Arrays of HgCdTe infrared photodetectors have been developed to satisfy stringent performance requirements for use in NASA’s Tropospheric Emission Spectrometer. The design of these detectors improves (relative to prior such arrays) manufacturing yield, current-versus-voltage characteristics, and low-temperature performance. The design is of the doublelayer planar heterostructure (DLPH) type but differs from conventional DLPH designs in that it features a distinctive lateral current-collection configuration.
Figure 1 depicts the basic DLPH configuration of one of these detectors. An important aspect of the DLPH approach is a planar p-doped/n-doped device geometry that includes a wide-bandgap cap layer over a narrow-bandgap base layer (which is the active device layer). The layers are all initially grown n-doped in situ by molecular-beam epitaxy (MBE), thereby ensuring that the critical heterointerface is never exposed to the ambient contamination that has plagued HgCdTe photovoltaic infrared detectors in the past. Then, within the area selected for each detector, arsenic ions are implanted through the cap layer into an upper sublayer of the base layer (which becomes a p-doped sublayer) to form a planar p-on-n photodiode. The implantation step is followed by a two-step thermal anneal in Hg vapor, which is needed to activate the arsenic doping.
A thin passivating layer of polycrystalline CdTe is deposited over the cap layer, holes for ohmic contacts to the p-on-n diodes are etched in the CdTe, and then the ohmic contacts are deposited. The wafer is then overcoated with ZnS, which serves to increase the adhesion of the metal interconnections to be deposited subsequently. Holes are etched in the ZnS to expose the ohmic-contact metal, then the metal interconnections are deposited. Finally, the back side (the lower side in Figure 1) of the wafer is polished and antireflection coated and the wafer is diced into separate detector arrays.
For fabrication as described above, localized defects give rise to area-dependent detector performance, such that conventional DLPH detectors with areas <10–5 cm2 perform considerably better than do detectors with larger areas: this is because reducing the electrical p/n junction area of a diode reduces the probability that a given defect will coincide with the junction. By taking advantage of lateral current collection, which takes place within a few diffusion lengths of a junction, one can obtain, for each detector, a large optically sensitive area with one or more small junction(s). Figure 2 illustrates the difference between the conventional DLPH current-collection configuration and the lateral current- collection configuration.
With proper design, as in the present HgCdTe detector arrays, the optical collection area of a detector can be substantially larger than its electrical junction area. Instead of one diode, each detector pixel contains several diodes formed as small-area implants separated by a distance of the order of one minority-carrier diffusion length and electrically connected in parallel via their ohmic contacts. Following this approach to design and fabrication, optical detection areas can be an order of magnitude greater than electrical junction areas and the advantages of reduced dark current, relatively high resistance, and relatively low capacitance can be realized.
This work was done by Carl Bruce, W. McLevige, and K. Vural of Caltech for NASA’s Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Electronic Components and Systems category.
NPO-21002
This Brief includes a Technical Support Package (TSP).
Lateral-Current-Collection HgCdTe Infrared Detectors
(reference NPO-21002) is currently available for download from the TSP library.
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Overview
The document presents a technical support package detailing the advancements in lateral-current-collection HgCdTe infrared detectors developed for NASA's Tropospheric Emission Spectrometer (TES). The TES is an imaging Fourier Transform Spectrometer designed to measure a wide range of infrared-active molecules in the Earth's troposphere, including ozone, nitric oxide, nitrogen dioxide, and carbon dioxide, among others. Its primary function is to map the global three-dimensional distribution of these molecules, which are crucial for understanding atmospheric physics and chemistry.
The document outlines the specifications and design improvements of the HgCdTe infrared photodetectors used in TES. These detectors are engineered to meet stringent performance requirements, enhancing manufacturing yield, current-voltage characteristics, and low-temperature performance. The design employs a double-layer planar heterostructure (DLPH) configuration, which features a unique lateral current-collection approach, differing from conventional designs. This innovation aims to optimize the detectors' efficiency and sensitivity across a broad spectral range.
Each of the 64 pixels in the TES has its own preamplifier and signal chain, allowing for precise signal processing. The document describes the architecture of the preamplifier circuits, which are designed to prevent saturation under maximum signal conditions. The detectors operate at a temperature of 65K, and the document emphasizes the rigorous material quality requirements necessary for optimal performance.
Additionally, the document discusses potential future enhancements, such as the use of micro-lens technology, which could further reduce junction area and capacitance, improving the detectors' performance. The need for a better understanding of the spatial response of micro-lenses and their performance under narrow band illumination is highlighted as a critical area for further research.
Overall, this technical support package provides a comprehensive overview of the innovative technologies and methodologies employed in the development of HgCdTe infrared detectors for the TES, showcasing their significance in advancing atmospheric science and remote sensing capabilities. The work was conducted at the Jet Propulsion Laboratory under NASA's sponsorship, emphasizing the collaborative effort in pushing the boundaries of infrared detection technology.
