Ge:Ga Far-infrared Photoconductor with a Low Ga Concentration of 1×1014cm-3

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Abstract
Germanium doped with gallium (Ge:Ga) far-infrared photoconductors with a Ga concentration of 1×1014 cm-3 are fabricated for use in sensors for detecting extremely weak radiation, such as for astronomical observations by a space-borne cooled telescope. The performance of the photoconductor at low temperatures and under low background photon influxes to simulate operation in a space environment was evaluated. The responsivity of the new Ge:Ga photoconductor is approximately double that of the previous Ge:Ga photoconductors with a Ga concentration of 2×1014 cm-3 which was developed for astronomical observations using the Infrared Telescope in Space (IRTS) satellite. This increase in responsivity, which is proportional to the product of hole mobility, hole lifetime, and quantum efficiency, is interpreted as being due to the enlargement of both hole mobility and hole lifetime and also because of almost equal quantum efficiency due to the utilization of a metal cavity in which the photoconductor is mounted. The new photoconductor has a slow transient response to a step change in photon influx similar to that in the previous photoconductors, but its magnitude is larger than that of the previous ones. We found good agreement between the time constants estimated from curve-fitting to the response to a step change in photon influx and the transient time constants derived by analytical considerations based on the two-region model of the Ge:Ga photoconductor with ion-implanted ohmic contacts.