Stress evolution of Ge nanocrystals in dielectric matrices

Abstract

Germanium nanocrystals (Ge NCs) embedded in single and multilayer silicon oxide and silicon nitride matrices have been synthesized using plasma enhanced chemical vapor deposition followed by conventional furnace annealing or rapid thermal processing in N ambient. Compositions of the films were determined by Rutherford backscattering spectrometry and x-ray photoelectron spectroscopy. The formation of NCs under suitable process conditions was observed with high resolution transmission electron microscope micrographs and Raman spectroscopy. Stress measurements were done using Raman shifts of the Ge optical phonon line at 300.7 cm. The effect of the embedding matrix and annealing methods on Ge NC formation were investigated. In addition to Ge NCs in single layer samples, the stress on Ge NCs in multilayer samples was also analyzed. Multilayers of Ge NCs in a silicon nitride matrix separated by dielectric buffer layers to control the size and density of NCs were fabricated. Multilayers consisted of SiN :Ge ultrathin films sandwiched between either SiO or SiN by the proper choice of buffer material. We demonstrated that it is possible to tune the stress state of Ge NCs from compressive to tensile, a desirable property for optoelectronic applications. We also observed that there is a correlation between the stress and the crystallization threshold in which the compressive stress enhances the crystallization, while the tensile stress suppresses the process.

Fabrication and characterization of Si1-xGex nanocrystals in as-grown and annealed structures: A comparative study

“…This can be due to a better wetting of SiGe layer, which in turn reduces the free energy of change and reduces the nucleation barrier. It can also be argued that since heterogeneous nucleation occurs at preferential sites (as in our case), small nano-crystallites in as-grown MLs or even the crystallites which are under strain38,39,40,41 , will further reduce the surface energy and facilitate nucleation. c b a…”

“…the matrix will resist the NCs to expand freely. Thus, as a result of growing crystallites, strain is introduced at the interface between the matrix and NCs 38,39,54 , which in turn alters the bandgap ( Fig. 7(d)).…”

“…7(d)). Additionally, the role of thermal expansion coefficient of SiGe/ SiO2 and lattice mismatch between Si and Ge (4.2 % 55,31 ), that assists in the development of strain in structure, should be taken into account 38 . From the above discussion, it can be summarized that the annealing temperature does affect the structuring of the oxide matrix which in turn induces strain in the structure and therefore alters the interface morphology, hence inducing a change in the intensity ratio of peak I/N ( Fig.…”

Fabrication and characterization of Si1−xGex nanocrystals in as-grown and annealed structures: a comparative study

“…This can be due to a better wetting of SiGe layer, which in turn reduces the nucleation barrier. It can also be argued that since heterogeneous nucleation occurs at preferential sites (as in our case), small NCs in as-grown MLs or even the crystallites that are under strain [39–42] will further reduce the surface energy and facilitate nucleation.…”

“…The degree of matrix ordering determines the accommodation of the growing crystallites, i.e., the matrix will hinder the NCs to expand freely. Thus, as a result of growing crystallites, strain is introduced at the interface between the matrix and NCs [39–40 55], which in turn alters the bandgap (Fig. 7).…”