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In recent years there has been growing interest in the electronic properties of ‘few layer’ graphene films. Twisted layers, different stacking and register with the substrate result in remarkable unconventional couplings. These distinctive electronic behaviours have been attributed to structural differences, even if only a few structural determinations are available. Here we report the results of a structural study of bilayer graphene on the Si-terminated SiC(0001) surface, investigated using synchrotron radiation-based photoelectron diffraction and complemented by angle-resolved photoemission mapping of the electronic valence bands. Photoelectron diffraction angular distributions of the graphene C 1s component have been measured at different kinetic energies and compared with the results of multiple scattering simulations for model structures. The results confirm that bilayer graphene on SiC(0001) has a layer spacing of 3.48 Å and an AB (Bernal) stacking, with a distance between the C buffer layer and the first graphene layer of 3.24 Å. Our work generalises the use of a versatile and precise diffraction method capable to shed light on the structure of low-dimensional materials.

Carbon doping of In 0o53 Ga 0.47 As has been investigated in a production molecular beam epitaxy (MBE) system equipped with a CBr 4 gas line. Growth temperature and As 4 flux effects on surface morphology and efficiency of carbon doping are studied. Hole concentrations from 1.7 x 10 17 to 1.3 x 10 20 cm -3 have been obtained. A first set of samples shows no significant difference in mobility for comparable beryllium- and carbon-doping levels either in solid source production (Riber MBE49) or in gas source research (Riber MBE32) systems. Best results for carbon doping are obtained with a low As 4 flux and a growth temperature in the 400-425 °C range. A nearly ideal emitter-base I(V) characteristic has been obtained for a doping level of 8 x 10 19 cm -3 , which is promising for high-frequency InP-based HBTs issue.

We investigate GaInP/GaAs high electron mobility transistor (HEMT) epitaxial structures grown in a production molecular beam epitaxy (MBE) system, with different interface commutation procedures. These structures have been designed to be very sensitive to the interface quality. They have been characterized by Hall measurements and photoluminescence. GaP- and GaInAs-type interfaces can be distinguished. Best mobility results occur for few second or even no growth interruption, demonstrating the very low As memory effect of this large MBE system.

Semiconductor PbSe/CdSe core-shell nanocrystals (NCs) in a double barrier tunnel junction have been investigated by means of scanning tunneling spectroscopy at low temperature. From the analysis of the differential conductance peak position as a function of the potential distribution in both potential barriers, we demonstrate a unipolar transport regime for a large amount of NCs. The same charge carriers are injected on both sides of the zero-conductance gap, and the peaks observed at higher energy arise from the charging of the NCs. Similar results are obtained for CdSe/CdS dot-in-rod NCs, indicating that the addition of a shell favors transitions between different charge states rather than single particle excited states. Further characterization of the PbSe/CdSe core-shell NCs by x-ray photoemission spectroscopy reveals that the variations in the transport properties from NC to NC are explained by the occurrence of unprotected PbSe facets that have different orientations in the junction.

MBE-growth conditions of mixed As/P and P-based heterostructures are studied. The objective is to improve their optical and structural characteristics. Growths were performed on multiwafer production system. Successive campaign data and maintenance procedure are presented

The rapid setup of the molecular beam epitaxy (MBE) growth process for GaInP/GaAs heterostructure bipolar transistor (HBT) in an all solid-source multiwafer production MBE system is reported. The first 100×100 μm2 large area test structures enable determination of the static gain, which is above 200. rf measurements on 8×10 μm2 devices exhibit a cut-off frequency fT of 34 GHz. Although most publications concerning production of such devices are related to metalorganic chemical vapor deposition grown material, we intend to demonstrate that MBE is a very efficient method for high volume production of GaInP/GaAs HBT. In particular we emphasize the fact that MBE growth for production of benchmark HBT devices is a high yield process that is attainable in a very short time. In order to optimize device characteristics further work focuses on metallurgical and electrical emitter/base interface studies by means of photoluminescence and I/V measurements on processed PN junctions.