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Stress-induced splitting of emission lines from excitonic molecules in CdS and ZnO
Journal of Luminescence 12/13 (1976) 569 573 0 North-Holland Publishing Company
STRESS-INDUCED SPLITTING OF EMISSION LINES FROM EXCITONIC MOLECULES IN CdS AND ZnO Yusaburo SEGAWA and Susumu NAMBA The Institute of Physical and Chemical Research, Wako-shi, Saitama, Japan
Splitting of spontaneous emission lines in laser excited wurtzite-type CdS and ZnO is observed under an uniaxial stress at 1.8 K. Energy shifts and polarization characteristics of these lines are successfully accounted for by an energy scheme of the excitonic molecule. Other models are found to be inapplicable to the observed polarization characteristics of these lines.
1. Introduction In the past few years, there have been extensive studies on various aspects of high excitation density effects in solids. The nature of an excitonic molecule has been one of the main points of interest to be clarified. However, only a few works [1] have been reported so far concerning the behaviour of the excitonic molecule under external perturbations, since the emission lines under the high density excitations are usually considerably broadened. For the study of these broad emission lines, the measurements of the uniaxial stress effect are quite suitable, because the exciton levels undergo a very large energy shift or splitting effect under the uniaxial stress 121 as compared with much smaller perturbations by the magnetic or electric field. We have observed the stress-induced splitting of the spontaneous emission lines which appear under the high density excitation in wurtzite-type CdS and ZnO crystals [3]. These lines (M) were previously suggested to arise from the excitonic molecule in CdS [4] and ZnO [5].Our results give new experimental evidence for ascribing these emission lines conclusively to the radiative decay of the excitonic molecule. Other models for the origin of these emissions are found to be inapplicable to the observed polarization characteristics of the split lines under stress.
2. Experimental results The uniaxial stress measurements were made on the emission spectra of CdS and ZnO single crystals excited by a pulsed N2 laser at 1.8 K. The emission spectra excited by a Hg lamp and the reflection spectra were also recorded for comparison. 569
570
Y. Segawa, S. Namba
/ Stress-induced splitting of excitonic
molecule emission
The samples were carefully oriented into parallelepipeds (in size of about 3 X 3 X 2 mm3). The emission spectra were observed at an as-grown surface or a cleaved surface. The force exerted on the sample was generated by a hydraulic system. Although measurements were made for several orientations, the splitting of the emission lines was observed only at the particular geometry where the external uniaxial stress a was applied perpendicularly to the C axis and the k vector of the emitted light was parallel to the C axis (al C and k I C). Fig. I shows the emission and reflection spectra on the (0001) plane of the CdS crystal during the uniaxial compression in the geometry of a I C and k C. The spectra are completely unpolarized as should be for the geometry of k C in the Strain free crystals. By applying the uniaxial Stress, both the M line and the reflection spectra were found to split, as seen in the figure, into two components with E a and El a. In the reflection Spectra, the higher energy component is polarized with E a, while in the M line, the polarization of the split components is reversed with the F I a component on the higher energy side. The bound exciton lines, Ii and 12, simply shifted to the lower energy side without splitting. Splitting has also been observed in the M line of ZnO in the same geonietry, as shown in fig. 2. The splitting is qualitatively similar to that in CdS with regard to the polarization of the split components. On the other hand, the splitting of the free exciton emission (Ex) observed under the Hg light excitation is in the opposite direction, i.e., the same as the reflection peak. The bound exciton lines: 12, 14, 16, 18, 19 and 16(2) also simply shifted to the lower energy o~C k,,C
Eno
—
N 2LASER
Hg LAMP
I
V’/’ ~
E~cr’
i~-+-I
i’
+
~/,
2~0~
-
A
~
~/ \~
o~gicm~
M
2.53
2.54
A
~/\~_/~Jl2~
2.55 253 PHOTON
254
255 254
255
2.56
ENERGY (eV)
Fig. 1. The changes of the emission and the reflection spectra of CdS 2. The at 1.8 intensities K. The intensity of the spectra of the laser are normalized light on thes~ithrespect surface of the to sample the A iso about components. 0.2 MW/cm
Y.
Segawa,
S. Namba /Stress-induced splitting of excitonic molecule emission a~jC kllC
—
________ LJL ‘\L’~.. N
2LASER
Hg
LAMP
E~o
2O~/r1
“S.”
--S. 5”
>-
I—
z
/
Li.j I—
Lu 0
z
_______ o z
f
(1)
Lu
_________ M 16
3.35
z -‘5
I—
_________
__________
Ix
16 xlOO
3.36
3.36
PHOTON
2
-J U U-
0kg/cm
3.37 ENERGY
Ex
A
3.38
3.37
B
3.38
339
(eV)
Fig. 2. The changes of the emission and the reflection spectra of ZnO at 1.8 K. The intensity of the laser light is about 1 MW/cm2. The intensities of the spectra are normalized to theE II components.
3.381-
-
Re~
~-4s
-
>6)
Re
CdS
2.555
P
k/IC
-Os
-.5
1371-
ZnO
2.550
-
lj
.—.
Eiio’
o--o
Ei~
‘i”
S
2
3
w
2.545 >-
~
M
Ui
0
1
2
3
-~
0
1
PRESSURE (lOikg/cm2)
Fig. 3. Stress-dependence of the M lines, the bound exciton lines, the free exciton lines and the reflection spectra in (‘dS and ZnO. In reflection data, experimental points correspond to the reflection minima. The letters S and W represent the strong and weak intensities of the split components.
571
572
Y. Segawa S. Namba / Stress-induced splitting of excitonic molecule emission
side without splitting. Only a bound exciton line (lx) with the smallest binding energy shows the same splitting pattern as that of the free exciton, as shown in fig. 2. As a summary of these results, we plotted in fig. 3, the splittings of the M lines and the reflection spectra in CdS and ZnO against the applied pressure. 3. Discussion The behaviour of the free excitons and the bound excitons under the uniaxial stress can well be explained by taking account of the stress and spin-exchange effects in excitons [2,6]. As for the M line, however, the observed polarization characteristics cannot be explained in the one-exciton scheme, since the polarization of the split components is opposite to that of the free exciton states. The only possible explanation for this is to introduce the scheme of the excitonic molecule. By assuming that the M line is caused by the transition with the excitonic molecule ground state as the initial state and the stress-split free exciton states as the final state, the reversed polarization pattern of the M line can well be understood. This interpretation is consistent with the form of the excitonic molecule wave functions derived by Hanamura [7]. According to his calculation, the excitonic molecule emission takes place between the nondegenerate ground state F 1 of the molecule and the F5 state of the free exciton, so that the observed splitting is ascribable only to that of the free exciton level. Furthermore, the wave functions have a form of F5~F5~ ~ ~ the radiative transition which cmits theE 1°photon (F5x) only decay to the F5~(F a) exciton state. These calculations also support our interpretation. If we assume that the final state of the transitions is the crystal ground state, as in the case of the free or the bound exciton, one expects a similar splitting patterns to that of the original free exciton in contrary to the observed splitting of the M line. In conclusion, we have brought forward new experimental evidence for the presence of the excitonic molecule in CdS and ZoO from the stress-induced splitting of the spontaneous emission spectra. We have shown that the reversed splitting behaviour of the M line is well accounted for by assuming that the final state of the transition is the free exciton state.
Acknowledgement The authors are very grateful to Professor T. Koda for fruitful discussions. The authors are also indebted to Professor E. Hanamura for communication of his results prior to publication.
References Ill J.L. Merz, R.A. Faulkner and P.J. Dean, Phys. Rev. 188 (1969) 1228. 121 D.W. Langer, R.N. Euwema, K. Era and T. Koda, Phys. Rev. B2 (1970) 4005.
Y. Segawa, £ Namba / Stress-induced splitting of excitonic molecule emission
573
[31 Y. Segawa and S. Namba, Solid State Commun. 17 (1975) 489. [41 S. Shionoya, H. Saito, E. Hanamura and 0. Akimoto, Solid State Commun. 12 (1973) 223. [5] J.M. Hvam, Phys. Stat. Sol. (b) 63 (1974) 511. [6] J.P. Woerdman, Solid State Commun. 13 (1973) 949. [71 E. Hanamura, J. Phys. Soc. Japan 39 (1975) 1506.
Discussion
a
C. Benoit Ia Guilaume: Very good experiment. The only drawback is that you have to use bulk material, then giving rather broad luminescence lines. U. Rossler: Can you explain why you don’t observe a splitting of the bound exciton under uniaxial stress? Y. Segawa: Recently, Dr. Woerdman showed theoretically in Solid State Communications that the exciton trapped by a neutral center should not split under the uniaxial stress. S. Shionoya: It seems to me that the speaker has just forgotten to mention a very important thing. I remember the speaker told me that the observed manner of the stress-induced splitting can be interpreted only by assuming that an exciton is left in the final state of the luminescence process, i.e. by assuming that the line is due to an excitonic molecule. The observed manner cannot be interpreted by assuming any kinds of bound excitons for which nothing is left in the final state.
STRESS-INDUCED SPLITTING OF EMISSION LINES FROM EXCITONIC MOLECULES IN CdS AND ZnO Yusaburo SEGAWA and Susumu NAMBA The Institute of Physical and Chemical Research, Wako-shi, Saitama, Japan
Splitting of spontaneous emission lines in laser excited wurtzite-type CdS and ZnO is observed under an uniaxial stress at 1.8 K. Energy shifts and polarization characteristics of these lines are successfully accounted for by an energy scheme of the excitonic molecule. Other models are found to be inapplicable to the observed polarization characteristics of these lines.
1. Introduction In the past few years, there have been extensive studies on various aspects of high excitation density effects in solids. The nature of an excitonic molecule has been one of the main points of interest to be clarified. However, only a few works [1] have been reported so far concerning the behaviour of the excitonic molecule under external perturbations, since the emission lines under the high density excitations are usually considerably broadened. For the study of these broad emission lines, the measurements of the uniaxial stress effect are quite suitable, because the exciton levels undergo a very large energy shift or splitting effect under the uniaxial stress 121 as compared with much smaller perturbations by the magnetic or electric field. We have observed the stress-induced splitting of the spontaneous emission lines which appear under the high density excitation in wurtzite-type CdS and ZnO crystals [3]. These lines (M) were previously suggested to arise from the excitonic molecule in CdS [4] and ZnO [5].Our results give new experimental evidence for ascribing these emission lines conclusively to the radiative decay of the excitonic molecule. Other models for the origin of these emissions are found to be inapplicable to the observed polarization characteristics of the split lines under stress.
2. Experimental results The uniaxial stress measurements were made on the emission spectra of CdS and ZnO single crystals excited by a pulsed N2 laser at 1.8 K. The emission spectra excited by a Hg lamp and the reflection spectra were also recorded for comparison. 569
570
Y. Segawa, S. Namba
/ Stress-induced splitting of excitonic
molecule emission
The samples were carefully oriented into parallelepipeds (in size of about 3 X 3 X 2 mm3). The emission spectra were observed at an as-grown surface or a cleaved surface. The force exerted on the sample was generated by a hydraulic system. Although measurements were made for several orientations, the splitting of the emission lines was observed only at the particular geometry where the external uniaxial stress a was applied perpendicularly to the C axis and the k vector of the emitted light was parallel to the C axis (al C and k I C). Fig. I shows the emission and reflection spectra on the (0001) plane of the CdS crystal during the uniaxial compression in the geometry of a I C and k C. The spectra are completely unpolarized as should be for the geometry of k C in the Strain free crystals. By applying the uniaxial Stress, both the M line and the reflection spectra were found to split, as seen in the figure, into two components with E a and El a. In the reflection Spectra, the higher energy component is polarized with E a, while in the M line, the polarization of the split components is reversed with the F I a component on the higher energy side. The bound exciton lines, Ii and 12, simply shifted to the lower energy side without splitting. Splitting has also been observed in the M line of ZnO in the same geonietry, as shown in fig. 2. The splitting is qualitatively similar to that in CdS with regard to the polarization of the split components. On the other hand, the splitting of the free exciton emission (Ex) observed under the Hg light excitation is in the opposite direction, i.e., the same as the reflection peak. The bound exciton lines: 12, 14, 16, 18, 19 and 16(2) also simply shifted to the lower energy o~C k,,C
Eno
—
N 2LASER
Hg LAMP
I
V’/’ ~
E~cr’
i~-+-I
i’
+
~/,
2~0~
-
A
~
~/ \~
o~gicm~
M
2.53
2.54
A
~/\~_/~Jl2~
2.55 253 PHOTON
254
255 254
255
2.56
ENERGY (eV)
Fig. 1. The changes of the emission and the reflection spectra of CdS 2. The at 1.8 intensities K. The intensity of the spectra of the laser are normalized light on thes~ithrespect surface of the to sample the A iso about components. 0.2 MW/cm
Y.
Segawa,
S. Namba /Stress-induced splitting of excitonic molecule emission a~jC kllC
—
________ LJL ‘\L’~.. N
2LASER
Hg
LAMP
E~o
2O~/r1
“S.”
--S. 5”
>-
I—
z
/
Li.j I—
Lu 0
z
_______ o z
f
(1)
Lu
_________ M 16
3.35
z -‘5
I—
_________
__________
Ix
16 xlOO
3.36
3.36
PHOTON
2
-J U U-
0kg/cm
3.37 ENERGY
Ex
A
3.38
3.37
B
3.38
339
(eV)
Fig. 2. The changes of the emission and the reflection spectra of ZnO at 1.8 K. The intensity of the laser light is about 1 MW/cm2. The intensities of the spectra are normalized to theE II components.
3.381-
-
Re~
~-4s
-
>6)
Re
CdS
2.555
P
k/IC
-Os
-.5
1371-
ZnO
2.550
-
lj
.—.
Eiio’
o--o
Ei~
‘i”
S
2
3
w
2.545 >-
~
M
Ui
0
1
2
3
-~
0
1
PRESSURE (lOikg/cm2)
Fig. 3. Stress-dependence of the M lines, the bound exciton lines, the free exciton lines and the reflection spectra in (‘dS and ZnO. In reflection data, experimental points correspond to the reflection minima. The letters S and W represent the strong and weak intensities of the split components.
571
572
Y. Segawa S. Namba / Stress-induced splitting of excitonic molecule emission
side without splitting. Only a bound exciton line (lx) with the smallest binding energy shows the same splitting pattern as that of the free exciton, as shown in fig. 2. As a summary of these results, we plotted in fig. 3, the splittings of the M lines and the reflection spectra in CdS and ZnO against the applied pressure. 3. Discussion The behaviour of the free excitons and the bound excitons under the uniaxial stress can well be explained by taking account of the stress and spin-exchange effects in excitons [2,6]. As for the M line, however, the observed polarization characteristics cannot be explained in the one-exciton scheme, since the polarization of the split components is opposite to that of the free exciton states. The only possible explanation for this is to introduce the scheme of the excitonic molecule. By assuming that the M line is caused by the transition with the excitonic molecule ground state as the initial state and the stress-split free exciton states as the final state, the reversed polarization pattern of the M line can well be understood. This interpretation is consistent with the form of the excitonic molecule wave functions derived by Hanamura [7]. According to his calculation, the excitonic molecule emission takes place between the nondegenerate ground state F 1 of the molecule and the F5 state of the free exciton, so that the observed splitting is ascribable only to that of the free exciton level. Furthermore, the wave functions have a form of F5~F5~ ~ ~ the radiative transition which cmits theE 1°photon (F5x) only decay to the F5~(F a) exciton state. These calculations also support our interpretation. If we assume that the final state of the transitions is the crystal ground state, as in the case of the free or the bound exciton, one expects a similar splitting patterns to that of the original free exciton in contrary to the observed splitting of the M line. In conclusion, we have brought forward new experimental evidence for the presence of the excitonic molecule in CdS and ZoO from the stress-induced splitting of the spontaneous emission spectra. We have shown that the reversed splitting behaviour of the M line is well accounted for by assuming that the final state of the transition is the free exciton state.
Acknowledgement The authors are very grateful to Professor T. Koda for fruitful discussions. The authors are also indebted to Professor E. Hanamura for communication of his results prior to publication.
References Ill J.L. Merz, R.A. Faulkner and P.J. Dean, Phys. Rev. 188 (1969) 1228. 121 D.W. Langer, R.N. Euwema, K. Era and T. Koda, Phys. Rev. B2 (1970) 4005.
Y. Segawa, £ Namba / Stress-induced splitting of excitonic molecule emission
573
[31 Y. Segawa and S. Namba, Solid State Commun. 17 (1975) 489. [41 S. Shionoya, H. Saito, E. Hanamura and 0. Akimoto, Solid State Commun. 12 (1973) 223. [5] J.M. Hvam, Phys. Stat. Sol. (b) 63 (1974) 511. [6] J.P. Woerdman, Solid State Commun. 13 (1973) 949. [71 E. Hanamura, J. Phys. Soc. Japan 39 (1975) 1506.
Discussion
a
C. Benoit Ia Guilaume: Very good experiment. The only drawback is that you have to use bulk material, then giving rather broad luminescence lines. U. Rossler: Can you explain why you don’t observe a splitting of the bound exciton under uniaxial stress? Y. Segawa: Recently, Dr. Woerdman showed theoretically in Solid State Communications that the exciton trapped by a neutral center should not split under the uniaxial stress. S. Shionoya: It seems to me that the speaker has just forgotten to mention a very important thing. I remember the speaker told me that the observed manner of the stress-induced splitting can be interpreted only by assuming that an exciton is left in the final state of the luminescence process, i.e. by assuming that the line is due to an excitonic molecule. The observed manner cannot be interpreted by assuming any kinds of bound excitons for which nothing is left in the final state.