Photonic systems based on complementary metal oxide semiconductor (CMOS) technology require the integration of passive and active photonic devices. The integration of waveguides and photodetector is one of the most important technologies. We report a Ge p-i-n photodetector that is monolithically integrated with silicon oxynitride and silicon nitride waveguides. All processes and materials are CMOS compatible and can be implemented in the current integrated circuit process technology. The small size of the devices results in low absolute dark current. The waveguide-coupled Ge devices show high efficiency (~90%) over a wide range of wavelengths well beyond the direct band gap of Ge, resulting in a responsivity of 1.08 A/W for 1550 nm light. The device speed of 7.2 GHz at 1V reverse bias is strongly affected by the capacitance of the probe pads. The high-performance of the devices at low voltage (≤ 1V) facilitates the integration with CMOS circuits.
©2007 Optical Society of America
Monolithic integration of photonic components with electronics has been pursued for more than two decades. While limited integration of III-V optical components has been successfully demonstrated and progresses towards commercialization, its main limitation is the inability to add complex electronic components to the devices. Silicon material, on the other hand, provides the ideal platform for electronic components but until recently was limited as few optical components have been developed for this platform. In the last few years, there has been growing interest in photonic devices based on Si-compatible materials [1, 2]. An increasing number of publications are dealing with a variety of photonics components on silicon [3-5]. The full CMOS compatibility for discrete devices such as waveguides, modulators, filters, and photodetectors has been recently established [6, 7]. These devices, by being embedded in the current Si-CMOS chip, may enable the realization of an electronicphotonic integrated circuit on Si platform. Although being a critical component of optical interconnects on the chip, the development of high-performance waveguide-integrated Ge photodetectors on Si CMOS platform has remained an imperative but unaccomplished task so far. Ge photodetectors epitaxially grown on a Si substrate have demonstrated promising performances in previously published reports [8-10], most of which were discrete components for operation with normally incident light. In this paper, we demonstrate high-speed and high-efficiency Ge p-i-n photodetectors monolithically integrated with top coupled waveguides. The small size of the waveguide-integrated devices results in low absolute dark current. The integration of photodetectors with waveguides can overcome a trade-off problem between the efficiency and bandwidth of the photodetector by having a photon-absorption path and a carrier-collection path perpendicular to each other. Therefore, the waveguide-integrated Ge photodetector can achieve the performance beyond the level possible with free-space illumination, especially at longer wavelength where absorption in Ge is less efficient.
2. Fabrication processes and device structure
The structure of a waveguide-integrated Ge photodetector device is schematically shown in Fig. 1. A germanium photodetector is fabricated first and a waveguide is placed on top of the photodetector. The light couples from the waveguide to the photodetector by evanescent wave coupling.
For device fabrication, Ge films were grown epitaxially on Si p+ substrates (0.01-0.03 Ω∙cm) in an ultrahigh vacuum chemical vapor deposition (UHV-CVD) reactor (~ 5×10-9 mbar). We used a two step growth for the Ge films. A 60 nm Ge buffer layer was grown at 360 °C followed by 1.1 μm thick Ge layer grown at 730 °C [11-12]. Cyclic thermal annealing between 650 °C and 850 °C was conducted in order to reduce threading dislocation density. A 200 nm poly-Si layer was deposited on the Ge film by low pressure chemical vapor deposition (LPCVD), followed by a phosphorous implant into the poly Si layer at a dose of 5×1015/cm2 at 85 keV. The phosphorous was activated by a rapid thermal processing (RTP) at 750 °C for 5 minutes. Germanium vertical p-i-n diodes were defined by dry-etching.
A 1.9 μm deep trench was etched underneath the location of the waveguide to reduce optical losses into bulk Si. A plasma enhanced (PE) CVD silicon dioxide was then deposited, followed by chemical-mechanical polishing (CMP) to planarize the top surface. After opening an oxide window to expose the top poly silicon surface of the photodetector, silicon oxynitride (SiON) or silicon nitride (SiN) waveguide layers were deposited by PECVD. The waveguides were then patterned and etched. The single mode SiON waveguide has a refractive index of 1.8 and dimensions of 0.9 μm × 0.9 μm. The single mode SiN waveguide has a refractive index of 2.2 and is 0.9 μm wide and 0.4 μm high. A 2 μm-thick SiO2 upper cladding layer was deposited, followed by the opening of the contact holes. Fifty nanometers of titanium and 1.5 μm aluminum with 2% silicon were then deposited by sputtering deposition. After the metal contact pads were patterned and etched, the wafers were annealed at 400 °C for 30min in N2/H2 forming gas.
3. Device performance results
The I-V characteristics of a waveguide integrated Ge p-i-n diode, under dark environment and with 1550 nm illumination coupled to the detector through the waveguide, are shown in Fig. 2. The device has a very good rectifying characteristic with low absolute leakage current. The optical power coupled into the detector through the waveguide is 65 μW. The photocurrent, Iph, is obtained by subtracting the dark current from the total current under illumination. The ratio of the photocurrent to the dark current is well above 100:1 for a reverse bias below 1 V at 65 μW input optical power, as shown by the I-V curve under 1550nm illumination. The photocurrent is flat over a wide range of reverse bias voltage, and nearly full DC photocurrent can be achieved at zero bias voltage. The full DC photocurrent at zero bias shows that a built-in electric field is already established within the Ge layer without applying a bias, indicating good intrinsic Ge materials quality . The Ge photodiodes have absolute leakage currents well below 1 μA at < 0.5 V reverse bias. A leakage current of <1 μA is generally considered to be acceptable for a high-speed receiver design where the transimpedance amplifier (TIA) noise becomes the main contributor to the total noise . The leakage current density is 0.41 A/cm2 at 0.5V reverse bias. The higher leakage current density compared to our previous report  is mainly due to the peripheral leakage which dominates the dark current in small area devices.
The responsivity of the waveguide-integrated photodetector is defined as the photocurrent (Iph) divided by the guided optical power at the input port of the detector (Pin) (see the inset of Fig. 3). We have already mentioned how to measure the photocurrent, Iph, in the previous paragraph. The optical power that arrived at the input port of the waveguide-integrated photodetector, Pin, is estimated from the measured transmitted optical power of the reference waveguide, Pout,ref, and the measured transmission loss of the waveguide. The responsivity R is then given by
where αWG is the waveguide transmission loss in units of dB/cm and l is the distance in cm from the photodetector input to the end of the waveguide (as shown in the inset of Fig. 3). The waveguide transmission loss αWG is measured using a cut-back method by evaluating nearby paper clip structures on the same chip. The measured transmission loss of SiON waveguides was 6.9 ± 0.7 dB/cm and that of SiN waveguides was 2.2 ± 0.4 dB/cm, both measured at 1550nm. The responsivity given by Eq. 1 includes the coupling loss between the waveguide and the photodetector, but not the propagation loss in the waveguide and the coupling loss from the optical fiber to the waveguide, because they are not intrinsic properties of the waveguide-coupled detector. In fact, with optimization the waveguide loss can be decreased to <1dB/cm and a fiber-waveguide coupling loss of ≤1 dB can be achieved.
Figure 3 shows the dependence of responsivity on the detector length for both the SiON and SiN waveguides with 1550 nm illumination coupled through the waveguides. As we previously investigated with Si photodetector and low/high index contrast SiON waveguides , the use of the higher index waveguide leads to a higher coupling efficiency and faster coupling. Being consistent with the trend, the responsivity for the SiN coupled detector leveled off at 1.08A/W compared to 0.96 A/W for the SiON waveguide. The responsivity of the waveguide coupled detectors is significantly higher than that of the discrete, normallyincident p-i-n Ge photodetectors where the responsivity of a 1.1 μm thick Ge detector with an absorption coefficient of 4000/cm at 1550nm is estimated to be 0.45 A/W assuming no surface reflection. Coupling from a waveguide to the Ge photodetector can reach efficiencies of 90% or higher without being limited by the intrinsic layer thickness of the device. The results indicate that most photons from the waveguide couple to the Ge intrinsic layer mainly around the input port of the photodetector and travel mostly in the longitudinal direction until being fully absorbed. Our results show that efficient coupling can be achieved with short coupling lengths, allowing the use of small devices that are advantageous for high speeds and low leakage currents.
The wavelength dependency of the responsivity, shown in Fig. 4, does not show the roll-off at 1540 nm that is typical for normally incident illumination . The responsivity is flat through the full range of our tunable laser up to 1570 nm for a 10 μm long device. As a result of coupling from the waveguide, we expect that the efficiency of the Ge detectors can be high even at longer wavelength where photon absorption is much less efficient. For example, if we assume all photons couple to the mode in the Ge intrinsic layer and travel in longitudinal direction, a 50 μm long Ge detector would achieve a responsivity of 0.76 A/W at 1650nm, compared to 0.02 A/W responsivity expected from the same device with surface-normal illumination .
The speed of the photodetectors was measured using an impulse response measurement. A train of 1-ps long optical pulses with a center wavelength of 1550nm, produced by a mode-locked erbium fiber laser, was coupled into the waveguides. The laser pulse was guided through the waveguide and then absorbed by the Ge photodetector, generating an impulse response of the photocurrent [Fig. 5(a)]. From the measured detector impulse response, we obtained the system’s frequency response transfer function by Fourier transform of the impulse response. Figure 5(b) shows the frequency response for different reverse bias voltages. In the inset of Fig. 5(b), the 3dB frequency is plotted against the reverse bias voltage. Without bias, the 3dB frequency is 6.6 GHz, suitable for a bit rate of >10Gb/s. With increasing reverse bias, the 3dB frequency reaches 7.5GHz at 3V. The speed of the photodetector was RC-limited by ~ 400 fF capacitance, which mainly resulted from the large probe pads used for easy probing. Therefore, if integrated with TIA, the bonding pads are no longer necessary and the Ge photodetector devices will achieve much higher bandwidth. Overall, our waveguide integrated Ge photodetector can be operated at a low reverse bias such as 0.1 V with a low absolute leakage current of 60 nA, a high responsivity of 1.08 A/W, and a bandwidth of 7.2 GHz.
We developed Ge photodetectors monolithically integrated with silicon oxynitride and silicon nitride waveguides for top-coupled photodetectors. High responsivity (~ 1.08 A/W) and highspeed (> 10 Gb/s) performances were obtained. The Si CMOS-compatible detector devices retain their high performance even at low operation voltages, thus satisfying the low-voltage requirement of CMOS circuits. The devices have leakage currents that are low enough to meet the requirement of high-speed receiver designs.
Early portion of this work was supported by Pirelli Lab, S.p.A. This research was sponsored under the Defense Advanced Research Projects Agency’s (DARPA) EPIC program supervised by Dr. Jagdeep Shah in the Microsystems Technology Office (MTO) under Contract No. HR0011-05-C-0027. We would also like to thank Samerkhae Jongthammanurak for support in Ge growth, and Andrew T.S. Pomerene and Joseph Giunta from BAE Systems for support of the CMP process.
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