Ref HAL: hal-00554481_v2
Ref Arxiv: 1101.2078
DOI: 10.1364/OL.36.002203
WoS: 000291722100010
Ref. & Cit.: NASA ADS
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56 Citations
Résumé:

We compare the quality factor values of the whispery gallery modes of microdisks incorporating GaN quantum dots (QDs) grown on AlN and AlGaN barriers by performing room temperature photoluminescence (PL) spectroscopy. The PL measurements show a large number of high Q factor (Q) resonant modes on the whole spectrum which allows us to identify the different radial mode families and to compare them with simulations. We report a considerable improvement of the Q factor which reflect the etching quality and the relatively low cavity loss by inserting QDs into the cavity. GaN/AlN QDs based microdisks show very high Q values (Q > 7000) whereas the Q factor is only up to 2000 in microdisks embedding QDs grown on AlGaN barrier layer. We attribute this difference to the lower absorption below bandgap for AlN barrier layers at the energies of our experimental investigation.

Ref HAL: hal-00545172_v1
DOI: 10.1063/1.3056657
WoS: 000262970900019
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10 Citations
Résumé:

We present experimental results obtained on wurtzite epitaxial GaN layers grown on sapphire and SiC substrates. Thanks to a set of samples with different values of the residual strain, we demonstrate that the high dislocation density enhances the spin relaxation rate through the Elliott-Yafet mechanism. This fact is validated by the T-1 temperature dependence of the spin-relaxation times. The influence of the valence-band structure on the hole-spin relaxation is also highlighted. In particular, a decrease in the hole-spin relaxation rate, accompanied by a strong polarization rate (similar to 50%) of the differential reflectivity signal (Delta R/R), is observed when the splitting Delta E-AB between the heavy-hole and the light-hole bands is larger than the broadening Gamma(A) of the A excitonic transition. On the contrary, the overlap of the A and B resonances for Gamma(A)>Delta E-AB is responsible for a decrease in the Delta R/R polarization rate (similar to 10%) and an enhancement of the spin relaxation rate.

Ref HAL: hal-00545170_v1
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Résumé:

The present study focused on ZnO thin films fabricated by sol-gel process and spin coated onto Si (100) and quartz substrates. ZnO thin films have a hexagonal wurtzite structure with a grain diameter about 50nm. Optical properties were determined by photoluminescence (PL) and absorption spectroscopy. The absorption spectrum is dominated by a sharp excitonic peak at room and low temperatures. At room temperature, two transitions were observed by PL One near to the prohibited energy band in ultraviolet (UV) region and the other centered at 640 nm, characteristic of the electronic defects in the band-gap. The spectrum at 6 K is dominated by donor-bound exciton lines and donor-acceptor pair transition. LO-phonon replica and two-electron satellite transitions are also observed. These optical characteristics are a signature of high-quality thin films. (C) 2008 Elsevier Ltd. All rights reserved.

Ref HAL: hal-00545165_v1
DOI: 10.1063/1.3197035
WoS: 000269850300034
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13 Citations
Résumé:

The carrier recombination processes and the relaxation dynamics of spin-polarized excitons in cubic epitaxial GaN have been investigated by time-resolved photoluminescence and pump-probe spectroscopy experiments. We evidence a very fast exciton spin relaxation tau(S)similar to 0.3 ps that is influenced by both the high defect concentration and the strong electron-hole exchange interaction. Spectral oscillatory features of the transient reflectivity are observed for negative delays that allow us to determine the exciton optical dephasing time T-2.

Ref HAL: hal-00545164_v1
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By performing non-degenerate pump-probe experiments, we study the relaxation dynamics of spin-polarized excitons in wurtzite epitaxial gallium nitride (GaN). Characteristic times of the different spin-relaxation channel (electron, heavy- and light- or split-off-hole spin flips) for different samples as a function of temperature are extracted. We show that the high dislocation density increases the spin relaxation of electrons and holes through the defect-assisted Elliot-Yafet mechanism. (C) 2009 Elsevier B.V. All rights reserved.

Ref HAL: hal-00545158_v1
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We performed time- and polarization-resolved pump-and-probe experiments on several differently strained GaN wurtzite epilayers.. Thanks to a selective spectral excitation of spin-polarized A-excitons at various temperatures and a fitting procedure considering the different A and B spin exciton states, we show that the Elliott-Yafet mechanism is the dominant spin-relaxation process. It origins from the high dislocation density in all of our GaN epilayers.

Ref HAL: hal-00389907_v1
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