Introductory Remarks

Edward D. Palik    Institute for Physical Science and Technology University of Maryland College Park, Maryland

I INTRODUCTION

The two previous volumes of Handbook of Optical Constants of Solids (HOC I and HOC II) contained about 85 materials. The present volume adds about 58 more materials, roughly divided into equal numbers of metals, semiconductors, and insulators. A long list of potential materials was circulated among possible critiquers. They chose and/or recommended the ones presented here. There are still some materials omitted because of a lack of critiquers. These include Bi, B, Ba, Ca, Cd, Pb, Zn, Zr, ZnO, HgS, HgSe, PbxSn1–xSe, PbO, MoS2, Cd3As2, GaS, LiOH, NaBr, Nal, KF, LiCl, LiBr, ZrO2, BaO, and NiO. Note that 43 of the 92 chemical elements are done and 143 of the 100,000 (or more) chemical compounds. We have omitted glasses generally, which are covered to some extent by Efimov [1] and Tropf et al. [2].

There is some loose use of the words semiconductor and insulator in HOC I and HOC II based primarily on the band gap being less or greater than 3 eV. AgCl, AgBr, Agl, BSO, and BGO are considered as semiconductors because their electrical properties can be controlled by doping during growth of the crystals, but they may end up in either category. Diamond is invariably thought of as an insulator, although natural diamonds can be found with semiconducting properties.

II THE CHAPTERS

The chapters are meant to describe experimental techniques for measuring n and k.

In Chapter 2, Parker et al. discuss the determination of n and k by differential Fourier transform spectroscopy in the far infrared. This is an elegant way to obtain n and k from the reflection amplitude and phase and has been applied to many semiconductors and insulators. It is of interest to compare these newer optical constants with those in HOC I or HOC II, which have generally been obtained by Kramers-Kronig (K-K) analysis or classical oscillator fit.

In Chapter 3, Mandelis describes the use of photoacoustic spectroscopy to determine k. While this chapter stresses semiconductors, several variations of the technique have also found widespread use for other materials.

In Chapter 4, Briggs discusses photothermal deflection as a method of determining k. It is done with two intense beams, one to heat up a small spot on a sample and a second to detect bending of the sample as it expands locally.

In Chapter 5, Boukari and Lagakos describe the determination of n and k by Brillouin scattering. While not usually thought of as a technique for determining optical constants, there are novel ways to determine n an k in highly absorbing materials and n in transparent slabs and thin films.

In Chapter 6, Auslender and Hava develop models for calculating n and k in n-Si for the infrared region for free-electron concentrations from 1016 to 1020 cm−3.

In Chapter 7, Palik collects a number of optical parameters for all the materials in HOC I, HOC II, and HOC III including crystal structure, space group symmetry, unit cell dimensions, number of molecules/unit cell, optic and acoustic irreducible representation, transverse and longitudinal lattice vibration frequencies, plasma frequency, band gap, and d.c. dielectric constant.

There were suggestions for chapters on the loss-tangent method at low frequencies and on the prism minimum-deviation technique (still the best way to determine n), but these could not be done at this time.

III THE CRITIQUES

The critiques are meant to be the critiquer's own judgment of the best room-temperature values of n and k over the widest spectral range. As the volumes have progressed, we have expanded the range a little with a few critiques including above and below room-temperature data. Sometimes there are two overlapping sets of data from two different laboratories which can be compared. This sometimes happens in the far infrared where reflectivity is K-K analyzed and/or a classical oscillator model is applied. Generally, it is not a good idea to give the reader a choice of data—it is the job of the critiquer to make the choice. Sometimes, however, a comparison sheds light on the problems arising in the measurement of optical constants such as wavelength calibration and determination of absolute reflectivity. Recently, spectroscopic ellipsometry has grown into a wide-spread technique for the 1.5–6 eV region, probably supplanting K-K analysis of reflectivity.

We have returned to a few materials done in previous volumes because more data have become available. Therefore, we revisit Al2O3, SixGe1−x and Si.

IV THE TABLES

The tables of n and k have become more individual to meet the needs of the individual critiquers. We maintain the columns labeled eV, cm−1, µm, n, k and proceed down the column in decreasing eV. Reference brackets [1] start at the top of a given data column and are understood down the column until a new data set occurs, which is then labeled [2], for example. We promoted exponential notation especially of k, ideally for every column, but have not yet reached that stage of uniformity.

To facilitate editing the tables for the Optical Society of America, we have requested that the exponential notation be used at every row, rather than omitting the exponent until it changes value somewhere down the column.

V THE FIGURES OF THE TABLES V

The critiquers have taken more leeway with the figures, since they had to produce them with whatever graphics software they had available. In previous editions, all the figures were prepared by the same artist who kept the style uniform. Now critiquers can use data points (usually for n and for k). Data points can be connected with lines, or just lines can be used (usu-ally — for n and — for k). In the past, both n and k were plotted on the same figure with the same log-log scale. Since the extent of k determined the number of decades of the log scale for the ordinate, (10−7 < k < 101), n was squeezed. Also, for solids with a lot of vibration bands, this region got very busy. Now some critiquers have separated n and k plots to show the structure more clearly.

The drawbacks in having each critiquer make his own figure are the many problems of font size, tick-mark position, and border thickness, remembering that the figure will be reduced in published form.

VI CORRECTIONS, ADDITIONS, AND COMMENTS

A Optical Constants of CdS

A correction and addition to the data for CdS from HOC II needs to be made according to the critiquer, L. Ward. Data of Dutton1 near the band edge has been added. The extinction coefficient for both Ec and E⊥c has been determined by transmission measurements of thin slabs of different thicknesses. These can be compared to data of Cardona and Harbeke2 for bulk-oriented samples obtained from K-K analysis of reflectivity with polarized light. Data of Khawaja and Tomlin3 for films deposited on quartz substrates (assumed to be polycrystal-line) were obtained from transmittance and reflectance measurements; these data have been reassessed to correct an error in reading the wavelength scale. The results are presented in Table I. While above the band gap the results for k are comparable, below the band gap where absorption decreases orders of magnitude, the results differ drastically among the three data sets. We suspect that K-K analysis of reflectivity in Cardona and Harbeke gives poor results for k « 1 compared to transmittance of slabs in Dutton. For the polycrystalline films in Khawaja and Tomlin, it is not obvious why k is so much larger compared to the data of Dutton below the band gap. Perhaps there was significant light scattering from defected...