Fast X-ray reflectivity measurements using an X-ray pixel area detector at the DiffAbs beamline, Synchrotron SOLEIL

This paper describes a method for rapid measurements of the specular X-ray reflectivity signal using an area detector and a monochromatic, well collimated X-ray beam (divergence below 0.01°), combined with a continuous data acquisition mode during the angular movements of the sample and detector. In addition to the total integrated (and background-corrected) reflectivity signal, this approach yields a three-dimensional mapping of the reciprocal space in the vicinity of its origin. Grazing-incidence small-angle scattering signals are recorded simultaneously. Measurements up to high momentum transfer values (close to 0.1 nm−1, also depending on the X-ray beam energy) can be performed in total time ranges as short as 10 s. The measurement time can be reduced by up to 100 times as compared with the classical method using monochromatic X-ray beams, a point detector and rocking scans (integrated reflectivity signal).

Confirmation of no Structural and Chemical Changes in Curie Temperature Variable Co Ultrathin Films by Electric Field

A cobalt ultrathin film, which shows a large change in Curie temperature by an electric field application, has been studied by x-ray reflectometry with applying electric fields. The cobalt film was made by the sputtering method on top of a Pt buffer layer, and capped with a MgO layer. X-ray reflectometry shows that the change in Co thickness caused by the applied voltage up to ±10 V was less than 0.06 Å. The reflectivity signal intensity shows a characteristic kink at the Co

MODULATIONS IN LOW-TEMPERATURE TRANSIENT REFLECTIVITY MEASUREMENTS

Periodic modulations that appear in the low-temperature transient reflectivity signal of a GaAsP / AlGaAs single quantum well is studied. Similar anomalous oscillations are also reported in layered manganite [K. Kouyama et al., J. Phys. Soc. Jpn.76 (2007) 123702(1–3).] We show that such periodic modulations are caused by changes in the linear reflectivity of the sample during transient reflectivity measurements. Studies carried out on reflectivity of different materials under identical conditions show that these modulations on the true transient reflectivity signal are caused by condensation of residual gases on the surface of quantum well. Methods to obtain reliable transient reflectivity data are also described.

Spin Dependent Pump Probe Reflection Spectroscopy in Bulk InP Crystal

The change of the reflectivity signal is modeled by taking into account of the spin dependent band filling and band gap renormalization effects with the carriers density from 5×1016/cm3to 2×1017/cm3. The calculated results show the reflectivity signal is a very sensitive function of the carrier density. The most interesting feature is that the reflectivity change from photo-bleaching into photo-absorption enhancement is observed with increasing pump photon energy. The theoretical predictions obtained in this work are consistent with the corresponding experiments.

Study of depth-dependent radiation-induced defects using coherent acoustic phonon spectroscopy

ABSTRACTHere we study the effect of radiation-induced point defect distributions on the optical reflectivity signal in GaAs using coherent acoustic phonon spectroscopy. We demonstrate that the presence of point defects significantly modifies the optical response, allowing estimation of the depth-dependent defect distribution in a nondestructive and noninvasive manner. We show that the observed changes are dependent on defect-induced changes to the electronic structure, namely defect-induced band tailing of the direct 1.43eV band edge. This provides a method for subsurface investigations on the complex interaction between different defects species and optoelectronic structure.

Pump-Probe Study of Bi1-xSbx Alloys

ABSTRACTBi1-xSbx alloys undergo a semimetal-semiconductor transition at x = 0.065 and 0.22 and are semiconducting between these limits. We investigate this system by means of femtosecond time-resolved experiments, and observe an optical response similar to those in Bi, Sb, Te and Ti2O3. A previously-reported model describes the mechanism of generation of coherent lattice vibrations via the optically excited carriers in this class of materials. As predicted by the model, the modulated reflectivity signal ΔR/R in Bi.88Sb.12 shows a beating oscillatory behavior with the Bi-Bi and Bi-Sb A1g vibrational frequencies. Model-based calculations of the ΔR background relaxation time at different Sb compositions provide understanding of the carrier and lattice dynamics throughout the transition.

Noncontact True temperature measurement

A laser pyrometer has been developed for acquiring the true temperature of a levitated sample. The laser beam is first expanded to cover the entire cross-sectional surface of the target. For calibration of such a system, the reflectivity signal of an ideal 0.95cm diameter gold-coated sphere (reflectivity = .99) is used as the reference for any other real targets. The emissivity of the real target can then be calculated. The overall system constant is obtained by passively measuring the radiance of a blackbody furnace (emissivity = 1.0) at a known, arbitrary temperature. Since the photo sensor used is highly linear over the entire operating temperature range, the true temperature of the target can then be computed. Preliminary results indicate that true temperatures thus obtained are in excellent correlation with thermocouple measured temperatures.