The pitch
There is significant interest in developing low-cost visible infrared (IR) sensors for a variety of applications such as imaging sensors for defense, homeland security, and commercial applications (Figure 1). There are several other technologies such as indium gallium arsenide (InGaAs), indium antimonide (InSb), and mercury cadmium telluride (HgCdTe) that cover different parts of the IR spectrum. HgCdTe IR focal-plane arrays are being developed for 3–5 μm and 8–14 μm applications. InSb is being used for 3–5 μm applications.
Similarly InGaAs offers an attractive approach for visible near-IR sensors that can cover spectral bandwidths up to 1.8 μm. Silicon germanium (SiGe) offers a low-cost alternative for developing visible near-IR sensors that will not require any cooling and can operate from 0.4 μm to 1.6 μm. In addition, there are new approaches that can further increase the spectral sensitivity to 2 μm and beyond. This technology has the potential for a small unit cell size and can be built with megapixel visible near-IR cameras.
The characteristics that make IR sensors appropriate for defense applications also benefit many industrial and scientific applications. Several applications for environmental monitoring and control use IR sensors. For example, low-cost IR-based systems can be used for night vision for vehicles, for robot vision, and for medical thermography for cancer and tumor detection during diagnosis and surgery. With higher volume and lower cost these sensors and cameras will also be used in cell phones for a variety of defense and commercial applications. The market for these sensors is projected to grow over the next decade.
Signal-to-noise ratio as a function of dark current (current that flows in a photodetector when not receiving any light) for various detection target temperatures (radiances) for a SiGe detector array.
The technology
SiGe-based visible-IR arrays offer a low-cost alternative for developing near-IR sensors that will not require any cooling and can operate in the visible and near-IR (VIS-NIR) bands. To efficiently access the VIS-NIR band for various applications, high-Ge-content layers are required. Magnolia Optical Technologies’ SiGe pin (large intrinsic region sandwiched between p-doped and n-doped semiconductor regions) photodiodes are fabricated with 1.7-μm thick Ge layers grown directly on 15 cm Si substrates, using low-pressure chemical vapor deposition in an epitaxial growth system. After Ge growth, standard complementary metal oxide semiconductor processes are used to deposit and pattern a dielectric film to open windows to the Ge surface. Then poly-Si is deposited and implanted with phosphorus to form the n + top contact. The samples are annealed to out-diffuse the phosphorus into the underlying Ge to form a vertical pin junction in the germanium (Figure 2). As the technology for SiGe detector arrays is developed, further development will be needed for fabricating high-density large format SiGe VIS-NIR focal-plane arrays.
The company is also exploring integrating SiGe VIS-NIR detectors with Si- microelectromechanical systems devices. The potential advantage of integrating SiGe devices with silicon-based bolometers is the use of the silicon-based process which makes it possible for the integration to be accomplished on large silicon wafers. Magnolia is evaluating the integration process using both VOx-based microbolometers and amorphous silicon-based microbolometer arrays that are sensitive in the 8–14 μm region for thermal imaging applications. Bolometers convert the IR signal into change of resistance that is detected.
A cross-sectional schematic of a Ge-on-Si photodiode for a visible near-infrared photodiode.
Opportunities
Magnolia Optical Technologies is developing SiGe-based imaging sensors for defense and commercial applications. We are exploring potential applications and teaming with commercial partners and investors for commercializing visible near-infrared sensors. This technology is scalable to large format arrays and sensors. Source: Ashok K. Sood, Magnolia Optical Technologies Inc., 52-B, Cummings Park, Suite 314, Woburn, MA 01801, USA; tel. 781-503-1200 ext. 105; email aksood@magnoliaoptical.com; www.magnoliaoptical.com.
