Elsevier

Surface Science

Volume 603, Issue 1, 1 January 2009, Pages 190-196
Surface Science

Electronic structure of bismuth terminated InAs(1 0 0)

https://doi.org/10.1016/j.susc.2008.10.042Get rights and content

Abstract

Deposition of Bi onto (4 × 2)/c(8 × 2)-InAs(1 0 0) and subsequent annealing results in a (2 × 6) surface reconstruction as seen by low electron energy diffraction. The Bi condensation eliminates the original (4 × 2) surface reconstruction and creates a new structure including Bi-dimers. This surface is metallic and hosts a charge accumulation layer seen through photoemission intensity near the Fermi level. The accumulation layer is located in the bulk region below the surface, but the intensity of the Fermi level structure is strongly dependent on the surface order.

Introduction

Indium arsenide is a narrow band-gap semiconductor with unique properties such as very high electron mobility and high carrier densities. An important characteristic for this semiconductor is that it forms a charge accumulation layer in the surface region. The (1 1 0) surface has been found to form such layers after adsorption of various metals [1], [2], [3] while they are directly found on the pristine (1 0 0) and (1 1 1) surfaces [4]. It was suggested that an accumulation layer on the (1 0 0) surface is induced by donor-like intrinsic surface states whose density is determined by the surface reconstruction [5], [6]. Theoretical calculations confirm the importance of native defects and the proportionality between host anion and cation covalent radii on the ‘averaged band edge’ for accumulation layer formation [7]. An experimental study of InAs(1 0 0) has shown that the charge density is an order of magnitude higher on the As-terminated surface compared to the In-terminated surface, due to differences in the Fermi level pinning [4]. The accumulation layer may also be induced by impurity ad-atoms or through adsorbed ordered overlayers. In Ref. [8], it was shown that one monolayer of Pb on the In-rich InAs(1 0 0) surface leads to a (2 × 1)/(1 × 4)-Pb/InAs(1 0 0) surface reconstruction, and a strong accumulation layer is present. The observed intensity was much stronger than from the pristine surface, although there was no additional band bending. This was explained as Pb acting as sub-surface donors giving an improved debye screening that moves the total charge closer to the surface.

A general property of III-V (1 0 0) surfaces is that their surface reconstructions depend on the surface composition and thermal treatment [9], [10]. An example is the indium-rich (4 × 2)/c(8 × 2) reconstruction, recently solved by a surface X-ray diffraction (SXDR) study by Kumpf [11] and the arsenic-rich (2 × 4)-InAs(1 0 0) surface described by the β(2 × 4) reconstruction (including adsorbed As-dimer in the top layer and another As-dimer in the third layer) [12]. For both surfaces, angle-resolved photoemission studies have revealed several surface states [13], [14]. A Bi-induced (2 × 6)/c(2 × 12)-Bi/InAs(1 0 0) reconstruction was recently observed above 1 ML Bi coverage, and a structural model was proposed including two layers with dimers [15].

In the present study we show a direct dependence between the surface order and the bismuth induced accumulation layer on the In-terminated InAs(1 0 0), using low energy electron diffraction (LEED) and angle-resolved photoemission spectroscopy (ARPES). Evaporating a thin layer of bismuth is not sufficient to create the accumulation layer, while removing excess bismuth and creating the Bi-induced (2 × 6)/c(2 × 12) surface reconstruction, the accumulation layer is seen as a weak photoemission peak above the conduction band minimum (CBM). The presence of an accumulation layer at the surface is important to further the understanding of the surface stabilization and development of proper surface models. ARPES was also used to study the bismuth induced surface states in the valence band, with special interest in the top layer dimer bismuth state.

Section snippets

Experimental

The experiments were performed at Beamline 33, MAX-lab synchrotron radiation facility in Lund, Sweden. The photon energy range of the beamline is 15–200 eV and the end station is equipped with a goniometer-mounted angle-resolving analyzer with a variable angular resolution [16], [17]. InAs(1 0 0) samples were cut from n-type single crystal wafers (Wafer Technology Ltd., UK) doped with S with carrier concentration of 4.4 × 1016 cm−3. In situ cleaning comprised initial degassing, cycles of simultaneous

Results and discussion

After preparation, the clean InAs(1 0 0) surface produced a sharp and well-defined (4 × 2)/c(8 × 2) LEED pattern as shown in Fig. 1a, in agreement with previous observations [6], [18]. After condensation of 3.5 ML Bi, the LEED pattern was weak and blurry (4 × 1)/(1 × 3) (Fig. 1b), with a ×three-symmetry along the [0 1 1] direction (the extra spots are marked by arrows), and a weak but visible ×four-symmetry along the [0 1¯ 1] direction. The ×4 symmetry was observed around 43 eV electron energy and is not

Conclusion

We have studied the electronic structure of the Bi-induced (2 × 6) reconstruction on InAs(1 0 0). A surface state related to dimer bond state has been observed, and the experimental data supports the dimer model proposed by Laukkanen [15]. Deposited Bi layers form a weak (4 × 1)/(1 × 3) structure with electronic similarities to the (2 × 6) structure, indicating an early dimer formation. We also observed the creation of a charge accumulation layer below the (2 × 6) reconstruction.

Acknowledgments

We would like to thank Dr. Balasubramanian Thiagarajan and Stefan Singer for their assistance during the experiments and helpful discussions, as well as to the assistance from the MAX-lab staff. We kindly acknowledge financial support from the Swedish Research Council (VR) and the Göran Gustafsson Foundation.

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