Strain-enabled ultra-broadband Ge-based photonic devices for low-cost integration in PICs

Photonic-integrated circuits (PICs) have become one of the most promising solutions to the burgeoning global data communication and are being envisioned to have revolutionary impact in many other emerging elds. This outlook requires future PICs to be signicantly more broadband and cost-effective. The current germanium (Ge)-based active photonic devices in PICs are thus facing a new bandwidth-cost trade-off. Here, we demonstrate ultra-broadband, high-eciency Ge photodetectors up to 1,630 nm operation wavelength and Ge 0.99 Si 0.01 electro-absorption (EA) modulator arrays with an operating range of ~100 nm from 1,525 to 1,620 nm, using a CMOS-compatible recess-type silicon nitride (SiN x ) stressor. The broadband operation could facilitate a wide (>100 nm) window for low-cost Ge modulator-detector co-integration, requiring only a single step of Ge epitaxy and two different SiN x depositions. The broad modulation and co-integration coverage can be entirely shifted to shorter (~1,300 nm) and longer (>1,700 nm) wavelengths with small amounts of Si or tin (Sn) alloying. This proof-of-concept work provides a pathway for PICs towards future low-cost and high-data-capacity communication networks, immediately accessible by designers through foundries.


Introduction
The demand for high-speed and large-capacity data communication has surged in the past decade, primarily due to the advent of insatiable needs in the emerging elds of high-de nition video streaming, cloud computing, internet-of-things, augmented reality, and many other applications. Photonic-integrated circuits (PICs) have thus been becoming one of the most promising solutions to potentially alleviate the speed, power, and bandwidth bottlenecks of its electronic counterparts at short-reach level interconnects (i.e. rack-to-rack, board-to-board, chip-to-chip and intra-chip) for high-performance computing and data center networks, by leveraging the state-of-the-art ber-optic communication infrastructure and silicon (Si) CMOS technology 1,2 . Beyond optical interconnects, PICs have recently been envisioned to have revolutionary impact on future sensing 3,4 , quantum computing 5,6,7 , arti cial intelligence 8,9,10 , and neuroscience 11,12 , to name a few. The burgeoning global data demand and future widespread utilization require future PICs to be more broadband and cost-effective. For the common optical transceiver PICs, a straightforward solution to the broadband operation is to utilize the short-reach nature of the data center networks. Unlike conventional long-haul ber-optic tele-communication, the ber link length is much shorter (in the range of meters or below) among PIC-incorporated optical modules in data centers. The shorter length leaves a broader wavelength window and consequently higher capacity potential for lowloss transmission (e.g. intrinsic loss <0.4 dB/km from ~1,300 to 1,700 nm 13 ), using the commonlyadopted wavelength-division multiplexing (WDM) scheme. One may exploit this wider window for future broadband PICs; however, this would give rise to new challenges for the development of existing photonic devices in PICs. For example, Ge, due to its CMOS-compatibility, considerable absorption coe cient 14 , and signi cant Franz-Keldysh effect 15 around 1,550 nm, has become an indispensable material in PICs for high-performance photodetectors 16,17,18,19,20 and electro-absorption (EA) modulators 21,22,23,24,25,26 . With years of development, Ge has shown an optimal adaptability to foundry processes. As of now, Ge photodetectors are ubiquitous in the process design kits (PDKs) of most silicon photonics foundries, and Ge EA modulators are also being actively included. Compared to the Si counterparts, Ge EA modulators have exhibited a smaller device footprint and lower power consumption than Mach-Zehnder modulators 27,28 , as well as a wider operating spectrum and superior temperature insensitivity than microring (MR) modulators 29,30 . Furthermore, MR modulators need additional control electronics and consequently increased power consumption to stay on resonance. However, the operating spectra of both devices remain limited compared to the potential low-loss window. The quantum e ciency of Ge photodetectors drastically drops beyond the C-band (>~1,570 nm); while Ge EA modulators generally exhibit a narrow optical bandwidth of 30 nm (~1,610 -1,640 nm) depending on the applied bias and insertion loss. Although GeSn 31 epitaxy and separate steps of GeSi epitaxy with different Si compositions can be viable for broadband photo-detection and EA modulation, it would inevitably lead to process complexity with increased cost. Likewise, heterogeneous integration via die bonding 32 or transfer printing 33 demands multiple bandgap-varied materials and stringent yield control. Another side effect from the bandwidth limitation is the operating spectra incompatibility between the Ge photodetectors and EA modulators. As discussed, Ge photodetectors display an optimal performance up to the C-band; whereas Ge EA modulators work most e ciently in the L-band (~1,610 nm). This poses a challenge to integrate both devices into a single transceiver PIC to fully harness their individual performance potential.
Similar drawbacks of process complexity and integration cost would remain if multiple epitaxial steps are employed. Therefore, enabling a broadband operation and co-integration of Ge-based photodetectors and EA modulators in a single epitaxial growth is of paramount importance for future high-capacity and lowcost PICs.
Prior studies have adopted a single epitaxy step for platforms that are potentially suitable for the modulator-detector co-integration and their broadband operation. Chaisakul et al. 34 realized a Ge/SiGe multi-quantum-well (MQW)-based platform in a single epitaxy step and demonstrated a Ge optical interconnect with co-integrated photodetectors and EA modulators. However, the epi-layer stack is too thick (~10 µm) for practical front-end-of-line integration and the resulting link optical bandwidth remains limited (~20 nm). Additionally, the Si-incorporated MQW design makes the platform di cult for a broader operating wavelength coverage. Furthermore, Littlejohns et al. 35 employed a single step of GeSi rapid melting growth on a tree-like structure and realized a GeSi strip array with varied inter-strip but uniform intra-strip Ge compositions. However, this technique lacks further device demonstration.
Here, we demonstrate an approach of utilizing a recess-type silicon nitride (SiN x ) stressor for Ge photodetectors and Ge 0.99 Si 0.01 EA modulator arrays with ultra-broad optical bandwidths. The SiN x stressor induces mechanical strain in the Ge-based materials for their bandgap tuning and the broadband device operation. The recessed stressor con guration at sidewalls prominently enhances both the strain magnitude and uniformity and leads to a wider operating optical bandwidth. As a proof of concept, recessed tensile (~600 MPa) SiN x -strained waveguide-shaped Ge-on-insulator (w-Ge-OI) metalsemiconductor-metal (MSM) photodetectors exhibit an extension of the photocurrent roll-off by ~70 nm to 1,612 nm, with a ~3.3× enhancement on the absorption coe cient. The concept is easily transferrable to conventional Ge photodetectors on-Si or -silicon-on-insulator (-SOI) wafers. The broadband EA modulation is achieved in device arrays utilizing the Ge 0.99 Si 0.01 strain tunability dependent on the Ge 0.99 Si 0.01 width with an identical SiN x stressor. By varying the Ge 0.99 Si 0.01 width from 0.4 to 4.0 µm with the similar 600-MPa tensile stressor, the operation of Ge 0.99 Si 0.01 EA modulator arrays on-SOI spans from ~1,525 to ~1,620 nm, with an extinction-ratio-to-insertion-loss (ER/IL) ratio above 1.3 at a reverse bias of -6 V. Incorporating an additional separate compressive (~1 GPa) stressor could extend the total coverage from ~1,460 to ~1,620 nm. In addition, the wider operating coverages of both devices are expected to facilitate a broadband (>100 nm, ~1,520-1,630 nm) Ge modulator-detector co-integration, by applying compressive (1 GPa) and tensile (600 MPa) stressors on the modulator arrays and photodetectors, respectively. Both, the broadband operations and the co-integration require only a single step of Ge-based growth and a maximum of two different SiN x depositions, one tensile and the other compressive, enabling simple and low-cost integration. Furthermore, the Si composition can be increased to 4% to shift the entire broad operating range to ~1,300 nm, while Sn can also be alloyed into Ge for an operating coverage beyond 1,700 nm. The mature SiN x -based process 36,37 makes this approach readily accessible in the state-of-the-art foundries. Compared to other Ge strain engineering techniques 38,39,40 , the recessed SiN x stressor maintains both CMOS-compatibility and small device footprint for compact integration. Foreseeably, the operating bandwidth can be further extended by using a SiN x stressor with higher stress. This technique provides a pathway for future broadband and low-cost PICs with cointegrated Ge-based EA modulator and photodetector arrays ( Fig. 1) and can be potentially applied to other semiconductor material platforms (e.g. III-V, 2D) for bandgap engineering and corresponding diversi ed electronic and photonic applications.

Results And Discussion
Recessed SiN x stressor for enhanced and uniform mechanical strain To present a wider adaptability of this technique, as a proof of concept, the broadband photodetection and EA modulation were demonstrated on Ge-on-insulator (Ge-OI) and Ge 0.99 Si 0.01 on SOI platforms, respectively. The Ge-OI photodetectors are rst discussed. In contrast to the technical maturity of the Geon-SOI platform, the Ge-OI platform is recently gaining attention for both advanced electronic 41 and photonic 42,43,44 applications. In this work, the Ge-OI was formed on a 200-mm Si (100) wafer via Ge-on-Si epitaxy, bonding, and layer transfer, using silicon dioxide (SiO 2 ) as the intermediate insulator layer 45 .
The detailed fabrication process is discussed in Supplementary section S1. The bonding and layer transfer approach could facilitate a scalable Ge-OI fabrication to any wafer diameter and Ge thickness, with a superior Ge quality than that from the direct Ge-on-Si epitaxy 46 . Its low process thermal budget (~300 ºC) enables a back-end-of-line electronic-photonic integration. It is without doubt that both the photodetectors and modulators can be demonstrated interchangeably on the other platform. The Ge-OI platform was also used to study the effect of the recessed SiN x stressor on its induced Ge mechanical strain (Fig. 2). A tensile stressor is placed in trenches at both sides of w-Ge-OI and in contact with Ge via the waveguide sidewalls (Fig. 2a); while deeper trenches are created into the underlying SiO 2 as the recessed con guration to accommodate the stressor (Fig. 2b). The modelling was performed using a nite element method, with a detailed description in Supplementary section S2. The results are shown in Fig. 2c, d. It was found that the transverse tensile strain () in Ge is enhanced with the use of the recessed SiN x stressor, especially at the bottom portion of Ge closer to the buried SiO 2 . The strain uniformity is thus improved simultaneously. This increased uniformity is due to the recessed stressor con guration that allows the tensile stress to be applied on both the sidewalls of SiO 2 and Ge to pull on both materials.
The constraint to the sidewall stressor from the underlying SiO 2 and Si bulk is therefore alleviated, enabling a uniform Ge strain pro le without selective substrate removal to form suspended Ge were used and lead to distinct photon penetration depths of ~9 and ~89 nm 47 , respectively, into Ge, facilitating Raman spectra revealing depth-dependent strain information. For this reason, the w-Ge was thinned down to ~100 nm, allowing a full Ge thickness coverage for the strain acquisition at the laser excitation of 785 nm.
The micro-Raman spectra on 0.5 µm-wide w-Ge-OIs are shown in Fig. 2h. The laser spot diameter of ~1 µm at both 532 and 785 nm ensures a complete transverse coverage of the waveguide structures. It can be observed that the peak LO phonon frequencies of all spectra are red-shifted compared to that of bulk Ge (~300.8 cm -1 ), indicating that a tensile strain is induced in Ge as expected. It is important to mention that the widths of the w-Ge-OI sidewall trenches are within 10 µm to accommodate the SiN x stressor, and the corresponding Raman spectra exhibit a consistent Ge tensile strain independent of the trench width ( Supplementary Fig. S10). The narrow trench width with a signi cant strain effect could envision a compact device integration. The Raman spectra reasonably overlap at both 532 and 785 nm for w-Ge-OIs with the recessed stressor, while only a partial overlap below ~300 cm -1 was seen for that with the nonrecessed stressor. The broader full-width-at-half-maximum (FWHM, ~12.90 cm -1 ) of the spectrum for the non-recessed w-Ge-OI at 785 nm, compared to the recessed structure (~9.04 cm -1 ), is attributed to the strain non-uniformity rather than strain-induced crystal quality deterioration 48 . This conclusion is made based on the gradual blue shift of the peak LO phonon frequencies with time, observed from some earlybatch tensile-strained w-Ge-OIs ( Supplementary Fig. S9), where the SEM inspection reveals delaminated stressors from the w-Ge-OI sidewalls. The matching Raman spectra with respect to an un-strained w-Ge-OI (~300 cm -1 ), after the stressor delamination, verify that the stressor has not caused any Ge crystal quality degradation. Therefore, the phenomena in Fig. 2h suggests an improved strain uniformity in Ge vertically along the z-axis as seen in Fig. 2d. Furthermore, it can be inferred that the Ge is compressively strained at the bottom part of Ge closer to the Ge/SiO 2 interface, since the deeper-penetrating Raman spectrum at 785 nm (Fig. 2h, upper panel) exhibits a shoulder at ~305 cm -1 while the spectrum at 532 nm does not. The compressive strain might be due to both the substrate constraint from the SiO 2 -on-Si bulk and the tensile strain at the top portion of w-Ge, which collectively exerts a compressing force to the bottom part of w-Ge. The reason for no compressive strain shown in the nite element simulation pertains probably to the thinner substrate thickness (100 µm) employed in the simulation than a 200-mm Si wafer (~725 µm), which alleviates the substrate constraint. The compressive Raman shoulder was suppressed with the adoption of the recessed stressor (Fig. 2h, lower panel), suggesting a strain enhancement at the bottom part of Ge. This agrees well with the mechanism of the recessed stressor from the simulation (Fig. 2c, d). As the tensile stressor at both sidewalls essentially pulls the sandwiched Ge in the middle for a tensile strain, the adhesion of the stressor to the sidewalls becomes critical. It is noteworthy that oxygen (O 2 ) plasma treatment signi cantly enhances the duration of the stressor adhesion ( Supplementary Fig. S9). Additional micro-Raman measurements on 2 µm-wide w-Ge-OIs nd that the compressive strain is accumulated close to sidewalls at the bottom part of w-Ge-OI, which was turned into tensile strain with the use of the recessed stressor ( Supplementary Fig. S7), thus improving the Ge strain uniformity with an enhanced magnitude. The mechanism can be similarly applied to the recessed compressive SiN x stressor for the magnitude and uniformity enhancement of the compressive strain.
Broadband strained Ge-OI MSM photodetectors To investigate the effect of the recessed SiN x stressor on the performance of Ge photo-detection, metalsemiconductor-metal (MSM) photodetectors were developed based on the strained w-Ge-OI structures, with a schematic shown in Fig. 3a. As a reference, MSM detectors on w-Ge-OIs without stressor were also fabricated. All w-Ge-OIs have identical Ge width of 1 µm and thickness of 400 nm. The detailed fabrication for all the devices is illustrated in Supplementary section S8. It is noteworthy that Al 2 O 3 is employed in the fabrication, serving both as an interlayer (~1 nm) at the metal/Ge interface and an etchstop layer (~20 nm) between the SiN x and Ge. The former unpins the Fermi level of Ge from its valence band edge 49 for an elevated Schottky barrier height (Supplementary Fig. S12) and consequently a suppressed dark current, while the latter avoids an undesired etching of Ge during the SiN x removal at its top 50 . A top-view SEM image of a recessed SiN x -strained device is shown in Fig. 3b,  Considering the Ge effective absorption area under the optical ber tip (tip diameter ~9 µm) without metal shielding, an optical responsivity of ~1.6 A/W was obtained at 2 V, corresponding to an internal quantum e ciency (IQE) of ~129% (Fig. 3c). Compared to that of the reference detectors without stressor, the IQE is signi cantly enhanced ( Supplementary Fig. S14). As the only difference in fabrication between the two devices lies in the introduction of SiN x , the IQE enhancement can be attributed to the defect and trap states in the SiN x , due to its nonstoichiometric (nitrogen-rich) nature from both the ellipsometry measurement ( Supplementary Fig. S4) and the low SiH 4 /NH 3 ratio used in the deposition (Supplementary   Table S2 Fig. S15). The IQE >100% indicates the existence of a photocurrent gain. The gain is unlikely originated from the interfacial charge as previously reported for MSM photodetectors 54 that induces an injection of extra image carriers into the device for a charge neutrality. This argument is based on the observation from the electric eld simulation (Supplementary Fig. S16) indicating that the carrier transport in Ge had reached its saturation velocity (~6×10 6 cm/s) at a low bias of 0.3 V, implying a full collection of photon-generated carriers and a saturated IQE. However, this is inconsistent with the increasing, and < 100%, IQE with an increasing bias observed in Fig. 3c and Supplementary Fig. S14. This suggests that the photocurrent is hindered by the Al 2 O 3 barrier and the barrier does not provide a gain mechanism to the photocurrent. Further observation nds that the gain can be attributed to the avalanche multiplication in Ge, since the electric eld at 2 V (~180 kV/cm), compared to that in literature 55 , is su ciently high to impose avalanche ampli cation ( Supplementary Fig. S16). (both at right). As expected, a uniform of ~0.42% is obtained for the recessed SiN x -strained w-Ge-OI, while the non-recessed stressor results in a two-step pro le of ~0.35% at the upper part of Ge and ~0.23% at the lower part. Combining these results, the calculated Ge bandgap edges at the respective simulated ( Fig. 3g) reasonably match with that observed from the photocurrent spectra in Fig. 3d. Subsequently, the simulated was used to calculate the absorption coe cient (a) enhancement of Ge, based on the photocurrent spectra. The detailed calculation is discussed in Supplementary section S10. Compared to the a (2,265 cm -1 ) of the w-Ge-OI without stressor, the a (7,474 cm -1 ) is enhanced by ~3.3× for the recessed SiN x -strained w-Ge-OI at 1,612 nm, which is comparable with that of In 0.53 Ga 0.47 As material at the same wavelength 58 . This suggests Ge photodetectors could reach a similar optoelectronic performance as InGaAs detectors and extend its application into prevailing III-V-dominated elds such as short-wave infrared imaging. In addition, the 3.3× increase on a implies a ~70% reduction on the photon penetration depth (∝ 1/a) in Ge. The detector could consequently become more compact (a ~70% reduction in length and capacitance) with a commensurate photon absorption, alleviating the common responsivity-bandwidth trade-off. A maximum 3.3× enhancement of the 3-dB bandwidth can be expected, provided that the device is limited by RC delay. It is also worthwhile to note that the equivalent a (6,000 cm -1 ) for the w-Ge-OI without stressor at 1,500 nm is extended by 120 nm towards 1,620 nm for the recessed SiN x -strained w-Ge-OI, covering almost the entire Cand L-bands. This indicates that the performance of a Ge photodetector at 1,620 nm with the use of the recessed SiN x stressor can be comparable to that at 1,500 nm without stressor. More channels at the L-band could thus be incorporated into the PIC design using the WDM scheme for a higher data capacity.
Broadband strained Ge 0.99 Si 0.01 EA modulator arrays The ultra-broadband EA modulation is demonstrated on a Ge 0.99 Si 0.01 on SOI platform via the design and fabrication of SiN x -strained waveguide-integrated Ge 0.99 Si 0.01 modulator arrays. The reason for the ~1% Si incorporation in Ge is to position the broadband modulation with both tensile (~600 MPa) and compressive (~1 GPa) SiN x stressors within the available tunable laser spectrum (1,510-1,610 nm) of our measurement system. Fig. 4a shows a schematic of an individual modulator and its working principle utilizing the Franz-Keldysh (FK) effect. The inset shows an optical microscope image of a fabricated device. Waveguide-shaped Ge 0.99 Si 0.01 mesas (~300 nm-thick, 50 µm-long) were placed on top of Si -OI waveguides (~250 nm-thick) with tapers on both ends for e cient optical coupling into and out of the modulators (Fig. 4b). The Si waveguides are cross-sectionally in rib shape (Fig. 4c (right)) with a slab thickness of 80 nm to allow for a bottom contact of the modulators and form the recessed trenches for the SiN x stressor for enhanced and uniform ( Fig. 2d-f, h) Supplementary  Fig. S17. To evaluate the modulator performance, optical transmission measurements (see Methods and Supplementary Fig. S18) were performed as a function of DC voltage bias. The lower maximum transmission at a wider Ge 0.99 Si 0.01 width is due to the higher insertion loss (IL, ~5 dB) from the corresponding wider Ge 0.99 Si 0.01 mesa (Fig. 4d). The extinction ratio (ER) spectra were then generated based on the transmission contrast of the modulators between 0 (on-state) and a reverse bias of -4 V (off-state) (Fig. 4e). A signi cant red-shift of the ER spectra is observed for the tensile-strained Ge 0.99 Si 0.01 modulators with a decreasing Ge 0.99 Si 0.01 width. The spectra ranges are consistent with the Ge 0.99 Si 0.01 bandgap edge wavelengths from ~1,520 to ~1,570 nm at the corresponding widths, obtained from both the tting of the optical transmission spectra (Fig. 4d) using the generalized FK model 59 , and the deformation potential theory calculation (Supplementary section S11) according to the respective pro les (Fig. 4c (left panel)). Similarly, an increasing compressive can be expected with a decreasing Ge 0.99 Si 0.01 width, which explains the observable blue-shift of the ER spectra for the compressive-strained modulators. The spectra are not completely visible due to the limited measurement window.
The operating wavelength range of individual modulators is important for the arrayed broadband modulation. Here, the ER/IL ratio, as gure-of-merit (FOM), of 1.3 is considered to determine the modulator operating coverage. This is because, besides ER, IL also increases with decreasing wavelength due to the increased Ge-based direct-band absorption. The value of 1.3 is reasonable among the values (0.8-1.7) reported in prior studies 24,25,26,60 and foundry PDK standards for high-speed optical interconnect applications 61 . Using this criterion, the operating coverage of a tensile Ge 0.99 Si 0.01 modulator (50-µm long, 0.7-µm wide) extends from ~1,580 to ~1,610 nm, where the IL is <6.8 dB. The operating optical bandwidth of individual modulators generally spans ~27.5 ± 3.5 nm at -6 V within the measurement window and is independent of the strain. The operating wavelength coverages for modulators beyond the measurement window were estimated based on the corresponding bandgap edge wavelengths obtained from the optical transmission measurements with the span of the determined average optical bandwidth. The individual spectra overlap and form an ultra-broadband wavelength coverage ranging from ~1,460 to ~1,620 nm at -6 V, where the compressive and tensile modulators cover from ~1,460 to ~1,530 nm and ~1,525 to ~1,620 nm, respectively (Fig. 4f). The modulator insertion loss can be reduced to ~2 dB via the optimization of top Si electrodes and taper coupling design 62 , which could further increase the ER/IL performance and reduce the operating voltage and modulation dynamic power of the modulator.

Broadband Ge-based modulator-detector co-integration
To investigate the feasibility of the Ge-based modulator-detector co-integration in a single step of Gebased epitaxy (Fig. 1), a 1-GPa recessed compressive SiN x stressor was introduced to an array of Ge waveguide-mesas on Si-OI waveguides similar to the design in Fig. 4. Similar optical transmission measurements were performed. As expected, the transmission started to increase at a shorter wavelength with decreasing mesa width, indicating an increase of the Ge bandgap due to the higher compressive strain in a narrower mesa (Fig. 5a). The FK model tting (Fig. 5a, solid curves) agrees well with the transmission spectra (circled dots) and extracts the strain and bandgap information in Ge (Supplementary Table 2). The ER spectra were then calculated (see Methods), as shown in Fig. 5b. The predicted operating coverage for the Ge modulator array ranges from ~1,520 to 1,630 nm. Overlaying with the absorption spectrum (Fig. 3d) of the tensile-strained Ge photodetector provides a broad wavelength window of over 100 nm for the Ge modulator-detector co-integration. The small footprint of the SiN x stressor (Supplementary section S7) provides the convenience to separately use tensile and compressive stressors on the photodetectors and modulator arrays, respectively, for the co-integration. Additionally, with an increasing Si incorporation, the bandgap of GeSi shrinks, which will shift the broad co-integration bandwidth to shorter wavelengths. Bandgap (see Methods) and FK model calculation nds that 1% Si incorporation will shift the co-integrated optical bandwidth range to ~1,460-1,575 nm, covering Eand C-bands (Fig. 5c). A 4% Si incorporation further shifts the co-integrated optical wavelength range to Oand E-bands (1,300-1,450 nm), where the FK effect remains signi cant since the direct-band transition dominates at the Γ-valley.

Conclusion
In conclusion, strained Ge photodetectors and Ge 0.99 Si 0.01 EA modulator arrays have been demonstrated with ultra-broad operating wavelength range. The photodetectors demonstrate a considerable Ge absorption coe cient (>4,000 cm -1 ) up to 1,630 nm and the Ge 0.99 Si 0.01 modulator arrays exhibit an operating wavelength range of ~100 nm where the ER/IL >1.3, from ~1,525-1,620 nm. The broadband operation also enables the co-integration of Ge-based photodetectors and EA modulators using one Ge composition, with an overlapping operating window over 100 nm. Si incorporation up to 4% is expected to shift the broad modulation and co-integration coverages to as short as 1,300 nm, and Sn incorporation to longer wavelengths beyond 1,700 nm. The broadband device operations are realized by simple and costeffective process integration requiring only a single-step Ge-based epitaxy and recessed-type SiN x stressors, both of which are matured processes in state-of-the-art foundries. In particular, the recessed feature of the stressor results in a uniform and enhanced strain pro le in the Ge-based materials to further extend the device operating coverage. This work paves the way towards high-data-capacity and low-cost PICs for future dense wavelength-division multiplexing (DWDM) computing and data center networks as well as a myriad of other emerging applications.

Micro-Raman measurement and its calibration
Micro-Raman measurements were performed using commercial systems (WITec UHTS 300, Renishaw inVia, and HORIBA LabRAM HR) at longitudinal optical (LO) mode via confocal backscattering con guration at laser emitting wavelengths of 532 and 785nm. The objective lens used are 100×, with numerical apertures at 0.9 (for WITEC and HORIBA) and 0.75 (for Renishaw). The diameter of the laser spots is ~1 µm. The backscattered signal went through a spectrometer and was collected by CCD detectors to form Raman spectra. The output power of the lasers was set to have a negligible heating effect on the samples. The spectra at different system and wavelengths were aligned based on their Si peaks (~520.6 cm -1 ) for comparison. The obtained spectra were tted by Lorentzian function to determine their peak positions for further analysis (Fig. S3b).

Photodetector characterization
The current-voltage (I-V) characteristics of the photodetectors were measured by an Aim-TTi QL355TP power supply and a Keysight 34450A multimeter. To measure the optical response, a TUNICS T100S-HP/CL tunable laser covering Cand L-bands (1500 to 1630 nm) was utilized as the light source. Light was out-coupled into a Corning SMF-28 single-mode silica glass ber to illuminate on the devices. The dark I-V curves were also collected as a function of temperature ranging from 293 to 353 K.
EA modulator array performance characterization and ER and GeSi bandgap calculation Optical transmission measurement was performed using an optical vector analyzer (OVA), comprising a tunable laser (1,510-1,610 nm) and a detector from Luna technologies (Luna Innovations Inc.), as seen in Supplementary Fig. S18. Light was butt-coupled into and out of the guiding Si -OI waveguides to modulators via cleaved facets. Difference in the transmitted optical power was recorded between a modulator at 0 V and a reference Si -OI waveguide identical to that in the modulator as the device insertion loss. Electrical probes were probed on the device metal pads to source DC voltage bias during the optical transmission measurement to obtain the device extinction ratio (ER). For the ER calculation in Fig. 5b, an electric eld contrast between 40 (off-state) and 10 (built-in eld, on-state) kV/cm was considered in the FK modelling, which corresponds to reverse bias voltages of -3 and 0 V, respectively. The built-in electric eld of 10 kV/cm is due to the Fermi-level equalization between Ge and Si substrate.