- Email: [email protected]
Electroluminescence in rare earth-doped Y2O3 powder layers
Journal of Luminescence 12/13 (1976) 911 915 © North-Holland Publishing Company
ELECTROLUMINESCENCE IN RARE EARTH-DOPED Y203 POWDER LAYERS S. TANAKA, Y. MARUYAMA, H. KOBAYASFII and H. SASAKURA Department of Electronics, Tottori, University, Tottori, Japan
3~powder to blue in casewith of Electroluminescence has been observed in rare earth-doped Y203 layers variousThe emission colorsofspanning from red in case2 of Eu Tm3t brightness about I afLspectrum and efficiency of about x 1O~were obtained for Y 203 Th~with dc excitation under the electric field of 3 5 X iQ~V/cm. Samples were also responsive to ac excitation. To investigate the emission mechanism, the transient behaviour of the emission has been measured.
1. Introduction 3+) doped in proper host riiaterials give sharp line spectra whkh earth ionsof(L11 are Rare characteristic transitions in the (4f)’~configuration and cover the ultraviolet to infrared region [11.Several investigations were reported on Ln3~-dopedZnS for the use of electroluminescence, and the possibility for the flat type of color displays was also proposed [2 4]. However, the high efficiencies in this electroluminescent ZnS: Ln3~have not been obtained because of low solubilities of Ln3~into ZnS [5]. Although a number of phosphor host materials for Ln3~have been examined in photoluminescence and cathodoluminescence, few studies on host materials except ZnS have been made in electroluminescence. The present work is concerned with a systematic investigation of the electroluminescence using Y 203, as a host material, which shows outstanding emission characteristics in photoluminescence and cathodoluminescence [61.
2. Experiments Samples used were prepared by the following procedures. Y203 and Ln203 powders were dissolved in a 10% hot hydrochloric acid solution. Subsequently, oxalates were precipitated using ammonium oxalate as a precipitant.The oxalates were fired at 1100°Cfor 3 hr in air. The concentration of rare 3+ earth powders ions was about 1 at.%. Electroluminescence samples were composed the an Y203: 100pm thick, sandwiched between an Al-plateofand Sn0 Ln 2-coated glass whose 911
.
912
S. Tanaka et al/Electroluminescence in RE-doped Y
203 powder layers
2. Electroluminescence measurements were made using dc-, acarea pulse was 3voltages X 4 mm of about 500 V. Photoluminescence measurenients were made by and a cw mercury lamp of 100 W or a pulse N 2 laser with a peak power of 5kW. Emission spectra were obtained, with a Hitachi-139 monochromator and were recorded using a lock-in amplifier and a boxcard integrator.
3. Results and discussions 3+, Electroluminescence and photoluminescence spectra Y203 doped withThe Eu Dy3+, Er3~,Tb3+ and Tm3+ are shown in figs. la, b, c, dfor and e, respectively.
electroluminescence samples were excited with ac voltages of 1 kHz and 450 500 V rms. The photoluminescence samples were excited with 365 nm light from the mercury ~iJDOT)~2O3E&
~
k~½4F~6H~
~203Er~’
~3LJjoTm3
400
500
600 700 WAVE LENGTH (nm)
fig. 1. Electroluminescence (solid line) snd photoluminescence 3~. Ln
800
(dashed line) spectra for
Y 203
S. Tanaka et al/Electroluminescence in RE-doped Y
203 powder layers
913
lamp. The electroluminescence and photoluminescence spectra show narrow line spectra caused by trivalent rare earth ions [11.No remarkable differences between 3+ the electroluminescence and photoluminescence gives rise to a red emission arising from the 5D spectra 7F have been found. Y203 : Eu 0 2 transition peaking at 612 nm. 3~gives rise to a yellow emission arising from the 4F 6H 3~gives rise to a green emission arising13/2 from the Dy 9/2—~ transition peaking at transition 572 nm. Y203 : Erat 552 nm and 564nm. Y peaking 3~gives rise to a 203 Tb green enlission arising from the 5D 7F 43~gives5 rise transition peaking at 544 nmfrom and yields to a blue emission arising the the 1G brightest 3H emission. Y203 Tm 4—~ 6 transition peaking at 455 rim. In this way the various emissions spanning a spectrum from red to blue have been observed by the suitable choice of rare earth ions. In fig. 2 are shown the voltage dependences of current density and brightness for 3+ in electroluminescence, where the samples were excited by dc voltage with theTbSn0 2 electrode negative-biased. The current density across 2theatsample 500 V. The increases begins exponentially appliedvoltages voltagesofand is about pA/cm emission to occurwith at applied about 300 V,10corresponding to the average electric field of about 3 X l0~V/cm, and increases exponentially. The brightness was about 1 fL. The efficiency was found to be 2 X 10 “ at higher applied voltages. Similar experiments were performed on the samples other than Y 34 : Tbfor and almost the same results were obtained. No essential differences were203 found dc and ac excitations except tor the brightness which was about 10 tL with using ac excitation. Two possible excitation mechanisms of electroluminescence for the rare earthdoped Y 203 may be expected; (1) the direct impact excitation of rare earth ions by hot electrons accelerated by a high electric field in the Y203 host [4], (2) the energy —*
—~
I
I I
2
~iO2~
i0
F:
~~II~10
200
300
400
500
APPLIED VOLTAGE V(V)
Fig. 2. Brightness and current density versus dc applied voltage for Y 203 Th~.
914
S. Tanaka et al./Electroluminescence in RE-doped Y
2 03 powder layers
(a) P L
~brad
0
band emission (450nm)
50
100
I
150
200
150
200
TIME (i.isec) (b) EL
0
50
100 TIME (psec)
3~emission and the broad band emission after pulse excitaFig. 3. tion forTransient Y behavior 3~,(a) inofphotolurninescence the Eu and (b) in electroluminescence. 2 03 Eu
transfer from Y 203 to the rare earth ions, the energy levels of Y203 being excited by some other process [7]. In order to investigate the excitation mechanism, the transient behavior of emission inThe photoluminescence for 34 has been observed. results are shownand in electroluminescence figs. 3a and b. In photoluminescence, a broad band emission peaking at about 450 nm has been observed Eu just after excitation. This broad band emission decreases with 20 3Ops. The emission arising from Eu3+ peaking at about 612 nm increases after the excitation, reaches the maximum within 20 3Ops and then increases slowly. In electroluminescence, the emission arising from Eu3~reaches the maximum within 20 3Ops and then decreases slowly, similarly to the photoluminescence results. The broad band emission was not observed. The time delay required to reach the maximum was approximately 20 3Ops for both electroluminescence and photoluminescence emissions. These experimental results suggest that first the Y 203 host is excited and subsequantly the energy is transferred from the Y203 host to the rare-earth ions. However, the excitation mechanism of the Y203 host is not clear yet.
References [11 See for example, G.l-I. Dieke, Spectra and energy levels of rare earth ion in crystals (John Wiley, New Yoik, 1968) p. 18.
S. Tanaka et al./Electroluminescence in RE-doped Y
203 powder layers
915
(2] E.W. Chase, R.T. Heppelewhite, D.C. Krupka and D. Kahng, J. Appi. Phys. 40 (1969) 2512. [31 M.S. Waite and A. Vecht, Appl. Phys. Letters 19 (1971) 471. [41 H. Kobayashi, S. Tanaka, H. Sasakura andY. Hamakawa, Japan. J. Appl. Phys. 13 (1974) 264. [5] H.H. Woodbury, Physics and Chemistry of II VI Compounds (North-Holland, Amsterdam, 1967) p. 230. [61 See for example, N.C. Chang, J. Appl. Phys. 34 (1963) 3500. [7] S.Z. Toma and D.T. Palumbo, 3. Electrochem. Soc. 117 (1970) 236.
Discussion A.T. Vink: It your idea of excitation via the host lattice is correct, one would expect a constant ratio between photoluminescence and electroluminescence efficiency. On the other hand, if direct impact excitation of the rare earth ion occurs, one would expect similarity of the efficiency with for instance rare earths in II VI compounds. Could you comment on this? H. Kobayashi: The efficiencies of electroluminescence and photoluminescence were measured to be about 10~ and 102 10~, respectively. If the excitation of rare earth ions comes from the energy transfer from the excited Y2O3 host, the excitation efficiency ofthe Y203 host may be obtained to be 101_10 2 by using the above measured value. However, this high efficiency excitation of the Y2O3 host seems to be difficult to understand. Therefore, there exist some possibilities in understanding that the excitation arises from the impact excitation of rare earth ions, as is the case of lI—VI compound. However, other observation seems to give strong evidence for energy transfer from the Y203 host to the rare earth ions. C. Paracchini: Does there exist, in your case, any relation between the current luminescence ratio and the excitation model? H. Kobayashi: The relation between the luminescence intensity B and the current I can be given by B I”, where n = 2—4. There seems to exist some relation between the currentluminescence ratio and excitation model, but we could not derive any obvious conclusion from these observation. 3~ions are known to have J.L. Sommerdijk: With other rare earth concentrations probably higher efficiencies can conbe obtained, at least for some rare earth ForReduction example, of thethe Tm siderable concentration quenching at ions. 1 at.%. Tm34 concentration in Y 3~may therefore be useful. 203: TmA. Halperin: In your curves describing the log intensity of the EL versus the voltage you get curved lines. Have you tried to describe them as function of i/.~/i7~ Such curves may give straight lines, possibly differing in slope in different voltage regions, and may provide more information. I-I. Kobayashi: Yes, we have tried. However, no clear results were obtained.
ELECTROLUMINESCENCE IN RARE EARTH-DOPED Y203 POWDER LAYERS S. TANAKA, Y. MARUYAMA, H. KOBAYASFII and H. SASAKURA Department of Electronics, Tottori, University, Tottori, Japan
3~powder to blue in casewith of Electroluminescence has been observed in rare earth-doped Y203 layers variousThe emission colorsofspanning from red in case2 of Eu Tm3t brightness about I afLspectrum and efficiency of about x 1O~were obtained for Y 203 Th~with dc excitation under the electric field of 3 5 X iQ~V/cm. Samples were also responsive to ac excitation. To investigate the emission mechanism, the transient behaviour of the emission has been measured.
1. Introduction 3+) doped in proper host riiaterials give sharp line spectra whkh earth ionsof(L11 are Rare characteristic transitions in the (4f)’~configuration and cover the ultraviolet to infrared region [11.Several investigations were reported on Ln3~-dopedZnS for the use of electroluminescence, and the possibility for the flat type of color displays was also proposed [2 4]. However, the high efficiencies in this electroluminescent ZnS: Ln3~have not been obtained because of low solubilities of Ln3~into ZnS [5]. Although a number of phosphor host materials for Ln3~have been examined in photoluminescence and cathodoluminescence, few studies on host materials except ZnS have been made in electroluminescence. The present work is concerned with a systematic investigation of the electroluminescence using Y 203, as a host material, which shows outstanding emission characteristics in photoluminescence and cathodoluminescence [61.
2. Experiments Samples used were prepared by the following procedures. Y203 and Ln203 powders were dissolved in a 10% hot hydrochloric acid solution. Subsequently, oxalates were precipitated using ammonium oxalate as a precipitant.The oxalates were fired at 1100°Cfor 3 hr in air. The concentration of rare 3+ earth powders ions was about 1 at.%. Electroluminescence samples were composed the an Y203: 100pm thick, sandwiched between an Al-plateofand Sn0 Ln 2-coated glass whose 911
.
912
S. Tanaka et al/Electroluminescence in RE-doped Y
203 powder layers
2. Electroluminescence measurements were made using dc-, acarea pulse was 3voltages X 4 mm of about 500 V. Photoluminescence measurenients were made by and a cw mercury lamp of 100 W or a pulse N 2 laser with a peak power of 5kW. Emission spectra were obtained, with a Hitachi-139 monochromator and were recorded using a lock-in amplifier and a boxcard integrator.
3. Results and discussions 3+, Electroluminescence and photoluminescence spectra Y203 doped withThe Eu Dy3+, Er3~,Tb3+ and Tm3+ are shown in figs. la, b, c, dfor and e, respectively.
electroluminescence samples were excited with ac voltages of 1 kHz and 450 500 V rms. The photoluminescence samples were excited with 365 nm light from the mercury ~iJDOT)~2O3E&
~
k~½4F~6H~
~203Er~’
~3LJjoTm3
400
500
600 700 WAVE LENGTH (nm)
fig. 1. Electroluminescence (solid line) snd photoluminescence 3~. Ln
800
(dashed line) spectra for
Y 203
S. Tanaka et al/Electroluminescence in RE-doped Y
203 powder layers
913
lamp. The electroluminescence and photoluminescence spectra show narrow line spectra caused by trivalent rare earth ions [11.No remarkable differences between 3+ the electroluminescence and photoluminescence gives rise to a red emission arising from the 5D spectra 7F have been found. Y203 : Eu 0 2 transition peaking at 612 nm. 3~gives rise to a yellow emission arising from the 4F 6H 3~gives rise to a green emission arising13/2 from the Dy 9/2—~ transition peaking at transition 572 nm. Y203 : Erat 552 nm and 564nm. Y peaking 3~gives rise to a 203 Tb green enlission arising from the 5D 7F 43~gives5 rise transition peaking at 544 nmfrom and yields to a blue emission arising the the 1G brightest 3H emission. Y203 Tm 4—~ 6 transition peaking at 455 rim. In this way the various emissions spanning a spectrum from red to blue have been observed by the suitable choice of rare earth ions. In fig. 2 are shown the voltage dependences of current density and brightness for 3+ in electroluminescence, where the samples were excited by dc voltage with theTbSn0 2 electrode negative-biased. The current density across 2theatsample 500 V. The increases begins exponentially appliedvoltages voltagesofand is about pA/cm emission to occurwith at applied about 300 V,10corresponding to the average electric field of about 3 X l0~V/cm, and increases exponentially. The brightness was about 1 fL. The efficiency was found to be 2 X 10 “ at higher applied voltages. Similar experiments were performed on the samples other than Y 34 : Tbfor and almost the same results were obtained. No essential differences were203 found dc and ac excitations except tor the brightness which was about 10 tL with using ac excitation. Two possible excitation mechanisms of electroluminescence for the rare earthdoped Y 203 may be expected; (1) the direct impact excitation of rare earth ions by hot electrons accelerated by a high electric field in the Y203 host [4], (2) the energy —*
—~
I
I I
2
~iO2~
i0
F:
~~II~10
200
300
400
500
APPLIED VOLTAGE V(V)
Fig. 2. Brightness and current density versus dc applied voltage for Y 203 Th~.
914
S. Tanaka et al./Electroluminescence in RE-doped Y
2 03 powder layers
(a) P L
~brad
0
band emission (450nm)
50
100
I
150
200
150
200
TIME (i.isec) (b) EL
0
50
100 TIME (psec)
3~emission and the broad band emission after pulse excitaFig. 3. tion forTransient Y behavior 3~,(a) inofphotolurninescence the Eu and (b) in electroluminescence. 2 03 Eu
transfer from Y 203 to the rare earth ions, the energy levels of Y203 being excited by some other process [7]. In order to investigate the excitation mechanism, the transient behavior of emission inThe photoluminescence for 34 has been observed. results are shownand in electroluminescence figs. 3a and b. In photoluminescence, a broad band emission peaking at about 450 nm has been observed Eu just after excitation. This broad band emission decreases with 20 3Ops. The emission arising from Eu3+ peaking at about 612 nm increases after the excitation, reaches the maximum within 20 3Ops and then increases slowly. In electroluminescence, the emission arising from Eu3~reaches the maximum within 20 3Ops and then decreases slowly, similarly to the photoluminescence results. The broad band emission was not observed. The time delay required to reach the maximum was approximately 20 3Ops for both electroluminescence and photoluminescence emissions. These experimental results suggest that first the Y 203 host is excited and subsequantly the energy is transferred from the Y203 host to the rare-earth ions. However, the excitation mechanism of the Y203 host is not clear yet.
References [11 See for example, G.l-I. Dieke, Spectra and energy levels of rare earth ion in crystals (John Wiley, New Yoik, 1968) p. 18.
S. Tanaka et al./Electroluminescence in RE-doped Y
203 powder layers
915
(2] E.W. Chase, R.T. Heppelewhite, D.C. Krupka and D. Kahng, J. Appi. Phys. 40 (1969) 2512. [31 M.S. Waite and A. Vecht, Appl. Phys. Letters 19 (1971) 471. [41 H. Kobayashi, S. Tanaka, H. Sasakura andY. Hamakawa, Japan. J. Appl. Phys. 13 (1974) 264. [5] H.H. Woodbury, Physics and Chemistry of II VI Compounds (North-Holland, Amsterdam, 1967) p. 230. [61 See for example, N.C. Chang, J. Appl. Phys. 34 (1963) 3500. [7] S.Z. Toma and D.T. Palumbo, 3. Electrochem. Soc. 117 (1970) 236.
Discussion A.T. Vink: It your idea of excitation via the host lattice is correct, one would expect a constant ratio between photoluminescence and electroluminescence efficiency. On the other hand, if direct impact excitation of the rare earth ion occurs, one would expect similarity of the efficiency with for instance rare earths in II VI compounds. Could you comment on this? H. Kobayashi: The efficiencies of electroluminescence and photoluminescence were measured to be about 10~ and 102 10~, respectively. If the excitation of rare earth ions comes from the energy transfer from the excited Y2O3 host, the excitation efficiency ofthe Y203 host may be obtained to be 101_10 2 by using the above measured value. However, this high efficiency excitation of the Y2O3 host seems to be difficult to understand. Therefore, there exist some possibilities in understanding that the excitation arises from the impact excitation of rare earth ions, as is the case of lI—VI compound. However, other observation seems to give strong evidence for energy transfer from the Y203 host to the rare earth ions. C. Paracchini: Does there exist, in your case, any relation between the current luminescence ratio and the excitation model? H. Kobayashi: The relation between the luminescence intensity B and the current I can be given by B I”, where n = 2—4. There seems to exist some relation between the currentluminescence ratio and excitation model, but we could not derive any obvious conclusion from these observation. 3~ions are known to have J.L. Sommerdijk: With other rare earth concentrations probably higher efficiencies can conbe obtained, at least for some rare earth ForReduction example, of thethe Tm siderable concentration quenching at ions. 1 at.%. Tm34 concentration in Y 3~may therefore be useful. 203: TmA. Halperin: In your curves describing the log intensity of the EL versus the voltage you get curved lines. Have you tried to describe them as function of i/.~/i7~ Such curves may give straight lines, possibly differing in slope in different voltage regions, and may provide more information. I-I. Kobayashi: Yes, we have tried. However, no clear results were obtained.