Subpicosecond (570 fs) Response Of p-i-n Traveling Wave Photodetector Using Low-temperature -grown

Traveling wave photodetectors (TWPD) have shown high-speed and highefficiency performance [1,2]. By distributing the RC elements and matching the microwave and optical velocities the RC time constant is no longer the bandwidth-limiting factor. Moreover, the trade-off between efficiency and bandwidth, inevitable for conventional vertical photodetectors, can be eliminated. Low-temperature-grown (LTG) GaAs material has been widely utilized for high speed photodetectors [3,4]. The detector bandwidth can easily be improved due to the short carrier life time of the LTG-GaAs material. For such detectors the impulse response is no longer limited by the carrier transit time but by the much shorter carrier recombination time. In this work, we incorporated a LTG-GaAs absorption layer in a p-i-n traveling wave photodetector (TWPD). The device was successfully fabricated and our results show that the performance can be enhanced by taking advantage of both the short carrier lifetime of LTG-GaAs and the high bandwidth efficiency product of a TWPD. Figure 1 shows the structure of the device (top) and the cross section of waveguide (bottom). The layers (bottom of fig. 1) were grown in a MBE system. The LTG-GaAs absorption layer (170 nm) was grown at 215 "C, and the substrate was subsequently insitu annealed at 590 "C for 10 minutes. The nand players were deposited at 570 "C. The device fabrication followed standard p-i-n photodetector processing [ 11. A polyimide layer was spun on the detector for passivation. Coplanar waveguide (CPW) metalization was used for connection to the nand pcontacts. The electrical impulse response was measured by pump-probe electro-optic (EO) sampling. For the optical excitation we used 100 fs pulses from a modelocked Tisapphire laser operating at 800 nm. After edge-coupling into the optical waveguide, the photocurrent is generated by exciting photocarriers in the LTG GaAs layer. A small LiTaO, crystal was placed on top of the CPW lines to probe the time evolution of the signal. As shown in fig. 2, the FWHM of the measured impulse response is 570 fs, corresponding to a -3dB bandwidth of 520 GHz. A external D.C. quantum efficiency of 8% was measured. Simulations have been performed to further study the device performance. The distributed photocurrent is excited by the optical wave propagating along in the p-i-n region [ 5 ] . At the output of the photodetector, the electrical wave is collected. There are three factors that will effect the impulse response. The fast carrier recombination in the LTGaAs, the velocity mismatch between optical and electrical waves, and the microwave loss