Hyun-Dam Jeong

A semi-empirical study of the chemisorbed state of benzene on Si(100)-(2 × 1)

Abstract The chemisorbed state of benzene on Si(100)-(2 × 1) has been studied using a cluster model and semi-empirical PM3 calculations. Starting from four different initial sites as the possible adsorption sites, the most stable structure is obtained as the adsorption at the pedestal site, where one benzene molecule interacts with two silicon dimers of the surface. This stable adsorption of benzene on the pedestal site has not been proposed previously, but is in good accord with the experimental results. The carbon atoms bonded to silicon dimers have some sp 3 characters, in accordance with the experimental results of high resolution electron energy loss spectroscopy, while the others preserve the hybridization state in a free molecule. In addition, the saturation coverage of benzene predicted from the adsorbed state is in agreement with the experimental results.

Newly Synthesized Silicon Quantum Dot−Polystyrene Nanocomposite Having Thermally Robust Positive Charge Trapping

Striving to replace the well known silicon nanocrystals embedded in oxides with solution-processable charge-trapping materials has been debated because of large scale and cost effective demands. Herein, a silicon quantum dot-polystyrene (SiQD-PS) nanocomposite (NC) was synthesized by post-functionalization of hydrogen-terminated silicon quantum dots (H-SiQDs) with styrene using a thermally induced surface-initiated polymerization approach. The NC contains two miscible components: PS and SiQD@PS which, respectively, are polystyrene and polystyrene chains-capped SiQDs. Spin-coated films of the nanocomposite on various substrate were thermally annealed at different temperatures and subsequently used to construct metal-insulator-semiconductor (MIS) devices and thin film field-effect transistors (TFTs) having a structure of p-Si++/SiO2/NC/pentacene/Au source-drain. Capacitance-voltage (C-V) curves obtained from the MIS devices exhibit a well-defined counterclockwise hysteresis with negative fat band shifts, which was stable over a wide range of curing temperatures (50-250 °C). The positive charge trapping capability of the NC originates from the spherical potential well structure of the SiQD@PS component while the strong chemical bonding between SiQDs and polystyrene chains accounts for the thermal stability of the charge trapping property. The transfer curve of the transistor was controllably shifted to the negative direction by varying applied gate voltage. Thereby, this newly synthesized and solution processable SiQD-PS nanocomposite is applicable as charge trapping materials for TFT based memory devices.

Adsorbed state of thiophene on Si(100)‐(2×1) surface studied by electron spectroscopic techniques and semiempirical methods

The adsorbed state of thiophene on Si(100)‐(2×1) surface at 300 K has been investigated using low‐energy electron diffraction (LEED), Auger electron spectroscopy (AES), and ultraviolet photoelectron spectroscopy (UPS). (2×1) LEED pattern at 300 K is sustained after the saturated exposure of thiophene, and the saturation coverage is estimated to be ∼0.6 by AES, suggesting that thiophene molecule is chemisorbed molecularly on the Si(100) surface most likely by σ bonds between C and Si atoms. UPS spectrum for the chemisorbed thiophene shows not only the π orbital shift but also the σ orbital shift. Semiempirical PM3 calculations based on the cluster model propose that the thiophene molecule adsorbs on the Si(100)‐(2×1) surface by forming di‐σ bonds between C atoms of thiophene and Si atoms of the surface.

Tuning Optical Properties of Si Quantum Dots by π-Conjugated Capping Molecules

The absorption and photoluminescence (PL) properties of silicon quantum dots (QDs) are greatly influenced by their size and surface chemistry. Herein, we examined the optical properties of three Si QDs with increasing σ-π conjugation length: octyl-, (trimethylsilyl)vinyl-, and 2-phenylvinyl-capped Si QDs. The PL photon energy obtained from as-prepared samples decreased by 0.1-0.3 eV, while the PL excitation (PLE) extended from 360 nm (octyl-capped Si QDs) to 400 nm (2-phenylvinyl-capped Si QDs). A vibrational PL feature was observed in all samples with an energy separation of about 0.192±0.013 eV, which was explained based on electron-phonon coupling. After soft oxidization through drying, all samples showed blue PL with maxima at approximately 410 nm. A similar high-energy peak was observed with the bare Si QD sample. The changes in the optical properties of Si QDs were mainly explained by the formation of additional states arising from the strong σ-π conjugation and QD oxidation.

Observation of Negative Charge Trapping and Investigation of Its Physicochemical Origin in Newly Synthesized Poly(tetraphenyl) silole Siloxane Thin Films

A new kind of organic-inorganic hybrid polymer, poly(tetraphenyl)silole siloxane, was invented and synthesized for realization of its unique charge trap properties. The organic portions consisting of (tetraphenyl)silole rings were responsible for negative charge trapping, while the Si-O-Si inorganic linkages provided the intrachain energy barrier for controlling electron transport. The polysilole siloxane dielectric thin films were fabricated by spin-coating and curing of the polymers, followed by characterization with spectroscopic ellipsometry (SE), near edge X-ray absorption fine structure spectroscopy (NEXAFS), and photoemission spectroscopy (PES). The abrupt increase in density and decrease in thickness of the thin film at a curing temperature of 100 °C was attributed to a thermodynamically preferred state in the nanoscopic arrangement of the polymer chains; this was due to cofacial π-π interactions in a skewed manner between peripheral phenyl groups of the (tetraphenyl)silole rings of the adjacent polymer chains. Using the NEXAFS spectrum to assess high electron affinity, the LUMO energy level of the dielectric thin film cured at 150 °C was positioned 1 eV above the Fermi energy level (E(F)). The electron trapping of the dielectric thin films was confirmed from the positive flat band shift (ΔV(FB)) in the capacitance-voltage (C-V) measurements performed within the metal-insulator-semiconductor (MIS) device structure, which strongly verified the polymer design concept. From the simple kinetics model of the electron transport, it was proposed that the flat band shift (ΔV(FB)) or trap density of the negative charges (|ρ|) was logarithmically proportional to the decay constant (β) for the electron-tunneling process. When a phenyl group of a silole ring in a polymer chain was inserted into the two available phenyl groups of another silole ring in another polymer chain, the electron transfer between the groups was enhanced, decreasing the trap density of the negative charges (|ρ|). For the thermodynamically preferred state generating the high refractive index, the distance between the two phenyl groups of the adjacent polymer chains was estimated to be in the range of 0.27-0.36 nm.

Tuning of refractive indices and optical band gaps in oxidized silicon quantum dot solids.

This laboratory has initiated compelling research into silicon quantum dot (Si QD) solids in order to utilize their synergetic benefits with quantum dot solids through fabrication of Si QD thin films. The issues of oxidation concerning the Si QD thin films were confirmed using Fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS). The refractive index value of the Si QD thin film at a 30 degrees C curing temperature was 1.61 and 1.45 at 800 degrees C due to complete oxidation of the Si phases. The optical band gap values of 5.49-5.90 eV corresponded to Si phases with diameters between 0.82 and 0.74 nm, dispersed throughout the oxidized Si QD thin films and modeled by Si molecular clusters of approximately 14 silicon atoms. The photoluminescence (PL) energy (2.64-2.61 eV) in the proposed Si QD thin films likely originated from the Si horizontal lineO bond terminating the Si molecular clusters.

Deducing nanopore structure and growth mechanisms in porogen-templated silsesquioxane thin films

Adjusting the functional group of a porogen is found to have a tremendous effect on the pore structre of porous low dielectric constant films with silsesquioxane as the matrix precursor. The pore size and interconnection length measured by positronium annihilation lifetime spectroscopy can be used to deduce the pore shape and its evolution with porosity from templates of isolated porogen molecules through film percolation. Inert, self-linkable, and amphiphilic porogens are demonstrated to randomly aggregate three-dimensionally, linearly polymerize, and form micelles, respectively.

Newly observed temperature and surface ligand dependence of electron mobility in indium oxide nanocrystals solids.

We developed a new class of organic surface ligands; 2-aminopyridine (2AP), 4-aminobenzoic acid (4ABA), and benzoic acid (BA); for use in the solution ligand exchange of nanocrystals (NCs) in the presence of nitric acid (HNO3). Here, colloidal NCs synthesis is used for the first time. These short, air-stable, easy-to-model ligands bind to the surface of the indium oxide nanocrystal (In2O3 NC) and provide the electrostatic stabilization of NC semiconductor dispersions in N,N-dimethylformamide, allowing for a solution-based deposition of NCs into thin-film transitors (TFTs). The shorter organic ligands greatly facilitate electronic coupling between the NCs. For example, thin films made from 2AP-capped In2O3 NCs exhibited a high electron mobility of μ≈9.5 cm2/(V·s), an on-off current ratio of about >10(7), and a subthreshold swing of 2.34 V/decade. As the ligand length decreased, the electron mobility increased exponentially. Furthermore, we also report on the temperature-dependent behavior of the electron transport of In2O3 NCs films, in the case in which thin films were cured at 150 °C, as the 2AP, BA, and 4ABA ligand molecules were sustained on the NC. We demonstrated a hopping transport mechanism instead of a band-like transport, and the thermally activated carrier transport process governed the charge transport in our In2O3 NC thin-film solid.

Ferroelectric/Dielectric Double Gate Insulator Spin-Coated Using Barium Titanate Nanocrystals for an Indium Oxide Nanocrystal-Based Thin-Film Transistor.

Barium titanate nanocrystals (BT NCs) were prepared under solvothermal conditions at 200 °C for 24 h. The shape of the BT NCs was tuned from nanodot to nanocube upon changing the polarity of the alcohol solvent, varying the nanosize in the range of 14-22 nm. Oleic acid-passivated NCs showed good solubility in a nonpolar solvent. The effect of size and shape of the BT NCs on the ferroelectric properties was also studied. The maximum polarization value of 7.2 μC/cm(2) was obtained for the BT-5 NC thin film. Dielectric measurements of the films showed comparable dielectric constant values of BT NCs over 1-100 kHz without significant loss. Furthermore, the bottom gate In2O3 NC thin film transistors exhibited outstanding device performance with a field-effect mobility of 11.1 cm(2) V(-1) s(-1) at a low applied gate voltage with BT-5 NC/SiO2 as the gate dielectric. The low-density trapped state was observed at the interface between the In2O3 NC semiconductor and the BT-5 NCs/SiO2 dielectric film. Furthermore, compensation of the applied gate field by an electric dipole-induced dipole field within the BT-5 NC film was also observed.

Characterization of the CTAB/NaSal/TEOS Solid Films – Prospective Candidates for Application as Low-Dielectric-Constant Interlayers

We present a complex investigation of the structure and mechanical and dielectric properties of the CTAB/NaSal/TEOS films spin-coated on silicon wafers. These novel films can be used as the low-dielectric-constant interlayers in microelectronics. It is shown that the films have average disordered structure: they consist of a great number of several hundred nanometer-sized domains, which contain mutually parallel cylindrical tubes, having diameters of the order of 5 nm and length of several hundred nanometers. The mechanical characteristics of our films are found to be rather high with respect to those of other films for low-dielectric-constant applications. Minimal hardness and Youngs modulus are 2.7 and 29.6 GPa respectively. The value of dielectric constant is around 3, and, in spite of chemical antimoisture absorption treatment, some small amount of water is present in the film.