Response of medium and fast diffusers to adapted high- high-temperature processes

In the following, some excerpts from the collaboration with Matthias Blazek in the frame of his diploma thesis, supervised by the author, are presented; for more details, the reader is referred to [38].

Intrinsic gettering based on re-precipitation of transition metals is only useful after the application of high-temperature steps like emitter diffusion. The driving force of the re-precipitation is the decreasing metal solubility in the silicon bulk with decreasing sample temperature. However, since it takes time for metal clusters to nucleate and grow, the precipitation is only successful if an optimal balance between solid solubility and diffusiv-ity, both determined by the temperature, is reached.

To elucidate the microscopic evolution of the plethora of transition metals which are pre-sent in standard mc-Si wafers, we set up two sets of experiments:

In a first approach, the macroscopic influence of differently designed high-temperature steps was investigated on the carrier lifetime of mc-Si wafers which had been intention-ally contaminated in the silicon melt with only one element. Thus, the wafer properties were dominated by the behavior of only one metal species; by applying the same proc-esses, the reaction of each metal could be specified.

Secondly, in the next section, the microscopic evolution of individual precipitates during different high-temperature steps is presented by means of synchrotron measurements.

Figure 5.8: Design of the high-temperature processes T1 - T3.

The wafers for the first investigation were neighboring samples taken from the middle of the ingots “Fe mc” (contaminated with 200 ppmw Fe), “Cr mc” (40 ppmw Cr) and “Ni mc”

(40 ppmw Ni), all manufactured for the research project CrystalClear at SINTEF, Norway.

While Fe and Cr belong to the intermediate diffusers in silicon, Ni diffuses very fast and possesses a high solubility.

At first, the wafers (format 125 x 125 mm²) were cut into pieces of 50 x 50 mm², cleaned and chemically polished³³. Two samples per ingot were set apart to be used as

“as-grown” reference for the following study.

The remainder of the samples was divided into four groups, each of which contained two wafers from each ingot. The first group was phosphorus-diffused at 825°C for 60 min., aiming at a sheet resistivity of 80 Ω/sq. Afterwards, the emitter was etched back³⁴.

To the other three groups, different high temperature treatments were applied. The tem-perature-time profiles of these processes T1, T2 and T3 are sketched in Figure 5.8.

Ramps T1 and T2 started with a high-temperature step at 900°C for 10 min., intended to imitate the contact firing at the end of the solar cell process³⁵, followed either by a fast ramping down to 550°C and quick extraction from the furnace, or a low-temperature an-neal for 90 min. at 600°C and subsequent extraction at the same temperature. While profile T1 imitates standard processes without intrinsic gettering, profile T2 is supposed to show the improvement due to metallic re-precipitation. Alternatively, temperature ramp T3 starts for 60 minutes at 760°C, continued with the low-temperature anneal at 600°C for 90 min. This step is intended to minimize the precipitate dissolution and maximize the intrinsic gettering without losing the capability to permit emitter diffusion.

These high-temperature steps were applied to the differently contaminated wafers at the same time under constant Argon flow in a cleanroom tube furnace. Thus, it was ensured that the processing conditions were exactly the same for each group.

After extraction, all samples (including the as-grown wafers and the P-diffused samples) were chemically cleaned and surface passivated with a high quality SiN coating on both sides. The minority carrier lifetime was measured by means of micro-photoconductance decay (µ-PCD) scanning and by QSSPC³⁶.

The results of the spatially resolved examination via µ-PCD are displayed in Figure 5.9 and the average values of the QSSPC lifetime of the two simultaneously processed sam-ples from the same ingot are summarized in Figure 5.10.

33 Cutting by M. Schwarzkopf, cleaning, polishing and emitter etch-back by M. Kwiatkowska.

34 P-diffusion and the high-temperature steps were done with the help of H. Lautenschlager.

35 The dedicated furnace used for these experiments is not capable of ramping up and down as fast as usual contact firing schemes. Nevertheless, the initial short high temperature successfully dissolves some of the trapped impurities, imitating usual high-temperature steps.

36 Measurements performed by M. Blazek.

Figure 5.9: Minority carrier lifetime maps obtained with the help of µ-photoconductance decay (µ-PCD) measurements on 50 x 50 mm² samples from ingots “Fe mc”, “Cr mc”

and “Ni mc”. The scaling of each image starts at 0 µs while the maximum value is de-noted in the corner at the bottom to the right.

Figure 5.10: Carrier lifetime measured via QSSPC at an injection density of 1x10¹⁵ cm^-3 on mc-Si samples taken from the middle of the intentionally contaminated ingots “Fe mc”, “Cr mc” and “Ni mc”, to which different temperature processes had been applied.

For explanation, see text.

The three transition metals exhibit three different responses to our temperature treat-ments. At the beginning, both the Fe and Cr wafers have very low carrier lifetimes <1 µs, while the Ni contaminated sample shows an extraordinarily high value above 50 µs in spite of the contamination. Again, this finding can be explained by the formation of large Ni precipitates under the optimal conditions (for Ni) during the cool-down after crystalli-zation, for which we also obtained indications in the synchrotron measurements de-scribed in section 5.2.1 (see Figure 5.5) which had been carried out on samples from the same ingot. On the other hand, our observations suggest that Fe and Cr have not had the chance to accumulate during cool-down, thus being relatively homogeneously distrib-uted. While the Fe-contaminated sample in the as-grown state shows signs of intrinsic gettering (the carrier lifetime at crystal defects is higher than in the grains), the Cr-contaminated sample hardly does. The reason may once more be found in the diffusivity and the solubility of Cr, which are both slightly below the Fe-values, hampering the pre-cipitation process.

After the short application of the 900°C step (profile T1), the picture changes completely:

While the Cr-sample experiences a noticeable lifetime enhancement, the carrier lifetime of both Fe and Ni samples suffers due to the significant metal dissolution in the silicon bulk without having the possibility to re-precipitate. In fact, we have made the experi-ence that intrinsic gettering of Cr is not a very delicate process – almost any high tem-perature leads to a lifetime increase from the admittedly very low initial values in mc-Si material [128].

The low-temperature anneal of the following step T2 at 600°C especially assists in the re-precipitation of Fe, which is in good accordance with results presented in literature [127].

The Cr-wafer also profits from the anneal, while the Ni sample shows a very low lifetime similar to the outcome of ramp T1. This indicates that the Ni out-diffusion during the high

temperature cannot be completely reversed within the low-temperature anneal for 90 minutes.

The temperature profile T3 presents the best option for intrinsic gettering of both Fe and Cr. In addition, the carrier lifetime of the Ni samples is not as deteriorated as after steps T1 and T2. The long, relatively low temperature treatment seems to keep the dissolution of all transition metals at an acceptable level while the subsequent low-temperature an-neal leads to intrinsic gettering as intended.

For comparison, the potential of external gettering without making use of precipitation is depicted in the result of the P-diffused wafers. The carrier lifetimes of all wafers doubles that of the best intrinsic gettering step T3. Nevertheless, the success of step T3 espe-cially for the intermediate diffusers Fe and Cr is considerable. Only the re-precipitation of Ni is difficult to enforce which is due to its high solubility in silicon.

To summarize, the three transition metals respond very differently to the high tempera-ture treatment. While the intrinsic gettering of the medium diffusers Fe and Cr can be relatively well controlled with the help of adapted temperature-time profiles, the Ni atoms tend to exhibit a contrary behavior, which would make it difficult to bring about a re-precipitation of all transition metals at the same time (e.g. when a multicrystalline silicon material contained a noticeable amount of each of them).

Therefore, the dissolution and re-precipitation of Ni clusters is examined in more detail in microscopic analyses of individual precipitate evolution with the help of synchrotron measurements in the following section.

5.3.2 Assessment of the Ni atom redistribution during high