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Abstract
Photonic integrated circuits (PICs) based on gallium nitride (GaN) platforms have been widely explored for various applications at C-band (1530 nm∼1565 nm) and visible light wavelength range. However, for O-band (1260 nm∼1360 nm) commonly used in short reach/cost sensitive markets, GaN-based PICs still have not been fully investigated. In this article, a microring resonator with an intrinsic Q-factor of ∼2.67 × 104 and an extinction ratio (ER) of 35.1 dB at 1319.9 nm and 1332.1 nm, is monolithically integrated with a transverse electric-polarized focusing grating coupler and a ridge waveguide on a GaN-on-sapphire platform. This shows a great potential to further exploit the optical properties of GaN materials and integrate GaN-based PICs with the mature GaN active electronic and optoelectronic devices to form a greater platform of optoelectronic-electronic integrated circuits (OEICs) for data-center and telecom applications.
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1. Introduction
In recent decades, photonic integrated circuits (PICs) have become a technology trend for various applications such as quantum and nonlinear photonics on a platform with a small form factor [1]. Till now, most PICs are built on material platforms with a small bandgap (1.11 eV∼1.42 eV), like gallium arsenide (GaAs), indium phosphide (InP) and silicon (Si) platforms [2–4]. For some short wavelength applications, a semiconductor with a wider band gap is essential. Gallium nitride (GaN) with a large band gap (∼3.4 eV) is advantageous for its relatively mature material growth techniques and fabrication process developed for high electron mobility transistors (HEMTs) and light emitting diodes (LEDs). Thus, GaN materials can be a good platform for various applications, such as photonic cavities [5–7], nonlinear optics [8–11] and acoustic applications [12–14], which have great potential of further integration with GaN electronic or optoelectronic devices for PICs with versatile functionalities.
Similar to electrical circuits where resistor, capacitor and inductor are basic components, there are also fundamental building blocks in PICs such as optical waveguide, grating coupler and resonator [2], that can form a simple and useful circuit for on-chip bio-sensing [15], nonlinear optics [16] and quantum photonics [17] applications. To enable high performance in sensing, communication, and a stronger interaction between acoustic and optic waves, a photonic integrated circuit with low-loss waveguides, high-coupling-efficiency couplers, high-quality-factor (Q) resonators is desperately needed. However, most studies on GaN semiconductor are focused on active devices such as HEMTs, LEDs and photodiodes. Whereas, research efforts on optical passive devices are still not enough, hindering the development of GaN PICs.
In addition, GaN is a good material for nonlinear optics by virtue of its large intrinsic χ(2) (16 ± 7 pm/V) and χ(3) values, excellent thermal properties, and a relative large bandgap for parametric conversion and frequency doubling without suffering from two-photon absorption at telecom wavelengths [5,8–11]. Only until 2011, GaN-based PICs at C-band started to be developed for nonlinear optics to generate second harmonics [8]. Later on, a four-wave-mixing (FWM) process was demonstrated without multiphoton absorption in 2019 [9], and optical parametric oscillation(OPO) process was demonstrated in 2022 [11]. But most works are still focused on C-band (1530 nm∼1565 nm).
In contrast, for short reach/cost sensitive markets, passive-optical networks (PONs) and high-speed Ethernet transmission, O-band (1260 nm∼1360 nm) is often used as the main choice for optical channels in the current and next generation data center networking (DCN), which demands increasingly high-volume data. This is mainly due to the zero dispersion of the standard single mode optical fiber at the telecommunication band. In the case of high transmission speed (such as Gb/s), dispersion can introduce signal distortion, which limits the highest achievable data rate. O-band can eliminate the additional cost of dispersion management such as digital signal processing (DSP) units. Therefore, O-band is preferred for applications in high-speed, cost-sensitive, high-volume markets.
As known, PICs can assemble various photonic components (laser diodes, amplifiers, waveguides, photodiodes, etc.) together on a single chip to minimize the coupling loss and assembly costs. In this way, there is a direct reduction in energy consumption, which leads to savings in both energy costs and operating expenses (OpEx). Therefore, in future of a cloud-based world, the combination of the O-band wavelength and photonic integrated circuits (PICs), namely “O-band + PICs”, will be an obvious trend for these applications due to its low dispersion and low cost advantages.
Except for the application in short-reach PONs, O-band is also preferred for biosensors due to its lower water optical absorption than the one at 1.55 µm [15].
In this paper, we introduce an O-band PIC consisting of a waveguide, grating couplers and a resonator with high extinction ratio (ER) based on a GaN-on-Sapphire platform. ER is an important parameter for a filter, a modulator and a receiver. For example, resonators functioning as filters with a high ER provide a better filter band profile. Detailed structure, schematic and performance of each component in this GaN PICs are also given. Without adopting complicated device structure and extra passivation, the influences of intrinsic defects of GaN materials can stand out and will not be covered by other factors. This GaN-based PIC can also provide a starting point for further optimization to achieve a good platform for investigating interaction between optical and acoustic waves.
2. Experimental methods
2.1 Sample information
The un-intentionally doped (un-doped) GaN sample is grown on a 2-inch, 430 µm-thick c-plane sapphire substrate by a Metalorganic Vapor Phase Epitaxy (MOVPE) method. First, a 25-nm thick low-temperature (LT) GaN nucleation layer is grown at 550°C as an initial layer for GaN epitaxy to provide nucleation centers and promote lateral growth of the film by reducing interfacial free energy between the film and the substrate [18]. Then an un-doped GaN layer with a thickness of around 430 nm is further grown at 1050°C. Epitaxial structure is shown in Fig. 1 (a). To evaluate the surface morphology of grown sample, atomic force microscope (AFM) measurement is taken by an Asylum Research MFP-3D Stand Alone AFM. Scanning areas of 2 µm × 2 µm and 5 µm × 5 µm are displayed in Fig. 1 (b), showing small surface roughness root-mean-square (RMS) values of 0.36 nm and 0.59 nm, respectively. The dark spots in GaN surface are caused by the termination of screw dislocations, which results from the coalescence of three-dimensional islands formed by nucleation layer. A step-flow growth can be observed in Fig. 1 (b), indicating a two-dimensional layer-by-layer growth mode. The X-ray diffraction (XRD) measurement of ω-2θ full scan and rocking curves of (002) and (102) crystal plane of GaN crystal are conducted by a SmartLab Rigaku high-resolution X-ray diffractometer and shown in Fig. 1 (c) and (d). For GaN film, narrow full width at half-maximum (FWHM) values of the XRD (002) and (102) rocking curves are ∼396 arcsec and ∼811 arcsec, respectively, comparable with typical values of the “low-temperature deposited buffer layer technology” invented by three Nobel Laureates Akasaki, Amano [18] and Nakamura [19]. The threading dislocation and edge dislocation density can be calculated as 3.154 × 108 cm-2 and 2.661 × 109 cm-2, respectively, according to the formula given in [20]. Therefore, there are still some room to further optimize the growth condition for better crystal quality.
Fig. 1. Sample information of GaN-on-Sapphire: (a) Epitaxial structure; (b) AFM images of sample surface within a range of 2 µm × 2 µm and 5 µm × 5 µm; (c) XRD Omega-2theta full scan curve; (d) Rocking curve of (002) and (102) crystal plane of GaN film.
2.2 Device design and fabrication
The device design is shown in Fig. 2. For the grating coupler, it adopts a focusing shape to save chip area. The duty cycle of the grating coupler is defined as the ratio of width of un-etched region compared to width of each period. There are 40 periods of trenches in total and the width of each period is 800 nm. We vary the duty cycle from 20% to 80%. A 500 µm-length waveguide is fabricated to connect the input and output coupler. For the microring resonator, we define the waveguide width to be 1.8 µm, which can also be used for coupling acoustic wave simultaneously in future [13]. The radius of the resonator is 100 µm. To enable efficient light coupling between waveguide and resonator, a pulley structure is adopted. As shown in Fig. 2(c), the pulley structure has a 18° angular curvature for a long-enough coupling length between waveguide and resonator. The coupling gap varies from 100 nm to 600 nm. For GaAs, InP and Si devices, light coupling between the fiber and the chip can be either edge coupling or grating coupling [21]. Edge coupling is beneficial for a large bandwidth and insensitivity to wavelength of incident light. Thus, it is a good scheme for materials that can be easily cleaved and polished, including GaAs, InP and Si, etc. However, GaN on sapphire substrate is difficult to cleave and hence achieve a smooth cleaving surface without polishing. Compared to the edge coupler, grating coupler possesses the advantages of compact size, large coupling tolerance, and simplified fabrication steps without the need for deep trench etching and dicing [22]. In addition, small-form-factor and compact PICs are always preferred. Therefore, for GaN-on-Sapphire PICs, vertical grating coupling is a more fabrication-friendly and compact scheme compared to edge coupling.
Fig. 2. Our GaN O-band PIC design: (a) 3D schematics of the whole PIC; (b) Cross-sectional view of grating couplers connecting with a waveguide in the y-z plane; (c) Top-view of microring resonator coupling with a bent waveguide.
To ensure a proper design, we first conduct a finite-difference time-domain (FDTD) simulation for GaN grating coupler. We choose a grating period of 800 nm with a duty cycle of 50%. Here 400 nm-width and 120 nm-depth trenches are etched to form grating couplers on GaN (refractive index: 2.34). For sapphire substrate, the refractive index is set to be 1.8193. The tilted angle of single-mode fiber transmitting incident light is 15° and diameter of fiber core is 9 µm. Simulated area and refractive index distribution are shown in Fig. 3(a). Our results show that, as duty cycle varies from 20% to 50%, the minimum loss reduces gradually from 13.85 dB at 1298.97 nm to 9.72 dB at 1309.6 nm. Whereas, as the duty cycle increases from 50% to 80%, the minimum loss increases with the corresponding wavelength further red-shifted, as shown in Fig. 3 (b).
Fig. 3. Simulation of designed grating coupler: (a) Simulated region and its refractive index distribution with a GaN film thickness of 430 nm; (b) Coupling efficiency of grating coupler with different duty cycles.
For the simulation of the microring resonator, we adopt a waveguide width of 1.8 µm, a height of 430 nm, and a bending radius of 100 µm. The gap between the coupling waveguide and the resonator is 100 nm. We first use Lumerical simulation to obtain coupling efficiency and dispersion coefficients. The transmission spectrum of micro-ring resonator can be calculated by Matlab simulation. The simulated coupling region, mode distribution in the bent waveguide, transmission spectrum within the O-band range of 1260∼1360 nm and two typical dips in spectrum are displayed in Fig. 4 (a)∼(d). Extinction ratio can reach up to 35 dB at 1318.92 nm according to the simulation results. The simulated free spectral range (FSR) is around 1.01 nm.
Fig. 4. Simulation of designed micro-ring resonator: (a) Simulated region; (b) Mode profile within the bent waveguide; (c) Transmission spectrum at 1260∼1360 nm; (d) Two typical dips in transmission spectrum.
To verify the device design and fabricate PICs, we dice a 2-inch GaN-on-Sapphire sample into a small piece (size: 2 cm × 2 cm) and fabricate devices in our clean room. The fabrication procedure is a pretty standard one for GaN semiconductor. First, a layer of 250 nm thick plasma-enhanced chemical vapor deposition (PECVD) oxide is deposited on GaN-on-Sapphire sample as the dry etching mask, followed by a layer of chromium (Cr) metal (thickness ∼42 nm) deposition. E-beam resist is coated on Cr metal which is further patterned by wet etching. In this way, device patterns can be transferred onto oxide dry etching mask and PECVD oxide mask is etched by F-ion based induced coupled plasma (ICP). Active region of PIC is further formed by Cl-ion based ICP dry etching of the 430 nm GaN film onto sapphire substrate. After this step, micro-ring resonator and waveguide have been formed. To form GaN grating coupler, 40 periodic shallow trenches with width of 400 nm and depth of 120 nm are further etched on GaN sector areas which are connected with the end of optical waveguide. E-beam resist itself serves as dry etching mask in this step. Therefore, only two lithography steps are needed to form the whole PICs.
The fabricated GaN PIC is displayed in Fig. 5. Three components including GaN grating coupler, ridge waveguide and microring resonator are fabricated on GaN-on-Sapphire. Figure 5 (a) is the optical microscope image of our fabricated PIC taken by an Olympus BX51 Microscope. To check smaller feature-sized patterns and cross-sections, scanning electron microscope (SEM) images are also taken by Zeiss GeminiSEM 300. Figure 5 (b) shows the cross-sectional SEM images of GaN waveguide with a width of 1 µm and height of 430 nm. The sidewall is almost vertical, indicating an anisotropic etching condition. Figure 5 (c) is a top-view SEM images of a grating coupler. Figure 5(d) shows the zoom-in SEM images of the gap between resonator and waveguide.
Fig. 5. Fabricated GaN PIC with three components: (a) Optical microscope image; (b) Cross-sectional view SEM images of GaN waveguide; (c) Top-view SEM image of grating coupler; (d) Top-view SEM image of a 100 nm gap between waveguide and microring resonator.
2.3 Test setup
The measurement setup is shown in Fig. 6. We use a wavelength tunable semiconductor laser (Santec TSL-550) connected with a single-mode fiber for device transmission property characterization. The incident light is set to transverse electric (TE) polarization using a polarization controller (Conquer, KG-CIR-3-1550-FC), and shined onto a grating coupler connected at one end of waveguide. The light evanescently couples into the resonator, and then couples out into a single-mode fiber from the output grating coupler. The received light is launched into a Santec multi-channel power meter (MPM-210 H). The optical loss caused by optical fiber can be deduced from the signal as background noise, similar to the de-embedding of RF cables used in microwave device test. The sample is placed on a three-dimensional transitional sample stage. Optical fibers are clamped with two fixtures and positioned at 15° off vertical directions to the surface of grating coupler for shining light onto one coupler as input signal and collecting light from the other coupler as output signal.
Fig. 6. Test setup: (a) Schematic of the whole optical path; (b) Photo of sample stage and (c) Side-view photo of fibers positioned onto the sample under test.
3. Results and discussions
3.1 Grating couplers
Grating couplers are fabricated with different duty cycles to compare the minimum coupling loss (corresponding to the maximum coupling efficiency) and corresponding wavelengths. As shown in Fig. 7, the period of grating coupler is fixed at 800 nm, while as the duty cycle increases from 20% to 50%, the minimum coupling loss varies from 40.68 dB to 33.50 dB with the corresponding wavelength changing from 1285.57 nm to 1300 nm for two couplers (namely one input coupler and one output coupler) and a 500 µm-length waveguide. When duty cycle is 60% and 80%, the minimum coupling losses are 31.36 dB (at 1312.76 nm) and 26.4 dB (at 1350 nm) for two couplers. Here, we choose 50% as the duty cycle of our grating coupler used for GaN PICs working at O-band. The 3 dB bandwidth of grating coupler is around 35∼40 nm, which is comparable to the typical value of III-V grating couplers. To extract the coupling loss value for each coupler, the loss caused by the 500 µm-length waveguide needs to be deduced. Thus, measuring the propagation loss of optical waveguide is required.
Fig. 7. (a) Coupling loss of two GaN focusing grating couplers with different duty cycles; (b) tilted-view SEM images of a grating coupler with a 20% duty cycle and (c) Zoom-in cross-sectional view SEM images.
The propagation loss of the ridge waveguide can be measured by a cut-back method. Here, different-length ridge waveguide with 50% duty cycle grating couplers at the two ends are fabricated and measured. The propagation loss is as large as 47 dB/cm, which is possibly caused by rough sidewall surfaces resulted from non-optimized etching condition and absorption centers such as oxygen vacancies and crystal defects (threading dislocations, point defects) in GaN materials [23,24]. Compared with typical results of 0.9 dB/cm (for TE) or 1.5 dB/cm (for TM) reported in [23], there is still much room to improve the waveguide fabrication process. Since the waveguide sidewall roughness is controlled by the etching rate of ICP, we can further fine-tune the etching recipe (such as ratio of Cl2/BCl3, RF power, pressure, etc.) to achieve a better etching result. Except for reducing scattering loss, we also need to further optimize the growth conditions to achieve a GaN epitaxial film with higher crystal quality to reduce the absorption loss.
According to the above measurement, the propagation loss of a 500 µm-length waveguide is 2.35 dB. Therefore, the minimum coupling losses of each grating coupler can be extracted to be 19.17 dB to 15.58 dB when duty cycle varies from 20% to 50%, and 14.51 dB to 12.03 dB when duty cycle varies from 60% to 80%. The coupling loss of our experimental value is much larger than the simulated value of 9.72 dB at 1309.6 nm, indicating a non-optimized fabrication condition. The cross-sectional view of grating coupler with 20% duty cycle is displayed in Fig. 7(c). The sidewall is not as vertical as the one of waveguide, which is attributed to the non-optimized etching condition in the second step where e-beam resist itself serves as dry etching mask. Compared with the best results of -4.1 dB/coupler at 1610 nm and -9.7 dB/coupler at 1570 nm reported in literature [25], our results of -15.58 dB/coupler at 1300 nm and -12.5 dB/coupler at 1350 nm are still lower. The excess coupling loss of grating couplers can be attributed to two factors: rating directionality and mode mismatch [26]. For rating directionality, part of the optical power incident on grating coupler is diffracted towards the sapphire substrate. To avoid this, poly-silicon over-layer or back-reflectors embedded in sapphire substrate can be adopted to improve rating directionality. For mode mismatch, the optical intensity radiated by grating has an exponential-decay shape with a reduced overlap with incident or collection fiber. Here we still need to carefully tune the fabrication process including etching conditions to get smooth sidewall for the trenches in grating coupler. Moreover, for etching depth and total GaN epi thickness, there is also a room to be further optimized.
3.2 Microring resonators
In Fig. 8, transmission spectrum of two grating couplers, a coupling waveguide and a resonator is displayed upon a 100 nm gap, with a zoom-in view of one resonance plotted in Fig. 8(b) to extract the Q factor. The resonant peaks of the microring resonator are represented by dips in the transmission spectrum. ER can reach up to a maximum value of 35.1 dB at 1319.9 nm. At 1332.1 nm, loaded Q factor of resonator is fitted to be as high as 1.335 × 104. The intrinsic Q can be estimated to be 2.67 × 104, twice of the intrinsic one considering the near critical coupling condition [27]. FSR and group index are calculated as 1.083 nm and 2.56, respectively. Compared with the simulated ER of 35 dB and FSR of 1.01 nm, the experimental values are pretty close, indicating a well-fabricated microring resonator. This is comprehensible because we use Cr-metal plus PECVD SiO2 as dry etching mask to ensure a good coverage of protected GaN area in the initial GaN etching step.
As the gap between waveguide and resonator increases from 100 nm to 600 nm, ER decreases from 35 dB to 0.35 dB, which means the coupling strength between resonator and waveguide changes from near critical coupling to under coupling. The decrease of ER can be attributed to the fact that less maximum power can be coupled into microring resonator as the gap increases. When the radius of resonator is fixed, the coupling coefficient (thus the coupled power) is closely related with gap and follows an exponential raw [28]. As shown in Fig. 9, the fitted ER also follows an exponential law of y = y0+ A*exp(-k0·x), where y0= -0.36, A = 74.05, k0= 0.0075 can be extracted. Since ER usually has a polynomial relationship with coupling coefficient in either under coupling or over coupling region [29], it can be approximated as a linear function of coupling coefficient when coupling coefficient is much smaller than 1. Therefore, ER can have an exponential relationship with gap when the coupling coefficient is small and coupling is within the under coupling region. But the physics behind such an exponential relation still needs to be explored in future.
To better evaluate the performance of GaN resonator in this work, we make two comparisons, namely with other GaN resonators (as shown in Table 1) and with O-band resonators on other material platforms (as shown in Table 2).
Table 1. Comparison of Q and ER values of GaN resonators
Table 2. Comparison of microring resonators based on other materials at O-band
To the best of our knowledge, almost all reported GaN resonators works are in C-band. The intrinsic Q factors and ER values in literature are listed in Table 1. For early research work in 2015, both coating top flowable oxide onto GaN resonator to achieve a high Q of 7 × 104 in [30] and the importance of using high-crystal-quality GaN on sapphire for further improving the Q value in [31] are proposed and investigated by researchers. Later on, in 2018, two-step polymer (thickness∼ 690 nm) is adopted for bonding GaN-on-silicon sample onto silicon substrate and removing the native substrate in [32]. However, due to large sidewall roughness (18.3 nm), tilted sidewall (67°) and light leakage into substrate, highest Q factor and ER for resonator are only 4.81 × 103 and 17.8 dB, respectively. This further shows the importance of smooth sidewall to improve light confinement. In 2019, a layer of 250 nm-thick 250-nm atomic layer deposition (ALD) deposited Al2O3 and another layer of 2000-nm evaporated Al2O3 are coated on GaN surface to ensure a refractive index match between cladding and substrate in [9]. This leads to a high Q value (1.37 × 105) for their devices. Similarly, in 2022, a GaN epitaxial film grown on silicon substrate is bonded to SiO2-on-Silicon substrate and the original native silicon substrate is removed [10]. A 2.5 µm-thick PECVD SiO2 is then deposited to reduce the refractive index mismatch between waveguide and cladding interfaces. SiO2 cladding layer also reduces the impact of sidewall roughness on the coupling between waveguide and microring resonator and improves its quality factor to 2.3 × 105. Therefore, this method of coating dielectric cladding layer is very useful and can also be adopted in our future work.
The best result occurs in 2022 and a highest Q value (2.5 × 106) has been reported for GaN-on-Sapphire, which is attributed to high GaN crystal quality and optimized GaN waveguide etching conditions [11]. The FWHM of (002) and (102) XRD rocking curve for 1 µm thick GaN film are 115 arcsec and 351.7 arcsec, which are almost three times smaller than our sample. This means there are less dislocations and absorption centers existing in their sample. Therefore, a 50 nm-thick AlN buffer layer and a thick GaN layer (1 µm) can ensure a high crystal quality and better light confinement as mentioned in [11]. Except for adopting GaN sample with high epitaxial crystal quality, dry etching condition especially the gas ratio of BCl3 among Cl2/Ar/BCl3 mixture is also optimized to be 30% for achieving a smooth sidewall of waveguide.
In this work, our resonator shows a Q value at the order of 104. This is also the first work of GaN-based PICs working at O-band. Further improving GaN crystal quality to reduce defects density and enhancing the waveguide sidewall smoothness by optimizing dry etching condition, and applying oxide layer as the top cladding are three possible ways to further achieve a higher Q value. Compared with other GaN resonators, this work has the highest ER value, which can be attributed to the small gap of 100 nm between bent waveguide and micro-ring resonators. As the gap increases, ER will reduce rapidly, as shown in Fig. 9.
At the same time, we also compare our resonators with the ones working at O-band based on other materials, as shown in Table 2. We note that for the Si microring work in Refs. [33] and [34], extra loss is intentionally introduced to obtain a suitable Q value for high-speed modulation. But for different silicon nitride materials in [35,36,37], our work shows decent Q value and much larger ER value. Therefore, for GaN materials, we do not need to deal with the tradeoff between Q and ER value encountered in [35].
4. Conclusions
In this work, we have demonstrated a GaN-based PIC working at O-band with a simple fabrication process and good performance. Our GaN resonator exhibits an intrinsic Q factor of 2.67 × 104 and an ER of 35.1 dB. Our work shows the highest ER value among all GaN resonators. The Q value still has room for further improvement compared with other GaN resonators at C-band and other platforms by further growing a thicker GaN with a better crystal quality, optimizing the dry etching conditions, and applying SiO2 or Al2O3 top-cladding layer. This GaN PIC can serve as a good platform for nonlinear optics, bio-sensing, photon-phonon interaction, etc. It can also be applicable to high-volume data-center and telecom in near future.
Funding
Southern University of Science and Technology (Y01236254, Y01236154); Shenzhen Fundamental Research and Discipline Layout project (K23795010).
Acknowledgments
The author would like to thank e-beam lithography steps finished by Tianjin H-chip Technology Group Corp., technical support of conventional nanofabrication steps by Micro and Nanofabrication Facility of SUSTech’s Core Research Facilities (CRF). The author would also like to thank Dr. Rui Gu, Ms. Qiuxia Hu from CRF and Dr. Shi Shi from School of Environment in SUSTech for their kind help in AFM, XRD and SEM measurements.
Disclosures
The authors have no relevant financial interests in this article and no potential conflicts of interest to disclose.
Data availability
Data will be made available by the corresponding author on reasonable request.
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