Study of PtSi/Si(100) Interfaces by Ballistic -Electronemission Microscopy

ABSTRACTThe PtSi/Si interface is of technological interest for Schottky barrier infrared detectors. We are studying PtSi/Si heterostructures using ballistic -electron-emission microscopy (BEEM), an STM-based technique that uses the STM tip to inject hot electrons at a particular energy into the metal overlayer. The BEEM technique allows imaging of the Schottky barrier with good spatial resolution (of the order of tens of nanometers) and allows the measurement of the hot electron attenuation length in the metal overlayer. Our results indicate a Schottky barrier of 0.87 eV for PtSi/Si n-type, and an attenuation length of 4 nm for electrons with an energy of 1 eV above the metal Fermi level. The attenuation length we measure is a convolution of the electron elastic and inelastic mean free path lengths.We have also used an ac BEEM technique to observe inelastic scattering events at the metalsemiconductor interface in PtSi/Si(100) n-type. There are several features visible in the spectrum, including one at 1040 meV which we attribute to optical phonon-assisted electron-hole pair creation near the metal- semiconductor interface in analogy to a feature we have observed at the same energy in the Au/Si(100) ac BEEM spectrum. Higher-energy features appear at 1230 meV and 1300 meV. Similar features appeared in the Au/Si(100) spectrum at 1120 meV and 1230 meV.We also suggest that the traditional assumption of momentum conservation parallel to the Schottky barrier interface is unnecessary to obtain a quadratic turn on of the BEEM current above the threshold. If electrons are elastically scattered at the interface so that momentum is not conserved, the increase in the ratio of the density of states in the semiconductor to those in the metal will also give a quadratic turn on even when the band structure is much more complicated than a nearly free electron model. This model also explains why the simple square-root dependence of the photoresponse on wavelength above threshold observed in all metal/semiconductor Schottky barriers despite the complications in band structure.