Paper TF+AS+EM+NS+SS-ThM10
Structure and Optical Properties of Nanocrystalline Hafnium Oxide Thin Films Made by Sputter-Deposition

Thursday, October 31, 2013, 11:00 am, Room 104 A

Hafnium oxide (HfO_2­) is a unique material characterized by excellent chemical and physical properties. It is a wide band gap ( 5.5 eV) material, which makes it attractive for optoelectronics since it transparent from the ultraviolet to the mid-infrared region. Being a promising high-k dielectric, HfO₂ has a strong potential to replace silicon oxide as the insulator in CMOS devices. HfO₂ exhibits various polymorphs; the thermodynamic stability and phase existence depends on the temperature and pressure conditions. Controlled growth and manipulation of specific crystal structures of HfO₂ at the nanoscale dimensions has important technological implications. The present work entails a detailed analysis of growth behavior, microstructure, and optical properties of monoclinic HfO₂ films as a function of growth temperature. In addition, the effect of post-deposition annealing temperature was also studied. HfO₂ thin films were grown by RF magnetron sputtering onto silicon (100) and quartz substrates by keeping power, pressure, and flow of Ar and O₂ and their ratio (70:30) constant, but varying growth temperature from room temperature to 600 C. A thorough characterization was performed through scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), and grazing incidence x-ray diffraction (GIXRD). Optical properties were evaluated using spectrophotometric and ellipsometry measurements. GIXRD data revealed a well oriented structure along (-111) as temperature increases, and an evident crystallization temperature at 300 C. The grain sizes measured were in the range of 15 to 20 nm; grain sizes increased with temperature. While SEM and AFM analyses also indicate the grain size with increasing temperature, roughness of the films exhibits a decreasing tendency with increasing temperature. Optical data revealed a double band gap for temperatures higher than RT as well as an increase in band gap with increasing growth temperature. The band gap for at HfO₂ thin films grown at RT was found to be 5.7 eV. The band gap increased to values of ~6.2 eV with increasing growth temperature. A correlation between growth conditions, microstructure and optical properties of nanocrystalline HfO₂ thin films is discussed.