U.S. patent application number 16/027733 was filed with the patent office on 2018-11-08 for lcd and organic el display.
The applicant listed for this patent is CANON KABUSHIKI KAISHA, Japan Science and Technology Agency, TOKYO INSTITUTE OF TECHNOLOGY. Invention is credited to Masahiro HIRANO, Hideo HOSONO, Toshio KAMIYA, Kenji NOMURA, Hiromichi OTA.
Application Number | 20180323313 16/027733 |
Document ID | / |
Family ID | 34975867 |
Filed Date | 2018-11-08 |
United States Patent
Application |
20180323313 |
Kind Code |
A1 |
HOSONO; Hideo ; et
al. |
November 8, 2018 |
LCD AND ORGANIC EL DISPLAY
Abstract
A switching element of LCDs or organic EL displays which uses a
thin film transistor device, includes: a drain electrode, a source
electrode, a channel layer contacting the drain electrode and the
source electrode, wherein the channel layer comprises
indium-gallium-zinc oxide having a transparent, amorphous state of
a composition equivalent to InGaO.sub.3(ZnO).sub.m (wherein m is a
natural number less than 6) in a crystallized state, and the
channel layer has a semi-insulating property represented by an
electron mobility of more than 1 cm.sup.2/(Vsec) and an electron
carrier concentration is less than 10.sup.18/cm.sup.3, a gate
electrode, and a gate insulating film positioned between the gate
electrode and the channel layer.
Inventors: |
HOSONO; Hideo; (Kanagawa,
JP) ; HIRANO; Masahiro; (Tokyo, JP) ; OTA;
Hiromichi; (Aichi, JP) ; KAMIYA; Toshio;
(Kawasaki-shi, JP) ; NOMURA; Kenji; (Tokyo,
JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Japan Science and Technology Agency
CANON KABUSHIKI KAISHA
TOKYO INSTITUTE OF TECHNOLOGY |
Kawaguchi-Shi
Tokyo
Tokyo |
|
JP
JP
JP |
|
|
Family ID: |
34975867 |
Appl. No.: |
16/027733 |
Filed: |
July 5, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13243244 |
Sep 23, 2011 |
10032931 |
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16027733 |
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12504116 |
Jul 16, 2009 |
10032930 |
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13243244 |
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10592431 |
Sep 11, 2006 |
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PCT/JP2005/003273 |
Feb 28, 2005 |
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12504116 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 29/78693 20130101;
C23C 14/28 20130101; C23C 14/3414 20130101; H01L 21/02565 20130101;
H01L 21/02631 20130101; C23C 14/086 20130101; H01L 21/02554
20130101; H01L 29/78696 20130101; C23C 14/0021 20130101; H01L
29/7869 20130101; H01L 27/1225 20130101 |
International
Class: |
H01L 29/786 20060101
H01L029/786; H01L 27/12 20060101 H01L027/12; H01L 21/02 20060101
H01L021/02; C23C 14/00 20060101 C23C014/00; C23C 14/34 20060101
C23C014/34; C23C 14/28 20060101 C23C014/28; C23C 14/08 20060101
C23C014/08 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 12, 2004 |
JP |
2004-071477 |
Nov 10, 2004 |
JP |
2004-325938 |
Claims
1. A Liquid Crystal Display (LCD) including: a substrate; and
switching elements formed on the substrate; wherein the substrate
is one of a glass substrate, a plastic substrate or a plastic film,
and wherein each of the switching elements comprises a thin film
transistor comprising: a gate terminal formed over the substrate; a
gate insulating film formed on the gate terminal; an amorphous
oxide channel layer formed on the gate insulating film, the
amorphous oxide channel layer including In, Ga, Zn and O; a drain
terminal formed on the amorphous oxide channel layer, the drain
terminal partially overlapping with the gate terminal; and a source
terminal formed on the amorphous oxide channel layer and apart from
the drain terminal, the source terminal partially overlapping with
the gate terminal.
2. The LCD according to claim 1, wherein the switching element is
operable in a normally off type.
3. The LCD according to claim 1, wherein the amorphous oxide
channel layer is formed such that an electron mobility of the
amorphous oxide channel layer increases with an electron carrier
concentration of the amorphous oxide channel layer.
4. The LCD according to claim 1, wherein each thin film transistor
further comprises a top-gate structure.
5. The LCD according to claim 1, wherein each thin film transistor
further comprises a bottom-gate structure.
6. The LCD according to claim 1, wherein each of the drain terminal
and the source terminal comprises a first layer contacting the
amorphous oxide channel layer and a second layer formed on the
first layer, the second layer comprising metal.
7. An organic Electroluminescence (EL) display including: a
substrate; and switching elements formed on the substrate; wherein
the substrate is one of a glass substrate, a plastic substrate or a
plastic film, and wherein each of the switching elements comprises
a thin film transistor comprising: a gate terminal formed over the
substrate; a gate insulating film formed on the gate terminal; an
amorphous oxide channel layer formed on the gate insulating film,
the amorphous oxide channel layer including In, Ga, Zn and O; a
drain terminal formed on the amorphous oxide channel layer, the
drain terminal partially overlapping with the gate terminal; and a
source terminal formed on the amorphous oxide channel layer and
apart from the drain terminal, the source terminal partially
overlapping with the gate terminal.
8. The organic EL display according to claim 7, wherein the
switching element is operable in a normally off type.
9. The organic EL display according to claim 7, wherein the
amorphous oxide channel layer is formed such that an electron
mobility of the amorphous oxide channel layer increases with an
electron carrier concentration of the amorphous oxide channel
layer.
10. The organic EL display according to claim 7, wherein each thin
film transistor further comprises a top-gate structure.
11. The organic EL display according to claim 7, wherein the thin
film transistor further comprises a bottom-gate structure.
12. The organic EL display according to claim 7, wherein each of
the drain terminal and the source terminal comprises a first layer
contacting the amorphous oxide channel layer and a second layer
formed on the first layer, the second layer comprises metal.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 13/243,244 filed Sep. 23, 2011, which is a
divisional application of U.S. patent application Ser. No.
12/504,116 filed on Jul. 16, 2009, which is a divisional
application of U.S. patent application Ser. No. 10/592,431, filed
on Sep. 11, 2006, now abandoned, which is a 371 of International
Application No. PCT/JP05/03273, filed on Feb. 28, 2005, which
claims the benefit of priority from the prior Japanese Patent
Application Nos. 2004-071477, filed on Mar. 12, 2004 and
2004-325938 filed on Nov. 10, 2004, the entire contents of all
which are incorporated herein by reference.
BACKGROUND
Technical Field
[0002] The present invention relates to amorphous oxides and thin
film transistors.
Background Art
[0003] A thin film transistor (TFT) is a three-terminal element
having a gate terminal, a source terminal, and a drain terminal. It
is an active element in which a semiconductor thin film deposited
on a substrate is used as a channel layer for transportation of
electrons or holes and a voltage is applied to the gate terminal to
control the current flowing in the channel layer and switch the
current between the source terminal and the drain terminal.
Currently, the most widely used TFTs are
metal-insulator-semiconductor field effect transistors (MIS-FETs)
in which the channel layer is composed of a polysilicon or
amorphous silicon film.
[0004] Recently, development of TFTs in which ZnO-based transparent
conductive oxide polycrystalline thin films are used as the channel
layers has been actively pursued (Patent Document 1). These thin
films can be formed at low temperatures and is transparent in
visible light; thus, flexible, transparent TFTs can be formed on
substrates such as plastic boards and films.
[0005] However, known ZnO rarely forms a stable amorphous phase at
room temperature and mostly exhibits polycrystalline phase;
therefore, the electron mobility cannot be increased because of the
diffusion at the interfaces of polycrystalline grains. Moreover,
ZnO tends to contain oxygen defects and a large number of carrier
electrons, and it is thus difficult to decrease the electrical
conductivity. Therefore, it has been difficult to increase the
on/off ratio of the transistors.
[0006] Patent Document 2 discloses an amorphous oxide represented
by Zn.sub.xM.sub.yIn.sub.zO.sub.(x+3y/2+3z/2) (wherein M is at
least one element selected from Al and Ga, the ratio x/y is in the
range of 0.2 to 12, and the ratio z/y is in the range of 0.4 to
1.4). However, the electron carrier concentration of the amorphous
oxide film obtained herein is 10.sup.18/cm.sup.3 or more. Although
this is sufficient for regular transparent electrodes, the film
cannot be easily applied to a channel layer of a TFT. This is
because it has been found that a TFT having a channel layer
composed of this amorphous oxide film does not exhibit a sufficient
on/off ratio and is thus unsuitable for TFT of a normally off type.
[0007] Patent Document 1: Japanese Unexamined Patent Application
Publication No. 2003-298062 [0008] Patent Document 2: Japanese
Unexamined Patent Application Publication No. 2000-044236
SUMMARY
[0009] An object of the present invention is to provide an
amorphous oxide having a low electron carrier concentration and to
provide a thin film transistor having a channel layer composed of
such an amorphous oxide.
[0010] The present invention provides: (1) an amorphous oxide
having an electron carrier concentration less than
10.sup.18/cm.sup.3. In the present invention, the electron carrier
concentration of the amorphous oxide is preferably
10.sup.17/cm.sup.3 or less or 10.sup.16/cm.sup.3 or less.
[0011] The present invention also provides: (2) an amorphous oxide
in which electron mobility thereof increases with the electron
carrier concentration.
[0012] The present invention also provides: (3) the amorphous oxide
according to item (1) or (2) above, in which the electron mobility
is more than 0.1 cm.sup.2/(Vsec).
[0013] The present invention also provides: (4) the amorphous oxide
according to item (2) or (3) above, exhibiting degenerate
conduction. Note that "degenerate conduction" used herein is
defined as a state in which the thermal activation energy for
temperature dependency of electrical resistance is 30 meV or
less.
[0014] Another aspect of the present invention provides: (5) the
amorphous oxide according to any one of items (1) to (4) above, in
which the amorphous oxide is a compound that contains at least one
element selected from Zn, In, and Sn as a constituent and is
represented by
[(Sn.sub.1-xM4.sub.x)O.sub.2]a[In.sub.1-yM3.sub.y).sub.2O.sub.3]b.[(Zn.su-
b.1-zM2.sub.z)O]c (wherein 0.ltoreq.x.ltoreq.1,
0.ltoreq.y.ltoreq.1, 0.ltoreq.z.ltoreq.1; x, y, and z are not
simultaneously 1; 0.ltoreq.a.ltoreq.1, 0.ltoreq.b.ltoreq.1,
0.ltoreq.c.ltoreq.1, and a+b+c=1; M4 is a group IV element (Si, Ge,
or Zr) having an atomic number smaller than that of Sn; M3 is Lu or
a group III element (B, Al, Ga, or Y) having an atomic number
smaller than that of In; and M2 is a group II element (Mg or Ca)
having an atomic number smaller than that of Zn).
[0015] In the present invention, the amorphous oxide according (5)
above may further contain at least one element selected from group
V elements (V, Nb, and Ta) M5 and W.
[0016] Another aspect of the present invention provides: (6) a thin
film transistor including the amorphous oxide according to any one
of (1) to (4) above, in which the amorphous oxide is a single
compound represented by [(In.sub.1-yM3.sub.y).sub.2O.sub.3]
[(Zn.sub.1-xM2.sub.x)O].sub.m (wherein 0.ltoreq.x.ltoreq.1;
0.ltoreq.y.ltoreq.1; x and y are not simultaneously 1; m is zero or
a natural number less than 6; M3 is Lu or a group III element (B,
Al, Ga, or Y) having an atomic number smaller than that of In; and
M2 (Mg or Ca) is a group II element having an atomic number smaller
than that of Zn) in a crystallized state or a mixture of the
compounds with different values of m. M3 is, for example, Ga, and
M2 is, for example Mg.
[0017] The present invention also provides the amorphous oxide
according to any one of (1) to (6) above formed on a glass
substrate, a metal substrate, a plastic substrate, or a plastic
film. The present invention also provides a field effect transistor
including a channel layer composed of the amorphous oxide described
above. The field effect transistor of the present invention is
characterized in that the gate insulating film is one of
Al.sub.2O.sub.3, Y.sub.2O.sub.3, and HfO.sub.2 or a mixed crystal
compound containing at least two of these compounds.
[0018] Another aspect of the present invention provides: (7) a
transparent semi-insulating amorphous oxide thin film comprising
In--Ga--Zn--O, in which the composition in a crystallized state is
represented by InGaO.sub.3(ZnO).sub.m (wherein m is a number less
than 6 and 0<x.ltoreq.1), the electron mobility is more than 1
cm.sup.2/(Vsec) and the electron carrier concentration is less than
10.sup.18/cm.sup.3.
[0019] Furthermore, the present invention also provides: (8) a
transparent semi-insulating amorphous oxide thin film comprising
In--Ga--Zn--Mg--O, in which the composition in a crystallized state
is represented by InGaO.sub.3(Zn.sub.1-xMg.sub.xO).sub.m (wherein m
is a number less than 6 and 0<x.ltoreq.1), the electron mobility
is more than 1 cm.sup.2/(Vsec) and the electron carrier
concentration is less than 10.sup.18/cm.sup.3. Moreover, the
present invention also provides a method for forming the
transparent semi-insulating amorphous oxide thin film in which an
impurity ion for increasing the electrical resistance is not
intentionally added and the deposition is conducted in an
atmosphere containing oxygen gas.
[0020] A thin-film transistor according to another aspect of the
present invention includes a source electrode, a drain electrode, a
gate electrode a gate insulating film and a channel layer, in which
the channel layer contains an amorphous oxide having an electron
carrier concentration of less than 10.sup.18/cm.sup.3. Preferably,
the electron carrier concentration of the amorphous oxide is
10.sup.17/cm.sup.3 or less or 10.sup.16/cm.sup.3 or less. The
amorphous oxide is an oxide containing In, Ga, and Zn, in which the
atomic ratio In:Ga:Zn is 1:1:m (m<6). Alternatively, the
amorphous oxide is an oxide including In, Ga, Zn, and Mg, in which
the atomic ratio In:Ga:Zn.sub.1xMg.sub.x is 1:1:m (m<6), wherein
0<x.ltoreq.1.
[0021] The amorphous oxide is selected from In.sub.xGa.sub.1-x
oxides (0.ltoreq.x.ltoreq.1), In.sub.xZn.sub.1-x oxides
(0.2.ltoreq.x.ltoreq.1), In.sub.xSn.sub.1-x oxides
(0.8.ltoreq.x.ltoreq.1), and In.sub.x(Zn, Sn).sub.1-x oxides
(0.15.ltoreq.x.ltoreq.1).
[0022] In a thin film transistor of the present invention, a
material in which the electron mobility increases with the electron
carrier concentration can be used as the amorphous oxide.
[0023] According to the present invention, an amorphous oxide
having a low electron carrier concentration can be provided, and a
thin film transistor including a channel layer composed of such an
amorphous oxide can be provided.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] FIG. 1 is a graph that shows the relationship between the
oxygen partial pressure during the deposition and the electron
carrier concentration of an In--Ga--Zn--O amorphous oxide deposited
by a pulsed laser deposition method.
[0025] FIG. 2 is a graph that shows the relationship between the
electron carrier concentration and electron mobility of an
In--Ga--Zn--O amorphous oxide film formed by a pulsed laser
deposition method.
[0026] FIG. 3 is a graph that shows the relationship between the
oxygen partial pressure during the deposition and the electrical
conductivity of an In--Ga--Zn--O amorphous oxide deposited by a
high-frequency sputtering method.
[0027] FIGS. 4A-4C are graphs showing changes in electron
conductivity, electron carrier concentration, and electron mobility
of InGaO.sub.3(Zn.sub.1-xMg.sub.xO).sub.4 deposited by pulsed laser
deposition against x.
[0028] FIG. 5 is a schematic illustration showing a structure of a
top gate TFT element.
[0029] FIG. 6 is a graph showing a current-voltage characteristic
of a top gate TFT element.
[0030] FIG. 7 is a schematic illustration showing a pulsed layer
deposition device.
[0031] FIG. 8 is a schematic illustration showing a sputter
deposition device.
DETAILED DESCRIPTION
[0032] An amorphous oxide of the present invention is characterized
in that the electron carrier concentration is less than
10.sup.18/cm.sup.3. A thin film transistor (TFT) of the present
invention is characterized in that an amorphous oxide having an
electron carrier concentration less than 10.sup.18/cm.sup.3 is used
in the channel layer.
[0033] For example, as shown in FIG. 5, the TFT is made by forming
a channel layer 2 on a substrate 1 and a gate insulating film 3, a
gate electrode 4, a source electrode 6, and a drain electrode 5 on
the channel layer 2. In this invention, an amorphous oxide having
an electron carrier concentration less than 10.sup.18/cm.sup.3 is
used in the channel layer.
[0034] The structure of the TFT to which the present invention can
be applied is not limited to the staggered structure (top-gate
structure) shown in FIG. 5 in which a gate insulating film and a
gate terminal (electrode) are sequentially stacked on a
semiconductor channel layer. For example, the TFT may have an
inverted staggered structure (bottom-gate structure) in which a
gate insulating film and a semiconductor channel layer are
sequentially stacked on a gate terminal. The electron carrier
concentration mentioned above is a value measured at room
temperature. Room temperature is, for example, 25.degree. C. and,
in particular, is appropriately selected from the range of about
0.degree. C. to about 40.degree. C.
[0035] The electron carrier concentration of the amorphous oxide of
the present invention need not be less than 10.sup.18/cm.sup.3 all
through the range of 0.degree. C. to 40.degree. C. For example, it
is sufficient if the carrier electron concentration is less than
10.sup.18/cm.sup.3 at 25.degree. C. When the electron carrier
concentration is reduced to 10.sup.17/cm.sup.3 or less and more
preferably to 10.sup.16/cm.sup.3 or less, TFTs of a normally off
type can be obtained in high yield. The electron carrier
concentration can be determined by hall-effect measurement.
[0036] In the present invention, "amorphous oxide" is defined as an
oxide that shows a halo pattern in an X-ray diffraction spectrum
and exhibits no particular diffraction line. The lower limit of the
electron carrier concentration of the amorphous oxide of the
present invention is not particularly limited as long as the oxide
can be used as the TFT channel layer. The lower limit is, for
example, 10.sup.12/cm.sup.3.
[0037] Thus, in the present invention, the starting materials,
composition ratio, production conditions, and the like of the
amorphous oxide are controlled as in the individual examples
described below so as to adjust the electron carrier concentration
to 10.sup.12/cm.sup.3 or more but less than 10.sup.18/cm.sup.3.
Preferably, the electron carrier concentration is adjusted to
10.sup.13/cm.sup.3 to 10.sup.17/cm.sup.3, and more preferably
10.sup.15/cm.sup.3 to 10.sup.16/cm.sup.3.
[0038] The electron mobility is preferably 0.1 cm.sup.2/(Vsec) or
more, more preferably 1 cm.sup.2/(Vsec) or more, and most
preferably 5 cm.sup.2/(Vsec) or more when measured at room
temperature. The amorphous oxide exhibits increased electron
mobility as the electron carrier concentration increases. The
conductivity thereof tends to exhibit degenerate conduction.
Degenerate conduction is defined as a state in which the thermal
activation energy for temperature dependency of electrical
resistance is 30 meV or less.
[0039] Starting Materials for Amorphous Oxide
[0040] The amorphous oxide of the present invention contains at
least one element selected from Zn, In, and Sn as a constituent
component and is represented by
[(Sn.sub.1-xM4.sub.x)O.sub.2]a.[(In.sub.1-yM3.sub.y).sub.2O.sub.3]b.[(Zn.-
sub.1-zM2.sub.z)O]c [0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1,
0.ltoreq.z.ltoreq.1; x, y, and z are not simultaneously 1;
0.ltoreq.a.ltoreq.1, 0.ltoreq.b.ltoreq.1, 0.ltoreq.c.ltoreq.1, and
a+b+c=1; M4 is a group IV element (Si, Ge, or Zr) having an atomic
number smaller than that of Sn; M3 is Lu or a group III element (B,
Al, Ga, or Y) having an atomic number smaller than that of In and
M2 is a group II element (Mg or Ca) having an atomic number smaller
than that of Zn. The amorphous oxide may further contain at least
one element selected from group V elements M5 (V, Nb, and Ta) and
W. In this description, the group II, III, IV, and V elements in
the periodic table are sometimes referred to as group 2, 3, 4, and
5 elements, respectively; however, the meaning is the same.
[0041] The electron carrier concentration can be further decreased
by adding at least one element that can form a compound oxide, the
at least one element being selected from a group 2 element M2 (M2:
Mg or Ca) having an atomic number smaller than that of Zn; Lu and a
group 3 element M3 (M3: B, Al, Ga, or Y) having an atomic number
smaller than that of In; a group 4 element M4 (M4: Si, Ge, or Zr)
having an atomic number smaller than that of Sn; and a group 5
element M5 (M5: V, Nb, and Ta) or W.
[0042] The elements M2, M3, and M4 having atomic numbers smaller
than those of Zn, In, and Sn, respectively, have higher ionicity
than Zn, In and Sn; thus, generation of oxygen defects is less
frequent, and the electron carrier concentration can be decreased.
Although Lu has a larger atomic number than Ga, the ion radius is
small and the ionicity is high, thereby achieving the same
functions as those of M3. M5, which is ionized at a valency of 5,
strongly bonds to oxygen and rarely causes oxygen defects. Tungsten
(W), which is ionized at a valency of 6, strongly bonds to oxygen
and rarely causes oxygen defects.
[0043] The amorphous oxide applicable to the present invention is a
single compound having a composition in a crystallized state
represented by
[(In.sub.1-yM3.sub.y).sub.2O.sub.3][(Zn.sub.1-xM2.sub.x)O].sub.m
(wherein 0.ltoreq.x.ltoreq.1; 0.ltoreq.y.ltoreq.1; x and y are not
simultaneously 1; m is zero or a number or a natural number less
than 6; M3 is Lu or a group 3 element (B, Al, Ga, or Y) having an
atomic number smaller than that of In; and M2 is a group 2 element
(Mg or Ca) having an atomic number smaller than that of Zn] or a
mixture of compounds with different values of m. M3 is, for
example, Ga. M2 is, for example, Mg.
[0044] The amorphous oxide applicable to the present invention is a
unitary, binary, or ternary compound within a triangle with apexes
of SnO.sub.2, In.sub.2O.sub.3, and ZnO. Among these three
compounds, In.sub.2O.sub.3 has high amorphous formation capacity
and can form a completely amorphous phase when In.sub.2O.sub.3 is
deposited by a vapor phase method while adding approximately 0.1 Pa
of water into the atmosphere.
[0045] ZnO and SnO.sub.2 in some cases do not form an amorphous
phase by themselves; however, they can form an amorphous phase in
the presence of In.sub.2O.sub.3 as a host oxide. In particular, of
binary compositions containing two of the above-described three
compounds (compositions located on the side of the triangle), the
In--Zn--O system can form an amorphous film when In is contained in
an amount of about 20 at % or more, and the Sn--In--O system can
form an amorphous film when In is contained in an amount of about
80 at % or more by a vapor phase method.
[0046] In order to obtain an In--Zn--O amorphous film by a vapor
phase method, about 0.1 Pa of steam may be introduced into the
atmosphere. In order to obtain an In--Sn--O-- system amorphous film
by a vapor phase method, about 0.1 Pa of nitrogen gas may be
introduced into the atmosphere. For the ternary composition,
Sn--In--Zn, containing the three compounds, an amorphous film can
be obtained by a vapor phase method when In is contained in an
amount of about 15 at % in the above-described composition range.
Note that "at %" herein indicates atomic percent with respect to
the metal ions other than oxygen ions. In particular, for example,
"the In--Zn--O system containing about 20 at % or more of In" is
equivalent to In.sub.xZn.sub.1-x (x>0.2).
[0047] The composition of the amorphous oxide film containing Sn,
In, and/or Zn may contain additional elements as described below.
In particular, at least one element that forms a compound oxide,
the at least one element being selected from a group 2 element M2
(M2: Mg or Ca) having an atomic number smaller than that of Zn, Lu
or a group 3 element M3 (M3: B, Al, Ga, or Y) having an atomic
number smaller than that of In, and a group 4 element M4 (M4: Si,
Ge, or Zr) having an atomic number smaller than that of Sn may be
added. The amorphous oxide film of the present invention may
further contain at least one element that can form a compound
oxide, the at least one element being selected from group 5
elements (M5: V, Nb, and Ta) and W.
[0048] Addition of the above-described elements will increase the
stability of the amorphous film and expands the composition range
that can give an amorphous film. In particular, addition of highly
covalent B, Si, or Ge is effective for stabilization of the
amorphous phase, and a compound oxide composed of ions with largely
different ion radii can stabilize the amorphous phase. For example,
in the In--Zn--O system, a stable amorphous film is rarely obtained
at room temperature unless the range of In content is more than
about 20 at %. However, by adding Mg in an equivalent amount to In,
a stable amorphous film can be obtained at an In content of more
than about 15 at %.
[0049] An example of the amorphous oxide material that can be used
in the channel layer of the TFT of the present invention is
described next. The amorphous oxide that can be used in the channel
layer is, for example, an oxide that contains In, Ga, and Zn at an
atomic ratio satisfying In:Ga:Zn=1:1:m, wherein m is a value less
than 6. The value of m may be a natural number but is not
necessarily a natural number. This applies to "m" referred to in
other sections of this description. The atomic ratio can be
considered as equivalent to a molar ratio.
[0050] A transparent amorphous oxide thin film whose composition in
a crystallized state is represented by InGaO.sub.3(ZnO).sub.m
(wherein m is a number less than 6) maintains a stable amorphous
state at high temperatures not less than 800.degree. C. when the
value of m is less than 6. However, as the value of m increases,
i.e., as the ratio of ZnO to InGaO.sub.3 increases and the
composition approaches to the ZnO composition, the composition
tends to be more crystallizable. Thus, the value of m is preferably
less than 6 for the channel layer of the amorphous TFT. A desired
amorphous oxide can be obtained by adjusting the composition of the
target material (e.g., a polycrystalline material) for deposition,
such as sputtering deposition or pulsed laser deposition (PLD), to
comply with m<6.
[0051] In the amorphous oxide described above, Zn in the
composition ratio of InGaZn may be replaced by Zn.sub.1-xMg.sub.x.
The possible amount of Mg for replacement is within the range of
0<x.ltoreq.1. When the replacement with Mg is conducted, the
electron mobility of the oxide film decreases compared to a film
containing no Mg. However, the extent of decrease is small, and the
electron carrier concentration can be decreased compared to when no
replacement is conducted. Thus, this is more preferable for the
channel layer of a TFT. The amount of Mg for replacement is
preferably more than 20% and less than 85% (0.2<x<0.85 in
term of x) and more preferably 0.5<x<0.85.
[0052] The amorphous oxide may be appropriately selected from In
oxides, In.sub.xZn.sub.1-x oxides (0.2.ltoreq.x.ltoreq.1),
In.sub.xSn.sub.1-x oxides (0.8.ltoreq.x.ltoreq.1), and In.sub.x(Zn,
Sn).sub.1-x oxides (0.15.ltoreq.x.ltoreq.1). The ratio of Zn to Sn
in the In.sub.x(Zn, Sn).sub.1-x oxides may be appropriately
selected. Namely, an In.sub.x(Zn, Sn).sub.1-x oxide can be
described as In.sub.x(Zn.sub.ySn.sub.1-y).sub.1-x oxide, and y is
in the range of 1 to 0. For an In oxide containing neither Zn nor
Sn, In may be partly replaced by Ga. In this case, the oxide can be
described as an In.sub.xGa.sub.1-x oxide (0.ltoreq.x.ltoreq.1).
[0053] Method for Producing Amorphous Oxide
[0054] The amorphous oxide used in the present invention can be
prepared by a vapor phase deposition technique under the conditions
indicated in the individual examples below. For example, in order
to obtain an InGaZn amorphous oxide, deposition is conducted by a
vapor phase method such as a sputtering (SP) method, a pulsed laser
deposition (PLD) method, or an electron beam deposition method
while using a polycrystalline sinter represented by
InGaO.sub.3(ZnO).sub.m as the target. From the standpoint of mass
productivity, the sputtering method is most suitable.
[0055] During the formation of an In.sub.2O.sub.3 or In--Zn--O
amorphous oxide film or the like, oxygen radicals may be added to
the atmosphere. Oxygen radicals may be added through an oxygen
radical generator. When there is need to increase the electron
carrier concentration after the film formation, the film is heated
in a reducing atmosphere to increase the electron carrier
concentration. The resulting amorphous oxide film with a different
electron carrier concentration was analyzed to determine the
dependency of the electron mobility on the electron carrier
concentration, and the electron mobility increased with the
electron carrier concentration.
[0056] Substrate
[0057] The substrate for forming the TFT of the present invention
may be a glass substrate, a plastic substrate, a plastic film, or
the like. Moreover, as described below in EXAMPLES, the amorphous
oxide of the present invention can be formed into a film at room
temperature. Thus, a TFT can be formed on a flexible material such
as a PET film. Moreover, the above-mentioned amorphous oxide may be
appropriately selected to prepare a TFT from a material that is
transparent in visible light not less than 400 nm or infrared
light.
[0058] Gate Insulating Film
[0059] The gate insulating film of the TFT of the present invention
is preferably a gate insulating film composed of Al.sub.2O.sub.3,
Y.sub.2O.sub.3, HfO.sub.2, or a mixed crystal compound containing
at least two of these compounds. When there is a defect at the
interface between the gate insulating thin film and the channel
layer thin film, the electron mobility decreases and hysteresis
occurs in the transistor characteristics. Moreover, leak current
greatly differs according to the type of the gate insulating film.
Therefore, a gate insulating film suitable for the channel layer
must be selected.
[0060] Use of an Al.sub.2O.sub.3 film can decrease the leak
current. Use of an Y.sub.2O.sub.3 film can reduce the hysteresis.
Use of a high dielectric constant HfO.sub.2 film can increase the
field effect mobility. By using a film composed of a mixed crystal
of these compounds, a TFT having small leak current and hysteresis
and large field effect mobility can be produced. the process for
forming the gate insulating film and the process for forming the
channel layer can be conducted at room temperature; thus, a TFT of
a staggered or inverted staggered structure can be formed.
[0061] Transistor
[0062] When a field effect transistor includes a channel layer
composed of an amorphous oxide film having an electron carrier
concentration of less than 10.sup.18/cm.sup.3, a source terminal, a
drain terminal, and a gate terminal disposed on the gate insulating
film, the current between the source and drain terminals can be
adjusted to about 10.sup.-7 A when a voltage of about 5V is applied
between the source and drain terminals without application of a
gate voltage. The theoretical lower limit of the electron carrier
concentration is 10.sup.5/cm.sup.3 or less assuming that the
electrons in the valence band are thermally excited. The actual
possibility is that the lower limit is about
10.sup.12/cm.sup.3.
[0063] When Al.sub.2O.sub.3, Y.sub.2O.sub.3, or HfO.sub.2 alone or
a mixed crystal compound containing at least two of these compounds
is used in the gate insulating layer, the leak voltage between the
source gate terminals and the leak voltage between the drain and
gate terminals can be adjusted to about 10.sup.-7 A, and a normally
off transistor can be realized.
[0064] The electron mobility of the oxide crystals increases as the
overlap of the s orbits of the metal ion increases. The oxide
crystals of Zn, In, and Sn having large atomic numbers exhibit high
electron mobility of 0.1 to 200 cm.sup.2/(Vsec). Since ionic bonds
are formed between oxygen and metal ions in an oxide, electron
mobility substantially comparable to that in a crystallized state
can be exhibited in an amorphous state in which there is no
directionality of chemical bonding, the structure is random, and
the directions of the bonding are nonuniform. In contrast, by
replacing Zn, In, and Sn each with an element having a smaller
atomic number, the electron mobility can be decreased. Thus, by
using the amorphous oxide described above, the electron mobility
can be controlled within the range of about 0.01 cm.sup.2/(Vsec) to
20 cm.sup.2/(Vsec).
[0065] In a typical compound, the electron mobility decreases as
the carrier concentration increases due to the dispersion between
the carriers. In contrast, the amorphous oxide of the present
invention exhibits increased electron mobility with the increasing
electron carrier concentration. The physical principle that lies
behind this phenomenon is not clearly identified.
[0066] Once a voltage is applied to the gate terminal, electrons
are injected into the amorphous oxide channel layer, and current
flows between the source and drain terminals, thereby allowing the
part between the source and drain terminals to enter an ON state.
According to the amorphous oxide film of the present invention,
since the electron mobility increases with the electron carrier
concentration, the current that flows when the transistor is turned
ON can be further increased. In other words, the saturation current
and the on/off ratio can be further increased. When the amorphous
oxide film having high electron mobility is used as the channel
layer of a TFT, the saturation current can be increased and the
switching rate of the TFT can be increased, thereby achieving
high-speed operation.
[0067] For example, when the electron mobility is about 0.01
cm.sup.2/(Vsec), the material can be used in a channel layer of a
TFT for driving a liquid crystal display element. By using an
amorphous oxide film having an electron mobility of about 0.1
cm.sup.2/(Vsec), a TFT that has performance comparable or superior
to the TFT using an amorphous silicon film and that can drive a
display element for moving images can be produced.
[0068] In order to realize a TFT that requires large current, e.g.,
for driving a current-driven organic light-emitting diode, the
electron mobility is preferably more than 1 cm.sup.2/(Vsec). Note
than when the amorphous oxide of the present invention that
exhibits degenerate conduction is used in the channel layer, the
current that flows at a high carrier concentration, i.e., the
saturation current of the transistor, shows decreased dependency on
temperature, and a TFT with superior temperature characteristics
can be realized.
EXAMPLES
Example 1: Preparation of Amorphous In--Ga--Zn--O Thin Film by PLD
Method
[0069] A film was formed in a PLD device shown in FIG. 7. In the
drawing, reference numeral 701 denotes a rotary pump (RP), 702
denotes a turbo molecular pump (TMP), 703 denotes a preparation
chamber, 704 denotes en electron gun for RHEED, 705 denotes a
substrate holder for rotating and vertically moving the substrate,
706 denotes a laser entrance window, 707 denotes a substrate, 708
denotes a target, 709 denotes a radical source, 710 denotes a gas
inlet, 711 denotes a target holder for rotating and vertically
moving the target, 712 denotes a by-pass line, 713 denotes a main
line, 714 denotes a turbo molecular pump (TMP), 715 denotes a
rotary pump (RP), 716 denotes a titanium getter pump, and 717
denotes a shutter. In the drawing, 718 denotes ionization gauge
(IG), 719 denotes a Pirani gauge (PG), 720 denotes a Baratron gauge
(BG), and 721 denotes a deposition chamber.
[0070] An In--Ga--Zn--O amorphous oxide semiconductor thin film was
formed on a SiO.sub.2 glass substrate (#1737 produced by Corning)
by a pulsed laser deposition method using a KrF excimer laser. As
the pre-deposition treatment, the substrate was degreased with
ultrasonic waves in acetone, ethanol, and ultrapure water for 5
minutes each, and then dried in air at 100.degree. C.
[0071] An InGaO.sub.3(ZnO).sub.4 sinter target (size: 20 mm in
dia., 5 mm in thickness) was used as the polycrystalline target.
This target was prepared by wet-mixing the starting materials,
In.sub.2O.sub.3:Ga.sub.2O.sub.3:ZnO (each being a 4N reagent), in a
solvent (ethanol), calcining (1000.degree. C., 2 h) the resulting
mixture, dry-milling the calcined mixture, and sintering the
resulting mixture (1550.degree. C., 2 h). The electrical
conductivity of the target obtained was 90 (S/cm).
[0072] The ultimate vacuum of the deposition chamber was adjusted
to 2.times.10.sup.-6 (Pa), and the oxygen partial pressure during
the deposition was controlled to 6.5 (Pa) to form a film. The
oxygen partial pressure inside the chamber 721 was 6.5 Pa, and the
substrate temperature was 25.degree. C. The distance between the
target 708 and the substrate 707 for deposition was 30 (mm). The
power of the KrF excimer laser entering from the entrance window
706 was in the range of 1.5 to 3 (mJ/cm.sup.2/pulse). The pulse
width was 20 (nsec), the repetition frequency was 10 (Hz), and the
beam spot diameter was 1.times.1 (mm square). A film was formed at
a deposition rate of 7 (nm/min).
[0073] The resulting thin film was subjected to grazing incidence
x-ray diffraction (thin film method, incident angle: 0.5.degree.),
but no clear diffraction peak was observed. Thus, the In--Ga--Zn--O
thin film obtained was assumed to be amorphous. The X-ray
reflectance was determined, and the pattern was analyzed. It was
observed that the root mean square roughness (Rrms) of the thin
film was about 0.5 nm, and the film thickness was about 120 nm. The
results of the fluorescence X-ray showed that the metal composition
ratio of the thin film was In:Ga:Zn=0.98:1.02:4. The electrical
conductivity was less than about 10.sup.-2 S/cm. The electron
carrier concentration and the electron mobility were presumably
about 10.sup.16/cm.sup.3 or less and about 5 cm.sup.2/(Vsec),
respectively.
[0074] Based on the analysis of the optical absorption spectrum,
the energy width of the forbidden band of the amorphous thin film
prepared was determined to be about 3 eV. Based on these values, it
was found that the In--Ga--Zn--O thin film had an amorphous phase
close to the composition of the crystals of InGaO.sub.3(ZnO).sub.4,
had fewer oxygen defects, and was a flat, transparent thin film
with low electrical conductivity.
[0075] Specific description is now presented with reference to FIG.
1. FIG. 1 shows a change in electron carrier concentration of the
oxide formed into a film against changes in oxygen partial pressure
when an In--Ga--Zn--O transparent amorphous oxide thin film
represented by InGaO.sub.3(ZnO).sub.4 in an assumed crystal state
is formed under the same conditions as in this EXAMPLE.
[0076] As shown in FIG. 1, the electron carrier concentration
decreased to less than 10.sup.18/cm.sup.3 when the film was formed
in an atmosphere at a high oxygen partial pressure of more than 4.5
Pa under the same conditions as this example. In this case, the
temperature of the substrate was maintained substantially at room
temperature without intentional heating. The substrate temperature
is preferably less than 100.degree. C. when a flexible plastic film
is used as the substrate.
[0077] By further increasing the oxygen partial pressure, the
electron carrier concentration was further decreased. For example,
as shown in FIG. 1, the number of the electron carriers of the
InGaO.sub.3(ZnO).sub.4 thin film deposited at a substrate
temperature of 25.degree. C. and an oxygen partial pressure of 5 Pa
decreased to 10.sup.16/cm.sup.3.
[0078] The thin film obtained had an electron mobility exceeding 1
cm.sup.2/(Vsec), as shown in FIG. 2. However, according to the
pulsed laser deposition method of the present invention, the
surface of the film deposited will have irregularities at an oxygen
partial pressure of 6.5 Pa or more, and thus, it is difficult to
use the thin film as a channel layer of a TFT. Therefore, by using
an In--Ga--Zn--O transparent amorphous oxide thin film having a
composition of InGaO.sub.3(ZnO).sub.m (m is less than 6) in a
crystal state prepared by a pulsed laser deposition method in an
atmosphere having an oxygen partial pressure exceeding 4.5 Pa,
preferably exceeding 5 Pa, but less than 6.5 Pa, a normally off
transistor can be prepared.
[0079] The electron mobility of this thin film was more than 1
cm.sup.2/(Vsec), and the on/off ratio thereof was increased to over
10.sup.3. As is described above, in forming an InGaZn oxide film by
a PLD method under the conditions set forth in this example, the
oxygen partial pressure is preferably controlled to not less than
4.5 Pa but less than 6.5 Pa. Whether an electron carrier
concentration of 10.sup.18/cm.sup.3 is realized depends on the
conditions of the oxygen partial pressure, the configuration of the
deposition device, the materials for deposition, the composition,
and the like.
[0080] Next, in the above-described device at an oxygen partial
pressure of 6.5 Pa, an amorphous oxide was made and a top-gate
MISFET element shown in FIG. 5 was formed. In particular, a
semi-insulating amorphous InGaO.sub.3(ZnO).sub.4 film having a
thickness of 120 nm for use as a channel layer (2) was formed on a
glass substrate (1) by the above-described method for making the
amorphous In--Ga--Zn--O thin film.
[0081] On this film, InGaO.sub.3(ZnO).sub.4 having a high
electrical conductivity and a gold film each 30 nm in thickness
were deposited by a pulsed laser deposition method while
controlling the oxygen partial pressure inside the chamber to less
than 1 Pa. A drain terminal (5) and a source terminal (6) were
formed by a photolithographic method and a lift-off method.
[0082] Lastly, an Y.sub.2O.sub.3 film (thickness: 90 nm, relative
dielectric constant: about 15, leak current density: 10.sup.-3
A/cm.sup.2 upon application of 0.5 MV/cm) for use as a gate
insulating film (3) was deposited by an electron beam deposition
method, and gold was deposited on the Y.sub.2O.sub.3 film. A gate
terminal (4) was formed by a photolithographic method and a
lift-off method.
[0083] Evaluation of Characteristics of MISFET Element
[0084] FIG. 6 shows the current-voltage characteristics of MISFET
elements measured at room temperature. Since the drain current
I.sub.DS increased with the drain voltage V.sub.DS, the channel was
proved to be an n-type semiconductor. This is consistent with the
fact that the amorphous In--Ga--Zn--O semiconductor is of an
n-type. I.sub.DS was saturated (pinch-off) at V.sub.DS=about 6 V,
which was a typical behavior for semiconductor transistors. The
gain characteristic was determined, and the threshold value of the
gate voltage V.sub.GS when V.sub.DS=4 V was applied was about -0.5
V. Upon application of V.sub.G=10 V, current of
I.sub.DS=1.0.ltoreq.x.ltoreq.10.sup.-5 A flowed. This is because
carriers were induced in the In--Ga--Zn--O amorphous semiconductor
thin film, i.e., an insulator, due to the gate bias. The on/off
ratio of the transistor exceeded 10.sup.3. The field effect
mobility was determined from the output characteristics. As a
result, a field effect mobility of about 7 cm.sup.2(Vs).sup.-1 was
obtained in the saturation region.
[0085] The same measurements were carried out on the element while
irradiating the element with visible light, but no change in
transistor characteristics was observed. According to the present
example, a thin film transistor having a channel layer exhibiting a
low electron carrier concentration, a high electrical resistance,
and high electron mobility can be realized. Note that the
above-described amorphous oxide showed excellent characteristics in
that the electron mobility increased with the electron carrier
concentration and that degenerate conduction was exhibited.
[0086] In this example, the thin film transistor was formed on the
glass substrate. Since the film can be formed at room temperature,
a substrate such as a plastic board or a film can be used. The
amorphous oxide obtained in this example absorbs little visible
light; thus, a transparent, flexible TFT can be made.
Example 2: Formation of Amorphous InGaO.sub.3(ZnO) and
InGaO.sub.3(ZnO).sub.4 Oxide Films by PLD Method
[0087] In--Zn--Ga--O amorphous oxide films were deposited on glass
substrates (#1737 produced by Corning) by using polycrystalline
sinters represented by InGaO.sub.3(ZnO) and InGaO.sub.3(ZnO).sub.4
as the targets by a PLD method using KrF excimer laser. The same
PLD deposition device as shown in EXAMPLE 1 was used, and the
deposition was conducted under the same conditions. The substrate
temperature during the deposition was 25.degree. C.
[0088] Each film obtained thereby was subjected to grazing
incidence x-ray diffraction (thin film method, incident angle:
0.5.degree.) for the film surface. No clear diffraction peak was
detected. The In--Zn--Ga--O films prepared from the two targets
were both amorphous.
[0089] The In--Zn--Ga--O amorphous oxide films on the glass
substrates were each analyzed to determine the x-ray reflectance.
Analysis of the pattern found that the root mean average roughness
(Rrms) of the thin film was about 0.5 mm and that the thickness was
about 120 nm. Fluorescence x-ray analysis (XRF) showed that the
ratio of the metal atoms of the film obtained from the target
composed of the polycrystalline sinter represented by
InGaO.sub.3(ZnO) was In:Ga:Zn=1.1:1.1:0.9 and that the ratio of the
metal atoms of the film obtained from the target composed of the
polycrystalline sinter represented by InGaO.sub.3(ZnO).sub.4 was
In:Ga:Zn=0.98:1.02:4.
[0090] The electron carrier concentration of the amorphous oxide
film obtained from the target composed of the polycrystalline
sinter represented by InGaO.sub.3(ZnO).sub.4 was measured while
changing the oxygen partial pressure of the atmosphere during the
deposition. The results are shown in FIG. 1. By forming the film in
the atmosphere having an oxygen partial pressure exceeding 4.5 Pa,
the electron carrier concentration could be decreased to less than
10.sup.18/cm.sup.3. In this case, the temperature of the substrate
was maintained substantially at room temperature without
intentional heating. When the oxygen partial pressure was less than
6.5 Pa, the surface of the amorphous oxide film obtained was
flat.
[0091] When the oxygen partial pressure was 5 Pa, the electron
carrier concentration and the electrical conductivity of the
amorphous oxide film obtained from the target composed of the
polycrystalline sinter represented by InGaO.sub.3(ZnO).sub.4 were
10.sup.16/cm.sup.3 and 10.sup.-2 S/cm, respectively. The electron
mobility was presumably about 5 cm.sup.2/(Vsec). Based on the
analysis of the optical absorption spectrum, the energy width of
the forbidden band of the amorphous thin film prepared was
determined to be about 3 eV. The electron carrier concentration
could be further decreased as the oxygen partial pressure was
increased from 5 Pa.
[0092] As shown in FIG. 1, the In--Zn--Ga--O amorphous oxide film
deposited at a substrate temperature of 25.degree. C. and an oxygen
partial pressure of 6 Pa exhibited a decreased electron carrier
concentration of 8.times.10.sup.15/cm.sup.3 (electrical
conductivity: about 8.times.10.sup.-3 S/cm). The resulting film was
assumed to have an electron mobility of more than 1
cm.sup.2/(Vsec). However, according to the PLD method,
irregularities were formed in the surface of the film deposited at
an oxygen partial pressure of 6.5 Pa or more, and thus it was
difficult to use the film as the channel layer of the TFT.
[0093] The relationship between the electron carrier concentration
and the electron mobility of the In--Zn--Ga--O amorphous oxide film
prepared from the target composed of the polycrystalline sinter
represented by InGaO.sub.3(ZnO).sub.4 at different oxygen partial
pressures was investigated. The results are shown in FIG. 2. When
the electron carrier concentration increased from
10.sup.16/cm.sup.3 to 10.sup.20/cm.sup.3, the electron mobility
increased from about 3 cm.sup.2/(Vsec) to about 11 cm.sup.2/(Vsec).
The same tendency was observed for the amorphous oxide film
prepared from the target composed of the polycrystalline sinter
represented by InGaO.sub.3(ZnO).
[0094] An In--Zn--Ga--O amorphous oxide film formed on a
polyethylene terephthalate (PET) film having a thickness of 200
.mu.m instead of the glass substrate also showed similar
characteristics.
Example 3: Formation of In--Zn--Ga--O Amorphous Oxide Film by SP
Method
[0095] Formation of a film by a high-frequency SP method using
argon gas as the atmosphere gas is described. The SP method was
conducted using the device shown in FIG. 8. In the drawing,
reference numeral 807 denotes a substrate for deposition, 808
denotes a target, 805 denotes a substrate holder equipped with a
cooling mechanism, 814 denotes a turbo molecular pump, 815 denotes
a rotary pump, 817 denotes a shutter, 818 denotes an ionization
gauge, 819 denotes a Pirani gauge, 821 denotes a deposition
chamber, and 830 denotes a gate valve. A SiO.sub.2 glass substrate
(#1737 produced by Corning) was used as the substrate 807 for
deposition. As the pre-deposition treatment, the substrate was
degreased with ultrasonic waves in acetone, ethanol, and ultrapure
water for 5 minutes each, and then dried in air at 100.degree.
C.
[0096] An InGaO.sub.3(ZnO).sub.4 polycrystalline sinter (size: 20
mm in dia., 5 mm in thickness) was used as the target material. The
sinter was prepared by wet-mixing the starting materials,
In.sub.2O.sub.3:Ga.sub.2O.sub.3:ZnO (each being a 4N reagent), in a
solvent (ethanol), calcining (1000.degree. C., 2 h) the resulting
mixture, dry-milling the calcined mixture, and sintering the
resulting mixture (1550.degree. C., 2 h). The target 808 had an
electrical conductivity of 90 (S/cm) and was in a semi-insulating
state.
[0097] The ultimate vacuum inside the deposition chamber 821 was
1.times.10.sup.-4 (Pa). The total pressure of the oxygen gas and
the argon gas during the deposition was controlled at a
predetermined value within the range of 4 to 0.1.times.10.sup.-1
(Pa), and the oxygen partial pressure was changed in the range of
10.sup.-3 to 2.times.10.sup.-1 (Pa) by changing the partial
pressure ratio of the argon gas and oxygen. The substrate
temperature was room temperature, and the distance between the
target 808 and the substrate 807 for deposition was 30 (mm). The
current injected was RF 180 W, and the deposition rate was 10
(nm/min).
[0098] The resulting film was subjected to grazing incidence x-ray
diffraction (thin film method, incident angle=0.5.degree.) for the
film surface, but no clear diffraction peak was observed. Thus, the
In--Zn--Ga--O thin film obtained was proved to be amorphous. The
X-ray reflectance was determined, and the pattern was analyzed. It
was observed that the root mean square roughness (Rrms) of the thin
film was about 0.5 nm, and the film thickness was about 120 nm. The
results of the fluorescence X-ray showed that the metal composition
ratio of the thin film was In:Ga:Zn=0.98:1.02:4.
[0099] The electrical conductivity of the amorphous oxide film
obtained by changing the oxygen partial pressure in the atmosphere
during the deposition was measured. The results are shown in FIG.
3. As shown in FIG. 3, the electrical conductivity could be
decreased to less than 10 S/cm by forming the film in an atmosphere
at a high oxygen partial pressure exceeding 3.times.10.sup.-2
Pa.
[0100] By further increasing the oxygen partial pressure, the
number of electron carriers could be decreased. For example, as
shown in FIG. 3, the electrical conductivity of an
InGaO.sub.3(ZnO).sub.4 thin film deposited at a substrate
temperature of 25.degree. C. and an oxygen partial pressure of
10.sup.-1 Pa was decreased to about 10.sup.-10 S/cm. An
InGaO.sub.3(ZnO).sub.4 thin film deposited at an oxygen partial
pressure exceeding 10.sup.-1 Pa had excessively high electrical
resistance and thus the electrical conductivity thereof could not
be measured. However, extrapolation was conducted for the value
observed from a film having a high electron carrier concentration,
and the electron mobility was assumed to be about 1
cm.sup.2/(Vsec).
[0101] In short, a normally off transistor having an on/off ratio
exceeding 103 could be made by using a transparent amorphous oxide
thin film which was composed of In--Ga--Zn--O prepared by a sputter
deposition method in argon gas atmosphere at an oxygen partial
pressure more than 3.times.10-2 Pa, preferably more than
5.times.10-1 Pa, and which was represented by InGaO3(ZnO)4 (m is a
natural number less than 6) in a crystallized state.
[0102] When the device and starting materials set forth in this
example are used, the oxygen partial pressure during the sputter
deposition is, for example, in the range of 3.times.10.sup.-2 Pa to
5.times.10.sup.-1 Pa. The electron mobility of the thin films
prepared by the pulsed laser deposition method and the sputtering
method increases with the number of the conduction electrons, as
shown in FIG. 2.
[0103] As described above, by controlling the oxygen partial
pressure, oxygen defects can be reduced, and therefore the electron
carrier concentration can be reduced. Unlike in the polycrystalline
state, in the amorphous state, there is essentially no grain
interface; therefore, an amorphous thin film with high electron
mobility can be obtained. Note that when a polyethylene
terephthalate (PET) film having a thickness of 200 .mu.m was used
instead of the glass substrate, the resulting
InGaO.sub.3(ZnO).sub.4 amorphous oxide thin film exhibited similar
characteristics.
Example 4: Formation of In--Zn--Ga--Mg--O Amorphous Oxide Film by
PLD Method
[0104] Formation of an InGaO.sub.3(Zn.sub.1-xMg.sub.xO).sub.4 film
(0<x<1) on a glass substrate by a PLD method is described.
The same deposition device shown in FIG. 7 was used as the
deposition device. A SiO.sub.2 glass substrate (#1737 produced by
Corning) was prepared as the substrate for deposition. As the
pre-deposition treatment, the substrate was degreased with
ultrasonic waves in acetone, ethanol, and ultrapure water for 5
minutes each, and then dried in air at 100.degree. C.
[0105] An InGa(Zn.sub.1-xMg.sub.xO).sub.4 (0<x<1) sinter
(size: 20 mm in dia., 5 mm in thickness) was used as the target.
The target was prepared by wet-mixing the starting materials,
In.sub.2O.sub.3:Ga.sub.2O.sub.3:ZnO:MgO (each being a 4N reagent),
in a solvent (ethanol), calcining (1000.degree. C., 2 h) the
resulting mixture, dry-milling the calcined mixture, and sintering
the resulting mixture (1550.degree. C., 2 h).
[0106] The ultimate vacuum inside the deposition chamber was
2.times.10.sup.-6 (Pa), and the oxygen partial pressure during the
deposition was 0.8 (Pa). The substrate temperature was room
temperature (25.degree. C.), and the distance between the target
and the substrate for deposition was 30 (mm). The power of the KrF
excimer laser was 1.5 (mJ/cm.sup.2/pulse), the pulse width was 20
(nsec), the repetition frequency was 10 (Hz), and the beam spot
diameter was 1.times.1 (mm square). The deposition rate was 7
(nm/min).
[0107] The resulting film was subjected to grazing incidence x-ray
diffraction (thin film method, incident angle: 0.5.degree.) for the
film surface, but no clear diffraction peak was observed. Thus, the
In--Zn--Ga--Mg--O thin film obtained was proved to be amorphous.
The surface of the resulting film was flat.
[0108] The dependency on the value x of the electrical
conductivity, electron carrier concentration, and electron mobility
of In--Zn--Ga--Mg--O amorphous oxide films deposited in atmosphere
at an oxygen partial pressure of 0.8 Pa was investigated by using
targets of different x values. Note that a high-resistance
amorphous InGaO.sub.3(Zn.sub.1-xMg.sub.xO).sub.m film could be
obtained at an oxygen partial pressure of less than 1 Pa as long as
the polycrystalline InGaO.sub.3(Zn.sub.1-xMg.sub.xO).sub.m (m is a
natural number less than 6; 0<x.ltoreq.1) was used as the
target.
[0109] The results are shown in FIG. 4. The results showed that the
electron carrier concentration of an amorphous oxide film deposited
by a PLD method in an atmosphere at an oxygen partial pressure of
0.8 Pa could be reduced to less than 10.sup.18/cm.sup.3 when the
value x was more than 0.4. The electron mobility of the amorphous
oxide film with x exceeding 0.4 was more than 1 cm.sup.2/(Vsec). As
shown in FIG. 4, when a target in which Zn was substituted with 80
at % Mg was used, the electron carrier concentration of the film
obtained by the pulsed laser deposition method in an atmosphere at
an oxygen partial pressure of 0.8 Pa could be reduced to less than
10.sup.16/cm.sup.3.
[0110] Although the electron mobility of these films is low
compared to that of Mg-free films, the degree of decrease is small,
while the electron mobility at room temperature is about 5
cm.sup.2/(Vsec), i.e., higher than that of amorphous silicon by one
order of magnitude. When deposition is conducted under the same
conditions, the electrical conductivity and the electron mobility
both decrease with an increase in Mg content. Thus, the Mg content
is preferably more than 20 at % but less than 85 at %
(0.2<x<0.85 in terms of x), and more preferably
0.5<x<0.85.
[0111] An InGaO.sub.3(Zn.sub.1-xMg.sub.xO).sub.4 (0<x<1)
amorphous oxide film formed on a polyethylene terephthalate (PET)
film having a thickness of 200 .mu.m instead of the glass substrate
also showed similar characteristics.
Example 5: Formation of In.sub.2O.sub.3 Amorphous Oxide Film by
PLD
[0112] Formation of an indium oxide film is now described. The
deposition device shown in FIG. 7 was used as the deposition
device. A SiO.sub.2 glass substrate (#1737 produced by Corning) was
prepared as the substrate for deposition. As the pre-deposition
treatment, the substrate was degreased with ultrasonic waves in
acetone, ethanol, and ultrapure water for 5 minutes each, and then
dried in air at 100.degree. C.
[0113] An In.sub.2O.sub.3 sinter (size: 20 mm in dia., 5 mm in
thickness) was used as the target. The target was prepared by
calcining the starting material In.sub.2O.sub.3 (a 4N reagent)
(1000.degree. C., 2 h), dry milling the calcined material, and
sintering the resulting material (1550.degree. C., 2 h).
[0114] The ultimate vacuum inside the deposition chamber was
2.times.10.sup.-6 (Pa), and the oxygen partial pressure during the
deposition was 5 (Pa). The steam partial pressure was 0.1 (Pa), and
200 W was applied to the oxygen radical generator to produce oxygen
radicals. The substrate temperature was room temperature. The
distance between the target and the substrate for deposition was 40
(mm). The power of the KrF excimer laser was 0.5
(mJ/cm.sup.2/pulse), the pulse width was 20 (nsec), the repetition
frequency was 10 (Hz), and the beam spot diameter was 1.times.1 (mm
square). The deposition rate was 3 (nm/min).
[0115] The resulting film was subjected to grazing incidence x-ray
diffraction (thin film method, incident angle: 0.5.degree.) for the
film surface, but no clear diffraction peak was observed. Thus, the
In--O thin film obtained was proved to be amorphous. The film
thickness was 80 nm. The electron carrier concentration and the
electron mobility of the In--O amorphous oxide film obtained were
5.times.10.sup.17/cm.sup.3 and about 7 cm.sup.2/(Vsec),
respectively.
Example 6: Formation of In--Sn--O Amorphous Oxide Film by PLD
[0116] Deposition of an In--Sn--O amorphous oxide film having a
thickness of 200 .mu.m by a PLD method is described. A SiO.sub.2
glass substrate (#1737 produced by Corning) was prepared as the
substrate for deposition. As the pre-deposition treatment, the
substrate was degreased with ultrasonic waves in acetone, ethanol,
and ultrapure water for 5 minutes each, and then dried in air at
100.degree. C.
[0117] An In.sub.2O.sub.3--SnO.sub.2 sinter (size: 20 mm in dia., 5
mm in thickness) was prepared as the target by wet-mixing the
starting materials, In.sub.2O.sub.3--SnO.sub.2 (a 4N reagent), in a
solvent (ethanol), calcining the resulting mixture (1000.degree.
C., 2 h), dry milling the calcined mixture, and sintering the
resulting mixture (1550.degree. C., 2 h). The composition of the
target was (In.sub.0.9Sn.sub.0.1).sub.2O.sub.3.1 polycrystal.
[0118] The ultimate vacuum inside the deposition chamber was
2.times.10.sup.-6 (Pa), the oxygen partial pressure during the
deposition was 5 (Pa), and the nitrogen partial pressure was 0.1
(Pa). Then 200 W is applied to the oxygen radical generator to
produce oxygen radicals. The substrate temperature during the
deposition was room temperature. The distance between the target
and the substrate for deposition was 30 (mm). The power of the KrF
excimer laser was 1.5 (mJ/cm.sup.2/pulse), the pulse width was 20
(nsec), the repetition frequency was 10 (Hz), and the beam spot
diameter was 1.times.1 (mm square).
[0119] The deposition rate was 6 (nm/min). The resulting film was
subjected to grazing incidence x-ray diffraction (thin film method,
incident angle: 0.5.degree.) for the film surface, but no clear
diffraction peak was observed. Thus, the In--Sn--O thin film
obtained was proved to be amorphous. The electron carrier
concentration and the electron mobility of the In--Sn--O amorphous
oxide film obtained were 8.times.10.sup.17/cm.sup.3 and about 5
cm.sup.2/(Vsec), respectively. The film thickness was 100 nm.
Example 7: Formation of In--Ga--O Amorphous Oxide Film by PLD
Method
[0120] Deposition of an indium gallium oxide is described next. A
SiO.sub.2 glass substrate (#1737 produced by Corning) was prepared
as the substrate for deposition. As the pre-deposition treatment,
the substrate was degreased with ultrasonic waves in acetone,
ethanol, and ultrapure water for 5 minutes each, and then dried in
air at 100.degree. C.
[0121] A (In.sub.2O.sub.3).sub.1-x--(Ga.sub.2O.sub.3).sub.x (x=0 to
1) sinter was prepared as the target (size: 20 mm in dia., 5 mm in
thickness). For example, when x=0.1, the target was an
(In.sub.0.9Ga.sub.0.1).sub.2O.sub.3 polycrystalline sinter. This
target was obtained by wet-mixing the starting materials,
In.sub.2O.sub.3--Ga.sub.2O.sub.3 (4N reagent), in a solvent
(ethanol), calcining the resulting mixture (1000.degree. C., 2 h),
dry-milling the calcined mixture, and sintering the resulting
mixture (1550.degree. C., 2 h).
[0122] The ultimate vacuum inside the deposition chamber was
2.times.10.sup.-6 (Pa), and the oxygen partial pressure during the
deposition was 1 (Pa). The substrate temperature during the
deposition was room temperature. The distance between the target
and the substrate for deposition was 30 (mm). The power of the KrF
excimer laser was 1.5 (mJ/cm.sup.2/pulse), the pulse width was 20
(nsec), the repetition frequency was 10 (Hz), and the beam spot
diameter was 1.times.1 (mm square). The deposition rate was 6
(nm/min).
[0123] The resulting film was subjected to grazing incidence x-ray
diffraction (thin film method, incident angle: 0.5.degree.) for the
film surface, but no clear diffraction peak was observed. Thus, the
In--Ga--O thin film obtained was proved to be amorphous. The film
thickness was 120 nm. The electron carrier concentration and the
electron mobility of the In--Ga--O amorphous oxide film obtained
were 8.times.10.sup.16/cm.sup.3 and about 1 cm.sup.2/(Vsec),
respectively.
Example 8: Preparation of TFT Element (Glass Substrate) Using
In--Zn--Ga--O Amorphous Oxide Film
[0124] Atop-gate TFT element shown in FIG. 5 was prepared. First,
an In--Zn--Ga--O amorphous film 120 nm in thickness for use as a
channel layer (2) was formed on a glass substrate (1) by a method
of preparing the In--Ga--Zn--O amorphous oxide film according to
EXAMPLE 2 at an oxygen partial pressure of 5 Pa while using a
polycrystalline sinter represented by InGaO.sub.3(ZnO).sub.4 as the
target.
[0125] An In--Ga--Zn--O amorphous film having high electrical
conductivity and a gold film each 30 nm in thickness were deposited
on the In--Ga--Zn--O amorphous film by a PLD method while
controlling the oxygen partial pressure inside the chamber to less
than 1 Pa, and a drain terminal (5) and a source terminal (6) were
formed by a photolithographic method and a lift-off method.
[0126] Lastly, an Y.sub.2O.sub.3 film (thickness: 90 nm, relative
dielectric constant: about 15, leak current density: 10.sup.-3
A/cm.sup.2 upon application of 0.5 MV/cm) for use as a gate
insulating film (3) was formed by an electron beam deposition
method, and gold was deposited on the Y.sub.2O.sub.3 film. A gate
terminal (4) was formed by a photolithographic method and a
lift-off method. The channel length was 50 .mu.m and the channel
width was 200 .mu.m.
[0127] Evaluation of Characteristics of TFT Element
[0128] FIG. 6 shows the current-voltage characteristic of the TFT
element measured at room temperature. Since the drain current
I.sub.DS increased with the drain voltage V.sub.DS, the channel was
found to be of an n-conductivity type. This is consistent with the
fact that the amorphous In--Ga--Zn--O oxide film is an n-type
conductor. I.sub.DS was saturated (pinch-off) at about V.sub.DS=6
V, which was a typical behavior for semiconductor transistors. The
gain characteristic was determined, and the threshold value of the
gate voltage V.sub.GS when V.sub.DS=4 V was applied was about -0.5
V. Upon application of V.sub.G=10 V, current of
I.sub.ds=1.0.times.10.sup.-5 A flowed. This is because carriers
were induced in the In--Ga--Zn--O amorphous semiconductor thin
film, i.e., an insulator, due to the gate bias. The on/off ratio of
the transistor exceeded 10.sup.3. The field effect mobility was
determined from the output characteristics. As a result, a field
effect mobility of about 7 cm.sup.2(Vs).sup.-1 was obtained in the
saturation region.
[0129] The same measurements were carried out on the element while
irradiating the element with visible light, but no change in
transistor characteristics was observed. Note that the film can be
used as a channel layer of a TFT by controlling the electron
carrier concentration of the amorphous oxide to less than
10.sup.18/cm.sup.3. An electron carrier concentration of
10.sup.17/cm.sup.3 or less was more preferable, and an electron
carrier density of 10.sup.16/cm.sup.3 or less was yet more
preferable.
Example 9: Preparation of TFT Element Using In--Zn--Ga--O Amorphous
Oxide Film
[0130] A top-gate TFT element shown in FIG. 5 was prepared. In
particular, an In--Zn--Ga--O amorphous oxide film 120 nm in
thickness for use as a channel layer (2) was formed on a
polyethylene terephthalate (PET) film (1) by a deposition method of
EXAMPLE 2 in an atmosphere at an oxygen partial pressure of 5 Pa
using a polycrystalline sinter represented by InGaO.sub.3(ZnO) as
the target.
[0131] An In--Zn--Ga--O amorphous oxide film having high electrical
conductivity and a gold film each 30 nm in thickness were deposited
on the In--Zn--Ga--O amorphous oxide film by the PLD method at an
oxygen partial pressure inside the chamber of less than 1 Pa, and a
drain terminal (5) and a source terminal (6) were formed by a
photolithographic method and a lift-off method.
[0132] Lastly, a gate insulating film (3) was formed by an electron
beam deposition method and gold is deposited thereon. A gate
terminal (4) was then formed by a photolithographic method and a
lift-off method. The channel length was 50 .mu.m and the channel
width was 200 .mu.m. Three types of TFTs with the above-described
structure were prepared using Y.sub.2O.sub.3 (thickness: 140 nm),
Al.sub.2O.sub.3 (thickness: 130 nm) and HfO.sub.2 (thickness: 140
nm), respectively.
[0133] Evaluation of Characteristics of TFT Element
[0134] The current-voltage characteristic of the TFT element
measured at room temperature was similar to one shown in FIG. 6.
Namely, since the drain current I.sub.DS increased with the drain
voltage V.sub.DS, the channel was found to be of an n-conductivity
type. This is consistent with the fact that the amorphous
In--Ga--Zn--O amorphous oxide film is an n-type conductor. I.sub.DS
was saturated (pinch-off) at V.sub.DS=about 6 V, which was a
typical behavior for semiconductor transistors. When V.sub.g=0 V,
current of I.sub.ds=10.sup.-8 A flowed, and when V.sub.g=10 V,
current of I.sub.ds=2.0.ltoreq.x.ltoreq.10.sup.-5 A flowed. This is
because carriers were induced in the In--Ga--Zn--O amorphous oxide
thin film, i.e., an insulator, due to the gate bias. The on/off
ratio of the transistor exceeded 10.sup.3. The field effect
mobility was determined from the output characteristics. As a
result, a field effect mobility of about 7 cm.sup.2(Vs).sup.-1 was
obtained in the saturation region.
[0135] The element formed on the PET film was inflected at a radius
of curvature of 30 mm, and the same transistor characteristic was
measured. No change in transistor characteristic was observed.
[0136] The TFT including the gate insulating film made from the
Al.sub.2O.sub.3 film also showed similar transistor characteristics
to those shown in FIG. 6. When V.sub.g=0 V, current of
I.sub.ds=10.sup.-8 A flowed, and when V.sub.g=10 V, current of
I.sub.ds=5.0.times.10.sup.-6 A flowed. The on/off ratio of the
transistor exceeded 10.sup.2. The field effect mobility was
determined from the output characteristics. As a result, a field
effect mobility of about 2 cm.sup.2(Vs).sup.-1 was obtained in the
saturation region.
[0137] The TFT including the gate insulating film made from the
HfO.sub.2 film also showed similar transistor characteristics to
those shown in FIG. 6. When V.sub.g=0 V, current of
I.sub.ds=10.sup.-8 A flowed, and when V.sub.g=10 V, current of
I.sub.ds=1.0.times.10.sup.-6 A flowed. The on/off ratio of the
transistor exceeded 10.sup.2. The field effect mobility was
determined from the output characteristics. As a result, a field
effect mobility of about 10 cm.sup.2(Vs).sup.-1 was obtained in the
saturation region.
Example 10: Preparation of TFT Element Using In.sub.2O.sub.3
Amorphous Oxide Film by PLD Method
[0138] A top-gate TFT element shown in FIG. 5 was prepared. First,
an In.sub.2O.sub.3 amorphous oxide film 80 nm in thickness for use
as a channel layer (2) was formed on a polyethylene terephthalate
(PET) film (1) by the deposition method of EXAMPLE 5.
[0139] An In.sub.2O.sub.3 amorphous oxide film having high
electrical conductivity and a gold layer each 30 nm in thickness
were formed on this In.sub.2O.sub.3 amorphous oxide film by the PLD
method at an oxygen partial pressure inside the chamber of less
than 1 Pa while applying zero voltage to the oxygen radical
generator. A drain terminal (5) and a source terminal (6) were then
formed by a photolithographic method and a lift-off method.
[0140] Lastly, an Y.sub.2O.sub.3 film for use as a gate insulating
film (3) was formed by an electron beam deposition method, and gold
was deposited on the Y.sub.2O.sub.3 film. A gate terminal (4) was
formed by a photolithographic method and a lift-off method.
[0141] Evaluation of Characteristics of TFT Element
[0142] The current-voltage characteristics of the TFT element
formed on the PET film were measured at room temperature. Since the
drain current I.sub.DS increased with the drain voltage V.sub.DS,
the channel was found to be of an n-conductivity type. This is
consistent with the fact that the amorphous In--O amorphous oxide
film is an n-type conductor. I.sub.DS was saturated (pinch-off) at
V.sub.DS=about 5 V, which was a typical behavior for semiconductor
transistors. When V.sub.g=0 V, current of 2.times.10.sup.-8 A
flowed, and when V.sub.g=10 V, current I.sub.ds=2.0.times.10.sup.-6
A flowed. This is because carriers were induced in the In--O
amorphous oxide thin film, i.e., an insulator, due to the gate
bias. The on/off ratio of the transistor was about 10.sup.2. The
field effect mobility was determined from the output
characteristics. As a result, a field effect mobility of about 10
cm.sup.2(Vs).sup.-1 was obtained in the saturation region.
[0143] The TFT element formed on a glass substrate showed similar
characteristics. The element formed on the PET film was inflected
at a radius of curvature of 30 mm, and the same transistor
characteristics were measured. No change in transistor
characteristics was observed.
Example 11: Preparation of TFT Element Using In--Sn--O Amorphous
Oxide Film by PLD Method
[0144] A top gate TFT element shown in FIG. 5 was prepared. In
particular, an In--Sn--O amorphous oxide film 100 nm in thickness
for use as a channel layer (2) was formed on a polyethylene
terephthalate (PET) film (1) by a deposition method of EXAMPLE
6.
[0145] An In--Sn--O amorphous oxide film having high electrical
conductivity and a gold film each 30 nm in thickness were deposited
on this In--Sn--O amorphous oxide film by the PLD method at an
oxygen partial pressure inside the chamber of less than 1 Pa while
applying zero voltage to the oxygen radical generator. A drain
terminal (5) and a source terminal (6) were formed by a
photolithographic method and a lift-off method.
[0146] Lastly, an Y.sub.2O.sub.3 film for use as a gate insulating
film (3) was formed by an electron beam deposition method and gold
was deposited thereon. A gate terminal (4) was then formed by a
photolithographic method and a lift-off method.
[0147] Evaluation of Characteristics of TFT Element
[0148] The current-voltage characteristic of the TFT element formed
on the PET film was measured at room temperature. Since the drain
current I.sub.DS increased with the drain voltage V.sub.DS, the
channel was found to be of an n-conductivity type. This is
consistent with the fact that the amorphous In--Sn--O amorphous
oxide film is an n-type conductor. I.sub.DS was saturated
(pinch-off) at V.sub.DS=about 6 V, which was a typical behavior for
semiconductor transistors. When V.sub.g=0 V, current of
5.times.10.sup.-8 A flowed, and when V.sub.g=10 V, current of
I.sub.ds=5.0.times.10.sup.-5 A flowed. This is because carriers
were induced in the In--Sn--O amorphous oxide thin film, i.e., an
insulator, due to the gate bias. The on/off ratio of the transistor
was about 10.sup.3. The field effect mobility was determined from
the output characteristics. As a result, a field effect mobility of
about 5 cm.sup.2(Vs).sup.-1 was obtained in the saturation
region.
[0149] The TFT element formed on a glass substrate showed similar
characteristics. The element formed on the PET film was inflected
at a radius of curvature of 30 mm, and the same transistor
characteristics were measured. No change in transistor
characteristics was observed.
Example 12: Preparation of TFT Element Using In--Ga--O Amorphous
Oxide Film by PLD Method
[0150] Atop gate TFT element shown in FIG. 5 was prepared. In
particular, an In--Ga--O amorphous oxide film 120 nm in thickness
for use as a channel layer (2) was formed on a polyethylene
terephthalate (PET) film (1) by the deposition method of EXAMPLE
7.
[0151] An In--Ga--O amorphous oxide film having high electrical
conductivity and a gold film each 30 nm in thickness were formed on
this In--Ga--O amorphous oxide film by the PLD method at an oxygen
partial pressure inside the chamber of less than 1 Pa while
applying zero voltage to the oxygen radical generator. A drain
terminal (5) and a source terminal (6) were formed by a
photolithographic method and a lift-off method.
[0152] Lastly, an Y.sub.2O.sub.3 film for use as a gate insulating
film (3) was formed by an electron beam deposition method and gold
was deposited thereon. A gate terminal (4) was then formed by a
photolithographic method and a lift-off method.
[0153] Evaluation of Characteristics of TFT Element
[0154] The current-voltage characteristic of the TFT element formed
on the PET film was measured at room temperature. Since the drain
current I.sub.DS increased with the drain voltage V.sub.DS, the
channel was found to be of an n-conductivity type. This is
consistent with the fact that the amorphous In--Ga--O amorphous
oxide film is an n-type conductor. I.sub.DS was saturated
(pinch-off) at V.sub.DS=about 6 V, which was a typical behavior for
semiconductor transistors. When V.sub.g=0 V, current of
1.times.10.sup.-8 A flowed, and when V.sub.g=10 V, current of
I.sub.ds=1.0.times.10.sup.-6 A flowed. This corresponds to the
induction of electron carriers inside the insulator, In--Ga--O
amorphous oxide film by the gate bias. The on/off ratio of the
transistor was about 10.sup.2. The field effect mobility was
determined from the output characteristics. As a result, a field
effect mobility of about 0.8 cm.sup.2(Vs).sup.-1 was obtained in
the saturation region.
[0155] The TFT element formed on a glass substrate showed similar
characteristics. The element formed on the PET film was inflected
at a radius of curvature of 30 mm, and the same transistor
characteristics were measured. No change in transistor
characteristics was observed.
[0156] It should be noted that, as described in EXAMPLES above, the
film can be used as a channel layer of a TFT by controlling the
electron carrier concentration to less than 10.sup.18/cm.sup.3. The
electron carrier concentration is more preferably
10.sup.17/cm.sup.3 or less and yet more preferably
10.sup.16/cm.sup.3 or less.
INDUSTRIAL APPLICABILITY
[0157] The amorphous oxide of the present invention can be used in
semiconductor devices such as thin film transistors. The thin film
transistors can be used as switching elements of LCDs and organic
EL displays and are also widely applicable to see-through-type
displays, IC cards, ID tags, etc.
* * * * *