Energy band alignments at metal/molecular layer/semiconductor and metal/quantum dot interfaces using Ballistic Electron Emission Microscopy

In the studies presented here, we used Ballistic Electron Emission Microscopy (BEEM), a version of Scanning Tunneling Microscopy in a three terminal configuration, to quantify the local Schottky barrier height and to image the local hot-electron transmittance at buried interfaces described below.;In the search to replace or advance silicon based technology, advances in molecular electronics promote the possibility to use molecular electronic components to enhance integrated circuits. Therefore, the studies of the electronic transport properties of junctions containing molecular layers are of great interest. In our study, we used Au/GaAs Schottky junctions which have molecular layers with large electric dipoles inserted between the Au and GaAs layers, for which the molecules are neither well organized nor close-packed, rather forming monolayers which turn out to present pinholes. We used Ballistic Electron Emission Microscopy to directly characterize the inhomogeneity of the buried molecular layer. One important result that we found in these nanometer resolution studies of Au/molecular film/GaAs junctions is that the electronic transport is made through the pinholes rather than tunneling through the molecules. We also found that the effective Schottky barrier height at the pinholes is modified by the surrounding molecular film with a dipole, consistent with a theoretical model based on macroscopic measurements of the same types of samples.;In a separate study, we present the electronic measurements of the position-dependent energy of the quantum-confined conduction band minimum in the wetting layer to the sides and at the back side of cleaved In0.4Ga 0.6As quantum dots using cross-sectional ballistic electron emission microscopy over a Schottky contact made to a cleaved quantum dot sample. We show that we can distinguished the depth at which the "high point" occurs in the conduction band profile beneath the probe tip, by applying a reverse bias to the Schottky contact and measuring the resulting effect on the measured local Schottky barrier height. We found that this high point occurs at the backside of the In0.4Ga 0.6As quantum dots in our measurements. Using these results, we estimated the physical size of the quantum dot in the z direction. Also, by comparing the measurements with the tip located to the side to those over the quantum dots we found that the conduction band minimum of the quantum-confined wetting layer is ∼ 90 meV below the GaAs conduction band minimum and that this offset is not substantially modified by Fermi level pinning effects.;Finally, we present several results concerning Ballistic Electron Emission Microscopy measurements on Au/GaAs samples taken at low temperature. We find that at 80K the BEEM current on these samples become suppressed over time, and propose that this is due to a build up of the injected hot electrons into the sample near the end of the depletion region. Our interpretation is that the injected electrons accumulate in the semiconductor due to increase in the resistance of the sample at low temperature, which prevents the injected electrons from easily reaching the ohmic contact on the sample.