Nong-Moon Hwang

Charged clusters in thin film growth

Abstract A cauliflower structure is a granular film composed of spherical particles similar in size, each with numerous nanoscale nodules on its surface. The structure is produced during certain chemical vapour deposition (CVD) processes for diamond and silicon thin film growth. A classical account in terms of atomic unit deposition fails to explain the growth of such a cauliflower structure, as it requires a gas phase of much higher supersaturation than for onset of diffusion controlled growth. Another interesting and somewhat puzzling phenomenon encountered during a diamond CVD process is that while diamond is depositing on a graphite substrate, carbon atoms in the graphite itself are etched away into the vapour phase; that is, experience evaporation. Again, an elementary kinetic barrier mechanism fails to explain such CVD deposition of a less stable diamond phase combined with simultaneous evaporation of a stable graphite phase. In order to account for such puzzling CVD phenomena and others, a theory of charged clusters has been developed over the past decade as a new paradigm for thin film growth. The theory and its applications are reviewed in this work.

Microstructural evidence of abnormal grain growth by solid-state wetting in Fe-3%Si steel

In this investigation, the mechanism of abnormal grain growth in Fe-3%Si steel was based on the microstructure evolution at the growth front of grains undergoing the abnormal growth. The most striking feature in the growth of abnormal grains was the penetration along the grain boundary of neighboring grains. This is energetically possible if the energy of the penetrated grain boundary is higher than the sum of the energy of two other grain boundaries shared by the penetrating abnormal grain. Along the growth front of an investigated abnormally growing grain, 15 out of 1381 triple junctions showed the clear microstructural evidence of the grain boundary penetration by the abnormal grain. Misorientation measurements of 34 penetrated grain boundaries using electron backscattered diffraction showed that not a single boundary has a low angle, implying that the penetrated grain boundaries have the high energy. These results are best explained by the abnormal grain growth with solid-state wetting.

Effect of sintering temperature on the secondary abnormal grain growth of BaTiO3

Abstract When BaTiO3 specimens containing a small amount of excess TiO2 were sintered for more than 15 h, some grains grew abnormally to several millimeters in size. This phenomenon may be referred to as the secondary abnormal grain growth (SAGG) because it occurred from a large and uniform grain structure (average grain size: 70 μm) after completion of primary abnormal grain growth. SAGG was observed only at a very narrow temperature range between 1360 and 1370°C, where the solid–liquid interface structure was atomically smooth. Almost all the secondary abnormal grains contained the (111) double twin, which provides the persistent twin-plane re-entrant edge (TPRE). During SAGG, the growth of matrix grains was strongly suppressed and material transfer occurred preferentially to the grains with a (111) double twin.

The mechanism of TiO2 deposition by direct current magnetron reactive sputtering

Thin films are generally thought of as being the product of a reaction between the surface and atoms and/or molecules in the gas phase. However, a relatively new theory, the theory of charged clusters (TCC), suggests that charged clusters nucleate in the gas phase and become the growth unit for a thin film. The aim of this study was to determine whether or not TiO2 thin film deposition by DC reactive sputtering occurs via this mechanism. TiO2 was deposited on unheated transmission electron microscopy grids to observe TiO2 clusters, as well as glass and silicon substrates to observe the resulting thin films. The results showed that TiO2 clusters were indeed produced in the chamber of a direct current reactive sputtering system. Furthermore, these clusters were observed as close as 50 mm away from the target. Clusters 3 nm and <2 nm in diameter were found 250 mm and 50 mm away from the target, respectively The cluster size was found to have a direct effect on the film deposited. Smaller clusters produced a facetted crystalline anatase film whereas larger clusters produced an amorphous film.

Charged nanoparticles in thin film and nanostructure growth by chemical vapour deposition

The critical role of charged nanoclusters and nanoparticles in the growth of thin films and nanostructures by chemical vapour deposition (CVD) is reviewed. Advanced nanoparticle detection techniques have shown that charged gas-phase nuclei tend to be formed under conventional processing conditions of thin films and nanostructures by thermal, hot-wire and plasma CVD. The relation between gas-phase nuclei and thin film and nanostructure growth has not been clearly understood. In this review it will be shown that many films and nanostructures, which have been believed to grow by individual atoms or molecules, actually grow by the building blocks of such charged nuclei. This new growth mechanism was revealed in an attempt to explain many puzzling phenomena involved in the gas-activated diamond CVD process. Therefore, detailed thermodynamic and kinetic analyses will be made to draw the conclusion that the well-known phenomenon of deposition of less stable diamond with simultaneous etching of stable graphite should be an indication of diamond growth exclusively by charged nuclei formed in the gas phase. A similar logic was applied to the phenomenon of simultaneous deposition and etching of silicon, which also leads to the conclusion that silicon films by CVD should grow mainly by the building blocks of charged nuclei. This new mechanism of crystal growth appears to be general in many CVD and some physical vapour deposition (PVD) processes. In plasma CVD, this new mechanism has already been utilized to open a new field of plasma-aided nanofabrication.

The formation of pentagon-heptagon pair defect by the reconstruction of vacancy defects in carbon nanotube

The reconstruction process of vacancy hole in carbon nanotube is investigated by tight-binding molecular dynamics simulations and by ab initio total energy calculations. In the molecular dynamics simulation, a vacancy hole is found to reconstruct into two separated pentagon-heptagon pair defects. As the result of reconstruction, the radius of the carbon nanotube is reduced and the chirality of the tube is partly changed. During the vacancy hole healing process, the formation of pentagonal and heptagonal rings is proceeded by the subsequent Stone-Wales [Chem. Phys. Lett. 128, 501 (1986)] transformation.

Formation and development of dislocation in graphene

The formation and development processes of dislocation in graphene are investigated by performing tight-binding molecular dynamics (TBMD) simulation and ab initio total energy calculation. It is found that the coalescence of pentagon-heptagon (5-7) pairs with vacancy defects induces the formation of dislocation due to the separation of two 5-7 pairs. In TBMD simulations, adatoms are ejected and evaporated from graphene surface so that the dislocation is developed. It is observed that diffusing carbon atoms nearby dangling bonds help non-hexagonal rings change into stable hexagonal rings. These results might give some ideas for the control of structural properties by inducing defect structures.

Spontaneous generation of negatively charged clusters and their deposition as crystalline films during hot-wire silicon chemical vapor deposition*

The hot-wire silicon chemical vapor deposition (CVD) was approached by the new concept of the theory of charged clusters (TCC). The role of a hot wire is to enhance the rate of negative surface ionization producing negative ions. These ions induce nucleation and produce negatively charged silicon clusters, which deposit as polycrystalline films at low temperatures. During the deposition of silicon, an appreciable amount of negative current (~nA/cm2) was measured, and clusters, a few nanometers in size, were captured and observed by transmission electron microscopy (TEM). The effect of bias on the deposition behavior of the clusters indicated that most of the clusters were negatively charged. In order to deposit films with a large grain size with a high mobility, both the generation of neutral clusters and the cluster size should be minimized. A working pressure of 0.3 Torr and a wire temperature of 1800 °C were found to be optimal. Under these conditions, the film with grain size of almost 1 μm could be deposited with a mobility of 175 cm2/Vsec.

Generation of Charged Nanoparticles during Synthesis of ZnO Nanowires by Carbothermal Reduction

We measured the size distribution of positively and negatively charged nanoparticles generated abundantly in the gas phase during the syntheses of ZnO nanowires by a carbothermal reduction process using a differential mobility analyzer. Under the conditions where these charged nanoparticles were not generated, no nanowires could be grown. The evolution of ZnO nanostructures on the substrate was studied with in-situ measurements of the size distribution of charged nanoparticles with varying reactor temperature and oxygen flow rate. The results suggest that the electrostatic energy arising from charged nanoparticles would play a critical role in the growth of ZnO nanowires.

The role of pentagon–heptagon pair defect in carbon nanotube: The center of vacancy reconstruction

We show that pentagon–heptagon (5–7) pair defects in carbon nanotube play an important role as the center of vacancy reconstruction using tight-binding molecular dynamics simulations and ab initio total energy calculations. Single vacancy defect diffuses toward and coalesces with 5–7 pair defects and the coalescence structure is reconstructed into a new and more stable 5–7 pair defect plus an adatom by an exchange mechanism. In the case of four single vacancy defects, the vacancy defects coalesce with 5–7 pair defects and form defect structures with nonhexagonal rings. Finally, these defective structures reconstruct into two new 5–7 pair defects.