Electrical Properties of Junctions between Hg and Si(111) Surfaces Functionalized with Short-Chain Alkyls

Abstract

Metal-semiconductor junctions between Hg and chemically modified n-and p-Si(111) surfaces have been prepared and analyzed using current-voltage and differential capacitance-voltage methods. To understand the role of the interfacial dipole on interfacial charge transfer, silicon surfaces were modified with either nonstoichoimetric oxide (SiO x ), terminal monohydride, short (C n H 2n+1 -, n ) 1, 2, 3) saturated alkyl chains, or propynyl (CH 3 -CtC-) groups. X-ray photoelectron spectra of the modified Si electrode surfaces taken before and after exposure to Hg contacts showed no evidence of irreversible chemical interactions between the Si and the Hg. Hg/Si contacts made using H-terminated Si(111) surfaces exhibited Schottky junctions having barrier heights (Φ b ) that were consistent with the known surface electron affinity of Si and the work function of Hg. In contrast, Si coated with a thin, chemically grown oxide formed Hg/Si junctions having barrier heights suggestive of Fermi level pinning. Si(111) surfaces modified with methyl groups yielded Hg junctions having barrier heights in accord with expectations based on the electron affinity (3.67 eV) and surface dipole (0.38 eV) measured on such surfaces by photoemission spectroscopy, attesting to the degree of chemical control that can be exerted over the barrier heights of such systems by surface functionalization methods. Incomplete coverages of functional groups produced by alkylation with ethyl or iso-propyl groups did not greatly impact the observed values of Φ b relative to Φ b values observed for CH 3 -terminated Si(111) surfaces. However, the observed variation in Φ b between nominally identical samples increased as the number of carbons in the functionalizing alkyl group increased. Junctions between Hg and Si(111) surfaces modified with propynyl groups showed nearly identical behavior to that of CH 3 -Si(111)/Hg contacts, both in average Φ b values and standard deviation between samples. The behavior of Si/Hg interfaces modified with short organic functional groups is consistent with the efficacy and utility of passivated surfaces in modifying the properties of surface-based Si devices.

Molecular Electronics at Metal/Semiconductor Junctions. Si Inversion by Sub-Nanometer Molecular Films

“…In case of very thin insulators, the junction is similar to an abrupt, one-sided p + -n junction 9 . Earlier reverse bias C-V measurements 12,13 that were confirmed by us (see (Fig. 3).…”

Molecules on Si: Electronics with Chemistry

“…Experimentally, though, the junction was found to be ohmic. [19,141] For p-SiÀH/Hg an effective SBH of 0.85 eV is found. [19,141] Binding a methyl group to the Si radically changes the situation and n-SiÀCH 3 /Hg is now strongly inverted with a SBH of 0.8 eV, while the p-SiÀCH 3 /Hg junction is close to ohmic (barrier <0.1 eV).…”

“…Remarkably, an MIS structure with an alkyl monolayer insulator shows reproducibility and uniformity that are superior to what is obtained with inorganic oxides of similar width [40,56] and for which, even if the alkyl monolayer width is shrunk down to a C 1 (ÀCH 3 ) layer, MIS-like behavior persists. [19,20] The dielectric strength of alkyl monolayers on either Si [57,58] or Au [59] is about 10-20 MV cm À1 , twice that of bulk SiO 2 , [41] 10 times that of bulk polyethylene, [57] and comparable to that of alkyl monolayers on SiOx. [60] The excellent insulating properties of alkanes allowed their use as gate insulators for FET devices.…”

“…[44,66] Transport across very short alkyl monolayers, directly bound to Si, and characterized by spectroscopy, [33,67] was studied with Hg, [19] Au [2] and other metals as second contact. [68] In other work powerful techniques for characterizing Si MOMS were used and, in part, adapted.…”

“…The difference between Hg work function (f ¼ 4.49 eV) [19,29] and Si electron affinity (x ¼ 4.05) [83] yields a theoretical Schottky barrier height (SBH, as defined in Eq. (1)) of $0.4 eV for the n-SiÀH/Hg junction.…”