The beam heat load and the pressure in the vacuum chamber of the cold bore superconducting undulator installed at ANKA (ANgstrom source KArlsruhe) have been monitored for almost two years. Possible sources of the observed heat load could be synchrotron radiation from upstream magnets, image currents, electron and ion bombardment. In this paper, the various possible contributions to the heat load are discussed and compared with experimental results. The dynamic pressure increases nonlinearly with the average beam current. The current where it assumes a maximum varies both with the bunch intensity and with the initial vacuum pressure. A correlation between the heat load and the dynamic pressure has been observed. This study suggests that electron bombardment could explain the beam heat load and pressure rise observed for a bunch length of 10 mm.
In this contribution the authors investigate the temperature-dependent conduction band structure of GaAs1−xNx for different nitrogen contents. An analysis of their experimental photoreflectance data based on the two-band version of the band anticrossing model shows that with decreasing temperature the energy of the effective nitrogen level EN in GaAsN epilayers shifts significantly to higher energies. Simultaneously, the coupling parameter CNM between the nitrogen states and the host conduction band also rises to higher values.
InAs quantum dot (QD) layers grown by molecular-beam epitaxy were investigated by transmission electron microscopy (TEM) and photoluminescence (PL) spectroscopy. To achieve the highest possible In concentration in the QDs, InGaAs (instead of GaAs) cap layers with different In concentrations were deposited after the growth of the InAs QD layer. We combine different TEM techniques to determine the shape, size, and composition of the QDs. By applying a post-processing procedure, we are able to reconstruct the In concentration in the QDs which is measured too low in TEM due to the embedding of the QDs in material with lower In concentration and averaging along the finite TEM sample thickness. The determination of the composition of the layers on an atomic scale shows that the In concentration in the QDs increases in growth direction and reaches values up to 90%. Redistribution of indium during the InGaAs cap layer growth leads to a decrease of the In concentration in the cap layer with respect to the nominal In concentration. The observed redshift of the PL peak with increasing In concentration in the cap layer is attributed to the enlargement of island size and the change of the strain in the QD layers.
We use room-temperature photoreflectance spectroscopy to investigate the influence of indium on the electronic structure of Ga 1Ϫx In x N y As 1Ϫy /GaAs multiple quantum wells. To fit our experimental data, a semiempirical theoretical model based on the band anticrossing Hamiltonian is successfully applied. Thus, we can extract some information about the Hamiltonian, in particular, the dependence of the coupling parameter C NM on the In concentration in GaInNAs. C NM is shown to decrease with increasing indium mole fraction, confirming theoretical predictions.
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