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Lattice location of implanted As in ZnO
Superlattices and Microstructures 42 (2007) 8–13 www.elsevier.com/locate/superlattices
Lattice location of implanted As in ZnO U. Wahl a,b,∗ , E. Rita a,b , J.G. Correia a,b , A.C. Marques b,c , E. Alves a,b , J.C. Soares b , The ISOLDE Collaboration c a Instituto Tecnol´ogico e Nuclear, Estrada Nacional 10, 2686-953 Sacav´em, Portugal b Centro de F´ısica Nuclear da Universidade de Lisboa, Avenida Prof. Gama Pinto 2, 1649-003 Lisboa, Portugal c CERN-PH, 1211 Geneva 23, Switzerland
Available online 4 June 2007
Abstract Radioactive 73 As ions were implanted into a ZnO single crystal at room temperature with 60 keV up to a fluence of 2 × 1013 cm−2 . Subsequently, the angular emission channeling patterns of emitted conversion electrons were recorded by means of a position-sensitive detector in the as-implanted state and following annealing up to 900 ◦ C, and were compared to simulated emission yields for a variety of different lattice sites. We find that As does not occupy substitutional O sites, but mainly occupies the substitutional Zn sites. The fraction of As on O sites was at most a few per cent. Arsenic in ZnO is thus an interesting example of an impurity in a semiconductor where the major impurity lattice site is determined by atomic size and electronegativity rather than its position in the periodic system. Possible consequences with respect to the role of arsenic as a p-type dopant in ZnO are being discussed. c 2007 Elsevier Ltd. All rights reserved.
Keywords: Lattice location; Ion implantation; ZnO:As; Emission channeling
1. Introduction Recently many authors have reported successful p-type doping of ZnO using the group V element As as a dopant [1–13]. However, since there exists a relatively large mismatch between ˚ and O2− (1.38 A), ˚ it was argued that As should have a low the ionic radii of As3− (2.22 A) solubility substituting for O in ZnO [14], raising doubts about whether it can act as an efficient ∗ Corresponding author at: Instituto Tecnol´ogico e Nuclear, Estrada Nacional 10, 2686-953 Sacav´em, Portugal. Tel.: +351 219946085; fax: +351 219941525. E-mail address: [email protected] (U. Wahl).
c 2007 Elsevier Ltd. All rights reserved. 0749-6036/$ - see front matter doi:10.1016/j.spmi.2007.04.052
U. Wahl et al. / Superlattices and Microstructures 42 (2007) 8–13
9
chemical p-type dopant. On the other hand, it has been suggested from theoretical considerations that the p-type character of As might actually not be due to simple chemical AsO acceptors but might result from the formation of more complicated defect complexes, possibly involving As on Zn sites (AsZn ) [15,16] which interacts with neighbouring Zn vacancies [16]. A crucial experiment to test the feasibility of this hypothesis is to determine the possible lattice sites of As in ZnO. We have recently carried out emission channeling experiments using the radioactive isotope 73 As implanted into ZnO [17], which provided direct evidence that As indeed prefers to occupy Zn sites following ion implantation. While in our previous paper we only briefly addressed specific issues of relevance to the perspectives of p-type doping of ZnO, this topic will be discussed in more detail here. 2. Experimental In emission channeling experiments [18], the lattice location of radioactive impurities is determined by detecting the angular distribution of the emitted charged particles from the decay (α, β − , β + or conversion electrons) around major crystallographic directions of a single crystal. Quantitative information about occupied lattice sites is then obtained by comparing the experimental to theoretical emission patterns for different lattice sites using appropriate fitting procedures. In our case we used the radioactive isotope 73 As (t1/2 = 80.3 d), which decays by means of electron capture (EC) into the excited state 73m Ge (499 ms). The de-excitation into the 73 Ge ground state is accompanied by the emission of 42.3 and 52.1 keV conversion electrons. Since only a small nuclear recoil of 0.54 eV is transferred to the 73 Ge atom during the EC decay and the atomic displacement energy of ZnO is 57 eV [19], the lattice location of 73m Ge should not be substantially altered with respect to 73 As. The sample was a commercially available ZnO [0001] crystal from Eagle Picher Technologies grown by the seeded chemical vapor transport method. Implantation of 73 As at room temperature into the polished O-face of the sample was carried out at the ISOLDE facility of CERN with 60 keV, a beam spot size of 1 mm diameter, fluence of 2.0 × 1013 cm−2 and under an angle of 7◦ from the surface normal. Using the SRIM-2003 simulation package [20] it was estimated that under these conditions the depth profile of the implanted ions is near-Gaussian with a projected ˚ and straggling of 11 nm (110 A). ˚ The peak 73 As concentration should be range of 24 nm (240 A) 18 −3 around 7.6 × 10 cm , i.e. 180 ppm with respect to either the Zn or O concentrations. Using the atomic displacement energy of 57 eV quoted above, it is estimated that each implanted ion creates around 435 Zn or O vacancies. Measurements of the angular-dependent emission of the conversion electrons were performed using a modified version of the energy- and position-sensitive Si detector system described in Ref. [21], the major modification being the use of a water-cooled detector [22] in order to improve the noise behaviour for detection of the relatively low-energy (40–50 keV) conversion electrons. More experimental details and brief descriptions of how to calculate theoretical emission patterns for different lattice sites and fit them to the experimental data can be found in Refs. [17,21]. 3. Results Fig. 1 [left hand side, panels (a)–(d)] shows the angular-dependent emission patterns of conversion electrons in the vicinity of the [0001], [−1102], [−1101], and [−2113] directions of the ZnO sample measured in the as-implanted state. Direct comparison of the experimental patterns to the best fit results, assuming only As on substitutional Zn sites and on random
10
U. Wahl et al. / Superlattices and Microstructures 42 (2007) 8–13
Fig. 1. Angular distribution of conversion electrons from 73 As → 73 Ge in ZnO measured in the room-temperature asimplanted state around the [0001] (a), [−1102] (b), [−1101] (c) and [−2113] (d) axis. (e)–(h) Best fits of the channeling patterns for each direction, corresponding to 78%, 62%, 85% and 73% of probe atoms at substitutional Zn sites. Note that only the [−1102], [−1101] and [−2113] patterns are suitable for distinguishing between substitutional Zn, substitutional O and interstitial T sites. The [0001] direction is characterized by mixed rows of Zn and O atoms and the interstitial T sites are also located within these rows, so that the same [0001] emission patterns are obtained for As on SZn , SO or T sites.
sites (right hand side, panels (e)–(h)), shows a very good correspondence. Whereas all patterns following 300 and 600 ◦ C annealing could be very well fitted assuming only As on Zn sites and on random sites, this was no longer the case following 900 ◦ C annealing. As is described in more detail in Ref. [17], in this case satisfactory fits could only be obtained by including in ˚ from interstitial T sites. In order to addition a fraction of As located within 0.04 nm (0.4 A) quantify fractions of emitter atoms on substitutional O sites and interstitial T sites, all patterns measured after the different annealing steps were fitted allowing for possible fractions of emitter ˚ from atoms on substitutional Zn sites, on substitutional O sites and on or within 0.04 nm (0.4 A) interstitial T sites. In addition, the root mean square (rms) displacement of emitter atoms from the substitutional Zn sites was optimized. The resulting fractions, obtained from an average of
U. Wahl et al. / Superlattices and Microstructures 42 (2007) 8–13
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Fig. 2. Fit results for the fractions of probe atoms as a function of annealing temperature allowing for As on three different regular lattice sites: substitutional Zn (SZn ), substitutional O (SO ) and interstitial T. The sum of these three regular fractions is also shown. The fraction of probe atoms accounting for the difference between the sum fraction and 100% is assumed to be located on random sites, i.e. sites which produce an isotropic contribution to the electron emission patterns.
the measurements of the four different crystal directions, are shown in Fig. 2. For all annealing temperatures the majority of As was found to occupy substitutional Zn sites, and only following vacuum annealing at 900 ◦ C did a significant fraction (28%) occupy sites close to the tetrahedral interstitial T site. On the other hand, the fitted fractions of emitter atoms on O sites were 5% at most in the as-implanted state, and either zero or slightly negative following all annealing steps. While this might indicate that a small fraction of As atoms initially occupied O sites, the experimental uncertainty due to statistical or systematic errors is of the same order of magnitude, which is for instance illustrated by the fact that the fits yielded a small negative AsO fraction following the last annealing step. The average displacements of the probe atoms on Zn sites from the ideal substitutional Zn position as obtained from the fits are shown in Fig. 3. As can be seen, displacements are ˚ somewhat larger than the thermal vibration amplitude of the Zn atoms, u 1 (Zn) = 0.080 A. 4. Discussion Our experiment clearly shows that the favoured lattice sites for As in ZnO are the substitutional Zn sites. As we have proposed previously [17], we attribute the Zn site character to the large size mismatch of As3− with O2− but the good match between As3+ and ˚ vs 0.60 A), ˚ its electronegativity of 2.0, which is closer to Zn (1.6) than to O (3.5), Zn2+ (0.58 A and its character as a semimetal, all of which make it energetically favorable for the As impurity to be incorporated on Zn sites [15,16]. The interesting issue of whether AsO , as is widely assumed, or AsZn –2VZn complexes, as suggested by Limpijumnong et al. [16], are responsible for the acceptor action in As-doped ZnO cannot be fully answered by our experiments. On the one hand, while the preference of As for substitutional Zn sites is clearly a strong argument in favour of the model put forward by Limpijumnong et al., we cannot fully confirm their structural model since the channeling effect is not suitable for characterizing the immediate neighbourhood of the probe atoms and we hence cannot probe whether the substitutional AsZn is paired with one or more vacancies. What we find is that the rms displacements of the probe atoms from the ideal substitutional Zn sites are
12
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Fig. 3. Best fit values for the one-dimensional root mean square (rms) displacements of probe atoms from substitutional Zn sites, determined from room temperature measurements following the annealing steps at different temperatures. Note that channeling experiments are only sensitive to the components of the rms displacements perpendicular to the corresponding channeling direction. The dotted and dashed lines indicate the rms displacements due to thermal vibrations of Zn and O atoms given in the literature.
relatively small, thus excluding the possibility of large lattice relaxations other than breathing mode relaxations. On the other hand, while we have proven that the majority of As prefers to occupy Zn sites, we do not have any information with respect to its electrical activity. Only a small number of publications reporting on p-type ZnO:As have specified the electrically active As concentration in their samples [2–4,12], while in all other cases either the As concentration or the electrical properties were not characterized quantitatively. From the limited number of data available it seems that no more than 5%–13% of incorporated As can be electrically activated. The maximum concentration of As in our implanted sample (around 7.6 × 1018 cm−3 ) is comparable with the As concentration used in most other studies. Although we are sure that in our case following annealing less than 5% of the implanted As occupied O sites, it cannot be excluded that by using different doping techniques around 10% of As might be incorporated on O sites and thus be responsible for the observed p-type conductivity. Our experiment also shows that a significant fraction of probe atoms (almost 30%) was found ˚ of interstitial T sites following vacuum annealing at 900 ◦ C. It is known within 0.04 nm (0.4 A) that annealing ZnO above 500 ◦ C under O-poor conditions can result in O loss and thus change the stoichiometry in the near-surface region [14]. Given the relatively shallow depth profile of the implanted As, it seems possible that some of the implanted probe atoms have reacted with O vacancies or Zn interstitials, causing changes in the local structure of As-related complexes which remove the As atoms from Zn sites. This should be investigated further by studying the lattice location of As following different annealing procedures, e.g. under O2 , N2 or in air, and possibly also via annealing at higher temperatures. We would like to point out that there also exists the possibility to do emission channeling experiments for two of the other group V candidates for p-type doping of ZnO. The lattice location of phosphorus can be studied using the radioactive β − emitters 32 P (t1/2 = 14.3 d) or 33 P (25.3 d) and the lattice location of antimony using 122 Sb (2.7 d), 124 Sb (60.3 d), or 126 Sb (12.4 d). While phosphorus is not available at the ISOLDE facility, laser-ionized beams of antimony have been produced recently [23] and we are therefore planning to perform lattice location of this element in ZnO.
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5. Conclusions We have provided direct evidence that the large majority of implanted As atoms in ZnO occupy substitutional Zn sites. While our experimental data do not rule out the possibility that small fractions (<5%) of As may have existed on O sites, or that larger fractions can be incorporated on O sites by using other doping techniques, the fact that As is not easily incorporated on O sites by ion implantation and annealing certainly does not encourage the widespread belief in its role as a possible chemical p-type dopant on O sites. On the other hand, our results clearly provide more credibility to the suggestion of Limpijumnong et al. that the acceptor action of As in ZnO might be due to the formation of AsZn in complexes with neighboring Zn vacancies. The fact that vacuum annealing at 900 ◦ C changed the lattice position for a significant fraction of the probe atoms indicates strongly that the nature of As-related complexes can be influenced by the interaction with point defects which can be promoted, for instance, via annealing under O-poor conditions. Acknowledgments This work was supported by the Portuguese Foundation for Science and Technology (FCT, project PDCT-FP-FNU-50145-2003) and by the European Union through its 6th Framework (RII3-EURONS, contract no. 506065). E. Rita and A.C. Marques acknowledge their fellowships from FCT, Portugal. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
Y.R. Ryu, S. Zhu, D.C. Look, J.M. Wrobel, H.M. Jeong, H.W. White, J. Cryst. Growth 216 (2000) 330. Y.R. Ryu, T.S. Lee, H.W. White, Appl. Phys. Lett. 83 (2003) 87. Y.R. Ryu, T.S. Lee, J.H. Leem, H.W. White, Appl. Phys. Lett. 83 (2003) 4032. D.C. Look, G.M. Renlund, R.H. Burgener, J.R. Sizelove, Appl. Phys. Lett. 85 (2004) 5269. W. Lee, M. Jeong, J. Myoung, Appl. Phys. Lett. 85 (2004) 6167. V. Vaithianathan, B. Lee, S.S. Kim, Appl. Phys. Lett. 86 (2005) 062101. W. Lee, M.C. Jeong, S.W. Joo, J.M. Myoung, Nanotechnology 16 (2005) 764. T.S. Jeong, M.S. Han, J.H. Kim, C.J. Youn, Y.R. Ryu, H.W. White, J. Cryst. Growth 275 (2005) 541. A. Dadgar, A. Krtschil, F. Bertram, S. Giemsch, T. Hempel, P. Veit, A. Diez, N. Oleynik, R. Clos, J. Christen, A. Krost, Superlatt. Microstruct. 38 (2005) 245. T.H. Moon, M.C. Jeong, W. Lee, J.M. Myoung, Appl. Surf. Sci. 240 (2005) 280. Y.R. Ryu, T.S. Lee, J.A. Lubguban, H.W. White, Y.S. Park, C.J. Youn, Appl. Phys. Lett. 87 (2005) 153504. G. Braunstein, A. Muraviev, H. Saxena, N. Dhere, V. Richter, R. Kalish, Appl. Phys. Lett. 87 (2005) 192103. A. Krtschil, A. Dadgar, N. Oleynik, J. Bl¨asing, A. Diez, A. Krost, Appl. Phys. Lett. 87 (2005) 262105. S.J. Pearton, D.P. Norton, K. Ip, Y.W. Heo, T. Steiner, J. Vac. Sci. Technol. B 22 (2004) 932. C.H. Park, S.B. Zhang, S.H. Wei, Phys. Rev. B 66 (2002) 073202. S. Limpijumnong, S.B. Zhang, S.H. Wei, C.H. Park, Phys. Rev. Lett. 92 (2004) 155504. U. Wahl, E. Rita, J.G. Correia, A.C. Marques, E. Alves, J.C. Soares, The ISOLDE Collaboration, Phys. Rev. Lett. 95 (2005) 215503. H. Hofs¨ass, G. Lindner, Phys. Rep. 210 (1991) 121. J.M. Meese, IEEE Trans. Nucl. Sci. NS 19 (1972) 237. J.F. Ziegler, Nucl. Instrum. Methods B 219 (2004) 1027. U. Wahl, J.G. Correia, A. Czermak, S.G. Jahn, P. Jalocha, J.G. Marques, A. Rudge, F. Schopper, A. Vantomme, P. Weilhammer, The ISOLDE Collaboration, Nucl. Instrum. Methods A 524 (2004) 245. A.C. Marques, U. Wahl, J.G. Correia, M.R. Silva, A. Rudge, P. Weilhammer, J.C. Soares, The ISOLDE Collaboration, Nucl. Instrum. Methods A 572 (2007) 1056. R. Catherall, V.N. Fedosseev, U. K¨oster, J. Lettry, G. Suberlucq, B.A. Marsh, E. Tengborn, The ISOLDE Collaboration, Rev. Sci. Instrum. 75 (2004) 1614.
Lattice location of implanted As in ZnO U. Wahl a,b,∗ , E. Rita a,b , J.G. Correia a,b , A.C. Marques b,c , E. Alves a,b , J.C. Soares b , The ISOLDE Collaboration c a Instituto Tecnol´ogico e Nuclear, Estrada Nacional 10, 2686-953 Sacav´em, Portugal b Centro de F´ısica Nuclear da Universidade de Lisboa, Avenida Prof. Gama Pinto 2, 1649-003 Lisboa, Portugal c CERN-PH, 1211 Geneva 23, Switzerland
Available online 4 June 2007
Abstract Radioactive 73 As ions were implanted into a ZnO single crystal at room temperature with 60 keV up to a fluence of 2 × 1013 cm−2 . Subsequently, the angular emission channeling patterns of emitted conversion electrons were recorded by means of a position-sensitive detector in the as-implanted state and following annealing up to 900 ◦ C, and were compared to simulated emission yields for a variety of different lattice sites. We find that As does not occupy substitutional O sites, but mainly occupies the substitutional Zn sites. The fraction of As on O sites was at most a few per cent. Arsenic in ZnO is thus an interesting example of an impurity in a semiconductor where the major impurity lattice site is determined by atomic size and electronegativity rather than its position in the periodic system. Possible consequences with respect to the role of arsenic as a p-type dopant in ZnO are being discussed. c 2007 Elsevier Ltd. All rights reserved.
Keywords: Lattice location; Ion implantation; ZnO:As; Emission channeling
1. Introduction Recently many authors have reported successful p-type doping of ZnO using the group V element As as a dopant [1–13]. However, since there exists a relatively large mismatch between ˚ and O2− (1.38 A), ˚ it was argued that As should have a low the ionic radii of As3− (2.22 A) solubility substituting for O in ZnO [14], raising doubts about whether it can act as an efficient ∗ Corresponding author at: Instituto Tecnol´ogico e Nuclear, Estrada Nacional 10, 2686-953 Sacav´em, Portugal. Tel.: +351 219946085; fax: +351 219941525. E-mail address: [email protected] (U. Wahl).
c 2007 Elsevier Ltd. All rights reserved. 0749-6036/$ - see front matter doi:10.1016/j.spmi.2007.04.052
U. Wahl et al. / Superlattices and Microstructures 42 (2007) 8–13
9
chemical p-type dopant. On the other hand, it has been suggested from theoretical considerations that the p-type character of As might actually not be due to simple chemical AsO acceptors but might result from the formation of more complicated defect complexes, possibly involving As on Zn sites (AsZn ) [15,16] which interacts with neighbouring Zn vacancies [16]. A crucial experiment to test the feasibility of this hypothesis is to determine the possible lattice sites of As in ZnO. We have recently carried out emission channeling experiments using the radioactive isotope 73 As implanted into ZnO [17], which provided direct evidence that As indeed prefers to occupy Zn sites following ion implantation. While in our previous paper we only briefly addressed specific issues of relevance to the perspectives of p-type doping of ZnO, this topic will be discussed in more detail here. 2. Experimental In emission channeling experiments [18], the lattice location of radioactive impurities is determined by detecting the angular distribution of the emitted charged particles from the decay (α, β − , β + or conversion electrons) around major crystallographic directions of a single crystal. Quantitative information about occupied lattice sites is then obtained by comparing the experimental to theoretical emission patterns for different lattice sites using appropriate fitting procedures. In our case we used the radioactive isotope 73 As (t1/2 = 80.3 d), which decays by means of electron capture (EC) into the excited state 73m Ge (499 ms). The de-excitation into the 73 Ge ground state is accompanied by the emission of 42.3 and 52.1 keV conversion electrons. Since only a small nuclear recoil of 0.54 eV is transferred to the 73 Ge atom during the EC decay and the atomic displacement energy of ZnO is 57 eV [19], the lattice location of 73m Ge should not be substantially altered with respect to 73 As. The sample was a commercially available ZnO [0001] crystal from Eagle Picher Technologies grown by the seeded chemical vapor transport method. Implantation of 73 As at room temperature into the polished O-face of the sample was carried out at the ISOLDE facility of CERN with 60 keV, a beam spot size of 1 mm diameter, fluence of 2.0 × 1013 cm−2 and under an angle of 7◦ from the surface normal. Using the SRIM-2003 simulation package [20] it was estimated that under these conditions the depth profile of the implanted ions is near-Gaussian with a projected ˚ and straggling of 11 nm (110 A). ˚ The peak 73 As concentration should be range of 24 nm (240 A) 18 −3 around 7.6 × 10 cm , i.e. 180 ppm with respect to either the Zn or O concentrations. Using the atomic displacement energy of 57 eV quoted above, it is estimated that each implanted ion creates around 435 Zn or O vacancies. Measurements of the angular-dependent emission of the conversion electrons were performed using a modified version of the energy- and position-sensitive Si detector system described in Ref. [21], the major modification being the use of a water-cooled detector [22] in order to improve the noise behaviour for detection of the relatively low-energy (40–50 keV) conversion electrons. More experimental details and brief descriptions of how to calculate theoretical emission patterns for different lattice sites and fit them to the experimental data can be found in Refs. [17,21]. 3. Results Fig. 1 [left hand side, panels (a)–(d)] shows the angular-dependent emission patterns of conversion electrons in the vicinity of the [0001], [−1102], [−1101], and [−2113] directions of the ZnO sample measured in the as-implanted state. Direct comparison of the experimental patterns to the best fit results, assuming only As on substitutional Zn sites and on random
10
U. Wahl et al. / Superlattices and Microstructures 42 (2007) 8–13
Fig. 1. Angular distribution of conversion electrons from 73 As → 73 Ge in ZnO measured in the room-temperature asimplanted state around the [0001] (a), [−1102] (b), [−1101] (c) and [−2113] (d) axis. (e)–(h) Best fits of the channeling patterns for each direction, corresponding to 78%, 62%, 85% and 73% of probe atoms at substitutional Zn sites. Note that only the [−1102], [−1101] and [−2113] patterns are suitable for distinguishing between substitutional Zn, substitutional O and interstitial T sites. The [0001] direction is characterized by mixed rows of Zn and O atoms and the interstitial T sites are also located within these rows, so that the same [0001] emission patterns are obtained for As on SZn , SO or T sites.
sites (right hand side, panels (e)–(h)), shows a very good correspondence. Whereas all patterns following 300 and 600 ◦ C annealing could be very well fitted assuming only As on Zn sites and on random sites, this was no longer the case following 900 ◦ C annealing. As is described in more detail in Ref. [17], in this case satisfactory fits could only be obtained by including in ˚ from interstitial T sites. In order to addition a fraction of As located within 0.04 nm (0.4 A) quantify fractions of emitter atoms on substitutional O sites and interstitial T sites, all patterns measured after the different annealing steps were fitted allowing for possible fractions of emitter ˚ from atoms on substitutional Zn sites, on substitutional O sites and on or within 0.04 nm (0.4 A) interstitial T sites. In addition, the root mean square (rms) displacement of emitter atoms from the substitutional Zn sites was optimized. The resulting fractions, obtained from an average of
U. Wahl et al. / Superlattices and Microstructures 42 (2007) 8–13
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Fig. 2. Fit results for the fractions of probe atoms as a function of annealing temperature allowing for As on three different regular lattice sites: substitutional Zn (SZn ), substitutional O (SO ) and interstitial T. The sum of these three regular fractions is also shown. The fraction of probe atoms accounting for the difference between the sum fraction and 100% is assumed to be located on random sites, i.e. sites which produce an isotropic contribution to the electron emission patterns.
the measurements of the four different crystal directions, are shown in Fig. 2. For all annealing temperatures the majority of As was found to occupy substitutional Zn sites, and only following vacuum annealing at 900 ◦ C did a significant fraction (28%) occupy sites close to the tetrahedral interstitial T site. On the other hand, the fitted fractions of emitter atoms on O sites were 5% at most in the as-implanted state, and either zero or slightly negative following all annealing steps. While this might indicate that a small fraction of As atoms initially occupied O sites, the experimental uncertainty due to statistical or systematic errors is of the same order of magnitude, which is for instance illustrated by the fact that the fits yielded a small negative AsO fraction following the last annealing step. The average displacements of the probe atoms on Zn sites from the ideal substitutional Zn position as obtained from the fits are shown in Fig. 3. As can be seen, displacements are ˚ somewhat larger than the thermal vibration amplitude of the Zn atoms, u 1 (Zn) = 0.080 A. 4. Discussion Our experiment clearly shows that the favoured lattice sites for As in ZnO are the substitutional Zn sites. As we have proposed previously [17], we attribute the Zn site character to the large size mismatch of As3− with O2− but the good match between As3+ and ˚ vs 0.60 A), ˚ its electronegativity of 2.0, which is closer to Zn (1.6) than to O (3.5), Zn2+ (0.58 A and its character as a semimetal, all of which make it energetically favorable for the As impurity to be incorporated on Zn sites [15,16]. The interesting issue of whether AsO , as is widely assumed, or AsZn –2VZn complexes, as suggested by Limpijumnong et al. [16], are responsible for the acceptor action in As-doped ZnO cannot be fully answered by our experiments. On the one hand, while the preference of As for substitutional Zn sites is clearly a strong argument in favour of the model put forward by Limpijumnong et al., we cannot fully confirm their structural model since the channeling effect is not suitable for characterizing the immediate neighbourhood of the probe atoms and we hence cannot probe whether the substitutional AsZn is paired with one or more vacancies. What we find is that the rms displacements of the probe atoms from the ideal substitutional Zn sites are
12
U. Wahl et al. / Superlattices and Microstructures 42 (2007) 8–13
Fig. 3. Best fit values for the one-dimensional root mean square (rms) displacements of probe atoms from substitutional Zn sites, determined from room temperature measurements following the annealing steps at different temperatures. Note that channeling experiments are only sensitive to the components of the rms displacements perpendicular to the corresponding channeling direction. The dotted and dashed lines indicate the rms displacements due to thermal vibrations of Zn and O atoms given in the literature.
relatively small, thus excluding the possibility of large lattice relaxations other than breathing mode relaxations. On the other hand, while we have proven that the majority of As prefers to occupy Zn sites, we do not have any information with respect to its electrical activity. Only a small number of publications reporting on p-type ZnO:As have specified the electrically active As concentration in their samples [2–4,12], while in all other cases either the As concentration or the electrical properties were not characterized quantitatively. From the limited number of data available it seems that no more than 5%–13% of incorporated As can be electrically activated. The maximum concentration of As in our implanted sample (around 7.6 × 1018 cm−3 ) is comparable with the As concentration used in most other studies. Although we are sure that in our case following annealing less than 5% of the implanted As occupied O sites, it cannot be excluded that by using different doping techniques around 10% of As might be incorporated on O sites and thus be responsible for the observed p-type conductivity. Our experiment also shows that a significant fraction of probe atoms (almost 30%) was found ˚ of interstitial T sites following vacuum annealing at 900 ◦ C. It is known within 0.04 nm (0.4 A) that annealing ZnO above 500 ◦ C under O-poor conditions can result in O loss and thus change the stoichiometry in the near-surface region [14]. Given the relatively shallow depth profile of the implanted As, it seems possible that some of the implanted probe atoms have reacted with O vacancies or Zn interstitials, causing changes in the local structure of As-related complexes which remove the As atoms from Zn sites. This should be investigated further by studying the lattice location of As following different annealing procedures, e.g. under O2 , N2 or in air, and possibly also via annealing at higher temperatures. We would like to point out that there also exists the possibility to do emission channeling experiments for two of the other group V candidates for p-type doping of ZnO. The lattice location of phosphorus can be studied using the radioactive β − emitters 32 P (t1/2 = 14.3 d) or 33 P (25.3 d) and the lattice location of antimony using 122 Sb (2.7 d), 124 Sb (60.3 d), or 126 Sb (12.4 d). While phosphorus is not available at the ISOLDE facility, laser-ionized beams of antimony have been produced recently [23] and we are therefore planning to perform lattice location of this element in ZnO.
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5. Conclusions We have provided direct evidence that the large majority of implanted As atoms in ZnO occupy substitutional Zn sites. While our experimental data do not rule out the possibility that small fractions (<5%) of As may have existed on O sites, or that larger fractions can be incorporated on O sites by using other doping techniques, the fact that As is not easily incorporated on O sites by ion implantation and annealing certainly does not encourage the widespread belief in its role as a possible chemical p-type dopant on O sites. On the other hand, our results clearly provide more credibility to the suggestion of Limpijumnong et al. that the acceptor action of As in ZnO might be due to the formation of AsZn in complexes with neighboring Zn vacancies. The fact that vacuum annealing at 900 ◦ C changed the lattice position for a significant fraction of the probe atoms indicates strongly that the nature of As-related complexes can be influenced by the interaction with point defects which can be promoted, for instance, via annealing under O-poor conditions. Acknowledgments This work was supported by the Portuguese Foundation for Science and Technology (FCT, project PDCT-FP-FNU-50145-2003) and by the European Union through its 6th Framework (RII3-EURONS, contract no. 506065). E. Rita and A.C. Marques acknowledge their fellowships from FCT, Portugal. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
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