Sputtered multicomponent amorphous dielectrics for transparent electronics

Phys. Status Solidi A 206, No. 9, 2149–2154 (2009) / DOI 10.1002/pssa.200881799 p s sapplications and materials science

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Sputtered multicomponent amorphousdielectrics for transparent electronicsLuıs Pereira*,1, Pedro Barquinha1, Goncalo Goncalves1, Anna Vila2, Antonis Olziersky2, Joan Morante2,Elvira Fortunato1, and Rodrigo Martins1

1 Materials Science Department, CENIMAT/I3N and CEMOP/UNINOVA, Faculdade de Ciencias e Tecnologia, Universidade Nova deLisboa, Campus de Caparica, 2829-516 Caparica, Portugal

2 Department of Electronics, University of Barcelona, Martı I Franques 1, 08028 Barcelona, Spain

Received 19 February 2009, revised 30 March 2009, accepted 1 April 2009Published online 13 August 2009

PACS 77.55.+f, 81.15.Cd

∗ Corresponding author: e-mail [email protected], Phone: +351 212 948 525, Fax: +351 212 941 365

In this work, we present the structural and electrical propertiesof HfO2, HfO2 + SiO2, and HfO2 + Al2O3 dielectric compositelayers deposited by sputtering without any intentional substrateheating. The films were deposited on glass and 〈100〉 crystallinesilicon (c-Si) substrates from ceramic targets by using argon(Ar) and oxygen (O2) as sputtering and reactive gases,respectively. The incorporation of SiO2 and Al2O3 into hafniawas obtained by co-sputtering and it was controlled by adjustingthe ratio of r.f. power applied between the targets. The HfO2

films present a microcrystalline structure, when deposited atroom temperature (RT). The lowest leakage current in c-Si MIS

(Metal-Insulator-Semiconductor) structures (below 109 A/cm2

at 10 V on films with a thickness around 180 nm) was obtainedfor an Ar/O2 ratio of 14:1 sccm, and further increase in O2

flow does not enhance the electrical characteristics. The co-deposition of SiO2 or Al2O3 with hafnia has a strong influenceon the structure of the resulting films since they becomeamorphous. The leakage current in MIS structures incorporatingthese multi-component dielectrics is reduced at least by a factorof 2, which is accompanied by an increase on the band gap. Thedielectric constant is decreased due to the lower values for SiO2

and Al2O3.

© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction The definition for transparent elec-tronics is directly related with the devices based on wideband gap oxide semiconductors. It has been boosted by thetremendous development achieved on thin film transistors(TFTs) by integrating such semiconductor oxide as activelayer. Materials such as zinc oxide, indium oxide, and binaryor ternary mixtures involving these materials have beenwidely used [1–7] in many applications. A conspicuousfeature is that the devices based on these oxides showa remarkable electrical performance, even when they areprocessed at room temperature (RT) [3, 5, 8]. This is dueto the reason that amorphous oxide semiconductors (AOS)possess a particular conduction mechanism with carriermobility close to that of crystalline materials [9, 10].

Multicomponent oxides lead to TFTs with field-effectmobility above 100 cm2/V/s. This makes them suitablefor application on active matrix backplanes for liquidcrystal or organic light emitting diodes displays. However,nearly all of the proposed TFTs use the dielectrics (gate

insulator), which require high temperature processing. A bestexample is the multilayer compound with alternate layers ofaluminum oxide (Al2O3) and titanium oxide (TiO2), knownfamiliarly as ATO, or thermal SiO2 [1–7]. These dielectricsare not compatible with conventional flexible substrates,where the processing temperature is restricted to below200 ◦C. Transparent and flexible electronics demands for gatedielectrics that can be processed at low temperature. Thismakes the high k dielectric materials, which can be processedat RT, as an automatic choice. Further, the possibility ofmaintaining high gate capacitance using thicker layers is anadded advantage.

The dielectric material need to accomplish the followingimportant points: first of all, a wide band gap, largerthan the semiconductor is desirable, preferentially withfavorable conduction band offset, to avoid strong gateleakage. A good interface is required, which can beeasily achieved using amorphous dielectrics with smoothsurfaces. Among the available high k dielectrics, hafnium

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2150 L. Pereira et al.: Sputtered multicomponent amorphous dielectrics for transparent electronics

oxide (HfO2, shortly known as hafnia) is proven to fulfillthe aforementioned requirements. However, hafnia showmicrocrystalline structure, even when processed at lowtemperature [11, 12]. This may represent an issue whenpreparing bottom-gate devices due to surface roughness.Besides that, grain boundaries are undesirable sincethey are preferential leakage paths for gate current ontransistors [13].

The foregoing discussion conclude that low temperatureprocessing techniques must be used and sputtering emergesas the best candidate, since it allows the deposition ofalmost all kind of materials without any intentional substrateheating. Besides, this technique may be used for thedeposition of both active layer and electrodes. It maybe noteworthy that the authors have earlier successfullyemployed sputtering technique to develop various electronicdevices without any substrate heating [3, 5, 8]. In the presentwork, an attempt is made to optimize the HfO2 multi-component amorphous dielectric structures co-sputteredwith SiO2 or Al2O3. Taking into account the wide band gap(∼9 eV) of Al2O3 and SiO2, the resulting insulator shouldhave a larger band gap than pure HfO2 (<6 eV). A typicalvalue of the active layer used in TFTs is varied between3 and 3.4 eV. The influence of SiO2 and/or Al2O3 contenton the structural and electrical properties of hafnia has beenexplored aiming their application as gate dielectrics on AOS-based TFTs.

2 Experimental The samples were prepared by r.f.magnetron sputtering from a cylindrical (2′′ diameter) HfO2

target at a pressure of 0.3 Pa with an r.f. power of 150 Wand using different Ar/O2 flow rates. The incorporation ofSiO2 and Al2O3 was achieved by co-sputtering the differentceramic targets by adjusting the r.f. power. Glass and p-type〈100〉 crystalline silicon (c-Si), which were cleaned in anHF solution and de-ionized water, were used as substrates.The films with 180 nm thickness were deposited without anyintentional heating of the substrates. X-ray diffraction (XRD)was performed using Rigaku DMAX III-C diffractometer.The surface microstructure was analyzed an atomic forcemicroscopy (AFM; Asylum MFP 3D) operated in AC modeusing a Si tip with a radius smaller than 10 nm on a cantileverwith a 42 N/m spring constant. The optical spectra wereobtained by Spectroscopic Ellipsometry (SE) using a Jobin-Yvon UVISEL ellipsometer on a spectral range between 1.5and 6.0 eV and with an angle of incidence of 70◦. Aluminumcontacts were made by e-beam evaporation on the back of theSi substrates and over the deposited films (using a shadowmask) for electrical characterization. J–V curves were plottedusing a Keithley 617 programmable electrometer.

3 Results3.1 HfO2 films The XRD patterns obtained from the

deposited hafnia thin films are shown comparatively inFig. 1 as a function Ar/O2 ratio. The films are confirmedas microcrystalline HfO2 with monoclinic phase, showing(−111) and (111) diffraction peaks at 2θ ∼ 28.3 and 31.7◦,

Figure 1 XRD patterns of HfO2 films prepared with differentAr/O2 ratio. The sample deposited with substrate bias is alsopresented for comparison.

respectively. This is the typical low temperature phasereported earlier for sputtered HfO2 [11, 14]. The introductionof O2 reduces the preferential growth of the monoclinic phaseby reducing the corresponding peak intensity. This reductionis accompanied by the detection of the orthorhombic phase(as observed from TEM analysis that is not shown here),which is meta-stable and not very commonly observed onsputtered films. However, it is well known fact that themonoclinic to orthorhombic transformation can occur inpolycrystalline bulk material under high pressure [15, 16].Hence, this phase transformation is attributed to the higherO2 flow that induces the substrate bombardment by negativeions, supplying enough energy to activate this change inphase. The peak centered ∼28.3◦ is slightly moved to theright when adding O2 to the gas mixture, which is probablyindicating the presence of a compressive stress.

In order to confirm the structural modification, thesubstrate was r.f. biased when depositing a sample with anAr/O2 ratio of 14:1. The XRD pattern obtained from thisbiased substrate (Fig. 1) confirms the dominant orthorhombicphase. The AFM microstructures (Fig. 2) show entirelydifferent morphology associated to these two phases, with alow RMS roughness value (2.9 nm) obtained from dominantorthorhombic phase, suggesting that the grain shape is alsodifferent.

The effect of O2 flow was also studied by SE. Afour layer model was elaborated to simulate the measureddielectric function of HfO2 films [11]. The first layer isa c-Si reference, whereas the second is a SiO2 reference

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Figure 2 AFM surface topography (1 × 1 �m2) of hafnia filmsdeposited using a Ar/O2 ratio of 14:1 sccm: a) without, and b) withsubstrate bias.

that simulates the formation of an interfacial layer betweenthe deposited film and the substrate. The SiO2 referenceis included because the interfacial reaction may result onsome kind of non-stoichiometric SiO2 or a mixture of itwith a hafnium silicate [17]. This approach was already usedwith satisfactory results [18]. On the third layer, dispersionformula (Tauc–Lorentz dispersion combined with an extraoscillator for sub-gap absorption detection) was used todescribe the optical response of the hafnia. The last one is avolume mixture of 50% dispersion formula and 50% voidsto describe the surface roughness.

As shown Fig. 3a, the increase in O2 flow leads to aslight decrease on the refractive index (n) of sputtered HfO2

films that becomes more notorious for higher O2 flow. Atthis point it is possible to speculate that this is associatedwith the mixed phase structure and also with substratebombardment by negative oxygen ions that are known to

Figure 3 a) Refractive index and b) extinction coefficient obtainedfrom HfO2 samples prepared with different Ar/O2 ratio.

cause some oxygen vacancies by breaking metal–oxygenbonds [19]. These defects create some gap states close tothe conduction band and are responsible for the increase inthe extinction coefficient for energies below the absorptionedge (Fig. 3b) [20].

The n is increased when the substrate is biased,corroborating the presence of just one phase. This is probablyindicating that the reduction in n observed for higherO2 flow may indeed result from the co-existence of twophases. Despite high negative ion bombardment, the useof a minimum O2 flow is necessary to compensate theoxygen vacancies in the film. This is further evidenced bythe electrical characterization through J–V curves in MISstructures on c-Si (Fig. 4). The introduction of O2 stronglyreduces the leakage current when comparing with the sampleproduced without O2. However, further increase in O2 flowleads to the increase in leakage current. This is attributed

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Figure 4 J–V characteristics of HfO2 samples deposited withdifferent Ar/O2 ratio.

to the negative ion bombardment of the film as explainedearlier. Besides, the presence of two different phases canoriginate highly defective grain boundaries that can act asleakage paths [13]. So, the addition of O2 is important butneeds to be adjusted in order to get the optimal insulatingproperties. Based on this discussion, the 14:1 ratio of Ar/O2

flow is chosen as the best.On other hand, when biasing the substrate (where only

orthorhombic phase exists), the leakage current does notchange significantly. This is supporting the aforementionedidea that the co-existence of two phases contributes tothe increase in leakage current. However, it suggests thatthe oxygen ion bombardment is harmful when comparedto the argon bombardment that dominates under substratebiasing. That is, positive ion bombardment seems to formmore compact and more insulating films.

3.2 Co-sputtered HfO2 + SiO2 and HfO2 + Al2O3

films When HfO2 is mixed with SiO2 by co-sputteringan important structural change occurs: the films becomeamorphous as evidenced by the XRD patterns shown inFig. 5a. The Ar/O2 ratio was fixed as 14:1 sccm. This meansthat the addition of SiO2 blocks the crystallization of HfO2

films deposited at RT. The RMS surface roughness (1.26 nm)is lower than that was obtained for the polycrystallinehafnia (4.5 nm), which can be associated with the amorphousstructure. The mixture of SiO2 with HfO2 decreases thecrystallization enthalpy per mol, turning the crystallizationof hafnia harder to achieve [21]. This represents an importantadvantage aiming the application of these dielectrics to TFTs,since amorphous films could provide smooth interfaces andless superficial trap states.

The XRD patterns (Fig. 5a) suggest that the resultingstructure of HfO2 films co-sputtered with Al2O3 is alsoamorphous. In this case, the AFM show that the RMS surfaceroughness (0.34 nm) is lower than for HfO2 + SiO2 (Fig. 5c).

Figure 5 a) XRD patterns obtained from the HfO2 samples co-sputtered with SiO2 and Al2O3; AFM microstructures (1 × 1 �m2)obtained from b) HfO2 + SiO2 film and c) HfO2 + Al2O3.


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Figure 6 Optical properties of HfO2 samples obtained by co-sputtering: a) refractive index and b) extinction coefficient ofHfO2 + SiO2 films.

This can be attributed to the difference in the effect of theion bombardment on the morphology of co-sputtered films.Seemingly the surface damaging on the HfO2 + SiO2 filmsis higher than on HfO2 + Al2O3 ones.

The SE data depicted in Fig. 6a show that the refractiveindex of HfO2/SiO2 films is located between those for pureHfO2 and SiO2, confirming the incorporation of silica. Thatis, n is moving closer to that of SiO2 as its content increases.Also the band gap is higher than HfO2, changing from 5.31to 5.56 eV. This is also evidenced by the shift in absorptionedge toward higher energy as the SiO2 content is increased(Fig. 6b). This is an important result, since it suggestsa reaction between HfO2 and SiO2 during deposition andconsequently the formation of a hafnium silicate. In fact,the band gap of the resulting film would not increase ifSiO2 did not react with HfO2 (and for instance formingclusters). Nevertheless, one open question still remains: didall sputtered SiO2 react with HfO2 or do still have some

Figure 7 Optical properties of HfO2 samples obtained by co-sputtering: a) refractive index and b) extinction coefficient ofHfO2 + Al2O3 films.

SiO2 clusters in the co-sputtered films? The efforts are underprogress to address this question by some chemical analysisand the results are expected to be published soon.

The same trend is observed for HfO2 mixed with Al2O3

(Fig. 7). In this case the power applied to the Al2O3 target waschanged and it is possible to see that n also moves toward theAl2O3 reference for more effective (more energetic) Al2O3

sputtering. The energy gap also changes, increasing from5.31 eV (for pure HfO2) to 5.48, 5.59, and 5.61 eV, for therespective r.f. power of 150, 200, and 250 W. This means thatthe formation of hafnium aluminates is enhanced as moreAl2O3 is being added to the films.

The J–V curves show that the leakage current is slightlylower on MIS structures with hafnium silicate than whenusing as dielectric pure HfO2 films, deposited under thesame conditions (Fig. 8). This means that the incorporationof a higher band gap oxide, associated with an amorphousstructure helps to improve the insulating characteristics of the

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Figure 8 J–V characteristics of co-sputtered HfO2/SiO2 films incomparison with pure HfO2 films with the same thickness (around180 nm).

produced films. However, as expected, the dielectric constantis decreased by more than 10% (from 24.1 to 21.4) due tothe lower value for pure SiO2 (we measured a value around4 for the films deposited on the same conditions).

4 Conclusions Hafnium oxide films were depositedby r.f. magnetron sputtering without any intentional substrateheating. The films are polycrystalline with a dominantmonoclinic phase. The introduction of oxygen duringdeposition is found to be effective in controlling theinsulating properties of the films as observed by J–Vcharacteristics recorded on MIS structures. However, theuse of high oxygen flow leads to severe negative ionbombardment of the substrate, which is reflected in adegrading the film properties. The best Ar/O2 flow ratio wasfound to be 14:1 sccm.

Amorphous hafnia-based multicomponent dielectricswere obtained by co-sputtering HfO2 with SiO2 or Al2O3.These oxides were chosen due to its higher gap andpossibility of formation of hafnium silicates/aluminates.The formation of these new dielectric composites issupported by the obtained physical characteristics, whichremain in between the ones known for the pure materialsused and because the overall band gap of the resultingcomposite increases. Besides the good insulating propertiesthe amorphous structure obtained is extremely important forprocessing TFT using channel layers with thicknesses below100 nm, where highly smooth surfaces are required.

Acknowledgements This work was funded by EuropeanCommunity through FP6 project MULTIFLEXIOXIDES (NMP3-CT-2006-032231) and by the Portuguese Science Founda-tion (FCT-MCTES) through projects PTDC/CTM/23943/2006,PTDC/EEA-ELC/64975/2006. The authors would also like to thankPortuguese Science Foundation (FCT-MCTES) for the fellowshipsSFRH/BD/17970/2004 and SFRH/BD/27313/2006 given to two ofthe authors (Pedro Barquinha and Goncalo Goncalves).

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Sputtered multicomponent amorphous dielectrics for transparent electronics