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Chemomechanical polishing methods and surface metrology of cadmium zinc telluride substrates suitable for IRFPA production

High quality substrates are a critical requirement for the epitaxial growth of high performance Group III-V (GaSb, InP, GaAs) or II-VI (MCT) based IR detector structures. Techniques such as MBE, LPE or MOCVD used to grow complex multilayer structures where the bulk and surface characteristics of the underlying substrate strongly influences the quality of the deposited device layers and thus the performance of the detector fabricated too. And whilst for Group III-V substrates, ‘epitaxy ready’ surfaces are very much the norm with no surface pre-treatment required prior to growth, much debate exists over whether it is possible to prepare Cadmium Zinc Telluride (CZT) substrates in the same way with commercially available substrates often etched or pre-polished prior to epitaxial growth. In this work we discuss the key factors which determine the ‘epi-readiness’ of a compound semiconductor substrate and identify the challenges which are associated with polishing ternary materials when compared with binary compounds. We also discuss the role which bulk CZT crystal quality plays on chemo-mechanical polish surface quality and the importance of substrate selection and screening steps to ensure that high quality substrates are obtained. We also compare the maturity level of infrared substrate materials such as GaSb with CZT and propose how CZT substrates can be chemo-mechanically polished in a production setting with the same tool sets as used to produce volume quantities of III-V compound semiconductor substrates in epitaxy ready form, as opposed to smaller scale, bespoke finishing processes. A forward looking view is given on the potential to align II-VI and III-V substrate manufacturing technologies in a way that supports product standardization and conformity at a time when demands for IR specification substrates is forecast to increase. Not only in quantity but also size where more stringent bulk and surface quality is required if state of the art, very large format detectors are to be delivered.