Dysprosium substituted Ce:YIG thin films with perpendicular magnetic anisotropy for silicon integrated optical isolator applications

In this report, dysprosium substituted Ce 1 Y 2 Fe 5 O 12 (Ce:YIG) thin films (Dy:CeYIG) with perpendicular magnetic anisotropy (PMA) are successfully 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-plane magnetic saturation field. Dy 2 Ce 1 Fe 5 O 12 and Dy 2 Ce 1 Al 0.4 Fe 4.6 O 12 thin films show PMA dominated by magnetoelastic effects due to thermal mismatch strain. These films further exhibit high Faraday rotation and low optical loss. Our work demonstrates that Dy substitution is an effective way to induce PMA in Ce:YIG thin films without compromising their magneto-optical figure of merit, making this material promising for self-biased transverse electric mode optical isolator applications.


I. INTRODUCTION
Integrated optical isolators or circulators are essential components of photonic integrated circuits (PICs) and optical communication systems. 1,2 Optical isolators allow one-way transmission of light, protecting sensitive components such as lasers and amplifiers from harmful reflections. Optical isolation can be achieved by breaking the time-reversal symmetry of light propagation using nonreciprocal properties of magneto-optical materials. At the telecommunication wavelengths, cerium doped yttrium iron garnet (Ce:YIG) is one of the best magneto-optical (MO) materials because of its large magneto-optical activity and low optical loss, which produces an exceptional magneto-optical figure of merit (FOM, defined as the ratio 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 as the guiding layer, 4 by bonding epitaxial Ce:YIG films onto semiconductor waveguides [5][6][7][8][9] or by monolithically depositing polycrystalline Ce:YIG with YIG seed layers on silicon photonic devices. 2,[10][11][12] Recent advances have led to waveguide optical isolators with performances approaching that of bulk optical isolators, 11,12 qualifying them as viable isolation solutions for PIC applications.
However, the vast majority of the waveguide magneto-optical isolators reported so far operate only for the transverse magnetic (TM) polarization. 13 As most semiconductor lasers emit into the transverse electric (TE) mode, TE mode isolators are essential for many applications. To achieve TE mode isolators without polarization rotators, 9,14,15 the magneto-optical material must be ARTICLE scitation.org/journal/apm incorporated into a horizontally asymmetric waveguide structure to generate a NRPS. One approach to create such in-plane (IP) asymmetry is to use a magneto-optical rib waveguide made of a garnet with an out-of-plane (OP) easy axis of magnetization, where a domain wall is placed at the center of the rib. A nonreciprocal TEmode isolator based on the approach was theoretically analyzed 16 and later experimentally demonstrated. 17 A TE-mode NRPS can also be produced by placing a magneto-optical cladding on one side of a waveguide. For example, a TE-mode Mach-Zehnder Interferometer (MZI) isolator was fabricated in a Si:H waveguide with a Ce:YIG garnet bonded on one sidewall. The device shows a maximum isolation of 17.9 dB at a wavelength of 1561.7 nm, although it requires an out-of-plane magnetic field to orient the magnetization of the garnet. 18 In our previous work, we experimentally demonstrated a monolithically integrated TE mode isolator with a maximum isolation of 30 dB, again necessitating an out-of-plane magnetic field to bias the garnet film. 12 For TE isolators based on the asymmetric magneto-optical cladding configuration, it is desirable to develop a magneto-optical material which exhibits perpendicular magnetic anisotropy (PMA) to facilitate operation with a low applied magnetic field or even self-biased operation. Meanwhile, the material must exhibit a high magneto-optical FOM to ensure low-loss optical isolation. Ce:YIG thin films grown on garnet or silicon substrates show an in-plane easy axis of magnetization and require magnetic fields as 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 inplane tensile strains in Ce:YIG and magnetic anisotropy due to magnetoelastic effects. Due to the positive magnetoelastic coefficient of Ce:YIG, the tensile strain favors in-plane magnetization and increases the OP saturation field. 10,19,20 On the other hand, PMA may be achieved in a substituted garnet with negative magnetostriction grown on Si. This has been observed in Bi-substituted Dy 3 Fe5O 12 (DyIG) thin films whose OP hysteresis loops exhibit a squareness ratio (remanence/saturation, Mr/Ms) of 1 21,22 and a tunable coercivity by incorporating nonmagnetic ions. 23 However, optical loss in the material was not reported. The negative magnetostriction coefficient of DyIG suggests that Dy substitution may compensate or even overcome the positive magnetoelastic response of Ce ions in Ce:YIG films, 24 leading to PMA DyCeYIG thin films with large magneto-optical coefficients essential for TE mode device integration.
In this paper, we report the fabrication of Dy-substituted Ce:YIG films with PMA on silicon via a single-step growth method. We also investigated Al substitution on the Fe sites, which lowers the shape anisotropy and further promotes PMA. 24 The structural, magnetic, and optical properties of the films were systematically characterized with varying Dy concentrations as detailed in Secs. II-IV.

II. EXPERIMENTAL METHODS
74 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 prior study, the YIG seed layer crystallizes first and then serves as a template to promote the crystallization of the substituted Dy:CeYIG film. 10 The SOI substrate consisted of a 220 nm Si layer on top of a 3 μm SiO 2 cladding layer. Targets of Ce 1 Y 2 Fe5O 12 , Ce 1 Dy 2 Fe5O 12 , and Ce 1 Dy 2 Al 1 Fe 4 O 12 were ablated by using a 10 Hz, 248 nm Compex Pro KrF excimer laser. The laser fluence was 2.5 J/cm 2 and the distance between the target and the substrate was 55 mm. The substrate temperature was 400 ○ C and oxygen pressure was 10 mTorr during the deposition process. The laser pulse ratios were chosen to yield Ce 1 DyxY 2−x Fe5O 12 films with x = 2.00, 0.87, and 0, and Ce 1 Dy 2 Al 0.42 Fe 4.58 O 12 films. The stoichiometries were verified by Xray photoelectron spectroscopy. The compositions are designated as Dy20, Dy08, Dy00, and Dy20-Al04. The films were subsequently rapid thermal annealed at 850 ○ C at an oxygen gas partial pressure of 2 Torr for 3 min. The crystal structure was characterized by Xray diffraction (XRD) (Shimadzu XRD-7000 X-ray diffractometer) with a Cu-Kα radiation source. Atomic force microscopy (AFM) was used to determine the film surface morphology. Room temperature in-plane (IP) or OP magnetic hysteresis loops were measured by vibrating sample magnetometry (VSM). The MO hysteresis loops were measured by using a custom-built Faraday effect characterization system 19 with field and light propagation directions out of plane.
To accurately measure the optical loss, 220 nm thick, 500 nm wide Si channel waveguides were fabricated on SOI wafers by standard planar processes. The Si waveguide structures were first formed using an Elionix ELS-F125 electron beam lithography (EBL) system to expose a negative HSQ (hydrogen silsesquioxane) resist followed by reactive ion etching (RIE) with Cl 2 gases. A 600 nm thick SiO 2 layer was deposited onto the patterned wafer to isolate the optical modes from interacting with the overlaid MO materials except at predefined window regions. The SiO 2 layer was first formed by spinning a 400 nm HSQ layer followed by rapid thermal annealing at 800 ○ C for 5 min, then followed by PECVD of another SiO 2 layer of 200 nm. The process yields a planarized SiO 2 top surface facilitating subsequent window opening. Windows with different lengths were patterned into the SiO 2 cladding layer to expose the Si waveguide surface by RIE with a gas mixture of CHF 3 and Ar. 12 Ce:DyIG/YIG thin films were then deposited on the waveguides. The transmission loss was characterized on a fiber butt coupled waveguide test station. The propagation loss of Ce:DyIG thin films was calculated from the measured propagation loss and simulated confinement factors in Si and Ce:DyIG.

III. FILM STRUCTURE AND MAGNETIC PROPERTIES
Figures 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 characteristic peaks of polycrystalline garnet thin films for both the YIG seed layer and the Dy:CeYIG layers. The lattice constant of Dy:CeYIG increases with Dy content, from 12.37 Å in Dy00 to 12.41 Å in Dy20. This is because the ionic radius of Y 3+ ions (1.02 Å) is smaller than that of Dy 3+ ions (1.083 Å). Substituting Fe 3+ ions with Al 3+ decreases the lattice constant given the smaller radius of the Al 3+ ion (0.535 Å) compared to that of the Fe 3+ ion (0.645 Å). The lattice constant of the 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 average crystal grain size of the films was estimated using the Scherrer equation to be 51-66 nm. The root mean square (RMS) surface roughness of sample Dy20-Al04/YIG is 0.6 nm, as shown in Fig. 1(d). The phase image in Fig. 1(e) clearly shows the grain boundaries, from which an average grain size of 64 nm is inferred, consistent with the XRD results. Figures 2(a)-2(e) show the room temperature magnetic hysteresis loops of YIG and Dy:CeYIG/YIG films. By subtracting the contribution of the top YIG layer (∼136 emu/cm 3 ), saturation magnetization (Ms) of the Dy00 film (i.e., the film without Dy substitution) reaches 161 emu/cm 3 , slightly higher than that of YIG. The Ms decreases to 114 emu/cm 3 in Dy20 and further drops to 68 emu/cm 3 in Dy20-Al04 as shown in Fig. 2(f). In the ferrimagnetic Dy:CeYIG lattice, the tetrahedral Fe 3+ (3/formula unit) and Ma are the magnetic moments of dodecahedral Ce 3+ , dodecahedral Dy 3+ , tetrahedral Fe 3+ , and octahedral Fe 3+ sites, respectively. The different temperature dependence of the Fe and Ce sublattices leads to a higher saturation magnetization at higher temperatures in Dy00. 19 The antiparallel alignment between M Dy c and M d contributes to the reduction of Ms when Dy 3+ substitutes for nonmagnetic Y 3+ . 25 The decrease of Ms in Dy20-Al04 is due to nonmagnetic Al 3+ ions substituting for Fe 3+ ions in the tetrahedral sites. 26 The magnetic hysteresis loops of the Dy:CeYIG/YIG films in Fig. 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/YIG bilayer shows isotropic behavior with both IP and OP saturation fields of ∼2.5 kOe. The Dy20/YIG film exhibits a coercivity of 230 Oe, which increases to 380 Oe for Dy20-Al04. The net anisotropy in polycrystalline Dy:CeYIG thin films includes contributions from shape anisotropy, magnetoelastic anisotropy, and magnetocrystalline anisotropy. The last term is negligible due to random alignment of the polycrystalline grains as confirmed by XRD. For YIG thin films, the uniaxial anisotropy Ku(YIG) is calculated to be (10.4 ± 0.3) × 10 4 erg cm −3 , based on the difference in areas between the OP and IP magnetic hysteresis loops, 27 which is comparable to the calculated shape anisotropy of 2πMs 2 = 11.6 × 10 4 erg cm −3 , indicating that the dominant contribution to magnetic anisotropy in YIG is shape anisotropy.
For the sample Dy08/YIG, the isotropic behavior indicates that the stress-induced anisotropy is balanced by the shape anisotropy in Dy08 and the top YIG layer. Assuming the two layers are magnetically coupled, the total uniaxial anisotropy can be considered as the volume average of the top layer Ku(YIG) and the bottom layer Ku(Dy08). The YIG and Dy08 have a similar shape anisotropy and the Dy08 is assumed to have an additional magnetoelastic anisotropy K λ (Dy08) opposite in sign to the shape anisotropy. The measured net anisotropy of the bilayer is Kυ = K λ −2πMs 2 ≅ 0, where K λ (Dy08) = ζK λ with ζ being the ratio of the thickness of the bilayer:thickness of the Dy08 = 1.57. We estimate the stress induced anisotropy K λ (Dy08) = 17.2 × 10 4 erg cm −3 . The stress induced anisotropy K λ is given by 24 where λs is the magnetostriction coefficient for a polycrystalline material, Y is the Young's modulus of the film (YIG: 200 GPa = 2 × 10 12 dyn cm −2 ), ν is the Poisson's ratio (YIG:0.29), α f and αs are the linear thermal expansion coefficients of the film and the substrate, respectively (YIG:10.4 × 10 −6 K −1 , Si:3 × 10 −6 K −1 ), and ΔT is the temperature difference (equal to 830 ○ C in our case). We estimate the magnetostriction coefficient of Ce 1 Dy 0.87 Y 1.13 Fe5O 12 to be λs ≈ −6.6 × 10 −6 .

IV. MAGNETO-OPTICAL AND OPTICAL PROPERTIES
The 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 previous reports of polycrystalline Ce:YIG thin films 19,20 possibly due to the use of the top YIG seed layer in this work. With Dy substitution, the films show higher Faraday rotations (−2830 ○ /cm for Dy08 and −2840 ○ /cm for Dy20 at 1550 nm), whereas Al substitution results in a reduction of Faraday rotation to −2080 ○ /cm in Dy20-Al04/YIG compared to Dy20/YIG at 1550 nm. Wavelengthdependent Faraday rotation measurements [ Fig. 3(e)] reveal that the Ce:DyIG films, similar to Ce:YIG, exhibit reduced Faraday rotation at longer wavelengths. The observation indicates that the enhanced Faraday rotation of Ce:DyIG in the near infrared region compared to YIG is also mainly due to the electric dipole transition of the Ce 3+ 4f 1 electron to tetrahedral Fe 3+ in the Ce:DyIG. 3,28 A possible explanation of the enhanced Faraday rotation in Dy-substituted samples is summarized as follows. Dy introduces positive Faraday rotation in Dy 3 Fe5O 12 crystals at a wavelength of 1.06 μm of +310 ○ /cm 29 which is expected to counteract the negative Faraday rotation in Ce:YIG and lower the magnitude of Faraday rotation. However, substituting Dy also increases the lattice constant due to the larger ionic radius of Dy 3+ vs Y 3+ , which may stabilize more Ce ions in the 3+ valence state and thus increase Faraday rotation. 28,30 Moreover, Al 3+ ions preferentially substitute into the tetrahedral sites in garnet lattices, 26 thus diminishing the electric dipole transition between Ce 3+ and tetrahedral Fe 3+ and the Faraday rotation in the Dy20-Al04 sample.
An advantage of characterizing hysteresis via Faraday rotation is that the contribution of the top YIG layer is small (YIG:θ f ∼ 200 ○ /cm, Ce:YIG: θ f ∼ −3000 ○ /cm at 1550 nm) so that the Faraday hysteresis loop originates primarily from the bottom layer Dy:CeYIG, unlike the VSM data where the magnetic signal of YIG is comparable to that of the Dy:CeYIG. The OP saturation field is ∼3 kOe in Dy00 and decreases to 600 Oe in Dy08. Dy20/YIG and Dy20-Al04/YIG exhibit square loops indicating perpendicular magnetic anisotropy. Figure 3(f) shows the ratio θr/θs and coercivity obtained from the Faraday hysteresis loops. A squareness ratio of 1 is obtained in Dy20-Al04/YIG with the highest coercivity of 500 Oe.
The PMA of Ce:DyIG films is derived from the negative magnetostriction contributed by the Dy 3+ combined with tensile strain induced during cooling after rapid thermal annealing (RTA). The tensile strain results from both volume changes during crystallization and the thermal mismatch between the film and the substrate. To illustrate the effects of annealing, Dy20-Al04/YIG films were grown using single-step and two-step deposition methods at two different RTA temperatures. The results are summarized in Fig. 4. For two-step deposition, a YIG seed layer was first deposited at 400 ○ C followed by RTA at 800 ○ C, and then the Dy20-Al04 film was deposited at 650 ○ C with a cooling rate of 1 ○ C/min to allow for strain relaxation. The OP Faraday loop has low squareness and a saturation field of 1.4 kOe, much smaller than that of Ce:YIG (2-3 kOe) fabricated using a two-step deposition process. 19 However, the film grown using the single-step deposition process (where Dy:CeYIG is grown first followed by YIG deposition and annealing of the bilayer) shows a square OP hysteresis loop indicating PMA, and an OP coercivity of ∼1 kOe is obtained with an optimal RTA temperature of 900 ○ C, as shown in Fig. 4(b). Although the same concentration of Dy is present, the difference in magnetic anisotropy of the one-step and two-step deposited films reveals that the microstructure and strain state depend on the fabrication method. In order to determine the optical transmission loss of Dy:CeYIG/YIG films, 500 nm wide straight SOI waveguides were fabricated with window lengths ranging from 50 μm to 4 mm and a fixed width of 10 μm [ Fig. 5(a)]. By measuring the waveguide loss with different window lengths, the optical loss of Si/Dy:CeYIG/YIG can be determined. A cross-sectional SEM image of the window area with the garnet cladding layer is shown in Fig. 5(b). The transmission loss as a function of window lengths is plotted in Fig. 5(c). The garnet/Si waveguide loss is thereby determined to be 39.9 ± 0.9 dB/cm. The optical loss of Dy:CeYIG films is inferred by accounting for the confinement factors in the garnet film. The waveguide modal profile of the Hx field amplitude is simulated using the finite element software COMSOL Multiphysics and plotted in Fig. 5(d). The simulated confinement factor in the Dy:CeYIG film is 41%, corresponding to an optical loss of ∼91 dB/cm in Dy20. The loss value gives a magnetooptical FOM of 31 ○ /dB. This value is only slightly lower than that of polycrystalline Ce:YIG films on Si with in-plane anisotropy (38 ○ /dB). 12 We note that strain relaxation at the edges of the garnet film in Fig. 5(b) affects the local magnetic anisotropy and domain state, but this does not influence the optical absorption measurements.

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

V. CONCLUSION
A magneto-optical thin film with perpendicular magnetic anisotropy, Dy-substituted Ce:YIG, was grown on silicon and SOI substrates by PLD using a YIG top seed layer. By replacing Y ions with Dy, perpendicular magnetic anisotropy is obtained as a result of magnetoelastic anisotropy, while maintaining a Faraday rotation up to −2840 ○ /cm in the film and a high magneto-optical figure of merit of 31 ○ /dB, comparable to that of CeYIG with in-plane anisotropy. Moreover, the top seed layer process enables direct contact between Dy:CeYIG and the Si waveguide core to enhance the device nonreciprocal phase shift. The tunable anisotropy and excellent magnetooptical performance of Dy:CeYIG films provide opportunities for new magneto-optical devices. In particular, the high remanence and coercivity in the out of plane direction make Dy:CeYIG a promising material candidate for magnet-free, self-biased TE mode optical isolators.