Takahiro Kawamura

High-Quality SiO2/Si Interface Formation and Its Application to Fabrication of Low-Temperature-Processed Polycrystalline Si Thin-Film Transistor

Improvement of SiO2/Si interface quality and its effect on the performance of low-temperature-processed polycrystalline silicon thin-film transistors (poly-Si TFTs) are investigated. Two gate SiO2 formation conditions for realizing the density of interface states (Dit) at mid-gap of 1.4×1011 and 4.1×1010 cm-2eV-1 were applied to a 425°C TFT fabrication process using electron cyclotron resonance (ECR) plasma enhanced chemical vapor deposition (PECVD). By reducing Dit, reduction of threshold voltage from 1.97 to 1.12 V, reduction of sub-threshold swing from 303 to 250 mV/decade and increase of mobility from 196 to 309 cm2V-1s-1 were observed. The analysis of TFT characteristics indicated the decrease of both deep and shallow level trap states. As a result, not only threshold voltage and sub-threshold swing, but also the mobility of the poly-Si TFT was significantly improved. The results indicate that low-temperature process technologies for forming a high-quality SiO2/Si interface are important for next-generation high-performance poly-Si TFTs.

Effect of water impurity in CsLiB6O10 crystals on bulk laser-induced damage threshold and transmittance in the ultraviolet region.

We investigate the effect of water impurity in a CsLiB(6)O(10) (CLBO) crystal on the ultraviolet properties of the bulk laser-induced damage threshold (LIDT) and transmittance. The water impurity was eliminated by heating the CLBO sample with dimensions of 5 mm x 5 mm x 10 mm at 150 degrees C in an ambient atmosphere and subsequently in a dry atmosphere. The bulk LIDT of the sample after heat treatment improved by about 1.6-fold compared with that before heat treatment.

Two-Dimensional In0.4Ga0.6As/GaAs Quantum Dot Superlattices Realized by Self-Organized Epitaxial Growth

We report on the realization of two-dimensional (2D) In0.4Ga0.6As/GaAs quantum dot superlattices (QDSLs) by self-organized epitaxial growth. The conditions for the formation of extended states or minibands are analyzed by treating QD arrays as disordered systems. Ordered quantum dot (QD) arrays are fabricated on GaAs (311)B substrates. High density and small size are achieved by decreasing the growth temperature. A large red shift of the photoluminescence (PL) peak energy and a dramatic narrowing of the linewidth are found when the dots become smaller and closer. The exciton coherence length in the high-density ordered QD array is confirmed to be much larger than the QD diameter by PL decay time measurements and by using beryllium impurities as scattering centers. As a comparison, the incoherent exciton motion dominated by nonresonant tunneling is discussed. The transition from coherent to incoherent, including the intermediate state, and the localization of excitons are demonstrated by various mechanisms.

Thermal conductivity of SiC calculated by molecular dynamics

We calculated the thermal conductivity of SiC by molecular dynamics simulation and investigated the effects of impurities on the thermal conductivity of SiC. We used Tersoff potential to express the structure of a SiC crystal. Thermal conductivity was obtained using Green–Kubos equation. The results show that the thermal conductivities of perfect 2H-, 3C-, 4H-, and 6H-SiC polytypes were in the range of 260 to 420 W/(m·K) and that the thermal conductivity of 3C-SiC was the largest among the polytypes. The thermal conductivities of 4H-SiC decreased with an increase in impurity concentration above 1.0×1017 to 1.0×1018 1/cm3.

The Procedure to Realize Two-Dimensional Quantum Dot Superlattices: From Incoherently Coupled to Coherently Coupled Quantum Dot Arrays

We will describe, from both the theoretical and experimental points of view, a procedure leading to the realization of twodimensional quantum dot superlattices (2D QDSLs). Two kinds of laterally coupled quantum dot arrays in which the exciton motions are coherent and incoherent are studied by photoluminescence (PL) measurements. The formation of minibands with coherent exciton motion is demonstrated in the large red shift of the PL peak energy as well as the dramatic narrowing of the PL linewidth. Moreover, we indicate, using excitation-dependent PL spectra as an example, that the exciton coherence can be reduced by various scattering mechanisms.

Mechanism for enhanced single-crystal GaN growth in the C-assisted Na-flux method

First-principles molecular dynamics simulations are used to examine the effect of C addition in Na-flux growth of GaN. The mechanism for suppression of polycrystalline growth and the enhancement of single-crystal growth was identified by systematically calculating activation free energies for the formation and dissociation of C–N bonds. The energy barrier for C–N dissociation in a Ga–Na melt is ≥3 eV; thus, dissociation is inhibited and the growth of polycrystals is suppressed. However, at kink sites at a Na/GaN interface with excess Ga atoms, the barrier is only ~1.0 eV, allowing C–N dissociation and growth of GaN single crystals.

Molecular beam epitaxy growth of GaN under Ga-rich conditions investigated by molecular dynamics simulation

Molecular beam epitaxial growth of GaN under Ga-rich conditions was simulated using a classical molecular dynamics method. We investigated nitrogen incorporation into the growth surface and the initial growth process using two kinds of simulation models: the Ga adlayer model and Ga droplet model. The simulation of the Ga adlayer model showed that the injected N atom diffused through the Ga adlayer and nucleation occurred in the solid/liquid interface. The simulation of the Ga droplet model showed that the injected N atom diffused on the bare GaN crystal surface and nucleation occurred at the edge of the Ga droplet. In the both simulations, scattering of injected N atoms on the surface of the Ga layer was often observed. Because Ga atoms in the Ga layer were intensively moving compared with that in the GaN crystal, injected N atoms were probably scattered by collisions with the Ga atoms in the Ga layer.

Structural Analysis of Carbon-Added Na–Ga Melts in Na Flux GaN Growth by First-Principles Calculation

We investigated the fundamentals of the effect of C addition on Na flux GaN growth by first-principles calculation. We simulated C-added Na–Ga melts using molecular dynamics (MD) simulations to examine the local melt structure around a N atom. We also calculated C–N bond energy using constrained MD simulations. Results show that a N atom bonded to a C atom and there were no Ga atoms around the N atom because C–N bond energy was larger than Ga–N bond energy. This is the reason for the suppression of heterogeneous nucleation by C addition. It was also found that the C–N bond energy was affected by surrounding Ga atoms and that the C–N atomic distance increased with the Ga coordination number around the N atom.

Strain energy analysis of screw dislocations in 4H-SiC by molecular dynamics

We simulated screw dislocations with the Burgers vector parallel to the [0001] direction in 4H-SiC by a classical molecular dynamics method. A stable structure of an extended dislocation generated by the dissociation of a screw dislocation was identified by calculating the strain energy caused by dislocation cores and stacking faults. As a result, we conclude that the most expected structure of the extended dislocation is made of partial dislocations with the Burgers vector b = 1/2c + 1/2c (c is equal to the thickness of one period in the c-axis direction of 4H-SiC) and the stacking fault that is parallel to the a-plane, and that the distance between the dislocation cores is less than about 44 A.

Growth of Bulk Nitrides from a Na Flux

The history of research and development of the Na flux method for GaN single crystal growth from its discovery in 1994 until now are summarized. Today this method becomes one of the important growth measures of high quality GaN crystals. The development of the Na flux method starting from clarification of the growth mechanism, then, showing a method of controlling nucleation by adding carbon, reporting a high quality crystallization technique by using solution stirring, and, finally, latest data on the point seeding are described.