Schottky-Barrier Origin

It is more likely that surface states exist but that different metals can affect the band bending. An analysis based on this model has been carried out by 

Wronski and Carlson (1977). The results shown in Fig, 21 indicate that the barriers on several metals parallel those of crystalline Si. The analysis clearly shows that the unmodified work-function model cannot be applied directly, but a significant linear dependence on metal work function was observed. The analysis indicated an interface density of 2 X 10 cm. This is similar to the surface-state density reported by Jackson et al. (1983a). 

Because of the close parallel in barrier results of a-Si H and crystalline Si, it is worthwhile to examine other models used for crystalline Si. One such model has been termed the effective work-function model (Freeouf, 1980). This model is based on the fact that atomic interactions occur for almost all metal-Si interfaces studied to date. Thus the work-function model should be modified to use the work function of the metallic states adjoining the semiconductor. This model thus must include the chemistry of the interface (Grunthanercfa/., 1980 RubloffcM/., 1981). Since similar atomic interactions are observed for metals on a-Si H as on crystalline Si, a similar model may apply. Unfortunately, the theoretical details of the model have not been 

In the laboratory the Schottky-barrier structure will continue to be significant in many experimental structures. Of significant use is the Schottky barrier in DLTS and, obviously, capacitance and photoconductivity measurements, which have been described in this chapter and in Chapter 2 by Cohen. 

At this stage the critical issues regarding a-Si H Schottky barriers are addressed. The first is whether the forward-bias transport of undoped diodes is limited by diffusion or thermionic emission. This point may prove crucial to interpreting other experiments such as DLTS or frequency-dependent C- V to obtain gap-state densities. It seems from the experiments described to date, that well-prepared, well-characterized interfaces for at least some metals yield transport characteristics consistent with the thermionic emission theory. 

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Schottky barrier height modulation and stabilization as a "by product" of the original studies. Moreover, since these results are thought to follow from replacement reactions in the vicinity of the interface, they reflect the importance of surface structure as well as composition at semiconductor interfaces. 

The n-p junction was discussed in Section 7.4.1.2. In the original concept, this junction resulted from the transfer of electrons from one semiconductor to another. In the figures in Sec. 7.4, potential-distance relations for the junction of n and p semiconductors are shown. It is clear that here the transfer of one charge carrier from one semiconductor to the next in an uphill direction can be thought of as being opposed by the electrical potential hill shown. Such potential hills are termed Schottky barriers. ... 

W. W. Gartner, Phys. Rev. 116 84 (1959). Photoemission from solids basis of much of Gerischer s subsequent Schottky barrier theory. Origin of neglect of surface properties until 1980s. 

The previously described four-probe technique [43,44] allows a separate determination of the source and drain contact resistances. If contacts would behave as Schottky barriers, one would expect the voltage drop at source to be substantially higher than that at drain. This is what is indeed observed with bad contacts. However, good contacts show comparable drops at both electrodes. A possible origin of this behavior has been recently put forward [46]. The model assumes that the regions immediately adjacent to the electrodes are made of organic material of quality different from that of the rest of the conducting channel, with very low mobility. 

The second part of this chapter will describe some of the general aspects of the Schottky barrier and properties specific to a-Si H. The third part is devoted to the transport mechanisms and measurements, and the fourth describes the effects due to atomic structural properties of the interface. Next, the effects due to doping are addressed, and following that the origin of the Schottky barrier is discussed. By presenting the question of the origin of the barrier near the end of this chapter, we imply that it is an unsolved problem. The chapter is concluded with comments about applications and important problems to be addressed for further understanding. 

Chattopadhyay, R Ray Chaudhuri, B. 1992. Origin of the anomalous peak in the forward capacitance-voltage plot of a Schottky barrier diode. Solid-State Electronics, 35 875-878. 

Hie Hiales group has patented an original sensor architecture based on an array of CNTFETs with various metals for the electrodes (BondavaUi et al., 2008, 2010). As described in Section 10.3.3, the sensing mechanism of CNTFETs is mainly attributed to the modulation of the Schottky barrier built at the contacts between metallic electrodes and semiconducting SWCNTs. Assuming that the adsorption of gas at the contacts depends on its chemical affinity with the electrodes, each gas-metal couple should induce different modulation of the Schottky barrier. 

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