Growth Behavior, Lattice Expansion, Strain, and Surface Morphology

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Growth Behavior, Lattice Expansion, Strain, and Surface Morphology of Nanocrystalline, Monoclinic HfO2 Thin Films C. V. Ramana,*,† K. Kamala Bharathi,† A. Garcia,† and A. L. Campbell‡ †

Department of Mechanical Engineering, University of Texas at El Paso, El Paso, Texas 79968, United States Materials and Manufacturing Directorate (RX), Wright-Patterson Air Force Base (WPAFB), WPAFB, Ohio 45433, United States

‡

ABSTRACT: Nanocrystalline HfO2 films have been produced by sputter deposition under varying growth temperatures (Ts). The effect of Ts, which is varied from room temperature (RT) to 500 °C, on the structural characteristics of HfO2 films has been investigated employing X-ray diffraction (XRD), scanning electron microscopy (SEM), and atomic force microscopy (AFM). The results indicate that the monoclinic HfO2 nanocrystals are highly oriented along the (1̅11) direction with increasing Ts. The lattice expansion increases with a reduction in grain size (L), while minimum strain energy occurs at a maximum lattice expansion. The strain factor increases linearly from 2.4 to 4% with increasing Ts from RT to 500 °C. The corresponding surface roughness also increases linearly with Ts. L values increase from ∼10 to 50 nm with increasing Ts. The L−Ts data derived from XRD and SEM are in good agreement and exhibit a relationship, which follows an Arrhenius' relation. A thermally activated growth process with an activation energy of ∼0.12 eV is evident from the structural analysis of nanocrystalline HfO2 films. conditions, and postfabrication processes.1−23 From the viewpoint, the ability to tailor the properties so as to optimize performance requires a detailed understanding of the geometric structure, particularly at the nanoscale dimensions, of HfO2. Furthermore, stabilizing the specific phases with the desired crystallite size, strain, and distribution of grain size characteristics in a controlled way requires detailed understanding of the structural characteristics of nanocrystalline HfO2 films. The present work was, therefore, performed to determine the effect of growth temperature on the structural characteristics of HfO2 nanocrystalline films made by reactive sputter deposition. The results obtained are presented and discussed to derive an interesting size-strain-phase-morphology correlation, which provides a roadmap to tailor the properties and performance of nanocrystalline HfO2 films.

I. INTRODUCTION Hafnium oxide (HfO2) is a high-temperature refractory material with excellent physical and chemical properties, which makes them promising for a wide variety of technological applications.1−13 The outstanding chemical stability, electrical and mechanical properties, high dielectric constant, and wide band gap of HfO2 make it suitable for several industrial applications in the field of electronics, magneto-electronics, structural ceramics, and optoelectronics.1−6 HfO2 has been identified as one of the most promising materials for the nanoelectronics industry to replace SiO2 because it has a high dielectric constant and is expected to be stable in contact with Si.1−14 HfO2 exhibits various polymorphs.15−17 One stable monoclinic phase and four metastable phases, cubic, tetragonal, orthorhombic I, and orthorhombic II, have been identified for HfO2.16,17 The stable structure of HfO2 is monoclinic under normal conditions of temperature and pressure.14,15 It transforms into the tetragonal form when heated to temperatures higher than 1700 °C.14,15 Further transformation into the cubic polymorphic form having the fluorite structure takes place at 2700 °C.14,15 The high density (∼10 g/cm3) makes this compound attractive as a host cell, when activated with rare earths (Eu3+), for applications as scintillating materials and waveguides amplifiers.18 The controlled growth and manipulation of specific crystal structures at the nanoscale dimensions have important implications for the design and applications of HfO2.19,20 However, it is well-known that the optical, electrical, and electro-optic properties of HfO2 are highly dependent on the surface/interface structure, morphology, and chemistry, which in turn controlled by the fabrication technique, growth © 2012 American Chemical Society

II. EXPERIMENTS A. Fabrication. Nanocrystalline HfO2 films were deposited onto silicon (Si) (100) wafers by radio frequency magnetron sputtering. The Si(100) substrates were cleaned by a standard procedure outlined elsewhere.24,25 All of the substrates were thoroughly cleaned and dried with nitrogen before introducing them into the vacuum chamber, which was initially evacuated to a base pressure of ∼10−6 Torr. A hafnium (Hf) metal target (Plasmaterials Inc.) of 2 in. diameter and 99.95% purity was employed for reactive sputtering. The Hf target was placed on a 2 in. sputter gun, which was placed at a distance of 8 cm from the substrate. Argon (Ar) was used as a sputter gas, while Received: November 17, 2011 Revised: March 20, 2012 Published: April 26, 2012 9955

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oxygen (O2) was allowed during deposition for reactive growth to form Hf-oxide. The flow of the Ar and O2 and their ratio (70:30) were controlled using as MKS mass flow meters. Before each deposition, the Hf target was presputtered for 10 min using Ar alone with shutter above the gun closed. The deposition was carried out for 40 min with the sputtering power of 100 W. The deposition was carried out to obtain ∼90 nm thick films. The samples were deposited at different temperatures (Ts) varying from room temperature (RT) to 500 °C. The substrates were heated by halogen lamps, and the desired temperature was controlled by Athena X25 controller. B. Characterization. The grown HfO2 films were characterized by performing crystal structure, morphology, and strain analysis measurements. X-ray diffraction (XRD) measurements on HfO2 films were performed by using a Bruker D8 Advance X-ray diffractometer. All of the measurements were made ex situ as a function of growth temperature. XRD patterns were recorded using Cu Kα radiation (λ = 1.54056 Ǻ ) at RT. In addition to that, high-resolution scans of selected individual diffraction peaks were obtained. The high-resolution data of selected peaks were obtained with the step size of 0.001 degree per 1 s. The coherently diffracting domain size (Dhkl) was calculated from the integral width of the diffraction lines using the well-known Scherrer's equation after background subtraction and correction for instrumental broadening. The Scherrer equation26,27 is Dhkl = 0.9λ /β cos θ

Figure 1. XRD patterns of HfO2 films grown at various temperatures.

(1)

where Dhkl is the size, λ is the wavelength of the filament used in the XRD machine, β is the width of a peak at half of its intensity, and θ is the angle of the peak. Bragg's law was used to calculate the interplanar spacing, d(hkl), from 2Θ(hkl). Surface imaging and cross-sectional analysis were performed using a high-performance and ultra high-resolution scanning electron microscope (Hitachi S-4800). The secondary electron imaging was performed on HfO2 films grown on Si wafers using carbon paste at the ends to avoid charging problems. The grain detection, size analysis, and statistical analysis were performed using the software provided with the SEM. Surface imaging was performed also using an atomic force microscope (AFM) (Nanoscope IV-Dimension 3100 SPM system).

Figure 2. High-resolution XRD scans of the monoclinic (1̅11) peaks of HfO2 films. A shift in the peak position to a higher diffraction angle with increasing Ts is evident from the curves.

expansion, which is dominant with decreasing grain size (L). The full width at half-maximum (fwhm) derived from the highresolution scans of the (1̅11) peak as a function of Ts is shown in Figure 3. It is evident that the fwhm decreases with increasing Ts, which accounts for the increased L values with increasing Ts. L values determined using the measured fwhm and Scherer's equation are in the range of 5−45 nm, where particles occasionally seen with a size of 5 nm for films grown at RT are included, for a variation of Ts from RT to 500 °C.

III. RESULTS The XRD patterns of HfO2 films are shown in Figure 1. The patterns show the crystalline nature of the film and peaks corresponding to monoclinic structure (Figure 1). The observed peaks are as indexed in Figure 1. The peak at 28.1° that corresponds to the (11̅ 1) orientation is seen to be very broad for HfO2 films grown at lower temperature (RT−200 °C), indicating the presence of small crystallites, which may be embedded in an amorphous matrix. It is evident from the XRD plots that the peak intensity at 28.1° that corresponds to diffraction from (1̅11) planes increases with increasing Ts. This is indicative of an increase in the average grain size (L) and preferred orientation of the film along (1̅11). HfO2 films grown at Ts = 500 °C exhibit the well-oriented [along the (11̅ 1) direction] structure of the samples. The detailed, highresolution scans of the (1̅11) peak of HfO2 films are measured to obtain a further information on the growth process and crystal structure at the nanoscale dimensions. High-resolution scans of the (1̅11) peak is shown in Figure 2. A shift to the higher angle with increasing Ts can be noted from Figure 2. The shift in the peak position with Ts is attributed to the lattice

Figure 3. Variation of fwhm of the (1̅11) peak with Ts for HfO2 films. 9956

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The high-resolution SEM images of HfO2 films are shown in Figure 5. The effect of Ts on the surface morphology of HfO2

It should be noted that the XRD line broadening can occur due to small grain size as well as due to microstrain in thin films. It is, therefore, important to separate the contribution due to size and strain to obtain the reliable information about the grain size. However, the grain size and microstrain effects are interconnected in the line broadening of XRD peaks, which makes it difficult to separate the contributions. The size and the strain effects can be separated by the Williamson−Hall method,28 where the grain diameter and microstrain are related by: ⎛ sin θ ⎞ β cos θ 1 ⎟ = + ε⎜ ⎝ λ ⎠ λ D

(2)

where D is the grain size, λ is the wavelength of X-rays, β is the fwhm of the diffraction peak located at 2θ. In this method, a plot “β cos θ” versus “sin θ” fits to a straight line with the inverse of the intercept of the line on β cos θ axis gives crystalline grain size. The Williamson−Hall plots (not shown) for HfO2 films were made using the XRD data shown in Figure 1 to obtain the grain size values as a function of Ts. The values determined from such a procedure were in the range of 12−50 nm, which are found to be slightly higher than those determined from Scherrer's equation. The variation of d spacing for (11̅ 1) planes with Ts is considered to obtain quantitative information on the lattice expansion in nanocrystalline HfO2 films. The d spacing (d(11̅ 1)) versus Ts for HfO2 films is shown in Figure 4. Important to

Figure 5. High-resolution SEM images of HfO2 films as a function of Ts .

films is remarkable (Figure 5). The SEM images of HfO2 films grown at Ts = 200−500 °C show the fine microstructure and uniform distribution of dense particles spherical in shape. An increase in Ts results in changes in the crystal structure and morphology. The observed average grain sizes from SEM image are found to agree well with the calculated grain size using XRD data. The cross-sectional SEM image obtained on the HfO2−Si interface for HfO2 films grown at 500 °C is shown in Figure 6.

Figure 4. Variation of d(1̅11) spacing and lattice mismatch [Γ(%)] with Ts for HfO2 films. A direct, inverse linear relationship between lattice expansion and lattice strain as a function of Ts is evident.

recognize is that d(1̅11) decreases with increasing Ts. For a direct comparison, the strain factor (Γ), which is derived from the lattice mismatch, is also shown in Figure 4. The lattice strain factor in the HfO2 nanocrystalline films is calculated using the formula:4,29 Γ=

d(100)Si − d( 1̅ 11)sample d( 1̅ 11)sample

Figure 6. Cross-sectional SEM image obtained on the HfO2−Si interface. The HfO2, Si, and HfO2−Si interfacial regions are as indicated.

The regions of oxide film and Si substrate are as indicated. A sharp interface, within the limits of the resolution of instrument, is seen between the Si substrate and the oxide layer for HfO2 films grown at lower temperatures. However, formation of an interfacial layer at the Si−HfO2 interface is noted for HfO2 films grown at higher temperature (500 °C). The most remarkable feature of the HfO2 film growth is the oriented columnar structure on Si substrate (Figure 6). Perhaps, this could be the intrinsic behavior of HfO2 films to

% (3)

It can be seen that Γ (%) increases linearly with increasing Ts. The most important feature that can be noticed from Figure 4 is a direct, inverse relationship between lattice expansion and strain as a function of Ts. We noticed that the lattice expansion is larger in the smaller crystallites where the strain factor is at a minimum. 9957

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Figure 7. AFM images of nanocrystalline monoclininc HfO2 films. Increasing grain size and surface roughness with an increase in growth temperature is evident.

Figure 8. L−Ts relationship in HfO2 films. (a) The data derived from XRD (dark circles; Scherer's equation) and SEM (open circle) are shown. (b) Arrhenius plot of L−Ts data obtained from XRD (Williamson−Hall method). (c) Arrhenius plot of L−Ts data obtained from Williamson−Hall method.

mean-square (rms) roughness of HfO2 films increased from 1 to 3 nm with increasing Ts.

grow in this columnar structure. When this observation is coupled with XRD measurements, it seems that the columnar structure is in such a way so that the crystal grows in the (1̅11) direction with those crystal planes laying parallel to the Si(100) substrate.30 The AFM images of HfO2 films are shown in Figure 7. The representative images shown are for HfO2 films grown at 300 and 500 °C. The AFM analysis also indicates that the grain size increases with increasing Ts. A slight increase in surface roughness also is noted as a function of Ts. The root-

IV. DISCUSSION The observed growth behavior, crystal structure, grain size variation, lattice expansion, and surface morphology evolution in nanocrystalline HfO2 as a function of Ts can be explained as follows. The growth temperature, which is an important thermodynamic parameter, plays an important role, besides the 9958

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in Figure 8c,d. The data indeed fit to a linear relation. The activation energy for nanocrystalline, monoclinic HfO2 films determined from the slope of the linear plot is 0.12 eV. A direct comparison of this value with any other reports is not possible at this time since the data or such analysis is not available for nanocrystalline HfO2 films grown by sputtering or any other physical/chemical vapor deposition methods. However, this value (0.12 eV) is rather small but expected for nanocrystalline films with the grain sizes obtained, when compared to the data of some of the microcrystalline metal-oxide thin films.31−34 The activation energy obtained for nanocrystalline, monoclinic HfO2 films in the present work is, however, comparable to that reported for nanocrystalline, tetragonal WO3 films.31 Having established a functional, quantitative relationship between the growth temperature, surface morphology and grain size, the focus now is the crystal structure of nanocrystalline HfO2 films. XRD data indicate that the HfO2 films crystallize in monoclinic structure, which is expected based on the phasestability considerations.14,15 Stabilization or quenching of any of the high-temperature phases is not observed in nanocrystalline HfO2 films. However, the important observation that comes directly from XRD data (Figure 1) is the oriented growth of HfO2 films. The oriented growth of nanocrystalline HfO2 films along the (1̅11) direction is clearly seen in the XRD data. Furthermore, the degree of orientation increases with increasing Ts. As reported by Mukhopadhyay et al.,35 in monolcininc HfO2, the (1̅11) planes exhibit the lowest surface energy. Therefore, growth of monolcininc HfO2 films with crystallites in preferred (1̅11) texturing is thermodynamically preferred. Recently, Cisneros-Morales and Aita have grown nanocrystalline monoclinic HfO2 films by sputter deposition onto fused silica substrates and showed the dominance of (1̅11) orientation for crystal growth.4 A sequential annealing followed by furnace cooling was performed on sputtered HfO2 films of ∼1.7 μm to affect the crystalline growth and crystallite size ranging ∼5−45 nm. However, in this work, 90 nm thick nanocrystalline monoclinic HfO2 films were grown on Si(100) substrates by varying the substrate temperature resulting in crystallite sizes more or less on the same order. We, therefore, believe that a direct comparison of our results with those reported for sputter-grown annealed nanocrystalline monoclinic HfO2 films4 can be made possible. Coupled with the (11̅ 1)-oriented growth of HfO2, lattice expansion is also noted with the grain size reduction. These results can be understood as follows. Anisotropy exists in crystalline materials, and the strain energy densities will typically be different for different crystallographic directions. As shown by Ramana et al. for either sputter-deposited or pulsed-laser deposited oxides based on W−, V−, Mo−, Ni−Co−, and most recently for Ti−W, the growth will favor those orientations with low strain energy density.25,29,32−35 Therefore, increasing Ts favors the preferred orientation along (1̅11) while minimizing the strain energy in the HfO2 film. This characteristic behavior is very similar to results reported for sputter-grown, air-annealed (1̅11) HfO2 films.4 For monoclicninc nanocrystalline HfO2 films, CisnerosMorales and Aita have argued that the lattice expansion in ionic solids occurs either due to the reduction in cation charge state or repulsion of strong surface dipoles leading to a reduction in surface tension in small crystallites.4 They proposed that the surface dipole repulsion is responsible for the observed lattice expansion.4 Our results on the lattice expansion and strain in nanocrystalline, monoclinic HfO2 films with comparable grain size agree with the results reported by Cisneros-Morales and

reactive oxygen partial pressure, in deciding the microstructure as well as properties of materials resulting from vapor-transport deposition. If Ts is low so that the period of the atomic jump process of adatoms on the substrate surface is very large, then the condensed species may stay stuck to the regions where they are landing, thus leading to an amorphous or amorphous medium with embedded small but ordered nanoparticles.24,25,29 From the experimental results, it is very clear that for HfO2 films deposited at RT, the impinging flux may be just sticking to the Si(100) surface at its place of hitting with almost no surface diffusion since no well-defined long-range order or oriented growth is observed. However, with an increase in Ts, the adatom mobility on the Si surface increases. The small size grains, spherical in shape, observed in SEM images coupled with XRD data for films grown at 200 °C indicate that the situation is entirely different as compared to films grown at Ts = RT−100 °C. The presence of well-resolved peaks indicates the improved structural order resulting in the formation of nanocrystalline HfO2 films. This is due to increased diffusion of adatoms leading to a larger rate of atoms joining together and, hence, formation of nanocrystalline films as a result of an increase in Ts. The morphological changes, increase in grain size, and their distribution characteristics, as a result of further increase in Ts, can be attributed to the enhanced mobility of sputtered species of Hf impinging on the Si(100) surface. Similar to diffusion coefficient, the grain size (L) is typically observed to depend on temperature as:29−31 L = Lo exp( −ΔE /kBT )

(4)

where ΔE is the activation energy, kB is the Boltzmann constant, T is the absolute temperature (T = Ts + 273 K), and Lo is a pre-exponential factor that depends on the physical properties of the substrate deposit. At lower temperatures, the impinging species may not have sufficient energy for atomic jump process or to overcome the potential energy of the nucleation sites of the substrate.29 At higher Ts, the mobility of adatoms on the Si(100) substrate surface is generally higher. As a result, the diffusion distance of the adatom on the surface increases, and the collision process initiates the nucleation for more adatoms joined together resulting in increased L values.25,29 If we assume that the grain size is directly related to the surface diffusion of sputter-deposited species on the substrate surface, one would expect to see the increase in L with Ts in accordance with the above relation. Therefore, the L values determined from XRD and SEM measurements and the analyses are presented in Figure 8. The experimental data obtained and fitting to the first order exponential growth function are shown. The grain size values determined from XRD using Scherer's equation and SEM are compared in Figure 8a. The Arrhenius plot of XRD data is presented in Figure 8b. Similarly, the grain size values determined from Williamson− Hall method and the corresponding Arrhenius plot are presented in Figure 8c,d, respectively. Two important observations that can be noted from Figure 8 are as follows. A very good agreement between the measured L values from XRD and SEM data is the first. All of the data fit to an exponential function (Figure 8b,d) is the later. This latter feature indicates the thermally activated process of surface diffusion of the impinging flux, which accounts for the evolution of HfO2 film surface morphology and grain size as a function of Ts as seen in XRD, SEM, and AFM data. For thermal-activated process of sputter-grown nanocrystalline HfO2 films, the Arrhenius plots for the XRD data are shown 9959

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Aita although the growth conditions, substrates, and film thickness are different. A correlation between lattice expansion and strain factor can be noted from Figure 3. It is very clear that the lattice expansion is larger in the smaller crystallites where the strain factor is at a minimum. While a more detailed account of MO2 family oxides at the reduced dimensionality and implications for their property and performance tuning calls upon further investigations, the results presented in this work coupled with those reported in reference 4 suggests that the “phase and size” (i.e., monoclinic and comparable grain size) are the unique attributes providing the similar results on the ultramicrostructural properties and observed phenomena in HfO2.

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V. CONCLUSIONS Nanocrystalline HfO2 films were grown by sputter deposition and their structural characteristics were evaluated. Nanocrystalline, monoclinic HfO2 films exhibit highly oriented growth along the (1̅11) direction with increasing Ts. A correlation between L, d(1̅11), and Γ is noted. The lattice expansion (d(1̅11)) decreases with increasing L, while the corresponding strain (Γ) increases with L. The lattice expansion is in nanocrystalline HfO2 films due to a size reduction. The L−Ts data derived from XRD and SEM are in good agreement and exhibit a relationship, which follows an Arrhenius' relation. The results indicate that the nanocrystalline, monoclinic HfO2 film growth is by a thermally activated growth process with activation energy of 0.12 eV. The results could be useful to optimize the conditions while considering HfO2 films in their potential technological applications.

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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

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