Carrier lifetimes in Ge waveguides on Si are deduced from time-resolved pump-probe spectroscopy. For a 1 pm wide Ge waveguide, a lifetime of 1.6 ns is estimated for a carrier density of around 2 x10(19) cm(-3)

Absil, P

Lepage, G

Pantouvaki, M

Srinivasan, Srinivasan Ashwyn

Van Campenhout, J

Van Thourhout, Dries

Verheyen, P

English

Ghent University Academic Bibliography

   Abstract— Carrier lifetimes in Ge wavegdeduced from time-resolved pump-probe specµm wide Ge waveguide, a lifetime of 1.6 ns icarrier density of around 2×1019 cm-3.   Index Terms— Waveguide modulator; OptiSilicon photonics I. INTRODUCTION ilicon photonics is widely consideredsolution in solving the bandwidthbottleneck of electrical interconnects forsystems [1]. Ge waveguides (WG) with andiode integrated on a silicon photonics platfbe used as a building block for a number of dephotodetectors [2], Ge electro-absorption mwell as Ge variable optical attenuators (VOA(with high n-type doping and tensile strain[4]). In this work, we extract carrier waveguides grown on Si, using time-retransmission pump-probe spectroscopy. Thmethod and analysis enable benchmarkingoptimization of carrier-injection based deviceand lasers. In this work, we study the imrecombination on carrier lifetime in sub-microII. DESIGN AND FABRICATION OF GE PA set of Ge WGs with embedded p-i-nlength of 8 µm and varying width (400 nm, and 1 µm) are studied here. They were fab200 mm Si photonics platform [5]. Tadiabatically coupled to an SOI WG using pmode grating couplers with a peak wavelenwere used for coupling light to and from thGe WGs are 350 nm thick and grown on top olayer of the SOI wafer as shown in Fig. 1. Soperated in reverse bias as an electro-absorpt[3] but can in forward bias also be used as a Vprovided with  appropriate strain and high n-t  Carrier LifetimWS. A. Srinivasan1, 2, *, M. Pantouv12Photonics Research Group, Dept. of InforUnive*AuSuides on Si are troscopy. For a 1 s estimated for a cal interconnects;  as a potential -power density  future CMOS  embedded p-i-n orm (Fig. 1) can vices such as Ge odulators [3], as ) and Ge lasers ed active region lifetimes in Ge solved infrared is experimental  and subsequent s, such as VOAs pact of surface n Ge WGs. -I-N WG  diode having a 600 nm, 800 nm ricated in imec’s he Ge WG is oly-Si tapers. TE gth of 1595 nm e chip. The p-i-n f the recessed Si uch a device was ion modulator in OA or Ge laser, ype doping [4].  Fig. 1 Schematic of the Ge p-i-n dioabsorption modulator (1 µm wide) andfabricated Ge p-i-n WG [3]. Fig. 2 Measurement schematic with bBPF2) centered at probe wavelength noise from EDFA with respect to the prprobe signal power, pump signal poefficiency are shown in the inset. III. EXPERIMWe used a mode-locked femtoCalmer laser as the pump and a CWprobe [6]. The pump signal has 1548.57 nm with a pulse width of signal has a center wavelength ocoupled by the grating couplers inmode and the absorption is domina(~1580 nm) [3]. The probe signal ican be filtered out easily from the puband-gap of Ge. The pump is oper20 MHz with an average output powpeak energy of 400 pJ. A 10 dB attpump source to avoid any self-phasewell as two-photon absorption in thtransmission data is extracted frAnalyzer, which is triggered by thresolution of the measurement is limof the photodiode. e Assessment in Integraaveguide Devices aki1, P. Verheyen1, G. Lepage1, P. Absill, J. Van CampenThourhout2 Imec, Kapeldreef 75, Heverlee B-3001, Belgium mation Technology, Ghent University – imec & Center for Nano- and Brsity, St. Pietersnieuwstraat 41, 9000 Ghent, Belgium thor e-mail address: ashwyn.srinivasan@imec.be  1de used as a Ge electro- a microscope image of the and pass filters (BPF1 and to suppress the pump and obe signal. The spectrum of wer and grating coupler ENT  second fiber laser from  tunable laser source as the a center wavelength of 0.235 ps while the probe f 1600 nm. The light is to the TE-like waveguide ted by the Γ-hh band gap s set at 1600 nm so that it mp and is below the Γ-hh ated at a repetition rate of er of around 7.9 mW and enuator is placed after the  modulation in the fiber as e SOI WG. Time-resolved om a Tektronix Signal e pulsed laser. The time ited by the 15 ps rise time ted Ge hout1, D. Van iophotonics, Ghent  2IV. DISCUSSION AND RESULTS  When pumping the device under test, the pump signal will be absorbed creating a high density of electrons and holes. These carriers will decay in time due to recombination. The dynamics of these carriers are probed using a CW signal at 1600 nm which suffers from the pump-induced excess free carrier absorption (FCA) and optical bleaching effect (OBE). The power of the probe signal is monitored using a high-speed oscilloscope as shown in the inset of Fig. 3. From the excess loss, and using the equations for FCA and OBE from [4], we can estimate the average carrier concentration in the Ge WG as function of time (Fig. 3) and the carrier lifetimes at these carrier densities. The latter are shown in Fig. 4, for a carrier concentration of approximately 2×1019 cm-3, and for different Ge WG widths.  Fig. 3 Time resolved extracted carrier concentration in Ge WG from transmission measurements in 8 µm long device (inset) for different width. Fig. 4 Extracted carrier lifetimes for different Ge WG widths. The colored data points are the modeled carrier lifetimes assuming a fixed surface recombination velocity (Vs) for Ge/SiO2 and Ge/Si. The data above the measurement correspond to under-estimation of Vs and those below the measurement correspond to over-estimation of Vs for Ge/Si interface for a fixed Vs for Ge/SiO2 interface.   For such a carrier concentration, Auger recombination and radiative recombination from the direct and indirect band gap are expected to be small as compared to defect assisted SRH recombination and surface recombination at Ge/SiO2 and Ge/Si interfaces [8]. In this paper, we assume that the carriers generated by the pump are distributed uniformly across the length of the WG and that carrier lifetimes are not affected by diffusion effects. The dependence of the carrier lifetime on the WG width suggests that the Ge/SiO2 and Ge/Si interfaces in the side walls have a significant impact on the recombination rate. This impact is reduced as we increase the volume/surface-area ratio when using wider devices. Assuming that the lifetime is surface recombination limited, we extract the surface recombination velocity for each interface as shown in Fig. 4 using the related equation in [8]. In this extraction, we assume the top and side interfaces between Ge and SiO2 and the bottom and sloped interfaces between Ge and Si to behave similarly and fit the recombination velocities (Fig. 4). From this fitting, we estimate the extracted recombination velocity for the Ge/Si interface to be less than 2×104 cm/s and the recombination velocity for the Ge/SiO2 to be the range 1-5×103 cm/s. The recombination velocities for Ge/Si interface are in good agreement with those reported in [7, 9].  The contribution to carrier recombination at Ge/SiO2 sidewall layers can be suppressed in future devices by using better passivation coatings.  V. CONCLUSION We investigated carrier lifetimes of Ge p-i-n WG on Si with different widths using time-resolved infrared transmission pump-probe spectroscopy. The lifetime of free electrons and holes increases for wider devices, thus revealing the impact of side walls of the WG. For a 1 µm wide WG, we estimate the lifetime to be 1.6 ns. We also estimate the Ge/Si interface to have a recombination velocity of < 2×104 cm/s. ACKNOWLEDGEMENTS  This work was supported by imec’s industry-affiliation program on Optical I/O.  REFERENCES [1] D.A.B. Miller, et al. “Device Requirements for Optical Interconnects to Silicon Chips”, Proc. IEEE vol. 97, no. 7, pp. 1166-1185, Jul. 2009. [2] H. Chen, et al. “High responsivity low-voltage 28Gb/s Ge p-i-n photodetector with silicon contacts”, Journal of Light. Tech., 33(4), p.820-824, 2015. [3] S. Gupta, et al. “50GHz Ge Waveguide Electro-Absorption Modulator Integrated in a 220nm SOI Photonics Platform”, Optical Fiber Communication Conference, OSA Technical Digest, 2015, paper Tu2A.4. [4] J. Liu, et al. ‘Tensile strained, n-type Ge as a gain medium for monolithic laser integration on Si”, Optics Letters, 15(19), 11272-77, 2007. [5] P. Verheyen, et al. “Highly Uniform 25 Gb/s Si Photonics Platform for High-Density, Low-Power WDM Optical Interconnects”, in Advanced Photonics for Communications, OSA Technical Digest, 2014, paper IW3A.4. [6] R. Van Laer, et al. “Interaction between light and highly confined hypersound in a Silicon photonics nanowire”, Nature Photonics, 9, 199-203, 2015. [7] R. Geiger, et al. “Excess carrier lifetimes in Ge layers on Si”, Applied Physics Letters, 104(6), 062106, 2014. [8] L. A. Corden, et al. Diode Lasers and Photonic Integrated Circuits, (Wiley, New Jersey, 2012), Chapter 4.  [9] M. Virgilio et al. “Photoluminescence, recombination rate, and gain spectra in optically excited n-type and tensile strained germanium layers”, Journal of Applied Physics, 114(24), 243102, 2013.  

Carrier lifetime assessment in integrated Ge waveguide devices

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