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Multiplication of anion and cation electronic excitations in wide-gap KCl and CsCl crystals
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.Solid State Communications. Vol. 90. No. I I. pp. 741-744, 1994 Elsevtcr Scicm:¢ Ltd Printed in Great Britain.
i Pergamon
0038-
I098194 $7.00 +
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0038- IO98(94)EO163-6
MULTIPLICATION OF ANION AND CATION ELECTRONIC EXCITATIONS IN WIDE-GAP KCI AND CsC! CRYSTALS M. Kirm and I. Martinson Department of Physics, University of Lund, S-223 62 Lund, Sweden A. Lushchik Department of Physics, Tartu University, EE 2400 Tartu, Estonia and K. Kalder, R. Kink, Ch. Lushchik and A. L6hmus Institute of Physics, Estonian Academy of Sciences, EE 2400 Tartu, Estonia
(Received 6 December 1993; acceptedfor publication 22 February 1994 by B. Lundqvist) Luminescence of self-trapped excitons, extrinsic recombinational luminescence and crossluminescence on the excitation by synchrotron radiation (7.7-30 eV) have been investigated for KCI and CsCI crystals at 6-80 K. The following mechanisms of multiplication of electronic excitation are discussed: (i) decay of cation excitons with the formation of a double amount of anion excitons (KCI); (iS) photoionization of anions and cations with the subsequent formation of secondary excitons and electron-hole pairs by fast electrons.
1. INTRODUCTION THE ELEMENTARY mechanism of multiplication of eh.'ctronic excitations (MEE) was revealed many years ago for narrow-gap semiconductors [1]. A photon with the energy exceeding by several times the value of the energy gap (Ez), creates a hot electron and a hot hole whose energies are sufficient for the formation of a secondary electron-hole pair. As a result the quantum yield of the photoelectric effect grows from I to 3 in the region of photon energies by h v > ( 3 - 4 ) E ~ . The spectra of recombinational luminescence excitation by VUV-radiation have already been investigated in wide-gap ionic crystals with narrow valence bands. According to these investigations the threshold energy of the electron-hole mechanism of MEE is Ej~ ~ 2Ex. The quantum yield of luminescence is doubled in the region of hv > E~ (the so-called effect of photon multiplication) [2, 3]. Another mechanism of M EE has been detected in doped alkali iodides [2, 3]. It involves the creation, by hot photoelectrons, of secondary excitons which excite the emission of impurity centres. The threshold energy of this mechanism of MEE is E° = Eg + E~.
where E,a is the energy of anion exciton formation. Additional information on excitonic and electronhole mechanisms of MEE has been obtained through the investigation of excitation spectra for the luminescence of self-trapped exeitons (STE) [4]. The application of synchrotron radiation (SR) has led to a significant broadening of the spectral region of investigations and some new MEE mechanisms have been detected in solids [5-8]. Investigations of excitation spectra of crossluminescence, (Auger free luminescence) due to the recombination of electrons from the valence band with the holes from the cation core band have been carried out recently. The threshold energy of crossluminescence (CL) excitation in CsBr and CsCI coincides with the value of the ionization energy for cations, Exc ~ 14eV [9-1 I]. The aim of the present study is to investigate various mechanisms of M EE with the participation of anion EE as well as the cation ones in f.c.c. KCI and s.c. CsCI crystals. For this purpose we have investigated the excitation spectra for luminescence of STE, extrinsic recombinational luminescence and CL in the photon energy region up to 30eV.
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ANION AND CATION ELECTRONIC EXCITATIONS 2. EXPERIMENTAL
Single KCi and CsCI crystals were grown by the Stockbarger method after a special purification cycle involving a manyfold recrystallization from the melt. The content of impurity ions was on the level of 0.01 to I ppm [12. 13]. Experiments were carried out at beamline 52 in MAX-LAB at Lund (550MeV storage ring). The experimental setup has been briefly described in [14]. Synchrotron radiation (SR) is focussed on the entrance slit of a l-m normal-incidence vacuum monochromator equipped with a gold-coated 1200 lines mm -I grating. The samples are placed into a liquid helium cryostat evacuated by using a turbomolecular pump. The measurements of optical spectra were carried out by means of photomultipliers operating in the photon counting mode. To account for the spectral distribution of the intensity of SR and the intensity changes due to the variation of the current in the storage ring, a two-channel photon counting system was used. The monochromatized SR traversed a mesh coated with sodium salicylate and then fell on the sample in the cryostat. One of the photomultipliers detected the blue luminescence of sodium salicylate (reference signal for normalizing the signal from the sample) and the other the radiation from the sample. The STE luminescence was detected through an OS-I I filter (transparent for hu < 2.4eV) for KCI and an SZS-22 lilter (3.4 > h~, > 2.2eV) for the CsCI crystal, whereas CL was detected through a 35em grating monochromator (McPherson EU-700), operating in the 2000-8000 A region. The slit width of the primary monochromator was 200-400/zm (resolution !.7-3.3 A). 3. RESULTS In a KCI crystal at 4.2 K, the formation energy of anion excitons is Eea = 7.77 eV, the energy gap EKa = 8.69eV [12] and the formation energy of cation excitons Eec = 19.4 eV [I 5]. For CsCI these values are E,a = 7.86, Ega = 8.4 [13] and Ee¢ = 13.2eV [3, 161. Thus, for KCI E° < E,c, E~ < Eec, E,~/E~a > 2 while for CsCI E,~ > E¢,, E~ > E¢¢, E~¢/E,~ < 2. Thus, the elementary mechanisms of MEE should be significantly different for KCI and CsCI.
3. I. KC! crystal The peaks at 7.77 and 7.88eV in the reflection spectrum of the KCI crystal at 4.2 K correspond to the spin-orbit splitting of pSs excitons. Photons with h u > E~=8.69eV create separate electrons and holes. The STE emission (maximum at 2.31eV,
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Fig. I. Excitation spectra of 2.1-2.4 eV luminescence for a KCI crystal. The lower curve is multiplied by 2. Arrows show the formation energies of anion and cation excitons, electron-hole pairs and the combinations between them. bandwidth of 0.51eV) is the dominant one in the emission spectrum of a high-purity KCI crystal at 4.2K - see, e.g. [12]. Figure ! shows the excitation spectrum of STE luminescence for a KCI crystal at 6 K. The quantum efficiency of STE luminescence on the excitation by 7.75eV photons is about 0.05 at 4-15 K and this value decreases by tens of times on heating the crystal up to 30K. In the region of 7.7-19eV SR creates anion EE in the KCI crystal. Narrow peaks at 19.39, 19.69, 20.06, 20.27, 21.00 and 21.42eV are clearly manifested in the reflection spectrum of KCI [15]. These peaks correspond to the optical excitation and the ionization of the 3p6 shell of the K + ion. The narrow minima in the excitation spectrum of STE emission are strictly correlated with the reflectance maxima in the region of 19-22eV. The intensity of the STE emission increases in the region of anion EE formation in two stages: at 15-17 and 17-19eV (Fig. 1, open circles). The absorption constant increases with the increase of photon energy from 17 to 22 eV. Therefore, the depth of penetration of the excitation radiation into a crystal decreases and the efficiency of nonradiative decay of EE near the surface increases in this energy region. The quantum yield of the photoelectric effect decreases and the energy distribution of the emitted electrons changes in favour of slow electrons in the photon energy range from 14 to 18 eV in the KCI crystal at 300 K [8]. All the above-mentioned facts can be explained by the transformation of one absorbed photon, with the energy from 2E,.a = 15.5eV up t o Ega 4- Ee,, = 16.SeV, into two STE. This transformation takes place
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ANION AND CATION ELECTRONIC EXCITATIONS
directly or after intermediate proeesses: the creation of a hole and a fast electron, the formation of a secondary exciton by a fast electron and a recombination of an electron and a hole with the creation of another secondary exciton. The intensity of STE emission increases even more in the region hu > 2Ega = 1.75 eV. Photoionization of chlorine ions leads to the appearance of electrons with energies sufficient for the formation of secondary electron-hole pairs. We have succeeded in separating the electronic and electron-hole mechanisms of MEE by investigating the processes of MEE in the KCI crystal at various temperatures. A comparison of the excitation spectra of the emission with 2.1 < hu <_2.4eV at 6 and 50K shows that the thermal quenching of this emission proceeds differently at the excitation by 16 or 18eV photons: the emission of secondary STE (excitation by 16eV photons) is practically quenched at 50K, whereas the recombination of secondary electrons and holes near defects (excitation by 18 eV photons) leads to the appearance of a weak emission even at 50 K. The mean free path of free excitons before self-trapping is about 20-5 lattice constants in KCI at 5-80K [17]. Therefore, secondary excitons can neither inter-act with defects nor form near-defect-localized EE. The energy region of 19-22eV, where SR generates cation EE, is of special interest for us. In this region the efficiency of STE luminescence excitation at 10 K is at least twice as high as at the excitation by I I eV photons. The quantum yield rI is about 1.4 for the recombinational emission of TI + centres on the excitation of KCI-TI crystal by 2023 eV photons [5] and r/= 1.3 for photoelectric effect on the excitation of KCI by 22eV photons [81. Undoubtedly, the decay of cation EE in KC1 leads to the formation of a double number of anion EE. This process is the main rival to CL, which has not yet been observed in a KCI crystal. 3.2. CsCi crystal The peaks at 7.86, 7.94, 8.01, 8.09 and 8.19eV in the reflection spectrum of CsCI at 10 K correspond to the pSs and pSd excitons. Photons with hu> Ex,,=8.4eV create separate electrons and holes [13]. Narrow peaks in the reflection spectrum (29 K) of CsCI at 13.2-16.77eV are connected with the excitation and ionization of the 5p6 shell of the Cs + ion [16]. The emission bands at 4.6 and 5.2eV have been detected during the excitation of the CsCI crystal by X-rays. For a long time these bands have been interpreted as originating from the luminescence of STE. However, the threshold energy of the excitation
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Fig. 2. Excitation spectra of 4.6eV luminescence (a, b) and 2.2-3.4eV emission (c) for a CsCI crystal. for 4.6 and 5.2eV emissions is about 14eV [10, !1], and thus these emission bands correspond to CL. According to our data [13], the luminescence of STE in high-purity CsCI at 5K has the maximum at 2.85 eV. The mean free path of free excitons before self-trapping in CsCI is about 350 lattice constants at 5 K [17], and excitons either rise easily to the surface and decay there nonradiatively or decay with a typical emission near point defects. The emission of near-defect-localized EE can be observed even at high temperatures. Figure 2 shows the excitation spectra of CL in CsCI. The threshold energy of CL excitation is about 14 eV at 6 and 80 K. The variations in the efficiency of CL excitation in the region of 15-19.5eV can be explained by the changes of the absorption constant and the depth of penetration of radiation into a crystal as well as different temperature dependences of the mean free path of excitons and holes before localization. According to our data the efficiency of CL excitation increases sharply at hv > 27 eV, which is caused by the multiplication of cation EE: one photon with hu > 2Ego creates two holes in the cation core band. According to [8] the yield of the photoelectric effect decreases and the energy distribution of the emitted electrons in CsCI changes in favour of slow electrons in the region of 26-34eV, Besides CL (4.6-5.2 eV) the emission of STE with the maximum at 2.8-2.9eV has been observed during
744
ANION AND CATION ELECTRONIC EXCITATIONS
the excitation of our CsCI sample by X-rays or 20.5eV photons at 6K. Photons of 8eV excite STE emission and do not excite the emissions at 4.6 and 5.2eV. An impurity-defect emission at 3-3.7eV has been detected during the excitation of the crystal by 7.7 eV photons. Figure 2(c) shows that the efficiency of 2.2-3.4 eV emission varies insignificantly in the excitation range of 12-19 eV, decreasing only in the region of narrow reflectance peaks that correspond to the formation of cation excitons. The intensity of this emission increases sharply at 20-26eV. This effect is caused partly by the decrease of the absorption constant and partly by the formation of secondary anion excitons and electron-hole pairs by the fast photoelectrons created by the ionization of cations. In the region of 26-30eV the efficiency of anion EE emission decreases simultaneously with the sharp increase of the intensity of CL due to the participation of secondary cation EE. The threshold energies of these processes are Etc + Es~ and 2Ego. In a CsCI crystal Eec/Eea equals 1.69. Therefore, in contrast to KCI, the decay of cation EE with the formation of a double amount of anion EE is impossible for energetic reasons. Apparently, in CsCI the decay of one cation exciton leads to the formation of one anion exeiton and a packet of phonons. Nonradiative decay of cation excitons with the formation of Frenkel defects in anion and cation sublattices may be expected in a CsCI crystal as well. The ionization of cations takes place at hu > E~c = 14 eV. CL is observed following the recombination of an electron from the anion valence band with a hole from the cation core band. The recombination of an electron with an anion hole causes either STE luminescence or recombinational emission near defects and impurities. 4. CONCLUSIONS This work has shown that MEE processes are rather different for KCI and CsCI crystals, which have different ratios of anion and cation exciton creation energies ( E, d E~). Excitonic and electron-hole mechanisms of M EE with the threshold energies E° ~ Esa+ E,a and Er~ ~ 2Eta have been detected for KCI, but these mechanisms are not realized for a CsCI crystal with the low energy of cation exeiton formation. The decay of a one cation EE with the formation of two anion EE has been detected in KC! (E¢,/Eea > 2) but
Voi. 90, No. 11
not in CsC! crystal (E, clEe, < 2). The multiplication of cation EE with the threshold energy Et~c:~ 2Esc occurs in a CsCi crystal and causes a sharp increase of the CL excitation efficiency.
Acknowledgements - We are grateful to Professor I. Lindau for valuable discussions. This work has been supported by the Swedish Natural Science Research Council (NFR), the Royal Swedish Academy of Sciences, the Swedish Institute, The Crafoord Foundation and the Carl Trygger Foundation. REFERENCES W. Shockley, Czech J. Phys. BII, 81 (1961). E.R. IImas, G.G. Liidya & Ch.B. Lushchik, Opt. Spectrosc. (USA) 18, 255, 359 (1965). 3. E.R. Ilmas, R.A. Kink, G.G. Liidya & Ch.B. Lushchik, Bull. Acad. Sci. USSR. Phys. Ser. (USA) 29, 29 (1965). 4. N.S. Rooze, Soy. Phys. Solid State 17, 690 (1975). 5. S.N. lvanov, E.R. llmas, Ch.B. Lushchik & V.V. Mikhailin, Soy. Phys.-SolidState 15, 1053 (1973). 6. J.H. Beamont, A.J. Bourdilon & M.N. Kabler, J. Phys. C9, 2961 (1976). 7. M. Yanagihara, Y. Kondo & H. Kanzaki, J. Phys. Soc. Jpn 52, 4357 (1983). 8. H. Sugawara & T. Sasaki, J. Phys. Soc. Jpn 46, 132 (i 979). 9. Yu.M. Aleksandrov, I.L. Kuusmann, P.Kh. Liblik, Ch.B. Lushchik, V.N. Makhov, T.i. Syreishchikova & M.N. Jakimenko, Soy. Phys.-Solid State 29, 587 (1987). 10. i. Kuusmann, T. Kloiber, W. Laasch & G. Zimmerer, Rad. Effects and Defects in Solids il9-121, 21 (1991). 11. N.Yu. Kirikova & V.N. Makhov, Soy. Phys.Solid State 34, 1557 (1992). 12. Ch. Lushchik, J. Kolk, A. Lushchik, N. Lushchik, M. Tajirov & E. Vasil'chenko, Phys. Status Solidi (b) !14, 103 (1982). 13. K.U. lbragimov, A.Ch. Lushchik, Ch.B. Lushchik, A.G. Frorip & N.A. Jaanson, Soy. Phys.Solid State 34, 1690 (1992). 14. R, Kink, A. L6hmus, H. Niedrais, P. Vaino, S. Sorensen, S. Huldt & I. Martinson, Physica Scripta 43, 517 (1991). 15. G. Spriissel, M. Skibowski & V. Saile, Solid State Commun. 32, 1091 (I 979). 16. V. Saile & M. Skibowski, Phys. Status. Solidi (b) 50, 661 (1972). 17. K.U. lbragimov, A.Ch. Lushchik, Ch.B. Lushchik, A. Baimakhanov, E.A. Vasil'chenko & T.I. Savikhina, Soy. Phys.-Solid State 34. 1831 (1992). !. 2.
.Solid State Communications. Vol. 90. No. I I. pp. 741-744, 1994 Elsevtcr Scicm:¢ Ltd Printed in Great Britain.
i Pergamon
0038-
I098194 $7.00 +
.oo
0038- IO98(94)EO163-6
MULTIPLICATION OF ANION AND CATION ELECTRONIC EXCITATIONS IN WIDE-GAP KCI AND CsC! CRYSTALS M. Kirm and I. Martinson Department of Physics, University of Lund, S-223 62 Lund, Sweden A. Lushchik Department of Physics, Tartu University, EE 2400 Tartu, Estonia and K. Kalder, R. Kink, Ch. Lushchik and A. L6hmus Institute of Physics, Estonian Academy of Sciences, EE 2400 Tartu, Estonia
(Received 6 December 1993; acceptedfor publication 22 February 1994 by B. Lundqvist) Luminescence of self-trapped excitons, extrinsic recombinational luminescence and crossluminescence on the excitation by synchrotron radiation (7.7-30 eV) have been investigated for KCI and CsCI crystals at 6-80 K. The following mechanisms of multiplication of electronic excitation are discussed: (i) decay of cation excitons with the formation of a double amount of anion excitons (KCI); (iS) photoionization of anions and cations with the subsequent formation of secondary excitons and electron-hole pairs by fast electrons.
1. INTRODUCTION THE ELEMENTARY mechanism of multiplication of eh.'ctronic excitations (MEE) was revealed many years ago for narrow-gap semiconductors [1]. A photon with the energy exceeding by several times the value of the energy gap (Ez), creates a hot electron and a hot hole whose energies are sufficient for the formation of a secondary electron-hole pair. As a result the quantum yield of the photoelectric effect grows from I to 3 in the region of photon energies by h v > ( 3 - 4 ) E ~ . The spectra of recombinational luminescence excitation by VUV-radiation have already been investigated in wide-gap ionic crystals with narrow valence bands. According to these investigations the threshold energy of the electron-hole mechanism of MEE is Ej~ ~ 2Ex. The quantum yield of luminescence is doubled in the region of hv > E~ (the so-called effect of photon multiplication) [2, 3]. Another mechanism of M EE has been detected in doped alkali iodides [2, 3]. It involves the creation, by hot photoelectrons, of secondary excitons which excite the emission of impurity centres. The threshold energy of this mechanism of MEE is E° = Eg + E~.
where E,a is the energy of anion exciton formation. Additional information on excitonic and electronhole mechanisms of MEE has been obtained through the investigation of excitation spectra for the luminescence of self-trapped exeitons (STE) [4]. The application of synchrotron radiation (SR) has led to a significant broadening of the spectral region of investigations and some new MEE mechanisms have been detected in solids [5-8]. Investigations of excitation spectra of crossluminescence, (Auger free luminescence) due to the recombination of electrons from the valence band with the holes from the cation core band have been carried out recently. The threshold energy of crossluminescence (CL) excitation in CsBr and CsCI coincides with the value of the ionization energy for cations, Exc ~ 14eV [9-1 I]. The aim of the present study is to investigate various mechanisms of M EE with the participation of anion EE as well as the cation ones in f.c.c. KCI and s.c. CsCI crystals. For this purpose we have investigated the excitation spectra for luminescence of STE, extrinsic recombinational luminescence and CL in the photon energy region up to 30eV.
741
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ANION AND CATION ELECTRONIC EXCITATIONS 2. EXPERIMENTAL
Single KCi and CsCI crystals were grown by the Stockbarger method after a special purification cycle involving a manyfold recrystallization from the melt. The content of impurity ions was on the level of 0.01 to I ppm [12. 13]. Experiments were carried out at beamline 52 in MAX-LAB at Lund (550MeV storage ring). The experimental setup has been briefly described in [14]. Synchrotron radiation (SR) is focussed on the entrance slit of a l-m normal-incidence vacuum monochromator equipped with a gold-coated 1200 lines mm -I grating. The samples are placed into a liquid helium cryostat evacuated by using a turbomolecular pump. The measurements of optical spectra were carried out by means of photomultipliers operating in the photon counting mode. To account for the spectral distribution of the intensity of SR and the intensity changes due to the variation of the current in the storage ring, a two-channel photon counting system was used. The monochromatized SR traversed a mesh coated with sodium salicylate and then fell on the sample in the cryostat. One of the photomultipliers detected the blue luminescence of sodium salicylate (reference signal for normalizing the signal from the sample) and the other the radiation from the sample. The STE luminescence was detected through an OS-I I filter (transparent for hu < 2.4eV) for KCI and an SZS-22 lilter (3.4 > h~, > 2.2eV) for the CsCI crystal, whereas CL was detected through a 35em grating monochromator (McPherson EU-700), operating in the 2000-8000 A region. The slit width of the primary monochromator was 200-400/zm (resolution !.7-3.3 A). 3. RESULTS In a KCI crystal at 4.2 K, the formation energy of anion excitons is Eea = 7.77 eV, the energy gap EKa = 8.69eV [12] and the formation energy of cation excitons Eec = 19.4 eV [I 5]. For CsCI these values are E,a = 7.86, Ega = 8.4 [13] and Ee¢ = 13.2eV [3, 161. Thus, for KCI E° < E,c, E~ < Eec, E,~/E~a > 2 while for CsCI E,~ > E¢,, E~ > E¢¢, E~¢/E,~ < 2. Thus, the elementary mechanisms of MEE should be significantly different for KCI and CsCI.
3. I. KC! crystal The peaks at 7.77 and 7.88eV in the reflection spectrum of the KCI crystal at 4.2 K correspond to the spin-orbit splitting of pSs excitons. Photons with h u > E~=8.69eV create separate electrons and holes. The STE emission (maximum at 2.31eV,
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Fig. I. Excitation spectra of 2.1-2.4 eV luminescence for a KCI crystal. The lower curve is multiplied by 2. Arrows show the formation energies of anion and cation excitons, electron-hole pairs and the combinations between them. bandwidth of 0.51eV) is the dominant one in the emission spectrum of a high-purity KCI crystal at 4.2K - see, e.g. [12]. Figure ! shows the excitation spectrum of STE luminescence for a KCI crystal at 6 K. The quantum efficiency of STE luminescence on the excitation by 7.75eV photons is about 0.05 at 4-15 K and this value decreases by tens of times on heating the crystal up to 30K. In the region of 7.7-19eV SR creates anion EE in the KCI crystal. Narrow peaks at 19.39, 19.69, 20.06, 20.27, 21.00 and 21.42eV are clearly manifested in the reflection spectrum of KCI [15]. These peaks correspond to the optical excitation and the ionization of the 3p6 shell of the K + ion. The narrow minima in the excitation spectrum of STE emission are strictly correlated with the reflectance maxima in the region of 19-22eV. The intensity of the STE emission increases in the region of anion EE formation in two stages: at 15-17 and 17-19eV (Fig. 1, open circles). The absorption constant increases with the increase of photon energy from 17 to 22 eV. Therefore, the depth of penetration of the excitation radiation into a crystal decreases and the efficiency of nonradiative decay of EE near the surface increases in this energy region. The quantum yield of the photoelectric effect decreases and the energy distribution of the emitted electrons changes in favour of slow electrons in the photon energy range from 14 to 18 eV in the KCI crystal at 300 K [8]. All the above-mentioned facts can be explained by the transformation of one absorbed photon, with the energy from 2E,.a = 15.5eV up t o Ega 4- Ee,, = 16.SeV, into two STE. This transformation takes place
Voi. 90, No. ! 1
ANION AND CATION ELECTRONIC EXCITATIONS
directly or after intermediate proeesses: the creation of a hole and a fast electron, the formation of a secondary exciton by a fast electron and a recombination of an electron and a hole with the creation of another secondary exciton. The intensity of STE emission increases even more in the region hu > 2Ega = 1.75 eV. Photoionization of chlorine ions leads to the appearance of electrons with energies sufficient for the formation of secondary electron-hole pairs. We have succeeded in separating the electronic and electron-hole mechanisms of MEE by investigating the processes of MEE in the KCI crystal at various temperatures. A comparison of the excitation spectra of the emission with 2.1 < hu <_2.4eV at 6 and 50K shows that the thermal quenching of this emission proceeds differently at the excitation by 16 or 18eV photons: the emission of secondary STE (excitation by 16eV photons) is practically quenched at 50K, whereas the recombination of secondary electrons and holes near defects (excitation by 18 eV photons) leads to the appearance of a weak emission even at 50 K. The mean free path of free excitons before self-trapping is about 20-5 lattice constants in KCI at 5-80K [17]. Therefore, secondary excitons can neither inter-act with defects nor form near-defect-localized EE. The energy region of 19-22eV, where SR generates cation EE, is of special interest for us. In this region the efficiency of STE luminescence excitation at 10 K is at least twice as high as at the excitation by I I eV photons. The quantum yield rI is about 1.4 for the recombinational emission of TI + centres on the excitation of KCI-TI crystal by 2023 eV photons [5] and r/= 1.3 for photoelectric effect on the excitation of KCI by 22eV photons [81. Undoubtedly, the decay of cation EE in KC1 leads to the formation of a double number of anion EE. This process is the main rival to CL, which has not yet been observed in a KCI crystal. 3.2. CsCi crystal The peaks at 7.86, 7.94, 8.01, 8.09 and 8.19eV in the reflection spectrum of CsCI at 10 K correspond to the pSs and pSd excitons. Photons with hu> Ex,,=8.4eV create separate electrons and holes [13]. Narrow peaks in the reflection spectrum (29 K) of CsCI at 13.2-16.77eV are connected with the excitation and ionization of the 5p6 shell of the Cs + ion [16]. The emission bands at 4.6 and 5.2eV have been detected during the excitation of the CsCI crystal by X-rays. For a long time these bands have been interpreted as originating from the luminescence of STE. However, the threshold energy of the excitation
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Fig. 2. Excitation spectra of 4.6eV luminescence (a, b) and 2.2-3.4eV emission (c) for a CsCI crystal. for 4.6 and 5.2eV emissions is about 14eV [10, !1], and thus these emission bands correspond to CL. According to our data [13], the luminescence of STE in high-purity CsCI at 5K has the maximum at 2.85 eV. The mean free path of free excitons before self-trapping in CsCI is about 350 lattice constants at 5 K [17], and excitons either rise easily to the surface and decay there nonradiatively or decay with a typical emission near point defects. The emission of near-defect-localized EE can be observed even at high temperatures. Figure 2 shows the excitation spectra of CL in CsCI. The threshold energy of CL excitation is about 14 eV at 6 and 80 K. The variations in the efficiency of CL excitation in the region of 15-19.5eV can be explained by the changes of the absorption constant and the depth of penetration of radiation into a crystal as well as different temperature dependences of the mean free path of excitons and holes before localization. According to our data the efficiency of CL excitation increases sharply at hv > 27 eV, which is caused by the multiplication of cation EE: one photon with hu > 2Ego creates two holes in the cation core band. According to [8] the yield of the photoelectric effect decreases and the energy distribution of the emitted electrons in CsCI changes in favour of slow electrons in the region of 26-34eV, Besides CL (4.6-5.2 eV) the emission of STE with the maximum at 2.8-2.9eV has been observed during
744
ANION AND CATION ELECTRONIC EXCITATIONS
the excitation of our CsCI sample by X-rays or 20.5eV photons at 6K. Photons of 8eV excite STE emission and do not excite the emissions at 4.6 and 5.2eV. An impurity-defect emission at 3-3.7eV has been detected during the excitation of the crystal by 7.7 eV photons. Figure 2(c) shows that the efficiency of 2.2-3.4 eV emission varies insignificantly in the excitation range of 12-19 eV, decreasing only in the region of narrow reflectance peaks that correspond to the formation of cation excitons. The intensity of this emission increases sharply at 20-26eV. This effect is caused partly by the decrease of the absorption constant and partly by the formation of secondary anion excitons and electron-hole pairs by the fast photoelectrons created by the ionization of cations. In the region of 26-30eV the efficiency of anion EE emission decreases simultaneously with the sharp increase of the intensity of CL due to the participation of secondary cation EE. The threshold energies of these processes are Etc + Es~ and 2Ego. In a CsCI crystal Eec/Eea equals 1.69. Therefore, in contrast to KCI, the decay of cation EE with the formation of a double amount of anion EE is impossible for energetic reasons. Apparently, in CsCI the decay of one cation exciton leads to the formation of one anion exeiton and a packet of phonons. Nonradiative decay of cation excitons with the formation of Frenkel defects in anion and cation sublattices may be expected in a CsCI crystal as well. The ionization of cations takes place at hu > E~c = 14 eV. CL is observed following the recombination of an electron from the anion valence band with a hole from the cation core band. The recombination of an electron with an anion hole causes either STE luminescence or recombinational emission near defects and impurities. 4. CONCLUSIONS This work has shown that MEE processes are rather different for KCI and CsCI crystals, which have different ratios of anion and cation exciton creation energies ( E, d E~). Excitonic and electron-hole mechanisms of M EE with the threshold energies E° ~ Esa+ E,a and Er~ ~ 2Eta have been detected for KCI, but these mechanisms are not realized for a CsCI crystal with the low energy of cation exeiton formation. The decay of a one cation EE with the formation of two anion EE has been detected in KC! (E¢,/Eea > 2) but
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not in CsC! crystal (E, clEe, < 2). The multiplication of cation EE with the threshold energy Et~c:~ 2Esc occurs in a CsCi crystal and causes a sharp increase of the CL excitation efficiency.
Acknowledgements - We are grateful to Professor I. Lindau for valuable discussions. This work has been supported by the Swedish Natural Science Research Council (NFR), the Royal Swedish Academy of Sciences, the Swedish Institute, The Crafoord Foundation and the Carl Trygger Foundation. REFERENCES W. Shockley, Czech J. Phys. BII, 81 (1961). E.R. IImas, G.G. Liidya & Ch.B. Lushchik, Opt. Spectrosc. (USA) 18, 255, 359 (1965). 3. E.R. Ilmas, R.A. Kink, G.G. Liidya & Ch.B. Lushchik, Bull. Acad. Sci. USSR. Phys. Ser. (USA) 29, 29 (1965). 4. N.S. Rooze, Soy. Phys. Solid State 17, 690 (1975). 5. S.N. lvanov, E.R. llmas, Ch.B. Lushchik & V.V. Mikhailin, Soy. Phys.-SolidState 15, 1053 (1973). 6. J.H. Beamont, A.J. Bourdilon & M.N. Kabler, J. Phys. C9, 2961 (1976). 7. M. Yanagihara, Y. Kondo & H. Kanzaki, J. Phys. Soc. Jpn 52, 4357 (1983). 8. H. Sugawara & T. Sasaki, J. Phys. Soc. Jpn 46, 132 (i 979). 9. Yu.M. Aleksandrov, I.L. Kuusmann, P.Kh. Liblik, Ch.B. Lushchik, V.N. Makhov, T.i. Syreishchikova & M.N. Jakimenko, Soy. Phys.-Solid State 29, 587 (1987). 10. i. Kuusmann, T. Kloiber, W. Laasch & G. Zimmerer, Rad. Effects and Defects in Solids il9-121, 21 (1991). 11. N.Yu. Kirikova & V.N. Makhov, Soy. Phys.Solid State 34, 1557 (1992). 12. Ch. Lushchik, J. Kolk, A. Lushchik, N. Lushchik, M. Tajirov & E. Vasil'chenko, Phys. Status Solidi (b) !14, 103 (1982). 13. K.U. lbragimov, A.Ch. Lushchik, Ch.B. Lushchik, A.G. Frorip & N.A. Jaanson, Soy. Phys.Solid State 34, 1690 (1992). 14. R, Kink, A. L6hmus, H. Niedrais, P. Vaino, S. Sorensen, S. Huldt & I. Martinson, Physica Scripta 43, 517 (1991). 15. G. Spriissel, M. Skibowski & V. Saile, Solid State Commun. 32, 1091 (I 979). 16. V. Saile & M. Skibowski, Phys. Status. Solidi (b) 50, 661 (1972). 17. K.U. lbragimov, A.Ch. Lushchik, Ch.B. Lushchik, A. Baimakhanov, E.A. Vasil'chenko & T.I. Savikhina, Soy. Phys.-Solid State 34. 1831 (1992). !. 2.