Infrared (IR) detectors characterized by a combination of (1) high-quantum-efficiency photoexcitation of inter-valence-subband transitions of charge carriers and (2) high-mobility conduction-band transport of the thus-excited charge carriers have been proposed in an effort to develop focal-plane arrays of such devices for infrared imaging. Like many prior quantum-well infrared photodetectors (QWIPs), the proposed devices would be made from semiconductor heterostructures. In order to obtain the combination of characteristics mentioned above, the proposed devices would be designed and fabricated in novel InAs/GaSb superlattice configurations that would exploit a phenomenon known in the semiconductor art as type-II broken-gap band offset.

Previously tested GaInSb/InAs type-II strained-layer-superlattice devices have shown the potential to offer optical properties comparable to, degrees of uniformity greater than, and tunneling currents and Auger recombination less than, those of HgCdTe IR photodetectors. Moreover, the GaInSb/InAs type-II strained-layer devices have been shown to be capable of operation at normal incidence. The operation of the GaInSb/InAs type-II strained-layer devices involves transitions from valence subbands in GaInSb to conduction subbands in InAs. The spatial separation of wave functions involved in the transition results in reduced oscillator strengths. Therefore, to obtain adequate quantum efficiency, it is necessary to grow thick, high-quality superlattices - a task that is challenging because GaInSb and InAs are lattice-mismatched. The development of the proposed devices would implement an alternative approach to exploitation of some of the same basic principles as those of the GaInSb/InAs type-II strained-layer devices.

This Energy-Band Diagram of a GaSb/InAs QWIP depicts the energy levels involved in the exploitation of (1) inter-valence-subband transitions in GaSb for detection and (2) transport in the InAs conduction band. The relevant energy bands and the energies at their edges are denoted as follows: EC for conduction band; EV for valence band; hh1 and hh2 for heavy-hole bands 1 and 2, respectively; and lh1 for light-hole band 1.

It is useful to compare the proposed GaSb/InAs QWIPs with prior GaAs/AlGaAs QWIPs, which utilize n doping. Because quantum-mechanical selection rules in n-doped QWIPs forbid inter-conduction-subband transitions induced by normally incident light, optical coupling gratings are needed to achieve acceptable quantum efficiencies in such QWIPs. On the other hand, inter-valence-subband transitions in p-doped QWIPs can absorb normally incident photons with high quantum efficiency, so that optical coupling gratings are not needed.

An InAs/GaSb multi-quantum well structure according to the proposal would comprise p-doped GaSb quantum wells embedded in InAs barriers (see figure). The inter-valence-subband transitions in the GaSb wells would absorb normally incident photons with high quantum efficiency. Although the InAs layers would be barriers to ground-state electrons, a device of this type would take advantage of the high electron mobility in the InAs conduction band for transporting excited electrons between the GaSb quantum wells. This would be made possible by the type-II broken-gap band offset between InAs and GaSb: The edge of the valence band of GaSb is approximately 0.15 eV higher than the conduction-band edge of InAs. Therefore, electrons in the p-doped GaSb quantum wells that were photoexcited to the uppermost subband (denoted the heavy-hole 1 or hh1 subband) could easily escape into the conduction band in InAs layers.

This work was done by David Ting, Sarath Gunapala, and Sumith Bandara 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 Electronics/Computers 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:

Innovative Technology Assets Management
JPL
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Refer to NPO-30426, volume and number of this NASA Tech Brief Issue, and the Page Number.

Topics:
Electronics & Computers Imaging and visualization Logistics Mobility Optics Photonics Semiconductors
This Brief includes a Technical Support Package (TSP).
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Inter-Valence-Subband/Conduction-Band-Transport IR Detectors

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Photonics Tech Briefs Magazine

This article first appeared in the October, 2004 issue of Photonics Tech Briefs Magazine (Vol. 28 No. 10).

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Overview

The document is a Technical Support Package from NASA's Jet Propulsion Laboratory, detailing advancements in Inter-Valence-Subband/Conduction-Band-Transport infrared (IR) detectors. It focuses on the development of high-performance solid-state devices utilizing a closely lattice-matched material system comprising InAs, GaSb, and AlSb. This system allows for various band alignments, including type-I, type-II staggered, and type-II broken-gap configurations, which are essential for creating novel heterostructure devices.

The primary focus is on the GaInSb/InAs type-II strained layer superlattice design, which promises optical properties comparable to traditional materials like HgCdTe, while offering advantages such as better uniformity, reduced tunneling currents, and suppressed Auger recombination. The document highlights the challenges associated with achieving high quantum efficiency in these devices due to the spatial separation of wave functions in the transitions between valence and conduction bands. To overcome these challenges, the authors propose an innovative structure that incorporates GaSb quantum wells embedded in InAs barriers, leveraging the excellent absorption characteristics of GaSb and the high electron mobility of InAs.

The proposed detector structure utilizes inter-valence-subband transitions in p-type GaSb quantum wells, which enhances absorption quantum efficiency. The design allows for efficient photocurrent transport, as electrons photoexcited in the GaSb quantum wells can easily escape into the InAs conduction band. This configuration is advantageous compared to traditional type-II strained layer superlattice designs, as it employs lattice-matched materials, simplifying growth requirements.

The document also compares the proposed GaSb/InAs detector structure with GaAs/AlGaAs-based quantum well infrared photodetectors (QWIPs). It notes that while n-type QWIPs face limitations due to quantum mechanical selection rules, the proposed structure combines the benefits of both n-QWIPs and p-QWIPs, achieving strong normal-incidence inter-valence subband absorption and high mobility electron transport.

Overall, the document presents a promising approach to developing high-performance infrared detectors, emphasizing the potential of the InAs/GaSb material system in advancing aerospace-related technologies. It serves as a resource for further research and technological applications in the field of infrared detection.