We have fabricated monolithic silicon avalanche photodiodes capable of 10 Gbps operation at a wavelength of 1550 nm. The photodiodes are entirely CMOS process compatible and comprise a p-i-n junction integrated with a silicon-on-insulator (SOI) rib waveguide. Photo-generation is initiated via the presence of deep levels in the silicon bandgap, introduced by ion implantation and modified by subsequent annealing. The devices show a small signal 3 dB bandwidth of 2.0 GHz as well as an open eye pattern at 10 Gbps. A responsivity of 4.7 ± 0.5 A/W is measured for a 600 µm device at a reverse bias of 40 V.
© 2013 Optical Society of America
The silicon photonics platform offers the integration of optical and electrical components on a common substrate . The primary application for this technology is evolving to be its use for short reach optical interconnection . The strength of the platform lies in the ubiquity of complimentary-metal-oxide semiconductor (CMOS) manufacturing, which allows for high volume processing of photonic circuits on silicon-on-insulator (SOI) wafers. Historically, the weakness of the silicon platform has been difficulty in the development of active devices: e.g. lasers and detectors, which are often incorporated using a hybrid material based approach. For example, III-V semiconductor materials bonded on SOI may be used to integrate laser sources  while direct germanium growth on silicon can provide photo-detection . These approaches are well-documented to achieve high performance but require the compromise of increased fabrication complexity. Monolithic silicon approaches are desired for all active device functionality, and while monolithic lasers that output enough optical power are difficult to envisage in the near-term, all-silicon photo-detection at a wavelength of 1550 nm may be achieved in a manner described below.
There is a fundamental challenge in using the same material for a waveguide and an absorptive photo-detector. In the case of silicon-based waveguides there exists a relatively low optical absorption at wavelengths at or close to 1550 nm, making SOI a suitable waveguide structure. All-silicon photodiodes for use at such wavelengths can be created by the introduction of deep electronic levels in the silicon bandgap. Such devices typically consist of a p-i-n junction integrated with a waveguide, with deep-levels (usually associated with lattice defects) selectively introduced into the intrinsic region of the junction. The deep-levels may be formed via ion implantation, a standard CMOS process allowing for high control over the position and concentration of the absorptive volume. This approach was demonstrated using divacancy defects in straight SOI waveguides, and cavity structures consisting of micro-ring resonators [5–7]. Similarly, interstitial clusters introduced by ion implantation of silicon have been utilized by Geis et al. to the same end . Those diodes achieved high speed operation, with a small signal bandwidth of 35 GHz and a quantum efficiency above unity suggesting avalanche multiplication. More recent work by Grote et al. demonstrated waveguide detectors at 10 Gbps with error free operation .
Avalanche photodiodes (APDs) useful for integration with silicon waveguides have been shown to provide acceptable performance through the use of an absorptive germanium volume, however germanium APDs must overcome the challenge of excessive dark current. This is often achieved with a separate absorption and multiplication layer (SAM structure) to exploit the strengths of each material. Germanium provides a highly absorbing material while low noise carrier multiplication occurs in silicon.
Despite the reports of waveguide integrated germanium/silicon hybrid avalanche detection [10,11], work in the general area of APDs for silicon photonic circuits has been limited. It is clear however that silicon is an excellent material to use in the fabrication of APDs due to its favourable ionization coefficient ratio. In this paper we report results from a monolithic silicon waveguide integrated avalanche photodiode. The detectors are potentially sensitive to a broad range of wavelengths, beyond the limit of germanium-based APDs. They are made sensitive to infrared light through ion implantation and subsequent annealing, in a fully CMOS compatible process. They also offer a significantly increased response over previously reported (non-avalanche) monolithic silicon photodiodes and achieve an open eye pattern at 10 Gbps. Such devices represent a viable alternative to hybrid approaches for optical interconnects.
2. Experimental procedure
2.1 Device fabrication
The photodiodes were fabricated at CEA-LETI using 193 nm deep-ultraviolet lithography. The SOI consisted of a 220 nm thick silicon layer over a 2 µm layer of buried silicon dioxide (BOX). Waveguides were defined with a 170 nm etch and had a nominal width of 500 nm. A secondary etch of 70 nm defined gratings used to couple light into the waveguides. The detectors were formed on the waveguides with a p-i-n junction created by boron and phosphorous implants with a target concentration of 5 x 1019 cm−3. A 1 µm oxide cladding covered the waveguides and a 560 nm thick Al/Cu metal layer provided electrical probe contacts to the diode. Deep-levels were introduced into the waveguide (the intrinsic volume of the p-i-n structure) using selective ion implantation, specifically the implantation of boron at a dose of 5 x 1012 cm−2 and at an energy of 500 keV. Following the implantation the devices were thermally annealed at 200 °C for 5 minutes in atmospheric conditions. A cross-sectional diagram with device dimensions is presented in Fig. 1. Detector lengths (dimension into the page in Fig. 1) ranged from 200 µm to 800 µm with an optical absorption of 8.6 dB/mm. Waveguide loss prior to implantation was 0.7 ± 0.1 dB/mm and was determined by measuring the throughput loss of various waveguide lengths.
2.2 Device characterization
Light was coupled into and out of the waveguide using a single mode fiber angled at 10 degrees with respect to the surface normal of a relief grating. The total throughput loss of an input grating coupler, waveguide and output coupler is approximately 15 dB (dominated by the grating coupling loss). The small signal frequency response of the device was measured using a lightwave network analyzer rated for 20 GHz and operating at 1550 nm. A source-measure unit and a 20 GHz bias tee were used to apply a bias voltage and measure the photocurrent. A bit pattern generator driving an optical amplitude modulator followed by an erbium doped fiber amplifier was used to generate a 10 Gbps optical non-return-to-zero signal incident on the photodiode. An equivalent time sampling oscilloscope was subsequently used to measure the eye pattern of the generated photocurrent.
Power linearity, current-voltage and wavelength response measurements were taken with a tunable laser in tandem with a built-in optical power attenuator. The measurement wavelength in the case of these DC measurements was 1565 nm (except in the case of the wavelength response measurement), which is the peak response of the grating coupler.
3. Experimental results
3.1 DC characteristics
The current-voltage characteristic of a 600 µm long photodiode is shown in Fig. 2. The power launched at the input grating was 1 mW. Estimated waveguide coupled power is 200 µW. This provides an estimated internal responsivity of 4.7 ± 0.5 A/W at 40 V reverse bias.
In the case of these specific detectors, it is not straight forward to assign a gain figure. Figure 2 shows there is no photocurrent plateau from which the primary photocurrent or unity gain can be assigned. In an attempt to at least provide an estimate of a gain figure, we created a two-dimensional model of the device using the semiconductor simulation software Atlas by Silvaco Inc . Device characteristics were fixed using the fabrication parameters described in section 2.1. In Fig. 3 we plot the electric field distribution as a function of reverse bias and show a non-uniform field between the center of the waveguide (x = 0) and the adjacent waveguide slab region (x > 250 nm, x < −250 nm), which introduces a gradual onset of impact ionization across the device. With a reverse bias of 10 V applied, the junction is entirely depleted with an associated field of 5x104 Vcm−1 and we assume the primary current to have peaked. Although some impact ionization occurs even before 10 V (the cause of the smoothly increasing photocurrent even at relatively low bias), we thus have conservatively chosen 10 V to represent unity gain.
The power linearity of the photodiode is shown in Fig. 4, where the photodiode current versus launched laser power is plotted for a wavelength of 1565 nm. After subtracting the device dark current, the photocurrent is a linear function of optical power across the measured range.
The wavelength response of the APD was measured from 1520 to 1600 nm with the result dominated by the response of the gratings used to couple light into and out of the waveguides. Figure 5 shows the wavelength spectra for a 200 µm long photodiode at a reverse bias of 40 V. Both the photodiode current and the transmitted optical power are shown, each normalized to their respective maximum response for ease of comparison. The photodiode current as a function of wavelength follows the general trend of the optical power transmitted (i.e. both peak near 1568 nm), however the optical power shows a sharper roll-off as compared to the photocurrent. This is a result of the fact that the optical power (measured externally in this case) is subject to two grating couplers (in and out). Although the photodiode response cannot be disentangled cleanly from the grating response, it is clear that a significant optical response is seen across the S, C and L bands. With an alternative coupler, we expect operation at longer wavelengths as previously reported for this class of silicon photodiode at 1744 nm  and 1900 nm .
3.2 Frequency response
The small signal frequency response is shown in Fig. 6. In this case a 600 µm photodiode was measured while varying the reverse bias voltage. At 40 V reverse bias there is a noteworthy drop in response, however below 35 V (not shown) there is no significant change in the electrical bandwidth of the device. We attribute the drop at 40 V to the increased multiplication factor at higher voltages. Using 10 V as unity gain the 600 µm photodiode in Fig. 2 shows a gain of 330 at a reverse bias of 35 V while operating at 10 Gbps. The 3 dB bandwidth is 2.0 GHz. The frequency response of both 200 µm and 800 µm long detectors were measured and no discernible difference was observed. This suggests that the limitation is not imposed by the device geometry.
The devices have a robust operation characteristic at 10 Gbps with a reverse bias of 35 V. The pattern-lock feature on an equivalent time sampling oscilloscope was used to capture the photocurrent waveform representing the transmitted binary data. In Fig. 7, the time domain trace of the received photocurrent at 10 Gbps with the detector operating at 35 V is shown together with an open eye pattern.
3.3 Temperature dependency
An important aspect to APD operation is temperature sensitivity. Photodiodes in optical interconnect applications should have a wide range of operational temperature. Relative to silicon, germanium photodiodes suffer from excessive thermal generation of carriers due to a smaller band gap. Consequently, thermoelectric cooling must be employed to maintain a stable operational temperature. We have measured the photocurrent of the present photodiodes at temperatures up to 70 °C shown in Fig. 8 for a 200 µm long device. As the temperature increases from 20 °C to 70 °C the measured photocurrent increases by a factor of 1.4. This is likely a consequence of the physical phenomenon which governs the operation of these deep-level enhanced photodiodes, which requires carrier excitation through a combined optical and thermal generation effect . The leakage current was also observed to increase across the same temperature range by a factor of 2.4.
This performance as a function of temperature suggests the possibility of operation without the requirement for thermo-electric cooling.
We have demonstrated monolithic silicon waveguide integrated avalanche photodiodes. The devices operate with an open eye pattern at 10 Gbps and are fabricated with standard CMOS processes. A detector with a length of 600 µm achieved a DC responsivity of 4.7 ± 0.5 A/W at a reverse bias of 40 V. The detectors show linear operation as a function of optical power, an improved response when increasing temperature from 20 °C to 70 °C and are sensitive to wavelengths across the S, C and L telecommunication bands, and perhaps up to 2000 nm.
The authors would like to thank Dan Deptuck, Jessica Zhang and Bob Mallard of CMC Microsystems for facilitating device design and assistance with frequency response measurements, Jack Hendriks of Western University for ion implantation, Professor Gerald Buller of Heriot-Watt University for discussion. The authors acknowledge the support of CMC Microsystems and the Natural Sciences and Engineering Research Council of Canada.
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