Composition and Morphology of A Si(111) Surface with a SiO₂ Surface Film of Different Thicknesses

Abstract—

The composition, morphology, and electronic structure of a SiO₂ nanofilm of different thicknesses created by thermal oxidation on the Si(111) surface are studied in this work. It is shown that up to a thickness of 30–40 Å the film has an island character. At d ≥ 60 Å, a homogeneous continuous film of SiO₂ with stoichiometric surface roughness, which does not exceed 1.5–2 nm, is formed. Regardless of the thickness of the SiO₂ films, the noticeable interdiffusion of atoms at the SiO₂–Si interface is not observed. The regularities of changes in the composition, the degree of surface coverage, and the energy of plasma oscillations with a change in the thickness of SiO₂/Si(111) films are determined in the range from 20 to 120 Å.

INTRODUCTION

Heterofilm structures of the SiO₂/Si type and multilayer systems based on them are widely used and promising for the creation of new solid-state electronic devices, in particular, in the development of microwave metal—oxide—semiconductor field-effect transistors (MOSFETs), integrated circuits, memory elements and displays, photoconverters, solar cells, etc. Such structures are mainly created by thermal oxidation, ion-plasma deposition, and ion implantation. At present, the composition, structure, and properties of SiO₂/Si films of different thicknesses obtained by different methods are well studied [1–5]. In this case, most homogeneous nanofilms (d ≤ 50–60 Å) of SiO₂, as in the case of metal silicides [6–9], were obtained by low-energy ion implantation in combination with annealing [4, 5]. The presence in thin films of SiO₂ and metal silicides of excess silicon atoms or clusters leads to a significant change in their physical properties [10–17]. In the case of thin SiO₂/Si films (d ≤ 10 nm), the diffusion of Si atoms into SiO₂ can occur. However, there are still no reliable data on the dynamics of changes in the morphology, composition, crystal and electronic structure of SiO₂/Si nanofilms with a thickness from d ≈ 20 Å to 100 Å obtained by thermal oxidation. Solving this problem is the main goal of this work.

EXPERIMENTAL

The objects of study were amorphous films of SiO₂ created on the Si(111) surface by thermal oxidation in a dry-oxygen atmosphere. Investigation of the composition, electronic structure, emission and optical properties was carried out using the methods of Auger electron spectroscopy (AES), characteristic electron-energy-loss spectroscopy (CEELS), ultraviolet photoelectron spectroscopy (UVES) on the same ultrahigh vacuum device at Р = 10^–7 Pa. The surface morphology and crystal structure were studied using a standard SUPRA-40 scanning electron microscopy, atomic force microscopy (XE-200) and Raman spectrometer. The depth-distribution profiles of atoms were recorded by the AES method in combination with surface etching with Ar⁺ ions. Films with thicknesses of d = 20, 40, 60, 80, 100, and 500 Å are mainly used. Before the study, the samples were evacuated at T = 900 K for 4–5 hours at a pressure of at least 10^–7 Pa.

RESULTS AND DISCUSSION

Figure 1 shows a SEM image of the SiO₂/Si film surface with d ≈ 20 Å. It can be seen that the film has an island character.

Fig. 1.

SEM image of a Si film surface with SiO₂ film 20 Å thick.

The average surface dimensions of the islands are 40–50 nm. Further studies showed that with an increase in the oxidation time, the sizes of the islands increase, starting from d ≈ 40 Å, the boundaries of neighboring islands overlap and a continuous SiO₂ film is formed. Figure 2 shows the Auger spectra of the SiO₂/Si(111) film of different thickness, recorded in the region E ≈ 70–100 eV.

Fig. 2.

Auger electron spectra of Si(III) with a SiO₂ nanofilm of different thickness d: (1) 0; (2) 20; (3) 40; (4) 80 Å.

The Auger spectra were recorded at the energy of the primary beam electrons E_p = 2500 eV. Already at d = 20 Å, a low-intensity SiO₂ peak appears in the Auger spectrum with E ≈ 76 eV. As d increases, the intensity of the SiO₂ peak increases and reaches its maximum value at d ≈ 60 Å, while the intensity of the Si peak decreases to zero (within the sensitivity of the Auger spectrometer).

As can be seen from Fig. 3, the intensity ${{I}_{{{\text{Si}}{{{\text{O}}}_{2}}}}}$ up to d ≈ 40 Å increases almost linearly, while in the range d = 40–80 Å, it increases exponentially. For d > 80 Å, the intensity ${{I}_{{{\text{Si}}{{{\text{O}}}_{2}}}}}$ barely changes, and the Auger peak of pure Si disappears completely. An analysis of the spectra of Auger electrons together with the SEM images showed that, in the range of d ≈ 0–40 Å, the linear growth of ${{I}_{{{\text{Si}}{{{\text{O}}}_{2}}}}}$ is mainly associated with an increase in the size of the surface islands, i.e., with the degree of Si surface coverage by SiO₂ islands. Since at d ≈ 40 Å a continuous film of SiO₂ starts to form, the increase in I_SiO2 in the range d ≈ 40–80 Å is explained by a decrease in the effect of the substrate (silicon) on the yield of secondary electrons. At d ≥ 80 Å, Auger electrons only escape from the SiO₂ film. It can be seen from these data that in the case of thin films of SiO₂/Si, there is no noticeable diffusion of substrate atoms into the oxide film.

Fig. 3.

Dependence of the intensity of the Auger peak of SiO₂ (E = 76 eV) on the thickness of the SiO ₂ film.

The CEEL spectra also provide rich information about the composition of the surface and the density of the state of valence electrons. In particular, measurement of the plasmon energy can serve to identify samples. Figure 4 shows the CEELS of SiO₂/Si(111) films of different thicknesses.

Fig. 4.

CEELS of Si with a SiO₂ film with a thickness of d: (1) 0; (2) 20; (3) 40; (4) 60 Å.

It can be seen that, as the film thickness increases, the contribution of Si (substrate) to the electron yield with characteristic losses decreases, and that of SiO₂ increases accordingly. For ${{d}_{{{\text{Si}}{{{\text{O}}}_{2}}}}}$ = 20 Å in the spectrum, along with the intense peaks of plasma oscillations (hω_s, hω_v, 2hω_s) Si, a low-intensity peak of the bulk plasmon SiO₂ with E ≈ 22 eV appears. As d increases, the intensity of this peak increases, while the intensity of the Si peaks decreases. Starting from d = 40 Å, one more peak is detected at Е = 14.5 eV, which is associated with excitation of the SiO₂ surface plasmon. Apparently, excitation of the surface plasmon SiO₂ occurs after the formation of a continuous film. At d ≈ 60 Å, the CEEL spectrum characteristic of bulk SiO₂ films is completely established.

Figure 5 shows the Raman spectra of Si(111) with a SiO₂ film 100 Å thick. It can be seen that at ~520 cm^–1 a very intense peak of the optical phonon mode of pure single-crystal Si is found. The peak of the optical-phonon mode of Si of the second order (∼943–980 cm^–1) is strongly broadened, which is due to the presence of a thin amorphous SiO₂ layer on the Si surface.

Fig. 5.

Raman spectra of Si(111) with a SiO₂ film 100 Å thick.

CONCLUSIONS

Based on analysis of the results obtained in the work, the following conclusions can be drawn: SiO₂ films obtained by the thermal oxidation of Si to a thickness of ∼40 Å have an island character, at d ≥ 60 Å a continuous amorphous homogeneous film is formed. In all cases, SiO₂ films have good stoichiometry and there is no interdiffusion of atoms at the SiO₂/Si(111) interface. Even at a thickness of 20 Å, a peak at ΔE ≈ 22 eV appears in the CEEL spectrum, which is characteristic of the SiO₂ bulk plasmon, and at a thickness of 40 Å, a surface plasmon.