Adsorption of hydrogen and hydrocarbon molecules on SiC(001)

Abstract

Adsorption of hydrogen and hydrocarbon molecules on semiconductor surfaces plays a key role in surface science and technology. Most studies have employed silicon (Si) as a substrate because of its paramount technological importance and scientific interest. However, other semiconductor substrates are gaining an increasing interest as well. Silicon carbide (SiC), which is a material with very special properties allowing developments of novel devices and applications, offers particularly fascinating new degrees of freedom for exceptional adsorption behaviour. For example, a very unusual hydrogen-induced metallization of a SiC(001) surface has been reported and hydrogen molecules show very different adsorption behaviour on different SiC(001) reconstructions although the latter exhibit very similar surface dimers. In marked contrast to the Si(001) surface, the adsorption of hydrocarbon molecules on SiC(001) can yield structurally well-defined adlayers in favourable cases which may have large potential for organic functionalization. We review and discuss theoretical ab initio results on conceivable adsorption scenarios of atomic and molecular hydrogen as well as acetylene, ethylene, butadiene, benzene and cyclohexadiene on various reconstructions of the SiC(001) surface. The main emphasize is on a detailed understanding of these adsorption systems and on identifying the physical origin of the particular adsorption behaviour. The results will be discussed in the light of related adsorption events on the Si(001) surface and in comparison with available experimental data.

Introduction

The interaction of atoms and molecules with semiconductor surfaces is of high fundamental interest and large technological importance in a number of fields ranging from semiconductor technology to biophysics. For example, atomic hydrogen plays a crucial role in crystal growth, surface etching, passivation and protection. Surfaces covered by atomic or molecular hydrogen serve as prototype systems for an understanding of surface reactivity. Organic layers covalently attached to semiconductor surfaces open up entirely new fields of applications. The combination of organic chemistry and semiconductor technology has the potential to realize customized devices for a multitude of special purposes, e.g., in molecular electronics and microelectronics as well as in chemical and biological sensing (cf. [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]).

Due to the outstanding importance of Si in semiconductor science and technology, most studies of hydrogen and hydrocarbon adsorption carried out to date have concentrated on Si substrates with particular emphasis on the Si(001)-(2×1) surface. A wealth of knowledge on structural, chemical, electronic and vibrational properties of such systems has been gained by innumerable investigations. Organic functionalization of the Si(001)-(2×1) surface by the attachment of organic molecules or layers (see, e.g., Fig. 1) and its prospects for technological applications have been described in comprehensive reviews by Wolkow [3] and Bent [6]. Much less is known about hydrocarbon molecule adsorption on cubic SiC surfaces.

SiC is a material of large fundamental interest. It is a compound group-IV semiconductor composed of a 1:1 stoichiometry ratio of Si and C exhibiting many of the advantages of its elemental components. At the same time, SiC has a number of salient features which strongly discern it from elemental group-IV semiconductors. While Si and diamond are purely covalent, SiC has a relatively large ionicity due to the different strengths of the Si and C potentials. Therefore, Si atoms behave like cations and C atoms behave like anions in SiC. The bonds in SiC are asymmetric rather than symmetric since they are strongly ionic. The ionicity of SiC gives rise to an appreciable charge transfer from Si to C. As a consequence, SiC occurs in about 200 polytypes depending on the stacking sequence of Si–C double layers along the crystal c-axis (cf. [12], [13]) while diamond and Si crystallize in the diamond structure. The most important polytypes of SiC are cubic or hexagonal. The lattice constant of cubic SiC (4.36 Å) is about 20% smaller than that of Si (5.43 Å) and some 22% larger than that of diamond (3.57 Å).

SiC is also a material of high technological potential, in particular for advanced applications (see, e.g., [14], [15], [16]). Its cubic and hexagonal polytypes have wide band gaps, ranging from 2.4 to 3.3 eV, and a very high thermal stability making SiC especially suitable for high-temperature, high-frequency, high-power, and high-speed electronic devices and sensors [16], [17]. In addition SiC is characterized by its chemical inertness and very high hardness qualifying it as an especially attractive material to operate under harsh environmental conditions [18]. Furthermore, it is one of the best biocompatible materials, very promising for biosensing and biophysics applications [19], [20], [21].

The basic physical properties of SiC bulk crystals and their clean surfaces, as well as the wide spectrum of their applications, have been described in great detail in a host of reviews (cf. [14], [15], [16], [17], [18], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31]). The cubic SiC(001) surface, described in more detail in Section 3, is of particular interest both from a fundamental as well as an applied point of view. In addition, it shows the closest general resemblance to the Si(001) surface. Therefore, the SiC(001) surface with varying termination, stoichiometry, and reconstruction is the substrate surface considered in this review.

Generally, it is well-known from previous studies that the surface atomic structure, the topology of surface dangling bonds and the high directionality of adsorbate–substrate interactions play a crucial role in adsorption on semiconductor surfaces. In this regard, the SiC(001) surface offers particularly interesting new degrees of freedom. It shows several reconstructions which are characterized by surface lattice constants that differ substantially from that of Si(001). This geometrical fact is of paramount importance for the adsorption properties of atomic and molecular adlayers on the SiC(001) surface, as compared to Si(001) or C(001). Likewise, the band gap of cubic SiC (2.4 eV) is much larger than the gap of Si (1.1 eV). As a consequence, hydrogen and hydrocarbon molecules adsorbed on different reconstructions of the SiC(001) surface exhibit unique and surprising properties. In particular, they appear to be very promising for making well-defined organic–semiconductor hybrid systems which could become useful for organic functionalization. Depending on their shape, size and composition, the organic molecules can impart special properties of an organic material to the semiconductor surface allowing for the design and creation of new capabilities in optical, electronic, and mechanical function as well as in chemical and biological activity (cf. [1], [2], [3], [4], [5], [6], [7], [11].) Also in this respect, SiC appears to be superior to Si due to its biocompatibility, the very good match of SiC band gaps to the HOMO–LUMO gaps of many organic molecules and the good structural match of SiC surface lattice constants to characteristic bond lengths in small hydrocarbon molecules. As a consequence, organic functionalization of SiC surfaces is more recently attracting increasing interest also in the fields of biotechnology, biosensing, bioelectronics and biomedical applications (cf. [32], [33]).

Both Si and SiC surfaces have characteristics that enable organic molecules to be attached by a number of different chemical reactions. These attachment chemistries provide interesting information on the reactivity of semiconductor surfaces and demonstrate how organic molecules can pass on their properties useful for a variety of applications. For instance, a full monolayer of bifunctional molecules reacting at the surface by only one of the functional groups can leave available the other group for subsequent layering chemistry. In particular, a sensing response can occur if a species of interest binds to the end group causing transduction of signal through the organic layer to the semiconductor where all capabilities of microelectronic circuitry become available. The end product would be a chemical or biological sensor [6]. Several different approaches to functionalize Si surfaces with organic molecules, in particular by cycloaddition reactions such as [2+2] and [4+2] (Diels–Alder) processes have been discussed (cf. [3], [5], [6], [7], [11], [34], [35], [36]). Cycloaddition reactions are used in organic chemistry to form new C–C bonds and new carbon rings. In such reactions two π bonded molecules come together to form a new cyclic molecule, thereby losing two π bonds and making two new σ bonds. Cycloadditions are designated by how many π electrons of each reactant molecule are involved in the reaction process. In this paper, we will address the reaction of a selected group of small hydrocarbon molecules such as acetylene, ethylene, butadiene, benzene and cyclohexadiene with different reconstructions of the SiC(001) surface to illustrate the general principles governing hydrocarbon adsorption on SiC(001).

Incorporating new functionalities at semiconductor surfaces by adsorption of atomic or molecular layers requires a detailed understanding of the surface reactivity and the structural as well as electronic properties of the adsorbate systems. First-principles approaches have turned out to provide powerful predictive tools allowing one to follow reactions of atoms or molecules with surfaces on a microscopic level and to determine optimal adsorption configurations and related electronic properties. This way, the electronic factors determining bond breaking and bond making at the surface can be tracked, giving access to reaction mechanisms, transition states, barrier heights and adsorption energies [37], [38], [39]. Many of these properties have been determined more recently by ab initio investigations of adsorption processes of atomic and molecular hydrogen and hydrocarbon molecules on various reconstructions of the SiC(001) surface. Understanding adsorption processes on SiC(001) on the basis of these results is in the focus of the current review. The main goal is to provide a microscopic picture of the atomic and electronic structure and the chemical bonding in such systems in which hydrogen or organic adlayers are covalently bound to SiC surfaces, as well as of adsorption processes that lead to the stable formation of these systems.

The paper is organized as follows. In Section 2 we briefly address the theoretical framework that has been used to investigate adsorption on SiC(001) by ab initio calculations. Section 3 summarizes basic structural properties of several reconstructions of SiC(001) which are used as substrates in the following sections. Section 4 addresses adsorption of atomic hydrogen on SiC(001)-(3×2) which can give rise to hydrogen-induced surface metallization or to semiconducting hydrogenated structures depending very sensitively on H coverage of the surface. This topic is still controversially discussed in the literature. Section 5 deals with a variety of dissociative adsorption processes of molecular hydrogen on several reconstructions of the SiC(001) surface and on Si addimer nanolines. Before discussing nondissociative adsorption of hydrocarbon molecules on various SiC(001) reconstructions in the following sections, their basic gas phase properties are summarized in Section 6. Acetylene and ethylene adsorption on SiC(001)-(2×1) and SiC(001)-(3×2) is considered in 7 Acetylene and ethylene on SiC(001)-(2×1), 8 Acetylene and ethylene on SiC(001)-(3×2), respectively. Very significant differences occur for the two different substrate configurations concerning structural, electronic and chemical properties. 9 Benzene adsorption on Si(001)-(2×1), 10 Benzene adsorption on SiC(001)-(3×2) are devoted to benzene adsorption on Si(001)-(2×1) and on SiC(001)-(3×2), respectively, which gives rise to chemically active systems in both cases. Section 11 addresses two examples of cycloaddition reactions of butadiene and cyclohexadiene on SiC(001)-c(2×2). A short summary and an outlook conclude the paper in Section 12.