Norio Ookubo

Effects of thermal annealing on porous silicon photoluminescence dynamics

Photoluminescence (PL) spectra and decay dynamics were studied for the spontaneously oxidized porous Si with subsequent various thermal annealing procedures. The PL decay was highly nonexponential and well described by the stretched‐exponential function. The PL lifetime was shorter for the higher PL photon energy, but at the same photon energy it decreased by an order of magnitude by the thermal annealing in N2 gas, in parallel with the large PL intensity decrease. This PL quenching upon the annealing is presumably ascribable to both the structural changes and dangling bond formations in porous Si, as suggested by ESR measurements and the annealing experiments in H2 gas.

Simplified sum-over-states calculations and missing-orbital analysis on hyperpolarizabilities of benzene derivatives

Abstract The simplified sum-over-states (SOS) and missing-orbital analysis methods are introduced. The former aims to calculate the molecular polarizabilities in the framework of ab initio SCF theory, and the latter to analyze the calculated polarizabilities. The utility of the combination of these methods is demonstrated for the benzene derivatives, e.g. p -nitroaniline (PNA) and p -cyanoaniline (PCA). The calculated first-order hyperpolarizabilities (β) agree well with those of both CPHF calculations and experimental results. The missing-orbital analysis shows that β for PCA is contributed by both forward and backward charge transfer (CT) related to “out-of-plane” and “in-plane” orbitals, respectively. In contrast, β for PNA is confirmed to be explained fully by the conventional two-state model characterized by strong forward CT.

Depth‐dependent porous silicon photoluminescence

Porous silicon showing a two‐layer structure is studied by stepwise peeling the surface layer to clarify the nonuniformity in photoluminescence (PL) emission as a function of depth. The upper layer is amorphous and luminesces at higher energy and efficiency. The deeper the depth or the lower the excitation energy Eex, the lower the PL peak Ep; but, at a low Eex, Ep is not sensitive to the depth. Both intrinsic PL emission and the variation of penetration depth with Eex contribute to the linear dependence of Ep on Eex, which is in contrast to the cases of a‐Si:H and siloxene exhibiting thermalization gaps. The total PL excitation spectrum, the integrated PL intensities versus Eex, saturates rather than exhibiting a peak. Its leading edge profile is similar to that for the absorption spectrum, unchanged by the depth, and described by an Urbach tail with energy of 0.26 eV, which is 4–5 times larger than that of a‐Si:H. The results can be understood based on silicon clusters embedded in amorphous silicon inco...

High-spatial-resolution scanning capacitance microscope using all-metal probe with quartz tuning fork

The scanning capacitance microscope (SCM) reported here uses a frequency modulation (FM) technique to control the distance between the sample and an all-metal probe. The probe was attached to a quartz tuning fork in a configuration minimizing the perturbation due to the probe. The FM-SCM yields two images of ∂C∕∂V and ∂C∕∂Z signals, where C is capacitance sensed by the probe, Z the probe–sample distance, and V a bias voltage, respectively. On a cross section of a field effect transistor, the two-dimensional p–n junction locus was observed with a spatial resolution better than 5nm in the ∂C∕∂V image. The ∂C∕∂Z images of polysilicon gate electrodes and highly doped source/drain regions have higher contrast than the ∂C∕∂V images.

Molecular hyperpolarizability components βzzz and βzxx under directional extensions of π conjugation: missing-orbital analysis and simplified sum-over-states calculations

Abstract The application of missing-orbital analysis to molecular hyperpolarizability β obtained by a simplified sum-over-states calculation, allows one to study the effects of the π conjugation extension on both diagonal β zzz and off-diagonal β zxx components for C 2v molecules (the z -axis being the principal one). Extension of the z -length in aminonitropolyacetylenes by increasing the number of acetylene units increases β zzz but β zxx remains unchanged. Conversely, β zxx increases significantly with only a slight increase in β zzz if the π conjugation is extended mainly in the x -direction for 1,3-diamino-4,6-dinitrobenzene by replacing benzene with a fused-ring or inserting acetylene units between the substituents and benzene. These are ascribable to changes in the charge-transfer structure by the π extension.

Missing-orbital analysis on hyperpolarizability β of organic molecules. Enhancement of the off-diagonal component βzxx using multiple substitutions

Abstract The first-order hyperpolarizability components, βzzz and βzxx, for C2v molecules (the z axis being the principal axis) are analyzed after simplified sum-over-states calculations. Compared with p-nitroaniline (PNA), βzxx is three times enhanced by x-extended π conjugation realized by a bulky substrate in 9-amino-10-nitroanthracene (ANA) and by multiple substitutions in 1,5-diamino-2,4-dinitrobenzene (DDB). While ANA shows βzzz unchanged because its z-directed charge transfer (CT) is similar to PNA, DDB has a β zzz 1 3 of PNA, which is reduced by two weak CTs introduced by two ortho-positioned donor-acceptor pairs on both sides of the z axis.

Shear-mode scanning capacitance microscope

Scanning capacitance microscope (SCM) is developed using an all-metallic probe, whose distance from the sample is controlled by detecting the shear-force drag on the laterally oscillating probe. The oscillatory motion of the probe is electromechanically excited and detected. Using this SCM, a set of images of topography, dC/dV, and dC/dX is simultaneously obtained, where C and V are, respectively, capacitance and applied voltage between the probe and the sample, and X is the coordinate along probe tip oscillation. The SCM developed shows sensitivity for dC/dV higher than the conventional SCM. The dC/dX image clearly indicates the built-in depletion region due to the p–n junction.

Missing-orbital analysis of molecular hyperpolarizability β calculated by a simplified sum-over-states method. Enhancement of the off-diagonal component βzxx

Abstract The off-diagonal component of the first-order hyperpolarizability tensor βzxx of C2v molecules, e.g. substituted polyynes and benzenes, has been calculated by a simplified sum-over-states method. The missing-orbital analysis of the calcualted results reveals that excitations from a2 and b2 orbitals contribute cooperatively to βzxx, suggesting an insufficiency of the two-state model to describe βzxx. Increasing the number of occupied a2 orbitals enhances the magnitude of βzxx. For the C2v molecule with a2 orbitals widely extended in the x direction, βzxx can exceed βzzz, as found in 9-amino-10-cyanoanthracene having a βzxx about twice as large as βzzz.

Rapid surface topography using a tapping mode atomic force microscope

In an atomic force microscope (AFM), it is possible to scan at high speeds without sacrificing resolution if the imaging is accomplished by combining the rapidly varying signal from the vibrating cantilever, which indicates the detailed surface features, with the more slowly varying feedback control signal to the piezotube. Scanning speed in this case is limited by the fundamental resonance of the cantilever—not, as in conventional AFM, by the feedback bandwidth—and about 10 s is required to image a surface area of 21 μm2 for 512×512 scanning points.

HYPERPOLARIZABILITIES OF SI CLUSTERS : MODEL CALCULATIONS TO INTERPRET SECOND-HARMONIC GENERATION FROM SIO2/SI(111) INTERFACE

Abstract We have evaluated molecular hyperpolarizability β of Si clusters either with or without a dangling bond normal to Si(111) surface, and discussed the results in relation to the second-harmonic-generation (SHG) measurement from SiO 2 /Si(111) interface. The simplified sum-over-states calculation and missing-orbital analysis are extended to handle an open shell system, i.e. a dangling bond. In the presence of the dangling bond, β is caused by charge-transfers from the cluster interior to the dangling bond, while its absence by hydrogen termination greatly reduces β. This is consistent with the experiment that SHG is reduced by termination of interfacial Si atoms by annealing under hydrogen ambient.