C. M. Garner

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 ...

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 the surface states and oxidation of group IV semiconductors

Photoemission utilizing synchrotron radiation has been used to study the electronic structure of surfaes and/or interfaces in the photon energy range up to 400 eV. The Si (111) surface exhibits well‐established filled surface states which disappear after about 103 langmuirs of O2. At this exposure, only a broad shoulder has appeared on the high‐binding‐energy side of the Si 2p levels. This shoulder continues to grow in magnitude until much higher exposures, about 108 langmuirs of O2, when a 2.6‐eV shifted peak begins to grow, indicating a higher state of oxidation of the Si. After exposure to activated oxygen, to stimulate the oxidation process, the chemical shift of the Si 2p level is found to be 3.7 eV, which is the same as observed in bulk SiO2. Similar results were obtained for the oxidation of the Ge(111) surface.

Valence band studies of clean and oxygen exposed GaAs(100) surfaces

We found, by correlating band bending, ultraviolet photoemission spectroscopy, and partial yield spectroscopy measurements, that Fermi level pinning at midgap of n-type GaAs(110) is caused by extrinsic states. The exact nature of these states is not yet clear, but the surfaces with Fermi level pinning were strained as evidence by a smeared valence band emission. This smearing was removed by as little as one oxygen per 104 to 105 surface atoms. This implies that the oxygen has very long range effects in causing spontanesous but small rearrangement of the surface lattice and removing surface strains. When about 5% of a monolayer of oxygen is adsorbed, a major change in the electronic structure takes place. Again, the oxygen coverage is very small, which suggests long range effects now leading to a fairly large rearrrangement of the surface lattice. Finally, from comparing the oxygen induced emission for exposures greater than 107 L O2, with the spectra from gas photoemission measurements on molecular oxygen, we suggest that the oxygen is chemisorbed as a molecule on the (110) surface of GaAs.

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.

Abrupt Ga1−xAlxAs‐GaAs quantum‐well heterostructures grown by metalorganic chemical vapor deposition

Multiple‐quantum‐well Ga1−xAlxAs‐GaAs heterostructures grown by metalorganic chemical vapor deposition have been analyzed by Auger electron spectroscopy combined with simultaneous argon‐ion sputter etching. The chemical‐interface widths of the Ga0.45Al0.55As‐GaAs heterojunctions are determined to be ≲17 A. In addition, no Al is detected in the GaAs quantum wells.

Photoemission studies of the electronic structure of III–V semiconductor surfaces

Abstract The photoemission technique using synchroton radiation in the photon energy range 5–450 eV has been applied to the study of the electronic structure of some III–V semiconductor surfaces, prepared by cleavage in situ under ultrahigh vacuum conditions, ≲ 10 −11 Torr. For p-type GaAs(110), the Fermi level is pinned at the top of the valence band and thus no filled surface states extend into the band-gap. The situation is more complicated for n-type GaAs(110), where band bending easily can be introduced by extrinsic effects (impurities, cleavage quality, etc.) and push the Fermi level down to about midgap. Chemical shifts of inner core levels (3d for Ga and As) are used to obtain information on the bonding site of oxygen on the (110) surface. GaAs(110) can be exposed to atmospheric pressure of molecular oxygen without breaking the bonds between the surface atoms and the bulk. Oxygen is predominantly bonded to the As atoms on the surface. The oxidation behavior is strikingly different for GaSb(110) with formation of gallium and antimony oxides on the surface directly upon oxygen exposure. Heavier oxidation of GaAs(110) and breaking of the surface bonds will also be reported.

Studies of Surface Electronic Structure and Surface Chemistry Using Synchrotron Radiation

Photoemission studies for hv < 400 eV are used to illustrate some of the advantages of synchrotron radiation for studying the surface electronic structure and chemistry of solids. Studies of GaAs and Pt are used to illustrate the following advantages (all of which depend on a source of radiation with a continuously tunable photon energy): (1) both valence and core states can be examined, (2) photon energies can be chosen so that the electron escape length can be minimized, restricting the studies to the first several atomic or molecular layers of the solids, (3) since the escape depth minimum is rather shallow the matrix element for excitation from a given set of levels may be maximized consistent with probing the last few layers, (4) when two sets of levels are degenerate or near degenerate in energy, e.g. the solid valence levels and the orbits of an adsorbed gas, the photon energy dependence of the matrix element can be used to separate the photoemission from the two states, and (5) good energy resolution (often 0.2 eV or better) can be obtained. For GaAs, studies of the surface valence electronic structure (hv ≈ 21 eV) in conjunction with LEED results indicated that rearrangement of the atoms within the last unit cell of the surface plays an important role in determining the electronic structure associated with this last layer of the crystal. In fact, it appears that measurement of this electronic structure may prove to be one of the most sensitive ways of determining the details of the atomic rearrangement. Studies of the Ga and As 3d core shifts on chemisorption of oxygen indicate that electrons are transferred from the As to the oxygen. The next step in oxidation, the formation of true As and Ga oxides, has also been followed using the core shifts. CO adsorbed on metals has been studied extensively previously using photoemission methods; however, neither the CO 3σ levels nor the shake up structure has been clearly seen in the past due to background produced by the strong Pt 5d valence photoemission. Making use of the Cooper minimum in the 5d excitation cross section at hv = 150 eV, the CO 3σ and shake up structures have been clearly seen for the first time.