Generation of charged nanoparticles and their contribution to growth of gallium nitride in the thermal chemical vapor deposition process

Abstract

The possibility that GaN charged nanoparticles might be generated during the synthesis of GaN nanostructures was examined in an atmospheric-pressure chemical vapor deposition (CVD) process using a differential mobility analyzer combined with a Faraday cup electrometer. With Ga2O3 precursor and NH3 gas for a GaN synthesis, both positively and negatively charged nanoparticles were confirmed in the reactor of the CVD process. The process parameters, such as the reactor temperature and the gas flow rate affected not only the growth behavior of GaN nanostructures but also the size distribution of charged nanoparticles. No GaN nanostructures were produced when these charged nanoparticles were not detected in the gas phase. Under the condition where the CNPs are abundantly generated which are considered as building blocks of nanostructures, it is expected that the nanostructure evolution can be controlled by the electric field near the substrate. In Chapter II, to investigate the effect of electric field on the GaN growth behavior, various electric biases of ground, float, direct current (DC), and alternative current (AC) were applied on the stainless substrate holder where the sapphire or Si substrate was loaded during the atmospheric-pressure CVD and metal organic CVD processes. Not only the nanostructure evolution but also the deposition rate on the substrates were affected by the applied electric biases. In the CVD process where the CNPs were generated, the electric field was suggested as an important processing parameter. In Chapter III, GaN growth behavior on various metal substrates was investigated during gallium oxide reduction process of atmospheric-pressure CVD. The metal substrates were Ti, Ta, Cu, Mo, W, Fe, Ni, and Pt in order of increasing charge transfer rate (CTR). On the group of lower CTR (from Ti to W), GaN nanowires were synthesized, while thin films were made on the group of higher CTR (Fe, Ni, Pt). GaN coverage on the metal substrates of lower CTR were relatively poor than of higher CTR as evaluated by the field emission scanning electron microscopy and the mass change measurements. As an important factor to understand the growth behavior, the correlation between CTR and nanostructure evolution was discussed. Finally, as an independent topic, the role of charges in the nanostructure evolution by units of CNPs was studied in Chapter IV. In order for crystals to grow by the fusion or coalescence of nanoparticles, an unusually high rate of atomic diffusion must be accompanied. There have been some evidences implying that such a high rate of diffusion might come from the electric charge carried by the nanoparticles. In Chapter IV. 7, the effect of charge on the nanoparticle-based crystallization was studied by comparing the deposition behavior of charged Si nanoparticles between electrically grounded and floated Si substrates. Under the same processing condition, nanowires grew on the floated substrate whereas nanoparticles grew on the grounded substrate or a dense film grew on the floated substrate whereas a porous film grew on the grounded substrate. In Chapter IV. 8, the nanostructure evolution was studied using the in-situ TEM observation of Au nanoparticles on the TEM grid. Au nanoparticles of bellow ~ 15 nm were sputtered on each TEM grid of the pure carbon and SiO membrane, where the SiO membrane was less conducting (more insulating) than the carbon membrane. After several tens of minutes of beam incidence time, the Au nanoparticles on the SiO membrane started to shake and coalesce, while the nanoparticles on the carbon membrane were not much changed. It is noticeable, when the nanoparticles of the SiO membrane coalesced each other to form a larger particle, local diffusion occurred on the nanoparticle surface. Accordingly, the large particle had smooth surface and was void free. This result indicates the crystal of void free nanostructure and smooth surface might be formed by the charge enhanced diffusion of atoms.