Figure 2 shows SEM cross-sections of the three barrier materials as deposited. For all materials investigated, a homogeneous coverage with a thickness of 50 nm was achieved. The W–Ti layer showed maximal roughness of R
a = 2.5 nm. The other layers are characterized by smaller R
a values being below the sensitivity of the profilometer used. Figure 1a shows the TaN barrier deposited, for which polycrystalline structure could be identified from SEM. The microstructure of TiN is depicted in Fig. 1b whereas the W–Ti alloy layer (Fig. 1c) shows a more pronounced columnar structure. Such columnar microstructures might provide efficient pathways for the lithium ions near the grain boundaries deteriorating the respective properties of the barrier.
The surface stress of the barrier layers was determined by wafer bow measurements and evaluated by means of the Stoney equation as mentioned above. Measurements of the stress of the layers lead to pressure values ranging from +3.2 × 108 to −1.9 × 109 Pa (see Table 4). High positive (tensile stress) or negative (compressive stress) values indicate a high concentration of dislocations or defects introduced during the deposition process [18]. Consequently, this might lead to reduced performance of the barrier. Whereas W–Ti and TiN showed compressive stress, TaN exhibited values pointing to tensile stress.
Table 4 Summary of the structural and mechanical properties of the barrier layers tested
Cyclic voltammetry
To determine the electrochemical activity of the barrier materials cyclic voltammetry (Fig. 3) measurements between 50 mV and 1.5 V versus Li/Li+ were carried out. The two refractory metal nitrides (cf. the curves drawn in black and blue color) showed no distinct peaks up to 1.5 V. This points to a sufficiently high electrochemical stability of the corresponding surfaces. On the other hand, for the tungsten titanium alloy (see the red curve in Fig. 3) two shallow reduction peaks were detected. As no corresponding oxidation reaction can be determined at the step speed of 30 µV s−1 applied, a CV measurement with increased scan rate was performed: peaks showed up at 0.37 and 0.65 V, respectively; moreover, an oxidation peak was detectable. In contrast to the nitride, the pure metallic tantalum revealed two pronounced reduction peaks and a broad oxidation peak directly pointing to lithiation and de-lithiation of the barrier. It is worth to mention that in both cases the voltage difference between oxidation and reduction peak is relatively large pointing to slow charge transfer kinetics; the peak current ratio, being much larger than one, as well as the dependence of the peak current on the scan rate points to irreversible processes taking place [19].
Charging/discharging
To evaluate the efficiency of the barrier materials to block Li ion migration across the different layers the materials were subjected to charge/discharge tests in Si-based batteries. We used the standardized battery cycling program mentioned above. The capacities referring to charging and discharging the cells were recorded as a function of cycle number. Figure 4 shows the change of the current (a) and the voltage (b) of the test cells (see step 8 of Table 2). At a process time of 121 h a charging current of 712 µA was applied with a maximum allowed decrease in voltage of 20 mV. For all samples the limit in current could not be reached and the 20 mV threshold applied limits the corresponding currents. After another 6 h the discharging step was started with the potential limited to 1 V. The silicon sample remained at this potential for several hours before relaxation to the open circuit voltage (OCV) took place. Although the samples with the barriers revealed very small voltage drops, their electrical relaxation behavior turned out to be differently. After the final charge/discharge cycle a 24-h rest step was applied after which all samples reached their OCVs.
Figure 5 shows the charge and the discharge capacities of the cells with the barriers as well as those of the reference cell without any barrier. The (charge) capacities directly correspond to the efficiency of the barrier materials tested. The lower the capacities measured the more efficient the barrier is, i.e., the more effectively the barrier blocks Li ion transfer. The reference cell without any barrier layer (Si, see right axis of Fig. 5) served to illustrate the blocking effect of the barriers. The cell having no barrier inside shows a continuous increase in capacity during cycling finally reaching values of ca. 235 µAh cm−2 (cycle no. 10). Upon discharge (see the dotted line) a very similar behavior is seen although the corresponding capacities are slightly lower (216 µAh cm−2). This difference can be explained by a partially irreversible uptake of Li ions by silicon as well as the formation of the passivating SEI [20]. Mechanical destruction, i.e., detachment of lithiated silicon could also lead to irreversible capacity losses. All cells equipped with barriers showed charging capacities below 9 × 10−3 µAh cm−2 (see left axis in Fig. 5), i.e., almost two orders of magnitude lower than the charge capacity of the cell without any barrier. Generally speaking, expect for the W–Ti and Ta system the barriers are able to block Li ion transport but there are noteworthy differences.
Despite its columnar microstructure the best performance in terms of any ability to block Li ion transfer was observed for the TiN barrier (see Fig. 2b). In the first cycle the capacity observed under charging conditions shows a larger irreversible value as compared to the other cycles. This behavior can be explained by the formation of a passivating layer that consumes some electrolyte, conductive salt and, thus, lithium ions. It is, thus, not necessarily related to the efficiency of the barrier. Most likely, the composition of the layer formed cannot be compared with the SEI at Si surfaces [21]. The corresponding discharge curve of the cell equipped with TiN reveals a capacity being lower than 1 × 10−3 µAh cm−2; because of the restrictions in sensitivity of the test equipment used; the limit was 75 nA, thus, very low currents cannot be determined with high precision. The capacity of a TiN layer is supposed to depend linearly on the layer thickness, for TiN values of 0.02 Li per formula were calculated [14]. Here, considering the discharge capacity experimentally a value of 0.002 Li atoms per TiN formula unit was found which is significantly lower than the calculated result (see Table 5). The value corresponding to the charge capacity turned out to be only slightly higher (0.003 Li). The difference points to some irreversible loss of Li at the barrier. These ions might reduce the efficiency of the blocking layer.
Table 5 Charge and discharge capacities (average values) and Li uptake per formula unit
Cells with Ta and TaN show higher capacities while the discharge capacities are, at least for TaN, again below the detection limit. Once again some amount of Li was irreversibly consumed during charging. In both cases the charge capacity slightly increases when the cells are cycled for more than three times. We found that during charging the Li uptake of Ta is ca. 0.09 Li, this value is in good agreement with literature data [14]. During discharging the value reduces to 0.02 Li per Ta. Thus, we assume that the charging-discharging procedure is only partially reversible affecting the overall efficiency of the barrier. For TaN, values of 0.05 Li per TaN (charging) and 0.009 Li (discharging) were found.
Compared to the other barrier layers W–Ti shows the highest charge and discharge capacities. Also in this case an irreversible redox reaction took place. Up to 25% of the charge carriers can be reversibly stored in the cell with a W–Ti barrier. Therefore, W–Ti seems to be an inadequate barrier in lithium-ion batteries; the uptakes for Li per formula unit are 0.15 (charging) and 0.04 (discharging), respectively.
Mass Spectrometry. Time of flight-secondary ion mass spectroscopy is a powerful tool to determine diffusion profiles in thin (<1 µm) layers. Most importantly, measurement artifacts have to be carefully excluded. For example, the roughness of the surface may broaden interfaces. Another more problematic measurement failure is the so-called tailing caused by ion mixing during sputtering [22]. To distinguish which profile originates from diffusion and which one stems from ion mixing comparative measurements on samples with a non-diffusive component are necessary. An inherent behavior of a silicon surface being in contact with an electrolyte is the SEI formation. The SEI is a mixture of organic and inorganic components. Especially for organic components diffusion into the layer material can be regarded as negligible. Therefore, such components can be used as an internal reference to study and to rule out the influence of ion mixing.
There is still another artefact known for its high impact for measurements of positively charged secondary ions. Because of the positive charge of the primary ions the analyte is pushed deeper into the bulk material. In particular, this is the case when one deals with an electric isolator, as TaN is. An increasing signal at the subsequent interface gives a hint to this phenomenon. In the next section we will present the results of ToF SIMS experiments on samples which were electrochemically charged and discharged for 10 cycles.
In the case of TaN (see Fig. 6a) a high intensity for Li+ and CHF+, which represents the SEI layer, can be found at the beginning, see the sputter time up to 40 s. While the Si signal is negligible the intensity for the barrier layer Ta+ continuously increases and reaches a plateau at 180 s. A sputter period of approximately 140 s corresponds to the 50-nm thick TaN layer. In general, each material each material behaves differently which makes a correct depth calibration almost impossible.
Note that the Li+ intensity remains on a relatively high level in the region of the TaN barrier; this behavior has, most probably, ascribed to the artefact described above and seems to be caused by the low conductivity of TaN. A slight peak at the silicon interface can be noticed before the Li+ signal decreases to the noise level. The slope of the Li+ signal at about 50 s is very similar to that of the signal corresponding to the SEI, which reached the noise level shortly after the Ta signal has reached its plateau regime. The observations indicates that the barrier material seems to effectively block Li ion transport. Thus, ToF SIMS indeed gives evidence that TaN might be used as a suitable barrier.
For the TiN barrier (see Fig. 6c) we observed a sharp drop of both the Li+ signal and the SEI signal after ca. 30 s; simultaneously a rise of the Ti+ signal is seen. This drop points to a narrow interface region. The Li+ level in the barrier is significantly lower compared to the situation seen for TaN. Most likely, this difference has to be ascribed to the different conductivities of the two barrier materials; the electrical resistance is 20 µΩcm for TiN and 135 µΩcm for TaN, respectively. Based on the ToF-SIMS traces we identify both TaN and TiN, in particular, as relatively dense barrier materials.
For W–Ti (see Fig. 6b) the ToF-SIMS signal for Li+ behaves quite different compared to tantalum nitride (Fig. 6a) and titanium nitride (Fig. 6c). We noticed that the signal does not follow the SEI signal in shape; in particular, the slopes of the two signals differ at the beginning. Most importantly, the Li+ signal does not reach the noise level neither in the barrier nor in the region of bulk silicon.
For the Ta layer our CV experiments indicate that Ta reacts with Li. After the electrochemical test procedure the surface showed planar dendrites. The visualization of the ion signal of our ToF SIMS experiments (cf. Fig. 7a) also reveals that tantalum electrochemically reacts with Si as a high silicon concentration could be determined. From the corresponding ToF-SIMS depth profile shown in Fig. 7b it can, thus, be concluded that Ta is not a suitable barrier since it reacts with both parts of the diffusion couple.
To quantify Li in-diffusion for the W–Ti layer in Fig. 8 the smoothed Li profile determined via ToF-SIMS (see black line) is plotted. For comparison, the profiles obtained by approximating the signal with a Gaussian solution of Fick′s second law (see red line) as well as the complementary error function (see blue line) is also shown. As expected, the experimentally probed Li ion diffusion profile cannot be approximated by these functions properly. They represent profiles with ideal boundary conditions. Several origins may explain the deviation from, e.g., an ideal Gaussian profile. For instance, the ToF-SIMS measurement itself might lead to a distorted shape of the signal as mentioned above. Moreover, the charge/discharge loop used to simulate battery conditions is another source of trouble. During each discharge step Li ions are pulled back leaving areas depleted in Li behind. Hence, the Li profile becomes steeper as one would expect for a pure bulk diffusion process. The fits shown in Fig. 8 were used to roughly estimate the underlying diffusion coefficients. The two functions shown are characterized by the following values D = 1.6 × 10−19 cm2 s−1 (error function) and D = 2.2 × 10−19 cm2 s−1 (Gaussian function), respectively. Taken together, in contrast to TaN and TiN the results for W–Ti clearly show that it is not a suitable barrier to block lithium ion migration in silicon-based microbatteries. For comparison, Li self-diffusion in amorphous Li1.5Si prepared electrochemically has to be characterized by D values in the order of 10−9 cm2 s− 1 [23].