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P-type single-crystalline ZnO films obtained by (N,O) dual implantation through dynamic annealing process
Superlattices and Microstructures 100 (2016) 468e473
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Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
P-type single-crystalline ZnO films obtained by (N,O) dual implantation through dynamic annealing process Zhiyuan Zhang, Jingyun Huang*, Shanshan Chen, Xinhua Pan, Lingxiang Chen, Zhizhen Ye** State Key Laboratory of Silicon Materials, Cyrus Tang Center for Sensor Materials and Applications, School of Materials Science & Engineering, Zhejiang University, Hangzhou 310027, People's Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 July 2016 Received in revised form 1 October 2016 Accepted 2 October 2016 Available online 3 October 2016
Single-crystalline ZnO films were grown on a-plane sapphire substrates by plasmaassisted molecular beam epitaxy technique. The films have been implanted with fixed fluence of 120 keV N and 130 keV O ions at 460 C. Hall measurements show that the dually-implanted single-crystalline ZnO films exhibit p-type characteristics with hole concentration in the range of 2.1 1018e1.1 1019 cm3, hole mobilities between 1.6 and 1.9 cm2 V1 s1, and resistivities in the range of 0.353e1.555 U cm. The ZnO films exhibit (002) (c-plane) orientation as identified by the X-ray diffraction pattern. It is confirmed that N ions were effectively implanted by SIMS results. Raman spectra, polarized Raman spectra, and X-ray photoelectron spectroscopy results reflect that the concentration of oxygen vacancies is reduced, which is attributed to O ion implantation. It is concluded that N and O implantation and dynamic annealing play a critical role in forming p-type singlecrystalline ZnO films. © 2016 Elsevier Ltd. All rights reserved.
Keywords: p-type single-crystalline ZnO films Oxygen vacancy Molecular beam epitaxy Ion implantation Dynamic annealing
1. Introduction ZnO is a wide band gap semiconductor with large exciton binding energy (60 meV), which has been studied in shortwavelength optoelectronic devices as a promising alternative to GaN [1e3]. However, the lack of stable, reproducible ptype ZnO film is still a challenge to obtain useful devices because of native donor defects such as oxygen vacancies [4]. So it is necessary to overcome the difficulties of the low solubility of p-type dopants and the effect of self-compensation caused by intrinsic donor defects, and the realization of stable p-type ZnO films mainly depends on choosing a suitable acceptor and doping method. So people have devoted much effort to achieving this aim [5e7]. Group V elements have attracted significant attention as p-type dopants and N is regarded as the most promising p-type dopant of ZnO films attributing to similar radius and electronic structure between N and O [8,9]. Reynolds et al. have achieved a mobility of 0.4 cm2 V1 s1 at a hole concentration of 3.6 1018 cm3 using N as p-type dopant [10]. In order to achieve highequality p-type ZnO films and overcome the low solubility of p-type dopants, ion implantation is a useful method to introduce a precise dopant concentration. However, ion implantation can cause lattice disorder and damage [11,12]. In the process of annealing, the implanted N ions lying in the interstitial positions can occupy the sites of oxygen
* Corresponding author. ** Corresponding author. E-mail address: [email protected] (J. Huang). http://dx.doi.org/10.1016/j.spmi.2016.10.006 0749-6036/© 2016 Elsevier Ltd. All rights reserved.
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vacancies or substitute for the lattice oxygen atoms through a kick-out mechanism [13,14]. So it is necessary to perform annealing to remove lattice disorder and activate dopants. There are different kinds of annealing to activate dopants and remove disorder including post-implantation annealing and dynamic annealing. It is reported that performing dynamic annealing can reduce the types of disorder caused by ion implantation which can't be reached by post-implantation annealing [15]. Since p-type ZnO films first attracted attention because of being promising material for ultraviolet optoelectronic devices, a large number of studies have focused on choosing different kinds of acceptors and doping methods, whose aim has been to produce as many holes as possible. However, an important problem that also needs to be studied is that a certain percentage of holes produced by acceptors in ZnO films are compensated by native donor defects, especially oxygen vacancies. There are few reports of studies paying attention to restraining the density of oxygen vacancies to contribute to better p-type characteristics of ZnO films and measuring the variation of the density of oxygen vacancies by experiment. In the work reported here, we not only paid attention to choosing acceptor and doping method but also devoted effort to decreasing density of oxygen vacancies. So in our study, in order to suppress and reduce the amount of native donor defects, especially oxygen vacancies, O ions were introduced by implantation to occupy the sites of oxygen vacancies and to reduce the effect of selfcompensation, increasing stable ZneO bonding [16]. In order to confirm the decreasing of density of oxygen vacancies, Raman spectra and X-ray photoelectron spectroscopy (XPS) measurements were performed. In this paper, molecular beam epitaxy (MBE), ion implantation and dynamic annealing have been combined to obtain ptype single-crystalline ZnO films. The effects of N and O ion implantation and the dynamic process on the p-type conductivity have been discussed. 2. Material and methods ZnO film was grown on a-plane sapphire substrate by plasma-assisted MBE technique. Elemental zinc (6 N grade) and oxygen radio frequency (RF) plasma (O2 gas of 6 N grade) were used as the sources. The a-plane sapphire substrates were cleaned ultrasonically with acetone, ethanol, and deionized water for 10 min at room temperature, respectively. Prior to beginning growth, the substrate was thermally cleaned by heating at 300 C in the preparation chamber for 3 h and then treated at 250 C for 20 min in oxygen plasma exposure in the growth chamber with further cleaning at 800 C for 30 min under ultrahigh vacuum achieved by a liquid nitrogen supply. After these cleaning processes, a ZnO buffer layer was first grown at 300 C for 10 min and annealed at 850 C for 5 min. Then the top ZnO layer was deposited at 800 C for 4 h and annealed at 850 C for 5 min for better film crystallinity and smoother surface, during which the Zn cell temperature, RF excitation power and oxygen flow rate were fixed at 300 C, 350 W and 1.0 sccm, respectively. After growth, the substrate temperature was decreased slowly at a rate of 5 C per minute. In order to achieve the concentration of N and O ions reaches 1021 cm3, the as-grown ZnO films were implanted with 120 keV N ions to fluence at 2.0 1016 cm2 and 130 keV O ions to fluence at 2.0 1016 cm2 at 460 C [17]. The thickness of ZnO film was about 380 nm, which is regarded as total thickness of the implanted layers and the projected range was calculated to be 190 nm by the TRIM code. N and O concentrations simulated by SRIM predict skew-Gaussian profiles with maximum implanted concentration at a depth of 190 nm for both. In order to make sure the effect of N and O ion implantation, dynamic annealing at 460 C was performed on unimplanted ZnO film. The electrical properties were examined by Hall-effect measurements using the Van der Pauw configuration at room temperature. The crystalline structure was analyzed by X-ray diffraction (XRD) measurements with a Cu Ka radiation source (l ¼ 1.54056 Å). The full spectrum and O 1s core-level spectrum of ZnO film were analyzed by X-ray photoelectron spectroscopy (XPS) and calibrated by the C 1s peak. The depth profile of implanted ZnO film was investigated by secondary ion mass spectroscopy (SIMS). What spectroscope is used for Raman measurements ? The room-temperature Raman spectra were performed on a Raman spectroscope using a 514 nm excitation wavelength from an Ar laser. The room-temperature polarized Raman spectra were performed on a Raman spectroscope using a 532 nm excitation wavelength. 3. Results and discussion Hall measurements were performed on all implanted and unimplanted ZnO films before and after dynamic annealing and their results are summarized in Table 1. To make sure of the reliability of the results, Hall measurements are repeated several times and similar results are obtained. The unimplanted ZnO films were measured to be n-type with high mobility prior to implantation. After implantation and dynamic annealing, the dually-implanted ZnO films exhibited p-type characteristics with N fluence at 2.0 1016 cm2, O fluence at 2.0 1016 cm2. As we regarded this, annealing at elevated temperature reduced lattice disorder caused by implanted ions with high energy to improve the quality of ZnO films and activated substitutional p-type acceptors so that p-type ZnO films had high mobility together with low resistivity, and O ions were introduced to occupy the sites of oxygen vacancies, reducing the effect of self-compensation and improving the hole concentration [18]. It is concluded from the results that performing dynamic annealing is possibly better at achieving p-type ZnO films contrary to post-implantation annealing and O ion implantation contributes to forming p-type single-crystalline ZnO films. P-type characteristics of the dually-implanted ZnO films have shown some degradation after three months. Fig. 1 shows the qe2q XRD patterns of implanted and unimplanted ZnO films before and after dynamic annealing. In XRD pattern, only ZnO (002) and (004) peaks exist beside the sapphire (110) peak and ZnO (002) is dominant for all samples, which
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Table 1 Electrical results of unimplanted and implanted ZnO films. Samples (N cm2)
Temp
Resistivity
Mobility
Carrier concentration
(O cm2)
( C)
(U cm)
(cm2 V1 s1)
(cm3)
0.14 0.012
43 45.5
1.0 1018 1.0 1019
n n
0.353
1.6
1.1 1019
p
1.555
1.9
2.1 1018
p
Unimplanted Unimplanted 2.0 1016 2.0 1016 2.0 1016 2.0 1016
460 460
Carrier type
460
Fig. 1. X-ray diffraction patterns of the implanted and unimplanted ZnO films before and after dynamic annealing.
suggests that after implantation, the ZnO film is single-crystalline in nature [19] and c-plane orientation is obtained. The sapphire (110) peak isn't observed in the dually-implanted ZnO films XRD pattern, which attributes to impact of lattice disorders and defects existing in the dually-implanted ZnO films caused by high energy ion implantation and weak intensity of the sapphire (110) peak. And the sapphire (110) peak of unimplanted ZnO films is enhanced by dynamic annealing, which reflects the quality of ZnO films is improved. Despite a small increase in the bandwidth of the ZnO (002) peak after implantation, it is observed that the positions of peaks in the XRD pattern don't change and it is concluded that the amount of implanted N and O ions is so little that the effect falls below the XRD detection limit on the crystal quality of the ZnO films. Fig. 2 shows the XPS results of the implanted single-crystalline ZnO films, which exhibit indexed peaks that correspond to C, O, and Zn. However, the peak associated with N isn't seen in the XPS spectra. Because of the low amount of the implanted N
Fig. 2. XPS spectrum of the implanted ZnO films.
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Fig. 3. SIMS spectrum of the implanted ZnO films.
ions contrary to other elements, the concentration is too low to be measured by the XPS technique. In order to make sure N ions were effectively implanted, it was necessary to perform secondary ion mass spectroscopy (SIMS) measurements on implanted single-crystalline ZnO films. As shown in Fig. 3, N signal is clearly seen in the SIMS results, which means that N ions were successfully introduced in the single-crystalline ZnO films. It is observed that the concentration of N ions changes little with depth, which is mainly attributed to diffusion of N ions when samples were performed dynamic annealing. However, because of the variation of concentrations of Zn and O, it is deduced that there are still lattice disorders and defects existing in the single-crystalline ZnO films caused by high energy ion implantation. Fig. 4 shows the room-temperature Raman spectra of unimplanted and implanted single-crystalline ZnO films. Wurtzite structure ZnO has two formula units in the primitive cell belonging to the space group of C6v4. It is reported that frequencies of ZnO films contain A (LO) ¼ 574 cm1 mode about the fundamental optical mode [20]. A Raman shift peak at 574 cm1 is clearly seen in the Raman spectrum of umimplanted ZnO films. However, there isn't any Raman peak at 574 cm1 in the spectrum of implanted ZnO films. And the Raman shift peak around 430 cm1 is clearly seen in the Raman spectrum of umimplanted ZnO films and implanted ZnO films, which corresponds to the stretching bond of Zn2þeO2. The A (LO) mode is reported to correspond to oxygen vacancies in ZnO films [21,22] and this is supported by the discussion of the XPS results. In order to make sure of the A (LO) mode of wurtzite structure ZnO, polarized Raman measurements with polarization configuration of z(x,x)z were performed on unimplanted and implanted single-crystalline ZnO films, as shown in Fig. 5. The x, y, z axes are aligned along the ½1210, ½1210, and [0001]. The peak at 574 cm1 is assigned to phonon mode of A (LO), which disappears in the spectrum of implanted ZnO films. So with the disappearance of the Raman shift peak, it can be concluded that the concentration of oxygen vacancies has been reduced and implanted O ions have occupied the sites of oxygen vacancies, reducing the effect of self-compensation caused by intrinsic donor defects and resulting in increasing of stable ZneO bonds. The XPS results for the O 1s level of unimplanted and implanted single-crystalline ZnO films are shown in Fig. 6 and Fig. 7, which are used to probe the formation of oxygen vacancies and verify the deduction associated with the Raman results. It is
Fig. 4. Raman spectra of the unimplanted and implanted ZnO films.
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Fig. 5. Polarized Raman spectra of the unimplanted and implanted ZnO films.
Fig. 6. The O 1s XPS spectrum of the unimplanted ZnO films.
Fig. 7. The O 1s XPS spectrum of the implanted ZnO films.
obvious that the O 1s peaks of unimplanted and implanted single-crystalline ZnO films exhibit asymmetry which means that more than one kind of oxygen atom bonding configuration existing in the single-crystalline ZnO films. So the O 1s peak can be divided into two Gaussian peaks. The centers of the two Gaussian peaks, OⅠ peak and OⅡ peak are located at 530 ± 0.4 eV and 531.5 ± 0.4 eV, respectively. The OⅠ peak is associated with O2 ions in the ZnO crystal lattice surrounded by Zn2þ [23]. There
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are reports exhibiting that with the increasing of loss of oxygen, the intensity of the OⅡ peak is enhanced and the OⅡ peak is associated with oxygen vacancies [24,25]. It is obviously observed that the intensity of the OⅡ peak decreases with the implantation of O ions. Combined with the previous Raman results, the XPS results are well consistent with the Raman spectral analysis and that the OⅡ peak is associated with oxygen vacancy in our study. Therefore, one can depend upon the concentration of absorbed oxygen to verify the trend of the concentration of oxygen vacancies. According to the rules of quantitative analysis, the ratio of oxygen vacancies to total oxygen reflects the trend [26]. It shows that with the implantation of O ions, the ratio decreases from 0.314 to 0.12 comparing unimplanted single-crystalline ZnO films with implanted singlecrystalline ZnO films. So it can be concluded that the concentration of oxygen vacancies decreases with the implantation of O ions and it contributes to reducing the effect of self-compensation and the realization of p-type single-crystalline ZnO films. 4. Conclusion Single-crystalline ZnO films were implanted with 120 eV N ions at a fluence of 2.0 1016 cm2, and 130 eV O ions at a fluence of 2.0 1016 cm2, both at 460 C, which were grown on a-plane sapphire by plasma-assisted molecular beam epitaxy technique. Hall measurements show that the single-crystalline ZnO films dually implanted with fixed fluences of N and O ions change from n-type characteristics to p-type characteristics, with hole concentration in the range of 2.1 1018e1.1 1019 cm3, hole mobilities between 1.6 and 1.9 cm2 V1 s1, and resistivities in the range of 0.353e1.555 U cm. The high hole concentration suggest that the N ions are activated as dopants and the mobility reflects that dynamic annealing is critical to reducing lattice disorder caused by implantation. Raman results, polarized Raman results, and XPS results exhibit that O ion implantation plays a necessary role in reducing the concentration of oxygen vacancies in single-crystalline ZnO films which contributes to p-type behavior. It is concluded that molecular beam epitaxy, ion implantation and dynamic annealing are combined to obtain p-type single-crystalline ZnO films successfully. Acknowledgements This work was financially supported by National Natural Science Foundation of China (91333203), Natural Science Foundation of Zhejiang Province, China (LY14E020006). References an, V. Avrutin, S.J. Cho, H. Morkoc, A comprehensive review of ZnO materials and devices, J. [1] Ü. Ӧzgür, Y.I. Alivov, C. Liu, A. Teke, M.A. Reshchikov, S. Dog Appl. Phys. 98 (2005) 041301. [2] J.G. Reynolds, C.L. Reynolds, Progress in ZnO acceptor doping: what is the best strategy? Adv. Condens. Matter Phys. (2014) 457058. [3] B.Y. Zhang, B. Yao, Y.F. Li, Z.Z. Zhang, B.H. Li, C.X. Shan, D.X. Zhao, D.Z. Shen, Investigation on the formation mechanism of p-type Li-N dual-doped ZnO, Appl. Phys. Lett. 97 (2010) 222101. [4] B. Lin, Z. Fu, Y. Jia, Green luminescent center in undoped zinc oxide films deposited on silicon substrates, Appl. Phys. Lett. 79 (2001) 943. [5] P. Ding, X.H. Pan, Z.Z. Ye, Realization of p-type non-polar a-plane ZnO films via doping of Na acceptor, Solid State Commun. 156 (2013) 8e11. [6] Y.J. Zeng, Z.Z. Ye, W.Z. Xue, Dopant source choice for formation of p-type ZnO: Li acceptor, Appl. Phys. Lett. 88 (2006) 062107. [7] F.X. Xiu, Z. Yang, L.J. Mandalapu, D.T. Zhao, J.L. Liu, W.P. Beyermann, High-mobility Sb-doped p-type ZnO by molecular-beam epitaxy, Appl. Phys. Lett. 87 (2005) 152101. [8] D.C. Look, D.C. Reynolds, C.W. Litton, R.L. Jones, D.B. Eason, G. Cantwell, Characterization of homoepitaxial p-type ZnO grown by molecular beam epitaxy, Appl. Phys. Lett. 81 (2002) 1830. [9] L. Gong, Z.Z. Ye, J.G. Lu, In-N codoped p-type ZnMgO thin films with bandgap engineering, Vacuum 85 (2010) 365e367. [10] J.G. Reynolds, C.L. Reynolds, A. Mohanta, J.F. Muth, J.E. Rowe, D.E. Aspnes, Shallow acceptor complexes in p-type ZnO, Appl. Phys. Lett. 102 (2013) 152114. [11] A.Y. Azarov, B.G. Svensson, A. Hallen, X.L. Du, A.Y. Kuznetsov, Effect of composition on damage accumulation in ternary ZnO-based oxides implanted with heavy ions, J. Appl. Phys. 108 (2010) 033509. [12] K. Lorenz, E. Alves, E. Wendler, O. Bilani, W. Wesch, M. Hayes, Damage formation and annealing at low temperatures in ion implanted ZnO, Appl. Phys. Lett. 87 (2005) 191904. [13] A. Catellani, A. Ruini, G. Cicero, A. Calzolari, First principles description of the electronic properties of doped ZnO, Phys. Status Solidi B 250 (2013) 2106e2109. [14] J.Y. Gao, R. Qin, G.F. Luo, J. Lu, Y. Leprince-Wang, H.Q. Ye, Z.M. Liao, Q. Zhao, D.P. Yu, First-principles study of the formation mechanisms of nitrogen molecule in annealed ZnO, Phys. Lett. A 374 (2010) 3546e3550. [15] S.J. Pearton, D.P. Norton, K. Ip, Y.W. Heo, T. Steiner, Recent progress in processing and properties of ZnO, Prog. Mater. Sci. 50 (2005) 293e340. [16] A. Powell, W.E. Spicer, J.C. McMenamin, Location of the Zn 3d states in ZnO, Phys. Rev. Lett. 27 (1971) 97e100. [17] M.A. Myers, M.T. Myers, M.J. General, J.H. Lee, L. Shao, H. Wang, P-type ZnO thin films achieved by Nþ ion implantation through dynamic annealing process, Appl. Phys. Lett. 101 (2012) 112101. [18] C.O. Kim, D.H. Shin, S. Kim, S. Choi, K. Belay, Effect of (O, As) dual implantation on p-type doping of ZnO films, J. Appl. Phys. 110 (2011) 103708. [19] Z.Z. Zhi, Y.C. Liu, B.S. Li, X.T. Zhang, Y.M. Lu, D.Z. Shen, X.W. Fan, Effects of thermal annealing on ZnO films grown by plasma enhanced chemical vapour deposition from Zn(C2H5)2 and CO2 gas mixtures, J. Phys. D. 36 (2003) 719e722. [20] C. Bundesmann, N. Ashkenov, M. Schubert, D. Spemann, T. Butz, E. M.Kaidashev, M. Lorenz, M. Grundmann, Raman scattering in ZnO thin films doped with Fe, Sb, Al, Ga, and Li, Appl. Phys. Lett. 83 (2003) 1974. [21] M. Naeem, S. Qaseem, I.H. Gul, A. Maqsood, Study of active surface defects in Ti doped ZnO nanoparticles, J. Appl. Phys. 107 (2010) 124303. [22] C.J. Youn, T.S. Jeong, M.S. Han, J.H. Kim, Optical properties of Zn-terminated ZnO bulk, J. Cryst. Growth 261 (2004) 526e532. [23] L. Jing, Z. Xu, X. Sun, J. Shang, W. Cai, The surface properties and photocatalytic activities of ZnO ultrafine particles, Appl. Surf. Sci. 180 (2001) 308e314. [24] J.C.C. Fan, J.B. Goodenough, X-ray photoemission spectroscopy studies of Sn-doped indium-oxide films, J. Appl. Phys. 48 (1977) 3524. [25] X.Q. Wei, B.Y. Man, M. Liu, C.S. Xue, H.Z. Zhuang, C. Yang, Blue luminescent centers and microstructural evaluation by XPS and Raman in ZnO thin films annealed in vacuum, N2 and O2, Phys. B 388 (2007) 145e152. [26] F. Li, X.C. Liu, R.W. Zhou, H. Ming Chen, S.Y. Zhuo, E.W. Shi, Strong correlation between oxygen vacancy and ferromagnetism in Yb-doped ZnO thin films, J. Appl. Phys. 116 (2014) 243910.
Contents lists available at ScienceDirect
Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
P-type single-crystalline ZnO films obtained by (N,O) dual implantation through dynamic annealing process Zhiyuan Zhang, Jingyun Huang*, Shanshan Chen, Xinhua Pan, Lingxiang Chen, Zhizhen Ye** State Key Laboratory of Silicon Materials, Cyrus Tang Center for Sensor Materials and Applications, School of Materials Science & Engineering, Zhejiang University, Hangzhou 310027, People's Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 July 2016 Received in revised form 1 October 2016 Accepted 2 October 2016 Available online 3 October 2016
Single-crystalline ZnO films were grown on a-plane sapphire substrates by plasmaassisted molecular beam epitaxy technique. The films have been implanted with fixed fluence of 120 keV N and 130 keV O ions at 460 C. Hall measurements show that the dually-implanted single-crystalline ZnO films exhibit p-type characteristics with hole concentration in the range of 2.1 1018e1.1 1019 cm3, hole mobilities between 1.6 and 1.9 cm2 V1 s1, and resistivities in the range of 0.353e1.555 U cm. The ZnO films exhibit (002) (c-plane) orientation as identified by the X-ray diffraction pattern. It is confirmed that N ions were effectively implanted by SIMS results. Raman spectra, polarized Raman spectra, and X-ray photoelectron spectroscopy results reflect that the concentration of oxygen vacancies is reduced, which is attributed to O ion implantation. It is concluded that N and O implantation and dynamic annealing play a critical role in forming p-type singlecrystalline ZnO films. © 2016 Elsevier Ltd. All rights reserved.
Keywords: p-type single-crystalline ZnO films Oxygen vacancy Molecular beam epitaxy Ion implantation Dynamic annealing
1. Introduction ZnO is a wide band gap semiconductor with large exciton binding energy (60 meV), which has been studied in shortwavelength optoelectronic devices as a promising alternative to GaN [1e3]. However, the lack of stable, reproducible ptype ZnO film is still a challenge to obtain useful devices because of native donor defects such as oxygen vacancies [4]. So it is necessary to overcome the difficulties of the low solubility of p-type dopants and the effect of self-compensation caused by intrinsic donor defects, and the realization of stable p-type ZnO films mainly depends on choosing a suitable acceptor and doping method. So people have devoted much effort to achieving this aim [5e7]. Group V elements have attracted significant attention as p-type dopants and N is regarded as the most promising p-type dopant of ZnO films attributing to similar radius and electronic structure between N and O [8,9]. Reynolds et al. have achieved a mobility of 0.4 cm2 V1 s1 at a hole concentration of 3.6 1018 cm3 using N as p-type dopant [10]. In order to achieve highequality p-type ZnO films and overcome the low solubility of p-type dopants, ion implantation is a useful method to introduce a precise dopant concentration. However, ion implantation can cause lattice disorder and damage [11,12]. In the process of annealing, the implanted N ions lying in the interstitial positions can occupy the sites of oxygen
* Corresponding author. ** Corresponding author. E-mail address: [email protected] (J. Huang). http://dx.doi.org/10.1016/j.spmi.2016.10.006 0749-6036/© 2016 Elsevier Ltd. All rights reserved.
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vacancies or substitute for the lattice oxygen atoms through a kick-out mechanism [13,14]. So it is necessary to perform annealing to remove lattice disorder and activate dopants. There are different kinds of annealing to activate dopants and remove disorder including post-implantation annealing and dynamic annealing. It is reported that performing dynamic annealing can reduce the types of disorder caused by ion implantation which can't be reached by post-implantation annealing [15]. Since p-type ZnO films first attracted attention because of being promising material for ultraviolet optoelectronic devices, a large number of studies have focused on choosing different kinds of acceptors and doping methods, whose aim has been to produce as many holes as possible. However, an important problem that also needs to be studied is that a certain percentage of holes produced by acceptors in ZnO films are compensated by native donor defects, especially oxygen vacancies. There are few reports of studies paying attention to restraining the density of oxygen vacancies to contribute to better p-type characteristics of ZnO films and measuring the variation of the density of oxygen vacancies by experiment. In the work reported here, we not only paid attention to choosing acceptor and doping method but also devoted effort to decreasing density of oxygen vacancies. So in our study, in order to suppress and reduce the amount of native donor defects, especially oxygen vacancies, O ions were introduced by implantation to occupy the sites of oxygen vacancies and to reduce the effect of selfcompensation, increasing stable ZneO bonding [16]. In order to confirm the decreasing of density of oxygen vacancies, Raman spectra and X-ray photoelectron spectroscopy (XPS) measurements were performed. In this paper, molecular beam epitaxy (MBE), ion implantation and dynamic annealing have been combined to obtain ptype single-crystalline ZnO films. The effects of N and O ion implantation and the dynamic process on the p-type conductivity have been discussed. 2. Material and methods ZnO film was grown on a-plane sapphire substrate by plasma-assisted MBE technique. Elemental zinc (6 N grade) and oxygen radio frequency (RF) plasma (O2 gas of 6 N grade) were used as the sources. The a-plane sapphire substrates were cleaned ultrasonically with acetone, ethanol, and deionized water for 10 min at room temperature, respectively. Prior to beginning growth, the substrate was thermally cleaned by heating at 300 C in the preparation chamber for 3 h and then treated at 250 C for 20 min in oxygen plasma exposure in the growth chamber with further cleaning at 800 C for 30 min under ultrahigh vacuum achieved by a liquid nitrogen supply. After these cleaning processes, a ZnO buffer layer was first grown at 300 C for 10 min and annealed at 850 C for 5 min. Then the top ZnO layer was deposited at 800 C for 4 h and annealed at 850 C for 5 min for better film crystallinity and smoother surface, during which the Zn cell temperature, RF excitation power and oxygen flow rate were fixed at 300 C, 350 W and 1.0 sccm, respectively. After growth, the substrate temperature was decreased slowly at a rate of 5 C per minute. In order to achieve the concentration of N and O ions reaches 1021 cm3, the as-grown ZnO films were implanted with 120 keV N ions to fluence at 2.0 1016 cm2 and 130 keV O ions to fluence at 2.0 1016 cm2 at 460 C [17]. The thickness of ZnO film was about 380 nm, which is regarded as total thickness of the implanted layers and the projected range was calculated to be 190 nm by the TRIM code. N and O concentrations simulated by SRIM predict skew-Gaussian profiles with maximum implanted concentration at a depth of 190 nm for both. In order to make sure the effect of N and O ion implantation, dynamic annealing at 460 C was performed on unimplanted ZnO film. The electrical properties were examined by Hall-effect measurements using the Van der Pauw configuration at room temperature. The crystalline structure was analyzed by X-ray diffraction (XRD) measurements with a Cu Ka radiation source (l ¼ 1.54056 Å). The full spectrum and O 1s core-level spectrum of ZnO film were analyzed by X-ray photoelectron spectroscopy (XPS) and calibrated by the C 1s peak. The depth profile of implanted ZnO film was investigated by secondary ion mass spectroscopy (SIMS). What spectroscope is used for Raman measurements ? The room-temperature Raman spectra were performed on a Raman spectroscope using a 514 nm excitation wavelength from an Ar laser. The room-temperature polarized Raman spectra were performed on a Raman spectroscope using a 532 nm excitation wavelength. 3. Results and discussion Hall measurements were performed on all implanted and unimplanted ZnO films before and after dynamic annealing and their results are summarized in Table 1. To make sure of the reliability of the results, Hall measurements are repeated several times and similar results are obtained. The unimplanted ZnO films were measured to be n-type with high mobility prior to implantation. After implantation and dynamic annealing, the dually-implanted ZnO films exhibited p-type characteristics with N fluence at 2.0 1016 cm2, O fluence at 2.0 1016 cm2. As we regarded this, annealing at elevated temperature reduced lattice disorder caused by implanted ions with high energy to improve the quality of ZnO films and activated substitutional p-type acceptors so that p-type ZnO films had high mobility together with low resistivity, and O ions were introduced to occupy the sites of oxygen vacancies, reducing the effect of self-compensation and improving the hole concentration [18]. It is concluded from the results that performing dynamic annealing is possibly better at achieving p-type ZnO films contrary to post-implantation annealing and O ion implantation contributes to forming p-type single-crystalline ZnO films. P-type characteristics of the dually-implanted ZnO films have shown some degradation after three months. Fig. 1 shows the qe2q XRD patterns of implanted and unimplanted ZnO films before and after dynamic annealing. In XRD pattern, only ZnO (002) and (004) peaks exist beside the sapphire (110) peak and ZnO (002) is dominant for all samples, which
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Z. Zhang et al. / Superlattices and Microstructures 100 (2016) 468e473
Table 1 Electrical results of unimplanted and implanted ZnO films. Samples (N cm2)
Temp
Resistivity
Mobility
Carrier concentration
(O cm2)
( C)
(U cm)
(cm2 V1 s1)
(cm3)
0.14 0.012
43 45.5
1.0 1018 1.0 1019
n n
0.353
1.6
1.1 1019
p
1.555
1.9
2.1 1018
p
Unimplanted Unimplanted 2.0 1016 2.0 1016 2.0 1016 2.0 1016
460 460
Carrier type
460
Fig. 1. X-ray diffraction patterns of the implanted and unimplanted ZnO films before and after dynamic annealing.
suggests that after implantation, the ZnO film is single-crystalline in nature [19] and c-plane orientation is obtained. The sapphire (110) peak isn't observed in the dually-implanted ZnO films XRD pattern, which attributes to impact of lattice disorders and defects existing in the dually-implanted ZnO films caused by high energy ion implantation and weak intensity of the sapphire (110) peak. And the sapphire (110) peak of unimplanted ZnO films is enhanced by dynamic annealing, which reflects the quality of ZnO films is improved. Despite a small increase in the bandwidth of the ZnO (002) peak after implantation, it is observed that the positions of peaks in the XRD pattern don't change and it is concluded that the amount of implanted N and O ions is so little that the effect falls below the XRD detection limit on the crystal quality of the ZnO films. Fig. 2 shows the XPS results of the implanted single-crystalline ZnO films, which exhibit indexed peaks that correspond to C, O, and Zn. However, the peak associated with N isn't seen in the XPS spectra. Because of the low amount of the implanted N
Fig. 2. XPS spectrum of the implanted ZnO films.
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Fig. 3. SIMS spectrum of the implanted ZnO films.
ions contrary to other elements, the concentration is too low to be measured by the XPS technique. In order to make sure N ions were effectively implanted, it was necessary to perform secondary ion mass spectroscopy (SIMS) measurements on implanted single-crystalline ZnO films. As shown in Fig. 3, N signal is clearly seen in the SIMS results, which means that N ions were successfully introduced in the single-crystalline ZnO films. It is observed that the concentration of N ions changes little with depth, which is mainly attributed to diffusion of N ions when samples were performed dynamic annealing. However, because of the variation of concentrations of Zn and O, it is deduced that there are still lattice disorders and defects existing in the single-crystalline ZnO films caused by high energy ion implantation. Fig. 4 shows the room-temperature Raman spectra of unimplanted and implanted single-crystalline ZnO films. Wurtzite structure ZnO has two formula units in the primitive cell belonging to the space group of C6v4. It is reported that frequencies of ZnO films contain A (LO) ¼ 574 cm1 mode about the fundamental optical mode [20]. A Raman shift peak at 574 cm1 is clearly seen in the Raman spectrum of umimplanted ZnO films. However, there isn't any Raman peak at 574 cm1 in the spectrum of implanted ZnO films. And the Raman shift peak around 430 cm1 is clearly seen in the Raman spectrum of umimplanted ZnO films and implanted ZnO films, which corresponds to the stretching bond of Zn2þeO2. The A (LO) mode is reported to correspond to oxygen vacancies in ZnO films [21,22] and this is supported by the discussion of the XPS results. In order to make sure of the A (LO) mode of wurtzite structure ZnO, polarized Raman measurements with polarization configuration of z(x,x)z were performed on unimplanted and implanted single-crystalline ZnO films, as shown in Fig. 5. The x, y, z axes are aligned along the ½1210, ½1210, and [0001]. The peak at 574 cm1 is assigned to phonon mode of A (LO), which disappears in the spectrum of implanted ZnO films. So with the disappearance of the Raman shift peak, it can be concluded that the concentration of oxygen vacancies has been reduced and implanted O ions have occupied the sites of oxygen vacancies, reducing the effect of self-compensation caused by intrinsic donor defects and resulting in increasing of stable ZneO bonds. The XPS results for the O 1s level of unimplanted and implanted single-crystalline ZnO films are shown in Fig. 6 and Fig. 7, which are used to probe the formation of oxygen vacancies and verify the deduction associated with the Raman results. It is
Fig. 4. Raman spectra of the unimplanted and implanted ZnO films.
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Fig. 5. Polarized Raman spectra of the unimplanted and implanted ZnO films.
Fig. 6. The O 1s XPS spectrum of the unimplanted ZnO films.
Fig. 7. The O 1s XPS spectrum of the implanted ZnO films.
obvious that the O 1s peaks of unimplanted and implanted single-crystalline ZnO films exhibit asymmetry which means that more than one kind of oxygen atom bonding configuration existing in the single-crystalline ZnO films. So the O 1s peak can be divided into two Gaussian peaks. The centers of the two Gaussian peaks, OⅠ peak and OⅡ peak are located at 530 ± 0.4 eV and 531.5 ± 0.4 eV, respectively. The OⅠ peak is associated with O2 ions in the ZnO crystal lattice surrounded by Zn2þ [23]. There
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are reports exhibiting that with the increasing of loss of oxygen, the intensity of the OⅡ peak is enhanced and the OⅡ peak is associated with oxygen vacancies [24,25]. It is obviously observed that the intensity of the OⅡ peak decreases with the implantation of O ions. Combined with the previous Raman results, the XPS results are well consistent with the Raman spectral analysis and that the OⅡ peak is associated with oxygen vacancy in our study. Therefore, one can depend upon the concentration of absorbed oxygen to verify the trend of the concentration of oxygen vacancies. According to the rules of quantitative analysis, the ratio of oxygen vacancies to total oxygen reflects the trend [26]. It shows that with the implantation of O ions, the ratio decreases from 0.314 to 0.12 comparing unimplanted single-crystalline ZnO films with implanted singlecrystalline ZnO films. So it can be concluded that the concentration of oxygen vacancies decreases with the implantation of O ions and it contributes to reducing the effect of self-compensation and the realization of p-type single-crystalline ZnO films. 4. Conclusion Single-crystalline ZnO films were implanted with 120 eV N ions at a fluence of 2.0 1016 cm2, and 130 eV O ions at a fluence of 2.0 1016 cm2, both at 460 C, which were grown on a-plane sapphire by plasma-assisted molecular beam epitaxy technique. Hall measurements show that the single-crystalline ZnO films dually implanted with fixed fluences of N and O ions change from n-type characteristics to p-type characteristics, with hole concentration in the range of 2.1 1018e1.1 1019 cm3, hole mobilities between 1.6 and 1.9 cm2 V1 s1, and resistivities in the range of 0.353e1.555 U cm. 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