T. Kendelewicz

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.

Surface structure of MBE-grown α-Fe2O3(0001) by intermediate-energy X-ray photoelectron diffraction

Abstract We have used intermediate-energy X-ray photoelectron diffraction to determine the surface structure of epitaxial α-Fe 2 O 3 (0001) grown on α-Al 2 O 3 (0001). Comparison of experiment with quantum mechanical scattering theory reveals that the surface is Fe-terminated, and that the first four layer spacings are −41, +18, −8, and 47% of the associated bulk values, respectively. These results agree reasonably well with the predictions of molecular mechanics and spin-density functional theory previously reported in the literature for the Fe-terminated surface. However, we find no evidence for an O-terminated surface predicted to be stable by spin-density functional theory.

On the Fermi level pinning behavior of metal/III–V semiconductor interfaces

We present the results of a systematic study designed to investigate the physical mechanisms involved in Fermi level pinning on III–V semiconductor surfaces. This study compares the results of surface sensitive photoemission spectroscopy performed during the initial stages of Schottky barrier formation (sub to several monolayer coverages of metals, as well as nonmetals) to the results of device electrical measurements for thick metal coverages (∼1000 A). These results give strong support for the basic premises of the unified defect model (UDM), as first proposed in 1979. However, several revisions were found to be important in correcting and/or sharpening several details of the UDM. As proposed in 1979, we find that the same native defect energy levels are created independent of the foreign atoms deposited on the clean GaAs surface. However, the small but finite range in pinning positions in GaAs and InP and the correlation that high (low) barrier heights are formed for high (low) electronegativity metals...

Structure of the Ag on Si(111) 7 × 7 interface by means of surface exafs

Polarization dependent surface extended X-ray absorption fine structure (SEXAFS) measurements are used to determine the structure of the Ag on Si(111)7 × 7 system at the early stages (< 3 monolayers (ML)) of interface formation. At room temperature (RT) Ag is found to initially (< 0.5 ML) chemisorb in the threefold hollow site, approximately 0.7 A above the outermost Si layer with an average Ag-Si distance of 2.48±0.05 A. Above monolayer coverage the SEXAFS spectrum is dominated by the Ag-Ag distance indicating Ag island formation on the surface. Upon heating (200 ⩽T⩽ 600°C) a (√3 ×√3)R30° LEED pattern is observed. At the lowest coverage ( < 0.7 ML) this pattern is determined to arise from Ag atoms which are embedded in the threefold hollows, ~ 0.7 A below the first and above the second Si layer, with a Ag-Si distance of 2.48 ± 0.04 A. At higher coverage (

Electrical study of Schottky barrier heights on atomically clean and air‐exposed n‐InP(110) surfaces

1 ML) Ag clusters are found to grow on this interface with the same Ag-Ag distance as in Ag metal. Our results are discussed in the context of previous experimental and theoretical results.

Schottky barriers on atomically clean n‐InP (110)

We report here a systematic study of the electronic properties of Schottky barrier diodes fabricated on atomically clean, as well as air‐exposed n‐InP (110) surfaces. Using the current‐voltage (I‐V) measuring technique, we found the barrier heights of 0.33 eV for Ni, Al, Sn, Mn, 0.43 eV for Pd, Cu, Au, Cr, and 0.54 eV for Ag. Contrary to earlier reports based on a limited amount of data, the results of this study do not show a simple relationship between the chemical reactivity and the Schottky barrier height. The large differences between the electrical characteristics of diodes prepared on clean surfaces and those prepared on air‐exposed surfaces were also not found.

The mechanisms of Schottky barrier pinning in III–V semiconductors: Criteria developed from microscopic (atomic level) and macroscopic experiments

We report the results of a systematic study of the Schottky barrier heights on n‐type InP (110). The measurements were performed on thick diodes and thin metal overlayers grown at room temperature under ultrahigh vacuum conditions on cleaved surfaces. A large number of metals, ranging from strongly reactive Ni, Al, Cr, Mn, and Pd to less reactive or nonreactive Ag, Au, Cu, Ga, and Sn have been used to correlate the barrier height with the interfacial chemistry. The barrier heights have been determined using the current‐voltage characteristics of diodes and the band bending related shifts in the core level spectra for the thin overlayers. Soft x‐ray photoemission spectroscopy has also been used to characterize the chemical reactions on a microscopic scale during the initial stages of the interface formation. Our results show several notable exceptions from the systematics relating the barrier heights with the interfacial chemistry. These results provide a critical test of currently considered models of Sch...

The Si(111)/Cu interface studied with surface sensitive techniques

Abstract For the sake of perspective, an overview is given of the development of concepts concerning the mechanism involved in Schottky barrier (SB) formation. Until about 1972 principally “macroscopic” data (e.g., I – V and C – V electrical measurements) were available. More recently “atomic” level microscopic tools have been increasingly applied experimentally to the problem of understanding SB formation. The most popular models for the III–V semiconductors are examined in terms of the metal:III–V chemistry including its correlation with barrier height and/or the effect of metal thickness. Experimentally it is found that, for most metals, the Schottky barrier pinning is completed with the deposition of less than a monolayer of metal. Most importantly, the Fermi level pinning position at these low metal coverages is found to correspond well with the SB height obtained from I – V measurements from carefully prepared samples with thicknesses of about 1000 A. On the other hand, the metal:III–V chemistry appears to have little effect on the SB height. For example, four metals - Ag, Au, Cu, and Pd - have very different chemistry (varying from essentially no reaction for Ag to a very strong reaction for Pd); however, they give almost identical SB heights. After comparison of experimental data with various currently popular models, only a refined version of the united defect model is found consistent with the available data.

The advanced unified defect model and its applications

The formation of the Cu/Si interface is described on the basis of joint photoemission (valence band and Si 2p core levels) and Auger lineshape (SiL2,3VV) analysis. The system is characterized by an extended mixed phase where a silicidelike compound of average stoichiometry Cu3Si is formed and appears to be stable for an extended range of Cu coverages and annealing temperatures. The intermixing is strongly temperature dependent, but the chemical reaction between Cu and the top layers of Si can proceed even at 100 K.

Annealing of intimate Ag, Al, and Au–GaAs Schottky barriers

The advanced unified defect model (AUDM) is presented for GaAs interfaces with metals, other semiconductors, and insulators. The key defect is the ASia antisite, which is also responsible for EL-2 and semi-insulating GaAs. The energy levels of the Asca antisite (0.75 and 0.52 eV) correspond well with the previously identified energy levels (0.75 and 0.5 eV) of the unified defect model (UDM). Using the AUDM, it is shown that a wide range of experimental data can now be explained. The Fermi level position on GaAs(001) MBE surfaces and its dependency on As or Ga are explained. The changes in Schottky barrier height of LaB6/GaAs, AI/GaAs, and Au/GaAs on annealing are explained in terms of Asia antisite density near the interface being increased or decreased due to the annealing. For A1 and Au this is correlated with the metal/semiconductor interfacial chemistry, and a predictive capability is developed in terms of net increase or decrease of As at the interface due to this reaction. The Fermi level pinning behavior at low temperature is also explained in terms of this model. True understanding of the surfaces and interfaces of 3-5 semiconductors is still in its infancy. In 1979 the unified defect model (UDM) was proposed to explain the behavior of interfaces of 3-5 compounds with metals and nonmetals. The key to this model was a suggestion, based on a wide range of experimental data, that the Fermi level at such interfaces could be determined by 3-5 lattice defects. In 1979 very little was known about 3-5 defects in the UDM. These were assigned to As and Ga vacancies. Much more is known today about defects in GaAs. As a result we present an advanced unified defect model (AUDM) in which the key defect is the same Asia antisite which provides EL-2 and semi-insulating GaAs. Based on this, a number of experimental observations which have given difficulty in the past can now be explained.