The Study Of Preparation And Properties Of Magnesium Oxide As High K Dielectric

With the rapid d eVelopment of large scale integrated circuit, feature size of Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET) which is the key part of integrated circuits is scaling down according to Moore's law. How e Ver, when the thickness of SiO2 gate dielectric in MOSFET is reduced to nanometer magnitude, the electron tunneling is becoming serious enough to endanger the stability and reliability of d eVices. Therefore, it is necessary now to find some novel dielectrics with higher static dielectric contant (K) to substitute the traditional SiO2 gate dielectric. With high K dieletrics used in MOSFET, relatively thicker physical gate thickness can be used to suppress the tunneling effect while keeping the same and higher capacitance.Magnesium oxide (MgO) has been widely used as optical films, buffer layers for growing oriented high-Tc superconducting thin films, large-area substrates, plasma display d eVices, high K dielectric materials, etc. because of its electrical insulating, chemically inert, mechanical flexibility, high temperature stability, thermal conductance and high secondary electron emission properties. MgO was chosen as a candidate of gate dielectrics due to its chemically inert, a wide band-gap (7.8 eV), and its higher K value (about 9.8) for bulk material. MgO can also be used to replace SiO2 in MOSFET with a new-type dielectric layer. Accurate optical and electrical properties, and film thickness of MgO as dielectric layer are crucial for the fabrication and understanding the optical and electronic d eVices and these properties are therefore the focus of this dissertation.The MgO thin films (～10 nm) were deposited by metal organic chemical vapor deposition (MOCVD) and R.F. magnetron sputtering on Si and sapphire substrates. The samples were grown at different substrate temperatures and some samples were annealed at 650℃in O2 for 20 min to density the films and reduce the defects. The composition, structural properties, and surface morphology of the MgO thin films were studied by_X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and scanning electron microscope (SEM). Spectroscopic ellipsometry (SE) and atomic force microscopy (AFM) were also employed to determine the film thickness and the refractive index n.Firstly, the MgO films were successfully prepared on p-Si (100) and sapphire (0001) substrates at 350-700℃by MOCVD. (C5H5)2Mg, N2 and O2 with ultra higher purity were chosen as the organometallic precusor, carrier gas, and oxidant, respectively. The composition of the MgO thin films deposited at different substrate temperatures were investigated by XPS which can determine the chemical bond of Mg-O in lattice in MgO and in Mg(OH)2 or MgCO3 possibly caused by exposing MgO films to the air atmosphere duo to carbon dioxide and water vapor absorbed. The optical transmittance spectra showed that the average transmittance in the visible light range for the films was over 85 %.SEM image showed that the surface morphology of the MgO thin films_deposited on Si at 450℃is very smooth. A Cauchy model was used to fit experimental SE data to generate the refractive index n and thickness of MgO films. The thickness of MgO films deposited on Si at 600℃and 700℃were also confirmed by AFM, which were in good agreement with those measured by SE.Secondly, the MgO films were successfully deposited on p-Si (100) and sapphire (0001) substrates by R.F. magnetron sputtering in comparison with the samples by MOCVD. XPS data showed almost the same results as the samples prepared by MOCVD. XRD pattern of the MgO film deposited at 80℃for 120 min and annealed in O2 at 650℃for 20 min indicated that the MgO thin film is amorphous. Equally, we obtained the refractive index and thickness of the MgO thin films by SE and confirmed by AFM.The electrical properties (dielectric constant K, correct capacitance Cc, equivalent oxide thickness LEOT, mid band gap Dit value for MgO-Si calculated, and the leakage current density J) of the MgO thin films were studied using capacitance-voltage (C-V), conductance-voltage [G(ω)-V], and leakage current density-electrical field (J-E) measurements based on Au-MgO-Si (MOS) capacitor structures. We also studied the effects of thermal annealing in O2 at 650℃and supercritical carbon dioxide (scCO2) treated at lower temperature (150℃) on the electrical properties of MgO thin films. scCO2 was chosen due to its negligible surface tension. With the novel property, it can carry the oxidant H2O to transport in the MgO films and the MgO-Si interface to react with the Mg dangling bonds and passivate the traps. The high frenqency C-V measurements showed that the capacitances in accumulation region were dispersion with the measuring frenquency which is possibly attributed to the gate leakage current and the series resistance. How eVer, this phenomenon can be eliminated by employing the three-element circuit model duo to considering the influence of leakage current and the series resistance. The extracted capacitances are frenqency-independent after performed the correction at two different frequencies. The corrected dielectric constant (Kc) and the equivalent oxide thickness (LEOT) were calculated according to the extracted capacitance. The calculated Kc of the pristine MgO films deposited by MOCVD on Si at 350℃and 450℃were about 7.3 and 7.6 respectively which were lower than that of the samples deposited at 600℃and 700℃(about 7.8 and 8.2 respectively). This may arise from the carbon contamination in the MgO films caused by the insufficient oxidation of the organic group in the metalorganic precursor at lower substrate temperature. The LEOT were calculated to be about half of that of MgO thin films.The negative shift of the C-V curves from zero and the hysteresis for most as-deposited samples indicates that there are some amounts of positive charges in MgO films and MgO-Si interface as a result of processing. How eVer, the significant improvement were achi e Ved after thermal annealed in O2 at 650℃and treated in scCO2 at 150℃in terms of the smaller or negligible shift of the C-V curves from zero and the hysteresis. The Dit was measured by G-V measurements according to the conductance method and was also corrected by the three-element circuit model. The approximate mid band gap Dit value for MgO-Si calculated by the extracted(Gp/ω)max at 10k Hz was at the magnitude of 1011 cm-2 eV-1 which was reduced to the magnitude of 1010 cm-2 eV-1 after scCO2 treatment. This is comparable to the Dit values for Si-SiO2 after forming gas anneal. The leakage current density versus electrical field (J-E) measurements show that the leakage current densities for the films deposited on Si at 350℃and 450℃are one magnitude higher than that of the samples deposited at 600℃and 700℃which are about 10-8 A/cm2. This is also the indicative of more defects in the films deposited at lower substrate temperature. The Dit values for the films deposited at 600℃and 700℃are reduced to 10-9 A/cm2 and 10-10 A/cm2 after thermal annealing in O2 and scCO2 treatment due to the further reaction of oxidant with the Mg dangling bonds to passivate the traps. The films deposited on Si by rf sputtering are inferior to those prepared by MOCVD in spite of the C-V behavior, Dit values, and J-E measurements.