We have investigated the temperature dependence of photoluminescence ͑PL͒ properties of a number of self-organized InAs/GaAs heterostructures with InAs layer thickness ranging from 0.5 to 3 ML. The temperature dependence of InAs exciton emission and linewidth was found to display a significant difference when the InAs layer thickness is smaller or larger than the critical thickness around 1.7 ML. The fast redshift of PL energy and an anomalous decrease of linewidth with increasing temperature were observed and attributed to the efficient relaxation process of carriers in multilayer samples, resulting from the spread and penetration of the carrier wave functions in coupled InAs quantum dots. The measured thermal activation energies of different samples demonstrated that the InAs wetting layer may act as a barrier for the thermionic emission of carriers in high-quality InAs multilayers, while in InAs monolayers and submonolayers the carriers are required to overcome the GaAs barrier to escape thermally from the localized states. ͓S0163-1829͑96͒06440-5͔Spontaneous formation of three-dimensional ͑3D͒ islands during Stranski-Krastanov-like growth of highly strained InAs layers on GaAs substrates by molecular-beam epitaxy ͑MBE͒ has been proposed as a promising way for fabricating high-quality InAs quantum dots ͑QD's͒ in GaAs.1-6 When the thickness of an InAs layer is beyond a critical thickness of around 1.7 ML, the structure is usually composed of an InAs wetting layer and conelike InAs islands deposited on it. The density, size distribution, uniformity, and coverage of such InAs islands were found to be growth condition dependent, and have been investigated by TEM and atomic force microscopy ͑AFM͒ in recent years. [7][8][9] The optical studies 10-13 revealed its excellent radiative recombination, which usually gives a broadband with a reported full width at half maximum ͑FWHM͒ in the range of 50-130 meV. Recent work by Lubyshev et al. 13 presented the unusual temperature dependence of exciton energy in InAs multilayer structures. An anomalous decrease of the FWHM was detected and explained in terms of the tunneling process between InAs dots.In this paper, we studied the exciton relaxation and thermal activation in InAs multilayer structures via the analysis of cw photoluminescence ͑PL͒ data under different temperatures. It is found that the temperature dependence of the exciton energy and linewidth is significantly different from that obtained in InAs monolayers and submonolayers. The unusual temperature behavior in InAs multilayers is associated with the relaxation effect of carriers, resulting from the spread and penetration of the wave functions of carriers in coupled InAs QDs. In the study of the thermal activation process, we found that potential barriers for InAs excitons to escape thermally from the localized states are different for different structures. For high-quality InAs multilayers, the barrier could be the wetting layer, while in the case of monolayers or submonolayers the carriers are required to ove...

Tungsten transport is investigated in WEST long pulse L-mode plasmas operated with the strike point on the actively cooled upper tungsten divertor. The pulses are mostly heated by lower hybrid waves. It is experimentally found that tungsten does not centrally accumulate throughout these ∼ 30 s reproducible discharges despite large normalised electron density gradients R/Ln e. To explain these observations, turbulent and neoclassical transport of electrons and tungsten ions are computed with GKW [1] and NEO [2, 3] respectively. Additionally, interpretative integrated modelling simulations are also performed to keep data coherency despite the lack of measurements of some quantities such as the Ti profiles. Modelled R/Ln e are found consistent with interferometry inversions and the tungsten peaking factor R/Ln W remains comparable to R/Ln e due to dominant turbulent diffusivities inside r/a = 0.3−0.8. In the central region r/a < 0.3 neoclassical W transport dominates but the convective velocities are several order of magnitudes lower compared to plasmas with toroidal rotation velocities induced by a neutral beam injection (NBI) torque. Finally, nitrogen is seeded in these pulses leading to an enhanced energy content which is consistent with stabilised ion temperature gradient modes from dilution.

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