It has been proposed to modify the basic structure of an nBn infrared photodetector so that a plain electron-donor-type (n-type) semiconductor contact layer would be replaced by a graded n-type III–V alloy semiconductor layer (i.e., ternary or quarternary) with appropriate doping gradient. The abbreviation “nBn” refers to one aspect of the unmodified basic device structure: There is an electron-barrier (“B”) layer between two n-type (“n”) layers, as shown in the upper part of the figure. One of the n-type layers is the aforementioned photon-absorption layer; the other n-type layer, denoted the contact layer, collects the photocurrent.
The basic unmodified device structure utilizes minority-charge-carrier conduction, such that, for reasons too complex to explain within the space available for this article, the dark current at a given temperature can be orders of magnitude lower (and, consequently, signal-to-noise ratios can be greater) than in infrared detectors of other types. Thus, to obtain a given level of performance, less cooling (and, consequently, less cooling equipment and less cooling power) is needed. [In principle, one could obtain the same advantages by means of a structure that would be called “pBp” because it would include a barrier layer between two electron-acceptor-type (p-type) layers.] The proposed modifications could make it practical to utilize nBn photodetectors in conjunction with readily available, compact thermoelectric coolers in diverse infrared-imaging applications that could include planetary exploration, industrial quality control, monitoring pollution, firefighting, law enforcement, and medical diagnosis.
The modifications are meant to address an aspect of the basic unmodified device structure that limits the performance advantages to photons having wavelengths less than either of two specific values: 3.4 or 4.4 μm. These values correspond to bandgaps associated with two specific semiconductor alloy compositions (InAs or InAsSb, respectively), either of which could be used in the photon-absorption layer. For reasons that, once again, are too complex to describe within the space available for this article, these two compositions are the only ones that afford the energy-band structures needed to obtain the desired combination of adequate photo-generated current and reduction of dark current with AlSb barrier. For other values, depending on the type of energy-band alignment, there arise, in the B layer, a valence-band well for holes. Undesirably, holes will be trapped in the valence-band well, with consequent reduction of collectable hole photocurrent through tunneling of holes to the conduction band and buildup of charge at the Bn (i.e., at the barrier and n-contact) interface.
The lower part of the figure depicts the energy-band structure for one of the proposed modified device structures. In this case, the plain n-type contact layer would be replaced with a graded III–-V alloy layer with proper doping gradient from undoped to doped n contact. The valence band potential dip can be eliminated under proper bis condition. In a different modification, not shown in the figure, the plain photon-absorption layer could be replaced with a chirped strain-layer superlattice. In either modification, the graded or chirped structure would provide an energy ramp that would serve as a smooth path for transport of minority charge carriers. The valence-band hole trap and the associated undesired effects would be eliminated.
This work was done by Sarath D. Gunapala, David Z. Ting, Cory J. Hill, and Sumith V. Bandara of Caltech for NASA’s Jet Propulsion Laboratory.
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Refer to NPO-45550, volume and number of this NASA Tech Briefs issue, and the page number.