Cerrutti, L

Hattasan, Nannicha

Rodriguez, JB

Roelkens, Günther

Tournié, E

Van Thourhout, Dries

English

Ghent University Academic Bibliography

Heterogeneous integration of GaSb based photodetector on Silicon-on Insulator waveguide circuits for shortwave-infrared sensing applications N. Hattasan1, L. Cerutti2, J.B. Rodriguez2, E. Tournié2, D. Van Thourhout1, G. Roelkens1 1Photonics Research Group-UGent/imec, Sint-Pietersnieuwstraat 41, 9000 Gent, Belgium 2Université Montpellier 2 – CNRS, UMR 5214, Place Bataillon, 34095 Montpellier, France   We report our work on the heterogeneous integration of GaSb-based epitaxy on silicon-on-insulator (SOI) waveguide circuits for shortwave-infrared sensing applications. We demonstrate the integration of p-i-n GaSb photodetectors on SOI waveguides. We use an evanescent coupling approach to couple light from the SOI waveguide to photodetector.  The integration of active material onto SOI is based on die-to-wafer adhesive bonding using DVS-BCB as the bonding agent.  Here, we present design, simulation results and fabrication details. The measurement results are also discussed. Introduction Spectroscopy is a well-known technique to examine composition and concentration of the substances by measuring and analyzing the transmission/absorption spectra. Several sensing applications in environment and biomedical field are realized by using this technique, where the system is used to detect bio-molecules or gases with their characteristic absorption lines in near and mid-infrared wavelength region. Nevertheless, such a system consists of source, detector and passive components that are bulky, expensive and unsuitable for many applications e.g. implantable sensors. Photonic integration offers an attractive economic solution with significant miniaturization. Photonic integrated circuits with silicon-on-insulator (SOI) platform provide compact and cost effective solution due to the access to CMOS fabrication technology, which has steadily matured over the last decades.  Hence, high quality SOI passive circuits can be easily designed and fabricated in wafer scale. Moreover, its high omni-direction refractive index contrast allows realization of the miniaturized photonic integrated circuits. Although SOI have several advantages, light generation is difficult due to its indirect bandgap transition. Heterogeneous integration of active devices and SOI integrated circuit is therefore needed to realize fully functional integrated circuit and speed up the development process. There are several techniques for integration, such as molecular bonding, metal bonding and adhesive bonding. In our work, we employ adhesive bonding technique using divinylsiloxane-benzocyclobutene (DVS-BCB) as a bonding agent [1]. This technique provides several advantages. For example, it does not require an ultra clean surface unlike the molecular bonding technique. Moreover, the DVS-BCB maintains good adhesion with some topography on both epitaxy and SOI wafer.    In this paper, we present our work on heterogeneous integration of p-i-n GaSb p-i-n  photodetector on SOI waveguide circuit. The design of the device and fabrication process is described.  The measurement results are also presented. Device designs Figure 1a shows schematic of the integrated device. It consists of SOI waveguide and photodetector on top of the SOI waveguide.  Light from SOI waveguide is evanescently coupled into photodetector waveguide when the phase matching occurs. Figure 1b shows simulation results of absorbed power dependence on the intrinsic region thickness of the photodiode. Clearly, the absorption efficiency is strongly dependent on the bonding thickness, i.e. on the phase matching condition. The simulation is done via CAMFR [2] which is based on frequency-domain eigenmode expansion techniques.       (a)                                                                                                       (b)  Figure 1: a) Schematic of the device b) Simulation results of the device where the absorbed power depends on intrintic region thinkness and BCB thickness. Fabrication process The fabrication process starts with cleaning of both SOI and GaSb epitaxy. SOI is cleaned with standard clean-1 solution containing NH4OH:H2O2:H2O. GaSb expitaxy is cleaned with acetone and IPA. For the heterogeneous integration, DVS-BCB is first diluted with mesitylene in 2:3 v/v. The SOI substrate then is spin-coated with DVS-BCB diluted solution at 3 krpm for 40s to obtain ~200nm bonded thickness. The sample is then baked at 150°C on a hotplate for 3 minutes to let mesitylene evaporate before transferring the GaSb die onto the SOI substrate. Bonding is performed at the same temperature, after which the sample is cured at 250°C for 1 hour. After the bonding process, the complete GaSb substrate is removed by using the combination of mechanical grinding and chemical etching using a mixture of CrO3, HF and H2O in 1:1:3 v/v at room temperature. InAsSb is used as an etch stop layer. This layer is later removed by selective wet etching with a 2:1 v/v citric acid and hydrogen peroxide mixture. The etching rate is ~100nm/minute [3]. The photodiode mesa is formed by using ICP-RIE (inductively coupled plasma reactive ion etching) with CH4 and H2 gas mixture. The etch rate is ~3.6 nm/minute. Figure 2 shows etching results of a dummy sample (GaSb substrate) with Ti as a hard mask.  The sidewall roughness is relatively high, as seen in Fig. 2a. The etching profile is also shown in Fig. 2b. The slope is 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.60.00.20.40.60.81.0  Absorbed power fraction (a.u.)Intrinsic region thickness (µm)BCB thickness (µm) BCB=0.05 BCB=0.1 BCB=0.15 BCB=0.2 BCB=0.3Photodetector Si waveguide estimated to be 101 degree. Ti/Au and NiGe/Au are used to form ohmic contact on both p and n-type GaSb, respectively. The photodiode island is defined by using wet etching in tartaric solution (tartaric acid/HCl/H2O2/H2O) with 0.5µm/minute etching rate [3]. DVS-BCB is used for device passivation. Figure 3a shows cross section diagram of the realized device with the detector on top of the SOI waveguide. Figure 3b is the SEM cross section image of the fabricated device .The etching profile is found to be similar to that of the dummy sample (Figure 2a). The bonding thickness of the device is estimated to be 265 nm            (a)                                                                                      (b)  Figure 2 SEM image of dummy sample (GaSb substrate) (a) showing side wall roughness (b) showing etching profile       (a)                                                                           (b)  Figure 3 a) Cross section diagram of the realized device b) SEM image of the device  BCB Si p-GaSb n-GaSb Ti/Au Ti/Au GeNiAu GeNiAu Si waveguide ~265 nm BCB thickness P-i-n GaSb Contact GaSb Platinum Ti mask Measurement Results Figure 4 shows V-I characteristic measured at room temperature. The high leakage current is attributed to ICP etching which results in high roughness on the mesa sidewall. Light with varied input powers at wavelength 1.57µm is coupled into the device via grating coupler to measure photoresponse. As depicted in Fig. 3, the device illustrates good photoresponse (~0.4A/W) as a function of the applied voltage.  -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0-120-100-80-60-40-20020406080  Current (µA)Voltage (V) 0mW 0.5mW 1mW 1.5mW 2mW 2.5mW 3mW 3.5mW 4mw Figure 4 V-I characteristic of the device at different light input powers. The power level indicates output power from tunable laser.  In conclusion, we demonstrate the first generation of integrated p-i-n GaSb on SOI waveguide. It shows good responsivity of ~0.4A/W. Lower dark current can be obtained by optimizing the fabrication process such as chemical passivation. The successful demonstration of this device yields significant step toward shortwave-infrared integrated circuit.  References [1] G. Roelkens et al., “III-V/silicon photonics for optical interconnects,” Laser & Photonics Rev. 4, no. 6, 751-779, 2010. [2] http://camfr.sourceforge.net [3] O. Dier et al., “Selective and non-selective wet-chemical etchants for GaSb-based materials,” Semicond. Sci. Technol., vol. 19, 1250–1253, 2004.  

Heterogeneous integration of GaSb photodetector on silicon-on insulator waveguide circuits for shortwave-infrared sensing applications

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Heterogeneous integration of GaSb photodetector on silicon-on insulator waveguide circuits for shortwave-infrared sensing applications

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