ALBERT

Notes: Optical response due to the photo excited carrier or exciton dynamics in a one-dimensional (1D) dimethylglyoxime Pt complex [Pt(dmg)2] has been investigated by femtosecond pump–probe spectroscopy. Measurements were made at several excitation energies between the exciton state and the free electron hole pair state. Induced absorption was observed in the low-energy side of the exciton band at any excitation energy. The spectral shape of the induced absorption changed with time between 0.1 and 0.3 ps after instantaneous rise up. These results can be explained by the formation and thermalization processes of the relaxed excited state on the adiabatic potential surface. The unthermalized relaxed excited state is formed in a very short time (〈50 fs). From the Raman spectrum, the frequency of the intermolecular stretching mode along the chain was estimated to be 80 cm−1 (T∼400 fs). These facts suggest that, in contrast to other 1D systems such as polydiacetylenes (PDA) and halogen bridged mixed valence metal complexes (MX), the formation of the relaxed excited state is caused not by the stretching vibration along the 1D chain. We propose the possibility that the triggering of the primary relaxation is caused by a high frequency (〉600 cm−1) intramolecular mode which is not in the direction of the 1D chain. © 1999 American Institute of Physics.

Notes: Ga0.5In0.5P layers have been grown by organometallic vapor-phase epitaxy using various values of input V/III ratio for two phosphorus precursors, phosphine, the conventional precursor, and tertiarybutylphosphine (TBP), a newly developed, less-hazardous precursor. For growth on nominally (001) GaAs substrates misoriented by 3° (and in some cases by 0° or 6°) to produce [110] steps on the surface at a growth temperature of 620 °C, the Cu–Pt-type ordering is found to be strongly affected by the input flow rate of the phosphorus precursor (V/III ratio). For decreasing input partial pressures below 3 Torr for PH3 and 0.75 Torr for TBP the low-temperature photoluminescence (PL) peak energy increases indicating a lower degree of order. This is confirmed by transmission electron diffraction results. The decrease in the degree of order corresponds to a decrease in the concentration of [1¯10]-oriented P dimers on the surface, as indicated by surface photoabsorption spectroscopy results. These data indicate that the reduction in ordering is caused by the loss of the (2×4) reconstructed surface during growth. The difference in the behavior for PH3 and TBP is interpreted as due to the lower pyrolysis efficiency of PH3. The surface structure measured using high-resolution atomic force microscopy indicates that the [110] steps produced by the intentional misorientation of the substrate are bunched to produce supersteps approximately 30–40 A(ring) in height for the lowest V/III ratios. The step height decreases markedly as the input phosphorus partial pressure increases from 0.4 to 0.75 Torr for TBP and from 1 to 3 Torr for PH3. This corresponds to a change from mainly monolayer to predominantly bilayer steps in the vicinal regions between bunched supersteps. Stabilization of the bilayer steps is interpreted as due to formation of the (2×2) reconstruction on the (111)B step edges. The degree of order is an inverted U-shaped function of the flow rate of the phosphorus precursor. Thus, use of very high input V/III ratios is also found to reduce the degree of order in the Ga0.5In0.5P layers. These high input phosphorus flow rates are found to result in a monotonic increase in the density of [1¯10]-oriented P dimers on the surface. This decrease in order is believed to be related to a change in the structure of kinks on the [110] steps at high V/III ratios. © 1996 American Institute of Physics.

Notes: The spatial distribution of backscattered electrons is studied for an Au target with a new Monte Carlo simulation including the discrete energy loss process. The new model is based on the Mott cross section for elastic scattering and the Vriens cross section for inelastic scattering. Some results are compared with the ones by the old Monte Carlo simulation which is based on the screened Rutherford cross section and the Bethe law. The new model results in a larger peak near the incident point and a more gradual decrease of the background part of the radial distribution. The incident energy dependence of the spatial distribution of backscattered electrons is shown for 0° and 45° incident angles. The calculated spatial distribution of low-loss backscattered electrons indicates the possibility of very high resolution even in the backscattered electron image. The exit angle dependence of the penetration depth of backscattered electrons is also studied with the new simulation. © 1995 American Institute of Physics.

Notes: Interdiffusion at the InGaAs/InAlAs interfaces which are implanted with Si ions has been studied. Quantum well (QW) structures are grown on InP(Fe) substrates by metalorganic vapor phase epitaxy. Each sample has three InGaAs wells of 2.6, 5.9, and 17.6 nm in thickness and is separated by 24-nm-thick InAlAs barrier layers with each other. The samples are implanted with Si ions at uniform Si densities ranging from 1.8×1017 to 3.9×1019 cm−3 over the QWs, and then annealed under various annealing conditions. The photoluminescence (PL) peak energy from each well is monitored to study the intermixing at the interface. Blue shifts in the PL peak energy are found to occur almost in the early stage of thermal annealing (within 15 s) above the critical Si dose of 2–3×1018 cm−3. The saturated value of the energy shift is determined mostly by the Si density but hardly dependent on the annealing temperature and time. It is concluded that defects formed by Si ion implantation enhance the thermal interdiffusion of Ga and Al atoms at the InGaAs/InAlAs interface, which finishes when implantation-induced defects are annealed out.

Notes: A natural monolayer {111} superlattice (the CuPt ordered structure) is formed spontaneously during organometallic vapor phase epitaxial (OMVPE) growth of Ga0.52In0.48P. The extent of this ordering process is found to be a strong function of the input partial pressure of the phosphorus precursor during growth due to the effect of this parameter on the surface reconstruction and step structure. Thus, heterostructures can be produced by simply changing the flow rate of the P precursor during growth. It is found, by examination of transmission electron microscope (TEM) and atomic force microscope (AFM) images, and the photoluminescence (PL) and PL excitation (PLE) spectra, that order/disorder (O/D) (really less ordered on more ordered) heterostructures formed by decreasing the partial pressure of the P precursor during the OMVPE growth cycle at a temperature of 620 °C are graded over several thousands of Å when PH3 is the precursor. The ordered structure from the lower layer persists into the upper layer. Similarly, D/O structures produced by increasing the PH3 flow rate yield PL spectra also indicative of a graded composition at the heterostructure. The grading is not reduced by a 1 h interruption in the growth cycle at the interface. Similar heterostructures produced at 670 °C using tertiarybutylphosphine (TBP) as the P precursor show a totally different behavior. Abrupt D/O and O/D heterostructures can be produced by abruptly changing the TBP flow rate during the growth cycle. PL and PLE studies show distinct peaks closely corresponding to those observed for the corresponding single layers. TEM dark field images also indicate that the interfaces in both for D/O and O/D heterostructures are abrupt. The cause of the difference in behavior for TBP and PH3 is not clear. It may be related to the difference in temperature. © 1997 American Institute of Physics.

Notes: Surface photoabsorption (SPA) measurements were used to clarify the Cu–Pt ordering mechanism in Ga0.5In0.5P layers grown by organometallic vapor phase epitaxy. The Cu–Pt ordering is strongly affected by the growth temperature and the input partial pressure of the phosphorus precursor, i.e., the V/III ratio. SPA was used to measure the concentration of [1¯10]-oriented phosphorus dimers on the surface, which are characteristics of the (2×4) reconstruction, as a function of the growth temperature and V/III ratio. The degree of order decreases markedly with increasing growth temperature above 620 °C at a constant V/III ratio of 40 [tertiarybutylphosphine (TBP) partial pressure of 50 Pa]. This corresponds directly to a decrease of the P-dimer concentration on the surface. Below 620 °C, the degree of order decreases as the growth temperature decreases, even though the concentration of P dimers increases. This is most likely due to the slow migration of adatoms on the surface during growth. The degree of order is found to decrease monotonically with decreasing V/III ratio in the range from 160 to 8 at 670 °C. This corresponds directly to the decrease of the P-dimer concentration on the surface. The direct correlation of the [1¯10]-oriented phosphorus dimer concentration and the degree of order with changes in both temperature (≥620 °C) and V/III ratio suggests that the (2×4) surface reconstruction is necessary to form the Cu–Pt structure, in agreement with published theoretical studies. The physical structure of the surface of these Ga0.5In0.5P layers was also characterized, using atomic force microscopy. For growth at 670 °C and a V/III ratio of 160, the structure of the layers growth on exactly (001) oriented GaAs substrates consists of islands surrounded mainly by bilayer (approximately 6 A(ring)) steps. As the V/III ratio is reduced, the step height transforms to one monolayer. Exclusively monolayer steps are formed at a V/III ratio of 8. This is interpreted in terms of the stabilization of the bilayers by formation of the (2×2) reconstruction on the (111)B step face at high V/III ratios. © 1996 American Institute of Physics.

Notes: Susceptibility and high field magnetization have been measured for Y(Co1−xMx)2 (M=Al, Fe, and Ni). YCo2 exhibits a metamagnetic transition at 69 T. Ni doping in YCo2 increases the critical field (Bc), while Fe doping decreases it. These changes can be elucidated with the band picture of metamagnetism. Bc of YCo2 exhibits a positive shift proportional to the square of temperature. In the paramagnetic region of Y(Co1−xAlx)2 with x≤0.11, the susceptibility is enhanced with Al and a sharp metamagnetic transition with lower Bc is observed. The susceptibility becomes maximum at a finite temperature Tmax. Bc in the ground state is proportional to Tmax. These experimental results are discussed with a new theory based on a spin fluctuation model. The susceptibility and the metamagnetic transition are found to be very sensitive to pressure.

Notes: Magnetic properties of amorphous carbon materials, prepared by pyrolysis of trialkylboranes, were examined using the superconducting quantum interference device and the electron spin resonance (ESR) methods. It is found that the observed magnetic properties could be greatly affected by pyrolysis temperature and the analysis of the magnetization curves observed in the magnetic field range 0–1 T has shown that these samples are basically of a superparamagnetic character. The ESR measurements have also supported these aspects. © 1996 American Institute of Physics.

Notes: An x-ray-diffraction technique for nondestructively determining the depth-dependent fraction of a phase in the surface layer of a polycrystalline solid is presented using the data at low angles of incidence above the critical angle for the total reflection of x rays. The fraction, which is expressed as a polynomial in depth, is given by solving the equation of the kinematic diffracted intensity under the ratios of diffracted intensities from the phase to the bulk measured with multiple angles of incidence. The method utilizes the fact that at a lower angle of incidence the diffracted intensity from a deep layer attenuates more steeply and also the degree of the attenuation largely varies depending upon the angle of incidence. An application of the method to ground plates of yttria-doped tetragonal zirconia polycrystals revealed that the fraction of monoclinic phase induced by the machining was maximum (∼0.2) near the surface and decreased gradually in the deeper layer up to a few microns.

Notes: Radiation-induced defects in a new second harmonic generation material, lithium vanadate single crystal, have been studied by electron paramagnetic resonance (EPR) and optical absorption measurements. EPR detects two intense signals due to intrinsic O− and impurity-associated CO−3 trapped-hole centers at 77 K, but no such signal in the case of trapped-electron centers. The EPR signal of impurity Fe3+ ions is suppressed by 77 K irradiation. Low-temperature irradiation produces three optical absorption bands peaking at 3.14, 2.0, and 1.56 eV. It has been concluded that some of the free electrons produced by irradiation are trapped by Fe3+ ions at low temperatures. After annealing at room temperature all the optical absorption bands disappear, although the EPR signals of CO−3 trapped-hole centers and unidentified electron-type centers at g=1.96 remain stable up to 330 K. © 1995 American Institute of Physics.