SiGe electro-absorption modulators for applications at 1550nm

Abstract

A novel SixGe₁-x, electro-absorption modulator design is experimentally demonstrated. The device is waveguide integrated, butt-coupled into high index contrast Si/SiO2 waveguides. 0.75% Silicon concentration in the alloy is optimized for 1550nm applications. With its 400nm height, 600nm width, and 50,pm length, the device has a footprint smaller than 30[mu]m². Low effective driving voltage, <2.5V, is needed to achieve an extinction ratio of 5.2dB in the broad 1510-1555nm wavelength operation range. At 1550nm, an extinction ratio of 6.5dB is achieved with an applied effective bias of -2.5V. High frequency measurements determine the device can reach a 3dB frequency of 1.2GHz. Electrical characterization of the device shows high series resistance (~15k[omega]) which is caused by fabrication over-etching during metal contact deposition. Series resistance reduction to ~100[omega] would allow the device to reach the predicted 3dB frequency of 100GHz with 10dB extinction ratio. A pseudo-linear relation is found between the achieved extinction ratio and the applied effective bias. The ratio between these two quantities, the modulation efficiency, can be considered as a new figure of merit of the device. The slope of this pseudo-linear relation measures 2.2dB/V for extinction ratio values ranging between 0 and 5.5dB. In terms of modulation depth it is equivalent to a slope of 40%/V in the range 0.5V-2V. Finally, an ultra-low power consumption per bit of 34fJ/bit is measured for a capacitance of 11fF and an effective applied reverse bias of 2.5V.