Semiconductor Surface and Interface States

The electronic structure of surfaces, which can be determined by various electron/ion spectroscopies including ultraviolet photoemission spectroscopy (UPS), is dependent on the surface atomic geometry. Thus, it is possible to determine the surface atomic structure via structure-dependent theoretical analyses of these experimental spectra. Because of the complexity of such systems, a simple but accurate theoretical scheme is required. We show that the valence electronic states and atomic structures of semiconductor surfaces—both clean and with chemisorbed species as well as relaxed and/or reconstructed—can be determined using a tight-binding model. Such calculations together with UPS data show that the annealed Si(111)7×7 surface contains about 25% vacancies. For GaAs(110), we have found tha the Ga atoms move into, and As atoms move out of the surface with small bond length distortions so that a plane through them makes an angle of about 20° with the surface. We show that H chemisorbed on Si(111) has two distinct structural phases: a monohydride phase at low coverages, and a rather unexpected trihydride phase at high coverages.

Surface science goes back to the beginning of the 19th century. It was, however, only with the development of ultra high vacuum (UHV) technology in the 1960,s that reproducible experimental results from surfaces on the atomic scale could be obtained. At about the same time high speed computers appeared which made it possible to apply realistic computations of surface properties. The last decade ha8 seen a large progress in surface 8cience and the field is under rapid development. This is largely due to the continuous development of the experimental and the theoretical methods. Many basic properties of surfaces are now in principle understood. Much more research is however necessary to achieve detailed descriptions, in particular of complex systems such as interfaces, overlayers and multilayers. A coordinated interplay between theory and experiment is desirable to achieve an efficient progress. In this article we will illustrate the present understanding of semiconductor surfaces as achieved mainly by photoelectron 8pectroscopy using synchrotron radiation. We will choose three examples, namely results from (i) core level studies on a clean surface, (ii) valence band studies of a clean surface, and (iii) core level and valence band studies on an adsorbate system. Before presenting these results we will summarise the present understanding of the atomic and the electronic structure of crystalline semiconductor surfaces.

Electronic structure of semiconductor surfaces and interfaces

Low-energy cathodoluminescence spectroscopy (CLS) is a powerful new technique for characterizing the electronic structure of semiconductor surfaces and ‘‘buried’’ metal–semiconductor interfaces. This extension of a more conventional electron microscopy technique provides information on localized states, deep-level defects, and band structure of new compounds at interfaces below the free solid surface. Specifically, CLS provides direct identification of metal-induced interface states which evolve in energy and density with multilayer metal coverages of the particular metal, extrinsic surface states due to lattice disruption, as well as bulk defect levels—all of which can play a role in Schottky barrier formation. From the energy dependence of spectral features, one can distinguish interface versus bulk state emission and assess the relative spatial distribution of states below the free surface. From the dependence of spectral intensity on injection level, one can spatially resolve large differences in recombination dynamics and band bending across semiconductor surfaces, depending on their detailed morphology. Unlike surface-science techniques sensitive to only the outer few monolayers, low-energy CLS reveals the electronic structure of buried interfaces and their changes with thermal processing at overlayer thicknesses which permit bulk chemical interactions to occur. For metal–semiconductor interfaces, these CLS spectral features provide a new perspective on physical mechanisms of Schottky barrier formation.

The theory of the electronic structure of clean metal and semiconductor surfaces is reviewed, starting from an effective one-electron Schrodinger equation. Methods for solving the Schrodinger equation at surfaces are briefly described, and the effects of the surface on the electronic wavefunctions are discussed using simple models. The results of detailed calculations of the surface electronic structure of s-p bonded metals, transition metals and semiconductors are reviewed, with an emphasis on the effect of the local environment on the density of states. Properties like the work function and surface energy depend on the surface electronic structure, and their variation with material and surface is discussed; the surface energy contains an important contribution from the interaction between electrons, and this will be considered in some detail. The change in electronic structure compared with the bulk leads to changes in atomic structure, with surface reconstruction on semiconductor and some metal surfaces, and this is also discussed. The interplay between theory and experiment is very important in surface studies, and the theoretical surface energy bands reviewed compare well with experimental photoemission results; this comparison has proved particularly useful for understanding surface reconstructions.

Photoelectron spectroscopy of surface states on semiconductor surfaces

Theoretical studies of the electronic structure of semiconductor surfaces

Electronic Structure of Semiconductor Surfaces and Interfaces

In these lectures I will survey some of the theoretical results, which have been obtained in the study of the electronic structure of semiconductor surfaces and interfaces in the last few years. After the first pioneering works at the beginning of the seventies (Applebaum and Hamann 1973, Bortolani et al., 1973, Jones 1975) our description of the electronic properties of these systems has become more and more accurate and for some specific systems a remarkable agreement between theory and experiments has been achieved.

Our present understanding of the electronic structure of semiconductor surfaces is reviewed. It is shown that photoemission and inverse photoemission are ideal techniques for probing occupied and unoccupied electronic states, respectively. All quantum numbers of an electron can be determined, i.e., energy, momentum, spin and angular symmetries. For simple systems, such as clean ordered surfaces with a small unit cell it is possible to understand the electronic structure from first-principles calculations. For complex systems, such as encountered during oxidation and dry etching one is restricted to measuring the properties determined by short-range order. Core level spectroscopy with synchrotron radiation is able to determine the oxidation state and the local bonding of surface and interface atoms.

Chapter 3 - Surface and Interfacial Recombination in Semiconductors

Low energy cathodoluminescence spectroscopy (CLS) is a powerful new technique for characterizing the electronic structure of semiconductor surfaces and interfaces. CLS provides information on localized states, deep level surface and interface defects, and compound formation at semiconductor interfaces. This electron microscopy technique provides direct identification of reaction-induced metal/semiconductor interface states, which evolve in energy and density with multilayer metal coverages of the particular metal, and extrinsic surface states due to lattice disruption. All of these interface states can play a role in Schottky barrier formation. Unlike surface science techniques sensitive to only the outer few monolayers, low energy CLS reveals electronic structure of "buried" interfaces.

3915. Atomic and electronic structure of semiconductor surfaces. (USA)

Theory of semiconductor surfaces

On the electronic structure of semiconductor surfaces, interfaces and defects at surfaces or interfaces