Franco Dinelli

GaSb quantum dot morphology for different growth temperatures and the dissolution effect of the GaAs capping layer

We compare the characteristics of GaSb quantum dots (QDs) grown by molecular beam epitaxy on GaAs at temperatures from 400°C to 490°C. The dot morphology, in terms of size, shape and density, as determined by atomic force microscopy on uncapped QDs, was found to be highly sensitive to the growth temperature. Photoluminescence spectra of capped QDs are also strongly dependent on growth temperature, but for samples with the highest dot density, where the QD luminescence would be expected to be the most intense, it is absent. We attribute this to dissolution of the dots by the capping layer. This explanation is confirmed by atomic force microscopy of a sample that is thinly capped at 490°C. Deposition of the capping layer at low temperature resolves this problem, resulting in strong QD photoluminescence from a sample with a high dot-density.

Ultrasonic force microscopy: detection and imaging of ultra-thin molecular domains.

The analysis of the formation of ultra-thin organic films is a very important issue. In fact, it is known that the properties of organic light emitting diodes and field effect transistors are strongly affected by the early growth stages. For instance, in the case of sexithiophene, the presence of domains made of molecules with the backbone parallel to the substrate surface has been indirectly evidenced by photoluminescence spectroscopy and confocal microscopy. On the contrary, conventional scanning force microscopy both in contact and intermittent contact modes have failed to detect such domains. In this paper, we show that Ultrasonic Force Microscopy (UFM), sensitive to nanomechanical properties, allows one to directly identify the structure of sub-monolayer thick films. Sexithiophene flat domains have been imaged for the first time with nanometer scale spatial resolution. A comparison with lateral force and intermittent contact modes has been carried out in order to explain the origins of the UFM contrast and its advantages. In particular, it indicates that UFM is highly suitable for investigations where high sensitivity to material properties, low specimen damage and high spatial resolution are required.

Nanometre scale 3D nanomechanical imaging of semiconductor structures from few nm to sub-micrometre depths

Multilayer structures of active semiconductor devices (1), novel memories (2) and semiconductor interconnects are becoming increasingly three-dimensional (3D) with simultaneous decrease of dimensions down to the few nanometres length scale (3). Ability to test and explore these 3D nanostructures with nanoscale resolution is vital for the optimization of their operation and improving manufacturing processes of new semiconductor devices. While electron and scanning probe microscopes (SPMs) can provide necessary lateral resolution, their ability to probe underneath the immediate surface is severely limited. Cross-sectioning of the structures via focused ion beam (FIB) to expose the subsurface areas often introduces multiple artefacts that mask the true features of the hidden structures, negating benefits of such approach. In addition, the few tens of micrometre dimension of FIB cut, make it unusable for the SPM investigation.