MBE Growth of Hg1-xCdxTe on Cadmium Zinc Telluride Substrates

A dual Chamber RISER 32P Molecular beam epitaxy (MBE) system was used to grow high quality HgCdTe epilayers. For growth, the critical parameters are: 1) accurate determination and control of real substrate temperature 2) precise identification of the Hg/Te ratios 3) CdZnTe substrates with reproducible surface finish and structural characteristics. Because the substrate holder rotates during growth, the junction of spring-loaded thermocouple does not contact the molyblock leading to irreproducible temperature measurements. We modified the manipulator thermocouple design and its contact with the molyblock and the real molyblock temperature is measured to an accuracy of +/- 0.5 degrees C even if we use different molyblocks. During the complete growth run of 4 - 5 hours (growth rate 1.5 - 2 mu m/h), the manipulator thermocouple temperature was kept constant as opposed to the temperature ramping reported by other groups. With the above modifications, we do not require to monitor the substrate temperature using a pyrometer, which needs emissivity and refractive index values. We also do not regularly need the melting (calibration) points of 3 standard metals (In, Pb/Sn & Sn): we only use Pb/Sn eutectic alloy (melting point 183 degrees C) to routinely check the temperature calibration within the growth window. HgCdTe epilayer growth experiments are carried out in the temperature range of 175 - 185 degrees C for different Hg/Te ratios (80-110) for high quality epilayers. The Hg/Te ratio is found to be higher for a rough CdZnTe substrate than for a smooth one. HgCdTe epilayers grown on 30 mm x 30 mm CdZnTe substrates show FWHM < 30 arc sec with composition and thickness uniformity < 1.5%.