Session JC43 - Industrial Devices, Materials, and Processes.
ORAL session, Tuesday morning, March 23
Knollwood Room B, Omni Hotel

[JC43.01] \mu grad(Un) or q grad(Dn)?

A question similar to the title of this abstract was asked by Landsberg(P.T.Landsberg, J. Appl. Phys. 56), 1119(1984). fifteen years ago. We have revisited this question in the context of spherical harmonic expansion (SHE) of the Boltzmann transport equation. Two different formulations for the current density taking the form of drift-diffusion are permissible. In one formulation, the mobility appears outside the gradient operator for the diffusion term, and in the other, the diffusion coefficient stays inside the operator. Both formulations contain a mobility in the drift term but these two mobilities are different from each other. The transport coefficients appearing in these formulations can be explicitly expressed in terms of coefficients of the SHE of the carrier distribution function which can be extracted from the Monte Carlo (MC) simulation. However, because of nonlocal effects associated with rapidly varying electric fields existing in today's advanced semiconductor devices, it is not easy to model these transport coefficients as functions of macroscopic state variables. Nevertheless, such a modeling attempt will be made based on the MC simulation data of \textstyle n^+-n-n^+ structures.

[JC43.02] Current-Voltage Characteristics in Real GaAs Schottky Barrier Diodes

Many fabricated Schottky diodes exhibit significant deviations from the theoretically calculated I-V characteristics of ideal Schottky diodes. Attempts have been made to account for this deviation using such phenomena such as interface states or surface state densities. In a paper published in 1992, Maeda et al. used the interfacial layer model to analyze the non-ideal I-V characteristics of a GaAs Schottky barrier. We show here how non-ideal behavior can be explained by considering surface leakage currents as well as material resistance. The typical measurement of Schottky diodes is the ideality factor which can be obtained from measurements of dV/dlnI. By taking into account device resistance and shunt leakage paths with physically appropriate parameters, a relation between dV/dlnI and voltage can be obtained which yields a better understanding of transport across the interface(s) of real Schottky diodes.

[JC43.03] Multi-Dimensional Quantum Tunneling and Transport Using the Density-Gradient Model

We show that quantum effects are likely to significantly degrade the performance of MOSFETs as these devices are scaled below 100 nm channel length and 2 nm oxide thickness over the next decade. A general and computationally efficient electronic device model including quantum effects would allow us to monitor and mitigate these effects. Full quantum models are too expensive in multi-dimensions. Using a general but efficient PDE solver called PROPHET, we implemented the density-gradient (DG) quantum correction to the industry-dominant classical drift-diffusion (DD) model. The DG model efficiently includes quantum carrier profile smoothing and tunneling in multi-dimensions and for any electronic device structure. We show that the DG model reduces DD model error from as much as 50% down to a few percent in comparison to thin-oxide MOS capacitance measurements. We also show the first DG simulations of simultaneous gate oxide tunneling and transverse current flow in ultra-scaled MOSFETs. The advantages of rapid model implementation using the PDE solver approach will be demonstrated, as well as the applicability of the DG model to any electronic device structure.

[JC43.04] 3D solution of Schrödinger's equation including a full bandstructure

The self-consistent quantum simulation of electron states involving high energy levels in semiconductor structures poses a considerable numerical challenge. The simple effective mass approximation breaks down at sufficently high energies, requiring the inclusion of realistic bandstructures. Atomistic models are a possible solution, however, the computational costs involved in these models are prohibitively high for the large geometries required by a full-scale device simulation. To maintain the convenience of a Schrödinger equation based on envelope wave functions, a continuum description of the semiconductor, combined with the correct electronic energy-momentum relation E(p) for the conduction band can be introduced to achieve a quantum description of electronic states. The kinetic energy operator in the resulting 3D Schrödinger equation is an infinite order differential operator. A Fourier approach yields an efficient solution of the corresponding eigenvalue problem for structures like quantum dots. To achieve self-consistency, Schrödinger's equation is then coupled to a 3D Poisson equation, and a predictor-corrector type iteration approach is used to solve the combined system. Several preliminary applications of the simulation method are presented.

[JC43.05] Real-Space Treatment of Electron-Electron and Electron-Impurity Interactions in Monte Carlo Particle-Based Simulators

In modern deep-submicron devices, for achieving optimum device performance, the doping densities must be quite high. This necessitates a careful treatment of both the short-range and the long-range electron-electron (e-e) and electron-impurity (e-i) interactions. The short-range portions are commonly accounted for by including them in the k-space portion of the Monte Carlo transport kernel. Major problems with this approach are: (1) the requirement for frequent re-evaluation of the scattering rates during the simulation process, to take into account the changes in the distribution function and the screening length, and (2) the double-counting of the long-range force.

Herein, we propose a new method for employing e-e and e-i interactions in particle based simulators, which eliminates these two problems. The method naturally incorporates the multi-ion contributions, local distortions in the scattering potential due to the movement of the free charges, and carrier-density fluctuations. The doping dependence of the low-field electron mobility, obtained from 3D resistor simulations, closely follows the experimental results.

[JC43.06] Modelling Metallic Microstructure: Orientation Dependence

Mean curvature grain boundary motion (MCGBM) describes the general behavior of interface motion for a wide variety of materials, but it cannot explain the faceting which occurs in anisotropic metals due to the dependence of the grain boundary mobility on the orientation of both the crystalline grains and the interface. We recently modified our Grain3d code, which uses Gradient Weighted Moving Finite Elements (GWMFE) to accurately model time-dependent 3D metallic microstructure, to allow for an orientation-dependent grain boundary energy (ODGBE). We use a simplified ODGBE fit to LDA calculations reflecting the grain boundary twist, tilt, and asymmetry. While the gross features of the evolution under MCGBM and using an ODGBE are the same, steps and facets result from the ODGBE, along with grain boundary angles not permitted within MCGBM. Initial microstructures were obtained both from experimental data and via Monte Carlo evolution of a discrete effective classical spin (Potts) model on the elements of the unstructured tetrahedral grid generated by the LANL LaGriT code. We also present premlinary results incorporating stress/strain dependence to yield a realistic model for grain boundary motion, and comparisons with experimental data on texture evolution during annealling.

[JC43.07] Ab-Initio Pseudopotential Calculations of Boron Diffusion in Silicon

First-principles calculations of formation and migration energies of dopant atoms and native defects in semiconductors have been recently shown to be a very useful input to improve semiconductor process simulations. One example for this is the widely accepted first-principles model for B diffusion and clustering in Si by Zhu et al.\ that is used by numerous groups to predict the transient enhanced diffusion (TED) after B implantation.(See, e.g., J.\ Zhu, in ``Defects and Diffusion in Silicon Processing''), ed.\ by T.\ Diaz de la Rubia, S.\ Coffa, P.\ A.\ Stolk, and C.\ S.\ Rafferty (Mater.\ Res.\ Soc.\ Proc.\ 469, Pittsburgh, PA 1997), p.\ 151; M.\ J.\ Caturla, M.\ D.\ Johnson, T.\ D.\ de la Rubia, Appl.\ Phys.\ Lett.\ 72, 2736 (1998). The recent coupling of the nudged elastic band method with first-principles methods results in a powerful tool which allows a more systematic and reliable search for diffusion paths and migration barriers.

Using such a method, we studied the diffusion of B in Si once again and found a qualitatively new diffusion mechanism with lower barriers. The effects of the different charge states of B and the implications of the new diffusion mechanism on the prediction of junction depths and B activation in future technologies will also be discussed.

[JC43.08] Luminescent properties of pulsed laser deposited Eu-activated Y2O3 thin film phosphors

In this paper we report the growth, structural and cathodoluminescent (CL) characterization of europium activated yttrium oxide (Eu:Y2O3) thin films. The level of europium activator in the host lattice of yttrium oxide was four percent by weight. The Eu:Y2O3 films were grown in-situ using a pulsed laser deposition technique. Our results have shown that Eu:Y2O3 films can grow epitaxially on (100) LaAlO3 substrates under optimized deposition parameters. The epitaxial growth of Eu:Y2O3 films on LaAlO3, which has wide lattice mismatch with film material, is explained by considering the atom positions in the lattices of the film and the substrate. The CL measurements carried out on these films have indicated that highly crystalline Eu:Y2O3 films give out an intense CL emission at 611 nm. The observed transition in CL spectrum has been explained on the basis of selection rules for Eu3+ ions located at two different (C2 and S6) sites in Y2O3 host lattcie.

[JC43.09] Scanning Capacitance Microscopy and Spectroscopy Study of 2D Dopant Profile in Ultra-Shallow p-n Junction

The two-dimensional(2D) dopant profiling in ultra-shallow p-n junctions is attracting much attention in the past few years due to its critical role in fabrication of VLSI devices. However, so far no method has been well-established for 2D dopant profiling with sufficient spatial resolution and dynamic range, nor there is a reliable method to verify such a technique. Here we report our recent approach to determining 2D dopant profiles of MOSFET devices with p-n junction less than 100 nm. The work includes the setup of scanning capacitance spectroscopy on a scanning probe microscopy system, which allows us to perform the measurement of not only the spatial variation of capacitance, but also its bias voltage dependence. With this system, we have studied 2D dopant profiles of MOSFET devices with p-n junctions as small as 60 nm. Furthermore, we compare the results with those obtained by using the technique combining dopant selective etching and AFM imaging. We will discuss the validity of SCM/SCS measurements in such small devices, as well as the influence of surface treatments and probe shape to the spatial resolution and dynamic range.

[JC43.10] Surface hydrodynamics on a freely standing layer of a polymer material

The dispersion relation and the power spectrum of the surface modes on a surface active freely standing film of a concentrated polymer solution are studied with a two component fluid model of a viscoelastic material. The diagram of bending modes is obtained as an asymptotic analysis of the dispersion equation when bending rigidity modulus is the main elastic effect on the layer interface.

[JC43.11] A Reliable Approach to Modelling Multiple Scattering of Radiation in Isotropic and Anisotropic Media

The ability to model the angular dependence of radiation reflected off of a slab of material comprised of scatterers of multifold (isotropic, dipole, quardupole) natures can lead to very rewarding applications in the optical probing of materials (including biological media). It also could possibly augment calculations in neutron transport modelling in N.Engineering. The work of this paper is based on radiative transfer theory and the mathematical simplifications for the analysis of radiative transfer formulated by S. Chandrasekhar. In this paper, the H-function corresponding to the first three Legendre polynomial contributions to the radiative transfer phase function are symmetrically analyzed and modelled. These H-functions directly determine the intensities of back reflected and scattered radiation and are obtained by solving the nonlinear Fredholm integral equations (International Journal of Theoretical Physics (Radiative Transfer Single-Scattering Albedo Estimation...), Vol. 36,4, April 1997) appropriate for the H-functions.\qquad Multiple scattering problems which involve a longitudinally attenuated optical source lead to linear Fredholm integral equations which can be more directly solved for radiation intensity. Methods to solve both the nonlinear and linear Fredholm integral equations will be presented and compared with older results.

Part J of program listing