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The interface passivation process based on post-oxidation, high temperature anneals in nitric oxide (NO) is well established for SiO2 on (0001) 4H-SiC. The NO process results in an order of magnitude or more reduction in the interface state density near the 4H conduction band edge. However, trap densities are still high compared to those measured for Si / SiO2 passivated with post-oxidation anneals in hydrogen. Herein, we report the results of studies for 4H-SiC / SiO2 undertaken to determine the effects of additional passivation anneals in hydrogen when these anneals are carried out following a standard NO anneal. After NO passivation and Pt deposition to form gate contacts, post-metallization anneals in hydrogen further reduced the trap density from approximately 1.5 × 1012 cm−2eV−1 to about 6 × 1011 cm−2eV−1 at a trap energy of 0.1 eV below the band edge for dry thermal oxides on both (0001) and (11–20) 4H-SiC.
We present results of a comprehensive set of first-principles total-energy calculations of native and impurity-defect complexes in ZnO and use these results to elucidate the problems that occur in efforts to achieve p-type doping. The analysis naturally leads to new approaches that are likely to overcome the difficulties. The results provide detailed explanations of recent puzzling observations made in attempts to produce p-type ZnO.
We report time-dependent simulations of the evolution
of atoms, molecules, and solids in the presence of intense
electromagnetic radiation using the density functional
theory. In the case of the ionic degrees of freedom we
find that selective breaking of strong bonds may be possible
at off-resonant infrared frequencies by a novel “concerted
kick” mechanism. In the case of the electron response
we find the following: free atoms and ions under intense
infrared light respond with high harmonics in the X-ray
regime; for a free molecule (Si2), we predict
an unusual third harmonic response to a UV pulse centered
at a frequency equal to the primary electronic excitation
of the molecule; for a semiconductor (Si), we find several
odd harmonics in response to a continuous wave of subgap
infrared radiation. Prospects for future calculations are
Results are reported for the passivation of interface states near the conduction band edge in n-4H-SiC using post-oxidation anneals in nitric oxide, ammonia and forming gas (N2/5%H2). Anneals in nitric oxide and ammonia reduce the interface state density significantly, while forming gas anneals are largely ineffective. Results suggest that interface states in SiO2/SiC and SiO2/Si have different origins, and a model is described for interface state passivation by nitrogen in the SiO2/SiC system. The inversion channel mobility of 4H-SiC MOSFETs increases with the NO annealing.
For molecular electronics, Boltzmann's equation is no longer valid for simulating device characteristics. We resent the first fully ab initio simulation of a molecular device that has already been studied ex erimentally, namely a benzene-1,4-dithiolate molecule between gold electrodes. The theoretical I-V curve has the same overall sha e as the ex erimental curve —reflecting the electronic structure of the molecule in the resence of the electric field — but the absolute value of the current is very sensitive to contact chemistry and geometry. In articular the resence ol a single gold atom between the molecule and the electrode surface reduces the conductance by more than an order of magnitude. Re lacement of the single gold atom by an aluminum atom, whose p orbitals cou le more effectively to the molecule's T orbitals, increases the conductance by about an order of magnitude. We have also studied the olarization effects induced by a third terminal (gate) on the I-V characteristics of the above device. In articular, we have found that current gain due to the gate bias can be achieved at reasonable gate fields. Finally the effect of current-induced forces on the device will be discussed.
In this paper we will present several new theoretical results on the properties of oxygen atoms in bulk crystalline silicon. Specifically, these properties will include (1) oxygen migration - where we will suggest that the conventional adiabatic-barrier model for oxygen migration may not be valid for this system; (2) oxygen catalysis - where we will demonstrate that certain oxygen configurations can act as “catalysts” to reactions that form silicon broken bond defects; and (3) oxygen aggregation - where we will introduce a new mechanism for the initial stages of aggregation and oxidation within the bulk of crystalline silicon.
We have carried out systematic state-of-the-art calculations using density-functional theory, norm-conserving pseudopotentials, and large supercells in order to investigate the diffusion mechanisms of B, P, As, and Sb in Si under both equilibrium and non-equilibrium concentrations of intrinsic point defects. In addition, we have developed a theory for the non-equilibrium concentrations of the relevant diffusing species from which expressions for the activation energies may be derived. In equilibrium, we find that vacancies and self-interstitials mediate the diffusion of B, P, and As with comparable activation energies, but we show from our non-equilibrium diffusion calculations that these impurities have a dominant interstitial component. Sb diffusion, on the other hand, is mediated primarily by vacancies. We also find that the direct exchange mechanism plays only a minor role in all cases.
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