E. Dimakis

Biaxial strain and lattice constants of InN (0001) films grown by plasma-assisted molecular beam epitaxy

We present a systematic study, using high resolution x-ray diffraction, of the in-plane a and out-of-plane c lattice parameters of high quality InN films grown by molecular beam epitaxy on GaN∕Al2O3 (0001) substrates. It is found that their values are dependent on the nucleation and growth conditions. Films nucleated in a two- or three-dimensional growth mode exhibit biaxial compressive or tensile strain, respectively. The linear dependence of c on a is consistent with biaxial strain being present in the films. A biaxial strain relaxation coefficient of 0.43±0.04 is deduced. The values of the lattice constants for the case of strain-free InN are estimated to be in the ranges c=5.699±0.004A and a=3.535±0.005A.

Coaxial multishell (In,Ga)As/GaAs nanowires for near-infrared emission on Si substrates.

Efficient infrared light emitters integrated on the mature Si technology platform could lead to on-chip optical interconnects as deemed necessary for future generations of ultrafast processors as well as to nanoanalytical functionality. Toward this goal, we demonstrate the use of GaAs-based nanowires as building blocks for the emission of light with micrometer wavelength that are monolithically integrated on Si substrates. Free-standing (In,Ga)As/GaAs coaxial multishell nanowires were grown catalyst-free on Si(111) by molecular beam epitaxy. The emission properties of single radial quantum wells were studied by cathodoluminescence spectroscopy and correlated with the growth kinetics. Controlling the surface diffusivity of In adatoms along the NW side-walls, we improved the spatial homogeneity of the chemical composition along the nanowire axis and thus obtained a narrow emission spectrum. Finally, we fabricated a light-emitting diode consisting of approximately 10(5) nanowires contacted in parallel through the Si substrate. Room-temperature electroluminescence at 985 nm was demonstrated, proving the great potential of this technology.

Enhanced near-green light emission from InGaN quantum wells by use of tunable plasmonic resonances in silver nanoparticle arrays

Two-dimensional arrays of silver nanocylinders fabricated by electron-beam lithography are used to demonstrate plasmon-enhanced near-green light emission from nitride semiconductor quantum wells. Several arrays with different nanoparticle dimensions are employed, designed to yield collective plasmonic resonances in the spectral vicinity of the emission wavelength and at the same time to provide efficient far-field scattering of the emitted surface plasmons. Large enhancements in peak photoluminescence intensity (up to a factor of over 3) are measured, accompanied by a substantial reduction in recombination lifetime indicative of increased internal quantum efficiency. Furthermore, the enhancement factors are found to exhibit a strong dependence on the nanoparticle dimensions, underscoring the importance of geometrical tuning for this application.

Physical model of InN growth on Ga-face GaN (0001) by molecular-beam epitaxy

A consistent physical model of the growth of InN on GaN (0001) by radio-frequency plasma-assisted molecular-beam epitaxy is presented. Four distinct regimes of InN growth are observed due to the temperature dependence of indium adatoms’ mobility and of the InN decomposition rate. At substrate temperatures higher than 450°C, indium adatoms are highly mobile and a self-regulating mechanism of InN islands’ diameter takes place, so that a stoichiometric N:In atomic ratio on the top face of the islands is established. As a result, two-dimensional growth is possible only with In∕N atomic ratio on the substrate surface equal to unity. The self-regulating mechanism could be exploited to engineer self-organized nanostructures.

Misfit accommodation of compact and columnar InN epilayers grown on Ga-face GaN (0001) by molecular-beam epitaxy

The interfacial structural properties of compact InN films and of noncoalesced three-dimensional InN islands, grown by molecular-beam epitaxy on Ga-face GaN/Al2O3 (0001) substrates, were investigated by transmission electron microscopy. Compact film growth was accomplished employing an InN nucleation layer, grown at low substrate temperatures. A 60° misfit dislocation network effectively accommodated the lattice mismatch in the InN/GaN interface in both cases of epilayers. The lattice constants of InN were determined by electron diffraction analysis, revealing a 0.28% larger in-plane parameter of the compact InN film relative to the corresponding lattice parameter of the InN islands. This is attributed to thermal tensile strain developed during post-growth cooling down of the epilayers, which also compensated the remaining compressive strain originating from the in-plane lattice mismatch of InN and GaN.

Control of the polarity of molecular-beam-epitaxy-grown GaN thin films by the surface nitridation of Al2O3 (0001) substrates

The nitridation of Al2O3 (0001) substrate surfaces by radio-frequency nitrogen plasma has been investigated. A 1.5-nm-thick surface nitride layer occurred for 100 min nitridation at high substrate temperature, while the nitridation appeared to be limited to a surface atomic plane at low temperature. In-plane lattice constant relaxation was observed in both cases. A high nitridation temperature resulted into a Ga face and a low temperature to N-face polarity of overgrown GaN films. However, low temperature nitridation and an AlN buffer layer also produced a Ga-face polarity. The results are consistent with low formation energy of AlN/sapphire films with Ga-face polarity.

Plasmon-enhanced light emission based on lattice resonances of silver nanocylinder arrays

Diffractive arrays of silver nanocylinders are used to increase the radiative efficiency of InGaN/GaN quantum wells emitting at near-green wavelengths. Large enhancements in luminescence intensity (up to a factor of nearly 5) are measured when the array period exceeds the emission wavelength in the semiconductor material. The experimental results and related numerical simulations indicate that the underlying mechanism is a strong resonant coupling between the light-emitting excitons in the quantum wells and the plasmonic lattice resonances of the arrays. These excitations are particularly well suited to light-emission-efficiency enhancement, compared to localized surface plasmon resonances at similar wavelengths, due to their larger scattering efficiency and larger spatial extension across the sample area.

Plasma-assisted MBE growth of quaternary InAlGaN quantum well heterostructures with room temperature luminescence

We report on the growth by rf plasma-assisted molecular beam epitaxy (RF-MBE) of high quality quatemary In_xAl_1-xGa_1-x-yN/GaN quantum well (QW) heterostructures, showing room temperature photoluminescence and lasing under optical pumping.

Shell-doping of GaAs nanowires with Si for n-type conductivity

We demonstrate the potential of using Si as n-type dopant in GaAs nanowires grown by molecular beam epitaxy. The amphoteric behavior of Si that typically accompanies the vapor-liquid-solid growth mode is adequately controlled when a shell doping scheme is utilized instead, i.e. when a Si-doped GaAs shell layer is grown conformally around the undoped GaAs nanowire core in the vapor-solid mode. The incorporation site of Si was evaluated by Raman spectroscopy, and correlated with the growth conditions of the doped shell. In that way, we identified a growth window that ensures the incorporation of Si as donor, and obtained donor concentrations up to 1 × 1019 cm−3, with the compensation level by Si acceptors remaining below 10%. Finally, resistivity measurements on planarized shell-doped nanowire ensembles were employed to probe the doping efficiency and the surface depletion of free-carriers. The achievement of n-type conductivity for nanowires is essential for the realization of functional devices, and is particularly significant when a dopant as well understood and advantageous as Si is employed.Graphical abstract

Plasmon enhanced light emission from InGaN quantum wells via coupling to chemically synthesized silver nanoparticles

Chemically synthesized single-crystal silver nanoparticles are used to demonstrate plasmon enhanced visible light emission from nitride semiconductor quantum wells. For ease of assembly and testing, the nanoparticles are embedded onto the surface of flexible resin films, which are then simply adhered on top of the light emitting samples. Large enhancements in photoluminescence efficiency are correspondingly measured at emission wavelengths near the nanoparticle plasmonic resonance. At the same time, when samples emitting at a sufficiently far detuned wavelength are used, the measured efficiency is not affected by the nanoparticles, which confirms the plasmonic origin of the observed enhancement.