ALBERT

Notes: The presence and evolution of traps in undoped semi-insulating (SI) InP obtained by high temperature annealing (900 °C for 90 h) in poor or rich phosphorus atmosphere has been studied by means of photoinduced current transient spectroscopy. Six traps named A1 to A6 having activation energies ranging from 0.2 to 0.6 eV have been detected in three samples submitted to the same annealing process. The samples differ in the Fe concentration of the starting material and the applied phosphorus pressure in the annealing process. A comparison of the corresponding photoinduced current transient spectroscopy spectra shows that among the observed traps, the 0.2 eV one can be related to a phosphorus deficiency, and the 0.3 eV and 0.4 eV traps could be due to an excess of phosphorus during annealing. Moreover, the trap corresponding to iron (0.6 eV) has been observed in all the studied samples. © 1996 American Institute of Physics.

Notes: Operation is demonstrated of a field-effect transistor made of transparant oxidic thin films, showing an intrinsic memory function due to the usage of a ferroelectric insulator. The device consists of a high mobility Sb-doped n-type SnO2 semiconductor layer, PbZr0.2Ti0.8O3 as a ferroelectric insulator, and SrRuO3 as a gate electrode, each layer prepared by pulsed laser deposition. The hysteresis behavior of the channel conductance is studied. Using gate voltage pulses of 100 μs duration and a pulse height of ±3 V, a change of a factor of two in the remnant conductance is achieved. The dependence of the conductance on the polarity of the gate pulse proves that the memory effect is driven by the ferroelectric polarization. The influence of charge trapping is also observed and discussed. © 1996 American Institute of Physics.

Notes: The early state of spinodal decomposition was studied by small angle neutron scattering in the critical mixture of the isotopic blend deutero-polystyrene/polystyrene (d-PS/PS) of equal molecular volume of 1.42×106 cm3/mol in a temperature range 12 K≤||Tc−T||≤82 K. This process can be described by the relaxation between two static structure factors, S(Q) representing the equilibrium values of the system in the mixed state and at the temperature where phase separation occurs. The time evolution of the relaxation process is described by the dynamical structure factor, L(Q,t) which depends on the dynamic properties of the mixture. It will be shown that the static structure factor of a mixed system can also be determined in the unstable two-phase region during the early state of spinodal decomposition. Consistent values for the Flory–Huggins parameter were found in comparison with a lower molecular d-PS/PS sample and, therefore, a lower critical temperature which was even smaller than the phase separation temperatures of the present system.The observed time evolution of the fluctuation modes is nonexponential. Therefore, it was originally supposed that internal modes of the coil come into play. The analysis of the data with an ansatz by Akcasu, which takes internal modes into account showed, however, that the phase separation in the experimental range of wave number and time is dominated by the centre of mass diffusion as in the C–H–C case and the nonexponential behavior was attributed to a time dependent increase of the "range'' of the Onsager coefficient. A range of the Onsager coefficient larger than the radius of gyration of a single coil is predicted in case of entangled polymers. However, no time dependence was predicted so far. The evaluated diffusion constants follow an Arrhenius behavior and are consistent with earlier studies. They show a D0∝N−2 scaling consistent with reptation. A further result is the observation of a second order peak in the structure factor already in the early times of spinodal decomposition. So far, this was only attributed to the late state of spinodal decomposition. © 1996 American Institute of Physics.

Notes: Plane-wave ray tracing has been performed through 2-D random media with Gaussian and exponential autocorrelation functions of the slowness perturbations. The standard deviation is ɛ, correlation length a and propagation distance L; computations have been performed for ɛ= 1 and 3 per cent and for L/a= 2.5 to 60. Up to L/a= 5 or 10, regular focusing and defocusing of the rays is observed. Then, irregular behaviour develops, apparently without foci, but with increasing deviations q from the straight rays of a homogeneous medium. Two neighbouring rays at the top of a rectangular cross-section usually arrive far from each other at the bottom, and rays can even turn. The standard deviation σ of q, normalized by L, is approximately σ/L=ɛ(L/a)1/2.From the traveltimes of the rays at the bottom of the cross-section, the first arrivals were determined and compared with the first arrivals according to the Huygens method of Podvin & Lecomte (1991). Both ray tracing and the Huygens method are high-frequency methods, but their traveltimes do not always agree. The reason is that the Huygens method gives first arrivals including diffractions around the slow parts of the structure, whereas ray tracing, in the sense of initial-value ray tracing, gives only transmitted rays. As a consequence, traveltimes calculated by ray tracing are systematically late with respect to the Huygens-method traveltimes. The difference becomes significant for L/a greater than 10 to 20, it increases with ɛ, and it is more pronounced for exponential than for Gaussian media. For instance, the velocity shift (with respect to the inverse of the volume average of the slowness) for an exponential medium, L/a= 60 and ɛ= 3 per cent, is –0.5 per cent according to ray tracing, whereas it is + 1.3 per cent according to the Huygens method. For ɛ= 1 per cent the corresponding numbers are 0 and +0.3 per cent, i.e. the difference between ray tracing and the Huygens method is still significant.A conclusion from our calculations is that, even in the mildly laterally heterogeneous earth models of seismic tomography, ray tracing may give traveltimes that are significantly biased, i.e. overestimated. Another result is that, in random media, ray-tracing first-arrival traveltimes often have much stronger short-scale spatial variations than Huygens-method traveltimes which include wavefront healing due to diffractions. Both effects, traveltime bias and short-scale variations, limit the applicability of ray tracing already for weak random media.