Metal and Semiconductor Surfaces in a Vacuum

Semiconductor Electrochemistry, Zweite Auflage. Rudiger Memming. 

With respect to the position of the Fermi level is composed of two parts, namely the chemical potential /Xe and an electrostatic term i.e. we then have (compare with Fig. 2.1) 

Provided that there is no additional surface charge, fj, is a pure bulk term which is independent of any electrostatic potential. The term is the contribution of surface dipoles [1, 2] (Fig. 2.1). Such a dipole can be caused by an unsymmetrical distribution of charges at the surface because there is a certain probability for the electrons to be located outside the surface. In the case of compound semiconductors, dipoles based on the surface structure caused by a particular ionic charge distribution occur. These effects depend on the crystal plane and on the reconstruction of the surface atoms [3, 4]. These dipole effects also influence the electron affinity and ionization energy. In the case of metals, the work function is a directly measurable quantity, and for semiconductors it is calculable from ionization measurements. However, the relative contributions of fi and ex are not accessible experimentally and data given in the literature are based on theoretical calculations (see e.g. ref. [1]). 

The situation becomes even more complex if surface states, i.e. additional energy levels within the bandgap, are present as illustrated in Fig. 2.2. In general, two types of 

---

Surface SHG is a viable method to monitor adsorption and desorption of molecules on well-defined metal and semiconductor surfaces in ultra-high vacuum UHV).2.5,9 Figure 3 is an example, showing how the SHG responds to the adsorption of CO on Cu 100). 0 The sample properly cleaned and kept at 1i 0 K in UHV was allowed to be exposed to CO, and SHG with a Nd YAG laser becim was used to monitor the adsorption of CO. It is known that at T - 140°K, CO adsorbs on Cu(IOO) only at the top sites. The adsorption kinetics is then likely to obey the simple Langmuir model. 

Schottky effect A reduction in the work function of a substance when an external accelerating electric field is applied to its surface in a vacuum. The field reduces the potential energy of electrons outside the substance, distorting the potential barrier at the surface and causing field emisslorL A similar effect occurs when a metal surface is in contact with a semiconductor rather than a vacuum, when It Is known as a Schottky barrier. The effect was discovered by the German physicist Walter Schottky (1886-1976). 

Figure 4.28 shows an example where STM recognizes the individual metal atoms in an alloy, thus revealing highly important structural information on the atomic level. The technique does not require a vacuum, and can in principle be applied under in situ conditions (even in liquids). Unfortunately, STM only works on well-defined, planar, and conducting surfaces such as metals and semiconductors, and not on oxide-supported catalysts. For the latter surfaces, atomic force microscopy offers better perspectives. 

The common example of real potential is the electronic work ftmction of the condensed phase, which is a negative value of af. This term, which is usually used for electrons in metals and semiconductors, is defined as the work of electron transfer from the condensed phase x to a point in a vacuum in close proximity to the surface of the phase, hut heyond the action range of purely surface forces, including image interactions. This point just outside of the phase is about 1 pm in a vacuum. In other dielectric media, it is nearer to the phase by e times, where e is the dielectric constant. 

Similar effects can also occur in surface electronic structure when a moiety is weakly physisorbed onto the surface. The surface core-level shifts measured at the vacuum interface are reduced when atoms or molecules are physisorbed onto the surface. Changes may also occur in the valence electronic structure upon physisorption, such as the disappearance of intrinsic surface states on metals and semiconductors. 

In metals and semiconductors Ps formation is negligible. The low excitation energy of electrons, asO Eq. (27.5) shows, hinders this process. However, a special kind of positronium might form even in these cases. This is the so-called surface positronium. As mentioned above, some positrons can reach the surface during their thermalization. These antiparticles are reemitted into the vacuum or get trapped on the surface. Some of them form Ps at the solid-vacuum interface. 

---