Tunable infrared detector with epitaxial silicide/silicon heterostructures

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Abstract
Epitaxial CoSi2/Si/CoSi2 and CoSi2/Si/poly‐PtSi heterostructures were grown with molecular‐beam epitaxy onto Si(111). Characterization of the heterostructures with scanning tunneling microscopy, Rutherford backscattering spectrometry, and transmission electron microscopy revealed very high structural quality. We report on the application of these heterostructures to a wavelength‐tunable infrared detector. It consists of two back‐to‐back Schottky contacts separated by the thin (1000–2000 Å) undoped Si spacer layer. The different Schottky barrier heights which photocreated charge carriers in the silicides have to surmount can be used to control the cutoff wavelength by simply varying the applied bias across the structure. Photoelectric measurements of so‐called symmetrical sensors made of CoSi2/Si/CoSi2 where both silicides contribute equally to the photocurrent, yielded a bias dependence of the cutoff energy three times as large as predicted by the conventional Schottky effect. In this case, the observed tunability of the cutoff energy can be explained only by considering ballistic transport of photocreated carriers (holes and electrons) in the silicon. Different mean free path lengths of hot electrons and holes in Si lead to a strongly bias‐dependent ratio of the collected photoelectrons and photoholes. Photocurrents measured in asymmetrical sensors made of CoSi2/Si/PtSi were found to change phase as a function of light energy at a constant bias. This change of photocurrent direction can also be understood with the proposed energy band diagram and ballistic transport of hot carriers in Si. This kind of device showed a tunability of the cutoff energy between 0.3 and 0.5 eV.