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Abstract
Heterogeneous integration through low-temperature die bonding is a promising technique to enable high-performance III-V photodetectors on the silicon nitride (Si3N4) photonic platform. Here we demonstrate InGaAs/InP modified uni-traveling carrier photodiodes on Si3N4 waveguides with 20 nA dark current, 20 GHz bandwidth, and record-high external (internal) responsivities of 0.8 A/W (0.94 A/W) and 0.33 A/W (0.83 A/W) at 1550 nm and 1064 nm, respectively. Open eye diagrams at 40 Gbit/s are demonstrated. Balanced photodiodes of this type reach 10 GHz bandwidth with over 40 dB common mode rejection ratio.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Photonic integration has been proven to enable an increasing number of applications in telecommunications [1], bio sensing [2], quantum information sciences [3], and microwave photonics [4]. As a complementary platform to silicon-on-insulator and InP-based photonic integrated circuits, the silicon nitride (Si3N4) photonic platform has attracted wide attention in recent years due to its unique optical properties that are difficult to achieve in other material systems [5]. First, Si3N4 strip waveguides have demonstrated high power handling properties and a propagation loss below 0.1 dB/cm [5,6], even with very small bend radius [7]. Two photon absorption in the telecommunication band is virtually zero because of the large bandgap of Si3N4 (5.3 eV). Second, using Si3N4 waveguides there have been numerous demonstrations of frequency comb generation [8, 9], supercontinuum generation [10,11], and wavelength conversion [12]. Moreover, the fact that the transparency window of the Si3N4 platform ranges from visible to mid-infrared wavelengths makes it possible to generate an octave-spanning frequency comb, which is a crucial component in a chip-scale optical synthesizer [13].
To date, devices for efficient on-chip light generation, amplification, and detection continue to be dominated by group III-V semiconductors. While the Si3N4 platform lacks monolithic active devices, heterogeneously integrated III-V components have the potential to complement the platform and enhance its functionality. For optical detectors, only few heterogeneous photodetectors on Si3N4 waveguides have been published to date, and are summarized in Table 1. Recently, we have demonstrated heterogeneously integrated InGaAs/InP PIN photodiodes (PDs) with high responsivity and low dark current using adhesive die bonding [14]. The high responsivity was enabled by using a bonding window on the Si3N4 waveguide with reduced silica top cladding thickness to improve the coupling efficiency from the waveguide into the PD absorber.
Table 1. Optical detectors on Si3N4 waveguides at 1550 nm wavelength reported in the literature.
In this paper we demonstrate heterogeneously integrated modified uni-traveling carrier (MUTC) PDs on Si3N4 waveguides that achieve both, high responsivity and high bandwidth.
In contrast to a PIN PD, the MUTC PD can achieve higher bandwidth since the carrier transit time component of the bandwidth is mainly based on fast electrons. By using a P-type doped absorber and a transparent electron drift layer, a higher transit time limited bandwidth with the same capacitance as in a PIN PD can be achieved [18]. By minimizing the bonding layer thickness, our PDs achieve record-high external (internal) responsivities of 0.8 A/W (0.94 A/W) and 0.33 A/W (0.83 A/W) at 1550 nm and 1064 nm, respectively. Balanced PD pairs of this type have a bandwidth of 10 GHz and over 40 dB common mode rejection ratio (CMRR).
2. Device design and fabrication
Figure 1(a) shows the Si3N4/Si chip with patterned waveguides and the bonding window before die bonding. The Si3N4 waveguides were deposited by low pressure chemical vapor deposition. The chip size is 8.4 × 6 mm2 with a bonding window size of 6.5 × 5.5 mm2. A schematic cross-section of the waveguide outside the bonding window is shown in Fig. 1(d). The waveguide core has a thickness of 400 nm and widths of 1, 2 or 4 µm and is buried in silica with 3 µm top and 4 µm lower cladding. Inside the bonding window the silica top cladding was selectively reduced to 60-70 nm [Fig. 1(e)] to achieve efficient evanescent coupling from the waveguide into the PD absorber.
Fig. 1. Microscope pictures of Si3N4/Si chip with (a) patterned waveguides and bonding window, and (b) heterogeneously integrated III-V die; (c) fabricated PDs on Si3N4/Si chip; cross-sectional views of (d) Si3N4 waveguide outside the bonding window, (e) Si3N4 waveguide inside the bonding window, and (f) PD on Si3N4; (g) epitaxial structure of PD with doping concentrations in cm-3; (h) microscope pictures of single PD, and (i) balanced PDs with circuit representation; (j) SEM picture of the PD’s cross-section.
The PD’s epitaxial structure is shown in Fig. 1(g). The layers were grown on semi-insulating InP substrate and consist of a 150 nm-thick N-type doped InP contact layer, a 400 nm P-type doped InP contact layer, a depleted InP electron drift layer, and a 450 nm-thick InGaAs depleted and un-depleted absorption layer. According to our simulation, the carrier transit time-limited bandwidth for this MUTC PD is 40 GHz. Similar to the bonding process used in Ref. [15], an adhesive bonding technique including SU-8 spin coating (SU-8 thickness 70 nm), soft bake at 110℃, UV exposure followed by a 40-minutes outgas, and curing at 130℃ under 10 psi for 60 min was adopted to integrate a 4.5 × 3.5 mm2 InGaAs/InP die onto the bonding window as shown in Fig. 1(b). Then, a mixture of hydrochloric acid and DI water was used to remove the InP substrate. MUTC PDs were fabricated as double mesa structures by conventional dry and wet etching techniques. Ground-signal-ground (GSG) radio frequency (RF) probe pads were deposited and connected to the PD N-contact through an Au electro-plated air-bridge. Figure 1(c) shows the finished MUTC PDs with single (Fig. 1(h)) and balanced [Fig. 1(i)] PDs fabricated on the same chip. A scanning electron microscope (SEM) picture of the PD’s cross-section with a 4 µm-wide waveguide and 69 nm-thick SU-8 layer is shown in Fig. 1(j).
3. Results and discussion
Typical dark currents were around 20 nA at 8 V reverse voltage. Figure 2 shows the current-voltage characteristics for a pair of balanced PDs. It should be mentioned that the dark currents of our heterogeneous PDs are at the same level or below as similar devices on native substrate [18].
One of the benefits of the Si3N4 platform is its wide transparency window. To this end we characterized the responsivities of the PDs at wavelengths of 1550 nm and 1064 nm. The waveguides were input-coupled by single mode tapered fibers with a spot size of 2.5 µm optimized for 1550 nm and 1064 nm, respectively. The Si3N4 waveguides have inverse tapers at the chip facets to aid with the input coupling. According to our simulation, the fiber-chip coupling loss due to mode mismatch is 0.7 dB at 1550 nm and 4 dB at 1064 nm, respectively. Based on these values, the blue solid lines in Fig. 3 illustrate the PD’s external (fiber-coupled) responsivities corresponding to 100% internal quantum efficiency. The red symbols in Fig. 3 show the measured data of different PDs. For a 30 µm-long PD, the measured external responsivities are 0.8 A/W and 0.33 A/W at 1550 nm and 1064 nm, respectively. Once we take the fiber input coupling loss into account, the internal responsivity is as high as 0.94 A/W and 0.83 A/W corresponding to 75% and 96% internal quantum efficiencies at 1550 nm and 1060 nm, respectively. The polarization dependence loss was measured to be only 0.67 dB and 0.26 dB at 1550 nm and 1064 nm, respectively. It should be mentioned that the responsivity has only a weak dependence on PD length between 20 µm and 60 µm. For a 10 µm-long PD, the external responsivity can be as high as 0.68 A/W at 1550 nm which indicates that most of the light is absorbed in the first 10 µm. Moreover, we found that the PD responsivity did not show any significant dependence on waveguide widths between 1 and 4 µm.
Fig. 3. Measured and simulated external responsivities for PDs of various lengths at 1550 nm (a), and 1064 nm (b) wavelength.
In order to further study device characteristics, we used a commercial software (RSoft Beamprop) and estimated the responsivity based on the total optical loss in the photodiode. In the simulation, we used 7000 cm-1 (9000 cm-1) and 870890 cm-1 (841632 cm-1) as the absorption coefficients for InGaAs and gold at 1550 nm (1060 nm), respectively, and assumed all other layers to be transparent. The small scattering loss (0.07 dB) due to the discontinuity of the Si3N4 waveguide at the edge of the bonded photodiode layers was also taken into account. By assuming that all other photon loss is contributing to the photocurrent, we obtain best case estimates for the responsivity that are illustrated by the black solid lines in Fig. 3 after considering the fiber-chip coupling loss. While the simulated curve matches well the measured data at 1064 nm [Fig. 3(b)], we found that the simulation overestimates the responsivity by 20% at 1550 nm and PD lengths larger than 40 µm. We believe that this discrepancy can be explained by the difference in optical mode intensity distributions in the PD. In Fig. 4, the mode intensity distributions in the PD at 1550 nm and 1064 nm are shown. As the index contrast between Si3N4 and silica is similar at these two wavelengths, the mode distributions in the PD region are strongly dependent on the wavelength. For our 400 nm thick Si3N4 waveguide, the mode at 1550 nm is less confined in the waveguide and extends significantly towards the top gold contact. The proximity with the metal creates additional optical loss, which, however, does not contribute to the photocurrent.
Fig. 4. Simulated mode intensity distributions in the PD at 1550 nm (left panel) and 1064 nm (right panel).
Using an optical heterodyne setup with modulation depth close to 100%, the frequency responses of the PDs were measured (Fig. 5). For a 10 × 30 µm2 PD, the bandwidth can reach 20 GHz at 8 V reverse bias with 2 mA photocurrent. Based on the measured capacitance of 69 fF we estimated a resistance-capacitance (RC) limited bandwidth at 50 Ω load of 23 GHz with a series resistance of 50 Ω. Most likely, the relatively large series resistance can be attributed to the P-contact resistance and the sheet resistance in the P-type InP contact layer which was unintentionally over-etched by 30%. We expect that the series resistance can be further reduced by optimizing the etch process and increasing the doping concertation in the contact layer. Assuming a series resistance of 20 Ω, a bandwidth of 25 GHz can be predicted. For larger sized PDs, the bandwidth further decreased due to the RC limitation. It should be mentioned that a reverse bias of 8 V was necessary to achieve maximum bandwidth in all measurements.
Fig. 5. Measured frequency responses of various sized PDs at 1550 nm with 8 V reverse bias and 2 mA photocurrent.
To evaluate our photodiodes for use in high-speed digital optical links we also measured the non-return-zero eye diagram by using a 40 Gbit/s pseudo random binary sequence generator and a 40 GHz Mach-Zehnder (MZ) modulator as the signal source. The pattern length was 231-1. The output of our photodiode was connected to a high-speed sampling oscilloscope through a bias-T and a short RF cable. Figure 6 shows the detected eye diagrams using a PD with 10 × 30 µm2 active area. We recorded a clearly opened eye pattern which demonstrates the PD’s high-speed capability for 40 Gbit/s systems.
Fig. 6. Detected 40 Gbit/s eye diagram at 0.5 mA (left panel) and 2 mA (right panel) average photocurrents.
It is well known that balanced photodiodes with high CMRR can suppress common mode noise, and therefore can help to increase the signal to noise ratio. To measure the balanced photodetector performance, the optical heterodyne signal was split into two branches before being launched into the waveguides of both PDs, PD1 and PD2, through a fiber array. We used variable optical delay lines in both branches to adjust the RF phase of the modulated optical signal to be either in-phase (common mode) or out-of-phase (differential mode). Figure 7 shows the frequency responses in differential mode for a 10 × 30 µm2 balanced PD pair at 2 mA photocurrent. The bandwidth is 10 GHz, half of the bandwidth of a single PD, which is expected due to the doubled capacitance of the PD pair. A similar measurement at 1064 nm showed not significant difference in bandwidth (Fig. 7).
Fig. 7. Measured frequency responses of balanced PDs at 1550 nm and 1064 nm at 8 V reverse bias. Each data point was taken in differential mode.
To measure CMRR we replaced the optical heterodyne source by a MZ modulator that was driven with a 10 GHz tone. CMRR was calculated by subtracting the measured RF power in common mode from the RF power in differential mode. Figure 8 shows the RF powers in common and differential modes as measured with an electrical spectrum analyzer with a CMRR of more than 40 dB indicating excellent symmetry between PD1 and PD2.
Fig. 8. Common and differential mode powers of balanced PDs at 10 GHz, 8 V reverse bias, 2 mA photocurrent for each PD, and 1550 nm wavelength.
4. Summary
MUTC PDs heterogeneously integrated onto Si3N4 waveguides were fabricated and characterized. The internal responsivity of a PD with 20 GHz bandwidth is as high as 0.94 A/W. Balanced PDs have 20 nA dark current, 10 GHz bandwidth, and over 40 dB CMRR. Based on their excellent performance, we believe that our MUTC photodiodes are promising candidates for high-speed Si3N4 photonic integrated circuit applications.
Funding
Defense Advanced Research Projects Agency (HR0011-15-C-0055); Air Force Office of Scientific Research (FA 9550-17-1-0071).
Disclosures
The authors declare no conflicts of interest.
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