Electrically-driven Mach-Zehnder interferometer type InGaAsP photonic-wire optical switches have been demonstrated using a III-V-on-insulator structure bonded on a thermally oxidized Si with an Al2O3/InP bonding interfacial layer which enables strong wafer bonding and low propagation loss. Lateral p-i-n junctions in the InGaAsP photonic-wire waveguides were formed by using ion implantation for changing refractive index in the InGaAsP waveguide through carrier injection. Optical switching with 10 dB extinction ratio was achieved with driving current of 200 µA which is approximately 10 times smaller than that of Si photonic-wire optical switch owing to larger free-carrier effect in InGaAsP than that in Si.
© 2012 OSA
A low-power-consumption optical switch on Si photonics platform is expected to be one of fundamental building blocks for routing optical packets in future photonic network because of its scalability and manufacturability through the complementary metal oxide semiconductor (CMOS) compatible process. In particular, a Mach-Zehnder interferometer (MZI) type Si photonic-wire switch or modulator using lateral p-i-n diodes has been widely developed [1–4] because of its wide bandwidth operation and high fabrication tolerance. Although optical switching based on carrier plasma effect  was achieved with injection current of a few mA on Si photonics platform, further reduction in drive current will be required for large scale integration. As is well known, direct band gap semiconductors such as InGaAsP have larger carrier-induced refractive index change than Si , enabling more efficient optical switching. However, as compared with a Si photonic-wire waveguide [7–9], a conventional InP-based deeply-etched waveguide [10,11] is not suitable for large scale integration because its weak optical confinement in the vertical direction prevents a bending radius of the waveguide from being scaled down to be less than a few μm . The high aspect ratio in the deeply-etched waveguide is also not compatible to fine lithography in the standard CMOS process. To overcome these problems, we have investigated III-V CMOS photonics platform , which enables monolithic integration with high-performance III-V semiconductor based CMOS transistors and InP-based photonic-wire waveguide devices  on a III-V on insulator (III-V-OI) structure bonded on a thermally oxidized Si wafer as shown in Fig. 1 .
The strong optical confinement of the III-V-OI wafer enables drastic reduction of the sizes of III-V photonic devices as like Si photonic-wire devices. A Si-based waveguide itself does not allow active functionalities, while InP-based photonic-wire enables active/passive integration. In addition, InGaAs MOSFETs exhibiting superior performance than Si MOSFETs have been successfully demonstrated owing to its high electron mobility [14–17]. The International semiconductor roadmap 2012 expects that Si MOSFETs will be replaced by InGaAs MOSFETs in the future technology nodes for logic large-scaled integrated circuits (LSIs) . The III-V-OI bonded on a thermally oxidized Si wafer also enables us to use the standard CMOS process owing to its high thermal stability, as compared with benzocyclobutene (BCB) based bonded wafers. Thus, III-V CMOS photonics is a promising platform of electronic-photonic integrated circuits (EPICs) outperforming Si photonics with respect to not only photonics but also electronics. Using this platform, a sharp bend waveguide with 5-µm bend radius and an ultra-small arrayed waveguide grating multiplexer have been demonstrated .
In this paper, we have demonstrated electrically-driven Mach-Zehnder interferometer type InGaAsP photonic-wire optical switches fabricated on III-V CMOS photonics platform using the CMOS compatible process. Owing to large refractive index change by current injection through a lateral p-i-n structure formed by ion implantation, optical switching with 10 dB extinction ratio was obtained with driving current of 200 μA, which is approximately 10 times smaller than that of Si photonic-wire optical switches.
2. Carrier-induced index change in InGaAsP
Carrier-induced effects that contribute to refractive index change in InGaAsP are bandfilling effect, bandgap shrinkage and free carrier plasma effect . In Si, on the other hand, only free carrier plasma effect contributes to refractive index change since Si is an indirect band gap semiconductor. Therefore, InGaAsP photonic-wire optical switches can be driven by lower current compared to Si. To calculate the amount of refractive index change in InGaAsP, we used the theoretical model depicted in Ref. 6 and Ref. 20, which can take into account bandfilling, bandgap shrinkage, and plasma effect.
Free-carrier-induced optical absorption change related to intraband transition is called free carrier plasma effect. From the absorption change, we can calculate the index change using Kramers-Kronig relation . According to the Drude model, refractive index change by free carrier plasma effect is given by 22]
As shown in Fig. 2(a) , band filling effect is related to interband transition. When free carriers are injected to the conduction band, amount of band edge absorption is reduced. This absorption change is given by 22] is the bandgap energy, is the probability of a conduction band state of energy being occupied by an electron, is the probability of a valence band state of energy being occupied by an electron, and is the constant that depends on the material. and are given by the Fermi-Dirac distribution functions. To obtain the in In1-xGaxAsyP1-y, we used the equation shown in Ref. 20:
Bandgap shrinkage caused by free carriers can be calculated by 6]
Finally, we have calculated index change in InGaAsP that considers all these three effects. Figure 2(b) shows the index change at 1.55-μm wavelength as a function of carrier concentration. At carrier concentration of 1 × 1017 cm−3, index change in InGaAsP (λg = 1.25 μm) is predicted to be approximately four times larger than that in Si. Hence, we can expect that driving current of InGaAsP photonic-wire switches is much lower than that of Si optical switches.
3. Propagation loss in InGaAsP photonic-wire waveguides
We have evaluated propagation loss in InGaAsP photonic-wire waveguides. Previously we have used O2 plasma assisted direct wafer bonding to fabricate the III-V-OI wafer. However, O2 plasma irradiation of the InGaAsP surface caused the InGaAsP oxide formation, which might result in increase in propagation loss of InGaAsP photonic-wire waveguides. In addition, the surface energy of the bonded interface with the O2 plasma activated bonding was still not enough for CMOS processes. To solve these problems, we have examined an atomic layer deposited (ALD) Al2O3 bonding interfacial layer for direct wafer bonding of an InGaAsP/InP wafer and a Si wafer. Figure 3 shows the bonding process.
Firstly, 5.5-nm-thick Al2O3 layer was deposited on a 2-inch InGaAsP (λg = 1.25μm)/InP wafer and a SiO2/Si wafer by ALD at 200 °C. The 2.5-μm-thick SiO2 buried oxide on Si was formed by wet oxide of the Si wafer. After surface cleaning, the ALD Al2O3 deposited wafers were bonded manually and annealed at 330 °C for 15 min. Thus, the wafers were bonded without any plasma irradiation. Figure 4(a) is an infrared (IR) image of the bonded wafer. We can find that most part of the wafer was successfully bonded. Figure 4(b) shows the surface energy of the bonded interfaces with ALD Al2O3 interlayer and O2 plasma irradiation. The surface energy was evaluated by the crack opening method .
The surface energy of the ALD Al2O3 interlayer bonding with no plasma irradiation was approximately twice of that of the O2 plasma activated interface. Then, the InP substrate was selectively etched by HCl. Figure 5(a) shows the cross-sectional transmission electron microscopy (TEM) image of the bonded wafer. No interfacial oxide layer was observed at the bonded interface as shown in Fig. 5(a) because of the plasma-less wafer bonding process.
To improve the bonding interface further, we have introduced 25-nm-thick InP layers on the top and bottom of the InGaAsP layer. Figure 5(b) shows the bonded interface which has the 25-nm-thick InP layer between the InGaAsP and Al2O3 layers. The root-mean-square (RMS) roughness at the bonded interface extracted from the TEM image was improved from 0.38 nm to 0.16 nm by the InP layer, which is probably attributable to chemical stability of InP against chemical reaction during the ALD process because InP contains single Group-V atom as compared with InGaAsP. The InP interlayer is also expected to reduce optical scattering at the bottom and top of the waveguide owing to the graded index change structure.
To confirm the effect of these improvements in wafer bonding, we have fabricated waveguides and measured propagation loss. A fabrication process after bonding is as follows. After patterning by photolithography, the waveguide mesa was formed by reactive ion etching (RIE) with CH4/H2 and O2 gases. The waveguide width was 2 μm. Then, 500-nm-thick SiO2 passivation layer were deposited. We have also introduced Al2O3 passivation layer to suppress the optical scattering from the sidewall by the graded index change structure . The fabricated devices were measured with a continuous-wave (CW) laser source at 1.55-μm wavelength. The CW light was input into the waveguides by a lensed fiber. The output power was monitored by an infrared (IR) camera and an InGaAs power meter. Figure 6 shows the structure of the fabricated waveguides and their propagation losses evaluated by the cut-back method.
The propagation loss in the waveguide with the ALD Al2O3 bonding interlayer (1) was 1.7dB/mm, which was approximately two times smaller than that of the photonic wire waveguide on the III-V-OI wafer with O2 plasma treatment reported in Ref. 19. The propagation loss was reduced further from 1.7 dB/mm to 0.8 dB/mm by inserting the 25-nm-thick InP layer owing to the smooth interface between InP and Al2O3. Finally we achieved 0.4-dB/mm propagation loss by passivating the waveguide sidewalls with the 11-nm-thick Al2O3 layer before SiO2 passivation.
4. InGaAsP photonic-wire optical switch
We have fabricated InGaAsP photonic-wire optical switch on the III-V-OI wafer using the standard CMOS process. The 25-nm-InP layer on the top and bottom of the InGaAsP layer and Al2O3 passivation were introduced to minimize the propagation loss. Figure 7 shows the fabrication process of optical switches after waveguide formation.
Ion implantation of Si and Be was used to form lateral p-i-n structures at the MZI arms. Si ions were implanted at an acceleration energy of 15 keV and a dose of 2 × 1014 cm−2 for n + region and Be ions were implanted at an acceleration energy of 10 keV and a dose of 1 × 1015 cm−2 for p + region. To activate the implanted region, rapid thermal annealing (RTA) was performed at 600 °C for 10 s. In general, high temperature process is not acceptable for bonded wafers because outgassing from bonded interfaces generates voids . However, no void generation was observed in our samples even at 600 °C probably because the thick SiO2 box layer absorbed gas generated at the bonded interface . Then, 300-nm-thick SiO2 passivation layer was deposited. After opening contact holes for the n + and p + regions by BHF, Pt electrodes were formed by sputtering and lift-off process.
At first, we have evaluated the characteristics of 3-dB MMI couplers which are indispensable for constructing an MZI interferometer. Figure 8(a) is an image of a fabricated MMI coupler, whose width and length were 6 μm and 43 μm, respectively. Figure 8(b) is an image of output from the 3-dB MMI coupler monitored by an IR camera. The imbalance between the two output ports was approximately 0.4 dB.
Finally, we have measured switching characteristics of the optical switch. Figure 9 is a plan-view photograph of the fabricated InGaAsP photonic-wire optical switch.
The length of the phase sifter was 500 μm. We have evaluated the switching properties of the fabricated device by injecting current with forward bias to the p-i-n junction. Figure 10(a) is images of output power. When injected current was 0 μA, output light came from the cross port. When injected current was increased to 200 μA, output light was observed from the bar port. Figure 10(b) shows the output powers of the cross and bar ports as a function of injected current. We obtained optical switching with 10-dB extinction ratio at 200 µA driving current. This driving current is approximately 10 times smaller than that of Si-based optical switches [1–3] owing to the large carrier-induced refractive index change in InGaAsP.
We have successfully demonstrated the InGaAsP photonic-wire optical switch fabricated on III-V CMOS photonics platform. The propagation loss of the InGaAsP photonic-wire waveguide was reduced to be 0.4 dB/mm by an Al2O3/InP bonding interfacial layer and Al2O3 device passivation. Owing to larger carrier-induced refractive index change in InGaAsP than that in Si, the InGaAsP photonic-wire optical switch can be driven by lower current than silicon optical switches. The driving current required for optical switching was 200 µA, which is approximately 10 times smaller than that of Si photonic-wire optical switches.
This work was supported by Grant-in-Aid for Young Scientists (A) from MEXT.
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