Valery Yu. Davydov

Influence of Substrate Microstructure on the Transport Properties of CVD-Graphene

We report the study of electrical transport in few-layered CVD-graphene located on nanostructured surfaces in view of its potential application as a transparent contact to optoelectronic devices. Two specific surfaces with a different characteristic feature scale are analyzed: semiconductor micropyramids covered with SiO2 layer and opal structures composed of SiO2 nanospheres. Scanning tunneling microscopy (STM) and scanning electron microscopy (SEM), as well as Raman spectroscopy, have been used to determine graphene/substrate surface profile. The graphene transfer on the opal face centered cubic arrangement of spheres with a diameter of 230 nm leads to graphene corrugation (graphene partially reproduces the opal surface profile). This structure results in a reduction by more than 3 times of the graphene sheet conductivity compared to the conductivity of reference graphene located on a planar SiO2 surface but does not affect the contact resistance to graphene. The graphene transfer onto an organized array of micropyramids results in a graphene suspension. Unlike opal, the graphene suspension on pyramids leads to a reduction of both the contact resistance and the sheet resistance of graphene compared to resistance of the reference graphene/flat SiO2 sample. The sample annealing is favorable to improve the contact resistance to CVD-graphene; however, it leads to the increase of its sheet resistance.

Band Gap of Hexagonal InN and InGaN Alloys

We present results of photoluminescence studies of the band gap of non-intentionally doped single-crystalline hexagona InN layers and In-rich InxGa1-xN alloy layers (0.36 < x < 1). The band gap of InN is found to be close to 0.7 eV. This is much smaller than the values of 1.8 eV to 2.1 eV cited in the current literature. A bowing parameter of b ≈ 2.5 eV allows one to reconcile our and the literature data for the band gap values of InxGa1-xN alloys in the entire composition region.

Raman spectroscopy of disorder effects in AlxGa1−xN solid solutions

Abstract The results of Raman spectroscopic studies of the disorder effects in hexagonal Al x Ga 1− x N epitaxial layers grown by MBE and HVPE on different substrates for a large range of Al concentrations are presented. The width of the nonpolar phonon line with E 2 symmetry results from the inhomogeneous broadening due to spatial fluctuations in the Al content. The abnormally small broadening of the A 1 (TO) polar phonon mode for x or (1− x )≪1 and the large broadening for x ≅0.5–0.7 are attributed to the specific frequency dependence of the density of states for the branch with the directional dispersion in pure crystals. Thus the Raman spectrum is found to be highly sensitive to the composition of Al x Ga 1− x N epitaxial layers and its inhomogeneity. It is shown that in the estimation of the crystal composition, on the basis of Raman data, the influence of the homogeneous strain effects could be excluded via measuring a linear combination of two Raman line frequencies.

Electronic and structural properties of InN thin films grown by MOMBE on sapphire substrates

The development of high quality semiconductor thin films for different applications is a demanding problem in material science. InN has not been an intensively studied as AlN and GaN. There is relatively little information on the fundamental optical properties, charge carrier transport, and the properties and behavior of electrically active defects in the material. The absence of good-quality material lead even to conflicting data reported in the literature concerning the optical gap and band structure. In this publication it will be shown that InN thin films can be successfully grown using the MO MBE method. For the first time the proper choice of growth conditions allows to obtain good quality InN thin films with a charge carrier concentration as low as 8.8 X 1018 cm-3.