# Is it possible to electroplate from a gallium anode?

I have been looking at some papers on electro-deposition of iron gallium alloys and these all use solutions made up of Fe and Ga sulphates.

I am just curious as to why perhaps an anode made of one of the materials might not have been used as part of the process? I mention gallium specifically because the cost of the Ga sulphates is quite high, while the Ga metal is relatively cheap.

My impression is that it is typically difficult to electro-deposit gallium, but I think this was in specific reference to plating from solutions of the Ga salts, I have not yet come across plating from a Ga anode, and I'm curious about this.

• Electroplating is a still an art rather than science. Too many magical additives are required to get the desired results as you can see here in the Table for electrodeposition og Ga-Fe alloys sciencedirect.com/science/article/pii/S0924424714005214. Ga is perhaps too soft and two low of a melting point (30 C). Like aluminum it is prone to form insoluble hydroxides during anodic dissolution, if the pH is just not right. – M. Farooq Apr 26 at 13:23
• @M.Farooq thank you for your comment, I got that impression for electroplating more art than science. The paper you reference is based of a couple earlier works where they got the recipe for the electrolyte. I guess it's more often based on experience what they choose you use as additives. – Dave Apr 26 at 13:43

Yes, it is possible to electrodeposit gallium. The difficulties are: efficiency due to the need for low electrodeposition potentials, formation of extremely fine hydrogen bubbles which spoils the surface finish, the requirement to use extremely low temperatures and the melting/alloying of gallium with the metal it's plated on, and poor repeatability of the process. There doesn't seem to be a patent describing using a pure gallium anode.

The difficulties are described in these patents:

• US20110226630A1 - "Gallium electroplating methods and electrolytes employing mixed solvents":

... 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° 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.

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.

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.

As mentioned above, gallium is a low melting point material with a melting temperature of around 30° C. As a result, when electrodeposited out of aqueous electrodeposition solutions kept at about room temperature (20-25° 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$$^2$$. 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$$_2$$ 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$$_2$$ alloy.

Gallium may be electrodeposited from the electrodeposition solution at temperatures below −10° C., preferably below −20° C., most preferably below −30° 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$$^2$$, preferably above 10 mA/cm$$^2$$ and even above 20 mA/cm$$^2$$ 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° 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° C. (−115° 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° C., thus avoiding the melting, reaction and surface roughness problems described above.

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• US7507321B2 - "Efficient gallium thin film electroplating methods and chemistries":

... Gallium is a difficult metal to deposit without excessive hydrogen generation on the cathode because Ga plating potential is high. Hydrogen generation on the cathode causes the deposition efficiency to be less than 100% because some of the deposition current gets used on forming the hydrogen gas, rather than the Ga film on the substrate or cathode. Hydrogen generation and evolution also causes poor morphology and micro defects on the depositing films due to the tiny hydrogen bubbles sticking to the surface of the depositing film, masking the micro-area under them, and therefore impeding deposit on that micro-area. This causes micro-regions with less than optimum amount of Ga in the film stack.

Poor plating efficiencies inherently reduce the repeatability of an electrodeposition process because hydrogen generation phenomenon itself is a strong function of many factors including impurities in the electrolyte, deposition current densities, small changes on the morphology or chemistry of the substrate surface, temperature, mass transfer etc. As at least one of these factors may change from run to run, hydrogen generation rate may also change, changing the deposition efficiency.

Electrodeposition of Ga out of low pH aqueous electrolytes or solutions may suffer from low cathodic efficiencies arising from the presence of a large concentration of H+ species in such electrolytes. Therefore, hydrogen gas generation may be expected to lessen at higher pH values. However, as the pH is increased in the solution, Ga forms oxides and hydroxides which may precipitate as reported in the literature. Only at extremely alkaline pH values these oxides/hydroxides dissolve as soluble Ga species. Therefore, it becomes possible to electrodeposit Ga in a bath of pH>14 containing Ga salts as was done in prior-art techniques using high concentrations of KOH and NaOH in the bath formulation.

High concentrations of alkaline species, however, cause corrosion problems for the equipment as well as the cathode material itself. There is also a limit of the Ga amount that can be dissolved in the form of acidic Ga salts (GaCl3, Ga(NO3)3 etc) in such solutions before Ga starts to precipitate. Therefore, the pH needs to be adjusted again by further addition of alkaline species such as NaOH and KOH.

As pointed out above, solutions comprising a large molar amount of caustics are difficult to handle and they also have high viscosity. High viscosity makes the hydrogen bubbles formed on the cathode stick more to the cathode making it very difficult to remove them by stirring or other means of mass transfer. As explained above, such gas bubbles on the cathode surface increase defectivity of the deposited Ga layer.

...

• US8545689B2 - "Gallium electrodeposition processes and chemistries" and

• WO2012028416A2 - "Gallium and indium electrodeposition processes":

"... Current processes for electroplating metal containing thin films such as gallium and gallium alloys present numerous problems. These problems include, among others, low cathodic deposition efficiency due to excessive hydrogen generation, poor repeatability of the process, partly due to the poor cathodic efficiency, and the poor quality of the deposited films such as their high surface roughness and poor morphology.

Gallium is generally considered a difficult metal to deposit without excessive hydrogen generation on the cathode because gallium plating potential is relatively high. Hydrogen generation on the cathode causes the deposition efficiency to be less than 100% because some of the deposition current gets used on forming the hydrogen gas rather than for forming the gallium film on the substrate or cathode. Moreover, hydrogen generation and evolution is a causal factor for introducing porosity into the deposited film, thereby contributing to increased surface roughness and microdefects.

The plating efficiencies inherently reduce the repeatability of an electrodeposition process because hydrogen generation itself is a strong function of many factors including impurities in the electrolyte, deposition current density, small changes on the morphology or chemistry of the substrate surface, temperature, mass transfer, and the like.

...

Summary:

The present invention is generally directed to solutions and methods for electrodeposition of a substrate. In one aspect, the solution for electrodeposition of the metal containing film comprises a metal salt, wherein the metal is selected from the group consisting of gallium, indium, a combination thereof, and a combination of any of the preceding with copper; an acid selected from the group consisting of an alkane acid, an alkene acid, an aryl acid, a heterocyclic acid, an aromatic sulfonic acid, an aromatic sulfuric acid, hydrochloric acid, perchloric acid, and nitric acid; optionally a metalloid compound wherein the metalloid compound is selected from the group consisting of arsenic, antimony, bismuth, a combination thereof; and a solvent to dissolve said metal salt, wherein the pH of the solution is in a range selected from the group consisting of from about zero to about 2.6 and greater than about 12.6 to about 14, and wherein the solution is free of a complexing agent.

The method for electrodepositing a substrate comprises contacting

(i) a solution comprising a metal salt, wherein the metal is selected from the group consisting of gallium, indium, a combination thereof, and a combination of any of the preceding with copper; optionally an inorganic metalloid compound; further optionally an organic additive having at least one sulfur atom; and a solvent to dissolve said metal salt, wherein the solution is free of a complexing agent; and

(ii) a substrate; adjusting pH of the solution to a range selected from the group consisting of from about zero to about 2.6 and greater than about 12.6 to about 14; and applying a current to electroplate the substrate with a metal containing film.

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