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Abstract
We studied a high-speed electro-absorption optical modulator (EAM) of a Ge layer evanescently coupled with a Si waveguide (Si WG) of a lateral pn junction for high-bandwidth optical interconnect. By decreasing the widths of selectively grown Ge layers below 1 µm, we demonstrated a high-speed modulation of 56 Gbps non-return-to-zero (NRZ) and 56 Gbaud pulse amplitude modulation 4 (PAM4) EAM operation in the C-band wavelengths, in contrast to the L-band wavelengths operations in previous studies on EAMs of pure Ge on Si. From the photoluminescence and Raman analyses, we confirmed an increase in the direct bandgap energy for such a submicron Ge/Si stack structure. The operation wavelength for the Ge/Si stack structure of a Ge/Si EAM was optimized by decreasing the device width below 1-µm and setting the post-growth anneal condition, which would contribute to relaxing the tensile-strain of a Ge layer on a Si WG and broadening the optical bandwidths for Franz-Keldysh (FK) effect with SiGe alloy formation.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Silicon photonics has recently attracted much attention because it offers low cost, low power consumption, and high bandwidth for the optoelectronic solutions for applications ranging from telecommunications to chip-to-chip interconnects [1]. To realize an effective photonics-electronics convergence system [2–3], it is very important to achieve a high-speed and low- power optical modulator to be integrated with a Si based optical circuit.
Among the various Si optical modulators (Si-MODs) demonstrated so far, the Mach-Zehnder Si-MODs based on the free carrier plasma dispersion effect have been mostly reported for high-speed modulation and broad-wavelength operation [4–11]. However, carrier-depletion Si modulators need a relatively long phase-shifter or high driving voltage because of the weak plasma dispersion effect in Si [12] and they are not favorable for large-scale integration. It has been reported that ring-resonator type Si-MODs realize high-speed and low power operation. However, operation wavelength needs to be controlled with accuracy less than 1 nm, which makes practical use difficult [13–14].
To achieve a low power and high-density interconnect system, a very small capacitance of an optical modulator is required. A GeSi electro-absorption modulator (EAM) is promising because its electrical capacitance is much smaller than that of the Mach-Zehnder Si-MOD, and its length is about several tens micrometers [15–18]. In addition, a GeSi EAM structure is also applicable to a photodetector, which has the advantage for fabricating an integrated optical circuit. However, an epitaxial grown Ge layer on a Si substrate has a tensile strain as large as 0.2%, which reduces the direct bandgap energy to 0.77 eV, while the unstrained Ge layer has a 0.80 eV bandgap energy [19]. Therefore, it is necessary to apply the optimized composition of a GeSi layer to GeSi EAM to operate in the C band wavelength [15–18]. Recently, operation at C-band wavelengths has been reported for the Ge/Si stack structure of an EAM, but control of operation wavelength region had not been achieved [20,21].
In this paper, we study a high-speed Ge/Si EAM evanescently coupled with a Si waveguide of a pn junction for a large-bandwidth optical interconnect. By decreasing the Ge/Si stack width, we demonstrate a C-band wavelengths operation and a 56-Gbps NRZ and 56-Gbaud PAM4 high speed with 2.5-Vpp applied voltage. From the photoluminescence spectra, we confirmed that bandgap energy increased in the submicron width of a Ge/Si stack layer, which is consistent with operation in the C-band wavelengths for a Ge/Si EAM. The operation wavelength is optimized by changing the width of a Ge/Si stack structure.
2. Device structure and fabrication process
Figure 1 shows (a) a schematic cross-section of a Ge/Si EAM on a Si rib waveguide with a lateral pn junction and (b) a cross-sectional TEM (transmission electron microscope) image of a Ge/Si EAM. The Ge/Si EAM consists of an evanescently coupled structure with a Si rib waveguide [22]. The stack Ge/Si layer width was changed from 0.3 µm to 1.0 µm, and the Ge height was about 300 nm.
Fig. 1. (a) Cross-sectional schematic diagram and (b) top view image of Ge/Si EAM. (c) Cross-sectional TEM image of Ge/Si EAM.
Figure 2 shows the Ge/Si EAM fabrication process. The fabrication process started from a 300 mm-diameter silicon-on-insulator (SOI) wafer with 200-nm SOI thickness. Boron (B) and phosphorus (P) ions were implanted into the SOI layer and the wafers were annealed to form a lateral pn junction in an SOI layer. Then, a Si pedestal was patterned by immersion ArF lithography and dry etching. Subsequently, a 500 nm-thick epitaxial Ge was selectively grown on the Si pedestal by the ultra-high-vacuum chemical vapor deposition method. In this case, Ge thickness decreased from 500 nm to about 300 nm with decrease in the Ge width less than 1 µm because Ge growth of (001) crystalline orientation interferes with that of (111) and (311) crystalline orientation. In the Ge epitaxial growth process, we applied about 20-30nm-thick Si0.5Ge0.5 buffer layer and a pure Ge layer was grown on it. After the Ge epitaxial growth, the post annealing process at around 800 °C was applied to the wafer for 30 minutes to improve the crystallinity of the Ge layer on an SOI substrate. Then a 20 nm-thick Si-capping layer was also deposited onto a Ge layer to passivate the Ge surface. Next, we implanted B and P ions into a Ge layer at the doping density of 2×1018/cm3 and annealed the wafers to form a lateral p-i-n junction in the Ge layer. Then a SiO2 upper-clad layer was deposited, and contact-holes were formed by UV lithography and a dry-etching process. Finally, metal electrodes of Ti/TiN/Al layers were deposited and patterned.
Figures 3(a), (b), and (c) show a cross section of an electric field intensity contour map in the Ge/Si EAM at a 2.0 applied reverse bias voltage (Vdc), doping profile analysis results on cross section of Ge/Si EAM by scanning capacitance microscope, and an optical electric field intensity contour map. From T-CAD simulation, electric field intensity is more than 40 kV/cm at the interface between a Ge layer and a Si pn junction for the low applied voltage around 2.0 V. More than 40 kV/cm electric field intensity is enough to increase the absorption coefficient in GeSi [16]. Therefore, the Franz-Keldysh (FK) effect can be induced in the Ge layer on the SOI layer. In addition, it is expected that FK effect would be induced in a GeSi layer at the Ge/Si interface. In the EAM structure, the distance between p and n-type electrodes in a Ge layer affects the optical absorption coefficient in the Ge layer and electrical capacitance in the simulations. By optimizing the spacing of p- and n-type Ge electrodes around 0.2 µm, over 40 GHz of bandwidth can be obtained from T-CAD simulation.
Fig. 3. (a) Simulation result of electric field distribution for Ge/Si EAM at 2.0 V dc reverse bias, (b) analysis result of doping profile for Ge/Si EAM, and (c) simulation result of optical electric field contour map [21].
3. Optical transmission characteristics and analysis of Ge/Si EAM
Figure 4 shows the experimental results of optical transmission dependence on applied voltage for (a) a 0.3-µm, (b) a 0.6-µm, and (c) a 1.0-µm-wide, and 20-µm-long Ge/Si EAM. Optical transmission for the Ge/Si EAM is defined as difference between the optical transmission of a reference Si WG and the Ge/Si EAM connected with a Si WG in the same chip. It includes the optical coupling loss between a Si WG and a Ge/Si EAM. When increasing the reverse bias voltage for 0.3-µm wide and 20-µm-long Ge/Si EAM, the optical transmission power decreased by 4 to 7 dB at around a 1550-nm wavelength, which would originate from the bandgap shrinkage of the FK effect. On the other hand, a 1.0 µm-wide Ge/Si EAM operated at around 1580 nm to 1600 nm wavelengths, which is consistent with tensile-strained Ge bandgap energy on a Si [19]. The optical transmission spectra for 0.3 µm-wide and 0.6 µm-wide Ge/Si EAMs shift to shorter wavelengths than that of a 1.0-µm-wide GeSi EAM. From the I-V characteristics, the leakage current of the GeSi EAM is less than 1 µA, which would not affect the optical transmission loss.
Fig. 4. Optical transmission spectra for (a) 0.3µm width, (b) 0.6 µm width, and (c) 1.0 µm width of Ge/Si EAMs with 20µm length [21].
Figure 5 shows the figure-of-merit (FOM) of extinction ratio (ER) divided by insertion optical loss (IL) dependence on wavelength for the 40-µm long Ge/Si EAM with 0.3 µm width. The ER is defined as the ratio of the optical transmission at each applied voltage and that at 0 V. The FOM shows maximum value at the C-band wavelengths for the 0.3-µm wide Ge/Si EAM. Therefore, we chose the 0.3-µm wide Ge/Si EAM for the high-speed operation in the C-band wavelength.
Fig. 5. FOM (ER/IL) dependence on wavelength for 0.3-µm width of Ge/Si EAM with 40-µm length for applied voltages from 1 V to 4 V.
To investigate the cause of shorter wavelength operation of an EAM’s Ge/Si stack layer with decrease in the Ge layer width, we analyzed the direct bandgap energy by photoluminescence (PL) analysis. Figure 6 shows the PL spectra of Ge/Si EAM devices with different Ge widths, a blanket Ge layer on the same SOI wafer, and a bulk Ge substrate. As for a blanket Ge layer on the SOI substrate, the peak wavelength in the PL spectrum was observed at around 1586 nm, which shows a bandgap shrinkage is induced due to about a 0.2% tensile strain. On the other hand, the PL spectrum peak wavelengths shifted to 1543 nm for 0.6-µm-width and 1551 nm for 1.0-µm-width of the Ge/Si EAM devices. Therefore, the PL spectrum peak wavelength shifted to shorter wavelength for the submicron wide Ge/Si EAMs than that of the blanket Ge on Si. As for the unstrained bulk Ge substrate, the PL spectrum peak wavelength was 1532 nm. From the PL analysis, it is suggested that the direct bandgap energy increases with decrease in the Ge/Si width below 1.0 µm. As for a 0.3-µm wide Ge/Si EAM device, it is difficult to obtain the PL spectrum because it would have a smaller light-absorption area and more surface-area ratio. The Ge surface defect would cause the recombination of the generated carriers and their non-radiative decay.
We analyzed the Ge/Si EAM device by Raman spectroscopy with 457-nm-wavelength excitation laser to investigate the crystalline strain and SiGe mixed crystalline in the Ge/Si EAM. Figure 7 shows Raman spectra for 0.3µm to 1.0µm-wide Ge/Si EAM devices compared with those of bulk Ge and Si substrates. The Raman spectrum peak of Ge-Ge bonding was around 298 cm-1 for the Ge/Si EAM devices, which is comparable with that of unstrained bulk Ge. In addition, we observed a small broader peak originated from SiGe mixed crystalline at around 390 cm-1 for the Ge/Si EAM, while it was not observed for the bulk Ge. Therefore, a SiGe-mixed crystalline layer is formed by the annealing process after Ge epitaxial growth [23–24], and this would contribute to the shorter wavelength operation for the Ge/Si EAM as well as the relaxing of tensile-strain with decreasing the Ge/Si width [25].
Fig. 6. Measurement results for photoluminescence spectra for 0.3µm, 0.6µm, and 1.0 µm-wide Ge/Si EAM, a blanket Ge layer on Si, and a bulk Ge substrate.
Figure 8 shows a SIMS (secondary ion mass spectrometry) depth profile of Ge and Si atomic ratio in the Ge/Si stack structure for the 200 µm square pattern. At the interface between the Ge and SOI layers, interdiffusion of Ge and Si atoms occurred. In addition, optical mode field shrinks with decrease in the Ge/Si EAM width, which would contribute to the shorter wavelength operation owing to larger overlap between optical mode field and SiGe alloy in the Ge/Si stack structure in addition to relaxing the tensile strain in the Ge layer with decreasing the Ge/Si stack width. The SiGe alloying could also contribute to the optical bandwidth broadening and smaller temperature dependence of the Ge/Si EA modulator [26].
Fig. 7. Raman spectra for 0.3µm-width, 0.6µm-width, and 1.0µm-width Ge/Si EAM devices, bulk Ge, and bulk Si.
4. Absorption coefficient and band edge energy
Figure 9(a) shows absorption coefficient spectra under different electric field strengths for tensile-strained pure Ge. The spectra were determined from the experimentally obtained responsivity spectra for a pin Ge photodetector on Si measured applying several different reverse voltages [27]. The absorption coefficient increases with increasing the electric field strength at the L-band wavelengths more than 1600 nm.
Fig. 8. SIMS depth profile of Ge and Si atomic ratio in the Ge/Si stack structure for the 200 µm square pattern.
Figure 9(b) shows band edge energies as a function of Ge composition of SiGe calculated under the assumption that SiGe is lattice-matched with the Ge layer, taking into account the result in Fig. 7 that SiGe was formed mainly due to the diffusion of Si atoms into the Ge layer. For unstrained Si1-xGex, the changes in the energies of the valence and conduction band edges as a function of Ge composition x, Ev(x), and Ec(x) are described as [28–29],
Fig. 9. (a) Absorption coefficient dependence on wavelength with increase in applied electric field for tensile strained pure Ge. (b) Band edge energy dependence on Ge composition in SiGe with and without thermal stress.
5. High-speed characteristics
Next, we studied high-speed characteristics of the 40-µm-long Ge/Si EAM with 0.3-µm-width. Figure 10(a) shows an equivalent electric circuit model [18]. It consists of the pad capacitance Cpd, the series resistance Rs, the p-i-n junction resistance Rj, the p-i-n junction capacitance Cj, the capacitance due to the buried oxide (BOX) layer Cox, and the resistance due to the SOI and substrate Rsi. Using the S-parameters model parameters available by Keysight Advance Design System, we found the lumped element values of the equivalent electrical circuit to match the experimental data, as shown in Figs. 10(b) and (c). We calculated Cpd = 26.9 fF, Rs = 36.7 Ω, Rj = 3436.9 Ω, Cj = 12.7 fF, Cox = 56.2 fF, and Rsi = 853.2 Ω. We can, then, calculate the power of the Ge/Si EAM at 56 Gbps to be Cj*Vpp2/4 = 19.8 fJ/bit with the swing voltage Vpp = 2.5 V. Therefore, the Ge/Si EAM would realize much lower-power operation than that of the Si modulators.
Fig. 10. (a) Equivalent electric circuit, (b) S11 parameter at 2.0 Vdc Ge/Si EAM with 0.3-µm width and 40-µm length.
Figure 11 shows a frequency EO response for the Ge/Si EAM with 0.3-µm width and 40-µm length. Its frequency bandwidth was more than 67 GHz at 2.0 Vdc, which is enough for high-speed operation. Figure 12 shows a schematic diagram of measurement system for a high-speed Ge/Si EAM. A continuous wave (CW) laser light source with 1550 nm wavelength and optical power of 13 dBm was applied to measurement system. Polarization-controlled light was input into the Ge/Si EAM and amplified by Erbium Doped Fiber Amplifier (EDFA) to detect the optical signal by the sampling oscilloscope or the real-time oscilloscope. To drive the Ge/Si EAM at high-speed, digital analog converter (DAC) from the pulse pattern generator (PPG) was used to generate the high-speed electrical signal of 56 Gbps NRZ and 56 Gbaud PAM4. The high-speed electrical signal to drive the Ge/Si EAM was amplified to 2.5 Vpp amplitude by the electrical amplifier and dc bias voltage was applied through the bias T.
Fig. 12. Schematic diagram of high-speed measurement system for Ge/Si EAM with 0.3-µm width and 40-µm length.
Figures 13(a) and (b) show output waveforms of 56 Gbps NRZ with 231-1 pseudo-random bit sequence (PRBS) and 56 Gbaud PAM4 with 213-1 PRBS at 1550 nm wavelength with a 2.5 Vpp RF applied voltage and 2.0 Vdc for a 40-µm-long Ge/Si EAM with 0.3-µm-width. In Fig. 13(a) clear eye opening was obtained and the extinction ratio was 3.2 dB, which would contribute to the efficient optical interconnect. Insertion optical loss was about 4.0 dB with 0 Vdc. In Fig. 13(b), a 56 Gbaud-PAM4 output waveform is shown. PAM 4 optical signal was obtained with a real-time oscilloscope with 70 GHz bandwidth of a photodetector. Transmitter dispersion eye closure quaternary (TDECQ) value for PAM4 signal quality was less than 2 dB in case of standard 5 Tap feed-forward equalizer application. Therefore, the Ge/Si EAM is promising for a low-power and high-bandwidth optical interconnect [30].
Fig. 13. Output waveforms of (a) 56 Gbps NRZ with 231-1 PRBS and (b) 56 Gbaud PAM4 with 213-1 PRBS at 1550 nm wavelength.
6. Summary
We studied a high-speed EAM of a Ge layer evanescently coupled with a Si WG of a lateral pn junction for high-bandwidth optical interconnect. By decreasing the widths of selectively grown Ge layers below 1 µm, we demonstrated a high-speed modulation of 56 Gbps NRZ and 56 Gbaud PAM4 EAM operation in the C band wavelengths, in contrast to the L-band wavelengths operations in previous studies on EAMs of pure Ge on Si. From the photoluminescence and Raman analyses, we confirmed an increase in the direct bandgap energy for such a submicron Ge/Si stack structure. The operation wavelength for the Ge/Si stack structure of a Ge/Si EAM was optimized by decreasing the device width below 1-µm and setting the post-growth anneal condition, which would contribute to relaxing the tensile-strain of a Ge layer on a Si WG and broadening the optical bandwidths for FK effect with SiGe alloy formation.
Funding
New Energy and Industrial Technology Development Organization (JPNP13004).
Acknowledgements
The authors thank SCR members of AIST for their cooperation in the device fabrication.
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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