Master Degree / Yüksek Lisans Tezleri
Permanent URI for this collectionhttps://hdl.handle.net/11147/3008
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Master Thesis The Effects of Prior Nitridation Process of Silicon Surface and Different Metal Gates on the Capacitance Voltage Characteristics of Metal-Ta2o5 Mos Capacitor(Izmir Institute of Technology, 2007) Özben, Eylem Durğun; Güneş, Mehmet; Güneş, Mehmet; 04.05. Department of Pyhsics; 04. Faculty of Science; 01. Izmir Institute of TechnologyAccording to the 2004 International Technology Roadmap of Semiconductor (ITRS), for sub-micron technology, an equivalent oxide thickness (EOT) less than 1 nm is required. However, for such thickness levels, the native oxide SiO2 is unacceptable since it does not posses its inherited physical properties and results in high leakage current density resulting in reduced device performance. The replacement of SiO2 with high dielectric constant material (high-k) may eliminate such problems since it will allow the usage of thicker dielectric material. The leakage current will be reduced while maintaining the same levels of inversion charge. In this study, the electrical properties of metal-Ta2O5-Si MOS capacitor were investigated for devices prepared with different conditions. A prior nitridation process of silicon surface in N2O and NH3 gas before Ta2O5 was carried out to improve interface quality. In addition, different metal gates formed on the Ta2O5 oxide layer were also used in order to see the effects of top oxide-metal gate on the electrical properties of MOS capacitors. The metal gates used are Al, TiN and W. High frequency (1MHz) Capacitance-Voltage Spectroscopy was used to understand the effects of prior nitridation process and metal gates on the Ta2O5 high-k oxide properties. From the analysis of high frequency C-V curves, oxide capacitance, dielectric constant, EOT, leakage current density, conductance, flat band voltage VFB shift, mobile charge density, effective oxide charge and interface trap density Dit were obtained and compared with those of reference samples. Reference sample -1 has native oxide SiO2 and Al metal gate and Reference sample-2 has Ta2O5 oxide layer with unnitrided silicon surface. It has been found that, the replacement of SiO2 gate oxide with Ta2O5 oxide layer results in an increase in dielectric constant by several factors and using nitridation process prior to Ta2O5 oxide layer improves the interface properties. Many promising results were obtained for samples with W metal gates and nitrided silicon surface prior to formation of Ta2O5 oxide layer. It is potentially applicable to future MOS devices.Master Thesis The Effects of Deposition Conditions on the Low Energy Absorption Spectrum of Microcrystalline Silicon Thin Films Prepared by Hwcvd Method(Izmir Institute of Technology, 2005) Işık, Nebile; Güneş, Mehmet; Güneş, Mehmet; 04.05. Department of Pyhsics; 04. Faculty of Science; 01. Izmir Institute of TechnologyThe optical and electronic properties of hydrogenated microcrystalline silicon films deposited by HWCVD method were investigated using steady state photoconductivity (SSPC), dual beam photoconductivity (DBP) and transmission spectroscopy methods to understand the effects of deposition conditions such as silane concentration and filament temperature on the low absorption coefficient spectrum, alpha (h.). The alpha (h.) spectrum obtained from the detailed optical calculation using the relative DBP and transmission spectra were compared with that independently measured on the same samples using photothermal deflection spectroscopy (PDS) and constant photocurrent method (CPM) techniques. The results were found to be in agreement with those of PDS and CPM at higher energy part of spectrum. On the other hand some differences exist among the spectra at lower energies. These differences were discussed to be consistent with underlying the physics of these methods.The effect of silane concentration on the sub-bandgap alpha (h.) spectrum was found to be substantial. At highest SC of 10% the alpha (h.) spectrum similar to that of a-Si: H is obtained. As SC decreases to 7%, microcrystalline phase becomes dominant.Further decrease of SC, the low energy alpha (h.) decreases and given a minimum around SC of 5%. For the lower SC.s, highly crystalline .c-Si: H films are obtained but the alpha (h.) values increases to higher values indicating an increase in the defect densities present in the microstructure.The effect of filament temperature was investigated for a constant SC of 10%. It was found that at 1700 C and 1800 C, fully amorphous films are obtained but 1800 C results in higher alpha (h.) values at lower energies. At 1880 C, microcrystalline phase becomes dominant and the alpha (h.) spectrum becomes similar to that of single crystal silicon.Finally, due to inhomogeneous microstructure of uec-Si: H, there are left fringes on calculated alpha(h.) spectrum on same samples. The degree of the inhomogeneity was investigated by front and back ac illumination of DBP measurements. It was found that there exists a substantial differences on the spectra measured on the same sample indicating importance of inhomogeneous film growth on optoelectronic measurements and its evaluation.Master Thesis Low Temperature Photoconductivity of Hydrogenated Amorphous Silicon (a-Si:h) Thin Flims(Izmir Institute of Technology, 2003) Erdoğan, Gökhan; Güneş, Mehmet; Güneş, Mehmet; 04.05. Department of Pyhsics; 04. Faculty of Science; 01. Izmir Institute of TechnologyIn this study low temperature photoconductivity of undoped hydrogenated amorphous silicon(a-Si:H) thin films have been studied to investigate the effect of native and Staebler-Wronski defects. The study covers undoped a-Si:H films prepared by various deposition techniques such as DC glow discharge, RF-PECVD with and without H-dilution, RF magnetron sputtering and hot-wire(HW) CVD.In the annealed state, the samples were characterized using temperature dependence of dark conductivity, steady-state photoconductivity, .ph, versus light intensity at room temperature and steady-state photoconductivity versus temperature down to 90 0K at three different intensities. Activation energy ,EF, of the samples changes from 0.60 eV to 1.0 eV. .ph shows a few orders of magnitude higher values from the dark conductivity and its magnitude is sample dependent due to differences in deposition conditions. The intensity dependence of .ph ,., (.ph . F.) is close to unity and varies between 0.70 to 0.90, indicating recombination kinetics through the midgap defect states in the bandgap of a-Si:H. Low temperature photoconductivity versus 1000/T spectrum shows three distinctly different regions. In Region I, .ph decreases with temperature until a transition temperature. Then Region II begins, where .ph begins to increase resulting a peak in spectrum or remains to be unchanged until a second transition temperature to Region III, where .ph continuously decreases with T. Transition temperatures and the degree of increase in .ph in Region II is sample dependent. These results indicate the presence of at least two different types of midgap defect states in the bandgap and exponential tail state present in the annealed state.In the light soaked state, Staebler-Wronski effect (SWE) was investigated after exposing the samples to white light illumination of a few suns intensity. The characterization involves dark conductivity and steady-state photoconductivity at room temperature and .ph versus temperature down to 90 0K for different intensities. Dark conductivity values decreased a certain factor indicating a slight shift in EF through midgap. .ph values decreased substantially from its annealed values due to creation of Steabler-Wronski defects in the bandgap. The intensity dependence of .ph become almost equal and close to unity for all the films even it shows slight variation in the annealed state. The shape of low temperature photoconductivity spectra becomes almost the same for all samples even drastic differences were observed in the annealed state. The spectrum is mainly dominated by only two regions.Region I dominates from room temperature down to 170 0K, where .ph decreases with a constant slope as T decreases. After that temperature, Region II sets in. .ph remains to be constant until temperature used in this study. Region III can only be detected at higher intensity and temperatures lower than 90 0K. Results indicate that more defects around the midgap are created by light, which decrease .ph and relatively less defects are created away from midgap and closer to band edge, which improve .ph instead of decreasing it as temperature decreases. The defect states in Region I responsible for decreasing .ph are more likely that they are neutral silicon dangling bond defects ,D0, and those in Region II responsible for increasing .ph are non-D0 defect states. They act as photosensitising defects with a very low capture cross-sections for electrons. They could be charged silicon dangling bonds ,D+ and D-, or floating bonds results in defect models proposed for a-Si:H.Master Thesis Subgap Absorption Spectroscopy in Microcrystalline Silicon Thin Films(Izmir Institute of Technology, 2004) Göktaş, Oktay; Güneş, Mehmet; Güneş, Mehmet; 04.05. Department of Pyhsics; 04. Faculty of Science; 01. Izmir Institute of TechnologyIntrinsic hydrogenated microcrystalline silicon thin films prepared by VHF-PECVD and HW-CVD methods under different deposition conditions have been investigated using steady state photoconductivity method (SSPC), photothermal deflection spectroscopy (PDS) and dual beam photoconductivity (DBP) method, and transmission spectroscopy. Absorption spectra of the investigated thin films were measured in a wide energy region using PDS and DBP. A procedure, for the firs time, was used to calculate fringe free absolute absorption coefficient of thin films from DBP yield spectrum and simultaneously measured transmission signal. The results were found to be in agreement with those of PDS above the bandgap energy. However, there are differences between below the bandgap energy in the spectra of both methods. The differences are discussed to be consistent with the underlying physics of these methods. For some of investigated thin films there are remaining fringes in the .(h.) spectra measured using both methods. This is a strong indication of inhomogeneity present in the films in growth direction. DBP measurements were also performed for ac monochromatic light incident from substrate side in order to investigate the effect of inhomogeneous microstructure of the material on the absorption spectrum. It is found that some films have remaining fringes on their spectra for back ac measurements both for VHF-PECVD and HW-CVD grown thin films, whereas there is no remaining fringes observed for front ac measurements or vice versa. These findings are discussed to be an indication of inhomogeneity in growth direction which is already reported from TEM and Raman study. Sub-bandgap absorption coefficients .(0.8 eV) were correlated with the silane concentration, which is main parameter to change the microstructure of these films. It is found that the thin films that deposited in the transition region, where a transition from a fully amorphous growth to full microcrystalline growth occurs, have smaller absorption coefficients indicating that the thin films deposited at transition region have less defect density. However, thin films deposited at the highly crystalline region have the highest defect density due to etching effect of H during the deposition. These results are also consistent with reported ESR studies.Master Thesis Capacitance-voltage spectroscopy in metal-tantalum pentoxide (Ta-O)-silicon mos capacitors(Izmir Institute of Technology, 2005) Özdağ, Pınar; Güneş, Mehmet; Güneş, Mehmet; 04.05. Department of Pyhsics; 04. Faculty of Science; 01. Izmir Institute of TechnologyThe electronic properties of Al-Ta2O5-Si MOS capacitors with oxide layers prepared by RF magnetron sputtering with or without a prior nitridation process in N2O or NH3 gas environments at temperature ranges between 700 °C to 850 °C were investigated using Capacitance-Voltage (C-V) Spectroscopy to determine the quality of oxide layer and oxide-silicon interface. The theoretical ideal capacitance-voltage calculations were compared with the experimental capacitance-voltage results in order to evaluate effective oxide charges, Qeff , present inside Ta2O5 insulating layer and density of interface trap states, Dit, present at the Ta2O5-Si interface. In addition,dielectric constant, doping concentration, flat band voltage values were determined by using the experimental data. Finally, the effects of deposition conditions on Ta2O5 MOS capacitors were compared by using a reference sample of a MOS capacitor with native oxide SiO2. It has been found that dielectric constant value up to 12 have been reached for Ta2O5 insulating layers which increases the capacitance value several times than that of MOS capacitor with native oxide SiO2. The density of interface trap states, Dit, for unnitrided Ta2O5 MOS capacitors, values around 1.6x1012 cm-2 eV-1 have been detected which is much higher than that of MOS capacitor with native oxide SiO2. However, prior nitridation process enhances the interface properties and Dit values down to 2-5x1011 cm-2 eV-1 have been reached for the nitrided samples which is in the limits for MOS capacitors with high quality insulating layers. In addition, the effective oxide charges, Qeff, for unnitrided samples, values as high as 3x1012 cm-2 were detected. Even though nitridation process enhances interface properties, the effective oxide charges are found to be higher for nitrided samples. Best electrical and interface properties are obtained by nitridation process at 800 °C in N2O and NH3. It can be inferred that samples nitrided in N2O gas at 800 °C improves the dielectric constant above the level of SiO2 and decreases both Qeff and Dit levels to that of native oxide SiO2. These results show that a prior nitridation of p-silicon surface is a promising approach to improve both oxide and interface properties of Al-Ta2O5-Si MOS devices. However, further investigation is necessary to understand the nature of these oxide charges and interface properties of MOS devices with high dielectric constant oxide layers before integration into large scale fabrication.Master Thesis Sub-Gap Absorption Spectroscopy and Its Applications To Amorphous Semiconductor Materials(Izmir Institute of Technology, 2002) Akdaş, Deniz; Güneş, Mehmet; Güneş, Mehmet; 04.05. Department of Pyhsics; 04. Faculty of Science; 01. Izmir Institute of TechnologySubgap absorption spectroscopy is one of the most fundamental experimental tools to investigate absorption coefficient, a(hn), spectrum in amorphous semiconductors. The effects of the disorder and defect states can be observed in the a(hn) spectrum. For this goal, dual beam photoconductivity technique (DBP) has been established and applied to the hydrogenated amorphous silicon, (a-Si:H), thin films prepared by different deposition systems, both in the annealed and light soaked states. In the annealed state, the effects of the native defect states in a-Si:H films were studied using dark conductivity, steady state photoconductivity and the DBP technique. The samples showed different dark and photoconductivity values. The a(hn) spectrum obtained from the DBP measured at different bias light intensities shows three regions. At high energies, parabolic extended state absorption dominates. As energy decreases below the bandgap, the exponential valence band tail absorption appears. At energies below 1.4 eV, the subgap absorption due to the midgap defect states exhibits a shoulder in the spectrum. Absorption coefficient at a (1.2 eV) showed variation among the films. Photoconductivity and subgap absorption values could not be correlated directly. This implies that more than one type of native defects is present in a-Si:H. In the light soaked state, samples were left under a white light source of a few sun intensity. All the samples showed the Staebler-Wronski effect. However, the magnitude of degradation in photoconductivity is different for all the samples and is not directly proportional to the increased subgap absorption. It is inferred that both measurements are not controlled by the same defect states. As a summary, the DBP technique established in this thesis was found to be a reliable characterization tool to study amorphous and microcrystalline silicon films. The DBP results with those of photothermal deflection spectroscopy (PDS) and constant photocurrent method (CPM) techniques showed very good agr eement, implying that DBP is a reliable spectroscopic tool for future investigations.Master Thesis The Effects of Native and Light Induced Defects in the Optical and Electronic Properties of Hydrogenated Amorphous Silicon Germanium (a-Sige:h) Alloy Thin Films(Izmir Institute of Technology, 2005) Dönertaş Yavaş, Medine Elif; Güneş, Mehmet; Güneş, Mehmet; 04.05. Department of Pyhsics; 04. Faculty of Science; 01. Izmir Institute of TechnologyHydrogenated amorphous silicon-germanium alloy thin films (a-SiGe:H) of various germanium concentrations, are potential candidates meeting the requirements of high efficiency stacked solar cells and optoelectronic devices where a certain bandgap is necessary. In this thesis to obtain reliable information about the native and light induced defect states present in a-SiGe:H alloy thin films of various germanium concentrations SSPC, DBP, transmission spectroscopy and PDS techniques have been used. A procedure based on Ritter Weiser optical formulation has been applied to calculate fringe free absolute absorption coefficient spectra of a-SiGe:H alloy thin films of various Ge% from the yield DBP and simultaneously measured transmission signals for the first time. The results have been compared with those independently measured by PDS method.In the annealed state the effects of native defect states in a-SiGe:H alloy thin films of various Ge% have been investigated. For the a-SiGe:H alloy films with Ge concentration in the range of 10% to 30%, hntn-products for the photogenerated free electrons is the highest, therefore they serve as the best photoconductive absorber layer in the multijunction solar cells. The effect of Ge content in amorphous silicon network clearly indicates a systematic decrease in the bandgap with increasing Ge content. The E0v values are almost constant around 55meV for alloying up to 40% Ge. Finally the changes in the defect density present in the bandgap of alloy films are inferred from the α(1.0eV) measured by both PDS and low bias light DBP spectrum. The difference between PDS and low bias DBP spectra is attributed to the underlying physics of these methods. The best film with lowest defect density can be prepared with alloying Ge in the range from 10% to 40% Ge.In the light soaked state, samples were left under white light illumination (15 suns) for determined time intervals. SSPC measurements indicate that all samples exhibit certain degree of degradation in the magnitude of sph and hntn products. The rate of α(1.0 eV) decreases as Ge% increases in the light soaked state. Higher Ge content films (50%, 75%) show almost no degradation in sub-bandgap absorption. As the degradation slope of α(hν) and 1/η nτn product are not same for all samples it can be inferred that subgap absorption and photoconductivity measurements are not controlled by the same set of defects present in the bandgap.