National Bureau of Standards Grant NB 82 NAHA 3032


This work is concerned with the design and fabrication of an absolute radiometric detector operated over the 0.7 to 1.5 μm wavelength range. This application requires a semiconductor photodiode with high internal quantum efficiency and long term stability. Of many possible materials, germanium is chosen because high quality material is available, the fabrication processes are relatively straight forward, and a high quantum efficiency is achievable. The fabrication procedures for a germanium cell were developed. Two types of germanium photodiodes were fabricated and tested. In both photodiodes, a channel stop has been employed to reduce the lateral current due to surface inversion. Ion implantation is used to form the n+-p junction, the channel stop and the back surface field. To reduce the surface recombination, CVD Si02 was deposited for surface passivation. A Ti/Pd/Ag metal layer was then sputtered to make the interconnections. With this process, dark current as low as 0.35 mA/cm2 has been observed on a 2 Ω -cm substrate. The n+pp+ photodiodes had a considerably low quantum efficiency than the induced junction photodiodes. It is shown by computer simulation that the internal quantum efficiency, η, of an n+pp+ diode is strongly affected by the carrier lifetime, r, in the emitter and the surface recombination velocity, S, at the SiO2-Ge surface. The high quantum efficiency in the induced junction diodes can be attributed to the absence of implantation induced damage in the emitter, and an electric field near the surface, induced by the fixed charges of the SiO2 layer. With the induced junction structure, we have observed an internal quantum efficiency of 98.8% at 0.7 μm and of 97% at 1.5 μm.

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