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The growth of single-crystalline aluminium films on lithium fluoride and magnesium oxide thin film substrates
Thin Solid Films, 30 (1975) L7--L10 © Elsevier Sequoia S.A., Lausanne - - Printed in Switzerland
L7
Letter
The growth of single-cryst~niue aluminium films on lithium fluoride and magnesium oxide thin film substrates K. REICHELT and SRIRAMAN SRINIVASAN*
Institut fiir Festk6rperforschung, Kernforschungsanlage Jiilich, 517 Jiilich (West Germany) (Received July 8, 1975; accepted September 9, 1975)
The epitaxial growth of A1 on alkali halide and MgO substrates has been studied by some authors 1 -e. However, the problem of the reproducible production of large single-crystal A1 films in the (100) orientation has n o t yet been solved completely. In this letter a method is described by which A1 single-crystal films of large size can be deposited. Formed b y evaporation onto vacuum-deposited epitaxial LiF or MgO substrates, they are reproducible in desired orientation. The properties of such films were studied b y transmission electron microscopy and X-ray Laue techniques. The experiments were performed in a high vacuum system with a diffusion pump and liquid nitrogen trap. The pressure during the evaporation was less than 1 × 10 - 6 Tort. The substances LiF/A1 or LiF/MgO/A1 were evaporated sequentially onto the substrate with a multi-crucible electron-beam gun. Between the substrate holder and the source was a shutter which could be opened after the desired substrate temperature and evaporation rate were reached. The layer thickness and the deposition rate were measured with a quartz monitor. Cleavage faces of NaC1 were used for the deposition of (100) films. When other orientations were required, the corresponding sawed and polished NaC1 crystals were used. Finally all the A1 films less than 500 A thick were coated with a carbon-bearing layer. The NaC1 substrate was dissolved in water for the structure and perfection of the films to be investigated. The films less than 2000 A thick were floated on grids and studied under the electron microscope. Films that were thicker than 2000 A were studied by X-ray methods (Laue transmission pictures and rocking curves).
Experiments with LiF thin film substrates From previous studies 7 it is well known that under particular conditions LiF forms single-crystal films on NaC1 in parallel orientations.
*On study leave from the Department of Physics, Indian Institute of Technology, Madras, India.
L8
Fig. 1. Electron micrograph of (100)-oriented AI clusters grown on a LiF single-crystal film substrate. Deposition conditions: LiF, Ts = 300 °C, R = 40 A sec -1, d = 1000 A; A1, Ts = 420 °C, R = 8 A sec -1.
Fig. 2. Electron micrograph of (100)-oriented A1 clusters on LiF. The LiF is partly dissolved during the procedure of detaching the NaCl substrate. Rectangular LiF crystals are visible. (For deposition conditions see Fig. 1.)
Under these conditions, a b o u t 1000 A LiF was epitaxially evaporated onto NaC1 substrates. After that, A1 was evaporated at a rate of 10 A sec -1 with substrate temperatures ranging between 300 ° and 450 °C. The experiments show that A1 grows as a single crystal in parallel orientation on LiF at substrate temperatures above 420 °C. Figure 1 shows A1 clusters of a film with an average layer thickness of 100 A. The diffraction pattern shows exclusively (100) spots. In this respect it is different from epitaxial films of other materials (e.g. Au on NaC1) where films of similar thickness consist mostly of differently oriented clusters which reorient only with further growth and coalescence. Figure 2 shows A1
L9
Fig. 3. Electron micrograph of an epitaxial (100) Al film. (For deposition conditions see Fig. 1; thickness about 1500 A.) Fig. 4. Laue transmission pattern of an epitaxial (110) Al film (d = 4pm) grown on LiF.
Fig. 5. Electron micrograph of epitaxial (100) AI clusters grown on an epitaxial MgO/LiF thin film substrate. Deposition conditions: LiF and A1 see Fig. 1;MgO, Ts = 420 °C, R = 10 A see-1 , d = 1000 A. Fig. 6. Laue transmission pattern of an epitaxial (100) A1 film (d = 2 ~tm) grown on an MgO thin film substrate.
clusters w i t h p o r t i o n s o f t h e LiF substrate, w h i c h is o n l y partially dissolved in w a t e r o w i n g t o its p o o r solubility. Figure 3 s h o w s an A1 film 1 5 0 0 A t h i c k w h i c h was also g r o w n o n epitaxial LiF. T h e d i s l o c a t i o n d e n s i t y o f t h e films can be r e d u c e d b y annealing t h e m directly o n the substrate at a t e m p e r a t u r e o f a b o u t 5 0 0 °C a f t e r e v a p o r a t i o n f o r a few m i n u t e s . By f u r t h e r d e p o s i t i o n o f A1 at t h e given t e m p e r a t u r e , the
LIO films can be grown as thick as required. Figure 4 shows a Laue transmission pattern of a film (about 4 p m thick) in the (110} orientation. The crystal perfection of these films was determined with a double~rystal X-ray diffractometer b y measurement of rocking curves. The FWHM value was between 10 and 20 rain in these measurements.
Experiments with MgO thin film substrates In order to make transmission electron microscope studies easier, epitaxially grown thin MgO substrates were used. The conditions for depositing epitaxial MgO substrate films have been described in an earlier publication 7. After producing a substrate the AI was evaporated at a rate of 10 A sec-1 with different substrate temperatures ranging between 300 ° and 450 °C. In the case of the epitaxial growth of Al on thin epitaxial M g O substrates, it is found that at substrate temperatures above 400 °C epitaxial growth in parallel orientation occurs. Figure 5 shows oriented A1 clusters on an MgO(100) substrate. The interval of the moire pattern obtained from the lattice parameters of M g O and Al agrees well with the measured values (approximately 105 A ). The double spots seen in the diffraction pattern are due to M g O and Al films, respectively. As in the case of LiF substrates it is also possible to produce films of desired thickness and orientation. Figure 6 shows a Laue transmission pattern of a thick Al film (2 pro) grown on an M g O single~rystal film substmte in the (100) orientation. The authors wish to thank Dr. H. Wenzl for his support and interest and M. R e y for his help during the progress o f these experiments. 1 2 3 4 5 6 7
S. Ino, D. Watanabe and S. Ogawa, J. Phys. Soc. Jpn, 19 (1964) 881. S. Ogawa, S. Ino, T. Kato and H. Ota, J. Phys. Soe. Jpn, 21 (1966) 1963. J. D. Landry and E. N. Mitchell,J. Appl. Phys., 40 (1969) 3406. G. Dumpich and E. F. Wassermann, Surf. Sci., 33 (1972) 203. W. R. Wagner, Surf. Sci., 41 (1974) 591. A. K. Green, J. Dancy and E. Bauer, J. Vac. Sci. Technol., 7 (1969) 159. K. Reichelt,J. Cryst. Growth, 19 (1973) 258.
L7
Letter
The growth of single-cryst~niue aluminium films on lithium fluoride and magnesium oxide thin film substrates K. REICHELT and SRIRAMAN SRINIVASAN*
Institut fiir Festk6rperforschung, Kernforschungsanlage Jiilich, 517 Jiilich (West Germany) (Received July 8, 1975; accepted September 9, 1975)
The epitaxial growth of A1 on alkali halide and MgO substrates has been studied by some authors 1 -e. However, the problem of the reproducible production of large single-crystal A1 films in the (100) orientation has n o t yet been solved completely. In this letter a method is described by which A1 single-crystal films of large size can be deposited. Formed b y evaporation onto vacuum-deposited epitaxial LiF or MgO substrates, they are reproducible in desired orientation. The properties of such films were studied b y transmission electron microscopy and X-ray Laue techniques. The experiments were performed in a high vacuum system with a diffusion pump and liquid nitrogen trap. The pressure during the evaporation was less than 1 × 10 - 6 Tort. The substances LiF/A1 or LiF/MgO/A1 were evaporated sequentially onto the substrate with a multi-crucible electron-beam gun. Between the substrate holder and the source was a shutter which could be opened after the desired substrate temperature and evaporation rate were reached. The layer thickness and the deposition rate were measured with a quartz monitor. Cleavage faces of NaC1 were used for the deposition of (100) films. When other orientations were required, the corresponding sawed and polished NaC1 crystals were used. Finally all the A1 films less than 500 A thick were coated with a carbon-bearing layer. The NaC1 substrate was dissolved in water for the structure and perfection of the films to be investigated. The films less than 2000 A thick were floated on grids and studied under the electron microscope. Films that were thicker than 2000 A were studied by X-ray methods (Laue transmission pictures and rocking curves).
Experiments with LiF thin film substrates From previous studies 7 it is well known that under particular conditions LiF forms single-crystal films on NaC1 in parallel orientations.
*On study leave from the Department of Physics, Indian Institute of Technology, Madras, India.
L8
Fig. 1. Electron micrograph of (100)-oriented AI clusters grown on a LiF single-crystal film substrate. Deposition conditions: LiF, Ts = 300 °C, R = 40 A sec -1, d = 1000 A; A1, Ts = 420 °C, R = 8 A sec -1.
Fig. 2. Electron micrograph of (100)-oriented A1 clusters on LiF. The LiF is partly dissolved during the procedure of detaching the NaCl substrate. Rectangular LiF crystals are visible. (For deposition conditions see Fig. 1.)
Under these conditions, a b o u t 1000 A LiF was epitaxially evaporated onto NaC1 substrates. After that, A1 was evaporated at a rate of 10 A sec -1 with substrate temperatures ranging between 300 ° and 450 °C. The experiments show that A1 grows as a single crystal in parallel orientation on LiF at substrate temperatures above 420 °C. Figure 1 shows A1 clusters of a film with an average layer thickness of 100 A. The diffraction pattern shows exclusively (100) spots. In this respect it is different from epitaxial films of other materials (e.g. Au on NaC1) where films of similar thickness consist mostly of differently oriented clusters which reorient only with further growth and coalescence. Figure 2 shows A1
L9
Fig. 3. Electron micrograph of an epitaxial (100) Al film. (For deposition conditions see Fig. 1; thickness about 1500 A.) Fig. 4. Laue transmission pattern of an epitaxial (110) Al film (d = 4pm) grown on LiF.
Fig. 5. Electron micrograph of epitaxial (100) AI clusters grown on an epitaxial MgO/LiF thin film substrate. Deposition conditions: LiF and A1 see Fig. 1;MgO, Ts = 420 °C, R = 10 A see-1 , d = 1000 A. Fig. 6. Laue transmission pattern of an epitaxial (100) A1 film (d = 2 ~tm) grown on an MgO thin film substrate.
clusters w i t h p o r t i o n s o f t h e LiF substrate, w h i c h is o n l y partially dissolved in w a t e r o w i n g t o its p o o r solubility. Figure 3 s h o w s an A1 film 1 5 0 0 A t h i c k w h i c h was also g r o w n o n epitaxial LiF. T h e d i s l o c a t i o n d e n s i t y o f t h e films can be r e d u c e d b y annealing t h e m directly o n the substrate at a t e m p e r a t u r e o f a b o u t 5 0 0 °C a f t e r e v a p o r a t i o n f o r a few m i n u t e s . By f u r t h e r d e p o s i t i o n o f A1 at t h e given t e m p e r a t u r e , the
LIO films can be grown as thick as required. Figure 4 shows a Laue transmission pattern of a film (about 4 p m thick) in the (110} orientation. The crystal perfection of these films was determined with a double~rystal X-ray diffractometer b y measurement of rocking curves. The FWHM value was between 10 and 20 rain in these measurements.
Experiments with MgO thin film substrates In order to make transmission electron microscope studies easier, epitaxially grown thin MgO substrates were used. The conditions for depositing epitaxial MgO substrate films have been described in an earlier publication 7. After producing a substrate the AI was evaporated at a rate of 10 A sec-1 with different substrate temperatures ranging between 300 ° and 450 °C. In the case of the epitaxial growth of Al on thin epitaxial M g O substrates, it is found that at substrate temperatures above 400 °C epitaxial growth in parallel orientation occurs. Figure 5 shows oriented A1 clusters on an MgO(100) substrate. The interval of the moire pattern obtained from the lattice parameters of M g O and Al agrees well with the measured values (approximately 105 A ). The double spots seen in the diffraction pattern are due to M g O and Al films, respectively. As in the case of LiF substrates it is also possible to produce films of desired thickness and orientation. Figure 6 shows a Laue transmission pattern of a thick Al film (2 pro) grown on an M g O single~rystal film substmte in the (100) orientation. The authors wish to thank Dr. H. Wenzl for his support and interest and M. R e y for his help during the progress o f these experiments. 1 2 3 4 5 6 7
S. Ino, D. Watanabe and S. Ogawa, J. Phys. Soc. Jpn, 19 (1964) 881. S. Ogawa, S. Ino, T. Kato and H. Ota, J. Phys. Soe. Jpn, 21 (1966) 1963. J. D. Landry and E. N. Mitchell,J. Appl. Phys., 40 (1969) 3406. G. Dumpich and E. F. Wassermann, Surf. Sci., 33 (1972) 203. W. R. Wagner, Surf. Sci., 41 (1974) 591. A. K. Green, J. Dancy and E. Bauer, J. Vac. Sci. Technol., 7 (1969) 159. K. Reichelt,J. Cryst. Growth, 19 (1973) 258.