Eirini Sarigiannidou

Terahertz intersubband absorption in GaN/AlGaN step quantum wells

We demonstrate terahertz intersubband absorptions at frequencies of 2.1 THz (lambda approximate to 143 mu m) and 4.2 THz (lambda approximate to 70 mu m) in nitride-based semiconductor quantum wells. The structures consist of a 3 nm thick GaN well, an Al(0.05)Ga(0.95)N step barrier, and a 3 nm thick Al(0.1)Ga(0.9)N barrier. The absorption is detected at 4.7 K. The structure design has been optimized to approach a flat-band potential in the wells to allow for an intersubband absorption in the terahertz frequency range and to maximize the optical dipole moments

Strain distribution in GaN∕AlN quantum-dot superlattices

The two-dimensional strain distribution in a GaN∕AlN quantum-dot (QD) superlattice is measured from high-resolution transmission electron microscopy images using the geometrical phase analysis. The results are compared to elastic theoretical calculations using a combination of Fourier transform and Green’s function techniques. The GaN∕AlN system appears to be a model system for a comparison between theory and experiments as interdiffusion between GaN and AlN is negligible. We verify that for the case of a three-dimensional system, such as a QD, the biaxial strain approximation is not valid. Furthermore, we demonstrate that the presence of QDs induces a modulation in the strain state of the AlN barriers which is the driving force for the vertical alignment of the GaN QDs in the AlN matrix.

Selective Area Growth of Well-Ordered ZnO Nanowire Arrays with Controllable Polarity

Controlling the polarity of ZnO nanowires in addition to the uniformity of their structural morphology in terms of position, vertical alignment, length, diameter, and period is still a technological and fundamental challenge for real-world device integration. In order to tackle this issue, we specifically combine the selective area growth on prepatterned polar c-plane ZnO single crystals using electron-beam lithography, with the chemical bath deposition. The formation of ZnO nanowires with a highly controlled structural morphology and a high optical quality is demonstrated over large surface areas on both polar c-plane ZnO single crystals. Importantly, the polarity of ZnO nanowires can be switched from O- to Zn-polar, depending on the polarity of prepatterned ZnO single crystals. This indicates that no fundamental limitations prevent ZnO nanowires from being O- or Zn-polar. In contrast to their catalyst-free growth by vapor-phase deposition techniques, the possibility to control the polarity of ZnO nanowires grown in solution is remarkable, further showing the strong interest in the chemical bath deposition and hydrothermal techniques. The single O- and Zn-polar ZnO nanowires additionally exhibit distinctive cathodoluminescence spectra. To a broader extent, these findings open the way to the ultimate fabrication of well-organized heterostructures made from ZnO nanowires, which can act as building blocks in a large number of electronic, optoelectronic, and photovoltaic devices.

Effect of doping on the mid-infrared intersubband absorption in GaN/AlGaN superlattices grown on Si(111) templates

We report on the effect of Si doping on the mid-infrared intersubband absorption in GaN/AlGaN superlattices. For increasing doping levels, interband luminescence displays a blueshift and a broadening of the band edge caused by the screening of the internal electric field and band-filling effects. The intersubband absorption energy is mainly governed by many-body effects like exchange interaction and depolarization shift, which increase the e1–e2 subband separation. The ISB blueshift induced by many-body effects can be more than 50% of the e1–e2 transition energy.

Strain relaxation in GaN/AlxGa1-xN superlattices grown by plasma-assisted molecular-beam epitaxy

We have investigated the misfit relaxation process in GaN/AlxGa1−xN (x = 0.1, 0.3, 0.44) superlattices (SL) deposited by plasma-assisted molecular beam epitaxy. The SLs under consideration were designed to achieve intersubband absorption in the mid-infrared spectral range. We have considered the case of growth on GaN (tensile stress) and on AlGaN (compressive stress) buffer layers, both deposited on GaN-on-sapphire templates. Using GaN buffer layers, the SL remains almost pseudomorphic for x = 0.1, 0.3, with edge-type threading dislocation densities below 9 × 108 cm−2 to 2 × 109 cm−2. Increasing the Al mole fraction to 0.44, we observe an enhancement of misfit relaxation resulting in dislocation densities above 1010 cm−2. In the case of growth on AlGaN, strain relaxation is systematically stronger, with the corresponding increase in the dislocation density. In addition to the average relaxation trend of the SL, in situ measurements indicate a periodic fluctuation of the in-plane lattice parameter, which i...

Intraband emission at λ≈1.48μm from GaN∕AlN quantum dots at room temperature

We report on the intraband emission at room temperature from GaN∕AlN quantum dots grown by plasma-assisted molecular-beam epitaxy. The dots exhibit TM-polarized absorption ascribed to the intraband transition from the s to the pz shells. The pz-s intraband luminescence is observed at λ=1.48μm under optical excitation at λ=1.34μm perpendicular to the [0001] growth axis. The population of the pz state arises from Raman scattering by GaN A1 longitudinal optical phonons. Based on the emission spectral shape, we estimate that the homogeneous linewidth of the s-pz intraband transition is less than 4meV.

Linear alignment of GaN quantum dots on AlN grown on vicinal SiC substrates

We demonstrate linear alignment of self-assembled GaN quantum dots (QDs) grown by molecular beam epitaxy on AlN using vicinal SiC substrates. Under specific surface preparation, such substrates can present well-ordered steps. We also show that stepped AlN layers can be grown on such SiC substrates, the height and width of AlN step depending on growth parameters. Such steps induce a heterogeneous nucleation of QDs and promote an alignment along their edges. We demonstrate, by atomic force microscopy, the possibility of controlling the island spatial distribution and point out a lateral ordering of the dots.

Surfactant effect of gallium during the growth of GaN on AlN(0001¯) by plasma-assisted molecular beam epitaxy

The growth mode of N-face GaN deposited on AlN(0001¯) by plasma-assisted molecular beam epitaxy has been investigated. Based on reflection high-energy electron diffraction experiments, we demonstrate that for appropriate Ga fluxes and substrate temperature, a self-regulated 1‐ML-thick Ga excess film can be formed on the growing surface. Depending on the presence of this Ga monolayer, the growth can proceed following either the Stranski–Krastanow or the Frank Van der Merwe growth modes, hence enabling the synthesis of either quantum dots or quantum wells.

Thermal stability of the deep ultraviolet emission from AlGaN/AlN Stranski-Krastanov quantum dots

We report on the structural and optical properties of AlGaN/AlN quantum dot (QD) superlattices synthesized by plasma-assisted molecular-beam epitaxy. Modifying the composition and geometry of the QDs, the peak emission wavelength can be shifted from 320 nm to 235 nm while keeping the internal quantum efficiency larger than 30%. The efficient carrier confinement is confirmed by the stability of the photoluminescence (PL) intensity and decay time, from low temperature up to 100 K. Above this threshold, the PL intensity decreases and the radiative lifetime increases due to carrier thermalization. We also identified the intraband electronic transition between the ground level of the conduction band and the first excited state confined along the growth axis (s-pz).

New HTS 2G Round Wires

Key components for the power transmission and distribution for the future grids are superconducting cables. The energy network in highly populated areas will soon fully benefit from this technology, in particular for the retrofitting of the existing cables. However retrofitting the existing grid in underground structures (pipes, pits, turn), requires cables as small a diameter as possible. The HTS cable core is generally manufactured by laying tapes around a former with a diameter of about 25 mm to prevent any damage to the tape during the cabling and the handling operations. However in this configuration, the former cannot be used to transport the current. This triggered a new approach in cable design involving round 2G HTS wires with a diameter from 1 to 2 mm. They can be assembled to build a small cable core with a high ampacity. The benefits of this new wire design on power cables are discussed. The first results on different manufacturing processes and the characterizations of the 2G HTS wires are presented.