Thin Solid Films 520 (2011) 1115–1119

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An orientation competition in yttria-stabilized zirconia thin films fabricated by ionbeam assisted sputtering deposition

Z. Wang a,b,⁎, Z.J. Zhao a,b, B.J. Yan a,b, Y.L. Li a,b, F. Feng c, K. Shi c, Z. Han c

a School of Physics, Beijing Institute of Technology, Beijing 100081, People's Republic of Chinab Key Laboratory of Cluster Science (Beijing Institute of Technology), Ministry of Education of China, Beijing 100081, People's Republic of Chinac Applied Superconductivity Research Center, Department of Physics, Tsinghua University, Beijing 100084, People's Republic of China

⁎ Corresponding author at: School of Physics, Beijing I100081, People's Republic of China.

E-mail address: wangzhi@bit.edu.cn (Z. Wang).

0040-6090/$ – see front matter © 2011 Elsevier B.V. Alldoi:10.1016/j.tsf.2011.09.038

a b s t r a c t

a r t i c l e i n f o

Article history:Received 22 January 2011Received in revised form 10 July 2011Accepted 22 September 2011Available online 29 September 2011

Keywords:IBADCoated conductorYSZOrientation competition

A previously found orientation competition in ion beam sputtered yttria-stabilized zirconia thin films wasstudied in detail. The effects of sputtering energy and deposition angle were analyzed in ion sputteredfilms without assisting ions bombardment. It is found that for normally deposited films, (001) and (011) ori-entations are favored at low and high sputtering energy respectively. For inclined substrate deposited films,as deposition angle increases, (001), (011) and (111) orientations are advantaged in turn. The results can beattributed to the in-plane energy exchange of deposition atom and adatoms. In ion beam assisting depositedYSZ films of low assisting ions energy and current, a (001) oriented biaxial texture is gradually induced as ionenergy increased. In the case of ion beam assisted inclined deposition of 45°, (001) orientation is enhancedand two preferential in-plane orientations are found coexist.

nstitute of Technology, Beijing

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© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Ion beamassisted deposition (IBAD), defined as thebombardment ofgrowing thin films with energetic ions, can fabricate biaxially texturedbuffer layers of coated conductors on non-textured substrates withoutepitaxial relationships between substrates and films [1,2]. Much exper-imental work has been done to study the effects of an assisting ion beam(AIB) on film properties [3–5]. A biaxial grain alignment formed by theoff-normal bombardment of an AIB has been reported on metal [6,7],alloy [8], nitride [9], and oxide films [10]. Various mechanisms of biaxialtexture development, such as selective resputtering [11], shadowing[12], and anisotropic damage [10], were proposed and modeled bycomputer simulations [13–15].

We have studied the effects of experimental parameters on thegrowth of IBAD yttria stabilized zirconia (YSZ) films. Recently, a compar-ative study showed a competition of (001) and (011) orientations in YSZfilms [16]. In the films deposited without assisting ion bombardment(non-IBAD), (011) alignment is advantaged and random in-plane texturedevelops.While in IBAD films, (001) alignment is advantaged and an AIBinduces a biaxial texture. In this paper, this orientation competition pro-cess is studied in detail by other methods. The effects of the depositionatom energy and inclined angle of substrates on this competition innon-IBAD films are studied. We also studied the process of gradually in-ducing a (001) oriented biaxial texture by an AIB through enhancingassisting ion energy. A competition of two in-plane orientation in ion

assisted inclined deposition films was found and is to be analyzed inthis paper.

2. Experimental details

For the deposition of YSZbuffer layers, an ion beamassisted sputteringdeposition system with two sputtering ion sources and one assisting ionsource was employed. The experimental facility can be found in Ref.[16]. Deposition atoms were sputtered by two beams of Ar+ from a12.5%Y–87.5%Zr (atom ratio) alloy target and deposited on 10×10×0.05 mm3 sized glass substrates. The substrates were bombarded withan AIB for one minute before deposition in order to clean the surface.No additional heating was applied on the substrates and depositionoccurs within ambient temperature. A background vacuum of 1.0×10−4 Pa was reached before gasses flowing into the chamber. Argonflows of 15 sccm and 25 sccm were maintained for the discharge in thesputtering and assisting ion sources. An oxygen flow of 40 sccm wasmaintained towards near the samples to ensure full oxidation of thefilms. The chamber pressure was about 4.0×10−2 Pa in the process ofdeposition.

In order to study the orientation competition process, we fabricated4 groups of YSZ samples. The deposition configuration of each group isshown in Fig. 1. Groups A and B were non-IBAD samples. For group A,atoms sputtered by different sputtering energy Es were deposited nor-mally on substrates for the thickness of about 800 nm. The thicknessesof all the films were measured by an XP-1 surface profilometer ofAMBIOS. For group B, atoms sputtered by the same energywere depos-ited at different inclined angles of substrate (defined between the depo-sition atom flux axis and the substrate normal line) for the same time.

i

i

Deposition atom flux

Assisting ion flux

Substrate normal

Substrate

55º

55º

10º45º

a)

d)

b) c)

Fig. 1. Deposition configurations of different groups of YSZ films. (a): Group A, normally deposited non-IBAD films under different sputtering energy; (b) group B, inclined depositednon-IBAD films with different inclined angle i; (c) group C, normally deposited IBAD films under low assisting ion energy and at the low beam current of 0.10 mA/cm2 (the optimalbeam current is 0.50 mA/cm2), the incidence angle of AIB is 55°; and (d): group D, 45° inclined deposited IBAD films with low assisting energy and flux; the incidence angle of AIB is10°.

1116 Z. Wang et al. / Thin Solid Films 520 (2011) 1115–1119

Groups C and D were IBAD samples. For group C, the deposition ada-toms with the same deposition energy and flux were bombarded byan Ar+ beam of low energy and low current. The incidence angle ofAIB is 55°. For group D, atomswere instead normally deposited at an in-clined angle of 45°. The incidence angle of AIB is 10°. The mainexperimental parameters of each group are listed in Table 1.

To characterize the orientation and texture of the films, we ap-plied X-ray diffraction (XRD). The XRD analyses for the films werecarried out using a D/MAX-RB system equipped with Cu-Kα1 source.The XRD θ–2θ scans revealed the film alignment. The grain size of thefilms was reflected by the full width at the half maximum (FWHM)values of the corresponding peaks. The quality of in-plane texture ofthe films was characterized by the YSZ (111) reflection measuredby the ϕ-scans in the standard Bragg case.

3. Results and discussion

In Ref. [16], we found (011) orientation dominates the alignmentin non-IBAD YSZ films under sputtering energies Es of 1500 eV. Infact an obvious orientation exists in non-IBAD YSZ as Es varies. Fig. 2(a) shows XRD patterns of θ–2θ scan of the non-IBAD YSZ films de-posited with different Es (group A, Fig. 1(a)). A competition of (001)and (011) orientations can be seen as Es varies. At Es of 900 eV,(002) peaks are higher than (022) peaks. As Es increases, the diffrac-tion intensity of the (002) peak decreases, whereas the intensity ofthe (022) peak increases and dominates the alignment of non-IBADsamples above 1200 eV. So the deposition of a dominating (011) ori-entation can be sustained by high sputtering energy. As Es increases,the full widths of half maximum (FWHMs) of (022) peaks in Fig. 2(b), can be found decrease, which reflects the size of the (011)aligned grains, increases with Es increasing. So we can say that higher

Table 1Experimental parameters.

Group A B C D

Sputtering ion current density (mA/cm2) 3.2 3.2 3.2 3.2Sputtering ion energy (Es) (eV) 900–1500 1000 1000 1000Substrate inclined angle(°) 0 0–70 0 45Assisting ion current density (mA/cm2) 0.10 0.10Assisting ion energy (Ea) (eV) 50–200 100, 200Assisting ion incidence angle (°) 55 10Deposition time (min) 50–75 90 60 90

sputtering energy can help the (011) aligned grains grow. The ϕ-scanresults of three typical samples are shown in Fig. 2(c). There is nopeak in every curve, and similar results were found in other sample,which shows that there is no preferential in-plane orientation in thenormally deposited non-IBAD YSZ films.

Fig. 3 shows the θ–2θ patterns of the non-IBAD YSZ films deposit-ed with different inclined angles i (group B, Fig. 1(b)). A competitionof (001) and (011) orientations can be found when i lies between0 and 45°. As i increases, the diffraction intensity of the (002) peakdecreases, whereas the intensity of the (022) peak increases anddominates the alignment of the samples at i=45∘. When i≥50°,(001) orientation can be neglected, while a competition occurs be-tween (011) and (111) orientations, and as i increases, the diffractionintensity of the (022) peak decreases, whereas the intensity of the(111) peak increases and dominates the alignment at i=70°.

From the above discussion, we can conclude that there is a growthcompetition among different orientations in both normally or in-clined deposited non-IBAD YSZ films. For films of normal deposition,(001) and (011) orientations are advantaged at low and high sputter-ing energy region respectively. For inclined deposition films, as the iincreases, (001), (011), and (111) orientations were advantaged inturn. The results can be attributed to the energy exchange betweendeposition atoms and adatoms. The energy of new-coming depositionatoms was transported to adatoms through collision, and the surfacemobility of the adatoms was enhanced consequently. The higher thesputtering energy, the higher the energy of deposition atoms, andthen the higher the mobility of the adatoms. For the face centeredcubic crystal as YSZ, according to the in-plane atom arrangement,the surface binding energy of (011) plane is higher than that of(001) plane. Therefore, (011) orientation is the lower energy statethan (001) and advantaged at higher sputtering energy. So the XRDintensity and grain size increase with Es increasing. The atoms are de-posited normally in non-IBAD films of group A, therefore the energyexchange is isotropic in the plane parallel to substrate. So there isno factor inducing preferential in-plane orientation, and only randomin-plane orientation can develop, as shown in Fig. 2(c).

For the atoms of inclined deposition, the part of the momentumparallel to substrate is reserved and the surface mobility of the ada-toms was enhanced. The higher the inclined angle, the higher the mo-bility of the adatoms. Based on the in-plane atom arrangement, forthe surface binding energy of the low-index crystal plane, we canknow that (111) plane is the highest, (011) plane takes the secondplace, and (001) plane is the lowest. So (001), (011), and (111)

a)

b)

c)

Fig. 2. XRD results of the non-IBAD YSZ films of group A deposited with different sputtering energy Es. (a): Patterns of θ–2θ scan; (b): FWHMs of (022) peaks as a function of Es; and(c): ϕ-scan results at YSZ (111) reflection.

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orientations were advantaged in turns as the inclined angle increases,as shown in Fig. 3.

IBAD-YSZ film deposited under optimal condition has a perfect(001) orientation and other orientation can be neglected, as shownin Fig. 4 (a), which is also a results of orientation competition ofIBAD films. The optimal parameters are ion beam current of0.5 mA/cm2 and ion energy of 300 eV. An obvious orientation compe-tition under low assisting ion energy and current (group C) is shownin Fig. 4. Panel (b) shows the XRD patterns of θ−2θ scan of the IBADsamples with different Ei at assisting ion current of 0.10 mA/cm2. As Eiincreases, the intensity of (002) peak increases and that of (022) peakdecreases, which indicates that the growth advantage is transferredgradually by an assisting ions from (011) to (001) orientation. A per-fect in-plane preferential orientation can be seen in the XRD ϕ-scanresults of the optimal IBAD samples shown in Fig. 4(c). Fig. 4(d)shows the ϕ-scan results of group C. We can see as Ei increases, four

Fig. 3. XRD θ–2θ results of the non-IBAD films of group B deposited with different in-clined angle.

peaks emerge and become higher, which indicates a process of assist-ing ion beam inducing preferential in-plane texture gradually with Eiincreasing under weak beam region.

The in-plane effect of assisting is anisotropic. This anisotropic ef-fect breaks the in-plane symmetry and induces preferential in-planeorientation. The construction of preferential in-plane orientationneeds higher energy than random orientation does. Assisting ion en-ergy is much higher than the energy of deposition atoms, and theadatoms of IBAD films have higher surface mobility. The effects ofAIB on the growing films mainly include selective resputtering andanisotropic damage. In the low ion energy and flux region as in ourexperiments, the selective resputtering effect can be neglected, andanisotropic damage is the main effect. The (001) aligned grains withtheir (111) planes facing assisting ions are favored by anisotropicassisting ions. In low ion current region, as Ei increase, a (001)-orientedbiaxial texture was induced little by little, as shown in Fig. 4. Underoptimal parameters, a perfect biaxial texture can be obtained.

Fig. 5 shows the results of ion beam assisted deposited YSZ filmswith the inclined angle of 45° (group D, Fig. 1(d)). Panel (a) showsthe θ–2θ scan results of IBAD sample of normal and inclined deposi-tion at Es=1000 eV and assisting ion current of 0.10 mA/cm2. As acontrast, the results of IBAD films normally deposited with the sameion energy and current are also shown in Fig. 5. The deposition timeof normally and inclined deposited films is 60 and 90 min respective-ly, so the thickness of the two films is nearly the same. We can seethat at the Ei of both 100 eV and 200 eV, (002) peak in inclined depos-ited sample is much higher than that of normal deposition. This resultshows that inclined deposition can remarkably enhance the (001)orientation.

The upper curve in Fig. 5(b) is the ϕ-scan results of inclined depo-sition sample with Ei=200 eV. There are 8 peaks in the curve. Theresult of ion assisted normal deposition (shown in Fig. 4(c)), was nor-malized by the peak positions and peak value of inclined deposition(upper curve), and is shown in Fig. 5(c) as middle curve. We subtractthe middle curve from the upper curve, and get the lower curve. Inother words, the upper curve is the sum of the middle and lowercurve. The middle and lower curves have the characteristics of prefer-ential in-plane orientation. So we can say that there are two main in-plane orientations (shown by middle and lower curves respectively)in this inclined deposition films. The (001) axes of both in-plane ori-entation are aligned along substrate normal line. Interestingly, the

a) c)

b) d)

Fig. 4. XRD results of the IBAD-YSZ films. (a): θ–2θ scan result of the sample deposited at optimal parameters; (b): θ–2θ scan results of samples of group C deposited with differentassisting ion energy at low beam current (0.10 mA/cm2); (c): ϕ-scan result at YSZ (111) reflection of the sample deposited at optimal parameters; and (d): ϕ-scan results of sam-ples of group C. The optimal parameters are ion beam current of 0.5 mA/cm2 and ion energy of 300 eV.

1118 Z. Wang et al. / Thin Solid Films 520 (2011) 1115–1119

valleys of lower curve lie at the position of the peaks of middle curve,which indicates that the angle between the in-plane basic axes ofgrains of the two orientations is 45°, as shown in Fig. 6.

The enhancement of (001) orientation and the preferential in-plane orientation can be attributed to the effects of depositionatoms and assisting ions. For ion assisted inclined deposition, besidea part of momentum of deposition atoms preserved along substratesurface, a part of the assisting ion energy is transported to the grow-ing films. Therefore the surface mobility of the adatoms is higher thanthat in groups A, B, and C. So a type of biaxial texture, as a structure oflower energy state than any fiber texture, can develop. Furthermore,the anisotropic in-plane effect of deposition atoms and AIB can helppreferential in-plane orientations develop.

The process of the development of the two preferential in-plane ori-entations is shown in Fig. 7. Deposition atom flux and assisting ion fluxdetermine a plane vertical to the substrate. The influence of depositionatoms and assisting ions is to make some low index axes aligned in thisplane. For the deposition incidence of 45°, the deposition favors the

a)

Fig. 5. XRD results of the film of ion assisted inclined deposition sample (group D). (a) pattsame ion energy and current are also shown; (b): analysis of the ϕ-scan results at YSZ (001)middle curve: the normalized result of normal deposition IBAD films (200 eV, 0°, shown insubtracting the middle curve from the upper curve, indicating the other preferential in-pla

orientation in which (011) axis is aligned along deposition atom flux,(001) axis is aligned normally, (010) axes and (100) axis is parallel tosubstrate, as shown in Fig. 7(a). This is one of the two coexisting in-plane orientations. The anisotropic damage effect of assisting ions fa-vors the other orientation shown in Fig. 7(b), in which (001) and(111) axes lie in the vertical plane, though the incidence of the AIB isnot 55°. Obviously, the angle between the in-plane basic axes of thetwo orientations is 45°. The (001) axes of both in-plane orientation liealong the normal line of substrates, and (001) orientation is enhancedconsequently. In our experiments, the energy and current of AIB ismuch lower the optimal value, so both the two in-plane orientationscan form. A deeper study is needed for a more clear understanding ofthis kind of competition between in-plane orientations.

4. Conclusions

An orientation competition was studied in 4 groups of ion sputteredYSZ films. The results of this competition are different under different

b)

erns of θ–2θ scan; As a contrast, the results of IBAD films normally deposited with thereflection; upper curve: the ϕ-scan result of inclined deposition sample (200 eV, 45°);Fig. 4(c)), indicating one preferential in-plane orientation; lower curve: the result of

ne orientation.

{010}’

{010}

{100} {100}’

{001}{001}’

45o

{010}

{100}

{001}

{010}’

{100}’

{001}’

Fig. 6. Two (001)-oriented preferential in-plane orientations coexisting in the film of ion assisted inclined deposition samples (group D). The angle between the corresponding in-plane basic axes of the two orientations is 45°.

45o

45o

{010}

{001}{100}

{011}

{010}

{001}

{100}

{111}

Deposition atoms

Assistingions

Deposition atoms

Assistingnsio

a) b)

Fig. 7. Development of the two preferential in-plane orientation coexisting in the filmof ion assisted inclined deposition samples (group D). Deposition atom flux and assist-ing ion flux determined a plane vertical to the substrate. (a): A orientation affected bydeposition atoms, in which (011) axis is aligned along deposition atom flux. (b): Theother orientation affected by assisting ions, in which (011) axis is aligned in the verticalplane.

1119Z. Wang et al. / Thin Solid Films 520 (2011) 1115–1119

experimental configurations and parameters. In non-IBAD YSZ films,(001) and (011) orientations are advantaged at low andhigh depositionenergy respectively. For inclined substrate deposited films, (001), (011)and (111) orientations are advantaged in turn. In IBAD YSZ films of lowassisting ion energy and current, a (001) oriented biaxial texture wasgradually induced by increasing ion energy. In the case of ion assistedinclined deposition of 45°, (001) orientation is enhanced and two in-plane preferential orientation can coexist. Maybe the direct factors

affecting the results of competition are surface binding energy and theanisotropy (or isotropy) of the main effects of assisting ions and depo-sition atoms.

Acknowledgments

The research is financially supported by National Natural ScienceFoundation of China (Project 51002010 and Project 50802047).

References

[1] Y. Iijima, N. Tanabe, Y. Ikeno, O. Kohno, Phys. C 185–189 (1991) 1959.[2] V. Matias, B.J. Gibbons, D.M. Feldmann, Phys. C 460–462 (2007) 312.[3] L. Dong, L.A. Zepeda-Ruiz, D.J. Srolovitz, J. Appl. Phys. 89 (2001) 4105.[4] R.T. Brewera, H.A. Atwater, Appl. Phys. Lett. 80 (2002) 3388.[5] S. Gnanarajan, N. Savvides, Thin Solid Films 350 (1999) 124.[6] L.S. Yu, J.M.E. Harper, J.J. Cuomo, D.A. Smith, Appl. Phys. Lett. 47 (1985) 932.[7] Tao Feng, Bingyao Jiang, Sun Zhuo, Xi Wang, Xianghuai Liu, Appl. Surf. Sci. 254

(2008) 1565.[8] F. Pan, X.W. Li, Y.Q. Ding, F. Zeng, Surf. Coat. Technol. 201 (2007) 4832.[9] W. Ensinger, Nucl. Instrum. Meth. Phys. Res. B 106 (1995) 142.

[10] K.G. Ressler, N. Sonnenberg, M.J. Cima, J. Am. Ceram. Soc. 80 (1997) 2637.[11] R.M. Bradley, J.M.E. Harper, D.A. Smith, J. Appl. Phys. 60 (1986) 4160.[12] N. Sonnenberg, A.S. Longo, M.J. Cima, B.P. Chang, K.G. Ressler, P.C. McIntyre, Y.P.

Liu, J. Appl. Phys. 74 (1993) 1027.[13] M. Matsuoka, S. Isotani, S. Miyake, Y. Setsuhara, K. Ogata, N. Kuratani, J. Appl.

Phys. 80 (1996) 1177.[14] L. Dong, D.J. Srolovitza, Appl. Phys. Lett. 75 (4) (1999) 584.[15] W. Oleszkiewicza, P. Romiszowskib, Vacuum 70 (2003) 347.[16] Z. Wang, B.J. Yan, F. Feng, H. Chen, K. Shi, Z. Han, Phys. C 470 (2010) 622.