[0001] The present invention relates to an ultra-low resistivity heterofilm which has a
hetero-structure.
[0002] According to the present invention there is provided an ultra-low resistivity heterofilm
having a hetero-structure composed of polarized and non-polarized Langmuir-Blodgett
(LB) films stacked on one another and sandwiched between conductive films, the ultra-low
resistivity heterofilm having a resistance value not higher than that of metal in
the direction of the film surface thereof at a temperature not lower than room temperature.
[0003] The polarized film(s) may be stacked on the non-polarised film(s) or vice-versa
and the hetero-structure sandwiched between conductive films so that the resistance
of the film in the direction of the film surface becomes much lower than that of metal
films.
[0004] Generally, a dielectric is also an insulator. A dielectric material according to
the invention which shows a resistivity much lower than metals at a temperature no
lower than room temperature has nor previously existed.
[0005] As produced according to the invention, if a two-dimensional potential well having
a depth of several tens of Angstroms (Å) can be formed by polarised and non-polarised
heterofilms and the potential well filled with an electron gas, generation of a two-dimensional
low resistivity potential well can be expected.
[0006] It is necessary to form a two-dimensional electrically-conductive well having a
depth of several tens Å filled with an electron gas and to produce a structure in
which the two-dimensional conductive well is sandwiched between dielectric films.
[0007] The Z-type or A-type LB film can be used as a polarized film. It has large polarization
substantially the same as saturation polarization of a ferroelectric even if no voltage
is applied. In order to form a uniform potential well over a large area, it is necessary
to smooth a surface on which a film is deposited. To this end, a SiO₂ film on a silicon
wafer can be used. Also, Ta₂O₅ film, ZrO₂ film, glass substrate, and plastic substrate
can be used.
[0009] Fig. 1 is a schematic view showing an embodiment of the dielectric ultralow resistivity
heterofilm according to the present invention; Fig. 2 is a sectional view taken on
line II-II of Fig. 1; Fig. 3 is a schematic view of the resistance measurement of
the ultralow resistivity heterofilm according to the present invention; Fig. 4 is
a view showing the voltage drop characteristics of the ultralow resistivity heterofilm
according to the present invention; Fig. 5 is a view showing the voltage characteristics
of a sample in which the ultralow resistivity heterofilm according to the present
invention is not existing; Fig. 6 is a view showing the resistance characteristics
of the ultralow resistivity heterofilm according to the present invention; Fig. 7
is a view showing a comparison of the resistivity between the ultralow resistivity
heterofilm according to the present invention and metal; Fig. 8 is a view showing
the temperature characteristics of the ultralow resistivity heterofilm according to
the present invention; and Fig. 9 is a view showing the switching characteristics
of the ultralow resistivity heterofilm according to the present invention.
[0010] Next, referring to the accompanying drawings, description will be made as to examples
of the dielectric ultralow resistivity heterofilm according to the present invention.
[Example 1]
[0011] In this example, first, a thin evaporated film 3 of aluminum is formed on a silicon
wafer 1 having, on its surface, an insulating SiO₂ film 2 (thickness: about 5000Å)
as shown in Figs. 1 and 2. The thickness and width of the evaporated Al film 3 are
several hundreds Å and 10 mm respectively, and the resistance value thereof measured
across its opposite ends separated away by 30 mm from each other is about 600 Ω. Next,
the evaporated Al film 3 is coated, by an LB method, with an LB heterofilm 4 which
is composed of an arachidic acid LB film 4-a constituted by 4-6 single molecular layers
and an LB film 4-b of 2-pentadecyl-7,7′,8,8′ tetracyanoquinodimethane (C₁₅·TCNQ) constituted
by 4-6 single molecular layers. Further, the LB heterofilm 4 is coated with a thin
evaporated film 5 of gold so that the dielectric ultralow resistivity heterofilm having
a structure of [Al / LB heterofilm / Au] is formed with respect to the perpendicular
to the LB heterofilm 4 according to the present invention. In this ultralow resistivity
heterofilm, the evaporated Al film and Au film are short-circuited so that potentials
of the Al and Au films are maintained in equipotentially. Finally, nine gold electrodes
6 are evaporated on the Au film 5, as the measurement terminals. Here, the arachidic
acid LB film (non-polarized Y-type film) shows very small polarization, while the
LB film of C₁₅·TCNQ (polarized Z-type film) shows large polarization.
[0012] Fig. 3 shows a circuit for measuring resistivity by using a four-point probe technique.
A current is made to flow into/from a power source 8 through the outermost pair of
electrodes 6-1 and 6-9 of the nine gold electrodes formed, through evaporation, on
the dielectric ultra low resistivity heterofilm of the present invention, and a voltage
drop
v across another pair of electrodes 6-a and 6-b is measured by a voltmeter 10 to thereby
obtain the resistance value of the dielectric ultralow resistivity heterofilm across
the electrodes 6-a and 6-b. In this case, the internal resistance of the voltmeter
10 is sufficiently high, and therefore the voltage drop
v across the electrodes 6-a and 6-b can be accurately measured. An ammeter 9 measures
a current I flowing across the outermost electrodes 6-1 and 6-9. Since the insulating
SiO₂ film 2 is so thick to be 5000Å as to have a very good insulating property, the
current flowing across the electrodes 6-1 and 6-9 passes through the very thin evaporated
Al film 3, the LB heterofilm 4, and the evaporated Au film 5. The resistance value
R of the foregoing thin film across the electrodes 6-a and 6-b separated about 3.3
mm can be obtained through the following expression (1).
R = v / I (1)
[0013] Fig. 4 shows voltage drops among nine electrodes with currents of 0.16, 0.55 and
1.1 A as parameters, measured with respect to samples (Si-5L) of the dielectric ultralow
resistivity heterofilm using the LB heterofilm constituted by the arachidic acid LB
film and the C₁₅·TCNQ LB film each constituted by five single molecular layers (5L)
according to the present invention. As seen in the figure, across the electrodes 6-2
through 6-8, the voltage drop is generally small, and when the voltage drop across
adjacent electrodes is converted into a resistance value by using the expression (1),
the resistance value is about 10⁻² to 10⁻³Ω while the value varies slightly depending
on specific positions of the adjacent. electrodes. On the other hand, the voltage
drop is large across the electrodes 6-1 and 6-2 and across the electrodes 6-8 and
6-9, which may be caused by the contact resistance between the electrode and the LB
film.
[0014] Fig. 5 shows voltage drops measured with respect to samples (Si-0L) in which only
the LB heterofilm is eliminated from the sample shown in Figs. 1 and 2. The voltage
drop generated when each of three kinds of currents 1, that is, I=0.026A (0L-1), I=0.013A
(0L-2), and 1=0.0026 (0L-3), was made to flow was proportional to a distance from
an electrode 6-7 when the measurement was performed from the electrode 6-7. Across
the outermost electrodes 6-1 and 6-7, the resistance is about 28 Ω and the resistance
value across adjacent electrodes is 4.4 Ω. This resistance value is substantially
equal to that of the evaporated film of Al/Au just underneath the electrodes.
[0015] Fig. 6 shows a comparison of the resistance value across adjacent electrode terminals
obtained from the results of Figs. 4 and 5 between the Si-0L having no LB heterofilm
and Si-3L, Si-4L, and Si-5L each having the LB heterofilm. The resistance value is
reduced to 10⁻³ times only by interposition of the LB film having only a thickness
of 189Å (Si-3L), 252Å (Si-4L), or 315Å (Si-5L) between the Al and Au evaporated films.
This fact shows that the current passes in the inside of the surface of the very thin
LB heterofilm.
[0016] Since the thickness of the LB heterofilm is known, the resistivity of the LB heterofilm
can be obtained from the thickness, the width of the electrode, and the interval between
the electrodes. Fig. 7 shows the values of the resistivity plotted with respect to
the current flowing in the LB film. The values within a range of 10⁻⁸ to 10⁻⁹ Ωcm
were obtained, and each of the values was 10⁻³ ∼ 10⁻⁴ times the illustrated value
of metal (M) (about 10⁻⁵ Ωcm).
[0017] From the experiments described above, it has been found that the dielectric heterofilm
having the LB film according to the present invention has a resistance value much
lower than that of metal.
[Example 2]
[0018] The resistivity of the dielectric heterofilm constituted by the LB films according
to the present invention hardly changes in a range of from the room temperature to
about 80 °C. Fig. 8 shows an example as to the sample of Si-4L. The temperature of
the silicon substrate was measured a thermocouple. The resistivity is about 8.6 x
10⁻⁸ Ωcm (4L-1). The temperature rise is caused by heat generated from the sample
in a way of making an applied voltage high. Also, current values at various temperatures
are shown (4L-2). From this experiment, it can clearly be seen that no current passes
the silicon wafer of the substrate. This is because the resistivity of silicon rapidly
decreases with temperature and therefore if the current passes in the silicon wafer,
the resistivity cannot be kept constant as illustrated in the drawing but it must
decrease with the temperature.
[Example 3]
[0019] A current of about 1A is flowing in the LB ultralow resistivity heterofilm according
to the present invention as shown in Fig. 8, and if converted, the current value corresponds
to a current density having a large value of 400,000 A/cm². Further, at this time,
the temperature rises to 80 °C as shown in Fig. 8. This LB heterofilm, however, was
never damaged. Moreover, even if the applied voltage was further increased in order
to increase the current, a switching phenomenon as shown in Fig. 9 was caused to thereby
rapidly decrease the current, and the current did not increase more.
[0020] Fig. 9 shows an example of the switching phenomenon, and shows a current I (5L-8)
and a voltage drop (5L-9) between the adjacent electrode terminals 6-8 and 6-9, with
respect to an applied voltage V. As apparent form the drawing, the current rapidly
falls from 1.3A to 3 x 10⁻⁴A, and at the same time the voltage drop rises to 15.5V.
The voltage drop is substantially equal to a voltage applied from the power source
to the sample at this point of time. If the applied voltage is lowered, the current
rapidly increases again (at the point of the applied voltage of 2V) so as to return
to the original value. At the same time, also the resistance value decreases so as
to return to the original one. That is, as the applied voltage increases/decreases,
the current changes through a course of 0-a-b-c-d-a-0, and, on the other hand, the
voltage drop changes through a course of 0-e-f-b-g-h-a-e-0. Such a switching phenomenon
was generated even when the experiment was repeated again and again. Further, the
same phenomenon was observed also with respect to the sample of Si-4L.
[0021] Various applications in the future of the dielectric ultralow resistivity heterofilm
using the LB films according to the present invention can be considered. Finally,
the characteristics of the heterofilm according to the present invention are summarized
in Table 1. All the characteristics are values obtained at a temperature not lower
than room temperature.
Table 1
material |
Si-3L |
Si-4L |
Si-5L |
Thickness of LB heretofilm [Å] |
189 |
252 |
315 |
Resistance between two adjacent electrodes (width 10mm, distance 3.3mm) [Ω] |
0.024 |
0.012 |
0.0037 |
|
0.0004 |
0.001 |
Resistivity of LB heterfilm [Ωcm] |
1.5x10⁻⁷ |
8.5x10⁻⁸ |
3.9x10⁻⁸ |
|
3.4x10⁻⁹ |
1.0x10⁻⁸ |
Switching current [A] |
|
0.91 |
1.3 |
Maximum current dentisty [A/cm³] |
2.4x10⁴ |
3.6x10⁵ |
4.1x10⁵ |
1. An ultra-low resistivity heterofilm having a hetero-structure composed of polarized
(4-a) and non-polarized (4-b) dielectric films stacked on one another and sandwiched
between conductive films (3,5), the ultra-low resistivity heterofilm having a resistance
value not higher than that of metal in the direction of the film surface thereof at
a temperature not lower than room temperature.
2. A heterofilm as claimed in claim 1, wherein the conductive films (3,5) are short-circuited
to maintain their potentials thereof in equipotential.
3. A heterofilm as claimed in claim 1 or claim 2, wherein at least the polarized dielectric
film (4-a) is a Langmuir Blodgett's film of Z-type or A-type.
4. A heterofilm as claimed in any of claims 1 to 3, wherein at least the non-polarized
dielectric film (4-b) is a Langmuir Blodgett's film of Y-type.
5. A heterofilm as claimed in any of claims 1 to 4, wherein a potential well is generated
in the inside of the heterofilm and the potential well is filled with an electron
gas so as to form a two dimensional conductive plane.
6. A heterofilm as claimed in any of claims 1 to 5, the ultra-low resistivity heterofilm
being formed on an insulating film (2).
7. A heterofilm as claimed in claim 5, the insulating film is at least one of SiO₂
film, Ta₂O₅ film, ZrO₂ film, glass substrate, and plastics substrate.
8. A heterofilm comprising:
a silicon wafer (1) on which an insulating film (2) is formed;
a thin evaporated Al film (3) formed on the insulating film;
a thin heterofilm (4), formed on the thin evaporated Al film, composed of polarized
(4-a) and non-polarized (4-b) dielectric films stacked on one another; and
a thin evaporated Au film (5) being formed on the heterofilm.
9. A heterofilm as claimed in claim 8, wherein the Al film and Au film are short-circuited
to maintain their potentials thereof in equipotential.
10. A method of manufacturing an ultra-low resistivity heterofilm comprising the steps
of:
forming an insulating film (2) on a silicon wafer (1);
forming a thin evaporated Al film (3) on the insulating film;
forming a thin heterofilm (4) on the thin evaporated Al film, the thin heterofilm
being composed of polarized (4-a) and non-polarized (4-b) dielectric films stacked
on one another; and
forming a thin evaporated Au film (5) on the heterofilm.
11. A method as claimed in claim 10, further comprising the step of:
shorting the Al film (3) and Au (5) film to maintain the potentials thereof in equipotential.