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Abstract
Silicon photonics for data communication requires key components in the O-band (1260 nm−1310 nm). However, very few studies report silicon integrated magneto-optical thin films operating at this wavelength range. In this study, we report a method to fabricate polycrystalline Bi2Tb1Fe5O12 thin films on silicon substrates for O-band nonreciprocal photonic device applications. The films are fabricated by magnetron sputtering at room temperature followed by rapid thermal annealing for crystallization. Pure garnet phase is stabilized by a Y3Fe5O12 seed layer on silicon. The film deposited on silicon-on-insulator (SOI) waveguides showed saturation Faraday rotation of −3300 ± 183 deg/cm, propagation loss of 53.3 ± 0.3 dB/cm and a high figure of merit of 61.9 ± 3.8 deg/dB at 1310 nm wavelength, demonstrating promising potential for O-band integrated nonreciprocal photonic devices.
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1. Introduction
The rapid development of 5 G/6 G technologies and the Internet of Things (IoT) has led to an increased demand for high-speed data communication [1–4]. This demand has boosted the development of silicon photonic integrated circuits (PICs) for optical interconnect applications. The O-band (1260 nm−1310 nm) has gained prominence due to its advantage of reduced dispersion compared to the C-band (1530 nm−1565 nm). A variety of on-chip integrated silicon photonic devices operating in the O-band have been developed [1,5]. In order to ensure low noise operation of lasers and amplifiers, optical isolators and circulators are required in silicon photonic systems [6,7]. However, such devices are still bulky based on single crystals fabricated by liquid phase epitaxy (LPE). With the successful development of lasers on silicon, bulk nonreciprocal photonic devices cannot be adapted to a photonic integrated circuit (PIC) [8–11]. Therefore, it is desired to develop integrated O-band nonreciprocal photonic devices.
So far, only a few reports studied integrated nonreciprocal photonic materials and devices in the O-band. The principles of nonlinearity and space-time modulation have been employed in nonreciprocal devices [12–15]. Such device may be used in the O-band, despite most of the reports are focusing on C band applications. Considering magneto-optical devices, it is important to develop silicon integrated magneto-optical thin films with large Faraday rotation angle and low optical loss at the O-band [3,16–19]. Although Ce doped yttrium iron garnet (Ce:YIG) thin films were developed for C-band integrated optical isolators and circulators, these materials show high optical loss in the O-band [20]. Bi-substituted rare-earth-doped iron garnet (Bi:RIG) thin films are widely used in bulk nonreciprocal photonic devices in the O-band [21–23]. The primary contribution to the Faraday effect observed in Bi:RIG arises from the energy level splitting resulting from the coupling between excited states and spin-orbit interactions [24]. The fabrication techniques of Bi:RIG thin films include pulsed laser deposition (PLD) [4], magnetron sputtering and Liquid Phase Epitaxy (LPE) [22,25]. Several research groups have employed these methods to deposit high figure of merit (FoM), single-phase polycrystalline Bi-doped garnet films onto silicon substrates [26–28]. The bismuth concentration, material figure of merit is still inferior compared to single crystals (Bi0.6Tb2.4Fe5O12) [26,29]. Meanwhile, it is unclear whether a high quality Bi:RIG thin film can be deposited on silicon waveguides.
In this study, we demonstrate Bi2Tb1Fe5O12 (BiTbIG) films grown on silicon by sputtering with high Bi concentration, strong Faraday rotation and high figure of merit. Using room temperature magnetron sputtering and rapid thermal annealing processes, we fabricated polycrystalline BiTbIG thin films on silicon with pure garnet phase. The Faraday rotation measured at 1310 nm reached −3300 ± 183 deg/cm. The propagation loss of BiTbIG thin films on silicon-on-insulator (SOI) waveguides was 53.3 ± 0.3 dB/cm at 1310 nm, yielding a high figure of merit of 61.9 ± 3.8 deg/dB. These results demonstrat silicon integrated BiTbIG is a promising candidate for O-band integrated nonreciprocal photonic devices.
2. Experiment details
YIG and BiTbIG thin films were deposited on silicon (100) and silicon on insulator (SOI) substrates using magnetron sputtering. Stoichiometric YIG and BiTbIG ceramic targets were fabricated by solid state reaction of 99.999% pure Tb2O3, Y2O3, Bi2O3, and Fe2O3 powders. The target material was held in a stationary position. The substrate rotated at a speed of 30 rpm. During the sputtering process, radio frequency power of 90 W was applied. The Ar pressure and flow rate were maintained at 0.5 Pa and 50 sccm respectively. The deposition rates for YIG and BiTbIG were measured to be 0.55 nm/min and 1.08 nm/min respectively.
A 50 nm thick YIG seed layer was first deposit on Si, followed by a rapid thermal annealing process at 850 °C, oxygen partial pressure of 2 Torr for a duration of 300 s. BiTbIG films were subsequently deposited on the YIG film at room temperature. The film was crystallized by annealing at different temperatures ranging from 650 °C to 850 °C. The oxygen partial pressure during annealing was varied from 10 mTorr to 2 Torr.
Crystal structure of the films was characterized by X-ray diffraction (XRD) with Cu-Kα radiation source using a Rigaku Ultima IV instrument. Room temperature in-plane and out-of-plane magnetic hysteresis loops were measured using a vibrating sample magnetometer (VSM). X-ray photoelectron spectroscopy (XPS) was measured on a Physical Electronics PHI Quantera Scanning X-ray Microprobe using monochromatic Al Kα radiation. The Faraday rotation hysteresis was measured on a customized Faraday effect characterization system [30]. The refractive indices and extinction ratio of YIG and BiTbIG were characterized by ellipsometry. SOI waveguides were prepared by photolithography and etching. Part of the films were deposited on SOI waveguides for material loss characterization. The propagation loss of the fundamental transverse magnetic (TM) mode was measured at 1310 nm wavelength using the cut-back method on a polarization-maintaining, fiber-butt-coupled system. The material loss of YIG and BiTbIG was then calculated using the confinement factors simulated by Comsol Multiphysics.
3. Results and discussion
Figure 1(a) and (c) present the X-ray diffraction patterns of BiTbIG thin films fabricated under different temperatures and oxygen partial pressure. Figure 1(a) shows the diffraction patterns as a function of annealing temperature. The oxygen partial pressure (PO2) during annealing was 1 Torr. The diffraction patterns show well-crystallized diffraction peaks of polycrystalline garnet phase for both the YIG and BiTbIG layers. The ionic radius of Bi3+ (1.11 Å) is larger than that of Tb3+ (0.923 Å) and Y3+ (1.015 Å), resulting in expansion of the lattice constant compared to YIG due to the incorporation of Bi3+ ions into the crystal lattice [31,32]. Zoom-in of the BiTbIG (420) diffraction peak was shown in Fig. 1(b). The (420) peak shifts to higher angles as the annealing temperature increases, indicating smaller lattice constants for higher annealing temperatures. The lattice constants of the BiTbIG films annealed at 650 °C, 700 °C, 750 °C, 800 °C and 850 °C were calculated to be 12.541 ± 0.007 Å, 12.534 ± 0.009 Å, 12.524 ± 0.012 Å, 12.518 ± 0.003 Å, and 12.514 ± 0.003 Å, respectively, as shown in the inset of Fig. 1(b). Bi is a temperature-sensitive element, and the decrease in lattice constant may be attributed to the precipitation of Bi [33]. Figure 1(c) shows the diffraction patterns as a function of annealing oxygen partial pressure when fixing the annealing temperature at 650 °C. Zoom-in of the (420) peak is shown in Fig. 1(d). The lattice constants for BiTbIG annealed under oxygen partial pressures of 10 mTorr, 500 mTorr, 1 Torr, 1.5 Torr, and 2 Torr were calculated to be 12.521 ± 0.005 Å, 12.538 ± 0.005 Å, 12.541 ± 0.007 Å, 12.527 ± 0.010 Å, and 12.529 ± 0.006 Å respectively, as shown in the inset of Fig. 1(d). The lattice constants first increase then decrease with oxygen partial pressure. We will later see how this is related to the valence states of Tb ions. It is noteworthy that the lattice constants of Tb3Fe5O12 and Bi3Fe5O12 are 12.436 Å and 12.626 Å, respectively [34,35]. The fabricated films showed lattice constants between these two extremes due to the Vegard’s law. Furthermore, we conducted experiments of the direct deposition of BiTbIG thin films without a seed layer. However, the films failed to crystallize into the YIG phase.
Fig. 1. X-ray diffraction patterns of BiTbIG/YIG films in a 2θ range from 25° to 40°. The blue squares and red dots indicate garnet peaks originated from BiTbIG and YIG respectively. (a) X-ray diffraction patterns of BiTbIG films annealed at various temperatures. (b) The zoomed-in spectrum of the (420) diffraction peaks. The inset shows the calculated lattice constants of BiTbIG films. (c) X-ray diffraction patterns of BiTbIG films annealed at various oxygen partial pressures. (d) The zoomed-in spectrum of the (420) diffraction peaks. The inset shows the calculated lattice constants of BiTbIG films.
Figure 2 shows the XPS characterization results on BiTbIG films. The C1s peak position of surface adsorbed carbon was fixed at 284.8 eV, serving as a reference for the calibration of the binding energy. Figure 2(a) illustrates the Bi (4f) XPS spectra for films fabricated under an oxygen partial pressure of 1 Torr at 650 °C. It is evident that the binding energy of Bi 4f electrons remains unaltered with oxygen partial pressure, indicating consistent trivalent (3+) state. Whereas the Tb ions showed mixed valence states as indicated by their 3d electron spectra in Fig. 2(b). The Tb 3d5/2 and Tb 3d3/2 peaks were observed at 1243.8 eV and 1276.2 eV, respectively. Compared to pure Tb3+ peaks are 1241 eV for 3d5/2 and 1276 eV for 3d3/2 [36], the peak position in our sample indicates mixed valence states of Tb ions. We fitted the spectrum using Gaussian peaks. For Tb 3d5/2, the peaks observed at 1240.1 eV and 1242.5 eV were attributed to Tb3+ and Tb4+ states, respectively. For Tb 3d3/2, the peaks at 1275.4 eV and 1276.9 eV were attributed to the Tb3+ and Tb4+ states respectively [37]. By analyzing the ratio of the combined fitted peak areas of the 3d5/2 and 3d3/2 states of Tb3+, the content of Tb3+ was determined to be 54%. The inset of Fig. 2(b) indicates the calculated Tb3+ concentration as a function of annealing oxygen partial pressure and annealing temperature. While rare earth elements are commonly found in the trivalent state, Tb3+ (0.923 Å) has only one electron above the half-filled 4f shell, making it susceptible to oxidation to Tb4+ (0.76 Å) [38–40]. It is noteworthy that this trend agrees with the trend of lattice constant variation as shown in Fig. 1(d), suggesting the lattice constant variation is mainly caused by the valence state variation of Tb ions. Figure 2(c) and (d) shows the XPS spectra of Tb 3d in samples annealed at different temperatures and oxygen partial pressures. Figure 2(e) and (f) shows the valence state of Bi in samples annealed at different temperatures and oxygen partial pressures remained trivalent state. Figure 2(g) and (h) shows the Fe 2p XPS spectra of BiTbIG films. The Fe 2p3/2 and Fe 2p1/2 peaks show similar binding energies as a function of oxygen partial pressure during annealing, indicating similar valence states of Fe in all samples.
Fig. 2. High resolution XPS spectra of (a) BiTbIG (b) Tb 3d of BiTbIG film annealed at 650 °C, 1 Torr oxygen partial pressure; Gaussian peaks are used to fit the peaks from Tb4+ and Tb3+ ions. (c) Tb 3d peaks of BiTbIG film annealed at various oxygen partial pressure. (d) Tb 3d peaks of BiTbIG film annealed at various temperature. (e) Bi 4f peaks of BiTbIG film annealed at various oxygen partial pressure. (f) Bi 4f peaks of BiTbIG film annealed at various temperature. (g) Fe 2p peaks of BiTbIG film annealed at various oxygen partial pressure. (h) Fe 2p peaks of BiTbIG film annealed at various temperature.
Next, we measured the magnetic hysteresis of BiTbIG thin films at room temperature, as shown in Fig. 3(a) and 3(b). All films showed easy-plane magnetic anisotropy. The saturation magnetization decreases as increasing annealing temperature. Under the annealing condition at 650 °C, the thin film shows the highest saturation magnetization of 104 emu/cm3. For comparison, the saturation magnetization literature reported of Bi3Fe5O12 was 135 emu/cm3 [41]. Varying the annealing oxygen partial pressure does not lead to a substantial change of the saturation magnetization or magnetic anisotropy. Room temperature Faraday rotation (FR) hysteresis loops of the BiTbIG thin films at 1310 nm wavelength were shown in Fig. 3(c) to Fig. 3(e). Figure 3(c) and Fig. 3(d) show the FR as a function of annealing temperature. The oxygen partial pressure was 1 Torr. The sample annealed at 650 °C exhibited the maximum saturation Faraday rotation up to −3300 ± 180 deg/cm at 1310 nm wavelength. The Faraday rotation of single-crystal Bi3Fe5O12 and Tb3Fe5O12 at 1310 nm was reported to be approximately −12000 deg/cm and 270 deg/cm [42,43]. This sample showed the largest lattice constant, suggesting lower annealing temperature may be beneficial for Bi3+ incorporation into the garnet lattice [44,45]. Across the measured wavelength range, the material showed saturation Faraday rotation of (−5.2×λ+10000) (deg/cm)/nm, where λ represents the wavelength (1100 nm−1500 nm). This dispersion was larger compared to literature reports on other Bi doped iron garnet materials [25,46]. This phenomenon could be attributed to the higher doping concentration of Bi in the thin films. Figure 3(e) and 3(f) show the FR as a function of annealing oxygen partial pressure. The annealing temperature was 650 °C. Faraday rotation of the films was dependent on the annealing oxygen partial pressure. Deviation from optimal oxygen partial pressure, either too low or too high, can induce alterations in the Bi content, thereby resulting in a reduction in the observed Faraday rotation values [47]. Based on other reports, it was observed that higher Bi content leads to larger saturation Faraday rotation angles in thin film samples [47,48].
Fig. 3. (a)Magnetic hysteresis as a function of annealing temperature of BiTbIG films. (b) Magnetic hysteresis as a function of annealing oxygen partial pressure of BiTbIG films. (c)FR hysteresis loops of BiTbIG films annealed at different temperatures measured at λ=1310 nm. (d) Saturation FR of BiTbIG films in the O band. (e) FR hysteresis loops of BiTbIG films annealed at different oxygen partial pressure measured at λ=1310 nm. (f) Saturation FR of BiTbIG films in the O band.
In order to evaluate the optical loss of BiTbIG, we deposited 49 nm thick seed layer of YIG grown on SOI waveguides, followed by the deposition of a 98 nm thick layer of BiTbIG. Figure 4(a) shows the optical microscope image of SOI waveguides used to measure the optical loss of BiTbIG. The pink region represents the deposition of YIG/BiTbIG on a Si waveguide, while the cyan region represents the deposition of YIG/BiTbIG on a SiO2 substrate. The waveguide width was 3.5 µm.The cyan line represents the waveguide. Only the fundamental TM mode was excited by a polarization maintaining fiber. The simulated mode profile was shown in Fig. 4(b). The confinement factor for BiTbIG film is simulated to be 34.5%, while that for YIG films and Si were 21% and 20% respectively. The propagation loss of the waveguide can be calculated as:
Fig. 4. (a) Optical microscope image of the SOI waveguides used for loss characterization. (b) The |E| field distribution of the fundamental TM mode of a BiTbIG/YIG/SOI waveguide. (c) Measured propagation loss of BiTbIG/YIG/SOI waveguides using the cutback method. The BiTbIG was annealed at 650 °C and PO2 = 1 Torr.
Figure 5(a) and 5(b) shows the measured αBiTbIG for films annealed at different oxygen partial pressure and temperature, respectively. αBiTbIG first decrease then increase with oxygen partial pressure. A minimum loss value of 53.3 ± 0.3 dB/cm was observed at an oxygen partial pressure of 1 Torr. For different annealing temperatures, higher annealing temperature leads to higher loss. But too low temperature below 650 °C resulted in amorphous films. Based on the measured FR in Fig. 3 and absorption loss in Fig. 5, we can calculate the FoM of BiTbIG films at 1310 nm:
The calculated FoM of BiTbIG thin films as a function of PO2 and annealing temperature is shown in Fig. 5(c) and 5(d) respectively. At annealing temperature of 650 °C and an oxygen partial pressure of 1 Torr, the BiTbIG thin film achieved the highest FoM of 61.9 ± 3.8 deg/dB. This value is significantly higher than other reports [49,50]. According to other reports, Tb3Fe5O12 thin films have been experimentally characterized using the ellipsometer testing method, revealing a FoM value of 310 deg/dB at a wavelength of 1550 nm [26]. However, this method may not be compared with characterization results on waveguides. These results demonstrate sputter deposited BiTbIG on silicon is promising for O band nonreciprocal photonic device applications. Additionally, we can mitigate the influence of the thick YIG seed layer by improve the fabrication process or utilizing a single-step deposition method with a YIG top seed layer [51].
Fig. 5. (a) Transmission loss of BiTbIG at different oxygen partial pressure. (b) Transmission loss of BiTbIG at different different annealing temperatures. (c) FoM of BiTbIG at different oxygen partial pressure. (d) FoM of BiTbIG at different annealing temperatures.
4. Conclusions
In summary, we demonstrate sputter deposited Bi2Tb1Fe5O12 on silicon as a promising candidate for O-band integrated nonreciprocal photonic devices. Phase pure, BiTbIG films with high Bi concentration can be stabilized using a YIG seed layer on silicon. The annealing temperature and oxygen partial pressure play critical role of controlling the valence states of Tb ions, which consequently influences the Faraday rotation and propagation loss. Optimum Faraday rotation up to 3300 ± 183 deg/cm and figure of merit up to 61.9 ± 3.8 deg/dB is demonstrated in BiTbIG films at 1310 nm wavelength, paving the way for future developments of O-band optical isolators and circulators.
Funding
Ministry of Science and Technology of the People's Republic of China (2021YFB2801600); National Natural Science Foundation of China (51972044, 52021001, 52102357, U22A20148); Science and Technology Department of Sichuan Province (99203070).
Acknowledgments
This work was supported by the Ministry of Science and Technology of the People’s Republic of China (MOST) (Grant No. 2021YFB2801600), National Natural Science Foundation of China (NSFC) (Grant Nos. U22A20148, 51972044, 52021001 and 52102357), Sichuan Provincial Science and Technology Department (Grant No. 99203070).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
Data availability
All data from this study are available from the corresponding authors upon reasonable request.
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