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Dysprosium substituted Ce:YIG thin filmswith perpendicular magnetic anisotropyfor silicon integrated optical isolatorapplicationsCite as: APL Mater. 7, 081119 (2019); https://doi.org/10.1063/1.5112827Submitted: 04 June 2019 . Accepted: 05 August 2019 . Published Online: 22 August 2019

Yan Zhang, Qingyang Du, Chuangtang Wang, Wei Yan, Longjiang Deng, Juejun Hu, Caroline A. Ross,and Lei Bi

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Dysprosium substituted Ce:YIG thin filmswith perpendicular magnetic anisotropyfor silicon integrated opticalisolator applications

Cite as: APL Mater. 7, 081119 (2019); doi: 10.1063/1.5112827Submitted: 4 June 2019 • Accepted: 5 August 2019 •Published Online: 22 August 2019

Yan Zhang,1 Qingyang Du,2 Chuangtang Wang,1 Wei Yan,1 Longjiang Deng,1 Juejun Hu,2 Caroline A. Ross,2and Lei Bi1,a)

AFFILIATIONS1National Engineering Research Center of Electromagnetic Radiation Control Materials, University of Electronic Scienceand Technology of China, Chengdu 610054, People’s Republic of China

2Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge,Massachusetts 02139, USA

a)Author to whom correspondence should be addressed: bilei@uestc.edu.cn

ABSTRACTIn this report, dysprosium substituted Ce1Y2Fe5O12 (Ce:YIG) thin films (Dy:CeYIG) with perpendicular magnetic anisotropy (PMA) aresuccessfully deposited on silicon and silicon-on-insulator waveguides by pulsed laser deposition. The structural, magnetic, and magneto-optical properties of Dy:CeYIG films are investigated. We find that increasing dysprosium concentration leads to a decreased out-of-planemagnetic saturation field. Dy2Ce1Fe5O12 and Dy2Ce1Al0.4Fe4.6O12 thin films show PMA dominated by magnetoelastic effects due to thermalmismatch strain. These films further exhibit high Faraday rotation and low optical loss. Our work demonstrates that Dy substitution isan effective way to induce PMA in Ce:YIG thin films without compromising their magneto-optical figure of merit, making this materialpromising for self-biased transverse electric mode optical isolator applications.

© 2019 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license(http://creativecommons.org/licenses/by/4.0/). https://doi.org/10.1063/1.5112827., s

I. INTRODUCTION

Integrated optical isolators or circulators are essential compo-nents of photonic integrated circuits (PICs) and optical communi-cation systems.1,2 Optical isolators allow one-way transmission oflight, protecting sensitive components such as lasers and amplifiersfrom harmful reflections. Optical isolation can be achieved by break-ing the time-reversal symmetry of light propagation using nonrecip-rocal properties of magneto-optical materials. At the telecommuni-cation wavelengths, cerium doped yttrium iron garnet (Ce:YIG) isone of the best magneto-optical (MO) materials because of its largemagneto-optical activity and low optical loss, which produces anexceptional magneto-optical figure of merit (FOM, defined as theratio of Faraday rotation (FR) to optical loss).3 The optical isolator

structure based on magneto-optical nonreciprocal phase shift(NRPS) effects was demonstrated using epitaxial Ce:YIG films asthe guiding layer,4 by bonding epitaxial Ce:YIG films onto semicon-ductor waveguides5–9 or by monolithically depositing polycrystallineCe:YIG with YIG seed layers on silicon photonic devices.2,10–12

Recent advances have led to waveguide optical isolators with per-formances approaching that of bulk optical isolators,11,12 qualifyingthem as viable isolation solutions for PIC applications.

However, the vast majority of the waveguide magneto-opticalisolators reported so far operate only for the transverse mag-netic (TM) polarization.13 As most semiconductor lasers emit intothe transverse electric (TE) mode, TE mode isolators are essen-tial for many applications. To achieve TE mode isolators withoutpolarization rotators,9,14,15 the magneto-optical material must be

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incorporated into a horizontally asymmetric waveguide structure togenerate a NRPS. One approach to create such in-plane (IP) asym-metry is to use a magneto-optical rib waveguide made of a gar-net with an out-of-plane (OP) easy axis of magnetization, where adomain wall is placed at the center of the rib. A nonreciprocal TE-mode isolator based on the approach was theoretically analyzed16

and later experimentally demonstrated.17 A TE-mode NRPS can alsobe produced by placing a magneto-optical cladding on one side ofa waveguide. For example, a TE-mode Mach-Zehnder Interferome-ter (MZI) isolator was fabricated in a Si:H waveguide with a Ce:YIGgarnet bonded on one sidewall. The device shows a maximum isola-tion of 17.9 dB at a wavelength of 1561.7 nm, although it requiresan out-of-plane magnetic field to orient the magnetization of thegarnet.18 In our previous work, we experimentally demonstrated amonolithically integrated TE mode isolator with a maximum iso-lation of 30 dB, again necessitating an out-of-plane magnetic fieldto bias the garnet film.12 For TE isolators based on the asymmet-ric magneto-optical cladding configuration, it is desirable to developa magneto-optical material which exhibits perpendicular magneticanisotropy (PMA) to facilitate operation with a low applied mag-netic field or even self-biased operation. Meanwhile, the materialmust exhibit a high magneto-optical FOM to ensure low-loss opticalisolation.

Ce:YIG thin films grown on garnet or silicon substrates showan in-plane easy axis of magnetization and require magnetic fieldsas high as ∼2 kOe to saturate the film in the out-of-plane (OP)direction.10 For Ce:YIG thin films deposited on silicon substrates,the large difference in thermal expansion coefficients induces in-plane tensile strains in Ce:YIG and magnetic anisotropy due tomagnetoelastic effects. Due to the positive magnetoelastic coeffi-cient of Ce:YIG, the tensile strain favors in-plane magnetizationand increases the OP saturation field.10,19,20 On the other hand,PMA may be achieved in a substituted garnet with negative mag-netostriction grown on Si. This has been observed in Bi-substitutedDy3Fe5O12 (DyIG) thin films whose OP hysteresis loops exhibita squareness ratio (remanence/saturation, Mr/Ms) of 121,22 and atunable coercivity by incorporating nonmagnetic ions.23 However,optical loss in the material was not reported. The negative magne-tostriction coefficient of DyIG suggests that Dy substitution maycompensate or even overcome the positive magnetoelastic responseof Ce ions in Ce:YIG films,24 leading to PMA DyCeYIG thin filmswith large magneto-optical coefficients essential for TE mode deviceintegration.

In this paper, we report the fabrication of Dy-substitutedCe:YIG films with PMA on silicon via a single-step growth method.We also investigated Al substitution on the Fe sites, which low-ers the shape anisotropy and further promotes PMA.24 The struc-tural, magnetic, and optical properties of the films were systemat-ically characterized with varying Dy concentrations as detailed inSecs. II–IV.

II. EXPERIMENTAL METHODS74 nm thick Dy:CeYIG thin films with a 42 nm thick YIG top

seed layer were grown on (100) Si and silicon-on-insulator (SOI)substrates by pulsed laser deposition (PLD). According to our priorstudy, the YIG seed layer crystallizes first and then serves as a tem-plate to promote the crystallization of the substituted Dy:CeYIG

film.10 The SOI substrate consisted of a 220 nm Si layer on top ofa 3 μm SiO2 cladding layer. Targets of Ce1Y2Fe5O12, Ce1Dy2Fe5O12,and Ce1Dy2Al1Fe4O12 were ablated by using a 10 Hz, 248 nm Com-pex Pro KrF excimer laser. The laser fluence was 2.5 J/cm2 and thedistance between the target and the substrate was 55 mm. The sub-strate temperature was 400 ○C and oxygen pressure was 10 mTorrduring the deposition process. The laser pulse ratios were chosento yield Ce1DyxY2−xFe5O12 films with x = 2.00, 0.87, and 0, andCe1Dy2Al0.42Fe4.58O12 films. The stoichiometries were verified by X-ray photoelectron spectroscopy. The compositions are designatedas Dy20, Dy08, Dy00, and Dy20-Al04. The films were subsequentlyrapid thermal annealed at 850 ○C at an oxygen gas partial pressureof 2 Torr for 3 min. The crystal structure was characterized by X-ray diffraction (XRD) (Shimadzu XRD-7000 X-ray diffractometer)with a Cu-Kα radiation source. Atomic force microscopy (AFM) wasused to determine the film surface morphology. Room temperaturein-plane (IP) or OP magnetic hysteresis loops were measured byvibrating sample magnetometry (VSM). The MO hysteresis loopswere measured by using a custom-built Faraday effect characteri-zation system19 with field and light propagation directions out ofplane.

To accurately measure the optical loss, 220 nm thick, 500 nmwide Si channel waveguides were fabricated on SOI wafers by stan-dard planar processes. The Si waveguide structures were first formedusing an Elionix ELS-F125 electron beam lithography (EBL) systemto expose a negative HSQ (hydrogen silsesquioxane) resist followedby reactive ion etching (RIE) with Cl2 gases. A 600 nm thick SiO2layer was deposited onto the patterned wafer to isolate the opticalmodes from interacting with the overlaid MO materials except atpredefined window regions. The SiO2 layer was first formed by spin-ning a 400 nm HSQ layer followed by rapid thermal annealing at800 ○C for 5 min, then followed by PECVD of another SiO2 layer of200 nm. The process yields a planarized SiO2 top surface facilitatingsubsequent window opening. Windows with different lengths werepatterned into the SiO2 cladding layer to expose the Si waveguidesurface by RIE with a gas mixture of CHF3 and Ar.12 Ce:DyIG/YIGthin films were then deposited on the waveguides. The transmissionloss was characterized on a fiber butt coupled waveguide test station.The propagation loss of Ce:DyIG thin films was calculated from themeasured propagation loss and simulated confinement factors in Siand Ce:DyIG.

III. FILM STRUCTURE AND MAGNETIC PROPERTIESFigures 1(a)–1(c) show X-ray diffraction spectra for Dy:CeYIG

films with a YIG top seed layer. A sharp peak located at 2θ = 32.96○

is attributed to the substrate Si (200). All films show characteristicpeaks of polycrystalline garnet thin films for both the YIG seed layerand the Dy:CeYIG layers. The lattice constant of Dy:CeYIG increaseswith Dy content, from 12.37 Å in Dy00 to 12.41 Å in Dy20. This isbecause the ionic radius of Y3+ ions (1.02 Å) is smaller than that ofDy3+ ions (1.083 Å). Substituting Fe3+ ions with Al3+ decreases thelattice constant given the smaller radius of the Al3+ ion (0.535 Å)compared to that of the Fe3+ ion (0.645 Å). The lattice constant ofthe top YIG seed layer is 12.30 Å. A wide range θ−2θ scan in Fig. 1(c)confirms that the film consists of a pure garnet phase. The averagecrystal grain size of the films was estimated using the Scherrer equa-tion to be 51–66 nm. The root mean square (RMS) surface roughness

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FIG. 1. X-ray diffraction patterns of dysprosium and aluminum substituted Ce:YIG with a top YIG seed layer in a 2θ range from (a) 25○ to 40○ and (c) 15○ to 60○. (b) Thezoomed-in spectrum of the (420) diffraction peaks. AFM images showing (d) height and (e) phase maps.

of sample Dy20-Al04/YIG is 0.6 nm, as shown in Fig. 1(d). The phaseimage in Fig. 1(e) clearly shows the grain boundaries, from whichan average grain size of 64 nm is inferred, consistent with the XRDresults.

Figures 2(a)–2(e) show the room temperature magnetic hys-teresis loops of YIG and Dy:CeYIG/YIG films. By subtracting the

contribution of the top YIG layer (∼136 emu/cm3), saturation mag-netization (Ms) of the Dy00 film (i.e., the film without Dy sub-stitution) reaches 161 emu/cm3, slightly higher than that of YIG.The Ms decreases to 114 emu/cm3 in Dy20 and further drops to68 emu/cm3 in Dy20-Al04 as shown in Fig. 2(f). In the ferrimag-netic Dy:CeYIG lattice, the tetrahedral Fe3+ (3/formula unit) and

FIG. 2. Room temperature magnetization hysteresis loops for dysprosium and aluminum substituted Ce:YIG/YIG bilayers: (a) YIG; (b) Dy00; (c) Dy08; (d) Dy20; (e) Dy20-Al04.(f) Saturation magnetization as a function of Dy3+ concentration.

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octahedral Fe3+ (2/formula unit) are coupled antiferromagneti-cally and the dodecahedral sites (Dy, Ce) couple with the octahe-dral Fe3+. Above the compensation temperature (220 K for bulkDy3Fe5O12), the net magnetic moment M is given by M = Md −(MCe

c + MDyc + Ma), where MCe

c , MDyc , Md, and Ma are the magnetic

moments of dodecahedral Ce3+, dodecahedral Dy3+, tetrahedralFe3+, and octahedral Fe3+ sites, respectively. The different temper-ature dependence of the Fe and Ce sublattices leads to a higher satu-ration magnetization at higher temperatures in Dy00.19 The antipar-allel alignment between MDy

c and Md contributes to the reduction ofMs when Dy3+ substitutes for nonmagnetic Y3+.25 The decrease ofMs in Dy20-Al04 is due to nonmagnetic Al3+ ions substituting forFe3+ ions in the tetrahedral sites.26

The magnetic hysteresis loops of the Dy:CeYIG/YIG films inFig. 2 indicate an out-of-plane easy axis for the Dy20 and Dy20-Al04 bilayer films and in-plane for YIG and Dy00. The Dy08/YIGbilayer shows isotropic behavior with both IP and OP satura-tion fields of ∼2.5 kOe. The Dy20/YIG film exhibits a coerciv-ity of 230 Oe, which increases to 380 Oe for Dy20-Al04. The netanisotropy in polycrystalline Dy:CeYIG thin films includes con-tributions from shape anisotropy, magnetoelastic anisotropy, andmagnetocrystalline anisotropy. The last term is negligible due to ran-dom alignment of the polycrystalline grains as confirmed by XRD.For YIG thin films, the uniaxial anisotropy Ku(YIG) is calculatedto be (10.4 ± 0.3) × 104 erg cm−3, based on the difference in areasbetween the OP and IP magnetic hysteresis loops,27 which is com-parable to the calculated shape anisotropy of 2πMs

2 = 11.6 × 104

erg cm−3, indicating that the dominant contribution to magneticanisotropy in YIG is shape anisotropy.

For the sample Dy08/YIG, the isotropic behavior indicates thatthe stress-induced anisotropy is balanced by the shape anisotropyin Dy08 and the top YIG layer. Assuming the two layers are mag-netically coupled, the total uniaxial anisotropy can be considered asthe volume average of the top layer Ku(YIG) and the bottom layerKu(Dy08). The YIG and Dy08 have a similar shape anisotropy andthe Dy08 is assumed to have an additional magnetoelastic anisotropyKλ(Dy08) opposite in sign to the shape anisotropy. The measurednet anisotropy of the bilayer is Kυ = Kλ−2πMs

2 ≅ 0, where Kλ(Dy08)= ζKλ with ζ being the ratio of the thickness of the bilayer:thicknessof the Dy08 = 1.57. We estimate the stress induced anisotropyKλ(Dy08) = 17.2 × 104 erg cm−3. The stress induced anisotropy Kλis given by24

Kλ = −32λs

Y1 − ν(αf − αs)ΔT,

where λs is the magnetostriction coefficient for a polycrystallinematerial, Y is the Young’s modulus of the film (YIG: 200 GPa= 2 × 1012 dyn cm−2), ν is the Poisson’s ratio (YIG:0.29), αf and αsare the linear thermal expansion coefficients of the film and the sub-strate, respectively (YIG:10.4 × 10−6 K−1, Si:3 × 10−6 K−1), and ΔTis the temperature difference (equal to 830 ○C in our case). We esti-mate the magnetostriction coefficient of Ce1Dy0.87Y1.13Fe5O12 to beλs ≈ −6.6 × 10−6.

IV. MAGNETO-OPTICAL AND OPTICAL PROPERTIESThe room-temperature OP Faraday rotation hysteresis loops

of Dy:CeYIG/YIG films are plotted in Fig. 3. The Faraday rotation

of the Dy00 film reaches −2430○/cm at 1550 nm, lower than pre-vious reports of polycrystalline Ce:YIG thin films19,20 possibly dueto the use of the top YIG seed layer in this work. With Dy sub-stitution, the films show higher Faraday rotations (−2830○/cm forDy08 and −2840○/cm for Dy20 at 1550 nm), whereas Al substi-tution results in a reduction of Faraday rotation to −2080○/cm inDy20-Al04/YIG compared to Dy20/YIG at 1550 nm. Wavelength-dependent Faraday rotation measurements [Fig. 3(e)] reveal that theCe:DyIG films, similar to Ce:YIG, exhibit reduced Faraday rotationat longer wavelengths. The observation indicates that the enhancedFaraday rotation of Ce:DyIG in the near infrared region comparedto YIG is also mainly due to the electric dipole transition of the Ce3+

4f 1 electron to tetrahedral Fe3+ in the Ce:DyIG.3,28 A possible expla-nation of the enhanced Faraday rotation in Dy-substituted samplesis summarized as follows. Dy introduces positive Faraday rotation inDy3Fe5O12 crystals at a wavelength of 1.06 μm of +310○/cm29 whichis expected to counteract the negative Faraday rotation in Ce:YIGand lower the magnitude of Faraday rotation. However, substitut-ing Dy also increases the lattice constant due to the larger ionicradius of Dy3+ vs Y3+, which may stabilize more Ce ions in the3+ valence state and thus increase Faraday rotation.28,30 Moreover,Al3+ ions preferentially substitute into the tetrahedral sites in gar-net lattices,26 thus diminishing the electric dipole transition betweenCe3+ and tetrahedral Fe3+ and the Faraday rotation in the Dy20-Al04sample.

An advantage of characterizing hysteresis via Faraday rotationis that the contribution of the top YIG layer is small (YIG:θf ∼200○/cm, Ce:YIG: θf ∼ −3000○/cm at 1550 nm) so that the Fara-day hysteresis loop originates primarily from the bottom layerDy:CeYIG, unlike the VSM data where the magnetic signal of YIGis comparable to that of the Dy:CeYIG. The OP saturation fieldis ∼3 kOe in Dy00 and decreases to 600 Oe in Dy08. Dy20/YIGand Dy20-Al04/YIG exhibit square loops indicating perpendicularmagnetic anisotropy. Figure 3(f) shows the ratio θr/θs and coerciv-ity obtained from the Faraday hysteresis loops. A squareness ratioof 1 is obtained in Dy20-Al04/YIG with the highest coercivity of500 Oe.

The PMA of Ce:DyIG films is derived from the negative mag-netostriction contributed by the Dy3+ combined with tensile straininduced during cooling after rapid thermal annealing (RTA). Thetensile strain results from both volume changes during crystal-lization and the thermal mismatch between the film and the sub-strate. To illustrate the effects of annealing, Dy20-Al04/YIG filmswere grown using single-step and two-step deposition methods attwo different RTA temperatures. The results are summarized inFig. 4. For two-step deposition, a YIG seed layer was first depositedat 400 ○C followed by RTA at 800 ○C, and then the Dy20-Al04film was deposited at 650 ○C with a cooling rate of 1 ○C/min toallow for strain relaxation. The OP Faraday loop has low square-ness and a saturation field of 1.4 kOe, much smaller than thatof Ce:YIG (2–3 kOe) fabricated using a two-step deposition pro-cess.19 However, the film grown using the single-step depositionprocess (where Dy:CeYIG is grown first followed by YIG deposi-tion and annealing of the bilayer) shows a square OP hysteresisloop indicating PMA, and an OP coercivity of ∼1 kOe is obtainedwith an optimal RTA temperature of 900 ○C, as shown in Fig. 4(b).Although the same concentration of Dy is present, the difference inmagnetic anisotropy of the one-step and two-step deposited films

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FIG. 3. Faraday rotation (FR) hysteresis loops of dysprosium and aluminum substituted CeYIG/YIG bilayers: (a) Dy00; (b) Dy08; (c) Dy20; (d) Dy20-Al04. Curves of differentcolors correspond to measurements taken at different wavelengths. (e) Faraday rotation of the films as functions of wavelength. (f) The ratio between the remanent andsaturation Faraday rotation (black symbols) and coercivity (red symbols) as functions of composition.

reveals that the microstructure and strain state depend on thefabrication method.

In order to determine the optical transmission loss ofDy:CeYIG/YIG films, 500 nm wide straight SOI waveguides werefabricated with window lengths ranging from 50 μm to 4 mm anda fixed width of 10 μm [Fig. 5(a)]. By measuring the waveguide losswith different window lengths, the optical loss of Si/Dy:CeYIG/YIGcan be determined. A cross-sectional SEM image of the win-dow area with the garnet cladding layer is shown in Fig. 5(b).The transmission loss as a function of window lengths is plot-ted in Fig. 5(c). The garnet/Si waveguide loss is thereby deter-mined to be 39.9 ± 0.9 dB/cm. The optical loss of Dy:CeYIG

films is inferred by accounting for the confinement factors in thegarnet film. The waveguide modal profile of the Hx field ampli-tude is simulated using the finite element software COMSOL Mul-tiphysics and plotted in Fig. 5(d). The simulated confinement fac-tor in the Dy:CeYIG film is 41%, corresponding to an opticalloss of ∼91 dB/cm in Dy20. The loss value gives a magneto-optical FOM of 31○/dB. This value is only slightly lower thanthat of polycrystalline Ce:YIG films on Si with in-plane anisotropy(38○/dB).12 We note that strain relaxation at the edges of the gar-net film in Fig. 5(b) affects the local magnetic anisotropy anddomain state, but this does not influence the optical absorptionmeasurements.

FIG. 4. Faraday rotation (FR) hysteresis loops at 1310 nmof (a) Dy20-Al04 deposited by a two-step method (black)and a single-step method (red). (b) The effect of annealingat 850 ○C (black) and 900 ○C (red) on Faraday rotation ofDy20-Al04.

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FIG. 5. (a) Optical microscope image, (b) SEM cross-sectional image, and (d) simulated Hx field distributionof Si/Dy20 waveguides. (c) The transmission loss for a500 nm wide Si/Dy20 waveguide.

V. CONCLUSIONA magneto-optical thin film with perpendicular magnetic

anisotropy, Dy-substituted Ce:YIG, was grown on silicon and SOIsubstrates by PLD using a YIG top seed layer. By replacing Y ionswith Dy, perpendicular magnetic anisotropy is obtained as a result ofmagnetoelastic anisotropy, while maintaining a Faraday rotation upto −2840○/cm in the film and a high magneto-optical figure of meritof 31○/dB, comparable to that of CeYIG with in-plane anisotropy.Moreover, the top seed layer process enables direct contact betweenDy:CeYIG and the Si waveguide core to enhance the device nonre-ciprocal phase shift. The tunable anisotropy and excellent magneto-optical performance of Dy:CeYIG films provide opportunities fornew magneto-optical devices. In particular, the high remanence andcoercivity in the out of plane direction make Dy:CeYIG a promis-ing material candidate for magnet-free, self-biased TE mode opticalisolators.

ACKNOWLEDGMENTSThe authors are grateful for support by the National Natural

Science Foundation of China (NSFC) (Grant Nos. 51522204 and61475031) and Ministry of Science and Technology of the People’sRepublic of China (MOST) (Grant No. 2016YFA0300802). Q.D.,J.H., and C.A.R. acknowledge support from the National ScienceFoundation (NSF) (Grant No. 1607865).

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APL Mater. 7, 081119 (2019); doi: 10.1063/1.5112827 7, 081119-7

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