Long wavelength metal-semiconductor-metal photodetectors with Ti/Au and indium-tin-oxide electrodes

Metal-semiconductor-metal (MSM) photodiodes have been shown to be excellent detectors for optoelectronic integrated circuits (OEIC´s) due to their potential for high speed, ease of fabrication, and compatibility with FET process technologies (1). A major disadvantage of MSM detectors is their low responsivity due to the blocking of incoming optical radiation by opaque metal electrodes. Since the signal-to-noise ratio of an optical receiver is strongly dependent on the responsivity of the photodetector, it is important to devise techniques to maximize responsivity. Backside illumination has been shown to increase responsivity since there are no interfering electrodes on the back of the substrate (2). Another avenue to improve this factor is through the use of transparent electrodes for MSMs. This has been successfully applied to GaAs MSMs operating at 850 nm (3). In this paper, we present a study of InAlAs/InGaAs MSMs utilizing transparent indium tin oxide (ITO) and Ti/Au electrodes. Transmission in the ITO is as high as 99% which has resulted in a responsivity of 0.76 A/W for ITO MSMs which is double the responsivity of 0.39 A/W obtained for Ti/Au MSMs