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In situ transmission electron microscopy studies of radiation damage in copper indium diselenide
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 242 (2006) 686–689 www.elsevier.com/locate/nimb
In situ transmission electron microscopy studies of radiation damage in copper indium diselenide S.E. Donnelly a
a,*
, J.A. Hinks a, P.D. Edmondson a, R.D. Pilkington a, M. Yakushev b, R.C. Birtcher c
Institute for Materials Research, Rm 106 Cockroft Building, University of Salford, Manchester M5 4WT, UK b Department of Physics, University of Strathclyde, Glasgow G4 0NG, UK c Materials Science Division, Argonne National Laboratory, Argonne, IL 60439, USA Available online 23 September 2005
Abstract The ternary semiconductor, CuInSe2 (CIS), is a promising semiconductor material for use in photovoltaic applications. Of particular interest is the high tolerance of this material to bombardment by energetic particles. This is of particular importance for photovoltaic applications in outer space where the lifetime of CIS-based solar cells has been found to be at least 50 times that of those based on amorphous silicon. In this paper we report on studies of the build-up of radiation damage in CIS during irradiation with Xe ions in the energy range 100–400 keV. Room temperature experiments indicate that dynamic annealing processes prevent the build-up of high levels of damage. However, for irradiation at a temperature of 50 K, the behaviour changes drastically with the material amorphising at low fluences. This effect is discussed in terms of defect mobility. 2005 Elsevier B.V. All rights reserved. PACS: 61.43.Dq; 61.80.Jh; 61.82.Fk; 68.37.Lp Keywords: Copper indium diselenide; Ion irradiation; TEM; Amorphisation
1. Introduction Radiation-hard semiconductor materials have been sought for a number of years due to the increased use of photovoltaics in space. However, radiation hardness also has importance in terrestrial applications in any environment in which materials are being subjected to damage caused by irradiation with energetic particles. Typical examples of radiation-induced problems include: reduced solar cell lifetimes due to amorphisation of the absorber layers and electronic device failure in environments such as those encountered during nuclear decommissioning. Copper indium diselenide (CuInSe2 or CIS) is a ternary compound (AI BII CVI 2 ) of chalcopyrite structure. It has
*
Corresponding author. Tel.: +44 161 295 5392; fax: +44 161 295 4382. E-mail address: [email protected] (S.E. Donnelly).
0168-583X/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.08.089
recently attracted a great deal of attention not only due to its radiation hardness [1] but also because of its photovoltaic efficiencies of up to 17.7% [2] (increasing to 19.2% for Cu(In,Ga)Se2 [3]). A short absorption distance [4] helps it to achieve a high specific power per unit weight and CIS thin-film based devices have relatively low production costs. It is these qualities, in particular its radiation hardness, which make it an ideal candidate for a variety of terrestrial and extraterrestrial applications. Although there has been considerable research into the electronic and optical properties of CIS, much less work has been carried out on the microstructural properties of this material. Of particular interest are the mechanisms responsible for the remarkable resistance of this material to damaging irradiation. CIS, whether prepared in single crystal or thin-film form, contains large populations of intrinsic defects which control the electro-optical characteristics [5]. Clearly,
S.E. Donnelly et al. / Nucl. Instr. and Meth. in Phys. Res. B 242 (2006) 686–689
687
displacing irradiation will, in general, introduce additional defects into the material; however, the reduction in performance of devices based on CIS is significantly less severe than that which would be produced in silicon devices under the same conditions. Unfortunately, little is known of the reasons for this radiation resistance. Clearly, a fundamental understanding of the material characteristics related to the defect structures would be of great importance in that it may offer the possibility of producing thin film CIS so as to achieve higher efficiency photovoltaic devices with an even greater degree of radiation resistance. This work reports some preliminary results on TEM observations of CIS irradiated, in situ, with energetic Xe ions at both room temperature and low temperature. 2. Experimental Single crystal CIS was grown using the standard Bridgeman growth technique that has been described in detail elsewhere [6]. Specimens were cut with a diamond saw before being prepared for transmission electron microscopy (TEM) either by electrochemical jet polishing or by ionbeam milling. The former method used a mixture of 30 ml HF, 30 ml acetic acid, 200 ml nitric acid, 300 ml methanol, 1 ml bromine and 8 ml phosphoric acid. The electrolyte was held at a temperature of 30 C and the jet nozzles and holder for the sample were PVC. The latter method involved mechanical polishing and dimpling using abrasives down to 1 lm followed by ion beam thinning at low energy (2.5 keV) and glancing angle (5). Specimens were then examined and irradiated in the IVEM/Accelerator Facility at Argonne National Laboratory [7]. The microscope in this facility is a Hitachi H-9000 (TEM) operating at 300 keV in which the ion beam is oriented 30 from the microscope axis. Specimens were irradiated with Xe+ ions at energies in the range 50– 400 keV. In addition to normal photographic recording, images from a Gatan 622 video camera and image-intensification system were viewed with total magnifications of approximately 2 million, and recorded on video tape with a time resolution of 33 ms (1/30 s – a single video frame). 3. Results
Fig. 1. Bright-field TEM micrographs of CIS (a) unirradiated (electron diffraction pattern inset); (b) following irradiation, at room temperature, with 100 keV ions to a fluence of 1014 ions/cm2 and (c) following irradiation, at room temperature, with 100 keV ions to a fluence of 4 · 1015 ions/cm2. See text for details. Note: scale marker applies to all three panels.
3.1. Room temperature experiments Fig. 1(a) shows the structure observed by TEM in a CIS specimen prepared by electrochemical thinning with its surface normal close to the 3-fold symmetric (2 2 1) zone axis. Prior to the irradiation experiments, it was unclear whether this contrast arose from surface roughness due to a non-uniform electrochemical etch or whether it was due to internal extended defects such as dislocations or stacking faults. Note that the symmetry of the defect structure reflects the symmetry of the crystal and was recorded with the electron beam tilted 1 or 2 from the
h2 2 1i zone axis. The inset diffraction pattern was recorded with the electron beam exactly aligned with the h2 2 1i zone axis. On beginning to irradiate the specimen with 100 keV Xe ions, in situ in the TEM, the anomalous defect structure was seen to disappear rapidly with complete disappearance by a fluence of approximately 1013 ions/cm2. At this stage the appearance of the specimen was similar to the micrograph in Fig. 1(b) which was, in fact, recorded after a fluence of 1014 ions/cm2. Much of the contrast in this image appears to be due to extended defects – probably
688
S.E. Donnelly et al. / Nucl. Instr. and Meth. in Phys. Res. B 242 (2006) 686–689
Fig. 2. Selected area diffraction patterns of CIS (a) unirradiated, (b) irradiated, at a temperature of 50 K, with 400 keV Xe+ ions to a fluence of 2.5 · 1013 ions/cm2 and (c) irradiated, at a temperature of 50 K, with 400 keV Xe+ ions to a fluence of 4 · 1013 ions/cm2. See text for details.
dislocation loops – which are seen to form, as a result of ion impacts and then move and disappear under subsequent irradiation. Qualitatively, this behaviour under the ion beam is very similar to that observed in Au and very different from that seen for heavy ion irradiation of Si where small static amorphous zones are formed by each impacting ion. It is important to note that, whether the initial 3-fold symmetric structure was due to a textured surface or internal extended defects, its disappearance by a fluence of 1013 ions/cm2 implies a very high mobility of defects either at the surface or in the bulk or both under displacing irradiation. Under continued irradiation, the defect formation and disappearance continued and there was little significant change to the appearance of the specimen apart from the eventual appearance of small defects identified by contrast changes on passing through focus as being bubbles, voids or precipitates with an inner potential less than that of the matrix. Fig. 1(c) shows these features under conditions of slight overfocus (about 1000 nm) at a dose of 4 · 1015 ions/cm2. Given that a very similar distribution of Xe bubbles would be seen in this fluence in other crystalline materials irradiated with Xe ions, this is the most likely interpretation. Note that even at this high fluence the material was still clearly crystalline with a diffraction pattern almost indistinguishable from that inset in Fig. 1(a), apart from slightly more diffuse scattering. 3.2. Low-temperature experiments Preliminary low-temperature experiments had indicated (results not shown here) that the CIS was at least partially amorphizing under Xe irradiation. However, it was impossible to produce specimens sufficiently thin to ensure significant levels of radiation damage through the thickness of the CIS TEM specimens using 100 keV Xe ions. In subsequent experiments, the incident ion energy was therefore increased to 400 keV and specimens were prepared by dimpling and ion-beam milling for the low-temperature experiments. This yielded a varying thickness profile toward the region where the specimen was perforated, with sufficient thin material around the hole to enable selected area diffraction to be carried out on regions thin enough to have
significant levels of damage across the entire thickness of the CIS. Fig. 2(a) shows an electron diffraction pattern from monocrystalline CIS, this time with a presumed (4 2 1) orientation, prior to irradiation with 400 keV Xe+ ions. Fig. 2(b) shows the same area following irradiation with 400 keV Xe+ ions to a fluence of 2.5 · 1013 ions/cm2. In total contrast to the specimens irradiated at room temperature the specimen is now largely amorphous and after a small increase in fluence to 4 · 1013 ions/cm2 the specimen is seen to be fully amorphous (Fig. 2(c)). 4. Discussion The very significant difference in the behaviour of CIS under irradiation at room temperature and at 50 K indicates that the defect or defects whose mobility prevents the build-up of damage to amorphousness at room temperature are immobile (or at least significantly less mobile) at 50 K. Unfortunately, little experimental information exists on defect formation and migration energies in CIS; however, Zhang et al., in a theoretical study [8], have reported that it is easier to create a neutral Cu vacancy in CIS than a cation vacancy in an analogous II–VI binary compound and that the formation energy of the Cu vacancy (VCu) is approximately 0.6 eV. At room temperature the Cu interstitial is known to migrate significant distances in CIS [9] but unfortunately no information exists on the mobility of the VCu; however either or both of these defects may play a role. 5. Conclusions We have presented some preliminary experiments on radiation damage in CIS at room temperature and at 50 K. In particular, we have reported here the first observation of ion-induced amorphisation of CIS at low temperature. Knowledge of the behaviour of CIS under displacing irradiation at low temperatures is of importance for the use of this material for photovoltaics in space. Although CIS is valued for its radiation hardness, this study has shown that
S.E. Donnelly et al. / Nucl. Instr. and Meth. in Phys. Res. B 242 (2006) 686–689
this property may disappear when in space and shadowed from the sun. Further work is underway to determine the threshold temperature for ion induced amorphization, and also to determine the stability (thermal and under ion irradiation) of amorphous CIS when subjected to irradiation with light ions as well as heavy ions. Acknowledgments We acknowledge funding from INTAS (2001-0283) that has made this collaborative project possible. Two of us (S.E.D., P.D.E.) would also like to acknowledge financial support from the Materials Science Division at Argonne National Laboratory that has enabled us to make extended visits to that laboratory.
689
References [1] G.A. Landis, A.F. Hepp, in: Proc. European Space Power Conf., Florence, 1991, p. 517. [2] T.J. Gillespie, C.H. Marshall, M.A. Contreras, J. Keane, Sol. Energ. Mat. Sol. Cells 59 (1999) 28. [3] K. Ramanathan et al., Prog. Photovolt. 11 (2003) 225. [4] O. Mah, Fundamentals of photovoltaic materials, National Solar Power Research Institute, Inc., 1998, p. 8. [5] F.A. Abou-Elfotouh, H. Moutinho, A. Bakry, T.J. Coutts, L.L. Kasmerski, Sol. Cells 30 (1991) 151. [6] R.D. Tomlinson, Sol. Cells 16 (1986) 17. [7] C.W. Allen, L.L. Funk, E.A. Ryan, S.T. Ockers, Nucl. Instr. and Meth. B 40/41 (1989) 553. [8] S.B. Zhang, Su-Huai Wei, A. Zunger, H. Katayama-Yoshida, Phys. Rev. B 57 (16) (1998) 9642. [9] V. Nadazdy, M. Yakushev, E.D. Djebbar, A.E. Hill, R.D. Tomlinson, J. Appl. Phys. 84 (8) (1998) 4322.
Nuclear Instruments and Methods in Physics Research B 242 (2006) 686–689 www.elsevier.com/locate/nimb
In situ transmission electron microscopy studies of radiation damage in copper indium diselenide S.E. Donnelly a
a,*
, J.A. Hinks a, P.D. Edmondson a, R.D. Pilkington a, M. Yakushev b, R.C. Birtcher c
Institute for Materials Research, Rm 106 Cockroft Building, University of Salford, Manchester M5 4WT, UK b Department of Physics, University of Strathclyde, Glasgow G4 0NG, UK c Materials Science Division, Argonne National Laboratory, Argonne, IL 60439, USA Available online 23 September 2005
Abstract The ternary semiconductor, CuInSe2 (CIS), is a promising semiconductor material for use in photovoltaic applications. Of particular interest is the high tolerance of this material to bombardment by energetic particles. This is of particular importance for photovoltaic applications in outer space where the lifetime of CIS-based solar cells has been found to be at least 50 times that of those based on amorphous silicon. In this paper we report on studies of the build-up of radiation damage in CIS during irradiation with Xe ions in the energy range 100–400 keV. Room temperature experiments indicate that dynamic annealing processes prevent the build-up of high levels of damage. However, for irradiation at a temperature of 50 K, the behaviour changes drastically with the material amorphising at low fluences. This effect is discussed in terms of defect mobility. 2005 Elsevier B.V. All rights reserved. PACS: 61.43.Dq; 61.80.Jh; 61.82.Fk; 68.37.Lp Keywords: Copper indium diselenide; Ion irradiation; TEM; Amorphisation
1. Introduction Radiation-hard semiconductor materials have been sought for a number of years due to the increased use of photovoltaics in space. However, radiation hardness also has importance in terrestrial applications in any environment in which materials are being subjected to damage caused by irradiation with energetic particles. Typical examples of radiation-induced problems include: reduced solar cell lifetimes due to amorphisation of the absorber layers and electronic device failure in environments such as those encountered during nuclear decommissioning. Copper indium diselenide (CuInSe2 or CIS) is a ternary compound (AI BII CVI 2 ) of chalcopyrite structure. It has
*
Corresponding author. Tel.: +44 161 295 5392; fax: +44 161 295 4382. E-mail address: [email protected] (S.E. Donnelly).
0168-583X/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.08.089
recently attracted a great deal of attention not only due to its radiation hardness [1] but also because of its photovoltaic efficiencies of up to 17.7% [2] (increasing to 19.2% for Cu(In,Ga)Se2 [3]). A short absorption distance [4] helps it to achieve a high specific power per unit weight and CIS thin-film based devices have relatively low production costs. It is these qualities, in particular its radiation hardness, which make it an ideal candidate for a variety of terrestrial and extraterrestrial applications. Although there has been considerable research into the electronic and optical properties of CIS, much less work has been carried out on the microstructural properties of this material. Of particular interest are the mechanisms responsible for the remarkable resistance of this material to damaging irradiation. CIS, whether prepared in single crystal or thin-film form, contains large populations of intrinsic defects which control the electro-optical characteristics [5]. Clearly,
S.E. Donnelly et al. / Nucl. Instr. and Meth. in Phys. Res. B 242 (2006) 686–689
687
displacing irradiation will, in general, introduce additional defects into the material; however, the reduction in performance of devices based on CIS is significantly less severe than that which would be produced in silicon devices under the same conditions. Unfortunately, little is known of the reasons for this radiation resistance. Clearly, a fundamental understanding of the material characteristics related to the defect structures would be of great importance in that it may offer the possibility of producing thin film CIS so as to achieve higher efficiency photovoltaic devices with an even greater degree of radiation resistance. This work reports some preliminary results on TEM observations of CIS irradiated, in situ, with energetic Xe ions at both room temperature and low temperature. 2. Experimental Single crystal CIS was grown using the standard Bridgeman growth technique that has been described in detail elsewhere [6]. Specimens were cut with a diamond saw before being prepared for transmission electron microscopy (TEM) either by electrochemical jet polishing or by ionbeam milling. The former method used a mixture of 30 ml HF, 30 ml acetic acid, 200 ml nitric acid, 300 ml methanol, 1 ml bromine and 8 ml phosphoric acid. The electrolyte was held at a temperature of 30 C and the jet nozzles and holder for the sample were PVC. The latter method involved mechanical polishing and dimpling using abrasives down to 1 lm followed by ion beam thinning at low energy (2.5 keV) and glancing angle (5). Specimens were then examined and irradiated in the IVEM/Accelerator Facility at Argonne National Laboratory [7]. The microscope in this facility is a Hitachi H-9000 (TEM) operating at 300 keV in which the ion beam is oriented 30 from the microscope axis. Specimens were irradiated with Xe+ ions at energies in the range 50– 400 keV. In addition to normal photographic recording, images from a Gatan 622 video camera and image-intensification system were viewed with total magnifications of approximately 2 million, and recorded on video tape with a time resolution of 33 ms (1/30 s – a single video frame). 3. Results
Fig. 1. Bright-field TEM micrographs of CIS (a) unirradiated (electron diffraction pattern inset); (b) following irradiation, at room temperature, with 100 keV ions to a fluence of 1014 ions/cm2 and (c) following irradiation, at room temperature, with 100 keV ions to a fluence of 4 · 1015 ions/cm2. See text for details. Note: scale marker applies to all three panels.
3.1. Room temperature experiments Fig. 1(a) shows the structure observed by TEM in a CIS specimen prepared by electrochemical thinning with its surface normal close to the 3-fold symmetric (2 2 1) zone axis. Prior to the irradiation experiments, it was unclear whether this contrast arose from surface roughness due to a non-uniform electrochemical etch or whether it was due to internal extended defects such as dislocations or stacking faults. Note that the symmetry of the defect structure reflects the symmetry of the crystal and was recorded with the electron beam tilted 1 or 2 from the
h2 2 1i zone axis. The inset diffraction pattern was recorded with the electron beam exactly aligned with the h2 2 1i zone axis. On beginning to irradiate the specimen with 100 keV Xe ions, in situ in the TEM, the anomalous defect structure was seen to disappear rapidly with complete disappearance by a fluence of approximately 1013 ions/cm2. At this stage the appearance of the specimen was similar to the micrograph in Fig. 1(b) which was, in fact, recorded after a fluence of 1014 ions/cm2. Much of the contrast in this image appears to be due to extended defects – probably
688
S.E. Donnelly et al. / Nucl. Instr. and Meth. in Phys. Res. B 242 (2006) 686–689
Fig. 2. Selected area diffraction patterns of CIS (a) unirradiated, (b) irradiated, at a temperature of 50 K, with 400 keV Xe+ ions to a fluence of 2.5 · 1013 ions/cm2 and (c) irradiated, at a temperature of 50 K, with 400 keV Xe+ ions to a fluence of 4 · 1013 ions/cm2. See text for details.
dislocation loops – which are seen to form, as a result of ion impacts and then move and disappear under subsequent irradiation. Qualitatively, this behaviour under the ion beam is very similar to that observed in Au and very different from that seen for heavy ion irradiation of Si where small static amorphous zones are formed by each impacting ion. It is important to note that, whether the initial 3-fold symmetric structure was due to a textured surface or internal extended defects, its disappearance by a fluence of 1013 ions/cm2 implies a very high mobility of defects either at the surface or in the bulk or both under displacing irradiation. Under continued irradiation, the defect formation and disappearance continued and there was little significant change to the appearance of the specimen apart from the eventual appearance of small defects identified by contrast changes on passing through focus as being bubbles, voids or precipitates with an inner potential less than that of the matrix. Fig. 1(c) shows these features under conditions of slight overfocus (about 1000 nm) at a dose of 4 · 1015 ions/cm2. Given that a very similar distribution of Xe bubbles would be seen in this fluence in other crystalline materials irradiated with Xe ions, this is the most likely interpretation. Note that even at this high fluence the material was still clearly crystalline with a diffraction pattern almost indistinguishable from that inset in Fig. 1(a), apart from slightly more diffuse scattering. 3.2. Low-temperature experiments Preliminary low-temperature experiments had indicated (results not shown here) that the CIS was at least partially amorphizing under Xe irradiation. However, it was impossible to produce specimens sufficiently thin to ensure significant levels of radiation damage through the thickness of the CIS TEM specimens using 100 keV Xe ions. In subsequent experiments, the incident ion energy was therefore increased to 400 keV and specimens were prepared by dimpling and ion-beam milling for the low-temperature experiments. This yielded a varying thickness profile toward the region where the specimen was perforated, with sufficient thin material around the hole to enable selected area diffraction to be carried out on regions thin enough to have
significant levels of damage across the entire thickness of the CIS. Fig. 2(a) shows an electron diffraction pattern from monocrystalline CIS, this time with a presumed (4 2 1) orientation, prior to irradiation with 400 keV Xe+ ions. Fig. 2(b) shows the same area following irradiation with 400 keV Xe+ ions to a fluence of 2.5 · 1013 ions/cm2. In total contrast to the specimens irradiated at room temperature the specimen is now largely amorphous and after a small increase in fluence to 4 · 1013 ions/cm2 the specimen is seen to be fully amorphous (Fig. 2(c)). 4. Discussion The very significant difference in the behaviour of CIS under irradiation at room temperature and at 50 K indicates that the defect or defects whose mobility prevents the build-up of damage to amorphousness at room temperature are immobile (or at least significantly less mobile) at 50 K. Unfortunately, little experimental information exists on defect formation and migration energies in CIS; however, Zhang et al., in a theoretical study [8], have reported that it is easier to create a neutral Cu vacancy in CIS than a cation vacancy in an analogous II–VI binary compound and that the formation energy of the Cu vacancy (VCu) is approximately 0.6 eV. At room temperature the Cu interstitial is known to migrate significant distances in CIS [9] but unfortunately no information exists on the mobility of the VCu; however either or both of these defects may play a role. 5. Conclusions We have presented some preliminary experiments on radiation damage in CIS at room temperature and at 50 K. In particular, we have reported here the first observation of ion-induced amorphisation of CIS at low temperature. Knowledge of the behaviour of CIS under displacing irradiation at low temperatures is of importance for the use of this material for photovoltaics in space. Although CIS is valued for its radiation hardness, this study has shown that
S.E. Donnelly et al. / Nucl. Instr. and Meth. in Phys. Res. B 242 (2006) 686–689
this property may disappear when in space and shadowed from the sun. Further work is underway to determine the threshold temperature for ion induced amorphization, and also to determine the stability (thermal and under ion irradiation) of amorphous CIS when subjected to irradiation with light ions as well as heavy ions. Acknowledgments We acknowledge funding from INTAS (2001-0283) that has made this collaborative project possible. Two of us (S.E.D., P.D.E.) would also like to acknowledge financial support from the Materials Science Division at Argonne National Laboratory that has enabled us to make extended visits to that laboratory.
689
References [1] G.A. Landis, A.F. Hepp, in: Proc. European Space Power Conf., Florence, 1991, p. 517. [2] T.J. Gillespie, C.H. Marshall, M.A. Contreras, J. Keane, Sol. Energ. Mat. Sol. Cells 59 (1999) 28. [3] K. Ramanathan et al., Prog. Photovolt. 11 (2003) 225. [4] O. Mah, Fundamentals of photovoltaic materials, National Solar Power Research Institute, Inc., 1998, p. 8. [5] F.A. Abou-Elfotouh, H. Moutinho, A. Bakry, T.J. Coutts, L.L. Kasmerski, Sol. Cells 30 (1991) 151. [6] R.D. Tomlinson, Sol. Cells 16 (1986) 17. [7] C.W. Allen, L.L. Funk, E.A. Ryan, S.T. Ockers, Nucl. Instr. and Meth. B 40/41 (1989) 553. [8] S.B. Zhang, Su-Huai Wei, A. Zunger, H. Katayama-Yoshida, Phys. Rev. B 57 (16) (1998) 9642. [9] V. Nadazdy, M. Yakushev, E.D. Djebbar, A.E. Hill, R.D. Tomlinson, J. Appl. Phys. 84 (8) (1998) 4322.