C. Y. Su

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...

Photoelectron spectroscopic determination of the structure of (Cs,O) activated GaAs (110) surfaces

p‐GaAs (110) surfaces activated to negative electron affinity (NEA) have been examined with photoelectron spectroscopy. A typical activated GaAs surface is found to consist of both a layer of oxygen bonded to GaAs and a (Cs+,O−2) layer. The GaAs‐O layer was not anticipated prior to this work. A GaAs‐O‐Cs dipole plus the polarization of a Cs+‐O−2‐Cs+ sandwich layer is proposed to explain the NEA condition based on the structure of activated surfaces found in this work. The identification of the O‐GaAs bonding layer explains the better yield achieved by the two‐step activation process compared to that achieved by a single‐step process. Possible optimization of the activation process by forming the O‐GaAs layer and the (Cs+,O−2) layer is also discussed. Adsorption of OH from the residual gas in an ultrahigh vacuum chamber is identified as one degradation mechanism of the GaAs photocathodes.

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.

Development and confirmation of the unified model for Schottky barrier formation and MOS interface states on III-V compounds ☆

The object of the work reported here was to develop an understanding on an atomic basis of the interactions between semiconductors and metal or oxygen overlayers which determine the electronic characteristics of the interface, e.g. the Schottky barrier heights and the density and the energy position of states at oxide-semiconductor interfaces. The principal experimental tool used by ourselves was photoemission excited by monochromatized synchrotron radiation (10 eV<hv<300 eV). Extreme surface sensitivity is obtained by tuning the synchrotron radiation so that the minimum escape depth is obtained for the excited electrons of interest. In this way only the last two or three atomic layers of the solid are sampled. By changing hv, core levels or valence bands can be studied. The Fermi level position Efs at the surface can be directly determined using a metallic reference. GaAs, InP and GaSb were studied. On a properly cleaved surface there are no surface states in the semiconductor band gap—thus, no pinning of Efs. Pinning of Efs can then be monitored as metal or oxygen is added to the surface, starting from submonolayer quantities. Two striking results are obtained: (1) the pinning position is independent of the adatom, whether it is oxygen or one of a wide range of metals, and (2) the pinning is completed by much less than a monolayer of adatoms. These results cannot rationally be explained by the hypothesis that the pinning is due to the levels produced directly by the adatoms. Rather, they suggest strongly that the adatoms disturb the semiconductor surface indirectly, forming defect levels. This is supported by the appearance of the semiconductor atoms in the metal and by the disordering of the semiconductor surface by submonolayer quantities of oxygen. Since these basic experiments have been reported previously they are only briefly reviewed here. When metal or oxygen is added under very gentle conditions, the following levels are formed (all energies are relative to the conduction band minimum). Semiconductor Acceptor Donor GaAs 0.65 eV 0.85 eV InP 0.45 eV 0.1 eV GaSb 0.5 eV Below VBM Full-size table Table options View in workspace Download as CSV where VBM denotes the valence band maximum.

Effect of low‐intensity laser radiation during oxidation of the GaAs(110) surface

Considerable interest in the use of laser irradiation as a means of processing (annealing, cleaning, alloying, etc.) semiconductor materials, as well as the importance of the oxygen surface chemistry on III‐V semiconductors, has led us to study the effect of low‐intensity (?3 W/cm2) laser radiation on the oxidation behavior of (110) surfaces of GaAs cleaved in UHV (<10−10 Torr). The oxygen sticking probability in the submonolayer coverage range has been increased by a factor of 103 (from a probability of ∠10−9 to ∠10−6) by uniform irradiation of the GaAs surface with a continuous wave Ar+ laser (λ = 5145 A) during the oxygen exposure. We find the data cannot be explained in terms of either heating of the surface or excitation of the oxygen by the laser radiation; it appears that the most likely explanation of the phenomena is an increase in the density of electrons and/or holes at the surface. A limiting step in the oxygen uptake process is the breakup of the oxygen molecule; this dissociation would be in...

Column III and V elements on GaAs (110): Bonding and adatom‐adatom interaction

Column V elements adsorbed on GaAs (110) exhibit strikingly different behavior than adsorbed column III elements in terms of the overlayers bonding to the semiconductor, the effect on the semiconductor surface lattice, and long‐range order. A strong interaction between adatoms is found with submonolayer coverings of column III metals which leads to the formation of flat raftlike metallic patches. Unlike the column III metals, Sb adsorption produces large changes in the electronic states and atomic arrangement of the semiconductor surface lattice. An elementary view of the factors responsible for the different characteristics and effects of these overlayers on GaAs (110) is given, but its extension to other adatoms is shown to be limited. These results have strong relevance to current theoretical models of Al and Ga overlayers on GaAs (110), as well as to molecular beam epitaxy and Schottky barrier formation.

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.