Ultrahigh-responsivity waveguide-coupled optical power monitor for Si photonic circuits operating at near-infrared wavelengths

A phototransistor is a promising candidate as an optical power monitor in Si photonic circuits since the internal gain of photocurrent enables high responsivity. However, state-of-the-art waveguide-coupled phototransistors suffer from a responsivity of lower than 103 A/W, which is insufficient for detecting very low power light. Here, we present a waveguide-coupled phototransistor operating at a 1.3 μm wavelength, which consists of an InGaAs ultrathin channel on a Si waveguide working as a gate electrode to increase the responsivity. The Si waveguide gate underneath the InGaAs ultrathin channel enables the effective control of transistor current without optical absorption by the gate metal. As a result, our phototransistor achieved the highest responsivity of approximately 106 A/W among the waveguide-coupled phototransistors, allowing us to detect light of 621 fW propagating in the Si waveguide. The high responsivity and the reasonable response time of approximately 100 μs make our phototransistor promising as an effective optical power monitor in Si photonic circuits.

layer. The width and the rib height were 400 nm and 150 nm for a single-mode operation at a 1.3 m wavelength. Grating couplers for the transverse electric (TE) mode were integrated with the Si waveguide for fiber coupling. After forming SiO2 cladding by chemical vapor deposition (CVD) on the Si waveguide, chemical mechanical polishing (CMP) was carried out for surface planarization. To form the p-Si (acceptor concentration NA = 5×10 17 cm -3 ) and p + -Si (NA = 1×10 20 cm -3 ) regions, boron ions were implanted.
Then, an InP epitaxial wafer consisting of a p-type 30-nm-thick In0.53Ga0.47As layer (NA = 5×10 16 cm -3 ) and etch-stop layers was bonded onto the Si waveguide with an Al2O3 bonding layer formed by atomic layer deposition (ALD) 1 . After bonding, the InP wafer and the etch-stop layers were selectively removed by wet etching. The 40-nm-thick SiO2 hard mask formed by CVD was patterned by electron-beam (EB) lithography and inductively coupled plasma (ICP) etching. Then, the InGaAs layer was patterned by wet etching. After removing the SiO2 hard mask, a 10-nm-thick Al2O3 passivation layer was formed by ALD. Finally, the contact windows for source and drain (S/D) were opened, and Ni/Au contact pads as metal S/D 2,3 were formed by EB evaporation and lift-off. The Ni/Au contact pad for the gate was also formed simultaneously. Figure S2 shows a planview microscopy image of the fabricated device. The contact pad for the Si waveguide gate was formed away from the S/D region owing to the limitation of the original Si photonic circuit design.

II. Band structure and electric field distribution
The operation principle of a phototransistor with the metal source and drain is depicted in Fig. S3. Since n-type transistor is considered here, the Schottky contact with large barrier height for holes are assumed. When no light is injected, the transistor is off, resulting in a low drain current. When the transistor channel is irradiated by light, photogenerated holes accumulate in the channel, pushing the conduction band and valence band down. As a result, the transistor turns on, and more drain current flows through the channel. In this way, the photocurrent is amplified through the change in the transistor conduction.

III. Measurement of electron mobility of InGaAs transistor
We prepared the InGaAs transistor shown in Fig. S6 for mobility measurement. From the drain current (Id) -gate voltage (Vg) characteristics in Fig. S7a, we extracted the fieldeffect electron mobility as where is the channel length, is the channel width, and is the gate capacitance.
Here, was 20 m, and was 30 m. was calculated to be 0.298 F/cm 2 by taking into account the 6-nm-thick SiO2 and 10-nm-thick Al2O3 layers. Figure S7b shows the extracted as a function of Vg. The peak value of the electron mobility was approximately 608 cm 2 /Vs. Note that there is room for improvement in electron mobility through the process optimization due to the high electron mobility of InGaAs 3 .

IV. Propagation loss of the Si waveguide
We evaluated the propagation loss of the Si waveguide and the coupling loss of the grating coupler at a wavelength of 1305 nm to obtain the optical power injected into the phototransistor. We prepared the Si waveguide of various waveguide lengths from 1296 µm to 12251 µm, as shown in Fig. S8a. A continuous wave (CW) light from a tunable laser was coupled into the Si waveguide through the grating coupler, and the output power from the Si waveguide was coupled again to a single-mode fiber through another grating coupler. The output power was measured using an InGaAs optical power monitor. The polarization of the input light was tuned to the transverse-electric mode using a polarizer.
A variable attenuator was also inserted between the tunable laser and the polarizer to adjust the input power. The insertion loss from the tunable laser to the power meter without the device under test was measured to be 5.2 dB. Figure S8b shows the  Fig. S8b, the propagation loss of the Si waveguide and the coupling loss of the grating coupler were extracted to be 2.14 dB/cm and 7.5 dB, respectively, as shown in Fig. S8c.
Since the length of the Si waveguide from the grating coupler to the PD is 95 m, the total insertion loss, which is dominated by the coupling loss of the grating coupler, is 7.52 dB.

VI. Measurement of noise equivalent power and specific detectivity
The noise power density spectrum of an InGaAs phototransistor was evaluated by the Fourier transform of the dark current waveform measured using a waveform generator/fast measurement unit (Agilent, B1530A) of a semiconductor device analyzer (Agilent Technologies, B1500A). Figure S10 shows the measured noise power density spectrum when Vd and Vg were 0.5 V and 1.0 V, respectively. As shown in Fig. S10, the noise power density was proportional to 1/f 2 , where f is a frequency. According to the method described in Ref. 4, the noise equivalent power (NEP) was extracted by integrating the noise power density from 0.1 Hz to 10 kHz. The specific detectivity was then obtained from the NEP, where the area of the phototransistor was the product of the waveguide width (0.4 m) and InGaAs length (30 m).