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Cesium-trihalogen-plumbates a new class of ionic semiconductors
Journal of Luminescence 18/19 (1979) 385—388 © North-Holland Publishing Company
CESIUM-TRIHALOGEN-PLUMBATES A NEW CLASS OF IONIC SEMICONDUCTORS D. FROHLICH, K. HEIDRICH, H. KUNZEL, G. TRENDEL and J. TREUSCH Institut fur Physik. Universität Dortmund, 46 Dortmund, Fed. Rep. Germany
Electronic and optical properties of perovskite type CsPbCl
1, CsPbBr1 and their mixed crystals, are reported. Measurements of absorption, reflectivity and two photon excited luminescence are explained in the framework of an empirical LCAO band model.
1.
Introduction
Semiconducting compounds of the valence-type IV—VI, hI—Vu, TV—Vu2, I—IV—V113, such as the lead chalcogenides, thallous halides, lead halides, and alkali—trihalogeno—plumbates, respectively, differ essentially from averagevalence 4-compounds. Due to the cationic 6s-shell (of lead or thallium) the band structure is inverted in such a way, that the cationic s-electrons dominate the uppermost valenceband, whereas the cationic p-states form the lowest conduction band. The band edge exciton is almost purely intracationic, the energy gap is comparatively small. We find high values of the electronic dielectric constant going along with high ionic polarizability, and as a consequence strong electron— phonon coupling. The cesium—trihalogeno—plumbates form a particularly interesting member of this class of semiconductors, although there is only few literature about their optical properties up to now [11. CsPbCI1 and CsPbBr1, with which we will deal in this contribution, crystallize in a slightly distorted perovskite lattice. The Pb~-ionsits in the center of a cube, the corners of which are formed by Cstions, whereas six nearest neighbor halogen-ions sit on the facecentered positions. On the basis of the close electronic relationship of the Pb~ (Cl Br )6 complex to the simple Thallous halides, an empirical LCAO-model was proposed [11 which completely neglects the electronic levels of cesium (fig. la). Absorption and reflectivity measurements in the spectral range from 2.3—5 eV were explained in the framework of this model. Fig. 2 shows an absorption spectrum of CsPbCI3 and CsPbBr1 on improved samples, which reinforces our assignment that the sharp peaks 1 and 2 are due to intracationic exciton transitions, whereas the broad peaks, 3,3’ are due to a different mechanism, namely a charge transfer transition from the anion ,
385
D. Frohlich ci a!. / C.s-trihalogeno -plumbate .semiconductor.c
t86
a
5
0
[sPbBr
3
X
100
r
111
h
1’[s~6s
~
R
CsPbBr~
sPbCL3
Fig. 1. (a) Empirical LCAO band model for CsPbBr~.Arrows indicate ionic energy levels corrected by Madelung energies; (b) energetic positions of reflection minima versus ratio x of composition for CsPbCl~~Br~
p-level to the conduction band. In fig. lb the energetic positions of reflection minima are given versus ratio of composition of the CsPbCl~,~Br,mixed crystal. There is a characteristic difference in behaviour between structures I and 2 on the one side and structure 3 on the other side. Although the charge transfer type transitions 3 are of the amalgamation type, too, as a consequence of the rather broad anionic p-band [2], we clearly distinguish the s.o. splitting of bromine (0.4eV), and that of chlorine (0.1 eV) for the pure crystals, a fact that supports our assignment. From a Kramers—Kronig analysis of the reflectivity spectra of CsPbCI~and
photonenerçjy
I eV)
Fig. 2. Absorption spectra of CsPbCh and CsPBBr~.
D. Fröhlich et al./Cs-trihalogeno-plumbate semiconductors
387
CsPbBr3, exciton binding energies EB were deduced assuming a Wannier-series: E~°”°~ = 67 E~SPbBr3=
meV,
37 meV.
These values compare to those of fcc-thallous halides, being about five times larger than those of sc thallous halides. This is mainly due to the fact, that the static dielectric constant has a value of about 10 [3], comparable to that proposed for fcc-thallous halides [41, but considerably smaller than that of sc thallous halides. The luminescence spectrum of CsPbCI1 was measured after excitation with a frequency doubled Neodym-YAG laser at 2.34 eV. As one easily sees from fig. 2, a two-photon excitation process generates chlorine p-holes and lead p-electrons via transitions 3, which in contrast to transitions 1 and 2 are two-photon allowed. These excited states decay to the excitonic states at the X- and R-point, respectively, of the Brillouin zone. We assign the luminescence line at 2.963 eV (see fig. 3) to the ls—R-exciton, the line at 2.958 eV to a bound exciton. Both of these lines show satellite structures which are almost equidistant. An explanation, however, is as yet hazardous, since the knowledge of the phonon spectrum of CsPbCI3 is very unsatisfactory [31. The intensity of the line groups described above goes quadratically with excitation intensity as it should for two-photon excitation. Our assignment is strengthened by measurements of one-photon excited luminescence, where the excitation energy (3.038, 3.031, 3.023, 3.015 eV)
CsPbC.13
29~5
2,952 2,95B Photonenergy cv)
2 963
Fig. 3. Two-photon excited luminescence spectra of CsPbCI3. Excitation energy is 2.34eV. Ex citation intensity is increased by a factor 1.5 in b, 4.5 in c, as compared to a. Scale is reduced by a factor 400 in c as compared to a and b.
388
D. Fröhlich ci aL/Cs-trihalogeno-plumbate .semiconductor,s
was close to resonance with the is-exciton absorption peak at 2.969 eV. Here a sharp emission line at 2.97 eV was measured with a broad shoulder to lower energies. Since reabsorption plays a greater role in two-photon excited luminescence, which stems from the bulk, the larger red shift of the free-exciton luminescence line in the latter case is understandable. The bound exciton line was not yet observed with close-to-resonance excitation. The luminescence line at 2.945 eV shows exponential behaviour with increasing excitation intensity. We assign it to a stimulated process the character of which is as yet unclear. From the energetic distance to the Is-exciton (~E 25 meV) an exciton—exciton scattering process is unlikely, since it should occur at an energy lower than E1~ ~EB. This process would correspond to a transition ls + ls—* photon + 2s. A phonon-assisted stimulated emission of the is-exciton seems to be possible, the LO-phonon energy being 28 meV [3]. From the stability of the line shape against variation of excitation intensity, and from its energetic position we would suggest, that also an electron hole-liquid may be evidenced. The comparatively strong electron—phonon coupling may very well favorize this channel as suggested by Beni and Rice [5]. Measurement of gain and temperature dependence are in preparation to decide the open questions concerning this line. In CsPbBr3 and the mixed crystal, CsPbCl~~ no stimulated emission could be observed, whereas the luminescence due to the band edge excitonic states could be followed over the whole range of mixing.
Acknowledgements We are grateful to R. Ulbrich, D. Engemann and R. Kenklies for their cooperation.
References ~1j K. Keidrich, H. Kunzel. and J. Treusch, Solid State Commun. 25 (l97tt)~S7and references therein [2] Y. Onodera, Y. Toyozawa, J. Phys. Soc. Japan 24 (1968) 341. [31 S. Hirotsu, J. Phys. Soc. Japan ~1 (1971) 552; Phys. Lett. 41A (1972) 55. [4] J. Treusch, Phys. Rev. Lett. 34 (1975) 1343. [5] 6. Beni, T.M. Rice. Phys. Rev. Lett. 37 (1976) 874.
CESIUM-TRIHALOGEN-PLUMBATES A NEW CLASS OF IONIC SEMICONDUCTORS D. FROHLICH, K. HEIDRICH, H. KUNZEL, G. TRENDEL and J. TREUSCH Institut fur Physik. Universität Dortmund, 46 Dortmund, Fed. Rep. Germany
Electronic and optical properties of perovskite type CsPbCl
1, CsPbBr1 and their mixed crystals, are reported. Measurements of absorption, reflectivity and two photon excited luminescence are explained in the framework of an empirical LCAO band model.
1.
Introduction
Semiconducting compounds of the valence-type IV—VI, hI—Vu, TV—Vu2, I—IV—V113, such as the lead chalcogenides, thallous halides, lead halides, and alkali—trihalogeno—plumbates, respectively, differ essentially from averagevalence 4-compounds. Due to the cationic 6s-shell (of lead or thallium) the band structure is inverted in such a way, that the cationic s-electrons dominate the uppermost valenceband, whereas the cationic p-states form the lowest conduction band. The band edge exciton is almost purely intracationic, the energy gap is comparatively small. We find high values of the electronic dielectric constant going along with high ionic polarizability, and as a consequence strong electron— phonon coupling. The cesium—trihalogeno—plumbates form a particularly interesting member of this class of semiconductors, although there is only few literature about their optical properties up to now [11. CsPbCI1 and CsPbBr1, with which we will deal in this contribution, crystallize in a slightly distorted perovskite lattice. The Pb~-ionsits in the center of a cube, the corners of which are formed by Cstions, whereas six nearest neighbor halogen-ions sit on the facecentered positions. On the basis of the close electronic relationship of the Pb~ (Cl Br )6 complex to the simple Thallous halides, an empirical LCAO-model was proposed [11 which completely neglects the electronic levels of cesium (fig. la). Absorption and reflectivity measurements in the spectral range from 2.3—5 eV were explained in the framework of this model. Fig. 2 shows an absorption spectrum of CsPbCI3 and CsPbBr1 on improved samples, which reinforces our assignment that the sharp peaks 1 and 2 are due to intracationic exciton transitions, whereas the broad peaks, 3,3’ are due to a different mechanism, namely a charge transfer transition from the anion ,
385
D. Frohlich ci a!. / C.s-trihalogeno -plumbate .semiconductor.c
t86
a
5
0
[sPbBr
3
X
100
r
111
h
1’[s~6s
~
R
CsPbBr~
sPbCL3
Fig. 1. (a) Empirical LCAO band model for CsPbBr~.Arrows indicate ionic energy levels corrected by Madelung energies; (b) energetic positions of reflection minima versus ratio x of composition for CsPbCl~~Br~
p-level to the conduction band. In fig. lb the energetic positions of reflection minima are given versus ratio of composition of the CsPbCl~,~Br,mixed crystal. There is a characteristic difference in behaviour between structures I and 2 on the one side and structure 3 on the other side. Although the charge transfer type transitions 3 are of the amalgamation type, too, as a consequence of the rather broad anionic p-band [2], we clearly distinguish the s.o. splitting of bromine (0.4eV), and that of chlorine (0.1 eV) for the pure crystals, a fact that supports our assignment. From a Kramers—Kronig analysis of the reflectivity spectra of CsPbCI~and
photonenerçjy
I eV)
Fig. 2. Absorption spectra of CsPbCh and CsPBBr~.
D. Fröhlich et al./Cs-trihalogeno-plumbate semiconductors
387
CsPbBr3, exciton binding energies EB were deduced assuming a Wannier-series: E~°”°~ = 67 E~SPbBr3=
meV,
37 meV.
These values compare to those of fcc-thallous halides, being about five times larger than those of sc thallous halides. This is mainly due to the fact, that the static dielectric constant has a value of about 10 [3], comparable to that proposed for fcc-thallous halides [41, but considerably smaller than that of sc thallous halides. The luminescence spectrum of CsPbCI1 was measured after excitation with a frequency doubled Neodym-YAG laser at 2.34 eV. As one easily sees from fig. 2, a two-photon excitation process generates chlorine p-holes and lead p-electrons via transitions 3, which in contrast to transitions 1 and 2 are two-photon allowed. These excited states decay to the excitonic states at the X- and R-point, respectively, of the Brillouin zone. We assign the luminescence line at 2.963 eV (see fig. 3) to the ls—R-exciton, the line at 2.958 eV to a bound exciton. Both of these lines show satellite structures which are almost equidistant. An explanation, however, is as yet hazardous, since the knowledge of the phonon spectrum of CsPbCI3 is very unsatisfactory [31. The intensity of the line groups described above goes quadratically with excitation intensity as it should for two-photon excitation. Our assignment is strengthened by measurements of one-photon excited luminescence, where the excitation energy (3.038, 3.031, 3.023, 3.015 eV)
CsPbC.13
29~5
2,952 2,95B Photonenergy cv)
2 963
Fig. 3. Two-photon excited luminescence spectra of CsPbCI3. Excitation energy is 2.34eV. Ex citation intensity is increased by a factor 1.5 in b, 4.5 in c, as compared to a. Scale is reduced by a factor 400 in c as compared to a and b.
388
D. Fröhlich ci aL/Cs-trihalogeno-plumbate .semiconductor,s
was close to resonance with the is-exciton absorption peak at 2.969 eV. Here a sharp emission line at 2.97 eV was measured with a broad shoulder to lower energies. Since reabsorption plays a greater role in two-photon excited luminescence, which stems from the bulk, the larger red shift of the free-exciton luminescence line in the latter case is understandable. The bound exciton line was not yet observed with close-to-resonance excitation. The luminescence line at 2.945 eV shows exponential behaviour with increasing excitation intensity. We assign it to a stimulated process the character of which is as yet unclear. From the energetic distance to the Is-exciton (~E 25 meV) an exciton—exciton scattering process is unlikely, since it should occur at an energy lower than E1~ ~EB. This process would correspond to a transition ls + ls—* photon + 2s. A phonon-assisted stimulated emission of the is-exciton seems to be possible, the LO-phonon energy being 28 meV [3]. From the stability of the line shape against variation of excitation intensity, and from its energetic position we would suggest, that also an electron hole-liquid may be evidenced. The comparatively strong electron—phonon coupling may very well favorize this channel as suggested by Beni and Rice [5]. Measurement of gain and temperature dependence are in preparation to decide the open questions concerning this line. In CsPbBr3 and the mixed crystal, CsPbCl~~ no stimulated emission could be observed, whereas the luminescence due to the band edge excitonic states could be followed over the whole range of mixing.
Acknowledgements We are grateful to R. Ulbrich, D. Engemann and R. Kenklies for their cooperation.
References ~1j K. Keidrich, H. Kunzel. and J. Treusch, Solid State Commun. 25 (l97tt)~S7and references therein [2] Y. Onodera, Y. Toyozawa, J. Phys. Soc. Japan 24 (1968) 341. [31 S. Hirotsu, J. Phys. Soc. Japan ~1 (1971) 552; Phys. Lett. 41A (1972) 55. [4] J. Treusch, Phys. Rev. Lett. 34 (1975) 1343. [5] 6. Beni, T.M. Rice. Phys. Rev. Lett. 37 (1976) 874.