Multiple-quantum-well (MQW) AlxGa1-xAs/GaAs infrared photodetectors that are better suited [relative to prior AlxGa1-xAs/GaAs quantum-well infrared photodetectors (QWIPs)] for operation under low-background conditions are undergoing development. These devices are expected to function at a temperature of 30 K without nonlinear effects or delayed responses. Even at this low temperature, the readout electronic circuits for imaging arrays of these photodetectors are expected to work smoothly — that is, with no freezeout of charge carriers. Large focal-plane arrays of these detectors should be relatively inexpensive because they could be fabricated by use of mature techniques for the growth and processing of AlxGa1-xAs/GaAs.
Some background information is prerequisite to a meaningful description of the present MQW photodetectors: The operation of QWIPs is based on photoexcitation of electrons between ground and first-excited-state subbands of quantum wells, which are formed by stacking alternate layers of two different, high-bandgap semiconductor materials (e.g., AlxGa1-xAs and GaAs). The discontinuity in bandgaps between the two materials gives rise to quantized subbands in the potential wells associated with conduction bands. The parameters of the layers are chosen so that photoexcited charge carriers can escape from the potential wells and be collected as photocurrent. Thus, in principle, a QWIP operates similarly to an extrinsic bulk photoconductor.
Electrons in the subbands of the isolated quantum wells in a typical prior QWIP can be visualized as electrons attached to impurity states in bulk photoconductors. As a photogenerated electron leaves the active doped quantum-well region, it leaves behind a hole that constitutes an increment of space charge. For operation under low-background conditions, QWIPs are designed to have extremely low tunneling currents and extremely low thermionically emitted dark currents at low temperature (30 K). Hence, in the event of low background irradiance, the high resistivity of the active region (a consequence of the large thickness of the barriers between wells) can lead to a delay in refilling the wells. This delay, in turn, results in a decrease in responsivity with an increase in the frequency of intensity modulation of the infrared radiation that one seeks to detect. This frequency response is similar to that associated with dielectric relaxation in bulk photoconductors. This completes the background information.
The developmental MQW based Block Intersubband Detectors (BID) are designed to suppress the deleterious effects described above. A device of this type (see figure) includes a heavily doped MQW emitter section with barriers that are thinner than in prior QWIP 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, large tunneling current through the whole device is suppressed by a relatively thick, undoped AlxGa1-xAs layer between the MQW emitter section and the positive ohmic contact. [This layer is similar to the thick, undoped silicon layers used in the block impurity band (BIB) type.]
This work was done by Sarath Gunapala, Sumith Bandara, John K. Liu, Sir B. Rafol, David Ting, and Jason Mumolo of Caltech forNASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP)free on-line at www.nasatech.com/tsp under the Electronics & Computers category.
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