R. Greef

Optical properties of porous silicon films

Abstract The optical constants of porous silicon layers formed in wafers with resistivities of 0.01–25 Omega cm and densities of 20%–75% were determined by ellipsometry in the wavelength range 0.28–0.7 microm and by reflectance and transmission from 0.6 to 2.5 microm. The IR refractive index was found to decrease as the density was reduced and an effective medium model was used to obtain density values from the optical data. Good agreement was obtained with gravimetric densities for 0.01 Omega cm samples on the assumption of a mixture of crystalline silicon and air, but material of higher resistivity showed apparently reduced optical densities because of the presence of some oxidized silicon. The optical measurements indicate that the material is a chemical mixture (SiO x ) at high oxygen concentrations. e 1 and e 2 spectra showed that 0.01 Omega cm samples had better crystallinity than higher resistivity samples and this was confirmed by the observation of amorphous-like bands in photoluminescence spectra of the latter material.

In situ monitoring of sputtered zinc oxide films for piezoelectric transducers

Abstract We report the application of in situ ellipsometry to monitor the growth by RF reactive sputtering of zinc oxide thin film piezoelectric transducers on silicon substrates. To establish the effect of substrate temperature on film quality and piezoelectric activity, films were grown at substrate temperatures of 100–300°C. Films of refractive index as high as 1.940 are achieved compared with refractive index of 1.99 for bulk zinc oxide crystals. Under optimum deposition conditions, transparent homogenous films of high piezoelectric activity are formed from the initial stage of growth. A simple single layer model is used to predict the in situ ellipsometry measurement data for films deposited under optimum conditions, while a two layer model (duplex model) is needed to model the ellipsometry measurement data for rough films formed under conditions away from the optimum. Films grown at the optimum substrate temperature of 200°C were smooth and of reproducibly good quality, having high piezoelectric activity. Although the refractive index remained constant during growth, the rate of growth was not quite linear, and was found to increase slowly with time according to a weakly quadratic law. X-ray diffraction and pulse echo techniques were used to establish that films deposited close to 200°C have good oriented structure and high piezoelectric activity. We have thus shown that a strong correlation exists between the optical properties measurable with a simple in situ ellipsometer, and the appearance of highly oriented films of zinc oxide having excellent piezoelectric activity.

Photosensitivity of lead germanate glass waveguides grown by pulsed laser deposition.

We report very large photoinduced refractive-index changes Dn, of the order of ~10(2), in lead germanate glass waveguides grown by pulsed-laser deposition. The magnitude of Dn was derived from measurements of diffraction efficiency for gratings written by exposure to 244-nm light through a phase mask, whereas the sign of Dn was determined from ellipsometric data. Results are shown for films grown under oxygen pressures ranging from 1 chi 10(-2) to 6 chi 10(-2)mbars (1.33mbars=1 Torr).

In-situ dual-wavelength and ex-situ spectroscopic ellipsometry studies of strained SiGe epitaxial layers and multi-quantum well structures

In-situ dual-wavelength ellipsometry and ex-situ spectroscopic ellipsometry have been used to study strained Si 1-x Ge x /Si multilayers. Reference dielectric function spectra of strained Si 1-x Ge x with 0.06<x<0.29 have been obtained for the first time and an interpolation procedure based on the strain dependence of the dominant critical point energies developed. Good agreement with composition and thickness values from corroborative techniques was obtained when the effects of strain were taken into account

High-Aspect ratio silicon pillars fabricated by electrochemical etching and oxidation of macroporous silicon

Abstract A technique for fabricating sub-micron free-standing silicon pillars has been developed. The silicon pillars have a high packing density, and aspect ratios over 50:1 can easily be achieved. Silicon wafers of 3–5 Ω cm resistivity is patterned with regular windows on a silicon nitride layer by conventional photolithography. Inverted pyramids are formed within the windows after orientation-dependent KOH etching. During photo-assisted electrochemical etching in 2.5% ethanoic hydrofluoric acid, the photo-generated holes are collected at the sharp tips of the inverted pyramids where the directional etching of macropores is localized. The bulk silicon left between the close-packed macropores is oxidised. Free-standing pillars are then formed by subsequent wet etching of the silicon dioxide.

Reactive ion etching of silicon carbide (SixC1−x)

Abstract The reactive ion etching of low pressure chemical vapour deposited SixC1 −x layers (x = 0.5 and 0.8) on oxide covered Si(100) wafers has been investigated using in-situ ellipsometry, mass spectrometry and optical emission spectroscopy. Etch rates using SF 6 O 2 and F2/Ar/O2 plasmas have been monitored and the conditions identified under which the etch rate of SiC relative to Si can be varied between ≈ 0.2 and ≈ 5 and SiC relative to SiO2 from ≈ 1 to ≈ 4. Etch products have been identified and an etch mechanism proposed that accounts for the observed variation in etch rates with percentage O2 for SF 6 O 2 plasmas.

Ellipsometry in electrochemistry: a spectrum of applications

Abstract Most electrochemical applications of ellipsometry are dynamic in nature, and this has meant that monochromatic ellipsometry is to date the dominant mode of operation. When measurements can be made in situ , ellipsometry is a powerful and versatile diagnostic tool, and this paper highlights a number of diverse applications to support this view. Examples are given of the kinetics of growth of thick, uniform films, with a strong gradient of density, very thin films, and nucleating surfaces. Some of the problems that can be met in interpreting ellipsometric results are indicated, with solutions where available. Other optical techniques which can be used as useful adjuncts to ellipsometry are also mentioned.

Characterisation of rough silicon surfaces using spectroscopic ellipsometry, reflectance, scanning electron microscopy and scattering measurements

Abstract A critical comparison of several techniques for the evaluation of imperfections in silicon-on-sapphire (SOS) wafers leads to the adoption of a model of the SOS surface based on an effective medium approximation treatment of the ellipsometric results.

Ellipsometric and electrochemical characterisation of the nucleation of mercury on glassy carbon

Abstract In previous papers on the nucleation and growth of mercury droplets on an inert substrate, the connection between the optical parameters and the population and size of the nuclei has been established. Some new results are presented allowing a test of both the electrochemical nucleation theory and the optical theory. There is excellent agreement between the nucleation densities determined independently by the two methods. The optical results moreover are very sensitive to another parameter, the size dispersion in the nuclei, which cannot be obtained with certainty by electrochemical techniques. The ellipsometric technique is therefore a powerful one for obtaining nuclear populations independently of electrochemical theory. Some results are also given for electrochemical growth of solid nuclei, suggesting that the technique may be of wider applicability than just to liquid metals.

An analysis of a "dead layer" in the emitter of n/sup +/pp/sup +/ solar cells

In this paper, the results obtained from fabrication and characterizations of n/sup +/pp/sup +/ solar cells are presented. The existence of a dead layer in the emitter was inferred regarding to the poor quantum efficiency of the cells under short wavelength region. The effects of this layer on the solar cell performance have been analyzed using the theoretical model and demonstrated in the term of low collection efficiency near the top surface of the emitter and high front surface recombination velocity.