Joel A. Appelbaum

Theory of reconstruction induced subsurface strain — application to Si(100)

Abstract The stress produced by surface layer reconstruction on semiconductors can produce sizable elastic distortions of deeper layers. These are calculated using a standard bulk model of interatomic force constants for the pairing model of 2 × 1 reconstruction on the (100) surface of C, Si and Ge. Kinematic analysis for the predicted geometry with subsurface distortions is shown to give good agreement with LEED intensity data for Si. This reconciles the LEED results, for which the simple pairing model fails completely, with spectroscopic measurements, which are best fit by the pairing model.

Exchange Model of Zero-Bias Tunneling Anomalies

Exchange model of zero bias tunneling anomalies, discussing Hamiltonian, interference magnetic scattering and metal junctions

Electronic structure of the Cu(111) surface

Abstract Self-consistent electronic structure calculations are reported on bulk Cu, and 3- and 5-layer Cu films. These yield a size insensitive work function, φ = 5.0±.1 eV, and a surface energy of 0.75 eV, in agreement with experiment. Good size convergence of the film potential permits the construction of a self-consistent potential for an 11-layer Cu(111) film, whose spectral properties we studied. A prominent p-like surface band was found within 0.1 eV of experiment, serving as a check on the surface potential.

Hydrogen chemisorption on the 100 (2 × 1) surfaces of Si and Ge

Abstract This paper combines a theoretical study of the Si(100) surface having a monolayer of atomic hydrogen chemisorbed to it with an experimental study of the analogous Ge(100) and Ge(110) surfaces. In the theoretical work the underlying (100) silicon surface is taken to be reconstructed according to the Schlier-Farnsworth-Levine pairing model with the hydrogen located on the unfilled tetrahedral bonds of this structure. Self-consistent calculations of the electronic potential, charge density, spectrum, and occupied surface density of states are carried out. The force on the hydrogen atoms is then calculated using the Hellman-Feynman theorem. This force is found to be close to zero, confirming that the hydrogen atoms are indeed at the equilibrium position for the chosen silicon geometry. Features in the calculated photoemission spectrum for the Si(100) 2 × 1 : H surface are discussed in terms of related features in the photoemission spectrum of Si(111) : H, but are found not to agree with the previously measured photoemission spectrum of Si(100) 2 × 1 : H. Measured photoemission and ion-neutralization spectra for Ge(100) 2 × 1 : H agree in their major features with what is calculated for Si(100) 2 × 1 : H, however, suggesting that the Ge(100) 2 × 1 : H surface is reconstricted according to the pairing model. Similarly, measured spectra for clean Ge(100) 2 × 1 agree with calculations for the row dimerized Si(100) surface.

Theory of the energy distribution of ions produced in the field ion microscope

Abstract The secondary structure (Jason peaks) in the ion energy distribution (IED) of the field ion microscope is described theoretically. The process of field ionization is treated as one of electron tunneling from the ionizing atom to a flat single-crystal face of the microscope tip within the Bardeen tunneling approach. The potential used has the correct three-dimensional form near the center of the ionizing atom and it reduces correctly to the sum of field and image terms far from the surface. Close to the surface the potential is parametrized in a simple one-dimensional form. The electron scattering by the crystal is represented by a complex amplitude reflection coefficient R , which is evaluated semiempirically using LEED data. The semi-empirical procedure is based on the dispersion relation satisfied by R . Numerical applications to W (011) surface for the imaging gases He, Ne and H 2 are reported. The theory accounts for the existence of pronounced secondary structure in observed IEDs and calculations reproduce the observed secondary peak positions within 1.0 eV rms. The improvement in the description of peak positions, relative to previous theory, is attributed mainly to the inclusion of the phase of R . The sensitivity of measured IED peaks to the phase is demonstrated and it is proposed that field ion spectroscopy can be used to measure the phase of electron reflection at crystal surfaces.

The effect of disordering on the spectral properties of partial monolayers of H on Si(111)

Abstract Ultraviolet photoemission measurements are reported for a H covered Si(111) surface for which the H coverage ranged from a fraction to a full monolayer. These measurements reveal striking differences depending whether the Si(111) substrate is kept at room temperature (RT) or 150°C. In particular, the 150°C sample UPS spectral series shows monotonic growth with little line shape change while the RT series shows significant line shape modification with coverage. These results are interpreted as island growth at 150°C and disordered adsorption at RT. Theoretical model calculations are carried out of the electron density of states of a fractional monolayer of H chemisorbed to Si(111) that reproduce the essential features of the RT data and confirm the role of disordering there.

Quantum physics and chemistry of surfaces

Abstract Within the last 15 years, exceptional progress has been made in the experimental study of solid surfaces. Detailed surface-spectroscopic informa. tion is now available on a wide range of surfaces, with semiconductor and transition metal surfaces studied most.1 In response to this experimental effort a later-starting—but at present equally vigorous—theoretical effort has been undertaken.2

Willingness to pay and the compensation threshold

Abstract Willingness to pay is formally defined as the maximum price a consumer will pay for one of a set of alternatives. This concept and a related concept, the compensation threshold, are shown to be useful in the context of discrete choice theory.

Electronic Structure of Si and Ge Surfaces

Self-consistent calculations of the potential, charge density, and electronic energy level structure are reported for the (111) surfaces of Si and Ge, and for Si(111) with adsorbed monolayers of H and A. Several bands of occupied surface states are found in each case, and their relationship to the surface geometry and to chemical bonding concepts is discussed.