Comparison on low excess carrier lifetime samples

In order to obtain silicon samples with lower carrier lifetimes, two samples were made of Czochralski (Cz) material (degraded state), one out of high quality Cz and the other

one of industrial grade Cz material. Both samples had a resistivity of 1.3 Ω cm and were passivated with a 70 nm PECVD SiN_x layer.

Fig. 11.4: Effective excess carrier lifetimes of a 1.3 Ω cm p-type high quality Cz sample measured using the four different carrier lifetime measurement techniques.

Fig. 11.5: Effective excess carrier lifetimes of a 1.3 Ω cm p-type industrial grade Cz sample measured using the four different carrier lifetime measurement techniques. The QSS-PL data have been calibrated based on the measured QSS-PC data.

For the high-quality Cz sample (Fig. 11.4) the QSS-PC and QSS-PL technique again agree very well, covering a broad injection range of approximately seven orders of magnitude. The TR-PC data deviate significantly from these data, especially at lower

11.1 Photoconductance vs. Photoluminescence 143

injection densities and should not be used with such lower lifetime samples. This deviation can easily be understood since the flash time constant is not small enough in comparison to the measured carrier lifetime, rendering the transient approximation invalid. The µW-PCD data deviate slightly from the QSS-PC data, but the overall shape of the curve is maintained.

Below an injection density of approximately 5×10¹³ cm^-3 the QSS-PC and µW-PCD data seem to flatten out (probably due to trapping of minority carriers (see Chap. 4.5)), while the QSS-PL data continue to decrease by more than one magnitude due to the insufficient passivation quality of the used SiN_x layer at very low injection densities.

In Fig. 11.5 the measurement data for the lower lifetime industrial grade Cz material are presented. Again, the TR-PC data deviate significantly from the QSS-PC data, which in turn agree quite well with the µW-PCD data. It was not possible to calibrate the PL signal using the self-consistent method, which is shown in Fig. 11.6.

Fig. 11.6: Effective excess carrier lifetimes of the 1.3 Ω cm p-type industrial grade Cz sample showing the problems when trying to perform a self-consistent calibration of the QSS-PL data. The differences result from the limited bandwidth of the used preamplifier on the one hand and from the need to use high excitation frequencies on the other hand. To obtain a reliable calibration, the PL signal was calibrated based on the QSS-PC measurement data.

Due to the limited bandwidth of the used preamplifier for the PL signal, the PL measurement signal gets distorted, what can be seen from the different resulting curves for different settings for the used excitation frequency (f_SC) when performing the self-consistent calibration. One solution would be to lower the excitation frequency so that

the bandwidth of the preamplifier is not affecting the signal anymore. However, the excitation frequency cannot be lowered that much due to the fact that the sample has to be partly in transient mode for the self-consistent calibration to work. To avoid these problems, the PL signal was calibrated for this sample by comparing the calculated effective lifetimes with those measured by the QSS-PC method. This behavior has been observed with many other samples with carrier lifetimes below approximately 100 µs at an injection density of 1×10¹⁴ cm^-3.

Below an injection density of 1×10¹⁴ cm^-3 the measurement artifacts of the QSS-PC and the TR-PC due to trapping of minority carriers are obvious. The µW-PCD data are thought to be less prone to these artifacts due to the use of an external bias light, which drains the trap centers continuously. Since the QSS-PL is not affected by these artifacts in principle (see Chap. 4.5), the true effective carrier lifetimes can again be measured for very low injection densities.

Since the maximal reachable injection density for PL measurements is only limited by the used illumination source, substituting the used LED with an appropriate laser system would greatly improve the accuracy of the PL calibration, since it could be aligned to highly reliable PC data from the mid or high injection range.

11.1.3 Conclusion

Four different techniques for determining the effective excess carrier lifetime of silicon samples were investigated. The commonly used quasi-steady-state photoconductance (QSS-PC) and the microwave-detected photoconductance decay (µW-PCD) were compared in detail with the transient photoconductance (TR-PC) and the recently introduced quasi-steady-state photoluminescence (QSS-PL).

Five different samples, covering a large carrier lifetime range from several milliseconds to a few microseconds were used for this in-depth analysis.

For silicon samples with high carrier lifetimes (> 200 µs at 1×10¹⁴ cm^-3), all four investigated lifetime measurement techniques agree very well. Only for one sample the µW-PCD data deviated significantly. The combined data from the QSS-PC and (self-consistent calibrated) QSS-PL measurements resulted in a wide injection range of seven orders of magnitude to be accessible.

For silicon samples with lower carrier lifetimes (< 100 µs at 1×10¹⁴ cm^-3), the TR-PC measurement technique cannot be used due to the not fast enough decaying flash intensity, while again the others (QSS-PC, QSS-PL and µW-PCD) agree very well.

Care has to be taken when trying to do a self-consistent calibration of the PL signal on

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such samples, since the limited bandwidth of the preamplifier can disturb these measurements.

As had been observed before, the measurements artifacts that influence photoconductance-based lifetime measurements at low injection densities are not observed for the photoluminescence-based technique. This important advantage can be used in order to reach very low injection densities (< 1x10¹⁰ cm^-3) for a subsequent in-depth analysis of the injection-dependent carrier lifetime.