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The monitoring photodiode is the key building block for an optical triplexer at wavelengths of 1310, 1490, and 1550 nanometers. The InGaAs/InP photodetectors were proposed and fabricated to be monolithically integrated with AlGaAs/GaAs optical waveguides using total internal reflection coupling. The metal coplanar waveguides on top of the polyimide planarization and passivation layer were then connected to illustrate the high speed monitoring functions. The full width half maximum of the temporal response and 3-dB bandwidth for the optical waveguide coupled photodiodes demonstrated 29.5 ps and 11 GHz, respectively. The bit error rate performance of this integrated photodiode at 10 Gbit/s with 27-1 long pseudo-random bit sequence NRZ input data also showed error-free operation.
©2010 Optical Society of America
1. Introduction
In the latest decade, the market of fiber optical communication expands rapidly for signal transmission such as data, voice, and video content. In a fiber-to-the-home (FTTH) system, the passive optical networks (PON) are usually utilized to directly connect the service providers and household users through fiber deployment. The original asynchronous transfer mode (ATM)-based PON system only specified 1300-nm wavelength for upstream and 1500-nm wavelength for downstream transmissions. To leverage the vast bandwidth in optical fibers, the downstream wavelengths can be extended to 1550 nm and 1490 nm, individually, for analog RF modulated video signal and digital data [1], which is called the triple-play service [2]. The photodiodes are one of the essential components for an optical triplexer at wavelengths of 1310, 1490, and 1550 nm for monitoring functions.
Wavelength division multiplexing (WDM) systems are currently being deployed for optical data transmission applications in telecommunications and local area networks. Compact hybrid techniques could be one of solutions for high-speed interconnection [3–5]. However, the monolithic optoelectronic integrated circuits (OEICs) are of considerable interests for future optical communication systems in the 1.3-1.6 μm wavelength range due to the small size, good reliability, ease of assembly, and significant reduction in interconnections for high speed applications [6].
Presently, GaAs is the most well developed compound semiconductor material for electronic devices due to its higher drift mobility [7]. Most 10-Gbps application-specific integrated circuits (ASICs) have been implemented on GaAs and SiGe technologies with 0.5-μm line width [8]. Also InGaAs p-i-n photodiode is the most commonly utilized for the fiber optic communication networks. For some OEIC applications, it is useful to use GaAs substrates instead of InP because of the mature fabrication technology for electronic components on GaAs.
Of the various waveguide-photodiode integration approaches [9], total-internal-reflection (TIR) mirror vertical coupling [10] was regrowth-free and applicable to single and double heterostructure waveguides. The novel technique from Bossi et al. [10] was demonstrated using a 5-GHz bandwidth on GaAs based optical waveguides integrated with metal-semiconductor-metal (MSM) photodetector within the 0.8-μm wavelength range. Therefore, it is highly desirable to develop higher bandwidth for OC-192 applications using the monolithic integration of InGaAs photodiode devices operating in the 1.3-to-1.6-μm wavelength range with AlGaAs/GaAs waveguides for optical signal routing and the potential for high-speed ASICs on GaAs substrates.
The InGaAs photodetectors connected to metal coplanar waveguides were then proposed to be monolithically integrated with AlGaAs/GaAs optical waveguides by TIR coupling for optical triplexing applications, as shown in Fig. 1 . To complement the material capability and achieve efficient monolithic integration, it is also highly desirable to establish a specialized dry etch processing technology for photonic integrated circuit (PIC) applications. The inductively coupled plasma (ICP) system and chemically assisted ion beam etcher (CAIBE) [11] were used for InGaAs based photodetectors and GaAs based optical waveguides/TIR, respectively. The ICP system is based on a standard reactive ion etching (RIE) configuration with a modified chamber. This modified chamber contains the inductively coupled plasma source. The plasma is formed inside a dielectric chamber that is encircled by an inductive coil. As the magnetic field is induced, a high density plasma is generated. Ion current density and plasma density are controlled by the inductive source power. And CAIBE etching is generally done by exposing a solid surface to ions and reactive gas simultaneously through physical mechanism (ion bombardment) to enhance the surface reaction of the solid with the reactive gas.
Fig. 1 A AlGaAs/GaAs optical waveguide was coupled with InGaAs/InP p-i-n photodiodes by metal coplanar waveguide using total internal reflection mirror (Insets: SEM picture for the waveguide coupled photodiode and layer structures for photodetector and optical waveguide)
2. Design
In this paper, we used a thin 1.7-μm InP buffer layer, grown by solid source molecular beam epitaxy [12], to suppress the dislocations between In0.53Ga0.47As/InP photodiodes and AlGaAs/GaAs five-layer optical waveguides, with its layer structure for the ridge waveguide and p-i-n photodiode illustrated in the inset of Fig. 1. The GaAs based optical waveguide could direct light to the p-i-n InGaAs photodetector by an etched, vertically deflecting TIR mirror. A novel metal coplanar waveguide was then connected to the photodetector to form the high-speed interconnect as shown in Fig. 1. A scanning electron microscope (SEM) picture of the waveguide coupled photodiode was also shown in the inset of Fig. 1. A side view of the beam path for the waveguide coupled photodiode is shown in Fig. 2 . The two polarizations of transverse-electric (TE) and transverse-magnetic (TM) modes in AlGaAs/GaAs ridge wave-guides are completely reflected from TIR mirrors, which means that there is no polarization dependent loss (PDL) from TIR.
The five-layer AlGaAs/GaAs optical waveguides were theoretically designed for fairly round mode to achieve a better control of birefringence. The single mode condition of 4-μm wide waveguides could be maintained with the etch depth lower than 1.7 μm. The birefringence of 9x10−4 and its variation of 4x10−5 were demonstrated at the triplexing wavelengths between 1310 and 1550 nm with etch depths from 1.5 μm to 1.7 μm, as shown in Fig. 3 .
Fig. 3 Birefringence as a function of wavelengths for AlGaAs/GaAs optical waveguides with the same width (w) of 4 μm for different etch depths (h) of 1.5 μm, 1.6 μm, and 1.7 μm.
The major limitations imposed on the speed of p-i-n photodetector are due to the junction capacitance for the RC time constant and the transit-time for the average time required for photo-induced carriers to cross the depletion region. Generally, the overall speed of a photodetector is determined by both its intrinsic bandwidth and its RC circuit-limit bandwidth. At very high frequencies the transit-time of carriers crossing the depletion region begins to dominate the response time and transit-time effects will cause anther roll-off in the photodiode’s response. Considering the rectangular time interval used to define the bandwidth B, the roll-off follows a sin(x)/x response curve. The product between the risetime of the temporal pulse response on the photodetector and its corresponding bandwidth is around 0.443 [13,14]. The overall photodetector bandwidth can be expressed as:
where B is the bandwidth. R (50 ohm) is the series resistance from bulk, contact and load. (8.85x10−12 F/m) and (14.1) [15] are the free space and relative permittivity of InGaAs, respectively. A is the detector area and is the depletion region. (4.8x106 cm/s) [15] is the hole saturation velocity of InGaAs.The transit time considerations suggested that a very thin intrinsic layer is needed to achieve fast response. However, as the intrinsic layer thickness was decreased, the capacitance was increased. Therefore, for any given detector area there was an optimum intrinsic layer thickness that yields the maximum bandwidth. When the intrinsic thickness was fixed, the smaller detector could always achieve higher bandwidth. Besides bandwidth, the absorption coefficient was also an important factor that could affect the optical performance.
For high-speed interconnection study, three different areas (706, 2827, and 4417 μm2)of photodetectors were attached to ground-signal-ground (GSG) coplanar waveguides for capacitance characterization, which could be measured by the network analyzer. The linear regression between the capacitance and areas could estimate the capacitance contributed only by the GSG coplanar waveguide. Three common coplanar waveguides are discussed: air-bridges on semi-insulating substrates, air-bridges with Si3N4 on n-doped substrates, and planarization/encapsulation by polyimide-PI2771 (negative acting).
3. Fabrication
In0.53Ga0.47As/InP p-i-n photodetectors were fabricated by etching mesas using ICP reactors. The Pt, p-contact metal, was used as the etch mask. A plasma containing Cl2/BCl3 at a substrate bias voltage of −100 V, inductive power of 500 W, pressure of 5 mTorr, and substrate temperature of 170°C were used to achieve highly anisotropic etching. A low value of the substrate bias was used to reduce the ion damage and hence minimize the leakage current in detectors. The inset of Fig. 4 shows a SEM picture of an etched mesa. The dark current, as shown in Fig. 4, was 38 nA at −5V bias for a 30-μm diameter detector. After the contact formation, Si3N4 was deposited using plasma-enhanced chemical vapor deposition (PECVD) for passivation of the photodiode and dielectric isolation for the metal coplanar waveguide.
Fig. 4 The dark current versus applied voltage on the detector and the inset SEM shows the mesa etched by ICP reactor with Pt as the mask.
Since this is the multi-layer processing, the alignment is extremely important to control the integrated component position accuracy and maintain the high optical performance. Therefore, the alignment tolerance is very critical to waveguide coupled photodiode process development. Verniers were utilized to insure that every pattern forms a line in different layers, basically a fraction of a micron. Figure 5 showed the novel vernier tool used for alignment purposes.
The InP buffer layer, suppressing the dislocation between the GaAs and InP substrates shown in the inset in Fig. 1, was then removed using wet etching to open the windows for the TIR mirror and ridge waveguide. The CAIBE was utilized to achieve a highly anisotropic etch for the TIR mirrors and optical waveguides in GaAs substrates. In CAIBE, a tilt angle of the mirrors was set by adjusting the wafer orientation with respect to the incident ion beam. The argon ion energy was 200 V. The choice of masks was very important. It should be easy to process and should be stripped off completely after a dry etch. Currently, we developed the OCG-OiR 897-21 MK photoresist as our tilt angle mask. The etched profile, shown in Fig. 6 , was done on the planar GaAs wafer for illustration. This TIR was developed for efficient coupling integration between optical waveguides and photodetectors. In Fig. 6, the top area of the angled surface was rough. This was caused by the mask erosion during etching. In our designs, we positioned the optical mode below the top interface. Therefore, the high coupling performance between TIR mirror and the photodetector was not affected. As tilt mirror angle, being larger than the TIR, kept increasing, the tolerance for tilt-etch-window opening would get tighter. This could create difficulties in processing, especially for the edge quality of the photoresist mask during TIR CAIBE etching. A 30° angle of the mirrors was set by adjusting the wafer orientation with respect to the incident ion beam. AlGaAs/GaAs ridge waveguides were still etched using CAIBE.
The main cause of roughness in the etching of compound semiconductors is the difference between the desorption rates of the reaction products. The desorption rate can be roughly estimated from the melting points (MPs) and boiling points (BPs) of the reactive products of etching. If we use chlorine as the reactive gas, the difference between the MPs of InClx and PClx is rather large, indicating inhomogeneous desorption, resulting in a rough surface due to excessive InClx sticking to the etched surface. Therefore, the main approach to achieve smooth dry etching of InP is to reduce the difference between the desorption rates of the reaction products. The ICP system is one of the preferred methods to achieve this goal, by using a high ion density to compensate this large thermal desorption difference and low bias voltage to reduce the ion damage. On the other hands, the MPs of GaClx and AsClx are quite similar. Therefore, angled ion beam assisted etching had been used in conjunction with a variety of lithographic techniques to produce structures in GaAs and AlGaAs with controlled sidewall geometries. The slope of the etched wall is determined by the angle at which the sample is tilted with respect to the ion beam. The mirror coupling can be used to avoid epitaxial regrowth by coupling waveguides and detectors on the same wafer surface. This approach significantly simplifies the process, while maintaining the small detector size and flexibility to insert layers between waveguides and absorbers.
To reduce the photodetector pad capacitance for high speed applications, the metal coplanar waveguides were fabricated on semi-insulating substrate by pyrolytic-photoresist [16]. The pyrolyzation process was simply a bake on a standard laboratory hot-plate that was ramped from room temperature to 320°C in air. The pyrolytic-photoresist could subsequently be removed by oxygen plasma stripping to leave behind the three-dimensional lifted-off metal thin-films of free-standing and convex-shaped air-bridges. Thus 1.5 μm-thick Au air bridges with a span of 30 μm and a vertical clearance were successfully achieved. Figure 7 (a) is a SEM picture for the air bridge on semi-insulating substrate, which demonstrated the air-bridge contact pads via the pyrolyzation process for 150 μm pitch GSG Cascade-Allessi coplanar waveguide probes. The air-bridge coplanar waveguide on n-doped substrate with Si3N4 as the dielectric material is shown in Fig. 7(b). A photo-definable polyimide layer (Dupont PI 2771) was used for passivation/planarization prior to the deposition of the metal coplanar waveguides, as shown in Fig. 7(c). The polyimide-planarized metal coplanar waveguides connected to InGaAs/InP photodetectors, monolithically integrated with AlGaAs/GaAs optical waveguides by total internal reflection coupling, were successfully demonstrated and its SEM picture was shown in the inset of Fig. 1.
4. Results and discussions
The waveguide coupled photodiode was characterized with DC- and RF-measurements for optical performance illustration. If the reflection loss at the photodiode surface was insignificant, the quantum efficiency η for the photodiode can be described as follows [15]:
whereα is the absorption coefficient for the i-region of the InGaAs photodiode (α :0.68x104 cm−1 at 1550-nm wavelength [15]).The higher thickness of i-region would facilitate the more absorption in the photodiode. To have reasonable sensitivity, a 1.4-μm thick i-region of the InGaAs photodiode was selected to meet the quantum efficiency over 0.6. Therefore, the detector area from the waveguide coupled photodiode by reflector and coplanar waveguide could be demonstrated on 30x30 μm2 with a 3-dB bandwidth greater than 10 GHz.
The spectral photoresponse of the diode was measured from 1480 to 1580 nm. The light source was supplied by a Photonetics tunable laser. The photo-responsivity and quantum efficiency performance for the p-i-n detectors in free-space illumination were 0.7 A/W and 56%, respectively, at 1550-nm wavelength without any antireflection coating.
The laser light was also coupled to the waveguide via a ~12-μm mode diameter of the SMF-28 fiber and then reflected to the detector by TIR. The mode mismatch conversion loss between SMF-28 fiber and 4-μm wide AlGaAs/GaAs waveguide was around 5 dB. The reflective loss between the effective indices of fiber and GaAs from the Fresnel equations at normal incidence was estimated as 0.7 dB. The measured responsivity range, excluding the Fresnel effect and mode conversion in the power budget, was then between 0.42 to 0.6 A/W, as shown in Fig. 8 . Due to the equipment limitation, the responsivity for 1310-nm wavelength was estimated to be 0.4 A/W [17]. Therefore, our 0.42-A/W responsivity, excluding any coupling losses, at 1550-nm wavelength from the waveguide coupled photodiode could be derived to 0.34 internal quantum efficiency and showed that the total loss from the optical waveguide and TIR reflector was ~2.5 dB after considering the quantum efficiency 0.6 from the 1.4-μm thick i-region photodiode. The propagation loss from AlGaAs/GaAs optical waveguides was verified usiang the Fabry-Perot resonance method to have 0.98 dB/cm. The waveguide coupled detector length was around 6 mm. The coupling loss from the TIR reflector was estimated to be 1.9 dB. The tilt surface smoothness and angle with respect to the incident ion beam can be further optimized for better responsivity. In particular, the roughness of the side walls is higher than the roughness of the top and bottom interfaces and that causes additional propagation loss for TE mode as its field is much higher at the side walls [18]. That is the reason why the propagation loss is typically higher for TE modes than for TM modes. Therefore, the PDL obtained from the waveguide coupled photodetector was measured around 0.4 dB.
The optical waveguide 9x10−4 modal birefringence is within the optimization range of semiconductor optical guided-wave devices [19]. The thin buffer layer is especially beneficial for subsequent processing with improved prospects for integration with devices and circuits in the underlying GaAs based material. Our dark current data from thin InP buffer (1.7 μm) detectors are comparable to that of thicker InP buffer (4 μm) devices on GaAs substrates [20] and also suitable for high bit rate applications. A picosecond fiber laser (Pri Tel, Inc.) was used to measure the temporal response of 30 x 30 μm2 photodetectors. The full width half maximum (FWHM) of the laser pulse is 1 ps at 1543.5 nm. A peak power of 1.5 kW was generated with a 66 MHz repetition rate. The FWHMs of the measured photocurrents are 29.5 ps for 30 x 30 μm2 detectors using −10 V reverse bias shown in the inset of Fig. 9 . After Fourier transform was carried on the characterized temporal response, a 3-dB bandwidth of 11 GHz was obtained for the frequency response curve as shown in Fig. 9. Even though the 0.42-A/W responsivity at 1550-nm wavelength from the waveguide coupled photodiode was not as good as Bossi’s data, 0.56 A/W at 840-nm wavelength, our speed response was suitable for 10-Gb/s OC-192 applications. The bit error rate performance of this device at 10 Gbit/s with 27-1 long pseudo-random bit sequence (PRBS) non-return to zero (NRZ) input data showed error-free operation.
Fig. 9 The frequency response of a 30 by 30 μm square detector with 4-μm width waveguide at −10 V detector bias and 1543.5-nm wavelength (temporal response inset)
S11, a vector scattering parameter measuring the fractional reflected signal from a device can be found as the reflection coefficient seen at port 1 and represented as the impedance parameters when port 2 is terminated in a matched load [21]. The negative imaginary part of the complex impedance, derived from the S11 reflection coefficient, is equal to 1/(2πfC) (f: frequency; C: capacitance). Capacitance of 0.2 pF for the integrated waveguide photodiode was then obtained. For simplicity, the GSG coplanar waveguide capacitance was only characterized on photodiodes, excluding the optical waveguides. The capacitance versus detector area for three different high-speed coplanar waveguides is shown in Fig. 10 . The linear regression from the pad capacitance comparison demonstrated that the metal coplanar waveguides on top of the polyimide planarization could achieve the capacitance of 185 fF.
Fig. 10 Pad capacitance comparison between air-bridge and polyimide coplanar waveguides, squares: polyimide coplanar waveguide, triangles: air-bridges with Si3N4 on n-doped substrate, circles: air-bridges on semi-insulating substrate
The typical serial bulk resistance is around 50 ohm for the photodiode speed response. If the InGaAs alloy lattice matched to the InP substrates was considered, the 3-dB bandwidth in Eq. (1) for 30x30-μm2 detector area could achieve 14.2 GHz at 1.4-μm i-region thickness and 4.8x106-cm/s saturation hole carrier velocity. From our measurement data, the InGaAs alloy lattice mismatched to the GaAs substrate at the same detector area could only achieve 11-GHz bandwidth, which implied that the lattice-mismatch-induced dislocation traps and generation-recombination would make the saturation velocity reduce to 3.6x106 cm/s from Eq. (1). These two curves for InGaAs photodiodes at two different substrates, InP and GaAs, for the bandwidth versus intrinsic thickness at 30x30-μm2 detector areas are shown in Fig. 11 .
Fig. 11 InGaAs photodiode bandwidth comparison between the substrates of InP and GaAs (14.2 and 11 GHz bandwidths, respectively, for InGaAs matched to InP substrate and InGaAs mismatched to GaAs substrate)
A thicker (>1.7 μm) InP buffer layer may be suitable for suppressing the dislocation propagation caused by the lattice mismatch (~3.8%) between the InP and GaAs to further improve the speed response and dark current for optical interconnections.
5. Conclusions
InGaAs/InP photodetectors were monolithically integrated with AlGaAs/GaAs five-layer optical waveguides through TIR coupling. The electrical signal from the photodetector was then directed by the polyimide-planarized metal coplanar waveguides with 0.2-pF low capacitance. A 3-dB bandwidth of 11 GHz at 1543.5-nm wavelength, dark current of 38 nA and responsivity of 0.42 A/W under −5 V bias were demonstrated for optical interconnection applications. By combining metal coplanar waveguide techniques with the integrated receiver module, the high speed and high density interconnects with monolithically or hybrid integrated amplifier circuits can be readily achieved.
Acknowledgments
The authors would like to thank Dr. F. G. Johnson, Dr. J. V. Hryniewicz, and Dr. G. A. Porkolab for their technical assistance and discussions. This paper was supported by the Joint Program for Advanced Electronic Materials at the Laboratory for Physical Sciences, U. S. A. and the National Science Council of the Republic of China, Taiwan.
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