Theory, fabrication and characterization of quantum well infrared photodetectors

Abstract The microscopic physics, device physics, and system performance of quantum well infrared photodetectors (QWIPs) are reviewed. QWIPs which respond to normally incident radiation without the need for an optical grating are of particular interest because they can be fabricated with fewer process steps. Recent demonstrations of n-type QWIPs (n-QWIPs) which show a significant detectivity of 4×1010 cm Hz /W without the use of an optical grating are discussed here. This detectivity is significant because it is large enough for focal plane array (FPA) performance to be limited by the uniformity of processing rather than the size of the single pixel detectivity. Studies of the microscopic physics of quantum wells are summarized to elucidate the physical origin of the intersubband absorption of normally incident radiation. The selection rules for intersubband absorption by holes in a p-doped 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 both the conduction intersubband absorption and the valence band-to-conduction band absorption. It is also shown that uniaxial strain does not have a large effect on the strength or the selection rules of intersubband absorption because the Hamiltonian describing uniaxial strain has the same (tetragonal) symmetry as that describing the confinement of carriers in the quantum wells along the growth direction. Also reviewed are device models which yield analytical expressions for the number of, and the distance over which, carriers are depleted from quantum wells under conditions of insufficient carrier injection. This carrier depletion becomes important when the incident photon flux is large or when the QWIP operating temperature is low. Uniformity of QWIP device parameters is important in determining the ultimate array signal-to-noise ratio (SNR). Examples of high-resolution X-ray diffraction methods used to find the layer width variations of QWIPs grown by molecular beam epitaxy are reviewed. The spread of the measured full-width at half-maxima (FWHM) of superlattice diffraction peaks with the diffraction order was used with Bragg’s Law to obtain the measured layer width variation in the growth direction. A theoretical study of different noise mechanisms which contribute to QWIP performance was carried out. A key result is that, when the SNR is limited by either fixed pattern noise or thermal leakage arrival noise, the largest expected QWIP SNR occurs when the number of quantum wells in the QWIP is at the optimal value of about η1−1, where η1 is the quantum efficiency of a QWIP having only one quantum well. Common QWIP designs used in industry are evaluated. In particular, different physical models for the leakage (sequential tunneling, thermionic and thermionic field assisted leakage) are reviewed. A new result is a physical model, derived from the Kronig–Penney model, for the tunneling leakage in existing QWIP designs in which the confinement barrier is a semiconductor superlattice. The tunneling leakage in such QWIPs is shown to vary exponentially with the average (rather than the full) height of the superlattice barrier.