Residual absorption in infrared materials

Residual absorption in infrared materials

Marvin Hass and Bernard Bendow

The sources of the residual absorption of ir materials in their transparent region between 2,um and 10gm arereviewed with emphasis upon recent developments. On the longer wavelength side of this spectral region,many insulating crystals are limited by intrinsic multiphonon absorption, and, on the shorter wavelengthside, all insulating crystals are limited by extrinsic effects associated with impurities, defects, and surfaces.During the last few years, the general character of the intrinsic multiphonon absorption has become suffi-ciently well understood so that its frequency and temperature dependence can be accounted for in both ionicand covalent crystals. The nature of extrinsic absorption is more complicated as it arises from a numberof sources. In some cases, the measured absorption coefficient can be attributed with some certainty to aspecific origin. In others, especially for the ionic crystals at shorter wavelengths, the origin of the extrinsicresidual absorption is not known and may be limited by the experimental measurement techniques, at leastin some of the better crystals.

Introduction

Infrared transmitting materials have long been em-ployed as optical windows and components in ir sys-tems. For most uses of this nature, the presence ofabsorption to the extent of a few percent is not detri-mental to system performance. However, the emer-gence of high power cw lasers has led to a need for irmaterials having extremely low levels of absorption, sothat passage of large power through transmissive com-ponents will not lead to degradation of the beam bythermal lensing or fracture from the absorbed heat. Inpractice, the choice of a particular window materialdepends not only upon its absorption coefficient butalso upon its mechanical, thermal, environmental, andother properties. However, for a particular material,the absorption coefficient appears to show the greatestsample-to-sample variation, and considerable im-provements over existing materials might be possible.In this review, attention will be focused entirely uponthe origins of the absorption in ir transmitting materi-als.

Common ir window materials are ionic crystals suchas the alkali halides, alkaline earth fluorides, andsemiconductors such as the III-V and II-VI compounds.

M. Hass is with U.S. Naval Research Laboratory, Washington, D.C.20375; B. Bendow is with Deputy for Electronic Technology (RADC),Solid State Sciences Division, Hanscom AFB, Massachusetts01731.

Received 18 January 1977.

They are characterized by a highly transparent region(absorption level of 10-3 cm'- or less) between thereststrahlen band at long wavelengths (associated withlattice motion) and the fundamental absorption edgeat small wavelengths (associated with the motions ofelectrons). Extensive experimental work has beencarried out concerning the residual absorption of irmaterials in this transparent region and on the highfrequency side of the reststrahlen tail which extendsinto the transparent region. This tail, which is associ-ated with intrinsic lattice multiphonon absorption, hasbeen found to display clear trends with respect to itsfrequency and temperature dependence. One finds, inmost cases, a relatively smoothly varying lattice tailspectrum as a function of frequency in ionic solids, whilesemiconductors display structure on the lattice tail.The classification and interpretation of the observedtrends have aided in distinguishing intrinsic from ex-trinsic features in spectra and in deducing the limitingintrinsic absorption properties of ir window materi-als.

At higher frequencies in the transparent region, theresidual absorption is extrinsic in character and couldbe associated with a number of different mechanisms.By considering the possible absorption mechanisms inconjunction with the experimental data, it has becomepossible to gain some idea of the various factors whichlimit the absorption in this extrinsic region.

This paper will first consider recent progress in theelucidation of the absorption mechanisms determiningthe intrinsic lattice multiphonon absorption in trans-parent materials. This will be followed by a discussionof the extrinsic absorption in various ir materials andpotentialities for the future.

2882 APPLIED OPTICS / Vol. 16, No. 11 / November 1977

Intrinsic Multiphonon Absorption

It has long been recognized that multiphonon pro-cesses provide the principal contribution to the intrinsicresidual absorption in the 2-10-Atm regime in purewide-gap materials.' The considerations involved inpredicting, measuring, and interpreting multiphononabsorption in this regime have been described in a va-riety of previous review papers on the subject (see Refs.2-5). We will here only briefly mention past work,concentrating instead on some recent developments inthe field and discussing their general implications.

Multiphonon absorption results from the combinedeffect of two interrelated mechanisms: the electricmoment M associated with the distorted charge densitycharacteristic of real materials to which light couplesdirectly; and the anharmonic interionic potential Vwhich, on a microscopic basis, stems from the interac-tions between electronic charge densities. Once anynumber of optically active modes are excited via theinteraction with M, they may further dissipate energythrough anharmonic interactions. The general natureof the frequency () dependence of the absorptioncoefficient a(o) may be understood on the basis of en-ergy conservation combined with the strengths ofmultiple interaction processes. If the number ofchannels available for absorption does not increase veryrapidly with order, the strength of an nth order processwill vary as y'n where, depending on the mechanisminvolved, y = mo2 or vo2, where mo and vo are momentand potential interaction parameters, respectively. Forsmall enough y, the lowest order process (involving theminimum number of phonons required by energy con-servation) will dominate the absorption in a given re-gime. These considerations suggest that the absorptionin the nth phonon regime an yn (with n = co/coo,where coo is a typical optical phonon frequency). Thus,we expect an exponential-like dependence for the ab-sorption, a A exp(-Bw/oo). Since the number ofavailable interaction channels necessarily increases withincreasing order, one expects some enhancement rela-tive to an exponential decrease in a(X) for o/co >> 1.Turning to temperature dependence, one expects thatthe ratio of absorptions

n+- N(.o) + 1 T for kT >> Two,An

where N(x) is the Bose-Einstein (thermal occupancy)function. A more detailed analysis4 indicates

[N(wo) + 1]n

N(w) + 1

where co = wo(T) contains an implicit T dependenceas well. The approximate dependence suggested by theabove discussion leads to6

a(co,T) c N wo l-, exp(- Bco/wo).N(w) + 1

The results of more detailed treatments of multiphononabsorption indicate that the above expression is ap-propriate only under conditions where selection rule andphonon density-of-states effects are negligible.

Moreover, it is, strictly speaking, valid only over re-stricted frequency and temperature ranges. Fortu-nately, many ionic and partially ionic materials do in-deed display multiphonon spectra which conform, atleast qualitatively, to the approximate behavior de-scribed above (i.e., exponential-like frequency depen-dence, Bose-Einstein type multiphonon T dependence).Rather extensive experimental evidence corroboratesthese observations.17-10 Typical examples of observedfrequency and temperature dependence are illustratedin Figs. 1 and 2.

An unresolved question is what is the relative con-tribution of the two mechanisms, nonlinear momentsand anharmonicity. Unfortunately, the predicted wand T dependences arising from both are qualitativelysimilar in the many phonon regime.4 On a more de-tailed level, one can show that the effect of nonlinearmoments on the shape of the spectrum depends criti-cally on the relative sign between the linear and non-linear portions of the moment.1"-'4 Thus, althoughnonlinear moments may be large in highly covalentsolids, their relative sign appears to be such that theireffect on a is a relatively weak. Moreover, the value ofmo appears to be equal to or smaller than vo, so that theanharmonic mechanism dominates the spectral shape.1However, according to the conventional choice of signfor the nonlinear moments in ionic solids such as alkalihalides, their effect on a(w) should be significant, e.g.,inducing a distinct interference dip in a(o). Such dipshave not been identified in measured spectra; in fact,theoretical predictions omitting the nonlinear momentsare in better agreement over a wide range of a; thanthose including them (with the conventional choice ofsign). And although Mills and Maradudin12 use justthese observations to argue for the reversed sign foralkali halides, at this juncture the question of the rela-tive contributions of linear vs nonlinear moments,and/or nonlinear moments vs anharmonicity to a(w,T)has yet to be resolved in a satisfactory fashion.

A perhaps more significant problem from a practicalstandpoint is the nature and origins of structural fea-tures in a(cw,T). The dominance of structure in themultiphonon spectra of semiconducting solids has longbeen known.14 However, although many of the theo-retical treatments15 -'8 of a were capable of accountingfor structure in principle, they were not easily employedfor this purpose in practice. To alleviate this state ofaffairs, Boyer et al.19 developed a hybrid approachwhich combined the realistic crystalline phonon densityof states with the exact single-particle matrix elementsfor a Morse potential; selection rule effects were omittedentirely. Not surprisingly, their calculations revealedthat the smoothness of ao() for most alkali halides re-sulted from the smoothness in the one-phonon densityof states p(w). However, for various crystals such as KI,which display a distinct gap between the acoustic andoptic portions of p, noticeable structure is predicted ina(w), even at higher temperatures. These results couldbe anticipated on general grounds, since a involves asum of convolutions p,, which directly reflect the extentof structure in p. Moreover, the theory of convolutions

November 1977 / Vol. 16, No. 11 / APPLIED OPTICS 2883

0

zw

-ILLw00zo -20_

0C)m

0

K Mg F3

-4 F-

xA 12 03

x\x

X

,\ \

\ \\

\\

\ \ \\\\ \\ \

\ \ \ \\

\ coF2 \ MgF2

IMgo

2 3

- -2

FREQUENCY ( 103 l)

Fig. 1. Measured absorption coefficients vs frequency for typicalionic solids (from Ref. 9). Symbols indicate experimental data.

implies that structure in p tends to wash out progres-sively in p with increasing n.

The broadness of the density of states in ionic solidsis associated, at least in part, with the large anharmo-nicity characteristic of these solids. The anharmonicityalso induces broader features in a(o) at higher T byessentially broadening the anharmonic density of states.The convolution formalism also indicates an inherentsmoothing at high T, where p(-co) _ p(co). The lattereffects combine to suppress structure in a(co) at in-creasing T in the same manner as for lower order spec-tra.

What about the role of selection rules? The argu-ments regarding smoothing of Pn with increasing nsuggest that k selection will play a relatively minor rolefor large n. Indeed, the majority of the measuredspectra for ionic solids show little deviation from ex-ponential behavior for n 3 4. Nevertheless, since mostexperiments were performed at room temperature orabove on materials with broad p(c)'s, it was appropriateto extend the measurements to situations where theseconditions were not met. Harrington et al.

2 0 haveshown recently that structure is in fact clearly mani-fested in the multiphonon spectra of various alkali ha-lides, especially at low T. The theoretical work ofDuthler 2 ' explains the observation of peaks in frequencyregimes corresponding to odd numbers of optical pho-nons, and their absence for even numbers, on the basisof selection rules for the rock salt structure. The rulestems for a factor fn associated with an nth order pro-cess of the form

........................ I.*. .7=.. .

400 600 800TEMPERATURE (K)

Fig. 2. Absorption coefficient vs temperature for selected alkalihalides; isolated points indicate experimental data; curves are theo-

retical fits from Eq. (4) (Compiled from Barker et al. in Ref. 1).

fn = 1 + (-l)n+l cos2 E Pik,jk

where 'Pik is the relative phase of the atomic motion ina unit cell, for mode j and wave vector k. For rocksaltp 0° for optical phonons and so - 90° for acoustic

phonons throughout the zone. Since the most signifi-cant effect on a is due to the higher frequency opticalphonons, the selection rule dominating the spectrumis that associated with optical phonons, namely, fn 0 °for n even and fn 1 for n odd. The experimentalobservations do indeed indicate the existence of athree-phonon peak, coupled with the absence of two-or four-phonon peaks, for alkali halides which displaystructure in a(co). Results for NaI, for example, areindicated in Fig. 3. In this case as in others, there isqualitative but not quantitative agreement betweentheory and experiment. Nevertheless, the existence ofthe predicted selection rules appears to be well con-firmed by the experiments.

Although marked structure in the spectrum is limitedin the ionic case to just certain materials and sufficientlylow temperatures and is thus mostly of interest from afundamental standpoint, the structural features areindeed dominant for the case of most semiconductors.Recent experiments22 have indicated persistence ofstructure in a() down to lower absorption levels (<10-2 cm-') and higher temperatures (T > 300 K),especially among the more covalent of these materials.Typical data for the case of Si at and above room tem-perature are indicated in Fig. 4. A number of interre-


9

8

7

6

4

3

2

l

100

\ cco NAI

00a

0 0E 0

0

I- 0W a.

w * 00 u *~~~~~~~~~~ LiU

Li.. ~~~~~~~~~~~~~010 0 0

00

200 300 400 5~~000

0

ou29Kexeimn-2Teoy10`

FREQUENCY (cm-')

Fig. 3. Absorption coefficient vs frequency for NaI. Solid ointsindicate experimental data for two samples at 80 K, open points atroom temperature. Continuous curve is the theoretical prediction

at 80 K (from Ref. 20).

z owX

LL000

-1

a-

m

0Cn

-3 II.

Fig

SILICON

lated factors determine the spectral structure for suchmaterials. First, the phonon density of states for thesemiconductors tend to display a number of well-sep-arated peaks in contrast to fewer and broader peaks forthe case of ionic materials. Another factor is thesmallness of the anharmonic broadening, which enablesmany phonon features in the spectrum to remain dis-tinct at elevated temperatures. It should be remarkedthat, even in partially ionic semiconductors such asZnSe, one may already discern anharmonic broadeningof many phonon peaks at elevated T in the three-pho-non regime.

What about selection rules? A theory developed tointerpret the semiconductor data, including density-of-states effects but omitting selection rules, providesa good account of the structure in the multiphononspectrum of these materials.22 The observation thatselection rules play a secondary role for the semicon-ductors is consistent with Duthler's2 3 preliminaryfindings, which indicate that strong many phonon se-lection rules do not arise for diamond and zincblende-type crystals.

In summary, recent work has corroborated the earlierobservations of exponential-like behavior of the in-trinsic multiphonon absorption a() for most ionicmaterials over extended frequency and temperatureranges. For certain ionic materials, with gaps in thephonon density of states, noticeable structure is mani-fested in a(o) at sufficiently low temperature. More-over, in crystals with the rocksalt structure, well-definedselection rules dictate either the absence or appearanceof various multiphonon features. In the case of semi-conductors, the spectrum is dominated by structuralfeatures, even at high temperatures. Theoreticalanalysis of the data indicates that the structure stemsprincipally from the phonon density of states ratherthan from selection rules. At this time the question ofthe relative contribution to a(c,T) from linear andnonlinear moments remains unanswered. Calculationsperformed with and without nonlinear moments for thesemiconductors, e.g., do not indicate sufficient differ-ences in the predictions to resolve the issue. Despitethe lingering uncertainty on this issue, however, it seemsfair to say that the principal features of the multiphononspectra of most materials are now well understood.

Extrinsic Absorption440 K On the shorter wavelength side of the ir spectral re-

gion from 2 Am to 10 Am the residual absorption of all295 K40 insulating materials is limited by extrinsic processes.

On the longer wavelength side, the absorption of some410 K ir materials such as CaF2 and Si is dominated by in-

trinsic effects, while others such as KBr are completelydominated by extrinsic effects. As noted in the intro-duction, the choice of a particular ir material for a spe-cial application depends not only upon its absorption

I | | ' ' ' 9 coefficient but upon other properties as well. A dis-0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 cussion of this from the users point of view has been

FREQUENCY (103 cm'1) given recently. 2 4 The last comprehensive review of

4. Absorption coefficient vs frequency for Si at selected laser window materials was given by Deutsch2 5 coveringtemperatures. the situation up to 1973, with emphasis on window


materials for CO2 (10.6-gm) and CO (5.3-gm) lasers.2 5

A survey of window materials for HF chemical lasers(2.7 gim) and DF chemical lasers (3.8 gim) has beenpublished by Harrington et al. 26 This present reviewwill cover recent developments in uncovering the originsof the residual absorption in ir materials with a criticaldiscussion of the available data and the potential forfuture improvement.

In most cases, the distinction between intrinsic andextrinsic absorption can be readily made with the aidof the exponential law discussed in the previous sectionor by studying the temperature dependence of the ab-sorption.'9 However, the separation between the sep-aration of the absorption between an intrinsic and ex-trinsic part cannot always be easily made, as will bediscussed later in more detail. Greater difficulties areencountered in assigning the origins of the residualabsorption in the extrinsic region because of both ex-perimental difficulties and uncertainty in the originsthemselves. At the extremely low absorption levels of10-4 cm-' and below, that are now being reported in anumber of materials, not only do the measurementsthemselves become more difficult to undertake, butsurface absorption can contribute a large share of thetotal absorption. Consequently, the study of low-lossabsorption in many state-of-the-art materials becomesnot only a question of obtaining measurements them-selves but also whether such measurements are limitedin some way by the techniques. Several interrelatedproblems are involved, including measurement tech-niques, surface absorption, and bulk absorption. A fewremarks about each of these will be given before a de-tailed consideration of the residual absorption of variousclasses of materials is undertaken.

The measurement of low-loss absorption coefficientsin materials has been reviewed by Skolnik,27 and recentdevelopments are covered by Hordvik 2 8 in this issue.Basically, methods capable of high sensitivity, abilityto discriminate between surface and bulk effects, in-sensitivity to scattering and continuous high resolution,and broad spectral coverage are desired. Unfortu-nately, no single method fulfills all these requirements.Spectral emittance, differential methods, and Fourierspectroscopy provide broad spectral coverage but arelimited in discriminating between surface and bulk ef-fects as well as having sensitivity problems at low valuesof the absorption coefficients. Laser calorimetry29 andphotoacoustic techniques 3 0 have been developed in thepast few years so that adequate sensitivity and abilityto discriminate between surface and bulk effects cannow be carried out but are limited to laser wavelengthswhere sufficient power is available.

Surface absorption and the associated problems ofabsorption in thin film coatings are also of primaryimportance in characterizing high transparency ir ma-terials. Often AR or protective coatings are requiredfor the practical utilization of ir materials. While adiscussion of such surface and thin film absorption liesbeyond the scope of this review, it cannot be completelyignored. In addition, both bulk and surface absorptionbands at about the same wavelength are often present

when they are associated with a molecule, such as water,or arise from light single ions. Of the various tech-niques for establishing the characteristics of surfaceabsorption, internal reflection spectroscopy (IRS) isparticularly useful as broad wavelength coverage canbe obtained with moderate sensitivity. Typical IRSspectra show surface water absorption near 3 gim, CHhydrocarbon absorption near 3.3 gm and an additionalband near 6 gm, and bands near 9 gm associated withoxygen containing molecular complexes.25 Surfaceabsorption of this nature is common for a wide varietyof materials. The presence of surface absorption shouldat least alert the investigator to the possible presenceof such absorption bands in the bulk material or inter-fering with the bulk measurement. A recent develop-ment has been the analysis of IRS spectra to revealquantitative information about surface absorption.3 0

Similar progress has been also forthcoming using el-lipsometric methods.

Finally, a few comments about bulk absorption. Thismay arise from several sources such as substitutionalmolecular impurities, metallic inclusions, or carrierabsorption. Attention in this review will be focused onpossible absorption mechanisms in various classes ofmaterials.

The alkali halides, alkaline earth fluorides, and oxidesform an important class of ir materials. The principalsource of the residual bulk absorption in these materialsis believed largely associated with molecular ion sub-stitutional impurities, although other mechanisms suchas macroscopic inclusions have been considered. Oneway of approaching the problem in these insulatingcrystals is by studying the absorption at 1.06 gim, whichis at an energy several times the fundamental vibra-tional frequency of even the lightest hydrogen con-taining impurities. The absorption from molecularimpurities in reasonably good crystals, i.e., those havinglittle or no observable fundamental molecular complexabsorption, should be extremely low. In addition, aswill be mentioned later, the absorption associated withsmall metallic inclusions would be higher at this shortwavelength than further out in the ir. Consequently,absorption measurements at this wavelength should beindicative of crystals which might have even lower ab-sorption further out in the ir. The bulk absorptioncoefficients of a number of crystals studied by Davissonat 1.06 m is shown in Table I.3' It is interesting to notethat a few of these do show absorption levels of 10-5cm-' or below, and this represents a practical goal at thepresent time.

The absorption of crystals in the spectral region near2.7 m and 3.8 m has been studied because of theavailability of high power lasers in this region. Workon a variety of crystals has been reported by Harringtonet al.2 6 and by Hass et al.32 Results on the best crystalsin this ir region can be illustrated in Fig. 5 which showsmeasurements at 1.06 ,gm, 2.7 gim, 3.8 gim, and 5.3 gim,in which multiline lasers centered at the wavelengthsindicated have been employed. Note that bulk ab-sorption levels of 10-4 cm- seem to have been reportedat Alabama and also by Hordvik and Skolnik.9 At 5.3


Table I. Bulk Absorption Coefficients at 1.06 jim

AbsorptionDimensions Coefficients

Crystala Source (cm X cm X cm) (cm- 1 )

LiF Harshaw 1.3 X 1.3 X 10.3 6 X 10-5LiF Optovac 1.3 X 1.3 X 10.3 8 X 10-4NaF Optovac 1.3 X 1.3 X 9.8 5 X 10-4

NaF NRL #11 1.7 X 1.8 X 5.1 5 X 10-5NaF NRL-96D 1.3 X 1.3 X 8.4 3 X 10-5NaCl Harshaw 1.4 X 1.4 X 10.3 7 X 10-6KCl NRL B-219 2 X 2 X 7.7 7 X 10-6KBr NRL B-305 1.5 X 1.5 X 7.9 <3 X 10-6KBr NRL B-212 1.2 X 1.3 X 6 5 X 10-6CaF2 Harshaw 1.3 X 1.3 X 10.3 6 X 10-5CaF2 Optovac 1.3 X 1.3 X 10.3 1 X 10-5CaF2 Hughes 1.0 X 1.0 X 7.45 4 X 10-5SrF 2 Harshaw 1.3 X 1.3 X 10.3 1 X 10-5SrF 2 Optovac 1.3 X 1.3 X 9.9 7 X 10-5BaF 2 Harshaw 1.3 X 1.3 X 10.3 3 X 10-5BaF 2 Optovac 1.3 X 1.3 X 10.3 1 X 10-5

a The chlorides and bromides were chemically polished in HCl andHBr, respectively. NaF was chemically polished in dilute Mn(NO3 )2+ HF solution. The alkaline earth fluorides and LiF were mechan-ically polished by rubbing.till dry on Pellon Pan-W pad using LindeC with water as lubricant. The Hughes CaF2 crystal was press-forged86% at 9000 C.

EI-S10z

w-4LIO

00z0I-

0*ni:n.4

i 2 3 4 5 6

WAVELENGTH (,m)

Fig. 5. Absorption coefficient of KCl and BaF2 at various wave-lengths as determined by laser calorimetry. 3 2 Very recent mea-surements on other crystals show the absorption band near 3-4 Jm

reduced in magnitude or even undetectable.

gim, the Hordvik and Skolnik measurements are also inthe range of 10-4 CM-1. However, there have been twocrystals measured by Deutsch 2 5 and one crystal of KCImeasured by Allen and Rudisill 3 3 which have levels inthe range of 10-5 cm1 or comparable to those found at1.06 gim.

Crystals of high quality, such as those studied in Fig.1, had approximately equal absorption coefficients at2.7 gim and 3.8 gAm, and this is difficult to explain on thebasis of an OH band near 3 gum. However, very recentmeasurements at Alabama34 on a Hughes KCl crystalhave shown that levels at least as low as 5 X 10-5 cm-1can be obtained at 3.8 gim, and new measurements atNRL at 2.7 gm on CaF 2 and BaF 2 have indicated ab-sorption levels of 10-5 cm-1 may have been achieved.3 5

Consequently, the situation regarding the apparentabsorption band implied in Fig. 5 may be altered byfuture work. In the case of the oxides such as MgO andyttrium oxide glasses, the presence of an OH band in the3-gm region is consistent with the measured absorptioncoefficients.2 6 In the case of A1203, an absorption levelof about 10-4 cm-' has been reported in this volume andis quite encouraging. 9

In the region near 5.3 gim, multiline CO lasers areavailable, and a number of measurements have beenreported. One problem in this spectral region is thata multiline output might predominate at longer wave-lengths under some operating conditions of the laser,and contributions from an absorption band near 6 gimmight be present. Most investigators have found ab-sorption levels in the region of 10-4 cMi 1 or above, ex-cept that a few crystals have shown levels of 10-5 CM-1or below, and these represent an important case of whatmay be achievable. An absorption band near 6 gim isseen on surfaces and crystals grown in air. This is closeto the deformation band wavelength of water. At 5.3gim, the intrinsic absorption for CaF 2 is about 2 X 10-4

cm-1 , and this has been observed. In the case of SrF2the intrinsic absorption is about 2 X 10-5 cm-1 , and thisis being approached. 2 5

The spectral region near 10.6 ,m has been studiedintensively with fixed wavelength and tunable CO2 la-sers.3 6-38 The absorption of KC1 on this region is par-ticularly complex because, in the better crystals, in-trinsic multiphonon, bulk extrinsic impurity,39 andsurface contributions can be of comparable magnitude.The spectrum in this region as obtained, using tunablelaser calorimetry, is shown in Fig. 6 along with the ex-pected intrinsic multiphonon absorption. The bulkabsorption at 10.6 gm has been found to be very closeto the expected intrinsic limit of 7 X 10-5 cmn-1 .3 6

,37

Surface absorption is quite large. Another way of dis-tinguishing between intrinsic and extrinsic effects is bystudying the temperature dependence of the absorptionas indicated in Fig. 7.19 Here it can be seen that theabsorption in the poorer crystals is essentially temper-ature independent, while those in good crystals show amuch larger temperature dependence characteristic ofintrinsic multiphonon absorption. This has beenstudied in greater detail by Rowe and Harrington. 4 0 Inall these measurements, surface effects are important,


0.0008 E 0Z 0.0006 0 _Ii 0 0.00022 - \ 0O 0 0 coU- w0U 0.0004z0

In ~~0 000

0.0002 0 0 000

0.0001 10.8 10.4 10.0 9.6

WAVE LENGTH (microns)

Fig. 6. Effective absorption coefficient for a pure crystal of KCI.3 6

Surface absorption has not been subtracted out. The solid line de-notes the expected bulk intrinsic multiphonon absorption. The band

near 9,um is largely associated with surface effects.

although they can be reduced by chemical pol-ishing.4 '

The II-VI and III-V compounds such as CdTe, ZnSe,and GaAs have been of interest because of low absorp-tion at 10.6 gm and good mechanical properties andresistance to environmental degradation. The ab-sorption of the better samples of ZnSe is close to theintrinsic limit of 2 X 10-4 at 10.6 ,gm. However, thereis still difficulty in establishing the true experimentalvalue of the bulk absorption coefficient due to thepresence of a strong surface component.42

Infrared measurements on some of the crystals in-dicate bulk absorption bands, especially in the 6-gimregion.43 The wings of these bands may contribute tothe observed absorption in the CO laser region at 5.3 gm.Away from any of these resonances, the absorption levelis still 10-4 cm-' or above, and the reason for this hasnot yet been established.

In the case of CdTe, attention has been focused at10.6 ,gm. For this material, metallic inclusions havebeen observed4 4 ,45 whose size is of the order of 50 A.The residual absorption has been attributed to suchmetallic inclusions, and a simple estimate of the ab-sorption expected is in accord with the experimentalresults.4 5 However, a characteristic wavelength de-pendence is also expected (absorption coefficient 1/X2for spheres),46 but this has not been studied in detail.In other crystals of CdTe, carrier absorption is believedto be the limiting factor.

Of the III-V materials which are useful as ir windowmaterials, GaAs is notable. The absorption near 10.6Am in the best materials is above the intrinsic limit.Above room temperature, the absorption at 10.6 gim canincrease sharply, which is characteristic of excitationof carriers to a level within the gap.47 Absorption hasalso been observed due to impurities such as Fe and Crat shorter wavelengths.25

The group IV elements, C (diamond), Ge, and Si havebeen employed as ir window materials for some time.Since these are not ionic crystals, there is no irreststrahlen absorption, but there is intrinsic multi-phonon absorption, impurity effects, and carrier effects.

0.005

0.002

E

0

I-zF 0.001

0z0 0.0005

0U,M

0.0001

300 500 700TEMPERATURE ( K)

Fig. 7. Temperature dependence of the absorption coefficient ofvarious crystals of KCI at 10.6 ,um.19 The purer crystal shows thelowest absorption coefficient and a temperature dependence expectedfor an origin associated with intrinsic multiphonon effects (as indi-cated by the solid lines for two different assumptions). The othercrystals display extrinsic absorption with a less well marked tem-perature dependence. The impure crystals were grown some yearsago and are not representative of the best crystals now available from

these sources.


5

_ 0.2zw

w 0.1.80z005cr

0.02

0.010 10 20 30 40

RESISTIVITY (ohn-cm)

Fig. 8. Absorption coefficient of Ge at 10.6 rim.represents a best fit calculation by Bishop and Gibspoints are experimental measurements by Capron an

lightly n-doped samples have the lowest absorption

Diamond has long been employed as broadband win-dows for detector materials although this is some mul-tiphonon and impurity absorption. Germanium andSi have also been widely employed in ir optics at wave-lengths longer than that corresponding to theirbandgap. At 10.6 gim, the absorption in Si is limited bymultiphonon and oxygen impurity effects. At shorterwavelengths, new measurements indicate that multi-phonon behavior is evident down to about 4 gim at alevel in the low 10-4 -cm-1 range.9 At still shorterwavelengths, the absorption coefficient of Si is ap-proximately constant. Very pure samples of Si can beobtained so that carrier absorption need not be observedat room temperature. The situation in Ge is different.Germanium still is a good medium-loss window near10.6 grm,48 although the absorption through the 3-10-gmregion can be primarily determined by the carrier con-centration. The absorption coefficient of Ge in the3-10-gm region can be expressed as25'49 a(T) = alatt +

NAe + PAh, where alatt is a term involving lattice, sur-face, and other contributions not arising from bulkcarriers. Here N and P are electron and hole concen-trations, with Ae and Ah being the electron and holecross sections. The electron cross section arises fromfree carrier absorption, while the hole cross sectionstems from intravalence band transitions. In the

spectral region under consideration, the hole crosssection is much greater than the electron cross section.Since the product of the hole and electron concentra-tions is a constant for any temperature, the absorptionactually can be decreased by reducing the hole con-centration, and this can be accomplished by n- (elec-tron) doping. The results of such a procedure are il-lustrated in Fig. 8 in which it can be seen that the ab-sorption at 10.6 gm of Ge can be reduced by n-dopingand that the absorption coefficient as a function ofcarrier concentration can be described fairly well by theusual semiconductor statistics for electron and holeconcentrations.49 In addition to the variation withcarrier concentration, a characteristic temperaturedependence is expected, and this is currently beingstudied.50 Similar considerations hold for Si, but be-

+ + cause of the greater bandgap, excitation of carriersacross the gap does not occur to any significant extentat room temperature.

Finally, it can be said that residual extrinsic absorp-tion can be well understood in some materials such asGe over the ir region from 2 gim to 10 gim. In othermaterials, part of the extrinsic absorption can be ac-counted for although not studied in detail. Perhapsmost challenging is the question of residual absorption

50 60 on the short wavelength side of this region. All insu-lating materials are dominated by extrinsic effects in

The solid line this region, although there are indications that ex-on,

49 while the tremely low absorption coefficients in the 10-6 cm-1 ord Brill.

48 The below may have already been obtained, at least for a few

n coefficient. crystals.

Concluding Remarks

The sources of residual absorption in ir materials inthe 2-10-gm region have been considered. This ab-sorption can be divided into an intrinsic part associatedwith multiphonon effects and an extrinsic part associ-ated with impurities, defects, and surfaces. The fre-quency and temperature dependence of intrinsic mul-tiphonon absorption can be accounted for in a varietyof materials, although there are some fundamentalquestions still remaining. The origins of the extrinsicportion can be accounted for, in many cases, althoughdetection of the true lower limit may be prevented incertain instances by current measurement tech-niques.

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Transparent Solids ( (Plenum, New York, 1975).2. A. R. Hilton, J. Electron. Mater. 2, 211 (1973).3. B. Bendow, J. Electron. Mater. 3, 101 (1974).4. B. Bendow, in Solid State Physics, H. Ehrenreich, F. Seitz, and

D. Turnbull, Eds. (Academic, New York, 1976).5. B. Bendow, in Annual Reviews of Materials Science, R. A.

Huggins, Ed. (Annual Reviews, Palo Alto, 1977).6. B. Bendow, Appl. Phys. Lett. 23, 133 (1973).7. T. F. Deutsch, J. Phys. Chem. Solids 34, 2091 (1973).


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B 6, 2614 (1976).11. B. Bendow, S. P. Yukon, and S. C. Ying, Phys. Rev. B 10, 2286

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CALL FOR PAPERSfor the

THIRD NASA CONFERENCEon

RADIATION ENERGY CONVERSION

The Third NASA Conference on Radiation Energy Conversion will beheld on 26, 27 and 28 January 1978, at NASA Ames Research Center,Moffett Field, California.

Prior conferences in this series have restricted attention to possiblemeans of efficient conversion of laser radiation to electrical or othertypes of useful work as would be needed for space power transmission.This third conference on energy conversion expands the area of cov-erage to include those space-related topics which are receiving in-creased consideration by NASA in the Space Shuttle era. The adventof large area structures, such as solar concentrators, in the weight-lessness of space, replenishment of expendables, and extended periodsof man-in-space activity, allows serious consideration of many novelpossibilities. These include the use of solar energy for terrestrial powerneeds, for pumping of lasers, and, perhaps in conjunction with coherentradiation, the enhancement of chemical and other processes usefulin space. The conference will provide a forum for the considerationof such novel space activities and, where applicable the presentationof research results. Papers are solicited in the areas of:Laser Energy Conversion

Techniques have evolved for achieving reasonably efficient con-version of high average power coherent radiation which would beused In laser power transmissions. Research results on these, aswell as novel concepts, will be covered.

Space Power Alternatives for Space and Terrestrial UseExtensive studies have been made on the Satellite Solar PowerStation as a means to use the unique possibilities afforded byspace to provide terrestrial energy. Many alternative ways to ac-complish this goal appear reasonable and will be explored.

Radiation-Enhanced Chemistry and ProcessesThe availability of direct or concentrated solar radiation, alone or Inconjunction with laser radiation, may allow the alteration of reac-tion rates, efficiencies, etc. of chemical or other processes whichare important to space usage and habitation. This mode of radia-tion energy conversion will be discussed.

Radiation Pumped LasersThe Importance of a space viable laser to the above areas Is un-questionable. Concentrated solar, and other, radiation may affordthe incoherent energy necessary to attain inversion.Specific concepts in the above, and other related topics that would

uniquely benefit from construction, assembly, or operation in space,should be submitted in a 500-word summary. Only unclassified papersare acceptable. Camera-ready full length papers, complete with fig-ures, will be due 15 January 1978. These will be incorporated into aNASA report documenting the conference proceedings. Summariesshould be sent to Kenneth W. Billman, NASA Ames Research Center,Mail Stop 230-3, Moffett Field, California, 94035 (telephone: 415-965-5233).


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Residual absorption in infrared materials