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CIS bilayers
Thin Solid Films 431 – 432 (2003) 46–52
Re-crystallisation and interdiffusion in CGSyCIS bilayers M. Bodegard*, O. Lundberg, J. Lu, L. Stolt ˚ ˚ ¨ Solar Center, Uppsala University, P.O. Box 534, 751 21 Uppsala, Sweden Angstrom
Abstract Stacked layers of CuGaSe2 and CuInSe2 grown in slightly Cu-rich conditions were compared to corresponding CuGaSe2 and CuInSe2 single layers using cross-section SEM and TEM analysis as well as XRD. All samples were grown both on Mo coated soda-lime glass substrates and on Mo coated glass substrates with an Al2 O3 barrier blocking Na outdiffusion. We found a difference in both grain structure and Ga–In interdiffusion behaviour depending on if Na was present or not during growth. In the Na-free case we found evidence of recrystallisation of the underlying CuGaSe2 layer during the subsequent CuInSe2 growth. In this case also the interdiffusion of In and Ga was enhanced as compared to the sample grown in the presence of Na. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: CuInSe2; CuGaSe2; Cu(In,Ga)Se2; Interdiffusion; XRD
1. Introduction The use of Gay(GaqIn) profiles has been found to increase the efficiency of Cu(In,Ga)Se2 thin film solar cells. By introducing a Ga gradient in the film with higher Ga concentration near the back contact, a back surface field is created. The effect of this back surface field has been observed as an increase mainly in the Voc of the solar cells w1x. Additionally, when thinning down the Cu(In,Ga)Se2 film, the back surface field has been found to be efficient in reducing Voc and fill factor losses caused by the limited thickness of the sample w2x. In order to optimise the Ga profiles, the interdiffusion and intermixing properties of In-rich and Ga-rich Cu(In,Ga)Se2 compounds must be taken into account. This paper deals with the extreme case of pure CuInSe2 deposited on top of pure CuGaSe2, both with and without Na presence during growth. In previous studies, In–Ga interdiffusion, or CuInSe2, CuGaSe2 alloying, has been studied either with Na w3x, or without Na w4,5x. In our normal coevaporation process the deposition starts Cu-rich and the film will stay Cu-rich until the last minutes of the deposition. Therefore the CuInSe2 and CuGaSe2 thin films in this work have been deposited *Corresponding author. Tel.: q46-18-471-72-49; fax: q46-18555-095. E-mail address: [email protected] (M. Bodegard). ˚
in slightly Cu-rich conditions, in order to study the In– Ga interdiffusion at the relevant deposition parameters, a prerequisite for optimising the Ga concentration near the back contact in our deposition process. 2. Experimental CuInSe2 and CuGaSe2 films were fabricated by coevaporation from elemental sources at constant evaporation rates. CuInSe2 layers with thickness of 0.5 mm were deposited on top of CuGaSe2 layers with the same thickness. The CuInSe2 was deposited on top of CuGaSe2 immediately after the CuGaSe2 deposition, without cool-down and with maintained Se vapour pressure. The deposition was carried out in a high vacuum system at approximately 10y6 mbar, using a quadrupole mass spectrometer to control the metal rates. The Se source was not controlled by the mass spectrometer, but was kept at a constant temperature during the evaporation. The substrate temperature during growth was 510 8C. Soda-lime glass substrates were used, both with and without an Al2O3 barrier preventing Na outdiffusion from the glass. The Al2O3 barrier was deposited by ALCVD to a thickness of approximately 250 nm. The evaporation time for one single layer of CuInSe2 or CuGaSe2 was 16 min, hence the total evaporation time was 32 min. As reference samples for the materials analysis, also single layers of CuInSe2 and
0040-6090/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0040-6090(03)00252-9
M. Bodegard ˚ et al. / Thin Solid Films 431 – 432 (2003) 46–52
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Fig. 1. SEM cross-sections of the CuInSe2 and CuGaSe2 single layers. a and b are CuInSe2 layers and; c and d are CuGaSe2 layers. a and c show samples without Na and; b and d show samples grown in the presence of Na.
CuGaSe2 were fabricated. Since there is room for up to three substrates in each run in our evaporation system, both samples with and without the barrier were deposited in the same run in each experiment. All films were made with a Cuy(InqGa) ratio of approximately 1.1 as confirmed with EDS analysis. SEM and TEM imaging was made on all samples. For TEM analysis the samples were finally thinned down with ion milling. XRD analysis was performed with a Philips D5000 parallel beam diffractometer, using Cu Ka radiation. 3. Results and discussion 3.1. CuGaSe2 vs. CuInSe2 grain size The Cu-rich CuInSe2 and CuGaSe2 layers have a clear difference in grain size, as can be seen in Fig. 1. The width of the CuGaSe2 grains is less than 250 nm, and the height is equal to the film thickness of 0.5 mm, whereas the CuInSe2 grows with grains which in width are equal to, or even wider than, the film thickness.
These results are true for films both with and without Na, although there is a difference in the surface roughness of the films. Na-containing films have grains with flat upper surfaces, which also can be seen in Fig. 1. 3.2. SEM and TEM analysis of the stacked layers SEM images of CuInSe2 deposited on top of CuGaSe2, with and without Na, are shown in Fig. 2. The film without Na has grains, which extend from the bottom of the film to the top of the film whereas the film with Na has smaller grains at the bottom of the film and larger grains at the top of the film with a boundary in between. Fig. 3 shows TEM images of the same sample without Na that was shown in Fig. 2a. As can be seen, the TEM analysis confirms that the large grains in the SEM images are really single crystals, but with some defects. There is a tendency also for larger grains in the top layer of the Na-containing film, see Fig. 2b, but we have not observed any grains extending all the way down to the Mo layer in this case. Reasons for the non-interrupted grains could be both epitaxial
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Fig. 2. SEM cross-sections of the bilayer samples with CuInSe2 grown on top of CuGaSe2 . a shows the sample without Na and b shows the sample with Na.
growth of the CuInSe2 grains on top of the CuGaSe2 grains, or recrystallisation of the CuGaSe2 film during the subsequent CuInSe2 deposition. A comparison between the single layer CuGaSe2 films with and without Na and the bottom part of the stacked CuGaSe2 y CuInSe2 layers reveals a difference in the width of the grains, i.e. in the Na-free case, the grains do not only extend all the way from the back to the front of the film, they are also wider. Epitaxy would not change the width of the grains in the underlying layer and we therefore conclude that recrystallisation, or some kind of grain growth has occurred. 3.3. XRD analysis In order to analyse the XRD spectra of the different layers accurately, the thickness of each of the layers must be taken into account. Since the angle of diffraction of the (1 1 2) peak is lower than for the (2 0 4, 2 2 0) peaks, the intensity of the (1 1 2) peak will be overestimated as compared to the (2 0 4, 2 2 0) peaks. This is explained by angular dependence of the diffraction
intensity of a sample of limited thickness. The ratio of the diffracted intensities Gx, which come from a layer of thickness x to the incoming X-ray intensity is, according to w6x, given by: B
C
y2mx sin u
GxsD1ye
E
F
(1)
G
where m is the absorption coefficient and u is the angle of diffraction. Since the mass absorption coefficient of a compound is the weighted fraction of the mass absorption coefficients of its constituents, the following equations for the mass absorption coefficients of CuInSe2 and CuGaSe2 are valid:
BmE
C F s D r GCuInSe2
BmE BmE BmE MCuC FCuqMInC F q2MSeC F DrG D r GIn D r GSe
MCuqMInq2MSe
Fig. 3. TEM cross-section of the bilayer CuInSe2 grown on top of CuGaSe2 without Na, corresponding to a.
(2)
M. Bodegard ˚ et al. / Thin Solid Films 431 – 432 (2003) 46–52 Table 1 Mass absorption coefficients (myr) values and atomic weight (M) values for Cu, In, Ga and Se from w6x Element 2
myr (cm yg) M (gymol)
Cu
In
Ga
Se
52.7 63.54
252 114.8
63.3 69.72
82.8 78.96
and
BmE
C F s D r GCuGaSe2
BmE BmE BmE MCuC FCuqMGaC F q2MSeC F DrG D r GGa D r GSe
MCuqMGaq2MSe
. (3)
The myr values w6x are given in Table 1. Hence from Eqs. (2) and (3), the myr value of CuInSe2 is 134.88 cm2 yg and the value for CuGaSe2 is 71.56 cm2 yg. The absorption coefficient m is obtained by multiplying with the densities of CuInSe2 and CuGaSe2, respectively to be 134.88Ø5.77s778ycm for CuInSe2 and 71.56Ø5.57s398ycm for CuGaSe2. Using Eq. (1) and the calculated values for the absorption coefficients, the fraction of X-ray diffracted intensity for a thin film as compared to specimen of infinite thickness can be obtained for different diffraction angles. Table 2 shows the results for the (1 1 2) and (2 0 4, 2 2 0) peaks of 0.5 mm CuInSe2 and CuGaSe2, respectively. The influence on the ratio between the intensities for the (1 1 2) peak and the (2 0 4, 2 2 0) peaks, due to the decrease in thickness, is a factor of approximately 1.5 for CuInSe2 and 1.6 for CuGaSe2. This means that the ratio of the peak height of the (1 1 2) peak and the (2 0 4, 2 2 0) peak(s) will be overestimated with 1.5, or 1.6, as compared to an infinitely thick sample. Another effect to consider for the underlying layer in the analysis of the bilayers is the absorption of X-rays in the upper layer. The attenuation of X-ray intensity caused by a 0.5 mm thick layer of CuInSe2 is calculated using Eq. (4) (4)
IsI0eymx
where I0 is the intensity of the incident beam and x is
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the distance through the sample which will vary with the angle. For the (1 1 2) peak the X-ray angle of incidence is 13.98 for pure CuInSe2, which will give a value of x for a 0.5 mm thick sample of 2.14 mm. The intensity of the beam after transit through the CuInSe2 layer twice is hereby 0.72 times its original intensity. For the (2 2 0) and (2 0 4) peaks, the attenuation is 0.81 times. The measured ratio of the (1 1 2) and the (2 0 4, 2 2 0) peaks after absorption in a 0.5 mm thick CuInSe2 is 0.89 times the real value. Taking both the error caused by the limited thickness of the CuGaSe2 and the error caused by absorption in the CuInSe2 into account, the ratio between the (1 1 2) and the (2 0 4, 2 2 0) peaks of the CuGaSe2 will be overestimated approximately 1.4 times. 3.3.1. XRD analysis of single layers XRD spectra of Cu-rich CuInSe2 and CuGaSe2 films, with and without Na, are shown in Fig. 4. There is a clear trend that although the grain size according to the SEM and TEM analyses for each of the compounds is the same, with and without Na, the grain orientation differs. The resulting peak height intensities from the XRD measurements are shown in Table 3. Since the (2 2 0, 2 0 4) peaks are well resolved for CGS, the intensities are given separately for each orientation in the table, whereas the sum of the peaks is used for CIS. All the samples have high values of the ratio between the intensities of the (1 1 2) peak and the (2 0 4, 2 2 0) peaks, as compared to JCPDS standards (JCPDS 401487 and JCPDS 35-1100). From now on the higher value of the ratio between the (1 1 2) peak and the (2 0 4, 2 2 0) is referred to as (1 1 2) orientation. The values of the (1 1 2) orientation is also given in Table 3. For CuGaSe2, the (1 1 2) orientation is defined as the ration between the (1 1 2) and the (2 2 0) peak. There is a clear correlation between higher (1 1 2) orientation and smoothness of the films. This indicates that more grains grown in the presence of Na as compared to in the absence of Na, be it CuInSe2 or CuGaSe2, have (1 1 2) planes as upper surfaces. 3.3.2. XRD analysis of Cu-rich CuInSe2 layers on top of Cu-rich CuGaSe2 layers X-ray diffraction spectra of the two bilayer samples with CuInSe2 deposited on top of CuGaSe2 are shown in Fig. 5.
Table 2 Calculated values of the ratio of the XRD peak intensities between a 0.5 mm thick sample and a sample of infinite thickness for the (1 1 2) and (2 0 4, 2 2 0) peaks
Intensity Gx (CuInSe2) Intensity Gx (CuGaSe2)
(1 1 2) peak
(2 0 4, 2 2 0) peak
Gx(1 1 2)yGx(2 0 4, 2 2 0)
0.283 0.153
0.187 0.097
1.51 1.58
In the rightmost column the ratio of these values are given.
M. Bodegard ˚ et al. / Thin Solid Films 431 – 432 (2003) 46–52
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Fig. 4. XRD spectra of the single layers. To the left, the (1 1 2) peaks and to the right the (2 0 4, 2 2 0) peaks. The dotted lines are from the samples with and the solid lines are from the samples without Na. The peaks at lower angles correspond to CuInSe2 and the peaks at higher angles to CuGaSe2.
Table 3 X-ray diffraction peak height intensities of various diffraction peaks for CuInSe2 and CuGaSe2
CuInSe2 Na CuInSe2 no Na CuGaSe2 Na CuGaSe2 no Na
(1 1 2)
(2 0 4, 2 2 0)
110 000 36 000 94 000 37 000
445 1050
(2 2 0)
(2 0 4)
(1 1 2)y(2 0 4, 2 2 0)
(1 1 2)y(2 0 4)
247 (165) 34 (23) 102 348
294 732
319 (201) 50 (32)
The last two columns show the ratio between the peak height intensities, where the number in parenthesis is corrected for the limited thickness of the sample.
Fig. 5. XRD spectra of the bilayer samples, the (1 1 2) peaks to the left and the (2 0 4, 2 2 0) peaks to the right. The dotted lines are from the samples with and the solid lines are from the samples without Na.
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Table 4 Peak height intensities for the (1 1 2) and (2 0 4, 2 2 0) XRD spectra shown in Fig. 5
In-rich CIGS, Na In-rich CIGS, no Na Ga-rich CIGS, Na Ga-rich CIGS, no Na
(1 1 2)
(2 0 4, 2 2 0)
46 000 10 000 15 000 18 000
200 250 600
(2 2 0)
(2 0 4)
(1 1 2)y(2 0 4, 2 2 0)
(1 1 2)y(2 0 4)
230 (153) 40 (27) 200 550 (overlap)
200 400 (overlap)
30 (21)
75 (54) 45 (32)
Also the ratio between the peaks is calculated. The values in parentheses are corrected for the limited thickness of the sample and for absorption in the upper layer. The In-rich Cu(In,Ga)Se2 peaks come from the CuInSe2 layers after in-diffusion of Ga. Accordingly, the Ga-rich Cu(In,Ga)Se2 peaks are the remainders of the CuGaSe2 layers after in-diffusion of In.
First, it can be observed that in both cases, interdiffusion of In and Ga has occurred, but to a smaller extent in the sample with Na. Comparing the XRD spectra with the spectra from the single layers, it is clear that there is no non-alloyed CuInSe2 or CuGaSe2 in any of the bilayer samples. Both spectra exhibit a two-peak structure, but the peaks are broader than in the single layers. The results are consistent with two more distinct phases, one more In-rich and one more Ga-rich, with a region of graded composition in between. The main difference between the XRD spectra of the two samples is the distances between these peaks. In the sample without Na, the distance between XRD peaks corresponding to the In-rich ‘phase’ and the Ga-rich ‘phase’ is much smaller than in the sample with Na. This means that the in-diffusion of In into the CuGaSe2 and outdiffusion of Ga to the CuInSe2 has been larger in the Na-free sample. The lower intensity of the XRD peak corresponding to the In-rich ‘phase’ in the Na-free sample as compared to the sample with Na is further evidence of increased interdiffusion of In and Ga. As calculated above, the (1 1 2) orientation of a CuInSe2 sample with a thickness of 0.5 mm will be overestimated by a factor of approximately 1.5, whereas the (1 1 2) orientation of a 0.5 mm thick CuGaSe2 film under a 0.5 mm thick CuInSe2 film will be overestimated approximately 1.4 times. The peak height intensities and the (1 1 2) orientation of the In-rich and Ga-rich compounds, respectively are shown in Table 4. The values in parentheses are corrected. There is an overlap in the (2 2 0, 2 0 4) peaks in the Na-free sample, because the resulting Ga-rich compound has these peaks very close to each other, but the peaks have been deconvolved and used for the calculation. It was observed in Section 3.2 that in the Nacontaining bilayers, the grain size differed between the bottom (Ga-rich) and the top (In-rich) part and that there was a clear border between the two parts of the film. From a direct comparison of the ratio between the peak intensities of the (1 1 2) peaks and the (2 0 4, 2 2 0) peaks in the bilayers, it is clear that there is also a difference in orientation between the lower and the upper part of the sample. Both these observations are consistent with a two-layer structure.
In the Na-free bilayers the SEM as well as the TEM observations show no difference in grain size between the upper and lower part of the sample. From the XRD analysis, the result from the calculation of the (1 1 2) orientation is that there is no difference in the (1 1 2) orientation of the In-rich and the Ga-rich compounds within the errors of the experiments. These observations are in agreement with a one-layer structure. However, it can be argued that in the Na-free single layers, the difference in (1 1 2) orientation between CuGaSe2 and CuInSe2 is also very small, so in this case the XRD result alone is not any proof that the grains extend all the way through the film. 4. Conclusion Results from SEMyTEM analysis of Cu-rich stacked CuGaSe2 yCuInSe2 bilayers indicate a difference in behaviour between samples with and samples without presence of Na during growth. SEM images of the film grown in the presence of Na show a clear two-layer structure. In contrast, a CuGaSe2 yCuInSe2 bilayer grown in the same deposition run, but using a substrate with a barrier blocking outdiffusion of Na from the substrate has grains which extends from the bottom to the top of the film. In addition, the grain width in the bottom part of the resulting film is larger than the single layer CGS reference sample grown under the same conditions, strongly indicating a recrystallisation or some kind of grain growth of this layer during the subsequent deposition of the CIS layer. XRD analysis of grain orientation confirms that there is a clear difference between the bilayers with and without Na. In both cases the results indicate two CIGS phases, one more In-rich and one more Ga-rich with a graded composition in between. In the sample grown without Na, the interdiffusion of In and Ga is higher than in the sample with Na. An estimation of (1 1 2) orientation of the grains indicate that in the Na-containing film there is a difference in orientation between the In-rich and the Ga-rich CIGS phase, whereas in the Nafree film, the difference in film texture between the two phases is negligible or very small.
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Acknowledgments This work was partly funded by the European Commission, project no ENK5-CT-2000-00331 (PROCIS), ˚ ¨ Solar Center, which is and partly by the Angstrom financially supported by the Foundation for Strategic Environmental Research (MISTRA) and the Swedish Energy Agency. References w1x M. Bodegard, ¨ L. Stolt, Proceeding ˚ O. Lundberg, J. Malmstrom, of 28th IEEE Photovoltaics Specialists Conference, Anchorage, Alaska, 2000, 450–453.
w2x O. Lundberg, M. Bodegard, ¨ J. Malmstrom, L. Stolt, Prog. ˚ Photovolt: Res. Appl. 11 (2003) 77–88. w3x M. Marudachalam, H. Hichri, R.W. Birkmire, J.M. Schultz, A.B. Swartzlander, M.M. Al-Jassim, Proceeding of 25th IEEE Photovoltaics Specialists Conference, Washington D.C., 1996, 805–807. w4x T. Walter, H.-W. Schock, Thin Solid Films 224 (1993) 74–81. w5x D.J. Schroeder, G.D. Berry, A. Rockett, Appl. Phys. Lett. 69 (1996) 4068–4070. w6x B.D. Cullity, Elements of X-ray Diffraction, vol. 1, first ed., Addison-Wesley Publishing Company, Inc, 1956.
Re-crystallisation and interdiffusion in CGSyCIS bilayers M. Bodegard*, O. Lundberg, J. Lu, L. Stolt ˚ ˚ ¨ Solar Center, Uppsala University, P.O. Box 534, 751 21 Uppsala, Sweden Angstrom
Abstract Stacked layers of CuGaSe2 and CuInSe2 grown in slightly Cu-rich conditions were compared to corresponding CuGaSe2 and CuInSe2 single layers using cross-section SEM and TEM analysis as well as XRD. All samples were grown both on Mo coated soda-lime glass substrates and on Mo coated glass substrates with an Al2 O3 barrier blocking Na outdiffusion. We found a difference in both grain structure and Ga–In interdiffusion behaviour depending on if Na was present or not during growth. In the Na-free case we found evidence of recrystallisation of the underlying CuGaSe2 layer during the subsequent CuInSe2 growth. In this case also the interdiffusion of In and Ga was enhanced as compared to the sample grown in the presence of Na. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: CuInSe2; CuGaSe2; Cu(In,Ga)Se2; Interdiffusion; XRD
1. Introduction The use of Gay(GaqIn) profiles has been found to increase the efficiency of Cu(In,Ga)Se2 thin film solar cells. By introducing a Ga gradient in the film with higher Ga concentration near the back contact, a back surface field is created. The effect of this back surface field has been observed as an increase mainly in the Voc of the solar cells w1x. Additionally, when thinning down the Cu(In,Ga)Se2 film, the back surface field has been found to be efficient in reducing Voc and fill factor losses caused by the limited thickness of the sample w2x. In order to optimise the Ga profiles, the interdiffusion and intermixing properties of In-rich and Ga-rich Cu(In,Ga)Se2 compounds must be taken into account. This paper deals with the extreme case of pure CuInSe2 deposited on top of pure CuGaSe2, both with and without Na presence during growth. In previous studies, In–Ga interdiffusion, or CuInSe2, CuGaSe2 alloying, has been studied either with Na w3x, or without Na w4,5x. In our normal coevaporation process the deposition starts Cu-rich and the film will stay Cu-rich until the last minutes of the deposition. Therefore the CuInSe2 and CuGaSe2 thin films in this work have been deposited *Corresponding author. Tel.: q46-18-471-72-49; fax: q46-18555-095. E-mail address: [email protected] (M. Bodegard). ˚
in slightly Cu-rich conditions, in order to study the In– Ga interdiffusion at the relevant deposition parameters, a prerequisite for optimising the Ga concentration near the back contact in our deposition process. 2. Experimental CuInSe2 and CuGaSe2 films were fabricated by coevaporation from elemental sources at constant evaporation rates. CuInSe2 layers with thickness of 0.5 mm were deposited on top of CuGaSe2 layers with the same thickness. The CuInSe2 was deposited on top of CuGaSe2 immediately after the CuGaSe2 deposition, without cool-down and with maintained Se vapour pressure. The deposition was carried out in a high vacuum system at approximately 10y6 mbar, using a quadrupole mass spectrometer to control the metal rates. The Se source was not controlled by the mass spectrometer, but was kept at a constant temperature during the evaporation. The substrate temperature during growth was 510 8C. Soda-lime glass substrates were used, both with and without an Al2O3 barrier preventing Na outdiffusion from the glass. The Al2O3 barrier was deposited by ALCVD to a thickness of approximately 250 nm. The evaporation time for one single layer of CuInSe2 or CuGaSe2 was 16 min, hence the total evaporation time was 32 min. As reference samples for the materials analysis, also single layers of CuInSe2 and
0040-6090/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0040-6090(03)00252-9
M. Bodegard ˚ et al. / Thin Solid Films 431 – 432 (2003) 46–52
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Fig. 1. SEM cross-sections of the CuInSe2 and CuGaSe2 single layers. a and b are CuInSe2 layers and; c and d are CuGaSe2 layers. a and c show samples without Na and; b and d show samples grown in the presence of Na.
CuGaSe2 were fabricated. Since there is room for up to three substrates in each run in our evaporation system, both samples with and without the barrier were deposited in the same run in each experiment. All films were made with a Cuy(InqGa) ratio of approximately 1.1 as confirmed with EDS analysis. SEM and TEM imaging was made on all samples. For TEM analysis the samples were finally thinned down with ion milling. XRD analysis was performed with a Philips D5000 parallel beam diffractometer, using Cu Ka radiation. 3. Results and discussion 3.1. CuGaSe2 vs. CuInSe2 grain size The Cu-rich CuInSe2 and CuGaSe2 layers have a clear difference in grain size, as can be seen in Fig. 1. The width of the CuGaSe2 grains is less than 250 nm, and the height is equal to the film thickness of 0.5 mm, whereas the CuInSe2 grows with grains which in width are equal to, or even wider than, the film thickness.
These results are true for films both with and without Na, although there is a difference in the surface roughness of the films. Na-containing films have grains with flat upper surfaces, which also can be seen in Fig. 1. 3.2. SEM and TEM analysis of the stacked layers SEM images of CuInSe2 deposited on top of CuGaSe2, with and without Na, are shown in Fig. 2. The film without Na has grains, which extend from the bottom of the film to the top of the film whereas the film with Na has smaller grains at the bottom of the film and larger grains at the top of the film with a boundary in between. Fig. 3 shows TEM images of the same sample without Na that was shown in Fig. 2a. As can be seen, the TEM analysis confirms that the large grains in the SEM images are really single crystals, but with some defects. There is a tendency also for larger grains in the top layer of the Na-containing film, see Fig. 2b, but we have not observed any grains extending all the way down to the Mo layer in this case. Reasons for the non-interrupted grains could be both epitaxial
M. Bodegard ˚ et al. / Thin Solid Films 431 – 432 (2003) 46–52
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Fig. 2. SEM cross-sections of the bilayer samples with CuInSe2 grown on top of CuGaSe2 . a shows the sample without Na and b shows the sample with Na.
growth of the CuInSe2 grains on top of the CuGaSe2 grains, or recrystallisation of the CuGaSe2 film during the subsequent CuInSe2 deposition. A comparison between the single layer CuGaSe2 films with and without Na and the bottom part of the stacked CuGaSe2 y CuInSe2 layers reveals a difference in the width of the grains, i.e. in the Na-free case, the grains do not only extend all the way from the back to the front of the film, they are also wider. Epitaxy would not change the width of the grains in the underlying layer and we therefore conclude that recrystallisation, or some kind of grain growth has occurred. 3.3. XRD analysis In order to analyse the XRD spectra of the different layers accurately, the thickness of each of the layers must be taken into account. Since the angle of diffraction of the (1 1 2) peak is lower than for the (2 0 4, 2 2 0) peaks, the intensity of the (1 1 2) peak will be overestimated as compared to the (2 0 4, 2 2 0) peaks. This is explained by angular dependence of the diffraction
intensity of a sample of limited thickness. The ratio of the diffracted intensities Gx, which come from a layer of thickness x to the incoming X-ray intensity is, according to w6x, given by: B
C
y2mx sin u
GxsD1ye
E
F
(1)
G
where m is the absorption coefficient and u is the angle of diffraction. Since the mass absorption coefficient of a compound is the weighted fraction of the mass absorption coefficients of its constituents, the following equations for the mass absorption coefficients of CuInSe2 and CuGaSe2 are valid:
BmE
C F s D r GCuInSe2
BmE BmE BmE MCuC FCuqMInC F q2MSeC F DrG D r GIn D r GSe
MCuqMInq2MSe
Fig. 3. TEM cross-section of the bilayer CuInSe2 grown on top of CuGaSe2 without Na, corresponding to a.
(2)
M. Bodegard ˚ et al. / Thin Solid Films 431 – 432 (2003) 46–52 Table 1 Mass absorption coefficients (myr) values and atomic weight (M) values for Cu, In, Ga and Se from w6x Element 2
myr (cm yg) M (gymol)
Cu
In
Ga
Se
52.7 63.54
252 114.8
63.3 69.72
82.8 78.96
and
BmE
C F s D r GCuGaSe2
BmE BmE BmE MCuC FCuqMGaC F q2MSeC F DrG D r GGa D r GSe
MCuqMGaq2MSe
. (3)
The myr values w6x are given in Table 1. Hence from Eqs. (2) and (3), the myr value of CuInSe2 is 134.88 cm2 yg and the value for CuGaSe2 is 71.56 cm2 yg. The absorption coefficient m is obtained by multiplying with the densities of CuInSe2 and CuGaSe2, respectively to be 134.88Ø5.77s778ycm for CuInSe2 and 71.56Ø5.57s398ycm for CuGaSe2. Using Eq. (1) and the calculated values for the absorption coefficients, the fraction of X-ray diffracted intensity for a thin film as compared to specimen of infinite thickness can be obtained for different diffraction angles. Table 2 shows the results for the (1 1 2) and (2 0 4, 2 2 0) peaks of 0.5 mm CuInSe2 and CuGaSe2, respectively. The influence on the ratio between the intensities for the (1 1 2) peak and the (2 0 4, 2 2 0) peaks, due to the decrease in thickness, is a factor of approximately 1.5 for CuInSe2 and 1.6 for CuGaSe2. This means that the ratio of the peak height of the (1 1 2) peak and the (2 0 4, 2 2 0) peak(s) will be overestimated with 1.5, or 1.6, as compared to an infinitely thick sample. Another effect to consider for the underlying layer in the analysis of the bilayers is the absorption of X-rays in the upper layer. The attenuation of X-ray intensity caused by a 0.5 mm thick layer of CuInSe2 is calculated using Eq. (4) (4)
IsI0eymx
where I0 is the intensity of the incident beam and x is
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the distance through the sample which will vary with the angle. For the (1 1 2) peak the X-ray angle of incidence is 13.98 for pure CuInSe2, which will give a value of x for a 0.5 mm thick sample of 2.14 mm. The intensity of the beam after transit through the CuInSe2 layer twice is hereby 0.72 times its original intensity. For the (2 2 0) and (2 0 4) peaks, the attenuation is 0.81 times. The measured ratio of the (1 1 2) and the (2 0 4, 2 2 0) peaks after absorption in a 0.5 mm thick CuInSe2 is 0.89 times the real value. Taking both the error caused by the limited thickness of the CuGaSe2 and the error caused by absorption in the CuInSe2 into account, the ratio between the (1 1 2) and the (2 0 4, 2 2 0) peaks of the CuGaSe2 will be overestimated approximately 1.4 times. 3.3.1. XRD analysis of single layers XRD spectra of Cu-rich CuInSe2 and CuGaSe2 films, with and without Na, are shown in Fig. 4. There is a clear trend that although the grain size according to the SEM and TEM analyses for each of the compounds is the same, with and without Na, the grain orientation differs. The resulting peak height intensities from the XRD measurements are shown in Table 3. Since the (2 2 0, 2 0 4) peaks are well resolved for CGS, the intensities are given separately for each orientation in the table, whereas the sum of the peaks is used for CIS. All the samples have high values of the ratio between the intensities of the (1 1 2) peak and the (2 0 4, 2 2 0) peaks, as compared to JCPDS standards (JCPDS 401487 and JCPDS 35-1100). From now on the higher value of the ratio between the (1 1 2) peak and the (2 0 4, 2 2 0) is referred to as (1 1 2) orientation. The values of the (1 1 2) orientation is also given in Table 3. For CuGaSe2, the (1 1 2) orientation is defined as the ration between the (1 1 2) and the (2 2 0) peak. There is a clear correlation between higher (1 1 2) orientation and smoothness of the films. This indicates that more grains grown in the presence of Na as compared to in the absence of Na, be it CuInSe2 or CuGaSe2, have (1 1 2) planes as upper surfaces. 3.3.2. XRD analysis of Cu-rich CuInSe2 layers on top of Cu-rich CuGaSe2 layers X-ray diffraction spectra of the two bilayer samples with CuInSe2 deposited on top of CuGaSe2 are shown in Fig. 5.
Table 2 Calculated values of the ratio of the XRD peak intensities between a 0.5 mm thick sample and a sample of infinite thickness for the (1 1 2) and (2 0 4, 2 2 0) peaks
Intensity Gx (CuInSe2) Intensity Gx (CuGaSe2)
(1 1 2) peak
(2 0 4, 2 2 0) peak
Gx(1 1 2)yGx(2 0 4, 2 2 0)
0.283 0.153
0.187 0.097
1.51 1.58
In the rightmost column the ratio of these values are given.
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Fig. 4. XRD spectra of the single layers. To the left, the (1 1 2) peaks and to the right the (2 0 4, 2 2 0) peaks. The dotted lines are from the samples with and the solid lines are from the samples without Na. The peaks at lower angles correspond to CuInSe2 and the peaks at higher angles to CuGaSe2.
Table 3 X-ray diffraction peak height intensities of various diffraction peaks for CuInSe2 and CuGaSe2
CuInSe2 Na CuInSe2 no Na CuGaSe2 Na CuGaSe2 no Na
(1 1 2)
(2 0 4, 2 2 0)
110 000 36 000 94 000 37 000
445 1050
(2 2 0)
(2 0 4)
(1 1 2)y(2 0 4, 2 2 0)
(1 1 2)y(2 0 4)
247 (165) 34 (23) 102 348
294 732
319 (201) 50 (32)
The last two columns show the ratio between the peak height intensities, where the number in parenthesis is corrected for the limited thickness of the sample.
Fig. 5. XRD spectra of the bilayer samples, the (1 1 2) peaks to the left and the (2 0 4, 2 2 0) peaks to the right. The dotted lines are from the samples with and the solid lines are from the samples without Na.
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Table 4 Peak height intensities for the (1 1 2) and (2 0 4, 2 2 0) XRD spectra shown in Fig. 5
In-rich CIGS, Na In-rich CIGS, no Na Ga-rich CIGS, Na Ga-rich CIGS, no Na
(1 1 2)
(2 0 4, 2 2 0)
46 000 10 000 15 000 18 000
200 250 600
(2 2 0)
(2 0 4)
(1 1 2)y(2 0 4, 2 2 0)
(1 1 2)y(2 0 4)
230 (153) 40 (27) 200 550 (overlap)
200 400 (overlap)
30 (21)
75 (54) 45 (32)
Also the ratio between the peaks is calculated. The values in parentheses are corrected for the limited thickness of the sample and for absorption in the upper layer. The In-rich Cu(In,Ga)Se2 peaks come from the CuInSe2 layers after in-diffusion of Ga. Accordingly, the Ga-rich Cu(In,Ga)Se2 peaks are the remainders of the CuGaSe2 layers after in-diffusion of In.
First, it can be observed that in both cases, interdiffusion of In and Ga has occurred, but to a smaller extent in the sample with Na. Comparing the XRD spectra with the spectra from the single layers, it is clear that there is no non-alloyed CuInSe2 or CuGaSe2 in any of the bilayer samples. Both spectra exhibit a two-peak structure, but the peaks are broader than in the single layers. The results are consistent with two more distinct phases, one more In-rich and one more Ga-rich, with a region of graded composition in between. The main difference between the XRD spectra of the two samples is the distances between these peaks. In the sample without Na, the distance between XRD peaks corresponding to the In-rich ‘phase’ and the Ga-rich ‘phase’ is much smaller than in the sample with Na. This means that the in-diffusion of In into the CuGaSe2 and outdiffusion of Ga to the CuInSe2 has been larger in the Na-free sample. The lower intensity of the XRD peak corresponding to the In-rich ‘phase’ in the Na-free sample as compared to the sample with Na is further evidence of increased interdiffusion of In and Ga. As calculated above, the (1 1 2) orientation of a CuInSe2 sample with a thickness of 0.5 mm will be overestimated by a factor of approximately 1.5, whereas the (1 1 2) orientation of a 0.5 mm thick CuGaSe2 film under a 0.5 mm thick CuInSe2 film will be overestimated approximately 1.4 times. The peak height intensities and the (1 1 2) orientation of the In-rich and Ga-rich compounds, respectively are shown in Table 4. The values in parentheses are corrected. There is an overlap in the (2 2 0, 2 0 4) peaks in the Na-free sample, because the resulting Ga-rich compound has these peaks very close to each other, but the peaks have been deconvolved and used for the calculation. It was observed in Section 3.2 that in the Nacontaining bilayers, the grain size differed between the bottom (Ga-rich) and the top (In-rich) part and that there was a clear border between the two parts of the film. From a direct comparison of the ratio between the peak intensities of the (1 1 2) peaks and the (2 0 4, 2 2 0) peaks in the bilayers, it is clear that there is also a difference in orientation between the lower and the upper part of the sample. Both these observations are consistent with a two-layer structure.
In the Na-free bilayers the SEM as well as the TEM observations show no difference in grain size between the upper and lower part of the sample. From the XRD analysis, the result from the calculation of the (1 1 2) orientation is that there is no difference in the (1 1 2) orientation of the In-rich and the Ga-rich compounds within the errors of the experiments. These observations are in agreement with a one-layer structure. However, it can be argued that in the Na-free single layers, the difference in (1 1 2) orientation between CuGaSe2 and CuInSe2 is also very small, so in this case the XRD result alone is not any proof that the grains extend all the way through the film. 4. Conclusion Results from SEMyTEM analysis of Cu-rich stacked CuGaSe2 yCuInSe2 bilayers indicate a difference in behaviour between samples with and samples without presence of Na during growth. SEM images of the film grown in the presence of Na show a clear two-layer structure. In contrast, a CuGaSe2 yCuInSe2 bilayer grown in the same deposition run, but using a substrate with a barrier blocking outdiffusion of Na from the substrate has grains which extends from the bottom to the top of the film. In addition, the grain width in the bottom part of the resulting film is larger than the single layer CGS reference sample grown under the same conditions, strongly indicating a recrystallisation or some kind of grain growth of this layer during the subsequent deposition of the CIS layer. XRD analysis of grain orientation confirms that there is a clear difference between the bilayers with and without Na. In both cases the results indicate two CIGS phases, one more In-rich and one more Ga-rich with a graded composition in between. In the sample grown without Na, the interdiffusion of In and Ga is higher than in the sample with Na. An estimation of (1 1 2) orientation of the grains indicate that in the Na-containing film there is a difference in orientation between the In-rich and the Ga-rich CIGS phase, whereas in the Nafree film, the difference in film texture between the two phases is negligible or very small.
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Acknowledgments This work was partly funded by the European Commission, project no ENK5-CT-2000-00331 (PROCIS), ˚ ¨ Solar Center, which is and partly by the Angstrom financially supported by the Foundation for Strategic Environmental Research (MISTRA) and the Swedish Energy Agency. References w1x M. Bodegard, ¨ L. Stolt, Proceeding ˚ O. Lundberg, J. Malmstrom, of 28th IEEE Photovoltaics Specialists Conference, Anchorage, Alaska, 2000, 450–453.
w2x O. Lundberg, M. Bodegard, ¨ J. Malmstrom, L. Stolt, Prog. ˚ Photovolt: Res. Appl. 11 (2003) 77–88. w3x M. Marudachalam, H. Hichri, R.W. Birkmire, J.M. Schultz, A.B. Swartzlander, M.M. Al-Jassim, Proceeding of 25th IEEE Photovoltaics Specialists Conference, Washington D.C., 1996, 805–807. w4x T. Walter, H.-W. Schock, Thin Solid Films 224 (1993) 74–81. w5x D.J. Schroeder, G.D. Berry, A. Rockett, Appl. Phys. Lett. 69 (1996) 4068–4070. w6x B.D. Cullity, Elements of X-ray Diffraction, vol. 1, first ed., Addison-Wesley Publishing Company, Inc, 1956.