An AlxGa1—x As/GaAs quantum-well infrared photodetector (QWIP) of the blocked-intersubband-detector (BID) type, now undergoing development, features a chirped (that is, aperiodic) superlattice. The purpose of the chirped superlattice is to increase the quantum efficiency of the device.
A somewhat lengthy background discussion is necessary to give meaning to a brief description of the present developmental QWIP. A BID QWIP was described in "MQW Based Block Intersubband Detector for Low-Background Operation" (NPO-21073), NASA Tech Briefs Vol. 25, No. 7 (July 2001), page 46. To recapitulate: The BID design was conceived in response to the deleterious effects of opera-tion of a QWIP at low temperature under low background radiation. These effects can be summarized as a buildup of space charge and an associated high impedance and diminution of responsivity with increasing modulation frequency.The BID design, which reduces these deleterious effects, calls for a heavily doped multiple-quantum-well (MQW) emitter section with barriers that are thinner than in prior MQW devices. The thinning of the barriers results in a large overlap of sublevel wave functions, thereby creating a miniband. Because of sequential resonant quantum-mechanical tunneling of electrons from the negative ohmic contact to and between wells, any space charge is quickly neutralized. At the same time, what would otherwise be a large component of dark current attributable to tunneling current through the whole device is suppressed by placing a relatively thick, undoped, impurity-free AlxGa1—x As blocking barrier layer between the MQW emitter section and the positive ohmic contact. [This layer is similar to the thick, undoped AlxGa1–x As layers used in photodetectors of the blocked-impurity-band (BIB) type.]
Notwithstanding the aforementioned advantage afforded by the BID design, the responsivity of a BID QWIP is very low because of low collection efficiency, which, in turn, is a result of low electrostatic-potential drop across the superlattice emitter. Because the emitter must be electrically conductive to prevent the buildup of space charge in depleted quantum wells, most of the externally applied bias voltage drop occurs across the blocking-barrier layer. This completes the background discussion.
In the developmental QWIP, the periodic superlattice of the prior BID design is to be replaced with the chirped superlattice, which is expected to provide a built-in electric field. As a result, the efficiency of collection of photoexcited charge carriers (and, hence, the net quantum efficiency and thus responsivity) should increase significantly.
This work was done by Sarath Gunapala, David Ting, 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. NPO-30510
This Brief includes a Technical Support Package (TSP).
Chirped-Superlattice, Blocked Intersubband QWIP
(reference NPO-30510) is currently available for download from the TSP library.
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Overview
The document is a Technical Support Package from NASA's Jet Propulsion Laboratory, detailing the Chirped-Superlattice, Blocked Intersubband Quantum Well Infrared Photodetector (QWIP). It outlines the advancements in infrared detection technology, particularly focusing on the design and functionality of QWIPs, which are crucial for various aerospace applications.
QWIPs utilize quantum wells to enhance their photodetection capabilities. The document explains that electrons in the subbands of isolated quantum wells behave similarly to electrons in bulk photoconductors, where photogenerated electrons can create a space-charge buildup that impedes further electron entry from the opposite electrode. This phenomenon can lead to reduced responsivity at high optical modulation frequencies, particularly in low-background irradiance conditions. To address this issue, the document describes a new multi-quantum well (MQW) structure that separates the active quantum well region from blocking barriers, allowing for quicker refilling of space-charge buildup through sequential resonant tunneling.
The document also compares QWIPs with Block Impurity Band (BIB) detectors, which are silicon-based and have specific cutoff wavelengths determined by the impurities used. BIB detectors require cooling to cryogenic temperatures (8-10 K) for operation, while the MQW-based QWIPs offer the advantage of tunable cutoff wavelengths through band gap engineering of GaAs/AlGaAs materials, making them suitable for shorter wavelength operations (e.g., 10 or 15 microns).
Additionally, the document includes figures illustrating the general frequency response of QWIPs and the energy band diagram of the GaAs/AlGaAs blocked intersubband detector. It highlights the noise spectral density of a QWIP focal plane array, emphasizing the lack of 1/f noise above 10 mHz, which is beneficial for slow modulation and scanning strategies in space-borne applications.
Overall, the document serves as a comprehensive overview of the technological advancements in QWIPs, their operational principles, and their advantages over competing technologies, positioning them as a significant development in infrared detection for aerospace and other applications. Further information and resources are available through NASA's Scientific and Technical Information Program Office.
