E. Y. Lee

In situ study of epitaxial CoSi2/Si(111) by ballistic‐electron‐emission microscopy

We have performed in situ ballistic‐electron‐emission microscopy (BEEM) and spectroscopy (BEES) at 77 K on epitaxial CoSi2/Si(111) films grown by molecular beam epitaxy. The transport in the silicide was found to be essentially ballistic. With the help of atomic‐resolution scanning tunneling microscopy it has been found that the BEEM current depends significantly on the surface electronic structure. On strain‐relaxed layers, hot electron scattering at individual interfacial dislocations has been observed for the first time by BEEM. Apart from the surface‐ and dislocation‐induced contrast variations, the BEEM current is generally homogeneous.

In situ ballistic‐carrier spectroscopy on epitaxial CoSi2/Si(111) and Si(100)

In situ ballistic‐electron‐emission microscopy (BEEM) and spectroscopy (BEES) have been performed at 77 K on CoSi2/Si(100) and Si(111) grown by molecular beam epitaxy (MBE). Scattering at individual interface dislocations and point defects gives rise to a localized increase of the BEEM current on n‐Si(111) and a decrease on p‐Si(111) in agreement with a kinematic interpretation. On n‐Si(100), (110)‐oriented grains exhibit a Schottky barrier of 0.58±0.04 V compared to 0.74±0.04 V on (100)‐oriented CoSi2. The magnitude of the BEEM current strongly depends on the epitaxial orientation on Si(100) and is comparable for CoSi2(100)/n‐Si(100) and CoSi2/n‐Si(111).

Surface effects in ballistic-electron-emission microscopy

Abstract In situ ballistic-electron-emission microscopy (BEEM) and scanning tunneling spectroscopy have been performed at 77 K on CoSi 2 Si (111) films grown by molecular beam epitaxy. Different atomic surface structures induce significant variations of the BEEM current. For the first time periodic surface structures could be imaged at atomic resolution by BEEM. This surface effect is explained by the energy distribution of the injected electrons, which is influenced by surface-induced variations of the tunneling barrier height. Quantum size effects in the local density of states can be correlated with subtle features in the BEEM spectra.

Ballistic-electron emission microscopy as a probe for surface science

Abstract In situ ballistic-electron emission microscopy (BEEM) and spectroscopy (BEES) have been performed at 77 K on epitaxial CoSi 2 Si (100) and Si(111). BEEM images reflect the atomic-scale periodicity of the surface topography. This effect is due to variations of the energy distribution of the tunneling electrons on an atomic scale. The ability of BEEM to image individual interfacial point defects and dislocations allows us to study the angular distribution of the tunneling electrons as well, in particular its dependence on the atomic structure of the sample and the tip. Our results show that with BEEM useful information can be obtained about the tunneling process on an atomic scale, which is complementary to the one accessible in a conventional STM experiment.

Electron transmission probability across CoSi2/n-Si(111) interfaces measured with a scanning tunneling microscope

In an in situ UHV study at 77 K, a scanning tunneling microscope was used in a new constant-height mode of ballistic-electron-emission microscopy to measure the electron transmission probability across epitaxial CoSi2 films and across CoSi2/n-Si(111) interfaces. The conventional ballistic-electron-emission microscopy, carried out in the constant-current mode, was found to give artifacts at high tip voltages (< −2 V with sample grounded), which could be eliminated in the constant-height mode with simultaneous scanning tunneling spectroscopy.

ELECTRONIC TRANSPORT IN EPITAXIAL FILMS STUDIED BY SCANNING PROBE TECHNIQUES

Hot electron transport in epitaxial CoSi2/Si heterostructures has been studied in situ by ballistic electron emission microscopy (BEEM) at 77 K. At CoSi2/Si(111) interfaces elastic scattering at interfacial defects could be imaged with spatial resolution on a nanometer scale. Hot electron injection across CoSi2/n-Si(111) interfaces was found to be favored by interface scattering, in contrast to hot hole injection across CoSi2/p-Si(111), in agreement with expectations based on the projected Si band structure. Inelastic scattering within the metal film was found to become dominant at energies above ~4 eV, leading to a strong sensitivity of the BEEM current on film thickness variations. This could be exploited to generate maps of the film thickness distribution, facilitating the interpretation of quantum size effects (QSEs) which appear both in BEEM and in scanning tunneling spectroscopy. The variation of the BEEM current was found to be entirely due to scattering effects at CoSi2/Si(111) interfaces, the Schottky barrier height being constant. At CoSi2/Si(100) interfaces, however, a pronounced decrease in the Schottky barrier height was observed at certain interfacial defects, some of which are not yet identified.

ATOMIC SCALE VARIATIONS OF THE STM TUNNELING DISTRIBUTION OBSERVED BY BALLISTIC-ELECTRON-EMISSION MICROSCOPY

In this work we present an experimental method which allows to investigate experimentally the energy and momentum distribution of the carriers which are injected by an STM tip into a metallic sample. We perform in situ ballistic-electron-emission microscopy (BEEM) and spectroscopy (BEES) simultaneously with atomic-resolution STM. The experiments are done at 77K on epitaxial CoSi2 films on Si(111) and Si(100). The potential of the method to improve on our understanding of tunneling on an atomic scale will be illustrated by two important findings: For the first time we have directly observed variations of the tunneling distribution on an atomic scale. The BEEM data also give evidence for a pronounced forward focussing of the angular distribution of the tunneling electrons.