Characterisation of the interface of high-K praseodymium oxide thin films on silicon surfaces

Further scaling down of microelectronic devices requires a thickness of the SiO₂ gate oxide (Figure 86a) of 1 nm, which leads to a large leakage current. Thus, a severe challenge facing semiconductor technology is to replace the SiO₂ gate dielectric in CMOS transistors. Candidates must have a high dielectric permittivity K, a large band gap, and exhibit high thermal stability and low interface state density in contact with silicon [1]. Rare-earth oxides, particularly praseodymium oxide Pr₂O₃, which show several properties that meet these requirements, are therefore considered as promising candidates.

With feature sizes reaching the nanoscale, surfaces and interfaces begin to dominate the silicon device performance, due to the increasing surface to volume ratio. Owing to the complexity of these structures and the difficulties in characterising such low dimensional systems, the Pr-oxide/Si(001) interface is not well understood despite its importance. Therefore, in this work, we have investigated the growth of Pr-oxide on Si(001) at the atomic scale and from the very early stages. We used grazing-incidence X-ray diffraction (GIXRD) and photoelectron spectroscopy (XPS) in situ, with the sample kept in ultra high vacuum (UHV), complemented by other techniques such as scanning tunnelling microscopy (STM) and transmission electron microscopy (TEM).

Pr-oxide thin films with thicknesses up to 4 nm were prepared by molecular beam epitaxy on atomically clean Si(001) surfaces. Figure 86b shows a TEM cross section of one of our 3 nm thick Pr-oxide films. An interfacial layer is observed but cannot be identified with TEM. The atomic structure (cf. Figure 86c) was determined by GIXRD based on 20 non-equivalent Si crystal truncation rods (CTRs). Four of them are presented in Figure 87a. The result of the structural refinement reveals a 0.5 nm interfacial epitaxial layer of cubic Pr₂O₃, which exhibits a 3x1 superstructure, on top of which a disordered layer forms.

Fig. 86: High-K oxide / Si(001) interface studied at the atomic scale. a) Schematic cross-section of a CMOS transistor. The square indicates the region of interest in our study. b) Cross sectional TEM picture of a 3 nm thick Pr-oxide film on Si(001), which indicates the presence of an interfacial layer. c) Refined atomic structure based on GIXRD data.

The high resolution STM topograph presented in Figure 87b displays the surface morphology of the Si surface after the deposition of 0.1 nm of Pr-oxide. Dimer rows of the 2x1 reconstruction of the Si(001) surface are visible between the oxide clusters, which appear bright in the topograph. A line profile is presented in the bottom inset of Figure 87b. The height difference between the areas marked by the first and second arrows from the left corresponds to a Si atomic step (~ 0.14 nm). The presence of small domains containing dimer rows aligned in two perpendicular directions indicates the formation of small patches of holes separated by atomic steps. Such surface morphology suggests a reaction of Si with the Pr-oxide, leading to the removal of some of the Si atoms in the topmost layer.

Fig. 87: a) Four CTRs measured in the case of a 1 nm thick Pr-oxide film. Each rod is labelled by its (HK) values. The red symbols correspond to the integrated intensities extracted from the experimental data and the blue curves correspond to the best fit from the structural refinement analysis. b) STM topography of a 0.1 nm thick film. 30 x 20 nm2, Us = 2 V, It = 0.50 nA. c) Core-level XPS spectra of Pr4d and Si2p states, collected at different a, from a 1 nm thick Pr-oxide film. Experimental data points and simulated curves are plotted together to show the evolution of the ratio I(Si2p-silicate)/I(Pr4d) versus .

Figure 87c exhibits the core level XPS spectra of the Pr4d and Si2p states collected at different emission angles , i.e. the angles between the sample surface and the outgoing photoelectrons, from a 1 nm thick Pr-oxide film using a photon energy of 2.5 keV. The spectra are background subtracted and normalised to the Pr4d integrated peak intensities. The Si2p component appearing at ~ 102.7 eV corresponds to Si-Si bonds in a silicate (Si-O-Pr) [2]. The I(Si2p-silicate)/I(Pr4d) ratio measured at = 4° suggests the Pr₂Si₂O₇ stoichiometry. The fact that this intensity ratio decreases as increases, i.e., when the measurement is more bulk sensitive, supports the existence of a thin Si-free Pr-oxide layer at the interface identified by GIXRD.

With the help of GIXRD and XPS we identified a crystalline Pr-oxide layer at the high-k-oxide/Si interface, hidden underneath a Pr₂Si₂O₇ silicate structure(on top of which cubic Pr₂O₃ eventually grows upon further deposition [3]). These findings may be relevant for the growth of other dielectrics such as Gd₂O₃, Nd₂O₃, Y₂O₃, and even HfO₂ on silicon.

References

[1] International Technology Roadmap for Semiconductors, 2005 ed., http://public.itrs.net.
[2] D. Schmeisser, H.-J. Müssig, J. Dabrowski, Appl. Phys. Lett. 85, 88 (2004).
[3] T. Schroeder et al., Appl. Phys. Lett. 85, 1229 (2004).

Principal Publication and Authors

L. Libralesso (a), T.-L. Lee (a), C. Dubourdieu (b), J. Zegenhagen (a).
(a) ESRF
(b) LMGP / CNRS – INPG (France)