Surface States and Barrier Height of Metal-Semiconductor Systems

Abstract

The dependence of the barrier height of metal-semiconductor systems upon the metal work function is derived based on the following assumptions: (1) the contact between the metal and the semiconductor has an interfacial layer of the order of atomic dimensions; it is further assumed that this layer is transparent to electrons with energy greater than the potential barrier but can withstand potential across it. (2) The surface state density (per unit area per electron volt) at the interface is a property only of the semiconductor surface and is independent of the metal. The barrier height φBn is defined here as the energy needed by an electron at the Fermi level in the metal to enter the conduction band of the semiconductor. With the above assumptions, the barrier height for n-type semiconductor-metal contacts is found to be a linear combination of the metal work function φm and a quantity φ0 which is defined as the energy below which the surface states must be filled for charge neutrality at the semiconductor surface. The energy φ0 is measured from the edge of the valence band. For constant surface state density the theoretical expression obtained is φBn=γ(φm−χ)+(1−γ)(Eg−φ0)−Δφn,where χ and Eg are electron affinity and the band gap of the semiconductor, respectively, Δφn is the image force barrier lowering, and γ is a weighting factor which depends mainly on the surface state density and the thickness of the interfacial layer. The theoretical expression is compared to the presently available φBn VS φm data for Si, GaP, GaAs, and CdS, by fitting the data to straight lines using the method of least squares. The best straight-line fit was obtained for the GaP data, with probable error limits on the slope and intercept of ±0.03 and ±0.13 eV, respectively. The parameter γ in the theoretical expression is found to range from 0.07 for GaAs to almost unity for the CdS data reported by Goodman indicating weak and strong dependence of the surface barrier height on the metal work function, respectively. The value of φ0 is roughly a third of the respective band gap energies for Si, GaP, and GaAs, and the surface state density for these semiconductors is found to be in the range 1013−1014 states/cm2/eV, for the experiments cited. Excessive scatter in the data points for the CdS data of Mead and Spitzer casts doubt on the significance of a straight-line fit for this case. The data of Goodman for CdS obey the Schottky theory for a metal-semiconductor barrier, but this agreement requires a value of the electron affinity χ which is different from the vacuum-photothreshold value measured by other authors.

Au/N‐type Si Schottky‐barrier contact and oxidation kinetics in Au‐contaminated and thermally oxidized N‐type Si (001) surfaces

“…Thus, the height of the Au/n-Si Schottky barrier ( B ) was calculated to be 0.70 eV from the energy gap E g (1.11 eV). This value is close to the former analysis (0.73-0.76 eV) and the reported Schottky-barrier height (0.79 eV) [10] of the Au/n-Si Schottky contact and supports the survival of the Au/n-Si Schottky contact even after thermal oxidation. Figure 4 shows the relationship between the thermal oxide thickness and the oxidation time from 1023 to 1173 K. At these…”

“…On the basis of the frequencydependent AC SPV, a band diagram of Au/n-Si Schottky contact was proposed and the Au/n-Si Schottky barrier was calculated to be 0.73-0.76 eV, [9] which is in good agreement with the previously reported result (0.79 eV). [10] Accordingly, Schottky-barrier-type AC SPV is quantitatively identified in the Au/n-Si Schottky contact [9] as well as the Cu/n-Si Schottky contact, [11] differing from an AC SPV occurrence mechanism originated in metal-induced negative oxide charge described by (AlOSi) − network and/or AlO 2 − in ntype Si. [3,4,12] Omori et al [9] speculated that Au may exist as clusters both on the surface of native oxide and/or the SiO 2 /Si interface.…”

Schottky barriers in carbon nanotube-metal contacts

“…However, it has been observed for many semiconductors that the Schottky barrier height has only a small dependence on the work function of the metal contact. 48 Thus, the Schottky barrier heights do not follow the simple relations in Eqs. (2) and (3) but instead have a much weaker dependence on the metal work function, a phenomenon known as Fermilevel pinning.…”

Nanoscale Schottky barrier visualization utilizing computational modeling and ballistic electron emission microscopy

“…When the metal and the semiconductor surfaces come in contact, the large conduc- Cowley and Sze described the discrepancy between the Schottky-Mott model and the experimental data by an interface state creating a dipole between the metal and the semiconductor [56]. Dipoles that are created only a few atomic layers into the silicon have very large electric fields which cause the Fermi level of the semiconductor to be pinned at the interface.…”