This chapter discusses the recent observations of the impact of surface effects on the optical properties of GaN-based NWs. It focuses on three aspects: (1) the existence of specific exciton and donor quantum states related to surfaces; (2) the intrinsic non-radiative activity of surfaces; (3) the impact on electron-hole pair recombination dynamics of the physical and chemical environment of the NWs. The non-radiative recombination at the surface of nitride NWs is generally believed to occur and that the band bending resulting from the pinning of Fermi level on surface states is generally believed to control access of carriers to those surface states, whereas it would also efficiently separate photo-generated electrons and holes. In particular, holes are believed to be driven toward the surface. The photochemical activity of this surface, most certainly related to photocarrier transport from and toward surface states, has strong effects on the optical properties of the NWs.

Crystal-phase engineering has emerged as a novel method of bandgap engineering, made feasible by the high surface-to-volume ratio of nanowires. There remains intense debate about the exact characteristics of the band structure of the novel crystal phases, such as wurtzite GaAs, obtained by this approach. We attack this problem via a low-temperature angle-dependent magneto-photoluminescence study of wurtzite/zinc-blende quantum disks in single GaAs nanowires. The exciton diamagnetic coefficient is proportional to the electron-hole correlation length, enabling a determination of the spatial extent of the exciton wave function in the plane and along the confinement axis of the crystal-phase quantum disks. Depending on the disk nature, the diamagnetic coefficient measured in Faraday geometry ranges between 25 and 75 μeV/T2. For a given disk, the diamagnetic coefficient remains constant upon rotation of the magnetic field. Along with our envelope function calculation accounting for excitonic effects, we demonstrate that the electron effective mass in wurtzite GaAs quantum disks is heavy, mostly isotropic and results from mixing of the two lower-energy conduction bands with Γ7 and Γ8 symmetries. Finally, we discuss the implications of the results of the angle dependent magneto-luminescence for the likely symmetry of the exciton states. This work provides important insight in the band structure of wurtzite GaAs for future nanowire-based polytypic bandgap engineering.

Long-lived indirect excitons, artificially produced by applying a bias to arsenide double quantum wells (DQWs), are currently the subject of intense investigations. The separation of electron and hole wave functions across the central barrier indeed yields a situation where the recombination time can be made much larger than other relaxation and interaction times between the excitons, to be considered, here, as composite bosonic particles. For example, exciton rings have been observed away from the excitation spot and their formation mechanism and instabilities were described and explained [1]. The current trend of research is the spatial manipulation of such artificial dipolar particles [2], the formation of condensates of them using traps [3] and the study of their coherence properties [4]. Due to the presence of strong internal electric fields, group-III nitride polar single quantum wells naturally induce long-lived excitons. The latter have, in addition, much larger binding energies, smaller Bohr radii and larger total effective masses than their counterparts in arsenide DQWs. Therefore, they appear to be good candidates for the study of dense gases of dipolar exciton, with expected higher temperature of quantum degeneracy. In this context, we present spatially-resolved and time-resolved photoluminescence experiments on GaN/(Al,Ga)N QWs, showing exciton radiative lifetimes of tens of microseconds. We study in detail the dynamics and range of their relaxation/diffusion along the growth plane, as a function of temperature. We discuss the results in terms of the potentiality of these systems for the creation and manipulation of dipolar exciton condensates. [1] L.V. Butov et al. Phys. Rev. Lett. 92, 117404 (2004). [2] A.G. Winbow, Phys. Rev Lett. 106, 196806 (2011). [3] A.T. Hammack et al., Phys. Rev. Lett. 96, 227402 (2006). [4] A.A. High et al., Nature 483, 584 (2012).

GaN nanowires (or nanocolumns) constitute a fascinating case of unstrained, nearly defect-free semiconductor systems. Their diameters (20-100 nm) are not small enough to induce 1D quantum confinement. Nevertheless some of their properties are clearly modified by the nanometric sizes. I will present the current state of understanding of (maybe) the most unimportant but certainly the most elusive -therefore interesting- of those properties : The vast majority of photoluminescence spectra of GaN nanowires reported in the literature present a doublet near 3.45 eV which is not observed (at least not with such an intensity) in bulk or epitaxial GaN. Since its first detailed observations in 2000, most discussions on this doublet tend to discard a number of hypotheses, but nearly none has ever risked any specific explanation, except that it is "most probably related to some surface defect". Yet a quite simple mechanism, involving the physics of donor-bound excitons has been suggested in 2009 by myself and co-workers at EPFL and I have gathered in the past years a number of quite supporting experimental and theoretical elements, if not definitive proof. I will propose these elements for an open discussion, along with the results of attempts by other groups to refute our hypothesis.

ZnCdO nanowires with up to 45% Cd are demonstrated showing room temperature photoluminescence (PL) down to 2.02 eV and a radiative efficiency similar to that of ZnO nanowires. Analysis of the microstructure in individual nanowires confirms the presence of a single wurtzite phase even at the highest Cd contents, with a homogeneous distribution of Cd both in the longitudinal and transverse directions. Thermal annealing at 550 C yields an overall improvement of the PL, which is blue-shifted as a result of the homogeneous decrease of Cd throughout the nanowire, but the single wurtzite structure is fully maintained.

Time-resolved photoluminescence of high-aluminum-content (Al,Ga)N quantum wells and epilayers emitting in the 215-255 nm range.

We compute by envelope function calculations the binding energy EB of donor atoms in thin slabs of semiconductor bounded by air, accounting for the dielectric mismatch between air and the semiconductor. We detail how EB depends on the donor-site and on the thickness of the slab. We show that due to the competition between surface and dielectric mismatch effects, EB does not monotonically decrease from the center to the surface of the nanostructures. Finally, we discuss our results in regard to recent photoluminescence experiments performed on ensemble and single GaN nanowires.