Theoretical models of Schottky-barrier height formation are reviewed. A particular emphasis is placed on the examination of how these models agree with general physical principles. New concepts on the relationship between interface dipole and chemical bond formation are analyzed, and shown to offer a coherent explanation of a wide range of experimental data.

Recent advances in Schottky barrier concepts

The structure and properties of metal-semiconductor interfaces

Core-level binding-energy shifts at surfaces and in solids

The reduction and removal of surface oxides from GaAs substrates by atomic layer deposition (ALD) of Al2O3 and HfO2 are studied using in situ monochromatic x-ray photoelectron spectroscopy. Using the combination of in situ deposition and analysis techniques, the interfacial “self-cleaning” is shown to be oxidation state dependent as well as metal organic precursor dependent. Thermodynamics, charge balance, and oxygen coordination drive the removal of certain species of surface oxides while allowing others to remain. These factors suggest proper selection of surface treatments and ALD precursors can result in selective interfacial bonding arrangements.

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GaAs interfacial self-cleaning by atomic layer deposition

Modulation spectroscopy is a powerful method for the study and characterization of a large number of semiconductor configurations, including bulk/thin film, microstructures (heterojunctions, quantum wells, superlattices, quantum dots), surfaces/interfaces and actual device structures in addition to semiconductor growth/processing. Furthermore, the influence of external perturbations such as temperature, electric fields, hydrostatic pressure, uniaxial stress, etc. can be investigated. This optical technique utilizes a very general principle of experimental physics, in which a periodically applied perturbation (either to the sample or probe) leads to sharp, derivative-like spectral features in the optical response of the system. Because of the richness of the derivative-like spectra, the information in the lineshape fits, room temperature performance and relative simplicity of operation this method is becoming increasingly more important as a tool to study these materials and structures. This article will review developments in the field during the last decade.

Modulation spectroscopy of semiconductors: bulk/thin film, microstructures, surfaces/interfaces and devices

Photoemission spectra of 3d core levels excited with synchrotron radiation reveal a multicomponent substructure which increases in complexity with oxygen exposures over the range 106–1014L (langmuir). Spectral changes are already evident for Ga at 104 L, and for As near 106 L. Two oxide components shifted by 0.45 and 1 eV relative to the bulk Ga‐3d core level are evident throughout the exposure range, but shift to 0.8 and 1.4 eV for 1014 L. With increasing exposure the As‐3d core level develops a sequential set of shifted components at 0.8, 2.3, 3.2, and 4.2 eV relative to the bulk position in GaAs, which are attributed to single through fourfold coordinated bond formation to oxygen. Both surface and bulk‐sensitive core spectra reveal a nearly equally intense oxide substructure, which indicates that contrary to previous notions subsurface oxidation is the dominant mechanism throughout the exposure range. The core spectra furthermore indicate preferential Ga oxidation—which suggests that separate Ga and As...

The oxidation of GaAs(110): A reevaluation

Angle-resolved photoemission techniques were used to study GaAs(100) surfaces grown by molecular-beam epitaxy. Three well-ordered surfaces with the $c(4\ifmmode\times\else\texttimes\fi{}4)$, $c(2\ifmmode\times\else\texttimes\fi{}8)$, and $c(6\ifmmode\times\else\texttimes\fi{}4)$ reconstructed surface structures and a disordered As-covered (1\ifmmode\times\else\texttimes\fi{}1) surface were investigated in the photon-energy range of 12-70 eV. All major sharp peaks in the normal-emission spectra were found to be transitions from bulk valence bands; therefore, transitions due to surface states were either weak or broad. The variations in bulk-peak intensities for different surface structures were related to surface diffraction effects. For photon energies greater than about 20 eV, primary-cone peaks for transitions from the upper bulk valence bands to a broadened and shifted free-electron-like final band were identified, and energy dispersion relations for the upper valence bands were determined along the [100] direction. Surface umklapp transitions were quite strong due to the presence of many short surface reciprocal-lattice vectors. To characterize these surfaces, angle-integrated photo-emission measurements of valence bands and core levels were made, and the positions of the Fermi level for these surfaces were determined.

Angle-resolved photoemission studies of GaAs(100) surfaces grown by molecular-beam epitaxy

A systematic investigation of Bi on n-type GaP(110) with scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), and ballistic-electron-emission microscopy (BEEM) is presented. The first 3 \AA{} of Bi grows in a quasiordered monolayer, forming alternating chain and vacancy segments along the Ga-P zigzag chains with a periodicity of about 23 \AA{}, which is consistent with the observed 6\ifmmode\times\else\texttimes\fi{}1 low-energy electron diffraction (LEED) pattern. Additional Bi atoms aggregate to form \ensuremath{\simeq}10-\AA{}-high clusters, which suggests a Stranski-Krastanov growth mode. The STS results show that the Bi monolayer is semiconductorlike with a band gap of about 0.55 eV; in contrast, the clusters exhibit metallic character. A 50-\AA{} Bi film exhibited monocrystalline and atomically flat regions \ensuremath{\simeq}1000 \AA{} in extent, which are delineated from similar adjacent regions by height differences equal to a biatomic step. LEED shows an ordered, two-domain hexagonal surface structure that consists of a close-packed-hexagonal arrangement of Bi atoms, as observed by STM. BEEM reveals a uniform Schottky-barrier height of 1.11\ifmmode\pm\else\textpm\fi{}0.02 eV at all measured positions across the sample surface.

Structural and electronic properties of the Bi/GaP(110) interface.

The crystallographic relationships, growth morphology, chemical activity, and electronic properties of Ag deposited at room temperature on GaAs(100) c(2×8) and (4×6) surfaces were investigated Ag(110) growth was observed independent of growth rates The growth is three‐ dimensional (nucleated) and the interfaces are abrupt Stabilization of the Fermi level occurs beyond Ag coverages of 10 A, is uncorrelated with the appearance of the metallic Ag phase at ∠05 A and appears to be dependent on the formation of atomiclike interfacial states near the bottom of the bandgap Schottky barrier heights of 083 and 097 eV were determined for Ag on the c(2×8) and (4×6) surfaces, respectively The results are at variance with current Schottky barrier models

Crystallographic relationships and interfacial properties of Ag on GaAs(100) surfaces

Electroreflectance measurements have been performed on the GaAs-${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Al}}_{x}\mathrm{As}$ superlattice, and the observed structure has been related to interband transitions at different points of the Brillouin zone. For increasing $x$ the energies of these transitions shift progressively to higher values with respect to those of bulk GaAs, indicating the effect of the superlattice potential on the band structure beyond the zone center. In contrast, similar shifts observed in ${\mathrm{In}}_{1\ensuremath{-}x}{\mathrm{Ga}}_{x}\mathrm{As}\ensuremath{-}\mathrm{Ga}{\mathrm{Sb}}_{1\ensuremath{-}y}{\mathrm{As}}_{y}$ only occur for $x$,$yg0.2$, an anomaly arising from its unusual periodic potential.

R. Ludeke

Papers

The oxidation of GaAs(110): A reevaluation

Angle-resolved photoemission studies of GaAs(100) surfaces grown by molecular-beam epitaxy

Structural and electronic properties of the Bi/GaP(110) interface.

Crystallographic relationships and interfacial properties of Ag on GaAs(100) surfaces

Observation of Superlattice Effects on the Electronic Bands of Multilayer Heterostructures