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Low-voltage cathodoluminescence of ZnS single crystals
Journal of Luminescence 16 (1978) 323—330 © North-Holland Publishing Company
LOW-VOLTAGE CATHODOLUMINESCENCE OF ZnS SINGLE CRYSTALS Shunri ODA, Kenji AKAGI, Hiroshi KUKIMOTO Imaging Science and Engineering Laboratory, Tokyo Institute of Technology, Nagatsuda, Midori-ku, Yokohama, 227, Japan and
Tadahisa NAKAYAMA NHK Broadcasting Science Research Laboratories, Kinuta, Setagaya-ku, Tokyo, 157, Japan (Received July 4, 1977)
Bright blue and green cathodoluminescence from tow resistivity ZnS crystals has been observed under the excitation of low-energy electron beams of several tens of volts; i.e., 40 ft at 50 V. Properties of the surface of the crystals are studied by the dependence of current and brightness on applied voltage and by the spectra of cathodoluminescence and photoluminescence.
1. Introduction Investigations of the low-voltage cathodoluminescence from phosphors are expected not only to be useful for the reduction of high voltages of existing cathode ray tubes but also to give an impact to the development of a new kind of flat panel color display in the future. Phosphor materials for these purposes are required to have the following two principal properties: first, their resistivity must be low enough for low energy electrons to land on their surfaces continuously without charging; and second, they must involve an adequate luminescent center. Recently one approach to the first requirement has been made by adding conductive ZnO powders to several kinds of insulating phosphors [1]. In the course of our studies of blue-emitting diodes [2,3] and our investigation of the reproducible preparation of low resistivity ZnS crystals [4], it has been found that both of the above-mentioned requirements can be satisfied by the Zn-extraction treatment of ZnS. Namely, resistivity as low as 10 ~lcm has been achieved reproducibly and the blue photoluminescence at room temperature has also been enhanced at the same time by the treatment. In this paper we present the first observation of the cathodoluminescence from 323
324
S. Oda et al. / Low voltage cathodoluminescence of ZnS single crystals
low-resistivity ZnS crystals excited by low energy electron beams (several tens of volts). Although powder phosphors are more practical, here we deal with the properties of single crystals in an attempt to clarify the fundamental aspects involved in the surfaces of these materials.
2. Experimental Low resistivity ZnS crystals with high photoluminescence efficiencies were prepared reproducibly by the method established in our previous work [4]. Undoped single crystals of ZnS grown from the melt were heat treated in a molten Zn Al (10 wt%) alloy at 1000 C for 4—20 hr by using a graphite container, as described in the previous paper, followed by rapid quenching. This treatment reduced the resistivities of as-grown crystals (~~~1012 ~lcm) down to 10—100 clcm depending upon the heat-treatment time and enhanced the blue photoluminescence at room temperature. The crystals were then cut and cleaved into dice with dimensions of 2 X 2 X 2 mm. After making ohmic contacts of an In—Hg alloy to two opposite surfaces, the dice were cleaved into two again. The final die (2 X2 X 1 mm), therefore, had one ascleaved surface to be exposed to electron beams and the opposite surface to be mounted on a diode header. In fig. 1 is depicted a configuration of an experimental device for the measurements of low-voltage cathodoluminescence of the sample. The structure is essentially the same as that of a simple vacuum tube with a hot-cathode and an anode. An ohmic electrode was connected to an anode lead. An oxide-coated tungsten filament of 0.1 mm in diameter was located in front of the sample surface at a distance of 5 mm. After this assembly was baked at 300 C for 3 hr in vacuo by using an ion pump and a sublimation pump, the tube was sealed off. Such a baking process was found to be essential to obtain stable and non-degradating emission. Otherwise, we were always confronted with an immediate deterioration of emission, presumably because the crystal surface was covered with oxygen gas or some kinds of voltatile impurities during operation, resulting in the formation of a highly-resistive or/and a non-emit-
filament
ZnS crystal
Vfv~senveiope
Fig. 1. Configuration of the device for the LCL measurement. A ZnS crystal is mounted on a diode header made of ceramics- A Ti getter filament is eliminated from the figure.
S. Oda et al.
/ Low voltage cathodoluminescence of ZnS single crystals
325
ting layer. Final pressure of 10—8 Torr or less was achieved by activating a titanium filament inside the tube which acts as a gas-getter. All the measurements of the cathodoluminescence were carried out under d.c. operation at room temperature. The photoluminescence measurements were also performed for the crystals mounted inside the tube with monochromated uv light from a 500 W xenon arc lamp. The luminescence spectra were measured by an S-20 photomultiplier through a lm-monochromator, and were corrected for monochromator dispersion and photomultiplier response.
3. Results and discussion 3.1. Current-voltage and brightness-voltage characteristics In fig. 2 are shown the specimen current I~,(analogous to the plate current in a vacuum tube) as a function of the applied bias V~,(the voltage between the cathode and the anode) at a fixed filament current of 60 mA, for samples(l), (2) and (3) with resistivities of about 10 &2cm and a sample (4) of about 100 f~cm.The solid
‘empty’
L
\,,/~ (I) 100
(2)
0
A0
/ 1.
.
(~)
0
I
A
I
a
a
a a
a
0
a
D
a a.
0 A
S —
Q
0
~
a 0 A
a
I 0
a
LO
0. 0
a
50 Vp
100 IV)
Fig. 2. Specimen current I~plotted as a function of applied bias V~(the crystal positive with respect to the filament).
326
S. Oda et aL
/ Low voltage cathodoluminescence of ZnS single crystals
curve labelled “empty” in the figure indicates the I~ V~,characteristics obtained for an empty tube, in which no sample was mounted on the diode header, operating at the same filament current. This was helpful for our estimating the beam impedance of our test tube, and hence for evaluating the effect of sample resistivity on the specimen current I~. The current for the 100 ~2cmsample starts to increase at about 45 V, which is much higher than the threshold voltage of 10 V for an empty tube. In other words, the resistivity of 100 ~2cmis still too high to eliminate the charging effect at the sample surface. Most of the samples with resistivities of about 10 clcm, on the other hand,have the threshold voltage of about 20 V. The sample (I), which was selected among several samples shows almost the same I~ V~characteristics as that of the empty tube, suggesting that the resistance of the sample is low enough for our purpose. The origin of the difference in the V~,characteristics between the specific sample (1) and most of all other samples with the same resistivities of 10 fkm (such as (2) and (3)) is not yet clear. However, it would be reasonable to assume that I~,for these samples is no more limited by their bulk resistance and is dependent upon the surface conditions of the crystals. —
—
—
.
0
100
~
:o0 ~,0 •0
•0
0
.
0
0
0
—
. 0
. 0
0 S
S
•~
0.)
-
0
50 Vp
00 (VI
Fig. 3. The dependence of LCL brightness L upon applied bias Vi,. Samples (1) and (3) correspond to those of fig. 2.
S. Oda et al.
/ Low voltage cathodoluminescence of ZnS single crystals
327
The brightness of the cathodoluminescence L as a function of the applied bias is shown in fig. 3 for the samples (1) and (3). Slightly above the threshold voltage for I~stable emission is observable and reaches about 100 ft of brightness at 60 V. It is noted that a brightness level of 40 ft observed at 50 V for these samples is high enough for their display applications. Another notable fact is that the L V,, characteristics for the samples (1) and (3) are almost the same, in spite of the remarkable difference in the l~, V~relation as seen in fig. 2. This implies the important role of surface conditions on the efficiency of the low-voltage cathodoluminescence. —
3.2. Cathodoluminescence and photoluminescence spectra In fig. 4 are shown the typical spectra of the low-voltage cathodoluminescence (LCL) at three levels of the specimen current. The spectral shape with the emission peak at around 465 nm and the half-width of 100 nm is almost the same as that of the photoluminescence and the electro-luminescence observed previously [2]. The nature of the LCL, therefore, is attributable to the self-activated emission due to the Al impurity. The spectra are shifted slightly towards the shorter wavelength direction with increasing current. This fact is also consistent with the nature of the donor-acceptor pair transition of the SA emission for low-resistivity ZnS suggested previously [2]. Further, we found that the luminous output is proportional to input power rather than input current. Such a tendency might be related to the change in the excitation depth from the surface, which is voltage dependent. The definite mechanism for this, however, is not clear yet. Among samples treated in several runs we happened to find a specific sample 3 LCL If.eOmA,r..\IP.I8o~A
400
500 WAVELENGTH (em)
~
600
Fig. 4. The LCL spectra ofsample (2) at three current levels of 180 IZA, 100 MA and 50 MA.
328
S. Oda et al.
/ Low
voltage cathodoluminescence of ZnS single crystals
which showed whitish LCL as shown in fig. 5. It is clearly seen that these spectra are composed of blue and green bands. One can easily assume that a trace of Cu impurity has been incorporated inadvertently into the crystal surface during the heat treatment of the crystal, resulting in the appearance of the green emission due to the donor (Al)—acceptor (Cu) pair transition. In order to evaluate the distribution of the green-emitting centers adjacent to the surface we measured the photoluminescence (PL) spectra for the sample (3) mounted in the tube under the excitation by the monochromated lights of 340 nm and 360 nm from a 500 W xenon arc lamp, which are shown in fig. 6. Also shown in the figure is the LCL spectrum, which is the same one in fig. 5, for comparison, Firstly, one can note that the green band in the spectra of PL is not so remarkable as for the LCL spectrum. Secondly, the band width of the spectrum (c) is narrower than that of the spectrum (b). These results can reasonably be explained on the basis of the penetration depth of excitation. One can estimate the penetration depth of the 70 eV electron beam to be less than 10 A [5], meanwhile as for the 340 nm light and the 360 nm light to be roughly 1000 A and 10 .tm, respectively, deduced from the absorption coefficient of ZnS. Taking into account the above estimation, one can interpret the green band of the LCL spectrum as due to the Cu impurities incorporated within several tens or hundreds of angstroms from the crystal surface. These considerations for the fact which was fortuitously observed suggest, in turn, the possibility of our obtaining the green LCL by the intentional doping of Cu impurity onto the surface of low-resistivity ZnS crystals. In fig. 7 is demonstrated the green LCL observed for the sample which were prepared by the adequate Cu-diffusion into the crystal surface.
3 LCL • 6OmA
(3)
lp• IBOpA
400
500 WAVELENGTH (em)
600
Fig. 5. The LCL spectra of sample (3) at three current levels of 180 MA, 100 MA and 50 MA.
I
S. Oda et aL Low voltage cathodoluminescence of ZnS single crystals
400
500 WAVeLENGTH
329
600 (em)
Fig. 6. The emission spectra of sample (3) excited by three different kinds of excitations; (a) LCL excited by 70 eV, (b) and (c) PL excited by 340 nm and 360 nm lights from a Xe arc lamp, respectively. A couple of arrows for each spectrum indicate the half-width.
It would be natural to say that powder phosphors are more practical for the applications of these materials. We have already found that several kinds of powder phosphors which show efficient blue, green and red emissions under the excitation of low-energy electron beams can be preparedby a similar method of Zn-extraction. In such a case, much intense attention should be paid towards the preparation and characterization of the surfaces. Detailed studies for the powder phosphors will be reported elsewhere.
LCL
400
500 WAVELENGTH
600 InmI
Fig. 7. The green emission of LCL from a heat treated ZnS:Cu, Al single crystal.
330
S. Oda et al.
/ Low voltage cathodoluminescence of ZnS single crystals
References [1] M. Hiraki, A. Kagami, 1. Hase, K. Narita and Y. Mimura, J. Luminescence 12/13 (1976) 941. [2] H. Kukimoto, S. Oda and H. Katayama, J. Luminescence 12/13 (1976) 923. [3] H. Katayama, S. Oda and H. Kukimoto, Appl. Phys. Lett. 27 (1975) 697. E~lS. Oda and H. Kukimoto, IEEE Trans. Electron Device, ED-24 (1977) 956. [5] E.G. McRae and H.D. Hagstrum, Treatise on Solid State Chemistry, ed. N.B. Hannay (Plenum Press, London, 1976) p. 57.
LOW-VOLTAGE CATHODOLUMINESCENCE OF ZnS SINGLE CRYSTALS Shunri ODA, Kenji AKAGI, Hiroshi KUKIMOTO Imaging Science and Engineering Laboratory, Tokyo Institute of Technology, Nagatsuda, Midori-ku, Yokohama, 227, Japan and
Tadahisa NAKAYAMA NHK Broadcasting Science Research Laboratories, Kinuta, Setagaya-ku, Tokyo, 157, Japan (Received July 4, 1977)
Bright blue and green cathodoluminescence from tow resistivity ZnS crystals has been observed under the excitation of low-energy electron beams of several tens of volts; i.e., 40 ft at 50 V. Properties of the surface of the crystals are studied by the dependence of current and brightness on applied voltage and by the spectra of cathodoluminescence and photoluminescence.
1. Introduction Investigations of the low-voltage cathodoluminescence from phosphors are expected not only to be useful for the reduction of high voltages of existing cathode ray tubes but also to give an impact to the development of a new kind of flat panel color display in the future. Phosphor materials for these purposes are required to have the following two principal properties: first, their resistivity must be low enough for low energy electrons to land on their surfaces continuously without charging; and second, they must involve an adequate luminescent center. Recently one approach to the first requirement has been made by adding conductive ZnO powders to several kinds of insulating phosphors [1]. In the course of our studies of blue-emitting diodes [2,3] and our investigation of the reproducible preparation of low resistivity ZnS crystals [4], it has been found that both of the above-mentioned requirements can be satisfied by the Zn-extraction treatment of ZnS. Namely, resistivity as low as 10 ~lcm has been achieved reproducibly and the blue photoluminescence at room temperature has also been enhanced at the same time by the treatment. In this paper we present the first observation of the cathodoluminescence from 323
324
S. Oda et al. / Low voltage cathodoluminescence of ZnS single crystals
low-resistivity ZnS crystals excited by low energy electron beams (several tens of volts). Although powder phosphors are more practical, here we deal with the properties of single crystals in an attempt to clarify the fundamental aspects involved in the surfaces of these materials.
2. Experimental Low resistivity ZnS crystals with high photoluminescence efficiencies were prepared reproducibly by the method established in our previous work [4]. Undoped single crystals of ZnS grown from the melt were heat treated in a molten Zn Al (10 wt%) alloy at 1000 C for 4—20 hr by using a graphite container, as described in the previous paper, followed by rapid quenching. This treatment reduced the resistivities of as-grown crystals (~~~1012 ~lcm) down to 10—100 clcm depending upon the heat-treatment time and enhanced the blue photoluminescence at room temperature. The crystals were then cut and cleaved into dice with dimensions of 2 X 2 X 2 mm. After making ohmic contacts of an In—Hg alloy to two opposite surfaces, the dice were cleaved into two again. The final die (2 X2 X 1 mm), therefore, had one ascleaved surface to be exposed to electron beams and the opposite surface to be mounted on a diode header. In fig. 1 is depicted a configuration of an experimental device for the measurements of low-voltage cathodoluminescence of the sample. The structure is essentially the same as that of a simple vacuum tube with a hot-cathode and an anode. An ohmic electrode was connected to an anode lead. An oxide-coated tungsten filament of 0.1 mm in diameter was located in front of the sample surface at a distance of 5 mm. After this assembly was baked at 300 C for 3 hr in vacuo by using an ion pump and a sublimation pump, the tube was sealed off. Such a baking process was found to be essential to obtain stable and non-degradating emission. Otherwise, we were always confronted with an immediate deterioration of emission, presumably because the crystal surface was covered with oxygen gas or some kinds of voltatile impurities during operation, resulting in the formation of a highly-resistive or/and a non-emit-
filament
ZnS crystal
Vfv~senveiope
Fig. 1. Configuration of the device for the LCL measurement. A ZnS crystal is mounted on a diode header made of ceramics- A Ti getter filament is eliminated from the figure.
S. Oda et al.
/ Low voltage cathodoluminescence of ZnS single crystals
325
ting layer. Final pressure of 10—8 Torr or less was achieved by activating a titanium filament inside the tube which acts as a gas-getter. All the measurements of the cathodoluminescence were carried out under d.c. operation at room temperature. The photoluminescence measurements were also performed for the crystals mounted inside the tube with monochromated uv light from a 500 W xenon arc lamp. The luminescence spectra were measured by an S-20 photomultiplier through a lm-monochromator, and were corrected for monochromator dispersion and photomultiplier response.
3. Results and discussion 3.1. Current-voltage and brightness-voltage characteristics In fig. 2 are shown the specimen current I~,(analogous to the plate current in a vacuum tube) as a function of the applied bias V~,(the voltage between the cathode and the anode) at a fixed filament current of 60 mA, for samples(l), (2) and (3) with resistivities of about 10 &2cm and a sample (4) of about 100 f~cm.The solid
‘empty’
L
\,,/~ (I) 100
(2)
0
A0
/ 1.
.
(~)
0
I
A
I
a
a
a a
a
0
a
D
a a.
0 A
S —
Q
0
~
a 0 A
a
I 0
a
LO
0. 0
a
50 Vp
100 IV)
Fig. 2. Specimen current I~plotted as a function of applied bias V~(the crystal positive with respect to the filament).
326
S. Oda et aL
/ Low voltage cathodoluminescence of ZnS single crystals
curve labelled “empty” in the figure indicates the I~ V~,characteristics obtained for an empty tube, in which no sample was mounted on the diode header, operating at the same filament current. This was helpful for our estimating the beam impedance of our test tube, and hence for evaluating the effect of sample resistivity on the specimen current I~. The current for the 100 ~2cmsample starts to increase at about 45 V, which is much higher than the threshold voltage of 10 V for an empty tube. In other words, the resistivity of 100 ~2cmis still too high to eliminate the charging effect at the sample surface. Most of the samples with resistivities of about 10 clcm, on the other hand,have the threshold voltage of about 20 V. The sample (I), which was selected among several samples shows almost the same I~ V~characteristics as that of the empty tube, suggesting that the resistance of the sample is low enough for our purpose. The origin of the difference in the V~,characteristics between the specific sample (1) and most of all other samples with the same resistivities of 10 fkm (such as (2) and (3)) is not yet clear. However, it would be reasonable to assume that I~,for these samples is no more limited by their bulk resistance and is dependent upon the surface conditions of the crystals. —
—
—
.
0
100
~
:o0 ~,0 •0
•0
0
.
0
0
0
—
. 0
. 0
0 S
S
•~
0.)
-
0
50 Vp
00 (VI
Fig. 3. The dependence of LCL brightness L upon applied bias Vi,. Samples (1) and (3) correspond to those of fig. 2.
S. Oda et al.
/ Low voltage cathodoluminescence of ZnS single crystals
327
The brightness of the cathodoluminescence L as a function of the applied bias is shown in fig. 3 for the samples (1) and (3). Slightly above the threshold voltage for I~stable emission is observable and reaches about 100 ft of brightness at 60 V. It is noted that a brightness level of 40 ft observed at 50 V for these samples is high enough for their display applications. Another notable fact is that the L V,, characteristics for the samples (1) and (3) are almost the same, in spite of the remarkable difference in the l~, V~relation as seen in fig. 2. This implies the important role of surface conditions on the efficiency of the low-voltage cathodoluminescence. —
3.2. Cathodoluminescence and photoluminescence spectra In fig. 4 are shown the typical spectra of the low-voltage cathodoluminescence (LCL) at three levels of the specimen current. The spectral shape with the emission peak at around 465 nm and the half-width of 100 nm is almost the same as that of the photoluminescence and the electro-luminescence observed previously [2]. The nature of the LCL, therefore, is attributable to the self-activated emission due to the Al impurity. The spectra are shifted slightly towards the shorter wavelength direction with increasing current. This fact is also consistent with the nature of the donor-acceptor pair transition of the SA emission for low-resistivity ZnS suggested previously [2]. Further, we found that the luminous output is proportional to input power rather than input current. Such a tendency might be related to the change in the excitation depth from the surface, which is voltage dependent. The definite mechanism for this, however, is not clear yet. Among samples treated in several runs we happened to find a specific sample 3 LCL If.eOmA,r..\IP.I8o~A
400
500 WAVELENGTH (em)
~
600
Fig. 4. The LCL spectra ofsample (2) at three current levels of 180 IZA, 100 MA and 50 MA.
328
S. Oda et al.
/ Low
voltage cathodoluminescence of ZnS single crystals
which showed whitish LCL as shown in fig. 5. It is clearly seen that these spectra are composed of blue and green bands. One can easily assume that a trace of Cu impurity has been incorporated inadvertently into the crystal surface during the heat treatment of the crystal, resulting in the appearance of the green emission due to the donor (Al)—acceptor (Cu) pair transition. In order to evaluate the distribution of the green-emitting centers adjacent to the surface we measured the photoluminescence (PL) spectra for the sample (3) mounted in the tube under the excitation by the monochromated lights of 340 nm and 360 nm from a 500 W xenon arc lamp, which are shown in fig. 6. Also shown in the figure is the LCL spectrum, which is the same one in fig. 5, for comparison, Firstly, one can note that the green band in the spectra of PL is not so remarkable as for the LCL spectrum. Secondly, the band width of the spectrum (c) is narrower than that of the spectrum (b). These results can reasonably be explained on the basis of the penetration depth of excitation. One can estimate the penetration depth of the 70 eV electron beam to be less than 10 A [5], meanwhile as for the 340 nm light and the 360 nm light to be roughly 1000 A and 10 .tm, respectively, deduced from the absorption coefficient of ZnS. Taking into account the above estimation, one can interpret the green band of the LCL spectrum as due to the Cu impurities incorporated within several tens or hundreds of angstroms from the crystal surface. These considerations for the fact which was fortuitously observed suggest, in turn, the possibility of our obtaining the green LCL by the intentional doping of Cu impurity onto the surface of low-resistivity ZnS crystals. In fig. 7 is demonstrated the green LCL observed for the sample which were prepared by the adequate Cu-diffusion into the crystal surface.
3 LCL • 6OmA
(3)
lp• IBOpA
400
500 WAVELENGTH (em)
600
Fig. 5. The LCL spectra of sample (3) at three current levels of 180 MA, 100 MA and 50 MA.
I
S. Oda et aL Low voltage cathodoluminescence of ZnS single crystals
400
500 WAVeLENGTH
329
600 (em)
Fig. 6. The emission spectra of sample (3) excited by three different kinds of excitations; (a) LCL excited by 70 eV, (b) and (c) PL excited by 340 nm and 360 nm lights from a Xe arc lamp, respectively. A couple of arrows for each spectrum indicate the half-width.
It would be natural to say that powder phosphors are more practical for the applications of these materials. We have already found that several kinds of powder phosphors which show efficient blue, green and red emissions under the excitation of low-energy electron beams can be preparedby a similar method of Zn-extraction. In such a case, much intense attention should be paid towards the preparation and characterization of the surfaces. Detailed studies for the powder phosphors will be reported elsewhere.
LCL
400
500 WAVELENGTH
600 InmI
Fig. 7. The green emission of LCL from a heat treated ZnS:Cu, Al single crystal.
330
S. Oda et al.
/ Low voltage cathodoluminescence of ZnS single crystals
References [1] M. Hiraki, A. Kagami, 1. Hase, K. Narita and Y. Mimura, J. Luminescence 12/13 (1976) 941. [2] H. Kukimoto, S. Oda and H. Katayama, J. Luminescence 12/13 (1976) 923. [3] H. Katayama, S. Oda and H. Kukimoto, Appl. Phys. Lett. 27 (1975) 697. E~lS. Oda and H. Kukimoto, IEEE Trans. Electron Device, ED-24 (1977) 956. [5] E.G. McRae and H.D. Hagstrum, Treatise on Solid State Chemistry, ed. N.B. Hannay (Plenum Press, London, 1976) p. 57.