- Email: [email protected]
Phonon detection by shallow electronic trap states in luminescent ZnO
Solid State Communications, Vol. 27, pp. 1105—1107. © Pergamon Press Ltd. 1978. Printed in Great Britain.
0038—1098/78/0915—1105 $02.00/0
PHONON DETECTION BY SHALLOW ELECTRONIC TRAP STATES IN LUMINESCENT ZnO R. Baumgartner, W. Eisfeld, G. Pauli, K.F. Renk and N. Riehl * Institut für Angewandte Physik, Universität Regensburg, 8400 Regensburg, West Germany (Received 25 April 1978 by M. Cardona) High-frequency acoustic phonons are detected in a ZnO crystal by phononinduced luminescence radiation. Our experiments indicate that this radiation is caused by recombination processes of carriers captured at very shallow traps with an ionization energy of about 4meV. Propagation of the phonons is studied by a time of flight method. IT IS KNOWN from glow curve experiments that shallow traps can occur in ZnO phosphors which show green luminescence [1] When a crystal after irradiation with UV-radiation is heated up trapped carriers are thermally excited to the band and recombine at activator states which leads to green luminescence emission. From these experiments binding energies of the carriers in the shallow traps down to about 10 meV were found [1]. It was proposed to use this system for the detection of high.frequency acoustic phonons [1]. From experiments in which changes in the glow curve due to neutral gas atom bombardment of a crystal surface were observed, detection of phonons with energies from 10meV up to the Debye energy of about 35meV was concluded [2] In this paper we report new experiments which indicate directly that phonon detection by means of shallow traps in a luminescent ZnO crystal is possible. We have studied phonon propagation by a time of flight experiment in which the phonon-induced green luminescence radiation was analysed with high time-resolution. In our experiment it was important that the phonons were injected in a crystal which was continuously optically pumped. In this way it was possible to maintam a permanent population of carriers in very shallow traps for which Continuous thermal emptying takes place. This method and the use of temperatures down to 1 .6 K allowed a detection of phonons with remark-
CB
_____________________________
.
.
ably lower energies than before, The principle of our experiment is shown in Fig. 1. Free carriers are created by optical pumping the crystal with UV-radiation near the bandgap energy (3.44 eV). At low temperatures carriers are captured by traps. We suggest that the carriers trapped during the optical pumping are holes rather than electrons because the ____________
*
Permanent address: Physik-Department, Technische Universität, 8046 Garching, West Germany.
ACTIVATOR
—
—
~
TRAP
4 VB
——
‘
PHONON
EF3
.
Fig. 1. Principle of phonon detection. n-type crystal contained free electrons without optical pumping. If a heat pulse [3] is injected in the optically pumped crystal phonons can excite holes from trap states to the valence band giving rise to green luminescence radiation due to recombination of free holes with electrons in activator centers. The experimental arrangement is shown in Fig. 2. The ZnO crystal (10 x 5 x 2 mm3) was immersed in liquid helium. A small area of the crystal surface opposite to the heater was optically pumped with the radiation of a mercury lamp. The pump light was absorbed in a thin layer near the crystal surface. A small penetration depth was obtained by using an appropriate UV-transmission filter (F 1 in Fig. 2) which transmitted only radiation of a small wavelength range. Release of the trapped carriers by infrared radiation was prevented by a metal shield around the crystal and, by transmitting the pump light through a CuSO4-solution and, by using quartz light-pipes to guide the light beams. The broad-band luminescence radiation (wavelength about 500 nm) was directly detected with a photomultiplier after passing a cut-off filter (F2) which blocked radiation with wavelengths shorter than about 450nm. The photomultiplier signal was time-analysed with a phonon counting technique. Because the signal rates were quite small
1105
1106
PHONON DETECTION IN LUMINESCENT ZnO OPTICAL PUMP
IeF2 IR-SHIELOfI
Vol. 27, No. 11
SIGNAL
~
I
[L~~i~ZnO
I I~I /
~
~
HEATER
Fig. 2. Experimental arrangement. By appropriate choice of the filter F~a small penetration depth of the pump radiation in the crystal is obtained. a high repetition rate for the heat pulses was necessary. We used therefore a special multigate analyzer [4] which can be triggered with a high repetition rate (up to 100 kl-lz) and which can measure signals at a large background rate. The result of a time of flight experiment is shown in Fig. 3. We find a time-dependent signal which is delayed with respect to the time of heat pulse injection. The signal shape indicates that the phonons are propagating diffusively, with a mean free path of about 0.5 mm. The diffusion is probably caused by elastic phonon scattering at crystal impurities. The onset of the signal occurs about
0 0
2
1.
6 TIME (~isec)
8
10
Fig. 3. Result of time of flight experiments at 2) twowere crystal temperatures Heat pulses 2OWmm injected with T. a repetition rate(power of 10kHz. The time resolution was 0.5 psec (heat pulse length t~= 0.5 jisec electronic gate width 0.5 j.isec). The inset shows the signal at T = 1 .6 K measured with higher time resolution (t~= 0.1 psec, gate width 0.1 jisec).
of t~and saturates at large values. From the linear increase at small phonon density we conclude that the traps are excited by single phonon excitation rather than by multi-phonon transition in which case a stronger that linear increase of the signal would be expected. At large phonon numbers (large t~)the peak signal S 0 700 nsec after heat pulse injection (inset of Fig. 3). This saturates due to the finite number of trap states. delay corresponds to the time of flight of transverse The nature of the trap center and its properties arc acoustic phonons through the 2mm thick crystal. The not known. We have obtained some informations by propagation direction was perpendicular to the crystal further experiments. We measured the dependence of c-axis, for which the velocity of sound is about 2.8 x l0~ the peak signal S0 on the heater temperature which can cm sec’ [5] We found no signal due to longitudinal be changed by varying the heat pulse power. We conphonons. This may be due to phonon focussing effects elude from our result that the ionization energy of the or due to selection rules for the excitation of hole traps traps is about 4meV (±1 meV) and that therefore the [6] detected phonons have a frequency near 1 THz. It is, Our method allows also for detection of phonons however, not clear whether the traps have a well dewithin the crystal. For this purpose the crystal was fined ionization energy or a distribution of different optically pumped with UV-radiation that penetrates ionization energies. the crystal. We observed the phonon-induced lumiIt was important in our experiment to illuminate nescence radiation from a volume near the heater. The the crystal continuously. After switching off the pump time dependence of this signal confirms that the phonons radiation no phonon-induced luminescence signal was generated in the heater propagate diffusively through found. This shows that the carriers remain trapped only the crystal. Immediately after phonon injection the for a short time (< 1 sec) in contrast to the traps observed signal increased to a peak value S~which was larger by glow curve experiments for which the lifetimes are than the maximum signal at the end of the crystal (Fig. very long (> 1 mm). We found by further experiments 3). We measured the dependence of S~on the number in which we changed the time between subsequent heat of injected phonons which was varied by changing the pulses that the lifetime of the carriers in the trap was in pulse length t~of the heat pulses. We found that the the order of io~sec. The carriers trapped in very shallow peak signal S0 increases linearly with t~for small values states may recombine directly at luminous centers. .
.
,
Vol. 27, No. 11
PHONON DETECTION IN LUMINESCENT ZnO
Our experiments show that the phonon-induced luminescence signals depend on the crystal temperature T. The signal at T = 1 .6K was remarkably larger than the signal at T= 4.2K as shown in Fig. 3. This is probably due to a larger population of trap stateswith carriers at the lower temperature. The crystal [7] which we used for the phonon detection was of n-type with a resistivity in the order of iO~~2-cmat helium temperature. We found for a crystal of higher resistivity, for which phonon detection was observed before [1] ,an electro-photoluminescence signal caused by the electric field of the current pulses used for heating the metal film. This effect completely predominated the stimulation of the traps by phonons. The electric field-induced signal showed no delay with respect to the current pulses and is probably due to impact
1107
ionization of the trapped carriers. In this experiment we found, independent on the position of the detector volume, a signal decay time of about 400 nsec which is the same value as found in experiments in which the traps are emptied by infrared radiation [81. In summary, we have demonstrated phonon detection in luminescent ZnO with a new method and we found a very shallow trap state. The further study of phonon propagation in this system could give more information on this trap center. It would also be interesting to extend our method to study trap centers with larger ionization energies. Acknowledgements thank W. Pretti for the ZnO crystal. Support by the We Deutsche Forschungsgemeinschaft is acknowledged. —
REFERENCES 1.
RIEHL N. & MULLER A.,Phys. Lett. 36A, 487 (1971).
2.
RIEHL N. & MULLER A., Phys. Status Solidi (a) 23,435 (1974); RIEI-JL N. & WENGERT R., Phys. Status Solidi(a) 28, 503 (1975); RIEHLN.,J. Luminesc. 12/13, 537 (1976).
3.
V. GUTFELD Ri. & NETHERCOT A.H.,Phys. Rev. Lett. 12,64(1964).
4.
EISFELD W. (to be published).
5.
BATEMAN T.B.,J. Appi. Phys. 33,3309 (1962).
6.
NARAYANAMURTI V., LOGAN R.A. & CHIN M.A.,Phys. Rev. Lett. 40,63(1978).
7.
The crystal was of good optical quality, delivered by Litton Company.
8.
KAMMERMAYER R., WITTWER V., EISENREICH N. & LUCHNER K., Solid State Commun. 19,461 (1976).
0038—1098/78/0915—1105 $02.00/0
PHONON DETECTION BY SHALLOW ELECTRONIC TRAP STATES IN LUMINESCENT ZnO R. Baumgartner, W. Eisfeld, G. Pauli, K.F. Renk and N. Riehl * Institut für Angewandte Physik, Universität Regensburg, 8400 Regensburg, West Germany (Received 25 April 1978 by M. Cardona) High-frequency acoustic phonons are detected in a ZnO crystal by phononinduced luminescence radiation. Our experiments indicate that this radiation is caused by recombination processes of carriers captured at very shallow traps with an ionization energy of about 4meV. Propagation of the phonons is studied by a time of flight method. IT IS KNOWN from glow curve experiments that shallow traps can occur in ZnO phosphors which show green luminescence [1] When a crystal after irradiation with UV-radiation is heated up trapped carriers are thermally excited to the band and recombine at activator states which leads to green luminescence emission. From these experiments binding energies of the carriers in the shallow traps down to about 10 meV were found [1]. It was proposed to use this system for the detection of high.frequency acoustic phonons [1]. From experiments in which changes in the glow curve due to neutral gas atom bombardment of a crystal surface were observed, detection of phonons with energies from 10meV up to the Debye energy of about 35meV was concluded [2] In this paper we report new experiments which indicate directly that phonon detection by means of shallow traps in a luminescent ZnO crystal is possible. We have studied phonon propagation by a time of flight experiment in which the phonon-induced green luminescence radiation was analysed with high time-resolution. In our experiment it was important that the phonons were injected in a crystal which was continuously optically pumped. In this way it was possible to maintam a permanent population of carriers in very shallow traps for which Continuous thermal emptying takes place. This method and the use of temperatures down to 1 .6 K allowed a detection of phonons with remark-
CB
_____________________________
.
.
ably lower energies than before, The principle of our experiment is shown in Fig. 1. Free carriers are created by optical pumping the crystal with UV-radiation near the bandgap energy (3.44 eV). At low temperatures carriers are captured by traps. We suggest that the carriers trapped during the optical pumping are holes rather than electrons because the ____________
*
Permanent address: Physik-Department, Technische Universität, 8046 Garching, West Germany.
ACTIVATOR
—
—
~
TRAP
4 VB
——
‘
PHONON
EF3
.
Fig. 1. Principle of phonon detection. n-type crystal contained free electrons without optical pumping. If a heat pulse [3] is injected in the optically pumped crystal phonons can excite holes from trap states to the valence band giving rise to green luminescence radiation due to recombination of free holes with electrons in activator centers. The experimental arrangement is shown in Fig. 2. The ZnO crystal (10 x 5 x 2 mm3) was immersed in liquid helium. A small area of the crystal surface opposite to the heater was optically pumped with the radiation of a mercury lamp. The pump light was absorbed in a thin layer near the crystal surface. A small penetration depth was obtained by using an appropriate UV-transmission filter (F 1 in Fig. 2) which transmitted only radiation of a small wavelength range. Release of the trapped carriers by infrared radiation was prevented by a metal shield around the crystal and, by transmitting the pump light through a CuSO4-solution and, by using quartz light-pipes to guide the light beams. The broad-band luminescence radiation (wavelength about 500 nm) was directly detected with a photomultiplier after passing a cut-off filter (F2) which blocked radiation with wavelengths shorter than about 450nm. The photomultiplier signal was time-analysed with a phonon counting technique. Because the signal rates were quite small
1105
1106
PHONON DETECTION IN LUMINESCENT ZnO OPTICAL PUMP
IeF2 IR-SHIELOfI
Vol. 27, No. 11
SIGNAL
~
I
[L~~i~ZnO
I I~I /
~
~
HEATER
Fig. 2. Experimental arrangement. By appropriate choice of the filter F~a small penetration depth of the pump radiation in the crystal is obtained. a high repetition rate for the heat pulses was necessary. We used therefore a special multigate analyzer [4] which can be triggered with a high repetition rate (up to 100 kl-lz) and which can measure signals at a large background rate. The result of a time of flight experiment is shown in Fig. 3. We find a time-dependent signal which is delayed with respect to the time of heat pulse injection. The signal shape indicates that the phonons are propagating diffusively, with a mean free path of about 0.5 mm. The diffusion is probably caused by elastic phonon scattering at crystal impurities. The onset of the signal occurs about
0 0
2
1.
6 TIME (~isec)
8
10
Fig. 3. Result of time of flight experiments at 2) twowere crystal temperatures Heat pulses 2OWmm injected with T. a repetition rate(power of 10kHz. The time resolution was 0.5 psec (heat pulse length t~= 0.5 jisec electronic gate width 0.5 j.isec). The inset shows the signal at T = 1 .6 K measured with higher time resolution (t~= 0.1 psec, gate width 0.1 jisec).
of t~and saturates at large values. From the linear increase at small phonon density we conclude that the traps are excited by single phonon excitation rather than by multi-phonon transition in which case a stronger that linear increase of the signal would be expected. At large phonon numbers (large t~)the peak signal S 0 700 nsec after heat pulse injection (inset of Fig. 3). This saturates due to the finite number of trap states. delay corresponds to the time of flight of transverse The nature of the trap center and its properties arc acoustic phonons through the 2mm thick crystal. The not known. We have obtained some informations by propagation direction was perpendicular to the crystal further experiments. We measured the dependence of c-axis, for which the velocity of sound is about 2.8 x l0~ the peak signal S0 on the heater temperature which can cm sec’ [5] We found no signal due to longitudinal be changed by varying the heat pulse power. We conphonons. This may be due to phonon focussing effects elude from our result that the ionization energy of the or due to selection rules for the excitation of hole traps traps is about 4meV (±1 meV) and that therefore the [6] detected phonons have a frequency near 1 THz. It is, Our method allows also for detection of phonons however, not clear whether the traps have a well dewithin the crystal. For this purpose the crystal was fined ionization energy or a distribution of different optically pumped with UV-radiation that penetrates ionization energies. the crystal. We observed the phonon-induced lumiIt was important in our experiment to illuminate nescence radiation from a volume near the heater. The the crystal continuously. After switching off the pump time dependence of this signal confirms that the phonons radiation no phonon-induced luminescence signal was generated in the heater propagate diffusively through found. This shows that the carriers remain trapped only the crystal. Immediately after phonon injection the for a short time (< 1 sec) in contrast to the traps observed signal increased to a peak value S~which was larger by glow curve experiments for which the lifetimes are than the maximum signal at the end of the crystal (Fig. very long (> 1 mm). We found by further experiments 3). We measured the dependence of S~on the number in which we changed the time between subsequent heat of injected phonons which was varied by changing the pulses that the lifetime of the carriers in the trap was in pulse length t~of the heat pulses. We found that the the order of io~sec. The carriers trapped in very shallow peak signal S0 increases linearly with t~for small values states may recombine directly at luminous centers. .
.
,
Vol. 27, No. 11
PHONON DETECTION IN LUMINESCENT ZnO
Our experiments show that the phonon-induced luminescence signals depend on the crystal temperature T. The signal at T = 1 .6K was remarkably larger than the signal at T= 4.2K as shown in Fig. 3. This is probably due to a larger population of trap stateswith carriers at the lower temperature. The crystal [7] which we used for the phonon detection was of n-type with a resistivity in the order of iO~~2-cmat helium temperature. We found for a crystal of higher resistivity, for which phonon detection was observed before [1] ,an electro-photoluminescence signal caused by the electric field of the current pulses used for heating the metal film. This effect completely predominated the stimulation of the traps by phonons. The electric field-induced signal showed no delay with respect to the current pulses and is probably due to impact
1107
ionization of the trapped carriers. In this experiment we found, independent on the position of the detector volume, a signal decay time of about 400 nsec which is the same value as found in experiments in which the traps are emptied by infrared radiation [81. In summary, we have demonstrated phonon detection in luminescent ZnO with a new method and we found a very shallow trap state. The further study of phonon propagation in this system could give more information on this trap center. It would also be interesting to extend our method to study trap centers with larger ionization energies. Acknowledgements thank W. Pretti for the ZnO crystal. Support by the We Deutsche Forschungsgemeinschaft is acknowledged. —
REFERENCES 1.
RIEHL N. & MULLER A.,Phys. Lett. 36A, 487 (1971).
2.
RIEHL N. & MULLER A., Phys. Status Solidi (a) 23,435 (1974); RIEI-JL N. & WENGERT R., Phys. Status Solidi(a) 28, 503 (1975); RIEHLN.,J. Luminesc. 12/13, 537 (1976).
3.
V. GUTFELD Ri. & NETHERCOT A.H.,Phys. Rev. Lett. 12,64(1964).
4.
EISFELD W. (to be published).
5.
BATEMAN T.B.,J. Appi. Phys. 33,3309 (1962).
6.
NARAYANAMURTI V., LOGAN R.A. & CHIN M.A.,Phys. Rev. Lett. 40,63(1978).
7.
The crystal was of good optical quality, delivered by Litton Company.
8.
KAMMERMAYER R., WITTWER V., EISENREICH N. & LUCHNER K., Solid State Commun. 19,461 (1976).