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IR-spectroscopy of crystals containing Jahn-Teller impurity centers
Infrared Phys. Vol. 29, No. 24, pp. 753-764, 1989 Printed in Great Britain. All rights reserved
Copyright
IR-SPECTROSCOPY JAHN-TELLER
OOZO-089Ij89 $3.00 + 0.00 IC 1989 Pergamon Press plc
OF CRYSTALS CONTAINING IMPURITY CENTERS
P. N. BUKIVSKY, Yu. P. GNATENKO, A. KH. ROZHKO and I. A. FARINA Institute of Physics Academy of Sciences of the Ukrainian SSR, Kiev-28, Prospect Nauki 46, U.S.S.R. (Received 7 October 1988)
Abstract-Using a novel IR-technique, the low-temperature spectra of CdTe, ZnTe and Cd,,,M~,osTe crystals, doped by Jahn-Teller Co*+ Impurity ions, were studied in the spectral range from 1 to 400 pm. The absorption spectrum regions and the energy positions of ZPL, corresponding to the transitions 4A2(4F)+4T,(P), T,(F), 4T2(F) and 2G were detected. The parameters of the spin-orbit interaction I, the crystal field parameter D,,., the parameters of electron-electron interaction B and C were obtained. It was found, that Jahn-Teller interaction differently affects the parameter i, for 4A2(F)-‘T,(P), q,(F) and 4T,(F) transitions and, consequently, the energy positions of ZPL. The electron-vibronic spectra were compared with phonon spectra.
1. INTRODUCTION
The introduction of impurity atoms may permit essential changes in the crystal physical properties, thus providing materials with the parameters necessary for various practical applications. Among the impurity atoms, elements with an unfilled electron 3d”-shell are of particular importance, the introduction of which may cause energy level system onset and optical transitions which cover a wide-range of the spectrum. The use of an up-to-date IR technique allows such transitions to be analysed well and to aquire information yielding crystal energy impurity spectrum and electron oscillation interactions with Jahn-Teller interaction peculiarities. Measurements in a distant IR region enable the study of oscillation centres of real crystals, this is very important for perfect comprehension of the above processes and is also of interest in itself. At present the scientific literature provides a wide choice of theoretical and experimental works including semiconductor crystal groups A” B”’ (mainly ZnS4’ crystals) alloyed by impurity atoms coz+ (I-7) In this work, using CdTe and ZnTe crystals as examples, investigation concerning the influence of impurity atoms Co on an energy structure and the oscillation spectrum of the crystals has been carried out. It resulted in experimental data yielding peculiarities of Jahn-Tellar interaction manifestation as well as a change in oscillation-spectrum in a low-frequency region for the above crystals. 2. EXPERIMENTAL
METHOD
CdTe and ZnTe crystals with various ions CO’+ (NC, = 5 x 10’8-1020cm-3) impurity concentrations were grown using the Bridgman method. Investigation of the absorption spectrum was carried out at T = 1.8-77 K. The preset temperature was maintained with the stabilization temperature system (accuracy = 0.05 K). Intracentral absorbtion spectrum of Co’+ ions in CdTe and ZnTe crystals and the oscillation spectrum of the crystals in the IR spectrum region were measured with a SDL-1 (i < 2 ,um) spectrometer and a Fourier-spectrometer IFS-l 13 (2 > 2 pm) respectively. The SDL- 1 monochromator width spectrum split and resolution Fourier-spectrometer did not exceed 1 cm-‘. 3. EXPERIMENTAL
RESULTS
AND
DISCUSSION
Co’+ ions substituting Cd2+ ions in CdTe crystals and in ZnTe-Zn2+ ions are in tetrahedral surrounding. As shown in the results from Ref. (7) the account of crystalline fields action (CF) of Td symmetry and spin-orbit interaction may lead to 4F main split and the excited 4P and 2G etc. levels of a free Co’+ ion. 153
754
P. N.
BUKIVSKY
et ul.
(cm-‘)
Y
11,600
12,500
50
-;
40
E 0 *
30
20
10
Fig. I. Absorption
spectrum
of CeTe:Co
crystals
at T = 4.5 K [transition
JA2(4F) -“T,(4P)].
Figures l-4 show energy levels of Co 2+ ions in these crystals and their optical spectrum of intracentral absorbtion, corresponding to their intracentral transition from the main state, which considering spin-orbit interaction has r,-symmetry on various excited states. Optical absorption for 4A,(4F) + 4T,(4P)
transition
The most intensive absorption adjoining the fundamental one that which is correlated with the impurity concentration Co in CdTe and is located in the region 1090&12300cm-‘. Such an absorption spectrum may be classified into 3 practically non-overlapping groups, each one of which consists of a great number of lines. The longest wave lines in each of these groups apparently corresponds to zero-phonon transitions from the main state r,(4A,(4F)) to 4T,(4P) term com-
Y
5900
(cm-‘) 5000
5400
4800
2.0
1.5
*=
1.0
0.5
I
1.7
1.9 A
Fig. 2. Co2+ ion absorption
spectrum
2.05
2.10
(pm)
in CdTe crystal
at T = 1.8 K [transition
4A2(4F) -4T,(4F)]
IR-spectroscopy
of crystals containing Jahn-Teller impurity centers
755
2.0 FREE
CF
Xi.;
EoN er 2
p
0.6
I
0
I
I
3800
2800
3200 Y
(cm-‘)
Fig. 3. Absorption spectrum of CdTe:Co crystals at T = 4.5 K [transition 4A2(4F)+4T2(4F)J,
ponents, split by spin-orbit interaction effects. At this in correspondence with the term diagram (see Fig. 1 caption) in the lowest-energy spectrum region two closely spaced lines may be expected (ZPL), governed by the transition effects to F, and Ts symmetry levels. Factually two lines are observed in this region with a maximum distance between each of 11 cm-‘. Magnetooptical investigations(*) indicate, that the lowest ZPL (v,, = 10947 cm-‘) corresponds to level transitions f, and ZPL with a maximum equal to 10959 cm-’ for rg transition. The other ZPL with maxima on the frequencies v = 11769 and v = 12275 cm-’ are due to the rf and r6 transitions correspondingly. The energy distance between ZPL allows the parameters of spin-orbit interaction to be defined as well as 4T, term energy position which is not disturbed by spin-orbit interaction v(~T, (‘P)). The values l&r = 332 f 2 cm-‘; v (4T,(4P)) = 11445 cm-’ are obtained from the calculations. Such a value exceeds twice the value )i 1= 167 cm-’ for CdTe: Co system, theoretically calculated with the effects of covalent bonds being considered(g’ and it is sufficiently more than l&l = 178 cm-’ for a free ion. It should be noted, that the increase of II,1 in comparison with 141 could be observed for other systems as well, for example, transient metals in CdS.“*’ A large value /,?,I_ provides that for 4T,(4P) term the reduction effects of spin-orbit interaction by Jahn-Teller interaction are practically absent. (‘I)On the other hand the correlation (established
0.4i
I 10,600
I 10,200
960Q Y
(cm-‘)
Fig. 4. Absorption spectrum of CaTe:Co crystals at T = 4.5 K (transition 4A2(‘F) -t *G).
156
P. N. BUKWSKY et al
as a result of a spectrum structure analysis) between the frequencies of zero-phonon transitions and static model split by spin orbit interaction is a testimony of a very weak disappearing influence of dynamic effects on the individually taken term components as well. Actually, with the Jahn-Teller effect for two of four ZPL present, governed by optical transitions to TX levels, shears of their positions to the side of low-energies on the energy value of Jahn-Teller stabilization should be expected. The lines frequency position, coupling with transition to F6 and r,-levels (for which the Jahn-Teller effect does not take place”‘) is independent from the value of electron oscillation coupling. ZPL frequency divergence from the static model of spin-orbit split would be the result of such a “selective” interaction which has not been proved empirically. The other less intensive lines (many of which are equidistant with respect to various ZPL), shifted from ZPL in the direction of high energies in each of the selected groups, are due to electron-oscillation transition (EOT). Energy distances of these lines from corresponding ZPL may be identified with frequencies, developed in the oscillation spectrum of crystals (it is mentioned below). It is noted that ZPL, associated with the transition to r$ level does not contain the electron-oscillation recurrencies at all.
Optical absorption ,for 4A2(4F) + 4T,(4F)
transition
Figure 2 shows the absorption spectrum. which corresponds to optical transition ‘A2 (4F) +4T,(4F) observed at 1.8 K. The long-wave section of this spectrum is enlarged to present the observed structure more vividly. It should be noted, that the temperature increase up to 4.5 K leads to a strong line diffusion on this spectrum section. At this a spectrum shape in 2 = 2.0 pm range, consisting of several lines on the background wide bands practically does not change. Crystal analyses of various concentrations and thicknesses has shown that in this spectrum there are no lines, the energetic position which are less than that of an observed one with maximum this line may be considered as a long-wave edge of the observed v = 4837 cm-‘. Therefore absorption. According to a given energy level diagram (Fig. 2) this line may be referred to zero-phonon transition on the lowest component 4T,(4F) term -r6 ZPL,(r,). A number of lines adjoining ZPL, (F,) from the high energy side and located in the region of v < 5000 cm -’ are due to an electron-oscillation transition with various crystal oscillations taking part. In the case of the highest energy maximum in an absorbtion spectrum located on the v = 5719 cm-’ frequency is taken as a transition defining the upper boundary of spinorbit split, then the spin-orbit interaction parameter value for 4T,(4F) term corresponds to & = 147 cm-‘. Thus & value for 4T,(4F)-term is less than i.,, defined experimentally for 4T,(4P) term and E. obtained as a result of theoretical calculations. ‘9) It appears that for the analysed ‘T,(4F) term spin-orbit interaction, supressed by the JahnTeller interaction may occur.“‘) The value of such an interaction may be roughly appreciated, ignoring the initial split of r, and r8 levels, in supposition that the line with maximum on the v = 5458 cm-’ frequency is stipulated by the transition to level. This level is separated from the upper r,-level, the position of which depends on the Jahn-Teller interaction value only by the effect of the latter one on /I value, by interval AEIT = 260 cm-’ equaling the energy of Jahn-Teller stabilization. And ZPL of the transition to rl level is overlapped with electron-oscillation background and is not distinguished vividly. Thus, the above estimations indicate that Jahn-Teller and spin-orbit interactions are values of the same order and the accurate structural discription of optical absorbtion spectra seems to be possible at simultaneous consideration of both disturbances. The use of 4T,(4F) term by spinorbit interaction as a high energy split boundary may lead to the same qualitative conclusion. To calculate crystal field (CF) parameter D, from the experimental data it is necessary to define energy distance between the main 4A,(4F) and non-disturbed by spin-orbit and Jahn-Teller interaction 4T,(4F) terms. Since the main Ts state participating in transition resulted from orbitally non-degenerated 4A,-term, the Jahn-Teller effect for Ts(4Az) state is weak”*’ and cannot contribute an essential error to the D, value. The D,, value decreases due to the Jahn-Teller effect for the 2: 15 cm-’ value. excited state, considering the above estimations AE,, cannot exceed AErr/ Hence using i.z = 147 cm-’ the energy distance 4T,(4F)-4A,(4F) may be defined, from which D,(T,) = 229 cm-‘.
IR-spectroscopy
of crystals containing Jahn-Teller impurity centers
Optical absorption for 4A,(4F) + 4T,(4F)
757
transition
Presented in Fig. 3 the absorbtion intracentral spectrum, corresponding to optical transition from r,(4A,(F)) state to spin-orbit components of 4T,(F)-term was measured at T = 4.5 K with the help of Fourier-spectrometer IFS-l 13. To define more accurately the line maximum position alongside with absorption spectrum its first derivative (AJjAn) was measured. A given spectrum absorption structure as compared to the spectrum, corresponding to optical transition 4A,(4F) -@T,(4P) is less expressed. It implies, that there exists a more essential electron-oscillation interaction value within the limits of a given term. Similar to the same consideration of the 4T,(4F) term, while analysing this structure in absorption spectrum the line with a v = 3121 cm-’ maximum may be selected, which in accordance with an energetic level diagram (see Appendix and Fig. 3) may be identified with optical transition to r, state. Apparently, both a band with v = 2880 cm-’ and the line, the energy position of which is equal to 2853 cm-’ are due to optical transitions to r6- and r$-state, respectively. The mutual state of both lines is defined by the total effect of spin-orbit and Jahn-Teller interaction. Since f, and f6 states are Crammer’s doublets, then knowing the energy distance between the lines, corresponding to these transitions, a spin-orbit interaction parameter may be defined. According to the energy levels diagram.“’ 21, = 241 cm-‘,
i.e. i, = 120 cm-‘.
As well as in the case of the transition to 4Tl(4F) term CF D, parameter may be defined, based upon an absorption spectrum corresponding to optical transition 4A,(4F) +“T,(4F). The energy distance 2970 cm-’ between r,(4A,) state and gravity centre of 4T,-term corresponds to IO D,, i.e. D,(4T,) = 297 cm-‘. Such an agreement of D, values, defined considering the transitions to various terms is apparently not random, but is the result of a small shift value of the position of nondisturbed by spin-orbit interaction 4Tl(4F) and 4T,(4F)-terms. It proves the applicability of the energetic diagram”’ and allows one to calculate the electron-electron oscillation parameter B. The energy distance 11457 cm-’ between r,(4A,)-state and gravity centre 4Tl(4P) term corresponds to 15B+ 12D,. Using the value D, = [D,(T,) + D,(T,)]/2 one can define the Rack parameter B = 525 cm-‘.
Optical absorption for 4A,(4F) -+ ‘G transition At impurity concentration Co*+ N 2 5 x 10’9cm-3 in the vicinity of 9200-10800cm-’ at low temperatures absorption is observed, the low intensity of which allows it to be referred to the forbidden rules of optical transition selection. From the high energy side this absorption joins the one, which is due to that described above transitions to 4T,(4P)-term. From the energy level diagram”’ one can assume, that absorption is to be expected in this vicinity, due to transition to split by CF and by spin-orbit interaction components of 2G-term of Co*+ ion. Figure 4 shows a spectrum associated with such transitions and a corresponding energy level diagram. The total energy split width calculated by using D, = 298 cm-’ makes up 1200 cm-‘, which corresponds to the vicinity width of an observed absorption. In a high energy spectrum region a structural absorption consisting of ZPL (v,,, = 10750 cm-’ and v,,, = 10414 cm-‘) and shifted to the vicinity of high frequencies of electron-oscillation absorption may be outlined. According to the energy diagram it may be supposed, that above ZPL corresponds to the transitions to spin-orbit components r7 and rf of the highest spaced *T,(*G) term. The results of earlier work”3’ show that mutual spacing of spin-orbit components of *T,-term are practically independent from the value and behaviour of Jahn-Tellar interaction, It provides the use of above lines to define spin-orbit coupling constant within the limits of a given term. The direct comparison with a given diagram gives 2, = 90 cm-‘. On the basis of A4 value and energy levels diagram it can be found that the energy position of the ‘T,(‘G) term corresponds to v = 10525 cm-‘. Using CF D, = 298 cm-’ parameter, obtained from the absorption spectrum analysis corresponding to the optical transitions to spin-orbit components of 4Tl(F) and 4T2(F) terms and knowing energy position of 2T2(2G) term, it is possible to evaluate the energy position of 2E; 2T,- and *A,-terms (v (2E) = 9886 cm-‘, v (*T,) = 9673 cm-’ and v(*Al) = 9375 cm-‘, respectively).
758
P. N.
BUKIVSKY
er al.
In
the absorption spectrum governed by optical transitions Ts(4Az) -+‘G, the line with 9840 cm-’ is observed. Therefore this line apparently corresponds to the optical transition to r,(*E) state, which is shifted by 46 cm ’ from the expected energy position v (‘E) = 9886 cm ’ on account of the JahnTeller interaction. From the high energy side of this line the equidistant structure of electron oscillation bands is observed. In the frequency region r = 9670 cm ’ where the transition to ‘T, (*G) terms should be expected, in absorption spectrum two weak lines are observed, which could be interpreted as transitions to spin-orbit components of rh and r$ symmetry of *T, term. The expected energy position of ‘A,(‘G) term v = 9375 cm ’ agrees with the energy position of a wide band absorption, which may be referred to the transitions to r,(‘A,)-state. Such a band width could be explained by fairly strong electron-oscillation interaction which has fully-symmetric and tetragonal’13) oscillations. Such an agreement of the designed energy terms positions, with the ones obtained experimentally, is another testimony to the selected energy level diagram validity of CO’+ ion in CdTe crystal. Since the energy distance 997 1 cm ’ between TX(JA, (F)) state and gravity centre of ‘G term corresponds to 4B + 3C + 12D,, it provides the estimation of the Rack parameters C = 1430 cm ‘. V max
=
Absorption,
stipulated
b_y oscillation
development
and electron-oscillation
interaction
in CdT~:co
The absorption spectrum structure indicates that the processes of electron-oscillation interaction play an important role in optical transitions. Since the ‘T,(P) state is three-fold degenerated, the interaction both with fully symmetrical and degenerated oscillations of t- and rz-symmetry may occur. As was mentioned above, in accordance with the Jahn-Teller theorem the interaction with fully symmetrical oscillations leads to a drop of a corresponding electron state by the value of Jahn-Teller interaction (A&). Allowing for such an interaction the electron states become the main vibronic states (n = 0). In an absorption spectrum zero-phonon lines, corresponding to transitions in the above vibronic states, are usually observed as well as a number of other lines. coupling with transitions to the excited electron-oscillations levels (n # 0).(‘4) From the short-wave side of ZPL a series of narrow absorption lines is observed, which certainly correspond to the transitions with various (acoustic and optical) oscillation participation i.e. vibronic structure of absorption spectrum. At low temperatures (T = 30 K) in the vicinity of a long-wave absorption edge for transition 4A2(4F) +4T,(4P) two weak lines are observed rather vividly (Fig. la). The distance between these lines correspond to that between ZFL, corresponding to transitions r, and r#’ levels, i.e. it is equal to 12cm-‘. It proves, that the indicated lines correspond to the transitions with a certain energy oscillation absorption, i.e. to transitions from the first vibronic level of the main state rg(4A2) Co” ion. On the basis of the relative position of vibronic states the energy is defined of oscillations, which participate in the process of absorption. and it makes up v = 56 cm-‘. Since the oscillation of a such energy is defined for a main state of Co*+ ion (orbital singlet of 4A2-symmetry), the oscillation energy is not practically lowered by the possible occurrence of the Jahn-Teller interaction. In principle, as shown in Ref. (I 5). the Jahn-Teller interaction may also occur for a singlet state (according to the orbital moment, but degenerated, according to a spin) of an impurity ion state, but it is rather weak and its value makes energy level decrease should not take place for up some cm -I. Therefore the electron-oscillation such a state. The development of oscillation with v = 56 cm-’ frequency in an absorption spectrum may be associated with the following conditions. The introduction of impurity substitution atoms into crystal lattice may lead to the violation of the transmittion symmetry, particularly for a perfect crystal. Accordingly the oscillations may be considered active in optical absorption, which usually do not participate in such processes, due to wave vector selection rules inhibition. Presence of impurity substitution atoms in a crystal causes oscillation spectrum development, the frequencies of which correspond to the frequencies of peak maxima in one-phonon density of states of a perfect in the absorption spectrum of impurity centres crystal. (i6) As well as the above frequencies pseudolocal oscillations seem to occur and their onset is caused by appreciable weakening of elastic constant crystals close to the impurity centre.‘15’ TO establish the nature of a discovered oscillation, being active in the absorption process, it is necessary to analyse the frequencies, which correspond to peak maximums in the one-phonon
IR-spectroscopy
of crystals containing Jahn-Teller impurity centers
759
density of a crystal state CdTe. The calculations g(v) for CdTe were carried out in Refs (17, 18), where it is shown, that in the low-frequency oscillation spectrum region there is rather an intensive peak, of frequency 34cm-‘, corresponding to a cross acoustic TA-phonon at the edge of the Brillouin zone (point x). The next maximum dependent g(v) corresponds to v = 53 cm-’ frequency, i.e. to a TA phonon with a wave vector K z 0.6 (point K of a Brillouin zone). Schematically the one-phonon density for states CdTe is shown at Appendix C, Fig. 1. Thus the complete analysis shows, that the oscillation value obtained (v = 56 cm-‘), which was active in the process of absorption corresponds to a cross acoustic phonon close to point K of the Brillouin zone to TA (K) phonon. The absorption in the region 11500-10980 cm-’ is governed by optical transitions with the simultaneous participation of both low-frequency acoustic oscillations and optical ones. In particular, in this spectrum region the lines are observed, the position of which with respect to ZPL (vmax= 10959 cm -‘) correspond to frequency values close to other maximum values in one-phonon density states. The absorption spectrum section in the vicinity of 12000-l 1760cm-’ is due to the optical transitions to the level of rf symmetry. The lines, located from the short-wave side of ZFL correspond to transitions to vibronic levels with participation of various oscillations. Since ZPL in a given case is a fairly well isolated (the nearest line is placed at a distance, greater than twice the halfwidth), it seems possible, on the basis of its half-width temperature dependence, to define the frequency oscillation, the interaction with which causes spectrum structure broadening at temperature increases. For optical transitions between descrete levels, as shown in Refs (19,20), the absorption temperature dependence of a half-width line within the limits of a one-mode model is defined by the following ration: H’(T) = H’(0) cth (hv/2kT),
(1)
where H(O)-a halfwidth of the line; at T = 0 K; hv-oscillation energy, responsible for temperature broadening of ZPL. The approximation of experimental dependence H(T) with the help of ratio (1) provided the oscillation frequency v = 36 cm-’ definition, that practically agrees with the frequency of the most intensive peak (v = 34 cm-‘) in a low-frequency region of the one-phonon state density CdTe. The result obtained proves, that ZPL broadening at an increase in temperature may result from the interaction with TA (X) phonons. Vibronic levels of the excited state, coupled with these oscillations do not appear in analysed spectrum structure absorption; since the nearest vibronic lines are away from ZPL at distances, exceeding TA (X) phonon frequency. But at the long-wave absorption edge of a given ZPL, as shown in Fig. l(b), when T = 15 K two weak lines may be observed. One of them corresponds to transitions with oscillation absorption, (v = 56 cm-‘) the frequency of which has been defined earlier for transitions to the lowest spin-orbital levels of rg and r, symmetry. Another line onset is caused by optical transitions with energy absorption frequency oscillation v = 36 cm-’ which agrees with frequency oscillation, obtained for an excited degenerated state. It is well known w that TA (X) phonon symmetry for zinc blende crystals (CdTe also refers to them) interacting with the substitution impurity centre is defined by r, and r2. For a state of r,-symmetry Jahn-Teller interaction, in principle, can be observed both with c-oscillations and the ones of a t,-symmetry. Since TA (X) is an oscillation, which does not develop in the spectrum structure, it should be considered that the spectrum structure, being close to ZPL is defined by interaction with c-symmetry oscillations. TA (X) phonons (v = 56 cm-‘) are the oscillations of this type, among which there are c-symmetry oscillations. It agrees well with data from Refs (3,21), where on the basis of the analyses it was shown, that a vibronic spectrum structure may be explained allowing for interaction with only the oscillation type of the indicated symmetry. To get data yielding various crystal oscillation participation in a case of intracentral Co:+ ions absorption some measurements of absorption spectrum in a FIR-spectrum region have been carried out. Figure 5 shows the results of such measurements for CdTe: Co(N = 5 x lOI cm-‘) where T = 4.2 K. As seen in Fig. 5, the absorption spectrum consists of a great number of lines of various intensity. The majority of lines may be identified with a well known crystal phonon spectrum of CdTe.(‘8~22*5) The narrow line (the halfwidth is approximately 2 cm-‘) with v = 56 cm-’ frequency, the intensity
760
I
I
I II I I
II
I
III
Y
Fig. 5. (a) Absorption spectrum of CdTe:Co energy position governed by electron-oscillation
]L~~PT,IPII
( cm-‘)
crystals in the FIR spectrum region at 7’ = 4.5 K: (b) line transitions with respect to corresponding ZFL for various Co>+ ion states.
of which increases with rise in impurity concentration corresponding to the oscillation, calculated earlier, when analysing the intracentre absorption spectrum of Co’+ ions, agrees well with a maximum of a one-phonon density crystal state CdTe. The temperature dependence measurement of an oscillation spectrum in a low-frequency region (< 56 cm ‘) shows that an T 2 50 K, when v < 34 cm -’ the absorption phonon is practically unchangeable. When v > 34 cm ’ a noticeable rise of a continuous absorption takes place, in the background a narrow absorption peak is observed (v = 56cm-‘). These results prove that a continuous background in a low-frequency spectrum region is governed by various energy acoustic oscillations absorption and the appearance of a narrow line (v = 56 cm-‘-by pseudolocal oscillation development. Detailed discussion of the oscillation spectrum is beyond the limits of the present work. Here we shall concentrate on some peculiarities of the oscillation structure in an intracentral absorption spectra, given in Figs 14. For this purpose in Fig. 5(b) the maximum position of the lines for each of the excited terms is shown, caused by the electron-oscillation transition in relation to the corresponding ZPL (Av). Firstly, it should be noted that vibration level absorption in the case of a transition to 4T,(4P) term, for which the Jahn-Tellar interaction failure is weak, most fully corresponds to the oscillation crystal spectrum of CdTe:Co. It proves, that in the electronoscillation interaction for optical transitions to the 4T,(4P) term, practically all oscillation crystal spectrum is taking part (quazilocal oscillation with v = 56 cm-’ energy included). The optical spectrum structure analysis, caused by transitions to other terms, also indicates the presence of electron-oscillation transitions with various frequency oscillation participating. The frequency interval Av from corresponding ZFL, in many cases differs from Av, observed at the transition to the 4T,(4P) term. Moreover at transition to a number of spin-orbit components ‘T,(4F) or 4T2(“F) terms (for example to rf levels) it is completely impossible to separate the individual lines in the vicinity of the electron-oscillation transitions, as a result of their strong broadening. Such absorption spectrum peculiarities indicate the presence of a multi mode Jahn-Teller effect for excited impurity centre states. One of the results of such an interaction is distinction for the terms 4T,(4F) and 4T,(4F) in a positron of line maxima of electron-oscillation bands in comparison with results, obtained for 4T,(4P) terms as well as when measuring the crystal oscillation spectrum. Thus, the analysis of the electron-oscillation spectrum structure of intracentral absorption of Co’+ ions in a CdTe crystal, shows that the electron-oscillation interaction behaviour is different both for various terms and for various spin-orbit components within the limits of one term. The difference in Av values for ZPL, stipulated by transitions to rb and r, levels for which the Jahn-Teller interaction does not occur, seems to indicate the necessity of considering the Jahn-Teller interaction for an excited term on the whole. Various parameter values of spin orbit interaction, obtained for various excited terms are another proof of it.
IR-spectroscopy
Intracentral
ions Co’+ absorption
of crystals containing Jahn-Teller impurity
centers
761
in Cd,_,YMn, Te
The measurement of Co*+ ions absorption spectrum for the 4A2(4F) +4T,(4P) transition in crystals Cd,,g,Mn,,,,Te indicates, that in a case of semimagnetic semiconductors, which are the substitution of solid solutions, the absorption spectrum structure may be maintained. However some peculiarities, which are different from CdTe crystals may be observed. Thus it was established: (i) a small spin-orbit interaction parameter rise; (ii) a change of energy position and line intensity of electron-oscillation transitions; (iii) energy distance increase between spin-orbit components rf; and (iv) r, symmetry for the 4T,(4P) term. The observed absorption spectrum structure is noticeably spread, which is governed by inhomogeneous broadening in solid substitution solutions. Parameters of the crystalline field (D,) and electron-electron interaction (B) actually do not change. Spin-orbit split value change for 4T,(4P) term in Cd,_,Mn,Te crystals as compared to CdTe is apparently governed by exchange interaction between local magnetic ions Co’+ and Mn*+ moments magnetic field, or by interaction of magnetic moments of Co’+ ions with effective fluctuating stipulated by the regulation of magnetic ions Mn’+. The change of energy position of electronoscillation bands may be associated with the one-phonon density change of a crystal state, when ions Mn*+ are being introduced into CdTe matrix while the semimagnetic semiconductor is being formed. Intracentral
Co2+ ions absorption
in ZnTe
crystals
The considered absorption spectrum of ZnTe:Co crystals (N = 10’8cm-3; T = 4.5 K, spectral vicinity 12500-10900cm~‘) corresponds to the optical transition between the main Ts level of 4A2(P) state and spin-orbit components of r$-, r,-, r;-, r,-symmetry of the excited 4T,(P) Co*+ state (Fig. 6). The intracentral absorption spectrum shape proves that the absorption in the range 11620-10900 cm-’ corresponds to optical transitions to the lowest spin-orbit components of the excited 4T,(P) state r$ and r,, while the absorption bands between 12200-l 1750 correspond to the transitions of higher spin-orbit energy levels of r$ and r6 symmetry. The experimentally observed split value for 4T,(P) of the Co*+ ion . state in the ZnTe crystal, constitutes approximately 1400cm-’ as shown earlier. In Fig. 6 the observed absorption spectrum structure cannot be explained within the framework of the astatic model of CF and requries the electron-oscillation interaction account. The observed difference of a relative ZPL position (520 cm-‘) coupling with transitions to spin-orbital levels of r6 and r$ symetry, from the expected one on the basis of a static model of CF (267 cm -I), as for the CeTe:Co crystals, is apparently governed by energy lowering of the Ts level as a result of the Jahn-Teller interaction. Since such a value shift is about the spinorbit interaction value, it proves an intermediate value of the Jahn-Teller interaction in ZnTe:Co crystals.
1.4 I
I
I
9000
1.5 I
1.6 I
1.7 1
1.6 I
I
I
7000
6000
1.9 I
2.0 I
2.1 I
2.2 2.3 I,
I
6000
xta,
Fig. 6. Absorption spectrum of ZnTe:Co crystals at T = 4.5 K [transition 4A2(4F)+4T,(4P)J.
162
P. N. BIJKIVSKYet al
As mentioned above, the absorption spectrum in the region of 12500-10900 cm-’ corresponds to transitions in spin-orbit components of the 4T,(P) state. In addition, the lines with maxima at 10986, 11791 and 12342 cm-’ are coupled with transitions to the main vibronic states, i.e. they are ZPL (ZPL (rt, r,), ZPL (rf) and ZPL (r,), respectively. Figure 6 shows that in an absorption spectrum one can observe a number of other lines, besides those associated with transitions to the excited electron-oscillation levels. Thus, from the short wave side of ZPL (r$, r,) there exist two lines the energy position of which proves the oscillations with energy of 218 and 134 cm -’ participating in an absorption action. The first value is close to the energy of LO phonon at a point r of the Brillouin sone, i.e. LO (T)-phonon, the second is close to the energy of LA (L)-phonon. In accordance with Ref. (16) LO (r)-phonon, interacting with the substitution impurity centre in crystals with zinkblende structure are fully-symmetrical oscillations, while LA (L)-phonons have a, and r,-symmetry. Thus among the oscillations indicated Jahn-Teller oscillations of c or r2 symmetry do not occur. So in a given case a vibronic state reduction (compression) is not observed. In the optical transition band to l-8 level two vibronic levels are also visible, which are away from ZPL (f 8”) by 205 and 79 cm-‘. The first line corresponds to the transition with LO (r) phonon participation, the second corresponds to a phonon with an energy level close to the energy of one of the maxima of the ZnTe crystal state one-phonon density spectrum, and apparently corresponds to TA (K) phonon.“” There are oscillations of c, r, and t2 symmetries among the TA (K) phonons. Since the energy position (79 cm -‘) of a corresponding vibronic line ZPL (rf) is comparatively too close to the spectrum maximum value of one phonon density state (73 cm-‘), apparently the appearance of a given vibronic band is associated with T, or L and T>symmetry phonons participation provided that Jahn-Teller interaction with such oscillations is rather small. In the case of the transition to spin-orbit component of rb symmetry only one ZPL (r,) line. broadening as compared to ZPL (rf, r,) and ZPL (r;) is hardly observed, which is located at the long-wave edge of the intensive total absorption spectrum. The comparison of the observed intracentral absorption Co 2+ ion spectrum in this work, with similar spectrum for CdTe(@ proves that there is a stronger interaction with full-symmetry oscillation in the case of ZnTe. As is shown in Ref. (13) the increase of the electron-oscillation force, assuming these oscillations may lead to a structure spectrum leveling, resulted from interaction with the Jahn-Teller oscillations. Therefore it should be expected that this condition is a cause of the absence of a more complex ZnTe:Co absorption spectrum structure. From the low-energy side of ZPL (r$, r,) absorption lines of a weak intensity (Fig. 6) may be observed, which are apparently associated with prohibition according to spin-orbit transitions between 4F and 2G Co ‘+ ions states. The lowest level of *G-state is 2A, symmetry level. Therefore a narrow-absorption line (v = 10672 cm ‘) is supposed to correspond to ZPL (‘A,). The temperature rise causes the sharp decrease of ZPL (‘A,) intensity, while the line remains symmetrical and the energy position at its maximum is independent from temperature. The other lines between ZPL (‘A,) and ZPL (r& r,) are coupled with transitions to electron-oscillation levels of *A, state. Their energy position comparative to ZPL (2A,) agrees with the local density spectral distribution of the ZnTe:Co crystals phonon state, which is obtained on the basis of luminiscence spectrum measurements stipulated by transitions between 4Tz and 4A2-states.‘23) Since the electron-oscillation structure spectrums for 4A2 and ‘A, states coincide it may be evidence of practically the same electron-oscillation interaction in two non-degenerated Co’+ ion states in ZnTe. ZPL (‘A,) temperature dependence analysis shows that its halfwidth H may be described by the dependence (I) from which the value hv = 37 cm-’ is derived. This value agrees with the first maximum position of the ZnTe:Co crystal one-phonon density state. That Huang Rhys factor, characterising the electron-oscillation interaction force is defined from the expression H(0) = 2 hv J%%-? and in a given case constitutes 0.003, which indicates a fairly weak electron-oscillation interaction in the case of optical transition between non-degenerated 4A, and 2A, levels of Co2+ ion. It should be noted that the value hv = 37 cm-’ obtained is close to the value 34 cm-‘, obtained when intracentral luminiscence crystal ZnTe:Mn spectrum was analysed.‘24’ In the latter case however the electron-oscillation interaction is strong (S = 29). The comparison of the absorption line temperature dependences for ZnTe:Co and CdTe:Co
IR-spectroscopy
of crystals containing Jahn-Teller
impurity centers
763
crystals, corresponding to the lowest spin-orbit *T, (P) states and the lines, corresponding to transitions between non-degenerated states, provides the establishment of some peculiarities of the Jahn-Teller electron-oscillation interaction found in a degenerated state. The temperature dependence of ZPL (rt, r,) intensity is similar to the one for the vibronic structure line; with the rise of the temperature ZPL (rf, r,) maximum shift observed on the long-wave side as well as the appearance of asymmetry on the low-energy side. These peculiarities are governed both by the vibronic nature of the main vibronic state and reduction of the excited vibronic states, due to the presence of the Jahn-Teller interaction for 4T,(P) states. 4.
CONCLUSIONS
The analysis of the experimental results obtained in the present work, when the low-temperature optical spectrum of CdTe, ZnTe, Cd,,, Mn,,,Te crystals, with Jahn-Teller Co*+ impurity ions were examined. It provides the separation of absorption spectrum regions, corresponding to the transitions between the main 4A,(4F) and excited 4T,(4P), 4T,(4F) and dT,(4F) states. ZPL are singled out in these spectra as well as the lines of electron-oscillation transitions with both phonons and pseudolocal low-frequency oscillations participation. The absorption spectrum structure analysis shows that Jahn-Teller interaction has practically no influence on the ZPL position and the spin-orbit interaction value in a case of transitions to the 4T,(P) state. But 1 = 332 cm-’ parameter of such an interaction is sufficiently higher than that I, = 178 cm-’ for a free ion. This parameter is sufficiently smaller 2, in a case of transition to the other terms. Besides, the relative position of spin-orbit components do not correspond to the expected one on the basis of the CF theory. The data obtained proves that the Jahn-Teller interaction for 4T,(4F) and 4T,(4F) terms suppress the spin-orbit interaction parameters to a great extent. The Jahn-Teller interaction developed for the states of f, symmetry causes the essential energy bias of corresponding ZPL relatively ZPL governed by transitions to the levels of Ts and r, symmetry. Both CF D, = 298 _t 3 cm-’ and electron-electron interaction, B = 525 cm-‘, C = 1430 cm’-’ parameters are defined assuming this situation. For Cd, _ ‘i Mn,rTe crystals the observed increase of parameters in comparison with CdTe crystals is apparently governed by exchange interaction manifestation between local magnetic moments of Co*+ and Mn2+ ions with an effective fluctuating magnetic field, which results from magnetic ions Mn2+ regulation and broadening of some lines in the absorption spectrum by inhomogeneous broadening, peculiar for solid substitution solutions. REFERENCES 1. B. Bersuker and V. 2. Polinger, Vibronic Interaction in Ma/ecules and Crysfals, p. 336. Nauka, Moscow (1983). 0. L. Natadze and A. T. Ruskin. News Acad. Sci. U.S.S.R.. Phvs. 140. 1846 (1976). A. V. Vasiliyew, B.-Z. Malkin, A. L. Natadze and A. T. R&kin, Zh. eksp. tear. F&. 71, 1192 (1976).
2. 3. 4. 5. 6. 7. 8.
9. 10. Il. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.
A. I. Ryskin, A. L. Natadze and S. A. Kazanskii, ZETA 64, 910 (1973). Yu. P. Gnatenko and A. Hh. Pozhko, ZOC 42, 1202 (1977). P. N. Babii, P. N. Bukiwskii, N. P. Gawaleshko and Yu. P. Gnatenko, Preprint Ph. Inst. Acad. Science Uk. SSR, No. 21 (1982). H. A. Weakliem. J. them. Phys. 36, 2117 (1962). P. N. Babii, W. M. Lichuk, A. I, Sawchuk, S. W. Melnichyk, S. W. Vatamanyk and N. P. Gawaleshcko, FTT 27, 1124 (1985). S. V. Melnichuk. V. M. Cramar and K. D. Tovstyuk, /‘@_&a Stutrcs Solidi 141b, 107 (1987). J. M. Langer and J. M. Baranowskii, Physica Status Solidi 44b, 155 (1971). F. S. Ham, P@s. Rm. 138, 1727 (1965). Yu. E. Perlin and B. C. Zukerblat, Eiect~onoseilla~ion Interaction E$eets in Optical Spectra o~~~purity Paramagnefic ions, p. 367. Kishinev (1974). Kikuo Cho., J. P&s Sot. Jup. 25, 1372 (1968). I. B. Bersuker, B. L. Wekhter and I. Ja. Ogurtsow, Csp. fir. Nauk. 116, 605 (1975). Yu. P. Gnatenko and A. Kh. Rozko, Letr. ZETF 24, 125 (1976). R. London, Proc. phys. Sot. 84, 379 (1964). P. Plumelle and M. Vandevyver, Physica Status Solidi 73, 271 (1976). J. M. Rowe, R. M. Nicklow, D. L. Price and K. Zanio, Phys. Reu. lOB, 671 (1974). J. D. Konitzer and J. J. Markham, J. them. Phys. 32, 843 (1960). J. J. Markham, Rev. Mod. Phys. 31, 956 (1959). S. M. Uba and J. M. Baranowskii, Phys. Rev. I7B, 69 (1978).
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22. M. Ja. Walakh and M. P. Licitsha, K. Electron. 22, 16 (1984). 23. A. P. Radlinski, J. Luminesc. 18/19, 147 (1979). 24. Yu. P. Gnatenko and A. I. Zhmuzko, UFZ 30, 843 (1985).
Copyright
IR-SPECTROSCOPY JAHN-TELLER
OOZO-089Ij89 $3.00 + 0.00 IC 1989 Pergamon Press plc
OF CRYSTALS CONTAINING IMPURITY CENTERS
P. N. BUKIVSKY, Yu. P. GNATENKO, A. KH. ROZHKO and I. A. FARINA Institute of Physics Academy of Sciences of the Ukrainian SSR, Kiev-28, Prospect Nauki 46, U.S.S.R. (Received 7 October 1988)
Abstract-Using a novel IR-technique, the low-temperature spectra of CdTe, ZnTe and Cd,,,M~,osTe crystals, doped by Jahn-Teller Co*+ Impurity ions, were studied in the spectral range from 1 to 400 pm. The absorption spectrum regions and the energy positions of ZPL, corresponding to the transitions 4A2(4F)+4T,(P), T,(F), 4T2(F) and 2G were detected. The parameters of the spin-orbit interaction I, the crystal field parameter D,,., the parameters of electron-electron interaction B and C were obtained. It was found, that Jahn-Teller interaction differently affects the parameter i, for 4A2(F)-‘T,(P), q,(F) and 4T,(F) transitions and, consequently, the energy positions of ZPL. The electron-vibronic spectra were compared with phonon spectra.
1. INTRODUCTION
The introduction of impurity atoms may permit essential changes in the crystal physical properties, thus providing materials with the parameters necessary for various practical applications. Among the impurity atoms, elements with an unfilled electron 3d”-shell are of particular importance, the introduction of which may cause energy level system onset and optical transitions which cover a wide-range of the spectrum. The use of an up-to-date IR technique allows such transitions to be analysed well and to aquire information yielding crystal energy impurity spectrum and electron oscillation interactions with Jahn-Teller interaction peculiarities. Measurements in a distant IR region enable the study of oscillation centres of real crystals, this is very important for perfect comprehension of the above processes and is also of interest in itself. At present the scientific literature provides a wide choice of theoretical and experimental works including semiconductor crystal groups A” B”’ (mainly ZnS4’ crystals) alloyed by impurity atoms coz+ (I-7) In this work, using CdTe and ZnTe crystals as examples, investigation concerning the influence of impurity atoms Co on an energy structure and the oscillation spectrum of the crystals has been carried out. It resulted in experimental data yielding peculiarities of Jahn-Tellar interaction manifestation as well as a change in oscillation-spectrum in a low-frequency region for the above crystals. 2. EXPERIMENTAL
METHOD
CdTe and ZnTe crystals with various ions CO’+ (NC, = 5 x 10’8-1020cm-3) impurity concentrations were grown using the Bridgman method. Investigation of the absorption spectrum was carried out at T = 1.8-77 K. The preset temperature was maintained with the stabilization temperature system (accuracy = 0.05 K). Intracentral absorbtion spectrum of Co’+ ions in CdTe and ZnTe crystals and the oscillation spectrum of the crystals in the IR spectrum region were measured with a SDL-1 (i < 2 ,um) spectrometer and a Fourier-spectrometer IFS-l 13 (2 > 2 pm) respectively. The SDL- 1 monochromator width spectrum split and resolution Fourier-spectrometer did not exceed 1 cm-‘. 3. EXPERIMENTAL
RESULTS
AND
DISCUSSION
Co’+ ions substituting Cd2+ ions in CdTe crystals and in ZnTe-Zn2+ ions are in tetrahedral surrounding. As shown in the results from Ref. (7) the account of crystalline fields action (CF) of Td symmetry and spin-orbit interaction may lead to 4F main split and the excited 4P and 2G etc. levels of a free Co’+ ion. 153
754
P. N.
BUKIVSKY
et ul.
(cm-‘)
Y
11,600
12,500
50
-;
40
E 0 *
30
20
10
Fig. I. Absorption
spectrum
of CeTe:Co
crystals
at T = 4.5 K [transition
JA2(4F) -“T,(4P)].
Figures l-4 show energy levels of Co 2+ ions in these crystals and their optical spectrum of intracentral absorbtion, corresponding to their intracentral transition from the main state, which considering spin-orbit interaction has r,-symmetry on various excited states. Optical absorption for 4A,(4F) + 4T,(4P)
transition
The most intensive absorption adjoining the fundamental one that which is correlated with the impurity concentration Co in CdTe and is located in the region 1090&12300cm-‘. Such an absorption spectrum may be classified into 3 practically non-overlapping groups, each one of which consists of a great number of lines. The longest wave lines in each of these groups apparently corresponds to zero-phonon transitions from the main state r,(4A,(4F)) to 4T,(4P) term com-
Y
5900
(cm-‘) 5000
5400
4800
2.0
1.5
*=
1.0
0.5
I
1.7
1.9 A
Fig. 2. Co2+ ion absorption
spectrum
2.05
2.10
(pm)
in CdTe crystal
at T = 1.8 K [transition
4A2(4F) -4T,(4F)]
IR-spectroscopy
of crystals containing Jahn-Teller impurity centers
755
2.0 FREE
CF
Xi.;
EoN er 2
p
0.6
I
0
I
I
3800
2800
3200 Y
(cm-‘)
Fig. 3. Absorption spectrum of CdTe:Co crystals at T = 4.5 K [transition 4A2(4F)+4T2(4F)J,
ponents, split by spin-orbit interaction effects. At this in correspondence with the term diagram (see Fig. 1 caption) in the lowest-energy spectrum region two closely spaced lines may be expected (ZPL), governed by the transition effects to F, and Ts symmetry levels. Factually two lines are observed in this region with a maximum distance between each of 11 cm-‘. Magnetooptical investigations(*) indicate, that the lowest ZPL (v,, = 10947 cm-‘) corresponds to level transitions f, and ZPL with a maximum equal to 10959 cm-’ for rg transition. The other ZPL with maxima on the frequencies v = 11769 and v = 12275 cm-’ are due to the rf and r6 transitions correspondingly. The energy distance between ZPL allows the parameters of spin-orbit interaction to be defined as well as 4T, term energy position which is not disturbed by spin-orbit interaction v(~T, (‘P)). The values l&r = 332 f 2 cm-‘; v (4T,(4P)) = 11445 cm-’ are obtained from the calculations. Such a value exceeds twice the value )i 1= 167 cm-’ for CdTe: Co system, theoretically calculated with the effects of covalent bonds being considered(g’ and it is sufficiently more than l&l = 178 cm-’ for a free ion. It should be noted, that the increase of II,1 in comparison with 141 could be observed for other systems as well, for example, transient metals in CdS.“*’ A large value /,?,I_ provides that for 4T,(4P) term the reduction effects of spin-orbit interaction by Jahn-Teller interaction are practically absent. (‘I)On the other hand the correlation (established
0.4i
I 10,600
I 10,200
960Q Y
(cm-‘)
Fig. 4. Absorption spectrum of CaTe:Co crystals at T = 4.5 K (transition 4A2(‘F) -t *G).
156
P. N. BUKWSKY et al
as a result of a spectrum structure analysis) between the frequencies of zero-phonon transitions and static model split by spin orbit interaction is a testimony of a very weak disappearing influence of dynamic effects on the individually taken term components as well. Actually, with the Jahn-Teller effect for two of four ZPL present, governed by optical transitions to TX levels, shears of their positions to the side of low-energies on the energy value of Jahn-Teller stabilization should be expected. The lines frequency position, coupling with transition to F6 and r,-levels (for which the Jahn-Teller effect does not take place”‘) is independent from the value of electron oscillation coupling. ZPL frequency divergence from the static model of spin-orbit split would be the result of such a “selective” interaction which has not been proved empirically. The other less intensive lines (many of which are equidistant with respect to various ZPL), shifted from ZPL in the direction of high energies in each of the selected groups, are due to electron-oscillation transition (EOT). Energy distances of these lines from corresponding ZPL may be identified with frequencies, developed in the oscillation spectrum of crystals (it is mentioned below). It is noted that ZPL, associated with the transition to r$ level does not contain the electron-oscillation recurrencies at all.
Optical absorption ,for 4A2(4F) + 4T,(4F)
transition
Figure 2 shows the absorption spectrum. which corresponds to optical transition ‘A2 (4F) +4T,(4F) observed at 1.8 K. The long-wave section of this spectrum is enlarged to present the observed structure more vividly. It should be noted, that the temperature increase up to 4.5 K leads to a strong line diffusion on this spectrum section. At this a spectrum shape in 2 = 2.0 pm range, consisting of several lines on the background wide bands practically does not change. Crystal analyses of various concentrations and thicknesses has shown that in this spectrum there are no lines, the energetic position which are less than that of an observed one with maximum this line may be considered as a long-wave edge of the observed v = 4837 cm-‘. Therefore absorption. According to a given energy level diagram (Fig. 2) this line may be referred to zero-phonon transition on the lowest component 4T,(4F) term -r6 ZPL,(r,). A number of lines adjoining ZPL, (F,) from the high energy side and located in the region of v < 5000 cm -’ are due to an electron-oscillation transition with various crystal oscillations taking part. In the case of the highest energy maximum in an absorbtion spectrum located on the v = 5719 cm-’ frequency is taken as a transition defining the upper boundary of spinorbit split, then the spin-orbit interaction parameter value for 4T,(4F) term corresponds to & = 147 cm-‘. Thus & value for 4T,(4F)-term is less than i.,, defined experimentally for 4T,(4P) term and E. obtained as a result of theoretical calculations. ‘9) It appears that for the analysed ‘T,(4F) term spin-orbit interaction, supressed by the JahnTeller interaction may occur.“‘) The value of such an interaction may be roughly appreciated, ignoring the initial split of r, and r8 levels, in supposition that the line with maximum on the v = 5458 cm-’ frequency is stipulated by the transition to level. This level is separated from the upper r,-level, the position of which depends on the Jahn-Teller interaction value only by the effect of the latter one on /I value, by interval AEIT = 260 cm-’ equaling the energy of Jahn-Teller stabilization. And ZPL of the transition to rl level is overlapped with electron-oscillation background and is not distinguished vividly. Thus, the above estimations indicate that Jahn-Teller and spin-orbit interactions are values of the same order and the accurate structural discription of optical absorbtion spectra seems to be possible at simultaneous consideration of both disturbances. The use of 4T,(4F) term by spinorbit interaction as a high energy split boundary may lead to the same qualitative conclusion. To calculate crystal field (CF) parameter D, from the experimental data it is necessary to define energy distance between the main 4A,(4F) and non-disturbed by spin-orbit and Jahn-Teller interaction 4T,(4F) terms. Since the main Ts state participating in transition resulted from orbitally non-degenerated 4A,-term, the Jahn-Teller effect for Ts(4Az) state is weak”*’ and cannot contribute an essential error to the D, value. The D,, value decreases due to the Jahn-Teller effect for the 2: 15 cm-’ value. excited state, considering the above estimations AE,, cannot exceed AErr/ Hence using i.z = 147 cm-’ the energy distance 4T,(4F)-4A,(4F) may be defined, from which D,(T,) = 229 cm-‘.
IR-spectroscopy
of crystals containing Jahn-Teller impurity centers
Optical absorption for 4A,(4F) + 4T,(4F)
757
transition
Presented in Fig. 3 the absorbtion intracentral spectrum, corresponding to optical transition from r,(4A,(F)) state to spin-orbit components of 4T,(F)-term was measured at T = 4.5 K with the help of Fourier-spectrometer IFS-l 13. To define more accurately the line maximum position alongside with absorption spectrum its first derivative (AJjAn) was measured. A given spectrum absorption structure as compared to the spectrum, corresponding to optical transition 4A,(4F) -@T,(4P) is less expressed. It implies, that there exists a more essential electron-oscillation interaction value within the limits of a given term. Similar to the same consideration of the 4T,(4F) term, while analysing this structure in absorption spectrum the line with a v = 3121 cm-’ maximum may be selected, which in accordance with an energetic level diagram (see Appendix and Fig. 3) may be identified with optical transition to r, state. Apparently, both a band with v = 2880 cm-’ and the line, the energy position of which is equal to 2853 cm-’ are due to optical transitions to r6- and r$-state, respectively. The mutual state of both lines is defined by the total effect of spin-orbit and Jahn-Teller interaction. Since f, and f6 states are Crammer’s doublets, then knowing the energy distance between the lines, corresponding to these transitions, a spin-orbit interaction parameter may be defined. According to the energy levels diagram.“’ 21, = 241 cm-‘,
i.e. i, = 120 cm-‘.
As well as in the case of the transition to 4Tl(4F) term CF D, parameter may be defined, based upon an absorption spectrum corresponding to optical transition 4A,(4F) +“T,(4F). The energy distance 2970 cm-’ between r,(4A,) state and gravity centre of 4T,-term corresponds to IO D,, i.e. D,(4T,) = 297 cm-‘. Such an agreement of D, values, defined considering the transitions to various terms is apparently not random, but is the result of a small shift value of the position of nondisturbed by spin-orbit interaction 4Tl(4F) and 4T,(4F)-terms. It proves the applicability of the energetic diagram”’ and allows one to calculate the electron-electron oscillation parameter B. The energy distance 11457 cm-’ between r,(4A,)-state and gravity centre 4Tl(4P) term corresponds to 15B+ 12D,. Using the value D, = [D,(T,) + D,(T,)]/2 one can define the Rack parameter B = 525 cm-‘.
Optical absorption for 4A,(4F) -+ ‘G transition At impurity concentration Co*+ N 2 5 x 10’9cm-3 in the vicinity of 9200-10800cm-’ at low temperatures absorption is observed, the low intensity of which allows it to be referred to the forbidden rules of optical transition selection. From the high energy side this absorption joins the one, which is due to that described above transitions to 4T,(4P)-term. From the energy level diagram”’ one can assume, that absorption is to be expected in this vicinity, due to transition to split by CF and by spin-orbit interaction components of 2G-term of Co*+ ion. Figure 4 shows a spectrum associated with such transitions and a corresponding energy level diagram. The total energy split width calculated by using D, = 298 cm-’ makes up 1200 cm-‘, which corresponds to the vicinity width of an observed absorption. In a high energy spectrum region a structural absorption consisting of ZPL (v,,, = 10750 cm-’ and v,,, = 10414 cm-‘) and shifted to the vicinity of high frequencies of electron-oscillation absorption may be outlined. According to the energy diagram it may be supposed, that above ZPL corresponds to the transitions to spin-orbit components r7 and rf of the highest spaced *T,(*G) term. The results of earlier work”3’ show that mutual spacing of spin-orbit components of *T,-term are practically independent from the value and behaviour of Jahn-Tellar interaction, It provides the use of above lines to define spin-orbit coupling constant within the limits of a given term. The direct comparison with a given diagram gives 2, = 90 cm-‘. On the basis of A4 value and energy levels diagram it can be found that the energy position of the ‘T,(‘G) term corresponds to v = 10525 cm-‘. Using CF D, = 298 cm-’ parameter, obtained from the absorption spectrum analysis corresponding to the optical transitions to spin-orbit components of 4Tl(F) and 4T2(F) terms and knowing energy position of 2T2(2G) term, it is possible to evaluate the energy position of 2E; 2T,- and *A,-terms (v (2E) = 9886 cm-‘, v (*T,) = 9673 cm-’ and v(*Al) = 9375 cm-‘, respectively).
758
P. N.
BUKIVSKY
er al.
In
the absorption spectrum governed by optical transitions Ts(4Az) -+‘G, the line with 9840 cm-’ is observed. Therefore this line apparently corresponds to the optical transition to r,(*E) state, which is shifted by 46 cm ’ from the expected energy position v (‘E) = 9886 cm ’ on account of the JahnTeller interaction. From the high energy side of this line the equidistant structure of electron oscillation bands is observed. In the frequency region r = 9670 cm ’ where the transition to ‘T, (*G) terms should be expected, in absorption spectrum two weak lines are observed, which could be interpreted as transitions to spin-orbit components of rh and r$ symmetry of *T, term. The expected energy position of ‘A,(‘G) term v = 9375 cm ’ agrees with the energy position of a wide band absorption, which may be referred to the transitions to r,(‘A,)-state. Such a band width could be explained by fairly strong electron-oscillation interaction which has fully-symmetric and tetragonal’13) oscillations. Such an agreement of the designed energy terms positions, with the ones obtained experimentally, is another testimony to the selected energy level diagram validity of CO’+ ion in CdTe crystal. Since the energy distance 997 1 cm ’ between TX(JA, (F)) state and gravity centre of ‘G term corresponds to 4B + 3C + 12D,, it provides the estimation of the Rack parameters C = 1430 cm ‘. V max
=
Absorption,
stipulated
b_y oscillation
development
and electron-oscillation
interaction
in CdT~:co
The absorption spectrum structure indicates that the processes of electron-oscillation interaction play an important role in optical transitions. Since the ‘T,(P) state is three-fold degenerated, the interaction both with fully symmetrical and degenerated oscillations of t- and rz-symmetry may occur. As was mentioned above, in accordance with the Jahn-Teller theorem the interaction with fully symmetrical oscillations leads to a drop of a corresponding electron state by the value of Jahn-Teller interaction (A&). Allowing for such an interaction the electron states become the main vibronic states (n = 0). In an absorption spectrum zero-phonon lines, corresponding to transitions in the above vibronic states, are usually observed as well as a number of other lines. coupling with transitions to the excited electron-oscillations levels (n # 0).(‘4) From the short-wave side of ZPL a series of narrow absorption lines is observed, which certainly correspond to the transitions with various (acoustic and optical) oscillation participation i.e. vibronic structure of absorption spectrum. At low temperatures (T = 30 K) in the vicinity of a long-wave absorption edge for transition 4A2(4F) +4T,(4P) two weak lines are observed rather vividly (Fig. la). The distance between these lines correspond to that between ZFL, corresponding to transitions r, and r#’ levels, i.e. it is equal to 12cm-‘. It proves, that the indicated lines correspond to the transitions with a certain energy oscillation absorption, i.e. to transitions from the first vibronic level of the main state rg(4A2) Co” ion. On the basis of the relative position of vibronic states the energy is defined of oscillations, which participate in the process of absorption. and it makes up v = 56 cm-‘. Since the oscillation of a such energy is defined for a main state of Co*+ ion (orbital singlet of 4A2-symmetry), the oscillation energy is not practically lowered by the possible occurrence of the Jahn-Teller interaction. In principle, as shown in Ref. (I 5). the Jahn-Teller interaction may also occur for a singlet state (according to the orbital moment, but degenerated, according to a spin) of an impurity ion state, but it is rather weak and its value makes energy level decrease should not take place for up some cm -I. Therefore the electron-oscillation such a state. The development of oscillation with v = 56 cm-’ frequency in an absorption spectrum may be associated with the following conditions. The introduction of impurity substitution atoms into crystal lattice may lead to the violation of the transmittion symmetry, particularly for a perfect crystal. Accordingly the oscillations may be considered active in optical absorption, which usually do not participate in such processes, due to wave vector selection rules inhibition. Presence of impurity substitution atoms in a crystal causes oscillation spectrum development, the frequencies of which correspond to the frequencies of peak maxima in one-phonon density of states of a perfect in the absorption spectrum of impurity centres crystal. (i6) As well as the above frequencies pseudolocal oscillations seem to occur and their onset is caused by appreciable weakening of elastic constant crystals close to the impurity centre.‘15’ TO establish the nature of a discovered oscillation, being active in the absorption process, it is necessary to analyse the frequencies, which correspond to peak maximums in the one-phonon
IR-spectroscopy
of crystals containing Jahn-Teller impurity centers
759
density of a crystal state CdTe. The calculations g(v) for CdTe were carried out in Refs (17, 18), where it is shown, that in the low-frequency oscillation spectrum region there is rather an intensive peak, of frequency 34cm-‘, corresponding to a cross acoustic TA-phonon at the edge of the Brillouin zone (point x). The next maximum dependent g(v) corresponds to v = 53 cm-’ frequency, i.e. to a TA phonon with a wave vector K z 0.6 (point K of a Brillouin zone). Schematically the one-phonon density for states CdTe is shown at Appendix C, Fig. 1. Thus the complete analysis shows, that the oscillation value obtained (v = 56 cm-‘), which was active in the process of absorption corresponds to a cross acoustic phonon close to point K of the Brillouin zone to TA (K) phonon. The absorption in the region 11500-10980 cm-’ is governed by optical transitions with the simultaneous participation of both low-frequency acoustic oscillations and optical ones. In particular, in this spectrum region the lines are observed, the position of which with respect to ZPL (vmax= 10959 cm -‘) correspond to frequency values close to other maximum values in one-phonon density states. The absorption spectrum section in the vicinity of 12000-l 1760cm-’ is due to the optical transitions to the level of rf symmetry. The lines, located from the short-wave side of ZFL correspond to transitions to vibronic levels with participation of various oscillations. Since ZPL in a given case is a fairly well isolated (the nearest line is placed at a distance, greater than twice the halfwidth), it seems possible, on the basis of its half-width temperature dependence, to define the frequency oscillation, the interaction with which causes spectrum structure broadening at temperature increases. For optical transitions between descrete levels, as shown in Refs (19,20), the absorption temperature dependence of a half-width line within the limits of a one-mode model is defined by the following ration: H’(T) = H’(0) cth (hv/2kT),
(1)
where H(O)-a halfwidth of the line; at T = 0 K; hv-oscillation energy, responsible for temperature broadening of ZPL. The approximation of experimental dependence H(T) with the help of ratio (1) provided the oscillation frequency v = 36 cm-’ definition, that practically agrees with the frequency of the most intensive peak (v = 34 cm-‘) in a low-frequency region of the one-phonon state density CdTe. The result obtained proves, that ZPL broadening at an increase in temperature may result from the interaction with TA (X) phonons. Vibronic levels of the excited state, coupled with these oscillations do not appear in analysed spectrum structure absorption; since the nearest vibronic lines are away from ZPL at distances, exceeding TA (X) phonon frequency. But at the long-wave absorption edge of a given ZPL, as shown in Fig. l(b), when T = 15 K two weak lines may be observed. One of them corresponds to transitions with oscillation absorption, (v = 56 cm-‘) the frequency of which has been defined earlier for transitions to the lowest spin-orbital levels of rg and r, symmetry. Another line onset is caused by optical transitions with energy absorption frequency oscillation v = 36 cm-’ which agrees with frequency oscillation, obtained for an excited degenerated state. It is well known w that TA (X) phonon symmetry for zinc blende crystals (CdTe also refers to them) interacting with the substitution impurity centre is defined by r, and r2. For a state of r,-symmetry Jahn-Teller interaction, in principle, can be observed both with c-oscillations and the ones of a t,-symmetry. Since TA (X) is an oscillation, which does not develop in the spectrum structure, it should be considered that the spectrum structure, being close to ZPL is defined by interaction with c-symmetry oscillations. TA (X) phonons (v = 56 cm-‘) are the oscillations of this type, among which there are c-symmetry oscillations. It agrees well with data from Refs (3,21), where on the basis of the analyses it was shown, that a vibronic spectrum structure may be explained allowing for interaction with only the oscillation type of the indicated symmetry. To get data yielding various crystal oscillation participation in a case of intracentral Co:+ ions absorption some measurements of absorption spectrum in a FIR-spectrum region have been carried out. Figure 5 shows the results of such measurements for CdTe: Co(N = 5 x lOI cm-‘) where T = 4.2 K. As seen in Fig. 5, the absorption spectrum consists of a great number of lines of various intensity. The majority of lines may be identified with a well known crystal phonon spectrum of CdTe.(‘8~22*5) The narrow line (the halfwidth is approximately 2 cm-‘) with v = 56 cm-’ frequency, the intensity
760
I
I
I II I I
II
I
III
Y
Fig. 5. (a) Absorption spectrum of CdTe:Co energy position governed by electron-oscillation
]L~~PT,IPII
( cm-‘)
crystals in the FIR spectrum region at 7’ = 4.5 K: (b) line transitions with respect to corresponding ZFL for various Co>+ ion states.
of which increases with rise in impurity concentration corresponding to the oscillation, calculated earlier, when analysing the intracentre absorption spectrum of Co’+ ions, agrees well with a maximum of a one-phonon density crystal state CdTe. The temperature dependence measurement of an oscillation spectrum in a low-frequency region (< 56 cm ‘) shows that an T 2 50 K, when v < 34 cm -’ the absorption phonon is practically unchangeable. When v > 34 cm ’ a noticeable rise of a continuous absorption takes place, in the background a narrow absorption peak is observed (v = 56cm-‘). These results prove that a continuous background in a low-frequency spectrum region is governed by various energy acoustic oscillations absorption and the appearance of a narrow line (v = 56 cm-‘-by pseudolocal oscillation development. Detailed discussion of the oscillation spectrum is beyond the limits of the present work. Here we shall concentrate on some peculiarities of the oscillation structure in an intracentral absorption spectra, given in Figs 14. For this purpose in Fig. 5(b) the maximum position of the lines for each of the excited terms is shown, caused by the electron-oscillation transition in relation to the corresponding ZPL (Av). Firstly, it should be noted that vibration level absorption in the case of a transition to 4T,(4P) term, for which the Jahn-Tellar interaction failure is weak, most fully corresponds to the oscillation crystal spectrum of CdTe:Co. It proves, that in the electronoscillation interaction for optical transitions to the 4T,(4P) term, practically all oscillation crystal spectrum is taking part (quazilocal oscillation with v = 56 cm-’ energy included). The optical spectrum structure analysis, caused by transitions to other terms, also indicates the presence of electron-oscillation transitions with various frequency oscillation participating. The frequency interval Av from corresponding ZFL, in many cases differs from Av, observed at the transition to the 4T,(4P) term. Moreover at transition to a number of spin-orbit components ‘T,(4F) or 4T2(“F) terms (for example to rf levels) it is completely impossible to separate the individual lines in the vicinity of the electron-oscillation transitions, as a result of their strong broadening. Such absorption spectrum peculiarities indicate the presence of a multi mode Jahn-Teller effect for excited impurity centre states. One of the results of such an interaction is distinction for the terms 4T,(4F) and 4T,(4F) in a positron of line maxima of electron-oscillation bands in comparison with results, obtained for 4T,(4P) terms as well as when measuring the crystal oscillation spectrum. Thus, the analysis of the electron-oscillation spectrum structure of intracentral absorption of Co’+ ions in a CdTe crystal, shows that the electron-oscillation interaction behaviour is different both for various terms and for various spin-orbit components within the limits of one term. The difference in Av values for ZPL, stipulated by transitions to rb and r, levels for which the Jahn-Teller interaction does not occur, seems to indicate the necessity of considering the Jahn-Teller interaction for an excited term on the whole. Various parameter values of spin orbit interaction, obtained for various excited terms are another proof of it.
IR-spectroscopy
Intracentral
ions Co’+ absorption
of crystals containing Jahn-Teller impurity
centers
761
in Cd,_,YMn, Te
The measurement of Co*+ ions absorption spectrum for the 4A2(4F) +4T,(4P) transition in crystals Cd,,g,Mn,,,,Te indicates, that in a case of semimagnetic semiconductors, which are the substitution of solid solutions, the absorption spectrum structure may be maintained. However some peculiarities, which are different from CdTe crystals may be observed. Thus it was established: (i) a small spin-orbit interaction parameter rise; (ii) a change of energy position and line intensity of electron-oscillation transitions; (iii) energy distance increase between spin-orbit components rf; and (iv) r, symmetry for the 4T,(4P) term. The observed absorption spectrum structure is noticeably spread, which is governed by inhomogeneous broadening in solid substitution solutions. Parameters of the crystalline field (D,) and electron-electron interaction (B) actually do not change. Spin-orbit split value change for 4T,(4P) term in Cd,_,Mn,Te crystals as compared to CdTe is apparently governed by exchange interaction between local magnetic ions Co’+ and Mn*+ moments magnetic field, or by interaction of magnetic moments of Co’+ ions with effective fluctuating stipulated by the regulation of magnetic ions Mn’+. The change of energy position of electronoscillation bands may be associated with the one-phonon density change of a crystal state, when ions Mn*+ are being introduced into CdTe matrix while the semimagnetic semiconductor is being formed. Intracentral
Co2+ ions absorption
in ZnTe
crystals
The considered absorption spectrum of ZnTe:Co crystals (N = 10’8cm-3; T = 4.5 K, spectral vicinity 12500-10900cm~‘) corresponds to the optical transition between the main Ts level of 4A2(P) state and spin-orbit components of r$-, r,-, r;-, r,-symmetry of the excited 4T,(P) Co*+ state (Fig. 6). The intracentral absorption spectrum shape proves that the absorption in the range 11620-10900 cm-’ corresponds to optical transitions to the lowest spin-orbit components of the excited 4T,(P) state r$ and r,, while the absorption bands between 12200-l 1750 correspond to the transitions of higher spin-orbit energy levels of r$ and r6 symmetry. The experimentally observed split value for 4T,(P) of the Co*+ ion . state in the ZnTe crystal, constitutes approximately 1400cm-’ as shown earlier. In Fig. 6 the observed absorption spectrum structure cannot be explained within the framework of the astatic model of CF and requries the electron-oscillation interaction account. The observed difference of a relative ZPL position (520 cm-‘) coupling with transitions to spin-orbital levels of r6 and r$ symetry, from the expected one on the basis of a static model of CF (267 cm -I), as for the CeTe:Co crystals, is apparently governed by energy lowering of the Ts level as a result of the Jahn-Teller interaction. Since such a value shift is about the spinorbit interaction value, it proves an intermediate value of the Jahn-Teller interaction in ZnTe:Co crystals.
1.4 I
I
I
9000
1.5 I
1.6 I
1.7 1
1.6 I
I
I
7000
6000
1.9 I
2.0 I
2.1 I
2.2 2.3 I,
I
6000
xta,
Fig. 6. Absorption spectrum of ZnTe:Co crystals at T = 4.5 K [transition 4A2(4F)+4T,(4P)J.
162
P. N. BIJKIVSKYet al
As mentioned above, the absorption spectrum in the region of 12500-10900 cm-’ corresponds to transitions in spin-orbit components of the 4T,(P) state. In addition, the lines with maxima at 10986, 11791 and 12342 cm-’ are coupled with transitions to the main vibronic states, i.e. they are ZPL (ZPL (rt, r,), ZPL (rf) and ZPL (r,), respectively. Figure 6 shows that in an absorption spectrum one can observe a number of other lines, besides those associated with transitions to the excited electron-oscillation levels. Thus, from the short wave side of ZPL (r$, r,) there exist two lines the energy position of which proves the oscillations with energy of 218 and 134 cm -’ participating in an absorption action. The first value is close to the energy of LO phonon at a point r of the Brillouin sone, i.e. LO (T)-phonon, the second is close to the energy of LA (L)-phonon. In accordance with Ref. (16) LO (r)-phonon, interacting with the substitution impurity centre in crystals with zinkblende structure are fully-symmetrical oscillations, while LA (L)-phonons have a, and r,-symmetry. Thus among the oscillations indicated Jahn-Teller oscillations of c or r2 symmetry do not occur. So in a given case a vibronic state reduction (compression) is not observed. In the optical transition band to l-8 level two vibronic levels are also visible, which are away from ZPL (f 8”) by 205 and 79 cm-‘. The first line corresponds to the transition with LO (r) phonon participation, the second corresponds to a phonon with an energy level close to the energy of one of the maxima of the ZnTe crystal state one-phonon density spectrum, and apparently corresponds to TA (K) phonon.“” There are oscillations of c, r, and t2 symmetries among the TA (K) phonons. Since the energy position (79 cm -‘) of a corresponding vibronic line ZPL (rf) is comparatively too close to the spectrum maximum value of one phonon density state (73 cm-‘), apparently the appearance of a given vibronic band is associated with T, or L and T>symmetry phonons participation provided that Jahn-Teller interaction with such oscillations is rather small. In the case of the transition to spin-orbit component of rb symmetry only one ZPL (r,) line. broadening as compared to ZPL (rf, r,) and ZPL (r;) is hardly observed, which is located at the long-wave edge of the intensive total absorption spectrum. The comparison of the observed intracentral absorption Co 2+ ion spectrum in this work, with similar spectrum for CdTe(@ proves that there is a stronger interaction with full-symmetry oscillation in the case of ZnTe. As is shown in Ref. (13) the increase of the electron-oscillation force, assuming these oscillations may lead to a structure spectrum leveling, resulted from interaction with the Jahn-Teller oscillations. Therefore it should be expected that this condition is a cause of the absence of a more complex ZnTe:Co absorption spectrum structure. From the low-energy side of ZPL (r$, r,) absorption lines of a weak intensity (Fig. 6) may be observed, which are apparently associated with prohibition according to spin-orbit transitions between 4F and 2G Co ‘+ ions states. The lowest level of *G-state is 2A, symmetry level. Therefore a narrow-absorption line (v = 10672 cm ‘) is supposed to correspond to ZPL (‘A,). The temperature rise causes the sharp decrease of ZPL (‘A,) intensity, while the line remains symmetrical and the energy position at its maximum is independent from temperature. The other lines between ZPL (‘A,) and ZPL (r& r,) are coupled with transitions to electron-oscillation levels of *A, state. Their energy position comparative to ZPL (2A,) agrees with the local density spectral distribution of the ZnTe:Co crystals phonon state, which is obtained on the basis of luminiscence spectrum measurements stipulated by transitions between 4Tz and 4A2-states.‘23) Since the electron-oscillation structure spectrums for 4A2 and ‘A, states coincide it may be evidence of practically the same electron-oscillation interaction in two non-degenerated Co’+ ion states in ZnTe. ZPL (‘A,) temperature dependence analysis shows that its halfwidth H may be described by the dependence (I) from which the value hv = 37 cm-’ is derived. This value agrees with the first maximum position of the ZnTe:Co crystal one-phonon density state. That Huang Rhys factor, characterising the electron-oscillation interaction force is defined from the expression H(0) = 2 hv J%%-? and in a given case constitutes 0.003, which indicates a fairly weak electron-oscillation interaction in the case of optical transition between non-degenerated 4A, and 2A, levels of Co2+ ion. It should be noted that the value hv = 37 cm-’ obtained is close to the value 34 cm-‘, obtained when intracentral luminiscence crystal ZnTe:Mn spectrum was analysed.‘24’ In the latter case however the electron-oscillation interaction is strong (S = 29). The comparison of the absorption line temperature dependences for ZnTe:Co and CdTe:Co
IR-spectroscopy
of crystals containing Jahn-Teller
impurity centers
763
crystals, corresponding to the lowest spin-orbit *T, (P) states and the lines, corresponding to transitions between non-degenerated states, provides the establishment of some peculiarities of the Jahn-Teller electron-oscillation interaction found in a degenerated state. The temperature dependence of ZPL (rt, r,) intensity is similar to the one for the vibronic structure line; with the rise of the temperature ZPL (rf, r,) maximum shift observed on the long-wave side as well as the appearance of asymmetry on the low-energy side. These peculiarities are governed both by the vibronic nature of the main vibronic state and reduction of the excited vibronic states, due to the presence of the Jahn-Teller interaction for 4T,(P) states. 4.
CONCLUSIONS
The analysis of the experimental results obtained in the present work, when the low-temperature optical spectrum of CdTe, ZnTe, Cd,,, Mn,,,Te crystals, with Jahn-Teller Co*+ impurity ions were examined. It provides the separation of absorption spectrum regions, corresponding to the transitions between the main 4A,(4F) and excited 4T,(4P), 4T,(4F) and dT,(4F) states. ZPL are singled out in these spectra as well as the lines of electron-oscillation transitions with both phonons and pseudolocal low-frequency oscillations participation. The absorption spectrum structure analysis shows that Jahn-Teller interaction has practically no influence on the ZPL position and the spin-orbit interaction value in a case of transitions to the 4T,(P) state. But 1 = 332 cm-’ parameter of such an interaction is sufficiently higher than that I, = 178 cm-’ for a free ion. This parameter is sufficiently smaller 2, in a case of transition to the other terms. Besides, the relative position of spin-orbit components do not correspond to the expected one on the basis of the CF theory. The data obtained proves that the Jahn-Teller interaction for 4T,(4F) and 4T,(4F) terms suppress the spin-orbit interaction parameters to a great extent. The Jahn-Teller interaction developed for the states of f, symmetry causes the essential energy bias of corresponding ZPL relatively ZPL governed by transitions to the levels of Ts and r, symmetry. Both CF D, = 298 _t 3 cm-’ and electron-electron interaction, B = 525 cm-‘, C = 1430 cm’-’ parameters are defined assuming this situation. For Cd, _ ‘i Mn,rTe crystals the observed increase of parameters in comparison with CdTe crystals is apparently governed by exchange interaction manifestation between local magnetic moments of Co*+ and Mn2+ ions with an effective fluctuating magnetic field, which results from magnetic ions Mn2+ regulation and broadening of some lines in the absorption spectrum by inhomogeneous broadening, peculiar for solid substitution solutions. REFERENCES 1. B. Bersuker and V. 2. Polinger, Vibronic Interaction in Ma/ecules and Crysfals, p. 336. Nauka, Moscow (1983). 0. L. Natadze and A. T. Ruskin. News Acad. Sci. U.S.S.R.. Phvs. 140. 1846 (1976). A. V. Vasiliyew, B.-Z. Malkin, A. L. Natadze and A. T. R&kin, Zh. eksp. tear. F&. 71, 1192 (1976).
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22. M. Ja. Walakh and M. P. Licitsha, K. Electron. 22, 16 (1984). 23. A. P. Radlinski, J. Luminesc. 18/19, 147 (1979). 24. Yu. P. Gnatenko and A. I. Zhmuzko, UFZ 30, 843 (1985).