GaAs/AlxGa1-xAs quantum-well infrared photodetectors (QWIPs) with broad-band responses are undergoing development. The broad-band responses are obtained by varying the depths and widths of the wells and the thicknesses of the barriers between the wells, as explained below.

A GaAs/AlxGa1-xAs QWIP contains layers of GaAs (the wells) alternating with layers of AlxGa1-xAs (0< x <1) (the barriers). Detection of a photon in a QWIP involves photoexcitation of electrons from the ground to the first excited quantum state of one of the wells. The excited electron can escape from the well and, if a suitable bias voltage is applied, then many such photoelectrons are collected as photocurrent.

Typically, a QWIP is of the bound-to-continuum or the bound-to-quasi-bound type. In the bound-to-continuum type, the first excited state lies above the top of the well. In the bound-to-quasi-bound type, the first excited state lies exactly at the top of the well. Each of these types offers a different advantage; the spectral width of photoresponse is broader in the bound-to-continuum type, while the bound-to-quasi-bound case offers the advantage of a smaller thermionic contribution to dark current.

The Spectral Responsivity of a QWIP containing repeating sequence of three different wells was much broader than that of a typical older QWIP containing multiple identical wells.

The height of each barrier, and thus the depth of an adjacent well, is determined by the mole fraction (x) of aluminum. The energy heights of ground and first excited states relative to the tops and bottoms of the wells, plus the spectral responses of QWIPs, are determined by the widths of the wells, the thicknesses and heights of the barriers, and the well-doping densities. These thicknesses and compositions can be chosen by design and implemented with established deposition techniques.

In a typical bound-to-quasi-bound QWIP of older design, all of the wells are of the same width and doping density, and all of the barriers are of the same thickness and the same aluminum content. The spectral width of its photoresponse is about 10 percent of its peak-response wavelength, which can be set at a value between 6 and 20 µm by suitable choice of layer thicknesses and x.

A device with broader spectral response can be constructed by stacking different quantum wells in a repeating sequence. Each sequence contains several quantum wells; the thicknesses and compositions of the barrier and well layers within each sequence can be tailored to optimize each well for a different pass band that partly overlaps one or more pass band(s) of the other wells in the sequence. The net effect of the multiple partial overlaps is a broader overall pass band for the QWIP device as a whole.

An experimental device of this type was made with 35 repeating three-well sequences. The wells in each sequence were designed for peak responses at wavelengths of 13.5, 14.3, and 15.5 µm, respectively. The measured spectral response of this device (see figure) was found to be about 4 times as broad as that of a typical bound-to-quasi-bound QWIP of older design for the same nominal peak wavelength

This work was done by Sumith Bandara and Sarath Gunapala of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at 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

Technology Reporting Office JPL Mail Stop 122-116 4800 Oak Grove Drive Pasadena, CA 91109 (818) 354-2240

Refer to NPO-20319, volume and number of this NASA Tech Briefs issue, and the page number.

This Brief includes a Technical Support Package (TSP).
Broad-Band GaAs/AlxGa1-xAs QWIPs

(reference NPO-20319) is currently available for download from the TSP library.

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This article first appeared in the April, 1999 issue of Electronics Tech Briefs Magazine.

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