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The optical absorption of orthorhombic thallous iodide
Solid State Communications,
Vol. 7, pp. 1023—1025. 1969.
Pergamon Press.
Printed in Great Britain
THE OPTICAL ABSORPTION OF ORTHORHOMBIC THALLOUS IODIDE* Robert Z. Bachracht Department of Physics and Materials Research Laboratory, University of Illinois, Urbana, Illinois
(Received 29 May 1969 by J./l. J
The optical absorption of strain reduced thallous iodide films has 1 is been measured. Strong absorption of the order of 10~cm observed and a number of relatively sharp lines appear. An exciton— longitudinal optical phonon coupled state sideband to the first exciton peak is observed.
THALLOUS iodide is an orthorhombic crystal under standard conditions with lattice parameters of 5.24, 4.57 and 12.92 A and space group D 2h ‘~ .‘ TlI has found extensive use as an i.r. window material, particularly mixed with TlBr in2 ahas compound KRS-5. reportedcalled on the static Recently dielectric Samara constant of Til and shown it has a temperature dependence anomaly similar to the other thallous halides. The low temperature static dielectric constant is of the order of 21. The optical dielectric constant is between 5.8 and 6.8. The measured transverse optical phonon frequency is 1.6 x 10~ cps3 and the longitudinal optical phonon energy is estimated from the LST relation to be 12 ±1 meV.
technique to be described elsewhere which resulted in a reduction in strain, and in the thallous halides, has produced optical spectra
Very little interband optical absorption work on TlI is reported in the literature, although there are several references to work and one photographic plate picture is published.4
Figure 1 shows the optical density over a range of photon energy from 2.8 to 6.8 eV and Fig. 2 shows the first peaks in detail. Til has an abundance of peaks which are summarized in Table 1. Of particular interest are the first two series of four non-hydrogenically spaced peaks. In Fig. 2 it is seen that the narrow first line has a sideband similar to that found in T1Br and T1C1 and separated by about 9meV from the main peak. The lack of resolution of the sideband indicates that the strain broadening is still large. The maximum absorption
of exceptional sharpness. The films of Til were evaporated by a conventional method and the thickness with a calibrated oscillator determined thickness monitor. The films crystal appeared to form completely in the orthorhimbic phase since no red absorption characteristic of the cubic phase was observed. The optical absorption was measured with a Cary 14R spectrophotometer and a low temperature Helium cryostat. The films were maintained in a dry nitrogen atmosphere during transfer from the evaporator to the cryostat.
In the course of a study of excitons in TlBr and T1C1,5 several films of Til were made by a * Work supported in part by the Advanced Research Projects Agency under contract SD131 and by the U.S. Army Research Office (Durham)
Current address: Bell Telephone Laboratories, Murray Hill, New Jersey.
1023
coefficient in the first peak is approximately iO~cm1. Most likely the transition in TlI is
1024
THE OPTICAL ABSORPTION OF ORTHORHOMBIC Til
Vol. 7, No. 14
Table 1 200
2
I3~4 5
A
A
E eV
1
4325
2.867
2 1’
4090 4312
3.031 2.875
3
3945
3.143
4 5
3870 3485
3.204 3.558
6 7
3302 3170
3.755 3.911
8 9 10 11
3125 2820 2760 2690
3.967 4.397 4.492 4.609
12
2550
4.862
13
2385
5.198
14 15
2315 2250
5.510 5.510
I.~ >-
z
l.~O .40 20 1.00 .80
ii23~
.60 440
3~0
5.20
PHOTON ENERG’r(eV)
Fic. 1. Optical absorption spectra at 4.7°K of orthorhombic thallous iodide.
I.8O~TI1/SRF 47 K ,,-
ISO
Til peak positions, 16 4.7°K. 1965
20
6.309
0.
o 900~~
600 2840
_________________________________________________________________
2920
3000
3.080
3160
3240
3320
3400
PHOTON ENrRGY (eV~
FIG. 2. Detail of first series including unresolved sideband to the first exciton peak. direct as it is in T1Br and T1C1. Unlike T1Br and T1C1, the peak shifts to higher energy as the temperature is lowered. At room temperature, all but the strongest peaks are washed out and even they are barely discernable. The origin of the two series of four lines is not clear, but they probably arise from band structure effects associated with the spin orbit splitting of the iodide. Most likely, the band structure of Til is qualitatively different from TlBr and T1C1 where the lowest excitation remains on the thallous sublattice. Til would be an interesting case for a band structure calculation since both thallium and iodine should be treated relativistically. Calculations could conceivably be done for both crystal phases since absorption measurements should be possible on the cubic phase at low temperatures by either
applying a high enough pressure or forming the
films above 180°Cand then rapidly quenching them. The sideband to the first peak observable in Fig. 2 at 2.880eV is thought to be an exciton—longitudinal optical phonon coupled 5 In this case, however, the main resonance state. may be between the higher exciton states and the LO-phonon rather than with the continuum as in TlBr and T1CI.
Vol. 7, No. 14
THE OPTICAL ABSORPTION OF ORTHORHOMBIC TIl
1025
REFERENCES 1. 2.
WYCKOFF R., Crystal Structures Vol. 1(1965). SAMARA G.A., Phys. Rev. 165, 959 (1968).
3. 4.
JONES et al, Proc. R. Soc. A261, 10 (1961). NIKITINE S., Progress in Semiconductors Vol. 6 (1962); NIKITINE S. et at Semiconductors Exeter (1962). BACHRACH R.Z. and BROWN F.C., Phys. Rev. Let:. 21, 685 (1968).
5.
mt. Conf. Phys.
Die Lichtabsorption in deforinationsarmen Thallojodidschichten wurde gemessen. Starke Absorption in der Grossenordnung von 10~cm~wird beobachtet, cine Reihe von verhaitnismassig ausgepragten Linien komrnen zum Vorschein. Eine Nebenbande eines gekoppelten Zustandes aus Excitonen und Langs-Lichtphononen wird and der 1. Excitoneaspitze beobachtet.
Vol. 7, pp. 1023—1025. 1969.
Pergamon Press.
Printed in Great Britain
THE OPTICAL ABSORPTION OF ORTHORHOMBIC THALLOUS IODIDE* Robert Z. Bachracht Department of Physics and Materials Research Laboratory, University of Illinois, Urbana, Illinois
(Received 29 May 1969 by J./l. J
The optical absorption of strain reduced thallous iodide films has 1 is been measured. Strong absorption of the order of 10~cm observed and a number of relatively sharp lines appear. An exciton— longitudinal optical phonon coupled state sideband to the first exciton peak is observed.
THALLOUS iodide is an orthorhombic crystal under standard conditions with lattice parameters of 5.24, 4.57 and 12.92 A and space group D 2h ‘~ .‘ TlI has found extensive use as an i.r. window material, particularly mixed with TlBr in2 ahas compound KRS-5. reportedcalled on the static Recently dielectric Samara constant of Til and shown it has a temperature dependence anomaly similar to the other thallous halides. The low temperature static dielectric constant is of the order of 21. The optical dielectric constant is between 5.8 and 6.8. The measured transverse optical phonon frequency is 1.6 x 10~ cps3 and the longitudinal optical phonon energy is estimated from the LST relation to be 12 ±1 meV.
technique to be described elsewhere which resulted in a reduction in strain, and in the thallous halides, has produced optical spectra
Very little interband optical absorption work on TlI is reported in the literature, although there are several references to work and one photographic plate picture is published.4
Figure 1 shows the optical density over a range of photon energy from 2.8 to 6.8 eV and Fig. 2 shows the first peaks in detail. Til has an abundance of peaks which are summarized in Table 1. Of particular interest are the first two series of four non-hydrogenically spaced peaks. In Fig. 2 it is seen that the narrow first line has a sideband similar to that found in T1Br and T1C1 and separated by about 9meV from the main peak. The lack of resolution of the sideband indicates that the strain broadening is still large. The maximum absorption
of exceptional sharpness. The films of Til were evaporated by a conventional method and the thickness with a calibrated oscillator determined thickness monitor. The films crystal appeared to form completely in the orthorhimbic phase since no red absorption characteristic of the cubic phase was observed. The optical absorption was measured with a Cary 14R spectrophotometer and a low temperature Helium cryostat. The films were maintained in a dry nitrogen atmosphere during transfer from the evaporator to the cryostat.
In the course of a study of excitons in TlBr and T1C1,5 several films of Til were made by a * Work supported in part by the Advanced Research Projects Agency under contract SD131 and by the U.S. Army Research Office (Durham)
Current address: Bell Telephone Laboratories, Murray Hill, New Jersey.
1023
coefficient in the first peak is approximately iO~cm1. Most likely the transition in TlI is
1024
THE OPTICAL ABSORPTION OF ORTHORHOMBIC Til
Vol. 7, No. 14
Table 1 200
2
I3~4 5
A
A
E eV
1
4325
2.867
2 1’
4090 4312
3.031 2.875
3
3945
3.143
4 5
3870 3485
3.204 3.558
6 7
3302 3170
3.755 3.911
8 9 10 11
3125 2820 2760 2690
3.967 4.397 4.492 4.609
12
2550
4.862
13
2385
5.198
14 15
2315 2250
5.510 5.510
I.~ >-
z
l.~O .40 20 1.00 .80
ii23~
.60 440
3~0
5.20
PHOTON ENERG’r(eV)
Fic. 1. Optical absorption spectra at 4.7°K of orthorhombic thallous iodide.
I.8O~TI1/SRF 47 K ,,-
ISO
Til peak positions, 16 4.7°K. 1965
20
6.309
0.
o 900~~
600 2840
_________________________________________________________________
2920
3000
3.080
3160
3240
3320
3400
PHOTON ENrRGY (eV~
FIG. 2. Detail of first series including unresolved sideband to the first exciton peak. direct as it is in T1Br and T1C1. Unlike T1Br and T1C1, the peak shifts to higher energy as the temperature is lowered. At room temperature, all but the strongest peaks are washed out and even they are barely discernable. The origin of the two series of four lines is not clear, but they probably arise from band structure effects associated with the spin orbit splitting of the iodide. Most likely, the band structure of Til is qualitatively different from TlBr and T1C1 where the lowest excitation remains on the thallous sublattice. Til would be an interesting case for a band structure calculation since both thallium and iodine should be treated relativistically. Calculations could conceivably be done for both crystal phases since absorption measurements should be possible on the cubic phase at low temperatures by either
applying a high enough pressure or forming the
films above 180°Cand then rapidly quenching them. The sideband to the first peak observable in Fig. 2 at 2.880eV is thought to be an exciton—longitudinal optical phonon coupled 5 In this case, however, the main resonance state. may be between the higher exciton states and the LO-phonon rather than with the continuum as in TlBr and T1CI.
Vol. 7, No. 14
THE OPTICAL ABSORPTION OF ORTHORHOMBIC TIl
1025
REFERENCES 1. 2.
WYCKOFF R., Crystal Structures Vol. 1(1965). SAMARA G.A., Phys. Rev. 165, 959 (1968).
3. 4.
JONES et al, Proc. R. Soc. A261, 10 (1961). NIKITINE S., Progress in Semiconductors Vol. 6 (1962); NIKITINE S. et at Semiconductors Exeter (1962). BACHRACH R.Z. and BROWN F.C., Phys. Rev. Let:. 21, 685 (1968).
5.
mt. Conf. Phys.
Die Lichtabsorption in deforinationsarmen Thallojodidschichten wurde gemessen. Starke Absorption in der Grossenordnung von 10~cm~wird beobachtet, cine Reihe von verhaitnismassig ausgepragten Linien komrnen zum Vorschein. Eine Nebenbande eines gekoppelten Zustandes aus Excitonen und Langs-Lichtphononen wird and der 1. Excitoneaspitze beobachtet.