Observation and Origin of Extraordinary Atomic Mobility at Metal-Semiconductor Interfaces at Low Temperatures

Extraordinarily high mobility of Si and Ge atoms at semiconductor (Si, Ge)-metal (Al) interfaces is observed at temperatures as low as 80 K during thin metal film deposition. In situ x-ray photoemission spectroscopic valence-band measurements reveal a changed chemical bonding nature of the semiconductor atoms, from localized covalentlike to delocalized metalliclike, at the interface with the Al metal. The resulting delocalized bonding nature of the interfacial semiconductor atoms brings about the observed extreme enhancement of their mobility. The finding opens avenues for tailoring reaction kinetics and phase transformations in nanostructured materials, as functional thin-film systems, at ultralow temperatures by dedicated interfacial design.

Figure 1

AES composition (elemental)-depth profiles of the (a) ${\mathrm{Al}}_{20-\mathrm{nm}}|{\mathrm{Si}}_{1-\mathrm{nm}}|\mathrm{Al}(111)$ and (b) ${\mathrm{Al}}_{20-\mathrm{nm}}|{\mathrm{Ge}}_{1-\mathrm{nm}}|\mathrm{Al}(111)$ specimens.

Figure 2

Cross-sectional EDX and HRTEM investigation of the ${\mathrm{Al}}_{20-\mathrm{nm}}|{\mathrm{Si}}_{1-\mathrm{nm}}|\mathrm{Al}(111)$ specimen. (a) Annular dark-field (ADF) TEM image. An EDX line scan of the Si-K signal from the top surface to the Al(111) substrate has been incorporated in the image. (b),(c) EDX mappings of the Si-K and Al-K signals, respectively, of the same location as in (a). (d) Cross-sectional HRTEM image, with its FFT shown in the inset.

Figure 3

Cross-sectional EDX and HRTEM investigation of ${\mathrm{Al}}_{20-\mathrm{nm}}|{\mathrm{Ge}}_{1-\mathrm{nm}}|\mathrm{Al}(111)$ specimen. (a) ADF TEM image. An EDX line scan of the Ge-K signal from the top surface to the Al(111) substrate has been incorporated in the image. (b),(c) EDX mappings of the Ge-K and Al-K signals, respectively, of the same location as in (a). (d) Cross-sectional HRTEM image, with its FFT shown in the inset.

Figure 4

(a) Evolution of the valence-band spectra (with E_F at 0 eV) of ultrathin Ge films on Al(111) with decreasing Ge thickness from in situ XPS measurements. The valence-band spectra of bulk $a\text{−}\mathrm{Ge}$ (100 ML) and bulk single-crystalline Ge ($c\text{−}\mathrm{Ge}$) are shown for comparison. All the spectra have been normalized with respect to their maximum height and a constant offset in intensity has been added in order to avoid any overlapping of the spectra. (b) Measured valence-band spectra of 1-ML and 2-ML Ge thin films grown on 0.6-nm amorphous-${\mathrm{Al}}_2{\mathrm{O}}_3/\mathrm{Al}(111)$.