Quantum-well infrared photodetectors (QWIPs) that include two-dimensional surface grating light couplers would be modified, according to a proposal to incorporate crossed slots. It should be possible to increase signal-to-noise ratios by suitable positioning and dimensioning of the slots, as explained below.
The figure depicts part of a typical GaAs/AlGaAs QWIP with a two-dimensional surface grating light coupler. The need for, and function of, a light coupler on a QWIP is described in the preceding article. In the case of a two-dimensional surface grating light coupler, there is an important effect incidental to the basic light-coupler function; the absorption of light inside the QWIP becomes concentrated into columns under the surface grating.
Because little or no light is absorbed in regions between the columns, material could be removed from these regions without reducing photocurrent much, if at all. This leads to the concept of slots. The positions and dimensions of the slots would be chosen to correspond to the low-absorption regions.
While the incorporation of slots would exert little or no effect on photocurrent, it would significantly affect noise. The dark current of a QWIP is proportional to its effective area. In the presence of slots, the effective area would be the cross-sectional area of the columns between the slots. In a typical case, this remaining area would be only one-fourth the original area; consequently, the dark current of the slotted QWIP would be only one-fourth the dark current of the unslotted QWIP. Inasmuch as the noise current of a QWIP is proportional to the square root of its dark current, the noise current of the slotted QWIP would be reduced to half that of the unslotted QWIP, resulting in a signal-to-noise ratio twice that of the unslotted QWIP at any wavelength and temperature.
Another anticipated benefit of slotting would be an increase in the fraction of incident light converted to polarization perpendicular to the planes bounding the quantum wells and thus an increase in light-coupling efficiency. The estimated net effect of slotting would be an enhancement of detectivity by a factor of 3 to 4.
This work was done by Sarath Gunapala, Sumith Bandara, John K. Liu, and Daniel Wilson of Caltech for NASA's Jet Propulsion Laboratory.
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