- Email: [email protected]
Resonant raman scattering on low energy excited states of Fe++ in Cd1-xFexSe
Solid State Communications, Printed in Great Britain.
Vol. 69, No. 5, pp.453-456,
1989.
0038-1098189 $3.00 + .oo Pergamon Press plc
RESONANT RAMAN SCATIERING ON LOW ENERGY EXCITED STATES OF Fe-
IN Cd,,FexSe
D. Scalbert, J. Cemo om, A. Mauger and C. Benoit i la Guillaume, Groupe de Physi ue %es Solides* de 1’Ecole Normale Sup&teure, Universite Parts V9 I, 2 place Jussieu, 75251 Paris Cedex 05, France. Institute
of Physics, (Received
A. tl cielski Polish Aca # emy of sciences, 02-668 Warsaw, Poland. 28 October
Al Lotnikow
32/46
1988 by M. Balkanski)
Raman scattering in &d Fe Se for x631 exhibit9 sharp excitations of Fe b&hs%t 13.0 and 17.6 cm-
scattering on polished and etched sam le with the k vector of the scattered photon pam Plel to the c axis (see insert of fi . 2); b1 back-scattering geometry on a cleaved ?ace containin the c axis It was used for the Raman resonant s ?udy to avo;$ uncertainties related to sub-band absorption [5] dependin upon the distance fPx ween the laser beam and the 9 ront surface of the sample. The experiments were done at 1.8 K without magnetic field.
I. Introduction. In the family of alloys named ‘semimagnetic semiconductors’ (SHSC), Mn substituted II-VI materials were the most commonly studied [I) More recently, Fe based SMSC have revealed very interesting pro@ies [23, related to the ene y spectrum of Fe (3d 1 ions in II-VI compoun 7 s: under the action of crustal field and spin-orbit coupling, they exhibit”a non ma netid singlet around state of summetru A. so tha 9 the magnetic @operties (Van ” Vleck ” pa)-amagnetism) &spend strongly upon the energy spectrum of the lowest excited states. They were determined throu h far infra-red absorption in CdTe:Fe [31 rsaman scatter-in is also a convenient way to determine such exe Btations [43 We have performed these experiments in Cd Fe Se, a c stal of Wurtzite structure, where 1% &fgonal ? ist$$-sion c8uses additional splitting in the Fe spectrum. Resonances in the Raman process have been studied in the vicinity of free and bound exciton transitions. 2. Samples
3. Experimental
results.
All the sam les s,tudied display;wo Raman peaks located at I j, cm- and 17.6 cm from the laser line (fig. 2) w&c-h we attribute to internal transitions of Fe ions from its ground state to khe+first and second excited state ( respectively and A +E 1. When the concentration of Fe i&e 1 es, tf?e Intensity of the Raman peaks -
15100
-/
Cd,_,Fe,Se
and appamtus.
Monocystals of Cd,_ Fe Se were grown b the modified Bridgman t&&d at the lnstitu Ye of Physics of the Polish Academ of Sciences, for nominal concentrations x=0.1, if, 3 and 5% ( x was not verified b micro robe analysis). The are not intentionally loped. l!te position of the lowest A exciton was determined at l.;e&f_;tgh the sha associated mflectivlty (Ffg. 1 ‘p. Photoluminescence is dominated by donor bound excitons (D”X). Raman scattering was performed with a double spectrometer, cooled GaAs photomultiplier and hoton countin detection. About 10 mW of DCtl B e laser mdiat Bon was used in two different con Yigumtions: a) right angle
* Unite Associse au Recherche Scieritifique.
Centm
National
de
012345 Concentration
CX 1
- Free exciton energ in Cd 1 Fe Se at 1.B K ?&ction of x ( the x sea ! e migh<%e &rected by a factor
la
453
454
RESONANT UMAN
SCATTERING
Vol. 69, No. 5
H = Ho +V,,+ US
+ y&+PSXH
With A the spln-orbit cou lin ammeter and Ii an eventual a lied magnet Pc f BP e d. Other notations am conven PPonal. me cystal field V, includes the cubic field term: ~r’~A,“(Y,o~g,~~+~10/7~‘h[Y,~~9,q~-Y,~*~~,~~]~ and the trigonal component: <3,
I
6
6
10 12 14 RAMAN SHIFT
I
I
16 18 ( cm-l
I
20 )
I
22
~“Yzo(6,#
+ 840Y4016,q)
I
24
Fig. 2 - Reman 8 ectm at 1.8 K for x= 1% laser wavenumber 14r! 94 cm-l and two different incident polarizations: a) ((c, b) I-c. increases, the Roman lines bvden and an additional line a pears at 25.6 cm consistently with the data of g . Hetman and al. [6)’ However, the identification of this line is not clear at the 100, = -5(3A4’/7+
84o/9)/(2&)
v = (3&%‘&~
+ 20 840/3)/(
l&n)
The on1 non-zero diagonal element of V, couples t and e s ! ates, and is equal to: v’ = G~“Js+5cr4>840/31
42 /(14&t)
which can also be written v’= QJ2, with Q the trigonal-field parameter defined in ref. [71. Since the splitting v between t levels is vey small, compared to the sepemtion lOD, between the e
absorption. The main purpose of this paper is to determine with accuracy the msition of the low energ excited levels of Fe which can be attrfbute 1 unambigousl to isolated ions, and consequent1 rovide reli r&Ile data for a fit with theory. To #o Phis, sam les wfth low Fe concentration wem selected Q x- 1% and 0.1% 1 and studied in the backscatterfng configumtion which provide good repmduciblllt . The narrowest line-width was obtained for 1hs ~~0.1% sample, under resonant conditfon, vith a FWHM for the A + about 1 cm’ ( fi . 3 1. The position!s o+ tk”til first exqted sta Pes are then obtafned ( 13 and 173 cm 1 with an accurac of the order of f 0.2 ;&fo’c;h”y am compared w I th theory in the next 4. Theory. The problem of 3d6 electron s stem In a trigonal c stal fleld has been investiga ! ed frl prior works [7? We write the Hamfltonian of a Fe Ion as:
-~ t-f T ,I
1
14734 cm-1
z
\
z
.\
6
6
10 RAHAN
I1
nJlPl
12 14 16 SHIFT ( cm-l
16
20
22
21
1
Fi 3 - Resonant Roman 8 ectm for x-O.1 X at 1.8 K ! or laser wavenumber P4695 and 14734 cm-l. The hatched Raman peak at 13 cm” comes over the,backgmund luminescence.
Vol. 69, No. 5
455
RESONANT RAMAN SCATTERING
state and the haycenter
of the t states, the effect of v on the 8 states is negli ible. Therefore we have arbitrarily chosen v-0 8 n! the energy of fhe e states ore determinfl by the pammeters 1OD A and v’. For Cd-Fe , we -[ion write, follo&g Since the en baycenter of the A, while the sepamtion primarily depends and E levels originates from v’, between the both i and v’ c% be determined unarnbigously from the fit of the e~erimental values of the energF;E[ 13 and 17.6 cm 1 which separate A frof$ . The result is h= -95.3 cm %l 101 This valug of WI is betweend l; of 103 cm- end the value 81 cm:, reported in ref. [7) Note this lower value 81 cm had been determined from the fi;itf the, ygnetic susceptibility, approximation whi%sKs”gnot justified,=s(nce “v? and h have the same order of magnitu@. The value of A should be about the same for Fe in all the II-VI c(rmpounds; with this respect, the value of 81 cm’ was considered es anomalous 71 indeed, ares well with I he values our value of Leo reported for Fe in“p nS, ZnTe, CdTe, c,rlS and ZnO, which are all in the range 93 - 100 cm . To make contact with a prior work [61 we have computed the energy A of the excited states (lobelled by 1) with respe4 t to the ground state, es a function of the magnetic fie d. The results
illustmted in fig. 4 reproduce the trends of the experiments1 data in fi .2 of ref. [63 In articular, in the geomet Ii gc \ he increase of Phe Raman shift versus H7 s well reproduced, as well as the eomety Ii_& t state is almosf H in close agreement with experiment. This b&havior is due to the fact that, in the trigonel-field function basis, the matrix between A, and elements of (L+2S).H -?& symmetry states are z&o, so that H ftIWd% tedt cou le the A ground state to the sta Pe to fir& order (in contrast b;aith H hence giving rise to the anisotropy of VaR’ Vleck pamma netism). The twofold degenemc of the E state a9 H-O is lifted bu the applicat ! on of the magnetic field. The splitting for w Ilc, however, is much smaller than the bandwidth L0.3meV) of ;Fs Rotttn eaks up to 15T, which explains thoh;; geen observed ex erimentall . litting, however is much Payer in t ii 8 case and should b observed especialllJ for the If I-c, two levels issuing from the khiiher leve located at about 4.1 meV above the g un state, accordin to fi . 4. However, the hi her Reman line reporte B in [6 1 is wound 3 meV a9 H-O and was observed only for Hlc. This casts doubt about its identification as the Ai+E2 transition [ 1 lj. 5. Resonant Raman scattering. Stu ,of the Raman resonances in the region of exciton7 c tmnsi tions was performed on the x-O.1 t sample, in order to have the narrowest resonances. Vey sharp resonances at the incident photon are observed on1 for the A,+ line on the en6 A4 he exciton lowes yY free exciton FX 1 t pe A 1 and bound to neutral donor DYt (fi . 5 I, while the A +E line becomes unobsenrab ?e because of the bdckground luminescence. A common feature in these two resonances is the creation of a hole with the angular momentum =J/2 in the intermediate state This t?12’ ebperiences the e%hange inter&ion In first with the Fe . H aS.j abxipation, we can neglect the’t!ikponents S’jand SJ of the excha e interaction, because non zero matrix elements “Bor these operators require
BOUND FREE
0
5
10
. 1
EXCITON
EXCITON
I
+
+ l
+
15
H (TI differences between the Fig. 4 - Calculated ene y! es of Fe*+ (of sgmmetrg three lowest excited sta A ,EendEatH=O)andthegmundstateAt WI $czof the applied magnetic field; _ Htic,
I I
14760 EXCITATION
- Intensity ~~npt~~&~O.iX
14710 WAVENUMBER
I
I
I
14660 ( cm-l
)
(in Log. scale) of the A +A at 1.8 K as a funtion bf th% ,
456
for
RESONANT RAMAN SCATTERING
the
hole
an
extra
intermediate
state
Vol. 69, No. 5
6. Conclusion. Raman scattering turns out to be a vey convenient wag to determine the spectnrm of low ene states of Fe++ in CdSe; the method can be easiy generalized to other II-VI compounds. In addit ! on through the study of resonance effects interest~n informations are gained abouf exchange ?nteraction between Fe++ substates and band states of the host lattice. Increasing x should allow the stu of Fe-Fe exchange interaction, bg an access to pa $ r eneyg spectrum.
s
1] for a review, see ‘Diluted Magnetic emiconductors’, ed. J. K. Furdgna and J. Kossut Vol. 25 in the series ‘Semiconductors Mid
Dobrowolski and J. Il. Baranowski, J. Phgs. C 19, 3605 ( 1986). 6] 0. Heiman, E. 0. Isaacs, A. Petrou, S. H. Bloom, Y. s hapira K. Smith ,R. Kershaw, K. Dwight, A. Wold and W. i&iat, 19th Int. Conf. on Semiconductor physics, Warsaw (Aug. 19BB), WeD-B-3.
observed: 0. Scalbert, J. Gaj, J. Cernogom A. Mauger, C. Benolt a la Guillaume and A. flgcle~ski, to be ublished. [121 #e above mentionned ayument is strictly val d for 0*X, where the two electrons are in a sin let spin state; for the resonance on free exe?ton of type A, one can arrive to a similar conclusion.
Vol. 69, No. 5, pp.453-456,
1989.
0038-1098189 $3.00 + .oo Pergamon Press plc
RESONANT RAMAN SCATIERING ON LOW ENERGY EXCITED STATES OF Fe-
IN Cd,,FexSe
D. Scalbert, J. Cemo om, A. Mauger and C. Benoit i la Guillaume, Groupe de Physi ue %es Solides* de 1’Ecole Normale Sup&teure, Universite Parts V9 I, 2 place Jussieu, 75251 Paris Cedex 05, France. Institute
of Physics, (Received
A. tl cielski Polish Aca # emy of sciences, 02-668 Warsaw, Poland. 28 October
Al Lotnikow
32/46
1988 by M. Balkanski)
Raman scattering in &d Fe Se for x631 exhibit9 sharp excitations of Fe b&hs%t 13.0 and 17.6 cm-
scattering on polished and etched sam le with the k vector of the scattered photon pam Plel to the c axis (see insert of fi . 2); b1 back-scattering geometry on a cleaved ?ace containin the c axis It was used for the Raman resonant s ?udy to avo;$ uncertainties related to sub-band absorption [5] dependin upon the distance fPx ween the laser beam and the 9 ront surface of the sample. The experiments were done at 1.8 K without magnetic field.
I. Introduction. In the family of alloys named ‘semimagnetic semiconductors’ (SHSC), Mn substituted II-VI materials were the most commonly studied [I) More recently, Fe based SMSC have revealed very interesting pro@ies [23, related to the ene y spectrum of Fe (3d 1 ions in II-VI compoun 7 s: under the action of crustal field and spin-orbit coupling, they exhibit”a non ma netid singlet around state of summetru A. so tha 9 the magnetic @operties (Van ” Vleck ” pa)-amagnetism) &spend strongly upon the energy spectrum of the lowest excited states. They were determined throu h far infra-red absorption in CdTe:Fe [31 rsaman scatter-in is also a convenient way to determine such exe Btations [43 We have performed these experiments in Cd Fe Se, a c stal of Wurtzite structure, where 1% &fgonal ? ist$$-sion c8uses additional splitting in the Fe spectrum. Resonances in the Raman process have been studied in the vicinity of free and bound exciton transitions. 2. Samples
3. Experimental
results.
All the sam les s,tudied display;wo Raman peaks located at I j, cm- and 17.6 cm from the laser line (fig. 2) w&c-h we attribute to internal transitions of Fe ions from its ground state to khe+first and second excited state ( respectively and A +E 1. When the concentration of Fe i&e 1 es, tf?e Intensity of the Raman peaks -
15100
-/
Cd,_,Fe,Se
and appamtus.
Monocystals of Cd,_ Fe Se were grown b the modified Bridgman t&&d at the lnstitu Ye of Physics of the Polish Academ of Sciences, for nominal concentrations x=0.1, if, 3 and 5% ( x was not verified b micro robe analysis). The are not intentionally loped. l!te position of the lowest A exciton was determined at l.;e&f_;tgh the sha associated mflectivlty (Ffg. 1 ‘p. Photoluminescence is dominated by donor bound excitons (D”X). Raman scattering was performed with a double spectrometer, cooled GaAs photomultiplier and hoton countin detection. About 10 mW of DCtl B e laser mdiat Bon was used in two different con Yigumtions: a) right angle
* Unite Associse au Recherche Scieritifique.
Centm
National
de
012345 Concentration
CX 1
- Free exciton energ in Cd 1 Fe Se at 1.B K ?&ction of x ( the x sea ! e migh<%e &rected by a factor
la
453
454
RESONANT UMAN
SCATTERING
Vol. 69, No. 5
H = Ho +V,,+ US
+ y&+PSXH
With A the spln-orbit cou lin ammeter and Ii an eventual a lied magnet Pc f BP e d. Other notations am conven PPonal. me cystal field V, includes the cubic field term: ~r’~A,“(Y,o~g,~~+~10/7~‘h[Y,~~9,q~-Y,~*~~,~~]~ and the trigonal component: <3,
I
6
6
10 12 14 RAMAN SHIFT
I
I
16 18 ( cm-l
I
20 )
I
22
~“Yzo(6,#
+
I
24
Fig. 2 - Reman 8 ectm at 1.8 K for x= 1% laser wavenumber 14r! 94 cm-l and two different incident polarizations: a) ((c, b) I-c. increases, the Roman lines bvden and an additional line a pears at 25.6 cm consistently with the data of g . Hetman and al. [6)’ However, the identification of this line is not clear at the 100, = -5(3A4’/7+
84o/9)/(2&)
v = (3&%‘&~
+ 20 840
l&n)
The on1 non-zero diagonal element of V, couples t and e s ! ates, and is equal to: v’ = G~“Js+5cr4>840/31
42 /(14&t)
which can also be written v’= QJ2, with Q the trigonal-field parameter defined in ref. [71. Since the splitting v between t levels is vey small, compared to the sepemtion lOD, between the e
absorption. The main purpose of this paper is to determine with accuracy the msition of the low energ excited levels of Fe which can be attrfbute 1 unambigousl to isolated ions, and consequent1 rovide reli r&Ile data for a fit with theory. To #o Phis, sam les wfth low Fe concentration wem selected Q x- 1% and 0.1% 1 and studied in the backscatterfng configumtion which provide good repmduciblllt . The narrowest line-width was obtained for 1hs ~~0.1% sample, under resonant conditfon, vith a FWHM for the A + about 1 cm’ ( fi . 3 1. The position!s o+ tk”til first exqted sta Pes are then obtafned ( 13 and 173 cm 1 with an accurac of the order of f 0.2 ;&fo’c;h”y am compared w I th theory in the next 4. Theory. The problem of 3d6 electron s stem In a trigonal c stal fleld has been investiga ! ed frl prior works [7? We write the Hamfltonian of a Fe Ion as:
-~ t-f T ,I
1
14734 cm-1
z
\
z
.\
6
6
10 RAHAN
I1
nJlPl
12 14 16 SHIFT ( cm-l
16
20
22
21
1
Fi 3 - Resonant Roman 8 ectm for x-O.1 X at 1.8 K ! or laser wavenumber P4695 and 14734 cm-l. The hatched Raman peak at 13 cm” comes over the,backgmund luminescence.
Vol. 69, No. 5
455
RESONANT RAMAN SCATTERING
state and the haycenter
of the t states, the effect of v on the 8 states is negli ible. Therefore we have arbitrarily chosen v-0 8 n! the energy of fhe e states ore determinfl by the pammeters 1OD A and v’. For Cd-Fe , we -[ion write, follo&g Since the en baycenter of the A, while the sepamtion primarily depends and E levels originates from v’, between the both i and v’ c% be determined unarnbigously from the fit of the e~erimental values of the energF;E[ 13 and 17.6 cm 1 which separate A frof$ . The result is h= -95.3 cm %l 101 This valug of WI is betweend l; of 103 cm- end the value 81 cm:, reported in ref. [7) Note this lower value 81 cm had been determined from the fi;itf the, ygnetic susceptibility, approximation whi%sKs”gnot justified,=s(nce “v? and h have the same order of magnitu@. The value of A should be about the same for Fe in all the II-VI c(rmpounds; with this respect, the value of 81 cm’ was considered es anomalous 71 indeed, ares well with I he values our value of Leo reported for Fe in“p nS, ZnTe, CdTe, c,rlS and ZnO, which are all in the range 93 - 100 cm . To make contact with a prior work [61 we have computed the energy A of the excited states (lobelled by 1) with respe4 t to the ground state, es a function of the magnetic fie d. The results
illustmted in fig. 4 reproduce the trends of the experiments1 data in fi .2 of ref. [63 In articular, in the geomet Ii gc \ he increase of Phe Raman shift versus H7 s well reproduced, as well as the eomety Ii_& t state is almosf H in close agreement with experiment. This b&havior is due to the fact that, in the trigonel-field function basis, the matrix between A, and elements of (L+2S).H -?& symmetry states are z&o, so that H ftIWd% tedt cou le the A ground state to the sta Pe to fir& order (in contrast b;aith H hence giving rise to the anisotropy of VaR’ Vleck pamma netism). The twofold degenemc of the E state a9 H-O is lifted bu the applicat ! on of the magnetic field. The splitting for w Ilc, however, is much smaller than the bandwidth L0.3meV) of ;Fs Rotttn eaks up to 15T, which explains thoh;; geen observed ex erimentall . litting, however is much Payer in t ii 8 case and should b observed especialllJ for the If I-c, two levels issuing from the khiiher leve located at about 4.1 meV above the g un state, accordin to fi . 4. However, the hi her Reman line reporte B in [6 1 is wound 3 meV a9 H-O and was observed only for Hlc. This casts doubt about its identification as the Ai+E2 transition [ 1 lj. 5. Resonant Raman scattering. Stu ,of the Raman resonances in the region of exciton7 c tmnsi tions was performed on the x-O.1 t sample, in order to have the narrowest resonances. Vey sharp resonances at the incident photon are observed on1 for the A,+ line on the en6 A4 he exciton lowes yY free exciton FX 1 t pe A 1 and bound to neutral donor DYt (fi . 5 I, while the A +E line becomes unobsenrab ?e because of the bdckground luminescence. A common feature in these two resonances is the creation of a hole with the angular momentum =J/2 in the intermediate state This t?12’ ebperiences the e%hange inter&ion In first with the Fe . H aS.j abxipation, we can neglect the’t!ikponents S’jand SJ of the excha e interaction, because non zero matrix elements “Bor these operators require
BOUND FREE
0
5
10
. 1
EXCITON
EXCITON
I
+
+ l
+
15
H (TI differences between the Fig. 4 - Calculated ene y! es of Fe*+ (of sgmmetrg three lowest excited sta A ,EendEatH=O)andthegmundstateAt WI $czof the applied magnetic field; _ Htic,
I I
14760 EXCITATION
- Intensity ~~npt~~&~O.iX
14710 WAVENUMBER
I
I
I
14660 ( cm-l
)
(in Log. scale) of the A +A at 1.8 K as a funtion bf th% ,
456
for
RESONANT RAMAN SCATTERING
the
hole
an
extra
intermediate
state
Vol. 69, No. 5
6. Conclusion. Raman scattering turns out to be a vey convenient wag to determine the spectnrm of low ene states of Fe++ in CdSe; the method can be easiy generalized to other II-VI compounds. In addit ! on through the study of resonance effects interest~n informations are gained abouf exchange ?nteraction between Fe++ substates and band states of the host lattice. Increasing x should allow the stu of Fe-Fe exchange interaction, bg an access to pa $ r eneyg spectrum.
s
1] for a review, see ‘Diluted Magnetic emiconductors’, ed. J. K. Furdgna and J. Kossut Vol. 25 in the series ‘Semiconductors Mid
Dobrowolski and J. Il. Baranowski, J. Phgs. C 19, 3605 ( 1986). 6] 0. Heiman, E. 0. Isaacs, A. Petrou, S. H. Bloom, Y. s hapira K. Smith ,R. Kershaw, K. Dwight, A. Wold and W. i&iat, 19th Int. Conf. on Semiconductor physics, Warsaw (Aug. 19BB), WeD-B-3.
observed: 0. Scalbert, J. Gaj, J. Cernogom A. Mauger, C. Benolt a la Guillaume and A. flgcle~ski, to be ublished. [121 #e above mentionned ayument is strictly val d for 0*X, where the two electrons are in a sin let spin state; for the resonance on free exe?ton of type A, one can arrive to a similar conclusion.