In the framework of this thesis Ballistic Electron Emission Microscopy/ Spectroscopy (BEEM/S) was used to investigate semiconductor heterostructures. Including organic semiconductors like titanylphthalocyanine (TiOPc) and hexa-peri hexabenzocoronene (HBC). Analyzing the BEEM spectra we find that the TiOPc increases the BEEM threshold voltage compared to reference Au/GaAs diodes. The energy dependent electron transmission properties of TiOPc were investigated too. 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. 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.