Novel metal-semiconductor-metal photodetectors on semi-insulating indium phosphide

Depositing Pd or Au on InP at substrate temperatures near 77 K has previously been found to significantly reduce the interaction between the metal and semiconductor upon formation of the interface. In this work, this technique was used to fabricate metal-semiconductor-metal photodetectors (MSMPD's) on semi-insulating (SI) InP substrates with superior characteristics compared to detectors formed using standard room temperature (RT) metal deposition. The low-temperature (LT) metallizations were patterned using a polyimide/SiO₂ lift-off mask, and a SiO antireflection coating was used to attain near-zero reflection at λ = 840 nm. The detectors had an active area of 200 μm×200 μm, and line widths and line spacings of 3 μm. Detectors having a LT-Pd/SI-InP structure had a dark current of 80 nA at 5 V, which was a factor of 4 lower than the dark current of conventional MSMPD's. Additionally, LT-Pd/SI-InP MSMPD's exhibited excellent saturation characteristics and a responsivity of 0.75 A/W. Detectors with an indium-tin-oxide (ITO)/LT-Au(200 angstroms)/SI-InP structure had a higher responsivity of 1.0 A/W, due to the relative transparency of this metallization. In contrast, MSMPD's with RT metallizations had poor saturation characteristics, consistent with the results of others. The difference in the illuminated characteristics of MSMPD's with RT and LT metallizations was due to a change in the internal photoconductive gain mechanism. In RT detectors, hole trapping at interface states near the cathode dominated the gain mechanism. In LT detectors, the difference in carrier transit-times dominated.