W. E. Spicer

New and unified model for Schottky barrier and III–V insulator interface states formation

For n- and p-doped III-V compounds, Fermi-level pinning and accompanying phenomena of the (110) cleavage surface have been studied carefully using photoemission at hv≲ 300 eV (so that core as well as valence band levels could be studied). Both the clean surfaces and the changes produced, as metals or oxygen are added to those surfaces in submonolayer quantities, have been examined. It is found that, in general, the Fermi level stabilizes after a small fraction of a monolayer of either metal or oxygen atoms have been placed on the surface. Most strikingly, Fermi-level pinning produced on a given semiconductor by metals and oxygen are similar. However, there is a strong difference in these pinning positions depending on the semiconductor: The pinning position is near (1) the conduction band maximum (CBM) for InP, (2) midgap for GaAs, and (3) the valence band maximum (VBM) for GaSb. The similarity in the pinning position on a given semiconductor produced by both metals and oxygen suggests that the states responsible for the pinning resulted from interaction between the adatoms and the semiconductor. Support for formation of defect levels in the semiconductor at or near the surface is found in the appearance of semiconductor atoms in the metal and in disorder in the valence band with a few percent of oxygen. Based on the available information on Fermi energy pinning, a model is developed for each semiconductor with two different electronic levels which are produced by removal of anions or cations from their normal positions in the surface region of the semiconductors. The pinning levels have the following locations, with respect to the VBM: GaAs, 0.75 and 0.5 eV; InP, 0.9 and 1.2 eV (all levels + 0.1 eV).

Unified defect model and beyond

The unified defect model has been successful in explaining a wide variety of phenomena as oxygen or a metal is added to the III–V surface. These phenomena cover a range from a small fraction of a monolayer of adatoms to practical III–V structures with very thick overlayers. The tenets of the unified defect model are outlined, and the experimental results leading to its formulation are briefly reviewed. InP levels 0.4 and 0.1 eV and GaAs levels 0.7 and 0.9 eV below the conduction‐band minimum (CBM) are associated with either missing column III or V elements. In InP, it has been found possible by a number of workers to ’’switch’’ between the two defect levels by variations in surface processing, temperature, and/or selection of the deposited atom. The need to apply the proper concepts for surface and interface chemistry and metallurgy is recognized, and the danger of using solely bulk concepts is emphasized. The reason for this is examined for certain cases on an atomic level. The need for new fundamental a...

The probing depth in photoemission and auger-electron spectroscopy

Abstract Electron scattering lengths, or escape depths, have been compiled from the literature for twenty different materials in the energy range 0–3000 eV. The energy dependence of the escape depth and its implications for interpretation of photoemission and Auger-electron data are discussed.

The advanced unified defect model for Schottky barrier formation

The advanced unified defect model (AUDM) for GaAs proposed in this paper can be looked upon as a refinement of the unified defect model (UDM) proposed in 1979 to explain Fermi level pinning on 3–5 compounds due to metals or nonmetals. The refinement lies in identifying the defect producing pinning at 0.75 and 0.5 eV above the valence band maximum as the AsGaantisite. Since the AsGaantisite is a double donor, a minority compensating acceptor is necessary. This is tentatively identified as the GaAsantisite. The concentration of As excess or deficiency due to processing or reactions at interfaces is particularly emphasized in this model. A wide range of experimental data is discussed in terms of this model and found to be in agreement with it. This includes the original data on which the UDM was based as well as more recent data including Fermi level pinning on the free-GaAs(100) molecular-beam epitaxy surface, Schottky barrier height for thick (∼ 1000 A) Ga films on GaAs, and the LaB6Schottky barrier height on GaAs(including thermal annealing effects). Of particular importance is the ability of this model to explain the changes in Schottky barrier height for Al and Au on GaAs due to thermal annealing and to relate these changes to interfacial chemistry.

The adsorption of CO, O2, and H2 on Pt: I. Thermal desorption spectroscopy studies☆

Thermal desorption spectroscopy (TDS) has been used to study the chemisorption of CO, O2, and h2 on Pt. It has been found that TDS is quite sensitive to local surface structure. Three single crystal and two polycrystalline Pt surfaces were studied. One single crystal was cut to expose the smooth, hexagonally close-packed plane of the fee Pt crystal (the (111) surface). The other two single crystals were cut to expose stepped surfaces consisting of smooth, hexagonally close-packed terraces six atoms wide separated by one atom high steps (the 6(111) × (100) and 6(111) × (111) surfaces). Only one predominant desorption state was observed for CO and H adsorbed on the smooth (111) single crystal surface, while two predominant desorption states were observed for these gases adsorbed on the stepped single crystal surfaces. The low temperature desorption states on the stepped surfaces are attributed to desorption from the terraces, while the high temperature desorption states are attributed to desorption from the steps. TDS of CO from the polycrystalline foils exhibited some desorption states which were similar to those observed on the stepped single crystal surfaces, indicating the presence of adsorption sites on the polycrystalline foils that were similar to the terrace and step sites on the stepped single crystals. In general, these results suggest a high density of defect sites on the polycrystalline foils which can not be attributed simply to adsorption at grain boundaries. Oxygen was found to adsorb well on the stepped single crystals and on the polycrystalline foils, but not on the smooth (111) single crystal, under the conditions of these experiments. This is attributed to a higher sticking probability for dissociative O2 adsorption at steps or defects than on terraces.

An Auger analysis of the SiO2‐Si interface

Auger electron spectroscopy has been used in conjunction with argon ion sputtering in a study of the chemical structure of the interface region between thermally grown SiO2 and the Si substrate. The distorting effects of the electron and ion beams are dealt with in detail, and we show how the beam parameters can be chosen to minimize instrumental artifacts in the Auger spectra and the chemical depth profiles of the SiO2‐Si interface. We discuss the SiO2‐Si interface in terms of a morphology model which includes a natural interface roughness and inclusions of silicon in the oxide close to the interface. The width of the interface of a 1000‐A oxide grown at 1200 °C in dry O2 on Si (100) is approximately 35 A.

UPS studies of the bonding of H2, O2, CO, C2H4 and C2H2 on Fe and Cu☆

UPS (ultraviolet photoemission spectroscopy) data of H2, O2, CO, C2H4 and C2H2 chemisorbed on polycrystalline Fe and Cu are presented. Together with data already available on Ni, this represents a comprehensive study of the adsorption of simple molecules on three representative metals of the first row transition metals. Adsorption of O2, or H2 on Fe gives rise to a single resonance level at 6 eV below EF. CO dissociatively adsorbs on Fe at room temperature but remains in molecular form at T = 110 K. O2 adsorbed on Cu produces two resonance levels at −5.5 eV and −1.5 eV respectively. In analyzing the chemicorbed CO, C2H4 and C2H2spectra of the three metals, we have considered the fact that the heat of adsorption of these gases is much higher on Fe or Ni than on Cu. Thus, even though the heat of adsorption of CO on Ni is a factor of two higher than on Cu, the chemisorbed CO spectra on these two metals are extremely similar. For the chemisorbed hydrocarbons, we have measured the π level bonding shifts and found that there is no correlation between the level shifts and the heats of adsorption. Application of Grimleys chemisorption model to calculate the chemisorption energy (using the π level shifts as input parameters) results in slightly better agreement between calculated and measured heats of adsorption. Such behavior suggests that the substrate contribution to the bonding is crucial in determining the heats of adsorption. From an examination of the difference spectra of the valence band of Fe and Cu, it is concluded that UPS should be able to supply information as to which group of the substrate orbitals participate in the bonding. However, much more experimental and theoretical work is required before such information can be made quantitative.

Photoemission Studies of High-Tc Superconductors: The Superconducting Gap

Over the last several years there have been great improvements in the energy resolution and detection efficiency of angle-resolved photoemission spectroscopy. These improvements have made it possible to discover a number of fascinating features in the electronic structure of the high transition temperature (Tc) superconductors: apparently bandlike Fermi surfaces, flat-band saddle points, and nested Fermi surface sections. Recent work suggests that these features, previously thought explainable only by one-electron band theory, may be better understood with a many-body approach. Furthermore, other properties of the high-Tc superconductors, which are difficult to understand with band theory, are well described using a many-body picture. Angle-resolved photoemission spectroscopy has also been used to investigate the nature of the superconducting pairing state, revealing an anisotropic gap consistent with a d-wave order parameter and fueling the current debate over s-wave versus d-wave superconductivity.

Quantum Efficiency and Radiative Lifetime in p‐Type Gallium Arsenide

A new method for the study of radiative recombination in uniformly doped semiconductors has been developed and applied to p‐type gallium arsenide. The bulk quantum efficiency is obtained by measurement of photoluminescence as a function of the penetration length of the exciting radiation. Using an estimated electron mobility, the total recombination lifetime of electrons (minority carriers) is found from this photoluminescence measurement and also independently by the measurement of surface photovoltage. The radiative lifetime is determined from the bulk quantum efficiency and the total electron lifetime. The model on which these measurements are based accounts for the principal factors affecting the intensity of photoluminescence, i.e., bulk quantum efficiency, surface recombination, and refraction and reflection of luminescence at the sample surface. The model is consistent with the experimental data for the p‐type gallium arsenide samples studied.The bulk quantum‐efficiency values obtained here are 8% ...

Vacuum ultraviolet photoelectron spectroscopy of (NH4)2S‐treated GaAs (100) surfaces

The surface chemistry and band bending of the ammonium sulfide‐treated GaAs (100) surface has been studied using surface‐sensitive synchrotron radiation photoemission spectroscopy. We find that the treatment leaves the GaAs surface terminated with roughly a monolayer of sulfur bonded to both As and Ga atoms. An n‐type barrier height of 0.8 eV is measured. The thermal stability of the various chemical components is studied and various issues of the passivating mechanism are discussed.