In theory, alloying ZnO with Cd it is possible to tune the bandgap from the visible to the UV spectral regions. The CdO Γ-Γ bandgap in the stable rock-salt phase is ~2.3 eV, but different calculations indicate that the bandgap is much lower in the wurtzite phase, 0.92-0.95 eV [1, 2]. The possibility tocontrol the bandgap also allows enhancing the carrier confinement through the development of heterostructures. Nevertheless, there are some problems related to the Cd solubility in the ZnO structure [3] and to the strong tendency of Cd to diffuse, which hinder the formation of high quality Zn1-xCdxO/ZnOheterostructures.In this work we present a detailed microstructure and optical analysis of high Cd content Zn1-xCdxO/ZnO multiple quantum well (MQW) nanowires grown by remote-plasma-enhanced metal-organic chemical vapor deposition. Six samples with ten Zn1-xCdxO/ZnO wells with widths from 0.7 to 10 nm are demonstrated, showing PL emissions ranging from 3.03 to 1.97 eV. The MQW nanowires with well thickness ≤ 2 nm show high radiative efficiencies compared with the thickest ones, consistent with the presence of quantum confinement. A single wurtzite phase and the MQW periodicity were confirmed by high resolution x-ray diffraction analysis. Finally, in order to quantify the Cd content and distribution along the structure, high resolution electron transmission microscopy and micro energy dispersive X-ray (micro-EDX) analysis were performed in single MQW-nanowires. A significant reduction of the Cd content is observed in the wells due to Cd interdiffusion to the barrier. Interdifussion effects modify the band strcuture and, consequently, the optical properties of the MQW heterostructure. These effects have to be considered in order to have a correct explanation of quantum confinement effects in Zn1-xCdxO/ZnO heterostructures.References[1] A. Schleife, C. Rödl , J. Furthmüller and F. Bechstedt, New J. Phys.,13, 085012 (2011).[2] A. Janotti, D. Segev and C. G. Van de Walle, Phys. Rev. B, 74, 045202 (2006).[3] K. Sakurai, T. Takagi, T. Kubo, D. Kajita, T. Tanabe, H. Takasu, Sz. Fujita and Sg. Fujita J. Cryst. Growth,514, 237 (2002).

In contrast to epitaxial layers, single-crystalline GaN can be grown in the form of nanowires on Si substrates. In addition, the sub-wavelength diameter of nanowires facilitates light extraction, which is advantageous for the realization of efficient nanowire-based light emitting diodes. However, the ionization energies of dopants are different in nanostructures compared to the bulk. This effect may seriously affect the operation of nanowire-baseddevices relying on a controlled doping [1]. First, the combination of surface and dielectric mismatch effects results in a strong and non-monotonic variation of the donor binding energy, when going from the core to the surface of a nanostructure [2]. Second, surface states pin the Fermi level at the surface of the nanowire. This causes the formation of internal electric fields and the depletion of nanowires, as demonstrated by photoconductivity experiments [3]. We emphasize that this depletion is incomplete at low temperature, as donor bound exciton luminescence is observed even for GaN nanowires with a diameter of 30 nm [4].In this work, we investigate the impact of surface and internal electric fields on the binding energy of donor atoms in GaN nanowires. Using Monte-Carlo simulations, we first examine the validity of the assumption that depletion leads to a parabolic potential across the section of nanowires. For instance, we show that for nanowires with a doping of 2 x 1015 cm-3 and with a diameter and a length of 80 nm and 1.5 μm, respectively, the potential cannot be described by a parabola. In contrast, for a doping of the order of 1017 cm-3, the potential is nearly parabolic and is well described using the Poisson equation, where one assumes a spatially homogeneous distribution of dopants.Using envelope function calculations, we then compute the binding energy of donors in thin GaN slabs bounded by air. We take into account surface, confinement, and the dielectric mismatch, similarly to what has been reported in Refs. [2,5]. Based on the evolution of the spatial extent of the electron wave function as a function of the electric field at the nanowire surface, we deduce the magnitude of the electric field required for the ionization of surface donors. In particular, we show that surface donors are ionized for doping densities typically larger than 3 x 1016 cm-3, irrespectively of the nanowire diameter. Finally, the result of our calculations is compared with low-temperature photoluminescence experiments on ensemble of nanowires.[1] M. Pierre et al., Nature Nanotechnology 5, 133, (2010).[2] P. Corfdir and P. Lefebvre, J. Appl. Phys. 112, 106104 (2012).[3] R. Calarco et al., Nano Letters 5, 981 (2005).[4] P. Corfdir et al., J. Appl. Phys. 105, 013113 (2009).[5] P. Corfdir et al., Phys. Rev. B 80, 153309 (2009).

We present spatially- and time-resolved photoluminescence experiments on polar GaN/AlGaN quantum well structures,showing exciton propagation distance of tens of micrometers from generation position and exciton radiative lifetimes oftens of microseconds. We study in detail the dynamics and range of their relaxation/diffusion along the growth plane, asa function of temperature.

The dynamics of the free-exciton capture by boron acceptors and phosphorus donors in diamond is observed in the picosecond range by time-resolved photoluminescence experiments at low temperature. The formation of boron-bound excitons is observed with a delay of 410 ps after the formation of free excitons. For phosphorus, this delay is 120 ps. This is the result of the free-exciton capture by B0 and P0 impurities. The lifetimes of boron- and phosphorus-bound excitons are measured and found to be equal to 270 and 70 ps, respectively. These values are about four orders of magnitude shorter than for the same impurities in silicon. Ei being the ionization energy of dopants, these results scale well with the E4i dependence of the Auger recombination rate expected for bound excitons in indirect band-gap semiconductors.

The aim of this work is to provide an overview on the recent advances in the selective area growth (SAG) of (In)GaN nanostructures by plasma assisted molecular beam epitaxy, focusing on their potential as building blocks for next generation LEDs.The first three sections deal with the basic growth mechanisms of GaN SAG and the emission control in the entire ultraviolet to infrared range, including approaches for white light emission, using InGaN disks and thick segments on axial nanocolumns. SAG of axial nanostructures is developed on both GaN/sapphire templates and GaN-buffered Si(111).As an alternative to axial nanocolumns, section 4 reports on the growth and characterization of InGaN/GaN core-shell structures on an ordered array of top-down patterned GaN microrods. Finally, section 5 reports on the SAG of GaN, with and without InGaN insertion, on semi-polar (11-22) and non-polar (11-20) templates. Upon SAG the high defect density present in the templates is strongly reduced as indicated by a dramatic improvement of the optical properties. In the case of SAG on nonpolar (11-22) templates, the formation of nanostructures with a low aspect ratio took place allowing for the fabrication of high-quality, non-polar GaN pseudo-templates by coalescence of these nanostructures.