Passivation, periodic trends, and selective attachment in organic functionalization of Ge(100)-2 x 1 [electronic resource]
Keywords: Chemical Engineering

Combining the current knowledge of microelectronics fabrication with the tailorability of organic materials is expected to facilitate progress in a number of fields including molecular electronics, nanoscale lithography, and biosensors. This work specifically probed the attachment of organic molecules to clean Group IV (100)-2 x 1 semiconductor surfaces, with emphasis on the Ge(100)-2 x 1 surface because of growing interest in germanium as an alternative to silicon in next-generation devices. To isolate interactions of the organic adsorbates with a well-defined surface, experiments were carried out under ultrahigh vacuum conditions, and reaction products were probed with in situ spectroscopic techniques, including infrared spectroscopy and X-ray photoelectron spectroscopy. Density functional theory quantum chemical calculations were also employed to model the surface reactions. A first step towards the use of functionalized surfaces in various applications is to establish a fundamental understanding of the reactivity of these surfaces toward different organic functional groups. To this end, periodic trends in the thermodynamics and kinetics of adsorption of Group IV-, V- and VI-containing molecules at Group IV (100)-2 x 1 surfaces were explored. In studying the reactivity of sulfur- versus oxygen-containing molecules at the Ge(100)-2 x 1 surface, it was found that the reactions of the sulfur-containing molecules with the Ge(100)-2 x 1 surface are both kinetically and thermodynamically more favorable than those of the oxygen-containing congeners with the surface. To investigate broader trends in adsorption at Group IV (100)-2 x 1 surfaces, the latter study was expanded and compiled with results from several literature studies. Trends toward stronger dative bonding on Si than on Ge as well as an increase in X--Ge dative bond strength to the left across Period 2 and down Group VI of the periodic table were identified. The results are rationalized in terms of donor atom electronegativity, acceptor atom electron affinity and orbital overlap between the bonding atoms. On the other hand, a decrease in ordinary covalent X--Ge bond strength was observed down Groups V and Group VI. Electronegativity difference and orbital overlap between the bonding atoms are used to explain these trends. The creation of a selectively attached monolayer where one reactive moiety remains available for subsequent reaction will likely be important for various deposition strategies used in fabricating molecular devices. Investigation into selective attachment was carried out through studies of the adsorption of bifunctional molecules at the Ge(100)-2 x 1 surface, with each study examining a different parameter affecting selective attachment. Studying the adsorption of glycine, which contains both carboxyl and amine groups, revealed that attachment through the carboxyl end is thermodynamically and kinetically more favorable than adsorption through the amino group, although a fraction of adducts also attach through the latter. Comparison of bifunctional molecules with one common and one distinct functional group allowed the effect of a second functional group on selective attachment to be examined. Both mercaptoethanol and mercaptamine adsorb through the thiol end via S--H dissociation; however, while the presence of a hydroxyl group in mercaptoethanol results in a homogeneous distribution of adducts adsorbed through S--H and O--H dissociation, the amine group in cysteamine leads to a mixture of products varying in degree of interaction between the amine group and the surface. Lastly, geometric and electronic effects on reactivity were examined through Ge(100)-2 x 1 surface reaction with phenylenediamine structural isomers, which vary in the spacing of two amine groups on a rigid benzene ring spacer. The study reveals that both geometric and electronic effects favor dual N--H dissociation of the meta isomer for which 85% of adducts are adsorbed through dual N--H dissociation, corresponding to an occupation of available surface sites of 96%. The fundamental studies on periodic trends in reactivity of organics at Ge(100)-2 x 1 indicated the stabilizing effect of sulfur attachment at Ge(100)-2 x 1. Consequently, disulfide adsorption on Ge(100)-2 x 1 was investigated as a possible passivation strategy. The study demonstrates that disulfides, which adsorb via S--S dissociation, may potentially constitute effective passivants for Ge. For all of the disulfides investigated, exposure of ambient air to the passivated surface did not lead to degradation of the thiolate adlayer, and in all cases the functionalized surfaces performed markedly better than the control (bare Ge(100)-2 x 1 surface). However, oxidation resistance is currently suboptimal for use in industry; use of disulfides to passivate Ge would likely require higher surface coverage of the adsorbate on Ge. Overall, the studies in this thesis provide insight into competition and selectivity, periodic trends, and surface passivation in adsorption of organic molecules on Ge(100)-2 x 1.