Metal-semiconductor interface formation

Compositional variations across interfaces between metals and various semiconductors have also been analyzed with the atom-probe. 

On a metal surface, silicide layers can be formed by two methods. In the first, Si atoms are vapor deposited by heating either a well degassed silicon wafer or a silicon rod to near its melting point. In the second method the metal is heated in 10 to 50 mTorr of silane for a desired length of time, usually about 10 to 60 s at a desired temperature, usually about 300 to 700°C. The first method is better suited for studying very early stages of silicide formation, the second more convenient for growing thick layers of silicides. Chemical vapor deposition or laser enhanced chemical vapor deposition may probably be used also, but have not yet been explored. 

Silicide layers can also be grown on Pt surfaces.247 The preferential facets of epitaxial growth are the 111. Atom-probe data reveal that the stoichiometry of the silicide phase is Pt2Si, and the Pt-Pt2Si interface is also very sharp. However, a small fraction of silicon atoms can diffuse into the Pt matrix. Formation of silicide layers on a nickel emitter surface is much more complicated where silicide layers of varying stoichiometries are formed.246,247 Owing to the statistical nature of the atom-probe data, identification of all the silicide phases in a nickel silicide layer is at best uncertain. 

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A surface sensitive version of the EXAFS technique has been attempted ten years ago, and has proven to be successful in a large variety of surface chemisorption and interface formation problems. In the following we recall very briefly what makes SEXAFS different from EXAFS and what is the specific information that can be withdrawn from the SEXAFS data, and address the problems of metal-metal interface formation, and metal-semiconductor interface formation with detailed examples. 

Structural information on the atomic arrangements at the early stage of formation of metal-metal, metal-semiconductor interfaces and semiconductor-semiconductor heterojunctions is needed along with the determination of the structure of the electron states in order to put on a complete experimental ground the discussion of the formation of solid-solid junctions. Amongst the structural tools that have been applied to the interface formation problem Surface-EXAFS is probably the best... 

The application we have in mind for the metal-polymer interfaces discussed in this book is primarily that where the polymer serves as the electroactive material (semiconductor) in an electronic device and the metal is the electric contact to the device. Metal-semiconductor interfaces, in general, have been the subject of intensive studies since the pioneering work of Schottky, Stromer and Waibel1, who were the first to explain the mechanisms behind the rectifying behaviour in this type of asymmetric electric contact. Today, there still occur developments in the understanding of the basic physics of the barrier formation at the interface, and a complete understanding of all the factors that determine the height of the (Schottky) barrier is still ahead of us2. 

Fig. 5.3. Formation of a bulk heterojunction and subsequent photoinduced electron transfer inside such a composite formed from the interpenetrating donor/acceptor network, plotted with the device structure for such a junction (a). The diagrams showing energy levels of an MDMO-PPV/PCBM system for flat band conditions (b) and under short-circuit conditions (c) do not take into account possible interfacial layers at the metal/semiconductor interface...

Around 1975, investigations of photoelectrochemical reactions at semiconductor electrodes were begun in many research groups, with respect to their application in solar energy conversion systems (for details see Chapter 11). In this context, various scientists have also studied the problem of catalysing redox reactions, for instance, in order to reduce surface recombination and corrosion processes. Mostly noble metals, such as Pt, Pd, Ru and Rh, or metal oxides (RUO2) have been deposited as possible catalysts on the semiconductor surface. This technique has been particularly applied in the case of suspensions or colloidal solutions of semiconductor particles [101]. However, it is rather difficult to prove a real catalytic property, because a deposition of a metal layer leads usually to the formation of a rectifying Schottky junction at the metal-semiconductor interface (compare with Chapter 2), as will be discussed below in more... 

The Schottky barrier is a fundamental property of metal/semiconductor interfaces. Although initial models used to describe the effect were posed as long ago as the 1930s and the problem has been addressed by the most up-to-date surface sensitive probes, a complete understanding has eluded us. Even though the details of barrier formation are not understood, much is known about the Schottky barrier on crystalline semiconductors. The transport properties are well described by various theoretical models, which are somewhat material dependent. For reviews of the properties of Schottky barriers on crystalline semiconductors, see Rhoderick (1978) or Sze (1969). 

The efforts to enhance efficiency in solar cells through use of metal nanoparticles should provide a general guide for the application of plasmonic concepts to water photoelectrolysis, although differences in device design, such as the presence of an electrolyte, suggest that there may be additional criteria. When metal nanoparticles are located at the semiconductor/electrolyte interface, considerations of stability, Fermi level, and band bending are essential. Secondary considerations may include the influence of catalytic effects and the possible formation of trap states at the metal/semiconductor interface. 

Finally, the investigation of noble metal bonding on semiconductor surfaces provides evidence that at moderate temperatures Cu diffuses easily into the Si surface whereas the penetration barrier for Ag is almost as large as its binding energy. The theoretical results help in the understanding of an important catalytic process in the synthesis of silicone polymers and shed light on the Cu/Si and Ag/Si interface formation. 

In the active state, the dissolution of metals proceeds through the anodic transfer of metal ions across the compact electric double layer at the interface between the bare metal and the aqueous solution. In the passive state, the formation of a thin passive oxide film causes the interfadal structure to change from a simple metal/solution interface to a three-phase structure composed of the metal/fUm interface, a thin film layer, and the film/solution interface [Sato, 1976, 1990]. The rate of metal dissolution in the passive state, then, is controlled by the transfer rate of metal ions across the film/solution interface (the dissolution rate of a passive semiconductor oxide film) this rate is a function of the potential across the film/solution interface. Since the potential across the film/solution interface is constant in the stationary state of the passive oxide film (in the state of band edge level pinning), the rate of the film dissolution is independent of the electrode potential in the range of potential of the passive state. In the transpassive state, however, the potential across the film/solution interface becomes dependent on the electrode potential (in the state of Fermi level pinning), and the dissolution of the thin transpassive film depends on the electrode potential as described in Sec. 11.4.2. 

The problems that one can address with the SEXAFS tool while studying the growth and formation of metal/semiconductor and/or semiconductor/semiconductor interfaces are ... 

Taking a general view of the above studies, we note that Chl-coated metal (platinum) electrodes commonly function as photocathodes in acidic solutions, although the photocurrent effcien-cies tend to be lower compared to systems employing semiconductors. This cathodic photoresponse may arise from a p-type photoconduc-tive nature of a solid Chi layer and/or formation of a contact barrier at the metal-Chl interface which contributes to light-induced carrier separation and leads to photocurrent generation. 

In spite of a great number of investigations aimed at the preparation of photocatalysts and photoelectrodes based on the semiconductors surface-modified with metal nanoparticles, many factors influencing the photoelectrochemical processes under consideration are not yet clearly understood. Among them are the role of electronic surface (interfacial) states and Schottky barriers at semiconductor / metal nanoparticle interface, the relationship between the efficiency of photoinduced processes and the size of metal particles, the mechanism of the modifying action of such nanoparticles, the influence of the concentration of electronic and other defects in a semiconductor matrix on the peculiarities of metal nanophase formation under different conditions of deposition process (in particular, under different shifts of the electrochemical surface potential from its equilibrium value), etc. 

The surface morphology of the PLD grown ZnO-based films is important for the interface quality of multilayer structures, including quantum wells with thickness of a few nanometer only, for the formation of metal-semiconductor Schottky contacts and for the optical emission properties. Therefore, the control and optimization of surface properties is essential for the successful application of ZnO thin films in related device configurations. 

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