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Epitaxial growth of cadmium films by the hot-wall technique
Thin Solid Films, 120 (1984) L73-L77
L73
Letter
Epitaxial growth of cadmium films by the hot-wall technique A. V. KULKARNI
D.B.J. CoNege, Chiplun 415 605 (India) S. BOSE R. Ruia College, Bombay 400 019 (India) R. PRATAP
Department of Physics, University of Bombay, Vidyanagari, Santacruz(E) (Received April 24, 1984; accepted
400 098 (India)
July 16, 1984)
I. Introduction Only a few attempts at preparing cadmium metal films by the conventional method of vacuum evaporation have been successful. Obtaining good coherent films has remained the principal difficulty which has hampered basic research on such filmslP3. Modifications of the conventional method have resulted only in polycrystalline films with quasi-whisker-like surface structures’ which caused the films to show poor reflectivity. In this letter we report the epitaxial growth of vacuum-evaporated cadmium films on a glass substrate by the hot-wall technique. In spite of the need for an ultrahigh vacuum for good epitaxy, the films were grown in a vacuum of 3 x lo-’ Torr. Care was taken to ensure suitable conditions for thermodynamic equilibrium (and minimum loss of material) of nucleated cadmium growth centres such that they develop into a well-ordered thin film structure 4. The hot-wall technique has been extensively reviewed by Lopez-Otero’. 2. Experimental details The hot-wall apparatus consisted of a Corning glass tube (height, 4cm; diameter, 1.5 cm) heated with a single resistance winding. The winding around the wall was widely spaced with respect to that at the bottom of the tube to provide differential heating. The glass substrate was radiation heated by the same resistance coil. A metal plate was placed on the back of the substrate for uniform heating of the deposition area. The apparatus was kept essentially simple to allow for subsequent modifications. Very small amounts (15-20 mg) of cadmium metal (purity, 99.9999%; obtained from Koch-Light Laboratories Ltd., Gt. Britain) were taken as the source material. The substrate was initially kept away from the tube. When the vacuum in the chamber reached 3.0 x lo-’ Torr, the substrate was brought to rest on the mouth of the tube to close it completely. Typical temperatures for the growth of epitaxial films on glass were as follows: substrate temperature, 60-80°C; wall temperature, 365-430 ‘C; source temperature, 375-450 “C. The average thickness of the films was 6 pm and the average growth rate was 30 pm h-l. 0040-6090/84/$3.00
0 Elsevier Sequoia/Printed
in The Netherlands
L74
LETTERS
(b)
4.0 pm
Fig. 1. Scanning electron micrographs of cadmium films deposited onto a glass substrate etching and (b) after etching, and (c) deposited onto a freshly air-cleaved NaCI( 100) substrate.
(a) before
L75
LETTERS
Cadmium films were also deposited substrates under similar growth conditions.
onto
freshly
air-cleaved
NaCl(lOO)
3. Results and discussion Films obtained on clean glass substrates showed a mirror-like shine at the back while the front surface had a dull appearance. As revealed by scanning electron microscopy (Fig. l(a)), the front surface has many randomly oriented plateletshaped crystallites (average size, 4.5 nm). A scanning electron micrograph of the bulk of the film after a layer a few tenths of a micron thick had been etched away (by a standard method) shows a smooth surface consisting of very large crystallites (Fig. l(b)) with the same orientation, as X-ray diffractometry of these samples reveals. The etched films showed good reflectivity. The formation of a number of randomly oriented thin crystallites on the unetched surface could possibly be due to the nucleation of cadmium on the highly epitaxial bulk of the film, at lower temperatures of the source and the substrate, after the heating coil was switched off. Results of X-ray diffraction measurements are given in Table 1. The measurements on the films as deposited onto glass substrates (Fig. 2(a)) show a sharp and intense (002) reflection. The (004) reflection is of moderate intensity, while all other reflections are totally absent. These two reflections were, in fact, so intense that the intensity of the incident X-rays had to be reduced considerably to record them on the chart. This clearly indicates the near perfection of epitaxial growth of cadmium along the c axis perpendicular to the plane of the substrate. The (004) reflection shows splitting owing to the Kcr doublet of the incident X-rays. TABLE X-RAY
hkl
I DIFFRACTION
MEASUREMENTS
d spacing ,from reJ 6
002 100 101 102 103 110
2.809 2.580 2.345 1.901 1.516 1.490
004
1.404
112
1.316
Cdjlms
ON EPITAXIAL
CADMIUM
FILMSa
as deposited onto glass
CdJiims on NaCI( 100)
Calculated dspacing (A)
Relurive in/ensify (%)
Calculated d spacing (A)
Relaiive intensify (%)
2.8100 -
100 -
2.8027 2.5709 2.3390 1.8973 1.5138 -
45 10 100 70 60 -
15 10
1.4028
3
1.3022
20
1.4033 (K u,) 1.4032 (K az) -
-
a With Co Ku radiation.
X-ray diffraction measurements on cadmium films grown on an NaCl(100) substrate show maximum intensity for the (101) reflection and significant intensities for the (102), (103) and (002) reflections (Fig. 2(b)). This indicates preferential growth of the (101) planes of cadmium on an NaCl(100) substrate. A scanning electron micrograph (Fig. l(c)) of such a film also suggests the growth of small crystallites with preferential orientation. Preferential orientation along [002] in cadmium films in the medium
L76
LETTERS
I
g 5
90
85
80
75
70
65
60
BRAGG
(4
ANGLE
55
50
45
40
35
28
t
90 (b)
I
I
85
80
75
70
65
60 BRAGG
ANGLE
55
50
45
40
35
28
Fig. 2. X-ray diffraction patterns (obtained with Co Ka radiation) of cadmium films (a) as deposited onto a glass substrate and (b) deposited onto a freshly air-cleaved NaCl(100) substrate. Aluminium peaks are due to the aluminium holder.
(0.02-0.3 urn) and high (1.5-2.8 urn) thickness ranges vacuum deposited onto glass substrates at elevated temperature has been reported by other researchers7S8. Epitaxial growth of thin (40-100 A) electrodeposits of cadmium on a copper singleHowever, no one has reported near crystal substrate has also been reported’. perfection of epitaxy and large crystallite size in cadmium deposits. In the hot-wall technique used by us, by maintaining a suitable differential heating we obtained cadmium films consisting of large crystal domains with [002] orientation only. However, preferential orientation of the (101) planes was obtained for films deposited onto NaCl(100) single crystals. We also presented in this letter a scanning electron microscopy study of the surface structure of as-deposited and etched cadmium films. We should like to thank Dr. M. J. Mehta, Tata Institute of Fundamental Research, Bombay, for extending help in X-ray diffraction measurements. A facility
LETTERS
L77
for scanning electron microscopy was provided by the Regional Sophisticated Instrumentation Centre, Indian Institute of Technology, Bombay. One of us (A.V.K.) gratefully acknowledges the award of a Teacher Fellowship by the University Grants Commission, New Delhi. 1 2 3 4 5 6 7 8 9
S. K. Bandopadhyay and A. K. Pal, J. Mater. Sci., 14 (1979) 2989. C. K. Ghosh, S. K. Bandopadhyay and A. K. Pal, Thin Solid Fi/ms, 76 (198 1) 3 13. S. K. Bandopadhyay and A. K. Pal, Indian J. Phys. A, 55 (1981) 270. L. 1. Maissel and R. Glang (eds.), Handbook of Thin Film Technology, McGraw-Hill, 1970. A. Lopez-Otero, Thin Solid Films, 49 (1978) 3. Powder Diffraction File, ASTM, Philadelphia, PA, Card 5-0674. S. Sen and S. P. Sen Gupta, J. Vat. Sci. Technol., 16 (1979) 42. N. C. Halder, Z. Naturforsch., 34A (1979) 176. A. Goswami, Indian J. Pure Appl. Phys., 7(1969) 232.
New York,
L73
Letter
Epitaxial growth of cadmium films by the hot-wall technique A. V. KULKARNI
D.B.J. CoNege, Chiplun 415 605 (India) S. BOSE R. Ruia College, Bombay 400 019 (India) R. PRATAP
Department of Physics, University of Bombay, Vidyanagari, Santacruz(E) (Received April 24, 1984; accepted
400 098 (India)
July 16, 1984)
I. Introduction Only a few attempts at preparing cadmium metal films by the conventional method of vacuum evaporation have been successful. Obtaining good coherent films has remained the principal difficulty which has hampered basic research on such filmslP3. Modifications of the conventional method have resulted only in polycrystalline films with quasi-whisker-like surface structures’ which caused the films to show poor reflectivity. In this letter we report the epitaxial growth of vacuum-evaporated cadmium films on a glass substrate by the hot-wall technique. In spite of the need for an ultrahigh vacuum for good epitaxy, the films were grown in a vacuum of 3 x lo-’ Torr. Care was taken to ensure suitable conditions for thermodynamic equilibrium (and minimum loss of material) of nucleated cadmium growth centres such that they develop into a well-ordered thin film structure 4. The hot-wall technique has been extensively reviewed by Lopez-Otero’. 2. Experimental details The hot-wall apparatus consisted of a Corning glass tube (height, 4cm; diameter, 1.5 cm) heated with a single resistance winding. The winding around the wall was widely spaced with respect to that at the bottom of the tube to provide differential heating. The glass substrate was radiation heated by the same resistance coil. A metal plate was placed on the back of the substrate for uniform heating of the deposition area. The apparatus was kept essentially simple to allow for subsequent modifications. Very small amounts (15-20 mg) of cadmium metal (purity, 99.9999%; obtained from Koch-Light Laboratories Ltd., Gt. Britain) were taken as the source material. The substrate was initially kept away from the tube. When the vacuum in the chamber reached 3.0 x lo-’ Torr, the substrate was brought to rest on the mouth of the tube to close it completely. Typical temperatures for the growth of epitaxial films on glass were as follows: substrate temperature, 60-80°C; wall temperature, 365-430 ‘C; source temperature, 375-450 “C. The average thickness of the films was 6 pm and the average growth rate was 30 pm h-l. 0040-6090/84/$3.00
0 Elsevier Sequoia/Printed
in The Netherlands
L74
LETTERS
(b)
4.0 pm
Fig. 1. Scanning electron micrographs of cadmium films deposited onto a glass substrate etching and (b) after etching, and (c) deposited onto a freshly air-cleaved NaCI( 100) substrate.
(a) before
L75
LETTERS
Cadmium films were also deposited substrates under similar growth conditions.
onto
freshly
air-cleaved
NaCl(lOO)
3. Results and discussion Films obtained on clean glass substrates showed a mirror-like shine at the back while the front surface had a dull appearance. As revealed by scanning electron microscopy (Fig. l(a)), the front surface has many randomly oriented plateletshaped crystallites (average size, 4.5 nm). A scanning electron micrograph of the bulk of the film after a layer a few tenths of a micron thick had been etched away (by a standard method) shows a smooth surface consisting of very large crystallites (Fig. l(b)) with the same orientation, as X-ray diffractometry of these samples reveals. The etched films showed good reflectivity. The formation of a number of randomly oriented thin crystallites on the unetched surface could possibly be due to the nucleation of cadmium on the highly epitaxial bulk of the film, at lower temperatures of the source and the substrate, after the heating coil was switched off. Results of X-ray diffraction measurements are given in Table 1. The measurements on the films as deposited onto glass substrates (Fig. 2(a)) show a sharp and intense (002) reflection. The (004) reflection is of moderate intensity, while all other reflections are totally absent. These two reflections were, in fact, so intense that the intensity of the incident X-rays had to be reduced considerably to record them on the chart. This clearly indicates the near perfection of epitaxial growth of cadmium along the c axis perpendicular to the plane of the substrate. The (004) reflection shows splitting owing to the Kcr doublet of the incident X-rays. TABLE X-RAY
hkl
I DIFFRACTION
MEASUREMENTS
d spacing ,from reJ 6
002 100 101 102 103 110
2.809 2.580 2.345 1.901 1.516 1.490
004
1.404
112
1.316
Cdjlms
ON EPITAXIAL
CADMIUM
FILMSa
as deposited onto glass
CdJiims on NaCI( 100)
Calculated dspacing (A)
Relurive in/ensify (%)
Calculated d spacing (A)
Relaiive intensify (%)
2.8100 -
100 -
2.8027 2.5709 2.3390 1.8973 1.5138 -
45 10 100 70 60 -
15 10
1.4028
3
1.3022
20
1.4033 (K u,) 1.4032 (K az) -
-
a With Co Ku radiation.
X-ray diffraction measurements on cadmium films grown on an NaCl(100) substrate show maximum intensity for the (101) reflection and significant intensities for the (102), (103) and (002) reflections (Fig. 2(b)). This indicates preferential growth of the (101) planes of cadmium on an NaCl(100) substrate. A scanning electron micrograph (Fig. l(c)) of such a film also suggests the growth of small crystallites with preferential orientation. Preferential orientation along [002] in cadmium films in the medium
L76
LETTERS
I
g 5
90
85
80
75
70
65
60
BRAGG
(4
ANGLE
55
50
45
40
35
28
t
90 (b)
I
I
85
80
75
70
65
60 BRAGG
ANGLE
55
50
45
40
35
28
Fig. 2. X-ray diffraction patterns (obtained with Co Ka radiation) of cadmium films (a) as deposited onto a glass substrate and (b) deposited onto a freshly air-cleaved NaCl(100) substrate. Aluminium peaks are due to the aluminium holder.
(0.02-0.3 urn) and high (1.5-2.8 urn) thickness ranges vacuum deposited onto glass substrates at elevated temperature has been reported by other researchers7S8. Epitaxial growth of thin (40-100 A) electrodeposits of cadmium on a copper singleHowever, no one has reported near crystal substrate has also been reported’. perfection of epitaxy and large crystallite size in cadmium deposits. In the hot-wall technique used by us, by maintaining a suitable differential heating we obtained cadmium films consisting of large crystal domains with [002] orientation only. However, preferential orientation of the (101) planes was obtained for films deposited onto NaCl(100) single crystals. We also presented in this letter a scanning electron microscopy study of the surface structure of as-deposited and etched cadmium films. We should like to thank Dr. M. J. Mehta, Tata Institute of Fundamental Research, Bombay, for extending help in X-ray diffraction measurements. A facility
LETTERS
L77
for scanning electron microscopy was provided by the Regional Sophisticated Instrumentation Centre, Indian Institute of Technology, Bombay. One of us (A.V.K.) gratefully acknowledges the award of a Teacher Fellowship by the University Grants Commission, New Delhi. 1 2 3 4 5 6 7 8 9
S. K. Bandopadhyay and A. K. Pal, J. Mater. Sci., 14 (1979) 2989. C. K. Ghosh, S. K. Bandopadhyay and A. K. Pal, Thin Solid Fi/ms, 76 (198 1) 3 13. S. K. Bandopadhyay and A. K. Pal, Indian J. Phys. A, 55 (1981) 270. L. 1. Maissel and R. Glang (eds.), Handbook of Thin Film Technology, McGraw-Hill, 1970. A. Lopez-Otero, Thin Solid Films, 49 (1978) 3. Powder Diffraction File, ASTM, Philadelphia, PA, Card 5-0674. S. Sen and S. P. Sen Gupta, J. Vat. Sci. Technol., 16 (1979) 42. N. C. Halder, Z. Naturforsch., 34A (1979) 176. A. Goswami, Indian J. Pure Appl. Phys., 7(1969) 232.
New York,