Chemical surface treatments during fabrication have been proposed to reduce surface leakage currents in focal-plane arrays (FPAs) of quantum-well infrared photodetectors (QWIPs). For reasons explained below, if most or all of the surface leakage currents could be eliminated by such treatments, then total dark currents could be reduced to about 1/4 or 1/5 of their original values.
The surface leakage current of a QWIP is a component of its dark current and is proportional to its circumference. The other components of the dark current of a QWIP originate within the bulk of its material and are proportional to its area. By fundamental geometry, the ratio between circumference and area is inversely proportional to width. Thus, as a QWIP is made narrower to incorporate it into a FPA, the ratio between its surface leakage current and the other components of its dark current increases. At the typical lateral dimensions (40 by 40 µm) of a QWIP in an FPA, the surface leakage current is about 4 times the total of all the other components of dark current; that is, the surface leakage current constitutes about 4/5 of the dark current.
Since the earliest days of solid-state electronics, it has been recognized that surfaces and interfaces play a decisive role. Solid-state electronic devices are uniquely sensitive to electrically active surface sites, even at concentrations as small as one part per million. Electrically active surface sites originate in unsatisfied covalent bonds. By satisfying all covalent surface bonds, one could shift surface electron-energy states out of the band gap and into the valence and conduction bands, thereby reducing the surface leakage current. The proposed surface chemical treatments are intended to satisfy the covalent bonds. They include treatment of open surfaces with inorganic sulfides (e.g., LiS2, NH4S, or Na2S·9H2O) or using NH4OH wet chemical etch to define mesas and passivating open areas with SiN.
This work was done by Sarath D. Gunapala, Mani Sundaram, Jin S. Park, Sumith V. Bandara, and John K. Liu of Caltech for NASA's Jet Propulsion Laboratory.For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the Electronic Components and Circuits category, or circle no. 136 on the TSP Order Card in this issue to receive a copy by mail ($5 charge).
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Refer to NPO-19669, volume and number of this NASA Tech Briefs issue, and the page number.