M.M. Giangregorio

Plasmonic gallium nanoparticles on polar semiconductors: interplay between nanoparticle wetting, localized surface plasmon dynamics, and interface charge.

Ga nanoparticles supported on large band gap semiconductors like SiC, GaN, and ZnO are interesting for plasmon-enhanced UV-emitting solid-state devices. We investigate the influence of the polarity of the SiC, GaN, and ZnO wurtzite semiconductors on the wetting of Ga nanoparticles and on the resulting surface plasmon resonance (SPR) by exploiting real time plasmonic ellipsometry. The interface potential between polar semiconductors (SiC, GaN, and ZnO) and plasmonic nanoparticles (gallium) is shown to influence nanoparticle formation dynamics, geometry, and consequently the SPR wavelength. We invoke the Lippman electrowetting framework to elucidate the mechanisms controlling nanoparticle dynamics and experimentally demonstrate that the charge transfer at the Ga nanoparticle/polar semiconductor interface is an intrinsic method for tailoring the nanoparticle plasmon resonance. Therefore, the present data demonstrate that for supported nanoparticles, surface and interface piezoelectric charge of polar semiconductors also affects SPR along with the well-known effect of the media refractive index.

Exploring and rationalising effective n-doping of large area CVD-graphene by NH3.

Despite the large number of papers on the NH3 doping of graphene, the achievement of stable n-doped large area CVD (chemical vapor deposition) graphene, which is intrinsically p-doped, is still challenging. A control of the NH3 chemisorption and of the N-bond configuration is still needed. The feasibility of a room temperature high pressure NH3 treatment of CVD graphene to achieve n-type doping is shown here. We use and correlate data for (a) sheet resistance, R(sh), and the Hall coefficient, R(H), in van der Pauw configuration, acquired in real time during the NH3 doping of CVD-graphene on a glass substrate, (b) optical measurements of the effect of doping on the graphene Van Hove singularity point at 4.6 eV in the dielectric function spectra by spectroscopic ellipsometry, and of (c) N-bond configuration by XPS to better understand and, finally, control the NH3 doping of graphene. The discussion is focused on the thermal and time stability of the n-doping after air exposure. A chemical rationale is provided for the NH3 n-doping based on the interaction of (i) NH3 with intrinsic oxygen functionalities and defects of CVD graphene and of (ii) C-NH2 doping centers with acceptor species present in the air.

Interrelation between microstructure and optical properties of erbium-doped nanocrystalline thin films

Abstract Nanocrystalline silicon thin films codoped with erbium, oxygen and hydrogen have been deposited by co-sputtering of Er and Si. Films with different crystallinity, crystallite size and oxygen content have been obtained in order to investigate the effect of the microstructure on the photoluminescence properties. The correlation between the optical properties and microstructural parameters of the films is investigated by spectroscopic ellipsometry. PL response of the discussed structures covers both the visible wavelength range (a crystallite size-dependent photoluminescence detected for 5– 6 nm sized nanocrystals embedded in a SiO matrix) and near IR range at 1.54 μm (Er-related PL dominating in the films with 1– 3 nm sized Si nanocrystals embedded in a-Si:H). It is demonstrated that the different PL properties can be also discriminated on the basis of ellipsometric spectra.

The Role of Microstructure in Luminescent Properties of Er-doped Nanocrystalline Si Thin Films*

In this contribution, we present a structural and photoluminescence (PL) analysis of Er-doped nanocrystalline silicon thin films produced by rf magnetron sputtering method. We show the strong influence of the presence of nanocrystalline fraction in films on their luminescence efficiency at 1.54 µm studied on a series of specially prepared samples with different crystallinity, i.e., percentage and sizes of Si nanocrystals. A strong increase, by about two orders of magnitude, of Er-related PL intensity in these samples with lowering of the Si nanocrystal sizes from 7.9 to about 1.5 nm is observed. The results are discussed in terms of the sensitization effect of Si nanocrystals on Er ions.

Influence of crystals distribution on the photoluminescence properties of nanocrystalline silicon thin films

Nanocrystalline silicon thin films doped with erbium were produced by reactive magnetron RF sputtering. Their structural and chemical properties were studied by X-ray diffractometry at grazing incidence, micro-Raman, spectroscopic ellipsometry and Rutherford Backscattering Spectroscopy, respectively. Films with different crystalline fraction and crystallite size were deposited. Since the luminescence efficiency of Er-doped nc-Si films is strongly influenced by the microstructure and impurity content (i.e. H, O, Er), the photoluminescence characteristics are discussed in terms of the microstructure. The novelty of these films, if compared to usually investigated structures with the nanocrystals embedded in SiO2, is their relative high conductivity, which makes them attractive for device applications.

Effect of Buffer Design on AlGaN/AlN/GaN Heterostrucutres by MBE

The effect of the buffer layers on the subsequent GaN epitaxial layers and electrical properties of AlGaN/AlN/GaN heterojunction structures nitrided at various temperatures was investigated. For AlN buffer layers, two different growth conditions of AlN buffer layers were introduced to avoid Al droplets. We found that etch pit density and structural quality of GaN epitaxial layer strongly depends on the growth conditions of AlN buffer layers. When using a double buffer layer (low temperature GaN on high temperature AlN) for 200 °C nitridation, the etch pit density was measured to high 10 7 cm -2 in GaN epitaxial layers. Furthermore, we observed that electrical properties of AlGaN/AlN/GaN heterostructures depend on growth conditions of buffer layers and nitridation temperatures. The mobility in Al 0.33 Ga 0.67 N/AlN/GaN structures grown on single AlN buffer layers for 200 °C nitridation were 1300 cm 2 /Vs at a sheet charge of 1.6×10 13 cm -2 . Using the double buffer layer for 200 °C nitridation, the mobility increased to 1587 cm 2 /Vs with a sheet charge of 1.25×10 13 cm -2 .

THE STRUCTURE AND PHOTOLUMINESCENCE OF ERBIUM DOPED NANOCRYSTALLINE SILICON THIN FILMS PRODUCED BY REACTIVE MAGNETRON SPUTTERING

We have produced and studied undoped and erbium-doped nanocrystalline silicon thin films in order to evaluate the erbium influence on the film microstructure and how this correlates with the photoluminescence properties. Films were grown by reactive RF sputtering. For the doped films metallic erbium was added to the c-Si target. The structural parameters and the chemical composition of the different samples were investigated by X-ray in the grazing incidence geometry, Raman spectroscopy, ellipsometry and Rutherford Back Scattering. The effect of the nc-Si/SiOx matrix ,i.e., nc-Si volume fraction and the presence of SiO and/or SiO2 phases, on the erbium photoluminescence efficiency is discussed.