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Photoluminescence of AgGaS2 and CuGaS2 doped with rare-earth impurities
Journal of Physics and Chemistry of Solids 64 (2003) 1801–1805 www.elsevier.com/locate/jpcs
Photoluminescence of AgGaS2 and CuGaS2 doped with rare-earth impurities Sho Shirakataa,*, Tomoaki Terasakoa, Eiji Niwab, Katashi Masumotob a Faculty of Engineering, Ehime University, Matsuyama, Ehime 790-8577, Japan The Research Institute for Electric and Magnetic Materials, Sendai, Miyagi 982-0807, Japan
b
Abstract Photoluminescence (PL) related to rare-earth (RE) impurities (Ho, Er and Eu) in AgGaS2 and CuGaS2 crystals has been studied. In Ho-doped AgGaS2 and CuGaS2, two series of PL lines are observed in 1.86 – 1.92 eV region and 2.24 eV region, and they are assigned to 5F3 – 5I7 and 5S2 – 5I8 transitions of the Ho3þ ion, respectively. Similarly, in Er-doped AgGaS2 and CuGaS2, Er3þ-related two PL series are observed: 1.83 – 1.88 eV region (4F9/2 – 4I15/2) and 2.22– 2.26 eV region (4S3/2 – 4I15/2). For both Ho and Er impurities, the profile of the PL spectrum in AgGaS2 is complex, and PL exhibited large number of lines compared with that in CuGaS2. The differences in PL spectra between this two compounds are related to the crystal field at the cation site and the local atomic arrangement of the RE impurities. This work also refers to the PL band at 2.28 eV observed for the Eudoped AgGaS2 crystal. q 2003 Elsevier Ltd. All rights reserved.
1. Introduction Much attention has been paid to the rare-earth (RE) related luminescence in semiconductors. RE impurities such as Er, Yb and Nd exhibit temperature-stable sharp luminescence lines due to the intra-4f transition, and thus, they are expected as new luminescent centers in semiconductor light-emitting devices. AgGaS2 is the wide-gap member of I– III – VI2-type chalcopyrite semiconductor with an energy gap of 2.73 eV. Because of the large non-linear optical coefficient and the large optical anisotropy, AgGaS2 is known to be promising for non-linear optical applications [1]. Large uniform single crystals have been grown from the melt for such applications [2]. The RE impurities in AgGaS2 are interesting because we expect new optical functions such as the wavelength conversion using both the emission and the optical non-linearity. For the first time observation of RE-related luminescence in chalcopyrite semiconductors, the authors have * Corresponding author. Tel.: þ81-89-927-9772; fax: þ 81-89927-9789. E-mail address: [email protected] (S. Shirakata).
doped Yb in several I – III – VI2 semiconductors and observed Yb-related photoluminescence (PL) [3]. In AgGaS2, series of sharp near-infrared PL lines (the 2F5/ 2 3þ 2 – F7/2 transition of Yb ) have been observed [3,4]. Also, 5 5 visible PL lines ( F3 – I7 and 5S2 – 5I8 transitions of Ho3þ) in the Ho doped AgGaS2 single crystal have been observed [5,6]. The authors have been studying the fine structure of RE-related PL in AgGaS2 from the view point of the crystal field and the local atomic arrangement of RE ions. For such purpose, Yb has been doped into AgGaS2, CuGaS2, AgGaSe2 and CuGaSe2, and dependence of PL lines on the different host materials has been studied [3,4]. On the other hand, Sato et al. [7 – 9] reported PL of various RE impurities in CuAlS2 with large band-gap of 3.49 eV from the view point of the stable visible emission in the wide-gap semiconductor. In this study, the authors would like to present visible emissions from the trivalent Ho and Er ions in both AgGaS2 and CuGaS2 crystals. The difference of RE-related PL spectra between the compounds is discussed in terms of the crystal field and the local atomic arrangement of RE impurities. This work also refers to the PL band at 2.28 eV observed in the Eu-doped AgGaS2 crystal.
0022-3697/03/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0022-3697(03)00246-4
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2. PL of AgGaS2 and CuGaS2 doped with Ho and Er The undoped AgGaS2 single crystal was grown by a selfseeding vertical gradient freezing (VGF) method [2]. The undoped CuGaS2 single crystal was grown by the chemical vapor transport method using iodine as a transport agent. The doping of Ho and Er was performed by the thermal diffusion method. Undoped AgGaS2 or CuGaS2 crystals with a vacuum-evaporated thin RE film on the surface were sealed into a quartz ampoule together with a small amount of sulphur. They were annealed at 800 8C for two days to perform the solid state diffusion. For Ho- and Er-doped AgGaS2 and CuGaS2, PL was excited by one of the blue-green lines of an Arþ laser (514.5– 457.9 nm). PL was analyzed by a grating monochromator (either Ritsu MC-50L or SPEX 1000M), and detected by a photomultiplier with a GaAs photo-cathode. Fig. 1 shows PL spectra at 77 K of AgGaS2:Ho and AgGaS2:Er. For AgGaS2:Ho, series of very sharp PL lines were observed at 1.86 – 1.92 eV (L-series). Weak PL peaks were also observed at 2.23– 2.28 eV (H-series). Based on the Dieke diagram [10], L- and H-series PL lines have been assigned to the 5F3 – 5I7 and 5S2 – 5I8 transitions in the 4f10 manifold of the Ho3þ ion, respectively [5,6]. For AgGaS2:Er, the PL spectrum exhibited sharp series of PL lines in both 1.83 –1.89 and 2.21 –2.27 eV regions, and they are assigned to the 4F9/2 – 4I15/2 and 4S3/2 – 4I15/2 transitions of the Er3þ ions, respectively, based on the Dieke diagram [10]. Similarly, low-temperature PL spectra of CuGaS2:Ho and CuGaS2:Er have been measured. In general, both Hoand Er-related PLs in CuGaS2 were much weaker than those in AgGaS2. This result shows that the solid solubility of RE ions in AgGaS2 is much higher than that in CuGaS2. This result is similar to the case of Yb [3,4]. The series of PL lines
Fig. 1. PL spectra at 77 K of AgGaS2:Ho and AgGaS2:Er.
are observed in the 1.86 – 1.90 eV region for CuGaS2:Ho (the 5F3 – 5I7 transition of Ho3þ) and in the 1.84 – 1.88 eV region for CuGaS2:Er (the 4F9/2 – 4I15/2 transition of Er3þ), similar to the case of AgGaS2. However, no PL line was observed in the 2.2– 2.3 eV region at low temperature for both CuGaS2:Ho and CuGaS2:Er, and thus, the 5S2 – 5I8 transition of Ho3þ and the 4S3/2 – 4I15/2 transition of Er3þ were not observed in CuGaS2. This is mainly because of high PL intensity of the defect-related PL in CuGaS2 in the 2.2– 2.3 eV region. The well known infrared luminescence of the 4I13/2 – 4I15/2 transition of Er3þ was not observed for both AgGaS2:Er and CuGaS2:Er in the 0.8 eV region. In Fig. 2, high-resolution PL spectra at 8 K of AgGaS2: Ho and CuGaS2:Ho are shown for the 1.86 –1.91 eV region (the 5F3 – 5I7 transition of Ho3þ). For AgGaS2:Ho, the profile of the PL spectrum is very complex and as many as 40 sharp PL lines are found. Full-width at half-maximum (FWHM) values of PL lines are small (0.4– 2.0 meV). Based on the excitation wavelength and temperature, PL spectra in AgGaS2:Ho is composed of PLs from three types of Ho centers with different local atomic arrangements [5]. These three types of PL lines are indicated by closed circles, open circles and open triangles in Fig. 2. In CuGaS2, the energy range of the PL distribution is similar to that in AgGaS2. However, the PL profile is different. The number of the PL lines in CuGaS2 is about 20, which is a half of that in AgGaS2. PL spectra of CuGaS2:Ho are clearly divided into two regions: the region A (high energy series: 1.885 – 1.895 eV) and the region B (low energy series: 1.858 – 1.874 eV), as can be seen in Fig. 2. The intense PL lines at around 1.882 eV in AgGaS2:Ho (denoted by open triangles in Fig. 2) are absent in CuGaS2:Ho. Thus, the PL spectrum of AgGaS2:Ho is much more complex than that of CuGaS2:Ho. Two reasons
Fig. 2. PL spectra at 8 K of AgGaS2:Ho and CuGaS2:Ho.
S. Shirakata et al. / Journal of Physics and Chemistry of Solids 64 (2003) 1801–1805
are considered for this: (i) in CuGaS2:Ho, the number of the types of Ho3þ centers with different atomic arrangements is smaller than that of AgGaS2:Ho, and (ii) the crystal field at the cation site is larger for AgGaS2 than for CuGaS2 where Ho ions occupy, and thus, the crystal field splitting of AgGaS2 is larger than that of CuGaS2. In Fig. 3, high-resolution PL spectra of AgGaS2:Er and CuGaS2:Er are shown for the 1.83– 1.89 eV region (the 4F9/2 – 4I15/2 transition of Er3þ). The PL spectrum of AgGaS2:Er at 8 K exhibited more than 20 very sharp PL lines with small FWHM values (0.6– 1.4 meV). At 80 K, FWHM of PL lines increases and PL spectrum becomes broader. In addition, two changes in the PL spectrum are noted: (i) the change of relative PL intensity, and (ii) the appearance of new PL lines in the high-energy region
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(1.875– 1.887 eV: denoted as HES in Fig. 3). Based on the energy of PL lines, the PL spectrum of AgGaS2:Er is composed of two series of PL lines: one series denoted as 1– 8 and another series denoted as 10 –80 in Fig. 3. The PL profiles of the two series are in good agreement with each other except for the energy difference of 4 meV. It can be considered that these PL series are from the two of the levels of the excited state (4F9/2) to the eight levels of ground state (4I15/2). For the 4I15/2 multiplet, number of Stark levels in D2d symmetry is eight [9]. In fact, lines 10 – 70 exhibited hotline behavior at low temperature (8– 60 K). This result shows that the electron population of the higher level of the excited state increases with increasing temperature. At 77 K, we can see the relative intensity of 10 – 70 line increases compared with that at 8 K, as can be seen in Fig. 3.
Fig. 3. PL spectra of AgGaS2:Er and CuGaS2:Er.
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The PL spectrum at 77 K of CuGaS2:Er is shown in Fig. 3, and it is compared with that of AgGaS2:Er. The PL spectrum of CuGaS2:Er exhibited five PL peaks (a– e in Fig. 3) in the high energy region (1.872– 1.878 eV) and three PL peaks (f– h in Fig. 3) in the low energy region (1.846– 1.851 eV). The number of the peak is eight, which happens to agree with the number of Stark levels of the ground state [9]. The smaller number of PL lines and the narrow energy range of the PL series for CuGaS2:Er than that of AgGaS2:Er may be due to the weaker crystal field at the Er site compared with that of AgGaS2. Further study is in progress in order to make this point clear.
3. PL of Eu-doped AgGaS2 The Eu impurity was introduced into AgGaS2 using two different methods. For both the methods, source crystal of AgGaS2 was synthesized in advance by the sulphurization of the Ag– Ga melt at 1100 8C. The first method (method I) is that the AgGaS2 powder was mixed with the EuS powder (AgGaS2 þ 0.02EuS) and it was pressed into a pellet. The pellet was annealed at 900 8C for 4 days in a closed quartz ampoule in a sulphur atmosphere in order to thermally diffuse Eu into AgGaS2 by the solid state reaction (sintering). The second method (method II) is that the melt-growth was done using the pressed pellet (AgGaS2 þ 0.02EuS: similar to the method I) as a source material. The pellet was sealed into the quartz ampoule. First, the ampoule was set in the vertical furnace and held at 900 8C for 3 days for the sintering. Next, furnace temperature was set at 1060 8C in order to melt the pellet, and the ampoule was held at this temperature for 1 day for homogenization of the melt. Then the ampoule was lowered down at a rate of 18 mm/day in the temperature gradient of 10 8C/cm, and the crystal was grown. PL spectra at 77 K of AgGaS2:Eu excited by the 325 nm line of a He– Cd laser are shown in Fig. 4 together with that of the source AgGaS2 poly-crystal used for the sample preparation. Fig. 4(a) shows the PL spectrum of the source AgGaS2 crystal. It exhibited an exciton-related emission at 2.69 eV, a PL peak due to donor – acceptor pair emission at 2.62 eV and a broad PL band due to impurities or intrinsic defects at 2.46 eV, similar to the PL spectra in single crystal of AgGaS2 grown by VGF [11]. The PL spectrum of the AgGaS2:Eu prepared by the sintering method (method I) is shown in Fig. 4(b). PL exhibited a dominant intense green PL peak at 2.28 eV with FWHM of 0.14 eV. PL intensity is similar to that of the 2.46 eV PL of the source AgGaS2 crystal (Fig. 4(a)). Although, the exciton-related PL (2.69 eV) and the D – A pair PL (2.62 eV) can be seen, their intensities are very weak (about 1/1000 of the 2.28 eV PL). Similar PL peak at 2.28 eV is observed for AgGaS2:Eu grown from melt (method II), as can be seen in Fig. 4(c). PL intensity is also comparable with that of AgGaS2:Eu
Fig. 4. PL spectra at 77 K of AgGaS2:Eu.
prepared by method I. These results show that PL peak at 2.28 eV, is related to the Eu impurity in AgGaS2. Considering the broad nature of the PL peak at 2.28 eV, we tentatively assign this PL to the 4f65d – 4f7 transition of the Eu2þ ion in AgGaS2. We can see the resemblance of the Eu2þ-related PL in CaGa2S4:Eu (peak energy is 2.21 eV and FWHM is 0.15 eV) [12].
4. Conclusions PL of RE impurities in AgGaS2 and CuGaS2 crystals has been studied. In Ho-doped AgGaS2 and CuGaS2, two series of PL lines are observed in 1.86– 1.92 and 2.24 eV regions, and they are assigned to 5F3 – 5I7 and 5S2 – 5I8 transitions of the Ho3þ ion, respectively. For AgGaS2:Er and CuGaS2:Er, two PL series are observed: 1.83 , 1.88 eV region (4F9/2 – 4I15/2) and 2.22 –2.26 eV region (4S3/2 – 4I15/2). The difference in the fine structure of the PL spectra between the compounds is related to; (i) the difference of the crystal field, and (ii) RE centers with different local atomic arrangements. In AgGaS2:Eu, a broad PL band at 2.28 eV was observed, and the PL peak was tentatively ascribed to the 4f65d– 4f7 transition of Eu2þ ion in AgGaS2.
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Acknowledgements The authors would like to thank Mr A. Miyata for technical assistance. References [1] D.S. Chemla, P.J. Kupecek, D.S. Robertson, R.C. Smith, Opt. Commun. 3 (1971) 29. [2] E. Niwa, K. Masumoto, J. Cryst. Growth 192 (1998) 354. [3] S. Shirakata, K. Ishii, S. Isomura, Proceedings of the Eighth International Conference on Ternary and Multinary Compounds, in: S.I. Radautsan, C. Schwab (Eds.), Shtiintsa Press, Kishinev, 1990, p. 164. [4] S. Shirakata, S. Isomura, Jpn. J. Appl. Phys. 37 (1998) 776. [5] T. Terasako, K. Hashimoto, Y. Nomoto, S. Shirakata, S. Isomura, E. Niwa, K. Masumoto, J. Lumin. 87–89 (2000) 1056.
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[6] T. Terasako, K. Hashimoto, S. Shirakata, S. Isomura, E. Niwa, K. Masumoto, Proceedings of the 12th International Conference on Ternary and Multinary Compounds, Jpn. J. Appl. Phys. Suppl. 39-1 (2001) 108. [7] K. Sato, Y. Kimura, T. Shimizu, T. Ohgoh, Y. Kudo, Cryst. Res. Technol. 31 (1996) 713. [8] T. Nishi, Y. Kimura, K. Sato, J. Lumin. 87–89 (2000) 1105. [9] J. Nakahara, K. Itoh, K. Sato, S. Yamamoto, J. Lumin. 87–89 (2000) 1112. [10] G.H. Dieke, H.M. Crosswhite, Appl. Opt. 2 (1963) 675. [11] E. Niwa, T. Yasuda, K. Masumoto, M. Issiki, Y. Segawa, Proceeding of the 11th International Conference on Ternary and Multinary Compounds, Salford, 1997, in: R.D. Tomlinson, A.E. Hill, R.D. Pilkington (Eds.), Institute of Physics Conference Series No. 152, Institute of Physics Publishing, UK, 1998, p. 473. [12] T.E. Peters, J.A. Baglio, J. Electrochem. Soc. 119 (1972) 230.
Photoluminescence of AgGaS2 and CuGaS2 doped with rare-earth impurities Sho Shirakataa,*, Tomoaki Terasakoa, Eiji Niwab, Katashi Masumotob a Faculty of Engineering, Ehime University, Matsuyama, Ehime 790-8577, Japan The Research Institute for Electric and Magnetic Materials, Sendai, Miyagi 982-0807, Japan
b
Abstract Photoluminescence (PL) related to rare-earth (RE) impurities (Ho, Er and Eu) in AgGaS2 and CuGaS2 crystals has been studied. In Ho-doped AgGaS2 and CuGaS2, two series of PL lines are observed in 1.86 – 1.92 eV region and 2.24 eV region, and they are assigned to 5F3 – 5I7 and 5S2 – 5I8 transitions of the Ho3þ ion, respectively. Similarly, in Er-doped AgGaS2 and CuGaS2, Er3þ-related two PL series are observed: 1.83 – 1.88 eV region (4F9/2 – 4I15/2) and 2.22– 2.26 eV region (4S3/2 – 4I15/2). For both Ho and Er impurities, the profile of the PL spectrum in AgGaS2 is complex, and PL exhibited large number of lines compared with that in CuGaS2. The differences in PL spectra between this two compounds are related to the crystal field at the cation site and the local atomic arrangement of the RE impurities. This work also refers to the PL band at 2.28 eV observed for the Eudoped AgGaS2 crystal. q 2003 Elsevier Ltd. All rights reserved.
1. Introduction Much attention has been paid to the rare-earth (RE) related luminescence in semiconductors. RE impurities such as Er, Yb and Nd exhibit temperature-stable sharp luminescence lines due to the intra-4f transition, and thus, they are expected as new luminescent centers in semiconductor light-emitting devices. AgGaS2 is the wide-gap member of I– III – VI2-type chalcopyrite semiconductor with an energy gap of 2.73 eV. Because of the large non-linear optical coefficient and the large optical anisotropy, AgGaS2 is known to be promising for non-linear optical applications [1]. Large uniform single crystals have been grown from the melt for such applications [2]. The RE impurities in AgGaS2 are interesting because we expect new optical functions such as the wavelength conversion using both the emission and the optical non-linearity. For the first time observation of RE-related luminescence in chalcopyrite semiconductors, the authors have * Corresponding author. Tel.: þ81-89-927-9772; fax: þ 81-89927-9789. E-mail address: [email protected] (S. Shirakata).
doped Yb in several I – III – VI2 semiconductors and observed Yb-related photoluminescence (PL) [3]. In AgGaS2, series of sharp near-infrared PL lines (the 2F5/ 2 3þ 2 – F7/2 transition of Yb ) have been observed [3,4]. Also, 5 5 visible PL lines ( F3 – I7 and 5S2 – 5I8 transitions of Ho3þ) in the Ho doped AgGaS2 single crystal have been observed [5,6]. The authors have been studying the fine structure of RE-related PL in AgGaS2 from the view point of the crystal field and the local atomic arrangement of RE ions. For such purpose, Yb has been doped into AgGaS2, CuGaS2, AgGaSe2 and CuGaSe2, and dependence of PL lines on the different host materials has been studied [3,4]. On the other hand, Sato et al. [7 – 9] reported PL of various RE impurities in CuAlS2 with large band-gap of 3.49 eV from the view point of the stable visible emission in the wide-gap semiconductor. In this study, the authors would like to present visible emissions from the trivalent Ho and Er ions in both AgGaS2 and CuGaS2 crystals. The difference of RE-related PL spectra between the compounds is discussed in terms of the crystal field and the local atomic arrangement of RE impurities. This work also refers to the PL band at 2.28 eV observed in the Eu-doped AgGaS2 crystal.
0022-3697/03/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0022-3697(03)00246-4
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2. PL of AgGaS2 and CuGaS2 doped with Ho and Er The undoped AgGaS2 single crystal was grown by a selfseeding vertical gradient freezing (VGF) method [2]. The undoped CuGaS2 single crystal was grown by the chemical vapor transport method using iodine as a transport agent. The doping of Ho and Er was performed by the thermal diffusion method. Undoped AgGaS2 or CuGaS2 crystals with a vacuum-evaporated thin RE film on the surface were sealed into a quartz ampoule together with a small amount of sulphur. They were annealed at 800 8C for two days to perform the solid state diffusion. For Ho- and Er-doped AgGaS2 and CuGaS2, PL was excited by one of the blue-green lines of an Arþ laser (514.5– 457.9 nm). PL was analyzed by a grating monochromator (either Ritsu MC-50L or SPEX 1000M), and detected by a photomultiplier with a GaAs photo-cathode. Fig. 1 shows PL spectra at 77 K of AgGaS2:Ho and AgGaS2:Er. For AgGaS2:Ho, series of very sharp PL lines were observed at 1.86 – 1.92 eV (L-series). Weak PL peaks were also observed at 2.23– 2.28 eV (H-series). Based on the Dieke diagram [10], L- and H-series PL lines have been assigned to the 5F3 – 5I7 and 5S2 – 5I8 transitions in the 4f10 manifold of the Ho3þ ion, respectively [5,6]. For AgGaS2:Er, the PL spectrum exhibited sharp series of PL lines in both 1.83 –1.89 and 2.21 –2.27 eV regions, and they are assigned to the 4F9/2 – 4I15/2 and 4S3/2 – 4I15/2 transitions of the Er3þ ions, respectively, based on the Dieke diagram [10]. Similarly, low-temperature PL spectra of CuGaS2:Ho and CuGaS2:Er have been measured. In general, both Hoand Er-related PLs in CuGaS2 were much weaker than those in AgGaS2. This result shows that the solid solubility of RE ions in AgGaS2 is much higher than that in CuGaS2. This result is similar to the case of Yb [3,4]. The series of PL lines
Fig. 1. PL spectra at 77 K of AgGaS2:Ho and AgGaS2:Er.
are observed in the 1.86 – 1.90 eV region for CuGaS2:Ho (the 5F3 – 5I7 transition of Ho3þ) and in the 1.84 – 1.88 eV region for CuGaS2:Er (the 4F9/2 – 4I15/2 transition of Er3þ), similar to the case of AgGaS2. However, no PL line was observed in the 2.2– 2.3 eV region at low temperature for both CuGaS2:Ho and CuGaS2:Er, and thus, the 5S2 – 5I8 transition of Ho3þ and the 4S3/2 – 4I15/2 transition of Er3þ were not observed in CuGaS2. This is mainly because of high PL intensity of the defect-related PL in CuGaS2 in the 2.2– 2.3 eV region. The well known infrared luminescence of the 4I13/2 – 4I15/2 transition of Er3þ was not observed for both AgGaS2:Er and CuGaS2:Er in the 0.8 eV region. In Fig. 2, high-resolution PL spectra at 8 K of AgGaS2: Ho and CuGaS2:Ho are shown for the 1.86 –1.91 eV region (the 5F3 – 5I7 transition of Ho3þ). For AgGaS2:Ho, the profile of the PL spectrum is very complex and as many as 40 sharp PL lines are found. Full-width at half-maximum (FWHM) values of PL lines are small (0.4– 2.0 meV). Based on the excitation wavelength and temperature, PL spectra in AgGaS2:Ho is composed of PLs from three types of Ho centers with different local atomic arrangements [5]. These three types of PL lines are indicated by closed circles, open circles and open triangles in Fig. 2. In CuGaS2, the energy range of the PL distribution is similar to that in AgGaS2. However, the PL profile is different. The number of the PL lines in CuGaS2 is about 20, which is a half of that in AgGaS2. PL spectra of CuGaS2:Ho are clearly divided into two regions: the region A (high energy series: 1.885 – 1.895 eV) and the region B (low energy series: 1.858 – 1.874 eV), as can be seen in Fig. 2. The intense PL lines at around 1.882 eV in AgGaS2:Ho (denoted by open triangles in Fig. 2) are absent in CuGaS2:Ho. Thus, the PL spectrum of AgGaS2:Ho is much more complex than that of CuGaS2:Ho. Two reasons
Fig. 2. PL spectra at 8 K of AgGaS2:Ho and CuGaS2:Ho.
S. Shirakata et al. / Journal of Physics and Chemistry of Solids 64 (2003) 1801–1805
are considered for this: (i) in CuGaS2:Ho, the number of the types of Ho3þ centers with different atomic arrangements is smaller than that of AgGaS2:Ho, and (ii) the crystal field at the cation site is larger for AgGaS2 than for CuGaS2 where Ho ions occupy, and thus, the crystal field splitting of AgGaS2 is larger than that of CuGaS2. In Fig. 3, high-resolution PL spectra of AgGaS2:Er and CuGaS2:Er are shown for the 1.83– 1.89 eV region (the 4F9/2 – 4I15/2 transition of Er3þ). The PL spectrum of AgGaS2:Er at 8 K exhibited more than 20 very sharp PL lines with small FWHM values (0.6– 1.4 meV). At 80 K, FWHM of PL lines increases and PL spectrum becomes broader. In addition, two changes in the PL spectrum are noted: (i) the change of relative PL intensity, and (ii) the appearance of new PL lines in the high-energy region
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(1.875– 1.887 eV: denoted as HES in Fig. 3). Based on the energy of PL lines, the PL spectrum of AgGaS2:Er is composed of two series of PL lines: one series denoted as 1– 8 and another series denoted as 10 –80 in Fig. 3. The PL profiles of the two series are in good agreement with each other except for the energy difference of 4 meV. It can be considered that these PL series are from the two of the levels of the excited state (4F9/2) to the eight levels of ground state (4I15/2). For the 4I15/2 multiplet, number of Stark levels in D2d symmetry is eight [9]. In fact, lines 10 – 70 exhibited hotline behavior at low temperature (8– 60 K). This result shows that the electron population of the higher level of the excited state increases with increasing temperature. At 77 K, we can see the relative intensity of 10 – 70 line increases compared with that at 8 K, as can be seen in Fig. 3.
Fig. 3. PL spectra of AgGaS2:Er and CuGaS2:Er.
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The PL spectrum at 77 K of CuGaS2:Er is shown in Fig. 3, and it is compared with that of AgGaS2:Er. The PL spectrum of CuGaS2:Er exhibited five PL peaks (a– e in Fig. 3) in the high energy region (1.872– 1.878 eV) and three PL peaks (f– h in Fig. 3) in the low energy region (1.846– 1.851 eV). The number of the peak is eight, which happens to agree with the number of Stark levels of the ground state [9]. The smaller number of PL lines and the narrow energy range of the PL series for CuGaS2:Er than that of AgGaS2:Er may be due to the weaker crystal field at the Er site compared with that of AgGaS2. Further study is in progress in order to make this point clear.
3. PL of Eu-doped AgGaS2 The Eu impurity was introduced into AgGaS2 using two different methods. For both the methods, source crystal of AgGaS2 was synthesized in advance by the sulphurization of the Ag– Ga melt at 1100 8C. The first method (method I) is that the AgGaS2 powder was mixed with the EuS powder (AgGaS2 þ 0.02EuS) and it was pressed into a pellet. The pellet was annealed at 900 8C for 4 days in a closed quartz ampoule in a sulphur atmosphere in order to thermally diffuse Eu into AgGaS2 by the solid state reaction (sintering). The second method (method II) is that the melt-growth was done using the pressed pellet (AgGaS2 þ 0.02EuS: similar to the method I) as a source material. The pellet was sealed into the quartz ampoule. First, the ampoule was set in the vertical furnace and held at 900 8C for 3 days for the sintering. Next, furnace temperature was set at 1060 8C in order to melt the pellet, and the ampoule was held at this temperature for 1 day for homogenization of the melt. Then the ampoule was lowered down at a rate of 18 mm/day in the temperature gradient of 10 8C/cm, and the crystal was grown. PL spectra at 77 K of AgGaS2:Eu excited by the 325 nm line of a He– Cd laser are shown in Fig. 4 together with that of the source AgGaS2 poly-crystal used for the sample preparation. Fig. 4(a) shows the PL spectrum of the source AgGaS2 crystal. It exhibited an exciton-related emission at 2.69 eV, a PL peak due to donor – acceptor pair emission at 2.62 eV and a broad PL band due to impurities or intrinsic defects at 2.46 eV, similar to the PL spectra in single crystal of AgGaS2 grown by VGF [11]. The PL spectrum of the AgGaS2:Eu prepared by the sintering method (method I) is shown in Fig. 4(b). PL exhibited a dominant intense green PL peak at 2.28 eV with FWHM of 0.14 eV. PL intensity is similar to that of the 2.46 eV PL of the source AgGaS2 crystal (Fig. 4(a)). Although, the exciton-related PL (2.69 eV) and the D – A pair PL (2.62 eV) can be seen, their intensities are very weak (about 1/1000 of the 2.28 eV PL). Similar PL peak at 2.28 eV is observed for AgGaS2:Eu grown from melt (method II), as can be seen in Fig. 4(c). PL intensity is also comparable with that of AgGaS2:Eu
Fig. 4. PL spectra at 77 K of AgGaS2:Eu.
prepared by method I. These results show that PL peak at 2.28 eV, is related to the Eu impurity in AgGaS2. Considering the broad nature of the PL peak at 2.28 eV, we tentatively assign this PL to the 4f65d – 4f7 transition of the Eu2þ ion in AgGaS2. We can see the resemblance of the Eu2þ-related PL in CaGa2S4:Eu (peak energy is 2.21 eV and FWHM is 0.15 eV) [12].
4. Conclusions PL of RE impurities in AgGaS2 and CuGaS2 crystals has been studied. In Ho-doped AgGaS2 and CuGaS2, two series of PL lines are observed in 1.86– 1.92 and 2.24 eV regions, and they are assigned to 5F3 – 5I7 and 5S2 – 5I8 transitions of the Ho3þ ion, respectively. For AgGaS2:Er and CuGaS2:Er, two PL series are observed: 1.83 , 1.88 eV region (4F9/2 – 4I15/2) and 2.22 –2.26 eV region (4S3/2 – 4I15/2). The difference in the fine structure of the PL spectra between the compounds is related to; (i) the difference of the crystal field, and (ii) RE centers with different local atomic arrangements. In AgGaS2:Eu, a broad PL band at 2.28 eV was observed, and the PL peak was tentatively ascribed to the 4f65d– 4f7 transition of Eu2þ ion in AgGaS2.
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