Fabrication and characterization of high-k dielectric films in mos structure

The fact that the speed of semiconductor devices increases incrementally when their size decreases, indicates that we have gotten close to the limits in semiconductor technology. Especially in metal-oxide-semiconductor field-effect transistor (MOSFET) and metal-insulator-semiconductor field-effect transistor (MISFET) technologies from electronic devices, thickness of insulating layers has reached limit values. Thickness is the most important factor when identifying leakage current in gate electrodes. In MOSFETs, high amounts of leakage current passes through thin (~2 nm) SiO2 layers used as gate dielectrics. Therefore, it is foreseen that SiO2 can change places with thicker oxide layers with high-κ. In this study, electrical, optical and structural characteristics of Si-based metal oxide semiconductor structures, such as vanadium pentoxide (V2O5), titanium dioxide (TiO2), cerium dioxide (CeO2) and tungsten trioxide (WO3), composed of high-κ dielectric oxides, which are fabricated through the dip coating technique by being synthesized with the sol gel method, have been investigated in order to identify the characteristics of the materials, which are alternative to SiO2. The thesis study started with the investigation of Al/SiO2/p-c-Si/ohmic Al structures. SiO2, thermally grown on p type silicon (p-Si), with different resistivity, in the form of metal (=Aluminum, Al)/SiO2/p-c-Si/Ohmic Al structure was investigated through admittance analysis. Admittance measurement results of MOS capacitors showed an apparent increase in inversion area, depending on the frequency, that cannot be explained with conventional MOS theory. Inversion charges can respond to the ac. excitation, which is above 50 Hz. This indicates the existence of another electron generation mechanism rather than typical diffusion, generation-recombination mechanisms within a depletion region of Si or Si/SiO2 interface states. Somehow positive charge generation causes the Si bands to be bent in humid environment and in this way it causes the p-type Si semiconductor to be inverted to the n-type semiconductor. A conducting channel is formed both beneath and beyond of the gate electrode. This channel comprised of RC network - where R= resistance and C= capacitance, leads to the decrease in relaxation time and to the frequency response of 100 kHz ac signal. Ion drift along the oxide surface and the proton diffusion models were proposed as the source of the conducting channel to understand the abnormal frequency dispersion in the inversion area of MOS structure. The inevitable local defects at the interface result in change of MOS capacitor properties. Therefore, determination of the interface density of states (DOS) is necessary. In this study, DOS amount was determined by comparing the capacitance measured at low (10-2 Hz) and high (106 Hz) frequencies, which is called high-low frequencies capacitance method. Measurement was performed in two different ways; in the first measurement, low and high frequency C-V measurements were done consecutively, in the other measurement low and high frequency measurements were done simultaneously in a single cycle. It was understood that there was no significant difference between two measurements. After investigating the MOS structures, fabricated with SiO2, high dielectric materials to replace SiO2 were investigated due to the problems resulting from thickness of the silicon dioxide. All high-κ dielectric materials were fabricated through sol gel method, grown through dip coating method and each oxide was subjected to the anneling at different temperatures. The first investigated high-κ dielectric material was V2O5. Structural, optical and electrical analysis of the structures, obtained from V2O5 sols, which were fabricated with two different formulas were carried out. V2O5, obtained through sol gel method, has a MOS structure. V2O5 films, laminated structure of which was proven through XRD measurements [1], are generated from V2O55.nH2O sol, synthesized at a specific water level, and as a result of the current-voltage (I-V) measurements it has been revealed that it has an anisotropic conductivity. Conductivity values differ by the vertical or lateral current measurements and the conductivity values are formed as electronic or ionic centered accordingly. The entire electrical conductivity water ratio of V2O5.nH2O sols depends on the n value and reduced V4+ ion amount. Here, the n value was obtained at the basal distance, which was determined through the XRD measurements and it is about 4-6 [2], and V4+ ions were obtained through FTIR analysis. Electronic conductivity overrides in waterless V2O5.0,5H2O sol; however non-stoichiometric V2O5 is a covalent compound and its electronic characteristics come out due to the bounce between V4+, V5+ ions and this is called small polaron model. Arheniuss-type activation energy (EA) between 0,3-0,5 eV was obtained through the thermal I-V measurements and this result indicates that there is an ionic conductivity rather than electronic conductivity demonstrating the conductivity provided through the proton diffusion in V2O5 film. Obtained through the sol-gel method from the second formula, V2O5 Sol was grown through dip coating technique over c-Si. TG-DTA analysis of the dried V2O5, bond structure measurement of V2O5/p-Si structure through FTIR device and Visible-UV transmittance measurement over the samples deposited on the glass were carried out and the optical bandgap value was determined as a result of optic measurement. In this way, structural and optical features of the generated V2O5 film were analyzed. I-V and frequency-dependent admittance measurements of Ag/V2O5/(p-n)c-Si structure were carried out at room temperature and within dark/light ambient and electrical analysis of the structure was conducted. In the light of the analysis, a thin and naturally formed SiO2 layer between V2O5 and c-Si semiconductors existed and semiconductor-insulating-semiconductor structure [SIS] was constituted rather than a metal-insulating-semiconductor [MOS] structure. Additionally, negative capacitance was observed in the devices grown on n-c-Si under high reverse bias and low frequency conditions. Titanium dioxide is another high-κ material examined in this study. Transport and storage properties of sol-gel synthesized, namely dip coating technique, titanium dioxide (TiO2) thin film over c-Si has been investigated by means of I-V and admittance analysis within different ambient (dark/light). Silver (Ag) was selected as front metal electrode considering its barrier features to charge carriers as well as the work function of anatase TiO2 film. As indicated by FTIR analysis and supported by electrical analysis, Ag/TiO2/c-Si [MOS] structure is actually within Ag/TiO2/ natural silicon dioxide (SiO2)/c-Si [SIS] structure. Consequently, the electrical features of the film were interpreted in terms of SIS diode by looking at C-V features. Cerium dioxide (CeO2) material, the next examined oxide in the study, was fabricated in accordance with the sol-gel method. Some part of the fabricated CeO2 sol was dried, and it was used in TG-DTA analysis. With the prepared CeO2 sol, CeO2 thin films were grown on p/n-c-Si and FTO-(fluorine doped tin oxide) coated glass by using the dip coating method and these films were subject to annealing at different temperatures. Through the FTIR analysis of the obtained CeO2/(p-n)c-Si structure, bonding types were determined and through the visible- UV measurements of the samples grown on FTO coated glass, optical features of the film were determined. Ag metal was used as front contact for electrical measurements and the structure was turned into Ag/CeO2/c-Si. Current, voltage and admittance measurements were conducted and analyzed under the same conditions as other oxides. As a result of the analysis, it was determined that CeO2 film acted like an insulator and a MOS structure was obtained. WO3 which was fabricated through the sol-gel method and grown by dip coating technique, is the last oxide examined in this study. Visible-UV conductivity of the WO3 films that were grown on ITO-(indium tin oxide) coated glass and annealed at different temperatures were measured and the optical bandgap values were calculated. For WO3/c-Si structures, WO3 films grown on c-Si and annealed at different temperatures, Ag was used as front contact metal and the structures were turned into Ag/WO3/c-Si. The electrical characterization of the obtained structures was determined as a result of I-V and frequency-based admittance measurements carried out in dark and light ambient. As a result of the analysis, it has been understood that the WO3 film behave like a semi-insulator.