- Email: [email protected]
Annealing behavior of impurities and defects in keV Er-implanted ZnO bulk single crystals
Nuclear Instruments and Methods in Physics Research B 304 (2013) 1–4
Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Annealing behavior of impurities and defects in keV Er-implanted ZnO bulk single crystals Chuan-Lei Jia a,⇑, Tong Zhang b a b
Department of Physics, China University of Mining and Technology, Xuzhou, Jiangsu 221116, PR China College of Science, China University of Petroleum, Qingdao, Shandong 266580, PR China
a r t i c l e
i n f o
Article history: Received 1 January 2013 Received in revised form 14 February 2013 Available online 9 April 2013 Keywords: Ion implantation Rutherford backscattering spectroscopy ZnO
a b s t r a c t We have investigated the effect of implantation and annealing temperatures on crystalline quality, disorder recovery and dopant distribution in ZnO bombarded with Er ions using Rutherford backscattering/channeling spectrometry. The channeling results indicate that the damage retains a low level in as-implanted samples due to the dynamic annealing effect during implantation at 600 °C. It is also found that the implantation disorder is well recovered when the samples are annealed at 1000 °C for 30 min. The results also demonstrate that many Er ions diffuse towards the surface during the whole annealing program. In particular, Er is distributed almost randomly after annealing at 1000 °C for 30 min. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction The direct band-gap semiconductor, ZnO has attracted a great deal of interest due to its prospects in electronic and optoelectronic applications such as gas sensors, light emitting devices, photodiodes, etc. [1–3]. The research on ZnO materials and devices has been reviewed in recent publications [4–7]. However, despite intensive research for several decades, as in any semiconductor, there are some challenging issues for ZnO, e.g., the difficulty of p-type doping and the role of compensating native defects [8–10]. In this regards, doped ZnO films have been studied comprehensively, owing to their improved properties in comparison with pure ZnO. Ion implantation, as a powerful technique in semiconductor processing, has been widely employed for introducing any given impurity with accurate control of depth profile [11]. Normally, various types of defects would be created by implantation even though ZnO is an irradiation-hard material; thereby a thermal treatment is usually required to reduce lattice damage to a satisfiable level [12]. Moreover, it has been shown that ZnO exhibits dynamic annealing characteristics to some extent when the implantation is carried out at appropriate temperature. Furthermore, the annealing behaviors of defects and impurities are affected by the implantation parameters, such as ion species, energy and fluences [13]. Up to now, it is believed that the physics mechanisms on defects evolvement and annealing effect are still immature and continuing [14]. Recently, there are many reports on ZnO bombarded with Er ions, in which the Er ion presents itself optical ⇑ Corresponding author. E-mail address: [email protected] (C.-L. Jia). 0168-583X/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nimb.2013.03.055
activeness with light emitting in infrared and green spectral region [15–17]. In the previous work, the issues mainly focus on the characterizations of optical properties. However, both implantation and annealing parameters play significant roles on these optical properties. In this work, we further study ZnO single crystals implanted with Er ions. The RBS/C measurements are performed to study the effects of implantation and annealing parameters on lattice crystalline quality, damage recovery and dopant distribution. 2. Experimental details Wurtzite (0 0 0 1) ZnO single crystals were implanted with keV Er+ ions at 600 °C in a special target chamber on Extrion 400 Ion Implanter. During implantation, all samples were titled by 7° off the ion beam direction to prevent channeling effect. In order to obtain a box-like Er distribution profile, four different energies with different fluences of Er were implanted in the same samples. In the present work, 200, 150, 100 and 50 keV with fluences of 4.2 1014, 1.4 1014, 1.4 1014 and 0.7 1014 cm2 were applied, respectively. Some samples were subsequently annealed in Ar atmosphere for 30 min at different temperatures, i.e. 800 °C, 900 °C, 1000 °C, respectively. All samples were investigated using RBS/C spectra to analyze the defects and impurity profiles induced by implantation. A 3.05 MeV He beam was used to obtain channeling yields by RBS along [0 0 0 1] direction and the backscattered ions were detected by a detector at 170° from the beam direction with energy resolution of 12 keV on a 4 MV dynamitron accelerator. Here the RBS/C spectra were collected with typical beam currents of 4 nA for channeling and 2 nA for random, respectively.
2
C.-L. Jia, T. Zhang / Nuclear Instruments and Methods in Physics Research B 304 (2013) 1–4
3. Results and discussion Fig. 1 shows the Er depth profile according to TRIM calculations. It is seen from Fig. 1 that a box-like Er profile was predicted by implantation with quadruple ion energies and fluences. For total fluences of 7.7 1014 cm2, the Er are calculated to be uniformly distributed with 0.2 at.% concentration in width of 40 nm. RBS/Channeling spectrometry has been widely used for analyzing the surface layer of solids, which allows for the quantitative determination of the concentration and structure of defects, depth profile of impurity atoms, surface and interface configuration, lattice strain and deformation and overall quality of crystals and crystalline layers [18]. It has attracted many attentions for its advantages, i.e., nondestructiveness, high depth resolution and good sensitivity for heavy elements etc. Generally, three parts included in RBS/C spectra, i.e., random, channeling and damage, should be collected for characterizing the damage properties. In
Fig. 1. The Er depth profiles in ZnO according to TRIM calculations.
addition, in order to evaluate the disorder from O atoms, the NRA-channeling measurements were performed with 3.05 MeV 4 He+ beams to match with O residence. The RBS/C measurements are also analyzed by RUMP simulation, which is valid to determine both the stoichiometry and depth profile of element in a compound matrix [19]. Random and [0 0 0 1]-aligned RBS spectra were shown in Fig. 2, where the insets show the Er signal windows. As can be seen the [0 0 0 1]-aligned yields below the surface from all implanted samples are higher than those from the virgin, which is due to the direct interaction of the channeled ions with displaced lattice atoms from the row into the random positions. Furthermore, it is clearly the damage created by implantation retains to a low level since the aligned yield is well below that from the random, which in in accordance with the excellent resist-radiation feature of ZnO [20]. The crystal lattice disorder can be quantified by the minimum channeling yield vmin, which is the ratio of backscattering yields at channeling condition to that at random. From a comparison of the areas of elements (Zn, O, and Er) distributions obtained in channeling versus random geometries, it is possible to make estimates of the fraction of interstitial corresponding ions. In virgin ZnO crystal, along [0 0 0 1] atomic rows, consist of alternative Zn and O atoms, which means the vmins’ of Zn and O are expected to be the similar in the direction. The values of minimum channeling yields vmin are 2.96% and 3.04% for Zn and O, respectively, which shows the excellent crystallinity of pure ZnO. For as implanted samples, the damage increases the minimum yields vmin (Zn) and vmin (O) to be 11.61% and 17.09%, respectively, indicating more defects generated by implantation. It is worth to mention that the relative low values may be due to the dynamic annealing effect during implantation at 600 °C, in comparison with the previous reports on Er implanted ZnO at RT [21]. After thermal treatment at 1000 °C, the values measured for the minimum yields vmin (Zn) and vmin (O) are 5.22% and 4.2%, respectively, attributing to the considerable decrease of implant damage by annealing. However, the recovery is not perfect, suggesting that the optimum annealing temperature may be higher
Fig. 2. Random and [0 0 0 1]-aligned RBS spectra recorded in Er-implanted ZnO samples: (a) as-implanted; (b) annealed at 800 °C; (c) annealed at 900 °C; and (d) annealed at 1000 °C. The insets show the Er signal windows in corresponding random spectrum.
3
C.-L. Jia, T. Zhang / Nuclear Instruments and Methods in Physics Research B 304 (2013) 1–4
than 1000 °C. During the whole annealing program, both parameters vmin(Zn) and vmin(O) decrease gradually with increasing temperature, indicating that both ions are subjected to similar types of lattice disorder. The experimental values of minimum yield are shown in Table 1. The inset windows in Fig. 2 exhibit the Er signals corresponding to RBS random spectra. Note, it is noticeable that Er diffuses towards the surface during thermal annealing. In particular, part of the implanted Er is lost after annealing at 1000 °C, resulting to narrower profiling. This observation is also consistent with previous studies of rare-earth ions implanted in ZnO [21,22]. The vmin (Er) of as implanted samples is 49.45%, which is higher than the value of 30% in Er-implanted ZnO at RT reported by Alves et al. [21], which reveals a different effect of irradiation temperatures. Simultaneously, the value of minimum yield vmin (Er) increases strongly with annealing temperatures. Especially, the maximum value is 88.9% for 1000 °C-annealed sample, suggesting that most of Er atoms are located at interstitial rather than substitutional sites. The dependences of experimental values vmin versus annealing temperatures are shown in Fig. 3. For both elements Zn and O, the linear dependences of vmin versus annealing temperatures are observed from fitted profiles. A simulation is performed to give the disorder-depth profiles with RBS/C spectra. Normally, the channel or energy scale can be converted into depth by the method of the mean energy approximation. The fraction of displaced Zn atoms at depth Zn can be approximately evaluated by the relation [23],
nZn vch ðx Þvr ðxn Þ d ðxn Þ ðn ¼ 1; 2; 3::::Þ ¼ min n r min 1 vmin ðxn Þ nZn h
ð1Þ
Table 1 The minimum channeling yields vmin measured along [0 0 0 1]-aligned direction. Samples
vmin (Zn)
vmin (O)
vmin (Er)
Pure as-implanted Annealed at 800 °C Annealed at 900 °C Annealed at 1000 °C
0.0296 0.1161 0.0996 0.0714 0.0522
0.0304 0.1709 0.1208 0.0930 0.0420
0.4945 0.5712 0.8447 0.8890
Fig. 3. The dependences of minimum yields vmin versus annealing temperatures for three elements: (a) Zn; (b) O; and (c) Er.
where nZn d ðxn Þ is the displaced Zn density at corresponding depth xn below the surface, the symbol nZn h is referred to Zn concentration of 4.2 1022 cm3 in bulk ZnO, vch min ðxn Þ is ratio of aligned count to random count at corresponding depth xn in implanted sample, and vrmin ðxn Þ is the de channelled fraction caused by displaced atoms r nZn d ðxn1 Þ in the depth increment of xn xn1 . The parameter vmin ðxn Þ is usually approximated by [23],
vrmin ðxn Þ ¼ vrmin ðxn1 Þ þ ½1 vrmin ðxn1 ÞrZn nZn d ðxn1 ÞDx
ð2Þ
Where rZn is the scattering cross section of Zn to 3.05 MeV He beam (rZn = 4.55 1019 cm2), Dx equals xn – xn1. At surface, i.e., x0 = 0, the value of vrmin ð0Þ is equal to the minimum yield vvmin ð0Þ of virgin sample. By Eqs. (1) and (2), one can determine the value of nZn d ðx0 Þ and hence vvmin ðx1 Þ. The procedure is iterated to calculate paramev ters of nZn d ðxn Þ and vmin ðxnþ1 Þ in depth increment of xn+1 xn, where
Fig. 4. The calculated damage distribution in Er-implanted ZnO samples: (a) as-implanted; (b) annealed at 800 °C; (c) annealed at 900 °C; and (d) annealed at 1000 °C.
4
C.-L. Jia, T. Zhang / Nuclear Instruments and Methods in Physics Research B 304 (2013) 1–4
Fig. 5. The calculated Er depth profiles in Er-implanted ZnO samples: (a) as-implanted; (b) annealed at 800 °C; (c) annealed at 900 °C; and (d) annealed at 1000 °C.
nZn d ðxn Þ is assumed to be constant. Thus, a calculation procedure is performed to evaluate the disorder-depth distribution. Fig. 4 shows the calculated damage profiles, where the damage ratio is related to Zn the term nZn d =nh . In as-implanted sample, the maximum disorder level is around 20%. It is seen, the disorder decreases considerably to a low level during thermal annealing. The Er distribution in implanted samples can then be evaluated from the random spectrum by the equation [23],
NEr i ¼
1 Ai dE ðDxÞi HZn ½ZnO Zn
rZn rEr
in interstitial sites, in comparison with the previous studies on Er-implanted ZnO at RT. Acknowledgments The authors thank Dr. Mengbing Huang for the help of implantation and RBS/C measurements in IBL lab at SUNY at Albany. This work is supported by the Fundamental Research Funds for the Central Universities (Grant No. 2012QNA49).
ð3Þ
wherenEr i is the Er concentration in depth increment (Dx)i, here (Dx)i being equal to xn xn1. The value of Ai can be calculated from RBS spectrum, denoting the area of backscattered signal in width (Dx)i. The symbol HZn is the surface height corresponding to Zn signal from RBS spectrum. dE is the energy per channel. ½ZnO Zn is the stopping cross section factor for backscattered He ions from Zn atoms, which can be defined by the mean energy approximation. The symbols rZn and rEr are the average differential scattering cross sections of elements Zn and Er, respectively. Normally, the value of rZn =rEr is approximately given by the ratio of Z Zn =Z Er where ZZn and Z Er are the atomic numbers of Zn and Er, respectively. A calculation procedure is performed to evaluate the Er ions distributions, which are shown in Fig. 5. Notice that the similar Er profiles are observed in Fig. 5(a) and (b). However, the different annealing behavior is exhibited after annealing at temperatures higher than 800 °C, shown as Fig. 5(c) and (d). Here at the higher annealing temperatures, most of the Er ions diffuse towards the surface and is distributed almost randomly. 4. Conclusions Er ions were implanted into ZnO bulk single crystals with quadruple energies and fluences at 600 °C. The samples were annealed at selected temperatures ranging from 800 to 1000 °C in Ar. Damage recovery, crystalline quality and Er+ ion distributions were studied by RBS/C. The results present the effects of irradiation and annealing temperatures on the damage evolution and Er distribution. The observations suggest that implantation at 600 °C would induce a lower damage level, but may retain more Er ions
References [1] S.J. Pearton, D.P. Norton, K. Ip, Y.W. Heo, T. Steiner, J. Vac. Sci. Technol., B 22 (2004) 932. [2] M.-W. Ahn, K.-S. Park, J.-H. Hee, J.-G. Park, D.-W. Kim, K.J. Choi, J.-H. Lee, S.-H. Hong, Appl. Phys. Lett. 93 (2008) 263103. [3] K. Kodama, T. Uchino, J. Appl. Phys. 111 (2012) 093525. [4] Ü. Özgür, Y.I. Alivov, C. Liu, A. Teke, M.A. Reshchikov, S. Dog˘an, V. Avrutin, S.-J. Cho, H. Morkoç, J. Appl. Phys. 98 (2005) 041301. [5] N. Izyumskaya, V. Avrutin, Ü. Özgür, Y.I. Alivov, H. Morkoç, Phys. Status Solidi B 244 (2007) 1439. [6] U. Ozgur, D. Hofstetter, H. Morkoc, Proc. IEEE 98 (2010) 1255. [7] J. Wu, D. Xue, Sci. Adv. Mater. 3 (2011) 127. [8] A. Janotti, C.G. Van de Walle, Phys. Rev. B 76 (2007) 165202. [9] M.D. McCluskey, S.J. Jokela, J. Appl. Phys. 106 (2009) 071101. [10] L.J. Brillson, Y. Dong, F. Tuomisto, B.G. Svensson, A.Y. Kuznetsov, D. Doutt, H.L. Mosbacker, G. Cantwell, J. Zhang, J.J. Song, Z.Q. Fang, D.C. Look, Phys. Status Solidi C 9 (2012) 1566. [11] S.O. Kucheyev, J.S. Williams, S.J. Pearton, Mater. Sci. Eng. 33 (2001) 51. [12] A. Audren, A. Hallén, M.K. Linnarsson, G. Possnert, Nucl. Instr. Meth. Phys. Res., Sect. B 268 (2010) 1842. [13] J. Lee, J. Metson, P.J. Evans, U. Pal, D. Bhattacharyya, Appl. Surf. Sci. 256 (2010) 2143. [14] P.T. Neuvonen, L. Vines, V. Venkatachalapathy, A. Zubiaga, F. Tuomisto, A. Hallén, B.G. Svensson, A.Y. Kuznetsov, Phys. Rev. B 84 (2011) 205202. [15] A.K. Pradhan, L. Douglas, H. Mustafa, Appl. Phys. Lett. 90 (2007) 072108. [16] Y. Liu, Q. Yang, C. Xu, J. Appl. Phys. 104 (2008) 064701. [17] E.F. Pecora, T.I. Murphy, L.D. Negro, Appl. Phys. Lett. 101 (2012) 191115. [18] L.C. Feldman, J.M. Mayer, S.T. Picraux, Materials Analysis by Ion Channeling, Academic Press, New York, 1982. [19] L.R. Doolittle, Nucl. Instr. Meth. Phys. Res., Sect. B 9 (1985) 344. [20] K. Lorenz, E. Alves, E. Wendler, O. Bilani, W. Wesch, M. Hayes, Appl. Phys. Lett. 87 (2005) 191904. [21] E. Alves, E. Rita, U. Wahl, J.G. Correia, T. Monteiro, J. Soares, C. Boemare, Nucl. Instr. Meth. Phys. Res., Sect. B 206 (2003) 1047. [22] S.M.C. Miranda, M. Peres, T. Monteiro, E. Alves, H.D. Sun, T. Geruschke, R. Vianden, K. Lorenz, Opt. Mater. 33 (2011) 1139. [23] W.-K. Chu, J.W. Mayer, M.-A. Nicolet, Backscattering Spectrometry, Academic Press, New York, 1978.
Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Annealing behavior of impurities and defects in keV Er-implanted ZnO bulk single crystals Chuan-Lei Jia a,⇑, Tong Zhang b a b
Department of Physics, China University of Mining and Technology, Xuzhou, Jiangsu 221116, PR China College of Science, China University of Petroleum, Qingdao, Shandong 266580, PR China
a r t i c l e
i n f o
Article history: Received 1 January 2013 Received in revised form 14 February 2013 Available online 9 April 2013 Keywords: Ion implantation Rutherford backscattering spectroscopy ZnO
a b s t r a c t We have investigated the effect of implantation and annealing temperatures on crystalline quality, disorder recovery and dopant distribution in ZnO bombarded with Er ions using Rutherford backscattering/channeling spectrometry. The channeling results indicate that the damage retains a low level in as-implanted samples due to the dynamic annealing effect during implantation at 600 °C. It is also found that the implantation disorder is well recovered when the samples are annealed at 1000 °C for 30 min. The results also demonstrate that many Er ions diffuse towards the surface during the whole annealing program. In particular, Er is distributed almost randomly after annealing at 1000 °C for 30 min. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction The direct band-gap semiconductor, ZnO has attracted a great deal of interest due to its prospects in electronic and optoelectronic applications such as gas sensors, light emitting devices, photodiodes, etc. [1–3]. The research on ZnO materials and devices has been reviewed in recent publications [4–7]. However, despite intensive research for several decades, as in any semiconductor, there are some challenging issues for ZnO, e.g., the difficulty of p-type doping and the role of compensating native defects [8–10]. In this regards, doped ZnO films have been studied comprehensively, owing to their improved properties in comparison with pure ZnO. Ion implantation, as a powerful technique in semiconductor processing, has been widely employed for introducing any given impurity with accurate control of depth profile [11]. Normally, various types of defects would be created by implantation even though ZnO is an irradiation-hard material; thereby a thermal treatment is usually required to reduce lattice damage to a satisfiable level [12]. Moreover, it has been shown that ZnO exhibits dynamic annealing characteristics to some extent when the implantation is carried out at appropriate temperature. Furthermore, the annealing behaviors of defects and impurities are affected by the implantation parameters, such as ion species, energy and fluences [13]. Up to now, it is believed that the physics mechanisms on defects evolvement and annealing effect are still immature and continuing [14]. Recently, there are many reports on ZnO bombarded with Er ions, in which the Er ion presents itself optical ⇑ Corresponding author. E-mail address: [email protected] (C.-L. Jia). 0168-583X/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nimb.2013.03.055
activeness with light emitting in infrared and green spectral region [15–17]. In the previous work, the issues mainly focus on the characterizations of optical properties. However, both implantation and annealing parameters play significant roles on these optical properties. In this work, we further study ZnO single crystals implanted with Er ions. The RBS/C measurements are performed to study the effects of implantation and annealing parameters on lattice crystalline quality, damage recovery and dopant distribution. 2. Experimental details Wurtzite (0 0 0 1) ZnO single crystals were implanted with keV Er+ ions at 600 °C in a special target chamber on Extrion 400 Ion Implanter. During implantation, all samples were titled by 7° off the ion beam direction to prevent channeling effect. In order to obtain a box-like Er distribution profile, four different energies with different fluences of Er were implanted in the same samples. In the present work, 200, 150, 100 and 50 keV with fluences of 4.2 1014, 1.4 1014, 1.4 1014 and 0.7 1014 cm2 were applied, respectively. Some samples were subsequently annealed in Ar atmosphere for 30 min at different temperatures, i.e. 800 °C, 900 °C, 1000 °C, respectively. All samples were investigated using RBS/C spectra to analyze the defects and impurity profiles induced by implantation. A 3.05 MeV He beam was used to obtain channeling yields by RBS along [0 0 0 1] direction and the backscattered ions were detected by a detector at 170° from the beam direction with energy resolution of 12 keV on a 4 MV dynamitron accelerator. Here the RBS/C spectra were collected with typical beam currents of 4 nA for channeling and 2 nA for random, respectively.
2
C.-L. Jia, T. Zhang / Nuclear Instruments and Methods in Physics Research B 304 (2013) 1–4
3. Results and discussion Fig. 1 shows the Er depth profile according to TRIM calculations. It is seen from Fig. 1 that a box-like Er profile was predicted by implantation with quadruple ion energies and fluences. For total fluences of 7.7 1014 cm2, the Er are calculated to be uniformly distributed with 0.2 at.% concentration in width of 40 nm. RBS/Channeling spectrometry has been widely used for analyzing the surface layer of solids, which allows for the quantitative determination of the concentration and structure of defects, depth profile of impurity atoms, surface and interface configuration, lattice strain and deformation and overall quality of crystals and crystalline layers [18]. It has attracted many attentions for its advantages, i.e., nondestructiveness, high depth resolution and good sensitivity for heavy elements etc. Generally, three parts included in RBS/C spectra, i.e., random, channeling and damage, should be collected for characterizing the damage properties. In
Fig. 1. The Er depth profiles in ZnO according to TRIM calculations.
addition, in order to evaluate the disorder from O atoms, the NRA-channeling measurements were performed with 3.05 MeV 4 He+ beams to match with O residence. The RBS/C measurements are also analyzed by RUMP simulation, which is valid to determine both the stoichiometry and depth profile of element in a compound matrix [19]. Random and [0 0 0 1]-aligned RBS spectra were shown in Fig. 2, where the insets show the Er signal windows. As can be seen the [0 0 0 1]-aligned yields below the surface from all implanted samples are higher than those from the virgin, which is due to the direct interaction of the channeled ions with displaced lattice atoms from the row into the random positions. Furthermore, it is clearly the damage created by implantation retains to a low level since the aligned yield is well below that from the random, which in in accordance with the excellent resist-radiation feature of ZnO [20]. The crystal lattice disorder can be quantified by the minimum channeling yield vmin, which is the ratio of backscattering yields at channeling condition to that at random. From a comparison of the areas of elements (Zn, O, and Er) distributions obtained in channeling versus random geometries, it is possible to make estimates of the fraction of interstitial corresponding ions. In virgin ZnO crystal, along [0 0 0 1] atomic rows, consist of alternative Zn and O atoms, which means the vmins’ of Zn and O are expected to be the similar in the direction. The values of minimum channeling yields vmin are 2.96% and 3.04% for Zn and O, respectively, which shows the excellent crystallinity of pure ZnO. For as implanted samples, the damage increases the minimum yields vmin (Zn) and vmin (O) to be 11.61% and 17.09%, respectively, indicating more defects generated by implantation. It is worth to mention that the relative low values may be due to the dynamic annealing effect during implantation at 600 °C, in comparison with the previous reports on Er implanted ZnO at RT [21]. After thermal treatment at 1000 °C, the values measured for the minimum yields vmin (Zn) and vmin (O) are 5.22% and 4.2%, respectively, attributing to the considerable decrease of implant damage by annealing. However, the recovery is not perfect, suggesting that the optimum annealing temperature may be higher
Fig. 2. Random and [0 0 0 1]-aligned RBS spectra recorded in Er-implanted ZnO samples: (a) as-implanted; (b) annealed at 800 °C; (c) annealed at 900 °C; and (d) annealed at 1000 °C. The insets show the Er signal windows in corresponding random spectrum.
3
C.-L. Jia, T. Zhang / Nuclear Instruments and Methods in Physics Research B 304 (2013) 1–4
than 1000 °C. During the whole annealing program, both parameters vmin(Zn) and vmin(O) decrease gradually with increasing temperature, indicating that both ions are subjected to similar types of lattice disorder. The experimental values of minimum yield are shown in Table 1. The inset windows in Fig. 2 exhibit the Er signals corresponding to RBS random spectra. Note, it is noticeable that Er diffuses towards the surface during thermal annealing. In particular, part of the implanted Er is lost after annealing at 1000 °C, resulting to narrower profiling. This observation is also consistent with previous studies of rare-earth ions implanted in ZnO [21,22]. The vmin (Er) of as implanted samples is 49.45%, which is higher than the value of 30% in Er-implanted ZnO at RT reported by Alves et al. [21], which reveals a different effect of irradiation temperatures. Simultaneously, the value of minimum yield vmin (Er) increases strongly with annealing temperatures. Especially, the maximum value is 88.9% for 1000 °C-annealed sample, suggesting that most of Er atoms are located at interstitial rather than substitutional sites. The dependences of experimental values vmin versus annealing temperatures are shown in Fig. 3. For both elements Zn and O, the linear dependences of vmin versus annealing temperatures are observed from fitted profiles. A simulation is performed to give the disorder-depth profiles with RBS/C spectra. Normally, the channel or energy scale can be converted into depth by the method of the mean energy approximation. The fraction of displaced Zn atoms at depth Zn can be approximately evaluated by the relation [23],
nZn vch ðx Þvr ðxn Þ d ðxn Þ ðn ¼ 1; 2; 3::::Þ ¼ min n r min 1 vmin ðxn Þ nZn h
ð1Þ
Table 1 The minimum channeling yields vmin measured along [0 0 0 1]-aligned direction. Samples
vmin (Zn)
vmin (O)
vmin (Er)
Pure as-implanted Annealed at 800 °C Annealed at 900 °C Annealed at 1000 °C
0.0296 0.1161 0.0996 0.0714 0.0522
0.0304 0.1709 0.1208 0.0930 0.0420
0.4945 0.5712 0.8447 0.8890
Fig. 3. The dependences of minimum yields vmin versus annealing temperatures for three elements: (a) Zn; (b) O; and (c) Er.
where nZn d ðxn Þ is the displaced Zn density at corresponding depth xn below the surface, the symbol nZn h is referred to Zn concentration of 4.2 1022 cm3 in bulk ZnO, vch min ðxn Þ is ratio of aligned count to random count at corresponding depth xn in implanted sample, and vrmin ðxn Þ is the de channelled fraction caused by displaced atoms r nZn d ðxn1 Þ in the depth increment of xn xn1 . The parameter vmin ðxn Þ is usually approximated by [23],
vrmin ðxn Þ ¼ vrmin ðxn1 Þ þ ½1 vrmin ðxn1 ÞrZn nZn d ðxn1 ÞDx
ð2Þ
Where rZn is the scattering cross section of Zn to 3.05 MeV He beam (rZn = 4.55 1019 cm2), Dx equals xn – xn1. At surface, i.e., x0 = 0, the value of vrmin ð0Þ is equal to the minimum yield vvmin ð0Þ of virgin sample. By Eqs. (1) and (2), one can determine the value of nZn d ðx0 Þ and hence vvmin ðx1 Þ. The procedure is iterated to calculate paramev ters of nZn d ðxn Þ and vmin ðxnþ1 Þ in depth increment of xn+1 xn, where
Fig. 4. The calculated damage distribution in Er-implanted ZnO samples: (a) as-implanted; (b) annealed at 800 °C; (c) annealed at 900 °C; and (d) annealed at 1000 °C.
4
C.-L. Jia, T. Zhang / Nuclear Instruments and Methods in Physics Research B 304 (2013) 1–4
Fig. 5. The calculated Er depth profiles in Er-implanted ZnO samples: (a) as-implanted; (b) annealed at 800 °C; (c) annealed at 900 °C; and (d) annealed at 1000 °C.
nZn d ðxn Þ is assumed to be constant. Thus, a calculation procedure is performed to evaluate the disorder-depth distribution. Fig. 4 shows the calculated damage profiles, where the damage ratio is related to Zn the term nZn d =nh . In as-implanted sample, the maximum disorder level is around 20%. It is seen, the disorder decreases considerably to a low level during thermal annealing. The Er distribution in implanted samples can then be evaluated from the random spectrum by the equation [23],
NEr i ¼
1 Ai dE ðDxÞi HZn ½ZnO Zn
rZn rEr
in interstitial sites, in comparison with the previous studies on Er-implanted ZnO at RT. Acknowledgments The authors thank Dr. Mengbing Huang for the help of implantation and RBS/C measurements in IBL lab at SUNY at Albany. This work is supported by the Fundamental Research Funds for the Central Universities (Grant No. 2012QNA49).
ð3Þ
wherenEr i is the Er concentration in depth increment (Dx)i, here (Dx)i being equal to xn xn1. The value of Ai can be calculated from RBS spectrum, denoting the area of backscattered signal in width (Dx)i. The symbol HZn is the surface height corresponding to Zn signal from RBS spectrum. dE is the energy per channel. ½ZnO Zn is the stopping cross section factor for backscattered He ions from Zn atoms, which can be defined by the mean energy approximation. The symbols rZn and rEr are the average differential scattering cross sections of elements Zn and Er, respectively. Normally, the value of rZn =rEr is approximately given by the ratio of Z Zn =Z Er where ZZn and Z Er are the atomic numbers of Zn and Er, respectively. A calculation procedure is performed to evaluate the Er ions distributions, which are shown in Fig. 5. Notice that the similar Er profiles are observed in Fig. 5(a) and (b). However, the different annealing behavior is exhibited after annealing at temperatures higher than 800 °C, shown as Fig. 5(c) and (d). Here at the higher annealing temperatures, most of the Er ions diffuse towards the surface and is distributed almost randomly. 4. Conclusions Er ions were implanted into ZnO bulk single crystals with quadruple energies and fluences at 600 °C. The samples were annealed at selected temperatures ranging from 800 to 1000 °C in Ar. Damage recovery, crystalline quality and Er+ ion distributions were studied by RBS/C. The results present the effects of irradiation and annealing temperatures on the damage evolution and Er distribution. The observations suggest that implantation at 600 °C would induce a lower damage level, but may retain more Er ions
References [1] S.J. Pearton, D.P. Norton, K. Ip, Y.W. Heo, T. Steiner, J. Vac. Sci. Technol., B 22 (2004) 932. [2] M.-W. Ahn, K.-S. Park, J.-H. Hee, J.-G. Park, D.-W. Kim, K.J. Choi, J.-H. Lee, S.-H. Hong, Appl. Phys. Lett. 93 (2008) 263103. [3] K. Kodama, T. Uchino, J. Appl. Phys. 111 (2012) 093525. [4] Ü. Özgür, Y.I. Alivov, C. Liu, A. Teke, M.A. Reshchikov, S. Dog˘an, V. Avrutin, S.-J. Cho, H. Morkoç, J. Appl. Phys. 98 (2005) 041301. [5] N. Izyumskaya, V. Avrutin, Ü. Özgür, Y.I. Alivov, H. Morkoç, Phys. Status Solidi B 244 (2007) 1439. [6] U. Ozgur, D. Hofstetter, H. Morkoc, Proc. IEEE 98 (2010) 1255. [7] J. Wu, D. Xue, Sci. Adv. Mater. 3 (2011) 127. [8] A. Janotti, C.G. Van de Walle, Phys. Rev. B 76 (2007) 165202. [9] M.D. McCluskey, S.J. Jokela, J. Appl. Phys. 106 (2009) 071101. [10] L.J. Brillson, Y. Dong, F. Tuomisto, B.G. Svensson, A.Y. Kuznetsov, D. Doutt, H.L. Mosbacker, G. Cantwell, J. Zhang, J.J. Song, Z.Q. Fang, D.C. Look, Phys. Status Solidi C 9 (2012) 1566. [11] S.O. Kucheyev, J.S. Williams, S.J. Pearton, Mater. Sci. Eng. 33 (2001) 51. [12] A. Audren, A. Hallén, M.K. Linnarsson, G. Possnert, Nucl. Instr. Meth. Phys. Res., Sect. B 268 (2010) 1842. [13] J. Lee, J. Metson, P.J. Evans, U. Pal, D. Bhattacharyya, Appl. Surf. Sci. 256 (2010) 2143. [14] P.T. Neuvonen, L. Vines, V. Venkatachalapathy, A. Zubiaga, F. Tuomisto, A. Hallén, B.G. Svensson, A.Y. Kuznetsov, Phys. Rev. B 84 (2011) 205202. [15] A.K. Pradhan, L. Douglas, H. Mustafa, Appl. Phys. Lett. 90 (2007) 072108. [16] Y. Liu, Q. Yang, C. Xu, J. Appl. Phys. 104 (2008) 064701. [17] E.F. Pecora, T.I. Murphy, L.D. Negro, Appl. Phys. Lett. 101 (2012) 191115. [18] L.C. Feldman, J.M. Mayer, S.T. Picraux, Materials Analysis by Ion Channeling, Academic Press, New York, 1982. [19] L.R. Doolittle, Nucl. Instr. Meth. Phys. Res., Sect. B 9 (1985) 344. [20] K. Lorenz, E. Alves, E. Wendler, O. Bilani, W. Wesch, M. Hayes, Appl. Phys. Lett. 87 (2005) 191904. [21] E. Alves, E. Rita, U. Wahl, J.G. Correia, T. Monteiro, J. Soares, C. Boemare, Nucl. Instr. Meth. Phys. Res., Sect. B 206 (2003) 1047. [22] S.M.C. Miranda, M. Peres, T. Monteiro, E. Alves, H.D. Sun, T. Geruschke, R. Vianden, K. Lorenz, Opt. Mater. 33 (2011) 1139. [23] W.-K. Chu, J.W. Mayer, M.-A. Nicolet, Backscattering Spectrometry, Academic Press, New York, 1978.