Focal-plane arrays of quantum-well infrared photodetectors (QWIPs) featuring adjacent pixels sensitive to different colors have been proposed. An array of this type would make it possible to image the same scene in multiple wavelength bands simultaneously on the same focal plane, without need for moving parts or for complex optics to split light into wavelength bands and make the light in each band impinge on a separate detector array optimized for that band. Hence, these arrays would make it possible to develop a new generation of spectral imagers that would be smaller, lighter, and less costly, relative to spectral imagers now or previously in use.

The figure is a schematic cross section of one pixel of a four-color array according to the proposal. The pixel would be divided into four adjacent sub-pixels, each sub-pixel optimized for one of the four desired wavelength bands. All the sub-pixels would contain identical stacks of four multiple-quantum-well (MQW) photodetectors (for all four bands), but as described below, functional electrical connections would be made to only the one MQW photodetector that was optimized for the wavelength band assigned to a given sub-pixel.

Sub-Pixel Stacks would contain identical semiconductor layers. They would differ in their light-coupling two-dimensional diffraction gratings and electrical contacts.

Each MQW photodetector in a stack would comprise 30 spatial periods of layers of GaAs quantum wells separated by AlxGa1-xAs barriers; the parameters of the layers would be chosen to maximize sensitivity in the designated wavelength bands. The photodetectors for the different wavelength bands would be separated by intermediate contact layers.

Fabrication of the array would begin with the growth of a wafer comprising all of the MQW, contact, and ancillary GaAs and AlxGa1-xAs semiconductor layers. Next, the pixels and sub-pixels would be defined by photolithographic processing, including masking, etching, chemical vapor deposition, and deposition of metal. The wavelength band for each sub-pixel stack would be delineated by use of a deep groove etch to make contact with the intermediate contact layers of the MQW photodetector for that band while short-circuiting the contact layers of the MQW photodetectors for the other bands. Short-circuiting would be effected by forming grids of gold-coated, reflective etched lines. In addition to serving as shorting conductors, these grids would constitute two-dimensional diffraction gratings that would be optimized for coupling light into the MQWs. For multicolor QWIPs, the grating grooves also serve to deactivate redundant quantum-well stacks (see figure). To ensure sufficient groove depth to penetrate inactive quantum-well stacks, as well as to provide light coupling, three-quarter or five-quarter wavelength groove depths have been used. [The need for such light couplers and the use of two-dimensional diffraction gratings to satisfy this need was described in "Cross-Grating Coupling for Focal-Plane Arrays of QWIPs" (NPO-19657) NASA Tech Briefs, Vol. 22, No. 1 (January 1998), page 6a.]

This work was done by Sumith Bandara, Sarath Gunapala, John K. Liu, David Ting, Sir B. Rafol, and Jason Mumolo of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Electronic Components and Systems category.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to

Intellectual Property group
JPL
Mail Stop 202-233
4800 Oak Grove Drive
Pasadena, CA 91109
(818) 354-2240

Refer to NPO-21084, volume and number of this NASA Tech Briefs issue, and the page number.


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