UVM Theses and Dissertations
Format:
Print
Author:
Zhou, Lan
Dept./Program:
Materials Science Program
Year:
2011
Degree:
Ph. D.
Abstract:
Sputter Deposition (SD) and Pulsed Laser Deposition (PLD) are modern thin film deposition techniques that are used to make thin films for a wide range of applications, such as optical coatings and magnetic recording devices. A detailed physical understanding of the fundamental processes occurring during the deposition is essential for tailoring film morphology and mechanical properties to meet the requirements for advanced devices.
In this dissertation, real-time growth studies using synchrotron-based x-ray scattering are performed to investigate the growth dynamics during SD and PLD: (i) The first part of the dissertation presents studies of sputter-deposited amorphous WSi₂films and WSi₂/Si multilayers. It is found that sudden changes in roughness and stress occur when the background argon gas pressure is varied above a critical value. Both transitions are consistent with aggregation of nanoclusters in the sputtered flux, which is proposed to be intrinsic to the sputtering process at high vapor density and hence in sputter deposition ofnearly all metals. Formation of nanoclusters in the gas phase causes increased surface roughness due to the larger' particle size and tensile stress in the film as nanoparticles merge together.
(Ii) In the second part, a study of Ge(OOl) homoepitaxial growth by PLD is presented. We find that high instantaneous flux in PLD initially produces a high density of 2D nanoscale islands, which continue to self-assemble during the recovery time between deposition bursts. The islands are found to have significant anisotropic in-plane strain, on the order of 2.5-4.0% relative to the bulk value. Strong correlations of nanometer scale islands are observed along the dimer bond direction. We explain these observations in terms of the properties of the Ge(001)-(2x1) reconstructed surface where anisotropic surface stress is tensile along the surface dimers and compressive perpendicular to them. The strain relaxation persist for multilayer growth, and are explained by a model with alternating (2x 1) and (1 x2) reconstructions in each level leading to a compensation of the stress, and partially stabilization of the metastable domains.
The observed effects are related to the high vapor density in SD and the high instantaneous deposition rate in PLD. Both processes result in formation of nanoscale structures, either by aggregation in the plasma/gas phase (SD) or by self-assembly on the growth surface (PLD).
In this dissertation, real-time growth studies using synchrotron-based x-ray scattering are performed to investigate the growth dynamics during SD and PLD: (i) The first part of the dissertation presents studies of sputter-deposited amorphous WSi₂films and WSi₂/Si multilayers. It is found that sudden changes in roughness and stress occur when the background argon gas pressure is varied above a critical value. Both transitions are consistent with aggregation of nanoclusters in the sputtered flux, which is proposed to be intrinsic to the sputtering process at high vapor density and hence in sputter deposition ofnearly all metals. Formation of nanoclusters in the gas phase causes increased surface roughness due to the larger' particle size and tensile stress in the film as nanoparticles merge together.
(Ii) In the second part, a study of Ge(OOl) homoepitaxial growth by PLD is presented. We find that high instantaneous flux in PLD initially produces a high density of 2D nanoscale islands, which continue to self-assemble during the recovery time between deposition bursts. The islands are found to have significant anisotropic in-plane strain, on the order of 2.5-4.0% relative to the bulk value. Strong correlations of nanometer scale islands are observed along the dimer bond direction. We explain these observations in terms of the properties of the Ge(001)-(2x1) reconstructed surface where anisotropic surface stress is tensile along the surface dimers and compressive perpendicular to them. The strain relaxation persist for multilayer growth, and are explained by a model with alternating (2x 1) and (1 x2) reconstructions in each level leading to a compensation of the stress, and partially stabilization of the metastable domains.
The observed effects are related to the high vapor density in SD and the high instantaneous deposition rate in PLD. Both processes result in formation of nanoscale structures, either by aggregation in the plasma/gas phase (SD) or by self-assembly on the growth surface (PLD).