significant potential. Piezoelectric polymers, ceramics, and composites offer advantage for strain measurements. Smart materials have the added requirement for actuation, and these piezoelectric materials can do "double duty." Active and passive taggants placed in structures offer the promise of low-cost sensing; their potential is only beginning to be explored.
LWIR sensor materials must satisfy a multitude of criteria in addition to their wavelength sensitivity. The performance of a sensor cannot surpass that determined through an understanding of a material's fundamental limitations, which include such factors as carrier interaction, absorption cross-section, and electron-phonon couplings, as well as material stability. In practice, maximum theoretical performance is rarely attained for large LWIR arrays due to the limitations of the materials synthesis and processing operations. For example, detector performance is extremely sensitive to defects and inhomogeneities that are not "fundamental" but which can be ubiquitous unless the materials growth and processing is extremely well controlled. Improvements in LWIR sensor materials thus depend to some extent on the development of LWIR and other sensor technologies that can be used for intelligent process control. This is particularly true for those processes that require atomic scale control during the growth process.
Cost is not a fundamental physical limit, but it does impose significant practical constraints. A key consideration is the selection of materials systems that are inherently robust and producible. In terms of operating costs, materials that provide sensors with a uniform response are preferable, as are sensors whose performance does not degrade over time.
Examples of specific materials R&D needs for the three LWIR photodetector materials systems (mercury-cadmium-telluride, multiple-quantum-well, and strained-layer-superlattice) are summarized below. In addition, LWIR bolometers offer attractive alternatives to photodetectors for certain applications, and the committee encourages continued development of those systems.
The quality of LWIR (MCT) has improved over the last decade, and continued incremental improvements may eventually yield temporally stable, uniform detector arrays. However, materials instabilities result in major materials and growth challenges. Very sophisticated materials-processing techniques are being employed to produce low-defect LWIR material. These processes can only be effective if there are sensors for in situ process control. Sensors that detect melt temperature and pressure of the constituent elements (especially mercury) during liquid-phase epitaxy processing are under development. Sensors developed for in situ process control can be applied to molecular beam epitaxy organometallic vapor-phase epitaxy and to metal-organic chemical vapor deposition processing.
Growth of MCT is made more difficult by a paucity of suitable lattice-matched substrates. The most commonly used substrate is CdTe, which is expensive and difficult to produce in large sizes needed for arrays. Development of a low-cost, producible alternative material represents a high-risk, high-payoff research opportunity. Another high-payoff, high-risk opportunity is the development of processing techniques for growth on nonlattice-matched substrates.
The background-limited performance of LWIR sensors based on multiple-quantum-well technology is lower than MCT. Arrays of GaAs/AlGaAs heterostructures exceed the performance MCT arrays for selected applications due to their superior response uniformity. Further improvements in detectivity of quantum-well detectors will require higher efficiency in the optical coupling of the incident radiation to the detector material and an increase in the carrier lifetime. Several promising approaches are under way to improve the optical coupling efficiency, but the definitive solution has not yet been found. At this time, it is not clear how to increase carrier lifetime, and thus this is an area requiring additional basic research.