P. W. Chye

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).

Synchrotron radiation studies of electronic structure and surface chemistry of GaAs, GaSb, and InP

The oxidation properties of GaAs(110), GaSb(110), and InP(110) have been studied with the photoemission technique using synchrotron radiation. One part of the work consisted of an investigation of the chemical shifts in the core levels upon adsorption of oxygen in submonolayer quantities. For GaAs and InP, oxygen removed electrons preferentially from the surface column V elements (As and P), leaving the column III elements (Ga and In) unaffected; whereas, for GaSb, both constitutents were involved in the oxidation suggesting a breaking of bonds between the surface atoms and the rest of the crystal. The second part of the study consisted of a careful investigation of the Fermi level (Ef) pinning in the GaAs(110) surface. Three different samples and many cleaves were studied. For one sample, Ef was always at the bulk position. On the other two crystals, both pinned and unpinned cases were found on various cleaves. The unpinned samples had sharp electron distribution curves (EDC’s), while the pinned samples ...

Surface and interface states on GaAs(110): Effects of atomic and electronic rearrangements

Research during the last year has led to a better understanding of the electronic and atomic structure of the (110) surfaces of III–V semiconductors. In this paper we will briefly review these new developments as well as point out areas where agreement has been found between various experimental results presented in the literature. It is now generally agreed that there are no intrinsic surface states in the band gap on GaAs and the smaller band‐gap materials (e.g., GaSb, InAs, and GaSb) and that Schottky barrier pinning must be due to states produced when the metal adlayer is applied. Particular attention is focused in this paper on the large surface rearrangement which takes place on the (110) GaAs surface and effects of the strain which may be produced in joining this rearranged surface layer to the rest of GaAs crystal. It is pointed out that this may lead to variations in the surface rearrangement which can produce variations in the valence electronic structure at the surface. Such variations are show...

The surface electronic structure of 3–5 compounds and the mechanism of Fermi level pinning by oxygen (passivation) and metals (Schottky barriers)☆

The surface electronic structure, sorption of oxygen, and effect of metallic overlayers on cleaved (110) surfaces of GaAs, GaSb, and InP has been studied using photoemission spectroscopy for 2 < hv < 120 eV. 1 or GaAs and InP. oxygen is found to chemisorb on the column 5 surface atoms if O2 in the unexcited (i.e., ground) state is used; however, such treatment produces bulk Ga and Sb oxides on GaSb. Excited O2 (thought to be the long lived singlet state) produces bulk oxides on GaAs but not InP. The observed behavior is found to correlate well with the heats of forlnation of the respective semiconductors. No intrinsic surface states are found in the band of these semiconductors. This is attributed to electronic and atomic rearrangement which sweeps these states from the band gap. However, extrinsic surface states due to defects or impurities can exist in the band gap. Thus, only on sufficiently perfect surfaces is there no pinning of the Fermi level at the surface. As oxygen or metals are added to the surface, extrinsic states arc induced which pin the Fermi level; the pinning is completed with the addition of much less than a monolayer, i.e., it remains approximately unchanged for layers arbitrarily thick. From detailed studies of the surface valence bands and core levels of the semiconductors, it is deduced that the pinning is due to defects produced by the oxygen or metals. Detailed mechanisms are suggested and discussed, Favorable correlations are found between pinning positions with a submonolayer amount of oxygen and those for “device” oxides. It is suggested that the Schottky barrier pinning on 3–5 materials is due principally to defects induced by deposition of the metal and that these defects are closely related to those formed by oxygen.

Fundamental studies of III–V surfaces and the (III–V)-oxide interface

Work using photoemission with synchrotron radiation (8 eV < hv < 300 eV) to investigate chemisorption and the formation of thin oxide layers on GaAs is reviewed and preliminary correlations with thick oxides are made. An advantage of synchrotron radiation is that it allows examination of both core and valence electrons of the last few molecular layers of the solid. By studying both the core levels and the valence band as a function of oxygen exposure, insight can be obtained into the bonding of oxygen and into oxide formation on an atomic scale. It is found that oxygen can bind to the Group V elements on the (110) surface without the necessity of breaking III–V bonds. Thus, it is suggested that optimum bonding of oxides or other insulators to III–V surfaces will be through the Group V elements. The stability of the surface against the breaking of III–V bonds and the formation of bulk oxides is found to correlate very well with the heat of formation of the III–V compound. For example InP is more stable than GaAs. This appears to correlate with the ease of forming metal-oxide-semiconductor (MOS) or metal-insulator-semiconductor (MIS) type devices. Even when it is possible to bond oxygen to a surface without the necessity of breaking III–V bonds, Fermi level pinning is produced by submonolayer quantities of oxygen. This is associated with defects induced by the strain produced by oxygen chemisorption. The pinning position is near the conduction band minimum and near mid-gap for (110) InP and (110) GaAs surfaces respectively. A striking correlation is found between these pinning positions and those found for thick oxides on practical device structures. This suggests that the pinning mechanism may be the same for both types of interfaces. It is suggested that lattice defects play a key role in the pinning. A novel approach to forming the oxide or insulation layer for MOS or MIS structures is suggested.

New phenomena in Schottky barrier formation on III–V compounds

The possibility of a defect mechanism is proposed to explain the Fermi‐level pinning in Schottky barriers on III–V semiconductor surfaces. This suggestion comes from the results of photoemission spectroscopy applied to the study of formation of metal–semiconductor barrier heights. Changes in the electronic structure and composition of the interface are studied. For Au metal overlayers on the (110) surfaces of GaAs, GaSb, and InP, the Fermi‐level pinning occurs at 0.1 or less of a monolayer coverage. Furthermore, the pinning position is roughly independent of the choice of adatom: Cs, Al, Au, or O. The partial yield structure disappears at about monolayer coverage of Au. The Au valence band had the characteristic shape for atomiclike Au, indicating that the metal was dispersed homogeneously on the surface without the formation of thick islands. Deposition of Au onto the GaSb surface causes the compound to decompose. The antimony segregates to the surface and leaves behind a nonstoichiometric interface. The...

Photoemission studies of clean and oxidized Cs

Abstract Photoemission studies of the oxidation of Cs at ∼ 140 K have been carried out. Four different states of oxygen were observed. Definitive identification of three of the four states has been achieved in two steps: (1) comparison of the multiplet structure of free oxygen ions and the multiplet structure in the photoemission spectra of oxygen sorbed in Cs (i.e., comparison of the energy separations of the various oxygen levels involved); and (2) comparison of experimentally measured binding energies of various states of oxygen with calculated binding energies of known oxygen species inserted in Cs. The oxygen species identified are O 2− , O 2 2− and O 2 − . The multiplet structure of O 2 2− in Cs is identical to that expected for free O 2 2− with no differential relaxation of orbital energies. In contrast, significant differential relaxation in orbital energies is observed for O 2 − in Cs. This difference is attributed to the closed-shell and open-shell configurations of O 2 2− and O 2 − , respectively. The oxidation-induced changes in the Cs 4 d , 5 s and 5 p core levels and the NOO and OVV Auger transitions have also been studied in detail. A discussion of the oxidation of Cs, based on the oxygen species identified definitively, is also given.

The oxidation of Cs—uv photoemission studies

The oxidation of cesium has been studied using ultraviolet photoemission spectroscopy. Upon exposure of a fresh cesium film to oxygen, a very narrow peak appears in the energy distribution curves (EDC’s) about 2.6 eV below the Fermi level Ef and grows with increasing exposure. This peak is associated with oxygen ions dissolved in the cesium metal below the surface. After 3×10−5 Torr sec of exposure, additional structure begins to appear. This is associated with the precipitation of cesium oxides. The structure associated with the oxides changes with increasing oxygen exposure indicating the appearance of different oxides. The oxide penetrates appreciably to the surface only after strong oxide buildup has taken place beneath the surface. A sharp minimum of 0.7 eV is found in the work function over a relatively small range of oxygen exposure.

Investigation of the mechanism for Schottky barrier formation by group III metals on GaAs(110)

New evidence for a defect mechanism which is responsible for pinning states within the band gap on the (110) surfaces of the III–V compounds is presented. Investigations of column III metals on both n‐ and p‐type GaAs revealed a systematic difference in surface Fermi energy stabilization in the gap with p‐type samples pinning 0.25 eV below n‐type samples. Several current models and theories of Schottky barriers are discussed in terms of both the results given in this paper and previously reported data.

Photoemission studies of the initial stages of oxidation of GaSb and InP

Abstract Synchrotron and HeI radiation were used to study the chemical shifts and valence band structural changes on the GaSb and InP (110) surfaces during the initial stages of oxidation. Chemical shifts indicating anion and cation oxide formation were observed for both GaSb and InP, even when unexcited oxygen was used. The valence band spectra were very sensitive to small amounts of oxygen, and it was possible to identify some oxygen-induced structures as being characteristic of anion or cation oxide. Extinction of surface excitons was also observed at low oxygen exposures.