Optical properties of semiconductor-metal composite thin films in the infrared region C. L. Nagendra and James L. Lamb Germanium:silver 1Ge:Ag2 composite thin films having different concentrations of Ag, ranging from 7% to 40%, have been prepared by dc cosputtering of Ge and Ag. The films’ surface morphology and optical properties have been characterized using transmission electron microscopy and infrared spectrophotom- etry. It is seen that, although the films that contain lower concentrations of Ag have islandlike morphology 1i.e., Ag particles distributed in a Ge matrix2, the higher metallic concentration films tend to have a symmetric distribution of Ag and Ge. The optical constants 1i.e., refractive index n and absorption index k2 derived from the measured optical properties show a semiconductor behavior even as high as 40% of Ag concentration, beyond which the metallic properties dominate over the entire infrared spectrum. Comparison of the n and k data with the two well-known effective medium theories, namely, the Maxwell-Garnett theory and the Bruggeman theory, shows that both theories have limited scope in predicting the optical properties of semiconductor–metal composite films in the infrared region. However, an empirical polynomial equation can simulate the experimental data at all wave numbers of the IR spectrum. Key words: Optical, dielectric, inhomogeneous, infrared, effective mean field theory, composite films, germanium, silver. 1. Introduction Present-day scientific and technological develop- ments demand the use of novel optical materials that exhibit unique optical properties not observed in conventional materials. Investigations have focused on developing either new optical materials or efficient ways of tailoring the properties of existing materials within the scope of the available preparation tech- niques. The latter approach is often simpler and easier to implement. To this end, thin-film deposi- tion techniques, namely, evaporation, 1,2 ion-beam- assisted depositions, 3 and sputtering, 4 have been uti- lized to prepare composite@inhomogeneous dielectric thin films successfully. 1–4 The resulting thin films with unique optical properties have found applica- tions in optical thin-film devices. 1,2,5 Similarly, dielec- trics have been codeposited with different metals that lead to a whole family of Cermet films 4,6,7 with applications in solar energy conversion devices. 8,9 Both dielectric–dielectric and metal–dielectric compos- ite films have been widely investigated with emphasis on the optical properties in the solar and near- infrared spectral regions. However, to the best of our knowledge, except for a few remarks, 8 there are neither any studies on the optical properties of these materials in the middle and far-infrared regions, nor any attempt to determine the possible combinations of composite@inhomogeneous materials for the infra- red region. Nevertheless, a growing use of the IR spectrum for various scientific and technological mis- sions, either in space or on the ground, demands the use of such novel materials. In view of this our investigation has been organized to study the infrared optical properties of composite thin films systemati- cally. In this paper, starting with a brief description of practical combinations of composite optical materials in the IR region, we present experimental results of the preparation and characterization of a few specific composite thin films. The scope of the phenomeno- logical and empirical theories to account for the experimental results is also discussed. When this research was performed both authors were with the Center for Space Microelectronics Technology, Jet Propulsion Labo- ratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, California 91109. The permanent address for C. L. Nagendra is the Indian Space Research Organization, Satellite Center, Airport Road, Vimanapura, Bangalore 560017, India. Received 10 January 1994; revised manuscript received 30 June 1994 0003-6935@95@193702-09$06.00@0. r 1995 Optical Society of America. 3702 APPLIED OPTICS @ Vol. 34, No. 19 @ 1 July 1995
Transcript
Optical properties of semiconductor-metalcomposite thin films in the infrared region
C. L. Nagendra and James L. Lamb
Germanium:silver 1Ge:Ag2 composite thin films having different concentrations of Ag, ranging from 7% to40%, have been prepared by dc cosputtering of Ge and Ag. The films’ surface morphology and opticalproperties have been characterized using transmission electron microscopy and infrared spectrophotom-etry. It is seen that, although the films that contain lower concentrations of Ag have islandlikemorphology 1i.e., Ag particles distributed in a Ge matrix2, the higher metallic concentration films tend tohave a symmetric distribution ofAg andGe. The optical constants 1i.e., refractive index n and absorptionindex k2 derived from themeasured optical properties show a semiconductor behavior even as high as 40%of Ag concentration, beyond which the metallic properties dominate over the entire infrared spectrum.Comparison of the n and k data with the two well-known effective medium theories, namely, theMaxwell-Garnett theory and the Bruggeman theory, shows that both theories have limited scope inpredicting the optical properties of semiconductor–metal composite films in the infrared region.However, an empirical polynomial equation can simulate the experimental data at all wave numbers ofthe IR spectrum.Key words: Optical, dielectric, inhomogeneous, infrared, effective mean field theory, composite films,
germanium, silver.
1. Introduction
Present-day scientific and technological develop-ments demand the use of novel optical materials thatexhibit unique optical properties not observed inconventional materials. Investigations have focusedon developing either new optical materials or efficientways of tailoring the properties of existing materialswithin the scope of the available preparation tech-niques. The latter approach is often simpler andeasier to implement. To this end, thin-film deposi-tion techniques, namely, evaporation,1,2 ion-beam-assisted depositions,3 and sputtering,4 have been uti-lized to prepare composite@inhomogeneous dielectricthin films successfully.1–4 The resulting thin filmswith unique optical properties have found applica-
When this research was performed both authors were with theCenter for Space Microelectronics Technology, Jet Propulsion Labo-ratory, California Institute of Technology, 4800 Oak Grove Drive,Pasadena, California 91109. The permanent address for C. L.Nagendra is the Indian Space Research Organization, SatelliteCenter, Airport Road, Vimanapura, Bangalore 560017, India.Received 10 January 1994; revised manuscript received 30 June
tions in optical thin-film devices.1,2,5 Similarly, dielec-trics have been codeposited with different metals thatlead to a whole family of Cermet films4,6,7 withapplications in solar energy conversion devices.8,9Both dielectric–dielectric andmetal–dielectric compos-ite films have been widely investigated with emphasison the optical properties in the solar and near-infrared spectral regions. However, to the best of ourknowledge, except for a few remarks,8 there areneither any studies on the optical properties of thesematerials in the middle and far-infrared regions, norany attempt to determine the possible combinationsof composite@inhomogeneous materials for the infra-red region. Nevertheless, a growing use of the IRspectrum for various scientific and technological mis-sions, either in space or on the ground, demands theuse of such novel materials. In view of this ourinvestigation has been organized to study the infraredoptical properties of composite thin films systemati-cally.In this paper, starting with a brief description of
practical combinations of composite optical materialsin the IR region, we present experimental results ofthe preparation and characterization of a few specificcomposite thin films. The scope of the phenomeno-logical and empirical theories to account for theexperimental results is also discussed.
2. Infrared Composite Optical Thin Films
From a critical review of the optical properties of IRmaterials,10–12 it can be observed that there is apaucity of optical materials for the IR region, whichare not only good absorbers but alsomedium reflectors.Such interesting optical properties can be expectedfrom semiconductor–metal composite materials forwhich many possible combinations exist. Examplesinclude Ge:Ag, Ge:Au, Si:Au, and Si:Ag. Thesemate-rials can be used for applications as broadband infra-red absorbers in optical, opto-electronic, and thermalcontrol systems. In the present investigation, thepreparation and characterization of the Ge:Ag combi-nation in thin-film form are discussed.
3. Preparation
Composite films of Ge:Ag having uniform concentra-tions ofAg that vary from 7% to 40%were prepared bydc magnetron cosputtering of Ge and Ag. The sput-tering process utilized a 50-mm-diameter planar99.999% pure Ag target mounted on a dc sputter gunmanufactured by U.S., Inc., and a two-piece 38-mm-diameter doughnut-shaped 99.9999% pure Ge targetmounted on a research S-gun manufactured by Sput-tered Films, Inc. The targets 1from InternationalAdvanced Materials, Suffern, N.Y.2 were oriented at a45° angle to a rotating substrate plane to achieve ahigh degree of uniformity over the substrate geometry.Uncooled substrates were mounted at a distance of150 mm from the targets. The sputtering chamberwas pumped by a turbomolecular pump to a basepressure of less than 1024 Pa and backfilled withargon.The concentrations of Ag and Ge were determined
by the ratios of the sputtering yield rates ofAg and Geunder a given set of experimental conditions, namely,argon flow rate, input power, and chamber pressure.The sputtering@deposition rates of Ag and Ge werearrived at by measurement of the thicknesses of filmsdeposited over a known amount of time. The thick-nesses were measured using a Tencor Alpha Talystep250 profilometer with a measurement accuracy ofbetter than 0.5 nm. Thus the sputtering setup wascalibrated for different sputtering rates against inputpower. The nominal deposition rates under 4.4 Pa ofargon pressure with 5 SCCM 1SCCM denotes cubiccentimeter per minute at STP2 of argon flow and 1 Wof power applied were 0.12 nm@min for Ge and 0.5nm@min for Ag. One can obtain the desired metallicconcentration@volume fractions by monitoring andcontrolling the input powers to the Ge and Ag targetsto obtain the desired proportions. Prior temperaturemeasurements indicate that the uncooled substrateswill equilibrate at #90 °C.For characterization of IR optical properties, films
deposited on CdTe and soda-lime glass were used withnominal thicknesses of 0.5 µm deposited. The filmssputtered on thin soda-lime glass microslide coverslips were used for TEM and stress pattern observa-
tions. It should be noted that in all the films aneutral stress state was observed, indicating that thefilms are mechanically stable.
4. Characterization
The surface characterization of the thin films of Ge:Agwas carried out using transmission electron micros-copy 1TEM2. The rear sides of the coated thin micro-slide samples were ground to reduce the substratethickness to a few micrometers after which they wereetched in an argon ion-beam milling system at 5 kVand 0.5 mA, with a 12° incident angle, to achieveelectron transparency in a freestanding film. Themicrosamples were then transferred to a substrateholder for microscopic observation utilizing a Topcon002B transmission electron microscope.The optical reflectance and transmittance of the
samples were characterized using a Beckmann IRspectrophotometer,Model 4800. The instrumentwasa priori calibrated for 100% transmittance and forreflectance against a standard Ge sample in thereflectance mode before carrying out transmittanceand reflectance measurements of the samples. Formeasurements of reflectance, a 10° specular reflec-tance accessory was made use of. From the mea-sured reflectance and transmittance, one can deducethe optical constants, i.e., refractive index n andabsorption index k, by using an inverse method ofsynthesis13,14 in which the R and T equations thatconnect the optical constants of the films to themeasured optical properties, the thickness of the filmand substrate optical constants, are solved by anumerical iteration technique. This procedure hasbeen applied to determine the optical constants offilms having metallic concentrations as high as 25%.For films having 40% metallic content with little
transmission, a reflectance measurement-based spec-trophotometric techniquewas adapted.15 In this tech-nique the measured reflectances R from the virginfilm andR8 from the film deposited with a transparentlayer are utilized,15,16 and the optical constants areevaluated.In these studies ZnS deposited by thermal evapora-
tion was used as the transparent layer over theopaque virgin film of interest. The refractive index ofthe ZnS layer is determined a priori over the IRspectrum from 4000 to 700 cm21, and then one canevaluate the optical constants of the film using thereflectance measurement-based spectrophotometrictechnique. To determine n and k from 700 to 320cm21, we carried out theoretical simulation to1R 2 RTh22 # 1025. RTh, the computed value ofreflectance, for preset values of n and k is obtainedfrom the equation
RTh 531n 2 n022 1 k24
31n 1 n022 1 k24, 112
where n and k are the optical constants of opaque filmand n0 is the refractive index of the medium.
One can obtain the experimental n and k data for agiven composite film by characterizing three filmsprepared under identical experimental conditions,evaluating the n and k data by the appropriatetechniques for each, and then averaging the results.In all cases, the scatter in n and k over the threesets of data is within 0.05 throughout the IR spec-trum.
The transmission electron micrographs of Ge:Ag com-posite films and the associated diffractograms areshown in Figs. 11a2–11c2. The calculated ratios of theobserved d spacings in all the diffraction patternsagree with the standard d-spacing ratios of Ag.
Fig. 1. Transmission electronmicrographs and the correspondingdiffraction patterns 1shown in the insets2 for Ge:Ag compositefilms: 1a2 13%Ag, 1b2 25%Ag, 1c2 40%Ag.
Micrographs of films having higher concentrations ofAg 125% and 40%2 exhibit d-spacing images with a fewcrystalline defects such as stacking faults. On theother hand, the Ge phase exhibits no crystallinefeatures, which conformswith earlier investigations.17These observations clearly indicate that Ag crystal-lites are distributed in an amorphous Ge matrix.It is also obvious from the micrographs that theparticle size and density of Ag increase as the concen-tration of Ag is increased. At lower concentration113%2, the particle size is as low as 20 Å, whereas theyare in the range of 100–150 and 200–300 Å for filmshaving 25% and 40% concentrations of Ag, respec-tively.It can be observed that, at lower Ag content, Ag
particles are sparsely distributed in the Ge matrix.This is similar to the morphology observed forNi@Al2O3 composites by Craighead and Buhrman.6However, at higher volume fractions of Ag there is atendency for a symmetric distribution of Ag and Ge,similar to those of Au@Al2O3 and Ag@MgO compositefilms.7 In light of these observations, it may beobvious to anticipate that the Maxwell-Garnetttheory18 should reasonably account for the opticalproperties of the Ge:Ag composite films having lowerconcentrations ofAg, and the effective medium theorythat is due to Bruggeman,19 which is based on asymmetrical distribution of the components of thecomposite materials, should adequately describe theoptical properties of the composite films having higherconcentrations of Ag 125% and 40%2. These are dis-cussed in detail in Subsection 5.B.
B. Optical Properties
The results of the optical constants n and k derivedfrom the measured optical properties are presented inFigs. 2, 3, 4, and 5 for films having 7%, 13%, 25%, and40% volume fractions of Ag, respectively. For thesake of convenience of presentation, these films arerepresented as F1, F2, F3, and F4. The measuredoptical properties R and T are also shown in Figs.21a2–51a2 for comparison. It can be seen from thefigures that the addition ofAg results in an increase ofn and k. The addition of even 7% ofAg is sufficient toincrease n by 15% and allow more than 30% absorp-tion. This property can be exploited to producehigh-index Ge films with controlled absorption thatcan be utilized in many applications such as neutraldensity filters, beam splitters, and semitransparenthigh reflecting layers in Fabry–Perot etalons.At lower metallic concentrations@volume fractions
1F1 and F22, the optical constants exhibit a weakdispersive behavior whereas films having higher vol-ume fractions 1F3 and F42 are highly dispersive in thespectral region below 1000 cm21. In addition to this,a resonance behavior centered between 700 and 500cm21 is also observed in F3 films and becomes morepronounced at higher volume fractions. It is evidentfrom Figs. 3 and 4 that the films with a Ag metalliccontent as high as 25% still retain the semiconductingbehavior 1n . k2, but with enhanced absorption.
On the other hand, F4 films 140% of Ag2 show somemetallic behavior. The characteristic feature of me-tallic optical properties 1i.e., k . n2 is seen below1000 cm21. Examination of the optical properties ofother families of composite thin films, namely,Ni@Al2O3, Au@Al2O3, Au@MgO, and Ag@Al2O3,7 indi-cates that such a feature is rarely observed except in
Fig. 2. Optical properties of Ge:Ag composite film with 7%Ag: 1a2measured reflectance 1R2 and transmittance 1T2, 1b2 compari-son of the experimentally deduced n and k with those from theMGT, 1c2 comparison of the experimentally deduced n and k withthose from the EMT.
Fig. 3. Optical properties of Ge:Ag composite film with 13%Ag: 1a2measured reflectance 1R2 and transmittance 1T2, 1b2 comparison ofthe experimentally deduced n and k with those from the MGT, 1c2comparison of the experimentally deduced n and k with those fromthe EMT.
Fig. 4. Optical properties of Ge:Ag composite film with 25%Ag: 1a2measured reflectance 1R2 and transmittance 1T2, 1b2 comparison ofthe experimentally deduced n and k with those from the MGT, 1c2comparison of the experimentally deduced n and k with those fromthe EMT.
the case of Ag@Al2O3 having 60% of Ag. Thus, theexhibition of metalliclike optical properties at me-dium metallic concentrations may be unique to thefamily of semiconductor–metal composite systems.In order to study the scope of the effective medium
theories to explain the experimentally observed opti-
cal properties, we carried out theoretical calculationsutilizing the associated equations from the Maxwell-Garnett theory 1MGT218 and the effective mediumtheory 1EMT2 of Bruggeman.19 The deduction of nand k in the case of the MGT is straightforward.20However, in the Bruggeman theory it involves either
the solution of a quadratic equation for the averagedielectric constant 7e8,21 or a simultaneous nonlinearequation that connects the real and imaginary partsof 7e8 to the dielectric functions of the constituents.The simultaneous nonlinear equation can be solved
Fig. 5. Optical properties of Ge:Ag composite film with 40%Ag: 1a2measured reflectance 1R2 and transmittance 1T2, 1b2 comparison ofthe experimentally deduced n and k with those from the MGT, 1c2comparison of the experimentally deduced n and k with those fromthe EMT.
by the Newton–Raphson iterative method,22 which isunambiguous and straightforward. Adapting the lat-ter approach, the real and imaginary parts of thecomposite dielectric function were derived fromwhichthe n and k are evaluated. The Ge optical constantswere set to 4.0 1 j0.0 over the entire IR spectrum14000–320 cm212 whereas one can evaluate the dielec-tric constant of Ag by use of a Drude dielectricfunction,23
e 5 2vp
2
v1v 1 jvt2, 122
where vp is the plasma frequency and vt is the inverseof the relaxation time.
vp is set equal to 1.25 3 104 cm21,23 and vt is givenby vt 5 vf@2pcr, where vf is the Fermi velocity ofelectrons in Ag, c is the velocity of light, and r is theradius of the Ag particles in the composite film.Many investigators24–26 have recognized the impor-tance of defining vt in terms of vf and r so that theeffect of a reduced electron mean-free path that is dueto limited particle size is taken into account. Inthese calculations, r is obtained from the TEM analy-sis and vf is set equal to 1.39 3 108 [email protected] results of the theoretical calculations are pre-
sented in the 1b2 and 1c2 components of Figs. 2–5 alongwith those of the experiments for easy comparison.From these figures it can be generally noted that,regardless of the nature of the composite film, eitherthe MGT or the EMT 1with one exception2 can be usedto predict that the films have lower and lower absorp-tion as the wave number decreases and they becomenearly transparent 1k , 0.052 at the far-infraredregion, whereas experimentally the reverse is thecase. The exception occurs for F4 films for which theEMT can be used to predict an increase in n and kvalues with a decrease of wave number, which is inagreement with the experiment. The experimen-tally observed transition from semiconductor to metalin F4 films is also supported by the EMT results, eventhough the transition region and the optical con-stants do not match the experimental results quanti-tatively. Although the experimental results showthe predicted transition occurring at 1250 cm21, thetheory shows a broad transition region that spreadsfrom 3000 to 1250 cm21. It is important to add thatthe MGT does not indicate any such features. Thesuccess of the EMT in explaining some of the fea-tures, at least qualitatively, may be attributed to thefact that the EMT is not prejudiced toward a systemof any particular composition and takes into account,in a mean field way, the interactions between therandomly dispersed constituent particles. On theother hand, the MGT is confined to situations inwhich one component predominates in concentrationover the other, making the theory inapplicable to F4films. From the TEM studies presented in Subsec-tion 5.A, it can be recalled that the F4 film exhibits a
competitive morphology on which the EMT is devel-oped.A fair degree of agreement between theory and
experiment can be observed in F1 films 17% concentra-tion of Ag2, both in the case of the MGT and the EMT.The discrepancy is limited to only the far-infraredregion in which the films are more absorbing inpractice. The agreement between theory and experi-ment is none too satisfactory in F3 films, whereas thedegree of agreement in F2 films is between that of F1and F3. From the theoretical calculations and theircomparison with experiments, it can be summarized
Fig. 6. Variation of 1a2 refractive index n and 1b2 absorption index kof Ge:Ag composite films with respect to metallic volume fraction,at different wave numbers, as calculated from a fourth degreepolynomial equation.
that the two inhomogeneous theories, MGT and EMT,are not highly successful in explaining the featuresthat are observed experimentally. This may not betotally unexpected in the context of the results of theoptical properties of composite films in the visible andinfrared regions.6,7 From these studies it can begenerally observed that for Au@Al2O3 and Ni@Al2O3composite films having medium metallic concentra-tions 114–20%2, k values predicted either by the MGTor the EMT are lower than the experimental values inthe near-infrared region 110,000–3330 cm212, whereasat higher volume fractions 126% in Au@Al2O3 andNi@Al2O32, the n and k predicted by the EMT arehigher than the experimental values. These discrep-ancies can also be seen in the present theoreticalresults.In order to explain all the observed experimental
results, a comprehensive approach is required inwhich there should be scope to consider the contribu-tions not only from the individual components butalso from the interactions between the metal and thesemiconductor materials. The mutual semiconduc-tor–metal interaction may be responsible for thedispersion of the optical constants and the resonancebehavior that is predominantly seen in higher metal-lic concentration films and at the far-infrared region.In the absence of an appropriate theoretical scheme
to determine the optical constants of binary semicon-ductor–metal composite systems precisely, an at-tempt has been made to determine the theoreticalform of the equation that can yield the n and k valuesfor any given volume fraction of metal at any wavenumber. For this, a polynomial equation of thefollowing type is assumed:
1nk2 5 oi51
m
1Pi
Qi2 Fi21, 132
where Pi and Qi are the polynomial coefficients and Fis the volume fraction@concentration of the metal inthe composite thin film.Using the experimental results of n and k at a given
wave number and for different volume fractions, wedetermined polynomial coefficients by which the poly-nomial equation would give the best fit to the experi-mental results. For this purpose, a computer pro-gram based on the Vandermonde matrix method wasutilized.28 Polynomial coefficients were determinedfor all wave numbers starting from 4000 to 320 cm21.
It is seen that a fourth degree polynomial equation1m 5 52 can satisfactorily account for the experimen-tally determined n and k data. Typical polynomialcoefficients that correspond to different wave num-bers are listed in Table 1 and the associated polyno-mial curves for n and k are presented in Figs. 61a2 and61b2, respectively. This approach has been used tosimulate the n and k of graded-index semiconductor–metal composite films and to study the optical proper-ties of graded-index composite films. These resultsare presented in detail elsewhere.29
6. Conclusions
Germanium:silver composite thin films have beensuccessfully prepared by simple dcmagnetron sputter-ing. The experimental studies indicate that Ge incombination with Ag produces films having a widerange of optical characteristics starting from lowreflectance@absorptance to high reflectance@absorp-tance that can be controlled by themetallic concentra-tion. These films may have interesting applicationssuch as selective infrared reflectors and high effi-ciency infrared absorbers.The semiconductor–metal composite films give scope
to study themechanism of transition from semiconduc-tor to metal. This type of transition has been ob-served in relation to optical properties at the infraredin our investigation.The available inhomogeneous theories 1EMT and
MGT2 have limited scope to explain the observedfeatures of the optical characteristics of semiconduc-tor–metal composite thin films. Asatisfactory expla-nation can emerge either by enlarging the scope of theexisting theories or adapting a comprehensive ap-proach that takes into account the possible interactionsbetween the metal and semiconductor materials.
The research described in this paper was performedby the Center for Space Microelectronics Technology,Jet Propulsion Laboratory, California Institute ofTechnology, under the sponsorship of the NationalAeronautics and Space Administration, Office of Ad-vanced Concepts and Technology. One of the authors1CLN2 thanks the Department of Science and Technol-ogy and the Indian Space Research Organization ofthe government of India and theUnited StatesAgencyfor International Development for the necessary finan-cial support and encouragement. Also, he is thank-ful to the Academy for Educational Development,U.S.A., for logistic support and encouragement.The authors gratefully acknowledge the efforts of
Thomas George and Thomas Pike of the Jet Propul-sion Laboratory, California Institute of Technology,in measuring the TEM properties of the coatings, andGhanimAl-Jumaily also of the Jet Propulsion Labora-tory for critical comments and suggestions.
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