- Email: [email protected]
Observation of the internal luminescence of Cr2+(3d4) in GaAs under hydrostatic pressure
Solid State Communications, Vol. 46, No. 4, pp. 359-362, 1983. Printed in Great Britain.
0038-1098/83 / 160359-04503.00/0 Pergamon Press Ltd.
OBSERVATION OF THE INTERNAL LUMINESCENCE OF Cr2÷(3d 4) IN GaAs UNDER HYDROSTATIC PRESSURE B. Deveaud and G. Picoli CNET (LAB/ICM) 22301 Lannion, France and Y. Zhou* and G. Martinez t Laboratoire de Physique des Solides, associ6 au CNRS, Universit6 Pierre et Marie Curie, 4, place Jussieu 75230 Paris Cedex 05, France
(Received 29 November 1982 by M. Cardona) Observation of the internal luminescence of Cr2+(3d 4) in GaAs under hydrostatic pressure is reported. The activation of this luminescence is correlated to the lifting of the degeneracy between the excited level of Cr 2+ and the conduction band of the host material. The possible mechanisms giving rise to this luminescence are discussed. 1. INTRODUCTION
2. EXPERIMENTAL RESULTS
CHROMIUM IN GaAs has been intensively studied due to the technological importance of semi-insulating chromium doped substrates. A lot of experimental techniques have been used and the existence of an acceptor level (Cr 2÷) at mid-gap is very well established. Mainly characterized by EPR [1 ], this level has been studied in absorption, DLTS, DLOS, photoluminescence and photoconductivity experiments, which result in a quite consistent scheme [2-5]. However, the expected internal transition (ST2 ~ SE) of Cr 2÷ has only been observed in absorption [2] and not in luminescence [6]. The precise location of the fundamental Cr 2÷ level is at 0.775 eV below the conduction band edge [7] at 4 K whereas the zero-phonon transition is observed in absorption at 0.820 eV [8]. The excited state (SE) of Cr 2+ is then degenerate with the conduction band (45 meV above the conduction band minimum), a fact confirmed by the observation of this zero-phonon line in photoconductivity [9]. This resonance between the SE state and the conduction band makes this luminescence highly unlikely. So, we have lifted this degeneracy by applying an hydrostatic presssure. This is also possible by alloying GaAs with AlAs [10]. We have observed for the first time the internal luminescence of Cr 2+ in GaAs. We also discuss the possible internal processes involved in that radiative recombination.
GaAs samples (n type) with 1 0 : cm -3 chromium were obtained by diffusion [7] or by doping during the growth (semi-insulating SUMITOMO samples). The pressure is obtained by compression of helium gas in a cell merged in a liquid nitrogen bath. In the range of pressures used for that experiment, the pressure is purely hydrostatic. The luminescence was excited by the 514.5 nm line of an argon laser, passed through a 0.3 m focal length monochromator and detected with a cooled PbS cell. The optical path was maintained under a primary vacuum. The results are corrected for the response function of the experimental set up. The results obtained under hydrostatic pressure are very similar for both types of GaAs : Cr samples and are perfectly reversible. One typical result is shown in Fig. 1. The corresponding absorption measurements under hydrostatic pressure have already been reported [I 1 ] and gave a pressure coefficient of about 3.6 meV kbar -1 for the excited SE level relative to the valence band. As the gap of GaAs has a pressure coefficient of about 12 meV kbar we expect to lift their degeneracy around 6 kbar. We indeed observe a significant increase of the luminescence intensity which is linear up to 8 kbar and then rises very rapidly above this pressure. We also notice a change of the shape of the spectrum together with the activation of the luminescence. The high pressure curve is very similar to that observed for the internal transition of Cr 2+ in ZnSe [12].
* On leave from the Changchung Institute of Physics, Chang Chung, China.
3. DISCUSSION OF THE RESULTS
t On leave to SNCI 166 X, 25 av. des Martyrs, 38042 Grenoble, France.
Experiments on GaAs under hydrostatic pressure as well as in GaA1As show clearly that it is possible to 359
360
OBSERVATION OF Cr2+(3d 4) IN GaAs
Vol. 46, No. 4
A
6
77K
>I-03 Z
12.6 k bar
iii i-Z
3 12.15
11.5 k bar
11 k bar
10.35 k bar
9 k bar
4.2 k bar 2 k bnr
5000
6000
70~ ENERGY(cm -1 )
Fig. 1. Luminescence spectra of a SUMITOMO sample at different hydrostatic pressures. The bottom curve is for 2 kbar, the upper curve for 12.6 kbar. The same intensity scale is used for all pressures, observe the internal luminescence of the Cr2+ impurity as soon as the degeneracy of the excited 5E level with the conduction band of the host material is lifted. As in the case of ZnSe :Cr, the luminescence band is Stokesshifted with respect to the absorption band [11 ] due to the difference between the Jahn-Teller coupling strength of the excited and ground state of Cr2+, An estimation of the Jahn-Teller energy for the ground state can be obtained by taking the mean value of the width
of the two bands and gives a value close to 600 cm -1. This value, compared to the absorption results on the zero-phonon line [8], leads us to assume that the J a h n Teller effect in the SE state is negligible in front of that of the ST2 ground state. The assymetric character of the luminescence band (low energy shoulder) which is not observed in absorption (Gaussian band) is also characteristic of this impurity state in zinc-blende compounds. To our
Vol. 46, No. 4
OBSERVATION OF Cr2+(3d 4) IN GaAs
C r 2+ (3d z, )
C r 3+(3d3)
tt
t2
tt
e
t'TI
t
e
t'T 2
ft
t,
tit
It
e
t 5
ST 2
a)
E
b)
t '
conduction
t it
361
t
tt
t/
vatence band
Cr3+(t'r') -..- CrZ+(SE} -.-- hV + e C.B. c)
?
tt
o
CrZ+(Sr2)--,--Cr2+(SE)-~ h v ÷(e-h)pair
d)
Fig. 2. One electron schemes for Cr 3+ : 4 TI, 4 7"2 (a), Cr 2+ : 5 T2, 5~7 (b), and possible luminescence process (c), Cr3+ + electron in the conduction band as a starting point or (d), Cr 2÷ + electron-hole pair (see text for details). knowledge, this character has never been fully interpreted. Kaminska et ai. [I 2] have tentatively attributed it to the existence of a non zero Jahn-Teller effect of the excited SE state. However, they assume, to reproduce the experimental spectra, that the selection rules are not the same for absorption and for emission. This procedure is not correct. We have calculated the band shape for both processes by writing properly, following Longuet-Higgins et al. [13] and Muramatsu [14] the vibronic wave functions of the system. We assume a Jahn-TeUer coupling only with a E mode and do not include spin-orbit effects. For a very wide range of Jahn-TeUer energies, we never got a large dissymetry between absorption and emission band shapes as experimentally observed. This dissymetry is certainly due to an additional perturbation in the ground state: either a coupling to a
r2 phonon mode or to spin-orbit coupling. The former solution is unlikely because if the r2 phonon coupling were important, it would have been revealed in the fundamental state by EPR measurements [ 1 ] which is not. So, it is likely that the spin-orbit coupling is responsible for the observed dissymetry. A point difficult to explain is the distortion of the luminescence band as the pressure is increased. This distortion cannot be explained by the vibronic coupling as no change upon pressure of the electron-phonon coupling is observed in the photo-ionisation transition [7]. It can neither be explained by a change in the spin-orbit coupling as the shift in the maximum of the band (600 cm- 1) is too large to be accounted for in such a way. Another interesting question is raised by these experiments, concerning the physical process which gives rise to the luminescence. To discuss this point it is
362
OBSERVATION OF Cr2+(3d 4) IN GaAs
necessary to consider the one electron scheme (as calculated for instance by Hemstreet and Dimmock [15]). Typical scheme for the excited and ground states of Cr 3÷ and Cr 2+ are displayed on Fig. 2(a) and (b). The luminescence that we are studying corresponds to the recombination of a t2 electron of Cr2÷(SE) on a e orbital to get the ground state (ST2) see Fig. 2(b). The question arises thus to know how this SE state is obtained. The direct mechanism which involves an internal absorption process within the 3d 4 multiplet is unlikely since for this range of excitation energies it is spin forbidden. An alternative mechanism would involve the trapping of a free electron by a Cr 3÷ in the 4T 1 state going with an energy transfer process promoting an electron from the e to t2 orbitals [see Fig. 2(c)]. Besides the fact that this mechanism would not necessary preserve the spin and could give rise to non-radiative transitions, it is directly proportional to the number of Cr 3+ centers, which in our n-type samples is certainly very low. We are then left with a luminescence due to the creation of electron-hole pairs, which interacting with the Cr 2+ centers lead to the quasi-simultaneous processes [see Fig. 2(d)]. (1) Cr2+(ST2) + hole in the valence band ~ Cr3+(4T2: hole on an e orbital), (2) Cra+(4T2) + electron in the conduction band -> Cr2+(sE: electron on a t 2 orbital). a
The final step is then the internal recombination of Cr 2+.
which leads to this luminescence is believed to be mainly due to the capture of an electron-hole pair on a Cr 2÷ center.
Acknowledgements - We wish to thank Dr B. Lambert for many helpful discussions during this work. It has been supported in part by the DRET under contract No. 80-1372. REFERENCES
I. 2. 3. 4. 5. 6.
7. 8. 9. 10. 11.
4. CONCLUSION We have shown experimentally that the internal luminescence of C r 2+ in GaAs is inhibited by the degereracy of the excited level of this impurity charge state with the conduction band of the material. This degeneracy can be lifted either by applying an hydrostatic pressure or by alloying. The interactions giving rise to the shape of the band have been discussed. The process
Vol. 46, No. 4
12. 13. 14. 15.
J.J. Krebs & G.H. Stauss,Phys. Rev. B16,971 (1977). D. Bois & P. Pinard, Phys. Rev. B9,4171 (1974). G. Picoli, B. Deveaud & D. Galland, J. Phys. 42, 133(1981). A. Nouailhat, F. Litty, S. Loualiche, P. Leyral & G. Guillot, J. Phys. 43,815 (1982). D. Look, Solid State Commun. 24, 825 (1977). The well known 0.839 eV luminescence in GaAs is attributed now to the internal transition of a Cr2÷ donor complex of C3v symmetry, the trigonal field being sufficient to lift the degeneracy of the excited level of this complex with the conduction band [3]. G. Martinez, A.M. Hennel, W. Szuszkiewicz, M. Balkanski & B. Clerjaud,Phys. Rev. B23, 3920 (1981). B. Clerjaud, A.M. Hennel & G. Martinez, Solid State Commun. 33,983 (1980). L. Eaves, P.J. Williams & C. Uihlein, J. Phys. C" Solid State Phys. 14, L693 (1981). B. Deveaud, B. Lambert, H. L'Haridon & G. Picoli, Z Lumin. 24•25, 273 (1981). A.M. Hennel ,W. Szuszkiewicz, M. Balkanski, G. Martinez & B. Clerjaud, Phys. Rev. 23, 3933 (1981). M. Kaminska, J.M. Baranowski, S.M. Uba & J.T. Vallin, J. Phys. C12, 2197 (1979). H.G. Longuet-Higgins, U. Opik, M.H.L. Pryce & R.A. Sack, Proc. Roy. Soc. London A244, 1 (1958). S. Muramatsu,J. Phys. Soc. Japan. 50, 1645 (1981). L.A. Hemstreet & J.O. Dimmock,Phys. Rev. B20, 1527 (1979).
0038-1098/83 / 160359-04503.00/0 Pergamon Press Ltd.
OBSERVATION OF THE INTERNAL LUMINESCENCE OF Cr2÷(3d 4) IN GaAs UNDER HYDROSTATIC PRESSURE B. Deveaud and G. Picoli CNET (LAB/ICM) 22301 Lannion, France and Y. Zhou* and G. Martinez t Laboratoire de Physique des Solides, associ6 au CNRS, Universit6 Pierre et Marie Curie, 4, place Jussieu 75230 Paris Cedex 05, France
(Received 29 November 1982 by M. Cardona) Observation of the internal luminescence of Cr2+(3d 4) in GaAs under hydrostatic pressure is reported. The activation of this luminescence is correlated to the lifting of the degeneracy between the excited level of Cr 2+ and the conduction band of the host material. The possible mechanisms giving rise to this luminescence are discussed. 1. INTRODUCTION
2. EXPERIMENTAL RESULTS
CHROMIUM IN GaAs has been intensively studied due to the technological importance of semi-insulating chromium doped substrates. A lot of experimental techniques have been used and the existence of an acceptor level (Cr 2÷) at mid-gap is very well established. Mainly characterized by EPR [1 ], this level has been studied in absorption, DLTS, DLOS, photoluminescence and photoconductivity experiments, which result in a quite consistent scheme [2-5]. However, the expected internal transition (ST2 ~ SE) of Cr 2÷ has only been observed in absorption [2] and not in luminescence [6]. The precise location of the fundamental Cr 2÷ level is at 0.775 eV below the conduction band edge [7] at 4 K whereas the zero-phonon transition is observed in absorption at 0.820 eV [8]. The excited state (SE) of Cr 2+ is then degenerate with the conduction band (45 meV above the conduction band minimum), a fact confirmed by the observation of this zero-phonon line in photoconductivity [9]. This resonance between the SE state and the conduction band makes this luminescence highly unlikely. So, we have lifted this degeneracy by applying an hydrostatic presssure. This is also possible by alloying GaAs with AlAs [10]. We have observed for the first time the internal luminescence of Cr 2+ in GaAs. We also discuss the possible internal processes involved in that radiative recombination.
GaAs samples (n type) with 1 0 : cm -3 chromium were obtained by diffusion [7] or by doping during the growth (semi-insulating SUMITOMO samples). The pressure is obtained by compression of helium gas in a cell merged in a liquid nitrogen bath. In the range of pressures used for that experiment, the pressure is purely hydrostatic. The luminescence was excited by the 514.5 nm line of an argon laser, passed through a 0.3 m focal length monochromator and detected with a cooled PbS cell. The optical path was maintained under a primary vacuum. The results are corrected for the response function of the experimental set up. The results obtained under hydrostatic pressure are very similar for both types of GaAs : Cr samples and are perfectly reversible. One typical result is shown in Fig. 1. The corresponding absorption measurements under hydrostatic pressure have already been reported [I 1 ] and gave a pressure coefficient of about 3.6 meV kbar -1 for the excited SE level relative to the valence band. As the gap of GaAs has a pressure coefficient of about 12 meV kbar we expect to lift their degeneracy around 6 kbar. We indeed observe a significant increase of the luminescence intensity which is linear up to 8 kbar and then rises very rapidly above this pressure. We also notice a change of the shape of the spectrum together with the activation of the luminescence. The high pressure curve is very similar to that observed for the internal transition of Cr 2+ in ZnSe [12].
* On leave from the Changchung Institute of Physics, Chang Chung, China.
3. DISCUSSION OF THE RESULTS
t On leave to SNCI 166 X, 25 av. des Martyrs, 38042 Grenoble, France.
Experiments on GaAs under hydrostatic pressure as well as in GaA1As show clearly that it is possible to 359
360
OBSERVATION OF Cr2+(3d 4) IN GaAs
Vol. 46, No. 4
A
6
77K
>I-03 Z
12.6 k bar
iii i-Z
3 12.15
11.5 k bar
11 k bar
10.35 k bar
9 k bar
4.2 k bar 2 k bnr
5000
6000
70~ ENERGY(cm -1 )
Fig. 1. Luminescence spectra of a SUMITOMO sample at different hydrostatic pressures. The bottom curve is for 2 kbar, the upper curve for 12.6 kbar. The same intensity scale is used for all pressures, observe the internal luminescence of the Cr2+ impurity as soon as the degeneracy of the excited 5E level with the conduction band of the host material is lifted. As in the case of ZnSe :Cr, the luminescence band is Stokesshifted with respect to the absorption band [11 ] due to the difference between the Jahn-Teller coupling strength of the excited and ground state of Cr2+, An estimation of the Jahn-Teller energy for the ground state can be obtained by taking the mean value of the width
of the two bands and gives a value close to 600 cm -1. This value, compared to the absorption results on the zero-phonon line [8], leads us to assume that the J a h n Teller effect in the SE state is negligible in front of that of the ST2 ground state. The assymetric character of the luminescence band (low energy shoulder) which is not observed in absorption (Gaussian band) is also characteristic of this impurity state in zinc-blende compounds. To our
Vol. 46, No. 4
OBSERVATION OF Cr2+(3d 4) IN GaAs
C r 2+ (3d z, )
C r 3+(3d3)
tt
t2
tt
e
t'TI
t
e
t'T 2
ft
t,
tit
It
e
t 5
ST 2
a)
E
b)
t '
conduction
t it
361
t
tt
t/
vatence band
Cr3+(t'r') -..- CrZ+(SE} -.-- hV + e C.B. c)
?
tt
o
CrZ+(Sr2)--,--Cr2+(SE)-~ h v ÷(e-h)pair
d)
Fig. 2. One electron schemes for Cr 3+ : 4 TI, 4 7"2 (a), Cr 2+ : 5 T2, 5~7 (b), and possible luminescence process (c), Cr3+ + electron in the conduction band as a starting point or (d), Cr 2÷ + electron-hole pair (see text for details). knowledge, this character has never been fully interpreted. Kaminska et ai. [I 2] have tentatively attributed it to the existence of a non zero Jahn-Teller effect of the excited SE state. However, they assume, to reproduce the experimental spectra, that the selection rules are not the same for absorption and for emission. This procedure is not correct. We have calculated the band shape for both processes by writing properly, following Longuet-Higgins et al. [13] and Muramatsu [14] the vibronic wave functions of the system. We assume a Jahn-TeUer coupling only with a E mode and do not include spin-orbit effects. For a very wide range of Jahn-TeUer energies, we never got a large dissymetry between absorption and emission band shapes as experimentally observed. This dissymetry is certainly due to an additional perturbation in the ground state: either a coupling to a
r2 phonon mode or to spin-orbit coupling. The former solution is unlikely because if the r2 phonon coupling were important, it would have been revealed in the fundamental state by EPR measurements [ 1 ] which is not. So, it is likely that the spin-orbit coupling is responsible for the observed dissymetry. A point difficult to explain is the distortion of the luminescence band as the pressure is increased. This distortion cannot be explained by the vibronic coupling as no change upon pressure of the electron-phonon coupling is observed in the photo-ionisation transition [7]. It can neither be explained by a change in the spin-orbit coupling as the shift in the maximum of the band (600 cm- 1) is too large to be accounted for in such a way. Another interesting question is raised by these experiments, concerning the physical process which gives rise to the luminescence. To discuss this point it is
362
OBSERVATION OF Cr2+(3d 4) IN GaAs
necessary to consider the one electron scheme (as calculated for instance by Hemstreet and Dimmock [15]). Typical scheme for the excited and ground states of Cr 3÷ and Cr 2+ are displayed on Fig. 2(a) and (b). The luminescence that we are studying corresponds to the recombination of a t2 electron of Cr2÷(SE) on a e orbital to get the ground state (ST2) see Fig. 2(b). The question arises thus to know how this SE state is obtained. The direct mechanism which involves an internal absorption process within the 3d 4 multiplet is unlikely since for this range of excitation energies it is spin forbidden. An alternative mechanism would involve the trapping of a free electron by a Cr 3÷ in the 4T 1 state going with an energy transfer process promoting an electron from the e to t2 orbitals [see Fig. 2(c)]. Besides the fact that this mechanism would not necessary preserve the spin and could give rise to non-radiative transitions, it is directly proportional to the number of Cr 3+ centers, which in our n-type samples is certainly very low. We are then left with a luminescence due to the creation of electron-hole pairs, which interacting with the Cr 2+ centers lead to the quasi-simultaneous processes [see Fig. 2(d)]. (1) Cr2+(ST2) + hole in the valence band ~ Cr3+(4T2: hole on an e orbital), (2) Cra+(4T2) + electron in the conduction band -> Cr2+(sE: electron on a t 2 orbital). a
The final step is then the internal recombination of Cr 2+.
which leads to this luminescence is believed to be mainly due to the capture of an electron-hole pair on a Cr 2÷ center.
Acknowledgements - We wish to thank Dr B. Lambert for many helpful discussions during this work. It has been supported in part by the DRET under contract No. 80-1372. REFERENCES
I. 2. 3. 4. 5. 6.
7. 8. 9. 10. 11.
4. CONCLUSION We have shown experimentally that the internal luminescence of C r 2+ in GaAs is inhibited by the degereracy of the excited level of this impurity charge state with the conduction band of the material. This degeneracy can be lifted either by applying an hydrostatic pressure or by alloying. The interactions giving rise to the shape of the band have been discussed. The process
Vol. 46, No. 4
12. 13. 14. 15.
J.J. Krebs & G.H. Stauss,Phys. Rev. B16,971 (1977). D. Bois & P. Pinard, Phys. Rev. B9,4171 (1974). G. Picoli, B. Deveaud & D. Galland, J. Phys. 42, 133(1981). A. Nouailhat, F. Litty, S. Loualiche, P. Leyral & G. Guillot, J. Phys. 43,815 (1982). D. Look, Solid State Commun. 24, 825 (1977). The well known 0.839 eV luminescence in GaAs is attributed now to the internal transition of a Cr2÷ donor complex of C3v symmetry, the trigonal field being sufficient to lift the degeneracy of the excited level of this complex with the conduction band [3]. G. Martinez, A.M. Hennel, W. Szuszkiewicz, M. Balkanski & B. Clerjaud,Phys. Rev. B23, 3920 (1981). B. Clerjaud, A.M. Hennel & G. Martinez, Solid State Commun. 33,983 (1980). L. Eaves, P.J. Williams & C. Uihlein, J. Phys. C" Solid State Phys. 14, L693 (1981). B. Deveaud, B. Lambert, H. L'Haridon & G. Picoli, Z Lumin. 24•25, 273 (1981). A.M. Hennel ,W. Szuszkiewicz, M. Balkanski, G. Martinez & B. Clerjaud, Phys. Rev. 23, 3933 (1981). M. Kaminska, J.M. Baranowski, S.M. Uba & J.T. Vallin, J. Phys. C12, 2197 (1979). H.G. Longuet-Higgins, U. Opik, M.H.L. Pryce & R.A. Sack, Proc. Roy. Soc. London A244, 1 (1958). S. Muramatsu,J. Phys. Soc. Japan. 50, 1645 (1981). L.A. Hemstreet & J.O. Dimmock,Phys. Rev. B20, 1527 (1979).