Nuclear Instruments and Methods in Physics Research B 375 (2016) 5–7
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Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Discrepancy between ambient annealing and H+ implantation in optical absorption of ZnO Jinpeng Lv a,⇑, Chundong Li b a b
College of Astronautics, Nanjing University of Aeronautics and Astronautics, Nanjing, Jiangsu, People’s Republic of China School of Materials Science and Engineering, Harbin Institute of Technology, Harbin, Heilongjiang, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 6 January 2016 Received in revised form 14 March 2016 Accepted 16 March 2016 Available online 23 March 2016 Keywords: ZnO Particle implantation Defect Visible absorption
a b s t r a c t The discrepancy between sub-bandgap absorption in ZnO induced by thermal annealing and H+ implantation is investigated in this study for the first time. Results indicate that nonreductive annealing-induced optical absorption is independent of annealing ambient, and can be assigned to VO, whereas the absorption centers caused by H+ implantation and H2 annealing are primarily associated with VO and ionized Zni. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction As a promising wide-band gap semiconductor, ZnO has attracted considerable attention for potential applications ranging from optoelectronics to photocatalysis [1]. Its practical application depends on a firm understanding of the relationship among defect chemistry, processing, and properties. Therefore, it is important to gain further insights into many unresolved fundamentals of ZnO, for example, the defect origin of visible band optical absorption. It is well observed that the white (powders) or transparent (single crystal) appearance of ZnO can be transformed to pale yellow or even black by energetic particle implantation, high-temperature treatment, and mechanical ball milling [2–4], corresponding to the so-called blue absorption band that extends across from the optical edge to the blue-green spectral region. However, the application of ZnO as window display materials, such as transparent conductive film, the buffer layer of solar cells, and the pigment of thermal control coatings, commonly require a high light transmittance and low absorption, whereas an intensive and broad visible optical absorption is pivotal for its light photocatalytic activity. Despite extensive effort, the mechanism of optical transition behind this blue-band absorption is still unclear. Various hypotheses have been proposed to explain this coloration, including oxygen vacancy (VO), interstitial zinc (Zni), defect complex, and even extrinsic NO and H impurities [2,5–7]. Furthermore, to the best of ⇑ Corresponding author. E-mail address:
[email protected] (J. Lv). http://dx.doi.org/10.1016/j.nimb.2016.03.034 0168-583X/Ó 2016 Elsevier B.V. All rights reserved.
our knowledge, the discrepancy between the blue-band absorption caused by thermal equilibrium annealing and nonequilibrium energetic particle implantation has rarely been studied. In this study, by using a combination of absorption spectrum, electron spin resonance (ESR), and Raman-scattering characterizations, we compared the blue-band absorption characteristics of thermally treated and H-implanted ZnO, and then we tentatively clarified the discrepancy of the optical absorption between thermal annealing and H+ implantation. This study furthers our understanding of the fundamentals of ZnO and resolves the longstanding controversies about this unique material.
2. Experimental section Commercial ZnO powders (Aladdin) with a mean particle size of 300–500 nm were used in this study. Thermal treatments were implemented by annealing pristine ZnO in air, N2, H2, or O2 ambient for 1 h. The H+ and electron implantations were performed in vacuum with energies of 90 and 70 keV and fluences of 3 1015 and 2 1016 cm2, respectively. The ultraviolet–visible (UV–vis) near infrared (NIR) absorption properties were investigated by diffuse reflectance spectroscopy using a Lambda 950 spectrophotometer. The ESR spectra were recorded on a JES-FA200 EPR (electron paramagnetic resonance) spectrometer operating in the X-band frequency (9.056 GHz) with a field modulation frequency of 100 kHz and microwave power of 1 mW. The room temperature (RT) micro-Raman spectra
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J. Lv, C. Li / Nuclear Instruments and Methods in Physics Research B 375 (2016) 5–7
of the samples were recorded on a JY-HR800 micro-Raman spectrometer using a 458-nm wavelength Ar+-ion laser.
3. Results and discussions Fig. 1 illustrates the reflectance spectra of pristine ZnO, thermally annealed, and H+-implanted samples. After thermal treatments, the reflectivity at the infrared wave band experiences a considerable augment, because of the removal of the absorbed hydroxyl groups within ZnO. It can be observed that both annealing and H+ implantation produced an apparent sub-bandgap absorption that extends from the band edge to green spectral regions. It is found that the air-, N2-, and O2-annealed ZnOs exhibit similar optical absorption features. Unexpectedly, beside the 395nm (3.14 eV) absorption peak, a broad shoulder peak around 430 nm (2.88 eV) appeared in H2-annealed samples. Although the origin of the blue absorption is potentially related to oxygendeficient defects [8], high-temperature O2 annealing still produces remarkable optical absorption. In addition, obvious coloration was also observed after annealing ZnO single crystals in a phosphorus vapor ambient, because P-atom capturing evaporated O atoms and thus left VO behind [9]. Interestingly, the absorption curve caused by H+ implantation shows a distinct shape from that of thermally treated samples: The different ambient annealed samples possess the same absorption center located at 3.14 eV, whereas the absorption peak of H+-implanted ZnO is centered at approximately 420 nm (2.95 eV). Furthermore, the absorption curve of H+-implanted sample is less steep but more broad. Because of the complicacy of defect physics in ZnO, the view of H impurity rather than native point defect responsible for the controversies of ZnO have become popular in recent studies [6,10]. Hþ O is such a case and was supposed to account for the blue-band absorption [6]. In order to explore whether H impurity was involved in the blue-band absorption, an electron-implanted sample was presented, showing a broad absorption centered around 2.88 eV, which overlapped the shoulder peak of H2-annealed sample. The appearance of the blue absorption after electron implantation, as well as by other ion implantation [3,11], suggests that the optical absorption centers are mainly attributed to the intrinsic defects. In order to compare the influences of thermal treatment and H+ implantation on the defect structure of ZnO, the Raman scattering modes are normalized by the intensity of the E2 (high) peak (Fig. 2). The scattering peak around 584 cm1 is usually attributed to the E1 or A1 LO phonon mode due to Frohlich scattering, which could be generated by lattice defects, for example, VO [12]. After thermal
Fig. 2. Raman spectra of a pristine ZnO, H+-implanted ZnO, and different atmosphere-annealed samples. The inset shows the Raman image of H+-implanted sample.
treatment, the E1 (LO) phonon mode exhibits a clear enhancement in thermally annealed samples and rises significantly with increasing annealing temperature. Notably, the variation of the E1 (LO) peak shows a good agreement with the blue-band absorption. Previous studies found that the E1 (LO) peak is potentially correlated to a VO defect [1]. This conclusion has been supported by the disappearance of the E1 (LO) mode after oxygen plasma exposure in a recent study [13]. After H+ implantation, the E1 (LO) peak was significantly enhanced. Furthermore, the peak also blue-shifted from 584 to 575 cm1. Such a drastic variation of the E1 (LO) peak was also observed in ZnO irradiated by other particles [14]. It has been well documented that interstitial zinc particles were created and accumulated during photon, electron, and ion implantation in ZnO [15–17]. Besides, the out-diffusion of interstitial Zn was observed in rapidly annealed and thermochemically reduced ZnO, which led to semiconducting to metallic transition [18]. In accordance with those results, accumulation of interstitial zinc after H+ implantation can be observed directly from the Raman image of ZnO, as shown in the insets of Fig. 2. Hence, it is reasonable to surmise that considerable amount of ionized Zni together with VO was created in particle-implanted and H2-annealed samples. Compared with thermally annealed sample, the resonant
Fig. 1. (a) Diffuse reflectance spectra of thermally annealed ZnO; the inset shows the corresponding visible band absorption spectra. (b) Comparison of the absorption curves of ZnO treated by different conditions. An electron-implanted ZnO is also presented for comparison.
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contributes to the broadening of the blue absorption band. Besides, compared with H+ implantation, we note that the O-ion implantation-induced absorption band, centered around 407 nm, is also narrower [11], which could be attributed to the partial capture and oxidation of Zni defects. 4. Conclusions
Fig. 3. EPR spectra of thermally treated and H+-implanted ZnO samples; the spectra were taken at 77 K under full-wavelength light illumination for 2 min.
intensification of the E1 (LO) peak caused by H+ implantation is several times stronger. This can be attributed to the fact that particle implantation is a nonequilibrium transient heating process accompanied with serious crystalline implantation damage, whereas thermal treatment is a dynamic equilibrium process for defect creation and recovery. Consequently, implantation introduced localized states, such as charged VO, Zni, and H, cut the long-range lattice ordering, and activated the forbidden modes [14], resulting in an abrupt vibration disorder and blue shift of the E1 (LO) mode. ESR is also used to detect defect variation in ZnO after different treatments. As shown in Fig. 3, two characteristic resonances at g = 2.002 and 1.957 can be observed in pristine ZnO. As proposed by Erdem et al. [19,20], these two signals represent the core and shell defects in ZnO, respectively. Interestingly, after thermal annealing, two new peaks at g = 1.998 and 2.017 appeared in samples annealed at different ambient conditions, whereas H+ implantation creates the g = 1.998 resonance solely. By comparing the relative intensities of the signal at g = 1.998 in different atmosphere-annealed ZnO, it can be found that this signal also follows the intensities of the blue-band absorption. As is well known, the spin signal at g = 1.998 is associated with Vþ O defects. Thus, in combination with Raman results, it is conclusive to correlate the blue-band absorption to Vþ O -related defects. The signal at approximately g = 2.017 was tentatively attributed to lithium or some other unknown impurities. It is well known that the shape of the reflectance spectrum is dependent on the energy and width of the band-to-band transition (including excitonic effects) and defect-related absorption [21,22]. It is evident from Fig. 1b that thermal treatment-induced blue absorption shows sharper peaks centered at 395 nm, while the absorption bands caused by H+ and electron implantation show peaks at 420 and 430 nm, respectively. As the energy of the band maximum is mainly determined by the steepness of the intrinsic band-to-band absorption, which is influenced by the band-toband transition width or exciton width, the electron- and hole-scattering rates depend on the concentration of defects in the sample. Thus, the band of irradiated samples exhibit less-steep high-energy tails than the band of nonreductive annealed samples, because of its higher defect concentration. It is reasonable that ion implantation- as well as H2 annealing-introduced interstitial zinc
The visible band optical absorption of ZnO powders after different ambient annealing and H+ implantation has been investigated in this study. It is found that thermal annealing-induced optical absorption peaks are centered around 395 nm, whereas H+ implantation-caused absorption peaks are located at 420 nm. Results indicated that the blue optical absorption exhibits intimate relationships with the E1 (LO) Raman peak as well as the g = 1.998 spin resonance, indicating that the VO defect dominates the absorption. Furthermore, obvious accumulation of Zni was also found from Raman results after H+ implantation. Thus, we surmise that nonreductive annealing-induced optical absorption in ZnO is independent of annealing ambient, and can be attributed to the VO defect, whereas absorption centers created by H+ implantation are primarily composed of VO and ionized Zni. Acknowledgments This study was financially supported by the National Natural Science Foundation of China (No. 11275054) and the International Science & Technology Cooperation Program of China (2015DFR50400). References [1] Ü. Özgür, Ya.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. [2] F.A. Selim, M.H. Weber, D. Solodovnikov, K.G. Lynn, Phys. Rev. Lett. 99 (2007) 085502. [3] S.K. Neogi, S. Chattopadhyay, A. Banerjee, S. Bandyopadhyay, A. Sarkar, R. Kumar, J. Phys. Condens. Matter 23 (2011) 205801. [4] P.K. Giri, S. Bhattacharyya, Dilip K. Singh, R. Kesavamoorthy, B.K. Panigrahi, K. G.M. Nair, J. Appl. Phys. 102 (2007) 093515. [5] W.E. Vehse, W.A. Sibley, F.J. Keller, Y. Chen, Phys. Rev. 167 (1968) 828–836. [6] M.H. Weber, N.S. Parmar, K.A. Jones, K.G. Lynn, J. Electron. Mater. 39 (2010) 573–576. [7] N.Y. Garces, N.C. Giles, L.E. Halliburton, G. Cantwell, D.B. Eason, D.C. Reynolds, D.C. Look, Appl. Phys. Lett. 80 (2002) 1334–1336. [8] R.M. de la Cruz, R. Pareja, R. Gonzalez, L.A. Boatner, Y. Chen, Phys. Rev. B 45 (1992) 6581–6586. [9] L.E. Halliburton, N.C. Giles, N.Y. Garces, Ming Luo, Chunchuan Xu, Appl. Phys. Lett. 87 (2005) 172108. [10] T.S. Bjørheim, S. Erdal, K.M. Johansen, K.E. Knutsen, T. Norby, J. Phys. Chem. C 116 (2012) 23764–23772. [11] S. Pal, A. Sarkar, S. Chattopadhyay, Chakrabarti Mahuya, D. Sanyal, P. Kumar, D. Kanjilal, T. Rakshit, S.K. Ray, D. Jana, in: Nucl. Instrum. Methods Phys. Res. Sect. B 311 (2013) 20–26. [12] S.K.S. Parashar, B.S. Murty, S. Repp, S. Weber, E. Erdem, J. Appl. Phys. 111 (2012) 113712. [13] M.A. Gluba, N.H. Nickel, N. Karpensky, Phys. Rev. B 88 (2013) 24520. [14] Z.Q. Chen, M. Maekawa, A. Kawasuso, S. Sakai, H. Naramoto, J. Appl. Phys. 99 (2006) 093507. [15] E.H. Khan, M.H. Weber, M.D. McCluskey, Phys. Rev. Lett. 111 (2013) 017401. [16] D.C. Look, J.W. Hemsky, J.R. Sizelove, Phys. Rev. Lett. 82 (1999) 2252–2255. [17] G. Brauer, W. Anwand, W. Skorupa, J. Kuriplach, O. Melikhova, C. Moisson, H. von Wenckstern, H. Schmidt, M. Lorenz, M. Grundmann, Phys. Rev. B 74 (2006) 045208. [18] C.Y. Hsu, Appl. Phys. Lett. 103 (2013) 242103. [19] E. Erdem, J. Alloys Compd. 605 (2014) 34–44. [20] P. Jakes, E. Erdem, Phys. Status Solidi RPL 5 (2011) 56. [21] Kwang Joo Kim, Young Ran Park, in: Appl. Phys. Lett. 78 (2001) 475–477. [22] R. Tena-Zaera, C. Martínez-Tomás, C.J. Gómez-García, V. Muñoz-Sanjosé, Cryst. Res. Technol. 41 (2006) 742–747.
