Akitomo Tachibana

Electronic energy density in chemical reaction systems

The energy of chemical reaction is visualized in real space using the electronic energy density nE(r) associated with the electron density n(r). The electronic energy density nE(r) is decomposed into the kinetic energy density nT(r), the external potential energy density nV(r), and the interelectron potential energy density nW(r). Using the electronic energy density nE(r) we can pick up any point in a chemical reaction system and find how the electronic energy E is assigned to the selected point. We can then integrate the electronic energy density nE(r) in any region R surrounding the point and find out the regional electronic energy ER to the global E. The kinetic energy density nT(r) is used to identify the intrinsic shape of the reactants, the electronic transition state, and the reaction products along the course of the chemical reaction coordinate. The intrinsic shape is identified with the electronic interface S that discriminates the region RD of the electronic drop from the region RA of t...

Quantum chemical study of parasitic reaction in III-V nitride semiconductor crystal growth

We have discussed the gas-phase parasitic reactions in M(CH3)3/H2/NH3 systems following the elimination of methane by carrying out ab initio quantum chemical calculations, where M denotes Al, Ga, or In. It is clearly shown that the Al source gases enhance reactivity, and the adduct-derived chain compounds grow successively with high exothermicity. We have concluded that the strong Al–N coordination interaction contributes remarkably to the stabilization of the reaction system. In the presence of excess ammonia, we have proved that potential energy barrier of the methane elimination is reduced considerably. The methane elimination by reaction of carrier H2 gas with M(CH3)3 is also exothermic.

Quantum chemical mechanism in parasitic reaction of AlGaN alloys formation

The mechanism of parasitic reactions among trimethylaluminum (TMA), trimethylgallium (TMG), and NH3 in atmospheric pressure (AP) MOVPE for growth of AlGaN is theoretically studied using the quantum chemical method. The calculations show that metal–nitrogen chain growth reaction easily proceeds through the successive reactions of ‘complex formation with NH3’ and ‘CH4 elimination by the bimolecular mechanism’. Additionally, a parasitic reaction in APMOVPE using other raw material is also investigated. The calculated result shows that small change of raw material raises activation energy of parasitic reaction, and, thus, the parasitic reaction is suppressed. This result suggests a way to improve APMOVPE by a suitable choice of substituent.

Quantum Chemical Studies of Gas Phase Reactions between TMA, TMG, TMI and NH3

The parasitic reactions among trimethylaluminum (TMA), trimethylgallium (TMG), and NH3 obstruct the growth of AlGaN alloys in atmospheric pressure (AP) MOVPE. In this paper, we present quantum chemical calculations for the reaction of M(CH3)3 + NH3 systems (M = Al, Ga, and In for TMA, TMG, and TMI, respectively) in the gas phase. The calculations show that TMA helps the parasitic reactions to progress by orbital interaction effect. The detailed energetics and the possible reaction mechanisms of the parasitic reaction processes are discussed.

Quantum chemical study of gas-phase reactions of trimethylaluminium and triethylaluminium with ammonia in III-V nitride semiconductor crystal growth

Abstract We have discussed the gas-phase parasitic reactions in MR 3 /H 2 /NH 3 (M=Al, Ga, In; R=CH 3 , C 2 H 5 ) systems following the elimination of methane or ethane in terms of the regional density functional theory by carrying out ab initio quantum chemical calculations. It is clearly shown that Al source gases enhance reactivity for both of R=CH 3 and C 2 H 5 . Furthermore, the difference in reactivity between M(CH 3 ) 3 /H 2 /NH 3 and M(C 2 H 5 ) 3 /H 2 /NH 3 systems is discussed. We found that the substitution of ethyl groups for methyl in metal source strengthens the M–N coordination interaction, and the tendency to reduce activation energies of the elimination reaction is observed. However, the steric repulsion between ethyl groups and ammonia, which hardly takes place for R=CH 3 , raises activation energies in the presence of excess ammonia.

High-spin electronic interaction of small lithium and sodium cluster formation in the excited states

We have carried out the MRCI ab initio calculations for small lithium and sodium clusters, and elucidate the interaction between atoms in various high-spin electronic states, in terms of the quantum mechanical energy densities based on the regional density functional theory [Tachibana, J. Chem. Phys. 115, 3497 (2001)]. When the separated two electronic drop regions, where the electronic kinetic-energy density is positive, connect to each other, it is observed that ratios of occupation on configurations change rapidly in the Li2 molecule. These results are considered as one of the evidences that valence electrons can move around both two Li atoms freely in the meaning of classical mechanics. The shape of electronic drop region depends strongly on the electronic state and represents the characteristics of interaction clearly, and the electronic tension density also gives new images of microscopic electronic stresses. Furthermore, we have clarified the most stable structures of Li3 and Li4 for the high-spin ...

First-principle theoretical study on epitaxial crystal growth of GaN

We have investigated the electronic characteristics in the Ga and N adsorption processes of crystal growth in gallium nitride (GaN) by first-principle density functional calculation. Potential energy curves along the Ga and N adsorption processes for the surface orientations (0001), (0001), (1100), and (1120) were obtained. We have observed the obvious superiority of (0001) orientation in growth rate. We have discussed the local electronic processes in the crystal growth of GaN in terms of the variety of energy densities introduced by the regional density functional theory.

Theoretical study of penetration reaction of fluorine atoms and ions into hydrogen-terminated Si(111) thin film

Abstract The interaction of fluorine atoms and ions with hydrogen-terminated Si(111) thin film has been studied using periodic ab initio electronic structure calculations. Penetration of fluorine atoms into the Si surface is found to be quite easy, and an interstitial fluorine atom can be situated directly behind the SiH bond while the original lattice structure, as well as the SiH bond itself, remains intact. The fluorine atom in this position is negatively charged by accumulating electron charges from the nearest silicon atoms. The penetration reaction of the fluorine ions into the Si surface is also found to be easy.

First-principle theoretical study on the dynamical electronic characteristics of electromigration in the bulk, surface and grain boundary

New formula for the driving force of electromigration have been found using new concepts of (1) kinetic energy density, (2) tension density, and (3) effective charge tensor density. The new “dynamic” wind charge tensor density Z⇊a dynamic wind(r) is revealed over and above the conventional “static” wind charge tensor density Z⇊a static wind(r). Some numerical analysis will be demonstrated for possible application to electromigration reliability problems of ULSI devices where extremely high current densities should be maintained through ultra thin film interconnects. Key issues in this study are the comparative study of the dynamical electronic characteristics for the electromigrating regions in the bulk, surface, and grain boundary.

Quantum chemical study of aluminum CVD reaction for titanium nitride (111) surface with terminal fluorine

Abstract Reaction mechanisms and interface structures in aluminum (Al) chemical vapor deposition (CVD) have been investigated in terms of the surface termination models for TiN by ab initio quantum chemical approach. The B3LYP density functional calculations and coupled-cluster calculations were carried out by using the local structural models in order to investigate reaction mechanisms, and we found Al can be deposited on F-terminated TiN (111) surface via the substitution of hydrogen for terminal fluorine. Energetics of the reaction processes has also been discussed by the local density approximation (LDA) with the periodic boundary models. The result that total reaction enthalpy is exothermic has been obtained for the local structural models calculations and the periodic boundary calculations. In particular, a series of reactions by the successive attack of AlH 3 proceed with no energy barrier for the periodic models.