Electroconductive thin film of organic charge transfer complexes of bisethylenedithiatetrathiafulvalene

An organic thin film and process for making the same, having electroconductivity, semiconductivity or superconductivity. The film is made of vapor-deposited bisethylenedithiatetrathiafulvalene (BEDT-TTF) by heating BEDT-TTF at a pressure of 10.sup.-2 Torr or below and at a temperature not higher than 260.degree. C. The temperature of the substrate on which the vapor is deposited is held at a lower temperature than the vapor. A thin film produced under these temperature and pressure conditions contains substantially no decomposition product. The electroconductivity of the film can be adjusted by selecting the substrate used for vapor-deposition of the film and the electron acceptor used as a dopant of the film. In order to achieve a vapor-deposited film with a high degree of orientation, silicon wafer is preferably used as a substrate for the film.

BACKGROUND OF THE INVENTION 
1. Field of the Industrial Utility 
The present invention relates to an organic film having semiconductivity. 
electroconductivity or even superconductivity. More particularly, the 
present invention relates to an electroconductive organic thin film made 
of a vapor-deposited bisethylene dithiatetrathiafulvalene (BEDT-TTF) film 
doped with an electron acceptor, as well as a process for producing such 
an organic thin film. 
Further, the present invention relates to a vapor-deposited BEDT-TTF film 
that can be used as an electroconductive organic thin film, as well as a 
process for producing such vapor-deposited film. 
2. Description of Related Art 
Organic thin films can be produced by various techniques such as a 
Langmuir-Blodgett (LB) method, vapor deposition, an ionized cluster beam 
(ICB) method and a molecular beam epitaxy (MBE) method. Using these 
methods, semiconductive or electroconductive organic thin films have been 
prepared from tetrathiofulvalene (TCNQ), 
tetrathiofluvalene-tetracyanoquinodimethane (TTF-TCNQ), 
metal-phthalocyanine, etc., and insulator films form aliphatic acids, etc. 
Because of their nature as electric conductors or semiconductors, organic 
thin films having electroconductivity have potential use in electronics, 
opto-electronics, energy conversion and other fields of application. In 
particular, organic thin films having high conductivity hold promise for 
use in many applications including micro-wiring, striplines, sensors, 
display devices, memories and switching devices. However, no organic thin 
films that have high electroconductivity like metals are yet to be 
realized. 
It has recently been proposed that an organic semiconductor or conductor 
film be prepared by a process that includes the steps of forming a film of 
an electron-donating organic compound such as a fulvalene containing a 
chalcogen atom in the molecule on an electrode substrate, and oxidizing 
said film by electrolysis (Unexamined Published Japanese Patent 
Application No. 289013/1989). According to a specific example disclosed in 
this prior patent, BEDT-TTF as an electron-donating organic compound was 
vacuum-deposited on a platinum-evaporated glass substrate, and gold was 
vacuum-deposited on the resulting BEDT-TTF film to form an electrode, and 
subsequently, the electrode was electrolytically oxidized with tetraethyl 
ammonium perchlorate used as an electrolyte, whereby a film having 
conductivity of 3.times.10.sup.-3 S/cm was obtained. 
However, this organic thin film and all others that are produced by the 
conventional methods have conductivity lower than that of metals. Further, 
the temperature dependency of the conductivity of prior art conductive 
organic thin films does not show a metallic behavior. 
Another problem with the prior art is that the melting point and the 
decomposition point of BEDT-TTF are so close to each other that it is 
extremely difficult to vaporize BEDT-TTF without generating decomposition 
products. If it were possible to prepare a vapor-deposited BEDT-TTF 
containing no decomposition products, an organic thin film having high 
conductivity could be produced by doping said BEDT-TTF film with an 
electron acceptor. However, no method has so far been proposed that is 
directed to forming a vapor-deposited BEDT-TTF film having such high 
quality and purity. Accordingly, there has been no report published on the 
possibility of using the vapor-deposited BEDT-TTF film to make an organic 
thin film that shows high conductivity or even superconductivity. 
SUMMARY OF THE INVENTION 
It is, therefore, an object of the present invention to provide an organic 
thin film having semiconductivity and electroconductivity, in particular, 
an organic thin film that shows high or metal-like conductivity and even 
superconductivity. 
Another object of the present invention is to provide a vapor-deposited 
BEDT-TTF film of high purity in which the generation of decomposition 
products is prevented or minimized. The structure of BEDT-TTF is given 
below as structure (I): 
##STR1## 
As a result of the intensive studies conducted to attain these objects, the 
present inventors found that a BEDT-TTF film could be formed on a 
substrate by heating BEDT-TTF at a temperature of up to 260.degree. C. in 
vacuo (10.sup.-2 to 10.sup.-3 Torr or even below) and that an 
electroconductive organic thin film could be obtained by doping the 
resulting vapor-deposited BEDT-TTF film with an electron acceptor. 
It has hitherto been held that the boiling point and the decomposition 
temperature of BEDT-TTF are so close to each other that upon heating, it 
hardly vaporizes but will simply decompose. However, to one's surprise, if 
BEDT-TTF is vapor-deposited in vacuo at a temperature not higher than 
260.degree. C. which is the decomposition point as determined from 
thermogravimetric (TG)-differential thermal analysis (DTA), preferably at 
a low temperature of 250.degree. C. or below, a satisfactory thin film of 
BEDT-TTF can be formed on a substrate without letting the BEDT-TTF 
decompose. Further, a vapor-deposited BEDT-TTF film that does not contain 
any decomposition product can be efficiently formed on a substrate in a 
tubular vapor-depositing vessel with a temperature gradient being created 
along the tube wall. 
While various conductive substrates can be employed, the present inventors 
found that a dense BEDT-TTF film having a high degree or orientation and 
that the major axes of the fine crystals of BEDT-TTF oriented 
perpendicular to the substrate surface could be obtained by using a 
silicon substrate. 
Further, by combining this film forming step with the step of doping with 
an electron acceptor, an organic thin film having semiconductivity or 
electroconductivity or even superconductivity can be obtained, if the 
doping method, the dopant concentration and the dopant type are properly 
selected. 
The present invention has been accomplished on the basis of these findings. 
Thus, in accordance with the present invention, there is provided an 
electroconductive organic thin film that is made of a vapor-deposited 
bipethylenedithiatetrathiafulvalene (BEDT-TTF) film doped with an electron 
acceptor. 
Also provided in accordance with the present invention is a process for 
producing an electroconductive organic thin film, which process comprises 
the steps of heating/BEDT-TTF to a predetermined temperature of up to 
260.degree. C. under vacuum, forming a BEDT-TTF film on a substrate held 
at a temperature less than the temperature to which said BEDT-TTF is 
heated, and doping the BEDT-TTF film with an electron acceptor. 
Further, in accordance with the present invention, there are provided a 
process for producing a vapor-deposited BEDT-TTF film, which process 
comprises the steps of heating BEDT-TTF to a predetermined temperature of 
up to 260.degree. C. under vacuum, and forming a BEDT-TTF film on a 
substrate held at a temperature less than the temperature to which said 
BEDT-TTF is heated, as well as the vapor deposited BEDT-TTF film formed by 
said process. 
Other objects, features and characteristics of the present invention, as 
well as the methods of operation and function of the related elements of 
the structure, and the combination of parts and economies of manufacture, 
will become more apparent upon consideration of the following detailed 
description and the appended claims with reference to the accompanying 
drawings all of which form a part of this specification.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
According to the general procedure of vacuum deposition, a sample in a 
vacuum enclosure held in high vacuum is heated to vaporize and its 
molecules are allowed to condense without involving any chemical reaction 
to form a vapor-deposited film on a substrate held at a predetermined 
temperature. Because of its operating principle, vacuum deposition is 
applied to materials that are solid at ordinary temperatures and pressures 
and that will not decompose upon heating. 
As is apparent from the results of thermogravimetric (TG)-differential 
thermal analysis (DTA) shown in FIG. 1, it has been held that BEDT-TTF has 
a boiling point (Tm=258.degree. C.) and a decomposition temperature 
(260.degree. C.) which are so close to each other that upon heating it 
hardly vaporizes but will simply decompose. As a matter of fact, if one 
attempts to form a vapor-deposited BEDT-TTF film on a substrate by the 
usual vacuum deposition technique, a volatile decomposition product will 
be generated and incorporated in the resulting vapor-deposited film. Even 
if such a film is doped with an electron acceptor, no practical conductive 
film can be formed. 
However, to one's surprise, if BEDT-TTF is heated under vacuum from room 
temperature to a predetermined temperature of up to 260.degree. C. and if 
a BEDT-TTF film is formed on a substrate held at a temperature lower than 
the temperature to which the BEDT-TTF is heated, a vapor-deposited 
BEDT-TTF film with a reduced content of the decomposition product can be 
obtained. 
In accordance with the present invention, a thin BEDT-TTF film can be 
formed with an ordinary thermal deposition apparatus, preferably a tubular 
vessel, by a vacuum deposition method or MBE method in which the thin-film 
forming material is heated to vaporize in high vacuum and the vapor is 
condensed on a substrate to form a thin film. 
Stated more specifically, BEDT-TTF is heated in vacuo at a pressure of 
10.sup.-2 Torr or below, preferably at 10.sup.-3 to 10.sup.-10 Torr, more 
preferably in high vacuum at a pressure of 10.sup.-5 to 10.sup.-10 Torr, 
and at a temperature of up to 260.degree. C., preferably between 
180.degree. and 250.degree. C., whereby a vapor-deposited BEDT-TTF film is 
formed on a substrate. 
If the heating temperature exceeds 260.degree. C., BEDT-TTF will decompose. 
The temperature at which the starting BEDT-TTF is heated can be made lower 
than 260.degree. C. by reducing the pressure in the system below 10.sup.-5 
Torr. For instance, if vapor deposition is performed in a high-vacuum 
atmosphere at a pressure of about 10.sup.-10 Torr, the temperature at 
which the starting BEDT-TTF is heated can be lowered to about 
180.degree.-200.degree. C. 
Further, a vapor-deposited BEDT-TTF film containing no decomposition 
products can be obtained efficiently by carrying out the above-described 
procedure in a tubular vapor-depositing vessel, with a temperature 
gradient being created along the tube wall. 
As shown specifically in FIG. 2, the fine crystals or powder of BEDT-TTF in 
a feed cell 3 is put into a tubular vapor-depositing vessel (vacuum 
enclosure) 1. Substances 1-5 are placed in the vessel 1 along the length 
of the tube wall at given distances from the feed cell 3. The vessel 1 is 
set horizontally within a tubular furnace 2. 
After evacuating the vessel, the feed cell is heated to a predetermined 
temperature less than 260.degree. C. and held at that temperature for a 
given period. The thickness of vapor-deposited film can be adjusted by 
controlling such factors as the distance between the feed cell and each 
substrate, the temperature to which the feed cell is heated, the time for 
which said temperature is maintained, and the degree of vacuum. 
The distance between the feed cell and each substrate and the temperature 
at which the substrates are heated are two important factors for obtaining 
a vapor-deposited BEDT-TTF film of good quality that does not contain 
decomposition products. If the distance between the feed cell and each of 
the substrates increases to lower the substrate temperature, a yellow 
decomposition product is likely to be deposited on the substrates. A 
vapor-deposited BEDT-TTF film of good quality has a red to orange color 
and can be readily identified with the eye. 
Therefore, vapor-deposited BEDT-TTF film of good quality can be efficiently 
produced if the distance to each substrate and the substrate temperature 
that insure the formation of an orange-colored vapor-deposited film are 
determined by preliminary testing with the apparatus shown in FIG. 2 in 
connection with the temperature at which the feed cell is heated and the 
temperature gradient along the wall of the tubular vessel. FIG. 3 shows a 
typical temperature profile along the wall of the quartz tube in the 
vapor-deposited film forming apparatus shown in FIG. 2, and one can see 
from this graph that under the conditions of vapor deposition used in 
Example 1 to be described below, an orange-colored thin film of good 
quality that did not contain decomposition products could be formed on 
substrates 1-3 which were placed at distances in the range of 5-15 cm from 
the feed cell. 
There is no particular limitation on the substrate that can be used in the 
present invention and illustrative examples include substrates made of ITO 
glass, NESA glass, silicon, electroconductive polymer films or thin films, 
conductive LB films, carbon, graphite, etc. Among these, silicon 
substrates (silicon wafers) made of (100) silicon (n- or p-type) and (111) 
silicon (n- or p-type) are preferred since, their use is effective in 
producing a vapor-deposited film characterized by a significant 
improvement in the orientation of the BEDT-TTF crystals. 
Vapor-deposited BEDT-TTF films are usually obtained as films in which the 
micro crystallites of BEDT-TTF are randomly oriented and agglomerated, and 
even if they are oxidized or doped electrolytically, the molecular 
orientation in the crystal still remains random and the failure to achieve 
high conductivity causes substantial difficulty in obtaining a 
superconductive film having small residual resistance. FIG. 5 is a 
scanning electron microscopic (SEM) picture of a cross section of 
vapor-deposited BEDT-TTF film formed on an indium oxide coated glass 
substrate, and it clearly shows the tendency of random orientation in the 
crystalline structure of the film. 
It was found that the orientation of the BEDT-TTF crystallites was markedly 
improved when a silicon wafer was used as the substrate compared to when 
oxide conductor coated glass substrates such as ITO glass and NESA glass 
were used. It was also found that the vapor-deposited BEDT-TTF film 
prepared with a silicon wafer used as a substrate had the major axes of 
the micro crystallites of BEDT-TTF oriented perpendicular to the substrate 
surface. FIG. 4 is a SEM picture of a cross section of a vapor-deposited 
BEDT-TTF film formed on a silicon wafer substrate, and the film is 
obviously a dense film having the major axes of the fine crystals of 
BEDT-TTF oriented perpendicular to the substrate surface. The thus 
obtained vapor-deposited BEDT-TTF film having a high degree of orientation 
is morphologically preferred for doping with an electron acceptor. 
The vapor-deposited BEDT-TTF film produced by the process of the present 
invention is an insulator per se but an electroconductive or even 
superconducting organic thin film can be obtained if this BEDT-TTf film is 
doped with an electron acceptor by a suitable method such as a vapor-phase 
method, a liquid-phase method, an electrochemical method or an 
ion-implantation method. 
The process for producing an electroconductive organic thin film in 
accordance with the present invention comprises the following two basic 
steps: 
(1) heating BEDT-TTF in vacuo at a pressure of 10.sup.-2 Torr or below at a 
temperature not higher than 260.degree. C. so as to form a vapor-deposited 
BEDT-TTF film on a substrate; and 
(2) doping the vapor-deposited BEDT-TTF film with an electron acceptor. 
Steps (1) and (2) are usually performed consecutively but if the doping 
method adopted permits, an electron acceptor may be doped as the 
vapor-deposited film is formed. 
Examples of the electron acceptor that may be used in the present invention 
include: trihalide anions such as I.sub.3.sup.-, Br.sub.3.sup.-, 
IBr.sub.2.sup.-, ICl.sub.2.sup.- and I.sub.2 Br.sup.- that are represented 
by X.sub.3.sup.- (X is a halogen atom); anions that are represented by 
MX.sub.2.sup.- (M is a metal atom; and X is a halogen atom or a pseudo 
halide such as AuI.sub.2.sup.-, AuIB.sub.r.sup.-, AuBr.sub.2.sup.-, 
Cu(NCS).sub.2.sup.-, Ag(NCS).sub.2.sup.-, Au(NCS).sub.2.sup.-, 
Cu(NCSe).sub.2.sup.-, Cu(NCO).sub.2.sup.- and Au(CN).sub.2 ; and other 
anions such as NO.sub.3.sup.-, BF.sub.4.sup.-, ClO.sub.4.sup.-, 
ReO.sub.4.sup.- and PF.sub.4.sup.-. Among these anions, X.sub.3.sup.- and 
MX.sub.2.sup.- are particularly preferred for the purpose of obtaining an 
organic superconducting thin film. 
By doping the vapor-deposited BEDT-TTF film with these electron acceptors, 
an organic thin film that shows conductivity or superconductivity can be 
obtained. 
While electron acceptors can be doped by various methods including an 
electrochemical method and a vapor-phase method, the use of an 
electrochemical method is preferred. Doping by an electrochemical method 
can be performed in the usual manner with a known electrochemical 
crystallizing apparatus. 
FIGS. 6a and 6b show a vapor-deposited BEDT-TTF film 61 that is produced on 
a substrate made of an indium oxide layer 62 coated on glass 63. This 
vapor-deposited BEDT-TTF film can advantageously be doped with an electron 
acceptor by an electrochemical method using a reactor 71 (see FIG. 7) for 
preparing an electron acceptor containing electrolytic solution 72 and a 
reactor (see FIG. 8) for performing doping with an electron acceptor. In 
the reactor shown in FIG. 8, an electroconductive substrate 85 having a 
vapor-deposited thin BEDT-TTF film is retained on the anode side and a 
platinum plate 86 is used as cathode, with the liquid electrolyte being a 
supporting electrolyte that contains one or more of the anions listed 
above an which is dissolved in an organic solvent solution that has strong 
polarity and that is resistant to oxidation and reduction. 
Electroconductivity can be adjusted by properly selecting the type of 
anion and the degree of doping. 
No report has heretofore been published on the preparation of 
vapor-deposited BEDT-TTF films of high quality that are free from the 
generation of decomposition products. This is because it has been held 
that BEDT-TTF has a melting point and a decomposition temperature that are 
so close to each other that it cannot be vaporized without decomposition. 
However, a satisfactory thin BEDT-TTF film can be formed on a substrate by 
heating BEDT-TTF in vacuo at a temperature of up to 260.degree. C. In this 
case, a vapor-deposited BEDT-TTF film having a high degree of orientation 
can be obtained by using a silicon substrate. 
Further, by properly selecting process parameters such as the type of 
electron acceptor used as a dopant, the electroconductivity of the thin 
film to be obtained can be adjusted in such a way as to produce a 
conductive organic thin film that shows metal-like properties. 
EXAMPLES 
The following examples are provided for the purpose of further illustrating 
the present invention but are in no way to be taken as limiting. 
EXAMPLE 1 
Five glass plates (10.times.20.times.1 mm) coated with indium oxide 
(In.sub.2 O.sub.3) were used as substrates for depositing a thin BEDT-TTF 
film on each of these substrates by the following procedure. 
As shown in FIG. 2, a feed cell 3 charged with 50 mg of a BEDT-TTF powder 
and the five In.sub.2 O.sub.3 coated glass substrates 1-5 were placed in a 
quartz reaction tube 1, which in turn was set in a two-zone tubular 
furnace 2. The quartz tube was evacuated with a vacuum pump capable of 
attaining an ultimate pressure of 10.sup.-5 Torr. The feed cell was heated 
from room temperature to 250.degree. C. and held at 250.degree. C. for 30 
min. The resulting temperature profile in the area of the reaction tube 
where the substrates were placed was as shown in FIG. 3. 
After cooling the tubular furnace 2 to room temperature, the quartz tube 
was taken out of the furnace and checked for the color and morphology of 
the vapor-deposited film on each of the substrates. An orange-colored thin 
film of good quality that did not contain decomposition products was 
obtained on substrates 1-3 that were placed at distances in the range of 
5-15 cm from the feed cell, whereas a yellow thin film containing 
decomposition products was obtained on substrates 4 and 5 that were placed 
at distances in the range of from 15 to 20 cm from the feed cell. Since 
the FTIR spectrum of each orange-colored vapor-deposited film was in 
agreement with that of the starting BEDT-TTF powder as shown in FIG. 9, 
this film was identified as being composed of BEDT-TTF. 
Comparison between an X-ray diffraction of the orange-colored 
vapor-deposited film (FIG. 10) and that of the starting BEDT-TTF powder 
(FIG. 11) shows that the vapor-deposited film had an improved degree of 
orientation. Observations with an optical microscope and a scanning 
electron microscope (SEM) showed that the orange-colored vapor-deposited 
film was composed of agglomerated micro crystallites of BEDT-TTF and that 
it had good quality in terms of the absence of pinholes. 
Similar experiments were conducted to prepare vapor deposited BEDT-TTF 
films using NESA glass or an ITO-coated polyester film as a substrate in 
place of the indium oxide coated glass substrate. Again, an orange-colored 
vapor-deposited BEDT-TTF film of good quality was obtained on each of the 
substrates placed at positions 1, 2 and 3 in FIG. 2. 
EXAMPLE 2 
A feed cell charged with 30 mg of BEDT-TTF and a substrate in the form of 
n-type silicon (100) wafer (resistance, 0.01-0.02.OMEGA..cm) that measured 
1.times.3 cm were placed in a quartz reaction tube in such a way that the 
distance between the feed cell and the substrate was 10 cm. The reaction 
tube was then set in a two-zone tubular furnace. 
The quartz tube was degassed and evacuated to a pressure of 10.sup.-3 Torr 
and thereafter, the feed cell was heated up to 250.degree. C. at a rate of 
ca. 2-3.degree. C./min and held at 250.degree. C. for 30 min. The 
temperature of the substrate was 175.degree. C. After cooling the tubular 
furnace to room temperature by standing, the substrate was taken out of 
the furnace and found to have a light brown vapor-deposited film thereon. 
Similar results were obtained when a p-type silicon (100) wafer, a p-type 
silicon (111) wafer and an n-type silicon (111) wafer were used as 
substrates. 
The FTIR spectrum of these thin films were in agreement with that of the 
starting BEDT-TTF powder, verifying the formation of the desired 
vapor-deposited film of BEDT-TTF without its decomposition. Observation 
with an optical microscope and a scanning electron microscope (SEM) showed 
that each of the vapor-deposited films was composed of agglomerated micro 
crystallites of BEDT-TTF and that they had satisfactory quality in terms 
of the absence of pinholes. 
FIG. 4 is a SEM picture of a cross section of one of the vapor-deposited 
BEDT-TTF films obtained in Example 2. The lower layer (black portion) in 
FIG. 4 is part of the substrate and the central layer (white portion) is 
part of the vapor-deposited BEDT-TTF film. The white line on the right 
side of FIG. 4 represents a scale and its length corresponds to 10 .mu.m. 
One can see from this SEM picture that the vapor-deposited film was a 
dense film having the major axes of the fine crystals of BEDT-TTF oriented 
perpendicular to the substrate surface. The thickness of this film was as 
4-5 .mu.m. 
EXAMPLE 3 
A vapor-deposited BEDT-TTF film was prepared as in Example 2 except that 
the substrate was an indium oxide coated glass plate (10.times.20.times.1 
mm). FIG. 5 is a SEM picture of a cross section of the vapor-deposited 
BEDT-TTF film obtained in Example 3. The lower layer (black portion) in 
FIG. 5 is part of the substrate and the central layer (white portion) is 
part of the vapor-deposited BEDT-TTF film. As is clear from FIG. 5, 
compared to the thin films obtained in Example 2 using a silicon wafer as 
the substrate, the vapor-deposited BEDT-TTF film obtained in Example 3 
using the indium oxide coated glass substrate was rather random in 
orientation and somewhat less dense in structure. The film thickness was 
found to be within the range of 5-7 .mu.m. 
EXAMPLE 4 
As in Example 1, an In.sub.2 O.sub.3 coated glass substrate was placed in a 
quartz reaction tube in such a way that the distance between the feed cell 
and the substrate was 10 cm. The reaction tube was then set in a two-zone 
tubular furnace. 
After evacuating the reaction tube to 10.sup.-5 Torr, the feed cell was 
heated to 250.degree. C. and held at that temperature for 30 min. The 
substrate temperature was 175.degree. C. As a result, an orange-colored 
thin film could be formed on the substrate. The thickness of this film was 
2-3 .mu.m. 
Using a reactor shown by 71 in FIG. 7, 70 mg of CuSCN, 125 mg of KSCN and 
210 mg of 18-crown-6 were dissolved in 100 ml of ethanol in a nitrogen gas 
atmosphere to thereby prepare a liquid electrolyte 72. As shown in FIG. 8, 
the substrate 85 having the vapor-deposited BEDT-TTF film was retained 
with a metal clip 84 on the anode side and a do constant current of 10 
.mu.A was allowed to flow between the two electrodes 81 and 82 in a 
thermostatic chamber (not shown) held at 20.degree. C. About 10 h later, 
the vapor-deposited film began to turn from orange to black, indicating 
the occurrence of a charge transfer reaction. After a 24 hour reaction, 
the substrate was recovered, washed with ethanol and dried. 
An X-ray diffraction of the resulting black thin film is shown in FIG. 12, 
from which two kinds of peaks are identified, one assignable to 
.kappa.-(BEDT-TTF).sub.2 Cu(NCS).sub.2 and the other assignable to 
unreacted BEDT-TTF. 
When the dc magnetization of the black thin film was measured, the Meissner 
effect was evident at 8K and this also verifies that the film was made of 
.kappa.-(BEDT-TTF).sub.2 Cu(NCS).sub.2. The black thin film had a 
conductivity of 0.1 S/cm at room temperature. 
EXAMPLE 5 
The highly oriented, vapor-deposited BEDT-TTF film that was prepared in 
Example 2 using an n-type silicon (100) wafer as a substrate was doped 
with an electron acceptor by an electrolytic method. As in Example 4, 70 
mg of CuSCN, 126 mg of KSCN and 210 mg of 18-crown-6 were dissolved in 100 
ml of ethanol in a nitrogen gas atmosphere in a reactor shown by 71 in 
FIG. 7, whereby a liquid electrolyte 72 was prepared. Subsequently, as 
shown in FIG. 8, the silicon wafer substrate 85 having the vapor-deposited 
BEDT-TTF film was retained with a metal clip 84 on the anode side and a dc 
constant current of 10 .mu.A was allowed to flow between the two 
electrodes 81 and 82 in a thermostatic chamber (not shown) held at 
20.degree. C. After 5-day electrolytic doping, the substrate with the 
doped film was recovered, washed by dipping in ethanol and dried. An X-ray 
diffraction of the dried film shown in FIG. 13. exhibits distinct (h00) 
peaks that are only assignable to .kappa.-(BEDT-TTF).sub.2 Cu(NCS).sub. 2. 
When the dc magnetization of the resulting Cu(NCS).sub.2.sup.- doped 
BEDT-TTF film was measured, the Meissner effect was evident at 9K. This 
thin film had a conductivity of 10 S/cm at room temperature. 
EXAMPLE 6 
The vapor-deposited BEDT-TTF film that was prepared in Example 3 using an 
indium oxide coated glass substrate was doped with an electron acceptor by 
an electrolytic method as in Example 5 so as to form a Cu(NCS).sub.2.sup.- 
doped BEDT-TTF film on the substrate. 
When the dc magnetization of the resulting Cu(NCS).sub.2.sup.- doped 
BEDT-TTF film was measured, the Meissner effect was evident at 7K. This 
film had a lower superconducting transition temperature (Tc) than the 
conductive organic thin film prepared in Example 5, which may be 
ascribable to the relatively low orientation and density of the 
vapor-deposited film. The thin film obtained in Example 6 had a 
conductivity of 0.1 S/cm at room temperature. 
EXAMPLE 7 
A silicon wafer substrate having an I.sub.3.sup.- doped BEDT-TTF film was 
obtained by performing electrochemical doping as in Example 5 except that 
243 mg of tetrabutyl ammonium triiodide in 100 ml of ethanol was used as 
an electrolyte. When the dc magnetization of resulting I.sub.3.sup.- doped 
BEDT-TTF film was measured after annealing at the temperature of liquid 
nitrogen, the Meissner effect was evident at 8K. This film had a 
conductivity of 1 S/cm at room temperature. 
EXAMPLE 8 
As in Example 4, a vapor-deposited BEDT-TTF film was formed on an In.sub.2 
O.sub.3 coated glass substrate and the film was subsequently doped with 
iodine by an oxidation method. The resulting doped thin film had a 
conductivity of 10 S/cm at room temperature, which increased to 10.sup.3 
S/cm at the temperature of liquid nitrogen. 
EXAMPLE 9 
A vapor-deposited BEDT-TTF film was formed on a substrate as in Example 4 
except that the substrate was ITO glass. Subsequently, the film was doped 
with iodine by electrolytic oxidation. The resulting doped film had a 
conductivity of 200 S/cm at room temperature which increased to 10.sup.4 
S/cm at the temperature of liquid nitrogen. 
While the invention has been described in connection with what is presently 
considered to be the most practical and preferred embodiment, it is to be 
understood that the invention is not limited to the disclosed embodiment, 
but, on the contrary, is intended to cover various modifications and 
equivalent arrangements included within the spirit and scope of the 
appended claims.