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Raman scattering of Zn doped CuGaS2 layers grown by vapor phase epitaxy
Journal of Physics and Chemistry of Solids 66 (2005) 1982–1986 www.elsevier.com/locate/jpcs
Raman scattering of Zn doped CuGaS2 layers grown by vapor phase epitaxy T. Terasako a,b,*, S. Iida b,1, H. Ichinokura b, A. Kato b a
Faculty of Engineering, Ehime University, 3 Bunkyo-cho, Matsuyama 790-8577, Japan b Nagaoka University of Technology, Kamitomioka, Nagaoka 940-2188, Japan
Abstract Raman spectra for non-site-selectively and site-selectively Zn-doped CuGaS2 layers grown by vapor phase epitaxy (VPE) were investigated. Although an appearance of characteristic Raman line(s) related with the doped Zn atom was not seen, an enhancement of the Raman intensity ratio of the highest LO mode to the A1 mode (ILO/IA1) was observed. The site-selectively Zn-doped layers with p-type conductivity exhibited larger ILO/IA1 ratio compared to those with n-type conductivity. The observed correlation between the ILO/IA1 ratio and the peak energy of the photoluminescence characteristic for Zn-doped p-type samples (L emission) suggests that the enhancement of ILO/IA1 is due to the increase of Zn atom substituting Ga site (ZnGa) which is acting as an acceptor. q 2005 Elsevier Ltd. All rights reserved. Keywords: A. Semiconductors; B. Epitaxial growth; C. Raman spectroscopy; D. Luminescence
1. Introduction The I–III–VI2 chalcopyrite semiconductor CuGaS2 having a direct energy gap of 2.49 eV at room temperature is a possible candidate for new functionality devices reflecting characteristic nature of the ternary compound [1,2]. For this kind of application, the first prerequisite is to establish epitaxial growth technique, which includes conductivity control by impurity doping. Ooe et al. found experimental evidence for the first time that Zn is really an amphoteric impurity [3,4] based on detailed studies of photoluminescence (PL) involving timeresolved (TR) spectra during decay in (CuGaS2)1Kx–(2ZnS)x (xZ0.00025K0.05) bulk crystals. Recently conductivity control has been achieved for Zn doped CuGaS2 layers on GaP substrates grown by vapor phase epitaxy based on atomic layer growth technique (AL-VPE) [5–7]. This paper reports Raman scattering spectra observed for non-site-selectively and site-selectively Zn doped CuGaS2 layers. Relation between Raman and PL spectra is discussed in
* Corresponding author. Address: Faculty of Engineering, Ehime University, 3 Bunkyo-cho, Matsuyama 790-8577, Japan. Tel./fax: C81 89 927 9789. E-mail address: [email protected] (T. Terasako). 1 Present address: Shizuoka Institute of Science and Technology, Fukuroi 437-8555, Japan.
0022-3697/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2005.09.036
terms of the site occupation of Zn impurities with reference to the reports of Ooe et al. [3,4]. 2. Experimental 2.1. Epitaxial layers Non-site-selectively rather heavily Zn-doped CuGaS2 layers on semi-insulating GaAs(100) substrates were grown by vapor phase epitaxy (VPE) using CuCl, GaCl3 and H2S(10%)CAr sources with metallic Zn, and Ar carrier gas. The growth system is the same as that reported previously [8], except for putting the dopant Zn source boat at upstream part of the CuCl source boat. The substrate temperature was about 500 8C. Feeding ratio of Cu source to summation of Cu and Ga sources, which is denoted hereafter by ‘[Cu]/([Cu]C[Ga])’, was changed from 0.09 to 0.60 by changing GaCl3 source temperature (TGaCl3) from 35 to 77 8C under the fixed CuCl source temperature (TCuCl) of 530 8C. Zn source temperature (TZn) was changed in the range from 280 to 510 8C. Feeding ratio of Zn source to summation of Cu and Ga sources ([Zn]/ ([Cu]C[Ga])) was in the range from 8!10K6 to 0.86. Detail of the growth condition was given in Ref. [9]. All the non-siteselectively Zn doped layers exhibited p-type conduction and their resistivity values were in the range of 20–1.2!104 Ucm. Site-selectively Zn-doped layers were grown based on the vapor phase atomic layer epitaxial (AL-VPE) technique [7,10]. The used substrate was n-GaP(100) and source materials were
T. Terasako et al. / Journal of Physics and Chemistry of Solids 66 (2005) 1982–1986
CuCl, diethylgalliumchloride (DEGaCl), metallic Zn and H2S(10%)CAr. The substrate temperature was 535 or 550 8C. These source materials were supplied under partially separated (that is, simultaneous Cu and Ga supply and H2S supply) or completely separated (that is, all source supplies are separated) alternate feeding condition. The growth system with separate Cu and Ga feeding route configuration was given in Ref. [7,10]. For both cases of completely and partially separated alternate source feeding, Zn was introduced either (a) by placing Zn in Ga supply route, or (b) in Cu supply route, and supplied simultaneously with Ga (i.e. GaCZn) or Cu (i.e. CuCZn). Source temperatures of CuCl and DEGaCl were changed between 500 and 550 8C, and between 26 and 30 8C, respectively. The [Cu]/([Cu]C[Ga]) ratio was changed from 0.744 to 0.855. The source temperature of metallic Zn was 350 or 375 8C. The [Zn]/([Cu]C[Ga]) ratio was in the range from 0.03 to 0.05. These site-selectively Zn doped layers grown by the Cu and Ga supply routes exhibited p- and n-type conductivity, respectively. Resistivity values for p-type layers were in the range from 3.9!102 to 6.7!103 Ucm. On the other hand, resistivity values for the n-type layers grown by completely separated condition and those by partially separated condition were between 1.9!106 and 3.6!109 Ucm and between 2.2 and 3.9!102 Ucm, respectively. Details on the growth system and conditions were given in Refs. [5,6].
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assigned to those observed on the undoped layer and no appearance of new Raman line(s) is seen for the Zn-doped VPE layer. The strongest Raman line assigned to the A1 mode phonon is purely due to vibration of S atoms in the c-plane [12,13]. According to the rigid-ion model calculation of Koshel and Bettini [14], the frequencies of highest E(LO) and B2(LO) mode Raman lines are strongly related to the reduced mass of the III–VI bond [12,15]. Since these highest E(LO) and B2(LO) mode Raman lines cannot be resolved, these Raman modes are denoted hereafter by ‘highest LO mode’. The intensity ratios of the highest LO mode to the A1 mode (denoted hereafter as ILO/ IA1) for the non-site-selectively Zn-doped VPE layers are much larger than that for the undoped VPE layer. When the TGaCl3 decreases from 77 to 54 8C (accordingly the [Cu]/([Cu]C[Ga]) ratio increases from 0.12 to 0.54), the ILO/IA1 ratio increases from 0.12 to 0.54. This tendency of the ILO/IA1 ratio increase with Zn concentration was also found for the Zn-doped bulk crystals [3,4] prepared by the iodine transport method. 3.2. Raman scattering for site-selectively Zn-doped layers Fig. 2(a) shows typical Raman spectra of n- and p-type siteselectively Zn-doped layers grown by the partially separated alternate source feeding mode. One can easily notice that the
2.2. Characterization For Raman scattering measurements, the 514.5 nm or the 488.0 nm radiation from an ArC laser (NEC, GLG3400) was irradiated on the surface of the layers. Collected scattered light was dispersed by a 1-m double-monochromator (JOBIN YVON, U-1000) and detected by a photomultiplier (HAMAMATSU, R943-02) coupled with a photon-counter (HAMAMATSU, C-1230). The Raman scattering measurements were carried out at room temperature and the samples were exposed to the air. Spectral resolution was 1–2 cmK1 and laser output power was 200 mW. The 435.8 nm line of a super high-pressure Hg-lamp was used for steady state PL measurements. The collected luminescence was dispersed with a 1-m monochromator (Nalumi, RM-23) and detected by a photomultiplier (HAMAMATSU, R943-02) with a photon-counter (HAMAMATSU, C767). For 77 K measurements, the samples were immersed in liquid nitrogen. The Zn concentrations were checked using an energy dispersive X-ray (EDX) microanalyzer. The conduction type of all the layers was checked by thermo-power measurement. X-ray diffraction measurements revealed that all the layers are of chalcopyrite phase. 3. Results 3.1. Raman scattering for non-site-selectively Zn-doped layers Fig. 1 shows Raman spectra of an undoped layer and nonsite-selectively Zn-doped VPE layers under different TGaCl3s. Zn concentrations of doped layers were in the range of a few at.%. All Raman lines of the Zn-doped VPE layer can be
Fig. 1. Raman spectra of (a) an undoped VPE layer, and (b)–(e) non-siteselectively Zn-doped VPE layers grown under the fixed source temperatures of CuCl (TCuClZ530 8C) and metallic Zn (TZnZ280 8C) at various GaCl3 source temperatures (TGaCl3s). Values of [Cu]/([Cu]C[Ga]) ratio corresponding to the TGaCl3s are indicated in parentheses. These Raman spectra were taken under irradiation of the 514.5 nm line. Vibration mode assignments for the Raman lines are based on the data reported by Carlone et al. [11].
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Fig. 3. Peak energy and FWHM for non-site-selectively Zn-doped VPE layers vs. GaCl3 source temperature (TGaCl3). The inset shows PL spectra of three layers grown under different growth conditions (top: TZnZ510 8C and TGaCl3Z 54 8C, middle: TZnZ280 8C and TGaCl3Z54 8C, bottom: TZnZ280 8C and TGaCl3Z65 8C). Fig. 2. (a) Typical Raman spectra of n- and p-type site-selectively Zn-doped VPE layers grown by the partially separated alternate source feeding mode. Values of [Cu]/([Cu]C[Ga]) ratio of the n- and p-type layers are 0.828 and 0.855, respectively. These Raman spectra were taken under irradiation of the 488.0 nm line. Asterisks indicate spontaneous emission lines from the ArC laser. (b) Raman intensity ratio of the highest LO mode to the A1 mode (ILO/IA1) for site-selectively Zn-doped VPE layers vs. [Cu]/([Cu]C[Ga]).
ILO/IA1 ratio of the p-type layer is larger than that of the n-type layer. Fig. 2(b) shows ILO/IA1 for the site-selectively Zn-doped layers as a function of [Cu]/([Cu]C[Ga]). EPMA measurements revealed that the Zn concentration in these layers are lower than the detection limit (0.01 at.%) which is approximately two to three orders smaller than the Zn concentrations in the non-site-selectively Zn-doped layers. For the p-type layers, the ILO/IA1 ratio shows an abrupt increase in the range of [Cu]/([Cu]C[Ga])Z0.74–0.78 and increases gradually from higher [Cu]/([Cu]C[Ga]) than 0.79. The ILO/IA1 ratios for the p-type layers are approximately twice larger than those for the n-type layers. 3.3. Relation between Raman and photoluminescence spectra Fig. 3 shows PL peak energy and full-width at halfmaximum (FWHM) value for non-site-selectively Zn-doped
layers as a function of TGaCl3. The inset shows PL spectra of three VPE layers grown under different conditions. For TZnZ 280 8C, when the TGaCl3 increases from 54 to 77 8C, the emission shifts from 2.10 to 2.30 eV and its FWHM decreases from 160 to 70 meV. On the other hand, for TZnZ510 8C, the PL peak energy fluctuates in the range of 2.05–2.11 eV, and the FWHM decreases from 170 to 155 meV as the TGaCl3 increases from 35 to 74 8C. Fig. 4 shows the relation between ILO/IA1 and PL peak energy. For TZnZ280 8C, the ILO/IA1 ratio decreases as the PL peak shifts towards higher energies. On the contrary, no correlation between PL peak energy and Raman intensity has been observed for the layers grown at TZnZ510 8C.
4. Discussion According to Elliot et al., the condition required for impurity mode generation in a binary compound AB by impurity C is given by the following inequality [16]: 1K m O 3ð0ÞK3ðNÞ 0 m 3ð0Þ C 3ðNÞ
(1)
T. Terasako et al. / Journal of Physics and Chemistry of Solids 66 (2005) 1982–1986
Fig. 4. Raman intensity ratio of the highest LO mode to the A1 mode (ILO/IA1) for non-site-selectively Zn-doped VPE layers vs. PL peak energy.
where m and m 0 are reduced masses of AB and AC ion pairs, and 3(0) and 3(N) are static and high frequency dielectric constants of the host crystal AB, respectively. We have already suggested that this inequality is suitable for highest polar E and B2 modes of several I–III–VI2 chalcopyrite compounds [17]. On the assumptions that Ga–S (or Cu–S) and Zn–S bonds correspond to AB and AC ion pairs, respectively, and the difference in force constant between the both bonds can be neglected, we attempted to check whether the impurity mode associated with the ZnGa appears or does not. For the present case, the inequality relation becomes as follows; 1K m Z j1K1:021j Z 0:021! 3ð0ÞK3ðNÞ 0 m 3ð0Þ C 3ðNÞ Z
8:9K6:2 Z 0:178 8:9 C 6:2
(2)
where numerical value of reduced mass ratio m/m 0 of Ga–S to Zn–S ion pairs is roughly estimated to be 1.021, static and highfrequency dielectric constants for E mode of CuGaS2 are cited from Ref. [18]. This is contrary to the condition required for impurity mode generation. Further, we also checked the above approach to the case in which Zn atom substitutes Cu site (ZnCu). The inequality relation (1) was found not to hold for the both cases of ZnGa and ZnCu, indicating that impurity modes associated with these impurities do not appear. This is consistent with the experimental result in which no new line appears on the Raman spectra of both the p-type and n-type Zndoped layers. Instead of the new Raman line appearance associated with doped Zn atoms, the enhancement of ILO/IA1 was observed as mentioned above. Although Raman scattering is considered to be sensitive not only to the impurities but also to intrinsic defects, the Zn concentration dependence of ILO/IA1 described in Section 3.1 strongly suggests that the enhancement of ILO/ IA1 is introduced by the doped Zn atoms rather than the intrinsic defects. Both the decrease of TGaCl3 for the non-site-
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selectively Zn-doped layers and the increase of [Cu]/([Cu]C [Ga]) for the site-selectively Zn-doped layers are considered to cause the increase of ZnGa. Therefore, there is a possibility that the enhancement of ILO/IA1 indicated in Figs. 1 and 2 is induced by the increase of ZnGa. Ooe et al. reported an appearance of a shifting emission denoted as L emission in heavily Zn-doped (0.25–5 at.%) bulk crystals. The reported peak energy and FWHM of the L emission are 2.3–2.1 eV and 121–147 meV, respectively. The L emission is due to the electron transitions from donor levels of sulfur (VS) or their complexes with ZnGa to the ZnGa acceptor levels and its peak shift with increasing Zn concentration can be explained by the broadening of the latter level [4]. As seen in Fig. 3, peak energies for TZnZ280 8C compare fairly with those for the L emissions. In addition, the observed peak shift towards higher energies with increasing TGaCl3 at constant TZnZ280 8C can be consistently explained by the picture of the L emission. We believe that the emissions for TZnZ280 8C are regarded as the L emission. Therefore, the increase of ILO/IA1 with decrease in peak energy of the L emission shown in Fig. 4 suggests the increase of ZnGa contributes to the enhancement of ILO/IA1. For TZnZ510 8C, peak energies and FWHMs of the emissions (denoted by ‘L 0 ’ emission in our previous paper [9]) deviate considerably from those of the L emission. The weak TGaCl3 dependence shown in Fig. 3 implies that the luminescence center for the L 0 emission is rather complicated than that for the L emission. It was reported that the site-selectively Zn-doped layers with p-type conduction and those with the n-type conduction usually exhibit the L emission and the T emission, respectively [6]. The T emission is a usual donor-acceptor pair emission and its donor and acceptor are thought to be ZnCu and Ga vacancy (VGa), respectively [3]. The nonappearance of the L emission and the appearance of the T emission in n-type layers imply that the ZnGas are not major acceptors in the n-type layers. This is consistent with the fact that the ILO/IA1 ratios for the n-type layers are smaller than those for the p-type layers.
5. Conclusion In conclusion, influence of Zn atoms on Raman spectra of the non-site-selectively and site-selectively Zn-doped CuGaS2 VPE layers was investigated. No appearance of new Raman line(s) characteristic to Zn-doped CuGaS2 VPE layer was observed, which is expected from the condition required for impurity mode generation according to the criterion by Elliot et al. However, it was found for the non-site-selectively Zndoped layers that the Raman intensity ratio of the highest LO mode to the A1 mode (ILO/IA1) becomes larger with increasing Zn concentration. Even if the Zn source temperature is fixed, the ILO/IA1 ratio becomes larger with decreasing GaCl3 source temperature. The relation between ILO/IA1 and PL peak energy revealed that the enhancement of ILO/IA1 is due to the increase of Zn atom occupying Ga site (ZnGa), which is acting as an acceptor. In fact, for the site-selectively Zn-doped layers, the ILO/IA1 ratios for the p-type layers tend to be larger than those
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for the n-type layers. This is consistent with high density ZnGa acceptor formation in site-selectively Zn-doped layers. Acknowledgements We would like to thank Mr Y. Nakagaki for some of sample preparations and Raman scattering measurements. References [1] J.L. Shay, J.H. Wernick, Ternary Chalcopyrite Semiconductors: Growth, Electrical Properties, and Applications, Pergamon Press, Oxford, 1975. [2] S. Shirakata, T. Terasako, E. Niwa, K. Masumoto, Photoluminescence of AgGaS2 and CuGaS2 doped with rare-earth impurities, J. Phys. Chem. Solids 64 (2003) 1801–1805. [3] A. Ooe, S. Iida, Role of Zn impurities in CuGaS2 at relatively low concentrations revealed by photoluminescence measurements, Jpn. J. Appl. Phys. 29 (1990) 1484–1489. [4] A. Ooe, N. Tsuboi, S. Iida, Absorption and photoluminescence spectra of heavily Zn-doped CuGaS2 crystals, Jpn. J. Appl. Phys. 30 (1991) 2709–2717. [5] H. Kumakura, S. Iida, Y. Nakagaki, H. Uchiki, T. Matsumoto-Aoki, A. Kato, Observation of n- and p-type conduction of site-selectively Zndoped epitaxial layers of CuGaS2, Jpn. J. Appl. Phys. 39 (2000) 208–209 Suppl. 39-1. [6] S. Iida, H. Ichinokura, Y. Toyama, A. Kato, Photoluminescence of siteselectively Zn-doped CuGaS2/GaP epitaxial layers, J. Phys. Chem. Solids 64 (2003) 2017–2020. [7] S. Iida, N. Tsuboi, Some attempts of atomic layer epitaxy of CuGaS2 and CuAlS2, Ternary and Multinary Compounds in the 21st century, 1, IPAP Books, 2001, pp. 114–118.
[8] A. Yamauchi, H. Saito, H. Kinto, S. Iida, Vapor phase epitaxy of CuGaS2 using metal chloride and H2S sources, J. Cryst. Growth 99 (1990) 752–756. [9] T. Terasako, N. Tsuboi, H. Uchiki, S. Iida, Amphoteric Zn impurities in CuGaS2 epitaxial layers grown by vapor phase epitaxy using metal chlorides and H2S, Cryst. Res. Technol. 31 (1996) S753–S756. [10] J. Mitomo, H. Sato, T. Terasako, T. Matsumoto, H. Uchiki, S. Iida, Vapor phase epitaxy of CuGaS2 Using CuCl, dietylgalliumchloride and H2S sources under simultaneous and alternate feeding conditions, Inst. Phys. Conf. Ser. 152 (1998) 337–340. [11] C. Carlone, D. Olego, A. Jayaraman, M. Cardona, Pressure dependence of the Raman modes and pressure-induced phase changes in CuGaS2 and AgGaS2, Phys. Rev. B 22 (1980) 3877–3885. [12] G.D. Holah, J.S. Webband, H. Mongonery, Lattice dynamics of AgGaS2, J. Phys. C: Solid State Phys. 7 (1974) 3875–3890. [13] M. Bettini, Zone-centered phonons in ternary compounds of chalcopyrite structure, J. Phys. C: Solid State Phys. 72 (1975) 201–212. [14] W.H. Koshel, M. Bettini, Zone-centered phonons in AIBIIIS2 chalcopyrites, J. Phys. C: Solid State Phys. 72 (1975) 720–737. [15] A. Aneda, G. Bongiovanni, F. Rage, E. Fortin, M. Quintero, Raman spectra of CuGaS2KxSex (xZ0, 0.5, 1.0), Il Nuovo Cimento 2 (1983) 1950–1956. [16] R.J. Elliott, J.A. Krumhansl, P.L. Leath, The theory and properties of randomly disordered crystals and related physical systems, Rev. Mod. Phys. 46 (1974) 465–543. [17] T. Terasako, N. Tsuboi, H. Uchiki, S. Iida, Raman scattering and twophonon infrared transmission spectra of Cu(AlxGa1Kx)S2 crystals, Jpn. J. Appl. Phys. 36 (1997) 997–1005. [18] J. Baars, W.H. Koshel, Dielectric dispersion of CuGaS2 by infrared reflectivity analysis, Solid State Commun. 11 (1972) 1513–1517.
Raman scattering of Zn doped CuGaS2 layers grown by vapor phase epitaxy T. Terasako a,b,*, S. Iida b,1, H. Ichinokura b, A. Kato b a
Faculty of Engineering, Ehime University, 3 Bunkyo-cho, Matsuyama 790-8577, Japan b Nagaoka University of Technology, Kamitomioka, Nagaoka 940-2188, Japan
Abstract Raman spectra for non-site-selectively and site-selectively Zn-doped CuGaS2 layers grown by vapor phase epitaxy (VPE) were investigated. Although an appearance of characteristic Raman line(s) related with the doped Zn atom was not seen, an enhancement of the Raman intensity ratio of the highest LO mode to the A1 mode (ILO/IA1) was observed. The site-selectively Zn-doped layers with p-type conductivity exhibited larger ILO/IA1 ratio compared to those with n-type conductivity. The observed correlation between the ILO/IA1 ratio and the peak energy of the photoluminescence characteristic for Zn-doped p-type samples (L emission) suggests that the enhancement of ILO/IA1 is due to the increase of Zn atom substituting Ga site (ZnGa) which is acting as an acceptor. q 2005 Elsevier Ltd. All rights reserved. Keywords: A. Semiconductors; B. Epitaxial growth; C. Raman spectroscopy; D. Luminescence
1. Introduction The I–III–VI2 chalcopyrite semiconductor CuGaS2 having a direct energy gap of 2.49 eV at room temperature is a possible candidate for new functionality devices reflecting characteristic nature of the ternary compound [1,2]. For this kind of application, the first prerequisite is to establish epitaxial growth technique, which includes conductivity control by impurity doping. Ooe et al. found experimental evidence for the first time that Zn is really an amphoteric impurity [3,4] based on detailed studies of photoluminescence (PL) involving timeresolved (TR) spectra during decay in (CuGaS2)1Kx–(2ZnS)x (xZ0.00025K0.05) bulk crystals. Recently conductivity control has been achieved for Zn doped CuGaS2 layers on GaP substrates grown by vapor phase epitaxy based on atomic layer growth technique (AL-VPE) [5–7]. This paper reports Raman scattering spectra observed for non-site-selectively and site-selectively Zn doped CuGaS2 layers. Relation between Raman and PL spectra is discussed in
* Corresponding author. Address: Faculty of Engineering, Ehime University, 3 Bunkyo-cho, Matsuyama 790-8577, Japan. Tel./fax: C81 89 927 9789. E-mail address: [email protected] (T. Terasako). 1 Present address: Shizuoka Institute of Science and Technology, Fukuroi 437-8555, Japan.
0022-3697/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2005.09.036
terms of the site occupation of Zn impurities with reference to the reports of Ooe et al. [3,4]. 2. Experimental 2.1. Epitaxial layers Non-site-selectively rather heavily Zn-doped CuGaS2 layers on semi-insulating GaAs(100) substrates were grown by vapor phase epitaxy (VPE) using CuCl, GaCl3 and H2S(10%)CAr sources with metallic Zn, and Ar carrier gas. The growth system is the same as that reported previously [8], except for putting the dopant Zn source boat at upstream part of the CuCl source boat. The substrate temperature was about 500 8C. Feeding ratio of Cu source to summation of Cu and Ga sources, which is denoted hereafter by ‘[Cu]/([Cu]C[Ga])’, was changed from 0.09 to 0.60 by changing GaCl3 source temperature (TGaCl3) from 35 to 77 8C under the fixed CuCl source temperature (TCuCl) of 530 8C. Zn source temperature (TZn) was changed in the range from 280 to 510 8C. Feeding ratio of Zn source to summation of Cu and Ga sources ([Zn]/ ([Cu]C[Ga])) was in the range from 8!10K6 to 0.86. Detail of the growth condition was given in Ref. [9]. All the non-siteselectively Zn doped layers exhibited p-type conduction and their resistivity values were in the range of 20–1.2!104 Ucm. Site-selectively Zn-doped layers were grown based on the vapor phase atomic layer epitaxial (AL-VPE) technique [7,10]. The used substrate was n-GaP(100) and source materials were
T. Terasako et al. / Journal of Physics and Chemistry of Solids 66 (2005) 1982–1986
CuCl, diethylgalliumchloride (DEGaCl), metallic Zn and H2S(10%)CAr. The substrate temperature was 535 or 550 8C. These source materials were supplied under partially separated (that is, simultaneous Cu and Ga supply and H2S supply) or completely separated (that is, all source supplies are separated) alternate feeding condition. The growth system with separate Cu and Ga feeding route configuration was given in Ref. [7,10]. For both cases of completely and partially separated alternate source feeding, Zn was introduced either (a) by placing Zn in Ga supply route, or (b) in Cu supply route, and supplied simultaneously with Ga (i.e. GaCZn) or Cu (i.e. CuCZn). Source temperatures of CuCl and DEGaCl were changed between 500 and 550 8C, and between 26 and 30 8C, respectively. The [Cu]/([Cu]C[Ga]) ratio was changed from 0.744 to 0.855. The source temperature of metallic Zn was 350 or 375 8C. The [Zn]/([Cu]C[Ga]) ratio was in the range from 0.03 to 0.05. These site-selectively Zn doped layers grown by the Cu and Ga supply routes exhibited p- and n-type conductivity, respectively. Resistivity values for p-type layers were in the range from 3.9!102 to 6.7!103 Ucm. On the other hand, resistivity values for the n-type layers grown by completely separated condition and those by partially separated condition were between 1.9!106 and 3.6!109 Ucm and between 2.2 and 3.9!102 Ucm, respectively. Details on the growth system and conditions were given in Refs. [5,6].
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assigned to those observed on the undoped layer and no appearance of new Raman line(s) is seen for the Zn-doped VPE layer. The strongest Raman line assigned to the A1 mode phonon is purely due to vibration of S atoms in the c-plane [12,13]. According to the rigid-ion model calculation of Koshel and Bettini [14], the frequencies of highest E(LO) and B2(LO) mode Raman lines are strongly related to the reduced mass of the III–VI bond [12,15]. Since these highest E(LO) and B2(LO) mode Raman lines cannot be resolved, these Raman modes are denoted hereafter by ‘highest LO mode’. The intensity ratios of the highest LO mode to the A1 mode (denoted hereafter as ILO/ IA1) for the non-site-selectively Zn-doped VPE layers are much larger than that for the undoped VPE layer. When the TGaCl3 decreases from 77 to 54 8C (accordingly the [Cu]/([Cu]C[Ga]) ratio increases from 0.12 to 0.54), the ILO/IA1 ratio increases from 0.12 to 0.54. This tendency of the ILO/IA1 ratio increase with Zn concentration was also found for the Zn-doped bulk crystals [3,4] prepared by the iodine transport method. 3.2. Raman scattering for site-selectively Zn-doped layers Fig. 2(a) shows typical Raman spectra of n- and p-type siteselectively Zn-doped layers grown by the partially separated alternate source feeding mode. One can easily notice that the
2.2. Characterization For Raman scattering measurements, the 514.5 nm or the 488.0 nm radiation from an ArC laser (NEC, GLG3400) was irradiated on the surface of the layers. Collected scattered light was dispersed by a 1-m double-monochromator (JOBIN YVON, U-1000) and detected by a photomultiplier (HAMAMATSU, R943-02) coupled with a photon-counter (HAMAMATSU, C-1230). The Raman scattering measurements were carried out at room temperature and the samples were exposed to the air. Spectral resolution was 1–2 cmK1 and laser output power was 200 mW. The 435.8 nm line of a super high-pressure Hg-lamp was used for steady state PL measurements. The collected luminescence was dispersed with a 1-m monochromator (Nalumi, RM-23) and detected by a photomultiplier (HAMAMATSU, R943-02) with a photon-counter (HAMAMATSU, C767). For 77 K measurements, the samples were immersed in liquid nitrogen. The Zn concentrations were checked using an energy dispersive X-ray (EDX) microanalyzer. The conduction type of all the layers was checked by thermo-power measurement. X-ray diffraction measurements revealed that all the layers are of chalcopyrite phase. 3. Results 3.1. Raman scattering for non-site-selectively Zn-doped layers Fig. 1 shows Raman spectra of an undoped layer and nonsite-selectively Zn-doped VPE layers under different TGaCl3s. Zn concentrations of doped layers were in the range of a few at.%. All Raman lines of the Zn-doped VPE layer can be
Fig. 1. Raman spectra of (a) an undoped VPE layer, and (b)–(e) non-siteselectively Zn-doped VPE layers grown under the fixed source temperatures of CuCl (TCuClZ530 8C) and metallic Zn (TZnZ280 8C) at various GaCl3 source temperatures (TGaCl3s). Values of [Cu]/([Cu]C[Ga]) ratio corresponding to the TGaCl3s are indicated in parentheses. These Raman spectra were taken under irradiation of the 514.5 nm line. Vibration mode assignments for the Raman lines are based on the data reported by Carlone et al. [11].
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Fig. 3. Peak energy and FWHM for non-site-selectively Zn-doped VPE layers vs. GaCl3 source temperature (TGaCl3). The inset shows PL spectra of three layers grown under different growth conditions (top: TZnZ510 8C and TGaCl3Z 54 8C, middle: TZnZ280 8C and TGaCl3Z54 8C, bottom: TZnZ280 8C and TGaCl3Z65 8C). Fig. 2. (a) Typical Raman spectra of n- and p-type site-selectively Zn-doped VPE layers grown by the partially separated alternate source feeding mode. Values of [Cu]/([Cu]C[Ga]) ratio of the n- and p-type layers are 0.828 and 0.855, respectively. These Raman spectra were taken under irradiation of the 488.0 nm line. Asterisks indicate spontaneous emission lines from the ArC laser. (b) Raman intensity ratio of the highest LO mode to the A1 mode (ILO/IA1) for site-selectively Zn-doped VPE layers vs. [Cu]/([Cu]C[Ga]).
ILO/IA1 ratio of the p-type layer is larger than that of the n-type layer. Fig. 2(b) shows ILO/IA1 for the site-selectively Zn-doped layers as a function of [Cu]/([Cu]C[Ga]). EPMA measurements revealed that the Zn concentration in these layers are lower than the detection limit (0.01 at.%) which is approximately two to three orders smaller than the Zn concentrations in the non-site-selectively Zn-doped layers. For the p-type layers, the ILO/IA1 ratio shows an abrupt increase in the range of [Cu]/([Cu]C[Ga])Z0.74–0.78 and increases gradually from higher [Cu]/([Cu]C[Ga]) than 0.79. The ILO/IA1 ratios for the p-type layers are approximately twice larger than those for the n-type layers. 3.3. Relation between Raman and photoluminescence spectra Fig. 3 shows PL peak energy and full-width at halfmaximum (FWHM) value for non-site-selectively Zn-doped
layers as a function of TGaCl3. The inset shows PL spectra of three VPE layers grown under different conditions. For TZnZ 280 8C, when the TGaCl3 increases from 54 to 77 8C, the emission shifts from 2.10 to 2.30 eV and its FWHM decreases from 160 to 70 meV. On the other hand, for TZnZ510 8C, the PL peak energy fluctuates in the range of 2.05–2.11 eV, and the FWHM decreases from 170 to 155 meV as the TGaCl3 increases from 35 to 74 8C. Fig. 4 shows the relation between ILO/IA1 and PL peak energy. For TZnZ280 8C, the ILO/IA1 ratio decreases as the PL peak shifts towards higher energies. On the contrary, no correlation between PL peak energy and Raman intensity has been observed for the layers grown at TZnZ510 8C.
4. Discussion According to Elliot et al., the condition required for impurity mode generation in a binary compound AB by impurity C is given by the following inequality [16]: 1K m O 3ð0ÞK3ðNÞ 0 m 3ð0Þ C 3ðNÞ
(1)
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Fig. 4. Raman intensity ratio of the highest LO mode to the A1 mode (ILO/IA1) for non-site-selectively Zn-doped VPE layers vs. PL peak energy.
where m and m 0 are reduced masses of AB and AC ion pairs, and 3(0) and 3(N) are static and high frequency dielectric constants of the host crystal AB, respectively. We have already suggested that this inequality is suitable for highest polar E and B2 modes of several I–III–VI2 chalcopyrite compounds [17]. On the assumptions that Ga–S (or Cu–S) and Zn–S bonds correspond to AB and AC ion pairs, respectively, and the difference in force constant between the both bonds can be neglected, we attempted to check whether the impurity mode associated with the ZnGa appears or does not. For the present case, the inequality relation becomes as follows; 1K m Z j1K1:021j Z 0:021! 3ð0ÞK3ðNÞ 0 m 3ð0Þ C 3ðNÞ Z
8:9K6:2 Z 0:178 8:9 C 6:2
(2)
where numerical value of reduced mass ratio m/m 0 of Ga–S to Zn–S ion pairs is roughly estimated to be 1.021, static and highfrequency dielectric constants for E mode of CuGaS2 are cited from Ref. [18]. This is contrary to the condition required for impurity mode generation. Further, we also checked the above approach to the case in which Zn atom substitutes Cu site (ZnCu). The inequality relation (1) was found not to hold for the both cases of ZnGa and ZnCu, indicating that impurity modes associated with these impurities do not appear. This is consistent with the experimental result in which no new line appears on the Raman spectra of both the p-type and n-type Zndoped layers. Instead of the new Raman line appearance associated with doped Zn atoms, the enhancement of ILO/IA1 was observed as mentioned above. Although Raman scattering is considered to be sensitive not only to the impurities but also to intrinsic defects, the Zn concentration dependence of ILO/IA1 described in Section 3.1 strongly suggests that the enhancement of ILO/ IA1 is introduced by the doped Zn atoms rather than the intrinsic defects. Both the decrease of TGaCl3 for the non-site-
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selectively Zn-doped layers and the increase of [Cu]/([Cu]C [Ga]) for the site-selectively Zn-doped layers are considered to cause the increase of ZnGa. Therefore, there is a possibility that the enhancement of ILO/IA1 indicated in Figs. 1 and 2 is induced by the increase of ZnGa. Ooe et al. reported an appearance of a shifting emission denoted as L emission in heavily Zn-doped (0.25–5 at.%) bulk crystals. The reported peak energy and FWHM of the L emission are 2.3–2.1 eV and 121–147 meV, respectively. The L emission is due to the electron transitions from donor levels of sulfur (VS) or their complexes with ZnGa to the ZnGa acceptor levels and its peak shift with increasing Zn concentration can be explained by the broadening of the latter level [4]. As seen in Fig. 3, peak energies for TZnZ280 8C compare fairly with those for the L emissions. In addition, the observed peak shift towards higher energies with increasing TGaCl3 at constant TZnZ280 8C can be consistently explained by the picture of the L emission. We believe that the emissions for TZnZ280 8C are regarded as the L emission. Therefore, the increase of ILO/IA1 with decrease in peak energy of the L emission shown in Fig. 4 suggests the increase of ZnGa contributes to the enhancement of ILO/IA1. For TZnZ510 8C, peak energies and FWHMs of the emissions (denoted by ‘L 0 ’ emission in our previous paper [9]) deviate considerably from those of the L emission. The weak TGaCl3 dependence shown in Fig. 3 implies that the luminescence center for the L 0 emission is rather complicated than that for the L emission. It was reported that the site-selectively Zn-doped layers with p-type conduction and those with the n-type conduction usually exhibit the L emission and the T emission, respectively [6]. The T emission is a usual donor-acceptor pair emission and its donor and acceptor are thought to be ZnCu and Ga vacancy (VGa), respectively [3]. The nonappearance of the L emission and the appearance of the T emission in n-type layers imply that the ZnGas are not major acceptors in the n-type layers. This is consistent with the fact that the ILO/IA1 ratios for the n-type layers are smaller than those for the p-type layers.
5. Conclusion In conclusion, influence of Zn atoms on Raman spectra of the non-site-selectively and site-selectively Zn-doped CuGaS2 VPE layers was investigated. No appearance of new Raman line(s) characteristic to Zn-doped CuGaS2 VPE layer was observed, which is expected from the condition required for impurity mode generation according to the criterion by Elliot et al. However, it was found for the non-site-selectively Zndoped layers that the Raman intensity ratio of the highest LO mode to the A1 mode (ILO/IA1) becomes larger with increasing Zn concentration. Even if the Zn source temperature is fixed, the ILO/IA1 ratio becomes larger with decreasing GaCl3 source temperature. The relation between ILO/IA1 and PL peak energy revealed that the enhancement of ILO/IA1 is due to the increase of Zn atom occupying Ga site (ZnGa), which is acting as an acceptor. In fact, for the site-selectively Zn-doped layers, the ILO/IA1 ratios for the p-type layers tend to be larger than those
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for the n-type layers. This is consistent with high density ZnGa acceptor formation in site-selectively Zn-doped layers. Acknowledgements We would like to thank Mr Y. Nakagaki for some of sample preparations and Raman scattering measurements. References [1] J.L. Shay, J.H. Wernick, Ternary Chalcopyrite Semiconductors: Growth, Electrical Properties, and Applications, Pergamon Press, Oxford, 1975. [2] S. Shirakata, T. Terasako, E. Niwa, K. Masumoto, Photoluminescence of AgGaS2 and CuGaS2 doped with rare-earth impurities, J. Phys. Chem. Solids 64 (2003) 1801–1805. [3] A. Ooe, S. Iida, Role of Zn impurities in CuGaS2 at relatively low concentrations revealed by photoluminescence measurements, Jpn. J. Appl. Phys. 29 (1990) 1484–1489. [4] A. Ooe, N. Tsuboi, S. Iida, Absorption and photoluminescence spectra of heavily Zn-doped CuGaS2 crystals, Jpn. J. Appl. Phys. 30 (1991) 2709–2717. [5] H. Kumakura, S. Iida, Y. Nakagaki, H. Uchiki, T. Matsumoto-Aoki, A. Kato, Observation of n- and p-type conduction of site-selectively Zndoped epitaxial layers of CuGaS2, Jpn. J. Appl. Phys. 39 (2000) 208–209 Suppl. 39-1. [6] S. Iida, H. Ichinokura, Y. Toyama, A. Kato, Photoluminescence of siteselectively Zn-doped CuGaS2/GaP epitaxial layers, J. Phys. Chem. Solids 64 (2003) 2017–2020. [7] S. Iida, N. Tsuboi, Some attempts of atomic layer epitaxy of CuGaS2 and CuAlS2, Ternary and Multinary Compounds in the 21st century, 1, IPAP Books, 2001, pp. 114–118.
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