Tomoyuki Kawashima

Drilled alternating-layer three-dimensional photonic crystals having a full photonic band gap

We propose a three-dimensional photonic crystal structure having a wide full photonic band gap in the optical regime, which can be fabricated by an alternating-layer deposition and etching (drilling) process. This fabrication process is much simpler than that previously reported. The combination of current lithographic technology and autocloning bias-sputtering deposition is a promising way of realizing these photonic crystals.

A new fabrication technique for photonic crystals: Nanolithography combined with alternating-layer deposition

We propose two photonic crystal structures that can be created by combining nanolithography with alternating-layer deposition. Photonic band calculations suggest that a drilled alternating-layer photonic crystal combining two-dimensional (2D) alternating multilayers and an array of vertically drilled holes may achieve a full photonic bandgap. In addition, a 3D/2D/3D cross-dimensional photonic crystal, which sandwiches a 2D photonic crystal slab between three-dimensional (3D) alternating-layer photonic crystals, should provide better vertical confinement of light than a conventional index guiding slab. Fabrication techniques based on existing technologies (electron beam lithography, bias sputtering, and low-pressure ECR etching) require very few process steps. Our preliminary fabrication suggests that, by refining these technologies, we will be able to realize photonic crystals.

Electrical transport properties of isolated carbon nanotube/Si heterojunction Schottky diodes

A detailed study of the electrical transport properties of Pd contacted carbon nanotube (CNT)/Si heterojunctions is presented. The CNT with a diameter ranging from 1.2 to 2.0 nm on n-type Si substrates showed rectifying behavior with the ideality factor of 1.1–2.2 and turn on voltage of 0.05–0.34 V. The current-voltage characteristics of the CNT/n+-Si diodes were investigated in the temperature range from 50 to 300 K. The transition from thermionic emission to tunneling process was seen in the forward current around 150 K and the Schottky barrier height at Pd/CNT interface is estimated to be 0.3–0.5 eV.

Control of Growth Modes by Carbon Mediation in Formation of Ge Quantum Dots on Si(100)

Control and mechanism analysis of a Ge quantum dot (QD) formation on a Si(100) substrate by using carbon (C)-mediated c(4 × 4) surface reconstruction (SR) and solid-phase epitaxy (SPE) methods is demonstrated. The Si surface was reconstructed via the formation of C–Si bonds before Ge deposition in the SR method, while the QDs were formed by annealing of an amorphous Ge/C/Si heterostructure in the SPE method. In the SR method, the QDs grew in the Volmer–Wever (VW) mode at C = 0.25 and 0.50 monolayer (ML), and in the Stranski–Krastanov (SK) mode at C = 0.75 ML. The VW-mode QDs were formed owing to the c(4 × 4) surface reconstruction that acted like a virtual partition for Ge nucleation. At C = 0.5 and 0.75 ML, it was confirmed that the C–Ge bonds were formed near the Ge/Si interface because the unreacted excessive C atoms remained at the Ge/Si interface. The formation of C–Ge bonds induced the strain relief of the Ge layer and acted to change the growth mode to the SK mode at C = 0.75 ML. On the other hand, in the SPE method, the QDs grew in the VW mode at C = 0.1 and 0.25 ML due to the Ge aggregation, and in the SK mode at C ≥ 0.4 ML. It was found that a large number of C–Ge bonds owing to the incorporation of C into the Ge layer during the SPE induced the formation of a wetting layer. Therefore, the reduction of the strain energy in the Ge layer occurred at the low C coverage and induced the transition to the SK mode. These results suggest that the growth modes of the QDs via C mediation are controllable in both methods by changing the amount of C used as the mediation layer owing to the change in the C binding states at the Ge/Si interface or in the Ge layer.

Control of VW and SK growth modes in Ge quantum dot formation on Si(100) via carbon mediation

Control and mechanism analysis of Ge quantum dot (QD) formation on Si(100) by using two carbon (C) mediated methods, c(4×4) surface reconstruction (SR) and solid-phase epitaxy (SPE), was demonstrated for the first time. Si surface was reconstructed via the formation of C-Si bonds in advance of Ge deposition in SR method, QDs grew in Volmer-Wever mode due to the preferential nucleation on uncarbonized Si surface. Ge QDs were formed by annealing an amorphous Ge/C/Si heterostructure in SPE method, QDs grew in Stranski-Krastanov mode due to the incorporation of C-Ge bonds. Investigations, in this work, clarified that both c(4×4) surface reconstruction and strain relief played important roles through the analyses of surface morphology and C binding states.

Growth of V-doped ZnO Thin Films with Two-layer Structure on a-Al 2 O 3 by RF Magnetron Sputtering

Drastic transition in c-axis orientation for V-doped ZnO (VZO) thin film grown on an a-face Al2O3 substrate was investigated. Through observations of TEM cross-sectional views, VZO films with two-layer structure were found. As a result of X-ray diffraction analyses changing the film thickness, the layer located immediately above the substrate was oriented along the c-axis direction, while the upper layer consisted of a mixture of c-axis and [10-11] direction. The initial thin layer grew coherently with compressive strain caused by V doping, and the extension of c-axis lattice constant was released gradually with thickening the VZO film up to about 200-nm thick. The crystal orientation tilted to [10-11] direction after the strain was fully relaxed. Conclusively, gradually relaxed thick layer affected crystal orientation of the upper layer.