Novel Metal-Semiconductor-Metal Photodetectors on Semi-Insulating Indium Phosphide
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J.W. PALMER * and W.A. ANDERSON ** * Semiconductor Test Products, Keithley Instruments, Inc., 28775 Aurora Road, Cleveland, Ohio, 44139-1891 ** Department of Electrical and Computer Engineering, State University of New York at Buffalo, 217C Bonner Hall, Amherst, NY 14260
ABSTRACT Depositing Pd or Au on InP at substrate temperatures near 77K 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 lowtemperature (LT) metallizations were patterned using a polyimide/Si0 2 lift-off mask, and a SiO antireflection coating was used to attain near-zero reflection at X=840nm. The detectors had an active area of 200gm x 200pm, and line widths and line spacings of 3pm. Detectors having a LTPd/SI-InP structure had a dark current of 8OnA at 5V, 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-tinoxide (ITO)/LT-Au(200A)/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.
INTRODUCTION Metal-semiconductor-metal photodetectors (MSMPD's) are prime candidates for integration into opto-electronic circuits. They are easy to fabricate, compatible with planar processing techniques, and have very high switching speeds. MSMPD's usually consist of two rectifying contacts with interdigitated fingers and have a transit-time limited photocurrent.[1] Ideally, when the applied voltage is high enough that the carrier transit-time is less than the carrier lifetime, the photocurrent saturates and the detector acts like a current source. However, most MSMPD's reported have a quasi-saturated regime in which the photocurrent increases slowly with the applied voltage. Such behavior indicates the presence of an internal gain,[2] and the' extent to which this occurs varies depending on the details of the device fabrication. Relatively little has been written about MSMPD's on InP. Standard metallizations to InP result in a low electron barrier height of about 0.5eV on undoped material (ND = 1015cm"3) due to interactions between the metal and InP. One paper [3] reported a MSMPD using a standard Au metallization on semi-insulating (SI) InP to attain a dark current of
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