In general, electrochemical compound or alloy deposition is very complex, involving thermodynamic and kinetic problems. A special case is the so called underpotential deposition. Here, the less noble constituent of the alloy is deposited at potentials more positive than its standard reduction potential. The energy is brought about by the free energy of compound formation. For ZnSe a plating range can be calculated (at standard activities, potentials vs. NHE):

Zn2+	+	2 e -			--->	Zn			E0 = -0.76 V	(1)
H2SeO3	+	4 H+	+	4 e-	--->	Se	+	3 H2O	E0 = 0.74 V	(2)
Zn	+	Se			--->	ZnSe			dG = -163 kJ/mol	(3)
Zn2+	+	Se	+	2 e-	--->	ZnSe			Edep = -0.10 ...... -0.76 V	(4)

Within the potential range of -0.1 to -0.76 V the deposition of ZnSe as single phase with different Zn activities is possible. Outside this range an additional phase (Zn or Te / Se) would be formed. Futhermore, within that range Zn deposition should only be possible into the chalcogenide compound (eq.(4)), autocontrolling stoichiometry (an excess of Zn is rather unlikely).
However, as a result of the rather negative standard reduction potential of Zn²⁺/Zn complications may arise leading to H₂ as well as to H₂Se/HSe^- formation. According to the Pourbaix-like diagram (Fig.1) the decomposition of ZnSe into Zn and H₂Se (HSe^- at pH > 4.75) should not take place at potentials more positive than -1.2 V (pH 0). Furthermore we have to consider the formation of H₂ and the reduction of Se once formed under plating conditions. The formation of hydrogen is thermodynamically possible throughout the deposition range of ZnSe. However, most of the substrates used in photovoltaic (or light emitting devices and laser) have a relatively high hydrogen overpotential (kinetic hindrance) and the rate of H₂ formation is rather small. Nevertheless it is an unwanted side reaction leading to a low Faraday efficiency and furthermore effecting film quality (e.g. porosity).
Due to the fact that the Se reduction not only falls within the ZnSe stability range but is also possible at more positive potentials than the reduction of Zn²⁺/Zn and furthermore that the reduction of HSeO₃^- directly to HSe^- is possible at potentials more positive than the beginning of ZnSe formation, HSe^- can be formed within the ZnSe plating range. In solution, HSe^- can react with Zn²⁺ and/or HSeO₃^- :

HSe-	+	Zn2+		--->	ZnSe	+	H+	(5)
2 HSe-	+	HSeO3-	+	3 H+	--->	3 Se	+	3 H2O	(6)

Se and ZnSe so formed in solution can precipitate on the electrode. During real plating very frequently an excess of Se in the films is encountered. Since the Se reduction to HSe^- / H₂Se falls within that range, a stripping of excess Se in the ZnSe films should be possible. This is in good agreement with plating experiments, where the best stoichiometry is obtained at deposition potentials close to -0.8 V where the Zn formation starts leading to an excess of Zn (E < -0.8 V).
As seen during plating experiments, we further have to consider zinc formation as an additional phase once the standard reduction potential of Zn²⁺/Zn is reached (at -0.76 V). Of course, already formed ZnSe would not decompose into Zn and Se. In the case of diffusion controlled electrodeposition with D_[HSeO3]^- = D_[Zn²⁺] (thus i_Se = i_Zn) and (infinite) fast formation of ZnSe out of the elements Zn and Se, only ZnSe would be formed until -1.2 V.

However, in real plating beside thermodynamics also kinetic factors such as adsorption, crystal growth, etc. play an important role. The most prominent case is the electrodeposition of CdTe. Here, the adsorption of Cd²⁺ plays an important role. It causes the growth of smooth films of stoichiometric CdTe over a wide range of deposition parameters (precursor concentration, pH, Deposition potential). Without the addition of 'brighteners' mirror like surfaces are obtained. Thus the autoregulative effect of Cd²⁺ adsorption is very important. In contrast, electrodeposition of ZnSe is more difficult (than the respective Cd chalcogenide semiconductors) and the possible range of deposition parameters is much more limited. At the same time Zn²⁺ adsorption is less pronounced.
As already outlined before, the real plating range for producing stoichiometric crystalline thin films of ZnSe is much more limited than the range given by the Pourbaix - like diagram (Fig.1).
The plating potential range suitable for stoichiometric ZnSe deposition is close to the beginning of Zn formation (at -0.8 V). Beside the problems with the weak Zn²⁺ adsorption, additionaly the formation of Se (by reaction (6)) limits the possible potentials for plating ZnSe. A rather high pH (5.5) and lower deposition potential (-0.95 V vs. Ag/AgCl) is required for forming ZnSe films that are close to stoichio-metric composition but still having an excess of Se (52 at%). From Fig 2. a general trend can be deduced: a higher deposition potential results in a higher Zn content in the ZnSe film, which is in general agreement with what is expected from the analysis of the Pourbaix-like diagramm (Fig.1).
Whereas electrodeposited CdTe films exhibit a shiny, mirrorlike surface, as deposited thick ZnSe films had a fairly light scattering surface. Therefore, a leveling effect caused by metal salt adsorption seems to be absent in the case of ZnSe. The addition of NTA to the plating solution did improve the surface characteristics of ZnSe films, but also led to the reduction of the SnO₂ (TCO substrate). The adsorption of Zn²⁺ (Cd²⁺ in the case of CdTe / CdSe) seems to be vital for obtaining stoichiometric films. If adsorption sites were occupied by other species (i.e. HSeO₃^- or the brightener) the rate of Zn²⁺ reduction was decreased compared to that of SeO₂ reduction thus Se clusters could be formed. It seems to be very likely that once the Se clusters have reached a certain size, they react only very slowly to give ZnSe.
Typical transmission an reflection spectra of an electrodeposited ZnSe film are given in Fig.3:

As mentioned above, thicker films are fairly light scattering. However, thin films are sufficiently smooth to give an interference pattern. Although the latter somewhat obscures the bandgap absorption, a value around 2.8 eV can be found from the spectra.

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