Semiconductor Barrier Height

SOLAR CELL PARAMETERS AND DESIGN CONSIDERATIONS 3.1. Metal-Semiconductor Barrier Height 

The maximum power output from an SBSC is directly proportional to the open circuit voltage of the cell under illumination (eqn 31) assuming F and J to be constant. From equations 2 and 28 it may be shown that 

For MIS SBSCs in which a thin interfacial oxide layer is present ( ) is replaced by From equation 35 it may be seen that in 

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The theoretical approach to band lineups outlined above, yields a metal/semiconductor barrier height of the form [53] ... 

Schliiter, M. 1978. Chemical trends in metal-semiconductor barrier heights. Phys Rev B 17 5044-5047. 

Some care must be exercised when using the reverse saturation current obtained from the semilogarithmic current voltage plot and equation 12 to determine the metal-semiconductor barrier height c()g. Card and Rhoderick have shown that if the interfacial oxide is sufficiently thick so that the electron tunnelling transmission coefficient is no longer unity then the reverse saturation current is reduced to a value equal to the product of the reverse saturation current when no interfacial layer is present and the transmission coefficient of the interfacial oxide> that is... 

Since T (6) is less than unity, the effective barrier height is higher than the barrier height when no interfacial oxide is present. Card and Rhoderick have attempted to quantify T (6) in terms of the thickness and work function of the interfacial oxide in gold n-type silicon MIS SBs. However, calculations based on their experimental data showed that for oxide thicknesses between 0.8 nm and 2.6 nm the work function properties did not necessarily resemble those of bulk silicon dioxide and made theoretical cal culation of T (6) impossible. Because of this, c > is best thought of as simply that value of metal-semiconductor barrier height which when used in conjunction with equations 11 and 12 provides an accurate prediction of the forward bias current voltage relationship. 

Fig. 9. Schottky barrier band diagrams (a) a rare situation where the metal work function is less than the semiconductor electron work affinity resulting in an ohmic contact (b) normal Schottky barrier with barrier height When the depletion width Wis <10 nm, an ohmic contact forms.

Table 7. Schottky Barrier Heights for Metals on Compound Semiconductors...

The degree of surface cleanliness or even ordering can be determined by REELS, especially from the intense VEELS signals. The relative intensity of the surface and bulk plasmon peaks is often more sensitive to surface contamination than AES, especially for elements like Al, which have intense plasmon peaks. Semiconductor surfaces often have surface states due to dangling bonds that are unique to each crystal orientation, which have been used in the case of Si and GaAs to follow in situ the formation of metal contacts and to resolve such issues as Fermi-level pinning and its role in Schottky barrier heights. 

In this context it should be mentioned that the height of the Schottky barrier depends on the proc iure of metal deposition and also on the pretreatment. Aspnes and Heller have investigated for instance metal-semiconductor contacts produced by depositing Ru, Rh or Pt as 400 A thick films. They found barrier heights for the metal in contact with air, of 0.6 eV for Ru on Ti02, which decreased to zero in the presence of hydrogen. These results are consistent with those of Yamamoto et al. . ... 

The Schottky-Mott theory predicts a current / = (4 7t e m kB2/h3) T2 exp (—e A/kB 7) exp (e n V/kB T)— 1], where e is the electronic charge, m is the effective mass of the carrier, kB is Boltzmann s constant, T is the absolute temperature, n is a filling factor, A is the Schottky barrier height (see Fig. 1), and V is the applied voltage [31]. In Schottky-Mott theory, A should be the difference between the Fermi level of the metal and the conduction band minimum (for an n-type semiconductor-to-metal interface) or the valence band maximum (for a p-type semiconductor-metal interface) [32, 33]. Certain experimentally observed variations of A were for decades ascribed to pinning of states, but can now be attributed to local inhomogeneities of the interface, so the Schottky-Mott theory is secure. The opposite of a Schottky barrier is an ohmic contact, where there is only an added electrical resistance at the junction, typically between two metals. 

Cowley AM, Sze SM (1965) Surface states and barrier height of metal-semiconductor systems. J Appl Phys 36 3212-3220... 

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