The Study Of Optical And Electrical Properties Of High K Dielectric Poly(Vinylidene Fluoride-trifluoroethylene) Copolymer

Inorganic silicon based semiconductors, especially the integrated circuits, have been the backbone of the microelectronic industry for over half century. Under the driving pace of Moore's Law, integrated circuits progressed from the early 1960's "Small-Scale-Integration" to the present "Ultra-Large-Scale-Integration". As the downscaling of microelectronic devices continues, conventional silicon-based materials have reached a thickness limitation which leads to unacceptably high leakage currents and degradation of carrier mobility in the channel. The further development in silicon-based semiconductors is also being prohibited by the complicated fabrication process and high cost. Numerous researches have been focused on replacing SiO₂ with a physically thicker layer of a material that has a higher static dielectric constant (K) (for SiO₂ K = 3.9). Meanwhile, there have been growing demands in applications requiring large area, flexibility, low-temperature processing and especially low-cost. The ease of fabrication of organic material-based devices such as organic thin film transistors (OTFT) and the ability to process large active areas at low temperatures have been the driving force for the applications in sensors, low-end smart cards, radio-frequency identification tags (RFIDs), etc. In addition, the mechanical flexibility of organic semiconductors makes them more compatible for use with plastic substrates for lightweight and foldable applications.Polyvinylidene fluoride (PVDF) and its copolymer with trifluoroethylene [P(VDF-TrFE)] have been widely used as ultrasound and audio frequency transducers, sensors, actuators, etc. because of their strong ferroelectricity, piezoelectricity, and pyroelectricity. As a ferroelectric material, P(VDF-TrFE) copolymer can be used in nonvolatile random-access memories which are based on metal-ferroelectric-semiconductor field-effect transistor structures. As a organic high K dielectric, P(VDF-TrFE) can also be used to replace SiO₂ with a physical thicker layer in MOSFET and accomplishing all-organic large area display based on OTFT. Accurate optical and electrical properties and film thickness of the P(VDF-TrFE) copolymer as ferroelectric and dielectric thin films are crucial for the fabrication and understanding the organic electronic and optical devices and these properties are therefore the focus of this dissertation.Thin films of 50/50 mol% P(VDF-TrFE) copolymer were prepared by spin casting from methylethylketone (MEK) solutions onto single crystal Si, SiO₂, and quartz slides substrates. The films were optically transparent in the 1.5-4.5 eV (830-280 nm) photon energy range and found to be uniform and smooth as determined using near ultraviolet-visible spectrum and atomic force microscopy, respectively. The optical properties were investigated using spectroscopic ellipsometry (SE) and the extracted film thicknesses were from about 8.6 nm to 249.8 nm obtained by changing solution concentration of 0.3-4.0 wt% and spin speed of 2000-8000 rpm. A Cauchy model was used to fit SE data to obtain the refractive index in 1.5-4.5 eV (830-280 nm) that was found to decrease for thinner films. The effects of substrates on the refractive index were investigated by P(VDF-TrFE) films with about the same thickness but deposited on bare silicon and on thin (native oxide < 1nm) and thick SiO₂ (about 60 nm) and the results showed that different substrates do not significantly affect the refractive index. The surface roughness will cause about 0.3 nm decrease in thickness and 0.004 increase in refractive index for 8.6 nm P(VDF-TrFE) film when the roughness is approximated using a Bruggeman effective medium approximation (BEMA) layer on top of the P(VDF-TrFE) film along with complementary data from atomic force microscopy. It is concluded that the roughness layer does not need to be included in the optical properties determination except for films thinner than 20 nm when the highest accuracy is desired. SE performed at several sensitive angles of incidence has revealed slight optical anisotropy for films thicker than 120 nm. When a unixial anisotropic fit was used the MSE was lowered from 15.4 to 5.1. The difference of n extracted from fitting SE data using isotropic model at 3.0 eV (415 nm) between 65°and 75°is 0.0033. Thermal annealing under vacuum can remove the residual solvent and also improve the crystallinity of P(VDF-TrFE). Annealing in～7×10^-4 Pa vacuum densified the films as evidenced by an increase in the refractive index and anisotropy with higher temperature and longer annealing time and a decrease in the film thickness. The improvement in the degree of structural order and the crystallinity after annealing was confirmed by XRD patterns. The diffracted intensity of the peak at 2θ= 18.5°-20.5°range that derives from the (110) and (200) reflections of the polar crystallineβphase of P(VDF-TrFE) increaseed with annealing time.The electrical properties of P(VDF-TrFE) films were studied using capacitance-voltage (C-V) and conductance-voltage [G(ω)-V] measurements based on Al/P(VDF-TrFE)(/SiO₂)/Si (MPS or MPOS) capacitor structures. The shift of the C-V curves beyond about -1.0 V from zero and small hysteresis indicates that there are small amounts of positive charges in P(VDF-TrFE)-Si interface as a result of processing. The dielectric constant K was 7.3 for 56 nm P(VDF-TrFE) film and the approximate mid band gap At value for Si-P(VDF-TrFE) was 5.2×10¹² cm^-2eV^-1 which is comparable to the At value for Si-SiO₂ before forming gas anneal. The study of dependence of the K on film thickness showed that K increases with the film thickness from 5.2 for 26 nm film to 8.0 for 247 nm film, and reaches a level value near K = 7.8 at about 120 nm that is in agreement with the study of the optertical properties. In addition the change of K with film thickness is greater in the thin film regime. The dependence of K on thermal annealing time under a vacuum of～7×10^-4 Pa at 125℃was determined for～160 nm thick P(VDF-TrFE) films deposited on～39 nm SiO₂ thermally grown on Si substrates, and the results indicate that K increases sharply during the beginning of the anneal, but levels after about 24 h (K = 9.2), and finally reaches K = 9.7 at 120 h apparently with full densification. The leakage current density versus electrical field (J-E) measurements indicate that the leakage current densities for both 32 and 56 nm P(VDF-TrFE) films are higher than those for SiO₂ and the dielectric breakdown voltages [7.4 and -7.6 V for 56 nm P(VDF-TrFE) film and 4.6 and -7.4 V for 32 nm film, respectively] lower than those for SiO₂ (25 and -24 V).In conventional semiconducting devices, a high capacitance dielectric is normally desirable, as it reduces the operating voltage required to turn on the device. Organic thin film transistors (OTFT's) were fabricated with poly(ο-methoxyaniline) (POMA) and naphthalenetetracarboxylic diimide derivative (NDA-n1) as the active semiconductor layers, P(VDF-TrFE) as the gate dielectric, heavily doped silicon as the substrate, and vacuum evaporated gold lines as the source and drain contacts. The output characteristics of the two OTFTs showed the drain current (I_SD) versus drain voltage (V_SD) at different gate voltages (V_G). An increasing positive I_SD for increasing negative V_G and positive V_SD indicates that both POMA and NDA-n1 exhibit p-channel semiconducting properties. The field effect mobilities of the fabricated OTFTs are calculated in the linear range to be 7×10^-5 cm² V^-1 s^-1 for POMA and 3×10^-4 cm² V^-1 s^-1 for NDA-n1. Moderate temperature annealing in high vacuum has improved the device mobility by several orders, yielding evidence for a hopping mechanism for charge transport in POMA and NDA-n1. The lower mobility in OTFTs with P(VDF-TrFE) as the gate dielectric is possibly due to the higher polarity in P(VDF-TrFE) which can affect the local morphology and the distribution of electronic states in the active layer, and the increased localized sites render charge transport difficult. Two alternate gate dielectric layers for the OTFT were chosen to prove this hypothesis: a non-polar low-K dielectric polyethylene (PE) and a polar conventional dielectric SiO₂. The mobility was increased when PE and SiO₂ substituting P(VDF-TrFE) as a gate dielectric. The highest mobility was 1×10^-2 for POMA/PE OTFT and 6.3×10^-3 cm²V^-1s^-1 for NDA-n1/PE OTFT, respectively. It must be noted that not all low- K insulators provide the advantages seen for PE. The use of this non-polar layer will also increase hydrophobicity of the surface and reduces wetting. A balance in between low-K and wetting properties in dielectric films has to be considered.SiO₂ films thermally grown on single crystal Si in high temperature O₂ were etched using nonaqueous HF/pyridine solutions in supercritical CO₂ (SCCO₂). scCO₂ was chosen because of its low viscosity, relative nonreactivity, and negligible surface tension. The etch rate of SiO₂ films were studied in the solutions with HF concentration up to 1000μM at 1.38×10⁷ Pa and at 35℃, 45℃, and 55℃. The results showed that the etch rate increases as a function of both HF concentration and temperature, with temperature having the greater effect. The highest etch rate was slightly above 50 A/min which was obtained using a 1000μM HF solution at 55℃which would enable an effective controlled etch process. Capacitance versus voltage, conductance versus voltage, and leakage current measurements were performed on capacitor structures fabricated after SiO₂ regrowth on completely etched Si surfaces. The electronic results revealed no systematic differences of etched and unetched samples with various etch time/concentration and that the Si-SiO₂ interface of completely etched samples was comparable to the unetched control sample in terms of interface electronic charge and states and leakage current.Spin cast films of P(VDF-TrFE) copolymer deposited on bare Si and SiO₂ coated Si substrates were annealed and treated in scCO₂, and the dielectric properties of the films before and after treatments were studied using capacitance-voltage and conductance-voltage techniques on thin film capacitor structures. Treating annealed P(VDF-TrFE) films in scCO₂ initially decreased K and the refractive index n to as-deposited values that increased and stabilized after reannealing. No systematic differences in interface charges and states were found between treated and untreated samples and with various substrates.