Infrared sensor arrays with 3-12 μm cutoff wavelengths in heteroepitaxial narrow-gap semiconductors on silicon substrates

The authors describe heteroepitaxy of IV-VI lead chalcogenide narrow-gap semiconductor (NGS) as well as II-VI materials (CdTe) on Si substrates. Epitaxy is achieved by using stacked intermediate CaF₂ -BaF₂ bilayers to overcome the large lattice and thermal expansion mismatch. The authors use lead chalcogenides (PbS, PbTe, Pb_1-xEu_xSe, and Pb_1-xSn_x Se) rather than Hg_1-xCd_xTe (MCT) as IR-sensitive NGS material because growth and fabrication techniques are much easier and compositional homogeneity much less critical with lead salts, while maximum sensitivities are comparable to those of MCT. The high permittivity of lead salts yields much more fault-tolerant devices due to the effective shielding of charges resulting from defects. The authors have fabricated linear sensor arrays on Si substrates with cutoff wavelengths ranging from 3 to above 12 μm. The sensitivities of the best PbTe on Si sensors (cutoff 5.5 μm) are already comparable to those of MCT with the same cutoff wavelengths, while those of the first Pb_1-xSn_xSe devices are a factor of 2-5 below. Although many fabrication steps are rather crude and far from being optimized, this considerable improvement is easily possible