Theoretical models of Schottky-barrier height formation are reviewed. A particular emphasis is placed on the examination of how these models agree with general physical principles. New concepts on the relationship between interface dipole and chemical bond formation are analyzed, and shown to offer a coherent explanation of a wide range of experimental data.

Recent advances in Schottky barrier concepts

Almost all metal-semiconductor or Schottky contacts exhibit rectifying behaviour which is caused by a depletion layer on the semiconductor side of the interface. The electronic properties of a Schottky diode are characterised by its barrier height which is the energy difference between the top of the valence band of the semiconductor and the Fermi level at the interface. The physical mechanism primarily determining barrier height is provided by the decay of the metal's electron wavefunctions into the semiconductor in the energy range between the top of the valence band and the Fermi level where the metal conduction band overlaps the semiconductor band gap. These wavefunction tails are described as the continuum of metal-induced gap states (MIGS) which are derived from the virtual gap states of the complex semiconductor band structure.

https://iopscience.iop.org/article/10.1088/0034-4885/53/3/001/pdf

On the physics of metal-semiconductor interfaces

Density functional supercell calculations of the Schottky barrier heights (SBH) of metal germanides and silicides on Si or Ge find that these vary with the facet, unlike those of elemental metals. In addition, silicides and germanides show a stronger dependence of their SBHs on the work function than those of elemental metals, as seen experimentally. Both effects are beyond the standard metal induced gap states model. NiSi2 is found to have a much lower SBH on n-Si(100) than on n-Si(111), as seen experimentally. It is shown how such results can be used to design lower SBH contacts for n-Ge, which are needed technologically. The SBHs of the better behaved Si/silicide interfaces can be used to benchmark the behavior of the less well behaved Ge-germanide interfaces for this purpose. The dependence of the SBH of epitaxial Pb-Si(111) on its reconstruction is also covered.

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Face Dependence of Schottky Barriers Heights of Silicides and Germanides on Si and Ge

We report the formation of PtSi nanowires, PtSi/Si/PtSi nanowire heterostructures, and nanodevices from such heterostructures. Scanning electron microscopy studies show that silicon nanowires can be converted into PtSi nanowires through controlled reactions between lithographically defined platinum pads and silicon nanowires. High-resolution transmission electron microscopy studies show that PtSi/Si/PtSi heterostructure has an atomically sharp interface with epitaxial relationships of Si[110]//PtSi[010] and Si(111)//PtSi(101). Electrical measurements show that the pure PtSi nanowires have low resistivities approximately 28.6 microOmega.cm and high breakdown current densities>1x10(8) A/cm2. Furthermore, using single crystal PtSi/Si/PtSi nanowire heterostructures with atomically sharp interfaces, we have fabricated high-performance nanoscale field-effect transistors from intrinsic silicon nanowires, in which the source and drain contacts are defined by the metallic PtSi nanowire regions, and the gate length is defined by the Si nanowire region. Electrical measurements show nearly perfect p-channel enhancement mode transistor behavior with a normalized transconductance of 0.3 mS/microm, field-effect hole mobility of 168 cm2/V.s, and on/off ratio>10(7), demonstrating the best performing device from intrinsic silicon nanowires.

Single crystalline PtSi nanowires, PtSi/Si/PtSi nanowire heterostructures, and nanodevices.

The authors review the band line-ups and band offsets between semiconductors, dielectrics, and metals, including the theory, experimental data, and the chemical trends. Band offsets have been critical in the choice of high dielectric constant oxides to be used in advanced metal oxide semiconductor field effect transistors. It turns out that band offsets are also critical in the theory of doping limits, design of transparent conducting oxides, organic semiconductors, and electrodes to use in light emitting devices, photovoltaic cells, and photochemical cells. It is shown how band line-ups can be understood in terms of charge neutrality levels. These are also related to states due to interstitial hydrogen.

Band offsets, Schottky barrier heights, and their effects on electronic devices

The work function of 13 polycrystalline transition metal silicides was measured by photoemission in uhv. Their values are discussed in relationship to their Schottky barrier heights on n-Si. While there appears to be a weak correlation for a certain group of transition metal silicides, the values of the 5d-noble metal silicides including some of the lattice matched Ni silicides appear to be completely uncorrelated. Experimental values of work functions are compared to the values proposed previously by Freeouf.

F. Greuter

Papers

Work function and barrier heights of transition metal silicides