HfO2 as gate dielectric on Si and Ge substrate

Hafnium oxide HfO2 has been considered as an alternative to silicon dioxide SiO2 in future nano-scale complementary metal-oxide-semiconductor (CMOS) devices since it provides the required capacitance at the reduced device size because of its high dielectric constant. HfO2 films are currently deposited by various techniques. Many of them require high temperature annealing that can impact device performance and reliability. In this research, electrical characteristics of capacitors with HfO2 as gate dielectric deposited by standard thermal evaporation and e-beam evaporation on Si and Ge substrates were investigated. The dielectric constant of HfO2 deposited by thermal evaporation on Si is in the range of 18-25. Al/HfO2/Si MOS capacitors annealed at 450°C show low hysteresis, leakage current density and bulk oxide charges. Interface state density and low temperature charge trapping behavior of these structures were also investigated. Degradation in surface carrier mobility has been reported in Si field-effect-transistors with HfO2 as gate dielectric. To explore the possibility of alleviating this problem we have used germanium (Ge) substrate as this semiconductor has higher carrier mobility than Si. Devices fabricated by depositing HfO2 directly on Ge by standard thermal evaporation were found to be too leaky and show significant hysteresis and large shift in flatband voltage. This deterioration in electrical performance is mainly due to the formation of unstable interfacial layer of GeO2 during the HfO2 deposition. To minimize this effect, Ge surface was treated with the beam of atomic nitrogen prior to the dielectric deposition. The effect of surface nitridation, on interface as well as on bulk oxide, trap energy levels were investigated using low temperature C-V measurements. They revealed additional defect levels in the nitrided devices indicating diffusion of nitrogen from interface into the bulk oxide. Impact of surface nitridation on the reliability of Ge/HfO2/Al MOS capacitors has been investigated by application of constant voltage stress at different voltage levels for various time periods. It was observed that deeper trap levels in nitrided devices, found from low frequency and low temperature measurements, trap the charge carrier immediately after stress but with time these carriers detrap and create more traps inside the bulk oxide resulting in further devices deterioration. It is inferred that though nitrogen is effective in reducing interfacial layer growth it incorporates more defects at interface as well as in bulk oxide. Therefore, it is important to look into alternative methods of surface passivation to limit the growth of GeO2 at the interface

Epitaxial functional oxide integration on germanium

Germanium, with its higher hole and electron mobility is a potential candidate to replace silicon as a channel material in a field effect transistor in the future. The integration of high quality crystalline oxides on semiconductors still remains a challenge due to lattice defects, a lattice constant mismatch as well as a possible thermodynamic instability between the thin film and the substrate. In this work we report the integration of functional oxides on germanium, which exhibit a wide variety of useful physical properties such as ferromagnetism, superconductivity or ferroelectricity which are of high interest for future electronic devices as i.e. for the development of a ferroelectric field-effect transistor. The focus of this thesis lies on the study of the high-[kappa] and ferroelectric material barium titanate, grown on germanium (001) by using an oxide molecular beam epitaxy machine. Further characterization techniques as x-ray diffraction, x-ray reflectivity, x-ray photoelectron spectroscopy, atomic force microscopy and electrical measurements are used to study the properties of the oxide films and to obtain a deeper understanding of their interface qualities with the substrate. This research contributes significantly for the development of a ferroelectric field-effect transistor and oxide heterostructures on germanium in general.Physic

Development of an integrated photonic sensor for monitoring hazardous organics

In this study, an integrated photonic sensor has been designed and fabricated for the purpose of monitoring hazardous organics in the environment. The operation of the sensor is based on the principles of the Mach-Zehnder interferometry. The sensor consists of a patterned waveguide structure made of phosphosilicate core and silica cladding. LPCVD processes were developed to produce undoped cladding and doped core silicon dioxide films. Diethylsilane (DES), Trimethylphosphite (TMP) and oxygen were used as the precursors for the deposition process. The effects of the O2/DES ratio and deposition temperature on the properties Of SiO2 films were investigated. It was observed that the oxide films deposited in the temperature range 550- 700 ºC followed an Arrhenius behavior with an apparent activation energy of 10 KCal/mol. It was found that the optimum conditions to synthesize underlying silicon dioxide films were 775 ºC, 200 mTorr and oxygen to DES flow ratio was 10: 1. After 20 hours deposition time under such conditions, the oxide film was ~15 μm thick, stress was compressive with a value of ~20 Mpa, and the refractive index was 1.458. The effect of TMP flow rate on the properties of PSG films was also studied. The composition and refractive index of PSG films varied with temperature and TMP flow rate. Optimum conditions to deposit PSG core layer were found to be at 600 ºC, 250 mTorr, 2 sccm TMP flow rate and an oxygen to DES flow ratio of 10:1. After 5 hours deposition time under such conditions, Phosphosilicate layer was about 7 μm thick, stress was compressive with a value of I Mpa, the refractive index was 1.466, and phosphorous oxide was ~7.5 wt%. The growth rate varied with TMP flow rate and exhibited a maximum value of 254 Å/min at 2.5 sccm and 600 ºC. PECVD processes were used to synthesize the upper cladding silicon dioxide films. The deposition conditions were 250 ° C, 900 mTorr, N2O flow rate 900 sccm, and 400 sccm SiH4 (3%) flow rate. After 30 minutes deposition time under such conditions, the oxide film had a thickness of 1.2 μm, stress was compressive with a value of ~110 Mpa, and the refractive index was 1.453. For all deposits, FTIR spectroscopy showed that no carbon was present in the deposits while UV/visible spectroscopy indicated better than 99% optical transmission. Optical analysis proved this integrated photonic sensor could be used as a prototype to monitor the hazardous organics in the environment

Reactions of Pyridazine and Some Gaseous Oxides on the GE(100) Surface

Ph.DDOCTOR OF PHILOSOPH

A comprehensive review of ZnO materials and devices

The semiconductor ZnO has gained substantial interest in the research community in part because of its large exciton binding energy (60 meV) which could lead to lasing action based on exciton recombination even above room temperature. Even though research focusing on ZnO goes back many decades, the renewed interest is fueled by availability of high-quality substrates and reports of p-type conduction and ferromagnetic behavior when doped with transitions metals, both of which remain controversial. It is this renewed interest in ZnO which forms the basis of this review. As mentioned already, ZnO is not new to the semiconductor field, with studies of its lattice parameter dating back to 1935 by Bunn [Proc. Phys. Soc. London 47, 836 (1935)], studies of its vibrational properties with Raman scattering in 1966 by Damen et al. [Phys. Rev.142, 570 (1966)], detailed optical studies in 1954 by Mollwo [Z. Angew. Phys.6, 257 (1954)], and its growth by chemical-vapor transport in 1970 by Galli and Coker [Appl. Phys. Lett.16, 439 (1970)]. In terms of devices, Au Schottky barriers in 1965 by Mead [Phys. Lett.18, 218 (1965)], demonstration of light-emitting diodes (1967) by Drapak [Semiconductors 2, 624 (1968)], in which Cu2O was used as the p-type material, metal-insulator-semiconductor structures (1974) by Minami et al. [Jpn. J. Appl. Phys.13, 1475 (1974)], ZnO∕ZnSe n-p junctions (1975) by Tsurkan et al. [Semiconductors 6, 1183 (1975)], and Al∕Au Ohmic contacts by Brillson [J. Vac. Sci. Technol.15, 1378 (1978)] were attained. The main obstacle to the development of ZnO has been the lack of reproducible and low-resistivity p-type ZnO, as recently discussed by Look and Claflin [Phys. Status Solidi B241, 624 (2004)]. While ZnO already has many industrial applications owing to its piezoelectric properties and band gap in the near ultraviolet, its applications to optoelectronic devices has not yet materialized due chiefly to the lack of p-type epitaxial layers. Very high quality what used to be called whiskers and platelets, the nomenclature for which gave way to nanostructures of late, have been prepared early on and used to deduce much of the principal properties of this material, particularly in terms of optical processes. The suggestion of attainment of p-type conductivity in the last few years has rekindled the long-time, albeit dormant, fervor of exploiting this material for optoelectronic applications. The attraction can simply be attributed to the large exciton binding energy of 60 meV of ZnO potentially paving the way for efficient room-temperature exciton-based emitters, and sharp transitions facilitating very low threshold semiconductor lasers. The field is also fueled by theoretical predictions and perhaps experimental confirmation of ferromagnetism at room temperature for potential spintronics applications. This review gives an in-depth discussion of the mechanical, chemical, electrical, and optical properties of ZnO in addition to the technological issues such as growth, defects, p-type doping, band-gap engineering, devices, and nanostructures

Transition and Rare-Earth metal oxides coatings: Surface characterization, Thermo-mechanical properties, and chemical reactivity

This thesis describes a number of scientific studies investigating different interesting properties of rare earth metal oxides desired in different applications via combined experimental measurements and accurate density functional theory (DFT) calculations. The electronic, structural, mechanical and thermodynamic properties of cubic lanthanide sesquioxides are first reported, with a particular focus on the most common dioxide in the lanthanide family, ceria (CeO2). This is followed an investigation into the effect of Hf and Zr dopants on the reduction energies of pure ceria. The reduction enthalpies of Ce1-xHfxO2 and Ce1-xZrxO2 and Ce1-2xHfxZrxO2 solid solutions are computed as a function of the reduction extent (x). Alloying with Hf and Zr is found to systematically reduce the energies required to remove oxygen atoms from bulk of ceria. The computed coefficients in the Born-Huang criterion infer a mechanical stability of all cubic lanthanide sesquioxides. Acquired electronic parameters encompass Bader’s atomic charges and Partial Density of States (PDOS). An important part of the thesis focuses on the catalytic capacity of CeO2 in acting as a stand-alone environmental catalyst toward the decomposition of a series of chlorinated volatile organic compounds, namely chloroethene, chloroethane and chlorobenzene. Guided by recent experimental measurements, the pyrolytic and oxidative decomposition of selected chlorinated compounds have been modelled on the most stable ceria surface, CeO2 (111). Dissociative addition (surface-assisted fission of the C-Cl bond) and direct elimination pathways (departure of a stable hydrocarbon entity with the co-adsorption of H and Cl atoms on the surface) assume comparable importance. Fission of the C-Cl bond over oxygen vacancies systematically necessitates lower energy barriers in reference to perfect surfaces. We have illustrated that observed catalytic deactivation in the experiment is attributed to the profound stability of adsorbed hydrocarbon adduct. Decomposition of an adsorbed phenyl moiety proceeds via addition of oxygen molecules to partially reduced surfaces. A simplified kinetic model plots the temperature-conversion profiles for the three compounds against corresponding experimental profiles, where a reasonable agreement has been attained. Surfaces of terbium dioxide (TbO2) possess an important catalytic feature in that they are capable of producing hydrogen by splitting water molecules. We have computed a large array of thermo-mechanical properties including heat capacities, bulk modules and thermal expansions of bulk TbO2 as a function of temperatures and pressures based on the quasi-harmonic approximation (QHA) approach. Our calculated lattice constant and band gap were in good agreement with analogous experimental findings. A surface truncated along the (111), terminated with O atoms and with oxygen vacant site (111): O+1Vo incurs a higher thermodynamic stability across all values of oxygen chemical potential. Nonetheless, in the vicinity of the lean-limit of chemical potential the surface terminated with Tb atoms (111): Tb becomes more stable. The implications of these geometries on OH-H fission reactions have been discussed. Magnetron sputtered CeO2 films as optically transparent materials, deposited onto crystalline silicon substrates at various oxygen-argon mixture gas, have been intensively studied and characterized by correlating their structural and chemical bonding states. All the thin films exhibit a polycrystalline character with cubic fluorite – structure for cerium dioxide along (111), (200) and (222) orientations. The XPS survey scans of the CeOx coatings revealed that that Ce, O, C elements are present in all of the obtained spectra of the studied films. XPS analysis demonstrated that the atomic percentages of Ce and O atoms increase as oxygen-argon mixture increases. Two oxidation states of CeO2 and Ce2O3 are present in the films prepared at lower oxygen/argon flow ratios; whereas the films are completely oxidized into CeO2 as the oxygen/argon flow ratio increase. Reflectance data obtained from UV-Vis examinations were utilized to calculate the optical constants such as absorption coefficient (α), the real and imaginary parts of the dielectric function (ε1, ε2), the refractive index (n) and the extinction coefficient (k). Our analysis indicates that the CeO2 films display indirect optical band gaps residing in the range of 2.25 - 3.1 eV. We utilized DFT calculations to estimate optical constants of a CeO2 cluster at ground state. The computed electronic density of states (DOSs) of the optimized unit cell of CeO2 yields a band gap that agrees well with the corresponding experimental value. The measured and DFT-computed absorption coefficient (α) exhibit a similar trend with similar values in the wavelength range from 100 to 2500 nm. Overall, a satisfactory correlation between the theoretical and experimental findings is demonstrated. Spinal oxides of CuxCo3-xO4 thin films as one of metal mixed oxide systems, synthesized by sol-gel method and annealed at various temperatures ranging from º200 to º500 with interval 100, are deeply studied and characterized by various structural and optical characterization techniques. XRD data indicates that as annealing progresses, all the coatings possess a crystalline phase of Cu0.56Co2.44O4 (ICSD 78-2175) with preferential orientation along (400) reflection plane. Optical analysis reveals that the solar selectivity of the studied films improves as the annealing progresses. Bader’s charge analysis calculated by DFT implemented in VASP code points out that the Cu and Co atoms in all the stoichiometries hold positive charges whereas the O atoms are linked with negative charges. Our model reveals a covalent character for Cu-Co bond in all the system and ionic characteristics for Cu-O and Co-O bonds. Finally, the influence of the variation in the Hubbard parameter U on the activation and reaction energies on CeO2-catalyzed reactions is studied. This has been achieved by surveying the change in activation and reaction energies for reactions underpinning the partial and full hydrogenation of acetylene over the CeO2 (111) surface. A positive correlation between the U values and reaction and activation energies reported. It is suggested that kinetic modeling against experimental profiles of products could be used as an approach to optimize the U value