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Defects in CuIn(Ga)Se2 solar cell material characterized by positron annihilation: post-growth annealing effects
Physica B 273}274 (1999) 930}933
Defects in CuIn(Ga)Se solar cell material characterized 2 by positron annihilation: post-growth annealing e!ects F. BoK rner!,*, J. Gebauer!, S. Eichler!, R. Krause-Rehberg!, I. Dirnstorfer", B.K. Meyer", F. Karg# !Fachbereich Physik, Martin-Luther-Universita( t Halle-Wittenberg, D-06108 Halle (Saale), Germany "I. Physikalisches Institut, Justus-Liebig-Universita( t, Heinrich-Buw-Ring 16, D-35392 Gie}en, Germany #Siemens Solar GmbH, PF 460705, D-80807 Mu( nchen, Germany
Abstract Thin (&1 lm) CuIn(Ga)Se layers were grown by the rapid thermal processing technique under In-rich conditions. 2 The as-grown samples were slightly p-type but highly compensated. They showed strong positron trapping in vacancies, indicated by a large valence annihilation parameter S. In order to identify the vacancies, we applied a novel PAS-method. In that, we compare the element-speci"c high-momentum part of the annihilation momentum distribution f (p) in CuInSe with f (p) from the pure elements constituting the material (i.e. with Cu, In and Se). The results provide direct 2 evidence that the vacancies in our as-grown CuInSe layers are related to Cu vacancies. Annealing under Ar atmosphere 2 did not alter the annihilation characteristics, i.e. it did not a!ect the vacancies. However, after annealing in air the samples become more heavily p-type and less compensated, whereas S is drastically reduced. PAS measurements as a function of temperature revealed that this e!ect is not due to a reduction of the vacancy concentration but due to the additional presence of negatively charged ions introduced by the annealing process. According to recent results, these ions are attributed to oxygen acceptors O . These new acceptors account for the increase of p-type conductivity after air S% annealing. ( 1999 Elsevier Science B.V. All rights reserved. Keywords: CuInSe ; Cu vacancies; Post-growth annealing; Positron annihilation 2
1. Introduction The chalcopyrite semiconductor CuInSe (CIS) is a 2 promising material for high-e$ciency photovoltaic applications. The defect physics of that system is known to be complicated due to the large number of possible intrinsic defects and defect complexes. However, understanding of the material requires knowledge about native defects which are expected to dominate its properties. Although much e!ort has been devoted to the investigation of di!erent electrical levels, little is known on the
* Corresponding author. Tel.: #49-345-5525570; fax: #49345-5527160. E-mail address: [email protected] (F. BoK rner)
nature of the defects producing these levels. Recent theoretical calculations indicate that Cu vacancies (< ) C6 should be the most abundant defect in CuInSe due to a 2 low formation energy of < [1]. C6 CIGS-layer grown under In-rich conditions are commonly used for solar cell applications. Such layers are highly defective and heavily compensated as indicated by, e.g., photoluminescence (PL). Post-growth annealing of CIGS-layers in oxygen or air atmosphere at temperatures of about 200}4003C is of great bene"t to the solar cell e$ciency [2,3]. However, the structural changes leading to this improvement are unknown. In this work we investigate CuIn(Ga)Se (CIGS) thin solar cell layers 2 using the method of positron annihilation spectroscopy (PAS) which provides structural sensitivity especially for vacancies and negative ions.
0921-4526/99/$ - see front matter ( 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 0 5 5 7 - 8
F. Bo( rner et al. / Physica B 273}274 (1999) 930}933
2. Experimental details The CIGS samples studied in this work were grown by the rapid thermal processing (RTP) technique. The "rst step consists of a deposition of Cu, Ga, In and Se on a soda-lime glass substrate by sputtering and evaporation. The precursor was then annealed in a RTP-furnace at about 5003C. The Ga/Cu ratio was "xed at 10%. The "nal thickness of the CIGS-layer was about 1 lm. Details about the growth process can be found elsewhere [4]. Annealing was carried out in a conventional furnace at 4003C under air or Ar atmosphere. The positron annihilation experiments were done with the slow positron beam system (POSSY) and the Doppler-coincidence setup in Halle. The POSSY consists of a 22NaCl positron source (0.5 GBq), a linear accelerator (up to 40 keV) and a Ge-c-detector. The resolution of the detector is 1.5 keV FHWM at 514 keV (85Sr). The sample temperature could be varied from 20 to 600 K. The Doppler coincidence setup consists of two Ge-c-detectors. The resolution of the system is 1.15 keV with a peak to background ratio of 105. Details about both setups can be found elsewhere [5]. During di!usion in a crystal a positron can be trapped by a vacancy. This results in a narrowing of the 511 keV annihilation line compared to defect free material. Experimentally, the annihilation line is characterized by the line shape parameters S and =. S is the relative fraction of counts in the center of the Doppler broadened annihilation spectrum (511$0.8 keV), = is the relative fraction of counts in the wings of the spectrum (2.76 keV( DE -511 keVD(3.96 keV). Positron trapping at vacancy c defects results in an increase (decrease) of S (=) since annihilation at defects occurs mainly with low momentum electrons. Every kind of annihilation site, e.g. bulk, surface, di!erent defects, yields characteristic (=, S) values.
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Positrons are sensitive for negative or neutral openvolume defects (vacancies, complexes containing vacancies, etc.). Vacancies were found in CIS bulk crystals by positron lifetime spectroscopy [7]. However, the identi"cation was di$cult due to the large number of possible vacancy defects and the insensitivity of positron lifetime spectroscopy to the chemical surrounding of a vacancy. Fig. 1 shows the S parameter as a function of incident positron energy for the as-grown CIGS layer and the layers annealed in Ar atmosphere at 4003C or in air at 3003C and 4003C. S is normalized to the value we assign to bulk CIGS (see below). The S parameter pro"le re#ects the structure of the layered CIGS/Mo/glass solar cell system. Up to positron energies of 18 keV (corresponding to about 1 lm mean implantation depth) the S parameter can be assigned to the CIGS layer. The variation of S at low energies (E(5 keV) is due to positron trapping at the surface. For higher incident energies, S is due to annihilation in the Mo/glass substrate and is therefore not relevant for this study. Annealing in air at 4003C distinctly reduces the S parameter of the CIGS layer. This decrease can only be explained as a lost of positron trapping at vacancies. This implies that the as-grown CIGS layer contains a high density of vacancy defects. The relative change of S is in the order 2%. However, annealing under Ar at 4003C or under air below ¹"4003C did not change the annihilation signal in the CIGS layer at all. This indicates that a reaction with oxygen at higher temperatures is responsible for the change of the annihilation parameter. A S versus = analysis [8] showed that the defect distribution within all CIGS-layers is homogeneous for the high-temperature air-annealed sample this means especially that the oxygen reacts throughout the whole layer. This fact is also indicated by the constant S parameter (Fig. 1). This is in agreement with the results of Niki et al. [9], who detected Oxygen by SIMS throughout a CIS layer after similar air annealing. Moreover, the vacancy
3. Results and discussion The annealing of In-rich CIGS layer in air atmosphere reduces the high compensation of the charge carriers in the In-rich CIGS layers. This e!ect can be observed by PL. The dependence on the excitation power of the PL peak energy was reduced from 10 to 1 meV/dekade after annealing at 4003C in air [6]. This e!ect was not seen after annealing the layers in Ar atmosphere. The reduction of compensation is con"rmed by Hall-measurements. The hole concentration of the as-grown In-rich layers is in the order of 1016 cm~3. After annealing the layer in air this concentration increases to 5]1018 cm~3. A more e!ective p-doping of the sample can be caused by an increasing number of acceptors or by a reduction of donors. The e!ect of the air annealing was interpreted as being likely due to a reduction of the donor density [6].
Fig. 1. The S parameter is shown as a function of the incident positron energy for the CuIn(Ga)Se /Mo/glass solar-cell layer 2 structure in the as-grown state (j), annealed in air at 3003C (n) and at 4003C (v) and annealed in Ar at 4003C (e).
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F. Bo( rner et al. / Physica B 273}274 (1999) 930}933
type is the same in the as-grown, Ar-annealed and lowtemperature air-annealed layers. Niki et al. [9] found a very similar e!ect, i.e. the average positron lifetime decreases in CIS-layers after annealing at 4003C under air. It was concluded that a structural change happens, i.e. the divacancies postulated to exist in as-grown CIS would change to monovacancy}oxygen complexes (e.g. V }V PV }O ) now dominating positron C6 S% C6 S% trapping. In order to get further information about the charge state of the defects detected by positrons and the whole defect structure in the CIGS-layers we performed measurements as a function of temperature from 20}600 K (Fig. 2). In the as-grown layer, S decreases only very slightly as a function of temperature. This increase can be explained by thermal lattice expansion. There are two possibilities to explain this behavior. First, the vacancy defects obviously present are neutrally charged, thus positron trapping at vacancies would be independent of temperature. Secondly, the vacancies are negatively charged but their density is so large that all positrons are trapped. Than, the S parameter is independent of temperature too although positron trapping at negative vacancies is expected to increase strongly with decreasing temperature. In contrast to as-grown material, the S parameter shows a distinct dependence on temperature in annealed CIGS. S is almost constant at ¹(300 K and increases strongly when the temperature is further increased. This behavior can only be explained if a second defect is present which traps positrons only at low temperatures but with a S parameter much lower than that of the vacancies. Such defects are commonly attributed to iontype acceptors which trap positrons in their attractive, shallow potential only at low temperatures. Since these defects do not have an open volume, the annihilation parameter are similar to the values found in defect-free material. Therefore, we attribute the S parameter at low
Fig. 2. (a) S parameter of the CIGS layers as a function of the measurement temperature for as-grown (j) and air-annealed (v) CIGS. (b) S parameter versus the = parameter for as grown and air annealed CIGS. The S parameters are the same ones as in part (a).
temperatures (¹(200 K) to the bulk value, used also for the normalization of the data. The strong increase of S at higher temperatures shows that vacancies are still present after annealing. The S versus = plot on the right-hand side of Fig. 2 shows that the vacancy type is similar to in as-grown and annealed CIGS since the data fall on a similar linear variation. The S parameter for the vacancies in as-grown CIGS must be at least 3% larger than the bulk value (Fig. 2). This is in the upper range expected for a monovacancy according to general knowledge. Indeed, Niki et al. [9] interpreted their lifetime results in as-grown CIS as being due to divacancies. However, the S parameter is not only dependent upon the open volume like the positron lifetime, i.e. it depends also on the chemical environment. Thus, we cannot de"nitely discriminate between mono- and divacancies in our samples. The behavior of the S parameter in annealed CIGS is typical when positrons are trapped at neutral vacancies and negative ions. An according "t to the data (solid line in Fig. 2a) yields a binding energy to the negative ions of 93$20 meV, in agreement to the value 88$5 meV found in electron irradiated CIS bulk material [7]. A natural question is then the nature of the negative ions. Because the ions appear only after annealing in air they must be related to the oxygen known to penetrate the layer. In a recent work, Su-Huai Wei et al. [10] showed by theoretical calculations that oxygen on a Se site acts as deep acceptor although oxygen is formally isoelectronic. Thus, oxygen on a Se site is a perfect candidate for the negative ions detected by positrons. We conclude that the change in compensation ratio after air annealing is probably not alone due to a reduction of the donor concentration (e.g. V ) but rather due to an inS% crease of the O acceptor density. S% The remaining problem is the nature of the vacancy defects which could not be addressed by the measurements above. For this reason we performed measurements with the Doppler broadening coincidence technique. With that technique, the annihilation momentum distribution can be measured up to rather high momenta due to a strong reduction of the background. It is known that the shape of the annihilation momentum distribution at high momenta (p *12]10~3m c) deL 0 pends only on the chemical nature of the annihilation site because high-momentum core electrons retain their element speci"c properties even when atoms form a solid. In Fig. 3, the annihilation momentum distribution measured at room temperature is shown for as-grown and air-annealed CIGS and several reference samples. The spectra are normalized by taking the ratio to a GaAs : Zn reference sample free from positron trapping. As references for the chemical environment we measured the single components of the chalcopyrite compound, i.e. Cu, In and Se. The references were annealed prior to the measurements, conventional positron
F. Bo( rner et al. / Physica B 273}274 (1999) 930}933
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4. Conclusions Native defects in CuIn(Ga)Se were observed by 2 means of positron annihilation. As grown samples are characterized by a high density of vacancy defects which are shown to be related to Cu vacancies. After annealing under air at 4003C negative ions acting as shallow positron traps were found. These ions are most probably O acceptors. The decrease of compensation in S% CuIn(Ga)Se annealed under air can thus be explained as 2 being due to an increasing density of acceptors making the material more p-type.
Fig. 3. Annihilation momentum distribution of the single elements Cu (r), In (m), Se (.) and the chalcopyrite CuInSe 2 compound in the as-grown (h) and the annealed (L) state. The data are normalized by taking the ratio to a GaAs : Zn reference sample free from positron trapping.
lifetime spectroscopy con"rmed that they are free from positron trapping at vacancies. Provided that the high momentum distribution depends only on the chemical nature of the annihilation site, it should be possible to "nd contributions from the single elements also in the spectra from CIGS. Thus, a "t of the momentum distribution according to f (p)"I ]f (p)#I ] CIGS C6 C6 I/ f (p)#I ]f (p) was done. Here, f (p) is the normalized I/ S% S% i annihilation momentum distribution for material i whereas the I characterizes its weight (RI "1). Indeed, i i the spectra for as-grown and annealed CIGS could be well composed of the single-element contributions as indicated by the solid lines (Fig. 3). The resulting weights are I "0.079, I "0.086 and I "0.835 for as grown C6 I/ S% CIGS and I "0.205, I "0.132 and I "0.663 for C6 I/ S% annealed material. Note that absolute values of these weight factors are meaningless because they depend on the intensity of the spectra which is not only a function of the chemical environment but depends also on the lattice structure and interatomic distances. Only relative changes between di!erent samples can be interpreted. In our data, obviously the fraction of Cu and, less distinctly, of In increases in annealed CIGS whereas the contribution from Se decreases. According to the results above, the annihilation momentum distribution in annealed CIGS should be close to that of the bulk. Thus, at the vacancy in as-grown CIGS annihilation takes place in a Cu-de"cient environment, i.e. the vacancies should be related to V . This is in accordance with the theoretical calcuC6 lations of Zhang et al. [1].
Acknowledgements This work was supported by the Bundesland SachsenAnhalt and in part by the Deutsche Forschungsgemeinschaft.
References [1] S.B. Zhang, Su-Huai Wei, A. Zunger, H, KatayamaYoshida, Phys. Rev. B 57 (1998) 9642. [2] R.J. Matson, R. Nou", K.J. Bachmann, D. Cahen, Appl. Phys. Let. 50 (1987) 158. [3] E. Moons, D. Gal, J. Beier, G. Hodes, D. Cahen, L. Kronik, L. Burnstein, B. Mishori, Y. Shapira, D. Hariskos, H.W. Schock, Sol. Energy Mater. Sol. Cells 35 (1996) 73. [4] V. Probst, F. Karg, J. Rimmasch, W. Riedl, W. Stetter, H. Harms, O. Eibl, in: D.C. Ginley, A. Schock, H.W. Eberspacher, C. Peterson, T.M. Wada, T. Pittsburgh (Eds.), Thin Films for Photovoltaic and Related Device Applications, Material Research Society 1996, p. 165. [5] R. Krause-Rehberg, H.S. Leipner, Positron annihilation in semiconductors, Springer, Verlag, Berlin, 1999. [6] I. Dirnstorfer, D.M. Ho!mann, D. Meister, B.K. Meyer, W. Riedl, F. Karg, J. Appl. Phys. 85 (1999) 1423. [7] A. Polity, R. Krause-Rehberg, T.E.M. Staab, M.J. Puska, J. Klais, H.J. MoK ller, B.K. Meyer, J. Appl. Phys. 83 (1998) 71. [8] L. Liszkay, C. Corbel, L. Baroux, P. HautojaK rvi, M. Bayhan, A.W. Brinkmann, S. Tatarenko, Appl. Phys. Lett. 64 (1994) 1380. [9] S. Niki, P. J. Fons, A. Yamada, R. Suzuki, T. Ohdaira, S. Ishibashi, H. Oyanagi, Proceedings of the 11th International Conference on Ternary and Multinary Compounds. Institute of Physics Publishing, 1998, p. 221. [10] Wei. Su-Huai, S.B. Zhang, A. Zunger, J. Appl. Phys. 85 (1999) 7214.
Defects in CuIn(Ga)Se solar cell material characterized 2 by positron annihilation: post-growth annealing e!ects F. BoK rner!,*, J. Gebauer!, S. Eichler!, R. Krause-Rehberg!, I. Dirnstorfer", B.K. Meyer", F. Karg# !Fachbereich Physik, Martin-Luther-Universita( t Halle-Wittenberg, D-06108 Halle (Saale), Germany "I. Physikalisches Institut, Justus-Liebig-Universita( t, Heinrich-Buw-Ring 16, D-35392 Gie}en, Germany #Siemens Solar GmbH, PF 460705, D-80807 Mu( nchen, Germany
Abstract Thin (&1 lm) CuIn(Ga)Se layers were grown by the rapid thermal processing technique under In-rich conditions. 2 The as-grown samples were slightly p-type but highly compensated. They showed strong positron trapping in vacancies, indicated by a large valence annihilation parameter S. In order to identify the vacancies, we applied a novel PAS-method. In that, we compare the element-speci"c high-momentum part of the annihilation momentum distribution f (p) in CuInSe with f (p) from the pure elements constituting the material (i.e. with Cu, In and Se). The results provide direct 2 evidence that the vacancies in our as-grown CuInSe layers are related to Cu vacancies. Annealing under Ar atmosphere 2 did not alter the annihilation characteristics, i.e. it did not a!ect the vacancies. However, after annealing in air the samples become more heavily p-type and less compensated, whereas S is drastically reduced. PAS measurements as a function of temperature revealed that this e!ect is not due to a reduction of the vacancy concentration but due to the additional presence of negatively charged ions introduced by the annealing process. According to recent results, these ions are attributed to oxygen acceptors O . These new acceptors account for the increase of p-type conductivity after air S% annealing. ( 1999 Elsevier Science B.V. All rights reserved. Keywords: CuInSe ; Cu vacancies; Post-growth annealing; Positron annihilation 2
1. Introduction The chalcopyrite semiconductor CuInSe (CIS) is a 2 promising material for high-e$ciency photovoltaic applications. The defect physics of that system is known to be complicated due to the large number of possible intrinsic defects and defect complexes. However, understanding of the material requires knowledge about native defects which are expected to dominate its properties. Although much e!ort has been devoted to the investigation of di!erent electrical levels, little is known on the
* Corresponding author. Tel.: #49-345-5525570; fax: #49345-5527160. E-mail address: [email protected] (F. BoK rner)
nature of the defects producing these levels. Recent theoretical calculations indicate that Cu vacancies (< ) C6 should be the most abundant defect in CuInSe due to a 2 low formation energy of < [1]. C6 CIGS-layer grown under In-rich conditions are commonly used for solar cell applications. Such layers are highly defective and heavily compensated as indicated by, e.g., photoluminescence (PL). Post-growth annealing of CIGS-layers in oxygen or air atmosphere at temperatures of about 200}4003C is of great bene"t to the solar cell e$ciency [2,3]. However, the structural changes leading to this improvement are unknown. In this work we investigate CuIn(Ga)Se (CIGS) thin solar cell layers 2 using the method of positron annihilation spectroscopy (PAS) which provides structural sensitivity especially for vacancies and negative ions.
0921-4526/99/$ - see front matter ( 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 0 5 5 7 - 8
F. Bo( rner et al. / Physica B 273}274 (1999) 930}933
2. Experimental details The CIGS samples studied in this work were grown by the rapid thermal processing (RTP) technique. The "rst step consists of a deposition of Cu, Ga, In and Se on a soda-lime glass substrate by sputtering and evaporation. The precursor was then annealed in a RTP-furnace at about 5003C. The Ga/Cu ratio was "xed at 10%. The "nal thickness of the CIGS-layer was about 1 lm. Details about the growth process can be found elsewhere [4]. Annealing was carried out in a conventional furnace at 4003C under air or Ar atmosphere. The positron annihilation experiments were done with the slow positron beam system (POSSY) and the Doppler-coincidence setup in Halle. The POSSY consists of a 22NaCl positron source (0.5 GBq), a linear accelerator (up to 40 keV) and a Ge-c-detector. The resolution of the detector is 1.5 keV FHWM at 514 keV (85Sr). The sample temperature could be varied from 20 to 600 K. The Doppler coincidence setup consists of two Ge-c-detectors. The resolution of the system is 1.15 keV with a peak to background ratio of 105. Details about both setups can be found elsewhere [5]. During di!usion in a crystal a positron can be trapped by a vacancy. This results in a narrowing of the 511 keV annihilation line compared to defect free material. Experimentally, the annihilation line is characterized by the line shape parameters S and =. S is the relative fraction of counts in the center of the Doppler broadened annihilation spectrum (511$0.8 keV), = is the relative fraction of counts in the wings of the spectrum (2.76 keV( DE -511 keVD(3.96 keV). Positron trapping at vacancy c defects results in an increase (decrease) of S (=) since annihilation at defects occurs mainly with low momentum electrons. Every kind of annihilation site, e.g. bulk, surface, di!erent defects, yields characteristic (=, S) values.
931
Positrons are sensitive for negative or neutral openvolume defects (vacancies, complexes containing vacancies, etc.). Vacancies were found in CIS bulk crystals by positron lifetime spectroscopy [7]. However, the identi"cation was di$cult due to the large number of possible vacancy defects and the insensitivity of positron lifetime spectroscopy to the chemical surrounding of a vacancy. Fig. 1 shows the S parameter as a function of incident positron energy for the as-grown CIGS layer and the layers annealed in Ar atmosphere at 4003C or in air at 3003C and 4003C. S is normalized to the value we assign to bulk CIGS (see below). The S parameter pro"le re#ects the structure of the layered CIGS/Mo/glass solar cell system. Up to positron energies of 18 keV (corresponding to about 1 lm mean implantation depth) the S parameter can be assigned to the CIGS layer. The variation of S at low energies (E(5 keV) is due to positron trapping at the surface. For higher incident energies, S is due to annihilation in the Mo/glass substrate and is therefore not relevant for this study. Annealing in air at 4003C distinctly reduces the S parameter of the CIGS layer. This decrease can only be explained as a lost of positron trapping at vacancies. This implies that the as-grown CIGS layer contains a high density of vacancy defects. The relative change of S is in the order 2%. However, annealing under Ar at 4003C or under air below ¹"4003C did not change the annihilation signal in the CIGS layer at all. This indicates that a reaction with oxygen at higher temperatures is responsible for the change of the annihilation parameter. A S versus = analysis [8] showed that the defect distribution within all CIGS-layers is homogeneous for the high-temperature air-annealed sample this means especially that the oxygen reacts throughout the whole layer. This fact is also indicated by the constant S parameter (Fig. 1). This is in agreement with the results of Niki et al. [9], who detected Oxygen by SIMS throughout a CIS layer after similar air annealing. Moreover, the vacancy
3. Results and discussion The annealing of In-rich CIGS layer in air atmosphere reduces the high compensation of the charge carriers in the In-rich CIGS layers. This e!ect can be observed by PL. The dependence on the excitation power of the PL peak energy was reduced from 10 to 1 meV/dekade after annealing at 4003C in air [6]. This e!ect was not seen after annealing the layers in Ar atmosphere. The reduction of compensation is con"rmed by Hall-measurements. The hole concentration of the as-grown In-rich layers is in the order of 1016 cm~3. After annealing the layer in air this concentration increases to 5]1018 cm~3. A more e!ective p-doping of the sample can be caused by an increasing number of acceptors or by a reduction of donors. The e!ect of the air annealing was interpreted as being likely due to a reduction of the donor density [6].
Fig. 1. The S parameter is shown as a function of the incident positron energy for the CuIn(Ga)Se /Mo/glass solar-cell layer 2 structure in the as-grown state (j), annealed in air at 3003C (n) and at 4003C (v) and annealed in Ar at 4003C (e).
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type is the same in the as-grown, Ar-annealed and lowtemperature air-annealed layers. Niki et al. [9] found a very similar e!ect, i.e. the average positron lifetime decreases in CIS-layers after annealing at 4003C under air. It was concluded that a structural change happens, i.e. the divacancies postulated to exist in as-grown CIS would change to monovacancy}oxygen complexes (e.g. V }V PV }O ) now dominating positron C6 S% C6 S% trapping. In order to get further information about the charge state of the defects detected by positrons and the whole defect structure in the CIGS-layers we performed measurements as a function of temperature from 20}600 K (Fig. 2). In the as-grown layer, S decreases only very slightly as a function of temperature. This increase can be explained by thermal lattice expansion. There are two possibilities to explain this behavior. First, the vacancy defects obviously present are neutrally charged, thus positron trapping at vacancies would be independent of temperature. Secondly, the vacancies are negatively charged but their density is so large that all positrons are trapped. Than, the S parameter is independent of temperature too although positron trapping at negative vacancies is expected to increase strongly with decreasing temperature. In contrast to as-grown material, the S parameter shows a distinct dependence on temperature in annealed CIGS. S is almost constant at ¹(300 K and increases strongly when the temperature is further increased. This behavior can only be explained if a second defect is present which traps positrons only at low temperatures but with a S parameter much lower than that of the vacancies. Such defects are commonly attributed to iontype acceptors which trap positrons in their attractive, shallow potential only at low temperatures. Since these defects do not have an open volume, the annihilation parameter are similar to the values found in defect-free material. Therefore, we attribute the S parameter at low
Fig. 2. (a) S parameter of the CIGS layers as a function of the measurement temperature for as-grown (j) and air-annealed (v) CIGS. (b) S parameter versus the = parameter for as grown and air annealed CIGS. The S parameters are the same ones as in part (a).
temperatures (¹(200 K) to the bulk value, used also for the normalization of the data. The strong increase of S at higher temperatures shows that vacancies are still present after annealing. The S versus = plot on the right-hand side of Fig. 2 shows that the vacancy type is similar to in as-grown and annealed CIGS since the data fall on a similar linear variation. The S parameter for the vacancies in as-grown CIGS must be at least 3% larger than the bulk value (Fig. 2). This is in the upper range expected for a monovacancy according to general knowledge. Indeed, Niki et al. [9] interpreted their lifetime results in as-grown CIS as being due to divacancies. However, the S parameter is not only dependent upon the open volume like the positron lifetime, i.e. it depends also on the chemical environment. Thus, we cannot de"nitely discriminate between mono- and divacancies in our samples. The behavior of the S parameter in annealed CIGS is typical when positrons are trapped at neutral vacancies and negative ions. An according "t to the data (solid line in Fig. 2a) yields a binding energy to the negative ions of 93$20 meV, in agreement to the value 88$5 meV found in electron irradiated CIS bulk material [7]. A natural question is then the nature of the negative ions. Because the ions appear only after annealing in air they must be related to the oxygen known to penetrate the layer. In a recent work, Su-Huai Wei et al. [10] showed by theoretical calculations that oxygen on a Se site acts as deep acceptor although oxygen is formally isoelectronic. Thus, oxygen on a Se site is a perfect candidate for the negative ions detected by positrons. We conclude that the change in compensation ratio after air annealing is probably not alone due to a reduction of the donor concentration (e.g. V ) but rather due to an inS% crease of the O acceptor density. S% The remaining problem is the nature of the vacancy defects which could not be addressed by the measurements above. For this reason we performed measurements with the Doppler broadening coincidence technique. With that technique, the annihilation momentum distribution can be measured up to rather high momenta due to a strong reduction of the background. It is known that the shape of the annihilation momentum distribution at high momenta (p *12]10~3m c) deL 0 pends only on the chemical nature of the annihilation site because high-momentum core electrons retain their element speci"c properties even when atoms form a solid. In Fig. 3, the annihilation momentum distribution measured at room temperature is shown for as-grown and air-annealed CIGS and several reference samples. The spectra are normalized by taking the ratio to a GaAs : Zn reference sample free from positron trapping. As references for the chemical environment we measured the single components of the chalcopyrite compound, i.e. Cu, In and Se. The references were annealed prior to the measurements, conventional positron
F. Bo( rner et al. / Physica B 273}274 (1999) 930}933
933
4. Conclusions Native defects in CuIn(Ga)Se were observed by 2 means of positron annihilation. As grown samples are characterized by a high density of vacancy defects which are shown to be related to Cu vacancies. After annealing under air at 4003C negative ions acting as shallow positron traps were found. These ions are most probably O acceptors. The decrease of compensation in S% CuIn(Ga)Se annealed under air can thus be explained as 2 being due to an increasing density of acceptors making the material more p-type.
Fig. 3. Annihilation momentum distribution of the single elements Cu (r), In (m), Se (.) and the chalcopyrite CuInSe 2 compound in the as-grown (h) and the annealed (L) state. The data are normalized by taking the ratio to a GaAs : Zn reference sample free from positron trapping.
lifetime spectroscopy con"rmed that they are free from positron trapping at vacancies. Provided that the high momentum distribution depends only on the chemical nature of the annihilation site, it should be possible to "nd contributions from the single elements also in the spectra from CIGS. Thus, a "t of the momentum distribution according to f (p)"I ]f (p)#I ] CIGS C6 C6 I/ f (p)#I ]f (p) was done. Here, f (p) is the normalized I/ S% S% i annihilation momentum distribution for material i whereas the I characterizes its weight (RI "1). Indeed, i i the spectra for as-grown and annealed CIGS could be well composed of the single-element contributions as indicated by the solid lines (Fig. 3). The resulting weights are I "0.079, I "0.086 and I "0.835 for as grown C6 I/ S% CIGS and I "0.205, I "0.132 and I "0.663 for C6 I/ S% annealed material. Note that absolute values of these weight factors are meaningless because they depend on the intensity of the spectra which is not only a function of the chemical environment but depends also on the lattice structure and interatomic distances. Only relative changes between di!erent samples can be interpreted. In our data, obviously the fraction of Cu and, less distinctly, of In increases in annealed CIGS whereas the contribution from Se decreases. According to the results above, the annihilation momentum distribution in annealed CIGS should be close to that of the bulk. Thus, at the vacancy in as-grown CIGS annihilation takes place in a Cu-de"cient environment, i.e. the vacancies should be related to V . This is in accordance with the theoretical calcuC6 lations of Zhang et al. [1].
Acknowledgements This work was supported by the Bundesland SachsenAnhalt and in part by the Deutsche Forschungsgemeinschaft.
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