Organic and inorganic surface modification of semiconductors for electronic and energy conversion devices [electronic resource]
Keywords: Chemical Engineering

Over the past 50 years, astonishing progress has been made in the microelectronics industry by continual scaling of semiconductor devices to smaller dimensions. In addition to the microelectronics industry, semiconductor materials are finding important use in the rapidly growing renewable energy field. These include both conventional inorganic semiconductors such as silicon and also organic semiconductors. Development of new materials and interface engineering has attracted special attention in recent years so that the future progress of the microelectronics and renewable energy industries can continue. Interactions at the sub-nanometer length scale are now of great importance and this has spurred the study of the atomic-level phenomena that govern interfacial bonding and chemical reactions occurring directly at semiconductor interfaces. In this work, a combined experimental and theoretical approach is used to investigate the reactions of organic functional groups, as well as inorganic molecules, on silicon (Si), germanium (Ge), and titanium dioxide (TiO2) surfaces. The reconstructed Si(100)-2 x 1 and Ge(100)-2 x 1 surfaces offer well-defined substrates, ideal for fundamental reactivity studies. Investigation of the reactivity of amino acids on the Si(100)-2 x 1 and Ge(100)-2 x 1 surfaces was carried out theoretically to provide insight into the behavior of simple biological molecules with multifunctional moieties for potential application in development of biosensors. The density functional theory (DFT) studies of these systems reveal several unique properties not exhibited by organic functionalities previously considered in the literature. These include pericyclic ene reactions of the imine functional group of arginine and the cyclic imine of the imidazole side chain of histidine. Both of these reactions involve formation of Si-N and Ge-N dative bonds which are significantly stronger than any previously observed on Si(100)-2 x 1 and Ge(100)-2 x 1, respectively. Because germanium is of interest for next generation electronics and methods to passivate the surface are not currently well established, studies were carried out on surface passivation and deposition strategies for modification of germanium surfaces. Formation of alkanethiolate self-assembled monolayers (SAMs) as well as growth of TiO2 by atomic layer deposition (ALD) were investigated. The investigation was carried out by employing a range of experimental techniques and DFT. Surface passivation studies of the Ge(100) and Ge(111) via halogenation and thiolation routes reveals that the quality and stability of 1-alkanethiolate SAMs, formed at the halogenated Ge surfaces, depend on the concentration, solvent, and the crystallographic orientation of the substrate. Moreover, synchrotron radiation photoemission spectroscopy (SR-PES) investigation of this system complemented by DFT calculations shows that the resulting Ge thiolates are thermally stable up to 150 °C, with the majority of surface thiolates converted to sulfide and carbide upon annealing to 350 °C under UHV conditions. Studies of ALD of TiO2 (as a high-[Kappa] dielectric material) at brominated Ge surfaces reveal an accelerated growth rate for the first 15 ALD cycles at 300 °C. Furthermore, the data suggest that TiO2 films are deposited with no interfacial oxide layer at 300 °C following the desorption of bromine from the Ge surface. Moreover, blocking of the TiO2 ALD precursors via utilization of 1-alkanethiolate SAMs is demonstrated and it is found to be a function of crystallographic orientation of the Ge substrate. Atomic layer deposited TiO2 formed on halogenated Ge surfaces is also subjected to a vacuum annealing study, where film thickness dependent TiO2 reduction is shown upon annealing to 700 °C. DFT calculations represents that an understanding of the thermodynamic and kinetic factors governing the reactivity of titanium tetrachloride (TiCl4) and water ALD precursors is essential for controlling the TiO2 growth rate as well as the creation of a desired chemical functionality at the interface. Studies evaluating novel deposition strategies of quantum dots on TiO2 semiconductor surfaces were also carried out to explore strategies for forming more efficient quantum dot sensitized solar cells (QDSSCs). To compete with cheap electricity provided by fossil fuels in the present, new efforts are needed to fabricate low-cost photovoltaic devices that can harvest photons more efficiently, and QDSSCs are one class of "third generation" photovoltaics that show promise. However, performance in such devices is reduced by electron-hole recombination at the interface between the QD and the semiconductor (such as TiO2) and the interface between the QD and the hole conductor. Therefore, detailed understanding of the modification of interfacial properties of these nanostructured devices stands out as one of the main challenges in the development of more efficient solar cells. This work has been carried out to evaluate the benefit of organic surface modification on cadmium sulfide (CdS)-based QDSSCs with solid-state hole conductors. Both liquid-phase (successive ionic layer adsorption and reaction (SILAR)) and gas-phase (ALD) deposition techniques were used to grow the CdS QDs. The results demonstrate that the use of phosphonate-based SAMs may enhance the overall efficiency of the solar cells.