Fabrication scheme reduces PIN dark current by order of magnitude

Researchers at Bell Laboratories, the research arm of Lucent Technologies (Murray Hill, NJ), have developed a wafer-bonding method that has allowed them to decrease the dark current in silicon/indium gallium arsenide (Si/InGaAs) PIN photodetectors by an order of magnitude. The previous bonding method involved placing the Si and InGaAs wafers in contact under 1 to 10 MPa of pressure at a temperature of 650°C, which produced substantial stress at the heterointerface. In the new method, the two wafer surfaces were first cleaned and flattened within atomic tolerances such that x-ray photoemission spectroscopy indicated less than 0.2 monolayers of carbon, oxygen, fluorine, and chlorine on the surfaces, and atomic force microscopy measured root-mean-square roughnesses of less than 1.5 Å. The two surfaces were then placed in contact for Van der Waals bonding under a weight of about 5 lb. The 650° temperature annealing was not applied in this case until after the weight was removed, which reduced thermal expansion stress and enabled elastic accommodation of thin device layers. The improved heterointerface resulted in a minimum dark current of 180 pA under a reverse bias of -10 V, representing an order-of-magnitude improvement over the 2-nA minimal dark current of devices fabricated to date.