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Electroluminescence of Ho3+ ions in a ZnO varistor-type structure
Solid State Communications,
Vol. 89, No. 10, p. 859-863 1994 Efscvicr SC& Ltd Printed in Great Britain. All rights -cd 0038-1098/94 56.00 + .OO
Pergamon
O&38-1098(93)EO120-M
ELECTROLUMINESCENCE
OF Ho3+ IONS IN A ZnO VARISTOR-TYPE
STRUCTURE
S. Bachir, J. Kossanyi and J.C. Ronfard-Haret Laboratoire
des Materiaux Moleculaires, CNRS, 2-8, rue H. Dunant, F 94320, Thiais, France (Received 14 October 1993 by G. Bastard) The characteristic luminescence of Ho3+ ions has been observed in a ZnO varistor-type structure containing holmium oxide. This huninescence evidences the presence of hot electrons as responsible for the excitation of the trivalent rare earth ions. The relationships between the emitted light intensity, the applied potential and the current flow were studied. They show that hot electrons can be generated at a relatively low voltage which corresponds to the prebreakdown region of the varistor. The results are consistent with the model of electrical breakdown at the grain boundaries. In addition, the possibility to use zinc oxide varistors as electroluminescent devices is demonstrated.
1. INTRODUCTION ZINC OXIDE varistors are polycrystalline ceramic devices which exhibit a rapid increase of their electrical conductivity with increased applied voltage [l-3]. They are manufactured by sintering ZnO powders with small amounts of other metal oxides. Among these additives, cobalt or manganese oxide for instance, can result in substitution for zinc atoms into the crystal lattice [2-41, whereas others such as rare earth (RE) oxides which act as grain growth inhibitors, are localized in intergranular layers or in aggregates at the grain corners [4-71. In our previous studies on the electroluminescence of RE-doped polycrystalline ZnO electrodes in contact with aqueous electrolytes in an electrochemical cell [&-IO], we have characterized the luminescence of RE3+ ions, and proposed a direct electron impact excitation mechanism to explain the observed variations of the emitted light intensity vs both the applied polarization potential and the RE concentration. In this mechanism, the electrons are generated at the ZnO/electrolyte interface. They are accelerated by the intense electric field present in the semiconductor until they gain enough energy to impact excite the RE3+ ions. The similarities of the observed current/voltage characteristics of the electroluminescent cell [7, 1l] with those of varistors, as well as the results of structural studies obtained by scanning electron micrography [4-71, by energy dispersive spectroscopy [5, 71 and by X-ray analysis [4] on both
RE3+-doped ZnO varistors and RE3+-doped ZnO electrodes lead us to question on the ability of the RE3+-doped ZnO varistors to emit light. To our knowledge the present study is the first report on the luminescence of a trivalent RE3+ ion in a varistor-type structure. 2. EXPERIMENTAL Zinc oxide (Koch-Light; 99.99% pure) and holmium oxide (Ho,03) (Rhone-Poulenc) powders are mixed together in the presence of a small amount of ethanol in an agathe mortar. Pellets are made by pressing the mixture of the two oxides in a Specac press (4 ton cm-*). They are sintered for 5 h at 1150°C under atmospheric pressure in an Adamel Lhormargil furnace. After sintering the diameter of the pellets is ca. 1 cm and their thickness 1 mm. The sintered discs are then electroded by covering both surfaces with InGa alloy and mounted in a bakelite holder where the electric contacts are achieved by means of copper wires. For luminescence measurements, the device is put in place of the usual cell holder of a Perkin-Elmer MPF-44 spectrofluorimeter. The pellet is located in such a way that the image of its edge is focussed on the entrance slit of the emission monochromator. 3. RESULTS AND DISCUSSION The current-voltage characteristics recorded under d.c. mode in the 0-250V potential range, of
859
IN A VARISTOR-TYPE
ELECTROLUMINESCENCE
860 2.5
Vol. 89, No. 10
varistor power law equation Va
i=
0c
0
STRUCTURE
0
50
100 150 Voltage (V)
200
250
Fig. 1. Current/voltage characteristics of a 3 at % Ho3+-doped ZnO disc between 0 and 240 V d.c.. Disc thickness 1 mm, diameter 1 cm. The solid line is a least squares fit to relation (1). a 3 at % Ho3+-doped ZnO pellet, presented in Fig. 1, shows a weak non-ohmic behaviour. Increasing the applied voltage without current control, usually results in a permanent burnout of the system, indicating an important energy excess as expected for a breakdown process, whereas no degradation seems to occur for long operating time (ca. 100 h) at voltages lower than 200V. A fit to the empirical
450
500
550
600
(1)
’
where i is the current flowing through the device, V the applied voltage, C a constant and (Ythe nonlinear coefficient, gives cr = 1.18. The greater the value of cy, the better the device [3]; the above given value of (Y corresponds to a bad device. However, it should be emphasized that this value is obtained in the prebreakdown region of the i-V curve where the (Y values are usually low [3], and where equation (1) is not a good representation of the varistor characteristic [ 11. For good devices in the breakdown region, (Yvalues as high as 100 indicating a strong non-linear i-V characteristic can be obtained [l-3]. But in order to avoid the burnout of the device as observed for higher applied voltages, the experiments were limited to values lower than 250V which correspond to the prebreakdown region. Nevertheless, the cy value, different from unity, describes a non-ohmic system, and our goal was not to obtain a good varistor, but to observe the emission of light. Effectively, such an emission is observed, and its
/uu
tixl
Wavelength
/su
800
/ nm
Fig. 2. Emission spectrum recorded for a 3 at % HO 3+-doped ZnO disc under 18OV d.c. potential.
Vol. 89, No. 10 ELECTROLUMINESCENCE
IN A VARISTOR-TYPE
1600 1400 1200 ,000: 800 600
0
50
100 Voltage
150
200
z
:: m
250
(V)
Fig. 3. Variation of the intensity B (in arbitrary units) of the light, observed at 550 nm, emitted by a 3 at % Ho3+-doped ZnO disc vs the applied potential. Solid circles: d.c. voltage, open circles: a.c. rms voltage. The solid and broken lines are least squares fittings of the experimental points to relation (2). spectrum has been recorded for an applied voltage of 18OV d.c. (Fig. 2). This spectrum is identical to that reported [6] for Ho3+-doped polycrystalline ZnO electrodes under anodic bias, in contact with an aqueous electrolyte in an electrochemical cell. It shows at 490, 550, 660 and 770nm the patterns characteristic of the ‘F3 -t5Z8, ‘S2 -+5Z8,‘F3 +5ZT and ‘S2 d5ZT transitions of the Ho3+ ion, respectively [12]. No pattern attributable to ZnO itself [9- 11, 13-161 could be observed under these conditions. The symmetrical structure of the system makes it able to operate in both d.c. and a.c. mode, and the same spectrum is obtained under 50 Hz a.c. conditions with the same applied voltage. No variation with time of the emitted light intensity has been detected for operating times as long as 100 h. The variation of the intensity B of the emitted light vs the applied potential in the 60-200 V range is reported in Fig. 3 for both d.c. and 50 Hz a.c. driving voltages. For a given potential the system is found to be less luminous under a.c. operating mode. Figure 3 shows that the intensity of the emitted light increases by three orders of magnitude when the potential is increased from 60 to 240 V d.c. Using the intensity of the emitted light instead of that of the electrical current, and fitting the experimental values to an empirical varistor-like power law v aI c
B= 0
(2)
one finds o1d.c. = 4.8 for the d.c. mode and Q~&,~, = 2.9 for the a.c. mode. At this point the device can be considered as both a varistor and a new electroluminescent system of metal-semiconductor-metal (M-S-M) structure
STRUCTURE
861
(as compared to the usual metal-insulator-semiconductor-insulator-metal (M-I-S-I-M) structure of most ZnS electroluminescent devices [ 171). Electroluminescence in ZnO varistors [I 3, 151 or in ZnO diodes [14, 161 was already observed. The typical band-gap and subband-gap emissions of ZnO were reported, evidencing the production of holes in the valence band of the semiconductor. We did not observe, in the present study, the characteristic luminescence pattern of ZnO as we did when the anodically biased RE-doped polycrystalline ZnO electrodes were in contact with an aqueous electrolyte in an electrochemical cell [9, lo]. This is probably the consequence of the relative intensities of the luminescence arising from the Ho3+ ion as compared to that of ZnO itself. For RE-doped ZnO electrodes, the excitation of the trivalent RE3+ ions was shown to originate from a hot-electron impact process [7-lo]. In the present case, a demonstration of the direct-impact mechanism, similar to that used by Krupka for the ZnS : Tb3+ system [18] or by us for the Tm3+-doped ZnO electrode [B] is not possible; the intensity of the light emitted by the 5F3 level (5F3-‘Is and ‘F3 d5Z, transitions) is too weak for accurate measurements. We have shown that for RE3+-doped polycrystalline ZnO, in the absence of any codopant, the recombination of electron/hole pairs (e-/h’) does not give rise to RE3+ luminescence. Both light induced band to band excitation of ZnO [19] and electrochemical injection of holes in the valence band [IO] lead to the creation of e-/h+ pairs in the semiconductor, and induce the luminescence of only ZnO. Thus, the observed RE3+ luminescence is not a consequence of such e-/h+ recombinations. The applied voltage, corresponding to the prebreakdown region of the i-V curve of the varistor, leads to an average electric field lower than 2 kVcm_’ . This value is considerably lower than that required for producing hot electrons [20]. But it was shown that, in polycrystalline ZnO, the electric field is inhomogeneous [21]. ZnO grains are conducting and do not support the applied field. Depending upon both the size of the grains and the thickness of the pellet, an average electric field equal to 1 kVcm-’ could give fields as high as lo6 V cm-’ at the grain boundaries where hot electrons can be thus generated. It has been shown from structural studies [4-71 that the RE3+ ions do not substitute for ZnZf ions in the semiconductor lattice but remain localized in intergranular layers or at the grain corners. Thus, the Ho3+ luminescence evidences the presence of both hot electrons and Ho3+ ions at the grain boundaries, and this is in good agreement with an electrical
862
ELECTROLUMINESCENCE
0.1 t 0.1
i (A*)
IN A VARISTOR-TYPE
STRUCTURE
0.1 t.._.,...l,,.,...l. 0.06 0.08 0.1
10
Vol. 89, No. 10
0.12
0.14
0.16
0.015
0.02
0.025
v-l/Z
Fig. 4. Dependence of the emitted light intensity B in arbitrary units, observed at 550nm vs the electric current flowing through the pellet under d.c. mode for a 3 at % Ho3+-doped ZnO disc. breakdown process at the ZnO grain boundaries proposed for varistors [22]. The disagreement between the OLvalues obtained from current and luminescence measurements lead us to question about the relationship existing between the intensity of the emitted light and the current. Figure 4 shows the dependence of the emitted light intensity B vs the current in d.c. mode. No simple relation can be deduced. An attempt to use the power law [relation (1) or (2)] to describe the current dependence of the intensity of the emitted light gives a non-linear coefficient (exponent) equal to 4.07. This value is obviously equal to the ratio old,Jo = 4.8/1.18. The description of the behaviour of the electroluminescent systems is usually achieved in terms of their B-V characteristics. Two variations have been mainly proposed [20] the Destriau law:
B=
B,expH~,/Vl, (3)
the Alfrey-Taylor
law:
B = B, exp[-( V,,/V)‘/2].
(4) Our results (Fig. 5) fit reasonably better the Alfrey-Taylor equation than the Destriau law. This could be a consequence of the double Schottky barriers located at the grain boundaries of ZnO varistors [l-3, 21, 221. But, as quoted by Allen [20], both the observed B and the applied V are not primary quantities so that it is difficult to draw firm conclusions from B-V laws. Furthermore, both relations (3) and (4) refer to homogeneous systems, while polycrystalline ZnO electrical properties are determined by the gross ceramic structure and the localized conduction processes which occur between the grains [21].
0.1 t... 0
0.005
0.01 l/V
Fig. 5. Destriau (lower) and Alfrey-Taylor (upper) plots of the intensity of the emitted light vs the applied potential for a 3 at % Ho3+-doped ZnO disc. The straight lines are least squares fittings of the experimental points to relations (3) and (4). Except our studies on RE3+-doped ZnO electrodes [6-lo], the use of ZnO as a host matrix to observe the electroluminescence of trivalent RE3+ ions received only little attention [23-251. By comparison, RE3+-doped ZnS or ZnSe have been and still are the subject of a very large number of studies [ 171.The ZnO results are disappointing, and difficult to interpret. Most of the articles report the same broad, more or less structured spectra centered around 550nm which has been attributed wrongly [23] to transitions between donor levels of the RE and acceptor levels of ZnO. Only our recent studies report the emission spectra characteristic of transitions between the 4f levels of the trivalent RE3+ ions embedded in ZnO [6-lo]. Unfortunately the presence of the liquid/solid interface causes serious limitations to their potential use as electroluminescent displays. However, the present study shows that the liquid/ solid interface is not a required condition for realizing electroluminescent displays using ZnO as a host matrix and RE3+ ions as luminescent centres. 4. CONCLUSION The luminescence
of the trivalent
Ho3+ ions,
Vol. 89, No. 10 ELECTROLUMINESCENCE
IN A VARISTOR-TYPE
acting as probes in polycrystalline ZnO, evidences the presence of hot electrons in the conduction mechanism of RE-doped ZnO varistors. These hot electrons are produced by electrical breakdown at the ZnO grain boundaries. The present report shows also that ZnO varistors containing RE3+ ions can be considered as new electroluminescent devices, and that the study of their luminescent properties can be used for a better understanding of their electrical behaviour. REFERENCES 1.
L.M. Levinson & H.R. Philipp, Ceram. Bull. 65,
2.
639 (1986). K. Eda, IEEE Electrical Insulation Magazine 5, 28 (1989).
3. 4.
T.K. Gupta, J. Am. Ceram. Sot. 73,1817 (1991). K. Mukae, K. Tsuda & I. Nagasawa, Jpn J.
5.
P. Williams, O.L. Krivanek, G. Thomas & M. Yodogawa, J. Appl. Phys. 51, 3930 (1980). D. Kouyate, J.C. Ronfard-Haret & J. Kossanyi,
6. 7. 8.
10.
’
5512 (1985).
B.W. Thomas & D. Walsh, Electronics Lett. 9,
15.
A. Miralles, A. Cornet, A. Herms & J.R. Morante, Mater. Sci. Eng. A109, 201 (1989). S. Takata, T. Minami & H. Nanto, J. Luminesc.
16. 17.
t ;:
362 (1973).
40 & 41, 794 (1988). Electroluminescence; Proc. Sixth Int. Workshop on Electroluminescence, El Paso, Texas, U.S.A., May 11-13 1992 (Edited by V.P. Singh & J.C.
McClure), Cinco Puntos Press, El Paso, Texas (1992). D.C. Krupka, J. Appl. Phys. 43, 476 (1972). D. Kouyate, J.C. Ronfard-Haret, P. Valat, J. Kossanyi, U. Mammel & D. Oelkrug, J.
22.
Luminesc. 46, 329 (1990). J.W. Allen, J. Luminesc. 23, 127 (1981). L.M. Levinson & H.R. Philipp, J. Appl. Phys. 46, 1332 (1975). G. Blatter & F. Greuter, Phys. Rev. B34, 8555
23.
(1986). S. Bhushan,
&J. Kossanyi,
24.
D. Kouyate, J.C. Ronfard-Haret
& J. Kossanyi,
25.
J. Luminesc. 50, 205 (1991).
G.E. Pike, S.R. Kurtz, P.L. Gourley, H.R. Philipp & L.M. Levinson, J. Appl. Phys. 57,
14.
D. Kouyate, J.C. Ronfard-Haret
J. Electroanal Chem. 319, 145 (1991).
133, 1607 (1986).
(1969).
l3
J. Mater. Chem. 2, 727 (1992).
J.C. Ronfard-Haret, K. Azuma, S. Bachir, D. Kouyate and J. Kossanyi J. Mater. Chem. (accepted for publication). S. Bachir, K. Azuma, J.C. Ronfard-Haret, D. Kouyate & J. Kossanyi, Chem. Phys. Lett. 213,
D. Fichou & J. Kossanyi, J. Electrochem. Sot.
12* E.W. Chase, R.T. Hepplewhite, D.C. Krupka & D. Kahng, J. Appl. Phys. 40, 849
Appl. Phys. 16, 1361 (1977).
54 (1993).
9.
11.
863
STRUCTURE
B.R.
Kaza
Pramana 11, 67 (1978). S. Bhushan, A.N. Pandey Luminesc. 20, 29 (1979).
& A.N.
Pandey,
& B.R. Kaza, J.
L.N. Tripathi, B.R. Chaubey & C.P. Mishra,
Acta Phys. Pol. A59, 15 (1981).
Vol. 89, No. 10, p. 859-863 1994 Efscvicr SC& Ltd Printed in Great Britain. All rights -cd 0038-1098/94 56.00 + .OO
Pergamon
O&38-1098(93)EO120-M
ELECTROLUMINESCENCE
OF Ho3+ IONS IN A ZnO VARISTOR-TYPE
STRUCTURE
S. Bachir, J. Kossanyi and J.C. Ronfard-Haret Laboratoire
des Materiaux Moleculaires, CNRS, 2-8, rue H. Dunant, F 94320, Thiais, France (Received 14 October 1993 by G. Bastard) The characteristic luminescence of Ho3+ ions has been observed in a ZnO varistor-type structure containing holmium oxide. This huninescence evidences the presence of hot electrons as responsible for the excitation of the trivalent rare earth ions. The relationships between the emitted light intensity, the applied potential and the current flow were studied. They show that hot electrons can be generated at a relatively low voltage which corresponds to the prebreakdown region of the varistor. The results are consistent with the model of electrical breakdown at the grain boundaries. In addition, the possibility to use zinc oxide varistors as electroluminescent devices is demonstrated.
1. INTRODUCTION ZINC OXIDE varistors are polycrystalline ceramic devices which exhibit a rapid increase of their electrical conductivity with increased applied voltage [l-3]. They are manufactured by sintering ZnO powders with small amounts of other metal oxides. Among these additives, cobalt or manganese oxide for instance, can result in substitution for zinc atoms into the crystal lattice [2-41, whereas others such as rare earth (RE) oxides which act as grain growth inhibitors, are localized in intergranular layers or in aggregates at the grain corners [4-71. In our previous studies on the electroluminescence of RE-doped polycrystalline ZnO electrodes in contact with aqueous electrolytes in an electrochemical cell [&-IO], we have characterized the luminescence of RE3+ ions, and proposed a direct electron impact excitation mechanism to explain the observed variations of the emitted light intensity vs both the applied polarization potential and the RE concentration. In this mechanism, the electrons are generated at the ZnO/electrolyte interface. They are accelerated by the intense electric field present in the semiconductor until they gain enough energy to impact excite the RE3+ ions. The similarities of the observed current/voltage characteristics of the electroluminescent cell [7, 1l] with those of varistors, as well as the results of structural studies obtained by scanning electron micrography [4-71, by energy dispersive spectroscopy [5, 71 and by X-ray analysis [4] on both
RE3+-doped ZnO varistors and RE3+-doped ZnO electrodes lead us to question on the ability of the RE3+-doped ZnO varistors to emit light. To our knowledge the present study is the first report on the luminescence of a trivalent RE3+ ion in a varistor-type structure. 2. EXPERIMENTAL Zinc oxide (Koch-Light; 99.99% pure) and holmium oxide (Ho,03) (Rhone-Poulenc) powders are mixed together in the presence of a small amount of ethanol in an agathe mortar. Pellets are made by pressing the mixture of the two oxides in a Specac press (4 ton cm-*). They are sintered for 5 h at 1150°C under atmospheric pressure in an Adamel Lhormargil furnace. After sintering the diameter of the pellets is ca. 1 cm and their thickness 1 mm. The sintered discs are then electroded by covering both surfaces with InGa alloy and mounted in a bakelite holder where the electric contacts are achieved by means of copper wires. For luminescence measurements, the device is put in place of the usual cell holder of a Perkin-Elmer MPF-44 spectrofluorimeter. The pellet is located in such a way that the image of its edge is focussed on the entrance slit of the emission monochromator. 3. RESULTS AND DISCUSSION The current-voltage characteristics recorded under d.c. mode in the 0-250V potential range, of
859
IN A VARISTOR-TYPE
ELECTROLUMINESCENCE
860 2.5
Vol. 89, No. 10
varistor power law equation Va
i=
0c
0
STRUCTURE
0
50
100 150 Voltage (V)
200
250
Fig. 1. Current/voltage characteristics of a 3 at % Ho3+-doped ZnO disc between 0 and 240 V d.c.. Disc thickness 1 mm, diameter 1 cm. The solid line is a least squares fit to relation (1). a 3 at % Ho3+-doped ZnO pellet, presented in Fig. 1, shows a weak non-ohmic behaviour. Increasing the applied voltage without current control, usually results in a permanent burnout of the system, indicating an important energy excess as expected for a breakdown process, whereas no degradation seems to occur for long operating time (ca. 100 h) at voltages lower than 200V. A fit to the empirical
450
500
550
600
(1)
’
where i is the current flowing through the device, V the applied voltage, C a constant and (Ythe nonlinear coefficient, gives cr = 1.18. The greater the value of cy, the better the device [3]; the above given value of (Y corresponds to a bad device. However, it should be emphasized that this value is obtained in the prebreakdown region of the i-V curve where the (Y values are usually low [3], and where equation (1) is not a good representation of the varistor characteristic [ 11. For good devices in the breakdown region, (Yvalues as high as 100 indicating a strong non-linear i-V characteristic can be obtained [l-3]. But in order to avoid the burnout of the device as observed for higher applied voltages, the experiments were limited to values lower than 250V which correspond to the prebreakdown region. Nevertheless, the cy value, different from unity, describes a non-ohmic system, and our goal was not to obtain a good varistor, but to observe the emission of light. Effectively, such an emission is observed, and its
/uu
tixl
Wavelength
/su
800
/ nm
Fig. 2. Emission spectrum recorded for a 3 at % HO 3+-doped ZnO disc under 18OV d.c. potential.
Vol. 89, No. 10 ELECTROLUMINESCENCE
IN A VARISTOR-TYPE
1600 1400 1200 ,000: 800 600
0
50
100 Voltage
150
200
z
:: m
250
(V)
Fig. 3. Variation of the intensity B (in arbitrary units) of the light, observed at 550 nm, emitted by a 3 at % Ho3+-doped ZnO disc vs the applied potential. Solid circles: d.c. voltage, open circles: a.c. rms voltage. The solid and broken lines are least squares fittings of the experimental points to relation (2). spectrum has been recorded for an applied voltage of 18OV d.c. (Fig. 2). This spectrum is identical to that reported [6] for Ho3+-doped polycrystalline ZnO electrodes under anodic bias, in contact with an aqueous electrolyte in an electrochemical cell. It shows at 490, 550, 660 and 770nm the patterns characteristic of the ‘F3 -t5Z8, ‘S2 -+5Z8,‘F3 +5ZT and ‘S2 d5ZT transitions of the Ho3+ ion, respectively [12]. No pattern attributable to ZnO itself [9- 11, 13-161 could be observed under these conditions. The symmetrical structure of the system makes it able to operate in both d.c. and a.c. mode, and the same spectrum is obtained under 50 Hz a.c. conditions with the same applied voltage. No variation with time of the emitted light intensity has been detected for operating times as long as 100 h. The variation of the intensity B of the emitted light vs the applied potential in the 60-200 V range is reported in Fig. 3 for both d.c. and 50 Hz a.c. driving voltages. For a given potential the system is found to be less luminous under a.c. operating mode. Figure 3 shows that the intensity of the emitted light increases by three orders of magnitude when the potential is increased from 60 to 240 V d.c. Using the intensity of the emitted light instead of that of the electrical current, and fitting the experimental values to an empirical varistor-like power law v aI c
B= 0
(2)
one finds o1d.c. = 4.8 for the d.c. mode and Q~&,~, = 2.9 for the a.c. mode. At this point the device can be considered as both a varistor and a new electroluminescent system of metal-semiconductor-metal (M-S-M) structure
STRUCTURE
861
(as compared to the usual metal-insulator-semiconductor-insulator-metal (M-I-S-I-M) structure of most ZnS electroluminescent devices [ 171). Electroluminescence in ZnO varistors [I 3, 151 or in ZnO diodes [14, 161 was already observed. The typical band-gap and subband-gap emissions of ZnO were reported, evidencing the production of holes in the valence band of the semiconductor. We did not observe, in the present study, the characteristic luminescence pattern of ZnO as we did when the anodically biased RE-doped polycrystalline ZnO electrodes were in contact with an aqueous electrolyte in an electrochemical cell [9, lo]. This is probably the consequence of the relative intensities of the luminescence arising from the Ho3+ ion as compared to that of ZnO itself. For RE-doped ZnO electrodes, the excitation of the trivalent RE3+ ions was shown to originate from a hot-electron impact process [7-lo]. In the present case, a demonstration of the direct-impact mechanism, similar to that used by Krupka for the ZnS : Tb3+ system [18] or by us for the Tm3+-doped ZnO electrode [B] is not possible; the intensity of the light emitted by the 5F3 level (5F3-‘Is and ‘F3 d5Z, transitions) is too weak for accurate measurements. We have shown that for RE3+-doped polycrystalline ZnO, in the absence of any codopant, the recombination of electron/hole pairs (e-/h’) does not give rise to RE3+ luminescence. Both light induced band to band excitation of ZnO [19] and electrochemical injection of holes in the valence band [IO] lead to the creation of e-/h+ pairs in the semiconductor, and induce the luminescence of only ZnO. Thus, the observed RE3+ luminescence is not a consequence of such e-/h+ recombinations. The applied voltage, corresponding to the prebreakdown region of the i-V curve of the varistor, leads to an average electric field lower than 2 kVcm_’ . This value is considerably lower than that required for producing hot electrons [20]. But it was shown that, in polycrystalline ZnO, the electric field is inhomogeneous [21]. ZnO grains are conducting and do not support the applied field. Depending upon both the size of the grains and the thickness of the pellet, an average electric field equal to 1 kVcm-’ could give fields as high as lo6 V cm-’ at the grain boundaries where hot electrons can be thus generated. It has been shown from structural studies [4-71 that the RE3+ ions do not substitute for ZnZf ions in the semiconductor lattice but remain localized in intergranular layers or at the grain corners. Thus, the Ho3+ luminescence evidences the presence of both hot electrons and Ho3+ ions at the grain boundaries, and this is in good agreement with an electrical
862
ELECTROLUMINESCENCE
0.1 t 0.1
i (A*)
IN A VARISTOR-TYPE
STRUCTURE
0.1 t.._.,...l,,.,...l. 0.06 0.08 0.1
10
Vol. 89, No. 10
0.12
0.14
0.16
0.015
0.02
0.025
v-l/Z
Fig. 4. Dependence of the emitted light intensity B in arbitrary units, observed at 550nm vs the electric current flowing through the pellet under d.c. mode for a 3 at % Ho3+-doped ZnO disc. breakdown process at the ZnO grain boundaries proposed for varistors [22]. The disagreement between the OLvalues obtained from current and luminescence measurements lead us to question about the relationship existing between the intensity of the emitted light and the current. Figure 4 shows the dependence of the emitted light intensity B vs the current in d.c. mode. No simple relation can be deduced. An attempt to use the power law [relation (1) or (2)] to describe the current dependence of the intensity of the emitted light gives a non-linear coefficient (exponent) equal to 4.07. This value is obviously equal to the ratio old,Jo = 4.8/1.18. The description of the behaviour of the electroluminescent systems is usually achieved in terms of their B-V characteristics. Two variations have been mainly proposed [20] the Destriau law:
B=
B,expH~,/Vl, (3)
the Alfrey-Taylor
law:
B = B, exp[-( V,,/V)‘/2].
(4) Our results (Fig. 5) fit reasonably better the Alfrey-Taylor equation than the Destriau law. This could be a consequence of the double Schottky barriers located at the grain boundaries of ZnO varistors [l-3, 21, 221. But, as quoted by Allen [20], both the observed B and the applied V are not primary quantities so that it is difficult to draw firm conclusions from B-V laws. Furthermore, both relations (3) and (4) refer to homogeneous systems, while polycrystalline ZnO electrical properties are determined by the gross ceramic structure and the localized conduction processes which occur between the grains [21].
0.1 t... 0
0.005
0.01 l/V
Fig. 5. Destriau (lower) and Alfrey-Taylor (upper) plots of the intensity of the emitted light vs the applied potential for a 3 at % Ho3+-doped ZnO disc. The straight lines are least squares fittings of the experimental points to relations (3) and (4). Except our studies on RE3+-doped ZnO electrodes [6-lo], the use of ZnO as a host matrix to observe the electroluminescence of trivalent RE3+ ions received only little attention [23-251. By comparison, RE3+-doped ZnS or ZnSe have been and still are the subject of a very large number of studies [ 171.The ZnO results are disappointing, and difficult to interpret. Most of the articles report the same broad, more or less structured spectra centered around 550nm which has been attributed wrongly [23] to transitions between donor levels of the RE and acceptor levels of ZnO. Only our recent studies report the emission spectra characteristic of transitions between the 4f levels of the trivalent RE3+ ions embedded in ZnO [6-lo]. Unfortunately the presence of the liquid/solid interface causes serious limitations to their potential use as electroluminescent displays. However, the present study shows that the liquid/ solid interface is not a required condition for realizing electroluminescent displays using ZnO as a host matrix and RE3+ ions as luminescent centres. 4. CONCLUSION The luminescence
of the trivalent
Ho3+ ions,
Vol. 89, No. 10 ELECTROLUMINESCENCE
IN A VARISTOR-TYPE
acting as probes in polycrystalline ZnO, evidences the presence of hot electrons in the conduction mechanism of RE-doped ZnO varistors. These hot electrons are produced by electrical breakdown at the ZnO grain boundaries. The present report shows also that ZnO varistors containing RE3+ ions can be considered as new electroluminescent devices, and that the study of their luminescent properties can be used for a better understanding of their electrical behaviour. REFERENCES 1.
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