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Infrared luminescence of ZnSe : Cr crystals
Journal of Luminescence 12/13 (1976) 701 705 o North-Holland Publishing Company
INFRARED LUMINESCENCE OF ZnSe: Cr CRYSTALS G. GREBE
“,
G. ROUSSUS and H.-J. SCHULZ
Institut für Elektronenmikrosko pie am Frits-Haber-Institut der Max-Planck-Gesellschaft, Berlin-Dahlem, Germany
At 4 K, the emission of ZnSe Cr crystals around 4500 cm displays a zero-phonon 5A 5E) —~ 5B 5T doublet and vibronic Structures. The radiative transition is 2~whose symmetry is reduced from Td to D,d by 1( the static2(Jahn 2) at Teller sub-efstitutional fect. The excitation Cr spectrum of this band indicates band-to-band transitions, photoionization of centres and transitions to Cr2+ excited states. These latter are fitted in the Tanabe Sugano approximation. Additional structured emission bands near 6000 and 10 000 cm are investigated.
The properties of Cr-doped ZnSe have been the subject of various papers in recent years. By means of EPR measurements charge states Cr+ [1] and Cr2+ [21 were detected. Later, optical absorption in the near infrared was studied [3,4]. Supplementing previous mvestigations [5] of the IR emission of Cr-doped ZnS, the luminescence of ZnSe : Cr crystals (cf. [6]) was studied in the spectral range from 3800 to 11 000 cm 1 at temperatures between T = 2 K and T = 300 K. Electron diffraction patterns of crushed grains of the melt-grown crystals were cubic with some twinning and a minimal degree of stacking faults At T 4.2 K, the spectrum (fig. 1) consists of three structured bands. The band in the lower energy range displays two lines of weak intensity at 4964 cm 1 and 4971 cm if (fig. 2). In analogy to our interpretation of the IR emission of ZnS : Cr near 5000 cm 1, this band can be attributed to the transition 5A 5E) 1( 5B 5T 2~ions in D 2( 2) at Cr 2d symmetry (cf. [5]). The two lines correspond to zero 5B phonon transitions into the spin orbit terms F5 resp. F1, ~‘2 of the 2 ground state. At absorption measurements with the same crystal, in accordance with Vallin et al. [3] at T = 4.2 K only one zero phonon line appears which has with 4973 cm 1 approximately the energy of 5Bthe high energy emission line. Transitions from the F5 term (and the F4 term) of 2 can, in absorption, only be observed at higher temperatures; in emission, however, even at T = 4.2 K, transitions to F5 can be recorded with an intensity comparable to that of transitions into the lowest spin orbit term F1, F2. This is in good agreement with the interpretation of the emission and ab‘~‘.
—
—~
* **
Present address: Institut für Atom- und FestkOrperphysik, Freie Universitat Berlin. Courtesy of Dr. R.R. Reeber. Vacuum wave numbers throughout. 701
702
G. Grebe eta!. wove Length (pmI 2.826 21. 22 20
/ Infrared luminescence of ZnSe:
1.8
1.6
1L
Cr crystals
1.2
:
10
/1\\
I ~ \E T~42K
-
:~ , ,~
l~80
1.000
5000
6000
7000
8000
9000 —‘
10000 1 wove number (cm
I
l’ig. 1. Emission spectrum of a ZnSe : Cr crystal (330 ppm Cr). Excitation range: t~ = 7700 40 000 cm The spectrum has been corrected for the spectral response characteristics of the PbS detector and for the spcctral efficiency curve of the grating. The emission bands have been normalized by multiplication with the indicated factors.
sorption line doublets for ZnS : Cr [5]. In view of the comparable value of ionicity for ZnS and ZnSe [7], the approximation of the static crystals field theory proves to be similarly suitable for both substances. The vibronic emission band with maximum at 4746 cm 1 displays a shoulder at 4915 cm 1 and further maxima at 4868 cm 1 and 4779 cm Thus, vibronic ener~.
wave length (pm] 2~/. 212
C
4—
210
.
2~8
206
201.
.
202
. —
3
wove number [cm 1]
Fig. 2. Structure of the 4500 cm 1 emission band of a ZnSe : Cr crystal (330 ppm Cr). Excitation range: v 7700 ... 40 000 cm* Corrections as with fig. 1.
G. Grebe eta!.
I Infrared luminescence of ZnSe:
Cr crystals
703
gies of about 52,99 and 188cm 1 follow as does the displacement of the main maximum by about 221 cm l A value of 49 cm 1 is also found in the vibronic absorption spectrum and in the far infrared transmission with Cr-doped ZnSe [3]. An energy of 96 cm 1 is, however, only found in the far infrared region, there even at non-Cr-doped ZnSe. For reasons of selection rules in Td symmetry, coupling with lattice modes of E- and/or T2-symmetries, respectively, is to be expected. Thus, the phonon energy of 52 cm~may be attributed to an impurity-induced lattice mode, here the TA(L) phonon. The 99 cm value corresponds to a TA1(K) mode. At 188 cm 1, a single-phonon coupling with LA(K) or LA(X) as well as a two-phonon process with the TA1(K) mode may be involved. Since near 221 cm 1 the highest peak occurs in the phonon density of states diagram [81, our main emission maximum can be interpreted as TO(L) coupling. At 4.2 K, the emission band which is next higher in energy (cf.band fig. also 1) is displays of rather 2~centre. This weak intensity compared with the band of the Cr two lines at 6319 cm 1 and at 6326 cm The structured emission band centered at 6128 cm also has shoulders, viz, near 6194 and 6258 cm l~The vibronic displacements of 63, 127, and 193 cm 1 seem to exclude phonons of the undisturbed ZnSe lattice, so that rather a quasi-local mode and possibly its multiples will be involved. The entire band cannot be recorded at higher temperatures. On the other hand, at 80 K a band with maximum at 7000 cm 1 is found (cf. [6]). At 4.2 K and at 80 K, ZnSe crystals doped for comparison with Cu or with (Li, Fe) display, it is true in this energy region IR emission bands; these bands are, however, distinctly different from one another and from those of the ZnSe : Cr crystals. An unambiguous identification of this band is thus not yet possible. The intriguing fact that the splitting of its zero phonon doublets is again 7 cm 1 suggests an explanation in terms of transitions into the 5B 2+ ground state from another 5D sub-term ex2 Cr periencing a different lattice interaction. The strongest band of the spectrum (cf. fig. 1) centered near 10000 cm 1 decreases strongly at T> 80 K. At 4.2 K, a zero phonon line is recorded at 10 702 cm Towards lower energies, a phonon satellite range is annexed with structures at 10650, 10610, 10 540, and 10485 cm This time the shifts with respect to the zero phonon line match with the lattice phonons as can be substantiated by the density-of-states diagram of ZnSe [81.Moreover, additional structures occur on the low-energy tail of the 10 000 cm 1 band: at 9700,8390, and 7530 cm l~With another crystal which is very weakly doped with Cr, pronounced peaks can be resolved here. Since an emission in the 10 000 cm 1 range is also observed with crystals doped by other transition elements (e.g. Cu or Fe, Li) it may be suspected that a native defect present in all these cases gives rise to this emission. A corresponding absorption could not be detected with the Cr-doped crystals. At T = 4.2 K, the excitation spectrum has been recorded for the 4500 cm 1 luminescence band (fig. 3). The dominating part is in the high photon energy range. Next to the narrow peaks at 22 800 and 22 250 cm 1, representmg the band gap ~.
~.
704
G. Grebe et aL
,
I Infrared luminescence of ZnSe:
wave length ] pm] 12 11 10 09 0,8
07
06
Cr crystals
05
04
~ 120
120
3
~~0~0~T42K
8000
10000
12000
14000
Fig. 3. Fxcitation spectrum of the 4500 cm Cr).
16000
18000
20000
22000
21.000
wave number [ m
1
1]
emission band of a ZnSe : Cr crystal (660 ppm
and exciton regions, respectively, a strong absorption tailing into the gap gives rise to the broader maxima near 21 200 and 19 000 cm l~This latter range closely resembles the dominating photo-ionization region known for ZnS: Cr [5j. Hence, the same types of processes will be involved in the excitation mechanism here: liberation of carriers into the conduction band and subsequent capture and radiative recombination at the Cr2~centre. Towards lower energies, peaks at 16 400, 14 950, and 12 950 cm 1 as well as a shoulder near 11 500 cm 1 follow. Various ways to fit this excitation structure (cf. [9]) by adjusting the parameters Dq, B, and C in the strong field coupling approximation [lOj have been tried among which an attribution of the most pronounced peak at 14950 cm 1 to the Dq-independent upper 3T 3H) level in the Tanabe 1( Sugano diagram was most successful. Details of this procedure will be given elsewhere. The closest fit has been obtained with: cubic crystal field parameter Dq = 480 cm Racah parameter B = 510 cm 1, Racah parameter C = 3053 cm
1,
The comparable values estimated for ZnS : Cr were [5,9]: Dq510,
B=500,
C2850cm
l~
G. Grebe et al.
/ Infrared luminescence of ZnSe:
Cr crystals
705
The described procedure does not yield the faint excitation structure near 9000 cm 1, In this region, however, changes in intensity and structure have been observed which are dependent on optical irradiation and other details of the prehistory of the samples so that slow charge transfer processes seem to indicate that a different type of centre could be implied here, e.g. Cr’ (cf. [5]). The general features of the proposed fitting procedure and the given set of parameters agree, in view of the crude approximations used, convincingly with the ZnS: Cr data and the known evidence from other transition metal impurities in the II VI compounds.
References [1] R.S. Title, Phys. Rev. 133 (1964) A1613. [2] M. DeWit, A.R. Reinberg, W.C. Holton and T.L. Estle, Bull. Am. Phys. Soc., Set. 2 10 (1965) 329. [3] J.T. Vallin, GA. Slack, S. Roberts and A.E. Hughes, Solid State Conimun. 7 (1969) 1211; J.T. Vallin, GA. Slack, S. Roberts and A.E. Hughes, Phys. Rev. B2 (1970) 4313. [4] B. Nygren, J.T. Vallin and G.A. Slack, Solid State Commun. 11(1972) 35. [51G. Grebe and l-l.-J. Schulz, Z. Naturforsch. 29a (1974) 1803. [6j G. Grebe, H. Nelkowski and G. Roussos, Verhandl. DPG (VI) 8 (1973) 306; G. Roussos, Diplomarbeit, Technische Universitat Berlin (1974). [7] J.C. Phillips, Rev. Mod. Phys. 42 (1970) 317. [8] K. Kunc, Ann. Phys. 8 (1973/74) 319. [9J G. Grebe and H.-J. Schulz, Phys. Stat. Sol. (b) 54(1972) K 69. [101 Y. Tanabe and S. Sugano, J. Phys. Soc. Japan 9 (1954) 753, 766.
—~
Discussion
D.S. McClure: I did not see the familiar quintet quintet transition in your excitation spectra. Does it give rise to an emission band? H.-J. Schulz: This transition can be seen in absorption as a broad band near 5500 cm with our crystals. The excitation spectrum has not yet been extended into this range for experimental reasons. According to the model, the quintet quintet absorption transition must give rise to the corresponding emission band centred near 4500 cm 1
—~
INFRARED LUMINESCENCE OF ZnSe: Cr CRYSTALS G. GREBE
“,
G. ROUSSUS and H.-J. SCHULZ
Institut für Elektronenmikrosko pie am Frits-Haber-Institut der Max-Planck-Gesellschaft, Berlin-Dahlem, Germany
At 4 K, the emission of ZnSe Cr crystals around 4500 cm displays a zero-phonon 5A 5E) —~ 5B 5T doublet and vibronic Structures. The radiative transition is 2~whose symmetry is reduced from Td to D,d by 1( the static2(Jahn 2) at Teller sub-efstitutional fect. The excitation Cr spectrum of this band indicates band-to-band transitions, photoionization of centres and transitions to Cr2+ excited states. These latter are fitted in the Tanabe Sugano approximation. Additional structured emission bands near 6000 and 10 000 cm are investigated.
The properties of Cr-doped ZnSe have been the subject of various papers in recent years. By means of EPR measurements charge states Cr+ [1] and Cr2+ [21 were detected. Later, optical absorption in the near infrared was studied [3,4]. Supplementing previous mvestigations [5] of the IR emission of Cr-doped ZnS, the luminescence of ZnSe : Cr crystals (cf. [6]) was studied in the spectral range from 3800 to 11 000 cm 1 at temperatures between T = 2 K and T = 300 K. Electron diffraction patterns of crushed grains of the melt-grown crystals were cubic with some twinning and a minimal degree of stacking faults At T 4.2 K, the spectrum (fig. 1) consists of three structured bands. The band in the lower energy range displays two lines of weak intensity at 4964 cm 1 and 4971 cm if (fig. 2). In analogy to our interpretation of the IR emission of ZnS : Cr near 5000 cm 1, this band can be attributed to the transition 5A 5E) 1( 5B 5T 2~ions in D 2( 2) at Cr 2d symmetry (cf. [5]). The two lines correspond to zero 5B phonon transitions into the spin orbit terms F5 resp. F1, ~‘2 of the 2 ground state. At absorption measurements with the same crystal, in accordance with Vallin et al. [3] at T = 4.2 K only one zero phonon line appears which has with 4973 cm 1 approximately the energy of 5Bthe high energy emission line. Transitions from the F5 term (and the F4 term) of 2 can, in absorption, only be observed at higher temperatures; in emission, however, even at T = 4.2 K, transitions to F5 can be recorded with an intensity comparable to that of transitions into the lowest spin orbit term F1, F2. This is in good agreement with the interpretation of the emission and ab‘~‘.
—
—~
* **
Present address: Institut für Atom- und FestkOrperphysik, Freie Universitat Berlin. Courtesy of Dr. R.R. Reeber. Vacuum wave numbers throughout. 701
702
G. Grebe eta!. wove Length (pmI 2.826 21. 22 20
/ Infrared luminescence of ZnSe:
1.8
1.6
1L
Cr crystals
1.2
:
10
/1\\
I ~ \E T~42K
-
:~ , ,~
l~80
1.000
5000
6000
7000
8000
9000 —‘
10000 1 wove number (cm
I
l’ig. 1. Emission spectrum of a ZnSe : Cr crystal (330 ppm Cr). Excitation range: t~ = 7700 40 000 cm The spectrum has been corrected for the spectral response characteristics of the PbS detector and for the spcctral efficiency curve of the grating. The emission bands have been normalized by multiplication with the indicated factors.
sorption line doublets for ZnS : Cr [5]. In view of the comparable value of ionicity for ZnS and ZnSe [7], the approximation of the static crystals field theory proves to be similarly suitable for both substances. The vibronic emission band with maximum at 4746 cm 1 displays a shoulder at 4915 cm 1 and further maxima at 4868 cm 1 and 4779 cm Thus, vibronic ener~.
wave length (pm] 2~/. 212
C
4—
210
.
2~8
206
201.
.
202
. —
3
wove number [cm 1]
Fig. 2. Structure of the 4500 cm 1 emission band of a ZnSe : Cr crystal (330 ppm Cr). Excitation range: v 7700 ... 40 000 cm* Corrections as with fig. 1.
G. Grebe eta!.
I Infrared luminescence of ZnSe:
Cr crystals
703
gies of about 52,99 and 188cm 1 follow as does the displacement of the main maximum by about 221 cm l A value of 49 cm 1 is also found in the vibronic absorption spectrum and in the far infrared transmission with Cr-doped ZnSe [3]. An energy of 96 cm 1 is, however, only found in the far infrared region, there even at non-Cr-doped ZnSe. For reasons of selection rules in Td symmetry, coupling with lattice modes of E- and/or T2-symmetries, respectively, is to be expected. Thus, the phonon energy of 52 cm~may be attributed to an impurity-induced lattice mode, here the TA(L) phonon. The 99 cm value corresponds to a TA1(K) mode. At 188 cm 1, a single-phonon coupling with LA(K) or LA(X) as well as a two-phonon process with the TA1(K) mode may be involved. Since near 221 cm 1 the highest peak occurs in the phonon density of states diagram [81, our main emission maximum can be interpreted as TO(L) coupling. At 4.2 K, the emission band which is next higher in energy (cf.band fig. also 1) is displays of rather 2~centre. This weak intensity compared with the band of the Cr two lines at 6319 cm 1 and at 6326 cm The structured emission band centered at 6128 cm also has shoulders, viz, near 6194 and 6258 cm l~The vibronic displacements of 63, 127, and 193 cm 1 seem to exclude phonons of the undisturbed ZnSe lattice, so that rather a quasi-local mode and possibly its multiples will be involved. The entire band cannot be recorded at higher temperatures. On the other hand, at 80 K a band with maximum at 7000 cm 1 is found (cf. [6]). At 4.2 K and at 80 K, ZnSe crystals doped for comparison with Cu or with (Li, Fe) display, it is true in this energy region IR emission bands; these bands are, however, distinctly different from one another and from those of the ZnSe : Cr crystals. An unambiguous identification of this band is thus not yet possible. The intriguing fact that the splitting of its zero phonon doublets is again 7 cm 1 suggests an explanation in terms of transitions into the 5B 2+ ground state from another 5D sub-term ex2 Cr periencing a different lattice interaction. The strongest band of the spectrum (cf. fig. 1) centered near 10000 cm 1 decreases strongly at T> 80 K. At 4.2 K, a zero phonon line is recorded at 10 702 cm Towards lower energies, a phonon satellite range is annexed with structures at 10650, 10610, 10 540, and 10485 cm This time the shifts with respect to the zero phonon line match with the lattice phonons as can be substantiated by the density-of-states diagram of ZnSe [81.Moreover, additional structures occur on the low-energy tail of the 10 000 cm 1 band: at 9700,8390, and 7530 cm l~With another crystal which is very weakly doped with Cr, pronounced peaks can be resolved here. Since an emission in the 10 000 cm 1 range is also observed with crystals doped by other transition elements (e.g. Cu or Fe, Li) it may be suspected that a native defect present in all these cases gives rise to this emission. A corresponding absorption could not be detected with the Cr-doped crystals. At T = 4.2 K, the excitation spectrum has been recorded for the 4500 cm 1 luminescence band (fig. 3). The dominating part is in the high photon energy range. Next to the narrow peaks at 22 800 and 22 250 cm 1, representmg the band gap ~.
~.
704
G. Grebe et aL
,
I Infrared luminescence of ZnSe:
wave length ] pm] 12 11 10 09 0,8
07
06
Cr crystals
05
04
~ 120
120
3
~~0~0~T42K
8000
10000
12000
14000
Fig. 3. Fxcitation spectrum of the 4500 cm Cr).
16000
18000
20000
22000
21.000
wave number [ m
1
1]
emission band of a ZnSe : Cr crystal (660 ppm
and exciton regions, respectively, a strong absorption tailing into the gap gives rise to the broader maxima near 21 200 and 19 000 cm l~This latter range closely resembles the dominating photo-ionization region known for ZnS: Cr [5j. Hence, the same types of processes will be involved in the excitation mechanism here: liberation of carriers into the conduction band and subsequent capture and radiative recombination at the Cr2~centre. Towards lower energies, peaks at 16 400, 14 950, and 12 950 cm 1 as well as a shoulder near 11 500 cm 1 follow. Various ways to fit this excitation structure (cf. [9]) by adjusting the parameters Dq, B, and C in the strong field coupling approximation [lOj have been tried among which an attribution of the most pronounced peak at 14950 cm 1 to the Dq-independent upper 3T 3H) level in the Tanabe 1( Sugano diagram was most successful. Details of this procedure will be given elsewhere. The closest fit has been obtained with: cubic crystal field parameter Dq = 480 cm Racah parameter B = 510 cm 1, Racah parameter C = 3053 cm
1,
The comparable values estimated for ZnS : Cr were [5,9]: Dq510,
B=500,
C2850cm
l~
G. Grebe et al.
/ Infrared luminescence of ZnSe:
Cr crystals
705
The described procedure does not yield the faint excitation structure near 9000 cm 1, In this region, however, changes in intensity and structure have been observed which are dependent on optical irradiation and other details of the prehistory of the samples so that slow charge transfer processes seem to indicate that a different type of centre could be implied here, e.g. Cr’ (cf. [5]). The general features of the proposed fitting procedure and the given set of parameters agree, in view of the crude approximations used, convincingly with the ZnS: Cr data and the known evidence from other transition metal impurities in the II VI compounds.
References [1] R.S. Title, Phys. Rev. 133 (1964) A1613. [2] M. DeWit, A.R. Reinberg, W.C. Holton and T.L. Estle, Bull. Am. Phys. Soc., Set. 2 10 (1965) 329. [3] J.T. Vallin, GA. Slack, S. Roberts and A.E. Hughes, Solid State Conimun. 7 (1969) 1211; J.T. Vallin, GA. Slack, S. Roberts and A.E. Hughes, Phys. Rev. B2 (1970) 4313. [4] B. Nygren, J.T. Vallin and G.A. Slack, Solid State Commun. 11(1972) 35. [51G. Grebe and l-l.-J. Schulz, Z. Naturforsch. 29a (1974) 1803. [6j G. Grebe, H. Nelkowski and G. Roussos, Verhandl. DPG (VI) 8 (1973) 306; G. Roussos, Diplomarbeit, Technische Universitat Berlin (1974). [7] J.C. Phillips, Rev. Mod. Phys. 42 (1970) 317. [8] K. Kunc, Ann. Phys. 8 (1973/74) 319. [9J G. Grebe and H.-J. Schulz, Phys. Stat. Sol. (b) 54(1972) K 69. [101 Y. Tanabe and S. Sugano, J. Phys. Soc. Japan 9 (1954) 753, 766.
—~
Discussion
D.S. McClure: I did not see the familiar quintet quintet transition in your excitation spectra. Does it give rise to an emission band? H.-J. Schulz: This transition can be seen in absorption as a broad band near 5500 cm with our crystals. The excitation spectrum has not yet been extended into this range for experimental reasons. According to the model, the quintet quintet absorption transition must give rise to the corresponding emission band centred near 4500 cm 1
—~