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Ion-beam-induced modifications of thin film structures and formation of metastable phases

Citation

Tsaur, Bor-Yeu (1980) Ion-beam-induced modifications of thin film structures and formation of metastable phases. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechETD:etd-10102006-094500

Abstract

The influences of energetic ion bombardment on the composition and structure of thin film materials and the utilization of ion-beam-mixing techniques to produce compounds and alloys are reported in this thesis.

Motivated by recent experimental observations that ion bombardment can induce alteration of atomic distributions in composite materials, a systematic study of ion-induced modification of interface profile and structure has been carried out. By bombarding through a thin transition-metal film deposited on a Si substrate, significant atomic mixing was observed near the metal-Si interface at dose levels of ~10(15)cm(-2). The atomic mixing led to the formation of well-defined silicide phases which are identical to those obtained by normal thermal treatment. A macroscopic model based on collision-cascade mixing and radiation-enhanced diffusion mechanisms was proposed to account for the ion-induced interface reaction. The mixing process and its products were found to be strongly influenced by the implantation conditions such as ion energy, mass, fluence and sample temperature, as well as by the intrinsic properties of target material such as thermal diffusivity and radiation stability. Ion-beam-mixing at higher ion doses (~10(16)cm(-2)) led to the formation of more Si-rich phases or disordered metal-Si layers which are difficult to form or, in some cases, inaccessible by normal thermal process. The phenomenon of ion-induced silicide formation is similar to that observed in thermal annealing except that the radiation stability of phase structure is important in determining whether a silicide phase is formed or a disordered metal-Si mixture is formed. (Chapter 2)

The ion-beam-mixing process was then utilized to investigate the production of nonequilibrium (or metastable) phases. A metastable silicide phase of a stoichiometry Pt2Si3 has been obtained by heat treating a Si-rich amorphous Pt-Si alloy layer produced by ion-beam-mixing of thin PtSi (or Pt) films on Si. The Pt2Si3 phase is absent in the equilibrium phase diagram of Pt-Si and has not been reported before. X-ray diffraction analysis established the crystal structure of Pt2Si3 to be hexagonal with lattice parameters a = 3.841 Å, c = 11.924 Å and with 10 atoms per unit cell. The metastable phase was found to exhibit a superconducting transition onset at about 4.2K and to become completely superconductive at temperature below 3.6K. For the first time, a compound of this structure was observed to be a superconductive material. The transformation behavior of the ion-induced amorphous Pt2Si3 alloy has been studied by using resistivity measurements. The amorphous to metastable crystalline transformation occurred at ~400°C as indicated by an abrupt decrease of resistivity. The metastable phase then gradually decomposed into an equilibrium PtSi and Si mixture at temperatures above 550°C. The kinetics of amorphous to crystalline transformation have been determined by isothermal treatment over the temperature interval 376-392°C. The results were interpreted in terms of a classical nucleation and growth mechanism with a t4 (time) dependence and an apparent activation enthalpy of 4.69 eV (108 kcal/mole). The microstructures of the alloys at various stages of transformation were studied by transmission electron microscopy and diffraction. The results were found to correlate well to the phase transformation behavior observed by resistivity measurements. (Chapters 3 and 4)

To further investigate the production of metastable phases by ion-beam mixing, experiments have been performed in the simple eutectic system of Au-Si. An amorphous alloy with a uniform composition Au-28 at .% Si (~Au5Si2) was formed by bombarding through a thin Au film on Si, a result distinctly different from that obtained in normal thermal treatment. Upon thermal annealing, the amorphous phase transforms into a metastable crystalline phase at ~100°C, which then gradually decomposes into an equilibrium Au and Si mixture at higher temperatures. The present observations were compared with those obtained previously by rapid quenching techniques. The comparison of metastable phase formation in the Pt-Si and Au-Si systems revealed a correlation between the existence of metastable phases and eutectic compositions, as well as the importance of sample temperature during implantation for direct observation of metastable phases. (Chapter 5)

As an extension of compound formation by ion-beam-mixing of a thin layer on a thick substrate, we investigated the mixing of thin deposited layers as a scheme for producing compounds or alloys of desirable compositions. Multiple-layered samples consisting of thin alternate layers of two elements were prepared by sequential vacuum deposition of the two components onto an inert substrate such as Si02 or Al2O3. The relative thicknesses of the individual layers were adjusted such that the average film composition was equal to a fixed, predetermined value. Ion bombardment was then performed to homogenize the layers on an atomic scale. Formation of supersaturated Ag-Cu and Au-Co solid solutions over a wide range of composition has been achieved. Extensions of alloy solubility and formation of amorphous phases have been obtained in the almost completely immiscible systems of Ag-Ni and Cu-Ta, respectively. The present scheme may promise to be a new technique for producing metastable phases which are difficult to form or unattainable by conventional rapid quenching techniques. (Chapter 6)

Finally, we consider the possibility of using ion-beam-mixing techniques as an alternative approach to direct high dose implantation for material surface modification. Comparison between ion-beam-mixing and high dose implantation is made to demonstrate the "effectiveness" of ion-beam mixing in incorporating Au or Ag in single crystal Cu substrates. A simple diffusion model for the evolution of ion-beam-mixing of a thin surface layer on a thick substrate was then proposed. The model predicts the influence of implantation conditions and material properties on the redistribution of the thin surface layers. From a practical point of view, the technique of introducing foreign species by ion-beam-mixing process exhibits many attractive advantages over direct implantation because of significantly lower ion doses required and the ability to use simple ion sources. (Chapter 7).

Item Type:Thesis (Dissertation (Ph.D.))
Degree Grantor:California Institute of Technology
Division:Engineering and Applied Science
Major Option:Electrical Engineering
Thesis Availability:Restricted to Caltech community only
Research Advisor(s):
  • Mayer, James W. (advisor)
  • Nicolet, Marc-Aurele (advisor)
Thesis Committee:
  • Mayer, James W. (chair)
  • Nicolet, Marc-Aurele
  • Vreeland, Thad
  • Johnson, William Lewis
Defense Date:21 April 1980
Record Number:CaltechETD:etd-10102006-094500
Persistent URL:http://resolver.caltech.edu/CaltechETD:etd-10102006-094500
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:4010
Collection:CaltechTHESIS
Deposited By: Imported from ETD-db
Deposited On:19 Oct 2006
Last Modified:26 Dec 2012 03:04

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