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.
Cadmium Zinc Telluride (Cd1-xZnxTe or CZT) is a compound semiconductor substrate material that has been used for infrared detector (IR) applications for many years. CZT is a perfect substrate for the epitaxial growth of Mercury Cadmium Telluride (Hg1-xCdxTe or MCT) epitaxial layers and remains the material of choice for many high performance IR detectors and focal plane arrays that are used to detect across wide IR spectral bands. Critical to the fabrication of high performance MCT IR detectors is a high quality starting CZT substrate, this being a key determinant of epitaxial layer crystallinity, defectivity and ultimately device electro-optical performance. In this work we report on a new source of substrates suitable for IR detector applications, grown using the Travelling Heater Method (THM). This proven method of crystal growth has been used to manufacture high quality IR specification CZT substrates where industry requirements for IR transmission, dislocations, tellurium precipitates and copper impurity levels have been met. Results will be presented for the chemo-mechanical (CMP) polishing of CZT substrates using production tool sets that are identical to those that are used to produce epitaxy-ready surface finishes on related IR compound semiconductor materials such as GaSb and InSb. We will also discuss the requirements to scale CZT substrate manufacture and how with a new III-V like approach to both CZT crystal growth and substrate polishing, we can move towards a more standardized product and one that can ultimately deliver a standard round CZT substrate, as is the case for competing IR materials such as GaSb, InSb and InP.
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