A lot goes into the meteorological surveys that bring you the daily weather forecast. Predicting trends using weather satellites began in the 1950s and the debut infrared imaging observation satellite launched in 1960. These first-ever images of Earth’s cloud cover from space played a pioneering role in weather forecasting.

Since then, much has evolved. There’s now a heightened demand for precise prediction of localized phenomena, particularly severe weather events such as thunderstorms, strong winds, fog, and hail, which profoundly affect both the economy and public safety worldwide. The emergence of drone-based weather monitoring has emerged as a promising frontier, driven by advancements in drone technology. Equipped with specialized sensors such as infrared sensors, drones are capable of conducting high-resolution, direct temperature measurements, significantly enhancing our ability to forecast and respond effectively to severe weather events.

However, one aspect remains unchanged—the substantial cost associated with equipping weather drones with highly sensitive infrared (IR) imaging systems. This financial barrier limits the widespread deployment of IR-enabled weather drones, consequently restricting the quantity of data available to meteorologists and consequently affecting the quality of trend forecasting.

Traditionally, highly sensitive IR focal plane arrays suitable for meteorology are manufactured from Mercury Cadmium Telluride (MCT) thin film materials, requiring their growth onto a substrate. Conventionally, the preferred substrate has been a semiconductor called Cadmium Zinc Telluride (CZT), as its lattice structure aligns with that of MCT, enabling high-quality epitaxial growth. However, CZT has limitations, being fragile, restricted to small dimensions, and expensive to produce.

The team at TMOS (Transformative Meta-Optical Systems) from the University of Western Australia has embraced an innovative approach to fabricating MCT infrared detectors as published in The Journal of Applied Physics. They’ve developed a buffer layer of Cadmium Telluride incorporating nanostructured dislocation reduction structures, mimicking the lattice structure and material quality of CZT.

This buffer layer can be applied onto more durable and cost-effective substrates such as Silicon (Si) and Gallium Arsenide (GaAs) to serve as an alternative composite substrate, facilitating high-quality growth of MCT.

The result of this approach is a more cost-effective MCT IR detector capable of withstanding environmental stressors more effectively, making it ideal for various applications, including meteorological observations using drones.

Lead author Wenwu Pan says, “To make wide-spread space-based infrared imaging more practical, we need a technology similar to Si-CMOS, which uses large, high-quality substrates for material growth and allows for the production of numerous imaging sensors per wafer through multi-wafer batch processing. The main challenge lies in the significant lattice mismatch between MCT and these alternative substrates, resulting in a high density of material defects, which are pixel/performance killers. Using a superlattice-based buffer to grow high-quality MCT on large alternative substrates addresses this problem.”

Chief Investigator Lorenzo Faraone says, “Integrating MCT detectors with optical metasurfaces is expected to significantly enhance infrared sensing systems for earth observation and remote sensing by providing higher performance and sensors with new capabilities such as polarization sensitivity. Integrating metasurfaces with CZT substrates is problematic in comparison to alternative substrates like Si and GaAs. This is because integration necessitates a complex substrate thinning process to couple the metasurface and imaging array. In addition, metasurfaces such as on-pixel-aligned metalens technology enables the use of smaller detector areas, which provides higher performance and/or higher operating temperature. It would have a high industry impact if the combined effects of defect reduction on non-CZT substrates and the integration of a metalens technology results in device performance comparable to or even surpassing that of detectors grown on CZT substrates.

For more information about this research, contact connect@tmos.org.au

Structural properties of MBE-grown CdTe (133)B buffer layers on GaAs (211)B substrates with CdZnTe/CdTe superlattice-based dislocation filtering layers.

Wenwu Pan, Shuo Ma, Xiao Sun, Shimul Kanti Nath, Songqing Zhang, Renjie Gu, Zekai Zhang, Lorenzo Faraone, and Wen Lei

Journal of Applied Physics, 8th Mary 2023

The ever-present demand for high-performance HgCdTe infrared detectors with larger array size and lower cost than currently available technologies based on lattice-matched CdZnTe (211)B substrates has fuelled research into heteroepitaxial growth of HgCdTe and CdTe buffer layers on lattice-mismatched alternative substrates with a (211)B orientation. Driven by the large lattice mismatch, the heteroepitaxial growth of (Hg)CdTe can result in (133)B-orientated material, which, however, has been less explored in comparison to (211)B-oriented growth. Herein, we report on the structural properties of heteroepitaxially grown single-crystal II–VI CdTe (133)B-oriented buffer layers on III–V GaAs (211)B substrates. Azimuthal-dependent x-ray double-crystal rocking curve measurements for the CdTe buffer layers show that the full-width at half-maximum value obtained along the GaAs [111] direction is narrower than that obtained along the GaAs [011] direction, which is presumably related to the in-plane anisotropic structural characteristics of the grown CdTe layers. By incorporating strained CdZnTe/CdTe superlattice-based dislocation filtering layers (DFLs), a significant improvement in material quality has been achieved in (133)B-orientated CdTe buffer layers, including a reduced etch pit density in the low-105 cm−2 range and improved surface roughness. These results indicate that the CdTe (133)B DFL buffer layer process is a feasible approach for growing high-quality CdTe and HgCdTe materials on large-area, low-cost alternative substrates.