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Metal-insulator phase transition in VO2: Influence of film thickness and substrate
Thin Solid Films, 150 (1987) 11-14
11
ELECTRONICS AND OPTICS
M E T A L - I N S U L A T O R PHASE T R A N S I T I O N IN VO2: I N F L U E N C E O F FILM THICKNESS AND SUBSTRATE E. V. BABKIN, A. A. CHARYEV, A. P. DOLGAREV AND H. O. URINOV
L. V. Kirensky Institute of Physics, U.S.S.R. Academy of Sciences, Siberian Branch, Krasnoyarsk 660036
(U.S.S.R.) (Received September 11, 1985; revised June 6, 1986; accepted January 28, 1987)
V O 2 films were prepared by the pyrolysis of vanadium acetylacetonate on different substrates. It is shown that the temperature Tk of the metal-insulator phase transition of these films is always lower than that in the bulk material and depends on the choice of substrate. The value of Tk for these films on the same substrate decreases monotonically with decreasing film thickness. These effects are related to substrate-film interactions.
1. INTRODUCTION
The metal-insulator phase transition (MIPT) in compounds of the 3d metals has received much attention and has been the subject of many investigations in recent years. An understanding of the nature of this phenomenon, extensive experimental data and a reliable technology have created conditions for the practical use of materials with an M I P T 1. However, in spite of the progress in this direction, the understanding of the properties of films showing an M I P T remains uncertain. In our opinion, one way to resolve this uncertainty is to utilize the dimensional phenomenon in films, where the contribution of the surface energy to the general energy can significantly alter the phase equilibrium and the properties of thin layers. The phase transition temperature T~ and the ratio between the "metal conductivity" and the "insulator conductivity" at the M I P T are important characteristics of a given material. VO 2 with a bulk M I P T temperature Tk° of 340 K 2 and a ratio between the metal conductivity and the insulator conductivity of 103-10 `* (ref. 2) is the most convenient material in practice. The influence of the substrate on the M I P T in VO 2 films has been investigated in a number of studies3'4; nevertheless, for the systematic classification of the results it is necessary to widen these investigations by changing the substrate class and investigating the thickness dependence of the effects observed. The results of an investigation of the influence of the substrate and VO2 film thickness on the M I P T temperature are given in this paper.
0040-6090/87/$3.50
© ElsevierSequoia/Printedin The Netherlands
12
E. BABKIN et
al.
2. EXPERIMENTAL PROCEDURE
The V O 2 films were prepared by the pyrolysis of vanadium acetylacetonate in a closed volume. The advantage of this method is that precipitation occurs at lower temperatures than with other chemical methods, so that it does not limit the selection of the substrate. Mica, sitall (ceramics based on silica, calcium and sodium oxides), alumina ceramics, single crystals of lithium fluoride and magnesium oxide were used as substrates. The optimum temperature of the substrate was 630 K and the pressure of oxygen in the synthesis chamber was maintained at 6.7 Pa in all technological processes. The deposition rate was 0.1/am min- 1. The thickness of the films obtained was monitored using X-ray fluorescence spectroscopy. Electrical measurements were carried out at a constant current. Contacts 5 mm apart were made by applying silver paste and then annealing at 310 K. 3. RESULTS AND DISCUSSION
Our investigation showed that the M I P T temperature Tk for VO2 films is always lower than that for the bulk material (Table I). The results for the temperature dependence of the electrical resistance for films with thicknesses of 0.47-0.51 I,tm and on different substrates are shown in Fig. 1. All the films show a hysteresis of the order of 2-4 K at the MIPT. Results are shown for heating of the samples. For comparison, the temperature dependence of the electrical resistance of a single crystal is also given (curve 5) 2. The change in the ratio of the metal conductivity to the insulator conductivity for films on different substrates may be a consequence of the change in M I P T temperature resulting from changes in the electronic band structure. The M I P T temperature on the same substrate decreases monotonically as the film thickness decreases; thus the M I P T temperatures of films on mica and magnesium oxide substrates of thickness 0.02 jam and 0.01 Arm are 317+1 K and 316+1 K respectively. The thickness dependence of the M I P T temperature for films on a magnesium oxide substrate is shown in Fig. 2. The influence of the substrate is greatest in this thickness range and the dependence of the M I P T temperature on the thickness of the films may be compared with calculation. It is natural to connect the results with the influence of substrate-film interactions on the phase transition; these interactions may affect the energy balance contribution to o/d, where cr is the surface energy density and d is the film thickness. Physically, this may mean that, as the film growth occurs at a temperature above the MIPT, the high temperature tetragonal phase is stabilized by the substrate-film interactions so that the reconstruction of the crystal structure at the phase transition is difficult. Phase stabilization in thin films because of surface effects is a widely observed phenomenon in thin film physics 5'6. Following ref. 5, let us consider phenomenologically the states of the film and the bulk material at the phase transition:
Ft(7~,) = F:(Tk)
(1)
Fl°(Tk °) ---- F2°(Tk °)
(2)
13
M E T A L ~ I N S U L A T O R P H A S E T R A N S I T I O N S IN V O 2
TABLE I METAL-INSULATOR PHASE TRANSITION TEMPERATURES FOR V O 2 FILMS ON DIFFERENT SUBSTRATESa
Substrate
Tk(K)
MgO LiF AI20 3 Sitall Mica
320+ 323 + 325 + 333 _ 333 +
Filmthickness(~n) 1 1 1 1 1
0.51 0.50 0.50 0.47 0.49
a For a single-crystal VO2 the temperature is 340___1 K.
5
3,01
L~ >,
335 ~.~
3 330
-
2 325
- -
) 320 -
I I
0 2.9
I 3.0
I 3.1
I 3.2
I 3.3
3./,
T - l ( x 103 K -1)
I~e I I I I I I I I I I 0.2 0.6 1.0 1./` 1.8 2./`
d-lt~tm-11
Fig. 1. ThetemperaturedependenceofthespecificelectricresistanceofVO2 films with thicknesses in the range 0.47-0.51 pm deposited onto different substrates: curve 1, MgO; curve 2, LiF; curve 3, A1203; curve 4, sitall. Curve 5: data for a VO2 single crystal2. Fig. 2. Thickness dependence of the MIPT temperature of VO2 films on an MgO substrate. where F is the free energy density, the superscript 0 characterizes the bulk material properties and the subscripts 1 and 2 correspond to the metallic and insulator phases respectively. The equations of state for the film and the bulk material m a y be written in the form ffl
G2
UI+~-TkS1 =U2+~---TkS2
(3)
U 1-Tk0sl
(4)
=
U2--Tk°S2
where U is the internal energy density and S is the entropy. Obviously, the form of the equation for phase equilibrium (eqn. (3)) places a lower limit on the film thickness; this means that the contribution of the surface energy to the total free energy must be small and must not change the thermodynamic properties of the film material. F o r the limitation on the applicability of the
E. BABKINet al.
14
model, comparison of experimental results with calculated data should be made. It is not difficult to obtain an expression for the film phase transition temperature from eqns. (3) and (4): AGTk 0 Tk = Tk ° q- - -
dQ
(5)
where Act -- rr 2 - a l is the difference in the surface energy densities of the two phases and Q = Tk°(St --$2) is the heat of the phase transition. The experimental results shown in Fig. 2 are in approximate agreement with the model. It may be assumed that eqn. (5) is correct for films of thickness 3 p,m and greater. The broken line is calculated using eqn. (5). F r o m the diagram, the difference in surface energy density of the two phases can be evaluated. Using the value Q = 1.02 kcal mol 1 (ref. 7) for the heat of the phase transition in VO2, we obtain for films on a magnesium oxide substrate Art ~ 2 x 10 v e r g c m 2. ACKNOWLEDGMENTS
We should like to thank G. A. Petrakovsky and V. G. Pynko for discussions about the results of this work. REFERENCES 1 A . A . Bugaev, E. P. Zacharchenja and F. A. Chudnovsky, Fasovyiperehodmetall-poluprovodnik i ego primenenie, Nauka, Leningrad, 1979. 2 Y. Bando, K. Nagasawa, Y. Kato, and T. Takada, Jpn. J. Appl. Phys., ?¢(1969) 633. 3 I. Balberg, B. Abeles and Y. Arie, Thin Solid Films, 24 (1974) 307. 4 K.-D. Ufert, Phys. Status Solidi A, 34 (1976) K 83. 5 Ju. F. Komnik, Fizika metallischeskihplenok, Atomizdat, Moscow, 1979. 6 E.V. Babkin, K. P. Koval and V. G. Pynko, Thin Solid Films, 117 (1984) 217. 7 N . F . Mott, Metal-Insulator Transitions, Nauka, Moscow, 1979.
11
ELECTRONICS AND OPTICS
M E T A L - I N S U L A T O R PHASE T R A N S I T I O N IN VO2: I N F L U E N C E O F FILM THICKNESS AND SUBSTRATE E. V. BABKIN, A. A. CHARYEV, A. P. DOLGAREV AND H. O. URINOV
L. V. Kirensky Institute of Physics, U.S.S.R. Academy of Sciences, Siberian Branch, Krasnoyarsk 660036
(U.S.S.R.) (Received September 11, 1985; revised June 6, 1986; accepted January 28, 1987)
V O 2 films were prepared by the pyrolysis of vanadium acetylacetonate on different substrates. It is shown that the temperature Tk of the metal-insulator phase transition of these films is always lower than that in the bulk material and depends on the choice of substrate. The value of Tk for these films on the same substrate decreases monotonically with decreasing film thickness. These effects are related to substrate-film interactions.
1. INTRODUCTION
The metal-insulator phase transition (MIPT) in compounds of the 3d metals has received much attention and has been the subject of many investigations in recent years. An understanding of the nature of this phenomenon, extensive experimental data and a reliable technology have created conditions for the practical use of materials with an M I P T 1. However, in spite of the progress in this direction, the understanding of the properties of films showing an M I P T remains uncertain. In our opinion, one way to resolve this uncertainty is to utilize the dimensional phenomenon in films, where the contribution of the surface energy to the general energy can significantly alter the phase equilibrium and the properties of thin layers. The phase transition temperature T~ and the ratio between the "metal conductivity" and the "insulator conductivity" at the M I P T are important characteristics of a given material. VO 2 with a bulk M I P T temperature Tk° of 340 K 2 and a ratio between the metal conductivity and the insulator conductivity of 103-10 `* (ref. 2) is the most convenient material in practice. The influence of the substrate on the M I P T in VO 2 films has been investigated in a number of studies3'4; nevertheless, for the systematic classification of the results it is necessary to widen these investigations by changing the substrate class and investigating the thickness dependence of the effects observed. The results of an investigation of the influence of the substrate and VO2 film thickness on the M I P T temperature are given in this paper.
0040-6090/87/$3.50
© ElsevierSequoia/Printedin The Netherlands
12
E. BABKIN et
al.
2. EXPERIMENTAL PROCEDURE
The V O 2 films were prepared by the pyrolysis of vanadium acetylacetonate in a closed volume. The advantage of this method is that precipitation occurs at lower temperatures than with other chemical methods, so that it does not limit the selection of the substrate. Mica, sitall (ceramics based on silica, calcium and sodium oxides), alumina ceramics, single crystals of lithium fluoride and magnesium oxide were used as substrates. The optimum temperature of the substrate was 630 K and the pressure of oxygen in the synthesis chamber was maintained at 6.7 Pa in all technological processes. The deposition rate was 0.1/am min- 1. The thickness of the films obtained was monitored using X-ray fluorescence spectroscopy. Electrical measurements were carried out at a constant current. Contacts 5 mm apart were made by applying silver paste and then annealing at 310 K. 3. RESULTS AND DISCUSSION
Our investigation showed that the M I P T temperature Tk for VO2 films is always lower than that for the bulk material (Table I). The results for the temperature dependence of the electrical resistance for films with thicknesses of 0.47-0.51 I,tm and on different substrates are shown in Fig. 1. All the films show a hysteresis of the order of 2-4 K at the MIPT. Results are shown for heating of the samples. For comparison, the temperature dependence of the electrical resistance of a single crystal is also given (curve 5) 2. The change in the ratio of the metal conductivity to the insulator conductivity for films on different substrates may be a consequence of the change in M I P T temperature resulting from changes in the electronic band structure. The M I P T temperature on the same substrate decreases monotonically as the film thickness decreases; thus the M I P T temperatures of films on mica and magnesium oxide substrates of thickness 0.02 jam and 0.01 Arm are 317+1 K and 316+1 K respectively. The thickness dependence of the M I P T temperature for films on a magnesium oxide substrate is shown in Fig. 2. The influence of the substrate is greatest in this thickness range and the dependence of the M I P T temperature on the thickness of the films may be compared with calculation. It is natural to connect the results with the influence of substrate-film interactions on the phase transition; these interactions may affect the energy balance contribution to o/d, where cr is the surface energy density and d is the film thickness. Physically, this may mean that, as the film growth occurs at a temperature above the MIPT, the high temperature tetragonal phase is stabilized by the substrate-film interactions so that the reconstruction of the crystal structure at the phase transition is difficult. Phase stabilization in thin films because of surface effects is a widely observed phenomenon in thin film physics 5'6. Following ref. 5, let us consider phenomenologically the states of the film and the bulk material at the phase transition:
Ft(7~,) = F:(Tk)
(1)
Fl°(Tk °) ---- F2°(Tk °)
(2)
13
M E T A L ~ I N S U L A T O R P H A S E T R A N S I T I O N S IN V O 2
TABLE I METAL-INSULATOR PHASE TRANSITION TEMPERATURES FOR V O 2 FILMS ON DIFFERENT SUBSTRATESa
Substrate
Tk(K)
MgO LiF AI20 3 Sitall Mica
320+ 323 + 325 + 333 _ 333 +
Filmthickness(~n) 1 1 1 1 1
0.51 0.50 0.50 0.47 0.49
a For a single-crystal VO2 the temperature is 340___1 K.
5
3,01
L~ >,
335 ~.~
3 330
-
2 325
- -
) 320 -
I I
0 2.9
I 3.0
I 3.1
I 3.2
I 3.3
3./,
T - l ( x 103 K -1)
I~e I I I I I I I I I I 0.2 0.6 1.0 1./` 1.8 2./`
d-lt~tm-11
Fig. 1. ThetemperaturedependenceofthespecificelectricresistanceofVO2 films with thicknesses in the range 0.47-0.51 pm deposited onto different substrates: curve 1, MgO; curve 2, LiF; curve 3, A1203; curve 4, sitall. Curve 5: data for a VO2 single crystal2. Fig. 2. Thickness dependence of the MIPT temperature of VO2 films on an MgO substrate. where F is the free energy density, the superscript 0 characterizes the bulk material properties and the subscripts 1 and 2 correspond to the metallic and insulator phases respectively. The equations of state for the film and the bulk material m a y be written in the form ffl
G2
UI+~-TkS1 =U2+~---TkS2
(3)
U 1-Tk0sl
(4)
=
U2--Tk°S2
where U is the internal energy density and S is the entropy. Obviously, the form of the equation for phase equilibrium (eqn. (3)) places a lower limit on the film thickness; this means that the contribution of the surface energy to the total free energy must be small and must not change the thermodynamic properties of the film material. F o r the limitation on the applicability of the
E. BABKINet al.
14
model, comparison of experimental results with calculated data should be made. It is not difficult to obtain an expression for the film phase transition temperature from eqns. (3) and (4): AGTk 0 Tk = Tk ° q- - -
dQ
(5)
where Act -- rr 2 - a l is the difference in the surface energy densities of the two phases and Q = Tk°(St --$2) is the heat of the phase transition. The experimental results shown in Fig. 2 are in approximate agreement with the model. It may be assumed that eqn. (5) is correct for films of thickness 3 p,m and greater. The broken line is calculated using eqn. (5). F r o m the diagram, the difference in surface energy density of the two phases can be evaluated. Using the value Q = 1.02 kcal mol 1 (ref. 7) for the heat of the phase transition in VO2, we obtain for films on a magnesium oxide substrate Art ~ 2 x 10 v e r g c m 2. ACKNOWLEDGMENTS
We should like to thank G. A. Petrakovsky and V. G. Pynko for discussions about the results of this work. REFERENCES 1 A . A . Bugaev, E. P. Zacharchenja and F. A. Chudnovsky, Fasovyiperehodmetall-poluprovodnik i ego primenenie, Nauka, Leningrad, 1979. 2 Y. Bando, K. Nagasawa, Y. Kato, and T. Takada, Jpn. J. Appl. Phys., ?¢(1969) 633. 3 I. Balberg, B. Abeles and Y. Arie, Thin Solid Films, 24 (1974) 307. 4 K.-D. Ufert, Phys. Status Solidi A, 34 (1976) K 83. 5 Ju. F. Komnik, Fizika metallischeskihplenok, Atomizdat, Moscow, 1979. 6 E.V. Babkin, K. P. Koval and V. G. Pynko, Thin Solid Films, 117 (1984) 217. 7 N . F . Mott, Metal-Insulator Transitions, Nauka, Moscow, 1979.