Indium tin oxide coating increases Schottky-barrier photodetector sensitivity
Ilesanmi Adesida and colleagues at the University of Illinois (Urbana, IL), with collaborators at Bell Communications Research (Red Bank, NJ), have enhanced the barrier heights to reduce dark current in vertical Schottky-barrier photodetectors and increased device responsivity by applying indium tin oxide (ITO) coatings to reduce optical reflection. The epitaxially grown, lattice-matched structures consist of a 25-nm-thick iron-doped indium phosphide (Fe:InP) buffer layer, a 300-nm-thick n+-silicon-doped In¥ohmic-contact layer, a 1.0-µm-thick InGaAs absorption layer, a 50-nm-thick InGaAs/InAlAs graded layer, and a 50-nm-thick InAlAs Schottky-barrier enhancement layer, all grown on an Fe:In¥substrate.
The mesa-shaped active region of each device was defined by a citric-acid etch, then topped with a 320-nm-thick layer of ITO and a 500-nm-thick layer of silicon nitride (Si3N4). The ITO acts as an electrode and enhances the barrier; in conjunction with the Si3N4 layer, it also acts as an antireflection coating. The researchers fabricated a family of photodetectors ranging from 15 to 100 µm in diameter. Responsivities ranged between 0.55 and 0.60 A/W for 1.31-µm illumination and between 0.56 and 0.58 A/W for 1.55-µm illumination. At a wavelength of 1.55 µm, the 15-µm device displayed a 3-dB cutoff frequency of 25 GH¥with a bias voltage of 10 V. Adesida plans to fabricate a bottom-illuminated device, which he expects will increase responsivity by 2 or 3 dB.