Ti-catalyzed HfSiO4 formation in HfTiO4 films on SiO2 studied by Z-contrast scanningelectron microscopyElizabeth Ellen Hoppe, Massiel Cristina Cisneros-Morales, and Carolyn Rubin Aita Citation: APL Materials 1, 022108 (2013); doi: 10.1063/1.4818171 View online: http://dx.doi.org/10.1063/1.4818171 View Table of Contents: http://scitation.aip.org/content/aip/journal/aplmater/1/2?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Thermal annealing effects of SrTiO 3 film on Si(100) AIP Conf. Proc. 1554, 112 (2013); 10.1063/1.4820297 Addendum to “Phase selection and transition in Hf-rich hafnia-titania nanolaminates” (on SiO2) [J. Appl.Phys.109, 123523 (2011)]: Hafnon formation J. Appl. Phys. 111, 109904 (2012); 10.1063/1.4719968 Electrical and material characterizations of HfTiO4 flash memory devices with post-annealing J. Vac. Sci. Technol. A 29, 06B102 (2011); 10.1116/1.3653970 Thermal stability of TiN/HfSiON gate stack structures studied by synchrotron-radiation photoemissionspectroscopy Appl. Phys. Lett. 97, 262903 (2010); 10.1063/1.3532846 Impact of titanium addition on film characteristics of Hf O 2 gate dielectrics deposited by atomic layer deposition J. Appl. Phys. 98, 054104 (2005); 10.1063/1.2030407

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APL MATERIALS 1, 022108 (2013)

Ti-catalyzed HfSiO4 formation in HfTiO4 films on SiO2studied by Z-contrast scanning electron microscopy

Elizabeth Ellen Hoppe, Massiel Cristina Cisneros-Morales,and Carolyn Rubin Aitaa

Department of Chemistry and Biochemistry, University of Wisconsin-Milwaukee,P.O. Box 413, Milwaukee, Wisconsin 53201, USA

(Received 28 February 2013; accepted 24 May 2013; published online 13 August 2013)

Hafnon (HfSiO4) as it is initially formed in a partially demixed film of hafniumtitanate (HfTiO4) on fused SiO2 is studied by atomic number (Z) contrast highresolution scanning electron microscopy, x-ray diffraction, and Raman spectroscopyand microscopy. The results show exsoluted Ti is the catalyst for hafnon formation bya two-step reaction. Ti first reacts with SiO2 to produce a glassy Ti-silicate. Ti is thenreplaced by Hf in the silicate to produce HfSiO4. The results suggest this behavioris prototypical of other Ti-bearing ternary or higher order oxide films on SiO2 whenfilm thermal instability involves Ti exsolution. © 2013 Author(s). All article content,except where otherwise noted, is licensed under a Creative Commons Attribution 3.0Unported License. [http://dx.doi.org/10.1063/1.4818171]

Group IVB (Zr, Hf) titanate films have long been considered as replacement dielectrics forSiO2 in integrated circuits.1, 2 In addition, a recent study shows the potential of HfxTi1-xO4 films astailorable optical band gap materials.3 At the same time, theoretical4 and experimental5 studies areconcerned with the titanate’s structural stability vis a vis phase separation and the potential reactionof the demixed products with the substrate to form an interfacial layer that limits long term deviceperformance. We previously reported that orthorhombic (o) HfTiO4 in sputter deposited films onfused SiO2 substrates demixes upon annealing to form monoclinic (m) HfO2 and rutile (r) TiO2.6, 7

When demixing occurs at high temperature (1273 K), crystalline hafnon (HfSiO4) with a zirconarchetype lattice is formed,8 indicating a reaction between the film and the substrate. This last resultis surprising because HfSiO4 does not form in a HfO2-on-SiO2 film subjected to a similar annealingregimen.8–10 The question of Ti as a catalyst in HfSiO4 formation was therefore raised.

In this study, we use scanning electron microscopy (SEM) with atomic number (Z) contrast toshow that Ti is indeed involved at the earliest stages of hafnon formation in a HfTiO4-on-SiO2 film,i.e., when Hf-O-Si nearest-neighbor bonding is initially observed even before crystalline hafnon isdetected. To this end, a 0.3 μm thick HfTiO4 film was grown on an unheated fused SiO2 substrateby sequential deposition of 5 nm HfO2 and 4 nm TiO2 bilayers (0.51 mole fraction HfO2) frommetal targets by reactive sputter deposition using 0.8 Ar/0.2 O2 discharges.3, 7, 9 Although this filmthickness is much greater than that used for electronic applications, we chose it here for two reasons:(1) it is used for optical applications3 and (2) it provides sufficient material on which to performaccurate SEM analysis. The latter is especially important since we are comparing Si concentrationobtained from the film distinct from the SiO2 substrate. Annealing in air at 1173 K for 192 h wascarried out to produce the desired partially demixed structure, verified by x-ray diffraction (XRD)and Raman spectroscopy.

Double angle XRD was carried out using unresolved Cu Kα radiation (λx-ray = 0.1542 nm).The diffractometer was calibrated using an unstressed Si powder standard with (111)Si at 2�

= 28.42◦. Low resolution data were acquired over the 15◦ ≤ 2� ≤ 70◦ range. High resolution

aElectronic mail: aita@uwm.edu

2166-532X/2013/1(2)/022108/6 © Author(s) 20131, 022108-1

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022108-2 Hoppe, Cisneros-Morales, and Aita APL Mater. 1, 022108 (2013)

18 20 22 24 26 28 30 32 34

Inte

nsity

(ar

b. u

.)

2Θ (deg)

HT H T HS

100 CPS

FIG. 1. High resolution XRD pattern of annealed, partially demixed o-HfTiO4 on SiO2. Vertical lines indicate standard peakpositions of m-HfO2 (H),11 r-TiO2 (T),12 o-HfTiO4 (HT),13 and HfSiO4 (HS).14

data were acquired from the average of 5 scans of the 17◦ ≤ 2� ≤ 34◦ range. Raman shift spectrawere acquired using a confocal (RENISHAW) Raman spectrometer equipped with a 2.5 μm beamdiameter, Ne-He laser (633 nm) excitation source and an optical microscope. The spectrometer’sspectral resolution was 0.5 cm−1. Twenty scans were collected for each spectrum to ensure a goodsignal-to-noise ratio. Optical imaging was carried out in conjunction with collecting Raman spectra.

A Hitachi S-4800 SEM equipped with a Bruker energy dispersive spectrometer was used insecondary electron (SE) mode to examine film topography. Polarized-diffuse backscatter electron(PDBSE) mode was used to study the distribution of chemical elements in specific topographicalfeatures. The sample (i.e., film + substrate) was coated with 2 nm of Ir and secured to a flat Alholder with Cu tape to reduce charging in the electron beam. Energy dispersive spectroscopy (EDS)element composition maps and point scans were acquired in PDBSE mode at an applied voltage of11 keV.

The XRD pattern of the as grown nanolaminate was similar to that reported in Refs. 3,6, and 7,and primarily attributed to nanocrystalline o-HfTiO4. Figure 1 shows the sample’s high resolutionXRD pattern after a 1173 K, 192 h anneal. The dashed lines indicate standard peak positions of therelevant phases. All major peaks attributed to m-HfO2,11 r-TiO2,12 o-HfTiO4,13 and HfSiO2

14 arewithin the 17◦ < 2� < 33◦ range. A broad peak centered at 2� ∼ 21.5◦ is from the substrate. Abare substrate subjected to the same annealing regimen as the film showed no other peaks. The XRDpattern of the partially demixed film shows unreacted o-HfTiO4 and the demixed phases, m-HfO2

and r-TiO2 shifted from their standard positions due to respective Ti, Hf doping at the equilibriumlevel.10 Notably, diffraction from crystalline HfSiO4 is absent.

An optical microscope image of the sample (Fig. 2(a)) shows characteristic round featuresassociated with demixing.6–8 The Raman shifts acquired from a feature and the surrounding matrixare shown in Fig. 2(b). The standard shift positions characteristic of short-range order in m-HfO2,15

r-TiO2,16 o-HfTiO4,17 and HfSiO218 are indicated as bars above the graph. Many bands cannot

be assigned to only one phase, but most can be attributed to the same phases identified by XRD.However, several bands that occur in the feature’s spectrum and are absent from the matrix’s spectrumare unequivocally assigned to HfSiO4.18 In the feature, these bands are at 210, 348, 982, and1018 cm−1 compared to their standard positions (and corresponding phonons) at 215 cm−1 (Eg),350 cm−1 (Eg), 984 cm−1 (A1g), and 1018 cm−1 (B1g), indicated by asterisks in Fig. 2(b). Hf-O-Sishort range order characteristic of hafnon is detected by Raman spectroscopy, whereas the longrange order of crystalline hafnon is not detected by XRD. These results indicate that hafnon hasbeen captured at its inception in the sample.

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022108-3 Hoppe, Cisneros-Morales, and Aita APL Mater. 1, 022108 (2013)

20 µm

(a)

(b)

100 250 400 550 700 850 1000 1150

***

HS

T

H

HT

Raman shift (cm-1)

Inte

nsity

(ar

b.u.

)

F

M

*

FIG. 2. (a) Optical micrograph annealed, partially demixed o-HfTiO4 on SiO2 showing round features with the matrix.(b) Raman shift spectra of feature (F) and matrix (M). Vertical lines indicate standard shifts of m-HfO2 (H),15 r-TiO2 (T),16

o-HfTiO4 (HT),17 and HfSiO4 (HS).18 Asterisks indicate shifts unequivocally from HfSiO4.

Figure 3(a) shows a SEM PDBSE image of a rounded feature surrounded by the matrix. Z-contrast can be observed. The darker gray areas represent regions of lower average atomic numberthan the lighter regions.19 A network of fine cracks likely due to annealing stress is observedthroughout the sample. Figures 3(b)–3(d) are EDS Z maps for Ti, Si, and Hf. Greater brightnessindicates a greater concentration of the detected element. It can be seen that the elemental distributionsare not homogeneous throughout the images. Comparison with Fig. 3(a) shows that regions of highTi concentration coexist with Ti-depleted regions within the feature. Hf concentration is depleted inthe regions of high Ti concentration within the feature. Si concentration increases above the matrixonly in Ti-depleted regions within the feature.

Figure 4(a) is an SE image corresponding to the PDBSE image in Fig. 3(a). Three areas areencircled: (I) from the matrix, and (II) and (III) from the feature. Figures 4(b)–4(d) are high resolutionSE images from each area. Area I (Fig. 4(b)) shows that the matrix is composed of small crystallites,corresponding to the uniform distribution of Ti, Hf, and Si in the maps shown in Fig. 3. Area II(Fig. 4(c)) contains a glassy network (labeled “A”) coexisting with large crystallites (labeled “B”).Area III (Fig. 4(d)) contains a very large crystallite surrounded by a cluster of smaller ones. Acomparison of Figs. 4(c) and 4(d) with the elemental maps shows that within the feature: (1) Ticoncentration is enhanced in the large crystallites. (2) Si and Hf concentrations are depleted in thelarge crystallites. (3) Si concentration is enhanced in the glassy network compared to the matrix.

EDS Z scans taken from points on the encircled areas in Fig. 4(a) yield data about Ti distributionin the different morphological structures shown in Figs. 4(b)–4(d). Most radiative emission energiesfrom Si, Hf, O, and Ti species overlap. However Ti kα1,2 emission at 4.51 keV20 can be unequivocallymonitored, yielding a signal from the large crystallite in area III that is twice as strong as from apoint in the matrix (area I) and three times as strong as a point in the glassy network (area II).In agreement with the elemental maps, these data indicate that Ti concentration is enriched in thecrystallite but depleted in the glassy network compared to the matrix.

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022108-4 Hoppe, Cisneros-Morales, and Aita APL Mater. 1, 022108 (2013)

(b) Ti(a) PDBSE

(c) Si (d) Hf

FIG. 3. (a) PDBSE SEM image of a feature and surrounding matrix. (b)-(d) EDS Z maps for Ti, Si, and Hf, respectively.

FIG. 4. (a) SE SEM image of a feature and surrounding matrix, as shown in Fig. 3(a). High resolution SE images of:(b) area I from matrix; (c) area II from feature showing glassy network (A) and large crystallite (B); (d) area III from featureshowing large crystallite within cluster of smaller crystallites.

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022108-5 Hoppe, Cisneros-Morales, and Aita APL Mater. 1, 022108 (2013)

The above results, combined with the fact that hafnon does not form in HfO2 films on SiO2

subjected to a similar annealing schedule, indicate that hafnon formation here is not simply a physicalmatter of bringing HfO2 and SiO2 constituents together and heating at high temperature. We proposethe following path leads to hafnon formation in HfTiO4 on SiO2. We previously showed10 thatdemixing in o-HfTiO4 initiates by a polymorphic second order phase transition to monoclinic (m)HfTiO4 (or m-Hf0.5Ti0.5O2) after 1 h at 1193 K. m-HfTiO4 is an elevated pressure phase unstableat atmospheric pressure at any temperature.21 In the films, m-HfTiO4 ultimately evolves into one ofthe demixed phases, m-HfO2, by Ti exsolution.10 We suggest here that Ti exsoluted from m-HfTiO4

catalyzes HfSiO4 formation by the following two-step reaction.First, Ti reacts with SiO2 at the substrate,22 and in the presence of O2 from the furnace atmo-

sphere, forms a glassy Ti-silicate denoted “A” in Fig. 4(c). The overall reaction for this step is givenby Eq. (1),

Ti + SiO2 + O2 → TiSiO4. (1)

This Ti-silicate possesses two important qualities: (1) its short-range order is isomorphous withor closely related to hafnon’s short-range order (i.e., based on a zircon archetype),23–27 and(2) it is unstable at 1173 K23, 24 and therefore is present only as a transient phase in the proposedreaction.

The second step to hafnon formation is the replacement of Ti by Hf in the glassy silicate. Thisreplacement is driven by the greater stability of Hf-O-Si bonding compared to Ti-O-Si bondingin an archetypical zircon structure.1, 26–28 The source of Hf for the replacement reaction is one ofthe demixing products, m-HfO2, as confirmed by a comparison of decreased m-HfO2/increasedHfSiO4 XRD peak intensities in the previously reported high temperature study of crystallinehafnon formation.8 Ti, thus exsoluted from the glassy silicate concentrates to form the large, Ti-rich crystallites observed in Figs. 4(c) and 4(d). The formation of these crystallites is concurrentwith a TiO2 peak in the XRD pattern given in Fig. 1. The reaction for this second step is givenby Eq. (2),

HfO2 + TiSiO4 → HfSiO4 + TiO2. (2)

The glassy structure of incipient hafnon is therefore a vestige of the glassy Ti-silicate. Inthe scenario described above, Ti species are “on the move” and therefore able to catalyze hafnonformation. The behavior exhibited by the HfTiO4-on-SiO2 system might indeed be prototypical ofother Ti-bearing ternary or higher order oxide films on SiO2 substrates when film thermal instabilityinvolves Ti exsolution.

Support from a Catalyst/Rockwell Automation Grant through the UWM Research Foundationand a UWM Research Initiation Grant is acknowledged.

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with Ti substitution on a Zr site in zircon, ZrSiO4, a sister material to HfSiO4.

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