Comment pubs.acs.org/JPCC
Reply to “Comment on 'Growth Behavior, Lattice Expansion, Strain, and Surface Morphology of Nanocrystalline, Monoclinic HfO2 Thin Films'” C. V. Ramana* and K. Kamala Bharathi Department of Mechanical Engineering, University of Texas at El Paso, El Paso, Texas 79968, United States
J. Phys. Chem. C 2012, 116 (18), 9955−9960. DOI: 10.1021/jp211109h J. Phys. Chem. C 2012, 116. DOI: 10.1021/jp3088868
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n a recent article,1 we reported on the relationship between crystallite size and lattice expansion in sputter-deposited HfO2 thin films. We respectfully submit this response to clarify the issues or concerns of Aita et al.2 with regard to our published work1 compared to their work3 as follows. Hafnium oxide (HfO2) has been the focus of much attention of the scientific community due to its high chemical stability and higher k-dielectric constant (k ∼ 20 at 300 K).4−6 The appropriate conduction band offset with silicon (ΔEC ∼ 1.5− 2.0 eV), high melting point (2700 °C), and high bulk modulus make HfO2 films more suitable as far as device fabrication is concerned.4−7 Due to its high refractive index, HfO2 has been considered as a suitable candidate for protective optical coatings.8,9 HfO2 films can also be integrated into dynamic random access memory (DRAM) capacitors and gas sensing devices.8,9 In addition, recently, Coey et al.10,11 and Hong et al.12 have observed ferromagnetism in HfO2 thin films fabricated by pulsed laser deposition, attributed to the ferromagnetism in HfO2 to the point defects in the lattice and the phenomenon of so-called “d0 magnetism”. According to the reports,10−12 the d0 ferromagnetism is due to oxygen vacancies (VO), which could produce oxygen holes, and the lattice defects arising should be the source of magnetism. However, while ferromagnetic properties in this dielectric material are expected to significantly enhance the potential utilization in the field of emerging quantum electronics and spin-based electronics, Rao et al.13 did not observe ferromagnetism in HfO2 films. The existence and physical origin of ferromagnetism in HfO2 thin films remains still a matter of debate. Clearly, this brief analysis presents the importance of HfO2-based materials for investigating the fundamental structure and electronic properties at the reduced dimensionality. Aita et al.3 have reported the connection between nanocrystallite size and near-edge optical behavior. Let us first discuss the issue of the typo about film thickness of Aita et al. that appeared in our article.1 We regret the inconvenience, in page 9959 of our article:1 the cited HfO2 film thickness of 1.7 μm is a typo error, and it should read as 0.17 μm. Two references (refs 4 and 29) are cited for eq 3 in our article.1 It should be corrected as ref 29 alone. In addition to that, we overlooked at the time of proofreading, that ref 4 in our article1 should read as follows: Cisneros-Morales, M. C.; Aita, C. R. Appl. Phys. Lett. 2010, 96, 191904-1−191904-3. We totally agree with Aita et al. that there are differences in the microdetails of both the works and results. Please note that © 2012 American Chemical Society
we clearly indicated that a more detailed account of MO2 family oxides at the reduced dimensionality and implications for their property and performance tuning need further investigation. One specific reason why we said that is we did not observe the relaxation to the bulk values. However, we were referring to the phenomenon of lattice expansion observed in our work when we were emphasizing the similarity. No doubt, there is lattice expansion noted in both the works. Obviously, one can infer the quantitative aspects and difference in these works by looking at the data (specifically d(−111) values) presented in these two articles.1,3 It should be noted that the experimental conditions and substrate materials are totally different in both the works. We have investigated and reported1 the effect of growth temperature, while Aita et al. air annealed the films to obtain crystalline quality films.3 The difference in the fabrication parameters are listed in Table 1 for the benefit of understanding and quick access to the data from both the works. Table 1. Comparison of Experimental Conditions physical/chemical parameter Sputtering target material Target dimensions Substrates Substrate temperature Postdeposition annealing Ar and O2 ratio Deposition rate Film thickness
Aita et al. (ref 3)
our work (ref 1)
Hf metal, purity not specified Not specified Fused silica No 573−1273 K
Hf-metal, 99.95% purity 2″ dia; 0.125″ thick Si(100) wafers RT−500 °C (in situ) No
80:20 2.0 nm/min 0.17 μm
70:30 2.25 nm/min 90 nm
We believe based on the earlier work1 that the characteristic feature of lattice expansion occurs specifically at the reduced crystal dimensions. However, as noted in our work, we did not see the relaxation to the bulk values in the range of Ts = RT− 500 °C or the crystallite dimensions measured. As stated by Aita et al.,2 stress in film plays a role. We did consider the fact and included such analysis to account for the stress−strain across the film. We have performed Williamson−Hall analysis to account for the chemistry and physics involved and Received: November 5, 2012 Published: December 10, 2012 26681
dx.doi.org/10.1021/jp310937u | J. Phys. Chem. C 2012, 116, 26681−26682
The Journal of Physical Chemistry C
Comment
A.; Dhar, S.; Shinde, S. R.; Venkatesan, T.; Lofland, S. E.; Schwarz, S. A. Appl. Phys. Lett. 2006, 88, 142505−1−142505−3. (14) Freund, L. B.; Suresh, S. Thin Film Materials - Stress, Defect Formation and Surface Evolution; Cambridge University Press, 2003. (15) Vemuri, R. S.; Noor-A-Alam, M.; Gullapalli, S. K.; Engelhard, M.; Ramana, C. V. Thin Solid Films 2012, 520, 1446−1450.
presented the results in ref 1. Also, we are aware of the fact that the peak broadening in XRD is usually a result of crystallite size reduction and/or strain development in the film and what factors could contribute to the homogeneous and inhomogeneous stress/strain.14,15 As such we believe that there is no scope for such ambiguity in the present case, i.e., what we observed and analyzed in our work. Furthermore, we believe that such relaxation to the bulk value is also dependent on the other factors such as film thickness since sputter deposition always starts with island morphology. The state of stress will be extremely high under such conditions and slowly relieves the stress as layers subsequently grow on the already added layers and/or being acted by thermal energy. We, therefore, believe that the postdeposition air annealing for sufficiently long time (1 h for all the temperatures considered in their work and 24 h for the 1273 K case) may be the factor that is responsible for such relaxation to occur very quickly as noted by Aita et al.3 So, these two works cannot be taken together to provide a unified theory except the fact that the lattice expansion occurs in nanocrystalline, monoclinic HfO2 films, although we do not agree in terms of relaxation to the bulk d-values. Perhaps, in our opinion, the difference in temperature to obtain crystalline quality films accounts for the observed difference in d(−111) values, specifically, the crystallite size (10.7 nm) at which d(−111) becomes insensitive to the crystallite size. However, we did not see such behavior in our case.1 We, therefore, believe that it is not just the crystallite dimensions but also the other factors such as film thickness, thermal treatment, and substrate materials that play a role in the lattice expansion of HfO2 films, which definitely calls upon further investigation.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Ramana, C. V.; Kamala Bharathi, K.; Garcia, A.; Campbell, A. L. J. Phys. Chem. C 2012, 116, 9955−9960. (2) Aita, C. R.; Cisneros-Morales, M. C.; Hoppe, E. E. J. Phys. Chem. C 2012, DOI: 10.1021/jp3088868. (3) Cisneros-Morales, M. C.; Aita, C. R. Appl. Phys. Lett. 2010, 96, 191904-1−191904-3. (4) Wilk, G. D.; Wallace, R. M.; Anthony, J. M. J. Appl. Phys. 2001, 89, 5243−5275. (5) Collin, M.; Charles, B. M. J. Phys. Chem. B 2004, 108, 15150− 15164. (6) Gordon, R. G.; Becker, J.; Hausmann, D.; Suh, S. Chem. Mater. 2001, 13, 2463−2464. (7) Afanasev, V. V.; Stesmans, A.; Chen, F.; Shi, X.; Campbell, S. A. Appl. Phys. Lett. 2002, 81, 1053−1055. (8) Lesser, M. Opt. Eng. 1987, 26, 911−915. (9) Niinisto, J; Putkonen, M.; Niinisto, L.; Stoll, S. L.; Kukli, K.; Sajavaara, T.; Ritala, M.; Leskela, M. J. Mater. Chem. 2005, 15, 2271− 2275. (10) Venkatesan, M.; Fitzgerald, C. B.; Coey, J. M. D. Nature 2004, 430, 630. (11) Coey, J. M. D.; Venkatesan, M.; Stamenov, P.; Fitzgerald, C. B.; Dorneles, L. S. Phys. Rev. B 2005, 72, 024450-1−24450-6. (12) Hong, N. H.; Sakai, J.; Poirot, N.; Brize, V. Phys. Rev. B 2006, 73, 132404-1−132404-4. (13) Rao, M. S. R.; Kundaliya, D. C.; Ogale, S. B.; Fu, L. F.; Welz, S. J.; Browning, N. D.; Zaitsev, V.; Varughese, B.; Cardoso, C. A.; Curtin, 26682
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