Theory, fabrication and characterization of quantum well infrared photodetectors

The microscopic physics, device physics, and system performance of quantum well infrared photodetectors (QWIPs) are reviewed. QWIPs which respond to n ormally incident radiation without the need for an optical grating are of p articular interest because they can be fabricated with fewer process steps. Recent demonstrations of n-type QWIPs (n-QWIPs) which show a significant d etectivity of 4 x 10(10) cm root Hz/W without the use of an optical grating are discussed here. This detectivity is significant because it is large en ough for focal plane array (FPA) performance to be limited by the uniformit y of processing rather than the size of the single pixel detectivity. Studi es of the microscopic physics of quantum wells are summarized to elucidate the physical origin of the intersubband absorption of normally incident rad iation. The selection rules for intersubband absorption by holes in a p-dop ed QWIP (p-QWIP) and electrons in an n-QWIP are reviewed. In particular, it is shown that the hole intersubband absorption is typically weaker than bo th the conduction intersubband absorption and the valence band-to-conductio n band absorption. It is also shown that uniaxial strain does not have a la rge effect on the strength or the selection rules of intersubband absorptio n because the Hamiltonian describing uniaxial strain has the same (tetragon al) symmetry as that describing the confinement of carriers in the quantum wells along the growth direction. Also reviewed are device models which yie ld analytical expressions for the number of, and the distance over which, c arriers are depleted from quantum wells under conditions of insufficient ca rrier injection. This carrier depletion becomes important when the incident photon flux is large or when the QWIP operating temperature is low. Unifor mity of QWIP device parameters is important in determining the ultimate arr ay signal-to-noise ratio (SNR). Examples of high-resolution X-ray diffracti on methods used to find the layer width variations of QWIPs grown by molecu lar beam epitaxy are reviewed. The spread of the measured full-width at hal f-maxima (FWHM) of superlattice diffraction peaks with the diffraction orde r was used with Bragg's Law to obtain the measured layer width variation in the growth direction. A theoretical study of different noise mechanisms wh ich contribute to QWIP performance was carried out. A key result is that, w hen the SNR is limited by either fixed pattern noise or thermal leakage arr ival noise, the largest expected QWIP SNR occurs when the number of quantum wells in the QWIP is at the optimal value of about eta(1)(-1), where n(1) is the quantum efficiency of a QWIP having only one quantum well. Common QW IP designs used in industry are evaluated. In particular, different physica l models for the leakage (sequential tunneling, thermionic and thermionic f ield assisted leakage) are reviewed. A new result is a physical model, deri ved from the Kronig-Penney model, for the tunneling leakage in existing QWI P designs in which the confinement barrier is a semiconductor superlattice. The tunneling leakage in such QWIPs is shown to vary exponentially with th e average (rather than the full) height of the superlattice barrier. (C) 20 00 Elsevier Science S.A. All rights reserved.