- Email: [email protected]
A new emission line in highly excited ZnO at low temperature
Solid State Communications,Vol. 15, pp. 883—887, 1974.
Pergamon Press.
Printed in Great Britain
A NEW EMISSION LINE IN HIGHLY EXCITED ZnO AT LOW TEMPERATURE C. Klingshirn Institut für Angewandte Physik der Universität, Erlangen-Nurnberg, D852 Erlangen and E. Ostertag and R. Levy Laboratoire de Spectroscopie et d’Optique du Corps Solide, (Groupe de recherche No 15 du CNRS), Université Louis Pasteur, Strasbourg (Received 30 May 1974 by E. Moliwo)
A new emission line from ZnO has been observed under strong u.v.-excitation at 4.2 K, which shifts to longer wavelengths with increasing excitation. The experimental results are described by a model involving the recombination of a bound exciton which transfers part of its energy to a free electron.
1. INTRODUCTION IN THE last few years, the investigation of the luminescence of highly excited crystals of ZnO brought up a number of emission processes in spontaneous and partially stimulated emission.115 The excitation of the crystals was produced either by single photon4’7”° or two photon absorption’2”5 or by electron beam bombardement.’35’6’8’9”1’13”4
as kinetic energy.8 (Ex—El). (5) The recombination of an exciton, which is bound to a neutral donor, with resulting excitation of the donor electron.’0 (6) The recombination of a free exciton, with simultaneous excitation of a second electron, bound to a neutral donor.6 In this work, a new emission line from ZnO has been observed at liquid helium temperature under high u.v.-excitation.
The six following emission processes have been described: (1) The recombination of a free exciton with simultaneous emission of n longitudinal optical phonons.1’2’56’9’~2’~5 (Ex-nLO: n = 0,1,2 . .). (2) The recombination of bound excitons with simultaneous emission of n longitudinal optical phonons.2’6’10’15 (Bound Ex—nLO: n = 1, 2,...).
Nitrogen
.
OMA
Scope
Lose ~ctor~od
(3) The inelastic scattering of two excitons, where one of them recombines radiatively, giving a part of its energy for the excitation or ionization of the other one.4’14 (Ex—Ex). (4) The inelastic scattering of a free exciton and a free electron, where the exciton recombines radiatively and gives a part of its energy to the remaining electron 883
075 a Spectromete
mO Son’~ein L~tEIH~ium FIG. 1. The experimental setup.
884
A NEW EMISSION LINE IN HIGHLY EXCITED ZnO
2. EXPERiMENTAL SETUP
Vol. 15, No. 5
Several crystals of different origins were investi-
The experimental setup is shown in Fig. 1. The source of excitation is a nitrogen laser (Lambda Physik), the emission of which peaks at a wavelength of 3371 .5A (3.678 eV) and has a duration of approximately 2.7 nsec. The laser beam is attenuated by means of neutral density filters and focused on the ZnO sample, which is immersed in liquid helium. The maximum power densityJimpinging on the crystal under thew conditions is about 5 MW/cm2. The luminescence of the sample is focused on the entrance slit of a 0.75 m spectrometer (Spex 1700) which displays the spectrum on the vidicon of an optical multichannel analyzer (OMA from SSRI, model 1205A/B). A spectral range from 3.385 to 3.265 eV was selected, covering the region of excitonic emission. The spectrum can be displayed on an oscilloscope and recorded with an X—Y recorder.
gated. In all of them the emission from bound excitons was found to be the dominant luninescence process in the excitation range 40 W/cm2 ~ J ~ 100 kW/cm2. In this region the intensity of the luminescence due to the recombination of bound excitons grows linearly with the excitation intensity J. At the higher excitation intensities(100 kW/cm2 ~ 5 MW/cm2) this emission tends to saturate slow’y. At the same excitation region new emission processes take place, which depend on the sample used, but can however be classified into three categories: In some samples the EX—EX line, located at 3731 A (3.323 eV) appears and shows a stimulated emission. In some others a number of different lines appear almost simultaneously between the free exciton and its LO phonon replica in a stimulated emission similar to the one already observed in CdS’6 and ZnO.10
3. EXPERIMENTAL RESULTS Finally, in several samples a broad emission line
_i Swr~e 1.4.2K~
increases. appears, bound which G=Figure isdepending equal Its located to the (hV)m~ on number the low islirn of shifted energy absorbed progressively side u.v.-phoofexcito tion tons tional lower intensities. per toexcitons. unit quantum If volume one energies 2 peak shows extrapolates and when this of time line the the at and generation different value isatom thus of proporexcita(hV)m~ rate G racy towards or 3.358 (± 0.002 GJ. eV 0of eV) ais value to the of~hv)o on energetic sample = position is (hV)m~of found. of the This 3.354 tons, which are bound a the substitutional of Lithe corresponds within the tb range of the measurement accu17 or Na.
28MW
J 1~ ~
______
I
__________I ______
3770
2NW
______
_____
3750 3730 3710 Wove~engthof Emissien
I
-
3590
The experimentally observed energy displacement LIE = (hv)0 (hP)max is plotted in Fig. 3 vs the
I
—
O5b~ 3570
A
FIG. 2. Four luminescence spectra, taken with the OMA, which show the new emission line at different excitation levels. The luminescence is measured in arbitiary units, which are equal for all spectra. The peak at 3692 A corresponds to a bound exciton.
213. With increasing as excitation can be aseen intensity Jon Fig. 2. a logarithmic As Fig. 4The theslope indicates law from of variation LIEshown J inscale. excitation intensity the line is also broadening slightly areaF ofthe emission line growswithJasF’~J”8”~9 with a slowly decreasing slope at higher excitation.
Vol. 15, No. 5 LU
A NEW EMISSION LINE IN HIGHLY EXCITED ZnO
__________________
~1 30 ~~Sompte ZP~h’~4~~358eV ci” ~ 20 oScim~eZ (hv)03,354eV-f— Cc ~ m~ ci SampLe Z’ (hv)~3358cpo’ C) I 9,”~__ 10 9~D T~4,2K a, ~
~ “2o
C—
LU
~
“~]mpleZP
rb. Units
~______
/
_________
A
—
___
SLope 2/3
oSori~teZ SampLe
-~
2
a,
885
io
__
/0/
‘vi
,0
U,
________
,o
~0,3
1 3 MWIcm2 Excitation Intensity J
10
~ 3
7°7T~
FIG. 3. The displacement LIE of the maximum of the
line for different crystals as a function of excitation intensity J. 4. DISCUSSION Hereafter a fairly simple model is presented, which can explain the experimental results: The broad emission line may be due to the recombination of a bound exciton, which interacts with a free electron. In this process, the exciton recombines radiatively and part of its energy is transferred to the remaining electron. At the high excitation intensities
0,3
1
3 ~
Excitation Intensity .1 FIG. 4. The variation of the area F of the emission line with the excitation intensity J. Therefore the energy of the emitted photon is given by hi’
=
(Eg
—
EEX
—
EB) + E~çj~ — Efn~~
If we replace Ekm by its mean value, then ~
=
E 1~1~ =
used, the electron gas is degenerate. The law of conservation of k does not apply here because localized states are involved. If we set the zero point of the energy scale at the top of the valence band, and use the following abbreviations Eg EE~ = EB E~, =
=
EF hi’
= =
width of the energy gap binding energy of the free exciton binding energy of a free exciton to a defect kinetic energy of the electron before the interaction kinetic energy of the electron after the interaction average kinetic energy of the electrons before the interaction energy of the quasi Fermi level of the electrons energy of the emitted photon
the energy conservation law is written as: (Eg
~
EB)+(Eg +Ekj~,) hv +(Eg +E(~).
10
~
~
Due to the Pauli principle, the following inequality holds: E~1~ ~ E~’. If the transition probability decreases rapidly with in. creasing (Ei~, EF) the following approximation will be valid: —
E~ Thus: (hi’)m~= (Eg EE~ E~) Er’. If the electron concentaration is designated by n then, since 2 h EF = ~(3ir2n)2’~3 =: an2’3 2m* —
—
—
~
one obtains: (hp)m~ (Eg ~ EB) an2’~3. Under the assumption that G is proportional to n, which —
—
—
~
has been confirmed at least at room temperature by photoconductivity measurements18 if finally results that
886
A NEW EMISSION LINE IN HIGHLY EXCITED ZnO lim (hP)m~ = (Eg
—EEX —EB)
=
(hi’)0
2”3 J2”3. LIE = (hi’)0 (hP)m~ G These two results are in good agreement with the experiment. Furthermore since the width of the energetic distribution of the occupied states is proportional to EF, it is understandable that the linewidth increases slightly with excitation.*
Vol. 15, No. 5
Several conditions must be fulfilled to make the occurence above described possible. There mustofbethe a sufficient numberprocess of impurities in
—
The experimentally observed variation ofF proportional toJ1’8 1~9 is also well explained by the fact, that the probability of the interaction grows with both, the density of bound exctions and the concentration of electrons. The highest excitation intensity yields an experimental value of LIE = 20 meV which in turn givesE~= 50meV and n 7•10i8 ~ A 3 sec~is demaximum generation rate G = 1030 cm duced from the maximum intensity of the impinging light (Jm~ 5 MW/cm2), the penetration depth of the light and the diffusion length of the electrons. This leads to a time constatit of 0.5 X 10~~ sec for the relaxation of the optically created free electrons and this holes into bound states. This is a reasonable value for process.’9 *
A complete evaluation of the line shape would involve the knowledge of the matrixelements associated with this process.
f At these concentrations of electrons the average spacing between electrons (~50 A) is still larger than the radius of the exciton (~12 A) so that the free u~dbound excitons are still good quasiparticles.
order to have enough bound excitons. The concentration of electron traps and recombination centers must be low enough so that the degenerate state of the electron gas can be reached at excitation levels which still do not damage the surface of the crystal. Finally no other emission process should show a stimulated behaviour before this process takes place. All these conditions explain, why this emission has not been observed in all the samples studied. other which explain the energy shiftTwo of this newmodels emission line could but not its energetic position are: the inelastic scattering of a free exciton and an electron, bound to a donor, or the radiative recombination of an excitonic molecule giving one free electron—hole pair and a photon. This second process may also be ruled out by the instability of excitonic 20 molecules in ZnO according to recent calculations. Acknowledgements We are indebted to Prof. G. Heiland and Dr. R. Helbig, who kindly gave us the ZnO crystals. Our thanks are also due to Prof. E. Mollwo and Dr. J.B. Grun for many stimulating discussions. [he work was supported by the CNRS and the Deutsche Forschungsgemeinschaft, who made the equipment available to us. —
REFERENCES 1.
NICOLL F.H.,Appl. Phys. Lett. 9, 13(1966).
2.
3.
PACKARD J.R., CAMPELL D.A. and TAIT W.C.,J. App!. Phys. 38, 5255 (1967). NICOLL F.H.,J. App!. Phys. 39,4469(1968).
4.
MAGDE D. and MAHR M.,Phys. Rev. Lett. 24, 890 (1970).
5.
IWAI S. and NAMBA S., App!. Phys. Lett. 16, 354(1970).
6. 7. 8.
HVAM J.M., Proc. 10th mt. Conf Phys. Semicond. Boston 1970. JOHNSTONW.D., Jr.,J. App!. Phys. 42, 2731 (1971). IWAI S. and NAMBA S.,Laser, p. 47 (2/1971).
9. 10. 11.
HVAMJ.M.,Phys. Rev. B4, 4459 (1971). GOTO T. and LANGERD.W.,J. App!. Phys. 42, 5066 (1971). SHEWCHUN J. et. a!., J. App!. Phys. 43, 545 (1972).
Vol. 15, No. 5
A NEW EMISSION LINE IN HIGHLY EXCITED ZnO
12.
BREDNIKOV S.L. et. a!., Opt. Spectrosc. 32, 442 (1972).
13. 14.
PACKARD J.R., TAIT W.C. and DIERSSEN G.M.,AppL Phys. Lett. 19, 338 (1971). HVAM J.M., Solid State Commun. 12, 95 (1973).
15.
KLINGSHIRN C., Solid State Commun. 13, 297 (1973).
16.
LEVY R. and GRUN J.B.,J. Lumin. 5,406(1972).
17.
HELBIG R. to be published.
18.
MOLLWO E. andPENSL G., Z. Phys. 228, 193 (1969).
19.
HElM U. and WIESNER P., Phys. Rev. Lett. 30, 1205 (1973) and private communication.
20.
HANAMURA E. private communication, to be published.
Eine neue.Emissionslinie von ZnO unter starker u.v.-Anregung bei 4.2 K wird beschrieben, die sich mit zunehmender Anregung zu langeren Wellen verschiebt. Die experimentellen Ergebnisse werden durch folgendes Model! erklärt: Em gebundenes Exciton rekombiniert und ubertragt dabei einen Teil seiner Energie auf em freies Elektron.
887
Pergamon Press.
Printed in Great Britain
A NEW EMISSION LINE IN HIGHLY EXCITED ZnO AT LOW TEMPERATURE C. Klingshirn Institut für Angewandte Physik der Universität, Erlangen-Nurnberg, D852 Erlangen and E. Ostertag and R. Levy Laboratoire de Spectroscopie et d’Optique du Corps Solide, (Groupe de recherche No 15 du CNRS), Université Louis Pasteur, Strasbourg (Received 30 May 1974 by E. Moliwo)
A new emission line from ZnO has been observed under strong u.v.-excitation at 4.2 K, which shifts to longer wavelengths with increasing excitation. The experimental results are described by a model involving the recombination of a bound exciton which transfers part of its energy to a free electron.
1. INTRODUCTION IN THE last few years, the investigation of the luminescence of highly excited crystals of ZnO brought up a number of emission processes in spontaneous and partially stimulated emission.115 The excitation of the crystals was produced either by single photon4’7”° or two photon absorption’2”5 or by electron beam bombardement.’35’6’8’9”1’13”4
as kinetic energy.8 (Ex—El). (5) The recombination of an exciton, which is bound to a neutral donor, with resulting excitation of the donor electron.’0 (6) The recombination of a free exciton, with simultaneous excitation of a second electron, bound to a neutral donor.6 In this work, a new emission line from ZnO has been observed at liquid helium temperature under high u.v.-excitation.
The six following emission processes have been described: (1) The recombination of a free exciton with simultaneous emission of n longitudinal optical phonons.1’2’56’9’~2’~5 (Ex-nLO: n = 0,1,2 . .). (2) The recombination of bound excitons with simultaneous emission of n longitudinal optical phonons.2’6’10’15 (Bound Ex—nLO: n = 1, 2,...).
Nitrogen
.
OMA
Scope
Lose ~ctor~od
(3) The inelastic scattering of two excitons, where one of them recombines radiatively, giving a part of its energy for the excitation or ionization of the other one.4’14 (Ex—Ex). (4) The inelastic scattering of a free exciton and a free electron, where the exciton recombines radiatively and gives a part of its energy to the remaining electron 883
075 a Spectromete
mO Son’~ein L~tEIH~ium FIG. 1. The experimental setup.
884
A NEW EMISSION LINE IN HIGHLY EXCITED ZnO
2. EXPERiMENTAL SETUP
Vol. 15, No. 5
Several crystals of different origins were investi-
The experimental setup is shown in Fig. 1. The source of excitation is a nitrogen laser (Lambda Physik), the emission of which peaks at a wavelength of 3371 .5A (3.678 eV) and has a duration of approximately 2.7 nsec. The laser beam is attenuated by means of neutral density filters and focused on the ZnO sample, which is immersed in liquid helium. The maximum power densityJimpinging on the crystal under thew conditions is about 5 MW/cm2. The luminescence of the sample is focused on the entrance slit of a 0.75 m spectrometer (Spex 1700) which displays the spectrum on the vidicon of an optical multichannel analyzer (OMA from SSRI, model 1205A/B). A spectral range from 3.385 to 3.265 eV was selected, covering the region of excitonic emission. The spectrum can be displayed on an oscilloscope and recorded with an X—Y recorder.
gated. In all of them the emission from bound excitons was found to be the dominant luninescence process in the excitation range 40 W/cm2 ~ J ~ 100 kW/cm2. In this region the intensity of the luminescence due to the recombination of bound excitons grows linearly with the excitation intensity J. At the higher excitation intensities(100 kW/cm2 ~ 5 MW/cm2) this emission tends to saturate slow’y. At the same excitation region new emission processes take place, which depend on the sample used, but can however be classified into three categories: In some samples the EX—EX line, located at 3731 A (3.323 eV) appears and shows a stimulated emission. In some others a number of different lines appear almost simultaneously between the free exciton and its LO phonon replica in a stimulated emission similar to the one already observed in CdS’6 and ZnO.10
3. EXPERIMENTAL RESULTS Finally, in several samples a broad emission line
_i Swr~e 1.4.2K~
increases. appears, bound which G=Figure isdepending equal Its located to the (hV)m~ on number the low islirn of shifted energy absorbed progressively side u.v.-phoofexcito tion tons tional lower intensities. per toexcitons. unit quantum If volume one energies 2 peak shows extrapolates and when this of time line the the at and generation different value isatom thus of proporexcita(hV)m~ rate G racy towards or 3.358 (± 0.002 GJ. eV 0of eV) ais value to the of~hv)o on energetic sample = position is (hV)m~of found. of the This 3.354 tons, which are bound a the substitutional of Lithe corresponds within the tb range of the measurement accu17 or Na.
28MW
J 1~ ~
______
I
__________I ______
3770
2NW
______
_____
3750 3730 3710 Wove~engthof Emissien
I
-
3590
The experimentally observed energy displacement LIE = (hv)0 (hP)max is plotted in Fig. 3 vs the
I
—
O5b~ 3570
A
FIG. 2. Four luminescence spectra, taken with the OMA, which show the new emission line at different excitation levels. The luminescence is measured in arbitiary units, which are equal for all spectra. The peak at 3692 A corresponds to a bound exciton.
213. With increasing as excitation can be aseen intensity Jon Fig. 2. a logarithmic As Fig. 4The theslope indicates law from of variation LIEshown J inscale. excitation intensity the line is also broadening slightly areaF ofthe emission line growswithJasF’~J”8”~9 with a slowly decreasing slope at higher excitation.
Vol. 15, No. 5 LU
A NEW EMISSION LINE IN HIGHLY EXCITED ZnO
__________________
~1 30 ~~Sompte ZP~h’~4~~358eV ci” ~ 20 oScim~eZ (hv)03,354eV-f— Cc ~ m~ ci SampLe Z’ (hv)~3358cpo’ C) I 9,”~__ 10 9~D T~4,2K a, ~
~ “2o
C—
LU
~
“~]mpleZP
rb. Units
~______
/
_________
A
—
___
SLope 2/3
oSori~teZ SampLe
-~
2
a,
885
io
__
/0/
‘vi
,0
U,
________
,o
~0,3
1 3 MWIcm2 Excitation Intensity J
10
~ 3
7°7T~
FIG. 3. The displacement LIE of the maximum of the
line for different crystals as a function of excitation intensity J. 4. DISCUSSION Hereafter a fairly simple model is presented, which can explain the experimental results: The broad emission line may be due to the recombination of a bound exciton, which interacts with a free electron. In this process, the exciton recombines radiatively and part of its energy is transferred to the remaining electron. At the high excitation intensities
0,3
1
3 ~
Excitation Intensity .1 FIG. 4. The variation of the area F of the emission line with the excitation intensity J. Therefore the energy of the emitted photon is given by hi’
=
(Eg
—
EEX
—
EB) + E~çj~ — Efn~~
If we replace Ekm by its mean value, then ~
=
E 1~1~ =
used, the electron gas is degenerate. The law of conservation of k does not apply here because localized states are involved. If we set the zero point of the energy scale at the top of the valence band, and use the following abbreviations Eg EE~ = EB E~, =
=
EF hi’
= =
width of the energy gap binding energy of the free exciton binding energy of a free exciton to a defect kinetic energy of the electron before the interaction kinetic energy of the electron after the interaction average kinetic energy of the electrons before the interaction energy of the quasi Fermi level of the electrons energy of the emitted photon
the energy conservation law is written as: (Eg
~
EB)+(Eg +Ekj~,) hv +(Eg +E(~).
10
~
~
Due to the Pauli principle, the following inequality holds: E~1~ ~ E~’. If the transition probability decreases rapidly with in. creasing (Ei~, EF) the following approximation will be valid: —
E~ Thus: (hi’)m~= (Eg EE~ E~) Er’. If the electron concentaration is designated by n then, since 2 h EF = ~(3ir2n)2’~3 =: an2’3 2m* —
—
—
~
one obtains: (hp)m~ (Eg ~ EB) an2’~3. Under the assumption that G is proportional to n, which —
—
—
~
has been confirmed at least at room temperature by photoconductivity measurements18 if finally results that
886
A NEW EMISSION LINE IN HIGHLY EXCITED ZnO lim (hP)m~ = (Eg
—EEX —EB)
=
(hi’)0
2”3 J2”3. LIE = (hi’)0 (hP)m~ G These two results are in good agreement with the experiment. Furthermore since the width of the energetic distribution of the occupied states is proportional to EF, it is understandable that the linewidth increases slightly with excitation.*
Vol. 15, No. 5
Several conditions must be fulfilled to make the occurence above described possible. There mustofbethe a sufficient numberprocess of impurities in
—
The experimentally observed variation ofF proportional toJ1’8 1~9 is also well explained by the fact, that the probability of the interaction grows with both, the density of bound exctions and the concentration of electrons. The highest excitation intensity yields an experimental value of LIE = 20 meV which in turn givesE~= 50meV and n 7•10i8 ~ A 3 sec~is demaximum generation rate G = 1030 cm duced from the maximum intensity of the impinging light (Jm~ 5 MW/cm2), the penetration depth of the light and the diffusion length of the electrons. This leads to a time constatit of 0.5 X 10~~ sec for the relaxation of the optically created free electrons and this holes into bound states. This is a reasonable value for process.’9 *
A complete evaluation of the line shape would involve the knowledge of the matrixelements associated with this process.
f At these concentrations of electrons the average spacing between electrons (~50 A) is still larger than the radius of the exciton (~12 A) so that the free u~dbound excitons are still good quasiparticles.
order to have enough bound excitons. The concentration of electron traps and recombination centers must be low enough so that the degenerate state of the electron gas can be reached at excitation levels which still do not damage the surface of the crystal. Finally no other emission process should show a stimulated behaviour before this process takes place. All these conditions explain, why this emission has not been observed in all the samples studied. other which explain the energy shiftTwo of this newmodels emission line could but not its energetic position are: the inelastic scattering of a free exciton and an electron, bound to a donor, or the radiative recombination of an excitonic molecule giving one free electron—hole pair and a photon. This second process may also be ruled out by the instability of excitonic 20 molecules in ZnO according to recent calculations. Acknowledgements We are indebted to Prof. G. Heiland and Dr. R. Helbig, who kindly gave us the ZnO crystals. Our thanks are also due to Prof. E. Mollwo and Dr. J.B. Grun for many stimulating discussions. [he work was supported by the CNRS and the Deutsche Forschungsgemeinschaft, who made the equipment available to us. —
REFERENCES 1.
NICOLL F.H.,Appl. Phys. Lett. 9, 13(1966).
2.
3.
PACKARD J.R., CAMPELL D.A. and TAIT W.C.,J. App!. Phys. 38, 5255 (1967). NICOLL F.H.,J. App!. Phys. 39,4469(1968).
4.
MAGDE D. and MAHR M.,Phys. Rev. Lett. 24, 890 (1970).
5.
IWAI S. and NAMBA S., App!. Phys. Lett. 16, 354(1970).
6. 7. 8.
HVAM J.M., Proc. 10th mt. Conf Phys. Semicond. Boston 1970. JOHNSTONW.D., Jr.,J. App!. Phys. 42, 2731 (1971). IWAI S. and NAMBA S.,Laser, p. 47 (2/1971).
9. 10. 11.
HVAMJ.M.,Phys. Rev. B4, 4459 (1971). GOTO T. and LANGERD.W.,J. App!. Phys. 42, 5066 (1971). SHEWCHUN J. et. a!., J. App!. Phys. 43, 545 (1972).
Vol. 15, No. 5
A NEW EMISSION LINE IN HIGHLY EXCITED ZnO
12.
BREDNIKOV S.L. et. a!., Opt. Spectrosc. 32, 442 (1972).
13. 14.
PACKARD J.R., TAIT W.C. and DIERSSEN G.M.,AppL Phys. Lett. 19, 338 (1971). HVAM J.M., Solid State Commun. 12, 95 (1973).
15.
KLINGSHIRN C., Solid State Commun. 13, 297 (1973).
16.
LEVY R. and GRUN J.B.,J. Lumin. 5,406(1972).
17.
HELBIG R. to be published.
18.
MOLLWO E. andPENSL G., Z. Phys. 228, 193 (1969).
19.
HElM U. and WIESNER P., Phys. Rev. Lett. 30, 1205 (1973) and private communication.
20.
HANAMURA E. private communication, to be published.
Eine neue.Emissionslinie von ZnO unter starker u.v.-Anregung bei 4.2 K wird beschrieben, die sich mit zunehmender Anregung zu langeren Wellen verschiebt. Die experimentellen Ergebnisse werden durch folgendes Model! erklärt: Em gebundenes Exciton rekombiniert und ubertragt dabei einen Teil seiner Energie auf em freies Elektron.
887