Contains reports on two research projects.Joint Services Electronics Programs (U.S. Army, U.S. Navy, and U.S. Air Force) under Contract DA 28-043-AMC-02536(E)U.S. Air Force (ESD Contract AFl9(628)-6066)Sloan Found for Basic Research (M.I.T. Grant

Fertel, Jeanne H.

Parrish, J. F.

Perry, C. H.

Contains reports on three research projects.Joint Services Electronics Programs (U. S. Army, U.S. Navy, and U. S. Air Force)under Contract DA 28-043-AMC-02536(E)U. S. Air Force Cambridge Research Laboratories Contract AF 19(628)-6066National Aeronautics and Space Administration (Grant NGR-22-009-237

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Optical and Infrared Spectroscopy

VII. OPTICAL AND INFRARED SPECTROSCOPYAcademic and Research StaffProf. C. H. PerryGraduate StudentsT. G. Davis D. J. Muehlner J. F. ParrishJeanne H. Fertel N. E. TornbergA. REFLECTION MEASUREMENTS ON POTASSIUM HALIDESIN THE FAR INFRARED REGION1. IntroductionMost ionic crystals have strong refelction bands, known as Reststrahlen bands, inthe far infrared region. These are due to lattice vibrations. The lattice resonanceshave been observed at temperatures down to 5 0 K for several potassium halides, bymeasuring the reflection from single crystals at an incident angle of 100.The following crystals have been investigated: KC1, KBr, and KI.2. Experimental ProcedureThe interferometer and detector used for this measurement have been describedelsewhere. 1 The reflectivity values indicated below are relative to the reflectivity of afront-aluminized reference mirror.3. Experimental ResultsThe measured reflectivity spectral profiles are shown in Fig. VII-l for T = 5, 80*,1950, and 300'K, except for KC1 which was not investigated at 195 0 K. The main absorp-tion peak can be identified as the transverse optical fundamental of frequency, WT, atthe zone center. Also, all of the alkali halides show a sideband on the high-frequencyside of the main peak.The Reststrahlen bands of KCI, KBr, KI have been fitted to a classical dispersionformula by machine programming. We found that this could not be done withoutassuming the existence of a second sideband between the main peak and observed side-band. The dispersion constants obtained in this way are tabulated in Table VII-1.Here, Si denotes the oscillator strength, and y.i the damping constant of oscillator i.1 -11All frequencies are in cm ;-1 L is the frequency of the longitudinal optical fundamental*This work was supported in part by the Joint Services Electronics Programs(U.S. Army, U.S. Navy, and U.S. Air Force) under Contract DA 28-043-AMC-02536(E),the U.S. Air Force (ESD Contract AFl9(628)-6066), and the Sloan Found for BasicResearch (M.I.T. Grant).39QPR No. 85KBr300'KKI300 K) 80 100 120 140 160 180 200 220Fig. VII- 1.z 600L50_ 4030202OI0 -60 80 100 120 140 160 180 200 220 240 60 80 00 120 140 160 180 200 220 240 60KBr KBr195°K 80'KKI KI195' K 80K40 60 80 100 120 140 160 180 200 220 40 60 80 100 120 140 160 180 200 220 40WAVE NUMBERS (cm- )Reflectivity of potassium halides at various temperatures.Measured.80 100 120 140 160 180 200 220 24060 80 100 120 140 160 180 200 220Calculated by using the classical dispersion analysis.40Table VII-1. Dispersion constants obtained from the classical oscillator model.ST T YT S 1  w Y 1 S2 2 Y2 o L LST e*/eKC1 (3000) 215 143 5 55 189.5 31 41 210.5 14.5 2.22 4.6 203 206 .77KC1 (90) 230 151 2 35 193 30 40 222 17 2.22 4.61 216 216KC1 (10) 230 150 1.0 35 193 30 40 222 17 2.22 4.64 215 216KBr (3000) 172 113 5 42 150 28 36.5 169 15 2.43 4.9 158 161 .74KBr (195) 170 117 1.5 39 147 30 30 172 15 2.43KBr (90) 173 120 0.8 30 152 28 25 174 15 2.43KBr (10) 178 122 0.1 24 157 28 28 178 23 2.43KI (3000) 147 103.5 6.6 50 136 28 48 148 36 2.82 5.08 135 138 .71KI (195) 151 106 3.5 44 138 27 38 149 25 2.82KI (90) 155 108 0.5 24 141 28 38 151 25 2.82KI (10) 161 110 0.2 15 144 28 41 153 25 2.82(VII. OPTICAL AND INFRARED SPECTROSCOPY)at the zone center, and wL(LST) is that same quantity calculated from the Lyddane-Sacks-Teller relation:oWL(LST) = /Eo/E T (1)The effective charge on an ion, e , is obtained from the Szigetti relation2 4rrN *2 (E O,+2) 2T =9m e E -E, (2)o 00where N is the number of ion pairs per unit volume, and m is the reduced mass of anion pair.We have tentatively identified the frequency wl with the multiphonon compensationTO + TA at the zone boundary, and w2 with TO + LA.All of the values of E that we used were obtained from the CRC tables. The valuesooof E° were obtained from the refelctivity spectra extrapolated to w = 0.We would like to thank Professor Alexander Smakula and Mr. J. Kalnajs, of the Cen-ter for Materials Science and Engineering, M.I.T., for the samples.Jeanne H. Fertel, C. H. PerryReferences1. C. H. Perry, R. Geick, and E. F. Young, J. Appl. Opt. 5, 1171 (1966).B. LATTICE VIBRATIONS IN CdS/Se MIXED CRYSTALS1. IntroductionThe infrared transmittance and reflectance of pure crystals of CdS and CdSe havebeen studied and reported several years ago. More recently, the infrared transmit-tance and reflectance of the hexagonal mixed crystals of CdS/Se have been studied, andseveral preliminary reports have been published or informally exchanged betweenresearchers at the Massachusetts Institute of Technology and elsewhere. At M.I.T, sev-eral mixed crystals of CdSxSe(l x) (.29<x<.93) were studied in transmission at temper-tures down to that of liquid Helium, and in reflection at temperatures down to that ofliquid nitrogen, with the use of the FS520 Fourier spectrophotometer. A preliminaryanalysis of these results is presented here.2. Discussion of ResultsThe CdS/Se systems appear to form good mixed crystals with hexagonal unit cellslike pure CdS and CdSe. Such a unit cell for CdS with the lattice constants "a" and "c"QPR No. 85(VII. OPTICAL AND INFRARED SPECTROSCOPY)labeled is shown in Fig. VII-2. A quantitative analysis of 7 samples of CdS/Se determinedthe relative weights of Cd and Se in each sample. These two weights for each sampleI8 s81M EFig. VII-2. Hexagonal CdS.were then used to calculate independent values of "x" in the empirical formulaCdS Se( . For all but one sample (sample H), these two values of x agreed withinx(1-x)1% of each other, thereby showing that the crystals were indeed stoichiometric mixturesof CdS and CdSe. X-ray measurements on 5 of these samples were performed to obtainthe lattice constants "a" and "c". All of the diffraction peaks were sharp, indicatingunique lattice constants and no evidence of clustering, thereby confirming that theCdS/CdSe systems formed good mixed crystals for the range of concentrations studied.Plots of a, c, and a2c (volume of unit cell) against x are shown in Figs. VII-3, VII-4,and VII-5, established for the pure samples; thus it is shown that the lattice parametersvary uniformly and nearly linearly with concentration. Moreover, the conclusion thatCdS/Se does form good mixed crystals has also been confirmed by other researcherswho measured these and other properties such as fluorescence of the CdS/Se sys-tem.1-6, 8-10The reflection spectra of the mixed crystals of CdS/Se show at least two opticallyactive lattice vibrations. Figure VII-6 shows the reflectivity of one of the samplesQPR No. 854.2504.200CdSFig. VII-3.7.0006.9006.800PURE CdSe/+------- HPURE CdS6. 7000 0 1CCdSPlot of a against x.zD(D120Fig. VII-4. Plot of c against x.2H+GDzPURE CdS1Fig. VII-5. Plot of a c (volume of unit cell) against x.QPR No. 85(VII. OPTICAL AND INFRARED SPECTROSCOPY)100zuJiu50wu- - PURE CdSe- SAMPLE B CdS/Se (.93/.07)100 200 300WAVE NUMBER (cm-1Fig. VII-6. Reflectivity vs frequency at room temperature.compared with pure CdSe at room temperature. Whereas the reflectivity of mixed crys-tals of the alkali halides and NiO/CoO show only a single peak whose frequency varieswith the atomic concentration of the crystal's constituents 5-7; the reflectivity of CdS/Seshows two distinct peaks whose frequencies remain nearly constant but whose intensitiesvary with the atomic concentration parameter x. Thus, the mixed II-VI compound showssome of the properties of the covalent-type bonding characteristic of the mixed III-Vcompounds like GaAs/P, but without the clustering found in GaAs/P.3 ' 9 Also, Verleurand Barker 1 0 have shown that some of the theoretical tools used to analyze the GaAs/Psystem may also be applied to the CdS/Se system.Table VII-2 shows the principal room-temperature multiphonon absorption bands of-iTable VII-2. Principal absorption bands, v > 300 cm-1CdS CdSe-l -1334 cm-1 310 cm-1345-355 360388 405404 470415498518QPR No. 85(VII. OPTICAL AND INFRARED SPECTROSCOPY)-lpure CdS and CdSe above 300 cm-1.tion spectrum of one of the typical100zz0500Figure VII-7 shows the room-temperature absorp-mixed crystals of CdS/Se. As can be seen bySAMPLE r) CdS/Se (.79/.21)AT ROOM TEMPERATURE300 400 500 600WAVE NUMBER (cm - 1Fig. VII-7. Sample D CdS/Se (.79/.21) at room temperature.comparing this graph with Table VII-2, the absorption of the mixed crystal looks muchlike the product of the two pure absorption curves.Although the experimental evidence shows that CdS and CdSe do form good,uniform mixed crystals, the infrared optical properties appear to behave as if deter-mined by two nearly independent crystal lattices, one being due to the CdS, and the otherto the CdSe. At present, a theoretical analysis of the experimental measurements of thetransmission and reflection of the mixed crystals of CdS/Se, based upon the observa-tions presented in this report and the classical oscillator model, is being completed.We would like to thank Dr. S. S. Mitra, of the University of Rhode Island, andDr. D. Reynolds, of Wright-Patterson Air Force Base, Ohio, for the samples, and tothank J. A. Kalnajs of the Center for Materials Science and Engineering, M.I.T., for thex-ray analysis.J. F. Parrish, C. H. PerryReferences1. M. Balkanski, R. Beserman, and J. M. Besson, "Phonon Processes in Mixed Crys-tals of CdS CdSey)," Solid State Communs. 4, 201-204 (1966).2. M. Balkanski, J. M. Besson, and R. Le Toullec, "Dispersion Curves of Phonons inHexagonal Cadmium Sulfide Obtained by Infrared Spectroscopy," Proc. InternationalUnion of Pure and Applied Physics, Paris, 1965.QPR No. 85(VII. OPTICAL AND INFRARED SPECTROSCOPY)3. Y. S. Chen, W. Shockley, And G. I. Pearson, "Lattice Vibration Spectra ofGaAs P (l) Single Crystals," Phys. Rev. 151, 648-656 (1966).4. R. Geick and C. H. Perry, "Lattice Vibrational Properties of Hexagonal CdSe,"J. Appl. Phys. 37, 1994-1997 (1966).5. P. J. Gielisse, J. N. Plendl, L. C. Mausar, R. Marshall, S. S. Mitra, R. Mykolajewycz,and A. Smakula, "Infrared Properties of NiO and CoO and Their Mixed Crystals,"J. Appl. Phys. 36, 2446-2450 (1965).6. R. Marshall, S. S. Mitra, P. J. Gielisse, and J. N. Plendl, "Optical Properties ofTransition Metal Oxides," Proc. International Union of Pure and Applied Physics,Paris, 1965.7. Jeanne H. Fertel, Private communication.8. S. S. Mitra and R. Marshall, "Multiphonon Infrared Absorption in II-VI Semiconduc-tors," Proc. International Union of Pure and Applied Physics, Paris, 1965.9. R. F. Potter and D. L. Stierwalt, "Reststrahlen Frequencies for Mixed GaAs Sb(ly )System," Proc. International Union of Pure and Applied Physics, Paris, 1965.10. H. W. Verleur and A. S. Barker, "Optical Phonons in Mixed Crystals of CdSexS(1-x)Ph. D. Thesis, New York University, 1965 (unpublished).QPR No. 85

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Optical and Infrared Spectroscopy

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