Waveguide avalanche photodiode shows promise for 1550 nm

Researchers at the University of Texas (Austin, TX) and Lucent Technologies (Holmdel, NJ) have used a side-illuminated waveguide structure to fabricate a waveguide-confined indium gallium arsenide/indium aluminum arsenide avalanche photodiode (APD). They hope that the device structure, which combines the attributes of waveguides with APD design, will provide fast, sensitive photodiodes for the 1.55-µm telecommunications wavelength range. The device was constructed based on a separate absorption, charge, and multiplication (SACM) scheme and achieved a unity-gain bandwidth of 27 GHz and a gain-bandwidth product of 120 GHz.

Indium aluminum arsenide was chosen for the multiplication region and cladding material of the device because of its transparency at 1.55 mm and low excess-noise characteristics. The device layers were grown on an indium phosphide substrate and buffer layer by molecular-beam epitaxy. Dark current in the completed waveguide SACM APD remained below 50 nA at 90% of breakdown, and the resonant-cavity bandwidth was estimated to be greater than 40 GHz. Bandwidth also appeared to be transit-time limited, because the maximum bandwidth of 27 GHz was independent of area for devices less than 100 µm². Contact Geoffrey Kinsey at [email protected].