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While increasing the Cd content in ZnCdO can lead to a very large nominal change in bandgap, incorporating high Cd concentrations has shown to be extremely difficult and can enhance the formation of different types of defects. Indeed, the tendency that Cd has to segregate and form clusters can yield alloy disorder which in turn can impact the carrier recombination paths, thus limiting the total radiative efficiency. In order to analyze the different recombination mechanisms and their dynamics we have used time-resolved photoluminescence (TRPL) as a function of temperature in dense ensembles of ZnCdO nanowires grown by MOCVD [1]. The nanowires were 1-3 µm-long and 100-200 nm-wide and contain different Cd contents, ranging from 0 to 45%, and were analyzed as-grown and after exposure to different rapid thermal annealing (RTA) cycles [2]. The comparison between the decay time of as-grown and thermally annealed ZnO nanowires revealed a higher lifetime after annealing, providing clear evidence of a reduction of non-radiative channels. In the case of the ternary ZnCdO nanowires, the carrier lifetime is larger than in the Cd-free case. This larger lifetime may be explained to arise from the localization of the photogenerated excitons at potential fluctuations produced by alloy disorder. As in the case of ZnO, annealing also yields a higher carrier lifetime in ZnCdO, indicative of the removal of non-radiative recombination paths. Temperature-dependent TRPL confirmed this exciton localization behaviour in the ternary nanowires. Indeed radiative emission was observed at higher temperatures in the ternaries, especially after thermal annealing, due to the enhanced carrier lifetime. In the case of the ZnO, even after thermal annealing where the non-radiative paths have been largely removed, increasing the temperature produces a rapid collapse of the PL likely due to the absence of exciton localization, as in the case of the ZnCdO nanowires. References: [1] K. Yamamoto, T. Tsuboi, T. Ohashi, T. Tawara, H. Gotoh, A. Nakamura, and J. Temmyo, J. Cryst. Growth 312, 10 (2010). [2] T. Ohasi, K. Yamamoto, A. Nakamura and J. Temmyo, J. J. of Applied Physics, vol.47, pp.2961 (2008).

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TRANSITION FROM STRONG COUPLING TO EXCITONIC LASING IN A ZNO MICROCAVITY

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Ref Arxiv: 1106.0183
DOI: 10.1063/1.3593032
WoS: 000291041600005
Ref. & Cit.: NASA ADS
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23 Citations
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The lasing operation of a ZnO planar microcavity under optical pumping is demonstrated from T=80 K to 300 K. At the laser threshold, the cavity switches from the strong coupling to the weak coupling regime. A gain-related transition, which appears while still observing polariton branches and, thus, with stable excitons, is observed below 240K. This shows that exciton scattering processes, typical of II-VI semiconductors, are involved in the gain process.

Ref HAL: hal-00437967_v1
DOI: 10.1103/PhysRevB.80.033303
WoS: 000268617800017
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109 Citations
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We study localized surface plasmons in strong coupling with excitons of organic semiconductors. The plasmons are localized in lithographed silver nanodisks organized in square array. A giant Rabi splitting energy of 450 meV is obtained, and typical behaviors of mixed states, i.e., anticrossing of their energies and crossing of their linewidths, are observed. Three-dimensional finite-difference time-domain simulations and coupled oscillator calculations are used to analyze and corroborate the experimental results.

Ref HAL: hal-00390003_v1
DOI: 10.1063/1.3062742
WoS: 000262534100013
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63 Citations
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A detailed study of excitons in unstrained GaN nanocolumns grown by plasma assisted molecular beam epitaxy on silicon substrates is presented. The time-integrated and time-resolved photoluminescence spectra do not depend significantly on the (111) or (001) Si surface used. However, an unusually high relative intensity of the two-electron satellite peak of the dominant donor-bound exciton line is systematically observed. We correlate this observation with the nanocolumn morphology determined by scanning electron microscopy, and therefore propose an interpretation based on the alteration of wave functions of excitonic complexes and of donor states by the proximity of the semiconductor surface. This explanation is supported by a model that qualitatively accounts for both relative intensities and time decays of the photoluminescence lines.

Ref HAL: hal-00354869_v1
DOI: 10.1002/adfm.200801387
WoS: 000263640600018
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22 Citations
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A new fluorescent silsequioxane precursor with tuned optical properties and controlled aggregation properties is designed. The two cyclohexyl moieties introduced in the molecular structure allow the formation of very good quality films. The J-aggregated structure is transcribed into the solid by photoacidcatalyzed hydrolysis–polycondensation. Aggregation of the chromophores is reducedandhighlyfluorescentmaterials are obtained.The photoacid generator liesonthe surface of thehomogeneouslayer of the sol–gelprecursor. Thisphase separation presents several advantages, including UV protection of the chromophore and easy removal of the PAG. The first example of chemical amplification in the photolithography of the conjugated silsesquioxane precursor is demonstrated. As hydrolysis–polycondensation could be achieved in a controlled way by UV exposure, chemically amplified photolithography is achieved by irradiating a composite film (_110nm thick) on silicon wafer by using a copperTEMgrid as shadowmask. Thepattern is produceduniformly on a miscroscopic scale of 3 mm, the photopatterned pixels remaining highly fluorescent.Thesizes of the photolithographedpixels correspondto the sizes of the rectangular holes of the 300_75 mesh grid (hole: 63 mm_204 mm).