Junichiro Kōga

Simulational study of liquid germanium under pressure

Abstract Tight-binding molecular dynamics simulations are carried out for liquid germanium with seven different densities. The obtained pair distribution functions are in good agreement with recent experiments. By studying the positions of the first peaks in the pair distribution functions as well as the bond-angle distribution functions, we clarify the change in the local structure of liquid germanium upon compression. We conclude that when the pressure is increased from the low-pressure region to the high-pressure region, the local structure of liquid germanium changes from a ‘disordered’ β-Sn structure to a structure that is closer to the β-Sn structure.

Light emission properties of amorphous silicon quantum dots

We theoretically study the light emission properties of amorphous silicon quantum dots (a-Si QDs). From an analysis of the radiative tunneling rate, we first assert that the dominant recombination process of a-Si QDs smaller than 2.4 nm in diameter is a direct band-to-band recombination. Our tight-binding calculations for direct band-to-band recombination reproduce the peak energies of experimental photoluminescence (PL) due to Park et al. This fact indicates that the PL origin is mainly the direct band-to-band recombination, which in other words indicates that the blueshift of the PL peak energy is attributed to quantum confinement rather than to the so-called spatial confinement effect related to radiative tunneling.

Theoretical study on the relation between structural and optical properties in Si nanostructures

Abstract Two types of model silicon (Si) nanostructures in zero, one, and two dimensions are calculated. The model of the first type is the one adopted in most previous works, and its atomic configuration has high point-group symmetries; we refer to this model as a ‘high-symmetry’ model. On the other hand, our model of the second type is a ‘low-symmetry’ model, and its atomic configuration has no point-group symmetries. Since it is unlikely that realistic nanostructures have high symmetries, our model is more realistic for light-emitting Si. We calculate, in the tight-binding scheme, the electronic states, energy gaps, and the radiative recombination rates for these two models in zero, one, and two dimensions. We show that our ‘low-symmetry’ model yields a radiative recombination rate greater than that of a ‘high-symmetry’ model for zero- and one-dimensional systems. On the other hand, we show that the behavior of the radiative recombination rates for two-dimensional systems differs greatly from the case of zero- and one-dimensional systems. We show that, for two-dimensional systems, the radiative recombination rate is higher for the high-symmetry model. This result is interpreted in terms of a simple argument based on the phases of the wave functions for these systems.

Positional dependence of optical absorption in silicon nanostructure

Abstract We present a theoretical study of the optical absorption process of a 9×9 Si quantum wire. We calculate the imaginary part of the dielectric constant e2 and the contribution to e2 due to three Si atoms located in different positions using the non-orthogonal tight-binding method. From these calculations, we clearly find for the first time that the optical absorption below 3.4 eV tends to occur in the inner region of the 9×9 Si quantum wire.

Structural relaxation and its effects on photoluminescence properties in Si quantum wires

We calculate structural relaxation for a Si quantum wire in order to determine the mechanism of the efficient photoluminescence (PL) from Si. We find that structural relaxation, which has been ignored in most previous works, has a strong correlation to the PL properties of Si. The total energy of the quantum wire is optimized within the framework of the tight-binding (TB) approximation. The resultant structures for the ground and excited states have different band gaps, which accounts for the luminescence Stokes shift observed in experiments. We also find that the relaxation greatly improves the oscillator strength for the transition between band edges, which gives a great improvement in the luminescence efficiency.

Theoretical study of electronic states and visible photoluminescence from silicon nanostructures

Abstract Two types of model silicon (Si) nanostructure are taken into account. The first one is a ‘Si nanostructure without point-group symmetry’, while the second one is an amorphous Si nanostructure. We perform tight-binding electronic state calculations for these systems, and elucidate their potential applicability as light-emitting devices. Our calculations show that both model structures have high radioative recombination rates compared to those obtained from conventional model Si nanostructures, and are good candidates for Si-based light-emitting devices.