| Research Abstract |
The objective of this study is to obtain high-dielectric-constant (high-k) thin films on Si which have an SiO_2 equivalent oxide thickness (EOT) of less than 2 nm for gate insulator application of next generation MOSFETs. First, various high-k thin films, such as ZrO_2, HfO_2, CeO_2, Y_2O_3, La_2O_3, Pr_2O_3, Sm_2O_3 and so on, were fabricated on Si substrate by the molecular beam deposition (MBD) technique and characterized. For HfO_2 and La_2O_3 thin films, EOT values of 0.88 nm and 1.5 nm were obtained, respectively. It was also found that the La_2O_3 film was easily damaged by the humidity in the air. Hence, to suppress this damage, gate electrodes were fabricated as soon as the La_2O_3 film was deposited. With this technique, we fabricated MOSFETs using a La_2O_3 gate insulator which had an EOT of 0.88 nm. Normal transistor operation was confirmed for the fabricated devices. Next, metalorganic chemical vapor deposition (MOCVD) technique was developed for HfO_2 thin films growth on Si substrates. Hf[N(CH_3)_2]_4 and Hf[N(C_2H_5)_2)]_4 which contain no oxygen nor chlorine as well as conventional Hf(O-t-C_4H_9)_4 were used as Hf precursors. Either O_2 or H_2O was used as an oxidant gas. We fabricated HfO_2 thin films on Si substrates by alternatively introducing Hf precursor and oxidant gas and found that the residual impurity concentrations and leakage current density are much lower for the HfO_2 films fabricated with H_2O than HfO_2 films fabricated with O_2. An EOT of the HfO_2 films grown with Hf[N(C_2H_5)_2]_4 and H_2O was as small as 1.8 nm and the leakage current density of the films at 1 V is less than 10^<-5> A/cm^2.
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