Corrugated quantum-well infrared photodetector (C-QWIPs) arrays are sensitive and high-resolution thermal imaging devices. They are not only suitable for a wide range of conventional military, scientific, and commercial applications but also offer new imaging capabilities, such as remote temperature sensing and polarization-sensitive detection. The C-QWIP arrays are easy to manufacture in large quantities and at a low cost. The new detector architecture thus greatly improves the quantum-well infrared technology.
Quantum-well infrared detectors emerge as a good candidate for imaging in the mid- and long-wavelength ranges. Based on the matured III-V material technology, QWIP arrays can be produced in high-resolution formats and be made to be sensitive in different wavelengths without affecting the material quality. Palm-sized cameras and cameras with noise-equivalent temperature differences less than 10 mK are commercially available. Basic mid-wave/long-wave and long-wave/very-long-wave QWIP dual-band cameras have also been demonstrated. This trend shows that the QWIP technology is maturing rapidly, and is expected to expand its market share in the near future.
Despite these promising developments, the production of QWIP arrays is not straightforward. A QWIP can only absorb light traveling at an oblique angle, and thus needs an additional light-coupling mechanism to sense normal incident light in the staring format. The conventional approach of using gratings at the top of detectors for light diffraction has several shortcomings. Diffraction is less efficient for small detector pixels, which limits the array's spatial resolution. Each grating design is only effective for a particular wavelength, not for broadband detection or dual-band detection. The fine features in the grating design require stringent electron-beam lithography, which greatly limits the production throughput and drives up costs. A better light-coupling scheme is thus needed.Continued
As shown in the figure, a C-QWIP uses total internal reflection to redirect light inside a detector. Using a special chemical etching technique, an array of V-grooves with slanted sidewalls can be etched into the detector's active volume. The unetched center island in the figure is reserved for contact bonding. The normal incident light reflects at these angled sidewalls and travels at a large oblique angle, allowing for efficient infrared absorption. Based on reflection rather than diffraction, the corrugated coupling scheme is both pixel-sized and wavelength-independent. These two important attributes improve the sensitivity of a typical array by an order of magnitude.
The new scheme is also particularly suitable for broadband detection and dual-band detection, the latter required for precision temperature sensing. A composite detector consisting of two C-QWIPs with their corrugations oriented in orthogonal directions can be used for polarization detection, further enhancing the detection capability. Since the corrugations are created during the detector's pixel separation, no extra processing steps or grating material layers are needed for the creation of these corrugations. The problems and restrictions associated with the grating approach can be completely eliminated. The production of C-QWIP arrays is thus extremely simple and of low cost. With the improved sensitivity and capability, the employment of the corrugated light coupling scheme greatly advances the quantum-well infrared technology.