# 30 GHz heterogeneously integrated capacitive InP-on-Si Mach-Zehnder modulators

Torrey Thiessen, Philippe Grosse, Jeremy Da Fonseca, Patricia Billondeau,

Bertrand Szelag, Christophe Jany, Joyce K S Poon, Sylvie Menezo

► To cite this version:

Torrey Thiessen, Philippe Grosse, Jeremy Da Fonseca, Patricia Billondeau, Bertrand Szelag, et al. 30 GHz heterogeneously integrated capacitive InP-on-Si Mach-Zehnder modulators. Optics Express, 2019, 27 (1), pp.102. 10.1364/oe.27.000102 . cea-04808877

HAL Id: cea-04808877

https://cea.hal.science/cea-04808877v1

Submitted on 28 Nov 2024

**HAL** is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
# 30 GHz heterogeneously integrated capacitive InP-on-Si Mach–Zehnder modulators

TORREY THIESSEN,<fnref n="1" />,<fnref n="2" /> PHILIPPE GROSSE,<fnref n="2" /> JEREMY DA FONSECA,<fnref n="2" /> PATRICIA BILLONDEAU,<fnref n="2" />, BERTRAND SZELAG,<fnref n="2" /> CHRISTOPHE JANY,<fnref n="2" /> JOYCE K. S. POON,<fnref n="1" /> AND SYLVIE MENEZO<fnref n="2" />,*

**Abstract:** We demonstrate high-bandwidth O-band Mach–Zehnder modulators with indium phosphide-on-silicon (InP-on-Si) capacitive phase shifters that are compatible with heterogeneous laser fabrication processes. An electro-optic conversion efficiency of 1.3 V·cm and a 3 dB bandwidth of up to 30 GHz was observed for a phase modulator length of 250 µm at a 0 V bias. Open eye patterns were observed at up to 25 Gb/s.

© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

## 1. Introduction

A challenge for optical transceivers in short-reach data communications is a mass-manufacturable, temperature-independent, power-efficient, high-speed optical transmitter. Mach–Zehnder modulators (MZMs) are often preferred over electro-absorption modulators (EAMs), since the latter typically require active thermal stabilization. EAMs rely on the steep change in absorption near the band-edge, which red-shifts as the temperature is increased [1]. Monolithically integrated silicon (Si) MZMs emerged as a promising technology [2] and are now of significant commercial interest [3]; however the modulation efficiency ($V_πL$) of typical lateral PN junction based Si MZMs is limited to about 2.5 V·cm. Si MZMs utilizing more complex geometries have been demonstrated, with $V_πL$ as low as 0.2-0.5 V·cm [4–6], but these often come at the expense of increased optical losses or limited bandwidth. Modulator performance on silicon platforms can be further improved by the heterogeneous integration of materials that demonstrate higher electro-optic (EO) modulation bandwidths (BWs) and efficiencies, such as $LiNbO_3$ [7] and EO polymers [8–10]. Although such $\chi^{(2)}$-based EO materials can support high BWs in excess of 100 GHz [11, 12] and a linear EO response, forming a complete transmitter would still require the additional use of III-V compounds for the laser.

For optical gain, as well as refractive index modulation, III-V quaternary alloys are excellent candidates for heterogeneous integration with Si. The low effective mass of electrons in these materials increases the plasma dispersion effect, which, when combined with the band filling effect, results in electron-induced refractive index changes that are 5 to 10 times larger in indium phosphide (InP) than in Si in the O and C bands [2, 13–15]. Heterogeneously integrated InP-on-Si capacitive phase modulators were first demonstrated for tuning the wavelength of a microring laser [16]. Recently, InGaAsP-on-Si capacitive MZMs were reported at 1550 nm [17, 18] and 1310 nm [19], with record-low $V_πL$ near 0.05-0.1 V·cm. These modulators have a similar geometry to the Si semiconductor-insulator-semiconductor capacitive (SISCAP) modulator in [4] and exploit the carrier accumulation on the Si-oxide interfaces for the effective index modulation. However, the EO BWs of the C-band devices were limited to < 4 GHz and modulation at 25 and 32 Gb/s required pre-emphasis and an optical amplifier. No high frequency measurements were reported for the O-band device.

In this article, we report on the first high speed heterogeneous InP-on-Si capacitive MZMs

<footnote n="1">¹Department of Electrical and Computer Engineering, University of Toronto, 10 King's College Road, Toronto, Ontario M5S 3G4, Canada</footnote>

<footnote n="2">²CEA-Leti, 17 rue des Martyrs, 38000 Grenoble, France</footnote>

<footnote>*sylvie.menezo@cea.fr</footnote>
<figure><img src="image_2.png" /><figcaption>Fig. 1. (a) Cross-section of hybrid InP-on-Si capacitive modulator platform. (b) SEM cross-section of a modulator with a $w_{top}$ of ~0 nm. (c) Optical mode supported by the hybrid InP-on-Si waveguide.</figcaption></figure>

for the O-band [20]. MZMs with 250 and 500 µm long phase-shifters have EO BWs of 30 and 11 GHz, respectively, IL of ~0.5-1 dB, and $V_πL$ of 1.3 V·cm. Open eye patterns at up to 25 Gb/s are demonstrated without optical amplifiers or pre-emphasis. The InP electrode of the MZM utilizes the same layer of the N-contact as in InP-on-Si lasers, and the modulator fabrication process is fully compatible with the heterogeneous InP-on-Si laser integration process [21, 22].

## 2. Device design

The cross-section and waveguide parameters of the nominal InP-on-Si phase modulator design are shown in Fig. 1(a). The hybrid waveguide phase-shifters are composed of a 400 nm or 500 nm wide ($w_{rib}$) P-doped Si rib with an N-doped InP slab separated by a thin layer of oxide ($t_{ox}$). The doping concentrations were $5×10^{17}$ cm⁻³, $2×10^{19}$ cm⁻³, and $3×10^{18}$ cm⁻³ for the P, P+, and N+ regions, respectively. The targeted oxide thickness between the Si and InP was 20 nm. The InP slab overhangs the rib waveguide to improve variation tolerance to the InP processing, which depends on the lithography alignment and the InP etching. Figure 1(b) shows a scanning electron micrograph (SEM) cross-section of a fabricated modulator with an InP overhang ($w_{top}$) of ~0 nm. Figure 1(c) shows the simulated mode profile of a 400 nm wide hybrid waveguide with an InP overhang of 300 nm at a wavelength of 1310 nm. The optical mode is primarily confined within the Si waveguide due to its higher refractive index.

Figure 2 shows the simulated EO efficiency, as given by the $V_πL$, the propagation loss due to free carrier absorption, and the high-frequency capacitance (simulated at 1 MHz), which were computed using Lumerical DEVICE and MODE. For the simulations in Fig. 2, the fixed waveguide parameters were set to $w_{rib} = 400$ nm, $t_{ox} = 20$ nm, and $w_{top} = 300$ nm. We used the data in [15] and [23] for the change in the real and imaginary parts of the InP refractive index, respectively. Figure 2(a) shows the simulated DC EO efficiency and propagation loss of a 400 nm wide InP-on-Si capacitive phase-shifter as a function of the oxide thickness. The oxide thickness between the InP and Si sets the device capacitance (Fig. 2(b)). A higher capacitance, due to a thinner oxide layer, leads to a larger carrier density change per volt at the oxide interfaces and a higher EO efficiency. The $V_πL$ shown in Fig. 2 is computed using the effective index difference between a 0 V and 4 V bias to the device, and the absorption loss is taken as the
<figure><img src="image_3.png" /></figure>

<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /><figcaption>Fig. 2. Simulated efficiency, absorption loss, and high-frequency capacitance as a function of (a)-(b) oxide thickness, (c)-(d) Si rib waveguides width, and (e)-(f) InP overhang.</figcaption></figure>

average between a 0 V and 4 V bias. The $V_π-L$ varies between 0.6 and 1.2 V·cm and is roughly inversely proportional to $t_{ox}$. To further increase the device efficiency, the oxide spacer can be replaced with a high-$\kappa$ dielectric [16, 17, 24].

Figures 2(c) and 2(d) show the simulated DC EO efficiency, propagation loss, and high-frequency capacitance of the hybrid waveguide with a 20 nm thick oxide spacer as a function of the Si rib width. A rib width of 400-500 nm provides a balance between the EO efficiency, loss, and capacitance (bandwidth). In narrower waveguides, the optical confinement within the waveguide is reduced, leading to lower efficiency and higher propagation losses. As the waveguide width is increased, the device capacitance increases but the carrier density at the interface is unchanged. Thus, the slight increase in efficiency, due to a better mode overlap between the optical mode and accumulated carriers, comes with an increase in capacitance which may limit the RC time constant. Figures 2(e) and 2(f) show the modulator characteristics as a function of the InP overhang. Once the InP overhang exceeds ~300 nm, the modulator performance becomes nearly independent of $w_{top}$.

### 3. Device fabrication

The devices were fabricated on a 200 mm silicon-on-insulator (SOI) wafer. Prior to waveguide etching, the Si was implanted P and P+ in two steps. The etched waveguide was then encapsulated in low-temperature deposited SiO$_2$, which was planarized and thinned down to a targeted thickness of 20 nm using chemical mechanical polishing (CMP). Post-CMP wafer-wide ellipsometry measurements indicated that the SiO$_2$ thickness where the InP wafer was bonded ranged from
<figure><img src="image_9.png" /><figcaption>Fig. 3. Optical micrograph of the fabricated 500 µm long MZM.</figcaption></figure>

10 nm to 40 nm. An InP substrate with an epitaxially grown 300 nm thick N+ doped InP layer was then directly bonded to the SOI at room temperature and annealed at relatively low temperatures. Following the removal of the InP substrate, the N+ doped InP was etched to form the N electrode of the modulator. The electrical contacts were formed on the electrodes, which were then encapsulated in benzocyclobutene (BCB). Vias were formed in the BCB to connect a top metal layer with the electrical contacts.

## 4. Mach–Zehnder modulators

The capacitive phase-modulators were integrated into MZMs. Two sets of devices were evaluated, one with a nominal rib width of 400 nm and length of 250 µm and one with a nominal rib width of 500 nm and length of 500 µm. The devices were driven as lumped elements, without traveling-wave electrodes. An optical micrograph of the fabricated 500 µm MZM is shown in Fig. 3. The MZM is composed of two 3 dB multimode interference (MMI) couplers, a path imbalance $\Delta L$ using 400 nm wide Si rib waveguides, and a hybrid InP-on-Si phase-shifter section. The path imbalance was introduced to ease the testing.

To form the hybrid waveguide, a 60 µm long, linear, adiabatic taper was used to introduce the InP slab over the Si waveguide. The transition losses due to the adiabatic tapers were checked by comparing the transmission spectra of cascaded chains of 2, 4, and 8 transitions from passive Si waveguide to the hybrid InP-on-Si section to equivalent lengths of hybrid InP-on-Si waveguide. The spectra showed no distinguishable difference, suggesting that the transition losses were negligible.

With the spectrum of the input/output grating couplers removed, the device insertion loss was ~0.5 dB and 1 dB for the MZM with 250 µm and 500 µm long phase shifters, respectively. Assuming insertion losses of 0.1 dB for the MMIs, the propagation loss of the hybrid InP-on-Si waveguide was 12-16 dB/cm. This is in good agreement with the simulated value of about 12 dB/cm in Fig. 1(c) plus about 2.5 dB/cm due to the sidewall roughness of the Si waveguide.

### 4.1. DC characterization

Figures 4(a) and 4(b) show the shift in the transmission spectrum as a function of a DC bias voltage applied to the top arm of the 250 µm long ($w_{rib} = 400$ nm) and 500 µm long ($w_{rib} = 500$ nm) MZMs, respectively. The response of the grating couplers has been removed from the data in Figs. 4(a) and 4(b). The DC MZM transmission spectra showed extinction ratio (ERs) around 25 and 35 dB for the 250 and 500 µm long modulators, respectively, suggesting that the losses in the two arms of the modulator were nearly balanced. The measured phase shift per unit length as a function of voltage is plotted for the two devices in Fig. 4(c) (solid lines) and agrees well with simulation (dashed lines). We extract the $V_πL$ by considering the wavelength shift of the
<figure><img src="image_10.png" /><figcaption>Fig. 4. The DC tuning spectra of (a) the 250 µm long MZM and (b) the 500 µm long MZM, respectively, when a voltage is applied to the top arm. The response of the grating couplers has been removed from the data in (a) and (b). (c) Accumulated phase shift per mm as a function of applied voltage for the measured (solid lines) and simulated (dashed lines) hybrid waveguides. (d) Measured and simulated capacitance of the 400 nm wide hybrid waveguide, as a function of applied voltage, for an oxide thickness of 20 nm. (e) Measured and simulated capacitance of the 500 nm wide hybrid waveguide, as a function of applied voltage, for an oxide thickness of 20 nm. The measurement with a lower modulation frequency shows a higher capacitance in depletion.</figcaption></figure>

transmission minima near 1310 nm when the bias is varied between 0 V and 4 V. For both modulators, the measured $V_πL$ was 1.3 V·cm.

Figures 4(d) and 4(e) compare the measured and simulated capacitance per unit length for the 400 nm wide and 500 nm wide capacitors, respectively, at a frequency of 1 MHz and an oxide thickness of 20 nm. The capacitance increases with increasing bias as the capacitor reaches accumulation. Although the capacitance decreases in reverse bias (i.e., in depletion), which can support a higher BW, this comes at the cost of a decrease in efficiency. In both devices, the measured capacitance was higher than the simulated value. It is unlikely that the discrepancy is due solely to a difference in the simulated and fabricated oxide thickness, as the device efficiency agrees quite well with simulation. The low-temperature deposited TEOS $SiO_2$ used in the modulator is expected to have a higher dielectric constant than thermally grown $SiO_2$ [25], which could contribute to a higher than expected capacitance in both the depletion and accumulation regimes. The measured capacitance of the 500 nm wide capacitor at a modulation frequency of 100 kHz, which is included in Fig. 4(e), is higher in depletion than the measurement at 1 MHz, indicating that there are traps at the oxide interface. This may further account for some of the discrepancy between the simulated and measured capacitance. Lastly, the difference in the simulated and measured capacitance is roughly twice as large for the capacitor with $w_{rib} = 500$ nm than for the capacitor with $w_{rib} = 400$ nm, suggesting there may be some fabrication non-uniformity.

The access resistance of the 250 µm and 500 µm long modulators were about 38 Ω and 22 Ω, respectively, with roughly half coming from the P contact. The EO 3 dB BWs of the devices, which are equivalent to the electrical 6 dB BWs due to the conversion from optical power to voltage at the receiver, can be approximated by $f_{3dB} = \sqrt{3}/(2\pi RC)$ [26]. Using the measured
<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /></figure>

<figure><img src="image_14.png" /><figcaption>Fig. 5. The measured electrical S11 and EO S21 of (a) the 250 µm long modulator and (b) the 500 µm modulator. The EO 3 dB BWs in (a) and (b) are 30 GHz and 11 GHz with a 0 V bias applied and 26 GHz and 9 GHz with a 2 V bias, respectively. (c) The eye pattern of the 500 µm long modulator biased at 0 V and driven with 2.4 Vpp at 10 Gb/s using the PRBS-9 pattern. No optical amplification or pre-emphasis was used. The ER and OMA were 4.8 dB and -2.3 dBm, respectively. (d) The eye pattern of the 250 µm long modulator biased at 0 V and driven with 4 Vpp at 25 Gb/s using the PRBS-9 pattern. The ER and OMA are 2.3 dB and -5.1 dBm, respectively.</figcaption></figure>

capacitances in Figs. 4(d) and 4(e), the RC limited EO 3 dB BWs of the 250 µm and 500 µm long modulators with a 0 V bias are about 29 GHz and 17 GHz, respectively.

## 4.2. High-frequency characterization

Figures 5(a) and 5(b) show the measured scattering parameters, the electrical S11 and EO S21, of the MZMs at 0 V and 2 V bias taken with a 67 GHz vector network analyzer (Agilent N4373C) with the input laser wavelength tuned to the MZM -3 dB transmission point. No optical amplifiers were used in the measurement. The top arm of the modulator was driven without termination. The EO S21 is plotted as half of the electrical S21 [26]. The 3 dB frequencies of the 250 and 500 µm long modulators were 30 GHz and 11 GHz with a 0 V bias and 26 GHz and 9 GHz with a 2 V bias, respectively. In forward bias, the devices were more efficient, but their BWs were reduced due to the increased capacitance. The peaking in the EO S21 of the 250 µm modulators at low frequencies (~3 GHz) may have been caused by reflected electrical power, which increased the effective voltage across the MZM. In the longer device, the peaking is no longer present.

Figures 5(c) and 5(d) show the measured eye patterns of the MZMs using a 2⁹⁻¹ pseudo-random binary sequence (PRBS) signal, without optical amplification or pre-emphasis. For both eye diagrams, the laser power was 15 mW. The laser was set to a wavelength corresponding to -3 dB from the MZM transmission maximum. The optical output from the MZM was directly detected using a 12 GHz photoreceiver (New Focus 1544) connected to a 80 GHz BW sampling oscilloscope (Keysight 86100D, 86116C module). For the eye patterns in Fig. 5, the MZMs were differentially driven. The large EO BWs of the devices enabled high-frequency operation without pre-emphasis. Figure 5(c) shows the eye diagram from the 500 µm long modulator operating at 10 Gb/s when biased at 0 V and driven at 2.4 Vpp. The extinction ratio and optical modulation amplitude (OMA) were 4.8 dB and -2.3 dBm, respectively. Figure 5(d) shows the eye diagram
from the 250 µm long modulator operating at 25 Gb/s when biased at 0 V and driven at 4 Vpp.
The ER and OMA were 2.3 dB and -5.1 dBm, respectively.

**5. Discussion**

Compared to prior works on hybrid InP-on-Si capacitive MZMs [17, 18], we have achieved higher
EO BW operation (30 GHz and ≥ 25 Gb/s) without pre-emphasis. The high EO BW was enabled
by a lower series resistance and a lower capacitance, though the latter led to a lower EO efficiency.
The ERs of the MZMs are consistent with the DC VπL, the lengths of the hybrid waveguide
sections, and the applied voltage at 10 Gb/s, but lower than expected at 25 Gb/s. Inferring from
the DC EO phase shift efficiency, the maximum achievable DC ER for the 500 µm long modulator
shown in Fig. 5(c) with a drive voltage of Vpp = 2.4 V is 5.3 dB, which is close to the measured
value of 4.8 dB. The maximum achievable DC ER for the 250 µm long modulator shown in
Fig. 5(d) with a drive voltage of Vpp = 4 V is 4.4 dB, much higher than the measured ER of
2.3 dB. This is most likely due to the limited bandwidth of the photoreceiver (12 GHz), since the
efficiency roll-off of the modulator should not be significant at 25 Gb/s.

The performance of these devices can be significantly improved in the future. Using a silicide
on the P contact would roughly halve the access resistance of the device, enabling higher BWs
without sacrificing capacitance. Optimization of the layer thicknesses and waveguide geometry
can provide higher efficiencies by improving the optical mode overlap with the accumulation
region. The mode overlap can also be improved by using a higher index III-V quaternary alloy,
which would more evenly distribute the mode between the two materials. Alternatively, the
efficiency could be improved by introducing a III-V quaternary alloy that has been optimized
to provide the highest index change per electron. To reduce the Vπ of the device, longer phase
modulation sections can be used in conjunction with traveling wave electrodes. Lastly, the use of
an alternative dielectric material between the Si and InP electrodes, with improved thickness
accuracy, could reduce the device variability.

**6. Conclusion**

In summary, we have demonstrated the first high BW InP-on-Si capacitive MZMs operating in
the O-band. The hybrid phase modulators achieved an efficiency of 1.3 V·cm and optical losses
of 12-16 dB/cm. An EO 3 dB BW of up to 30 GHz was achieved and open eye patterns were
observed at up to 25 Gb/s. The performance of these devices can be significantly improved in
future fabrication iterations, to achieve both higher efficiency and higher EO BWs. Importantly,
the fabrication is compatible with hybrid laser integration, and laser integration is currently
underway.

**References**

1. I. Vurgaftman, J. Meyer, and L. Ram-Mohan, "Band parameters for III-V compound semiconductors and their alloys," J. Appl. Phys. **89**, 5815-5875 (2001).

2. G. T. Reed, G. Mashanovich, F. Gardes, and D. Thomson, "Silicon optical modulators," Nat. Photonics **4**, 518–526 (2010).

3. F. Boeuf, S. Crémer, E. Temporiti, M. Feré, M. Shaw, C. Baudot, N. Vulliet, T. Pinguet, A. Mekis, G. Masini, H. Petiton, P. L. Maitre, M. Traldi, and L. Maggi, "Silicon photonics R&D and manufacturing on 300-mm wafer platform," J. Light. Technol. **34**, 286–295 (2016).

4. M. Webster, C. Appel, P. Gothoskar, S. Sunder, B. Dama, and K. Shastri, "Silicon photonic modulator based on a MOS-capacitor and a CMOS driver," in *CSICs*, (2014), pp. 1–4.

5. J. Fujikata, S. Takahashi, M. Takahashi, M. Noguchi, T. Nakamura, and Y. Arakawa, "High-performance MOS-capacitor-type Si optical modulator and surface-illumination-type Ge photodetector for optical interconnection," *Jpn. J. Appl. Phys.* **55**, 04EC01 (2016).

6. Z. Yong, W. D. Sacher, Y. Huang, J. C. Mikkelsen, Y. Yang, X. Luo, P. Dumais, D. Goodwill, H. Bahrami, P. G.-Q. Lo, E. Bernier, and J. K. S. Poon, "U-shaped PN junctions for efficient silicon Mach-Zehnder and microring modulators in the O-band," *Opt. Express* **25**, 8425–8439 (2017).
7. C. Wang, M. Zhang, B. Stern, M. Lipson, and M. Lončar, "Nanophotonic lithium niobate electro-optic modulators," Opt. Express **26**, 1547-1555 (2018).

8. S. Koeber, R. Palmer, M. Lauermann, W. Heni, D. L. Elder, D. Korn, M. Woessner, L. Alloatti, S. Koenig, P. C. Schindler et al., "Femtojoule electro-optic modulation using a silicon-organic hybrid device," Light. Sci. & Appl. **4**, e255 (2015).

9. S. Wolf, H. Zwickel, W. Hartmann, M. Lauermann, Y. Kutuvantavida, C. Kieninger, L. Altenhain, R. Schmid, J. Luo, A. K.-Y. Jen et al., "Silicon-organic hybrid (SOH) Mach-Zehnder modulators for 100 Gbit/s on-off keying," Sci. Reports **8**, 2598 (2018).

10. C. Kieninger, Y. Kutuvantavida, D. L. Elder, S. Wolf, H. Zwickel, M. Blaicher, J. N. Kemal, M. Lauermann, S. Randel, W. Freude, L. R. Dalton, and C. Koos, "Ultra-high electro-optic activity demonstrated in a silicon-organic hybrid modulator," Optica **5**, 739-748 (2018).

11. P. O. Weigel, J. Zhao, K. Fang, H. Al-Rubaye, D. Trotter, D. Hood, J. Mudrick, C. Dallo, A. T. Pomerene, A. L. Starbuck, C. T. DeRose, A. L. Lentine, G. Rebeiz, and S. Mookherjea, "Bonded thin film lithium niobate modulator on a silicon photonics platform exceeding 100 GHz 3-dB electrical modulation bandwidth," Opt. Express **26**, 23728-23739 (2018).

12. L. Alloatti, R. Palmer, S. Diebold, K. P. Pahl, B. Chen, R. Dinu, M. Fournier, J.-M. Fedeli, T. Zwick, W. Freude, C. Koos, and J. Leuthold, "100 GHz silicon-organic hybrid modulator," Light. Sci. & Appl. **3**, e173 (2014).

13. R. Soref and B. Bennett, "Electrooptical effects in silicon," IEEE J. Quantum Electron. **23**, 123-129 (1987).

14. B. R. Bennett, R. A. Soref, and J. A. D. Alamo, "Carrier-induced change in refractive index of InP, GaAs and InGaAsP," J. Quantum Electron. **26**, 113-122 (1990).

15. L. Chusseau, P. Martin, C. Brasseur, C. Alibert, P. Hervé, P. Arguel, F. Lozes-Dupuy, and E. V. K. Rao, "Carrier-induced change due to doping in refractive index of InP: measurements at 1.3 and 1.5 µm," Appl. Phys. Lett. **69**, 3054-3056 (1996).

16. D. Liang, X. Huang, G. Kurczveil, M. Fiorentino, and R. Beausoleil, "Integrated finely tunable microring laser on silicon," Nat. Photonics **10**, 719-722 (2016).

17. J.-H. Han, F. Boeuf, J. Fujikata, S. Takahashi, S. Takagi, and M. Takenaka, "Efficient low-loss InGaAsP/Si hybrid MOS optical modulator," Nat. Photonics **11**, 486 (2017).

18. T. Hiraki, T. Aihara, K. Hasebe, K. Takeda, T. Fujii, T. Kakitsuka, T. Tsuchizawa, H. Fukuda, and S. Matsuo, "Heterogeneously integrated III-V/Si MOS capacitor Mach-Zehnder modulator," Nat. Photonics **11**, 482 (2017).

19. J.-H. Han, S. Takagi, and M. Takenaka, "High-efficiency O-band Mach-Zehnder modulator based on InGaAsP/Si hybrid MOS capacitor," in *Optical Fiber Communication Conference*, (Optical Society of America, 2017), p. W3E.2.

20. S. Menezo, T. Thiessen, P. Grosse, J. K. S. Poon, C. Jany, J. D. Fonseca, B. Szelag, B. Charbonnier, G. El-Zammar, O. Lemonnier, P. Bilondeau, S. Malhouitre, B. Montmayeul, and L. Sanchez, "High-speed heterogeneous InP-on-Si capacitive phase modulators," in *Optical Fiber Communication Conference*, (Optical Society of America, 2018), p. Tu3K.4.

21. B. B. Bakir, C. Sciancalepore, A. Descos, H. Duprez, D. Bordel, L. Sanchez, C. Jany, K. Hassan, P. Brianceau, V. Carron, and S. Menezo, "Heterogeneously integrated III-V on silicon lasers," ECS Transactions **64**, 211-223 (2014).

22. H. Duprez, A. Descos, T. Ferrotti, C. Sciancalepore, C. Jany, K. Hassan, C. Seassal, S. Menezo, and B. B. Bakir, "1310 nm hybrid InP/InGaAsP on silicon distributed feedback laser with high side-mode suppression ratio," Opt. Express **23**, 8489-8497 (2015).

23. O. K. Kim and W. A. Bonner, "Infrared reflectance and absorption of N-type InP," J. Electron. Mater. **12**, 827-836 (1983).

24. X. Huang, D. Liang, C. Zhang, G. Kurczveil, X. Li, J. Zhang, M. Fiorentino, and R. Beausoleil, "Heterogeneous MOS microring resonators," in *IPC*, (2017), pp. 121-122.

25. O. F. Rocha, C. E. Viana, L. C. D. Gonçalves, and N. I. Morimoto, "Electrical characteristics of PECVD silicon oxide deposited with low TEOS contents at low temperatures," in *Microelectronics Technology and Devices SBMICRO 2004*, (The Electrochemical Society, 2004), pp. 295-300.

26. G. Ghione, *Semiconductor Devices for High-Speed Optoelectronics* (Cambridge University Press, 2009), chap. 6.