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Development of CuInS2 solar cell material by PAC: bulk phases, thin films, and nuclear reaction doping
Nuclear Instruments North-Holland
and Methods
in Physics Research
Nuclear lnstrwnents 8 Methods in Physics Research
863 (1992) 231-235
Sectton
B
Development of CuInS, solar cell material by PAC: bulk phases, thin films, and nuclear reaction doping Chr. Dzionk, M. Briissler and H. Metzner Hahn - Meitner-Institut, Bereich Schwerionenphysik, 1000 Berlin 39, Germany
of CuInS*. Perturbed angular correlations (PAC) of y-rays using “‘In nuclear probes are utilized for the material development Three applications are presented which are a suitable tool to contribute to this complex: i) Using metallic In containing “‘In as one of the starting materials, single- and multiphase bulk samples were grown from the melt and studied by PAC, which leads to a complete data set of PAC signatures for all In-containing phases of the Q-In-S system that are stable at room temperature, as “‘In is controlled by PAC observing the phase well as to the phase relations of CuIn$. ii) The preparation of thin films containing formation. In first experiments, a thin Cu-In film was sulphurized, and the evaporation of sulphur out of the InS compound was “‘In afterwards in CuInS, thin films and studied. iii) The method of nuclear reaction doping offers the possibility to incorporate bulk samples earlier produced for other purposes.
1. Introduction With a direct energy gap of Eg = 1.5 eV the chalcopyrite semiconductor CuInS, is a promising material for photovoltaic devices. However, the best conversion efficiency obtained until now for solar cells based on CuInS, bulk material is only 9.7% [l]. Using CuInS, as an absorber material, Mitchell et al. [2] have recently achieved a value of 7.3% in a thin film solar cell. In both of these cases the results were obtained with heterogeneous samples containing precipitates of In, In,S3 or Cu,_,S. This shows that for the development of this material accurate investigations about the phase formation are necessary. Therefore, the knowledge of the phase relations of CuInS, in the Cu-In-S phase diagram is important. Recently we have reported first applications of perturbed angular correlations (PAC) of y-rays using “‘In nuclear probes to study phases and phase relations in the Cu-In-S system [3]. The PAC method yields information on the local environment of nuclear probe atoms in solids. We use the fact that the “‘In nuclei are sensitive to the electric field gradient (EFG) at the site of the probe in a non-cubic lattice structure [4]. The “‘In probes are incorporated in the sample with a concentration of approximately 10” cmm3, detecting one EFG for each crystallographically equivalent “‘In-site in a phase. Having a noncubic lattice structure at room temperature, most of the investigated phases in the Cu-In-S system can be characterized by one or more well defined EFGs, which are identified by two quantities: the strength (parameter v,) and the symmetry (parameter 77). For more details about PAC see e.g. ref. [4]. 0168-583X/92/$05.00
0 1992 - Elsevier
Science
Publishers
In this paper we present three applications for the PAC method concerning the material development of CuInS,: i) Single- and multiphase bulk samples were prepared using In enriched with “‘In. A complete set of PAC parameters for all In-containing phases in the Cu-In-S system that are stable at room temperature was drawn up. With this set of data we are able to identify phases appearing in other experiments. The phase relations in the system are studied. ii) In thin film experiments “‘In probes are used for the observation of the phase formation. iii) By means of nuclear in reaction doping “‘In atoms can be incorporated CuInSz samples which were earlier produced for other purposes. In the following sections the three applications are discussed including examples and the experimental techniques.
2. Investigation
of bulk phases
Our investigations of the bulk phases in the Cu-InS system are described in detail in other publications [3,5,6], so that only a concise presentation is given here. All sample preparations presented in this section were performed using metallic In enriched with “‘In. Consequently the PAC spectra produce a true image of all In-containing phases of the specimen. For details of sample preparations see ref. [3]. The experiment aimed at two points: first, the PAC parameters of all In-containing compounds in the CuIn-S system which are stable at room temperature were determined by preparing the corresponding single phase samples and characterizing them by PAC. In a
B.V. All rights reserved
232
Chr. Dzionk et al. / Decelopment of CuInS, solar cell material
second step the phase relations of CuInS, at room temperature were studied. Multiphase samples were grown from the melt and homogenized to establish thermal equilibrium. The resulting phases have been identified by means of their PAC parameters. This shows which phases can coexist with CuInS, in equilibrium leading to the tie-lines between CuInS, and the respective phases in the Cu-In-S phase diagram. Fig. 1 shows the Cu-In-S phase diagram indicating the prepared compounds and the determined phase relations. Next to CuInS,, CuIn,S, and all Cu-In phases which are stable at room temperature were prepared. Thereby it could be shown that CuIn,, which by then was only known from thin film experiments, also exists as a bulk phase. In the In-S diagram only In,& was produced because the parameters of the other phases were known [7,8]. A table of all PAC parameters can be found in’ref. [6]. The results of the prepared multiphase samples (points A to E in fig. 1) lead to seven new tie-lines; another eight are supposed to exist with high probability (dotted lines). This shows that CuInS, can be produced in equilibrium with nearly all known phases in the diagram.
3. Thin film experiments Several methods for the preparation of CuInS, thin films have been applied up to date. We consider two techniques to be very promising for a precise control of thin film formation in order to yield high efficiency solar cell material: a) The “reactive annealing” method [9]. There, CuIn films are exposed to an atmosphere of hydrogen sulphide and annealed at appropriate temperatures. In this case, the influence of parameters such as annealing time and temperature and the influence of the initial Cu : In ratio on the resulting film composition should be examined in detail. b) The “three source evaporation” method [lo]. In this case, Cu, In and S are evaporated from appropriate sources. Even though the method fulfills the requirements for well controlled CuInS, thin film preparation, there may occur some difficulties concerning the evaporation of elementary sulphur [lo]. In order to apply the PAC method to the control of thin film formation, we have designed an UHV chamber for film preparation and in situ PAC measure-
Fig. 1. Cu-In-S phase diagram at room temperature. The points A to E mark the studied multiphase samples to gain information represented by the tie-lines. The dotted lines are proposed ones. PAC signatures of the on the phase relations of CuInS,, underlined phases are known.
Chr. Dzionk et al. / Decelopment of CuInS, solar cell material
ments. The chamber is provided with three Knudsen cells as evaporation sources. Two techniques can be used to incorporate “‘In in the thin films: The In starting material for evaporation can be homogeneously doped with the “‘In probe atoms using a technique described earlier [3]. Therefore all In-containing phases produced are enriched with “‘In and so they are distinguishable by PAC. The other way is to deposit “‘In probes direct at interfaces. Since interfaces are very important for the properties of thin film devices, the possibility to particularly investigate them is one of the advantages of the PAC method. In the following we report our first experiments. Using the reactive annealing method, a 12.5 nm Cu-In film with a 1: 1 atomic ratio, the In homogeneously doped with “‘In, was deposited on a glass substrate and then transferred into a quartz tube which was placed in a furnace at 400 o C. It has then been exposed to a H,S atmosphere for 2 h. Fig. 2 shows the PAC spectrum of the resulting film. For 70% of the probe atoms cubic symmetry at the site of the “‘In atoms is found which can be related to the CuInS, phase [3], for the other 30% there is an EFG with the parameters vo = 22.6(2) MHz and 77= 0, which belongs to a phase that has not yet been reported. In order to avoid the problems connected with the evaporation of elementary sulphur, we investigated the possibility of using InS as starting material. First, we prepared a sample of InS with little In excess by melting In doped with “‘In in a sulphur atmosphere for several hours followed by tempering at 560°C for one day. The PAC measurement of this source mate-
0.021
I’
1
Thin
film
containing
233
InS
and
In
0.00
2 y
-0.05
-0.10 I
I
0
100
200 time
400
300 [ns]
Fig. 3. PAC spectrum of a 3 urn film evaporated at 800°C from a sample with starting composition InssS,,. Two noncubic components are found, which belong to In and Ins.
rial showed the existence of the phases In and InS in the ratio 6: 94, as it was expected from the phase diagram. This material was then used to evaporate three films each of them about 3 urn thick onto glass substrates, varying the source temperature from 760 OC up to 900°C. The PAC spectra of the thin films showed again the appearance of the phases In and InS (fig. 31, but with a higher In to InS ratio compared to the source material. A PAC measurement of the source material after the evaporation process showed virtually single-phase In,&, which means a more sulphur-rich composition compared with the starting material. Nevertheless, an increasing Ins: In ratio with increasing source temperature has been found (fig. 4). This shows
0.00 InS /
In
ratio
-0.04 Q
1.50
g-O.06
_-:oy , -0.08
0
200
L time
,I
i? 1 .oo e
0.50
-
400
[nt]
Fig. 2. PAC spectrum of a 125 nm Cu-In thin film with a Cu: In ratio of 1: 1, annealed in H,S atmosphere at 400 o C for 2 h. It shows cubic symmetry for 70% of the probes, related to CuIn&, while for 30% there is an EFG with the parameters ho = 22.6(2) MHz and n = 0 (unknown phase up to now).
0.00’
750
800 evaporation
850 temperature
900 PC]
Fig. 4. The ratio of Ins/In on the substrates when evaporating In,,S,, as a function of evaporation temperature.
Chr. Dzionk et al. / Development of CulnS, solar cell material
233
J
I..
0
100
300
200 time
[ns]
Fig. 5. PAC spectrum of a CuInS, sample irradiated with a 90 MeV “He beam. Radiation damages were annealed at 700 ’ C. The spectrum shows cubic symmetry with a slight decrease of anisotropy with time which is probably an effect of randomly distributed lattice defects.
that the sulphur content of the evaporated layer can be varied by a change of temperature.
4. Nuclear reaction doping
This type of experiment offers the opportunity to incorporate “‘In into the samples by means of heavy ion induced nuclear reactions. In that way specimens containing CuInS, and other In-containing phases can
be studied which were not prepared using “‘In. This method makes it possible to characterize samples produced by other groups, too. Both bulk samples and thin films can be used. The defects created during irradiation can be removed by annealing. The experiments are performed at the heavy ion accelerator facility VICKSI at the Hahn-Meitner-Institut in Berlin. Two ways of producing the “‘In were developed: In-containing bulk material is irradiated by a 3He beam of 90 MeV. The “‘In isotopes in the sample are converted to “‘In via the main nuclear reaction channels ‘151nt3He, p6n) and “51n(3He, 7n). The PAC spectrum of a CuInS, sample activated in this way, annealed at 700 ‘C, is shown in fig. 5. It indicates that nearly 100% of the probes are on lattice sites with cubic symmetry, as it is expected for the In-site in the CuInS, chalcopyrite structure [3]. The decrease of anisotropy can be explained by randomly distributed lattice defects. The spectrum is similar to that measured for a CuInS, sample prepared with In containing “lIn (see ref. [3]). In the other case a 90 MeV ‘*Ne beam is used to produce “IIn in a 2 Frn thin Nb-foil by the main reaction channels 93Nb(22Ne, 4n) and y3Nb(22Ne, p3n), both after decays leading to “‘In. The Nb-foil is located before the sample (see fig. 6). The product nuclei receive kinetic energies up to 18 MeV. Therefore they leave the foil and are stopped in the sample in depths of up to 5 pm. This offers the opportunity to examine thin films. Direct beam passage can be avoided by taking advantage of the different scattering angles for the 22Ne beam and the recoil products due to their different energy and atomic number [ll]. Because the
water
cooling 22Ne
IPI
i
Fig. 6. Geometry of the nuclear doping experiment of thin film samples using a ‘*Ne beam. The product nuclei, which decay to the PAC probe “‘In, are implanted, while the primary beam passes the sample and therefore does not cause radiation damage.
Chr. Dzionk et al. / Development of CuInS, solar cell material
“iIn probes are incorporated also in phases which do not contain inactive In, in this experiment the PAC is also sensitive to Cu-S phases.
5. Conclusions
We have demonstrated that the method of perturbed angular correlations with “‘In probes is a suitable method to contribute to the material development of CuInS,. A complete set of PAC parameters for all In-containing phases of the bulk system at room temperature is now available. Further, the phase relations of CuInS, were investigated. Relying on these measurements the formation of CuInS, thin films has been studied. In first experiments the sulphurization of a Cu-In film was carried out, where a new phase was found, and the possibility of evaporating sulphur from the InS compound was explored. Moreover, we demonstrated the possibility of radioactively doping CuInS, thin film or bulk samples that were prepared for other purposes using a nuclear reaction doping technique with high energy *‘Ne or ‘He beams.
Acknowledgement
We thank K. Dietz for supplying the CuInS, sample used for the nuclear reaction doping experiment.
235
References
[l] H.J. Lewerenz, H. Goslowsky, K.-D. Husemann and S. Fiechter, Nature 321 (1986) 687. [2] K.W. Mitchell, G.A. Pollock and A.V. Mason, Conf. Record of the 20th IEEE Photovoltaic Specialist Conf., Las Vegas, 1988, vol. 2, p. 1542. [3] M. Briissler, H. Metzner, K.-D. Husemann and H.J. Lewerenz, Phys. Rev. B38 (1988) 9268. [4] W. Witthuhn and W. Engel, in: Hyperfine Interactions of Radioactive Nuclei, ed. J. Christiansen (Springer, Heidelberg, 1983) p. 205. [5] Th. Wichert, W. Witthuhn, H. Metzner and R. Sielemann, in: Submicroscopic Studies of Defects in Semiconductors, ed. G. Langouche (North-Holland, Amsterdam, in press). [6] H. Metzner, M. Briissler, K.-D. Husemann and H.J. Lewerenz, submitted for publication (1991). [7] H. Haas and D.A. Shirley, J. Chem. Phys. 58 (1973) 3339. [8] M. Frank, F. Gubitz, W. Ittner, W. Kreische, A. Labahn, B. Roseler and G. Weeske, Z. Naturforsch, 41a (1986) 104. [9] C.D. Lokhande and G. Hodes, Solar Cells 21 (1987) 215. [lo] Y.L. Wu, H.Y. Lin, C.Y. Sun, M.H. Yang and H.L. Hwang, Thin Solid Films 168 (1989) 113. [ll] M. Briissler, Doctoral thesis, FU Berlin (1989).
and Methods
in Physics Research
Nuclear lnstrwnents 8 Methods in Physics Research
863 (1992) 231-235
Sectton
B
Development of CuInS, solar cell material by PAC: bulk phases, thin films, and nuclear reaction doping Chr. Dzionk, M. Briissler and H. Metzner Hahn - Meitner-Institut, Bereich Schwerionenphysik, 1000 Berlin 39, Germany
of CuInS*. Perturbed angular correlations (PAC) of y-rays using “‘In nuclear probes are utilized for the material development Three applications are presented which are a suitable tool to contribute to this complex: i) Using metallic In containing “‘In as one of the starting materials, single- and multiphase bulk samples were grown from the melt and studied by PAC, which leads to a complete data set of PAC signatures for all In-containing phases of the Q-In-S system that are stable at room temperature, as “‘In is controlled by PAC observing the phase well as to the phase relations of CuIn$. ii) The preparation of thin films containing formation. In first experiments, a thin Cu-In film was sulphurized, and the evaporation of sulphur out of the InS compound was “‘In afterwards in CuInS, thin films and studied. iii) The method of nuclear reaction doping offers the possibility to incorporate bulk samples earlier produced for other purposes.
1. Introduction With a direct energy gap of Eg = 1.5 eV the chalcopyrite semiconductor CuInS, is a promising material for photovoltaic devices. However, the best conversion efficiency obtained until now for solar cells based on CuInS, bulk material is only 9.7% [l]. Using CuInS, as an absorber material, Mitchell et al. [2] have recently achieved a value of 7.3% in a thin film solar cell. In both of these cases the results were obtained with heterogeneous samples containing precipitates of In, In,S3 or Cu,_,S. This shows that for the development of this material accurate investigations about the phase formation are necessary. Therefore, the knowledge of the phase relations of CuInS, in the Cu-In-S phase diagram is important. Recently we have reported first applications of perturbed angular correlations (PAC) of y-rays using “‘In nuclear probes to study phases and phase relations in the Cu-In-S system [3]. The PAC method yields information on the local environment of nuclear probe atoms in solids. We use the fact that the “‘In nuclei are sensitive to the electric field gradient (EFG) at the site of the probe in a non-cubic lattice structure [4]. The “‘In probes are incorporated in the sample with a concentration of approximately 10” cmm3, detecting one EFG for each crystallographically equivalent “‘In-site in a phase. Having a noncubic lattice structure at room temperature, most of the investigated phases in the Cu-In-S system can be characterized by one or more well defined EFGs, which are identified by two quantities: the strength (parameter v,) and the symmetry (parameter 77). For more details about PAC see e.g. ref. [4]. 0168-583X/92/$05.00
0 1992 - Elsevier
Science
Publishers
In this paper we present three applications for the PAC method concerning the material development of CuInS,: i) Single- and multiphase bulk samples were prepared using In enriched with “‘In. A complete set of PAC parameters for all In-containing phases in the Cu-In-S system that are stable at room temperature was drawn up. With this set of data we are able to identify phases appearing in other experiments. The phase relations in the system are studied. ii) In thin film experiments “‘In probes are used for the observation of the phase formation. iii) By means of nuclear in reaction doping “‘In atoms can be incorporated CuInSz samples which were earlier produced for other purposes. In the following sections the three applications are discussed including examples and the experimental techniques.
2. Investigation
of bulk phases
Our investigations of the bulk phases in the Cu-InS system are described in detail in other publications [3,5,6], so that only a concise presentation is given here. All sample preparations presented in this section were performed using metallic In enriched with “‘In. Consequently the PAC spectra produce a true image of all In-containing phases of the specimen. For details of sample preparations see ref. [3]. The experiment aimed at two points: first, the PAC parameters of all In-containing compounds in the CuIn-S system which are stable at room temperature were determined by preparing the corresponding single phase samples and characterizing them by PAC. In a
B.V. All rights reserved
232
Chr. Dzionk et al. / Decelopment of CuInS, solar cell material
second step the phase relations of CuInS, at room temperature were studied. Multiphase samples were grown from the melt and homogenized to establish thermal equilibrium. The resulting phases have been identified by means of their PAC parameters. This shows which phases can coexist with CuInS, in equilibrium leading to the tie-lines between CuInS, and the respective phases in the Cu-In-S phase diagram. Fig. 1 shows the Cu-In-S phase diagram indicating the prepared compounds and the determined phase relations. Next to CuInS,, CuIn,S, and all Cu-In phases which are stable at room temperature were prepared. Thereby it could be shown that CuIn,, which by then was only known from thin film experiments, also exists as a bulk phase. In the In-S diagram only In,& was produced because the parameters of the other phases were known [7,8]. A table of all PAC parameters can be found in’ref. [6]. The results of the prepared multiphase samples (points A to E in fig. 1) lead to seven new tie-lines; another eight are supposed to exist with high probability (dotted lines). This shows that CuInS, can be produced in equilibrium with nearly all known phases in the diagram.
3. Thin film experiments Several methods for the preparation of CuInS, thin films have been applied up to date. We consider two techniques to be very promising for a precise control of thin film formation in order to yield high efficiency solar cell material: a) The “reactive annealing” method [9]. There, CuIn films are exposed to an atmosphere of hydrogen sulphide and annealed at appropriate temperatures. In this case, the influence of parameters such as annealing time and temperature and the influence of the initial Cu : In ratio on the resulting film composition should be examined in detail. b) The “three source evaporation” method [lo]. In this case, Cu, In and S are evaporated from appropriate sources. Even though the method fulfills the requirements for well controlled CuInS, thin film preparation, there may occur some difficulties concerning the evaporation of elementary sulphur [lo]. In order to apply the PAC method to the control of thin film formation, we have designed an UHV chamber for film preparation and in situ PAC measure-
Fig. 1. Cu-In-S phase diagram at room temperature. The points A to E mark the studied multiphase samples to gain information represented by the tie-lines. The dotted lines are proposed ones. PAC signatures of the on the phase relations of CuInS,, underlined phases are known.
Chr. Dzionk et al. / Decelopment of CuInS, solar cell material
ments. The chamber is provided with three Knudsen cells as evaporation sources. Two techniques can be used to incorporate “‘In in the thin films: The In starting material for evaporation can be homogeneously doped with the “‘In probe atoms using a technique described earlier [3]. Therefore all In-containing phases produced are enriched with “‘In and so they are distinguishable by PAC. The other way is to deposit “‘In probes direct at interfaces. Since interfaces are very important for the properties of thin film devices, the possibility to particularly investigate them is one of the advantages of the PAC method. In the following we report our first experiments. Using the reactive annealing method, a 12.5 nm Cu-In film with a 1: 1 atomic ratio, the In homogeneously doped with “‘In, was deposited on a glass substrate and then transferred into a quartz tube which was placed in a furnace at 400 o C. It has then been exposed to a H,S atmosphere for 2 h. Fig. 2 shows the PAC spectrum of the resulting film. For 70% of the probe atoms cubic symmetry at the site of the “‘In atoms is found which can be related to the CuInS, phase [3], for the other 30% there is an EFG with the parameters vo = 22.6(2) MHz and 77= 0, which belongs to a phase that has not yet been reported. In order to avoid the problems connected with the evaporation of elementary sulphur, we investigated the possibility of using InS as starting material. First, we prepared a sample of InS with little In excess by melting In doped with “‘In in a sulphur atmosphere for several hours followed by tempering at 560°C for one day. The PAC measurement of this source mate-
0.021
I’
1
Thin
film
containing
233
InS
and
In
0.00
2 y
-0.05
-0.10 I
I
0
100
200 time
400
300 [ns]
Fig. 3. PAC spectrum of a 3 urn film evaporated at 800°C from a sample with starting composition InssS,,. Two noncubic components are found, which belong to In and Ins.
rial showed the existence of the phases In and InS in the ratio 6: 94, as it was expected from the phase diagram. This material was then used to evaporate three films each of them about 3 urn thick onto glass substrates, varying the source temperature from 760 OC up to 900°C. The PAC spectra of the thin films showed again the appearance of the phases In and InS (fig. 31, but with a higher In to InS ratio compared to the source material. A PAC measurement of the source material after the evaporation process showed virtually single-phase In,&, which means a more sulphur-rich composition compared with the starting material. Nevertheless, an increasing Ins: In ratio with increasing source temperature has been found (fig. 4). This shows
0.00 InS /
In
ratio
-0.04 Q
1.50
g-O.06
_-:oy , -0.08
0
200
L time
,I
i? 1 .oo e
0.50
-
400
[nt]
Fig. 2. PAC spectrum of a 125 nm Cu-In thin film with a Cu: In ratio of 1: 1, annealed in H,S atmosphere at 400 o C for 2 h. It shows cubic symmetry for 70% of the probes, related to CuIn&, while for 30% there is an EFG with the parameters ho = 22.6(2) MHz and n = 0 (unknown phase up to now).
0.00’
750
800 evaporation
850 temperature
900 PC]
Fig. 4. The ratio of Ins/In on the substrates when evaporating In,,S,, as a function of evaporation temperature.
Chr. Dzionk et al. / Development of CulnS, solar cell material
233
J
I..
0
100
300
200 time
[ns]
Fig. 5. PAC spectrum of a CuInS, sample irradiated with a 90 MeV “He beam. Radiation damages were annealed at 700 ’ C. The spectrum shows cubic symmetry with a slight decrease of anisotropy with time which is probably an effect of randomly distributed lattice defects.
that the sulphur content of the evaporated layer can be varied by a change of temperature.
4. Nuclear reaction doping
This type of experiment offers the opportunity to incorporate “‘In into the samples by means of heavy ion induced nuclear reactions. In that way specimens containing CuInS, and other In-containing phases can
be studied which were not prepared using “‘In. This method makes it possible to characterize samples produced by other groups, too. Both bulk samples and thin films can be used. The defects created during irradiation can be removed by annealing. The experiments are performed at the heavy ion accelerator facility VICKSI at the Hahn-Meitner-Institut in Berlin. Two ways of producing the “‘In were developed: In-containing bulk material is irradiated by a 3He beam of 90 MeV. The “‘In isotopes in the sample are converted to “‘In via the main nuclear reaction channels ‘151nt3He, p6n) and “51n(3He, 7n). The PAC spectrum of a CuInS, sample activated in this way, annealed at 700 ‘C, is shown in fig. 5. It indicates that nearly 100% of the probes are on lattice sites with cubic symmetry, as it is expected for the In-site in the CuInS, chalcopyrite structure [3]. The decrease of anisotropy can be explained by randomly distributed lattice defects. The spectrum is similar to that measured for a CuInS, sample prepared with In containing “lIn (see ref. [3]). In the other case a 90 MeV ‘*Ne beam is used to produce “IIn in a 2 Frn thin Nb-foil by the main reaction channels 93Nb(22Ne, 4n) and y3Nb(22Ne, p3n), both after decays leading to “‘In. The Nb-foil is located before the sample (see fig. 6). The product nuclei receive kinetic energies up to 18 MeV. Therefore they leave the foil and are stopped in the sample in depths of up to 5 pm. This offers the opportunity to examine thin films. Direct beam passage can be avoided by taking advantage of the different scattering angles for the 22Ne beam and the recoil products due to their different energy and atomic number [ll]. Because the
water
cooling 22Ne
IPI
i
Fig. 6. Geometry of the nuclear doping experiment of thin film samples using a ‘*Ne beam. The product nuclei, which decay to the PAC probe “‘In, are implanted, while the primary beam passes the sample and therefore does not cause radiation damage.
Chr. Dzionk et al. / Development of CuInS, solar cell material
“iIn probes are incorporated also in phases which do not contain inactive In, in this experiment the PAC is also sensitive to Cu-S phases.
5. Conclusions
We have demonstrated that the method of perturbed angular correlations with “‘In probes is a suitable method to contribute to the material development of CuInS,. A complete set of PAC parameters for all In-containing phases of the bulk system at room temperature is now available. Further, the phase relations of CuInS, were investigated. Relying on these measurements the formation of CuInS, thin films has been studied. In first experiments the sulphurization of a Cu-In film was carried out, where a new phase was found, and the possibility of evaporating sulphur from the InS compound was explored. Moreover, we demonstrated the possibility of radioactively doping CuInS, thin film or bulk samples that were prepared for other purposes using a nuclear reaction doping technique with high energy *‘Ne or ‘He beams.
Acknowledgement
We thank K. Dietz for supplying the CuInS, sample used for the nuclear reaction doping experiment.
235
References
[l] H.J. Lewerenz, H. Goslowsky, K.-D. Husemann and S. Fiechter, Nature 321 (1986) 687. [2] K.W. Mitchell, G.A. Pollock and A.V. Mason, Conf. Record of the 20th IEEE Photovoltaic Specialist Conf., Las Vegas, 1988, vol. 2, p. 1542. [3] M. Briissler, H. Metzner, K.-D. Husemann and H.J. Lewerenz, Phys. Rev. B38 (1988) 9268. [4] W. Witthuhn and W. Engel, in: Hyperfine Interactions of Radioactive Nuclei, ed. J. Christiansen (Springer, Heidelberg, 1983) p. 205. [5] Th. Wichert, W. Witthuhn, H. Metzner and R. Sielemann, in: Submicroscopic Studies of Defects in Semiconductors, ed. G. Langouche (North-Holland, Amsterdam, in press). [6] H. Metzner, M. Briissler, K.-D. Husemann and H.J. Lewerenz, submitted for publication (1991). [7] H. Haas and D.A. Shirley, J. Chem. Phys. 58 (1973) 3339. [8] M. Frank, F. Gubitz, W. Ittner, W. Kreische, A. Labahn, B. Roseler and G. Weeske, Z. Naturforsch, 41a (1986) 104. [9] C.D. Lokhande and G. Hodes, Solar Cells 21 (1987) 215. [lo] Y.L. Wu, H.Y. Lin, C.Y. Sun, M.H. Yang and H.L. Hwang, Thin Solid Films 168 (1989) 113. [ll] M. Briissler, Doctoral thesis, FU Berlin (1989).