S. K. Lahiri

Gd2O3, Ga2O3(Gd2O3), Y2O3, and Ga2O3, as high-k gate dielectrics on SiGe: A comparative study

In this paper we report a comparative study of the electrical properties of some oxides e.g. Gd 2 O 3 , Ga 2 O 3 (Gd 2 O 3 ), Y 2 O 3 and Ga 2 O 3 as gate dielectric for strained Si 0.74 Ge 0.26 MOS devices. The deposited films have been characterized using EDAX, SIMS and ellipsometry. The Au / oxide / SiGe MIS structures using these insulators were examined using I-V, C-V and G-V techniques. Though Gd 2 O 3 and Y 2 O 3 showed highest resistivity and breakdown strength, Ga 2 O 3 (Gd 2 O 3 ) was found to be most effective for surface passivation of SiGe giving lowest interface state density while pure Ga 2 O 3 was incapable of passivating SiGe surface.

Microwave plasma oxidation of gallium nitride

Abstract Low temperature oxidation of gallium nitride epilayer by microwave oxygen plasma treatment has been characterized by secondary ion mass spectroscopy and X-ray photoelectron spectroscopy (XPS). In the initial stage the oxide layer grows almost linearly with time while after approximately a thickness of 10 nm the growth rate saturates. The chemical shift of 1.2 eV in the Ga 2p and 3d peak in XPS data is observed upon oxidation, indicating the formation of Ga2O3 on the GaN surface.

Electrical trimming of ion-beam-sputtered polysilicon resistors by high current pulses

Phosphorus doped polysilicon resistors have been fabricated from microcrystalline silicon films which were deposited by ion beam sputtering using an argon ion beam of diameter 3 cm, energy 1 keV and current density 7mA/cm/sup 2/, with a deposition rate of 100-120 /spl Aring//min. The resistors, having a sheet resistance of 70 /spl Omega//square and a carrier concentration of 7.5/spl times/10/sup 19/ cm/sup -3/, were stressed with current pulses of width 10 /spl mu/s and duty cycle 0.6% for 5 min. There was a steady decrease of resistance with increasing pulse current density above a threshold value 5/spl times/10/sup 5/ A/cm/sup 2/. A maximum fall of 27% was observed for a 95 /spl mu/m long resistor. The current-voltage characteristics were also recorded during the trimming process. The trimming characteristics were simulated using a small-signal resistivity model of Lu et al. (1983). and the I-V characteristics by a large-bias conduction model. A close fitting of the experimental data with the theoretical values needed an adjustment of some grain boundary parameters for the different pulse current densities used for stressing. The nature of variation of the grain boundary parameters indicates that the rapid Joule heating of the grain boundaries due to current pulses passivates the grain boundary interfaces, at lower currents above the threshold, and then, at higher values of currents, causes zone melting and gradual recrystallization of the disordered boundary layers and subsequent dopant segregation. It confirms the mechanism suggested in the physical model of Kato et al. (1982). The role played by the field-enhanced diffusivity and electromigration of dopant ions, due to the high instantaneous temperature of the grain boundaries, has also been discussed. The pulse trimming technique is simple and does not cause damage to the adjacent components on a monolithic chip. >

Effect of reactive‐ion bombardment on the properties of silicon nitride and oxynitride films deposited by ion‐beam sputtering

Silicon nitride and oxynitride films of very low hydrogen content have been deposited on silicon at low temperatures (150–200 °C) using ion‐beam sputtering. A dual‐ion‐beam sputtering technique, making simultaneous use of an energetic argon‐ion beam to sputter silicon nitride from a target and a low‐energy oxygen or nitrogen ion beam to react with the sputtered films on the substrate, has been employed to control the composition of the films. A precise control of film composition independent of deposition rate has been achieved through the control of oxygen/nitrogen ion‐beam parameters and gas flow ratios. The films have been characterized by the measurement and study of refractive index, chemical etch rate, infrared absorption, and x‐ray photoelectron spectra. A direct correlation between film properties with oxygen content has been obtained for silicon oxynitride films. The electrical properties have been studied by the measurement of the characteristics of metal‐insulator‐semiconductor capacitors fabri...

Optical and dielectric properties of junctionlike CdS nanocomposites embedded in polymer matrix

CdS nanocomposites have been grown in a polymer (polyvinyl alcohol) matrix using a simple chemical bath deposition process. Transmission electron micrographs of nanocomposites grown at different solution temperatures revealed the formation of isolated as well as junctionlike structures. X-ray and selected area electron diffraction patterns show that the nanocomposites are polycrystalline with cubic CdS phase. Optical band gaps of nanocomposite films are found to decrease (3.26–2.86eV) with the increase in bath temperature from 70to90°C. Photoluminescence spectra show strong green emission attributed to the Cd2+ or Cd+ ion-related recombination via moderately deep trap states. The nanocomposites show an enormous enhancement of dielectric constant in polyvinyl alcohol matrix over a frequency range of 40Hz–10MHz.

TEOS-based PECVD of silicon dioxide for VLSI applications

Silicon dioxide films deposited from tetraethylorthosilicate (TEOS) using plasma-enhanced chemical vapour deposition (PECVD) are reviewed. The effect of the presence of oxygen on the film deposition rate and mechanism and the physical properties of the films, particularly the step coverage properties (conformality), are discussed in detail. Structural characterisation of the films has been carried out via etch rate measurements, infrared transmission spectroscopy, X-ray photoelectron spectroscopy (XPS) and Auger and secondary ion mass spectroscopy (SIMS) analysis. Electrical properties, i.e. resistivity, breakdown strength, fixed oxide charge density, interface state density and trapping behaviour, have been evaluated using metal-oxide-semiconductor (MOS) structures fabricated using the deposited oxides. Films deposited by microwave plasma-enhanced decomposition of TEOS in the presence of oxygen have been found to be comparable with standard silane-based low-pressure chemical vapour deposition (LPCVD) and PECVD oxides. It has been shown that films deposited on thin native oxides grown by either in situ plasma oxidation or low-temperature thermal oxidation exhibit excellent electrical properties.

Modal profiles in Ti:LiNbO3 two-waveguide and three-waveguide couplers by effective-index-based matrix method

Abstract Graded-index optical channel waveguides like Ti-indiffused LiNbO 3 waveguides have been modelled by effective refractive index profile along the lateral direction, discretising the profile and then using the matrix method. This effective-index-based matrix method, which was earlier used to compute the modal propagation constants of Ti:LiNbO 3 channel waveguides and directional couplers [Fib. Integrat. Opt. 17 (1998) 139] and the bending loss [J. Opt. Commun. 155 (1998) 125], has been established in this paper as a computationally fast, analytical method to compute electric field and modal intensity profiles in single and coupled waveguides. In order to handle coupled waveguides consisting of three waveguides, the concept of overlap integral has been used to analyse power coupling to the middle waveguide for applications in power splitting. Some of the computed intensity profiles have been compared with experimental data obtained by the authors. Although computation has been performed only for TE and TM polarisations, the method has the inherent capability of handling arbitrary polarisation.

A large-bias conduction model of polycrystalline silicon films

There exists a need for a large-bias conduction model of polysilicon films used in VLSI/ULSI and in high power integrated circuits. A large-bias conduction model has been developed by extending the emission-based models of Lu et al. (1983) and Mandurah et al. (1981) valid for small-bias, small-signal conditions. The following large-bias effects have been taken into account: (1) asymmetry of potential distribution around grain boundaries and (2) avalanche multiplication of carriers in the grain boundary layers at high electric fields. Since the exact nature of the grain boundary material is not yet known, and there is no direct method for determining the model parameters relating to grain boundaries, these were extracted by the parametric fitting of resistance versus temperature data of polysilicon resistors near room temperature with the above small-signal resistivity models modified by including Fermi-Dirac distribution. The model has been validated with experimental data on the current-voltage characteristics of ion-beam sputtered polysilicon resistors of different sizes and aspect ratios. The dependence of model parameters relating to grain boundary scattering and avalanche multiplication on the dimensions of resistors have been explained physically. The increased kink effect in polysilicon TFTs may also be predicted from the present theory. Some results on the I-V characteristics of polyresistors trimmed by high current pulses have been discussed qualitatively in the light of the present model. Although the model involves numerical integrations and a few iterations, it is reasonably fast in execution. >

Analysis of directional coupler electro-optic switches using effective-index-based matrix method

Abstract The effective-index-based matrix method (EIMM) has been used to simulate the characteristics of integrated-optic directional coupler switch (both the uniform-Δ β and reversed-Δ β types) based on electro-optic (EO) effect. The characteristics are derived from the distributions of optical power and electrical modulating field within the device. The analysis was focused on directional coupler switching devices made by the diffusion of titanium in lithium niobate (Ti:LiNbO 3 ) substrates and incorporation of suitable electrodes. The simulated results are found to match well with available experimental results and other numerical simulation results obtained from the literatures. Although the computations are performed for Ti:LiNbO 3 waveguides, the model is applicable to arbitrary graded-index waveguides with the known refractive index profile and electro-optic coefficient. EIMM is found computationally well efficient and considerably faster than beam propagation method (BPM).

Modelling of titanium indiffused lithium niobate channel waveguide bends: a matrix approach

Abstract An analytical model for computation of bending loss of Ti:LiNbO 3 channel waveguide bends has been presented. The analytical steps involved are as follows. The 2D refractive index profile over the cross-section of Ti:LiNbO 3 waveguide is first transformed to 1D effective-index profile along the lateral direction. A conformal mapping technique is then used to transform the effective-index profile of the waveguide bend to that of an equivalent straight waveguide. A stair-case type step-index profile is generated from the equivalent effective-index profile in lateral direction by partitioning the latter into a large number of thin sections of varying refractive indices. The overall transfer matrix of the step-index layered structure so obtained may be computed by the progressive multiplication of individual 2×2 transfer matrices relating the field components in adjacent layers. The excitation efficiency of the wave in the guiding layer shows a resonance peak around the mode propagation constant. The full-width-half-maximum (FWHM) of this peak determines the power attenuation coefficient of the bent waveguide. The losses due to the discontinuity of the curvature are also computed. The computed results for different bends including S-bends are in good agreement with the published experimental data. The computation using the model is quite fast and versatile to consider arbitrary waveguide dimensions, Ti-film thickness, diffusion parameters and wavelength of light for both TE and TM polarizations. The model, in principle, is not limited to Ti:LiNbO 3 channel waveguides only but is valid for any arbitrary graded-index channel waveguide bends provided that the refractive index profile and the wavelength dependence of the refractive index are known.