טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentKeren Rotberg
SubjectThe Stability of Surface Planes in Sapphire
DepartmentDepartment of Materials Science and Engineering
Supervisor Full Professor Kaplan Wayne D.
Full Thesis textFull thesis text - English Version


Abstract

The equilibrium crystal shape (ECS or Wulff shape) represents relative surface energies for solids, and can be studied in ceramics by equilibrating internal cavities. Previous studies observed five stable facet planes in the ECS of sapphire: c{0001}, r{012}, s{101}, a{110}, and p{113}. The m{100}facet plane was not observed and tended to break up into a ‘hill-and-valley’ morphology, although theoretical calculations predicted it to have a low surface energy. The equilibrated surface structure is known to be affected by impurities, dopants, and components of the environment used to equilibrate the surface.  For example, the presences of oxygen, hydrogen, or Si are expected to cause either macroscopic or microscopic reconstruction of sapphire surfaces.

In this study, two phenomena were investigated: the macroscopic surface stability of sapphire with respect to the m-plane in the presence of impurities and varying atmospheres, and the intersection of holes in thin sapphire films with the sapphire ECS. 

Sapphire substrates with the m-plane parallel to the free surface were annealed in air at low and high impurity contents, and in an Ar2 atmosphere. The morphology of the surface was examined by high resolution scanning electron microscopy, atomic force microscopy, and in cross-section using transmission electron microscopy (TEM) from specimens prepared using the focused ion beam ‘in-situ lift-out’ technique. The air annealed m-plane surfaces decomposed into a ‘hill-and-valley’ morphology, unlike the Ar2 annealed surfaces, suggesting that the m-plane may be thermodynamically stable below a certain impurity level or in the presence of hydrogen. To investigate the intersection of the sapphire ECS with thin films, small (<500nm) holes were formed in m and a-plane sapphire TEM samples prepared by ion-milling methods. The shape of the holes was monitored by TEM during several thermal treatments in air. The final faceted shape of the holes was analyzed using a computer program designed to analyze the relative (metastable) energy of holes in thin films which intersect the ECS.