Study the structural properties of Ge nanoparticles formed by ion implantation and thermal annealing in nitride-base dielectric matrices
This thesis investigates the formation of Ge nanoparticles (NPs) in amorphous Si3N4 and SiOxNy by ion implantation and thermal annealing. The structural properties of the NPs were determined using a combination of laboratory and synchrotron based techniques including cross-section transmission electron microscopy (TEM), x-ray diffraction (XRD), Rutherford backscattering spectrometry (RBS), Raman spectroscopy measurements and x-ray absorption spectroscopy (XAS). The motivation for this research is that NPs synthesized from Group IV semiconductors, including Ge, show potential for novel electronic and optoelectronic devices. Ge was chosen as the material for this study, mainly due to its very large bulk exciton Bohr radius (Rb~ 10 nm) and consequently small R/Rb ratio (where R is the NP radius), which increases the quantum confinement regime for optoelectronic applications. Also, Ge NPs embedded in thin dielectric films have exhibited impressive charge storage capabilities, useful in non-volatile memory (NVM) applications. While many studies have focused on the SiO2 matrix, there have been few experimental studies on the growth of Ge NPs in a silicon nitride and silicon oxynitride matrices. The main focus of this project was on the formation of Ge NPs in different Si3N4-based matrices and examining the short range atomic structure of embedded NPs as function of NP size using extended x-ray absorption fine structure (EXAFS). Specifically, four different matrices have considered: PECVD Si3N4, LPCVD Si3N4, PECVD SiO1.67N0.14 and PECVD SiO1.12N0.37. Size evolution and structural properties of NPs were examined as function of implantation conditions and the host matrix. Ge NPs were formed in plasma enhanced chemical vapour deposition (PECVD) Si3N4. Precipitations occurred for Ge concentrations of ≥6 at.%, which suggests the solubility limits of this order. NP size was influenced by Ge concentration and annealing temperature. NP size increased from 2.4 to 4 nm - for samples annealed at 900 ºC for 1 hour- when concentration was increased from 9 to 12 at.%. Moreover, as annealing temperature was increased from 700 to 900 ºC NP size increased from 3 to 4.5 nm (for 12 at.% Ge samples). In general NP diameters were much small compared to SiO2 matrix due to the N content of the system, low diffusivity and large interfacial energy between Ge atoms and Si3N4-based matrices. Unlike PECVD Si3N4, ion beam synthesis of Ge in LPCVD Si3N4 layers resulted in the formation of SiGe NPs. NP size altered from 2.8 to 3.2 nm for 12 at.% Ge concentration samples after annealing temperature was increased from 700 to 900 ºC. Also, NP size increased from 2.3 to 3.2 nm for samples annealed at 900 ºC for 1 hour when Ge concentration was increased from 9 to 12 at.%. Complementary XAS techniques (in particular EXAFS), allowed us a precise and better understanding of the structure of embedded SiGe NPs. Since the lattice difference between Ge and SiGe NPs (less than 4%) could not be distinguished by TEM. A phase transition of the matrix was observed for LPCVD and PECVD Si3N4 matrices after annealing at 1100 ºC. This significantly lowered crystallization temperature of un-implanted layers (1600-1800 ºC). Formation of nanosilicide particles, change of stoichiometry and implantation induced-damage are found to be the most likely causes of crystallization. Ge NPs were formed in SiO1.67N0.14 matrix for different Ge concentrations and annealing temperatures. For Ge concentrations from 9 to 12 at.% annealed at 900 ºC for 1 hour, NP size increased from 3.7 to 4.5 nm. Also, when annealing temperature was increased from 700 to 900 ºC NP size increased from 4.1 to 4.5 for 12 at.% Ge concentration. We observed that slight amount of the N in the system had a significant effect on NP size similar to PECVD Si3N4. Unlike other examined systems, cavities were formed near the implanted region in SiO1.12N0.37 matrix for different concentrations and annealing temperatures, which could be correlated to the matrix structure and composition or vacancy events after ion implantation. The size, origin and properties of formed cavities are subjects of future work. No phase transition (or long-range diffusivity of Ge atoms) was observed for SiOxNy matrices after annealing at 1100 ºC.
Ion implantation, Ge nanoparticles, X-ray absorption spectroscopy, XANES and EXAFS
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