AVS2009 Session SS2-FrM: Semiconductor Surfaces and Interfaces II: Si

The grafting of organic molecules on semiconductor surfaces initiated by UV light has become an efficient means to tailor the chemical and physical properties of surfaces of materials, enabling their integration with various applications of the devices. In situ photoelectron yield experiments were performed in the spectral range from 3 to 6 eV on alkene liquid/nanocrystalline diamond (NCD) interfaces. N-Alkenes carrying different terminal functional groups: trifluoroacetamide-protected 1-aminodec-1-ene (TFAAD) and 10-N-Boc-aminodec-1-ene (tBoc) were used to investigate the dependence of photoelectron yield on the energy of the molecular acceptor level. Amorphous carbon was used in addition to NCD to study the influence of the substrate electronic structure on the photoelectron yield threshold. The photochemical attachment of TFAAD on H-terminated NCD surfaces at various incident photon energies was characterized by X-ray photoelectron spectroscopy (XPS) to investigate the correlation between the excitation energy and the reaction efficiency. These measurements reveal that the photochemical reaction on carbon surfaces is initiated via the photoejection of electrons from the solid valence band into the acceptor levels of the alkenes. SEM images of patterned molecular layers on two H-terminated single crystal diamonds (SCD, type Ib and IIb) with different hole mobilities reveal much sharper transition between functionalized and non-functionalized regions on the sample with much lower hole mobility (type Ib SCD). However, the surface coverage of grafted alkenes as characterized by XPS is quite similar on these two surfaces. These data imply that while the photoemission of electrons controls the reaction efficiency, the holes left in the substrates can diffuse and control the grafting sites.

Recent data exploring the mechanism on silicon will also be presented. Our comparison of silicon and diamond suggests that there are some common factors underlying the ability to graft alkenes onto various semiconductors, but also some important differences. These results suggest that photochemical grafting may be broadly applicable to a wide range of materials, and that a fundamental understanding of the mechanism facilitates the design and synthesis of well defined functional interfaces.

Organophosphonic acid self-assembled monolayers on oxide surfaces have seen growing use in electrical and biological sensor applications [1-2]. Molecular order in organophosphonic acid SAMs is highly desirable for reproducible electronic properties of these modified surfaces. In this regard, packing and order of the SAMs is important, as it influences the electron transport measurements. In this study, we examined the order of the phosphonate films deposited on silicon oxide surface by the Tethering By Aggregation and Growth (T-BAG) method [3] using various state-of-art surface characterization tools. Near edge x-ray absorption fine structure (NEXAFS) spectroscopy is used to study the order of a methyl- and hydroxyl- terminated phosphonate SAMs in vacuum and sum frequency generation (SFG) spectroscopy is used to study their order in aqueous condition. X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) on these samples confirm the presence of chemically intact monolayer phosphonate films. NEXAFS spectroscopy confirmed a considerable degree of molecular order in the octadecylphosphonic acid (ODPA) and 11‑hydroxyundecylphosphonic acid (PUL) films with a tilt angle of 37^° and 47^° respectively. In situ SFG studies in deuterated water were conducted to determine the order of these films under biologically relevant conditions. The ODPA film showed the peaks for terminal methyl units, which are expected for ordered films. PUL films also showed a considerable degree of alignment indicated by resonances of the methylene unit next to the terminal hydroxyl group. These studies indicate that well ordered SAMs with methyl or hydroxyl termination can be prepared on oxide surfaces using phosphonate headgroups. These surfaces can be subsequently used to anchor biomolecules for biomaterial and biosensor applications.

[1] A. Cattani-Scholz, D. Pedone, M. Dubey, S. Neppl, B. Nickel, P. Feulner, J. Schwartz, G. Abstreiter, M. Tornow, Acs Nano 2008, 2, 1653.

[2] H. Klauk, U. Zschieschang, J. Pflaum, M. Halik, Nature 2007, 445, 745.

[3] E. L. Hanson, J. Guo, N. Koch, J. Schwartz, S. L. Bernasek, Journal of the

American Chemical Society 2005, 127, 10058

The growth of glycine film by thermal evaporation on Si(111)7×7 at room temperature has been studied by X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). In contrast to the common carboxylic acids, glycine is found to adsorb on the 7×7 surface through dissociation of the N-H bond (instead of the O-H bond). The presence of a transitional adlayer between the first adlayer and the zwitterionic multilayer is identified by its characteristic N 1s XPS features attributable to intermolecular N-to-HO hydrogen bond. An intramolecular proton transfer mechanism is proposed to account for the adsorption process through the amino group. We also demonstrate that the observed transitional adlayer can be used as a flexible platform to “catch-and-release” biomolecules with compatible H-bonding active sites in a controllable and reversible manner. Intricate evolution of the surface adsorption arrangement during the initial growth has also been monitored by STM. Early evidence of collective assembly of glycine into novel structures in the nanometer length scale could be found.

The adsorption of organic molecules on semiconductor surfaces is of special interest with respect to surface functionalisation and its use in molecular electronics. Due to the localized electronic states of a semiconductor surface, its reactivity strongly correlates with the local electronic properties of the dangling-bond states. Dissociative adsorption of H₂ on Si(001), e.g., shows pronounced site-selective reactivity at steps or preadsorbed atomic hydrogen [1].

In this study, we test the concept of site-selective reactivity at locally distorted configurations for more complex, organic molecules. On that account, the adsorption of ethylene on clean and hydrogen precovered Si(001) surfaces has been investigated by means of scanning tunneling microscopy. On the clean surface, two ethylene adsorption geometries were identified with ethylene adsorbed on one and two dimers, respectively. The latter adsorption geometry shows significantly lower reactivity and has not been observed so far. Preadsorption of atomic hydrogen and the concomitant distortion of the electronic states is found to increase the reactivity of this two-dimer adsorption pathway by almost two orders of magnitude. Its site selective reactivity thus surpasses that of the one dimer configuration on the clean surface.

The results are rationalized in the framework of a precursor mediated adsorption process. Our experiments indicate that the conversion barrier between precursor and final chemisorbed state can be efficiently controlled by changing the local electronic structure of the surface. Thus, locally distorted dangling-bond configurations allow for the control of site-selective reactivity also in the case of barrierless, non-dissociative adsorption of an organic molecule.

[1] M. Dürr and U. Höfer, Surf. Sci. Rep. 61, 465 (2006)

Simulated Scanning Tunneling Microscopy images, based on first-principles calculations, have been used to characterize adsorption products of organic molecules on semiconductor surfaces and assign molecular structures to specific features in experimental STM images. Three examples are presented: 1) Styrene molecular lines on the H-terminated Si(100)-2x1 surface system exhibit several novel molecular conductance phenomena. First-principles calculations show that the phenyl ring orientation at chain ends are fluxional, favoring structures with the terminal ring arranged perpendicular to the molecular line. Simulated STM images show that the tunneling current depends strongly on the phenyl ring orientation, since it controls the coupling between the charged dangling bond and the styrene π system. The perpendicular orientation shows higher conductivity than the parallel one, increasing the apparent height of the molecule at the end of the row. This is consistent with experimental observations, while the simulated images of the parallel-ring geometry are not. Because such subtle changes in molecular structure control the flow of electrical currents, STM can be used to distinguish these conformations. 2) The bonding configuration of styrene attached to the bare Si(111)-7x7 surface is not known from spectroscopic measurements, though two likely possibilities have been identified. Matching simulated empty-state and filled-state STM images to new experimental observations provides strong evidence that the attachment of styrene to Si(111)-7x7 is by a [4+2] cycloaddition, involving both the external –CH=CH₂ and a C=C inside the phenyl ring. A comparison experimental images to theoretical predictions of the bias dependence for the two binding structures is critical to this identification. 3) Three unsaturated organic molecules – styrene, phenyl acetylene, and benzaldehyde – were attached to the H-terminated Si(111) surface through analogous radical chain reactions. Both simulated and experimental STM images of the three molecules show significant differences in apparent height, despite small differences in physical height. Thus STM is sensitive to the functional group used to link the molecule to the surface. The simulations also suggest a new scanning protocol that can enhance the contrast among molecules. The so-called “local constant height” STM images, which probe the spatial variation of conductance, show distinctive features for the three molecules that can be used to tell the molecules apart very easily.

The behavior of defects within silicon can be changed significantly by controlling the chemical state at nearby surfaces or solid interfaces. Experiments have shown that certain chemical treatments change the ability of a free surface to act as a “sink” for point defects such as interstitials. When the surface is made chemically active, this ability rises. The surface can then remove Si interstitials selectively over impurity interstitials and such behavior can be kinetically quantified through an annihilation probability. Although annihilation probabilities for interstitials have been measured under various conditions for free surfaces, very little understanding exists for the corresponding quantity at solid-solid interfaces. Understanding of interface influence on interstitial annihilation is very important in fabrication of advanced transistors for post-implantation damage removal and dopant activation. The present work seeks to develop scientific understanding of interface activity for Si interstitial annihilation and measure the annihilation probabilities at interfaces between silicon and several kinds of oxides and nitrides. Diffusion of isotopically labeled Si (mass 30) in Si host lattice was used as a marker for elucidating how changes in the Si-SiO₂ interface affected Si self-diffusion after annealing. Marked differences are observed among the various interfaces. Continuum simulations of the measured SIMS profiles were subsequently employed to quantify annihilation rates at the Si-SiO₂ interface.