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Formation of Zn-doped CuInSe2 films by thermal annealing using dimethylzinc
ARTICLE IN PRESS
Journal of Crystal Growth 310 (2008) 794–797 www.elsevier.com/locate/jcrysgro
Formation of Zn-doped CuInSe2 films by thermal annealing using dimethylzinc M. Sugiyamaa,, A. Kinoshitaa, A. Miyamaa, H. Nakanishia, S.F. Chichibub a
b
Department of Electrical Engineering, Tokyo University of Science, 2641 Yamazaki, Noda 278-8510, Japan Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba, Sendai 980-8577, Japan Received 10 August 2007; received in revised form 17 November 2007; accepted 28 November 2007 Communicated by D.W. Shaw Available online 3 December 2007
Abstract Copper indium diselenide (CuInSe2:CIS) pn-homojunction diodes were fabricated by the thermal diffusion of Zn into the p-type CIS films at 300 1C for 5 min using a dimethylzinc [(CH3)2Zn:DMZn] vapor. This method does not require any additional processing equipment since the diffusion can be carried out subsequent to the selenization of a Cu–In precursor using organoselenium liquid, such as diethylselenide [(C2H5)2Se:DESe]. A donor-to-acceptor pair emission attributable to Zn impurity was observed in the low-temperature photoluminescence spectrum. From the capacitance–voltage characteristics, the depletion layer width and diffusion potential of the junction were estimated as 300 nm and 0.6–0.7 V, respectively. The method is highly advantageous for the development of low-cost solar modules. r 2007 Elsevier B.V. All rights reserved. PACS: 73.20.At; 73.30.+y; 73.40.Lq Keywords: A1. Growth models; A3. Physical vapor deposition processes; B2. Semiconducting ternary compounds
1. Introduction Chalcopyrite-structure Cu(In,Ga)(S,Se)2 alloys [1] have attracted attention as a promising candidate for the lightabsorbing medium of high conversion efficiency (Z) lowcost solar cells. In fact, polycrystalline CuIn1–xGaxSe2 (CIGS)-based solar modules with Zffi13% have been put into practical use. They generally consist of a layered structure such as ZnO/CdS/CIGS/Mo on a soda–lime– glass (SLG) substrate. The CdS buffer layer on the CIGS layer is usually deposited by the chemical bath deposition (CBD) method, and is considered to play important roles in preventing the CIGS layer from damaging during the sputtering deposition of the ZnO layer and in forming a pn-homojunction on the CIGS surface due to the Cd diffusion [2,3]. However, the use of CdS involves handling
Corresponding author.
E-mail address: [email protected] (M. Sugiyama). 0022-0248/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2007.11.172
difficulties and environmental concerns since Cd is highly toxic and CdS is deposited by complex wet processes. The use of CdS can be avoided by fabricating a pn junction through thermal diffusion of a donor impurity into the p-type CIGS layer. Zn is a suitable element for the donor impurity because Zn is safer than Cd. In addition, Zn-doped CIGS layers can be formed by simple dry processes such as chemical vapor deposition, thermal evaporation, and thermal diffusion. However, few experimental results have been reported for the fabrication of pn junction using Zn-doped CuInSe2 (CIS) bulk crystals [4–8] or CIGS films [9,10]. The authors have proposed the use of diethylselenide [(C2H5)2Se:DESe] for the selenization growth of CIS and CIGS films [11–14]. Since DESe is a liquid at room temperature and is stored under atmospheric pressure in a stainless-steel bubbler, the risk of a widespread leakage is less likely than that of the case of H2Se. Similarly, dimethylzinc [(CH3)2Zn:DMZn] is an attractive organozinc liquid as the diffusion source for the Zn-doped CIGS layers
ARTICLE IN PRESS M. Sugiyama et al. / Journal of Crystal Growth 310 (2008) 794–797
3. Results and discussion Single-phase polycrystalline CIS films with a thickness of approximately 2.0 mm were successfully formed by selenization using DESe. The films were composed of densely packed, 1–3-mm-diameter columnar grains. The films adhered well to the Mo/SLG substrate, which was confirmed by the peeling test. Before determining the optimized temperature and time for the fabrication of pn junctions by the thermal diffusion of Zn, critical effects of the use of DMZn was first examined: thermal annealing was carried out for up to 60 min to obtain Zn-doped thick CIS. The annealing temperature was varied between 300 and 500 1C, which was limited by the decomposition temperature of DMZn and
ZnIn2Se4
CIS
(116, 312)
Cu16In9
(220, 204)
CuInSe2 Zn compounds Mo Cu16In9
Sequentially stacked 20-period Cu/In layers with a total thickness of approximately 670 nm were used as a precursor. The precursors were evaporated using two K-cells on unintentionally heated Mo-coated SLG substrates. The gross thickness of the precursor was controlled by in situ monitoring of the thickness of each layer with a quartz crystal oscillator. The precursors were selenized using DESe at 515 1C for 90 min under atmospheric pressure. The flow rates of DESe and N2 carrier gases were 35 mmol/min and 2 L/min, respectively. Details of the evaporation and selenization equipments have been described in our previous reports [12–14]. Subsequent to the selenization growth, the CIS films were annealed at 300–500 1C for 3–60 min under the DMZn stream in the same reactor. The flow rates of DMZn and N2 carrier gases were 60 mmol/min and 2 L/min, respectively. X-ray diffraction (XRD) measurement was carried out for evaluating the lattice constants and crystal phases. It was also used to examine whether the film exhibited phase separation. Electron probe microanalysis (EPMA) was carried out to evaluate the solid concentrations of the constituent atoms. The depth profile of Zn in the CIS films was measured using glow discharge optical emission spectroscopy (GDOES). Photoluminescence (PL) was excited by the 532.0 nm line of a frequency-doubled quasi-cw Nd:YAG laser (60 mW) and dispersed by a 50cm-focal-length grating monochromator. The phase-sensitive detection was carried out using a liquid-N2-cooled Ge photodetector. The current–voltage (I–V) and capacitance– voltage (C–V) measurements were carried out on the CIS pn junction.
(112)
2. Experimental procedure
the softening temperature of an SLG substrate. XRD patterns of the Zn-annealed CIS films are shown as a function of annealing temperature in Fig. 1(a). To examine whether extra phases are formed in the films, the signals near the (1 1 2) diffraction angle are magnified in Fig. 1(b). The films annealed between 300 and 350 1C exhibited the XRD peaks due to single-phase CIS solid solutions. We note that these films exhibited low-resistivity n-type conductivity, which was confirmed by the resistivity measurement and the hot-point probe analysis. Conversely, the films annealed between 400 and 500 1C exhibited XRD peaks due to Cu16In9 and Zn compounds such as ZnSe and ZnIn2Se4. It should be noted that extra phases were not found in the CIS films annealed at 500 1C for 60 min without DMZn flow (data not shown). Therefore, hightemperature annealing in a DMZn vapor had undesirable effects on the CIS films: Cu16In9 compounds might be formed due to the thermal decomposition of CIS films and the decomposed Se and In might react with Zn. It should be noted that Zn concentration in the CIS film annealed at 300 1C for 60 min was measured by EPMA to be approximately 3%, which was far higher than the appropriate doping level for fabricating pn junctions. Therefore, the annealing time in DMZn flow was varied between 3 and 5 min to obtain a Zn-doped layer, since the diffusion depth of Zn was estimated to be 400 and 550 nm for the diffusion times of 3 and 5 min, respectively, according to the experimental report on the thermal diffusion depth of Zn into CIGS films [9]. The depth profile of Zn in the CIS film annealed at 300 1C for 5 min is shown in Fig. 2. As shown, weak Zn signal is seen to spread deeply into CIS. However, this is caused by the irregular surface of the polycrystalline
XRD INTENSITY [arb. units]
by thermal annealing. This method does not require any additional processing equipment since the diffusion can be carried out subsequent to the selenization growth of CIGS. This is highly advantageous for the development of the low-cost solar modules. In this article, both the industrial and physical advantages of using a DMZn source for the formation of a Zn-doped CIS layer are shown.
795
ZnSe
500 °C 450 °C 400 °C 350 °C 300 °C as grown 30
40
50
26 27
2θ [deg.] Fig. 1. (a) XRD patterns of Zn-annealed CIS films as a function of annealing temperature and (b) the signals near the (1 1 2) diffraction peak angle are magnified to show the extra phases.
ARTICLE IN PRESS M. Sugiyama et al. / Journal of Crystal Growth 310 (2008) 794–797
796
18K P3 GDOES INTENSITY [arb. units]
CuInSe2
P3 P2
CuInSe2 P2
P1 18K
P1
100%
3min anneal
PL INTENSITY [arb. units]
5min anneal
PL INTENSITY [arb. units]
SLG
Zn-diffused layer
5min. anneal
0
500
1000
1500
Wood’s anomaly
CIS layer 2000
THICKNESS [nm]
as grown
Fig. 2. Depth profile of Zn in the CIS film annealed at 300 1C for 5 min.
0.85
CIS, which gives rise to inhomogeneous sputtering during the GDOES measurement. Therefore, the Zn diffusion depth is estimated to be 400–500 nm, and a linearly graded junction is formed at the diffusion front. The value of diffusion depth is approximately the same as the estimated value: the Zn diffusion depth can be controlled by varying the annealing time. Low-temperature PL spectra of the undoped and Zndoped CIS films are shown in Fig. 3(a) as a function of annealing time. The annealing temperature was maintained at 300 1C. PL peaks labeled P1, P2, and P3 in the order of high to low energy were observed. We note that the spectral discontinuity at 0.88 eV is due to Wood’s anomaly of the diffraction grating. Since P1 and P2 are observed in both Zn-doped and undoped films, they may originate from intrinsic defect levels of CIS. Conversely, P3 is considered to originate from Zn-related impurity levels, because the peak was observed only in the Zn-doped films. We should mention that deep-level emissions attributable to carbon contamination were not observed; the result was similar to that of the CIS epilayers grown by metalorganic vapor phase epitaxy using DESe as a Se source [15]. Low-temperature PL spectra of the Zn-doped CIS film annealed at 300 1C for 5 min are shown in Fig. 3(b) as a function of relative excitation power. Peaks P1 and P3 are attributed to independent donor–acceptor pair (DAP) transitions, because the peaks shifted to the higher energy [16] as the excitation power increased. Conversely, the peak energy of P2 was independent of the excitation power. Considering the defect and impurity levels in CIS [17], P1 is assigned to the transition from the Se-vacancy level to the Cu-vacancy level (VSe, VCu). Similarly, P2 is assigned to the transition of electrons in the conduction band to the Se at the In-site level (e, SeIn) and/or the one in the In at the Se-site level to the valance band (InSe, h). The donor or the acceptor responsible for P3 emission has not been identified
0.90
0.95
0.1%
1.00 0.85 0.90 PHOTON ENERGY [eV]
5min anneal 0.95
1.00
Fig. 3. (a) Low-temperature PL spectra of the Zn-doped and undoped CIS films as a function of annealing time and (b) low-temperature PL spectra, as a function of excitation power of the Zn-doped CIS film, which was annealed at 300 1C for 5 min.
yet: the number of reports on the defect and impurity levels of Zn-doped CIS is limited. Applying the analytical method proposed by Zacks and Halperin [16], the ionization energies of the shallower and deeper levels are estimated to be 30 and 70 meV, respectively. Considering the defect and impurity levels of Zn-doped CuGaS2 [18,19], Zn at the Cu-Site (ZnCu) is considered to form the deeper level. However, further studies are required to clarify the origins of Zn-related DAP emission. The surface region of Zn-doped CIS films typically exhibited n-type conductivity in the hot-point probe analysis. The result implied the formation of a CIS pn junction. Although the photovoltaic effect was so weak that it was difficult to calculate the conversion efficiency, the I–V characteristics of a Zn-doped CIS/Mo/SLG structure revealed rectifying properties, as shown in Fig. 4(a). A representative 1/C3–V curve of a Zn-doped CIS/Mo/SLG structure is shown in Fig. 4(b). It is known that inverse cube of the depletion layer capacitance (1/C3) changes linearly with the applied voltage in a linearly graded junction [20]. Therefore, the result might imply that the formation of the homojunction is attributable to the thermal diffusion of Zn into the CIS film. From the linear extrapolation of the data between the reverse voltage of 0 and 2 V, the diffusion potential (VD) is estimated to be 0.6–0.7 V. By means of the C–V depth profiling technique [20,21], the depletion layer width of the Zn-doped layer is estimated to be 300 nm. According to the result, band alignment of the Zn-diffused CIS is proposed in the inset of
ARTICLE IN PRESS M. Sugiyama et al. / Journal of Crystal Growth 310 (2008) 794–797
was observed in the low-temperature PL spectra. From the capacitance–voltage characteristics, the depletion layer width and the diffusion potential of the pn junction were estimated as 300 nm and 0.6–0.7 V, respectively. These results indicate that DMZn is a suitable doping source for the thermal diffusion process in fabricating CIS pn junctions for low-cost CIGS-based solar cells.
CURRENT [mA]
2.0 1.5 1.0 0.5 0
Acknowledgments
-1
0
1
2
VOLTAGE [V]
8
VD = 0.6-0.7eV
Zn diffusion:400-500nm CBM p-CIS Fermi level
6
VBM
1/C3 [ x1034 F-3]
depletion layer: 300nm
4
2
CuInSe2/Mo 5min anneal -2
-1
VD 0
1
The authors would like to thank A. Umezawa, T. Yasuniwa, M. Soda, M. Abe, T. Kato, and Dr. F. B. Dejene for their help in the experiments. They also thank T. Mitani at the Central Service Facilities for Research, Keio University, for stimulating discussions and continuous encouragement. References
n-CIS
0
797
2
VOLTAGE [V] Fig. 4. Representative (a) I–V and (b) 1/C3–V curves of the Zn-doped CIS/Mo/SLG, which is a component of a solar cell structure. The inset of (b) shows the typical band alignment of the Zn-diffused CIS film.
Fig. 4(b). It should be noted that band alignment of a homojunction is dominated by both the intrinsic properties of Zn-diffused CIS and extrinsic properties such as interface defects and/or grain boundaries. Therefore, there are certain difficulties in drawing the band alignment. However, both the experimental diffusion depth of Zn atoms (400–500 nm) and depletion layer width (300 nm), which were estimated by GDOES and C–V measurements, respectively, nearly agreed with the designed values. 4. Conclusions Vapor-phase Zn doping was carried out by the thermal diffusion of Zn into p-type CuInSe2 films at 300 1C for 3–5 min using a DMZn vapor. Zn-related DAP emission
[1] J.L. Shay, J.H. Wernick, Ternary Chalcopyrite Semiconductors: Growth, Electronic Properties, and Applications, Pergamon, Oxford, 1975. [2] T. Nakada, A. Kunioka, Appl. Phys. Lett. 74 (1999) 2444. [3] T. Nakada, Thin Solid Films 361–362 (2000) 346. [4] P. Migliorato, J.L. Shay, H.M. Kasper, S. Wagner, J. Appl. Phys. 46 (1975) 1777. [5] P.W. Yu, Y.S. Park, J.T. Grant, Appl. Phys. Lett. 28 (1976) 214. [6] B. Tell, S. Wagner, P.M. Bridenbaugh, Appl. Phys. Lett. 28 (1976) 454. [7] B. Tell, P.M. Bridenbaugh, J. Appl. Phys. 48 (1977) 2477. [8] M. Benabdeslem, N. Benslim, L. Bechiri, L. Mahdjoubi, E.B. Hannech, G. Nouet, J. Crystal Growth 274 (2005) 144. [9] S. Nishiwaki, T. Satoh, Y. Hashimoto, S. Shimakawa, S. Hayashi, T. Negami, T. Wada, Sol. Energy Mater. Sol. Cells 77 (2003) 359. [10] T. Sugiyama, S. Chaisitsak, A. Yamada, M. Konagai, Y. Kudriavtsev, A. Godines, A. Villegas, R. Asomoza, Jpn. J. Appl. Phys. 39 (2000) 4816. [11] S.F. Chichibu, M. Sugiyama, M. Ohbasami, A. Hayakawa, T. Mizutani, H. Nakanishi, T. Negami, T. Wada, J. Crystal Growth 243 (2002) 404. [12] T. Yamamoto, M. Nakamura, J. Ishizuki, T. Deguchi, S. Ando, H. Nakanishi, Sf. Chichibu, J. Phys. Chem. Solids 64 (2003) 1855. [13] M. Sugiyama, F.B. Dejene, A. Kinoshita, M. Fukaya, Y. Maru, T. Nakagawa, H. Nakanishi, V. Alberts, S.F. Chichibu, J. Crystal Growth 294 (2006) 214. [14] M. Sugiyama, A. Kinoshita, M. Fukaya, H. Nakanishi, S.F. Chichibu, Thin Solid Films 515 (2007) 5867. [15] S. Chichibu, Appl. Phys. Lett. 70 (1997) 1840. [16] E. Zacks, A. Halperin, Phys. Rev. B 6 (1972) 3072. [17] L.L. Kazmerski, Jpn. J. Appl. Phys. Suppl. 32-3 (1993) 25. [18] S. Shirakata, S. Chichibu, S. Matsumoto, S. Isomura, Jpn. J. Appl. Phys. 33 (1994) 345. [19] S. Chichibu, S. Shirakata, S. Isomura, Y. Harada, M. Uchida, S. Matsumoto, H. Higuchi, J. Appl. Phys. 77 (1995) 1225. [20] S.M. Sze, Physics of Semiconductor Devices, Wiley, New York, 1981. [21] C.P. Wu, E.C. Douglas, C.W. Mueller, IEEE Trans. Electron Devices 22 (1975) 319.
Journal of Crystal Growth 310 (2008) 794–797 www.elsevier.com/locate/jcrysgro
Formation of Zn-doped CuInSe2 films by thermal annealing using dimethylzinc M. Sugiyamaa,, A. Kinoshitaa, A. Miyamaa, H. Nakanishia, S.F. Chichibub a
b
Department of Electrical Engineering, Tokyo University of Science, 2641 Yamazaki, Noda 278-8510, Japan Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba, Sendai 980-8577, Japan Received 10 August 2007; received in revised form 17 November 2007; accepted 28 November 2007 Communicated by D.W. Shaw Available online 3 December 2007
Abstract Copper indium diselenide (CuInSe2:CIS) pn-homojunction diodes were fabricated by the thermal diffusion of Zn into the p-type CIS films at 300 1C for 5 min using a dimethylzinc [(CH3)2Zn:DMZn] vapor. This method does not require any additional processing equipment since the diffusion can be carried out subsequent to the selenization of a Cu–In precursor using organoselenium liquid, such as diethylselenide [(C2H5)2Se:DESe]. A donor-to-acceptor pair emission attributable to Zn impurity was observed in the low-temperature photoluminescence spectrum. From the capacitance–voltage characteristics, the depletion layer width and diffusion potential of the junction were estimated as 300 nm and 0.6–0.7 V, respectively. The method is highly advantageous for the development of low-cost solar modules. r 2007 Elsevier B.V. All rights reserved. PACS: 73.20.At; 73.30.+y; 73.40.Lq Keywords: A1. Growth models; A3. Physical vapor deposition processes; B2. Semiconducting ternary compounds
1. Introduction Chalcopyrite-structure Cu(In,Ga)(S,Se)2 alloys [1] have attracted attention as a promising candidate for the lightabsorbing medium of high conversion efficiency (Z) lowcost solar cells. In fact, polycrystalline CuIn1–xGaxSe2 (CIGS)-based solar modules with Zffi13% have been put into practical use. They generally consist of a layered structure such as ZnO/CdS/CIGS/Mo on a soda–lime– glass (SLG) substrate. The CdS buffer layer on the CIGS layer is usually deposited by the chemical bath deposition (CBD) method, and is considered to play important roles in preventing the CIGS layer from damaging during the sputtering deposition of the ZnO layer and in forming a pn-homojunction on the CIGS surface due to the Cd diffusion [2,3]. However, the use of CdS involves handling
Corresponding author.
E-mail address: [email protected] (M. Sugiyama). 0022-0248/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2007.11.172
difficulties and environmental concerns since Cd is highly toxic and CdS is deposited by complex wet processes. The use of CdS can be avoided by fabricating a pn junction through thermal diffusion of a donor impurity into the p-type CIGS layer. Zn is a suitable element for the donor impurity because Zn is safer than Cd. In addition, Zn-doped CIGS layers can be formed by simple dry processes such as chemical vapor deposition, thermal evaporation, and thermal diffusion. However, few experimental results have been reported for the fabrication of pn junction using Zn-doped CuInSe2 (CIS) bulk crystals [4–8] or CIGS films [9,10]. The authors have proposed the use of diethylselenide [(C2H5)2Se:DESe] for the selenization growth of CIS and CIGS films [11–14]. Since DESe is a liquid at room temperature and is stored under atmospheric pressure in a stainless-steel bubbler, the risk of a widespread leakage is less likely than that of the case of H2Se. Similarly, dimethylzinc [(CH3)2Zn:DMZn] is an attractive organozinc liquid as the diffusion source for the Zn-doped CIGS layers
ARTICLE IN PRESS M. Sugiyama et al. / Journal of Crystal Growth 310 (2008) 794–797
3. Results and discussion Single-phase polycrystalline CIS films with a thickness of approximately 2.0 mm were successfully formed by selenization using DESe. The films were composed of densely packed, 1–3-mm-diameter columnar grains. The films adhered well to the Mo/SLG substrate, which was confirmed by the peeling test. Before determining the optimized temperature and time for the fabrication of pn junctions by the thermal diffusion of Zn, critical effects of the use of DMZn was first examined: thermal annealing was carried out for up to 60 min to obtain Zn-doped thick CIS. The annealing temperature was varied between 300 and 500 1C, which was limited by the decomposition temperature of DMZn and
ZnIn2Se4
CIS
(116, 312)
Cu16In9
(220, 204)
CuInSe2 Zn compounds Mo Cu16In9
Sequentially stacked 20-period Cu/In layers with a total thickness of approximately 670 nm were used as a precursor. The precursors were evaporated using two K-cells on unintentionally heated Mo-coated SLG substrates. The gross thickness of the precursor was controlled by in situ monitoring of the thickness of each layer with a quartz crystal oscillator. The precursors were selenized using DESe at 515 1C for 90 min under atmospheric pressure. The flow rates of DESe and N2 carrier gases were 35 mmol/min and 2 L/min, respectively. Details of the evaporation and selenization equipments have been described in our previous reports [12–14]. Subsequent to the selenization growth, the CIS films were annealed at 300–500 1C for 3–60 min under the DMZn stream in the same reactor. The flow rates of DMZn and N2 carrier gases were 60 mmol/min and 2 L/min, respectively. X-ray diffraction (XRD) measurement was carried out for evaluating the lattice constants and crystal phases. It was also used to examine whether the film exhibited phase separation. Electron probe microanalysis (EPMA) was carried out to evaluate the solid concentrations of the constituent atoms. The depth profile of Zn in the CIS films was measured using glow discharge optical emission spectroscopy (GDOES). Photoluminescence (PL) was excited by the 532.0 nm line of a frequency-doubled quasi-cw Nd:YAG laser (60 mW) and dispersed by a 50cm-focal-length grating monochromator. The phase-sensitive detection was carried out using a liquid-N2-cooled Ge photodetector. The current–voltage (I–V) and capacitance– voltage (C–V) measurements were carried out on the CIS pn junction.
(112)
2. Experimental procedure
the softening temperature of an SLG substrate. XRD patterns of the Zn-annealed CIS films are shown as a function of annealing temperature in Fig. 1(a). To examine whether extra phases are formed in the films, the signals near the (1 1 2) diffraction angle are magnified in Fig. 1(b). The films annealed between 300 and 350 1C exhibited the XRD peaks due to single-phase CIS solid solutions. We note that these films exhibited low-resistivity n-type conductivity, which was confirmed by the resistivity measurement and the hot-point probe analysis. Conversely, the films annealed between 400 and 500 1C exhibited XRD peaks due to Cu16In9 and Zn compounds such as ZnSe and ZnIn2Se4. It should be noted that extra phases were not found in the CIS films annealed at 500 1C for 60 min without DMZn flow (data not shown). Therefore, hightemperature annealing in a DMZn vapor had undesirable effects on the CIS films: Cu16In9 compounds might be formed due to the thermal decomposition of CIS films and the decomposed Se and In might react with Zn. It should be noted that Zn concentration in the CIS film annealed at 300 1C for 60 min was measured by EPMA to be approximately 3%, which was far higher than the appropriate doping level for fabricating pn junctions. Therefore, the annealing time in DMZn flow was varied between 3 and 5 min to obtain a Zn-doped layer, since the diffusion depth of Zn was estimated to be 400 and 550 nm for the diffusion times of 3 and 5 min, respectively, according to the experimental report on the thermal diffusion depth of Zn into CIGS films [9]. The depth profile of Zn in the CIS film annealed at 300 1C for 5 min is shown in Fig. 2. As shown, weak Zn signal is seen to spread deeply into CIS. However, this is caused by the irregular surface of the polycrystalline
XRD INTENSITY [arb. units]
by thermal annealing. This method does not require any additional processing equipment since the diffusion can be carried out subsequent to the selenization growth of CIGS. This is highly advantageous for the development of the low-cost solar modules. In this article, both the industrial and physical advantages of using a DMZn source for the formation of a Zn-doped CIS layer are shown.
795
ZnSe
500 °C 450 °C 400 °C 350 °C 300 °C as grown 30
40
50
26 27
2θ [deg.] Fig. 1. (a) XRD patterns of Zn-annealed CIS films as a function of annealing temperature and (b) the signals near the (1 1 2) diffraction peak angle are magnified to show the extra phases.
ARTICLE IN PRESS M. Sugiyama et al. / Journal of Crystal Growth 310 (2008) 794–797
796
18K P3 GDOES INTENSITY [arb. units]
CuInSe2
P3 P2
CuInSe2 P2
P1 18K
P1
100%
3min anneal
PL INTENSITY [arb. units]
5min anneal
PL INTENSITY [arb. units]
SLG
Zn-diffused layer
5min. anneal
0
500
1000
1500
Wood’s anomaly
CIS layer 2000
THICKNESS [nm]
as grown
Fig. 2. Depth profile of Zn in the CIS film annealed at 300 1C for 5 min.
0.85
CIS, which gives rise to inhomogeneous sputtering during the GDOES measurement. Therefore, the Zn diffusion depth is estimated to be 400–500 nm, and a linearly graded junction is formed at the diffusion front. The value of diffusion depth is approximately the same as the estimated value: the Zn diffusion depth can be controlled by varying the annealing time. Low-temperature PL spectra of the undoped and Zndoped CIS films are shown in Fig. 3(a) as a function of annealing time. The annealing temperature was maintained at 300 1C. PL peaks labeled P1, P2, and P3 in the order of high to low energy were observed. We note that the spectral discontinuity at 0.88 eV is due to Wood’s anomaly of the diffraction grating. Since P1 and P2 are observed in both Zn-doped and undoped films, they may originate from intrinsic defect levels of CIS. Conversely, P3 is considered to originate from Zn-related impurity levels, because the peak was observed only in the Zn-doped films. We should mention that deep-level emissions attributable to carbon contamination were not observed; the result was similar to that of the CIS epilayers grown by metalorganic vapor phase epitaxy using DESe as a Se source [15]. Low-temperature PL spectra of the Zn-doped CIS film annealed at 300 1C for 5 min are shown in Fig. 3(b) as a function of relative excitation power. Peaks P1 and P3 are attributed to independent donor–acceptor pair (DAP) transitions, because the peaks shifted to the higher energy [16] as the excitation power increased. Conversely, the peak energy of P2 was independent of the excitation power. Considering the defect and impurity levels in CIS [17], P1 is assigned to the transition from the Se-vacancy level to the Cu-vacancy level (VSe, VCu). Similarly, P2 is assigned to the transition of electrons in the conduction band to the Se at the In-site level (e, SeIn) and/or the one in the In at the Se-site level to the valance band (InSe, h). The donor or the acceptor responsible for P3 emission has not been identified
0.90
0.95
0.1%
1.00 0.85 0.90 PHOTON ENERGY [eV]
5min anneal 0.95
1.00
Fig. 3. (a) Low-temperature PL spectra of the Zn-doped and undoped CIS films as a function of annealing time and (b) low-temperature PL spectra, as a function of excitation power of the Zn-doped CIS film, which was annealed at 300 1C for 5 min.
yet: the number of reports on the defect and impurity levels of Zn-doped CIS is limited. Applying the analytical method proposed by Zacks and Halperin [16], the ionization energies of the shallower and deeper levels are estimated to be 30 and 70 meV, respectively. Considering the defect and impurity levels of Zn-doped CuGaS2 [18,19], Zn at the Cu-Site (ZnCu) is considered to form the deeper level. However, further studies are required to clarify the origins of Zn-related DAP emission. The surface region of Zn-doped CIS films typically exhibited n-type conductivity in the hot-point probe analysis. The result implied the formation of a CIS pn junction. Although the photovoltaic effect was so weak that it was difficult to calculate the conversion efficiency, the I–V characteristics of a Zn-doped CIS/Mo/SLG structure revealed rectifying properties, as shown in Fig. 4(a). A representative 1/C3–V curve of a Zn-doped CIS/Mo/SLG structure is shown in Fig. 4(b). It is known that inverse cube of the depletion layer capacitance (1/C3) changes linearly with the applied voltage in a linearly graded junction [20]. Therefore, the result might imply that the formation of the homojunction is attributable to the thermal diffusion of Zn into the CIS film. From the linear extrapolation of the data between the reverse voltage of 0 and 2 V, the diffusion potential (VD) is estimated to be 0.6–0.7 V. By means of the C–V depth profiling technique [20,21], the depletion layer width of the Zn-doped layer is estimated to be 300 nm. According to the result, band alignment of the Zn-diffused CIS is proposed in the inset of
ARTICLE IN PRESS M. Sugiyama et al. / Journal of Crystal Growth 310 (2008) 794–797
was observed in the low-temperature PL spectra. From the capacitance–voltage characteristics, the depletion layer width and the diffusion potential of the pn junction were estimated as 300 nm and 0.6–0.7 V, respectively. These results indicate that DMZn is a suitable doping source for the thermal diffusion process in fabricating CIS pn junctions for low-cost CIGS-based solar cells.
CURRENT [mA]
2.0 1.5 1.0 0.5 0
Acknowledgments
-1
0
1
2
VOLTAGE [V]
8
VD = 0.6-0.7eV
Zn diffusion:400-500nm CBM p-CIS Fermi level
6
VBM
1/C3 [ x1034 F-3]
depletion layer: 300nm
4
2
CuInSe2/Mo 5min anneal -2
-1
VD 0
1
The authors would like to thank A. Umezawa, T. Yasuniwa, M. Soda, M. Abe, T. Kato, and Dr. F. B. Dejene for their help in the experiments. They also thank T. Mitani at the Central Service Facilities for Research, Keio University, for stimulating discussions and continuous encouragement. References
n-CIS
0
797
2
VOLTAGE [V] Fig. 4. Representative (a) I–V and (b) 1/C3–V curves of the Zn-doped CIS/Mo/SLG, which is a component of a solar cell structure. The inset of (b) shows the typical band alignment of the Zn-diffused CIS film.
Fig. 4(b). It should be noted that band alignment of a homojunction is dominated by both the intrinsic properties of Zn-diffused CIS and extrinsic properties such as interface defects and/or grain boundaries. Therefore, there are certain difficulties in drawing the band alignment. However, both the experimental diffusion depth of Zn atoms (400–500 nm) and depletion layer width (300 nm), which were estimated by GDOES and C–V measurements, respectively, nearly agreed with the designed values. 4. Conclusions Vapor-phase Zn doping was carried out by the thermal diffusion of Zn into p-type CuInSe2 films at 300 1C for 3–5 min using a DMZn vapor. Zn-related DAP emission
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