Growth and properties of Cu 3 N films and Cu 3 N /γ ′ - Fe 4 N bilayersD. M. Borsa, S. Grachev, C. Presura, and D. O. Boerma Citation: Applied Physics Letters 80, 1823 (2002); doi: 10.1063/1.1459116 View online: http://dx.doi.org/10.1063/1.1459116 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/80/10?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Ge interface engineering using ultra-thin La2O3 and Y2O3 films: A study into the effect of deposition temperature J. Appl. Phys. 115, 114102 (2014); 10.1063/1.4868091 Field emission properties of Si tip arrays coated with N-doped SrTiO 3 thin films at different substratetemperature J. Appl. Phys. 105, 013312 (2009); 10.1063/1.3056180 Optical, structural, and electrical properties of Mg 2 Ni H 4 thin films in situ grown by activated reactiveevaporation J. Appl. Phys. 100, 063518 (2006); 10.1063/1.2349473 High temperature electrical properties of highly epitaxial CaCu 3 Ti 4 O 12 thin films on (001) LaAlO 3 Appl. Phys. Lett. 82, 2317 (2003); 10.1063/1.1565702 Epitaxial growth of Y 2 O 3 films on Si(100) without an interfacial oxide layer Appl. Phys. Lett. 71, 903 (1997); 10.1063/1.119683

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

128.138.73.68 On: Fri, 19 Dec 2014 23:59:36

Growth and properties of Cu 3N films and Cu 3NÕg8-Fe4N bilayersD. M. Borsa,a) S. Grachev, C. Presura, and D. O. BoermaNuclear Solid State Physics, Materials Science Centre, University of Groningen, Nijenborgh 4,9747 AG Groningen, The Netherlands

~Received 5 November 2001; accepted for publication 22 January 2002!

Copper nitride films were grown by molecular-beam epitaxy of copper in the presence of nitrogenfrom a radio-frequency atomic source on~001! g8-Fe4N/(001)MgO or directly on MgO substrates.The structural properties of the Cu3N films were found to be very dependent on the substrate andon the deposition temperature. At optimal growth conditions, the Cu3N films grow epitaxial on bothsubstrates. The Cu3N films grown on MgO were characterized optically to be insulators with anenergy gap of 1.65 eV. Ong8-Fe4N, Cu3N films with a thickness of only 6 nm, were grown asclosed layers, epitaxial and rather smooth~root-mean-square roughness of 0.7 nm!. This materialhas ideal properties to be used as a barrier in low resistance magnetic tunnel junctions. ©2002American Institute of Physics.@DOI: 10.1063/1.1459116#

Transition metal nitrides show a wide variety of proper-ties and applications. Among them, copper nitrides have at-tracted considerable attention as a new material for opticalstorage devices, based on the rather low decompositiontemperature.1–3

According to theoretical calculations,4 Cu3N is an insu-lator, with an energy gap of;0.9 eV, whereas experimentalvalues ranging from 0.8 to 1.9 eV were found. The structureof Cu3N is, in itself, rather interesting. With the cubic anti-ReO3 structure, the face-centered-cubic close-packed sitesare vacant and, therefore, it is possible to insert another me-tallic atom~e.g., Cu and Pd!. With the insertion of Cu atoms,the Cu3N lattice undergoes a small (;1.13%) expansion,and the Cu4N phase is formed.5,6 Contrary to Cu3N, thiscompound is believed to be a conductor. Similarly, Cu3NPdcompounds were synthesized, also with metallic character.4,7

Up until now, films of Cu3N or Cu3N/Cu were reportedto be grown by different methods, such as rf magnetronsputtering,1,3,8–12dc sputtering,13 or ion-assisted deposition.2

The obtained films were polycrystalline on substrates likePt/MgO, Al2O3 , Si, or glass and amorphous on MgO andSrTiO3 .8 The properties of the films were found to be verydependent on substrate and growth conditions.

In this letter, the epitaxial growth and properties of thinfilms of Cu3N on g8-Fe4N/MgO and MgO substrates arereported. This material, with a rather low energy gap, mightbe used as a barrier material in low resistance magnetic tun-nel junctions. Additionally, the excellent match betweenCu3N, an insulator, andg8-Fe4N, a ferromagnetic conduc-tor, opens the perspective of developing an all-nitride, all-epitaxial magnetic tunnel junction.

Thin films of copper nitride were grown in a ultrahighvacuum~UHV! chamber~base pressure of 10210 mbar) bymolecular-beam epitaxy~MBE! of copper in the presence ofatomic nitrogen obtained from a rf atomic source. As sub-strates, we used~001! g8-Fe4N thin films grown on~001!MgO. The iron nitride films were grown in the same cham-ber, in a similar fashion as the copper nitride films. In previ-

ous work, we proved that this method is very effective togrow epitaxial iron nitride films.14,15As reported earlier, theoptimal substrate temperature for the growth ofg8-Fe4Nfilms on~001! MgO substrates was 400 °C. At such substratetemperatures, the obtained films were of high crystal qualityand rather smooth@for a ;33 nm film the root-mean-square~rms! roughness was 0.4 nm#. Based on the very good latticematch, theg8-Fe4N phase~cubic, 3.795 Å! is an ideal sub-strate for the epitaxial growth of Cu3N ~cubic, 3.817 Å!.

Additionally, Cu3N films were also grown directly on~001! MgO substrates, in a similar way. Here the lattice mis-match is;10%. In all cases, before growth, the MgO sub-strates were cleaned by annealing at 600 °C in 1026 mbarO2. The copper was evaporated from a Knudsen cell, atrather low deposition rates~70–130 s/Å! on g8-Fe4N/MgOsubstrates heated at 150 °C and on MgO substrates heated at150 °C or 250 °C. The rf atomic source was operated withpure nitrogen at a gas pressure of 531022 mbar and alsowith mixtures of nitrogen and hydrogen. The rf power ap-plied was 60 W. The pressure in the UHV chamber duringgrowth was 1027 mbar.

The crystal structure was investigated by means of x-raydiffraction in a standardu–2u geometry~XRD! and also byperforming texture measurements in a Philips X’Pert MRDsystem@two-dimensional-XRD~2D-XRD!#. Both measure-ments were done using the CuKa radiation (l51.54 nm).The surface morphology was probed with a NanoScope IIIaatomic force microscope~AFM!. The optical properties wereobtained using a Woollam variable angle spectroscopy ellip-someter. The thickness of the films was measured by Ruth-erford backscattering spectroscopy.

With only nitrogen in the rf atomic source, the optimaldeposition temperature to obtain epitaxial layers was foundto be 150 °C ong8-Fe4N substrates and 150 °C– 250 °C onMgO. The upper limit of the deposition temperature is dic-tated by the N uptake in the sample: at too high temperaturesno nitrogen is taken up and a pure Cu layer is grown. Also,the formation of Cu3N films was found not to be criticallydependent on the growth rate. Contrary to the growth ofa!Electronic mail: d.borsa@phys.rug.nl

APPLIED PHYSICS LETTERS VOLUME 80, NUMBER 10 11 MARCH 2002

18230003-6951/2002/80(10)/1823/3/$19.00 © 2002 American Institute of Physics This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

128.138.73.68 On: Fri, 19 Dec 2014 23:59:36

g8-Fe4N,15 we found that the formation of pure Cu3N filmsis less effective when hydrogen is present in a mixture withnitrogen in the rf atomic source, as compared with the casewhen only nitrogen is used. Possibly, part of the needed ni-trogen for the formation of Cu3N combines with hydrogen,and finally desorbs as NH3.

Typical results of the XRD measurements for differentCu3N samples are shown in Fig. 1, as follows: Fig. 1~a! forthe bilayer structure, 17 nm Cu3N/15 nmg8-Fe4N/MgO withthe copper nitride film grown at 150 °C, Fig. 1~b! for a 18.6nm Cu3N film grown at 150 °C on MgO and~c! for a 15.7nm Cu3N film grown at 250 °C also on MgO. When Cu3N isgrown on ag8-Fe4N underlayer, the~002! reflections over-lap, which results in a slightly broader peak at 2u547.76°.However, Cu3N can be recognized by the presence of arather intense~001! peak at 2u523.3°, as compared with theone ofg8-Fe4N ~for Cu3N the intensity ratio of the~001! and~002! reflections is 1.16 whereas forg8-Fe4N this ratio isonly 0.028!. With bare MgO substrates, the~002! and ~001!reflections of Cu3N are clearly visible inu–2u scans. In allscans, only (00k) reflections corresponding to copper nitrideand iron nitride are present. Therefore, we conclude that allcopper nitride films have a well-defined orientation with the@100# direction normal to the~001! substrate.

To determine the in-plane orientation of the film withrespect to the crystal orientation of the substrate, we per-formed 2D-XRDc –w pole scans with 2u fixed at the~111!

reflection of Cu3N. Note that also the Cu3N ~111! reflectionis very close to the~111! reflection ofg8-Fe4N. The resultscorresponding to the three samples are presented in Figs.1~d!–1~f!. As shown in Fig. 1~d!, only four sharp peaks cor-responding to the~111! reflections of both Cu3N andg8-Fe4N are present. Therefore, the copper nitride filmgrown at a deposition temperature of only 150 °C, on amatching substrate, is epitaxial and monocrystalline with thesame symmetry as the iron nitride layer with respect to theMgO substrate~no in-plane rotation!. For the films grown onMgO substrates, the corresponding pole scans are shown inFigs. 1~e! and 1~f!. In this case, at the same deposition tem-perature of 150 °C as for Cu3N films on g8-Fe4N ~but on adifferent substrate!, a more complex pattern with eight~111!intensity maxima is present@Fig. 1~e!#. The intensity inbe-tween the maxima is due to background. This pattern corre-sponds to an epitaxial film, with two preferred in-plane ori-entations, instead of only one. At higher depositiontemperatures, (250 °C), the Cu3N films are epitaxial andmonocrystalline@Fig. 1~f!#. The difference in structure of theCu3N films on MgO obtained at different deposition tem-peratures could be explained on the basis of the high mis-match present between the film and the substrate~;10%!combined with a higher mobility of the Cu atoms at a higherdeposition temperature. For the Cu3N films grown on a MgOsubstrate we found from the 2u values corresponding to the~002! and ~111! reflections of Cu3N, that a 18.6 nm thickfilm grown at 150 °C, is fully relaxed, whereas the 15.7 nmthick film grown at 250 °C, shows a slight tetragonal distor-tion, with a small in-plane contraction and an out-of-planeexpansion.

The surface morphology of Cu3N films of differentthickness~6 nm and 17 nm! grown ong8-Fe4N substrateswas studied in air with AFM in contact mode. For a thick-ness of;17 nm, the Cu3N films were found to be rough ona nm scale@Fig. 2~a!#. However, for the Cu3N film of 6 nmthickness, the surface was rather smooth.@Fig. 2~b!#. Despitethe rather low deposition temperature, the surface of the filmhas a uniform granular appearance with a rms roughness ofonly 0.7 nm. The difference in smoothness between the thin~6 nm! and the thicker~17 nm! Cu3N film could be explainedassuming the Stranski–Krastanov growth mode, with a criti-cal thickness for the transition from the 2D growth regime~layer by layer! to the three-dimensional~3D! growth regime~islands! between 6 and 17 nm. Similar values~and higher!of the critical thickness for the transition from 2D to 3Dgrowth were observed also in different systems~eg.

FIG. 1. X-Rayu–2u scans:~a! 17 nm Cu3N/15 nmg8-Fe4N/MgO grown at150 °C,~b! 18.6 nm Cu3N/MgO grown at 150 °C,~c! 15.7 nm Cu3N/MgOgrown at 250 °C. The corresponding polec–w scans are shown in~d!, ~e!,and ~f!. In all pole scans, the 2u angle is fixed for the~111! reflection ofCu3N.

FIG. 2. AFM topographical images of Cu3N films of different thickness (t)grown under similar conditions ong8-Fe4N substrates.~a! t517 nm, 131 mm2 image ~vertical range 25 nm; rms roughness;2.6 nm!. ~b! t56 nm, 131 mm2 image~vertical range 10 nm; rms roughness;0.7 nm!.

1824 Appl. Phys. Lett., Vol. 80, No. 10, 11 March 2002 Borsa et al.

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

128.138.73.68 On: Fri, 19 Dec 2014 23:59:36

YBa2Cu3O7-d!. Additional x-ray photoelectron spectroscopymeasurements showed, as expected, that the Cu3N filmsgrow as complete layers for such thin films.

Optical measurements were performed at room tempera-ture in the 0.8–4.5 eV energy range. All spectra were takenat an angle of incidence of 70° and a fixed polarization angleof 45°. Figure 3~a! shows the measured ellipsometric param-etersc and D for a Cu3N thin film ~;18.6 nm! grown at150 °C on MgO. The experimental data were fitted takinginto account the thickness of the Cu3N film and the opticalproperties of the MgO substrate, with Lorentzian dielectricfunctions. The results of the fit are shown together with theexperimental data. From the fit, the real (e1! and imaginary(e2! parts of the dielectric function were calculated16 @Fig.3~b!#. The dependence ofe1 ande2 on energy, typical for aninsulator, agrees well with previously reported data. How-ever, in our case, the magnitude ofe1 and e2 is slightlyhigher than the values reported~all for much thicker films!.This might be an effect correlated with the thickness of thefilms, which is much lower in our case. From the dependenceof the absorption coefficient on energy as derived from ourdata@inset Fig. 3~b!# we estimate an energy gap of 1.65 eV.This value is in the range of data previously measured forthicker Cu3N films grown on different substrates. For theCu3N film grown on MgO at 250 °C, similar results wereobtained, although here a small absorption peak was presentbelow the optical gap. Such an effect, also seen in othermaterials,17 and known as the Urbach tail, might be due toabsorption by impurities~in our case Cu!.

Additional resistivity measurements confirmed the insu-lating character of all the Cu3N films. Values in the order of;1000V cm were measured. Due to the complexity of amultilayer system for optical spectroscopy measurementsand also because of lack of optical data forg8-Fe4N films,we could only characterize optically the Cu3N films growndirectly on MgO substrates.

In summary, this letter reports on the epitaxial growth ofCu3N/g8-Fe4N bilayers and Cu3N layers on~001! MgO sub-strates by MBE of copper in the presence of nitrogen from arf atomic source. The crystal structure of the films was foundto be dependent on the deposition temperature, as well as onthe substrate. Films of Cu3N grow as closed epitaxial andmonocrystalline layers on epitaxialg8-Fe4N. Moreover,Cu3N films with a thickness in the range of interest for tun-nel junction applications were found to be rather smooth~rms roughness;0.7 nm!. On MgO substrates, the films arealso epitaxial. If higher deposition temperatures (250 °C) areused, the films are also monocrystalline.

The Cu3N films grown on MgO substrates were charac-terized optically to be insulators, with an optical energy gapof 1.65 eV. The present growth technique proved its potentialin growing high quality insulating films, which might begood candidates for barriers in magnetic tunnel junctions.

The authors thank S. A. Koch~Department of AppliedPhysics, University of Groningen! for performing the AFMmeasurements. This work is part of the research program ofthe Foundation for Fundamental Research on Matter—FOM.

1T. Maruyama and T. Morishita, Appl. Phys. Lett.69, 890 ~1996!.2M. Asano, K. Umeda, and A. Tasaki, Jpn. J. Appl. Phys., Part 129, 1985~1990!.

3T. Nosaka, M. Yoshitake, A. Okamoto, S. Ogawa, and Y. Nakayama, Appl.Surf. Sci.169, 358 ~2001!.

4U. Hahn and W. Weber, Phys. Rev. B53, 12684~1996!.5J. Blucher and K. Bang, Mater. Sci. Eng., A117, L1 ~1989!.6T. Maruyama and T. Morishita, J. Appl. Phys.78, 4104~1995!.7U. Zachwieja and H. Jacobs, J. Less-Common Met.170, 185 ~1991!.8S. Terada, H. Tanaka, and K. Kubota, J. Cryst. Growth94, 567 ~1989!.9Z. Q. Liu, W. J. Wang, T. M. Wang, S. Chao, and S. K. Zheng, Thin SolidFilms 325, 55 ~1998!.

10K. J. Kim, J. H. Kim, and J. H. Kang, J. Cryst. Growth222, 767 ~2001!.11T. Nosaka, M. Yoshitake, A. Okamoto, S. Ogawa, and Y. Nakayama, Thin

Solid Films348, 8 ~1999!.12D. Wang, N. Nakamine, and Y. Hayashi, J. Vac. Sci. Technol. A16, 2084

~1998!.13L. Maya, J. Vac. Sci. Technol. A11, 604 ~1993!.14S. Grachev, D. M. Borsa, S. Vongtragool, and D. O. Boerma, Surf. Sci.

482, 802 ~2001!.15D. M. Borsa, S. Grachev, J. W. J. Kerssemakers, and D. O. Boerma, Appl.

Phys. Lett.79, 994 ~2001!.16C. Presura, D. van der Marel, M. Dischner, C. Geibel, and R. K. Kremer,

Phys. Rev. B62, 16522~2000!.17A. T. M. van Gogh, D. G. Nagengast, E. S. Kooij, N. J. Koeman, J. H.

Rector, and R. Griessen, Phys. Rev. B63, 195105~2001!.

FIG. 3. ~a! Measured ellipsometric parametersC andD ~thick dotted line!together with the fit of the experimental data~thin solid line! for a Cu3N thinfilm (;18.6 nm) grown at 150 °C on a MgO substrate.~b! The real (e1) andimaginary (e2) parts of the dielectric function. The inset shows the absorp-tion coefficient~a! from which the optical energy gap is estimated to be;1.65 eV.

1825Appl. Phys. Lett., Vol. 80, No. 10, 11 March 2002 Borsa et al.

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

128.138.73.68 On: Fri, 19 Dec 2014 23:59:36