Bibliothek

Notizen: 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.

Notizen: 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.

Notizen: Intensities of x-ray microbeams formed by the 250-mm-long hollow glass pipes of inner diameters of 27.4, 23.0, and 18.8 μm have been theoretically investigated, by taking account of the slope distribution of microprojections (surface roughness) on the pipe inner wall, using the Monte Carlo method. The intensities for all the pipes calculated on the supposition that each pipe inner wall is perfect (i.e., zero rms of the slope distribution) have been much greater than the experimental values in the x-ray energy region from 6.93 to 19.6 keV. Assuming the slope rms from 2.5×10−4 rad to 3.3×10−3 rad, the calculated results have agreed with the experimental values. Discussions on the results for all the pipes are given in relation to the x-ray anomalous dispersion, the penetration of x rays, the intensity distribution on the x-ray sources used, undulation of the pipes, and the presence of microdust in the pipes.

Notizen: X-ray microbeams have been produced by utilizing hollow glass pipes. It has been observed that the intensities of ten x-ray lines (from 6.93 to 19.6 keV) transmitted through 250-mm-long hollow glass pipes of inside diameters of 27.4, 23.0, and 18.8 μm range from 1.9 to 2500 times greater than intensities reduced by inverse-square attenuation.

Notizen: Substrate orientation strongly affects Cu–Pt ordering in Ga0.5In0.5P layers grown by organometallic vapor phase epitaxy. In situ surface photoabsorption (SPA) measurements were used to measure the concentration of [1¯10]-oriented P dimers, characteristic of the (2×4) reconstructed surface, as a function of substrate misorientation from (001). For substrates misoriented toward either [110] [or (111)A] or [1¯10] [or (111)B], the P-dimer concentration is found to decrease systematically as the misorientation angle increases from 0° to 15.8° at 620 °C with tertiarybutylphosphine (TBP) partial pressures of 10, 50, and 200 Pa. The P-dimer concentrations on substrates misoriented toward [110] are higher than for those misoriented toward [1¯10]. The Ga0.5In0.5P layers were found to form the Cu–Pt structure during growth. The degree of order, determined from 20 K photoluminescence measurements, shows a strong correlation with the concentration of [1¯10]-oriented P dimers. The data also clearly show the effect of step structure on Cu–Pt ordering. They indicate that [110] steps formed by misorientation toward [1¯10] enhance ordering while [1¯10] steps formed by misorientation toward [110] retard ordering. © 1996 American Institute of Physics.

Notizen: Cu–Pt ordering is widely observed in Ga0.5In0.5P layers grown by organometallic vapor phase epitaxy. The degree of order is a strong function of the input partial pressure of the phosphorus precursor, i.e., the V/III ratio, during growth. By observing the surface structure using in situ surface photoabsorption (SPA) measurements, the concentration of [1¯10]-oriented P dimers, characteristic of the (2×4) reconstructed surface, has been measured as a function of the growth conditions. For growth at 670 °C, the P-dimer concentration is found to increase systematically as the input tertiarybutylphosphine pressure is increased from 10 to 200 Pa. This corresponds directly to a monotonic increase in the degree of order, measured using transmission electron microscopy and low-temperature photoluminescence. These data strongly suggest that the (2×4) surface reconstruction is necessary for formation of the Cu–Pt structure. The step structure at the surface was also observed for these layers using atomic force microscopy. For high V/III ratios the structure of the layers grown on exactly (001) oriented GaAs substrates consists of islands surrounded mainly by bilayer (5.7 A(ring)) steps. As the V/III ratio is reduced, the step height transforms to 2.8 A(ring) (one monolayer). © 1996 American Institute of Physics.

Notizen: Ongoing spontaneous postsynaptic potentials (SPSPs) were intracellularly recorded at 34-36°C from hippocampal CA3 neurons in slices obtained from postnatal days (P) 0 – 6 and 7 – 31. SPSPs occurred randomly, and their frequency distribution was fitted by a single exponential function. They were little affected by kynurenic acid, but were reversibly blocked by bicuculline, implying that they were mediated by GABAA receptors. The mean amplitude was 4.53 ± 0.89 mV in control conditions and 4.07 ± 0.79 mV in kynurenic acid. In kynurenic acid (with CsCl-filled microelectrodes), SPSPs reversed polarity at 2.4 ± 2 mV. When tetrodotoxin (1 μM) was added to kynurenic acid solution, GABAA-mediated miniature postsynaptic potentials (MPSPs) were recorded. Under these conditions large events disappeared. The mean amplitude of MPSPs was 2.51 ± 0.43 mV. The mean frequency decreased from 2.96 ± 1.04 Hz in kynurenic acid to 0.4 ± 0.15 Hz in kynurenic acid plus tetrodotoxin. In contrast with P0 – P6, at P7 – P31 SPSPs were significantly affected by kynurenic acid. The mean amplitude of SPSPs shifted from 4.71 ± 0.82 mV in control conditions to 3.79 ± 0.76 mV in kynurenic acid. At this developmental stage, the reversal potential of GABAA-mediated SPSPs shifted towards more negative values (-23.7 ± 1.3 mV). Addition of tetrodotoxin to kynurenic acid solution abolished larger events and revealed GABAergic MPSPs. The mean amplitude of MPSPs was 2.72 ± 0.5 mV, a value very close to that observed at P0 – P6. Synaptic currents were recorded at 22 – 24°C from voltage-clamped CA3 pyramidal neurons (at P6) using the tight-seal whole-cell recording technique. Cells were dialysed with CsCl solution and held at -70 mV. Spontaneous GABAA-mediated miniature postsynaptic currents (MPSCs) were recorded in the presence of kynurenic acid and tetrodotoxin. The decay time of MPSCs was fitted with a single exponential and was 29 ± 3 ms. No correlation was found between the peak amplitude of individual events and their rise or decay time constant. The mean amplitude distribution of MPSCs was 12 ± 4.3 pA. In outside-out patches from acutely dissociated CA3 hippocampal neurons, GABA (50 μM) activated single-channel events of 24 and 35 pS conductance. Therefore one quantal current represents the simultaneous opening of five to seven GABAA receptor channels on the postsynaptic cell. These data show also that in the immediate postnatal period spontaneous glutamatergic potentials are poorly developed and start appearing towards the end of the first postnatal week concomitant with the shift of GABA from the depolarizing to the hyperpolarizing direction.