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Structural characterization of thin ZnS films by X-ray diffraction
Thin Solid Films, 90 (1982) 339-343 PREPARATION AND CHARACTERIZATION
339
S T R U C T U R A L C H A R A C T E R I Z A T I O N O F T H I N ZnS FILMS BY X-RAY DIFFRACTION * V. P. TANNINENAND T. O. TUOMI Helsinki University of Technology, Laboratory of Physics, SF-02150 Espoo 15 (Finland)
(ReceivedAugust 18, 1981; acceptedSeptember22, 1981)
Structural properties of electroluminescent Z n S : M n thin films grown by atomic layer epitaxy, which emit yellow light with an external efficiency as high as 2~, are investigated. The films grown at 500 °C have a strong preferred orientation: one-half of the 00.1 plane normals are aligned within 7 ° from the normal of the glass substrate. The length of coherently diffracting domains (crystallite size) in the direction of the normal obtained from the Fourier analysis of a line profile ranges from 30 to 160 nm. The average relative strain in the same direction is calculated to be 10-3. The estimated dislocation density is about 101 o cm-2.
1. INTRODUCTION The only X-ray diffraction studies reported for ZnS thin films prepared under various conditions are crystal phase and preferred orientation analyses. No diffraction line profile studies for microstructural parameters such as crystallite size, microstrain and dislocation density have yet been carried out. In previous studies ZnS films were found to be amorphous 1, randomly oriented cubic 2, preferentially oriented cubic 3' 4 and preferentially oriented hexagonalt In this work thin ZnS films grown by atomic layer epitaxy (ALE) 6 are investigated. In addition to the phase constitution and texture studies, X-ray diffraction line profile analyses are carried out. This investigation is a part of a larger project, the purpose of which is to understand the basic mechanism of electroluminescence (EL) in compound semiconductor thin films. 2. EXPERIMENTALDETAILS 2.1. Atomic layer epitaxy process and film structures
In the ALE method 6 a compound thin film is grown directly to its final form in a sequential process in which one atomic layer is formed in each reaction step. To be compatible with ALE it is essential that the compound should have a binding energy * Paper presented at the Fifth International Thin Films 21-25, 1981.
0040-6090/82/0000-0000/$02.75
Congress,
Herzlia-on-sea, Israel, September
© ElsevierSequoia/Printedin The Netherlands
340
v . P . TANNINEN, T. O. TUOMI
which is higher than those of the reacting components. The growth rate during this process is determined by the number of surface reaction steps provided that the dose of the source vapour supplied to the reaction is sufficient for full coverage. One essential difference compared with conventional growth methods is that there are never free compound molecules on the surface. The exchange reaction in the ALE technique occurs in analogy with the CVD process. The structures of films prepared by ALE for the X-ray diffraction studies are presented in Fig. 1. The thicknesses of the ZnS films varied from 10 to 500 nm and those of the substrate films from 10 to 50 nm. The growth temperature of the samples was 350 or 500 °C.
i (a)
i
i (b)
zos TazOm
I
I
ZnS AI~01 SnO,
(c)
II
I
ZnS
Ta20
I
(d)
Fig. 1. Film structures prepared by ALE on a glass substrate. The thicknesses ofthe ZnS films rangefrom 10 to 500 n m and those of the substrate films from 10 to 50 nm.
2.2. Recording of X-ray diffraction patterns The X-ray diffraction patterns of ZnS films were measured with a conventional horizontal Siemens diffractometer (R = 170 mm) using Cu Kat radiation. The effect of the Kct2 component on the broadening of an X-ray line profile was eliminated by using a Johansson-type quartz crystal in the incident beam and placing a narrow separating slit in the exact position of the K % component of the focusing circle. All diffraction peaks were step scanned with an interval of 0.01 °. For computer calculations intensity distributions were recorded with a tape writer. The specific orientation of the films was studied by measuring a onedimensional pole distribution curve of the crystallographic [001] direction. The specimen tilt angle ~bvaried from - 2 5 ° to 25 °. 3. RESULTS
3.1. Crystal structure and specific orientation Our crystal structure studies showed that ZnS films grown at 500 °C by the ALE method have a high degree of hexagonality 7. This is somewhat surprising because the cubic-to-hexagonal phase transition temperature is reported to occur in the range 960-1250 °C a-i 1 The specific orientation of the microcrystallites in the films clearly depends on the film growth temperature in the range 350-500 °C and the substrate materials, as seen in Fig. 2 and Fig. 3 respectively. For comparison, curves measured with a randomly oriented polycrystalline sample (curve a) and with a silicon crystal (curve d) are plotted in Fig. 2. It is seen from Fig. 2 that the higher the growth temperature, the larger the degree of texture orientation is. The strongest preferred orientation is found for sample b in which the substrate is a Ta2Os layer 50 nm thick on glass (Fig. 3, curve b). In all samples the addition of an SnO2 layer lowered the degree of preferred orientation.
STRUCTURAL CHARACTERIZATION OF THIN
ZnS
341
FILMS
I
110
'.
. . . . . . . . . . . . . . . .
~
i
1.0
. . . . . .
\\\\
g F0.5!
\.
......
~_ - 0.5
' ~ ' \ . \ x... "'-.~.&.~. "\
o
00
10"
,
". . . .
~ZiZi
I
o
20°
"~-
0°
10"
I
Iqbl
20*
Fig. 2. One-dimensional pole density distribution curves obtained from randomly oriented ZnS powder (curve a), from ZnS film (type (a) in Fig. 1) grown at 350 °C (curve b), from ZnS film (type (a) in Fig. 1) grown at 500 °C (curve c) and from a silicon single crystal (curve d). Fig. 3. One-dimensional (00.1) pole density distribution curves obtained from sample types (a), (b), (c) and (d) (see Fig. 1) (curves a, b, c and d respectively) grown at 500 °C.
3.2. Crystallite size and microstrain The broadening effects of small coherent domain size D and microstrain ~n on the X-ray line profile are separated in this study by a single-line Fourier method. This method was developed by Gangulee 12 for analysing highly oriented thin films from which only the first-order diffraction profile is statistically adequate for mathematical analysis. It was shown by Gangulee that if the functional form of (%2) is approximated as C/n, where C is a constant and n is a harmonic number, acceptable separation of the two components can be achieved. The results obtained using this approximation are presented in Tables I and II for three types of samples. They were calculated from the average values of 6/D (D t), where 6 is a distance whose magnitude is inversely proportional to the Fourier period and n3 is the real averaging distance normal to the hkl planes, from the average value of C (D2) and from the slope of the Fourier coefficients using the method of Mitra and Mathur 13 (D3). The agreement between the three methods is good. Figure 4 shows a crystallite size distribution curve which is obtained from the Fourier coefficients using a second TABLE I VALUES D I , D 2 AND D 3 OF THE CRYSTALLITE SIZE C A L C U L A T E D USING THREE METHODS (SEE TEXT) FOR THE THREE SAMPLES OF FIG. 1
Sample
D 1 (nm)
D 2 (nm)
D 3 (nm)
a b c
91 82 81
112 93 103
90 65 88
TABLE II VALUES OF THE MICROSTRAIN CORRESPONDING TO THE CRYSTALLITE SIZE VALUES OF TABLE I
Sample
(e(D 1))
(e(D2))
(/~(D3))
a b c
0.77 x 10- 3 1.60x 10 -3 0.90x 10 -3
1.47 x 10- 3 1.90x 10 -3 1.53 × 10 -3
0.74 x 10- 3 1.04x 10 -3 1.05 x 10 -a
342
v.P.
TANNINEN,
T. O. TUOMI
derivative m e t h o d d e v e l o p e d b y M i g n o t a n d R o n d o t 14 a n d G a n e s a n et al. is with the a i d of the m e t h o d of G a n g u l e e . The average size of the crystallites in the direction p e r p e n d i c u l a r to the s a m p l e surface is r a t h e r large, a b o u t 90 nm. -//
i
i
,
,
5 •-: t~
~-
3
~2
.//-'",,
/
1
/
0
//
\.. I
l
~0
80
I
1.0
I
160
Olnra)
Fig. 4. CrystaUite size distribution curve obtained from sample type (a) (see Fig. 1) grown at 500 °C. The ZnS layer was 400 nm thick. D is the crystallite dimension in the direction perpendicular to the sample surface. 3.3. Dislocation density T h e d i s l o c a t i o n density estimates for the Z n S films were p e r f o r m e d in this s t u d y using a t h e o r y of W i l l i a m s o n a n d S m a l l m a n 16 a n d the same a p p r o x i m a t i o n s as for thin m e t a l films 17. T h e results c a l c u l a t e d from the crystallite size a n d the average m i c r o s t r a i n for three s a m p l e types are p r e s e n t e d in T a b l e III.
TABLE III DISLOCATIONDENSITIESp OFSAMPLESa~ b ANDc CALCULATEDFROMTHECRYSTALLITESIZE(TABLE0 ANO FROMTHEMICROSTRAIN(TABLEn)
p(DO (cm -2) p(D2) (cm -2) p(D3) (cm -2) p{~(Dl)} (ere -2) p{e(D2)} (cm -2) p{t(Dz)} (cm -z)
Sample a
Sample b
Sample c
3.6 x 101° 2.4 x 101° 3.7 x 101° 1.1 x 101° 4.2 x 101° 1.1 x 101°
4.4 x 10t° 2.8 x 101° 3.9 x 10t° 4.9 x 10~° 7.3 x 10l° 1.9 x 101°
4.6 × 101° 3.5 x 101° 7.1 x 10l° 1.6 x 101° 4.5 x 101° 2.1 x 101°
4. DISCUSSION T h i n Z n S films studied in this w o r k are used in electroluminescent devices 7. T h e y c o n t a i n a p p r o x i m a t e l y 0.5 at.% M n as a n active impurity. The films emit yellow light with a l u m i n a n c e o f as m u c h as 9000 cd m - 2 a n d an external efficiency of up to 1 0 1 m W -1 in films 3 5 0 n m thick. Little a t t e n t i o n has been p a i d to the d e p e n d e n c e of the electrical a n d optical p r o p e r t i e s of Z n S on the crystal p h a s e a n d on the m i c r o s t r u c t u r a l p a r a m e t e r s . T h e crystal structure of o u r s a m p l e s g r o w n at a relatively low t e m p e r a t u r e o f 350 °C was m a i n l y cubic 7. The E L intensity o b t a i n e d from these s a m p l e s was very weak. H o w e v e r , from the samples g r o w n at 500 °C we get the m a x i m u m value o f E L as m e n t i o n e d above• T h e X - r a y analysis of these films s h o w e d t h a t they are m a i n l y h e x a g o n a l 7. T h e degree of preferred o r i e n t a t i o n of the microcrystallites in the films was so high t h a t o n e - h a l f of the 00.2 poles are aligned
STRUCTURAL CHARACTERIZATION OF THIN
ZnS FILMS
343
within 7 ° from the substrate normal. Therefore the high EL efficiency can be partly associated with the phase change and the increase in the perfection of the ZnS film. However, we consider that the useful current for EL increases with increasing crystallite size because the current that does not cause EL is expected to flow along the crystallite boundaries. The diffraction profiles obtained from the low temperature samples were not statistically good enough for mathematical analysis but they showed a marked broadening due to the small crystallite size or the presence of microstrains. Therefore it is very probable that, in the films in which EL is weak, the crystallite size is much smaller than that in the very bright films. It is obvious that the degree of specific orientation, the hexagonality and the microstructural parameters such as crystallite size and microstrain affect the EL intensity. However, it is not clear at present whether they are interdependent or whether each parameter must have a certain value as a prerequisite for a high EL brightness. It is hoped that the preparation of more perfect films, together with a systematic quantitative evaluation of their structure, will give more information about this problem. ACKNOWLEDGMENTS
The authors wish to thank M. Oikkonen for his computer calculations and Lohja Corporation for financial support and preparing the thin film structures. REFERENCES 1 J. Hall and Ferguson, J. Opt. Soc. Am., 45 (1955) 714. 2 P. Coldberg and J. Nickerson, J. Appl. Phys., 34 (1963) 1601. 3 E. Soxman and G. Steele, NTIS Rep. AD-437866, 1963 (National Technical Information Service, U.S. Department of Commerce) (Contract NONR 4165(00)). 4 E. Soxman, JANAIR 700903, Tech. Rep. EL-7, 1979 (Sigmatron Inc.). 5 Z.K. Kun, D. Leksell, P. R. Malmberg, J. Murphy and L. J. Scienkiewicz, J. Electron. Mater., 10 (1981) 287. 6 T. Suntola, J. Antson, A. Pakkala and S. Lindfors, Society for Information Display Int. Syrup., San Diego, May 1980, Dig. Tech. Pap. XI, 1980, p. 108. 7 V.P. Tanninen, T. O. Tuomi, R. O. T6rnqvist, T. S. Suntola, J. O. Antson, A. J. Pakkala and S. G. Lindfors, Proc. 8th Int. Vacuum Congr., Cannes, September 22-26, 1980, Vol. 1, Thin Films, in Vide, Couches Minces, Suppl., 201 (1980) 410. 8 H.E. Swanson and R. K. Fuyat, Natl. Bur. Stand. (U.S.), Circ., 14 (II) (1953) 539. 9 A.M. Gurvich, Introduction to the Physical Chemistry of Crystal Phosphors, Nauka, Moscow, 1966. 10 C. Kittel, Introduction to Solid State Physics, Wiley, New York, 1971. 11 E.T. Allen, J. L. Grenschaw and H. F. Merwin, Am. J. Sci., 34 (1912) 341. 12 A. Gangulee, J. Appl. Crystallogr., 7 (1974) 434. 13 G. B. Mitra and B. K. Mathur, J. Appl. Crystallogr., 8 (1975) 543 ; 9 (1976) 352. 14 J. Mignot and D. Rondot, Acta Metall., 23 (1975) 1321. 15 P. Ganesan, H. K. Kuo, A. Saavedra and R. J. De Angelis, J. Catal., 52 (1978) 310. 16 G.K. Williamson and R. E. Smallman, Philos. Mag., 1 (1956) 34. 17 S. Suchitra, R. K. Nandi and S. P. Sen Gupta, J. Phys. D, 10 (1977) L 139.
339
S T R U C T U R A L C H A R A C T E R I Z A T I O N O F T H I N ZnS FILMS BY X-RAY DIFFRACTION * V. P. TANNINENAND T. O. TUOMI Helsinki University of Technology, Laboratory of Physics, SF-02150 Espoo 15 (Finland)
(ReceivedAugust 18, 1981; acceptedSeptember22, 1981)
Structural properties of electroluminescent Z n S : M n thin films grown by atomic layer epitaxy, which emit yellow light with an external efficiency as high as 2~, are investigated. The films grown at 500 °C have a strong preferred orientation: one-half of the 00.1 plane normals are aligned within 7 ° from the normal of the glass substrate. The length of coherently diffracting domains (crystallite size) in the direction of the normal obtained from the Fourier analysis of a line profile ranges from 30 to 160 nm. The average relative strain in the same direction is calculated to be 10-3. The estimated dislocation density is about 101 o cm-2.
1. INTRODUCTION The only X-ray diffraction studies reported for ZnS thin films prepared under various conditions are crystal phase and preferred orientation analyses. No diffraction line profile studies for microstructural parameters such as crystallite size, microstrain and dislocation density have yet been carried out. In previous studies ZnS films were found to be amorphous 1, randomly oriented cubic 2, preferentially oriented cubic 3' 4 and preferentially oriented hexagonalt In this work thin ZnS films grown by atomic layer epitaxy (ALE) 6 are investigated. In addition to the phase constitution and texture studies, X-ray diffraction line profile analyses are carried out. This investigation is a part of a larger project, the purpose of which is to understand the basic mechanism of electroluminescence (EL) in compound semiconductor thin films. 2. EXPERIMENTALDETAILS 2.1. Atomic layer epitaxy process and film structures
In the ALE method 6 a compound thin film is grown directly to its final form in a sequential process in which one atomic layer is formed in each reaction step. To be compatible with ALE it is essential that the compound should have a binding energy * Paper presented at the Fifth International Thin Films 21-25, 1981.
0040-6090/82/0000-0000/$02.75
Congress,
Herzlia-on-sea, Israel, September
© ElsevierSequoia/Printedin The Netherlands
340
v . P . TANNINEN, T. O. TUOMI
which is higher than those of the reacting components. The growth rate during this process is determined by the number of surface reaction steps provided that the dose of the source vapour supplied to the reaction is sufficient for full coverage. One essential difference compared with conventional growth methods is that there are never free compound molecules on the surface. The exchange reaction in the ALE technique occurs in analogy with the CVD process. The structures of films prepared by ALE for the X-ray diffraction studies are presented in Fig. 1. The thicknesses of the ZnS films varied from 10 to 500 nm and those of the substrate films from 10 to 50 nm. The growth temperature of the samples was 350 or 500 °C.
i (a)
i
i (b)
zos TazOm
I
I
ZnS AI~01 SnO,
(c)
II
I
ZnS
Ta20
I
(d)
Fig. 1. Film structures prepared by ALE on a glass substrate. The thicknesses ofthe ZnS films rangefrom 10 to 500 n m and those of the substrate films from 10 to 50 nm.
2.2. Recording of X-ray diffraction patterns The X-ray diffraction patterns of ZnS films were measured with a conventional horizontal Siemens diffractometer (R = 170 mm) using Cu Kat radiation. The effect of the Kct2 component on the broadening of an X-ray line profile was eliminated by using a Johansson-type quartz crystal in the incident beam and placing a narrow separating slit in the exact position of the K % component of the focusing circle. All diffraction peaks were step scanned with an interval of 0.01 °. For computer calculations intensity distributions were recorded with a tape writer. The specific orientation of the films was studied by measuring a onedimensional pole distribution curve of the crystallographic [001] direction. The specimen tilt angle ~bvaried from - 2 5 ° to 25 °. 3. RESULTS
3.1. Crystal structure and specific orientation Our crystal structure studies showed that ZnS films grown at 500 °C by the ALE method have a high degree of hexagonality 7. This is somewhat surprising because the cubic-to-hexagonal phase transition temperature is reported to occur in the range 960-1250 °C a-i 1 The specific orientation of the microcrystallites in the films clearly depends on the film growth temperature in the range 350-500 °C and the substrate materials, as seen in Fig. 2 and Fig. 3 respectively. For comparison, curves measured with a randomly oriented polycrystalline sample (curve a) and with a silicon crystal (curve d) are plotted in Fig. 2. It is seen from Fig. 2 that the higher the growth temperature, the larger the degree of texture orientation is. The strongest preferred orientation is found for sample b in which the substrate is a Ta2Os layer 50 nm thick on glass (Fig. 3, curve b). In all samples the addition of an SnO2 layer lowered the degree of preferred orientation.
STRUCTURAL CHARACTERIZATION OF THIN
ZnS
341
FILMS
I
110
'.
. . . . . . . . . . . . . . . .
~
i
1.0
. . . . . .
\\\\
g F0.5!
\.
......
~_ - 0.5
' ~ ' \ . \ x... "'-.~.&.~. "\
o
00
10"
,
". . . .
~ZiZi
I
o
20°
"~-
0°
10"
I
Iqbl
20*
Fig. 2. One-dimensional pole density distribution curves obtained from randomly oriented ZnS powder (curve a), from ZnS film (type (a) in Fig. 1) grown at 350 °C (curve b), from ZnS film (type (a) in Fig. 1) grown at 500 °C (curve c) and from a silicon single crystal (curve d). Fig. 3. One-dimensional (00.1) pole density distribution curves obtained from sample types (a), (b), (c) and (d) (see Fig. 1) (curves a, b, c and d respectively) grown at 500 °C.
3.2. Crystallite size and microstrain The broadening effects of small coherent domain size D and microstrain ~n on the X-ray line profile are separated in this study by a single-line Fourier method. This method was developed by Gangulee 12 for analysing highly oriented thin films from which only the first-order diffraction profile is statistically adequate for mathematical analysis. It was shown by Gangulee that if the functional form of (%2) is approximated as C/n, where C is a constant and n is a harmonic number, acceptable separation of the two components can be achieved. The results obtained using this approximation are presented in Tables I and II for three types of samples. They were calculated from the average values of 6/D (D t), where 6 is a distance whose magnitude is inversely proportional to the Fourier period and n3 is the real averaging distance normal to the hkl planes, from the average value of C (D2) and from the slope of the Fourier coefficients using the method of Mitra and Mathur 13 (D3). The agreement between the three methods is good. Figure 4 shows a crystallite size distribution curve which is obtained from the Fourier coefficients using a second TABLE I VALUES D I , D 2 AND D 3 OF THE CRYSTALLITE SIZE C A L C U L A T E D USING THREE METHODS (SEE TEXT) FOR THE THREE SAMPLES OF FIG. 1
Sample
D 1 (nm)
D 2 (nm)
D 3 (nm)
a b c
91 82 81
112 93 103
90 65 88
TABLE II VALUES OF THE MICROSTRAIN CORRESPONDING TO THE CRYSTALLITE SIZE VALUES OF TABLE I
Sample
(e(D 1))
(e(D2))
(/~(D3))
a b c
0.77 x 10- 3 1.60x 10 -3 0.90x 10 -3
1.47 x 10- 3 1.90x 10 -3 1.53 × 10 -3
0.74 x 10- 3 1.04x 10 -3 1.05 x 10 -a
342
v.P.
TANNINEN,
T. O. TUOMI
derivative m e t h o d d e v e l o p e d b y M i g n o t a n d R o n d o t 14 a n d G a n e s a n et al. is with the a i d of the m e t h o d of G a n g u l e e . The average size of the crystallites in the direction p e r p e n d i c u l a r to the s a m p l e surface is r a t h e r large, a b o u t 90 nm. -//
i
i
,
,
5 •-: t~
~-
3
~2
.//-'",,
/
1
/
0
//
\.. I
l
~0
80
I
1.0
I
160
Olnra)
Fig. 4. CrystaUite size distribution curve obtained from sample type (a) (see Fig. 1) grown at 500 °C. The ZnS layer was 400 nm thick. D is the crystallite dimension in the direction perpendicular to the sample surface. 3.3. Dislocation density T h e d i s l o c a t i o n density estimates for the Z n S films were p e r f o r m e d in this s t u d y using a t h e o r y of W i l l i a m s o n a n d S m a l l m a n 16 a n d the same a p p r o x i m a t i o n s as for thin m e t a l films 17. T h e results c a l c u l a t e d from the crystallite size a n d the average m i c r o s t r a i n for three s a m p l e types are p r e s e n t e d in T a b l e III.
TABLE III DISLOCATIONDENSITIESp OFSAMPLESa~ b ANDc CALCULATEDFROMTHECRYSTALLITESIZE(TABLE0 ANO FROMTHEMICROSTRAIN(TABLEn)
p(DO (cm -2) p(D2) (cm -2) p(D3) (cm -2) p{~(Dl)} (ere -2) p{e(D2)} (cm -2) p{t(Dz)} (cm -z)
Sample a
Sample b
Sample c
3.6 x 101° 2.4 x 101° 3.7 x 101° 1.1 x 101° 4.2 x 101° 1.1 x 101°
4.4 x 10t° 2.8 x 101° 3.9 x 10t° 4.9 x 10~° 7.3 x 10l° 1.9 x 101°
4.6 × 101° 3.5 x 101° 7.1 x 10l° 1.6 x 101° 4.5 x 101° 2.1 x 101°
4. DISCUSSION T h i n Z n S films studied in this w o r k are used in electroluminescent devices 7. T h e y c o n t a i n a p p r o x i m a t e l y 0.5 at.% M n as a n active impurity. The films emit yellow light with a l u m i n a n c e o f as m u c h as 9000 cd m - 2 a n d an external efficiency of up to 1 0 1 m W -1 in films 3 5 0 n m thick. Little a t t e n t i o n has been p a i d to the d e p e n d e n c e of the electrical a n d optical p r o p e r t i e s of Z n S on the crystal p h a s e a n d on the m i c r o s t r u c t u r a l p a r a m e t e r s . T h e crystal structure of o u r s a m p l e s g r o w n at a relatively low t e m p e r a t u r e o f 350 °C was m a i n l y cubic 7. The E L intensity o b t a i n e d from these s a m p l e s was very weak. H o w e v e r , from the samples g r o w n at 500 °C we get the m a x i m u m value o f E L as m e n t i o n e d above• T h e X - r a y analysis of these films s h o w e d t h a t they are m a i n l y h e x a g o n a l 7. T h e degree of preferred o r i e n t a t i o n of the microcrystallites in the films was so high t h a t o n e - h a l f of the 00.2 poles are aligned
STRUCTURAL CHARACTERIZATION OF THIN
ZnS FILMS
343
within 7 ° from the substrate normal. Therefore the high EL efficiency can be partly associated with the phase change and the increase in the perfection of the ZnS film. However, we consider that the useful current for EL increases with increasing crystallite size because the current that does not cause EL is expected to flow along the crystallite boundaries. The diffraction profiles obtained from the low temperature samples were not statistically good enough for mathematical analysis but they showed a marked broadening due to the small crystallite size or the presence of microstrains. Therefore it is very probable that, in the films in which EL is weak, the crystallite size is much smaller than that in the very bright films. It is obvious that the degree of specific orientation, the hexagonality and the microstructural parameters such as crystallite size and microstrain affect the EL intensity. However, it is not clear at present whether they are interdependent or whether each parameter must have a certain value as a prerequisite for a high EL brightness. It is hoped that the preparation of more perfect films, together with a systematic quantitative evaluation of their structure, will give more information about this problem. ACKNOWLEDGMENTS
The authors wish to thank M. Oikkonen for his computer calculations and Lohja Corporation for financial support and preparing the thin film structures. REFERENCES 1 J. Hall and Ferguson, J. Opt. Soc. Am., 45 (1955) 714. 2 P. Coldberg and J. Nickerson, J. Appl. Phys., 34 (1963) 1601. 3 E. Soxman and G. Steele, NTIS Rep. AD-437866, 1963 (National Technical Information Service, U.S. Department of Commerce) (Contract NONR 4165(00)). 4 E. Soxman, JANAIR 700903, Tech. Rep. EL-7, 1979 (Sigmatron Inc.). 5 Z.K. Kun, D. Leksell, P. R. Malmberg, J. Murphy and L. J. Scienkiewicz, J. Electron. Mater., 10 (1981) 287. 6 T. Suntola, J. Antson, A. Pakkala and S. Lindfors, Society for Information Display Int. Syrup., San Diego, May 1980, Dig. Tech. Pap. XI, 1980, p. 108. 7 V.P. Tanninen, T. O. Tuomi, R. O. T6rnqvist, T. S. Suntola, J. O. Antson, A. J. Pakkala and S. G. Lindfors, Proc. 8th Int. Vacuum Congr., Cannes, September 22-26, 1980, Vol. 1, Thin Films, in Vide, Couches Minces, Suppl., 201 (1980) 410. 8 H.E. Swanson and R. K. Fuyat, Natl. Bur. Stand. (U.S.), Circ., 14 (II) (1953) 539. 9 A.M. Gurvich, Introduction to the Physical Chemistry of Crystal Phosphors, Nauka, Moscow, 1966. 10 C. Kittel, Introduction to Solid State Physics, Wiley, New York, 1971. 11 E.T. Allen, J. L. Grenschaw and H. F. Merwin, Am. J. Sci., 34 (1912) 341. 12 A. Gangulee, J. Appl. Crystallogr., 7 (1974) 434. 13 G. B. Mitra and B. K. Mathur, J. Appl. Crystallogr., 8 (1975) 543 ; 9 (1976) 352. 14 J. Mignot and D. Rondot, Acta Metall., 23 (1975) 1321. 15 P. Ganesan, H. K. Kuo, A. Saavedra and R. J. De Angelis, J. Catal., 52 (1978) 310. 16 G.K. Williamson and R. E. Smallman, Philos. Mag., 1 (1956) 34. 17 S. Suchitra, R. K. Nandi and S. P. Sen Gupta, J. Phys. D, 10 (1977) L 139.