HgCdTe composition determination using spectroscopic ellipsometry during molecular beam epitaxy growth of near-infrared avalanche photodiode device structures

The application of spectroscopic ellipsometry (SE) for real-time composition determination during molecular beam epitaxy (MBE) growth of Hg1-xCdxTe alloys with x > 0.5 is reported. Techniques previously developed for SE determination of composition in long-wavelength infrared (LWIR) HgCdTe have been successfully extended to near-infrared HgCdTe avalanche photodiode (APD) device structures with x values in the range of 0.6-0.8. Ellipsometric data collected over a spectral range of 1.7-5 eV were used to measure depth profiles of HgCdTe alloy composition through the use of an optical model of the growth surface. The optical model used a dielectric-function database collected through the growth of a set of HgCdTe calibration samples with x ranging from 0.6 to 0.8. The sensitivity of this SE method of composition determination is estimated to be Deltax similar to 0.0002 at x = 0.6, which is sufficiently low to sense composition changes arising from flux variations of less than 0.1%. Errors in composition determination because of Hg-flux variations appear to be inconsequential, while substrate-temperature fluctuations have been observed to alter the derived composition at a rate of -0.0004/degreesC. By comparing the composition inferred from SE and postgrowth 300 K IR transmission measurements on a set of APD device structures, the run-to-run precision of the SE-derived composition (at x = 0.6) is estimated to be +/-0.0012, which is equivalent to the precision achieved with the same instrumentation during the growth of mid-wavelength infrared (MWIR) HgCdTe alloys in the same MBE system.