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We present a compact 1.3 × 4 μm2 Germanium waveguide photodiode, integrated in a CMOS compatible silicon photonics process flow. This photodiode has a best-in-class 3 dB cutoff frequency of 45 GHz, responsivity of 0.8 A/W and dark current of 3 nA. The low intrinsic capacitance of this device may enable the elimination of transimpedance amplifiers in future optical data communication receivers, creating ultra low power consumption optical communications.
© 2011 Optical Society of America
1. Introduction
The success of the silicon electronics industry is primarily due to its high yield, low cost, high volume manufacturing and the doubling of transistor density every two to three years. Although the density of transistors continues to grow exponentially, the bandwidth of electrical IO does not. The poor scaling of data communication may lead to a performance bottleneck in future generation CPUs. Furthermore, in order to continually improve CPU performance manufacturers are now using chip multi-processor technology, placing increased demand on data communication bandwidth. Complementary metal oxide semiconductor (CMOS) compatible silicon photonics has been identified as the most likely candidate for future generation data communication interconnects both in the back plane of high performance computing nodes and within an optically interconnected multi-core processor.
A key component for CMOS compatible silicon photonics is a photodiode capable of detecting light in the near infrared. Recently there have been a number of demonstrations of vertical waveguide Ge p-i-n on silicon photodiodes with device areas ranging from (800–60 μm2) [1–10]. In order to create a “receiverless” system (eliminating the need for a transimpedance amplifier (TIA)), ∼ 1 fF capacitance photodiodes will be required [11]. Hence, compact Ge photodiodes with best-in-class performance metrics are highly desirable. Here we present an ultra compact (5.2 μm2) Germanium waveguide photodiode which has been integrated into Sandia National Laboratories CMOS compatible silicon photonics process [12–20]. Due to it’s ultra low intrinsic capacitance of only 1.2 fF, this compact photodiode may enable direct driving of a transistor gate, eliminating the need for a TIA and drastically reducing power consumption in CMOS compatible data communication links. Due to its low capacitance and small device area this photodiode has low dark current (3 nA) and is capable of operating at data rates in excess of 40 Gbps. Furthermore, we show through a design of experiments reducing the dimensions of vertical n-i-p photodiodes leads to the most sensitive and highest bandwidth device.
2. Diode fabrication
The typical approach to vertically illuminated Ge detectors on Si has been to utilize a top-down approach where blanket epitaxy is grown over the entire wafer area and then features are defined by lithography and etching [21–24]. The optoelectronic devices fabricated with such an approach are dominated by recombination at dislocations introduced to relieve the misfit strain. We have taken a bottom-up approach, where Ge epitaxy grows only in exposed windows (selective area growth) of a field oxide. This approach lends itself to dislocation reduction because the dislocations that form are much closer to the window edge and can terminate on this surface [25].
Using selective area growth in an oxide trench on top of a Si pedestal, Germanium waveguide photodiodes have been integrated into Sandia’s CMOS compatible silicon photonics process flow as follows: High Density Plasma (HDP) oxides are used for initial cladding of the silicon and nitride waveguides to an approximate 1 μm thickness, after which patterning of oxide windows for Ge growth is completed. The selective Ge epitaxy consists of a low temperature buffer layer grown at 400 °C and in-situ doped with boron to a carrier concentration of ∼ 1E18 cm−3 followed by a growth at 600 °C and pressure of 10 mTorr to fill and overgrow the oxide windows, allowing for chemical-mechanical polish (CMP) planarization. The final Germanium thickness was targeted at 0.6 μm. A high resolution transmission electron micrograph (TEM) cross-section of the overgrown oxide trenches is shown in Fig. 1.
Fig. 1 TEM cross-section of selective area epitaxially grown Ge structure showing overgrowth before CMP and low threading dislocation defect density.
Phosphorous was implanted and activated to a concentration of 1E19 cm−3 to form the n-type layer and top contact of the diode, deposition of a capping oxide to completed the formation of the vertical n-i-p diode structure. Rapid thermal anneal (RTA) activation of the implants is done at 630 °C for 30 seconds. The n-i-p Ge diode’s anode is contacted with parallel contacts in the Si pedestal which was implanted with Boron and activated to a concentration of 1E19 cm−3 prior to growth. Ohmic electrical contact to the active silicon and germanium regions was accomplished with a two step contact process: First Ti/TiN sputter depositions were used to form titanium/silicon as well as titanium/germanium contacts to all n+ and p+ regions in both materials and second, vias were filled with tungsten. CMP planarization completed the contact formation. Metal electrical interconnects were formed by deposition and patterning of a 1 μm Ti/TiN/AlCu/TiN stack. The final optical cladding is a plasma enhanced chemical vapor deposition (PECVD) oxide, which was deposited to a thickness of 2.5 μm prior to bond pad opening for electrical connection. A device schematic and scanning electron micrograph (SEM) cross section of a completed Ge photodiode are shown in Fig. 2(a) and (b) respectively.
Fig. 2 (a) Schematic of Germanium waveguide n-i-p photodiode. (b) SEM cross-section of final selective area epitaxially grown Ge photodiode with final electrical contacts.
3. Experimental results
Ge photodiodes with widths ranging from 1.3 to 5.3 μm and lengths ranging from 4 to 64 μm were fabricated. The fabricated photodiodes were characterized for dark current (idark), responsivity (ℛ), 3 dB bandwidth (f3dB) and noise equivalent power (NEP), with results summarized in Table 1.
Table 1. Summary of the measured characteristics of integrated Ge photodiodes for different width (W) x length (L) geometries.
The dark current was measured as a function of reverse bias, with the lowest measured dark current densities of ∼ 40 mA/cm2 at 1 V reverse bias. From these measurements we find a linear increase in dark current density with width of the photodiode. This indicates an increase in threading dislocation density for wider photodiodes, likely due to greater defect annihilation at the side of the oxide trench for the narrower devices. It was previously reported that the threading dislocation density can be decreased by more than an order of magnitude by adding a post growth anneal step to the process [25–28]. Thus it is likely that with further process development, an added anneal step will reduce dark current densities to ∼1 mA/cm2. Fig. 3(a) shows the measured dark current density at 1 V reverse bias as a function of diode width where each point represents an average of 11 measurements and Fig. 3(b) shows a typical set of IV curves for a photodiode 1.3 μm wide with lengths ranging from 4–64 μm.
Fig. 3 (a) Measured linear trend between dark current density at 1 V reverse bias and photodiode width. Each point is the average of 11 separate measurements. (b) Typical set of IV curves for photodiode 1.3 μm wide for various lengths.
The fiber to waveguide coupling losses and waveguide propagation losses up to the photodiode were calibrated out of the responsivity measurements. We find that the maximum responsivity of the n-i-p photodiodes has a slight dependence on diode width, due to the increased coupling efficiency of the silicon waveguide mode to the modes of the wider diodes. As expected, there is also an increase in diode responsivity with device length as seen in Fig. 4(a). Interestingly, since the increase in diode responsivity due to both larger width and length is accompanied by an increase in dark current which increases shot noise more rapidly than responsivity, the larger diodes have a higher NEP and are therefore less sensitive detectors. This result can be seen in Fig. 4(b) and leads us to the conclusion that the smaller the photodiode the more sensitive the detector and the lower the detector intrinsic capacitance.
Fig. 4 (a) Measured responsivity for 1.3 μm wide Ge waveguide photodiodes with various lengths, measured with −10 dBm of TM polarized light (b) NEP as a function of wavelength for the same 1.3 μm wide Ge photodiodes, measured for −10 dBm of TM polariled light.
A large built in electric field is highly desirable for CMOS compatible photodiodes. Large internal electric fields enable operation at the carrier velocity saturation bandwidth with low applied reverse bias voltages. Maintaining low voltage operation is critical for detector compatibility with current and future CMOS voltage levels and allows for low power dissipation and low dark currents. In Germanium, field strengths on the order of 1E4 V/cm are required to reach carrier velocity saturation in holes [29]. If we use
where kB is the Boltzmann constant, T is temperature in kelvin, q is the electron charge, NA is the density of acceptors in the p region, ND is the density of donors in the n region and ni is the intrinsic carrier concentration, to estimate the built in potential (Vbi). A value of ∼1E4 V/cm for the built in field is calculated. This result suggests that high speed, carrier velocity saturation bandwidth operation for moderate to large optical powers can be obtained with low applied reverse bias voltages in this device. Evidence of a strong built in field can be seen in Fig. 5(a) where no roll off of the photocurrent is observed at 0V bias and an open circuit voltage of 0.4 V is measured.Fig. 5 (a) Dark current-voltage (IV) and Illuminated (λ = 1.53 μm) current-voltage (LIV) characteristics of a 1.3 x 4 μm2 Ge photodiode (b) Bandwidth of 1.3 × 4 μm2 Ge photodiode at 2 V reverse bias, measured using an optical heterodyne technique showing a 3-dB roll off frequency of 45 GHz. Inset shows saturation of bandwidth at 0.8 V reverse bias for a photocurrent of 300 μA.
The 3 dB bandwidth was measured using the heterodyne note of two ultra-stable, narrow line-width Agilent 81600B lasers. The lasers were combined with a 50/50 beam splitter with the wavelength one laser was held constant at 1530 nm and the other was stepped in 100 MHz increments up to 50 GHz, beginning at 1530 nm. The power of each laser was kept equal and such that the total DC photocurrent was 300 μA. The RF power extracted from the photodiode from GSG probes was then measured on an Agilent E4419B RF power meter with a 50 GHz power meter head. The measurement was calibrated to the device under test (DUT) using an Agilent E8364B 50 GHz vector network analyzer. The results of the measurement were fit to a single pole frequency response, which was used to determine the device 3 dB bandwidth. Fig. 5(b) illustrates a typical measured data and curve fit. We found that the largest measured bandwidth was 45 GHz, for the 1.3 × 4 μm2 detector with larger area diodes having slightly lower bandwidths. This result is better than the best bandwidth previously reported for this class of detector [5]. Furthermore, we found that for a relatively large photocurrent of 300 μA the bandwidth of the diodes saturated below 1V reverse bias (see Fig. 5(b) inset), another strong indication of a large internal electric field. Although the bandwidth roles off at 0V bias due to carrier screening effects for the relatively large photocurrent of 0.3 mA, we found that with a photo-current of 30 μA the photodiode had a bandwidth of 37 GHz at 0V bias, giving further confirmation that low voltage operation is possible. This result demonstrates that these devices can be operated at high speed using current and future CMOS voltage levels.
4. Conclusions
We have fabricated and characterized integrated Ge waveguide photodiodes in a process flow which is fully compatible with Sandia’s CMOS electronics and active silicon photonics platforms. We find that reducing photodiode dimensions to 1.3 × 4 μm2 leads to the most sensitive, lowest capacitance and highest bandwidth integrated photodiode with a dark current of only 3 nA, and a responsivity of 0.8 A/W. To the best of the authors knowledge, this result demonstrates a device which has best-in-class bandwidth performance and the lowest intrinsic device capacitance (∼1 fF) thus far reported in the literature. This ultra compact integrated photodiode may enable “receiverless” optical links with ultra low power dissipation in future data communication systems.
Acknowledgments
Funding for this work was provided by Sandias Laboratory Directed Research and Development (LDRD) program and the Department of Defense. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.
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