Multiple-quantum-well AlxGa1 - xAs photodetectors that exploit transitions of electrons from quantum-well bound states to quasi-bound states are undergoing development for use at wavelengths from 6 to 25 µm. These photodetectors are intended to provide detectivities that are higher (signal-to-noise ratios that are higher) than those of predecessor quantum-well infrared photodetectors that exploit transitions of electrons from bound to continuum states.
Noise in a photodetector is associated with dark current — a component of current that flows whether or not illumination is present. The three mechanisms that contribute to dark current in a multiple-quantum-well photodetector are (1) temperature-independent quantum-mechanical sequential tunneling through the barriers between the wells, (2) thermally assisted quantum-mechanical tunneling through the last barrier into continuum states, and (3) classical thermionic emission. The problem, then, is to maximize the photocurrent/dark-current ratio; this must be done by tailoring (a) the depth of the wells (by suitable choice of the composition parameter x in the well and barrier layers) and (b) widths of the wells and barriers (by suitable choice of the thicknesses of the well and barrier layers) to obtain a multiple-quantum-well structure that is optimum for the purpose.
In the development of the predecessor quantum-well infrared photodetectors (QWIPs), a large part of the strategy was to narrow the quantum wells to push the second bound quantum state (first excited state) into the continuum to obtain strong bound-to-continuum intersubband absorption. The major advantage of a bound-to-continuum QWIP is that a photoexcited electron can escape from a quantum well to the continuum transport states without having to tunnel through a barrier (see Figure 1). As a result, the bias needed to collect photoelectrons efficiently can be reduced from the bias needed to collect photoelectrons via quantum tunneling as in some prior photodetectors, thereby decreasing the component of dark current attributable to sequential quantum tunneling. Moreover, because the photoelectrons do not have to tunnel through the barriers, the barriers can be thickened to reduce ground-state sequential tunneling to effect a further decrease in dark current.
In a bound-to-continuum QWIP, the energy barrier, ET, for thermionic emission is about 10 to 15 meV less than the energy, EP, needed for intersubband photoionization; that is, the extended state (or virtual level) lies 10 to 15 meV above the tops of the barriers between wells. A bound-to-quasi-bound QWIP (a device of the present type) differs from a bound-to-continuum QWIP in that the barriers are raised by 10 to 15 meV so that the first excited state lies at the tops of the barriers. The additional barrier height reduces thermionic emission over the barriers, thereby reducing the dark current. In addition, the bound-to-quasi-bound transition maximizes intersubband absorption while maintaining excellent electron transport.
As shown in Figure 2, the dark current in an experimental bound-to-quasi-bound QWIP was found to be about 1/12 that of a comparable bound-to-continuum QWIP. As a result, the signal-to-noise ratio of the bound-to-quasi-bound device is about 3.5 times that of the bound-to-continuum device.
This work was done by Sarath D. Gunapala, Jin S. Park, Gabby Sarusi, and John K. Liu of Caltech for NASA's Jet Propulsion Laboratory. 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
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