In the framework of this thesis Ballistic Electron Emission Microscopy/Spectroscopy (BEEM/S) was used to investigate semiconductor heterostructures.
Especially heterostructures including organic semiconductors like titanylphthalocyanine (TiOPc) and hexa-peri hexabenzocoronene (HBC).
Au/TiOPc/GaAs diodes were investigated by BEEM/S.
From the BEEM images it can be concluded that our MBE grown samples are very homogeneous in comparison to organic films manufactured by evaporation.
All features visible in the BEEM images of our samples correlate exclusively with the granular structure and the topographic features of the Au-film and cannot be correlated to the organic film underneath.
Analyzing the BEEM spectra we find that the TiOPc increases the BEEM threshold voltage compared to reference Au/GaAs diodes.
The barrier height measured on the Au-TiOPc-GaAs heterostructure is Vb 1.2 eV, whereas the barrier on the Au/GaAs diode is 0.9 eV.
In addition, the derivative of the BEEM spectra shows multiple features in the energy regime above the lowest unoccupied molecular orbital (LUMO) level.
Using the first derivative of the BEEM spectra signatures of the L- and X-valleys of GaAs can be seen.
On conventional Au-GaAs diodes, the influence of higher valleys is normally seen as an increased slope in the first derivative of the spectrum, as the higher valleys open additional transport channels into the collector electrode.
The positions of the higher valleys in GaAs can now be used to calibrate the energy scale.
Hence we are sure that the measured Schottky barrier height really is the barrier height at the Au/TiOPc interface.
Temperature studies show an increase of the SBH from 1.2 eV at room temperature to 1.5 eV at T=10 K.
The energy dependent electron transmission properties of TiOPc were investigated too.
It was found, that the transmission and the mean free path both increase with energy, but exhibit a different behavior as a function of temperature.
While the decreasing mean free path with increasing temperature suggests, that the mean free path is dominated by phonon scattering processes, the TiOPc transmission increases with increasing temperature, which suggests that the transmission is dominated by impurity scattering processes. Assuming dominant ionized impurity scattering at the interfaces, and dominant phonon scattering in the bulk, however, our findings can be consistently explained.
To analyze the data, a model calculation for the BEEM current through the Au/TiOPc/GaAs heterostructure was implemented.
It was found that at higher energies, the ballistic current through Au/TiOPc/GaAs heterostructure can not be described precisely with the heterostructure extension of the Bell-Kaiser model for mainly two reasons: First, the influence of the GaAs L and X-valleys was not included into the calculations, and second, modelling the TiOPc with a simple rectangular potential barrier is not accurately enough.
Additionally, an approach for the determination of the attenuation length of TiOPc as a function of energy has been successfully introduced.
Finally, the Schottky barrier heights of Au/HBC/GaAs heterostructures were investigated by BEEM. Between T=300 K and T=10 K, the Schottky barrier height at the Au/HBC interface increases from 1.3 eV at T=300 K to 1.56 eV at T=10 K. Simultaneously, the Fermi level pinning at the HBC/GaAs interface becomes systematically deeper, starting with a position of 1.2 eV above the GaAs conduction band at T=300 K and ending at 1.4 eV at T=10 K, which is close to the valence band of GaAs. The high barrier at the HBC-GaAs interface makes this material a promising interfacial layer for increasing the open circuit voltage of GaAs Schottky barrier solar cells.