Thin Solid Films 520 (2012) 6631–6635

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Electrical and optical properties of nanocrystalline yttrium-doped hafnium oxidethin films

M. Noor-A-Alam, K. Abhilash, C.V. Ramana ⁎Department of Mechanical Engineering, University of Texas at El Paso, El Paso, TX 79968, USA

⁎ Corresponding author. Tel.: +1 915 747 8690; fax:E-mail address: rvchintalapalle@utep.edu (C.V. Ram

0040-6090/$ – see front matter © 2012 Elsevier B.V. Alldoi:10.1016/j.tsf.2012.06.080

a b s t r a c t

a r t i c l e i n f o

Article history:Received 13 October 2011Received in revised form 21 June 2012Accepted 25 June 2012Available online 2 July 2012

Keywords:Y-doped hafniaThin filmsStructureElectrical conductivityBand gap

Yttrium-doped hafnium oxide (YDH) films have been produced by sputter-deposition by varying the growthtemperature (Ts) from room-temperature (RT) to 400 °C. The electrical and optical properties of YDH filmshave been investigated. Structural studies indicate that YDH films grown at Ts=RT−200 °C were amorphousand those grown at 300–400 °C are nanocrystalline. The crystalline YDH films exhibit the high temperaturecubic phase of HfO2. Spectrophotometry analysis indicates that all the YDH films are transparent. The bandgap of YDH films was found to be in the range of 6.20–6.28 eV. Frequency variation of frequency dependentresistivity indicates the hopping conduction mechanism operative in YDH films. While the electrical resistivity(ρac) is ~1 Ω-m at low frequencies (100 Hz), ρac decreases to ~10−4 Ω-cm at higher frequencies (1 MHz).

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Hafnium oxide (HfO2) is a high temperature refractory materialwith excellent physical, electronic and chemical properties [1–10].The outstanding chemical stability, electrical andmechanical properties,high dielectric constant (high-k), and wide band gap of HfO2 make itsuitable 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 dielectric toreplace SiO2 in nano-electronics.

Hafnium oxide exhibits various polymorphs; monoclinic, tetragonaland cubic [9,10]. HfO2 exhibits monoclinic phase under normal condi-tions of temperature and pressure [9,10]. At temperatures higher than1700 °C, HfO2 transforms to tetragonal phase. Transformation to cubicpolymorph having the fluorite structure occurs at 2600 °C [11]. Similarto zirconia (ZrO2), doping a small amount of yttria (Y2O3) stabilizes theHfO2 cubic phase (Fm3m space group) [12,13]. The overall cubicsymmetry of HfO2 is preserved down to room temperature in yttria-doped hafnia (YDH), which belongs to the group of fluorite-structured(CaF2) [11] oxygen superionic conductors. Interest in yttria-dopedZrO2 and HfO2 materials is generated due to a wide variety of theirhigh temperature applications involving solid oxide fuel cells, oxygensensors etc. [12,13]. In addition, YDH is used as thermal barrier coatingsto protect the metallic components such as combustion cans, blades,vanes, etc. of the hot sections of aerospace and land-based gas turbines.Furthermore, high temperature cubic hafnia phase stabilized byY-doping has been shown to exhibit increased dielectric constant

+1 915 747 5019.ana).

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compared to that in monoclinic phase [14–20]. While alloying HfO2

with SiO2 forms amorphous hafnium silicate and prevents the crystalli-zation of pure hafnia, thermodynamic stability of hafnium silicate islimited [20]. Thermodynamic stability of YDH is relatively higher andcan be utilized in those electronic and mechanical applications requir-ing operation at higher temperatures [20]. Furthermore, doping yttriaincreases the oxygen vacancy concentration along with cubic phasestabilization of hafnia. These vacancies in turn increase the ionicconductivity and make YDH useful as a super-ionic conductor, whichmakes YDH promising electrolyte for application in solid oxide fuelcells [11]. Therefore, doped hafnia provides a platform to explore thefundamental science and future technological applications.

The fundamental, microscopic characteristics such as the opticalproperties, electrical conductivity mechanism of YDH films are notwell understood, specifically at the reduced dimensions, compared tothat of best studied YSZ. Fujimori et al. reported the cubic–tetragonalphase change of Y-doped HfO2 solid solution prepared by arc meltingof mixed powders [19]. Dai et al. investigated yttria-doped hafnia(YDH) epitaxial films on silicon where the interfacial reaction anddiffusion were noted [20]. Yang et al. reported an abrupt YDH/GaAs(001) interfaces for epitaxial films of yttrium-doped cubic HfO2

films bymolecular beam epitaxy [21]. In this case, authors have demon-strated the dielectric constant enhancement from ~27 for undopedHfO2 to ~32 for yttrium-doped HfO2. Kita et al. studied Y-doped HfO2

films with variable yttrium concentration. The films were grown byco-sputter deposition employing Y and Hf targets [17]. Rauwel et al.observed that the optimal concentration of ~6% to 10% yttrium yieldsamaximumdielectric constant (~22) in the YDH films grown by organ-ometallic chemical vapor deposition [22]. Majumder et al. reported thestructural phase transformations in Y-doped HfO2 films grown by

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atomic layer deposition (ALD) [23]. Post-deposition annealing at 600 °Cor higher was claimed to obtain high dielectric cubic structural phase ofHfO2. However, while most of the earlier efforts are directed towardsthe dielectric constant, the ultra-microstructure and electronic propertyrelationships in YDH thin films further need to be explored. It is, there-fore, of interest to investigate the correlation between microstructure,electrical and optical properties in YDH films as a function of growthconditions which is the impetus for the present work. The effect ofgrowth temperature on the microstructure, electrical and optical prop-erties of YDH films made by radio-frequency magnetron sputtering isinvestigated in detail and the results are presented and discussed inthis paper.

2. Experimental details

Yttrium-doped hafnium oxide (YDH) thin films were deposited ontosilicon (Si) (100) wafers and optical grade quartz substrates by radio-frequency magnetron sputtering. All the substrates were thoroughlycleaned and dried with nitrogen before introducing them into the vac-uum chamber, which was initially evacuated to a base pressure of~0.21 mPa. YDH target (5 cm diameter, 0.32 cm thick) (PlasmaterialsInc.) was used for sputtering. The content of yttria in YDH wasmaintained at 7.5 mol% (7.5 YDH). The YDH target was placed on asputter gun, which was correspondingly placed at a distance of 8 cmfrom the substrate. Sputtering was performed at a pressure of0.5 mPa. A sputtering power of 30 Wwas initially applied to the targetwhile introducing high purity argon (Ar) into the chamber causing plas-ma ignition. Once ignited, the power was increased to 100 W to depositthe films. The Ar gas flow (50 sccm) was controlled using MKS massflow meter. Before each deposition, the YDH-target was pre-sputteredfor 10 min using Ar keeping the shutter above the gun closed. Deposi-tion was made to obtain a ~90 nm thick film. The deposition rate was~3 nm/min. The samples were deposited at different temperatures(Ts) varying from room temperature (RT) to 400 °C. The substrateswere heated by halogen lamps and the desired temperature was con-trolled by Athena X25 controller. The film fabrication conditions aresummarized in Table 1.

X-ray diffraction (XRD) measurements were performed using aBruker D8 Advance X-ray diffractometer. All the measurements weremade ex-situ as a function of Ts. XRD patterns were recorded using CuKα radiation (λ=1.54056 Ǻ) at RT. Surface imaging analysis wasperformed using a Hitachi S-4800 high-performance and ultra high res-olution scanning electron microscope (SEM). The SEM operating volt-age is 15 kV. The secondary electron imaging was performed on YDHfilms grown on Si wafers using carbon paste at the ends to avoid charg-ing problems. The grain detection, size-analysis and statistical analysiswere performed using the software provided with the SEM. The opticalproperties of YDH films were evaluated using spectrophotometric opti-cal transmission and reflectance measurements using Cary 5000 UV–VIS-NIR double-beam spectrophotometer. Films grown on opticalgrade quartz substrates were employed for optical property measure-ments. There were no significant structural differences as compared tofilms grown on Si substrates as verified both in XRD and SEM analyses.

Table 1YDH film fabrication conditions.

Deposition parameter Set value

Base pressure ~0.21 mPaSputtering pressure ~0.50 PaAr gas flow 50 sccmTarget YDH (5 cm×0.32 cm)Substrate Si, quartzSubstrate temperature RT−400 °CTarget substrate distance 8 cmRF power 100 WFilm thickness ~90 nm

Electrical conductivitymeasurementswere carried out employing a LCRmeter (HP 4192A).

3. Results and discussion

The XRD patterns of YDH films are shown in Fig. 1 as a function of Ts.The XRD curves of YDH films exhibit the peaks corresponding to cubicstructure of HfO2. It is evident (Fig. 1) that the peak at ~30.28°, whichcorresponds to diffraction from (111) planes, is rather broad for filmsgrown at Ts=RT−200 °C indicating the presence of nanocrystallineparticles embedded in an amorphous matrix. The intensity and thewidth of the (111) peak increase with increasing Ts. This is indicativeof an increase in the average crystallite-size and preferred orientationof the film along (111). Monoclinic and tetragonal phases consist ofseveral shorter Hf\O bond lengths (2.0–2.1 Å) compared to cubicHfO2 (2.37 Å) [24]. As a consequence, the strain energy because of thesizemismatch becomes significant inmonoclinic and tetragonal phases.On the other hand, the distortion is quite less in cubic phase with theoversized Y3+ doping, whichmakes cubic HfO2 stable even at low tem-perature. Furthermore the structural relaxation plays another role inthe stabilization of cubic phase of HfO2 by Y2O3 dopant [24]. Our resultsare in good agreement with those reported by Lee et al. [24]. Theaverage crystallite size calculated is approximately 20 and 25 nm forthe crystalline, cubic YDH films grown at 300 and 400 °C, respectively.

SEM images of nanocrystalline, cubic YDH films are shown in Fig. 2.The fine microstructure and uniform distribution characteristics of theparticles are evident in themicrographs (Fig. 2). The SEM images repre-sent the characteristic triangular shapedmorphology of nanocrystallineYDHfilms. Based on the observations of XRD, SEMand data analysis, theeffect of temperature on the microstructure evolution of YDH films canbe explained as follows. If temperature is low such that the period of theatomic jump process of adatoms on the substrate surface is very large,the condensed species may stay stuck to the regions where they are

Fig. 1. XRD patterns of YDH films grown at various substrate temperatures. YDH filmsgrown at various substrate temperatures exhibit the cubic phase as indexed. The PDF ofcubic hafnia is presented for comparing the data of YDH films obtained in the presentwork.

Fig. 2. SEM images of nanocrystalline, cubic YDH films grown at 300 °C (a) and 400 °C(b). The characteristic triangular morphology of the films is evident.

Fig. 3. Room temperature AC resistivity values of the YDH films at 100 Hz. A line isdrawn to guide the eye.

Fig. 4. Frequency dependent resistivity of YDH films. The dispersion relation is shownwith increasing frequency.

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landing thus leading to an amorphous YDH films. The adatommobilityon the surface increases with increasing Ts [25]. The XRD and SEM dataanalyses indicate that 300 °C is the critical temperature to promote thegrowth of nanocrystalline, cubic YDH films. This temperature is lowerthan that reported for crystalline, cubic YSH films obtained by co-sputtering of Y and Hf metals [17]. Kita et al. [17] have reported thatpost-deposition annealing at higher temperatures is needed to obtaincrystalline, cubic YDH films. Similarly, post-deposition annealing at600 °C or higher is required for realizing the high dielectric cubicphase hafnia films when grown by atomic layer deposition (ALD) [23].

The variation of electrical resistivity, measured at room tempera-ture, with Ts is shown in Fig. 3. The frequency dependence of the electri-cal resistivity (ρac) of YSH films is shown in Fig. 4. The total conductivityof the films can be expressed by the relationship [26,27]:

σ tot ¼ σ0 Tð Þ þ σ ω; Tð Þ ð1Þ

The first term at the right hand side of Eq. (1) is the dc conductivitydue to the band conduction, which is frequency independent. On theother hand, the second term of the equation is purely a representationof the ac conductivity due to the electron hopping between the Hf4+

and (Y3+) ions. The hopping mechanism in HfO2 films is attributed tothe presence of oxygen vacancies. The vacancy level forms below theconduction band and it can also trap one or two electrons [28]. Atrapped electron causes the adjacent Hf ions to distort asymmetrically,pulling down a singly degenerate B1 state from the conduction band[28]. This state can be occupied by 1 or 2 electrons [28]. Resistivitydecrease (by three orders of magnitude) with increasing frequencyfrom 100 Hz to 1 MHz is attributed due to the hopping mechanism inYDH films. Whereas, while ρac~1 Ω-m at low frequencies (100 Hz), itdecreases to ~10−4 Ω-cm at higher frequencies (1 MHz).

The observed dispersion in the resistivity is due to the contributionfrom the Hf, Y and O ions to the relaxation. At low frequency regime,hopping of electrons between the localizedHf or Y ions increases subse-quently the resistivity decreases. At high frequencies, the hopping ofelectron could not follow the applied field and hence becomes almostconstant (Fig. 4). The frequency variation of electrical resistivity datawas fit to the following equation [27]:

ρ ¼ ρ0−ρ∞ð Þ1þ ωtð Þ2 1−αð Þ� � ð2Þ

where ρ∞ is the resistivity value at 1 MHz, ρ0 is the resistivity value at20 Hz, τ is the mean relaxation time (4.1–2.9 μs for Ts=RT−400 °C)and α is the spreading factor about the mean relaxation time (0.56–0.64 for Ts=RT−400 °C). The calculated mean relaxation time valuesare found to decrease from 4.1 μs to 2.9 μs with increasing Ts is due tothe increase in resistivity.

The optical transmission spectra of YDH films grown at various Tsare shown in Fig. 5. The spectra of all the YDH films (Fig. 5) exhibitvery high transparency in the spectral region except where the inci-dent photons are absorbed across the band gap (Eg). This observationindicates the high-quality and transparent nature of YDH films withalmost zero absorption losses. Wood et al. investigated the opticalproperties of cubic hafnia stabilized with yttria [29]. They reportedthat the cubic YDH single crystals exhibit transparence in a widerange, from ultraviolet to mid-infrared range. The optical absorption

Fig. 5. Optical transmission curves of YDH films. The curves indicate that YDH films arehighly transparent.

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coefficient, α, of the YDH films is evaluated using the standard proce-dure taking the film thickness into account [30,31]. To determine theband gap (Eg), absorption data analysis is performed employing thepower law [30–33]:

αhvð Þ ¼ B hv−Eg� �n ð3Þ

where, hν is the energy of the incident photon, B the absorption edgewidth parameter, Eg the band gap, and n the exponent. The absorptiondata and the plots obtained for YDH films are shown in Fig. 6. It isevident that (αhν)2 vs. hν results in linear plots suggesting directallowed transitions across Eg of YDH films. Eg values were calculatedby linear regression fit to the data at high absorption (linear) region(inset, Fig. 6) to zero absorption. Band gap of YDH films slightlyincreases from 6.20±0.03 to 6.28±0.03 eV with increasing Ts fromRT to 400 °C. Several groups have reported theoretical investigationsof phase stabilization by divalent doping (Gd, Dy, Si, Sc, Ge, Sn, Ti, P)and the band structure calculation in cubic HfO2 [17,33–35]. Doping ofSn, Ge or P shows defect levels within the energy gap [33] whereasthe Y-doping has not shown any defect level because valence electronsof the Y atom are close in energy to that of the Hf atom [33]. In addition,theoretical and experimental [34] investigations indicate that the opti-cal band gap for cubic HfO2 is a direct band gap and the value lies around5.53 to 6.1 eV [17,33–35] depending on the preparation method. In the

Fig. 6. (αhυ)1/2 versus energy plots for YDH films as a function of Ts. Inset shows thelinear regression fit to the high absorption region to obtain the band gap value asindicated.

present case, the observed Eg value is found to be higher than that of thereported value. It must be emphasized that the Eg value is sensitive tothe microstructure; it is influenced by various factors such as surface/interface structure, grain size, crystal quality, lattice parameters, latticestrain, carrier concentrations, defect structure, and chemical composi-tion. XRD measurements clearly indicate that the lattice constantincreased due to the Y doping.We, therefore, believe that increased lat-tice constant and the subsequent strain developed in YDH films causethe Eg enhancement compared to that of pure, cubic HfO2. It has beenreported [36,37] that the band gap (Eg) enhancement in nanostruc-tured materials is due to two factors, quantum confinement and strain.As a result, although the approximation was proposed for sphericalnanocrystals, the variation in band gap energy can be written as:

Eg ¼ Eg bulkð Þ þ ΔEg Ið Þ þ ΔEg IIð Þ ð4Þ

The ΔEg (I) and ΔEg (II) terms account for the quantum and straininduced effects, respectively. Clearly, the Eg enhancement in the pre-sent case is not due to ΔEg (I) term since the band gap is essentiallythe same for the amorphous and crystalline YDH films. Since theother experimental conditions are similar, Eg enhancement in generalcompared to cubic hafnia must be associated with the lattice param-eter expansion, which leads to the strain in the films. In other words,the second term (ΔEg (II)) of the above equation is at play to inducethe observed changes.

4. Conclusions

Nanocrystalline YDH films have been produced by sputter-deposition under controlled temperature and pressure and theirstructural, electrical and optical properties have been evaluated.Structural investigations indicate that YDH films crystallize in cubicphase, which is due to yttria stabilizer. The intensity and the widthof the peak at 30.28° which corresponds to diffraction from (111)planes are seen to increase with increasing substrate temperature(Ts). The SEM images of YDH films indicate the fine microstructureand uniform distribution of dense particles. Thermally-driven growthprocess of YDH films is evident from the XRD and SEM results. Theoptical absorption measurements indicate that the Eg of YDH filmsis in the range of 6.20–6.28 eV. No evidence of quantum confinementeffects is seen in YDH films. Frequency variation of AC resistivity isseen to exhibit dispersion which reveals the hopping conductionmechanism.

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