CuInSe2 films produced by graphite box annealing of multilayer precursors

Thin Solid Films 339 (1999) 44±50

CuInSe2 ®lms produced by graphite box annealing of multilayer precursors S.N. Kundu, M. Basu, S. Chaudhuri, A.K. Pal* Department of Materials Science, Indian Association for the Cultivation of Science, Calcutta 700 032, India Received 11 September 1997; accepted 10 July 1998

Abstract Copper indium diselenide ®lms were prepared by graphite box annealing of stacked elemental layer (SEL) precursors. Effects of annealing temperature and ambient atmosphere on the microstructural, optical and electrical properties were studied critically. PL measurements were also carried out to ®nd information on the defect levels. From the above studies, the optimum condition for the deposition of high quality CuInSe2 ®lms by this technique was determined. q 1998 Elsevier Science S.A. All rights reserved. Keywords: Copper indium diselenide ®lm; Graphite box annealing; Stacked elemental layer precursor

1. Introduction During the last decade, a tremendous amount of effort [1± 4] has been directed towards the fabrication of high-ef®ciency copper indium diselenide (CIS)-based solar cells. Ef®ciencies greater than 17% have already been achieved [2] with the CIS-based absorber layer produced by coevaporation technique. This achievement and the near zero degradation of the solar cells have made CIS-based material one of the most viable photovoltaic materials for terrestrial applications. A number of techniques were utilized [1±16] to prepare the CIS-based absorber layer which would have large grains with lesser amount of selenide subphases. The coevaporation technique, although having an edge over most of techniques for the production of a high-quality CIS layer, is being questioned for its suitability as regard to the cost and scalability for commercial production. The search for a low-cost preparation technique for large-scale applications is still going on to make the CIS-based solar cells viable for commercial applications. In this regard, the works carried out at the Newcastle Photovoltaic Application Center [11± 16] on graphite box annealing and rapid thermal processing of SEL are worth mentioning. We report here the results of an investigation on the synthesis of CuInSe2 ®lms by graphite box annealing of

* Corresponding author. Tel.: 191-33-469371; fax: 191-33-473-2805; e-mail: [email protected]

SEL precursors and characterization of the above synthesized ®lms by measuring the microstructural, optical and electrical properties.

2. Experimental The precursors (SEL) were deposited at a system pressure of ,10 25 Pa in a conventional 20-inch diameter vacuum coating unit ®tted with three appropriate evaporation sources for evaporating Cu, In and Se. Alumina crucibles heated indirectly by tungsten heaters were used for evaporating Cu and In while a Ta muf¯ed boat was used for Se Ê/ evaporation. The ¯uxes of the individual sources (Cu ,1 A Ê /s and Se ,2 A Ê /s) could be monitored by three s, In ,1 A quartz crystal thickness monitors. The substrate could be rotated for uniformity of the thickness of the deposit. The ¯uxes of the individual elements were controlled at appropriate rates for obtaining the coevaporated ®lms with predetermined composition. For the SEL precursor, the layers were deposited in sequence of In/Cu/Se with predetermined thicknesses of the individual layers in order to attain the required composition of the synthesized ®lm. The precursors were deposited onto sodalime glass substrates (with or without Mo coating) at room temperature (300 K), and they were placed in an appropriate rectangular graphite box for annealing in a cylindrical (6.5 cm diameter) quartz chamber either in vacuum or in argon (purity ,99.99%) atmosphere. The quartz chamber could be heated by an electronically controlled furnace.

0040-6090/98/$ - see front matter q 1998 Elsevier Science S.A. All rights reserved. PII S0040-609 0(98)01073-6

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3. Results and discussion 3.1. XRD and SEM studies Fig. 1 shows the schematic diagram of the stacked layers used in this study. We have found that the initial layer of indium on glass or Mo-coated glass has profound impact on the adhesion of the synthesized CuInSe2 ®lm. In case of Mocoated glass substrate, the initial In layer in the precursor served to protect the Mo coating from reacting with Se. Annealing of the stacked elemental layer precursor in closed graphite box was carried out in vacuum and in argon atmosphere (,1 atm). A couple of selenium shots (total amount of Se ,1 g) were placed inside the graphite box during annealing. The annealing temperature was varied between 620 and 730 K. It was observed that there was substantial material loss if the precursor was annealed in vacuum at temperatures greater than 650 K. It may also be noted that no complete synthesis occurred for annealing temperature lower than 650 K (for 45 min duration), and loss of selenium was always observed as was evident from the condensation of selenium in the glass trap connected to the evacuation line. Thus, we preferred to synthesize the ®lm by annealing the precursor in argon atmosphere. We tried to deposit the elemental layers at room temperatures (300 K). As the sequential deposition of the layers were going on, a substantial rise in the substrate temperature (.370 K) was observed. This culminated in solid-state reactions at the interfaces of the elemental layers and one would expect to observe the formation of binaries like CuIn, CuSe and In2Se3, the formation of which was favoured at lower temperatures. Indeed, the XRD pattern of the as-deposited precursor indicated (Fig. 2a) the presence of binary phases (In2Se3, CuIn, CuSe) only. Fig. 3 shows the SEM micrographs of the ®lms obtained by annealing at various temperatures. The ®lms

Fig. 1. Schematic diagram of stacked elemental layer (SEL) structure.

Fig. 2. XRD of as-deposited precursor and CuInSe2 ®lms synthesized at various annealing temperatures: (a) as-deposited; (b) annealed at 690 K; (c) annealed at 710 K.

obtained by annealing the SEL at temperatures between 570±620 K for 45 min indicated that molten selenium came out on the top as droplets (Fig. 3a) and no complete synthesis could be obtained even after annealing for prolonged time (,2 h). As the annealing temperature was increased to 650 K (Fig. 3b) and above (Fig. 3c), synthesis of CuInSe2 ®lms with increased grain growth could be obtained. The SEM micrographs for ®lms synthesized by annealing at various temperatures are shown in Fig. 3b±e . It can be seen from the XRD of the ®lm synthesized at 650 K for 45 min (Fig. 2b) that the synthesis is not yet complete. One can still observe the peaks for the binaries like In2Se3, CuIn, CuSe along with the intense peak for (112), (204) and (116/312) of chalcopyrite CuInSe2. As we increased the annealing temperature to 710 K to anneal the precursor for 1 h in argon atmosphere, the synthesized ®lm showed (Fig. 3d) the best morphology as regards the grain growth and chalcopyrite CIS phase. The grains of 3.5 mm were visible from the SEM micrograph (Fig. 3d). The corresponding XRD (Fig. 2c) indicated nearly unblemished diffractogram for a polycrystalline CuInSe2 ®lm. This would mean that annealing at 710 K would facilitate the formation of the ternary chalcopyrite CuInSe2 compound through the multistep formation of binaries and their reaction with Se vapour, leading towards the formation of CuInSe2 via Cu2 Se 1 In2 Se3 ˆ CuInSe2 [14]. If the annealing temperature was increased further beyond 735 K, loss of material similar to vacuum annealed ®lm was observed and no further distinguishable grain growth could be detected (Fig. 3e). Additional peaks for Cu2Se subphase began to reappear. This may basically be due to loss of In in the form of volatile

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Fig. 3. SEM micrographs of ®lms synthesized at various annealing temperatures: (a) annealed at 620 K; (b) annealed at 650 K; (c) annealed at 670 K; (d) annealed at 710 K; (e) annealed at 735 K.

subphases of InxSey and as such law of mass action would favour the formation of CuxSey subphases during the synthesis at temperatures above 735 K. Surface roughness (s o) of the ®lms synthesized at various annealing temperature was evaluated by measuring the diffuse re¯ectances [17] in the ®lms which are shown in Fig. 4a. It can be seen that the surface roughness increases sharply with the increase in grain size. The band gap of the ®lms synthesized at temperatures of 660±710 K varied between 0.97 and 0.98 eV. The ®lms synthesized at temperatures at and above 735 K showed lower band gap (#0.925 eV). The lowering of band for ®lms synthesized at temperatures at and above 715 K is due to the appearance of

Cu2Se subphases which is evident from the XRD studies discussed above. The ®lms synthesized at the optimum annealing temperature ,710 K had higher refractive index (n) than those for ®lms treated at other annealing temperatures (Fig. 4b). Thus, it may be observed that the optimum condition for depositing high quality CuInSe2 ®lms by graphite box annealing is to anneal the precursor in argon atmosphere at temperature ,710 K for 1 h. We also tried to synthesize CuInSe2 ®lms by the above graphite box annealing technique but without putting additional Se in the graphite box. For this, we deposited the last Se layer (on the top) with substantially higher thickness than that would be required for obtaining stoichiometric compo-

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Fig. 4. (a) Variation of surface roughness (s o) with annealing temperature. (b) Variation of refractive index (n) with wavelength of the incident photons for ®lms synthesized at various annealing temperatures.

sition in the stack. We annealed these SEL at 440 K for 1 h in argon atmosphere. It can be seen from the SEM micrographs (Fig. 5a,b) and the XRD pattern (Fig. 6) that largegrained CuInSe2 ®lms could be also obtained by this technique. The surface of these synthesized ®lms looked a bit shiny and the micrographs (Figs. 5a,b) clearly indicated the existence of Cu22xSe subphases located on the grains and at the grain boundaries. The XRD pattern also showed additional peaks for Cu22xSe and CuSe2 subphases at 2u ,44.588 and 35.518, respectively. Interestingly, when we annealed this ®lm again at 710 K in vacuum for 10 min, the shiny surface disappeared and the peak for the Cu2Se subphase also became nonexistent. This may be due to the fact that Cu2Se subphase would be volatile at higher temperature and thus would tend to leave the surface when evacuated. We then tried to achieve the same in one annealing bout. We annealed the precursor containing excess Se at 710 K for 45 min in argon atmosphere and then evacuated the quartz chamber while annealing was going on for additional 5 min. It can be seen from the micrograph (Fig. 5c) that the synthesized ®lms did not show the existence of Cu22xSe subphases and the XRD pattern reverted back to the one similar to that shown in Fig. 2c. 3.2. Dark conductivity measurements The ®lms synthesized at optimum condition were polycrystalline and as such grain boundary effects would dominate the electron transport processes in these ®lms. Thus the determination of grain boundary barrier height and density of trap states at the grain boundaries will be of immense importance for the evaluation of the quality of the large-

Fig. 5. SEM micrographs for ®lms synthesized with excess Se in the top layer: (a) ®lm synthesized at 710 K in argon atmosphere.; (b) enlarged vision of (a) showing the secondary growth on the grains; (c) ®lm synthesized at 710 K in Ar atmosphere for 45 min followed by vacuum annealing for 5 min at the same annealing temperature.

grained CuInSe2 ®lms synthesized as above. The ®lms were highly photoconducting. The dark and photo conductivity in the ®lms were measured by the four probe technique in the temperature range of 140±350 K. The variation of conductivity with temperature in dark and when illuminated is shown in Fig. 7. It can seen that there are two distinct temperature zones in the above variation. The high temperature zone is characterized by the presence of large activation energy (0.15 eV) which indicates that the electron transport process will mainly be governed by thermal activation of the

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sd ˆ …q2 Dn†=…2pm*e kT†1=2 exp…2Ebd =kT† ˆ sod T 21=2 exp…2Ebd =kT† ˆ nqmo exp…2Ebd =kT†

(1)

where mo ˆ Dq=…2pm*e kT†1=2 and s odˆ(Dnq 2)/(2p m*k) 1/2, q and me * are the electronic charge and the effective mass of the carrier respectively. T is the lattice temperature, k is the Boltzmann constant, n is the free carrier concentration and D is the average grain diameter of the crystallites in the ®lm. In the low temperature region when the grain boundary scattering will p predominate over other types of scattering, a plot of lns d/ T versus 1/T (Fig. 7) will give the barrier height (Ebd) in the dark. The Ebd value evaluated from the slope of this plot ,0.043 eV. The density of trap states (Qt) at the grain boundaries was evaluated from this Ebd value while the carrier concentration was evaluated from C±V measurements. The value of Qt obtained is ,2 £ 1010 cm 22 for the ®lms synthesized at temperatures ,670 K. 3.3. Photoconductivity measurements

Fig. 6. XRD spectrum of the ®lm corresponding to the micrograph Fig. 5a.

charge carriers. The activation energy for the lower temperature zone is quite small (0.044 eV). The grain boundary barrier height in dark (Ebd) may be obtained [18] from the following expression of the dark conductivity (s d) assuming monovalent trapping states lying below the Fermi level (Ef):

p p Fig. 7. Plots of ln(s d T) and ln(sf T) versus 1/T for a representative CuInSe2 ®lm synthesized at optimum annealing condition.

The photoconductivity (sf ) of the CuInSe2 ®lms was measured at different illumination levels within 20-130 mW/cm 2. The conductivity (s d) was seen to be modulated with the variation of the intensity of the incident radiation through an increase (Dn) in carrier concentration and decrease in barrier height (DEb). Thus, we can rewrite Eq. (1) in the modi®ed form as:

sw ˆ sow T 21=2 exp…2Ebw =kT†

(2)

The barrier heights at different illumination levels (f ) could p be evaluated from the slope of the plot of lns f T versus 1/ T (Fig. 7). It may be noted that the barrier height decreases with increasing illumination level which resulted in the creation of increased amount of minority carriers. These minority carriers easily ¯ow down the barrier to neutralize the trap states at the grain boundaries resulting in the decrease in barrier height. It may be noted that the barrier height becomes quite small (Fig. 8) at higher intensities. This would mean that all the occupied trap states at the grain boundary region would be neutralized by the electrons created by photoexcitation at higher illumination level. Thus, it would mean that under high illumination level the effect of grain boundaries may be reduced substantially. This result is in conformity with our previous observation on polycrystalline CuInSe2 ®lms prepared by three source coevaporation technique [19] and that of Muraska et al. on polycrystalline silicon ®lm [20]. Kazmerski and Shieh [21] reported the photoconductive response or decay time for some ternary compound semiconductors including CuInSe2 ®lms. They also indicated that the magnitude of the above parameter depended on the density of trapping centers available for the current density induced by the illumination. The photo-induced current decreased approximately exponentially with the removal of illumination. They also observed

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Fig. 8. Variation of the barrier height (Eb) with the intensity of illumination (f ).

a decrease in the density of trap states at the grain boundaries with recrystallization. The variation of the change in barrier height (DEbf ) with intensity of illumination (f ) may be expressed as [22] as: Ebf ˆ ctf

(3)

where c is a constant, t is the minority carrier lifetime and corresponds to the rate of detrapping of the donor levels present at the grain boundaries of the ®lm. Thus we see that the change in the barrier height will depend on the intensity of illumination and the intensity dependence of the minority carrier lifetime (t ). If we assume that t / fg , then

DEbf / fg11

(4)

so that the plot of lnDEbf versus lnf gives (Fig. 9) the value of g ˆ 0:08. This indicates that in our range of measurement the dependence of minority carrier life time on the

Fig. 9. Plot of ln(DEb) versus lnf .

Fig. 10. PL spectra for two representative ®lms of CuInSe2 synthesized at 710 K for: (a) 60 min; (b) 45 min.

incident photon energy (f ) is quite weak and DEbf is proportional to f . 3.4. PL measurements Photoluminescence (PL) measurements were carried out at 79 K, using 300 mW Xe arc lamp as the excitation source, 3/4 meter monochromator and Hamamatsu photomultiplier as detector. The samples were excited with 650 nm radiation. Fig. 10 shows the PL spectra for two representative CuInSe2 ®lms synthesized at 710 K for 60 min (Fig. 10, curve a) and for 45 min (Fig. 10, curve b). One can see that the PL spectrum is dominated by a near-bandedge peak which is broad, asymmetric and centered at ,1275 nm (,0.97 eV). This main peak may be attributed to the free-to-bound excitonic emission as peak positions did not change with excitation energy. There was a smaller peak at ,937±940 nm due to the volume and surface defects. It may be noted that the position of the smaller peak did not change with the wavelength of the exciting photons although they would correspond to different penetration depths. It is now believed that In-rich CuInSe2 ®lms generally contain CuIn3Se5 which tends to segregate at the surface and possibly in grain boundaries [23]. CuIn3Se5 is known to have n-type conductivity and a direct band gap of ,1.3 eV. The ®lms under consideration have Cu/In ,0.9 as indicated by the EDAX measurements. The observed luminescence spectrum at 1.31±1.32 eV energy region may thus be considered to be arising out of the presence of CuIn3Se5 overlayer in the ®lms.

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4. Conclusion

References

CuInSe2 ®lms were synthesized by graphite box annealing of stacked elemental layers deposited onto glass and Mo-coated glass substrates. The following general remarks may be made from the experimental observations.

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1. High-quality CuInSe2 ®lms could be obtained by annealing the stacked elemental layer of In/Cu/Se in a closed graphite box in argon atmosphere. The optimum annealing temperature and time are 710 K and 1 h, respectively. 2. Annealing above 735 K resulted in material loss during synthesis. 3. Annealing in vacuum at temperatures at and above 670 K resulted in excess material loss in course of synthesis of CuInSe2 ®lm. 4. The grain boundary scattering plays a dominant role in controlling the electron transport processes in these ®lms. The barrier height and the density of trap states are found to be 0.043 eV and 2£10 10 cm 22 respectively. 5. The ®lms are highly photoconducting. The grain boundary barrier height decreases with the increase in illumination level. 6. The variation of the barrier height with the intensity of illumination indicated a weak dependence of the minority carrier life time on the intensity of incident radiation. 7. PL spectra indicated near-band-edge emission at 1275 nm for the best ®lm synthesized by graphite box annealing. Another smaller peak in the deep energy region (,936 nm) might be assigned to the formation of CuIn3Se5 overlayers for In-rich ®lms. Acknowledgements The authors wish to acknowledge with thanks the ®nancial assistance by the Ministry of Non-Conventional Energy Sources, Govt. of India, for executing this programme. Two of us (S.N.K. and M.B.) wish to thank the Council of Scienti®c and Industrial Research, Government of India, for granting them fellowships.