Superhardness effects of heterostructure NbN/TaN nanostructured multilayersJunhua Xu, Masao Kamiko, Yaomin Zhou, Ryoichi Yamamoto, Geyang Li, and Mingyuan Gu

Citation: Journal of Applied Physics 89, 3674 (2001); doi: 10.1063/1.1353809 View online: http://dx.doi.org/10.1063/1.1353809 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/89/7?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Vibrational spectroscopy and microwave dielectric properties of Ca 5 x Ba x Nb 2 Ti O 12 and Ca 5 x Ba x Ta 2Ti O 12 ceramics J. Appl. Phys. 98, 084105 (2005); 10.1063/1.2112179 Low-loss Ca 5 x Sr x A 2 Ti O 12 [ A = Nb , Ta ] ceramics: Microwave dielectric properties and vibrationalspectroscopic analysis J. Appl. Phys. 97, 104108 (2005); 10.1063/1.1897065 Alternating stress field and superhardness effect in TiN/NbN superlattice films J. Vac. Sci. Technol. A 20, 674 (2002); 10.1116/1.1460887 Internally shunted sputtered NbN Josephson junctions with a TaNx barrier for nonlatching logic applications Appl. Phys. Lett. 78, 99 (2001); 10.1063/1.1337630 Nanoindentation hardness, abrasive wear, and microstructure of TiN/NbN polycrystalline nanostructuredmultilayer films grown by reactive magnetron sputtering J. Vac. Sci. Technol. A 16, 3104 (1998); 10.1116/1.581466

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JOURNAL OF APPLIED PHYSICS VOLUME 89, NUMBER 7 1 APRIL 2001

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Superhardness effects of heterostructure NbN ÕTaNnanostructured multilayers

Junhua Xu,a) Masao Kamiko, Yaomin Zhou, and Ryoichi YamamotoInstitute of Industrial Science, University of Tokyo, 7-22-1 Roppongi, Minato-ku, Tokyo 106-8558, Japan

Geyang Li and Mingyuan GuState Key Laboratory of Metal Matrix Composites, Shanghai Jiaotong University, Shanghai, 200030 China

~Received 11 October 2000; accepted for publication 14 January 2001!

Although superhardness effects have been extensively investigated for epitaxial ceramicnanomultilayer films with the same crystal structures in the last decade, those for multilayers withdifferent crystal structures have been seldom studied. In this article, NbN/TaN nanomultilayers havebeen designed and deposited by reactive magnetron sputtering. The results showed that the crystalstructures of NbN and TaN are face-centered cubic and hexagonal in superlattice films, respectively,and the lattice plane~111! of NbN is coherent with the~110! of TaN, i.e.,$111% fcc-NbNi$110%h-TaN.The results of microhardness measurement showed that the superhardness effects of NbN/TaNmultilayers exist in a wide range of modulation period from 2.3 to 17.0 nm. This phenomenon isdifferent from that of epitaxial ceramic multilayers where the maximum hardness usually takesplace at a modulation period of 5.0–10.0 nm. It is proposed that the coherent stresses and thestructural barriers~fcc/hexagonal! to dislocation motion between NbN and TaN layers are the mainreasons for the high-hardness value in a wide range of modulation periods. ©2001 AmericanInstitute of Physics.@DOI: 10.1063/1.1353809#

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I. INTRODUCTION

Research on the mechanical properties of nanostructmultilayer films and superlattice films is of great interesince there are strength and hardness anomalies in somthese films.1–11 Several theories have been proposed inliterature to explain the mechanisms of the superhardneffects. These are:~1! dislocation generation and mobilitmechanisms according to the Hall–Petch or Orowmodel;12,13 ~2! the supermodulus effect model,5 where hard-ness anomalies may be the result of a supermodulus effethe line tension of dislocation;~3! the coherent strainmodel,6,8 where the coherency strain in multilayers playsrole in the hardness anomalies; and~4! the elastic modulusdifference model,14,15 where the hardness anomalies comfrom the difference of the elastic modulus between theperlattice layers. As for the ceramic multilayer systemsvestigated, the superhardness effects of epitaxial superlafilms with the same crystal structure have been reported.5–11

The models of the superhardness effects are based on mlayers with the same crystal structure and the same sliptems of dislocation.14,15

In this article nanostructured multilayers with differecrystal structures between constituent materials have bdesigned and deposited. The relationship between the mstructures and superhardness effects has been investigNbN and TaN are chosen as the constituent materials ofmultilayers in this experiment. It is well known that boNbN and TaN have two kinds of crystal structures, i.e., facentered cubic~fcc! and hexagonal. The lattice constantsthe face-centered-cubic structure of NbN and TaN are 0.

a!Electronic mail: jhxu@iis.u-tokyo.ac.jp

3670021-8979/2001/89(7)/3674/5/$18.00

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and 0.440 nm, respectively. The lattice mismatchonly 0.2%. The hexagonal lattice parameters of NbN aTaN are a50.2968 nm,c50.5548 nm anda50.5192 nm,c50.2908 nm, respectively, which shows a very large diffence. The research results of single-layer films of NbN aTaN deposited by magnetron sputtering showed that thebility crystal structures of NbN and TaN are hexagonal. Bcause the ratio ofa/c is very different between NbN andTaN, we can guess that it is difficult for NbN and TaNgrow in the epitaxial model in NbN/TaN multilayers. It ipossible to synthesize multilayers with different crysstructures and different slip systems of dislocation.

II. EXPERIMENTAL PROCEDURE

NbN/TaN ceramic nanostructured multilayer films wedeposited using the SPC-350 magnetron sputtering syswhich has three targets including one dc magnetron cathand two rf magnetron cathodes. Sputtering targets were pNb ~99.9%! and Ta~99.9%!, which were mounted on the dand rf cathodes, respectively. After base pressures of31024 Pa were obtained, Ar and N2 were bled into thechamber through two separate gas manifolds. A mixAr–N2 gas was used for reactive sputtering with an Ar ptial pressure of 3.431021 Pa and a N2 partial pressure of0.431021 Pa. Ground and polished stainless-steel wafwere used as deposition substrates. They were ultrasonicleaned in chemical solvents before being mounded ontosubstrate holder in the chamber. Modulation structures wobtained by rotating the substrate holder, letting samplesNb and Ta targets alternately. The designed modulation rwas l TaN/ l NbN51:1. The modulation period and modulatioratio were obtained through exact control of the stopp

4 © 2001 American Institute of Physics

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time in front of the Nb and Ta targets. The mixture films~Nb, Ta!N were obtained by rotating the substrate holder aspeed of 20 rounds per minute. In order to improve thehesion between the film and the substrate, a titanium unlayer of 200 nm was first deposited on the substrate andalternately deposited with NbN and TaN layers. The soupower of the Ta and Nb targets was 100 W and 0.13400 V, respectively. The deposition rate was 0.32 nm/sTaN and 0.30 nm/s for NbN. The total thickness of tmultilayer films was 2.0mm. The substrates were heated400 °C before deposition and the deposition temperaturebelow 70 °C.

The hardness of the multilayer films was measured usan MHT-1 microhardness tester. A Knoop diamond tipdenter was used at a load of 25 gf for 15 s. This indgeometry was chosen because of its large ratio of the dianal length to the depth, which can decrease the effect ofsubstrate hardness on the hardness of the films. Ten indtions were made at each load. The crystal structure ofmultilayer films was determined by x-ray diffraction~XRD!in a D/max-3A diffractometer with CuKa radiation. TheJEM-200CX transmission electron microscope was usedstudy the cross-sectional microstructure of the samples.

III. EXPERIMENTAL RESULTS

A. Microstructure analysis

Figure 1 shows the low-angle XRD spectra of the NbTaN multilayers with modulation periods of 2.3, 11.1, a21.5 nm. It is shown that the interfaces of the multilayerssharpness from the numbers and the shape of low-aXRD peaks. There is obvious composition modulation asmall modulation period of 2.3 nm. The results of the XRspectra of single-layer NbN and TaN films with the samdeposition condition as the multilayers showed that singlayer TaN and NbN films are of hexagonal structure. Tlattice constants of TaN and NbN area50.5234 nm, c50.2975 nm,c/a50.57 anda50.3036,c50.556 nm,c/a51.83, respectively. Shown in Fig. 2 are the XRD spectraNbN/TaN multilayers with modulation periods from 0 t73.2 nm. It is shown that the characterizations of XRD sptra of NbN/TaN multilayers are similar to those of singllayer TaN films~Fig. 2!, with strong ~110! and ~300! tex-

FIG. 1. Low-angle x-ray diffraction of NbN/TaN multilayers at differenmodulation periods.

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tures. The peak position of~110! and ~300! lattice planesalmost does not move with the change of modulation periWe can only determine that the structures of TaN layersmultilayers are hexagonal in Fig. 2, but do not know whkinds of crystal structures NbN layers are. It is possible tthe hexagonal NbN structure is ‘‘epitaxially stabilized’’ bthe hexagonal TaN. However, it is unlikely because the raof c/a of hexagonal lattice constants of NbN and TaN isdifferent. Another possibility is that the diffraction lines othe NbN layers are overlapped by that of the TaN films.Fig. 2, we can find an XRD peak~where 2u is 40.0°! whenthe modulation period is 73.2 nm, which may be the~200!peak of cubic NbN. This needs to be verified by other eperiments.

The bright-field transmission electron microsco~TEM! micrograph and electron diffraction patterns of tcross-sectional NbN/TaN multilayers with a modulation priod L511.1 nm are shown in Figs. 3~a! and 3~b!. The TEMimage indicates that the interfaces are planar and the mlation structure is clear in the multilayers. The TaN layeappear darker and the NbN layers are brighter becaustheir different average atomic numbers. It is found thatelectron diffraction rings of NbN/TaN multilayers are a combination of the cubic NbN and hexagonal TaN electron dfraction circles. It can be confirmed that the diffraction peaof cubic NbN are overlapped by that of hexagonal TaN asmall modulation period, but with the increase of the modlation period, the~200! peak of cubic NbN can be seenL573.2 nm.

FIG. 2. High-angle x-ray diffraction spectra of NbN/TaN multilayersmodulation periods from 0 to 73.2 nm.

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Figure 4 shows the high-resolution TEM~HRTEM! pho-tograph of NbN/TaN multilayers that gave the electron dfraction patterns in Fig. 3. It can be found that the lattfringes penetrate through the interfaces. This means thtakes coherent growth between TaN and NbN layers. Fdislocations were found in the coherent interfaces. Thetice fringes are distorted at the interfaces, which indicathat the NbN/TaN multilayers are still a coherent growmodel atL511.1 nm. The lattice spacing measured froFig. 4 is 0.2568 nm. According to the lattice spacingd(110)50.2617 nm for TaN andd(111)50.2535 nm for NbN,

FIG. 3. Cross-sectional TEM photographs~a! and electron diffraction~b! ofNbN/TaN multilayers at a modulation period of 11.1 nm.

FIG. 4. HRTEM micrograph of NbN/TaN multilayers with a modulatioperiod 11.1 nm.

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the coherent relationship between~111! of the cubic NbNand ~110! of the hexagonal TaN is determined. That is,

$111% fcc-NbNi$110%h-TaN.

The lattice mismatch is 3.18% in this case.

B. Microhardness test

Shown in Fig. 5 is the microhardness of the NbN/Tamultilayers as a function of the modulation period. The haness of the mixture films is 29.0 GPa, which is almostsame as that of rule of mixtures of single-layer NbN and Tfilms. The Knoop microhardness values are found to increrapidly with the increase of the modulation period and reaa maximum of 51.0 GPa atL52.3 nm. The hardness stayshigh values (HK546.5– 50.0 GPa) withL52.3– 17.0 nm.The hardness decreases rapidly atL.17.0 nm, and the hardness is 34.9 GPa atL522.8 nm. Further increasingL resultsin a relatively slow decrease in hardness and the hardne30.0 GPa atL573.2 nm, which also is comparable with thvalue calculated from the rule of mixtures.

IV. DISCUSSION

The results of microstructures and mechanical properobserved from our experiments showed two charactertions.~1! It takes heterostructure coherent interfaces betwNbN and TaN layers. It is known that both NbN and Tahave cubic structures and the lattice constants are almossame. But, the growth mode of NbN and TaN does not tthe epitaxial method with the same crystal structure in NbTaN multilayers. Instead, there is a heterostructure coherelationship between cubic NbN layers and hexagonal Tlayers in multilayers.~2! The superhardness effects taplace at a relatively wide modulation period from 2.3 to 17nm. This phenomenon is different from the superhardneffects of epitaxial ceramic multilayers, where the maximuhardness usually takes place at a modulation period of 510.0 nm.5–11

In recent years, it has been reported that the superhness effects of ceramic multilayers and superlattice filmainly exist in epitaxial multilayers with the same cryststructures. The theoretical models of superhardness menisms can only explain the superhardness effects in s

FIG. 5. Microhardness of NbN/TaN multilayers as a function of the modlation period.

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ceramic system multilayers~NbN/TiN, VN/TiN, TaN/TiN,etc.!, in which the hardness peak occurred from 5.0 to 1nm, a short modulation period. Hence, superhardness efthat take place at a wide modulation period in NbN/Tamultilayers may be the combination of three factors aslows:

A. Effect of elastic modulus difference

According to Koehler’s theory,14 the elastic modulus dif-ference between NbN and TaN layers can lead to thecrease of hardness in these multilayers. The maximum sstress enhancement caused by the modulus differencNbN/TaN multilayers is2,14

tmax5RGB sinu/8p, ~1!

whereR5(GA2GB)/(GA1GB). GA and GB are the shearmodulus ofA andB materials, respectively, andGA is largerthan GB . u is the smallest angle between the interface athe glide planes of crystal with smaller elastic constants.using Schmid’s law, the shear stresstmax is converted toyield stresss:

s5tmax/m, ~2!

wherem, the Taylor factor, is 0.3.15 The maximum hardnesenhancement of NbN/TaN multilayers can be estimated frH53s, wheres is yield stress. WithGNbN5132.0 GPa,16

GTaN5196.0 GPa,17 and with angleu at 45°,14 the maximumhardness enhancement of NbN/TaN multilayers causedthe modulus difference is 7.2 GPa. From the results, iknown that the modulus difference between NbN and Tlayers plays a small role in the hardness enhancemenNbN/TaN multilayers.

B. Effect of coherent strains

The coherent strain theory18,19 predicted that alternatingcoherent stress fields, which exist in multilayers or suphardness films because of lattice mismatch, can inhibitlocation motion, leading to the enhancement of strengthhardness of films. From the HRTEM photograph shownFig. 4, it is known that heterostructure coherent interfaexist between NbN and TaN layers. Dislocations were sdom found, which means there are still coherent strainsthe interfaces. According to the Cahn and Kato model,18,19

the maximum predicted shear yield stress enhancementhree-dimensional alternating stress fields was

tmax5~1/6!1/2AEe, ~3!

where A is the composition modulation amplitude,E isYoung’s modulus, ande is the coherency strain forA51. Asan approximate estimate, we calculate the hardness enhament in one-dimensional NbN/TaN multilayers through E~3! deduced from three-dimensional composition modution. Using A50.5, E5533.5 GPa ~the average elasticmodulus of NbN and TaN!, e50.0318 and H53s53(tmax/m) yield a maximum hardness enhancement33.9 GPa. It is obvious that coherent stresses play a mimportant role than the modulus difference in hardnesshancement in this system.

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The total hardness enhancement of NbN/TaN multilaers due to the modulus difference and coherent stress mois about 41.1 GPa, which is larger than the experimenmaximum hardness enhancement of 20.062.5 GPa. Thehardness enhancement calculated by three-dimensional sfields will be larger than that of one-dimensional strefields, which is the real stress state in NbN/TaN multilayeThis is the reason that the experimental hardness enhament is smaller than the calculated value.

C. Effect of different slip systems of dislocations

In NbN/TaN multilayers, the structures of NbN and Talayers are cubic and hexagonal, respectively. Cubic NbNers have more slip systems of dislocations than TaN layerhexagonal structures. The slip systems of dislocation areferent in NbN and TaN layers. When the dislocations mofrom one layer to the other, the dislocations cannot direcpenetrate the interfaces because of the different slip systeOne possibility is that a secondary slip system is activatethe other layer at high stress levels, allowing glide acrossinterface. Another possibility is the stress concentrationcause of dislocation pileup near the interface of one laywhich leads to dislocation nucleation in the next layer.15 Inspite of this possibility, the strength and hardness of the mtilayers will increase. It can be seen that there are obvicomposition modulations atL52.3 nm from low-angle XRDof NbN/TaN multilayers. Hence, the NbN and TaN layestill keep cubic and hexagonal structures, respectively,have a sharp interface because NbN and TaN layers aremiscible. This leads to maintaining superhardness at a vsmall modulation period. Comparing the epitaxial superltice films with the same crystal structures, the superhardnof NbN/TaN multilayers can also maintain a larger modution period of 17.0 nm, because of the different crystal strtures and the different slip systems of dislocations. But, wfurther increase of the modulation period, the decreasehardness occurs because the dislocations can generatemove in layers of smaller elastic modulus.

V. CONCLUSIONS

In order to study the effects of heterostructure multilaers on mechanical properties, NbN/TaN superlattice filhave been deposited by rf magnetron sputtering. The resof microstructure analysis showed that the crystal structuof NbN and TaN are face-centered cubic and hexagoin superlattice films, respectively, and the lattice pla~111! of NbN is coherent with ~110! of TaN, i.e.,$111% fcc-NbNi$110%h-TaN. The results of microhardness mesurement show that the superhardness effects of NbN/multilayers exist in a wide range of modulation periods fro2.3 to 17.0 nm. This phenomenon is different from thatepitaxial ceramic multilayers where the maximum hardnusually takes place at a modulation period of 5.0–10.0 nmis proposed that the coherent stresses and structural ba~fcc/hexagonal! to dislocation motion between NbN and Talayers are the main reasons for high-hardness valueswide range of modulation periods.

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