In situ compositional control of advanced HgCdTe-based IR detectors

The application of a feedback control system for maintaining a desired compositional profile during Hg1-xCdxTe epitaxy is reported. A spectroscopic ellipsometer monitored the optical properties of Hg1-xCdxTe films during deposition by molecular beam epitaxy. A library of optical constants was established from in situ measurements of multiple layers of varying composition. The compositions were subsequently determined ex situ using Fourier transform infrared spectroscopy. This work represents an extension of the compositional range of this control system to measure the x-values of Hg1-xCdxTe sensitive to long-, mid-, and short-wavelength infrared (LWIR, MWIR, and SWIR) radiation (0.16 < x < 0.6). The extension of the useful range of Hg1-xCdxTe compositions, which can be accurately determined in situ. has enabled epitaxy of a greater variety of detector structures and more complex device architectures. A proportional control algorithm, utilizing this library, adjusted the x-value in real time through changes in the temperature of the CdTe effusion cell. The application of this control system is demonstrated by measuring the compositional profile of a three layer Hg1-xCdxTe structure suitable for use as an MWIR/SWIR detector. (C) 1999 Elsevier Science B.V. All rights reserved.