Study of the transition from the ideal Si(111)–H(1×1) surface to the (7×7) reconstruction by HREELS, UPS and LEED

doi:10.1016/S0039-6028(01)01647-8

Surface Science

Volume 497, Issues 1–3, 20 January 2002, Pages 247-253

https://doi.org/10.1016/S0039-6028(01)01647-8 Get rights and content

Abstract

The temperature-induced evolution of the Si(1 1 1)–H(1×1) surface towards the (7×7)-reconstruction has been studied by means of UPS, HREELS and LEED techniques. We found that H atom desorption occurs at about 500 °C, and the full development of the (7×7)-phase, checked by both UPS and LEED, occurs around 700 °C. In the valence band spectra, the (7×7)-related restatom peak is present already for annealing temperature of 465 °C, while the adatom-related peak starts to appear only around 600 °C and fully develops at 700 °C. A band bending is detected and its behavior discussed in relation to the surface state evolution. In the first annealing steps, the desorption-induced defects cause the shift of the surface Fermi level and the consequent upwards band bending, which reaches the maximum value of 0.47 eV for annealing to 465 °C. The subsequent evolution of the dangling-bond states into the (7×7) surface states determines the band bending decrease till 0.1 eV.

Introduction

The Si(1 1 1) surface is widely known to present several types of reconstruction, depending on the surface preparation and temperature. The equilibrium structure of Si(1 1 1) below 870 K is the (7×7)-phase, described by the dimer-adatom-stacking-fault (DAS) model [1]. Cleaved Si(1 1 1) surface is instead characterized by the (2×1) metastable phase, described by the Pandey π-chain model [2], [3], while a whole range of other different metastable reconstructions have been observed upon annealing-quenching treatments [4], [5], [6], [7]. One of the key issues in the study of surface reconstructions is to elucidate their formation process, starting from the ideal bulk-terminated surface. A Si(1 1 1) surface very akin to the ideal one can be obtained by hydrogen saturation of the surface dangling bonds, through a proper ex situ procedure [8]. The annealing of the Si(1 1 1)–H(1×1) surface at increasing temperatures induces the hydrogen desorption and the subsequent transition to the equilibrium (7×7)-phase. This system can therefore be studied as a model of ideal-to-reconstructed surface transitions. The Si(1 1 1)–H surface can be obtained also by in situ hydrogenation of the Si(1 1 1)(7×7) surface. Several works studied its thermal evolution upon hydrogen desorption, reporting the formation of local reconstruction such as the (2×2), c(2×4) and $(3 × 3)$ together with (7×7) DAS regions at temperatures around 500 °C and the subsequent formation of the (7×7) reconstruction at temperatures between 650 and 800 °C [9], [10].

In this work, we studied the Si(1 1 1)–H(1×1)-to-Si(1 1 1)(7×7) transition by means of UPS, HREELS and LEED. In particular, we focused on the evolution of the surface electronic properties during the structural transition. In the initial stage of hydrogen desorption (below 500 °C), the presence of isolated dangling bonds pins the Fermi level, causing an upward band bending in the n-type semiconductor. For annealing temperature above 500 °C, hydrogen desorption is completed and the surface begins to evolve towards the (7×7)-phase. The band bending decreases, while the electronic surface states proper of the (7×7)-phase gradually develop. In particular, the adatom-related electronic state, which is located at the Fermi level and determines the surface metallic character, clearly appears only after annealing at 700 °C. The link between the degree of structural local order and the semiconducting character of the surface is discussed, suggesting that the position of the adatom surface state relative to the Fermi level strongly depends on the presence of adatom-vacancy defects on the surface.

The paper is organized as follows: After the experimental section (Section 2), in Section 3 we report on the experimental findings. In particular, the LEED pattern and HREEL spectrum evolution as a function of annealing temperature is described in Section 3.1, while the corresponding evolution of the UPS valence band is reported in Section 3.2. In Section 4 the experimental results are discussed and the evolution of the surface electronic and dielectric properties during the reconstruction process is described. Some conclusions are given in Section 5.

Section snippets

Experimental

Measurements were performed in UHV conditions (base pressure 1×10⁻¹⁰ mbar) on an ex situ hydrogenated Si(1 1 1) surface. The Si sample was n-doped, with n=5×10¹⁵ cm⁻³. The etching procedure is well known to produce highly ordered hydrogenated surfaces with low density of defects [8], that can be introduced into the UHV system with no substantial contamination. The sample quality and cleanliness was checked by means of UPS and HREELS. The sample temperature was varied by directly heating the

LEED and HREELS evolution

In Fig. 1 we show the energy loss spectra of the Si(1 1 1)–H surface at increasing annealing temperatures. The RT spectrum is characterized by two main peaks located at 79 and 262 meV, which are associated to the Si–H bending and stretching modes, respectively [11], [12]. The small peaks at 99 and 132 meV can be most probably associated to the Si–OH and the Si–O–Si modes, respectively [11], [12], indicating, together with a very small peak at 365 meV, the presence of a tiny amount of O and C

Discussion

In the initial steps of the annealing process, only a small fraction of hydrogen atoms desorbs leaving unsaturated Si atoms at the surface. For temperature lower than 500 °C, the LEED pattern does not change, showing that no reconstruction occurs. The few unsaturated surface atoms therefore can be viewed as isolated restatoms. The LEED pattern is in fact not affected by these hydrogen vacancies, as the LEED technique is quite insensitive to local defects. The energy level of the partially

Conclusions

In conclusion, we have studied the electronic and dielectric properties of the Si(1 1 1) surface during the transition from the ideal bulk-truncated (1×1) surface, obtained by H-passivation, to the (7×7) reconstruction. The observed band bending behavior has been related initially to the presence of defect states, induced by the hydrogen desorption, and subsequently to the gradual formation of the surface bands. We showed that the electronic properties of the surface are strongly influenced by

Acknowledgements

This work has been partially supported by INFM project PAIS-SiCSAF.

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Study of the transition from the ideal Si(111)–H(1×1) surface to the (7×7) reconstruction by HREELS, UPS and LEED

Abstract

Introduction

Section snippets

Experimental

LEED and HREELS evolution

Discussion

Conclusions

Acknowledgements

Surf. Sci.

Surf. Sci.

Surf. Sci. Lett.

Surf. Sci. Rep.

Surf. Sci. Lett.

Phys. Rev. Lett.

Phys. Rev. B

Phys. Rev. B

Phys. Rev. Lett.

Phys. Rev. Lett.

Study of the transition from the ideal Si( $1 1 1$ )–H(1×1) surface to the (7×7) reconstruction by HREELS, UPS and LEED