Selex Galileo have selected the ORS eye for their Interdiffused Multilayer Process or IMP by MOCVD
Jan 2010
Selex Galileo have selected the ORS eye for their Interdiffused Multilayer Process or IMP by MOCVD The ORS was chosen for our expertise in Cadmium Telluride and Mercury Telluride (CMT) and our customised software package that enables accurate fitting of layer thicknesses and composition of this complex material.
IMP is a method of growing infra-red detecting material via MOCVD using II-VI materials, namely Cadmium Telluride (CdTe) & Mercury Telluride (HgTe). The resultant material is called Cadmium Mercury Telluride (CMT) in the UK and Mercury Cadmium Telluride (MCT) everywhere else. The material will detect at selectable wavelengths in the infra-red, according to its “x-factor” , the ratio CdTe / (CdTe + HgTe) and is a value between 0 and 1. In a typical structure there will be several different x regions within the material, thus tuning the material to a variety of wavelengths or wavelength bands – the choice will depend on the application; e.g. night vision, looking through smoke, space applications, etc.
In standard semiconductor materials, when you lay down a layer of material, the interface between it and the previous layer remains where its put; thus as you lay down layer after layer of material you build up your structure and produce something that, in cross-section looks like a layered structure. However, in this system the material is considerably softer than other compound semiconductors. If you cleave off CMT from its underlying GaAs substrate the remaining wafer is flexible; yet it remains single crystal!.
Growing this material directly with the correct x factor is not straightforward in MOCVD. So a technique has been developed of growing a succession of alternate thin layers of CdTe and HgTe and allowing the inherent softness of these materials to then subsequently mix together (or interdiffuse) to produce the CMT of the correct x factor. The interdiffusion process can actually be seen – if you switch off all the growth and keep the temperature constant, then you will see the reflectance immediately start to change – this is due to the mixing of the CdTe/HgTe material and the subsequent change in the refractive index to the resulting mixed CMT material. A typical structure is a GaAs wafer, followed by several microns of CdTe buffer and then many microns of CMT. A standard growth run takes around 9 hours to complete, making it one of the longest MOCVD processes.
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