P. R. Skeath

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...

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.

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.

Models of column III and V elements on GaAs (110): Application to MBE

Models of ordered Sb overlayers on GaAs (110) are presented and discussed in terms of simple chemical considerations and the available data. The basic character (directional, covalent) of the Sb–GaAs bond is expected to apply to other column V elements on GaAs. Recent developments from theory in the understanding of column III metal–GaAs (110) bonding are summarized and compared to experimental data. It is proposed that the tendency for column III metals to bond nondirectionally on the (110) surface, while column V elements bond directionally, is highly relevant to molecular beam epitaxy (MBE) on all surfaces of III–V compounds. The basis of this proposal is the requirement that a column III atom must have several neighboring column V atoms before it can hybridize and form directional covalent bonds, while no such requirement of hybridization is necessary for the column V elements to bond to the surface directionally. One important result is that the column V element is initially responsible for establish...

Systematics on the electron states of silicon d-metal interfaces

We discuss the trends in photoemission spectra from some silicon d‐metal interfaces: Si (111)–Au, Si (111)–Ag, Si (111)–Ni, Si (111)–Pd, Si (111)–Pt. In particular, we discuss the correlation between the chemical processes taking place at the interface and those in the formation of bulk silicon d‐metal compounds. Although a general correlation is found, some important differences are seen; in particular, no evidence of the formation of strictly stoichiometric silicides in the interface is found even for the cases in which well defined bulk silicides are known to be stable. We discuss also the role of the hybridization between d‐electrons of the metal and (sp) electrons of silicon in the formation of intermixed interface phases. The considerations presented here can be useful as guidelines to interpret future results on other reactive silicon d‐metal interfaces.

The unified model for Schottky barrier formation and MOS interface states in 3–5 compounds

Abstract The object of the work reported here has been to develop an understanding on an atomic basis, of the interaction between semiconductor and metals or oxygen overlayers which determine the electronic characteristics of the interface, e.g. the Schottky barrier height of metal-semiconductor interface or the density and the energy position of oxide-semiconductor interface states. The principal experimental tool used has been photoemission excited by monochromatized synchrotron radiation (10 III–V Acceptor (missing atom) Donor (missing atom) GaAs 0.65 eV (As) 0.85 eV (Ga) InP 0.45 eV (P) 0.1 eV (In) GaSb 0.5 eV (Sb) below VBM (Ga) These results explain why Schottky barrier gates will provide useful FETs on n-GaAs but not n-InP. Likewise they predict that MOS or MIS gates will be practical for n-InP but not n or p GaAs. Studies of the oxygen surface chemistry find the As oxides to be unstable and P oxides to be stable — reinforcing the prediction. Recent work of others is reviewed and alternate identification of the missing atoms in the defects is discussed. Some of the new process possibilities opened up by this work are considered.

Bonding of Al and Ga to GaAs(110)

The interfacial electronic states associated with Al and Ga overlayers on cleaved GaAs (110) surfaces are studied by ultraviolet photoemission spectroscopy (UPS) and low energy electron diffraction (LEED). Deposition of Al or Ga can produce valence‐band spectra nearly devoid of new structure. Sb produces quite significant changes in the surface valence band and is shown as a contrasting example. Pseudopotential calculations from the literature for Al‐ and Ga‐ordered overlayers are found to be in somewhat better agreement with the data than the corresponding tight‐binding calculations, particularly in the adatom states which lie above the GaAs valence‐band maximum (VBM), although neither calculation gives a good description of the observed overlayer states. It is shown that Al and Ga most likely form two‐dimensional clusters or rafts on the surface at submonolayer coverage which are not in registry with the GaAs surface lattice. A comparison is made with results by other workers for Al, Ga, and In on GaAs ...

Oxygen adsorption on the GaAs(110) surface

The adsorption of nonexcited molecular oxygen on cleaved GaAs(110) surfaces at room temperature has been studied using photoemission techniques. Detailed analysis of the oxygen‐induced structure in the valence‐band region revealed two different forms of adsorbed oxygen. Adsorption in the first form saturates at a very low coverage (∠0.01 monolayer) and is probably associated with defect sites. Adsorption in the second form occurs at normal surface sites and produces measurable chemical shifts in Ga‐3d and As‐3d core levels. The nature of the second form of oxygen has been further investigated with core level studies of surfaces oxidized at room temperature and subsequently heated to a high temperature. Annealing to moderately high temperature (∠370°C) causes transfer of oxygen from As–O bonds to form additional Ga–O bonds. Fast heating to high temperature (430°–450°C) leads to desorption of roughly half of the oxygen atoms and all of the chemically affected As atoms, while little change in the Ga‐3d core ...