High-temperature phase stability of hafnium aluminate films for alternative gate dielectrics

Abstract

Hafnium aluminate films with different compositions were deposited at room temperature by jet vapor deposition. The as-deposited films were amorphous. After annealing at 1100 °C, the microstructure of the films was analyzed by high-resolution transmission electron microscopy, electron diffraction and electron energy loss spectroscopy (EELS). The crystalline phase in pure ${\mathrm{HfO}}_2$ films was monoclinic. With an increase in the Al content in the films, the amount of metastable ${\mathrm{HfO}}_2$ with a tetragonal distorted fluorite structure increased. In addition, the grain sizes decreased, making the detection of crystallization by x-ray diffraction difficult. No crystalline ${\mathrm{Al}}_2{\mathrm{O}}_3$ phase could be unambiguously detected in electron diffraction patterns in films with up to 30 mol % ${\mathrm{Al}}_2{\mathrm{O}}_3.$ However, an Al-rich intergranular phase was identified by EELS. Films with ∼64 mol % ${\mathrm{Al}}_2{\mathrm{O}}_3$ crystallized as tetragonal ${\mathrm{HfO}}_2$ and metastable cubic ${\mathrm{Al}}_2{\mathrm{O}}_3$ with the spinel structure. The complex microstructures of the films should be considered in the interpretation of their dielectric and electrical properties after high temperature anneals.