Comment on “Growth Behavior, Lattice Expansion, Strain, and Surface

Comment pubs.acs.org/JPCC

Comment on “Growth Behavior, Lattice Expansion, Strain, and Surface Morphology of Nanocrystalline, Monoclinic HfO2 Thin Films”: Implications for Undesirable Polaron Formation Carolyn Rubin Aita,* Massiel Cristina Cisneros-Morales, and Elizabeth Ellen Hoppe Department of Chemistry and Biochemistry, University of WisconsinMilwaukee, P.O. Box 413, Milwaukee, Wisconsin 53201, United States

J. Phys. Chem. C 2012, 116 (18), 9955−9960. DOI: 10.1021/jp211109h J. Phys. Chem. C 2012, 116. DOI: 10.1021/jp310937u

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amana et al.1 recently reported a relationship between crystallite size and lattice expansion in sputter-deposited monoclinic HfO2 thin films. They state that their results agree with our previously published results on the same topic.2 However, a comparison of references 1 and 2 does not support this claim. In our study, nanocrystalline monoclinic HfO2 films were sputter deposited on fused silica substrates, air annealed at several temperatures from 573 to 1273 K to affect crystallite growth, and analyzed by X-ray diffraction (XRD) and optical spectrophotometry. Nanocrystallites preferentially grew with (−111) planes oriented parallel to the substrate. Our key results showing the relationship between the (−111) interplanar spacing, d(−111), and the nanocrystallite dimension in the growth direction, D(−111), as estimated using the Scherrer relationship, are summarized here in both graphical (Figure 12) and numerical (Table 1) form to aid the reader with the following analysis.

Table 1. Coordinates of D(−111) and d(−111) Data Points in Figure 1a D(−111) (nm)

d(−111) (nm)

1 2 3 4 5 6

6.7 7.7 9.2 15.5 29.2 38.4

0.3177 0.3169 0.3160 0.3148 0.3148 0.3148

a

d(−111) = 0.3148 nm for an unstressed, monoclinic standard perfect lattice (ref 3).

d(−111) > 0.3148 nm yields the dashed line in Figure 1. The intersection of regression and saturation lines yields the point D(−111) = 10.7 nm, d(−111) = 0.3148 nm. D(−111) = 10.7 nm therefore represents the smallest crystallite for which there is no lattice expansion due to crystallite size, assuming a continued linear dependence of d(−111) on D(−111) as the point of saturation is approached. At the other extreme, note that, when D(−111) = 15.5 nm, d(−111) has already achieved the infinite-size crystal value. We conclude therefore that the onset of bulk lattice behavior occurs in crystallites with 10.7 nm ≤ D(−111) ≤ 15.5 nm. In the study of Ramana et al.,1 films were deposited on Si substrates at growth temperatures from room to 773 K and analyzed by XRD. Their results differ from ours as stated above in three basic ways. First, their data show no saturation of d(−111) with increasing D(−111) (compile the data in Figures 4 and 8 in ref 1). Second, their data show very large values of d(−111) compared to those given in Table 1 and Figure 1 for a comparable D(−111) range (compile the data in Figures 4 and 8 in ref 1). Third, their d(−111) data never achieve the infinite-size lattice value of 0.3148 nm, even at the upper end of the range in which D(−111) is ∼40−45 nm (see Figure 8 in ref 1). On the basis of these differences, we contest the claim of Ramana et al.1 that the results of these two studies agree despite the fact that “growth conditions, substrates, and film thickness are different”. Contrary to the assertion of Ramana et al.,1 these two studies cannot be taken together to yield universal information about nanocrystallite HfO2 films.

Figure 1. d(−111) versus D(−111) for all film states discussed in reference 2. The dashed line is a linear regression analysis for data in which d(−111) decreases linearly with increasing D(−111). The solid horizontal line indicates d(−111) = 0.3148 nm in a perfect lattice.3 The intersection of these lines indicates the smallest value at which d(−111) becomes insensitive to crystallite size, 10.7 nm. Numbers adjacent to the data points indicate coordinate values given in Table 1. (Adapted from ref 2.)

Figure 1 and Table 1 show that d(−111) versus D(−111) decreases linearly with increasing crystallite size for small crystallites and saturates for larger crystallites. The saturation value, d(−111) = 0.3148 nm, is indicated as a solid horizontal line in Figure 1. This value is significant because it is the value of d(−111) for an unstressed bulk (i.e., infinite-size) monoclinic crystal.3 A linear regression analysis using points for which © 2012 American Chemical Society

point

Received: September 6, 2012 Revised: October 31, 2012 Published: November 9, 2012 26679

dx.doi.org/10.1021/jp3088868 | J. Phys. Chem. C 2012, 116, 26679−26680

The Journal of Physical Chemistry C

Comment

Ramana et al.1 state that their films are in various states of stress depending upon the growth temperature. We suggest that film stress, which would affect both the position of the (−111) XRD peak (hence d(−111)) and its width (hence D(−111)), might not be completely accounted for in their study. This suggestion is based on the fact that their data for d(−111) never relax to the unstressed crystal value, even in relatively large crystallites. HfO2 has for some time been under consideration as a high dielectric constant gate insulator in integrated circuits.4 However, theoretical calculations show that hole and electron small polaron states in a well-ordered monoclinic HfO2 lattice reside within the energy band gap and are therefore undesirable.5−9 These states are observed in well-crystallized monoclinic HfO2 films as a low energy shoulder on the fundamental optical absorption edge.2,10 Reference 2 reports an important connection between nanocrystallite size and nearedge optical behavior. Specifically, the intensity of the shoulder decreases with increasing lattice expansion. This result is consistent with the inhibition of small polaron formation due to increased nearest-neighbor distance. (Alternatively, one can envision HfO2 as becoming more covalent as its lattice expands11 and therefore less likely to support small polaron formation.) We therefore stress that an accurate determination of the range of the crystallite size over which lattice expansion occurs provides a powerful tool for tailoring HfO2 films in which small polaron formation is suppressed via size effects. In addition to the above comment, the following inaccuracies appear in reference 1 with respect to reference 2: (1) The film thickness in reference 2 is 170 nm, not 1.7 μm as stated by Ramana et al.1 Correction of this order-ofmagnitude error is important because it nullifies the claim of Ramana et al.1 that the combined results of references 1 and 2 are independent of film thickness. (2) Equation 3 in reference 1 is cited as coming from reference 2. However, this equation does not come from our work, and this citation to our work should be removed. (3) The citation for reference 2 is incorrect and should appear as it does in this Comment.

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(7) Muñoz Ramo, D.; Gavarton, J. L.; Shluger, A. L.; Bersuker, G. Microelectron. Eng. 2007, 84, 2362−2365. (8) Shluger, A. L.; McKenna, K. P.; Sushko, P. V.; Muñoz Ramo, D.; Kimmel, A. V. Modell. Simul. Mater. Sci. Eng. 2009, 17, 084004−1− 084004−21. (9) Muñoz Ramo, D.; Sushko, P. V.; Shluger, A. L. Phys. Rev. B 2012, 85, 024120−1−024120−10. (10) Hoppe, E. E.; Sorbello, R. S.; Aita, C. R. J. Appl. Phys. 2007, 101, 123534−1−123534−6. (11) Guangshe, L.; Boerio-Goates, J.; Woodfield, B. F. Appl. Phys. Lett. 2004, 85, 2059−2061.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by AceLab discretionary funds. REFERENCES

(1) Ramana, C. V.; Kamala Bharathi, K.; Garcia, A.; Campbell, A. L. J. Phys. Chem. C 2012, 116, 9955−9960. (2) Cisneros-Morales, M. C.; Aita, C. R. Appl. Phys. Lett. 2010, 96, 191904−1− 191904−3. (3) Joint Committee on Powder Diffraction Standards Card No. 78− 0050. (4) See, for example: Wilk, G. D.; Wallace, R. M.; Anthony, J. M. J. Appl. Phys. 2001, 89, 5243−5275. (5) Gavartin, J. L.; Muñoz Ramo, D.; Shluger, A. L.; Bersuker, G.; Lee, B. H. Appl. Phys. Lett. 2006, 89, 082908−1−082908−3. (6) Muñoz Ramo, D.; Shluger, A. L.; Gavarton, J. L.; Bersuker, G. Phys. Rev. Lett. 2007, 99, 155504−1−155504−4. 26680

dx.doi.org/10.1021/jp3088868 | J. Phys. Chem. C 2012, 116, 26679−26680