Evidence for Fermi-energy pinning relative to either valence or conduction band in Schottky barriers

Abstract

Using the temperature dependence of Schottky-barrier heights in epitaxial silicide-silicon diodes, we show that the metal Fermi level at the interface is pinned either relative to the Si conduction band or to the Si valence band. The contribution of the semiconductor to the interface states is restricted to the semiconductor band nearest in energy. We then discuss the effect of the metal on the interface states and propose different models of Schottky-barrier formation that may account for these results.Despite decades of intense study on metalsemiconductor contacts, the enigma of Schottky-barrier formation still challenges basic solid-state research. The first explanation, given by Schottky, ' was simply based on the difference of the work function between the metal and the semiconductor. This model did not account for the low dependence of Schottky-barrier heights (SBH) on metal work function and the deviations were explained by Bardeen in terms of semiconductor interface states. In this approach, the Fermi level is pinned in the semiconductor band gap by electronic states, so that the SBH is equal to the energetic depth of the pinning states below the conduction band. The origin and the nature of these states still remain enigmatic; the theoretical attempts are often questionable because of a crucial lack of experimental evidence. Some authors have argued that the SBH is nearly independent of the metal and it has been currently advanced that the pinning states are associated with the semiconductor (SC) alone and are related to metal-induced gap states, ' native or antisite defects at or near the SC surface. On the other hand, various correlations have been made with heats of formation, polarizability, ionicity, and even the anion work function in III-V compounds with intent to propose again an effective work-function model. The diversity of models and invoked assumptions, which can be easily explained by the lack of precise data on SBH, imply that the most basic aspects of Schottky-barrier formation have not yet been established.In this paper, we therefore present new and original data which are relevant for the understanding of the Fermi energy pinning in Schottky diodes. Our results throw new light on the formation mechanisms of the Schottky barrier (SB) and show that, in a metal-semiconductor contact, the Ez pinning is possible to either the valence band or the conduction band of the semiconductor. In our study, we used silicide-silicon heterostructures which appear to be ideal candidates for investigating the SB formation for two reasons: from a metallurgical point of view, the silicide layer is epitaxial on Si, so the interface is free of interfacial layer, and the contact is really intimate; from an electrical point of view, the SBH measurements are highly reproducible and the great variety of sil-icides allows the SBH values to scan the entire Si band gap. ' Among these silicides, special attention is devoted to ErSi2 and CoSi2 because of their opposite behavior when deposited on Si: while the former le...

Electron reflection and interference in the GaAs/AlAs-Al Schottky collector resonant-tunneling diode

“…If the change in E fe /e is neglected, this implies V s changes in the same manner as E g /e, with the metal Fermi energy pinned relative to the GaAs valence band. This tends to support a conclusion drawn in a study of epitaxial silicide-silicon diodes, 15 which stated that the semiconductor contribution to the interface states that pin the Fermi energy of a metal-semiconductor junction is dominated entirely by the nearest semiconductor band. In this case the result implies the interface states have valence-band wave functions.…”

Nanoscale mapping of the W/Si(001) Schottky barrier

“…10,11 In addition to interface dipoles, other effects, such as defects, foreign species, and compositional fluctuations, can also affect the spatial distribution of the Schottky barrier height on the nanometer length scale. [12][13][14][15] Powerful techniques to probe metal to semiconductor interfaces with nanoscale precision are ballistic electron emission microscopy (BEEM) and ballistic hole emission microscopy (BHEM). 16 BEEM and BHEM have been used to measure the Schottky barrier height of many different metals to semiconductor interfaces.…”

CoSi 2 -coated Si nanocrystal memory

“…However when CoSi 2 connects with Si, the actual Fermi level of CoSi 2 is pinned around Si valance band, together with those interface defect states. 26,27 Owing to the pinning effect and high density of states around the Fermi level, CoSi 2 -coated Si nanocrystal memory can achieve larger storage capacity, more uniform program/erase, and stable retention performance.…”

“…24 The formation of TiSi 2 on Si nanocrystals by annealing is much more difficult and requires much higher temperature due to fine-line effect. 25 Owing to high-density interface states between silicide and Si, the defects induced during original formation of Si nanocrystals may be pinned at the Fermi level of silicide, [26][27][28][29][30] leading to uniform programming/erasing among the devices on a chip. This was not recognized in our earlier TiSi 2 nanocrystal memory effort.…”