Infrared detectors have a wide array of applications in basic sciences, industry and for tactical military use. Most of the high-efficiency IR detection and imaging is carried out with photo-voltaic or photoconductor-type detectors, which are fabricated using narrow-bandgap semiconductors. In these detectors, the IR radiation is absorbed within the semiconductor by interaction with electrons and produce an electrical output signal resulting from the interaction. These semiconductor detectors exhibit both, excellent signal-to-noise performance and a very fast response, which are critical for military applications. However, to achieve this performance, IR detectors require cryogenic cooling. These cooling requirements are the main obstacle to the more widespread use of IR systems based on semiconductor photodetectors because they are bulky, heavy, expensive and inconvenient to use.
Another challenge with narrow bandgap semiconductors is that their performance is strongly influenced by the nature of their surface. Surface passivation of these semiconductors is critically important for optimal detector performance since any free surface with or without contamination will impact meaningful IR detection.
RMD is working on cutting-edge technologies to address these challenges and enable the advancement of infrared detectors for civilian and military applications with ongoing research in these areas:
Room Temperature IR Detectors
The IR cameras based on focal plane arrays (FPAs) require cryogenic cooling which add to the size, weight, power, and cost. The state-of-the-art detectors generally must operate below ~200 K to achieve reasonable signal-to-noise ratios. Lead Selenide (PbSe) is an excellent semiconductor for mid-wavelength infrared (MWIR) band. Recent work in academia and industry has included MWIR Lead Selenide (PbSe) detectors, which work at room temperatures. The bandgap of PbSe results in a cut-off frequency near 4.6 µm. However, a range of cut-off wavelengths across the MWIR band (3-5 µm) can be accessed by doping PbSe with other elements. RMD has demonstrated the growth and processing of high-quality doped PbSe semiconductors in order to achieve sensitive room-temperature detectors with tunable cut-offs. These devices have achieved detectivities up to of 1011 cm Hz1/2/W in the 3 to 5 µm range, measured at room temperature.
Conformal Passivation of HgCdTe-based Infrared Detectors
Due to the narrow bandgap of the HgCdTe semiconductor, the IR detector characteristics are largely influenced by the properties of the semiconductor surfaces. Surface passivation of the HgCdTe surface is critical to its performance, especially for the mesa-delineated focal plane arrays (FPA), which is characterized by large surface area and non-flat surface geometries of HgCdTe. The requirements of the desired passivant are stringent, where the passivant has to be an insulator, has to match the physical and chemical properties of HgCdTe and most importantly not contribute to generation/recombination of charge carriers under IR radiation.
RMD has developed a novel passivation technology specifically optimized for the mesa-delineated LWIR HgCdTe devices. RMD has demonstrated a unique Atomic Layer Deposition (ALD) process for passivating the complex HgCdTe surface morphology with cadmium telluride (CdTe) material, which is the most ideal material for HgCdTe passivation. The ALD process for CdTe is unique because the processing temperature is limited to below 75 C in order to prevent the Hg from depleting the semiconductor surface, which can otherwise be detrimental to the device’s performance. RMD’s ALD process can deposit insulating CdTe on complex HgCdTe surfaces with aspect ratio greater than 20:1 and the passivant itself has the desired 1:1 Cd:Te atomic stoichiometry, with an excellent control on thickness of within few nanometers.
Conformal Passivation of Super-lattice Infrared Detectors
The electronic properties and the periodic arrangement of indium arsenide/gallium antimonide (InAs/GaSb) Type-II strained layer superlattices (T2SLS) make them a superior alternative to the conventional HgCdTe sensors for longwave infrared (LWIR) detection. Some key benefits of the T2SLS over HgCdTe-based detectors:
- Tunability of the wavelength response by altering the film thicknesses as opposed to changing doping levels in HgCdTe
- Higher chemical stability of III-V compounds compared to the II-VI compounds (HgCdTe)
While these attributes make the SLS materials very promising for LWIR detectors, their performance is currently limited by high reverse leakage current caused by traps, dangling bonds, and conductive native oxides on the semiconductor surface caused during the mesa delineation.
RMD has developed a unique passivation process optimized for InAs/GaSb T2SLS mesa structure. The passivation process involves atomic layer deposition (ALD) of proprietary passivant applied at room temperature while ensuring that the semiconductor’s native oxides from the III-V compounds have been completely eliminated at the interface. The passivation process has resulted in dramatic reduction in dark current in T2SLS n- and p-type devices and has also improved device reliability.