Teruya Ishihara

Surface plasmon resonant interference nanolithography technique

We demonstrate a promising nanofabrication method, used to fabricate fine patterns beyond the diffraction limit, by employing surface plasmon polariton (SPP) resonance. Sub-100 nm lines were patterned photolithographically using surface plasmon polaritonic interference in the optical near field excited by a wavelength of 436 nm. The unperforated metallic mask approach which has corrugated surfaces on both sides is proposed for arbitrary patterning. The corrugated surface of the metallic mask on the illuminated side collects light through SPP coupling, and SPP on the exit side of metallic mask redistributes the light into nanoscale spatial distribution, which can be used to fabricate nanostructures. Preliminary numerical simulations support the experimental results.

Optical properties of PbI-based perovskite structures

Abstract Remarkable enhancement of the excitonic binding energy and oscillator strength is demonstrated in PbI-based layered perovskite compounds where quantum well layers of corner sharing PbI6 octahedra are sandwiched by alkylammonium barrier layers with a much smaller dielectric constant. The enhancement is quantitatively explained by the image charge effect. By changing the dimensionality of PbI6 networks, we observe a change in the exciton formation energy from 1.633 eV for three-dimensional networks to 342 eV for zero-dimensional isolated octahedra. This change is mainly ascribed to the quantum confinement effect.

Magnetoabsorption of the lowest exciton in perovskite-type compound (CH3NH3)PbI3

Magnetoabsorption spectra of (CH3NH3)PbI3 are investigated in the Faraday configuration up to 40 T at 4.2 K. The compound consists of three-dimensional networks of corner-sharing octahedra [PbI6]4−. The Zeeman splitting and the diamagnetic shift are observed, and the effective g-factor and the diamagnetic coefficient are 1.2 ± 0.1 and (2.7 ± 0.1) × 10−6eV/T2, respectively. From the diamagnetic shift, the Bohr radius, the binding energy and the reduced mass of the exciton are estimated to be 28 A, 37 meV and 0.12m0, respectively. The exciton in (CH3NH3)PbI3 is one of typical Wannier-type excitons with large radius, which is in contrast with the exciton in (C10H21NH3)2PbI4, consisting of two-dimensional networks of [PbI6]4−.

Exciton state in two-dimensional perovskite semiconductor (C10H21NH3)2PbI4

Abstract Reflection, luminescence and absorption spectra are measured in an exciton region of (C 10 H 21 NH 3 ) 2 PbI 4 . Crystal structure is determined to be of layered perovskite type by X-ray diffraction. PbI 4 layers are sandwiched by barrier layers consisting of alkylammonium chains. The exciton state has a large binding energy of 370 meV and is very stable even at room temperature. Its oscillator strength is estimated to be about 0.7 per a formula unit. The binding energy is too large to be explained only by the two-dimensional character of the exciton and is ascribed to the small dielectric constant of the barrier layer, which makes Coulomb interaction between an electron and a hole stronger.

Subwavelength photolithography based on surface-plasmon polariton resonance

The use of surface-plasmon polariton (SPP) resonance in the optical near field of a metallic mask to produce fine patterns with a resolution of subwavelength scale is proposed. Preliminary numerical simulations indicate that the critical resolution is mainly determined by the thickness of the metallic mask. The surface of the metallic mask on the illuminated side collects light through SPP coupling, and the interference of SPPs on the exit side of the metallic mask results in enhanced optical intensity with high spatial resolution, which can facilitate nanolithography efficiently by use of conventional photoresist with simple visible or ultraviolet light sources. Several schemes for sub-half-wavelength lithography based on SPPs are described. Inasmuch as the technique is not diffraction limited, nanostructures can be reproduced photolithographicly.

Exciton Features in 0-, 2-, and 3-Dimensional Networks of [PbI6]4- Octahedra

We have investigated reflection spectra at 4 K for single crystals of (CH 3 NH 3 ) 4 PbI 6 ·2H 2 O which consists of isolated [PbI 6 ] 4- octahedra, (C 10 H 21 NH 3 ) 2 PbI 4 and (CH 3 NH 3 )PbI 3 , which consist 2- and 3-dimensional networks of [PbI 6 ] 4- , respectively. The formation energy of the lowest exciton state increases from 1.633 eV to 3.4 eV with decreasing the dimensionality in the crystal structure, from the transfer energy between the adjacent octahedra is estimated to be about 0.5 eV. The on-site Coulomb energy in the octahedron is estimated to be 2.3 eV. We suggest that the large on-site Coulomb energy enhances the exciton binding energy, especially in the 2-dimensional system.

Photoconductivity and electroluminescence in lead iodide based natural quantum well structures

Abstract For the natural quantum well structures of (C6H5 - C2H4NH3)2 - PbI4 and (C6H5 - C2H4NH3)2(CH3NH3) - Pb2I7, step-like features in the absorption and photo-current spectra were identified as 2D band edge transitions from their temperature dependence. Photocurrent along the c-axis is much smaller than that along the well plane, revealing the large anisotropy of the electronic band structure, and shows a plateau in the current-voltage curve, which can be explained in terms of Wannier-Stark localization in the materials. Electroluminescence from free excitons was observed at a relatively low field (10 kV cm-1) along the wells below 200 K.

Dielectric confinement effect for exciton and biexciton states in PbI4-based two-dimensional semiconductor structures

Abstract We have studied the role of dielectric confinement for enhancing excitonic effects in layered PbI 4 -based compounds. In particular, we demonstrate that unusually strong biexciton effects are readily observable (binding energy on the order of 50 meV) and that the large excitonic binding (∼ 300 meV) can be modified by chemically adjusting the dielectric constant of the alkylammonium barrier layers.

Optical properties of photonic crystal slabs with an asymmetrical unit cell

Using the unitarity and reciprocity properties of the scattering matrix, we analyze the symmetry and resonant optical properties of the photonic crystal slabs (PCS) with non-trivial unit cell. We show that the reflectivity does not change upon the 180\ifmmode^\circ\else\textdegree\fi{}-rotation of the sample around the normal axis, even in PCS with asymmetric unit cell, whereas the transmissivity of the asymmetric PCS becomes asymmetrical if the diffraction or absorption are present. The PCS reflectivity peaks to unity near the quasiguided mode resonance for normal light incidence in the absence of diffraction, depolarization, and absorptive losses. For the oblique incidence the full reflectivity is reached only in symmetric PCS.

Fluorine‐enhanced thermal oxidation of silicon in the presence of NF3

The oxidation rate of silicon has been dramatically increased at temperatures between 600 and 800 °C by adding NF3 gas to a dry oxygen atmosphere. The chemical analysis of the resulting oxide has revealed that fluorine atoms in SiO2 are incorporated as Si–F bonds. The existence of such bonded‐fluorine atoms enhances the diffusion of oxidant through the oxide layer. The oxidation reaction rate in the SiO2‐Si interface is also remarkably increased because fluorine atoms in the interface create silicon dangling bonds and weakly positively charged silicon atoms as well. The kinetics of the fluorine‐enhanced oxidation has primarily been interpreted in terms of the Deal–Grove model.