U.S. patent application number 13/149381 was filed with the patent office on 2011-09-22 for gallium electroplating methods and electrolytes employing mixed solvents.
Invention is credited to Serdar Aksu, Bulent M. Basol, Jiaxiong Wang.
Application Number | 20110226630 13/149381 |
Document ID | / |
Family ID | 42164208 |
Filed Date | 2011-09-22 |
United States Patent
Application |
20110226630 |
Kind Code |
A1 |
Wang; Jiaxiong ; et
al. |
September 22, 2011 |
GALLIUM ELECTROPLATING METHODS AND ELECTROLYTES EMPLOYING MIXED
SOLVENTS
Abstract
An electrochemical deposition method and electrolyte to plate
uniform, defect free and smooth gallium films are provided. In a
preferred embodiment, the electrolyte may include a solvent that
comprises water and at least one monohydroxyl alcohol, a gallium
salt, and an acid to control the solution pH and conductivity. The
method electrodeposits a gallium film possessing sub-micron
thickness on a conductive surface. Such gallium layers are used in
fabrication of semiconductor and electronic devices such as thin
film solar cells.
Inventors: |
Wang; Jiaxiong; (Castro
Valley, CA) ; Aksu; Serdar; (Milpitas, CA) ;
Basol; Bulent M.; (Manhattan Beach, CA) |
Family ID: |
42164208 |
Appl. No.: |
13/149381 |
Filed: |
May 31, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12267488 |
Nov 7, 2008 |
7951280 |
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13149381 |
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Current U.S.
Class: |
205/170 ;
205/261 |
Current CPC
Class: |
C25D 3/54 20130101; C25D
5/10 20130101 |
Class at
Publication: |
205/170 ;
205/261 |
International
Class: |
C25D 5/10 20060101
C25D005/10; C25D 3/02 20060101 C25D003/02 |
Claims
1. An electrodeposition solution for electrodepositing a gallium
(Ga) thin film on a conductive surface, comprising: water; at least
one organic solvent with a room temperature viscosity of less than
or equal to 10 centipoise and a freezing point of lower than
-20.degree. C.; and a Ga source material that provides Ga ions.
2. The electrodeposition solution of claim 1, wherein a volume
ratio of the at least one organic solvent to water is in the range
of 0.1-10.
3. The electrodeposition solution of claim 2 wherein the at least
one organic solvent comprises a monohydroxyl alcohol.
4. The electrodeposition solution of claim 3, wherein the
monohydroxyl alcohol comprises at least one of methanol, a primary
alcohol, a secondary alcohol and a tertiary alcohol, wherein the
primary alcohol is selected from the group consisting of ethanol,
1-propanol, isobutanol, 1-pentanol, 1-hexanol and 1-heptanol, and
wherein the secondary alcohol is selected from the group consisting
of isopropyl alcohol, 2-butanol, 2-methyl-2-butanol and 2-hexanol,
and wherein the tertiary alcohol is selected from the group
consisting of tert-butanol and tert-amyl alcohol.
5. The electrodeposition solution of claim 4 further comprising a
pH adjustment agent wherein the solution has a pH value in the
range of 0-7.
6. The electrodeposition solution of claim 1, wherein the Ga source
comprises at least one of a dissolved Ga metal and a dissolved Ga
salt, wherein the Ga salt is selected from the group consisting of
Ga-chloride, Ga-sulfate, Ga-sulfamate, Ga-acetate, Ga-carbonate,
Ga-nitrate, Ga-perchlorate, Ga-phosphate, Ga-oxide, and
Ga-hydroxide.
7. The electrodeposition solution of claim 3, wherein the Ga source
comprises at least one of a dissolved Ga metal and a dissolved Ga
salt, wherein the Ga salt is selected from the group consisting one
of Ga-chloride, Ga-sulfate, Ga-sulfamate, Ga-acetate, Ga-carbonate,
Ga-nitrate, Ga-perchlorate, Ga-phosphate, Ga-oxide, and
Ga-hydroxide.
8. The electrodeposition solution of claim 5, wherein the pH
adjustment agent comprises at least one of an acid, an alkali metal
salt of the acid, and an alkali earth metal salt of the acid,
wherein the acid is selected from the group consisting of sulfamic
acid, citric acid, acetic acid, tartaric acid, maleic acid, boric
acid, malonic acid, succinic acid, phosphoric acid, oxalic acid,
formic acid, arsenic acid, benzoic acid, sulfuric acid, nitric
acid, hydrochloric acid, and amino acids, wherein the alkali metal
salt is selected from the group consisting of a lithium salt, a
sodium salt, a potassium salt, a rubidium salt, a cesium salt, and
wherein the alkali earth metal salt is selected from the group
consisting of a beryllium salt, a magnesium salt, a calcium salt, a
strontium salt, and a barium salt.
9. The electrodeposition solution of claim 1 further comprising an
organic additive, where the organic additive is selected from the
group consisting of surfactants, suppressors, levelers, and
accelerators.
10. A method of electrodepositing gallium (Ga) for manufacturing
solar cell absorbers, comprising the steps of: providing an
electrodeposition solution that comprises water, an organic solvent
with a room temperature viscosity of less than or equal to 10
centipoise and a freezing point of lower than -20.degree. C., and a
Ga source material that provides Ga ions; contacting the
electrodeposition solution with an electrode and a conductive
layer; establishing a potential difference between the electrode
and the conductive layer; and electrodepositing a Ga layer over the
conductive layer.
11. The method of claim 10, wherein a volume ratio of the organic
solvent to water is in the range of 0.05-99.
12. The method of claim 11, wherein the organic solvent is a
monohydroxyl alcohol.
13. The method of claim 12, wherein the pH value of the
electrodeposition solution is in the range of 0-7.
14. The method of claim 10, wherein the Ga source material
comprises at least one of a dissolved Ga metal and a dissolved Ga
salt, wherein the Ga salts is selected from the group consisting of
Ga-chloride, Ga-sulfate, Ga-sulfamate, Ga-acetate, Ga-carbonate,
Ga-nitrate, Ga-perchlorate, Ga-phosphate, Ga-oxide, and
Ga-hydroxide.
15. The method of claim 12, wherein the monohydroxyl alcohol is
selected from the group consisting of methanol, a primary alcohol,
a secondary alcohol and a tertiary alcohol, wherein the primary
alcohol is selected from the group consisting of ethanol,
1-propanol, isobutanol, 1-pentanol, 1-hexanol and 1-heptanol, and
wherein the secondary alcohol is selected from the group consisting
of isopropyl alcohol, 2-butanol, 2-methyl-2-butanol and 2-hexanol,
and wherein the tertiary alcohol is selected from the group
consisting of tert-butanol and tert-amyl alcohol.
16. The method of claim 13, wherein the pH adjustment agent
comprises at least one of an acid and an alkali metal salt of the
acid wherein the acid is selected from the group consisting of
sulfamic acid, citric acid, acetic acid, tartaric acid, maleic
acid, boric acid, malonic acid, succinic acid, phosphoric acid,
oxalic acid, formic acid, arsenic acid, benzoic acid, sulfuric
acid, nitric acid, hydrochloric acid, and amino acids and wherein
the alkali metal salt is selected from the group consisting of
lithium salt, sodium salt, potassium salt, rubidium salt, cesium
salt, beryllium salt, magnesium salt, calcium salt, strontium salt,
and barium salt.
17. The method of claim 10, wherein the potential difference
between the anode and the conductive layer is established in a
pulsing manner.
18. The method of claim 10, wherein the conductive surface is a
copper layer formed over a base layer.
19. The method of claim 18, further comprising depositing an indium
layer over the gallium layer.
20. The method of claim 19, further comprising reacting the copper,
gallium, and indium layers in presence of at least one of selenium
and sulfur to form a CIGS(S) solar cell absorber.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a Continuation of U.S. patent
application Ser. No. 12/267,488, filed Nov. 7, 2008, which is
incorporated herein by reference.
BACKGROUND
[0002] 1. Field of the Inventions
[0003] Embodiments of the present invention relate to thin film
gallium (Ga) electroplating methods and chemistries employing
electrolytes or solutions comprising mixtures of water and certain
classes of organic liquids. Such films have application in the
field of electronic devices such as solar cells.
[0004] 2. Description of the Related Art
[0005] Thin film solar cells have attracted much attention lately
because of their potential low cost. Thin film solar cells may
employ, as their light absorbing layer or absorber, polycrystalline
silicon, amorphous silicon, cadmium telluride (CdTe), copper indium
gallium selenide (sulfide) (CIGS(S)), etc. The processing methods
used for the preparation of thin film solar cell absorber layers
can generally be classified as dry and wet processes. The dry
processes include physical vapor deposition (PVD) and chemical
vapor deposition (CVD) techniques, which are usually well
developed, however, expensive. Wet processes include ink spraying
or printing, chemical bath deposition (CBD) and electrochemical
deposition (ED), also called electrodeposition or electroplating.
Among these methods, CBD is popular for the preparation of some
n-type semiconductor films like CdS, ZnSe, In--Se, etc. In ink
deposition processes, inks comprising nano-particles dispersed in a
solvent are deposited on a substrate. When the solvent evaporates
away, it leaves behind a precursor layer comprising the
nano-particles. The precursor layer is then sintered at high
temperatures to form the absorber.
[0006] Electrochemical deposition techniques can provide thin
precursor films which may then be converted into solar cell
absorbers. One recent application of electroplated copper (Cu),
indium (In) and gallium (Ga) films is in the formation of
Cu(In,Ga)(Se,S).sub.2 or CIGS(S) type layers, which are the most
advanced compound absorbers for polycrystalline thin film solar
cells. It should be noted that the notation (In, Ga) means all
compositions from 100% In and 0% Ga to 0% In and 100% Ga.
Similarly, (Se,S) means all compositions from 100% Se and 0% S to
0% Se and 100% S. Applying electrodeposition to the formation of a
CIGS(S) type absorber layer may involve a two-stage or two-step
processing approach comprising a precursor deposition step and a
reaction step. A thin In layer, for example, may be electroplated
on a Cu layer. A thin Ga film may then be formed on the In layer to
form a Cu/In/Ga stack precursor. The Cu/In/Ga precursor stack thus
obtained may then be reacted with selenium (Se) to form a CIGS
absorber. Further reaction with sulfur (S) would form a CIGS(S)
layer. The CIGS(S) absorber may be used in the fabrication of thin
film solar cells with a structure of "contact/CIGS(S)/buffer
layer/TCO", where the contact is a metallic layer such as a
molybdenum (Mo) layer, the buffer layer is a thin transparent film
such as a cadmium sulfide (CdS) film and transparent conductive
oxide (TCO) is a transparent conductive layer such as a zinc oxide
(ZnO) and/or an indium tin oxide (ITO) layer.
[0007] In a thin film solar cell employing a Group IBIIIAVIA
compound absorber such as CIGS(S), the cell efficiency is a strong
function of the molar ratio of IB/IIIA. If there are more than one
Group IIIA materials in the composition, the relative amounts or
molar ratios of these IIIA elements also affect the properties. For
a Cu(In,Ga)(S,Se).sub.2 or CIGS(S) absorber layer, for example, the
efficiency of the device is a function of the molar ratio of
Cu/(In+Ga), where Cu is the Group IB element and Ga and In are the
Group IIIA elements. Furthermore, some of the important parameters
of the cell, such as its open circuit voltage, short circuit
current and fill factor vary with the molar ratio of the IIIA
elements, i.e. the Ga/(Ga+In) molar ratio. In general, for good
device performance Cu/(In+Ga) molar ratio is kept at or below 1.0.
For ratios close to or higher than 1.0, a low resistance copper
selenide phase may form, which may introduce electrical shorts
within the solar cells. As the Ga/(Ga+In) molar ratio increases, on
the other hand, the optical bandgap of the absorber layer increases
and therefore the open circuit voltage of the solar cell increases
while the short circuit current typically may decrease. It is
important for a thin film deposition process to have the capability
of controlling both the molar ratio of IB/IIIA, and the molar
ratios of the Group IIIA components in the composition. Therefore,
if electrodeposition is used to introduce the Ga into the film
composition, it is essential that the electroplated Ga films have
smooth morphology and be free of defects such as pinholes. It
should be noted that the typical thickness of Ga layers to be
electroplated for CIGS(S) absorber formation is in the range of
50-300 nm and many prior art electroplated Ga layers display a
peak-to-valley surface roughness in the range of 50-500 nm, which
means that these films are very thick in some areas and very thin
in others.
[0008] In an application of electroplated Ga layers to solar cell
fabrication, the Ga layer may be electroplated to form precursor
stacks with structures such as Cu/In/Ga, Cu/Ga/In, etc. These
stacks may then be reacted at high temperature (typically in the
range of 400-600.degree. C.) with a Group VIA material such as Se
and S to form a CIGS(S) absorber layer. The absorber layer may then
be further processed to construct a solar cell. US Patent
Application with publication No. 20070272558, entitled "Efficient
Gallium Thin Film Electroplating Methods and Chemistries" filed by
the applicants of this application and incorporated herein by
reference, discloses new methods and chemistries to deposit Ga
films with high plating efficiency. Other work on electrodeposition
of Ga includes the publication by S. Sundararajan and T. Bhat (J.
Less Common Metals, vol.11, p. 360, 1966) who utilized electrolytes
with a pH value varying between 0 and 5. Other researchers
investigated Ga deposition out of high pH solutions comprising
water and/or glycerol. Bockris and Enyo, for example, used an
alkaline electrolyte containing Ga-chloride and NaOH (J.
Electrochemical Society, vol. 109, p. 48, 1962), whereas, P.
Andreoli et al.(Journal of Electroanalytical Chemistry, vol. 385,
page.265, 1995) studied an electrolyte comprising KOH and
Ga-chloride. While some of these previous works used very corrosive
solutions, i.e., pH=15, most of them were carried out under low
plating efficiencies in low pH electrolytes, the plating
efficiencies being typically 20% or lower. Glycerol, due to its
high boiling temperature has also been used in high temperature
(>100.degree. C.) preparation of electrodeposition chemistries
to plate molten globules of Ga--In alloys (see e.g. U.S. Pat. No.
2,931,758). Although, glycerol-based plating solutions may be
adequate to obtain Ga deposits in the form of thick molten globules
such deposits cannot be used in the formation solar cell absorbers
such as thin film CIGS(S) compounds. From the foregoing, there is a
need to develop Ga electrolytes and electrodeposition methods to
generate smooth, uniform and defect-free Ga thin films with high
plating efficiencies on surfaces of varying chemical composition.
This way Ga layers may be electroplated onto different cathode
surfaces for electronics applications, specifically for the
fabrication of high quality CIGS(S) type thin film solar cell
absorbers.
SUMMARY
[0009] An aspect of embodiments of the present invention is to
provide an electrodeposition solution for depositing a gallium (Ga)
thin film on a conductive surface. The electrodeposition solution
includes a solvent including an organic solution, such as a mixture
of at least one monohydroxyl alcohol and water and a Ga source. The
electrodeposition solution further includes at least one of an acid
and a salt for controlling the solution pH value and providing a
high ionic conductivity in the plating solution. This solution can
be used to plate Ga at a very low temperature.
[0010] Another aspect of embodiments of the present invention is to
provide a method of electrodepositing a Ga thin film on a
conductive surface. The method includes the steps of providing an
electrodeposition solution that includes a mixture of at least one
monohydroxyl alcohol and an aqueous solvent; a Ga source, and at
least one of an acid and a salt to control the solution pH value
and provide a good conductivity for plating; adjusting the pH of
the electrodeposition solution between 0 and 7, preferably between
1 and 3; contacting the solution with an anode and the conductive
surface; establishing a potential difference between the anode and
the conductive surface; and electrodepositing the Ga thin film on
the conductive surface. The freezing point of the electrodeposition
solution can be significantly lower than that of water, and thus
the electrodeposition solution of the present invention can be used
at low temperatures to prevent Ga melting and alloying with the
underlying materials and to obtain films with improved amounts of
surface roughness.
BRIEF DESCRIPTION OF THE DRAWING
[0011] FIG. 1 is a schematic illustration of a gallium film
electrodeposited on a conductive surface from an electrodeposition
solution.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0012] The embodiments of the present inventions provide methods
and electrodeposition solutions or electrolytes to electrodeposit
uniform, smooth and repeatable gallium (Ga) films. Through the use
of various aspects of the present inventions it is possible to form
micron or sub-micron thick Ga films on conductive surfaces from
solutions mixed with aqueous and organic solvents such as alcohols.
The present inventions may be used to form gallium films for
manufacturing solar cell absorbers. Electrodeposition solutions of
the embodiments of the present inventions may be used at very low
temperatures to improve the surface morphology of electroplated Ga
films.
[0013] FIG. 1 shows an exemplary gallium thin film 100 or layer
electrodeposited on a surface 102 of a conductive layer 104 from an
electrodeposition solution 106 using an electrodeposition method.
The gallium thin film 100 may be a part of a precursor stack, which
may include indium and copper layers. The conductive layer 104 may
be a solar cell base comprising a substrate and a contact layer
deposited on the substrate, or a precursor layer including at least
one of a gallium layer, indium layer and copper layer formed on the
base. During the electrodeposition process, the conductive layer
104 is brought into contact with the electrodeposition solution 106
and negatively polarized with respect to a positively polarized
electrode (not shown) that is also in contact with the
electrodeposition solution. A typical conductive layer 104 used by
embodiments of the inventions comprise at least one of Cu, Ga, In,
Mo, Ru, Ir and Os.
[0014] Gallium electrodeposition electrolytes and electrodeposition
methods for solar cell manufacturing processes have many more
stringent and special requirements than the electrodeposition
methods and solutions employed for many other commonly plated
metals such as Cu, Ni, Co, Pb, Sn, Ag, Au, Pt, and their alloys,
etc. This stems from the facts that; i) Ga is one of the lowest
melting point metals in existence, with a melting point of about
30.degree. C., ii) Ga has a high negative electrodeposition
potential and thus Ga electrodeposition efficiency is naturally low
since high electrodeposition potentials cause hydrogen generation,
in addition to Ga deposition, at the cathode surface in aqueous
electrolytes, iii) hydrogen bubbles generated on a cathode surface
form defects such as un-deposited regions unless such bubbles could
immediately be removed from the surface, iv) Ga has a tendency to
form low temperature melting alloys with many alloy-partner
materials such as In, Cu, Ag, Pb, Sn, etc. Furthermore, such alloys
may form during electrodeposition of Ga onto surfaces comprising
any of such alloy-partner materials.
[0015] Electrodeposition solutions employing glycerol are very
viscous and difficult to handle. The viscosity of glycerol at room
temperature is 1500 centipoise (cP) compared to the viscosity of
water, which is 1 cP. Gas bubbles such as hydrogen bubbles formed
on the electroplated (cathode) surface during Ga plating in viscous
electrolytes cannot be easily removed from that surface and
therefore cause voids and other defects in the electrodeposited
films. Such defects may be acceptable for some applications of
thick electrodeposited Ga globules. However, they cannot be
tolerated in electronic device applications such as solar cell
absorber formation applications where they cause compositional
non-uniformities, morphological non-uniformities, and pinholes
etc., all of which negatively impact the device performance.
[0016] Glycerol based plating solutions become more viscous as
their temperature is lowered and therefore the problems cited above
may get worse at lower temperatures. One other important point
about the electrodeposition process for Ga is its sensitivity to
the nature of the substrate surface on which the electrodeposition
is performed. For example, to form a Cu/In/Ga precursor stack, the
Ga film needs to be electrodeposited on an In surface. To form a
Cu/Ga/In precursor stack, on the other hand, Ga plating needs to be
performed on a Cu surface. One Ga electrodeposition solution that
performs well for plating Ga on a Cu surface may not perform well
for electrodepositing Ga on an In surface because the
electrodeposition efficiency of Ga on one surface may be very
different from its electrodeposition efficiency on another
surface.
[0017] As mentioned above, gallium is a low melting point material
with a melting temperature of around 30.degree. C. As a result,
when electrodeposited out of aqueous electrodeposition solutions
kept at about room temperature (20-25.degree. C.), it often forms
rough films comprising molten surface features, especially at high
electrodeposition current densities such as current densities
greater than about 5 mA/cm.sup.2. This is because even though the
electrodeposition solution may be at a temperature lower than the
melting point of Ga, the local temperature on the cathode surface
may actually exceed this melting point due to the heat generated by
the electrodeposition current. As further mentioned above, when Ga
is electrodeposited on surfaces of materials that easily form
alloys with Ga, molten droplets of Ga alloys with low melting
temperatures may be formed on such surfaces. If the Ga film is
electrodeposited over In and/or Cu, the local heating and Ga
melting may actually promote alloying between the plated Ga film
and the underlying In and/or Cu because there are low melting alloy
phases between Ga and these materials such as In--Ga alloy phases
and CuGa.sub.2 alloy phase. As a result, the surface roughness of
the deposit may further be increased due to the above mentioned
reaction and the formation of molten alloy phases. For example,
Mehlin et al. (Z. Naturforsch, vol. 49b, p.1597 (1994)) attributed
the rough morphology of their electroplated Ga layers to the
alloying of the electrodeposited Ga with the underlying Cu surface
of the cathode and the formation of a molten CuGa.sub.2 alloy.
[0018] Gallium may be electrodeposited from the electrodeposition
solution at temperatures below -10.degree. C., preferably below
-20.degree. C., most preferably below -30.degree. C., so that local
melting of the deposited Ga and its possible reaction with the
materials on the cathode surface are avoided. Furthermore, at these
low temperatures, the electrodeposition current densities may be
increased to levels above 5 mA/cm.sup.2, preferably above 10
mA/cm.sup.2 and even above 20 mA/cm.sup.2 without causing melting
and/or alloying on the cathode surface. As a result, the
electrodeposition rate and therefore the process throughput may be
increased while, at the same time, the deposited film roughness is
reduced. All of these benefits are important for the successful use
of electrodeposited Ga layers in thin film solar cell
manufacturing. For example, the melting point of methanol is
-97.degree. C. and the freezing point of a methanol/water mixture
is a function of the ratio of methanol to water in the
electrodeposition solution. A mixture of 75% methanol and 25%
water, for instance, has a freezing point of -82.degree. C.
(-115.degree. F.). This means that a Ga plating electrodeposition
solution comprising 75% methanol and 25% water may be operated at a
temperature as low as about -70-80.degree. C., thus avoiding the
melting, reaction and surface roughness problems described
above.
[0019] The electrodeposition solution may be used to electroplate
Ga thin films onto conductive surfaces with a considerably high
electrodeposition efficiency of greater than 40%. The electrolyte
solution may comprise water and an organic solvent with a room
temperature viscosity of less than about 10 cP, preferably less
than about 5 cP. Examples of such organic solvents include
monohydroxyl alcohols such as methanol, ethanol, and isopropyl
alcohol. These organic solvents also have very low freezing
points.
[0020] As well known in the field of chemistry, an alcohol is
defined to be a hydrocarbon derivative in which a hydroxyl group
(--OH) is attached to a carbon atom of an alkyl or substituted
alkyl group. If the alcohols have two (--OH) groups, such as
ethylene glycol and propylene glycol, they are classified as diols
or glycols. Glycerol or sugar alcohol has three (--OH) groups and a
boiling point of 290.degree. C. Glycols also have boiling points
close to 200.degree. C. Therefore, diols containing two (--OH)
groups or other organic compounds containing 3 or more (--OH)
groups may be useful for high temperature electrodeposition
solutions. However, as explained before, such organic compounds
have shortcomings including high viscosity giving rise to
defectivity in the electrodeposited thin layers. Furthermore, the
freezing point of glycerol is too high for the purpose of good
quality thin film Ga electrodeposition. The viscosities of ethylene
glycol, propylene glycol and diethylene glycol, which are all
diols, are 16 cP, 40 cP and 32 cP, respectively. Their freezing
points, on the other hand are about -13.degree. C., -59.degree. C.
and -10.degree. C., respectively. The viscosity and the freezing
point of glycerol, which has three (--OH) groups, are 1500 cP and
+18.degree. C., respectively.
[0021] The electrodeposition solutions of one embodiment of the
present inventions employ at least one monohydroxyl alcohol mixed
with water as solvent. Monohydroxyl alcohols contain only one
(--OH) or hydroxyl group and they include methanol, primary
alcohols (such as ethanol, 1-propanol, isobutanol, 1-pentanol,
1-hexanol, 1-heptanol), secondary alcohols (such as isopropyl
alcohol, 2-butanol, 2-methyl-2-butanol, 2-hexanol) and tertiary
alcohols (such as tert-butanol, tert-amyl alcohol). The viscosities
of monohydroxyl alcohols are typically below 10 cP, mostly below 5
cP, and their freezing points vary from -12.degree. C. for
2-methyl-2-butanol, to -126.degree. C. for 1-propanol. For example,
viscosities of methanol, ethanol, 1-propanol, isobutanol, and
isopropyl alcohol are 0.59 cP, 1.2 cP, 1.94 cP, 3.95 cP and 1.96
cP, respectively. Their respective freezing points are about
-97.degree. C., -114.degree. C., -126.degree. C., -108.degree. C.,
and -89.degree. C. As can be seen, these low viscosities and
extremely low freezing temperatures are very desirable properties
for thin Ga film electrodeposition.
[0022] The electrodeposition solution may further comprise an acid
and/or a salt to control the pH and increase the solution
conductivity. The electrodeposition solution may further include a
Ga source dissolved in the electrolyte, such as Ga chloride, Ga
sulfate, Ga sulfamate, Ga perchloride, Ga phosphate, Ga nitrate,
etc. Additional inorganic and organic acids and their alkali metal
(lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium
(Cs), and francium (Fr)) and/or alkali earth metal (beryllium (Be),
magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba) and
radium (Ra)) salts can be added to the electrodeposition solution
to provide a buffer to stabilize the solution pH and to increase
the conductivity of the electrodeposition solution. Concentrations
of additional organic or inorganic acids and/or their alkali metal
salts may not be high since the Ga salts in the composition also
provide some of the ionic conduction. Acids such as sulfamic acid,
citric acid, acetic acid, tartaric acid, maleic acid, boric acid,
malonic acid, succinic acid, phosphoric acid, oxalic acid, formic
acid, arsenic acid, benzoic acid, sulfuric acid, nitric acid,
hydrochloric acid, and amino acids, may be used. As stated above,
Li, Na, K, Rb, Cs, Be, Mg, Ca, Sr and Ba salts of these acids could
be added along with the acid to adjust the pH, provide buffering
and increase the electrodeposition solution conductivity. The
electrodeposition solution pH range may be acidic or basic, but is
preferably between 0 and 7.
[0023] The standard potential of Ga electrodeposition from aqueous
electrolytes is E.sup.0.sub.Ga(III)/Ga=-0.52 V. At this potential,
the hydrogen evolution is aggressive, especially in an acidic
aqueous solution. This is why the Ga electrodeposition processes
typically display low electrodeposition efficiencies in aqueous
acidic electrodeposition solutions. The mixture of an organic
solvent described in embodiments of the inventions reduces the
amount of water in the electrodeposition solution and thereby
reduces the tendency of hydrogen evolution from water and increases
the Ga electrodeposition efficiency. Because of the low viscosity
of the present electrodeposition solutions any hydrogen bubbles
formed on the cathode surface are easily swept away reducing or
eliminating defectivity in the electrodeposited Ga films. The
embodiments of the present inventions will now be further described
in the following example.
EXAMPLE
[0024] To demonstrate the wide range of capabilities of the
developed electrodeposition solution chemistries and techniques,
the electrodeposition conditions of the Ga layers were widely
varied using a factorial design with three factors and three
levels. The exemplary solvent was a mixture of methanol and
de-ionized water. The Ga source used was GaCl.sub.3. Sulfamic acid
was used in the electrodeposition solution to increase the ionic
conductivity. The three factors that were changed in the
experiments were: i) the volume ratios of methanol to water (M/W
ratio), ii) the concentration of GaCl.sub.3, and, iii) the
concentration of the sulfamic acid. The pH was kept in the range of
1.3 and 2. All of the electrodeposition tests were carried out
using a current density of 30 mA/cm.sup.2 for 150 seconds without
stifling the electrodeposition solutions. According to the
Faraday's Law, the total charge passed to the cathodes was 4.5
Coulombs/cm.sup.2. Therefore, a Ga film thickness of about 1.83
.mu.m was expected if the Ga electrodeposition efficiency were
100%. The anode was a platinum (Pt) mesh. The cathode surface
comprised a thin Cu layer. All of the solvent combinations resulted
in clear miscible solutions of methanol and water. The thickness of
the resultant Ga films was measured to evaluate the
electrodeposition efficiencies.
[0025] M/W ratio in the present example (or more generally the
organic solvent-to-water ratio of the electrodeposition solutions)
was found to be an important variable. This ratio may be in the
range of about 0.05-99, preferably in the range of about 0.1-10,
more preferably in the range of about 0.2-5. The Ga concentration
range in the electrolyte is preferably more than 0.1M. The maximum
concentration of Ga is determined by the amount of Ga source
dissolvable in the solvent with a specific M/W ratio, a typical
concentration being in the range of 0.2-0.6M. The sulfamic acid
concentration of the present example could be changed from zero to
about 0.5M. However, the preferred range of the acid concentration
in general is 0.05-0.2M. At higher concentrations of acid, for
example over 0.5 M, the Ga electrodeposition efficiency was found
to reduce to less than 10%. It should be noted that, within the
preferred ranges of the above variables, Ga layers may be
electrodeposited at electrodeposition efficiencies greater than 40%
using the electrodeposition solutions or electrolytes.
[0026] The results of the above experiments may be summarized as
follows: i) As the M/W ratio got higher, the electrodeposition
efficiency also got higher; ii) as the sulfamic acid concentration
became greater than 0.2M, the plating efficiency started to
decline, and iii) in general higher Ga concentration in the
electrodeposition solution yielded higher electrodeposition
efficiencies.
[0027] The Ga source in the electrodeposition solution of the
embodiments of the present inventions may comprise stock solutions
prepared by dissolving Ga metal into their ionic forms as well as
by dissolving soluble Ga salts, such as sulfates, chlorides,
acetates, sulfamates, carbonates, nitrates, phosphates, oxides,
perchlorates, and hydroxides in the solvent of the
electrodeposition solution. As mentioned above, the polar organic
solvents (monohydroxyl alcohols) are used in the formulation since
they need to be miscible with water and dissolve certain amount of
Ga salts, acids and their salts. Many primary, secondary or
tertiary monohydroxyl alcohols may also be used in place of or in
addition to the methanol used in the above example. These alcohols
include but are not limited to ethanol, 1-propanol, isobutanol,
1-pentanol, 1-hexanol, 1-heptanol, isopropyl alcohol, 2-butanol,
2-methyl-2-butanol, 2-hexanol, tert-butanol and tert-amyl alcohol.
The acids used in the embodiments of the present inventions may
cover a wide range including sulfamic acid, acetic acid, citric
acid, tartaric acid, maleic acid, boric acid, succinic acid,
phosphoric acid, oxalic acid, formic acid, arsenic acid, benzoic
acid, sulfuric acid, nitric acid, hydrochloric acid, and amino
acids, etc. The concentrations of the acids and their alkali metal
and alkali metal earth salts can be adjusted according to the pH
requirements of the solutions. The solution pH values can be widely
varied between acidic and basic ranges. The preferred range is a pH
of 0 to 7. A more preferred range is between 1 and 3. For the pH
values larger than 3, some acids with low pK.sub.a, i.e., maleic
acid, oxalic acid, and phosphoric acid, may be preferred to both
control the solution pH and at the same time complex the Ga.sup.3+
cations and avoid precipitation of Ga(OH).sub.3.
[0028] It should be noted that although the monohydrated alcohols
constitute the preferred ingredients in the Ga electrodeposition
solutions of embodiments of the present inventions, in certain
embodiments some other organic solvents with appropriate viscosity
and freezing point values may also be employed. These organic
solvents include, but are not limited to acetonitrile (viscosity of
about 0.35 cP and freezing point of about -45.degree. C.), acetone
(viscosity of about 0.32 cP and freezing point of about -95.degree.
C.), formaldehyde (viscosity of about 0.5 cP and freezing point of
about -117.degree. C.), and dimethylformimide (viscosity of about
0.9 cP and freezing point of about -61.degree. C.), butyronitrile
(viscosity of about 0.55 cP and freezing point of about
-112.degree. C.), docholoromethane (viscosity of about 0.41 cP and
freezing point of about -97.degree. C.), N-methyl-pyrrolidinone
(freezing point of about -23.degree. C.), .gamma.-Butyrolactone
(freezing point of about -43.degree. C.), 1-2-Dimethoxy-ethane
(viscosity of about 0.5 cP and freezing point of about -69.degree.
C.), and tetrahydrofuran (viscosity of about 0.5 cP and freezing
point of about -108.degree. C.). It should also be noted that other
organic ingredients may also be added to the electrodeposition
solution as long as they do not appreciably alter its desired
properties described previously. These additional organic
ingredients include, but are not limited to diols and alcohols with
three (--OH) groups.
[0029] Both direct current (DC) and pulsed or variable
voltage/current may be utilized during the electrochemical
deposition processes in embodiments of the present inventions. The
temperature of the electrodeposition solution may be in the range
of -120.degree. C. to +30.degree. C. depending upon the nature of
the organic solvent, the organic solvent-to-water volume ratio, and
the nature of the cathode surface. If the cathode surface comprises
materials that alloy easily at low temperature with Ga, then low
temperatures such as temperatures in the range of -120.degree. C.
to -20.degree. C., may be beneficially selected for the
electrodeposition solution.
[0030] The electrodeposition solutions of the embodiments of the
present inventions may comprise additional ingredients. These
include, but are not limited to, grain refiners, surfactants,
wetting agents, dopants, other metallic or non-metallic elements
etc. For example, organic additives such as surfactants,
suppressors, levelers, accelerators and the like may be included in
the formulation to refine its grain structure and surface
roughness. Organic additives include but are not limited to
polyalkylene glycol type polymers, propane sulfonic acids,
coumarin, saccharin, furfural, acrylonitrile, magenta dye, glue,
SPS, starch, dextrose, and the like.
[0031] Although the present inventions are described with respect
to certain preferred embodiments herein, modifications thereto will
be apparent to those skilled in the art.
* * * * *