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Abstract
The effects of rapid thermal annealing (RTA) on aluminum nitride (AlN) waveguides were investigated. For the AlN prepared by the sputtering, high temperature annealing for too long time may deteriorate the device performance, while a 6-7 times RTA with a 60 s annealing at a temperature of 800 °C would result in a lowest waveguide loss of about 0.76 dB/cm. After annealing, self-pumped four-wave mixing was performed on an 800-nm-wide, 5.8-mm-long waveguide. With a pump beam launched into the waveguide, signal and idler sidebands can be generated, which shows that RTA assisted sputtering grown AlN can have a potential to be applied for optical frequency comb generation.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Aluminum nitride (AlN) material has attracted significant attention as a promising wave-guiding material, which has been demonstrated for wavelength conversion [1–3], signal regeneration [4–7], logical operations [8], etc. Compared with silicon (Si) and silicon nitride (Si3N4), AlN has a broader transmission spectrum from ultraviolet to mid-infrared [9,10], with a moderate refractive index of about 2.12 for a single-crystalline film [11], which can result in a suitable waveguide size and high efficiency coupling with the external optical fiber. In addition, AlN waveguides can also produce significant second and third-order nonlinear effects [12–15]. The sputtered AlN film can realize a single-crystalline structure with the XRD test results showing a peak value of 36.04° and full width at half maxima (FWHM) = 0.16° [16]. Some recent investigation also shows that sputtered AlN on sapphire substrate can exhibit a better crystalline quality than MOCVD grown samples [17]. However, AlN waveguides grown by the sputtering may have a high propagation loss, which is not favorable for the optical nonlinear effects related applications [18]. The high loss may be due to the material absorption related with the hydrogen bonds during the material deposition process. Since some H2O and O2 remains in chamber as a residual gas, during the air exposure [19]. Thus, high-temperature annealing treatment can be used to reduce the transmission loss of AlN waveguides. It is demonstrated that AlN film decomposes when the temperature reaches 940 °C in the air environment [20]. Zavada et al. studied the relationship between different annealing temperatures and the disturbance of hydrogen in a nitrogen environment [21]. It is shown that when the annealing temperature is above 800 °C, the atomic lattice rearrangement phenomenon occurs in the hydrogen element. Recently, Dong et al. demonstrated that the propagation loss can be reduced more than half for the AlN waveguides on the mid-infrared region after thermal annealing at 400 °C for 2 hours in ambient gas environment [16]. For RTA, short time high temperature annealing can promote the outward diffusion of hydrogen to achieve a better passivation effect. As mentioned above, the material absorption loss may be related with the hydrogen bonds during the material deposition process. Thus RTA can be used for the efficient reduction of the waveguide loss [22,23]. Thus, rapid thermal annealing (RTA) technology can be applied for the performance improvement of AlN waveguide. Cao et al. used RTA to improve crystalline quality of AlN film [19]. However, the RTA effect on the AlN waveguides has not been investigated to the best of our knowledge.
In this following, the RTA effect on AlN waveguide was carried out to reduce the waveguide loss and the detailed influence of annealing temperature and times were studied. In order to check the high temperature RTA effects for the AlN waveguide and realize high efficiency fiber-to-waveguide coupling, the core layer was covered with cladding oxide layer. After annealing, a 5.8-mm-long waveguide was applied for the four-wave mixing and significant signal and idler wavelength could be observed.
2. Device preparation
A 2-µm-thick SiO2 buffering layer was prepared on a silicon wafer by plasma-enhanced chemical vapor deposition (PECVD). Then, a 1-µm-thick AlN film was deposited on the thermal oxide using reactive sputtering. After a 0.5-µm-thick PECVD oxide deposition and patterning as hardmask, the AlN waveguides with rib-structure were formed by partially etching with chlorine-based etchant gases, which can be expected to realize a low propagation loss compared with the conventional fully-etched structure. Followed by the removal of the remaining oxide on the top surface with buffered hydrofluoric acid (BHF), a fresh 2-µm -thick oxide was deposited as the cladding layer. By following the process of single-crystalline, the AlN should have the similar structure [16]. Figure 1(a) shows the cross-sectional illustration of the rib-waveguide with Fig. 1(b) for the scanning electron microscope image of the film.
Fig. 1. (a) Cross-sectional illustration of AlN waveguide and (b) scanning electron microscope image of the film.
RTA was carried out to the AlN chips with patterns of long waveguides and micro-rings, with varying rib width of 600 and 800 nm, respectively. The long waveguides have two lengths of 1.9 and 5.8 mm, with a bending radius of 110 µm. A 90° bending waveguide with such a radius can realize a simulated transmission loss of about 0.004 dB for a 600 or 800-nm-wide mesa, while the transmission loss cannot be significantly reduced with the further increase of the bending radius. RTA experiment was carried out in Annealsys’ facility. After the chip was put into the annealing furnace, N2 was delivered to the chamber at a flow rate of 80 sccm. The temperature was first raised to 500 °C within three minutes and kept for 90 s. Then, 800 or 1000 °C was reached within 3 min and kept for 60 s. After the furnace temperature quickly lowered to room temperature, the chip was taken out for further measurement.
The measurement setup was shown in Fig. 2 as in Refs. [24] and [25]. An ASE laser was accurately adjusted to TE or TM polarization by the fiber polarization controller (FPC), and then coupled into the AlN waveguide by a tapered fiber. After passing through the waveguide, output light was collected by a tapered fiber, then switched to the power-meter or optical spectrum analyzer (OSA), respectively.
Fig. 2. Schematic illustration of waveguide coupling test platform. ASE, amplified spontaneous emission; FPC, fiber polarization controller; OSA, optical spectrum analyzer.
3. Device characterization
With varying the number of annealing times and temperature, waveguide losses are compared in Fig. 3, which are obtained by the cut-back method. For the annealing temperature of 800 °C, the lowest loss of about 0.76 dB/cm can be obtained at a 7-times annealing for a 600-nm-wide waveguide, while it is about 0.9 dB/cm for a 6-times 1000 °C annealing. With the annealing times further increase, the loss becomes larger again. Waveguide loss may be affected by the combined effects such as roughness, crystalline morphology, and impurities. During the sputter process, AlN film will inevitably produce atom mismatch. Annealing can release the stress in the film and reduce the influence of stress on the crystal lattice, which helps to reduce threading dislocations and defects in the AlN epitaxial layer, and the crystalline morphology and impurities can be improved [26]. Meanwhile, annealing may increase the roughness and thus the scattering loss [16]. In the initial RTA process, the reduction of threading dislocations and defects in AlN epilayers may be more obvious, so the waveguide loss can be significantly reduced [27]. With the increase of the number of annealing times, the loss caused by scattering would play a main role, deteriorating the waveguide transmission performance. The Payne-Lacey Equation below can be used to analyze the influence factors of the roughness during annealing [28], while volume current method may be more accurate but requiring more efforts [29]:
Here, α is the scattering loss, σ represents the root-mean of surface roughness, K is the coefficient of waveguide structure and surface roughness distribution, λ represents the wavelength, d is the half-width of the waveguide, and n1 is the effective index of propagating light. Excessive thermal effect will cause negative impacts on quality of roughness, making K and σ worse [30,31]. Wafer bonding method can thus be applied for the roughness reduction and performance improvement [32]. It should also be mentioned that a narrow waveguide can realize a lower loss as shown in Figs. 3(a) and 3(b). The phenomenon can be mainly attributed to the sidewall scattering for the etched rib section [31]. As can be seen from the inset of Figs. 3(a) and 3(b), the mode field distributes less in the ridge area for a 600-nm-wide mesa, which can be less influenced by the sidewall roughness.Fig. 3. Influence of annealing temperature on the waveguide transmission loss with waveguide width of (a) 600 nm and (b) 800 nm. Inset: TE modal field for a (a) 600-nm-wide and (b) 800-nm-wide waveguide.
Besides, waveguide dispersion is also an important factor that should be considered for the nonlinear optics related applications. Micro-ring resonator can be used here [ Fig. 4(a)], with the measured spectral properties shown in Fig. 4(b), and the free spectral range (FSR) can be obtained. Then the group refractive index can be derived from FSR, and the dispersion characteristics can be calculated [33]:
where ngj is the group index of the jth order eigen-mode of the ring resonator at frequency v, R is the resonator radius, c is the speed of light in vacuum, and λ is the wavelength. Figure 4(b) shows FSR for 1 and 10 times RTA at different annealing temperature. The FSR decreases with further annealing, while group index becomes larger.Fig. 4. (a) Microscope image of the fabricated micro-ring resonator. (b) Measured micro-ring spectra for 1 and 10 times RTA at different annealing temperature.
The obtained dispersion coefficient D with varying wavelength can then be obtained from Eq. (3), as shown in Fig. 5. The dispersion gradually decreases with the wavelength. Particularly, the overall dispersion properties at 1000 °C is stronger than these at 800 °C. Furthermore, with the power of input light increase, the waveguide temperature will increase and the refractive index will change accordingly, which would cause a red-shift of the resonant wavelength. From this, the material thermal coefficient can be derived. Figure 6 shows the relationship between the shift of resonant wavelength and the incident power at a wavelength of 1550 nm for a micro-ring with waveguide width of 600 and 800 nm under different annealing condition. According to Alexander Gondarenko’s method [34], the thermal coefficient is calculated to be around 0.00133 nm/mW for annealing temperature of 1000 °C, while that for 800 °C is about 0.00286 nm/mW, which are smaller than the value of 0.01 nm/mW for Si3N4 [34], showing a better thermal performance.
Fig. 5. Dispersion coefficient with varying wavelength at different annealing temperature for 600 and 800-nm wide AlN waveguide.
4. Nonlinear application
After annealing, self-pumped four-wave mixing experiment was carried out for an 800-nm-wide, 5.8-mm-long AlN waveguide, as shown in the inset of Fig. 7(b). With a pump light launched into the waveguide, signal and idler sidebands can be generated. The interaction process of photons and waveguides can be analyzed with the degenerated four-wave-mixing (FWM), while the pump field inside the waveguide gives rise to two new frequency components. The relationship can be expressed as ws+wi = 2wp, where wp, ws, wi are the frequency of pump, signal, and idle photons, respectively [35]. Degenerated FWM at a specific wavelength can be accomplished through modification of the waveguide cross section to tailor the waveguide dispersion anomalous [36]. The 800-nm-wide waveguide used in the FWM experiment has a smaller dispersion coefficient of about -271 (ps/km/nm), whose fundamental mode profile for TE polarization is presented in the inset of Fig. 3(b).
Fig. 7. (a) Self-pumped FWM experiment for a 5.8-mm-long AlN waveguide, with the inset for the waveguide under normal long term annealing. (b) Intensity relationship between the input pump and output idle light. Inset: Microscope image for 600 and 800-nm-wide, 5.8-mm-long AlN waveguides.
For the waveguide gain, it can be obtained by
For the self-pumped four-wave mixing experiment, the light source in Fig. 2 was changed to a tunable laser and a 1550-nm wavelength was adopted. With changing the intensity of the input light, the output spectra can be obtained as in Fig. 7(a), where the signal and idler sidebands can be observed at 1559.56 and 1550.44 nm, respectively, meeting the phase-matching requirements as discussed above. With increasing the input light power, the intensity of the signal and idler light changes accordingly, as shown in Fig. 7(b). The conversion efficiency is about -73.6 dB, calculated by Pidle/Ppump with Ppump and Pidle for the pump and idle light power, respectively. Such a small conversion efficiency may be mainly caused by the broad spectral width of the input pump light and the high fiber-to-waveguide coupling loss (about 10 dB). Actually, for AlN waveguide with a propagation loss of 3.5 dB/cm, the frequency comb can be successfully generated [38]. With a better light source and improved coupling design, the conversion efficiency can be improved and some optical frequency comb application can be expected.
As a comparison, ordinary high-temperature annealing was also performed at an annealing temperature of 800 °C for 7 minutes, with the temperature rising and cooling process lasting for 6 hours. The waveguide then gained a high propagation loss of about 2.1 dB/cm. Self-pumped FWM experiment was also carried out with the recorded spectrum shown in the inset of Fig. 7(a). It can be seen that nonlinear phenomenon is not so obvious. With careful observation of the chip surface, it seems that the waveguide structure damaged due to the long-term annealing, indicating that RTA would be a better choice for the device performance improvement.
5. Conclusion
In summary, the loss of AlN waveguide could be reduced through 6 or 7 times RTA, while too long high temperature annealing would deteriorate the device performance. The waveguide dispersion characteristics and temperature coefficient were also analyzed. After annealing, an 800-nm-wide, 5.8-mm-long AlN waveguide was used for a self-pump four-wave mixing and the signal and idler sidebands can be generated. The conversion efficiency was about -73.6 dB, which was limited by the broad spectral width of the pump light and the high fiber-to-waveguide coupling loss. With the RTA, AlN based device can have an improved performance, which would help its potential nonlinear optics related applications.
Funding
National Natural Science Foundation of China (11774235, 61705130, 11933005, 11727812); Natural Science Foundation of Shanghai (17ZR1443400); Shanghai Rising-Star Program (19QA1406100); Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning; Open Fund of Key Laboratory of Intelligent Infrared Sensing, Chinese Academy of Sciences.
Acknowledgements
The authors thank Prof. Jifang Tao from Shandong University and Prof. Guoqiang Wu from Wuhan University for the helpful discussions.
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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