Proximity Fuze Applications

Proximity-fuzed weapons require low-cost uncooled infrared photodetectors in order to be economically mounted on projective weapons.

Typically, infrared photodetectors are designed for operation in either the 3-5 μm, (MWIR) or 8-14 μm wavelength ranges. These two ranges correspond to the transmission windows in the atmosphere where long-range detection is of interest. This is particularly important for military and security applications where surveillance and long-range monitoring are of strategic importance. On the other hand, the atmosphere absorbs extremely well in the 5-8 μm (LWIR) wavelength range. There is one application that is particularly useful for this wavelength. This is the U.S. Navy proximity fuze application that makes use of the short-range detection to avoid countermeasures particularly flares or other bright infrared sources designed to jam the detectors. The environmental scenario is shown in the figure below.

The current proximity fuze system utilizes four PbSe detectors operating in the MWIR. A twocolor system consisting of two MWIR and two LWIR photodetectors utilizing an algorithm can decrease the fuze failure rate by an order of magnitude.

To meet this need, InAsSb photodetectors have been developed for operation in the MWIR. A wide host of technological difficulties had to be overcome. The obstacles include:

• Control of the As composition to obtain the correct cutoff wavelength.
• Reduction of dislocations and defects to enhance device performance.
• Reduction of the Auger recombination mechanism which limits device performance at room temperature.
• Development of low impedance pre-amplifier circuit.

In developing the InAsSb photodetectors for the proximity fuze application, CQD has accomplished several technological firsts. This includes the:

• First demonstration of room temperature InAsSb/AlInSb double heterostructures operating in the 5-8 μm wavelength range.
• Successful demonstration of the InAsSb photodetectors under different scenarios mimicking the proximity fuze application.

Other firsts include the:

• First demonstration of room temperature InAsSb photoconductive devices operating up to 12 μm on GaAs substrate.
• First demonstration of InAsSb photovoltaic detectors operating at near room temperature on GaAs substrate.

Heteroepitaxial InSb/GaAs Focal Plane Arrays for Imaging

The detection of infrared signals has a variety of beneficial applications in the military, industry, the medical community, and the scientific community. Examples of military applications include target detection, LIDAR, and proximity fuzes. Examples of industry applications include industrial quality assurance, equipment diagnostics, and pollution monitoring. Examples of medical applications include blood glucose level monitoring and injury detection. Examples of scientific applications include measurement analysis and infrared astronomy. Of particular importance is thermal imaging. A wide range of applications exists partic ularly in industry and public service. Examples include facilities security, fire fighting, and surveillance. The primary drawback to thermal imaging systems is their high price resulting from the stringent processing requirements.

Images obtained from the InSb/GaAs FPA.

In order to make the thermal imaging cameras more affordable, the Center for Quantum Devices (CQD) pursued the development of InSb focal plane arrays on GaAs substrates in collaboration with Lockheed Martin�s Loral Fairchild division. The primary goal of the project was to develop InSb focal plane arrays (FPAs) on GaAs substrates. A limiting step in the production of the InSb focal plane arrays is the array fabrication. Because the substrate is InSb, it needs to be thinned to allow for backside illumination; otherwise, not light will reach the detector. This lapping and polishing step is very difficult due to the fragility of the InSb substrate. The use of GaAs substrate will eliminate this step. GaAs is transparent to the InSb operating wavelength, thus eliminating the need to lap and polish the substrate. The development of InSb on GaAs offers an alternative to the InSb substrate technology that would be inexpensive, have a higher yield, and take advantage of the well-established GaAs growth and processing technology.

Images obtained from the InSb/GaAs FPA.

Lattice-mismatched he teroepitaxy has attained interest in recent years for the development of novel devices to meet application needs. The growth of InSb on GaAs substrate is no exception. The 14 % lattice mismatch between the InSb epitaxial layer and the GaAs substrate proved not to be an insurmountable obstacle to overcome. The large lattice mismatch results in island or 3-D growth rather than the desired layer-bylayer growth that epitaxy requires. In other words, the material quality is degraded significantly taking on polycrystalline properties. Thus, the optimization of the material is of critical importance in obtaining device quality material. In spite of this disadvantage, the development of the best quality heteroepitaxial InSb allowed CQD and Lockheed Martin Loral Fairchild to successfully develop and deliver the first 256x256 and 640x480 InSb on GaAs substrate FPAs operating at 77 K. Captured images are shown in the figure below.

Last Updated 01/31/2007