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The excitation of large-amplitude plasma waves by intense few-cycle
laser pulses in thin overdense plasma layers is studied using
one-dimensional particle-in-cell simulation. P-polarized
pulses generate relativistic electron pulses (jets) at the irradiated
surface that penetrate the layer and then oscillate back and forth due
to reflection by self-generated space charge fields building up in
front of the surfaces. Counterpropagating plasmons of large amplitude
are excited and start to emit radiation at 2ωp
and other harmonics of the plasma frequency. The analogy to type III
solar radio emission that is driven by electron bursts from deeper
layers of the solar corona is pointed out; it highlights the present
topic as another example of laboratory astrophysics with lasers.
Characterization and quantitative analysis of stressed states of a series of W/C multilayers (10-40 periods prepared by pulsed laser deposition on Si (111) substrates of different thickness) were carried out by means of X-ray reflectometry, wide angle diffractometry and a novel laser mapping device. As the W/C multilayers were dedicated to technical applications as X-ray optics and subjected to optimization of stacking parameters (thickness and number of layers) for a long term (mechanical) stability also further investigations will be discussed. Comparison of wafer distortion as evaluated by laser scanning and strain of the W layer as deduced from X-ray diffraction let us conclude that W layers are under compressive and C layers under tensile stress. The investigation of the thermally stimulated relaxation behavior of the multilayers provided a confirmation of these results. Additional information could be obtained by comparative relaxation experiments under external mechanical constraints. Furthermore, we report on a self-organized process of structuring of the multilayers under investigation, which might be of interest also from a technical point of view. The entire surface area (diameter 2') could be converted from the smooth (as-deposited) to a structured (relaxed) state stable at room temperature. Investigations using optical and atomic force microscopy showed that the topology of the surface consists of a mountain range where the valleys are on the level of the as-deposited non-debonded surface and that long wrinkled ridges of about the same height run along arbitrary directions.
This article investigates the electronic transport properties of ZnO/ CdS/ Cu(In,Ga)Se2
heterojunction solar cells during and after illumination or forward bias in the dark.
We observe a relaxation of the open circuit voltage under constant illumination as well
as a relaxation
of the voltage drop over the device under constant forward bias
current in the dark. Both phenomena are accompanied by an increase of the sample
capacitance. We introduce a general quantitative model concept for the open circuit
voltage relaxation and related effects in heterojunction devices that explains the
phenomena as a consequence of the persistent capture of charge carriers within the
space charge region. We apply our concept to develop a specific quantitative model for
the observed metastablity in Cu(In,Ga)Se2 heterojunction solar cells.
We present a theoretical study of atomic structures, electrical properties and formation energies for a variety of possible reconstructions with 1×1 and 2×2 periodicity of the GaN(0001) and (0001) surfaces. We find that during MBE growth in the (0001) direction 2×2 structures become stable under N rich growth conditions while Ga rich environment should yield structures with 1×1 periodicity. Considering MBE growth on (0001) surfaces, among the investigated structures only those with 1×1 periodicity are predicted to be stable. During MOCVD growth, where H terminated surfaces may occur, only structures with lx1 periodicity are found to be stable for both growth directions.
Using methyltrichlorosilane diluted with hydrogen and a modified temperature program during the initial stage of the deposition we have improved the deposition process and obtained high quality heteroepitaxial films on Si (100) without any carbon buffer layer. The maximum temperature of 1200°C which is used for the substrate cleaning and film deposition is significantly lower than that used in the conventional deposition system SiH4/C3H8 of about 1400°C for the buffer layer and 1350°C for deposition.
The behaviour of 300 keV Ba ions implanted at room temperature with doses between 1015 and 1017 cm−2 in Mg single crystal and foils was investigated. The results show that the Ba ions do not occupy substitutions sites in Mg, either after the implantation or the annealing treatments. However, pronounced migration of Ba to the surface is observed above 380 °C. The remaining fraction overlaps with the aa-implanted distribution and forms small precipitates. This behaviour is not correlated with the recovery of the Mg lattice which is already complete at about 250 °C. The surface segregation of Ba delays the evaporation of Mg to temperatures near the melting point.