The size distributions of Si and ZnTe nanoparticles produced by low energy density ArF (193 nm) pulsed laser ablation into ambient gases were measured as a function of the gas pressure, P, and target-substrate separation, Dts. For both Si and ZnTe, the largest nanoparticles were found closest to the ablation target, and the mean nanoparticle size decreased with increasing Dts. For Si ablation into He, the mean nanoparticle diameter did not increase monotonically with gas pressure but reached a maximum near P = 6 Torr. High resolution Z-contrast transmission electron microscopy and energy loss spectroscopy revealed that ZnTe nanoparticles consist of a crystalline core surrounded by an amorphous ZnO shell; growth defects and surface steps are clearly visible in the crystalline core. A pronounced narrowing of the ZnTe nanocrystal size distribution with increasing Dts also was found. The results demonstrate that the size of laser-ablated nanoparticles can be controlled by varying the molecular weight and pressure of an ambient gas and that nanometer-scale particles can be synthesized. Larger aggregates of both ZnTe and Si having a “flakelike” or “weblike” structure were formed at the higher ambient gas pressures; for ZnTe these appear to be open agglomerates of much smaller (∼10 nm) particles.
We have calculated the nucleation region (NR) location of Si nanocrystal grains prepared by pulsed laser ablation (PLA) with fluence of 4 J/cm2 in 10 Pa gas at room temperature, and ambient gases were He, Ne, and Ar, respectively. Results of calculation indicated that NR width in Ne gas was narrowest, while it was widest in He gas. Maximum mean size of grains deposited on substrates under ablated spot, which were placed horizontally, was the smallest in Ne gas. It would be attribute to more effective energy transfer during the process of collision when atomic mass of Si and ambient gas Ne are more close to each other. In this work, an additional gas flow with the same element as ambient gas was introduced, which is vertical to the plume axis at different lateral positions above ablated spot.
ABSTRACTEpitaxial thin films of nitrogen-doped p-ZnTe were grown on single-crystal, semi-insulating GaAs substrates via pulsed laser ablation of a stoichiometric ZnTe target. Both low pressure nitrogen ambients and high vacuum were used. Results of in situ reflection high energy electron diffraction (RHEED) and time-resolved ion probe measurements have been compared with ex situ Hall effect and transmission electron microscopy (TEM) measurements. A strong correlation was observed between the nature of the film's surface during growth (2-D vs. 3-D, assessed via RHEED) and the ambient gas pressures employed during deposition. The extended defect content (assessed via cross-sectional TEM) in the region >150 nm from the film/substrate interface was found to increase with the ambient gas pressure during deposition, which could not be explained by lattice mismatch alone. At sufficiently high pressures, misoriented, columnar grains developed which were not only consistent with the RHEED observations but also were correlated with a marked decrease in Hall mobility and a slight decrease in hole concentration. Ion probe measurements, which monitored the attenuation and slowing of the ion current arriving at the substrate surface, indicated that for increasing nitrogen pressure the fast (vacuum) velocity-distribution splits into a distinct fast and two collisionally-slowed components or modes. Gas-controlled variations in these components mirrored trends in electrical properties and microstructural measurements.
Influence of Gas Sort on the Nucleation Region Width of Si Nanocrystal Grains Prepared by Pulsed Laser Ablation