Sensitivity of an evaporated PbS cell depends not only upon the content of atmospheric oxygen in the cell layer, but upon the temperature of the substrate during the deposition. Two methods of preparing the PbS layer are described in regard to the cell sensitivity. In one method, termed A-method, the substrate is kept at about 270°C by a jet of air before the deposition is begun; in the other, termed B-method, the deposition is begun on the substrate when its temperature is 400°C, then the substrate is gradually cooled to about 250°C by a jet of air. In both methods, the deposition is processed at 0.7_??_0.9mmHg pressure.
Optical properties of A-type cell by A-method are considerably different from those of B-type cell by B-method: the peak of sensitivity of the former is on the shorter wavelength side of, and a few times higher than, that of the latter.
As for the time constant, B-type cell's is 150 μsec. on the average which is twice as good as A-type cell's 300 μsec. average. However, the average values of NEP of A-type and B-type cells are 6.6×10
-12 W and 1.3×10
-11W respectively.
For spectrophotometric measurement, the time constant needs not to be very small, therefore the deposition of PbS on a substrate of comparatively low temperature as in A-method seems feasible for preparing a high sensitive photoconductive cell.
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