M. N. Afsar

The measurement of the properties of materials

This review covers approximately 15 years of development in the techniques used to measure dielectric properties of materials over the frequency range 1 MHz to 1500 GHz. An introductory section summarizes the broad development trends and is followed by short sections which deal with developments at a more detailed level. The approaches described include time- and frequency-domain methods; reflection, transmission, and resonant methods, guided and free-space methods; discrete-frequency and broad-band methods, especially Fourier Transform Spectroscopy. Measurements on magnetic materials are also briefly discussed.

Millimeter-wave dielectric measurement of materials

It is no longer necessary to use extrapolated microwave dielectric values when designing millimeter-wave components and systems. Very recently, highly accurate millimeter-wave (5- to 1/2-mm) data on complex dielectric permittivity and loss tangent have become available to engineers for a variety of materials such as common ceramics, semiconductors, crystalline, and glass materials. One quasi-optical measurement method has proved to be most accurate and reproducible, namely, dispersive Fourier transform spectroscopy (DFTS) applied to a polarizing interferometer. The openresonator method and the Mach-Zehnder-IMPATT spectrometer will also be described and compared. The fact that the dielectric loss increases with frequency in the millimeter, unlike the microwave, is an important feature of these data. Reliable measurements also reveal that the methods of preparation of nominally identical specimens can change the dielectric losses by a factor of three.

Precision millimeter-wave measurements of complex refractive index, complex dielectric permittivity, and loss tangent of common polymers

Our improved dispersive Fourier transform technique applied to a polarizing two-beam interferometer enables us to provide high-precision continuous spectra of complex refractive index, complex dielectric permittivity, and loss tangent of polyethylene, polypropylene, poly-4 methyl pentene-1 (TPX), polytetrafluorethylene, Plexiglas, acrylic, and nylon over the frequency range 50–300 GHz. The first four are nonpolar polymers and exhibit extremely low-loss characteristics. The last three are typical polar polymers, but they exhibit nearly 20 to 30 times higher loss compared to nonpolar polymers.

Precision millimeter-wave dielectric measurements of birefringent crystalline sapphire and ceramic alumina

A quasi-optical technique, namely, dispersive Fourier transform spectrometry, has now been improved to provide high-precision continuous data of complex refractive index, complex dielectric permittivity, and loss tangent of materials at millimeter wavelengths. The use of a polarizing two-beam interferameter is ideally suited for the measurement of birefringent materials. A massive biréfringent effect (δ∊′ 2.193) is now observed for crystalline sapphire (Al<inf>2</inf>O<inf>3</inf>). The bifringent effect for crystalline quartz is much smaller (δ∊′ ⋍ 0.2). New millimeter-wave results of commercially available ceramic alumina (Al<inf>2</inf>O<inf>3</inf>) are compared with those of crystalline sapphire.

Millimeter Wave Dielectric Properties of Materials

Highly accurate continuous spectra of the absorption coefficient and refractive index of some potentially useful materials have been made over the 60-420 GHz range. Measurements have been made on some common ceramic, semiconductor, crystalline and glass materials. The absorption coefficient of low loss materials increases with frequency which implies that microwave data cannot be used for the design of millimeter wave dielectric waveguides, devices, windows and quasi-optical elements. The data in this paper show the millimeter wave frequency dependence of tan δ, the real and imaginary parts of the dielectric permittivity and the optical constants, namely, the refractive index and absorption coefficient. The measurements have been made in a plane-wave Michelson interferometer operating as a polarizing, dispersive Fourier transform spectrometer. The accuracy and reproducability of the refractive index is six significant figures.

Millimeter and submillimeter wave measurements of complex optical and dielectric parameters of materials

These two promising millimeter wave materials were found to exhibit one order of magnitude higher absorption coefficient than the common low-loss ceramics such as alumina, fused silica and beryllia. A modular, polarizing, dispersive Fourier transform spectrometer capable of operating over the range 5 mm to 0.004 mm was used to provide a continuous spectrum of the refractive index and absorption coefficient to an accuracy of five decimal places and less than 1 percent, respectively.

Reliable far-infrared photoconductivity method to identify a variety of residual donors in epitaxial GaAs

The chemical identity of unintentional contaminants in ultra-high purity eipitaxial GaAs and related semiconductor crystals can be distinguished by submillimeter wave magneto-spectroscopy at low temperature. An improved method of identifying hydrogen-like donors such as sulfur, silicon, selenium and germanium has been developed for the purpose of correcting some mistaken identifications that have been published during the past ten years. The new method requires the development of an experimental “signature curve” for each contaminant by measuring the energy of its 1s to 2p (m=−1) transition at several values of magnetic field intensity. The energy of this transition at a given magnetic field intensity is different depending upon the nucleus to which the electron is bound in the 1s state. The validity of the improved method was tested by means of transmutation doping.

Ultimate method for unambiguous identification of all donors in epitaxial GaAs and related compounds

When epitaxial GaAs is grown by the method of molecular beam epitaxy (mbe) it would be p-type unless it is intentionally doped lightly during growth by using a particular substitutional donor atom. We have chosen the tin donor in this case to render the specimen n-type. Then the conventional far infrared photoconductivity technique is used to observe the 1s to 2p transition of the electron of the tin donor. The identity of the donor, the energy of the quantum transition as a function of applied magnetic field intensity, and the line shape characteristics of that particular donor then become unquestionable.

Submillimeter-wave measurements on liquid dielectrics

Recent advances in submillimeter-wave complex relative permittivity measurements on liquids are described. The developments include direct measurements of both real and imaginary parts of the complex relative permittivity both at spot frequencies using submillimeter-wave laser spectrometry and in the broad band using dispersive Fourier transform spectrometry.

An automated 60 GHz open resonator system for precision dielectric measurement

An automated open resonator system designed and constructed for precision measurement of loss tangent and dielectric permittivity of low absorbing materials at 60 GHz is reported. The use of a high-Q hemispherical Fabry-Perot cavity together with highly stabilized synthesized phase-locked Gunn oscillator sources and the superheterodyne receiver enabled a loss tangent value as low as 10 mu rad to be measured. The system is automated by means of a precision lock-in amplifier, a V-band Hewlett-Packard spectrum analyzer and a Hewlett-Packard Vectra computer system with analog-to-digital conversion accessories. The synthesizer allows the collection of data at very small steps over the complete Gaussian beam, and, together with a statistical fitting, the Q determination can be made very accurately.<<ETX>>