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Electrical measurements have been carried out on epitaxial FeSi2 layers on silicon substrates, the silicide thickness being either 180 or 350 Å. A direct gap of 0.85 eV was measured by optical absorption. Current-voltage characteristics of mesa-structures Cr/Fe/FeSi2/Si show a p-type semiconductor behavior. Capacitance-voltage and capacitance-temperature data at different frequencies indicate a large response of deep levels or interface states. Admittance spectroscopy yields the activation energy and capture cross section of two levels. Finally an energy-band diagram is proposed.

Investigation of the photoelectric properties of several metal/β-FeSi2/Si heterostructures is presented. For thin silicide samples (200 Å), the photocurrent follows a Fowler’s law with a threshold Φ1 lower than the silicide band gap Eg. For thicker silicide samples (2500 Å), the behavior of the photocurrent is different because the optical absorption within the silicide can no longer be neglected: a maximum of the photocurrent is observed instead at Eg. The variations of Eg and Φ1 with temperature are compellingly similar and show the strong effect of the electron-phonon coupling. We suggest that the threshold Φ1 corresponds to transitions between a trap localized near the heterojunction and the silicide conduction band.

Experimental photoresponses and electrical characteristics of metal/P-FeSi,/Si structures are presented. Three kinds of samples are compared: two with a thin epitaxial silicide layer (180A), preparedby two different methods, and one with a thick polycrystalline silicide layer (2500 A). The rectifying behaviour and the photoelectric response of the three kinds of samples are different. In the thin samples these properties are governed by those of the P-FeSi,/Si interface, whereas for thick samples bulk mechanisms dominate. Analysis of the photocurrent in one kind of thin sample shows that two contributions exist. Their intensities follow similar temperature behaviours but the two transition thresholds do not. These considerations allow assignment of the initial and final states of the transitions, and the upper threshold is shown to correspond to an internal photoemission effect at the P-FeSi,/Si interface. The conduction band offset is deduced from the difference between the two thresholds The valence band discontinuity is less than 50 meV between 360 K and 260 K, whereas it changes sign when the temperature decreases below 260 K, the two bandgaps becoming nested within each other. These properties are also discussed for the other kinds of sample and related to the mechanisms which are responsible for the electrical characteristics.

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