U.S. patent application number 11/611228 was filed with the patent office on 2007-05-03 for devices containing annealed stabilized silver nanoparticles.
This patent application is currently assigned to XEROX CORPORATION. Invention is credited to Yuning Li, Beng S. Ong, Yiliang Wu.
Application Number | 20070099357 11/611228 |
Document ID | / |
Family ID | 35148805 |
Filed Date | 2007-05-03 |
United States Patent
Application |
20070099357 |
Kind Code |
A1 |
Li; Yuning ; et al. |
May 3, 2007 |
DEVICES CONTAINING ANNEALED STABILIZED SILVER NANOPARTICLES
Abstract
An electronic device including in any suitable sequence: a
substrate; an optional insulating layer or an optional
semiconductor layer, or both the optional insulating layer and the
optional semiconductor layer; and an electrically conductive
element of the electronic device, wherein the electrically
conductive element comprises annealed silver-containing
nanoparticles, wherein the silver-containing nanoparticles are a
product of a reaction of a silver compound with a reducing agent
comprising a hydrazine compound in the presence of a thermally
removable stabilizer in a reaction mixture comprising the silver
compound, the reducing agent, the stabilizer, and an optional
solvent.
Inventors: |
Li; Yuning; (Mississauga,
Ontario, ON) ; Wu; Yiliang; (Mississauga, Ontario,
ON) ; Ong; Beng S.; (Mississauga, Ontario,
ON) |
Correspondence
Address: |
PATENT DOCUMENTATION CENTER
XEROX CORPORATION
100 CLINTON AVE., SOUTH, XEROX SQUARE, 20TH FLOOR
ROCHESTER
NY
14644
US
|
Assignee: |
XEROX CORPORATION
800 Long Ridge Road, PO Box 1600
Stamford
CT
06904-1600
|
Family ID: |
35148805 |
Appl. No.: |
11/611228 |
Filed: |
December 15, 2006 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10958937 |
Oct 5, 2004 |
|
|
|
11611228 |
Dec 15, 2006 |
|
|
|
Current U.S.
Class: |
438/149 ;
257/347; 438/154 |
Current CPC
Class: |
H01L 51/0021 20130101;
B22F 2998/00 20130101; Y10S 977/896 20130101; Y10S 977/81 20130101;
Y10S 977/932 20130101; Y10S 977/777 20130101; B22F 1/0062 20130101;
B22F 1/0018 20130101; H01B 1/22 20130101; B22F 9/24 20130101; H01L
51/105 20130101; Y10S 977/786 20130101; B82Y 30/00 20130101; Y10S
977/787 20130101; B22F 2998/00 20130101; B22F 1/0062 20130101 |
Class at
Publication: |
438/149 ;
438/154; 257/347 |
International
Class: |
H01L 21/84 20060101
H01L021/84; H01L 21/00 20060101 H01L021/00; H01L 27/12 20060101
H01L027/12; H01L 27/01 20060101 H01L027/01; H01L 31/0392 20060101
H01L031/0392 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0003] This invention was made with United States Government
support under Cooperative Agreement No. 70NANBOH3033 awarded by the
National Institute of Standards and Technology (NIST). The United
States Government has certain rights in the invention.
Claims
1. An electronic device comprising in any suitable sequence: a
substrate; an optional insulating layer or an optional
semiconductor layer, or both the optional insulating layer and the
optional semiconductor layer; and an electrically conductive
element of the electronic device, wherein the electrically
conductive element comprises annealed silver-containing
nanoparticles, wherein the silver-containing nanoparticles are a
product of a reaction of a silver compound with a reducing agent
comprising a hydrazine compound in the presence of a thermally
removable stabilizer in a reaction mixture comprising the silver
compound, the reducing agent, the stabilizer, and an optional
solvent.
2. A thin film transistor circuit comprising an array of thin film
transistors including electrodes, connecting conductive lines and
conductive pads, wherein the electrodes, the connecting conductive
lines, or the conductive pads, or a combination of any two or all
of the electrodes, the connecting conductive lines and the
conductive pads comprise annealed silver-containing nanoparticles,
wherein the silver-containing nanoparticles are a product of a
reaction of a silver compound with a reducing agent comprising a
hydrazine compound in the presence of a thermally removable
stabilizer in a reaction mixture comprising the silver compound,
the reducing agent, the stabilizer, and an optional solvent.
3. A thin film transistor comprising: (a) an insulating layer; (b)
a gate electrode; (c) a semiconductor layer; (d) a source
electrode; and (e) a drain electrode, wherein the insulating layer,
the gate electrode, the semiconductor layer, the source electrode,
and the drain electrode are in any sequence as long as the gate
electrode and the semiconductor layer both contact the insulating
layer, and the source electrode and the drain electrode both
contact the semiconductor layer, and wherein at least one of the
source electrode, the drain electrode, and the gate electrode
comprises annealed silver-containing nanoparticles, wherein the
silver-containing nanoparticles are a product of a reaction of a
silver compound with a reducing agent comprising a hydrazine
compound in the presence of a thermally removable stabilizer in a
reaction mixture comprising the silver compound, the reducing
agent, the stabilizer, and an optional solvent.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional application of co-pending
U.S. application Ser. No. 10/958,937 (filing date Oct. 5, 2004)
from which priority is claimed, the disclosure of which is totally
incorporated herein by reference.
[0002] Yiliang Wu et al., U.S. application Ser. No. 10/733,136
(Attorney Docket No. D/A3401), published as US 2005/0129843A1.
BACKGROUND OF THE INVENTION
[0004] Fabrication of electronic circuit elements using liquid
deposition techniques is of profound interest as such techniques
provide potentially low-cost alternatives to conventional
mainstream amorphous silicon technologies for electronic
applications such as thin film transistors (TFTs), light-emitting
diodes (LEDs), RFID tags, photovoltaics, etc. However the
deposition and/or patterning of functional electrodes, pixel pads,
and conductive traces, lines and tracks which meet the
conductivity, processing, and cost requirements for practical
applications have been a great challenge. Silver is of particular
interest as conductive elements for electronic devices because
silver is much lower in cost than gold and it possesses much better
environmental stability than copper. There is therefore a critical
need, addressed by embodiments of the present invention, for lower
cost methods for preparing liquid processable, stable
silver-containing nanoparticle compositions that are suitable for
fabricating electrically conductive elements of electronic
devices.
[0005] The following documents provide background information:
[0006] Pozarnsky et al., U.S. Pat. No. 6,688,494.
[0007] Lee et al., U.S. Pat. No. 6,572,673 discloses hydrazide as a
reducing agent at for example column 1, lines 52-53.
[0008] Heath et al., U.S. Pat. No. 6,103,868.
[0009] Wilcoxon, U.S. Pat. No. 5,147,841 discloses hydrazine as a
reducing agent at for example column 4, line 44.
[0010] G. Blanchet and J. Rodgers, "Printed Techniques for Plastic
Electronics" , Journal of Imaging Science and Technology, Vol. 47,
No. 4, pp. 296-303 (July/August 2003).
[0011] P. Buffat and J-P. Borel, "Size effect on the melting
temperature of gold particles", Physical Review A, Vol., 13, No. 6,
pp. 2287-2298 (June 1976).
[0012] C. Hayashi, "Ultrafine Particles", J. Vacuum Sci. Technol.
A, Vol. 5, No. 4, pp. 1375-1384 (July/August 1987).
[0013] S. B. Fuller, E. J. Wilhelm, and J. M. Jacobson, "Ink-Jet
Printed Nanoparticle Microelectromechanical Systems", Journal of
Microelectromechanical Systems, Vol. 11, No. 1, pp. 54-60 (February
2002).
[0014] X. Z. Lin, X. Teng, and H. Yang, "Direct Synthesis of
Narrowly Dispersed Silver Nanoparticles Using a Single-Source
Precursor", Langmuir, Vol. 19, pp. 10081-10085 (published on web
Nov. 1, 2003).
[0015] H. Hiramatsu and F. E. Osterloh, "A Simple Large-Scale
Synthesis of Nearly Monodisperse Gold and Silver Nanoparticles with
Adjustable Sizes and with Exchangeable Surfactants", Chem. Mater.,
Vol. 16, No. 13, pp. 2509-2511 (June 29, 2004; published on web May
28, 2004).
SUMMARY OF THE DISCLOSURE
[0016] In embodiments, there is provided a process comprising:
reacting a silver compound with a reducing agent comprising a
hydrazine compound in the presence of a thermally removable
stabilizer in a reaction mixture comprising the silver compound,
the reducing agent, the stabilizer, and an optional solvent, to
form a plurality of silver-containing nanoparticles with molecules
of the stabilizer on the surface of the silver-containing
nanoparticles.
[0017] In further embodiments, there is provided a process
comprising:
[0018] (a) reacting a silver compound with a reducing agent
comprising a hydrazine compound in the presence of a thermally
removable stabilizer in a reaction mixture comprising the silver
compound, the reducing agent, the stabilizer, and an optional
solvent, to form a plurality of silver-containing nanoparticles
with molecules of the stabilizer on the surface of the
silver-containing nanoparticles;
[0019] (b) isolating the plurality of silver-containing
nanoparticles with the molecules of the stabilizer on the surface
of the silver-containing nanoparticles; and
[0020] (c) preparing a composition including a liquid and the
plurality of silver-containing nanoparticles with molecules of the
stabilizer on the surface of the silver-containing
nanoparticles.
[0021] In other embodiments, there is provided a process
comprising:
[0022] (a) depositing a composition comprising a liquid and a
plurality of silver-containing nanoparticles with a stabilizer on a
substrate by a liquid deposition technique to form a deposited
composition, wherein the silver-containing nanoparticles are
obtained by reacting a silver compound with a reducing agent
comprising a hydrazine compound in the presence of a thermally
removable stabilizer in a reaction mixture comprising the silver
compound, the reducing agent, the stabilizer, and an optional
solvent; and
[0023] (b) heating the deposited composition to form an
electrically conductive layer comprising silver.
[0024] There is further provided in embodiments, a composition
comprising a liquid and a plurality of silver-containing
nanoparticles with a stabilizer, wherein the silver-containing
nanoparticles are a product of a reaction of a silver compound with
a reducing agent comprising a hydrazine compound in the presence of
a thermally removable stabilizer in a reaction mixture comprising
the silver compound, the reducing agent, the stabilizer, and an
optional solvent.
[0025] In additional embodiments, there is provided an electronic
device comprising in any suitable sequence:
[0026] a substrate;
[0027] an optional insulating layer or an optional semiconductor
layer, or both the optional insulating layer and the optional
semiconductor layer; and
[0028] an electrically conductive element of the electronic device,
wherein the electrically conductive element comprises annealed
silver-containing nanoparticles, wherein the silver-containing
nanoparticles are a product of a reaction of a silver compound with
a reducing agent comprising a hydrazine compound in the presence of
a thermally removable stabilizer in a reaction mixture comprising
the silver compound, the reducing agent, the stabilizer, and an
optional solvent.
[0029] In more embodiments, there is provided a thin film
transistor circuit comprising an array of thin film transistors
including electrodes, connecting conductive lines and conductive
pads, wherein the electrodes, the connecting conductive lines, or
the conductive pads, or a combination of any two or all of the
electrodes, the connecting conductive lines and the conductive pads
comprise annealed silver-containing nanoparticles, wherein the
silver-containing nanoparticles are a product of a reaction of a
silver compound with a reducing agent comprising a hydrazine
compound in the presence of a thermally removable stabilizer in a
reaction mixture comprising the silver compound, the reducing
agent, the stabilizer, and an optional solvent.
[0030] In yet other embodiments, there is provided a thin film
transistor comprising:
[0031] (a) an insulating layer;
[0032] (b) a gate electrode;
[0033] (c) a semiconductor layer;
[0034] (d) a source electrode; and
[0035] (e) a drain electrode,
[0036] wherein the insulating layer, the gate electrode, the
semiconductor layer, the source electrode, and the drain electrode
are in any sequence as long as the gate electrode and the
semiconductor layer both contact the insulating layer, and the
source electrode and the drain electrode both contact the
semiconductor layer, and
[0037] wherein at least one of the source electrode, the drain
electrode, and the gate electrode comprises annealed
silver-containing nanoparticles, wherein the silver-containing
nanoparticles are a product of a reaction of a silver compound with
a reducing agent comprising a hydrazine compound in the presence of
a thermally removable stabilizer in a reaction mixture comprising
the silver compound, the reducing agent, the stabilizer, and an
optional solvent.
BRIEF DESCRIPTION OF THE DRAWINGS
[0038] Other aspects of the present invention will become apparent
as the following description proceeds and upon reference to the
following figures which represent exemplary embodiments:
[0039] FIG. 1 represents a first embodiment of a thin film
transistor wherein the conductive layers were made using the
present silver-containing nanoparticles.
[0040] FIG. 3 represents a third embodiment of a thin film
transistor wherein the conductive layers were made using the
present silver-containing nanoparticles.
[0041] FIG. 4 represents a fourth embodiment of a thin film
transistor wherein the conductive layers were made using the
present silver-containing nanoparticles.
[0042] Unless otherwise noted, the same reference numeral in
different Figures refers to the same or similar feature.
DETAILED DESCRIPTION
[0043] Suitable silver compounds include organic and inorganic
silver compounds. In embodiments, the silver compounds include
silver acetate, silver carbonate, silver nitrate, silver
perchlorate, silver phosphate, silver trifluoroacetate, silver
benzoate, silver lactate, and the like, or mixtures thereof in any
suitable ratio.
[0044] The reducing agent for the silver compounds includes a
hydrazine compound. The hydrazine compound includes hydrazine and
any suitable derivatives (substituted at one or both nitrogen atoms
where each nitrogen atom can be substituted one or two times with
the same or different substituent), as well as salts and hydrates
of hydrazine and salts and hydrates of the hydrazine derivatives.
It is understood that the exemplary compounds described herein for
the hydrazine compound also include the hydrate form where
applicable. For example, the compound "hydrazine" includes
hydrazine hydrate and hydrazine not in hydrated form. Exemplary
examples of the hydrazine compound are as follows:
[0045] Hydrazine (H.sub.2HNH.sub.2);
[0046] Hydrazine salt such as for example hydrazine acid tartrate,
hydrazine monohydrobromide, hydrazine monohydrochloride, hydrazine
dichloride, hydrazine monooxalate, and hydrazine sulfate.
[0047] Hydrocarbyl hydrazine (e.g., RNHNH.sub.2 and RNHNHR and
RRNNH.sub.2) where one nitrogen atom is mono- or di-substituted
with R, and the other nitrogen atom is optionally mono- or
di-substituted with R, where each R is an independently selected
hydrocarbon group such as methyl ethyl, propyl, butyl,
hydroxyethyl, phenyl, benzyl, tolyl, bromophenyl, chloropehnyl,
nitrophenyl, xylyl, and the like. Illustrative examples of
hydrocarbyl hydrazine include for example, methylhydrazine,
tert-butylhydrazine, 2-hydroxyethylhydrazine, benzylhydrazine,
phenylhydrazine, tolylhydrazine, bromophenylhydrazine,
chlorophenylhydrazine, nitrophenylhydrazine, 1,1-dimethylhydrazine,
1,1-diphenylhydrazine, 1,2-diethylhydrazine, and
1,2-diphenylhydrazine.
[0048] Hydrocarbyl hydrazine salt (which is a salt of the
hydrocarbyl hydrazine described herein) such as for example
methylhydrazine hydrochloride, phenylhydrazine hydrochloride,
benzylhydrazine oxalate, butylhydrazine hydrochloride,
butylhydrazine oxalate salt, and propylhydrazine oxalate salt.
[0049] Hydrazide (e.g., RC(O)NHNH.sub.2 and RC(O)NHNHR' and
RC(O)NHNHC(O)R) where one or both nitrogen atoms are substituted by
an acyl group of formula RC(O), where each R is independently
selected from hydrogen and a hydrocarbon group, and one or both
nitrogen atoms are optionally mono- or di-substituted with R',
where each R' is an independently selected hydrocarbon group.
Illustrative examples of hydrazide are for example, formic
hydrazide, acethydrazide, benzhydrazide, adipic acid dihydrazide,
carbohydrazide, butanohydrazide, hexanoic hydrazide, octanoic
hydrazide, oxamic acid hydrazide, maleic hydrazide,
N-methylhydrazinecarboxamide, and semicarbazide.
[0050] Carbazate (or hydrazinocarboxylate) (e.g., ROC(O)NHNHR' and
ROC(O)NHNH.sub.2 and ROC(O)NHNHC(O)OR) where one or both nitrogen
atoms are substituted by an ester group of formula ROC(O), where
each R is independently selected from hydrogen and a hydrocarbon
group, and one or both nitrogen atoms are optionally mono- or
di-substituted with R', where each R' is an independently selected
hydrocarbon group. Illustrative examples of carbazate are for
example, methyl carbazate (methyl hydrazinocarboxylate), ethyl
carbazate, butyl carbazate, benzyl carbazate, and 2-hydroxyethyl
carbazate.
[0051] Sulfonohydrazide (e.g., RSO.sub.2NHNH.sub.2,
RSO.sub.2NHNHR', and RSO.sub.2NHNHSO.sub.2R) where one or both
nitrogen atoms are substituted by a sulfonyl group of formula
RSO.sub.2, where each R is independently selected from hydrogen and
a hydrocarbon group, and one or both nitrogen atoms are optionally
mono- or di-substituted with R', where each R' is an independently
selected hydrocarbon group. Iullustraive examples of
sulfonohydrazide are for example, methanesulfonohydrazide,
benzenesulfonohydrazine, 2,4,6-trimethylbenzenesulfonohydrazide,
and p-toluenesulfonohydrazide.
[0052] Other exemplary hydrazine compounds are for example
hydrazine acetate, aminoguanidine, thiosemicarbazide, methyl
hydrazinecarbimidothiolate, and thiocarbohydrazide.
[0053] Unless otherwise indicated, in identifying the substituents
for R and R' of the various hydrazine compounds, the phrase
"hydrocarbon group" encompasses both unsubstituted hydrocarbon
groups and substituted hydrocarbon groups. Unsubstituted
hydrocarbon groups may be for example a straight chain alkyl group,
a branched alkyl group, a cycloalkyl group, an aryl group, an
alkylaryl group, and an arylalkyl group. Exemplary alkyl groups
include for example methyl, ethyl, propyl, butyl, pentyl, hexyl,
heptyl, octyl, nonyl, decyl, undecyl, dodecyl, tridecyl,
tetradecyl, pentadecyl, cyclopentyl, cyclohexyl, cycloheptyl, and
isomeric forms thereof. Substituted hydrocarbon groups may be the
unsubstituted hydrocarbon groups described herein which are
substituted one, two or more times with for example a halogen
(chlorine, bromine, fluorine, and iodine), nitro, cyano, an alkoxy
group (e.g., methoxyl, ethoxyl, and propoxy), or a mixture thereof.
In embodiments, the hydrocarbon group may be optionally substituted
alkyl and optionally substituted aryl.
[0054] In embodiments, the hydrazine compound is other than a
hydrazine and a hydrazine salt; in other embodiments, the hydrazine
compound is other than a hydrazide; and in further embodiments, the
hydrazine compound is other than a hydrazine, a hydrazine salt, and
a hydrazide.
[0055] One, two, three or more reducing agents may be used. In
embodiments where two or more reducing agents are used, each
reducing agent may be present at any suitable weight ratio or molar
ratio such as for example from about 99(first reducing
agent):1(second reducing agent) to about 1(first reducing
agent):99(second reducing agent). The amount of reducing agent used
in the embodiments of the present invention is for example about
0.25 molar equivalent or more per mole of silver compound.
[0056] Any suitable stabilizer may be used which has the function
of minimizing or preventing the silver-containing nanoparticles
from aggregation in a liquid and optionally providing the
solubility or dispersibility of silver-containing nanoparticles in
a liquid. In addition, the stabilizer is thermally removable which
means that the stabilizer can be caused to dissociate from the
silver-containing nanoparticle surface under certain conditions
such as through heating. The heating may be accomplished to a
certain temperature such as for example below about 250 degree C.,
or below about 200 degree C., under normal atmospheric conditions
or at a reduced pressure of for example from several mbars to about
10.sup.-3 mbar. The thermal dissociation of the stabilizer from the
silver-containing nanoparticles at a temperature such as for
example lower than about 250 degree C. may result in the
evaporation of the stabilizer or decomposition of the stabilizer
into gaseous forms.
[0057] In embodiments, the stabilizer may be an organic stabilizer.
The term "organic" in "organic stabilizer" refers to the presence
of carbon atom(s), but the organic stabilizer may include one or
more non-metal heteroatoms such as nitrogen, oxygen, sulfur,
silicon, halogen, and the like. Exemplary organic stabilizers
include for instance thiol and its derivatives, amine and its
derivatives, carboxylic acid and its carboxylate derivatives,
polyethylene glycols, and other organic surfactants. In
embodiments, the organic stabilizer is selected from the group
consisting of a thiol such as for example butanethiol,
pentanethiol, hexanethiol, heptanethiol, octanethiol, decanethiol,
and dodecanethiol; an amine such as for example ethylamine,
propylamine, butylamine, penylamine, hexylamine, heptylamine,
octylamine, nonylamine, decylamine, and dodecylamine; a dithiol
such as for example 1,2-ethanedithiol, 1,3-propanedithiol, and
1,4-butanedithiol; a diamine such as for example ethylenediamine,
1,3-diaminopropane, 1,4-diaminobutane; a mixture of a thiol and a
dithiol; and a mixture of an amine and a diamine. Organic
stabilizers containing a pyridine derivative (e.g., dodecyl
pyridine) and/or organophosphine that can stabilize
silver-containing nanoparticles are also included as a stabilizer
in embodiments of the present invention.
[0058] In embodiments, the stabilizer is an organoamine such as for
example butylamine, pentylamine, hexylamine, heptylamine,
octylamine, nonylamine, decylamine, hexadecylamine, undecylamine,
dodecylamine, tridecylamine, tetradecylamine, diaminopentane,
diaminohexane, diaminoheptane, diaminooctane, diaminononane,
diaminodecane, diaminooctane, dipropylamine, dibutylamine,
dipentylamine, dihexylamine, diheptylamine, dioctylamine,
dinonylamine, didecylamine, methylpropylamine, ethylpropylamine,
propylbutylamine, ethylbutylamine, ethylpentylamine,
propylpentylamine, butylpentylamine, tributylamine, trihexylamine,
and the like, or mixtures thereof.
[0059] One, two, three or more stabilizers may be used. In
embodiments where two or more stabilizers are used, each stabilizer
may be present at any suitable weight ratio or molar ratio such as
for example from about 99(first stabilizer):1(second stabilizer) to
about 1(first stabilizer):99(second stabilizer). The amount of the
stabilizer used is for example about 1 or more molar equivalents
per mole of silver compound, or about 2 or more molar equivalents
per mole of silver compound, or about 10 or more molar equivalents
per mole of silver compound, or about 25 or more molar equivalents
per mole of silver compound.
[0060] In embodiments, the silver-containing nanoparticles may form
a chemical bond with the stabilizer. The chemical names of the
stabilizer provided herein are before formation of any chemical
bond with the silver-containing nanoparticles. It is noted that the
nature of the stabilizer may change with the formation of a
chemical bond, but for convenience the chemical name prior to
formation of the chemical bond is used.
[0061] The attractive force between the silver-containing
nanoparticles and the stabilizer can be a chemical bond and/or
physical attachment. The chemical bond can take the form of for
example covalent bonding, hydrogen bonding, coordination complex
bonding, or ionic bonding, or a mixture of different chemical
bondings. The physical attachment can take the form of for example
van der Waals' forces or dipole-dipole interaction, or a mixture of
different physical attachments.
[0062] The extent of the coverage of stabilizer on the surface of
the silver-containing nanoparticles can vary for example from
partial to full coverage depending for instance on the capability
of the stabilizer to stabilize the silver-containing nanoparticles
in the solvent. Of course, there is variability as well in the
extent of coverage of the stabilizer among the individual
silver-containing nanoparticles.
[0063] Any suitable solvent can be used for the reaction mixture
including for example organic solvents and/or water. The organic
solvents include for example hydrocarbon solvents such as pentane,
hexane, cyclohexane, heptane, octane, nonane, decane, undecane,
dodecane, tridecane, tetradecane, toluene, xylene, mesitylene, and
the like; alcohols such as methanol, ethanol, propanol, butanol,
pentanol and the like; tetrahydrofuran; chlorobenzene;
dichlorobenzene; trichlorobenzene; nitrobenzene; cyanobenzene;
acetonitrile; and mixtures thereof. One, two, three or more
solvents may be used. In embodiments where two or more solvents are
used, each solvent may be present at any suitable volume ratio or
molar ratio such as for example from about 99(first solvent):
1(second solvent) to about 1(first solvent):99(second solvent).
[0064] The reaction of the silver compound with the reducing agent
is carried out at a suitable temperature of for example from about
-50.degree. C. to about 200.degree. C., or from about 0.degree. C.
to about 150.degree. C., particularly at a temperature ranging for
example from about 20.degree. C. to about 120.degree. C.
[0065] The silver-containing nanoparticles have a particle size of
for example less than about 100 nm, less than about 50 nm, less
than about 25 nm, or less than about 10 nm. The particle size is
defined herein as the average diameter of silver-containing
particle core, excluding the stabilizer, as determined by
transmission electron microscopy ("TEM"). Generally, a plurality of
particle sizes may exist in the silver-containing nanoparticles
obtained from the preparation. In embodiments, the existence of
different sized silver-containing nanoparticles is acceptable.
[0066] In embodiments, the silver-containing nanoparticles are
composed of elemental silver or a silver composite. Besides silver,
the silver composite includes either or both of (i) one or more
other metals and (ii) one or more non-metals. Suitable other metals
include for example Al, Au, Pt, Pd, Cu, Co, Cr, In, and Ni,
particularly the transition metals for example Au, Pt, Pd, Cu, Cr,
Ni, and mixtures thereof. Exemplary metal composites are Au--Ag,
Ag--Cu, Au--Ag--Cu, and Au--Ag--Pd. Suitable non-metals in the
metal composite include for example Si, C, and Ge. The various
components of the silver composite may be present in an amount
ranging for example from about 0.01% to about 99.9% by weight,
particularly from about 10% to about 90% by weight. In embodiments,
the silver composite is a metal alloy composed of silver and one,
two or more other metals, with silver comprising for example at
least about 20% of the nanoparticles by weight, particularly
greater than about 50% of the nanoparticles by weight. Unless
otherwise noted, the weight percentages recited herein for the
components of the silver-containing nanoparticles do not include
the stabilizer.
[0067] Silver-containing nanoparticles composed of a silver
composite can be made for example by using a mixture of (i) a
silver compound (or compounds) and (ii) another metal salt (or
salts) or another non-metal (or non-metals) in the reaction.
[0068] The preparation of silver-containing nanoparticle
compositions, which are suitable for the preparation of conductive
elements for electronic applications can be carried out using all
or some of the following procedures: (i) addition of a scavenger to
the final reaction mixture from the preparation of
silver-containing nanoparticles to destroy excess reducing agent;
(ii) concentrating the reaction mixture by removing solvent; (iii)
adding the concentrated reaction mixture to a non-solvent (or vice
versa) to precipitate the silver-containing nanoparticles; (iv)
collecting the silver-containing nanoparticles by filtration or
centrifugation to result in isolated silver-containing
nanoparticles (with the stabilizer molecules on the surface of the
silver-containing nanoparticles); (v) dissolving or dispersing
(assisted by for example ultrasonic and/or mechanical stirring) the
isolated silver-containing nanoparticles (with molecules of the
stabilizer on the surface of the silver-containing nanoparticles)
in an appropriate liquid.
[0069] Silver-containing nanoparticle compositions can also be made
by mixing silver-containing nanoparticles with other metal or
non-metal nanoparticles.
[0070] In embodiments, it may be possible to form a
silver-containing nanoparticle composition (with stabilizer
molecules on the surface of the silver-containing nanoparticles)
suitable for forming conductive elements for electronic
applications without the need for the above described procedures to
isolate the silver-containing nanoparticles from the reaction
mixture. In such embodiments, the reaction mixture (optionally
augmented with another liquid which may be the same or different
from the solvent used in the reaction mixture) may be considered
the silver-containing nanoparticle composition.
[0071] The scavengers that can be used to destroy excess reducing
agent include for example ketone, aldehyde, carboxylic acid, or a
mixture thereof. Specific exemplary scavengers include acetone,
butanone, pentanone, formaldehyde, acetaldehyde, acetic acid, and
the like, or a mixture thereof.
[0072] Suitable non-solvents that can be used for the precipitation
of silver-containing nanoparticles include any liquids that are
mixable with the reaction solvent or solvents for the preparation
of silver-containing nanoparticles.
[0073] The liquid that can be used to disperse or dissolve
silver-containing nanoparticles to form a silver-containing
nanoparticle composition includes organic liquids or water. The
organic liquids include for example hydrocarbon solvents such as
pentane, hexane, cyclohexane, heptane, octane, nonane, decane,
undecane, dodecane, tridecane, tetradecane, toluene, xylene,
mesitylene, and the like; alcohols such as methanol, ethanol,
propanol, butanol and the like; tetrahydrofuran; chlorobenzene;
dichlorobenzene; trichlorobenzene; nitrobenzene; cyanobenzene;
acetonitrile; and mixtures thereof One, two, three or more liquids
may be used. In embodiments where two or more solvents are used,
each solvent may be present at any suitable volume ratio or molar
ratio such as for example from about 99(first liquid): 1(second
liquid) to about 1(first liquid):99(second liquid).
[0074] Exemplary amounts of the components of the silver-containing
nanoparticle composition are as follows. The silver-containing
nanoparticles and the stabilizer are present in an amount ranging
for example from about 0.3% to about 90% by weight, or from about
1% to about 70% by weight, the balance being the other components
of the composition such as the liquid.
[0075] In embodiments, the stabilizer present in the
silver-containing nanoparticle composition originated from the
reaction mixture for the preparation of silver-containing
nanoparticles; no stabilizer is added subsequently for the
formation of the silver-containing nanoparticles. In other
embodiments, the same or different stabilizer may be added
subsequently for the formation of the silver-containing
nanoparticles in an amount ranging for example from about 0.3% to
about 70% by weight based on the weight of the silver-containing
nanoparticle composition.
[0076] The silver-containing nanoparticle composition has a
stability (that is, the time period where there is minimal
precipitation or aggregation of the silver-containing
nanoparticles) of for example at least about 3 hours, or from about
3 hours to about 1 month, from about 1 day to about 3 months, from
about 1 day to about 6 months, from about 1 week to over 1
year.
[0077] The fabrication of an electrically conductive element from
the silver-containing nanoparticle composition ("composition") can
be carried out by depositing the composition on a substrate using a
liquid deposition technique at any suitable time prior to or
subsequent to the formation of other optional layer or layers on
the substrate. Thus, liquid deposition of the composition on the
substrate can occur either on a substrate or on a substrate already
containing layered material (e.g., a semiconductor layer and/or an
insulating layer).
[0078] The phrase "liquid deposition technique" refers to
deposition of a composition using a liquid process such as liquid
coating or printing, where the liquid is a solution or a
dispersion. The silver-containing nanoparticle composition may be
referred to as an ink when printing is used. Illustrative liquid
coating processes include for example spin coating, blade coating,
rod coating, dip coating, and the like. Illustrative printing
techniques include for example lithography or offset printing,
gravure, flexography, screen printing, stencil printing, inkjet
printing, stamping (such as microcontact printing), and the like.
Liquid deposition deposits a layer of the composition having a
thickness ranging from about 5 nanometers to about 5 millimeters,
preferably from about 10 nanometers to about 1000 micrometers. The
deposited silver-containing nanoparticle composition at this stage
may or may not exhibit appreciable electrical conductivity.
[0079] As used herein, the term "heating" encompasses any
technique(s) that can impart sufficient energy to the heated
material to cause the desired result such as thermal heating (e.g.,
a hot plate, an oven, and a burner), infra-red ("IR") radiation,
microwave radiation, or UV radiation, or a combination thereof.
[0080] Heating the deposited composition at a temperature of for
example below about 250.degree. C., or below about 200.degree. C.
or about 150.degree. C., causes the silver-containing nanoparticles
to form an electrically conductive layer which is suitable for use
as an electrically conductive element in electronic devices. The
heating temperature preferably is one that does not cause adverse
changes in the properties of previously deposited layer(s) or the
substrate (whether single layer substrate or multilayer substrate).
The heating is performed for a time ranging from for example about
1 second to about 10 hours, particularly from about 10 seconds to
about 1 hour. The heating is performed in air, in an inert
atmosphere for example under nitrogen or argon, or in a reducing
atmosphere for example under nitrogen containing from about 1 to
about 20 percent by volume hydrogen. The heating is performed under
normal atmospheric conditions or at a reduced pressure of for
example from several mbars to about 10.sup.-3 mbar.
[0081] Heating produces a number of effects. Prior to heating, the
layer of the deposited silver-containing nanoparticles may be
electrically insulating or with very low electrical conductivity,
but heating results in an electrically conductive layer composed of
annealed silver-containing nanoparticles which increases the
conductivity. In embodiments, the annealed silver-containing
nanoparticles may be coalesced or partially coalesced
silver-containing nanoparticles. In embodiments, it may be possible
that in the annealed silver-containing nanoparticles, the
silver-containing nanoparticles achieve sufficient
particle-to-particle contact to form the electrically conductive
layer without coalescence.
[0082] Heating may cause separation of the stabilizer and the
liquid from the silver-containing nanoparticles in the sense that
the stabilizer and the liquid are generally not incorporated into
the electrically conductive layer but if present are in residual
quantities. In embodiments, heating may decompose a portion of the
stabilizer to produce "decomposed stabilizer." Heating may also
cause separation of the decomposed stabilizer such that the
decomposed stabilizer generally is not incorporated into the
electrically conductive layer, but if present is in a residual
amount. Separation of the stabilizer, the liquid, and the
decomposed stabilizer from the silver-containing nanoparticles may
lead to enhanced electrical conductivity of the resulting
electrically conductive layer since the presence of these
components may reduce the extent of silver-containing nanoparticle
to silver-containing nanoparticle contact or coalescence.
Separation may occur in any manner such as for example a change in
state of matter from a solid or liquid to a gas, e.g.,
volatilization.
[0083] In embodiments, one or more of the stabilizer, decomposed
stabilizer, and the liquid is absent from the electrically
conductive layer. In embodiments, a residual amount of one or more
of the stabilizer, decomposed stabilizer, and the liquid may be
present in the electrically conductive layer, where the residual
amount does not appreciably affect the conductivity of the
electrically conductive layer. In embodiments, the residual amount
of one or more of the stabilizer, decomposed stabilizer, and the
liquid may decrease the conductivity of the electrically conductive
layer but the resulting conductivity is still within the useful
range for the intended electronic device. The residual amount of
each component may independently range for example of up to about
5% by weight, or less than about 0.5% by weight based on the weight
of the electrically conductive layer, depending on the process
conditions such as heating temperature and time. When heating
causes separation of the stabilizer and/or decomposed stabilizer
from the silver-containing nanoparticles, the attractive force
between the separated stabilizer/decomposed stabilizer and the
silver-containing nanoparticles is severed or diminished. Other
techniques such as exposure to UV radiation, microwave radiation,
or IR radiation may be used or combined with thermal heating to
accelerate the separation of the liquid and the stabilizer (and/or
the decomposed stabilizer) from the silver-containing
nanoparticles.
[0084] In embodiments, after heating, the resulting electrically
conductive layer has a thickness ranging for example from about 5
nanometers to about 5 millimeters, preferably from about 10
nanometers to about 1000 micrometers.
[0085] The conductivity of the resulting silver-containing element
produced by heating the deposited silver-containing nanoparticle
composition is for example more than about 0.1 Siemens/centimeter
("S/cm"), more than about 100 S/cm, more than about 500 S/cm, more
than about 2,000 S/cm, more than about 5,000 S/cm, more than about
10,000 S/cm, and more than about 20,000 S/cm as measured by
four-probe method.
[0086] The resulting conductive elements can be used as conductive
electrodes, conductive pads, conductive traces, conductive lines,
conductive tracks, and the like in electronic devices. The phrase
"electronic device" refers to macro-, micro- and nano-electronic
devices such as thin film transistor, organic light emitting
diodes, RFID tags, photovoltaic, and other electronic devices which
require conductive elements or components.
[0087] In embodiments, the advantages of the present chemical
method for preparing silver-containing nanoparticles are one or
more of the following: (i) single phase synthesis (where the silver
compound, the stabilizer, and the solvent form a single phase)
without the need for a surfactant; (ii) short reaction time; (iii)
low reaction temperatures of below about 100.degree. C.; (iv)
uniform particle size and narrow particle size distribution; (v)
stable silver-containing nanoparticle composition which can be
easily processed by liquid deposition techniques; (vi) relatively
inexpensive starting materials; and (vii) suitable for large-scale
production that would significantly lower the cost of
silver-containing nanoparticles.
[0088] In embodiments, the silver-containing nanoparticle
composition can be used in for example, but not limited to,
fabricating conductive components such as source and drain
electrodes in thin film transistor ("TFT").
[0089] In FIG. 1, there is schematically illustrated a TFT
configuration 10 comprised of a heavily n-doped silicon wafer 18
which acts as both a substrate and a gate electrode, a thermally
grown silicon oxide insulating layer 14 on top of which are
deposited two metal contacts, source electrode 20 and drain
electrode 22. Over and between the metal contacts 20 and 22 is an
organic semiconductor layer 12.
[0090] FIG. 2 schematically illustrates another TFT configuration
30 comprised of a substrate 36, a gate electrode 38, a source
electrode 40 and a drain electrode 42, an insulating layer 34, and
an organic semiconductor layer 32.
[0091] FIG. 3 schematically illustrates a further TFT configuration
50 comprised of a heavily n-doped silicon wafer 56 which acts as
both a substrate and a gate electrode, a thermally grown silicon
oxide insulating layer 54, and an organic semiconductor layer 52,
on top of which are deposited a source electrode 60 and a drain
electrode 62.
[0092] FIG. 4 schematically illustrates an additional TFT
configuration 70 comprised of substrate 76, a gate electrode 78, a
source electrode 80, a drain electrode 82, an organic semiconductor
layer 72, and an insulating layer 74.
[0093] The substrate may be composed of for instance silicon, glass
plate, plastic film or sheet. For structurally flexible devices,
plastic substrate, such as for example polyester, polycarbonate,
polyimide sheets and the like may be used. The thickness of the
substrate may be from amount 10 micrometers to over 10 millimeters
with an exemplary thickness being from about 50 micrometers to
about 2 millimeters, especially for a flexible plastic substrate
and from about 0.4 to about 10 millimeters for a rigid substrate
such as glass or silicon.
[0094] The gate electrode, the source electrode, and the drain
electrode are fabricated by embodiments of the present invention.
The thickness of the gate electrode layer ranges for example from
about 10 to about 2000 nm. Typical thicknesses of source and drain
electrodes are, for example, from about 40 nm to about 1 micrometer
with the more specific thickness being about 60 to about 400
nm.
[0095] The insulating layer generally can be an inorganic material
film or an organic polymer film. Illustrative examples of inorganic
materials suitable as the insulating layer include silicon oxide,
silicon nitride, aluminum oxide, barium titanate, barium zirconium
titanate and the like; illustrative examples of organic polymers
for the insulating layer include polyesters, polycarbonates,
poly(vinyl phenol), polyimides, polystyrene, poly(methacrylate)s,
poly(acrylate)s, epoxy resin and the like. The thickness of the
insulating layer is, for example from about 10 nm to about 500 nm
depending on the dielectric constant of the dielectric material
used. An exemplary thickness of the insulating layer is from about
100 nm to about 500 nm. The insulating layer may have a
conductivity that is for example less than about 10.sup.-12
S/cm.
[0096] Situated, for example, between and in contact with the
insulating layer and the source/drain electrodes is the
semiconductor layer wherein the thickness of the semiconductor
layer is generally, for example, about 10 nm to about 1 micrometer,
or about 40 to about 100 nm. Any semiconductor material may be used
to form this layer. Exemplary semiconductor materials include
regioregular polythiophene, oligthiophene, pentacene, and the
semiconductor polymers disclosed in Beng Ong et al., US Patent
Application Publication No. US 2003/0160230 Al; Beng Ong et al., US
Patent Application Publication No. US 2003/0160234 Al; Beng Ong et
al., US Patent Application Publication No. US 2003/0136958 Al; and
"Organic Thin Film Transistors for Large Area Electronics" by C. D.
Dimitrakopoulos and P. R. L. Malenfant, Adv. Mater., Vol. 12, No.
2, pp. 99-117 (2002), the disclosures of which are totally
incorporated herein by reference. Any suitable technique may be
used to form the semiconductor layer. One such method is to apply a
vacuum of about 10.sup.-5 to 10.sup.-7 torr to a chamber containing
a substrate and a source vessel that holds the compound in powdered
form. Heat the vessel until the compound sublimes onto the
substrate. The semiconductor layer can also generally be fabricated
by solution processes such as spin coating, casting, screen
printing, stamping, or jet printing of a solution or dispersion of
the semiconductor.
[0097] The insulating layer, the gate electrode, the semiconductor
layer, the source electrode, and the drain electrode are formed in
any sequence, particularly where in embodiments the gate electrode
and the semiconductor layer both contact the insulating layer, and
the source electrode and the drain electrode both contact the
semiconductor layer. The phrase "in any sequence" includes
sequential and simultaneous formation. For example, the source
electrode and the drain electrode can be formed simultaneously or
sequentially. The composition, fabrication, and operation of thin
film transistors are described in Bao et al., U.S. Pat. No.
6,107,117, the disclosure of which is totally incorporated herein
by reference.
[0098] The invention will now be described in detail with respect
to specific exemplary embodiments thereof, it being understood that
these examples are intended to be illustrative only and the
invention is not intended to be limited to the materials,
conditions, or process parameters recited herein. All percentages
and parts are by weight unless otherwise indicated. Room
temperature refers to a temperature ranging for example from about
20 to about 25 degrees C.
EXAMPLE 1
[0099] Silver acetate (0.167 g, 1 mmol) and 1-dodecylamine (3.71 g,
20 mmol) were first dissolved in toluene (100 mL) by heating at
60.degree. C. until silver acetate was dissolved. To this solution
was added a solution of phenylhydrazine (0.43 g, 4 mmol) in toluene
(50 mL) with vigorous stirring over a period of 10 min. The
resulting reaction mixture was stirred at 60.degree. C. for 1 hr
before cooling down to room temperature. Subsequently, acetone (10
mL) was added to the reaction mixture to destroy excess
phenylhydrazine. Solvent removal from the reaction mixture gave a
residue which was added to stirring methanol (100 mL) to
precipitate the crude silver nanoparticle products. The crude
silver nanoparticle product was isolated by centrifugation, washed
with acetone twice, and air-dried. It was then dispersed in
cyclohexane (2 mL) to form a dispersion of silver nanoparticles in
cyclohexane (with molecules of the 1-dodecylamine stabilizer on the
surface of the silver nanoparticles). This dispersion was suitable
for fabricating conductive elements for electronic devices.
[0100] To form a conductive thin film for conductivity measurement,
the dispersion of silver nanoparticles in cyclohexane (with the
1-dodecylamine stabilizer) was spin-coated on a glass substrate to
form a brownish thin film. The latter was heated on a hot plate at
about 120.degree. C. under ambient conditions, a shiny silver film
formed immediately upon heating. The thin-film conductivity of the
resulting silver film was about 23,000 S/cm as calculated from the
measurements using the conventional four-probe technique.
EXAMPLE 2
[0101] Silver acetate (0.167 g, 1 mmol) and 1-hexadecylamine (4.83
g, 20 mmol) were first dissolved in toluene (100 mL) by heating at
60.degree. C. until silver acetate was dissolved. To this solution
was added a solution of phenylhydrazine (0.43 g, 4 mmol) in toluene
(50 mL) with vigorous stirring over a period of 10 min. The
resulting reaction mixture was stirred at 60.degree. C. for 1 hr
before cooling down to room temperature. Subsequently, acetone (10
mL) was added to the reaction mixture to destroy excess
phenylhydrazine. Solvent removal from the reaction mixture gave a
residue which was added to stirring methanol (100 mL) to
precipitate the crude silver nanoparticle product. The crude silver
nanoparticle product was isolated by centrifugation, washed with
acetone twice, and air-dried. It was the dispersed in cyclohexane
(2 mL) to form a dispersion of silver nanoparticles in cyclohexane
(with molecules of the 1-hexadecylamine stabilizer on the surface
of the silver nanoparticles). This dispersion was suitable for
fabricating conductive elements for electronic devices.
[0102] To form a conductive thin film for conductivity measurement,
the dispersion of silver nanoparticles in cyclohexane (with the
1-hexadecylamine stabilizer) was spin-coated on a glass substrate
to form a brownish thin film. The latter was heated on a hot plate
at about 160.degree. C. under ambient conditions, a shiny silver
film formed immediately upon heating. The thin-film conductivity of
the silver film was about 26,000 S/cm as calculated from the
measurements using the conventional four-probe technique.
EXAMPLE 3
[0103] Silver acetate (0.167 g, 1 mmol) and 1-dodecylamine (3.71 g,
20 mmol) were first dissolved in toluene (100 mL) by heating at
60.degree. C. until silver acetate was dissolved. To this solution
was added a solution of benzoic hydrazide (benzoylhydrazine) (0.54
g, 4 mmol) in toluene (50 mL) with vigorous stirring over a period
of 10 min. The resulting reaction mixture was stirred at 60.degree.
C. for 1 hr before cooling down to room temperature. Subsequently,
acetone (10 mL) was added to the reaction mixture to destroy excess
benzoic hydrazide. Solvent removal from the reaction mixture gave a
residue which was added to methanol (100 mL) with stirring to
precipitate crude silver nanoparticle product. The crude silver
nanoparticle product was isolated by centrifugation, washed with
acetone twice, and air-dried. It was dispersed in cyclohexane (2
mL) to form a dispersion of silver nanoparticles in cyclohexane
(with molecules of the 1-dodecylamine stabilizer on the surface of
the silver nanoparticles). This dispersion was suitable for
fabricating conductive elements for electronic devices.
[0104] To form a conductive thin film for conductivity measurement,
the dispersion of silver nanoparticles in cyclohexane (with the
1-dodecylamine stabilizer) was spin-coated on a glass substrate,
and the resulting brownish film was heated on a hot plate at about
120.degree. C. for 1.5 hr under ambient conditions, The thin-film
conductivity of the resulting silver film was about 15,000 S/cm as
calculated from the measurements using the conventional four-probe
technique.
EXAMPLE 4
[0105] A bottom-contact thin film transistor, as schematically
shown by FIG. 1, was chosen to illustrate the use of
silver-containing nanoparticle composition as the conductive
electrodes of a thin-film transistor. The experimental device was
fabricated under ambient conditions, and comprised of an n-doped
silicon wafer with a thermally grown silicon oxide layer of a
thickness of about 110 nm thereon. The wafer functioned as the gate
electrode while the silicon oxide layer acted as the insulating
layer and had a capacitance of about 30 nF/cm.sup.2
(nanofarads/square centimeter), as measured using a capacitor
meter. The silicon wafer was first cleaned with oxygen/argon
plasma, isopropanol, air dried, and then immersed in a 0.1 M
solution of octyltrichlorosilane in toluene for about 20 min at
60.degree. C. Subsequently, the wafer was washed with toluene,
isopropanol and air-dried.
[0106] Stencil printing was used to deposit the silver-containing
nanoparticle composition on the modified wafer substrate. A
stainless stencil with a thickness of 13 .mu.m was positioned on
top of the wafer. A dispersion of silver-containing nanoparticle
composition of Example 1 in cyclohexane (30 wt %) was then painted
through the electrode features of the stencil with a fine paint
brush. After drying at room temperature for 1-5 min, the stencil
was removed. The printed silver-containing nanoparticle elements
were heated at 120.degree. C. on a hotplate under ambient
conditions. This resulted in the formation of shiny silver
electrodes. Subsequently, a semiconductor layer was deposited on
the electroded substrate using the polythiophene semiconductor of
the following Formula: ##STR1##
[0107] where n is a number of from about 5 to about 5,000. This
polythiophene and its preparation are described in Beng Ong et al.,
US Patent Application Publication No. US 2003/0160230 A1, the
disclosure of which is totally incorporated herein by reference.
The semiconductor polythiophene layer of about 30 nm to about 100
nm in thickness was deposited on top of the device by spin coating
of the polythiophene in dichlorobenzene solution at a speed of
1,000 rpm for about 100 seconds, and dried in vacuo at 80.degree.
C. for 20 hr, followed by annealing in a vacuum oven at
120-140.degree. C. for 10-30 min to induce high structural orders
of the semiconductor.
[0108] The evaluation of field-effect transistor performance was
accomplished in a black box at ambient conditions using a Keithley
4200 SCS semiconductor characterization system. The carrier
mobility, .mu., was calculated from the data in the saturated
regime (gate voltage, V.sub.G<source-drain voltage, V.sub.SD)
accordingly to equation (1)
I.sub.SD=C.sub.i.mu.(W/2L)(V.sub.G-V.sub.T).sup.2 (1) where
I.sub.SD is the drain current at the saturated regime, W and L are,
respectively, the semiconductor channel width and length, C.sub.i
is the capacitance per unit area of the insulating layer, and
V.sub.G and V.sub.T are, respectively, the gate voltage and
threshold voltage. V.sub.T of the device was determined from the
relationship between the square root of I.sub.SD at the saturated
regime and V.sub.G of the device by extrapolating the measured data
to I.sub.SD=0. An important property for the thin film transistor
is its current on/off ratio, which is the ratio of the saturation
source-drain current in accumulation regime over the current in
depletion regime.
[0109] The inventive device prepared in this manner showed very
good output and transfer characteristics. The output
characteristics showed no noticeable contact resistance, very good
saturation behaviour, clear saturation currents which are quadratic
to the gate bias. The device turned on at around zero gate voltage
with a sharp subthreshold slope. Mobility was calculated to be 0.08
cm Vs, and the current on/off ratio was about 10.sup.6-10.sup.7.
The performance of the inventive device was essentially the same as
that of a conventional bottom-contact TFT with vacuum deposited
silver electrodes.
[0110] It will be appreciated that various of the above-disclosed
and other features and functions, or alternatives thereof, may be
desirably combined into many other different systems or
applications. Various presently unforeseen or unanticipated
alternatives, modifications, variations or improvements therein may
be subsequently made by those skilled in the art which are also
intended to be encompassed by the following claims. Unless
specifically recited in a claim, steps or components of claims
should not be implied or imported from the specification or any
other claims as to any particular order, number, position, size,
shape, angle, color, or material.
* * * * *