M. R. Krause

Combined molecular beam epitaxy low temperature scanning tunneling microscopy system: Enabling atomic scale characterization of semiconductor surfaces and interfaces

A molecular beam epitaxy and low temperature scanning tunneling microscopy chamber have been integrated to characterize both compound and elemental semiconductor surfaces and interfaces. The integration of these two commercially available systems has been achieved using a custom designed sample transfer mechanism. The MBE growth chamber is equipped with electron diffraction and provides substrate temperature measurements and control by means of band-edge thermometry accurate to within ±0.5°C. In addition, the microscope can operate at temperatures as low as 4K and perform ballistic electron emission microscopy measurements. The chamber that houses the microscope includes a preparation chamber with an evaporation source for metals. The entire STM chamber also rests on an active vibration isolation table, while still maintaining an all ultrahigh vacuum connection to the MBE system.

Ostwald ripening of manganese silicide islands on Si(001)

The deposition of Mn onto Si(001) in the submonolayer regime has been studied with scanning tunneling microscopy to gain insight into the bonding and energetics of Mn with Si. The as-deposited Mn films at room temperature are unstructured. Upon annealing to 300–700 °C three-dimensional islands of Mn or MnxSiy form while between the islands the Si(001)-(2×1) reconstruction becomes visible. With increasing annealing time the density of islands per surface area decreases while the average height of the remaining islands increases. The large islands grow in size at the expense of the small ones, which can be understood in the context of Ostwald [Z. Phys. Chem. 34, 495 (1900)] ripening theory. The average island height shows a time dependence of H∼t1∕4, indicating that surface diffusion is the growth limiting process.

Ballistic electron transport properties of Fe-based films on Si(001)

Thickness dependent ballistic electron emission microscopy (BEEM) studies have been performed on Au∕Fe81C19∕Si(001) and Au∕Si(001) Schottky diodes at 80K. The Schottky height was measured to be 0.70±0.02eV for the Fe81C19∕Si(001) interface. Electron attenuation lengths were extracted from the slope of the semilog BEEM current versus the thickness of the Fe81C19 layers for electron energies ranging from 1.0to1.5eV. In this range the attenuation length was found to decrease with increasing energy from 4.1±0.9to2.5±0.6nm, which indicates that some electron-electron scattering is occurring in the metal overlayer. This decrease is slightly greater than predicted for a free electron gas system, resulting from the complex structure of the Fe81C19 film.

Hot electron transport across manganese silicide layers on the Si(001) surface

Ballistic electron emission microscopy (BEEM) has been performed on MnSi∕Si(001) Schottky diodes at 80K to study the hot electron transport properties. The BEEM spectra best fit the thermally broadening 5∕2 power law model with two threshold heights at 0.71 and 0.86eV, indicating a complex interface band structure. In addition, the normalized BEEM current in the MnSi overlayer was found to be approximately seven times less than is observed in Au∕Si(001) samples of similar thicknesses, indicating a larger amount of hot electron scattering in the MnSi∕Si(001) samples.

Measurement of the clustering energy for manganese silicide islands on Si(001) by ostwald ripening

The rate of growth during annealing of manganese silicide islands in the submonolayer coverage regime on the Si(001) surface has been measured by scanning tunneling microscopy. The fourth power of the growth rate is linearly dependent upon the annealing time, consistent with a diffusion limited Ostwald ripening mechanism for island growth. The growth rate has been determined for four different annealing temperatures to extract the activation energy for clustering, which has been found to be 2.6±0.2eV.