Light emission in polar group-III nitride quantum wells (QWs) optically pumped at densities significantly below the lasing threshold is usually interpreted in terms of excitons. Whereas thin GaN QWs with fast radiative lifetimes are employed in light-emitting devices, the excitons in thick QWs (typ. 7nm) are characterized by a non-zero dipole moment and long radiative lifetimes, due to the built-in electric field. These features result in many interesting properties of dipolar excitons: they can propagate over large distances and thus cool down to the lattice temperature before recombination, offering the possibility for studies of cold and dense gas of interacting bosons. In this context, two important issues must be addressed: the strength of the exciton-exciton interaction, and the excitonic Mott transition, that sets the maximum density of excitons. Combining spatially and time-resolved photoluminescence in different excitation geometries, we provide a set of experimental data showing that the emission linewidth is proportional to the emission energy in a wide range of carrier densities and temperatures. The proportionality coefficient is almost temperature independent. These results are compared to the existing data in various material systems and discussed in terms of the Mott transition and its criteria, exciton-exciton interaction and its possible contribution to the linewidth, as well as the role of disorder in the density-dependent emission spectra.

Excitons in quantum wells (QWs) based on group-III nitrides or ZnO are naturally indirect in real space, due to the strong internal electric field that exists along the growth axis. Within such excitons, the electron and hole are spatially separated, leading to (1) strong dipole moments and (2) long radiative lifetimes. Extensive studies of indirect excitons (IXs) in GaAs-based heterostructures have shown that a combination of these two features results in many interesting properties of IXs : they can propagate over large distances, they can be controlled in-situ by light or by external gate voltage, they can cool down to the lattice temperature before recombination, and form cold and dense gases of interacting bosons. Compared to traditional IXs in arsenide heterostrutures, IXs in GaN- or ZnO-based QWs have much larger binding energies and smaller Bohr radii. This allows exploring IX propagation up to room temperature, and over a much larger density range. Using spatially- and time-resolved photoluminescence experiments, we investigate the exciton transport in several QW samples based on GaN and ZnO. The IX emission is imaged, with spectral and temporal resolutions, along the sample plane. Therefore the corresponding spatial density profiles are obtained and monitored as a function of (i) temperature, (ii) the exciton density at the excitation spot, (iii) the substrate material and (iv) the excitation regime (continuous vs pulsed excitation). We provide a comprehensive analysis of the data combined with numerical modeling, and we show that the efficient propagation of IXs takes place in the high density regime where the in-plane disorder is efficiently screened by the dipole-dipole interaction. Under these latter conditions, exciton mobility is almost temperature-independent from 10K up to room temperature. This suggests that exciton-exciton interaction is by far the dominant scattering mechanism, compared to scattering by the interface disorder. However, exciton transport is maintained up to 300K only in the GaN/AlGaN QW that was epitaxially grown on a GaN substrate, since nonradiative processes dominate the transport in the case of a sapphire substrate, for which high densities of threading dislocations are present. We provide a detailed understanding of the physical mechanisms of IX transport and its temperature and density dependence, and we show that IXs in wide band-gap QWs constitute a promising system for the formation of collective bosonic states in semiconductors. Financial support : projects "INDEX" (FP7 PITN-GA-2011-289968) and "OBELIX" (ANR-15-CE30-0020-02).

In recent years indirect (dipolar) excitons have emerged as highly interesting objects for fundamental studies like Bose-Einstein condensation as well as a concept of excitonic devices for development of excitonic signal processing. An indirect exciton (IX) is a bound pair of an electron and a hole (e-h) in spatially separated quantum wells (QWs). Studies of IX have been concentrated almost entirely on (Al,Ga)As/GaAs coupled double QWs (CDQWs). Due to the large exciton binding energy polar heterostructures of Group III nitrides seem to be more suitable for observation of IX. In the present work we have studied IXs formed by a e-h pair in InGaN/GaN CDQWs. Very thin barrier of 1-2 atomic monolayer (ML) separating QWs enables a tunneling of electrons and holes after optical excitation to the adjacent QWs. The samples consisted of DQWs of In0.17Ga0.83N/In0.02Ga0.98N grown by MBE technique. Intended width of QWs was 2.6 nm and the barrier thickness varied between 1- 8 ML (025-2nm). GaN crystals were used as substrates. The XRD and TEM measurements showed that the obtained structures agree well with the intended ones. Modeling of their band structure using the NEXTNANO software revealed that the structures with the thinnest barriers (1-2 ML) formed one electron (hole) state located in the conduction (valence) band of adjacent QWs. The corresponding emission wavelength was λ≈510 nm. Simulations performed for structures with the thicker barriers demonstrated presence of e- and h-states in each QW and the emission at λ≈450nm. PL measured in these samples showed such an emission. For structures with 1-2 ML barrier we found a peak at λ≈505-520nm which we associated with IX in the system of CDQW. To prove strongly indirect character of the long-wavelength PL-band we performed TRPL characterization of the studied structures. At T=4K, the band at λ≈510nm showed the decay time, τD, approaching msec range. Whereas the band at λ≈450nm is characterized by τD of about 100 ns.

Excitons in nitride quantum wells (QWs) are naturally indirect due to the strong internal electric field: electron and hole within such excitons are spatially separated, leading to strong dipole moments and long radiative lifetimes. Extensive studies of indirect excitons (IXs) in GaAs-based heterostructures have shown that a combination of these two features results in many interesting properties of IXs: they can propagate over large distances, can be controlled in-situ by light and external gate voltage, cool down to the lattice temperature before recombination, and form cold and dense gas of interacting bosons. Compared to traditional IXs in arsenide heterostrutures, IXs in GaN QWs have much larger binding energies and smaller Bohr radii. This allows exploring IX propagation up to room temperature, and over a much larger density range. Using spatially- and time-resolved photoluminescence experiments, we have investigated the exciton transport in a 7 nm-wide GaN single QW sandwiched between Al0.19Ga0.81N barriers. The IX emission is imaged, with spectral and temporal resolutions, along the sample plane. Therefore the corresponding spatial density profiles are obtained and monitored as a function of (i) temperature, (ii) the exciton density at the excitation spot, (iii) the substrate material (GaN vs Sapphire) and (iv) the excitation regime (continuous vs pulsed excitation) [F. Fedichkin et al., Phys. Rev. B 91, 205424 (2015)]. We provide a comprehensive analysis of the data combined with numerical modeling, and we show that the efficient propagation of IXs takes place in the high density regime where the in-plane disorder is efficiently screened by the dipole-dipole interaction. Under these latter conditions, exciton mobility is almost temperature-independent from 10K up to room temperature. This suggests that exciton-exciton interaction is by far the dominant scattering mechanism, compared to scattering by the interface disorder. However, the exciton transport is maintained up to 300K only in the QW that was epitaxially grown on a GaN substrate, since nonradiative processes dominate the transport in the case of a sapphire substrate, for which high densities of threading dislocations are present.We provide a detailed understanding of the physical mechanisms of IX transport and its temperature and density dependence, and we show that nitride-hosted IXs constitute a promising system for the formation of collective bosonic states in semiconductors. Financial support: projects INDEX (FP7 PITN-GA-2011-289968) and OBELIX (ANR-15-CE30-0020-02).

Excitons in polar group-III nitride quantum wells (QWs) are naturally indirect, because electron and hole within such excitons are spatially separated. As a consequence, they have strong dipole moments and long radiative lifetimes. Extensive studies of indirect excitons (IXs) in GaAs-based heterostructures have shown, that a combination of these two features results in many interesting properties of IXs: they can propagate over large distances, can be controlled in-situ by light and external gate voltage, cool down to the lattice temperature before recombination, and form cold and dense gas of interacting bosons. Compared to traditional IXs in arsenide heterostrutures, IXs in nitride QWs have much larger binding energies and smaller Bohr radii. This allows exploring IX propagation up to room temperature, and over two orders of magnitude higher density range. Using spatially- and time-resolved photoluminescence experiments we examined temperature, density, substrate material (GaN vs Sapphire) and excitation regime (cw vs pulsed excitation) dependence of the exciton transport in 7 nm wide GaN QW sandwiched between Al0.19Ga0.81N barriers, the structure is characterized by internal electric field of order of 1 MV/cm. Comprehensive analysis of the data combined with numerical modeling show that in GaN QWs efficient propagation of IX takes place in the high density regime where the in-plane disorder is efficiently screened by the dipole - dipole interaction. Under these conditions, exciton mobility is almost temperature-independent from 10 K up to room temperature. This suggests that exciton-exciton interaction is by far dominant scattering mechanism, compared to scattering on the interface disorder. In this context, we argue that nitride-hosted IXs constitute a promising system for formation of collective bosonic states in semiconductors.

We report on the exciton propagation in polar ðAl; GaÞN=GaN quantum wells over several micrometers and up to room temperature. The key ingredient to achieve this result is the crystalline quality of GaN quantum wells grown on GaN substrate that limits nonradiative recombination. From the comparison of the spatial and temporal dynamics of photoluminescence, we conclude that the propagation of excitons under continuous-wave excitation is assisted by efficient screening of the in-plane disorder. Modeling within drift-diffusion formalism corroborates this conclusion and suggests that exciton propagation is still limited by the exciton scattering on defects rather than by exciton-exciton scattering so that improving interface quality can boost exciton transport further. Our results pave the way towards room-temperature excitonic devices based on gate-controlled exciton transport in wide-band-gap polar heterostructures.