C. Y. Kim

Structural Evolution and the Control of Defects in Atomic Layer Deposited HfO2–Al2O3 Stacked Films on GaAs

The structural characteristics and interfacial reactions of bilayered dielectric stacks of 3 nm HfO2/2 nm Al2O3 and 3 nm Al2O3/2 nm HfO2 on GaAs, prepared by atomic layer deposition (ALD), were examined during film growth and the postannealing process. During the postdeposition annealing (PDA) of the Al2O3/HfO2/GaAs structures at 700 °C, large amounts of Ga oxides were generated between the Al2O3 and HfO2 films as the result of interfacial reactions between interdiffused oxygen impurities and out-diffused atomic Ga. However, in the case of the HfO2/Al2O3/GaAs structures, the presence of an Al2O3 buffer layer effectively blocked the out-diffusion of atomic Ga, thus suppressing the formation of Ga oxide. Microstructural analyses showed that HfO2 films that were deposited on Al2O3/GaAs had completely crystallized during the PDA process, even at 700 °C, because of the Al2O3 diffusion barrier. Capacitance-voltage measurements showed a relatively large frequency dispersion of the Al2O3/HfO2/GaAs structure in accumulation capacitance compared to the HfO2/Al2O3/GaAs structure due to a higher interface state density. Conductance results revealed that the Al2O3 buffer layer on GaAs resulted in a significant reduction in gap states in GaAs. The induced gap state in the Al2O3/HfO2/GaAs structure originated from the out-diffusion of atomic Ga into the HfO2 film. Density functional theory calculations supported this conclusion.

Evidence of the C60/Cu contact formation after thermal treatment

The origin of the lowered electron injection barrier height of C60∕Cu was investigated by in situ ultraviolet photoelectron spectroscopy and x-ray photoelectron spectroscopy (XPS). The onset of the highest occupied molecular orbital level was shifted by 0.2eV toward high binding energy upon the heat treatment, resulting in the improved injection characteristics of the device. Moreover, an unexpected gap state has been observed at 1.2eV below the Fermi level. The XPS core-level spectra revealed that the chemical reaction between C60 and Cu at the interface induced the gap state after heat treatment. The gap state pinned the Fermi level close to the lowest unoccupied molecular orbital of C60. We obtained the complete energy level diagram of C60∕Cu before and after the heat treatment.

Change in the interfacial reaction of Hf-silicate film as a function of thickness and stoichiometry.

Medium energy ion scattering and high-resolution transmission electron microscopy are used to investigate the depth of the interfacial reaction of Hf-silicate film. The interfacial reaction is critically affected by the film thickness and the mole fraction of HfO(2) in silicate film. The interfacial compressive strain generated at the surface of the Si substrate is dependent on the film thickness during the postannealing process in film with a thickness of approximately 4 nm. Finally, the phase separation phenomenon demonstrates critically different behaviors at different film thicknesses and stoichiometries because the diffusion of Si from interface to surface is dependent on these factors. Moreover, the oxidation by oxygen impurity in the inert ambient causes SiO(2) top formation.

Nucleation Behavior of Atomic Layer Deposited SiO2 for Hf-Silicate Films

The nucleation and growth of SiO 2 , as a function of surface condition, was investigated using in situ medium-energy ion-scattering analysis. The amount of SiO 2 required for saturation on the thermal oxide and on the chemical oxide was found to be 0.75 and 3.84 X 10 14 Si/cm 2 , respectively. The growth of SiO 2 on the initial HfO 2 surface increased as a function of HfO 2 coverage and growth when the HfO 2 coverage over 1 monolayer (ML) becomes saturated when the coverage of SiO 2 is ∼7 X 10 14 Si/cm 2 , which corresponds to a coverage of 1 ML SiO 2 . The nucleation of SiO 2 greatly depends on the initial surface conditions. In addition, the nucleation of SiO 2 has a tendency to become saturated, regardless of the initial surface conditions.

Energy level alignment at C 60 /Hf using x-ray and ultraviolet photoelectron spectroscopy

We report the characterization of the interface formation of fullerene (C60) on hafnium (Hf) using x-ray and ultraviolet photoelectron spectroscopy (XPS and UPS). The valence band, C 1s and Hf 4f spectra were measured during the deposition of C60 on a clean Hf surface in a stepwise manner. After enough deposition of C60 layers on Hf, XPS measurements indicate that there is no chemical reaction between C60 on Hf, and band bending exists at the C60/Hf interface. From UPS measurements, the energy level difference between Fermi level of Hf and the onset of the highest occupied molecular orbital of C60 is 2.01 eV. The vacuum level of Hf was shifted toward low binding energy (0.95eV) as the C60 deposition. These valence spectra results indicate that there exist an interface dipole and small electron injection barrier at the C60/Hf interface. We provide the complete energy band diagram of the C60/Hf interface and confirm that a small electron injection barrier can be achieved by inserting a low work function metal in a C60 field-effect transistor.