The precipitation of red Se seems to be an unavoidable side reaction of indium selenide plating under the given conditions. Its quantification allows to verify the calculation in section 10.2.3 and to determine its importance in the electrodeposition process of indium selenide. The precipitation might not only be a Se4+ loss mechanism that limits the lifetime of the deposition bath but could
also effect the sample composition, if its particles can be incorporated in the deposited film.
Although the precipitate is formed by the comproportionation reaction 8.3, it requires the presence of an electrode to generate the necessary Se2− ions. Nine indium selenide samples have been plated
from a (9.47;1) bath at −0.60 V and 400 rpm RDE rotation speed. After each deposition a sample of the electrolyte has been taken. In order to study a possible incorporation of Se precipitate particles in the sample, the complete bath was centrifuged after the deposition of the 6th sample. The centrifuged red particles have been rinsed with water and afterwards analysed in ICP-MS (details of the experimental procedure in section 6.4) .
The atomic Se/In ratio of the precipitate is 4.6 ± 0.1. If the precipitate consists of In2Se3+Se like
the deposit it contains 55% (atomic) Se and 45% In2Se3. If In2Se3 is present it does not necessarily
have to form at the electrode, but could be from a slow reaction with In3+ ions in the bath. It
is also likely that residues of the bath stayed despite the washing of the precipitate and cause the indium contamination, because only about 100 µg of precipitate have been analysed. Therefore it is assumed that the In content in the precipitate does not influence the calculations in section 10.2.3. The results of the bath and sample composition are shown in figure 10.9. The measured Se concentration is close to the calculated value. Therefore most of the Se is plated on the sample and Se precipitation is only a small perturbation to it. Since the atomic In3+ concentration is ten times
higher than for Se4+ it is nearly constant throughout the depositions. The slight increase in the
measured In3+concentration can result from some evaporation of electrolyte. The deviation of Se4+
from its calculated concentration can be used to estimate the amount of Se precipitation, because it is the only Se loss that has not been considered in the calculation. After the 6th sample the measured Se4+ concentration is 4.9 ± 1.2 mg/l below the calculated value. Considering a constant
Se precipitation for all samples this results in 212 ± 52 µg Se precipitate per sample. This result is in the same order of magnitude as it has been predicted by the calculation in table 10.2. Since the table does not include a sample deposited at −0.60 V from a (9.47;1) bath, it can be assumed that its value is between the (9.47;1) / −0.70 V sample with 400 µg Se precipitate and the (3.79;1) / −0.60 V sample with 180 µg Se precipitate. Despite the aggregating amount of Se precipitate the plated sample does not show significantly more Se until the 6th sample which conflicts with a possible incorporation of Se precipitate in the bath. Instead the Se content rises after the Se precipitate has been removed from the bath. It could be another indication of the effect of an increasing Se content in the sample with decreasing relative Se concentration in the bath. For the last sample the bath concentration has already changed from the initial (11.1; 1.1) bath7 to a (11.8; 0.6) bath. On the
7The (11.1; 1.1) composition has been determined by ICP-MS. This bath was designed to be a (9.47; 1) bath.
The discrepancy can be explained by the hygroscopic property of H2SeO3 and a wrong coordination number of
10.2. Results and Discussion
Figure 10.9.: Series of indium selenide samples deposited from a (9.47; 1) In-Se bath at −0.60 V, 400 rpm, and 80 °C. Figure a) records the Se (squares) and In (triangles) concentrations in the bath after deposition of the sample noted on the x-axis. The open symbols are values calculated from the mass balance of material in the bath. The solid symbols have been measured by ICP-MS. The indium concentrations have been divided by 10 to fit on the same scale. The datapoint “bath1” is the fresh bath before samples were plated; “bath2” has been measured after Se precipitate has been removed by centrifuging and electrolyte loss has been replaced with fresh electrolyte. Figure b) notes the atomic Se/In ratio of the electrodeposited films measured by ICP-MS.
other hand the removal of Se precipitate by centrifuging has been a considerable disturbance to the bath, because it had to be cooled down to room temperature, was refilled several times and heated up again. This could explain the unexpected behaviour of increasing Se concentration after deposition of the 7th sample. Therefore the data points after precipitate removal must be interpreted with care. The constant composition of the film until the 6th sample is still not a proof that particles of Se precipitate are not incorporated in the sample. Adsorbed particles could further react with In3+
ions to In2Se3 and would not change the film composition. The question of particle incorporation
is of practical interest, e.g. Hirono plated indium layers in a suspension of fine Se powder. The resulting film contained 35% (at) Se [120]. In our case the visual appearance of the obtained films changed during the series. The samples got darker from the 1st to the 6th run indicating a change in surface morphology. After removing the Se precipitate sample 7 appeared light grey again. The surface morphology can be identified in the SEM pictures in figure 10.10. Sample 6 has a loose
concentration measured by ICP-MS can be explained if the actual coordination number of the used indium sulphate is smaller.
10. Electrochemistry of Indium Selenide
surface structure with pronounced round particles as they are expected, if Se precipitate adsorbs to the surface. The morphology of the 1st and 7th sample is more compact but also in this case particles are visible. EDX maps with an electron high voltage of 7 kV were recorded to determine the composition of the top 150 nm in the indium selenide layer [53]. Sample 6 in figure 10.10 shows an increased Se content in the EDX map at the spots where the round particles are located. Sample 7 that has been plated after the removal of Se precipitate shows a uniform composition of In2Se3
instead. Therefore the spots of Se concentration in sample 6 are not a quantification artifact of the rough morphology but a real indication for the adsorption of Se particles on the surface.
Figure 10.10.: SEM pictures and EDX composition maps of chosen indium selenide samples from the plating series in figure 10.9. The column label corresponds to the sample index in figure 10.9. The first row shows SEM pictures before the EDX mapping, the last row shows the same spot after the EDX measurement. Although a reduced electron voltage of 7 kV has been applied to record the EDX map, parts of the surface start to evaporate. The EDX maps show the atomic composition of Se and In.
It is surprising that the first sample also shows variations in the surface composition, because the amount of Se precipitate is low at the beginning of the series. This indicates that the particles are perhaps incorporated at the moment when they are formed near the surface and the precipitate in the bath does not longer contribute. In this case the absence of Se particles in sample 7 is not
10.2. Results and Discussion
caused by the removal of precipitate from the bath. It is possible that the storage of the bath and the removal procedure of the precipitate have changed the formation kinetics of the precipitate.
In summary it could be shown that Se particles can be adsorbed to the sample surface. Nevertheless the total composition of the plated film is not effected. It means either the particles only stick to the surface and are not incorporated in the indium selenide film, or if they are incorporated, they have previously reacted with In3+ to indium selenide. Additionally it has been shown that due to the low
bath concentration of Se4+ its composition is changed during plating. This depletion can effect the