Metal Oxide Silicon Structures Research Articles

Optimization of thermal oxidation for 4H-SiC and fabricate/characterize MOS capacitor

On the 4H-SiC substrate, C-face and Si-face oxide layers have been grown by thermal oxidation process and sputtering. The thermal oxidation temperature dependence of 4H-silicon carbide (SiC) is systematically investigated using capacitance-voltage (C–V) measurements. The oxidation quality and thickness vary according to the temperature and time duration of the thermal oxidation. The layers’ thicknesses are determined by atomic force microscopy (AFM), and the temperature range is between 800°C and 1110 °C. The primary reason to fabricate the Metal-Oxide-Silicon (MOS) capacitor is to know the thermal oxidation process and a working principle. In this paper, we optimize a thermal oxidation process and fabricate the MOS structure. Then we determine the various parameters such as flat band voltage (Vf b), Inversion threshold voltage (Vt), Surface depletion capacitance (Cdep), Oxide capacitance (Cox), the total capacitance of the device (Co), doping concentration (Nd), Depletion width (Xd), Maximum depletion width (Xdt) and Interface trap density (Dit). Finally, we analyze and discuss the MOS capacitance.

Gamma and proton irradiation effects and thermal stability of electrical characteristics of metal-oxide-silicon capacitors with atomic layer deposited Al2O3 dielectric

The radiation hardness and thermal stability of the electrical characteristics of atomic layer deposited Al2O3 layers to be used as passivation films for silicon radiation detectors with slim edges are investigated. To directly measure the interface charge and to evaluate its change with the ionizing dose, metal-oxide-silicon (MOS) capacitors implementing differently processed Al2O3 layers were fabricated on p-type silicon substrates. Qualitatively similar results are obtained for degradation of capacitance–voltage and current–voltage characteristics under gamma and proton irradiations up to equivalent doses of 30Mrad and 21.07Mrad, respectively. While similar negative charge densities are initially extracted for all non-irradiated capacitors, superior radiation hardness is obtained for MOS structures with alumina layers grown with H2O instead of O3 as oxidant precursor. Competing effects between radiation-induced positive charge trapping and hydrogen release from the H2O-grown Al2O3 layers may explain their higher radiation resistance. Finally, irradiated and non-irradiated MOS capacitors with differently processed Al2O3 layers have been subjected to thermal treatments in air at temperatures ranging between 100°C and 200°C and the thermal stability of their electrical characteristics has been evaluated. Partial recovery of the gamma-induced degradation has been noticed for O3-grown MOS structures. This can be explained by a trapped holes emission process, for which an activation energy of 1.38±0.15eV has been extracted.

Structural, compositional and electrical characterization of Si-rich SiOx layers suitable for application in light sensors

Metal-Oxide-Silicon (MOS) structures containing silicon nanoparticles (SiNPs) in three different gate dielectrics, single SiOx layer (c-Si/SiNPs-SiOx), two-region (c-Si/thermal SiOx/SiNPs-SiOx) or three-region (c-Si/thermal SiO2/SiNPs-SiOx/SiO2) oxides, were prepared on n-type (100) c-Si wafers. The silicon nanoparticles were grown by a high temperature furnace annealing of sub-stoichiometric SiOx films (x=1.15) prepared by thermal vacuum evaporation technique. Annealing in N2 at 700 or 1000°C leads to formation of amorphous or crystalline SiNPs in a SiOx amorphous matrix with x=1.8 or 2.0, respectively. The three-region gate dielectric (thermal SiO2/SiNPs-SiO2/SiO2) was prepared by a two-step annealing of c-Si/thermal SiO2/SiOx structures at 1000°C . The first annealing step was carried out in an oxidizing atmosphere while the second one was performed in N2. Cross-sectional Transmission Electron Microscopy and X-ray Photoelectron Spectroscopy have proven both the nanoparticle growth and the formation of a three region gate dielectric. Annealed MOS structures with semitransparent aluminum top electrodes were characterized electrically by current/capacitance–voltage measurements in dark and under light illumination. A strong variation of the current at negative gate voltages on the light intensity has been observed in the control and annealed at 700°C c-Si/SiNPs-SiOx/Al structures. The obtained results indicate that MOS structures with SiO1.15 gate dielectric have potential for application in light sensors in the NIR–Visible Light–UV range.

State Transition of a Defect Causing Random-Telegraph-Noise Fluctuation in Stress-Induced Leakage Current of Thin SiO2 Films in a Metal–Oxide–Silicon Structure

Dynamic fluctuation in stress-induced leakage current – called “variable stress-induced leakage current” – in a gate oxide of a metal–oxide–semiconductor structure was investigated. Variable stress-induced leakage current is attributed to random telegraph noise, which is associated with the state-transition of a single defect. To analyze the mechanism of the state-transition, dependence of state-transition probabilities on gate current and on temperature were investigated. These dependences indicate that the state-transition mechanism is a defect-structure transition by charge collision.

First-Principles Study on Intrinsic Point Defects in Rhombohedral LaAlO3 and Their Effects on Electrical Properties

Formation energies and energy levels of various neutral and charged defects in bulk rhombohedral LaAlO3 are calculated numerically in different equilibrium conditions under different oxygen partial pressures, using first-principles plane-wave calculations. The roles of such defects in electrical properties of LaAlO3 are also discussed. The results obtained by calculations show that the formation energy becomes low for oxygen interstitial if LaAlO3 is in an oxygen-rich condition. In contrast, an oxygen vacancy is easily formed in a condition where oxygen is deficient and metal elements are relatively rich, even if the oxygen partial pressure is high. These results explain well the concentration change of constituent elements in LaAlO3 under different treatment atmospheres. Furthermore, the calculations can interpret a large tunnelling current that is often observed in a metal–oxide–silicon structure using LaAlO3 as the oxide layer, since the O vacancy has an energy level near the valence band maximum of Si.

Analysis of electrically biased paramagnetic defect centers in HfO2 and HfxSi1−xO2/(100)Si interfaces

This study reports on the first experimental observations of electrically biased paramagnetic defects at 800 °C N2 annealed HfxSi1−xO2 (x=0.4, and 0.6)/(100)Si and HfO2/(100)Si interfaces in metal oxide silicon structures. These defects are examined by electrical-field controlled electron spin resonance (ESR) and correlated with capacitance-voltage (C-V) analysis. Distributions of ESR measured density of interface traps (ESR-Dit), Pb0 and Pb1, exhibit distinct charge humps and peaks in the Si bandgap with maximum defect density of 0.9–1.9×1012 cm−2 eV−1 in the Hf0.4Si0.6O2/Si interface. Three Pb0 and one Pb1 charged ESR-Dit peaks with density of 1.7–2.8×1012 cm−2 eV−1 are observed in the Hf0.6Si0.4O2/Si interface. Cross-sectional transmission electron microscopic images show decreasing interfacial layer (IL) thickness with increasing hafnium composition (x) at the HfxSi1−xO2/Si interface. The roughest IL observed at the HfO2/Si interface may have contributed to an ESR-Dit of Pb0 greater than 2×1013 cm−2 eV−1 and a pinned Fermi level near the midgap. It appears that the energy distributions of interface defects in HfxSi1−xO2/Si and HfO2/Si have different signatures compared to those at SiO2/Si interface, especially the charged peak near the midgap.

Theory of spin-dependent tunneling current in ferromagnetic metal–oxide–silicon structures

A theory is presented to calculate the spin-dependent tunneling current of ferromagnetic metal–oxide–silicon junctions. It is based on the theory of the tunneling process which was developed by Bardeen, and takes into account the specific properties of the barrier, as well as the material specifics on both sides of the barrier. It is applied to calculate the current–voltage characteristics of the spin-up and spin-down currents of the elemental ferromagnetic metals (Fe, Co, and Ni) connected to Si by a tunneling barrier.

Modeling observed capacitance–voltage hysteresis in metal–SiO 2-thin film organic semiconductor devices

Recently, metal–oxide–organic semiconductor capacitors (MOOSCAPs) and organic field effect transistors (OFETs) using organic semiconductor (OS) materials such as arylamino–poly-phenylene-vinylene (arylamino–PPV), pentacene, etc., have been reported of being subject to capacitance–voltage ( C– V) hysteresis. This hysteresis is exhibited with a simple parallel shift in the C– V. In this paper, the C– V hysteresis observed in these devices is modeled using a physics-based thermodynamic-variational approach and compared with the empirical values reported in literature. Our thermodynamic-variational model is based on minimizing the free energy of the device. The hysteresis effect is modeled as arising from residual polarization of the organic film at flat band which consequently produces a polarization charge layer at the SiO 2/OS interface. In the arylamino–PPV OS film, polarization is associated with redistribution of the molecular π-orbitals. Consequently, such residual polarization is expected to be of opposite polarity when the sweep direction is alternated. The change in polarity thus is expected to produce a shift in the flat band voltage which can be related simply and quantitatively to the charge density in the layer. In order to compare these MOOSCAP results with metal–oxide–silicon (MOS) TCAD, we performed 3-D TCAD simulations of a MOS structure where the p-Si layer is used to approximate the properties of an OS such as arylamino–PPV. In this MOS structure the polarization charge layer is approximated as a sheet charge at the p-Si/SiO 2 interface. This comparison signifies that hysteresis effects in a MOOSCAP structure can be estimated reasonably using TCAD simulations of the MOS structure.

Definition of curve fitting parameter to study tunneling and trapping of electrons in Si/ultra-thin SiO 2/metal structures

The tunneling of electrons through metal–oxide–silicon (MOS) structures with ultra-thin oxide is modeled using a linear model for the electron potential energy, an approach which simplifies the computation of both the interface potential and the field penetration distance in the substrate. The one-particle quantum problem is split into finding the metastable states induced by the internal field penetration in the substrate and the running states in the gate region. The two states are assumed to be connected by the condition for the continuity of the probability density at the substrate–dielectric interface. The electron probability current and the total gate current density are obtained for different gate voltages. As the model yields excellent fittings with experimental current–voltage ( I– V) data for MOS structures, it was further applied to constant current stressing analysis in order to obtain values for important electron trapping parameters in the oxide. The resultant estimates of the electron trapping cross-section fall in the range of other independent determinations in the literature.

Carrier Accumulation in Silicon-On-Insulator Structures Containing Ge Nanocrystals in the Burried SiO<sub>2</sub> Layer

Electro-physical properties of metal-oxide-silicon (MOS) structures and MOS transistors, prepared in the top silicon layer of silicon-on-insulator (SOI) structures containing Ge nanocrystals in the buried SiO2 layers, have been studied. It was obtained that carrier accumulation in MOS structures depend on the direction of built-in electrical field in MOS structures. Accumulation of the excess negative charges in the case of p-channel transistors is associated with electron trapping on Ge nanocrystals synthesized in the buried dielectric. In the case of n-channel transistor, positive charge related to the Si/SiO2 interface or to the charged oxide is accumulated. The Ge atoms diffused to the SiO2/Si interface can stimulate the formation of the excess positive charge.

Thomas–Fermi approximation for a two-dimensional electron gas at low temperatures

The main purpose of this article is to cast the quantum mechanical electron density, obtained via the Schrödinger–Poisson solver, into a classical form. The Thomas–Fermi (T–F) equation states that the electron density n is dependent on the electrostatic potential φ. The electrostatic potential is determined by Poisson’s equation. In order to account for quantum effects, such as confinement and tunneling, we set out to dervive an effective potential, φ*, that when used in place of Poisson’s potential in the original T–F equation will mimic the solution of the Schrödinger equation. This total potential is then used directly to calculate the electron density. Thus, φ* effectivley washes out the intricacies of the wave functions and yields the electron density. The validity of the T–F equation, for a two-dimensional electron gas at low temperatures, is demonstrated through the solution of a single-electron distribution in fixed square and triangular potential wells. Self-consistent inversion-layer charge densities for single-gate and double-gate metal–oxide–silicon structures are also presented.

Refractive index at infrared wavelengths and dielectric permittivity of pure and fluorinated silicon dioxide from measurements of their thin films deposited on Si

The Fourier-transform infrared (FTIR) spectra of silicon dioxide (SiO2) and fluorinated silicon dioxide (SiOF) deposited on bare Si wafers have been recorded and the interference fringes in the spectra have been used to estimate the refractive index, n, of the SiO2 and SiOF films. It is found from the FTIR spectra that n is reduced by 3.5% on doping with F atoms. The refractive indices and thicknesses of the films were also determined by ellipsometry for comparison. The complex capacitance, C*(ω) = C′ − iC″, of samples from the wafers has been measured in the frequency range 1 Hz–1.8 GHz with broadband dielectric spectroscopy. Two relaxation peaks, one centred at ∼1 kHz and the other in the 10–100 MHz range are observed in both the SiO2 and SiOF coated wafer samples. The measurements of C*(ω) are related to the frequency-dependent complex permittivity, ε*(ω) = ε′ − iε″ of the films with the use of simple equivalent circuit models. ε′ shows a reduction of 16% on fluorine addition. It is suggested that the origin of the high frequency peak is the RC network formed by the wafer substrate and the dielectric film, while the low frequency peak results from charged defects at the Si–SiO2 interface. The dielectric results are compared with the microwave dielectric measurements, made using a different method, on both bare Si wafers and wafers coated with dielectric films, which have been reported recently. The observed dispersion in C′ and peaks in C″ are a result of the layered metal–oxide–silicon structure of the samples.

Electrical properties of metal–oxide–silicon structures with LaAlO3 as gate oxide

In this paper, electrical investigations performed on complex metal–oxide–semiconductor (MOS) structures with amorphous LaAlO3 deposited as gate oxide on a rapid thermal processed SiO2 buffer layer are presented. Current–voltage (I–V) measurements at different temperatures and high frequency room-temperature capacitance–voltage (C–V) measurements indicate the presence of a non-equilibrium state in p-type Si (100) substrate under reverse polarization of the structure. The asymmetry in I–V characteristics is opposite to the expected one for the high-κ/interfacial layer stack behavior. Different indications for the non-equilibrium MOS state are inferred from the experimental data, and the extraction of MOS physical parameters by using the classical C–V method is shown to be unreliable. For small and large applied forward voltages defect-related charge transport and tunneling currents dominate, respectively.

Gate-oxide grown on the sidewalls and base of a U-shaped Si trench: effects of the oxide and oxide/Si interface condition on the properties of vertical MOS devices

We report on the SiO 2/Si interface condition and on its high electric field stress reliability in U-shaped trench gated metal-oxide-silicon (MOS) structures. The interface state density measured is ∼10 11 eV −1 cm −2, and using deep level transient spectroscopy the P b center is found to be the dominant interface defect. The degradation induced by electrical stressing is examined using changes in the charge pumping current in a U-trench-gated transistor. It is shown that the SiO 2/Si interface at the bottom corners of the trench is the weakest region in the MOS structure.

Low field stress induced double donor defect in metal oxide silicon structures

A variable frequency light pumping (VFLP) effect is presented which describes the energetic induced photon generation mechanism in the stress metal oxide silicon (MOS) system. It is shown that the VFLP occurs, when both the tunneling electrons and the induced photons are coherent. Both the tunneling electrons and the induced photons in the MOS system from the light pumping can gain the energy required to break the Si–O bonds and to excite interstitial oxygen and to lead then to create the intrinsic point defects in the oxide, and the surface silicon, and at the interfaces. Based on the hybrid sixfold ring of the continuous random network model for SiO 2, the intrinsic point defect generation under low field and its basic properties have been studied.

Effect of Annealing on the Characteristics of Silicon Implanted Nano/trap Crystal Memories

AbstractIn this paper we are presenting the results of silicon nano-trap memory fabricated by implanting high dose silicon into gate oxide of thickness 30 nm. The gate oxide was grown by dry oxidation. Capacitance versus voltage characteristics of MOS (metal oxide silicon) structures with silicon implanted samples annealed in nitrogen environment at a temperature of 950 °C show a memory window depending on the applied DC bias voltage. A memory window of 3V was obtained for an applied bias voltage of ± 10V. Annealing of the MOS structures in a furnace at a temperature of 800 °C for 30 minutes in oxygen resulted in complete loss or collapse of the memory window. Annealing the samples rapid thermally in oxygen environment at 800 oC for 30 seconds, resulted in a memory window of about 2 Volts for an applied voltage of ± 14V.

Nanoparticle-modified metal–oxide–silicon structure enhancing silicon band-edge electroluminescence to near-lasing action

With the insertion of SiO(2) nanoparticles in the oxide layer, near-lasing actions such as threshold behavior and resonance modes are observed at the Si bandgap energy of metal-oxide-silicon (MOS) structure. The threshold current is ~12 mA . The SiO(2) nanoparticles cause simultaneous localization of electrons and holes to enhance phonon-assisted radiative recombination. Electroluminescence at Si bandgap energy is increased to orders of magnitude larger than in similar MOS structures without SiO(2) nanoparticles. The efficient light emission at the Si bandgap energy indicates that a direct bandgap nature is not necessarily the basic requirement for radiative recombination.

Band alignments in metal–oxide–silicon structures with atomic-layer deposited Al2O3 and ZrO2

The energy barrier height Φ for electrons at the interfaces of various metals (Mg,Al,Ni,Cu,Au) with nanometer-thin Al2O3 and ZrO2 layers grown on (100)Si by atomic layer deposition has been directly measured using internal photoemission of electrons into the insulator. The behavior of the metal/Al2O3 contacts with increasing metal electronegativity XM resembles that of the metal/SiO2 interfaces with ideality factor dΦ/dXM≈1. The metal/ZrO2 contacts exhibit a less ideal behavior with dΦ/dXM≈0.75. The metal–silicon work function differences in structures with Al2O3 and ZrO2 insulators appear to be considerably larger than in the structures with thermally grown SiO2, suggesting the presence of a negative dipole layer at the metal/deposited oxide interface.

Gate tunneling currents in ultrathin oxide metal–oxide–silicon transistors

Carrier tunneling through ultrathin (1–3 nm) SiO2 layers in MOS (metal–oxide–silicon) structures is investigated using the Bardeen–Harrison transition probability method. Quantum mechanical wave function matching at the two abrupt potential boundaries of a trapezoidal Si/SiO2/Si barrier gives an electric-field dependent preexponential factor in the Wentzel–Kramers–Brillouin tunneling probability, which significantly affects the current–voltage characteristic at low fields. An analytical theory is employed to predict the relative importance of three elastic tunneling pathways (electrons, valence electrons, and holes) and two geometrical tunneling locations (channel region and source or drain overlap regions) in MOS transistors (MOSTs), showing (1) hole tunneling dominant in p+gate pMOST (p-channel MOST) at low gate voltages, and (2) overlap regions dominant prior to base-region inversion in both p+gate pMOST and n+gate nMOST (n-channel MOST). The analytic theory is used to analyze the experimental tunneling currents measured at the gate, source, well, and substrate terminals of sourced MOS capacitors to give the oxide thickness and impurity doping concentrations in the base and source regions.

Electrical characterization of Pb centers in (100)Si–SiO2 structures: The influence of surface potential on passivation during post metallization anneal

Capacitance–voltage measurements were made on Cr-gated metal–oxide–silicon structures with ultrathin (∼30 Å) thermal oxides. Using an empirical model, activation energies for the passivation of the Pb center were determined and found to be dependent on the charge state of the defect. Depassivation was found to occur at positive gate biases.