Tuning the reactivity of semiconductor surfaces by functionalization with amines of different basicity

Abstract

Surface functionalization of semiconductors has been the backbone of the newest developments in microelectronics, energy conversion, sensing device design, and many other fields of science and technology. Over a decade ago, the notion of viewing the surface itself as a chemical reagent in surface reactions was introduced, and adding a variety of new functionalities to the semiconductor surface has become a target of research for many groups. The electronic effects on the substrate have been considered as an important consequence of chemical modification. In this work, we shift the focus to the electronic properties of the functional groups attached to the surface and their role on subsequent reactivity. We investigate surface functionalization of clean Si(100)-2 × 1 and Ge(100)-2 × 1 surfaces with amines as a way to modify their reactivity and to fine tune this reactivity by considering the basicity of the attached functionality. The reactivity of silicon and germanium surfaces modified with ethylamine (CH(3)CH(2)NH(2)) and aniline (C(6)H(5)NH(2)) is predicted using density functional theory calculations of proton attachment to the nitrogen of the adsorbed amine to differ with respect to a nucleophilic attack of the surface species. These predictions are then tested using a model metalorganic reagent, tetrakis(dimethylamido)titanium (((CH(3))(2)N)(4)Ti, TDMAT), which undergoes a transamination reaction with sufficiently nucleophilic amines, and the reactivity tests confirm trends consistent with predicted basicities. The identity of the underlying semiconductor surface has a profound effect on the outcome of this reaction, and results comparing silicon and germanium are discussed.

Fig. 1.

Representative spectral regions in the MIR-FTIR studies of full coverage of ethylamine (EA) and aniline (An) (blue lines) on the Si(100)-2 × 1 surface, these same surfaces exposed to a saturating dose of TDMAT (red lines), and then heated to 500 k (black lines).

Scheme 1.

Reaction of a primary amine on a clean Si(100)-2 × 1 surface.

Fig. 2.

(A) IR spectrum (intensity scaled by a factor of 0.004) of an aniline multilayer on Ge(100) at 130 K. (B) An average of IR spectra taken following saturation exposure of aniline to Ge(100)-2 × 1 at 300 K. (C) IR spectrum calculated for intradimer N─H dissociated aniline on a two-dimer Ge₂₃H₂₄ cluster. Calculated frequencies are scaled by 0.96.

Fig. 3.

(A) Ge(3d), (B) C(1s) and (C) N(1s) photoelectron spectra taken after 260 L saturation exposure of aniline to Ge(100)-2 × 1 at room temperature. The Ge(3d) spectrum has been scaled by 1/15.

Scheme 2.

Reaction of transamination used to test the reactivity of amine-precovered semiconductor surfaces.

Fig. 4.

A summary of AES studies of transamination reaction for TDMAT on ethylamine- and aniline-precovered Si(100)-2 × 1 and of XPS investigation of the same reaction on aniline-precovered Ge(100)-2 × 1.