Dang, Phys. Rev. B 27 (1983) 6222. [7] J. Krustok, J. Madasson, K. Hjelet, H. Collan, J. Mater. Sci. Lett. 14 (1995) 1490. [8] S. Seto, A. Tanaka, Y. Masa, S. Dairaku, M. Kawashima, Appl. Phys. Lett. 53 (1988) 1532.
[9] J. Krustok, J. Madasson, K. Hjelet, H. Collan, J. Mater. Sci. Lett. 14 (1995) 1490. [10] H.Y. Shin, C.Y. Sun, Mater. Sci. Eng. B 52 (1998) 78. [11] H.Y. Shin, C.Y. Sun, J. Crystal Growth 186 (1998) 354. [12] B. Yang, Y. Ishikawa, T. Miki, Y. Doumae, T. Tomizono, M. Isshiki, J. Crystal Growth 159 (1996) 171. [13] S.H. Song, J. Wang, Y. Ishikawa, S. Seto, M. Isshiki, J. Crystal Growth 1726 (2002) 237. [14] J. Hamann, A. Burchard, M. Deicher, T. Filz, S. Lany, V. Ostheimer, F. trasser, H. Wolf, ISOLDE Collaboration,, Th. Wichert, J. Crystal Growth 214/215 (2000) 207. [15] S.H. Song, J. Wang, M. Isshiki, J. Crystal Growth 236 (2002) 165. [16] S.H. Song, J.F. Wang, G.M. Lelav, L. He, M. Isshiki, J. Crystal Growth 252 (2003) 102. [17] S. Seto, A. Tanaka, F. Takeda, K. Matsuura, J. Crystal Growth 138 (1994) 346. [18] N.C. Giles, J. Lee, D. Rajavel, C.J. Summers, J. Appl. Phys. 73 (1993) 4541. [19] J. Lee, N.C. Giles, D. Rajavel, C.J. Summers, Phys. Rev. B 49 (1994) 1668. [20] C.E. Barnes, K. Zanio, J. Appl. Phys. 46 (9) (1975) 3959. [21] B. Monemar, E. Molva, L.S. Dang, Phys. Rev. B 33 (1986) 1134. [22] J.P. Chamonal, E. Molva, J.L. Pautrat, L. Revoil, J. Crystal Growth 59 (1982) 297.